電化學製備氧化鋅及其結構與光學性質研究

Abstract

本研究於氯化鋅水溶液中，以電化學方式製備氧化鋅於透明導電玻璃上。首先，應用循環伏安法解析溶液與工作電位之關係，找出最適合沈積氧化鋅的條件，接著探討溶液組成及電鍍參數對鍍層材料性質和光學性質的影響。 研究結果顯示氧化鋅沈積的電位範圍約在－0.6至－1.0V(vs. Ag/AgCl)之間，過電位不足造成反應速率過慢，過電位過大則造成金屬鋅的沈積。定電位電鍍的過程中，電流值隨時間遞減，且趨於一飽和值，顯示氧化鋅半導體導電性不佳。於本研究中，藉由電化學所製備的氧化鋅為柱狀晶體，以c軸垂直基材成長，XRD結果顯示為多晶的氧化鋅鍍層具有(002)優選方位。晶體的緻密性影響氧化鋅鍍層粗糙度，粗糙度大造成散射嚴重，降低穿透率。工作電位、過氧化氫濃度及電鍍時間的增加皆有助於氧化鋅晶體的緻密性，但並無提升其光學性質。改採兩段式變電鍍製程不但能使晶體緻密，也降低了柱狀晶的尺寸，大幅提升了穿透率。由PL與PLE的結果發現一段式製程的發光位置、吸收邊際及能隙大小都有藍位移的情形，推測為內部存在大量間隙鋅原子所致，而兩段式製程則改善了這些問題，顯示兩段式電鍍製程可沈積較佳的氧化鋅鍍層。

Zinc oxide (ZnO) was electrodeposited from zinc chloride electrolyte on ITO glass. The potentials at which ZnO can be plated from a specific electrolyte were determined using cyclic voltammetry. The effects of electrolyte composition and electroplating parameters on the materials and optical properties of ZnO films were then systematically studied. Experimental results indicate that ZnO films can be plated at potentials ranging from -0.6 to -1.0V (vs. Ag/AgCl). The deposition rate was relatively small at low overpotentials, while metallic zinc was likely to form at high overpotentials. During potentiostatic deposition, the resultant current decreased with continued electroplating, and gradually reached a saturated value, signifying the semiconductor characteristic of ZnO. The ZnO electrodeposits consisted of columnar grains, which grew with c axis perpendicular to the substrate. Consequently, this polycrystalline ZnO film displayed a (002) texture as characterized by XRD. Dense ZnO crystals reduced the surface roughness of ZnO film, which, in turn, enhances the transmittance of the film due to less scattering effects. Although more dense ZnO films were plated by increasing overpotential, solution H2O2 concentration, and deposition time, these ZnO films exhibited insignificant improvement in optical properties. A two-step electroplating process was thus employed to prepare the ZnO film. This process enhanced the population density of columnar ZnO crystals, while reduced the size of the crystals. Both contributed to the enhanced transparency of ZnO films. The PL peak, absorption edge, and energy band gap of ZnO prepared by one-step process was found to have blue shift in PL and PLE spectrums, suggesting that the ZnO crystals contained significant amounts of interstitial zinc vacancies. The ZnO film fabricated by the two-step process showed less blue shift in PL and PLE spectrums. Apparently, the ZnO film with improved optical properties can be deposited using this two-step electroplating process.