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Y.-Z. Yoo

Bio: Y.-Z. Yoo is an academic researcher from Tokyo Institute of Technology. The author has contributed to research in topics: Pulsed laser deposition & Epitaxy. The author has an hindex of 7, co-authored 7 publications receiving 865 citations.

Papers
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Journal ArticleDOI
TL;DR: Combinatorial laser molecular-beam epitaxy method was employed to fabricate epitaxial ZnO thin films doped with all the 3d transition metal (TM) ions in a high throughput fashion as discussed by the authors.
Abstract: Combinatorial laser molecular-beam epitaxy method was employed to fabricate epitaxial ZnO thin films doped with all the 3d transition metal (TM) ions in a high throughput fashion The solubility behavior of TM ions was discussed from the viewpoints of the ionic radius and valence state The magneto-optical responses coincident with absorption spectra were observed for Mn- and Co-doped samples Cathodoluminescence spectra were studied for Cr-, Mn-, Fe-, and Co-doped samples, among which Cr-doped ZnO showed two sharp peaks at 297 eV and 371 eV, respectively, at the expense of the exciton emission peak of pure ZnO at 325 eV Different magnetoresistance behavior was observed for the samples codoped with n-type carriers Ferromagnetism was not observed for Cr- to Cu-doped samples down to 3 K

587 citations

Journal ArticleDOI
TL;DR: In this article, an alternating deposition (AD) method was employed for doping ZnO(Zn1−xCoxO) and Co, codoped Zn2+ in order to enhance homogeneous substitution.
Abstract: Co-doped ZnO(Zn1−xCoxO) and Co, Al codoped ZnO(Zn1−xCoxO:Al) films were grown on c-plane sapphire (0001) substrates by pulsed laser deposition using a KrF excimer laser. In order to enhance homogeneous substitution of Co2+ for Zn2+ in the ZnO film, an alternating deposition (AD) method was employed for doping. ZnO films doped with the same elements were grown without employing the AD method under the same fabrication conditions to compare with the corresponding AD films. Despite Co incorporation, the Zn1−xCoxO film by AD methods showed better crystallinity than pure ZnO film. Also, the crystallinity of Zn1−xCoxO:Al films by AD methods was better than those of Zn1−xCoxO and ZnO:Al films by conventional doping methods. Root mean square roughnesses of the films by AD methods were less than 1.6 nm. Absorption peaks caused by d–d transitions were observed at 1.88, 2.01, and 2.19 eV in Zn1−xCoxO films. The electric conductivity of Zn1−xCoxO:Al film by AD methods was comparable to that of ZnO:Al film by conventi...

121 citations

Journal ArticleDOI
TL;DR: In this article, ZnS films were grown on Si (100) at high temperatures by pulsed laser deposition using a KrF excimer laser and the growth temperature was varied from 200 to 700
Abstract: ZnS films were grown on Si (100) at high temperatures by pulsed laser deposition using a KrF excimer laser. The growth temperature was varied from 200 to 700 °C and all films were found to have a specific preferential orientation. With increasing Ts, growth rate decreased but the quality of the film improved. The highest quality ZnS film was obtained at 700 °C. The presence of ZnS+ ions among the ablation products of a ZnS target was verified by laser desorption time of flight mass spectroscopy measurements. ZnO was formed by thermal oxidation of ZnS and the films showed strong near band-edge emission at 3.26 eV.

56 citations

Journal ArticleDOI
TL;DR: In this article, the s−d exchange interaction between the conducting s electron spins and the d electron spins localized at the magnetic transition metal impurities was investigated to investigate the s-d exchange effect.
Abstract: Epitaxial ZnO thin films co-doped with 3d transition metal (TM) (TM=Cr, Mn, Fe, Co, Ni and Cu) and 1 mol % Al were fabricated as a series of oxide-diluted magnetic semiconductors by pulsed-laser-deposition method. Magnetoresistance (MR) of the films was measured to investigate the s–d exchange interaction between the conducting s electron spins and the d electron spins localized at the magnetic TM impurities. A variety of MR behaviors were observed depending on the different TM impurities. It is deduced that the negative MR behavior in the vicinity of zero field is originated from an electron weak-localization effect. Caused by the s–d exchange interaction, the increase of Thomas–Fermi radius Rs and the decrease of spin-disorder scattering with increasingly aligned spins of the TM ion impurities are responsible respectively for the positive and negative MR in the higher magnetic filed.

43 citations

Journal ArticleDOI
TL;DR: In this article, a high throughput combinatorial laser molecular-beam epitaxy method was used for the fabrication of epitaxial ZnO films doped with Co and the solubility limit of Co was determined.

36 citations


Cited by
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Journal ArticleDOI
TL;DR: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature.
Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

10,260 citations

Journal ArticleDOI
TL;DR: In this paper, a review of recent results in developing improved fabrication processes for ZnO devices with the possible application to UV light emitters, spin functional devices, gas sensors, transparent electronics, and surface acoustic wave devices is given.
Abstract: A review is given of recent results in developing improved fabrication processes for ZnO devices with the possible application to UV light emitters, spin functional devices, gas sensors, transparent electronics, and surface acoustic wave devices. There is also interest in integrating ZnO with other wide band-gap semiconductors, such as the AlInGaN system. In this article, we summarize recent progress in controlling n- and p-type doping, materials processing methods, such as ion implantation for doping or isolation, Ohmic and Schottky contact formation, plasma etching, the role of hydrogen in the background n-type conductivity of many ZnO films, and finally, the recent achievement of room-temperature ferromagnetism in transition-metal (Mn or Co)-doped ZnO. This may lead to another class of spintronic devices, in which the spin of the carriers is exploited rather than the charge as in more conventional structures.

656 citations

Journal ArticleDOI
TL;DR: In this article, the lattice constant of c axis of wurtzite Zn1−xCoxO follows Vergard's law for 0
Abstract: We report on the high-temperature ferromagnetism in Co-doped ZnO films fabricated by the sol–gel method above 350 K. The lattice constant of c axis of wurtzite Zn1−xCoxO follows Vergard’s law for 0

644 citations

Journal ArticleDOI
TL;DR: In this article, a review of the recent progress in the theoretical and experimental studies of ZnO-and GaN-based DMSs is presented, focusing on the structural, optical, and magnetic properties of these materials.
Abstract: The observation of ferromagnetism in magnetic ion doped II–VI diluted magnetic semiconductors (DMSs) and oxides, and later in (Ga,Mn)As materials has inspired a great deal of research interest in a field dubbed “spintronics” of late, which could pave the way to exploit spin in addition to charge in semiconductor devices. The main challenge for practical application of the DMS materials is the attainment of a Curie temperature at or preferably above room temperature to be compatible with junction temperatures. Among the studies of transition-metal doped conventional III–V and II–VI semiconductors, transition-metal-doped ZnO and GaN became the most extensively studied topical materials since the prediction by Dietl et al., based on mean field theory, as promising candidates to realize a diluted magnetic material with Curie temperature above room temperature. The underlying assumptions, however, such as transition metal concentrations in excess of 5% and hole concentrations of about 1020 cm−3, have not gotten as much attention. The particular predictions are predicated on the assumption that hole mediated exchange interaction is responsible for magnetic ordering. Among the additional advantages of ZnO-and GaN-based DMSs are that they can be readily incorporated in the existing semiconductor heterostructure systems, where a number of optical and electronic devices have been realized, thus allowing the exploration of the underlying physics and applications based on previously unavailable combinations of quantum structures and magnetism in semiconductors. This review focuses primarily on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs. One of the desirable outcomes is to obtain carrier mediated magnetism, so that the magnetic properties can be manipulated by charge control, for example through external electrical voltage. We shall first describe the basic theories forwarded for the mechanisms producing ferromagnetic behavior in DMS materials, and then review the theoretical results dealing with ZnO and GaN. The rest of the review is devoted to the structural, optical, and magnetic properties of ZnO- and GaN-based DMS materials reported in the literature. A critical review of the question concerning the origin of ferromagnetism in diluted magnetic semiconductors is given. In a similar vein, limitations and problems for identifying novel ferromagnetic DMS are briefly discussed, followed by challenges and a few examples of potential devices.

616 citations

Journal ArticleDOI
Feng Pan1, Cheng Song1, X. J. Liu1, Yuchao Yang1, Fei Zeng1 
TL;DR: In this paper, the magnetic properties and intrinsic ferromagnetism of transition-metal (TM)-doped ZnO films, which are typical diluted magnetic oxides used in spintronics, are discussed.
Abstract: This review article first presents a summary of current understanding of the magnetic properties and intrinsic ferromagnetism of transition-metal (TM)-doped ZnO films, which are typical diluted magnetic oxides used in spintronics. The local structure and magnetic behavior of TM-doped ZnO are strongly sensitive to the preparation parameters. In the second part, we discuss in detail the effects of doping elements and concentrations, oxygen partial pressure, substrate and its orientation and temperature, deposition rate, post-annealing temperature and co-doping on the local structure and subsequent ferromagnetic ordering of TM-doped ZnO. It is unambiguously demonstrated that room-temperature ferromagnetism is strongly correlated with structural defects, and the carriers involved in carrier-mediated exchange are by-products of defects created in ZnO. The third part focuses on recent progress in TM-doped ZnO-based spintronics, such as magnetic tunnel junctions and spin field-effect transistors, which provide a route for spin injection from TM-doped ZnO to ZnO. Thus, TM-doped ZnO materials are useful spin sources for spintronics.

614 citations