Fermi level depinning of Ge Schottky contacts using single crystalline MgO

摘要

Fermi level (FL) pinning at the Ge valence band results in a high Schottky barrier height (SBH) for all metal-Ge contacts [1]. Even though a few groups have demonstrated depinning of the FL by various methods, an explicit explanation of the underlying mechanism is still unclear. One of the most conjectured origins is the metal-induced-gap-states (MIGS) [2], which are energy states in the bandgap of the semiconductor due to the tailing of the metal electron wave functions into the semiconductor. If MIGS plays the dominant role in FL pinning, inserting an ultrathin oxide (UTO) or insulator between the metal and Ge should depin the FL since the UTO can block the tailing of the metal wave function into Ge and thus reduce the MIGS formation. More importantly, the depinning should be increasingly effective when the UTO thickness is increased until a saturation thickness is reached, at which all MIGS are eliminated. Another possible origin of FL pinning is due to the surface states, which arise from unsatisfied dangling bonds or other defects on the semiconductor surface [3]. Depinning could be achieved by passivating those surface states before metal deposition. If surface states play the dominant role in FL pinning, the depinning effect should have a rather weak dependence on the interfacial insulator thickness. However, so far, no experimental results provide an unambiguous deduction of the FL pinning mechanism, mostly due to the lack of an explicit characterization of a high quality metal/Ge or metal/ultrathin oxide/Ge junction, which ideally should be single crystalline, atomically smooth and free of process-induced defects or intermixing. We report the Schottky characteristics of high quality metal/MgO-Ge junctions with the ultrathin MgO epitaxially grown on Ge. We find the depinning effect displays a weak dependence on the MgO thickness, indicating the interface states due to the native defects on Ge surface are likely to play the dominant role in FL pinning.