The emission from a light emitting diode (LED) that is emitted under the metal electrode cannot escape into free space. A current blocking layer (CBL) is used to address this issue by forcing the current to flow laterally under the electrode reducing the emission absorbed and hence increasing the overall efficiency of the LED. In this paper a new method to fabricate Schottky and isolating CBLs in GaN LED are investigated. Optical and electrical measurements of these vertical LEDs with and without CBL show different light output powers at identical current densities. The results of this study indicate that CBLs could also be used to suppress the efficiency droop effect for GaN LEDs.

We demonstrate an easy way of decorating MWCNTs with CdS and CuxSy using the solvothermal method.

R. E. Smalley (1985 Nobel Laureate for discovery of C60) provided the humanity’s top ten important issues for next 50 years in 2005, which are energy, water, food, environment, poverty, terrorism & war, disease, education, democracy, and population[1]. In the past five years, the people has realized that with exhaustion of fossil fuel energy, global warming, population growth, and etc., the energy challenge & revolution is the most important issue. In order to solve this problem, many governments in the world have been developing the very important projects dealing with collecting, emitting, and manipulating energy. This paper reviewed the recent progress of these projects such as generation of renewable energies and save energies, especially focus on thin film solar cells, LEDs and smart power and grid, which including III-V materials on Si or ceramic substrates for high efficiency thin film solar cell and high power, high brightness and high efficiency LEDs, as well as smart control of mixture of electric, thermal and other energies. The Building Integrated Photovoltaics (BIPV) and City Integrated LEDs (CILED) will be most important manufacturing in this century.

Thin film gallium nitride (GaN) scintillators have been produced by MOCVD and made neutron-sensitive by applying an enriched lithium-6 fluoride (6LiF) conversion layer. The 6Li(n,α) reaction produces both alpha and triton particles, which have very penetration depths in GaN. The range and energy deposition characteristics of these particles in GaN have been simulated. Alpha-induced scintillation was measured in silicon-doped GaN using an americium-241 (241Am) source. The thermal neutron responses of the 6LiF-coated GaN scintillator were tested using two thermal neutron sources, an 241Am-Be source inside a graphite pile and a reactor source. The scintillator was found to have a linear response to thermal neutron flux level over a range of more than three orders of magnitude.

Nonpolar a-plane InN/GaN heterostructures were grown by plasma assisted molecular beam epitaxy. The growth of nonpolar a- plane InN / GaN heterostructures were confirmed by high resolution x-ray diffraction study. Reflection high energy electron diffraction patterns show the reasonably smooth surface of a-plane GaN and island-like growth for nonpolar a-plane InN film, which is further confirmed by scanning electron micrographs. An absorption edge in the optical spectra has the energy of 0.74 eV, showing blueshifts from the fundamental band gap of 0.7 eV. The rectifying behavior of the I-V curve indicates the existence of Schottky barrier at the InN and GaN interface. The Schottky barrier height (φb) and the ideality factor (η) for the InN/GaN heterostructures found to be 0.58 eV and 2.05 respectively.

Orientation patterned (OP)-GaAs crystals are attractive materiasl for mid-infrared and terahertz lasers sources, using non linear optics frequency conversion from shorter wavelength sources. The optical propagation losses are critical to the fabrication of these sources; among the causes of optical losses the generation of defects and the incorporation of impurities must play a relevant role. The control of the incorporation of impurities and defects is, therefore, crucial to improve the performance of the OP-GaAs crystals as non linear optical materials. We present herein a cathodoluminescence (CL) analysis of OP-GaAS crystals intentionally doped with Si, in order to understand the incorporation paths of Si in the OP-GaAs crystals.

Combining interdisciplinary fields of nonlinear optics, dynamic holography, and photoelectrical phenomena, we developed the optical measurement technologies for monitoring the spatial and temporal non-equilibrium carrier dynamics in wide bandgap semiconductors at wide range of excitations (1015 to 1020 cm-3) and temperatures (10 to 800 K).

We explored advantages of non-resonant optical nonlinearities, based on a short laser pulse induced refractive or absorption index modulation (Δn and Δk) by free excess carriers. This approach, based on a direct correlation between the electrical and optical processes, opened a possibility to analyze dynamics of electrical phenomena in “all-optical” way, i.e. without electrical contacts.

Carrier diffusion and recombination processes have been investigated in various wide band gap materials - differently grown GaN, SiC, and diamonds - and their key electrical parameters determined, as carrier lifetime, diffusion coefficient, diffusion length and their dependences on temperature and injected carrier density. The studies provided deeper insight into nonradiative and radiative recombination processes in GaN crystals, revealed diffusion-driven long nonradiative carrier lifetimes in bulk GaN and SiC, disclosed impact of delocalization in InGaN layers, and suggested a trap-assisted Auger recombination in highly-excited InN. Injection and temperature dependent diffusivity revealed a strong contribution of carrier-carrier scattering in diamond and bandgap renormalization in SiC.

We demonstrate the highly reflective broadband a-Si distributed Bragg reflector fabricated by oblique angle deposition. By tuning the refractive index of a-Si film, the high index contrast material system was achieved. The broadband reflective characteristics of a-Si distributed Bragg reflector were investigated by calculation and fabrication. The broad stop band (Δλ/λ=33.7%, R>99%) with only a five-pair a-Si distributed Bragg reflector was achieved experimentally at center wavelength of 650, 980, and 1550 nm. The size-, feature- and substrate-independent method for highly reflective Bragg reflectors was realized by simple oblique angle evaporation.

In this paper the thermoelectric properties–the Seebeck coefficient, the electrical conductivity and the power factor – of GaN and InGaN thin films grown by Metal Organic Vapor Deposition (MOCVD) are reported. The Seebeck coefficient and power factor of InGaN decreases with increasing indium content, although the electrical conductivity shows an inverse behavior. P-type doped samples demonstrated the highest Seebeck coefficient (637 μV/K in GaN:Mg, 1200 μV/K in InGaN:Mg) but the lowest power factor (0.1x10-4 W/m-K for GaN:Mg, 0.4x10-4 W/m-K for InGaN:Mg). The Seebeck coefficient of the doped GaN thin films decreased linearly with log of the carrier concentration. GaN:Si exhibited a maximum power factor of 9.1x10-4 W/m-K with a carrier concentration of 1.6x1018 cm-3, and In0.1Ga0.9N exhibited a maximum power factor of 109x10-4 W/m-K with a carrier concentration of 1.2x1018 cm-3. The results also indicate that GaN and InGaN-based materials could potentially be useful materials for TE applications at high temperatures.

The lead-salt epitaxial membrane embedded with two dimensional hexagonal air holes lattice array was used to develop the mid-infrared (mid-IR) photonic crystal surface emitting laser. For the initial proof-of-concept research work, we demonstrated intensive photonic crystal coupled mid-IR vertical light emissions from the designed structure under cryogenic temperature range. In order to realize room temperature operation, we modified the photonic bands structure, aligned the photonic crystal coupled mode with the gain spectrum of active material at 300 K. As a result, under pulsed mode optical excitation, multi-mode room temperature mid-IR photonic crystal lasing emissions were achieved at wavelength ∼3.5 μm recently. With further optimization, a practical lead-salt continuous-wave operating room temperature surface emitting photonic crystal laser is anticipated soon, and this will explore promising applications in a wide variety of fields.

We apply infrared spectroscopic ellipsometry (IRSE) in combination with near-infrared to vacuum-ultraviolet ellipsometry to study the concentration and mobility of holes in a set of Mg-doped In-polar InN samples of different Mg-concentrations. P-type behavior is found in the IRSE spectra for Mg-concentrations between 1x1018 cm-3 and 3x1019 cm-3. The free-charge carrier parameters are determined using a parameterized model that accounts for phonon-plasmon coupling. From the NIR-VUV data information about layer thicknesses, surface roughness, and structural InN layer properties are extracted and related to the IRSE results.

A clear PL spectrum was observed from β-FeSi2 grains on gold (Au)-coated (100)Si substrates, and indicated the formation of crystal with the same high quality level as the β-FeSi2 on a copper (Cu)-coated Si substrate. Moreover, the temperature dependence of photoluminescence peak intensities showed lower density of the nonradiative recombination center in β-FeSi2 grains on Au-coated Si substrates than that of β-FeSi2 film on Cu-coated Si. Au was not detected in β-FeSi2 grains by STEM-EDX observation, while Cu was observed in the grains and grain boundaries of β-FeSi2 and rolled as non-radiative recombination center.

We have investigated an InAs channel Hall-bar structure with ferromagnetic spin injector in one of the current terminals. After magnetizing the Fe electrode, spin polarized electrons are injected through the edge of the isolation mesa structure and the anomalous Hall voltage is observed, when electrons are injected from the ferromagnetic terminal. However, when electrons are injected from the non-magnetic metal (Ti/Au) of opposite terminal, the Hall voltage disappeared to the variation error level due to the fabrication imperfections. This result suggests the possibility that out-of-plane spin injection from the channel edge lead to perpendicular nuclear magnetic field. It is presumably caused by nuclear spin polarization in InAs channel near the spin source edge through Overhauser effect. The estimated internal magnetic field was 2000 Gauss.

Resonant coupling of an optical mode confined within a microcavity and an emitter is the basic prerequisite for the observation of Bose-Einstein condensation phenomena and the development of novel optical devices based on cavity polaritons.

We demonstrate highly spatially resolved 2” wafer characterization of the reflectivity and emission properties of a nitride based multi quantum well semi microcavity (i.e. structure without top Bragg reflector) to verify resonant regions. Photoluminescence and reflectivity spectra recorded at the same positions on the wafer exhibit a strong spatial dependence of the multi quantum well emission and the center wavelength of the stop band of the bottom Bragg reflector across the sample. Resonance, i.e., matching of the emission and the center wavelength of the stop band, is found in a region 8 mm off the center of the wafer.

The thickness profile across the AlInN/GaN Bragg reflector and multi quantum well layers was obtained by x-ray mappings over the full wafer. A perfect correlation between the local optical properties and the x-ray thickness distribution is found. Additional transmission electron microscopy investigations indicate a complete crack free structure and smooth interfaces between the layers within the Bragg reflector making the structure appropriate for strong coupling applications.

To have better light-emitting performance, semiconductor-metal periodic photonic waveguides can generate stable wavelengths. This work constructs a multi-parameter model to compute the backward-wave mode-coupling coefficients, which are important to the analysis and performance of photonic devices. For such a semiconductor-metal hybrid structure, a proper photonic technique needs to be utilized to solve this computational complexity.

Numerical results demonstrate how the materials of metal gratings, the corrugation amplitudes of metal gratings, and the metallic aluminum mole fraction can affect the coupling coefficients. Further physical interpretation and discussion can support and explain the above results. The results can help engineers decide the values of parameters used in fabrication. Future work and applications will be proposed.

Shockley-Queisser detailed balance theory predicts that under one sun a semiconductor with its bandgap in the range of 1.0 – 1.6 eV can potentially achieve an energy conversion efficiency > 30%. Therefore, the conversional wisdom would suggest looking for a semiconductor with a bandgap in this range for a single junction solar cell. Here we explore an alternative way of selecting the absorber material for PV, which allows using semiconductors with much larger bandgaps, in conjunction with new device architecture. Specifically, our device is based on an array of core-shell semiconductor nanowires, such as ZnO-ZnSe, where the two components exhibit type II band alignment. Our approach relies on the most basic property of a type II heterojunction, i.e., the staggered band alignment, that provides the function of charge separation, as in the case of dye-sensitized solar cell or (organic) bulk heterojunction solar cell. However, they differ in two important aspects: (1) the current structure is all inorganic, thus, expected to offer better chemical and photo- stability; and (2) In this approach, the interfacial transition provides an effective absorption or photo-response threshold that can be much lower than that of either component. In this work, using a ZnO-ZnSe core-shell nanowire array, we report the observation of the key signatures associated with the type II optical transition, and the demonstration of a solar cell based on the core-shell nanowire array.

We have investigated electronic band-gap states in AlGaN/GaN hetero-structures with different growth conditions of GaN buffer layers from a viewpoint of Carbon impurity incorporation into GaN, using photoluminescence (PL), capacitance-voltage (C-V) and steady-state photo-capacitance spectroscopy (SSPC) techniques. The Carbon incorporation was found to be enhanced with decreasing the growth temperature of the GaN buffer layer between 1120 and 1170 °C. Acting in concert, three specific deep levels located at ~2.07, ~2.70, and ~3.23 eV below the conduction band were found to become dense significantly at the low growth temperature. Therefore, these levels are probably attributable to Ga vacancies and/or Carbon acceptors produced by the Carbon impurity incorporation, and are likely in conjunction with each other.

In-situ wafer curvature measurements were used to study the effect of Si doping on intrinsic growth stress during the metalorganic chemical vapor deposition (MOCVD) growth of AlxGa1-xN (x=0-0.62) layers on SiC substrates. Post-growth transmission electron microscopy (TEM) characterization was used to correlate measured changes in stress with changes in film microstructure. Si doping was found to result in the inclination of edge-type threading dislocations (TDs) in AlxGa1-xN which resulted in a relaxation of compressive stress and generation of tensile stress. The experimentally measured stress gradient was similar to that predicted by an effective climb model. Dislocation inclination resulted in a reduction in the TD density for Si-doped layers compared to undoped AlxGa1-xN likely due to increased opportunities for dislocation interaction and annihilation. The TD density, which increased with increasing Al-fraction, was found to significantly alter the stress gradients in the films. Film stress was also observed to play a role in TD inclination. In undoped AlxGa1-xN, TD inclination was observed only when the film grew under a compressive stress while in Si-doped AlxGa1-xN, TD inclination was observed independent of the sign or magnitude of the film stress. Si dopants are believed to alter the concentration of surface vacancies which gives rise to dislocation jog via a surface-mediated climb mechanism.

Hydrogen passivation was applied to the initial epitaxial growth of n-type β-FeSi2 thin films on p-type Si(111) substrates by facing-targets direct-current sputtering (FTDCS) in order to reduced the formation of interface states and terminate dangling bonds in the β-FeSi2 films, and the passivation effects were studied on basis of the electrical evaluation results of the formed n-type β-FeSi2/p-type Si heterojunction photodiodes. The initial growth was made at different gas inflow H2/Ar ratios ranging from 0 to 0.2. The photodetection performance of the photodiode fabricated at the ratio of 0.2 was markedly improved as compared to those of the other samples. The quantum efficiency and detectivity were 2.08 % and 1.75 × 1010 cm√Hz/W, respectively. The sample exhibited the minimum junction capacitance density of 9.2 nF/cm2. The enhanced photodetective performance should be mainly because dangling bonds that act as trap centers for photocarriers are effectively inactivated by the passivation.

In this study, we propose copper oxide (CuOx) edge-termination for GaN-based Schottky barrier diodes (SBDs) with low turn-on voltage. CuOx fabricated by thermal oxidization of sputtered Cu film at 275°C consisted mainly of Cu2O which is known as a p-type semiconductor. We applied CuOx edge-termination to GaN SBDs with tantalum (Ta) Schottky electrode which has low work function of 4.25 eV. The experimental results of current-voltage characteristics insisted that CuOx edge-termination structure was effective to increase breakdown voltage of GaN SBDs with keeping low turn-on voltage of 0.29 V at 10 A/cm2.