Ge pMOSFETs with GeO_x Passivation Formed by Ozone and Plasma Post Oxidation

A comparison study on electrical performance of Ge pMOSFETs with a GeO_x passivation layer formed by ozone post oxidation (OPO) and plasma post oxidation (PPO) is performed. PPO and OPO were carried out on an Al₂O₃/n-Ge (001) substrate followed by a 5-nm HfO₂ gate dielectric in situ deposited in an ALD chamber. The quality of the dielectric/Ge interface layer was characterized by X-ray photoelectron spectroscopy and transmission electron microscopy. The PPO treatment leads to a positive threshold voltage (V_TH) shift and a lower I_ON/I_OFF ratio, implying a poor interface quality. Ge pMOSFETs with OPO exhibit a higher I_ON/I_OFF ratio (up to four orders of magnitude), improved subthreshold swing, and enhanced carrier mobility characteristics as compared with PPO devices. A thicker Al₂O₃ block layer in the OPO process leads to a higher mobility in Ge transistors. By comparing two different oxidation methods, the results show that the OPO is an effective way to increase the interface layer quality which is contributing to the improved effective mobility of Ge pMOSFETs.