利用原子氣相沉積技術成長氧化鋅磊晶薄膜與 n型氧化鋅/ p型氮化鎵異質介面發光二極體之研究

Characteristics of ZnO epilayers and n-ZnO/p-GaN hetrojunction light-emitting diodes grown by atomic layer deposition

陳星兆(Hsing-Chao Chen)

指導教授 ： 陳敏璋

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.03260

摘要

本論文研究使用原子層沉積技術所成長之氧化鋅磊晶薄膜其晶體結構、光激發光以及電激發光之性質。以原子層沉積技術所成長之氧化鋅薄膜在高溫退火後其結晶方向為高度c-軸取向，且於穿透式電子顯微鏡(TEM)的觀察下並無明顯的貫穿式差排(threading dislocation)出現。而其發光性質良好，自發輻射(spontaneous emission)之主要發光波長為378nm的紫外光，缺陷發光強度約為紫外光強度之千分之一。光激發受激輻射(optically-pumped stimulated emission)之起始值(threshold)為33.3kW/cm2，且此起始值隨著鋁原子摻雜濃度的升高而逐漸降低，此現象可歸因於random lasing之產生，而此random lasing現象的發生及來源可藉由兩種不同摻雜模式之鋁摻雜氧化鋅薄膜其所表現出不同的光學性質來佐證。
為研究其電激發光之性質，本研究將氧化鋅磊晶薄膜以原子層沉積技術成長於p型氮化鎵上以製作n-ZnO/p-GaN異質接面發光二極體(LED)。此發光二極體於順向偏壓下發出391nm之紫外光及425nm之藍光，其中391nm之紫外光為氧化鋅薄膜所產生，而425nm之藍光則源自於p型氮化鎵。另外本研究亦發現，若於此發光二極體背面鍍上金屬反射層，則氧化鋅薄膜所發之紫外光隨著電流的上升而呈現出非線性上升的現象，可歸因為amplified spontaneous emission的發生。
另一方面，將此n-ZnO/p-GaN異質接面發光二極體操作在逆向偏壓時可觀察到白光的產生。此白光為波長分別為430nm的藍光與550nm的黃光混合而成，其於色域圖上之座標為(0.31,0.36)與理想之白光座標(0.33,0.33)十分接近。另外若於氧化鋅薄膜中摻雜鋁原子，可在此n-ZnO/p-GaN異質接面發光二極體於逆向偏壓操作下觀察到波長範圍為370nm-396nm之紫外光，此紫外光是由p型氮化鎵所產生，其發光波長隨著空乏區電場強度的增加而紅位移，其成因為excitonic Franz-Keldysh所導致。
由上述結果可以得知，以原子層沉積技術所成長之氧化鋅磊晶薄膜具有良好的晶體品質及光學性質，可以應用於下一世代的高效率發光元件。

關鍵字

原子層沉積 ； 氧化鋅 ； 貫穿式差排 ； 受激輻射 ； 發光二極體 ； 紅位移

並列摘要

High-quality ZnO epilayers were grown on the (0002) sapphire substrate by atomic layer deposition (ALD) and treated by post-deposition thermal annealing. The highly c-axis orientated, well crystallized ZnO epilayers and dominant ultraviolet (UV) near-band-edge emission with negligible defect-related bands were manifested by the X-ray diffraction pattern, transmission electron microscopy images and photoluminescence measurement. A low-threshold (33.3 kW/cm2) optically-pumped stimulated emission was observed in the ZnO epilayers. It was found that the threshold gradually decreased with the increase in the Al doping concentration in the ZnO films. The onset of random lasing was proposed to be the reason of the reduction in the threshold of stimulated emission.
The high-quality ZnO epilayers were grown by ALD on the p-type GaN and treated by rapid thermal annealing to form the n-ZnO/p-GaN heterojunction light-emitting diodes (LEDs). The LEDs exhibited a competition between the electroluminescnce (EL) from the n-ZnO (λ=391 nm) and p-GaN (λ=425 nm). On the other hand, the effect of external feedback on the UV EL from n-ZnO/p-GaN heterojunction LEDs was also studied. The super-linear increase in the UV EL intensity from ZnO with the injection current was observed, attributed to the occurrence of amplified spontaneous emisson in the ZnO epilayer.
White-light EL from the n-ZnO/p-GaN hetrojunction LEDs operated at reverse breakdown bias was also reported. The EL spectrum was composed of the blue light at 430 nm and broad yellow band around 550 nm. The chromaticity coordinate of the EL spectrum was estimated to be (0.31,0.36), very close to (0.33,0.33) of the standard white light. Significant ultraviolet (UV) electroluminescence (EL) coupled with a red shift from n-ZnO/p-GaN hetrojunction LEDs diodes was observed under the reverse breakdown bias. The reverse breakdown is attributed to the type II band alignment with large band offsets between n-ZnO and p-GaN heterojunction, which facilitates electron tunneling from the valence band in p-GaN to the conduction band in n-ZnO.Theoretical calculations were carried out to fit the EL spectra, indicating that the excitonic Franz-Keldysh effect under a large electric field in the depletion region of p-GaN is responsible for the red shift in the UV EL spectra.
The crystal structures and optical properties of ZnO epilayers and the n-ZnO/p-GaN hetrojunction LEDs grown by ALD were studied in this thesis. The results indicate that the ALD technique is a very promising approach to the growth of high-quality ZnO epilayers for the next-generation UV photonic devices.

並列關鍵字

ALD ； ZnO ； threading dislocation ； stimulated emission ； light emitting diode ； red-shift.

參考文獻

[8] Y. Liu, B. P. Zhang, N.T. Binh, Y. Segawa, Appl. Phys. B 79, 83 (2004).

[9] W. Y. Shiao, C. Y. Chi, S. C. Chin, C. F. Huang, T. Y. Tang, Y. C. Lu, Y. L. Lin, L. Hong, F. Y. Jen, C. C. Yang, B. P. Zhang, and Y. Segawa, J. Appl. Phys. 99, 054301 (2006).

[15] Hsieh H.H. and Wu C.C., Appl. Phys. Lett. 89 041109 (2006)

[19] Janotti and C. G Van de Walle, Rep. Prog. Phys. 72, 126501 (2009)

[22] Klingshirn, Phys. Stat. Sol. (b) 71, 547 (1975).