Porous silicon thin film gas sensor
- PDF / 407,096 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 53 Downloads / 263 Views
Porous silicon thin film gas sensor I. Ferreira, E. Fortunato and R. Martins, Departamento de Ciência dos Materiais, CENIMAT, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa and CEMOP/UNINOVA, Quinta da Torre 2828-114 Caparica, Portugal.
ABSTRACT The performances of amorphous and nano-crystalline porous silicon thin films as gas detector are pioneer reported in this work. The films were produced by the hot wire chemical vapour deposition (HW-CVD). These films present a porous like-structure, which is due to the uncompensated bonds and oxidise easily in the presence of air. This behaviour is a problem when the films are used for solar cells or thin film transistors. For as gas detectors, the oxidation is a benefit, since the CO, H2 or O2 molecules replace the OH adsorbed group. In the present study we observe the behaviour of amorphous and nano-crystalline porous silicon thin films under the presence of ethanol, at room temperature. The data obtained reveal a change in the current values recorded by more than three orders of magnitude, depending on the film preparation condition. This current behaviour is due to the adsorption of the OH chemical group by the Si uncompensated bonds as can be observed in the infrared spectra. Besides that, the current response and its recover time are done in few seconds.
INTRODUCTION Several authors demonstrate the possibility to use crystalline porous silicon (PS) for the detection of vapors from organic compounds such as ethanol at room or higher temperatures [1,2,3]. Other materials like ZnO [3], SnO2 [3], Bi2(MoO4)3 [4] have been also tested as ethanol gas sensor. Nevertheless, they are sensitive only at temperatures around 400-600ºC. As far as doped or undoped films produced by hot wire chemical vapor deposition technique it concerns, in certain deposition conditions a porous-like film structure is obtained [5]. In these films it was observed the presence of a strong absorption Si-O vibration mode (around 1050-1150cm-1) after the films being exposed to atmospheric pressure. That information led us to conclude about a sponge-like film structure with a high density of uncompensated Si bonds where oxygen or hydroxyl groups are adsorbed. By etching the film with an HF:H2O solution it was confirmed that the Si-O vibration modes disappear, appearing again later on [6]. From that it emerges the idea of possible alcohol detection, as it happens in this work, where we show the possibility to use porous silicon thin films produced by HW-CVD technique as ethanol vapor detector with high sensitivity at room temperature. EXPERIMENTAL The porous silicon doped and undoped thin films were obtained by HW-CVD technique. Details concerning the deposition system used to produce the films analyzed in this work are described elsewhere [5]. Silane gas diluted in hydrogen in the proportion of 5sccm (SiH4):500sccm (H2), a total gas pressure of 0.5Torr, filament temperature (Tf) of 1850ºC and a A26.7.1
substrate temperature of 200ºC was used to the production of intrinsic porous Si:H
Data Loading...
