International Journal of High Speed Electronics and Systems Vol. 20, No. 3 (2011) 521 525 World Scientific Publishing Company DOI: 10.1142/S0129156411006817 INTEGRATION OF N- AND P-CONTACTS TO GaN-BASED LIGHT EMITTING DIODES WENTING HOU, THEERADETCH DETCHPROHM and CHRISTIAN WETZEL Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 8th Street Troy, New York 12180, United States houw2@rpi.edu Low-resistance Ohmic contacts are essential for the fabrication of electrical devices. While low contact resistance has been achieved to p-type layers or n-type layers separately, contacts are likely to degrade when both types need to be integrated into a single fabrication process, in particular when prior mesa etching is required. We present a solution to the problem, resulting in low-resistance Ohmic contacts on n-type GaN layers without post-deposition thermal anneal, while maintaining the quality of typical p-type contacts. We implement an integrated process for both, n- and p-contacts, involving an oxygen pretreatment to fabricate light emitting diodes with lower series resistance in the contacts and lower voltage drop at high current when compared to separately optimized contacts. Keywords: GaN; light emitting diode; Ohmic contact; rapid thermal anneal. 1. Introduction Low-resistance Ohmic contacts are essential for the fabrication of efficient electrical devices, such as light emitting diodes (LEDs). Low contact resistance has readily been achieved to p-type layers or n-type layers separately 1-3, yet we find problems when both types of contacts have to be integrated in the same device. A commonly used Ohmic contact to n-type GaN is a layer sequence of Ti/Al/Ti/Au, followed by rapid thermal annealing (RTA) in nitrogen ambient at a high temperature of 750 C to 900 C. 1 Specific contact resistances as low as 8 10-6 Ω cm 2 in bilayer Ti/Al contacts have been achieved when a post-deposition high temperature thermal annealing step at 900 C for 30 seconds is being applied. 1 However, in our experimental findings, such a step will degrade the quality of any p-contact that has been formed prior to the n-contact. In particular, we find that the p-contact resistance can increase by up to two orders of magnitude. The formation of Ohmic contacts to p-type GaN typically requires RTA in an oxygen ambient at a somewhat lower temperature, from 400 C to 600 C. 2,3 In turn, we find that this thermal anneal in an oxygen ambient degrades the quality of the n-contact if it is formed prior to the p-contact. Here we present a solution to this problem, which results in lowresistance Ohmic contacts on n-type GaN layers without post-deposition thermal anneal and without conflict to a p-contact anneal. 521
522 W. Hou, T. Detchprohm & C. Wetzel 2. Contact on n-type GaN The GaN-basephase epitaxy (MOVPE) on c-plane sapphire substrate. The n-type GaN is Si doped to samples used in this study are grown by metalorganic vapor 3 10 18 cm -3. Fig. 1. Illustration of the process of the mesa etched n-gan samples. 2.1. Experimental procedures Four approaches are applied plied and evaluated on samples from the same epi run (Fig. 1). Samples are etched through the LED structure and into the n-gan layer by inductive coupled plasma / reactive ion etching (ICP/RIE) in all four approaches. The n-metal layers Ti/Al/Ti/Au with thickness of 20 nm/100 nm/45 nm/55 nm respectively, are evaporated on the etched n-gan by e-beam evaporation in approaches 1, 2, and 3. In approach 4, samples are annealed in oxygen ambient at 550 C for 1 minute in RTA equipment before the metal evaporation. In approaches 2 and 3, samples are annealed in nitrogen ambient at 750 C for 1 minute after metal deposition. In approach 3, the sample is then annealed in oxygen ambient at 550 C for 1 minute, as required for Ohmic p-contact formation. 2.2. Results and discussion Standard current-voltagee (I-V) measurements at room temperature are performed on multiple samples of the etched n-gan wafers as prepared by the four approaches. Typical data is shown in Fig. 2. The size of the metal contacts is 140 µm 100 µm, with a space of 5 µm between the contact pads. Only the as-deposit n-metal on etched n-gan exhibits a nonlinear I-V behavior (approach 1: open circles), resulting in a high resistance. After thermal anneal in N 2 at 750 C for 1 minute, the I-V behavior becomes linear, indicating an Ohmic n-contact (approach 2: open triangles). A specific contact resistance of 6.94 10-5 Ω/cm 2 is achieved after the nitrogen anneal. However, the I-V behavior deteriorates after the annealed n-contact is later annealed in oxygen (approach 3: stars),
Integration of N- and P-Contacts to GaN-based Light Emitting Diodes 523 100 Current (ma) 50 0-50 -100 approach 1 approach 2 approach 3 approach 4 5 µm between two TLM pads -2.0-1.5-1.0-0.5 0.0 0.5 1.0 1.5 2.0 Voltage (V) Fig. 2. Current-voltage characteristics of the different contacts on etched n-gan. which is a requirement for the Ohmic p-contact. 2,3 This indicates that the annealed n-contact degrades during the p-metal anneal in oxygen ambient. The specific contact resistance for the n-contact degrades to 8.52 10-5 Ω/cm 2, which is more than 20% higher than the contact without the p-metal anneal. Generally, we cannot revert the sequence to do p-contacts first, since the high temperature anneal required for the subsequent n-contact is found to be destructive to prior prepared p-contact. The I-V behavior of sample of approach 4 with the pre-deposition surface treatment is linear, even without any high temperature anneal in nitrogen after metal deposition (approach 4: open squares). The quality of the n-contact with our novel pre-deposition treatment is similar or even better than the quality of the n-contact after nitrogen anneal in high temperature. The specific contact resistance is 3.78 10-5 Ω/cm 2. The I-V characterization results show that the RTA in oxygen assists the formation of an Ohmic contact to n-type GaN. A possible explanation can be given by the following considerations. Oxygen is known to act as a donor in GaN and related compounds 4. It is therefore possible that the annealing in oxygen induces an n-type doping at the surface which should reduce the thickness of the surface depletion layer. This would allow for an easier tunneling of electrons through the barrier promoting an Ohmic contact behavior. A surface sensitive materials analysis by x-ray photoelectron spectroscopy is underway and may reveal more details of this process. 3. LED Fabrication Process Standard LED structures 6 are grown on top of n-type GaN by MOVPE. 3.1. Experimental procedures LEDs are fabricated using two different processes (Fig. 3). Mesa etching is performed in both processes by ICP/RIE, followed by a piranha clean. In the standard process, n-metal
524 W. Hou, T. Detchprohm & C. Wetzel Fig. 3. Illustration of the LED fabrication process. evaporation is performed, followed by the n-metal annealing in nitrogen ambient at 750 C for 1 minute. The p-contact is evaporated afterwards. The LED sample is then annealed in oxygen at 550 C for 1 minute to form the Ohmic p-contact. Unfortunately, this annealing step is found to degrade the previously formed n-contact, as approach 3 shows in the previous paragraph. In our novel process, the p-metal evaporation is performed first, followed by annealing in oxygen ambient in RTA. This annealing step for the p-contact also serves as the surface treatment on n-gan before n-metal deposition, as sample 4 discussed in the previous paragraph. The n-contact is evaporated afterwards, without post-deposition anneal. 3.2. Results and discussion More than ten LED dies fabricated with those different processes as described in section 3.1 are evaluated. The average of the I-V curves is shown in Figure 4, in both linear scale and logarithmic scale. LEDs fabricated with the new process show a lower series Fig. 4. Current-voltage characteristics of LEDs fabricated with different processes: (a) linear scale; (b) log scale.
Integration of N- and P-Contacts to GaN-based Light Emitting Diodes 525 resistance and lower voltage drop at currents higher than 50 ma compared to the LEDs fabricated with the standard process. The voltage at 100 ma is about 0.7 V lower for the LEDs fabricated with the new process. At -5 V the reverse current has been reduced by about 3 orders of magnitude. This indicates significant improvements in the performance of the n-type contacts. Apparently, the RTA annealing in oxygen as generally used for p-metal annealing also assists in the formation of the Ohmic n-contact without the need for a post-deposition anneal. Therefore, in absence of any post-deposition annealing for n-contacts, degradation of any of the previously formed contact layers is no longer a problem. 4. Conclusion In this work, we presented a novel approach to form Ohmic contacts on n-type GaN without post-deposition anneal and without conflict to the p-contact anneal. A new surface treatment consisting of RTA in an oxygen ambient is found to create a highlydoped n-gan surface at the metal-semiconductor interface, that is found to assist the formation of Ohmic contacts to n-gan. The performance of the n-contact utilizing this surface treatment without post-deposition anneal is found to be similar, or even better than the n-contact annealed in RTA at high temperature and without surface treatment. The absence of this post deposition annealing avoids any possible degradation to the p-contact. The new LED fabrication process with p-metal deposition and annealing after the mesa etching, prior to n-metal deposition shows smaller series resistance and a lower voltage across the LED at high current. Acknowledgments This work was supported by a DOE/NETL Solid-State Lighting Contract of Directed Research under DE-EE0000627. References 1. M. E. Lin, Z. Ma, F. Y. Huang, Z. F. Fan, L. H. Allen, and H. Morkoc, "Low-Resistance Ohmic Contacts on Wide Band-Gap GaN," Appl. Phys. Lett. 64 (8), 1003-1005 (1994). 2. J. K. Ho, C. S. Jong, C. C. Chiu, C. N. Huang, K. K. Shih, L. C. Chen, F. R. Chen, and J. J. Kai, "Low-resistance ohmic contacts to p-type GaN achieved by the oxidation of Ni/Au films," J. Appl. Phys. 86 (8), 4491-4497 (1999). 3. D. H. Youn, M. S. Hao, H. Sato, T. Sugahara, Y. Naoi, and S. Sakai, "Ohmic contact to p-type GaN," Jpn. J. Appl. Phys. Part 1 - Regul. Pap. Short Notes Rev. Pap. 37 (4A), 1768-1771 (1998). 4. C. Wetzel, T. Suski, J. W. Ager, E. R. Weber, E. E. Haller, S. Fischer, B. K. Meyer, R. J. Molnar, and P. Perlin, "Pressure induced deep gap state of oxygen in GaN," Phys. Rev. Lett. 78 (20), 3923-3926 (1997). 5. C. Wetzel, T. Salagaj, T. Detchprohm, P. Li, and J. S. Nelson, "GaInN/GaN growth optimization for high-power green light-emitting diodes," Appl. Phys. Lett. 85 (6), 866-868 (2004).