POLAR SURFACES OF ZINC OXIDE CRYSTALS

Polar surfaces have been prepared by cleavage in ultrahigh vacuum. The electrical conductivity at these surfaces has been studied between 90 and 600 °K. Changes occur by annealing in vacuum and by adsorption of atomic hydrogen or of oxygen. Additional information is obtained from cleaving crystals under mercury and measuring the current versus voltage relation. Furthermore, changes of the reflectance of light have been observed which are induced by adsorption of atomic hydrogen or of oxygen. For clean polar surfaces the conductivity must be below an upper limit of 4 × 10-12 A/V. Transient heating in vacuum produces a high surface conductivity of 10-4 A/V on the Zn surface whereas the O surface does not change. Atomic hydrogen acts much faster on the O surface than on the Zn surface creating an accumulation layer of high conductivity. By contrast, the depletion layer caused by the adsorption of oxygen is more evident on the. Zn surface. The results of the optical measurements support these data. An atomic model of the polar surfaces is discussed. Several arguments concerning the distribution of electrons at these surfaces are considered in connection with the idea of dangling bonds and with compensation of the internal dipole moment of the wurtzite structure. © 1969.