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標題: | 利用原子層沉積技術沉積氧化鋁與氧化鋅薄膜於矽奈米點與其光電元件之研究 Characteristics of Si nanodots with Al2O3 and ZnO films deposited by atomic layer deposition and the applications on optoelectronic devices |
作者: | Fu-Hsiang Su 蘇富祥 |
指導教授: | 陳敏璋(Miin-Jang Chen) |
關鍵字: | 原子層沉積技術,氧化鋁,氧化鋅,矽奈米點, Si nanodots,Al2O3,ZnO,atomic layer deposition,PL,EL, |
出版年 : | 2008 |
學位: | 碩士 |
摘要: | 摘要
本論文研究矽奈米點的光電性質與其在光電元件上之應用,並利用原子層沉積技術沉積氧化鋁和氧化鋅薄膜於矽奈米點上,研究其光電性質之變化。本論文可分為五個主題,第一個主題研究矽奈米點內的載子陷阱,藉由光激發光(photoluminescence, PL)量測在不同溫度下,不同尺寸的奈米點與不同種類的矽基板的發光頻譜與強度。發現越大的矽奈米點,會有越多載子釋放的現象,而電洞是主要釋放出的載子。在低溫的情況下,電洞陷阱與電子電洞液體(electron-hole liquid)是相互競爭的機制。隨著溫度的上昇,電洞由矽奈米點中的電洞陷阱釋放出來,會顯著的影響PL的強度。 第二個主題為研究利用原子層沉積技術沉積氧化鋁薄膜於矽奈米點上的1表面鈍化效果。由原子層沉積技術沉積的氧化鋁薄膜可在矽奈米點上提供良好的表面鈍化效果以增加PL的發光強度。此外,矽奈米點中居多數的電洞陷阱與少量的電子陷阱對於不同種類的矽基板的試片會造成不同的PL特性。此外,我們發現利用Buffer-Oxide-Etch (BOE)去除二氧化矽的過程,會對電洞與電子陷阱造成不同的影響,進而影響不同種類的矽基板的PL性質。 第三主題是 n-type ZnO/SiO2-Si nanodots-SiO2/p-Si 異質接面在光偵測器上的應用。具有適當厚度的氧化鋅薄膜可以作為為抗反射層與透明導電層,使得光偵測器可達到152%的光電轉換效率。其原因為高能的電子在穿遂經過厚的氧化層後進入氧化鋅內產生impact ionization,進而碰撞出更多的電子電洞對以導致大於100%的量子效率。 第四主題是高效率n-type ZnO/SiO2-Si nanodots-SiO2/p-Si 異質接面矽發光二極體。由二氧化矽包圍的矽奈米點具有載子侷限(carrier localization)的效應,可以有效提高發光效率。此外,氧化鋅薄膜可以作為此異質接面的電子注入層,具有適當厚度的氧化鋅可以作為為抗反射層,不僅是降低矽基板與空氣間的反射率以提高光取出效率,更因其高電子濃度與高能隙能量,可以大幅的提升矽發光二極體的外部量子效率,其最佳的外部量子效率可達 3.3×10-4。 第五主題為研究在室溫及低溫下,矽奈米點的光學增益(optical gain)。由於矽奈米點產生的載子侷限的效應,提高了自發發光(spontaneous emission)的強度,進而造成光學增益。我們利用可變式光激發長度(variable stripe length, VSL)的量測方式,發現具有矽奈米點試片的光激發光強度會隨激發長度成指數型成長,成功的觀測到矽奈米點光學增益的現象。作為抗反射層的氧化鋅薄膜可以提高光學增益的大小。 Abstract This thesis presents the optical characteristics of the Si nanodots together with of Al2O3 and ZnO thin films deposited by atomic layer deposition (ALD). This thesis is divided into five topics. The first topic investigated the carrier traps in Si nanodots. The Si nanodots with different sizes grown on n-type and p-type silicon substrates present various temperature-dependent photoluminescence (PL) characteristics. The hole traps and electron-hole liquid were two carrier competition mechanisms at low temperature, resulting in the significant change of PL intensity with the nanodot size and substrate type. The dominant hole traps in the Si nanodots capture large amount of minority carriers, and thus reduces the PL intensity of the n-type Si substrate at low temperature. As a result, the PL intensity at low temperature decreases dramatically with the size of the Si nanodots. The hole emission from Si nanodots into the n-type Si substrate leads to the significant increase in the PL intensity with temperature at the temperature greater than 40K. The second topic is the surface passivation of Al2O3 thin films deposited by ALD on Si nanodots. The Al2O3 thin films deposited by ALD on Si nanodots causes the enhancement of PL intensity after the Buffer-Oxide-Etch (BOE) treatment, indicating the excellent surface passivation effect to suppress the nonradiative recombination. The BOE treatment results in the formation of surface defects and remove of a great amount of hole traps in the Si nanodots. For the thermal SiO2/Si nanodot/p-Si samples, the PL vs. temperature characteristics are sensitive to surface defects rather than the hole traps at low temperature. As for the thermal SiO2/Si nanodot/n-Si samples, the PL vs. temperature characteristics are very sensitive to the decrease in concentration of hole traps at low temperature. The third topic is the efficient n- ZnO/SiO2-Si nanodots-SiO2/p-Si heterojunction photodetector. The n-type ZnO:Al layer acts as the transparent conductive oxide (TCO) and anti-reflection coating (ARC) layer to increase the quantum efficiency of the photodetectors. For samples with the smaller Si nanodots, the hot electrons tunneling through the thicker oxide into ZnO may cause impact ionization in ZnO, resulting in the quantum efficiency of the photodetector greater than 100%. Efficient n-ZnO/SiO2-Si nanodots-SiO2/p-Si heterojunction photodetector with quantum efficiency up to 152% was achieved. The fourth topic is the efficient n-ZnO/SiO2-Si nanodots-SiO2/p-Si heterojunction light-emitting diodes (LEDs). The n-type ZnO:Al layer acts as the TCO, ARC, and electron injection layer to increase the external quantum efficiency of the Si LEDs. Because of the carrier confinement and surface passivation effects, the Si nanodots surrounded by SiO2 contributes to the increase in the light-emitting efficiency of Si LEDs. Efficient n-ZnO/SiO2-Si nanodots-SiO2/p-Si heterojunction LED with external quantum efficiency up to 3.32×10-4 was achieved at room temperature. The fifth topic investigated the optical gain in the Si nanodots. Super-linear increase in the PL intensity and the spectral narrowing with the excitation length was observed at room temperature using the variable stripe length (VSL) measurement, which may be attributed to the amplified spontaneous emission in the Si nanodots. The ZnO:Al layer in the n-ZnO/SiO2-Si nanodots-SiO2/p-Si structure enhances the optical gain by decreasing the reflectance of pumping light. Optical gain at the wavelength corresponding to the Si bandgap energy was achieved in the Si nanodots embedded in the SiO2 matrix at room temperature. The optical gain results from the enhancement in spontaneous emission rate caused by carrier localization in Si nanodots as well as the population inversion at high excitation intensity. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/40807 |
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