J. H. Neave

Dynamics of film growth of GaAs by MBE from Rheed observations

Detailed observations have been made of the intensity oscillations in the specularly reflected and various diffracted beams in the RHEED pattern during MBE growth of GaAs, GaxAl1−xAs and Ge. The results indicate that growth occurs predominantly in a two-dimensional layer-by-layer mode, but there is some roughening, which is enhanced by deviations from stoichiometry and the presence of impurities. In the case of the GaAs (001) −2×4 reconstructed surface a combination of dynamic and static RHEED measurements has provided firm evidence for the presence of one-dimensional disorder features as well as surface steps.

Structure and stoichiometry of {100} GaAs surfaces during molecular beam epitaxy

Abstract Relationships between surface structures on {100} GaAs substrates and incident fluxes of Ga and As 4 have been examined in detail over the temperature range 300–950 K. Surface reconstruction effects observed by RHEED have been related to surface stoichiometry, and the stability criteria for several reconstructed surfaces determined. Reconstruction observed during autoepitaxial growth of GaAs from beams of As 4 and Ga is not a simple function of the incident fluxes, since effects associated with the thermal stability of the substrate can have important consequences. The results are discussed in relation to surface kinetic information obtained from modulated beam measurements, and to some current theories of surface reconstruction and growth step behaviour. Results on surface topography are used to illustrate the ambiguities of streaked RHEED patterns. It is shown that streaks of constant intensity normal to the shadow edge cannot be used as the sole criterion by which to define the presence of a flat surface.

Dynamic RHEED observations of the MBE growth of GaAs

Detailed measurements have been made of the specular beam intensity in RHEED patterns from static and growing GaAs surfaces. The basic parameters investigated were substrate temperature and electron beam azimuth. The results have provided further understanding of growth dynamics and surface disorder, respectively. There is a significant trend away from two-dimensional growth at the higher temperatures, which also correspond to more Ga-rich surface structures. Conversely, surface disorder is apparently greater during growth at the lower temperatures, where the structure is As-rich. The static As-stable 2×4 surface is, however, the most ordered and the most closely two-dimensional. It has also been shown that ordered, two-dimensional growth can be initiated from excess Ga adatom populations.

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.

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.

Some observations on Ge:GaAs(001) and GaAs:Ge(001) interfaces and films

Ge:GaAs(001) and GaAs:Ge(001) interfaces and thin films have been prepared by molecular beam epitaxy (MBE) and studied in situ by reflection electron diffraction (RHEED), angle‐resolved photoemission spectroscopy (ARPES), and core level photoemission. The film structures have also been examined by transmission electron microscopy (TEM). Ge films grown on GaAs(001) substrates have a c(2×2) surface symmetry and are free of extended defects and domain structures. ARPES measurements of Ge(001)‐c(2×2) surfaces in the photon energy range 20–50 eV show the presence of two surface states and dispersing bulk states. Valence bands along the ΓX symmetry direction are derived on the basis of a free‐electronlike final state. The quality of the Ge–GaAs(001) interface however, is determined by the GaAs substrate reconstruction. Ge deposition on an intrinsic (2×4) reconstructed surface produces an abrupt interface, but when a c(4×4) chemisorption phase is used the distribution of arsenic across the interface region is di...

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.

The dependence of Auger electron yield on primary beam energy at normal and glancing incidence

Abstract Auger electron yields as a function of primary beam energy have been measured using a retarding field analyser at normal and glancing primary beam incidence under conditions of both constant beam current and constant current through the target to earth. The functional dependencies observed under the different operating conditions are shown to be related to the energy distributions of back diffused inelastically scattered electrons which occur under the various conditions. The contribution of the primary beam to the Auger yield is shown to be small, and using a very simple model good agreement is obtained between theoretical and observed yields. A comparison is also made between results from the substrate material (silicon) and from a sub-monolayer surface film (carbon).