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 $\rho$, and low-field magnetoresistance $\frac{\Delta \rho }{\rho }$ 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 ${\mu }_H$, but $N_D-N_A$ was the same; this indicates that some deep-lying centers are present in this sample. At low $T$ the magnetic field dependence of $\rho$ of this sample was in good agreement with the theory of impurity conduction modified by Shklovskii. $\frac{\Delta \rho }{\rho }$ 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 ${\mu }_H$ 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 $T$, $\frac{\Delta \rho }{\rho }$ changes from positive to negative as the room-temperature carrier concentration $n_0$ reaches 2 × ${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Above this carrier concentration $\frac{\Delta \rho }{\rho }$ is negative and reaches a maximum value at $n_0\simeq 1\times {10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$. $\frac{\Delta \rho }{\rho }$ disappears when $n_0$ exceeds the concentration of the true metallic state $n_{\mathrm{cb}}\simeq 5\times {10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The closeness of the $n_0$ value at which the negative $\frac{\Delta \rho }{\rho }$ has its maximum value and the critical concentration $N_c\simeq \mathrm{}(3-4)\mathrm{}\times {10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$ at which the metal-nonmetal transition is observed indicates that these two phenomena are related.