RELAXATION OF PHOTOGENERATED CARRIERS UNDER HE, H-2 CO2 AND O-2 ON ZNO

The relaxation process of photogenerated carriers was investigated usi ng conductivity measurement on ZnO under He, H-2, CO(2 )and O-2. The p rocess was well explained with the rate constant of reaction or recomb ination of hole and electron, k(h) and k(e) (k(h) > k(e)), respectivel y. Generally, k(h) increased with the pressure of the gases. The slope of k(h) with respect to the pressure increased in the order of H-2 le ss than or equal to He < CO2, while k(h) of O-2 was sensitive to the h istory of the sample. The relaxation process on ZnO which was exposed to oxygen at 298 K and 573 K was observed during the illumination at 2 98 K and it was found that the rate constant of hole decreased with il lumination time. From the result, it was suggested that the rate const ant of photogenerated excess carriers was affected by the surface barr ier of the semiconductor.