The Evolution of Periodic Step Arrays on Si by Surface Diffusion
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The Evolution of Periodic Step Arrays on SI by Surface Diffusion Mary E. Keeffe, C.C. Umbach and Jack M. Blakely Materials Science & Eng.,Cornell University , Ithaca NY, 14853. Abstract Periodic step arrays on Si(001) surfaces have been produced using ghotolithography, reactive ion etching and vacuum annealing. These have been studied by optical diffraction, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). The periodically varying step density on these arrays has been examined by STM. For small deviations from (100) along the [110] zone, single atomic steps dominate, while at larger angles biatomic steps are the most common; at intermediate angles the steps are of mixed character and there is some evidence for a range of unstable orientations. Interesting differences in the ratio of the areas of the two types of terrace (2x1) reconstructions are observed for the minima and maxima of the quasisinusoidal surfaces; these differences may be due to stresses produced by the step arrays or to differences in the line tensions associated with the two different types of steps on reconstructed Si(001) surfaces. The observations will be compared to the predictions of capillarity theory for isotropic materials. t high temperatures surface diffusion leads to a decay in amplitude of these surface gratings probably by mutual annihilation of atomic steps at the extrema. The overall rate of this process has been followed by monitoring the change in the distribution of intensity in the diffraction pattern from the grating using a He-Ne laser while the sample is annealed in UHV. With some simplifying assumptions, the intensity distribution can be directly related to the grating amplitude. The experiments are being performed for a range of grating spacings (to allow identification of the dominant transport process from scaling laws) and for a range of temperatures. The relationship between the 'macroscopic' observations of the shape development and the STM results will be explored. I.
Introduction
Atomic diffusion at surfaces is believed to be the prime mechanism by which surface structural rearrangements occur at elevated temperatures. It plays a key role in the formation of epitaxial overlayers and in many surface chemical reactions. As with bulk atomic diffusion, the motion of surface atoms is generally believed to be an activated process and to involve surface point defects, adatoms, vacancies or complexes of these entities. In this paper we will report on some observations of atomic transport at Si surfaces where net mass transport takes place. The experiments involve the measurement, at a number of different temperatures, of the shape and the rate of evolution of non-equilibrium surface morphologies produced by lithography and annealing methods. The surface diffusivities are determined by monitoring, with optical techniques, the rate of decay of the amplitudes of sinusoidally shaped gratings on single crystal surfaces. Scanning tunneling microscopy studies on such periodic surfaces reveal details
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