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The combination of an ultrawide band gap and controllable $n$-type conductivity makes monoclinic gallium sesquioxide a promising material for high-power electronics. However, this technological development will require accurate knowledge about the identity and properties of prominent deep-level defects in the material. This work explores close-associate Ga-O divacancies. Owing to the low symmetry of the crystal structure, divacancies can potentially occur in a plethora of crystallographically inequivalent configurations. Hybrid functional calculations were performed to shed light on the relative stability of different divacancy configurations, the energy barriers for transformation between them, and trends in their electrical properties.

The evolution of electrical resistance as function of defect concentration is examined for the unipolar n-conducting oxides CdO, β-Ga 2 O 3 , In 2 O 3 , SnO 2 and ZnO in order to explore the predictions of the amphoteric defect model. Intrinsic defects are introduced by ion irradiation at cryogenic temperatures, and the resistance is measured in-situ by current–voltage sweeps as a function of irradiation dose. Temperature dependent Hall effect measurements are performed to determine the carrier concentration and mobility of the samples before and after irradiation. After the ultimate irradiation step, the Ga 2 O 3 and SnO2 samples have both turned highly resistive. In contrast, the In 2 O 3 and ZnO samples are ultimately found to be less resistive than prior to irradiation, however, they both show an increased resistance at intermediate doses. Based on thermodynamic defect charge state transitions computed by hybrid density functional theory, a model expanding on the current amphoteric defect model is proposed.

Deep-level transient spectroscopy measurements on β-Ga 2 O 3 crystals reveal the presence of three defect signatures labeled E 2 a, E 2 b, and E 3 with activation energies at around 0.66 eV, 0.73 eV, and 0.95 eV below the conduction band edge. Using secondary ion mass spectrometry, a correlation between the defect concentration associated with E 3 and the Ti concentration present in the samples was found. Particularly, it is found that E 3 is the dominant Ti-related defect in β-Ga 2 O 3 and is associated with a single Ti atom. This finding is further corroborated by hybrid functional calculations that predict Ti substituting on an octahedral Ga site, denoted as Ti GaII, to be a good candidate for E 3. Moreover, the deep level transient spectroscopy results show that the level previously labeled E 2 and attributed to Fe substituting on a gallium site (Fe Ga) consists of two overlapping signatures labeled E 2 a and E 2 b. We tentatively assign E 2 a and E 2 b to Fe substituting for Ga on a tetrahedral or an octahedral site, respectively.

The effect of lattice anisotropy on the diffusion of hydrogen (H)/deuterium (2H) in β-Ga2O3 was investigated using secondary ion mass spectrometry (SIMS) and hybrid-functional calculations. Concentration-depth profiles of 2H-implanted single crystals show that 2H can diffuse along the direction perpendicular to the (010) surface at temperatures as low as 300 °C, whereas diffusion along the direction perpendicular to the (-201) surface occurs only around 500 °C. For both directions, the evolution of the 2H concentration–depth profiles after heat treatments can be modeled by trap-limited diffusion. Moreover, the traps can be present in the as-received crystals or created during ion implantation. Comparison of the experimentally obtained binding energy for 2H to the trap (2.3 ± 0.2 eV) with the binding energies determined from first-principles calculations suggests that intrinsic point defects (e.g., V Ga ib) or defect complexes (e.g., V Ga ( 2 ) V O ( 2 )) are excellent candidates for the trap and will play a crucial role in the diffusion of H or 2H in β-Ga2O3.

Reliable single-photon emission is crucial for realizing efficient spin-photon entanglement and scalable quantum information systems. The silicon vacancy (VSi) in 4H-SiC is a promising single-photon emitter exhibiting millisecond spin coherence times, but suffers from low photon counts, and only one charge state retains the desired spin and optical properties. Here, we demonstrate that emission from VSi defect ensembles can be enhanced by an order of magnitude via fabrication of Schottky barrier diodes, and sequentially modulated by almost 50% via application of external bias. Furthermore, we identify charge state transitions of VSi by correlating optical and electrical measurements, and realize selective population of the bright state. Finally, we reveal a pronounced Stark shift of 55 GHz for the V1′ emission line state of VSi at larger electric fields, providing a means to modify the single-photon emission. The approach presented herein paves the way towards obtaining complete control of, and drastically enhanced emission from, VSi defect ensembles in 4H-SiC highly suitable for quantum applications., npj Quantum Information, 5 (1), ISSN:2056-6387

We investigate the migration mechanism of the carbon vacancy (V-C) in silicon carbide (SiC) using a combination of theoretical and experimental methodologies. The V-C, commonly present even in state-of-the-art epitaxial SiC material, is known to be a carrier lifetime killer and therefore strongly detrimental to device performance. The desire for V-C removal has prompted extensive investigations involving its stability and reactivity. Despite suggestions from theory that V(C )migrates exclusively on the C sublattice via vacancy-atom exchange, experimental support for such a picture is still unavailable. Moreover, the existence of two inequivalent locations for the vacancy in 4H-SiC [hexagonal, V-C(h), and pseudocubic, V-C(k)] and their consequences for V-C migration have not been considered so far. The first part of the paper presents a theoretical study of V(C )migration in 3C- and 4H-SiC. We employ a combination of nudged elastic band (NEB) and dimer methods to identify the migration mechanisms, transition state geometries, and respective energy barriers for V(C )migration. In 3C-SiC, V-C is found to migrate with an activation energy of E-A = 4.0 eV. In 4H-SiC, on the other hand, we anticipate that V-C migration is both anisotropic and basal-plane selective. The consequence of these effects is a slower diffusivity along the axial direction, with a predicted activation energy of E-A = 4.2 eV, and a striking preference for basal migration within the h plane with a barrier of E-A = 3.7 eV, to the detriment of the k-basal plane. Both effects are rationalized in terms of coordination and bond angle changes near the transition state. In the second part, we provide experimental data that corroborates the above theoretical picture. Anisotropic migration of V-C in 4H-SiC is demonstrated by deep level transient spectroscopy (DLTS) depth profiling of the Z(1/2) electron trap in annealed samples that were subject to ion implantation. Activation energies of E-A = (4.4 +/- 0.3) eV and E-A = (3.6 +/- 0.3) eV were found for V-C migration along the c and a directions, respectively, in excellent agreement with the analogous theoretical values. The corresponding prefactors of D-0 = 0.54 cm(2)/s and 0.017 cm(2)/s are in line with a simple jump process, as expected for a primary vacancy point defect. Funding Agencies|Research Council of Norway; University of Oslo through the frontier research project FUNDAMeNT [251131]; University of Oslo through the Norwegian Micro- and Nanofabrication Facility [NorFAB 245963]; Fundacao para a Ciencia e a Tecnologia (FCT) [UID/CTM/50025/2019]; FEDER funds through the COMPETE 2020 Program; NATO SPS programme [985215]; Swedish Energy Agency Energimyndigheten project [43611-1]; Swedish Government Strategic Research Area in Materials Science (AFM)

Hybrid functional calculations reveal the Zn-O divacancy in ZnO, consisting of adjacent Zn and O vacancies, as an electrically active defect exhibiting charge states ranging from $2+$ to $2-$ within the band gap. Notably, the divacancy retains key features of the monovacancies, namely the negative-\textit{U} behavior of the O vacancy, and the polaronic nature of the Zn vacancy. The thermodynamic charge-state transition levels associated with the negative-\textit{U} behavior $\varepsilon$($0$/$2-$), $\varepsilon$($-$/$2-$) and $\varepsilon$($0$/$-$) are predicted to occur at 0.22, 0.42 and 0.02 eV below the conduction band minimum, respectively, resulting in \textit{U} = $-$0.40 eV. These transition levels are moved closer to the conduction band and the magnitude of \textit{U} is lowered compared to the values for the O vacancy. Further, the interaction with hydrogen has been explored, where it is shown that the divacancy can accommodate up to three H atoms. The first two H atoms prefer to terminate O dangling bonds at the Zn vacancy, while the geometrical location of the third depends on the Fermi-level position. The calculated electrical properties of the divacancy are in excellent agreement with those reported for the E4 center observed by deep-level transient spectroscopy, challenging the O vacancy as a candidate for this level., Comment: 8 pages, 5 figures

This work explores the luminescence properties of self-trapped holes and impurity-related acceptors using one-dimensional configuration coordinate diagrams derived from hybrid functional calculations. The photoluminescence spectrum of as-grown β-Ga 2O 3 typically consists of a broad band in the wavelength region from ultraviolet to green and is often dominated by an impurity independent ultraviolet band that is commonly attributed to self-trapped holes. Here, we use the self-trapped hole as a benchmark to evaluate the accuracy of the theoretical defect luminescence spectra and estimate the optical properties of Mg Ga, Be Ga, Ca Ga, Cd Ga, Zn Ga, Li Ga, and N O acceptor impurities, as well as their complexes with hydrogen donors. We also explore V Ga acceptors complexed with hydrogen and Si Ga donor impurities. The results show that these defects can give rise to broad luminescence bands peaking in the infrared to visible part of the spectrum, making them potential candidates for the defect origin of broad luminescence bands in β-Ga 2O 3.

Results from hybrid density functional theory calculations on the thermodynamic stability and optical properties of the Zn vacancy ($V_{\text{Zn}}$) complexed with common donor impurities in ZnO are reported. Complexing $V_{\text{Zn}}$ with donors successively removes its charge-state transition levels in the band gap, starting from the most negative one. Interestingly, the presence of a donor leads only to modest shifts in the positions of the $V_{\text{Zn}}$ charge-state transition levels, the sign and magnitude of which can be interpreted from a polaron energetics model by taking hole-donor repulsion into account. By employing a one-dimensional configuration coordinate model, luminescence lineshapes and positions were calculated. Due to the aforementioned effects, the isolated $V_{\text{Zn}}$ gradually changes from a mainly non-radiative defect with transitions in the infrared region in \textit{n}-type material, to a radiative one with broad emission in the visible range when complexed with shallow donors., Comment: 9 pages, 5 figures

This work explores the Zn vacancy in ZnO using hybrid density functional theory calculations. The Zn vacancy is predicted to be an exceedingly deep polaronic acceptor that can bind a localized hole on each of the four nearest-neighbor O ions. The hole localization is accompanied by a distinct outward relaxation of the O ions, which leads to lower symmetry and reduced formation energy. Notably, we find that initial symmetry-breaking is required to capture this effect, which might explain the absence of polaronic hole localization in some previous hybrid density functional studies. We present a simple model to rationalize our findings with regard to the approximately equidistant thermodynamic charge-state transition levels. Furthermore, by employing a one-dimensional configuration coordinate model with parameters obtained from the hybrid density functional theory calculations, luminescence lineshapes were calculated. The results show that the isolated Zn vacancy is unlikely to be the origin of the commonly observed luminescence in the visible part of the emission spectrum from \textit{n}-type material, but rather the luminescence in the infrared region., 9 pages, 4 figures

Diffusion of lithium (Li) in uniformly gallium (Ga)-doped monocrystalline bulk zinc oxide (ZnO) is studied over a wide temperature range (500–1150 ° C) and is demonstrated to be dictated by the distribution of Ga. Below 800 ° C, the indiffusion of Li from a Li-doped ZnO sputtered film into n + single crystalline ZnO yields an abrupt and compensated Li-doped box region with the Li concentration matching the free-electron concentration, in accordance with several previous experimental and theoretical reports. However, experimental observations of Li-diffusion at higher temperatures reveal a dissociative diffusion mechanism for heat treatments up to 1150 ° C. By employing a reaction-diffusion model that includes both Li and Ga, a dissociation energy of 4.6 eV is obtained from the experimental Li diffusion data. This is in excellent agreement with theoretical results for the dissociation of ( Li Zn Ga Zn ) 0 ( 4.8 eV) into Li i + and (Ga ZnV Zn ) − and suggests that this neutral and stable acceptor-donor pair prevails in Li- and Ga-doped ZnO.Diffusion of lithium (Li) in uniformly gallium (Ga)-doped monocrystalline bulk zinc oxide (ZnO) is studied over a wide temperature range (500–1150 ° C) and is demonstrated to be dictated by the distribution of Ga. Below 800 ° C, the indiffusion of Li from a Li-doped ZnO sputtered film into n + single crystalline ZnO yields an abrupt and compensated Li-doped box region with the Li concentration matching the free-electron concentration, in accordance with several previous experimental and theoretical reports. However, experimental observations of Li-diffusion at higher temperatures reveal a dissociative diffusion mechanism for heat treatments up to 1150 ° C. By employing a reaction-diffusion model that includes both Li and Ga, a dissociation energy of 4.6 eV is obtained from the experimental Li diffusion data. This is in excellent agreement with theoretical results for the dissociation of ( Li Zn Ga Zn ) 0 ( 4.8 eV) into Li i + and (Ga ZnV Zn ) − and suggest...