Some Electrical Properties of Zinc Oxide Semiconductor

Abstract

Measurements of the dark electrical conductivity σ, and Hall coefficient were made on sintered spectroscopically pure zinc oxide powder samples over a temperature range from 100°K to 625°K and at room temperature on zinc oxide crystals containing lead impurities, using both the usual dc potentiometer-probe method and an ac (4000 cps) set. In this reproducible range, lnσ is not linear with 1/T for the sintered samples but exhibits maxima occurring at higher T and σ values, the higher the sintering temperature; the electronic carrier concentration of the different samples, calculated from the Hall data, indicates thermal ionization of 1.6×1016 to 9.0×1017 donors/cm3 lying 0.017 to 0.045 ev below the conduction band, with increasing ionization energy and increasing donor concentration, in general, occurring for higher sintering temperatures. Conductivity measurements are also made on sintered samples from 300°K to 1040°K using the ac (4000 cps) voltmeter-ammeter method; above about 650°K, lnσ versus 1/T is linear and in agreement with the results of previous workers with ionization energies varying from 1.4 to 2.4 ev. Back reflection x-ray patterns indicate an increase in the lattice spacing after sintering; the change can be correlated with the presence of interstitial zinc atoms. The forbidden gap is taken to be about 3 ev, corresponding to an ultraviolet absorption edge and luminescence peak at 3.2 ev; the yellowing of worked or highly conducting or heated samples is attributed to a shift of this absorption edge into the visible. Room temperature impedance measurements show a frequency dependent reactance and resistance attributable to capacitive shunting of the high resistivity grain boundaries (ν∼107 cps). It is shown that the low frequency conductivity of the sintered samples is determined by the conductivity and relative thickness of the grain boundary material while the Hall coefficient depends mainly on the carrier concentration in the semiconducting grains; uncritical use of low frequency and dc results thus would lead to spuriously low values of the conductivity and mobility in relation to the true properties of the grains.