744 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 4, JULY/AUGUST 2002 400-nm InGaN–GaN and InGaN–AlGaN Multiquantum Well Light-Emitting Diodes S. J. Chang, C. H. Kuo, Y. K. Su, Senior Member, IEEE, L. W. Wu, J. K. Sheu, T. C. Wen, W. C. Lai, J. F. Chen, Member, IEEE, and J. M. Tsai Abstract—The 400-nm In Ga N–GaN MQW light-emit- ting diode (LED) structure and In Ga N–Al Ga N LED structure were both prepared by organometallic vapor phase epi- taxy. It was found that the use of Al Ga N as the material for barrier layers would not degrade crystal quality of the epitaxial layers. It was also found that the 20-mA electroluminescence in- tensity of InGaN–AlGaN multiquantum well (MQW) LED was two times larger than that of the InGaN–GaN MQW LED. The larger maximum output intensity and the fact that maximum output in- tensity occurred at larger injection current suggest that AlGaN barrier layers can provide a better carrier confinement and effec- tively reduce leakage current. Index Terms—AlGaN, light-emitting diode, multiquantum well, organometallic vapor phase epitaxy. I. INTRODUCTION R ECENTLY, tremendous progress has been achieved in GaN-based blue and green light-emitting diodes (LEDs) [1], [2]. These blue/green LEDs have already been extensively used in full-color displays and high-efficient light sources for traffic light lamps. Although these blue/green LEDs are already commercially available, it is still difficult to achieve LEDs emitting at even shorter wavelength regions, such as ultraviolet (UV) region [3]–[8]. Short wavelength emitters are of interest for various fluorescence-based chemical sensing applications, high efficiency lighting, flame detection, and possibly optical storage applications. Conventional nitride-based multiquantum well (MQW) LEDs use InGaN as the material for well layers and GaN as the material for barrier layers. To achieve a short wavelength emitter, one needs to reduce the indium compo- sition in the well layers so as to increase its bandgap energy. However, a reduction in indium composition in the well layers will result in a small bandgap discontinuity at the well/barrier interfaces. Thus, the quantum well depth in the MQW active region will become smaller and the carrier confinement effect will be reduced. As a result, severe carrier leakage problem might occur in the short wavelength InGaN–GaN MQW LEDs. One possible way to solve this problem is to use Manuscript received March 29, 2002; revised May 9, 2002. This work was supported by the National Science Council under Grant NSC 90-2215-E-008-043 and Grant NSC 90-2112-M-008-046. S. J. Chang, C. H. Kuo, Y. K. Su, L. W. Wu, T. C. Wen, W. C. Lai, and J. F. Chen are with the Institute of Microelectronics & Department of Electrical Engineering, National Cheng KungUniversity, Tainan 701, Taiwan, R.O.C. J. K. Sheu is with the Optical Science Center, National Central University, Chung-Li 320, Taiwan, R.O.C. J. M. Tsai is with the South Epitaxy Corporation, Hsin-shi 744, Tainan, Taiwan, R.O.C. Digital Object Identifier 10.1109/JSTQE.2002.801677. AlGaN or AlGaInN as the barrier layers instead of GaN [9]. The quaternary AlGaInN permits an extra degree of freedom by allowing independent control of the bandgap and lattice constant. Thus, the use of quaternary AlGaInN for barrier layers could potentially offer better carrier confinement while minimizing lattice mismatch issues. However, it is much more difficult to grow high-quality AlGaInN than AlGaN. Since the bandgap energy of AlGaN is also larger than that of GaN, InGaN–AlGaN MQW should still be able to provide a better carrier confinement, as compared to InGaN–GaN MQW. Also, since the lattice constant of AlGaN is smaller while the lattice constant of InGaN is larger than that of GaN base layer, it is possible to achieve a strain compensated InGaN–AlGaN MQW on GaN with proper composition ratios in InGaN and AlGaN layers. As a result, we could increase the effective MQW critical thickness, and thus reduce the probability of relaxation occurred within the MQW active region. In this study, InGaN–GaN LED and InGaN–AlGaN LED will both be fabricated. The optical and electrical properties of these LEDs will be reported. II. EXPERIMENTS The InGaN–GaN MQW LED and InGaN–AlGaN MQW LED structures used in this study were both grown on -face 2-in sapphire (Al O ) (0001) substrates in a vertical low-pres- sure organometallic vapor phase epitaxy (OMVPE) reactor with a high-speed rotation disk [9]–[17]. Trimethylindium (TMIn), trimethylgallium (TMGa), trimethylaluminum (TMAl), and ammonia (NH ) were used as the source materials of In, Ga, Al, and N, respectively. Bicyclopentadienyl magnesium (Cp Mg) and silane (SiH ) were used as the -type and -type doping sources, respectively. Fig. 1(a) schematically depicts the InGaN–GaN MQW LED structure used in this study. The structure consists of a 30-nm-thick GaN nucleation layer grown at a low temperature of 560 C, a 4- m-thick Si-doped -GaN cladding layer, an InGaN–GaN MQW active region, a 50-nm-thick Mg-doped -Al Ga N cladding layer, and a 0.25- m-thick Mg-doped -contact layer. The InGaN–GaN MQW active region consists of five pairs of 3-nm-thick In Ga N well layers and 12-nm-thick GaN barrier layers. The sample structure of InGaN–AlGaN MQW LED was almost identical to that of the InGaN–GaN MQW LED. The only difference is that we used Al Ga N, instead of GaN, as the barrier layers in the active region. In other words, the active region of InGaN–AlGaN LED consists of five pairs of 3-nm-thick In Ga N well layers and 12-nm-thick 1077-260X/02$17.00 © 2002 IEEE