Charged Ga Vacancies Research Articles

Electrical conductivity improvement of charged Ga vacancies in wurtzite GaN

This investigation explores the complex role of Ga vacancy charged defect in the crystalline structure of wurtzite Gallium Nitride (GaN), highlighting a significant increase in electrical conductivity—contrary to conventional beliefs. Through advanced Density Functional Theory (DFT) calculations, the study reveals that specific configurations of Ga vacancy charged defect contribute to a sixfold increase in electrical conductivity in wurtzite GaN. However, it is crucial to note that N vacancy charged defects negatively impact these electrical properties. This detailed theoretical analysis sheds light on how these vacancy charged defects can positively affect conductivity and provides a novel perspective on charged defects in GaN, paving the way for the optimized design of GaN in the future.

High temperature annealing of n-type bulk β-Ga2O3: Electrical compensation and defect analysis—The role of gallium vacancies

The effect of high temperature annealing under O2 and N2 atmospheres on the electrical properties and defect formation on Sn doped n-type β-Ga2O3 bulk samples was investigated by electron paramagnetic resonance (EPR) spectroscopy. EPR, being a volume sensitive technique, probes the entire sample volume. Our results show an electrical compensation being correlated with the formation of a negatively charged Ga vacancy defect VGa2−. This VGa center is different from the one observed after particle irradiation. The associated shift of the Fermi level reveals the presence of Fe3+, Cu2+, and Cr3+, which are residuals related to the growth conditions. The 1100 °C N2 annealed sample is fully compensated, and the neutral donor resonance is no longer observed. Our results directly confirm the thermal instability of Ga and Sn in n-type conducting samples. No oxygen vacancy related centers were detected. We discuss the various Ga vacancy centers reported previously.

Experimental and theoretical study on the green persistent luminescence phosphor Ca5Ga6O14:Bi3+

In this study, we developed a type of green persistent luminescence (PersL) material Ca5Ga6O14:Bi3+, which could be excited by UV light ranging from 270 nm to 360 nm with peak at 325 nm and exhibited a broad afterglow band peaking at 515 nm with the duration of about 11 min, which can be ascribed to the electron transitions between the 6s orbital and the 6p orbitals of Bi3+ ion. The thermoluminescence investigation and the First-principle calculation results revealed that the 6s orbital of Bi3+ was about 0.2 eV above the valence band maximum, while the 6p orbitals were 0.6–1.0 eV below the conduction band minimum. Moreover, we found that O vacancies and −2 charged Ga vacancies could form defect levels just above valence band and serve as the hole-trap centers, playing the key role in the process of the afterglow of Ca5Ga6O14:Bi3+. The possible afterglow mechanism was discussed in detail. This work offers an efficient way to construct Bi3+ doped PersL phosphors with desired properties.

Low-Energy Ion-Induced Single-Event Burnout in Gallium Oxide Schottky Diodes

Low-energy ion-induced breakdown and single-event burnout are experimentally observed in β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Schottky diodes with voltages well below those of expected electrical breakdown. Fundamentally different responses were observed among alpha-particle, Cf-252, and heavy-ion irradiation. TCAD simulations suggest that ion-induced burnout can be triggered at high voltages as a result of a single ion strike, leading to impact ionization, voltage-induced charge separation accentuated by the low intrinsic hole mobility in β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , and breakdown. At significantly lower voltages, the cumulative buildup of displacement-damage-induced defects in β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> during high-fluence ion irradiation can lead to defect-driven breakdown due to the generation and motion of negatively charged Ga vacancies and O interstitials. First-principles calculations show that defect clusters can be formed that are much less resistive than the surrounding material. These clusters can be driven deeply into the device by the reverse bias, ultimately forming conduction paths that can facilitate the destruction of the device at reduced voltages.

Strain effects on the behavior of intrinsic point defects within the GaN/AlN interface

Defect behaviors in GaN-based compounds studied by using density functional theory have exhibited widely practical applications for optoelectronic devices. Ga is usually used to solubilize in AlN to release the lattice mismatch and the interface strain. In this study, we investigate the formation energies and diffusion barriers of intrinsic point defects within the GaN/AlN interface. The intrinsic defect with the lowest formation energy is positively charged N vacancy, and negatively charged Ga vacancy is the defect with second lower formation energy. Compared with the case in bulk GaN, the GaN/AlN interface promotes the generation of Ga vacancy and inhibits the generation of N vacancy. Meanwhile, the defect diffusion barrier of N vacancy within the GaN/AlN interface is higher than that in the bulk GaN. The hydrostatic compressive/ tensile strain significantly promotes the generation of these two vacancy defects, while biaxial tensile strain and the biaxial compressive strain slightly promote and inhibit the generation of these two vacancy defects, respectively. These interface and strain effects provide the significant information in understanding the electrical properties of GaN-based devices, especially under the extreme conditions. Our work therefore not only elucidates defect behaviors under different strains in GaN/AlN interface, but also paves the way for understanding and designing promising electronic devices with interface engineering.

The Study of Magnetism in Un-doped 3C-GaN: The First-Principles Calculations

We performed first-principles calculations to investigate the electronic structures and magnetic properties of neutral and charged Ga vacancies of 3C-GaN. Our results show that the neutral, −1 charged, and −2 charged Ga vacancies can introduce the magnetic moments of 3 μB, 2 μB, and 1 μB, respectively. We found that both −1 charged Ga vacancies and −2 charged Ga vacancies couple ferromagnetically, while the −1 charged Ga vacancies have lower formation energy compared with the −2 charged ones, which may be responsible for the unexpected ferromagnetism in the system. However, the neutral Ga vacancies couple antiferromagnetically, which cannot contribute to the ferromagnetism.

Probing nanoscale potential modulation by defect-induced gap states on GaAs(110) with light-modulated scanning tunneling spectroscopy

We investigated charged defects on an n-GaAs(110) surface using light-modulated scanning tunneling spectroscopy. Tunneling via a single defect-induced gap state under photoillumination was observed for the isolated atomic defects. Screened Coulomb potentials induced around a charged Ga vacancy and a step edge were visualized, for the first time, with a nanometer spatial resolution. Furthermore, the charge states of the individual defects were determined on the atomic level.

Introduction and recovery of Ga and N sublattice defects in electron-irradiated GaN

We have used positron annihilation spectroscopy to study the introduction and recovery of point defects introduced by 0.45 and $2\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ electron irradiation at room temperature in $n$-type GaN. Isochronal annealings were performed up to $1220\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We observe vacancy defects with specific lifetime of ${\ensuremath{\tau}}_{V}=190\ifmmode\pm\else\textpm\fi{}15\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ that we tentatively identify as N vacancies or related complexes in the neutral charge state in the samples irradiated with $0.45\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ electrons. The N vacancies are produced at a rate ${\ensuremath{\Sigma}}_{\mathrm{N}}^{0.45}\ensuremath{\simeq}0.25\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The irradiation with $2\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ electrons produces negatively charged Ga vacancies and negative nonopen volume defects (negative ions) originating from the Ga sublattice, at a rate ${\ensuremath{\Sigma}}_{\mathrm{Ga}}^{2.0}\ensuremath{\simeq}5\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The irradiation-induced N vacancies anneal out of the samples at around $600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, possibly due to the motion of the irradiation-induced N interstitials. Half of the irradiation-induced Ga vacancies anneal out of the samples also around $600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and this is interpreted as the isolated Ga vacancies becoming mobile with a migration barrier of ${E}_{M}^{V,\mathrm{Ga}}=1.8\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Interestingly, we observe a change of charge state of the irradiation-induced negative ions from $2\ensuremath{-}$ to $1\ensuremath{-}$ likely due to a reconstruction of the defects in two stages at annealing temperatures of about 600 and $700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The negative ions anneal out of the samples together with the other half of the Ga vacancies (stabilized by, e.g., N vacancies and/or hydrogen) in thermal annealings at $800--1100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Dominant intrinsic acceptors in GaN and ZnO**Invited paper.

Positron annihilation measurements reveal negatively charged Ga vacancies in n-type GaN and Zn vacancies in n-type ZnO. Positron trapping at other negative defects is not observed, indicating that cation vacancies are the dominant acceptors in these materials. The vacancy concentrations are the same as the total acceptor densities determined in Hall experiments, confirming the dominant role of the vacancy defects. The Ga vacancy in GaN is found as the main compensating centre over the range of four orders of magnitude of intentional oxygen doping.

Scanning Tunneling Microscopy and Barrier-Height Imaging of Subsurface Dopants on GaAs(110)

Subsurface dopant atoms are investigated by scanning tunneling microscopy (STM) and barrier height (BH) imaging. The obtained BH image shows a local reduction in BH -(1–2) eV at positively charged donor sites while it shows a local increase in BH (1–5) eV at negatively charged surface Ga vacancies. Our results indicate that the charge-sensitive imaging can be accomplished by utilizing the STM–BH technique. A simple one-dimensional simulation of tunneling current through a STM junction on the GaAs(110) surface indicates that the results of BH imaging well reflect the charge state of the subsurface dopant or charged surface defects under appreciable electron accumulation conditions.

Ga vacancies and grain boundaries in GaN

We have applied a low-energy positron beam to study epitaxial Si-doped GaN layers, where the grain size varies from 0.2 to 2–5 μm. Negatively charged Ga vacancies are found in n-type samples. Their concentration is independent of the grain size, suggesting that Ga vacancies exist in the grain interior. Positrons are observed to get trapped also at other negatively charged centers. The positron trapping rate at these defects correlates with the grain-boundary density. We attribute the observed shallow positron traps, which do not contain open volume, to negatively charged edge-type dislocations which define the grain boundaries.

Diffusion in isotopically controlled semiconductor systems

Isotopically controlled heterostructures of 28Si/natSi and Al71GaAs/Al69GaAs/71GaAs have been used to study the self-diffusion process in this elemental and compound semiconductor material. The directly measured Si self-diffusion coefficient is compared with the self-interstitial and vacancy contribution to self-diffusion which were deduced from metal diffusion experiments. The remarkable agreement between the Si self-diffusion coefficients and the individual contributions to self-diffusion shows that both self-interstitials and vacancies mediate Si self-diffusion. The Ga self-diffusion in undoped AlGaAs was found to decrease with increasing Al concentration. The activation enthalpy of Ga and Al diffusion in GaAs and of Ga diffusion in AlGaAs all lie in the range of (3.6±0.1)eV, but with different pre-exponential factors. The doping dependence of Ga self-diffusion reveals a retardation (enhancement) of Ga diffusion under p-type (n-type) doping compared to intrinsic conditions. All experimental results on the group-III atom diffusion are accurately described if vacancies on the group-III sublattice are assumed to mediate the Ga self- and Al–Ga interdiffusion in undoped AlGaAs and the Ga self-diffusion in Be- and Si-doped GaAs with an active dopant concentration of 3×1018cm−3. The doping dependence of Ga self-diffusion in GaAs provides strong evidence that neutral, singly and doubly charged Ga vacancies govern the self-diffusion process.

Enhanced and retarded Ga self-diffusion in Si and Be doped GaAs isotope heterostructures

Ga self-diffusion in undoped, Si- and Be-doped 71GaAs/ n at GaAs isotope heterostructures has been investigated at temperatures between 736°C and 1050°C. Concentration profiles of the Ga isotopes measured separately by secondary ion mass spectrometry reveal retarded (enhanced) self-diffusion when doped p-type (n-type), compared to intrinsic conditions. Electrochemical C–V profiling was performed to determine the free carrier concentrations before and after annealing. Detailed analysis of the doping dependence of Ga self-diffusion includes a compensation of Si donors by negatively charged vacancies and shows that one native defect, presumably the vacancy on the Ga sublattice, in the neutral, singly and doubly negatively charged states governs the self-diffusion process. The energy levels E V − − E v =0.42 eV and E V 2− − E v =0.60 eV of the singly and doubly negatively charged gallium vacancy have been deduced from the doping dependence of Ga self-diffusion. No significant contribution from the triply negatively charged Ga vacancy has been found. Our evaluation of the doping dependence of Ga self-diffusion is compared with literature data for the group-III atom diffusion in the AlGaAs material system. Seemingly inconsistent literature data can now be described consistently on the basis of our results. Finally, thermal equilibrium concentrations and transport coefficients of Ga vacancies in GaAs in their various charge states are calculated for different doping conditions taking into account the vacancy related energy levels and the thermal equilibrium concentration of neutral vacancies which were deduced from our self-diffusion data.

Suppression of superlattice intermixing by p-type doping

We report a systematic investigation of the effect of n- and p-type doping on the AlGaAs GaAs superlattice (SL) intermixing using photoluminescence. We find that the SL intermixing is not only enhanced by Si doping but also suppressed by Be doping. Although previous models have suggested the contribution of triply negatively charged Ga vacancies, V 3 − Ga , in n-type and intrinsic materials and of doubly positively charged Ga interstitials, I 2 + Ga , in p-type materials, we find that at 900°C the Al-Ga interdiffusion is mediated by singly negatively charged Ga vacancies, V − Ga , in both p- and n-type materials for n < 1 × 10 18 cm − 3 . These results are shown to be explained consistently in terms of the Fermi-level effect. Our results demonstrate that the thermal stability of AlGaAs GaAs heterostructures can be controlled, i.e., weakened or strengthened, by appropriate tuning of the Fermi level.

Quantification of Electrical Deactivation by Triply Negative Charged Ga Vacancies in Highly Doped Thin GaAs Layers

We present for the first time a theoretical approach to electrical deactivation by triply negative charged Ga vacancies (V Ga3-) in highly doped thin n-GaAs layers grown by molecular beam epitaxy, and quantify their deactivation under as-grown and annealed conditions. We also show that thinning of n-GaAs epitaxial layers results in low-level electrical deactivation. This effect is apparently caused by the fact that thinning of the doped layers results in lowering of the Fermi energy in the doped layers, and thereby inhibition of the generation of V Ga3- acceptors. Furthermore, we deduce from the results of this study the thermal equilibrium concentration of V Ga3- in intrinsic GaAs. The resulting expression is [V Ga3-(i)]=5.37×1031 exp (-4.64 eV/k BT) cm-3.

Theory of hydrogen-decorated gallium vacancies in GaAs and of their radiative complexes.

Complexes formed by neutral or charged Ga vacancies and one or more H atoms have been investigated by using a pseudopotential-local-density-functional approach. Stable and metastable configurations have been found with some hydrogen atoms saturating As dangling bonds around the Ga vacancy. In this case, the electronic energy levels of the vacancy, although slightly lowered toward the valence band, remain in the energy gap. When one H atom is located at a bond center position next to a H-decorated vacancy, a donor level appears in the energy gap. The donor-acceptor pairs formed by this H atom and the Ga vacancy can account for the low-energy luminescence bands reported in hydrogenated p-type GaAs.

Are the Models of the Triply Charged Gallium Vacancy and Doubly Charged Gallium Interstitial Alive or Dead?

AbstractWe have found that the measured diffusivity can be quite different than predicted by recent models of multiply-charged defects. Key problems dealing with the measurement and the interpretation of diffusion mechanisms are discussed. Using a few common variations in GaAs epilayer growth, we have obtained diffusivities which range over several orders of magnitude at the same temperature. Making use of the relatively weak In-As bond, we have used In as a marker to measure group III interdiffusion, DIII. DIII is consistent with the results of others using Al as a marker in n-type GaAs, but orders of magnitude smaller than predicted by the triply charged Ga vacancy, V3−Ga, model of Tan and Gosele. Although diffusion can be attributed to a negatively charged vacancy in n-type GaAs, In is found to often move by a kick-out mechanism in p-type GaAs. It appears likely that many early experiments with n- and p-type GaAs-AlAs interdiffusion were affected by large concentrations of Ga interstitials, IGa, caused by Fermi energy pinning at the growing surface. We present the first direct experimental evidence for the existence of a positively charged Ga interstitial. Our results, combined with those of others, suggest that VGa has a single negative charge associated with it. Because epilayer growth conditions appear to cause the point defect concentrations to deviate substantially from equilibrium, we conclude that the exact charge states of VGa and IGa still remain to be determined.

Thermal equilibrium concentrations of point defects in gallium arsenide

Expressions of the thermal equilibrium concentrations of point defects in GaAs, including the neutral and charged species, are derived. These expressions are explicit functions of well-defined thermodynamic quantities, which in turn yield explicit expressions for the reaction constant K in the usual As 4 pressure power law representation of the point defect thermal equilibrium concentrations. Such power laws have been of little quantitative value in the past, because values for K were not known. In the present derivation, emphases are placed upon the difference between the Gibbs free energies of an As atom in the interior of a GaAs crystal and in an As vapor phase molecule, and the role of the crystal Fermi level. Numerical values of the thermal equilibrium concentrations of the neutral and three negatively charged Ga vacancies (V 0 Ga, V − Ga, V 2− Ga and V 3− Ga), the neutral As vacancies V 0 As and the two neutral antisite de Ga 0 As and As 0 Ga have been obtained. The calculated thermal equilibrium concentration of the anion antisite defect As 0 Ga reaches a peak value of about 1 sx 10 17 cm −3 and is practically temperature independent, in agreement with experimental findings. The thermal equilibrium concentrations of the triply negatively charged Ga vacancy V 3− Ga, C eq v 3− Ga ( n), have been found to exhibit a temperature independence or a negati temperature dependence behavior under strong n-doping conditions. That is, the C eq v 3− Ga ( n) value is either unchanged or increases as the temperature is lowered. This C eq v 3− Ga ( n) property provided explanations to a number of outstanding experimental results, either requiring the interpretation that V 3− Ga has attained its thermal equilibrium concentration at the onset of each experiment, or requiring mechanisms involving point defect nonequilibrium phenomena. The calculated C eq v 3− Ga ( n) values are in agreement with available experimental results.

Simulation of the transient indiffusion-segregation process of triply negatively charged Ga vacancies in GaAs and AlAs/GaAs superlattices

In GaAs and AlAs/GaAs superlattice crystals containing n-type regions, several sets of recent experimental results obtained from diffusion studies require the interpretation that the responsible point defect species, the triply negatively charged Ga vacancy (VGa3−), has attained its thermal equilibrium concentration (CVGa3−eq) at the onset of an experiment. This could be due to either the fact that under heavy n-doping conditions CVGa3−eq is fairly temperature independent, or the fact that the transient process of populating VGa3− from an undersaturated to the appropriate CVGa3−eq value via indiffusion from the surfaces to the interior of the crystals is extremely rapid. We have simulated the transient process of populating VGa3− to the crystal interior. The experiments use crystals consisting of adjacent intrinsic and n-type regions for which CVGa3−eq values are different, leading to the simultaneous occurrence of VGa3− diffusion and segregation phenomena. A diffusion-segregation equation has been derived and subsequently used in the simulation calculations. The simulation results showed that, as long as n-type regions are involved, such transient processes are ineffective and therefore cannot explain the experimental requirement that VGa3− is already present in the appropriate CVGa3−eq(n) value at the onset of an experiment. On the other hand, the transient process is sufficiently rapid for the purely intrinsic crystal cases. These simulation results support our recent finding that the CVGa3−eq(n) values are essentially temperature independent, obtained via a thermodynamic treatment.

A study of Si outdiffusion from predoped GaAs

Experiments have been conducted to study the Si outdiffusion behavior in GaAs using predoped samples. The results showed that the Si diffusivity values are dependent on the As4 vapor-phase pressure in the ambient, and on the electron concentration in the crystal. It is concluded from these results that, in GaAs, diffusion of the Si donor species occupying Ga sites SiGa+ is governed by the triply negatively charged Ga vacancies, VGa3−. The present VGa3−-dominated SiGa+ outdiffusion diffusivity values are, however, larger than those obtained under Si indiffusion conditions by many orders of magnitude. A tentative explanation of this large difference is given in terms of an undersaturation of VGa3− in intrinsic GaAs during indiffusion experiments and of a supersaturation of VGa3− developed during the outdiffusion of Si from n-type, Si-doped GaAs.