Light Emission Enhancement of GaN-Based Photonic Crystal With Ultraviolet AlN/AlGaN Distributed Bragg Reflector

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 22, NOVEMBER 15, 2010 3189

Light Emission Enhancement of GaN-Based Photonic

Crystal With Ultraviolet AlN/AlGaN Distributed

Bragg Reflector

Cheng-Chang Chen, Jun-Rong Chen, Yi-Chun Yang, M.-H. Shih, and Hao-Chung Kuo, Senior Member, IEEE

Abstract—In this study, we demonstrated two-dimensional (2-D)

photonic crystal band-edge coupling operation in the ultraviolet

wavelength range. The light extraction enhancement was obtained

from the photonic crystal structure with an ultraviolet AlN/AlGaN

distributed Bragg reflector (UVDBR). The DBR provides a high

reflectivity of 85% with 15-nm stopband width. A fivefold

enhance-ment in photoluminescence emission was also achieved compared

with the emission from the unpatterned area on the same sample

at 374 nm wavelength. We also study the photonic crystal

band-edge coupling with finite-difference time-domain and plane-wave

expansion methods.

Index Terms—Band-edge coupling, photonic crystal, ultraviolet

distributed Bragg reflector (UVDBR).

I. I

D

IRECT wide-bandgap gallium nitride (GaN) and other

III-nitride-based semiconductors have attracted much

attention because of their potential applications, such as blue,

green, and ultraviolet (UV) LEDs and laser diodes (LDs)

[1]–[4]. The high reflectivity GaN-based distributed Bragg

reflector (DBR) is one of the key elements for GaN optical

de-vices, such as resonant cavity RCLEDs [5] and vertical-cavity

surface-emitting lasers (VCSEL) [6], [7]. Today, wide-bandgap

III-nitride-based material has been applied in flashlights and

area lighting to replace the traditional lighting sources. The

blue LD can serve as the light source of high-density data

storage. However, due to the applications, the efficiency of the

light source needs to be improved.

In general, there are two main methods to improve light

ex-traction with photonic crystal structures. One is the use of the

photonic bandgap (PBG) to inhibit the propagation of guided

modes [8]; the other is utilizing photonic crystal structure to

couple guided modes to radiative modes [9]–[13]. In this study,

Manuscript received May 28, 2010; revised September 21, 2010; accepted September 24, 2010. Date of publication October 04, 2010; date of current version November 10, 2010. This work was supported by the Center for Nanoscience and Technology, National Chiao Tung University and the Na-tional Science Council of the Republic of China, Taiwan under Contract NSC 96-2628-E009-017-MY3.

C.-C. Chen, J.-R. Chen, M. H. Shih, and H.-C. Kuo are with the Department of Photonics and the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30050, Taiwan (e-mail: mhshih@gate.sinica.edu.tw; hckuo@faculty.nctu.edu.tw).

M.-H. Shih and Yi-Chun Yang are with the Research Center for Applied Sci-ences, Academia Sinica, Nankang, Taipei 115, Taiwan (e-mail: mhshih@gate. sinica.edu.tw).

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org

Digital Object Identifier 10.1109/JLT.2010.2083634

Fig. 1. SEM image of the epitaxial structure from cross-sectional view. The ultraviolet AlN/AlGaN DBR contains 25 periods of=4 pairs.

we demonstrated light enhancement of GaN-based 2-D

pho-tonic crystals with the ultraviolet AlN/AlGaN DBR at room

temperature. This structure combines a 2-D photonic crystal

in-plane and a 1-D DBR in vertical direction, which is designed

for the UV wavelength region. The DBR structure has center

stopband at 375 nm and a width approximately 15 nm.

There-fore, it can be acted as a mirror to reflect light from the bottom

area and played the role as a lower refracted index layer to

con-trol the guided modes.

II. F

The 2-D photonic crystal square lattices were fabricated

in an ultraviolet GaN-based DBR (UVDBR) structure. This

AlN/AlGaN DBR structure was grown by a low pressure

metal-organic chemical vapor deposition (MOCVD) system. A

2- m-thick undoped GaN was first grown on a C-plane (0001)

sapphire substrate. Then 25-pair AlN/Al

Ga

N structure

was grown at 900 C, followed by a 200-nm undoped GaN

gain layer on the top of the epitaxial structure. The schematic

structure of the grown DBR is shown in Fig. 1. The structural

quality of the UVDBR layers is maintained by compensating

the compressive and tensile stress in each

pair. This

ap-proach results in the lowest elastic strain energy and allows the

3190 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 22, NOVEMBER 15, 2010

Fig. 2. (a) Schematic structure of the square photonic crystal patterns with a depth 500 nm and total square area of a width about 50m. (b) Top view SEM images of the photonic crystal structures with square lattices and circle unit cells.

growth of thick coherently strained DBR. The growth details

were reported in our previous works [14], [15].

To fabricate the photonic crystal lattices, a 300-nm Si N

layer and a 300-nm polymethylmethacrylate (PMMA) layer

were deposited as the masks during the process. The photonic

crystal square lattice patterns were defined on the PMMA

layer by E-beam lithography and the patterns were transferred

into Si N layer in reactive ion etching (RIE) with CHF /O

mixture. The structure was then etched by inductively coupled

plasma reactive ion etching (ICP-RIE) with Cl /Ar mixture.

The mask layers were removed at the end of processes. The

size of a fabricated photonic crystal pattern is approximately 50

m

50

m with a lattice constant

of 250 nm and a hole

radius of

. The etch depth of the holes is approximately

500 nm, which is pass through undoped GaN layer into DBR

region. The schematic structure of the fabricated photonic

crystals in AlN/AlGaN DBR is shown in Fig. 2(a). The top

view of an SEM image of a fabricated photonic crystal pattern

on the GaN-based structure is shown in Fig. 2(b).

III. C

M

Before characterizing the photonic crystal with the

AlN/AlGaN DBR structure, the high reflectivity in UV region

from the DBR layers was characterized. The UVDBR structure

is designed for the UV wavelength around 360 nm. The

thick-ness of AlN and AlGaN layers are 45 and 42 nm decided by the

formula

and

.

Here,

and

are refractive indexes of AlN and

AlGaN, which are 2.03 and 2.19, respectively. The black curve

in Fig. 3 is the simulated reflectivity spectrum from

transmis-sion matrix method for the UVDBR. The blue curve in Fig. 3

Fig. 3. Calculated (solid) and measured (dash) reflectivity spectra of ultraviolet AlN/AlGaN DBR measured at room temperature with a stopband width of about 15 nm and the center wavelength is 375 nm.

is the measured spectrum by an

and

spectrum analyzer

with a normal incident light from 300 to 420 nm wavelength.

The ultraviolet DBR has the highest reflectivity of 85% at the

center wavelength of 375 nm, with a stopband width of about

15 nm. A good agreement between simulation and experiment

was obtained for the UVDBR structure. The mismatch between

calculated and measured reflectivity spectra is attributed to

inaccuracy of material indexes in simulation and imperfection

of DBR fabrication.

To demonstrate the light enhancement from the photonic

crystal structure, the optical pumping was performed by using

a frequency-tripled Nd:YVO 355-nm pulsed laser with a pulse

width of 0.5 ns and a repetition rate of 1 kHz. (The device

was pumped by a normal incident laser beam with a spot size

of 50

m, which can cover the whole photonic crystal pattern

area. The light emission from the sample was collected by a

15 X objective lens through a multimode fiber and coupled into

a spectrometer with a charge coupled device detectors.

Fig. 4(a) shows the measured PL spectrum from the undoped

GaN layer. The gain peak of the undoped GaN is located around

360 nm wavelength. Fig. 4(b) shows the measured spectra from

the photonic crystal DBR structure (black curve) and

nonpat-terned region (red curve). The gray region in Fig. 4(a) and (b)

is the high reflection region of the bottom UVDBR structure.

The reflectivity of DBR might have very small degradation since

photonic crystals were etched into top four pairs of AlN/AlGaN

layers. The stopband region remains between 362 to 381 nm in

wavelength. A strong light emission from the photonic crystal

pattern was observed. A strong resonant peak was observed at

374 nm wavelength, which is inside the high reflection region,

as expect. A fivefold enhancement in photoluminescence

emis-sion was also achieved compared with the emisemis-sion from the

unpatterned area on the same sample, as shown in Fig. 4(b).

The slight misalignment between the GaN gain peak and the

stopband of UVDBR can be avoided by optimizing geometry of

the bottom DBR structure. The better performance from the UV

GaN photonic crystals can be expected with higher gain support

and better optical properties.

CHEN et al.: LIGHT EMISSION ENHANCEMENT OF GAN-BASED PHOTONIC CRYSTAL WITH ULTRAVIOLET ALN/ALGAN DISTRIBUTED BRAGG REFLECTOR 3191

Fig. 4. (a) Photoluminescence spectrum from the undoped GaN layer without the DBR. (b) Photoluminescence spectra of unpatterned (red) and patterned (black) GaN UVDBR structure at room temperature. This peak of the resonant mode is 374 nm wavelength and is located within stopband width (gray region).

IV. S

M

A

In order to understand optical modes of the photonic crystal

DBR structure, 3-D finite-difference time-domain (FDTD) and

plane-wave expansion (PWE) methods were used to performed

simulations for the fabricated structure [16], [17]. The left part

of Fig. 5(a) shows calculated band diagram (red) of the

pho-tonic crystal structure from the PWE method from normalized

frequency

0.6 to 0.8. The right part of Fig. 5(a) is

calcu-lated spectrum (blue) from FDTD simulation within the same

frequency region. The gray region in Fig. 5(a) indicates the high

reflection region due to the UVDBR at the bottom of photonic

crystal lattices. Several resonant modes were observed from the

FDTD spectrum around the UV region, which are labeled with

modes, A, B, C, and D. They are all bandedge modes of photonic

crystal lattices because the strong resonances can be observed

near these bandedges due to the flat photonic bands and their

slow group velocities. By comparing the band diagram and the

FDTD spectrum, the mode A, B, and D are bandedge modes at

symmetry point

, the mode C is a mode at M point. By

com-paring the measured and calculated spectra, the strong emission

at 374 nm wavelength, which is corresponded to a normalized

frequency of

, is verified to be the resonant mode A.

Fig. 5(b) shows the calculated in-plane

field profile of mode

A from the FDTD simulation. We also obtained same mode

profile from the PWE simulation. This in-plane mode profile also

Fig. 5. (a) Corresponding band diagram calculated by the PWE method (red) and the calculated luminescence spectra is presented (blue), corresponding dif-ferent modes, A, B, C, and D. (b) and (c) are top view and side view of the electric-field distribution, respectively, calculated by FDTD simulation.

confirms mode behavior of the bandedge mode at the

point.

Fig. 5(c) is the vertical

field profile of mode A along the

-axis. According to the vertical mode profile, the optical mode

is clearly located in the 200-nm undoped GaN layer due to the

high reflection of the UVDBR.

The UVDBR plays an important role to select emission

wave-length in the GaN-based photonic crystal structure. The gain peak

of the GaN without the DBR is around 360 nm wavelength, which

is outside the stopband of the DBR. However, the PL peak of GaN

on DBR is around 378 nm and the resonant modes are around

374 nm wavelength. There is a 14 nm wavelength difference

be-tween the GaN gain peak and the resonant mode. It is mainly

at-tributed to the reflection of the UVDBR mirror on the bottom of

photonic crystal lattices. This DBR effect in the GaN epitaxial

structure had been observed in our previous works [15], [18]. The

amplitude difference between mode A and other modes (mode B,

C, and D) in FDTD spectrum also prove stronger optical

enhance-ment within the high reflection region of DBR. The UVDBR also

reduces number of resonant modes by tuning its reflection

band-width. The mode reduction decreases energy waste in modes

out-side the DBR high reflection region. More emission behavior of

the GaN photonic crystals can be obtained with further studies

such as far-field pattern characterization [19], [20].

V. C

In this study, a strong emission from the GaN-based photonic

crystal with the AlN/AlGaN DBR structure was achieved in the

UV wavelength region. A fivefold enhancement in

photolumi-nescence emission was also observed. This enhancement results

from the coupling between electron-hole recombination in the

3192 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 22, NOVEMBER 15, 2010

top GaN gain layer and low group velocity modes at the

band-edge of

point. Experimental results show excellent agreement

with the FDTD and PWE simulations. Due to the larger

en-hancement of the devices, we believe the photonic crystal

struc-ture with bottom DBR mirror, which has the potential to light

sources for the future applications.

A

The authors would like to thank Prof. T.-C. Lu and Dr.

Z.-Z. Li at the National Chiao-Tung University.

R

[1] S. Nakamura, T. Mukai, and M. Senoh, “Candela-class high-bright-ness InGaN/AIGaN double-heterostructure blue-light-emitting,” Appl.

Phys. Lett., vol. 64, pp. 1687–1689, 1994.

[2] S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama, “High-bright-ness InGaN blue, green and yellow light-emitting diodes with quantum structures,” Jpn. J. Appl. Phys., vol. 34, pp. L797–L799, 1995. [3] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T.

Mat-sushita, Y. Sugimoto, and H. Kiyoku, “Room-temperature continuous-wave operation of InGaN multi-quantum-well-structure laser diodes with a long lifetime,” Appl. Phys. Lett., vol. 70, pp. 868–870, 1997. [4] S. Nakamura, “The roles of structural imperfections in InGaN-based

blue light-emitting diodes and laser diodes,” Science, vol. 281, pp. 956–961, 1998.

[5] M. Diagne, Y. He, H. Zhou, E. Makarona, A. V. Nurmikko, J. Han, K. E. Waldrip, J. J. Figiel, T. Takeuchi, and M. Krames, “Vertical cavity violet light emitting diode incorporating an aluminum gallium nitride distributed Bragg mirror and a tunnel junction,” Appl. Phys. Lett., vol. 79, pp. 3720–3722, 2001.

[6] T. C. Lu, S. W. Chen, L. F. Lin, T. T. Kao, C. C. Kao, P. Yu, H. C. Kuo, S. C. Wang, and S. Fan, “GaN-based two-dimensional surface-emit-ting photonic crystal lasers with AlN/GaN distributed Bragg reflector,”

Appl. Phys. Lett., vol. 92, pp. 011129-1–011129-3, 2008.

[7] T. C. Lu, C. C. Kao, H. C. Kuo, G. S. Huang, and S. C. Wang, “CW lasing of current injection blue GaN-based vertical cavity surface emit-ting laser,” Appl. Phys. Lett., vol. 92, pp. 141102-1–141102-3, 2008. [8] M. Boroditsky, T. F. Krauss, R. Coccioli, R. Vrijen, R. Bhat, and E.

Yablonovitch, “Light extraction from optically pumped light-emitting diode by thin-slab photonic crystals,” Appl. Phys. Lett., vol. 75, pp. 1036–1038, 1999.

[9] N. Eriksson, M. Hagberg, and A. Larsson, “,” IEEE J. Quantum

Elec-tron., vol. 32, pp. 1038–1047, 1996.

[10] A. L. Fehrembach, S. Enoch, and A. Sentenac, “Highly directive light sources using two-dimensional photonic crystal slabs,” Appl. Phys.

Lett., vol. 79, pp. 4280–4282, 2001.

[11] M. Rattier, H. Benisty, E. Schwoob, C. Weisbuch, T. F. Krauss, C. J. M. Smith, R. Houdre, and U. Oesterle, “Omnidirectional and com-pact guided light extraction from Archimedean photonic lattices,” Appl.

Phys. Lett., vol. 83, pp. 1283–1285, 2003.

[12] D. Delbeke, P. Bienstman, R. Bockstaele, and R. Baets, “Rigorous electromagnetic analysis of dipole emission in periodically corrugated layers: The grating-assisted resonant-cavity light-emitting diode,” J.

Opt. Soc. Amer., vol. B19, pp. 871–880, 2002.

[13] A. David, T. Fujii, R. Sharma, K. McGroddy, S. Nakamura, S. P. Den-Baars, E. L. Hu, C. Weisbuch, and H. Benisty, “Photonic-crystal GaN light-emitting diodes with tailored guided modes distribution,” Appl.

Phys. Lett., vol. 88, pp. 061124-1–061124-3, 2006.

[14] G. S. Huang, T. C. Lu, H. H. Yao, H. C. Kuo, S. C. Wang, C. W. Lin, and L. Chang, “Crack-free GaN/AlN distributed Bragg reflectors incor-porated with GaN/AlN superlattices grown by metalorganic chemical vapor deposition,” Appl. Phys. Lett., vol. 88, pp. 061904-1–061904-3, 2006.

[15] J. R. Chen, S. C. Ling, C. T. Hung, T. S. Ko, T. C. Lu, H. C. Kuo, and S. C. Wang, “High-reflectivity ultraviolet AlN/AlGaN distributed Bragg reflectors grown by metalorganic chemical vapor deposition,” J.

Crystal Growth, vol. 310, pp. 4871–4875, 2008.

[16] S. Noda, M. Yokoyama, M. Imada, A. Chutinan, and M. Mochzuki, “Polarization mode control of two-dimensional photonic crystal laser by unit cell structure design,” Science, vol. 293, pp. 1123–1125, 2001. [17] S.-H. Kwon, H.-Y. Ryu, G.-H. Kim, Y.-H. Lee, and S.-B. Kim, “Photonic bandedge lasers in two-dimensional square-lattice photonic crystal slabs,” Appl. Phys. Lett., vol. 83, pp. 3870–3872, 2003. [18] C. C. Chen, M. H. Shih, Y. C. Yang, and H. C. Kuo, “Ultraviolet

GaN-based microdisk laser with AlN/AlGaN distributed Bragg reflector,”

Appl. Phys. Lett., vol. 96, pp. 151115-1–151115-3, 2010.

[19] K. McGroddy, A. David, E. Matioli, M. Iza, S. Nakamura, S. DenBaars, J. S. Speck, C. Weisbuch, and E. L. Hu, “Directional emission control and increased light extraction in GaN photonic crystal light emitting diodes,” Appl. Phys. Lett., vol. 93, pp. 103502-1–103502-3, 2010. [20] S. W. Chen, T. C. Lu, Y. J. Hou, T. C. Liu, H. C. Kuo, and S. C.

Wang, “Lasing characteristics at different band edges in GaN photonic crystal surface emitting lasers,” Appl. Phys. Lett., vol. 96, pp. 071108-1–071108-3, 2010.

Cheng-Chang Chen was born in Shalu, Taiwan, in 1981. He received the

B.E. degree in physics from the National Chung-Cheng University, Minhsiung, Taiwan, in 2004, and the M.E. degree in photonics technologies from the National Tsing Hua University, Hsinchu, Taiwan, in 2006. He is currently working toward the Ph.D. degree in electrophysics at the National Chiao Tung University (NCTU), Hsinchu, Taiwan.

In 2007, he joined the Semiconductor Laser Technology Laboratory, NCTU, where he is currently with the Department of Photonics and the Institute of Electro-Optical Engineering. His current research interests include III-V semi-conductor materials and Si-based material for light-emitting devices and nanos-tructure for optical devices under the instruction of Prof. Hao-Chung Kuo and Prof. Min-Hsiung Shih.

Jun-Rong Chen was born in Taichung, Taiwan, on October 23, 1980. He

re-ceived the B.S. degree in physics from the National Changhua University of Education (NCUE), Changhua, Taiwan, in 2004, and the M.S. degree in op-toelectronics from the Institute of Photonics, NCUE, Taiwan, in 2006. He is currently working toward the Ph.D. degree in the Department of Photonics and the Institute of Electro-Optical Engineering, National Chiao Tung University (NCTU), Hsinchu, Taiwan.

In 2006, he joined the Semiconductor Laser Technology Laboratory, NCTU, where he was engaged in research on III-V semiconductor materials for light-emitting diodes and semiconductor lasers under the instruction of Prof. T.-C. Lu, Prof. H.-C. Kuo, and Prof. S.-C. Wang. His current research interests include III-nitride semiconductor lasers, epitaxial growth of III-nitride materials, and numerical simulation of III-V optoelectronic devices.

Yi-Chun Yang received the B.S. degree in applied physics from the National

University of Kaohsiung, Kaohsiung, Taiwan, in 2006.

In 2007, she joined the Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan, as a Research Assistant. Her current research interests include integrated photonic circuits, photonic crystal lasers and waveguides, and cavity quantum electrodynamics.

Min-Hsiung Shih received the B.S. degree in physics from the National Cheng

Kung University, Tainan, Taiwan, in 1995, the M.S. degree in physics from the National Tsing Hua University (NTHU), Taiwan in 1997, and the Ph.D. degree in electrical engineering/electrophysics from the University of Southern Cali-fornia, Los Angeles, in 2006.

In 2007, he joined the Research Center for Applied Sciences, Academia Sinica, Nankang, Taiwan, as an Assistant Research Fellow. His current re-search interests include integrated photonic circuits, photonic crystal lasers and waveguides, surface plasmonics, and cavity quantum electrodynamics.

Hao-Chung Kuo (S’98–M’99–SM’04) received the B.S. degree in physics

from the National Taiwan University, Taipei, Taiwan, in 1990, the M.S. degree in electrical and computer engineering from Rutgers University, Camden, NJ, in 1995, and the Ph.D. degree in electrical and computer engineering from the University of Illinois at Urbana-Champaign, Urbana, in 1999.

He has an extensive professional career both in research and industrial re-search institutions, which includes as follows: Rere-search Consultant with Lucent Technologies, Bell Labs, Holmdel, NJ, from 1995 to 1997, R&D Engineer with the Fiber-Optics Division, Agilent Technologies, from 1999 to 2001, and R&D Manager with LuxNet Corporation, from 2001 to 2002. Since September 2002, he has been with the National Chiao Tung University, Hsinchu, Taiwan, as a member of the faculty at the Institute of Electro-Optical Engineering. He is the author or coauthor of more than 60 papers published in various national and international journals and conference proceedings. His current research inter-ests include the epitaxy, design, fabrication, and measurement of high-speed In-Pand GaAs-based vertical-cavity surface-emitting lasers, as well as GaN-based lighting-emitting devices and nanostructures.