Growth of Si0.75Ge0.25 alloy layers grown on Si(001) substrates using step-graded short-period (Si-m/Ge-n)(N) superlattices

Short-period (Si-m/Ge-n)(N) superlattices (SSLs) are grown step by step on a Si(001) substrates by solid source molecular beam epitaxy. Using the step -graded SSLs as buffer layers, 2000 Angstrom uniform Si0.75Ge0.25 alloy lay ers are grown on the same substrates. The growth temperature of the SSLs an d uniform layers is 500 degreesC. In the SSLs layers, m and n are the numbe r of monolayers of Si and Ge, respectively. N is the period of (Si-m/Ge-n) bilayers. The samples grown are characterized by x-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM) as a fun ction of the step number of SSL layers. The SSLs show very smooth surfaces [the root-mean-square (rms) surface roughness is between 7 and 12 Angstrom] . A dramatic decrease in roughness is observed in the uniform Si0.75Ge0.25 alloy layers, when even a one-step SSL is used as a buffer layer. A noticea ble increase in rms roughness is seen in both SSL and alloy layers when the number of Ge monolayers is changed from one to two. AFM observation shows that the rms surface roughness behavior of the SSLs is reflected to their c orresponding top alloy layers. The residual strains in alloy layers are con siderably lower, with a maximum relaxation rate of about 80% for the sample with a seven-step SSL buffer. Cross-sectional TEM images show that straine d SSL buffer layers effectively deflect threading dislocations in the subst rate or confine the dislocations in the SSL buffer layers. (C) 2001 America n Institute of Physics.