Arsenic Vacancies Research Articles

Point defect thermal equilibria in GaAs

The thermal equilibrium concentrations of the six electrically neutral single point defect species in GaAs are expressed as explicit functions of well-defined thermodynamic quantities. The difference between the Gibbs free energies of an arsenic atom in the interior of a GaAs crystal and in an arsenic vapor phase molecule is emphasized. Numerical values of the thermal equilibrium concentrations of the gallium and arsenic vacancies and the two antisite defects are estimated. It is of interest that the calculated thermal equilibrium concentration of the anion antisite defect AsGa0 reaches a peak value of about 1 × 1017 cm−3 and is almost temperature independent. This computed value for AsGa0 is consistent with experimental findings.

Stoichiometry-dependent deep levels in Te-doped GaAs prepared by annealing under excess arsenic vapour pressure

Reports the photocapacitance (PHCAP) measurement results at 20 K of Te-doped GaAs crystals, which were rapidly cooled after annealing under various arsenic vapour pressures. The Ec-0.485 eV and Ec-0.492 eV deep donors which are peculiar to Te-doped GaAs are considered to arise from the presence of arsenic vacancies (VAs) and the donor impurity Te. The level density of the Ec-0.485 eV and Ec-0.492 eV donors decreases monotonically with increasing arsenic vapour pressure. The deep donors, which are classified into Ec-0.717 eV and Ec-0.737 eV levels, are considered to be associated with arsenic interstitial atoms (IAs), and their level density increases monotonically with increasing arsenic vapour pressure PAs(GaAs). The Ec-0.72 eV deep donor is commonly seen in various n-GaAs bulk crystals with different dopant impurities. Good spectral correspondences between the results of PHCAP, deep-level transient spectroscopy and deep-level photoluminescence (PL) measurements are also presented. The PHCAP and PL measurements show that the Ec-0.72 eV onset may originate from the presence of a greater number of IAs with a more strained configuration than the so-called EL2 level.

Deep levels in Te-doped GaAs prepared by annealing under controlled arsenic vapor pressure

This paper reports the photocapacitance (PHCAP) results of the deep levels associated closely with donor impurity atoms and arsenic vacancies in heavily Te-doped GaAs. The present PHCAP measurements detect two kinds of deep levels at 0.62–0.64 eV and 0.83 eV above the valence band preferentially formed under low arsenic vapor pressure, and show that the level density decreases monotonically with increasing arsenic vapor pressure. By varying the wavelength of irradiation light, the neutral level position of 0.62–0.64 eV ionized level is determined to be 1.38 eV below the conduction band. From the spectroscopic observation, it is concluded that the 0.62–0.64 eV+Ev level disappears in the absence of As vacancies and appears in the presence of the donor dopant Te. This level shows such a large value of Frank–Condon shift (dFC) as 0.25 eV.

Study of the incorporation of oxygen in Ga1−xAlxAs prepared by MOVPE

Abstract The influence of oxygen on the electronic properties of semiconductors is not well known. This is in part due to the fact that the analysis of this element at low concentrations is difficult; the determination of the lattice location is difficult as well. Using charged particle activation (CPA), one is able to analyse oxygen accurately below the ppm level. By combining CPA with channelling, one can also obtain information on the lattice position(s) occupied by the oxygen atoms. The method is described and discussed. Recent work on Ga1−xAlxAs prepared by vapour phase epitaxy with organometallic compounds will be presented as an example. It is shown in this work that oxygen can occupy at least two different sites: a tetrahedral interstitial site and another site, quite probably an arsenic site. The number of atoms on each site is governed by the number of arsenic vacancies and by the total oxygen concentration, which are both governed by the growth conditions.

A model for the Zn diffusion in GaAs by a photoluminescence study

To study the mechanism of zinc diffusion in GaAs, we diffused zinc from a ZnAs2 source into Si-doped GaAs samples (n ≊ 1.3 × 1018 cm−3) at different temperatures (from 575 °C up to 700 °C) in sealed evacuated quartz tubes. The samples are characterized by the depth profile of the photoluminescence at different temperatures. The photoluminescence spectra show characteristic emission associated to deep levels of gallium and arsenic vacancies. A detailed analysis of the spectra demonstrates the role played by vacancies in the Zn diffusion process. The spatial correlation between the luminescence spectra and the Zn concentration obtained from secondary ion mass spectroscopy measurements has been demonstrated.

Study of the lattice location of oxygen in semiconductors by combining channeling and charged particle activation

The influence of oxygen on the electronic properties of semiconductors is not well known. This is in part due to the fact that the analysis of this element at low concentrations is difficult; the determination of the lattice location is difficult as well. Using charged particle activation (CPA), one is able to analyze oxygen accurately below the ppm level. By combining CPA with channeling, one can also obtain information on the lattice position(s) occupied by the oxygen atoms. The method is described and discussed. Recent work on Ga 1− x Al x As prepared by vapour phase epitaxy with organometallic compounds will be presented as an example. It is shown in this work that oxygen can occupy at least two different sites: a tetrahedral interstitial site and another site, probably an arsenic site. The number of atoms on each site is governed by the number of arsenic vacancies and by the total oxygen concentration, which are both governed by the growth conditions.

Raman-scattering and optical studies of argon-etched GaAs surfaces.

We have studied the structual damage in low-energy argon-ion-bombarded (ion-etched) GaAs using Raman scattering and ultraviolet reflectivity. When combined with post-bombardment sequential chemical etching, the Raman results reveal a graded depth profile of the damage layer, with a nearly linear damage dropoff with depth. The total damage-layer thickness is about 600 A for high-fluence bombardment with 3.89-keV Ar{sup +} ions. The spectral effects produced by argon etching are very different from those produced by high-energy ion implantation. The longitudinal-optic Raman line seen for argon-etched GaAs is not shifted and broadened as in ion-implanted GaAs. More striking are the results of the reflectivity measurements. For argon-etched GaAs, the electronic interband peaks are both broadened and strongly red shifted relative to the crystal peaks; for ion-implanted GaAs, only the broadening occurs. Distinct nanocrystals, which account for the effects seen in ion-implanted GaAs, are evidently absent in argon-etched GaAs. Instead, the damage layer caused by argon etching appears to be characterized by a very high density of point defects, which previous work suggests may be arsenic vacancies.

Influence of the implantation of C+ ions on photoluminescence and electrical properties of GaAs

Ion implantation of carbon (C) into extremely pure GaAs grown by molecular- beam epitaxy is carried out over a wide range of C atomic concentrations [C], from 1×1016 to 5×1019 cm−3. The impurity levels in the implanted layer are investigated by using photoluminescence (PL) and Hall-effect measurements. Below the well-defined exciton luminescence lines, one broad emission band, namely [g-g], is found to be exclusively attributable to acceptor impurities and is dominant for [C] lower than 3×1017 cm−3. However, a decrease of its intensity and a ‘‘locking’’ of its emission energy shift is observed for [C] higher than this value. A carrier transport mechanism is found to be relevant to the C acceptor level for C atoms at As sites, and deep acceptors caused by residual radiation defects. The concentration of substitutional C atoms is nearly coincident with [C] up to 1×1017 cm−3, but the activation efficiency for [C]=1×1018 cm−3 decreases by about 13%. The decrease of overall PL intensity and the locking of the emission energy shift of [g-g] are the peculiar features in highly C+- implanted GaAs. Moreover, the location of a moderately deep acceptor level and its related-emission properties as a function of [C] strongly suggest the formation of complexes involving arsenic vacancies (VAs) and C impurities, or residual radiation defects.

Characterization of electron irradiation induced defects in δ-doped GaAs superlattice structures

We have studied the behaviour of electron irradiation induced defects in δ-doping GaAs superlattices, using capacitance-voltage and deep level transient spectroscopy. For the planar doping used (10 11SSi cm −2) with a distance of 70 å between each plane, we observed a delocalization of the carriers, in agreement with theoretical calculation. The defects created by the irradiation are identical to the defects created in bulk GaAs. However, we observed in addition two new defects which are tentatively ascribed to a complex formed between the silicon dopant and the arsenic vacancy.

GaAs homogeneity region and point defect concentration calculation in ternary systems Ga-As-electrically active impurity

Thermodynamic calculations of homogeneity region of Sn-, Te- and Si-doped GaAs were carried out to analyse equilibrium and supersaturated native point defect behaviour during crystallization and postcrystallization cooling. Arsenic Frenkel, Schottky and antisite defects are included in the model with the Frenkel defects being dominating. Besides decomposition Ga- (or As-) supersaturated nonstoichiometric solid solution which takes place in the two-phase region of the phase diagram at temperatures below 800–1000°C, a great amount ( ≈ 10 19cm -3) of arsenic vacancies and interstitials become overquilibric on postcrystallization cooling immediately after crystallization, which leads further to the high-temperature aggregation process. The great difference in point defect formation energies causes their transformation to each other and leads to significant decrease of increase of arsenic antisite concentration on cooling of Ga- or As-rich crystals respectively. Narrowing of low-temperature (850–1000°C) boundaries of GaAs homogeneity region on Sn and Si doping accomplished by a shift of congruent point to Ga-rich compositions stimulate Ga-supersaturated nonstoichiometric solid solution decomposion. The so-called superdilation phenomena may be due to the great amount of gallium interstitials formed at the very initial stage of the decomposition. Te-doping shifts GaAs homogeneity region to the As-rich compositions and makes lateral surfaces of existence region to twist around the axis of elongation of the latter.

Reverse contrast imaging in GaAs

“Reverse contrast imaging” is a technique that has been developed for imaging dislocation distributions in undoped semi-insulating LEC GaAs. This technique, which relies on the mapping of near-bandgap infrared transmission, is both rapid and non-destructive. Until now, no satisfactory mechanism has been proposed for the images except that it has been concluded that they are a result of an absorption process. In the present work we have investigated the rôle of scattering in the production of this image. We have previously described an optical apparatus which is sensitive to changes in refractive index and have shown how it can be used to image both growth striations and doping non-uniformities in n-type GaAs. In this method, changes of refractive index which produce small changes in paths of light beams transmitted by the sample, are directly imaged by an infra-red sensitive TV camera. Using this apparatus we have now demonstrated that the images in reverse contrast, which occur only when the sample is cooled below 150 K and when imaged with light of energy within ∽ 50 meV of the bandedge, have two components, one due to absorption and one due to scattering. The wavelength dependencies of these components are different; the scattering image is observable at longer wavelengths than the absorption image. We conclude that the scattering and absorption arise from the same defects. These absorb near bandedge light, are gettered by dislocations and the resulting concentration gradients are the cause of the light scattering. We tentatively associate these defects with arsenic vacancies.

Behavior of electron-irradiation-induced defects in GaAs.

In GaAs, electron irradiation is known to produce vacancy-interstitial pairs in the arsenic sublattice (${\mathit{V}}_{\mathrm{As}\mathrm{\ensuremath{-}}}$${\mathrm{As}}_{\mathit{i}}$). The associated levels are electron traps (labeled E1--E5), and hole traps (labeled H0 and H1). In addition, complexes (labeled H2--H5) involving the ${\mathrm{As}}_{\mathit{i}}$ and residual impurities are created in p-type GaAs. This different behavior between n- and p-type materials is found to be related to a difference in the mobility of ${\mathrm{As}}_{\mathit{i}}$ during the irradiation. The existence of the various levels observed for the ${\mathit{V}}_{\mathrm{As}}$-${\mathrm{As}}_{\mathit{i}}$ pair corresponds to a distribution in distance between ${\mathit{V}}_{\mathrm{As}}$ and ${\mathrm{As}}_{\mathit{i}}$. Most of the pairs are correlated in n-type material while in p-type material a large fraction of the pairs are uncorrelated. In order to verify this picture we have performed a study of the pair distribution versus the flux of irradiation in n- and p-type materials and versus the irradiation dose in p-type material. In p-type material, these studies confirm that the defect labeled H1 is a primary defect and the defects H2, H3, and H4 are complexes. The large diffusion length of ${\mathrm{As}}_{\mathit{i}}$ explains the observed creation rates and the annealing behaviors of these defects. In n-type material, a partial annealing of the defects E1--E5 is observed under high flux of irradiation because the mobility of ${\mathrm{As}}_{\mathit{i}}$ is then enhanced by the holes injected during the irradiation. Both this ionization-enhanced annealing and the thermal annealing (which occurs around 200 \ifmmode^\circ\else\textdegree\fi{}C) can be understood in detail. A careful analysis of the kinetics of the pair annihilation and in particular the asymptotic behavior of these kinetics allows the determination of the fraction of correlated pairs and the evaluation to some extent of the distribution in distance of the pairs. Finally these conclusions allow us to propose a microscopic model for the defect E3, resulting from the interaction of ${\mathrm{As}}_{\mathit{i}}$ located at an average distance of 8 \AA{} from the arsenic vacancy.

Quenching and recovery characteristics of the EL2 defect in GaAs under monochromatic-light illumination.

Quenching of the normal state and recovery from the metastable state of the EL2 defect in semi-insulating GaAs grown by the liquid-encapsulated Czochralski technique were studied under monochromatic-light illumination in the energy range 0.7\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}1.51 eV. The photoquenching data show two peaks at 1.125 and 1.30 eV. In addition, light with energy h\ensuremath{\nu}=1.46 eV is found to photoquench the normal state of EL2 in unannealed doped samples within 25 min. On the other hand, the photoinduced recovery data from the metastable state show a complex structure consisting of a broad band around 0.9 eV and multiple sharp peaks between 1.4 eV and the band edge. The magnitude as well as structure of recovery were found to be sample dependent. Both quenching and recovery data are in conflict with the available calculations and predictions reported for the isolated-arsenic-antisite model. The recovery peaks are coincident with the calculated arsenic vacancy (${V}_{\mathrm{As})}$ energy levels. Thus, the present results support the complex models involving ${V}_{\mathrm{As}}$. The optical recovery from the metastable state suggests that EL2 is a charge-controlled bistable defect. The sample dependence of most of the data indicates the existence of interactions between EL2 and other defects and traps.

On the mechanism of semi-insulating property formation in In-doped GaAs

The precision lattice parameter and density of Czochralski GaAs: In crystals doped to the level of 2×10 16-2.1×10 20 In atoms cm -3 were measured by Bond X-ray diffraction and comparative hydrostatic weighting methods, respectively. It appears that semi-insulating GaAs: In crystals always contain a remarkable amount of overstoichiometric arsenic interstitials ((5−10) × 10 18 atoms cm -3). On the other hand, low-resistivity GaAs: In samples turn out to be enriched by overstoichiometric arsenic vacancies in the concentration range of 0 to 6×10 18 cm -3. A thermodynamic model is developed according to which indium doping of GaAs up to concentrations of (2.2−10) ×10 19 atoms cm -3 provides the same native point defect concentration changes (including antisite As Ga-defect and EL2-related complex) as in the case of undoped melt composition changes of 48−54 at % As.

Effect of proton bombardment on GaAs luminescence

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Recovery from the Metastable EL2 Defect in GaAs Under Monochromatic Light Illumination.

AbstractThe infrared absorption technique is used to study the recovery of the EL2 metastable state in semi-insulating GaAs under monochromatic light illumination. The induced optical recovery is monitored after low intensity (≤ 2mW/cm2) irradiation in the energy range 0.7 ≤ hv ≤ 1.5 eV. The data exhibit a complex structure consisting of a broad band around 0.9 eV and a set of multiple sharp peaks between 1.44 and 1.5 eV. This recovery is strongly dependent on the sample, temperature and illumination time. The present results suggest that a) the existing data and theoretical predictions for the isolated AsGa antisite structure are not compatable with the optical recovery data, b) EL2 is affected dramatically by other defects (traps) present in the sample and c) the peaks observed in the optical recovery data are coincident with the arsenic vacancy energy levels and therefore the present results support the proposed complex models involving an arsenic vacancy.

Positron-annihilation spectroscopy of native vacancies in as-grown GaAs.

Native vacancy defects are studied in as-grown GaAs materials by positron-lifetime measurements. Direct evidence of native monovacancy-type defects is found in as-grown n-type Te-, Sn-, and Si-doped GaAs. However, the same evidence is not found in as-grown semi-insulating In- or Cr-doped GaAs or semi-insulating undoped GaAs or p-type GaAs(Zn). It is shown that the positron trapping and annihilation in the native defects are strongly dependent on the position of the Fermi level in as-grown n-type GaAs. It is concluded that the configurations of the native monovacancy defects in GaAs (Te or Sn) change with the position of Fermi level. Two Fermi-level-controlled transitions are found: one is located at 0.035\ifmmode\pm\else\textpm\fi{}0.015 eV and the other at 0.10\ifmmode\pm\else\textpm\fi{}0.02 eV below the conduction band. It is proposed that the two transitions correspond to two charge-state transitions of the arsenic vacancy ${V}_{\mathrm{As}{}^{2\mathrm{\ensuremath{-}}}\ensuremath{\rightarrow}{V}_{\mathrm{As}{}^{\mathrm{\ensuremath{-}}}}}$ and ${V}_{\mathrm{As}{}^{\mathrm{\ensuremath{-}}}\ensuremath{\rightarrow}{V}_{\mathrm{As}{}^{0}}}$, respectively.

Electrical activation of silicon coimplanted with nitrogen, phosphorus, or arsenic in semi-insulating GaAs substrates grown by the liquid encapsulated Czochralski method

We fit some existing data on the electrical activation of Si ions coimplanted with either N, P, or As ions in semi-insulating GaAs substrates grown by the liquid encapsulated Czochralski method. For one data set the activation anneal was done with proximity capped samples in a flowing H2 ambient, and for the other data sets anneals were done with dielectric capped samples. We are able to obtain good fits to all data with simple choices for the depth profiles of implanted ions if we adopt the following assumptions: (1) With no group V coimplants, the Si donors formed during a dielectric cap anneal, possibly SiGaVAs, are 100% ionized, while some of those formed during a proximity cap anneal in H2, possibly SiGaHAs, are less than 100% ionized and also serve to eliminate some arsenic vacancies. (2) All Si-related acceptors appear as BGaSiAs defects. (3) The observed enhancement of Si activation with coimplanted P is due to a decreased concentration of BGaSiAs acceptors whose formation is inhibited by P. (4) Coimplanted N or As does not inhibit the formation of BGaSiAs acceptors but, like coimplanted P at high doses, forms deep acceptors.

The effect of nonstoichiometry and polarity of the (111) plane on microtwin formation in ion-implanted GaAs

To study the effect of nonstoichiometry and polarity of the (111) plane on recrystallization of an implantation-induced amorphous layer, arsenic-implanted (100) and (111) A and B GaAs wafers were annealed under vacuum and specified arsenic vapor pressures. It has been found that the formation of microtwins is suppressed by annealing at the specified arsenic pressure. This suggests that microtwin formation is closely related to the arsenic deficiency or arsenic vacancies. Moreover, the result of direct implantation into (111) A (Ga) and B (As) surfaces indicates that the twins would preferentially grow on the (111) A plane, which has a smaller solid-phase regrowth rate than that of the (111) B plane.

Silicon and indium doping of GaAs: Measurements of the effect of doping on mechanical behavior and relation with dislocation formation

The effect of indium and silicon doping on dislocation formation in GaAs single crystals has been studied experimentally using dynamic compression tests and indentation rosettes. By direct measurements of the critical resolved shear stress (CRSS) in temperature range from 400 to 1100° C using dynamic compression tests, we found that at the melting point, the critical resolved shear stress of GaAs: In is twice that of undoped GaAs. More recent measurements show that at high temperatures ( T ⩾ 1000° C) the critical resolved shear stress of silicon-doped crystals is lower than that of undoped GaAs. Well-defined indentation rosettes were obtained at high temperatures from a Vickers indenter using small loads. Analysis of such rosettes confirm that the mechanical behavior of the GaAs crystals is not drastically affected by the presence of dopants. Reduction of dislocation densities in doped crystals is attributed to modifications introduced by the dopant in the equilibrium concentration of native defects at the melting point (particularly gallium and arsenic vacancies).