Polarized light emission from photonic crystal light-emitting diodes

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Polarized light emission from photonic crystal light-emitting diodes

Chun-Feng Lai, Jim-Yong Chi, Hsi-Hsuan Yen, Hao-Chung Kuo, Chia-Hsin Chao, Han-Tsung Hsueh, Jih-Fu Trevor Wang, Chen-Yang Huang, and Wen-Yung Yeh

Citation: Applied Physics Letters 92, 243118 (2008); doi: 10.1063/1.2938885

View online: http://dx.doi.org/10.1063/1.2938885

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/24?ver=pdfcov Published by the AIP Publishing

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Polarized light emission from photonic crystal light-emitting diodes

Chun-Feng Lai,1Jim-Yong Chi,2,3Hsi-Hsuan Yen,1Hao-Chung Kuo,1,a兲 Chia-Hsin Chao,3 Han-Tsung Hsueh,3Jih-Fu Trevor Wang,3Chen-Yang Huang,3and Wen-Yung Yeh3

1

Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu 300, Taiwan

2

Institute of Optoelectronic Engineering, National Dong-Hua University, Hualien 97401, Taiwan 3

Electronics and Opto-electronics Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan

共Received 7 April 2008; accepted 13 May 2008; published online 18 June 2008兲

We have experimentally studied polarization characteristics of the two-dimensional photonic crystal 共PhC兲 light-emitting diodes 共LEDs兲 using an annular structure with square lattice and observed a strong polarization dependence of the lattice constant and orientation of the PhC. The extracted light from the GaN PhC LEDs has P/S ratios of 5.5 共⬃85% polarization light兲 for light propagating in the⌫X direction and 2.1 共⬃68% polarization light兲 for the ⌫M direction, respectively. Based on the couple mode theory, the dependence of polarization behaviors on different lattice constant and orientation was found to be in good agreement with theoretical discussion. © 2008 American Institute of Physics. 关DOI:10.1063/1.2938885兴

Light-emitting diodes 共LEDs兲 have become ubiquitous in illumination and signal applications as their efficiency and power level improve. While the improvement of the basic characteristics will benefit the replacement of the conven-tional light sources, further improvement in other character-istics can bring about unique applications. One notable ex-ample is the polarized light emission which is highly desirable for many applications,1e.g., backlighting for liquid crystal displays and projectors. Several authors have reported polarized light emission for LED structures grown on non-polar or seminon-polar GaN substrates.2,3In the present study, we investigate the approach employing photonic crystals共PhCs兲 which do not require the growth on different orientation of sapphire or GaN substrates nor using specific wafer orienta-tions. PhC has been widely studied in recent years4–8for the enhancement of LED efficiency, but polarized light emission using PhC has not been investigated.

Due to valence band intermixing, the side emission of light from quantum well 共QW兲 structure is predominantly polarized in the TE direction 共along the wafer plane兲. The observed polarization ratio has been reported to be as high as 7:1 for GaN/InGaN QWs.9 For common GaN LED struc-tures grown along the c axis, access to this polarized light can only be gained by measurements taken from the edge of the sample.10,11 In this work, we use the PhC structure to access the polarized emission and measured their orientation dependence using a especially designed PhC structure to ex-tract the waveguided light. It is found that the PhC can be-have as a polarizer to improve the P/S ratio of the extracted electroluminescence 共EL兲 emission. The results of the P/S ratio for light propagating in different lattice orientation were found to be consistent with the results obtained using the PhC Bloch mode coupling theory.

The GaN-based PhC LED samples used in the present work are the same as described before.7,8Figure1shows the cross-sectional view of the GaN blue PhC LED structure, where an annular PhC region with an inner/outer diameter of 100/200␮m was fabricated. The lattice constants a and hole

diameters d of square lattice are 260 nm and 180 nm, respec-tively, wherein the etch depth is 120 nm, the same as in Ref.

8. The orientation of the PhC is fixed in space and the ratio of hole diameter d to lattice constant a is also fixed to 0.7 to provide the consistent band structure.

The polarization properties of the GaN blue PhC LEDs were measured at room temperature using a scanning optical microscopic system, the same as in Refs.7and8. Figure2共a兲

shows EL CCD image for the sample with square lattice constant a = 260 nm corresponding to a/␭=0.553. Inset in Fig. 2共a兲 are the photoluminescence 共PL兲 CCD image and the reduced Brillioun zone. The observed light emission is from the light propagation along the⌫M and ⌫X directions as reported before.8Furthermore, the extraction enhancement of the PhC LED chips was determined to be above 100% by mounting the dies on TO packages and using an integration sphere with Si photodiode, when compared to the GaN-based LED chips without PhC. A polarizer共Newport, 10LP-VIS-B兲

a兲_{Electronic mail: hckuo@faculty.nctu.edu.tw.} FIG. 1.共Color online兲 Schematic of the cross section of the annual structure

of GaN PhC LED used in this work.

APPLIED PHYSICS LETTERS 92, 243118共2008兲

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was placed on the GaN blue PhC LEDs for the EL measure-ments. Figure2共b兲presents CCD image of room temperature EL for samples biased at a drive current of 20 mA. The red dashed line indicates the polarization axis for the polarizer. Since the polarization direction of the light is perpendicular to its propagation directions, the light propagated in the di-rection align with the axis of the polarizer will be blocked. The luminescent signal emitted by the sample was collected by the 40⫻ objective lens of the confocal microscope and was transferred to a monochromator for␮-PL measurement through an optical fiber with a core diameter of 600␮m. Figure2共c兲shows the EL intensity as a function of the ori-entation of the polarizing filter placed between the GaN blue PhC LED and the spectrometer, at a drive current of 20 mA. The intensity at various angles was determined from image

taken under the same bias condition. Thus the polarization for different propagation direction can be determined, as shown in Fig. 2共c兲. It can be seen that there is a periodic variation of the EL intensity with angular orientation of the polarizer. This indicates that the light collected from the PhC LED is partially polarized, and the P/S ratios 共defined as P/S=I_max/I_min兲 were 5.5 and 2.1 for square lattice 共a = 260 nm兲 in the ⌫X and ⌫M directions, respectively, as shown in Fig. 2共d兲. Figure 2共d兲 also shows the P/S ratio measured in other samples with different period. For square-lattice PhC LEDs, P/S ratio in ⌫X orientation is larger than those in other orientations despite the lattice constants. In addition, for the same orientations, PhC LEDs with shorter lattice constant have higher P/S ratio.

The experimental results described above can be ex-plained by examining the electromagnetic field distributions of PhC Bloch modes. Field distributions of Bloch modes were calculated by plane wave expansion共PWE兲 method us-ing the structure with PhC sandwiched in between air and GaN materials. Figure3共a兲schematically shows the device structure where light is generated and extracted through PhCs. Due to the valence band mixing effects in QWs, guided light propagating in the GaN slab is nearly linear polarized in the transverse direction, as shown in Fig.3共b兲. For PhC a/␭=0.553, the field distributions for propagation in⌫X and ⌫M directions are shown schematically in Figs.

FIG. 2.共Color online兲共a兲 CCD EL images for lattice constants a=260 nm, inset of the PL CCD image, and the reduced Brillouin zone.共b兲 CCD EL images show polarization properties; the red line indicates the polarization axis of the polarizer.共c兲 Spectrally integrated EL intensity of the GaN PhC LED as a function of polarizer angle␪.共d兲 P/S ratio of different lattice constant as a function of orientation direction.

FIG. 3. 共Color online兲共a兲 Schematic of the light generating, propagating, and coupling to PhC Bloch modes. Electromagnetic field distributions for a waveguiding mode in the共b兲 plane slab guide mode and PhC Bloch modes in the共c兲 ⌫X and 共d兲 ⌫M directions of the frequency a/␭=0.553 and in the 共e兲 ⌫X and 共f兲 ⌫M directions of the frequency a/␭=0.872, respectively. Arrows indicate the electric field vectors in the plane, and black circles indicate the locations of lattice points.

243118-2 Lai et al. Appl. Phys. Lett. 92, 243118共2008兲

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3共c兲 and 3共d兲, respectively, where the arrows indicate the electric field vectors in the plane, and black circles indicate the locations of holes. The individual electric field distribu-tions are complicated, resulting in complicated polarization pattern. It can be seen that the field distribution in the ⌫X orientation has linearlike polarization behavior, and those in the ⌫M orientation has circularlike polarization.12 This be-havior can be inferred from the arrangement of the atoms relative to the propagation direction. For the ⌫X direction, the propagating wave sees the same atom arrangement in the planes perpendicular to the propagating direction from one lattice plane to plane, while in the ⌫M direction, the field distribution sees an alternately displaced atom arrangements from plane to plane. Such a staggered atom arrangement will tend to generate the field components normal to the polariza-tion plane. Based on the couple mode theory, the polarizapolariza-tion behavior of extracted light can follow the Bloch modes in PhCs and reveal the similar polarization characteristics. Therefore, the P/S ratio of light extracted through the ⌫X orientation would be higher than through the⌫M orientation. From the Bloch mode patterns in Fig. 3, the experimental polarization results can be realized and consistent with the above discussion.

At a/␭=0.872, the field distribution in the ⌫X orienta-tion also has more linearlike than circularlike behavior, and those in the⌫M orientation have stronger circularlike polar-ization, as shown in Figs. 3共e兲 and3共f兲. The degree of the polarization appears to be much weaker than that for a/␭ = 0.553. In order to discuss this observation, the P/S ratio as a function of normalized frequency was calculated. We em-ploy the PWE method to calculate the polarization properties 共P/S ratio兲 of the leaky modes in the ⌫X directions as a function of normalized frequency. In the calculation, the po-larization of the generated light is assumed to be TE polar-ized. The calculation was carried for each band along the⌫X direction up to the light line where the light becomes guided and its polarization is then the same as they were generated. As can be seen in Fig.4, the trend of the P/S ratio decreases with normalized frequency although the trend within each band is not uniform depending on the field distribution. De-tails of this discussion will appear in later publication. It can also be seen from Fig.4that by varying the fill factor of the lattice constant, the PhC can be designed to have higher ex-traction efficiency for TE polarization while discriminating the TM polarization. In such case, a very high P/S ratio 共⬎102_{兲 can be achieved. The maximum efficiency for the}

polarized emission that can be obtained in such case is equal to the TE portion of the total emission which is as high as 88% for a 7:1 P/S ratio.

In conclusion, we have experimentally demonstrated that surface emitted polarized light from GaN blue PhC LEDs. A P/S ratio of 5.5 共⬃85% polarization light兲 has been ob-served. The polarization characteristics are theoretically dis-cussed by couple mode theory. At lower normalized

fre-quency, PhC LED has better polarization property, and lattice orientation not only affects the extraction efficiency but also P/S ratio of radiative light. This polarization behavior sug-gests an efficient means to design and control the GaN blue PhC LEDs for polarized light emission.

The authors gratefully acknowledge Dr. S. C. Wang at National Chiao-Tung University共NCTU兲 in Taiwan for tech-nical support. This work is supported by the National Nano-technology Program of Taiwan, R.O.C., and in part by the National Science Council of the Republic of China under Contract Nos. NSC 2752-E-009-007-PAE, and NSC 95-2221-E-009-282.

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243118-3 Lai et al. Appl. Phys. Lett. 92, 243118共2008兲