L.-M. Peng

Dynamical RHEED from MBE growing surfaces

Abstract A practical computing procedure has been developed for calculating reflection high energy electron diffraction (RHEED) from molecular beam epitaxy (MBE) growing surfaces. A birth-death model is employed to describe the epitaxy growth on the surfaces, and the diffracion is treated dynamically. A typical calculation of RHEED intensity from a GaAs(001) MBE growing system consisting of a bulk substrate crystal and up to ten growing surface layers takes five seconds on a VAX 8800 system.

A treatment of RHEED from a rough surface of a crystal by an optical potential method

Abstract The general formalism of the optical potential method has been applied to the problem of reflection high energy electron diffraction (RHEED) from the rough surface of a crystal. For the case where short-range disorder dominates an explicit analytical expression for the effective absorption potential has been obtained, and procedures for evaluating this potential have been developed which utilize the conventional approaches and computer routines developed earlier for transmission high energy electron diffraction (THEED). This potential has been combined with dynamical RHEED calculations for a molecular beam epitaxy (MBE) growing surface. It is found that under certain circumstances the optical potential due to the roughness of the MBE growing surface has an appreciable effect on the shape of the corresponding RHEED rocking curve.

Studies on the etching and annealing behaviour of α−Al2O3(1012) surfaces by reflection electron microscopy

Abstract The effects of chemical etching and annealing on α-AI 2 O 3 (1012) surfaces have been studied using reflection electron microscopy (REM). Cleaved surfaces exhibit domains the contrast of which depends on whether the terminating ion is aluminium or oxygen. It is further observed that chemical etching followed by annealing at 1100°C gives atomically flat surfaces. It is also demonstrated that the technique of REM can provide very useful information on surface diffusion processes. A new method for measuring surface diffusion coefficients and binding energies is proposed. The surface diffusion coefficient of α-Al 2 O 3 (1012) determined to be 5.7 × 10 −1 cm 2 s −1 at 1100°C and 2.9 × 10 −13 cm 2 s 1 at 1150°C, giving D 0 = 8.7 × 10 −13 cm 2 s approximately 5.5 eV.

Evidence for the damping of coherence in inelastic scattering of high-energy electrons by crystals

Abstract We present experimental observations followed by theoretical simulations which confirm the existence of the recently reported [Phys. Lett. A 170 (1992) 111] mechanism of damping of coherence of high-energy electrons due to small-angle inelastic scattering in crystals. Our analysis shows that electron-electron inelastic interactions are likely to play a leading role in the deterioration of the contrast of energy-unfiltered electron micrographs of crystals of light elements such as silicon.

Direct determination of crystal and surface structures in THEED

Abstract Tensor theory of transmission high-energy electron diffraction (THEED) has been formulated based on a standard first-order perturbation theory. The theory has been applied to both a thin crystal film with perfect bulk structure and a crystal slab having its surface layer reconstructed, and has been combined with a direct scheme which reduces the complicated problem of crystal and surface structure determination to a linear least-squares problem. The crystal or surface structure can be obtained either by directly inverting and n × n matrix of the linear least-squares problem, where n is the number of parameters to be determined, or, in the case when the problem is under-determined, using a singular value decomposition method. The scheme allows the variances associated with the estimate of the unknown parameters to be readily obtained and thus assessment of the reliability of the determined parameters, and can be applied to many other cases where a perturbation treatment applies.

Tensor theories of high energy electron diffraction and their use in surface crystallography

Abstract A distorted wave approximation has been used to formulate tensor expressions for the scattering amplitude of high energy electrons by an assembly of atoms. When the tensor expressions are applied to either reflection high energy electron diffraction (RHEED) or transmission high energy electron diffraction (THEED) geometry, it is shown that the standard method of least-squares refinement can be used to solve surface structure directly.

A note on the general Bloch wave theory and boundary problems in RHEED and REM

Abstract Applications have been made of the Bloch wave theory to the Bragg case of high energy electron diffraction for a perfect extended surface and for an inclined surface including a regular ordered array of surface steps. A general discussion is given to the boundary problems concerning the difficulties arising from the deficiency in the number of boundary conditions for solving the reflection diffraction problem uniquely, and also from the restriction imposed by the original assumption that the crystal potential ceases suddenly at the external vacuum-crystal interface. The problem has been addressed by referring to a symmetric two beam Bragg case. A method for incorporating the influence of surface potential and structure has been proposed.

Bloch wave origin of surface resonance scattering in RHEED

Abstract Surface resonance scattering in RHEED is shown to result from Bloch wave degeneracy, which gives singular Bloch wave excitations and an anomaly in the surface reflected beam amplitudes. The minimum condition for the resonance to occur is that two reciprocal lattice rods are involved, and the involvement of bound states of atomic planes parallel to the surface is not necessary. The concept of Bloch wave degeneracy has been applied to explain resonance scattering occurring either in the selvedge or/and in the substrate. When the diffraction condition has been so chosen that the diffraction processes are dominated by two reciprocal lattice rods, the total electron wave function is shown to be highly localized at the surface. In a general case when many reciprocal lattice rods are involved, the total electron wave function may or may not be localized, depending on the diffraction condition

Reflection electron imaging of semiconductor multilayer materials

Abstract The technique of reflection electron microscopy (REM) has been applied to image both lattice-matched multilayer materials, such as GaAs/Al x Ga 1− x As multiple quantum wells (MQW), and lattice-mismatched strained-layer superlattices (SLS), such as GaAs/In x Ga x As SLS. The basic experimental arrangements are discussed. Applications have been made to examine semiconductor multilayer devices, such as GaAs/Al x Ga 1− x As MQW infrared detectors, and to investigate the various effects which influence the growth of high-quality multilayer structures. We demonstrated, using testing samples, that an In x Ga 1− x As epilayer with as low as 0.2% In concentration can be identified from the neighboring GaAs layers, and a group of GaAs/Al x Ga 1− x As MQW having a periodicity of about 2 nm can be resolved.

On the damping of coherence in the small-angle inelastic scattering of high-energy electrons by crystals

Abstract We study the effect of small-angle inelastic scattering on the propagation of high-energy electrons through crystals. Numerical integration of the kinetic equation for the density matrix shows that electron-electron inelastic collisions lead to the damping of the off-diagonal elements of the density matrix describing the coherence between the states of an electron on different branches of the dispersion surface. The attenuation distance is found to be comparable with the inelastic mean free path for excitation of the electron subsystem of the crystal. Our results demonstrate that in many cases the transition from dependent to independent Bloch wave model takes place far more rapidly than it was commonly assumed earlier.