EVOLUTION OF SURFACE-ROUGHNESS IN EPITAXIAL SI0.7GE0.3(001) AS A FUNCTION OF GROWTH TEMPERATURE (200-600-DEGREES-C) AND SI(001) SUBSTRATE MISCUT

The evolution of surface roughness in epitaxial Si0.7Ge0.3 alloys grow n on Si(001) as a function of temperature (200-600 degrees C), thickne ss (t=7.5-100 nm), and substrate miscut were investigated by atomic fo rce microscopy and quantified in terms of the height-difference correl ation function G(rho), in which rho is lateral distance and [G(rho-->i nfinity)](1/2) is proportional to the surface width. The films were de posited by ultrahigh vacuum ion-beam sputter deposition at 0.1 nm s(-1 ). Strain-induced surface roughening was found to dominate in alloys g rown on singular Si(001) substrates at T-s greater than or equal to 45 0 degrees C where [G(rho-->infinity)](1/2) initially increases with in creasing t through the formation of coherent islanding. The islands ar e preferentially bounded along [100] directions and exhibit 105 faceti ng. This tendency is enhanced, with much better developed [100] island s separated by deep trenches-of interest for growth of self-assembled nanostructures-in films grown on Si(001)-4 degrees[100]. Increasing th e film thickness above critical values for strain relaxation leads to island coalescence and surface smoothening. At very low growth tempera tures (T-s less than or equal to 250 degrees C), film surfaces roughen kinetically, due to limited adatom diffusivity, but at far lower rate s than in the higher-temperature strain-induced regime. Si0.7Ge0.3 all oy surfaces are smoother, while the films exhibit larger critical epit axial thicknesses, than those of pure Si films grown in this temperatu re regime. There is an intermediate growth temperature range, however, over which the alloy film surfaces remain extremely smooth even at th icknesses near critical values for strain relaxation. This latter resu lt is of potential importance for device fabrication. (C) 1996 America n Institute of Physics.