Nanoscale electrodeposition of germanium on Au(111) from an ionic liquid:: an in situ STM study of phase formation -: Part II.: Ge from GeCl4

The electrodeposition of germanium on Au(111) from the GeCl4 saturated ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate was investigated with in situ scanning tunneling microscopy and in part, qualitatively, with in situ I/U tunneling spectroscopy. Ge deposition starts in the UPD regime with the decoration of the gold steps by Ge at +1000 mV vs. Ge, then small islands that have initial heights of about 150 pm on average start growing at +950 mV vs. Ge. At +750 mV vs. Ge islands with average heights of 250 pm form, they can be removed reversibly at higher electrode potentials and holes with apparent depths between about 30 and 100 pm remain in the gold surface. Before bulk deposition of germanium starts, a thin rough layer with metallic behaviour is deposited on the surface. It can be stripped reversibly upon increasing the electrode potential to higher values. The overpotential deposition on this layer starts with nanoclusters of 10-40 nm in diameter and some nanometres in height. They can be stabilized for hours by proper selection of the electrode potential. In situ I/U tunneling spectroscopy of about 100 nm thick Ge films clearly shows semiconducting behaviour on the whole of the surface in the scanned area. The band gap of the bulk Ge determined by in situ I/U tunneling spectroscopy is 0.7 +/- 0.1 eV, in good agreement with the theoretical value of 0.67 eV for intrinsic microcrystalline germanium at 300 K. On a layer of approximately 20 nm in thickness we found both sites with semiconducting behaviour and sites with rather metallic behaviour.