ARTICLE IN PRESS
Microelectronics Journal 39 (2008) 1354–1355 www.elsevier.com/locate/mejo
Porous silicon thin film as CO sensor H.M. Martı´ nez, N.E. Rincon, J. Torres, J.E. Alfonso Grupo de Materiales con Aplicaciones Tecnolo´gicas-GMAT, Universidad Nacional de Colombia, A.A. 14490 Bogota´ DC, Colombia Available online 6 March 2008
Abstract This work presents the electric behavior of porous silicon (PS) thin films when the material’s surface is exposed to carbon monoxide. PS thin films were fabricated by the electrochemical anodization method of Si–c (1 0 0) substrates with resistivity 0:01 O cm. The samples were prepared at 20 min anodization time and 5 mA=cm2 anodization current. Aluminum electrodes were deposited on the surface of the material by high vacuum evaporation, such that the electric conduction was parallel to the substrate’s surface. The detector was placed in vacuum during 1 h and then CO was allowed into the vacuum chamber. Measurements of the I2V characteristic were carried out at atmospheric pressure, in vacuum and with CO. Changes in the resistance of the material, of about MO, were observed in the different samples, indicating that the material is sensitive to the presence of CO and therefore suitable as gas sensor. r 2008 Elsevier Ltd. All rights reserved. PACS: 72.20.Ee; 71.23.Cq; 73.40.Cg Keywords: Porous silicon; CO; Gas sensor
Porous silicon (PS) is a material with ample possibilities to be incorporated as optoelectronic device in integrated circuits using silicon technology, thanks to its intense photoluminescence signal in the visible region [1]; this material has been studied to obtain the material’s optical constants [2]. Other scientific studies have shown that PS is suitable for gas sensor and among many other applications [3]. The electric behavior of PS thin films prepared by electrochemical anodization, in the presence of carbon monoxide, is evaluated in this work. PS thin films were fabricated by electrochemical anodization of single crystal type p (boron) (1 0 0) silicon substrates with resistivity 0:01 O cm. A mix of hydrofluoric acid (HF) 10%) ðv=vÞ þ isopropyl alcohol (IPA) at 90% (v/v) was used as an electrolyte. A film of Al was deposited on back of the silicon by high vacuum evaporation, before the formation of the PS films. The PS samples were prepared using a 20 min anodization time and current densities of 5 and 15 mA=cm2 . After the anodization process the samples were rinsed twice with IPA, then rinsed with distilled de-ionized water; the water was removed by rinsing with acetone Corresponding author.
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[email protected] (H.M. Martı´ nez). 0026-2692/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.01.035
and finally the samples were allowed to dry out in the open. Aluminum electrodes were deposited on the PS surface by high vacuum evaporation at room temperature and base pressure of 2 105 mbar. The electrodes were made in such a way that the electric conduction is carried out parallel to the substrate surface and the distance between electrodes was reduced as much as possible by using a comb shaped configuration The samples were characterized through I2V measurements at atmospheric pressure, in vacuum at pressures of about 2 mbar and in CO. The electric characterization process was as follows: initially the I2V characteristic of the sample was obtained at atmospheric pressure; next the pressure was lowered to around 2 mbar and the I2V response was taken again. Finally the cryostat was closed and CO injected until reaching the atmospheric pressure. At this point the I2V response was taken once more, as well as 20 and 40 min later. A second test consisted of measuring the device’s resistance as a function of time using polarization voltage. Initially the chamber was evacuated down to 2 mbar; once that pressure was obtained CO was injected until reaching around the atmospheric pressure and then the value of the resistance was measured in the times.
ARTICLE IN PRESS H.M. Martı´nez et al. / Microelectronics Journal 39 (2008) 1354–1355
5 mA/cm2; 10% HF 15 mA/cm2; 40% HF
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Fig. 1. I2V characteristic for a porous silicon sample with aluminum electrodes. 10% HF and 5 mA=cm2 .
Fig. 2. Resistance for porous silicon samples with 5 mA=cm2 , 10% HF and 15 mA=cm2 , 40% HF.
Fig. 1 presents the sample’s electric responses (I2V characteristics) at atmospheric pressure, at 2 mbar and in CO atmosphere. Additionally, it shows the 20 and 40 min curves after CO was injected. This sample’s I2V response is characteristic of a Schottky type juncture. The behavior of the sample at atmospheric pressure is very similar to its behavior at the indicated vacuum, indicating that the air contained in the sample does not affect its electric behavior. A current increment for positive voltages is observed when CO is injected; this current keeps rising as the contact time between sample and CO increases. The current increased approximately one order of magnitude for 40 min after CO injection, compared to its value for the sample in vacuum. CO molecules passivated the PS structure by reducing on the material’s skeleton surface the great density of imperfections acting as traps for the circulating electrons. The response to the CO is more noticeable as exposure time increases since PS structure is very complex, packed with imperfections and tunnels such that CO takes a long time to reach deep inner locations. The response to CO for samples prepared at anodization currents greater than 10 mA=cm2 (high porosity) was low, possibly because of a slide on the PS skeleton due to high tensions suffered by the material in the drying process. The
resistivity change of the PS film is low in this case, which makes very difficult to detect any change of it when the sample is in contact with the gas. Fig. 2 presents the change with time for the resistance when the PS sample was exposed to carbon monoxide in the cryostat at a pressure of about 2 mbar. The resistance decreases exponentially with the exposure time. Resistance changes of up to an order of magnitude were found for PS thin films when their surfaces were exposed to carbon monoxide, indicating the suitability of this material as CO sensor. Response times for the samples are long, suggesting that more studies should be performed to lower this parameter. We thank COLCIENCIAS and the DIB from the Universidad Nacional for their economic support to carry on this work. References [1] L.T. Canham, Appl. Phys. Lett. 57 (1995) 313. [2] J. Torres, F. Castillejo, J.E. Alfonso, Brazilian J. Phys. 36 (2006) 1021. [3] Z.H. Mkhitaryan, A.A. Shatveryan, V.M. Aroutiounian, M. Ghulinyan, L. Pavesi, L.B. Kish, C.G. Granqvist, Physica E 38 (2007) 160.
