DETECTION OF PHOSPHINE AT THE PPM LEVEL BY SR1-YCAYFEO3-X THICK LAYERS PRETREATED WITH HIGHLY CONCENTRATED PHOSPHINE IN AIR - EVIDENCE OF A TRANSITION OF THE SURFACE CONDUCTIVITY FROM PARA-TYPE TO NORMAL-TYPE

Thick films and pellets of Sr1 - yCayFeO3 - x (y = 0, 0.5 and 1) have been tested as sensors for detecting gaseous phosphine. In the absence of PH3 pretreatment, the three compositions exhibit only small responses. All responses are characteristic of p-type semiconductivity. After pretreating with 2000 ppm PH3 for 3 h at 360 °C, the semi-conductivity of thick films of SrFeO3 - x remains of p-type, whereas those of thick films of Sr0.5Ca0.5FeO3 - x and CaFeO2.5 are changed into n-type. The responses of the latter two samples are large enough to allow ppm levels of PH3 to be detected in air. The change from p- to n-type semiconductivity is verified by Seebeck effect measurements. The pretreatment time is apparently too short to have any effect on the pellets. IR absorption spectra and X-ray microprobe analysis of thick films suggest that part of the samples is converted into phosphate during the PH3 pretreatment. Although the structure and composition of the bulk remains unaffected, as shown by X-ray diffraction, the surface of the grains is probably covered with thin layers of iron-containing phosphate, which control the transport and detection properties. © 1990.