Stages Of Molecular Beam Epitaxy Research Articles

Growing nanocrystalline silicon on sapphire by molecular beam epitaxy

It is established that an array of silicon nanoislands is formed on the surface of a sapphire substrate at the initial stages of molecular beam epitaxy. Silicon islands formed at low temperatures of the substrate (below 650°C) exhibit a pyramidal shape, while the islands grown at T > 650° possess a dome shape. The maximum density of islands was 2 × 1011 cm−2, their lateral dimensions were within 20 nm, and their heights did not exceed 3 nm.

Growth mode transitions and scaling behaviour at successive stages of molecular beam epitaxy

Recently developed kinetic model for homoepitaxial growth is extended to the case of heteroepitaxy (without lattice mismatch) by introducing different adatom surface mobilities in the first layer (heterodiffusion) and in all the next layers (self-diffusion). With this model the effect of two adatom mobilities as a function of the Schwoebel step-edge barrier is studied with an emphasis on the growth mode transitions. It is shown that the difference between homo- and heteroepitaxy is confined to the first few monolayers and is crucially sensitive to the ratio between the hetero- and self-diffusion coefficients: lower heterodiffusion coefficient with respect to that of self-diffusion improves essentially epitaxial growth and vice-versa. This is important for growing smooth ultrathin layers needed in modern nanotechnology. Island density kinetics in successive growing layers is studied and it is found that in smooth growth regime it acquires eventually (after deposition approx. 10 monolayers) a universal scaling form and corresponding scaling exponents have been determined.

Nucleation of misfit and threading dislocations during epitaxial growth of GaSb on GaAs(001) substrates

The initial stages of molecular beam epitaxy of GaSb on highly mismatched GaAs(001) substrates were investigated. Transmission electron microscopy was used to analyze the defect structure in GaSb islands and at their interfaces with GaAs(001) at different stages of growth. Based on experimental observations, we propose that the semiperiodic net of 90° misfit dislocations at the GaSb/GaAs(001) interface nucleate homogeneously at the leading edges of advancing {111} planes. After nucleation, they glide inwards along the interface plane to reach their equilibrium position. Threading dislocations in GaSb layers were directly correlated with the misfit dislocation net. We demonstrate that there are no threading defects in GaSb islands when their interfaces consist solely of 90° misfit dislocations, and that threading dislocations in the GaSb epilayer are all associated with minority 60° misfit dislocations nucleated in growing islands. The number of threading dislocations per unit area of the GaSb film is found to be independent of GaSb coverage, indicating that island coalescence does not substantially increase the number of 60° dislocations.

Hydrogen sulfide treatment of GaAs substrate and its effects on initial stage of ZnSe growth

Abstract Hydrogen sulfide (H 2 S) treatment of GaAs substrates was examined by means of in situ reflection high energy electron diffraction (RHEED) and ex situ atomic force microscope (AFM) observations. Its effects on initial stage of molecular beam epitaxy (MBE) of ZnSe were also investigated. According to the H 2 S treatment conditions, an atomically flat surface with (4 × 3) reconstruction or a faceted surface with (2 × 6) reconstruction was obtained. Clear RHEED intensity oscillations over 40 cycles were observed just after starting the growth on the (4 × 3) surface, showing two-dimensional (2D) nucleation, i.e., layer-by-layer growth of ZnSe. Etch pit density (EPD) of the epilayer was about 2.5 × 10 6 cm −2 , which was much lower than that obtained by the ever applied ammonium-sulfide ((NH 4 ) 2 S x ) treatment.

Interface composition and stacking fault density in II-VI/III-V heterostructures

The Zn/Se flux ratio employed during the early stages of molecular beam epitaxy of pseudomorphic ZnSe/GaAs(001) as well as lattice-matched ZnSe/In0.04Ga0.96As(001) heterostructures controls the density of the native stacking faults, which have been associated with the early degradation of blue-green lasers. In particular, the density of Shockley stacking fault pairs decreases by three to four orders of magnitude and that of Frank stacking faults by one order of magnitude in going from Zn-rich to Se-rich interfaces.

Anisotropic diffusion and island formation in the MBE growth of Si(001)

The effect of anisotropic diffusion on the process of island formation and growth during the early stages of molecular beam epitaxy of Si on Si(001) is studied. The theoretical treatment in terms of a rate-limiting step predicts strong growth anisotropy for the isotropic (1:1) and anisotropic (1000:1) diffusion models. Although, in the case of anisotropic diffusion, the direction of easy diffusion is perpendicular to the direction of preferential island growth, Monte Carlo simulations show that the experimental STM data on island densities and island shapes can be reproduced by both models. Elongate island formation is, in the first place, an effect of anisotropic lateral interactions, while the migration anisotropy has a minor effect on the island shapes. It somewhat suppresses shape anisotropy at low temperatures and slightly enhances that at high temperatures.