Strong localization of carriers in δ-doped GaAs structures

Abstract

Electrical conduction in Be \ensuremath{\delta}-doped GaAs structures grown by molecular-beam epitaxy was studied in the temperature range from 10 to 350 K. A Be \ensuremath{\delta}-doped layer and a 1-nm-thick GaAs spacer layer were grown at 520 \ifmmode^\circ\else\textdegree\fi{}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 \ifmmode^\circ\else\textdegree\fi{}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 \ensuremath{\delta}-doped well. Activation energies for the p-type conduction range around 100 meV, indicating strong localization of holes in \ensuremath{\delta}-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, $\frac{1}{2}{h/e}^{2}.$