High interfacial conductivity at amorphous silicon/crystalline silicon heterojunctions

We performed static coplanar conductance measurements as a function of temperature on samples consisting of n-type hydrogenated amorphous silicon (a-Si:H) deposited onto either glass or p-type crystalline silicon (c-Si). The conductance is found orders of magnitude higher and its activation energy is one order of magnitude lower when the a-Si:H film is deposited on c-Si. It is demonstrated both experimentally and with the help of numerical modeling that this high conductance is due to an electron-rich inversion layer in c-Si at the heterointerface. When a thin (3 nm) undoped silicon layer deposited under conditions that normally lead to polymorphous silicon is inserted at the interface, the coplanar conductance slightly increases, which is attributed to a small increase of the conduction band discontinuity at the interface.