InGaN/GaN LEDs with a Si-Doped InGaN/GaN Short-Period Superlattice Tunneling Contact Layer L.W. WU, 1 S.J. CHANG, 1,5 Y.K. SU, 1 T.Y. TSAI, 1 T.C. WEN, 1 C.H. KUO, 1 W.C. LAI, 1 J.K. SHEU, 2 J.M. TSAI, 3 S.C. CHEN, 4 and B.R. HUANG 4 1.—Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan 70101, Taiwan. 2.—Optical Science Center, National Central University, Chung-Li 320, Taiwan. 3.—South Epitaxy Corporation, Hsin-Shi, Tainan County 744, Taiwan. 4.— Institute of Electronics and Information Engineering and Department of Electronic Engineering, National Yunlin University of Science and Technology, Touliu 640, Taiwan. 5.—E-mail: changsj @mail.ncku.edu.tw Nitride-based light-emitting diodes (LEDs) with Si-doped n + -In 0.23 Ga 0.77 N/GaN short-period superlattice (SPS) tunneling contact top layer were fabricated. It was found that although the measured specific-contact resistance is around 1  10 -2 -cm 2 for samples with an SPS tunneling contact layer, the measured specific-contact resistance is around 1.5  10 0 -cm 2 for samples without an SPS tunneling contact layer. Furthermore, it was found that one could lower the LED-operation voltage from 3.75 V to 3.4 V by introducing the SPS struc- ture. It was also found that the LED-operation voltage is almost independent of the CP 2 Mg flow rate when we grow the underneath p-type GaN layer. The LED- output intensity was also found to be larger for samples with the SPS structure. Key words: InGaN/GaN, short-period superlattice (SPS), multiple-quantum well (MQW), light-emitting diode (LED) Journal of ELECTRONIC MATERIALS, Vol. 32, No. 5, 2003 Special Issue Paper 411 INTRODUCTION Poor ohmic contact at the metal/p-GaN interface is the major problem that led to light-emitting de- vices (LEDs) with limited performance. To achieve high-performance nitride-based LEDs, it is required to reduce contact resistance. Conventional nitride- based LEDs use semitransparent Ni/Au on Mg- doped GaN as the p-contact material. 1–4 However, the operation voltage of such LEDs is still high be- cause of the low Mg-ionization percentage. The low Mg-ionization percentage will result in a highly re- sistive top p-GaN layer and a large metal/p-GaN contact resistance in nitride-based LEDs. Thus, the doping concentration in the top p-GaN contact layer could be important in conventional nitride-based LEDs. Recent investigations have indicated that Mg-doped AlGaN/GaN strained-layer superlattice (SLS) can be used to increase the Mg-ionization per- centage. 5–8 Nitride-based LEDs with a low operation voltage were also demonstrated by using such an Al x Ga 1–x N/GaN SLS-contact layer. 9–11 Very recently, we have demonstrated low operation-voltage, nitride- based LEDs with an n + -InGaN/GaN short-period superlattice (SPS) tunneling contact layer. 12 By grow- ing such an SPS structure on top of the p-GaN cap layer, one could achieve a good “ohmic” contact through tunneling when the n + (InGaN/GaN)-p(GaN) junction was properly reverse biased. However, the tunneling efficiency of such an n + (InGaN/GaN)- p(GaN) junction could depend on the doping concen- tration on both sides of the junction. In this paper, nitride-based LED structures with and without an SPS tunneling contact layer were prepared. The opti- cal and electrical properties of LEDs with and with- out an SPS tunneling contact layer were studied. The influences of p-doping concentration on the LED per- formance will also be reported. EXPERIMENT Samples used in this study were all grown on (0001) sapphire (Al 2 O 3 ) substrates by a metal- organic chemical-vapor deposition (MOCVD) system using a high-speed rotating disk in a vertical- growth chamber. 13–27 Briefly, trimethylaluminum, trimethylgallium, trimethylindium, and ammonia were used as aluminum, gallium, indium, and nitro- gen sources, respectively. Biscyclopentadienyl mag- nesium (CP 2 Mg) and disilane (Si 2 H 6 ) were used as the p-type and n-type doping sources, respectively. Prior to growth, sapphire substrates were thermally baked at 1,100°C in hydrogen gas to remove sur- face contamination. A low-temperature, 30-nm-thick, (Received October 20, 2002; accepted December 17, 2002)