利用常壓噴射式電漿製備氧化鋅透明導電薄膜及其性質之研究

Abstract

本研究為在常壓下利用直流脈衝式電源產生的噴射式電漿製備氧化鋅透明導電薄膜，本實驗參考噴霧熱解法處理先趨物水溶液之方法，利用1.7 MHz超音波霧化器將氧化鋅水溶液霧化後，再通入載流氣體將此先趨物通入電漿下游反應，本製程結合噴霧熱解法設備便宜與常壓電漿具有高反應性自由基之優點，提供高沉積薄膜速率且可大面積製備氧化鋅薄膜的製程方法。 藉由改變電漿操作條件研究其對本製程之最適化條件，在本製程中發現電漿氣體使用氮氣製備之氧化鋅薄膜導電性比使用氧氣好；隨著電漿的施加電壓增加及氣體流量減少，電漿下游的溫度及反應性隨之增加，且低氣體流量使先驅物滯留時間較長，可使氯化鋅水溶液先形成鹼式氧化鋅再轉化成氧化鋅薄膜，本製程電漿最適化的條件為氮氣電漿在30 slm、275 V下操作，可製備出電阻率為13 Ω cm且在可見光範圍內平均穿透率為80%的氧化鋅透明導電薄膜。 透過研究氧化鋅薄膜可能的成長機制，欲改善氧化鋅薄膜的表面結構與基本性質。研究在不同沉積時間下之氧化鋅薄膜結構，當沉積時間為60秒以上可得到一較連續的薄膜，而當沉積時間為120秒時可得到氧化鋅薄膜下層為柱狀的緻密結構，但薄膜表面具有片狀結構的物質，透過XPS縱深分析發現表面含氯的成份較高，故推論此片狀結構為鹼式氯化鋅；透過氮氣電漿表面處理氧化鋅薄膜可減少表面的片狀結構，且隨著表面處理時間增加，由SEM影像可以觀察到薄膜表面的晶粒變大；利用熱退火處理本實驗製備的氧化鋅薄膜，對氧化鋅薄膜的性質並沒有太大的影響。藉由改變基材至電漿出口的距離與進料位置，發現會影響氧化鋅薄膜的晶粒大小，當進料位置為靠近電漿出口，則溫度較高且成核速率較快，可得到較小的晶粒，而當基材至電漿出口的距離越遠，表示薄膜成長時間較長可得到較大的晶粒。 探討改變反應先驅物的條件對氧化鋅薄膜之影響，隨著先驅物載流氣體流量增加，可通入越多的先趨物水溶液使沉積薄膜的速率增加，但也會使表面片狀結構增加，故當載流氣體流量為500 sccm時可以得到導電性最好的氧化鋅薄膜；也利用摻雜鋁或銦離子欲增加氧化鋅薄膜之導電性，但目前在本製程中此方法對氧化鋅薄膜之導電性並無太大的影響。

Deposition of transparent and conductive ZnO thin films by an atmospheric pressure plasma jet (APPJ) is studied. The APPJ used is sustained by a pulsed power source with a repetitive frequency up to 25 kHz using N2 or O2 as plasma gases. Nebulized zinc chloride solution is used as the precursor and is sprayed into the downstream of the plasma jet to deposit thin films on Si wafers or glass substrates. By using N2 plasmas, lower resistivity can be obtained comparing with those obtained using O2 plasmas. X-ray diffraction spectra show that the crystal structure changes with the operating parameters, namely plasma gas flow rate and the applied voltage, which influence the jet temperature and reactivity. Under an applied voltage of 275 V and a flow rate of 30 slm, dense and smooth films can be deposited. ZnO films with a resistivity of 13 Ω-cm and an average transmittance of 80% between 400 and 800 nm can be obtained. The growth mechanism of ZnO thin films deposited by the APPJ is proposed. It is found that upon exposure of the precursor to the plasma jet, sheet-like zinc hydroxide chloride (ZHC) are formed first, and is converted to zinc oxide if the jet temperature is high enough. Under relatively low temperature, the conversion of the precursor end at ZHC. The grain size of the films is greatly influenced by the nucleation and growth rate. High jet temperature leads to a larger number of the nuclei and results in smaller grain sizes and denser ZnO thin films. We study the influence of the carrier gas flow rate on the properties of ZnO thin films. The results show that the resistivity is lowest as carrier gas flow rate is 500 sccm. In our work, the effect of the Al or In doping on the electrical properties of ZnO thin films do not improve the conductivity of ZnO thin films.