Strong localization of carriers in δ-doped GaAs structures

Electrical conduction in Be δ-doped GaAs structures grown by molecular-beam epitaxy was studied in the temperature range from 10 to 350 K. A Be δ-doped layer and a 1-nm-thick GaAs spacer layer were grown at 520 °C, followed by growth of a 1-nm-thick GaAs layer with a high concentration of excess As and a 5-nm-thick nearly stoichiometric GaAs layer at a temperature close to 150 °C. At low Be doping concentrations, the conduction is n type and thermally activated, occurring in the 1-nm-thick low-temperature-grown GaAs layer. At higher Be doping concentrations, the conduction is p type and thermally activated, resulting from thermal excitation of localized holes to extended states in the δ-doped well. Activation energies for the p-type conduction range around 100 meV, indicating strong localization of holes in δ-doped wells. By further increasing the Be concentration, the activation energy decreases and eventually leads to the metal-like temperature dependence of the conduction at room temperature, suggesting a possible metal-insulator transition. High-temperature limits of the resistivity of insulating samples were found to be close to the value of the quantum unit of resistance, (1 / 2)h/e^2