利用原子層沉積技術成長氧化鋅薄膜於玻璃基材上之研究

Abstract

本文利用原子層沉積技術在玻璃基材上成長氧化鋅(ZnO)薄膜，探討其電性及光學性質。本論文分為三個不同主題，第一部份為氧化鋅作為透明導電膜之研究，結果顯示摻雜Al為2%的ZnO薄膜其最低電阻值為1.45×10-3Ω-cm，其穿透度約為74%以上，且此薄膜的光吸收邊界值為3.35eV，很接近ZnO的能隙3.37eV。此外我們發現利用ALD所製備的ZnO薄膜做為透明導電膜，最主要的機制是改善載子遷移率來降低其電阻率，這與其他的製程方法有效提高載子濃度是不同的。並觀察出傾向於（ ）方向成長且其晶粒形貌類似楔子狀之AZO薄膜比傾向往(0002)方向成長的ZnO薄膜其晶粒型貌類似柱狀，具有較高的電子遷移率(mobility)。 第二部份，我們主要研究如何利用ALD沉積高品質的ZnO薄膜於玻璃基材上。我們利用兩段式的成長方式成長ZnO薄膜，並藉由X-光繞射儀(X-ray diffraction, XRD)觀察得知其薄膜具有(0002)晶面擇優取向的晶體結構。並由光激發光頻譜(Photoluminescence, PL)的研究顯示，此薄膜關於defect的發光強度很低，主要發光波長是靠近ZnO能隙位置(near band gap)所發的紫外光，研究顯示我們已得到具有良好晶體品質的ZnO薄膜於玻璃基材上。 第三部份主要是將第二部份所得到的高品質ZnO薄膜做PL光學性質的量測，探討其發光性質。在變溫系統下的PL量測，觀測其LO(longitudinal optical)聲子與FX(free exciton)自由激子在不同溫度下互相作用的變化情形，另外我們發現在ZnO薄膜裡發生random rasing 的行為，其具有低的臨界強度值(threshold intensity)以產生受激輻射(stimulated emission)的現象，顯示使用ALD技術所成長的ZnO薄膜具有良好的光學品質。

This thesis presents the investigation of the electrical and optical properties of the ZnO films deposited by atomic layer deposition (ALD) on the glass substrates. The thesis is divided into three topics. The first topic is the deposition of ZnO films as transparent conductive oxide (TCO). The Al-doped ZnO film with a low resistivity of 1.45×10-3Ω-cm was obtained and the average optical transmittances of the films is in excess of 74%.The decrease in the resistance of ZnO films by the Al doping is caused by the increase in the mobility (＞200cm2/V-s),which may be attributed the the increase in the grain size with（ ）orientation as indicated in the X-ray diffraction(XRD) pattern. The second topic is the preparation of high-quality ZnO films on the glass substrate using the ALD technique.We used two-step approach to grow ZnO films with high (0002) orientation indicated by the XRD measurement. The PL spectrum of ZnO exhibits a strong, near-band-edge UV emission at 379 nm with negligible defect-related bands. The buffer layer and the deposition temperature are important factors for the the growth of ZnO films with high optical quality. The third topic is the study of the optical properties of the high-quality ZnO thin films grown by ALD on the glass substrate. The exciton emission and their multiple-phonon replicas were observed in the low-temperature PL spectrum. The optically pumped stimulated emission with a low threshold intensity (51kW/cm2) was also observed at room temperature. The low-threshold stimulated emission indicates the ZnO film grown by the ALD technique on the glass substrate has a high optical quality.