Study On Properties Of ZnSe Thin Films Deposited By Chemical Bath

Zinc selenide(ZnSe) is an important Ⅱ-Ⅵ group semiconductor material, and widely used as buffer layer on CdTe, CIGS and CZTS based solar cells device structure in recent years.Chemical bath deposition(CBD) is the main fabrication technique for the preparation of above mentioned materials. So ZnSe thin films are fabricated on glass substrate by CBD in this paper.Comprehensive theoretical investigation are given for the CBD wet chemical process, XRD, SEM and XPS technologies are used to investigate the structure, optical properties and annealing behavior in air and nitrogen atmosphere.Based on the basic principles of chemistry, comprehensive chemical equilibrium investigation were given on normally used CBD compound solution system as ZnSO4/NH4OH/N2H4/CSe(NH2)2. It was found that when the balanced ammonia concentration is above 1 mol/L,the residual zinc ions concentration is less than 3.467×10-10 times of the initial ones, equation of the pH value was also given on NH4OH/N2H4 double complex system, which clearly confirmed the formation of Zn(OH)2 precipitate, dependence of growth velocity on the concentration of the basic Zn and Se ions were discussed. All these results have great significance for the actual preparation of ZnSe as well as Cd S and ZnS buffer layers.SEM and XRD were used to investigate the structure properties. Results show that thin films have zinc-blende structure and(111) preferred orientation, mean grain size was around 200 nm.Transmission and reflection spectrum of the films were also measured by spectrophotometer,optical properties like absorption coefficient, extinction coefficient, refraction index and the energy gaps were investigated comprehensively in visible wavelength region. It was found that transmittance and reflectance decreased with the increase of film thickness, absorption coefficient in intrinsic absorption region is very high and increases with the decrease of wavelength. Obtained energy band gap values of 3.16, 3.14, 3.07 eV when film thickness are 150, 400, 630 nm,respectively.The structural, morphological and chemical compositional modification before and after annealing are investigated at different temperatures(473 K 、 573 K 、 673 K 、 773 K) in air atmosphere. Results show that mean grain size is improved with an increased annealing temperature below 573 K, apparent phase transformation from pure zinc blende structure of ZnSe to stable hexagonal phase of ZnO is observed when annealing at 673 K and 773 K. Relative atoms content of Se and Zn reduces and O content increases under increased annealing temperature.More loss of Se atoms is also found than Zn atoms under identical annealing temperature and noSe signal can be detected at 773 K by XPS. Mechanism is given and explains well to all of the results, which maybe due to the decomposition of ZnSe to Zn and Se under certain temperature,then react with oxygen to form ZnO and SeO2, respectively, and ZnO can be partially re-deposited again.