T. Figielski

Recombination at dislocations

Abstract A dislocation in semiconductors behaves as a recombination flaw having a large number of charge states. The effective cross section of the dislocation is therefore a variable parameter depending by the electrostatic interaction on the occupation factor. This property manifests itself through peculiarities of photoconductivity of dislocated crystals e.g. logarithmic type decay of excess current carriers at low temperature. The elementary centres of recombination are most probably dangling bonds as indicated by spin-dependent effects. The magnitude of the spin-dependent cross section can be explained only if one takes into account the exchange interaction between neighbouring dangling electrons on a dislocation.

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.

Compensation in GaAs crystals due to anti-structure disorder

Complete equilibrium of native point defects in a GaAs crystal being in contact with the liquid phase (melt) is considered. Equilibrium relations for the concentrations of antisite defects, GaAs and AsGa, in dependence on the melt composition are derived. It is argued that the AsGa defect is a double donor, the lower energy level of which corresponds to EL2, while the GaAs is a double acceptor that can be identified with “residual acceptor” having the levels at 78 meV and 200 meV above the valence band, which was discovered by Elliot et al. It is shown that the presence of antisite defects together with an excess of background shallow acceptors leads, under reasonable assumptions, to a nearly intrinsic behaviour of LEC-grown GaAs, if the As atom fraction in the melt ranges from 0.471 to 0.535, in accordance with observations of Holmes et al.

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.

Mechanism for the creation of antisite defects, during combined climb-glide motion of dislocations in sphalerite-structure crystals

Processes are considered which lead to a formation of irregular jogs in a dislocation core, e.g., when in GaAs two As atoms are absorbed at an already existing jog. Then, after glide, the dislocation leaves an arsenic antisite defect behind. This mechanism can give an account of a correlation between the spatial distributions of EL2 donors (that are most likely related to the AsGa defects) and dislocations in melt-grown GaAs crystals.

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.

Deep-level defects at lattice-mismatched interfaces in GaAs-based heterojunctions

Electrical properties of lattice-mismatch-induced defects in GaAs/GaAsSb and GaAs/InGaAs heterojunctions have been studied by means of an electron-beam-induced current (EBIC) in a scanning electron microscope and deep-level transient spectroscopy (DLTS). DLTS measurements, carried out with p-n junctions formed at the interfaces, revealed one electron trap and two hole traps induced by the lattice mismatch. The electron trap, at about Ec-0.68 eV, has been attributed to electron states associated with threading dislocations in the ternary compound. By comparing the concentration of this trap, revealed by DLTS, with EBIC results on the diffusion length, obtained for heterojunctions with different lattice mismatches, it is inferred that the minority-carrier lifetime is controlled by dislocations in the epilayer region close to the interface. Two new hole traps have been ascribed to defects associated with the lattice-mismatched interface of the heterostructures.

Remnant magnetoresistance in ferromagnetic (Ga,Mn)As nanostructures

Magneto-resistive nanostructures have been investigated. The structures were fabricated by electron beam lithography patterning and chemical etching from thin epitaxial layers of the ferromagnetic semiconductor (Ga,Mn)As, in shape of three nanowires joined in one point and forming three-terminal devices, in which an electric current can be driven through any of the three pairs of nanowires. In these devices, a novel magneto-resistive memory effect has been demonstrated, related to a rearrangement of magnetic domain walls between different pairs of nanowires in the device consisting in that its zero-field resistance depends on the direction of previously applied magnetic field. The nanostructures can thus work as two-state devices providing basic elements of nonvolatile memory cells. (Less)