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ZnO-based thin films and light-emitting diodes grown by ALD

Zinc oxide (ZnO) is a promising material for ultraviolet (UV) light-emitting diodes (LEDs) because of its large exciton binding energy up to 60 meV and direct bandgap energy of 3.37 eV. Many techniques have been used to grow high-quality ZnO films, including molecular beam epitaxy, metal-organic chemical vapor deposition, chemical vapor deposition, pulsed laser deposition, radio-frequency magnetron sputtering, and filtered cathodic vacuum arc technique. Atomic layer deposition (ALD) is another noteworthy method of growing high-quality ZnO films. The self-limiting and layer-by-layer growth of ALD offer many benefits including easy and accurate thickness control, conformal step coverage, high uniformity over a large area, low defect density, good reproducibility, and low deposition temperatures. Actually, ALD has been demonstrated as one of the promising techniques for preparing high-quality ZnO-based thin films and LEDs. 
A high-quality ZnO epilayer was grown on the c-Al2O3 substrate by ALD and treated by the post-deposition rapid thermal annealing (RTA) [1][2]. The layer-by-layer growth and low deposition temperature of ALD prevent the formation of columnar structures in the ZnO epilayer. A distorted ZnO layer at the ZnO/sapphire interface, which relaxes the misfit in ZnO, leads to a low threading dislocation density in the ZnO epilayer even though the lattice mismatch between ZnO and c-Al2O3 is up to 18%. Optically pumped stimulated emission with a low threshold value was achieved at room temperature. The good crystal quality and low-threshold stimulated emission indicate that the ALD technique is appropriate for preparing high-quality ZnO epilayers. 
The n-ZnO/p-GaN heterojunction LEDs have been fabricated by the growth of n-type ZnO epilayer using ALD on p-type GaN and the post-deposition RTA treatment [3]. TEM observations also reveal that the ZnO grows in good epitaxial relation with GaN and to almost a perfect single crystal with a very few dislocations. It has been found that the threading dislocations from the underlying layers reduce at the interface between GaN and ZnO, due to the layer-by-layer growth of ALD and the recrystallization during the RTA treatment. At a low injection current, light emission from the Mg acceptor levels in p-GaN is obvious. Increase in the forward bias causes dominant emission from n-ZnO. Competition between the ELs from ZnO and GaN is elucidated to result from the interfacial layer as well as the differences in the light emission efficiency and the carrier concentration in n-ZnO and p-GaN layers. The achievement of UV EL at a low DC injection current from ZnO indicates that the ZnO epilayers grown by the ALD technique are effectually applicable to the next-generation short-wavelength photonic devices. 
Significant white-light EL consisting of a blue light at 450 nm and a broad band around 550 nm was also observed from the n-ZnO/p-GaN hetrojunction LED at reverse breakdown bias [4]. The blue light comes from the Mg acceptor levels in p-GaN, and the broad band may be attributed to the deep-level states near the n-ZnO/p-GaN interface. The mixing color of the EL observed by human eyes located at the coordinate of (0.31,0.36) in chromaticity diagram, which is close to the standard white light. The type II n-ZnO/p-GaN heterojunction facilitates the electron tunneling from the occupied valence band in p-GaN, through the deep-level states near the ZnO/GaN interface, to the empty conduction band in n-ZnO, which is responsible for the reverse breakdown in n-ZnO/p-GaN hetrojunction.
Long-term stable p-type ZnO films were prepared on semi-insulating GaAs substrates using ALD and treated by post-deposition RTA in the ambience of oxygen gas [5]. The RTA treatment facilitates the diffusion of arsenic atoms from GaAs into ZnO and the activation of As-related acceptors, leading to significant decrease in the electron concentration and increase in the hole concentration. High-quality p-type ZnO films were achieved with a hole concentration as high as 3.44E+20 cm-3, resistivity as low as 7.51E-4 Ω-cm, long-term stability up to 180 days, together with a nearly defect-free PL spectrum at room temperature. 
All of these results indicate that ALD is a very promising technique for preparing high-quality ZnO thin films for the LEDs and lasers.
[1] H. C. Chen, M. J. Chen, M. K. Wu, Y.C. Cheng, F. Y. Tsai, “Low-threshold stimulated emission in ZnO thin films grown by atomic layer deposition,” IEEE Journal of Selected Topics in Quantum Electronics 14, 1053-1057 (2008)
[2] H. C. Chen, M. J. Chen, T.C. Liu, J. R. Yang, and M. Shiojiri, “Structure and stimulated emission of a high-quality zinc oxide epilayer grown by atomic layer deposition on the sapphire substrate,” Thin Solid Films 519, 536–540 (2010)
[3] H. C. Chen, M. J. Chen, M. K. Wu, W. C. Li, H.L Tsai, J. R. Yang, H. Kuan and M. Shiojiri, “UV electroluminescence and structure of n-ZnO/p-GaN heterojunction light-emitting diodes grown by atomic layer deposition,” IEEE Journal of Quantum Electronics 46, 265-271 (2010)
[4] H. C. Chen, M. J. Chen, Y. H. Huang, W. C. Sun, W. C. Li, J. R. Yang, H. Kuan, and M. Shiojiri, “White-light electroluminescence from n-ZnO/p-GaN heterojunction light-emitting diodes at reverse breakdown bias,” has been accepted for publication in the IEEE Transactions on Electron Device (2011)
[5] Yung-Chen Cheng, Ying-Shen Kuo, Yun-Hsiu Li, Jing-Jong Shyue, and Miin-Jang Chen, “Stable p-type ZnO films grown by atomic layer deposition on GaAs substrates and treated by post-deposition rapid thermal annealing,” Thin Solid Films 519, 5558–5561 (2011)


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Posted by: Miin-Jang Chen
Co-Authors: Jer-Ren Yang, National Taiwan University; Makoto Shiojiri, Kyoto Institute of Technology
August 14th, 2011 at 21:28

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