Electrodeposition of Protocrystalline Germanium from Supercritical Difluoromethane

We report results for the electrochemistry of the germanium(II) tri-halide anions, [GeCl3](-), [GeBr3](-) and [GeI3](-), in supercritical difluoromethane containing 60 mM [(NBu4)-Bu-n][BF4] at 19.1 MPa and 358 K. The voltammetry shows mass-transport-limited currents for reduction to germanium at gold on the first scan. There is no evidence of a germanium stripping peak and, on subsequent scans, the electrode slowly passivates with the deposition of approximately 0.4 mu m of material. The redox potentials for the reduction of the three tri-halides are in the order [GeCl3](-) < [GeBr3](-) <[GeI3](-), with the iodide being the most easily reduced complex. Electrodeposition of germanium onto TiN electrodes from supercritical difluoromethane at 19.1 MPa and 358 K, using either 16 mM [EMIM][GeI3] with 60 mM [EMIM][BF4] or 16 mM [(NBu4)-Bu-n][GeI3] with 60 mm [(NBu4)-Bu-n] [BF4], gave deposition rates of 2-3 mu m h(-1). Raman spectroscopy and transmission electron microscopy showed that the resulting germanium films were protocrystalline, containing nanocrystals of germanium embedded in an amorphous germanium matrix.