Organic Vapors Sensor Based on Dangling Bonds of Porous Silicon

In this paper, a porous silicon (PS) layer is investigated as a sensing material to detect the organic vapors with low concentration. The structure of the prepared sensor consists of thin Au /PS/n-Si/Au thick where the PS is etched photo -chemically. The current response of the sensor is governed by the partial depletion of silicon located between two adjacent (porous regions).This depletion is due to the charges trapped on dangling bonds associated with the silicon – porous silicon interface


Introduction
Although porous silicon was first discovered by Uhlir [1] in 1956, significant interest in the material is shown after the observation at room temperature of photoluminescence properties by Canham [2] .The porous silicon /silicon substrate junction has been used for sensing applications as gas sensors based upon the increase in current due to the dipole moment of the gas [3][4][5], and a humidity sensor based upon the changing current with humidity [6].In this paper , the current response of thin Au /PS/n-Si/Au sandwich structure in the presence of organic vapors such as ethanol and methanol which quantitatively discussed based upon the existence of the dangling bonds(DB) associated with the porous silicon

Experimental
The porous layer was prepared by photochemical etching process of a mirror-like (111) oriented silicon wafer of resistivity of 0.22 -0.38 ohm.cm in an HF (40%) with etching time of 35 min under 33.5W/cm 2 optical etching power density of tungsten halogen lamp.More details of this process can be found elsewhere [7,8]  The current-voltage characteristics was studied out by evaporating the back side of the sample ohmic contact using a thick gold electrode.On the top etched side of the sample , thin gold thin film of 10-15nm thickness we evaporated directly on the porous layer in order to produce (Schottkylike) diode.Fig. (1) shows, the measurement setup for vapor sensing .As shown in this figure , the gas vaporized from diluted organic solutions was warmed by a heater and was injected through a tube into a chamber with N 2 gas as a carrier gas .Every measurement was carried out after 60 sec exposure time to the gas .The I-V measurements were carried out using fine dc power supply and 616 kethily digital electrometer .4) and ( 5) show, the current response of the sandwich structure for 0 -5 V bias voltage against ethanol and methanol vapors evaporated from 0.2-0.4% solutions concentrations.The figures show a nonlinear shape with small slope below 2 volt , by increasing the applied voltage above this value the current increased rapidly .Such rectifying nature in the current response is caused by the Schottky junction between the Au thin film and the etched surface and this junction has a large value of the series resistance [9] and one can easily distinguish, that the current response PS sensor of ethanol is higher than that of methanol , this is probably due to that the silicon is a good absorber for the ethanol than methanol and this will lead to efficient passivation of the dangling bond of the PS [4,10,11].The electrical behavior of the sensor is analyzed with the aid of the model described by Stievenard and Deresmes [ 12], in which the absorption of the vapor is mainly occur by the Si-H bonds (dangling bonds).Based on the SEM

Voltage(v)
Current( A) 0.20% 0.40% micrographs of the etched surface , the distance of the crystalline silicon between two porous regions(d) is in the range from 5 to 12µm since the resistivity of crystalline silicon is much less than that of the porous silicon [ 11], therefore the current will pass in a zig-zag way within the crystalline silicon regions instead of the porous regions.At the interface between the silicon and the pore , there is a thin layer of silicon dioxide and the associated interface states , having a density δ(cm -2 ).Due to the charges trapped on the interface states , there is a depleted region in the crystalline silicon channel over the distance (W) .By assuming that the situation is symmetric and depending on the value of (w) , there is a central channels of width (d-2w), where d represent of the distance between two adjacent porous regions .The charge carriers can move when a potential is applied on the sandwich structure .The integration of Poisson , s equation leads to a simple relation between (w) and(δ) given by [12].
where N D is the doping concentration in the silicon wafer .Therefore the dc current can be controlled by δ.The effect of the gas is to passivat the active dangling bonds , perhaps through a screening mechanism, so that (W) decreases and the width of the channel increases .The density of the dangling bonds from electron paramagnetic (EPR) measurements [ 13]is about 20 10 DB/cm -3 and as the developed surface area of the PS layer of order 600m 2 /cm 3 , therefore the density of the dangling bonds is of order of 12 10 --- 13  10 cm -2 [2].As the initial doping concentration of the silicon wafer is of 2x 16 10 cm -3 , there are enough dangling bonds to passivate the free charge carriers .In our samples , (d )is in the range from 5 to 12µm.Using equation ( 1), we find that (w) is of order of few micron , so that the channel can be easily pinched

Conclusions
Based on the IV characteristics of the porous silicon sandwich structures, the Photosynthesized porous silicon which is prepared by the photochemical etching can be used as a sensor for low concentrations of an organic vapor in the range 0.2---0.4% .This study depends on the interaction of the absorbed vapor and the dangling bonds in the porous network.