奈米半導體元件之光電性質研究

Optoelectronic properties of nanocomposite devices based on ZnO and Si

陳勇廷(Yung-Ting Chen)

指導教授 ： 陳永芳

國立臺灣大學/理學院/物理研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.02936

摘要

在本論文中，我們研究半導體奈米材料的光學特性以及其在光電元件上的應用。實驗上，我們發現許多奈米複合材料新奇、有趣的特性，這些特性將在下述摘要中作說明。我們相信這些研究對光電元件未來的發展及應用有極大的幫助。本論文共包含五大主題，其摘要如下：

1.氧化鋁（溶膠-凝膠）與氧化鋅奈米柱組成的奈米複合材料之螢光特性-白光及藍光

氧化鋅是發光在紫外區域的直接能隙半導體，在光學元件中佔有相當重要的角色。然而，在氧化鋅奈米柱的表面往往具有相當大量的氧缺陷，氧缺陷的存在會造成紫外的能隙放光大幅度地減弱。因此，如何將氧化鋅奈米柱的氧缺陷作修補，對光電元件的發展是一個重要的議題。在這個主題中，我們提出了一種新方法使得氧化鋅奈米柱的能隙放光顯著地增強（約19倍），就是將溶膠-凝膠法製備的氧化鋁塗佈在氧化鋅奈米柱表面並作熱處理。除此之外，氧化鋁的缺陷放光亦有明顯地增強（約12倍）。此奈米複合材料之螢光涵蓋紫外光到可見光範圍（360-600奈米）。主要的機制是來自於氧化鋁中氧原子的擴散到氧化鋅表面修補表面缺陷。我們相信這種奈米複合材料有機會被用來發展白光或藍光發光元件。

2.在散亂分布的氧化鋅奈米梳狀結構的群組中，展現出費比-白洛共振增強隨機雷射的現象

我們在散亂分布的氧化鋅奈米梳狀結構的群組中，發現紫外光隨機雷射的行為。氧化鋅奈米梳具有特殊的幾何結構，有助於隨機雷射的發生及降低臨界激發光能量密度。每根奈米梳狀結構具有數個到十幾個分支，每個分支（約數個微米）是自然形成的費比-白洛共振腔。氧化鋅的螢光在費比-白洛共振腔內共振放大，再由分支的兩端射出。射出的光在散亂分布的氧化鋅奈米梳之間散射，當光走的路徑形成封閉回路且增益大於耗損時，隨機雷射隨即發生。有別於一般的隨機雷射，在這個系統中，費比-白洛共振效應也參與其中，因而我們得到相當窄的雷射光譜（半高寬約5奈米），其中包含數個尖端（半高寬約0.3奈米）。我們利用了陰極發光影像來分析費比-白洛共振。此外我們更進一步比較不同長度的費比-白洛共振腔，對雷射光譜的影響。

3.陽極氧化鋁輔助製備高感度及寬偵測波段之金屬/氧化物/半導體（金氧半）光檢測器

在本實驗中，我們將陽極氧化鋁製備在矽基金氧半光偵測器並且分析研究其光電特性。此光偵測器的光反應度以及光偵測範圍可隨外加偏壓的調變而作調整。在順向偏壓下，光檢測範圍涵蓋整個可見光及部分近紅外光波段（400-1150奈米），然而在逆向偏壓下，光檢測範圍變為紅外光波段（900-1150奈米，矽能隙附近的吸收波段）。此光偵測器在光通訊波段（850奈米）的反應度為0.24安培每瓦，外部量子效率約35%。傳統的矽基金氧半光偵測器工作在逆向偏壓下，而在本實驗中做出的光偵測器在順、逆偏壓下均有反應。最後，對這樣特殊的光反應特性，我們提出了明確的機制來解釋。

4.金奈米柱摻雜之石墨烯為透明電極製備金屬/氧化物/半導體（金氧半）光檢測器

本實驗中我們以石墨烯為透明電極製作矽基金氧半光偵測器並且分析研究其光電特性。我們利用化學氣相沉積法製作石墨烯，並在其上加入金奈米柱以增加石墨烯的導電度，因此提高金氧半光偵測器的光反應度。此外，矽奈米尖錐陣列基板比平面矽基板具有較好的抗反射效果，因此本實驗亦比較這兩種基板製成的金氧半光偵測器之光反應度。其中，以金奈米柱摻雜之石墨烯為透明電極及以矽奈米尖錐陣列基板做成的金氧半光偵測器具有極佳的外部量子效率（71%）。但由於矽奈米尖錐陣製作時具有大量表面缺陷，對製作成的金氧半光偵測器在短波長的光感應度有明顯的影響，且光反應時間也有變慢的現象。我們相信若將矽奈米尖錐陣列基板作表面缺陷修補，此金氧半光偵測器會有更好的效率及表現。

5.氧化鋅奈米柱/氮化鎵薄膜/聚(3-己烷噻吩)雙異質接面之紫外發光二極體

藉由氧化鋅奈米柱、氮化鎵薄膜、聚(3-己烷噻吩)形成的雙異質接面，在低注入電流密度下，此發光二極體的放光範圍包含紫外光（370–400 奈米）及可見光（400–700 奈米）。在高注入電流密度下，相較於可見光，紫外放光變得極強，且紫外放光強度隨注入電流密度呈現指數增加的趨勢，而且紫外光的半高寬小於16奈米。聚(3-己烷噻吩)對此紫外放光（來自於氮化鎵）有極重要的影響，主要原因是聚(3-己烷噻吩)與氮化鎵間的能帶排列。根據已發表的文獻，氧化鋅奈米柱/氮化鎵薄膜的發光元件，是沒有紫外光被發現的。因此，我們相信用類似的方式，將有機分子加入無機的發光二極體而形成雙異質接面，可有助於增加其發光效率或增加新的發光波段。

關鍵字

光激螢光光譜 ； 光檢測器 ； 發光二極體 ； 隨機雷射 ； 奈米半導體

並列摘要

In this thesis, we have reported the design, fabrication, and characterization of nanoscale semiconductors and optoelectronic devices. Several newly designed nanocomposites with intriguing properties have been discovered, which are described below. It is believed our studies shown here can serve as a key step for the further development of novel functional optoelectronic devices.

1.Giant white and blue light emission from Al2O3 and ZnO nanocomposites

A new and general approach enabling us to amplify not only the bandgap emission of ZnO nanorods but also the defect emission of Al2O3 is proposed. The light intensity of the band edge emission of ZnO nanorods can be improved by as much as 19 times after the decoration of Al2O3 layers. Moreover, white light emission arising from Al2O3 defects in ZnO/Al2O3 nanostructures also shows a large enhancement factor of 12 times. The underlying physics has been attributed to the diffusion of oxygen atoms from Al2O3 to ZnO nanorods. Our new strategy offers an alternative possibility to create strong white and blue light-emitting devices.

2.Enhanced random lasing in ZnO nanocombs assisted by Fabry-Perot resonance

The ultraviolet random lasing behavior of an ensemble of ZnO nanocombs has been demonstrated. It is found that the Fabry-Perot resonance induced by nanocomb geometry can greatly enhance random lasing action with a low threshold condition. Besides, the emission spectra exhibit few sharp lasing peaks with a full width at half maximum (FWHM) of less than 0.3 nm and a narrow background emission with a FWHM of about 5 nm. Cathodoluminescence mapping images are utilized to analyze the Fabry-Perot resonance phenomenon. The resonant effect on the lasing system is further confirmed by nanocombs with different resonant cavity lengths. The unique lasing behavior induced by the simultaneous occurrence of Fabry-Perot resonance and random laser action shown here may open up a new possibility for the creation of highly efficient light emitting devices.

3.Highly sensitive MOS photodetector with wide band responsivity assisted by nanoporous anodic aluminum oxide membrane

A new approach for developing highly sensitive MOS photodetector based on the assistance of anodic aluminum oxide (AAO) membrane is proposed, fabricated, and characterized. It enables the photodetector with the tunability of not only the intensity but also the range of the response. Under a forward bias, the response of the MOS photodetector with AAO membrane covers the visible as well as infrared spectrum; however, under a reverse bias, the near-infrared light around Si band edge dominates the photoresponse. Unlike general MOS photodetectors which only work under a reverse bias, our MOS photodetectors can work even under a forward bias, and the responsivity at the optical communication wavelength of 850nm can reach up to 0.24 A/W with an external quantum efficiency (EQE) of 35%. Moreover, the response shows a large enhancement factor of 10 times at 1050 nm under a reverse bias of 0.5V comparing with the device without AAO membrane. The underlying mechanism for the novel properties of the newly designed device has been proposed.

4.MOS photodetectors based on Au-nanorods doped graphene electrodes

By using Au-nanorods (Au-NRs) doped graphene as a transparent conducting electrode, Si-based metal-oxide-semiconductor (MOS) photodetectors (PDs) exhibit high external quantum efficiency (EQE) and fast response time. It is found that upon adding Au-NRs to the graphene, the significant increase in EQE is observed for both planar and Si-nanotips (Si-NTs) MOS PDs. The planar Si-based MOS PDs reveal a notable photoresponse with an EQE of 49% at the peak wavelength of 530 nm under zero bias, and an EQE of 66% at the peak wavelength of 600 nm under -0.4 V bias. For the Si-NTs MOS PD, it exhibits a relatively high EQE of 71% under -4 V bias due to the effect of light trapping arising from the nature of Si-NTs array.

5.Ultraviolet electroluminescence from hybrid inorganic/organic ZnO/GaN/poly(3-hexylthiophene) dual heterojunctions

Based on hybrid inorganic/organic n-ZnO nanorods/p-GaN thin film/poly(3-hexylthiophene)(P3HT) dual heterojunctions, the light emitting diode (LED) emits ultraviolet (UV) radiation (370 nm – 400 nm) and the whole visible light (400 nm -700 nm) at the low injection current density. Meanwhile, under the high injection current density, the UV radiation overwhelmingly dominates the room-temperature electroluminescence spectra, exponentially increases with the injection current density and possesses a narrow full width at half maximum less than 16 nm. Comparing electroluminescence with photoluminescence spectra, an enormously enhanced transition probability of the UV luminescence in the electroluminescence spectra was found. The P3HT layer plays an essential role in helping the UV emission from p-GaN material because of its hole-conductive characteristic as well as the band alignment with respect to p-GaN. With our new finding, the result shown here may pave a new route for the development of high brightness LEDs derived from hybrid inorganic/organic heterojuctions.

並列關鍵字

Photodetector ； LED ； Random Lasing ； PL ； Nanorod

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