Showing 1–50 of 87 results for author: Van de Walle, C

Native defects and erbium impurities in CaWO4

Authors: Minseok Choi, Mark E. Turiansky, BaiQing Zhao, Jeff D. Thompson, Chris G. Van de Walle

Abstract: We perform hybrid density functional calculation to study the energetics, electronic properties, optical transitions, and migration barriers of native defects in CaWO$_4$. Oxygen and calcium vacancies are most likely to form in the absence of doping, but interstitials could also incorporate. Tungsten-related defects are unlikely to be present. The positively charged $V_{\rm O}$ and the negatively… ▽ More We perform hybrid density functional calculation to study the energetics, electronic properties, optical transitions, and migration barriers of native defects in CaWO$_4$. Oxygen and calcium vacancies are most likely to form in the absence of doping, but interstitials could also incorporate. Tungsten-related defects are unlikely to be present. The positively charged $V_{\rm O}$ and the negatively charged $V_{\rm Ca}$ are likely to form complexes. Calculated optical transition levels indicate that experimentally observed absorption and emission peaks can be attributed mainly to oxygen-related defects. Calculations of migration barriers allow us to conclude that Ca$_i^{2+}$, $V_{\rm O}^{2+}$ and O$_i^{2-}$ are highly mobile, even below room temperature. We have also examined Er dopants, finding that erbium easily substitutes on the Ca site in a positive charge state. Erbium can form complexes with $V_{\rm Ca}$ and O$_i$, which would deactivate the Er. If Er is introduced by implantation, Er interstitials are likely present, which will produce emission that is prone to spectral diffusion and blinking. Our calculated properties of Er$_i$ explain why annealing at modest temperatures allows the interstitials to move into substitutional sites and point defects to move away, resulting in stable emission. △ Less

Submitted 23 May, 2026; originally announced May 2026.

Charge dynamics at nitrogen impurities and nitrogen-vacancy centers in diamond

Authors: Chandan Kumar Vishwakarma, J. K. Nangoi, Mark E. Turiansky, Chris G. Van de Walle

Abstract: The nitrogen-vacancy (NV) center in diamond is the prototype quantum defect that enables a variety of diamond-based quantum technologies. However, charge-state instability and spectral diffusion, often induced by substitutional nitrogen impurities (N$_{\rm C}$), remain key challenges for device performance. Here, we employ first-principles density functional theory calculations to quantitatively i… ▽ More The nitrogen-vacancy (NV) center in diamond is the prototype quantum defect that enables a variety of diamond-based quantum technologies. However, charge-state instability and spectral diffusion, often induced by substitutional nitrogen impurities (N$_{\rm C}$), remain key challenges for device performance. Here, we employ first-principles density functional theory calculations to quantitatively investigate nonradiative carrier capture processes mediated by multiphonon emission at both the NV center and the N$_{\rm C}$ impurity. For relevant cases, we also compute the rates of radiative and thermal emission processes. For N$_{\rm C}^0$ $\to$ N$_{\rm C}^-$, we obtain an electron capture coefficient of $2.2 \times 10^{-8}$ cm$^3$s$^{-1}$ at 300 K. Both the magnitude and temperature dependence are in excellent agreement with experimentally measured capture cross sections. Electron capture at N$_{\rm C}^+$ is even faster, with a capture coefficient of $1.0 \times 10^{-4}$ cm$^3$s$^{-1}$ at 300 K. For the NV center, we find that carrier capture rates involving only the ground states of NV$^0$ and NV$^-$ are negligibly slow. However, capture into the excited states (NV$^{0*}$ and NV$^{-*}$) is significantly faster. In particular, the capture coefficient for the hole capture process NV$^-$ $\to$ NV$^{0*}$ is as large as $1.8 \times 10^{-7}$ cm$^3$s$^{-1}$ and largely temperature-independent. Hole capture at NV$^-$ will thus occur via nonradiative capture into an excited state of NV$^{0}$ followed by fast radiative decay to the NV$^0$ ground state. Similarly, electron capture at NV$^0$ will occur via the NV$^0$ $\to$ NV$^{-*}$ $\to$ NV$^-$ pathway, but with a lower nonradiative capture coefficient ($2.1 \times 10^{-9}$ cm$^3$s$^{-1}$ at 300 K). Our calculated capture coefficients and rates provide essential information for analyzing charge-state dynamics. △ Less

Submitted 23 May, 2026; originally announced May 2026.

Comments: Submitted to Physical Review B

A defect in diamond with millisecond-scale spin relaxation time at room temperature

Authors: Sounak Mukherjee, Anran Li, Johannes Eberle, Sean Karg, Zi-Huai Zhang, Mayer M. Feldman, Yilin Chen, Mark E. Turiansky, Mengen Wang, Yogendra Limbu, Tharnier O. Puel, Yueguang Shi, Matthew L. Markham, Rajesh L. Patel, Patryk Gumann, Michael E. Flatte, Chris G. Van de Walle, Stephen A. Lyon, Nathalie P. de Leon

Abstract: Spin defects in diamond are promising platforms for quantum sensing. The longest electron spin relaxation times ($T_1$) at room temperature for solid-state defects are observed in nitrogen vacancy centers in diamond, which can reach 6.67 ms, and substitutional nitrogen ("P1 centers") in diamond, which exhibit a $T_1$ of 2 ms. No other solid-state defect has exhibited millisecond-scale spin relaxat… ▽ More Spin defects in diamond are promising platforms for quantum sensing. The longest electron spin relaxation times ($T_1$) at room temperature for solid-state defects are observed in nitrogen vacancy centers in diamond, which can reach 6.67 ms, and substitutional nitrogen ("P1 centers") in diamond, which exhibit a $T_1$ of 2 ms. No other solid-state defect has exhibited millisecond-scale spin relaxation times at room temperature thus far. Here, we characterize the spin properties of the WAR5 defect in diamond with pulsed electron spin resonance. The observed $T_1$ is one of the longest for solid-state spin defects: 0.97(27) ms at room temperature and 14.38(19) min at 4 K. The observed coherence time ($T_2$) is 246(7) $μ$s, which can be extended to 6.49(34) ms at 4 K with dynamical decoupling. Furthermore, we demonstrate optical spin polarization with a range of wavelengths from 405 nm to 500 nm and propose potential zero-phonon line candidates. △ Less

Submitted 7 March, 2026; originally announced March 2026.

Identification of the I$_{10}$ Donor in ZnO as a Sn--Li Complex with Large Hyperfine Interaction

Authors: Xingyi Wang, Sai Mu, Jeong Rae Kim, Ethan R. Hansen, Yaser Silani, Lasse Vines, Joseph Falson, Chris G. Van de Walle, Kai-Mei C. Fu

Abstract: Donor impurities in wide direct band gap semiconductors provide a promising platform for spin--photon quantum technologies by combining a donor spin qubit with optically addressable transitions. In ZnO, the shallow donor with the largest reported binding energy has long been associated with the I$_{10}$ bound exciton line, but its microscopic origin has remained unresolved. Here we demonstrate the… ▽ More Donor impurities in wide direct band gap semiconductors provide a promising platform for spin--photon quantum technologies by combining a donor spin qubit with optically addressable transitions. In ZnO, the shallow donor with the largest reported binding energy has long been associated with the I$_{10}$ bound exciton line, but its microscopic origin has remained unresolved. Here we demonstrate the controlled formation and identification of this donor as a Sn--Li complex through a combination of ion implantation, annealing, optical spectroscopy, and first-principles calculations. Resonant two-laser coherent population trapping measurements reveal an electron--$^{119}$Sn hyperfine interaction of $392 \pm 15$\,MHz, establishing a coupled electron--spin--1/2, nuclear--spin--1/2 system with one of the largest hyperfine couplings reported for shallow donors in semiconductors. Density functional theory calculations show that a nearest-neighbor Sn$_{\mathrm{Zn}}$--Li$_{\mathrm{Zn}}$ complex has favorable formation energetics, donor character with the electron localized on Sn, and an extrapolated hyperfine interaction consistent with experiment. The large donor binding energy and excited-state structure indicate enhanced thermal robustness of the optical transition relative to conventional group--III donors, while the strong hyperfine interaction enables fast electron--nuclear spin control and prospects for direct nuclear--spin--photon interfaces. We further observe efficient optically induced nuclear spin polarization, highlighting a path toward nuclear spin initialization. More broadly, our results reveal how a donor--acceptor complex can access previously unexplored regimes of shallow donor physics, extending the design space of quantum defects beyond isolated substitutional dopants. △ Less

Submitted 5 March, 2026; originally announced March 2026.

Comments: 10 pages, 7 figures

Strain effects on $n$-type doping in AlN

Authors: Haochen Wang, Chris G. Van de Walle

Abstract: Controllable doping in AlN and its alloys is essential for deep-ultraviolet light sources. Ionization energies for donors in AlN ($\mathrm{Si_{Al}}$, $\mathrm{S_N}$, $\mathrm{Se_N}$) are high. We report first-principles calculations demonstrating that strain engineering can result in a reduction in ionization energies. The donor levels for $\mathrm{S_N}$ and $\mathrm{Se_N}$ shift closer to the con… ▽ More Controllable doping in AlN and its alloys is essential for deep-ultraviolet light sources. Ionization energies for donors in AlN ($\mathrm{Si_{Al}}$, $\mathrm{S_N}$, $\mathrm{Se_N}$) are high. We report first-principles calculations demonstrating that strain engineering can result in a reduction in ionization energies. The donor levels for $\mathrm{S_N}$ and $\mathrm{Se_N}$ shift closer to the conduction-band minimum (CBM) under in-plane tensile strains, driven by a downward shift of the CBM. The most widely used donor, $\mathrm{Si_{Al}}$, forms a $DX$ center in AlN. We find that a 2.5% in-plane tensile strain (which would be induced by pseudomorphic growth on GaN in experiment) shifts the ($+/-$) transition level from 271 meV to 98 meV below the CBM, which would enhance the electron concentration by three orders of magnitude. These results demonstrate that strain engineering offers an effective route to enhance doping levels in AlN. △ Less

Submitted 3 March, 2026; originally announced March 2026.

Comments: 4 pages, 3 figures

High-Performance Near-Infrared Quantum Emission from Color Centers in hBN

Authors: Sean Doan, Sahil D. Patel, Yilin Chen, Jordan A. Gusdorff. Mark E. Turiansky, Luis Villagomez, Luka Jevremovic, Nicholas Lewis, Kenji Watanabe, Takashi Taniguchi, Lee C. Bassett, Chris Van de Walle, Galan Moody

Abstract: Color centers hosted in hexagonal boron nitride have emerged as a highly promising platform for single-photon emission and spin-photon technologies relevant to quantum communication and quantum networking. As a wide-bandgap van der Waals material, hBN can host optically active quantum defects across a broad spectral range. Here, we demonstrate a simple and scalable oxygen-plasma process that repro… ▽ More Color centers hosted in hexagonal boron nitride have emerged as a highly promising platform for single-photon emission and spin-photon technologies relevant to quantum communication and quantum networking. As a wide-bandgap van der Waals material, hBN can host optically active quantum defects across a broad spectral range. Here, we demonstrate a simple and scalable oxygen-plasma process that reproducibly creates single quantum emitters in hBN with blinking-free zero-phonon lines spanning the near-infrared from 700 up to 971 nm. These emitters combine MHz-level brightness, single-photon purity up to 99.9\%, and ultranarrow cryogenic linewidths down to 2.7~GHz under quasi-resonant excitation, placing them in a particularly attractive regime for quantum photonics. Photostability measurements further reveal resistance to photobleaching, sub-nm spectral stability over long timescales, and near-shot-noise-limited intensity fluctuations. Analysis of the phonon sidebands shows weak vibronic coupling and ZPL-dominated emission, with Debye--Waller factors approaching 50\%. Control experiments together with EDS elemental mapping support oxygen incorporation as a necessary ingredient in activating the NIR emitter population, while first-principles calculations identify O$_N$V$_N$ and O$_N$V$_N$H as the leading defect candidates. These results establish a high-performance NIR quantum-emitter platform in hBN for free-space quantum networking and future integrated quantum-photonic architectures. △ Less

Submitted 23 April, 2026; v1 submitted 18 December, 2025; originally announced December 2025.

Electrical Stability of Cr2O3/\b{eta}-Ga2O3 and NiOx/\b{eta}-Ga2O3 Heterojunction Diodes

Authors: Yizheng Liu, Haochen Wang, Carl Peterson, Chinmoy Nath Saha, Chris G. Van de Walle, Sriram Krishnamoorthy

Abstract: This work reports the electrical characteristics comparison study between Cr2O3 and NiOx based heterojunction diodes (HJD) on halide vapor phase epitaxy (HVPE) grown \b{eta}-Ga2O3 epitaxial layers. Both as-fabricated Cr2O3 and NiOx HJDs exhibited forward current density in a range of 130-150 A/cm^2 at 5 V with rectifying ratios >10^10 and a reverse leakage current density at 10^-8 A/cm^2 at -5 V.… ▽ More This work reports the electrical characteristics comparison study between Cr2O3 and NiOx based heterojunction diodes (HJD) on halide vapor phase epitaxy (HVPE) grown \b{eta}-Ga2O3 epitaxial layers. Both as-fabricated Cr2O3 and NiOx HJDs exhibited forward current density in a range of 130-150 A/cm^2 at 5 V with rectifying ratios >10^10 and a reverse leakage current density at 10^-8 A/cm^2 at -5 V. The differential specific on-resistance of Cr2O3 and NiOx HJDs was 12.01 mΩ*cm^2 and 12.05 mΩ*cm^2, respectively. Breakdown voltages of Cr2O3 HJDs ranged from 1.4-1.9 kV and 1.5-2.3 kV for NiOx HJDs. Theoretical band alignment between Cr2O3 and \b{eta}-Ga2O3 was calculated from first principles. The ambient exposed NiOx/HVPE \b{eta}-Ga2O3 HJDs forward current density degraded after 10 days while that of Cr2O3/HVPE \b{eta}-Ga2O3 HJDs remained nearly unchanged after the same amount of time. It was later confirmed that the ambient exposed sputtered NiOx sheet resistance (Rsh) degradation gave rise to the reduction of the forward current density of the NiOx based HJDs, and water (H2O) was qualitatively determined to be the agent attributed to the forward conduction degradation by measuring the Rsh of NiOx-on-sapphire reference wafer after exposing it to different environments. The Cr2O3/HVPE \b{eta}-Ga2O3 HJD also exhibited enhanced thermal stability compared to the NiOx/\b{eta}-Ga2O3 heterostructures at elevated temperatures. Interfacial nickel gallate (Ga2NiO4) phase formation expected from phase diagrams can explain the reduced thermal stability of NiOx/\b{eta}-Ga2O3 HJDs. This study indicates that Cr2O3 is a stable p-type oxide for the realization of robust multi-kV \b{eta}-Ga2O3 HJDs. △ Less

Submitted 11 December, 2025; originally announced December 2025.

Charge state equilibration of nitrogen-vacancy center ensembles in diamond: The role of electron tunneling

Authors: Audrius Alkauskas, Chris G. Van de Walle, Lukas Razinkovas, Ronald Ulbricht

Abstract: The charge state stability of nitrogen-vacancy (NV) centers critically affects their application as quantum sensors and qubits. Understanding charge state conversion and equilibration is critical not only for NV centers in diamond but also for defects and impurities in wide-bandgap materials in general. The mechanisms by which these centers change charge state upon optical or electronic excitation… ▽ More The charge state stability of nitrogen-vacancy (NV) centers critically affects their application as quantum sensors and qubits. Understanding charge state conversion and equilibration is critical not only for NV centers in diamond but also for defects and impurities in wide-bandgap materials in general. The mechanisms by which these centers change charge state upon optical or electronic excitation without the presence of mobile carriers remain unclear, potentially affecting the performance of applications ranging from phosphors to power electronics. Here, we elucidate this issue for the case of photoionization of NV center ensembles. Using pump-probe spectroscopy, we ionize negatively charged NV centers and monitor the recovery of $\NVm$ on timescales of up to several seconds. We find that the recovery rate depends strongly on the concentration of surrounding nitrogen donors. Remarkably, the equilibration dynamics exhibit no discernible dependence on temperature, ruling out thermally activated processes. The multiphonon-assisted electron tunneling model, supported by density-functional calculations, explains the measurements and identifies tunneling as the equilibration mechanism. △ Less

Submitted 30 November, 2025; originally announced December 2025.

Resonant states and nuclear dynamics in solid-state systems: the case of silicon-hydrogen bond dissociation

Authors: Woncheol Lee, Mark E. Turiansky, Dominic Waldhör, Byounghak Lee, Tibor Grasser, Chris G. Van de Walle

Abstract: Bond breaking in the presence of highly energetic carriers is central to many important phenomena in physics and chemistry, including radiation damage, hot-carrier degradation, activation of dopant-hydrogen complexes in semiconductors, and photocatalysis. Describing these processes from first principles has remained an elusive goal. Here we introduce a comprehensive theoretical framework for the d… ▽ More Bond breaking in the presence of highly energetic carriers is central to many important phenomena in physics and chemistry, including radiation damage, hot-carrier degradation, activation of dopant-hydrogen complexes in semiconductors, and photocatalysis. Describing these processes from first principles has remained an elusive goal. Here we introduce a comprehensive theoretical framework for the dissociation process, emphasizing the need for a non-adiabatic approach. We benchmark the results for the case of silicon-hydrogen bond dissocation, a primary process for hot-carrier degradation. Passivation of Si dangling bonds by hydrogen is vital in all Si devices because it eliminates electrically active mid-gap states; understanding the mechanism for dissociation of these bonds is therefore crucial for device technology. While the need for a non-adiabatic approach has been previously recognized, explicitly obtaining diabatic states for solid-state systems has been an outstanding challenge. We demonstrate how to obtain these states by applying a partitioning scheme to the Hamiltonian obtained from first-principles density functional theory. Our results demonstrate that bond dissociation can occur when electrons temporarily occupy the antibonding states, generating a highly repulsive excited-state potential that causes the hydrogen nuclear wavepacket to shift and propagate rapidly. Based on the Menzel-Gomer-Redhead (MGR) model, we show that after moving on this excited-state potential on femtosecond timescales, a portion of the nuclear wavepacket can continue to propagate even after the system relaxes back to the ground state, allowing us to determine the dissociation probability. Our results provide essential insights into the fundamental processes that drive carrier-induced bond breaking in general, and specifically elucidate hydrogen-related degradation in Si devices. △ Less

Submitted 29 November, 2025; originally announced December 2025.

Comments: 26 pages, 15 figures

Journal ref: Phys. Rev. B 113, 075304 (2026)

Cr2O3/\b{eta}-Ga2O3 Heterojunction Diodes with Orientation-Dependent Breakdown Electric Field up to 12.9 MV/cm

Authors: Yizheng Liu, Haochen Wang, Carl Peterson, James S. Speck, Chris Van De Walle, Sriram Krishnamoorthy

Abstract: We report the fabrication of Cr2O3/\b{eta}-Ga2O3 heterojunction diodes using reactive magnetron sputtering of Cr2O3 on highly doped \b{eta}-Ga2O3 bulk substrates along (100), (010), (001), (110), and (011) orientation dependence of high electric field handling capability in \b{eta}-Ga2O3. Additional relative permittivity values in (110) and (011) orientations of \b{eta}-Ga2O3 were computed by usin… ▽ More We report the fabrication of Cr2O3/\b{eta}-Ga2O3 heterojunction diodes using reactive magnetron sputtering of Cr2O3 on highly doped \b{eta}-Ga2O3 bulk substrates along (100), (010), (001), (110), and (011) orientation dependence of high electric field handling capability in \b{eta}-Ga2O3. Additional relative permittivity values in (110) and (011) orientations of \b{eta}-Ga2O3 were computed by using first-principles calculation methods for accurate apparent charge density (ND-NA) extraction and breakdown electric field analysis from capacitance-voltage measurements. The HJDs fabricated on n+ (110) exhibited breakdown electric fields >10 MV/cm up to 12.9 MV/cm, showing the highest experimentally observed parallel-plane junction electric field among \b{eta}-Ga2O3-based junctions. Breakdown electric fields among (100), (010), (001), and (011) orientations showed distinct distribution in the range of 5.13-5.26 MV/cm, 5.10-7.05 MV/cm, 2.70-3.33 MV/cm, and 3.88-4.38 MV/cm, respectively, validating the orientational dependence of parallel-plane junction electric field at breakdown in low-symmetry monoclinic \b{eta}-Ga2O3. The parallel-plane breakdown electric fields (EBr,||) reported in this work were extracted when the device experienced catastrophic breakdown at 100 mA/cm^2 current density compliance, and should not be confused with critical electric field (Ec) as a function of drift layer doping concentration, which accounts for electric-field dependent impact ionization coefficients in Si, SiC and GaN. This study can guide the choice of crystal orientation for high performance gallium oxide-based devices that require high electric field handling capability. △ Less

Submitted 25 November, 2025; originally announced November 2025.

A CN complex as an alternative to the T center in Si

Authors: J. K. Nangoi, M. E. Turiansky, C. G. Van de Walle

Abstract: We present a first-principles study of a carbon-nitrogen (CN) impurity complex in silicon as an isoelectronic alternative to the T center [(CCH)$_\mathrm{Si}$]. The latter has been pursued for applications in quantum information science, yet its sensitivity to the presence of hydrogen is still problematic. Our proposed complex has no hydrogen, thereby eliminating this issue. First, we show that th… ▽ More We present a first-principles study of a carbon-nitrogen (CN) impurity complex in silicon as an isoelectronic alternative to the T center [(CCH)$_\mathrm{Si}$]. The latter has been pursued for applications in quantum information science, yet its sensitivity to the presence of hydrogen is still problematic. Our proposed complex has no hydrogen, thereby eliminating this issue. First, we show that the CN complex is stable against decomposition into substitutional and interstitial defects. Next, we show that due to being isoelectronic to the T center, the CN complex has a similar electronic structure, and therefore could be used in similar applications. We assess several low-energy configurations of the CN complex, finding (CN)$_\mathrm{Si}$ to be stable and have the largest Debye-Waller factor. We predict a zero-phonon line (ZPL) of 828 meV (in the telecom S-band) and a radiative lifetime of 4.2 $μ$s, comparable to the T center. Due to the presence of a bound exciton, choice of the exchange-correlation functional and also supercell-size scaling of the ZPL and transition dipole moment require special scrutiny; we rigorously justify our extrapolation schemes that allow computing values in the dilute limit. △ Less

Submitted 1 November, 2025; originally announced November 2025.

Comments: Submitted to Physical Review B

Journal ref: Phys. Rev. B 113, L060101 (2026)

Design of high-mobility p-type GaN via the piezomobility tensor

Authors: Jie-Cheng Chen, Joshua Leveillee, Chris G. Van de Walle, Feliciano Giustino

Abstract: Gallium nitride (GaN) is a wide-bandgap semiconductor of significant interest for applications in solid-state lighting, power electronics, and radio-frequency amplifiers. An important limitation of this semiconductor is its low intrinsic hole mobility, which hinders the development of \textit{p}-channel devices and the large-scale integration of GaN CMOS in next-generation electronics. Prior resea… ▽ More Gallium nitride (GaN) is a wide-bandgap semiconductor of significant interest for applications in solid-state lighting, power electronics, and radio-frequency amplifiers. An important limitation of this semiconductor is its low intrinsic hole mobility, which hinders the development of \textit{p}-channel devices and the large-scale integration of GaN CMOS in next-generation electronics. Prior research has explored the use of strain to improve the hole mobility of GaN, but a systematic analysis of all possible strain conditions and their impact on the mobility is lacking. In this study, we introduce a piezomobility tensor notation to characterize the relationship between applied strain and hole mobility in GaN. To map the strain-dependence of the hole mobility, we solve the \textit{ab initio} Boltzmann transport equation, accounting for electron-phonon scattering and GW quasiparticle energy corrections. We show that there exist three optimal strain configurations, two uniaxial strains and one shear strain, that can lead to significant mobility enhancement. In particular, we predict room-temperature hole mobility of up to 164~\mob\ for 2\% uniaxial compression and 148~\mob\ for 2\% shear strain. Our methodology provides a general framework for investigating strain effects on the transport properties of semiconductors from first principles. △ Less

Submitted 8 August, 2025; originally announced August 2025.

Clarification of the Spontaneous Polarization Direction in Crystals with Wurtzite Structure

Authors: Simon Fichtner, Mohamed Yassine, Chris van de Walle, Oliver Ambacher

Abstract: The wurtzite structure is one of the most frequently found crystal structures in modern semiconductors and its inherent spontaneous polarization is a defining materials property. Despite this significance, confusion has been rampant in the literature with respect to the orientation of the spontaneous polarization inside the unit cell of the wurtzite structure, especially for the technologically ve… ▽ More The wurtzite structure is one of the most frequently found crystal structures in modern semiconductors and its inherent spontaneous polarization is a defining materials property. Despite this significance, confusion has been rampant in the literature with respect to the orientation of the spontaneous polarization inside the unit cell of the wurtzite structure, especially for the technologically very relevant III-N compounds (AlN, GaN, InN). In particular, the spontaneous polarization has been reported to either point up or down for the same unit cell orientation, depending on the literature source - with important implications for, e.g., the carrier type and density expected at interfaces of heterostructures involving materials with wurtzite-structure. This perspective aims to resolve this ambiguity by reviewing available reports on the direction of the energetically preferred polarization direction in the presence of external electric fields, as well as atomically resolved scanning transmission electron microscopy images. While we use ferroelectric wurtzite AlScN as a key example, our conclusions are generalizable to other compounds with the same crystal structure. We demonstrate that a metal-polar unit cell must be associated with an upward polarization vector - which is contrary to long-standing conventional wisdom. △ Less

Submitted 6 May, 2025; originally announced May 2025.

Journal ref: Appl. Phys. Lett. 125, 040501 (2024)

Impact of Absorption due to Zero-Field Splitting on Loss in Dielectrics: A Case Study in Sapphire

Authors: Mark E. Turiansky, Chris G. Van de Walle

Abstract: The coherence times of superconducting qubits are limited by loss mechanisms, whose microscopic origins have remained elusive. We propose a mechanism caused by transitions between zero-field-split states of paramagnetic impurities or defects. We derive the absorption cross section for a magnetic dipole transition and apply it to calculate the loss tangent. For Cr, Fe, and V impurities in sapphire,… ▽ More The coherence times of superconducting qubits are limited by loss mechanisms, whose microscopic origins have remained elusive. We propose a mechanism caused by transitions between zero-field-split states of paramagnetic impurities or defects. We derive the absorption cross section for a magnetic dipole transition and apply it to calculate the loss tangent. For Cr, Fe, and V impurities in sapphire, we find loss tangents at 4.5 GHz in the range of 10$^{-9}$-10$^{-8}$, comparable to the loss measured in experiments. This value suggests that magnetic loss may be a limiting factor in the coherence times of superconducting qubits. △ Less

Submitted 17 April, 2026; v1 submitted 6 April, 2025; originally announced April 2025.

Carbon in GaN as a nonradiative recombination center

Authors: Fangzhou Zhao, Hongyi Guan, Mark E. Turiansky, Chris G. Van de Walle

Abstract: Trap-assisted nonradiative recombination has been shown to limit the efficiency of optoelectronic devices. While substitutional carbon ($\mathrm{C_N}$) has been suggested to be a nonradiative recombination center in GaN devices, a complete recombination cycle including the two charge-state transition levels has not been previously described. In this work, we investigate the trap-assisted recombina… ▽ More Trap-assisted nonradiative recombination has been shown to limit the efficiency of optoelectronic devices. While substitutional carbon ($\mathrm{C_N}$) has been suggested to be a nonradiative recombination center in GaN devices, a complete recombination cycle including the two charge-state transition levels has not been previously described. In this work, we investigate the trap-assisted recombination process due to $\mathrm{C_N}$ in GaN, including multiphonon emission (MPE), radiative recombination, trap-assisted Auger-Meitner (TAAM) recombination, as well as thermal emission of holes. Our study shows the key role of TAAM processes at the high carrier densities relevant for devices. We also reveal the carrier-density regimes where thermal emission and radiative recombination are expected to play an observable role. Our results highlight that carbon concentrations exceeding $\sim$10$^{17}$ cm$^{-3}$ can have a noticeable impact on device efficiency, not just in GaN active layers but also in InGaN and AlGaN. Our comprehensive formalism not only offers detailed results for carbon but provides a general framework for assessing the multiple processes that participate in trap-assisted recombination in semiconductors. △ Less

Submitted 18 February, 2025; originally announced February 2025.

Distinguishing thermal fluctuations from polaron formation in halide perovskites

Authors: Bai-Qing Zhao, Xuan-Yan Chen, Chuan-Nan Li, Jinshan Li, Chris G. Van de Walle, Xie Zhang

Abstract: Recent angle-resolved photoelectron spectroscopy (ARPES) measurements of the hole effective mass in CsPbBr$_3$ revealed an enhancement of $\sim$50 % compared to the bare mass computed from first principles for CsPbBr$_3$ at $T = 0 K$. This large enhancement was interpreted as evidence of polaron formation. Employing accurate finite-temperature first-principles calculations, we show that the calcul… ▽ More Recent angle-resolved photoelectron spectroscopy (ARPES) measurements of the hole effective mass in CsPbBr$_3$ revealed an enhancement of $\sim$50 % compared to the bare mass computed from first principles for CsPbBr$_3$ at $T = 0 K$. This large enhancement was interpreted as evidence of polaron formation. Employing accurate finite-temperature first-principles calculations, we show that the calculated hole effective mass of CsPbBr$_3$ at $T = 300 K$ can explain experimental results without invoking polarons. Thermal fluctuations are particularly strong in halide perovskites compared to conventional semiconductors such as Si and GaAs, and cannot be ignored when comparing with experiment. We not only resolve the debate on polaron formation in halide perovskites, but also demonstrate the general importance of including thermal fluctuations in first-principles calculations for strongly anharmonic materials. △ Less

Submitted 11 February, 2025; v1 submitted 9 February, 2025; originally announced February 2025.

Characterization of Chromium Impurities in $β$-Ga$_2$O$_3$

Authors: Mark E. Turiansky, Sai Mu, Lukas Razinkovas, Kamyar Parto, Sahil D. Patel, Sean Doan, Ganesh Pokharel, Steven J. Gomez Alvarado, Stephen D. Wilson, Galan Moody, Chris G. Van de Walle

Abstract: Chromium is a common transition-metal impurity that is easily incorporated during crystal growth. It is perhaps best known for giving rise to the 694.3 nm (1.786 eV) emission in Cr-doped Al$_2$O$_3$, exploited in ruby lasers. Chromium has also been found in monoclinic gallium oxide, a wide-bandgap semiconductor being pursued for power electronics. In this work, we thoroughly characterize the behav… ▽ More Chromium is a common transition-metal impurity that is easily incorporated during crystal growth. It is perhaps best known for giving rise to the 694.3 nm (1.786 eV) emission in Cr-doped Al$_2$O$_3$, exploited in ruby lasers. Chromium has also been found in monoclinic gallium oxide, a wide-bandgap semiconductor being pursued for power electronics. In this work, we thoroughly characterize the behavior of Cr in Ga$_2$O$_3$ through theoretical and experimental techniques. $β$-Ga$_2$O$_3$ samples are grown with the floating zone method and show evidence of a sharp photoluminescence signal, reminiscent of ruby. We calculate the energetics of formation of Cr from first principles, demonstrating that Cr preferentially incorporates as a neutral impurity on the octahedral site. Cr possesses a quartet ground-state spin and has an internal transition with a zero-phonon line near 1.8 eV. By comparing the calculated and experimentally measured luminescence lineshape function, we elucidate the role of coupling to phonons and uncover features beyond the Franck-Condon approximation. The combination of strong emission with a small Huang-Rhys factor of 0.05 and a technologically relevant host material render Cr in Ga$_2$O$_3$ attractive as a quantum defect. △ Less

Submitted 31 December, 2024; originally announced January 2025.

Heterostructure Engineering for Wurtzite LaN

Authors: Andrew J. E. Rowberg, Sai Mu, Chris G. Van de Walle

Abstract: Wurtzite LaN (wz-LaN) is a semiconducting nitride with favorable piezoelectric and ferroelectric properties, making it promising for applications in electronics. We use first-principles density functional theory with a hybrid functional to investigate several features that are key for its use in heterostructures. First, for the purposes of growing wz-LaN on a substrate or designing a heterostructu… ▽ More Wurtzite LaN (wz-LaN) is a semiconducting nitride with favorable piezoelectric and ferroelectric properties, making it promising for applications in electronics. We use first-principles density functional theory with a hybrid functional to investigate several features that are key for its use in heterostructures. First, for the purposes of growing wz-LaN on a substrate or designing a heterostructure, we show that it can be lattice-matched with a number of cubic materials along their [111] axes. We also evaluate the bound charge at such interfaces, taking into account both the polarization discontinuity and the piezoelectric polarization due to pseudomorphic strain. Second, we investigate band alignments and assess the results for interfaces with zincblende-, rocksalt-, and perovskite-structure compounds, and with chemically similar wurtzite and rocksalt nitrides. Our results provide guidance for the development of electronic devices based on wz-LaN. △ Less

Submitted 30 December, 2024; originally announced December 2024.

Comments: 9 pages, 3 figures

Journal ref: J. Chem. Phys. 162, 194713 (2025)

First-principles theory of direct-gap optical emission in hexagonal Ge and its enhancement via strain engineering

Authors: Christopher A. Broderick, Xie Zhang, Mark E. Turiansky, Chris G. Van de Walle

Abstract: The emergence of hexagonal Ge (2H-Ge) as a candidate direct-gap group-IV semiconductor for Si photonics mandates rigorous understanding of its optoelectronic properties. Theoretical predictions of a "pseudo-direct" band gap, characterized by weak oscillator strength, contrast with a claimed high radiative recombination coefficient $B$ comparable to conventional (cubic) InAs. We compute $B$ in 2H-G… ▽ More The emergence of hexagonal Ge (2H-Ge) as a candidate direct-gap group-IV semiconductor for Si photonics mandates rigorous understanding of its optoelectronic properties. Theoretical predictions of a "pseudo-direct" band gap, characterized by weak oscillator strength, contrast with a claimed high radiative recombination coefficient $B$ comparable to conventional (cubic) InAs. We compute $B$ in 2H-Ge from first principles and quantify its dependence on temperature, carrier density and strain. For unstrained 2H-Ge, our calculated spontaneous emission spectra corroborate that measured photoluminescence corresponds to direct-gap emission, but with $B$ being approximately three orders of magnitude lower than in InAs. We confirm a pseudo-direct- to direct-gap transition under $\sim 2$\% [0001] uniaxial tension, which can enhance $B$ by up to three orders of magnitude, making it comparable to that of InAs. Beyond quantifying strong enhancement of $B$ via strain engineering, our analysis suggests the dominance of additional, as-yet unquantified recombination mechanisms in this nascent material. △ Less

Submitted 16 December, 2024; v1 submitted 11 December, 2024; originally announced December 2024.

Towards higher electro-optic response in AlScN

Authors: Haochen Wang, Sai Mu, Chris G. Van de Walle

Abstract: Novel materials with large electro-optic (EO) coefficients are essential for developing ultra-compact broadband modulators and enabling effective quantum transduction. Compared to lithium niobate, the most widely used nonlinear optical material, wurtzite AlScN offers advantages in nano-photonic devices due to its compatibility with integrated circuits. We perform detailed first-principles calculat… ▽ More Novel materials with large electro-optic (EO) coefficients are essential for developing ultra-compact broadband modulators and enabling effective quantum transduction. Compared to lithium niobate, the most widely used nonlinear optical material, wurtzite AlScN offers advantages in nano-photonic devices due to its compatibility with integrated circuits. We perform detailed first-principles calculations to investigate the electro-optic effect in $\mathrm{Al}_{1-x}\mathrm{Sc}_{x}\mathrm{N}$ alloys and superlattices. At elevated Sc concentrations in alloys, the EO coefficients increase; importantly, we find that cation ordering along the $c$ axis leads to enhanced EO response. Strain engineering can be used to further manipulate the EO coefficients of AlScN films. With applied in-plane strains, the piezoelectric contributions to the EO coefficients increase dramatically, even exceeding 251 pm/V. We also explore the possibility of EO enhancement through superlattice engineering, finding that nonpolar $a$-plane $\mathrm{(AlN)}_m/\mathrm{(ScN)}_n$ superlattices increase EO coefficients beyond 40 pm/V. Our findings provide design principles to enhance the electro-optic effect through alloy engineering and heterostructure architecture. △ Less

Submitted 21 October, 2024; v1 submitted 9 October, 2024; originally announced October 2024.

Comments: Six pages in main manuscript including the reference. Three figures in main manuscript, 10 figures in Supplementary Materials

Optical lineshapes for orbital singlet to doublet transitions in a dynamical Jahn-Teller system: the NiV$^{-}$ center in diamond

Authors: Rokas Silkinis, Vytautas Žalandauskas, Gergő Thiering, Adam Gali, Chris G. Van de Walle, Audrius Alkauskas, Lukas Razinkovas

Abstract: We apply density functional theory to investigate interactions between electronic and vibrational states in crystal defects with multi-mode dynamical Jahn-Teller (JT) systems. Our focus is on transitions between orbital singlet and degenerate orbital doublet characterized by $E \otimes (e \oplus e \oplus \cdots)$ JT coupling, which frequently occurs in crystal defects that are investigated for app… ▽ More We apply density functional theory to investigate interactions between electronic and vibrational states in crystal defects with multi-mode dynamical Jahn-Teller (JT) systems. Our focus is on transitions between orbital singlet and degenerate orbital doublet characterized by $E \otimes (e \oplus e \oplus \cdots)$ JT coupling, which frequently occurs in crystal defects that are investigated for applications in quantum information science. We utilize a recently developed methodology to model the photoluminescence (PL) spectrum of the negatively charged split nickel-vacancy center (NiV$^{-}$) in diamond, where JT-active modes significantly influence electron-phonon interactions. Our results validate the effectiveness of the methodology in accurately reproducing the observed 1.4 eV PL lineshape. The strong agreement between our theoretical predictions and experimental observations reinforces the identification of the 1.4 eV PL center with the NiV$^{-}$ complex. This study highlights the critical role of JT-active modes in affecting optical lineshapes and demonstrates the power of advanced techniques for modeling optical properties in complex systems with multiple JT-active frequencies. △ Less

Submitted 1 July, 2024; v1 submitted 15 June, 2024; originally announced June 2024.

Comments: 13 pages, 7 figures

Journal ref: Phys.Rev.B 110 (2024) 075303

Unveiling the Pockels Coefficient of Ferroelectric Nitride ScAlN

Authors: Guangcanlan Yang, Haochen Wang, Sai Mu, Hao Xie, Tyler Wang, Chengxing He, Mohan Shen, Mengxia Liu, Chris G. Van de Walle, Hong X. Tang

Abstract: Nitride ferroelectrics have recently emerged as promising alternatives to oxide ferroelectrics due to their compatibility with mainstream semiconductor processing. ScAlN, in particular, has exhibited remarkable piezoelectric coupling strength ($K^2$) comparable to that of lithium niobate (LN), making it a valuable choice for RF filters in wireless communications. Recently, ScAlN has sparked intere… ▽ More Nitride ferroelectrics have recently emerged as promising alternatives to oxide ferroelectrics due to their compatibility with mainstream semiconductor processing. ScAlN, in particular, has exhibited remarkable piezoelectric coupling strength ($K^2$) comparable to that of lithium niobate (LN), making it a valuable choice for RF filters in wireless communications. Recently, ScAlN has sparked interest in its use for nanophotonic devices, chiefly due to its large bandgap facilitating operation in blue wavelengths coupled with promises of enhanced nonlinear optical properties such as a large second-order susceptibility ($χ^{(2)}$). It is still an open question whether ScAlN can outperform oxide ferroelectrics concerning the Pockels effect -- an electro-optic coupling extensively utilized in optical communications devices. In this paper, we present a comprehensive theoretical analysis and experimental demonstration of ScAlN's Pockels effect. Our findings reveal that the electro-optic coupling of ScAlN, despite being weak at low Sc concentration, may be significantly enhanced and exceed LiNbO$_3$ at high levels of Sc doping, which points the direction of continued research efforts to unlock the full potential of ScAlN. △ Less

Submitted 18 October, 2024; v1 submitted 13 May, 2024; originally announced May 2024.

First-principles studies of Schottky barriers and tunneling properties at Al(111)/Si(111) and CoSi$_2$(111)/Si(111) interfaces

Authors: J. K. Nangoi, C. J. Palmstrøm, C. G. Van de Walle

Abstract: We present first-principles calculations of Schottky barrier heights (SBHs) at interfaces relevant for silicon-based merged-element transmon qubit devices. Focusing on Al(111)/Si(111) and CoSi$_2$(111)/Si(111), we consider various possible interfacial structures, for which we study the relaxations of the atoms near the interface, calculate the formation energies and Schottky barrier heights, and p… ▽ More We present first-principles calculations of Schottky barrier heights (SBHs) at interfaces relevant for silicon-based merged-element transmon qubit devices. Focusing on Al(111)/Si(111) and CoSi$_2$(111)/Si(111), we consider various possible interfacial structures, for which we study the relaxations of the atoms near the interface, calculate the formation energies and Schottky barrier heights, and provide estimates of the Josephson critical currents based on the WKB tunneling formalism as implemented in the Simmons/Tsu-Esaki model. We find that the formation energies and SBHs are very similar for all Al(111)/Si(111) structures, yet vary significantly for the CoSi$_2$(111)/Si(111) structures. We attribute this to the more covalent character of bonding at CoSi$_2$/Si, which leads to configurations with distinct atomic and electronic structure. Our estimated Josephson critical currents, which govern the behavior of merged-element transmons, provide insight into the trends as a function of Schottky-barrier height. We show that desirable qubit frequencies of 4-5 GHz can be obtained with a Si barrier thickness of about 5-10 nm, and demonstrate that the critical current density as a function of Schottky barrier height can be modeled based on the tunneling probability for a rectangular barrier. △ Less

Submitted 20 March, 2024; originally announced March 2024.

Comments: Submitted to Physical Review B

Journal ref: Phys. Rev. B 110, 035302 (2024)

Carrier confinement and alloy disorder exacerbate Auger-Meitner recombination in AlGaN ultraviolet light-emitting diodes

Authors: Nick Pant, Kyle Bushick, Andrew McAllister, Woncheol Lee, Chris G. Van de Walle, Emmanouil Kioupakis

Abstract: The quantum efficiency of AlGaN ultraviolet light-emitting diodes (LEDs) declines (droops) at increasing operating powers due to Auger-Meitner recombination (AMR). Using first-principles density-functional theory, we show that indirect AMR mediated by electron-phonon coupling and alloy disorder can induce bulk $C$ coefficients as large as $\sim10^{-31}$ cm$^6$/s. Furthermore, we find that the conf… ▽ More The quantum efficiency of AlGaN ultraviolet light-emitting diodes (LEDs) declines (droops) at increasing operating powers due to Auger-Meitner recombination (AMR). Using first-principles density-functional theory, we show that indirect AMR mediated by electron-phonon coupling and alloy disorder can induce bulk $C$ coefficients as large as $\sim10^{-31}$ cm$^6$/s. Furthermore, we find that the confinement of carriers by polarization fields within quantum wells severely relaxes crystal-momentum conservation, which exacerbates the rate of AMR over radiative recombination by an order of magnitude relative to the bulk. This results in a striking decrease in quantum efficiency at high power. Suppressing polarization fields and jointly increasing the well width would greatly mitigate AMR and efficiency droop. △ Less

Submitted 16 March, 2024; originally announced March 2024.

Journal ref: Appl. Phys. Lett. 125, 021109 (2024)

Dielectric Loss due to Charged-Defect Acoustic Phonon Emission

Authors: Mark E. Turiansky, Chris G. Van de Walle

Abstract: The coherence times of state-of-the-art superconducting qubits are limited by bulk dielectric loss, yet the microscopic mechanism leading to this loss is unclear. Here we propose that the experimentally observed loss can be attributed to the presence of charged defects that enable the absorption of electromagnetic radiation by the emission of acoustic phonons. Our explicit derivation of the absorp… ▽ More The coherence times of state-of-the-art superconducting qubits are limited by bulk dielectric loss, yet the microscopic mechanism leading to this loss is unclear. Here we propose that the experimentally observed loss can be attributed to the presence of charged defects that enable the absorption of electromagnetic radiation by the emission of acoustic phonons. Our explicit derivation of the absorption coefficient for this mechanism allows us to derive a loss tangent of $7.2 \times 10^{-9}$ for Al$_2$O$_3$, in good agreement with recent high-precision measurements [A. P. Read et al., Phys. Rev. Appl. 19, 034064 (2023)]. We also find that for temperatures well below ~0.2 K, the loss should be independent of temperature, also in agreement with observations. Our investigations show that the loss per defect depends mainly on properties of the host material, and a high-throughput search suggests that diamond, cubic BN, AlN, and SiC are optimal in this respect. △ Less

Submitted 27 February, 2024; originally announced February 2024.

Rational Design of Efficient Defect-Based Quantum Emitters

Authors: Mark E. Turiansky, Kamyar Parto, Galan Moody, Chris G. Van de Walle

Abstract: Single-photon emitters are an essential component of quantum networks, and defects or impurities in semiconductors are a promising platform to realize such quantum emitters. Here we present a model that encapsulates the essential physics of coupling to phonons, which governs the behavior of real single-photon emitters, and critically evaluate several approximations that are commonly utilized. Emis… ▽ More Single-photon emitters are an essential component of quantum networks, and defects or impurities in semiconductors are a promising platform to realize such quantum emitters. Here we present a model that encapsulates the essential physics of coupling to phonons, which governs the behavior of real single-photon emitters, and critically evaluate several approximations that are commonly utilized. Emission in the telecom wavelength range is highly desirable, but our model shows that nonradiative processes are greatly enhanced at these low photon energies, leading to a decrease in efficiency. Our results suggest that reducing the phonon frequency is a fruitful avenue to enhance the efficiency. △ Less

Submitted 13 February, 2024; originally announced February 2024.

A first-principles approach to closing the "10-100 eV gap" for charge-carrier thermalization in semiconductors

Authors: Dallin O. Nielsen, Chris G. Van de Walle, Sokrates T. Pantelides, Ronald D. Schrimpf, Daniel M. Fleetwood, Massimo V. Fischetti

Abstract: The present work is concerned with studying accurately the energy-loss processes that control the thermalization of hot electrons and holes that are generated by high-energy radiation in wurtzite GaN, using an ab initio approach. Current physical models of the nuclear/particle physics community cover thermalization in the high-energy range (kinetic energies exceeding ~100 eV), and the electronic-d… ▽ More The present work is concerned with studying accurately the energy-loss processes that control the thermalization of hot electrons and holes that are generated by high-energy radiation in wurtzite GaN, using an ab initio approach. Current physical models of the nuclear/particle physics community cover thermalization in the high-energy range (kinetic energies exceeding ~100 eV), and the electronic-device community has studied extensively carrier transport in the low-energy range (below ~10 eV). However, the processes that control the energy losses and thermalization of electrons and holes in the intermediate energy range of about 10-100 eV (the "10-100 eV gap") are poorly known. The aim of this research is to close this gap, by utilizing density functional theory (DFT) to obtain the band structure and dielectric function of GaN for energies up to about 100 eV. We also calculate charge-carrier scattering rates for the major charge-carrier interactions (phonon scattering, impact ionization, and plasmon emission), using the DFT results and first-order perturbation theory. With this information, we study the thermalization of electrons starting at 100 eV using the Monte Carlo method to solve the semiclassical Boltzmann transport equation. Full thermalization of electrons and holes is complete within ~1 and 0.5 ps, respectively. Hot electrons dissipate about 90% of their initial kinetic energy to the electron-hole gas (90 eV) during the first ~0.1 fs, due to rapid plasmon emission and impact ionization at high energies. The remaining energy is lost more slowly as phonon emission dominates at lower energies (below ~10 eV). During the thermalization, hot electrons generate pairs with an average energy of ~8.9 eV/pair (11-12 pairs per hot electron). Additionally, during the thermalization, the maximum electron displacement from its original position is found to be on the order of 100 nm. △ Less

Submitted 7 August, 2023; originally announced August 2023.

Comments: 23 pages, 20 figures. This LaTex file uses RevTex4.2 from APS

Trap-Assisted Auger-Meitner Recombination from First Principles

Authors: Fangzhou Zhao, Mark E. Turiansky, Audrius Alkauskas, Chris G. Van de Walle

Abstract: Trap-assisted nonradiative recombination is known to limit the efficiency of optoelectronic devices, but the conventional multi-phonon emission (MPE) process fails to explain the observed loss in wide-band-gap materials. Here we highlight the role of trap-assisted Auger-Meitner (TAAM) recombination, and present a first-principles methodology to determine TAAM rates due to defects or impurities in… ▽ More Trap-assisted nonradiative recombination is known to limit the efficiency of optoelectronic devices, but the conventional multi-phonon emission (MPE) process fails to explain the observed loss in wide-band-gap materials. Here we highlight the role of trap-assisted Auger-Meitner (TAAM) recombination, and present a first-principles methodology to determine TAAM rates due to defects or impurities in semiconductors or insulators. We assess the impact on efficiency of light emitters in a recombination cycle that may include both TAAM and carrier capture via MPE. We apply the formalism to the technologically relevant case study of a calcium impurity in InGaN, where a Shockley-Read-Hall recombination cycle involving MPE alone cannot explain the experimentally observed nonradiative loss. We find that, for band gaps larger than 2.5 eV, the inclusion of TAAM results in recombination rates that are orders of magnitude larger than recombination rates based on MPE alone, demonstrating that TAAM can be a dominant nonradiative process in wide-band-gap materials. Our computational formalism is general and can be applied to the calculation of TAAM rates in any semiconducting or insulating material. △ Less

Submitted 15 November, 2022; originally announced November 2022.

Coherent control of a nuclear spin via interactions with a rare-earth ion in the solid-state

Authors: Mehmet T. Uysal, Mouktik Raha, Songtao Chen, Christopher M. Phenicie, Salim Ourari, Mengen Wang, Chris G. Van de Walle, Viatcheslav V. Dobrovitski, Jeff D. Thompson

Abstract: Individually addressed Er$^{3+}$ ions in solid-state hosts are promising resources for quantum repeaters, because of their direct emission in the telecom band and compatibility with silicon photonic devices. While the Er$^{3+}$ electron spin provides a spin-photon interface, ancilla nuclear spins could enable multi-qubit registers with longer storage times. In this work, we demonstrate coherent co… ▽ More Individually addressed Er$^{3+}$ ions in solid-state hosts are promising resources for quantum repeaters, because of their direct emission in the telecom band and compatibility with silicon photonic devices. While the Er$^{3+}$ electron spin provides a spin-photon interface, ancilla nuclear spins could enable multi-qubit registers with longer storage times. In this work, we demonstrate coherent coupling between the electron spin of a single Er$^{3+}$ ion and a single $I=1/2$ nuclear spin in the solid-state host crystal, which is a fortuitously located proton ($^1$H). We control the nuclear spin using dynamical decoupling sequences applied to the electron spin, implementing one- and two-qubit gate operations. Crucially, the nuclear spin coherence time exceeds the electron coherence time by several orders of magnitude, because of its smaller magnetic moment. These results provide a path towards combining long-lived nuclear spin quantum registers with telecom-wavelength emitters for long-distance quantum repeaters. △ Less

Submitted 12 September, 2022; originally announced September 2022.

Journal ref: PRX Quantum 4, 010323 (2023)

Anisotropic-strain-enhanced hole mobility in GaN by lattice matching to ZnGeN$_2$ and MgSiN$_2$

Authors: Joshua Leveillee, Samuel Ponce, Nicholas L. Adamski, Chris G. Van de Walle, Feliciano Giustino

Abstract: The key obstacle toward realizing integrated gallium nitride (GaN) electronics is its low hole mobility. Here, we explore the possibility of improving the hole mobility of GaN via epitaxial matching to II-IV nitride materials that have recently become available, namely ZnGeN$_2$ and MgSiN$_2$. We perform state-of-the-art calculations of the hole mobility of GaN using the ab initio Boltzmann transp… ▽ More The key obstacle toward realizing integrated gallium nitride (GaN) electronics is its low hole mobility. Here, we explore the possibility of improving the hole mobility of GaN via epitaxial matching to II-IV nitride materials that have recently become available, namely ZnGeN$_2$ and MgSiN$_2$. We perform state-of-the-art calculations of the hole mobility of GaN using the ab initio Boltzmann transport equation. We show that effective uniaxial compressive strain of GaN along the $[1\bar{1}00]$ by lattice matching to ZnGeN$_2$ and MgSiN$_2$ results in the inversion of the heavy hole band and split-off hole band, thereby lowering the effective hole mass in the compression direction. We find that lattice matching to ZnGeN$_2$ and MgSiN$_2$ induces an increase of the room-temperature hole mobility by 50% and 260% as compared to unstrained GaN, respectively. Examining the trends as a function of strain, we find that the variation in mobility is highly nonlinear; lattice matching to a hypothetical solid solution of Zn$_{0.75}$Ge$_{0.75}$Mg$_{0.25}$Si$_{0.25}$N$_2$ would already increase the hole mobility by 160%. △ Less

Submitted 12 May, 2022; originally announced May 2022.

Probing the Optical Dynamics of Quantum Emitters in Hexagonal Boron Nitride

Authors: Raj N. Patel, David A. Hopper, Jordan A. Gusdorff, Mark E. Turiansky, Tzu-Yung Huang, Rebecca E. K. Fishman, Benjamin Porat, Chris G. Van de Walle, Lee C. Bassett

Abstract: Hexagonal boron nitride is a van der Waals material that hosts visible-wavelength quantum emitters at room temperature. However, experimental identification of the quantum emitters' electronic structure is lacking, and key details of their charge and spin properties remain unknown. Here, we probe the optical dynamics of quantum emitters in hexagonal boron nitride using photon emission correlation… ▽ More Hexagonal boron nitride is a van der Waals material that hosts visible-wavelength quantum emitters at room temperature. However, experimental identification of the quantum emitters' electronic structure is lacking, and key details of their charge and spin properties remain unknown. Here, we probe the optical dynamics of quantum emitters in hexagonal boron nitride using photon emission correlation spectroscopy. Several quantum emitters exhibit ideal single-photon emission with noise-limited photon antibunching, $g^{(2)}(0)=0$. The photoluminescence emission lineshapes are consistent with individual vibronic transitions. However, polarization-resolved excitation and emission suggests the role of multiple optical transitions, and photon emission correlation spectroscopy reveals complicated optical dynamics associated with excitation and relaxation through multiple electronic excited states. We compare the experimental results to quantitative optical dynamics simulations, develop electronic structure models that are consistent with the observations, and discuss the results in the context of ab initio theoretical calculations. △ Less

Submitted 21 January, 2022; originally announced January 2022.

Comments: 31 pages, 16 figures, 6 tables

Journal ref: PRX Quantum 3, 030331 (2022)

Role of carbon and hydrogen in limiting $n$-type doping of monoclinic (Al$_x$Ga$_{1-x}$)$_2$O$_3$

Authors: Sai Mu, Mengen Wang, Joel B. Varley, John L. Lyons, Darshana Wickramaratne, Chris G. Van de Walle

Abstract: We use hybrid density functional calculations to assess n-type doping in monoclinic (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys. We focus on Si, the most promising donor dopant, and study the structural properties, formation energies and charge-state transition levels of its various configurations. We also explore the impact of C and H, which are common impurities in metal-organic chemical vapor deposition… ▽ More We use hybrid density functional calculations to assess n-type doping in monoclinic (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys. We focus on Si, the most promising donor dopant, and study the structural properties, formation energies and charge-state transition levels of its various configurations. We also explore the impact of C and H, which are common impurities in metal-organic chemical vapor deposition (MOCVD). In Ga$_2$O$_3$, Si$_{Ga}$ is an effective shallow donor, but in Al$_2O_3$ Si$_{Al}$ acts as a DX center with a (+/-) transition level in the band gap. Interstitial H acts as a shallow donor in Ga$_2$O$_3$, but behaves as a compensating acceptor in n-type Al$_2O_3$. Interpolation indicates that Si is an effective donor in (Al$_x$Ga$_{1-x}$)$_2$O$_3$ up to 70% Al, but it can be compensated by H already at 1% Al. We also assess the diffusivity of H and study complex formation. Si$_{cation}$-H complexes have relatively low binding energies. Substitutional C on a cation site acts as a shallow donor in Ga$_2$O$_3$, but can be stable in a negative charge state in (Al$_x$Ga$_{1-x}$)$_2$O$_3$ when x>5%. Substitutional C on an O site (C$_O$) always acts as an acceptor in n-type (Al$_x$Ga$_{1-x}$)$_2$O$_3$, but will incorporate only under relatively O-poor conditions. C$_O$-H complexes can actually incorporate more easily, explaining observations of C-related compensation in Ga$_2$O$_3$ grown by MOCVD. We also investigate C$_{cation}$-H complexes, finding they have high binding energies and act as compensating acceptors when x>56%; otherwise the H just passivates the unintentional C donors. C-H complex formation explains why MOCVD grown Ga$_2$O$_3$ can exhibit record-low free-carrier concentrations, in spite of the unavoidable incorporation of C. Our study highlights that, while Si is a suitable shallow donor in ALGO alloys, control of unintentional impurities is essential to avoid compensation. △ Less

Submitted 23 January, 2022; v1 submitted 13 November, 2021; originally announced November 2021.

Comments: 17 pages, 13 figures

Journal ref: Physical Review B 105, 155201(2022)

Piezoelectric effect and polarization switching in Al$_{1-x}$Sc$_x$N

Authors: Haochen Wang, Nicholas Adamski, Sai Mu, Chris G. Van de Walle

Abstract: Aluminum nitride is piezoelectric and exhibits spontaneous polarization along the $c$-axis, but the polarization cannot be switched by applying an electric field. Adding Sc to AlN enhances the piezoelectric properties, and can make the alloy ferroelectric. We perform a detailed first-principles analysis of spontaneous and piezoelectric polarization. Comparisons between explicit supercell calculati… ▽ More Aluminum nitride is piezoelectric and exhibits spontaneous polarization along the $c$-axis, but the polarization cannot be switched by applying an electric field. Adding Sc to AlN enhances the piezoelectric properties, and can make the alloy ferroelectric. We perform a detailed first-principles analysis of spontaneous and piezoelectric polarization. Comparisons between explicit supercell calculations show that the virtual crystal approximation produces accurate results for polarization, but falls short in describing the phase stability of the alloy. We relate the behavior of the piezoelectric constant $e_{33}$ to the microscopic behavior of the internal displacement parameter $u$, finding that the internal strain contribution dominates in the Sc-induced enhancement. The value of $u$ increases with scandium concentration, bringing the alloy locally closer to a layered hexagonal structure. Our approach allows us to calculate the ferroelectric switching barrier, which we analyze as a function of Sc concentration and temperature based on Ginzburg-Landau theory. △ Less

Submitted 15 May, 2021; originally announced May 2021.

Comments: 7 pages, 6 figures

Journal ref: Journal of Applied Physics 130, 104101 (2021)

First-principles study of electron transport in ScN

Authors: Sai Mu, Andrew J. E. Rowberg, Joshua Leveillee, Feliciano Giustino, Chris G. Van de Walle

Abstract: We investigate the conduction-band structure and electron mobility in rocksalt ScN based on density functional theory. The first-principles band structure allows us to obtain band velocities and effective masses as a function of energy. Electron-phonon scattering is assessed by explicitly computing the $q$-dependent electron-phonon matrix elements, with the inclusion of the long-range electrostati… ▽ More We investigate the conduction-band structure and electron mobility in rocksalt ScN based on density functional theory. The first-principles band structure allows us to obtain band velocities and effective masses as a function of energy. Electron-phonon scattering is assessed by explicitly computing the $q$-dependent electron-phonon matrix elements, with the inclusion of the long-range electrostatic interaction. The influence of free-carrier screening on the electron transport is assessed using the random phase approximation. We find a notable enhancement of electron mobility when the carrier concentration exceeds 10$^{20}$ cm$^{-3}$. We calculate the room-temperature electron mobility in ScN to be 587 cm$^2$/Vs at low carrier concentrations. When the carrier concentration is increased, the electron mobility starts to decrease significantly around $n=10^{19}$ cm$^{-3}$, and drops to 240 cm$^2$/Vs at $n=10^{21}$ cm$^{-3}$. We also explore the influence of strain in (111)- and (100)-oriented ScN films. For (111) films, we find that a 1.0\% compressive epitaxial strain increases the in-plane mobility by 72 cm$^2$/Vs and the out-of-plane mobility by 50 cm$^2$/Vs. For (100) films, a 1.0\% compressive epitaxial strain increases the out-of-plane mobility by as much as 172 cm$^2$/Vs, but has a weak impact on the in-plane mobility. Our study sheds light on electron transport in ScN at different electron concentrations and shows how strain engineering could increase the electron mobility. △ Less

Submitted 15 May, 2021; originally announced May 2021.

Comments: 12 pages, 11 figures

Journal ref: Phys. Rev. B 104, 075118 (2021)

Hydride Conductivity in Nitride Hydrides

Authors: Andrew J. E. Rowberg, Chris G. Van de Walle

Abstract: Nitride hydrides are a largely unexplored class of materials with promising applications in solid-state hydrogen fuel cells. Here, we use first-principles calculations to characterize defects and ionic mobility in Sr$_2$LiH$_2$N (SLHN), a nitride hydride with high hydride conductivity. Calculating defect formation energies, we find that SLHN contains high concentrations of hydrogen interstitials (… ▽ More Nitride hydrides are a largely unexplored class of materials with promising applications in solid-state hydrogen fuel cells. Here, we use first-principles calculations to characterize defects and ionic mobility in Sr$_2$LiH$_2$N (SLHN), a nitride hydride with high hydride conductivity. Calculating defect formation energies, we find that SLHN contains high concentrations of hydrogen interstitials (H$_i$). H$_i^-$ migrates with very low energetic barriers, which, together with its low formation energy, implies that SLHN will have excellent hydride kinetics, potentially surpassing those of other known hydride electrolytes. Oxygen contamination is a concern, meaning that encapsulation will be critical. By direct analogy to the La/Sr-based oxyhydrides, which have similar crystal structures, we also investigate the La-based nitride hydride La$_2$LiHN$_2$ but find that it will be significantly less conductive, and thus not as technologically useful. Our findings buttress the exploration of SLHN and similar nitride hydrides for use in solid-state hydrogen fuel cells. △ Less

Submitted 10 May, 2021; originally announced May 2021.

Comments: 13 pages, 6 figures

Understanding Carbon Contamination in Proton Conducting Oxides

Authors: Andrew J. E. Rowberg, Michael W. Swift, Chris G. Van de Walle

Abstract: Carbon contamination is a significant concern for proton-conducting oxides in the cerate and zirconate family, particularly for BaCeO$_3$. Here, we use first-principles calculations to evaluate carbon stability in SrCeO$_3$, BaCeO$_3$, SrZrO$_3$, and BaZrO$_3$. The cerates require more carbon-poor environments to prevent carbonate formation, though this requirement can be loosened through the use… ▽ More Carbon contamination is a significant concern for proton-conducting oxides in the cerate and zirconate family, particularly for BaCeO$_3$. Here, we use first-principles calculations to evaluate carbon stability in SrCeO$_3$, BaCeO$_3$, SrZrO$_3$, and BaZrO$_3$. The cerates require more carbon-poor environments to prevent carbonate formation, though this requirement can be loosened through the use of more oxygen-poor growth conditions. Carbonate formation is not the only concern, however. We find that interstitial carbon has lower formation energies in the cerates relative to the zirconates, leading to higher carbon concentrations that compete with the desired oxygen vacancy formation. We also examine the mobility of carbon interstitials, finding that both migration barriers and binding energies to acceptors are lower in the cerates. As a result, the cerates are likely to degrade when exposed to carbon at operating temperatures. Our results show definitively why the cerates are less stable than the zirconates with respect to carbon and elucidate the mechanisms contributing to their instability, thereby helping to explain why alloying with zirconium will enhance their operational efficiency. △ Less

Submitted 10 May, 2021; originally announced May 2021.

Comments: 11 pages, 3 figures. Supporting Information: 5 pages, 2 figures

Vibrational and vibronic structure of isolated point defects: the nitrogen-vacancy center in diamond

Authors: Lukas Razinkovas, Marcus W. Doherty, Neil B. Manson, Chris G. Van de Walle, Audrius Alkauskas

Abstract: We present a theoretical study of vibrational and vibronic properties of a point defect in the dilute limit by means of first-principles density functional theory calculations. As an exemplar we choose the negatively charged nitrogen-vacancy center, a solid-state system that has served as a testbed for many protocols of quantum technology. We achieve low effective concentrations of defects by cons… ▽ More We present a theoretical study of vibrational and vibronic properties of a point defect in the dilute limit by means of first-principles density functional theory calculations. As an exemplar we choose the negatively charged nitrogen-vacancy center, a solid-state system that has served as a testbed for many protocols of quantum technology. We achieve low effective concentrations of defects by constructing dynamical matrices of large supercells containing tens of thousands of atoms. The main goal of the paper is to calculate luminescence and absorption lineshapes due to coupling to vibrational degrees of freedom. The coupling to symmetric $a_1$ modes is computed via the Huang-Rhys theory. Importantly, to include a nontrivial contribution of $e$ modes we develop an effective methodology to solve the multi-mode $E \otimes e$ Jahn-Teller problem. Our results show that for NV centers in diamond a proper treatment of $e$ modes is particularly important for absorption. We obtain good agreement with experiment for both luminescence and absorption. Finally, the remaining shortcomings of the theoretical approach are critically reviewed. The presented theoretical approach will benefit identification and future studies of point defects in solids. △ Less

Submitted 6 August, 2021; v1 submitted 8 December, 2020; originally announced December 2020.

Comments: 21 pages, 17 figures

Journal ref: Phys. Rev. B 104, 045303 (2021)

Nonrad: Computing Nonradiative Capture Coefficients from First Principles

Authors: Mark E. Turiansky, Audrius Alkauskas, Manuel Engel, Georg Kresse, Darshana Wickramaratne, Jimmy-Xuan Shen, Cyrus E. Dreyer, Chris G. Van de Walle

Abstract: Point defects in semiconductor crystals provide a means for carriers to recombine nonradiatively. This recombination process impacts the performance of devices. We present the Nonrad code that implements the first-principles approach of Alkauskas et al. [Phys. Rev. B 90, 075202 (2014)] for the evaluation of nonradiative capture coefficients based on a quantum-mechanical description of the capture… ▽ More Point defects in semiconductor crystals provide a means for carriers to recombine nonradiatively. This recombination process impacts the performance of devices. We present the Nonrad code that implements the first-principles approach of Alkauskas et al. [Phys. Rev. B 90, 075202 (2014)] for the evaluation of nonradiative capture coefficients based on a quantum-mechanical description of the capture process. An approach for evaluating electron-phonon coupling within the projector augmented wave formalism is presented. We also show that the common procedure of replacing Dirac delta functions with Gaussians can introduce errors into the resulting capture rate, and implement an alternative scheme to properly account for vibrational broadening. Lastly, we assess the accuracy of using an analytic approximation to the Sommerfeld parameter by comparing with direct numerical evaluation. △ Less

Submitted 22 November, 2020; v1 submitted 14 November, 2020; originally announced November 2020.

Radiative capture rates at deep defects from electronic structure calculations

Authors: Cyrus E. Dreyer, Audrius Alkauskas, John L. Lyons, Chris G. Van de Walle

Abstract: We present a methodology to calculate radiative carrier capture coefficients at deep defects in semiconductors and insulators from first principles. Electronic structure and lattice relaxations are accurately described with hybrid density functional theory. Calculations of capture coefficients provide an additional validation of the accuracy of these functionals in dealing with localized defect st… ▽ More We present a methodology to calculate radiative carrier capture coefficients at deep defects in semiconductors and insulators from first principles. Electronic structure and lattice relaxations are accurately described with hybrid density functional theory. Calculations of capture coefficients provide an additional validation of the accuracy of these functionals in dealing with localized defect states. We also discuss the validity of the Condon approximation, showing that even in the event of large lattice relaxations the approximation is accurate. We test the method on GaAs:$V_\text{Ga}$-$\text{Te}_\text{As}$ and GaN:C$_\text{N}$, for which reliable experiments are available, and demonstrate very good agreement with measured capture coefficients. △ Less

Submitted 6 August, 2020; originally announced August 2020.

Comments: 6 pages, 2 figures

Electronic structure and magneto-optical properties of silicon-nitrogen-vacancy complexes in diamond

Authors: Marcin Roland Zemła, Kamil Czelej, Paulina Kamińska, Chris G. Van de Walle, Jacek A. Majewski

Abstract: The silicon-vacancy (SiV) and nitrogen-vacancy (NV) centers in diamond are commonly regarded as prototypical defects for solid-state quantum information processing. Here we show that when silicon and nitrogen are simultaneously introduced into the diamond lattice these defects can strongly interact and form larger complexes. Nitrogen atoms strongly bind to Si and SiV centers and complex formation… ▽ More The silicon-vacancy (SiV) and nitrogen-vacancy (NV) centers in diamond are commonly regarded as prototypical defects for solid-state quantum information processing. Here we show that when silicon and nitrogen are simultaneously introduced into the diamond lattice these defects can strongly interact and form larger complexes. Nitrogen atoms strongly bind to Si and SiV centers and complex formation can occur. Using a combination of hybrid density functional theory (DFT) and group theory, we analyze the electronic structure and provide various useful physical properties, such as hyperfine structure, quasi-local vibrational modes, and zero-phonon line, to enable experimental identification of these complexes. We demonstrate that the presence of substitutional silicon adjacent to nitrogen significantly shifts the donor level toward the conduction band, resulting in an activation energy for the SiN center that is comparable to phosphorus. We also find that the neutral SiNV center is of particular interest due to its photon emission at $\sim$1530 nm, which falls within the C band of telecom wavelengths, and its paramagnetic nature. In addition, the optical transition associated with the SiNV$^0$ color center exhibits very small electron--phonon coupling (Huang--Rhys factor~=~0.78) resulting in high quantum efficiency (Debye-Waller factor = 46\%) for single-photon emission. These features render this new center very attractive for potential application in scalable quantum telecommunication networks. △ Less

Submitted 21 June, 2020; originally announced June 2020.

Comments: 12 pages, 9 figures, 2 tables

Journal ref: Phys. Rev. B 102, 115102 (2020)

Giant polarization charge density at lattice-matched GaN/ScN interfaces

Authors: Nicholas L. Adamski, Cyrus E. Dreyer, Chris G. Van de Walle

Abstract: Rocksalt ScN is a semiconductor with a small lattice mismatch to wurtzite GaN. Within the modern theory of polarization, ScN has a nonvanishing formal polarization along the [111] direction. As a result, we demonstrate that an interface between (0001) GaN and (111) ScN exihibts a large polarization discontinuity of $-$1.358 $\rm Cm^{-2}$. Interfaces between ScN and wurtzite III-nitrides will exhib… ▽ More Rocksalt ScN is a semiconductor with a small lattice mismatch to wurtzite GaN. Within the modern theory of polarization, ScN has a nonvanishing formal polarization along the [111] direction. As a result, we demonstrate that an interface between (0001) GaN and (111) ScN exihibts a large polarization discontinuity of $-$1.358 $\rm Cm^{-2}$. Interfaces between ScN and wurtzite III-nitrides will exhibit a high-density electron gas on the (000$\bar{1}$) GaN interface or a hole gas on the (0001) GaN interface, with carrier concentrations up to $8.5 \times 10^{14}$ cm$^{-2}$. The large polarization difference and small strain makes ScN a desirable choice for polarization-enhanced tunnel junctions within the III-nitride materials system. The large sheet carrier densities may also be useful for contacts or current spreading layers. △ Less

Submitted 19 December, 2019; v1 submitted 30 September, 2019; originally announced October 2019.

Comments: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Applied Physics Letters {\bf 115}, 232103 (2019) and may be found at https://aip.scitation.org/doi/10.1063/1.5126717

Journal ref: Appl. Phys. Lett. 115, 232103 (2019)

Limitations of In$_2$O$_3$ as a transparent conducting oxide

Authors: H. Peelaers, E. Kioupakis, C. G. Van de Walle

Abstract: Sn-doped In$_2$O$_3$ or ITO is the most widely used transparent conducting oxide. We use first-principles calculations to investigate the limitations to its transparency due to free-carrier absorption mediated by phonons or charged defects. We find that the main contribution to the phonon-assisted indirect absorption is due to emission (as opposed to absorption) of phonons, which explains why the… ▽ More Sn-doped In$_2$O$_3$ or ITO is the most widely used transparent conducting oxide. We use first-principles calculations to investigate the limitations to its transparency due to free-carrier absorption mediated by phonons or charged defects. We find that the main contribution to the phonon-assisted indirect absorption is due to emission (as opposed to absorption) of phonons, which explains why the process is relatively insensitive to temperature. The wavelength dependence of this indirect absorption process can be described by a power law. Indirect absorption mediated by charged defects or impurities is also unavoidable since doping is required to obtain conductivity. At high carrier concentrations, screening by the free carriers becomes important. We find that charged-impurity-assisted absorption becomes larger than phonon-assisted absorption for impurity concentrations above 10$^{20}$ cm$^{-3}$. The differences in the photon-energy dependence of the two processes can be explained by band-structure effects. △ Less

Submitted 31 July, 2019; originally announced July 2019.

Comments: 5+3 pages, 4+2 figures

Journal ref: Appl. Phys. Lett. 115, 082105 (2019)

Carbon dimer defect as a source of the 4.1 eV luminescence in hexagonal boron nitride

Authors: Mazena Mackoit-Sinkeviciene, Marek Maciaszek, Chris G. Van de Walle, Audrius Alkauskas

Abstract: We propose that the carbon dimer defect in hexagonal boron nitride gives rise to the ubiquitous narrow luminescence band with a zero-phonon line of 4.08 eV (usually labeled the 4.1 eV band). Our first-principles calculations are based on hybrid density functionals that provide a reliable description of wide band-gap materials. The calculated zero-phonon line energy of 4.3 eV is close to the experi… ▽ More We propose that the carbon dimer defect in hexagonal boron nitride gives rise to the ubiquitous narrow luminescence band with a zero-phonon line of 4.08 eV (usually labeled the 4.1 eV band). Our first-principles calculations are based on hybrid density functionals that provide a reliable description of wide band-gap materials. The calculated zero-phonon line energy of 4.3 eV is close to the experimental value, and the deduced Huang-Rhys factor of ${S \approx 2.0}$, indicating modest electron-phonon coupling, falls within the experimental range. The optical transition occurs between two localized $π$-type defects states, with a very short radiative lifetime of 1.2 nanoseconds, in very good accord with experiments. △ Less

Submitted 25 November, 2019; v1 submitted 4 July, 2019; originally announced July 2019.

Comments: 4 pages, 3 figures

Journal ref: Appl. Phys. Lett. 115, 212101 (2019)

Unusual Formation of Point Defect Complexes in the Ultra-wide Band Gap Semiconductor beta-Ga2O3

Authors: Jared M. Johnson, Zhen Chen, Joel B. Varley, Christine M. Jackson, Esmat Farzana, Zeng Zhang, Aaron R. Arehart, Hsien-Lien Huang, Arda Genc, Steven A. Ringel, Chris G. Van de Walle, David A. Muller, Jinwoo Hwang

Abstract: Understanding the unique properties of ultra-wide band gap semiconductors requires detailed information about the exact nature of point defects and their role in determining the properties. Here, we report the first direct microscopic observation of an unusual formation of point defect complexes within the atomic scale structure of beta-Ga2O3 using high resolution scanning transmission electron mi… ▽ More Understanding the unique properties of ultra-wide band gap semiconductors requires detailed information about the exact nature of point defects and their role in determining the properties. Here, we report the first direct microscopic observation of an unusual formation of point defect complexes within the atomic scale structure of beta-Ga2O3 using high resolution scanning transmission electron microscopy (STEM). Each complex involves one cation interstitial atom paired with two cation vacancies. These divacancy - interstitial complexes correlate directly with structures obtained by density functional theory, which predicts them to be compensating acceptors in beta-Ga2O3. This prediction is confirmed by a comparison between STEM data and deep level optical spectroscopy results, which reveals that these complexes correspond to a deep trap within the band gap, and that the development of the complexes is facilitated by Sn doping through the increase in vacancy concentration. These findings provide new insight on this emerging material's unique response to the incorporation of impurities that can critically influence their properties. △ Less

Submitted 1 July, 2019; originally announced July 2019.

Journal ref: Phys. Rev. X 9, 041027 (2019)

Defect identification based on first-principles calculations for deep level transient spectroscopy

Authors: Darshana Wickramaratne, Cyrus E. Dreyer, Bartomeu Monserrat, Jimmy-Xuan Shen, John L. Lyons, Audrius Alkauskas, Chris G. Van de Walle

Abstract: Deep level transient spectroscopy (DLTS) is used extensively to study defects in semiconductors. We demonstrate that great care should be exercised in interpreting activation energies extracted from DLTS as ionization energies. We show how first-principles calculations of thermodynamic transition levels, temperature effects of ionization energies, and nonradiative capture coefficients can be used… ▽ More Deep level transient spectroscopy (DLTS) is used extensively to study defects in semiconductors. We demonstrate that great care should be exercised in interpreting activation energies extracted from DLTS as ionization energies. We show how first-principles calculations of thermodynamic transition levels, temperature effects of ionization energies, and nonradiative capture coefficients can be used to accurately determine actual activation energies that can be directly compared with DLTS. Our analysis is illustrated with hybrid functional calculations for two important defects in GaN that have similar thermodynamic transition levels, and shows that the activation energy extracted from DLTS includes a capture barrier that is temperature dependent, unique to each defect, and in some cases large in comparison to the ionization energy. By calculating quantities that can be directly compared with experiment, first-principles calculations thus offer powerful leverage in identifying the microscopic origin of defects detected in DLTS. △ Less

Submitted 11 October, 2018; originally announced October 2018.

Comments: Main text and Supplementary Material

Sr$_3$Ir$_2$O$_7$F$_2$: Topochemical conversion of a relativistic Mott state into a spin-orbit driven band insulator

Authors: Christi Peterson, Michael W. Swift, Zach Porter, Raphaele J. Clement, Guang Wu, G. H. Ahn, S. J. Moon, B. C. Chakoumakos, Jacob P. C. Ruff, Huibo Cao, Chris Van de Walle, Stephen D. Wilson

Abstract: The topochemical transformation of single crystals of Sr$_3$Ir$_2$O$_7$ into Sr$_3$Ir$_2$O$_7$F$_2$ is reported via fluorine insertion. Characterization of the newly formed Sr$_3$Ir$_2$O$_7$F$_2$ phase shows a nearly complete oxidation of Ir$^{4+}$ cations into Ir$^{5+}$ that in turn drives the system from an antiferromagnetic Mott insulator with a half-filled J$_{eff}=1/2$ band into a nonmagnetic… ▽ More The topochemical transformation of single crystals of Sr$_3$Ir$_2$O$_7$ into Sr$_3$Ir$_2$O$_7$F$_2$ is reported via fluorine insertion. Characterization of the newly formed Sr$_3$Ir$_2$O$_7$F$_2$ phase shows a nearly complete oxidation of Ir$^{4+}$ cations into Ir$^{5+}$ that in turn drives the system from an antiferromagnetic Mott insulator with a half-filled J$_{eff}=1/2$ band into a nonmagnetic $J=0$ band insulator. First principles calculations reveal a remarkably flat insertion energy that locally drives the fluorination process to completion. Band structure calculations support the formation of a band insulator whose charge gap relies on the strong spin-orbit coupling inherent to the Ir metal ions of this compound. △ Less

Submitted 6 October, 2018; originally announced October 2018.

Comments: 10 pages, 7 figures

Journal ref: Phys. Rev. B 98, 155128 (2018)

Posner molecules: From atomic structure to nuclear spins

Authors: Michael W. Swift, Chris G. Van de Walle, Matthew P. A. Fisher

Abstract: We investigate "Posner molecules", calcium phosphate clusters with chemical formula Ca$_9$(PO$_4$)$_6$. Originally identified in hydroxyapatite, Posner molecules have also been observed as free-floating molecules $in$ $vitro$. The formation and aggregation of Posner molecules have important implications for bone growth, and may also play a role in other biological processes such as the modulation… ▽ More We investigate "Posner molecules", calcium phosphate clusters with chemical formula Ca$_9$(PO$_4$)$_6$. Originally identified in hydroxyapatite, Posner molecules have also been observed as free-floating molecules $in$ $vitro$. The formation and aggregation of Posner molecules have important implications for bone growth, and may also play a role in other biological processes such as the modulation of calcium and phosphate ion concentrations within the mitochondrial matrix. In this work, we use a first-principles computational methodology to study the structure of Posner molecules, their vibrational spectra, their interactions with other cations, and the process of pairwise bonding. Additionally, we show that the Posner molecule provides an ideal environment for the six constituent $^{31}\text{P}$ nuclear spins to obtain very long spin coherence times. $In$ $vitro$, the spins could provide a platform for liquid-state nuclear magnetic resonance quantum computation. $In$ $vivo$, the spins may have medical imaging applications. The spins have also been suggested as "neural qubits" in a proposed mechanism for quantum processing in the brain. △ Less

Submitted 15 November, 2017; originally announced November 2017.

Comments: 8 pages, 6 figures

Journal ref: Phys. Chem. Chem. Phys. 20, 12373 (2018)

Electron doping in $\text{Sr}_3\text{Ir}_2\text{O}_7$: collapse of band gap and magnetic order

Authors: Michael W. Swift, Zach Porter, Stephen D. Wilson, Chris G. Van de Walle

Abstract: The electron-doping-driven collapse of the charge gap and staggered magnetization of the spin-orbit-assisted Mott insulator Sr$_{3}$Ir$_{2}$O$_{7}$ is explored via first-principles computational methods. In the antiferromagnetic phase, the gap and magnetization are observed to decrease slowly with increasing doping, with an abrupt collapse of both the gap and the magnetization at an electron conce… ▽ More The electron-doping-driven collapse of the charge gap and staggered magnetization of the spin-orbit-assisted Mott insulator Sr$_{3}$Ir$_{2}$O$_{7}$ is explored via first-principles computational methods. In the antiferromagnetic phase, the gap and magnetization are observed to decrease slowly with increasing doping, with an abrupt collapse of both the gap and the magnetization at an electron concentration corresponding to 4.8\% substitution of Sr with La, in excellent agreement with experiment. Additionally, we describe the structural effects of electron doping in Sr$_{3}$Ir$_{2}$O$_{7}$ via a competition between the steric effect from smaller La atoms substituted within the lattice and the dominant doping-driven deformation-potential effect. Curiously, our first-principles calculations fail to capture the low-temperature structural distortion reported in the low-gap phase of Sr$_{3}$Ir$_{2}$O$_{7}$, supporting the notion that this distortion arises as a secondary manifestation of an unconventional electronic order parameter in this material. △ Less

Submitted 31 July, 2018; v1 submitted 11 November, 2017; originally announced November 2017.

Comments: 6 pages, 4 figures, PRB Rapid Communication (accepted version)

Journal ref: Phys. Rev. B 98, 081106 (2018)

Linear hyperfine tuning of donor spins in silicon using hydrostatic strain

Authors: John Mansir, Pierandrea Conti, Zaiping Zeng, Jarryd J. Pla, Patrice Bertet, Michael W. Swift, Chris G. Van de Walle, Mike L. W. Thewalt, Benoit Sklenard, Yann-Michel Niquet, John J. L. Morton

Abstract: We experimentally study the coupling of Group V donor spins in silicon to mechanical strain, and measure strain-induced frequency shifts which are linear in strain, in contrast to the quadratic dependence predicted by the valley repopulation model (VRM), and therefore orders of magnitude greater than that predicted by the VRM for small strains $|\varepsilon| < 10^{-5}$. Through both tight-binding… ▽ More We experimentally study the coupling of Group V donor spins in silicon to mechanical strain, and measure strain-induced frequency shifts which are linear in strain, in contrast to the quadratic dependence predicted by the valley repopulation model (VRM), and therefore orders of magnitude greater than that predicted by the VRM for small strains $|\varepsilon| < 10^{-5}$. Through both tight-binding and first principles calculations we find that these shifts arise from a linear tuning of the donor hyperfine interaction term by the hydrostatic component of strain and achieve semi-quantitative agreement with the experimental values. Our results provide a framework for making quantitative predictions of donor spins in silicon nanostructures, such as those being used to develop silicon-based quantum processors and memories. The strong spin-strain coupling we measure (up to 150~GHz per strain, for Bi-donors in Si), offers a method for donor spin tuning --- shifting Bi donor electron spins by over a linewidth with a hydrostatic strain of order $10^{-6}$ --- as well as opportunities for coupling to mechanical resonators. △ Less

Submitted 23 March, 2018; v1 submitted 2 October, 2017; originally announced October 2017.

Journal ref: Phys. Rev. Lett. 120, 167701 (2018)

Conditions for $T^2$ resistivity from electron-electron scattering

Authors: Michael Swift, Chris G. Van de Walle

Abstract: Many complex oxides (including titanates, nickelates and cuprates) show a regime in which resistivity follows a power law in temperature ($ρ\propto T^2$). By analogy to a similar phenomenon observed in some metals at low temperature, this has often been attributed to electron-electron (Baber) scattering. We show that Baber scattering results in a $T^2$ power law only under several crucial assumpti… ▽ More Many complex oxides (including titanates, nickelates and cuprates) show a regime in which resistivity follows a power law in temperature ($ρ\propto T^2$). By analogy to a similar phenomenon observed in some metals at low temperature, this has often been attributed to electron-electron (Baber) scattering. We show that Baber scattering results in a $T^2$ power law only under several crucial assumptions which may not hold for complex oxides. We illustrate this with sodium metal ($ρ_\text{el-el}\propto T^2$) and strontium titanate ($ρ_\text{el-el}\not\propto T^2$). We conclude that an observation of $ρ\propto T^2$ is not sufficient evidence for electron-electron scattering. △ Less

Submitted 17 January, 2017; originally announced January 2017.

Comments: 5 pages, 4 figures

Journal ref: Eur. Phys. J. B 90, 151 (2017)