Submonolayer Homoepitaxy Research Articles

Size Distribution and Scaling Behavior of InAlAs/AlGaAs Quantum Dots Grown on GaAs by Molecular Beam Epitaxy

We studied the size distribution and scaling behavior of self-assembled InAlAs/AlGaAs quantum dots (QDs) grown on GaAs in the Stranski–Krastanow (SK) mode by molecular beam epitaxy (MBE) at 480 and 510 °C as a function of InAlAs coverage. A scaling function of the volume was found for the first time in ternary alloy QDs. The function was similar to that of InAs/GaAs QDs, which agreed with the scaling function for a two-dimensional submonolayer homoepitaxy simulation with a critical island size of i = 1. However, a characteristic of i = 0 was also found as a tail in a large volume.

Island shapes, island densities, and stacking-fault formation onIr(111): Kinetic Monte Carlo simulations and experiments

Submonolayer homoepitaxy on $\mathrm{Ir}(111)$ is studied by a refined kinetic lattice Monte Carlo (KLMC) model and compared to results obtained from scanning tunneling microscopy experiments. The KLMC model not only considers individual atomic jumps on regular and stacking-fault sites, but also describes the cooperative motion of small adatom clusters, which determines the temperature-dependent probability of stacking-fault island formation. A complete catalog of diffusion processes at island edges is included that allows one to model the variations of island shapes with temperature. By taking input parameters for cluster and edge diffusion from experiments, calculated island densities as well as the probability of stacking-fault formation agree very well with experimental results for different temperatures. The comparison of simulated and experimental island shapes, however, reveals obvious differences. After systematic modifications of the event database for edge diffusion processes, all features of island shape evolution are well reproduced.

Growth coalescence shapes for islands during metal (100) homoepitaxy

During submonolayer homoepitaxy, instability in the shapes of growing two-dimensional islands can develop due to the diffusion-limited aggregation of deposited adatoms at their edges. However, in metal (100) systems, periphery diffusion is typically efficient, quenching this shape instability, and resulting in simple near-square or near-rectangular shapes of isolated islands. Despite this feature, growth coalescence shapes resulting from collision of two or more growing islands are nontrivial. These coalescence shapes are elucidated here by developing three complementary formulations: (i) suitable atomistic lattice-gas models analyzed by kinetic Monte Carlo simulation; (ii) deterministic rate equations for the dynamics of kinks along island step edges; and (iii) continuum theories for step-edge evolution. Characterization of coalescence shapes is important as they affect interlayer transport during multilayer growth. Such a characterization is also necessary to enable coarse-grained modeling of film growth with a realistic treatment of the evolution of island edge morphologies, e.g., using level-set methods.

Scaling of three-dimensional InN islands grown on GaN(0001) by molecular-beam epitaxy

The scaling property of three-dimensional InN islands nucleated on GaN(0001) surface during molecular-beam epitaxy (MBE) is investigated. Due to the large lattice mismatch between InN and GaN $(\ensuremath{\sim}10%),$ the islands formed from the Stranski-Krastanow growth mode are dislocated. Despite the variations in (residual) strain and the shape, both the island size and pair separation distributions show the scaling behavior. Further, the size distribution resembles that for submonolayer homoepitaxy with the critical island size $i=1,$ suggesting that detachment of atoms is not significant. The above results also indicate strain is insignificant in determining the nucleation and growth of dislocated islands during heteroepitaxy by MBE.

Island shape selection in Pt(111) submonolayer homoepitaxy with or without CO as an adsorbate.

The microscopic selection mechanisms of single-layer island shapes in Pt(111) homoepitaxy with or without minute amounts of CO adsorbate have been investigated theoretically. For clean growth, only triangular islands of a fixed orientation are obtained within a wide range of growth temperatures, with the orientation uniquely determined by a disparity in the rates of atom supply to an island corner site from the two island edges defining the corner. This novel picture is further corroborated by growth predictions in the presence of CO, whose preferential decoration of one type of the island edges reverses the intrinsic rate disparity for atom supply, thereby inverting the island orientation.

Island size scaling for submonolayer growth of InAs on GaAs(001)-(2×4):Strain and surface reconstruction effects

The submonolayer growth by molecular beam epitaxy of InAs on the GaAs(001)-$(2\ifmmode\times\else\texttimes\fi{}4)$ surface has been studied using rapid-quench scanning tunneling microscopy. InAs islands exhibiting the $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction are formed and show remarkably similar characteristics to GaAs submonolayer homoepitaxy on this surface. Detailed analysis of the islands indicates that strain plays a negligible role in their nucleation, and the $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction dominates both island growth and island anisotropy.

Kinetic Monte Carlo study of the effect of hydrogen on the two-dimensional epitaxial growth of Ni(100)

The effects of hydrogen on Ni~100! submonolayer homoepitaxy have been investigated by kinetic Monto Carlo ~KMC! simulations of the Ni growth process with and without the presence of a small hydrogen impurity. The two-dimensional KMC simulations are based upon rate constants for a set of uncorrelated Ni site-to-site hopping mobilities with and without the presence of hydrogen atoms which are found to act as a catalyst. The rate constant activation energies are determined by classical-potential total-energy calculations with semiclassical zero-point energy corrections for the hydrogen atom. This set of KMC simulations extends preliminary work on the activation energies which were reported along with their connection to available experiments in Ref. 1. We find that fast diffusion of H atoms occurs on the flat Ni~100! surface and the presence of these highly mobile H atoms is found to have significant effects on the mobility of lone Ni adatoms, and therefore also on the Ni islands which form during the epitaxial growth. The H atoms increase the mobility of lone Ni adatoms across the flat Ni~100! terrace by stabilizing the transition state for site-to-site hopping of the Ni adatom. While the effects of the H atom on the Ni mobility are primarily catalytic, the kinetically determined Ni island morphologies differ substantially as a function of H atom concentration over time periods which are long on the deposition time scale, and therefore the morphology differences can become frozen in place. Even quite low H atom coverage, by acting to increase lone Ni atom mobility, tends to decrease Ni island density which results in a corresponding increase in the average Ni island size. This suggests that the H atoms are acting as an antisurfactant in the Ni~100! homoepitaxy at low H coverage. @S0163-1829~99!06339-0#

Scaling properties of InAs/GaAs self-assembled quantum dots

We studied the scaling law for volume distribution of the self-assembled quantum dots grown by molecular-beam epitaxy of the Stranski-Krastanow mode. The scaling law was found to hold regardless of the annealing time and growth temperature. Also, we found the scaling law for the pair distribution of self-assembled quantum dots. Both the volume distribution and pair distribution agreed with the scaling functions for two-dimensional submonolayer homoepitaxy model with critical cluster size $i=1,$ which excludes adatom detachment from clusters.

Submonolayer homoepitaxy on Fe(1 0 0) studied by grazing ion–surface scattering: experiments and computer simulations

Grazing ion-surface scattering is used to study in real space and real time epitaxial growth processes. By example of 25 keV He +-ions scattered during submonolayer homoepitaxy on Fe(1 0 0), we show that an interpretation of experiments is straightforward by means of classical mechanics computer simulations. A fit of intensity and angular profile of the specular beam allow one to deduce basic quantities like the island density.

Flux-dependent scaling behavior in Cu(100) submonolayer homoepitaxy

The average separation of two-dimensional islands in Cu(100) submonolayer homoepitaxy as a function of the deposition flux at 213 K has been studied using spot profile analysis low-energy electron diffraction. As the flux decreases, a large change in the apparent critical island size, from one to a value between seven and 12 atoms, is obtained even though the temperature is held constant. This is shown to be consistent with a recently proposed dimer shearing mechanism which has a significant influence on the stability of islands with eight atoms or less.

Effects of hydrogen in Ni(100) submonolayer homoepitaxy

The effects of hydrogen in Ni(100) submonolayer homoepitaxy have been investigated by classical-potential total-energy calculations with semiclassical hydrogen zero-point-energy corrections. The fast diffusion of H on a flat Ni(100) surface is found to persist even when the surface is decorated with various intrinsic defects such as Ni adatoms, islands, and steps created during epitaxial growth. On the other hand, the presence of H significantly enhances the mobility of both Ni adatoms and Ni islands, resulting in a decrease in the Ni island density and a corresponding increase in the average island size. These results suggest that hydrogen will function as an antisurfactant in Ni(100) homoepitaxy. Connections to available experiments are made. {copyright} {ital 1997} {ital The American Physical Society}

Self-consistent rate theory of submonolayer homoepitaxy with attachment/detachment kinetics

The reversible nucleation and growth of two-dimensional islands during the submonolayer stage of epitaxial growth is studied with self-consistent rate equations and kinetic Monte Carlo simulations. In contrast to most previous work, we take account of the effects of both a finite energy barrier to the detachment of atoms from islands and a finite barrier to the incorporation of adatoms into islands. A correct boundary condition for the diffusion field at island edges is derived that takes account of these processes. For small detachment rates, quantitative agreement is obtained between the solutions to the rate theory and the simulations for the average monomer and island densities as a function of coverage. {copyright} {ital 1997} {ital The American Physical Society}

Island morphology and adatom energy barriers during homoepitaxy on Cu(001)

Island morphologies and island separations were determined from the two-dimensional diffraction patterns measured with high-resolution low-energy electron diffraction during sub-monolayer homoepitaxy on the Cu(001) surface. In the temperature range considered, islands were found to have square shapes with edges along the [110] direction indicating a low energy-barrier for diffusion along close packed step edges. By measuring the temperature dependence of the average island separation in the diffusion limited steady-state growth regime, the energy barrier for adatom diffusion and the binding energy for small islands were determined.

Growth and coalescence in submonolayer homoepitaxy on Cu(100) studied with high-resolution low-energy electron diffraction.

By measuring the dependence of the island separation [ital L] on the flux [ital F] during submonolayer epitaxy on Cu(100), the scaling exponent [ital p] in [ital L][similar to][ital F][sup [minus][ital p]/2] is determined in the steady-state and island coalescence regimes. In both regimes at low temperature (223 K), a crossover of [ital p] is observed from a low-flux value of 1/3 to a high-flux value of 1/2. At elevated temperatures (263--305 K), [ital p][similar to]3/5 is obtained. These results agree with classic nucleation theories and recent Monte Carlo simulations, and imply that the smallest stable island changes directly to a tetramer from a low-temperature dimer with increasing temperature. Dissociation energy calculations using the embedded-atom method support these results.