Contributions of sidewall illumination and current spreading to the light emission of InGaN-GaN light-emitting diode arrays

(1)

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 8, APRIL 15, 2006 983

Contributions of Sidewall Illumination and Current

Spreading to the Light Emission of InGaN–GaN

Light-Emitting Diode Arrays

Yi-Chen Yu, Chih-Hao Hsieh, Ghien-An Shih, Tzu-Yang Chiu, Dong-Ming Yeh, Chih-Feng Lu,

JianJang Huang, Member, IEEE, and C. C. Yang, Senior Member, IEEE

Abstract—We investigate the contribution of sidewall

illumi-nation in InGaN–GaN quantum-well-based light-emitting diode (LED) arrays with various cell radii. The intensity contribution from the array sidewall decreases with the increase of radius as the perimeter/area ratio is reduced. We then compare the effects of current spreading in the LED arrays of different sizes and conclude that the effect of current spreading needs to be given full consideration when the cell size in a microarray becomes larger. This letter provides a novel approach to calculate the intensity contributions of sidewall illumination and current spreading to GaN-based LED arrays.

Index Terms—Light-emitting diodes (LEDs), microarray, sidewall illumination.

I. INTRODUCTION

T

HE demand for solid-state lighting for general illumi-nation, backlights in flat-panel displays, and automotive lighting has pushed the development of high-power and highly efficient light-emitting diodes (LEDs). The low external ex-traction efficiency of LEDs is believed to be the bottleneck for high-power operations. More than 80% of light is totally re-flected from the semiconductor–air interface in a conventional LED structure. To resolve the drawback, some groups have proposed various designs of surface texturing to roughen the surface for improving light extraction [1], [2]. Furthermore, the idea of microarray was demonstrated to achieve higher extraction efficiency than the conventional broad-area devices [3]–[6]. For example, Jin et al. [3] reported an interconnected microdisk with an efficiency improvement of around 60%, when compared with conventional broad-area devices. Also, Huseh et al. [4] showed an around 40% improvement by etching up holes in the p-type mesa. In addition, Choi et al. [5] have shown a 60% improvement with a microring array structure. They all attributed the efficiency improvement of such microarrays over the conventional broad-area structures to the scattering of light from the sidewall perimeter. In all of those devices, the efficiency decreases as the size of the cells in the microarrays becomes larger. Despite the demonstration

Manuscript received January 10, 2006; revised February 4, 2006. This work was supported by the National Science Council of Taiwan under Grant NSC 94-2215-E-002-028.

The authors are with the Graduate Institute of Electro-Optical Engineering and Department of Electrical Engineering, National Taiwan University, Taipei 106, Taiwan (e-mail: [email protected]).

Digital Object Identifier 10.1109/LPT.2006.873461

of improvement in the external quantum efficiency, the identifi-cation of the real contribution, either from the enhancement of light extraction or the increase of internal quantum efficiency, in such an LED array is still unavailable. The internal quantum efficiency improvement, especially from the current spreading in the p-type layer, is also a key factor for increasing the output power of an LED array as the cell size becomes larger. The considerations of both sidewall illumination and current spreading conditions in the mesa area are essential in designing an LED array structure.

In the letter, we fabricate both microdisk and microring LED arrays with various dimensions. We also develop a novel process for p-type metal interconnections in the microdisk and microring structures. The sidewall of each cell is covered with a thick interconnect metal to eliminate sidewall illumination. We then calculate the intensity contributions of the effects of sidewall illumination and current spreading of different mi-croarrays of different cell sizes. The dimensions of LED arrays in this letter were designed to be much larger than previously published LED arrays in order to understand the upper limit of sidewall illumination.

II. DEVICEFABRICATION

The microring and microdisk samples were grown on -plane sapphire substrates. The material structure is composed of 25 nm of GaN buffer layer, 1.8 m of Si-doped n-type GaN layer, a five-period InGaN-GaN multiple quantum wells, and a 0.2- m p-type GaN layer. The spectral peak wavelength of photoluminescence is around 470 nm. The process of LED arrays started with inductively coupled plasma etching to define the mesa areas. We used Ni–Au for p-type and Ti–Al for n-type contacts. They were separately annealed to achieve ohmic contact. The p-type contact strip is 4 m in width and is 2 m away from the p-type mesa edge. On the subsequent steps, we first coated the sample with a 200-nm SiN insulating layer with plasma-enhanced chemical vapor deposition. The SiN layer is used to avoid the contact of the n-type GaN layer and the subsequent interconnect metal layer. Moreover, since the refractive index of SiN ( ) is between that of the air and GaN semiconductor ( ), the dielectric layer helps to increase light extraction. We then opened via holes on the SiN layer and evaporated a thick Ti (20 nm)/Au (560 nm) metal layer to connect microdisks/microrings p-type contact in the LED array.

(2)

984 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 8, APRIL 15, 2006

Fig. 1. (a) Cross-sectional view of microdisk with interconnect sidewall metal; (b) four-cell microdisk with sidewall metal (E-100-04); (c) four-cell microdisk (B-100-04); (d) four-cell microdisk with sidewall metal and star-shape p-type contacts/ITO layer (C-100-04).

TABLE I

NOMENCLATURE OFMICROARRAYSAMPLES

To study the microarray illumination, we designed five types of microdisk and microring structures. Microarrays, with side-walls either covered with thick interconnect metal or not, were fabricated for comparing the contributions from sidewall illu-mination. The scanning electron microscopy image in Fig. 1(a) shows a microdisk with its sidewall covered with metal. The slope of the p-type mesa is approximately 75 with respect to the wafer surface that ensures the complete elimination of sidewall optical output since most light reflected from the covered inter-connect metal will not reach the p-type mesa surface. We also fabricated microarrays with and without star-shaped p-type con-tacts and indium tin oxide (ITO) layer to understand the effects of current spreading. For those samples with star-shaped con-tacts, we sputtered and alloyed an additional 200-nm ITO layer on top of the p-type mesa to enhance current spreading prior to the deposition of SiN . The 4- m width star-shaped p-type metal strips were first coated to improve carrier flow across the ITO layer without blocking too much light. The nomenclature of those microarray structures is described in Table I. The pic-tures of four-cell microdisks are shown in Fig. 1(b), (c), and (d).

Fig. 2. Light output–current (L–I) curves of several types of microarrays under investigation.

Fig. 3. Output power densities of one-, four-, eight-cell microdisks.

III. CHARACTERIZATIONS OFMICROARRAYS

To compare the output power of those five types of microar-rays, output powers as functions of injection current of samples A, B, C, D, and E, all with four cells in an array and cell radius 100 m, were measured. As shown in Fig. 2, the sidewall il-lumination plays an important role in contributing total optical output power. The microring structure has the highest output power while the microdisk with sidewall covered with thick metal has the lowest one. In addition, the star-shaped p-type metal contact along with the ITO layer help spread out injected currents. Thus, the output power is higher than sample C-100-04 than that of sample E-100-04.

The dependence of the output power density, defined as the output power per unit p-type mesa area, on cell numbers of mi-crodisk arrays is plotted in Fig. 3. The injected current density is fixed at 0.08 A m for all of the microarrays under consider-ation and the radius of each cell is 100 m. Microdisks covered with sidewall metal show only little decrease of power densi-ties as the cell number increases. The decrease is attributed to a slightly higher series resistance of the p-type metal contact as the cell number increases. We believe the decrease is even more severe as part of the current density is used for sidewall illumi-nation. A decline is found for microdisks without sidewall metal (samples ).

(3)

YU et al.: CONTRIBUTIONS OF SIDEWALL ILLUMINATION AND CURRENT SPREADING 985

Fig. 4. (a) Power densities of one-cell LEDs with radius 100, 200, and 300m. (b) Contributions of sidewall illumination and current spreading [from (a)] at different cell size.

We next compare the output power with different cell dimen-sions. As shown in Fig. 4(a), sidewall illumination enhances the light extraction out of a one-cell LED under the same bias cur-rent density (0.07 A m ). The current spreading ( ) is not as significant as sidewall illumination when the cell ra-dius is within 200 m. However, the current spreading becomes critical when the cell radius is larger. For radius 300 m, the contribution from sidewall illumination is smaller than the ef-fect of current spreading since the aspect ratio (perimeter/area) is smaller. Proper current spreading in the large microdisk area becomes important.

Contributions of sidewall illumination and microdisk current spreading of those samples in Fig. 4(a) are plotted in Fig. 4(b). For devices with radius 100 m, sidewall illumination enhances optical output by 78% when compared with devices that the sidewall is completely blocked by thick metal (E-100-01). It then decreases to 25% as the radius is 300 m. In contrast, the improvement of efficiency by the effect of current spreading becomes more significant than that by the sidewall illumina-tion. Sample C-300-01 has 39% more output power than sample E-300-01.

IV. CONCLUSION

The illumination of LED microarrays is affected by several factors such as sidewall illumination, microdisk area, and cur-rent spreading. We compare the optical output power of several types of microarray structures. The ratio of sidewall illumina-tion to total output power can be calculated by comparing mi-croarrays with and without sidewall metal. The effect of cur-rent spreading becomes more significant than sidewall illumi-nation as the cell size becomes larger. The improvement of effi-ciency by the effect of current spreading is 39% while the side-wall contribution is 25% for cell radius 300 m. This letter pro-vides a novel approach to calculate the intensity contributions of sidewall illumination and current spreading to GaN-based LED arrays.

REFERENCES

[1] C. M. Tsai, J. K. Sheu, W. C. Lai, Y. P. Hsu, P. T. Wang, C. T. Kuo, C. W. Kuo, S. J. Chang, and Y. K. Su, “Enhanced output power in GaN-based LEDs with naturally texture surface grown by MOCVD,” IEEE Electron Device Lett., vol. 26, no. 7, pp. 464–466, Jul. 2005.

[2] K.-M. Chang, C.-H. Li, B.-S. Sheih, J.-Y. Yang, S.-W. Wang, and T.-H. Yeh, “A new simple and reliable method to form a textured Si surface for the fabrication of a tunnel oxide film,” IEEE Electron Device Lett., vol. 19, no. 5, pp. 145–147, May 1998.

[3] S. X. Jin, J. Li, J. Y. Lin, and H. X. Jiang, “InGaN/GaN quantum well interconnected microdisk,” Appl. Phys. Lett., vol. 77, no. 20, pp. 3236–3238, Nov. 2000.

[4] T. H. Hsueh, J. K. Sheu, H. W. Huang, J. Y. Chu, C. C. Kao, H. C. Kuo, and S. C. Wang, “Enhancement in light output of InGaN-based microhole array light-emitting diodes,” IEEE Photon. Technol. Lett., vol. 17, no. 6, pp. 1163–1165, Jun. 2005.

[5] H. W. Choi, C. W. Jeon, and M. D. Dawson, “InGaN microring light-emitting diodes,” IEEE Photon. Technol. Lett., vol. 16, no. 1, pp. 33–35, Jan. 2004.

[6] H. W. Choi, M. D. Dawson, P. R. Edwards, and R. W. Martin, “High ex-traction efficiency InGaN micro-ring light-emitting diodes,” Appl. Phys. Lett., vol. 83, no. 22, pp. 4483–4485, Dec. 2000.