Chen-I Hung

Surface modification of ZnO film by hydrogen peroxide solution

The effect of hydrogen peroxide (H2O2) treatment on the microstructure and luminescent properties of ZnO thin films has been investigated. Governed by high-resolution transmission electron microscopy and selected-area electron diffraction patterns, the oxygen radicals dissociated from H2O2 solution at room temperature and substantially changed the polycrystalline ZnO film into an insulator. In addition, the photoluminescence spectra showed that H2O2 solution had nearly no effect on the intensity of ultraviolet emission, whereas it significantly enhanced the intensity of deep-level emission. These observations strongly reveal the fact that the oxygen radicals penetrating into a ZnO film are reasonably speculated to occupy the interstitial sites to form oxygen interstitials Oi or fill the Zn vacancies to form antisite oxygen OZn defects. Because of these extra defects involved, an enhancement of the green light luminescence is significantly promoted in our ZnO samples after handling with H2O2 solution. Base...

Influence of quantum capture and tunneling mechanisms on the dark current of bound-to-continuum quantum-well infrared photodetectors

We present a theoretical model for the dark current of bound-to-continuum quantum-well infrared photodetectors (QWIPs), by considering the field-induced mixing effect, tunneling rate and phonon scattering rate between bound and continuum states. Using this model, we can see clearly how these mechanisms significantly influence the Fermi levels of bound and continuum electrons, and thus, the dark current. Nonlinear temperature dependence of the dark current at low temperature is predicted and discussed in detail. The simulated dark currents exhibit good agreement with the experimental results, without use of parameter fitting techniques.

Formation of Selective High Barrier Region by Inductively Coupled Plasma Treatment on GaN-Based Light-Emitting Diodes

By inductively coupled plasma (ICP) etching, a selective high barrier region (SHBR) was fabricated below the p-pad metal electrode for modifying the injection current distribution on p-type GaN of GaN-based light-emitting diodes (LEDs). Through the analysis of current noise power spectra, the samples with ICP etching treatment have excess nitrogen vacancies at the selectively etched surface of p-type GaN; thus, they have a lower hole concentration than the as-grown sample, resulting in a larger barrier height for carrier transport. With this SHBR, the light-output power for the LED chip measured at 20 mA was significantly increased by 12% as compared with that for the conventional LED chip. The light-output power increase could be attributed to a relative reduction in optical power absorption under the p-pad electrode and a higher density of current effectively injected into the active layer of the LED by the SHBR structure.

Light Output Enhancement of AlGaInP Light Emitting Diodes with Nanopores of Anodic Alumina

In this paper, we describe the improvement in the light extraction efficiency of AlGaInP light-emitting diodes (LEDs) upon use of a porous anodic alumina (PAA) film and pore widening. Acting as an intervening layer between air and the GaP window layer, PAA increased the critical angle to reduce the internal light reflection of LEDs. The nanopores of the PAA film provided a rough surface that enhanced the light output extraction. These factors resulted in the LEDs with PAA films exhibiting a light output power 29% higher than that of a conventional LED (without PAA film). Furthermore, following a pore-widening treatment for 40 min, the light output improvement increased to 39%. Therefore, the use of PAA film and a suitable pore-widening treatment increases the light extraction efficiency of LEDs.

Platinum Schottky contacts on single-crystal ZnO with hydrogen peroxide treatment

Platinum (Pt) Schottky contacts (SCs) on hydrothermal grown Zn-terminated (0001) ZnO substrates with the different hydrogen peroxide (H2O2) treatment time are investigated. Under the treatment in room temperature, effective SCs are made for 45 min and longer time and the electrical characteristics show the dependence on treatment time. The irregular humps on ZnO surface with roughness measured by atomic force microscope differ as the treatment time and roughness exhibits the large variation between 0.368 and 3.566 nm, indicating the etching effect and near-surface defects related to the lattice imperfections. The evaluated barrier height has the value at 0.89–0.96 eV with the saturation current density in the range of 2.21×10−10–3.31×10−9u2002A/cm2. The effective donor concentration calculated from capacitance-voltage (C-V) measurement decreases as treatment time, implying the widening of the space charge region after H2O2 treatment. The improved SC characteristics are attributed to the product of the wider b...

Microstructures and Microwave Dielectric Properties of (1-x)MgO·xBaO·TiO2 Ceramics

The microstructures and microwave dielectric properties of (1-x)MgOxBaOTiO2 ceramics (x=0.005, 0.01, 0.03, and 0.05) prepared by a conventional solid-state reaction method and sintered at temperatures from 1280°C to 1400°C were investigated. Some minor phases were observed in (1-x)MgOxBaOTiO2 under different sintering conditions. The dielectric constant and Q×f of the specimens increased with an increase in x. In the (1-x)MgOxBaOTiO2 system, the temperature coefficient of resonant frequency could be controlled by varying x and can lead to a zero τf. For practical applications, 0.97MgO0.03BaOTiO2 ceramics sintered at 1320°C have excellent microwave dielectric properties: er=20.6, Q×f=32600 GHz and τf=+4.47 ppm/°C.

RUPTURE OF THIN LIQUID FILM UNDER THE MAGNETIC FIELD

The nonlinear rupture of a thin liquid film on a horizontal plane is studied by taking into account the effect of a transversely applied uniform magnetic field load. First, a nonlinear evolution equation is derived for film thickness, h(x,t), and then solved numerically. The results show that the time of rupture of a liquid film is increasingly delayed as the magnitude of the magnetic field is increased. The dominant wave number is, however, independent of the magnetic field.

Investigation of Novel Microwave Surface-Acoustic-Wave Filter on Different Piezoelectric Substrates

A novel GHz-band low-loss large-bandwidth microwave microstrip, surface-acoustic-wave (SAW) filter used in spread spectrum communication systems was designed and fabricated. The 16 µm input/output-interdigital-transducer (IDT) LiNbO3 61.00 MHz SAW filter, with microwave square open-loop resonators, has an insertion loss of -3.987 dB. This device can also be used as a 1.064 GHz microwave microstrip SAW filter with four cross-coupled microstrip square open-loop resonators and two planar interdigital capacitors. It was found that this novel device has an insertion loss (S21) of -2.962 dB, a reflection loss (S11) of -25.497 dB and a 3 dB bandwidth of 800 MHz. The characteristics of these low-loss large-bandwidth (BW=80%) microwave microstrip SAW filters are influenced by the interaction between electromagnetic waves and piezoelectric SAWs. To confirm this claim, devices of identical design were fabricated on the three types of substrate: i) 128°-rotated YX-cut lithium niobate (LiNbO3) with a high electromechanical coupling coefficient (K2=5.3%), ii) GaAs S-I photo-piezoelectric material with a low electromechanical coupling coefficient (K2=0.07%) and iii) non-piezoelectric SiO2/Si material with a thickness of 9000 A. The performances of these devices were significantly different. We will apply the principle of SAWs and the equivalent circuit of IDT to prove our experimental results.

Self-consistent simulation on the modal gain of graded-index separate-confinement-heterostructure quantum-well lasers

The effective guide index directly causes the mutual influences in determining the complex refractive index of quantum well and the complex propagation constant of a guided mode. In this paper, self-consistent model (SCM) working on both density matrix theory and transfer matrix method is applied to investigate the modal gain of AlGaAs/GaAs graded-index separate-confinement-heterostructure single quantum-well (GRIN-SCH-SQW) lasers. Based on SCM, the simulated modal gain spectrum shows good agreement with the experimental result. Although varying with the thickness of SCH region or aluminum mole fraction, the percentage change of optical gain is much smaller than that of optical confinement factor. On the other hand, thin well width of QW results in a relative high optical gain but poor optical field confinement. Such opposing effects tend to balance each other and cause the modal gain almost insensitive to the well width change before 60 Å. Further increase of well thickness, the percentage change of optical gain is obviously larger than that of optical confinement factor. Therefore the optical gain becomes the dominant parameter that directly decreases the magnitude of modal gain.

Heat generation approximation in modulation-doped field-effect transistors by the energy relaxation between carriers and phonons

A combination of the nonlinear charge-control model, rate balance equations, and electron-phonon scattering is used to simulate the hot-electron transport and heat generation problem of modulation-doped field-effect transistors. Based on the present model, the trend of high-field transport can be well demonstrated. In addition, the heat generation inside the channel can also be estimated by the energy flux balance equation with the energy relaxation rate formulated by polar-optical phonon scattering mechanism. Scaling down the gate length to submicron range, more heat power is generated inside the channel. However, the heat generation ratio reveals that hot electrons carry over half of the energy density gained from the applied electric field directly into drain region. For 0.1 μm gate length, more than 90% energy density is achieved. By reducing the gate length into the deep submicron range, the thermal impact on the drain side becomes more serious and complicated than the one found inside the channel.A combination of the nonlinear charge-control model, rate balance equations, and electron-phonon scattering is used to simulate the hot-electron transport and heat generation problem of modulation-doped field-effect transistors. Based on the present model, the trend of high-field transport can be well demonstrated. In addition, the heat generation inside the channel can also be estimated by the energy flux balance equation with the energy relaxation rate formulated by polar-optical phonon scattering mechanism. Scaling down the gate length to submicron range, more heat power is generated inside the channel. However, the heat generation ratio reveals that hot electrons carry over half of the energy density gained from the applied electric field directly into drain region. For 0.1 μm gate length, more than 90% energy density is achieved. By reducing the gate length into the deep submicron range, the thermal impact on the drain side becomes more serious and complicated than the one found inside the channel.