D. A. Woolf

Analysis of molecular‐beam epitaxial growth of InAs on GaAs(100) by reflection anisotropy spectroscopy

The molecular‐beam epitaxial growth of InAs on GaAs(100) was investigated in situ using reflection anisotropy spectroscopy (RAS) and simultaneously reflection high‐energy electron diffraction. The RAS spectra of the GaAs c(4×4) and (2×4) and the InAs (4×2) and (2×4) reconstructions are reported. During InAs deposition, the RAS signal shows significant changes for InAs coverages as low as 1/6 of a monolayer. At this coverage surface reconstructions are responsible for the signal variation. For InAs coverages larger than four monolayers, the RAS signal is essentially determined by the anisotropic roughness of the three‐dimensional growing surface. This is verified using a three‐layer model which gives an excellent description of the experimental spectra at large coverages.

Growth of InxGa1−xAs on GaAs (001) by molecular beam epitaxy

Abstract As a precursor to investigating the growth of In x Ga 1 − x As on Si, a series of 2.8 ± 0.2 μ m thick films of various compositions ( x = 0, 0.13, 0.56 and 1.0) have been deposited by molecular beam epitaxy onto (001) GaAs on-axis substrates, using a minimized As 4 flux and a range of growth temperatures. In addition, a series of layers were grown at a fixed temperature of 350°C over the whole composition range. It was found that the electrical and structural characteristics of InAs and In 0.56 Ga 0.44 As were very similar, in both cases the layers were n-type, and growth below 330° produced a rapid deterioration of crystalline quality with the associated dislocations/defects being electrically active. In addition, the characteristics of GaAs and In 0.13 Ga 0.87 As were also found to be very similar. Both were doped with Si to make them conduct and showed a transition to insulating behaviour on reducing the growth temperature below ∼ 480±10°C; above this temperature good mobilities were obtained. Increasing the growth temperature for all the In containing alloys produced a gradual increase in surface roughness up to the point at which In accumulation on the surface occurred (∼410°C for InAs, ∼510°C for In 0.56 Ga 0.44 As and ∼550°C for In 0.13 Ga 0.87 As). Growth at 350°C showed a rapid deterioration in crystalline quality as x approached 0.5, as can be illustrated by a pronounced maximum in the X-ray diffraction peak width.

Surface reconstructions of GaAs(111)A and (111)B: A static surface phase study by reflection high‐energy electron diffraction

GaAs(111)A and (111)B static surface phase maps have been generated under a variety of substrate temperature and incident As4 flux conditions ranging from, respectively, 400–700 °C, and from 1×1014 to 1×1016 molecules cm−2 s−1. For the case of GaAs(111)A only a (2×2) reconstruction was observed. However, four GaAs(111)B surface reconstructions were identified below a critical As4 flux of JcAs4≂5×1015 molecules cm−2 s−1, viz.: (2×2); (1×1)LT; (√19×√19); and, (1×1)HT. Above JcAs4 the (√19×√19) surface phase was quenched, such that the (1×1)LT and (1×1)HT structures merged to form a single (1×1) phase. The transitions to and from each of these surface phases were found to be reversible, occurring at very specific substrate temperatures for a given incident As4 flux. The activation energies (eA) characterizing the reversible surface phase transitions were measured and compared with those on the GaAs(100) surface.

The molecular beam epitaxial growth of GaAs/GaAs(111)B: doping and growth temperature studies

A series of investigations are presented which address various aspects of the growth, by molecular beam epitaxy, of n‐type (Si doped) on‐axis GaAs/GaAs(111)B. In situ characterization by reflection high‐energy electron diffraction has identified four surface phases on the static (zero growth rate) surface, and three reconstructions which occur, depending upon the substrate temperature, during growth. The n‐type doping properties of GaAs/GaAs(111)B epilayers have been compared with n‐GaAs/GaAs(100) structures. Hall effect and low‐temperature photoluminescence measurements have demonstrated that it is possible to dope GaAs/GaAs(111)B with Si in the 6×1014 to 1018 cm−3 range. A variable growth temperature study is also presented which examines the surface structural, electrical, optical, and surface morphological properties of n‐GaAs/GaAs(111)B grown in the 400 to 650 °C temperature range. The onset of electrical conduction, and optically active material, was found to be directly related to changes in the dynamic surface structure. The variable growth temperature study also revealed a temperature regime within which it was possible to significantly improve the surface morphology of on‐axis GaAs/GaAs(111)B structures whilst retaining good electrical and optical properties.

The molecular beam epitaxial growth of InAs on GaAs(111)B- and (100)-oriented substrates: a comparative growth study

Molecular beam epitaxy has been used to simultaneously grow epitaxial InAs onto (111)B- and (100)-oriented GaAs substrates, under a variety of growth conditions. Despite the large lattice mismatch InAs was observed to grow upon GaAs(111)B in a two-dimensional (layer-by-layer) manner, from onset of nucleation to micrometre thicknesses as indicated by clear, well streaked 2*2 reconstructed RHEED patterns throughout the deposition and smooth post-growth scanning tunnelling micrographs. The comparative study of InAs grown upon GaAs(100) followed the expected Stranski-Krastanow growth mode. Systematic variations of the epilayer thickness, substrate temperature and concentration of intentional n-type (Si) dopant, were carried out and the samples measured ex situ by the Hall effect and double-crystal X-ray diffraction. Despite the two-dimensional growth mode, the InAs/GaAs(111)B system was found to be electrically and structurally inferior to the InAs/GaAs(100) system for growth temperatures below 450 degrees C. However, above this temperature the systems became comparable in terms of both the measured electron mobilities and epilayer Bragg peak widths.

Optical monitoring of the development of InAs quantum dots on GaAs(001) by reflectance anisotropy spectroscopy

Reflectance anisotropy spectroscopy (RAS) in combination with reflection high‐energy electron diffraction (RHEED) was used to study in situ the initial steps of molecular beam epitaxial growth of InAs on GaAs(001). Due to the large lattice mismatch InAs is known to grow in Stranski–Krastanov mode leading to the formation of quantum dots after the transition from two‐ to three‐dimensional growth mode. In this article the precise determination of the growth mode transition and the subsequent development of the islands have been of particular interest. During the growth of the two‐dimensional InAs layer, the RHEED‐pattern changed from the c(4×4) of the clean GaAs to a (1×3) surface reconstruction. Accordingly, the RAS‐spectra, taken every 0.2 ML, indicate changes of the As‐dimer configuration. At 1.8 ML (spotty RHEED‐pattern) a saturation of the intensity of the dimer related RAS‐signal around 2.6 eV was found. The relaxation of the InAs layer and the formation of the quantum dots was followed by time‐resolved RAS at 2.6 and 4 eV. It is shown here, that the time constant of this process, the thickness of the InAs wetting layer and the equilibrium morphology of the islands are strongly temperature dependent. The remaining equilibrium InAs wetting layer thickness at the surface was estimated to be about 1 ML (0.8 ML at 625 K and 1.2 ML at 725 K).

Residual strain measurements in thick InxGa1−xAs layers grown on GaAs (100) by molecular beam epitaxy

The final stages of strain relief in the lattice mismatched InxGa1−xAs/GaAs(100) system are addressed by the examination of the residual strain in thick films (∼3 μm), grown by molecular beam epitaxy, across the entire compositional range. These results are compared with the observed variations in both the growth mode and material quality, and related to available theories. It is found that measurements are not consistent with a degradation of material quality that is simply misfit dependent, or to an abrupt change from two‐dimensional (2‐D) to three‐dimensional (3‐D) growth in the relaxation stage. Instead, the results seem to be more consistent with a continuous change from 2‐D to 3‐D growth between x=0 and x=0.4 (it is wholly 3‐D above x=0.4). In addition, the large residual strains observed around x=0.5 are related to the poor material quality (possibly through work hardening) at this composition, which is in turn due to problems peculiar to the growth of mismatched alloys.

Optical second harmonic generation from Si(111) 1 × 1−As and Si(100)2 × 1−As

Abstract Optical second harmonic generation (SHG) from well-characterised As/Si systems under UHV conditions has been measured. The azimuthal dependence of the SH signal, combined with dispersion studies using pulsed dye and Ti/sapphire lasers, is used to distinguish surface and bulk contributions. The first absolute values of the SH response from well-characterised Si surfaces are also reported.

(2×4)/c(2×8) to (4×2)/c(8×2) transition on GaAs(001) surfaces

We present scanning tunneling microscopy data illustrating the evolution of the decapped GaAs(001) surface following annealing in stages from 450 to 540 °C. After annealing at 450 °C a (2×4) reconstruction is formed by kinked rows of two As dimer unit cells. Following annealing in the 475–500 °C range small isolated regions of (4×2) reconstruction are visible, with a considerable increase in disorder of the remaining (2×4) reconstructed areas. Annealing at higher temperatures causes the (4×2) structure to become increasingly dominant. We have noted significant differences in the surface morphology as a function of annealing time. Our images of the (4×2) surface are similar to those recently reported by other groups but we propose a new structural model.

The characterization of the growth of sub-monolayer coverages (1200th to 1 monolayer) of Si and Be on GaAs(001): A reflectance anisotropy spectroscopy and reflection high-energy electron diffraction study

The techniques of reflectance anisotropy spectroscopy (RAS) and reflection high-energy electron diffraction (RHEED) have been employed, for the first time in concert, to characterize the growth of sub-monolayer coverages of both Si and Be deposited onto theGaAs(001)-c(4 × 4) surface. The RHEED observations enabled the RAS spectra collected for a series of Si and Be coverages, in the range 0.005 to 1.000 monolayer (ML), to be interpreted in terms of changes in the sample surface structure. For the case of Si/GaAs(001) the following series of surface reconstructions were observed with increasing Si coverage:c(4 × 4),c(4 × 4)(1 × 2), (1 × 2),(1 × 2)(3 × 1) and, (3 × 1). During the deposition ofBeGaAs(001), c(4 × 4),c(4 × 4)(1 × 2),c(4 × 4)(1 × 3),(1 × 2)(1 × 3), (1 × 3), and (1 × 2) reconstructions were noted to evolve. The fact that unique, but highly reproducible, RAS signatures were obtained for each of these surface phases demonstrates the applicability of a combined RHEED/RAS system for monitoring sub-monolayer heteroepitaxial growth with a surface sensitivity of the order of1200th of a monolayer.