Electrical properties of neutron-transmutation-doped GaAs below 450 K

Abstract

Various concentrations of Ge and Se donors were introduced into GaAs crystals by means of neutron transmutation doping. Three kinds of GaAs crystals were used: undoped, Cr-doped crystals, and a high-purity epitaxial layer. Hall coefficient $R$, resistivity $\ensuremath{\rho}$, and low-field magnetoresistance $\frac{\ensuremath{\Delta}\ensuremath{\rho}}{\ensuremath{\rho}}$ were measured between 1.4 and 450 K. Good agreement was found between the measured concentrations of added donors and the values expected from the neutron-capture cross sections and the neutron fluences used. The analysis of the temperature dependence of the carrier concentration of the epitaxial sample gave somewhat smaller values for ${N}_{D}$ and ${N}_{A}$, the concentration of donors and acceptors, than the analysis of the $T$ dependence of ${\ensuremath{\mu}}_{H}$, but ${N}_{D}\ensuremath{-}{N}_{A}$ was the same; this indicates that some deep-lying centers are present in this sample. At low $T$ the magnetic field dependence of $\ensuremath{\rho}$ of this sample was in good agreement with the theory of impurity conduction modified by Shklovskii. $\frac{\ensuremath{\Delta}\ensuremath{\rho}}{\ensuremath{\rho}}$ of this sample was positive to the lowest $T$ (1.4 K) and had two peaks; one at about 50 K corresponds to a maximum of ${\ensuremath{\mu}}_{H}$ and the second one at about 4.2 K corresponds to the temperature at which band conduction and impurity conduction are of equal magnitude. At low $T$ all the undoped and Cr-doped crystals had a negative magnetoresistance whose magnitude increases with decreasing $T$. At low $T$, $\frac{\ensuremath{\Delta}\ensuremath{\rho}}{\ensuremath{\rho}}$ changes from positive to negative as the room-temperature carrier concentration ${n}_{0}$ reaches 2 \ifmmode\times\else\texttimes\fi{} ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Above this carrier concentration $\frac{\ensuremath{\Delta}\ensuremath{\rho}}{\ensuremath{\rho}}$ is negative and reaches a maximum value at ${n}_{0}\ensuremath{\simeq}1\ifmmode\times\else\texttimes\fi{}{10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. $\frac{\ensuremath{\Delta}\ensuremath{\rho}}{\ensuremath{\rho}}$ disappears when ${n}_{0}$ exceeds the concentration of the true metallic state ${n}_{\mathrm{cb}}\ensuremath{\simeq}5\ifmmode\times\else\texttimes\fi{}{10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. The closeness of the ${n}_{0}$ value at which the negative $\frac{\ensuremath{\Delta}\ensuremath{\rho}}{\ensuremath{\rho}}$ has its maximum value and the critical concentration ${N}_{c}\ensuremath{\simeq}(3\ensuremath{-}4)\ifmmode\times\else\texttimes\fi{}{10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ at which the metal-nonmetal transition is observed indicates that these two phenomena are related.