In this paper, we report a comparison between CdS deposition by a conventional batch reactor and a newly developed continuous-flow microreactor. This microreactor setup makes use of a micromixer for efficient mixing of the reactant streams and helps in controlling the homogeneous reaction before the solution impinges on a substrate. Transmission electron microscopy analysis indicated that an impinging flux without the formation of nanoparticles could be obtained from this reactor at a short residence time. The surface morphology of the deposited films clearly indicated an improvement of film smoothness and coverage over films deposited from a batch process. Highly oriented nanocrystalline CdS films were obtained from the continuous-flow microreactor in contrast to poor crystalline films from the batch process. This new approach could be adopted for the deposition of other compound semiconductor thin films at low temperatures using a solution-based chemistry with improved control over the processing chemistry.

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Chemical Bath Deposition (CBD) has been primarily used in the process of fabricating Cu (In, Ga) Se 2 and CdTe based thin film photovoltaics. Normally carried out as a batch process, the most significant feature of CBD is its ability to deposit thin films at low temperature. However, a major drawback of CBD is the formation of particles which generate waste and create defects in devices. Moreover, it is necessary to better understand the role of particle formation and deposition in CBD thin film growth process. We have developed a continuous impinging flow microreactor for CBD. This novel reactor provides the capability to control the particle size and size distribution before impinging on the substrate and thus helps us in differentiating the molecule-by-molecule growth from the cluster-by-cluster growth. It can also be used to deposit compound semiconductor thin films for electronic devices with improved film quality and minimum waste production. Our continuous impinging flow microreactor setup makes use of a micromixer for efficient mixing of the reactant streams.CdS semiconductor thin films were successfully deposited on oxidized silicon substrates using this novel microreactor setup. In this study, comparisons of nanostructured thin films deposited by a batch reactor and a continuous impinging flow microreactor were performed by various characterization techniques. The surface morphology of the deposited films, carried out by AFM, SEM and Dektak surface profiler, clearly indicated an improved film quality from the microreactor. As seen in Fig. 1a, the microreactor produced a dense and uniform nanocrystalline CdS film. On the other hand, the batch reactor produced groups of non-continuous CdS nanocrytals on the substrate with the same amount of time (Fig. 1b). The CBD solutions were characterized by collecting samples at certain time using TEM copper grids covered with thin lacey carbon film. No nanoparticles could be observed from the impinging flow solution. In contrast, the batch solution produced agglomerates of nanoparticles over 0.1 um in size with individual particles up to 10 nm in diameter as shown in Fig. 2.