Paper TF-ThP6
Sputter Deposition of Atomically Smooth ZnO Films with Buffer Layers Crystallized via Nitrogen Mediation

Thursday, November 1, 2012, 6:00 pm, Room Central Hall

Zinc oxide (ZnO) is a promising oxide semiconductor for optoelectronic devices because of its attractive properties such as wide direct band gap (3.3 eV), high exciton binding energy (58 meV), and material abundance. For realizing optoelectronic devices utilizing such attractive properties, a fabrication method of high quality crystalline ZnO films is essential. We have recently demonstrated a novel fabrication method of ZnO films utilizing nitrogen mediated crystallization (NMC), where the crystal nuclei density can be controlled because the nitrogen atoms suppress crystallization of ZnO films [1]. By using NMC-ZnO films as buffer layers, we have succeeded in high-quality epitaxial growth of ZnO films on sapphire substrates by RF magnetron sputtering [2]. However, for device applications such as light emitting diodes (LED), there still remains a need for improvement of properties of ZnO films. Here we apply off-axis sputtering to epitaxial growth of ZnO films on NMC-ZnO buffer layers aiming at reduction of negative-ion bombardment that causes serious damage during oxide deposition. NMC-ZnO buffer layers were fabricated by RF magnetron sputtering. The used gas was N₂-Ar and the total pressure was 0.3 Pa. ZnO ceramic targets were used. The applied RF power was 100 W and the deposition temperature was 700^oC. The thickness of the buffer layers was 10 nm. On NMC-ZnO buffer layers, ZnO films were deposited by off-axis RF magnetron sputtering at 700^oC. Ar-O₂ was used and the total pressure was 0.67 Pa. The applied RF power was 60 W. The film thickness was 1μm. After deposition, the films were annealed in a furnace at 1000^oC for 3h in air. The surface of as-deposited ZnO films fabricated by off-axis sputtering on NMC-ZnO buffer layers has subnm scale corrugation and the RMS roughness is 0.28 nm, being significantly small compared with 1.02 nm for the films fabricated by conventional on-axis sputtering. Moreover, the annealed surface has 0.26-nm-high steps corresponding to one molecular layer of ZnO. We have demonstrated fabrication of atomically flat ZnO films by using NMC-ZnO buffer layers together with the off-axis sputtering.

[1] N. Itagaki, K. Kuwahara, K. Nakahara, D. Yamashita, G. Uchida, K. Koga, and M. Shiratani, Appl. Phys. Express 4 (2011) 011101.

[2] K. Kuwahara, N. Itagaki, K. Nakahara, D. Yamashita, G. Uchida, K. Kamataki, K. Koga, and M. Shiratani, Thin Solid Films 520 (2012) 4674.