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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 11, JUNE 1, 2009
Improving Light Output Power of the GaN-Based Vertical-Injection Light-Emitting Diodes by Mg+ Implanted Current Blocking Layer Min-An Tsai, Peichen Yu, Member, IEEE, J. R. Chen, J. K. Huang, C. H. Chiu, H. C. Kuo, Senior Member, IEEE, T. C. Lu, Member, IEEE, S. H. Lin, and S. C. Wang, Life Member, IEEE
Abstract—A method for forming a current blocking layer (CBL) by ion implantation in GaN-based vertical-injection light-emitting diodes (VI-LEDs) was proposed. It was found that the use of CBL in VI-LEDs can effectively reduce the current crowding effect and enhance the light output power. The uniform emission intensity distribution of VI-LEDs with CBL was demonstrated by electroluminescence measurements. Experimental results show that the wall-plug efficiency was enhanced by 12.3% at an injection current of 20 mA, compared to that of VI-LEDs without CBL, and by 56.2% compared to that of conventional LEDs. The device simulation results reveal that the current path can be blocked by CBL, resulting in high light extraction efficiencies and large current densities within the effective emission region of active layers. Index Terms—Current blocking layer (CBL), current crowding, ion implantation, GaN, vertical-injection light-emitting diodes (VILEDs), wall-plug efficiency (WPE).
I. INTRODUCTION
T
HE III–nitride materials have wide bandgap covering the emission wavelengths from ultraviolet to infrared and have been extensively investigated in past ten years [1]–[3]. Recently, high-brightness GaN-based light-emitting diodes (LEDs) have become a promising candidate for applications in flashlight, traffic signals, backlight source for liquid crystal displays, direct-view large-area signage, and solid-state lighting [4], [5]. Although the external quantum efficiency and illumination of high-brightness GaN-based LEDs have been significantly improved, it still cannot replace the traditional light sources. To extend the applications of nitride-based light sources, it is necessary to improve the reliability and overcome the nonuniform light distribution caused by current Manuscript received December 04, 2008; revised January 22, 2009. First published March 10, 2009; current version published May 08, 2009. This work was supported in part by the National Science Council in Taiwan under Grant NSC96-2221-E-009-095-MY3 and Grant NSC96-2628-E-009-017-MY3. M.-A. Tsai is with the Department of Electrophysics, National Chiao-Tung University, Hsinchu 30010, Taiwan and also with the Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu 30010, Taiwan. P. Yu, J. R. Chen, J. K. Huang, C. H. Chiu, H. C. Kuo, T. C. Lu, and S. C. Wang are with Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu 30010, Taiwan (e-mail: yup@faculty. nctu.edu.tw;
[email protected]). S. H. Lin is with the Department of Electrophysics, National Chiao-Tung University, Hsinchu 30010, Taiwan (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2009.2016431
crowding effect. In general, the relatively low hole mobility of the p-type GaN layer results in severe current crowding effect in conventional GaN-based LEDs, which have coplanar pand n-contacts for current injection due to insulating sapphire substrates [6]. Consequently, the high resistivity in p-type GaN layer restricts the current spreading at lateral direction of the LED structures, leading to nonuniform light emission intensity distribution. Therefore, the photon generation rate is decreased due to the presence of high current density in a localized region. To solve the problem caused by current crowding effect, current spreading through the lateral structure can be improved by using various techniques, such as transparent conductive electrodes, superlattice tunneling contacts [7], and insulating current blocking layer (CBL) [8]. The conventional methods for fabricating the CBL requires regrowth or deposition of SiO for insulation, which are relatively complex and cannot produce a planar insulating layer. In this work, we propose a novel vertical-injection LED (VI-LED) structure with CBL embedded in the p-type GaN layer. The CBL is formed by utilizing a Mg implantation method [9], [10], which provides a planar insulating layer due to the requirement of wafer bonding in the VI-LED structure. The performance of the VI-LED with CBL shows uniform light emission intensity distribution and improved light output power since the CBL effectively prevents the injected carriers from recombination under the n-pad electrode. Furthermore, the characteristics of the conventional LEDs and VI-LEDs without CBL are also discussed and analyzed. II. EXPERIMENTS The schematic sketch of the fabricated InGaN–GaN VI-LEDs with CBL embedded in the p-type GaN layer is illustrated in Fig. 1. First, a conventional LED structure was grown on a c-plane sapphire substrate by metal–organic chemical vapor deposition. The epitaxial LED structure consisted of a 1.7- m-thick heavily doped n-type GaN layer, followed by ten-pair InGaN–GaN multiple quantum wells (MQWs) with a total thickness of 90 nm, and a 0.2- m-thick p-type GaN layer. The detail information of this epitaxial structure can be found in [11]. Second, a 100-nm-thick SiN layer was deposited by plasma-enhanced chemical vapor deposition, where a CBL region with a mask diameter of 90 m was defined using photolithography and dry etching, followed by the evaporation of a 20-nm-thick metal layer. Next, the insulating CBL embedded
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TSAI et al.: IMPROVING LIGHT OUTPUT POWER OF THE GaN-BASED VI-LEDs BY Mg
Fig. 1. Schematic sketch of the InGaN–GaN VI-LED with CBL embedded in the p-type GaN layer by utilizing the ion implantation technique. The red dotted line shows the current spreading path.
Fig. 2. Light emission pattern of the InGaN–GaN VI-LED with CBL. The n-pad electrode is not yet deposited. (a) The probe touches the surface of the implanted area. (b) The probe touches the surface of the nonimplanted area.
in the localized p-GaN layer was formed by the Mg implantation method. The Mg ions were implanted with ion energy of cm . Then the structure 80 keV and a concentration of was bonded to an Au-coated Si substrate at a temperature of 350 C, resulting in an alloy of Au and Sn to form adhesion between the LED structure and Si substrate. Since silicon has a higher thermal conductivity ( 1.457 W/cm C) than that of sapphire ( 0.35 W/cm C) at room temperature, the host Si substrate also functions as a heat sink. After wafer-bonding, the sapphire substrate was removed by a laser lift-off process using a KrF excimer laser (Lambda Physick LPX200) at 248-nm wavelength with a pulsewidth of 25 ns [12]–[14]. The undoped GaN was also removed by an inductively coupled plasma reactive ion etcher. Subsequently, the mesa with an area of the 12 mil 12 mil was defined by using standard photolithography and dry etching, and then passivated with SiN , and followed by surface passivation with silicon dioxide (SiO ) for electric isolation. Finally, a patterned Cr–Pt–Au electrode was deposited on n-GaN layer. The diameter of the n-pad was 100 m. III. RESULTS AND DISCUSSION Fig. 2 shows the light emission pattern of an InGaN–GaN VI-LED with CBL, where the n-pad electrode is not yet deposited on top of the VI-LEDs. Fig. 2(a) shows the emission pattern as the probe touches the surface of the implanted area, while Fig. 2(b) shows the emission pattern as the probe touches the surface of the nonimplanted area. The light emission pattern demonstrates that the CBL have been embedded into the p-GaN successfully. To further understand the internal current distribution of the VI-LED structures with and without CBL, we
IMPLANTED CBL
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Fig. 3. Calculated 2-D vector plot of the current density and lateral current density distribution within the active region for LEDs (a) without and (b) with CBL. The blue line between the p-type and n-type contacts represents the position of MQW region.
Fig. 4. Calculated 2-D distribution of the radiative recombination rate within the MQW active regions of LEDs (a) without and (b) with CBL.
employed advanced device simulation software APSYS which is based on two-dimensional (2-D) finite-element analysis, which solves self-consistent Schrödinger, Poisson, the current continuity, the carrier energy transport, the heat transfer, and drift-diffusion equations [15]. Fig. 3 shows the calculated 2-D vector plot of the current density and the current density distribution within the MQW active region for VI-LEDs (a) without and (b) with CBL. The blue line between the p-type and n-type contacts represents the position of MQW region. As shown in Fig. 3(a), without CBL, the distribution of the injection current is uniform through the MQW region and full device structure, which is the main benefit of the VI-LEDs. However, the emission light from the active region covered by n-type contact cannot be extracted efficiently. On the contrary, the injection current path of the VI-LED with CBL is blocked and separated into two current paths, as shown in Fig. 3(b). Fig. 4 shows the 2-D distribution of calculated radiative recombination rate within the active regions for VI-LEDs (a) without and (b) with CBL. By comparing Fig. 4(a) and (b), it is clearly observed that the effective emission region at the lateral direction is located outside the regions of CBL. Moreover, as shown in Fig. 3, under the same driving current, the current density within the emission region of VI-LED with CBL is larger than that without CBL. Therefore, the VI-LED with CBL not only enhances the light extraction efficiency but also provides a large current density within the effective emission region of the active layers. Finally, Fig. 5 shows the typical current–voltage ( – ) and light output power–current ( – ) characteristics of three kinds of packaged LEDs, including 15 mil 15 mil conventional GaN-based LEDs, 12 mil 12 mil VI-LEDs without CBL,
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 11, JUNE 1, 2009
use of CBL slightly increases the resistivity, the output power of the VI-LEDs with CBL achieves 19.18 mW under an injection current of 20 mA, corresponding to a WPE of 22.8%. The design of CBL in InGaN–GaN LEDs not only enhances the WPE but also provides larger current density within the effective emission region of the active layers. ACKNOWLEDGMENT (a)
The authors thank Epistar Corporation for their technical support and Dr. T. C. Hsu for helpful suggestions.
(b)
2
Fig. 5. Comparison of the L–I –V characteristics of the 15 mil 15 mil conventional LEDs, the 12 mil 12 mil VI-LEDs without CBL, and the 12 mil 12 mil VI-LEDs with CBL. (a) I –V curves, and (b) I –L curves.
2
2
and 12 mil 12 mil VI-LEDs with CBL. The characterization was performed for ten randomly selected devices of each kind with resolutions of 1 pW, 1 pA, and 1 pV for photodetection, injection current, and applied bias, respectively. As shown in Fig. 5(a), the measured forward voltages at an injection current of 20 mA are 4.2, 3.7, and 3.8 V for VI-LEDs with CBL, VI-LEDs without CBL, and conventional LEDs, respectively. The VI-LEDs without CBL have a lower forward voltage than that of conventional LEDs since the vertical-injection design reduces the resistivity and increases the thermal conductivity. The slightly increased forward voltage for the VI-LED with CBL is attributed to the CBL design which reduces the current path and the lateral-current spreading in the n-type GaN layer, giving rise to a slightly increased resistivity. As shown in Fig. 5(b), the respective light output powers at an injection current of 20 mA are 19.2, 15.0, and 11.1 mW for VI-LEDs with CBL, VI-LEDs without CBL, and conventional LEDs, corresponding to wall-plug efficiencies (WPEs) of 22.8%, 20.3%, and 14.6%, respectively. The WPE of VI-LEDs with CBL is enhanced by 12.3% at an injection current of 20 mA, compared to that of VI-LEDs without CBL, and by 56.2% compared to that of conventional LEDs. The increased WPE and light output power result from the use of CBL, which effectively enhances the light extraction efficiency and provides a large current density within the effective emission region of the active layers. IV. CONCLUSION The VI-LEDs with CBL embedded in the p-type GaN layer by using the ion implantation was proposed in order to provide an insulating region beneath the pad cathode region. The experimental results show that the light output power and WPE of the InGaN–GaN LEDs can be obviously improved by using the vertical-injection design and CBL simultaneously. Although the
REFERENCES [1] E. Fred Schubert, Light-Emitting Diodes. Cambridge, U.K.: Cambridge Univ. Press. [2] A. Zukauskas, M. S. Shur, and R. Gaska, Introduction to Solid-State Light.. New York: Wiley. [3] J. T. Chu, H. W. Huang, C. C. Kao, W. D. Liang, F. I. Lai, C. F. Chu, H. C. Kuo, and S. C. Wang, “Fabrication of large-area GaN-based lightemitting diodes on Cu substrate,” J. Appl. Phys., vol. 44, no. 4B, pp. 2509–2511, 2005. [4] Y. C. Lin, S. J. Chang, Y. K. Su, C. S. Chang, S. C. Shei, J. C. Ke, H. M. Lo, S. C. Chen, and C. W. Kuo, “High power nitride based light emitting diodes with Ni/ITO p type contacts,” Solid-State Electron., vol. 47, pp. 1565–1568, 2003. [5] H. X. Jiang, S. X. Jin, J. Li, J. Shakya, and J. Y. Lin, “III-nitride blue microdisplays,” Appl. Phys. Lett., vol. 78, pp. 1303–1305, 2001. [6] Y. J. Lee, J. M. Hwang, T. C. Hsu, M. H. Hsieh, M. J. Jou, B. J. Lee, T. C. Lu, H. C. Kuo, and S. C. Wang, “Enhancing the output power of GaN-based LEDs grown on wet-etched patterned sapphire substrates,” IEEE Photon. Technol. Lett., vol. 18, no. 10, pp. 1152–1154, May 15, 2006. [7] J. K. Sheu, J. M. Tsai, S. C. Shei, W. C. Lai, T. C. Wen, C. H. Kou, Y. K. Su, S. J. Chang, and G. C. Chi, “Low-operation voltage of InGaN/GaN light-emitting diodes with Si-doped In Ga N/GaN short-period superlattice tunneling contact layer,” IEEE Electron Device Lett., vol. 22, no. 10, pp. 460–462, Oct. 2001. [8] C. Huh, J. M. Lee, D. J. Kim, and S. J. Park, “Improvement in light-output efficiency of InGaN/GaN multiple-quantum well light-emitting diodes by current blocking layer,” J. Appl. Phys., vol. 92, pp. 2248–2250, 2002. [9] B. J. Pong, C. J. Pan, Y. C. Teng, G. C. Chi, W.-H. Li, and K. C. Lee, “Structural defects and microstrain in GaN induced by Mg ion implantation,” J. Appl. Phys., vol. 83, pp. 5992–5996, 1998. [10] S. J. Pearton, C. B. Vartuli, J. C. Zolper, C. Yuan, and R. A. Stall, “Ion implantation doping and isolation of GaN,” Appl. Phys. Lett., vol. 67, pp. 1435–1437, 1995. [11] Y. J. Lee, H. C. Kuo, T. C. Lu, and S. C. Wang, “High light-extraction GaN-based vertical LEDs with double diffuse surfaces,” IEEE J. Quantum Electron., vol. 42, no. 12, pp. 1196–1201, Dec. 2006. [12] C. F. Chu, C. C. Yu, H. C. Cheng, C. F. Lin, and S. C. Wang, “Comparison of p-side down and p-side up GaN light-emitting diodes fabricated by laser lift-off,” J. Appl. Phys., vol. 42, no. 2B, pp. L147–L150, 2003. [13] C.-F. Chu, F.-I. Lai, J.-T. Chu, C.-C. Yu, C.-F. Lin, H.-C. Kuo, and S. C. Wang, “Study of GaN light-emitting diodes fabricated by laser lift-off technique,” J. Appl. Phys., vol. 95, pp. 3916–3922, 2004. [14] C. H. Chiu, C. E. Lee, C. L. Chao, B. S. Cheng, H. W. Huang, H. C. Kuo, T. C. Lu, S. C. Wang, W. L. Kuo, C. S. Hsiao, and S. Y. Chen, “Enhancement of light output intensity by integrating ZnO nanorod arrays on GaN-based LLO vertical LEDs,” Electrochem. Solid-State Lett., vol. 11, pp. H84–H87, 2008. [15] APSYS 2008, Crosslight Software. Burnaby, Canada.
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