Short-period (Si14 / Si0.75 Ge0.25)20 Superlattices for the Growth of High-quality Si0.75 Ge0.25

In the present experiment, we have grown 2500-Å thick Si0.75Ge0.25 alloy layers on Si(001) substrate by MBE process using a short-period (Si14/Si0.75Ge0.25)20 superlattice (SL) as buffer layers. In the SL layers, first a layer of 14 monolayers (MLs) of Si (thickness about 20Å) then a thin layer of Si0.75Ge0.25 (thickness 5-6Å) were grown. This Si/(Si0.75Ge0.25) bilayers were repeated for 20 times. The buffer layers were grown at different temperatures from 300-400°C and the alloy layers were then grown at 500°C on the buffer layers. The alloy layer showed low residual strain (about -0.16%) and smooth surface (rms roughness ~15Å) with 300°C grown SL buffer. Low temperature growth of Si in SL layer introduces point defects and low temperature growth of Si1-xGex in SL layer reduces the Ge segregation length, which leads to strained SL layer formation. Strained layers are capable to make barrier for the propagation of threading dislocations and point defect sites can trap the dislocations.