C. H. Lee

Evidence of oxygen vacancy enhanced room-temperature ferromagnetism in Co-doped ZnO

The annealing effects on structure and magnetism for Co-doped ZnO films under air, Ar, and Ar∕H2 atmospheres at 250°C have been systematically investigated. Room-temperature ferromagnetism has been observed for the as-deposited and annealed films. However, the saturation magnetization (Ms) varied drastically for different annealing processes with Ms∼0.5, 0.2, 0.9, and 1.5μB∕Co for the as-deposited, air-annealed, Ar-annealed, and Ar∕H2-annealed films, respectively. The x-ray absorption spectra indicate all these samples show good diluted magnetic semiconductor structures. By comparison of the x-ray near edge spectra with the simulation on Zn K edge, an additional preedge peak appears due likely to the formation of oxygen vacancies. The results show that enhancement (suppression) of ferromagnetism is strongly correlated with the increase (decrease) of oxygen vacancies in ZnO. The upper limit of the oxygen vacancy density of the Ar∕H2-annealed film can be estimated by simulation to be about 1×1021cm−3.

ZnO−ZnS Heterojunction and ZnS Nanowire Arrays for Electricity Generation

Vertically aligned ZnO-ZnS heterojunction nanowire (NW) arrays were synthesized by thermal evaporation in a tube furnace under controlled conditions. Both ZnO and ZnS are of wurtzite structure, and the axial heterojunctions are formed by epitaxial growth of ZnO on ZnS with an orientation relationship of [0001](ZnO)//[0001](ZnS). Vertical ZnS NW arrays have been obtained by selectively etching ZnO-ZnS NW arrays. Cathodoluminescence measurements of ZnO-ZnS NW arrays and ZnS NW arrays show emissions at 509 and 547 nm, respectively. Both types of aligned NW arrays have been applied to convert mechanical energy into electricity when they are deflected by a conductive AFM tip in contact mode. The received results are explained by the mechanism proposed for nanogenerator.

Near UV LEDs made with in situ doped p-n homojunction ZnO nanowire arrays.

Catalyst-free p-n homojunction ZnO nanowire (NW) arrays in which the phosphorus (P) and zinc (Zn) served as p- and n-type dopants, respectively, have been synthesized for the first time by a controlled in situ doping process for fabricating efficient ultraviolet light-emitting devices. The doping transition region defined as the width for P atoms gradually occupying Zn sites along the growth direction can be narrowed down to sub-50 nm. The cathodoluminescence emission peak at 340 nm emitted from n-type ZnO:Zn NW arrays is likely due to the Burstein-Moss effect in the high electron carrier concentration regime. Further, the electroluminescence spectra from the p-n ZnO NW arrays distinctively exhibit the short-wavelength emission at 342 nm and the blue shift from 342 to 325 nm is observed as the operating voltage further increasing. The ZnO NW p-n homojunctions comprising p-type segment with high electron concentration are promising building blocks for short-wavelength lighting device and photoelectronics.

Direct growth of high-rate capability and high capacity copper sulfide nanowire array cathodes for lithium-ion batteries

A general solution method for the growth of highly ordered large-scale Cu2S nanowire arrays onto the copper metal current collector substrates has been developed. The electrochemical behaviors of Cu2S nanowire array cathodes for lithium-ion battery applications reveal that they exhibit stable lithium-ion insertion/extraction reversibility, high reversible lithium storage capacity, long cycle life and outstanding rate capability. The superb electrochemical performance can be attributed to the nanowire arrays having increased reaction sites, improved cycle life in the face of mechanical strain and efficient charge transport. With the simplicity of fabrication and good electrochemical properties, the Cu2S nanowire arrays are promising cathode materials for the practical use in the next generation lithium-ion batteries.

Oriented growth of large-scale nickel sulfide nanowire arrays via a general solution route for lithium-ion battery cathode applications

A general solution method for the oriented growth of large-scale Ni3S2nanowire arrays has been developed. The controlled oxidation scheme by combining ethylenediamine-chalcogens and hydrazine in alkali solution has been shown to have great advantages for the fabrication of metal chalcogenides with fewer instrumental limitations. This method is reliable and works in mild template-free conditions for the production of single-crystalline nanowire arrays. It provides a convenient route for the large-scale growth of pure-phase metal chalcogenide nanowire arrays on metal substrates. The electrochemical measurement results of Ni3S2nanowire arrays for lithium-ion battery electrode applications reveal that they have high reversible lithium storage capacity, long cycle life, good cyclic stability and high charge/discharge rate. With the simplicity of fabrication and good electrochemical performance, Ni3S2nanowire arrays are promising cathode materials for lithium-ion batteries.

Origin of ferromagnetism in ZnO∕CoFe multilayers: Diluted magnetic semiconductor or clustering effect?

Epitaxial growth of (0001) oriented [ZnO(20A)∕Co0.7Fe0.3(xA)]25 multilayers (MLs) with nominal thickness x=1, 2 and 5 has been prepared on α-Al2O3 (0001) substrate by ion-beam sputtering. The magnetic properties over a temperature range of 6–350K and structures probing by x-ray absorption spectroscopy (XAS) are reported. Above room-temperature ferromagnetism has been observed for x=1 and x=2 MLs, while superparamagnetic behavior dominates for x=5 ML. The field-cooled magnetization-temperature M(T) curves of x=1 and x=5 MLs can be fitted by a standard three-dimensional (3D) spin-wave and a Curie–Weiss model, respectively. For x=2 ML, however, neither a 3D spin-wave nor a Curie–Weiss model, but a combination of the two fits the M-T curve. The XAS studies together with the magnetic measurements further reveal that x=1 sample behaves as a diluted magnetic semiconductor (DMS) ML, while x=2 ML shows a mixed structure consisting of a minor component of DMS and a major component of CoFe clusters. A predominant cl...

Inversion-channel GaN metal-oxide-semiconductor field-effect transistor with atomic-layer-deposited Al2O3 as gate dielectric

Inversion n-channel GaN metal-oxide-semiconductor field-effect-transistors (MOSFETs) using atomic-layer-deposited Al2O3 as a gate dielectric have been fabricated, showing well-behaved drain I-V characteristics. The drain current was scaled with gate length (varying from 1to16μm), showing a maximum drain current of ∼10mA∕mm in a device of 1μm gate length, at a gate voltage of 8V and a drain voltage of 10V. At a drain voltage of 0.1V, a high Ion∕Ioff ratio of 2.5×105 was achieved with a very low off-state leakage of 4×10−13A∕μm. Both MOSFET and MOS capacitor showed very low leakage current densities of 10−8A∕cm2 at biasing fields of 4MV∕cm. The interfacial density of states was calculated to be (4–9)×1011cm−2eV−1 near the midgap.

Influence of crystal structure on the perpendicular magnetic anisotropy of an epitaxial CoPt alloy

By molecular beam epitaxy CoPt1.1 alloys were simultaneously prepared on Mo seeding layers on Al2O3(11–20), (1–102), and (1–100) substrates, respectively. Distinct crystal structures and chemical ordering of the CoPt1.1 alloys were observed for substrate temperatures of 300 and 400 °C. Structural and magnetic observations for CoPt1.1 alloys grown on separate sapphire substrates show that the appearance of the ordered L11(111) phase results in an enhancement of the perpendicular magnetic anisotropy and Kerr rotations in the CoPt alloys.

Observation of bias-dependent low field positive magneto-resistance in Co-doped amorphous carbon films

We report a considerable intrinsic positive magnetoresistance (PMR) effect in Co-doped amorphous carbon films by radio frequency magnetron sputtering. The kind of PMR effect is bias dependence and its ratio reaches a peak at a particular voltage. At room temperature, the maximum PMR ratio is about 10% among these samples. The x-ray absorption spectroscopy and Raman spectra results support the appearance of the bias-dependent PMR effect strongly depends on the sp2 states and Co dopants. A phenomenological model related to orbital Zeeman splitting has been proposed to describe the resistance, which is controlled by voltage and magnetic field.

Influence of crystal structure on the magnetoresistance of Co/Cr multilayers

Epitaxial Co/Cr multilayers, and single‐crystal Co thin films etc. have been grown on MgO and Al2O3 substrates with Cr and Mo as buffer layers by molecular beam epitaxy technique. From the structure and magnetoresistance studies, we have found that the ferromagnetic anisotropy of resistance (AMR) is strongly influenced by the buffer layer, but with negligible effect due to the variation of the structure of Co films. The AMR of Co film on Cr buffer layer is quite small (0.1%); however, the MR of Co/Cr multilayers is almost one order larger than the AMR of Co film on Cr buffer layer. An enhancement factor of 4 for the MR in Co/Cr multilayers by the interface roughness has been observed. This suggests that the effect due to the spin dependent scattering at the interfacial regions of the superlattice is larger than that due to the spin dependent scattering in the ferromagnetic layers for the MR in the Co/Cr multilayer system.