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Andriy Zakutayev

Andriy Zakutayev appears in the imported research catalog. Authorship, coauthor and topic links are available while profile ownership is still unclaimed.

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cond-mat.mtrl-sciphysics.app-phArtificial Intelligencecond-mat.dis-nneess.SYphysics.comp-phSystems and Control

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Published work

22 published item(s)

preprint2026arXiv

Born-Qualified: An Autonomous Framework for Deploying Advanced Energy and Electronic Materials

Autonomous science is transforming how we discover materials and chemical systems for advanced energy technologies. However, many initially promising systems never reach deployment. This "valley of death" stems from optimization that prioritizes laboratory metrics over industrial viability. We propose a new strategy: "born-qualified" autonomous development, which embeds manufacturability, cost, and durability constraints from the outset. This approach is enabled by four pillars, including the development of multi-objective metrics, causal models, a modular infrastructure, and embedding manufacturing in the discovery loop. Realizing this vision will require sustained, community-wide commitment, but the potential return on that investment is commensurate with the scale of the challenge.

0 citations0 reviewsNo code linkedOpen access
preprint2025arXiv

Towards a deeper fundamental understanding of (Al,Sc)N ferroelectric nitrides

Density Functional Theory (DFT) calculations, within the virtual crystal alloy approximation, are performed, along with the development of a Landau-type model employing a symmetry-allowed analytical expression of the internal energy and having parameters being determined from first principles, to investigate properties and energetics of Al1-xScxN ferroelectric nitrides in their hexagonal forms. These DFT computations and this model predict the existence of two different types of minima, namely the 4-fold-coordinated wurtzite (WZ) polar structure and a 5-times paraelectric hexagonal phase (to be denoted as H5), for any Sc composition up to 40%. The H5 minimum progressively becomes the lowest energy state within hexagonal symmetry as the Sc concentration increases from 0 to 40%. Furthermore, the model points out to several key findings. Examples include the crucial role of the coupling between polarization and strains to create the WZ minimum, in addition to polar and elastic energies, and that the origin of the H5 state overcoming the WZ phase as the global minimum within hexagonal symmetry when increasing the Sc composition mostly lies in the compositional dependency of only two parameters, one linked to the polarization and another one being purely elastic in nature. Other examples are that forcing Al1-xScxN systems to have no or a weak change in lattice parameters when heating them allows to reproduce well their finite-temperature polar properties, and that a value of the axial ratio close to that of the ideal WZ structure does imply a large polarization at low temperatures but not necessarily at high temperatures because of the ordered-disordered character of the temperature-induced formation of the WZ state. Such findings should allow for a better fundamental understanding of (Al,Sc)N ferroelectric nitrides, which may be used to design efficient devices operating at low voltages.

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preprint2022arXiv

A Landau-Devonshire Analysis of Strain Effects on Ferroelectric Al1-xScxN

We present a thermodynamic analysis of the recently discovered nitride ferroelectric materials using the classic Landau-Devonshire approach. The electrostrictive and dielectric stiffness coefficients of Al1-xScxN with wurtzite structure (6mm) are determined using a free energy density function assuming a hexagonal parent phase (6/mmm), with the first order phase transition based on the dielectric stiffness relationships. The results of this analysis show that the strain sensitivity of the energy barrier is one order of magnitude larger than that of the spontaneous polarization in these novel wurtzite ferroelectrics, yet both are less sensitive to strain compared to classic perovskite ferroelectrics. These analysis results reported here explain experimentally reported sensitivity of coercive field to elastic strain/stress in Al1-xScxN films, and would enable further thermodynamic analysis via phase field simulation and related methods.

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preprint2022arXiv

Gallium Oxide Heterojunction Diodes for Improved High-Temperature Performance

$β$-Ga${_2}$O${_3}$ based semiconductor devices are expected to have significantly improved high-power and high-temperature performance due to its ultra-wide bandgap of close to 5 eV. However, the high-temperature operation of these ultra-wide-bandgap devices is usually limited by the relatively low 1-2 eV built-in potential at the Schottky barrier with most high-work-function metals. Here, we report heterojunction p-NiO/n-$β$-Ga${_2}$O${_3}$ diodes fabrication and optimization for high-temperature device applications, demonstrating a current rectification ratio of more than 10${^6}$ at 410°C. The NiO heterojunction diode can achieve higher turn-on voltage and lower reverse leakage current compared to the Ni-based Schottky diode fabricated on the same single crystal $β$-Ga${_2}$O${_3}$ substrate, despite charge transport dominated by interfacial recombination. Electrical characterization and device modeling show that these advantages are due to a higher built-in potential and additional band offset. These results suggest that heterojunction p-n diodes based on $β$-Ga${_2}$O${_3}$ can significantly improve high-temperature electronic device and sensor performance.

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preprint2022arXiv

Ionic Bonds Control Ferroelectric Behavior in Wurtzite Nitrides

Ferroelectricity enables key integrated technologies from non-volatile memory to precision ultrasound. Wurtzite ferroelectric Al1-xScxN has recently attracted attention because of its robust ferroelectricity and Si process compatibility in addition to being the first known ferroelectric wurtzite. However, the origin and control of ferroelectricity in wurtzite materials is not yet fully understood. Here we show that the local bond ionicity, rather than simply the change in tetrahedral distortion, is key to controlling the macroscopic ferroelectric response, according to our coupled experimental and computational results. Across the composition gradient in Sc < 0.35 range and 140-260 nm thickness in combinatorial thin films of Al1-xScxN, the pure wurtzite phase exhibits a similar c/a ratio regardless of the Sc content, due to elastic interaction with neighboring crystals. The coercive field and spontaneous polarization significantly decrease with increasing Sc content despite this invariant c/a ratio, due to the more ionic bonding nature of Sc-N relative to the more covalent Al-N bonds, supported by DFT calculations. Based on these insights, ionicity engineering is introduced as an approach to reduce coercive field of Al1-xScxN for memory and other applications and to control ferroelectric properties in other wurtzites.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

Ternary Wide Band Gap Oxides for High-Power Electronics Identified Computationally

As electricity grids become more renewable energy-compliant, there will be a need for novel semiconductors that can withstand high power, high voltage, and high temperatures. Wide band gap (WBG) semiconductors tend to exhibit large breakdown field, allowing high operating voltages. Currently explored WBG materials for power electronics are costly (GaN), difficult to synthesize as high-quality single crystals (SiC) and at scale (diamond, BN), have low thermal conductivity ($β$-Ga$_2$O$_3$), or cannot be suitably doped (AlN). We conduct a computational search for novel semiconductors across 1,340 known metal-oxides using first-principles calculations and existing transport models. We calculate the Baliga figure of merit (BFOM) and lattice thermal conductivity ($κ_L$) to identify top candidates for n-type power electronics. We find 40 mostly ternary oxides that have higher $κ_L$ than $β$-Ga$_2$O$_3$ and higher n-type BFOM than SiC and GaN. Among these, several material classes emerge, including 2-2-7 stoichiometry thortveitites and pyrochlores, II-IV spinels, and calcite-type borates. Within these classes, we propose In$_2$Ge$_2$O$_7$, Mg$_2$GeO$_4$, and InBO$_3$ as they are the most favorable for n-type doping based on our preliminary evaluation and could be grown as single crystals or thin film heterostructures. These materials could help advance power electronic devices for the future grid.

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preprint2022arXiv

The role of disorder in the synthesis of metastable zinc zirconium nitrides

In materials science, it is often assumed that ground state crystal structures predicted by density functional theory are the easiest polymorphs to synthesize. Ternary nitride materials, with many possible metastable polymorphs, provide a rich materials space to study what influences thermodynamic stability and polymorph synthesizability. For example, ZnZrN2 is theoretically predicted at zero Kelvin to have an unusual layered "wurtsalt" ground state crystal structure with compelling optoelectronic properties, but it is unknown whether this structure can be realized experimentally under practical synthesis conditions. Here, we use combinatorial sputtering to synthesize hundreds of ZnxZr1-xNy thin film samples, and find metastable rocksalt-derived or boron-nitride-derived structures rather than the predicted wurtsalt structure. Using a statistical polymorph sampler approach, it is demonstrated that although rocksalt is the least stable polymorph at zero Kelvin, it becomes the most stable polymorph at high effective temperatures similar to those achieved using this sputter deposition method, and thus corroborates experimental results. Additional calculations show that this destabilization of the wurtsalt polymorph is due to configurational entropic and enthalpic effects, and that vibrational contributions are negligible. Specifically, rocksalt- and boron-nitride-derived structures become the most stable polymorphs in the presence of disorder because of higher tolerances to cation cross-substitution and off-stoichiometry than the wurtsalt structure. This understanding of the role of disorder tolerance in the synthesis of competing polymorphs can enable more accurate predictions of synthesizable crystal structures and their achievable material properties.

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preprint2022arXiv

Theoretical insights for Improving the Schottky-barrier Height at the Ga$_2$O$_3$/Pt Interface

In this work we study the Schottky barrier height (SBH) at the junction between $β$-Ga$_2$O$_3$ and platinum, a system of great importance for the next generation of high-power and high-temperature electronic devices. Specifically, we obtain interfacial atomic structures at different orientations using our structure matching algorithm and compute their SBH using electronic structure calculations based on hybrid density functional theory. The orientation and strain of platinum are found to have little impact on the barrier height. In contrast, we find that decomposed water (H.OH), which could be present at the interface from Ga$_2$O$_3$ substrate preparation, has a strong influence on the SBH, in particular in the ($\overline{2}$01) orientation. The SBH can range from $\sim$2 eV for the pristine interface to nearly zero for the full H.OH coverage. This result suggests that SBH of $\sim$2~eV can be achieved for the Ga$_2$O$_3$($\overline{2}$01)/Pt junction using the substrate preparation methods that can reduce the amount of adsorbed water at the interface.

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preprint2020arXiv

Growth and Characterization of Homoepitaxial $β$-Ga$_2$O$_3$ Layers

$β$-Ga$_2$O$_3$ is a next-generation ultra wide bandgap semiconductor (E$_g$ = 4.8 eV to 4.9 eV) that can be homoepitaxially grown on commercial substrates, enabling next-generation power electronic devices among other important applications. Analyzing the quality of deposited homoepitaxial layers used in such devices is challenging, in part due to the large probing depth in traditional x-ray diffraction (XRD) and also due to the surface-sensitive nature of atomic force microscopy (AFM). Here, a combination of evanescent grazing-incidence skew asymmetric XRD and AFM are investigated as an approach to effectively characterize the quality of homoepitaxial $β$-Ga$_2$O$_3$ layers grown by molecular beam epitaxy at a variety of Ga/O flux ratios. Accounting for both structure and morphology, optimal films are achieved at a Ga/O ratio of $\sim$1.15, a conclusion that would not be possible to achieve by either XRD or AFM methods alone. Finally, fabricated Schottky barrier diodes with thicker homoepitaxial layers are characterized by $J-V$ and $C-V$ measurements, revealing an unintentional doping density of 4.3 $\times$ 10$^{16}$ cm$^{-3}$ - 2 $\times$ 10$^{17}$ cm$^{-3}$ in the epilayer. These results demonstrate the importance of complementary measurement methods for improving the quality of the $β$-Ga$_2$O$_3$ homoepitaxial layers used in power electronic and other devices.

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preprint2020arXiv

Synthesis of ferroelectric LaWN3 -- the first nitride perovskite

Next generation telecommunication technologies would benefit from strong piezoelectric and ferroelectric response in materials that are compatible with nitride radio-frequency electronic devices. Ferroelectric oxides with perovskite structure have been used in sensors and actuators for half a century, and halide perovskites transformed photovoltaics research in the past decade, but neither of them is compatible with nitride semiconductors. Nitride perovskites, despite numerous computational predictions, have not been experimentally demonstrated and their properties remain unknown. Here we report the experimental realization of the first nitride perovskite: lanthanum tungsten nitride (LaWN3). Oxygen-free LaWN3 thin films in a polar perovskite structure are confirmed by spectroscopy, scattering, and microscopy techniques. Scanning probe measurements confirm a large piezoelectric response and strongly suggest ferroelectric behavior, making it the first stable nitride ferroelectric compound. These results should lead to integration of LaWN3 with nitride semiconductors for wireless telecommunication applications, while enabling synthesis of many other predicted nitride perovskites.

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preprint2020arXiv

Thin Film Growth Effects on Electrical Conductivity in Entropy Stabilized Oxides

Entropy stabilization has garnered significant attention as a new approach to designing novel materials. Much of the work in this area has focused on bulk ceramic processing, leaving entropy-stabilized thin films relatively underexplored. Following an extensive multi-variable investigation of polycrystalline (Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$)O thin films deposited via pulsed laser deposition (PLD), it is shown here that substrate temperature and deposition pressure have strong and repeatable effects on film texture and lattice parameter. Further analysis shows that films deposited at lower temperatures and under lower oxygen chamber pressure are $\sim$40x more electrically conductive than otherwise identical films grown at higher temperature and pressure. This electronic conductivity is hypothesized to be the result of polaron hopping mediated by transition metal valence changes which compensate for oxygen off-stoichiometry.

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preprint2020arXiv

Wide band gap chalcogenide semiconductors

Wide band gap semiconductors are essential for today's electronic devices and energy applications due to their high optical transparency, as well as controllable carrier concentration and electrical conductivity. There are many categories of materials that can be defined as wide band gap semiconductors. The most intensively investigated are transparent conductive oxides (TCOs) such as ITO and IGZO used in displays, carbides and nitrides used in power electronics, as well as emerging halides (e.g. CuI) and 2D electronic materials used in various optoelectronic devices. Chalcogen-based (S, Se, Te) wide band gap semiconductors are less heavily investigated but stand out due to their propensity for p-type doping, high mobilities, high valence band positions (i.e. low ionization potentials), and broad applications in electronic devices such as CdTe solar cells. This manuscript provides a review of wide band gap chalcogenide semiconductors. First, we outline general materials design parameters of high performing transparent conductors. We proceed to summarize progress in wide band gap (Eg > 2 eV) chalcogenide materials, such as II-VI MCh binaries, CuMCh2 chalcopyrites, Cu3MCh4 sulvanites, mixed anion layered CuMCh(O,F), and 2D materials, among others, and discuss computational predictions of potential new candidates in this family, highlighting their optical and electrical properties. We finally review applications of chalcogenide wide band gap semiconductors, e.g. photovoltaic and photoelectrochemical solar cells, transparent transistors, and diodes, that employ wide band gap chalcogenides as either an active or passive layer. By examining, categorizing, and discussing prospective directions in wide band gap chalcogenides, this review aims to inspire continued research on this emerging class of transparent conductors and to enable future innovations for optoelectronic devices.

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preprint2019arXiv

High-throughput fabrication and semi-automated characterization of oxide thin film transistors

High throughput experimental methods are known to accelerate the rate of research, development, and deployment of electronic materials. For example, thin films with lateral gradients in composition, thickness, or other parameters have been used alongside spatially-resolved characterization to assess how various physical factors affect material properties under varying measurement conditions. Similarly, multi-layer electronic devices that contain such graded thin films as one or more of their layers can also be characterized spatially in order to optimize the performance. In this work, we apply these high throughput experimental methods to thin film transistors (TFTs), demonstrating combinatorial device fabrication and semi-automated characterization using sputtered Indium-Gallium-Zinc-Oxide (IGZO) TFTs as a case study. We show that both extrinsic and intrinsic types of device gradients can be generated in a TFT library, such as channel thickness and length, channel cation compositions, and oxygen atmosphere during deposition. We also present a semi-automated method to measure the 44 devices fabricated on a 50x50mm substrate that can help to identify properly functioning TFTs in the library and finish the measurement in a short time. Finally, we propose a fully automated characterization system for similar TFT libraries, which can be coupled with high throughput data analysis. These results demonstrate that high throughput methods can accelerate the investigation of TFTs and other electronic devices.

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preprint2016arXiv

Pathway to oxide photovoltaics via band-structure engineering of SnO

The prospects of scaling current photovoltaic technologies to terawatt levels remain uncertain. All-oxide photovoltaics could open rapidly scalable manufacturing routes, if only oxide materials with suitable electronic and optical properties were developed. A potential candidate material is tin monoxide (SnO), which has exceptional doping and transport properties among oxides, but suffers from a low adsorption coefficient due to its strongly indirect band gap. Here, we address this shortcoming of SnO by band-structure engineering through isovalent but heterostructural alloying with divalent cations (Mg, Ca, Sr, Zn). Using first-principles calculations, we show that suitable band gaps and optical properties close to that of direct-gap semiconductors are achievable in such SnO based alloys. Due to the defect tolerant electronic structure of SnO, the dispersive band-structure features and comparatively small effective masses are preserved in the alloys. Initial Sn1-xZnxO thin films deposited by sputtering exhibit crystal structure and optical properties in accord with the theoretical predictions, which confirms the feasibility of the alloying approach. Thus, the implications of this work are important not only for terawatt scale photovoltaics, but also for other large-scale energy technologies where defect-tolerant semiconductors with high quality electronic properties are required.

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preprint2016arXiv

Synthesis of a mixed-valent tin nitride and considerations of its possible crystal structures

Recent advances in theoretical structure prediction methods and high-throughput computational techniques are revolutionizing experimental discovery of the thermodynamically stable inorganic materials. Metastable materials represent a new frontier for studies, since even simple binary non ground state compounds of common elements may be awaiting discovery. However, there are significant research challenges related to non-equilibrium thin film synthesis and crystal structure predictions, such as small strained crystals in the experimental samples and energy minimization based theoretical algorithms. Here we report on experimental synthesis and characterization, as well as theoretical first-principles calculations of a previously unreported mixed-valent binary tin nitride. Thin film experiments indicate that this novel material is N-deficient SnN with tin in the mixed II/IV valence state and a small low-symmetry unit cell. Theoretical calculations suggest that the most likely crystal structure has the space group 2 (SG2) related to the distorted delafossite (SG166), which is nearly 0.1 eV/atom above the ground state SnN polymorph. This observation is rationalized by the structural similarity of the SnN distorted delafossite to the chemically related Sn3N4 spinel compound, which provides a fresh scientific insight into the reasons for growth of polymorphs of the metastable material. In addition to reporting on the discovery of the simple binary SnN compound, this paper illustrates a possible way of combining a wide range of advanced characterization techniques with the first-principle property calculation methods, to elucidate the most likely crystal structure of the previously unreported metastable materials.

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preprint2016arXiv

Understanding and Control of Bipolar Doping in Copper Nitride

Semiconductor materials that can be doped both n-type and p-type are desirable for diode-based applications and transistor technology. Copper nitride (Cu3N) is a metastable semiconductor with a solar-relevant bandgap that has been reported to exhibit bipolar doping behavior. However, deeper understanding and better control of the mechanism behind this behavior in Cu3N is currently lacking in the literature. In this work, we use combinatorial growth with a temperature gradient to demonstrate both conduction types of phase-pure, sputter-deposited Cu3N thin films. Room temperature Hall effect and Seebeck effect measurements show n-type Cu3N with 10^17 electrons/cm^3 for low growth temperature (~35 degrees C) and p-type with 10^15-10^16 holes/cm^3 for elevated growth temperatures (50-120 degrees C). Mobility for both types of Cu3N was ~0.1-1 cm^2/Vs. Additionally, temperature- dependent Hall effect measurements indicate that ionized defects are an important scattering mechanism in p-type films. By combining X-ray absorption spectroscopy and first-principles defect theory, we determined that V_Cu defects form preferentially in p-type Cu3N while Cu_i defects form preferentially in n-type Cu3N. Based on these theoretical and experimental results, we propose a kinetic defect formation mechanism for bipolar doping in Cu3N, that is also supported by positron annihilation experiments. Overall, the results of this work highlight the importance of kinetic processes in the defect physics of metastable materials, and provide a framework that can be applied when considering the properties of such materials in general.

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preprint2015arXiv

Accelerated development of CuSbS2 thin film photovoltaic device prototypes

Development of alternative thin film photovoltaic technologies is an important research topic due to the potential for low-cost, large-scale fabrication of high-efficiency solar cells. Despite the large number of promising alternative absorbers and corresponding contacts, the rate of progress is limited by complications that arise during solar cell fabrication. One potential solution to this problem is the high-throughput combinatorial method, which has been extensively used for research and development of individual absorber and contact materials. Here, we demonstrate an accelerated approach to development of thin film photovoltaic device prototypes based on the novel CuSbS2 absorber, using the device architecture employed for CuInxGa(1-x)Se2 (CIGS). The newly developed three-stage, self-regulated CuSbS2 growth process enables the study of PV device performance trends as a function of phase purity, crystallographic orientation, layer thickness of the absorber, and numerous back contacts. This exploration results in initial CuSbS2 device prototypes with ~1% conversion efficiency; currently limited by low short-circuit current due to poor collection of photoexcited electrons, and a small open-circuit voltage due to a cliff-type conduction band offset at the CuSbS2/CdS interface (suggested by first-principles calculations). Overall, these results illustrate the potential of combinatorial methods to accelerate the development of thin film photovoltaic devices with novel absorbers.

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preprint2015arXiv

Combinatorial Insights into Doping Control and Transport Properties of Zinc Tin Nitride

ZnSnN2 is an Earth-abundant analog to the III-Nitrides with potential as a solar absorber due to its direct bandgap, steep absorption onset, and disorder-driven bandgap tunability. Despite these desirable properties, discrepancies in the fundamental bandgap and degenerate \emph{n}-type carrier density have been prevalent issues in the limited amount of literature available on this material. Using a combinatorial RF co-sputtering approach, we have been able to explore a growth-temperature-composition space for Zn(1+x)Sn(1-x)N(2) over the ranges 35-340 degrees C and 0.30-0.75 Zn/(Zn+Sn). In this way, we were able to identify an optimal set of deposition parameters for obtaining as-deposited films with wurtzite crystal structure and carrier density as low as 1.8 x 10^(18) cm^(-3). Films grown at 230 degrees C with Zn/(Zn+Sn) = 0.60 were found to have the largest grain size overall (70 nm diameter on average) while also exhibiting low carrier density (3 x 10^(18) cm^(-3)) and high mobility (8.3 cm^(2) V^(-1) s^(-1)). Furthermore, we report evidence of a Burstein-Moss shift widening the apparent bandgap as cation composition becomes increasingly Sn-rich, and tunable carrier density as a function of cation composition (lower carrier density for higher Zn content), which suggests the formation of defect complexes. Collectively, these findings provide important insight into the fundamental properties of the Zn-Sn-N material system, and also highlight the potential to utilize ZnSnN2 for photovoltaics.

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preprint2015arXiv

CuSbSe2 photovoltaic devices with 3% efficiency

Recent technical and commercial successes of existing thin film solar cell technologies motivates exploration of next-generation photovoltaic (PV) absorber materials. Of particular scientific interest are compounds like CuSbSe$_2$, which do not have the conventional tetrahedral semiconductor bonding. Here, we demonstrate 1.5 μm thick CuSbSe$_2$ PV prototypes prepared at 380-410°C by a self-regulated sputtering process using the conventional substrate device architecture. The p-type CuSbSe$_2$ absorber has a 1.1 eV optical absorption onset, ~$10^{5}$ cm$^{-1}$ absorption coefficient at 0.3 eV above the onset, and a hole concentration of ~10$^{17}$ cm$^{-3}$. The promising >3% energy conversion efficiency (Jsc = 20 mA/cm$^2$, FF = 0.44, Voc = 0.35 V) in these initial devices is limited by bulk recombination that limits photocurrent, device engineering issues that affect fill factor, and a photovoltage deficit that likely results from the non-ideal CuSbSe2/CdS band offset.

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preprint2015arXiv

Effects of disorder on carrier transport in Cu$_2$SnS$_3$

In recent years, further improvements in the efficiency of Cu$_2$ZnSn(S,Se)$_4$ photovoltaic devices have been hampered due to several materials issues, including cation disorder. Cu$_2$SnS$_3$ is a promising new absorber material that has attracted significant interest in recent years. However, similar to CZTS, Cu$_2$SnS$_3$ displays cation disorder. In this work, we develop synthetic techniques to control the disorder in Cu$_2$SnS$_3$ thin films. By manipulating the disorder in this material, we observe crystal structure changes and detect improvements in the majority carrier (hole) transport. However, when the minority carrier (electron) transport was investigated using optical pump terahertz probe spectroscopy, minimal differences were observed between the ordered and disordered Cu$_2$SnS$_3$. By combining these results with first-principles and Monte Carlo theoretical calculations, we are able to conclude that even ostensibly "ordered" Cu$_2$SnS$_3$ displays minority carrier transport properties corresponding to the disordered structure. The presence of extended planar defects in all samples, observed in TEM imaging, suggests that disorder is present even when it is not detectable using traditional structural characterization methods. The results of this study highlight some of the challenges to the further improvement of Cu$_2$SnS$_3$-based photovoltaics, and have implications for other disordered multinary semiconductors such as CZTS.

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preprint2015arXiv

Effects of Low Temperature Annealing on the Transport Properties of Zinc Tin Nitride

ZnSnN2 has recently garnered increasing interest as a potential solar absorber material due to its direct bandgap that is predicted to be tunable from 1.0-2.1 eV based on cation disorder. One important challenge to the further development of this material for photovoltaics (PV) is to reliably synthesize films with carrier density less than or equal to 10^17 electrons/cm^3. In this work, we perform a systematic annealing study on compositionally-graded Zn-Sn-N thin films to determine the effects on carrier density and transport of such post-growth treatment. We find that annealing up to 6 hr under an activated nitrogen atmosphere results in a reduction in carrier density by ~80% for zinc-rich films, and by ~50% for stoichiometric films. However, we also find that annealing reduces mobility as a function of increasing annealing time. This result suggests that initial film disorder hampers the benefits to film quality that should have been gained through annealing. This finding highlights that carefully managed initial growth conditions will be necessary to obtain PV-quality ZnSnN2 absorber films.

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preprint2014arXiv

Electronic band structure and ambipolar electrical properties of Cu2O based semiconductor alloys

Tuning the opto-electronic properties through alloying is essential for semiconductor technology. Currently, mostly isovalent and isostructural alloys are used (e.g., group-IV and III-V), but a vast and unexplored space of novel functional materials is conceivable when considering more complex alloys by mixing aliovalent and heterostructural constituents. The real challenge lies in the quantitative property prediction for such complex alloys to guide their experimental exploration. We developed an approach to predict compositional dependence of both band-structure and electrical properties from ab-initio calculations by extending conventional dilute defect model to higher (alloy) concentrations. Considering alloying of aliovalent (Mg, Zn, Cd) cations and isovalent anions (S, Se) into Cu2O, we predict tunability of band-gap energies and doping levels over a wide range, including the type conversion from p- to n-type. Initial synthesis and characterization of Zn and Se substituted Cu2O support the defect model, suggesting these alloys as promising novel oxide semiconductor materials.

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