G.R. Bell

Nucleation and growth mechanisms during MBE of III-V compounds

Abstract In this paper we discuss our recent results on the homoepitaxial growth of GaAs from beams of Ga and As 2 ( 4 ) and of InAs on GaAs from In and As 2 ( 4 ) beams. Experimental measurements are based on in-situ reflection high energy electron diffraction (RHEED) and scanning tunnelling microscopy (STM), while their analysis relies on kinetic Monte Carlo (KMC) simulations and the application of rate equations. We emphasise the comparative behaviour on all three low-index substrate orientations and the importance of surface reconstruction. Wherever possible we use an atomistic approach and in the homoepitaxial studies we are principally concerned with nucleation effects occurring at the sub-monolayer stage, including site-specific adatom incorporation and arsenic molecule dissociation pathways, especially the essential requirement of a mobile precursor state. Some results for vicinal plane growth are also presented. In the case of the InAs system, we have investigated strain relaxation processes using STM images, while specifically for growth on GaAs(001)-2×4 and c(4×4) surfaces we have studied the formation of so-called self assembled quantum dots (SADs) in relation to the classical Stranski–Krastanov mechanism and the associated strain effects. Although it is frequently stated that this accounts fully for dot formation, we have found many anomalies and believe the process, and indeed the final structures, to be much more complex than generally believed.

Transformation kinetics of homoepitaxial islands on GaAs(001)

Abstract Kinetic Monte Carlo simulations, in situ scanning tunneling microscopy, and mean-field rate equations are used to characterize the atomistic nucleation, growth, and structural transformation kinetics of homoepitaxial islands on GaAs(001)-(2×4). After an induction period, islands are formed that do not adopt the reconstruction of the substrate, but transform into β2(2×4) structures as they grow. Comparison of measured and simulated island statistics at several coverages reveals that the unreconstructed islands initially grow slowly in size and rapidly in number, whereas the transformed islands have an appreciably higher growth rate but appear much more gradually.

Direct observation of anisotropic step activity on GaAs(001)

Abstract Scanning tunnelling microscopy has been used to study the differences between A-type and B-type atomic steps on vicinal GaAs(001) surfaces miscut 1° towards either (111)A or (111)B. Two situations involving mass transport at the surface have been considered: sub-monolayer homoepitaxial growth and the phase transition from the (2×4) to c(4×4) surface reconstruction. In both cases, B-type steps are found to be more favourable as sites for the growth of new material and hence have a stronger influence on the overall surface morphology than A-type steps.

Quantitative comparison of surface morphology and reflection high-energy electron diffraction intensity for epitaxial growth on GaAs

Abstract The relationship between developing surface morphology and specular spot intensity in reflection high-energy diffraction (RHEED) has been investigated during the homoepitaxial growth of GaAs on various surface orientations. The ‘morphological’ quantities of step density and effective coverage have been measured directly by means of rapid-quench scanning tunnelling microscopy (STM). For growth on the singular (001), (110) and (111)A surfaces, the step density, layer coverage and RHEED intensity oscillate with a period of one monolayer and little damping. The morphological quantities are always in phase, while the phase of the RHEED intensity depends on the diffraction conditions. On vicinal GaAs(001), the RHEED intensity oscillations are rapidly damped, as are oscillations of the morphological quantities, but again the phase of the RHEED intensity can be varied with respect to the other two. In all cases on the (001) surface, a well-ordered (2×4) surface reconstruction develops very rapidly in the growing layer. Implications for theories of RHEED intensity oscillations are discussed.

Nanoscale effects of arsenic kinetics on GaAs(001)-(2×4) homoepitaxy

Abstract The early stages of homoepitaxy on the GaAs (001)-(2×4) surface have been studied by scanning tunnelling microscopy (STM). Detailed island statistics have been obtained from STM images and show a clear dependence on both the arsenic species used and the incident arsenic flux. These results are explained by considering the effects of the macroscopic arsenic kinetics (As 2 and As 4 ) on island nucleation and growth at the nanometre scale.

A new mechanism for reentrant behaviour in semiconductor epitaxy : a reflection high-energy electron diffraction study of the growth of GaAs(111)A thin films

Abstract Reflection high-energy electron diffraction (RHEED) intensity oscillations recorded along high-symmetry azimuths during the homoepitaxial growth of GaAs(111)A thin films by molecular-beam epitaxy show reentrant behaviour, with oscillations occurring at high and low temperatures but disappearing at intermediate values over a ∼10°C temperature window. In off-symmetry azimuths, however, the oscillations persist at all temperatures. Scanning tunnelling microscopy images indicate growth-mode changes with temperature consistent with the RHEED results. RHEED patterns and rocking curves provide further evidence for a model based on a transition between two-dimensional layer-by-layer growth and step propagation.

Monte Carlo Simulation of GaAs(001) Homoepitaxy

By carrying out Monte Carlo simulations based on the two-species atomic-scale kinetic growth model of GaAs(001) homoepitaxy and comparing the results with scanning tunneling microscope images, we show that initial growing islands undergo the structural transformation before adopting the proper beta2(2x4) reconstruction.