P. J. Dobson

Reflection high‐energy electron diffraction oscillations from vicinal surfaces—a new approach to surface diffusion measurements

A simple extension of the reflection high‐energy electron diffraction oscillation technique to vicinal surfaces provides a means of studying surface diffusion during molecular beam epitaxial growth. The basis of the method is described and some preliminary results for Ga diffusion during the growth of GaAs films with (001) 2×4 and 3×1 reconstructed surfaces are presented.

Rheed studies of heterojunction and quantum well formation during MBE growth-from multiple scattering to band offsets

Abstract The basic concepts and first-order growth model derived from the RHEED intensity oscillation technique are described and the limitations imposed by the experimentally demonstrated multiple-scattering nature of the diffraction process are indicated. Despite these restrictions the value of the technique is illustrated in relation to growth mechanism studies, heterojunction and quantum well interface formation and as a process control monitor.

Current understanding and applications of the RHEED intensity oscillation technique

Abstract The problem of reflection high energy electron diffraction (RHEED) and electron scattering from smooth and growing surface is briefly reviewed. Evidence is given that strong electron beam penetration and multiple scattering effects are present under the conditions used to observe intensity oscillations in RHEED during growth by molecular beam epitaxy (MBE). A survey is made of the predominant RHEED oscillation features, i.e. damping, increases in amplitude, transient behaviour, phase differences and the appearance of harmonics. These features can be related to growth and diffraction processes.

Effects of diffraction conditions and processes on rheed intensity oscillations during the MBE growth of GaAs

The RHEED intensity oscillation technique has received wide-spread attention for the study of MBE growth dynamics, but insufficient consideration has been given to the diffraction conditions and processes involved. We report here a systematic investigation of the intensity oscillation behaviour as a function of diffraction parameters (azimuth, incidence angle, specular and non-specular beams), with constant growth conditions for GaAs films on GaAs (001) substrates.We show that many reported anomalies attributed to growth effects, such as phase differences and periodicity variations, can be accounted for entirely by diffraction events, provided it is realised that multiple scattering processes are the dominant cause of RHEED intensity variations during growth.The technique can provide valuable information on growth behaviour, but only if diffraction-dependent effects are first eliminated.

Kinetic limitations to surface segregation during MBE growth of III–V compounds: Sn in GaAs

The incorporation of Sn as a dopant in GaAs has been studied in the temperature range of 500°–650° C, over a wide range of Ga and As fluxes, the latter being incident as either As4 or As2 molecules. The results are explained in terms of a surface segregation model in which the behaviour at high growth temperatures (above ∼600 °C) approaches thermal equilibrium, but growth at lower temperatures involves a kinetic limitation to the segregation process.

Dynamic effects in rheed from MBE grown GaAs(001) surfaces

Abstract Reflection high energy electron diffraction measurements of the intensity of (elastically) diffracted beams as a function of the incident angle have been carried out for an exactly oriented GaAs(001) surface, using a primary beam energy of 12.5 keV. Different surface reconstructions were prepared in-situ by molecular beam epitaxy and the measurements of the diffracted intensities were made at temperatures and As2-fluxes where these structures were stable. Results for the (0,0) beam in the [ 1 10], [110] and [010] azimuths are reported for the 2 × 4, 3 × 1 and c(4 × 4) structures and in addition for fractional and integral order beams for the 2 × 4 surface. In general, very large intensity variations (up to a factor of 104) are observed. The results are discussed in terms of diffraction processes involving primary Bragg, secondary Bragg and surface resonances, and refraction effects introduced by both bulk and surface “inner potentials” are included. The data cannot be explained using the kinematic approximation, but they are a consequence of multiple scattering events. The reconstructed surface layer appears to have a dominant influence. A complete analysis of the results would require full dynamical calculations based on realistic surface models, but they are not yet available. Finally, the significance of the results to recent observations of oscillations in diffracted intensity during thin film growth by molecular beam epitaxy is considered.

Silicon doping of MBE-grown GaAs films

Two concentration ranges of silicon doping in MBE-grown GaAs films have been investigated in some detail. In lightly doped films, with a free-electron concentration of ≈1016cm−3, low-temperature photoluminescence spectra have been analysed to develop a model to account for spectral features previously attributed to Ge and Si acceptor levels.In heavily doped films, a maximum free-electron concentration of ≈7×1018 cm−3 has been obtained, which is only rather weakly dependent on growth conditions and the nature of the arsenic species (As2 or As4). Transmission electron microscopy has shown that no significant precipitation effects occur when higher Si fluxes are used but there is evidence for autocompensation. The maximum PL intensity (300 K) is found at a lower free electron concentration then with Sn-doped films, and is more sharply peaked, but there is no evidence for an anomalous Moss-Burstein shift.

Experimental confirmation of a sum rule for room‐temperature electroabsorption in GaAs‐AlGaAs multiple quantum well structures

Measurements of the electric field dependence of room‐temperature photocurrent in a GaAs‐A10.4Ga0.6As multiple quantum well p‐i‐n diode have been carried out in the wavelength range 650–920 nm. Calculations of the field dependence of absorption from the photocurrent spectra show that the recently derived sum rule for electroabsorption in quantum wells holds to within 0.3%, for electric fields as high as 2×105 V/cm, provided that full account is taken of changes in absorption across the entire spectral region.

Far infrared reflectivity of semiconductor superlattice and multi-quantum well structures

Abstract Measurements by dispersive Fourier transform spectroscopy of the amplitude and phase of the reflectivity of GaAs/Al x Ga 1- x As MQW and superlattice specimens in the spectral region 75–500 cm -1 are presented. The MQW data satisfy an average-medium description, and a least-squares fit gives values for reststrahl-region parameters of the layers. The superlattice results show pronounced free-carrier effects in addition to reststrahl structure.

The occurrence of sharp exciton-like features in low temperature photoluminescence spectra from MBE grown GaAs

Abstract Evidence is presented that a series of sharp exciton-like features 5 to 10 meV below the free exciton feature in the low temperature photoluminescence spectra for MBE grown GaAs can be obtained even for substrate temperatures up to 620°C and for fluxes of As 2 , contrary to a recent study. N-type doping reduces or eliminates the exciton-like features. We discuss their origin.