Photoluminescence of a high quality CuInSe2 single crystal

A CuInSe2 (CIS) bulk single crystal grown by the directional freezing method from the melt at 1250 degrees C shows superior crystal quality, as evidenced by its (p-type) electrical properties. Photoluminescence at 1.5 K shows a strong and well resolved spectrum with fine structures never before observed from this compound. Free exciton emission exhibits a doubler structure which can either be described as a splitting due to the uniaxial crystal field or as a polariton. The energy gap of the CIS semiconductor as determined from the temperature dependence of the free exciton line is 1.058 eV. Two moderate free-to-bound transitions are assigned to V-Cu and Cu-In accepters. A weak PL peak corresponding to a deep level is interpreted as arising from the Se-i acceptor. Strong phonon replicas are also observed for the first time in a CIS bulk single crystal. The phonon wavelength of 218-237 cm(-1) is in good agreement with the reported Raman result of 233 cm(-1) for the LO phonon.