Growth of Transparent and Conductive Polycrystalline (0001)‐ZnO Films on Glass Substrates Under Low‐Temperature Hydrothermal Conditions

Abstract

Flat panel display technology seems to be an ever-expanding fi eld developing into a multibillion dollar market. A set of technical solutions involve a transparent conducting fi lm (TCF) that is today still dominated by indiumtinoxide (ITO). In a race to fi nd alternatives that would avoid the indium pitfalls, mainly due to its increasing price and limited natural availablity, replacement materials have been extensively investigated. This work demonstrates that by exploiting basic principles of crystal growth in geometrically constrained conditions, zinc oxide (ZnO) could easily be utilized for this purpose. ZnO layers were grown on inexpensive glass substrates via lowtemperature citrateassisted hydrothermal (HT) method. It was shown that in the nucleation stage the crystal growth can be effi ciently controlled by spatially confi ned oriented growth (SCOG) mechanism to produce smooth and dense (0001) oriented polycrystalline ZnO fi lms with superb optical properties. Our products show optical transparency of 82% and surprisingly low sheet resistance for undoped ZnO, only in the order of few 100 Ω sq − 1 . We believe that a very high degree of selforganization between the ZnO crystals in our polycrystalline fi lms grown under controlled SCOG conditions is main reason for the highest so far reported transparency to conductivity ratio for undoped ZnO thin fi lm ceramics.conductivity which meet the high standards for many photonic and/or electronic applications.

Comparison of chemical bath-deposited ZnO films doped with Al, Ga and In

“…S1). The films exhibit a spatially confined, oriented growth mode, typical for ZnO grown by this technique [9]. In the early stages of the CBD, the polycrystalline seed layer promotes the growth of randomly oriented crystals.…”

“…This concept was used by several groups to deposit intrinsic [9] and doped ZnO thin films [10][11][12][13][14]. The key to obtain dense materials rather than nanorods was in all approaches the use of citrate as structuredirecting agent, which selectively decelerates crystal growth along the ZnO c-axis [9,15].…”

(0 0 2)-oriented growth and morphologies of ZnO thin films prepared by sol-gel method

“…Many techniques have been utilized to prepare ZnO thin films, such as metal organic chemical vapor deposition, pulsed laser deposition, sputtering, hydrothermal, sol-gel method, etc. [8][9][10][11][12][13][14][15][16][17][18]. Due to the low cost and simple equipment, sol-gel method has been extensively applied to prepare ZnO thin films.…”

Mg-doped ZnO films prepared by chemical bath deposition

“…As seen in many investigations on solution-deposited ZnO under similar conditions [28,29], Cit 3-is a strong crystal morphology-controlling agent for obtaining a Zn-terminated (0001) ZnO surface texture. The strong adsorption onto the {0001} ZnO plane arises from the free rotation around the single bonds of the three carboxylate groups, which allows the Cit 3-to adopt to the geometry of the zinc atoms of the {0001} plane [28,30] (Fig. 6a).…”