A.C. Wright

High resolution transmission electron microscopy (HRTEM) of interfaces in epitaxial ZnSeyS1 − y grown by MOCVD

Abstract The preparation of cross-sectional samples of a number of epitaxially grown, binary and ternary materials within the system ZnSe 1 − y S y (0 ⪕ y ⪕ 1) on a variety of single crystalline substrates is reported. A combination of chemical polishing and fast atom bombardment (FAB) provides samples down to ca. ⪕ 100 A thickness and these are suitable for study by high resolution (lattice imaging) transmission electron microscopy (HRTEM). The integrity of the structure and composition of epitaxial layers appears to be maintained during thinning and our techniques allow the selection and subsequent thinning of particular regions of interfaces. The lattice images obtained at 100 and 120 keV reveal structural details of the interfaces and defects in the bulk of the epiaxial layers.

High resolution and conventional transmission electron microscopy in the characterisation of thin films and interfaces involving II–VI materials

Abstract The use of high resolution transmission electron microscopy (HRTEM) in the characterisation of semiconductor interfaces is reviewed. The importance of employing particular imaging conditions in the electron microscope to reveal and characterise structural and compositional features will be highlighted. The application of these combined experimental and theoretical approaches to the elucidation of specific problems in the II–VI field will then be discussed. We show how the presence of compound formation, e.g. Ga 2 Se 3 at interfaces between ZnSe epitaxial layers prepared by metalorganic vapour phase epitaxy (MOVPE) and single crystal GaAs substrates has been confirmed by employing [100] projection instead of the more usual [110] direction for cross-sectional studies. Also, we show how conventional TEM has been employed to study the relation between structural defects in ZnSe films and specific low temperature photoluminescence peaks such as Y 0 . We also show recent results on growth procedures of II–VI multilayer structures using HRTEM as the main characterisation tool.

Detection of compound formation at the ZnSe/GaAs interface using high resolution transmission electron microscopy (HRTEM)

Abstract Experimental high resolution transmission electron microscopy (HRTEM) and theoretical image simulations have been used to confirm the presence of compound formation at interfaces between ZnSe epitaxial layers prepared by metalorganic vapour phase epitaxy (MOVPE) and single crystal GaAs substrates. Visibility of the compound in HRTEM is enhanced using the [100] projection instead of the more usual [110] direction for cross-sectional studies. Electron image simulations indicate that HRTEM has the capability to detect the occurrence of compound formation down to thicknesses of one unit cell. Our experimental results show compound layer thicknesses of 2 nm.

MOVPE growth of magnesium cadmium sulphide: rocksalt or sphalerite?

Abstract Epitaxial MgS has been grown by the MOVPE process using bis(methylcyclopentadienyl)magnesium and tertiarybutylthiol precursors on both GaP and GaAs substrates. For substrate temperatures between 450 and 550°C epitaxial layers possessing the rocksalt (NaCl) crystal form were produced. At lower temperature polycrystalline layers resulted. The absence of the previously reported sphalerite (zincblende) phase can be attributed to a combination of the high substrate temperatures employed and the choice of sulphur precursor rather than the effect of substrate mismatch. Additionally dimethyl cadmium was used to produce magnesium cadmium sulphide layers. Low concentrations of cadmium ( x ≤ 0.08) resulted in homogeneous Mg 1− x Cd x S alloy epilayers. For x ≥ 0.14 polycrystalline material was produced. Similar isovalent Mg doping of hexagonal Cd 1− y Mg y S produced an upper limit of y ≤ 0.18 for single crystal alloy growth.

Polarity determination in 〈100〉-orientated GaSb by high-resolution transmission electron microscopy at 300 kV

Abstract The polarity or orientation of 〈110〉 projected single crystals of tetrahedral compound semiconductors has been determined by high-resolution electron microscopy (HREM) at 300 kV. The method has been applied to GaSb and computer simulations show that the image contrast is the reverse of that expected intuitively. Similar results are predicted for GaAs.

On the presence of holes in stearic acid Langmuir-Blodgett films studied by dark field electron microscopy

Transmission electron microscopy using the dark field mode of imaging has been demonstrated as a useful method for the structural characterization of Langmuir-Blodgett (LB) thin films and for revealing the presence of holes in such films. In this paper, we show that the presence of such holes may be related to the nature of the substrate used to support the LB films and that some substrates, notably amorphous carbon, may be unsuitable for this purpose. In addition, real-time electron microscopy studies reveal that holes may, in some materials at least, be created by the action of the electron beam.

Thermal decomposition of di-tertiarybutyl selenide and dimethylzinc in a metalorganic vapour phase epitaxy reactor

The thermal decomposition of di-tertiarybutyl selenide (DtBSe), both alone and in the presence of dimethylzinc (DMZn), was investigated using ex-situ Fourier transform infrared (FTIR) absorption spectroscopy in a low-pressure metalorganic vapour phase epitaxy (LP-MOVPE) reactor. The decomposition of DtBSe alone, yields isobutene as the major product, with a much smaller proportion of isobutane detected. Pyrolysis of DMZn in dihydrogen in the presence of DtBSe is very similar to pyrolysis of DMZn alone in dihydrogen with methane the exclusive product. This indicates that co-pyrolysis of the DMZn/DtBSe mixture occurs via radical attack by H on DMZn and largely independent pyrolysis of DtBSe via a β-hydrogen elimination reaction. Traces of the intermediate tertiarybutyl selenol ( t BuSeH) were also detected. The small difference observed in the decomposition behaviour of the DtBSe-DMZn mixture in a dihydrogen compared to a helium ambient further indicate that the pyrolysis processes are independent. These conclusions are supported by PM3 semi-empirical molecular orbital calculations, which indicate that the most likely pathway for unimolecular dissociation of DtBSe is via β-hydrogen elimination with C-Se bond homolysis only likely to be an effective competing mechanism at higher growth temperatures and reactor pressures.

High-resolution lattice-imaging study of twin boundaries in epitaxial films of ZnSe grown by metal-organic vapour-phase epitaxy

Abstract Twin boundaries found to occur in ZnSe epitaxial layers grown by metal-organic vapour phase epitaxy on {100} Ge substrates have been studied using high-resolution transmission electron microscopy. These twin boundaries are of the {111} type and, because of the polarity of the sphalerite (ZnSe) structure, can in principle exist as three different sub-types: neutral, P-type and N-type. Comparison of experimentally recorded images with extensive computer simulations of all three types of twin boundary show that while lattice images unique to each twin type can be found, it is possible to confuse the various types when the effects of crystal misorientation are considered. The present results, however, indicate that only the neutral type of twin boundary has been found so far in these epitaxial layers.

Anomalous twin boundaries in epitaxial zinc selenide grown on (100)oriented GaAs substrates by molecular beam epitaxy

Abstract Anomalous ‘bent’ microtwins lying in a 〈111〉 direction and containing high concentrations of Ga and As have been observed in ZnSe epitaxial layers grown on (100)-oriented GaAs substrates by molecular beam epitaxy at 410°C. The bend radius R meas of 0·928 × 10−5 m compares favourably with a theoretical value based on the use of expressions applicable to a bimetallic strip comprised of GaAs and ZnSe.

Mass-spectrometric study of the pyrolysis reactions in the MOVPE of Ga2Se3 by in-situ gas sampling

An investigation of the thermal decomposition reactions of trimethylgallium (Me3Ga) and hydrogen selenide (H2Se) using hydrogen as a carrier gas involved in the growth of Ga2Se3 (and/or GaSe) was undertaken, using a simple mass spectrometric sampling system on a conventional atmospheric pressure metalorganic vapour phase epitaxy reactor, MOVPE, reactor. The mass fragments corresponding to the unreacted precursor species as well as stable volatile hydrocarbon products were monitored using a quadrupole mass spectrometer as the temperature of the susceptor, the VIIII ratio and the precursor mole fractions were varied. The only gas phase hydrocarbon product detected was methane. Possible mechanisms for the reaction of Me3Ga with hydrogen selenide have been investigated theoretically and the implications of these results for the growth of Ga2Se3 (and/or GaSe) epilayers are discussed.