On the universality of inversion layer mobility in Si MOSFET´s: Part II-effects of surface orientation

For part I see ibid., vol.41, no.12, pp.2357-62 (1994). This paper reports the studies of the inversion layer mobilities in n-channel MOSFET´s fabricated on Si wafers with three surface orientations ((100), (110), and (111)) from the viewpoint of the universal relationship against the effective field, E_eff(=q(N_dpl+η·N_s )/εSi). It is found that the universality does hold for the electron mobilities on (110) and (111), when the value of η is taken to be 1/3, different from the electron mobility on (100), where η is 1/2. Also, the E_eff dependence of the electron mobility is found to differ among (100), (110), and (111) surfaces. This is attributed to the differences in the E_eff dependence of the mobility limited by surface roughness scattering among the orientations. The origins of E_eff and η are discussed on the basis of the relaxation time approximation for a 2DEG (2-dimensional electron gas). While the surface orientation dependence of η in phonon scattering can be understood in terms of the subband occupation, it is found that the theoretical formulation of surface roughness scattering, used currently, needs to be refined in order to explain the differences in E_eff dependence and the value of η among the three orientations