p 型導電氧化物薄膜之高密度電漿製程開發

p-type conductive oxide films deposited by high-density plasma processes

溫朝光(Chao-Kuang Wen)

指導教授 ： 莊東漢 ； 陳勝吉

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2021年)

https://doi.org/10.6342/NTU202103687

摘要

要製作透明pn 光電元件，p 型導電氧化物之開發為重要之研究議題。然而，p 型導電氧化物之發展緩慢，進而限制透明pn 元件之開發。傳統磁控濺鍍技術之濺鍍物種離化率很低(<10%)，不易製作出高導電性之p 型氧化物薄膜。故本研究利用高功率脈衝磁控濺鍍系統高離化率之優勢(Cu+及Ni2+將更容易與氧氣反應)選用無毒性、成本較低且有潛力製作成p 型透明導電薄膜的Cu2O 及NiO 作為研究對象，本研究第一部分採用高功率脈衝磁控濺鍍系統鍍製 Cu2O 薄膜。第一系列之實驗調變氧流率比在2.5%至50%的範圍，探討氧流率比對於CuxO 薄膜之顯微結構及光電性質的影響。研究發現，CuxO 薄膜之光電性質與薄膜之成分及結晶性有關聯性。在氧流率比12.5%~35%之間，獲得以Cu2O 相為主之薄膜。在光電性質方面，當氧流率比低於10%，由於薄膜中的主要相為Cu，所以呈現n 型傳導；隨著氧流率比增加至15%以上，薄膜的主要相由Cu 轉變為Cu2O，此時傳導型式由n 型轉為p 型；同時，薄膜之穿透率顯著提升。當氧流率比為20%時，可得到較佳的p 型電導率，其值0.4 S×cm-1。第二系列之實驗藉由調變HiPIMS 之工作週期(duty cycle)=ton/(ton+toff)，探討duty cycle 對Cu2O 薄膜的顯微結構和光電性質的影響。結果顯示，當duty cycle 降低(尖峰功率密度提高)時，Cu2O 薄膜之沉積速率顯著下降，但結晶性及可見光平均穿透率皆獲改善；同時，薄膜的傳導型式由n 型轉變為p 型。當duty cycle=2.44%時，可獲得最佳之p 型電導率，其值為3 S×cm-1。
為解決單純 HiPIMS 系統沉積速率過慢的問題，本研究第二部分運用疊加型高功率脈衝磁控濺鍍{Superimposed High power impulse magnetron sputtering, 疊加型HiPIMS [HiPIMS+中頻(MF)]}系統鍍製純NiO 及Cu2O 薄膜。首先，探討不同電源模式對單純NiO 及Cu2O 薄膜之影響。研究發現，與單純HiPIMS 模式相比，疊加型HiPIMS 模式鍍製之薄膜的沉積速率顯著改善[甚至高於傳統直流磁控濺鍍(DCMS)模式]；同時，HiPIMS 電源之高離化率優勢仍可被保留。另外，增加MF 持續工作時間，則薄膜之沉積速率將再上升。進一步增加氧流率薄膜之載子濃度再提升致使電阻率下降。最後，將氣氛之工作壓力調降至2mTorr 時，有更好之p 型電性質。而對Cu2O 薄膜而言，除可大幅提升薄膜之沉積速率外，且薄膜之p 型導電性也被改善，即使增加MF 之工作時間，薄膜仍維持良好之p 型導電性。本研究發現，採用疊加型HiPIMS 系統可鍍製出兼具高導電性及高沉積速率之p 型Cu2O 薄膜，深具應用於光電元件的潛力。
為再提升NiO 薄膜之p 型導電性，本研究第三部分運用疊加型HiPIMS (HiPIMS+MF)混合射頻(rf)電源系統鍍製NiO-Cu 薄膜。其中鎳靶採用疊加型HiPIMS 電源並調控duty cycle 為2%，而銅靶則施加射頻電源。研究發現，當工作氣氛中的氧流率比(fo2)控制在70%，工作壓力設定於5 mTorr 時，在NiO 薄膜中摻雜Cu 可有效提升薄膜之p 型電導率(電洞載子濃度上升及電阻率下降)，但結晶性及載子遷移率會降低。進一步增加氧流率比可再提高薄膜之載子濃度及降低電阻率。另外，調降氣體工作壓力，亦有助於p 型導電性的提升。當採用HiPIMS+MF 3X 模式鍍膜，氧流率比及工作壓力分別調控在100%及2 mTorr 時，於Cu 含量為6.96%可獲得較佳p 型導電性之NiO-Cu 薄膜，其載子濃度及電阻率可達1.02×1021cm-3 及9.15×10-3 Ω-cm。此可歸因於薄膜中Ni2+離子被Cu+離子所置換及產生大量Ni2+空位使電洞濃度大幅上升所致。此外，採用疊加型HiPIMS (HiPIMS+MF 3X)模式可大幅提升薄膜沉積速率至0.021 nm/s，遠高於單純HiPIMS 模式下的0.011 nm/s，甚至高於傳統DCMS 的0.019 nm/s。
本研究最後一部分將 p 型Cu2O 薄膜鍍製於ITO 基板上量測其紫外光感測能力。探討不同氧流率比、不同濺鍍電源模式及後退火後對光感測能力之影響。研究顯示，採用HiPIMS+MF3X 模式，在高fO2 (25%或35%)時，有較佳的光電流反應；此外，若改以單純HiPIMS 及傳統DCMS 模式鍍製薄膜可獲得較好的光電流反應，這是由於薄膜內缺陷濃度較低，結晶性較佳的關係。最後，將初鍍膜進一步後退火處理發現，當退火溫度Ta=100℃時，有本研究較佳的IUV/IDark 比值，在HiIPIMS+MF 3X 及fO2=35%條件下，其值為1.53；而採用單純HiPIMS 及fO2=17.5%條件下，IUV/IDark 比值可提升至2.40。

關鍵字

疊加型高功率脈衝磁控濺鍍 ； p型氧化物薄膜 ； 沉積速率 ； p型導電性 ； 紫外光感測

並列摘要

The fabrication of p-n transparent optoelectronic devices, the development of p-type TCOs is an important issue. However, the progress of p-type TCOs is slow, hance the development of transparent p-n devices has been restricted. Due to the typical ionization rate of sputter species in conventional direct current magnetron sputtering (DCMS) is quite low (less than 10%). It is hard to fabricate the p-type TCOs with high conductivity. In this study, the advantage of high-power impulse magnetron sputtering (HiPIMS) with high ionization rates (the Cu+ or Ni2+ ions will easier react with oxygen) was utilized to deposit the Cu2O and NiO films which are nontoxic nature, low-cost production and has the potential to make p-type transparent conductive films.
In the first part of this study, the Cu2O films deposited by HiPIMS system. The first series of the experiment, the variation of oxygen flow ratio between 2.5% to 50% was conducted. The influence of oxygen flow ratio on the microstructure and optoelectronic properties was investigated. The results show that the optoelectronic properties of CuxO films are dependent on their composition and crystallinity. When oxygen flow ratio ranges from 12.5% to 35%, Cu2O-dominated films are achieved. In terms of optoelectronic properties. At oxygen flow ratio lower than 10%, the majority phase in the films are Cu. Thus, the film’s conductivity type is n-type. With oxygen flow ratio increasing to above 15%, the majority phase in the films changing from Cu to Cu2O. the films conductivity type changes from n-type to p-type. Meanwhile, the transmittance of films improves significantly. When the oxygen flow ratio is at 20%, a better p-type conductivity of 0.4 S/cm is achieved. The second series of the experiment, the effect of duty cycles on the microstructure and optoelectronic properties was investigated. It is found that when decreases the duty cycle (higher peak power density), the deposition rate of films obviously reduces. But, both crystallinity and the average transmittance in visible are improved. Meanwhile, the conductivity type transforms from n-type to p-type. When duty cycle is at 2.44%, the optimal p-type conductivity is achieved, its value is 3 S/cm.
In order to solve the low deposition rare in the pure HiPIMS deposition. The second part of this study, the superimposed HiPIMS system [i.e. HiPIMS+Middle-frequency (MF)] was used to deposit pure NiO and Cu2O films. Firstly, the influence of sputtering mode on the deposition rate and properties of pure NiO and Cu2O films was investigated. The results show that compare with pure HiPIMS mode, the deposition rate of superimposed HiPIMS mode is significantly improved. Meanwhile, the advantages of HiPIMS with high ionization rates are retained. On the other hand, the deposition rate continuous rises with increasing the MF pulse duration. Further increasing the oxygen flow ratio, the film’s carrier concentration rises leading to the reduce of resistivity. Finally, reducing the working pressure to 2 mTorr. The p-type conductivity of films further promoted. For the Cu2O films deposited by superimposed HiPIMS system, besides greatly increasing the deposition rate of the films, the p-type conductivity of the films is also improved. Even if increasing the MF pulse duration, the films still maintain better p-type performances. It was found that p-type Cu2O films having both high conductivity and high deposition rate can be achieved by superimposed HiPIMS, which possess significant potential for applications in the optoelectronic devices field.
In order to further improve p-type conductivity of NiO films. The third part of this study, NiO-Cu films were deposited by hybrid superimposed HiPIMS system and rf power supply. HiPIMS system with duty cycle of 2% was supplied to the nickel target and rf power was supplied to the copper target. It was found that the addition of Cu in NiO films is an effective method to improve the p-type conductivity of NiO films (i.e. an increase in hole concentration and a decrease in resistivity) when the oxygen flow ratio and working pressure are set at 70% and 5 mTorr, respectively. However, both the film’s crystallinity and carrier mobility drop. Upon increasing the oxygen flow ratio, the carrier concentration of the films rises and the resistivity drops further. Alternatively, the p-type conductivity of NiO-Cu films improves further when the working pressure decreases. When the Cu content and oxygen flow ratio are fixed at 6.96% and 100%, the optimal p-type conductivity of NiO-Cu films deposited by superimposed HiPIMS (HiPIMS+MF 3X) mode is achieved when the working pressure is decreased to 2 mTorr. Its values of carrier concentration and resistivity are 1.02×1021 cm-3 and 9.15×10-3 Ω·cm respectively. This is due to the substitution of Ni2+ by Cu+ ions and the large amount of Ni2+ vacancies in the films, resulting in the hole concentration rising significantly. In addition, the deposition rate of this film increases markedly to 0.021 nm/s, which is much higher than the 0.011 nm/s by pure HiPIMS mode and even higher than 0.019 nm/s that is achieved using conventional DCMS.
In the last part of this study, the p-type Cu2O films were deposited on ITO glass substrates applied for the UV light sensing capability. The effects of oxygen flow ratio, sputtering mode and post-annealing on UV light sensing were investigated. The results show that the photocurrent response is better by HiPIMS+MF 3X mode with higher oxygen flow ratio (fO2=25% or 35%). Moreover, the photocurrent response can be further improved when the pure HiPIMS mode and DCMS modes were used. This is due to the defect concentration level is lower in the films, leading to the increase in crystallinity of the films. Finally, it is found that the better IUV/IDark ratio can be achieved when the films were annealed at Ta=100℃. Under the conditions of HiPIMS+MF 3X and fO2=35%, the value of IUV/IDark ratio is 1.53. It is improved to 2.40 as the pure HiPIMS and fO2=17.5% conditions were used.

並列關鍵字

Superimposed high power impulse magnetron sputtering ； p-type oxide films ； Deposition rate ； p-type conductivity ； UV light sensing

參考文獻

參考文獻

[1] Hong Xiao, “Introduction to semiconductor manufacturing technology” 2nd ed., SPIE press, p.2 (2012).

[2] Hong Xiao, “Introduction to semiconductor manufacturing technology” 2nd ed., SPIE press, pp.3-5 (2012).

[3] K. Baedeker, Ann. Phys. (Leipzig) 22 (1907) 749.

[4] H. Kim, A. Pique, J. S. Horwitz, H. Mattoussi, H. Murata, Z. H. Kafafi, D. B. Chrisey, Appl. Phys. Lett. 74 (1999) 3444.