Tuned Tilt of Epitaxial Crystals
- PDF / 431,204 Bytes
- 4 Pages / 604.8 x 806.4 pts Page_size
- 35 Downloads / 178 Views
d tilt, for example, the regularly stepped surface of Figure l b offers controlled densities of ledge sites for catalysis studies, 2 which may be modified at choice by ledge growth of impurities specifically to ledge sites or into the terraces themselves. In other areas the stacking becomes important. For example, when Cu 3 Au orders, its (110) planes alternate with CuAu and pure Cu compositions. At the (110) surface, the order-disorder phase transition, which now excites considérable interest, 3 is affected by the coupling of
(a)
Figure 1. In molecular beam epitaxial growth (a), atoms fall on the terraces and diffuse to the step edges to accrete in a process termed step flow growth. The cohérent tilt process (b) causes the deposited planes to tilt during epitaxial growth, with the same tilt maintained coherently over the entire surface. Crystals with tilts over 7° précise to 0.T, hâve been created this way.
thèse chemical différences to the surface. No one yet knows what will happen when a tilted surface forces both alternatives into surface sites, but this can clearly be achieved in the laboratory by using coherently tilted epilayers. V a r i o u s p r o s p e c t i v e t e c h n i c a l applications also présent themselves, for example in the préparation of oned i m e n s i o n a l f e a t u r e s 4 by a c c r e t i o n onto regular step-edges, or in the application of a position-dependent tilt to the optics of focusing éléments that employ Bragg ref lection. Requirements for Cohérent Tilt It was my great pleasure to see the physical conditions required to produce an epilayer with controlled tilt worked out mainly by a talented graduate student, Rui-Rui Du (now at Princeton U n i v e r s i t y ) after we had found tilted m a t e r i a l s in r a r e - e a r t h films grown for applications to magnetism. 5 At first the prospects for deliberately t a i l o r i n g tilt look hopeless. Even if the epilayer wanted to tilt at a spécial angle, there was usually symmetry to be broken, or the material would und e r g o o p p o s i t e tilts in différent régions, resulting in a useless polycrystal. In any case, why would the material prefer t h a t p a r t i c u l a r tilt about the chosen axis? In reality, the controlling factors are each reasonably clear, but there are several of them so the outcome is less easily visualized. Three largely independent ingrédients will be described hère. Epitaxial Misfit Epitaxial misfit provides the réservoir of energy that drives the tilt process. There are two parts to the energy équation. 6 First, the interfacial energy obviously dépends on how the atoms are a r r a n g e d in the initial epilayers. Quite frequently the new species may adopt the same structure as the substrate for one or more layers, which are then termed pseudomorphic. The same term is often used when the epilayer takes its equilibrium lattice structure but is stretched until its atomic spacing matches that of the substrate. The resulting interface is then cohérent and this generally gives an energy m i n i m u m ,
Data Loading...
