Changhong Chen

Structural, electrical, and terahertz transmission properties of VO2 thin films grown on c-, r-, and m-plane sapphire substrates

The structure, metal-insulator transition (MIT), and related Terahertz (THz) transmission characteristics of VO2 thin films obtained by sputtering deposition on c-, r-, and m-plane sapphire substrates were investigated by different techniques. On c-sapphire, monoclinic VO2 films were characterized to be epitaxial films with triple domain structure caused by β-angle mismatch. Monoclinic VO2 β angle of 122.2° and the two angles of V4+–V4+ chain deviating from the am axis of 4.4° and 4.3° are determined. On r-sapphire, tetragonal VO2 was determined to be epitaxially deposited with VO2 (011)T perpendicular to the growth direction, while the structural phase transformation into lower symmetric monoclinic phase results in (2¯11) and (200) orientations forming a twinned structure. VO2 on m-sapphire has several growth orientations, related with the uneven substrate surface and possible inter-diffusion between film and substrate. Measurements of the electrical properties show that the sample on r-sapphire has MIT ...

VO2 multidomain heteroepitaxial growth and terahertz transmission modulation

We report the epitaxial relationship of VO2 thin-films on c-plane sapphire and their terahertz transmission modulation with temperature. The films exhibit a triple-domain structure caused by the lattice mismatch between monoclinic VO2 and sapphire hexagon. The epitaxial relationship is determined to be VO2[010]∥Al2O3[0001] and VO2(2¯02)∥Al2O3{112¯0}, with the in-plane lattice mismatch of 2.66% (tensile) along [2¯02] and the out-of-plane lattice mismatch of −2.19% (compressive). Terahertz measurements revealed that VO2 films have over fourfold change in transmission during the metal-insulator transition, indicating a strong potential for terahertz wave switching and modulation applications.

TiO2 nanotubes infiltrated with nanoparticles for dye sensitized solar cells

We present a detailed study of the infiltration of titanium dioxide (TiO(2)) nanotubes (NTs) with TiO(2) nanoparticles (NPs) for dye sensitized solar cells (DSSCs). The aim is to combine the merits of the NPs high dye loading and high light harvesting capability with the NTs straight carrier transport path and high electron collection efficiency to improve the DSSC performance. On infiltrating NTs with TiCl(4) solution followed by hydrothermal synthesis, 10 nm size NPs were observed to form a conformal and dense layer on the NT walls. Compared with the bare NT structure, dye loading of this mixed NT and NP structure is more than doubled. The overall photon conversion efficiencies of the fabricated DSSCs are improved by 152%, 107%, and 49% for 8, 13, and 20 µm long NTs, respectively. Electron transport and recombination parameters were extracted based on electrochemical impedance spectroscopy measurements. Although a slight reduction of electron lifetime was observed in the mixed structures due to enhanced recombination with a larger surface area, the diffusion length is still significantly longer than the NT length used, suggesting that most electrons are collected. In addition to dye loading and hence photocurrent increment, the photovoltage and filling factor were also improved in the mixed structure due to a low serial resistance, leading to the enhancement of the overall efficiency.

Gate-field-induced phase transitions in VO2: Monoclinic metal phase separation and switchable infrared reflections

In a metal-oxide-semiconductor VO2 active layer under uniaxial stress, gate-field-induced phase transitions are revealed by strongly field-dependent Raman scattering and infrared reflections. A metal-insulator transition (MIT) is demonstrated by a strongly correlated monoclinic metal phase separation that percolates, thereby making the reflections switchable. In addition, the MIT occurs at a gate voltage around 3.36V, much lower than the threshold of a structural phase transition (SPT). Hence, the MIT is easily controlled by the gate field to avoid the SPT-caused fatigue and breakdown in high-speed operation.

Influence of defects on structural and electrical properties of VO2 thin films

We present the structural and electrical properties of (011) preferred polycrystalline (Poly) and multidomain (020) epitaxial (Epi) VO2 thin films grown at different temperature (Ts) and on different substrates with variable defects. These defects cause variation in strain, metal-insulator transition (MIT) temperature (TMIT), activation energy (ΔEa), and charge carrier type in insulating phase. Both the Poly- and Epi-VO2 behave n-type conductivity when grown at relative low TS. As TS increases, defects related acceptor density increases to alter conductivity from n- to p-type in the Poly-VO2, while in the Epi-VO2 donor density increases to maintain n-type conductivity. Moreover, the strain along monoclinic am axis dramatically reverses from tensile to compressive in both the Poly- (848 K < TS < 873 K) and Epi-VO2 (873 K < TS < 898 K), and eventually approaches to a constant in the Poly-VO2 (TS ≥ 898 K) in particular. TMIT decreases with increasing the carrier density independent of the conductive type in ...

Changes in VO2 band structure induced by charge localization and surface segregation

Vanadium vacancies introduce acceptor doping with hole localization, while oxygen vacancies cause electron localization and donor doping. As deposition temperature increases, donor concentration stays constant, whereas acceptor concentration significantly increases, leading to enhanced (011) lattice-plane compression and surface segregation. Localized charges result in shifts of O 1s and V4+ 2p core levels toward higher binding energies, and O 2p and V4+ 3d valence bands toward the Fermi level, but egπ bands lifting and a1g bands splitting energies are both insensitive to charge localization. Particularly, band-gap energy decreases with increasing V–V pair distance, and is significantly reduced by band tailing.

Tuning the properties of VO2 thin films through growth temperature for infrared and terahertz modulation applications

Results are reported on tuning the electrical and optical properties of sputter-deposited vanadium dioxide (VO2) thin films through control of substrate growth temperature (Ts). As Ts increases from 550 to 700 °C, the morphology changes from granular to smooth film and finally to rough film. X-ray diffraction shows the presence of VO2 along with additional weak features related to the presence of non-stoichiometric phases. Electrical measurements show the phase transition to change from abrupt to gradual as both the below- and above-transition resistivities vary with Ts. The transition and hysteresis dependences observed in electrical resistivity are similarly observed in infrared transmission. Terahertz transmission measurements show that high conductivity above the phase transition is more important in achieving high modulation depth than obtaining high resistivity below the transition. We attribute changes in the electrical and optical properties to the formation of V and O vacancies, which result in diverse valence states from the ideal V4+ of VO2. Low Ts produces material with V5+ states resulting in higher resistivity in both the insulating and metallic phases. Alternatively, high Ts introduces material with V3+ states leading to lower resistivity in the insulating phase but slightly higher resistivity in the metallic phase.

Optical phonons assisted infrared absorption in VO2 based bolometer

Optical phonons assisted infrared absorption in VO2 based bolometer is demonstrated to be free, low, or over damping oscillation over different spectral ranges depending on the passivation thickness. In particular, it will become saturated due to the over damping oscillation in the spectral range corresponding to the absorption bands from strong phonon vibrations. The device reaches a peak absorbance of 99.9% at wavelength of 9.1μm and shows a broadband absorption, independent of metal-insulator transition and radiation incident angle up to 30° in the long wavelength (8–14μm) infrared region.

Partial strain relaxation by stacking fault generation in InGaN multiple quantum wells grown on (11¯01) semipolar GaN

We have investigated the structural properties and relaxation phenomenon of InGaN multiple quantum wells (QWs) on (11¯01) semipolar GaN templates grown on patterned (001) silicon substrates by selective area growth technique. Our studies by transmission electron microscopy and x-ray diffraction reciprocal space mapping reveal that QWs emitting light at 540 nm experience significant strain relaxation along the in-plane [11¯02¯] direction by the generation of an array of basal stacking faults (BSF). The generation of BSFs in 540 nm QWs could be an important factor limiting its luminescence efficiency.

Effect of free-carrier concentration on the phase transition and vibrational properties of VO2

The effects of native defect doping concentration on the phase transition properties of vanadium dioxide thin films are investigated. The onset temperature of the metal-insulator transition is found to depend on the free-carrier concentration and to correlate with an abrupt change in the temperature dependence of the vibrational energies of the V-O related Raman band. A phase diagram is proposed identifying insulating, intermediate, and conducting regimes. The intermediate region is attributed to a mixed phase. V C 2011 American Institute of Physics. [doi:10.1063/1.3626032] Vanadium dioxide exhibits a reversible, first-order metal insulator phase transition (MIT) at temperature TMIT � 350 K. 1 The transformation is associated with a structural phase transition (SPT) from low-temperature monoclinic, in which the material may be characterized as either a narrow gap insulator or semiconductor, to high-temperature tetragonal rutile, in which the material is metallic. 2 Upon undergoing the MIT, the electrical conductivity (r )o f VO2 changes by as much as 4-5 orders of magnitude and the optical properties exhibit significant variations. 1 Several factors are known to affect the phase transition temperature of VO2, and various electrical, thermal, and optical devices have been proposed based on this material. 3‐6 Despite extensive research, little work has been devoted to the effects of native defect-related doping on the properties of VO2. 7 We describe electrical and Raman studies of VO2 deposited on silicon substrates at different temperatures TS. The n- and p-type behaviors are attributed to O and V vacancies, respectively. 7 We find that variations in free-carrier concentration have a profound effect on the temperature at which the MIT begins and on the vibrational properties. We develop a phase diagram for doped VO2 based on the conductivity and Raman spectrum across the phase transition.