Admittance measurements of acceptor freezeout and impurity conduction in Be-doped GaAs.

Abstract

Carrier freezeout and impurity conduction in lightly doped p-type GaAs have been studied in two groups of ${\mathit{p}}^{\mathrm{\ensuremath{-}}}$-type GaAs--undoped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As--${\mathit{p}}^{+}$-type GaAs capacitors. For one group the substrate doping was ${\mathit{N}}_{\mathit{S}}$\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$; for the other ${\mathit{N}}_{\mathit{S}}$\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. From the temperature dependence of the appearance of a Gray-Brown dip in the capacitance-voltage curves at fixed frequency and temperature, the separation of the Be acceptor from the valence band, ${\mathit{E}}_{\mathit{A}}$, is \ensuremath{\sim}20 meV for both groups. The activation energy for ac transport in ${\mathit{p}}^{\mathrm{\ensuremath{-}}}$-type GaAs, ${\mathit{E}}_{\mathit{I}}$, is determined from admittance measurements at fixed bias and variable temperature and frequency. For the lighter-doped samples, ${\mathit{E}}_{\mathit{I}}$=17--23 meV, which is consistent with the value of ${\mathit{E}}_{\mathit{A}}$. For the heavier-doped samples, ${\mathit{E}}_{\mathit{I}}$\ensuremath{\sim}6 meV; ac transport is either through a well-defined impurity band separated by \ensuremath{\sim}6 meV from the energy level of an isolated acceptor, or by activated hopping.