Kinetics of persistent photoconductivity inAl0.3Ga0.7As andZn0.3Cd0.7Se semiconductor alloys

Abstract

The kinetics of persistent photoconductivity (PPC) in ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As and ${\mathrm{Zn}}_{0.3}$${\mathrm{Cd}}_{0.7}$Se has been investigated. The PPC relaxation behaviors in both materials can be well described by stretched-exponential functions, ${\mathit{I}}_{\mathrm{PPC}}$(t)=${\mathit{I}}_{\mathrm{PPC}}$(0)exp[-(t/\ensuremath{\tau}${)}^{\mathrm{\ensuremath{\beta}}}$] (\ensuremath{\beta}1). For ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As, the relaxation-time constant \ensuremath{\tau}, as a function of the relative photoexcited electron concentration n, is measured through the variation of the excitation photon dose in the temperature region T\ensuremath{\ge}10 K. At low temperatures, we found that in ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As, \ensuremath{\tau} decreases and reaches a minimum value as n increases in the low-concentration region but it increases with increasing n in the higher-concentration region. Such a turning-over behavior observed in ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As is believed to be due to the crossover from a nondegenerate to a degenerate regime as the electron concentration increases.At higher temperatures, \ensuremath{\tau} observed in ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As decreases monotonically with increasing electron concentration, which is consistent with the fact that the degenerate carrier concentration is more difficult to attain at higher temperatures. The PPC-buildup transients in ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As and ${\mathrm{Zn}}_{0.3}$${\mathrm{Cd}}_{0.7}$Se have also been measured and formulated at different conditions and are shown to be very different. These results have shown that the PPC-buildup transients contain information not only about electron excitation but also electron recapture. The photoionization cross section of DX centers, ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}\mathit{X}}$, in ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As has been obtained from the PPC-buildup-transient measurements. The experimental results indicate that the transport properties in ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As are controlled by DX centers as expected, but in II-VI semiconductor alloys in the low-electron-concentration region they are governed nonetheless by tail states induced by compositional fluctuations.