利用原子層沉積技術成長光學薄膜於溝槽式矽基板之研究

Study of Optical Thin Films Deposited by Atomic Layer Deposition on Silicon Trench

孫啟元(Chi-Yuan Sun)

指導教授 ： 陳敏璋

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.02834

摘要

廣泛用於製作動態隨機存取記憶體（Dynamic Random Access Memory, DRAM）的溝槽式矽基板(silicon trench)，具有高深寬比(50:1)的奈米結構。本論文探討使用原子層沉積系統(atomic layer deposition, ALD)，成長氧化鋁(Alumina, Al2O3)及氧化鋅(Zinc Oxide, ZnO)等薄膜材料於溝槽式結構，並進行一系列之相關光學性質研究。
我們利用熱氧化製程以及之後的氧化層蝕刻，對溝槽矽結構進行擴孔。藉由調整氧化的時間，可以得到不同尺寸的溝槽矽結構。我們進一步使用ALD系統成長氧化鋁薄膜作為表面鈍化層，並進行它們的光激發光(photoluminescence, PL)行為研究。在低溫下，溝槽式矽相對於一般平面的矽基板，其擁有較高的電子電洞液滴(electron-hole liquid)以及電子電洞電漿(electron-hole plasma)的解離溫度，同時發光強度亦有所增強，而這現象在溝槽越大，也就是在溝槽與溝槽間的矽的厚度越小的試片中越加明顯。
另外，在平面基板上，我們於不同的溫度下以ALD系統分別成長了氧化鋯(Zirconium Oxide, ZrO2)及氧化鋁作為矽基板的表面鈍化層(surface passivation layer)，並將之製作成穿隧式金氧半導體發光二極體(Metal-oxide-semiconductor (MOS) tunneling diodes)，對它們進行電激發光(electroluminescence, EL)與光激發光的研究，試圖找出較適合作為表面鈍化層的材料以及其成長條件。我們發現以在250˚C溫度下成長的氧化鋯作為MOS元件的氧化層，具有最佳的發光強度。
最後，我們使用ALD系統成長氧化鋅於溝槽式二氧化矽(SiO2)結構，而該結構是由溝槽矽結構在高溫下進行長時間的熱氧化所得。使用二氧化矽作為成長氧化鋅的基材的目的，是因為二氧化矽相對於氧化鋅，擁有較低的折射率(refractive index)，有利於形成光波導(waveguide)架構，而其高能隙(bandgap)亦可以減少其對於氧化鋅的放光的吸收，這些特點有助於改善成長於其上的氧化鋅的發光表現。利用ALD技術成長於二氧化矽溝槽式結構的氧化鋅，擁有一近似奈米管(nanotube)之結構。而其發光強度亦勝於以相同條件在平面二氧化矽層上成長的氧化鋅薄膜，並擁有較低的雷射閥值(lasing threshold)。同時我們亦在其發光頻譜上，觀察到一近似週期性強度變化的現象，這是因為ZnO奈米管本身所形成的Fabry-Perot共振腔結構所致。

關鍵字

原子層沉積技術 ； 氧化鋅 ； 溝槽式矽基板 ； 電子電洞液滴

並列摘要

Silicon trench, widely used in the fabrication of DRAM (Dynamic Random Access Memory), is the nanostructure with high aspect ratio (50:1). In this thesis, we present the study of the deposition of alumina (Al2O3) and zinc oxide (ZnO) thin films by atomic layer deposition (ALD) on trench structure and their optical properties.
By thermal oxidation and following etching of SiO2, pore widening of the silicon trench was achieved. Precise size control of the trench could also be achieved by adjusting the time of oxidation.
The temperature-dependent photoluminescence (PL) was studied after the deposition of Al2O3 thin film by ALD as the surface passivation layer on the silicon trench. The light emission from electron-hole liquid (EHL) and electron-hole plasma (EHP) could be enhanced in the silicon trench as compared to the ordinary flat silicon substrate. Not only the luminescence intensity of the EHL/EHP increases, but also the temperature for the dissociation of the EHL/EHP rises. This phenomenon is more obvious in the specimen with larger trench size (that is, smaller volume of the silicon between trenches.)
Zirconium oxide (ZrO2) and alumina was deposited by ALD at different temperature on the flat silicon substrate as the passivation layer to fabricate the metal-oxide-semiconductor (MOS) tunneling diodes. We compared their PL intensity and electroluminescence (EL) intensity and looked for the optimum condition of the deposition of oxide layer. The MOS device using 250˚C-deposited ZrO2 as the oxide layer can reach the best light-emitting efficiency.
Finally, we deposited ZnO on the SiO2 trench structure by ALD. The SiO2 trench structure is fabricated by thermal oxidation of the silicon trench for a long time. The reason of using SiO2 as the substrate for the deposition of ZnO instead of using silicon is the low refractive index and the high bandgap of SiO2 as compared to ZnO. The former is favorable to form waveguide structure and the latter prevents the absorption of the light emission from ZnO. The ZnO deposited by ALD on the SiO2 trench structure has nanotubes-like structure with stronger PL intensity and lower lasing threshold than the flat ZnO thin film deposited on the same condition. The periodic intensity variation in the PL spectra from the nanotubes-like structure was observed, which may result from micro-cavity effect due to the Fabry-Perot cavity caused by the SiO2 trench and the nanotubes-like ZnO structure.

並列關鍵字

atomic layer depositon ； zinc oxide ； silicon trench ； Fabry-Perot cavity ； electron hole liquid

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