발표제목: 금속 / 반도체 및 금속 / 산화물 / 반도체를 이용한 쇼트키 접합의 전류-전압 특성
I studied on the electrical properties of Au / n-GaN metal / semiconductor (MS) junction and Au / TiO2 / n-GaN metal/insulator/semiconductor (MIS) junction prepared by pulsed laser deposition (PLD) and magnetroning sputtering. All others parameters except oxygen pressure and deposition time for TiO2 deposition were fixed. For comparison, the titanium dioxide (TiO2) film was also prepared on Al2O3 (0001) substrate to investigate the resistance value of TiO2 itself with the oxy- gen pressure or with the deposition time (film thickness). The surface morphologies of TiO2 films and GaN substrates were observed by atomic force microscope. The root-mean-square(RMS) roughness values of TiO2 films were 1 ~ 7 nm and that of GaN substrate was ~ 3 nm. A quite large roughness of GaN substrate itself was thought to be due to lattice mismatch between GaN and Al2O3. As the oxygen pressure or the film thickness increased, the surface of TiO2 film be- came more rough. To make an ohmic contact, the Au electrode was deposited on TiO2 film by magnetron sputtering. The resistance of TiO2 film was estimated from the linear slope in the cur- rent-voltage curve. As the oxygen pressure or the film thickness increased, the resistance of TiO2 film was also increased. I used three different types of GaN films grown on Al2O3 substrates; (i) Si-doped 5 μm thick GaN film, (ii) undoped 5 μm thick GaN film, and (iii) Si-doped 0.2 μm thick GaN film. To obtain electrical data of GaN films, I carried out Hall effect measurement. The carrier concentrations of the GaN substrates were 0.9×10^{18}/cm^3 ~ 4.2 ×10^{18}/cm^3 . The cross section of TiO2 / GaN junction was measured by transmission electron microscope (TEM). TEM data suggested that TiO2 films deposited on GaN substrates at room temperature were an amorphous phase with some nano-size crystallites. The Ti / Al (Au) electrode was deposited on GaN films to make ohmic (Schottky) contacts, while the Au electrode was deposited on TiO2 films to ohmic contacts. The current-voltage characteristics of Au / GaN MS junctions and Au / TiO2 / GaN MIS junctions were investigated. As the oxygen pressure or the film thickness of TiO2 films increased, the rectification characteristics of Au / TiO2 / GaN MIS junctions were improved in comparison with them of Au / GaN MS junctions. Consequently, I concluded that the rectification characteris- tics of the Au / GaN junction can be improved by an insertion of an insulating TiO2 layer.
Thesis Advisor: Prof. Joonghoe Dho