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We study lithium-diffused and annealed GaAs by admittance spectroscopy in the frequency range $10\char21{}{10}^{6}\mathrm{Hz}$ and the temperature range 30\char21{}300 K. Li diffusion turns the GaAs semi-insulating but subsequent out-diffusion of Li increases the conductivity and makes the samples p type. It is demonstrated that the conduction in semi-insulating Li-diffused GaAs is due to thermally activated carriers in the valence-band percolating around insulating metallic precipitates. At high frequencies the ac conductivity is proportional to ${\ensuremath{\omega}}^{x},$ with x being close to unity value, independent of temperature. We suggest that the percolation may be due to metallic precipitates formed during in-diffusion of lithium and following cooling. After subsequent annealing the ac conductivity becomes proportional to ${\ensuremath{\omega}}^{s}$ at high frequencies with the value of s decreasing with increasing temperature. The temperature dependency of s suggests a correlated barrier hopping mechanism in a band of defects. We attribute these defects to gallium vacancies ${V}_{\mathrm{Ga}}$ and gallium antisites ${\mathrm{Ga}}_{\mathrm{As}}.$

Originally n-type Si-doped GaAs turns p-type after Li indiffusion and consequent annealing at 200-600°C. Temperature-dependent conductivity and Hall effect measurements carried out in the temperature range 30-300 K revealconduction via impurity bands made up of shallow acceptors in addition to the valence-band conduction. Li diffusion into GaAs reduces the free-carrier concentration which leads to electrical resistivity as high as 10 7 Ω cm. Annealing highly resistive samples at temperatures above 200 °C significantly decreases the room-temperature resistivity to 0,1-1 Ω cm, depending on the annealing temperature. For samples annealed at 300-400°C we observe a metallic-type conduction which contributes to the conduction even at room temperature. In samples annealed at 500-600 °C the temperature dependence of the resistivity indicates that impurity conduction sets in at temperatures below 100 K. We discuss this conduction change in relation to enhanced gallium vacancy (V G a ) and gallium antisite (Ga A s ) concentration in Li indiffused and annealed samples.