Nanoparticle-assisted growth of porous germanium thin films

Since the discovery of its room-temperature luminescence in 1990([1]) much attention has been focused on porous silicon, which is prepared typically by electrochemical anodization in a dilute hydrofluoric-acid solution. This fluorescence behavior is of interest because bulk silicon is an indirect bandgap semiconductor, in which crystal momentum is needed to excite an electron from the valence band to the conduction band. Quantum confinement effects of the nanowire skeleton and the state of the surface at the nanocrystal-oxide interface have been suggested to account for the visible luminescence of porous silicon.([2]) Relative to silicon, the exciton in germanium has a larger effective Bohr radius, which causes quantum size effects to be more easily achieved. Only a few studies, however, have been carried out on porous germanium because of the lack of adequate procedures for its preparation.([4]) Porous silicon is made up of a network of silicon nanowires. Nanowires can be grown by the vapor-liquid-solid (VLS) growth mechanism, which was first demonstrated by Wagner and Ellis,([15]) in which metal catalysts act as energetically favorable sites for the nanowires' growth. The catalyst particles rise up as the nanowire material precipitates from them. This method has been adopted recently to fabricate germanium nanowires by using gold nanoparticles as the seeds.([6-8]) Because of the structural instability that results from their high surface energy, an oxide sheath is always found to saturate the dangling bonds on the surface of the sp(3)-bonded covalent nanowires.([6,9,10]) It has been found that when the silicon nanowires are annealed in hydrogen gas (in order to remove the oxide sheath) the bare nanowires tend to agglomerate.([11]) Herein, we report a procedure for growing porous germanium thin films by high-density, inductively coupled plasma chemical vapor deposition (ICPCVD). Compared to other processes (see Table S1 in the Supporting Information),([6,8,12])