Effect of composition on properties of In2O3–Ga2O3 thin films

The In2O3–Ga2O3 mixed oxide polycrystalline thin films with various ratios of components were obtained by pulsed laser deposition. The effect of films composition on surface morphology, electrophysical and gas sensing properties and energies of adsorption and desorption of combustible gases was studied. The films with50%In2O3–50%Ga2O3 composition showed maximum gas response (∼25 times) combined with minimum optimal working temperature (∼530 °C) as compared with the other films. The optical transmittance of the films in visible range was investigated. For 50%In2O3–50%Ga2O3 films, the transmittance is higher in comparison with the other films. The explanation of the dependency of films behaviors on their composition was presented.The In2O3–Ga2O3 films were assumed to have perspectives as gas sensing material for semiconducting gas sensors.


Introduction
Oxides of transition metals have various applications. Some examples aresemiconductor gas sensors and conductive transparent films.Semiconductor gas sensors, are actually one of the most investigated groups of gas sensors. They are low cost, have considerable long-term stability, provide a variety of detectable gases and able to operate in harsh environments [1]. All these advantages allow one to use semiconductor gas sensors in numerous fields [2]. An important trend in a field of semiconductor gas sensor research is the finding of for novel gas sensing materials [3]. One of the achievements on that way is the use of mixed metal oxides. The main idea of that method is based on fundamental principle of the resistance variation of the sensing layer. It involves two important functions: the recognition and the transducer function. The recognition includes all the interactions between gas and solid. The transducer function is responsible for transformation of the chemical stimulus into an electrical signal. The most accepted explanation of mixed oxide gas sensing performance is the separation of those functions between two parts of the compound system [3]. One part of multicomponent system may be responsible for recognition having very high reactivity to target gas while the second part takes over the transducer duties. This effective combination allows optimizing both functions simultaneously, which lead to a possibility of sensors sensitivity increase [4]. The production of multicomponent films requires a technique with a good control and reproducibility of parameters of resulting films [5]. The pulsed laser deposition (PLD) method used in the present study satisfies these requirements very well. The uniform distribution of components in prepared films and low growth rates (5-10 nm/min) allow achieving very high reproducibility of a sensing element. Another important advantage of the PLD is the stoichiometric transfer of material from sputtering target to substrate, which enables very precise control over the composition of the film [6].
A huge variety of mixed oxides are employed as sensing materials in semiconductor gas sensors [1,4,7]. Some well-known examples are In 2 O 3 -SnO 2 (ITO), In 2 O 3 -Ga 2 O 3 -ZnO (IGZO), ZnO-SnO 2 , SnO 2 -WO 3 and many others. The In 2 O 3 -Ga 2 O 3 films among them are poorly investigated although they have good gas sensing properties [7,8]. Most of the available data show possibility of their application as conductive transparent films [9][10][11]. In [8] there is a study on gas sensing properties of In 2 O 3 -Ga 2 O 3 thin films made by magnetron sputtering from In-Ga eutectic alloy followed by annealing in air. Also there are results [12] on agglomerative single-electrode gas sensors with In 2 O 3 -Ga 2 O 3 sensing layer, coated on a platinum spiral from In(OH) 3 , Ga(OH) 3 sols. However, while perspectives of In 2 O 3 -Ga 2 O 3 thin films in a field of gas sensing are clearly marked, there is insufficient information to date on the electrical and gas sensing properties and methods of their production. The aim of this paper is the study of electrophysical, optical and gas-sensing properties of In 2 O 3 -Ga 2 O 3 thin films (made by PLD) with different ratios of components. Gas induced resistance response was investigated for the following combustible gases: alcohol, acetone, ammonia andliquefied petroleum gas (LPG).

Experimental
The In 2 O 3 -Ga 2 O 3 thin films were obtained in two consecutive steps: fabrication of bulk targets followed by pulsed laser deposition of the films. The targets were formed by pressing the mixture of In 2 O 3 and Ga 2 O 3 powders and Ga(NO 3 ) 3 ·8H 2 O. The obtained targets were annealed in natural air at 820 °C for 10 minutes to prepare them for subsequent pulsed laser deposition, which requires sufficient durability of targets [13]. The pulsed laser deposition was carried out with Nd:YAG pulsed laser «Melaz» at 1.06 µm in residual air with pressure 10 2 Pa. Pulse parameters: duration 16 ns, repetition rate 10 Hz, focal spot area 7.85 10 -5 cm 2 , laser pulse energy density 637 J/cm 2 . Substrate parameters: temperature 400 °C, material СТ 50-1 glass ceramics. The film thickness reaches 0.20-0.21 µm after 20 minutes of deposition. Compositions of the targets were 100%In 2 O 3 , 75%In 2 O 3 -25%Ga 2 O 3 , 50%In 2 O 3 -50%Ga 2 O 3 , 25%In 2 O 3 -75%Ga 2 O 3 and 100%Ga 2 O 3 . Surface morphology of the obtained films was investigated by atomic force microscopy (NT-MDT AFM «Solver Pro»). Optical transmittance spectra were obtained using spectrophotometer «SF-56» in the wavelength range 200-1100 nm. Study of electrophysical and gas-sensing properties of the films was conducted in the temperature range 150-750 °C on a special experimental setup (two-electrode method of resistance measurement) placed in a sealed chamber with natural air. The sample gas was injected into chamber in small portions by medical injector to achieve required concentration of that gas in the volume of the chamber. The resistance response S was calculated as a ratio of R 0 to R g , where R 0 and R g are resistances of the film in the natural air (base resistance) and in natural air with sample gas, respectively.
More details of the experiment may be found in [13,14].

Atomic force microscopy
Atomic-force microscopy (AFM) images of films surfaces allow calculating mean grain size, making assumptions of gas penetrability of films [15] (see table 2). In a field of semiconductor gas sensors it is well known that high base resistance usually leads to high gas response [1]. Results of gas-inducedresistance response study (discussed below) confirm that trend. One of important characteristics of films for semiconductor gas sensors is relative change of resistance per °C, because of usual problem with temperature-caused baseline instability [1,[16][17][18].Relative change of resistance per °C in temperature range 530-550 °C (range of maximum gas-induced resistance response)have values 1.0-1.5 % for films with different compositions. There is a peculiarity on all the temperature dependencies of resistance in the temperature range 270-400 °C -a region of positive temperature coefficient of resistivity. This behavior of resistance is wellknown for oxides of transition metals. It is explained by the change in the oxygen adsorption mechanism [13,19,20]. The transition from preferential adsorption of molecular oxygen O 2ˉ to preferential adsorption of atomic oxygen Oˉ and Oˉˉ occurs at these temperatures, which leads to increase in resistance. All films were obtained with identical deposition parameters, so the only explanation of the optical transmittance increase is the formation of new phase in films with 50%In 2 O 3 -50%Ga 2 O 3 composition. It may be InGaO 3 phase, which is known for its good optical transmittance [10].   Temperature dependencies of gas response on the aforementioned combustible gases diluted in natural air have a maximum at a certain temperature, which is typical for semiconducting gas sensors. Amplitude of response on acetone and ethanol for all investigated films in temperature range 500-600 °C exceeds 3-4 times amplitude of their response on ammonia and LPG. Table 3 represents data on amplitude of resistance response on acetone for all films and data on temperature of maximum response on that gas. Data for other gases are similar. The temperature of maximum response was determined by approximation of experimental results with Gauss function. The obtainedresults show that temperature of maximum response for all films differs no more than 7%. The temperature of maximum resistance response on ethanol and acetone for films with 50%In 2 O 3 -50%Ga 2 O 3 composition is lower than that temperature for other films. Moreover, their amplitude of resistance response on those gases is significantly higher. 3.5. Temperature dependencies of resistance response in coordinates ln(S)=f(1/kT) The temperature dependencies of gas response on combustible gases in coordinates ln(S)=f(1/kT) have two linear regions (see figure 5 for cases of acetone and ethanol). Slope angles for those regions are proportional to activation energies of resistance response, ΔE. Table 4 represents obtained values of ΔE for all investigated films. According to [1,21] the main factor, influencing amplitude of resistance response in low temperature region (ΔE LT ), is difference between energy of combustible gas adsorption (E R ) and energy of oxygen adsorption (E O ):

Temperature dependencies of gas-induced resistance response
The activation energy of resistance response in low temperature region (ΔE HT ) is determined by the difference between energy of combustible gas desorption (q R ) and energy of its oxidation products desorption (q RO ) [1,21].  The slope angles of high temperature linear regions of the dependencies in figure 5 are approximately equal. According to (2) it indicates that 50%In 2 O 3 -50%Ga 2 O 3 and 75%In 2 O 3 -25%Ga 2 O 3 films have approximately equal difference between energy of combustible gas desorption (q R ) and energy of its oxidation products desorption (q RO ). The slope angles of low temperature linear regions of those dependencies are significantly different. According to (3) [12], area of heterojunction between the crystallites of In 2 O 3 and Ga 2 O 3 [3]. The most likely option is the formation of InGaO 3 , because previously it was illustrated in [8] by the means of X-ray diffraction that In 2 O 3 -Ga 2 O 3 films contain several phases:In 2 O 3 , Ga 2 O 3 and InGaO 3 . It is obvious, that the highest concentration of InGaO 3 phase should be in films with composition close to 50%In 2 O 3 -50%Ga 2 O 3 . It was confirmed in [8].

Conclusion
Presented results demonstrate, that In 2 O 3 -Ga 2 O 3 thin films may be considered as promising materials for sensitive layers of semiconductor gas sensors and conductive transparent layers. Production of these films by pulsed laser deposition allows precise control of their composition and thickness. Considered films are sensitive to a number of flammable gases. The highest sensitivity of the films is observed for the case of acetone and ethanol vapors. Study of effect of films composition on their properties showed, that highest gas-induced resistance response have films with 50%In 2 O 3 -50%Ga 2 O 3 composition. In addition, these films have least temperature of maximum resistance response as compared with films with other compositions. Also, their optical transmittance in visible range have noticeable local maximum. The reason for all those phenomena may be the formation of InGaO 3 solid solution in films with composition close to 50%In 2 O 3 -50%Ga 2 O 3 .