EPITAXY OF SI SI1-XGEX HETEROSTRUCTURES WITH VERY SMALL ROUGHNESS USING A PRODUCTION-COMPATIBLE ULTRAHIGH-VACUUM CHEMICAL-VAPOR-DEPOSITION REACTOR/

Two different roughening mechanisms are studied using a production-rea dy epitaxial deposition system to grow various epilayers, including th e base layer of bipolar transistors. Surface roughness is measured usi ng atomic force microscopy and transmission electron microscopy is use d for cross-sectional analysis. Very smooth surfaces are routinely obt ained, with a root mean square roughness of 0.15 nm. However, two poss ible mechanisms are shown to increase surface roughness under certain conditions. Above a certain critical thickness, for growth temperature s higher than 525 degrees C and Ge compositions above 25%, strain redi stribution results in surface ripples with a typical wavelength of the order of 100 nm. In addition, we find that substrate contamination (C ,O) can produce wide depressions, several hundreds of nm wide and on a verage 10 nm deep in Si epilayers. The addition of a small concentrati on of Ge (below 15%) seems to produce more planar growth compared to t he same thickness (50 nm) of Si only. The origin of the different surf ace morphologies observed is discussed. (C) 1998 American Vacuum Socie ty. [S0734-211X(98)06802-4].