AVS 46th International Symposium
    Thin Films Division Thursday Sessions
       Session TF-ThM

Paper TF-ThM7
Deposited Porous Silicon on Insulator Substrates

Thursday, October 28, 1999, 10:20 am, Room 615

Session: Nanophase Thin Films
Presenter: S.J. Fonash, The Pennsylvania State University
Authors: A.K. Kalkan, The Pennsylvania State University
S.H. Bae, The Pennsylvania State University
H. Li, The Pennsylvania State University
S.J. Fonash, The Pennsylvania State University
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High porosity crystalline Si thin films have been directly deposited using a high density plasma approach at temperatures as low as 100°C. These films exhibit the same unique properties, such as visible luminescence and gas sensitivity, that are seen in electrochemically etched Si (i.e., porous Si). The ultimate advantage of our low temperature direct deposition approach is that now porous Si films can be obtained on any substrate including plastics. XRD identifies our films, deposited by ECR-PECVD, as crystalline with no preferential orientation. TEM shows our as-deposited porous Si films consist of a periodic array of uniformly sized rodlike columns normal to the substrate surface in a void matrix. A typical rod diameter is 80 Å and a typical rod separation is 30 Å. Unlike other previously obtained columnar films, these rodlike columns are not tapered but have a constant diameter. We have demonstrated this structure is fully controllable and have varied the porosity up to ~80% by varying the deposition conditions. We have also found that the porosity can be further increased by reducing the diameter of the Si columns by hydrogen ECR-plasma etch exposures after deposition. Red and orange photoluminescence has been observed from our high porosity films. Furthermore, a large and fast conductivity response to certain vapor or gas ambients has been found. In particular, in the case of humidity, a weak response was found up to a threshold humidity level. Above this level a steep exponential increase in conductivity of 3-4 orders of magnitude was observed. The onset of this steep increase was found to occur at higher humidity levels as the porosity is increased. A saturation is observed at relative humidity levels above 75%. Sensitivity for acetone and isopropyl alcohol was observed also for films in the higher porosity range. This implies the sensitivity for larger molecules may be enabled by increasing the void size.