Thickness Dependence of the Effective Masses in a Strained Thin Silicon Film

By comparing results obtained with the density-functional method, empirical pseudo-potential method, and empirical tight-binding method it is demonstrated that the conduction band structure is accurately described by the two-band k-p model. The later model is used to investigate the subband structure in ultra-thin (001) silicon films. It is demonstrated for the first time that the unprimed subbands with the same quantum number are not equivalent in ultra-thin films and develop different effective masses along [110] and [-110] directions. Using the two-band k-p model the dependence of the subband effective masses on strain and thickness is calculated. It is shown that the mass along tensile stress in [110] direction decreases with strain guaranteeing current enhancement in thin films. Shear strain also introduces large splitting between the unprimed subbands with the same n. Finally, the dependence of the effective masses in primed subbands is calculated and found to agree well with recent pseudopotential calculations.