Session F05: Defects and Doping in III-Vs and their Alloys

doped: a python package for solid-state defect and dopant calculations

Point defects are a universal feature of crystalline materials. Computational methods (DFT, quantum embedding, GW...) are widely used to predict defect behavior, before combining and comparing predictions with experimental measurements. However, there are many critical stages in the computational workflow for defects, which, when performed manually, not only leave room for human error but also consume significant researcher time and effort. Moreover, there are growing efforts to perform high-throughput investigations of defects in solids, necessitating robust, user-friendly and efficient software implementing this calculation workflow.

Here we report doped, our python package for the full generation, calculation setup, post-processing and analysis of defect supercell calculations.^2–5 The generation and thermodynamic analysis (i.e. defect formation energy diagrams, chemical potentials & stability region, doping analysis etc.) are agnostic to the underlying first-principles software, while input file generation is supported for several of the most widely-used DFT codes, including VASP, FHI-aims, CP2k, Quantum Espresso and CASTEP. A defect charge state prediction algorithm is implemented, which is shown to significantly outperform previous oxidation-state approaches in terms of both efficiency and completeness. Moreover, doped is built to be compatible with other computational toolkits for advanced defect characterisation, including ShakeNBreak⁶ for defect structure-searching, py-sc-fermi⁷ for in-depth concentration and Fermi level analysis, and CarrierCapture.jl⁸/nonrad⁹ for non-radiative recombination calculations. Its object-oriented python framework make it readily-usable in high-throughput architectures such as atomate(2) or AiiDA, with examples included in the documentation.

We will discuss the key features of doped for computational defect workflows, exemplified with relevant systems (CdTe, t-Se, Y₂Ti₂O₅S₂). We anticipate that doped will serve as a highly useful tool for computational defect researchers, being an efficient platform for conducting reproducible calculations of solid-state defect properties.   

Accelerating GW calculations of point defects with the defect-patched screening approximation

GW method is considered as an established ab initio tool for calculating defect levels. However, the GW simulation cost increases dramatically with the system size, and, unfortunately, large supercells are often required to model low-density defects that are experimentally relevant. In this work, we propose to accelerate GW calculations of point defects by reducing the simulation cost of the many-electron screening, which is the primary computational bottleneck. The random-phase approximation of many-electron screening is divided into two parts: one is the intrinsic screening, calculated using a unit cell of pristine structures, and the other is the defect-induced screening, calculated using the supercell within a small energy window. Depending on specific defects, one may only need to consider the intrinsic polarizability or include the defect contribution. This approach avoids the summation of many conductions states of supercells and significantly reduces the simulation time. We have applied it to calculating various point defects, including neutral and charged defects in two-dimensional and bulk systems with small or large energy gaps. The results consist with direct GW results, and the agreements are further improved at the dilute-defect limit, which is experimentally relevant but extremely challenging for direct GW simulations. This defect-patched screening approach clarifies the roles of defects in many-electron screening and can fast screen defect structures/materials for novel applications, including single-photon sources, quantum qubits, and quantum sensors.   

Characterizing 3D Defect Networks in GaAs Nanowires using Coherent X-ray Diffractive Imaging

Bragg Coherent Diffractive Imaging (BCDI) is a sophisticated X-ray technique proficient in nondestructively probing internal strain fields, defects, and 3D networks of dislocations within crystalline samples. Capitalizing on the coherent properties of X-rays, it attains nanoscale resolution, making it invaluable for materials science. In our recent study, we employed BCDI to investigate an as-grown GaAs nanowire, a material paramount in the semiconductor realm due to its optimal direct bandgap for photovoltaic applications and diverse morphological adaptability. Our analysis unveiled a multifaceted 3D network of dislocations and stacking faults, intriguingly manifesting a core-shell architecture in the wire without necessitating multiple growth phases or external treatments. This in-depth characterization elucidates the inherent defects and corresponding long-range strain, accentuating BCDI's prowess in achieving picometer strain resolution and detecting a broad spectrum of defects. This research underscores the potential of BCDI for holistic 3D analysis of nanostructured materials.   

Influence of Non-stoichiometry and Local Atomic Environments on Carrier Transport in GaAs1-x-yNxBiy Alloys

Due to the significant bandgap narrowing induced by the incorporation of dilute fractions of N and Bi into GaAs, dilute nitride-bismide alloys are of interest for optoelectronic devices operating in the near- to mid-infrared range. However, the low substrate temperatures (≤ 400°C) required to incorporate both N and Bi into GaAs during molecular-beam epitaxy (MBE) lead to non-stoichiometry, primarily due to excess arsenic incorporation. Thus, non-stoichiometry should be considered to understand the electronic properties of GaAsNBi. Here, we have investigated the influence of non-stoichiometry and local atomic environments on carrier transport in GaAs(N)Bi alloy films using local-electrode atom probe tomography (LEAP) and high-resolution x-ray diffraction (HRXRD) in conjunction with time-resolved THz photoconductivity measurements. Through direct measurement of the excess As concentration ([As]_excess) using LEAP, quantified by calibration using HRXRD, we demonstrate that [As]_excess increases with decreasing substrate temperature and observe that it is suppressed in layers with y_Bi > 0.035. The local concentrations of N and Bi, as well as Bi pair-correlations, are also quantified using LEAP. Using time-resolved THz photoconductivity measurements, we show that carrier transport is primarily limited by excess As, with the highest carrier mobilities for layers with y_Bi  > 0.035.   

Rare-earth impurities in III-V semiconductors and their alloys

Rare-earth impurities in semiconductors have long been studied theoretically and experimentally, with implications for both basic and applied sciences. For example, Er-doped GaAs, known for its sharp and temperature-independent 4f-intrashell emission near 1.55 μm, aligns with the lowest attenuation wavelength of silica-based optical fibers. Early experimental results suggested that Yb substitute on the In in InP, while Er was found to occupy interstitial sites. The location of the rare-earth impurity in the lattice of the semiconductor may well be related to how it modifies the electronic and optical properties of the host material. In this presentation, we discuss the interaction between rare-earth doping and III-V semiconductors, such as AlAs, GaAs, and InAs, and their alloys. Using first-principles calculations based on hybrid density functional theory, we explore the incorporation of La, Gd, Er, and Lu into interstitial and substitutional sites of the zinc blende lattice, considering possible charge states of the impurity other than the neutral state. The choice of rare-earth impurities covers the different occupations of the 4f shell, from completely unoccupied to half occupied, to fully occupied. We analyze the stability of the rare-earth impurity configuration not only through their formation energy but also through their migration barriers. Our calculations provide valuable insights into the behavior and properties of rare-earth elements as dopants in semiconductors, contributing to a better understanding of their potential to enhance the performance of III-V semiconductor devices.   

Onset of tetrahedral interstitial formation in GaAsN alloys

Due to the dramatic bandgap reductions induced by dilute N concentrations, dilute nitride alloys are useful for near-to-mid-infrared devices. However, several studies have linked non-substitutional N incorporation to diminished absorption and emission efficiencies. Meanwhile, most computational studies have focused on the relative stabilities of substitutional nitrogen (NAs) and (N-N)As and (N-As)As split interstitials, with minimal consideration of N tetrahedral interstitials (Ntetra). To date, (N-As)As and (N-N)As have been observed via channeling NRA (NRA/c) and x-ray photoelectron spectroscopy (XPS). However, direct detection of Ntetra has not been reported. Here, we probe N incorporation mechanisms using combined experimental and computational NRA and Rutherford Backscattering spectrometry (RBS) angular yield scans. For xN< 0.025, both NAs and split interstitials are observed. However, for xN ≥ 0.025, evidence for Ntetra emerges. We discuss the possible role of interacting strain fields between N atoms as the driving force for Ntetra incorporation. This work opens opportunities for consideration of Ntetra and its influence on the properties of a variety of highly mismatched alloys.   

Lateral strain in InGaAsSb epitaxial layers grown by LPE: Effect on structural and optical properties

In(0.145)Ga(0.855)As(y)Sb(1-y) epilayers were grown on GaSb(100) substrates by liquid phase epitaxy. Lattice mismatch was analyzed for different As concentrations (y) by high-resolution X ray diffraction (HR-XRD). Epilayers with 0.120 < y < 0.124 showed a positive lattice mismatch, leading to compresive strain. These samples present high crystalline quality and flat surfaces as measured by HR-XRD and atomic force microscopy (AFM). For InGaAsSb layers with As concentrations in the range 0.133 < y < 0.141 a negative lattice mismatch was observed, resulting in tensile strain. Structural defects in these InGaAsSb quaternary layers were clearly shown on the HR-XRD and on the AFM micrographs. Raman spectroscopy measurements on these layers also revealed that lateral strain has a direct impact on the intensities of the LO-like, phonon-plasmon, and disorder-activated LA modes. For all the In(0.145)Ga(0.855)As(y)Sb(1-y) epilayers, the photoluminescence (PL) spectra showed a bound exciton (BE) transition around 635 meV for layers under compresive strain, and BE transition around 637 meV for layers under tensile strain, with additional features observed in samples under tensile strain like donor-to-acceptor pair (DAP) recombination and defect-related emission bands. This study provides new insights into the effect of lateral strain on the crystalline and surface quality, and its effect on the optical properties of the quaternary alloys, which are relevant for novel optoelectronic applications.   

Electronic properties of InAlAs and InGaAs alloys containing a few percent of Bi

Adding a few percent of Bi to III-V conventional semiconductors such as GaAs, InAs, and AlAs, leads to drastic changes in their electronic properties. The band gaps decrease and the spin-orbit splitting increases by sizeable amounts, with a small lattice mismatch to the parent compound, enabling interesting heterostructure materials for optoelectronic applications. In this work, we present a theoretical study of the electronic and optical properties of Bi-alloyed InAlAs and InGaAs ternary alloys using hybrid functional HSE06 calculations. We investigate the effects of adding a few atomic percent of Bi to these alloys, analyzing the changes in the band gap, spin-orbit coupling, and band alignments. We find that Bi doping reduces the band gap and enhances the spin-orbit coupling in both InAlAs and InGaAs alloys, resulting in tunable band gaps and band alignments that are of interest to heterostructure devices. Our results provide useful insights for the design and optimization of dilute InAlAsBi and InGaAsBi quaternary alloys that are promising for infrared device applications.   

Defects in as-processed, irradiated, and stressed GaAs-based device structures

GaAs and its semiconductor alloys are of increasing interest for use in highly-scaled, high-frequency CMOS and 3-D ICs. In devices with insulating layers, charge trapping by defects in the gate oxide often dominates the device’s radiation response. In devices without these insulating layers, process maturity has generally decreased the impact of many well-understood defects leading to the emergence of a wider range of defects and impurities limiting device performance and reliability. In this work, we begin with a re-evaluation of the origin of observed significant increases in thermal generation rates for p-i-n-i-p GaAs structures attributed to depassivation of hydrogen-defect/impurity complexes during electron-beam irradiation. Density-functional-theory (DFT) calculations of defect energy levels and reaction pathways suggest depassivation of O_As-H complexes as a likely explanation. Turning to more contemporary GaAs/AlGaAs-based pseudomorphic high-electron mobility transistors, 1/f noise spectra show three prominent peaks associated with defect activation energies. Through a combination of DFT calculations and literature review, we discuss plausible origins of these peaks, including oxygen-based defects, DX centers, and other impurities.   

Atomic scale analysis of N dopants in InAs using DFT and X-STM

The band gap of most III-V semiconductors is strongly reduced with the introduction of only a few percent of N, even if the III-N alloy has a much bigger band gap. N impurities in InAs introduce an impurity state around 1 eV above the conduction band minimum, much deeper in the band than in other III-V materials. Topographic scanning tunneling spectroscopy measurements (STS) and areal spectroscopy measurements performed on N atoms up to two layers below the (110) surface of InAs show a reduction of the resonance energy of the N atom with increasing depth. This is attributed to tip induced band bending, pulling the N states up at positive bias and acting most strongly on surface N atoms. STS measurements obtained on undoped InAs and N-doped InAs show a band gap reduction of <0.1 eV. Spacial imaging of features corresponding to N dopants up to two layers below the surface are also compared to density functional theory simulations and show excellent correspondence. Spectroscopy maps of N atoms up to two layers below the surface provide a high resolution spatial and spectroscopic view of the N atoms. Here the characteristic shape of the N atoms in different layers below the surface is observed as an enhancement of the dI/dV signal compared to the InAs background. At energies above the enhancement a reduction of the dI/dV is observed, which has the same shape and size as the enhancement. This shows that the redistribution of density of states caused by the N impurities is mainly energetic in nature.   

Tuning van der Waals heterostructures and moiré materials with near-field electrostatics

Van der Waals heterostructures and moiré materials have demonstrated large tunability of their electronic and optoelectronic properties through the control of the interface, such as the local atomic registry, strain and rotation. Beyond the quantum mechanical coupling between adjacent layers, 2D materials can impact and be impacted by local electrostatic potential modulations. At typical interlayer distance of 2D-2D and 0D-2D heterostructures (3~5Å), near-field effects­–resulting from the high multipoles of the electronic density–can be dominant.

In this work, we demonstrate large and tunable near-field electrostatic effects of self-assembled organic layers and 2D materials, and develop a theory for describing and understanding of those effects. We derive analytical expressions for the in-plane potential superlattices and out-of-plane electric fields, and show their applicability in tuning the electronic structure of 2D materials and moiré structures.   

The mystery of the "invisible" Ga vacancy in GaAs

Irradiation of semiconductors with energetic particles results in displacement damage causing device and material degradation. Characterizing the formation and reactivity of atomic defects is imperative in developing an understanding necessary to mitigate detrimental effects in device performance. Primitive displacement damage in GaAs produces vacancies, divacancies, and interstitials. Interstitials are predicted to be mobile. The immobile As vacancy and divacancy defects are predicted to have defect levels as observed in experiments. However, despite also being predicted to be immobile and have defect levels, the Ga vacancy has yet to be observed in any experiment—raising the question: Where is the Ga vacancy? Using an atomistic-aware device modeling approach to explore the chemical evolution of defects in irradiated GaAs, we discover that a strong Coulomb attraction, stemming from a Fermi level shift, drives fast As interstitials to preferentially annihilate Ga vacancies. This dynamical process causes the Ga vacancy defect population to plummet below detectable limit—to disappear. This novel approach solves the mystery of the “invisible” Ga vacancy and reveals the importance of a multiscale approach to probe this experimentally inaccessible short-time regime.   

Trap-assisted Non-radiative Recombination of CN in GaN

The efficiency of optoelectronic devices is often constrained by trap-assisted non-radiative recombination. The conventional multiphonon emission process falls short in describing the measured nonradiative recombination rate for wider-band-gap semiconductors due to its negligibly small rate in materials with band gaps greater than 2.5 eV. It has been suggested that trap-assisted Auger-Meitner (TAAM) recombination can resolve this discrepancy [1]. In TAAM processes, a free electron or hole recombines with a defect state, exciting a second carrier to a higher energy state. C_N impurities in GaN are commonly introduced during the metal-organic chemical vapor deposition (MOCVD) growth process. While C_N has been suggested to be a nonradiative recombination center, the complete recombination cycle has not been described, particularly regarding the influence of the (+/0) level.  Implementing our recently developed first-principles methodology [1], we investigate the TAAM process of C_N in GaN in addition to the multiphonon capture process for both the (+/0) and (0/-) levels. Furthermore, we study the impact of thermal emission on the overall recombination rate. We find that re-emission of holes from C+ N into the valence band has significant impact on the overall rates at lower carrier densities.