Mobility enhancement in strained-Ge modulation-doped structures by planarization of SiGe buffer layers

Compressively strained-Ge channel p-type modulation-doped structures were fabricated on SiGe buffer layers planarized by chemical mechanical polishing (CMP). While the mobility of the sample without CMP was drastically reduced with decreasing channel thickness, the much higher mobility was maintained for the sample with CMP, suggesting that atomic-scale interface roughness as well as long-ranged roughness was significantly improved by CMP. As a result, mobility enhancement factors of 8 and 1.8 at 10 and 300 K, respectively, were obtained by CMP in the channel thickness of 7.5 nm, indicating that the planarization is very essential for realization of high-mobility strained-Ge structures. Dependence of the structure on the buffer fabrication method is also discussed.