Symposium EQ13—Nitride Materials—Synthesis, Characterization and Modeling

Hexagonal boron nitride (h-BN) is a unique 2D III nitride material. It is interesting for applications in electronics and opto-electronics because of its properties such as high thermal conductivity and a wide bandgap (~ 6eV), and its graphite-like 2-D structure makes it interesting for convenient lift-off of devices. AlN, on the other hand, is the most studied material for applications in the deep UV regime, but its high crystalline quality and p-type doping are challenging. To date there have been few reports of boron-rich BAlN alloys. These alloys may enable choice of the bandgap and facilitate lattice/strain engineering for applications. They would enhance emission efficiency in the deep UV for UV LEDs and allow for the achievement of an electron blocking layer.
In this work, we report the growth of boron rich BAlN alloys with Al content up to 17% on 2-inch sapphire substrates by Metal-Organic Vapor Phase Epitaxy (MOVPE). Characterization of the surface morphology of BAlN alloys shows a transition stage from a completely 2D to a 3D granular surface when the Al content is increased. A shift in the position of the 002 plane reflection peak to higher diffraction angles in the 2 theta-omega scan along with a decrease in intensity was observed, evidence of the formation of layered BAlN alloys. Cross sectional transmission electron microscopy (TEM) studies demonstrate the formation of a single BAlN hexagonal phase with the presence of wurtzite Al rich BAlN phases in a matrix of layered hexagonal B rich BAlN. A band-to-band transition around 5.86 eV has been observed, which shifted slightly to lower energy with increasing Al incorporation. The bowing parameter in boron rich BAlN alloy systems was evaluated to be around 0.65 ± 0.05 eV. This is encouraging evidence of boron rich BAlN alloy formation, motivating further exploration of growth conditions and the study of fundamental BAlN properties for applications in deep UV optoelectronics.

Understanding lattice dynamics is crucial for effective thermal management in high-power electronic devices because phonons dominate thermal transport in most semiconductors. This study utilizes complementary inelastic X-ray and neutron scattering techniques and reports the temperature-dependent phonon dynamics of α-GaN, one of the most important third-generation power semiconductors. A prominent Matryoshka phonon dispersion is discovered with the scattering tools and confirmed by the first-principles calculations. Such Matryoshka twinning throughout the three-dimension reciprocal space is demonstrated to amplify the anharmonicity of the related phonon modes through creating abundant three-phonon scattering channels and cutting the phonon lifetime of affected modes by more than 50%. Such phonon topology effectively contributes to the reduction of the in-plane thermal transport, thus the anisotropic thermal conductivity of α-GaN. The results not only have significant implications for engineering the thermal performance and other phonon-related properties of α-GaN, but also offer valuable insights on the role of anomalous phonon topology in thermal transport of other technically important semiconductors.

We explore new transition metal nitride compounds using a combination of epitaxial layer growth, first-principles calculations, and measurements of electronic and optical properties as a function of composition and structure. (1) Rock salt structure: ScN has a E_g = 0.92 eV indirect bandgap and a carrier concentration that is controlled by F doping. CrN forms a charge-transfer insulator with E_g = 0.2+/-0.4 eV, but oxygen exposure results in a two-dimensional metallic CrN_xO_y surface layer. The addition of Al to form Sc_1-xAl_xN and Cr_1-xAl_xN compounds increases the bandgap with d E_g/dx = 2.75 and 2.81 eV, respectively. Ti_0.5Mg_0.5N has a 0.7-1.7 eV bandgap. However, a high density of N vacancies act as n-type dopants resulting in a highly degenerate semiconductor. The vacancy formation is suppressed when alloying with Al, yielding (Ti_0.5Mg_0.5)_1-xAl_xN alloys with E_g increasing with x. (2) Wurtzite structure: Alloying AlN with Sc causes a drop in the direct bandgap for Al_1-xSc_xN with d E_g/dx = -9.23 eV and a structural instability with a non-linear composition dependence in bond angle and length that causes the reported piezoelectric enhancement. (Ti_1-xMg_x)_0.25Al_0.75N exhibits a stability peak at x = 0.5 with a minimum in the lattice constant ratio c/a that is caused by a Fermi-level shift into the bandgap and a trend toward nondirectional ionic bonding, leading to a maximum in the expected piezoelectric stress constant.

Superconducting single-photon detectors used for quantum optics, LIDAR, and astronomy, have traditionally been fabricated in a meander with strip widths around 100 nm. The narrow width was designed to be similar to the diameter of a hot spot formed by an incident photon. However, recent work has shown strips as wide as 3 µm were able to detect single photons by fine tuning the film’s sputtering conditions^1–3. Wider detectors show improved signal-to-noise ratio, and higher fabrication yield when compared to traditional detectors of the same active area. The increased performance of these detectors obtained by material deposition optimization suggests that by porting this process to traditional-width detectors may increase their sensitivity at mid-infrared wavelengths. This has prompted the recent work towards wide detectors in nitrogen rich niobium nitride (NbN)^4,5, however this work has ignored any characterization of the atomic structure of the material, instead focusing on electrical characterization of the thin film only. Here we study thin film NbN using sputtering conditions similar to previous work and fabricate single photon detectors between 0.5 and 3 µm wide. We observe saturated photon counts at 1550-nm wavelength in a 2.5 µm wide strip with a large inductor patterned in series to prevent latching. We characterize the film’s atomic structure by performing x-ray diffraction (XRD) and transmission electron microscopy (TEM) and compare the results with RF-bias sputtered NbN⁶. We observe an increase in the 111-phase of NbN for the nitrogen rich films used to fabricate wide wire single photon detectors. These results suggest that the continued development of the NbN phase shown here would improve the detector performance. This optimization would allow for the detection of low energy photons in the mid-infrared⁷ and lead to the development of large area single-photon detectors for dark matter detection⁸, high energy physics⁹, or biomedical imaging¹⁰.
1. Korneeva, Y. P. et al. Optical Single-Photon Detection in Micrometer-Scale NbN Bridges. Phys. Rev. Applied 9, 064037 (2018).
2. Charaev, I. et al. Large-area microwire MoSi single-photon detectors at 1550 nm wavelength. Appl. Phys. Lett. 116, 242603 (2020).
3. Chiles, J. et al. Superconducting microwire detectors based on WSi with single-photon sensitivity in the near-infrared. Appl. Phys. Lett. 116, 242602 (2020).
4. Zolotov, P. I. et al. Dependence of Photon Detection Efficiency on Normal-State Sheet Resistance in Marginally Superconducting Films of NbN. IEEE Trans. Appl. Supercond. 31, 1–5 (2021).
5. Vodolazov, D. Y., Manova, N. N., Korneeva, Y. P. & Korneev, A. A. Timing Jitter in NbN Superconducting Microstrip Single-Photon Detector. Phys. Rev. Applied 14, 044041 (2020).
6. Dane, A. E., McCaughan, A. N., Zhu, D. & Zhao, Q. Bias sputtered NbN and superconducting nanowire devices. J. Phys. D Appl. Phys. (2017).
7. Verma, V. B. et al. Single-photon detection in the mid-infrared up to 10 μm wavelength using tungsten silicide superconducting nanowire detectors. APL Photonics vol. 6 056101 (2021).
8. Hochberg, Y. et al. Detecting Sub-GeV Dark Matter with Superconducting Nanowires. Phys. Rev. Lett. 123, 151802 (2019).
9. Polakovic, T., Armstrong, W., Karapetrov, G., Meziani, Z.-E. & Novosad, V. Unconventional Applications of Superconducting Nanowire Single Photon Detectors. Nanomaterials (Basel) 10, (2020).
10. Schaart, D. R., Ziegler, S. & Zaidi, H. Achieving 10 ps coincidence time resolution in TOF-PET is an impossible dream. Medical Physics vol. 47 2721–2724 (2020).

Despite the great promise of nitrides as functional materials, exploration of the nitrides chemical space has been relatively limited compared to oxides and other chalcogenides/pnictides. Recently, we used a suite of computational materials discovery and informatics methods to construct a large stability map of the inorganic ternary metal nitrides [W. Sun. et al., Nature Materials, 2019], revealing exciting opportunities for the discovery of novel nitride materials. Here, I will briefly review the computational methods we used to construct this stability map and will describe a new class of II_x-TM-N ternary nitride semiconductors experimentally realized from our 2019 search. However, since our 2019 search efforts, we have encountered new challenges to more comprehensively survey and design nitride materials. In this talk, I will examine these challenges and discuss our strategies for their solution: in particular, 1) search strategies for Zintl-based metal-rich nitrides which are relevant as permanent magnets, ammonia synthesis catalysts and superhard materials; 2) strategies for the facile synthesis of metastable nitrides based on computationally-guided ammonolysis and metathesis reactions, and 3) the pO₂ and pN₂ chemical potential boundaries between nitrides, oxynitrides and oxides.

Ternary nitrides are a novel and emerging class of semiconductors with potential impact within a wide range of applications, including solar cells, power electronics, and overall water-splitting [1]. Zn-IV-N₂, with IV = Sn, Ge or Si, represents an earth-abundant alternative to conventional III-nitrides, currently dominating the optoelectronic market and playing a vital role in a quest for record-high efficiency solar cells, specifically in multi-junction architectures. Zn-IV-N₂ semiconductors exhibit close similarities to the III-nitrides in terms of the functional properties, e.g., optical band gap tunability [2-4]. Such close similarities invite for both the knowledge transfer and combinational utilization of the III-nitride and Zn-IV-nitride systems. The main challenge for Zn-IV-N₂ in general, and for ZnSnN₂ in particular, is in controlling intrinsic and extrinsic defects, governing the carrier concentration (n).
Typically, for as-grown stoichiometric ZnSnN₂, n remains in the range of 5×10¹⁹ - 5×10²⁰ cm^-3, where possible defects causing the degeneracy have been identified by first-principles calculations; the tin-zinc antisite (Sn_Zn), the nitrogen vacancy (V_N), and the zinc interstitial (Zn_i), are all defects probable to occur and form shallow donors [5,6]. Furthermore, common impurities such as oxygen substituting for nitrogen (O_N), hydrogen interstitials (H_i) and substitutional hydrogen (H_N), also form donor states in ZnSnN₂ [5,6]. The prevailing route for controlling n is the fabrication of Zn-rich ZnSnN₂, effectively reducing and enhancing the formation probability of Sn_Zn donors and Zn_Sn acceptor [7], but the carrier mobility (µ) remains unacceptably low (< 10 cm² V^-1s^-1). Possible p-type dopants in ZnSnN₂ have also been identified by first-principles calculations, where Na_Zn and K_Zn are predicted to exhibit deep acceptor levels, while Li_Zn is predicted to be a shallow acceptor [8]. However, the corresponding experimental explorations are rare or missing.
Here, we report on ZnSnN₂ thin films grown by high-power impulse magnetron sputtering (HiPIMS), with carrier concentration reducing strategies, such as shallow acceptor doping with Li and off-stoichiometric growth. We thus extend our work on epitaxial ZnSnN₂ growth exhibiting a dislocation density of 1×10¹⁰-5×10¹⁰ cm^-3, i.e., comparable to III-nitrides grown without dislocation-reducing strategies [9], in order to obtain electrical control. Our preliminary results confirm that the Sn_Zn antisite is the dominant intrinsic defect, as n is reduced, and hence also the optical band gap (from ~1.78 eV to ~1.28 eV) due to reduced Burstein-Moss shift, when moving towards more Zn-rich conditions. Moreover, we confirm that Li acts as an acceptor in ZnSnN₂, reducing n in the stoichiometric film by an order of magnitude, while keeping µ at a reasonable high value, µ ~ 60 cm²V^-1s^-1. As predicted by Wang et al. [8], Li_Zn is a shallow acceptor in ZnSnN₂; however, in the interstitial configuration (Li_i), Li may act as a donor too. Nevertheless, even though both Li_Zn and Li_i are possible to occur in our samples, the significant reduction in n for the stoichiometric film suggests that the majority of the incorporated Li is in the form of Li_Zn acceptors. Thus, controlling the interplay between intrinsic and extrinsic defects may lead to a breakthrough in the development of ZnSnN₂.

[1] Schepf, R. R. et al., ACS Energy Letters, 5, 2027-2041, 2020
[2] Lahourcade, L. et al., Advanced Materials, 25, 2562-2566, 2013
[3] Kikkawa, S. et al., Solid State Communications, 112, 513-515, 1999
[4] Osinsky, A. et al., Proceedings 2000 IEEE/Cornell Conf. on High Perform. Devices, 2000
[5] Tsunoda, N. et al., Physical Review Applied, 10, 011001, 2018
[6] Chen, S. et al., Advanced Materials, 26, 311-315, 2013
[7] Fioretti, A. et al., Journal of Materials Chemistry C, 3, 11017, 2015
[8] Wang, T. et al., Physical Review B, 95, 205205, 2017
[9] Olsen, V. S. et al., Advanced Optical Materials, 21000015, 2021

Refractory nitrides (TiN, ZrN, Hf, etc.) and nitride alloys (Ti_1-xAl_xN, etc.) form a class of materials with high industrial relevance for hard coatings of cutting tools [1] or for plasmonic-based devices [2]. For these applications the major objectives are the material’s high-temperature thermal and elastic properties. A recently developed combination of quantum mechanical calculations with machine-learning interatomic potentials [3], is utilized to calculate materials elastic constants with high accuracy. On-the-fly training of these potentials allows us to perform the calculations with more than two orders less computational effort than using state-of-the-art ab initio molecular dynamics simulations. The derived elastic constants are used to calculate surface acoustic waves and Brillouin light scattering (BLS) spectra. The results are compared with experiments. Furthermore, we investigate high-temperature bcc phase of titanium and predict very weak temperature dependence of its elastic moduli [4], called Elinvar effect, similar to the behavior observed for so-called GUM alloys. However, the effect in bcc-Ti is intrinsic and therefore unique.


[1] See, for example, F. Tasnádi et al., Phys. Rev. B 85, 144112 (2012); F. Tasnádi et al., Appl. Phys. Lett. 97, 231902 (2010); D. Holec et al. Phys. Rev. B 90, 184106 (2014); F. Tasnádi et al., Mater. Des. 114, 484 (2017).
[2] U. Guler et al., Mater. Today 18, 227 (2015), Catellani et al., Phys. Rev. B 95, 15145 (2017); M. Kumar et al., ACS Photonics 3, 43 (2016); U. Guler et al., Faraday Discuss. 178, 71 (2015).
[3] I. S. Novikov et al., Mach. Learn.: Sci. Technol. 2 025002 (2021).
[4] A. Shapeev et al., New. J. Phys. 22, 113005 (2020).

The sintering of AlN ceramics has been intensely investigated and it is reasonably understood. The increasing possibilities offered by the outstanding properties of nanostructured ceramics underline the necessity to also understand the sintering process of AlN ceramic nanoscale powder. Here, we use atomistic molecular dynamics simulations to study the hot-press sintering of AlN nanoceramics. We employ nanopowder samples composed of nanoparticles with 8, 12, and 16 nm: namely, samples AlN-8, AlN-12, and AlN-16. Densification simulations are run for 6 ns under a hydrostatic pressure of 1 GPa and sintering temperature of 1,900 K. An extra simulation is performed for an 8 nm sized nanoparticle system at a lower pressure of 0.1 GPa (sample AlN-8-0.1). Results show that the sample AlN-8 reaches 99% densification, the highest of all systems. 96.2%, 95.6%, and 93.2% densifications are achieved for the samples AlN-8-0.1, AlN-12, AlN-16, respectively. Grain size distribution statistics shows that the AlN-8 system experience significant microstructural evolution from 3 to 6 ns of the sintering process developing a wide distribution of grain sizes from 4 to 15 nm with a large average grain size of ~11 nm. These results reveal a two-stage sintering process: (1) Initial swift densification that is rooted at the high diffusivity of surface atoms. (2) Intense grain growth driven by grain boundary migration. Moreover, a wurtzite-to-rocksalt structural phase transformation occurs at the nanoparticles contact points in the AlN-16 sample during the sintering. These exciting results offer nanoscale insights into the intriguing hot-press sintering of nanoscale ceramics and shed light on the densification and microstructural evolution processes.

SiAlON ceramics are solid solutions of Al and O in the Si₃N₄ lattice. They are used as wear parts and gas turbine engine blades, and more recently have shown promise for application as biomedical implants. The densification of SiAlONs is generally achieved using sintering additives and/or pressure-assisted sintering techniques. Traditionally, additives such as alumina and yttria powders are utilized. In this work, we investigate the use of organic precursors rather than powders in order to incorporate sintering additives at the atomic or molecular level. This presentation will highlight the densification, strength, elastic modulus, hardness, microstructure, and phase assemblage of SiAlONs fabricated via atomic-scale as well as conventional additives. It was found that the use of molecular precursors may enhance densification by providing a very fine scale distribution of solute atoms which are more readily available to the Si₃N₄ starting powder than atoms confined to powder particles.

Silicon nitride (SiN_x) is ubiquitous in photovoltaic technology as a dual-purpose passivation layer and antireflection-coating. It also is used as a durable coating/material in industrial settings as it is a high-strength ceramic. Recent work has explored nanoscale silicon nitride morphologies such as nanowires, nanopores, nanocomposites, and others. Specifically, nanoparticle (NP) SiN_x is a promising nano-scale form as it has applications in battery technology, passive radiative cooling films, and as passivation layer on silicon nanoparticles for improving photo-luminescence. Also hot-pressed and sintered SiN_x from a powder is an attractive route to high mechanical strength materials. These studies outline the need for SiN_x NP with a narrow size distribution, minimal aggregation, high purity, and high throughput. However, producing SiN_x nanoparticles with current top-down techniques does not meet the high yield and narrow size distribution requirements. Bottom-up techniques such as gas phase synthesis typically require very high temperatures and use ammonia as the nitrogen precursor as it has lower bonding energy than diatomic nitrogen. Ammonia, however, is caustic and requires several safety precautions. Nonthermal plasma NP synthesis has been shown to be a promising technique which meets these production requirements and can deposit uniform films over a wide area with tunable thickness and porosity. Here, we report SiN_x NP synthesis with this technique using a silane-nitrogen plasma. After synthesis, the particles are accelerated through a nozzle to sonic speeds to deposit ballistically onto a rastered substrate. Resulting SiN_x NP films show low visible absorption suggesting stoichiometric silicon nitride. This study shows that silicon nitride nanoparticles can be produced in this high-yield, scalable, and tunable synthesis process.
Acknowledgement: This work is supported by the Army Research Office under MURI project under W911NF-18-1-0240.

The 2019 report from Fichtner, et al.[1] of ferroelectric switching in Al_1-xSc_xN thin films further increased interest in this materials system that was already the darling of the piezoelectric thin film community. This system now promises both resonator-quality piezoelectric properties and the potential to enable a class of next-generation compute-in-memory hybrids if switching voltages can be sufficiently scaled. Hayden, et al.[2] recently expanded the family of AlN-based ferroelectrics to include Al_1-xB_xN and even unmodified AlN, adding to the excitement over potential for integrated functionality. The large switchable spontaneous polarization values, enormous coercive fields, and remarkably square hysteresis loops of AlN-based ferroelectrics draw sharp contrast to HfO₂-based ferroelectrics and highlight the need for better understanding the switching behaviors in these novel ferroelectrics to effectively leverage this switchable polarization.

Using a custom-built high power measurement capability in addition to commercial test equipment, we have studied the polarization reversal behavior of AlN-based ferroelectric thin films across more than 9 orders of magnitude in time. This talk summarizes our findings on the effects of chemistry (intentional alloying elements such as scandium and boron as well as unintentional contaminants such as oxygen), measurement temperature, film structure and stress state, and electrodes on switching dynamics. Combined with complementary measurements of Rayleigh behavior and polarization retention, these studies provide strong hints at the underlying mechanisms controlling ferroelectricity in these new AlN-based ferroelectrics.

[1] S. Fichtner, N. Wolff, F. Lofink, L. Kienle, and B. Wagner, “AlScN: A III-V semiconductor based ferroelectric,” J. Appl. Phys., 125, 114103 (2019). doi: 10.1063/1.5084945

[2] J. Hayden, M.D. Hossain, Y. Xiong, K. Ferri, W. Zhu, M.V. Imperatore, N. Giebink, S. Trolier-McKinstry, I. Dabo, and J.-P. Maria, “Ferroelectricity in boron-substituted aluminum nitride thin films,” Phys. Rev. Mater., 5, 044412 (2021). doi: 10.1103/PhysRevMaterials.5.044412

We present characterizations of the local atomic structure and nanoscale chemistry of ferroelectric aluminum scandium nitride thin films (Al_1−xSc_xN) using advanced transmission electron microscopy (TEM) techniques. An Al_1−xSc_xN (x = 0.36) film of ∼20 nm thickness was grown on a Pt(111)/Ti/SiO2/Si(100) substrate via pulsed DC co-sputtering. The homogeneous distribution of scandium and the formation of defects in the epitaxial growth of 2.1% lattice-mismatched AlScN on Pt are reported. We also examined the strain distribution across the AlScN/Pt interface. The “four-dimensional scanning TEM” (4D-STEM) technique was employed to systematically investigate the nanoscale order by measuring the average spacing between atoms within certain regions in the film and determining the strain. It was conclusively shown that AlScN crystallites preserve the piezoelectric wurtzite-type structure in the (001) preferential orientation normal to the substrate. The strain map confirms a significant increase in the out-of-plane component of the lattice parameter (∼9%) at the AlScN/Pt interface. The lattice parameter in the Pt template decreases as a function of distance from the Pt/Si interface. The study of the atomic crystal structure and the chemical composition of the AlScN thin film provides useful information that will aid future applications of this material in ferroelectric memories and microelectromechanical systems.

The potential of Ge as n-type dopant for AlGaN materials is currently under consideration. Ge is known to generate significantly less local tensile strain than silicon, whatever the Al mole fraction of the alloy. However, at high doping concentrations, the issue of the effective dopant incorporation and its spatial distribution is relevant. Formation of precipitates or diffusion of dopants along structural defects may occur [1]. Recently, we have demonstrated plasma-assisted molecular-beam epitaxy growth of Al_xGa_1−xN planar layers with Ge nominal concentrations up to 10²¹ cm^-3 [2]. Here, we present a structural analysis of Al_xGa_1−xN:Ge samples with x covering the full compositional range (0≤x≤1), together with the measurement of Ge concentration and its spatial distribution from the µm range down to the nm scale [3]. Our study is based on a combination of X-ray diffraction (XRD) and quantitative energy dispersive X-ray spectrometry (EDX) coupled to high-resolution scanning transmission electron microscopy (HR-STEM) and atom probe tomography (APT).

In Al_xGa_1−xN:Ge samples with x>0.4, top-view SEM observations reveal the presence of µm-size crystallites at the surface. The nature of such crystallites is pure Ge, as verified by EDX. Inside those layers, we have also identified Ge-rich inclusions with a size of tens of nanometers, generally associated with Ga-rich regions around structural defects. With these local exceptions, the Al_xGa_1−xN:Ge matrix presents homogenous Ge composition. The Ge concentration in the matrix, i.e. away from the precipitates, was obtained from the ratio of the EDX signal between doped and undoped samples. The Ge content in the binary AlN matrix is extremely low, and it increases linearly with the Ga mole fraction in ternary Al_xGa_1−xN, which suggests that the Ge incorporation takes place by substitution of Ga atoms. The maximum percentage of Ga sites occupied by Ge saturates around 1%. These solubility issues and the Ge segregation phenomena should play a role in the efficiency of Ge as n-type dopant, even at Al concentrations where Ge DX centers are not expected to manifest. In view of these results, the extracted solubility limit of Ge in Al_xGa_1−xN can have direct impact on the performance of Al_xGa_1−xN-based UV light emitting diodes. In principle, the Ge content should be kept below these limits to prevent a degradation of carrier transport due to scattering at structural defects. Co-doping or the use of surfactant species should be explored to attain Ge concentrations beyond the solubility limit.


References
[1] L. Amichi, et al., Nanotechnol. 31, 045702 (2020).
[2] R. Blasco, et al., J. Phys. D.: Appl. Phys. 52, 125101 (2019).
[3] C. Bougerol, et al., ACS Applied Materials & Interfaces 13, 4165 (2021).

The remarkable properties of InGaN/GaN nanowires (NWs) heterostructures emitting at 480 nm and grown by PA-MBE, namely a large amount of free surface favouring strain relaxation and the expected absence of extended defects make them attractive for the development of a new generation of highly efficient light emitting diodes (LEDs). However, as a counterpart of these potential advantages, the surface itself may act as a non-radiative recombination centre. To clarify this issue, the effect of surface passivation by deposition of an aluminium oxide material, using the Atomic Layer Deposition technique was investigated by time resolved photoluminescence experiments. It appears that the photoluminescence decay - which follows a power law - before passivation is markedly shorter than following passivation, emphasizing the role of the NW surface as a non-radiative recombination channel in the case of InGaN/GaN NWs heterostructures. Furthermore, Micro-photoluminescence spectroscopy experiments performed on InGaN/GaN NWs for laser excitation power density ranging from 10^-4 W.cm^-2 to 10⁶ W.cm^-2, exhibit a bell shape curve, typical of a defect saturation regime. As the power law dependence of the photoluminescence prevents from determining a single lifetime, the standard phenomenological ABC model cannot be used to describe it. Hence, we developed a theoretical model that accounts for the carrier generation, the radiative recombination, the capture of carriers by non-radiative defect states — leading to their saturation, as well as the Auger losses. The internal quantum efficiency and the nature of carrier recombination (monomolecular or bimolecular) are quantitatively assessed by this model, as a function of the excitation power.

The quality of III-nitride epilayers depends strongly on the characteristics of the substrate. The reduction of the number of extended defects in lattice mismatched epitaxial GaN layers grown on the most used substrates, namely sapphire and silicon, requires complex growth strategies that, despite the continuous advances, result in higher costs of the final device. The mismatch in thermal expansion coefficients of III-nitride layers and, for instance, Si substrate, is an additional source of extended defects associated with a strong substrate/layer chemical bonding. This motivates the exploration of alternative substrates, including the renewed interest in van der Waals epitaxy, where epitaxial growth is achieved through weak dipolar interactions. Materials such as graphite, mica, MoS₂ or GaSe, have layered structures bonded via weak van der Waals interaction and can be easily cleaved, presenting very flat surfaces and the absence of dangling bonds. When GaN is grown on them, these substrates allow a significant relaxation of the epilayer strain and facilitate, at the same time, layer detachment [1].

We study the case of GaN grown by molecular beam epitaxy on graphene and muscovite mica as possible van der Waals substrates, addressing epilayer morphology, crystallographic phase and orientation, surface potential and strain. We also study the changes experienced by the graphene substrate during growth: doping, strain and, under certain conditions, the intercalation of a self-limited bilayer of metal atoms below graphene [2, 3]. In the case of mica, we observe that part of the strain accumulated in the GaN layer during growth relaxes by the formation of three-dimensional structures in the shape of telephone cord buckles, straight blisters or by more complex arrangements. The characteristics of these structures are analyzed in relation to the compressive strain of the surrounding material, the achieved relaxation and layer adhesion [4].

[1] Kim, J., Bayram, C., Park, H., Cheng, C. W., Dimitrakopoulos, C., Ott, J. A., Reuter, K. B., Bedell, S. W., & Sadana, D. K. (2014). Principle of direct van der Waals epitaxy of single-crystalline films on epitaxial graphene. Nature Communications, 5, 1–7. https://doi.org/10.1038/ncomms5836
[2] Gruart, M., Feldberg, N., Gayral, B., Bougerol, C., Pouget, S., Bellet-Amalric, E., Garro, N., Cros, A., Okuno, H., & Daudin, B. (2020). Impact of kinetics on the growth of GaN on graphene by plasma-assisted molecular beam epitaxy. Nanotechnology, 31(11), 115602. https://doi.org/10.1088/1361-6528/ab5c15
[3] Feldberg, N., Klymov, O., Garro, N., Cros, A., Mollard, N., Okuno, H., Gruart, M., & Daudin, B. (2019). Spontaneous intercalation of Ga and In bilayers during plasma-assisted molecular beam epitaxy growth of GaN on graphene on SiC. Nanotechnology, 30(37), 375602. https://doi.org/10.1088/1361-6528/ab261f
[4] Daudin, B., Donatini, F., Bougerol, C., Gayral, B., Bellet-Amalric, E., Vermeersch, R., Feldberg, N., Rouvière, J.-L., Recio Carretero, M. J., Garro, N., Garcia-Orrit, S., & Cros, A. (2021). Growth of zinc-blende GaN on muscovite mica by molecular beam epitaxy. Nanotechnology, 32(2). https://doi.org/10.1088/1361-6528/abb6a5

III-Nitride epitaxy is fraught with many challenges ranging from phase separation to high defect densities to doping limitations. Traditional epitaxy has suggested that high temperatures are required to maintain long surface diffusion lengths and thus, high material quality. Unfortunately, high temperature environments also introduce significant complications and limitations in practically achievable compositions and doping ranges. Furthermore, huge variations in the effective metal-fluxes from one growth system to another at traditional epitaxy growth temperatures makes it difficult to achieve commercial scalability. By deviating from the normal high temperature epitaxy via the control of surface chemistry and adding intentional kinetic limitations on adatom diffusion, we can achieve significant improvements in the material quality of traditionally difficult materials to grow such as AlInN and InGaN. Non-phase separated InGaN has been grown throughout the miscibility gap and record Indium-content solar cells produced via a low temperature variant of molecular beam epitaxy called metal modulated epitaxy (MME). MME offers the ability to control major factors in phase separation: growth at low temperature avoids InGaN decomposition and indium desorption; high growth rates limit the lateral separation of surface adatoms; and fine-control of excess-metal dose per shutter cycle prevents extreme build-up of a metal adlayers and allows control over vertical segregation of indium and gallium. Likewise, highly luminescent AlInN has been produced that has x-ray diffraction (XRD) figures of merit 11 times better than previous literature. Furthermore, MME has been expanded to grow high crystal quality AlInN over the entire composition range. Additionally, low temperature MME AlInN growth near the composition that would be latticed matched to GaN (~18% In) shows XRD figures of merit rivaling the best in the literature. By modeling and controlling surface chemistry, especially vertical phase segregation, remarkably coherent superlattices can be obtained in AlGaN and InGaN ternary systems and can be maintained coherent for many microns thicknesses. MME achieves excellent uniformity and thickness control suitable for commercial use primarily because of low temperature growth while simultaneously achieving high quality films via enhanced adatom diffusion through its additional growth control parameter of excess-metal dose per shutter cycle.
Additionally, by using substantially lower growth temperatures than other methods, MME can enhance the doping concentration of traditionally difficult dopants such as magnesium and beryllium exceeding prior doping limits by more than 40 times and for more traditional dopants like Si and Ge by factors of 2 to 4. This has led to a new understanding of the acceptor energy wherein a wide acceptor energy band forms because of the Pauli Exclusion principle compared to a single isolated energy in traditionally doped cases. Using these highly advanced low temperature doping methods, tunnel junctions with record conductance and strong negative differential resistance have been demonstrated and used in GaN and AlGaN homojunction devices even up to 58% Al. These advances have enabled the first tunnel junction contacted III-Nitride solar cell with much higher uniformity and improved the 270 nm DUV optical output of LEDs by as much as 110%. Finally, we will demonstrate that by a combination of low temperature growth and control of surface chemistry, for the first time we have been able to demonstrate p-type aluminum nitride with hole concentrations exceeding 1x10¹⁸ cm^-3 and showed a first time heterojunction p-AlN/n-GaN diode with modest rectification showing future potential for future deep ultraviolet, high-power and high-frequency devices capable of operation in extreme radiation and heat environments.

Thanks to its ultra-wide band gap, AlGaN and AlN nanowires (NWs) are attractive candidates for deep UV light emitting diodes (LED). The absence of extended defects, such as dislocations, as well as the morphology of nitride NWs, are favorable to improve the dopants incorporation, the internal quantum efficiency and light extraction. In this context, some of us recently reported axial pn junction realization with Mg acceptors in the p-region and Si donors in the n-region [1]. Thus, it becomes crucial to understand and describe in detail the transport mechanisms at the origin of the current flow inside the individual nanodevices. The n-type doping of AlN nanowires remains a blind spot in recent research and is as important as its p-type counterpart in order to achieve the best performances for nanowire-based DUV LEDs.

In this work, we investigate the electrical properties of n-type AlN NWs grown by molecular beam epitaxy using several silicon fluxes. The current voltage measurements show that all the samples exhibit an Ohmic regime at low bias (< 0.2 V) and a space charge limited current regime assisted by traps for higher voltage (> 0.2 V). This regime occurs in semiconductor with moderate carrier density either because of a near-surface depletion or because of a weak ionization of relatively deep dopants. The conductivity reaches a maximum of 6.10^-6S/cm for intermediate Si flux before dropping down to 4.10^-9 S/cm at larger flux, in good agreement with high compensation effects observed in bulk AlN [2]. Temperature dependent measurements from 180 K to 600 K revealed that NW conductivity is a combination of the two identified silicon levels at 75meV and 270meV [3]. Due to a high surface level pinning energy of 2.1eV [4], the NW diameter is shown to be a key parameter of particular importance in order to not reach its full depletion, decreasing the free electrons concentration at thermal equilibrium and leading to poor conductivity. The full model and analysis will be presented and discussed in this presentation. Those results point towards the need for larger nanowires and/or surface passivation to ensure better electron injection, enabling the realization of efficient UV LEDs.

[1] A.-M. Siladie et al, Nano Lett. 19, 8357-8364 (2019)
[2] J. S. Harris et al, Appl. Phys. Lett. 112, 152101 (2018)
[3] M.H. Brenckenridge et al, Appl. Phys. Lett. 118, 112104 (2021)
[4] P. Reddy et al, J. Appl. Phys. 116, 123701 (2014)

Introduction of electrical dopant species in semiconductors generally results in increasing built-up strain for increasing dopant densities, till reaching the solubility limit and/or a strain relaxation stage associated with extended defect formation. By contrast doping in nanowires (NWs) obeys a specific regime, associated with their unique morphology. In the case of GaN NWs grown on Si (111) by molecular beam epitaxy, the eased strain relaxation due to the large amount of free surface favors the incorporation of Si dopant at concentrations higher than in layers [1]. Also, specific adatom surface diffusion and reconstruction-stabilized sidewall stoichiometry are affecting dopant incorporation. Hence, atom probe tomography [2] and energy-dispersive X-rays spectroscopy (EDX) [3] experiments at the nanoscale demonstrate that preferential Mg incorporation through the m-plane side walls of GaN NWs leads to a higher Mg density in the NW periphery, about ten times larger than in the NW core. Scanning spreading resistance microscopy (SSRM) experiments were performed to investigate the effect of inhomogeneous dopant distribution on the electrical transport properties of n-type and p-type GaN NWs at the nanoscale. In Si-doped GaN NWs a conductive core surrounded by a more resistive shell was put in evidence, which is assigned to the depletion resulting from Fermi level pinning on m-plane surface states. By contrast, in the case of Mg-doped GaN NWs, the SSRM is one order of magnitude smaller at the periphery than at the centre of the Mg-doped GaN. This experimentally demonstrates that peripheral Mg incorporation in GaN NWs indeed counterbalances the depletion resulting from Fermi level pinning on m-plane surface states, which results in preferential current flow in the highly doped shell. For the same samples as measured by SSRM, EDX reveals a Mg concentration of about 2-4 × 10²⁰ /cm³ in periphery, higher by more than one order of magnitude compared to the NW centre where the Mg content is below the experimental detection limit of 10¹⁹/cm³. The coexistence of these different electrical conduction channels, specific to NWs, opens new prospects on the full understanding of the opto-electrical properties of NW-based devices such as visible or UV light emitting diodes.
[1] Z. Fang et al, Nano Lett. 15, 6794 (2015)
[2] A.M. Siladie et al, Nanotechnology, 29, 255706 (2018)
[3] E. Aybeke et al, submitted

The search for novel semiconductors is increasingly important as the applications of semiconductors become more prevalent in modern society. This situation stimulates not only experimental but also computational explorations of as-yet-unreported semiconductors, typically using first-principles calculations. In such computational searches, reliable design principles, as well as accurate and efficient computational schemes, are key requirements for the successful identification of target materials and functionalities. In this talk, I will discuss the design and exploration of nitride semiconductors using first-principles calculations, with a focus on ternary zinc nitrides. Topics to be covered include the theoretical prediction and experimental verification of new nitride semiconductors such as CaZn2N2, SrZn2N2, and related alloys with direct band gaps covering most of the visible light range [1-3]. Computational methods for predicting fundamental and defect properties of semiconductors and their applications to nitride semiconductors will also be discussed [4].
[1] Y. Hinuma, T. Hatakeyama, Y. Kumagai, L. A. Burton, H. Sato, Y. Muraba, S. Iimura, H. Hiramatsu, I. Tanaka, H. Hosono, and F. Oba, Nat. Commun. 7, 11962 (2016).
[2] R. Kikuchi, K. Ueno, T. Nakamura, T. Kurabuchi, Y. Kaneko, Y. Kumagai, and F. Oba, Chem. Mater. 33, 2864 (2021).
[3] M. Tsuji, H. Hiramatsu, and H. Hosono, Inorg. Chem. 58, 12311 (2019).
[4] F. Oba and Y. Kumagai, Appl. Phys. Express 11, 060101 (2018).

While the III-N compounds revolutionized solid-state lighting, there is ongoing work to identify nitrides which enable technologies currently inaccessible with III-Ns, such as bridging the “green gap”. This has led to the investigation of increasingly complex materials which potentially have increased property tunability over the III-Ns, such as II-IV-N₂ compounds ZnGeN₂ and ZnSnN₂. Interest in these materials is driven by their cation site disorder, which may be harnessed to tune bandgap without a corresponding change in lattice parameter [1]. At the same time, it is increasingly clear that metastable nitrides are highly synthesizable compared to other compounds [2], increasing the diversity of nitride materials which can be utilized in emerging technologies.

While interest in the Zn-based II-IV-N₂ compounds has been high, Mg-IV-N₂ compounds (IV = Si, Ge, Sn) are substantially underexplored in comparison. However, these materials not only display the potential for bandgap tunability via cation disorder of II-IV-N₂ compounds, but also multiple low-energy metastable polymorphs, as exemplified by MgSnN₂ [3]. The wurtzite-type, ground-state polymorph of MgSnN₂ has a ~2.4 eV bandgap, but it can also form as a >2.9 eV, high dielectric constant rocksalt-type metastable phase. Here, we present ongoing work on the synthesis of the Mg-IV-N₂ family of compounds, seeking to understand and control polymorph formation to enable property tuning across the group.

First, we discuss our combinatorial approach to the synthesis of the Mg-IV-N₂ compounds, beginning with MgSnN₂ [3]. The metastable rocksalt-type polymorph of MgSnN₂ was found to form as a secondary phase to the wurtzite-type polymorph at substrate temperatures below 200 °C and at high Mg content on Si substrates, but also as part of a mixed wurtzite/rocksalt phase on GaN substrates at high temperatures. The close effective lattice match between GaN (001) and rocksalt MgSnN₂ (111) surfaces promotes the formation of the rocksalt at those high temperatures, an effect that we seek to leverage in ongoing work exploring control of polymorph formation via epitaxial strain energy of different substrates. We will discuss related phenomena in MgSiN₂ and MgGeN₂, and present trends across this family of materials, with broad lessons for development of emerging nitride semiconductors.

This material is based on work funded by a Director’s Postdoctoral Research Fellowship (LDRD) at
the National Renewable Energy Laboratory and by the Office of Science, Basic Energy Sciences, Material Sciences and Engineering Division.

[1] Schnepf et al, ACS Energy Letters (2020).
[2] Greenaway et al, Annual Review of Materials Research (2021).
[3] Greenaway et al, Journal of the American Chemical Society (2020).

In oxides, the oxygen-vacancy ordering often occurs, leading to the emergence of novel functional properties, as exemplified by YBCO. On the other hand, oxynitrides are expected to have functions beyond oxides due to the difference in nitrogen and oxygen (e.g., valence and polarizability), but there are no reports on anion-vacancy ordering. In my talk, a successful example of anion-vacancy order is shown by SrVO_2.2N_0.6, where anion-vacant layers appear periodically along the 111 direction when perovskite oxide SrV(IV)O3 powder is treated with NH3 at about 600°C. We suggest that the anion-deficient ordering is originated from the oxidation of V⁴⁺ to V⁵⁺ by N^_3-/O^2- exchange and the preference of pentavalent V for tetrahedral coordination. More surprisingly, when the same treatment was applied to a single-crystal SrVO₃ thin film, the direction of the vacancy planes (111, 112) and the periodicity (5-fold, 6-fold) changed depending on the substrate used. The biaxial strain effect was verified by first-principles calculations. Like oxide heterostructures, the oxynitride has a superlattice of insulating and metallic blocks. Given the abundance of perovskite families, this study provides new opportunities to design superlattices by chemically modifying simple perovskite oxides with tunable anion-vacancy patterns through epitaxial lattice strain.
T. Yamamoto et al., Nat. Commun. 11, 5923 (2020).
H. Kageyama et al., Nat. Commun. 9, 772 (2018).

Perovskite oxynitrides ABO₂N or ABON₂ (A=rare earth or alkaline earth metal; B=transition metal) are important heteroanionic materials that show high permittivities, photocatalytic activity under visible light and colossal magnetoresistance among other relevant properties. (1) Pseudocubic oxynitride perovskites derived from the Pm-3m aristotype are centrosymmetric and show symmetry lowering for tolerance factors (t) below 1. All anion sites show similar coordination environments with cations, which favours the partial or total disorder of nitrogen and oxygen. The observed high relative permittivities and ferroelectricity in compounds such as SrTaO₂N and BaTaO₂N have been ascribed to local electrical dipoles (2) from cis order of nitrides, induced by covalency. (3)

Hexagonal perovskites formed by sequences of cubic and hexagonal close packed AX₃ layers are stabilized for tolerance factors greater than 1 and show two well differentiated environments for anions, in sharing vertex and sharing faces of the BX₆ octahedra, providing the possibility of selective occupation by nitride and oxide. This talk will present results on the new polar compound BaWON₂ that is the first example of a hexagonal perovskite oxynitride. (4) It shows the 6H polytype and crystallizes in the non-centrosymmetric space group P6₃mc, with cell parameters a=5.85073(2) and c=14.53220(9) Å. The two anions N^3- and O^2- are completely ordered in the vertex shared and face shared positions of the WX₆ octahedra respectively. The preferred occupation of the more electronegative O atom at face-shared sites is a consequence of the weaker bonding for these positions, with W-X-W angles ≈90°, compared with corner-shared sites, with W-X-W angles≈180°. Total anion order, together with the large electrostatic repulsion between W⁶⁺ cations in the face shared octahedra, and second order Jahn-Teller effect induce highly distorted environments in three inequivalent WX₆ octahedra stabilizing the non-centrosymmetric structure. The spontaneous polarization, calculated from the crystal coordinates and formal electrical charges of all atoms, is ≈17.7 µC/cm², parallel to the c axis. BaWON₂ is a semiconductor with a band gap of 1.1 eV and a relative permittivity of 200-230 in the 10-100 K temperature range, and shows a second harmonic generation (SHG) signal under excitation with a Ti:sapphire laser. The observed remarkably large permittivity is consistent with anisotropic, multi-anionic bond network in the structure, which in turn shall promote anharmonicity of the bond dynamics and subsequently the observed non-linear optical response. This achievement contrasts with earlier results on ABO₂N perovskites with pseudocubic centrosymmetric structures that lead to large dielectric permittivity and piezoresponse under electric field bias, ascribed to field-response of nanopolar regions, but without long range polar order.
References
1. A.Fuertes, APL Mater. 2020, 8, 020903.
2. A.Hosono, Y.Masubuchi, S. Yasui, M. Takesada, T. Endo, M. Higuchi, M. Itoh and S. Kikkawa, Inorg. Chem. 2019, 58, 16752.
3. M.Yang, J.Oró-Solé, J.A. Rodgers, A. B. Jorge, A.Fuertes and J. P.Attfield, Nat. Chem., 2011, 3, 47.
4. J. Oró-Solé, I.Fina, C.Frontera, J.Gázquez, C.Ritter, M.Cunquero, P. Loza-Alvarez, S.Conejeros, P.Alemany, E.Canadell, J.Fontcuberta and A.Fuertes, Angew. Chem. Int. Ed. 2020, 59, 18395.

The prospect of combining members of the large family of heterovalent ternary nitride semiconductors with each other and with the III-nitrides opens up new avenues for band structure engineering, lattice matching with different substrates, and new applications. There is interest in ZnSnN₂, with a band gap in the range of 1-2 eV that tunes with the degree of disorder on the cation sublattice, for its application to single junction solar cells [1]. Alloys of ZnGeN₂ and GaN are of interest for photocatalytic applications such as water splitting [2].

A major aim of our work on developing the growth of these materials by MOCVD is to improve the efficiency of nitride LEDs at green and longer wavelengths by taking advantage of the large predicted type II band offsets of the Zn-IV-nitrides with the III-nitrides. Band offsets at the ZnGeN₂-GaN interface greater than 1 eV in both the conduction and valence bands have recently been confirmed experimentally [3]. Appropriate insertion of a few-unit-cell-wide Zn-IV-nitride layer into a conventional InGaN quantum well is predicted to improve the electron-hole wave function overlap substantially, thus increasing the emission efficiency while also providing the benefit of reducing the In content in the InGaN quantum well for emission at a given wavelength, compared to the conventional InGaN structure [4].

In order to realize these proposed LED structures, the growth temperature for the Zn-IV-nitride or alloy insertion layer should be matched to that for the InGaN quantum well. However, the temperatures for the growth of high quality ZnGeN₂ by sub-atmospheric-pressure MOCVD are substantially higher than those used for the growth of high quality InGaN [5]. On the other hand, the case is reversed for ZnSnN₂, and the growth temperature for producing high quality ZnSnN₂ layers is too low. ZnSn_xGe_(1-x)N₂ presents a solution to matching the optimal growth temperature of the insertion layer to InGaN; the growth window of the homovalent alloy is fortuitously broadened over that for pure ZnSnN₂.

Growth of thick layers of ZnSnN₂, as are necessary for other applications such as photovoltaics, is problematic by MOCVD. We find that if the precursors (in this work, DEZn, TMSn and NH₃) are flowed continuously with a DEZn-to-TMSn ratio that produces stoichiometric ZnSnN₂ for thin layers, for thicker layers Sn droplets accumulate on the growth surface and cause decomposition of the underlying ZnSnN₂, in a reversal of the process that produces vapor-liquid-solid growth of ZnSnN₂ [6]. Pulsing the Sn precursor is an effective route to preventing the accumulation of Sn droplets.

The temperature window for MOCVD growth of the heterovalent ZnGeN₂-GaN alloy is also broadened over that for pure ZnGeN₂. The incorporation rate of Zn into the film is enhanced by the addition of Ga, and, correspondingly, the incorporation of Ge is depressed.

This work was supported by funding from the U.S. Department of Energy (DE-EE0008718) and from the National Science Foundation (DMREF-SusChEM-1533957).

[1] R. A. Makin, K. York, S. M. Durbin, N. Senabulya, J. Mathis, R. Clarke, N. Feldberg, P. Miska, C. M. Jones, Z. Deng, L. Williams, E. Kiyoupakis, and R. J. Reeves, Phys. Rev. Lett. 122, 256403 (2019).
[2] T. Suehiro, M. Tansho, and T. Shimizu, J. Phys. Chem. C 121, 27590 (2017).
[3] M. R. Karim, B. A. Noesges, B. H. D. Jayatunga, M. Zhu, J. Hwang, W. R. L. Lambrecht, L. J. Brillson, K. Kash, and H. Zhao, J. Phys. D: Appl. Phys. 54, 245102 (2021).
[4] L. Han, K. Kash, and H. Zhao, J. Appl. Phys. 120, 103102 (2016), and subsequent work.
[5] M. R. Karim, B. H. D. Jayatunga, M. Zhu, R. A. Lalk, O. Licata, B. Mazumder, J. Hwang, K. Kash, and H. Zhao, AIP Advances 10, 065302 (2020).
[6] P. C. Quayle, G. T. Junno, K. He, E. W. Blanton, J. Shan, and K. Kash, Phys. Status Solidi B254, 1600178 (2017).

Nitrides are promising functional materials for a variety of applications. Despite their technological importance many promising theoretically predicted metal nitrides are yet to be discovered.[1] This is partly rooted in their challenging synthesis as compared to oxides. Non-equilibrium PVD in ultra-high vacuum conditions, such as reactive RF-magnetron sputtering, provides ideal prerequisites for the formation of novel metastable nitride thin films.[2] Furthermore, combinatorial sputtering approaches are extremely effective for a rapid screening of the associated complex synthesis phase-spaces.[3]

In this study, an epsilon-support vector regression machine learning (ML) algorithm was used in combination with density functional theory (DFT) calculations using Heyd–Scuseria–Ernzerhof (HSE) functionals to identify interesting candidates from computational materials databases for a combinatorial PVD screening.[4,5] Zn-V-N was selected as a promising material system for a comprehensive experimental phase screening mainly due to the presence of a potentially stable Zn₂VN₃ semiconducting phase with properties suitable for optoelectronic applications.[4,5]. We carried out a combinatorial PVD screening of the entire Zn-V-N phase space. RF co-sputtering of Zn and V targets was carried out in mixed plasma of Ar and N₂ to grow combinatorial libraries on plain glass substrates. N₂ was supplied directly to the sputter plasma to increase the N₂ dissociation rate and consequently increase the N chemical potential. Automated mapping characterization of the sample libraries was performed using X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS) and four-point probe techniques. In addition, selected samples were studied using hard X-ray photoelectron spectroscopy (HAXPES), Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERDA), photoluminescence (PL) and optical spectrophotometry.

XRD analysis revealed the presence of an orthorhombic Zn_1-xV_xN phase over a large compositional range from Zn₂VN₃ to ZnVN₂, with a narrow process window for single-phase Zn₂VN_3. The measured XRD patterns are consistent with the corresponding calculated patterns.[4] Following the combinatorial screening, we were able to improve the crystallinity and isolate the Zn₂VN₃ phase on larger substrates (51 x 51) mm². A combined RBS/ERDA analysis on these samples demonstrates that the material is stoichiometric with only trace amounts of O, well below 1 at. %. Further, XPS/HAXPES analysis confirms this result with Zn and V being present in oxidation states of +2 and +5, respectively, as expected for Zn₂VN₃. The Fermi level is located ~ 1 eV above valence band maximum indicating a weak p-type doping. Zn₂VN₃ thin films exhibit a resistivity of 0.42 Ωcm. Moreover, PL spectra measured at room temperature exhibits three distinctive peaks at 2.65 eV, 2.85 eV and 3.05 eV with a tail towards 2 eV. The latter is consistent with a significant amount of sub-band gap absorption measured using optical spectrophotometry. This could be due to the presence of anti-site defects and as well as scattering on amorphous phase impurities.

In conclusion, we performed an experimental screening of the Zn-V-N phase space and isolated the novel semiconducting Zn₂VN₃ phase. The experimental results are consistent with DFT calculations, which predict direct band gap of 2.35 eV and p-type conductivity for Zn₂VN₃. However, further studies are required to improve optoelectronic properties of the compound.

References:
[1] W. Sun et al. Nat. Mater. 18 (2019) 732–739.
[2] A. Zakutayev, J. Mater. Chem. A 4 (2016) 6742–6754.
[3] K. Alberi et al. J. Phys. D. Appl. Phys. 52 (2019) 013001.
[4] A. Jain et al. APL Mater. 1 (2013) 011002.
[5] S. Lany, Phys. Rev. B 87 (2013) 085112.

Zinc nitride (Zn₃N₂) has a small direct bandgap, is composed of only earth-abundant elements, and possesses characteristics such as a high mobility that make it a promising semiconductor material for optoelectronic applications. However, non-degenerate Zn₃N₂ is rare and the cause of the high charge carrier concentration remains unknown. Prior density functional theory calculations have predicted oxygen and hydrogen as being responsible for the strong n-type doping but this has not been conclusively verified experimentally. In this work, we studied the role of oxygen on the structural, electrical, and optical properties of Zn₃N₂ grown by plasma-assisted molecular beam epitaxy. The crystalline quality is optimized by tuning both flux and growth temperature, resulting in single phase (222) oriented Zn₃N₂ with a rocking curve full width at half maximum of 0.031°. In addition, we have developed an amorphous GaN capping layer that can be directly deposited onto air-sensitive Zn₃N₂, which shows excellent promise for protecting the material without adversely impacting its electronic properties. To analyze the influence of oxygen on the material properties, the amount of incorporated oxygen in the layer was tuned by changing the elemental base pressure in the growth chamber. Hall and temperature-dependent conductivity measurements reveal high mobilities ranging from 100 to 210 cm²V^-1s^-1, as well as a clear correlation between the oxygen concentration in the sample and the bulk charge carrier concentration. In particular, the carrier concentration of nominally oxygen-free Zn₃N₂ can be decreased from 10²⁰ cm^-3 to 10¹⁷ cm^-3 via controlled oxygen incorporation, without significantly impacting the high mobility of the material. Photothermal deflection spectroscopy shows a concurrent decrease in the bandgap with increasing oxygen concentration (decreasing free electron concentration), consistent with a reduced Burstein-Moss shift.

Copper nitride (Cu-N) and tantalum nitride (Ta-N) films have potential applications in photovoltaic solar cell technology and electronic devices, in general. These thin film materials are non-toxic and can be obtained from readily available low-cost precursors. Cu-N can be used in absorber layers of solar cells because of its high optical absorption coefficient and energy band gap that are almost ideal for efficient absorption of light in the solar spectrum. Cu-N films can be doped both p-type and n-type, they are defect tolerant, and their surfaces can be passivated by a native oxide. On the other hand, Ta-N films are chemically inert and corrosion resistant, which makes them very attractive for various functions in integrated circuits, including thin film resistors and diffusion barrier layers. Furthermore, carrier-selective contact materials, rectifying contacts, and Schottky diodes have been developed based on Ta-N. Recently, there has been interest in the Cu-Ta-N ternary semiconductor alloy system for electronic device applications, which has been the motivation for our work and interest in the binaries that comprise this ternary system. Our intention is to work on the fabrication and study applications of films from the ternary system after gaining a good understanding of the sputtering deposition of the related binaries as well as their properties.
In this work, Cu-N and Ta-N films were deposited on glass or Si substrates by reactive RF magnetron sputtering of Cu or Ta in a nitrogen-containing atmosphere. Films with thicknesses of several hundred nanometers were obtained by systematically varying the substrate temperature at a fixed N:Ar sputtering gas ratio or by varying the N:Ar gas ratio at a fixed substrate temperature. The film morphology was examined by both top-view and cross-sectional-view scanning electron microscopy, and the films’ chemical composition was established by energy-dispersive X-ray spectroscopy. Four-probe resistivity measurements and optical spectrophotometry were performed, and the optical band gaps were calculated by a standard Tauc’s plot technique. The crystallinity of the film’s material was studied by X-ray diffraction in a small-angle X-ray scattering geometry.
The film properties are influenced by the RF power used during the deposition, the substrate temperature, the overall deposition pressure, and the N:Ar gas ratio. The color of the Cu-N films that were obtained was reddish-brown with slight variation among the different types of samples, while the Ta-N films had a metallic luster for lower nitrogen gas fractions and were more brownish-transparent for higher nitrogen gas fractions. Both types of film surfaces were relatively smooth and uniform, with a granular structure, which contained dense pyramid-like grain shapes for Cu-N and irregular cauliflower-like shapes for Ta-N. The optical band gap of Cu-N films ranges from 1.3 to 1.4 eV and decreases with increasing the substrate temperature. For Ta-N, the band gap varies from 1.62 to 1.77 eV and shows an increasing trend with increasing the nitrogen concentration in the sputtering gas mixture.
This work is in progress and some of the results obtained so far appear to match published reports, while in other cases there are differences, or there is no published work that could be used for reference. All indications are that reactive RF sputtering can be used as an efficient, low-cost, and flexible method to fabricate Cu-N and Ta-N films and stacks of such films for device applications. We will report on these and any new results that are becoming available as well as on their analysis, and we expect that this work will be of significant interest to applications and the understanding of metal nitrides, in general.

The ability to tune electronic structure through cation ordering while maintaining lattice constants closely matched to III-Vs makes II-IV-N₂ materials promising for tuning band gaps for use in energy-relevant devices such as LEDs and PV. However, characterizing the degree of ordering in synthesized materials presents challenges due to the large number of variables besides ordering which also influence diffraction and spectroscopy techniques. Through simulations, we develop a picture of site disorder in ZnGeN₂ at short- and long-range, isolated from other structural effects as the state of the system relates to the energetics under nonequilibrium conditions. In ZnGeN₂, the non-isovalent character of the disordered species (Zn²⁺ and Ge⁴⁺) subjects the cation ordering to strong short-range order effects which influence band structure by decreasing the band gap relative to ordered ZnGeN₂ from 3.5 eV to 2.0 eV with small but non-dilute fractions of cations present as anti-site pairs. ZnGeN₂ exhibits pronounced discontinuities in enthalpy and order parameters, which correspond to a first-order phase transition and narrowed forbidden gap in the density of states as well as an onset in localized states in disordered material. We use Monte Carlo simulations to model cation disorder, which in turn use a cluster expansion to approximate the formation enthalpy of a cation configuration. Converged configurations are relaxed in volume and lattice parameters through Density Functional Theory as 1,024 atom supercells. We calculate nitrogen coordination as a short-range order parameter and the Bragg-Williams and stretching parameters as long-range order parameters. We then relate these order parameters with the mixing entropy, free energy of the system and electronic properties to draw relations between order and electronic structure.

The GaN ternary analog ZnGeN₂ is proposed as a green-to-amber emitter able to be integrated easily into current GaN light emitting diode (LED) heterostructures. However, growing ZnGeN₂ on GaN makes accurate material characterization difficult due to their similar optical and structural properties. This ambiguity motivates this work where we present ZnGeN₂ grown on lattice mismatched AlN templates and regrown AlN buffer layers by molecular beam epitaxy. We present highly accurate lattice parameters using multiple reflections and a least squares regression (a = 3.1962 A ± 0.0032 A and c = 5.2162 A ± 0.0015 A [c/a ratio = 1.632]) enabled by the full relaxation of ZnGeN2 and get a lattice-mismatch of 0.22%. We find the preferred growth temperature for ZnGeN₂ on AlN is 450 C by reflection high-energy electron diffraction, X-ray diffraction, and atomic force microscopy characterization. We also present photoluminescence attributed to ZnGeN₂ with a broad recombination band centered around 2.96 eV. Theory suggests that disordered material will emit light as a broad peak in the 2.5-3 eV region, suggesting that we have disordered material. Here we provide insight into the properties of ZnGeN₂ and provide direction for further studies modeling and characterizing GaN/ZnGeN₂ heterostructures.

Photoelectron spectroscopy (PES) has long been considered a standard characterization technique in the development of new semiconducting materials. Despite its mainstream use for the analysis of surface chemistry and electronic structure of new materials, conducting meaningful PES on nitrides is challenging. Handling nitride samples in air usually results in surface oxidation as well as the formation of adsorbate layers, which complicates and quite often prevents meaningful PES measurements.[1] Unfortunately, few labs possess the experimental infrastructure to perform clean (i.e. UHV) transfers from synthesis to characterization environments and sputter cleaning the samples often results in sputter damage or a reduction of the surface.

In this study, we show how a combination of X-ray photoelectron spectroscopy (XPS) and laboratory based hard X-ray photoelectron spectroscopy (HAXPES) was successfully used on ex-situ treated ternary zinc nitrides to complement accelerated phase discovery in Zn-V-N thin films deposited by combinatorial reactive co-sputtering.

The presented concepts can be applied to many different material systems. In particular, we discuss how to gain valuable insights for the design and discovery of new nitrides using chemical state analysis based on the Auger parameter (AP) concept.[2] The core level emission lines of many oxides and nitrides show considerable overlap, which makes analysis of nitride samples exhibiting surface oxides challenging. Auger lines are more sensitive to changes in the local chemical environment, which can make it easier to distinguish oxide and nitride phases. In addition, the AP can show considerable shifts upon the formation of new phases or changes in the microstructure, providing valuable, complimentary insights to conventional structural analysis (e.g. X-ray diffraction or Raman spectroscopy).[3]

For the Zn-V-N material system, a new potentially stable phase was discovered by XPS mapping characterization via analysis of the modified AP for Zn. The latter was calculated using the kinetic energy of the Zn LMM Auger line as well as the binding energy of the Zn 2p3/2 core level emission. The Zn AP showed a distinct shift for some samples (>0.3 eV), which directly correlated with the occurrence of a new phase, as confirmed by XRD and RBS.
In addition, we used a laboratory-based HAXPES system equipped with monochromatic Cr-Kα X-ray source (5414.7 eV) to perform measurements with an information depth of over 20 nm. In a contrast, an XPS measurement using monochromatic Al-Kα excitation (1486.7 eV) can probe only up to 10 nm deep.[3] Consequently, the results of the HAXPES analysis are muss less affected by the presence of surface oxides or adsorbates, which allowed us to not only to probe the chemical composition of the material, but more importantly investigate the Zn and V oxidation states as well as the doping of the material without the need of any sputter cleaning.
These results highlight how novel photoelectron spectroscopy techniques can produce important insights for the discovery and design of new nitride materials, even when the samples exhibit surface oxides or adsorbates. Specifically, measurements of the AP can be used to identify new phases and changes of the microstructure in previously underexplored material systems.
References
[1] G. Greczynski et al. Progress in Materials Science. (2020) 100591.
[2] C. D. Wagner, Faraday Discuss. Chem. Soc. 60, (1975) 291.
[3] S. Siol et al. Surf. Interface Anal. 52 (2020) 802–810.

The II-IV-N₂ class of heterovalent ternary nitrides have gained significant interest as alternatives to the III-nitrides for electronic and optoelectronic applications. In this study, we apply first-principles calculations based on density functional theory to systematically investigate the effects of structural distortions due to cation size mismatch on the cation sublattice disorder and the valence band structure in this class of materials. We find that larger size mismatch between the group-II and the group-IV cations results in stronger lattice distortions from the ideal hexagonal ratio, which in turn inhibits the propensity of these materials towards octet-rule violating cation disorder. We also demonstrate that the formation energy of a single cation antisite pair, which is fast and simple to calculate, is a strong indicator of a material’s propensity towards disorder. Furthermore, the breaking of in-plane symmetry leads to a splitting of the top three valence bands at Γ, which is also directly related to the magnitude of structural distortions. Our work demonstrates that the structural and functional properties of the II-IV-N₂ materials can be finely tuned through controllable structural distortions that stem from the choice of cations.

Ternary transition metal nitrides, AETMN₂ (AE = Sr, Ba; TM = Ti, Zr, Hf), are expected to exhibit unique electro-magnetic properties since they possess two-dimensional (2D) electron transport layers in their natural layered crystal structures, and their conduction band minimums are formed mainly of TM d_xy orbitals like SrTiO₃-based 2D electron gas system [1,2]. However, high purity bulk samples of AETMN₂ have not yet been synthesized, hindering their intrinsic physical properties. To overcome the problem, we herein propose a 2-step process for the synthesis of high purity AETMN₂. We realized high-purity polycrystalline bulks and investigated the electronic structure and electronic properties of AETMN₂.
Polycrystalline bulks of AETMN₂ were synthesized by the high-temperature solid-state reactions of 2AENH + 2TMN → 2AETMN₂ + H₂. Since the complete nitridation of Sr and Ba metals, i.e. SrN_x and BaN_x phases, by direct annealing under N₂ gas is difficult, we made AENH powders through the nitridation of AEH₂ hydrides as the starting precursors. Then, to achieve a high nitrogen chemical potential during the solid state reactions of AENH + TMN, the NaN₃ powder is used as N-source.
By optimizing the annealing temperature and stoichiometric ratio, high purity AETMN₂ bulks with phase purity more than 90 mol% were obtained. From diffuse reflectance spectra, the optical bandgap was estimated to be 1.7 eV (SrTiN₂), 2.0 eV (BaZrN₂), and 2.2 eV (BaHfN₂). The ρ-T of all AETMN₂ exhibit semiconducting behavior. Arrhenius plot of ln σ (1/ρ) versus 1000/T is nearly linear over the temperature range 100-300 K, and an activation energy (E_A) increased from 2.9 meV (SrTiN₂) to 116 meV (BaHfN₂). Seebeck coefficient of all AETMN₂ were negative and increased from -26 μV/K (SrTiN₂) to -117 μV/K (BaHfN₂), indicating that AETMN₂ are n-type semiconductor and the unintentional impurity doping is suppressed in BaHfN₂. We will discuss the difference of impurity incorporation in AETMN₂ by calculating the defect formation enthalpies of intrinsic defects and oxygen/hydrogen impurities in AETMN₂ at the conference.

[1] D. H. Gregory et al., Inorg. Chem. 37, 3775 (1998)., [2] I. Ohkubo et al., Chem. Mater. 26, 2532 (2014).

Nitride-, carbide- and boride-based alloys of light main-group elements are attractive due to unique combinations of properties ranging from optical transparency through high hardness to high-temperature stability and oxidation resistance. The properties, in the first place electrical conductivity, can be further modulated by addition of early transition metals. In this contribution, amorphous nitrogen-rich Hf(M)SiBCN thin films are investigated by combining magnetron sputtering of composite B₄C–Si–Hf–M targets in Ar + N₂ reactive atmosphere with ab-initio calculations [1]. First, we study the effect of the M choice and fraction on calculated mechanical properties and formation energy of crystalline MN and Hf_xM_1–xN nitrides. We discuss the dependence of formation energy on the crystal structure and on the distribution of Hf and M in the metal sublattice. The calculated mechanical properties of MN correlate with those measured on HfMSiBCN. The driving force towards N incorporation, monotonically decreasing with increasing periodic-table group number of M according to the calculated formation energy of MN, very well correlates with measured electrical conductivity and extinction coefficient of HfMSiBCN. Second, we use ab-initio molecular dynamics to model the amorphous HfMSiBCN materials themselves. The calculated band gap, localisation of states around the Fermi level and bonding preferences of M atoms (in particular their tendency to bind with N) also correlate with the measured metallicity and confirm the possibility of predicting the trends in characteristics of HfMSiBCN using those of MN. Third, we identify an optimum target composition leading to hard (>20 GPa) HfMSiBCN materials with a relatively high conductivity at a given extinction coefficient. The results are important for the design of transparent and/or conductive hard high-temperature coatings.

[1] M. Matas, M. Prochazka, J. Vlcek, J. Houska, Acta Mater. 206 (2021) 116628

Thermoelectric devices have the potential to contribute to energy harvesting in society by directly converting heat into electricity or vice versa. However, the conversion efficiency of thermoelectric devices of today is limited. The critical material-dependent parameter is the figure of merit (ZT = S²T/rk, where r is the electrical resistivity, S is the Seebeck coefficient and k is the total thermal conductivity). In this lecture, I present an overview of our experimental and theoretical investigations of CrN-, ScN-, and Ca₃Co₄O₉-based thin films. ScN thin films exhibit an anomalously high power factor (S²/r) for transition metal nitrides of 2.5-3.3×10^-3 W/mK² at 800 K. We have explained this result, from first-principles calculations, by nitrogen vacancies generating an asymmetric sharp feature in the density of states which allows low electrical resistivity with relatively large S. However, ScN has high thermal conductivity, thus its ZT is low (~0.2). To reduce lattice thermal conductivity, potential strategies are nanostructuring, alloying or nanoinclusion formation. We have modeled the thermal conductivity in pure ScN as a function of grain size using time-dependent effective potentials (TDEP), correlating the ideal thermal conductivity of an infinite ideal crystal with the thermal conductivity reduction of grain-size effects, and alloying trends in mixing thermodynamics of ScN-based solid solutions. Pure CrN exhibits n-type conduction with a high power-factor enabled by a high electron concentration thermally activated from N vacancies, and alloys can be made of rocksalt-Cr_1-xSc_xN. We have demonstrated that it can be rendered p-type by Al alloying in combination with N superstoichiometry.

References
1) P Eklund, S Kerdsongpanya, B Alling (invited review) J. Mater. Chem. C 4, 3905 (2016)
2) Y Du, J Xu, B Paul, P Eklund (invited review) Appl. Mater Today 12, 366 (2018)
3) A le Febvrier, N Van Nong, G Abadias, P Eklund Appl. Phys. Expr. 11, 051003 (2018)
4) S Kerdsongpanya, O Hellman, …, B. Alling, P Eklund Phys Rev B 96, 195417 (2017)
5) M. A. Gharavi, …, B. Alling P. Eklund, Materials Today Communications, 28 102493 (2021)

The nitride semiconductor materials GaN, AlN and InN revolutionized solid state lighting in the last two decades and are currently doing the same for high-speed and high-voltage electronics, and deep-ultraviolet photonics. As the semiconducting properties, it is becoming increasingly clear that this material family has far more to offer ranging from communication at high frequencies, to harsh environment and high voltage digital electronics, to quantum communications and computation. In this talk, I will present recent developments in the synthesis and integration of nitride semiconductors, magnets, ferroelectrics, and superconductors towards an exciting new platform that feeds off the many past successes of photonics and electronics with GaN.

A missing piece in the classical computation paradigm for nitride semiconductors was the p-channel transistors with high hole mobilities. This problem has found an exciting solution in the discovery of undoped polarization-induced 2-dimensional hole gases in epitaxially grown GaN quantum wells on AlN substrates. This new material-driven discovery is a key enabler for digital CMOS with wide-bandgap nitride semiconductor that can operate in harsh environments.

Substitution of Al or Ga of AlN or GaN with other group-III metals such as the transition metals Sc and Y, or the metal B is bringing in new physical properties into the nitride family. In particular, I will talk about a) the growth and properties of ScN, a narrow-bandgap semiconductor, and b) the impact of substitution of Al by Sc, Y, and B on the structural and electro-mechanical properties of AlN. These materials boost the piezoelectric properties of AlN so much so that for certain compositions ferroelectricity is observable in both sputtered and MBE grown layers.

The epitaxial growth of metallic and superconducting NbN and TiN, and its heterostructures with AlN and GaN polar semiconductors will be discussed. Epitaxial metal/AlN heterostructures are enabling new forms of integrated epitaxial bulk acoustic wave (epiBAW) filters with the potential to scale to far thinner structures and extend the speed of such devices. It is allowing the direct integration of such filters with AlN/GaN/AlN quantum well high-electron mobility transistors (HEMTs) for highly integrated RF systems. These epitaxial heterostructures are also finding use in superconducting single-photon detectors (SNSPDs) and in new forms of microwave qubits.

Integrated discovery workflows are effective tools for identifying candidate functional materials in underexplored chemistries. Nitrides represent one such chemical space: the number of known nitrides is an order of magnitude less than known oxides. At the same time, many of the known inorganic nitrides—and (Al, Ga, In)-N semiconductors in particular—are useful in thin-film technological applications. Thus, our team has developed a workflow for discovering new inorganic nitrides based on data-mining the materials project, performing structure prediction calculations, experimentally screening materials using combinatorial exploration, and performing targeted, epitaxial growth of compelling candidate compositions.

One family of new compounds, described as either MgTMN₂ (TM=group 4 transition metal) or Mg₂TMN₃ (TM=group 5 transition metal), were identified using such a discovery pipeline. These materials represent a class of medium-gap (E_G=0.9-2.4) semiconducting nitrides which adopt a rocksalt-derived crystal structure. This is a departure from normal structure/property relationships in functional inorganic nitrides, where compounds tend to belong to one of two primary families—hexagonal main-group metal nitride semiconductors or cubic transition-metal nitride superconductors. Furthermore, the material properties can be controlled with the metal composition in these ternary composition spaces. For example, electronic transport properties can be tuned using the cation Mg:TM ratio from metallic (TM-rich) to non-degenerately doped (Mg-rich).

We found that the lattice parameters of some these materials are compatible with the two primary families of technologically important nitrides, suggesting the possibility for epitaxial integration of these materials into functional devices. For example, we have observed epitaxial alignment when MgZrN₂ and Mg₂NbN₃ are grown via sputter-deposition onto MgO, GaN, and Al₂O₃ substrates, as well as underlying superconducting ZrN and NbN epilayers. Inspired by these results, we are now evaluating Mg₂NbN₃ as a barrier layer in tunnel junctions with NbN superconducting layers. The combination of elemental abundance, compelling properties, and structural compatibility highlights the potential of these new Mg-TM-N materials for integration with known nitrides.

The development of single crystal III-N semiconductor heterostructures has been central to the success of III-N materials in wide range of electronic and optoelectronic applications. With this fact in mind, we have pursued the ability to integrate materials with previously inaccessible properties, such as metals, superconductors, and piezoelectrics, into single crystal III-N heterostructures. New and improved III-N devices become possible with access to the larger design space offered by these new materials and properties.

We present our development of novel III-N heterostructure containing NbN grown by molecular beam epitaxy (MBE). NbN, a metal and superconductor with critical temperature around 15 K, has been shown to possess structural and chemical compatibility sufficient to enable epitaxial integration of NbN into III-N heterostructures. We have investigated the growth of both GaN and AlN on NbN thin films by MBE. Our systematic study of the nucleation and growth of AlN on NbN demonstrate that lattice and symmetry matching between AlN and NbN enables the growth of high crystal-quality AlN on NbN. Atomically smooth ultra-thin AlN films can be nucleated directly on NbN, and thicker AlN films grown on NbN exhibit [0 0 2] rocking curve FWHM values as low as 250 arcsecond, which we believe to be the lowest value reported for AlN grown on a metal.

The crystal quality of our AlN films significantly surpasses that of AlN films deposited on non-epitaxial metallic electrodes, which are commonly utilized to produce piezoelectric acoustic resonators. We utilize our AlN/NbN heterostructure to produce high-overtone bulk acoustic resonators (HBARs), and are investigating the coupling of NbN/AlN HBARs to qubits to create hybrid quantum systems.

In addition, we have developed epitaxial NbN/AlN Josephson junctions grown on low dielectric loss sapphire substrates. We are exploring the properties of these materials and devices, and the suitability of these structures for quantum computing applications. MBE grown single crystal AlN/NbN heterostructures are a promising platform for the development of high coherence time Josephson junctions for quantum computing due to the chemical purity and lack of structural defects at the MBE grown NbN/AlN interface. This is especially promising in light of the limit on qubit coherence time set by two-level systems in the amorphous barriers of Nb/Al/AlO_X/Nb Josephson junctions.

Experimental evidence of ferroelectricity of (Al_1-xSc_x)N films by Fichtner et al in 2019 open the novel applications of nitride films using ferroelectric properties [1]. It includes low power nonvolatile ferroelectric memories (FeRAMs) as well as resistance-type memories using ferroelectric tunnel junction (FTJ). The most important feature of the ferroelectricity of these nitrides is their large remanent polarization (P_r) after releasing an applied voltage that is more than 5 times larger than that of present-fashioned HfO₂-based ferroelectric films and more than 3 times larger than that of Pb(Zr,Ti)O₃ films presently used in FeRAMs. This large P_r has a potential to realize high density FeRAMs. High Curie temperature beyond 1000 ^oC is another feature of the ferroelectricity of nitrides that makes it possible to use at high temperatures.
In my presentation, we introduce our results of ferroelectricity of (Al_1-xSc_x)N films prepared by sputtering method. Obtained results are listed as follows [2,3].
1) P_r and coercive filed (E_c) values can be controlled by the crystal anisotropy of (Al_1-xSc_x)N films
2) Ferroelectricity was observed down to 9 nm in thickness. This means a fundamentally small decrease of Pr with decreasing film thickness.
3) Ferroelectricity was observed for the films deposited at low temperature including room temperature.
These results show that ferroelectric nitrides are novel and one of the most promising candidates for memory applications using their ferroelectricity.
[1] Fichtner, et al., J. Appl. Phys., 125, 114103 (2019).
[2] Yasuoka et al., J. Appl. Phys., 128, 114103 (2020).
[3] Yasuoka et al., Phys. Status Solidi (A), accepted.

Among nitride semiconductor materials, wurtzite phase scandium aluminum nitride (ScAlN) has the highest piezoelectric coefficient which is why it has been touted as a future material for many promising electronic applications including the radio frequency (RF) resonators [1-2]. A highly c-oriented ScAlN piezoelectric thin film could exhibits either aluminum/scandium (Al/Sc) or nitrogen (N) polarity. The use of thin films with Al/Sc-polarity or N-polarity could lead to different electronic property which potentially affect the performance of the developed devices. For example, integrating both N-polar and Al-polar layers in a solidly mounted resonator BAW (SMR-BAW) was found to enhance the ability of the device to work at higher frequency when compared with that using a single layer Al-polar thin film [3]. Since ScAlN has been widely used as filter material for wireless telecommunication devices, controlling the polarity of ScAlN-based thin films is expected to promote the development of broadband filter that can function at higher frequency range in 5G communication technology. As a method to control the polarity, addition of silicon (Si) into AlN has been reported to inverse the polarization direction of non-doped aluminum nitride (AlN) from Al-polar to N-polar [4]. Therefore, in this study, we propose the use of Si as dopant to control the polarity of ScAlN piezoelectric thin film.
All thin films were fabricated by using radio frequency (RF) magnetron sputtering system. The concentration of Sc and Si was controlled by adjusting the power of cathodes for each corresponding target during deposition process. Positive d₃₃value indicates that the thin film has Al/Sc-polarity while negative d₃₃ value indicates that the thin film has N-polarity. All Sc_xAl_1-xN thin films exhibited positive d₃₃ values which indicate that the thin films have Al-polarity. However co-addition of Sc and Si into AlN was found to resulted in thin films with inversed polarity. For example, co-addition of 30 at.% Sc and Si with concentration in the range of 8-20 at.% (Sc/Si < 4) was found to results in thin films that exhibited N-polarity. While co-addition of 10 at.% Si and Sc with concentration higher than 40 at.% (Sc/Si 4) yielded in thin film that showed Al-polarity. It is evident that the polarity inversion of (ScSi)_xAl_1-xN was found to be largely relies on the concentration of Sc, Si and their ratio. These results indicate that Si addition can also be used not only to inverse the polarity of AlN but also to control the polarization direction of ScAlN, particularly for Sc concentration that is less than 40 at.%. Other than changes in polarization direction, effect of co-addition of Sc and Si into AlN on changes in crystal structure, lattice parameters and surface state have been studied and will be discussed.
References:
[1] M. Akiyama, T. Kamohara, K. Kano, A. Teshigashara, Y. Takeuchi, N. Kawahara, Adv. Mater. (2009) 21, 593.
[2] M. Moreira, T. Torndahl, I. Katardajiev, T. Kubart, J. Vac. Sci. Technol. A 33 (2015) 021518.
[3] Mizuno, T. et al. In 2017 19th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS). 1891.
[4] S. A. Anggraini, M. Uehara, K. Hirata, H. Yamada, M. Akiyama, Scientific Reports (2020) 10, 4369.

Novel lead-free ferroelectric and pyroelectric materials such Aluminum Scandium Nitride (AlScN) show great potential for development of CMOS-compatible temperature and motion sensors. A key challenge is the understanding of the origin of pyroelectricity and the profile of the spontaneous polarization in AlScN, that could be, for example, related to thermal deformation and corresponding piezoelectric mechanical clamping. A powerful tool to analyze the spontaneous polarization in three dimensions with sub-micron resolution is the Laser Intensity Modulation Method (LIMM). In this study, we report on the LIMM tomography of ferro-/pyroelectric AlScN thin-films using Fabry-Perot quantum-cascade mid-infrared, and near-infrared lasers. We discuss the influence of Scandium on the thermal and pyroelectric properties of AlScN based on our LIMM measurements. In particular, we report on thermal conductivity levels of AlScN two to three orders of magnitude lower than for conventional AlN films, which we contribute to phonon/alloy scattering mechanisms. We also characterized the profile of the spontaneous polarization as a function of the sample temperature, ambient conditions, and bias-voltage. Our tomography results are complemented by thermal simulations, and advanced microscopic and spectroscopic techniques. Our findings gain insight into important material properties of AlScN and provide guidelines for engineering of novel lead-free pyroelectric devices.

Wurtzite ferroelectrics such as Al_1-xSc_xN are promising nitride materials for piezoelectric and ferroelectric applications in 5G communication and non-volatile memories. Finding the upper Sc concentration limit and the lower thickness limit, both of which reduce the coercive voltage of the materials system, is important for the applications due to the large coercive electric field of the Al_1-xSc_xN nitride alloys. However, rocksalt phase transition in Sc rich side and ferroelectric hysteresis degradation in thinner films hinder the efforts in decreasing coercive voltage. Combinatorial approaches have emerged in the past decade as important tools for rapidly screening new compositions and thicknesses, in the search for new and improved structures and properties of the materials. One of the common concerns voiced about combinatorial approaches is that a focus on chemistry and thickness de-emphasizes effects of microstructure, which is very important in piezoelectric and ferroelectric applications. This common concern implies that the properties of combinatorial libraries are often not representative of the structures and properties of traditionally-fabricated homogeneous samples.

In this study, we demonstrate Al_1-xSc_xN films deposited by combinatorial co-sputtering with ferroelectric response across the sample library rivaling that of single-composition samples [1] . The composition and thickness gradients in the combinatorial Al_1-xSc_xN library co-sputtered from Al and Sc metal targets range from x = 0.25 to 0.5 and 250 to 150 nm. XRD q -2q patterns show the wurtzite – rocksalt phase transition across the composition gradient. Even in this inhomogeneous library, the Al rich region (x = 0.25) exhibits a well saturated ferroelectric hysteresis loop and larger breakdown field. As expected, the Sc rich region (x = 0.5) side does not show fully opened hysteresis loop due to the larger leakage contribution and lower breakdown electric field. As for the thickness gradient, thinner region (150 nm) possesses large leakage current contribution to the hysteresis loop compared to the thicker side (250 nm) at the same composition. As a result of this study, the systematic comparison map among composition, thickness, crystal structure, microstructure, and electric characterization gives us insights of physics and chemistry behind the ferroelectricity in Al_1-xSc_xN nitride alloys with wurtzite structure.

[1] K. Yazawa et al, Appl. Phys. Lett., 118, 162903 (2021)

Behavior of matter at extreme conditions challenge accepted concepts within materials science [1]. Nitrides have been found to have the largest fraction of metastable phases, and very high accessible thermodynamic range of metastability [2]. Combining theoretical simulations with experiment and broadly varying external parameters, pressure, temperature and composition we report on several novel nitrides with fascinating crystallochemistry [3] and properties attractive for applications [4,5]. First, we explore the Re-N system at high pressure up to 1 TPa and compare thermodynamics of experimentally synthesized phases with those obtained in theoretical structure predictions and reported in the literature. We point out a remarkable observation: tough most of the synthesized Re nitrides are stable at the synthesis pressure, and their energies are below the convex hull formed by the predicted structures, they have not been identified by the state-of-the-art structure prediction algorithms.
Next, we pay attention to a serious drawback of the high-pressure synthesis: the discovered materials cannot be always recovered at ambient conditions and the quantity of the produced material does not allow to carry out a characterization of their physical and mechanic properties. We argue that theoretical simulations have important role in identifying materials of interest for various applications, and in guiding their quenching to ambient pressure. We report the synthesis of metallic, ultraincompressible and very hard rhenium nitride pernitride Re₂(N₂)(N)₂. Unlike known transition metals pernitrides, it contains both pernitride (N₂)⁴⁻ and discrete N³⁻ anions, which explains its exceptional properties. Importantly, Re₂(N₂)(N)₂, which was discovered via a reaction between rhenium and nitrogen in a diamond anvil cell at pressures from 40 to 90 GPa has been recovered at ambient conditions, and a route to scale up its synthesis has been developed [4].
Finally, we demonstrate that the diamond anvil cell technique can be used for synthesis of layered materials, precursors of novel 2D-materials. We illustrate this considering a triclinic phase of beryllium tetranitride tr-BeN₄ which upon decompression from the synthesis pressure of ~85 GPa to ambient conditions, transforms into a compound with atomic-thick BeN₄ layers interconnected via weak van der Waals bonds. Theoretical calculations for a single BeN₄ layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN₄ layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions [5].
In summary, the fundamental understanding of the physical principles behind the formation of high-pressure materials generated in our studies is essential for the accelerated knowledge-based design of novel materials.

[1] E. Bykova, et al., Nature Commun. 9, 4789 (2018).
[2] W. Sun et al., Sci. Adv. 2, e1600225 (2016).
[3] M. Bykov, et al., Angew. Chem. Int. Ed. 59, 10321 (2020).
[4] M. Bykov, et al., Nature Commun. 10, 2994 (2019).
[5] M. Bykov, et al., Phys. Rev. Lett. 126, 175501 (2021).

Transition metal nitrides are typically based on metals in low oxidation states and have many useful properties, for example, ZrN was recently discovered to be an outstanding ORR (Oxygen Reduction Reaction) catalyst for electrochemical applications [1]. However, stabilisation of solid nitrides of transition metals in high oxidation states, comparable to those in oxides and fluorides, has proved difficult due to the well-known inertness of nitrogen and the tendency to evolve dinitrogen gas. A high pressure method using sodium azide has recently been demonstrated to overcome these issues leading to the synthesis of a highly oxidised iron(IV) nitride, Ca4FeN4 [2].

[1] Y. Yuan et al, Nature Mat. 19, 282–286 (2020). https://doi.org/10.1038/s41563-019-0535-9

[2] S. D. Kloss et al, Nature Comm. 12, 571 (2021). https://doi.org/10.1038/s41467-020-20881-y

Conductive transition metal nitrides (TMN) have been considered for applications in micro- and opto-electronics such as diffusion barriers and ohmic contacts to III-nitrides; recently, they are revisited as alternative plasmonic materials, which are refractory, spectrally tunable, CMOS-compatible, and durable in high electric fields. The most prominent examples of TMNs are TiN and ZrN. The refractory character of TMN and their growth predominantly by reactive sputtering are simultaneously blessings and curses, as they limit the fabrication of nitride plasmonic nanostructures to top-down processes, so far. Despite of the spatial accuracy of top-down processing, such fabrication is time consuming, costly, and of limited scalability. In this work, alternative routes of fabrication of TMN nanostructures are critically reviewed and presented. In particular, we focus on the fabrication of nano-islands and dichroic nanowires supported on silicon wafers and silica, and colloidal nanoparticles. We emphasize on emerging techniques that may be applied on meso- or macro-scopic scales, such as nanosphere lithography (NSL) combined with ionized reactive magnetron sputtering, glancing angle deposition (GLAD), and laser ablation in liquids (LAL). We critically evaluate the assets and drawbacks of each individual fabrication technique, and we evaluate the optical and plasmonic performance of the produced nanostructures using finite difference time domain (FDTD) calculations. There is, unfortunately, a feature that is present in all the presented manufacturing schemes: TMN typically contain a considerable amount of point defects that are known to affect the properties of the host crystal. We provide thorough experimental evidence that in TMN nanostructures the point defects tend to be outdiffused to the surface, thus limiting the deterioration of their performance, at least from the optical and plasmonic point of view; in fact, we show that for many TMN and under certain conditions the polycrystalline samples may be equivalent, if not superior, to epitaxial samples. This feature adds to the feasibility of NSL, GLAD and LAL to produce efficient TMN nanostructures on large areas and/or in large volumes.

2D carbides and nitrides, known as MXenes, are among the most recent, but quickly expanding material families. The field is experiencing rapid growth with more than 1000 papers published on MXenes each year. Major breakthroughs have been achieved in the past 3-4 years, including the discovery of 2D M₅C₄ carbides with the twinned layers and CVD synthesis of MoSi₂N₄, representing a new family of 2D nitrides. Synthesis of dozens of predicted MXenes, demonstration of superconductivity in MXenes with specific surface terminations, stronger interactions with electromagnetic waves compared to metals, metallic conductivity combined with hydrophilicity and redox activity, have led to numerous applications. The outstanding mechanical properties of Ti₃C₂, combined with its conductivity and strong interfaces, have been used in polymer, ceramic, and metal matrix composites. The reversible redox activity of transition metal atoms in the outer layers of MXene flakes combined with high electronic conductivity have led to applications in a variety of batteries and electrochemical capacitors. MXenes are promising candidates for energy storage and related electrochemical applications, but applications in optoelectronics, plasmonics, electromagnetic interference shielding, electrocatalysis, medicine, sensors, or water purification are equally exciting. At the same time, the majority of studies were conducted using carbide MXenes. Relatively few nitrides (Ti₂N and Ti₄N₃) and carbonitrides (Ti₃CN and Ti₂C_0.5N_0.5) have been produced by etching MAX phases. A few more were synthesized by replacing carbon by nitrogen in 2D carbides using high-temperature retreatments. While synthesis of nitrides is more challenging compared to carbides, theoretical predictions of attractive optoelectronic and magnetic properties of 2D nitrides fuel further efforts.

Transition metal nitrides (TMNs) as a class of metallic interstitial compounds, by tuning chemical composition through suitable selection of parent metals and introducing different amounts of nitrogen atoms into the lattice of parent metals could allow TMNs to be either semiconducting or metallic. In the last 5 years, our team using different synthetic methods have successfully prepared TMNs which show great practical value to catalysis and sensors. The observations have been theoretically rationalized. Using both experimental and theoretical results, we have been analyzing the relationship of crystal and surface structure and have correlated it to the activity for the related applications. In particular, major breakthroughs from our lab involved the discovery that ZrN shows better catalytic property than Pt/C for oxygen reduction reaction ¹, Surface oxide-rich activation layer (SOAL) on Ni₂Mo₃N for rapid and durable oxygen evolution reaction ² and ternary metal nitride based fuel cell type methanol sensor ³.

References
Y. Yuan, J. Wang, S. Adimi; H. Shen, T. Thomas, R. Ma, J. P. Attfield, M. Yang, Zirconium nitride catalysts surpass platinum for oxygen reduction, Nature Materials 19, 282 (2020).
Y. Yuan, S. Adimi, X. Guo, T. Thomas, Y. Zhu, H. Guo, G. S. Priyanga, P. Yoo, J. Wang, J. Chen, P. Liao, J. P. Attfield, M. Yang, Surface oxide-rich activation layer (SOAL) on Ni₂Mo₃N for rapid and durable oxygen evolution reaction, Angewandte Chemie International Edition 59, 41, 18036 (2020).
C. Huang, S. Adimi, D. Liu, H. Guo, T. Thomas, J. P. Attfield, S. Ruan, F. Qu, M. Yang, Pt/WN based fuel cell type methanol sensor, Journal of Materials Chemistry A, 2021. DOI: 10.1039/D1TA02433G.

Most of the 14 rare-earth nitrides (REN) are intrinsic ferromagnetic semiconductors. Their magnetism resides in the 4f shell with a strong spin-orbit interaction that locks the spin and orbital magnetic moments together, a contrast to the quenched orbital magnetic moments in the d shell of transition-metal ferromagnets. The contribution of both spin and orbital magnetic moments delivers outrageous magnetisations across the series: coercive fields from 0.01T to 10 T, zero magnetic moment in otherwise conventional ferromagnetic SmN, composition tuning of magnetic moment, coercive field and the net angular momentum in (Gd,Sm)N and (Gd,Nd)N alloys. All of these promise enhanced efficacy in various device structures, for which the doping control of electronic conduction further broadens their technical potential.
The REN are a favourite playing field of theorists, but experimentalists are deterred by the reputation of nitride film growth as problematic. We began growing them in 2006, motivated by a prediction that some are half metals, to find that they are semiconductors. They are readily grown in a N₂ atmosphere of 10^-5 mbar; there is no need for activated nitrogen. Surprisingly a freshly deposited rare-earth film spontaneously breaks the enormously strong triple bond of N₂, a reaction which is itself of substantial chemical interest. We have developed growth of polycrystalline and epitaxial films, mixed rare-earth nitride alloys and superlattices, both stoichiometric and n-doped by nitrogen vacancies when grown at a reduced N₂ pressure.
Following a brief description the thin-film growth that is at the core of our programme, I will describe the highlights from our 15 years exploring these materials: their huge range of magnetic states, their coupled electronic-magnetic responses, a surprising incidence of heavy-fermion superconductivity and hints of a Kondo lattice state. The study is supported by DFT computations, a variety of laboratory-based transport and magnetisation measurements and X-ray spectroscopic investigations at various synchrotrons.

The rare-earth nitrides are associated with many remarkable magnetic behaviours. These range from the near-zero moment of ferromagnetically aligned samarium nitride (SmN), through the J = 0 ground state of europium nitride (EuN) to the more conventional magnetism of gadolinium nitride (GdN). In all cases, the electrical transport of these materials can be controlled over multiple orders of magnitude through n-type nitrogen vacancy doping. Thus they may be classed as intrinsic ferromagnetic semiconductors.

The coupling of gadolinium and samarium in a ternary alloy (Gd_xSm_1−xN) allows the independent tuning of the magnetic properties, as well as the electrical conduction. The magnetism comes from the localised 4f moments of the rare-earths and interpolates the magnetic behaviours of the two endpoints GdN and SmN, whose disparate behaviours offer a deal of scope in tuning the alloy magnetism. Moreover, the orbital dominated nature of the magnetic moment of SmN ensures that these alloys host magnetic moment (net m = 0) and angular momentum (net J = 0) compensation points. These features, added to the exhibition of perpendicular magnetic anisotropy offer significant benefits for magnetic recording applications. We will describe the features of these materials with reference to studies of their magnetization, XMCD (X-ray Magnetic Circular Dichroism) and electron transport, which establish their suitability for future device development.

The rare-earth nitrides are mostly intrinsic ferromagnetic semiconductors at cryogenic temperatures and have vast potential applications in magnetic memory (MRAM) structures. The 15 members of the series offer the selection of both saturation magnetisation and coercive field required for these devices. We will report layered MRAM structures with two compositions of Gd_xSm_1-xN, separated by an exchange-blocking layer, and describe their performance under (i) current applied through an insulating exchange-blocking layer (tunnelling MRAM geometry) and (ii) in-plane current in a metallic exchange-blocking layer (GMR geometry). Both the high-impedance tunneling and the low-impedance GMR devices show binary data storage action.

The magnetic properties of the rare-earth nitride series are determined largely by the occupation of the 4f electron shell. In a select few members of the series this shell also directly influences electrical transport. Here we discuss a combined experimental and computational study of nitrogen-vacancy doped SmN. We show that the material crosses an insulator-to-metal transition as samples are increasingly doped with nitrogen vacancies. At low doping levels, the transport can be understood as variable range hopping between defect states, while at high doping levels extended state metallic transport is seen. In the intermediary 'anomalous metallic' region where transport is in a mixed 4f-5d band, there are indications of more exotic ground states, such as unconventional superconductivity and a Kondo lattice. These may coexist with ferromagnetic order.

While the impressive flexibility and tunability of the perovskite crystal structure ABX₃ has allowed materials in this family to exhibit a very wide range of properties, nearly all reported perovskites have oxygen or halides as the anion. Nitride perovskites have long presented a synthetic challenge due to the difficulty of completely excluding oxygen during synthesis and the high oxidation states required for the A and/or B site cations. This phase space is nearly unexplored, and the greater covalency of nitrogen makes nitrides attractive for semiconducting and other functional applications in renewable energy.
Recently, we reported on the experimental realization of the first oxygen-free nitride perovskite—LaWN₃—as a thin film, crystallizing in polar space group R3c.[1] The oxygen-free character of LaWN₃ is determined by Auger Electron Spectroscopy, and the structure is determined by synchrotron based X-ray diffraction. LaWN₃ exhibits a large piezoelectric response and signs of ferroelectricity, making it the first stable nitride ferroelectric compound. This opens a host of possible avenues to target novel compounds for functionalization, including as potential multiferroics for spintronic applications.
Here we report on the synthesis and detailed investigation of the structure and electronic properties of thin film LaWN₃ and analogous novel nitride perovskites. The films were grown through using combinatorial radiofrequency plasma sputtering and characterized via spatially-resolved structural, electronic, and optical measurements, allowing correlation to the films’ composition. The combinatorial synthesis methods and high-throughput characterization used in this study allows rapid exploration of the phase space, accelerating the discovery of novel materials and pinpointing which have promising properties for functional applications.

[1] Talley, K.; Perkins, C.; Diercks, D.; Brennecka, G.; Zakutayev, A. arXiv:2001.00633, 2020.

Perovskite materials, those with a crystal structure related to the mineral perovskite and typically with the formula ABX₃, are well known across a number of fields, such as ultrasonics and photovoltaics. These fields use perovskites that have oxygen or halides on the X (anion) site of the perovskite structure, but there are relatively few perovskites that have nitrogen as the anion. In this work, new nitride perovskites of composition CeMoN₃ and CeWN₃ were grown as thin films using a high-throughput approach. These films were grown using reactive sputtering with a chemical gradient in cation stoichiometry. After growth, films were annealed under flowing nitrogen to explore the phases present at a number of different conditions. The crystallization path revealed a competing defect phase within the fluorite family of crystal structures, which is reported here for the first time and is similar to the fluorite family phase that is observed in the Pb(Zr,Ti)O₃ (PZT) system.. For CeMoN₃, the perovskite phase was grown using post growth annealing of thin films in a flowing nitrogen atmosphere at a temperature of 900°C for 10 minutes. For the CeWN₃ system, growing a perovskite phase was best accomplished during growth conditions that emphasized nitrogen reactivity. The fluorite-family phase was present for both compositions at high Ce content (well above the 1:1 perovskite stoichiometry) and when either temperature or nitrogen reactivity were low. A nitrogen plasma source offered a way to increase nitrogen reactivity at the surface of the film growth to induce growth, especially in CeWN₃. For both of these composition families, the high-throughput technique offered insight into the defects that form in off-stoichiometry regions of the film, which serve as a comparison to tell a more complete story of the crystallization of the perovskite phase. For both films, a crystallization path to the perovskite phase is presented and verified by x-ray diffraction. Preliminary property information was also gathered to demonstrate the changes after crystallization of the perovskite phase from the fluorite phase.

The ammonothermal process is a promising route for the synthesis and crystal growth of various nitride-based semiconductor materials with potential for technological applications. This method typically employs ammonobasic or ammonoacidid mineralizers in supercritical ammonia and was proven to be capable of growing a broad variety of nitrides like InN crystals and freestanding GaN wafers as well as a broad variety of further interesting nitride-based materials. Such chemical transport reactions for crystal growth require use of high-pressure autoclaves, typically operated at process parameters of up to 3 MPa and 600 °C and protection against corrosion provoked by the aggressive medium.

For a successful synthesis and crystal growth, not only a good knowledge of the process parameters and technological challenges is necessary, but also a profound understanding of the chemical processes that govern the feedstock dissolution, the material transport and the deposition in the crystallization zone. In order to obtain deeper insights, we have studied the chemistry of the ammonothermal crystal growth utilizing various mineralizers. Soluble intermediates and their influence on the crystal growth in supercritical ammonia depending on the chemical nature of the mineralizer as well as the pressure and temperature conditions will be discussed with emphasis on crystal growth of AlN, GaN, InN and the feasible synthesis of Zn₃N₂.

Wide bandgap GaN (E_g ~ 3.4 eV) with the intrinsic breakdown field E_br ~ 3.5 MV/cm and high electron mobilities µ_n ~ 1000 cm²/Vs possesses the Baliga figure-of-merit (FOM) at least 5X better than SiC and almost 1000X better than Si for power devices. Vertical GaN devices have been demonstrated with true avalanche breakdown [1]. The current key challenge to achieve high power operation for GaN vertical device with high breakdown voltage (V_br) and low on resistance (R_on) is on the development of thick GaN drift layer with low controllable doping (N_d-N_a). Key impurities in MOCVD GaN serving as charge compensation centers were systematically investigated. For example, mechanisms of Fe incorporation in MOCVD GaN were identified and addressed [2]. Laser-assisted MOCVD (LA-MOCVD) growth technique was used to enable fast GaN epitaxy while suppressing C impurity incorporation [3]. The effects of LA-MOCVD GaN growth parameters on GaN growth rate, impurity incorporation and charge transport properties will be discussed. GaN PN diodes with breakdown voltage ~ 3kV are demonstrated.

Expanded from III-nitrides, ternary II-IV-N₂ material system possesses unique properties beyond well-developed III-nitrides. Particularly, II-IV-N₂/III-N heterostructures with close lattice matching and large band offsets can potentially address existing challenges facing in pure III-nitride based devices. Closely lattice-matching ZnGeN₂/(In)GaN heterostructures have been demonstrated with large band offsets of >1 eV [4, 5]. In this talk, MOCVD developments of ZnGeN₂, ZnGeN₂/GaN heterostructures and InGaN/ZnGeN₂ quantum well structures will be discussed.

Acknowledgement:
The authors acknowledge the funding support from Advanced Research Projects Agency-Energy (ARPA-E) DE-AR0001036, U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Manufacturing Office, FY18/FY19 Lab Call. The authors also acknowledge the funding support from the U.S. Department of Energy (DE-EE0008718) and the National Science Foundation (DMREF-SusChEM-1533957).

[1] I. C. Kizilyalli, A. P. Edwards, H. Nie, D. Disney, D. Bour, IEEE Tran. on Elec. Dev., 60, 3067, 2013.
[2] Y. Zhang, Z. Chen, W. Li, H.-S. Lee, M. R. Karim, A. R. Arehart, S. A. Ringel, S. Rajan, H. Zhao, J. Appl. Phys., 127, 215707, 2020.
[3] Y. Zhang, Z. Chen, K. Zhang, Z. Feng, H. Zhao, Physica Status Solidi (RRL), 2100202 (2021). doi.org/10.1002/pssr.202100202
[4] M. R. Karim, B. H. D. Jayatunga, M. Zhu, R. A. Lalk, O. Licata, B. Mazumder, J. Hwang, K. Kash, and H. Zhao, AIP Advances 10, 065302, 2020.
[5] M. R. Karim, B. A. Noesges, B. H. D. Jayatunga, M. Zhu, J. Hwang, W. R. L. Lambrecht, L. J. Brillson, K. Kash, and H. Zhao, J. Phys. D: Appl. Phys., 54, 245102 (2021).

Due to its deep acceptor level, carbon doping can lead to highly resistive gallium nitride. But recent experiments indicate that other configurations of carbon become important in heavily C-doped GaN. Here we investigate the properties of heavily C-doped GaN grown by hydride vapor phase epitaxy using optical experiments and hybrid density functional theory (DFT) calculations. Prior work established that carbon acceptors (C_N) give rise to a yellow luminescence (YL) band near 2.2 eV, along with a blue luminescence band (BL) near 2.9 eV. Photoluminescence measurements show the YL band shifting as a function of carbon concentration, suggesting a change in the behavior of carbon species as carbon content increases. With hybrid DFT we calculate the electrical and optical behavior of multi-carbon complexes which may arise in heavily doped material. We compare the behavior of these complexes to the isolated centers, and find that the dicarbon donor-acceptor (C_Ga-C_N) complex is a candidate to explain the shift in the YL peak. Tricarbon complexes have modest binding energies but high formation energies, and also give rise to optical transitions inconsistent with the observed spectra. The split dicarbon interstitial on the gallium site is identified as a low-energy species with a large binding energy, that may act to compensate carbon acceptors. Local vibrational modes are calculated for carbon impurity centers and compared to recent experiments. Dicarbon and tricarbon complexes involving C_Ga and C_N exhibit modes only slightly higher than the isolated species, while carbon interstitials and related complexes give rise to vibrational modes significantly higher than C_Ga and C_N.

[1] Phys. Rev. B 104, 075201 (2021).

This work was supported by the ONR/NRL 6.1 Basic Research Program.

Controlling surface defects is a key to reduce efficiency losses in wurtzite (WZ) GaN-based optoelectronic and power electronic devices. Inverted pyramids or V-pits are a prominent and often observed extended defect in heteroepitaxially grown GaN surfaces. They are widely accepted to be detrimental for optoelectronic devices. Recently, there is an increasing interest in the effect of V-pits on the efficiency of power electronic devices. V-pits are commonly associated with threading dislocations, can reach lateral sizes as large as a few hundred nms, and are buried inside the buffer layer of the heteroepitaxial structures [1]. The buried nature of this defect makes experimental investigations challenging. Therefore, despite their importance, an in-depth understanding of their origin, size, and shape is still lacking.

In the present work, we combine first-principles calculations with elasticity theory and investigate thermodynamic stability and growth of V-pits at GaN (0001) surfaces. The nucleation and properties (e.g., size and shape) of these defects are governed by the complex interplay between dislocation’s strain and core energies, surface energies, and oversaturation. A prerequisite to accurately describe these defects is the identification of their absolute surface energies that constitute the inverted pyramid, i.e., polar, semipolar, and/or non-polar planes. Therefore, in the first step, we calculate roughly 300 different GaN surfaces having low index (0001), (10-1n), and (11-2n) orientations, where n=0,…, 3. Although absolute polar and semipolar WZ surface energies are ill-defined, the energies of the semipolar planes can be aligned with respect to those of polar ones. Furthermore, we exploit geometric similarities between {0001} WZ surfaces with higher symmetry {111} zincblende surfaces to calculate “absolute” polar surface energies. For these calculations, we employ our recently developed methodologies, which address the spontaneous polarization fields and allow us to identify accurate surface energetics and electronic structures of WZ surfaces [2, 3]. Based on these calculations we construct a ‘V-pit’ phase diagram, which describes the equilibrium size and shape of V-pits as a function of the ambient growth conditions, i.e., the Ga and H chemical potentials reflecting oversaturation. This diagram reveals that under typical MOCVD growth conditions, the GaN polar planes are intrinsically unstable against the formation of V-pits at sites where screw threading dislocations reach the surface. The defect formation is driven by the preferential decoration of the surfaces by hydrogen, which reduces the surface energies. The remaining surface energy is compensated by releasing the energy stored in the dislocation core and strain field. Our calculations predict equilibrium lateral sizes up to a few tenths of a nm. Although this is smaller than the experimentally observed maximum size, the size of the energetically favorable V-pit nucleus may be enhanced by surface kinetics, i.e., the presence of Ehrlich–Schwoebel barriers. On the other hand, under typical MBE growth conditions and the lack of hydrogen, V-pits are energetically unfavorable. Based on these results we will further discuss the electronic properties of the V-pit side facets and how they influence critical device parameters such as work functions and ionization energies.

This project has received funding from the ECSEL Joint Undertaking (JU) project UltimateGaN under grant agreement No 826392. The JU receives support from the European Union’s Horizon 2020 research and innovation program and Austria, Belgium, Germany, Italy, Slovakia, Spain, Sweden, Norway, Switzerland.

[1] S. Besendörfer et al., J. Appl. Phys. 127, 015701 (2020)
[2] S.-H. Yoo, et al., npj Comput. Mater. 7, 58 (2021)
[3] S.-H. Yoo, et al., Phys. Rev. Mater. 5, 044605 (2021)

One of the major challenges faced by the III-Nitride materials is caused by the absence of a suitable substrate, which results in high dislocation densities in epitaxially grown planar structures. Nanowire geometries are drawing the attention of the III-nitride community for their narrow diameter and small footprint, enabling efficient strain relaxation, thus dislocation free III-nitride nanowires. Yet, a solid understanding of the grow mechanisms for self-assembled III-Nitride nanowires is to be developed, despite many successfully demonstrated devices based on the III-nitride nanowires.

The self-assembled GaN nanowires/nanorods can be achieved via two major growth methods: one with a dielectric mask and one without a pre-deposited dielectric mask. Typically, the GaN nanowires grown with dielectric masks have metal (e.g. Ga) polarity, and have a growth rate comparable to their planar counterparts, while the ones grown directly on a sapphire substrate has a growth rate that is of orders of magnitude higher, and they are mostly with N-polarity.

In this study, we conducted a systematic study of self-assembled GaN nanowires on c-Sapphire substrates grown by metalorganic chemical vapor deposition (MOCVD) through X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), scanning electron microscopy and transmission electron microscope. Self-assembled GaN nanowires were grown on c-sapphire substrates in a Veeco Propel Power GaN MOCVD, using ammonia (NH₃), trimethylgallium (TMGa) and silane (SiH₄) as precursors. The substrates were kept in H₂ flow at 1060°C for 5 min to decontaminate the surface followed by a 10 min nitridation step at 1080°C. The temperature was then dropped to (1035 – 1050)°C for GaN nanowire growth. Growth was initiated with a nucleation step followed by a 15s growth interruption before 10 min nanowire growth with [NH₃]/[TMGa] = 6 – 24 and [SiH₄] = 0.8 -1.6 μmol/min. The substrate surfaces after the decontamination, nitridation and nucleation steps were studied by XPS and AFM to understand the initiation of nanowire growth. The polarities of as-grown structures were studied by SEM after 30 min etching by KOH solution at room temperature.

We confirmed that the nitridation treatment of the c-Sapphire substrate forms a thin layer of AlN that leads to the N-polarity of the subsequently grown GaN nanowires. The reported Al(NO)x complex associated with the metal-polar III-nitride growth¹, is not observed in the samples after nitridation step, and is consistent with the observation of predominate N-polar GaN nanowires. The nucleation step is essential for GaN nanowire growth, yet this step does not control the nanowire diameters as reported². We found that the broad distribution of nanowire lengths is caused by the different growth mechanisms. Typically, N-polar GaN nanowires' growth rate is 15 – 23 atomic bilayers/sec compared to 2 – 4 atomic bilayers/sec of the Ga-polar nanowires. The N-polar GaN nanowires were found with two distinctive growth rate region – one with (15±2) atomic bilayers/sec and one with (23±2) atomic layers/sec. TEM results suggest screw dislocations may be one of the causes of the growth rate differences.

Understanding the underlying growth mechanisms of N-polarity GaN nanowire growth and their fast growth rate will provide insides to the III-Nitride materials growth community and shine lights to explore the full potential of III-nitride nanowires.

References:
¹ S. Mohn, N. Stolyarchuk, T. Markurt, R. Kirste, M.P. Hoffmann, R. Collazo, A. Courville, R. Di Felice, Z. Sitar, P. Vennéguès, and M. Albrecht, Phys. Rev. Appl. 5, 1 (2016).
² R. Koester, J.S. Hwang, C. Durand, D. Le Si Dang, and J. Eymery, Nanotechnology 21, (2010).

Boron nitride (BN) is a structurally versatile insulator since it can be found in several crystalline structures with outstanding mechanical and electrical properties, making this material to be attractive for technological applications. Seeking to improve its features, first principles studies for the amorphous phase (a-BN) have been carried out by several groups, focusing on the electrical [1] and structural [2] properties, pressure-induced phase transformations [1], and hydrogenated a-BN [3]. In this work we shall contribute to the understanding of the a-BN by generating an amorphous structure by means of ab initio Molecular Dynamics on a 216-atom supercell, following the undermelt-quench approach which has proven to give good structures for structurally disordered materials [4, 5] and studying the vibrational density of states for the samples obtained.

[1] M. Durandurdu. Amorphous boron nitride at high pressure, Philosophical Magazine 96 (2016), pp. 1950–1964, DOI:10.1080/14786435.2016.1183830.
[2] D.G. McCulloch, D.R. McKenzie, C.M. Goringe. Ab initio study of structure in boron nitride, aluminum nitride and mixed aluminum boron nitride amorphous alloys, Journal of Applied Physics 88 (2000), pp. 5028–5032. DOI:10.1063/1.1316790.
[3] T.A. Üçhöyük, M. Durandurdu. Hydrogenated amorphous boron nitride: A first principles study, Journal of Non-Crystalline Solids 502 (2018), pp. 159–163 DOI:10.1016/j.jnoncrysol.2018.08.021.
[4] Z. Mata-Pinzón, A.A. Valladares, R.M. Valladares, A. Valladares. Superconductivity in Bismuth. A New Look at an Old Problem. PLoS ONE 11 (2016), e0147645. DOI:10.1371/journal.pone.0147645.
[5] D. Hinojosa-Romero, I. Rodríguez, A. Valladares, R.M. Valladares, A.A. Valladares. Ab initio Study of the Amorphous Cu-Bi System. MRS Advances 4 (2019), pp. 81–86. DOI:10.1557/adv.2019.83.

Developing p-type wide-bandgap semiconductors with bandgap energy corresponding to UV-C spectral region (> 280 nm) is still challenging. This issue motivated us to explore wide-bandgap p-type MnO quantum dots (QDs) with the bandgap of > 4 eV synthesized by femtosecond laser ablation in liquid (FLAL), which is a cost-effective technique. In this study, we demonstrate for the first time a pn-junction photodetector based on p-MnO QDs/n-GaN. As a part of this work, MnO target in ethanol was ablated by ultrafast Ti:Sapphire laser (at 800 nm and 75 MHz) to synthesize the wide-bandgap p-MnO QDs under ambient conditions. We subsequently demonstrate the p-type conductivity of the QDs using Kelvin probe (KP), as well as via field-effect transistor (FET) measurements. X-ray diffraction (XRD), transmission electron microscopy and Raman spectroscopy were also performed to ascertain the p-type MnO composition. Absorption spectroscopy findings indicated that the bandgap was in the 4−5 eV range. The solution-processed MnO QDs were simply spray-coated on Si-doped GaN to fabricate MnO/GaN-based solar-blind DUV photodetector. X-ray Photoelectron Spectroscopy (XPS) analysis reveals the heterojunction band alignment of p-MnO QDs/n-GaN and the band offset at the heterojunction interface, demonstrating a good band alignment between GaN and MnO QDs with type II. Electrical characterizations show a good photocurrent response, as well as self-powered characteristics. This study provides new insights into the use of p-type MnO QDs for III-nitride devices.

III-V-N alloys have been receiving much attention for optoelectronic device applications because of their attractive features, for example, large gap bowing and splitting of conduction band. In particular, GaPN alloys are candidate materials for intermediate band solar cells (IBSCs). Since some defects due to nitrogen atoms in III-V-N alloys are found to act as nonradiative recombination (NRR) centers, however, it is crucial to study non-radiative recombination processes in GaPN alloys for improving the efficiency of solar cells. In this study, therefore, we evaluate the carrier recombination process in GaPN alloys with various nitrogen concentrations by using an optical technique, two-wavelength excited photoluminescence (TWEPL).
The samples used in this work were grown by metalorganic chemical vapor deposition. A 300 nm-thick GaP buffer layer and a 500 nm-thick GaP_1-xN_x (x = 0.105%, 0.56%, 0.75%, 1.4% and 3.2%) were grown in sequence on 400 μm-thick GaP substrates with an orientation of (100). Trimethylgallium, phosphine, and dimethylhydrazine were used as Ga, P, and N sources, respectively.
The TWEPL measurements were carried out using above-gap excitation (AGE) and below-gap excitation (BGE) light. While the sample was excited by AGE light, BGE light was irradiated at the same point of AGE spot. We measured PL intensities with BGE (I_AGE+BGE) and without BGE (I_AGE), and derived the normalized PL intensity (I_N=I_AGE+BGE/I_AGE). We used two light sources for AGE; λ=374 nm, 3.32 eV and λ=532 nm, 2.33 eV for conduction band (CB) excitation and intermediate band (IB) excitation, respectively. For BGE, we used 0.81 eV (1532 nm), 0.93 eV (1340 nm), 1.17 eV (1064 nm), 1.27 eV (980 nm), and 1.46 eV (850 nm). The sample was immersed in liquid nitrogen (77 K) to prevent temperature rise effect due the excitation light sources.
First, we studied the nitrogen dependence of the integrated PL intensity by AGE and found that the PL intensity of 3.2% sample was the lowest in all the samples, indicating that 3.2% sample is the most significantly affected by NRR centers. The PL intensity of 1.4% sample was the highest and the samples with nitrogen concentration of 0.105%, 0.56% and 0.75% showed the PL intensity between 3.2% and 1.4%; however, the PL intensity of 0.105% sample was a little lower than 0.56% and 0.75% because isolated N-N pairs emissions are dominant for 0.105%.
Second, we investigated the BGE power dependence of normalized PL intensity (I_N) based on TWEPL method. For IB excitation, I_N value increased and became saturate for all the BGE sources, showing that some electrons are excited from valence band (VB) to NRR centers or from NRR centers to IB. The I_N value of 3.2% was higher than that of 1.4% sample. This is consistent with the integrated PL intensity result that 3.2% sample is significantly affected by NRR centers. For CB excitation, in contrast, I_N value was almost constant for 1.4% and 3.2% samples, which is clear evidence that the carrier relaxation and subsequent recombination processes are much different for CB and IB excitation. Therefore, we found that the reduction of NRR centers located between VB and IB is important for reducing the relaxation from CB to NRR centers, and thus improving the efficiency of CB excitation.
We will clarify the carrier recombination process in GaPN alloys with different nitrogen concentrations by combining the experimental results of time-resolved photoluminescence measurements.

To fully unleash the potential of a semiconductor, it is critical to 1) control its defect levels below the limits that the targeted applications can tolerate, 2) control its doping in both n-type and p-type, and 3) engineer the most effective carrier injection into the conduction and valence bands, i.e. excellent ohmic contacts. A DUV emitter is an epitome of devices where these requirements need to be met. Even for an application that can be successful in engineering only one band of the semiconductor, availability of adequate control of the other band expands the design and operation space of the device tremendously. I will discuss advances in fundamental science and technology development that promise solutions for DUV devices, power and RF electronics.

Aluminum Indium Nitride (AlInN) has shown great potential in the fields of optoelectronics, photovoltaics, and electronics. It has shown potential as an electron blocking layer and quantum well material in light emitting diodes (LEDs), as an absorbing material in solar cells, and as a barrier layer in high electron mobility transistors (HEMTs). Additionally, AlInN has a wide range of useful properties, including a tunable bandgap from 0.65 to 6.1 eV, a high absorption coefficient, high polarization coefficients to allow for high density 2D hole and electron gasses, and the ability to be grown lattice matched to GaN at a composition of about 18% indium. However, despite the proven useful applications and properties of the alloy, crystal quality issues have plagued this material. Due to the large mismatches between the two binary alloys, AlN and InN, a large miscibility gap exists for the ternary material. Additionally, the growth temperatures required to achieve state-of-the-art quality InN and AlN differ in extremes of ≥500 °C. At the preferred temperature required for superb AlN, it is difficult to incorporate indium into a film. Conversely, at growth temperatures that allow for high or complete indium incorporation, aluminum adatom mobility is traditionally extremely low. Thus, a nontraditional method must be employed to grow this challenging material.

For these reasons, Aluminum Indium Nitride was grown over the entire composition range from InN to AlN using a non-traditional method, Metal Modulated Epitaxy (MME). MME is an extremely low temperature modification of molecular beam epitaxy (MBE) performed at temperatures where desorption and thermal decomposition are negligible, utilizing cyclic, high metallic doses to establish a low bond strength surface layer resulting in higher adatom mobility. MME has shown great potential in the past for the growth of AlInN, demonstrating the ability to grow high indium content AlInN with state-of-the-art crystal quality and sub-nm RMS roughness. By utilizing pulsed cation fluxes with high metal fluxes, short metal doses, and a metal rich III/V ratio at substrate temperatures low enough to prevent In-N decomposition and In desorption, adatom kinetics have been improved, improving film structural and optical properties. Using this approach, MME has produced Al_0.3In_0.7N with X-ray rocking curves 11 times better than prior literature. This non-traditional approach was expanded to look at 9 compositions from 10% to 90% Al, each at multiple substrate temperatures. Because the optimal substrate temperature varies with composition, 27 samples ranging from: 10-40% Al were grown at 300, 350, and 400 ^oC; 50-70% Al were grown at 350, 400, and 450 ^oC; and 80-90% Al were grown at 400, 450, and 500 ^oC. High structural quality was observed over the entire composition range but varied with growth temperature for different compositions, with (0002) X-ray diffraction (XRD) rocking curve full-width at half-maximum (FWHM) values as low as 163 arc-sec even near the center of the composition range where phase separation is predicted to be problematic. Atomic force microscopy (AFM) measurements showed smooth surfaces throughout the composition range with RMS roughness’ ranging between 0.16 to 3 nm. Photoluminescence measurements of high indium content AlInN show promising luminescence at wavelengths traditionally difficult for efficient InGaN emission while the luminescence efficiency varies with composition and growth conditions.

The ever-increasing demand for “solar-blind” (insensitive to photons with wavelengths longer than ~285 nm) photodetectors (PDs) in healthcare, space science, and technology, and the military requires focusing on state-of-the-art PDs. Owing to its appealing properties of tunable wide bandgap (3.4 to 6.2 eV), high-temperature robustness, chemical and radiation hardness, Al_xGa_1-xN has emerged as the most suitable candidate for fabricating solar-blind PDs [1]. However, the “5S” requirements of high spectral selectivity, sensitivity, speed, signal-to-noise ratio, and stability for a commercial PD emphasize novel strategies to improve the performance parameters of AlGaN-based solar-blind PDs. In the last two decades, the localized surface plasmon resonance (LSPR) in metal nanoparticles (NPs) has proved to be a promising strategy to enhance the performance of semiconductor PDs [2]. The role of Al NPs has been investigated to improve the performance of AlGaN solar-blind PDs. However, the reported responsivity values were only 0.288 AW⁻¹ at 5 V [3] and 2.34 AW⁻¹ at 20 V [4]. Therefore, there is a need to explore other metal NPs that may prove better than Al NPs for AlGaN solar-blind PDs.
In this work, we report a significant enhancement in the performance parameters of Al_0.4Ga_0.6N solar-blind PDs mediated by LSPR in Pd NPs. The interdigitated electrode (IDE) geometry (width and inter-electrode spacing 20 μm) of metal-semiconductor-metal (MSM) PDs was fabricated on MOCVD grown Al_0.4Ga_0.6N epilayers. Pd NPs were deposited on one of the PDs using an integrated gas-phase setup and then annealed at 500 °C to obtain AlGaN PD with Pd NPs. On decorating the Al_0.4Ga_0.6N film by Pd NPs, a significant enhancement in the current at 500 nm (visible) and 280 nm (solar-blind) was observed. At −5 V, the photo-to-dark current ratio (PDCR) was found to increase by 99.7% and 71.7% at 500 and 280 nm, respectively. Similarly, at −10 V, the enhancement was 99.6% and 85.0% at the two wavelengths. The observed results are attributed to the increased absorption and hot-carrier generation by LSPR in the Pd NPs. As a result, the performance parameters of the PD, such as responsivity and specific detectivity, were found to improve significantly. At −10 V, the peak responsivity was found to increase by 75%, reaching a maximum value of 2.2 AW⁻¹ at 220 nm with an incident optical power of only 1.7 μW, indicating the ability to detect even weak signals. It was observed that the responsivity increased with the increasing bias, and an enhancement by 71% was observed in the responsivity at 280 nm for the PD with NPs. To compare the sensitivity of the PDs, the specific detectivity was calculated, and its variation with the incident wavelength was analyzed. At −10 V, the maximum value of specific detectivity was found to increase by 82%, and a peak value of 3.4×10¹³ Jones was calculated. In conclusion, a significant increase in the PDCR, peak responsivity, and specific detectivity by 85%, 75%, and 82%, respectively, was observed for Al_0.4Ga_0.6N solar-blind PD on decorating with Pd NPs.

References
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Threading dislocations constitute a long standing and highly debated issue in the field of III-nitride materials. Typical dislocation densities in nitride based devices can be as high as of 10⁸ to 10¹⁰ cm^-2. These defects have recently attracted considerable interest due to their detrimental role in power electronics. c-type screw dislocations are considered to be one of the major limiting factors in achieving the full potential of nitride based power electronic devices. Specifically, screw dislocations as well as the associated V-pits have been proposed to be the root cause of device leakage and breakdown [1,2]. An intriguing feature of these defects is that experimental evidences suggest that they are associated with open core structures, thus forming nanopipes with diameters of a few nm [3]. On the other hand, early as well as recent first principles calculations indicate that the formation of nanopipes is energetically unfavorable under typical growth conditions [4,5]. However, these calculations do not explicitly consider the effect of core and/or do not consider the effect of impurities such as hydrogen that is abundant in MOCVD growth or dopand species such as C and Mg.

In order to resolve the aforementioned discrepancies and provide an understanding on the formation and the properties of dislocation induced nanopipes in III-nitrides we combine first principles with large scale empirical potential calculations within the Implicit Boundaries Multiscale Scheme [6]. Using this approach we investigate the energetics, atomic structure and electronic properties of c-type screw dislocations in GaN and AlN. Based on these calculations we construct a screw dislocations’ phase diagram which describes the energetically most favorable core structures as function of the species’ chemical potentials. We explicitly consider the effect of H, C, and Mg as well as the formation of superscrew dislocations, i.e., dislocations with Burgers vector multiple lattice constants. Although nanopipes exhibit large free surface areas which increase the energy, they can be energetically favorable if the surface energy is compensated by the core and strain energy of the material removed to create the open core screw dislocation. Our calculations indicate that under MBE growth this requirement is not met and the formation of nanopipes is energetically highly unfavorable. However, a general trend that emerges from the aforementioned phase diagram is that nanopipes become energetically favorable at higher H chemical potentials. Indeed, under typical MOCVD or MOVPE growth conditions the cation and anion dangling bonds at the open core inner surfaces are passivated by NH_x molecules and H atoms, respectively. This reduces considerably the surface energy and the formation of nanopipes with hexagonal cross section and equilibrium diameters ranging from approx. 1 to 2 nm are energetically favorable. Based on these results we will discuss the electronic properties of the nanopipes, the diffusion and incorporation of C and Mg dopands at the open core structure as well as implications these defects may have on the efficiency of the electronic devices.

This project has received funding from the ECSEL Joint Undertaking (JU) project UltimateGaN under grant agreement No 826392. The JU receives support from the European Union’s Horizon 2020 research and innovation program and Austria, Belgium, Germany, Italy, Slovakia, Spain, Sweden, Norway, Switzerland.

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[6] L. Lymperakis et al., Phys. Rev. Lett. Phys. Rev. Lett. 93, 196401-1 (2004).

Recently, UV-C lights are getting explosive attention for UV sterilization. Al_xGa_1-xN is a direct bandgap material covering from 3.4~6.0 eV. It is one of the most promising one to realize semiconductor-based UV-C emitter. However, metal-rich growth of high-AlN content AlGaN by RF-MBE has known to be suffered from drawback due mainly to the difference in the bonding energy of Ga-N and Al-N [1,2]. In this study, high-AlN content Al_xGa_1-xN films were grown by RF-MBE. Impact of change in group-III to nitrogen supply ratio on their crystallinity and optical property are studied to phenomenologically understand the growth kinetics.
Al_xGa_1-xN films were grown on 425-nm-thick c-plane AlN on Al₂O₃ templates [3] at substrate temperature of 775°C. Beam equivalent pressures of Al and Ga sources were fixed at 2.0×10^-7 and 3.0×10^-7 Torr, respectively. N₂ gas flow rate was varied in between 0.3~2.0 sccm by fixing RF input power of 150 W. Growth time was controlled by the group-III source supply for 1 hour with maintaining the reactive nitrogen source supply during the heating and cooling processes above 300°C. Photoluminescence (PL) spectra were measured at room temperature (RT) using a frequency-quintupled (213 nm) Q-switched YAG:Nd laser. Reflectance spectra were measured at RT using a 25 W Deuterium and a 20 W Tungsten Halogen lamps.
XRD 2q-w pattern for the film with N₂=0.3~0.5 sccm only shows a (0002) diffraction peak of AlN. Diffraction peaks of GaN and AlGaN additionally appeared for the films with 0.7~2.0 sccm. Furthermore, the AlGaN peaks for 0.7~2.0 sccm exhibited a shoulder at higher and lower diffraction angle side, respectively. Al mole fraction x was measured by X-ray reciprocal space mapping to be 0.67~0.88, confirming the growth of high-AlN content AlGaN. Film thicknesses were assessed by the spectroscopic ellipsometry measurements to be 387~416 nm for AlN overlayers at N₂=0.3~0.5 sccm, 420~653 nm for AlGaN at N₂=0.7~2.0 sccm, and 4~73 nm for GaN grown on top of the AlGaN layer. PL spectra exhibited near-band-edge emission peaks for GaN at around 3.3~3.4 eV and for AlGaN at around 4.3~4.8 eV. Furthermore, the AlGaN peak for 1.0 and 2.0 sccm clearly shows double peaks. The reflectance spectra show the multiple interference fringes in which three periodic oscillations are overlapped, and the averaged effective bandgap energies of AlGaN layers were estimated to 5.09~5.43 eV.
The overall results can be understood by the growth model [1], in which the Al-N bond is favorably formed under the metal rich condition. The additional lower-Al-content AlGaN and GaN overlayers were found to be grown during the cooling process by nitriding the excess Al and Ga metals.

This work was supported in part by Takahashi Industrial and Economic Research Foundation. The authors grateful to Prof. A. Yoshikawa of Chiba University for his continuous encouragements.
[1] E. Iliopoulos and T. D. Moustakas, Appl. Phys. Lett. 81, 295 (2002). [2] M. Shirazi-HD et al., J. Appl. Phys. 123, 161581 (2018). [3] H. Fujikura et al., Appl. Phys. Express. 13, 025506 (2020)

The AlGaN/GaN heterostructure has been demonstrated to be a promising candidate for high power amplifiers in the microwave communication system and high-voltage power switches [1]. The combination of GaN device performance with the low cost of silicon substrates enhances the interest of this technology. Despite numerous improvements of GaN power components during the last decades, charge trapping effects at deep levels still lead to dynamic on-resistance and switching losses [2]. To detect these deep traps and determine in which part of the device structure they are located, capacitance deep level transient spectroscopy (DLTS) has been applied to GaN Schottky barrier diodes (SBDs) [3].

However, hole-like traps are frequently observed in DLTS spectra of devices with n-type AlGaN/GaN heterojunctions without any minority carriers. Many works refer to previous studies carried out in AlGaAs/GaAs systems to explain the nature of these DLTS signals. Hence, hole-like traps are often associated with surface states at the ungated SiN/AlGaN interface [4]. They are sometimes related to bulk traps in the AlGaN layer [5] or to the transfer of electrons by tunneling effect from the traps in the lower half of the AlGaN bandgap into the states in the GaN buffer [6].

To our knowledge, no deep investigation exists to clearly identify the origin of hole-like traps in AlGaN/GaN heterojunctions. A detailed understanding of the trapping mechanism involved in this anomalous DLTS signal is of fundamental importance to learn which defects affect device performance such as the gate-lag, for instance [5].

This work introduces a deep study on a hole-like signal which appears in DLTS spectra of AlGaN/GaN SBDs. A correlation between the amplitude of the peak and the reverse leakage current of the diode has been demonstrated and a mechanism is proposed to explain the formation of the hole-like signal. During the pulse voltage, electrons move by tunneling conduction from the two-dimensional electron gas (2DEG) towards the AlGaN layer where they are captured on deep donor states. When the temperature is increased and the reverse voltage is applied on the anode, the thermal energy removes electrons from the traps and the 2DEG is refilled by the reverse leakage current of the diode. The hole-like signal results from the return of electrons into the 2DEG.

The simulation of the DLTS peak using a model involving a transient time constant, which takes into account the thermal activation energy of the reverse leakage current of the diode, leads to the extraction of two deep levels in AlGaN. The concentration of the dominant trap depends on the ammonia partial pressure used to perform the AlGaN layer and was calculated at 4.1x10¹³ cm^-3 and 5.3x10¹³ cm^-3 for the two tested SBDs. With the help of secondary ion mass spectrometry studies, this trap was assigned to nitrogen antisite defects. The second trap was tentatively associated with Ga vacancy related defects.

The proposed model allows an explanation of the origin of traps reported in the literature. In addition, this work demonstrates that information can be extracted from a hole-like signal in AlGaN/GaN heterojunctions to access features of traps in AlGaN. We also demonstrated that the amplitude of the hole-like signal is an indication of the quantity of electrons which leave the 2DEG. Since the reduction of electrons in the 2DEG directly results in a decrease of device performance, the appearance of a hole-like signal in DLTS spectra of AlGaN/GaN heterojunctions should be thoroughly investigated.

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[2] M. Meneghini et al., IEEE Trans. Electron Devices 58, 2996 (2011)
[3] P. Hacke et al., J. Appl. Phys. 76, 304 (1994)
[4] T. Okino et al., IEEE Electron Device Lett. 25, 523 (2004)
[5] A. Y. Polyakov et al., ECS J. Solid State Sci. Technol. 6, S3034 (2017)
[6] K. L. Enisherlova et al., Russ. Microelectron. 48, 28 (2019)

AlGaN/GaN high electron mobility transistor (HEMT) are known to be promising devices for high voltage applications. But the voltage range of the commercially available devices is not exceeding 650 V. To achieve operating voltages above kV, several solutions have been tested. One of them is the use of a thinner GaN channel combined with an ultra-wide bandgap material as a buffer in order to reduce its impact on the overall breakdown voltage.
The device studied in this work is an AlN-based HEMT [1] grown by molecular beam epitaxy. It is composed of a 6 µm thick commercial AlN-on-sapphire template, a 190 nm AlN buffer, a 8 nm GaN channel, a 10 nm AlGaN barrier layer with 90% aluminum molar fraction to ensure a high sheet carrier density and a 5 nm in-situ grown SiN_x cap layer. Ti/Al/Ni/Au ohmic contacts are deposited on top of the barrier layer and Ni/Au is used to form the gate metal. Characterization by capacitance deep-level transient spectroscopy (DLTS) has been performed between 77 K and 500 K. A reverse voltage V_R from −1 V to −7 V has been applied between the gate and the ohmic contact. The filling pulse voltage V_P was fixed at −0.5 V.
For a reverse voltage V_R > −5 V, the depleted layer is under the gate. A DLTS peak can be observed at T_max = 123 K for a period width of T_W = 10 ms. The Arrhenius plot gives an activation energy of E_a = 0.18 eV and a capture cross-section of σ_n = 1.2×10⁻¹⁵ cm². The amplitude of the peak is saturating for a filling pulse time of t_P = 10 ms, indicating this is a point defect. It could be related to a N-vacancy defect [2][3]. For a reverse voltage V_R < −5 V, the depleted area extends under the SiN_x cap layer. At least five new peaks appear on the DLTS spectra at T_max = 158, 190, 294, 375, 443 K for the same period width of 10 ms. Their activation energies are consequently higher than the first identified defect. When the negative voltage is applied on the gate contact, the negative charges in the SiN_x are pushed back towards the ohmic contact and let positive charges on the gate side. The electrons of the channel are attracted by these positive charges and are captured by the defects in the AlGaN layer. This leakage mechanism has already been proposed for another AlGaN/GaN HEMT [4]. The peaks resulting from this phenomenon are not visible on the DLTS spectra for V_R > −5 V because the probed area is not under the SiN_x cap layer. It is expected that the large lattice mismatch between AlGaN and GaN layers, resulting from the high aluminum concentration of AlGaN, leads to the formation of defects. Additional levels which appear in DLTS spectra for V_R < −5 V could then be related to interface states or border traps in AlGaN.
The DLTS results enabled to highlight defects in the high aluminum concentration barrier layer and the leakage mechanisms occurring between the channel and the barrier. These results will help to improve critical properties of the AlN-based HEMT with a thin channel.

References:
[1] I. Abid et al., Micromachines 10, 690 (2019)
[2] L. Polenta et al., Appl. Phys. Lett. 76, 2086 (2000)
[3] Z-Q. Fang et al., Proc. 11th International Conference on Semiconducting and Insulating Materials, 35 (2000)
[4] P. Ferrandis et al., J. Phys. D: Appl. Phys. 53 185105 (2020)

We reveal the role of the sub-well centers and related carrier dynamics mechanisms in enhancing the DUV emission of AlGaN/AlGaN multiple quantum wells (MQWs) when AlN substrate is grown homoepitaxially using MOCVD, thus eliminating the dislocation effect. As a part of this work, a 400 nm thick AlN layer was initially regrown on a thick (0001) AlN substrate at 1250 °C, followed by a 20×Al_0.6Ga_0.4N/AlN superlattice (SL) layer. Subsequently, a 1 µm thick n-AlGaN layer was grown, followed by a nominal 5-period Al_0.4Ga_0.6N/Al_0.5Ga_0.5N (3 nm/12 nm) MQW active layer capped by a 30 nm p-Al_0.65Ga_0.35N electron blocking layer (EBL). Both plan-view and cross-sectional images obtained via scanning transmission electron microscopy (STEM) reveal dislocation defect-free epitaxial layers of very high crystalline quality, as well as ultra-thin Al-rich sub-quantum barrier layers. Energy dispersive x-ray (EDX) compositional analysis of the STEM cross-sections indicates that the formation of these layers can be ascribed to the diffusion of Al adatoms towards the top of the barriers, creating shallow sub-wells within the barrier regions. Advanced carrier dynamic analyses via photoluminescence (PL), power-dependent time-resolved PL (TRPL) spectroscopy and PL excitation measurements show that these sub-wells/sub-barriers resulted in additional carrier confinement and exciton localization centers. As a result, internal quantum efficiency is enhanced via staggered carrier repopulation into the MQWs to reach its maximum (~ 85%) and remains high (> 60%) after a slight droop at high carrier densities. We show that this slight efficiency droop is due to Auger recombination counteracted by a simultaneous increase in radiative recombination processes at high power density, demonstrating the role of sub-wells/barriers in efficiency enhancement.

The use of deep ultraviolet (DUV) light for sterilization is attracting much attention behind recent global situation regarding COVID-19. AlGaN is the most promising candidate material [1,2]. Recent developments of high quality AlN template [3] as well as AlN bulk substrate [4] acccelerate improvements of perforance of DUV emitters. Nevertheless the develpments of the template and substrate, precise control of conditions is still nessessary for growth of Al_xGa_1-xN by RF-MBE. In general, metal-rich condition is preferred to maintain high crystallinity and flat surface. Meanwhile, Al atoms tend to be stronger bonded to the surface, and Ga and N atoms are hardly incorporated into the alloy lattice even if Ga and N are sufficiently supplied [5]. Therefore, controll of Al flux is crucial to eliminate Al precipitation and to supress composition fluctuation in Al_xGa_1-xN alloy. In this study, AlN thin films were grown on AlN templates as functions of Al flux and growth temperature. Effects of the parameters on crystallinity and surface flatness are discussed.
AlN templates were 431-nm-thick c-plane AlN prepared on Al₂O₃ by halide vapor phase epitaxy (HVPE) [3]. 181 to 432-nm-thick AlN films were grown for 1 hour with N₂ gas flow rate of 0.3 and RF input power of 150 W. Substrate temperature was varied as T_s=775, 800, and 850 °C, and Al equilibrium vapor pressure was varied as Φ_Al =1.0×10^-7 Torr and 2.0×10^-7 Torr. The samples were evaluated by atomic force microscopy (AFM), laser microscopy, X-ray diffraction (XRD), and spectroscopic ellipsometry.
All the XRD 2θ-ω patterns predominatly show a (0002) diffraction peak of AlN. The patterns for Φ_Al =2.0×10^-7 Torr additionally show a (111) diffraction peak of Al metal. The peak appeared regardless of T_s, and the intensity decreased with increasing T_s. It was also confirmed by the laser microscope images that the amount of Al precipitaes decreased with increasing T_s. On the contrary, the films grown with Φ_Al =1.0×10^-7 Torr showed no tendency of Al precipitation at all T_s. The results imply that the growths with Φ_Al =2.0×10^-7 Torr were Al-rich conditions, and growths with Φ_Al =1.0×10^-7 Torr were stoichiometric or slightly N-rich condtions. Furthermore, the growth rate decreased for Φ_Al =2.0×10^-7 Torr at the lowest T_s of 775°C. It was suggested that exesss Al supply induced a fomation of Al layer, which are no longer contribute to the growth, and resulted in a decrease in film thickness [6]. Root mean square roughness obtained from the surface AFM images was improved from 3.7 nm at (Φ_Al , T_s)=(2.0×10^-7 Torr, 800°C) to 1.4 nm at (Φ_Al , T_s)=(1.0×10^-7 Torr, 800°C). The results indicate that the Al precipitaion degrade the surface flatness, and it must be elimineted to obtain crystallinity as is the same with the AlN template.

This work was supported in part by Takahashi Industrial and Economic Research Foundation. The authors grateful to Prof. A. Yoshikawa of Chiba University for his continuous encouragements.
[1] H. Hirayama, J. Appl. Phys. 97, 091101 (2005). [2] M. Kneissl and J. Rass, III–Nitride Ultraviolet Emitters (Springer International Publishing, Cham, 2016). [3] H. Fujikura et al., Appl. Phys. Express 13, 025506 (2020). [4] Y. Kumagai et al., Appl. Phys. Express 5, 055504 (2012). [5] M. Shirazi-HD et al., J. Appl. Phys. 123, 161581 (2018). [6] V. N. Jmerik et al., J. Cryst. Growth 354, 188 (2012).

Their large direct bandgap, high thermal stability and surprisingly large insensitivity against even large defect concentrations have made group-III-nitrides the alloy system of choice in manufacturing light emitting devices and make them highly attractive for high-power applications. Despite the amazing progress that has been achieved in understanding, controlling and processing these materials fundamental issues remain unresolved that severely limit device performance. Infamous examples are the efficiency droop in the green region of the optical spectrum, the spontaneous formation of extended defects during epitaxial growth such as V-pits, or the formation of long-range electrostatic fields due to spontaneous polarization.
Understanding the origin of the underlying fundamental mechanisms requires an in-situ observation of atomic-scale phenomena during epitaxial growth, which makes it hard and often even impossible to explore them by experimental techniques. Recent developments and progress in first principles calculations provide now a powerful tool to unravel the mechanisms that are the fundamental cause for limitations in these materials [1,2]. In the presentation a brief overview about key concepts of the novel approaches will be given. It will be further demonstrated how they help to uncover hitherto unknown surface phenomena such as frustrated hybridization [3] that explain the origin of droop as a phenomenon originating at the surface rather than in bulk.
[1] S.-H. Yoo, M. Todorova, D. Wickramaratne, L. Weston, C.G. Van de Walle, and J. Neugebauer, Finite-size correction for slab supercell calculations of materials with spontaneous polarization, npj Comput. Mater. 7, 58 (2021).
[2] S.-H. Yoo, L. Lymperakis, and J. Neugebauer, Efficient electronic passivation scheme for computing low symmetry compound semiconductor surfaces in density-functional theory slab calculations, Phys. Rev. Mater. 5, 044605 (2021).
[3] L. Lymperakis, T. Schulz, C. Freysoldt, M. Anikeeva, Z. Chen, X. Zheng, B. Shen, C. Chèze, M. Siekacz, X. Q. Wang, M. Albrecht, and J. Neugebauer, Elastically frustrated rehybridization: Origin of chemical order and compositional limits in InGaN quantum wells, Phys. Rev. Mater. 2, 011601(R) (2018).

In recent years, nitrogen-polar based devices have emerged as a potential source for novel and innovative developments in the field of GaN technology. With a different polarity, surface reactivity, and growth dynamic, N-polar GaN demonstrates notable advantages over Ga-polar GaN. The benefits of N-polar also span the typical compounds grown on Ga-polar GaN substrate. In_xGa_1-xN is especially interesting to investigate due to its attractive applications in optoelectronic devices. However, InGaN growth on GaN is challenging for several reasons, one of which concerns the conflicting values for the optimum growth temperatures of GaN and InGaN. To better examine such obstacles, this study explores InGaN growth on N-polar GaN by plasma assisted Molecular Beam Epitaxy (MBE) with high temperature. Samples were grown within a range of temperatures from 600° to 670°C, although typically InGaN films on Ga-polar GaN are grown around 100° less. The resulting N-polar InGaN samples achieved higher indium incorporation compared to Ga-polar growths. With further experimentation, the desirable growth conditions can be established and applied for development of higher performance InGaN devices.

A remarkable increase in the quantum efficiency of long-wavelength III-nitride light emitters has been demonstrated by AlN or AlGaN strain-compensating interlayers that cap In-rich InGaN quantum wells (QWs). However, the structural non-uniformity within these strain-compensated heterostructures, and its effect on the local and overall optical properties, has not yet been satisfactorily addressed.

In this work, we use aberration-corrected scanning transmission electron microscopy (STEM) combined with cathodoluminescence (CL) in the STEM to study optical inhomogeneities resulting from various atomic-scale localization centers in red-emitting AlN capped InGaN/GaN QW devices. By probing the spectral response of individual InGaN QWs with nanometer spatial precision, we show that a secondary blueshifted emission originates within the QW near to the AlN interlayer. On the other hand, the presence of criss-crossing inversion domains intrinsically linked to the strain fields surrounding V-pits result in further distinct blueshifts. Finally, high intensity luminescence can be directly correlated to In-rich quantum dots spontaneously nucleated at intersecting V-pit regions. This study highlights the importance of controlling thickness modulation of interlayers and threading dislocation densities to ensure compositional and optical homogeneity in strain-compensated In-rich optoelectronic devices targeted for practical applications. Our results also suggest the nanoscopic origins for the significant emission wavelength shift with injection-current observed in red-emitting devices.

In_xGa_1-xN light-emitting diodes (LEDs) with higher In content have reduced efficiency compared to their lower In counterparts. The origin of this reduction in efficiency is still not fully understood, although compositional fluctuations in In_xGa_1-xN quantum wells have been linked with the efficiency of LEDs. We combined atomic-resolved electron energy loss spectroscopy (EELS) imaging with first-principles multiscale computational models to obtain a statistical distribution of the compositional fluctuations in In_xGa_1-xN LEDs at different indium concentrations. By analyzing a large dataset of EELS data, we obtain the statistical distribution of the compositional fluctuations as a function of the In_xGa_1-xN composition. We show that although lower In content In_xGa_1-xN with x~18% LEDs mostly show compositional fluctuations in the limit of a random alloy, the distribution of compositional fluctuation drastically changes in In_xGa_1-xN LEDs with higher In content (≥24%)[1]. Therefore, a distinctly different mechanism for carrier localization can be expected for In_xGa_1-xN LEDs of different In content, which would ultimately affect the performance of the LEDs [2]. Our theoretical approach directly incorporates strain effects in the In_xGa_1-xN quantum wells, resulting from the epitaxial growth. Based on our theoretical model, we will showcase a new method to prevent compositional fluctuations in higher In content In_xGa_1-xN LEDs.

References:

1. Tara P. Mishra et al., Physical Review Materials 5, 024605 (2021).
2. C. M. Jones et al., Appl. Phys. Lett 111, 113501 (2017).

Extended defects in In-rich quantum wells (QWs) hamper the fabrication of efficient long-wavelength III-nitride light emitting diodes (LEDs). A detailed understanding on their nature, formation mechanism, and the resulting properties is crucial for the design of more efficient optoelectronic devices. With the advent of 5th order aberration correctors in scanning transmission electron microscopes (STEMs), the direct imaging of atom displacements becomes possible, allowing an unambiguous interpretation of the atomic structure of defects. However, at the moment there is no broad reference for STEM-based atomic-resolution defect classification such as those which exists for conventional TEM.

We demonstrate high-angle annular dark-field (HAADF) depth-sectioning as a convenient tool to distinguish between various defect types in In-rich QWs. We show that different planar defects – the dominant defect type emerging from the QWs – are identifiable by their unique atomic column overlap configurations. Through this method, we reveal that defects commonly identified as edge type dislocations in conventional QWs are instead isolated sub 5-nm type I₂ basal stacking faults (BSFs). Electron energy loss spectroscopy mapping of the QWs suggests the origins of these small diameter BSFs to be local compositional fluctuations, which have theoretically been predicted to increase in QWs with higher strain. Through the engineering of a strain relaxed architecture, we show that a pathway for defect-free long wavelength InGaN LEDs can be achieved.

Group III−V quaternary alloy semiconductors are attractive materials due to their capability of covering a relatively wide wavelength range at least between those values of their constituent binary compounds. These alloy systems are also attractive due to their capability of changing their bandgap energies even under a fixed lattice constant [1]. The latter nature furthermore opens a possibility of fabricating high−quality layers of these alloys lattice−matched to an underlying layer or a substrate with a low density of crystal defects.
Among a number of group III−V quaternary alloys, this study is focused on cubic Al_xIn_1−xN_ySb_1−y in terms of not only a wide range of its usable light wavelengths spanning from the ultraviolet to infrared regions but also its lattice matching ability to Si, GaAs, and GaN substrates. Furthermore, this alloy system can potentially be grown in the cubic phase over the entire composition range, since each of the four constituent binary compounds of this alloy system can be grown in the cubic phase [2, 3]. The large difference in the atomic radius between N and Sb should cause large energy gap bowing. This may lower the minimum bandgap energy value of this alloy system to below that of InSb, which has the lowest bandgap energy value among the four constituent binary compounds.
It should be noted that Sakai et al. calculated bandgap energies of a number of group III−V quaternary alloy semiconductors incorporating Nitrogen as group V element [4] by means of the dielectric method [5−8] using band parameters of the constituent binary compounds. Al_xIn_1−xN_ySb_1−y was also included in this pioneering and comprehensive work [4]. The band parameters for cubic group−III nitrides were scant at that time and more substantial data has become available in recent years including drastically revised parameters of InN. Furthermore, bandgap energies of cubic Al_xIn_1−xN_ySb_1−y have not been calculated by other methods to the best of authors’ knowledge.
In this study, direct and indirect bandgap energies of cubic Al_xIn_1−xN_ySb_1−y were calculated by means of the dielectric method [5−8]. We referred to a review article for the lattice constant and the direct and indirect bandgap values of the constituent cubic group−III nitrides, AlN and InN [3]. In contrast with Sakai et al.’s work [4], we show all energy set of interest: Γ−valley direct (E_Γ), X−Γ indirect (E_X), and L−Γ indirect (E_L) bandgap energies of this alloy system as contour maps. The transition types of this alloy system were also distinguished on the alloy composition space by simply comparing the calculated E_Γ, E_X, and E_L values.
The calculation results showed that cubic Al_xIn_1−xN_ySb_1−y converted its band structure from direct to indirect transition types by variation of x and y values. The bandgap range covered in the direct transition regime was between − 5.38 [eV] and 4.60 [eV], indicating that cubic Al_xIn_1−xN_ySb_1−y can be applicable for optical devices over a wide wavelength range longer than 270 [nm]. Al_xIn_1−xN_ySb_1−y lattice-matched to either Si or GaAs resulted in having only negative bandgap energies at all possible compositions. On the other hand, Al_xIn_1−xN_ySb_1−y lattice-matched to GaN was of direct transition type at all possible compositions, with bandgap energies ranging between 2.02 [eV] and 3.81 [eV].

1. Sadao Adachi, Properties of Semiconductor Alloys: Group-IV, III-V, and II-VI Semiconductors (WILEY, West Sussex, 2009).
2. I. Vurgaftman, J. R. Meyer, and L. R. Ram-Mohan, J. Appl. Phys. 89 5815 (2001) and references therein.
3. I. Vurgaftman and J. R. Meyer, J. Appl. Phys. 94 3675 (2003) and references therein.
4. S. Sakai, Y. Ueta, and Y. Terauchi, Jpn. J. Appl. 32 4413 (1993).
5. J. A. Van Vechten, Phys. Rev. 182 891 (1969).
6. J. A. Van Vechten, Phys. Rev. 187 1007 (1969).
7. J. A. Van Vechten, Phys. Rev. B1 3351 (1970).
8. Y. Ueta, Doctoral Dissertation (The University of Tokushima, March 1995).