H2-mediated reduction of GeO2 and chemical vapor deposition of polycrystalline Ge porous films

Abstract

Hydrogen reduction of metal oxides has received considerable attention owing to no direct emissions of pollutants or greenhouse gasses. In this study, the reduction behavior of GeO2 in H2 flow was investigated in the temperature range 800 K to 1200 K in order to grow Ge films/wires by chemical vapor deposition (CVD) technique at 723 K. A slight oxide reduction was observed at 800 K due to sluggish reaction kinetics. The mass loss from GeO2 increased and exceeded that corresponding to the removal of O2 in GeO2 as the temperature was raised to 1200 K. The excessive loss was attributed to the formation of gaseous GeO species predicted by the thermodynamic calculation. At 900–1000 K, the GeO2 reduction to Ge was nearly complete, close to the thermodynamic prediction. The experimental losses were higher than the predicted ones at 1100 and 1150 K. The discrepancy was explained by a mass-transport theory involving a boundary layer in the gas phase over the powder bed. At 1200 K, the reduction process was close to the equilibrium. It was found that the Ge-containing species in-situ generated by the upstream reaction of GeO2 with H2 at 1100 K and 1200 K sufficed to grow polycrystalline Ge porous films on a Si (400) substrate held downstream at 723 K. Ge wires were also obtained on the Si substrate that was initially covered with an Au film. This exploratory study demonstrates that polycrystalline Ge porous films and wires can be deposited on a Si substrate by CVD technique using environmentally friendly H2 and solid GeO2 precursors.