極化導致氮化銦鎵/氮化鎵奈米柱發光二極體陣列的光學特性

直接寬能隙化合物半導體氮化鎵在固態照明的領域已經被廣泛的研究。然而，低的光萃取效率是高功率固態照明發光二極體的瓶頸。再者，氮化銦鎵/氮化鎵量子井通常會承受由於晶格不匹配產的應力造成的內建電場。這個內建電場會導致電子電洞之間的波函數重疊下降，而產生較低的內部量子效益。奈米柱結構不僅可以釋放因為晶格不匹配造成的應力，也因為具有較高的面積體積比而有更高的光萃取效率。因此，我們開始研究氮化鎵奈米結構的元件。氮化鎵的奈米柱結構也同樣被廣泛的研究，但是大多數的研究僅侷限於材料特性。製作奈米柱發光二極體的製程困難，特別是高逆向電流的議題。我們發展了一個穩定的製程來製作氮化銦鎵/氮化鎵奈米柱發光二極體陣列。鈍化層和研磨的製成被結合應用在製作奈米柱結構上的p型電極。我們的奈米柱結構發光二極體的逆向電流在-5伏特只有奈安培等級。我們應用拉曼光譜來觀察奈米柱結構的應力釋放。與拉曼波數以及應力相關的公式被應用來估計氮化銦鎵量子井內的應力。為了研究製程產生的奈米柱側壁缺陷，我們研究了氮化銦鎵/氮化鎵奈米柱發光二極體陣列和平面發光二極體陣列的低溫電激發光頻譜。藉由低溫光激發光頻譜和電激發光頻譜的量測，奈米柱結構的光萃取效率也同樣被計算得知。我們更進一步發現，不同鈍化層材料的選擇會造成不同的應力存在於氮化銦鎵/氮化鎵量子井裡，而發光的特性也因此受到改變。這個結果經過拉曼光譜的檢測來了解不同鈍化層的應力並藉由理論計算來得知應力對能隙和極化的壓電電場的影響。在最後，我們也探討奈米柱結構發光二極體的效率下降效應。我們的模擬和量測不只考慮量子效益，而且也考慮電激發光頻譜，這樣會使得模擬的情形更貼近真實情況。我們結果顯示歐傑複合主宰了低電流注入下的效率下降效應。然而，在高電流注入下載子溢流是效率下降的主要原因。

關鍵字

奈米柱；氮化鎵；發光二極體；極化；化學機械研磨

並列摘要

Wide and direct band gap compound semiconductor material GaN has been studied extensively for the applications of solid state lighting. However, the low light-extraction efficiency is the bottleneck for high-power light-emitting diodes (LEDs). Moreover, the InGaN/GaN quantum well usually suffer from strong built-in electric field because of lattice mismatch induced strain. The built-in electric field can cause low internal quantum efficiency due to a reduced overlapping between the electron and hole wave functions. The nanorod structure can not only release the strain caused by the lattice mismatch, but also improve the extraction efficiency by the higher surface to volume ration. Thus, we begin to study GaN based nanorod structured devices. The GaN based nanord structure also have been studied widely but most of the research are restricted at material characterization. The process to fabricate nanorod structure LEDs is difficult especially on the large reversed current issue. We developed a stable process to fabricate InGaN/GaN based nanorod light emitting diode arrays. The combination method of passivation layer and polishing process was applied to fabricate p-type metal contact on the top of nanorod structure. The nanorod LED demonstrates a reverse current only nano ampere level under -5V bias voltage. We applied Raman spectroscopy to observe the strain relaxation of nanorod structure. The equation of the correlation between Raman wave-number and strain of InGaN is applied to estimate the strain within InGaN quantum wells. To study the process created side wall defect, the low-temperature electroluminescent (EL) spectra of InGaN/GaN nanorod arrays were also explored and compared with those of planar LEDs. The extraction efficiency of nanorod structure was also calculated by the low temperature PL and EL measurement. We further discovered that the choice of nanorod passivation materials results in the variation of strain in the InGaN/GaN quantum wells, and thus the corresponding change of light emission properties. The results were further investigated by performing Raman measurement to understand the strain of nanorods with different passivation materials and by calculating the optical transition energy of the devices under the influence of strain-induced deformation potential and the piezoelectric polarization field. At last but not least, the efficiency droop effect is also discussed in our nanorod structure LEDs. The simulation and measurement consider not only the quantum efficiency but also the EL spectra, which will make the simulation more close to the real situation. Our results show that Auger recombination dominates at low-level currents. However, the increase number of leakage carriers out of quantum wells is responsible for the efficiency droop at high current injection levels.

並列關鍵字

nanorod ； GaN ； LED ； polarization ； cmp

參考文獻

References of Chapter 1.

[1] W. Lee, J. Limb, J.-H. Ryou, D. Yoo, T. Chung, and R. D. Dupuis, “Influence of growth temperature and growth rate of p-GaN layers on the characteristics of green light emitting diodes,” J. Electron. Mater., vol. 35, no. 4, pp. 587–591, Apr. 2006.

[2] C. H. Liu, R. W. Chuang, S. J. Chang, Y. K. Su, L. W. Wu, and C. C. Lin, “Improved light output power of InGaN/GaN MQW LEDs by lower temperature p-GaN rough surface,” Mater. Sci. Eng. B, vol. 112, no. 1, pp. 10–13, Sep. 2004.

[3] T. Fujii, Y. Gao, R. Sharma, L. Hu, P. DenBaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett., vol. 84, no. 6, pp. 855–857, Feb. 2004.

[4] C. Huh, K. S. Lee, E. J. Kang, and S. J. Park, “Improved light-output and electrical performance of InGaN-based light-emitting diode by microroughening of the p-GaN surface,” J. Appl. Phys., vol. 93, no. 11, pp. 9383–9385, Jun. 2003.

國際替代計量

極化導致氮化銦鎵/氮化鎵奈米柱發光二極體陣列的光學特性

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