928 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL 19, NO 12, JUNE 15, 2007 Improving the Luminescence of InGaN–GaN Blue LEDs Through Selective Ring-Region Activation of the Mg-Doped GaN Layer Ray-Ming Lin, Jen-Chih Li, Yi-Lun Chou, Kuo-Hsing Chen, Yung-Hsiang Lin, Yuan-Chieh Lu, Meng-Chyi Wu, Hung Hung, and Wei-Chi Lai Abstract—In this study, we used the selective ring-region activation technique to restrain the surface leakage current and to monitor the luminescence characteristics of InGaN–GaN multiple quantum-well blue light-emitting diodes (LEDs) To access the current blocking region after forming a periphery high-resistance ring-region of the Mg-doped GaN layer and to reduce the degree of carrier trapping by the surface recombination centers, we deposited a titanium film onto the Mg-doped GaN epitaxial layer to form a high-resistance current blocking region To characterize their luminescence performance, we prepared LEDs incorporating titanium films of various widths of the highly resistive current blocking layer The hole concentration in the Mg-doped GaN epitaxial layer decreased from 3:45 1017 cm03 to 3:31 1016 cm03 after capping with a 250-nm-thick layer of titanium and annealing at 700 C under a nitrogen atmosphere for 30 Furthermore, the luminescence characteristics could be improved by varying the width of the highly resistive region of the current blocking area; in our best result, the relative electroluminescence intensity was 30% (20 mA) and 50% (100 mA) higher than that of the as-grown blue LEDs Index Terms—InGaN–GaN, light-emitting diodes (LEDs), selective activation systems as outdoor full-color displays, traffic lights, indicators, indoor illuminations, and liquid crystal display (LCD) backlights To reduce power consumption, these applications will require LEDs exhibiting high light output power and low forward voltage Because of the limited internal quantum efficiency and wastage through internal reflection of large amounts of their generated light, improvements in the quantum and light extraction efficiencies of LEDs are presently issues that must be addressed in the search for LEDs exhibiting higher performance and brightness A few reports have described the selective activation of Mg-doped GaN layers by capped Ti–Au [1] and Ni [2] films as an approach toward obtaining a current blocking layer under the p-electrode metal of LEDs and, consequently, enhancing the performance of the devices [3] To date, however, only a few reports have discussed methods to improve the external quantum efficiency by restraining the surface leakage current of InGaN–GaN blue LEDs In this letter, we describe the use of the selective ring-region activation technique to restrain the surface leakage current and to monitor the luminescence characteristics of InGaN–GaN multiple quantum-well (MQW) blue LEDs I INTRODUCTION II EXPERIMENT ALLIUM nitride (GaN)-based wide-bandgap semiconductors currently play important roles in many optoelectronic devices The bandgap energy of GaN is 3.4 eV at room temperature (RT); it forms a number of alloys having bandgaps ranging from 0.7 eV [with indium nitride (InN)] to 6.2 eV [with aluminum nitride (AlN)] Because of this wide range of bandgaps and its excellent electronic, optical, and thermal properties, GaN is becoming increasingly more attractive for use in a variety of applications High-brightness light-emitting diodes (LEDs) based on group III nitrides are of great interest for use in such The InGaN–GaN MQW blue LED wafers were grown on a c-plane sapphire substrate using metal–organic chemical vapor deposition The LED structure consisted of a 30-nm-thick low-temperature GaN buffer layer, a 2- m-thick lightly doped n-type GaN layer, a 2- m-thick highly doped n-type GaN layer, N (2 nm)/GaN (10 nm) MQWs, and a five pairs of In Ga 0.3- m-thick Mg-doped GaN layer After growth, the current blocking area was capped with a 250-nm-thick titanium film to form the high-resistance area The sample was then treated in a quartz furnace for selective activation of the Mg-doped GaN under a nitrogen atmosphere The annealing temperature and time were 700 C and 30 min, respectively After thermal annealing, the titanium film was removed using HF solution The hole concentration of the Mg-doped GaN epitaxial layer, as determined through Hall measurement, cm to cm after capdecreased from ping with the 250-nm-thick layer of titanium Next, a nickel film was used as the etching mask layer in an inductively coupled plasma (ICP) dry etching process The ICP process was used to etch through the Mg-doped p-type GaN and InGaN–GaN MQW to the n-type GaN layer The nickel film was then removed A Ni (5 nm)/Au(10 nm) transparent contact layer (TCL) was deposited on the p-type layer using an electron-beam evaporator; the sample was then thermally annealed at 500 C under an G Manuscript received November 10, 2006; revised March 17, 2007 This work was supported by the National Science Council of the Republic of China under Contract NSC 94-2215-E-182-004 R.-M Lin, J.-C Li, K.-H Chen, Y.-H Lin, and Y.-C Lu are with the Department of Electronic Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan 333, Taiwan, R.O.C (e-mail: rmlin@mail.cgu.edu.tw) Y.-L Chou and M.-C Wu are with the Institute of Electronics Engineering and Department of Electrical Engineering, National Tsing-Hua University, Hsinchu 300, Taiwan, R.O.C H Hung is with the Institute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, R.O.C W.-C Lai is with the Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, R.O.C 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.2007.898870 1041-1135/$25.00 © 2007 IEEE LIN et al.: IMPROVING THE LUMINESCENCE OF InGaN–GaN BLUE LEDs Fig Top-view illustration of the InGaN–GaN MQW blue LED chip formed using the selective ring-region activation technique The descriptor d refers to the width of the highly resistive region of the current blocking area; it is better defined as the spacing from the edge of the selective activation area to the edge of the LED chip Fig Luminescence images of conventional fully activated as-grown LEDs and selective-ring-region activated LEDs operated at 20 mA oxygen atmosphere for The Ti and Al metals used for the p-bonding pad and n-electrode were also deposited using the electron-beam evaporator Fig provides a schematic illustration of the InGaN–GaN blue MQW LED chips prepared by using the selective ring-region activation technique III RESULTS AND DISCUSSION Fig displays luminescence images, recorded at 20 mA, of a conventional fully activated as-grown LED (Sample A) and of the selective region-activated LEDs (Samples B-H) possessing different highly resistive regions The value of in Samples B-H ranged from 10 to 70 m, respectively, with a step interval of 10 m We observed light extraction from the entire mesa of the LED chip in Sample A, whereas it was centered in the selectively activated areas adjacent to the highly resistive regions in Samples B-H Note that the TCL was present in all of the LED samples Fig presents the forward current–voltage ( – ) characteristics of the LEDs The forward voltages of Samples A-H operated at 20 mA were 3.24, 3.34, 3.37, 3.40, 3.42, 3.54, 3.60, and 3.73 V, respectively: i.e., the forward voltage operated at 20 mA increased slightly upon increasing the width from 10 to 70 m The slight increase in forward voltage can be attributed to the reduction in the total of the p-GaN neutral region and the nonactivation selective ring-region of the Mg-doped GaN layer as a result of the presence of a high-resistance region which is used as a current blocking region [4]–[6] The series resistances of the samples are determined according to the slopes of versus plots, not shown here It is indeed seen that the series 929 Fig Forward I –V characteristics of LEDs plotted as the function of the space d Fig Reverse I –V characteristics of LEDs plotted as a function of the space d resistances, i.e., , are proportional to the inverse of the aperture area The reverse – characteristics of the LEDs are also shown in Fig The reverse leakage currents of the LEDs at V A m), A m), were A m), A m), A m), A m), A m), and A m) In general, the diode leakage current includes the junction leakage current and the surface leakage current Under reverse bias, the injection of minority carriers and the thermal generation of electron hole pairs in the space charge region primarily determine the current through the junction Both the Shockley equation and space charge layer generation are proper to the junction crosssectional area So the surface leakage current of the LEDs are clearly retrained after forming a periphery high-resistance region by using the selective ring-region activation technique Further mechanistic investigations of the surface leakage current in these LEDs are presently in progress The maximum breakdown voltage occurred when the width was 70 m and the reverse breakdown voltage decreased upon reducing the spacing In order to clarify the increase of light emission by the selective ring-region activation technique, electroluminescence (EL) relative intensity-injection current characteristics are measured, as shown in Fig The greatest EL intensity of the LEDs ( m) increased by 50% over that of the as-grown blue LEDs at the injection currents ranging from to 100 mA Fig displays the relative EL intensities as a function of the space of Samples A-H operated at 20 mA We observed that the EL intensity increased upon increasing the width from 10 930 Fig EL relative intensities of the InGaN–GaN MQW blue LEDs presented as a function of the injection current when operated at room temperature IEEE PHOTONICS TECHNOLOGY LETTERS, VOL 19, NO 12, JUNE 15, 2007 The typical reliability test of the variation in relative brightness as a function of the width is not shown here For each value of , we tested five samples and normalized the brightness variation to its initial reading During the reliability test, the driving current was 20 mA; the samples were analyzed at room temperature Despite of the heating effect due to the diode junction area decreasing and the current density increasing upon proceeding from Samples A to H, the brightness decayed by only 15%–20% after 500 h in the burn-in test The results of these reliability tests followed the same trend as did the brightness decays of Samples A-H This result indicates that the selective ring-region technique can be used to control the surface state damage of LEDs; because it does not affect the results of the reliability tests, such LEDs are reliable for use in commercial applications IV CONCLUSION Fig EL relative intensities of the InGaN–GaN MQW blue LEDs presented as a function of the space d when operated at 20 mA to 40 m, but then it decreased upon increasing from 50 to 70 m We observed the increase in the relative EL intensity for m at 20 mA; it was 30% higher than that sample of the as-grown bar-chip LEDs (averaged at 3.67 mW, 465 nm; derived by integrate sphere) Because of the presence of the high-resistance region of the current blocking region, we observed a current blocking area under the p-electrode metal and a periphery high-resistance ring-region of the Mg-doped GaN layer [4], [5] Thus, the current was dominated injected into the central-region of the p-GaN through the TCL; then, the parasitic optical absorption can be reduced due to the reduction in light absorption at the thick p-pad electrode, which reduced the number of carriers trapping by the surface recombination centers But as the -space is larger than 50 m, the more series resistance causes a higher voltage drop as the current passes through Therefore, the effective p-n junction voltage decreases from the edge to the center Because the amount of carrier injection depends on the voltage at a given position, most of the injection occurs near the highly resistive edge (not the central-region of the p-GaN) This phenomenon, current crowding, will be discussed in detail elsewhere As discussed above, reducing the aperture sizes will increase not only resistance in the device but also the ohmic thermal effect The current crowding and the considerable heating effect leads to a deterioration in the LED’s external quantum efficiency As a result, it indicates that the samples prepared by using the selective ring-region technique can be optimized to improve the external quantum efficiency of the as-grown conventional LED [6] In this study, we demonstrated a method to increase the external quantum efficiency of InGaN–GaN blue LEDs through selective ring-region activation of the Mg-doped GaN layer As a result, the hole concentration of the Mg-doped GaN epitaxial cm to cm layer decreased from after capping with a 250-nm-thick layer of titanium and annealing at 700 C under a nitrogen atmosphere for 30 The experimental result reveals that an efficient current blocking layer was produced under the p-electrode metal and a periphery high-resistance ring-region of the Mg-doped GaN layer The EL intensity of the selective ring-region activated LEDs was found to be greatly increased, compared to that of a as-grown LED due to the increase in current injection into the active layer of the LED structure and a reduced number of carriers trapped by the surface recombination centers Furthermore, we found that the EL intensity could be improved by varying a periphery spacing width of the high-resistance ring-regions; the greatest EL intensity of the blue LEDs increased by 50% over that of the as-grown blue LEDs at the injection currents ranging from to 100 mA ACKNOWLEDGMENT The authors would like to thank Dr Y.-C Lin and Prof S.-J Chang for their technical assistance REFERENCES [1] C.-M Lee, C.-C Chuo, Y.-C Liu, I.-L Chen, and J.-I Chyi, “InGaN–GaN MQW LEDs with current blocking layer formed by selective activation,” IEEE Electron Device Lett., vol 25, no 6, pp 384–386, Jun 2004 [2] C.-C Liu, Y.-H Chen, M.-P Houng, Y.-H Wang, Y.-K Su, W.-B Chen, and S.-M Chen, “Improved light-output power of GaN LEDs by selective region activation,” IEEE Photon Technol Lett., vol 16, no 6, pp 1444–1446, Jun 2004 [3] I Waki, H Fujioka, M Oshima, H Miki, and A Fukizawa, “Lowtemperature activation of Mg-doped GaN using Ni films,” Appl Phys Lett., vol 78, no 19, pp 2899–2901, 2001 [4] H W Jang and J.-L Leea, “Enhancement of electroluminescence in GaN-based light-emitting diodes using an efficient current blocking layer,” J Vac Sci Technol B, vol 23, no 6, pp 2284–2287, 2005 [5] Y.-B Lee, R Takaki, H Sato, Y Naoi, and S Sakai, “High efficiency GaN-based LEDs using plasma selective treatment of p-GaN surface,” Phys Stat Sol.(a), vol 200, no 1, pp 87–90, 2003 [6] C Huh, J.-M Lee, D.-J Kim, and S.-J Parkc, “Improvement in lightoutput efficiency of InGaNÕGaN multiple-quantum well light-emitting diodes by current blocking layer,” J Appl Phys., vol 92, no 5, pp 2248–2250, 2002 ... to the width of the highly resistive region of the current blocking area; it is better defined as the spacing from the edge of the selective activation area to the edge of the LED chip Fig Luminescence. ..LIN et al.: IMPROVING THE LUMINESCENCE OF InGaN? ??GaN BLUE LEDs Fig Top-view illustration of the InGaN? ??GaN MQW blue LED chip formed using the selective ring-region activation technique The descriptor... increasing the width from 10 to 70 m The slight increase in forward voltage can be attributed to the reduction in the total of the p-GaN neutral region and the nonactivation selective ring-region of the