Charge Transport and Space-Charge Formation in Cd1−xZnxTe1−ySey Radiation Detectors

Abstract

The electron- and hole-transport properties in cadmium zinc telluride selenide (CZTS) crystals are studied using a laser-induced transient-current technique with pulsed and dc bias. The internal electric field profile and velocity of surface recombination are determined by Monte Carlo simulations of electron and hole transient currents combined with a numerical solution of the drift-diffusion equation coupled with Poisson's equation. Electron and hole drift mobilities of ${\ensuremath{\mu}}_{e}$ = 830 ${\mathrm{cm}}^{2}$/Vs and ${\ensuremath{\mu}}_{h}$ = 40 ${\mathrm{cm}}^{2}$/Vs, respectively, are determined. We also develop a simple technique for evaluating surface recombination directly from measured current waveforms without the need for numerical simulation. The good quality of the prepared detector at pulsed bias, with electron- and hole-mobility-lifetime products of $(\ensuremath{\mu}\ensuremath{\tau}{)}_{e}$ = 1.9 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{2}$/V and $(\ensuremath{\mu}\ensuremath{\tau}{)}_{h}$ = 1.4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{2}$/V, respectively, are observed. The formation of a positive space charge, originating from hole injection combined with a recombination level, is found. We observe a significant position dependence of the lifetime of electrons and holes in dc bias due to hole injection. The experiment is successfully fitted by a simple model dominated by a single deep recombination level with an energy of ${E}_{t}={E}_{C}\ensuremath{-}0.73\phantom{\rule{0.2em}{0ex}}\mathrm{eV}$; concentration of 7.3 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$; and electron- and hole-capture cross sections of 3.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{2}$ and 6.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{2}$, respectively.