Small Full-width At Half-maximum Research Articles

Laser Modification and Characterization of LiMn2O4 Cathodes for Thin Film Lithium Ion Battery

In this paper, we report on laser modification and characterization of LiMn2O4 cathodes for thin film lithium ion battery. The as-deposited film was in amorphous state. When the laser power is over 60W, a peak at 2θ = 18.48° with d111 = 4.795 Å was observed, which suggests that the LiMn2O4 films were restructured and crystalline spinel films were formed. Increasing the laser power the full width at half maximum (FWHM) of the (111) peak was decreased and the intensity was increased, which implies that the laser power significantly improves the crystal quality of the films, and the smallest FWHM was gained at the laser power of between 80 W and 100 W. In electrochemical analysis of the cathode laser annealed at 80 W, the maximum discharge capacity is 25.3 μAh/cm2, which is twice larger than that of the battery with amorphous cathode films. Due to higher internal resistance, which originates from the difficult absorption reaction of the electrolyte and the cathode film melted or hardened in the laser annealing process, the coulombic efficiency was only 23.6% at the 1st cycle. Thus it is necessary to further improve the laser annealing process for fabrication of thin film battery.

Optimized grating as an ultra-narrow band absorber or plasmonic sensor

Lamellar gratings are investigated via temporal coupled-mode theory and numerical simulations. Total absorption can be achieved by an optimized grating with shallow grooves under normal incidence and the full width at half-maximum (FWHM) is only 0.4 nm. For certain wavelengths, the structure shows high absorption only within an ultra-narrow angle, which suggests that it can be used as a highly directional thermal emitter according to Kirchhoff's law. Besides, the resonant wavelength is sensitive to the refractive index of the environmental dielectric. The large sensitivity (1400 nm/RIU) and simultaneous small FWHM result in a huge figure-of-merit of 2300/RIU, which enables the structure to have great potential in plasmonic sensing.

Impact of Epi-Layer Quality on Reliability of GaN/AlGaN/GaN Heterostructure Field-Effect Transistors on Si Substrate

This paper reports the impact of epi-layer quality on the short-term reliability of GaN/AlGaN/GaN heterostructure field-effect transistors fabricated on Si substrates. At an early stage of 50 V high-temperature operating life tests, they exhibited a significant lowering of the forward breakdown voltage. The mean time to failure (MTTF) determined by this degradation mode was exponentially increased by decreasing full-width at half-maximum (FWHM) values of X-ray rocking curves. In small FWHM samples, extrapolated MTTF values exceeded 1×107 h at Tch=150°C. Alternatively, another degradation mode such as drain current degradation will dominate the long-term reliability at this temperature range. High-resolution transmission electron microscope and energy dispersive X-ray spectroscopy analyses revealed thinning or partial vanishment of GaN cap layer in a degraded sample. Gate current-voltage characteristics calculation based on the self-consistent Schrodinger-Poisson analysis suggested lowering of the effective barrier height by decreasing cap layer thickness is responsible for the Schottky degradation. When considering these experimental and theoretical findings, degradation mechanisms assuming diffusion of interfacial materials along dislocations are discussed.

Effects of incidence angle on the structure and properties of cathodic vacuum arc deposition MgO thin films

MgO thin films, as the protective layers for plasma display panels (PDP), were prepared by using cathodic vacuum arc deposition technique. The influences of deposition angle between −60° and 60° on film structure and properties were investigated. X-ray diffraction (XRD), ellipsometer, thermal field emission environment scanning electron microscopy (SEM) and UV-Visible spectrophotometry were used to study the properties of MgO thin films like crystallization, surface structures, thicknesses and refraction indices. Our results show that the thickness of MgO thin film decreases with the increase of incidence angle. This is confirmed by the transmittance spectra as well. The film deposited at 0° shows sharper diffraction peaks and smaller FWHMs (full width at half maximum) of both MgO (200) and (220), which means better crystallization quality of the film. The higher packing density is achieved on the 0° deposited film as well.

Radio-frequency superimposed direct current magnetron sputtered Ga:ZnO transparent conducting thin films

The utilization of radio-frequency (RF) superimposed direct-current (DC) magnetron sputtering deposition on the properties of gallium doped ZnO (GZO) based transparent conducting oxides has been examined. The GZO films were deposited using 76.2 mm diameter ZnO:Ga2O3 (5 at. % Ga vs. Zn) ceramic oxide target on heated non-alkaline glass substrates by varying total power from 60 W to 120 W in steps of 20 W and at various power ratios of RF to DC changing from 0 to 1 in steps of 0.25. The GZO thin films grown with pure DC, mixed approach, and pure RF resulted in conductivities of 2200 ± 200 S/cm, 3920 ± 600 S/cm, and 3610 ± 400 S/cm, respectively. X-ray diffraction showed all films have wurtzite ZnO structure with the c-axis oriented perpendicular to the substrate. The films grown with increasing RF portion of the total power resulted in the improvement of crystallographic texture with smaller full-width half maximum in χ and broadening of optical gap with increased carrier concentration via more efficient doping. Independent of the total sputtering power, all films grown with 50% or higher RF power portion resulted in high mobility (∼28 ± 1 cm2/Vs), consistent with observed improvements in crystallographic texture. All films showed optical transmittance of ∼90% in the visible range.

Image quality of microcalcifications in digital breast tomosynthesis: Effects of projection-view distributions

To analyze the effects of projection-view (PV) distribution on the contrast and spatial blurring of microcalcifications on the tomosynthesized slices (X-Y plane) and along the depth (Z) direction for the same radiation dose in digital breast tomosynthesis (DBT). A GE GEN2 prototype DBT system was used for acquisition of DBT scans. The system acquires PV images from 21 angles in 3° increments over a ±30° range. From these acquired PV images, the authors selected six subsets of PV images to simulate DBT of different angular ranges and angular increments. The number of PV images in each subset was fixed at 11 to simulate a constant total dose. These different PV distributions were subjectively divided into three categories: uniform group, nonuniform central group, and nonuniform extreme group with different angular ranges and angular increments. The simultaneous algebraic reconstruction technique (SART) was applied to each subset to reconstruct the DBT slices. A selective diffusion regularization method was employed to suppress noise. The image quality of microcalcifications in the reconstructed DBTs with different PV distributions was compared using the DBT scans of an American College of Radiology phantom and three human subjects. The contrast-to-noise ratio (CNR) and the full width at half maximum (FWHM) of the line profiles of microcalcifications within their in-focus DBT slices (parallel to detector plane) and the FWHMs of the interplane artifact spread function (ASF) in the Z-direction (perpendicular to detector plane) were used as image quality measures. The results indicate that DBT acquired with a large angular range or, for an equal angular range,with a large fraction of PVs at large angles yielded superior ASF with smaller FWHM in the Z-direction. PV distributions with a narrow angular range or a large fraction of PVs at small angles had stronger interplane artifacts. In the X-Y focal planes, the effect of PV distributions on spatial blurring depended on the directions. In the X-direction (perpendicular to the chestwall), the normalized line profiles of the calcifications reconstructed with the different PV distributions were similar in terms of FWHM; the differences in the FWHMs between the different PV distributions were less than half a pixel. In the Y-direction (x-ray source motion), the normalized line profiles of the calcifications reconstructed with PVs acquired with a narrow angular range or a large fraction of PVs at small angles had smaller FWHMs and thus less blurring of the line profiles. In addition, PV distributions with a narrow angular range or a large fraction of PVs at small angles yielded slightly higher CNR than those with a wide angular range for small, subtle microcalcifications; however, PV distributions had no obvious effect on CNR for relatively large microcalcifications. PV distributions affect the image quality of DBT. The relative importance of the impact depends on the characteristics of the signal and the direction (perpendicular or parallel) relative to the direction of x-ray source motion. For a given number of PVs, the angular range and the distribution of the PVs affect the degree of in-plane and interplane blurring in opposite ways. The design of the scan parameters of tomosynthesis systems would require proper consideration of the characteristics of the signals of interest and the potential trade-off of the image quality of different types of signals.

Simulation of Two-Dimensional Transverse Laser Cooling of Cesium Beam from Pyramidal Magneto-Optical Trap Atom Funnel

Simulation of two-dimensional (2D) transverse laser cooling of Cs atomic beam from pyramidal magneto-optical trap atom funnel (PMOTAF) conceived for atom lithography is presented. The results show that both the minimum full width at half maximum (FWHM) and the maximum peak value of the spatial profile of the atomic beam occur at the frequency detuning of optical molasses equals to -0.5 Г. Moreover, for each frequency detuning, an increase in the intensity of the optical molasses leads to smaller FWHM and higher peak value. The not negligible role of gravity on the atomic beam of sub-thermal longitudinal velocity along the horizontal direction is that every atomic trajectory possesses a parabolic motion either before or after laser cooling which leads to a noticeable displacement of the peak value at the observation plane with respect to the starting point.

Room-Temperature Epitaxial Growth of High-Quality m-Plane InAlN Films on Nearly Lattice-Matched ZnO Substrates

We have found that single-phase m-plane In0.24Al0.76N(1100) grows without phase separation in the layer-by-layer mode at room temperature from the initial stages of the growth. The full-width at half-maximum (FWHM) values of the 1100 X-ray rocking curves (XRCs) for the film with X-ray incident azimuths perpendicular to the c- and a-axes are 119 and 102 arcsec, respectively. m-plane In0.24Al0.76N films grew without accommodating misfit dislocations beyond the critical thickness on ZnO(1100) substrates, which is probably due to the large energy barrier for the initiation process of misfit dislocations and is responsible for the small FWHM values for XRCs.

Control of full width at half maximum of Coulomb oscillation in silicon single-hole transistors at room temperature

The full width at half maximum (FWHM), the sharpness of the Coulomb blockade oscillation in a single-hole transistor (SHT), has been controlled at room temperature by means of substrate capacitance control using substrate depletion and accumulation/inversion. When the substrate is depleted, the substrate capacitance is lower than when it is accumulated or inverted, resulting in a smaller FWHM. The SHT was fabricated on a thin buried oxide silicon-on-insulator substrate whose initial thickness was 10nm. Low temperature measurements have been performed on another SHT to support the results. The control of the sharpness in a single-charge transistor (SCT) may add further functionality to the SCT.

Resolution recovery in pinhole SPECT based on multi-ray projections: a phantom study

Low sensitivity can become a major problem when very small pinholes are used in SPECT imaging. Although a larger pinhole aperture will improve the sensitivity, this will be at the cost of the spatial resolution. With a view to improving the resolution-sensitivity trade-off, this paper explores an iterative reconstruction algorithm that models the pinhole aperture based on multi-ray projections. This new implementation was validated using simulated data and phantom experiments. Two approaches were investigated. Firstly, the pinhole aperture was modelled in both the forward and the back projector. Secondly, the dual matrix implementation was investigated by modelling the pinhole aperture only in the forward projector. The systematic error, the full-width at half-maximum (FWHM) and the statistical error were quantified using the simulated data. Experimental phantom data were acquired for visual comparison with the reconstructions obtained from the simulated data. For a predefined number of iterations, the systematic error, the FWHM and the statistical error could be decreased when the pinhole aperture was modelled during iterative reconstruction. For a fixed, predefined statistical error of +/-10%, smaller systematic errors and smaller FWHM were obtained when modelling the pinhole opening. When the dual matrix implementation was used, equivalent results could be obtained as when modelling the pinhole opening in both the forward and the back projector. The multi-ray method to accomplish resolution recovery during the reconstruction of pinhole SPECT projection images offers a better trade-off between spatial resolution and noise compared with a reconstruction which does not model the pinhole aperture.

Voltage gain dependence of the negative differential conductance width in silicon single-hole transistors

The full width at half maximum (FWHM) of the negative differential conductance (NDC) characteristics in room temperature (RT)-operating silicon single-hole transistors (SHTs) has been studied by experiments and calculations. It is found that when the voltage gain of the SHT is higher, sharper NDC and smaller FWHM are achieved. Lower drain coupling is considered to be the main reason for the small FWHM in a resonant tunneling system of RT-operating SHTs. FWHM of NDC of as small as 95mV has been obtained in a SHT with the gain of as high as 5.2 at RT, which is the highest value of the gain ever reported. The device is in the form of an ultranarrow wire channel metal oxide semiconductor field-effect transistor, which shows large Coulomb blockade oscillations at RT and has extremely small drain capacitance due to its ultranarrow channel structure. NDC can now be designed by device parameters, showing further potential for application to low-voltage, low-power NDC circuits.

ZnO growth on Si with low-temperature ZnO buffer layers by ECR-assisted MBE

High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 μm were grown at different growth temperatures of 350, 450 and 550 °C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 μm at 550 °C. A ZnO buffer layer with a growth temperature of 450 °C (450 °C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 °C-buffer sample (6.7 nm), the 550 °C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c-axis orientations of (0 0 2) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 °C-buffer sample was the narrowest as 0.209°, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 °C. Consistent with these results, the 450 °C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm −1 was found in Raman scattering spectra for all films with buffers, which is corresponding to the E 2 mode.

Tuning the performance of hybrid organic/inorganic quantum dot light-emitting devices

The luminescence of inorganic core-shell semiconductor nanocrystal quantum dots (QDs) can be tuned across much of the visible spectrum by changing the size of the QDs while preserving a spectral full width at half maximum (FWHM) as narrow as 30 nm and photoluminescence efficiency of 50% [Journal of Physical Chemistry B 101 (46) (1997) 9463] [1]. Organic capping groups, surrounding the QD lumophores, facilitate processing in organic solvents and their incorporation into organic thin film light-emitting device (LED) structures [Nature 370 (6488) (1994) 354] [2]. A recent study has shown that hybrid organic/inorganic QD-LEDs can indeed be fabricated with high brightness and small spectral FWHM, utilizing a phase segregation process which self-assembles CdSe(ZnS) core(shell) QDs onto an organic thin film surface [Nature 420 (6917) (2002) 800] [3]. We now demonstrate that the phase segregation process can be generally applied to the fabrication of QD-LEDs containing a wide range of CdSe particle sizes and ZnS overcoating thicknesses. By varying the QD core diameter from 32 Å to 58 Å, we show that peak electroluminescence is tuned from 540 nm to 635 nm. Increase in the QD shell thickness to 2.5 monolayers (∼0.5 nm) improves the LED external quantum efficiency, consistent with a Förster energy transfer mechanism of generating QD excited states. In this work we also identify the challenges in designing devices with very thin (∼5 nm thick) emissive layers [Chemical Physics Letters 178 (5–6) (1991) 488] [4], emphasizing the increased need for precise exciton confinement. In both QD-LEDs and archetypical all-organic LEDs with thin emissive layers, we show that there is an increase in the exciton recombination region width as the drive current density is increased. Overall, our study demonstrates that integration of QDs into organic LEDs has the potential to enhance the performance of thin film light emitters, and promises to be a rich field of scientific endeavor.

Continuous stimulated emission at T=293 K from separate-confinement heterostructure diode lasers with one layer of InAs quantum dots grown on vicinal GaAs(bd001) surfaces misoriented in the [010] direction in the active region

The electroluminescence and stimulated emission of lasers with one layer of InAs quantum dots (QD’s) grown in a single molecular-beam epitaxial process on vicinal GaAs(001) surfaces misoriented in the direction [010] by 2, 4 and 6° are investigated. It is discovered that an increase in the misorientation angle leads to a blue shift and a decrease in the full width at half maximum (FWHM) of the electroluminescence spectrum. This effect is attributed to a decrease in the size of the quantum dots and improvement in their size uniformity. A strong dependence of the threshold current density on the width of the spontaneous luminescence spectrum is discovered. The room-temperature threshold current density of the lasers with one layer of quantum dots and the spontaneous luminescence spectrum having the smallest FWHM (54 meV) equals 210 A/cm2.

Influence of lattice mismatch on the crystalline quality of metastable GaAsSb grown by metalorganic chemical vapor deposition

GaAs 1-x Sb x layers covering an immiscible compositional range (0.14 < x < 0.74) are grown either on GaAs or InP substrates by low-pressure metalorganic chemical vapor deposition. Although the controllability of the solid-phase composition is independent of the substrate material, the full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve is found to depend strongly on the lattice mismatch between the epi-layer and the substrate. A very small FWHM of 50 arcsec is obtained in closely lattice-matched GaAsSb layers grown on InP substrate, showing the importance of lattice matching in obtaining high crystalline quality epi-layers of metastable materials.

Photoluminescence Study of GaAs Grown on (001) Si

The optical properties of GaAs epilayers grown by organometallic vapour phase epitaxy (OMVPE) on (001) Si substrates are reported. Low-temperature photolumniescence (PL) indicated that an increase in GaAs layer thickness led to an increase in PL intensity, a decrease in the full width at half-maximum (FWHM) of the excitomic transitions and an increase in the ratio of the exciton to carbon impurity peak heights. A comparison of PL spectra, obtained from layers grown on nominally (001) and 2? off (001) Si substrates, indicated that the carbon impurity concentration is higher in epitaxial layers grown on nominally (001) Si. PL studies also indicated that the optical quality of the top GaAs epilayer is critically dependent on the growth parameters of the nucleation layers. The best PL response (i.e., greatest intensity, smallest FWHM and lowest impurity concentration) was obtained from layers in which the initial Al0.2Ga0.8As nucleation layer was deposited at a low V/III ratio of 10. It was also shown in this study that postgrowth thermal annealing and the incorporation of AlGaAs/GaAs superlattices resulted in a dramatic improvement in the optical quality of GaAs/Si epilayers.

Analysis of a GaAs metal-semiconductor-metal (MSM) photodetector with 0.1- mu m finger spacing

The temporal response of a GaAs metal-semiconductor-metal (MSM) photodetector with a finger spacing of 0.1 mu m is discussed. The intrinsic detector has a minor effect (25% increase) on the full width at half-maximum (FWHM) of the temporal response of the device and its parasitic circuit elements. The analysis indicates that a long time constant due to the decay of holes is solely responsible for this increase. The smallest FWHM for this detector is estimated to be less than 2.5 ps. >

Irradiation-induced texture variations and atomic displacements in Au and NbN layers

Irradiation of Au and NbN sputtered layers with 600 keV Ar2 + ions leads to a reduced orientation distribution of the [111] direction of the Au layers from a full width at half maximum (FWHM) of the orientation distribution of 12° to a FWHM of 6° and to only very small FWHM changes in the texture distributions of the NbN layers. However, the FWHM of the distribution of the reflections of the irradiated NbN layers is increased significantly from Δθ = 0.65 to 1.17° for one sample position. This is caused by small atomic displacements in the NbN lattice. The FWHM of the reflection distribution of the Au layers increased from Δθ = 0.60 to 0.65° after irradiation. Oblique incidence of Ar2 + ions causes, by absence of channeling, stronger distortions than perpendicular incidence.