High reflectivity and thermal-stability Cr-based Reflectors and n-type Ohmic Contact for GaN-based flip-chip light-emitting diodes Kuang-Po Hsueh, a * Kuo-Chun Chiang, a Charles J. Wang, b and Yue-Ming Hsin a a Department of Electrical Engineering, National Central University, Jhongli 32054, Taiwan, R.O.C. b LEDARTS OPTO Corporation, Taiwan, R.O.C. ABSTRACT We have investigated the thermal stability of three composite metals on their contact resistivities and luminous intensities for using as the reflector in flip-chip light-emitting diode (FCLED). The composite metals were simultaneously deposited on n-type GaN without alloy to form n-type Ohmic contact and simplify the process. The investigated composite metals were Ti/Al/Ti/Au (30/500/30/300 nm), Cr/Al/Cr/Au (30/500/30/300 nm) and Cr/Ti/Au (500/30/300 nm), respectively. The specific contact resistivity of Ti/Al/Ti/Au, Cr/Al/Cr/Au and Cr/Ti/Au on the n-type GaN Ohmic contact were changed from 5.4 10-4, 6.6 10-4 and 7.7 10-4 Ω-cm 2 to 5.3 10-4, 4.5 10-4 and 1.3 10-4 Ω-cm 2 respectively after 500 hours thermal stress at 150 C in the air. After 96 hours of thermal stress, the luminous intensities at 20 ma of these three structures were decreased 6.2%, 11.1% and 1.4%, respectively. Therefore, in addition to maintain good n-type ohmic contact and simplify the process, the Cr/Ti/Au composite metal demonstrates good thermal stability as a reflector in FCLED. Keywords : flip-chip light-emitting diode, GaN, FCLED, Cr, Al. *Electronic mail: s0541008@cc.ncu.edu.tw ; Fax : +886-3-4255830 Gallium Nitride Materials and Devices, edited by Cole W. Litton James G. Grote, Hadis Morkoc, Anupam Madhukar, Proc. of SPIE Vol. 6121, 61210V, (2006), 0277-786X/06/$15 doi: 10.1117/12.646154 Proc. of SPIE Vol. 6121 61210V-1
1. INTRODUCTION Group III-Nitride semiconductors are of great technological importance for the fabrication of optoelectronic devices, such as blue and ultraviolet light-emitting diodes (LEDs), laser diodes, and photodetectors. 1-3 One of the most important applications of GaN-based LEDs is solid-state lighting, which could replace incandescent bulbs and ultimately fluorescent lamps. Realization of high brightness and high power GaN-based LEDs are important to solid-state lighting applications, which require high extraction efficiency in LED structures. The flip-chip LEDs (FCLEDs) have been demonstrated the enhanced light extraction and reduced thermal dissipation. 4-8 However, two issues are important for FCLED to achieve high performances. One is the highly reflective p-type ohmic contact, which serves as the p-electrode and the mirror to reflect the light emitting from the active region. The other one is good thermal stability during packaging and operation. It is well known that the conventional p-gan contact is a Ni/Au bilayer composed of several hundred angstroms of each material annealed at around 500 C under nitrogen. This contact exhibits good electrical characteristics but is a poor reflector at visible wavelengths. In this paper, oxidized Ni/Au bilayer contacts were used to obtain high-quality p-gan Ohmic contacts, but added the composite metals for using as the reflector and bonding layer in FCLED. In addition, these composite metals were simultaneously deposited on n-type GaN without alloy to form Ohmic contact and simplify the process. The investigated composite metals were Ti/Al/Ti/Au, Cr/Al/Cr/Au and Cr/Ti/Au, respectively. The thermal stability of reflectivity and contact resistivity at 150 C in the air was studied. 2. DEVICE FABRICATION The InGaN/GaN MQW LED wafers were grown by metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates using a GaN buffer layer followed by a thickness of 3 µm, 5 10 18 cm -3 Si-doped, n-type GaN current spreading layer. The MQW active region consists of 5 periods of InGaN wells and Si doped GaN barriers. The structure was completed with a thickness of 50 nm and 2 10 17 cm -3 Mg-doped AlGaN layer, followed by a 50 nm 5 10 17 cm -3 Mg-doped GaN contact layer. The fabricated LEDs were designed to operate at 460 to 470 nm. The schematic device structure with flip-chip submount is shown in Fig. 1. Devices were fabricated using standard lithographic techniques to define features. The Mg acceptors in the p-gan and p-algan were activated by the furnace at 750 C for 30 minutes in nitrogen ambient. High density plasma etching was used to expose the n-type GaN current spreading layer. Ohmic p-type contacts were first formed by evaporating a Ni/Au metal stack which was annealed by the furnace at 500 ºC for 5 minutes under oxygen ambient. The size of the p-type contact is 200 285 µm 2. Following the p-type contacts, three different types of composite metals (including diffusion barrier, metal reflector and bonding pad layers) were formed on Proc. of SPIE Vol. 6121 61210V-2
top of the Ni/Au p-electrode. Three types of composite metals in this study, as shown in Fig. 1, were Ti/Al/Ti/Au (30/500/30/300 nm), Cr/Al/Cr/Au (30/500/30/300 nm) and Cr/Ti/Au (500/30/300 nm), respectively. In addition, these composite metals were simultaneously deposited on n-type GaN without alloy to form Ohmic contact and simplify the process. The deposition of composite metals was carried out by electron beam evaporation at a chamber pressure of 3 10-6 torrs. After metallization, devices were characterized for thermal stability under thermal stress at 150 C in the air. Current-voltage (I-V) measurements performed by the parameter analyzer (HP4156) were used to evaluate the contact resistances. The luminous intensity was measured by Instrument System Spectrometer (CAS140B) at wavelength of 460 nm to compare the relative reflectivity after thermal stress. The surface morphology and roughness of the contact metal were studied using an atomic force microscope (AFM). Sapphire GaN buffer M 1 M 2 Type I T i A l Type II C r A l Type III C r solder n-gan MQW p-algan p-gan submount M 3 T i C r T i M 4 A u A u A u Ni/Au p-electrode M1 : Diffusion Barrier Layer M2 : Metal Reflector Layer M3 : Bonding Pad M4 : Bonding Pad Fig. 1 Schematic cross section of the FCLED with the different composite metals in this study. 3. RESULTS Alumina is usually considered to achieve a good metal reflector in FCLED. For the type I of composite Ti/Al/Ti/Au, Al is good reflective material and could be formed Ti-Al alloy with the adjacent metal of Ti. And the Ti-Al alloy can prevent the Al diffusion. 9 The outside metal of Au could avoid the oxidization of Al. Due to Proc. of SPIE Vol. 6121 61210V-3
the similar characteristic on the n-type Ohmic contact, Cr was used to replace Ti to form type II of composite Cr/Al/Cr/Au for this study. 10 Another reason is the better thermal stability in Cr than Al at higher temperature operation. 11 Type III of composite Cr/Ti/Au was included to study the composite metal without Al. Figure 2 shows the specific contact resistances (ρ c ) of Ti/Al/Ti/Au, Cr/Al/Cr/Au and Cr/Ti/Au on n-type GaN contacts versus stress time at 150 C in the air obtained from TLM measurement. All the Ti/Al/Ti/Au, Cr/Al/Cr/Au and Cr/Ti/Au contacts show linear I-V curves with and without thermal stress. The measured data shows that ρ c decrease from 5.4 10-4, 6.6 10-4 and 7.7 10-4 Ω-cm 2 to 5.3 10-4, 4.5 10-4 and 1.3 10-4 Ω-cm 2, respectively, after 500 hours thermal stress. The specific contact resistivity of Cr/Ti/Au demonstrates the most reduction of 83.1% after 500 hours thermal stress while other two composite metals do not decrease significantly. This is due to the Al metal tends to oxidize during the thermal stress. ρ c (x10-4 Ω-cm 2 ) 9 8 7 6 5 4 3 2 1 0-100 0 100 200 300 400 500 600 Stress Time (hr) Ti/Al/Ti/Au Cr/Al/Cr/Au Cr/Ti/Au Fig. 2 The specific contact resistivities versus thermal stress time for three different composite metals on n-type GaN. (Metal contact area is 50µm 100µm.) Proc. of SPIE Vol. 6121 61210V-4
Table I The changes of the specific contact resistivities after thermal stress time for three different composite metals on n-type GaN. Metal Specific Contact Resistivity (W-cm 2 ) As-deposited 150 C / 500hours % Ti/Al/Ti/Au 5.40 10-4 5.32 10-4 -1.5 Cr/Al/Cr/Au 6.58 10-4 4.49 10-4 -31.8 Cr/Ti/Au 7.70 10-4 1.3 10-4 -83.1 Current (ma) 50 40 30 20 10 0-10 Current (ma) 40 30 Ti/Al/Ti/Au Cr/Al/Cr/Au Cr/Ti/Au 20 3.1 3. 2 3.3 3.4 3.5 Voltage (V) 2.6 2.8 3.0 3.2 3.4 Voltage (V) Ti/Al/Ti/Au Cr/Al/Cr/Au Cr/Ti/Au Fig. 3 Typical forward I-V characteristics of LEDs fabricated with three composite metals after 500 hours at 150 C in air. For comparison, the I-V behaviors without thermal stress () are also shown. Figure 3 shows the corresponding forward I-V characteristics of LEDs measured up to 3.5V. It shows the Proc. of SPIE Vol. 6121 61210V-5
increased series resistance of 2.8 and 0.8 Ω in both Ti/Al/Ti/Au and Cr/Al/Cr/Au metals after 500 hours thermal stress. Since the contact resistance on the n-type GaN is decreased, the degradation would result from the p-type Ohmic contact with Al oxidation. In addition, the root-mean-square surface roughness of Ti/Al/Ti/Au, Cr/Al/Cr/Au and Cr/Ti/Au on p-electrode shows the increase from 3.65, 7.17 and 3.67 nm to 4.23, 17.24 and 3.76 nm respectively after 150 C thermal stress. The least change in morphology in Cr/Ti/Au composite metal is due to the elimination of Al. The thermal stability of relative luminous intensity of FCLEDs using Ti/Al/Ti/Au, Cr/Al/Cr/Au and Cr/Ti/Au composite metal is showed in Fig. 4. The measured luminous intensities at 20 ma in these three structures (Ti/Al/Ti/Au, Cr/Al/Cr/Au and Cr/Ti/Au) without thermal stress are 27.6, 27.9 and 35.4 mcd respectively. The improvement of using metal reflector is significant while compared with the normal LED without flip-chip process (16.6 mcd). In addition, FCLED with Cr/Ti/Au demonstrates the highest luminous intensity than those with Al composite reflector; it is possible due to the wafer uniformity (7 mcd variation) and the existence of diffusion barrier layer. After 96 hours of thermal stress, the luminous intensities were decreased 6.2%, 11.1% and 1.4% for diodes with Ti/Al/Ti/Au, Cr/Al/Cr/Au and Cr/Ti/Au, respectively. The Cr/Ti/Au composite metal demonstrated the least reduction in luminous intensity and thus excellent thermal stability. Normalized Luminous lntensity @ 20 ma (%) 110 100 90 80 0 24 48 72 96 hours Ti/Al/Ti/Au Cr/Al/Cr/Au Cr/Ti/Au Fig. 4 Normalized luminous intensity versus thermal stress time for three FCLEDs operated at current of 20 ma. Proc. of SPIE Vol. 6121 61210V-6
Table II The changes of Normalized luminous intensity after thermal stress time for three different composite metals. Metal Normalized luminous intensity (%) As-deposited 150 C / 96hours % Ti/Al/Ti/Au 100 93.8-6.20 Cr/Al/Cr/Au 100 88.9-11.10 Cr/Ti/Au 100 98.6-1.40 4. CONCLUSION To summarize, three composite metals were investigated to form n-type Ohmic contacts and reflectors for FCLEDs. The specific contact resistivity of Ti/Al/Ti/Au, Cr/Al/Cr/Au and Cr/Ti/Au on the n-type GaN Ohmic contact were decreased from 5.4 10-4, 6.6 10-4 and 7.7 10-4 Ω-cm 2 to 5.3 10-4, 4.5 10-4 and 1.3 10-4 Ω-cm 2 respectively after 500 hours thermal stress at 150 C in the air. The luminous intensity at 20 ma of the FCLEDs in these three structures (Ti/Al/Ti/Au, Cr/Al/Cr/Au and Cr/Ti/Au) were significantly improved compared with the normal LED without flip-chip process. After 96 hours of thermal stress, the luminous intensities at 20 ma of these three structures were decreased 6.2%, 11.1% and 1.4%, respectively. Therefore, FCLEDs fabricated with the Cr/Ti/Au reflector demonstrated good n-type Ohmic contact and thermal stability in reflectivity. By using this process, the n-type Ohmic contact, the reflector and the bonding pads can be formed at the same time. It is not only to simplify the process and thus manufacture cost, but also maintain good thermal stability on Ohmic contact and luminous intensity. Acknowledgments The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract No. NSC 94-2215-E-008-005. Proc. of SPIE Vol. 6121 61210V-7
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