─ 1 ─1. IntroductionWide band-gap compound semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), are expected to im-prove the energy efciency of power electronic devices1, 2). Many researchers are actively studying to develop the devic-es1–8). One of the issues preventing the development is the for-mation of low-resistance metallic electrode on the semicon-ductors.Each material has its characteristic Fermi level, which is determined by the composition and chemical bond states of the constituent atoms. If two different materials come into contact to each other, some of the constituent electrons move from the material with higher Fermi level to the other. In the case of the contact of semiconductors to metals, the transfer of electrons forms a Schottky barrier at the interface. The barrier hinders the current of charge carriers across the interface, i. e., it is a kind of resistance. Thus, electric conduction through the interface generates Joule heat. The heat corresponds to the en-ergy loss at the interface and lowers the energy efciency of the devices. Even worse, the local heating at the interface de-teriorates the bond strength of the interface and shortens the service life of the devices. Consequently, the reduction of the contact resistance is inevitable issue for the development of the next-generation high-efciency power electronic devices. The contact resistance can be reduced by lowering the Schottky barrier height and by thinning the Schottky barrier width. The barrier height can be suppressed by forming a con-tact electrode made of an appropriate material. In most cases, precursor lms are deposited one after another on the semi-conductor substrate and then annealed. The heat treatment in-Articles日本大学生産工学部研究報告Ａ2019年 12 月 第 52 巻 第 ２ 号Control of Interfacial Nanostructure between Wide-Gap Semiconductors and Their ElectrodesMasakatsu MAEDA＊（Received December 30, 2018）The present study was conducted to solve two issues related to contact electrode formation of wide band-gap semiconductors, which are the candidates for the next-generation power electronic devices. One was to improve the mechanical properties retaining the good electrical properties of Ni-based electrodes on n-type silicon carbide. The mechanical properties are improved by suppressing the reaction which produces free-carbon. The harmful reaction can be successfully replaced by a reaction forming TiC. It is achieved by adding a titanium layer of an appropriate thickness on the nickel precursor layer and annealing at 1273 K for a very short time in vacuum. The strategy for control of the interfacial reaction is also described in detail. The other issue was the formation of low resistance contact electrode on p-type gallium nitride (GaN). In order to activate acceptor dopants of GaN in the vicinity of electrodes, a method to enhance hydrogen-atoms evacuation by applying voltage during annealing was developed and its effect was demonstrated. Ni and Pd electrodes were tested. Although Pd electrodes were expected to be better for hydrogen evacuation due to the high hydrogen permeability of Pd, Ni electrodes show higher evacuation rate and lower contact resistance. The difference in the resistance is considered to be caused by the return-back behavior of hydrogen, once evacuated from GaN and stored in the electrode.Keywords:Silicon Carbide, Gallium Nitride, Contact Electrode, Nanostructure Control＊Associate Professor, Department of Mechanical Engineering, College of Industrial Technology, Nihon University.
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