N. Maung

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.

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.

Effect of substrate autodoping on MOVPE-grown ZnSxSe1-x and ZnSe: Analysis by photoluminescence (PL) and secondary ion mass spectrometry (SIMS)

The effect of substrate outdiffusion on the optical properties and compositional integrity of high quality ZnSe and ZnSxSe1-x (0<x<1) grown by atmospheric pressure metalorganic vapour phase epitaxy (MOVPE) was investigated by low tenperature photoluminescence (PL) and secondary ion mass spectrometry (SIMS) depth profiling analyses. Substrate outdiffusion was found to occur in the ZnSe/GaAs and lattice mismatched ZnSxSe1-x(0 < x <1)/GaAS system but mot appreciably in the ZnSe/Ge and lattice matched ZnSxSe1-x(x = 0.06)/GaAs as indicated by diffusion compositional SIMS depth profiles respectively. Low temperature PL measurements indicated changes in the main excitonic recombination transitions for ZnSe/GaAs. In particular the emphasis changes from the IGa2 donor bound exciton emission for thin layers to the Ix native donor bound exciton emission for thick layers. The IGa2 donor bound exciton peak is also domain in the lattice mismatched ZnSexSe1-x (0< x <1).

Electrical and photo luminescence properties of ZnSe epitaxial layers grown on GaAs (100) by atmospheric pressure MOVPE: the role of substrate temperature

Epitaxial layers of ZnSe ranging in thickness from 5μm to 30 μm have been grown on GaAs (100) substrates over the temperature range 240° C to 340° C by atmospheric pressure MOVPE employing dimethylzinc and hydrogen selenide. An optimum growth temperature of 280 ± 5° C has been identified and when grown at this temperature the ZnSe epitaxial layers exhibit low resistivity (ρ298K ≤ 10 ohm · cm), a low compensation ratio (θ298K = 0.27), a carrier mobility (μ298K) of 250 ±10 cm2V-1s-1) and aren-type (n298K = 8.0 × 1014 cm-3). The ratio of photoluminescence intensity measured at 298K and at 12 K is high (104) and is dominated by a sharp emission due to excitons bound to neutral donors at 2.7956 eV. Mass spectrometric investigations of the chemical reactions occurring inside the reactor in the presence of the GaAs substrate indicate significant surface-controlled reactivity in the region of 280° C.

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.

Direct synthesis of II3-V2 compound semiconductors by the heterovalent exchange reaction

Abstract It is shown that it is possible to synthesize thin films of the II3-V2 compound Zn3As2 by the heterovalent exchange reaction (HER) of dimethylzinc in H2 with GaAs substrates at 350°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 α phase, which is stable below about 190°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.

A study of the mechanism of the reaction of trimethylgallium with hydrogen selenide

A study of the pre-deposition room temperature gas-phase reactions involved in the growth of Ga 2 Se 3 (and/or GaSe) using trimethylgallium (GaMe 3 ) and hydrogen selenide (H 2 Se) 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 GaMe 3 –H 2 Se 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 GaMe 3 with H 2 Se on the growth mechanism of Ga 2 Se 3 /GaSe and its role in determining epilayer morphology and microstructure are also discussed.

Dynamic vapour pressure measurements of di-tertiarybutyl sulphide using an ultrasonic monitor

Abstract 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.

Growth process studies by reflectance anisotropy spectroscopy on MOVPE ZnSe

Abstract 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.

On the origin of compositional fluctuations in ZnSe1-xSx alloys grown by metal-organic vapour-phase epitaxy

Periodic vertical modulations of the sulphur concentration have been observed in epitaxial layers of ZnSe1− x S x 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.