─ 5 ─nm) samples are almost one order of magnitude lower than those of the other two samples. The contact lms on these samples are completely removed by scratching. It should be noted that an inappropriate combination of thicknesses of the Ti and Ni layers deteriorates both the mechanical and electri-cal properties. The interfacial phenomena during the forma-tion process and the resultant performance of the contact lm are very sensitive to the thicknesses of the Ti and Ni layers. 2.4Short summaryThe present paper proposed and demonstrated the improve-ment of the mechanical properties of a Ni-based electrode for n-type SiC by the addition of a Ti layer. The following points became clear. (1) SiC/Ni samples achieve low-resistance Ohmic contact af-ter annealing at 1273 K for a short time. However, the me-chanical properties are signicantly deteriorated by the an-nealing.(2) SiC/C/Ti samples become Ohmic after annealing at 1273 K for a short time. The electrodes perform high mechani-cal properties. However, the electrical conductance is low-er than that of annealed SiC/Ni electrodes.(3) SiC/Ni/Ti samples also become Ohmic after annealing at 1273 K for a short time. Appropriate thicknesses of the Ti and Ni layers signicantly improve the mechanical proper-ties, retaining high electrical conductivity. 3.A new electrode formation process which activates acceptors in p-type GaN3.1Strategy for thinning the Schottky barrier at the metal/GaN interfaceThe commonly known process for contact electrode forma-tion on p-type GaN consists of two steps: deposition of four-layered precursor lm on GaN and subsequent annealing at approximately 900 K8). The rst layer of the precursor, be-ing adjacent to GaN, is, in most cases, Ni8). Ni has a work function as large as 5.15 eV, which is effective for lowering the Schottky barrier8). In addition, Ni does not react with ni-trogen. This character in the reactivity of Ni is important for electrodes on p-type GaN, since the consumption of nitrogen by the interfacial reaction forms nitrogen vacancies, which act as donor dopants, in GaN in the vicinity of the interface. On the other hand, Ni does not permeate hydrogen. Hydrogen ions will accumulate at the negative electrode by the voltage applied during annealing. However, further evacuation is dif-cult as far as the electrode does not allow hydrogen go through the electrode. Consequently, Pd is selected as the second candidate for the electrode material in this study. Pd shows the highest hydro-gen permeability among all metals19), a large work function being 5.12 eV, and sufcient nobleness against nitrogen. 3.2Experimental procedureThe p-type GaN used in the present study was a 2.0-µm-thick epitaxial lm grown by MO-CVD process on the (0001) plane of sapphire wafer with a 2.4-µm-thick undoped GaN buffer layer. The surface orientation and the Mg-doping con-centration were (0001) Ga-face and 3.0×1022 m–3, respectively. The GaN substrates were cut to 4.0-mm square size in the di-rections of [101-0] and [1-21-0] of the GaN. After ultrasonic cleaning of the GaN substrates with acetone, the electrode lms of Ni or Pd were deposited on the substrates by ra-dio-frequency magnetron sputtering. The gray areas in Fig. 6(a) show the size and layout of the electrodes on a GaN substrate. The electrodes were connected to a direct-current power sup-ply as schematically shown in Fig. 6(b) and installed in a tu-bular furnace. The heat treatment was implemented in a ow of high-purity (99.999%) nitrogen atmosphere at 673 K for 3.6 ks, applying 30 V along the [1-21-0] direction of GaN. The ef-fect of voltage application was investigated by comparing the samples annealed with and without the voltage application. The major carrier-type after the heat treatment was measured by the van der Pauw method. The interfacial microstructure after the heat treatment was analyzed by transmission electron microscopy (TEM). 3.3Results and discussionFig. 7 compares three types of the electric conduction pro-les between two Ni electrodes arranged along the [1-21-0] di-rection of GaN. The line connecting black circles is the prole Fig. 5 Relation between the electrical properties and mechanical properties of the samples prepared in the present study.
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