氮化鎵光偵測器的暗電流與激子效應

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姓名

葉立學(Li-Shei Yeh) 查詢紙本館藏

畢業系所

物理學系

論文名稱

氮化鎵光偵測器的暗電流與激子效應
(The dark current and the exciton effect of GaN p-i-n photodetectors)

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摘要(中)

在本論文中，我們研究氮化鎵（GaN）p-i-n光偵測器的暗電流，經過比較計算和實驗，我們發現暗電流主要由產生-複合和陷阱穿隧電流（間接穿隧）所主導。在逆偏壓低於8伏特時，暗電流是由產生-複合電流所主導。在高於8伏特時，暗電流呈指數增加的行為歸因於陷阱穿隧電流（間接穿隧）。陷阱能階為受體雜質能階（0.145電子伏特）。大於30伏特時，暗電流可能歸因於其他接近價帶的陷阱能階（0.24與0.35電子伏特）。
對於厚p層（700奈米）同質接面p-i-n光二極體，在363奈米波長處觀察到一個光響應率（responsivity）的峰值。而對於薄p層（50奈米）同質接面p-i-n光二極體，觀察到平緩的光響應率，不同於厚p層試片。因為薄p層抑制了空乏區電流與n側擴散電流中的激子（excitons）吸收現象。在光響應率上，我們發現室溫下有激子效應。在室溫下砷化鎵（GaAs）材料，從來沒有激子現象被觀察到。
我們亦研究p層超晶格（superlattices）結構與在i層中摻雜銦（In）結構的元件特性。p層超晶格（superlattices）結構能達到高電洞濃度和低電阻率。當操作在逆偏壓下，p層的高電洞濃度使得往p層空乏的程度較輕。所以較薄的p層便能同時達到高崩潰電壓及減少無用的吸收（不能轉換成光電流）。較短的p層擴散時間導因於較低的p層電阻率，然而由於我們的元件面積較大，使得元件的響應時間由RC延遲時間所主導，我們無法確定擴散時間有無改善。另一方面，在i層中摻雜銦（In）引入等電子（isoelectronic）雜質，導致暗電流比非有意摻雜試片高出一個次方。對於逆偏壓下的暗電流，等電子雜質效應的影響超過材料品質的改善。逆偏壓下暗電流沒有改善，但在順偏壓下，觀察到材料品質的改善（低的理想因子值約1.75與串聯電阻300歐姆）。
對於我們的氮化鎵（GaN）同質p-i-n光二極體，有下列幾個好的特性：（1）低暗電流密度0.3奈安培．公分-2 ，在-7.5伏特，（2）高響應率0.1 安培/瓦特 (353奈米) ，在0伏特，（3）快的響應時間7.1 奈秒，在0伏特。

摘要(英)

For this dissertation, we investigated the mechanism of the dark current for GaN p-i-n (PIN) photodetectors. The exciton effect to responsivity was also studied. We compared theoretical calculations with the experimental data. We found the dark current to be dominated by the generation-recombination current and the trap-assisted tunneling current. The dark current is dominated by the generation-recombination mechanism below 8V reverse bias. An exponential increase in the dark current beyond 8V can be attributed to the trap-assisted tunneling current (indirect tunneling). The traps mainly come from the acceptor impurity level (0.145eV). Beyond 30V, the dark current may be attributed to trap donor levels of 0.43eV and 0.7eV below the conduction band.
Two types of p-layers structures were also studied, one with a thick p-layer (700nm) and the other with a p-layer thickness of only 50nm. In the thick p-layer homojunction p-i-n photodiodes, we observed a peak for the responsivity-wavelength curve at 363nm. However, a flat responsivity curve was observed for the thin p-layer (50nm) homojunction p-i-n photodiodes. The thinner p-layer constricted the tip (exciton) absorption phenomenon of the depletion region current and the diffusion current on the n-side, because of the thin penetration depth of the GaN material and the short diffusion length of the p-type GaN. We determined the peak (exciton effect) of the responsivity from measurements and the calculations at room temperature. The GaAs material showed no such phenomenon because the exciton does not exist at RT.
The properties of the superlattices (SLs) in p-layer samples and the In-doped in i-layer samples are investigated. The SLs p-layer could achieve the high hole concentration and low resistivity. The high hole concentration of the p-layer GaN p-i-n leads a slight depletion in the p-layer slightly when operate at reverse bias. The high breakdown voltage could be achieved using the thin p-layer, that reduced useless absorption (could not be transferred to the photocurrent). The high hole concentration increased the breakdown voltage and simultaneously reduced useless absorption. The shorter p-layer diffusion time was owed to the lower resistivity of the p-layer. However we designed large geometry devices which caused that the response time dominated by the RC delay time. We could not be sure that the diffusion time had improved. On the other hand, the In-doped i-layer included isoelectronic impurities which induced the dark current of one order of magnitude greater than the unintentionally doped sample for reverse bias. The isoelectronic impurity effect was more than the material quality improvement for reverse dark current. An absence of improvement in the dark current was observed. However we did observe an improvement in the material quality (low ideality factor 1.75 and series resistance 0.3kΩ) for forward bias.
Our GaN homojunction p-i-n photodiode showed several good properties indicated as follows (1) a low dark current density of 0.3nA/cm2 at -7.5V, (2) a high responsivity of 0.1 A/W (353nm) at 0V, (3) a fast response time of 7.1 ns at 0V.

關鍵字(中)

★ 暗電流
★ 光偵測器
★ 氮化鎵

關鍵字(英)

★ photodiode
★ GaN
★ p-i-n
★ photodetectors
★ dark current

論文目次

Content
Abstract (in Taiwanese)……………………………………….….………..I
Abstract (in English)………………………………………….…………..III
Acknowledgements…………………………………………….…………V
Content………………………………….………………………………..VI
Table captions………………………………………………..………….VII
Figure captions………………………………………………………….VIII
1. Introduction…………………………………………………………….1
1.1 Introduction of GaN p-i-n photodetector…………………………1
1.2 The aim of this dissertation……………………………………….2
2. Experimental techniques and measurement setup……………………...6
2.1 Device processes………………………………………………….6
2.2 Measurement setup……………………………………………….8
2.3 Material characterization…………………………………………9
3. Characteristics of GaN p-i-n with a p-layer AlGaN/GaN superlattice..12
3.1 Motivation………………………………………………...……..12
3.2 Dark current and photocurrent characteristics…………..............13
3.3 Responsivity characteristics…………………….………………15
3.4 Response time measurement………………………….………...16
3.5 Capacitance-voltage measurement………………….…………..19
3.6 Summary……………………………………………….……….19
4. Characteristics of GaN p-i-n with In-doping in i-layer…….……….22
4.1 Introduction………………………………………….………….22
4.2 Current-voltage characteristics……………………….…………23
4.3 Characteristics of spectral photoresponse……….……………...25
4.4 Capacitance-voltage measurement……………….……………..25
4.5 Response time…………………………………………………..25
4.6 Summary………………………………………….…………….26
5. Dark current and exciton effect of GaN p-i-n photodetector…….….28
5.1 Introduction……………………………………………….…….28
5.2 Calculation and characteristics of dark current…………….…...29
5.3 Exciton effect of responsivity…………………….......................33
5.4 Summary……………………………………………….……….37
6. Conclusion............................................................................………...40
Bibliography.............................................................................................43
Publication List………..…………………………………..…….………84

參考文獻

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指導教授

紀國鐘(Gou-Chung Chi)

審核日期

2003-7-16

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