17 Search Results

Absmore » tract Chalcogen vacancies in the semiconducting monolayer transition-metal dichalcogenides (TMDs) have frequently been invoked to explain a wide range of phenomena, including both unintentional p-type and n-type conductivity, as well as sub-band gap defect levels measured via tunneling or optical spectroscopy. These conflicting interpretations of the deep versus shallow nature of the chalcogen vacancies are due in part to shortcomings in prior first-principles calculations of defects in the semiconducting two-dimensional TMDs that have been used to explain experimental observations. Here we report results of hybrid density functional calculations for the chalcogen vacancy in a series of monolayer TMDs, correctly referencing the thermodynamic charge transition levels to the fundamental band gap (as opposed to the optical band gap). We find that the chalcogen vacancies are deep acceptors and cannot lead to n-type or p-type conductivity. Both the (0/−1) and (−1/−2) transition levels occur in the gap, leading to paramagnetic charge states $S=1/2$ and S  = 1, respectively, in a collinear-spin representation. We discuss trends in terms of the band alignments between the TMDs, which can serve as a guide to future experimental studies of vacancy behavior.« less

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

We report corundum (Al_xGa_1-x)₂O₃ alloys have been proposed as a candidate ultrawide-bandgap oxide for a number of applications, but doping is unexplored. We examine the prospects for n-type doping with H, Si, Ge, Sn, Hf, Zr, and Ta in corundum (Al_xGa_1-x)₂O₃ alloys using first-principles calculations. All of the dopants are shallow donors in corundum Ga₂O₃. In the (Al_xGa_1-x)₂O₃ alloy, they transition from shallow to deep donors at Al compositions that are unique to each donor. Si and Hf remain shallow donors up to the highest Al contents in corundum (Al_xGa_1-x)₂O₃ alloys and are still shallow even as the (Al_xGa_1-x)₂O₃ bandgapmore » exceeds 6.5 eV. Finally, we address the detrimental role of cation vacancies as compensating deep acceptors and suggest that doping in a hydrogen-rich environment under cation-rich conditions can be used to overcome this problem.« 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

Hole localization is a root cause of difficulties in p-type doping of aluminum nitride. Herein, the stability of localized holes in AlN and their susceptibility to self-trapping within bulk material or in the presence of isovalent elements and acceptor impurities are calculated. It is found that self-trapped holes are metastable in bulk AlN, and also exhibit a very low barrier to detrapping. However, holes become trapped in the presence of acceptor impurities as well as isovalent elements such as B, In, and Sc, and may contribute to the broad luminescence observed in AlN alloys. It is also found that holemore » trapping contributes to the large ionization energies for acceptors in AlN. Most acceptors can also trap a second hole, becoming positively charged and further exacerbating difficulties in doping aluminum nitride p type. Furthermore, the stability of trapped holes is found to scale with atomic size mismatch to Al.« less

Semiconductor nanoplatelets (NPLs), with their large exciton binding energy, narrow photoluminescence (PL), and absence of dielectric screening for photons emitted normal to the NPL surface, could be expected to become the fastest luminophores amongst all colloidal nanostructures. However, super-fast emission is suppressed by a dark (optically passive) exciton ground state, substantially split from a higher-lying bright (optically active) state. Here, the exciton fine structure in 2-8 monolayer (ML) thick Cs_{n - 1}Pb_nBr_{3n + 1}NPLs is revealed by merging temperature-resolved PL spectra and time-resolved PL decay with an effective mass model taking quantum confinement and dielectric confinement anisotropy into account. Thismore » approach exposes a thickness-dependent bright-dark exciton splitting reaching 32.3 meV for the 2 ML NPLs. The model also reveals a 5-16 meV splitting of the bright exciton states with transition dipoles polarized parallel and perpendicular to the NPL surfaces, the order of which is reversed for the thinnest NPLs, as confirmed by TR-PL measurements. Accordingly, the individual bright states must be taken into account, while the dark exciton state strongly affects the optical properties of the thinnest NPLs even at room temperature. Significantly, the derived model can be generalized for any isotropically or anisotropically confined nanostructure.« less

The Rashba effect has been proposed to give rise to a bright exciton ground state in halide perovskite nanocrystals (NCs), resulting in very fast radiative recombination at room temperature and extremely fast radiative recombination at low temperature. In this paper we find the dispersion of the “Rashba exciton”, i.e., the exciton whose bulk dispersion reflects large spin–orbit Rashba terms in the conduction and valence bands and thus has minima at non-zero quasi-momenta. Placing Rashba excitonsin quasi-2D cylindrical quantum dots, we calculate size-dependent levels of confined excitons and their oscillator transition strengths. Here, we consider the implications of this model formore » two-dimensional hybrid organic–inorganic perovskites, discuss generalizations of this model to 3D NCs, and establish criteria under which a bright ground exciton state could be realized.« less

We investigate the properties of heavily C-doped GaN grown by hydride vapor phase epitaxy using both optical experiments and hybrid density functional theory calculations. Previous work has established that carbon acceptors (C_N) give rise to a yellow luminescence band near 2.2 eV along with a blue luminescence band near 2.9 eV. Photoluminescence measurements show the yellow band shifting as a function of carbon concentration, suggesting a change in the behavior of carbon species as carbon content increases. With hybrid density functional theory we calculate the electrical and optical behavior of carbon centers containing multiple carbon impurities, which may arise inmore » heavily doped material. We compare the behavior of these complexes to the isolated centers, and find that the dicarbon donor-acceptor (C_Ga–C_N) complex is a candidate to explain the shift in the yellow luminescence peak. Tricarbon complexes have high formation energies and modest binding energies, and also give rise to optical transitions that are inconsistent with the observed spectra. We also identify the split dicarbon interstitial on the gallium site as a low-energy species with a large binding energy that may act to compensate carbon acceptors. Here, local vibrational modes are calculated for carbon impurity centers, and we compare these results to recent experiments. Dicarbon and tricarbon complexes involving C_Ga and C_N exhibit modes that are only slightly higher than the isolated species, while carbon interstitials and related complexes give rise to vibrational modes much higher than C_Ga and C_N.« less

Attaining control over the electrical conductivity of gallium nitride through impurity doping is one of the foremost achievements in semiconductor science. Yet, unwanted contaminants and point defects continue to limit device performance, and experimental techniques alone are insufficient for elucidating the behavior of these unintentionally incorporated species. Methodological advancements have made first-principles calculations more powerful than ever and capable of quantitative predictions, though care must still be taken in comparing results from theory and experiment. In this Tutorial, we explain the basic concepts that define the behavior of dopants, unintentional impurities, and point defects in GaN. We also describe howmore » to interpret experimental results in the context of theoretical calculations and also discuss how the properties of defects and impurities vary in III-nitride alloys. Lastly, we examine how the physics of defects and impurities in GaN is relevant for understanding other wide-bandgap semiconductor materials, such as the II–IV-nitrides, boron nitride, and the transition metal nitrides.« less

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:VGa-TeAs and GaN:CN, for which reliable experiments are available, and demonstrate very good agreement with measured capture coefficients.