A Differential Scanning Calorimetry (DSC) Study on The Pyrolysis Mechanism of Zinc Oxide CVD Precursor, Zinc Acetylaceto
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A Differential Scanning Calorimetry (DSC) Study on The Pyrolysis Mechanism of Zinc Oxide CVD Precursor, Zinc Acetylacetonate Yuneng Chang, Junhsuan Hsieh, Chonan Wang, Liting Hong Lunghwa University of Science and Technology, Dept. of Chemical Engineering, No.300, Sec.1, Wanshow Rd., Gueishan, Taoyuan, 333, Taiwan, R.O.C. ABSTRACT In this study, we used differential scanning calorimetry (DSC) to study pyrolysis kinetics of zinc acetylacetonate (Zn(acac)2) in chamber ambient simulating real CVD condition, with heating rate from 10 to 20 oC/min. For Zn(acac)2 heated in pure N2, inert environment, DSC showed three endothermic peaks locating at 91-106 oC, 121-128 oC, and 135-142 oC, with peak area being 88 KJ/mol, 14 KJ/mol, and 18 KJ/mol, respectively. The DSC residuals analyzed by IR indicated all peaks might be originated from physical phenomena, since no compositional change observed. Further study by temperature programmed polarized optical microscope (POM) showed these peaks might be related to dehydration, phase transition, and melting of Zn(acac)2 and its hydrates. DSC in oxygen-containing ambient with O2 concentration from to 13 to 40 % showed three endothermic peaks and two additional exothermic broad peaks at higher temperature, 209-236 oC, and 330-400 oC. These exothermic peaks are assigned to oxidative decomposition of Zn(acac)2 and releasing of ligands. When oxygen concentration increased, peak area grew. Positions of these peaks depend on heating rate. Increasing the heating rate caused every peak to shift toward higher temperatures, with the first two endothermic peaks merged into one broad one. INTRODUCTION Compound semiconductor zinc oxide (ZnO), is an ultraviolet wavelength LED materials. ZnO films are used for transparent conductive films in flat panel displays and surface acoustic wave (SAW) devices for its high conductivity and piezoelectricity. Chemical vapor deposition (CVD) is a valuable technique for preparing zinc oxide films with electronic and optoelectronic purposes. We have successfully demonstrated an atmospheric MOCVD process using zinc acetylacetonate as precursor to deposit (002) ZnO films at 320 oC, with epitaxy like quality [1]. However, the complex nature of reaction mechanism and specifically precursor thermochemistry has laid limitations for scaling up ZnO CVD process. Focusing on precursor behavior during heating history, differential scanning calorimetry (DSC), DTA, and TGA have been considered. These thermal analysis techniques provide the solution by defining exact temperature positions for any energy/mass change involved transition during heating history. According to literature, Zn(acac)2 TGA study show that, under a ramp heating rate of 16 K/min, the weight loss by decomposition initiated at 325 K, reach the maximum (peak position) at 360, 385, 400 K, and ended by 560k [2]. Beech, G. [3] has discussed that the additional two peaks near the major fusion peaks was assigned to melting point of monohydrate (Zn(acac)2 -H2O), and a phase transition. Glavas [4] used DTA in
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