Crystalline oxides and semiconductors exhibit distinct yet complementary properties owing to their respective ionic and covalent natures. By electrically coupling oxides to semiconductors within epitaxial heterostructures, enhanced or novel functionalities beyond those of the constituent materials alone can potentially be realized. Key to electrically coupling crystalline oxides to semiconductors is controlling the physical and electronic structure at interfaces between the two materials. In this talk, we will discuss how stoichiometry can be exploited to control both physical and electronic structure at interfaces. Two prototypical interfaces, namely Ba_1-xSr_xTiO₃/ Ge and SrZr_xTi_1-xO₃/Ge, will be presented. In the case of Ba_1-xSr_xTiO₃/ Ge, we will discuss how strain can be engineered through stoichiometry to enable the re-orientable ferroelectric polarization of the former to be coupled to carriers in the latter [1]. In the case of SrZr_xTi_1-xO₃/Ge we will discuss how stoichiometry can be exploited to control the band offset at the interface [2]. Analogous to heterojunctions between III-V semiconducting materials, control of band offset, i.e. band-gap engineering, provides a pathway to electrically couple crystalline oxides to semiconductors to realize a host of material functionalities [1] J.H. Ngai et al. Appl. Phys. Lett. 104, 062905 (2014); [2] J. Moghadam et al. Adv. Mater. Interfaces 2, 1400497 (2015).

We use first principles calculations to model the surface adsorption and hydration of strontium bis(cyclopentadienyl) [Sr(Cp)2] for the deposition of strontium oxide, SrO, by atomic layer deposition (ALD). For initial calculations, we study the Sr(Cp)2 adsorption on a TiO2-terminted STO surface. The Sr(Cp)2 precursor is shown to adsorb on the TiO2-terminated surface, with the Sr atom sitting essentially where it should be in a bulk STO cell. Calculations show that the TiOnot;2 surface does not need to be hydrogenated precursor adsorption. We simulate a pulse of water by incrementally adding H and O atoms to the surface. The two H atoms bond to the Cp rings and cause them to detach from the surface. O placed on the surface prefers to be closest to the adsorbed Sr atom. The first principle calculations are compared with experimental observations for a Sr cyclopendienyl precursor, Sr(iPr3Cp)2 adsorbed onto the TiO2-terminated STO. High-resolution x-ray photoelectron spectroscopy shows adsorption of the Sr precursor on the TiO2-terminated STO after a single precursor dose. The adsorption of Sr on the TiO2-terminated STO surface is further confirmed by low-energy ion scattering spectroscopy (LEISS). This study suggests that Sr(Cp)2 precursors may be used for ALD growth on non-hydroxylated surfaces.

The interface between the band insulators SrTiO₃ (STO) and LaAlO₃ (LAO) is host to diverse phenomena including a quasi-two dimensional electron liquid (2DEL) with gate-tunable superconductivity, magnetism, and spin-orbit coupling. However, many basic questions remain about the mechanisms regulating conductivity switching and the role of the surface chemistry in dictating these properties. Here we report the observation of a fully reversible, >4 order of magnitude change in LAO/STO interface conductance in which nominally conductive interfaces are rendered insulating by solvent immersion and returned to a conductive state through exposure to light. Through coordinated electrical measurements and X-ray photoelectron spectroscopy, we determine that the metal-to-insulator transition is consistent with deprotonation of the hydroxylated LAO surface while reprotonation occurs via photocatalytic oxidation of adsorbed water. These observations suggest a strong connection between surface chemistry and 2DEL formation and point to a future route toward large-scale patterning of interfacial electronic devices.

SrTiO₃ is an archetypal functional metal oxide with defects that give rise to emergent properties that cannot be explained by the underlying perfect crystal. Electric conductivity, ferroelectricity, blue luminescence, and magnetism are all functional properties that can be tuned by generating and controlling ionic and electronic defects in SrTiO₃. In spite of the extensive theoretical and experimental work on the defects of this material, a comprehensive picture for its defect equilibria in a wide range of thermodynamic conditions spanned by temperature and anion and cation activities is still missing. In this work we provide a detailed study for all ionic and electronic defects of SrTiO₃. The formation energy of anion and cation vacancies, antisites on all sublattices, and small polarons are computed using density functional theory with an on-site Coulomb interaction term (DFT+U approach). In addition, using density functional perturbation theory (DFPT) and statistical thermodynamics, we account for the contribution of lattice vibrations (phonons) to the formation free energies of all defects. By imposing the charge neutrality condition, we construct defect equilibria diagrams that depict defect concentrations as a function of a thermodynamic force (temperature and/or chemical potential). The charged defects which deemed to be predominant under certain conditions, were selected for further examination of their electronic structure. In particular, we examine their local magnetic and electric dipole moments and relate these defect properties to the macroscopic properties of defective SrTiO₃. There has been several phenomenological observations related to SrTiO₃ that have been attributed tentatively to its defects. The framework we introduce in this study can be invoked to link each defect to the property that it causes.

The extraordinary wealth of physical properties in complex oxides offers a promising potential for developing new functionalities in devices that can address societal needs related to health, energy or information and communication technologies. Ferroelectrics are particularly attractive for their applications in nanoelectronics, communication devices, electro-mechanical systems and integrated photonics provided that they can be integrated on semiconductors. In this presentation, we will review our recent work on the molecular beam epitaxy of BaTiO₃ heterostructures on Si and strained-SiGe/Si substrates. Effects of process parameters on the microstructure, cationic composition, crystalline orientation and ferroelectricity will be discussed. The growth of nanostructures on patterned Si substrates will be presented. Finally, perspectives on integrating ferroelectric BaTiO₃ thin films in field-effect devices using a damascene scheme will be discussed.

Conducting characteristics of topological defects in ferroelectric materials, such as charged domain walls, engendered a broad interest on their scientific merit and the possibility of novel applications utilizing domain engineering. [1] At the same time, the problem of electron transport in ferroelectrics still remains full of unanswered questions, and becomes yet more relevant over the growing interest in ferroelectric semiconductors and new improper ferroelectric materials. We have employed self-consistent phase-field modeling to investigate the physical properties of a local metal-ferroelectric (Pb(Zr_0.2Ti_0.8)O₃) junction in applied electric field. We revealed an up to 10-fold local enhancement of electric field realized by large polarization gradient and over-polarization effects due to inherent non-linear dielectric properties of PZT. The effect is independent of bias polarity and maintains its strength prior, during and after ferroelectric switching. The observed field enhancement can be considered on similar grounds as increased doping level, giving rise to reduced switching bias and threshold voltage for charge injection, electrochemical and photoelectrochemical processes.
This research was sponsored by the Division of Materials Sciences and Engineering, Basic Energy Sciences, Department of Energy (YC, SVK, PM). Research was conducted at the Center for Nanophase Materials Sciences, which also provided support (AVI) and which is a DOE Office of Science User Facility. The phase-field simulation was performed in collaboration with Prof. Long-Qing Chen at Penn State, which is supported by the U.S. Department of Energy, Of#64257;ce of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-07ER46417(Chen). We thank Pu Yu and Ramamoorthy Ramesh for providing the PZT sample on which the I-V curves were recorded.
[1] J. Seidel, L. W. Martin, Q. He, Q. Zhan, Y. H. Chu, A. Rother, M. E. Hawkridge, P. Maksymovych, P. Yu, M. Gajek, N. Balke, S. V. Kalinin, S. Gemming, F. Wang, G. Catalan, J. F. Scott, N. A. Spaldin, J. Orenstein, and R. Ramesh, Nature Materials 8, 229 (2009)

Correlated band theory calculations based on the DFT+U approach are employed to investigate oxygen-deficient SrTiO3. We show that the appearance of magnetism in oxygen vacancies is not determined solely by the presence of a single oxygen vacancy, but by the density of free carriers and the relative proximity of the vacant sites. While an isolated vacancy behaves as a non-magnetic double donor, manipulation of the doping conditions allows the stability of a single donor state with emergent local moments. For clusters of vacancies, different kinds of Ti atomic orbital hybridization are described as a function of the doping level and defect geometry. Our description of the charged clusters widens the previous descriptions of mono and multi-vacancies and points out the importance of the controlled formation of defects for the realization of transition metal oxide based devices with a desirable magnetic performance.

Ba0.4Sr0.6TiO3 (BST) is a good ferroelectric material characterized by high (>6000) dielectric constant, has been used for tunable microwave devices. Our interest is whether we could identify possible processing routes that would introduce a coexisting magnetic component, i.e produce of multiferroics. In this work, we have epitaxially integrated ferroelectric BST thin films on MgO/TiN buffered Si (100) substrates using PLD. The phase formation was confirmed by XRD, TEM, XPS and Raman measurements. As deposited films show weak-ferromagnetic nature measured through SQUID magnetometry and electron spin resonance spectroscopy. We found that the magnetic moment decreased upon laser and oxygen annealing. Our study confirmed that the weak ferromagnetism originates from oxygen vacancies, not from external (unwanted) magnetic impurities. BST-based devices fabricated on silicon substrates are being tested for their ferroelectric and multiferroic properties as a function of temperature and magnetic field using our QUANTUM DESIGN PPMS coupled with Radiant technologies ferroelectric tester. We present and discuss the salient experimental findings.

Oxide thin films possess a great potential for technological applications, from synaptic transistors to diodes to catalysts, due to the high degree of tunability in their transport properties using simple parameters like temperature, magnetic field, strain, etc. Recent advances in layer-by-layer epitaxial growth techniques and engineering of the heterointerfaces have led to the realization of stoichiometric, single-crystalline, ultrathin oxides films with a plethora of exciting properties. NdNiO₃ thin films (15 u.c. thick) exhibit two phase transitions, upon cooling, from a paramagnetic metal (PM) to a paramagnetic insulator (PI) and from that to an antiferromagnetic insulator (AFI). While standard transport measurements can probe the average behavior, noise spectroscopy can provide valuable, complementary information about the microscopic charge carrier transport, structural defects, phase transitions, etc. From our noise studies on NdNiO₃ thin films, we find that the magnitude of 1/f noise at room temperature is 3-5 orders of magnitude higher than in typical disordered metallic films of similar resistivity. Results from noise studies across both the phase transitions in a set of NdNiO₃ thin films with varying strain values at the interface will be presented. We observe an intriguing time dependence in the noise characteristics in the AFI phase, where the probability density function fluctuates between a single-peak Gaussian behavior and a two-peak non-Gaussian behavior. The implications of these results will be presented. This work is supported by NSF-DMR 0847324.

Furnishing semiconductor materials with useful properties alien to them but inherent to functional oxides has been a main driving factor to develop materials integration strategies of crystalline oxides on semiconductors. Conversely, the epitaxial integration of high quality crystalline oxides on Si would enable a much faster dissemination of these materials into marketable technologies, lowering their costs by providing an affordable wafer scale platform. In this regard the combination of Si, being the most widely used commercial substrates, and SrTiO₃, serving as a substrate for the growth of many functional perovskite oxides, including superconducting, ferroelectric, pyroelectric, piezoelectric and (anti)ferromagnetic and multiferroic thin films, make the monolithic integration of SrTiO₃ on Si (001) highly desirable.
In this talk we will discuss how the challenges towards the development of a wafer scale virtual substrate can be addressed. By combining the advantages of molecular beam epitaxy and chemical beam epitaxy the requirements for the growth of a high quality template are met in an ideal way. We will show how carbonization of Si surfaces is avoided and discuss how structural quality and surface morphology of the films are affected by growth condition, film thickness and post growth anneals. The self-regulated growth of atomically smooth SrTiO₃ films at high growth rates is demonstrated, forming an ideal buffer layer for subsequent epitaxial growth. We will propose strategies, how the large thermal expansion mismatch of Si and SrTiO₃ can be utilized to create growth templates with continuously variable lattice parameter, key to empower strain-enabled functionality engineering of perovskite thin films on Si-based virtual substrates.
This work was supported by the Office of Naval Research through Grant No. N00014-11-1-0665.

The ability to effectively harvest visible sunlight for the purpose of photovoltaics, photoelectrochemistry and photochemical organics destruction is of clear importance in the energy and environmental landscapes. Several different classes of materials are under consideration for this purpose. Perovskite oxide semiconductors are of interest because of their superior stability in aqueous environments compared to that of traditional semiconductors. It is thus of interest to combine perovskite oxides with traditional low-gap semiconductors with the goal of utilizing the best properties of both classes of materials. To this end, we are investigating the MBE growth and properties of p-Sr_xLa_1-xCrO₃/n-Si(001). LaCrO₃ is an insulator. However, replacing La with Sr in LaCrO₃ dopes holes into the Cr 3dt_2g-derived top of the valence band, resulting in significantly enhanced p-type conductivity of a polaronic nature. X-ray based spectroscopies (XPS and XAS) and first-principles modeling reveal that Sr doping results in a split-off, unoccupied Cr 3dt_2g-derived band above the Fermi level which shifts in energy with increasing x. The material becomes metallic above x = ~0.5, but only when grown without large (~1%) in-plane tensile strain. In the semiconducting regime (x le; ~0.5), Sr_xLa_1-xCrO₃ exhibits the characteristics of a p-type transparent semiconducting oxide. p-Sr_0.1La_0.9CrO₃/n-Si(001) thus constitutes an attractive low-gap p-n junction with substantially more stability in aqueous media than Si based heterostructures without an oxide surface layer. In this talk, I will present our results on this material system.

Almost two decades ago, McKee and co-workers achieved a breakthrough in the direct epitaxial growth of single crystal perovskite SrTiO₃ (STO) on Si(001) using 1/2 monolayer (ML) of Sr deposited on a clean Si(001) 2×1 surface as a template. One of the surprises of this approach is that at 1/2 ML coverage, Sr atoms assume positions between Si dimer rows and inhibit the formation of an amorphous SiO₂ layer during the subsequent STO deposition in a relatively wide range of temperatures and oxygen partial pressures Using density functional theory, we investigate the oxidation stability of the Si(001) (2×1) reconstructed surface passivated by ½ monolayer of Sr. With Sr present, we find that the surface is indeed protected against oxidation. The presence of Sr delays the adsorption of O in the bridging site, and even when this site is occupied, O does not adsorb in the backbond, preventing the unwanted vertical growth of SiO₂. This provides the microscopic explanation of how ½ monolayer of Sr makes silicon surface oxidation resistant in a wide window of temperature and pressure.

Developments in thin film growth have led to the monolithic integration of single crystalline SrTiO₃ on silicon, opening up opportunities to combine the multifunctionalities of oxide systems with conventional semiconductor platforms. One of the benefits of perovskite oxides such as SrTiO₃ is the ability to support a high density of charges by chemical doping. However, devices made from oxide heterostructures tend to have low room temperature electron mobilities, compared with conventional semiconductors. In this work we demonstrate the growth of highly crystalline La-doped SrTiO₃ on silicon, and we measure its electrical and structural properties. We show that the conduction band offset between the oxide and silicon is tunable by varying the oxygen content at the SrTiO₃-Si interface. By changing the conduction band offset we present an approach toward moving the high carrier charge density in the oxide into the silicon.

Ferroelectric oxides integrated on semiconductor substrates are of particular interest for various silicon-based electronic and photonic devices. Among them, the perovskite BaTiO₃ could be an attractive candidate as the insulator of field effect transistors for low voltage/low power operations [1]. For this application, the control of the crystalline orientation of the ferroelectric tetragonal cell (c-axis versus a-axis growth) as a function of the fabrication parameters is a key issue.
In this study, a quantitative analysis of atomic structure images (HRTEM or STEM-HAADF) using the geometric phase analysis (GPA) is proposed in order to support the molecular beam epitaxy (MBE) growth strategy (growth temperature, oxygen pressure and cooling conditions) of epitaxial BaTiO₃ thin films (15 nm) grown on Si with the desired crystalline orientation [2]. An SrTiO₃ epitaxial buffer layer is grown before the BaTiO₃ epitaxy to reduce both thermal and lattice mismatches between BaTiO₃ and Si. With GPA, maps of the strain within the BaTiO₃ films with respect to the Si substrate reference are determined at the nanometric scale (1nm). From these maps, average profiles of the local lattice parameters within the BaTiO₃ films as a function of distance from the bottom of the SrTiO₃ buffer are deduced. This enables to evidence the tetragonality modifications within the BaTiO₃ films, tetragonality being defined as the ratio of the out-of-plane and in-plane parameters. The results are directly connected to the local cation stoichiometry profiles determined by electron energy loss spectroscopy (EELS). The particular case of the effect of the oxygen pressure during the MBE growth will be illustrated. The HRTEM work is performed on an image corrected Hitachi HF3300S microscope (I2TEM-Toulouse) and STEM-HAADF-EELS data are collected on a FEI Titan Low-Base 60-300 probe corrected microscope (Zaragoza).
[1] S. Salahuddin, S. Datta, Use of Negative Capacitance to Provide Voltage Amplification for Low Power Nanoscale Devices, Nano Letters. 8 (2008) 405
[2] L. Mazet, R. Bachelet, L. Louahadj, D. Albertini, B. Gautier, R. Cours, et al., Structural study and ferroelectricity of epitaxial BaTiO₃ films on silicon grown by molecular beam epitaxy, Journal of Applied Physics. 116 (2014) 214102

To date, the majority of research on crystalline oxides integrated with semiconductors has been based on strontium titanate, SrTiO₃ (STO), epitaxially grown on Si (001) by molecular beam epitaxy (MBE). Chemical routes, such as atomic layer deposition (ALD), have required a buffer layer consisting of four unit cells of STO that is grown by MBE. This STO provides a template upon which many crystalline perovskites form and the talk will address the key issues in growing layers and perovskite heterostructures with the desired ferroelectric properties or conductivity monolithically on STO/Si(001) using ALD. Applications for these films ranging from resistive switching in memory structures to conductor-ferroelectric-conductor layers for negative capacitance transistors will be presented.
Due to the thermal instability of the oxides of GeO₂ versus SiO₂ it is possible to grow crystalline perovskites directly on Ge(001) by ALD. Using this approach we have been able to deposit STO, BaTiO₃, SrHfO₃, and Sr(HfTi)O₃ directly on Ge(001). Since Ge exhibits higher hole and electron mobilities than Si, potentially enabling device operation at higher speed, our current work demonstrates the promise for ALD-grown crystalline oxides for advanced electronic applications in the near future, especially high-mobility Ge-based transistors. This talk will discuss the growth and properties of the perovskite layers directly on Ge(001), and will discuss the interface chemistry that likely controls the interfacial reactions that allow for crystalline film formation.

With the development of wireless communication technology there is a need for novel multi-functional and fast acting devices, e.g., for tunable and switchable Thin Film Bulk Acoustic wave Resonator (TFBAR) that will improve the front-end of the mobile devices. The most recent TFBAR devices are based on functional oxide materials (e.g., ferroelectrics) which calls for integration of the emerging (Functional oxides) fabrication methods with the existing semiconductor technology.
In this paper we discuss the growth and properties of a graphene/ferroelectric multi-layer structure on Silicon substrate and comment on its potential for TFBAR applications. The proposed TFBAR architecture utilises the dc field induced piezoelectricity in the active layer of ferroelectric material in paraelectric phase that allows the tuning and switching of the TFBAR to be controlled by external voltage.
A pre-cleaned high resistive Silicon substrate was covered by stack of three Ti(180nm)/Ru(215nm) bi-layers using magnetron sputtering to form Bragg reflectors that confines the generated acoustic wave in the active layers. The Ti and Ru were identified as materials with most appropriate thermal expansion coefficients with the substrate and the on grown layers, while their thicknesses were chosen to enable operation within 2 GHz-10 GHz frequency range. The growth of the Au/FE/Metal-graphene/FE/Pt was optimised to not disturb the properties of the high resistive Si substrate covered with all metals Bragg reflector multilayer structure. Introduction of the intermediate (between active layers) metal-graphene electrode provides the capability to apply control voltage to both (active FE) layers independently, hence excitation of different resonating modes in the structure. In order to achieve this, the electrodes in between each active layer have to be ultra-thin to minimise the losses in generated acoustic wave propagation. Therefore the thickness of the graphene/Ti bi-layer was less than 6nm. The material chosen for the active layers was barium strontium titanate (BST) with barium and strontium composition of 50/50. It has been grown by Pulsed Laser Deposition (PLD). The graphene (Gr) was grown by Chemical Vapour Deposition (CVD) and transferred using PMMA assisted wet-transfer method; while the Ti layer was grown on top of it by magnetron sputtering. The electrical resistivity of the Gr/Ti bi-layer structure was gradually reduced by more than an order of magnitude compared with that of the Ti electrode with same thickness.
Structural characterisation (SEM, TEM and XRD) analysis of the deposited multilayer structures was carried out and will be presented as well the results of the electrical measurement (at dc and microwave frequencies) of the graphene/metal and Bragg reflector structures, and the resulting TFBAR device.

Combining oxide heterostructures with conventional semiconductors is an attractive approach for integrating the diverse functionalities of complex oxides with technologically relevant materials platforms. 2D electron gases (2DEGs) formed at oxide interfaces, such as those between rare-earth titanates and SrTiO₃ (RTO-STO), are a prominent example of a well-studied oxide system with useful electronic properties. We present the integration of oxide 2DEGs based on titanate heterostructures with silicon and III-V semiconductors. High carrier densities are observed and the origin is pinpointed to the RTO-STO interface. The formation of 2DEGs with high mobility and carrier density on both semiconductors requires tailoring the growth conditions to ensure the structural and chemical abruptness of the oxide-semiconductor interface, as characterized using x-ray and electron diffraction. By combining this approach with the diverse array of possible III-V heterostructures, novel oxide-semiconductor devices should be possible.

Silicon based electronics have reached its limits in terms of materials and device scaling. High-k dielectrics are allowing the Si-CMOS technology to shrink the device pitch further. Now, the switching speed/performance of the channel is limited by the carrier injection from source material. Materials with high electron/hole mobilities are required to prevail over this issue. Germanium is one such channel material and can be integrated with well established silicon technology. Dielectrics or functional oxide/Ge interface stability is a concern. We studied the stability of various functional oxides like BaTiO₃, SrRuO₃, BaTiO₃, ZrO₂, Y2O₃, CeO₂, BaO, SrO, TiO₂, ZnO, Al₂O₃, V₂O₃ in contact with Ge. The criterion is the stability of these oxides against the formation of GeOx at the interface between functional oxide/Ge at 873 K. This study can serve as a route to select stable oxides to integrate with technologically important germanium.

Recent progress in the synthesis of layered complex oxide heterostructures with atomically abrupt interfaces on semiconductor substrates using techniques such as molecular beam epitaxy has enabled the integration of functional oxides with semiconductor-based devices. Interfacial coupling at these complex oxide-semiconductor interfaces provides a route to induce atomic-scale structural changes to produce novel functional properties. We present a detailed characterization of the BaTiO3-Ge interface using a combination of synchrotron x-ray diffraction, first principles theory and transmission electron microscopy. We observe symmetry breaking distortions in the interfacial BaTiO3 arising from structural coupling to the Ge substrate. We show how these distortions can be predicted from first principles calculations of the characteristic lattice modes of the oxide which couple to the Ge substrate and how this materials design approach can be used to guide the development of novel electronic and magnetic phases in complex oxide materials.

In this presentation we review our recent work on the properties and applications of MBE grown vanadium oxide based thin films and heterostructures.
The metal-insulator transition (MIT) in vanadium oxide compounds remains a topic of intense research due to the large change in resistivity observed around 170 K (V2O3) and 340 K (VO2). Our general focus is to tune the properties of the compounds in a way that could enable an easier electric field switching of the MIT transition.
A first appraoch is to grown ultrathin films on different substrates. For 4 nm V2O3 films grown on Al2O3 substrates it was observed that the MIT is nearly completely suppressed. However when the same films were grown on Cr2O3 / Al2O3 buffer layers, it was possible to conserve the transition albeit with some subtle changes. These results demonstrated the role of lattice deformations in the tuneability of the compound¹.
A second approach is to dope V2O3 with different metal ions. In the case of Cr doped V2O3, an abrupt decrease of the MIT was observed for low Cr doping (<3%) in contradition with the reported phase diagrams. However, upon subsequent vacuum annealing, the MIT was restored suggesting that oxygen excess was responsible for the observed behavior.
Next moving on to VO2 it turned out that the low oxygen pressure conditions used for MBE growth are not appropriate for the formation of VO2. Therefore an annealing recipe was developed that allows to transform V2O3 into VO2. Bulk like VO2 properties were obtained for films grown on Al2O3.
DC and AC pulsed electric field switching was performed on both systems from the insulating to the metallic state. For large dimensions and thick films the electric field induced switching is entirely driven by Joule heating. Both volatile and non-volatile features are present in the hysteresis region and their contribution is a function of the applied switching power.
Finally the first results obtained on switching VO2 / Si waveguides* will be presented*.
¹L. Dillemans et al., Applied Physics Letters 104, 071902 (2014).
* Work performed together with the teams of: KU Leuven Functional Nanosystems and the EU FP7 SITOGA project.

The integration of metal oxide based thermoelectric and magneto resistive thin films into silicon technology is mandatory for their application. In addition to the encouraging thermoelectric properties, the coupling of the magnetic moments of the Co spins and the quasi-two-dimensional electric transport properties of Ca₃Co₄O₉ can lead to novel functionalities and applications in thermomagnetic sensor devices. For the integration of these functionalities into silicon technology a diffusion and chemical reaction barrier is needed which allows epitaxial growth of Ca₃Co₄O₉ thin films on (001)-Si. In order to take advantage of Ca₃Co₄O₉ and other complex metal oxides ohmic contacts with minimal electrical contact resistivity are indispensible. We show in this work that thin film Ir-Ca₃Co₄O₉ ohmic contacts in parallel with low in-plain electrical resistivity of the Ca₃Co₄O₉ can be realized. For that, thin epitaxial films of Ca₃Co₄O₉ were grown by pulsed laser deposition (PLD) on epitaxial Ir metal thin films grown on yttria-stabilized zirconia (YSZ) buffered single crystalline (001)-Si substrates. The Ca₃Co₄O₉ films grow epitaxial with a 12-fold in-plane rotational symmetry on a (001)-Ir surface without intermediate phase at the interface proven by high resolution transmission electron microscopy (HRTEM). Moreover, we show that Ca₃Co₄O₉ thin films grow on (001)-YSZ with a 12-fold in-plane rotational symmetry, but rotated by a 15° off-set compared to the Ca₃Co₄O₉ in-plane orientation on (001)-Ir. This leads to well-defined high symmetry grain boundaries with low electrical resistivity where the Ca₃Co₄O₉ trace crosses the edge of the Ir metalized contact trace to the electrically insulating YSZ buffer layer. These observations can be explained by energetically preferred epitaxial growth directions of the pseudo hexagonal CoO₂ sublayers in monoclinic Ca₃Co₄O₉ onto cubic (001)-YSZ and (001)-Ir surfaces. It will be shown, that the resulting symmetric in-plane mutual orientation between the charge carrying CoO₂ sublayer domains determine the electrical charge transport characterized by minimal electrical in-plane resistivity. In addition, the influence of the azimuthal orientation on the Seebeck coefficient of Ca₃Co₄O₉ films has proven to be imperceptible.

ZnO/GaN-based semiconductor heterojunctions are widely used in LED and photovoltaics applications. Using a multifunction oxide material such as ferroelectric or multiferroic complex oxide to tune or switch the properties of the semiconductor heterojunction is a challenging research topic that can provide potentially significant benefits. We present our results of PbZr_0.2Ti_0.8O₃ ferroelectric oxide growth on epitaxial ZnO/GaN/c-Al₂O₃, ZnO/r-Al₂O₃, and ZnO/m-Al₂O₃ using off-axis rf-magnetron sputtering and electrical characterization of ferroelectric/semiconductor heterojunctions.

The oxygen evolution reaction (OER), characterized by a high overpotential (1.93 eV vs. SHE for anatase), is a major bottleneck in the photocatalytic splitting of water on anatase (TiO₂). Integrating TiO₂ with other materials to modify its surface reactivity is an appealing way to potentially get around this problem. In particular, TiO₂/ ferroelectric heterostructures would allow one to dynamically tune the surface reactivity of TiO₂ by changing the polarization of the ferroelectric substrate. We present here a density functional theory (DFT) study of the OER on a TiO₂ (001)/ BaTiO₃ (001) heterostructure. We find that the reaction&’s Gibbs free energy profile is polarization-dependent. Importantly, the most rate-limiting step of the OER changes from endothermic on unsupported anatase to highly exothermic for one of the polarizations of the substrate. We explain the modified surface reactivity of TiO₂ in terms of an interplay between the layer-by-layer induced polarization in the TiO₂ layers and the charge transfer between the surface and the adsorbate molecules. Additionally, we find that the reaction's Gibbs free energy profile exhibits a dramatic sensitivity to the number of monolayers of TiO₂ in the heterostructure: reducing the number of monolayers enhances the effect of the underlying polarization on the OER profile. Our results suggest that the polarization at the interface of TiO₂ and BaTiO₃ is a fixed property of the interface, independent of the number of monolayers of TiO₂, that can be used to potentially tune the reactivity of a nearby surface for catalysis, either by changing the number of TiO2 monolayers or by switching the direction of the substrate polarization.

Recent years efforts have been focused on how to utilize the ultrafast metal-insulator phase transition in vanadium dioxide materials for device applications. [1] A sharp (3-5 orders of magnitude) resistance change occurs across the phase transition of vanadium dioxide materials. How contact resistance between vanadium dioxide and the metal electrodes deposited on top changes from vanadium dioxide&’s room-temperature insulating phase to high-temperature metallic phase is important for the design and fabrication of any vanadium dioxide devices; however, it has not been studied yet to the best of our knowledge.
In this presentation, we report the contact resistance studies on high-quality vanadium dioxide materials using transmission line measurements. Metal electrodes with transmission line pattern were defined and deposited by lithography, e-beam evaporation, and lift-off process. The measured contact resistance shows orders of magnitude difference from the room-temperature insulating phase to high-temperature metallic phase of vanadium dioxide. The obtained contact resistance values are useful for practical fabrication as well as simulation of devices based on vanadium dioxide materials.
Reference
1. Zheng Yang et al, Oxide electronics utilizing ultrafast metal-insulator transitions, Annual Review of Materials Research 41, 337 (2011)

We report on the dielectric and interface properties of several functional oxides such as Al₂O₃, HfO₂, ZrO₂ and MgO deposited on nitride semiconductors. These nitride semiconductors such as GaN, InN, AlN, InGaN and AlGaN which act as virtual substrates for oxide growth, were grown epitaxially on Si(111) substrates by plasma assisted molecular beam epitaxy (PA-MBE) technique. Capacitance-voltage (C-V), leakage current (J-V) characteristics of Metal-Oxides-Nitride/Si(111) based most capacitors were compared in order find out the most suitable oxide for nitride semiconductors. The density of interface states (Dit) measured for all these MOS capacitors were compared with regard to their interface structure which were investigated in detail by high resolution x-ray photo electron spectroscopy (XPS). As the thermal stability of these oxides is serious concern owing to the very high device processing temperature (~1000°C) of wide band gap semiconductor, these oxides were annealed at various temperatures (700-950°C) and subsequently studied their interface structure using XPS in order study their thermal stability. We find that although most the oxides remain stable until 900°C, however there seems to significant reaction at the interface for the layers annealed at and above 950°C.
For further analysis, we will also compare these results with epitaxial Gd₂O₃ grown on these virtual substrates by MBE technique. Comparing all these results obtained as of now, we find that that atomic layer deposited HfO₂ exhibits the best electrical results in terms of their C-V characteristics and Interface states (~10¹¹ eV^-1cm^-2). However, the leakage current measured for Al₂O₃ exhibited the lowest value among all the oxides studied in this work.

Alumina (Al₂O₃) thin films were deposited onto well-cleaned Si <100> substrates by DC reactive magnetron sputtering. The films were subjected to post-deposition annealing in vacuum at 350, 550 and 750^oC. The composition of the annealed films was quantified using X-ray photoelectron spectroscopy (XPS) and the films annealed at 350^oC and 550^oC were found to be nearly stoichiometric whereas the film annealed at 750^oC was aluminium rich. X-ray diffraction analysis indicated that the annealed films were amorphous. The interface properties of Al₂O₃/Si were studied using scanning transmission electron microscopy (STEM) and the interface was found to be aluminium rich in all the annealed films. Al-Al₂O₃-Si MOS were fabricated on n-type Si <100> substrates and current-voltage (IV) measurements were carried out. The leakage current density was estimated as a function of input bias voltage and minimum leakage current of 9.04 x 10^-8 /cm² was observed for the film annealed at 550^oC. The films were found to exhibit schottky emission type of conductivity up to 0.5 MV/cm and space charge limited conduction process at higher electric fields. The correlation between the composition of the films and leakage current density was also studied and the results are discussed.

In the past decade metal oxide semiconductors have emerged as a fascinating class of electronic materials, exhibiting a wide-range of unique and technologically relevant properties. Thin-film transistors based on oxide semiconductors represent the most well-known application of the technology due to their tremendous potential for use in a continuously expanding range of large-area opto-electronics. However, like incumbent silicon-based technologies, the ultimate level of performance of oxide transistors is limited by the intrinsic properties of the semiconductor material used and as such the technology falls short in addressing the continuously increasing needs of emerging opto/electronics.
Here we report on exotic electron transport phenomena observed in low-dimensional layered metal oxide systems grown from solution, and their use for the development of advanced electronic device concepts. The work starts with the study of the formation of quantised energy states in ultra-thin layers of indium oxide grown via spin-casting at 200 °C. Optical absorption measurements reveal a characteristic widening of the optical band gap with reducing semiconductor layer thickness from ~43 nm down to ~3 nm in agreement with theoretical predictions for an infinite quantum-well. Through sequential deposition of indium oxide and gallium oxide, we demonstrate multilayer structures with controlled dimensionality and spatially-varying conduction band characteristics. The ability to grow such elaborate structures with nanometre precision via spin-casting is then explored for the fabrication quantum effect devices such as resonant tunneling diodes (RTDs) on large-area substrates. The RTDs exhibit robust room-temperature operation with symmetric negative differential conductance characteristics.
Based on the energy quantisation studies, we are also able to develop a new transistor concept by exploiting the enhanced electron transport properties observed in solution-processed metal oxide superlattices (OSLs). The latter structures consist of alternating ultra-thin (<5 nm) layers of different oxide semiconductors separated by atomically sharp interfaces. Prototype OSL transistors exhibit electron field-effect mobilities that are one order of magnitude greater (25-45 cm²/Vs) than single oxide semiconductor transistors (2-5 cm²/Vs). Charge transport studies reveal that this dramatic enhancement is the result of the formation of 2-dimensional electron gas systems in the vicinity of these solution-grown epitaxial-like oxide heterointerfaces. Finally, the method is explored for the development of high-electron mobility OSL transistors on large-area plastic substrates with superior mechanical flexibility. The OSL transistor concept described here can in principle extend to a whole range of other metal oxides, offering a universal approach in overcoming the intrinsic limitations of single semiconductor-based transistor technologies.

We report on the use of pulsed metal organic chemical vapor deposition (PMOCVD) method to overcome the longstanding challenge of growing high Mg content, high quality, single phase wurtzite Mg_xZn_1-xO (MgZnO) epitaxial film to realize photodetectors and emitters in the solar blind spectral window. A series of Mg_xZn_1-xO epitaxial films with variable Mg concentration were systematically deposited on sapphire substrates with and without AlN (~25 nm) buffer layer. The substrates were covered by low temperature grown thin (~30 nm) ZnO nucleation layer to grow high quality crystalline film by reducing lattice mismatch between the substrate and the film. The optical band gap for the films estimated using UV-visible transmission spectroscopy ranges from 3.24 eV to 4.50 eV, corresponding to the fraction of Mg between x=0.0 to x=0.51, as determined by Rutherford backscattering spectroscopy (RBS). The cathodoluminescence (CL) measurement has shown a linear blue shift in the spectral peak position of Mg_xZn_1-xO as a result of increasing Mg content. No multi-absorption band edge as well as CL band splitting were observed, suggesting the absence of phase segregation in the as grown films even with the highest Mg content (x=0.51). The crystal structure and phase purity of the films were further confirmed by XRD analysis. The AFM measurement on the films has shown a decreasing surface roughness with Mg content. To the best of our knowledge, the current result shows the highest Mg content (x=0.51), high quality, single phase wurtzite Mg_xZn_1-xO epitaxial film ever grown by MOCVD. The absence of phase separation in the current films, even with x as high as 0.51, unlike those grown via PLD and MBE, is believed to be due to the non-equilibrium behavior of the PMOCVD method, where the thermodynamic process is dominated by the kinetic one. In the growth the MgZnO films using PMOCVD, Zn and Mg metalorganic sources as well as oxygen gas are introduced to the reactor for 12 seconds to grow ~5.5Å thick MgZnO layer, a layer with a thickness comparable to the c-parameter of ZnO (~5.2 Å). Then, the Zn and Mg source valves are closed for 6 seconds leaving the oxygen gas to oxidize the grown layer. This periodic process cycles as many times as needed-similar to the traditional atomic layer deposition (ALD) methodshy;shy;-to grow the intended film thickness. The fast periodic transition steps between MgZnO layer growth and oxidation creates a far from equilibrium system in which radicals that are formed could not have sufficient time to reach their energy minimum. Consequently, high Mg incorporation into the films without phase separation.

We investigated temperature-dependent evolution of surface work function (W_S) distributions in epitaxial VO₂/TiO₂ thin films using Kelvin probe force microscopy. The coexistence of metallic and insulating domains in VO₂ thin films could influence their physical properties. Thus, there have been intensive research efforts to reveal geometric configuration of the domains in VO₂ while varying the sample temperature. Nearly grain-boundary-free VO₂ thin films allowed observation of metallic and insulating domains with distinct W_S values. The metallic fraction, estimated from W_S maps, described the evolution of the resistance based on a two-dimensional percolation model. The domain size was tens of nanometers at intermediate temperatures. The perimeter and area of the metallic domains followed power-law behaviors: a power exponent larger than 1/2 suggested that the metallic domains were fractal objects.

Effective integration of functional oxides (magnetic, ferroelectric, piezoelectric and other multi-functional materials) with semiconductors will lead to next-generation devices such as: new architectures that enable multiple and simultaneous interactions with the environment for multifunctional active sensors and controllers; energy harvesting and conversion devices as part of everyday items from clothing to sidewalks; integrated nonvolatile memories for harsh environments; and paradigm-shifting spintronics. Many properties of complex oxides are directional, thus requiring a specific plane alignment in a device, and most oxides have multiple stable structures for a given stoichiometry. Small changes in stoichiometry with the same unit cell structure can produce measurable differences in performance properties. While this sensitivity enables the possibility of tuning complex oxides for different and novel applications, precise control of stoichiometry and structure is required.
We will present commonalities of interface formation of several different metal oxides on SiC in order to raise questions about kinetically controlled interface formation. Through molecular beam epitaxy studies of oxide thin film heteroepitaxy, we have experimentally observed interesting and unexpected surface and interface bonding structures in both simple (MgO) and more complex (Fe₃O₄, Fe₂O₃, BTO, BaFe₁₂O₁₉) oxide films. 6H-SiC(0001) substrates are cleaned by an ex-situ hydrogen etch to produce a clean, atomically smooth, stepped surface with a radic;3×radic;3 R30° surface reconstruction, verified by reflection high energy electron diffraction (RHEED) and x-ray photoelectron spectroscopy (XPS). High quality, single crystalline MgO(111) is obtained with a smooth (RMS<0.5 nm) and a stepped morphology conformal to the underlying 6H-SiC, but inherently twinned due to the ionic nature of a (111) oriented rock salt structure. The engineered MgO surface is both effective and necessary to promote heteroepitaxy of both the hexagonal BaFe₁₂O₁₉, and the pseudo-hexagonal BTO(111) and various Fe_xO_y. For example, the BTO in BTO/MgO/6H-SiC grows in tension with a 5.3% lattice mismatch and is inherently twinned with a 6-fold symmetry due to 60° in-plane rotations, resulting epitaxy alignments of a BTO{111}//MgO{111}//6H-SiC{0001} out-of-plane relationship and a BTO{11(bar)0}//MgO{11(bar)0}//6H-SiC{002(bar)1} in-plane relationship. We have been able to demonstrate control of iron bonding-states in thin films of Fe₂O₃ and Fe₃O₄. This control has enabled production of single-crystal heterostructures Fe₃O₄/SiC, Fe₃O₄/MgO/SiC, and BaFe₁₂O₁₉/FeMgO/SiC. Experimental and modeling investigations of the polarity of interfaces and control of oxide bonding states in various Fe_xO_y films on SiC and MgO, has led to questions of kinetic control of nucleation and growth, and insight into the role of atomic hydrogen in both surface passivation and film formation mechanisms.

Ce and Bi substituted yttrium iron garnets (Y₃Fe₅O₁₂ / YIG) exhibit enhanced magneto-optical properties such as Faraday rotation and nonreciprocal phase shift (NRPS). As a result, these garnets have dominated the research in magneto-optic films for nonreciprocal photonics, imaging, recording, and spatial light modulators. However, integration of these thin films with semiconductor substrates requires a two-step deposition method in which an ultrathin garnet seedlayer must be grown prior to the deposition of the substituted garnet layer. YIG is the most commonly used seedlayer for this purpose. However, the polarization rotation in pure YIG is opposite to that in Bi and Ce substituted YIG. Hence, the YIG seedlayer will attenuate the performance of substituted YIG layers. Simulations of light travelling through garnet-clad semiconductor waveguides show that a YIG seedlayer as thin as 20nm decreases the interaction of light with the substituted YIG layer by 40%. Here, we present novel Bi substituted terbium iron garnets (Bi_xTb_3-xFe₅O₁₂ / Bi:TIG) directly grown on semiconductor substrates that rotate the polarization in the same direction as substituted YIG and hence are much more effective seedlayers for Ce:YIG and Bi:YIG than pure YIG.
YIG, TIG and Bi:TIG were grown on Si substrates using metallic targets via reactive sputtering. X-ray diffraction (XRD) indicated that single phase garnet was obtained after a rapid thermal anneal at 900°C for 2 minutes in oxygen. The magnetizations were measured using vibrating sample magnetometry (VSM) and the Faraday rotation was measured at 1545.2 nm. Using similar sputtering and annealing parameters, single phase Ce:YIG thin films were grown on the YIG, TIG and Bi:TIG seedlayers. Mode solver simulations of garnet clad Si waveguides show that the Faraday rotation of Ce:YIG grown on Bi:TIG seedlayer is 1.5 times greater than that of Ce:YIG grown on a YIG seedlayer.
Finite Difference Time Domain (FDTD) simulation shows a 40% reduction in the device length of the waveguide developed with Ce:YIG on Bi:TIG seedlayer compared to one with Ce:YIG on a YIG seedlayer. Furthermore, FDTD simulations also show that a waveguide device fabricated with alternating garnet layers with opposite Faraday rotations enables “push/pull” optical effects for an overall reduced device footprint.

Materials that undergo a metal-insulator transition (MIT) are potentially useful for a wide variety of applications including electronic and opto-electronic switches, memristors, sensors, and coatings. In most such materials, the MIT is driven by temperature. For applications in electronics, a reversible and reproducible means of externally modulating the transition is necessary. Niobium dioxide, NbO₂, is an MIT material whose phase transition temperature is very high (800°C). This makes it suitable for use in high temperature and high power electronic devices where variations in the ambient temperature will not affect the state of the device. However, the problem of how to induce the MIT in NbO₂ remains. In NbO₂, the MIT mechanism is primarily of the Peierls-type, in which the dimerization of the Nb atoms without electron correlation causes the transition from metallic to semiconducting. One possible way of inducing the MIT in Peierls-type materials is to alter the lattice constant using strain. By growing NbO₂ on a piezoelectric substrate, one can then use an external electric field to modulate strain in the material. One such piezoelectric material is GaN, which is a wide band gap semiconductor that has applications in LEDs, opto-electronics, and high-temperature/high-power electronics. The combination of NbO₂ and GaN would be potentially useful in resistive switching devices at elevated temperatures. In this work, we report on the epitaxial growth of NbO₂ films on GaN(0001) substrates using molecular beam epitaxy with an electron beam metal evaporation source. The films are found to grow with (110) out of plane orientation and an epitaxial relationship of NbO₂[1-10]//GaN[10-10] and NbO₂[001]//GaN[11-20]. Because of the symmetry mismatch between the film and substrate, three symmetry-related orientation domains form. The results of film characterization using in situ x-ray photoelectron spectroscopy and reflection high energy electron diffraction, and by ex situ in-plane x-ray diffraction and cross-sectional scanning transmission electron microscopy will be discussed.

Thin layers of insulating binary rare earth oxides (RE₂O₃) epitaxially grown on Si are considered promising alternatives to SiO₂ for their application as gate dielectrics in the future complimentary metal oxide semiconductor (CMOS) based devices. Despite their higher dielectric constants and excellent structural qualities, epitaxial RE₂O₃ s often exhibit poor electrical properties such as high leakage current densities, flat band instabilities, poor reliability etc. The degraded properties are related to electrically active defects, such as interface traps or fixed (Q_f) oxide charges generated either during the oxide layer growth or typical subsequent CMOS process steps. Here we will review different aspects of defect passivation which lead to significant improvements in the dielectric properties of epitaxial RE₂O₃ layers. As a model system we will consider Gd₂O₃ layers epitaxially grown on Si. MOS capacitors fabricated with Gd₂O₃ layers on Si surface display significant flat band instability due to the accumulation of Q_fs at the oxide/Si interface. The Q_fs originate from the unsaturated Si dangling bonds which capture electrons from the oxide layer preferably near the oxide/Si interface in order to become electrically neutral. We demonstrate that such defects can be passivated by depositing small amount of isoelectronic Ge or C prior to the oxide growth. Charge transfer from Si to the chemisorbed Ge or C and change in the Si surface reconstruction due to surface strain result in a significant reduction in the charge densities at the Si surface. We will show that this approach not only helps to improve the oxide/Si interface by lowering D_it and the Q_fs but can also act as a suitable way to tune the flat band voltage and hence the threshold voltage of CMOS devices in a controlled manner. Further, we will discuss about the improvement of dielectric properties of epitaxial RE₂O₃s by suitable dopant incorporation. In addition to Q_fs and higher densities of interface traps, the electrical properties of epitaxial RE₂O₃s on Si are degraded also by the presence of mobile (Q_m) oxide charges. Q_ms manifest themselves into hysteretic capacitance-voltage characteristics of the MOS structures. The flat band instability due to the presence of Q_ms restricts the applicability of epitaxial RE₂O₃s in CMOS based transistors. One common source of Q_ms in such ionic oxides is the omnipresent oxygen vacancies. We will discuss that nitrogen doping in epitaxial RE₂O₃s can be a potential method to reduce the Q_ms by citing the example of Gd₂O₃. Further, the leakage current densities in epitaxial Gd₂O₃ layers have been found to decrease significantly with an increase in nitrogen content, which also indicates that nitrogen doping can be an effective route to eliminate the adverse effects of oxygen vacancy induced defects in the epitaxial RE₂O₃s.

Monolithic integration of yttrium iron garnet (YIG) films into silicon-based devices is of high current interest for a wide range of applications in photonics and spintronics. However, growing high-purity YIG on non-garnet substrates has proven challenging owing to lattice mismatch. This typically results in incomplete crystallization of YIG and the presence of unwanted phases in the films. Here, we use an in situ transmission electron microscopy (TEM) laser-annealing method to directly visualize the crystallization of amorphous YIG thin films ranging in thickness from seven to 100 nm as deposited on SiO₂ TEM membranes. With real-space video-rate imaging, insight into the nanostructure crystallization process, grain-structure evolution, and crystal-growth velocities is obtained, while reciprocal-space electron diffraction studies are conducted in order to elucidate how different phases nucleate, grow, and are consumed. Through systematic laser-parameter studies, we find that a two-step variable-power annealing method is most effective for achieving the highest fraction of desired YIG phase. The results of this study demonstrate the feasibility of laser annealing as a processing route for YIG thin films. Further, development of the in situ method employed here provides a means for gaining additional understanding of the fundamentals of crystallization in these and other thin-film systems.

Ga₂O₃ and In₂O₃ are currently popular as, respectively, deep-UV large-breakdown and near-UV transparent-conducting materials. A natural development is the growth (typically but not necessarily epitaxial) of a solid solution (Ga_1-xIn_x)₂O₃ which would enable to combine the properties of two parent compounds and to export the band-engineering and nanostructuration concepts of other semiconductor systems to higher absorption and breakdown energies. Progress in this directions requires a knowledge of the miscibility, as well as the behavior of relevant properties (gap, specific volume, band offsets, etc.), of a solid-solution substitutional alloy composed, in a given proportion, of the two parent materials. Here we report the modeling of (Ga_1-xIn_x)₂O₃ over the entire range of x by ab initio density-functional-theory techniques. Previous results [1] for the low-x composition range are integrated in a comprehensive picture of the miscibility and attendant properties.
The main result is that the alloy will phase-separate in a large composition range, exhibiting a large and temperature-independent miscibility gap. On the low-x side, the favored phase is isostructural with β-Ga₂O₃, and on the high-x side it is isostructural with bixbyite In₂O₃. The miscibility gap opens between approximately 15% and 55% In content for the bixbyite alloy grown epitaxially on In₂O₃, and 15% and 85% the free-standing bixbyite alloy. Both assessments agree with the few available experiments. The gap and band offsets to the parent compounds also exhibit anomalies as function of x. Specifically, the offsets in epitaxial conditions are predominantly type-B staggered, but have opposite signs in the two end-of-range phases. The largest conduction and valence offsets for the bixbyite alloy are about 0.4 eV, and occur in the epitaxial phase around x=0.7-0.75.
Finally, we also address the gap value and the anisotropy of optical absorption of Ga₂O₃ using several beyond-DFT theoretical methods (hybrids, GW, SIC). The results are in good agreement with experiment [2], and show that anisotropy is due to a matrix element suppression at the valence band top.
[1] M. B. Maccioni, F. Ricci, and V. Fiorentini, Appl. Phys. Express 8, 021102 (2015); J. Phys. Conf. Ser. 566, 012016 (2014); to be published.
[2] F. Ricci, F. Boschi, A. Baraldi, A. Filippetti, M. Higashiwaki, A. Kuramata, V. Fiorentini, and R. Fornari, to be published.

The presence of a spontaneous polarization and the high and non-linear dielectric response of ferroelectric materials continue to be of interest for semiconductor device industry. Metal/ferroelectric/metal and a metal/ferroelectric/semiconductor stacks are the main components of devices based on the “functionality of the ferroelectric” but the two configurations impose very different boundary conditions on the ferroelectric. Interfacing a semiconductor is certainly expected to lead to significant deviation of the characteristics of the ferroelectric layer than what is anticipated of them in simple capacitor geometries. This is often neglected in experiments and typically overlooked in the analysis of data as properties of ferroelectrics could be influenced by the electronic and atomic properties of its interfaces with other materials.
In this study, we present theoretical results on the effect of semiconductor electrodes on the functional response of ferroelectric layers and superlattices. A computational approach using a thermodynamic theory of ferroelectrics in contact with semiconductor interfaces is undertaken. Stability limits of the polar phase with various anisotropies are explored, revealing the importance of the boundary conditions on device functionality and possible integration efforts of well-known ferroelectrics such as BaTiO₃ and Pb(Zr,Ti)O₃ (PZT) solid solutions. We rigorously show that electrical domains are inevitable and directly impact the carrier distributions in the semiconductor channel. However domains can be tailored, contrary to the desire to utilize a switchable single domain state, for power efficient binary logic field effect transistor (FET) devices. Efficient manipulation of carriers in the semiconductor via domain wall motion is theoretically shown [1, 2] and the results are compared to real device data [3] to explain certain characteristic features of domain related phenomena in FETs. This is followed by comments on the limit of use of such layers for the much desired “non-volatile memory” gates. The approach is then extended to ultra-thin ferroelectric layers used as tunnel junctions where we comment on the changes in the polar state of the FE layer under asymmetric electrodes [4].
1. I. B. Misirlioglu, M. Yildiz, K. Sendur, Domain control of carrier density at a ferroelectric-semiconductor interface, to appear in Scientific Reports, 2015.
2. I. B. Misirlioglu, C. Sen, M. T. Kesim and S. P. Alpay, Low voltage ferroelectric-paraelectric superlattices as gate materials for field effect transistors, submitted.
3. Sakai S. and Takahashi M., Recent Progress of Ferroelectric-Gate Field-Effect Transistors and Applications to Nonvolatile Logic and FeNAND Flash Memory,Materials 2010, 3, 4950-4964.
4.I. B. Misirlioglu, O. M. Moradi, C. Sen, Switchability of a ferroelectric tunnel junction: Competition between interface asymmetry and domain states, currently under preparation.

There have been considerable efforts to grow crystalline metal oxide thin films on semiconductors for applications in electronic devices. A subset of these applications depend on ferroelectricity in a thin film of crystalline metal oxide grown epitaxially on a semiconductor. This would enable the creation of, e.g., non-volatile field-effect transistors where the oxide polarization maintains the state of the system. Unlike most current approaches, we pursue oxides that are not ferroelectric in the bulk but become so when grown as an ultrathin film on a semiconductor. Experimental realization of these systems is feasible with present epitaxial growth methods.
We use density functional theory to first examine the interface of a monolayer of ZrO₂ and the Si (001) surface. We find a number of stable structures with a range of positive and negative out-of-plane polarizations. We present geometrical and electronic analyses of these structures. We then investigate aspects of switching between states of differing polarization. First, we examine the transition energy barriers between metastable states. Second, we analyze the thermodynamically likely structures by mapping the system to an anisotropic lattice model with parameters determined ab initio. With a basic understanding of the energetics and dynamics of differently polarized domains and switching in a monolayer of ZrO₂ on silicon, we propose that a monolayer of ZrO₂ can be used as a buffer layer to induce ferroelectricity in thin perovskite oxides such as SrTiO₃ on Si, which will couple the polarization of the oxide to the silicon carrier density. We analyze the layer-by-layer polarization profile of the Si - 1ML ZrO₂ - SrTiO₃ system for various thicknesses of the oxide and therefore demonstrate the feasibility of this proposal.

Memristive nanodevices based on redox reactions and electrochemical phenomena in metal oxides represent a disruptive technology for the semiconductor industry toward several applications such as data storage, logic, analog circuits as well as novel computing paradigms. These devices, which are based on a two terminal structure (metal/oxide/metal) and exhibit a reversible change in their resistance upon electrical stimuli, are non-volatile, with low power consumption, switching times from ms down to sub-nanoseconds, and scalability down to nm scale or atomic level. While initially these devices were mainly explored for resistive switching memory devices for high density data storage, recently an increasing interest has been directed to alternative applications and with the aim to integrate novel functionalities on a CMOS platform following the More than Moore approach. Indeed, memristive devices can be fabricated by using materials and low temperature processes compatible with monolithic integration into Si based technology. Among the various proposed approaches, the memristive technologies based on the formation and dissolution of nanoscaled filamentary paths in a ionic conductive oxide are the most promising; in these systems the oxide as well as the electrodes and interfaces play a significant role in the switching mechanism depending on the selected material stacks. After an overview on recent advancements in oxide memristive device technology for various target applications, this talk will address resistive switching in HfO_x based devices as a model system to highlight switching mechanisms, material engineering through doping and scaling issues [1,2,3]. The investigated devices rely on the TiN/TiO_x/HfO_x/Pt and TiN/TiO_x/HfO_x/TiO_x/Ti/TiN structures, with HfO_x films deposited by atomic layer deposition. Beside the binary oxide, we investigated the effect of the Al doping concentration on the switching properties in TiN/TiO_x/AlHfO/Pt devices. The switching properties are analysed in micro- to nanoscale devices as well as through conductive atomic force microscopy. Beside the standard digital memory operation, the possibility to tune the device resistance in an analog way is explored to address the use of HfO_x memristive devices to build artificial synapses for neuromorphic architectures, i.e. devices that can emulate the potentiation and depression of the communication strength of biological synapses [4]. To achieve this goal different pulse scheme operations and material stacks have been characterized. Our results evidence that the non linear kinetic of the switching mechanism can be efficiently exploited to achieve an analog synaptic behavior and that these devices are promising for spike based computation paradigms.
[1] S. Brivio et al., Nanotechnology 25, 385705 (2014); [2] J. Frascaroli et al., ACS Nano 9, 2518 (2015);
[3] J. Frascaroli et al., Microelect. Engin. 147, 104 (2015); [4] E. Covi et al.,. Microelectr. Engin.147 41 (2015)

Preparation of perovskite oxide thin films at very low temperatures, over large areas and compatible with low-cost and flexible substrates offers great industrial potential. The degree of crystal perfection required for these perovskites varies according to specific application. However, in general, high temperature thermal treatments and textured growth are required for improved functional properties. This adds complexity in the materials processing in device integration. The unique characteristics of conformality, atomic scale control and low temperature deposition that atomic layer deposition (ALD) technique offers can have direct technological applications: well controlled interfaces, smaller and more demanding structures (3D substrates), increased density of devices and integration in multilayer systems. However, ALD of ternary oxides such as perovskite oxides is still in its early stages. Among the perovskite oxides, BiFeO₃ (BFO) holds great potential as stable, lead-free material that simultaneously presents ferroelectric and magnetic order at room temperature. In the literature, resistive-switching and photovoltaic responses have also been reported. Interestingly, A- and B- site engineering (ABO₃) can strongly modify their properties such as leakage current, ferroelectric properties and even the band gap. From a theoretical point of view, Bi-based perovskites (i.e. BiCoO₃) are very interesting materials as they offer large variations in crystal symmetries, polarity, magnetic and ferroelectric properties (Ps=175 µC/cm² and band gap of 0.9 eV), however, most of them can only be prepared at high-pressure high-temperature conditions and with low Co content.
Here we study for the first time the low temperature epitaxial stabilization (<3000C) of Bi_1-x(Fe,Co)_xO₃ (BFCO) thin film solid solutions by ALD. By x-ray photoelectron spectroscopy and piezoelectric force microscopy we demonstrate that ALD allows stabilization and compositional tuning of ferroelectric BFCO solid solution with high concentrations of Co. Structural properties have been evaluated by x-ray diffraction and scanning transmission electron microscopy. Spectroscopic ellipsometry measurements have also been performed to identify modifications in the band gap from 2.7 eV to 2.1 eV.

The recent realization of electro-optic (EO) devices based on the Pockels effect in BaTiO₃ (BTO) thin films on Si motivates further development of the ferroelectric oxide. Due to the challenges presented by epitaxial integration of oxides on semiconductors, molecular beam epitaxy (MBE) has served as the primary deposition method for EO-active BTO films on Si. To compare and contrast films of varied morphology and crystalline orientation, we deposited BTO thin films on Si by a two-step process: initial deposition of a 4 nm-thick SrTiO₃ (STO) buffer layer by MBE is followed by BTO deposition via MBE, sputtering, or pulsed laser deposition (PLD). In contrast to polycrystalline BTO deposited directly on Si by sputtering, x-ray diffraction confirms epitaxial films with rocking curve widths as low as 0.5° for MBE-, 0.7° for PLD-, and 2° for sputtered-BTO on STO-buffered Si. A linear EO response was confirmed for all crystalline BTO films by analysing induced rotation (δ) of the polarization of a laser beam (lambda; = 1550 nm) transmitted through lithographically defined electrodes. To detect small values of δ, a sinusoidal, alternating voltage (f = 17.3 kHz) was applied to gain sensitivity using a lock-in amplifier. Effective Pockels coefficients of 105, 2, and 18 pm/V were measured for MBE-, PLD-, and sputtered--BTO, respectively, illustrating that the magnitude of the EO response is highly sensitive to the crystalline quality of the active material. Transmission electron microscopy study provides insight into the microscopic origin of the variability. Furthermore, the ferroelectric domain configuration as well as the relative orientation between the crystalline domains and applied electric field play a crucial role in determining the magnitude of the observed EO response. These geometrical effects are analysed by comparing films of similar crystalline quality with varied domain structure and electrode configuration. The results are complemented by second harmonic generation (SHG) using femtosecond pulses. This noninvasive nonlinear optical technique reveals the symmetry and domain structure of ferroelectric BTO. Understanding of the strong influence of BTO structure on EO response enables the development of BTO/Si integrated nano-photonics devices with optimal EO performance

Ferroelectric materials such as BaTiO3 have been studied for emerging non-volatile memory applications. However, most of the previous work has focused on this material when it was deposited on insulting oxide substrates such as SrTiO3. Unfortunately, this substrate is not suitable for CMOS-based microelectronics applications. This motivated us to carry out the present work. We have studied the resistive switching behavior in BaTiO3/La0.7Sr0.3MnO3 (BTO/LSMO) heterostructures integrated with semiconducting substrates Si (100) using pulsed laser deposition1,2. Current-Voltage (I-V) measurements were conducted on BTO (500nm)/LSMO (25nm) devices at 200K, with the compliance current of 10mA. Here, Pt was used as a top electrode and LSMO served as bottom electrode. A few important observations are noted: (a) broad hysteresis in forward and reverse voltage sweeps -ideal for memory applications, (b) the ratio of high resistance to low resistance state is ~ 600, significantly higher than some of the literature values (~100, see at Nature Nanotech., 7, 101 (2012)) -important for switching devices, (c) the device is stable (endurance) at least up to 50 cycles. However, we found that hysteretic behavior was collapsed after 36 cycles upon oxygen annealing of the device at 1 atmospheric pressure, 200o C for 1 hour, inferring the important role of oxygen vacancies in the resistive switching behavior of BTO/LSMO device. In addition, we are studying the correlation between ferroelectricity and resistive switching in these devices using local probe technique PFM. The comprehensive experimental data will be presented and discussed.
1Ferroelectric and ferromagnetic properties of BaTiO3 thin films on Si (100); Srinivasa Rao Singamaneni, J. Appl. Phys., 116, 094103 (2014); 2Magnetic properties of BaTiO3/La0.7Sr0.3MnO3 thin films integrated on Si(100), Srinivasa Rao Singamaneni, Fan Wu, J T Prater and Jay Narayan, J. Appl. Phys., 116, 224104 (2014).

Impurity doping is a well-known method to increase carrier concentration of transparent conducting oxides (TCOs). However, the doping sacrifices the carrier mobility, since doped impurities work as scattering centers and decrease electron-collision length. Recently, we have reported that addition of metal nanoparticles (NPs) circumvents a tradeoff between carrier concentration and mobility of TCO films. The work function difference at the interface causes electron donation from metal NPs to oxide matrix. Once electrons are transferred to the matrix, electrons can travel freely without impurity-scattering. Cu is an ideal choice as metal NPs, since its low work function is suitable for electron donation and it is more abundant than other metals. However, Cu NPs are easily converted to oxide NPs in air ambience and/or at high temperature. A drawback of easy oxidation is detrimental to employing Cu NPs as a donor of composite TCO films.
Herein, we report our recent results on the surface passivation of Cu NPs by Ni shell and the application of modified Cu NPs to highly conductive ZnO film without donor impurities. Cu@Ni core-shell NPs were successfully synthesized via a self-seeding polyol method with an assistance of microwave radiation. The multiple-twinned Cu@Ni core-shell nanoparticles were observed, and the particle sizes were about 30 nm characterized by HRTEM. Moreover, the 20 nm Cu core and 10 nm Ni shell were also observed via the chemical mapping facilitated by STEM-EDS. The ZnO: Cu@Ni NP composite films were fabricated on glass substrates by pre-mixing those Cu@Ni NPs and ZnO sol-gel solution with variable [Cu]/[Zn] ratios. By controlling the [Cu]/[Zn] ratio, we observed that the conductivity of ZnO film was increased by up to two-orders of magnitude. This phenomenon was attributed to the electron donation by Cu@Ni NPs, since low energetic barrier at the interface between Cu@Ni NPs enabled electrons to move toward the surrounding ZnO matrix. Cu@Ni NPs showed much thermal stability than monometallic Cu NPs. The conductivity of ZnO: Cu NP composite films degraded dramatically at 300 ^oC. In contrast, ZnO: Cu@Ni NP films maintained good conductivity even after the heat treatment, and XPS spectrum of the Cu core showed no sign of oxidation. This is due to the fact that the surface of Ni shell was oxidized preferentially and formed a uniform NiO_X thin layer passivating the entire Cu core.
In addition, we also explored the optical transmittance of the composite films. ZnO:Cu@Ni NP film exhibited higher transmittance over visible light than ZnO:Cu NP films. The surface plasmonic resonance (SPR) of Cu NPs was significantly dissipated by the Ni shell, since non-plasmonic Ni NPs (or NiO NPs) dampen the collective electron oscillation. This indicates that such ZnO: Cu@Ni NP composite films could provide a good figure-of-merit as conducting electrode of optoelectronic devices such as solar cells.

Thin-film transistors (TFTs) fabricated with p-type oxide semiconductors have recently attracted much attention as it is the essential part of complementary circuitry. The development of complementary circuitry based on oxide semiconductors was used to be hampered with the lack of p-type oxide semiconductors due to the poor hole mobility of these materials. In this work, we demonstrated a good quality p-type copper oxide thin film can be deposited by the radio-frequency (RF) reactive sputtering, followed by vacuum annealing. Device characteristics of transistors using p-type copper oxide as the active layer have been also investigated. The device performance of the copper oxide TFTs gated with SiN_x is comparable to that reported by others. Nevertheless, the device performance is significantly improved with high field-effect hole mobility and low threshold voltage when the transistors are gated with protein polyelectrolyte. The significant improvement of p-type copper oxide TFTs with protein as the gate dielectrics may shed light on the exploitation of complementary circuitry fabricated with oxide semiconductors.
Reference:
1. S.H. Chen, H.C. Liu, C.Y. Lee, J.Y. Gan, H.W. Zan, J.C. Hwang, Y.Y. Cheng, P.C. Lyu, Org. Electron. 24 (2015) 200.
2. C.Y. Hsieh, J.C. Hwang, T.H. Chang, J.Y. Li, S.H. Chen, L.K. Mao, L.S. Tsai, Y.L. Chueh, P.C. Lyu, Shawn S.H. Hsu, Appl. Phys. Lett. 103 (2013) 023303

Ferroelectric oxides integrated on a semiconductor substrate are of particular interest for their applications in nanoelectronics, electro-optic devices and electro-mechanical systems. Molecular beam epitaxy (MBE) allows the construction of the interface at the atomic level, which is of utmost importance to achieve the epitaxy. Among the ferroelectric oxides, c-axis oriented BaTiO₃ has been successfully grown on Si using SrTiO₃ as a buffer layer to overcome the lattice and thermal mismatch with Si. Compared to Si, the lattice mismatch of BaTiO₃ with Ge is very low (-0.4% for Ge vs 4% for Si), which allows the direct growth of the oxide on Ge. However, Ge generates tensile in-plane strain, which promotes in-plane polarization. The solid solution Si_1-xGe_x offers the ability to tune the lattice parameter of the substrate in order to keep a compressive stress while decreasing the lattice mismatch with BaTiO₃.
In this work, we investigated BaTiO₃ thin films grown on Si_1-xGe_x/Si substrates by MBE. At first, strained Si_1-xGe_x films were used to understand the effect of Ge presence and concentration on the BaTiO₃ growth. Barium and strontium passivation were investigated to optimize the direct growth of BaTiO₃ on strained-Si_1-xGe_x substrates without the use of a SrTiO₃ template. X-ray photoelectron spectrometry (XPS) was performed in situ at different stages of the growth to understand and control the interface oxidation state. In situ reflection high-energy electron diffraction (RHEED) and X-ray diffraction were performed and give valuable information on the crystallinity and morphology of the films. The interface and crystalline structure were characterized at the nanoscale using advanced transmission electron microscopy techniques. In this presentation, the impact of Ba or Sr passivation on the interface bonds formed during the deposition process as well as their influence on the crystallinity of the BaTiO₃ films and on the interfacial layer regrowth will be discussed.

Solution based thin film deposition techniques are well known for producing high quality organic as well as inorganic metal oxide thin films. In recent years, several mist based deposition techniques have been introduced to enable solution based deposition over large areas and non-planar substrates. Mist deposition involves the creation of a precursor mist and its subsequent condensation on the substrate. Mist deposition techniques to date have been limited by disadvantages such as the requirement for highly volatile precursors and the non-homogeneous distribution of the mist, both of which lead to inferior film thickness uniformity. To address these limitations, we employ a new mist deposition system with a novel mist creation technique consisting of an atomizer with two opposing precursor jets. The unique opposing configuration of the atomizer enables the formation of a highly uniform mist even from low volatility precursors. In this work, we address the question of whether this new mist deposition technique can produce high quality film of various thickness.
Amorphous aluminum phosphate (AlPO) thin films were deposited via mist deposition using a BENEQ ACS 200-101. All films were deposited at room temperature and pressure using precursors based on aqueous suspensions of aluminum phosphate inorganic clusters. The inorganic ligands decompose at low temperature with minimal volume change presenting a route to high density films at low temperature. Mist coating technique achieved thickness uniformity of 5% across a 150 mm Si substrate. This techniques result in ultra-smooth films with average surface roughness of less than 1 nm. Variation in film density and roughness as a function of annealing temperature was identical to spin coating technique. Film of various thickness were produced by changing the deposition time and precursor concentrations. Film thickness could also be scaled by multilayer depositions.
An aqueous precursor was used in a novel mist deposition technique to deposit high quality AlPO thin films of various thickness. Films were found to have high uniformity, density and low roughness. Based on these results, this novel mist based deposition technique appears to be a promising candidate for the next generation of thin film deposition techniques for large area electronics.

Antiferromagnets (AFM) have been shown to be a promising alternative to ferromagnets (FM) in spintronic applications. The reason stems from the fact that at high data storage densities stray #64257;elds may destroy the FM set states, while an AFM would be relatively insensitive to these stray #64257;elds and maintain its anisotropic magnetoresistance (AMR). Bulk La_0.4Sr_0.6MnO₃ lies on a 4-point phase boundary where ferromagnetic metal, paramagnetic metal, canted AFM metal, and A-type AFM metal phases meet at 240K. In strained films, this material has been shown to be a prime candidate for exploring parameter tuning to control AMR arising from shifting AFM ordering. We will present recent progress on tuning the exceptional AMR responses in this material using several different techniques. Our findings show that metastable AFM states can be controlled and may be of fundamental importance to making AFM spintronic devices a reality.

Transition metal oxides are potential candidates for various applications because of their widest range of properties. These functional oxide films on semiconductor can be used for system#8209;on-chip applications. However, integration of these oxide films on semiconductor remains a challenge for research community for past four decades. The best quality oxide films on Si have been deposited by techniques such as molecular beam epitaxy and pulsed laser deposition, which are expensive and not suitable for large area deposition. These oxide films on semiconductor deposited by sputtering, which is a simple, scalable and inexpensive technique, are of poor quality. Here, (100) and (111) textured yttria stabilized zirconia (YSZ) films were deposited by reactive direct current sputtering. In-situ stresses in these films were measured by multiple beam optical stress sensor (MOSS). Texture and stress in these films can be independently controlled by simply varying deposition rate. AFM images, XRD (theta;-2theta; and rocking curve) scans and electrical measurements show effect of stress on microstructure, texture and dielectric constant of these films respectively. The effect of stress on evolution of microstructure and texture, during annealing, is explained by a model. Interesting correlation among microstructure, texture and dielectric constant of these films is studied. The effect of stress on microstructure, texture and dielectric constant of YSZ/Si films is exploited for the first time to achieve better functionality.

Band alignment between materials and potential changes across thin films is of great importance for understanding electronic properties of multilayer structures and their resulting properties in devices. Although energy alignment and band bending has been studied for decades, an accurate profile of the potential across an ultrathin insulating layer (<10nm) has yet to be determined. In this work, studies of potential changes across metal-insulator-semiconductor (MIS) systems are realized using x-ray photoelectron spectroscopy (XPS) while applying an external bias perpendicular to the surface plane between metal and semiconductor. The use of an ultrathin conductive electrode (CVD graphene) and a thin dielectric enables the substrate to be analyzed by photoemission. We report on a graphene-SiO₂-Si structure, with an ultrathin SiO₂ layer (~6nm). The shift in binding energy of electrons photoemitted from different physical positions as a function of external bias provides a new method to follow changes in the potential across the dielectric. We also use marker atoms and layers at both interfaces and in multi-layered structures, as they can be identified and their “chemical” shifts quantified, yielding accurate information about potential across the film. To complement experimental studies, density functional theory (DFT) calculations on model MIM and MIS systems are also presented to help understand the underlying electronic structure and potential profile.

Volatile organic compounds (VOCs) are very harmful to human and numerous techniques has been applied to reduce the concentrations of these materials such as toluene, xylene, ethylbenzene, etc. Titanium dioxide (TiO₂) materials are generally known to decompose various VOCs under UV light but this VOC decomposition is observed only under UV light source lower than the wavelength of 365nm. In this study, metal doped or coupled TiO₂ nanoparticle samples were prepared by one-pot synthesis with various metal source and synthesis conditions. For the optical bandgap measurement of the prepared nanoparticles, the absorption spectra are measured by UV-visible spectrophotometer and the bandgap calculation are performed by Kubelka-Munk theory. Photocatalytic activity of metal-TiO₂ nanoparticles fabricated at various conditions were characterized by both the degradation of methylene blue in aqueous solutions and the degradation of gaseous VOCs in a closed chamber. . VOC removal abilities are quantified by the variation of various VOCs concentration in the closed chamber system with UVLED light source. Nanocomposites show significant enhancement of photocatalytic abilities and various VOC decomposition results depending on metal dopant species, concentration, and calcination temperature. Obtained results show that the as-prepared metal-TiO₂ nanocomposites have novel VOCs degradation abilities and could be applicable for filter media, air purifies, and other gaseous pollutant reduction.

Nitride/nitrogen oxides are widly used in phosphor converted white LEDs as the orange-red phosphor for improving its color-rendering index. At present, many oxide and several oxygen-free nitride phosphors have been studied. Among all these materials, owing to higher light efficiency, better thermal and chemical stability with a broad absorption band spanning from UV to the green region of the electromagnetic spectrum, (Sr,Ca)AlSiN₃:Eu²⁺ has been considered as one of the most promising red phosphors for white LEDs. But the harsh reaction conditions such as high pressure (190MPa) and high temperature (1900#8451;) restrict its commercial applications. For decrease reaction conditions and improve crystalline properties, Li₃N Flux was considered and investigated in this paper. Sr_0.7Ca_0.3AlSiN₃:Eu²⁺ had been successfully produced by the addition of Li₃N flux at 1800#8451; and 0.4MPa. The samples were identified by powder X-ray diffraction. The morphology of the products and the photoluminescence properties with different amount of Li₃N were also examined.

Amorphous In-Ga-Zn-O thin-film transistors (a-IGZO TFTs) have been intensively investigated for applications in next generation displays due to their superior transparency and high field-effect mobility compared with amorphous Si TFTs. Conventional a-IGZO TFTs fabricated by sputtering process have a limitation in requiring additional annealing process over 300^oC after deposition of a-IGZO active layers. This high thermal energy activates a-IGZO TFTs to show desirable semiconducting behavior by forming of chemical bonds and curing of defects from ion bombardment damage. However, the high temperature annealing is a barrier for a-IGZO TFTs to be fabricated on various flexible substrates such as polyethersulphone (PES) and polycarbonate (PC).
In this research, we propose a new method to activate sputtered a-IGZO TFTs by applying voltages to electrodes along with annealing at low temperature (150^oC). The a-IGZO active layer was deposited on the heavily doped p-type Si wafer with thermally oxidized SiO₂ of 1200 #8491;, and aluminum source and drain electrodes were sputtered on the a-IGZO active layer. Then, we applied various bias voltages to gate, source, and drain electrodes and annealed the sample at 150^oC for 1 hour simultaneously. Bias voltages applied to gate, source, and drain varied from -100 to +100 V, and the optimized gate, source, and drain voltages were +100, 0, and +10.1 V, respectively. The sample activated in the optimized condition exhibited improvements in electrical characteristics compared with the sample annealed at 300^oC for 1 hour: field-effect mobility increased from 7.33 to 7.50 cm²/Vs, on-off current ratio increased from 8.96 x 10⁷ to 2.31 x 10⁸, and the shift in V_on decreased from -0.4 to +0.2 V.
By controlling the bias voltages, the activation effect of applied electric field on samples can be controlled. The electrical energy compensates for insufficient thermal energy of low temperature, and we decreased the activation temperature from 300^oC to 150shy;^oC without degradation of electrical characteristics. Electrical and thermal treatment on the active layer at the same time induces local joule heating between source and drain selectively, and this efficient annealing focused on the narrow area does not damage the substrate. Therefore, this study demonstrates the possibility of electric field-aided activation at low temperature which could be the first step for future display.

Amorphous oxide semiconductor thin-film transistors typically require thermal activation above 300^oC to acquire acceptable electrical characteristics and stability. Although this temperature is necessary to control defect sites related to oxygen deficiencies, it is still too high for certain flexible substrates with low melting point, therefore it limits substrate options for application in flexible displays. Herein, we propose an effective simultaneous ultraviolet (UV)-thermal method for IGZO TFTs which not only enables low temperature activation but also improves electrical characteristics and stability. After deposition of IGZO films on p⁺-Si/SiO₂ substrate via sputtering, the films are irradiated with UV light of wavelengths of 185 nm and 265 nm and thermally annealed at 150^oC on hotplate simultaneously for 1 hour in air. Furthermore, to compare the effect of UV and thermal treatment orders, we conducted only-UV, only-thermal, UV after thermal, and thermal after UV treatment. Out of the five control devices, the simultaneous UV-thermal treated IGZO TFTs exhibited the most superior electrical characteristics than only-thermal (300^oC) treated devices including mobility (15.81 vs. 3.23 cm²/Vs), on-off ratio (1.03 x 10⁸ vs. 3.96 x 10⁷), and threshold voltage shift under positive bias stress (time = 10,000s; 7.2 vs. 11.2 V). These results indicate that the UV-thermal treatment leads to higher metal-oxide (M-O) bonds and lower defect sites such as oxygen vacancies and carbon residues in the bulk region. This is because UV irradiation with wavelength of 185 nm and 265 nm can promote generation of oxygen radical and decomposition of weak M-O bonds in IGZO films. In addition, thermal treatment promotes reactions between generated oxygen radical and IGZO metal lattice, as well as effectively rearranges the decomposed weak M-O bonds.

Recently, amorphous oxide semiconductor thin-film transistors (TFTs) are extensively investigated as next generation material due to their superior device performance such as high mobility and optical transparency. In fabrication of oxide TFTs, oxide thin film need activation process with annealing over 300°C. However, there is limitation to choose various flexible substrates in transparent flexible TFTs due to its low glass transition temperature. In order to solve this problem, method for low-temperature process should be suggested.
In this research, we investigated the method to lower annealing temperature for fabrication of In-Ga-Zn-O (IGZO) TFTs with rotating magnetic field. IGZO film was annealed on stirring hot plate with rotating magnetic field and thermal energy simultaneously for its activation. To verify the effect of rotating magnetic field, we controlled the rotation speed of magnet in stirring hot plate by 300, 500, 800 and 1150 rpm, respectively. The IGZO TFTs activated with 1150 rpm magnetic field under 150°C for 1 hour showed the relatively high electrical performance compared with those under 300°C for 1 hour: The field-effect mobility (mu;_FE) increased from 11.75 to 13.23 cm²/Vs, on-off ratio increased from 7.26 × 10⁷ to 2.54 × 10⁸, and a sub-threshold swing (S.S) decreased from 0.47 to 0.43 V/dec. Rotation of magnet in stirring hot plate can change the magnetic flux which can make electromagnetic induction. Also, this electromagnetic induction causes scattering of electrons which can aid the activation of IGZO film with reducing annealing temperature. In this result, we noticed IGZO TFTs activated with rotating magnetic field under 150°C for 1 hour have sufficiently high transfer characteristics compared with those under 300°C for 1 hour. Consequently, it can be diversified to choose flexible substrates by additional process with applying rotating magnetic field in the process of activating IGZO TFTs.

The ZnO thin films are grown on the p-Si for the hetero junction fabrication by pulsed laser deposition method. X-ray diffraction method is used to study the crystalline structure and grain size of grown ZnO thin film. The lattice constants of ZnO obtained from these X-ray peaks are found to be c = 5.187 Å and a = 3.605 Å. The average crystallite size of the ZnO thin film calculated from the full width at half maximum (FWHM) of diffraction peak using Debye-Scherrer formula found to be 22.6 nm. Optical properties of ZnO thin film have been studied by UV-visible and photoluminescence spectroscopy. The energy band gap of ZnO thin films calculated from the Tauc plot found to be 3.43 eV. In order to probe the energy transitions within ZnO and determine the defect structure the photoluminescence measurements were undertaken. At room temperature photoluminescence spectra of ZnO thin film grown on p-Si silicon, three emission peaks appears. In UV region the low near band-edge emission at about 376 nm has very low intensity peak and corresponds to the band gap of ZnO. The highly crystalline ZnO as shown by XRD and Raman analysis would be expected to generate strong near band emission. The diminished near band emission indicates rapid recombination of charge carriers in the defect energy states, which gives rise to intense deep-level emission at 747.8 nm (1.66 eV). This energy emission at 1.66 eV is almost equal to half the value of energy emitted in near band-edge emission (3.30 eV) calculated above. This clearly indicates that these mid gap defect states giving rise to emission at 747.8 nm are exactly at the center of the band gap of ZnO. These mid gap states may arise due to deep level defects in ZnO. Experimental observations confirmed that the deposited ZnO thin film has potential for sharp emission in the red region. High purity (99.999%) vacuum evaporated aluminum metal was used to make contacts to the n-ZnO and p-Silicon. The current-voltage characteristics of Al/n-ZnO/p-Si(100)/Al hetero junction are measured over the temperature range 60-300K have been analyzed on the basis of thermionic emission diffusion mechanism. The equivalent Schottky barrier height and diode ideality factor are determined by fitting of measured current-voltage data into thermionic diffusion equation. It is observed that the barrier height decreases and the ideality factor increases with decrease of temperature and the activation energy plot exhibit non-linear behavior at low temperature. These characteristics are attributed to the Gaussian distribution of barrier heights. The capacitance-voltage characteristics of Al/n-ZnO/p-Si(100)/Al hetero junction diode are also studied over wide temperature range. From the measured capacitance-voltage data the built in voltage and impurity concentration in n-type ZnO is estimated.

Cobalt ferrite (CFO) is one of the hard magnetic insulator materials with wide ranges of application such as magnetic recording media, ferrofluids and tunnel magnetoresistance. In this work, the CFO films were grown on silicon substrates with 300 nm amorphous silicon dioxide by pulsed laser deposition (PLD) under different annealing conditions. X-ray diffraction (XRD), atomic force microscopy (AFM), Raman spectra and X-ray absorption spectra (XAS) were carried out to investigate the correlation with the magnetism measured by superconducting quantum interference device (SQUID). The XRD reveals the CFO films are single orientation (111) in out-of-plane direction and AFM images display the roughness of the films is approximately 1 nm. All the results prove the high quality of the CFO films. The Raman spectra indicate that the Co ions can transfer from tetrahedral sites to octahedral sites with increasing the annealing pressure. In XAS measurements of the Fe L2 and L3 edges suggest that the Fe ions exchange the sites with the Co ions. The site exchange of Co and Fe ions leads to the change of saturation magnetization in the CFO films. Our experiments provide not only a way to control the magnetism of CFO films, but also a suitable magnetic layer for the integration in Si-based devices.

Zinc based amorphous semiconducting oxides (ASOs) have attracted a great deal of attentions as a promising alternative to Si based semiconductors for applications to high speed and flexible electronics. This is because the high carrier mobility of ASOs which shows ten times more than that of amorphous Si (a-Si) can be achieved with room temperature fabrication. In spite of advantages of ASOs, the realization of the device using ASOs is still limited and several problems to be overcome have been reported. Among them, the photo-bias instability of ASOs is considered as its critical bottleneck. To be specific, the threshold voltage (V_th) of ASO based TFT is fairly shifted by illumination and gate bias stress and long time over a few hours is required for its full recovery to the original state. Various models to explain V_th shift based on gate oxide/ASO interface trap or field induced oxygen vacancy migration have been suggested, the underlying mechanism still remains unclear.
In this study, we investigate the impact of hydrogen impurity in ASOs which is semi-intrinsic defect incorporated during device fabrication on photo-bias instabilities using density functional theory (DFT) calculation and experiments. Our calculation results show that the charge state of hydrogen has bistablity in ASOs depending on Fermi level (E_F) and hydrogen tends to trapping(detrapping) an electron when the positive(negative) gate bias is applied causing V_th shift. Furthermore, the thermal energy barrier for transition between cation and anion states exists implying long time for the recovery process of shifted V_th. It is also found that the illumination accelerates negative V_th shift by extracting an electron from anionic hydrogen.
To examine the DFT calculation results, we fabricated two kinds of TFTs using amorphous In-Zn-Sn-O (a-IZTO): one has thin Al₂O₃ layer above a-IZTO active layer for passivation of hydrogen incorporation into a-IZTO and the other does not. We observe the clear improvement of the photo-bias instabilities for the device with Al₂O₃ passivation layer showing much smaller V_th shift in comparison with the other one.

Sol-gels are widely used as precursors for the printing of metal oxide films. These films can be used as either semiconductors (ZnO, In₂O₃, hellip;), dielectrics (ZrO₂, SiO₂, hellip;), or conductors (ZnO:Al, In₂O₃:Sn, hellip;). As semiconductors, they show the best performance of any printable material, but often require temperatures in excess of 500 °C to fully convert. We show that by using a catalytic film beneath the sol-gel we can reduce the temperature at which these sol-gels convert. Specifically, we demonstrate that sol-gels formed from zinc acetate solutions in alcohols convert at lower temperatures on zirconia than on silica. We use X-ray photoelectron spectroscopy to quantify the conversion of the sol-gel by observing the shift in binding energy of the O1s peak as the film converts from a hydroxide to an oxide. We show that by 150 °C the zinc oxide is more fully converted on the zirconia than on the silica at 300 °C. Since both the zirconia and silica can act as a gate dielectric, we can exploit this catalytic effect and are able to achieve mobilities as high as 1.5 cm²/V-s, at plastic compatible temperature. Typical enhancement over non-catalytic conditions is >2 orders of magnitude. This work demonstrates the utility of catalytic layers to reduce the conversion temperature of sol-gels to metal oxides, thereby improving their electrical performances at lower temperatures.

InAs has generated much interest due to its small bandgap and high electron mobility quasi-two dimensional electron gas (2DEG) which forms at the surface of oxidized and atomically-clean InAs. The origin of the 2DEG remains unknown despite decades of research on the electronic properties of this model confined electron gas system. Electrical characterization of the 2DEG and its correlation to oxide chemistry and related defects which form at the oxide-semiconductor interface is key to understanding the factors controlling conductivity. This study perturbs the formation of the oxide through InAs surface termination in order to reveal dominate chemical reactions yielding the heterogeneous In- and As-based native oxide. Herein, we study the relationships between the 2DEG conductivity, specifically the electron density and mobility, and the oxide chemistry of 100nm InAs films terminated with In or As monolayers using Molecular Beam Epitaxy. X-ray photoelectron spectroscopy (XPS) was used to quantify the oxide chemistry and the electrical data were obtained using the Van der Pauw (VdP) Hall technique. We speculate that the observed trends in the 2DEG conductivity result from differences in the nature and densities of the predominate defects associated with different oxide chemistries and their formation. We show that by changing the relative abundance of In and As at the surface, the native oxide chemistry is modified as well as the 2DEG mobility and concentration. X-ray photoelectron spectroscopy revealed that different terminations resulted in varying degrees of hydroxide formation and different compositions and spatial heterogeneity of the key oxide components:, In₂O₃, As₂O₃ and As₂O₅. Overall, the surface termination impacts the 2DEG mobility and carrier concentration through the extent and homogeneity of oxygen incorporation during the formation of the oxide layer. The formation of In-hydroxide correlated positively with As₂O₅/As-OH formation. Furthermore, these two oxides negatively correlated with elemental As formation at the oxide surface. From the data, we hypothesize that In-hydroxide is an electron trap which reduces the sheet carrier concentration of the 2DEG.

Cerium doped yttrium iron garnet (Ce:YIG) thin films are promising candidates for integrated nonreciprocal photonic devices on silicon. The magneto-optical effect of Ce:YIG thin films is proportional to Ce ion concentrations. So far, the highest Ce concentration in Ce:YIG thin films deposited on Gd₃Ga₅O₁₂ substrates is Ce_2.5Y_0.5Fe₅O₁₂. However for thin films grown on silicon, the Ce concentration is limited to CeY₂Fe₅O₁₂. Therefore, it is highly desired to increase the Ce concentration in films deposited on silicon for strong magneto-optical effects and compact on-chip nonreciprocal photonic device applications.
In this study, we used combinatorial pulsed laser deposition (PLD) method to deposit Ce:YIG thin films with high Ce concentration on silicon. Thin films with record high Ce concentrations of Ce_1.3Y_1.7Fe₅O₁₂ were successfully grown on silicon, which showed about 3 fold enhancement of the Kerr rotation at 635 nm wavelength compared to previously reported Ce₁Y₂Fe₅O₁₂. Ce_xY_3-xFe₅O₁₂ films with Ce concentration varying from x=0.8 to x=1.4 were deposited on polycrystalline YIG thin film (60 nm) buffered Si substrates by PLD. X-ray diffraction shows that all films up to x=1.3 crystallized into the garnet phase, with the exception of x=1.4, which yielded partially amorphous phases. The lattice parameters of x=0(YIG seed layer), 0.8, 1.0, 1.2, 1.3 samples are 12.307 Å, 12.442 Å, 12.462 Å, 12.464 Å and 12.476 Å respectively. The monotonic increase of lattice constants with Ce concentration demonstrates the incorporation of Ce³⁺ into the crystal lattice. FTIR of all films featured characteristic absorption peaks of asymmetric stretching modes of the tetrahedron Fe-O bond of YIG, which peaked at around 558, 586 cm^-1 for the YIG seed layer. The peak positions monotonically red shifted to 545, 566 cm^-1 with Ce concentrations up to x=1.0, possibly due to the decrease of the Fe-O bond strength. For higher Ce concentrations, the peaks intensity gradually decreases and are convoluted with absorption peak from the YIG seed layer. Longitudinal magnetic-optic Kerr hysteresis loops were measured at the wavelength of lambda;=635 nm at room temperature. All Ce:YIG films show opposite signs of Kerr rotation compared to the YIG seed layer, and the Kerr rotation increase with the Ce concentration. Moreover, the Kerr rotation value is proportional to the lattice constants, which demonstrates the major contributor to magneto-optical effect is the Ce³⁺ ions. For Ce_1.3Y_1.7Fe₅O₁₂, the Kerr rotation is 3 and 16 times larger than the sample of x=1.0 and pure YIG thin films respectively. The highly Ce substituted YIG thin films with record high magneto-optical effect demonstrates the importance of seed-layer on stabilizing the garnet phase on silicon, which may promote on-chip integration of highly compact and low loss nonreciprocal photonic devices.

While the display market has been developed rapidly, thin film transistors(TFT) characteristics such as minimizing of the RC delay, high mobility and high stability become more important in backplane of display. Because of its high mobility, simpe device process, and scalability, oxide semiconductor TFTs have attracted much interst for high resolution active matrix display. Most of high mobility oxide semiconductor TFTs, however, suffered from the controlling of Vth and instabilities originated from the oxygen vacancy and hydrogens in both semiconductor and gate insulator. To control these speicies, optimization of oxygen partial pressure during the semiconductor deposition and gas ratio and plasma power of the PECVD process for the gate insulator deposition have been key technologies to obtain high perfromance oxide TFTs.
As one of other technologies in deposition of gate insulator, atomic layer depostion has been investigated. According to our previosu results,* TFTs with alumina GI deposited by plasma enhanced atomic layer deposition (PEALD) method resulted in better NBIS stability performance than those with alumina grown by thermal ALD (ALD) methode due to the minimization of H diffusion from gate insulaor to the active layer and inducing more stoichiometri alumina film. However, changes on TFT characteristics such as mobility, sub-threshold swing(SS), turn on voltage (Von) and instability can be induced by plasma damage. Therefor, it is very important to optimize the PEALD process conditions in oxide TFT. In this study, with manipulating plasma power, we deposited alumina for the GI by PEALD method to confirm the effect of plasma damage on IGZO TFT characteristics. As a result of the plasma power split into 40W, 60W, 90W, and 120W, we found out that Von shifted to zero, SS value decreased and hysterisis increased with power increase. However, Von value rapidly shifted to negative direction at plasma power of 120W or above. While our TFT showed very stable properties for NBIS and NBTS conditions, some TFT parameters changed in PBTS condition for various process conditions. We will suggest the way to optimize conditions of GI deposition process for high performance and high stability in oxide TFT.

We discuss the characteristics of structural and electrical properties of polycrystalline Al-doped ZnO (AZO) films with ZnO/Al₂O₃ (1.0 wt.%, 2.0 wt.% and 3.0 wt.%) composite targets, to clarify the dependence of the carrier transport on the Al-dopants contents. 500-nm-thick AZO films were deposited on glass substrates (@200 0C) by direct current (DC) magnetron sputtering with a DC power of 200 W.
The analysis of the data obtained by out-of-plane XRD measurements showed strong intensity of the wurtzite ZnO 0002 peak for all the AZO films. AZO films with an Al₂O₃ content of 1.0 wt. % exhibited a peak of a 10-11 plane of ZnO. With increasing Al₂O₃ contents up to 2.0 wt.%, we found the 0002 peak with the 10-11 peak of diminished intensity. With further increasing the Al₂O₃ contents up to 3.0 wt.%, the XRD patterns of the AZO films showed sharp 0002 peak with strong intensity together with the 10-11 reflection being completely absent.
The results of Hall effect measurements were as follows. An increase in the Al₂O₃ contents from 1.0 to 3.0 wt.% increased carrier concentration (N) from 4.57×10²⁰ to 8.89×10²⁰ cm^-3 monotonically. On the other hand, with increasing Al₂O₃ contents up to 2.0 wt.%, Hall mobility (µ_H) increased from 28.7 to 33.5 cm²/Vs. With further increasing up to 3.0 wt.%, µ_H decreased abruptly to 29.8 cm²/Vs. With increasing Al₂O₃ contents up to 2.0 wt.%, electrical resistivity gradually decreased, above which it changed little, as a result.
To obtain a better understanding of the above behavior of µ_H with Al₂O₃ contents, we investigated the changes in optical mobility (µ_opt) corresponding to the carrier mobility in grain bulk and in the contribution of grain boundaries (GBs) scattering to the carrier transport, which is defined by the ratio of µ_opt to the carrier mobility at GBs (µ_GB), µ_opt/µ_GB=(µ_opt-µ_H)/µ_H. µ_opt was calculated based on the Drude model with Tauc-Lorentz model using the experimental optical data.
We found that the value of µ_opt remained almost constant, asymp; 37 cm²/Vs, in cases of the Al₂O₃ contents of 1.0 and 2.0 wt.%. Further increase in the Al₂O₃ contents up to 3.0 wt.%, we found a drastically reduction of the value of µ_opt, 26.8 cm²/Vs. The decrease of µ_opt can be attributed to not only the increase of N but also the degradation of lattice symmetry of grain bulk due to the presence of Al interstitial, n-type defects, together with the generation of Zn vacancies as compensation. With increasing Al₂O₃ contents up to 2.0 wt.%, the values of µ_opt/µ_GB rapidly decreased. In our previous work, we discussed the relationship between the presence of growth mode of the 10-11 plane and µ_opt/µ_GB, which explains the above findings using the behavior of the out-of-plane XRD measurements results. On the other hand, we found that the value of m_opt is smaller than that of µ_H for AZO films with the Al₂O₃ contents of 3.0 wt.%. Further study on the cause should be needed.

SrTiO₃ belongs ABO₃ perovskite family, large-gap semiconductor & quantum paraelectric. Cubic at room temperature with interposition of TiO₆ octahedra & SrO₁₂ cuboctahedra, undergoes cubic-tetragonal structural phase transition at 105 K, driven by condensation of zone corner phonon involving rotation of oxygen octahedra. Proximity to ferroelectricity leads to large dielectric constant. Competitive or cooperative interplay between ferroelectric & antiferrodistortive distortions also link between superconducting order & two lattice instabilities attract attention. JT centers allows dynamical charge transfer, interband interactions, polaron-bipolaron formation. Dynamical covalency elicit structural instability in layered superconductors approach ferroelectrics. Magnetic transition elements doped STO with Cr⁴⁺ (d²), Cr⁵⁺ (d¹) & Fe⁴⁺ (high-spin d⁴) substituting host Ti⁴⁺exhibits JT distortion. RBS- ICh is used to determine structure, position of impurity atom to study defects of crystals & provides direct evidence for JT effect at Cr⁴⁺,Cr⁵⁺& Fe⁴⁺centers in STO lattice. Distinct masses of STO elements helps to probe impurities by ICh method to determine distortions of sub lattices without considering small ICh effect from impurities. 2.0 MeV He⁺ axial ion channeling is used to study JT lattice distortions of ion implanted STO. Angular ICh spectra of Sr & Ti sub lattices (crystal axes [110] & [100]) are obtained for pure & ion implanted STO crystals. Angular ICh spectra for Sr & Ti sublattices of pure STO have a nearly symmetric U shape, indicates perfect lattice. Though scattering effect of Cr & Fe in RBS spectra is screened by the effect from Ti, the increase of RBS yield in doped STO than pure STO, indicates the presence of lattice distortions in doped crystal. Experimental critical channeling angle psi;_c & ratio of minima of ICh-RBS yield Chi;_min for Sr and Ti sub lattices are used for quantitative determination of observed lattice distortions due to impurities.After Cr/Fe-doping and 10000C post-annealing defects still remain in sample. Absence of peak in minima of angular ICh spectra of doped crystals suggests, impurity Cr/Fe is not located in interstitial positions & does not form interstitial defects.Cr & Fe impurities mostly distort Ti sublattice. Narrowing of U shaped wells for Ti sublattice suggests displacement of Ti ions from ideal lattice sites. Displacements of Ti ions due to presence of Cr/Fe are calculated. Similar values of angles psi;_1/2 for U shaped wells for Sr sublattice indicates Cr/Fe impurity hardly causes displacements of Sr ions. Distortion of Ti sublattice means Cr/Fe is actually located in Ti positions.JT Cr⁴⁺& Fe⁴⁺ impurity could induce Raman-active localized oxygen vibrational mode, which does not involve motion of nearest Fe or Ti ions. Displacive phase transition is investigated by RBS- ICh, provides direct evidence of changes with temperature in thermal vibrational amplitude of lattice atoms across the structural phase transition.

Yttria stabilized zirconia (YSZ) is a well known buffer layer material for depositing functional oxides and also used as solid electrolyte in Solid Oxide Fuel Cells (SOFC). Controlling stress and microstructure in YSZ is vital for any of it&’s functionality. Radio frequency (RF) sputter deposition of YSZ on Pt/TiO₂/SiO₂/Si and Si was performed. The effect of growth parameters such as pressure and substrate temperature during deposition was analyzed. Influence of substrate on film micro structure and stress evolution is examined. Two stages in stress evolution are observed, stress is transformed from high compression at thickness <20 nm to tension or less compression at higher thickness. The stress transition is analyzed based on the growth factors. By using temperature ge; 500^oC or pressure of 500 mTorr, the second stage in stress can be evaded.
YSZ films with different microstructure were deposited just by tuning deposition conditions. Adatom incorporation into grain boundaries was the dominant source for the compressive stress and zipping of the neighboring grains to form a grain boundary was the cause of tensile stress. Substrate roughness was augmenting the physical screening effect, this resulted in porous/columnar microstructure and rough surface morphology, and hence the tensile stress developed even at lower film thickness. Substrate roughness is identified as a parameter to tune microstructure and stress evolution in thin films

Microstructure control during thin film deposition is crucial for fine-tuning of the functional properties. In this study, ZrO₂ thin films were deposited on Si by reactive DC sputtering, and a model is developed to predict microstructure based on deposition parameters. Understanding the microstructure evolution of ZrO₂ film is a prerequisite to study their functionality. Dense films of zirconia are required for buffer layer and solid electrolyte for a solid oxide fuel cell (SOFC) applications. Porous/columnar microstructure of zirconia is necessary for sensor applications, which require a large surface area.
The deposition rate is used as a key to control microstructure. Lower growth rate (<5nm/min) resulted in dense microstructure with equiaxial grains, whereas the high growth rate (>15 nm/min) produced columnar microstructure. A closer look at the evolution of the columns revealed an interesting correlation between growth parameters and microstructure. Mobility of the species/adatoms on the growth front is the decisive factor that reins microstructure. As the film grows column width increase and saturates at higher thicknesses. The width is also a function of growth flux density and rate of attachment of the adatoms on the growth front. The microstructure evolution and the effect of deposition conditions were quantified using the model and compared with the experimental results.

In this paper, we will report on the formation of HfO₂ layer on an HF-last Si(100) substrate by atomic layer deposition from tetrakis(dimethylamido)hafnium (TDMAH) and atomic oxygen generated by a microwave remote plasma. Transmission electron microscopy observations of HfO₂ /Si structures deposited at 100 and 300 ^oC revealed that 3~5-nm-thick amorphous HfSi_xO_y
layers were unintentionally formed preceded the growth of crystalline HfO₂ layers. To understand the mechanism of this unintentional growth of HfSi_xO_y, the depth profiles of Hf, O and Si elements were measured by X-ray photoelectron spectroscopy. It was found that Hf atoms deeply diffused into the Si substrate. From these results, suppression of Hf in diffusion to the Si substrate must be important to reduce the capacitance equivalent thickness of the metal-oxide-semiconductor capacitors. The roles of TDMAH and plasma-generated oxygen radical on the enhanced diffusion of Hf will be discussed in detail.

VO₂ thin films featuring a metal-insulator phase transition have attracted great interest for their wide tunability for infrared optical transmission and conductivity. Recently, the oxygen concentration is found to strongly influence the phase transition hysteresis of VO₂. Particularly, when the films are oxygen rich, the phase transition temperature increases and the hysteresis became significantly wider. Whether V⁵⁺ causes the change of phase transition hysteresis is unclear. A micro scale structural understanding of this phenomenon is lack. In this study, we carried out confocal Raman microscopy mapping to analyze the metal-insulator phase transition in oxygen rich VO₂ thin films.
VO2 thin films were grown by pulsed laser deposition from a vanadium metal target on silicon substrates under an oxygen partial pressure of 5 Pa at room temperature. The as-deposited amorphous thin films were subsequently annealed at 500^oC for one hour with different oxygen pressures to form the VO₂ phase. Two films annealed at P_O2=150 Pa(Sample A) P_O2=250 Pa(Sample B) respectively were analyzed by X-ray diffraction. The crystal size of the VO₂ phase is decreased from 120 nm in sample A to 78 nm in sample B with increasing the annealing oxygen partial pressure. Both samples were analyzed on a Reinishaw inVia micro Raman microscopy equipped with a nitrogen protected temperature stage. Mapping of the Raman spectrum intensity was carried out over a 20 mu;m by 20 mu;m region for both samples in a heating and cooling phase transition cycle from room temperature (25^oC) to 80 ^oC. Characteristic peaks at 194 cm^-1, 224 cm^-1 and 613 cm^-1 were observed in sample A at room temperature, which diminish with the insulator to metal phase transition at higher temperatures, indicating most vanadium ion is in the V⁴⁺ state and VO₂ phase. Although similar peaks and phase transition process is observed in sample B, additional V⁵⁺ characteristic peaks were observed at different regions. Three regions featuring either similar Raman spectroscopy as sample A, or additional peak at 144 cm^-1 due to layered structure of the V₂O₅ phase, or additional peaks at 163 cm^-1 due to the layered structure of V⁴⁺ in a V₂O₅ structure respectively were identified. Raman analysis of these regions were carried out during the phase transition process. Interestingly, although they show different structures, the phase transition hysteresis are equivalent, and are all significantly wider than that observed sample A. This result suggests that V⁵⁺ rich phase is unlikely to cause the wider metal-insulator transition hysteresis in VO₂. The decreased crystal size may be the dominant factor of VO₂ phase transition. This study provides valuable insight of phase transition process in oxygen rich VO₂ thin films.

Amorphous oxide semiconductor (AOS) based thin-film transistors (TFTs) have attracted as a promising alternative to amorphous silicon based TFTs due to their high mobility, low off current, and high transparency [1]. However, AOS TFTs have a critical issue on bias stability due to the interaction between ambient atmosphere and back channel layer [2]. To solve these issues, many studies have been studied using a vacuum and solution-processed passivation layer. Especially, solution-process has many advantages such as low cost, simplicity of process, and damage free of plasma [3]. In this study, we investigated the bias stability characteristic of AOS TFTs using solution-processed hafnium oxide (HfO_x) passivation layer.
We fabricated inverted staggered amorphous indium gallium zinc oxide (a-IGZO) TFTs by sputtering process. For the HfO_x passivation layer, a HfO_x solution was spin-coated onto the a-IGZO channel layer, and annealed in air at 160#8451; for 2 h. On the other hand, non-passivated TFT was also annealed additionally at 160#8451; for 2 h for same thermal treatment. To investigate the effect of HfO_x passivation layer, we performed the positive bias stress (PBS) test (V_GS = 20 V and V_DS = 10.1 V, applied 1000 s). The threshold voltage shift of HfO_x passivated TFT was 2.03 V, whereas non-passivated TFT was 6.27 V after PBS test. Therefore, solution-processed HfO_x layer has a potential to resolve the bias stability issue.
[1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature, 432, 488 (2004).
[2] X. Xu, L. Feng, S. He, Y. Jin, X, Guo, IEEE Electron Device Lett., 33, 1420-1422 (2012).
[3] S. Nam, D. S. Chung, J. Jang, S. H. Kim, C. Yang. S.-K. Kwon, and C. E. Park, J. Electrochem. Soc., 157, H90 (2010).

S12A7 (12SrOmiddot;7Al₂O₃) electride is a new oxide semiconductor, having an attractive electrical property [1]. The crystal structure is composed of cage-like local structure, which is similar to C12A7 (12CaOmiddot;7Al₂O₃). The cages are linked with each other three dimensionally, forming the framework of the crystal. Since the framework [Sr₂₄Al₂₈O₆₄]⁴⁺ has four positive charges per unit cell, the cages can engage the negative ions (mX^n-) to compensate the electrical charge. The whole crystal is denoted as [Sr₂₄Al₂₈O₆₄]⁴⁺(mX^n-). It is reported that S12A7 decomposes at very high temperature because it is metastable phase. Thermally stable, crystalline S12A7 crystal was synthesized successfully by encaging halogen ions. Furthermore, it was possible to replace a part of the halogen ions by electrons. As a result, a dense sintered body of S12A7 electride was obtained, in which the relative density was ~94%. It is expected that the work function of S12A7 electride would be lower than that of C12A7 electride (~2.4eV) since Sr has a lower electronegativity and a larger ionic radius in comparison with those of Ca.
[1] M.Miyakawa, N.Ueda, T.Kamiya, M.Hirano and H.Hosono, Journal of the Ceramic Society of Japan, 115[9]567-570, (2007)

Zinc Oxide (ZnO) is a highly degenerated wide band-gap semiconductor. The ZnO films should be one of the most potential candidates for transparent semi-conductive oxides as well as In-based films. ZnO films have been deposited by wet process using a tetrahydroxozincate(#8545;) ion because of its advantages with atmospheric pressure deposition. However, to achieve atmospheric pressure deposition is difficult in the case of using the solution that include tetrahydroxozincate(#8545;) ion because of its low formation energy. The solution process using a non-uniform nucleation or electric field should be one of the most possible techniques for atmospheric pressure deposition. Because the crystal-nucleation formation energy can be reduced by the tetrahydroxozincate(#8545;) ion.
The electrical properties of ZnO films deposited by solution process should be strongly affected by the morphology. Therefore, in order to obtain the high-quality ZnO films by solution process with high crystalline morphology, the deposition conditions should be precisely controlled. In this study, ZnO films were deposited on substrates by non-electrical or electrolytic depositions. In case of non-electrical deposition, the substrate was immersed in nitric acid with precisely controlled pH condition. On the other hand, the substrate and Au electrodes were immersed in nitric acid in zinc nitrate solution for electrolytic deposition.
The solution containing zinc ion was added in a basic solution to prepare a basic solution coating tetrahydroxozincate(#8545;) ions and having a pH of 13 or more. Next, this solution was added to the Zn ion containg solution in order to be the pH 12.6 or less. A basic solution and tetrahydroxozincate(#8545;) ion based solution were NaOH and Zn(NO₃)₂, respectively. XRD pattern shows ZnO(100)(002)(101).
In the case of the depositions by non-electrical process, the electron field-effect mobility of the ZnO-TFT was around 0.3 cm²/Vs, where the highest on/off ratio of ZnO-TFT was 10³.
Electrodes formed on Si substrate connected with the alternating current (AC) power supply. The power supply possesses AC with frequency of 1Hz, which make it possible to form high quality ZnO on the substrate. The electron field-effect mobility of the ZnO-TFT with the highest on/off ratio of 10³ was around 0.4 cm²/Vs in the case of the deposition by electrical deposition.

Recently, extremely high quality functional thin films are available, thanks to the evolution of thin film fabrication technology. These fabrication technologies are operated under vacuum condition in order to secure the safety in the use of active precursors and prevent the impurity contamination in the films. However, more than 20% energy is used to just operate vacuum pumps in a semiconductor factory, according to the report [1]. Therefore, the thin film fabrication technologies under atmospheric pressure have attracted attention in recent years, from the viewpoint of reducing an environmental load. Among them, we have developed mist chemical vapor deposition (Mist CVD), which is one of the solution based open-air atmospheric pressure process. According to the latest researches, quality of metal oxide thin films prepared by Mist CVD has been getting higher [2].
On the other hand, in general, thin films with few defects are an essential for the high-performance devices fabrication. For example, base substrate or bottom thin film of the same crystal-structure as the films are most effective for decreasing the defect in the films. The crystal structure of gallium oxide (α-Ga₂O₃), iron oxide (α-Fe₂O₃) and indium oxide (rh-In₂O₃) is corundum structure (Rc, No.167) equal to that of general-purpose sapphire substrate (α-Al₂O₃) [3]. Therefore, there is a possibility that these corundum oxide thin films are successfully fabricated with a high-crystalline and few defects on sapphire substrate. Actually, super high crystalline α-Ga₂O₃ and α-Fe₂O₃ were fabricated by direct growth on a sapphire substrate by mist CVD, in spite of those lattice mismatches around 5%. However, high-crystalline rh-In₂O₃ hasn&’t been obtained yet because of over 15% lattice mismatch between rh-In₂O₃ thin film and α-Al₂O₃ substrate. Then, a buffer layer (BL), such as α-Ga₂O₃ and α-Fe₂O₃, between them was installed in order to decrease the influence of stress from large lattice mismatch.
As a result, high crystalline rh-In₂O₃ thin films were obtained, as evident from numerous Laue fringes. From reciplocal spase mapping (RSM), there is no stress on each interface because all peaks were almost original position. From the results, it is suggested that the relaxation mechanism of the interface between rh-In₂O₃ thin film and BL are originated by the periodic structure. In the rh-In₂O₃, the lowest conduction band of a pure In-5s character is well known and it is promising materials for the realization of high mobility devices [4]. In our latest results, a hole mobility of high-crystalline rh-In₂O₃ was around 70 cm²/Vs.
We would like to report in detail and discuss it in the meeting.
[1] T. Huang: Solid State Technol., 51 [10], 30 (2008).
[2] T. Kawaharamura, Jpn. J. Appl. Phys., 53 (2014) 05FF08
[3] X.B. Zhou and J.Th.M. De Hosson, Scripta Metallurgica et Materialia, 28, p. 219, 1993
[4] F. Fuchs and F. Bechstedt, Phys. Rev. B, 77 (2008), 155107

Zinc oxide has been fascinating many researchers because of its wide range of applications arising due to its direct wide bandgap. Similarly, nickel oxide has generated interest recently as a semiconductor material for transparent electronics since its electrical conductivity can be tuned along with high optical transparency. The d-d transitions in the valence and conduction bands lead to a strong absorption in the visible spectrum giving a chemically stable pale green color to the oxide.
In this work, NiO and ZnO films as well as NiO/ZnO stacks with ohmic metal electrodes were deposited under optimum oxygen gas flows and temperatures, which will be reported in this paper. The structural and electrical properties of these intrinsic p-NiO and n-ZnO films were studied by energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), hot-probe measurement and current-voltage (I-V) measurements.
The effect of nanosecond UV laser irradiation, using the 4^th harmonic Nd:YAG laser line with lambda;=266nm, on the properties of the individual films was studied upon varying the laser irradiation and other conditions (laser energy density, number of pulses, film thickness and substrate type) . Changes in crystalline orientation and charge carrier type and concentrations are observed without any significant film degradation. Point defects arising from the laser irradiation such as vacancies and interstitials play a major role in modifying the electrical and optical properties. It is envisaged that the migration of O^2- ions in ZnO result in high resistance states and the interstitial defects arising from Ni²⁺ and O^2- ions in NiO result in low resistance states. The memrisistive properties of a ZnO/NiO stack were examined as well and could be interpreted in terms of the above-mentioned changes. Sweeping the device from reverse bias to forward bias continuously for a short period of time was found to result in a hysteresis loop with gradually increasing conductance of the ZnO/NiO stack.

Mist chemical vapor deposition (Mist-CVD) is a solution-based growth technique and can be performed even under atmospheric pressure using a simple and easy system configuration with low cost and environmental friendliness. [1] Mist-CVD is a suitable technique for preparing metal oxide films, such as gallium oxide (Ga₂O₃) and indium oxide (In₂O₃). [2, 3] However, it has not been fully clarified the growth mechanism of mist CVD yet.
In this study, we investigated the growth temperature dependence of Ga₂O₃ and In₂O₃ growth rates in mist CVD in order to clarify the growth mechanism.
Ga₂O₃ and In₂O₃ films were grown on c-plane sapphire substrates for 1 h by mist CVD. The growth temperature was varied between 300 and 650 °C. Gallium acetylacetonates and indium acetylacetonates were used as source materials for these growths. The acetylacetonates were solved in deionized water with a small amount of hydrochloric acid. The solution was atomized using an ultrasonic transducer at 2.4 MHz and the formed aerosols were transferred to a quartz tube using a carrier gas. Oxygen gas with the flow rates of 5.0 and 0.5 L/min was used as a carrier gas and a dilution gas, respectively.
Three different slopes were observed from the temperature dependence of Ga₂O₃ growth rate. In region I (growth temperature of 300 to 400 °C), the growth rate became faster with the increase of growth temperature. In region II (growth temperature of 400 to 550 °C), the growth rate was almost constant. In region III (growth temperature of 550 to 650 °C), the growth rate became lower with the increase of growth temperature. This tendency was similar to the dependence in the growth by the metal organic chemical vapor deposition (MOCVD). [4] Therefore, we attributed the regions I, II and III to surface reaction limited region, mass-transport limited region and desorption limited region, respecteively, as it is well known in the growth of MOCVD.
Two different slopes (region I and II) were observed in the case of In₂O₃ in the temperature range of 350 to 650 °C, which is the limit in our equipment setup. The results on the growth temperature dependence of Ga₂O₃ and In₂O₃ growth rates are compared. The results on the crystal structure and crystal quality of Ga₂O₃ and In₂O₃ in each growth temperature will also be discussed.
The authors would like thank Prof. Fujita and Dr. Kaneko of Kyoto University and Prof. Kawaharamura of Kochi University of Technology for technical supports of mist CVD growth. This work was partly supported by JSPS KAKENHI Grant Numbers #25706020, #25420341 and #25390071 and ALCA project of JST.
[1] T. Kawaharamura, Dr. Thesis, Faculty of Engineering, Kyoto University, Kyoto, (2008).
[2] D. Shinohara and S. Fujita, Jpn. J. Appl. Phys., 47, 7311 (2008).
[3] N. Suzuki, K. Kaneko and S. Fujita, J. Cryst. Growth, 364, 30 (2013).
[4] J. D. Ye, S. L. Gu, W. Liu, S. M. Zhu, R. Zhang, Y. Shi, Y. D. Zheng, X. W. Sun, G. Q. Lo, and D. L. Kwong, Appl. Phys. Lett. 90, 174107 (2007).

As recent display market has rapidly evolved into ultra-high resolution, technology of minimizing RC delay and establishing high mobility, high integrated, and reliable TFT has become very important. Accordingly, developing oxide TFT became necessary to produce highly stable and reliable backplane to realize high-resolution and large-area AMOLED for next generation. For that reason, low resistance electrode and self-aligned structured TFT should be applied in order to minimize RC delay which impedes transmission of electrical signal.
We examined on materials which act as gate insulator in top gate structure mimicking self-aligned structure. Deposition of SiO₂ by plasma-enhanced chemical vapor deposition (PECVD) as a gate insulator in top gate structure is limited in applying high temperature deposition or high temperature heat treatment after deposition of gate insulator because of control the number of carrier in oxide semiconductor. To solve this problem, we tried to apply dielectric material deposited by plasma-enhanced atomic layer deposition (PEALD) to prevent diffusion of hydrogen into channel area during the deposition process and to form high-performance insulator which is stoichiometric. In addition, to use copper as a gate electrode for low resistance, barrier properties of gate insulator toward gate electrode are very important in self-aligned structure with thin dielectric gate insulator, unlike bottom gate structure adopting SiN_x / SiO₂ double layer. Therefore, dielectric material with good barrier properties such as SiN_x should be used in self-aligned structure for sure. Consequently we fabricated top gate transistor with double-layered gate insulator of high k dielectric that doesn&’t cause diffusion of hydrogen, has no defect, and performs as a good Cu barrier as well. Adopting SiN_x to main material of gate insulator which is well known as a great barrier for Cu electrode, both of SiO₂ by PECVD and A_l2O₃ by PEALD films were deposited directly next to a-IGZO active layer. We also analyzed film characteristics and properties of the each transistor. In the case of applying A_l2O₃ deposited by PEALD as a first dielectric gate insulator forming interface with active layer, excellent characteristics of the transistor with SiO₂ as a bulk gate insulator were verified certainly. Moreover, the transistor with double-layered gate insulator (PEALD A_l2O₃ / PECVD SiN_x) including a bulk SiN_x instead of SiO₂ results in improvement on Vth control and stability.

Unique feature of metal oxide thin film transistors (TFT) is their transparency. In contrast to silicon based technology, they are suitable for numerous application including displays and sensors. In pursuit for highest performance, research is being focused on finding alternative materials. In this paper we report the fabrication of thin film transistors with zinc oxide channel and molybdenum doped indium oxide (IMO) electrodes, achieved by room temperature sputtering of the materials. For simplification of the fabrication process, commercially oxidized silicon substrates with 100 nm SiO2 was used to create common gate devices. Once channel and electrode material was known to be working, fabrication of TFT with IMO common gate electrode and hafnium oxide gate dielectric, grown by atomic layer deposition on glass substrate was performed. A set of devices was fabricated, with varying channel width and length from 5 µm to 300 µm in different deposition parameters, along with control substrate dedicated for material study. This approach allows to relate directly change in device key properties to deposition parameters. Output and transfer characteristics were then extracted to study the performance of thin film transistors. Organization of the data in respect to threshold voltage, turn on resistance, saturation current and other important parameters, enables to determine optimum fabrication process and optimum material properties. Additionally, measurements of supporting experimental data on transparency and spatial information of the spectral characteristics.

Metal oxide transistors are widely studied due to their transparency, which is making them ideal for display and sensor applications. In this paper we report the fabrication of thin film transistors (TFT) with zinc oxide channel and molybdenum doped indium oxide electrodes, achieved by room temperature sputtering of the materials. A set of devices were fabricated, with varying channel width and length from 5 mu;m to 300 mu;m. For process simplification common gate electrode TFT were used. This allowed to focus on channel, source and drain electrode properties, accelerating prototyping and sample preparation for glycerol experiments. Output and transfer characteristics were then extracted to study the performance of fabricated devices. Once the reference data is taken, thin film transistors were then exposed to the presence of glycerol. Observation of transient and permanent changes in the channel material and device performance due to the influence of glycerol, a low conductive medium, is reported. To ensure, that the measurement is conducted on a TFT, glycerol was investigated on dummy TFT contacts without ZnO channel material to, in order to extract its behavior to polarization voltage and current density. The thin film transistors were then studied with and without glycerol, with different soak times, and with different glycerol concentrations. Changes in saturation current, threshold voltage, turn-on resistance and other important transistor parameters were extracted. Comparison with other polyols for specific selectivity towards the number of carbons in the polyols chain is also reported.

Correlated electron systems exhibiting phase transitions hold greatly important clues for the development of smart sensors and phase change memories. Among them, vanadium dioxide (VO₂) has attracted much attention for its drastic metal-insulator transition (MIT) of several orders of magnitude change in resistance at around 340 K driven by a variety of externally applied forces. Controlling the MIT via an electric bias would be more practical and lead to exotic applications such as memristive devices. In this research, we investigated voltage bias-driven electronic phase switching from insulating to metallic states in VO₂ thin films having freestanding structures (FSS), compared with that in non-freestanding structures (N-FSS) clamped on substrate. By measuring the electrical power at switching under different thermal conditions, we found that thermal coupling of the microstructures distributing spatial temperature gradient strongly affected the efficiency of insulator-to-metal switching induced by the Joule effect. The critical electric power required for switching in the FSS is over two orders lower than that for the N-FSS. This indicates that proper design of thermal flow will create more effective switching and memristive devices. In this symposium, we will report the detail experimental results and the mechanism of low power switching in terms of heat flow.

Ferroelectric niobates with tetragonal tungsten bronze TTB-type structure have gained further interest due to the set of distinct crystalline sites and high anisotropy of their crystalline structure. Sr₂KNb₅O₁₅ is an unipolar ferroelectric lead free TTB material that belongs to the class of ceramic compounds that exhibit texturing by application of pressure or magnetization, piezoelectric, catalytic and dielectric properties. This work provides a deep insight on the structural thermal stability of the Sr₂KNb₅O₁₅ powder investigated by dielectric-permittivity properties at cryogenic temperatures of Sr₂KNb₅O₁₅ ceramic with microstructure based on nanosized grains. The correlations between thermal hysteresis of dielectric permittivity and non structural phase transitions are established. Nanosized grains of potassium and strontium niobate ceramic was prepared from conventional sintering of powder synthesized by chemical route using the modified polyol method. Crystallographic data was used in the understanding of the geometry and shape of nanosized grains. The set of major interface type grain-boundary was identified and indexed from micrograph analysis by Scanning Electron Microscopy. High temperature, impedance spectroscopy measurements were carried out, in the temperature range from room temperature up to 550 K. From impedance data analysis, a set of four relaxation phenomena were detected. Relaxation phenomena were addressed to the grain and grain-boundary interfaces. In this sense, three of relaxation phenomena were ascribed to the interfaces between grain boundaries. These interfaces phenomena define an excess of interfaces ascribed to the grain-boundary in nanosized ceramics. The relaxation frequency of each grain- boundary interface is discussed.

Display devices have become common due to the development of mobiles phones, next generation flat-screen TVs, personal computer monitors, etc. The essential element for such flexible and large area device is a thin-film transistor (TFT). Among four kind of TFTs, oxide-based TFTs employing oxide semiconductors such as indium gallium zinc oxide (IGZO) and zinc tin oxide (ZTO) are promising devices for active matrix display applications and sensor arrays because of their high mobility, visible light transparency, good uniformity and solution process ability. One of efficient printing techniques to enable drop-on-demand patterning is inkjet with large area and mass production for electrical devices. Inkjet printing with exact control of the drop volume and position can substitutes the production of complex patterns with expensive and chemical waste photolithographic processes.
Solution-processed ZTO TFTs that are easily inclined to make oxygen vacancies do not show good device performances. It also has poor leakage gate current with increasing drain current. Thus, if oxygen vacancies in the patterned ZTO thin film are adequately suppressed, electrical properties of TFTs will be improved. One of effective methods for controlling oxygen vacancies is a material doping considering the SEP, bandgap and electronegativity. The second is the control of annealing temperature. An appropriate control of zirconium doping concentration as suppressor is necessary because it has a low electronegativity (1.33) and the high bandgap(7 eV) and low SEP(-1.45 V) compared to the zinc and tin. Although a study on Zr mole ratio in solution processed ZTO TFTs was conducted previously, it is needed to optimize a doping concentration to obtain better electrical properties and to make patterned thin film to prevent gate leakage current through inkjet printing. So, we proposed a doping of Zr which is a stable material and carrier suppressor into ZTO system is expected to help in controlling the number of oxygen vacancies. We found the optimized doping mole ratio and annealing temperature of Zr into patterned ZTO system (ZZTO) in channel region to control for suitable oxygen vacancies.
Electrical characteristics of inkjet-printed ZZTO TFT are measured by semiconductor parameter analyzer and the prepared solutions are analyzed by TGA-DSC and thin films are analyzed by XRD and XPS. Mobility, on/off ratio, threshold voltage and sub-threshold voltage of a inkjet-printed 0.0025M ZZTO TFT at 500#8451; are 6.43cm²/Vs, 3.72 x 10⁸, 3.35V and 0.53V, respectively. Also we obtained a better bias stress.

Inkjet printing has attracted attention because of its potential for printed electronics. A piezo-type nozzle has been used for inkjet printing and is operated by the volume change with an electrical signal. An ink drop can be formed by the tensile force but the jetted drop is larger than the nozzle diameter. As a result, ink-jet printing is limited to micro-scale printing. The use of ink is confined to their viscosity and volatility. High viscosity ink cannot be spouted out from a tiny orifice and a high volatile ink induced to clog the nozzles. The electrohydrodynamic (EHD)-jet printing process is unique technique that can overcome these limits. In the present study, IZO TFTs were fabricated through EHD-jet with cone-jetting. The nozzle had an inner diameter of 100 mu;m, which is the smallest metallic nozzle without any surface treatment instead of an expensive and brittle glass needle. The EHD-fabricated IZO TFTs showed good characteristics at an annealing temperature of 300 and 400^oC. The positive and negative bias stability was also characterized.
IZO thin films were fabricated by the EHD-jet printing with metallic needles without any surface treatment instead of an expensive and brittle glass needles. The EHD-processing parameters were optimized to produce the cone-jet type jetting and improve the quality of the IZO thin films. Because of the high wettabillity between the IZO solution and SiO₂ surface, the solution was spread out to an IZO layer width of 1000 µm, even though the EHD-jet equipment was optimized to the cone-jet type for fine ejection. The EHD-printed IZO TFTs were annealed at 300, 400 and 500^oC to obtain the transistor characteristics. The EHD jet-printed IZO TFTs showed good electrical properties: mobility of 4.8 cm²/Vs, threshold voltage of 8.4 V, on-to-off current ratio of 10⁷, subthreshold slope of 1.2 V/dec at 400^oC. The positive and the negative bias stress of the EHD-jet printed IZO TFTs were characterized. No negative shifts in the EHD-printed IZO TFTs were observed at process temperatures of 300 and 400^oC under negative bias stress.

All inorganic quantum dot light emitting devices with solution processed transport layers are investigated. The device consists of an anode, a hole transport layer, a quantum dot emissive layer, an electron transport layer and a cathode. Indium tin oxide coated glass slides are used as substrates with the indium tin oxide acting as the transparent anode electrode. The transport layers are both inorganic, which are relatively insensitive to moisture and other environmental factors as compared to their organic counterparts. Nickel oxide acts as the hole transport layer, while zinc oxide nanocrystals act as the electron transport layer. The nickel oxide hole transport layer is formed by annealing a spin coated layer of nickel hydroxide sol-gel. On top of the hole transport layer, CdSe/ZnS quantum dots synthesized by hot injection method is spin coated. Finally, zinc oxide nanocrystals, dispersed in methanol, are spin coated over the quantum dot emissive layer as the electron transport layer. The material characterization of different layers is performed by using absorbance, Raman scattering, XRD, XPS and photoluminescence measurements. The completed device performance is evaluated by measuring the IV characteristics, electroluminescence and quantum efficiency measurements. The device turn on is around 4V with a maximum current density of ~200 mA/cm² at 9 V. The emission wavelength from the electroluminescence spectrum is 520 nm.

Polycrystalline ZnO has been applied to various electrical and optical applications such as gas sensors, piezoelectric transducers, phosphors, transparent conducting films, and varistors. To control the figure of merits of these devices it is essential to understand the behavior of defects and interface states in doped ZnO, especially in ZnO varistor (short for variable resistor). All of these applications are either affected by, or depend on, defects and grain boundary properties according to various dopants (Bi, V, Pr, Co, Mn, Ni, Cr, Cu, Li, P, Sbhellip;oxides). ZnO is strictly a non-stoichiometric material of Zn_1+xO in which Zn interstitials are too mobile to be stable at room temperature. The deep or shallow defects commonly found in nano- and bulk-ZnO are intrinsic zinc interstitial and oxygen vacancy, and extrinsic donors and acceptors. These defects controlled by doping elements and/or post-heat treatments also have improved the electrical and optical properties of ZnO. It is known that cobalt oxide added to ZnO improves the nonlinear current-voltage characteristics, which is presumably due to the formation of the Schottky-type double barriers at the grain boundaries. Also it is found that the presence of a large Bi, V or Pr ions induces the concentration of native defects of acceptor type in the grain boundary of doped ZnO. However, it is not known about the relationship between the defects and grain boundary properties in the simple three component system of Co₃O₄-doped ZnO-Zn₂BiVO₆ ceramics revealed good nonlinear current-voltage characteristics.
In our study, we investigated the effects of Co₃O₄ doping and Zn₂BiVO₆ additive in ZnO (it&’s a new varistor system developed by us) on the defects and grain boundary properties using dielectric functions (Z*, M*, ε*, Y*, and tanδ) including admittance and impedance spectroscopy. It will provide information about the relationship between the formations of donor or acceptor defects at the bulk or grain boundary for the double Schottky barrier. Experimentally Co-doped ZnO-Zn₂BiVO₆ sintered at 900°C in air have two kinds of defects (Zn interstitial and O vacancy) according to its doping levels, a single grain boundary (apparent activation energy E_a=0.6~1.1 eV), and a good varistor properties (nonlinear coefficient α>70). The origin of varistor behavior in Co-doped ZnO-Zn₂BiVO₆ system has been discussed in relation to grain boundary traps and defect chemistry based on the experimental results.

With a growing interest in flexible and transparent electronics it is important to develop methods of fabrication that can meet the demands relevant to this exciting area. As such laser processing of thin film materials on flexible substrates is posed to play a significant role. Characteristic attributes of laser processing include a relatively low thermal budget, rapid speed of processing, area selectivity and electronic property enhancement that in many cases cannot be achieved with conventional techniques. Therefore, in this work, we investigate the application of Excimer Laser Annealing of Indium Gallium Zinc Oxide for thin film transistors.
a-IGZO thin films (200 nm thick) were deposited by radio frequency RF magnetron sputtering at room temperature. A range of deposition parameters including applied RF power, oxygen in argon ratio, working pressure as well as target metallic composition was explored to fabricate suitable thin film material for TFTs fabrication. Then the IGZO thin films were subjected to nanosecond excimer laser annealing at room temperature in air using KrF excimer laser. One laser pulse was applied at moderate laser energy density (50 to 125 mJ/cm²). IGZO samples demonstrated a significant resistivity reduction upon processing, while maintaining the amorphous structure. Also, the IGZO resistivity was largely tuned via varying the deposition conditions and the target composition.
Thereafter, Top-gate-staggered TFT devices with 40-nm-thick a-IGZO active layer were fabricated. Excimer laser annealing with one pulse at rather low energy densities (15 - 60 mJ/cm²) provided TFT of improved performance. The optimised laser annealed TFTs (processed at 30 mJ/cm²) exhibited an on/off ratio of 2.8 #1093; 10⁷, a field-effect mobility of 3.4 cm²/Vs, a Von voltage of + 0.3 V, and a sub-threshold swing of 0.27 V/decade, compared to 6 #1093; 10⁶, 0.7 cm²/V s, - 0.1 V, and 0.28 V/decade respectively for the as-deposited TFTs. This improvement in the TFTs transfer characteristics could be attributed to reducing the resistivity of the channel layer (a-IGZO), and reducing the contact resistance between IGZO and the source/drain electrodes after processing.
Laser processing at the reported laser energy density in this work is very promising for TFTs fabrication on heat sensitive plastic substrates. Moreover, since the required laser energy to enhance IGZO TFTs characteristics is rather low, a high throughput of IGZO-TFTs could be realised, making laser processing a promising technique for large volume production of large area electronics on flexible substrates.

Recently, researches on the perovskite-structured oxide materials are rapidly increasing, and interesting physical properties of perovskite material have observed [1]. LaAlO₃/SrTiO₃ interface, for instance, shows various unique physical properties such as superconductivity, magnetic effect and high electron mobility. [2-4] Among them, 2-dimensional electron gas (2DEG) phenomenon at the interface of two insulating oxide has attracted many researchers&’ attention since the first report by H. Y. Hwang et al. in 2004 [4]. There are several hypotheses for the conductivity at the LaAlO₃/SrTiO₃ interface such as electron reconstruction, oxygen vacancy and intermixing [5-7]. The 2DEG phenomenon is observed at the crystalline(c)-LaAlO₃/SrTiO₃ [4] interface as well as the amorphous(a)-LaAlO₃/SrTiO₃ interface [8].
In this study, we investigated the crystallization behaviors of a-LaAlO₃ at the interface with SrTiO₃ under electron-beam irradiation. Interestingly, epitaxial crystallization from the interface occurred. Furthermore, it was found that the crystallization kinetics depends on the conductivity of the a-LaAlO₃/SrTiO₃ interface. The nucleation and epitaxial growth of the LaAlO₃ under e-beam irradiation might be induced by enhanced defect mobility by elastic interactions, rearrangement of unstable bonds and heating effect [9-11]. In this case, however, the effect of the conductivity of the interface on the crystallization kinetics would be additionally considered. Further, not only the phase transformation behavior but also thermodynamic stability upon the conductivity of the interface and irradiation spot distance from the interface were studied, which is important to the application of heterointerfaces.
All heterostructures were grown on TiO₂-terminated SrTiO₃ substrates by pulsed laser deposition (PLD) in an oxygen atmosphere. The cross-sectional TEM specimens were prepared by standard mechanical polishing (Struers; Labopol-5) and subsequent argon ion milling (Gatan; model 691). The observation of nucleation and crystallization behavior under electron-beam irradiation is performed in a scanning transmission electron microscope (STEM, TitanS80-300; FEI) operated at 300 kV.

For the past decades, the quest to replace the gate dielectric in field-effect transistor has created a strong momentum in the research field of integrating ultra-thin oxides on silicon substrates. Great scientific and technological breakthroughs were achieved, such as the replacement of SiO₂ with HfO₂ in advanced transistor designs. The knowledge and expertise gained during the development of high-k dielectrics cross-fertilized research areas beyond traditional complementary metal-oxide-semiconductor (CMOS) micro-electronics.
One particular example is the integration of high-quality, crystalline, complex oxide thin films such as strontium titanate on silicon substrates. Since the first methods for the epitaxial integration were demonstrated, the growth process has been carefully optimized and single crystalline layers were fabricated even on 200 mm substrate sizes. Linked to their crystal structure, complex oxides show a rich variety of physical properties such as ferroelectricity, magnetic behavior, or optical nonlinearities which are today not available in materials used in standard CMOS processes. The integration of “functional” oxides onto silicon is therefore a promising path to enable novel device concepts in various domains for future information processing, such as ultra-compact logic circuits, neuromorphic networks, or quantum computing.
The presentation will focus on the field of silicon photonics, which is a promising technology to handle the continuously increasing intra-chip data communication of advanced processor layouts. Many building blocks for functional photonic networks such as detectors or optical switches have already been realized using CMOS compatible microfabrication processes. However, other building blocks such as integrated optical isolators and modulators compatible with higher modulation formats are still missing or are limited in their performance. These missing building blocks nicely illustrate the potential of functional oxides with strong nonlinear and magneto-optical properties for silicon photonics, as they are routinely used in the form of bulk crystals to realize such components in non-integrated optical communication links.
The opportunities and challenges of combining functional oxides with integrated photonic devices and circuits will be discussed. Thereby, the focus will be on how various oxides relevant for optical applications can be integrated on silicon, and what kind of novel functionality and silicon photonic device types have been achieved. Examples are the fabrication of electro-optical switches based on oxides with metal-to-insulator transitions (e.g. VO₂), high-speed modulators based on oxides with strong Pockels coefficients (e.g. BaTiO₃), and optical isolators based on magnetic oxides. If correctly implemented, the integration of functional oxides will extend the toolbox of silicon photonic device design and enable a variety of promising and strongly needed photonic components and concepts.

The integration of photonic circuits into current CMOS compatible process technology is currently undergoing an intense development. In particular, the realization of electro-optic modulators as key element for optical on-chip communcation represents a major challenge.
Electro-optic modulators are based on a material whose refractive index is altered when applying an external electric field. Currently, great efford is focussed on silicon as active optical material. Due to its center symmetric crystal structure, it does not exhibit the so-called Pockels effect and hence alternative mechanisms (for example based on plasma dispersion) are used to change the refractive index, which ultimatively limits the device&’s bandwidth.[1]
Barium titanate has a non-center symmetric tetragonal unit cell at room temperature. It is known for its high dielectric constant, high piezo electric cofficients and very high pockels coefficients which makes it an interesting material for electro-optic modulators. However, the integration of barium titanate on silicon is challenging due to its substantial thermal expansion and lattice mismatches. While the deposition on buffered systems has been demonstrated by molecular beam epitaxy[2,3], deposition temperatures remain too high for CMOS compatible integration.
Here, we present a low temperature high vacuum chemical vapor deposition (HV-CVD) process of barium titanate um films on magnesium oxide (MgO) and strontium titanate (SrTiO₃) substrates using barium tri-isopropyl cyclopentadienyl, titanium isopropoxide and water as precursors at a substrate temperature of 400°C. We have optimized the precursor fluxes by combinatorial experiments to obtain right elemental ratios and grow stoichiometric barium titanate films. X-ray diffraction analysis indicates the epitaxial relationship between substrates and film; the growth rate for epitaxial films lies in the range of 30-50 nm/h. This is the first time that the chemical vapor deposition of epitaxial barium titanate films is reported at substrate temperatures that are compatible with today&’s CMOS technology. Currently, we are working on the deposition on buffered silicon substrates for a complete integration.
[1] G.T. Reed, G. Mashanovich, F.Y. Gardes, D.J. Thomson, Nat. Photonics 4, 518-526 (2010).
[2] S. Abel, T. Stöferle, C. Marchiori, C. Rossel, M.D. Rossell, R. Erni, D. Caimi, M. Sousa, A. Chelnokov, B.J. Offrein, J. Fompeyrine, Nat Commun 4, 1671 (2013).
[3] C. Xiong, W.H.P. Pernice, J.H. Ngai, J.W. Reiner, D. Kumah, F.J. Walker, C.H. Ahn, H.X. Tang, Nano Lett. 14, 1419-1425 (2014).

Germanium, with its higher hole and electron mobility is a potential candidate to replace silicon as a channel material in a field effect transistor in the future. The ferroelectric high-k dielectric barium titanate (BTO) can be integrated on germanium (001) due to the small lattice mismatch between BTO and Ge and could therefore be a potential candidate for a ferroelectric memory. However, for a successful device development, several obstacles have to be overcome. The ferroelectric polarization charge must be as close to the transistor channel as possible while simultaneously avoiding a low-permittivity interfacial oxide. The polarization must be normal to the semiconductor to effectively modulate the charge in the underlying semiconductor when the polarization is switched. Another major problem constitutes the formation of the gate, source and drain contacts, due to the resistivity of BaTiO₃ to chemical etching.
Here, we report the epitaxial growth of BTO on a germanium (001) substrate with a thin STO buffer layer, which imposes compressive strain on BTO and causes it to be out of plane polarized. The BTO film crystallizes as-deposited which is monitored by RHEED. XRD measurements of the BTO film indicate an out-of-plane ferroelectric polarization which can be confirmed by piezoresponse force microscopy. Using microwave impedance microscopy we could show that we can effectively modulate the charge in germanium; this charge modulation constitutes the field effect which is an important step towards the development of a ferroelectric-FET. First results of MOS capacitors will be reported, which were created through lithographically defined selected area epitaxy on germanium substrates in combination with focused-ion beam (FIB) and patterned Pt as a top contact. Current versus voltage behavior were extracted to determine the electrical properties of the epitaxial BaTiO₃ on Ge(001). We believe this work has advanced the realization of the ferroelectric field-effect transistor based on BTO on Ge.

Metal oxide semiconductor field effect transistors (MOSFET) containing hafnia-based high-k dielectrics have been successfully implemented at the 45nm technology node. The continual miniaturization of MOSFETs necessitates additional electrical performance improvements in both the gate dielectric and the channel material. Research and development efforts for effective incorporation of passivation layers (e.g., Al₂O₃) are currently underway so that high mobility channel materials (e.g., Ge) can be integrated into devices under the purview of the existing 300mm Si VLSI technology¹. In addition, lattice engineering of various metal oxides (e.g., HfO₂, ZrO₂) are being pursued² for stabilizing the structures (e.g, tetragonal) with predicted higher values of the dielectric constant k^3-5.
In this work, 300nm Ge layer is grown epitaxially on a graded SiGe buffer layer on top of a 300mm Si wafer. Using a non-plasma dry etching process, native Ge oxides are carefully removed. A passivation layer of amorphous Al₂O₃ (~1nm) is deposited via atomic layer deposition (ALD) to improve device drive current in both short channel and low equivalent oxide thickness regime. Various as-deposited, post deposition annealed (PDA) and deposited-annealed-deposited-annealed (DADA) thin-films of Hf_1-xZr_xO₂ (02 under similar ALD schemes.
The phase and texture of the thin-films are determined using grazing incident in plane x-ray diffraction and reciprocal space maps collected at the Cornell High Energy Synchrotron Source (CHESS) with 11.27 keV x-rays. Grazing incidence geometry with 0.3° incident angle allowed us to collect x-rays with diffraction volume mainly confined to the ultra-thin films and smaller contribution from the substrate. A primarily amorphous/nano-crystalline matrix of the as-deposited films changed to randomly aligned grains of polycrystalline MO₂ (M=Hf, Zr) after PDA processing. In contrast, the DADA films are highly textured. With increasing Zr content, the structure and fiber texture of the films changed from monoclinic (-111 ) to tetragonal (111). A smaller amount of Zr (x~0.33) is required for stabilizing the higher-k tetragonal phase in films grown on Al₂O₃ passivated Ge substrate as compared to the films grown on Si substrate (x~0.5).
References:
(1) K. Tapily, et al., ECS Trans. 61 (2), 89-96 (2014).
(2) K. Tapily, et al., ECS Trans. 45 (3), 411-420 (2012).
(3) X. Zhao, D. Vanderbilt, Phys. Rev. B 65 (23), 233106 (2002).
(4) X. Zhao, D. Vanderbilt, Phys. Rev. B 65 (7), 075105 (2002).
(5) A. A. Demkov, Phys. Status Solidi B 226, 57 (2001).

Photonic devices enabling light modulation and switching are of paramount importance for modern telecommunication. Their performances directly impact both the speed of data transmission and energy consumption. In that field, silicon-based electro-optic modulators are widely investigated because of their direct compatibility to CMOS fabrication processes. However, the trade-off between modulation speed and power consumption #822; characteristic of the plasma dispersion effect used in Si based modulators #822; intrinsically limits the performances of these devices. To overcome such limits, ferroelectric oxides with naturally strong electro-optical coefficients could be ideal candidates for high-speed modulators. It is thus of particular interest to implement such materials on silicon based photonic platforms.
It is however challenging to seamlessly implement ferroelectric oxides on silicon, both in terms of material growth and optoelectronic design. We will present methods to monolithically integrate BaTiO₃ (BTO) and Pb(Zr_x,Ti₁-x)O₃ (PZT) on silicon and to design devices that leverage their strong Pockels coefficient for efficient and fast electro-optical modulation. Specifically, we report epitaxial growth of thin films of these materials on silicon, using a SrTiO₃ template. We then design slot waveguide photonic devices and RF electrodes to both confine the optical mode and maximize the electro-optical overlap within the active ferroelectric layer. We will show how our designs and technologies allowed for the fabrication of low-loss BTO slot waveguides, compatible with the requirements of Si photonics devices. We further discuss on the latest experimental results, expected performances and future devices.
We acknowledge funding from the European Commission under project FP7-ICT-2013-11-619456 SITOGA.

Functional oxides with perovskite structure exhibit a broad range of physical properties that are weak or absent in silicon. For decades their integration onto silicon platform has been identified as an important route to improve and pursue the integration in nanoelectronic devices. Moreover the needs for low power consumption devices is triggering the search for materials with higher resistivity and low 1/f noise in remote sensor applications, bringing epitaxial oxides materials as very good candidates. To succeed, different barriers remain to be overcome since the first demonstration of the epitaxial growth of SrTiO3 epilayer onto silicon by Mc Kee et al. [1]. Using the SrTiO3/Si structure as pseudo substrate, it is then possible to take advantage of oxide physical properties, allowing the development of freestanding devices based on Micro Electromechanical Systems (MEMS) technologies with higher possibilities for strain engineering as compared with more classical solutions [2]. As physical properties are closely related to crystallographic order, only few examples of MEMS devices using perovskites have been demonstrated [3, 4]. It is also known that epitaxial strain strongly modulates oxide thin films properties competing with the chemical pressure induced by cation size in the crystalline structure, this playing, in the case of MEMS, a key role in the final characteristics.
For this purpose, strain analysis has been undertaken at each step of the growth of manganites on SrTiO3/Si pseudo substrates in order to get the initial strain state of the obtained heterostructures using X ray diffraction and TEM strain mapping analysis. Then the strain evolution and the impact on the released MEMS structures is studied by means of optical profilometry. Based on these fabricated MEMS devices, the determination of the piezoresistive coefficient under applied dynamic strain of La0.66Sr0.33MnO3 films epitaxially grown on STO/Si pseudo substrate, submitted to non-conventional strain configuration, has been performed. The observed electrical response can be described by the change in the double exchange mechanism. Finally these devices can be used as very efficient pressure sensor devices that takes also advantage of the high chemical stability of manganites for severe environments.
R. A. McKee et al., Phys. Rev. Lett. 81, 3014 (1991)
Lei Na et al., Nature Communications, 4, 1378 (2013)
S. Liu et al., Microelectron. Eng. 111, 101 (2013).
M. D. Nguyen et al., Adv. Nat. Sci. Nanosci. Nanotechnol. 2, 015005 (2011)

Pulsed Laser Deposition (PLD) has been established in recent years as a versatile thin film deposition technique for the near stoichiometric synthesis of materials including complex transition metal oxide thin films. These oxides offer a variety of exploitable properties and, when combined, new functionalities due to electronic reconstruction at interfaces. Despite this rich potential for use in electronics, actual industrial applications are relatively few as they rely on full control of thin film growth on atomic scale with substrate materials at sizes suitable for industrial applications, where silicon defines the standard.
Growth of oxide materials on silicon by PLD has shown to be challenging due to the ever-present native silicon oxide layer, which must be removed to obtain an epitaxial relation between the silicon substrate and the grown oxide. Yttrium-stabilized-zirconia (YsZ) has shown to be a very effective buffer layer as, besides its small lattice mismatch with silicon of about 5% zirconium potentially reduces the native oxide layer to form an epitaxial relation with the silicon. This demands highly optimized growth conditions which has been shown on small scale. Though no significant results on highly epitaxial crystalline growth of YsZ as buffer layer and subsequent functional oxides have been shown yet on silicon wafer scale (>4”) using PLD.
We present the latest results on optimized growth and characterisation of YsZ on 4” silicon wafers utilizing a TSST developed large-area PLD system. X-ray diffraction measurements show a high degree of epitaxy over the full wafer comparable to small scale PLD grown films. Subsequent growth of ferromagnetic La_0.67Sr_0.33MnO₃ (LSMO) and SrTiO₃ has been optimized, resulting in films with a high degree of epitaxy and clear (001) and (110) oriented highly crystalline domains. Aditionally, magnetic measurements on LSMO films show bulk-like magnetic behaviour, supporting this high crystalline quality. Furthermore, to obtain single crystalline orientation, buffer layers of CeO₂ and SrO/SrRuO₃ on YsZ are investigated, showing promising results. Due to an improved lattice match and/or altered crystal structure suppression of multiple phases in LSMO and SrTiO₃ is observed.
It is shown that optimized large area depositions settings do not trivially copy from small-scale settings. For both LSMO and STO, at typical small-scale temperature settings of up to 700°C, no or very little epitaxy is observed, where at elevated temperature, a high degree of epitaxy is obtained. As the full PLD plume is collected on the wafer unlike small-scale experiments, temperature dependent film stoichiometry is understood by recent improved understanding on the temporal and spatial dependence of the composition of the plasma. Here it is suggested that the oxidation of species in the plasma is a crucial mechanism in the stoichiometric reconstruction of the synthesized oxide thin films¹.
1. R. Groenen et al. Subm. APL. Mat.

The metal oxide Cr₂O₃ is a magnetoelectric and antiferromagnetic insulating material below its Néel temperature 307K. The ferromagnetic metallic La_0.7Sr_0.3MnO₃ (LSMO) has a Curie temperature of 350K-370K. Study of interface magnetic properties of LSMO/Cr₂O₃ can offer additional insights into magnetoelectric coupling at room temperature, which is essential for the next generation spintronic devices -a topic of current interest, particularly, when it is integrated with CMOS-compatible silicon substrate. Our goal is to investigate the magnetic and multiferroic properties of LSMO-Cr₂O₃ thin film heterostructures when epitaxially integrated them with silicon substrate, which is not reported by previous researchers. In this presentation, we show the epitaxial growth of LSMO/Cr₂O₃ heterostructures on yttria stabilized zirconia (YSZ)-buffered Si(001) using pulsed laser deposition. The X-ray diffraction (2#415; and Phi;) and TEM characterizations confirm that the films were grown epitaxially. The epitaxial relations can be written as [001]Si #8214; [001]YSZ #8214; [0001]Cr₂O₃ #8214; [111]LSMO and [100]Si #449; [100]YSZ #449; [10-10]Cr₂O₃ or [11-20]Cr₂O₃ #8214; [10-1]LSMO. In our previous study, we have reported that the Cr₂O₃ thin films exhibit in-plane ferromagnetic behavior, attributed to strain caused by defects such as oxygen vacancies [1]. The epitaxial growth of LSMO on Cr₂O₃ occurs at oxygen partial pressure <1x10^-2 Torr, which results in oxygen deficient LSMO. We have carried out a comprehensive study on this heterostructure as a function of LSMO thickness. Interestingly, as the LSMO thickness increased from 66nm to 528nm, while keeping the Cr₂O₃ thickness constant at 50nm, the magnetic moment was reduced and also the magnetic nature changed from ferromagnetic to super-paramagnetic. In addition, LSMO/Cr₂O₃ showed in-plane exchange bias with a maximum exchange bias filed of 87 Oe. These observations could arise from the multiferroic nature of Cr₂O₃ thin film, from the super-exchange interaction between ferromagnetic LSMO and antiferromagnetic Cr₂O₃ or from change in the magnetic anisotropy as a function of LSMO layer thickness. We are in the process of understanding this exciting observation through in-depth TEM and STEM-Z correlation with the proprties. We will present interesting magnetic, magnetotransport data as a function of LSMO thickness and EELS measurements at the interface of LSMO/Cr₂O₃.
[1]: S. Punugupati, J. Narayan, and F. Hunte, Applied Physics Letters 105, 132401 (2014).

While electric field induced ferroelectric switching has been extensively studied and broadly utilized, pure mechanical switching via flexoelectric effect has recently opened up an alternative method for domain writing due to their highly localized, electrically erasable and free of electrical damage characteristics. [1] Thus far, few studies have been made on the coupling effect of electro-mechanical switching in ferroelectric materials, presumably due to experimental difficulty in definition of the tip-surface contact area and distinguishing mechanical contribution from the electrical effect. Here we employed self-consistent phase-field modeling to investigate the bi-polar switching behavior of (001) oriented Pb(Zr_0.2Ti_0.8)O₃ thin film under concurrent electric and strain fields created via a piezoresponse force microscope probe. By separating the effects of electric field, homogeneous strain and strain gradient, we revealed that the homogeneous strain suppresses the spontaneous polarization and accordingly increases the coercive field, and the strain gradient favors unipolar switching and inhibit it in the reverse direction, thus causing lateral shifts in the polarization-electric voltage hysteresis loops. The observed switching behavior is rationalized by examining the free energy densities curves of the system. The calculated local stress distribution indicates that the longitudinal flexocoupling coefficients predominantly influence the flexoelectric switching. Our study necessitates further understanding of the electric, piezoelectric and flexoelectric contribution to the switching behavior in nanoscale ferroelectric oxides.
This research was sponsored by the Division of Materials Sciences and Engineering, Basic Energy Sciences, Department of Energy (YC, PM, SVK). Research was conducted at the Center for Nanophase Materials Sciences, which also provided support and which is a DOE Office of Science User Facility. Support was provided (QL) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division through the Office of Science Early Career Research Program. The phase-field simulation was performed in collaboration with Prof. Long-Qing Chen at Penn State, which is supported by the U.S. Department of Energy, Of#64257;ce of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-07ER46417(Chen).
[1] H. Lu, C.-W. Bark, D. Esque de los Ojos, J. Alcala, C. B. Eom, G. Catalan and A. Gruverman, Science 336, 59 (2012)

Ferroelectric perovskites are have been essential in the development of high strength capacitors and electro-optics. Currently, lead-based materials such and PZT, as well as lithium niobate, have been the staples of perovskite devices due to the electro-optical properties they possess. Both of these materials, however, have disadvantages that make them unsuitable for certain applications. PZT contains lead which is harmful to health and the environment, and also makes usage in high radiation environments such as particle colliders impossible due to to distortion of the ferroelectric hysteresis. Lithium niobate is a lead-free alternative to PZT, but the low electro-optical constant of lithium niobate requires long optical path lengths, making monolithic integration of lithium niobate impossible.
Barium strontium titanate (BSTO) offers a promising solution to this problem. Its very high dielectric constant of 3600 makes it suitable for capacitors, and its electro-optical coefficient is useful for modulators. It is also free of lead, indicating that it is likely to be able to withstand radiation.
BSTO has historically been grown by two methods, either by epitaxy or rf sputtering of a ceramic BSTO target. While these methods are effective for making thin films, they do not have high deposition rates necessary for making optically thick films of BSTO. We have developed a novel technique using co-sputtering of metal targets to achieve high deposition rates of BSTO films on dielectric substrates. Films were co-sputtered using separate metal Ba, Sr and Ti targets in an oxygen atmosphere followed by rapid thermal anneal for crystallization. This allows for the fabrication of thick films that can be used for optical devices, as well as allowing for the tuning of film stoichiometry simply by adjusting the plasma powers during sputtering. X-Ray diffraction of different compositions of BSTO shows that the lattice constant increases with the infusion of more barium as expected. Furthermore, there is a point at about 15% Ba to 85% Sr where the film goes from compressive to tensile stress. This would be the ideal composition of these films because it would mean they are under no stress and could be biased by an external field.
Films were characterized for their dielectric properties and for transparency. Impedance spectroscopy shows a very high dielectric constant of over 8000 in these films, and spectroscopic ellipsometry shows an index of about 2.0, with the more Sr rich films having a lower index than the Ba rich films, which is consistent with expected values for BSTO. Ellipsometry also shows that films are highly transparent, in fact below the detectable range of the instrument, making them ideal for optical applications. Films were exposed to radiation, which in fact increased the dielectric constant rather than reducing it, indicating that these films would function well in high radiation environments.