T. Wosiński

Identification of AsGa antisites in plastically deformed GaAs

AsGa antisite defects formed during plastic deformation of GaAs are identified by electron paramagnetic resonance (EPR) measurements. From photo‐EPR results it can be concluded that the two levels of this double donor are located near Ec −0.75 eV and Ev +0.5 eV. These values are coincident with the Fermi level pinning energies at Schottky barriers. The upper level can be related to the ’’main electron trap’’ EL2 in GaAs. Photoluminescence experiments before and after thermal annealing suggest that AsGa defects reduce the near band edge luminescence efficiency. A dislocation climb model is presented which is able to explain AsGa formation during dislocation movement. The production of AsGa antisites during dislocation motion under injection conditions in light emitting devices may thus be connected with degradation of the light output.

Carrier recombination at single dislocations in GaN measured by cathodoluminescence in a transmission electron microscope

We study radiative and nonradiative recombination at individual dislocations in GaN by cathodoluminescence performed in a transmission electron microscope. The dislocations are produced by indentation of dislocation free single crystals and have a-type Burgers vectors (b=1/3〈1120〉). They are aligned along 〈1120〉 directions in the basal plane. Our direct correlation between structural and optical properties on a microscopic scale yields two main results: (i) 60°-basal plane dislocations show radiative recombination at 2.9 eV; (ii) screw-type basal plane dislocations act as nonradiative recombination centers. We explain the nonradiative recombination by splitting this dislocation into 30° partials that have dangling bonds in the core. The dissociation width of these dislocations is <2 nm.

Raman and cathodoluminescence study of dislocations in GaN

Structural and optical properties of freshly created and in-grown dislocations in GaN single crystal are investigated by Raman and cathodoluminescence (CL) microscopy. The introduction of a high density of dislocations by micro-indentation is accompanied by the generation of intrinsic point defects. A high amount of VGa–impurity complexes is responsible for the decrease in the free electron concentration and the enhanced yellow luminescence around the indentation. A compressive stress induced by deformation is revealed by Raman scattering and CL. In-grown dislocations are decorated with a point defect atmosphere, leading to a reduction in the free carrier concentration around the dislocation.

Study of individual grown-in and indentation-induced dislocations in GaN by defect-selective etching and transmission electron microscopy

Abstract Vickers diamond indentation at 370°C has been employed to introduce dislocations into the plate-like (000-1) N-polar GaN single crystals. It has been established that using standard Vickers diamond indenter, well-defined ‘rosettes’ of defects are formed under 1.5–2 N load applied for 10 min. The resolved patterns of dislocation-related etch pits are formed using molten KOH–NaOH eutectic (E) at 200°C for 1.5–2 min. Individual grown-in dislocations are revealed by this E etch in the GaN matrix. Transmission Electron Microscopy confirmed the correlation of etch pits to individual dislocations emerging at the surface. Nano-crystalline material was found in the highly deformed central region of the indentation rosette. The structure of these nano-crystals was analyzed using electron diffraction. Speculative explanation on a phase transition induced by high local pressure is briefly discussed.

Misfit strain anisotropy in partially relaxed lattice-mismatched InGaAs/GaAs heterostructures

Partially relaxed InGaAs/GaAs heterostructures with a small lattice mismatch have been studied by means of atomic force microscopy and high-resolution x-ray diffractometry. Additionally, electron-beam induced current in a scanning electron microscope and transmission electron microscopy have been employed to investigate misfit dislocations formed at the (001) heterostructure interface. The measurements revealed a direct correlation between the surface cross-hatched morphology and the arrangement of interfacial misfit dislocations. The reciprocal lattice mapping and the rocking curve techniques employed for the samples aligned with either the or the [110] crystallographic direction perpendicular to the diffraction plane revealed anisotropic misfit strain relaxation of the InGaAs layers. This anisotropy results from an asymmetry in the formation of the α and β types of misfit dislocations oriented along the and [110] directions, respectively, which differ in their core structures. The misfit strain anisotropy causes a distortion of the unit cell of the layer and lowers its symmetry to orthorhombic.

DEEP LEVELS CAUSED BY MISFIT DISLOCATIONS IN GAASSB/GAAS HETEROSTRUCTURES

Two deep electron traps induced by lattice mismatch in relaxed GaAs1−xSbx layers (x=0% to 3%) grown by liquid phase epitaxy (LPE) on GaAs substrates have been revealed by means of deep‐level transient spectroscopy. One of the traps, that shows nonstandard, logarithmic capture kinetics and whose energy level is tied to the valence‐band edge, has been related to electron states associated with α dislocations. The other trap has been attributed to the EL2 defect and possible reasons of its unexpected formation in the LPE‐grown layers are briefly discussed.

Evidence for two energy levels associated with EL2 trap in GaAs

By photocapacitance technique, applied to n-type LEC-grown GaAs, two energy levels:Ev+0.45 eV andEc−0.75 eV are identified, for the first time, as being associated with the EL2 trap. As follows from the analysis of photo-EPR results on highly resistive GaAs crystals, the same energy levels can be attributed to the arsenic antisite defect, AsGA. In view of these findings, it is argued that the occupied EL2 level corresponds to the neutral charge state of AsGa defect.

Capture kinetics at deep-level defects in lattice-mismatched GaAs-based heterostructures

Two deep-level traps associated with lattice-mismatch induced defects in GaAs/InGaAs heterostructures have been revealed by means ofdeep-level transient spectroscopy (DLTS). An electron trap, at Ec � 0:64 eV, has been attributed to electron states associated with threading dislocations in the ternary compound while a hole trap, at Ev þ 0:67 eV, has been ascribed to misfit dislocations at the heterostructure interface. Detailed investigation ofthe dependence ofDLTSline amplitude and its shape on the filling time ofthe traps with charge carriers allowed us to specif y the type of electronic states related to both traps. In terms ofthe model ofelectronic states associated with extended defects, which takes into account the rate at which the states reach their internal electron equilibrium, we relate the electron trap to ‘‘localized’’ states and the hole trap to ‘‘bandlike’’ ones. r 2001 Elsevier Science B.V. All rights reserved.

Misfit dislocations and surface morphology of lattice-mismatched GaAs/InGaAs heterostructures

Abstract A regular network of 60° misfit dislocations aligned along two orthogonal 〈110〉 directions at the (001) interface of GaAs/InGaAs heterostructures with a small lattice-mismatch has been revealed by means of transmission electron microscopy and electron-beam induced current mode in a scanning electron microscope. The network of misfit dislocations has been also reproduced, in a form of a well-defined cross-hatch pattern on the surface of the structures, with atomic force microscopy. Almost one-to-one correspondence between the structure of misfit dislocations at the interface and the surface morphology clearly demonstrate that the cross-hatch development results primarily from misfit-dislocation generation.

Arsenic antisite defects as the main electron traps in plastically deformed GaAs

It is found from DLTS measurements that plastic deformation of GaAs single crystal creates a new kind of electron traps with an activation energy of 0.37 eV, and gives rise to an increase in the concentration of main electron traps with an energy of 0.80 eV. By comparing the concentrations of the main electron traps before and after deformation with analogous concentrations of AsGa paramagnetic centers, found by EPR experiments, it is concluded that the centers observed in both cases are of the same origin. A nonstandard feature of the main traps is discovered: linear dependence of the DLTS-peak amplitude on the logarithm of the filling-pulse duration time. This feature can be explained in terms of the barrier-limited capture rate, assuming the traps are arranged in rows.