SURFACE SMOOTHING INDUCED BY EPITAXIAL SI CAPPING OF ROUGH AND STRAINED GE OR SI1-XGEX MORPHOLOGIES - A RHEED AND TEM STUDY

The same Si/Ge/Si or Si/Si1 - xGex/Si structures grown at 400 degrees C on Si(0 0 1) are compared, either in real time by reflection high-en ergy electron diffraction (RHEED) or on the final product by transmiss ion electron microscopy (TEM). This allows us to follow interface morp hology variations during Si re-growth upon Ge containing layers of var ious Ge thicknesses or alloy x fractions. As shown by the passage from spotty to streaky RHEED patterns and by specular beam intensity oscil lation evolutions, the surfaces roughen systematically during strained Ge or Si1 - xGex (SiGe) growth and smooth rapidly during subsequent g rowth of 4 to 6 Si monolayers, at least in the elastic hut-clustering islanding range with {1 0 5} facets. With the help of TEM examinations , a coherent picture may be proposed for these surface smoothing obser vations: (i) A dominant mechanism in form of a quick Si surface diffus ion occurring initially on the Ge-strained surfaces. It ensures a hete regeneous Si accumulation towards the places of minimized misfit, i.e. , in the troughs of the Ge or SiGe morphologies. (ii) A slower Ge diff usion (as occuring on Si) depleting the emerging island crests and con tributing to an overall Ge surface termination (Ge surface segregation ) and to a complementary island smoothing. The latter mechanism, only important at low growth kinetics, favours the formation of alloyed int erfaces as a by-product of the island smoothing and lateral intermixin g. At high Si growth kinetics the former mechanism prevails leading to better preserved island morphologies and final interfaces appearing c hemically more abrupt but less flat. (C) 1998 Elsevier Science B.V. Al l rights reserved.