48 Search Results

The ability to achieve highly resistive beta-phase gallium oxide (β-Ga₂O₃) layers and substrates is critical for β-Ga₂O₃ high voltage and RF devices. To date, the most common approach involves doping with iron (Fe), which generates a moderately deep acceptor-like defect state located at E_C-0.8 eV in the β-Ga₂O₃ bandgap. Recently, there has been growing interest in alternative acceptors, such as magnesium (Mg) and nitrogen (N), due to their predicted deeper energy levels, which could avoid inadvertent charge modulation during device operation. In this work, a systematic study that makes direct correlations between the introduction of N using ion implantation and themore » observation of a newly observed deep level at E_C-2.9 eV detected by deep-level optical spectroscopy (DLOS) is presented. The concentration of this state displayed a monotonic dependence with N concentration over a range of implant conditions, as confirmed by secondary ion mass spectrometry (SIMS). With a near 1:1 match in absolute N and E_C-2.9 eV trap concentrations from SIMS and DLOS, respectively, which also matched the measured removal of free electrons from capacitance-voltage studies, this indicates that N contributes a very efficiently incorporated compensating defect. Density functional theory calculations confirm the assignment of this state to be an N (0/-1) acceptor with a configuration of N occupying the oxygen site III [N_O(III)]. The near ideal efficiency for this state to compensate free electrons and its location toward the midgap region of the β-Ga₂O₃ bandgap demonstrates the potential of N doping as a promising approach for producing semi-insulating β-Ga₂O₃.« less

Individually addressed Er³⁺ ions in solid-state hosts are promising resources for quantum repeaters, because of their direct emission in the telecom band and their compatibility with silicon photonic devices. While the Er³⁺ electron spin provides a spin-photon interface, ancilla nuclear spins could enable multiqubit registers with longer storage times. In this work, we demonstrate coherent coupling between the electron spin of a single Er³⁺ ion and a single I = 1/2 nuclear spin in the solid-state host crystal, which is a fortuitously located proton (¹H). We control the nuclear spin using dynamical-decoupling sequences applied to the electron spin, implementing one-more » 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 toward combining long-lived nuclear spin quantum registers with telecom-wavelength emitters for long-distance quantum repeaters.« less

The iodine vacancy (V_I) has frequently been discussed as a strong nonradiative recombination center in halide perovskites. This proposition was mainly based on the presence of charge-state transition levels in the band gap, as found in early first-principles calculations. In this work, we perform accurate hybrid-density-functional calculations for V_I in CsPbI₃, CsSnI₃, and CsGeI₃ and find that V_I does not have any transition levels in the band gap in CsPbI₃, in contrast to the results from calculations based on semilocal functionals. The iodine vacancy V_I does introduce levels in the band gap in CsSnI₃ and CsGeI₃, but our explicitly computedmore » nonradiative capture coefficients demonstrate that V_I has a negligible impact on nonradiative recombination. Our study corrects a misunderstanding of the role of V_I in the iodide-based perovskites, and shifts the focus toward identifying and mitigating actual recombination centers in order to further improve the optoelectronic performance.« less

Diffusion of the n-type dopant Sn in β-Ga ₂ O ₃ is studied using secondary-ion mass spectrometry combined with hybrid functional calculations. The diffusion of Sn from a Sn-doped bulk substrate with surface orientation (001) into an epitaxial layer is observed after heat treatments in the temperature range of 1050–1250 °C. Calculated formation energies of Sn-related and intrinsic defects show that the migration of Sn is mediated by Ga vacancies ( V _Ga ) through the formation and dissociation of intermittent mobile V _Ga Sn _Ga complexes. The evolution of the Sn concentration vs depth profiles after heat treatments can bemore » well described by a reaction–diffusion model. Using model parameters guided by the hybrid functional calculations, we extract a V _Ga Sn _Ga complex migration barrier of 3.0 ± 0.4 eV with a diffusion coefficient of 2 × 10 ⁻¹ cm ² /s. The extracted migration barrier is consistent with our theoretical predictions using the nudged elastic band method, which shows migration barriers of 3.42, 3.15, and 3.37 eV for the [100], [010], and [001] directions, respectively.« less

Pathways and energy barriers for the migration of Ga vacancies (V_Ga) and Ga interstitials (Ga_i) in β–Ga₂O₃ are explored using hybrid functional calculations and the nudged elastic band method. Considering β–Ga₂O₃ as primarily being an n-type semiconductor, we focus on defect charge states relevant under such conditions: $$V^{3 –}_{Ga}$$, Ga$$^{3+}_{i}$$, and Ga$$^{+}_{i}$$. Notably, we describe a mechanism by which V_Ga can transform between its different split configurations. In all cases, the intermediate state consists of a vacancy split between three Ga sites—a three-split vacancy—which enables passage over a significantly lower energy barrier. This is because it avoids the unfavorable simplemore » vacancy at the tetrahedral Ga site. The proposed mechanism lowers the overall barrier for $$V^{3–}_{Ga}$$ diffusion along the [001] crystal direction from 1.73 to 0.97 eV, whereas the 2.08 eV barrier for the [100] and [010] directions is unaffected. For Ga$$^{3+}_{i}$$, we obtain similar overall migration barriers of 0.72, 0.80, and 1.02 eV for the [010], [001], and [100] directions, respectively. Ga$$^{+}_{i}$$ exhibits a strong preference for diffusion within the large eight-sided channel; the overall migration barrier is 0.92 eV for the [010] direction, and 2.16 eV for the [001] and [100] directions. The limiting step for the two latter directions involves ionization of Ga$$^{+}_{i}$$ followed by a jump to an adjacent large eight-sided channel as Ga$$^{3+}_{i}$$. Finally, our results are discussed in light of experimental observations of thermally activated recovery processes in irradiated material.« less

Beryllium oxide (BeO) is a promising host for quantum defects because of its ultrawide band gap. We conducted comprehensive first-principles investigations of the native point defects in BeO using density functional theory with a hybrid functional. We found that the beryllium and oxygen vacancies are the most stable defects, whereas other native defects such as interstitials or antisites have high formation energies. We investigate the point defects as candidates for quantum defects by examining spin states and internal optical transitions. Here, the oxygen vacancy ($$V$$$^{+}_{O}$$) emerges as a suitable spin qubit or single-photon emitter; we also find its stability canmore » be enhanced by forming a (V_O – Li_Be)⁰ complex with a Li acceptor. The $$O$$$^{–}_{Be}$$ antisite also has desirable optical and spin properties. Overall, because of its desirable properties as a host material, BeO could be an excellent host for quantum defects, with $$V$$$^{+}_{O}$$, (V_O – Li_Be)⁰, and $$O$$$^{–}_{Be}$$ as prime candidates.« less

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, ZnGeN₂ and MgSiN₂. 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 ZnGeN₂ and MgSiN₂ results in the inversion of the heavy hole band and split-off hole band, thereby lowering the effective hole mass in themore » compression direction. We find that lattice matching to ZnGeN₂ and MgSiN₂ induces an increase in the room-temperature hole mobility by 50% and 260% as compared to unstrained GaN, respectively. Further, 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_0.75Ge_0.75Mg_0.25Si_0.25N₂ would already increase the hole mobility by 160%.« less

In recent years, the impressive photovoltaic performance of halide perovskites has been commonly attributed to their defect tolerance. This attribution is seemingly intuitive and has been widely promoted in the field, though it has not been rigorously assessed. In this Perspective, we critically discuss the proposition of defect tolerance in halide perovskites based on first-principles calculations. We show that halide perovskites actually do suffer from defect-assisted nonradiative recombination, i.e., they are not defect tolerant. The nonradiative recombination rates in halide perovskites are comparable to or even greater than those in more conventional semiconductors. We note that to obtain accurate defectmore » properties in halide perovskites, the level of theory and computational details are highly important, which was previously not sufficiently recognized. A distinctive feature of halide perovskites is that they can be grown with moderate defect densities using low-cost deposition techniques. But, similar to the case of conventional semiconductors, defect engineering is still key to improving the efficiency of perovskite solar cells.« less

Inmore » this study, we use hybrid density functional calculations to assess $n$-type doping in monoclinic $({\mathrm{Al}}_x{\mathrm{Ga}}_{1–x}{)}_2{\mathrm{O}}_3$ alloys. We focus on silicon, 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 carbon and hydrogen, which are common impurities in metal-organic chemical vapor deposition (MOCVD). ${\mathrm{Ga}}_2{\mathrm{O}}_3,{\mathrm{Si}}_{\mathrm{Ga}}$ is an effective shallow donor, but in ${\mathrm{Al}}_2{\mathrm{O}}_3{\mathrm{Si}}_{\mathrm{Al}}$ acts as a $\mathit{DX}$ center with a ( $+/–$) transition level in the band gap. Interstitial hydrogen acts as a shallow donor in ${\mathrm{Ga}}_2{\mathrm{O}}_3$ but behaves as a compensating acceptor in $n$-type ${\mathrm{Al}}_2{\mathrm{O}}_3$. Interpolation indicates that Si is an effective donor in $({\mathrm{Al}}_x{\mathrm{Ga}}_{1–x}{)}_2{\mathrm{O}}_3$ up to 70% Al, but it can be compensated by hydrogen already at 1% Al. We also assess the diffusivity of hydrogen and study complex formation. ${\mathrm{Si}}_{\mathrm{cation}}\text{–}\mathrm{H}$ complexes have relatively low binding energies. Substitutional carbon on a cation site acts as a shallow donor in ${\mathrm{Ga}}_2{\mathrm{O}}_3$, but can be stable in a negative charge state in $({\mathrm{Al}}_x{\mathrm{Ga}}_{1–x}{)}_2{\mathrm{O}}_3$ when $x>5\%$. Substitutional carbon on an oxygen site ( ${\mathrm{C}}_{\mathrm{O}}$) always acts as an acceptor in $n$-type $({\mathrm{Al}}_x{\mathrm{Ga}}_{1–x}{)}_2{\mathrm{O}}_3$, but will incorporate only under relatively oxygen-poor conditions. ${\mathrm{C}}_{\mathrm{O}}\text{–}\mathrm{H}$ complexes can actually incorporate more easily, explaining observations of carbon-related compensation in ${\mathrm{Ga}}_2{\mathrm{O}}_3$ grown by MOCVD. We also investigate ${\mathrm{C}}_{\mathrm{cation}}\text{–}\mathrm{H}$ complexes, finding they have high binding energies and act as compensating acceptors when $x>56\%$; otherwise the hydrogen just passivates the unintentional carbon donors. C-H complex formation explains why MOCVD-grown ${\mathrm{Ga}}_2{\mathrm{O}}_3$ can exhibit record-low free-carrier concentrations, in spite of the unavoidable incorporation of carbon. Our study highlights that, while Si is in principle a suitable shallow donor in $({\mathrm{Al}}_x{\mathrm{Ga}}_{1–x}{)}_2{\mathrm{O}}_3$ alloys up to high Al compositions, control of unintentional impurities is essential to avoid compensation.« less

Epitaxial Sc_xAl_1-xN thin films of ~100 nm thickness grown on metal polar GaN substrates are found to exhibit significantly enhanced relative dielectric permittivity (ε_r) values relative to AlN. ε_rvalues of ~17–21 for Sc mole fractions of 17%–25% ( x = 0.17–0.25) measured electrically by capacitance–voltage measurements indicate that Sc_xAl_1-xN has the largest relative dielectric permittivity of any existing nitride material. Since epitaxial Sc_xAl_1-xN layers deposited on GaN also exhibit large polarization discontinuity, the heterojunction can exploit the in situ high-K dielectric property to extend transistor operation for power electronics and high-speed microwave applications.