Debao Zhang

The role of Coulomb interaction in thermoelectric effects of an Aharonov-Bohm interferometer.

We investigate the thermoelectric effects of an Aharonov-Bohm (AB) interferometer with a quantum dot (QD) embedded in each of its arms, where the intra-dot Coulomb interaction between electrons in each QD is taken into account. Using Greens function methods and the equation of motion (EOM) technique, we find that the Seebeck coefficient and Lorenz number can be strongly enhanced when the chemical potential sweeps the molecular states associated with the Fano line-shapes in the transmission spectra, due to quantum interference effects between the bonding and antibonding molecular states. It is found that enhancement of the thermoelectric effects occurs between the two groups of conductance peaks in the presence of strong intra-dot Coulomb interaction-the reason being that a transmission node is developed in the Coulomb blockade regime. In this case, the maximum value of the Lorenz number approaches 10π(2)k(B)(2)/(3e(2)). Its thermoelectric conversion efficiency in the absence of phonon thermal conductance, described by the figure of merit ZT, approaches 2 at room temperature. Therefore, it may be used as a high-efficiency solid-state thermoelectric conversion device under certain circumstances.

Trap-assisted tunneling in AlGaN avalanche photodiodes

We fabricated AlGaN solar-blind avalanche photodiodes (APDs) that were based on separate absorption and multiplication (SAM) structures. It was determined experimentally that the dark current in these APDs is rapidly enhanced when the applied voltage exceeds 52 V. Theoretical analyses demonstrated that the breakdown voltage at 52 V is mainly related to the local trap-assisted tunneling effect. Because the dark current is mainly dependent on the trap states as a result of modification of the lifetimes of the electrons in the trap states, the tunneling processes can be modulated effectively by tuning the trap energy level, the trap density, and the tunnel mass.

Interface state density effect on the performance of graphene silicon heterojunction solar cell

A planar structure consisting of graphene layer as the hole transport material, and n-type silicon as substrate is simulated. The degradation of this cell caused by high interface state density has been carried out. The simulated results match published experimental results indicating the accuracy of the physics-based model. Using this model, the effect of interface state density as zero, 1×1016cm-2, 1×1017cm-2 on the photovoltaic performance has been studied. The obtained IV and EQE characteristic based on realistic parameters shows that the interface state playing a prominent role in graphene silicon schottky contact.

Humidity sensor based on perfect metamaterial absorber

We proposed a humidity sensor based on perfect metamaterial absorber. The sensor is composed of three layers, which are metallic particle array on the top, porous silicon in the middle layer and metallic film at the bottom. It is shown that the resonant wavelength displays significant red-shift with the increasing effective permittivity of porous silicon, which is influenced by the filling fraction of water condensation. Furthermore, the simulation results indicate that the refractive index sensitivity of absorber is high to 249 nm/RIU, which makes our structure be an ideal candidate for evaluating the humidity of environment.

Numerical simulation study of graphene silicon solar cell with boron diffusion layer

Two dimensional model of graphene silicon heterojunction solar cell with an boron doped surface layer is structured using Silvaco TCAD tools by accurate control of diffusion process. The introduction of inverse doped layer obviously improved the efficiency of heterojunction photovoltaic cell.

Light absorption enhancement by embedding aluminum nanodisk arrays in rear dielectric of ultra-thin film solar cells

In this work, in order to enhance the light absorption in one micron thick crystalline silicon solar cells, a back reflecting and plasmonic nanodisk scheme is proposed. We investigate the scattering properties of aluminum nanostructures located at the back side and optimize them for enhancing absorption in the silicon layer by using finite difference time domain simulations. The results indicate that the period and diameters nanoparticles, spacer layer have a strong impact on short circuit current enhancements. This finding could lead to improved light trapping within a thin silicon solar cell device.

Effects of LaNiO3 interlayer on the microstructures and electrical properties of Ba0.9Sr0.1Ti0.99Mn0.01O3 multilayer

The aim of this work was to investigate the effects of low-resistivity interlayer on the physical properties of periodic Ba0.9Sr0.1Ti0.99Mn0.01O3 (BSTM) multilayers prepared by a chemical solution deposition method. A LaNiO3 (LNO) layer was inserted into the periodic BSTM multilayer artificially to form a sandwiched configuration of BSTM/LNO/BSTM. The capacitances at low frequencies (<100 kHz) of the sandwiched multilayer are significantly enhanced compared to that of the pure BSTM multilayer. The space charge accumulated at the LNO layer was proposed to explain the enhancement based on Maxwell-Wagner (M-W) model. However, LNO interlayer leads to an increase in the leakage current. A non-Ohmic conduction region is observed for BSTM/LNO/BSTM multilayer when the electric field exceeds 100 kV/cm. The results offer a new approach to achieve dielectric films with high dielectric constant.

Infrared tunable dual-band polarization filter based on compound asymmetrical cross-shaped resonator

We propose an infrared tunable dual-band polarization filter in this paper. Based on the perfect absorption characteristic of the metal-dielectric-metal sandwich structure, the reflection spectrum performs as a filter. The calculated results show that different dual-band wavelengths are filtered while the incident light has different polarization which is parallel or vertical to the x axis. Moreover, it is found that the resonant wavelengths can be tuned independently and freely by adjusting the length of the corresponding rectangular strip. In addition, it is found that all of the intensities at the filter wavelengths are closed to zero, which implies the filter exhibits good filtering performance.

Rear located hemispherical silver nanoparticles for light trapping in thin film solar cells

In this work, we investigate the scattering and coupling efficiencies of the rear located hemispherical silver nanoparticles by using finite difference time domain simulations. The results indicate that the placement and diameters of silver nanoparticles have a strong impact on scattering efficiency. This finding could lead to improved light trapping within a thin silicon solar cell device.

Two dimensional simulation studies on graphene semiconductor junction solar cell

The photo voltaic behaviors of graphene based schottky solar cell have been investigated for silicon and GaAs substrate respectively by two dimensional model. GaAs heterojunction has a superior photo voltaic behavior.