Reflective and Low-Resistance Zn∕Rh Contacts to p-Type GaN for Flip-Chip Light-Emitting Diodes

Abstract

We report on the formation of high-quality ohmic contacts to p-type GaN ͑5 ϫ 10 17 /cm 3 ͒ using a Rh ͑100 nm͒ layer combined with a 3 nm thick Zn interlayer for high-power flip-chip light-emitting diodes ͑LEDs͒. The as-deposited sample produces a nonlinear behavior. However, the samples annealed at 430 and 530°C for 1 min in air become ohmic with a contact resistivity of ϳ10 −5 ⍀ cm 2 . Measurements show that the reflectivity of the samples annealed at 530°C is 73% at 460 nm. LEDs fabricated using the Zn/Rh contact layers give forward-bias voltages of 3.09-3.12 V at an injection current of 20 mA.High-performance flip-chip light-emitting diodes ͑LEDs͒ are of significant technological importance for applications in solid-state lighting. 1 In order to realize such LEDs, the development of p-type ohmic electrodes having high reflectivity as well as low contact resistance is crucial. Ag, Al, and Rh are known to be the best metallic reflectors. In particular, owing to its thermal stability, a Rh contact has been considered as a potentially important electrode for the fabrication of high-power flip-chip LEDs. Several attempts have been hitherto made to develop high-quality ohmic contacts to p-type GaN using Rh reflectors. 2,3 For example, Song et al., 2 showed that nonalloyed Rh ͑10 nm thick͒ contacts yield a low specific contact resistance of ϳ10 −5 ⍀ cm 2 , when the samples were two-step surface-treated using a buffered oxide etch solution prior to metal deposition. However, single Rh contacts suffer from thermal degradation when annealed at temperatures in excess of 300°C. 4 Recently, Lee et al. 3 used an 8 nm thick Pt interlayer to improve the ohmic behavior of Rh contacts and showed that Pt/Rh contacts give a specific contact resistance of 9.0 ϫ 10 −5 ⍀ cm 2 and reflectance of 62% at 460 nm when annealed at 500°C. In this work, to improve the thermal stability of single Rh contacts, a Zn layer is added inbetween a Rh layer and a GaN layer. Use of a Zn layer is advantageous since Zn atoms could act as an acceptor dopant 5 or transform into transparent conducting oxide, such as ZnO, when annealed in air. The results show that the Zn͑3 nm͒/Rh͑100 nm͒ contacts produce specific contact resistances as low as ϳ10 −5 ⍀ cm 2 , when annealed at 430 and 530°C for 1 min in air. GaN-based LEDs fabricated using the Zn/Rh ohmic contact layers give forward-bias voltages of 3.09-3.12 V at an injection current of 20 mA.Metallorganic chemical vapor deposition ͑MOCVD͒ was used to grow 2 m thick unintentionally doped GaN layers on ͑0001͒ sapphire substrates. This was followed by the growth of 1 m thick p-GaN:Mg layer ͑n a = 5 ϫ 10 17 /cm 3 ͒. The GaN layers were ultrasonically degreased with trichloroethylene, acetone, methanol for 5 min in each step, and then rinsed with deionized water. After cleaning the samples using a buffered oxide etch ͑BOE͒ solution for 20 min, 6 the GaN layers were blown dry by nitrogen gas. Circular transfer length model ͑CTLM͒ patterns were defined by the photolithographic technique to measure the specific...

Ohmic Contacts on p‐GaN

“…Ga‐Rh and Ga‐Pt phases were observed at the interface region. The Fermi levels were also found to be shifted towards the valence band after annealing of Zn/Rh contacts, which is probably due to the formation of acceptor‐like Ga vacancies near the GaN surface.…”

“…Hibbard et al employed a thin oxidized Ni/Au layer with a thicker Al or Ag overlayer, capped with Ni/Au, and obtained a resistance comparable with that of Ni/Au contacts, but with higher reflectivity due to the high reflectivities of Al and Ag . Zn/Rh and Pt/Rh contacts on p‐GaN were investigated in an attempt to obtain contacts with good conductance and high reflectance. Ga‐Rh and Ga‐Pt phases were observed at the interface region.…”

“…Zn/Rh and Pt/Rh contacts on p‐GaN were investigated in an attempt to obtain contacts with good conductance and high reflectance. Ga‐Rh and Ga‐Pt phases were observed at the interface region. The Fermi levels were also found to be shifted towards the valence band after annealing of Zn/Rh contacts, which is probably due to the formation of acceptor‐like Ga vacancies near the GaN surface.…”

Electrical and Interfacial Properties of Nonalloyed Ti/Au Ohmic and Pt Schottky Contacts on Zn-Terminated ZnO

“…5(b), exhibit a kinetic energy shift toward lower energy with increased sputtering time, which is indicative of an interfacial reaction of oxygen with Ti. 20,21) Although the surface consists of a hexagonal array of Zn 2þ ions, a subsurface layer of O À2 ions is partially exposed on the metal layer due to the small size of the Zn 2þ ions. 16,22) Therefore, out-diffusion of oxygen atoms is possible due to the existence of the exposed O À2 subsurface.…”

Transparent p-type layer with highly reflective Rh/Al p-type electrodes for improving the performance of AlGaN-based deep-ultraviolet light-emitting diodes

“…On the one hand, the Rh ensures an Ohmic contact. [29][30][31] On the other hand, the thin enough Rh layer can make the incident light directly reflect or transmit to the top Al layer with higher reflectance and then be reflected, thus improving the light extraction, as shown in Fig. 3(a).…”