2208
Choi et al
Electrical Conductivity of Cadmium Oxide Jae Shi Choi,' Young Hwan Kang, and Keu Hong Kim Department of Chemistry, Yonsei University, Seoul, Korea (Received May 19, 1976; Revised Manuscript Received August 3 1, 1977) Publication costs assisted by Yonsei Un/versity
The electrical conductivity of n-type polycrystalline CdO has been measured from 25 to 650 "C under oxygen pressures, Poz,of lod5to lo2 mmHg. Plots of log CT vs. T at constant oxygen pressure are found to be linear, and the conductivity dependence of oxygen pressure at the above temperature range is closely approximated The conductivities of specimens pretreated in Cd vapor at different temperatures are higher by u a than those of the specimens before treatment. This shows that the concentration of conduction electrons is associated with an excess of Cd or a deficiency of oxygen. It was also found that, for a given oxygen pressure, the slope of log CT vs. Tis positive up to about 400 "C, then becoming negative for further increase in temperature. This is explained by the increasing number of free electrons and by the effect of the scattering mechanism on the electrons which are presumed to originate from the nonstoichiometric excess of Cd atoms in interstitial positions or from the oxygen vacancy donors.
Introduction It is well known that cadmium oxide is a n-type semiconductor with semimetallic conductivity due to the fact that it has an excess of cadmium atoms in it. The physical properties of cadmium oxide along with its defect structure have been extensively investigated. Several papers have been published on compressed tablets'J! and sputtered film^^-^ of the oxide. Nevertheless, there is no agreement on whether the principal defects are cadmium interstitials or oxygen vacancies. For example, Cimino' and Faivre6 have investigated the defect structure of CdO by measuring the lattice parameter and concluded that Cd interstitials are the important defects. Koffyberg7also concluded that the principal defects in nonstoichiometric CdO are cadmium interstitials. however, found through lattice diffusion studies that the predominating defects are oxygen vacancies and showed that the proportion of vacancies can for pure CdO at be estimated, giving a value of 4.4 X 790 "C and 0.16 atm of 02.Lakshmanan4 also found from the optical and electrical properties of CdO films that the principal defects are oxygen vacancies. The purpose of the present investigation is to determine the principal defects in CdO and to explain the electrical conduction mechanism of the oxide by measuring its electrical conductivity under various conditions. Experimental Section Specpure cadmium rod 1.0 cm in diameter supplied by the Johnson-Mathey Co. was used in the preparation of CdO. The rod was cut into sheet, etched in dilute nitric acid, washed with distilled water, and then kept in a vacuum desiccator. Oxidation was carried out in air at 300 OC for 640 h or more in a platinum crucible with quenching in air. The prepared sample was pulverized in an agate mortar. After drying at 200 OC for several hours, it was compressed into pellets under a pressure at 600 kg/cm2 under vacuum. A sample from the complete oxidation of the cadmium sheet was identified as CdO by x-ray (Cu Ka) diffraction. The prepared samples were ground with no. 1/10 emery paper and then the pellets were cut into rectangular form with dimensions of 0.2 cm X 0.5 cm X 0.9 cm. Each sample was etched in 20% (NH4)2S208and dilute nitric acid and washed with distilled water, dried, and then annealed in air at 900 "C. After annealing, each was quenched rapidly to room temperature. The Journal of Physical Chemistry, Vol. 81, No. 23, 1977
TABLE I: Impurities in CdO Prepared from the Cadmium Metal Detected by Spectroscopic Analysis Element Impurities, ppm Element Impurities, ppm A1
5 4 1
