Vacancies In GaAs Research Articles

Vacancies and defect levels in III–V semiconductors

Using electronic structure calculations, we systematically investigate the formation of vacancies in III-V semiconductors (III = Al, Ga, and In and V = P, As, and Sb), for a range of charges (−3≤q≤3) as a function of the Fermi level and under different growth conditions. The formation energies were corrected using the scheme due to Freysoldt et al. [Phys. Rev. Lett. 102, 016402 (2009)] to account for finite size effects. Vacancy formation energies were found to decrease as the size of the group V atom increased. This trend was maintained for Al-V, Ga-V, and In-V compounds. The negative-U effect was only observed for the arsenic vacancy in GaAs, which makes a charge state transition from +1 to –1. It is also found that even under group III rich conditions, group III vacancies dominate in AlSb and GaSb. For InSb, group V vacancies are favoured even under group V rich conditions.

Tunable Field Control Over the Binding Energy of Single Dopants by a Charged Vacancy in GaAs

Local manipulation of electric fields at the atomic scale may enable new methods for quantum transport and creates new opportunities for field control of ferromagnetism and spin-based quantum information processing in semiconductors. We used a scanning tunneling microscope to position charged arsenic (As) vacancies in the gallium arsenide 110 [GaAs(110)] surface with atomic precision, thereby tuning the local electrostatic field experienced by single manganese (Mn) acceptors. The effects of this field are quantified by measuring the shift of an acceptor state within the band gap of GaAs. Experiments with varying tip-induced band-bending conditions suggest a large binding energy for surface-layer Mn, which is reduced by direct Coulomb repulsion when the As vacancy is moved nearby.

Thermally activated charge reversibility of gallium vacancies in GaAs

The dominant charge state for the Ga vacancy in GaAs has been the subject of a long debate, with experiments suggesting −1, −2, or −3 as the best answer. We revisit this problem using ab initio calculations to compute the effects of temperature on the Gibbs free energy of formation, and we find that the thermal dependence of the Fermi level and of the ionization levels lead to a reversal of the preferred charge state as the temperature increases. Calculating the concentrations of gallium vacancies based on these results, we reproduce two conflicting experimental measurements, showing that these can be understood from a single set of coherent local density approximation results when thermal effects are included.

Nitrogen related vacancies in GaAs based quantum well superlattices

The authors report on the influence of nitrogen incorporation on vacancies in GaAs based superlattices. The samples were molecular beam epitaxy grown on p-type GaAs substrates with the superlattice structure consisting of ten periods of quantum well material separated by GaAs buffers. Three different quantum well compositions were used, Ga0.63In0.37As, Ga0.63In0.37N0.01As0.99, and GaN0.01As0.99. Rapid thermal anneals were performed on each sample set. Positron spectroscopy was used for vacancy detection in the superlattice structure. Annealed GaNAs and GaInNAs superlattice samples were found to contain vacancy-type defects. A comparison with photoluminescence measurements shows that the detected vacancy-type defects are not optically active.

Self-vacancies in gallium arsenide: Anab initiocalculation

We report here a reexamination of the static properties of vacancies in GaAs by means of first-principles density-functional calculations using localized basis sets. Our calculated formation energies yields results that are in good agreement with recent experimental and {\it ab-initio} calculation and provide a complete description of the relaxation geometry and energetic for various charge state of vacancies from both sublattices. Gallium vacancies are stable in the 0, -, -2, -3 charge state, but V_Ga^-3 remains the dominant charge state for intrinsic and n-type GaAs, confirming results from positron annihilation. Interestingly, Arsenic vacancies show two successive negative-U transitions making only +1, -1 and -3 charge states stable, while the intermediate defects are metastable. The second transition (-/-3) brings a resonant bond relaxation for V_As^-3 similar to the one identified for silicon and GaAs divacancies.

Determination of the Gibbs free energy of formation of Ga vacancies in GaAs by positron annihilation

We determined the Gibbs free energy of formation---i.e., the formation enthalpy and entropy---as well as the charge state of Ga vacancies in n-type GaAs by directly probing the vacancy concentration as a function of annealing temperature, arsenic vapor pressure, and doping concentration using positron annihilation. The Ga vacancy concentration increases with doping concentration and arsenic vapor pressure, but decreases with temperature. Using equilibrium thermodynamics, we obtained a $\ensuremath{-}3e$ charge state of the Ga vacancy in n-doped GaAs as well as a formation enthalpy of $(3.2\ifmmode\pm\else\textpm\fi{}0.5)\mathrm{eV}$ and a formation entropy of $(9.6\ifmmode\pm\else\textpm\fi{}{1)k}_{B}$ for the uncharged vacancy state.

Direct identification of As vacancies in GaAs using positron annihilation calibrated by scanning tunneling microscopy

We report on the identification of native vacancies in GaAs by positron annihilation with a special emphasis on As vacancy-related defects. In annealed highly Si-doped GaAs, we observe a neutral vacancy defect with a positron lifetime \ensuremath{\tau} of 280--285 ps and a high intensity of the core annihilation, in contrast to Ga vacancies which exhibit a lifetime of \ensuremath{\sim}260 ps and a lower intensity of the core annihilation. This defect is identified by scanning tunneling microscopy measurements to be an As vacancy ${\mathrm{Si}}_{\mathrm{Ga}}$ donor complex. We find that the same defect is also present in low n-doped GaAs, where it was earlier assigned to a neutral As vacancy. The high positron lifetime is explained by a large outward lattice relaxation. Theoretical calculations of the momentum distribution employing free atomic wave functions are in good agreement with the experimental results, provided only relative changes are considered and an outward lattice relaxation is included which yields the experimental positron lifetime. These calculations also yield annihilation parameters for Ga vacancies, in good agreement with the experiment. Our results demonstrate that vacancies in both sublattices of GaAs can directly and unambiguously be identified by positron annihilation once the annihilation characteristics are calibrated by a complementary method. On the basis of this identification, the abundance of As vacancies in GaAs is discussed in terms of stoichiometry and formation energies.

Structure, kinetics, and vibrational properties of complexes formed by hydrogen and gallium vacancies in GaAs: A theoretical study

In the last years, complexes formed by atomic H and gallium vacancies ${(V}_{\mathrm{Ga}})$ have been investigated in order to account for donor-acceptor bands observed in the photoluminescence spectra of hydrogenated GaAs. New configurations for these complexes are investigated here, together with the kinetic properties of the H donors that affect the complex stability critically. In addition to the H donor, a peculiar ${V}_{\mathrm{Ga}}\ensuremath{-}\mathrm{H}$ complex has been shown to behave as a donor. The electronic levels in the energy gap and several H vibrational modes have been studied for the new and the old configurations of the $\mathrm{H}\ensuremath{-}{V}_{\mathrm{Ga}}$ complexes, as well as for the new donor. The new complexes are more stable than those previously studied and can well account for the photoluminescence results. The values of the H vibrational frequencies are quite different for the different types of complexes, which should permit a determination of the microstructure of the $\mathrm{H}\ensuremath{-}{V}_{\mathrm{Ga}}$ complexes by infrared spectroscopy.

Microscopic characterization of defects using scanning tunneling microscopy

Using a scanning tunneling microscope (STM), it is possible to get microscopic characterization of defects in semiconductors. The atomic resolution allows the study of local atomic configuration, interface roughness or defects migration. Local spectroscopy allows for the determination the local density of state associated with the defect and its charge state. Illustrations of these possibilities are presented concerning the roughness of GaSb–InAs or InP–GaInAs interfaces and InAs quantum boxes buried in GaAs, the diffusion of vacancy in silicon, silicon dopants in GaAs, the charge state of the arsenic vacancy in GaAs and finally the STM and scanning tunneling spectroscopy (STS) of the antisite AsGa in GaAs. Finally, a discussion is proposed about a basic problem: how tunneling current occurs through defects? There is a necessary coupling between the energy level associated with the defect and the conduction or valence bands. This coupling is analysed in terms of emission and capture rates usually used for electrical characterization of the defects. Some new experimental suggestions are also proposed concerning the determination of the emission rate of a defect using the STM technique.

Transient enhanced intermixing of arsenic-rich nonstoichiometric AlAs/GaAs quantum wells

The superlattice intermixing of arsenic-rich nonstoichiometric AlAs/GaAs quantum wells grown at low-substrate temperatures around 300 \ifmmode^\circ\else\textdegree\fi{}C is enhanced by several orders of magnitude relative to diffusion in stoichiometric structures grown at ordinary substrate temperatures. The transient enhanced intermixing is attributed to a supersaturated concentration of group-III vacancies grown into the crystal by low-temperature growth conditions. The enhanced diffusion decays during moderate-temperature annealing between 600 \ifmmode^\circ\else\textdegree\fi{}C and 900 \ifmmode^\circ\else\textdegree\fi{}C for annealing times between 30 and 1000 s. First-order and second-order decay kinetics were both found to agree equally well with diffusion data obtained from isochronal and isothermal annealing. However, both of these kinetics require a thermally activated annihilation enthalpy to explain temperature-insensitive behavior observed in the time-dependent diffusion coefficient. The activation enthalpy ${H}_{a}$ for the decay is between 1.4 and 1.6 eV, which is compared with the migration enthalpy ${H}_{m}=1.8\mathrm{eV}$ of the gallium vacancy in GaAs. For the strongest annealing, the diffusion length approaches the self-diffusion values observed in isotopic superlattices of stoichiometric GaAs.

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.

On the possible identification of defects using the autocorrelation function approach in double Doppler broadening of annihilation radiation spectroscopy

The recent revived interest in the use of double-Doppler broadening of annihilation radiation (D-DBAR) spectroscopy, which employs two Ge detectors in back-to-back geometry, has stemmed mainly from its potential in defect identification as a result of its elemental sensitivity through core annihilations in atoms at the defect site. Emphasis has thus largely concentrated on the high momentum spectral range. In contrast the present work emphasizes the need to also focus attention on the low momentum region of the D-DBAR spectra. It is argued that the improved resolving power of D-DBAR, in conjunction with spectral deconvolution, should give future 1D (one dimensional) momentum data approaching in quality those obtainable using 1D-ACAR (angular correlation of annihilation radiation), thus forming an alternative technique for observing the structure containing diffraction patterns that originate from annihilations with localized electron states at positron trapping defects. Rotation of the sample about a specified crystal axis, and the binning of events by angle, is suggested as a means of extending the technique to form a 2D- (two dimensional) DBAR counterpart to 2D-ACAR. The advantages of considering the real space positron electron wavefunction product AF (autocorrelation function), obtained by simple manipulation of the D-DBAR data in Fourier space, are outlined. In particular the possible visualization offered in real space of a defect's physical geometry, with the prospect of building up a library of contour patterns for future defect identification, is discussed, taking the silicon monovacancy in Si and the negative As vacancy in GaAs as examples.

Interdiffusion: A probe of vacancy diffusion in III-V materials

We have used the interdiffusion of a multiple quantum well sample due to a thin source of vacancies, as a probe, to simultaneously measure the interdiffusion coefficient, diffusion coefficient for group III vacancies in GaAs and the background concentration of these vacancies in a single experiment. We have shown that the interdiffusion at all temperatures is governed by a constant background concentration of vacancies in the material and that this background concentration is the concentration of vacancies in the substrate material. The measured vacancy concentration is around 2\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. This result shows that the vacancy concentrations in GaAs are not at thermal equilibrium concentrations as has been widely assumed. Rather it has value which is ``frozen in,'' probably at the GaAs crystal growth temperature. The activation energy found for the intermixing of InGaAs/GaAs is shown to be governed solely by the activation term for vacancy diffusion which is calculated to have an activation energy of 3.4\ifmmode\pm\else\textpm\fi{}0.3 eV.

Electronic configuration of the gallium and arsenic vacancies in GaAs

The gallium vacancy defect has been identified in the photoluminescence spectrum of irradiated gallium arsenide, and the arsenic vacancy by deep level transient spectroscopy. A model is presented which explains the optical observation of these defects.

Theory of Self-Diffusion in GaAs*

Ab initio molecular dynamics simulations are employed to investigate the dominant migration mechanism of the gallium vacancy in GaAs as well as to assess its free energy of formation and the rate constant of gallium self-diffusion. Our analysis suggests that the vacancy migrates by second nearest neighbour hops. The calculated self-diffusion constant is in good agreement with the experimental value obtained in 69 GaAs/ 71 GaAs isotope heterostructures and at significant variance with that obtained earlier from interdiffusion experiments in GaAlAs/GaAsheterostructures.

Arsenic Antisite-Arsenic Vacancy Complex and Gallium Vacancy in GaAs: A Kind of Bistability Pair of Intrinsic Defects?

Arsenic Antisite-Arsenic Vacancy Complex and Gallium Vacancy in GaAs: A Kind of Bistability Pair of Intrinsic Defects?

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.

Positron lifetime and 2D-ACAR studies of divacancies in Si

We have measured positron lifetime and Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) distributions of Floating-Zone grown (FZ) Si specimens containing divacancies (V2) with the definite charge states, V 2 0 , V 2 −1 or V 2 −2 from room temperature to about 10 K. These charge states are accomplished by an appropriate combination of dopant species, their concentration and irradiation doses of 15 MeV electrons. with reference to the currently accepted ionization level of divacancies. The positron lifetime of the negatively charged divacancy increases with temperature, while that of the neutral divacancy shows little change with temperature. The positron trapping rate, obtained from lifetime and 2D-ACAR measurements, increases markedly with decreasing temperature. This is found not only for the negative divacancies but also for the neutral divacancy. We need a model which explains this temperature dependence. The 2D-ACAR distribution from positrons trapped at divacancies shows nearly the same distribution for the different charge states, which differs considerably from the case of As vacancies in GaAs studied by Ambigapathy et al. We have observed a small but definite anisotropy in the distribution of trapped positrons in V 2 − using a specimen containing oriented divacancies.

Positron Spectroscopy of Defects in Semiconductors

At a vacant lattice cell positron-ion repulsion is reduced leading to positron trapping. This causes observable changes in annihilation characteristics: the positron lifetime increases and the positron-electron momentum distribution narrows. Positron trapping in semiconductors is analogous to carrier capture. Due to the long-range Coulomb interaction, the charge state of a vacancy has a strong effect on positron trapping. The lattice relaxation due to a charge-state transition of a vacancy is also seen in the positron lifetime. Thus ionization levels of vacancy defects can be determined. Applications are shown on native vacancies in GaAs. Positrons reveal As and Ga vacancies in bulk crystals. The As vacancies have negative, neutral and positive charge states in the upper part of the band gap, whereas Ga vacancies are negative. The EL2 defect has a vacancy in its metastable state. A vacancy is also seen in the deep ground state of the DX center in AlGaAs. Positron results thus support the vacancy-interstitial model for the structure of the EL2 and DX centers

Positron Trapping at a Negatively Charged as Vacancy in GaAs

Positron Trapping at a Negatively Charged as Vacancy in GaAs