Optical Absorption Characteristics of Cu-Ga-Se Thin Films Grown by Molecular Beam Deposition Method

摘要

Energy band gap and excitonic energy of high quality Cu-Ga-Se thin films with Cu/Ga-ratio ranging from 1.45 to 0.70 were determined by means of optical absorption measurement in the wavelength range 500 to 1500 nm at room temperature. The Cu-Ga-Se thin films were prepared by molecular beam deposition (MBD) method with 2-stage growth process onto soda-lime glass as well as Mo coated soda-lime glass substrate. During the growth process, power of the substrate heater and pyrometric detection of the Cu-Ga-Se surface temperature were monitored simultaneously. The Cu-Ga-Se thin firms with the thickness of 1.4-2.4 μm corresponding to Cu/Ga-ratio from 1.45 to 0.70 can be obtained with high precision. The optical transmittance and reflectance measurements were used to observe the optical absorption characteristics of the Cu-Ga-Se films from the Cu-rich (Cu/Ga=1.45) to the Cu-poor (Cu/Ga=0.70) composition. All optical absorption spectra (ranging from 600 to 800 nm) of the Cu-Ga-Se thin films were fit including excitonic absorption at room temperature. The free excitonic resonance energy of 1.689 eV and fundamental energy band gap of 1.696 eV were obtained at room temperature. Both energy band gap and free excitonic resonance energy of the Cu-Ga-Se thin films were found to be no significantly change with Cu/Ga-ratio from 1.45 to 0.70. The amount of free exciton was slightly decreased with decreasing of Cu/Ga-ratio. The results indicated that the Cu-Ga-Se polycrystalline thin films with excitonic grade could be fabricated by MBD method.