Evolution of Mesas on Si(111) Surface Under Sublimation: Nanofabrication through the Control of Atomic Steps
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Evolution of Mesas on Si(111) Surface Under Sublimation: Nanofabrication through the Control of Atomic Steps Kee-Chul Chang and Jack M. Blakely Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14850 ABSTRACT How the surface morphology of a low index crystal surface evolves under annealing below the roughening temperature is still being investigated both theoretically and experimentally. This research is needed to understand the stability of nanostructures and for the control of step array on surfaces for various technological applications. We have approached this problem from an experimental point of view by looking at the step morphology on the top of mesas patterned on Si(111) after flashing at 1200°C and annealing between 1000-1100°C. We find that after annealing, nanometer height ridges develop on the edges of the mesas except for one edge which becomes a source for steps advancing into the mesa top. The edge that does not develop a ridge is mainly determined by the initial miscut of the surface. We attempt to explain our results through step dynamics and present some applications of this phenomena in creating novel arrays of steps. INTRODUCTION After suitable heat treatment, single crystal surfaces cut close to a low index (close packed) plane tend to develop into a step and terrace structure. This is due to the fact that it is technologically almost impossible to orient the crystal exactly in the low index orientation during the cutting and polishing process, resulting in a miscut surface. An example of a Si(111) surface with step terrace structure is given in figure 1a. The steps can be thought of as discrete contour lines of the surface profile. Thus, by controlling the shape of the steps, the surface morphology can be manipulated. The main advantage of shaping the surface profile by this method lies in the fact that the mono-atomic step height is in the range of angstroms, so very fine height control over long (microns) distances can be realized. There are two different approaches to shaping the surface profile through the control of steps. One approach is to use various instabilities of steps on an unpatterned surface to spontaneously create the desired surface profile1. The second approach, which is the focus of our research, is to start with a patterned surface. The shape of the steps can be controlled by fabricating trenches and sidewalls which impede step motion. The simplest and most successful implementations of this approach were carried out by the Blakely group2 and the Homma group3 in the creation of large (10 microns and more) step-free surfaces in the bottom of patterned crater structures on Si(111) and (001) by annealing in ultra high vacuum. Our recent theoretical investigation into the processes leading to these step-free surfaces focused on the role of circular steps near the local minima and maxima that is created by the combination of patterning and the miscut. Using BCF (Burton, Cabrera and Frank) theory4, we have concluded that the key to understan
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