Nanocrystalline Si thin films with arrayed void-column network deposited by high density plasma

High porosity nanocrystalline Si thin films have been deposited using a hig h density plasma approach at temperatures as low as 100 degrees C. These fi lms exhibit the same unique properties, such as visible luminescence and ga s sensitivity, that are seen in electrochemically etched Si (i.e., porous S i). The nanostructure consists of an array of rodlike columns normal to the substrate surface situated in a void matrix. We have demonstrated that thi s structure is fully controllable and have varied the porosity up to simila r to 90% (as derived from optical reflectance) by varying the deposition co nditions. In particular, the impact of plasma power has been found to reduc e porosity by increasing the nuclei density and therefore the areal density of columns. Humidity sensors have been demonstrated based on the enhanced conductivity of our films (up to 6 orders of magnitude) in response to incr ease in relative humidity. Depending on the porosity, the conductivity-rela tive humidity behavior of our films shows variations which can be correlate d with the nanostructure. Also, these variations indicate that the dominant charge transport is limited by the dissociation of water into its ions at the column surfaces. (C) 2000 American Institute of Physics. [S0021-8979(00 )07813-0].