Kinetics of relaxation and recombination of nonequilibrium carriers in GaAs: Carrier capture by impurities

Abstract

The kinetics of relaxation and recombination processes of nonequilibrium and quasiequilibrium charge carriers in high-purity n- and p-type GaAs are investigated. Luminescence spectra taken in time windows positioned at the end of excitation pulses of increasing length and the shape of corresponding luminescence transients are found to depend strongly on pulse length up to 500 ns. It is thus shown that quasiequilibrium of the excited state under continuous excitation is reached only after relatively long times.Slow capture of minority charge carriers by ionized impurities is found to cause the delay. Capture cross sections of holes by the ionized effective-mass acceptor carbon ${\mathrm{C}}^{\mathrm{\ensuremath{-}}}$ and the ionized 167-meV deep ${\mathrm{Sn}}^{\mathrm{\ensuremath{-}}}$ acceptor are determined from the onset of the respective luminescence in n-type samples. Time-delayed spectra and luminescence transients taken after the end of an exciting pulse of 2 \ensuremath{\mu}s length visualize the stepwise relaxation of the electronic system from the nonthermal quasiequilibrium of an excited state to thermal equilibrium. In all n- and p-type samples investigated here free holes disappear first.Consequently, the conductivity type of p-type material inverts twice during recombination. Capture cross sections of free electrons by ionized donors and neutral acceptors and of free holes by ionized carbon acceptors are determined. The capture cross sections are \ensuremath{\sigma}(h\ensuremath{\rightarrow}${\mathrm{SnAs}}^{\mathrm{\ensuremath{-}}}$)=7.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ ${\mathrm{cm}}^{2}$, \ensuremath{\sigma}(h\ensuremath{\rightarrow}${\mathrm{C}}^{\mathrm{\ensuremath{-}}}$)=8.7\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}14}$ ${\mathrm{cm}}^{2}$, \ensuremath{\sigma}(e\ensuremath{\rightarrow}${D}^{+}$) =5.1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$, and \ensuremath{\sigma}(e\ensuremath{\rightarrow}${\mathrm{C}}^{0}$)=2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}16}$ ${\mathrm{cm}}^{2}$. A specially devised low-temperature cathodoluminescence system is used for these experiments. Electron pulses of varying length 1 ns<eqt<eq10 \ensuremath{\mu}s but constant rise and decay time of <eq200 ps and moderate to low power density (0.8${\mathrm{eqI}}_{\mathrm{exc}}$${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) are employed for excitation.