Low-resistance ohmic contacts to p-type GaN

Abstract

Low-resistance ohmic contacts with high transparency to p-type GaN have been developed by oxidizing Ni/Au thin films. Compared to the metallic Ni/Au contacts, the oxidized Ni/Au contacts exhibited lower specific contact resistance and much improved transparency. The transparency was from 65% to 80% in the wavelength of 450–550 nm. A specific contact resistance below 1.0×10−4 Ω cm2 was obtained by oxidizing Ni(10 nm)/Au(5 nm) on p-type GaN. The mechanism of low-resistance ohmic contact could be related to the formation of NiO.

High transparency low resistance oxidized Ni/Au–ZnO contacts to p-GaN for high performance LED applications

“…However, owing to the high resistivity of p-GaN, the injected current does not spread uniformly in an LED device: a thin semi-transparent Ni/Au (<10 nm) current spreading layer (CSL) is needed for this purpose at the expense of reduced light extraction (transparency of 30-40%) due to optical absorption [2]. The introduction of thin oxidized Ni/Au contact having enhanced optical transmittance of 60 -75% in the wavelength range of 400 -500 nm has partially alleviated this problem [3,4]. But oxidized Ni/Au CSL is known to suffer from poor reliability compared to N 2 -annealed Ni/Au contact [5].…”

“…Both thin Ni/Au(5 nm/5 nm) and thicker Ni/Au(10 nm/10 nm) are deposited on p-GaN using e-beam evaporation. The samples are then oxidized at 500 °C in oxygen ambient to form NiO with embedded Au matrix [3]. The deposition of AZO is carried out using a Perkin-Elmer 2400A RF sputtering system.…”

Transparent ZnO-Based Ohmic Contact to p-GaN

“…For GaN-based photonic devices, where the major losses of performance are due to poor conductivity of p-type subcontact region, causing the so-called "current crowding" problem, a highly conductive and transparent p-type electrode is a prerequisite. Among the investigated metallizations that would allow low-resistivity contact to p-GaN, oxidized Ni/Au bilayer was reported to give the lowest operating voltages [1][2][3]. For the transparent p-type electrode, optimization of Ni/Au metallization via reducing its thickness [4,5] has been achieved, however, at the cost of increase of the specific contact resistance.…”

Low Resistance Optically Transparent Contacts to p–type GaN Using Oxidized Ni/Au and ITO for LED Application

“…Ho et al reported that by oxidizing a thin bilayer of Ni/Au under air or water vapor, record low levels of ρ c can be achieved while increasing transparency. They measured ρ c = 2 x 10 -4 Ω-cm 2 for a Ni/Au (100 Å /50 Å) bilayer annealed at 500ºC in air and 60-70% transparency for the same film on a BK-7 glass substrate [2]. They described further reduction of ρ c for their oxidized Ni/Au (NiO/Au) contacts could be obtained by decreasing the metal thicknesses to 50 Å each [3].…”

“…These contacts exhibit ρ c values of around 1-2 x 10 -2 Ω-cm 2 [1]. Ho et al measured the level of optical transmission through a Ni/Au (100 Å /50 Å) bilayer on BK-7 glass, annealed at 500ºC under N 2 , as only around 30-35% between 400-500 nm [2]. While this amount of transparency may be enhanced by decreasing the metal thicknesses, these layers may be too thin to accommodate the high lateral current densities anticipated for the next generation of larger area LEDs.…”