A Unique Method of Forming GexSi1-x Thin-Films on Insulator

Abstract

A unique process of fabricating a strained layer Ge_xSi_1−x on insulator is demonstrated. Such strained heterostructures are useful in the fabrication of high-mobility transistors. This technique incorporates well-established silicon processing technology e.g., ion implantation and thermal oxidation. A dilute GeSi layer is initially formed by implanting Ge⁺ into a silicon-on-insulator (SOI) substrate. Thermal oxidation segregates the Ge at the growing oxide interface to form a distinct Ge_xSi_1−x thin-film with a composition that can be tailored by controlling the oxidation parameters (e.g. temperature and oxidation ambient). In addition, the film thickness can be controlled by the implantation fluence, which is important since the film forms pseudomorphically below 2×10¹⁶ Ge/cm². Continued oxidation consumes the underlying Si leaving the strained GeSi film encapsulated by the two oxide layers, i.e. the top thermal oxide and the buried oxide. Removal of the thermal oxide by a dilute HF etch completes the process. Strain relaxation can be achieved by either of two methods. One involves vacancy injection by ion implantation to introduce sufficient open volume within the film to compensate for the compressive strain. The other involves spontaneous relaxation of the film, which may depend, in part, upon the formation of GeO. If Ge is oxidized in the absence of Si, it evaporates as GeO(gas). Conditions under which this occurs will be discussed.