Plasma-Enhanced Atomic Layer Deposition and Vapor Phase Infiltration of ZnO - From Fundamental Growth Characteristics to Piezoelectric Films

Julian Pilz
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In this thesis, the growth of the semiconducting material ZnO by two methods - plasmaenhanced atomic layer deposition and vapor phase infiltration - is investigated. As ZnO is utilized in diverse applications such as UV-protection, gas sensors, or piezoelectrics, precise knowledge about the characteristics of the growth process is needed to obtain the desired properties for a specific application. Plasma-enhanced atomic layer deposition (PE-ALD) is a thin film technique which can deposit uniformal and conformal films with high thickness control at low temperatures. The presented studies show that PE-ALD is able to deposit ZnO with small amount of impurities as low as room temperature. Furthermore, by variation of the substrate temperature, ideal temperature regions for specific applications and the relationship between growth and resulting properties could be identified. In the beginning of the deposition, deviations from the ideal growth occur, which are identified as substrateenhanced island growth. The formation of crystallites is found to occur after this initial growth periode. The obtained knowledge about these growth characteristics is furthermore applied to piezoelectric devices. The piezoresponse of ZnO, sandwiched between electrodes, is hereby studied on both flexible and rigid substrates with a combination of macroscopic and scanning probe techniques. Vapor phase infiltration (VPI) is a technique for transforming polymers into hybrid organic/inorganic materials. It often uses the same precursors as ALD but instead of growing a thin film on a substrate, the polymer free volume is infiltrated with the precursors. In the thesis, the successfull infiltration of ZnO into polyisoprene is presented. Polyisoprene is an elastomeric polymer, a class of polymers which has not been widely studied as a substrate for VPI. The infiltration kinetics and the chemical mechanisms of this system are presented and it is shown that pre-heating of the polymer largely affects these due to changes in thickness and chemical structure. Concluding, the thesis gives fundamental insights into the growth characteristics for a future application of ZnO thin films or polymer/ZnO hybrids in diverse fields as well as a demonstration of ZnO in a piezoelectric device. 

Graz University of Technology
(Graz, Austria)
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