Simultaneous numerical solution of Schrödinger's and Poisson's equations is used to show that a narrow triangular Ge composition profile graded to a peak value of 100% with rise and fall lengths of just 2 nm can provide very effective subsurface hole confinement in a heterostructure Si1-xGex channel p metal-oxide-semiconductor field effect transistor (pMOSFET). This result is confirmed by analysis of MOS capacitor C-V curves for experimental devices fabricated using a very low thermal budget process on substrates grown by ultrahigh vacuum chemical vapor deposition. Unfortunately, transconductance measurements on experimental long-channel MOSFETs indicate that the peak low-field mobility of holes in the buried channel is just 132 cm2 V-1 s-1, slightly lower than that for a typical surface channel Si MOSFET. Buried channel hole mobilities up to 262 cm2 V-1 s-1 were obtained for reference devices fabricated using the same process with more conventional wide triangular composition profiles graded to a peak Ge content of 40%.

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Journal Journal of Vacuum Science and Technology A
Pawlowicz, C., Tarr, N.G, Berndt, L.P., Williams, R.L., Landheer, D., Xu, D.-X., … McAlister, S.P. (1998). Heterostructure Si1-xGex channel pMOSFETs with high Ge concentration. Journal of Vacuum Science and Technology A, 16(2), 864–867.