在可撓式基板製作氧化鋅光檢偵器之研究

A study for ZnO photodectors fabricated on flexible

吳誠智(Cheng-Zhi Wu)

指導教授 ： 姬梁文

國立虎尾科技大學/電機資訊學院/光電與材料科技研究所/碩士(2008年)

摘要

氧化鋅(Zinc Oxide, ZnO)為Ⅱ-Ⅵ族化合物半導體，具有直接能隙( Direct Bandgap ) 的能帶結構，能隙的大小為3.37 eV，所對應的波長範圍在紫外光區段，並具有高達60 meV的激子束縛能 (Exciton Binding Energy)。此外，由於ZnO具有高透光率以及低電阻率的特性。因此ZnO被認為是非常重要且具有未來性的光電材料。
本論文主題是以氧化鋅做為半導體薄膜，使用可撓式高分子做為基板，製作金屬-半導體-金屬(MSM)、選區主動層(Selective active layer)、雙層主動層(Double-layer)和奈米柱(Nanorod)相關之光偵測器。光偵測器之MSM電極為交叉指間( Interdigital, IDT )結構，以氧化鋅( ZnO )薄膜作為吸光層。我們以射頻磁控濺鍍法製作氧化鋅薄膜，而電極則是以電子束蒸鍍法製作；這兩種製程都屬於低溫度製程，非常適合在可撓性的基板製作元件，本研究中所採用的兩種可撓性基板分別為聚对苯二甲酸乙二酯(polyethylene terephthalate, PET)和聚苯醚砜(Polyethersulfone, PES)。
以濺鍍法於可撓式塑膠基板上沉積氧化鋅薄膜，並針對濺鍍後的薄膜，利用x-ray繞射儀(X-ray Diffraction, XRD)觀察晶體結構、掃描式電子顯微鏡(Scanning Electron Microscopy, SEM)觀察表面形態、原子力顯微鏡(Atomic Force Microscope, AFM) 粗糙度量測，紫外光/可見光分光光譜儀(UV/VIS spectrophotometer)量測穿透特性、能量散佈光譜儀(Energy Dispersive Spectrometer, EDS)分析元素含量、光激發螢光(Photoluminescence, PL)觀察薄膜品質、穿透式電子顯微鏡(transmission electron microscope, TEM)和選區電子繞射(selected area electron diffraction, SAED)觀察奈米柱品質，將量測所得的結果加以分析討論及驗証。
在元件製作方面，以氧化鋅薄膜製作金半金光檢測器、選區主動層光檢測器、雙層主動層光檢測器、奈米柱光檢測器四種元件，並採用鉑(Pt) 、銀(Ag)作為電極，使用電子束蒸鍍法沉積薄膜。再利用光、電特性俱佳的氧化鋅薄膜應用於可撓式塑膠基板上製作蕭特基二極體的光偵測器(photodetector)，以HP-4156C半導體參數分析儀和氙燈經由單色分光儀進行氧化鋅光檢測器分別量測元件的暗電流(dark current)、照光光電流(photo current)及光響應度(responsivity)大小，結果證實此光電元件適用於短波長紫外線感測器。
經過量測我們得到上面四種元件(MSM, Selective, Double-layer, Nanorod)接觸金屬為鉑的光暗電流比值在偏壓為1伏特下分別為：3.31*104、1.06*105、5.545*105與3.66*103；接觸金屬為銀的光暗電流比值在偏壓為1伏特下分別為：1.73*104、2.85*104、9.44*104與 5.82*103。我們做用了熱離子放射理論的公式來計算它們的蕭特基能障高度分別為：0.859與0.782。
在光響應方面，當偏壓為1 V與3 V，入射光波長為370 nm 時，分別對由四種元件(MSM, Selective, Double-layer, Nanorod)的金半半光檢測器作量測，可得到接觸金屬為鉑最大的光響應值分別為：0.0588 A/W、0.349 A/W (device 1), 0.0116 A/W、0.0506 A/W (device 2), 0.0726 A/W、0.402 A/W (device 3), 0.498 A/W、2.07 A/W (device 4) ;接觸金屬為銀最大光響應值分別為：0.473 A/W、1.33 A/W (device 1), 0.0128 A/W、0.0504 A/W (device 2), 0.0382 A/W、0.146 A/W (device 3), 4.13 A/W、7.68 A/W (device 4) 。由光暗電流比值跟光響應的分析，我們得到元件為Double-layer和接觸金屬鉑的光檢測器可以得到最高之光響應。
這些簡單卻具有發展潛力的可撓式氧化鋅光偵測器可配合其他的光電元件使用，並可作為未來光訊號處理及光電應用功能。這可以得到低成本之短波長紫外線光檢測器，使得這項技術未來在光電積體電路元件之應用極具發展潛力。

關鍵字

金屬-半導體-金屬(MSM)光偵測器 ； 氧化鋅 ； 奈米柱 ； 可撓式高分子基板 ； 射頻磁控濺鍍法 ； 電子束蒸鍍法

並列摘要

Zinc oxide is a II-VI semiconductor material with direct band-gap of 3.37 eV corresponding to the wavelength in the ultraviolet region. ZnO also has large exction binding energy (~60 meV). In addition, ZnO has low resistivity and high transparency in the visible region. As a result, ZnO is considered as a promising material for the application of the optoelectronics.
In this thesis, Metal-semiconductor-metal (MSM) and selective active layer ultraviolet (UV) photodetectors have been fabricated and studied over the last few years. Metal-semiconductor-metal (MSM) electrode was interdigital (IDT) structure, and ZnO mid-ultraviolet (mid-UV) potodetector was fabricated. The ZnO thin film was fabricated with radio frequency magnetron sputtering. The electrode was evaporated on the ZnO thin film by electron beam assisted evaporation. Radio frequency magnetron sputtering deposition system and electron beam assisted evaporation system were both operated at low temperature, so the flexible substrates could be used. ZnO thin film was deposited on PET, PES substrate by radio frequency magnetron sputtering method. The ZnO film was deposited on the flexible substrate by sputtering techniques. The ZnO thin film was then thermally annealed in order to achieve high transparency and decrease the concentration of oxygen vacancy. Finally, The ZnO thin films were analyzed by using XRD, FE-SEM, UV/VIS spectrophotometer, EDS, PL spectra, TEM and SAED.
Sequencely, the Ag, Pt metal was evaporated on the ZnO thin film to form the electrodes of ZnO MSM photodetector. The current-voltage behavior and responsivity of ZnO MSM photodetector were measured. Base on these results of photoelectric measurements, we concluded that the structure of ZnO / flexible substrates is suitable for short-wavelength UV photodetector.
We successfully fabricated MSM photodetector with structure of device 1: electrodes / ZnO / flexiable substrates, device 2: selective active layer photodetector with structure of electrodes / ZnO (selective) / flexiable substrates, device 3: double-Layer photodetector with structure of ZnO / electrodes / ZnO / flexiable substrates, device 4: nanorod photodetector with structure of electrodes / ZnO (nanorod) / flexiable substrates. The flexiable substrates of PET, PES were both used for MSM, selective active layer, double-Layer and nanorod photodetector, which can be used as building blocks for the future optical signal process and the optoelectronic applications.
The photo/dark contrast of Pt MSM photodetectors were 3.31 E+4, 1.06 E+5, 5.545 E+5 and 3.66 E+3 at 1 V bias, respectively ; The photo/dark contrast of Ag MSM photodetectors were 1.73 E+4, 2.85 E+4, 9.44 E+4 and 5.82 E+3 at 1 V bias, respectively. And I also use the thermionic emission theory to calculate Pt and Ag barrier height. The value was 0.859 and 0.782 eV, respectively.
The peak responsivity of those ZnO photodiodes four devices(MSM, Selective, Double-layer, Nanorod) occurs at around 370 nm and its value equals to 0.0588 A/W and 0.349 A/W (device 1), 0.0116 A/W and 0.0506 A/W (device 2), 0.0726 A/W and 0.402 A/W (device 3), 0.498 A/W and 2.07 A/W (device 4) for Pt contact electrode; 0.473 A/W and 1.33 A/W (device 1), 0.0128 A/W and 0.0504 A/W (device 2), 0.0382 A/W and 0.146 A/W (device 3), 4.13 A/W and 7.68 A/W (device 4) for Ag contact electrode when the sample was reversed biased at 1 V and 3 V, respectively . And which corresponds to quantum efficiencies of 19 %, 3.9 %, 24 % and 166 % for Pt contact electrode, respectively; quantum efficiencies of 159 %, 4.3 %, 13 % and 1384 % for Ag contact electrode, respectively. It could be found that device 3: double-Layer photodetector with structure of ZnO / Pt / ZnO / flexiable substrates was the most suitable electrode for ZnO MSM photodetectors among four device.
It is useful to develop low cost short wavelength UV photodetectors and has a great potential for the application of the short wavelength optoelectronic integrated circuit (OEIC).

並列關鍵字

Metal-semiconductor-metal(MSM) photodetectors ； ZnO ； flexible substrates ； PET ； PES

參考文獻

參考文獻

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