Chemical vapor transport of chalcopyrite semiconductors: CuGaS sub(2) and AgGaS sub(2)

Crystals of CuGaS sub(2) and AgGaS sub(2) with different isotopic compositions have been grown by chemical vapor transport (CVT) using iodine as the transport agent. Before performing the CVT growth, sulfur and copper were purified by sublimation and etching, respectively. super(109)Ag and the etched super(71)Ga isotopes were purified from oxides by vacuum annealing. Transparent yellow orange crystals of CuGaS sub(2) and greenish yellow crystals of AgGaS sub(2) were obtained in the shape of platelets, chunks, rods and needles in sizes of up to 8 mm (CuGaS sub(2)) and 30 mm (AgGaS sub(2)). These crystals were used to study their electronic, vibrational and thermodynamic properties. Higher excitonic states (n=2,3) were observed at low temperatures with wavelength-modulated reflectivity spectroscopy, thus proving an excellent surface and crystal quality. In addition, the experimentally determined non-monotonic temperature dependence of the excitonic energies can be well fitted by using two Bose-Einstein oscillators and their statistical factors, corresponding to characteristic acoustic and optical phonon frequencies. Isotopic shift of excitonic energies has also been successfully observed in these crystals.