Diffusion Effect between Schottky Metals and AlGaN/GaN Heterostructure during High Temperature Annealing Process
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1167-O05-06
Diffusion Effect between Schottky Metals and AlGaN/GaN Heterostructure during High Temperature Annealing Process Young-Hwan Choi, Jiyong Lim, Young-Shil Kim and Min-Koo Han School of Electrical Engineering and Computer Science, Seoul National University, 599 Gwanangno, Gwanak-gu, Seoul, 151-744, Korea ABSTRACT We have investigated the change of the Schottky contact surface and the interface between Schottky metals and AlGaN/GaN heterostructure after the annealing process for 35 min at 300 oC. The secondary ion mass spectroscopy (SIMS) and the scanning electron microscopy (SEM) show that the Schottky metals and AlGaN/GaN heterostructure interacted actively during the annealing process. The atoms in Schottky contact and AlGaN/GaN heterostructure diffused interactively and the surface roughness of Schottky contact was increased. After the annealing process for fabricated AlGaN/GaN High-Electron-Mobility Transistor (HEMT), the threshold voltage was shifted by +0.2 V and the leakage current was decreased by 40 %. INTRODUCTION AlGaN/GaN HEMTs (high-electron-mobility transistors) may be promising devices for microwave and high voltage applications due to wide band gap and low intrinsic carrier density of GaN and two-dimensional electron gas and high saturation velocity of AlGaN/GaN heterostructure [1-2]. AlGaN/GaN HEMTs for high voltage application would require a low leakage current and a high breakdown voltage. The leakage current of AlGaN/GaN devices is due to the poor interface between Schottky metal and AlGaN/GaN heterostructure. When GaN epitaxial structure is grown, dislocations are formed due to the lattice mismatch between GaN and substrate so a considerable amount of defects is formed at the surface of GaN wafer. These defects deteriorate Schottky contact and cause the leakage current of GaN based devices [3]. AlGaN/GaN heterostructure is also grown on the substrate such as SiC, sapphire and Si, so defects induced by the mismatch between GaN and substrate are formed on the surface of wafer. The high temperature annealing is a simple and effective process for the improvement of leakage current and breakdown voltage of AlGaN/GaN HEMTs [4-6]. It has been reported that the high temperature annealing after Schottky contact metallization reduces the trapping effect on AlGaN surface and/or GaN buffer layer [4] and decreases the density of Schottky metal/AlGaN interface states [5]. The passivation on the AlGaN/GaN HEMTs is also applied for the decrease of the leakage current [7-8]. However, the deposition process employing plasma for the passivation layer requires high temperature condition. AlGaN/GaN HEMTs are well known for the stable operation in the high temperature condition due to the material characteristics of GaN. But Schottky contact can be changed in high temperature environment and the change of Schottky contact affects the electrical characteristics of AlGaN/GaN devices. We have investigated the change of the Schottky contact surface and the interface between Schottky metal and AlGaN/GaN heteros
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