Zinc oxide (ZnO) is one of promising semiconductors for numerous (opto)electronic applications because of excellent properties such as a wide band gap, a high exciton binding energy, and large piezoelectric coupling coefficients. ZnO crystallizes in the wurtzite hexagonal structure, which forms the polar c-plane and non-polar a and m planes. ZnO thin films of different preferential crystal orientations exhibit different properties, which leads to various applications.
In ZnO films grown by chemical vapor deposition (CVD) techniques, the crystal orientation is mainly influenced by the orientations of the nucleation layer, which is, in turn, affected by diffusion of atoms on the substrate surface during film growth. Several parameters affect the diffusion process such as oxidation reactions, type of substrates, growth temperatures, and gas flow rates. In the growth of metal oxide films by CVD, oxidation rate can be controlled by varying the supply amounts of oxygen and metal sources.
Mist-CVD is one of the non-vacuum metal oxide film growth techniques1 capable of controlling the oxygen and metal supply amounts precisely. The precise control of the oxygen/metal ratios was realized by using the mist-CVD system with two solution chambers2 storing oxygen and metal sources (H2O and Zn precursor in this case) separately. In this work, we found strong dependences of the crystal orientations, growth rates, and surface morphologies on the [H2O]/[Zn] ratios in the ZnO films grown by the two chamber system. The equilibrium reaction of thermal decomposition of the Zn precursor was enhanced by addition of H2O leading to an initial increase in the growth rate with the [H2O]/[Zn] ratio. However, excessive addition of H2O caused a decrease in the growth rate. Moreover, H2O also influences the film crystal orientation. The diffusion of zinc and oxygen reactive atoms on the substrate surface during film formation was influenced by the [H2O]/[Zn] ratios, which resulted in different ZnO preferential growth orientation and film surface morphology.
The details about fabrication of the ZnO films using the mist-CVD system with two solution chambers and a plausible ZnO film growth mechanism with different [H2O]/[Zn] ratios will be presented in the conference.
 T. Kawaharamura, Jpn. J. Appl. Phys. 53, 05FF08 (2014).
 G.T. Dang, T. Yasuoka, Y. Tagashira, T. Tadokoro, W. Theiss, T. Kawaharamura, Appl.Phys. Lett. 113, 062102 (2018).
5:00 PM–7:00 PM Apr 23, 2019 (US - Arizona)
PCC North, 300 Level, Exhibit Hall C-E