Hole band anisotropy effect on on-state performance of biaxial compressive strained SiGe-based short channel QW pMOSFETs: Experimental observations

Qantum well (QW) FETs with compressively-strained SiGe channel are promising candidates for pMOSFET for future logic technology with scaled operating voltage [1-3]. High hole mobility observed in strained SiGe channel layer, as compared to Si, is expected to result in enhanced performance of these devices for deep submicron channel lengths. However, most of experimental results in literature so far, focusing on [011] channel direction on relaxed (100) Si bulk substrate have shown mobility degradation (hence drive current degradation) or marginal drive current enhancement at short channel regime in pseudomorphic SiGe based channels over Si control [4-7]. This has been attributed to effects of additional Coulomb scattering (from N_it and halo) and from neutral defects [4], as shown in Fig. 1. While these are indeed additional source of defects over Si counterpart inhibiting performance enhancement, highly anisotropic hole band structure with biaxial compressive strained SiGe channel could also play an important role on the ON-state performance degradation especially in deep submicron regime. In this paper we investigate the channel orientation dependence on the performance in an optimized Si_0.5Ge_0.5 QW channel. Strong drive current (mobility) enhancement is observed in [010] versus [011]. This directional dependence is further amplified at shorter channel lengths and lower temperature, and is explained on the basis of anisotropy in band structure.