D. W. Bullock

Activation energy for Ga diffusion on the GaAs(0 0 1)-(2×4) surface: an MBE-STM study

The pure migration of individual Ga atoms on the technologically important GaAs(0 0 1)-(2]4) reconstructed surface has been studied as a function of substrate temperature using a combined molecular beam epitaxy and scanning tunneling microscopy (STM) ultra-high vacuum, multi-chamber facility. We have successfully deposited 1 10 of a plane of Ga atoms onto a pristine GaAs surface under a constant As 4 beam equivalent pressure of 10~6 Torr, at various substrate temperatures. After deposition the substrate was quenched to room temperature and transferred to the surface analysis chamber for STM imaging. A plot of the number density of islands formed as a function of deposition temperature follows an Arrhenius relationship. Assuming either a pure one-dimensional di!usion model or a pure isotropic two-dimensional di!usion model, the activation energy for di!usion is 2.3 or 1.7 eV, respectively. ( 1999 Elsevier Science B.V. All rights reserved.

Enabling in situ atomic-scale characterization of epitaxial surfaces and interfaces

A custom designed sample handling system which allows the integration of a commercially available scanning tunneling microscope (STM) facility with a commercially available molecular beam epitaxy (MBE) facility is described. No customization of either the STM imaging stage or the MBE is required to implement this design.

Origins of GaN(0 0 0 1) surface reconstructions

The reconstructions of the Ga polarity GaN(0 0 0 1) surface with and without trace amounts of arsenic and prepared by molecular beam epitaxy (MBE) have been studied with in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). Various reconstructions are observed with RHEED by analyzing patterns while the substrate is exposed to a fixed NH3 flux or after depositing known amounts of Ga as a function of substrate temperature. In situ STM images reveal that only a few of these reconstructions yield long-range periodicity in real space. The controversial role of arsenic on Ga induced reconstructions was also investigated using two independent MBE chambers and X-ray photoelectron spectroscopy. � 2003 Elsevier B.V. All rights reserved.

Enabling electron diffraction as a tool for determining substrate temperature and surface morphology

The reconstruction transitions for the GaAs(001) surface have been identified as a function of the band gap-derived substrate temperature and As4 beam equivalent pressure. Surface morphology measurements using in situ scanning tunneling microscopy reveal that the surface spontaneously forms a random distribution of two-dimensional islands. The onset of island formation is coincident with the reflected high-energy electron diffraction pattern changing from the β to α subphase of the (2×4) reconstruction. An electron diffraction-based method for determining the substrate temperature and engineering the surface morphology with a desired amount of roughness is presented.

Time-evolution of the GaAs(0 0 1) pre-roughening process

The GaAs(0 0 1) surface is observed to evolve from being perfectly flat to a surface half covered with one-monolayer high spontaneously formed GaAs islands. The dynamics of this process are monitored with atomic-scale resolution using scanning tunneling microscopy. Surprisingly, pit formation dominates the early stages of island formation. Insight into the nucleation process is reported.

A UNION OF THE REAL-SPACE AND RECIPROCAL-SPACE VIEW OF THE GaAs(001) SURFACE

A union of the real-space and reciprocal space view of the GaAs(001) surface is presented. An optical transmission temperature measurement system allowed fast and accurate temperature determinations of the GaAs(001) substrate. The atomic features of the GaAs(001)-(2×4) reconstructed surface are resolved with scanning tunneling microscopy and first principles density functional theory. In addition, the 2D lattice-gas Ising model within the grand canonical ensemble can be applied to this surface to understand the thermodynamics. An algorithm for using electron diffraction on the GaAs(001) surface to determine the substrate temperature and tune the nanoscale surface roughness is presented.

Dynamics of spontaneous roughening on the GaAs(0 0 1)-(2×4) surface

Abstract The dynamics of a random distribution of spontaneously formed 2D GaAs islands are studied using scanning tunneling microscopy. The equilibrium concentration of islands is easily tuned from 0% to 50% coverage by only changing the As 4 overpressure. Images taken during the early stages of island formation reveal the roughening transition primarily occurs through an intermediate pit formation phase. Interestingly, pit formation in the middle of an otherwise pristine terrace is overwhelmingly preferred to atom detachment from the edges of the terraces.

Mapping the Spin-Injection Probability on the Atomic Scale

A spin-polarized electron current is injected into the GaAs(110) surface at 100 K by using a polycrystalline ferromagnetic Ni scanning tunneling microscopy (STM) tip. The injected electrons recombine to the valence band and emit circularly polarized radiation whose degree of light polarization is related to the polarization of the conduction-band electrons at the instant of recombination. When the polarized electrons are injected into clean, flat terraces an average polarization for the emitted radiation is found to be 6.79%, while over the 10-nm step region the polarization is reduced to 0.46%. This step scattering effect is studied further by varying the tunneling gap through adjusting the tunneling current. As the distance between the STM tip and sample decrease the spin-scattering effect of the step edge is enhanced.

Simultaneous surface topography and spin-injection probability

A spin-polarized electron current is injected into a p-type GaAs(110) surface at 100 K using a polycrystalline ferromagnetic Ni scanning tunneling microscope tip. The injected electrons recombine to the valence band and emit circularly polarized light, and the degree of the light polarization is related to the degree of the electron polarization at the instant of recombination. Details of how to simultaneously measure the surface topography and obtain a pixel-by-pixel map of the spin-injection probability are discussed. The degree of light polarization is found to change when the electrons are injected into a clean, flat terrace versus over a step. However, the terrace-to-step polarization differences are systematically reduced as the energy of the electron is reduced.

Role of aperiodic surface defects on the intensity of electron diffraction spots

A random distribution of two-dimensional gallium arsenide (GaAs) islands is found to effect the intensity of the electron diffraction pattern from the GaAs(001) surface. By utilizing the spontaneous island formation phenomenon as well as submonolayer deposition, the island coverage is systematically changed. It is found that the intensities of the one-, two-, and three-quarter-order diffraction spots of the [110] azimuth decrease as the concentration of islands increases. In addition, only in the presence of islands, does the intensity of the half-order spot decrease as the grazing angle of the electron beam is decreased. A simple quantitative model is developed that provides insight into how an aperiodic arrangement of islands effects the electron diffraction patterns.