Chen Chang-Le

Rectifying and Photovoltage Properties of ZnO:A1/p-Si Heterojunction

An A1-doped ZnO/p-Si heterojunction is fabricated by a laser molecular beam epitaxy technique. The abnormally high ideality factors (n ≫ 2) of the prepared heterojunction are observed in the interim bias voltage range. A theoretical model is proposed to understand the much higher ideality factor of the special heterojunction diode. The ZnO:A1 film shows metal-like conductivity with the electrical resistivity about 6.56 × 10−4 Ω·cm at room temperature. The temperature dependence of the photovoltage indicates that the photovoltaic effect of the A1-doped ZnO based heterojunction can be changed by the intrinsic metal-semiconductor transition at 120 K.

Leakage Current and Photovoltaic Properties in a Bi2Fe4O9/Si Heterostructure

A heterostructure composed of a Bi2Fe4O9 film and an n-type Si substrate is fabricated. The characteristics of leakage current density versus electric field are investigated and the leakage current density is about 6×10−6 A/cm2 at an electric field of 200 kV/cm at 300 K. A strong photovoltaic effect is observed when the heterostructure is exposed to a laser pulse with a wavelength of 532 nm and a power of 6 mW/mm2. It is found that the peak photovoltages initially increase with decreasing temperature, followed by a decrease at T < 210 K. These results reveal that the heterostructure is a promising candidate for photovoltaic devices that are compatible with Si integrated circuits.

Effect of rotating magnetic field on the solidification microstructures of Pb-Sn alloys

The influence of the rotating magnetic field (RMF) on the solidification process of Pb-Sn binary alloys is studied by comparing the solidification microstructures under the common condition and RMF condition. It is found that the RMF can completely eliminate the gravity induced macrosegregation, and result in dendrite fragmentation and promote solute diffusion velocity. These differences are regarded as the effect of complicated melt flow caused by RMF. Moreover, when the content of the primary phase is small, many spherical microstructures form under the RMF condition. The analyses indicate that these special microstructures are likely the conjunction action of melt flow and concentration and temperature field uniformity caused by RMF.

High-pressure phonon dispersion of copper by using the modified analytic embedded atom method

By using the Born—von Karman theory of lattice dynamics and the modified analytic embedded atom method, we reproduce the experimental results of the phonon dispersion in fcc metal Cu at zero pressure along three high symmetry directions and four off-symmetry directions, and then simulate the phonon dispersion curves of Cu at high pressures of 50, 100, and 150 GPa. The results show that the shapes of dispersion curves at high pressures are very similar to that at zero pressure. All the vibration frequencies of Cu in all vibration branches at high pressures are larger than the results at zero pressure, and increase correspondingly as pressure reaches 50, 100, and 150 GPa sequentially. Moreover, on the basis of phonon dispersion, we calculate the values of specific heat of Cu at different pressures. The prediction of thermodynamic quantities lays a significant foundation for guiding and judging experiments of thermodynamic properties of solids under high pressures.

The Photovoltaic Properties of BiFeO3La0.7Sr0.3MnO3 Heterostructures

An epitaxial BiFeO3/La0.7Sr0.3MnO3 (BFO/LSMO) multiferroic heterostructure is grown on an LaAlO3 (001) substrate by laser molecular beam epitaxy, and its photovoltaic properties are investigated. It is found that the photocurrent is significantly increased under illumination, and the short-circuit photocurrent has a linear relationship with the laser intensity. Furthermore, when the ferroelectric polarization of the BFO layer is switched, the short-circuit photocurrent and open-circuit voltage can be switched. These results are discussed by considering the contributions from the ferroelectric polarization and the electrode/film interface.

Dendrite to symmetry-broken dendrite transition in directional solidification of non-axially oriented crystals

In this paper, the morphological transition from dendrite to symmetry-broken dendrite is investigated in the directional solidification of non-axially-oriented crystals using a quantitative phase-field model. The effects of pulling velocity and crystal orientation on the morphological transition are investigated. The results indicate the orientation dependence of the symmetry-broken double dendrites. A dendrite to symmetry-broken dendrite transition is found by varying the pulling velocity at different crystal orientations and the symmetry-broken multiple dendrites emerge as a transition state for the symmetry-broken double dendrites. The state region during the transition can be well characterized through the variations of the characteristic angle and the average primary dendritic spacing.

Anomalous specific heat of charge-ordered La0.5Ca0.5MnO3

An exact treatment of the double exchange-Holstein model with an electron-phonon interaction γ coupled to magnetism is used to compute the heat capacity. The model is performed combining a mean-field approximation for the double exchange interaction and the Lang–Firsov transformation for the electron-phonon interaction. The calculated results of heat capacity point to the dominance of the lattice contribution, especially in the ferromagnetic regime. This behavior is in qualitative agreement with experimental findings.

Ferroelectricity in hexagonal YFeO3 film at room temperature

In this paper we report the leakage current, ferroelectric and piezoelectric properties of the YFeO3 film with hexagonal structure, which was fabricated on Si(111) substrate by a simple sol-gel method. The leakage current test shows good characteristics as the leakage current density is 5.4×10−6 A/cm2 under 5 V. The dominant leakage mechanism is found to be an Ohmic behavior at low electric field and space-charge-limited conduction at high electric field region. The P–E measurements show ferroelectric hysteresis loops with small remnant polarization and coercive field at room temperature. The distinct and switchable domain structures on the nanometer scale are observed by piezoresponse force microscopy, which testifies to the ferroelectricity of the YFeO3 film further.

Effect of Zn Doping on Transport Properties of La2/3 Sr1/3 Mn1 - x Znx O3 Films

Abstract La2/3Sr1/3Mn1 - xZnxO3 films (x = 0.05, 0.1, 0.3, and 0.5) were prepared using magnetron sputtering method, and the effect of Zn doping on transport properties of the films was studied. An analysis of X-ray diffraction showed that the main phase of the bulk target was orthorhombic and the films had better epitaxial character. It was found that the films with x =0.05 and x =0.1 exhibited typical insulator-metal transition. No transition of the films with x≥0.3 was observed and the dominant transport was variable-range hopping due to observable secondary phase ZnO. These could be attributed to the Zn doping effect on manganites.

Photoinduced Resistance Change in an Oxygen-Deficient La0.9Sr0.1MnO3−δThin Film

Photoinduced resistance change (ΔR/R) in an oxygen-deficient La0.9Sr0.1MnO3−δ thin film is studied. At room temperature, the resistance change of about 30% and response time of about 75 ns are observed under the illumination with a 532 nm laser pulse of 7 ns and light power of 750 mW. It is also found that ΔR/R changes with the light power. The phenomena are explained in terms of the photoinduced hole carriers and localized insulator-to-metal transition, which may have potential applications in optoelectronic devices.