A comprehensive model for the electrical nanocontact on germanium for scanning spreading resistance microscopy applications

摘要

Quantitative carrier profiling represents a key element in the process development of future nanoelectronic devices. During the last decade, scanning spreading resistance microscopy (SSRM) has evolved as the method of choice for two-dimensional carrier mapping due to its unique spatial resolution and high sensitivity when applied to silicon (Si)-based devices. While the electrical nanocontact between a SSRM probe and Si is well documented, the insight is insufficient to understand or make predictions about the properties of the SSRM contact in case of high-mobility germanium (Ge) samples. Therefore, we present in this paper a model describing this contact in more detail, taking into account the effects of the applied pressure as this leads to the formation of a β-Sn pocket right underneath the probe and a spatially non-homogeneous bandgap reduction in the underlying Ge. The resistance probed through the resulting Schottky contact is further influenced by the dimensions of the nanocontact and in particular by the spatial extent of the surface states which are present at the cross-sectional surface. To account for the low-bias Ⅳ-characteristics of n-Ge, the model also includes trap-assisted tunneling. Using this model, we are able to describe the role of probe properties such as probe radius and resistivity on the shape (steepness, saturation, non-linearity) of the Ge calibration curve. We demonstrate experimentally and by simulation that low-resistivity probes are indispensable for a high sensitivity when applying SSRM to highly doped samples.