Sensing Properties of WO 3 as Thin Film prepared by PVD Towards Ethanol Vapor

: Three thin films were prepared by PVD starting from WO 3 powder with thicknesses (998.7, 1620, 2240 nm) respectively on glass substrates under limited thermal and pressure conditions, studied their I-V characteristics and calculated the sensitivity for 100 ppm of ethanol vapor adsorption (The temperature of the films have been changed from 25°C to 350°C). A comparison among them was achieved at 300  C as an operating degree and found that the 1620 nm WO 3 has more sensitivity and has more power to adsorb for ethanol vapor on it.


INTRODUCTION
Thin film considers the most important invention in twenty century cause we use this technique to prepare and study the materials in micro or even in nano dimensions; without any errors depends on crystal defects or that huge surface of a bulk material….etc, in addition ; low cost; friend of environment and small size, all of these give thin film a rapid extensive in common photoelectrical and electronic industry.
By the time thin film get more attention specially in semiconductor in several technical and industrial application like as: corrosion protection, optical filter, anti-reflection coating, solar cells, anti-scratched layer, and sensors [1].

Nano-Technology:
Nanotechnology studies the molecules and compounds with dimensions not more than 100 nanometer as a grain size and has an accurate control in producing these materials and in inner reaction to direct these molecules to get a desired substance.
Nanotechnology gets a huge antecedence as the most cleaned produced technology for decreasing the industrial garbage, less industrial pollution, and more efficiency in all of electronic, agricultural, medicine, water treatment, and environment industries [2] [3].
Gas sensors considers as one of the best output of nanotechnology specially that made from metal oxide [4,5] for their properties like: sensitivity, working at high temperature (200-400C), good chemical flexibility, low cost, small size, low energy desperation, low limit detection (a few of ppm), stability, in addition that we can make it to detect more than one gas at the same time [6], so, we find that it is important to study sensors and their sensitive properties for organic vapors which that is very important in many industrial fields [7].

Principal of sensors working:
The principal of sensors working depends on a variation of resistance or electrical conductivity for metal oxide when some of pollutant gas particles affection on it [8,9] Gas sensors depend on physical or chemical adsorption and desorption reactions on the surface of the sensor material which caused some physical unique variation that is detected by series of several principals like conductivity, electrical polarization,….etc. The aim of our research is to prepare thin film of WO 3 by PVD, and study of its I-V characteristics in air and in 100 ppm of ethanol vapor by using KEITHLEY 237, then find which thickness has the best sensitivity to ethanol vapor.

EXPERIMENTAL PROCEDURE 2.1. Substrate and cleaning procedure [10]:
We used glass substrates Soda-lime that has 13% Na 2 O which dissolved in water and converting to NaOH as a thin film making a collapse in preparing procedure so we have to etch this film by washing the substrate with distilled water and apply the cleaning procedure as follows: 1. Put the substrate in 3% solution of normal detergent ultrasonically 5-7 min. 2. Immerse it in HCl 2M for 15 min 3. Washed with distilled water for 20 min 4. Dry it by hot air for 5 min

Preparation of thin film by PVD:
Three types of thin films prepared by PVD (Elltroava, Italy) with thickness 998.7, 1620, 2240 nm respectively, so we put a cleaned glass substrate in its location in the apparatus and vacuumed the chamber into 5х10 -4 mbar then we evaporated the substance to get a desired thickness. Table .1 shows the thicknesses and preparation conditions:

Method of deposition
The temperature during process The pressure in the chamber The rate of evaporation We choose evaporation rate 32 A/sec as a suitable velocity, and 400C as a temperature of the substrate for deposition to eliminate all any other factors may cause problems on the prepared film.

RESULTS AND DISSECTION : 3.1. I-V Characteristics for prepared films:
The conductivity measurements are carried out in a pressure-controlled chamber during heating the thin films in a gas flow apparatus equipped with an external controlled heating facility. The resistances of the thin films are measured by the two-probes method with sliver-copper electrode deposited on the films by chemical painting method. The nature of the contact is verified to be ohmic by I-V Characteristics. A thermocouple was attached to the thin films holders for monitoring and controlling the operation during conductance measurements. The resistance of the sensors in the presence of either pure air (R air ) or the different pollutants (R gas ) at different concentrations is monitored and stored in a PC. All films prepared from powdered metal oxides used underwent for the same conditions and the measurement of electrical parameters applied to the device KEITHLEY 237using the following parameters which shown in the table (2).

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ILCPA Volume 58 This way was used from the other for many materials to measure the sensitivity for humidity, gases, and vapor for many organic solvents [11,12].
The I-V characteristics for each prepared film is studied in air and in 100 ppm of ethanol vapor at different temperature ( 25-350 C) drawn as follows: We notice from the previous figures that all thin films are affected by change of applied temperatures during the measurements, and have an ohmic resistance, as increasing the temperature as increasing the current from A at 25C to a few parts of mA at 350C, and from these figures we can calculate the resistance in air and in 100 ppm of ethanol vapor as follows:   We notice that the sensitivity of WO 3 thin film which has 1620 nm as a thickness was increasing by increasing of temperature to reach a high value 458.72% at 300 C then it was decreasing by increasing of temperature so, we can say that a 300 C is an operating degree for this film, but the others WO 3 ( 998.7 and 2240 nm ) have as increase of temperature as increase of sensitivity till 350°C but not over 100%.
We found that the 1620 nm WO 3 thin film has a high sensitivity where the thickness, and the defects play a big role in sensitivity, so we get more defects on the surface at 1620 nm and these defects decrease by increasing the thickness to 2240 nm [14].

CONCLUSION:
Three thin films were prepared by PVD starting from WO 3 powder ( 998.7, 1620, 2240 nm), studied the I-V characteristic and calculated the sensitivity at 100 ppm of ethanol vapor and in comparison among them at 300 C as an operating degree and found that the 1620 nm WO 3 has more sensitivity and power to adsorb an ethanol vapor on it.