Doping and Raman characterization of boron and phosphorus atoms in germanium nanowires.

Impurity doping is the most important technique to functionalize semiconductor nanowires. The crucial point is how the states of impurity atoms can be detected. The chemical bonding states and electrical activity of boron (B) and phosphorus (P) atoms in germanium nanowires (GeNWs) are clarified by micro-Raman scattering measurements. The observation of B and P local vibrational peaks and the Fano effect clearly demonstrate that the B and P atoms are doped into the crystalline Ge region of GeNWs and electrically activated in the substitutional sites, resulting in the formation of p-type and n-type GeNWs. This method can be a useful technique for the characterization of semiconductor nanowire devices. The B-doped GeNWs showed an increasingly tapered structure with increasing B concentration. To avoid tapering and gain a uniform diameter along the growth direction of the GeNWs, a three step process was found to be useful, namely growth of GeNWs followed by the deposition of an amorphous Ge layer with high B concentration and then annealing.