Session B67: Advanced Approaches in Modeling and Simulation of Defects

Computational Approaches to Defects and Doping in Non-Ideal Semiconductors

To account for the complexities of real functional materials, we are developing computational approaches for the first-principles prediction of the defect and doping properties in highly nonideal semiconductor materials. These issues include nonequilibrium growth and process conditions, disorder, and off-stoichiometry resulting from impurities interacting with defects. Application areas include nitride-oxide solid solutions [1], Fermi level engineering in Ga₂O₃ [2], and transition metal oxides for solar thermochemical hydrogen production [3].

[1] J. Pan, J. Cordell, G.J. Tucker, A.C. Tamboli, A. Zakutayev, S. Lany, “Interplay between Composition, Electronic Structure, Disorder, and Doping due to Dual Sublattice Mixing in Nonequilibrium Synthesis of ZnSnN₂:O “, Adv. Mater. 1807406 (2019). DOI: 10.1002/adma.201807406

[2] A. Goyal, A. Zakutayev, V. Stevanovic, S. Lany, “Computational Fermi level engineering and doping-type conversion of Mg:Ga₂O₃ via three-step synthesis process“, J. Appl. Phys. 129, 245704 (2021). DOI: 10.1063/5.0051788

[3] S.L. Millican, J.M. Clary, C.B. Musgrave, S. Lany, “Redox defect thermochemistry of FeAl2O4 hercynite in water-splitting from first principles methods” (unpublished)   

Accurate Excitations of the NV- Defect in Diamond via Embedding with Auxilliary-Field Quantum Monte Carlo

Predictive capability for excitations in defects would enable better understanding of the defects that occur in materials, and significantly improve our ability to engineer defects for applications, such as qubits.  Dilute defect systems require large simulations and defects often host correlated states that require accurate and robust computations to treat.  We present high-accuracy first-principles auxiliary-field quantum Monte Carlo (AFQMC) calculations applied to large systems via an embedding approach. This system contains a set of charged states with both closed-shell, weakly correlated states as well as open-shell, strongly correlated states, including the experimentally well-characterized triplet and singlet excitations. This provides a challenging realistic system to thoroughly test the accuracy of our approach.  Utilizing the favorable scaling of AFQMC, we systematically study the convergence of our embedding approach by including up to hundreds of electrons in the embedded space.  We determined the most effective procedure for converging excitations, and the degree of locality of the excitations, with possible implications to many other embedding approaches. The computed excitation energies are in very good agreement with experiment. We anticipate this method will become a general approach for first-principles characterization of correlated defects.   

Ab-initio investigation of Er3+ defects in tungsten disulfide

We use density functional theory (DFT) to explore the physical properties of an Er_W point defect in monolayer WS₂. Our calculations indicate that electrons localize at the dangling bonds associated with a tungsten vacancy (V_W) and at the Er³⁺ ion site, even in the presence of a net negative charge in the supercell. The system features a set of intra-gap defect states, some of which are reminiscent of those present in isolated Er³⁺ ions. In both instances, the level of hybridization is low, i.e., orbitals show either strong Er or W character. Through the calculation of the absorption spectrum as a function of wavelength, we identify a broad set of transitions, including one possibly consistent with the Er^{3+ 4}I_15/2→⁴I_13/2 observed in other hosts. Combined with the low native concentration of spin-active nuclei as well as the two-dimensional nature of the host, these properties reveal Er:WS₂ as a potential platform for realizing spin qubits that can be subsequently integrated with other nanoscale optoelectronic devices.   

First-principles ionized-impurity scattering and charge transport in doped materials

Ionized impurity scattering governs charge transport in doped semiconductors. However, understanding of the interactions between electrons and ionized impurities relies heavily on simplified models. In this talk, we show an ab initio approach to compute the interactions between electrons and ionized impurities (or other charged defects) with quantitative accuracy [1]. Our approach includes both short- and long-range electron-defect (e-d) interactions on the same footing, takes the atomic structure of the defect into account, and allows for efficient computation and interpolation of the e-d matrix elements. With this novel tool in hand, we combine the e-d and electron-phonon interactions in the Boltzmann transport equation. We show calculations of the carrier mobility in a doped material (silicon) over a wide range of temperature and doping concentrations, spanning seamlessly the defect- and phonon-limited transport regimes. The individual contributions of the defect- and phonon-scattering mechanisms to the carrier relaxation times and mean-free paths are analyzed. The method presented in this talk provides a powerful tool to study electron interactions in doped semiconductors and oxides, with applications in electronics, energy, and quantum technologies.   

Machine Learning Defect Properties of Semiconductors

Defects and impurities in semiconductors can reduce photovoltaic absorption via nonradiative recombination of charge carriers or enhance absorption via intermediate bands. Defect levels in the band gap may also be used as qubits for quantum computing. Quick and accurate predictions of defect properties are thus desired in technologically important semiconductors, but are complicated by difficulties in sample preparation and assigning measured levels to specific defects, as well as by the expense of large-supercell first principles computations that involve charge corrections and advanced functionals. In this work, we address this issue by combining high-throughput density functional theory (DFT) with machine learning (ML) to develop predictive models for defect formation energy and charge transition levels in (a) ABX₃ halide perovskites, and (b) zincblende group IV, III-V and II-VI semiconductors. ML models utilize unique encoding of the defect atom’s elemental properties, coordination environment, and cheaper DFT properties, along with rigorous training using random forests, Gaussian processes, and neural networks. The extensive DFT datasets and best ML models are made available as online tools for easy prediction and screening across large semiconductor-defect chemical spaces.   

First-principles study of planar Humble defects in Ge and GeSi alloys

Planar defects are widespread in group IV elements. The Humble defect is a special type of {001} planar defect that has gained much attention in recent years since its experimental observation in Ge and Ge_0.8Si_0.2. A detailed study of the electronic properties of Humble defects is still lacking in the literature. In this work, we perform first-principles density functional theory (DFT) calculations to study Humble defects in both Ge and Ge_0.8Si_0.2. We further compare our theoretical results with Si L3 edge electron energy loss spectra (EELS) measured at room temperature, finding excellent agreement, but only when core-hole effects are accounted for. Moreover, a comparison of the EELS observed in the vicinity of the Humble defect versus in the bulk provides a fingerprint for different types of local atomic-bonding environments in Ge_0.8Si_0.2. Our DFT calculations reveal that Humble defects can enlarge the electronic band gap, and hence may potentially be used in band engineering. DFT studies using a hybrid functional, which provides an improved description of band gaps, are also presented.   

Prospects for n-type conductivity in cubic boron nitride

Cubic boron nitride is an ultra-wide-band-gap semiconductor with a range of useful properties, including high breakdown field, high thermal conductivity, and excellent chemical stability. These properties make cubic boron nitride a leading candidate for applications in power electronics, deep-ultraviolet optoelectronics, and quantum information science. Realizing these applications is predicated on the ability to achieve control over doping. By employing advanced first-principles calculations, we systematically explore group-IV (C, Si, and Ge) and group-VI (O, S, and Se) elements, as well as Li and F, as potential n-type dopants. We identify Si_B and O_N as the most promising dopants due to their low formation energy and resistance to self-compensation. However, we also find native boron vacancies, which are deep acceptors, can be a source of compensation. We examine vacancy migration and suggest that control over growth kinetics is necessary to achieve n-type conductivity in cubic boron nitride.   

Impurity Auger recombination in gallium nitride from first principles

Auger recombination is a nonradiative recombination mechanism in which the annihilation of an electron and a hole results in excitation of a third carrier to an excited state.  Direct and indirect Auger recombination in the bulk of a material is known to limit the performance of optoelectronic devices. Impurity Auger recombination, in which the recombination involves defects or impurities in the material, may also be important, but first-principles studies of it are lacking. Such a process has been suggested to occur in gallium nitride, which is an essential semiconductor material for applications including blue light emitting diodes (LEDs) and power electronics. Using our first-principles methodology, we calculate the impurity Auger rate for select impurities in gallium nitride and study its impact on efficiency of gallium-nitride-based LEDs.   

Quantum oscillatory interaction between isovalent centers in semiconductors

Interaction between isovalent centers is of great interest in device physics. We discovered a quantum oscillatory interaction based on the first principles calculations of two identical isovalent centers in C/Ge/Sn co-doped Si. The interaction is explained by Green's function's analysis and linear combination of atomic orbitals (LCAO) method. One point defect interacts with another by a product between the defect potentials and the summation term that characterizes the metallization process of the host lattice. The trend of the oscillation is an intrinsic property of the host. The interaction mechanism is further verified by the calculations of the isovalent pairs with different elements. Our works shed light on the precise control of defects in semiconductors.   

Molecular dopants in LiGaO2: N2, NO and O2 in Ga and Li vacancies

LiGaO₂ is an ultrawide band gap tetrahedrally bonded semiconductor (E_g = 5.6 eV) which was recently shown by first-principles calculations to be n-type dopable with Si or Ge. Here we investigate using N₂, NO, and O₂ molecules placed in either Ga or Li-vacancies as potential acceptors for p-type doping. Their optimal position and orientation relative to the lattice, as well as their transition levels and formation energy, are investigated. We use a realistic oxygen chemical potential based on the growth condition. The energy of formation of the (N₂)_Li is lower than that of (N₂)_Ga, which reflects the similar ordering of the vacancy formation energies.  Both (N₂)_Ga and (NO)_Ga show a negative U behavior, with transition levels of 1-/3 and 0/2-, respectively. Unfortunately, all transition levels are quite deep, with the lowest 0/1- transition level for the (N₂)_Li case occurring at approximately 1.57 eV above the VBM. As a result, p-type doping is impossible to achieve with these molecular dopants. We discuss their energy levels in relation to the corresponding Ga and Li vacancies.   

Tuning of Band Gap in Doped Diamonds

Diamond as a material of extreme properties has already proved to be a very promising candidate for a vast variety of electronic applications, such as power switches, Schottky diodes, FETs and photovoltaics. To harness diamond properties at best, it is then crucial to understand how its entangled geometric and electronic properties determine the band gap.

To this aim, we conducted a quantum mechanical research on diamond structure doped with five different atoms (Al, B, N, P, Si) in three different concentrations (0.78%, 1.85%, 6.25%).  We performed advanced geometric and electronic analyses on the ground state structures, such as orbital polarization, bond covalency and Hirshfeld charges; in this way, we understood how to direct the spatial electronic density in order to obtain the desired band gap width.  We also propose guidelines on how to tune the character (direct/indirect) of the band gap and how such character is governed by the electronic distribution.

The outcomes of our investigation provide a compact source of information for further use in experimental and technical fields and can lead to the creation of the new semiconducting materials.   

Interstitials of binary rock salt compounds

The energetic and mechanical stability of interstitial point defects in binary rock salt materials was studied using first-principles methods. A novel, stable, and energetically competitive interstitial site (base-interstitial) was identified for anion interstitials in rock salts. The formation energies of base-interstitial defects were compared with well-explored tetrahedral (body-interstitial) and split interstitials and were found to be highly competitive energetically. For alkali halides, the lowest formation energies are associated with the base-interstitial site together with the <110> split interstitial, and these are therefore the predominant interstitial sites. Electronic band structures are affected by the presence of interstitial defects in rock salt structures. In particular, the Fermi level is shifted below the valence band maxima for the body, base, and split interstitials in metal halides, indicating p-type conductivity. However, the Fermi level remains within the bandgap for metal monochalcogenides, indicating no preferred conductivity for base- and split-interstitial defects. The discovery of a new interstitial site affects our understanding of defects in binary rock salts, including structure and dynamics as well as associated thermodynamic and kinetic properties that are interstitial dependent.   

Hyperfine Constant and Binding Energy Calculation of Shallow Donor in Silicon from Pseudopotential and All-Electron Mixed Approach

Design of robust silicon-processing infrastructure based on spin qubits requires an accurate modeling of hyperfine coupling interaction and s manifold energy levels for donors in silicon. However, the well-known ab-initio methods have so far been lacking the accuracy in predicting these properties due to the large spatial extent of wavefunction of shallow donors in silicon. The present work employs density functional theory (DFT) with pseudopotential and all-electron mixed approach as well as hybrid functional working in tandem and permits efficient calculations for systems containing more than 4000 atoms. Remarkable accuracy in the prediction of hyperfine coupling interaction and s manifold energy levels has been achieved for single phosphorus donor in silicon, including: Fermi contact hyperfine constant (116.0MHz), quadratic Stark coefficient (-2.65×10^-3μm²/V²), binding energy of the ground state (44.8meV), and energies of  the excited 1sT₂ (35.9meV) and 1sE (34.5meV) states. Additionally, accurate computations of super-hyperfine parameters have been achieved, showing excellent agreement with the well-known Green’s functional approach.