Interdiffusion in platinum—tin oxide multilayered films

Abstract

Tin(IV) oxides doped with a small amount of noble metals are well known as gas sensors to detect inflammable gases [1]. Thus, the distribution of the noble metals in the SnO2 matrix plays an important role in determining the sensor characteristics. Therefore, the stability of these sensors depends partly on the interdiffusion between noble metals and SnO2 at their operating temperatures. However, direct measurement of the diffusion coefficient in the noble metaldoped SnO 2 is very difficult because the values of the diffusion coefficient are low at the sensor operating temperatures, usually less than 500 ° C. DuMond and Youtz [2] have developed a new technique to measure interdiffusivities down to 10-20cm2sec i. They achieved this by depositing compositionally modulated films of gold-copper alloys with an average interplanar distance of 10 nm. The diffusion coefficient is directly related to the observed rate of decay of the diffracted X-ray intensity. Similar experiments with a refined theory have been reported for crystalline and amorphous multilayered films [3, 4]. In this letter we present interdiffusivity measurements in polycrystalline Pt-SnO2 multilayered films with an interplanar distance 2 of about 3.5 nm and the resulting value of interdiffusivity as low as 9 x 10 -22 cm 2 sec ' at 500 ° C. A Pt-SnOx multilayered film, with a layer thickness of 0.88nm for platinum and 2.65nm for SnOx (2 = 0.88 + 2.65nm) and total thickness of about 110nm (31 pairs) was prepared by sequentially sputtering from two targets. Each layer thickness was controlled by the sputtering time using the deposition rate determined before formation of the film. The film was deposited on an optical flat Pyrex glass at temperatures lower than 90 ° C. The film was annealed at 500°C in air. Details of the ion-beam sputtering method have been described elsewhere [5]. Before going to the multilayered film, the structural change of as-deposited pure film was examined. X-ray diffraction analysis using nickel-filtered CuKa radiation showed that the platinum film (27 nm thick) was polycrystalline with a weak reflection, and that the SnOx film (114 nm thick) was amorphous. On heating the platinum film for 2 h, a sharp increase in intensity occurred, indicating a rapid grain growth. On the other hand, the crystalline SnOx film showed negligibly small peaks compared with those of the platinum film. The behaviour of pure films being thus observed, the annealing behaviour of the multilayered film was examined next. Fig. 1 shows the X-ray diffraction pattern from the multilayered film annealed for 16h. The chart shows a weak broad diffraction from the (1 0 1) plane of the SnOz (JCPDS 21-1250) at d ~ 0.226 nm (20 ,,~ 34.1 °) and the strong peak from the (1 1 1) reflection of the platinum (JCPDS 4-0802) at d = 0.226nm (20 = 39.8°). It is to be noted that the (1 1 1) peak of the S n O 2 at d = 0.236 nm (20 = 38.1 °) and the (1 1 1) reflection from the platinum overlap each other; the intensity of the former is less than 5% of that of the latter. Further annealing up to 144h changed the diffraction pattern little. Low-angle X-ray diffraction analysis gave rise to at least third-harmonic (000) satellite in as-deposited films. Fig. 2 shows the decrease of the first satellite intensity with the typical successive anneals. For the first few hours of annealing at 500 ° C the film experienced the crystallization, grain growth and absorption of oxygen, and this transient period exhibited a large decrease in the satellite intensity. After annealing for more than 16 h, only the first-harmonic (0 0 0) satellite remained.