Deyan Shen

Characterization of biaxial stress and its effect on optical properties of ZnO thin films

Biaxial stress of ZnO film deposited on quartz was measured by side-inclination x-ray diffraction technique, indicating that the film is subjected to a tensile stress. One part of the stress is induced by thermal mismatch between the ZnO and the quartz and increases with annealing temperature, while another part results from lattice mismatch and is about 1.03GPa. The optical band gap of the ZnO film shows a blueshift with increasing biaxial tensile stress, opposed to the change of the band gap with biaxial tensile stress for GaN. The mechanism of the stress-dependent band gap is suggested in the present work.

Nitrogen-related recombination mechanisms in p-type ZnO films grown by plasma-assisted molecular beam epitaxy

The recombination mechanisms of nitrogen-related emissions in p-type ZnO films were investigated by photoluminescence (PL) measurements. The enhanced peak at 3.260 eV was confirmed as a donor-acceptor pair (DAP) transition and the emission around 3.310 eV was assigned to the free electron to acceptor (FA) recombination by studying the temperature evolution of DAP and FA luminescence, calculating the energy level of the corresponding nitrogen acceptor, and measuring the decay time of DAP transition. By comparing the PL spectra of the samples with various hole concentrations, it can be found that as the hole concentration increases, the DAP peak significantly dominates the spectra. In addition, the acceptor binding energy is estimated to be about 120 meV from the FA transition, which is in good agreement with the value calculated by a hydrogenic acceptor model.

Realization of p-type conduction in undoped MgxZn1−xO thin films by controlling Mg content

Undoped MgxZn1−xO thin films with Mg content of 0⩽x⩽0.20 were grown on c-sapphire substrate by plasma-assisted molecular beam epitaxy. The MgxZn1−xO shows n-type conduction in Mg content of x⩽0.05, and the carrier concentration decreases slowly from 1018to1017cm−3 with increasing Mg content. However, as x⩾0.10, the MgxZn1−xO begins to show p-type conduction, and the carrier concentration goes down sharply to 1015cm−3 firstly and then increases slowly with increasing Mg content from 1015to1016cm−3. The mechanism of transformation from n to p type and change of the carrier concentrations with Mg content were investigated by photoluminescence and absorption measurements as well as first-principle calculation.

On the nature of the carriers in ferromagnetic FeSe

The optical and electrical properties of FeSe thin films are studied by both optical absorption and Hall measurements, which suggest that ferromagnetic FeSe is a metal instead of a semiconductor. No absorption gap is observed in the whole spectrum range from far infrared to ultraviolet. Temperature dependent transport measurement indicates that FeSe has a resistivity about 10−3Ωcm. It is also found that there is a transition from n-type conductivity at low temperatures to p-type conductivity at higher temperatures in FeSe, which is attributed to the two-carrier transport nature and the thermal activation of localized carriers in the thin film.The optical and electrical properties of FeSe thin films are studied by both optical absorption and Hall measurements, which suggest that ferromagnetic FeSe is a metal instead of a semiconductor. No absorption gap is observed in the whole spectrum range from far infrared to ultraviolet. Temperature dependent transport measurement indicates that FeSe has a resistivity about 10−3Ωcm. It is also found that there is a transition from n-type conductivity at low temperatures to p-type conductivity at higher temperatures in FeSe, which is attributed to the two-carrier transport nature and the thermal activation of localized carriers in the thin film.

Electronic and magnetic properties of FeSe thin film prepared on GaAs (001) substrate by metal-organic chemical vapor deposition

FeSe film was prepared on GaAs (001) substrate by low pressure metal-organic chemical vapor deposition. The x-ray diffraction measurement indicated that the sample was preferentially oriented with tetragonal structure. The structure relationship between FeSe epilayer and GaAs (001) substrate has been studied. The critical behavior in the temperature-dependent resistivity at ∼290K is close to the Curie temperature, which confirmed that the transformation from ferromagnetism to paramagnetism could be responsible for the critical behavior.

Study of multiple melting behaviour of syndiotactic polystyrene in β‐crystalline form

A series of syndiotactic polystyrene (SPS) samples in β-crystalline form were prepared by cooling from the melt at various rates. The effects of cooling rate from the melt, DSC heating rate and annealing on the multiple melting behaviours of β crystals were investigated by differential scanning calorimetry (DSC) and temperature modulated differential scanning calorimetry (TMDSC), from which the nature of the multiple melting behaviour was determined. The two melting endotherms of β-form crystals were considered to arise from the occurrence of simultaneous melting, recrystallization and remelting processes in the melting region. It is suggested that the lower melting endotherm is due to the melting of imperfect β crystals originally present in the sample, whereas the higher melting endotherm comes from the melting of recrystallized SPS crystals, ie more perfect β crystals that formed during the DSC scanning process. © 2000 Society of Chemical Industry

The effect of water absorption on the physical ageing of amorphous poly(ethylene terephthalate) film

Abstract Amorphous poly(ethylene terephthalate) (PET) films with different thermal histories were annealed at 30°C in a desiccator, in a humid atmosphere and in liquid water respectively, for investigation of enthalpy relaxation by differential scanning calorimetry (DSC). For the quenched sample annealed in a humid atmosphere, the evolution of the endothermic peak near the glass transition temperature ( T g ) with annealing time is remarkable. For the aged samples further annealed in the humid atmosphere and water, the shape of the endothermic peak changed from a single peak into broad double peaks with annealing time. However no such change of the endothermic peak can be observed for both the quenched and the aged samples further annealed in the desiccator. The experimental results showed that the change of the DSC curve was reversible, involved only with the physical process. The results may indicate that some ordered structure formed during the physical ageing process.

Carrier tunneling in a novel asymmetric quantum well structure

Based on the carrier tunneling effect, a novel asymmetric quantum well is designed to improve the carrier injection for a type I quantum well. Two type II quantum wells were chosen to provide the electrons and holes for the type I quantum well via carrier resonant tunneling, respectively. Under the condition that both electrons and holes have fast tunneling rate, the carrier recombination in the type I quantum well can be enhanced. In order to optimize the structure, the small coupling barrier width, energy level resonance and small energy level width should be chosen simultaneously.

Electrical properties of N-doped ZnO grown on sapphire by P-MBE

N-doped ZnO samples were grown on sapphire substrate by plasma-assisted molecular beam epitaxy (P-MBE) method. The electrical properties of the samples were investigated by temperature-dependent Hall measurement. It was found that the carrier concentration and mobility of the samples showed some unstable characteristics under different magnetic fields at a certain temperature. However, the conductivity was a much more stable parameter. By fitting the dependence of conductivity on temperature and theoretical calculating, it was believed that the conduction mechanism was possibly the mix of band and hopping conduction mechanisms. The coexistence of a huge density of donor and acceptor defects was considered to cause the instability of some electrical properties (carrier concentration and mobility) and the appearance of the hopping conduction mechanism in N-doped ZnO samples.

Recovery Behavior of Amorphous Poly(Ethylene Terephthalate) Film Uniaxially Drawn at Different Temperatures

Homogeneously deformed amorphous poly(ethylene terephthalate) (PET) film samples were obtained by uniaxial drawing at temperatures from near the glass transition temperature T g to far above T g. Isothermal dimensional recovery behavior was studied for the free stress state of the deformed sample. It was shown that the dimensional recovery behavior of samples drawn near T g was different from that of a sample drawn about 20°C above T g, indicating that the deformation mechanisms changed when the drawing temperature rose from near T g to far above T g. Recovery curves of samples drawn near T g show two distinct steps in log time scale; the second step follows a single exponential function. The infrared measurement of a drawn sample showed that the dichroic function decreased during recovery in the first step and was almost invariant at a very low value (<0.03) in the second step. However, the recovery curve of samples drawn about 20°C above T g demonstrated nearly a straight line in log time scale.