Improved Low Resistance Contacts of Ni/Au and Pd/Au to p-Type GaN Using a Cryogenic Treatment
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ABSTRACT A low resistance Ohmic contact to p-type GaN is essential for reliable operation of electronic and optoelectronic devices. Such contacts have been made using Ni/Au and Pd / Au contacts to p-type Mg-doped GaN (1.41x1017 cm-3) grown by metalorganic chemical vapor deposition ( MOCVD ) on ( 0001 ) sapphire substrates. Thermal evaporation was used for the deposition of those metals followed by annealing at temperatures of 400 ~ 700 oC in an oxygen and nitrogen mixed gas ambient, then subsequently cooled in liquid nitrogen which reduced the specific contact resistance from the range of 9.46~2.80x10-2 Ωcm2 to 9.84~2.65x10-4 Ωcm2 for Ni/Au and from the range of 8.35~5.01x10-4 Ωcm2 to 3.34~1.80x10-4 Ωcm2 for Pd/Au. The electrical characteristics for the contacts were examined by the current versus voltage curves and the specific contact resistance was determined by use of the circular transmission line method (cTLM). The effects of the cryogenic process on improving Ohmic behavior (I-V linearity) and reducing the specific contact resistance will be discussed from a microstructural analysis which reveals the metallurgy of Ohmic contact formation. INTRODUCTION Great interest exists in the III-nitride semiconductors since the successful development in growth of GaN based materials and operation of electronic and optoelectronic devices such as blue and green light emitting diodes (LEDs) and laser diodes (LDs) [1,2,3,4]. The formation of stable and reliable low resistance ohmic contacts to p-type GaN has been a problem in achieving good performance of those devices. For devices with large contact areas such as LEDs and LDs, the specific contact resistance ( ρc ) between 10-4 to 10-6 Ωcm2 is considered acceptable and for devices with smaller contact areas, values of ρc between 10-5 to 10-7 Ωcm2 are necessary [5]. Bilayer metal schemes such as Ni/Au and Pd/Au were studied by many groups [6,7,8]. These have been studied due to the stable electrical and thermal properties and the high work function which is one criteria to form low resistance Ohmic contacts to p-type materials. The effects of cryogenic cooling after heat treatment on the formation of Ni/Au and Pd/Au contacts are presented in this paper. We also compare these effects on forming Ni/Au and Pd/Au contacts annealed in a combined O2/N2 gas ambient. High temperature annealing may degrade homogeneity possibly caused by spiking of metals between themselves or between metal and semiconductor due to the differences in thermodynamic properties of materials. Annealing was conducted in an oxygen and nitrogen mixed gas ambient as reported by Y. Koide et al. [6]. This is to remove hydrogen atoms contained in Mg-doped GaN epilayers. Removing hydrogen atoms results in the increase of the hole concentration and decrease of the contact resistance. Annealing in nitrogen gas ambient is
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believed to avoid nitrogen vacancies which act as doners. We expected that cryogenic treatment would reduce disadvantages from the discrepancy in thermodynamic properties of materi
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