Electron Mobility and Shallow Impurity Levels in In- and Cu-Doped CdS

Abstract

Electrical-conductivity and Hall-effect measurements were made between 300 and 4.2 \ifmmode^\circ\else\textdegree\fi{}K on In-doped (\ensuremath{\sim} 6 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) CdS single crystals. Normal band conduction was observed between 300 and 20 \ifmmode^\circ\else\textdegree\fi{}K, while impurity-hopping conduction in a shallow-donor state (${E}_{D2}$) dominated the conduction mechanism for $T<20$ \ifmmode^\circ\else\textdegree\fi{}K. In the normal-band-conduction region, the best theoretical fit was obtained by using a two-shallow-impurity-level model. The result yields ${E}_{D1}=0.033$ eV for the In level with ${N}_{D1}=6\ifmmode\times\else\texttimes\fi{}{10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and ${E}_{D2}=0.007$ eV for a level with unknown origin with ${N}_{D2}=1.3\ifmmode\times\else\texttimes\fi{}{10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. The Hall mobility vs temperature was studied for both Cu- and In-doped CdS between 4.2 and 300 \ifmmode^\circ\else\textdegree\fi{}K. It was found that the longitudinal optical phonons dominate the scattering of electrons for $T>200$ \ifmmode^\circ\else\textdegree\fi{}K and ${\ensuremath{\mu}}_{L}\ensuremath{\cong}110({e}^{\frac{(300\ifmmode\pm\else\textpm\fi{}10)}{T}}\ensuremath{-}1)$ ${\mathrm{cm}}^{2}$/V sec. A brief discussion of the scattering mechanism for $T<100$ \ifmmode^\circ\else\textdegree\fi{}K is given for both In- and Cu-doped CdS.