Effect of ultrathin GeO_x interfacial layer formed by thermal oxidation on Al₂O₃ capped Ge

We propose a modified thermal oxidation method in which an Al₂O₃ capping layer is used as an oxygen blocking layer (OBL) to form an ultrathin GeO_x interfacial layer, and obtain a superior Al₂O₃/GeO_x/Ge gate stack. The GeO_x interfacial layer is formed in oxidation reaction by oxygen passing through the Al₂O₃ OBL, in which the Al₂O₃ layer could restrain the oxygen diffusion and suppress the GeO desorption during thermal treatment. The thickness of the GeO_x interfacial layer would dramatically decrease as the thickness of Al₂O₃ OBL increases, which is beneficial to achieving an ultrathin GeO_x interfacial layer to satisfy the demand for small equivalent oxide thickness (EOT). In addition, the thickness of the GeO_x interfacial layer has little influence on the passivation effect of the Al₂O₃/Ge interface. Ge (100) p-channel metal-oxide-semiconductor field-effect transistors (pMOSFETs) using the Al₂O₃/GeO_x/Ge gate stacks exhibit excellent electrical characteristics; that is, a drain current on-off (I_on/I_off) ratio of above 1× 10⁴, a subthreshold slope of ～ 120 mV/dec, and a peak hole mobility of 265 cm²/V· s are achieved.