Role of short-period superlattice buffers for the growth of Si0.75Ge0.25 alloy layers on Si(001) substrates

The short-period superlattices (SLs) consisting of (Si14Ge1)(20), (Si-8/Ge- 1)(30), (Si-11/Ge-2)(30), (Si-9/Ge-2)(30), (Si-8/Ge-2)(40) and (Si-7/Ge-2)( 40) were grown step-by-step on Si(0 0 1) substrates at 500 degreesC by mole cular beam epitaxy. Residual strain, surface roughness and dislocations of Si0.75Ge0.25, alloy layers with a thickness of 2000 Angstrom grown on the S Ls have been characterized by X-ray diffraction (XRD), atomic force microsc opy (AFM) and cross-sectional transmission electron microscopy (XTEM), resp ectively, as a function of step number of SLs. By inserting the first step (Si-14/Ge-1)(20), SL, a drastic decrease to about 10 Angstrom in root-mean- squared (RMS) surface roughness is observed compared to that of about 40 A of the sample grown without SLs. although the residual strain is almost the same. A noticeable change in RMS roughness is also observed when Ge is dep osited as 2-monolayers mode from I-monolayer mode. Residual strain in one-s tep and seven-step SL buffer samples are found to be about -0.38 and -0.15% , respectively. XTEM images verify that at various steps St buffers help to relax the strain of the upper alloy layer by introducing dislocations in t he buffer layers or deflecting them into the substrate. Furthermore. the al loy layers grown at 500 degreesC using one-step SLs which are grown at diff erent temperatures from 300 to 550 degreesC are also studied. At 300 degree sC, the alloy layer becomes about relaxed leaving behind only -0.08% residu al strain (relaxation rate 90%) with small RMS roughness of about 10 Angstr om. (C) 2001 Elsevier Science B.V. All rights reserved.