The electrical behavior of Pt3In7 and NiIn contacts to p-GaN

Abstract

Based on the criteria for the solid-state exchange reaction with p-GaN, we have investigated the intermetallic compounds, Pt 3 In 7 and NiIn, as possible ohmic contacts. The as-deposited contacts were found to be rectifying and using current-voltage characterization rapid thermal annealing of the contacts was shown to significantly decrease their resistance, with contacts annealed at 800°C for 1 min yielding the lowest resistance. Pt 3 In 7 contacts to p-GaN when annealed at 800°C for 1 min exhibited a specific contact resistance ( c ) of 9.5ϫ10 Ϫ2 ⍀ cm 2 while NiIn was more than an order of magnitude lower at 8.0ϫ10Ϫ3 ⍀ cm 2 . The NiIn contacts also show a lower specific contact resistance than the more traditionally used Ni/Au contacts ( c ϭ1.3ϫ10 Ϫ2 ⍀ cm 2 ) processed under the same conditions. Atomic force microscopy was used to examine the morphology of the reacted contacts. While the Ni/Au contacts showed the formation of deep spikes ͑Ͻ200 nm͒ after annealing the NiIn contact showed a much smoother reaction region with no evidence of spiking. © 1999 American Institute of Physics. ͓S0003-6951͑99͒04517-9͔GaN is a wide band gap semiconductor with tremendous promise for optical applications. When alloyed with InN and AlN it can be used to fabricate high efficiency optical devices such as light emitting diodes ͑LEDs͒ and lasers that operate in the visible ͑yellow to blue͒ and ultraviolet ͑uv͒ ranges.1,2 The addition of blue and green LEDs to the already available reds, oranges, and yellows completes the visible spectrum, allowing LEDs to be used for applications such as traffic lights and large flat full-panel displays.3,4 Additionally, blue and uv lasers will have a significant impact on high-density information storage using magnetic and optical media. 4 Aside from its applications in optoelectronics, GaN's wide band gap, good thermal conductivity, and chemical stability make it an ideal candidate for use in high temperature and high power electronic devices. 5,6 Despite the availability of commercial GaN based products, technical challenges still limit these devices. One major concern is the metal contacts to GaN semiconductors, because contacts with a high resistance can substantially reduce the performance of GaN based optical and electrical devices. In fact, the high resistance of the metal contact to p-type GaN is one of the most significant problems limiting laser diode performance. 4Currently Ni/Au is employed as an ohmic contact to p-GaN in most LED and laser applications. 7,8 Despite being successfully used in these devices, the contacts typically exhibit a specific contact resistance ( c ) in the 10 Ϫ3 -10 Ϫ2 ⍀ cm 2 range 9,10 and show poor thermal stability.11 The standard approach used to form ohmic contacts to p-GaN has been to select metals with large work functions such as Pd, Pt, Ni, and Au and depositing them in multilayer combinations. 10,12 However, none of these schemes has yielded a contact with the desired less than 10 Ϫ5 ⍀ cm 2 resistance typically considered device quality.In an atte...