Tight-Binding based SiGe Band Structure Calculations and Implication on Transport Saumitra Mehrotra, Abhijeet Paul, Gerhard Klimeck, and Mathieu Luisier School of Electrical and Computer Engineering, Network for Computational Nanotechnology, and Birck Nanotechnology Center, Purdue University, West Lafayette, IN, USA - 47907 Email: smehrotr@purdue.edu Abstract— This work presents a comprehensive analysis of the SiGe band structure using a Tight- Binding based approach within the virtual crystal approximation. We analyze the material properties of bulk relaxed SiGe and biaxially compressed strained systems. The simulation approach has been benchmarked against experimental data wherever possible. We further investigate the effect of process induced uniaxial strain in <100> SiGe/Si pMOS devices. It is found that uniaxial strain can further improve the performance of biaxially compressed SiGe/Si based pMOS devices by as much as 10% for high Ge% devices. Keywords- SiGe, biaxial, uniaxial strain, bandstructure. I. INTRODUCTION The continuous shrinking of channel lengths (Lc) in silicon CMOS devices to increase their performance has led to the exploration of new high mobility channel materials. Ge exhibits higher hole mobility compared to Si. However, due to the difficulties in fabricating a suitable dielectric in Ge based devices, the focus has been redirected towards SiGe. About 30% hole mobility enhancement relative to unstrained Si [1] have been demonstrated in biaxially compressed SiGe/Si channel materials. Strained SiGe pMOS devices are being considered as one of the designs for the ultimate CMOS [2]. So far device engineers have mostly employed the k•p method to model transport in bulk transistors.. Here we present a Tight-Binding (TB) based bandstructure calculation method in the virtual crystal approximation (VCA) [3] for bulk relaxed and strained SiGe/Si material systems. Section II provides a brief description of the TB-VCA model used for calculation of electronic structure. Section III discusses the bandgaps/bandedges. In Section IV we talk about the variations in electron and hole effective masses. Finally in Section V we talk about uniaxial strain as a performance booster in SiGe/Si pMOS devices. II. TIGHT-BINDING VCA MODEL SiGe is a binary alloy composed of Si and Ge atomic species. In the virtual crystal approximation the alloy is represented as an ‗averaged‘ atom which has the averaged properties of the individual atomic species. The sp3d5s* tight-binding parameters for Si and Ge [7] are scaled as below for Si 1-x Ge x (0<x<1) . Here E represents the onsite energy for the orbital σ while V represents the nearest neighbour σ1-σ2 orbital coupling energy. Ge Si SiGe E x E x E    . ). 1 (    (1) Ge Si SiGe V x V x V 2 1 2 1 2 1 . ). 1 (          (2) For the strained case the individual parameters are scaled according to Harrison‘s scaling rule [8]. Lattice Figure 1. Bandgaps computed for relaxed Si 1-x Ge x and Si 1-x Ge x /Si using TB-VCA model. Experimental data from [4][5][6].