D . C conductivity of In 2 O 3 : SnO 2 thin films and manufacturing of gas sensor

Compounds were prepared from In2O3 doped SnO2 with different doping ratio by mixing and sintering at 1000oC. Pulsed Laser Deposition PLD was used to deposit thin films of different doping ratio In2O3: SnO2 (0, 1, 3, 5, 7 and 9 % wt.) on glass and p-type wafer Si(111) substrates at ambient temperature under vacuum of 10-3 bar thickness of ~100nm. X-ray diffraction and atomic force microscopy were used to examine the structural type, grain size and morphology of the prepared thin films. The results show the structures of thin films was also polycrystalline, and the predominate peaks are identical with standard cards ITO. On the other side the prepared thin films declared a reduction of degree of crystallinity with the increase of doping ratio. Atomic Force Microscopy (AFM) measurements show the average grain size exhibit to change in non-systematic manner with the increase of doping ratio with tin oxide. The average grain size increases at doping ratios 1, 5 and 7 % from 52.48 to 79.12, 87.57, and 105.59 nm respectively and decreases at residual doping ratio. The average surface roughness increases from 0.458 to 26.8 nm with the increase of doping ratio. The gas sensing measurements of In2O3:SnO2 thin films prepared on p-Si to NO2 gas showed good sensitivity and Maximum sensitivity (50) obtained for In2O3:SnO2 prepared on p-Si at operating temperature 573 K and doping ratio 7 % and 9 %. Maximum speed of response time (8 sec) at operating temperature 573 K and doping ratio 1 %.


Introduction
One of the most important semiconductors is the so-called transparent conductive oxides (TCO), which are compound semiconductors composed of Oxygen combined with metal (i.e semiconductor oxides).Transparent Conducting Oxide (TCO) films have been used extensively in the optoelectronics industry because they exhibit high electrical conductivity, high optical transmittance in the visible region, and high reflectance in the infrared (IR) region [1].Although most research on TCO materials has been focused on the above oxides, there have been some efforts on making multicomponent oxides to improve the electrical conductivity and optical transparency of the films, such as In 2 O 3 -ZnO, In 2 O 3 -SnO 2 , Ga 2 O 3 -In 2 O 3 [2].In 2 O 3 :SnO 2 (ITO) thin films have been receiving significant attention in several applications due to their attractive properties such as high transmittance in visible region and unique electrical conductivity, which originates from its n-type highly degenerate semiconductor behavior with a wide band gap in the range between 3.5 and 4.3 Ev [3,4].ITO has been widely applied in various optoelectronic devices such as photovoltaic cells [5], liquid crystal displays [6] and gas sensors [7].In 2 O 3 -SnO 2 is the most used n-type semiconductor in gas sensing devices because of its capabilities to detect inflammable gases like CH 4 , H 2 , C 2 H 5 OH, CO and so on [8].Besides, Indium Tin oxide (In 2 O 3 :SnO 2 ) nano composites that exhibited superior thermal stability against grain growth have been reported [9].It is difficult to control the size and morphology of the oxide composites, which have important influence on their physical and chemical properties.There are many deposition techniques to obtain high quality ITO films such as pulsed laser deposition [10], sol-gel [11], RF and DC sputtering [12,13].
The current research devoted with preparation of ITO films by PLD on glass substrate and The morphology, structural and composition analysis of ITO films were examined and the sensing properties of un-doped In 2 O 3 and doped with different concentrations (1, 3, 5, 7 and 9) % of SnO 2 films deposited on p-type silicon wafer(111) are examined as a function of operating temperature and time to find the temperature dependence of the sensitivity for oxidizing gas (NO 2 ).

Experimental part
In 2 O 3 :SnO 2 is were prepared by quenching technique.Takes appropriate amount the of high purity (99.99)Indium oxide powder and doping with different percentages of tin oxide 99.9 % (are weighed using an electronic balance with the least count of (10 -4 gm) and put in a quartz ampoule (length ~ 25 cm and internal diameter ~ 8 mm) are heated to 1000 °C and let at this temperature for 8 hours.The temperature of the furnace was raised at a rate of 10 °C/min.During heating the ampoules are constantly agitated .This is done to obtain homogeneous compound.Si wafer cut in small pieces (1x1) cm 2 .Cleaning glass slides and Si wafer substrates were used which were subjected to several steps to remove any contamination such as dust, oily material, grease and some oxides using soap solution, then the glass slides and Si were placed in a clean beaker containing distilled water and with ethanol solution then the glass slides and Si were dried by blowing air.Thin films were deposited using pulsed lased deposition technique under vacuum of (10  6) Applied voltage: 220 V.The laser beam which coming from a window is incident on the target surface making an angle of 45° with it.The substrate is placed in front of the target with its surface parallel to that of the target.The structure of the prepared alloys and thin films was examined using X-ray diffraction (XRD).The present work x-ray diffractrometer type (Miniflex II), with Cu-K α x-ray tube (λ = 1.54056Ǻ) is used.The resistivity of pure In2O3 and In 2 O 3 :SnO 2 thin films with different composition ratios prepared on glass substrate estimated by DC measurements after depositing metal electrodes (Al) on the samples using appropriate masks.The method comprises a temperature controller oven.The films glass samples are heated in the oven from room temperature up to 473 K with step of 298K.Electrical resistance is then measured directly with digital electrometer.The resistivity is conventionally calculated from measured electrical resistance.The activation energy is calculated using equations.
The resistivity (ρ) of the films is calculated using the following equation: where R is the sample resistance, A is the cross section area of the films and L is the distance between the electrodes.The conductivity of the films was determined from the relation: ( The activation energies could be calculated from the plot of ln σ versus 1000/T according to equation where σ o is the minimum electrical conductivity at 0 °K, E a is the activation energy which corresponds to (E g /2) for intrinsic conduction, T is the absolute temperature and k B is the Boltzman's constant equal (8.617×10 −5 eVK -1 ) .By taking (Ln) of the two sides of equation we can get: From determination of the slope we can find the activation energy The conductivity type of the thin films is deduced using Hall measurement.Hall Effect measurements have been used in determining majority carrier concentrations, type of carrier and their mobility in thin film materials.The values of carrier concentration (n H ) and Hall mobility (μ H ) were calculated using equations: where RH is Hall coefficient, VH is Hall voltage, t is the sample of thickness, I is constant current, σ is conductivity, and B is magnetic field.
In this work, the gas responsivity tests performed at different operation temperature beginning from (room temperature, 373, 473 and 573) ºC.The time taken for the sensor to attain 90 % of the maximum increase in resistance on exposure to the target gas is the response time.The time taken for the sensor to get back 90 % of original resistance is the recovery time.The test was performed at various sensing temperatures with 6 V bias voltage.The sensitivity factor (S %) at different operating temperatures is calculated using equation: for oxidizing gases.

Results and discussion 1-Structural properties
The diagram of the X-ray diffraction spectra of In

3-The Electrical properties 3-1 D.C conductivity
Fig. 3 shows the variation of Ln(σ) with reciprocal temperature of In 2 O 3 thin films prepared with different doping ratios of SnO 2 .From these figures it is evident that than are more than one conduction mechanism and hence more than one activation energy which reflects the polycrystalline structures of the prepared thin films.The activation energies were estimated according to Eqs. (3 and 4) and listed in Table 1.Indeed two activation energies can be observed for the pure and doped samples with 1 %, 3 %, 7 % while three activation energies for (5 and 9 %).On the other hand is clear that the conductivity decreased as SnO 2 added to the host material but then get to rise.The continues addition of SnO 2 lead to reduce the conductivity.The activation energy exhibit to change in reverse manner to that of conductivity.This results was expected since the activation energy is half of energy gap and can be attributed to same reasons i.e. the increment of the activation energy related to the creation of new states in the band gap which lead to visual reduction of energy gap while the reduction of activation energy is related to compensation effect of the added dopant.According to Davis and Mott model 1979 [15] the tails of localized states should be rather narrow and extend a few length of tenths of an electron volt into the forbidden gap, and further more thus suggested of localized levels near the middle of the gap.This leads to different channels of conduction: Ea 1 is the activation energy required to transport electron from Fermi level to the extended states above the conduction band edge, Ea 2 is the activation energy required to transport electron from Fermi level to the localized below the conduction band edge.The increasing of doping ratio has significant effect on of the number on transport mechanisms of the In 2 O 3 : SnO 2 system.The variation of Ea of In 2 O 3 : SnO 2 thin films with doping ratio is illustrated in Table 1.It is clear from this table that the activation energies change in non regular manner with the increase of doping ratio.Indeed E a1 and E a2 increases as the dopant material added to the host system but then decreases with further addition of SnO 2 .The decreasing of activation energy with the increase of thickness is resulting from the effect of reduction of energy gap which in turn reduces the energy requires to transport the carriers from Fermi level to the conduction band.The appearance of third transport mechanism and third activation energy is related with reduction of degree of crystallinity which consequently reduces the grain size.It is clear from these figures that the resistance decrease with gas on for (pure and doped samples with 1% SnO 2 ).These figures show decreasing in the resistance value when their films expose to NO 2 gas, (Gas on), then the resistance value back upward at the closure of the gas (Gas off).While the doped samples with (3, 5, 7 and 9) wt.% SnO 2 show an increase of the resistance with (gas on).

4-1 Effect of operation temperature on the sensor
Table 3 shows the operating temperature dependence of sensitivity for pure In 2 O 3 and doped with different concentrations of SnO 2 , which are deposited on p-type silicon substrates.It is clear that the sensitivity change in non regular manner with both doping ratio and operating temperatures.In general the sensitivity increases with the increase of doping ration for temperatures 473 and 573 K while the sensitivity return to fall for low operating temperatures, i.e.R.T and 373 K.It is obvious the sensitivity values are lower that these values for gas sensors deposited on n-Si substrate.On the other hand it is clear that the sensitivity of the films increase with increasing of the operating temperature.Maximum point values for In 2 O 3 films doped with the ratio (7 and 9) % SnO 2 are seen at temperature of (573K) which known as optimal temperature.
At the optimal temperature, the activation energy may be enough to complete the chemical reaction.There is an increase and decrease in the sensitivity indicates the adsorption and desorption phenomenon of the gas.

4-2 Response and recovery times
Table 4 shows the relation between the response time and the recovery time with different Tin oxide doping ratios at different operating temperatures of the undoped and doped In 2 O 3 thin films.From the table can be observed increasing response time with increased of doping ration at room temperature, while it get to reduce at high operating temperatures (373, 473, and 573 K).Also the recovery time have the same manner, i.e it get to increase with doping ratio for low operating temperature i.e (R.T and 373 K) while it decreases with doping ratio for high operating temperatures (473 and 573 K).The previous figure shows that the 1% SnO 2 doped samples exhibits a fast response speed (8 s) with recovery time (30 s) at 573 K this referred that a low doping ratio is the best to achieve fast response sensor.The quick response sensor for NO 2 gas may be due to faster oxidation of gas.
2 O 3 thin films deposited on glass substrates prepared by the PLD technique with different doping concentrations of SnO 2 (1, 3, 5, 7 and 9) wt.% showed in Fig.1.All the peaks of XRD patterns were analyzed and indexed using JCDD data base and compared with standards (JCPDS-#06-0416).XRD diffractograms revealed that In 2 O 3 :SnO 2 films become polycrystalline when deposited at room substrate temperature and crystallize in a cubic bixbyite structure (In 2 O 3 ).The peaks observed for (2θ =30.4 o ) associated to the plane (222) and other planes related to the ITO system likes (400) (440) and (622).The preferential growth of the In 2 O 3 :SnO 2 films is the (222) plane and this orientation should be dependent on the deposition conditions.The similar result was reported using an evaporation method [14].

Fig. 2 :
Fig.2: AFM images for of pure In 2 O 3 thin film and In 2 O 3 doped with SnO 2 at different doping ratio.

Fig. 4 :
Fig. 4: Variation of resistance as a function of time for pure In 2 O 3 films deposited on p-Si at different operating temperatures.

Fig. 5 :Fig. 6 :
Fig. 5: Variation of resistance as a function of time for In 2 O 3 films deposited on p-Si doped with 1 wt %.SnO 2 ratio at different operating temperatures.

Fig. 7 :Fig. 8 :
Fig. 7: The variation of resistance as a function of time for In 2 O 3 films deposited on p-Si doped with 5 wt %.SnO 2 ratio at different operating temperatures.

Fig. 9 :
Fig. 9: Variation of resistance as a function of time for In 2 O 3 films deposited on p-Si with 9 wt %.SnO 2 ratio at different operating temperatures.

Conclusions 1 - 2 - 4 -
The prepared In 2 O 3 :SnO 2 alloys and thin films have polycrystalline structure with a cubic structure with a preferential orientation along (222) direction.The average diameter and average roughness change in reverse manner with the increase of in tin oxide concentration.Maximum diameter and average roughness obtained are 105.59 and 26.8 nm respectively.3-The results showed good correlation between structural, morphological and electrical properties was found.The increase of tin oxide to 3 and 5 % improves the crystallinity, increases the grain size and decreases the resistivity.Maximum sensitivity obtained from In 2 O 3 :SnO 2 /p-Si thin films gas sensor (50) for tin oxide concentration 7% at 573 K. 5-Minimum response time obtains from In 2 O 3 :SnO 2 /p-Si thin films gas sensor were 8 sec at 573 K.6-Increase of operating temperature enhanced the sensitivity of the prepared In 2 O 3 :SnO 2 thin films.