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Ohmic contacts to Mg-doped p-GaN grown by MOCVD  are studied using a circular transmission line model (TLM) to avoid the need for isolation. For samples which use a p-dopant activation anneal before metallization, no appreciable difference in the specific contact resistance, rc, as a function of different capping options is observed. However, a lower rc is obtained when no pre-metallization anneal is employed, and the post-metallization anneal simultaneously activates the p-dopant and anneals the contact. This trend is shown for Pt/Au, Pt, Pd/Pt/Au, and Ni/Au contacts to p-GaN. The rc 's for these metal contacts are in the range of 1.4–7.6 × 10-3 ohm-cm2 at room temperature at a bias of 10mA. No particular metallization formula clearly yields a consistently superior contact. Instead, the temperature of the contact has the strongest influence.
Detailed studies of the electrical properties of the Pt/Au contacts reveal that the I-V linearity improves significantly with increasing temperature. At room temperature, a slightly rectified I-V characteristic curve is obtained, while at 200°C and above, the I-V curve is linear. For all the p-GaN samples, it is also found that the sheet resistance decreases by an order of magnitude with increasing temperature from 25°C to 350°C. The specific contact resistance is also found to decrease by nearly an order of magnitude for a temperature increase of the same range. A minimum rc of 4.2 × 10-4 ohm-cm2 was obtained at a temperature of 350°C for a Pt/Au contact. This result is the lowest reported rc for ohmic contacts to p-GaN.
The III-V nitride-containing semiconductors InN, GaN, and AIN and their ternary alloys are the focus of extensive research for application to visible light emitters and as the basis for high temperature electronics. Recent advances in ion implantation doping of GaN and studies of the effect of rapid thermal annealing up to 1100 °C are making new device structures possible. Both p- and n-type implantation doping of GaN has been achieved using Mg co-implanted with P for p-type and Si-implantation for n-type. Electrical activation was achieved by rapid thermal anneals in excess of 1000 °C. Atomic force microscopy studies of the surface of GaN after a series of anneals from 750 to 1100 °C shows that the surface morphology gets smoother following anneals in Ar or N2. The photoluminescence of the annealed samples also shows enhanced bandedge emission for both annealing ambients. For the deep level emission near 2.2 eV, the sample annealed in N2 shows slightly reduced emission while the sample annealed in Ar shows increased emission. These annealing results suggest a combination of defect interactions occur during the high temperature processing.
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