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.
Zinc tin nitride (ZnSnN2) is attracting growing interest as a non-toxic and earth-abundant photoabsorber for thin-film photovoltaics. Carrier transport in ZnSnN2 and consequently cell performance are strongly affected by point defects with deep levels acting as carrier recombination centers. In this study, the point defects in ZnSnN2 are revisited by careful first-principles modeling based on recent experimental and theoretical findings. It is shown that ZnSnN2 does not have low-energy defects with deep levels, in contrast to previously reported results. Therefore, ZnSnN2 is more promising as …
Physical Properties and Thermal Stability of Zirconium Platinum Nitride Thin Films
ETH Zurich（苏黎世联邦理工学院）；Laboratory for Nanometallurgy, Department of Materials, ETH Zurich（苏黎世联邦理工学院材料系纳米冶金实验室）；Ion Beam Physics, Department of Physics, ETH Zurich（苏黎世联邦理工学院物理系离子束物理）
Ground state properties of SiC, AlN, GaN and InN in the zinc-blende and wurtzite structures are determined using an ab initio scheme. For the self-consistent field part of the calculations, the Hartree-Fock program Crystal has been used. Correlation contributions are evaluated using the coupled-cluster approach with single and double excitations. This is done by means of increments derived for localized bond orbitals and for pairs and triples of such bonds. A comparison between the ground state energies of the zinc-blende and wurtzite structure is made: At the Hartree-Fock level, it turns out …
A simple theoretical method for deducing the effective bond-orbital model (EBOM) of III-nitride wurtzite (WZ) semiconductors is presented. In this model, the interaction parameters for zinc-blende (ZB) structures are used as an initial guess for WZ structure based on the two-center approximation. The electronic band structure of III-nitride WZ semiconductors can hence be produced by utilizing this set of parameters modified to include effects due to three-center integrals and fitting with first-principles calculations. Details of the semi-empirical fitting procedure for constructing the EBOM H…
We report calculated, electronic and related properties of wurtzite and zinc blende gallium nitrides (w-GaN, zb-GaN). We employed a local density approximation (LDA) potential and the linear combination of atomic orbital (LCAO) formalism. The implementation of this formalism followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). The calculated electronic and related properties, for both structures of GaN, are in good agreement with corresponding, experimental data, unlike results from most previous ab initio calculations utilizing a density function…
Photoelectron spectroscopy is an important tool for the development of new materials. However, especially for nitride semiconductors, the formation of surface oxides, surface band bending as well as the lack of a suitable charge reference often prevent a robust analysis. Here, we perform a comprehensive chemical state analysis of the Zn-V-N phase space using the Auger parameter concept, which is less sensitive to such uncertainties. Phase-pure Zn2VN3, VN, and Zn3N2 samples are analyzed using XPS/HAXPES after transfer in inert-gas atmosphere. In addition, high-throughput chemical state analysis…
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 un…
Accurate values for polarization discontinuities between pyroelectric materials are critical for understanding and designing the electronic properties of heterostructures. For wurtzite materials, the zincblende structure has been used in the literature as a reference to determine the effective spontaneous polarization constants. We show that, because the zincblende structure has a nonzero formal polarization, this method results in a spurious contribution to the spontaneous polarization differences between materials. In addition, we address the correct choice of "improper" versus "proper" piez…
Developing a valence force field model for wurtzite semiconductors by exploiting similarities with [111]-oriented zinc blende systems: The case of wurtzite boron nitride, III-N materials and (B,In,Ga)N alloys
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…
The elasticity and piezoelectricity of zinc oxide (ZnO) crystals and single layers are investigated from the first-principles calculations. It is found that a ZnO thin film less than three Zn-O layers prefers a planar graphite-like structure to the wurtzite structure. ZnO single layers are much more flexible than graphite single layers in the elasticity and stronger than boron nitride single layers in the piezoelectricity. Single-walled ZnO nanotubes (SWZONTs) can exist in principle because of their negative binding energy. The piezoelectricity of SWZONTs depends on their chirality. For most Z…
Studies of Cr-doped III-V nitrides, dilute magnetic alloys, in the zinc-blende crystal structure are presented. The objective of the work is to investigate half-metallicity in Al(0.75)Cr(0.25)N, Ga(0.75)Cr(0.25)N and In(0.75)Cr(0.25)N for their possible application in the spin based electronic devices. The calculated spin polarized band structures, electronic properties and magnetic properties of these compounds reveal that Al0.75Cr0.25N and Ga0.75Cr0.25N are half-metallic dilute magnetic semiconductors while In0.75Cr0.25N is metallic in nature. The present theoretical predictions provide evid…
Possible crystalline modifications of chemical compounds at low temperatures correspond to local minima of the energy landscape. Determining these minima via simulated annealing is one method for the prediction of crystal structures, where the number of atoms per unit cell is the only information used. It is demonstrated that this method can be applied to covalent systems, at the example of boron nitride, using ab initio energies in all stages of the optimization, i.e. both during the global search and the subsequent local optimization. Ten low lying structure candidates are presented, includi…
Although Boron nitride (BN) is a well-known compound widely used for engineering and scientific purposes, the phase stability of its polymorphs, one of its most fundamental properties, is still under debate. The ab initio determination of the ground state of the BN polymorphs, such as hexagonal and zinc-blende, is difficult because of the elusive Van der Waals interaction, which plays a decisive role in some of the polymorphs, making quantitative prediction highly challenging. Hence, despite multiple theoretical studies, there has been no consensus on the ground state yet, primarily due to con…
Binary III-N nitride semiconductors with wurtzite crystal structure such as GaN and AlN have been long used in many practical applications ranging from optoelectronic to telecommunication. The structurally related ZnGeN2 or ZnSnN2 derived from the parent binary compounds by cation mutation (elemental substitution) have recently attracted attention, but such ternary nitride materials are mostly limited to II-IV-N2 compositions. This paper reports on synthesis and characterization of zinc niobium nitride Zn2NbN3 - a previously unreported II2-V-N3 ternary nitride semiconductor. The Zn2NbN3 thin f…
Many commonly used nearest neighbor tight binding models for cubic semiconductors often result in inaccurate band structures when transferred to hexagonal polytypes. The resulting bandgaps are systematically too small, and in some cases calculations may erroneously predict a metal for the wurtzite polytype. We have calculated a set of $\rm spds^*$ tight binding parameters for the hexagonal phases of non-nitride III-V and group V semiconductors fit to experimental data and empirical pseudopotential calculations. Our fitting procedure constrains the parameters to be as close as possible to those…
Structural and electronic properties of hypothetical zinc blende Tl(x)Ga(1-x)N alloys have been investigated from first principles. The structural relaxation, preformed within the LDA approach, leads to a linear dependence of the lattice parameter a on the Tl content x. In turn, band structures obtained by MBJLDA calculations are significantly different from the corresponding LDA results. The decrease of the band-gap in Tl-doped GaN materials (for x<0.25) is predicted to be a linear function of x, i.e. 0.08 eV per atomic % of thallium. The semimetallic character is expected for materials with …
The electronic properties of lanthanide (from Eu to Tm) impurities in wurtzite gallium nitride and zinc oxide were investigated by first principles calculations, using an all electron methodology plus a Hubbard potential correction. The results indicated that the 4f-related energy levels remain outside the bandgap in both materials, in good agreement with a recent phenomenological model, based on experimental data. Additionally, zinc oxide doped with lanthanide impurities became an n-type material, showing a coupling between the 4f-related spin polarized states and the carriers. This coupling …
The bandstructure of the zinc-blende phase of AlN, GaN, InN is calculated employing the exact-exchange (EXX) Kohn-Sham density-functional theory and a pseudopotential plane-wave approach. The cation semicore d electrons are treated both as valence and as core states. The EXX bandgaps of AlN and GaN (obtained with the Ga 3d electrons included as core states) are in excellent agreement with previous EXX results, GW calculations and experiment. Inclusion of the semicore d electrons as valence states leads to a large reduction in the EXX bandgaps of GaN and InN. Contrary to common belief, the remo…
In this study, by means of classical molecular dynamics simulations, we investigated the thermal transport properties of hexagonal single-layer, zinc-blend and wurtzite phases of BN, AlN, and GaN crystals, which are very promising for the application and design of high-quality electronic devices. With this in mind, we generated fully transferable Tersoff-type empirical inter-atomic potential parameter sets by utilizing an optimization procedure based on particle swarm optimization. The predicted thermal properties as well as the structural, mechanical and vibrational properties of all material…
A number of diverse bulk properties of the zincblende and wurtzite III-V nitrides AlN, GaN, and InN, are predicted from first principles within density functional theory using the plane-wave ultrasoft pseudopotential method, within both the LDA (local density) and GGA (generalized gradient) approximations to the exchange-correlation functional. Besides structure and cohesion, we study formation enthalpies (a key ingredient in predicting defect solubilities and surface stability), spontaneous polarizations and piezoelectric constants (central parameters for nanostructure modeling), and elastic …
The spontaneous polarization of wurtzite III-V nitrides XN (X=Al, Ga, In) and II-VI oxides YO (Y=Be, Zn) is investigated via first-principles computational methods. The modern treatment defines this quantity as the polarization difference between the investigated system and an appropriate reference state. We demonstrate that the trigonal structure can be used as a reference to determine the spontaneous polarization of wurtzite materials. We compare the current values with the widely-known zincblende results reported in the literature and find a very good agreement. It is shown that the electro…
While non-nitride III-V semiconductors typically have a zincblende structure, they may also form wurtzite crystals under pressure or when grown as nanowhiskers. This makes electronic structure calculation difficult since the band structures of wurtzite III-V semiconductors are poorly characterized. We have calculated the electronic band structure for nine III-V semiconductors in the wurtzite phase using transferable empirical pseudopotentials including spin-orbit coupling. We find that all the materials have direct gaps. Our results differ significantly from earlier {\it ab initio} calculation…
