S. M. Koch

Ion milled tips for scanning tunneling microscopy

Ion milling of electrochemically etched tungsten tips is shown to improve the characteristics for scanning tunneling microscopy. The primary mechanism for the enhancement of tip reliability is identified to be the removal of a residual oxide. A greatly decreased radius of curvature is also achieved without significantly changing the macrostructural geometry of the tip.

The growth of GaAs on Si by MBE

Abstract The growth mechanisms occuring during the heteroepitaxy of GaAs/Si have been studied using a number of characterization techniques. In situ monitoring of the growth by reflection high energy electron diffraction shows that three-dimensional growth occurs initially, but the GaAs islands eventually coalesce to form a continuous layer with a 2 × 4 surface reconstruction. Transmission electron microscopy of a 30 nm film shows that the boundaries between the merged islands are disordered. In addition, regions of amorphous or misoriented single crystal material at the GaAs/Si interface are apparent. We also discuss the implications of using the two-stage growth scheme that is typically followed for GaAs/Si growth. Ion channeling indicates that the material is compressively strained in the plane of the film during the initial low temperature growth, but the lattice relaxes upon heating to 575°C for the second step of the growth. Comparisons were made among films whose growths were initiated a different temperatures and then terminated with an overlayer grown at 575°C. Although the surface morphology degrades with increasing temperature, the photoluminescence intensity rises.

Effect of substrate surface structure on nucleation of GaAs on Si(100)

We study the effect of surface structure upon the nucleation of GaAs in molecular beam epitaxy growth on vicinal Si(100) surfaces. In general, cross‐sectional transmission electron microscopy reveals that nucleation of GaAs islands is associated with surface steps produced by the deliberate substrate misorientation. Furthermore, it is found that standard in situ cleaning of the Si surface prior to deposition can result in steps grouping together, producing local surface facets. GaAs nucleation then occurs on these facets, the nucleation sites being correspondingly further apart than for an equilibrium distribution of monolayer steps.

Atomic structure of the GaAs/Si interface

The atomic structure of the GaAs/Si(100) interface is studied using high resolution transmission electron microscopy. The samples studied were grown by molecular beam epitaxy using a conventional two‐step growth process of a relatively low‐temperature (405 °C) GaAs buffer layer followed by a higher temperature (575 °C) device layer. Following this growth procedure, the interface is found to be atomically rough with nonuniform distributions of steps, and to contain regions of disrupted crystallinity. These regions are found to be more prevalent in material which has undergone only low‐temperature (buffer layer) GaAs deposition. By using the atomic resolution microscope at the National Center for Electron Microscopy, Lawrence Berkeley Laboratories, we are also able to atomically image the same area of interface along orthogonal 〈011〉 directions, observing qualitatively similar structures.

Nucleation and initial growth of GaAs on Si substrate

The microstructure of thin layers of GaAs grown on Si substrates at low growth temperatures by molecular beam epitaxy was examined using transmission electron microscopy and MeV 4He+ ion channeling angular scan analysis. Crystalline island formation is observed at temperatures as low as 325 °C, with epitaxial orientation and distinct nucleation habits apparently tied to the symmetry of the misoriented substrate. For films with no exposure to temperatures above 405 °C, the planar strain is found to be compressive, up to a thickness of 100 nm.

Simulation of GaAs cluster formation on GaAs(001̄), AlAs(001̄), Si(001), and As1/Si(001) surfaces

Recently developed semiempirical potential energy functions for the Ga–As–Si and Ga–As–Al systems have been applied here to determine the excess formation energy for GaAs clusters on GaAs(001), AlAs(001), Si(001), and one atomic layer As‐covered Si(001) substrates as a function of cluster size and cluster shape by the Monte Carlo technique. Pyramidal type ledges on the GaAs clusters are found to be the favored ledge for the first three layers while an inverted‐pyramidal type ledge is also favored in certain cases for the As1/Si(001) substrate. Cluster formation at ledges is compared with cluster formation on a flat terrace for the Si(001) and the As1/Si(001) substrates.

The Growth of GaAs on Si by Molecular Beam Epitaxy

Successful growth of GaAs on Si has recently been demonstrated. This work is directed toward an understanding of the processes occurring during the growth and their effects on the quality of the GaAs epilayers. Reflection High Energy Electron Diffraction monitoring of the growth in situ shows that the islands that are initially formed coalesce into an epilayer with a 2×4 surface reconstruction. Ion channeling indicates that the crystallinity of the entire epilayer improves with coverage. Substantial reordering of the material occurs when buffer layers grown at low temperatures are annealed at 575°C before and during further growth at this temperature. Comparison of 300 nm layers differing in the growth temperature of the first 100 nm shows no variation in the crystallinity as determined by ion channeling. Surface morphology degrades, however, and 77K photoluminescence intensity rises with the initial growth temperature. The optical and structural properties of 2μm thick films are also discussed.

GaAs/Si Nucleation and Buffer Layer Growth

We investigate two aspects of the initial stages of GaAs/Si growth: the effects of Si misorientation on GaAs island nucleation, and the structural changes occurring upon heating the buffer layer to the final deposition temperature. We observe that the GaAs islands tend to nucleate at Si surface steps and grow along them. The shape of the islands depends on the degree and direction of the Si misorientation from (100). Island coalescence occurs as the film grows, such that a 500A film is almost completely continuous. In situ heating of a 30A film to 5750C for 15 minutes causes the islands to agglomerate, while similar treatment of the 500A film does not appear to expose more of the underlying substrate and in fact improves the film crystallinity.

Structural Characterization of Thin, Low Temperature Films of GaAs on Si Substrates

The technology of heteroepitaxially growing compound semiconductors, particularly GaAs, on Si has attracted an increasing volume of attention in the past 3–5 years. In the past two years there have been few major advances in the growth recipes, which all include a high temperature Si preheat, a two-step growth temperature profile, and the use of misoriented substrates. The need for mechanistic understanding of the effect of these parameters is crucial to advancing the state of the art beyond this current practice. This work focusses on the effect of the misorientation in inducing asymmetry in early stages of the molecular beam epitaxy of GaAs on Si substrates. The strain in the films is found to have greatly different rates of relaxation in the plane of the film when measured in the two orthogonal directions. This asymmetry persists to greater than 30 nm film thickness at 400°C. The nucleation morphology was also examined as a function of substrate misorientation. At low substrate tilts, nucleation density was sparse and there was not substantial ordering of the nuclei. As the tilt was increased, a distinct habit emerged where collections of nuclei were quite coherent along the steps for several 100 nm, with dimensions of the order of 10s of nm perpendicular to the steps. The density of steps with nuclei was also substantially less than that predicted by the widely accepted “array of double-height steps” used to explain the curious lack of anti-phase disorder in these films.

Structural Studies of Nucleation and the Initial Stages of Growth of Epitaxial Gaas on Si(100) Substrates

Nucleation and growth of GaAs fIlms deposited by Molecular Beam Epitaxy upon Si(l00) substrates are studied by transmission electron microscopy. The initial nucleation of GaAs consists of approximately hemispherical islands associated with steps upon the substratesurface and coherently strained to the substrate lattice. As the island size increases, crystalline defects relax the strain between epilayer and substrate. Island coalescence is a secondary defect generation mechanism. Subsequent growth of the epitaxial GaAs layer reveals a progressive deterioration of the GaAs/Si interface planarity and the growth and eventual dissolution of amorphous or misoriented crystalline regions at the GaAs/Si interface.