ZnO-Based Semiconductors as Building Blocks for Active Devices
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Semiconductors as Building Blocks for Active Devices
Toshio Kamiya and Masashi Kawasaki Abstract This article provides a review of materials and devices of wide-bandgap oxide semiconductors based on ZnO, highlighting the nature of the chemical bond. The electronic structures of these materials are very different from those of conventional covalently bonded semiconductors, owing to the ionic nature of the chemical bonds. Therefore, one needs to design and optimize fabrication processes and structures of active devices containing such materials, taking into account the peculiar defect formation mechanisms. A variety of active devices that have clear advantages over the conventional ones have been demonstrated, for example, ultraviolet light-emitting diodes, quantum Hall devices, and transparent and flexible thin-film transistors with high electron mobility, paving the way for future applications. The reasons behind the successes identify future challenges in research on oxide semiconductors.
Introduction Representative oxide semiconductors are simple oxides such as ZnO, In2O3, and SnO2. These materials are normally n-type conductors because of the electronic structures common to typical oxides: Specifically, the cations have filled d and empty s orbitals, and the conduction and valence bands are mainly formed by the empty metal s and filled O 2p orbitals, respectively. Owing to this electronic structure and ionic crystal nature, these oxides have unique properties such as large bandgaps, wide controllability of carrier concentration, and great flexibility in impurity ion doping while still retaining reasonably high carrier mobility. Such properties allow for the production of transparent electronic devices and circuits.1–6 However, most commercial applications of oxide semiconductors have been limited so far to passive devices such as transparent conducting films, sensors, photocatalysts, and varistors with the forms of polycrystalline films and ceramics. To realize active devices, a well-defined thin-film form is necessary. Here, we classify thin films into four categories:
single-crystalline (virtually free of grain boundaries and dislocations), epitaxially textured (crystallographically aligned but not single-crystalline), polycrystalline (randomly oriented crystallites), and amorphous. This article deals with the two extremes, namely, single-crystalline ZnO and amorphous In–Ga–Zn–O (a-IGZO). These materials are promising not only for such active devices as ultraviolet light-emitting diodes (UV-LEDs)5 and thin-film transistors (TFTs)6 but also for the exploration of new research topics such as defect management, quantum phenomena, and integration with oxides with other functionalities. In this review, the current status of device development is first introduced to make these issues clearer. Then the electronic structures and defects are explained in the context of future potential applications of oxide semiconductors.
Oxide Semiconductor Devices ZnO is a representative oxide semiconductor and has been investig
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