M. Mazzer

Determination of surface lattice strain in ZnTe epilayers on (100)GaAs by ion channeling and reflectance spectroscopy

We report on the direct measurements of surface lattice strain in ZnTe epitaxial layers on {100}GaAs substrates by ion channeling Rutherford backscattering spectrometry and low‐temperature (10 K) reflectance spectroscopy measurements. The measured ZnTe strain is the superposition of the expected thermal (tensile) strain and a thickness‐dependent residual compressive strain. Our data indicate that the removal of this residual strain is slower than the rate predicted by the equilibrium theory, following an apparent h−1/2 power‐law dependence on the epilayer thickness h, above ∼100 nm.

Mechanisms of strain relaxation in III-V semiconductor heterostructures

Abstract The known models describing the breakdown of coherency between layer and substrate in mismatched heterostructures are based on the isotropic elastic continuum approximation. As a matter of fact an internal contribution to the misfit accommodation, that is a deviation from the so-called “virtual crystal approximation”, is expected in ternary or more complex alloy structures. This effect is clearly seen in a set of In x Ga 1− x As / GaAs low misfit samples in the presence of misfit dislocations. The complete structural characterisation including the elastic distortion field and the dislocation density and distribution has been performed by means of Rutherford backscattering based techniques and double crystal X-ray diffraction.

Properties and structure of antiphase boundaries in GaAs/Ge solar cells

Abstract Antiphase boundaries (APBs) are particularly crystallographically simple but technologically troublesome boundaries that tend to occur in profusion in GaAs grown epitaxially on (100) oriented substrates of Ge or Si. APBs were found to form only in a narrow band near the periphery of square wafers of GaAs/Ge grown by metal organic chemical vapour deposition (MOCVD) to make solar cells for use on space satellites. Bands of APBs have continued to appear in a fraction of the wafers grown over a period of several years. The geometry of these bands is presented and the distribution and structure of the APBs within it was studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) electron beam induced current (EBIC) and cathodoluminesence (CL). TEM showed that APBs and misfit dislocations were present in the bands in earlier specimens while stacking faults were the only defects in areas outside the bands in more recent specimens. The significance of the formation of APBs only in a limited area and of their relatively large interface recombination velocities are discussed.

Structural study of (100)CdTe epilayers grown by MOVPE on ZnTe buffered and unbuffered (100)GaAs

Abstract We report on the structural assessment of metalorganic vapour phase epitaxy grown (100)-oriented CdTe epilayers on both (100)GaAs and (100) ZnTe GaAs . Ion channelling Rutherford backscattering spectrometry and cathodoluminescence (CL) measurements are used to study the effect of inserting a proper ZnTe buffer layer [G. Leo et al., J. Vac. Sci. and Technol. B 14 (1996) 1739] between CdTe and GaAs. The insertion of a ZnTe buffer layer improves the surface crystalline and optical quality of the CdTe: CL images show that non-radiative recombination regions, associated with extended defects, strongly decrease when a ZnTe buffer layer is used. Also, enhanced excitonic emissions are observed in the case of CdTe/ZnTe/GaAs samples.

SELF-ORGANIZED GROWTH OF ZNTE NANOSCALE ISLANDS ON (001)GAAS

The Stransky–Krastanow metalorganic vapor phase epitaxy growth of self-organized ZnTe islands on homoepitaxial (001)GaAs is demonstrated. The −7.4% lattice mismatch of the ZnTe/GaAs heterostructure leads to a strain-driven distribution of nanoscale ZnTe islands on top of a two-dimensionally (2D) grown wetting layer. Atomic force microscopy and Rutherford backscattering spectrometry are used to determine the island dimensions and the thickness of the wetting layer. The density of the islands, their average diameter, and aspect ratio turn out to be about 520 μm−2, 13.6 nm, and 0.20, respectively, for a 1.2 ML thick 2D layer. Furthermore, the average aspect ratio of the islands decreases by increasing the thickness of the wetting layer, as expected by the progressive extinction of the strain-driven island nucleation.

Ion beam analysis of mismatched epitaxial heterostructures

Abstract The structural characterization of mismatched epitaxial heterostructures requires at least the determination of the strain and of the dislocation density. In this paper it is shown that ion-channeling is a very suitable technique for measuring both of these properties. In particular an improved technique for measuring the absolute angular distance between any crystal direction is presented. It is shown that in the case of single layer heterostructures the achieved precision is comparable to or better than that of double crystal X-ray diffraction. Moreover by computing the lattice distortion caused by a misfit dislocation the dechanneling cross section and its dependence from the relative orientation of the channeling and the dislocation directions have been evaluated. The application of these techniques for studying the mechanisms of strain relaxation in the In x Ga 1− x As/GaAs system is presented.

Dechanneling by misfit dislocations in III–V semiconductor heterostructures

Abstract This work reports on the use of the dechanneling technique for the determination of the misfit dislocation distribution at the interface between mismatched III–V semiconductor heterostructures grown on (001) substrates. Planar channeling is employed because of the higher sensitivity with respect to the axial case. For low misfit heterostructures, dislocations are oriented along the [110] and [110] and the depth distribution is nearly planar and close to the chemical interface. We show that, for this kind of structure, dechanneling measurements allow one to determine the dislocation densities along the two quoted directions. This is done by using a fitting procedure based on a numerical simulation program of the RBS-channeling spectra. As a result of this procedure, an estimation of the dislocation distribution thickness can also be obtained, besides the ratio of the energy loss under channeling conditions to that under non-channeling conditions. Experimental data concerning a set of InxGa1−xAs/GaAs single layer samples having different thicknesses and indium concentrations are presented and discussed in the framework of this model. The dechanneling probability due to the misfit dislocations is obtained from the RBS-channeling spectra at several beam energies for all the samples. In some cases we note two distinct dechanneling probabilities for the two {110} channeling planes perpendicular to the interface. In light of the dechanneling cross section properties, this fact reflects the difference between the dislocation densities in the two directions.

Dechanneling cross section for misfit dislocations

Abstract This work reports on the use of the dechanneling technique for the determination of the misfit dislocation distribution at the interface between mismatched III–V semiconductor heterostructures grown on (001) substrates. In this case the dislocations are oriented along the [110] and [110] and the depth distribution is nearly planar and close to the chemical interface. The displacement field caused by one misfit dislocation is calculated in the frame of the linear elasticity theory. An analytical treatment of the dechanneling cross section dependence on both the dislocation line and Burgers vector orientation and on the beam direction in the channeling plane is performed within the Quere model. It is shown that (110) planar channeling is most sensitive to the dislocation orientation. Experimental data concerning a set of In x Ga 1− x As/GaAs single layer samples are presented and analyzed. The two {110} planes perpendicular to the interface show distinct dechanneling probabilities. In the light of the dechanneling cross section properties this fact reflects the difference between the dislocation densities in the two directions. The obtained dislocation densities are in agreement with those derived indirectly from strain measurements. It is shown that the sensitivity of the dechanneling technique is of the order of 10 4 dislocation lines cm −1 .

STRANSKI-KRASTANOW SELF-ORGANIZED GROWTH OF NANO-SCALE ZNTE ISLANDS ON (0 0 1)GAAS BY METALORGANIC VAPOUR PHASE EPITAXY

The Stranski–Krastanow (SK) growth by atmospheric pressure metalorganic vapour phase epitaxy of self-organized ZnTe nanoislands on homoepitaxial (0 0 1)GaAs is demonstrated. The −7.4% lattice mismatch of the ZnTe/GaAs heterostructure leads to a strain driven distribution of nanoscale ZnTe islands on top of a two-dimensionally (2D) grown wetting layer. Atomic force microscopy and Rutherford backscattering spectrometry are used to determine the island dimensions, the ZnTe mean coverage and the thickness of the 2D wetting layer. The island average density and diameter, as well as their aspect ratio are about 520 μm-2, 13.6 nm and 0.20, respectively, in the case of a 1.20 monolayer (ML) thick wetting layer and a growth rate of 0.074 ML s-1. Preliminary data on the effects of different growth rates on the island average densities are also reported.

Structural characterization techniques for the analysis of semiconductor strained heterostructures

A combined method for structural characterization of strained epitaxial heterostructures involving different techniques such as Rutherford backscattering spectrometry (RBS), multiple crystal X-ray diffractometry (MCD) and transmission electron microscopy (TEM) is presented. In order to obtain a complete characterization of the analysed structure, three different quantities are measured independently: the epilayer thickness, the density of misfit dislocations which may appear at the interface, and the significant components of the strain tensor, mainly the tetragonal distortion, affecting the epilayer lattice. In this way the thermodynamic state and the mechanisms of plastic deformation of the structures can be fully investigated. In this contribution we present and discuss the experimental results concerning a set of InP/GaAs samples having different layer thicknesses ranging from 5 to 500 nm. The thickness of the samples has been determined by RBS. Measurements of in-plane strain and tetragonal distortion have been performed by MCD and RBS-channelling respectively, finally TEM has been used for determining the defects densities and distribution.