IMPROVED CARRIER TRANSPORT IN STRAINED Si/SiGe DEVICES V. Palankovski, S. Dhar, H. Kosina, and S. Selberherr Institute for Microelectronics, TU Vienna, Gusshausstrasse 27–29, A-1040 Vienna, Austria Phone: +43-1-58801/36018, Fax: +43-1-58801/36099, Email: Dhar@iue.tuwien.ac.at Abstract Performance improvement in RFIC technology can be achieved by the introduction of novel materials and device structures. The SiGe/Si material system allows obtaining beneficial band structure and transport properties due to strain. This paper reviews recent theoretical and experimental achievements. Special focus is put on the description of the anisotropic majority/minority electron mobility in strained Si and SiGe layers as a function of doping and material composition. The Monte Carlo method is used for analyzing the transport properties of the strained Si/SiGe material system and for developing models for Technology Computer-Aided Design (TCAD) applications. Introduction In the last years, there has been enormous research in the area of materials compatible with Si technology and device structures for improving the speed of VLSI cir- cuits. SiGe has emerged as a promising material be- cause of its electrical and material properties. SiGe HBTs have found application in low-noise amplifiers and frequency dividers and have been combined with digital ICs (BiCMOS) for volume production. For the CMOS technology, although the SiGe channel has been used to enhance the performance of PMOS transistors, a desired improvement of the complimentary NMOS transistors is not achievable with SiGe. The replace- ment of the channel material by strained Si, which uti- lizes an underlying relaxed SiGe layer for its function- ing, renders a solution to the problem since it leads to enhancement of both the electron and hole mobilities. Strained Si/SiGe FETs exhibit superior performance for RF applications. Major developments have been re- ported by IBM [1, 2, 3, 4, 5, 6, 7, 8] and Daimler- Chrysler (DC) [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20], both for p- and n-type devices. Fig. 1 summa- rizes reported values for the cut-off frequencies f T and f max in the last years. In order to investigate and design strained SiGe (Si) de- vice structures, it is necessary to model the carrier mo- bilities in these devices. This paper discusses the recent theoretical and experimental achievements reported for to describe the doping and material composition depen- dence for the strained Si material. Physical Background It is well known that due to lattice mismatch, a pseudo- morphically grown SiGe (Si) layer on Si (relaxed SiGe) experiences a biaxial compressive (tensile)strain, pro- vided that the layer thickness is below the critical thick- 1998 1999 2000 2001 2002 2003 2004 Year 0 50 100 150 200 f T , f max [GHz] DC NMOS DC NMOS IBM NMOS IBM NMOS DC PMOS DC PMOS IBM PMOS IBM PMOS Figure 1: Cut-off frequencies f T (filled symbols) and f max (open symbols) of strained-Si FETs. ness. This strain leads to a modification of both the conduction and valence bands. It lifts the degeneracy of the light and heavy hole bands and lowers the spin- orbit band resulting in reduction of inter-band scatter- ing and improvement of hole mobility. Since the conduction band structure in SiGe is silicon like for Ge< 0.85, compressively straining SiGe leads to splitting of the 6-fold degenerate ∆ 6 -valleys in Si into 2-fold degenerate ∆ 2 valleys higher in energy and 4- fold degenerate ∆ 4 valleys lower in energy. The higher in-plane effective electron mass of in ∆ 4 valleys leads to a reduction of the electron mobility for strained SiGe. In the case of tensile strained Si, the direction of mo- tion of the splitting is reversed with the ∆ 4 valley mov- ing lower in energy and ∆ 4 higher. The lower in-plane effective mass of electrons in the ∆ 2 valleys and the re- duction of inter-valley phonon scattering lead to an en- hanced electron mobility. Fig. 2 shows the band align- ment of strained Si (SiGe) relative to relaxed SiGe and unstrained Si. The figure shows the strain-induced split- ting of the conduction and valence bands, together with the band edge discontinuities, as a function of the ger-