Influence of carrier density on the friction properties of silicon pn junctions

We present experimental results showing a significant dependence of the friction force on charge carrier concentration in a Si semiconductor sample containing p- and n-type regions. The carrier concentration was controlled through application of forward or reverse bias voltages in the p and n regions that caused surface band bending in opposite directions. Excess friction is observed only in the highly doped p regions when in strong accumulation. The excess friction increases with tip-sample voltage, contact strain, and velocity. The sample is an oxide-passivated Si (100) wafer patterned with arrays of 2-mu m-wide highly doped p-type strips with a period of 30 mu m in a nearly intrinsic n-type substrate. The countersurface is the tip of an atomic force microscope coated with conductive titanium nitride. The excess friction is not associated with wear or damage of the surface. The results demonstrate the possibility of electronically controlling friction in semiconductor devices, with potential applications in nanoscale machines containing moving parts.