The growth of erbium doped gallium arsenide by MBE at normal substrate temperatures (~580°C) is constrained by a solubility limit of 8×1017 cm−3. This is much less than is desirable for optical emitters using the forbidden 4f transitions of Er3+ to produce radiation at 1.54µm. We have developed an MBE technique where it is possible to produce spherical mesoscopic precipitates containing erbium as a matrix element within the gallium arsenide. Structural and analytical studies indicate that the precipitate is cubic (rock salt) erbium arsenide. The physical size of the precipitates is self limiting as a result of surface migration occurring during MBE growth. By adjusting the growth conditions it is possible to produce an array of uniform erbium arsenide quantum dots of a size chosen from the range 10–20Å. The dot density can be varied by changing the erbium flux.

Chemical Vapour Deposition (CVD) is now a well-established route to the growth of carbon nanotubes (CNT). Generally, a transition metal such as nickel dispersed as clusters on a nonreactive substrate acts as a catalyst for growth from carbon-bearing gases such as acetylene. While the diameter of the tubes is closely linked to the size of the metal clusters, the nature of the substrate itself influences the size and size distribution of the clusters themselves. Using Scanning Tunnelling Microscopy (STM) to study the clusters and high resolution Field Emission Scanning Electron Microscopy (FE-SEM) to study the resultant nanotubes, we show that nickel clusters created on TiN-coated SiC substrates have a much smaller size distribution than MoSi2-coated SiC substrates. This is reflected in the nanotube diameter after CVD growth. Raman spectroscopy and Transmission Electron Microscopy has also been used to provide further data on the nature of the nanotubes and clusters grown on these substrates.

Periodic vertical modulations of the sulphur concentration have been observed in epitaxial layers of ZnSe1-xSx grown by metal-organic vapour-phase epitaxy. High-spatial-resolution microanalysis has shown that the sulphur variation is of the order of 1 at.% with a period of about 44 +/- 0.1 nm. A detailed analysis of the possible causes of this modulation is presented which shows that periodic temperature fluctuations of the substrate are the most likely source.

A number of ZnS phosphor powders, both commercial and experimental, have been characterized by transmission electron microscopy (TEM). The microstructure of the phosphors has been compared with the light output (intensity and spectra) obtained when incorporated into an ac-driven electroluminescent test lamp. No evidence has been found in any samples examined by TEM of Cu2S precipitates. It was also found that the brightest emission is obtained when the phosphor particles have a very heavily multiply twinned structure together with a rough profile, and least output when they show the cubic phase. Examination of the electroluminescence output of individual phosphor grains indicates that emission is mostly from material close to the surface and suggests that field emission occurs from the surface which is aided by a marked surface roughness. Finally, the microstructure of a commercial ac powder phosphor sheet device was examined and also found to contain ZnS in the same heavily twinned form.

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Iodine ion milling was developed to overcome the deficiencies of using argon as a milling gas for certain compound semiconductors. Here it is shown that iodine gas introduces strong artefacts in some materials such as Zn3As2, a material occurring at some ZnSe/GaAs interfaces and that even efficient specimen cooling does not prevent this effect. Argon ion milling by contrast leaves the Zn3As2 layer intact. However, deliberate amorphisation of the Zn3As2 by iodine enables a clearer detection of its presence and to distinguish better the coverage of very thin ZnSe epilayers. These effects can be accounted for by simple thermodynamic calculations and enable the possibility for prediction as to whether reactive milling agents such as iodine will cause problems in specimen preparation. These results also indicate that iodine molecules rather than atoms are the principle milling species.

The microstructure of ZnSe single crystal substrate material grown by solid phase recrystallisation has been investigated. These substrates have potential for the homoepitaxial growth of superior II–VI blue–green emitting devices. Optical and scanning electron microscopy has been used to reveal the presence of considerable numbers of micron-sized tetrahedral shaped features within the crystals. Micro-Raman spectroscopy reveals these to be crystalline selenium precipitates. Transmission electron microscopy has been used to characterise the dislocation structure of these crystals and shows that careful polishing protocols must be used to prevent subsurface damage.

A study of the pre-deposition room temperature gas-phase reactions involved in the growth of Ga2Se3 (and/or GaSe) using trimethylgallium (GaMe3) and hydrogen selenide (H2Se) was undertaken, using a simple mass spectrometric sampling system on a conventional atmospheric pressure MOCVD reactor. The experimental studies were complemented by theoretical quantum chemical calculations which were used to predict the reaction thermochemistry and kinetics of the proposed reaction scheme. We have shown that the gas phase reaction of the GaMe3-H2Se mixture can be described by a simple reaction mechanism with no need for the participation of a stable Lewis acid-base adduct, although a transient adduct type species may be involved. The effect of the room temperature reaction of GaMe3 with H2Se on the growth mechanism of Ga2Se3/GaSe and its role in determining epilayer morphology and microstructure are also discussed.

It is shown that it is possible to synthesize thin films of the II3-V-2 compound Zn3As2 by the heterovalent exchange reaction (HER) of dimethylzinc in H-2 with GaAs substrates at 350 degrees C. These films are of a very high structural quality, being both epitaxial and highly ordered in nature as shown by an extensive transmission electron microscopy study. Using comparisons with simulated images and diffraction patterns, it is shown that the Zn3As2 formed is of the low-temperature cu phase, which is stable below about 190 degrees C. The lack of free gallium present on the surface suggests that methyl radicals are responsible for transport of exchanged gallium into the vapour phase. The high structural quality of the layers grown by this technique suggests that the HER is a promising method of circumventing the usual problems of mismatched epitaxial growth and that a wide range of thin-film materials may possibly be grown.

Di-tertiarybutyl sulphide is an attractive precursor for the growth, using metal-organic vapour-phase epitaxy (MOVPE), of sulphur-based wide band-gap II-VI compounds due to its lower volatility compared to other more commonly used sulphur sources and likely suppression of unwanted gas-phase pre-reactions. The vapour pressure of MOVPE precursors is a key parameter and needs to be precisely known in order to control the concentration of source materials entering the reactor. Only scattered and incomplete vapour pressure data exist for di-tertiarybutyl sulphide. The concentration under dynamic conditions was measured with an Epison concentration monitor and used to predict the saturated vapour pressures. The vapour pressure equation (log10p=8.559(±0.082)−(2338(±23)/T) was obtained and the measured vapour pressures were found to be consistently smaller than the reported literature values. The saturated vapour pressure for tertiarybutyl thiol under dynamic conditions has also been measured and new data are reported.

We have studied the magnetic properties of self-organized ErAs aggregates embedded in a GaAs matrix using telegraph noise spectroscopy. The electrical transport through the samples is mesoscopic and exhibits universal conductance fluctuations and two-level fluctuations (TLF's). A statistical analysis of a TLF allows us to measure the extremely small magnetization of an isolated ErAs cluster. Rotating the sample in a magnetic field reveals the sixfold magnetic anisotropy expected for ErAs with a rocksalt structure. At high temperatures the switching is thermally activated, while at low temperatures it is the tunneling of the magnetization which dominates. The magnetic-field dependence of the crossover temperature between the thermally activated and tunneling regimes can be explained with a simple model.

We have investigated the effect of the initial state of the GaAs substrate surface on the structural quality of ZnSe epilayers as grown by metal-organic vapour phase epitaxy (MOVPE). In contrast to the findings of many researchers using Molecular Beam Epitaxy (MBE), we have discovered that an extended exposure (15 min) of the GaAs substrate to a zinc flux (dimethylzinc in hydrogen) results in the formation of a well developed epitaxial Zn3As2 layer and that subsequent growth of ZnSe on this surface results in very high defect densities within the epilayer. This finding and other facts discussed herein indicate that the future direction for MOVPE of wide-gap ZnSe-related devices may be best served via homoepitaxy.

The growth of MgS on GaAs substrates by MOVPE has been previously reported using bis(methylcyclopentadienyl) magnesium (MeCp)2Mg along with various sulphur precursors. Although epitaxial growth is possible with hydrogen sulphide (H2S) and tertiarybutylthiol (tBuSH), they have both been shown to undergo severe parasitic gas phase reactions at room temperature and atmospheric pressure, resulting in MgS layers with poor surface morphology and crystallinity. Preliminary results on the growth of MgS/GaAs(0 0 1) have been reported previously [1]. Here we extend those and report ex situ mass spectrometric investigations of the reaction chemistry between bis(methylcyclopentadienyl) magnesium and tBuSH and an alternative sulphur precursor, ditertiarybutylsulphide (DtBS). No pre-reaction is observed at room temperature as with other sulphur precursors and MgS layers with the rocksalt structure can be grown at temperatures of ∼450 °C. A decline in MgS growth rate is observed with increasing temperature indicating the participation of a competitive parasitic reaction along with the growth reaction which becomes dominant at high temperatures. Mass spectrometric sampling supports the growth results and suggests tetiarybutylthiol as an intermediate in the growth process. © 1998 Chapman & Hall

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 DMZnDtBSe 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 (tBuSeH) 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.

The growth of ZnSe on GaAs by metal organic vapour phase epitaxy (MOVPE) has been studied using reflectance anisotropy spectroscopy (RAS). The RA spectra of ZnSe are significantly different for growth on initially Se- or Zn-exposed GaAs surfaces. The Se-terminated GaAs (001) RA spectrum has Se-dimer-related features at 3.3 and 5.1 eV, and the large, high energy peak dominates during ZnSe growth on this surface. Transmission electron microscopy (TEM) analysis has been used to show that these large RA signals arise from anisotropic surface corrugation of the growing ZnSe epilayer. Under initially Zn-stabilised growth conditions, the ZnSe epilayer RA spectrum is largely featureless, showing only a weak peak at 4.7 eV and a dip at 5.1 eV. The corresponding surface anisotropy is greatly reduced in comparison with growth from the initially Se-terminated surface. These observations reveal RAS to the an important technique for ensuring the desired initial GaAs surface since the grown ZnSe surface morphology is critically dependent on the pre-growth substrate surface treatment. However, as the characteristic ZnSe RA spectra are relatively insensitive to changes in substrate temperature and VI–II ratio, RAS is of more limited use as an in-growth surface probe for MOVPE-grown ZnSe.

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 degrees C epitaxial layers possessing the rocksalt (NaCl) crystal form were produced. At lower temperatures 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 less than or equal to 0.08) resulted in homogeneous Mg1 - xCdxS alloy epilayers. For x greater than or equal to 0.14 polycrystalline material was produced. Similar isovalent Mg doping of hexagonal Cd1 - yMgyS produced an upper limit of y less than or equal to 0.18 for single crystal alloy growth.

N-type ZnSe thin films have been grown by MBE on GaAs (001) surfaces and capped with an amorphous selenium layer. The Se cap was thermally desorbed under ultrahigh vacuum to recover the (2 × 1) and c(2 × 2) reconstructed surfaces. Selected metal contact formation was monitored using core and valence level photoelectron emission spectroscopy by in-situ exposure of the surface to heated sources of Au, Ag and Pb. In each case, lineshape analysis of emission spectra indicated a low level of interfacial mixing and provided an insight into the metal layer growth model. Both Au and Ag were found to grow in closely spaced islands of approximately equal height. The morphology of Au and Ag layers was confirmed by cross-sectional transmission electron microscopy. Monitoring of core and valence level emission peak positions allowed the determination of the metal-n-ZnSe Schottky barrier height, for a sufficiently thick metallic layer. Measurements on this wide-gap semiconductor, even at 300 K, were influenced by the presence of a surface photovoltage, which could be identified and subtracted for a fully-formed metallic layer. The n-type ZnSe Schottky barrier heights inferred from the relative Fermi level shifts (ΦBN(Au) = 1.74 eV, ΦBN(Ag) = 1.47 eV and ΦBN(Pb) = 1.25 eV), were found to scale with the metal work function for these three unreactive interfaces.

Cadmium-doped calcium sulphide has great potential as a broadband light source in both powder and thin film electroluminescent devices. We now report the first structural investigation of polycrystalline cadmium-doped calcium sulphide using a combination of transmission electron microscopy (TEM) with microanalysis, and X-ray diffraction. We show that the crystals formed by direct reaction of CaS and CdS under excess sulphur do not yield an even distribution of cadmium. We also show that the crystals are heavily dislocated with defect structures typical of rock-salt structures. We show that TEM can be successfully applied to the study of these moisture-sensitive materials without the formation of artefacts and that TEM should be a valuable tool for studying their thin film structures.