TitleGrowth, Characterization and Applications of Zinc Sulfide Thin Films by Solution-Based Processes
Publication TypeThesis
Year of Publication2017
AuthorsChiu, D
UniversityOregon State University
CityCorvallis, Or.
Thesis TypeDissertation

Zinc Sulfide (ZnS) thin film, with a wide band gap, has been used for many applications, such as buffer layer for CIGS solar cells, light emitting diodes and thin film electroluminescent devices. In this work, ZnS thin films were prepared using two different deposition processes. In the first method, ZnS thin films were deposited by using conventional chemical bath deposition (CBD) process. Micro-reactor assisted solution deposition (MASD) with a flow cell was used as the second method. Growth kinetics of ZnS thin films in CBD was analyzed using in-situ quartz crystal microbalance measurements, and ex-situ transmission electron microscopy (TEM) and scanning electron microscopy (SEM) measurements. The results from the TEM and SEM measurements suggest that the film growth follows a two-step process with the formation of the nuclei in the solution first, attachment to the surface, followed by aggregation of nanoparticles into half spheres on the surface of the substrate and finally half spheres connect to the neighbor half spheres, thereby forming a continuous film. The mechanism study, verified by the SEM images, shows that nucleation starts very early in the CBD process. The degree of supersaturation influences the growth rate and final surface morphology. Temperature-dependent growth rate in the linear growth region follows the Arrhenius equation with an estimated value of activation energy (Ea) to be around 36 KJ/mol. This value, which is considered low (less than 40 kJ/mol), indicates that the rate limiting step is more likely to be a physical process such as adsorption or diffusion, rather than a chemical process, which tends to have higher activation energies. In our study, the chemical bath is vigorously stirred so that the rate-limiting step is likely controlled by a physically adsorption mechanism. The continuous flow micro reactor was used to deposit ZnS thin films using various flow cells of different designs. The depositions were carried out on display glass of 1 inch wide by 3 inches long. Both analytical equations (Hagen-Poiseuille) and computational fluid dynamics were applied to determine proper height for the flow channel. COMSOL Multiphysics simulation of fluid flow along with particle tracer was carried out to find an optimum cut out radius for further study. The film thickness growth kinetics and solute concentration near the substrate surface was simulated using the COMSOL Multiphysics program with an assumption of laminar flow, transport of diluted species and a simplified first order reaction. An insert that mimics a cut out radius of 2.31 inches was fabricated using a 3D printer and installed in the flow cell to deposit ZnS thin films. ZnS thin films deposited using the flow cells with and without the 3D printed insert were investigated. The results were analyzed using plane-view and cross-sectional SEM images. The film thickness was determined by cross-sectional SEM image. The results indicated that the thickness uniformity was improved with the 3D printed insert. We found toward the end of the substrate, the ZnS thin film was not continuous due to the lower solution concentration caused by the depletion of reactants. New flow cell designs were proposed and COMSOL simulation was performed to examine the effectiveness of these flow cells. To demonstrate the utility of the ZnS thin films by solution-based processes, SnS and CuS thin films were deposited on top of the ZnS thin film to form SnS/CuS/ZnS layered precursor film then followed by selenziation at various temperatures in an attempt to produce CZTSSe absorber layers for CZTSSe thin film solar cells.