(Invited) Germanium-Tin P-Channel Field-Effect Transistor with Low-Temperature Si2H6 Passivation
In this paper, we discuss the research and development of using GeSn as the channel material of pFETs for enhancing the hole mobility and drive current. This includes GeSn planar pFETs, GeSn nanowire (NW) pFETs, as well as GeSn tunneling FETs (TFETs). Low-temperature Si2H6 passivation was developed to realize high-quality gate stack on GeSn.
GeSn Planar pFETs with Various Sn Compositions and Surface Orientations
GeSn (100)-oriented pFETs were demonstrated to have higher hole mobility than Ge pFETs -. With increasing Sn concentration in GeSn, the light hole effective mass at the G point is reduced , leading to higher hole mobilities. An important enabling process module for realization of high performance GeSn pFETs is the low-temperature Si2H6 passivation to achieve high-quality gate stack on GeSn -, as shown in Fig. 1 (a) and (b).
Further mobility enhancement can be achieved by a two-step Si2H6 passivation technique . In addition, various channel surface orientations for the GeSn pFETs have been investigated , with the (111) orientation giving a higher hole mobility for a given Sn content in the channel. Fig. 1 (c) compares the effective hole mobilities of Si2H6-passivated GeSn pFETs with various Sn compositions and surface orientations at an inversion carrier density Ninv of 1 × 1013 cm-2.
Uniaxially Compressive Strained GeSn Nanowire pFETs
Besides planar GeSn pFETs, uniaxially compressive strained GeSn NW pFETs were realized using a CMOS compatible top-down approach  (Fig. 2). The uniaxial compressive strain leads to a significant reduction of the hole effective mass and an increase in the separation between light hole and heavy hole bands of GeSn compared to unstrained and biaxially compressive strained GeSn, which could enhance hole mobility.
GeSn Tunneling FETs
With increasing Sn content, the band gap of GeSn is reduced and GeSn becomes a direct band gap material. GeSn is therefore promising for the design of the tunneling junction in tunneling field-effect transistor (TFET) -. By incorporating Sn into Ge to shift the Г valley down and reduce the bandgap, GeSn TFETs with enhanced drive current and reduced subthreshold swing (SS) were demonstrated (Fig. 3).