Gallium Antisite Defect Research Articles

Visualizing localized, radiative defects in GaAs solar cells

We have used a calibrated, wide-field hyperspectral imaging instrument to obtain absolute spectrally and spatially resolved photoluminescence images in high growth-rate, rear-junction GaAs solar cells from 300 to 77 K. At the site of some localized defects scattered throughout the active layer, we report a novel, double-peak luminescence emission with maximum peak energies corresponding to both the main band-to-band transition and a band-to-impurity optical transition below the band gap energy. Temperature-dependent imaging reveals that the evolution of the peak intensity and energy agrees well with a model of free-to-bound recombination with a deep impurity center, likely a gallium antisite defect. We also analyzed the temperature dependence of the band-to-band transition within the context of an analytical model of photoluminescence and discuss the agreement between the modeling results and external device parameters such as the open circuit voltage of the solar cells over this broad temperature range.

Discriminating a deep gallium antisite defect from shallow acceptors in GaAs using supercell calculations

For the purposes of making reliable first-principles predictions of defect energies in semiconductors, it is crucial to distinguish between effective-mass-like defects, which cannot be treated accurately with existing supercell methods, and deep defects, for which density functional theory calculations can yield reliable predictions of defect energy levels. The gallium antisite defect ${\mathrm{Ga}}_{As}$ is often associated with the 78/203 meV shallow double acceptor in Ga-rich gallium arsenide. Within a conceptual framework of level patterns, analyses of structure and spin stabilization can be used within a supercell approach to distinguish localized deep defect states from shallow acceptors such as ${\mathrm{B}}_{As}$. This systematic approach determines that the gallium antisite supercell results has signatures inconsistent with an effective mass state and cannot be the 78/203 shallow double acceptor. The properties of the Ga antisite in GaAs are described, total energy calculations that explicitly map onto asymptotic discrete localized bulk states predict that the Ga antisite is a deep double acceptor and has at least one deep donor state.

Application of the bounds-analysis approach to arsenic and gallium antisite defects in gallium arsenide

A recently developed bounds-analysis approach has been used to interpret density-functional-theory (DFT) results for the As and Ga antisites in GaAs. The bounds analysis and subsequent processing of DFT results for the As antisite yielded levels---defined as the Fermi levels at which the defect charge state changes---in very good agreement with measurements, including the \ensuremath{-}1/0 level which is within 0.1 eV of the conduction-band edge. Good agreement was also obtained for the activation energies to transform the $\mathrm{A}{\mathrm{s}}_{\mathrm{Ga}}$ from its metastable state to its stable state. For the Ga antisite, the bounds analysis revealed that the \ensuremath{-}1 and 0 charge states are hole states weakly bound to a localized \ensuremath{-}2 charge state. The calculated levels are in good agreement with measurements.

Gallium antisite defect and residual acceptors in undoped GaSb

Undoped GaSb samples were investigated by positron lifetime spectroscopy (PAS) and the coincident Doppler broadening (CDB) technique. PAS measurement indicated that there were monovacancy-type defects in undoped GaSb samples, which were identified to be predominantly Ga vacancy ( V Ga ) related defects by combining the CDB measurements. After annealing of these samples at 520 °C, positron shallow trapping have been observed and should be due to Ga Sb defects. Undoped GaSb is intrinsically p-type having a residual carrier density of 10 16– 10 17 cm − 3 . And the Ga Sb antisite defects are stable in the (0), (1-) and (2-) charge states and act as a double acceptor. Thus, we infer that Ga Sb antisite defects are the acceptor contributing to the p-type conduction for undoped samples.

Evidence of native gallium antisite defects in semi-insulating liquid-encapsulated Czochralski-grown GaAs

We report the observation by photoluminescence of the gallium antisite double acceptor in As-rich GaAs grown by the liquid-encapsulated Czochralski technique. In the semi-insulating material both arsenic antisite and gallium antisite defects are present simultaneously which must be taken into account for any electrical compensation model.

EL2 and gallium antisite defects in GaAs: Si

Photoluminescence (PL), Fourier-transform infrared (FTIR), and deep-level transient spectroscopy (DLTS) measurements have been made on various samples of silicon-doped liquid-encapsulated Czochralski-grown GaAs. All the samples show prominent PL peaks at 1.443 and 1.325 eV together with their longitudinal optic (LO) phonon peaks. The PL peak at 1.443 eV has been reported in the literature as being due to either GaAs or a boron-related defect. The FTIR results show the presence of BGa at 540.3 and 517.0 cm−1 and SiGa at 383.6 cm−1. We have observed that there is no correlation between the PL peak at 1.443 eV and BGa. Thus, we believe that this PL peak is related to the GaAs antisite defect. The presence of EL2 in the samples has been measured using DLTS. We have found that the intensity of the PL peak at 1.443 eV varies inversely with that of the EL2 peak. This relationship indirectly confirms that the 1.443 eV peak is due to the gallium antisite defect. The PL peak at 1.325 eV is significantly different from those reported in the literature for GaAs:Si. Measurements have also been made on samples of GaAs:Si annealed under different arsenic overpressures.

Electron-beam doping of Si into GaAs: The annealing behaviour of photoluminescence

Si impurities are introduced in (100)-oriented undoped semi-insulating GaAs at 50°C by using an electron-beam doping method from a Si impurity sheet, which is in contact with a GaAs surface. The Si surface is irradiated with a fluence of ∼ 5 × 10 17 electrons cm −2 at 7 MeV. Before and after annealing at 300°C for 20 min with a SiO 2-cap layer, the photoluminescence spectra at 77 K indicate two dominant peaks which are probably attributed to silicon acceptors, Si As, at 1.48 eV and gallium antisite defects, Ga As, at 1.445 eV. When the annealing temperature is higher than 600°C, the band-gap transition spectra appear at 1.51 eV. The hole concentration for a sample annealed at 700°C is measured to be 1–4 × 10 16 cm −3 with a nearly constant depth distribution in the range of 0.5–5 μm.

Native acceptor levels in Ga-rich GaAs

A photoluminescence, photocapacitance, and thermal annealing study of Ga-rich GaAs has revealed the complex behavior of acceptor levels at 68–77 and 200 meV above the valence band. The concentration of all levels is enhanced by Ga-rich growth conditions, however, only the 77- and 200-meV levels formed preferably in n-type GaAs are consistent with a double-acceptor model of the gallium antisite defect. In p-type GaAs the 68-meV level associated with a different single-acceptor defect is dominant. It is argued that the inhibited formation of double-acceptor GaAs defects in p-type crystals is caused by the Fermi-energy control of the defect formation.

Investigation of defects in gallium arsenide using positron annihilation

Grown-in defects in undoped GaAs, silicon-doped GaAs, and cadmium-doped GaAs have been investigated by positron-lifetime measurements. Trapping by vacancylike defects is observed in both undoped and silicon-doped GaAs, while no trapping occurs in cadmium-doped samples. The positron lifetime in these samples is 220 ps. The lifetime of trapped positrons in undoped GaAs is 290 ps, while in silicon-doped GaAs it is 260 ps. The 290-ps lifetime component is interpreted to arise from a monovacancy-impurity complex and the 260-ps component from annihilation at a substitutional silicon site. Room-temperature measurements on samples isochronally annealed up to 750\ifmmode^\circ\else\textdegree\fi{}C show removal of defects in undoped GaAs at approximately 500\ifmmode^\circ\else\textdegree\fi{}C. At higher annealing temperatures a new defect is created which yields a lifetime of only 225 ps. Similar observations pertain to silicon-doped GaAs except that no annealing stage is evident at 500\ifmmode^\circ\else\textdegree\fi{}C. It is proposed that the thermally created defect is a gallium antisite defect. Measurements in the (20-300)-K temperature range show that the trapping rate of the positrons increases roughly exponentially with increasing temperature in contrast to the case of silicon. A thermally activated trapping mechanism is proposed with activation energies appropriate to the defect in question. For the gallium antisite defect the activation energy is approximately 200 meV, about 60 meV for the substitutional silicon site, and 40 meV for the monovacancy-impurity complex.