SURFACE EVOLUTION ON VICINAL GAAS(001) SURFACES IN THE TRANSITION RANGE FROM 2-DIMENSIONAL TO STEP-FLOW GROWTH

We have used a 90 degrees double reflection high-energy electron diffr action (RHEED) setup to perform a comprehensive real-time study of the morphology of vicinal GaAs(001) surfaces during molecular beam epitax y. The technique allows to record RHEED intensities simultaneously in the [(1) over bar 10] and [110] azimuths and thus enables a detailed s tudy of anisotropy effects. Comparative measurements on surfaces with 2 degrees misorientation towards (111)Ga (A surface) and towards (1 (1 ) over bar 1)As (B surface), respectively, show that independent on th e step type and reconstruction anisotropy, recordings of the specular beam intensity in the azimuth perpendicular to the steps are clearly d ominated by the evolution of the staircase order whereas intensity rec ordings in the azimuth parallel to the steps reveal the evolution of t he step edge roughness. Measurements over a wide range of substrate te mperatures give insight in the competition between kinetic processes a nd thermodynamic equilibrium on a length scale accessible to RHEED. Fo r the A surface the transition between two-dimensional (2D) growth and step-flow growth occurs not only at higher temperature than on the B surface, but the disappearance of the intensity oscillations occurs al so at different substrate temperatures in different azimuths. The simi lar to 20 degrees C higher disappearance temperature in the [<(1over b ar>10] azimuth is explained with a model based on previous scanning tu nneling microscopy results which revealed an increasing elongation of the islands in [(1) over bar 10] direction with increasing substrate t emperature. The B surface is more isotropic and hence no difference in the transition temperature in the two azimuths could be detected. Dur ing growth in the transition range between 2D and step-flow growth we observe increased terrace width fluctuations on the B surface, whereas the A surface becomes more uniformly stepped. This demonstrates that in the kinetically controlled regime the anisotropic barrier height fo r downward diffusion of adatoms over step edges plays an important rol e for the evolution of the surface morphology. At elevated temperature the barrier height allows downward jumps of the adatoms over B-type s teps but not over A-type steps. At conditions close to the thermodynam ic equilibrium a kinetic smoothing is observed on the A as well as on the B surface indicating another mechanism to be effective with a chan ge of the energetics due to ordering of the steps in combination with a disordering of the reconstruction on the terraces. This surface is, however, metastable and recovers after growth interruption rapidly (at substrate temperatures >580 degrees C within less than 1s) to the equ ilibrium bunched surface. (C) 1997 American Institute of Physics.