Sharp Band Edge Research Articles

COMPACT DUAL BAND-REJECT UWB ANTENNA WITH SHARP BAND-EDGE FREQUENCY

A microstrip line fed dual band-reject ultra wideband antenna with sharp band edge frequency of 3.1{10.6GHz is presented. The antenna consists of a rectangular patch on the front side and a partial ground plane at the rear. A step is cut on the bottom edge of the patch for impedance matching. A split ring slot etched on the radiating patch rejects WiMAX (3.3{3.8GHz) band, and a pair of inverted S-shaped slot in the partial ground plane rejects WLAN (5{6GHz) band. In order to eliminate the radiation outside the FCC specifled 3.1{10.6GHz band, a rectangular slot is etched on the ground plane below the feed line. The antenna exhibits UWB band width of 109% except for the notch band. The radiation characteristics are consistent throughout the band. The performance of the antenna is analyzed both in the frequency domain and time domain to assess its suitability for ultra wideband communication. Pulse distortion of the antenna is investigated for both Rayleigh and Gaussian source pulse excitation.

Surfactant-Free Hydrothermal Synthesis of Ag/C Nanocables

We present a versatile technique based on nano-imprint lithography to fabricate high-quality semiconductor-polymer compound nonlinear photonic crystal (NPC) slabs. The approach allows one to infiltrate uniformly polystyrene materials that possess large Kerr nonlinearity and ultrafast nonlinear response into the cylindrical air holes with diameter of hundred nanometers that are perforated in silicon membranes. Both the structural characterization via the cross-sectional scanning electron microscopy images and the optical characterization via the transmission spectrum measurement undoubtedly show that the fabricated compound NPC samples have uniform and dense polymer infiltration and are of high quality in optical properties. The compound NPC samples exhibit sharp transmission band edges and nondegraded high quality factor of microcavities compared with those in the bare silicon PC. The versatile method can be expanded to make general semiconductor-polymer hybrid optical nanostructures, and thus it may pave the way for reliable and efficient fabrication of ultrafast and ultralow power all-optical tunable integrated photonic devices and circuits (C) 2012 Optical Society of America

Fabrication of semiconductor-polymer compound nonlinear photonic crystal slab with highly uniform infiltration based on nano-imprint lithography technique

We present a versatile technique based on nano-imprint lithography to fabricate high-quality semiconductor-polymer compound nonlinear photonic crystal (NPC) slabs. The approach allows one to infiltrate uniformly polystyrene materials that possess large Kerr nonlinearity and ultrafast nonlinear response into the cylindrical air holes with diameter of hundred nanometers that are perforated in silicon membranes. Both the structural characterization via the cross-sectional scanning electron microscopy images and the optical characterization via the transmission spectrum measurement undoubtedly show that the fabricated compound NPC samples have uniform and dense polymer infiltration and are of high quality in optical properties. The compound NPC samples exhibit sharp transmission band edges and nondegraded high quality factor of microcavities compared with those in the bare silicon PC. The versatile method can be expanded to make general semiconductor-polymer hybrid optical nanostructures, and thus it may pave the way for reliable and efficient fabrication of ultrafast and ultralow power all-optical tunable integrated photonic devices and circuits.

Observation of a blue shift in the optical response at the fundamental band gap in Ga1−xMnxAs

We report the observation of a sharp band-edge response in spectrally resolved differential reflectivity experiments on GaMnAs, in contrast to linear optical experiments in which large band-tail effects are known to dominate. The differential reflectivity response exhibits a blue shift relative to results in GaAs and LT-GaAs, consistent with the valence-band model of ferromagnetism. Our results demonstrate the utility of nonlinear optical techniques for studying the electronic structure of III-Mn-V diluted magnetic semiconductors.

Microwave Assisted Synthesis of ZnO Nano-Sheets and Their Application in UV-Detector

Here we report a low cost electrochemical aqueous solution method for synthesis of high quality two dimensional flat ZnO nano-sheets using standard home use microwave-oven. Independent studies by field emission scanning electron microscopy and high resolution transmission electron microscopy confirm the formation of very thin graphene-like nanosheets of ZnO. The high quality of the sheets is confirmed from a photoluminescence study, which shows the presence of a narrow and sharp band edge emission at 360 nm and the presence of a weak defect related emission peak in the visible region. The conductivity of the nanosheets increases dramatically upon illumination of UV lights as compared to the dark condition. The better crystal quality and excellent photo conductivity ensure these ZnO nanosheets can be used as potential UV detector. It is also found that both response and recovery times of ZnO nano-sheets are very fast and are comparable to those of nanowires and nanorods.

Excellent UV absorption in spin-coated thin films of oleic acid modified zinc oxide nanorods embedded in Polyvinyl alcohol

The aim of the study is to investigate the optical properties of spin-coated, highly transparent nanocomposite films of oleic acid modified ZnO (Zinc oxide) nanorods embedded in Polyvinyl alcohol (PVA) matrix. Pristine and oleic acid (OA) modified ZnO nanorods have been prepared by wet chemical synthesis and are characterized by X-ray diffraction, FESEM, TEM and FT–IR spectroscopy techniques. The optical properties of ZnO/PVA films are studied using UV–visible absorption and Photoluminescence (PL) spectroscopy. The results show that the optical absorption of the films in the UV region is quite high and more than 95% absorption is observed in films prepared from OA modified ZnO nanorods. The excellent UV absorption at around 300 nm offers prospects of applications of these films as efficient UV filters in this wavelength region. The PL spectrum of pristine ZnO nanorods shows almost white light emission whereas OA modified ZnO nanorods have a more intense peak centered in the blue region. The PL emission of OA modified ZnO/PVA film shows appreciable increase in intensity compared to the film obtained with pristine ZnO. The surface modification of ZnO by the polymer matrix removes defect states within ZnO and facilitates sharp near band edge PL emission at 364 nm.

The Fabrication of Ag Islands on AlN/GaN/AlN/Si(111) by Using Thermal Evaporator and Thermal Annealing Methods

In this paper, we studied growth of AlN/GaN/AlN on Si (111) by using plasma assisted molecular beam epitaxy (PA-MBE) system. The structural and optical characteristics of the sample have been investigated by using high resolution X-ray diffraction (HR-XRD), Raman spectroscopy and photoluminescence (PL). PL spectrum of the sample has shown sharp and intense band edge emission of GaN with the absence of yellow emission band, indicating good crystal quality of the sample. The silver (Ag) metal contact was then deposited on the sample followed by thermal treatment at 500°C and 700°C, respectively. Treated sample at 700°C showed good spherical Ag islands on sample compared to the treated sample at 500°C. The effect of Ag islands on the electrical characteristics of sample was also examined by usingI-Vmeasurement. The results showed that the treated sample at 700°C has decreased the photo-current of Schottky diode.

Electronic Structure Analyses of BN in Cubic and Hexagonal Phases

First principle calculations are performed on the structure, energy band gap, and dielectric properties of wurtzite and hexagonal BN by using a plane-wave pseudopotential method. It is found thath-BN has much narrower VB (valence band ) width and much sharper band edge than those ofw-BN. And the N 2sstates of the two phases of BN are dominant below 30.03 eV and the N 2pstates are dominant in the range between −20.62 and 20.32 eV.

Hydrothermally grown ZnO buffer layer for the growth of highly (4 wt%) Ga-doped ZnO epitaxial thin films on MgAl 2O 4 (1 1 1) substrates

Gallium (4 wt%) doped ZnO (GZO) thin films were deposited on hydrothermally grown ZnO buffered and non-buffered MgAl 2O 4 (1 1 1) substrates by RF magnetron sputtering technique at a growth temperature of 250 °C. The epitaxial ZnO buffer layer was deposited on the MgAl 2O 4 (1 1 1) substrate by a hydrothermal technique using aqueous solutions of zinc nitrate hexahydrate, ammonium nitrate and ammonium hydroxide at 90 °C. The effect of the ZnO buffer layer on the crystallinity, epitaxial nature, surface morphology, optical and electrical properties of the GZO thin films is investigated. X-ray diffraction and transmission electron microscopy showed that the hydrothermally grown ZnO buffer layer and GZO thin film grown on the hydrothermally grown ZnO buffered substrate were grown epitaxially with an orientation relationship of ( 0 0 0 1 ) [ 1 1 2 ¯ 0 ] GZO ‖ ( 1 1 1 ) [ 1 1 2 ¯ ] MgAl 2 O 4 . However, the GZO thin films grown on the non-buffered substrate are polycrystalline in nature with a hexagonal wurtzite phase. The room temperature photoluminescence spectra of the GZO epitaxial thin films grown on the buffered substrate revealed a sharp near band edge emission peak and a lower broad deep-level emission peak compared to the polycrystalline GZO thin film grown on a non-buffered substrate. The electrical resistivity of the GZO thin films is found to be proved from 4.69×10 −3 to 2.27×10 −3 Ω cm by introducing the hydrothermally grown ZnO buffer layer between the GZO thin film and MgAl 2O 4 (1 1 1) substrate.

Strong Room Temperature 505 nm Emission from Hexagonal Crack Free InGaN Thin Film on Si(111) Grown by MBE

GaN growth on Si(111) substrate typically routes to the initial high dislocation density and cracks because of the large discrepancy of lattice constant and thermal expansion coefficient between GaN and Si. This article reports the use of plasma-assisted molecular beam epitaxy (MBE) to grow crack-free InGaN on Si(111) substrate using an optimized GaN/AlN buffer layer. X-ray diffraction revealed that monocrystalline InGaN was obtained. To assess the morphology of the sample, scanning electron microscopy (SEM) and atomic force microscopy (AFM) have been employed. A layer-cracking problem was not observed in our samples. The root–mean–square (rms) roughness value of the surfaces sample is 10.34 nm on a 10 μm × 10 μm scan area. The film is then characterized by photoluminescence (PL) spectroscopy. PL measurement exhibits a sharp and intense band edge emission of InGaN with yellow luminescence (YL), red luminescence (RL) and near infrared luminescence (NIRL) peaks. To date, no approaches have been reported that YL is observed in samples grown by MBE.

Coalescence of magic sized CdSe into rods and wires and subsequent energy transfer

We report on the synthesis of CdSe magic-sized clusters (MSCs) and their evolution into 1D rod and wires retaining the diameter of the order of MSCs. At the beginning of the reaction, different classes of stable MSCs with band gaps of 3.02 eV and 2.57 eV are formed, which exhibit sharp band edge photoluminescence features with FWHM in the order of ∼13 nm. Reaction annealing time was carried out in order to monitor the shape evolution of the MSCs. We find that magic sized CdSe evolve into 1D rod and wires retaining the same diameter upon increasing annealing time. We observed the gradual emergence of new red shifted emission peaks during this shape evolution process, which emerge as a result of one dimensional energy transfer within the magic sized clusters during their subsequent transformation into rods and wires. The smallest, the second smallest sized MSC and the wires sequentially act as donors and acceptors during the size evolution from small MSCs to larger ones, and then eventually to wires. Steady-state and time-resolved luminescent spectroscopy revealed Forster resonance energy transfer (FRET) between the MSCs to the rods and wires.

Fabrication of Improved p-AgGaSe2/n-Si Heterojunction Solar Cells on Optimum Quality Thermally Evaporated AgGaSe2 Thin Films

ABSTRACTOptimum quality polycrystalline AgGaSe2 thin films were deposited on H-terminated n-Si substrates by controlled thermal evaporation method. The film deposition conditions were varied to optimize the structure and optoelectronic properties of AgGaSe2 thin films. X-ray diffraction (XRD) studies showed that all AgGaSe2 films were of chalcopyrite structure and while the films deposited at room temperature (300 K) had random grain orientation, the films deposited at higher substrate temperature (≥ 450K) showed preferred (112) orientation. The composition of the films were analyzed by electron probe microanalysis (EPMA) deposited at different substrate temperatures. The ultraviolet-visible (UV-Vis) spectra showed the optical bandgap of 1.80 eV, with sharper band edge for the films deposited at higher temperature. The films were p-type and the resistivities of the as deposited films at 300 and 650K were ~5×103 and ~200 Ω.cm respectively. p-AgGaSe2/n-Si heterojunction solar cells, having an active area of 0.18 cm2 without any antireflection coating were designed and fabricated. It was observed that the films deposited at 650K produced heterojunctions with significantly improved photovoltaic properties. The evidence of the barrier height modifications have been provided by C-V measurements. Under solar simulator AM1 illumination, the improved junction exhibited an efficiency of 5.2%, whereas the AgGaSe2 films deposited at 300K showed a lower efficiency of 2.1%.

High-quality CdTe nanowires: Synthesis, characterization, and application in photoresponse devices

High-quality straight and multiply kinked CdTe nanowires (NWs) were synthesized by the facile chemical vapor deposition method at 600 °C. The as-synthesized NWs were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive x-ray spectroscopy, and photoluminescence (PL) spectroscopy. The straight CdTe NWs have single crystalline zinc blende structure with growth direction along the ⟨111⟩ direction. Their PL spectra consist only sharp near band edge emission around 824.3 nm. The multiply kinked CdTe NWs contain one or more fixed (∼125.2°) angle joints; each arm of the kinked NWs is single crystalline with similar selected area electron diffraction pattern as that of the straight CdTe NWs. The two growth directions in the multiply kinked CdTe NWs are ⟨200⟩ and ⟨111⟩. Single straight and kinked CdTe NW photoresponse devices were fabricated and testified to have high photocurrent decay ratio, high responsivity, fast response time, and no decay tail under 633 nm He–Ne laser illumination. These straight and multiply kinked CdTe NWs may open up potential applications in the bottom-up integrated nanoelectronic and nanophotonic systems, such as photovoltaic and multiterminal nanodevices etc.

Surface Andreev bound states of the superfluid 3He B phase

Surface Andreev bound states of superfluid 3He have been detected by transverse acoustic impedance measurement. In the diffusive limit of the quasiparticle scattering off a wall, a singularity in the temperature dependence of the acoustic impedance appears at a particular temperature. At this temperature, pair excitations of the surface Andreev bound state quasiparticle and the propagating Bogoliubov quasiparticle drastically change their nature due to a very sharp band edge of the surface Andreev bound states, resulting in the singularity of the acoustic impedance. This temperature dependence was well reproduced by a theory which properly takes into account the surface Andreev bound states. In a partially specular wall boundary condition achieved by coating the wall with thin 4He layers, the singularity shifted to a higher temperature since the bandwidth became broader as the specularity increased. The coating with 4He influences only the states in the vicinity of the wall and thus the shift is further evidence that the transverse acoustic impedance senses the surface Andreev bound states.

Superhydrophobic and transparent ZnO thin films synthesized by spray pyrolysis technique

Superhydrophobic and transparent zinc oxide (ZnO) thin films were deposited by a simple and cost effective spray pyrolysis technique (SPT) onto the glass substrates at 723 K from an aqueous zinc acetate precursor solution. The solution concentration was varied from 0.1 to 0.4 M and its effect on structural, morphological, wetting and optical properties of ZnO thin films was studied. The synthesized films were found to be polycrystalline, with preferential growth along c-axis. A slight improvement in the crystallite size and texture coefficient is observed as the concentration of the solution is increased. SEM micrographs show the uniform distribution of spherical grains of about 60–80 nm grain size. The films were specular and highly transparent with average transmittance of about 85%. The spectrum shows sharp absorption band edge at 381 nm, corresponding to optical gap of 3.25 eV. The samples of texture coefficient less than 90% and roughness less than 75 nm are hydrophobic and above these values they become superhydrophobic in nature. The hydrophobicity coupled with high transmittance is of great importance in commercial application such as transparent self-cleaning surfaces, anti-fog, anti-snow, fluid microchips and microreactors.

An integrated compact coplanar waveguide high‐pass filter implemented on high resistivity silicon substrate

AbstractIn this article, the fabrication of a compact and high performance semi‐lumped coplanar waveguide high‐pass filter (CPW‐HPF) implemented on high resistivity silicon (HRS) substrate is proposed.The design procedure and the equivalent circuit of the proposed semi‐lumped CPW‐HPF is discussed. The filter structure is simple, and the performances are fairly good. This designed filter at cutoff frequency fc of 21.6 GHz has very good measured characteristics including a balance passband of S21 of −0.5 ± 0.3 dB and a sharp band edge rejection. Experimental results of the fabricated filter show a good agreement with the predicted results. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1150–1153, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25116

A Compact HTS Duplexer for Communication Application

A compact L-band high temperature superconducting (HTS) microstrip input duplexer for satellite communication application has been developed. The duplexer has two output channels with center frequencies at 1200 and 1250 MHz, respectively. It was fabricated on a single piece of 2-in double-sided YBCO thin film on LaAlO3 substrate with a dimension of 26 × 42 mm. The duplexer consists of a T-junction and two bandpass filters, i.e., a ten-pole filter and an eight-pole filter for the two transmission channels, respectively. Both of the two HTS bandpass filters were specially designed to suppress the second-harmonic spurious passband. Moreover, an HTS bandstop filter is added to the duplexer to improve the out-of-band rejection at a special frequency band. As a result of the preceding special designs, in addition to a very low insertion loss (0.2 dB), considerably sharp band edge (21 dB/MHz), and good voltage standing wave ratio (better than 1.4:1), much deeper out-of-band rejection (more than 75 dB) can be realized in a much wider frequency range, i.e., up to 3 GHz, which is almost three times above the center frequency of the passbands. Although all the elements of the duplexer are integrated within a single-piece YBCO film and the layout is compact, excellent isolation (higher than 80 dB) between the two channels has still been achieved.

Characterization of quaternary AlInGaN epilayers and polarization-reduced InGaN/AlInGaN MQW grown by MOCVD

We have demonstrated the growth of quaternary AlInGaN compounds at different growth temperatures and pressures with metalorganic chemical vapor deposition (MOCVD). The optical properties of the samples have been investigated by photoluminescence (PL) at different temperatures. The results show that the sample grown at higher temperature (850 °C) exhibits the best optical quality for its sharp band edge luminescence and weak yellow luminescence. The AlInGaN exhibited three-dimensional (3D) growth mode at higher pressure. The band edge emission almost disappeared. With the optimization of AlInGaN growth parameters, we replaced the traditional barrier in InGaN/GaN multiple quantum wells (MQWs) with AlInGaN barriers. The peak wavelength for the InGaN/AlInGaN-MQW based light emitting diodes (LEDs) was very stable at various injection current levels because of the polarization-matched InGaN/AlInGaN MQWs.

High Al-content Al xGa 1− xN epilayers grown on Si substrate by plasma-assisted molecular beam epitaxy

The microstructure and optical properties of Al x Ga 1− x N/GaN/AlN films on Si (1 1 1) substrate grown by plasma-assisted molecular beam epitaxy (MBE) have been studied and investigated. Reflection high energy electron diffraction (RHEED), scanning electron microscopy (SEM), high-resolution X-ray diffraction (HR-XRD), energy dispersive X-ray spectroscopy (EDS) line analysis and photoluminescence (PL) were used to investigate a reconstruction pattern, cross-section, mole fraction and crystalline quality of the heterostructure. By applying the Vegard's law, a high Al-mole fraction of Al x Ga 1− x N sample with value of 0.43 has been obtained and compared with EDS line analysis measurement value. PL spectrum has exhibited a sharp and intense band edge emission of GaN with the absence of yellow emission band, indicating good crystal quality of the Al x Ga 1− x N has been successfully grown on Si substrate.

Metal-insulator transition in cluster dynamical mean field theory: Intersite correlation, cluster size, interaction strength, and the location of the transition line

To gain insight into the physics of the metal insulator transition and the effectiveness of cluster dynamical mean field theory (DMFT) we have used one, two and four site dynamical mean field theory to solve a polaron model of electrons coupled to a classical phonon field. The cluster size dependence of the metal to polaronic insulator phase boundary is determined along with electron spectral functions and cluster correlation functions. Pronounced cluster size effects start to occur in the intermediate coupling region in which the cluster calculation leads to a gap and the single-site approximation does not. Differences (in particular a sharper band edge) persist in the strong coupling regime. A partial density of states is defined encoding a generalized nesting property of the band structure; variations in this density of states account for differences between the dynamical cluster approximation and the cellular-DMFT implementations of cluster DMFT, and for differences in behavior between the single band models appropriate for cuprates and the multiband models appropriate for manganites. A pole or strong resonance in the self energy is associated with insulating states; the momentum dependence of the pole is found to distinguish between Slater-like and Mott-like mechanisms for metal insulator transition. Implications for the theoretical treatment of doped manganites are discussed.