| Zinc sulfide (ZnS) is a wide, direct band gap Ⅱ-Ⅵ compound semiconductor with good transmittance in visible wavelength range; therefore it has very good application in photoelectric area. Furthermore, ZnS is one of the most important materials in the area of CIGS thin film solar cells. At present, various techniques have been employed to prepare ZnS thin films, but among them, chemical bath deposition (CBD) has demonstrated to be the most suitable method for depositing uniform and adherent thin films with a minimal thickness in large area, because it has low cost, does not require complicated equipments and vacuum system, especially its low deposition temperature and so on.This paper systematically studied the process parameters of ZnS thin film prepared by CBD technique. Experiments were carried out with hydrazine (N2H4) system, the surface morphology, composition, growth rate, structure and optical properties of ZnS thin films were observed and measured by field emission scanning electron microscope (FESEM), energy dispersive spectrum (EDS), X-ray diffraction (XRD) and UV-VIS spectrophotometer and so on. The influence of different zinc salts, different deposition temperatures and different deposition systems on structure and properties of ZnS thin films were investigated specifically.1. The influence of different deposition temperatures:influence of three different temperatures (50℃,70℃and90℃) on growth rate, structure and properties of ZnS thin films were studied, respectively. Results are shown:ZnS thin films deposited at90℃in the "static (without stirring the reaction bath)" solution are the best. Its growth rate is highest and Zn/S atomic ratio of the film is most close to1. ZnS films with low thickness deposited for1.5h to2.5h at70℃and90℃have the cubic structure only after single deposition. The average transmission of all films deposited at50℃,70℃and90℃is higher than90%. The direct band gap energy varies from3.93to4.06eV.2. The influence of different zinc salts:influence of four different zinc salts (ZnSO4, Zn(CH3COO)2, Zn(NO3)2and ZnCl2) on growth rate and quality of ZnS thin films. Results are shown:deposited from Zn(CH3COO)2and Zn(NO3)2are formed via clusters by clusters deposition. XRD and HRTEM results show that cubic ZnS films are obtained after single deposition, and the grain size of ZnS thin film deposited from ZnSO4for2.5h is about10nm. The average transmission of all films is greater than85%in the wavelength ranging from600to1100nm, and the transmission of the films deposited from ZnSO4or Zn(NO3)2for1.5,2and2.5h is greater than85%in the wavelength varying from340to600nm, which will enhance the blue response. The band gaps of all ZnS films are in the range of3.88-3.99eV. After annealing treatment, the mechanical and optical properties of the ZnS thin film deposited from ZnSO4are improved significantly.3. The influence of different deposition systems:influence of two different deposition systems (NH3system and NH3-N2H4system) on growth rate, structural and optical properties of ZnS thin films. Results are shown:with the deposition time increasing, the thicknesses of ZnS films from NH3system and NH3-N2H4system are increasing rapidly at the beginning, and then tend to saturation. Adding second complexing agent of N2H4changes the deposition mechanism of ZnS films, which attribute to the formation of heterogeneous ZnS films. The thicknesses of ZnS films deposited from NH3-N2H4system are thicker than that from NH3system. The transmission of ZnS films deposited from the two systems is higher than85%in visible range, especially that from NH3system (above95%). ZnS film (2.5h) deposited from NH3system is amorphous, but ZnS film (2.5h) deposited from NH3-N2H4is plymorphous. The result indicates that adding second complexing agent of N2H4improves the crystallinity of ZnS films. The direct band gap energy of ZnS film deposited from NH3-N2H4system (4.0eV) is higher than that from NH3system (3.98eV). |
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