Structural, electrical and optical properties of CuIn1-xGaxSe2 thin films fabricated by physical deposition techniques

In this study, the effect of anneling on structural, electrical and optical properties of CuIn1- xGaxSe2 (for x=0, 1 and x=0.5) thin films deposited on glass substrates have been investigated in detail. CuIn1-xGaxSe2 thin films were produced using physical vapor deposition techniques, such as thermal and electron beam deposition techniques. For the deposition of CuIn0.9Ga0.1Se2 thin films, both e-beam and thermal evaporation techniques were used based on various deposition parameters such as thickness and evaporation rate. On the other hand, for the deposition of CuIn0.5Ga0.5Se2 thin films, only the e-beam technique was preferred. The thickness of the deposited films was simultaniously monitored through thickness monitor during the deposition cycle and subsequently checked by surface profilometer (DEKTAK), which was found to be around 500 and 700 nm for CuIn0.9Ga0.1Se2 thin films, 1μm for CuIn0.5Ga0.5Se2. A number of characterization techniques have been employed in order to reveal the structural, electrical and optical properties of the films deposited on glass substrates. CuIn0.9Ga0.1Se2 thin films were deposited onto glass substrates at room temperature using thermal evaporation technique. During the deposition cycle, polycrystalline powder of CuIn0.9Ga0.1Se2 was used as an evaporation source. Thickness of the films were found to be around 500 and 700 nm for different two depositions. To determine the optical properties of the films, the transmission and reflectance measurements were carried out by using UV-VIS and NKD photospectrometers in the 300-1100 nm wavelength range. Based on the recorded reflectance and transmittance data, the band gap of the CuIn1-xGaxSe2 thin films were determined through the relation between absorption coefficient and photon energy. In addition, the effect of annealing on the band gap of the films was deduced with the help of the aforamentioned relation. Based on transmittance and reflectance measurement results, it was observed that there was a fluctuation in both following the films at different annealing temperatures. From the absorption and photon energy relation, energy band gaps were extracted. Energy band gaps were found to be as 1.14 and 1.25 eV for the as-grown and the film annealed at 450oC temperature for single phase CuIn0.9Ga0.1Se2 thin films, respectively. For the multi-phase CuIn0.9Ga0.1Se2 thin films, the energy band gaps were found to be as 1.43, 1.50, 1.58, 1.66 and 1.69 for as-grown, and films annealed at 150oC, 250oC, 350oC and 450oC, respectively. In order to investigate the structural properties and the nature of existing phases in the structure of the deposited films X-ray diffraction (XRD) measurements were performed. For CuIn0.9Ga0.1Se2 films, XRD measurments revealed that as-grown and the film annealed at 150oC had amorphous structure. Transition from amorphous state to polycrystalline state took place at 250oC anneling temperature. It was observed that with increasing annealing temperature there was a dramatic improvement in the crystallinity, deduced from the variation of peak intensity for certain plane directions. To determine the morphology and chemical nature of the surface of the deposited films, scanning electron microscopy (SEM) equiped with energy-dispersive-X-ray analysis (EDAX) unit, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) measurements were conducted. For CuIn0.9Ga0.1Se2 films, EDAX results showed that both the poly-crystalline powder used as evaporation source during deposition cycle and the deposited CuIn0.9Ga0.1Se2 thin films were not sthoichiometric, and there was a remarkable variation in the atomic percantage of constituent elements (Cu, In, Ga, and Se) following the post-annealing treatment. SEM measurments showed that the surface of both as-grown and annealed CuIn0.9Ga0.1Se2 films were consisted of large agglomerations, the number and size of which changed dramatically with annealing temperatures. These modifications with annealing temperature were also verified by AFM measurements, showing a good agreement between two different measurement results. For the photo-electical characterization of the films, temperature dependent conductivity and photoconductivity measurements were performed in the 80-400 K temperature range. From the conductivity versus temperature plot the activation energies of the CuIn1-xGaxSe2 films were evaluated. In addition, the resistivities of deposited films were measured through four-point-probe at room temperature. For single phase CuIn0.9Ga0.1Se2 thin films, temperature dependent photoconductivity measurements were conducted for as-grown and the film annealed at 250oC temperature under different light illumination intensities ( 17, 34, 55, 81 and 113 mW/cm2). Measurement results showed that with increasing illumination intensity there was a systematic increase in conductivity for annealed film. However, it was not the case for the as-grown films. Room temperature resistivities of as-grown and the film annealed at 250oC were calculated to be 6,1x10-3 and 6,5x105 Ω.cm, respectively. CuIn0.5Ga0.5Se2 thin films were deposited onto glass substrated at room temperature using electron beam technique. During the deposition cycle, poly-crystalline powder of CuIn0.5Ga0.5Se2 was used as an evaporation source. Thickness of the films were found to be around 1000 nm. The aforamentioned characterization measurements were conducted for the CuIn0.5Ga0.5Se2 thin film deposited by e-beam technique to investigate the structural, electrical and optical properties. EDAX results showed that source powder was nearly stoichiometric. In addition, it was observed that there was a deviation in stoichiometery of deposited films. The atomic percentage of Cu (copper) in the composition was found to be around %0,38 for as-grown film. Following the post-annealing process, a significant fluctuation in atomic percantage of constituent elements was observed. SEM measurements showed that the surface of both as-grown and annealed CuIn0.5Ga0.5Se2 films were consisted of large agglomerations, the number and size of which changed dramatically with annealing temperatures. These modifications with annealing temperature were also verified by AFM measurements, showing a good agreement between two different measurement results. CuIn0.5Ga0.5Se2, In4Se3, GaSe, In2Se3 and Se phases were resolved from the XRD spectra of the CuIn0.5Ga0.5Se2 films. Dominant peak was associated with GaSe phase with (1 0 3) preffered orientation. XRD results also revealed that as-grown films had an amorphous structure and transition to polycrystalline structure were starting at 250oC anealing temperature. Most instense crystalline form was observed for the film annealed at 450oC Temperature dependent photoconductivity measurements were performed for as-grown and CuIn0.5Ga0.5Se2 thin films annealed at 350oC and 450oC under the aforementioned light intensities. Results showed that there was a remarkable increase in conductivity following the increase in illumination intensity for all CuIn0.5Ga0.5Se2 thin films. Room temperature conductivities of this films were calculated to be 6.76x107, 3.38x107 and 2.19x105 Ω.cm for as-grown, the film annealed at 350 oC and 450 oC, respectively. Based on transmitance and reflectance measurement results, it was observed that there was a fluctuation in both following the films at different annealing temperatures. From the absorption and photon energy relation, energy band gaps were extracted, which were found to be as 1.72, 1.78, 1.80, 1.81, 1.91 and 1.92 eV for as-grown and films annealed at 150oC, 250oC, 350oC, 450oC and 550oC, respectively.