Frontier of transparent oxide semiconductors
Introduction
Transparent conductive oxides (TCOs) have been widely used as transparent “metallic” electrodes for solar cells and flat panel displays including liquid crystal displays (LCDs) and organic light emitting diodes (OLEDs) [1] because of their unique properties of coexistence of “optical transparency in visible region” and “controllability of electronic conduction from insulator to metal”. However, only passive functions of TCO materials have been utilized in these applications and there have been no practical optoelectronic applications by using their active functions so far. This definitely comes from the lack of good p-type TCOs, as suggested from the fact that a variety of active functions in semiconductors originate from p–n junction. In 1997, we reported for the first time CuAlO2 thin films as a p-type TCO along with a chemical design concept for exploration of p-type TCOs [2]. After that, a series of p-type TCOs based on Cu+-bearing oxides such as CuGaO2 [3] and SrCu2O2 [4] have been found as a consequence of material exploration efforts following the design concept. Discovery of the p-type TCOs has led to TCOs to a frontier in semiconductors, “transparent oxide semiconductors (TOSs)”. We have been making intensive efforts to open the new frontier of “transparent oxide optoelectronics” utilizing active function of TOSs. First of all, we think it very important to explore “better performance and novel functions in TOS materials” such as a further enhancement of conductivity in TCO thin films, expansion of transparent wavelength towards deep-UV region, better controllability of p-type conduction which may be realized either in TOS materials containing Cu+ ions or in new category of TOSs developed based on novel design concepts for p-type conduction, enhancement of light emission capability in short wavelength. It is also important to improve TOS device processes, especially film deposition process to grow epitaxial films of TOS. The process should be applicable to TOSs having complicated compositions and crystal structures. Finally, fabrication of TOS devices, whose performance should be unique derived from inherent features of TOSs and thus compete with conventional semiconductors devices, is essential to make “the oxide optoelectronics” really practical. In line with these considerations, we will introduce in this paper (1) highly conductive ITO thin film with atomically flat surface, (2) p-type TOS material ZnRh2O4, (3) deep-ultraviolet (DUV) transparent conductive oxide β-Ga2O3 thin film, (4) electrochromic oxyfuolide NbO2F, (5) reactive solid-phase epitaxy and its application to InGaO3(ZnO)m, (6) p-type semiconductor LaCuOS1−xSex epitaxial films capable of emitting UV- and purple-light, (7) p–n homojunction based on bipolar CuInO2, (8) transparent FET based on single-crystalline InGaO3(ZnO)5 films, and (9) UV-light emitting diode based on p–n heterojunction.
Section snippets
Highly conductive ITO thin film [5,6]
Sn3+ doped In2O3 (ITO) is the best known TCO and ITO films having a conductivity of ∼103 S cm−1 prepared by a sputtering deposition technique are commercially used as transparent electrodes in various devices such as LCD, PDP, OLED and solar cell. Although higher conductivity is more favorable in these devices, the maximum electrical conductivity attainable in ITO films had not been clarified yet. In order to examine the potential of ITO thin films as TCO, we have grown single-crystalline ITO
Reactive solid-phase epitaxy [16–18]
It is hard to grow single-crystalline thin film of complex oxides, especially those having layered structures or “natural superlattice” structures by a conventional vapor-phase epitaxy. Our finding, “reactive solid-phase epitaxy (R-SPE)”, is a unique and practical growth method for these compounds. Thermal annealing of a bi-layer on a substrate, composed of an extremely thin epilayer of an appropriate simple oxide or metal and an amorphous or polycrystalline layer of the complex oxide, leads to
p–n homojunction based on bipolar CuInO2 [19,20]
Our success in the developments of p-type delaffosite CuMO2 (M=Al, Ga, and Sc) and n-type AgInO2 makes it possible to form a heterojunction between materials having the same crystal structure (homostructual p–n junction). However, a junction composed of the same material, that is a homojunction is more desirable. Since several Cu+-delaffosite compounds are known to exhibit p-type conduction, the issue to be resolved is how to realize n-type conduction in these compounds. We selected CuInO2 as a
Conclusion
The discovery of p-type TCOs makes an epoch to open a world of “transparent oxide semiconductor (TOS)”. As fruits in the world, we expect innovative devices including blue and UV-LED/LD, transparent oxide FET and UV-sensor, which are highly reliable and environment friendly. To realize these applications, development of new TOS materials, especially p-type TOSs, is still a critical issue. Efforts in expanding the frontier of TOSs together with technology progresses in adjacent areas make the
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