Apparently Fermi level pinning presents a problem in these scenarios. There have been some works to unpin the FL, typically by ultrathin oxides or other layers. I suggest taking a look at the following:

Nishi's group, APL 2009 http://dx.doi.org/10.1063/1.3065990

Toriumi's group highly cited Appl Phys Exp from 2008 http://dx.doi.org/10.1143/APEX.1.051406

And Zhou et al. APL 2009 http://dx.doi.org/10.1063/1.3357423 

It's possible that easier approaches have been developed since.