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.