Decay kinetics of excitons and the electron-hole plasma in GaP: N

Abstract

The lifetime of excitons bound to nitrogen, ${\ensuremath{\tau}}_{e}$, and of the electron-hole plasma (EHP), ${\ensuremath{\tau}}_{p}$, in GaP: N was studied as a function of the following parameters: temperature (2-100 K), excitation intensity, and nitrogen-doping level. The photoluminescence was excited both selectively and above the gap by using a pulsed, tunable dye laser with photon flux varying in the range of ${10}^{14}$-${10}^{19}$ photons/${\mathrm{cm}}^{2}$ per pulse. The observed ${\ensuremath{\tau}}_{e}(T)$ for various excitation intensities was quantitatively fitted to a model which assumes the existence of saturable deep exciton and electron traps ("shunt paths"). An estimate of the concentration and capture cross section of these traps is obtained. In addition to the well-known exciton capture by the nitrogen, an additional free-electron---hole pair-capture process is observed. This occurs when the photoexcited electron-hole pair concentration is greater than that required for a Mott transition (so that no excitons are available). In crystals with nitrogen concentration greater than ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and under intense photoexcitation (resonantly with the $A$ line), the EHP is created and its lifetime is virtually temperature independent. In addition, ${\ensuremath{\tau}}_{e}\ensuremath{\ne}{\ensuremath{\tau}}_{p}$ for most of the temperature range studied. This indicates that particle exchange between EHP and exciton regions is small.