Investigation of low temperature solution-based deposition process for flexible electronics

The critical contribution of this dissertation is to provide a better understanding of the fundamental Chemical Bath Deposition (CBD) growth kinetic and mechanism for the well known II-VI semiconductor CdS using the newly developed continuous flow microreactor. This continuous flow microreactor provides the temporal resolution to control the homogeneous reaction of the chemical solution before it impinges on the substrate surface. This capability was used to decouple the homogeneous particle formation and deposition from the molecular level heterogeneous surface reaction to overcome the drawbacks associated with a conventional batch process. Transmission electron microscopy (TEM) analysis indicated an impinging flux without the formation of nanoparticles which could be obtained from this reactor in a short residence time. In addition, the reactor could be operated in a homogeneous particle formation regime. Size increasing CdS nanoparticles grown by homogeneous reaction were clearly observed from TEM and SEM micrographs by increasing the residence time from 1 to 280 sec using pre-heated precursor solutions. The formation of CdS nanorod and arrayed nanorod bundle structures using the CBD recipe were also observed in some areas and reported here for the first time. The growth kinetics were studied using a particle-free flux. The deposition results suggest that HS- ions formed through the thiourea hydrolysis reaction are the dominant sulfide ion source responsible for the CdS deposition rather than thioura itself that had been widely discussed in almost all of the previous literature. This finding could not be observed previously by a conventional CBD batch setup because all the reactant solutions were sequentially pulled into the reaction beaker and mixed all at once. An impinging flux without the formation of nanoparticles enables us to deposit extremely smooth and highly oriented nanocrystalline CdS semiconductor thin films at low temperature (80°C). Enhancement-mode functional thin film transistors with an effective mobility of mueff =1.46 cm2/V s, drain current on-to-off ratio of approximately 105 and turn-on voltage at 0 V were fabricated from the as-deposited films without any post annealing process. This microreactor could be adapted for the deposition of other compound semiconductor thin films such as highly transparent amorphous Indium Oxide (In2O3) thin films at low temperature (70°C) using chemical solution deposition and opens a low-cost avenue to fabricate thin film flexible electronics on polymeric substrates.