MBE grown short-period (Sim/Gen)N superlattices (SSLs) and its effect on the growth of uniform Si0.75Ge0.25/(SSLs)/Si(001) systems

Buffer layers are very important for the growth of Si1−xGex alloy layers. Here, short-period (Sim/Gen)N superlattices (SSLs) with various steps are grown on Si(001) substrates by MBE process. Alloy layers, 2000-Å-thick Si0.75Ge0.25, are grown on the SSL buffer layers. Besides, the alloy layers of the same thickness are deposited on the one-step (Si14/Ge1)20 SSL and Si buffers grown at different temperatures, from 300 to 500 °C. Residual strain, surface roughness and nature of dislocations of the samples are observed by X-ray diffraction (XRD), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM), respectively. The SSL layers show smooth surfaces. A sharp decrease of roughness is observed in uniform Si0.75Ge0.25 alloy layers, when SSL are used as buffer layers. The surface roughness of the alloy layers with SSL buffer layers follow the surface roughness of its corresponding buffer layers. Residual strain of the uniform alloy layers with SSL buffer layers is considerably lower. The alloy layer with one-step SSL buffer layer, grown at 300 °C, relaxes leaving behind only −0.08% residual strain and small rms roughness of ≈10 Å, whereas Si buffer grown at the same temperature reveals residual strain and rms roughness of about −0.41% and 24 Å, respectively.