Gas Sensor Using an Aluminium-Porous Silicon Junction Application to the Detection of Non-Zero Molecular Dipole Moment
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INTRODUCTION Since the publication of visible light emission out of porous silicon1 , a lot of studies have been devoted to this material, mainly on its optical properties. Only a few studies have investigated its electrical properties" . The results show that the conductivity is governed by the great concentration of surface states. Moreover, the dc characteristics is sensitive to moisture or methanol9' 10 and porous silicon can be used as a gas sensor. In this paper, we present the electrical behaviour of an aluminium-porous silicon junction versus the presence of a series of gases and we propose a simple model to explain quantitatively the observed phenomenon. Moreover, EPR measurements are used to study the interaction between dangling bonds and the adsorbed gas. EXPERIMENTAL RESULTS
Sample The porous layers were prepared from 2 inch diameter (100) oriented silicon wafers, B doped (8x10 18 cm"3) by electrochemical dissolution in an HF (40%) : ethanol (1:1) electrolyte under currents of 20 mA/cm 2 . The porosity was 45 % and the thickness was equal to 30 4im. On the back side, an ohmic contact was obtained using a p+ doped substrate. On the top side, we 599 Mat. Res. Soc. Symp. Proc. Vol. 358 ©1995 Materials Research Society
evaporated an aluminium pad (S=2xl0-3 cm 2 ) directly on the porous layer in order to produce "Schottky-like" diodes. Before the evaporation, the surface was cleaned by an HF treatment. The other electrical parameters (series resistance, ideality factor and current/voltage characteristics) are detailed elsewhere 8 . Response of the junction to moisture Figure 1 gives the variation of the dc current versus time for a relative humidity (RH) equal to 11 %, 54 %, 75 % and 100 %, measured at room temperature. The junction is under direct polarization (1 Volt). In any cases, the sensitivity of the current to gas is greater under forward bias than under reverse bias. The amplitude of the current before the excitation is of the order of a few nA. This low value is bound to the thickness of the porous layer and to the the great concentration of dangling bonds in the material. passivation of the free carriers 1 by 2 Indeed, our EPR measurements show a typical concentration of dangling bonds of 1012_10L3 cm-2 . We observe that, for a given RH, the current follows a transient, which is not exponential and a saturation regime is risen after typically 200-250 s. When the moisture is removed, we observe a decrease of the current. As for the excitation, the transient is not exponential. A plateau is reached after a typical time of 400 s. Finally, the phenomenon is reversible.
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Time(9) Figure 1: Response to moisture of an aluminium-porous silicon junction Response of the junction to ammoniac, acetone and propanol Figures 2 and 3 give the the response of the junction (excitation and desexcitation respectively ) to ammoniac, acetone or propanol. In each case, before the excitation, the cur
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