Evidence of Real-Space Transfer in Buried-Channel Ge$_{x}$C$_{1-x}$ Devices

We present experimental evidence of real-space transfer (RST) in buried-channel Ge$_{x}$C$_{1-x}$ p-type metal-oxide-semiconductor field effect transistors (MOSFET) containing a Si cap layer. The output characteristics of these devices reveal a negative differential resistance (NDR) below 150K, at the onset of the saturation regime. This observation indicates a charge transfer from Ge$_{x}$C$_{1-x}$ layer into the Si cap at sufficiently large drain bias values. The lower hole mobility of the Si cap with respect to the Ge$_{x}$C$_{1-x}$, translates into a drain current reduction, hence the observed NDR. Our low-field, temperature-dependent mobility measurements indeed reveal a higher effective carrier mobility in the buried-channel Ge$_{x}$C$_{1-x}$ layer with respect to a Si-reference sample, which suggests that the observed NDR is caused by RST of holes from the Ge$_{x}$C$_{1-x}$ into the Si layer.