| Summary: | 碩士 === 國立中山大學 === 材料科學研究所 === 94 === In this study , we investigate the relationship between growth condition and thin film quality of cadmium sulfide (CdS) thin films grown by chemical bath deposition (CBD) method. Thin film structures are analyzed by X-ray diffraction , morphology and grain size are obtained by scanning electronic microscopy , and film thickness measured by reflectance spectroscopy . In medium pH value ( pH=10.5) , 20 min deposition can achieve 450 nm film thickness . At lower pH (pH=8.5), low hydrolysis rate of thiourea limits the concentration of sulfur anion , film thickness is only 49 nm after one hour deposition at 70 ℃ , and the obtained thin film can not fully cover the substrate to have a pin-hole free film . At higher pH (pH=11), 40 min only deposites 100 nm film thickness. Strong binding between the cadmium ion and the complexing agent causes low free cadmium cation concentration in the solution, which limits the growing rate. By varying the growth pH condition, the difference in thickness is more than one order of magnitude.
In contrast to the film thickness relationship, grain size distribution suffers less influence from the pH consition. Our results indicate no explicitly relation between the grain size and solution pH condition. 20 minutes depositions result cadmium sulfide grain sizes to be 70±10nm , and 40 minutes result grain size to be 75±15nm . The results indicate that film thickness increases mainly due to the increasing of grain number, rather than the increasing of grain size .
By summarized the results, we propose that the deposition of cadmium sulfide film is initially formed homogeneous nucleation in the solution. The nuclea diffuse and adhere to the substrate. Intristic electric dipole momentum of the cadmium sulfide nano-particle provides an attractive force for the adhesion, and results preferred orientation. Meanwhile, the grain keeps growing up until the size saturated .
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