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 structur
es never before observed from this compound. Free exciton emission exh
ibits a doubler structure which can either be described as a splitting
due to the uniaxial crystal field or as a polariton. The energy gap o
f the CIS semiconductor as determined from the temperature dependence
of the free exciton line is 1.058 eV. Two moderate free-to-bound trans
itions are assigned to V-Cu and Cu-In accepters. A weak PL peak corres
ponding to a deep level is interpreted as arising from the Se-i accept
or. Strong phonon replicas are also observed for the first time in a C
IS 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.