─ 2 ─duces the interlayer reaction which forms aligned interface between the contact phase and the semiconductor substrate. On the other hand, the barrier thickness can be decreased by increasing the local carrier density of the semiconductor in the vicinity of the electrode by heavy-doping of donor or acceptor elements. For n-type SiC, NiSi is known as an excellent material for lowering the Schottky barrier9). It is commonly formed by deposition of Ni on SiC and subsequent annealing at tempera-tures higher than 1173 K3, 9, 10). The interfacial reaction during the heat treatment forms NiSi adjacent to SiC. However, NiSi is not the only one phase produced by the reaction. The most fatal product of the reaction is the free carbon, consisting of graphite and/or amorphous carbon. The free carbon deterio-rates the mechanical properties of the electrode. To suppress the formation of the free carbon, an element X of which afn-ity with carbon is high should be added in the deposited layer to absorb carbon atoms by forming X-carbide instead of the free carbon. A candidate of such element X is Ti, since it is a strong carbide former11). Furthermore, TiC has a low work function12), which is favorable for lowering the Schottky barri-er on n-type SiC. Although several papers report Ni-Ti elec-trode on SiC, the mechanical properties are scarcely dis-cussed13). The present study reports the mechanical properties of Ni-Ti contacts on n-type 4H-SiC in relation with electrical properties. TiC contact on SiC is also examined. Difculty in the formation of low-resistance electrode on p-type GaN arises from the following two points. One is that no conductive material can lower the Schottky barrier to a sufcient level, since the Fermi level of p-type GaN is very deep (7.49 eV under the vacuum level) 8). The other point is that only Mg can be used as the acceptor dopant for GaN. Mg can be doped up to 2×1025 m–3 in GaN, and further doping de-teriorates the crystal stability of GaN. Generally, the doping concentration is sufcient for thinning the Schottky barrier. However, the most of doped Mg is inactivated by hydrogen atoms inevitably entrapped during the MO-CVD crystal growth process of GaN. Most of the research papers, however, report the results of material seeking and their formation pro-cess6–8), i. e., the approach from the former point of the dif-culty. On the other hand, almost no approach is made from the latter point. In the present study, a method to evacuate hydro-gen by applying voltage during annealing is demonstrated. Since the inactivation of Mg acceptors occurs by electron transfer from hydrogen to Mg, hydrogen is ionized to plus. Therefore, hydrogen can be evacuated by applying voltage between the electrodes on GaN. Furthermore, the evacuation will be enhanced by increased mobility of atoms at high tem-peratures. In this way, the electrode annealing process is com-bined with voltage application. 2.Improvement of mechanical properties of NiSi electrode formed on n-type SiC2.1Strategy for control of interfacial reaction among SiC, Ni, and TiThe interfacial reaction between SiC and Ni is considered based on the Ni-Si-C ternary phase diagram. Fig. 1 shows the isotherm at 1173 K14). SiC and Ni cannot coexist in equilibri-um. Therefore, the interfacial reaction proceeds during anneal-ing to establish local equilibrium. Since Ni does not form any stable carbides nor dissolves carbon, the interfacial reaction produces Ni-Si compounds and the free carbon. Therefore, the formation of the free carbon is inevitable when only Ni reacts with SiC. Addition of Ti overcomes this problem. Formation of TiC proceeds spontaneously, since the formation reduces the chemical potential of carbon in the system signicantly. However, it has to be reminded that Ti can react also with Si. Naka et al. reported that the reaction between SiC and pure Ti forms Ti5Si3CX and Ti3SiC2 at the interface15). To avoid the formation of these products, the chemical potential of Ti at the reaction interface has to be lowered to a sufcient level. The chemical potential can be reduced by forming compound phases and/or by diluting Ti at the reaction interface. Thermo-dynamic calculations suggests that the formation of NiTi and Ni3Ti intermetallic compounds lowers down the chemical po-tential of Ti to an appropriate level16). On the other hand, it is effective to dilute Ti by placing a Ti-source at a portion distant Fig. 1 Isothermal section of Ni-Si-C temary phase diagram14). The broken line indicates the phase sequence reported by Gülpen et al.7).
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