Reflection High-energy Electron Diffraction Specular Intensity Research Articles

Determination of step-adatom kinetics by reflection high-energy electron diffraction during step-flow growth: Theory.

We propose a method for using reflection high-energy electron-diffraction (RHEED) measurements to determine step-adatom kinetics during growth by step flow. Analytical results are obtained for the adatom sticking coefficients at the steps in terms of the steady-state in-phase RHEED specular intensity at two distinct misorientations. Calculated results based on estimated values for these intensities in the step-flow regime indicate the presence of a pronounced barrier at the down step, providing support for previous conjectures. Extension of these results to lower temperatures, including the region below the critical temperature where the majority of RHEED studies reported have been made, should be possible but will involve calculations of a more complicated nature.

The surface evolution and kinetic roughening during homoepitaxy of GaAs (001)

Scanning tunneling microscopy studies have been performed on GaAs homoepitaxial films grown by molecular-beam epitaxy. Images show that in the earliest stages of deposition the morphology oscillates between one with two-dimensional islands and flat terraces. After the initial transient regime, the system evolves to a dynamical steady state. This state is characterized by a constant step density and as such the grown mode can be termed step flow. Comparison with reflection high-energy electron-diffraction (RHEED) shows that there is a direct correspondence between the surface step density and the RHEED specular intensity. Thick films (up to 1450 monolayers) display a slowly-increasing surface roughness. Analysis of the scaling properties and comparison with theories of film growth will be made.

Migration Enhanced Epitaxy of GaAs Studied by Reflection High Energy Electron Diffraction and Scanning Tunneling Microscopy

AbstractMigration enhanced epitaxy (MEE) of GaAs on (001) GaAs substrates was studied by reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). In MEE, Ga and As species are alternately deposited on the growing surface. Ga adatom migration can be enhanced by the low arsenic pressure environment. The STM study was performed ex-situ by the arsenic capping and decapping procedure. We have demonstrated the correlation between the peak RHEED specular intensity during MEE growth and the variation of the lateral step density on the surface, even though the surface stoichiometry changes repetitively during MEE. The peak RHEED intensity during MEE is inversely dependent on the surface step density. The MEE surface exhibited a lower step density than the MBE surface, as shown by both RHEED and STM. However, the MEE surface still exhibited a much higher step density than a well-annealed surface. Consequently we believe that to achieve an atomically flat interface, annealing at high temperature in an arsenic flux is still necessary even if MEE is employed.

Surface evolution during molecular-beam epitaxy deposition of GaAs.

Scanning tunneling microscopy studies have been performed on GaAs homoepitaxial films grown by molecular-beam epitaxy. Images show that in the earliest stages of deposition the morphology oscillates between one with two-dimensional islands and flat terraces. As growth proceeds there is a gradual coarsening of the surface features. Comparison with reflection high-energy electron diffraction (RHEED) leads us to propose that there is a direct correspondence between the surface step density and the RHEED specular intensity. As such, we associate the decay of the RHEED oscillation amplitude with a reduction in the temporal variation of the step density rather than the buildup of interface width.

Step-density variations and reflection high-energy electron-diffraction intensity oscillations during epitaxial growth on vicinal GaAs(001).

Systematic measurements and simulations are reported for the transition from growth by nucleation and accretion of two-dimensional islands to step advancement on misoriented GaAs(001) surfaces during epitaxial growth. The growth conditions have been chosen in order to satisfy as much as possible the underlying assumptions of a solid-on-solid model of epitaxial growth, namely, that the adatom mobility is isotropic (by using a surface misoriented toward the [010] direction), the effect of the As is not rate determining (by using a As/Ga ratio of 2.5), and that the presence of any surface reconstruction can be subsumed in effective migration parameters, i.e., that no explicit account of surface reconstruction is required if the reconstruction does not change (by maintaining the 2\ifmmode\times\else\texttimes\fi{}4 reconstruction). The diffraction conditions for reflection high-energy electron-diffraction (RHEED) measurements were chosen to eliminate as much as possible the contribution of well-known incoherent features to the specular intensity on GaAs(001). Using these growth and diffraction conditions, the parameters of a solid-on-solid model have been optimized by performing extensive simulations to quantitatively reproduce the measured misorientation-angle dependence and the Ga flux dependence of the growth-mode transition temperature. Since at the chosen diffraction conditions, the kinematic diffraction is insensitive to surface morphologies, we have modeled the growth-induced loss of intensity from the specular beam as being due to the steps on the surface, with each step acting as an individual source of scattering. Direct comparisons between the time-dependent density of surface steps of the simulated surfaces and the RHEED specular intensity profiles measured during growth reveal several qualitative and quantitative similarities. The most striking of these is that the two quantities show the same relative change of magnitude with time and temperature for a given misorientation and Ga flux. The implications of these comparisons are discussed for the growth dynamics of GaAs(001), for the scattering processes in RHEED, and the morphological sensitivity of RHEED for these diffraction conditions.

Validity of RHEED specular intensity measurements for structural studies of liquid metal surfaces: applications to liquid In and Sn

The authors propose a method of analysing the glancing angle dependence of the RHEED (reflection high-energy electron diffraction) specular intensity I( theta ) from liquid metal surfaces and apply it to liquid In and Sn, for which preliminary RHEED experiments have recently been performed. They use two models to derive a scattering potential that is supposed to cause the specular reflection of fast electrons. Calculations of I( theta ) based on these models reveal that the tail behaviour of the scattering potential on the vapour side essentially determines the characteristics of I( theta ) in the range of small glancing angle theta . The comparisons of these calculations with experiments suggest that the scattering potential in the liquid-vapour transition zone of liquid In results from neutral atoms, while the scattering potential due to ions and extended conduction electrons is plausible for liquid Sn even in the surface region. It is concluded from the present analyses that accurate experimental data for I( theta ) in a wider range of theta is needed to determine the full behaviour of the surface density profiles of liquid metals.