Investigation of GaN-based light-emitting diodes using double photonic crystal patterns

(1)

Investigation of GaN-based light-emitting diodes using double photonic

crystal patterns

H.W. Huang

a,b,⇑

_{, Fang-I Lai}

c,⇑

_{, S.Y. Kuo}

d

_{, J.K. Huang}

a,b

_{, K.Y. Lee}

b a

Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu 30050, Taiwan, ROC

b

Unilite Corporation, Chunan, Miaoli 350, Taiwan, ROC

c

Department of Photonics Engineering, Yuan Ze University, Chungli, Taoyuan 32003, Taiwan, ROC

d

Department of Electronic Engineering, Chang Gung University, Kwei-Shan, Taoyuan, 333, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 22 March 2010

Received in revised form 22 August 2010 Accepted 5 October 2010

Available online 3 November 2010 The review of this paper was arranged by E. Calleja

Keywords:

Gallium Nitride (GaN) Light-emitting diodes (LEDs) Photonic Crystal (PhC)

a b s t r a c t

GaN-based LEDs with photonic crystal (PhC) patterns on an n- and a p-GaN layer by nano-imprint lithog-raphy (NIL) are fabricated and investigated. At a driving current of 20 mA on Transistor Outline (TO)-can package, the light output power of the GaN-based LED with PhC patterns on an n- and a p-GaN layer is enhanced by a factor of 1.30, and the wall-plug efﬁciency is increased by 24%. In addition, the higher out-put power of the LED with PhC patterns on the and p-GaN layer is due to better crystal quality on n-GaN and higher scattering effect on p-n-GaN surface using PhC pattern structure.

1. Introduction

Recent developments of high performance GaN-based light-emitting diodes (LEDs) are dominated by both material tech-niques, such as metal organic chemical vapor deposition (MOC-VD) epitaxial growth, and device fabrication techniques. Thus, high brightness LEDs have been used in various applications, including backlight of large and small size flat-panel displays, traffic signal light and illumination lighting by white light LEDs [1,2]. In order to get higher brightness of LEDs, extensive re-search has been conducted. In epitaxial growth method, a num-ber of attempts have been made to reduce the dislocation effect using such strategies as the insertion of a micro-scale epitaxial lateral overgrowth (ELOG) layer over a SiO2 or SixNypattern on the GaN thin film [3,4], as well as the use of micro-scale pat-terned sapphire substrate (PSS) [5,6]. Moreover, high-quality GaN-based LEDs have been demonstrated on a micro-scale PSS by wet or dry etching[5,6], where the micro-scale patterns serve as a template for the ELOG of GaN and scattering centers for the

guided light in GaN-based LEDs structure. Both the epitaxial crystal quality and the light extraction efﬁciency were improved by utilizing a micro-scale PSS. Recently, MOCVD growth of GaN-based LEDs on PSSs with micro-scale and nano-scale pyramidal patterns has been reported and compared[7,8]. The LEDs grown on the nano-scale PSS showed more enhancements in EQE than those grown on micro-scale PSS. Furthermore, photonic crystal (PhC) is a promising technique due to the great improvement of light extraction efﬁciency[9–11]. However, PhC patterns were often performed by electron beam lithography, which takes a lot of time to accomplish the pattern on a device. Another high throughput and economic technique, nano-imprint lithography (NIL), was recommended to manufacture light-emitting devices with PhC or photonic quasi-crystal (PQC) structures[12]. In this letter, we report the nano-imprinting and epitaxial overgrowth techniques to fabricate GaN-based LEDs using PhC nano-patterns on an n- and a p-GaN layer for 2-in. mass production. As a re-sult, the intensity-current (L-I) measurements demonstrate that the light output power of a LED with PhC patterns on an n-and a p-GaN layer was higher than that of a conventional LED at 20 mA with standard device processing. In addition, the reli-ability test of normalized output power of LED with PhC patterns on an n- and a p-GaN layer only decreased by 5%. This results offer promising potential technique to enhance the light output power of commercial light-emitting devices using the technique of nano-imprint lithography.

⇑ Corresponding authors. Address: Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu 30050, Taiwan, ROC. Tel.: +886 37 586388; fax: +886 37 586677 (H.W. Huang), Tel.: +886 34 638800x7516; fax: +886 34 514281 (F.-I. Lai).

E-mail addresses:[email protected](H.W. Huang), ﬁ[email protected](F.-I. Lai).

Solid-State Electronics 56 (2011) 31–34

Contents lists available atScienceDirect

Solid-State Electronics

(2)

2. Experiments

All GaN-based LED samples are grown by MOCVD with a rotat-ing-disk reactor (Veeco) on a c-axis sapphire (0 0 0 1) substrate at a growth pressure of 200 mbar. The LED structure with a 100 nm-thick SiO2PhC rod pattern on an n-GaN layer structure was fabri-cated with a sapphire substrate consists of a 50 nm-thick GaN nucleation layer grown at 500 °C, a 2

l

m un-doped GaN buffer, a 1.0

l

m-thick Si-doped GaN buffer layer grown at 1050 °C. The de-tail of the nano-imprint process is described in Ref.[12]. Then, the LED structures with a SiO2PhC rod pattern overgrowth on an n-GaN layer were designed and grown on a 2 in. wafer. The re-grown LED structures consists of a 1.2

l

m-thick n-type Si-doped GaN layer grown at 1050 °C, an unintentionally doped InGaN/ GaN multiple quantum well (MQW) active region grown at 770 °C, a 50 nm-thick Mg-doped p-AlGaN electron blocking layer grown at 1050 °C, and a 220 nm-thick Mg-doped p-GaN contact layer grown at 1050 °C. The MQW active region consists of ﬁve periods of 3 nm/7 nm-thick In0.18Ga0.82N/GaN quantum well layers and barrier layers. Then, the LED with a SiO2PhC rod pattern on an n-GaN layer were performed PhC hole pattern on the p-GaN sur-face by NIL process. Fig. 1shows the schematic structure of the LED with double PhC patterns. The conventional LED structure con-sists of a 50 nm-thick GaN nucleation layer grown at 500 °C, a 2

l

m un-doped GaN buffer, a 2

l

m-thick Si-doped GaN buffer layer grown at 1050 °C, and afterward the same epitaxial structure as LEDs with PhC patterns on an n- and a p-GaN layer.

All LED samples, including the LED with double PhC patterns and the conventional LED, are fabricated using the standard LED processes with a mesa area of 265

l

m 265

l

m (LED chip area of 300

l

m 300

l

m).Fig. 2shows a scanning electron microscope (SEM) top-view image of a PhC pattern on the p-GaN surface based on triangular lattice. Additionally, in this study, the PhC nano-rod and hole diameters (D) and lattice constant (a) we used were approximately 370 nm and 550 nm, respectively. The ratio D/a is ﬁxed to 0.67. The height of SiO2PhC pattern is 200 nm and the PhC hole depth on the p-GaN is 100 nm.

3. Results and discussion

Fig. 3a plots the typical current–voltage (I–V) characteristics of a conventional LED, an LED with a PhC pattern on the n-GaN layer, and an LED with PhC patterns on the n- and p-GaN layer. It is found that the measured forward voltages under injection current 20 mA at room temperature for a conventional LED, an LED with a PhC

pattern on the n-GaN layer, and an LED with PhC patterns on the n- and p-GaN layer are 3.11, 3.08, and 3.14 V, respectively. There-fore, there is no inﬂuence on this type of devices by incorporating PhC structures into the LED. The light output is detected by cali-brating an integrating sphere with an Si photodiode on package de-vice, so that light emitted in all directions from the LED can be collected. The intensity-current (L-I) characteristics of the LEDs with and without PhC structures are shown inFig. 3b. At an injec-tion current of 20 mA and peak wavelength of 460 nm for Transis-tor Outline (TO)-can package, the light output powers of a conventional LED, an LED with a PhC pattern on the n-GaN layer, and an LED with PhC patterns on the n- and p-GaN layer on TO-can are given as 11.6, 13.5, and 15.1 mW, respectively. Hence, com-pared to the conventional LED, the enhancement percentages of the LED with a PhC pattern on the n-GaN layer, and the LED with PhC patterns on the n- and p-GaN layer are 16%, and 30%, respec-tively. The higher enhancement on the LED with a PhC pattern on the n-GaN layer may be owing to the scattering light from PhC layer onto the top direction and higher epitaxial crystal quality [13,14]to increase more light output power. In addition, the corre-sponding wall-plug efﬁciencies (WPE) of a conventional LED, an LED with a PhC pattern on the n-GaN layer, and an LED with PhC patterns on the n- and p-GaN layer are 19%, 22%, and 24%, respec-tively, which addresses a substantial improvement by the PhC pat-terns on an n- and a p-GaN layer as well at a driving current of 20 mA.

Fig. 4shows that temperature dependent photoluminescence (PL) used to determine the internal quantum efﬁciency (IQE) of

n-GaN InGaN/GaNMQW p-AlGaN p-GaN ITO

Cr/Pt/Au

SiO

₂

Sapphire

U-GaN

Cr/Pt/Au

Fig. 1. Schematic diagram of GaN-based LEDs with a PhC pattern on an n- and p-GaN layer.

Fig. 2. Top-view SEM image of a PhC pattern on a p-GaN layer. 32 H.W. Huang et al. / Solid-State Electronics 56 (2011) 31–34

(3)

the InGaN/GaN MQW structures. The IQE of LED samples with and without a PhC pattern on an n-GaN layer calculated from IQE = PL300K/PL90K, where PL300Kis the PL intensity at room temper-ature and PL90Kis the PL intensity at the 90 K which is the lowest temperature in our system. Assuming that the internal quantum efﬁciency equals unity at 90 K, we obtain an internal quantum efﬁ-ciency of LED with PhC pattern on an n-GaN layer to be 42%, which is higher than that of the conventional LED (IQE = 34%) at room

temperature [15]. The integrated intensity of the PL peak at 460 nm of MQW with a PhC pattern on an n-GaN layer was in-creased by 55%, compared to that MQW without a PhC pattern on an n-GaN layer at room temperature. The increase in IQE of MQW with a PhC pattern on an n-GaN layer can be attributed to reduction of defects, for example, screw and edge-type threading dislocation in the n-GaN and MQW layer. Based on the observed increases of 55% for EQE and 24% for IQE, the increase in light extraction efﬁciency was estimated to be 25% for LED with a PhC pattern on an n-GaN layer. The increase of 25% is attributed to the enhancement of light extraction from LED by the PhC patterned inside an n-GaN layer which scatters light onto the top surface [14].

During lifetime test, twenty chips of GaN-based LED were encapsulated and driven by 50 mA injection current at 55 °C of ambient temperature. As shown inFig. 5, it was found that forward voltage and normalized output power of LED with PhC structures on an n- and a p-GaN layer only decreased by 5% after continuity driving of 1000 h. The result indicates that using the PhC structures on an n- and a p-GaN performed by NIL technique is a reliable and promising method for device production.

4. Conclusion

GaN-based LEDs with PhC patterns on an n- and a p-GaN layer by nano-imprint lithography are fabricated and investigated. At a driving current of 20 mA on TO-can package, the light output power of a LED with a PhC pattern on n-GaN, a LED with PhC pat-terns on the n- and p-GaN layer are enhanced by 16% and 30%, respectively, compared with the conventional LED. After lifetime test of 1000 h (55 °C/50 mA), normalized output power of the LED with PhC patterns on an n- and a p-GaN layer only decreased by 5%.

References

[1] Koike M, Shibata N, Kato H, Takahashi Y. IEEE J Select Topics Quantum Electron 2002;8:271.

[2] Schubert EF. Light-Emitting Diodes. Cambridge: Cambridge University Press; 2003.

[3] Iida K, Kawashima T, Iwaya M, Kamiyama S, Amano H, Akasaki I, et al. J Cryst Growth 2007;298:265.

Soc 2006;153:G765.

[6] Oh TS, Kim SH, Kim TK, Lee YS, Jeong H, Yang GM, et al. Jpn J Appl Phys 2008;47:5333.

[7] Su YK, Chen JJ, Lin CL, Chen SM, Li WL, Kao CC. Jpn J Appl Phys 2008;47:6706. [8] Gao H, Yan F, Zhang Y, Li J, Zeng Y, Wang G. J Appl Phys 2008;103:014314-1. Fig. 3. (a) The current–voltage (I–V) characteristics, and (b) the light output power–

current (L–I) and wall-plug efﬁciency (WPE) characteristics of a conventional LED, an LED with a PhC pattern on the n-GaN layer, and an LED with PhC patterns on the n- and p-GaN layer.

2 4 6 8 10 12

103

104

Conventional LED

LED with a PhC on an n-GaN

PL Intensity (arb. units)

1000/T

Fig. 4. Arrhenius plots of integrated PL intensities for GaN-based LED with/without a PhC structure on an n-GaN layer.

Fig. 5. The lifetime test results of the LEDs with the PhC structures on an n- and a p-GaN layer under the driving condition of 50 mA/55 °C.

(4)

[9] Wierer JJ, Krames MR, Epler JE, Gardner NF, Craford MG, Wendt JR, et al. Appl Phys Lett 2004;84:3885.

[10] David A, Fujii T, Sharma R, McGroddy K, Nakamura S, DenBaars SP, et al. Appl Phys Lett 2006;88:061124-1.

[11] Byeon KJ, Hwang SY, Lee H. Appl Phys Lett 2007;91:091106-1.

[12] Huang HW, Lin CH, Lee KY, Yu CC, Huang JK, Lee BD, et al. Semicond Sci Technol 2009;24:085008-1.

[13] Matsubara H, Yoshimoto S, Saito H, Jianglin Y, Tanaka Y, Noda S. Science 2008;319:445.

[14] Chiu CH, Yen HH, Chao CL, Li ZY, Yu PC, Kuo HC, et al. Appl Phys Lett 2008;93:081108.