Characteristics of III-nitride photodiodes with self-assembled quantum dots

In this work, it has been demonstrated that metal–semiconductor–metal (MSM) photodiodes (PDs) with InGaN self-assembled quantum dots (QDs) were fabricated and compared with conventional InGaN MSM photodiodes. The scanning near-field optical microscope (SNOM) results revealed that such InGaN nanostructures could have better absorption for the near-field light with the wavelength of 457–514 nm. It was found that the InGaN QD photodiode with lower dark current can operate in the normal incidence mode; we could achieve a much larger photocurrent to dark current contrast ratio from MSM photodiodes with nanoscale InGaN quantum dots. It was also found that the measured responsivity of MSM photodiodes with QDs and without QDs approximated to the same in the range of 390–460 nm. Furthermore, the photodiodes with QDs showed higher spectral response than that of the photodiodes without QDs at wavelengths < 350 nm and > 480 nm.     

Electrical and optical properties of thin films of DNA:PEDOT

We report investigations of functionalized DNA:PEDT–PSS films. The electrical conductivity of the sample material at the room temperature was about (1–5) × 10⁻¹⁰ Ω⁻¹ cm⁻¹. The IV curves of the samples were linear and symmetrical in the region from the room temperature down to the liquid nitrogen temperature. The thermal activation energy of the conductivity near the room temperature was about 0.033 eV independently on the applied bias. The weak carrier trapping was identified by the Thermally Stimulated Current method, proving the fast recombination of light-generated carriers. Notably, by constant light excitation a “bistable” photoconduction below the room temperature was evidenced, i.e., upon excitation by a white light a remarkable increase of the photocurrent could be observed below 145–155 K by cooling the samples. Meanwhile by heating the photosensitivity remained increased up to 235–245 K. Such phenomenon could presumably be attributed to the light-induced changes of the sample material morphology and/or associated variation of carrier transport conditions.     

Synthesis,structure, and optical properties of Au–TiO2 composite thin films

Titanium dioxide (TiO₂) films with varying concentrations of gold particles were synthesized using pulsed DC magnetron sputtering, with the intent to develop infrared reflecting films for use on cars and planes to reduce solar heat load. Under our deposition conditions, the films are smooth (RMS roughness on the order of 1.0–2.0 nm) and consist of rutile TiO₂ with embedded gold. The average gold particle diameter on the sample surface was found to change from 60 to 200 nm as the volume fraction of gold in the films increased from 1.9 to 4.3% (3.5 to 7.9 mol% Au). The maximum reflectance of these films in the infrared region (800–2500 nm) is > 50%, compared with 30% for pure TiO₂. The Maxwell–Garnett equation does not model the reflectance data very well, due to the relatively large gold particle size. Instead, by assuming that the contribution of gold particles to the reflectance response is proportional to their projected areal fraction in an effective medium approximation, we were able to fit the observed reflectance data quite well.     

Comparison between synthesis techniques to obtain ZnO nanorods and its effect on dye sensitized solar cells

This paper reports additive-free, reproducible, low-temperature solution-based process for the preparation of crystalline ZnO nanorods by homogeneous precipitation from zinc acetate. Also, ZnO nanorod structured dye sensitized solar cells using ruthenium dye (Z907) have been fabricated and characterized. The formation and growth of zinc oxide nanorods are successfully achieved. We analyzed three different synthesis method using solution phase, autoclave and microwave. The calcination effects on the morphology of ZnO nanorods are also investigated. Analysis of ZnO nanorods shows that calcination at lower temperature is resulted in a nanorod growth. Additive-free, well-aligned ZnO nanorods are obtained with the length of 330–558 nm and diameters of 14–36 nm. The XRD, SEM, and PL spectra have been provided for the characterization of ZnO nanorods. Microwave-assisted ZnO nanostructured dye sensitized solar cell devices yielded a short-circuit photocurrent density of 6.60 mA/cm², an open-circuit voltage of 600 mV, and a fill factor of 0.59, corresponding to an overall conversion efficiency of 2.35% under standard AM 1.5 sun light.     

High frequency magnetic eigen excitations in a spin valve submitted to CPP DC current

We study the magnetization dynamics induced at low field by spin-transfer in a pillar-shaped spin valve. The spin valve is a square of 150 nm× 150 nm patterned from a film of IrMn 7 nm/CoFe, 2.4 nm/Ru, 0.8 nm/CoFe, 4.4 nm/Cu, 2.6 nm/CoFe, and 3.6 nm. The spin valve is studied in its anti-parallel state at 50 K. The high frequency voltage noise generated by the DC current flowing through the magneto-resistive device is used to identify the excitations induced by spin-transfer. Between an instability current of 1.72 mA and the switching current of 1.89 mA, we demonstrate the existence of pre-switch steady-state excitations, i.e. low amplitude precession. We study the frequency (10 GHz, red shift −1.46 GHz/mA) of this excitation, its line width (78–246 MHz), the power it carries (113 nW), and the current dependance thereof. We discuss those experimental findings using the formalism of Sun et al. and Valet et al., and show that the experimental behavior can be described by a macrospin approximation only at the very onset of the pre-switch excitations. The early saturation of the excitation power and the non-monotonic switching probability with the current are experimental indications that the pre-switch excitations are strongly non-uniform when approaching the switching current.     

Gas sensitive vapor grown carbon nanofiber/polystyrene sensors

A new class of conductive composites with good gas sensitivity was fabricated by filling polystyrene with vapor grown carbon nanofibers (VGCNF). A solution mixing/solvent removal procedure was used. VGCNFs form conductive networks at fiber loadings above the percolation limit within the matrix. Greatly improved conductivity is achieved relative to the same volume fraction of carbon black addition when these fibers are distributed to give reasonably uniform dispersions in the matrix. The high aspect ratios of these fibers (∼70–250 nm diameters and 5–75 μm lengths) assist in forming low wt.% percolation thresholds (below 1 wt.% fiber). Excellent gas sensitivity with 10⁴–10⁵ times higher than the original resistance value in many saturated organic vapors and a maximum resistance response of about 1.1 × 10⁵ times exposure to saturated THF vapor at 6.25 wt.% of VGCNF in the polystyrene matrix was observed. The maximum resistance response declined from about 2.0 × 10⁵ times at 15 °C to about 3.4 × 10⁴ times at 55 °C. These composites exhibited stable and reusable gas sensitivity to THF vapor. Carbon black/polystyrene composites exhibit a negative vapor coefficient (NVC) upon swelling caused by filler redistribution. In contrast, VGCNF/polystyrene composites are more stable, with much smaller NVC values due to their high aspect ratios and reinforcing effects which stabilize electrical percolation pathways. Thus, VGCNF/organic polymer composites are good gas sensor candidates for detecting organic vapors.     

Role of environmental and annealing conditions on the passivation-free in-Ga–Zn–O TFT

We examined the characteristics of passivation-free amorphous In–Ga–Zn–O thin film transistor (a-IGZO TFT) devices under different thermal annealing atmospheres. With annealing at higher temperature, the device performed better at the above-threshold operation region, which indicated the film quality was improved with the decrease of defects in the a-IGZO active region. The mobility, threshold voltage and subthreshold swing of a-IGZO TFT annealed at 450 °C was 7.53 cm²/V s, 0.71 V and 0.18 V/decade, respectively. It was also observed that the a-IGZO was conductive after thermal annealing in the vacuum, due to the ease of oxygen out-diffusion from the a-IGZO back channel. The oxygen deficiency resultantly appeared, and provided leaky paths causing electrical unreliability when TFT was turned off. In contrast, the annealing atmosphere full of O₂ or N₂ would suppress the oxygen diffusion out of the a-IGZO back channel. The worst V_th degradation of a-IGZO TFT after positive gate bias stress and negative gate bias stress (NGBS) was about 2 V and ? 2 V, respectively. However, the V_th shift in the NGBS testing could be suppressed to ? 0.5 V in vacuum chamber. Material analysis methods including X-ray photoelectron spectroscopy and scanning electron microscopy were used to investigate the change of a-IGZO film after different thermal annealing treatments. The variation of O 1s spectra with different annealing atmospheres showed the consistence with our proposed models.     

Current-induced butterfly shaped domains and magnetization switching in magnetic tunnel junctions

Patterned magnetic tunnel junctions (MTJs) with the layer structure of Ta (5 nm)/Ni₇₉Fe₂₁ (5 nm)/Cu (20 nm)/Ni₇₉Fe₂₁ (5 nm)/Ir₂₂Mn₇₈ (10 nm)/Co₇₅Fe₂₅ (4 nm)/Al (0.8 nm)-oxide/Co₇₅Fe₂₅ (4 nm)/Ni₇₉Fe₂₁ (20 nm)/Ta (5 nm) were fabricated using magnetron sputtering deposition and lithography. High tunnelling magnetoresistance ratios of 22 and 50% were obtained at room temperature before and after annealing, respectively. The evolution of leaf shaped images was observed via Lorentz transmission electron microscopy (LTEM) on the MTJs, which were deposited on a patterned and carbon-coated transmission electron microscopy grid. These leaf-like LTEM images correspond to a butterfly shaped domain structure that was confirmed by a micromagnetics simulation. When a large DC current or bias voltage was applied across the MTJ, the butterfly-like vortex domain structures could be induced to form in the free layer of the MTJ, resulting in a significant decrease of magnetization in the free layer. The existence of these butterfly shaped domains could be one of the major causes of the bias voltage dependence of the TMR ratio.     

Preparation and thermomechanical properties of Ag-PVA nanocomposite films

Metal–polymer hybrid nanocomposites have been prepared from an aqueous solution of polyvinyl alcohol (PVA) and silver nitrate (AgNO₃). The silver nanoparticles were generated in PVA matrix by the reduction of silver ions with PVA molecule at 60–70 °C over magnetic stirrer. UV–vis analysis, X-ray diffraction studies, transmission electron microscopy, scanning electron microscopy and current–voltage analysis were used to characterize the nanocomposite films prepared. The X-ray diffraction analysis reveals that silver metal is present in face centered cubic (fcc) crystal structure. Average crystallite size of silver nanocrystal is 19 nm, which increases to 22 nm on annealing the film at 150 °C in air. This result is in good agreement with the result obtained from TEM. The UV–vis spectrum shows a single peak at 433 nm, arising from the surface plasmon absorption of silver nanocolloids. This result clearly indicates that silver nanoparticles are embedded in PVA. An improvement of mechanical properties (storage modulus) was also noticed due to a modification of PVA up to 0.5 wt% of silver content. The current–voltage (I–V) characteristic of nanocomposite films shows increase in current drawn with increasing Ag-content in the films.     

Development of a very fast spectral response measurement system for analysis of hydrogenated amorphous silicon solar cells and modules

An important requirement for a very fast spectral response measurement system is the simultaneous illumination of the solar cell at multiple well defined wavelengths. Nowadays this can be done by means of light emitting diodes (LEDs) available for a multitude of wavelengths. For the purpose to measure the spectral response (SR) of amorphous silicon solar cells a detailed characterization of LEDs emitting in the wavelength range from 300 nm to 800 nm was performed. In the here developed equipment the LED illumination is modulated in the frequency range from 100 Hz to 200 Hz and the current generated by each LED is analyzed by a Fast Fourier Transform (FFT) to determine the current component corresponding to each wavelength. The equipment provides a signal to noise ratio of 2–4 orders of magnitude for individual wavelengths resulting in a precise measurement of the SR over the whole wavelength range. The difference of the short circuit current determined from the SR is less than 1% in comparison to a conventional system with monochromator.     

Electrical and photocurrent characteristics of Au/PVA (Co-doped)/n-Si photoconductive diodes

In this work, we investigate the some main electrical and photocurrent properties of the Au/PVA(Co-doped)/n-Si diodes by using current–voltage (I–V) measurements at dark and various illumination intensity. Two types of diodes with and without polyvinyl alcohol (PVA) (Co-doped) polymeric interfacial layer were fabricated and measured at room temperature. Results show that the polymeric interfacial layers and series resistance (R_s) strongly affect the main electrical parameters of these structures. Also, metal/polymer/semiconductor (MPS) diode with PVA (Co-doped) interfacial organic layer is very sensitive to the light such that the current in reverse bias region increase by 10³–10⁴ times with the increasing illumination intensity. The open circuit voltage V_oc and short-circuit current I_sc values of this MPS diode under 100 mW/cm² illumination intensity were found as 0.28 V and 19.3 μA, respectively.     

Preparation,spectroscopic characterization and energy transfer investigation of iron-chromium diffusion co-doped ZnSe for mid-IR laser applications

The spectroscopic characterization and energy transfer mechanism of iron-chromium co-doped ZnSe polycrystalline (Cr,Fe:ZnSe) were reported with dimension of 15 mm × 15 mm × 2 mm obtained by controlled post-growth thermal diffusion method. The infrared absorption is characterized by a strong broad-band centered at 1770 nm which can be attributed to the only spin-allowed transition ⁵T₂ → ⁵E within the 3d⁴ shell of Cr²⁺ ions. Photoluminescence spectrum shows a relatively strong broad emission band centered at 4.1 μm with a width of 0.8 μm (FWHM) under 1770 nm excitation at room temperature and reveals effective Cr²⁺ → Fe²⁺ energy transfer process. Room temperature photoluminescence decay about 8 μs was measured. All the results indicate that Cr,Fe:ZnSe could achieve laser operation at 3.7–4.5 μm via Cr²⁺ → Fe²⁺ energy transfer using a more convenient laser pump source in the near IR region.     

Fabrication of partially crystalline TiO2 nanotube arrays using 1, 2-propanediol electrolytes and application in dye-sensitized solar cells

We report the fabrication of self-organized partial crystalline TiO₂ nanotube arrays in 1, 2-propanediol containing fluoride ion. The influence of anodization parameters including NH₄F concentration, water content, anodization voltage and time on the morphology, diameter and length of TiO₂ nanotube were investigated in detail. The prepared TiO₂ nanotube has diameter in 30–120 nm and length in 0.6–3 μm. TiO₂ nanotube arrays are used as photoanode for the application in dye-sensitized solar cell and the photovoltaic performance of 1.91% is achieved with a TiO₂ nanotube sample of 2.2 μm in length combining with N719 dye, and the corresponding photovoltaic parameters of 3.6 mA cm^?2 in short circuit photocurrent density, 840 mV in open circuit potential, and 63.2% in fill factor.     

Formation and third-order optical nonlinearities of silver nano-crystals embedded bismuthate glasses

We report the synthesis of silver nano-crystals (Ag-NCs) composited bismuthate glasses through a single-step thermal reduction approach. The Ag-NCs show broad surface plasmon resonance (SPR) absorption bands with spectral coverage from visible to near infrared region. Bi₂O₃ is found to be the key factor to the formation of Ag-NCs, e.g. their number density varies significantly and the corresponding SPR red-shifts as Bi₂O₃ content increased. Third-order optical nonlinearities (TON) of the silver-bismuthate nanocomposites were investigated by Z-scan and pump-probe techniques at the wavelength of 800 nm. The experimental results revealed SPR-assisted behavior and ultrafast temporal response (less than 100 fs) of TON properties of the nanocomposites.     

MgO:Dy3 + nanophosphor: Self ignition route,characterization and its photoluminescence properties

For the first time series of MgO phosphors doped with different concentrations of Dy^3 + (1–9 mol%) were prepared by solution combustion method using glycine as a fuel. The final products were well characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. The powder X-ray diffraction patterns of the as-formed product show single cubic phase. The crystallite size estimated using Scherrer's method was found to be in the range 5–15 nm and the same was confirmed by transmission electron microscopy result. Photoluminescence properties of Dy^3 + (1–9 mol%) doped MgO for near ultra violet excitation (325 nm) was studied in order to investigate the possibility of its use in white light emitting diode applications. The emission spectra consists of intra 4f transitions of Dy^3 +, namely ⁴F_9/2 → ⁶H_15/2 (483 nm), and ⁴F_9/2 → ⁶H_13/2 (573 nm). Further, the emission at 573 nm shows strong yellow emission and can be applied to the yellow emission of phosphor for the application for near ultraviolet excitation. The intensity of yellow emission was attributed to intrinsic defects, especially oxygen-vacancies, which could assist the energy transfer from the MgO host to the Dy^3 + ions. The Commission International De I-Eclairage chromaticity co-ordinates were calculated from emission spectra, the values (x,y) were very close to the National Television System Committee standard value of white emission. Therefore, the present phosphor was highly useful for display applications.     

Tantalum oxide film prepared by reactive magnetron sputtering deposition for all-solid-state electrochromic device

Inorganic-solid-state electrolyte tantalum oxide thin films were deposited by reactive DC magnetron sputtering to improve the leakage and deterioration of traditional liquid electrolytes in electrochromic devices. O₂ at 1–20 sccm flow rates was used to deposit the tantalum oxide films with various compositions and microstructures. The results indicate that the tantalum oxide thin films were amorphous, near-stoichiometric, porous with a loose fibrous structure, and highly transparent. The maximum charge capacity was obtained at an oxygen flow rate of 3 sccm and 50 W. The transmission change of the Ta₂O₅ film deposited on a WO₃/ITO/glass substrate between colored and bleached states at a wavelength of 550 nm was 56.7%. The all-solid-state electrochromic device was fabricated as a multilayer structure of glass/ITO/WO₃/Ta₂O₅/NiO_x/ITO/glass. The optical transmittance difference of the device increased with increasing applied voltage. The maximum change was 66.5% at an applied voltage of ± 5 V.     

Growth of SiO2 hierarchical nanostructure on SiC nanowires using thermal decomposition of ethanol and titanium tetrachloride and its FTIR and PL property

Novel SiO₂ hierarchical nanostructure has been grown on SiC nanowires using thermal decomposition of a mixture of ethanol and titanium tetrachloride. Novel nanostructure was realized in one synthesis route. Based on SEM and TEM observations, the hierarchical nanostructure consists of core-shell SiC–SiO₂ main stem nanowire and a lot of SiO₂ nanorod branches grown on the main stem. A mean diameter of SiC central cores was about 40 nm and their lengths reach about 100 μm. The lengths and diameters of SiO₂ nanorod-like branches were ranged in 400–800 nm and 30–120 nm, respectively. The growth of core-shell SiC–SiO₂ nanowires obeyed vapor–liquid–solid mechanism and the SiO₂ nanorod-like branches grew via vapor–solid mechanism. The infrared absorption and photoluminescence properties of the grown nanostructure were investigated.     

Dielectric inspection of BaZr0.2Ti0.8O3 ceramics under bias electric field: A survey of polar nano-regions

The structure and dielectric properties of BaZr_0.2Ti_0.8O₃ ceramics prepared by citrate method were investigated. Structural analysis of the ceramics indicated a cubic perovskite structure and a fine-grained (about 250 nm) microstructure. The ceramics displayed a frequency dispersion of the dielectric loss and a slim polarization versus electric field (P–E) hysteresis loop. These results were related to the existence of polar nano-regions (PNRs) embedded in the non-polar matrix of the ceramics. The nonlinear dielectric properties under bias electric field were found to be dependent on holding time of applied bias field. The phenomenon was qualitatively explained with polarization reorientation of PNRs under the bias field. Fitting the dielectric constants under the bias field to a multipolarization mechanism model resolved the contribution of PNRs from the overall dielectric response. From the fitting, the polarization and size of PNRs in the ceramics were determined to be around 0.4 μC/cm² and 9 nm, respectively.     

Near infrared and mid infrared investigations of adsorbed phenol on HDTMAB organoclays

X-ray diffraction patterns (XRD), infrared Spectroscopy (IR) and near infrared spectroscopy (NIR) have been used to measure the adsorption of phenol on untreated montmorillonite and on hexadecyltrimethylammonium bromide (HDTMAB) modified montmorillonite. The mid infrared spectra indicate that both the surfactant molecule and phenol enter the interlayer of organoclays, replacing the interlayer cations. The higher concentration surfactant leads to a decrease in wavenumber of the bands of organoclays and to increase in intensity. The near infrared spectra (9000–4000 cm^?1) show a prominent band 8260 cm^?1, assigned to the combination result of the CH stretching vibrations of high concentration surfactant and absorbed phenol. The main band observed at 7090 cm^?1 is assigned to the first fundamental overtone of the OH stretching vibrations at 3415 cm^?1 for organoclay. The organoclays are characterised by prominent bands situated between 5900 and 5700 cm^?1. Both the higher concentration of organic molecules and adsorbed phenol causing the near infrared spectra of organic clays to be more complex for spectra in the region from 4700 to 5500 cm^?1. The main band of 4535 cm^?1 for montmorillonite shifts towards the lower wavenumber sites for higher concentration organoclay. The intensity of near infrared spectra generally rises with the value of surfactant concentration increasing, showing certain regularity. It is concluded that phenol is adsorbed to significantly greater amounts on the higher concentration organoclays.     

Transparent amorphous zinc oxide thin films for NLO applications

This review focuses on the growth and optical properties of amorphous zinc oxide (ZnO) thin films. A high quality ZnO films fabricated by dip-coating (sol–gel) method were grown on quartz and glass substrates at temperature equal to 350 K. The amorphous nature of the films was verified by X-ray diffraction. Atomic Force Microscopy was used to evaluate the surface morphology of the films. The optical characteristics of amorphous thin films have been investigated in the spectral range 190–1100 nm. Measurement of the polarized optical properties was shows a high transmissivity (80–99%) and low absorptivity (<5%) in the visible and near infrared regions at different angles of incidence. Linear optical properties were investigated by classic and Time-Resolved Photoluminescence (TRPL) measurements. Photoluminescence spectrum exhibits a strong ultraviolet emission while the visible emission is very weak. An innovative TRPL technique has enabled the measurement of the photoluminescence decay time as a function of temperature. TRPL measurements reveal a multiexponential decay behavior typical for amorphous thin films. Second and third harmonic generation measurements were performed by means of the rotational Maker fringe technique using Nd:YAG laser at 1064 nm in picosecond regime for investigations of the nonlinear optical properties. The obtained values of second and third order nonlinear susceptibilities were found to be high enough for the potential applications in the optical switching devices based on refractive index changes. Presented spectra confirm high structural and optical quality of the investigated zinc oxide thin films.