Gas Microsensors Based on Semiconductor Thin ...

2004 1st InternationalConference on Electrical and Electronics Engineering

Gas Microsensors Based on Semiconductor Thin Films of Zn0:Ga J. L. Godez-Vidal*. C. I. T, I. S. Universidad Autbnoma del Estado de Hidalgo Carretera Pachuca-Tulancingo Km 4.5, Pachuca, Hidalgo, 42076, MkXICO. A. Reyes-Barranca, M. de la L. Olvera, A. Maldonado. Depto. De Ingenieria ElCctrica, CINVESTAV-EN. Apdo. Postal 14-740, MBxico D. F. 07000, I d X I C O . Abstract. Four carbon monoxide microsensor based on semiconducting oxide Zn0:Ga has been developed. The results and analysis of the characterization in an atmosphere containing carbon monoxide (CO), are presented in this work. dilution CO is 50ppm. Zn0:Ga thin films were deposited at 450°C by the spray pyrolysis technique from a 0 . 2 M starting solution with a IGal/[ZnI= 3 at. YO.Microsensors with four different dimensions were designed: 20x20pm2, 20x40pm2, 20x60pm2 and 100x100pm2. Ohmic contacts were manufactured by thermal evaporation of aluminum on the top of the films. Both of gas microsensors and AI terminals were patterned by lift off. A surface resistance variation of several orders of magnitude was found in doped-gallium ZnO thin films when these were introduced into a camera with Oppm, lppm, 5ppm, 50ppm and lOOppm of CO. Thin films gas microsensors were tested at several temperatures and different CO concentrations. The measurement temperatures employed were 2OO0C, 250°C and 300°C. The surface of ZnO thin films doped with Ga was also characterized by AFM, showing a regular and uniform morpholgy. Keywords: Microsensors, thin films, zinc oxide, semiconductor oxide.

I. INTRODUCTION The objective of the present study is to obtain a detailed understanding of the effect of doping ZnO films with particularly its behaviour Ga, as a function of temperature and CO concentration, applied on gas microsensors with different areas. It is well know that gas sensing is a task very well done by thin films based on semiconductor oxides. Also semiconductor gas microsensors are used for many applications due to their low price, reliability and simple measurement processes, Thin films based on semiconductor oxides are very sensitive to a multitude of inorganic gases. The main detection process is the change of the oxygen concentration at the surface metal oxide, caused by the adsorption and heterogenous catalytic reaction of oxidizing, this tends to increase the surface resistance metal oxide, and when a reducting gas is present, such CO, it reacts with adsorbed oxygen to form gas COZ, then electrons are reinjented into the semiconductor oxide, causing a decrease in the surface resistance of the metal oxide. Doping with other elements or substances like promoters or catalysts had proved their importance since they help important to improve the sensitivity of these semiconducting oxides in presence of a gas. On the other hand, the tendency towards miniaturization makes necessary the design of microsensors,

0-7803-8531 -4/04/$20.00 02004 IEEE

such requirements as smaller area and lower energy consumption are satisfied [ 1-41 To improve today gas microsensors sensitivity, some catalyst like metals are commonly used, such as chromium, copper, platinum, paladium, silver and gallium, amog others. Doping with such metals improve the semiconductor oxide thin film behavior, mainly increasing the microsensor sensitivity. It is well known that when a semiconductor oxide is doped, its performance is improved having a better sensitivity at less reduction gas concentrations and lower temperatures. In this paper microsensors based on Zn0:Ga thin films deposited by the spray pyrolysis technique are reported. Four different microsensor were designed and characterized at room temperature, lOO"C, 150"C, 200"C, 250°C and 300°C in presence of gas CO, using concentration values of Oppm, lppm, Sppm, 50ppm and lOOppm CO. [S-81 11. GAS MICROSENSOR FABRICATION In this work, the performance of Zn0:Ga thin films deposited on soda-lime glass subsbates by the spray

pyrolysis technique is studied, starting ftom zinc pentanedionate and also, the performance of four Zn0:Ga based gas microsensors designed are studied. Four microsensor sizes, 20x20p2, 20x40p2,2 0 x 6 0 ~and ~ 100x100p2 were designed, (fig. 1), two square pads of 5 0 x 5 0 ~were ~ added to both gas microsensors ends for contact purposes. Gas microsensors reported here were designed using LASI6 program. [9] Fabrication was developed from the following steps a) aluminium was deposited on soda-lime glass substrates by evaporation technique. A1 terminals were pattemed by lift off. AI was etched by orthofosforic acid (H3P04) at 6OoCb) ZnO:Ga thin films were deposited by the spray pyrolysis technique and gas microsensors were photolitographically pattemed by lift off Zn0:Ga was etched by chloridric acid (HCl). The Layout and a picture of microsensors are shown in fig. 2. [IO] 111. MEASUREMENTS

Measurements of Zn0:Ga thin films resistance were made either to layers without any photolithographic process and patterned as microsensors, also. The resistance mesasurents were made with a Keithley 2001 multimeter, and the temperature of the system was controlled by an Euroterm temperature controller, model 847, The pads of the Zn0:Ga gas microsensors were contacted using tungsten microprobes, installed within the measurement chamber in order to make resistance measurements.

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Before the measurement is done, pressure in the chamber is lowered and then, the oxidizing gas {CO in our case) is introduced at the desired concentration. For the experiments carried out, concentrations of 1 ppm, 5 ppm, 50 ppm and 100 ppm were used, at different temperatures: 100 'C, 150 "C, 200 "C, 250 "C and 300 "C. Fig. 3 shows the surface resistance variation of 2nO:Ga thin films as a function of the CO concentration, measured at different temperatures. From fig. 3. We can see that at room temperature through 150°C there is no significant variation of the surface resistance for all CO concentrations used. However, at 200°C through 300°C an evident surface resistance decrease is noticed as the temperature is increased. Maximum resistance variation is registered at low CO concentration values in the order of 5ppm at 300"C, saturation behaviour is observed then as the CO concentration is increased. Also a surface resistance decrease is observed as temperature is increased.

On the other hand, comparison of the four gas microsensors

sizes measured at 300°C is shown in fig. 4. A decrease in the resistance of the ZnO layer of the gas microsensors is observed; this behaviour is expected, as the area of gas microsensors is smaller. The resistance is inversely proportional to area. We can see that the 100x100y2area gas microsensor has the highest resistance and it has the minimum resistance variation measured between both Oppm and lOOppm of CO concentrations, and the sensor with an area of 20x20p2 has the lower resistance measured for lOOppm of CO concentration. Also t h i s last microsensor has the greatest resistance change at low CO concentrations. ZnO-Ga thin film.

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AFM scanning was used to observe the Zn0:Ga thin film surface. Its surface micrograph is shown in fig. 5. There a rough and uniform surface and very well shaped grains are observed and few high grains are present. This characterization was made with a Quesant microscope, model 250.

IV.RESULTS AND DISCUSSION These Zn0:Ga thin films obtained have very acceptable quality, and as they were deposited by spray pyrolysis technique, the technique shows it reliability in the fabrication of thin filmsfor gas microsensors. Yet, they have a rough and uniform surface and very well shaped grains and few high grains that can be suitable for the expected application. The experiments show either, that a lower resistance can be attained when they are doped with Ga, as compared with those undoped films. From the resistance measurement to microsensors with different areas, it was demostrated that even if the sensor has a small area as 2Ox20p2,the ability in sensing a reducing gas is still present, having a two orders of magnitud resistance variation (105-106 n). NO resistance variations were appretiated at room temperature and 100 "C, but a resistence decrease is appreciated kom 150 "C and over. A resistance variation of one order of magnitude is noticed at 300 "C with low CO concentrations (fig 6a). For the microsensors with an area of 20x40p2,the resistance range falls between 106-108n,

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Compared with the smaller area microsensor, there is no significative resistance changes even at 200 "C. A resistance decrement with good behavior at 250 ' C with high CO concentrations is noticed (fig 6b). The microsensor with an area of 2 0 x 6 0 ~has ~ resistance values within the lo5 lo9 R range.Good behavior at 200 "C and 300°C is similar as the smaller ones, as well. Also a wider resistance variation is noticed when the film was set at 150 "C with low COconcentrations, having a decrease of one order of magnitude (fig 6c). On the other hand, the last area considered ( 1 0 0 ~ 1 0 0 has ~ ~ )a similar resistance variation as the last one with greater resistance variations are noticed at 250 "C and 300 "C (fig 6d). 20x40

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