M. R. Fahy

Inhibitions of three dimensional island formation in InAs films grown on GaAs (111)A surface by molecular beam epitaxy

A comparison has been made of the surface morphology of thin InAs films grown on GaAs (001) and (111)A substrates by molecular beam epitaxy using in situ reflection high energy electron diffraction and ex situ atomic force microscopy. InAs growth on (001) surface proceeds via the Stranski‐Krastanov mechanism, with three‐dimensional island formation beginning between one and two monolayers, but on the (111)A surface there is a two‐dimensional mode, independent of detailed growth conditions. This advantage accruing from the use of a novel index substrate provides the opportunity of fabricating a wide range of high quality heterostructures.

Reflection high‐energy electron diffraction study of the GaAs:Si:GaAs system

Surface structures occurring as a function of coverage during the deposition of Si on GaAs (001) and the further changes brought about by subsequent GaAs overgrowth using molecular beam epitaxy (MBE) have been studied with reflection high‐energy electron diffraction (RHEED). Deposition of Si in the presence of an As4 flux causes the surface reconstruction to change systematically from 2×4 to symmetric 3×1 via an asymmetric 3×1 stage. The process is reversed during the overgrowth of GaAs. The change in surface periodicity in the [110] direction from two‐fold to three‐fold is explained by a superpositioning model. The implications of this for the growth and incorporation mechanisms of Si on GaAs are discussed.

X‐ray characterization of Si δ‐doping in GaAs

High‐resolution triple‐axis x‐ray diffractometry has been used to examine the structural properties of a δ‐doped superlattice of sixty periods, each consisting of half a monolayer of Si and 500 A of GaAs, grown by molecular beam epitaxy (MBE) at 400 °C under an arsenic flux. The measurements indicated that the superlattice was of high structural quality. Using dynamical simulation, it was demonstrated that the period variation was equal to 3%, while the silicon spreading was no greater than 2 monolayers. It was possible to extract this information because of the high‐resolution diffractometer which produced the theoretical line shape and wide dynamic range. Using a simple model, it was deduced that virtually all Si atoms were located on Ga lattice sites.

Indium migration control on patterned substrates for optoelectronic device applications

Strained layer InGaAs/GaAs quantum wells have been grown by molecular‐beam epitaxy on patterned (100) GaAs substrates. Indium migration away from the facets of patterned mesas is shown to be strongly dependent upon the arsenic flux during growth. Based upon this effect we have grown structures in which the number of active quantum wells in adjacent areas of a segmented contact device can be varied.

Silicon incorporation behaviour in GaAs grown on GaAs (111)A by molecular beam epitaxy

We have made a systematic study of the effect of growth conditions on Si incorporation in GaAs layers grown on GaAs (111)A substrates. We show that it is dominated by the low incorporation coefficient of As4 on the GaAs (111)A surface. The site occupancy of the Si has been shown by local vibrational mode spectroscopy to be on Ga and As lattice sites to provide donor and acceptor character, respectively. The doping behaviour of Si for specific growth conditions may be predicted from the product of the As4:Ga flux ratio and the As4 incorporation coefficient at the growth temperature. The growth conditions for which Si acts as an acceptor produce films with poor surface morphology.

A scanning tunnelling microscopy study of the deposition of Si on GaAs(001); implications for Si δ-doping

Atomic resolution scanning tunnelling microscopy (STM) has been used to study the adsorption of Si on GaAs(001) surfaces, grown in situ by molecular beam epitaxy (MBE), with a view to understanding the incorporation of Si in δ-doped GaAs structures. Under the low-temperature deposition conditions chosen, the clean GaAs surface is characterized by a well-defined c(4 × 4) reflection high-energy electron diffraction (RHEED) pattern, a structure involving termination with two layers of As. Filled states STM images of this surface indicate that the basic structural unit, when complete, consists of rectangular blocks of six As atoms with the AsAs bond in the surface layer aligned along the [110] direction. Deposition of < 0.05 ML of Si at 400°C onto this surface shows significant disruption of the underlying structure. A series of dimer rows are formed on the surface which, with increasing coverage, form anisotropic “needle-like” islands which show no tendency to coalesce even at relatively high coverages (∼ 0.5 ML). The formation of these islands accompanies the splitting of the 12 order rods in the RHEED pattern along [110]. As the Si is known to occupy only Ga sites, the Si atoms displace the top layer As atoms of the c(4 × 4) structure, with the displaced As atoms forming dimers in a new top layer. The results are consistent with a recently proposed site exchange model and subsequent island formation for surfactant mediated epitaxial growth.

Incorporation of silicon during MBE growth of GaAs on (111)A substrates

Si-doped GaAs has been grown on (111)A and (111)A vicinal GaAs substrates and carrier concentrations measured for a range of Si fluxes and growth temperatures. The use of As 2 as opposed to As 4 has been examined. These results are discussed with respect to the growth mechanisms. Photoluminescence measurements have been made and compared with growth on an (001) substrate. The nature of the lattice site of incorporated Si is confirmed using local vibrational mode measurements

Reflection high energy electron diffraction intensity oscillation study of the growth of GaAs on GaAs(111)A

Abstract Reflection high energy electron diffraction (RHEED) intensity oscillations have been studied over a wide range of growth temperatures (300–600°C) and As:Ga flux ratios (2:1–15:1), during the growth of GaAs on singular GaAs(111)A substrates by molecular beam epitaxy (MBE). The oscillation period is strongly dependent on both growth temperature and the As:Ga flux ratio, in contrast to observations on GaAs(001). A growth rate equivalent to that expected from the supply of Ga is only measured (by RHEED) at high As:Ga ratios and/or low growth temperatures. This behaviour can be explained through a model in which arsenic has a low initial sticking coefficient, thus allowing the formation of free Ga at the beginning of growth, alongside two-dimensional (2D) GaAs island growth. Activation energies related to desorption and surface diffusion of the incident As 4 have been obtained.

Optical studies of the growth of single monolayer wide InAs quantum wells on GaAs by MBE

Abstract We report here on photoluminescence (PL) and photoluminescence excitation (PLE) investigations into the direct growth by molecular beam epitaxy (MBE) of a single monolayer (ML) InAs quantum well (QW) having GaAs barriers on a GaAs substrate. Our growth temperature studies show that there is appreciable In desorption during the growth of single MLs of InAs in the temperature range 420 to 540°C. Investigation into surface segregation of In using a simple structure consisting of two coupled 1 ML wide InAs QWs reveal that this phenomenon is negligible at 420°C although significant at higher temperatures. We find that the optimum growth temperature for the growth of a 1 ML wide InAs QW is 420°C.

Surface segregation effects of In in GaAs

Photoluminescence (PL), secondary‐ion mass spectroscopy (SIMS), and cross‐sectional transmission electron microscopy (TEM) measurements have been performed to assess surface segregation of In in GaAs during molecular‐beam epitaxial growth of InAs monolayers between GaAs layers. The InAs growth temperature at which In segregation is detectable depends on the characterization technique used; using PL it is above 420 °C, but from TEM and SIMS it is 420 and 340 °C, respectively. These results highlight the need for complementary information to provide a better understanding of the segregation phenomenon. SIMS data show that the total amount of In segregating and the extent of its distribution both increase with InAs deposition temperature.