Molecular Beam Epitaxy Experiments Research Articles

Growth instability due to lattice-induced topological currents in limited-mobility epitaxial growth models

The energetically driven Ehrlich-Schwoebel barrier had been generally accepted as the primary cause of the growth instability in the form of quasiregular moundlike structures observed on the surface of thin film grown via molecular-beam epitaxy (MBE) technique. Recently the second mechanism of mound formation was proposed in terms of a topologically induced flux of particles originating from the line tension of the step edges which form the contour lines around a mound. Through large-scale simulations of MBE growth on a variety of crystalline lattice planes using limited-mobility, solid-on-solid models introduced by Wolf-Villain and Das Sarma-Tamborenea in 2+1 dimensions, we show that there exists a topological uphill particle current with strong dependence on specific lattice crystalline structure. Without any energetically induced barriers, our simulations produce spectacular mounds very similar, in some cases, to what have been observed in many recent MBE experiments. On a lattice where these currents cease to exist, the surface appears to be scale invariant, statistically rough as predicted by the conventional continuum growth equation.

Heteroepitaxial growth of InAs on GaAs(0 0 1) by in situ STM located inside MBE growth chamber

The growth of InAs on GaAs(001) is of great interest primarily due to the self-assembly of arrays of quantum dots (QDs) with excellent opto-electronic properties. However, a basic understanding of their spontaneous formation is lacking. Advanced experimental methods are required to probe these nanostructures dynamically in order to elucidate their growth mechanism. Scanning tunneling microscopy (STM) has been successfully applied to many GaAs-based materials grown by molecular beam epitaxy (MBE). Typical STM–MBE experiments involve quenching the sample and transferring it to a remote STM chamber under arsenic-free ultra-high vacuum. In the case of GaAs-based materials grown at substrate temperatures of 400–600°C, operating the STM at room temperature ensures that the surface is essentially static on the time scale of STM imaging. To attempt dynamic experiments requires a system in which STM and MBE are incorporated into one unit in order to scan in situ during growth. Here, we discuss in situ STM results from just such a system, covering both QDs and the dynamics of the wetting layer.

Ge/Si quantum dot nanostructures grown with low-energy ion beam-assisted epitaxy

Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) experiments were performed to study growth modes induced by hyperthermal Ge + ion action during molecular beam epitaxy (MBE) of Ge on Si(100). The continuous and pulsed ion beams were used. These studies have shown that ion beam bombardment during heteroepitaxy leads to decrease in critical film thickness for transition from two-dimensional (2D) to three-dimensional (3D) growth modes, enhancement of 3D island density, and narrowing of island size distribution, as compared with conventional MBE experiments. Moreover, it was found that ion beam assists the transition from hut- to dome-shaped Ge islands on Si(100). The crystal perfection of Ge/Si structures with Ge islands embedded in Si was analyzed by Rutherford backscattering/channeling technique (RBS) and transmission electron microscopy (TEM). The studies of Si/Ge/Si(100) structures indicated defect-free Ge nanopaticles and Si layers for the initial stage of heteroepitaxy (five monolayers of Ge) in pulsed ion beam action growth mode at 350 °C. Continuous ion beam irradiation was found to induce dislocations around Ge clusters. The results of kinetic Monte Carlo (KMC) simulation have shown that two mechanisms of ion beam action can be responsible for stimulation of 2D–3D transition: (1) surface defect generation by ion impacts, and (2) enhancement of surface diffusion.

MODIFICATION OF GROWTH MODE OF Ge ON Si BY PULSED LOW-ENERGY ION-BEAM IRRADIATION

Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) experiments were performed to study growth modes induced by hyperthermal Ge + ion action during molecular beam epitaxy (MBE) of Ge on Si (100). The continuous and pulsed ion beams were used. These studies have shown that ion-beam bombardment during heteroepitaxy leads to decrease in critical film thickness for transition from two-dimensional (2D) to three-dimensional (3D) growth modes, enhancement of 3D island density and narrowing of island size distribution, as compared with conventional MBE experiments. The crystal perfection of Ge / Si structures with Ge islands embedded in Si was analyzed by Rutherford backscattering/channeling technique (RBS) and transmission electron microscopy (TEM). The results of Kinetic Monte Carlo (KMC) simulation have shown that two mechanisms of ion beam action can be responsible for stimulation of 2D–3D transition. They are: (1) surface defect generation by ion impacts and (2) enhancement of surface diffusion.

Molecular beam epitaxy

Molecular beam epitaxy (MBE) is a process for growing thin, epitaxial films of a wide variety of materials, ranging from oxides to semiconductors to metals. It was first applied to the growth of compound semiconductors. That is still the most common usage, in large part because of the high technological value of such materials to the electronics industry. In this process beams of atoms or molecules in an ultra-high vacuum environment are incident upon a heated crystal that has previously been processed to produce a nearly atomically clean surface. The arriving constituent atoms form a crystalline layer in registry with the substrate, i.e., an epitaxial film. These films are remarkable because the composition can be rapidly changed, producing crystalline interfaces that are almost atomically abrupt. Thus, it has been possible to produce a large range of unique structures, including quantum well devices, superlattices, lasers, etc., all of which benefit from the precise control of composition during growth. Because of the cleanliness of the growth environment and because of the precise control over composition, MBE structures closely approximate the idealized models used in solid state theory.This discussion is intended as an introduction to the concept and the experimental procedures used in MBE growth. The refinement of experimental procedures has been the key to the successful fabrication of electronically significant devices, which in turn has generated the widespread interest in the MBE as a research tool. MBE experiments have provided a wealth of new information bearing on the general mechanisms involved in epitaxial growth, since many of the phenomena initially observed during MBE have since been repeated using other crystal growth processes. We also summarize the general types of layered structures that have contributed to the rapid expansion of interest in MBE and its various offshoots. Finally we consider some of the problems that remain in the growth of heteroepitaxial structures, specifically, the problem of mismatch in lattice constant between layers and between layer and substrate. The discussion is phenomenological, not theoretical; MBE has been primarily an experimental approach based on simple concepts.

Self-organization of an ensemble of Ge nanoclusters upon pulsed irradiation with low-energy ions during heteroepitaxy on Si

Size distribution of Ge islands formed in the course of Ge heteroepitaxy on Si(111) was studied by scanning tunneling microscopy in experiments of two types: (i) conventional molecular beam epitaxy (MBE) and (ii) pulsed (0.5 s) irradiation with Ge ions of energy ≃200 eV at instants of time corresponding to a filling degree >0.5 for each monolayer. Experiments were performed at a temperature of 350°C. The pulsed ion-beam irradiation during heteroepitaxy leads to a decrease in the average size of Ge islands, an increase in their concentration, and a decrease in the root-mean-square deviation from the mean, as compared to the analogous values in conventional MBE experiments.

Combined use of computational intelligence and materials data for on-line monitoring and control of MBE experiments

This paper describes the combined use of computational intelligence procedures and materials data for monitoring and controlling the growth of thin films using molecular beam epitaxy (MBE). Given ellipsometry data ( Ψ and Δ) at a specific wavelength, a genetic algorithm-like method is used to solve an inverse problem, and estimate values of the complex refractive index and the deposition rate. Using a set of such values at different wavelengths, and combining the use of multiwavelength spectroscopic materials data and computational intelligence procedures, it is then possible to provide an optimal estimate of the composition of the material being deposited. Control of the film growth is then accomplished through adjustments of cell temperatures. This procedure is described in this paper, and examples of monitoring and control results are reported for the system of Al x Ga 1− x As film on GaAs substrate.

Simulations and experiments of SiC heteroepitaxial growth on Si(001) surface

Mechanism of SiC heteroepitaxial growth by the carbonization of the Si(001) surface was studied at the atomic scale using molecular dynamics (MD) simulations and molecular beam epitaxy (MBE) experiments. Heteroepitaxial growth of single crystal 3c-SiC on the Si(001) surface (3c-SiC[001]∥Si[001] and 3c-SiC[110]∥Si[110]) was observed in both the MD simulations and MBE experiments. Breaking of the Si—Si bonds and shrinkage of the [110] Si rows with C atoms are possible mechanisms for the heteroepitaxial growth of SiC on Si(001). Microscopic structures and mechanisms of the twin formations and pit formations are discussed. Ultraviolet light irradiation is proposed and confirmed to enhance the epitaxial growth of SiC in the MBE experiments.

Atomic layer doping for Si

We report on initial results of the potential of self-limiting surface reactions for the doping of Si layers using molecular beam epitaxy (MBE) and atmospheric pressure chemical vapor deposition (APCVD). In MBE experiments using Sb as an n-type dopant a self-limiting process is obtained at a coverage of half a monolayer. No evidence of a self-limiting process has yet been found for p-type doping using B 2H 6 above 400 °C. In the case of MBE growth at temperatures below 400 °C the B is only partly activated (10%–20%). In APCVD grown samples B surface coverage leads to significant growth inhibition of the subsequent deposition of Si from SiCl 2H 2. Finally, preliminary results of atomic layer doping using AsH 3 in APCVD indicate a self-limitation of chemisorption of AsH 3 at about 0.1 monolayer at a temperature of 600 °C; however, subsequent growth of Si leads to a smearing out of the As due to segregation and to the residence time of As in the system.