Ohmic and Highly Reflective Ag based contacts on p-GaN for Resonant Cavity Light Emitting Diodes
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Ohmic and Highly Reflective Ag based contacts on p-GaN for Resonant Cavity Light Emitting Diodes
V. Kumbham1, S. Kuchibhatla1, K. Lee1, L.E. Rodak1, V. Narang2, D. Korakakis1, and L.A. Hornak1 1
Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown ,WV 26506 2 Department of Physics, West Virginia University, Morgantown, WV 26506, USA
ABSTRACT: Low resistance and high reflectance ohmic contacts on p-type GaN were achieved using an Ag-based metallization scheme. The ohmic nature of the contacts can be obtained by annealing the contacts in an O2 ambient. However, Ag based contacts degrade due to agglomeration of Ag when annealed above 400 oC [1]. In this work a Ni (1 nm)/Ag (150 nm)/Pt (50 nm)/Ni (20 nm)/Au (50 nm) metallization stack is investigated to reduce Ag agglomeration. The inclusion of platinum as a diffusion barrier is expected to suppress excess oxygen diffusion into the Ag films thereby preventing Ag agglomeration and can also provide high thermal stability when compared to other metallization schemes. The reflectivity of this kind of metallization scheme is around 85-90 % in the wavelength range of 400-600 nm making it suitable for blue and green LEDs, with a specific contact resistivity value comparable to other well developed contacts to p-GaN. INTRODUCTION: The desire to replace conventional lighting by solid-state lighting has led to much interest in the development of high brightness Gallium Nitride (GaN) Light Emitting Diodes (LEDs). Efficient LED structures such as a flip chip configurations and Resonant Cavity LEDs (RCLEDs) typically need a highly reflective ohmic contact on p-GaN. RCLEDs are of great interest due their features such as high spectral purity and high emission intensity when compared conventional GaN based LEDs [2]. Au based contacts are not appropriate for the RCLEDs due to low reflectance in the blue region. Ag films have much higher reflectivity in the visible region when compared to Au films. Furthermore, metallic mirror films provide reflectivity over a wide wavelength range in comparison to the reflectivity of a Distributed Bragg Reflector (DBR) which is usually tuned to operate over a narrow wavelength range. The metal contacts play a key role in device performance such as power consumption, reliability and light extraction [3]. It is very difficult to achieve high performance Ag contacts on p-GaN because of poor adhesion and post annealing effects including agglomeration, oxidation, and thermal instability [4]. A wide variety of metallization schemes with high work function metals such as Ni [5-8] and Pt [9-11] have been employed to achieve low resistance ohmic contacts to p-type GaN. Among those schemes, Ni/Au is commonly used as a p-type ohmic electrode for GaN-based LEDs due to it’s reliably, low contact resistivity and high light transmittance [12-14]. In this work, with the metallization scheme we used, all the above mentioned problems were addressed and high quality ohmic and reflective contacts were
achieved. The impo
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