Low-resistance and thermally stable ohmic contact on p-type GaN using Pd/Ni metallization

Abstract

We report a low-resistance thermally stable ohmic contact on p-type GaN using a promising contact scheme of Pd/Ni. Specific contact resistance as low as 5.7×10−5 Ω cm2 was obtained from the Pd (30 Å)/Ni (70 Å) contact annealed at 500 °C under an oxidizing ambient. NiO that formed at the surface prevented Pd atoms from outdiffusing, promoting the formation of Pd gallides, Ga2Pd5 and Ga5Pd. This reaction produces Ga vacancies below the contact, leading to enhancement of the thermal stability as well as reduction of the contact resistivity.

Calculations and measurements of contact resistance of semi-transparent Ni/Pd contacts to p-GaN

“…This suggests that NiO acts as a barrier and prevents Pd from diffusing out toward the contact surface and prevents N from diffusing away from the p-GaN. This also suggests that Pd interacts with Ga to form Pd-gallides, 14 which would create Ga vacancies (acceptors) at the GaN surface, thus increasing hole concentrations and lowering contact resistance.…”

“…The structural analysis of the contact supports many of the observed characteristics of the classic oxidized Ni/Au contact and also of other semitransparent contacts. The only other Ni/Pd contact the authors found in the literature is a Pd/Ni contact studied by Jang, et al 14 In this work, Pd was deposited directly on the p-GaN, followed by Ni with several thickness ratios of Pd:Ni studied, giving r C values between 2 3 10 ÿ3 V cm 2 and 6 3 10 ÿ5 V cm 2 for 75 Å /25 Å and 30 Å /70 Å , respectively. While the overall thickness of our contact is greater than any of those studied by Jang et al, our metal ratio (100 Å /100 Å ) falls in the range of their samples and our r C data are also similar to theirs for anneals at the same temperature (450°C).…”

“…2 The ideal metal contact to p-GaN would have a work function greater than that of GaN (;6.5 eV to 7.5 eV). 3,4 For light emitting diodes (LEDs), extraction of the light from the device is paramount to device efficiency and several contact schemes using semitransparent contacts, including oxidized Ni/Au, [5][6][7][8][9][10][11][12] Ru/Ni, 13 Pd/Ni, 14 NiAu, and Al or Ag, 15 indium-tin-oxide-based, 16 and ZnObased, 17 have been reported. The most detailed studies have been reported on the oxidized thin Ni/Au system.…”

Scanning probe microscopy studies on the growth of palladium and nickel on GaN(0001)

“…For example, the formation of Pd gallides (Ga 2 Pd 5 and Ga 5 Pd) during annealing of Pd contacts on p-GaN results in enhanced thermal stability and reduced contact resistivity [3]. Control of the metal deposition conditions as well as the effective preparation of clean GaN surfaces is therefore crucial for the performance of the contacts.…”

“…Previous studies have focused on the improvement of ohmic contacts, such as Ti/Al and Pd/Al on n-type GaN, or Ni/Au, Ni/Pd/Au, Pd/Au on p-type GaN [1, 2]. Low-resistance, thermally stable ohmic contacts on p-type GaN have been reported using Pd/Ni and Pd/NiO based contacts [3,4]. However, little is known about the formation and properties of these contacts on the atomic scale.…”