Wei-Chih Tsai

Manufacturing method for n-type porous silicon based on Hall effect without illumination

A method for the fabrication of n-type porous silicon (n‐PS) is developed. The Hall effect is applied in the fabrication process. The majority carriers in n-type Si (electrons) are swept down by the Lorentz force. Enough holes continuously appear on the surface layer to participate in chemical reaction during the etching process. Illumination sources are not necessary in this method. Therefore, no illumination limit has to be concerned in the formation of deep PS layer. The morphology, porosity, and photoluminescence of the n‐PS prepared by the proposed method are investigated. Strong visible photoluminescence emissions are demonstrated on n‐PS at about 650nm.

White-light emissions from p-type porous silicon layers by high-temperature thermal annealing

In this study, the white-light emissions, including red, green and blue colors, appearing on the same porous silicon samples are originally introduced by a thermal-annealing method. The SEM, FTIR, and PL are discussed for different annealing temperature cases. The FTIR is used to monitor the chemical bonding structures of the PS samples under different annealing temperatures. The results show that the variation of chemical bonding relates to the variation of the emission wavelength. The emission intensities of the blue-green-light components are enhanced with the increase of annealing temperature. The PL spectra cover the entire visible region under the excitations of He-Cd laser beam, and a strong white-light emission can be observed by the naked eye at room temperature.

The evolution of tungsten oxide nanostructures from nanowires to nanosheets

The self-synthesis of tungsten oxide (W18O49) nanowires/nanosheets on sputtering-deposited tungsten films was obtained by thermal annealing in nitrogen under the thermal budgets of 750 °C for ≥2 h or ≥850 °C for 0.5 h. Experimental evidence of the nanomorphology transformation of tungsten oxide nanostructures from nanowires to nanosheets was presented, which can be attributed to the formation and re-crystallization of an amorphous interface layer between two neighboring parallel-growth nanowires linked together with a low angle misalignment along their growth directions.

Photoluminescence from n-type porous silicon layer enhanced by a forward-biased np-junction

A new approach for the fabrication of n-type porous silicon layer is proposed. A hole-rich p-layer is arranged underneath the n-layer, and the np-junction is under forward biased condition in the etching process. Therefore sufficient holes can drift straight-upward and pass across the np-junction from p-region to n-region to participate in electrochemical reaction during the etching process with an unfailing supply. Illumination is an optional hole-supplier in this approach, so the problem of illumination-depth limitation can be overcome. Strong visible photoluminescence emissions are demonstrated on the hole-poor n-type porous layer at about 650 nm.

Improvement of field emission characteristics of tungsten oxide nanowires by hydrogen plasma treatment

The use of hydrogen plasma (H-plasma) treatment to improve field emission (FE) characteristics of self-synthesized tungsten oxide nanowires (TONWs) is reported. With a H-plasma treatment under a working power of 200 W and a pressure of 500 mtorr for 20 s, improved FE characteristics with a turn-on field (4.7 V/μm at 10 μA/cm2) lower than those of the as-grown case by 23% and a reduction in the effective emission barrier of 0.72 eV were obtained, which is attributed to the reduction in oxygen adsorption, decrease in the wire length and density, and transition of TONWs surfaces from well crystalline into the amorphous phase.

Use of anisotropic laser etching to the top n-GaN layer to alleviate current- crowding effect in vertical-structured GaN-based light-emitting diodes

To equalize the resistance of all possible current paths in regular vertical-conducting metal-substrate GaN-based light-emitting diodes (VM-LEDs), an anisotropic laser etching to the surface layer (n-GaN) of 40mil VM-LEDs for improving light emission uniformity and light output power is proposed and demonstrated. The feasibility of the proposed scheme was verified by current and light emission distribution as well as light extraction rate simulations. In conjunction with a nonuniform excimer laser beam irradiation through a mask and rotation of the epitaxy wafer, VM-LEDs with a concave-surface n-GaN layer were also fabricated. Typical improvement in light output power by 38%–26% at an injection current of 350mA as compared to the one without anisotropic etching has been obtained.

Continuous-time photoelectron spectroscopy for monitoring monochromatic soft x-ray photodissociation of CF3Cl adsorbed on Si(111)−7×7

The continuous-time photoelectron spectroscopy was proposed to study the monochromatic soft x-ray photodissociation of CF3Cl molecules adsorbed on Si(111)−7×7. Evolution of adsorbed CF3Cl was monitored at two photon energies of 240 and 730eV to deduce the photolysis cross section as a function of energy. Dissociation of adsorbed CF3Cl by 240∕730eV photons is attributed to the C–Cl∕C–F bond scission following the excitation of Cl(2p)∕F(1s) core electron. Observation of time-dependent photodissociation also demonstrates the capability for “real-time” monitoring of the variations of electronic structure and chemical bonding of adsorbate in bond-selective photochemistry.

Light emission and negative differential conductance of n-type nanoporous silicon with buried p-layer assistance

The light-emission and current-voltage properties of n-type nanoporous silicon (n-NPS) with a hole assistance of buried p layer are explored. The influences of anodic current density on the formation, morphology, and properties of n-NPS are measured. Such n-NPS films have nanoscaled pores and high-aspect-ratio pillars. Since the anisotype junction is forward biased during the anodization process, many holes can drift straightupward from p layer and participate in the electrochemical reaction. At room temperature, high peak-to-valley current ratios of about 117.3 can be obtained in negative difference conductance region as well as strong visible light emissions are clearly observed under ultraviolet excitation.

Simulation and analysis of metamorphic high electron mobility transistors

In this paper, the metamorphic high electron mobility transistors (mHEMTs) are investigated numerically and compared with pseudo-morphic high electron mobility transistors (pHEMTs). The two-dimensional device simulator, MEDICI, is used to solve the Poissons equation and the electron/hole current continuity equations. The influences of @d-doping concentration and position, gate width, spacer thickness, etc. on the performances of HEMTs are explored. It shows clearly that mHEMTs have higher transconductances, drain currents and DC voltage swings than pHEMTs.

Preparation and optoelectronic properties of NiO/ZnO heterostructure nanowires

This study proposes the use of a ZnO-nanowire (ZnO-NW)-based heterojunction structure for applications of nano optoelectronic sensors and photovoltaic devices. Nano heterojunctions (NHJs) were formed via e-beam deposition of ptype nickel oxide (NiO) onto the vertical-aligned ZnO-NWs grown by hydro-thermal growth method. The dark J-V curve shows that the prepared NiO/ZnO-NWs NHJ has a diode-like behavior with a forward threshold voltage (Vth) of 1.2 V and a leakage current (Jr at -1V) of 0.02 μA/cm2, respectively. It also exhibits a superior response to UV (366 nm) and AM 1.5G light illuminations. The Vth and the photocurrents (i.e., Jr at -1V) under UV (366 nm @ 6 mW/cm2) and AM 1.5G light were 0.7 V/0.06 μA/cm2 and 0.5 V/ 3.2 μA/cm2, respectively, revealing an increase in the diode current of about 3× and 160×, respectively.