FEATURES OF EDGE ABSORPTION IN ZINC DIARSENIDE

Optical transitions near the intrinsic edge of perfect anisotropic ZnA s2 crystals were characterized by absorption, photoconductivity, and. short-circuit photocurrent spectral measurements. The position of the intrinsic edge for the E parallel to c polarization is found to be det ermined by an indirect, allowed excitonic transition for absorption co efficients alpha < 10 cm(-1) and by a direct, allowed excitonic transi tion for alpha > 10 cm(-1). The 5-K transmission spectra for E perpend icular to c exhibit a free-exciton series consisting of three peaks. T he exciton binding energy is found to be 17.5 meV. The excitonic band gap at 300 K is 0.875, 0.939, and 0.956 eV for the indirect, allowed d irect, and forbidden direct transitions, respectively, with temperatur e coefficients of -4.3 x 10(-4), -3.8 x 10(-4), and -3.3 x 10(-4) eV/K in the temperature range 80-300 K.