Physical mechanism determining Ge p- and n-MOSFETs mobility in high Ns region and mobility improvement by atomically flat GeOx/Ge interfaces

Hall measurements have been carried out for the Ge p-and n-MOSFETs with different substrate orientations and GeO
_x
/Ge interface qualities. It is found that the significant reduction of effective mobility in high surface carrier concentration (N
_s
) or high normal field in Ge MOSFETs is attributed partly to the N
_s
loss due to large amounts of interface states inside the valence and conduction bands of Ge. The GeO
_x
/Ge interface roughness is another reason limiting the high N
_s
mobility. It has been revealed that room temperature plasma post oxidation can realize Al
₂
O
₃
/GeO
_x
/Ge gate stacks with atomically-flat GeO
_x
/Ge interfaces. Ge MOSFETs with these interfaces have provided record high effective hole and electron mobility, which overcome the Si universal mobility in both low and high N
_s
regions.