Low Resistance Ohmic Contact on p -type GaN Grown by Plasma-Assisted Molecular Beam Epitaxy
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ABSTRACT Ohmic contacts on p-type GaN have been investigated. High quality GaN epilayers on cplane sapphire were prepared using plasma-assisted molecular beam epitaxy that utilized an inductively coupled rf nitrogen plasma source and solid source beams. The resulting film thickness and the doping concentration of the grown samples were in the range of 0.7-1.35 jim and 1018 -102 0/cm 3, respectively. The metallization consisted of high work function metal bilayers which included a combinations of 25 nm-thick Ni, Ti, Pt and/or Cr and 200 nm-thick Au on the highly p-doped GaN in a transmission line model pattern. Ohmic contacts were formed by alloying the bi-layers using rapid thermal annealing (RTA) at temperatures in the range of 300-700 'C for 1 min under nitrogen ambient. Current-voltage measurements showed that the contact resistance was as low as 1.2 x 10 -4 Q2_cm 2 for the sample having 1.4 x 10 specific 20 /cm 3 p-type doping concentration with a Cr/Au contact annealed at 500 'C for 1 min by RTA. Judging from the scanning Auger microscopy results and the glancing angle x-ray diffraction analysis, this resistance is attributed to Cr diffusion into the GaN layer.
INTRODUCTION Rapid progress in the GaN-based optoelectronic research has resulted in commercialization of blue/green light emitting diodes (LEDs) [1-2]. More recently, Nakamura et al. of Nichia Chemical have achieved InGaN multi-quantum well (MQW) structure blue/violet lasing under pulsed current injection [3]. Although continuous wave (cw) lasing operation has not yet been demonstrated, this pioneering work is certainly a great step toward the development of III-V nitride semiconductor lasers. To improve the device performance and to realize cw lasing, there are, however, a number of issues that have to be addressed. Among these, p-type ohmic contact is considered one of the most important. According to the I-V characteristics of the Nichia laser, the turn-on voltage of a Ni/Au metal contact was 34 V which could very well be due to the poor p-type contact properties. To ensure reliable laser operation it is, therefore, very important that a low resistive p-type ohmic contacts be developed. GaN epilayers grown with metalorganic chemical vapor deposition (MOCVD), in many cases, have shown better crystal quality compared with those grown with molecular beam epitaxy (MBE). However, p-type doping concentrations of the MOCVD-grown GaN are known to be lower than those of MBE because of the difficulty in removing the dangling hydrogen bond on the GaN surface [4]. Therefore, post-growth annealing process is essential in MOCVD 131 Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
to activate the p-type dopants in the GaN crystal [5], while no such procedure is necessary in MBE [6]. It is well known that the as-grown GaN films exhibit n-type nature. This is attributed to the vacancy formation due to improper material incorporation between Ga and N atoms. As a result, forming a metal ohmic contact on the n-type GaN is known to be relatively e
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