Nuofu Chen

Enhancement of conductivity and transmittance of ZnO films by post hydrogen plasma treatment

We studied the effects of hydrogen plasma treatment on the electrical and optical properties of ZnO films deposited by radio frequency magnetron sputtering. It is found that the ZnO H film is highly transparent with the average transmittance of 92% in the visible range. Both carrier concentration and mobility are increased after hydrogen plasma treatment, correspondingly, the resistivity of the ZnO H films achieves the order of 10(-3) cm. We suggest that the incorporated hydrogen not only passivates most of the defects and/or acceptors present, but also introduces shallow donor states such as the V-O-H complex and the interstitial hydrogen H-i. Moreover, the annealing data indicate that H-i is unstable in ZnO, while the V-O-H complex remains stable on the whole at 400 degrees C, and the latter diffuses out when the annealing temperature increases to 500 degrees C. These results make ZnO H more attractive for future applications as transparent conducting electrodes.

Enhancement of field emission of the ZnO film by the reduced work function and the increased conductivity via hydrogen plasma treatment

The ZnO films deposited by magnetron sputtering were treated by H/O plasma. It is found that the field emission (FE) characteristics of the ZnO film are considerably improved after H-plasma treatment and slightly deteriorated after O-plasma treatment. The improvement of FE characteristics is attributed to the reduced work function and the increased conductivity of the ZnO:H films. Conductive atomic force microscopy was employed to investigate the effect of the plasma treatment on the nanoscale conductivity of ZnO, these findings correlate well with the FE data and facilitate a clearer description of electron emission from the ZnO:H films.

Surface plasmon enhanced ultraviolet emission from ZnO films deposited on Ag∕Si(001) by magnetron sputtering

The ZnO films were grown on Ag/Si(001) substrates by sputtering Ag and ZnO targets successively in a pure Ar ambient. A significant enhancement of ZnO ultraviolet emission and a reduction of its full width of half maximum have been observed while introducing a 100 nm Ag interlayer between ZnO film and Si substrate. Furthermore, a complete suppression of the defect related visible emission was also found for the ZnO/Ag/Si sample. This improved optical performance of ZnO is attributed to the resonant coupling between Ag surface plasmon and ultraviolet emission of ZnO. (c) 2007 American Institute of Physics.

Effects of the morphology of ZnO/Ag interface on the surface-plasmon-enhanced emission of ZnO films

The effects of the surface morphology of Ag on the surface-plasmon-enhanced emission of ZnO films have been studied for a ZnO/Ag/Si system by photoluminescence spectroscopy and atomic force microscopy. The results indicate that the enhancement of ZnO ultraviolet emission is dependent on the deposition conditions of the Ag interlayers. By examining the dependence of the enhancement ratio of surface-plasmon-mediated emission on the characteristic parameters of Ag surface morphology, we found that the surface plasmon coupling to light is determined by both the Ag particle size and density.

Energy band alignment of SiO2/ZnO interface determined by x-ray photoelectron spectroscopy

Thin SiO2 interlayer is the key to improving the electroluminescence characteristics of light emitting diodes based on ZnO heterojunctions, but little is known of the band offsets of SiO2/ZnO. In this letter, energy band alignment of SiO2/ZnO interface was determined by x-ray photoelectron spectroscopy. The valence band offset ΔEV of SiO2/ZnO interface is determined to be 0.93±0.15u2002eV. According to the relationship between the conduction band offset ΔEC and the valence band offset ΔEV: ΔEC=EgSiO2−EgZnO−ΔEV, and taking the room-temperature band-gaps of 9.0 and 3.37 eV for SiO2 and ZnO, respectively, a type-I band-energy alignment of SiO2/ZnO interface with a conduction band offset of 4.70±0.15u2002eV is found. The accurate determination of energy band alignment of SiO2/ZnO is helpful for designing of SiO2/ZnO hybrid devices and is also important for understanding their carrier transport properties.

Magnetic Properties of FePt Nanoparticles Prepared by a Micellar Method

FePt nanoparticles with average size of 9 nm were synthesized using a diblock polymer micellar method combined with plasma treatment. To prevent from oxidation under ambient conditions, immediately after plasma treatment, the FePt nanoparticle arrays were in situ transferred into the film-growth chamber where they were covered by an SiO2 overlayer. A nearly complete transformation of L 10 FePt was achieved for samples annealed at temperatures above 700 °C. The well control on the FePt stoichiometry and avoidance from surface oxidation largely enhanced the coercivity, and a value as high as 10 kOe was obtained in this study. An evaluation of magnetic interactions was made using the so-called isothermal remanence (IRM) and dc-demagnetization (DCD) remanence curves and Kelly–Henkel plots (ΔM measurement). The ΔM measurement reveals that the resultant FePt nanoparticles exhibit a rather weak interparticle dipolar coupling, and the absence of interparticle exchange interaction suggests no significant particle agglomeration occurred during the post-annealing. Additionally, a slight parallel magnetic anisotropy was also observed. The results indicate the micellar method has a high potential in preparing FePt nanoparticle arrays used for ultrahigh density recording media.

Localized-Surface-Plasmon Enhanced the 357 nm Forward Emission from ZnMgO Films Capped by Pt Nanoparticles

The Pt nanoparticles (NPs), which posses the wider tunable localized-surface-plasmon (LSP) energy varying from deep ultraviolet to visible region depending on their morphology, were prepared by annealing Pt thin films with different initial mass-thicknesses. A sixfold enhancement of the 357 nm forward emission of ZnMgO was observed after capping with Pt NPs, which is due to the resonance coupling between the LSP of Pt NPs and the band-gap emission of ZnMgO. The other factors affecting the ultraviolet emission of ZnMgO, such as emission from Pt itself and light multi-scattering at the interface, were also discussed. These results indicate that Pt NPs can be used to enhance the ultraviolet emission through the LSP coupling for various wide band-gap semiconductors.

Tetragonal-tetragonal-monoclinic-rhombohedral transition: Strain relaxation of heavily compressed BiFeO3 epitaxial thin films

BiFeO3 films with in-plane compressive strain of −3.5% were deposited on oxygen-deficient La0.3Sr0.7MnO3-δ buffered SrTiO3(001) substrates. This highly strained BiFeO3 does not relax directly into its rhombohedral parent phase upon increasing the film thickness. Instead, a multi-step path involving structural transitions is observed. The misfit stress is first accommodated by the occurrence of true tetragonal BiFeO3 with c/a ratio of 1.23, then reduced by the transformation to the MC-type monoclinic structure, and finally alleviated through the MC-rhombohedral transition. Moreover, this process enables the formation of strain-driven morphotropic phase boundaries at a stress level much lower than the reported threshold of −4.5%.

Fabrication and temperature dependence of a GaInP/GaAs/Ge tandem solar cell

GaInP/GaAs/Ge tandem solar cells were fabricated by a MOCVD technique. The photoelectric properties of the solar cells were characterized by a current—voltage test method. The dependence of the solar cells characteristics on temperature were investigated from 30 to 170 °C at intervals of 20 °C. Test results indicated that with increasing temperature, Jsc of the cell increased slightly with a temperature coefficient of 9.8 (μA/cm2)/°C. Voc reduced sharply with a coefficient of −5.6 mV/°C. FF was reduced with a temperature coefficient of −0.00063/°C. Furthermore, the conversion efficiency decreased linearly with increasing temperature which decreased from 28% at 30 °C to 22.1% at 130 °C. Also, detailed theoretical analyses for temperature characteristics of the solar cell were given.

Enhancing the Photocurrent of Top-Cell by Ellipsoidal Silver Nanoparticles: Towards Current-Matched GaInP/GaInAs/Ge Triple-Junction Solar Cells

A way to increase the photocurrent of top-cell is crucial for current-matched and highly-efficient GaInP/GaInAs/Ge triple-junction solar cells. Herein, we demonstrate that ellipsoidal silver nanoparticles (Ag NPs) with better extinction performance and lower fabrication temperature can enhance the light harvest of GaInP/GaInAs/Ge solar cells compared with that of spherical Ag NPs. In this method, appropriate thermal treatment parameters for Ag NPs without inducing the dopant diffusion of the tunnel-junction plays a decisive role. Our experimental and theoretical results confirm the ellipsoidal Ag NPs annealed at 350 °C show a better extinction performance than the spherical Ag NPs annealed at 400 °C. The photovoltaic conversion efficiency of the device with ellipsoidal Ag NPs reaches 31.02%, with a nearly 5% relative improvement in comparison with the device without Ag NPs (29.54%). This function of plasmonic NPs has the potential to solve the conflict of sufficient light absorption and efficient carrier collection in GaInP top-cell devices.