Recent technological advancement in ZnO heterostructures has expanded the possibility of device functionalities to various kinds of applications. In order to extract novel device functionalities in the heterostructures, one needs to fabricate high quality films and interfaces with minimal impurities, defects, and disorder. With employing molecular-beam epitaxy and single crystal ZnO substrates, the density of residual impurities and defects can be drastically reduced and the optical and electrical properties have been dramatically improved for the ZnO films and heterostructures with Mg x Zn 1- x O. Here, we overview such recent technological advancement from various aspects of application. Towards optoelectronic devices such as a light emitter and a photodetector in an ultraviolet region, the development of p-type ZnO and the fabrication of excellent Schottky contact, respectively, have been subjected to intensive studies for years. For the former, the fine tuning of the growth conditions to make Mg x Zn 1- x O as intrinsic as possible has opened the possibilities of making p-type Mg x Zn 1- x O through NH3 doping method. For the latter, conducting and transparent polymer films spin-coated on Mg x Zn 1- x O was shown to give almost ideal Schottky junctions. The wavelength-selective detection can be realized with varying the Mg content. From the viewpoint of electronic devices, two-dimensional electrons confined at the Mg x Zn 1- x O/ZnO interfaces are promising candidate for quantum devices because of high electron mobility and strong electron-electron correlation effect. These wonderful features and tremendous opportunities in ZnO-based heterostructures make this system unique and promising in oxide electronics and will lead to new quantum functionalities in optoelectronic devices and electronic applications with lower energy consumption and high performance.
The authors gratefully acknowledge S. Akasaka, S. F. Chichibu, J. Falson, T. Fukumura, T. Makino, D. Maryenko, K. Nakahara, M. Nakano, A. Ohtomo, Y. Segawa, K. Tamura, K. Ueno, and H. Yuji. This work was partially supported by JSPS Grant-in-Aid for Scientific Research (S) No. 24226002 and for Young Scientists (A) (Grant No. 23686008), and by JST, CREST as well as by Asahi Glass Foundation. A. T. and M. K. were partly supported by the Japan Society for the Promotion of Science through the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program), initiated by the Council for Science and Technology Policy.
II. HOMOEPITAXIAL GROWTH
A. Reduction of impurities in ZnO substrate
B. Growth conditions of ZnO films
III. GRWOTH OF Mg x Zn 1- x O FILMS
A. Determination of Mg content
B. Mg dependence of physical parameter of Mg x Zn 1-xO
C. Residual carrier density
IV. SCHOTTKY CONTACT
A. PEDOT:PSS/ZnO junction
B. Schottky photodetector
V. LIGHT EMITTER
VI. TWO-DIMENSIONAL ELECTRON GAS
A. Formation mechanism
B. Depth profile of the 2DEG
C. 2DEG density tuning
D. Electron mobility of the 2DEG
E. Evolution of quantum Hall effect
F. Correlated 2DEG in Mg x Zn 1-xO/ZnO
G. Spin coherence time
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