1766 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 50, NO. 8, AUGUST 2003 Nitride-Based Green Light-Emitting Diodes With High Temperature GaN Barrier Layers L. W. Wu, S. J. Chang, Y. K. Su, Senior Member, IEEE, R. W. Chuang, T. C. Wen, C. H. Kuo, W. C. Lai, C. S. Chang, J. M. Tsai, and J. K. Sheu Abstract—High-quality InGaN–GaN multiquantum well (MQW) light-emitting diode (LED) structures were prepared by temperature ramping method during metalorganic chemical vapor deposition (MOCVD) growth. It was found that we could re- duce the 20-mA forward voltage and increase the output intensity of the nitride-based green LEDs by increasing the growth temper- ature of GaN barrier layers from 700 C to 950 C. The 20-mA output power and maximum output power of the nitride-based green LEDs with high temperature GaN barrier layers was found to be 2.2 and 8.9 mW, respectively, which were more than 65% larger than those observed from conventional InGaN–GaN green LEDs. Such an observation could be attributed to the improved crystal quality of GaN barrier layers. The reliability of these LEDs was also found to be reasonably good. Index Terms—Green light-emitting diode (LED), InGaN–GaN, multiple quantum well (MQW), reliability. I. INTRODUCTION W ITH a wide bandgap energy varying from 0.7 eV for InN to 6.3 eV for AlN, group III-nitrides are highly promising for the light-emitting diode (LED) applications in the wavelength range from green to ultraviolet [1], [2]. In fact, nitride-based blue/green LEDs grown by metalorganic chemical vapor deposition (MOCVD) are already commer- cially available. These nitride-based LEDs normally use an InGaN–GaN multiple quantum well (MQW) structure as the active light-emitting region. Although these nitride-based LEDs have been very successfully over the past decade, the progress of these LEDs is often limited by the fundamental problems of InGaN. For example, the optimal growth temperature of InGaN well layers is normally much lower than the optimal growth temperature of GaN barrier layers due to the low miscibility of InN in GaN [3]. It has been shown that the crystal quality of GaN layers grown at low temperatures is poor. On the other hand, the decomposition rate of ammonia is low at low temperature. As a result, low temperature grown InGaN and GaN layers tend to lack nitrogen atoms. It is also known that a higher equilibrium vapor pressure of nitrogen is required to prevent the dissociation of In-N bonds. The formation of in- Manuscript received January 17, 2003; revised June 2, 2003. The review of this paper was arranged by Editor P. Bhattacharya. L. W. Wu, S. J. Chang, Y. K. Su, R. W. Chuang, T. C. Wen, C. H. Kuo, W. C. Lai, and C. S. Chang are with the Institute of Microelectronics and De- partment of Electrical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, R.O.C. J. M. Tsai is with the South Epitaxy Corporation, Hsin-Shi 744, Taiwan, R.O.C. J. K. Sheu is with the Optical Science Center, National Central University, Chung-Li 320, Taiwan, R.O.C. Digital Object Identifier 10.1109/TED.2003.815150 dium droplets at temperatures below 800 C is another problem during the MOCVD growth of InGaN layers [4]. Keller et al. have previously reported that an extremely high V–III ratio is required to prevent the formation of indium droplets [5]. The conventional method to grow barrier and well layers of MQW in III–V compound semiconductors is by controlling the alkyl source flow into reactor during MOCVD growth. This is because the optimized growth conditions of barrier and well layers, which include growth temperature and growth pressure, are very similar. However, the growth of MQWs in III-nitrides is much more difficult since the optimal growth temperatures of InGaN well layers and GaN barrier layers are very different. In other words, the optimal growth temperature of GaN barrier layers (i.e., above 1000 C) is much higher than that of InGaN well layers. However, most people still select to grow the whole MQW region at a constant low temperature since it is difficult to change the growth temperature rapidly and accurately. As a result, we could only achieve low-tem- perature-grown InGaN–GaN MQW structure with poor crystal quality in our MOCVD system. Thus, optical properties of the low-temperature-grown InGaN–GaN MQW structure are also poor. Such poor optical properties of MQW structure could result in nitride-based LEDs with limited performance. This problem is more severe when we fabricate nitride-based green LEDs since we need to introduce more indium into the InGaN well layers in the MQW structure. Thus, if we could grow InGaN well layers and GaN barrier layers at different temperatures, we should be able to significantly improve the performance of nitride-based green LEDs. In this paper, we present the growth of InGaN/GaN MQW structures prepared by temperature ramping method. In other words, we grew InGaN well layers at a low temperature and GaN barrier layers at a high temperature. High brightness nitride-based green LEDs were also fabricated by such temperature ramping method. The physical and optoelectrical properties of the fabricated LEDs will also be discussed. II. EXPERIMENT Samples used in this study were all grown on (0001) sapphire substrates by an EMCORE D180 MOCVD system [6]–[14]. During the growth, chamber pressure was controlled at 100 Torr. Trimethylaluminum (TMAl), trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia were used as aluminum, gallium, indium, and nitrogen sources, respectively. Biscyclopentadienyl magnesium and disilane were used as the p- and n-type doping sources, respectively. Prior to the growth, sapphire substrates were first 0018-9383/03$17.00 © 2003 IEEE Authorized licensed use limited to: National Cheng Kung University. Downloaded on October 23, 2008 at 00:13 from IEEE Xplore. Restrictions apply.