Epitaxial Growth of ZrO 2 on GaN by MOMBE for High Dielectric Material Applications
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Epitaxial Growth of ZrO2 on GaN by MOMBE for High Dielectric Material Applications Xing Gu, Jinqiao Xie, Serguei Chevtchenko, Natalia Izyumskaya, Vataliy Avrutin, and Hadis MorkoÁ Dept. of Electrical and Computer Engineering, Virginia Commonwealth University, 601 W. Main Street, Richmond, VA, 23284 Abstract: Epitaxial growth of ZrO2 has been achieved on MOCVD grown GaN templates by oxides MBE using reactive H2O2 for oxygen and organometallic source for Zr. Utilizing a low temperature buffer followed by high temperature in-situ annealing and hightemperature growth of the final layer, monoclinic (100)-oriented ZrO2 thin films were achieved. The full width at half maximum of ZrO2 (100) rocking curve is 0.4 arc degree and the rms roughness for a 5µm by 5 µm AFM scan is 4≈. The use of epitaxially grown ZrO2 in the AlGaN/GaN HFET structure as a gate dielectric layer have resulted in the increase of the saturation-current density and pinch-off voltage as well as showed near symmetrical gate-drain I-V behavior.
Introduction: In scaled MOS devices the thickness of gate dielectric must be reduced to e.g. equivalent thickness around 0.8nm for the devices1, which is sufficiently thin for SiO2 to exhibit tunneling current. With a dielectric constant above 20, a wide band gap of ~5.8 eV, a high breakdown field of 15-20 MV/cm, and good thermal compatibility with silicon2, ZrO2 has been studied as a candidate for gate oxide. It is well known that the ZrO2 has three different crystal structures: the monoclinic structure (baddeleyite) is stable at low temperatures, as temperature rises to around 1400 K, ZrO2 undergoes a first order martensitic phase transition into the tetragonal structure, and further increase temperature in 2570 K will render the ZrO2 into the cubic structure (fluorite) stable at very high temperatures. Compared to monoclinic ZrO2, which commonly has a dielectric constant around 20~24, the tetragonal ZrO2 possesses a dielectric constant of up to 35~50 (Ref.3), despite the fact that polarization at the atomic level is reduced. Dopants such as Yttria can stabilize high- temperature forms of zirconium dioxide partially or fully at room temperature, making the Yttria-stabilized zirconia (YSZ) tetragonal or cubic (when fully stabilized) and thus rising dielectric constant effectively. Such YSZ thin films have already been realized by different epitaxy methods including MOCVD4, e-beam evaporation5,6 and pulsed organometallic beam epitaxy (POMBE)7. In other electronic device applications, ZrO2 has been used as gate dielectric for GaN based devices. The films in the latter case were obtained by using ultraviolet ozone oxidation of Zr metal8 or electron beam evaporation9. However, to the best of our knowledge, there has not been any report so far on the epitaxial growth of ZrO2 on GaN. ZrO2 is also expected to be a promising bridge material between complex perovskite
oxides and semiconductors, since it helps to accommodate the large discrepancy in crystal structure and lattice constant between the perovskite
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