Conduction in amorphousMg2Xcompounds (X=Geand Sn)

Amorphous films of ${\mathrm{Mg}}_2$Ge, ${\mathrm{Mg}}_2$Sn, and ${\mathrm{Mg}}_2{\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ ($x$ varying from 0 to 1) were prepared by sputtering at 77 °K. ${\mathrm{Mg}}_2$Ge and ${\mathrm{Mg}}_2$Sn, which are semiconductors in the crystalline state, showed semiconducting-type conduction in the amorphous state. The electrical resistivity of amorphous ${\mathrm{Mg}}_2{\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ is well fitted at low temperatures by the relation $\rho ={\rho }_0$ $\exp \left[{\left(\frac{T_0}{T}\right)}^{\frac{1}{4}}\right]$. The experimental value of the temperature coefficient $T_0$ is found to decrease exponentially with increasing $x$ (the Sn concentration). On the other hand, $T_0$ was found to be inversely proportional to the impurity concentration when doping $a-\mathrm{S}\mathrm{b}$ or $a-\mathrm{G}\mathrm{e}$ with impurities forming localized impurity states. The exponential decrease in the present case is believed to be caused by the progressive decrease of the pseudogap as ${\mathrm{Mg}}_2$Sn is substituted to ${\mathrm{Mg}}_2$Ge.