Ohmic Contact Formation to Doped GaN

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MBE, using Mg as the acceptor impurity (p=lx1018 cm- 3 ). Dopant levels and carrier concentrations for the n-GaN films varied. The nitride films were cleaned with a 50:50 HCI:H 2 0 dip and carefully pulled dry from the solution, prior to deposition of the metals. Any remaining cleaning solution was blown dry with N2. Where the equipment used for metallization provided the capability, the GaN samples were also thermally desorbed in vacuum at 700'C for 15 min prior to metal deposition. The Al layers were deposited in a conventional thermal evaporator; the Ti, Au and Au/Mg/Au layers were deposited by means of electron beam evaporation. Titanium nitride was grown by means of ion beam-assisted deposition (IBAD), using electron beam evaporation of Ti and purified N 2 activated by a Kaufman-type ion gun. Both the Ti and the TiN growth were performed at a substrate temperature of 350'C and a deposition rate of 10-15 A/min. All other metallizations were performed at room temperature. After deposition, I-V measurements were taken using tungsten probe tips and an HP 4145C Semiconductor Parameter Analyzer. Annealing treatments for most of the samples were performed in a flowing N2 atmosphere at successively higher temperatures using a Heatpulse 410 rapid thermal annealing (RTA) furnace. The TLM measurements were taken using the method described by Reeves and Harrison [14]. The mathematical assumptions and pattern geometry inherent in this model yield values for Pc that represent an upper limit; thus, the measured values are conservative assessments of performance. Analytical and high-resolution transmission electron microscopy were performed on the contact-GaN interfaces; conditions for the study were reported previously [15]. RESULTS AND DISCUSSION Al contacts on n-GaN Aluminum contacts (single Al layers 2500 A thick) deposited on Ge-doped n-GaN

(n=5x10' 9 cm- 3) were ohmic and exhibited low contact resistivity in the as-deposited condition.

The I-V relationship of the Al contacts on Ge:GaN was completely linear; from TLM measurements, the contact resistivity pc at room temperature was found to be 8.6xl0- 5 Q.cm 2 . These results compare very favorably with those reported by Foresi and2Moustakas [9,10], and, more2 3 recently, Lin et al. [16], who obtained Pc values of lxl0- Q.cm and approximately 1 ".cm respectively. In both cases, the carrier concentration of the unintentionally doped n-type GaN used was -1017 cm-3 However, as noted above, the n-GaN films used in the present study for these Al contact measurements were highly doped and thus had characteristics favorable for low-resistivity contact formation. The samples were annealed at 200, 350, 450, 550, and 650'C for 3 min at each temperature; TLM measurements were performed after every heat treatment. The effect of this heat treatment on the contact resistivity is plotted in Figure 1. From room temperature through 450'C, the behavior of the Al contacts was essentially unchanged from the as-deposited condition. However, the 550'C and 650'C anneals resulted in an overal