A.G. Taylor

Dynamics and kinetics of MBE growth

Abstract The epitaxial growth of semiconductor films from molecular beams in an ultra-high-vacuum environment has made available structures of unprecedented complexity and precision. In parallel, the application of surface science techniques and computer simulation to studies of surface reaction kinetics and growth dynamics has allowed very significant advances to be made in our understanding of epitaxial film growth at the atomic level. The experimental techniques which have been used most extensively are modulated beam relaxation spectroscopy (MBRS) and reflection high energy electron diffraction (RHEED). A summary of results from solid sources for both lattice matched and strained layer systems which illustrate our present level of knowledge is related to Monte Carlo growth simulations and an analytical treatment of non-linear nucleation kinetics. Particular attention is focussed on cation effects in the growth of III-V compounds over a wide temperature range. In addition to MBRS and RHEED, optical methods such as reflectance difference spectroscopy and surface photo-absorption have recently been introduced as in-situ diagnostic probes to study surface chemistry and growth morphology. The information derived from this type of measurement is considered in the context of growth models developed from the longer-established techniques.

Gas phase monitoring of reactions under InP MOVPE growth conditions for the decomposition of tertiarybutyl phosphine and related precursors

Abstract Ex-situ Fourier transform infrared spectroscopy has been used to study the decomposition of tertiarybutylphosphine (TBP), trimethylindium (TMIn) and mixtures of TBP and TMIn under MOCVD conditions using dihydrogen as a carrier gas. IR bands due to TBP, TMIn, phosphine, isobutene, isobutane, ethene and methane have been monitored as a function of susceptor temperature. The decomposition of TBP alone in dihydrogen is observed to commence at temperatures above 773 K and is accompanied by the formation of isobutene, phosphine and isobutane. The pyrolysis of TBP is observed to be complete at temperatures in excess of 973 K. For TMIn in dihydrogen, the only observable product at temperatures greater than 573 K is methane. For TBP in the presence of TMIn a room temperature reaction is observed, the only detachable product of which is methane. The implication is that TMIn reacts in some way with TBP, possibly forming an “adduct” or polymer; however, decomposition products from TBP are not observed until temperatures are in excess of 573 K, while decomposition is observed to be complete at temperatures of 873 K. Once again isobutene, isobutane and phospine formation accompanies the TBP decomposition at 573K. At temperatures in excess of 900 K, both methane and ethene were observed during both of these experiments and are assumed to arise via the decomposition of isobutene. The deposited product on the reactor wall was found to be InP and phosphorus (rhombohedral) by X-ray diffraction. Some mechanistic steps for these reactions are proposed. In addition, preliminary data for the decomposition of cyclohexylphosphine are presented.

Optical second harmonic generation studies of the nature of the GaAs(100) surface in air

Abstract We present in this paper a study of the non-linear optical response of the GaAs/air interface as measured by second harmonic generation. By careful choice of incidence and analysis electric vectors for the radiation the second harmonic response as a function of crystallographic orientation has been characterized for a GaAs(100) surface. The exact origin of the response remains unclear but comments on the surface specificity of the technique are made and the observed anisotropy in the data is explained in terms of the step density of the crystal surface.

Laser-surface diagnostics of GaAs growth processes II. Reflectance anisotropy studies of GaAs growth by MBE

Abstract The relationship between reflectance anisotropy (RA) signals and As or Ga coverage on GaAs (100) has been investigated by monitoring the RA response through a series of surface reconstructions generated under MBE growth conditions. Under As-rich conditions for which RHEED indicated c(4 × 4) reconstruction the substrate was heated to promote As 2 desorption and distinct desorption transients were observed superimposed upon an approximately monotonic variation in the RA signal which correlate with the observation of the c(4 × 4)As to (2 × 4), (2 × 4) to (3 × 1) and (3 × 1) to (4 × 2)Ga RHEED transitions. These phase transitions were observed to be reversible. It is suggested that these phase transitions involve three distinct types of As species each resulting from the unique nature of each reconstruction, and that the experiment performed here may be thought of as an “optical” thermal desorption experiment.

Structures, dynamics and vibrations of cyclic (H2O)3 and its phenyl and naphthyl derivatives

The cyclic water trimer shows a fascinating complexity of its intermolecular potential-energy surface as a function of the three intermolecular torsional coordinates: there are six isometric but permutationally distinct minimum-energy structures of C1 symmetry, which can interconvert by torsional motions via six isometric transition states, also of C1 symmetry. A second type of interconversion can occur through different torsional motions via two C3 symmetric transition structures, and a third interconversion type via a planar C3h symmetric transition structure. The equivalence of the six minima is broken if the ‘free’ H atom of one H2O molecule in the cluster is chemically substituted, yielding three distinct conformers, which occur in enantiomeric pairs. Not all three conformers are necessarily locally stable minima; this depends on the substituent. The phenol–(H2O)2, p-cyanophenol–(H2O)2, 1-naphthol–(H2O)2 and 2-naphthol–(H2O)2 clusters, which are the phenyl, p-cyanophenyl and naphthyl derivatives of (H2O)3, were examined by resonant two-photon ionization spectroscopy in supersonic beams. These clusters exhibit S0→ S1 vibronic spectra with very different characteristics. These reflect the number of cluster structures formed, their low-frequency intermolecular vibrations and indirectly give information about the cluster fluxionality.

A RHEED and reflectance anisotropy study of the MBE growth of GaAs, AlAs and InAs on GaAs(001)

Abstract MBE growth of GaAs, AlAs and InAs on GaAs(001) has been monitored in real time, using simultaneous reflection high-energy electron diffraction (RHEED) and reflection anisotropy (RA) measurements, the RA measurements all being made with a fixed wavelength laser source operating at 647 nm. For the growth of GaAs and AlAs on GaAs(100), RA oscillations which occurred during the bilayer growth are explained in terms of the variation in the anisotropic distribution of surface “dimer” bonds during the growth process. RA data obtained during the growth of InAs/GaAs heterostructures are presented and discussed in relation to the influence of surface roughening upon both the RA and RHEED measurements.

The Pyrolysis of Precursors for Gaas Mocvd Studied by In-situ and Ex-situ Fourier Transform Infared Spectroscopy

Abstract The decomposition of arsine at atmospheric pressure as studied by in-situ and ex-situ Fourier transform infrared spectroscopy (FTIR) is presented, comparing the techniques. Ex-situ spectroscopic and spectrometric evidence for the formation of methylarsine and dimethylarsine during the reaction of trimethylgallium and arsine, and in-situ spectroscopic evidence for the formation of diethylarsine during the reaction of triethylgallium and arsine is discussed. The observation of alkylarsines strongly implies a stepwise free radical reaction mechanism, however whether these processes are heterogeneous or homogeneous remains uncler.

Reflectance and reflectance anisotropy measurements of the photoelectrochemical deposition of gold on p-type gallium arsenide (100) electrodes

The photoelectrochemical deposition of gold on p-type GaAs has been monitored in situ via the total reflectance response and reflectance anisotropy of the nascent surface. It is demonstrated that the former method probes the macroscopic deposition rate while the latter method probes surface ordering in the growing layer. For the system studied here, the RA data are most consistent with a highly anisotropic initial gold layer, such as may be expected from a single-crystalline film. This proposal is shown to be in good agreement with electron diffraction data for the GaAs/Au interface, which indicate that the initial gold deposit adopts a (110) orientation in which the lattice parameter of the gold film matches that of the GaAs (100) substrate.

Optical monitoring of deposition and decomposition processes in MOCVD and MBE using reflectance anisotropy

Abstract A series of experiments have been performed in order to assess the ability of reflection anisotropy (RA) to follow the MBE growth of GaAs and AlAs on (001) GaAs, combined RA/RHEED measurements reveal a layer-by-layer growth mechanism as indicated by periodic RA oscillations. For the growth of the lattice mismatched GaAs/InAs heterostructure large RA transients are observed which are found to reach their maximum during the growth of the first monolayer and before the onset of 3D growth. RA studies have also been carried out during the MOCVD decomposition of triethylgallium (TEGa) on (001) GaAs at several temperatures ranging from 300 to 425°C. Results obtained suggest that TEGa begins to fully decompose at 350°C which is found to correlate with the formation of TEGa gas phase decomposition products. Results are also presented which may indicate a change in surface reconstruction under MOCVD conditions on moving from a Ga-rich to an As-rich surface state.

A RHEED and RA study of MEE growth

Migration enhanced epitaxy (MEE) has been generally accepted as an important low-temperature epitaxial growth technique. The main advantage of MEE over conventional molecular beam epitaxy (MBE) according to the accepted model is the enhanced migration of group III elements during growth as proposed by its inventor. Although there have been a number of reflection high-energy electron diffraction (RHEED) and optical studies of the MEE process, certain aspects of the mechanism are not well understood. Using the in-situ techniques of RHEED and reflectance anisotropy (RA), the response of a GaAs(100) surface to the deposition of Ga and to the alternate deposition of Ga and As2 or As4 has been studied. Whilst the well-known 4×X(X=2or6...) Ga-stabilised surface is obtainable after deposition of Ga (or during the Ga deposition phase of an MEE cycle) at high temperatures (e.g., 580°C), such a surface is highly disordered at lower temperatures (<530°C) and only obtainable at slow deposition rates at temperatures below 350°C. After a complete MEE cycle, the surface can only return to the 2×4 As-stabilised reconstruction if the temperature is 500°C or above. Therefore barriers must be present which prevent a well ordered surface being obtained at the end of each Ga and As deposition phase. The RA response to the MEE process is quite independent of temperature, but the RHEED specular beam intensity variation indicates that during the As deposition phase of an MEE cycle the surface morphology is under the control of As and changes in a way similar to MBE.