Parameter E0 Research Articles

A semiclassical model of dielectric relaxation in glasses

A semiclassical double-well model of dielectric relaxation current in glasses is extended to (i) nonzero temperature, (ii) complex bias histories, and (iii) strong electric fields. At finite temperature, thermal excitation yields a contribution linear in temperature, which adds to the temperature-independent contribution from tunneling from the zero-temperature case. Both contributions vary linearly with applied field and have a time dependence of 1∕tn. Experimental measurements in three different glasses are shown to agree with this prediction, and it is shown how to use such measurements to estimate the material parameters t0 and σ0. For complex bias histories, a principle of superposition is found, as observed experimentally, if the applied fields are weak compared to the material parameter E0, estimated to be on the order of 107V∕m. For an electric field pulsed periodically from 0 to E, the current can be decomposed into a fast contribution due to particles tunneling back and forth every cycle of the field, and a slow residual contribution whose time dependence is the same as that for a continuous bias, but whose magnitude is reduced by the duty cycle of the periodic bias. For a sinusoidal electric field, thermal excitation gives a contribution to the dielectric constant that varies linearly with temperature and has a real part that varies logarithmically with frequency and an imaginary part that varies as the inverse tangent of frequency. For tunneling, both parts are independent of temperature and vary approximately as the logarithm of frequency, a dependence observed experimentally and almost indistinguishable from that suggested by the 1∕tn current response to a step voltage. For strong electric fields, the current that flows after the field is removed is found to be dominated by particles that fell forward when the field was on. Since particles fall forward quickly but tunnel back slowly, even strong fields applied for a short period of time can produce a large, long-lasting return current. These analyses lead to a number of testable predictions, and should be useful for understanding the phenomenon of dielectric relaxation and its impact on electronic devices.

Optical Properties of Indium-Doped ZnO Films

Indium-doped ZnO (IZO) films with low In content (<6 at. %) were fabricated by rf helicon magnetron sputtering. The uniformity of the composites was confirmed by elemental analysis. The formation of an In–Zn–O solid solution was verified using X-ray diffraction (XRD) patterns. A wide, high-transmittance region (400–2000 nm) and >80% transmittance in the window of fiber optics telecommunication (1.30–1.55 µm) were observed. The incorporation of indium enhances the optical transmission in the designated visible and infrared wavelengths. The optical band gap shows a slight blue-shift with increasing In doping which can be explained by the Burstein–Moss effect. The Urbach tail parameter E0 increases with increasing indium content, which coincides with the increase in the full width half maximum (FWHM) of (0002) planes in XRD patterns. A decline in crystal quality with In incorporation in IZO films is also confirmed from photoluminescence (PL) spectra.

Effects of Indium Incorporation on the Optical Properties of ZnO Films

Transparent indium-doped ZnO (IZO) films with low In content (&lt;6at%) were fabricated through radio-frequency (rf) helicon magnetron sputtering. Formation of In-Zn-O solid solution was confirmed by X-ray diffraction (XRD) patterns. Incorporation of indium into ZnO films enhances the optical transmission in the visible wavelength. The optical band-gaps slightly increase from 3.25eV (ZnO) to 3.28eV (In0.04Zn0.96O) and to 3.30eV (In0.06Zn0.94O) due to Burstain-Moss effect. The Urbach tail parameter E0, which is believed to be a function of structural disorder, increases from 79meV (ZnO), to 146meV (In0.04Zn0.96O), and to 173meV (In0.06Zn0.94O), which is consistent with increase of Full-Width Half-Maximum (FWHM) in corresponding XRD patterns. Decreasing in crystal quality with increasing indium concentration is also confirmed by photoluminescence spectra.

Effect of lithium impurity on the opto-electrical properties of zinc oxide films

Lithium-doped zinc oxide (ZnO) films were prepared on C-plane sapphire substrates by the e-beam deposition technique in vacuum. The dependences of the optical absorption edge characteristics, structural, and photoelectrical properties on Li content (from 0 to 10 at.%) were investigated. It is found that the variations of the energy bandgap Eg, the spread in the tail of the band edge (parameter E0), the electrical resistivity ρ, and the lattice constant c with impurity concentration do not have monotonic character. Abrupt jumps of the optical and electrical characteristics were observed as the impurity concentration changed near 0.8 at.% Li. The significant enhancement of photoconductivity response for 0.8 at.% Li-doped ZnO films also correlates with the enhanced electrical resistivity up to 2 × 106 Ω cm and the appearance of a photoinjection current. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Band-tail parameter in polysilicon materials

We develop a band-tail model in polysilicon by analyzing the energy band structure on the basis of the band-tail parameter with Urbach exponential behavior due to the trap states and the strain in grain bulks and grain boundaries. A quantitative analytical formulation for the band-tail parameter E0 has been deduced by taking into account the carrier–impurity interaction, the carrier–phonon interaction, and the structural disorder. The dependence of E0 and the electric field F in the grain boundary of polysilicon on carrier concentration n0 and temperature T has been studied within the framework of the model. The results indicate that E0 and F increase with T. Furthermore, the increase in E0 at n0=1.0×1018 cm−3 is much larger than that in E0 at n0=1.0×1016 cm−3 at the same temperature. We have demonstrated that the theoretical results are consistent with the experiment, and our model is therefore valuable for both describing polysilicon materials and improving the performance of polysilicon devices.

Time-resolved photoluminescence studies of AlxGa1−xN alloys

The optical properties of AlxGa1−xN alloys with x varied from 0 to 0.35 have been investigated by picosecond time-resolved photoluminescence (PL) spectroscopy. Our results revealed that while the PL intensity decreases with an increase of Al content, the low-temperature PL decay lifetime increases with Al content. These results can be understood in terms of the effects of tail states in the density of states due to alloy fluctuation in the AlxGa1−xN alloys. The Al-content dependence of the energy-tail-state distribution parameter E0, which is an important parameter for determining optical and electrical properties of the AlGaN alloys, has been obtained experimentally. The PL decay lifetime increases with the localization energy and, consequently, increases with Al content. The implications of our findings to III-nitride optoelectronic device applications are also discussed.

Luminescence Spectra Of Superbright Blue and Green InGaN/AlGaN/GaN Light-Emitting Diodes

Electroluminescence spectra of superbright blue and green LEDs based on epitaxial InxGa1−xN/AlyGa1−yN/GaN heterostructures with thin quantum well active layers [1] were studied at currents J = 0.01-20 mA. Spectral maxima of blue and green LEDs are ωmax = 2.58-2.75 eV and ωmax = 2.38-2.45 eV, dependent on the active layer In content. The low energy tails of the spectra are exponential with the parameter E0 = 42-50 meV almost independent of the temperature. The high energy tails of the spectra are exponential with a temperature dependent parameter E1= 20-40 meV. Both parameters (E0, E1) are current independent at J &gt; 0.5 mA. The spectral band can be described by taking into account quantum size effects, impurities and electron-phonon interactions in active layers. A structure in the spectra was detected which can be described by the influence of light interference in the GaN layer on the sapphire substrate. Light intensity was a linear function of the drive current over the interval J = 1-20 mA, and was slightly temperature dependent. In the blue LEDs, the efficiency fall off at low currents (J &lt; 0.7 mA) had a I ~ J4-5 dependence at room temperature. The green LEDs showed no such dependence. The influence of tunnel effects on the efficiency at low currents is discussed. Tunnel radiation spectra with maxima moving with the voltage were detected at low currents in III-N structures.

Study of defect states in GaN films by photoconductivity measurement

Optical absorption by defect states in gallium nitride (GaN) films was studied by photoconductivity (PC) spectroscopy at room temperature. A number of undoped n-type and Mg-doped p-type samples were employed in the present study. The PC response per absorbed photon decreased by more than four orders of magnitude as GaN became p-type conducting. Furthermore, all the samples exhibit PC response at photon energies far below the band gap energy of GaN. The optical absorption increases with photon energy hν from 1.5 to 3.0 eV approximately as exp(hν/E0). The parameter E0 ranges from 180 to 280 meV, and is considerably smaller for the insulating p-type sample. For a p-type conducting sample, the PC response is flat between 0.7 and 1.4 eV. A model for the density of states distribution in the forbidden gap of GaN and the effect of Mg doping is proposed.

Surface reactivity of liquid metal with oxygen and its relationship with surface tension measurements: a kinetic-fluodynamic model

A kinetic and fluodynamic description of processes that may take place near the liquid-vapour interface in the presence of an inert carrier flow containing definite oxygen amounts is presented, in relation to surface tension measurements. The evaluation of the Thiele modulus, φ, and of the parameter e0 (which compares the magnitude of the fluxes of interest to and from the surface), allows definition of the regimes in which fast or slow reactions in the gas phase surrounding a drop of liquid metal are possible in the presence of variable oxygen concentrations. The kinetic features of the process are described in terms of these different reaction regimes, and on the basis of the thermodynamic stability of the possible oxidation products. Diagrams on a plane log φ2/log e0 are drawn, immediately allowing one to know the range of \(P_{O_2 }^0 \) and T in which the reaction considered is thermodynamically possible, and to foresee the evolution in this field from a kinetic point of view, as a function of \(P_{O_2 }^0 \) and T. Therefore, the treatment presented here (and applied to selected cases of technological interest) allows a “stability field” for a chosen oxide on the log φ2/log e0 plane to be defined, and the behaviour of the system from a kinetic-fluodynamic point of view to be predicted when the flow parameters and the geometry of the experimental set-up are known. In particular, the reactive gas fluxes at the liquid-vapour interface can be evaluated as a function of the different experimental conditions. Kinetic-fluodynamic diagrams for liquid metal-oxygen systems containing Pb, Sn, Zn, Cu and Al are presented, as well as an application of the model to surface tension measurements on liquid tin, which gives semi-quantitative confirmation of the theory developed here.

Experimental and Theoretical Cross Sections for Escherichia coli Mutants B, B/r, and B s-1 after Heavy-Ion Irradiation

Data for the inactivation of three Escherichia coli mutants by energetic heavy ions are fitted by the track theory of a one-hit detector in an extended target mode. The respective E0's are 46, 36.5, and 12.6 Gy for E. coli B, B/r, and Bs-1 and a0, the assumed target radius, is 0.5 microns for all three. The parameter E0, the D37 with gamma rays, is measured directly, while a0 is fitted to the data. It is significant that neither a point target model nor calculations with a0 = 0.2 and 1.0 micron fit the data. Our fitted target radius, a0, approximates the size of the bacterium. These results raise questions as to why the E. coli mutants are one-hit detectors, and concerning the differences in the E0's in relation to a mechanistic interpretation of cell killing.

Instrumental background in gamma-ray spectrometers flown in low Earth orbit

Techniques are presented for calculating the instrumental continuum background in gamma-ray spectrometers flow in low Earth orbit. Simple methods are developed for scaling from the better-understood measurements and calculations of background in balloon-borne instruments. Secondary radiations are found to scale with the parameter E0.7 defined as the integral of the particle energy raised to the 0.7 power times the incident proton spectrum. The new scaling methods give background results for the HEAO 3 gamma-ray spectrometer that compare well with measurements. The key results are predictions of the background, sensitivity and orbit effects for future spaceborne high-resolution gamma-ray spectrometers. The background for these instruments will be highly dominated from 0.1 to 2 MeV by the beta-decay component. Detector segmentation and pulse-shape discrimination beta-rejection techniques will therefore be critically important. Loss of observing time and sensitivity due to activation by trapped particles in the South Atlantic Anomaly (SAA) is strongly dependent on orbit inclination and altitude. The largest losses occur between 0.2 and 1.0 MeV. For the scientifically important energy range near 0.6 MeV, a 28°, 500 km orbit has up to 70% loss of observing time. For a 28° inclination orbit, acceptable time loss of less than 50% can only be achieved by flying under the SAA in a 300–400 km orbit. To reduce data loss to less than 20%, the > 250 MeV trapped proton daily fluence must be less than 2 × 104 proton cm−2, which occurs for orbits with inclinations less than 20° for 400 km altitude and less than 15° for 500 km altitude.

Microdosimetric considerations of effects of heavy ions on E. coli K-12 mutants.

The inactivation cross sections of E. coli K-12 recombination-deficient mutants, JC1553 (recA) and AB2470 (recB), for several MeV/u alpha-particles and N ions have been successfully analyzed by Katz's target theory in which radiosensitivity parameter E0 is assumed to be LET independent and equal to D37 for gamma-rays. For E. coli K-12 wild type, AB1157 (rec+, uvr+), however, it is impossible to interpret the inactivation cross section data by an LET-independent E0-value. In the latter case, as in the case of B. subtilis spore, it is necessary to assume that the radiosensitivity of the target for the core of a heavy ion is higher than that for delta-electrons. As well as Waligorski, Hamm and Katz's dose, the dose around the trajectory of an ion based on Tabata and Ito's energy deposition algorithm for electrons has been used in the course of analysis.

Crystalline potential for photoelectron scattering phase-shift calculations and X-ray absorption spectra of Ti in crystals

X-ray absorption spectra (XAS) of Ti in crystals of Ti metal, TiC, TiN and TiO2 rutile are calculated in EXAFS and XANES regions. Single, double and triple photoelectron scattering processes are included through the spherical-wave formalism and the crystalline muffin-tin (MT) potential model, which employs a Hartree-Fock exchange interaction between photoelectron and core electrons inside MT spheres. Overall agreement of the theoretical and experimental spectra is obtained on the absolute photon energy scale. This result enables one to treat the experimental XAFS without using adjustable parameter E0. The neutral-atoms approximation is shown to be adequate for the phase-shift calculations in XAS theory for both metals and insulators. A comparison of Ti K XAFS of Ti metal calculated by means of various exchange-correlation potentials for the photoelectrons inside MT spheres is carried out. The best results are shown to be provided by Hartree-Fock and Dirac-Hara exchange potentials.

Density of midgap states and Urbach edge in chemically vapor deposited hydrogenated amorphous silicon films

Intrinsic or lightly boron-doped amorphous silicon films were produced by chemical vapor deposition (CVD) from disilane at temperatures from 380 to 460 °C and at growth rates from 0.1 to 40 Å/s. The density of states (DOS) near the middle of the mobility gap was determined by space-charge-limited conduction on n-i-n structures and by steady-state capacitance-temperature spectroscopy on p-i-n structures. Very close agreement was found between the two techniques for intrinsic layers deposited under similar conditions. The DOS distribution is rather flat from 0.60 to 0.75 eV from the conduction-band edge and has a range of 1–6×1017 cm−3 eV−1 for intrinsic films, with the minimum occurring for depositions at 440 °C independent of growth rate or contaminants in the disilane. Diborane levels from 9 to 18 ppm in the gas phase reduces the DOS at 440 °C to 3–6×1016 cm−3 eV−1. The exponential absorption below the band edge in the range 1.4–1.6 eV was determined from primary photocurrent spectra of p-i-n structures. The Urbach edge parameter E0 ranged from 0.048 to 0.056 eV with the minimum also occurring for depositions at 440 °C, independent of growth rate and boron content. Comparison to glow-discharge a-Si:H films indicates that CVD a-Si:H films have a similar amount of structural disorder, determined from E0, but an inherently larger DOS deep in the gap. A model is proposed in which boron reduces the high midgap dangling bond density caused by insufficient weakly bonded hydrogen in the film as a consequence of the high deposition temperatures. The model is consistent with other measurements, such as hole collection width.

Photo-induced reversible change of Urbach edge in hydrogenated amorphous silicon

Photoconductivity spectra temperature dependence combining with transmission measurements have been formed on hydrogenated amorphous silicon. After exposure by AM1 light a reversible increase of 6% of the Urbach parameter E0(T,X) has been observed. This is a direct evidence for the fact that photo-induced changes in a-Si:H create not only metastable defects but also affect the topological disorder.

“Magnetochemical Series” for Lanthanoid Compounds

“Magnetochcmical series”, not only for ligands but also for central ions, can be drawn up in the case of alkali‐metal lanthanoid oxides or halides. To explain the magnetic behavior of these compounds use was made, inter alia, of the Angular Overlap (AO) model. The AO parameter e0(R) could be determined for some series of compounds (from susceptibility measurements). Taking into account the (calculated) overlap integrals S(R) and additional relationships afforded magnetochemical series of the ligands I−&lt; Br−&lt; Cl−&lt; F− ≈ O2− and of the lanthanoids. The sequence of the latter series Corresponds to the pattern of lanthanoid contraction.

Changes in the distribution of low-energy trapped protons associated with the April 17, 1965, magnetic storm

The absolute intensity of geomagnetically trapped protons in the energy ranges 0.52 ≤ Ep ≤ 4.0 Mev and 0.90 ≤ Ep ≤ 1.8 Mev has been measured with the solid-state proton detector on the University of Iowa low-altitude (initial apogee 2502 km, perigee 527 km) high-latitude (inclination 81°) satellite Injun 4 for the period from March 1 to May 31, 1965. A study of the temporal variations of these fluxes associated with the April 17, 1965, magnetic storm (sc 1313 UT April 17, main phase onset ∼0200 UT April 18) shows a general redistribution of these protons for L ≳ 2.5 and all sampled |B| ranges, which persisted for at least 36 days after the storm. The effect of the sudden commencement was a general depression in the intensities and a hardening of the energy spectrums, although the intensities recovered to their prestorm level during the initial phase. The major redistribution was apparently initiated by the polar substorm that began at ∼0620 UT on April 18 and continued long after (to 1100 UT) the substorm had subsided (∼0800 UT). During the recovery phase, a secondary peak developed in the intensity profile at L ∼ 3.5 for 0.52-Mev protons that had no counterpart at this energy at the equator (L. R. Davis, private communication, 1971). No such peak was observed for 0.9-Mev protons. The over-all effect of the storm on the steady-state distribution was a nonadiabatic one with an increase in intensities at L ≲ 3 and a decrease at L ≳ 3. The loss and/or gain of particles appears to be fractionally the same at all |B| values sampled here. The post-storm dependence of the spectral parameter E0 on L generally follows the relation E0 ∝ L−3, although the prestorm dependence cannot be described by a simple function at all L values. The appearance of a secondary peak and the behavior of the prestorm and post-storm spectrums are in qualitative agreement with the predictions of the bimodal diffusion model of Theodoridis et al. [1969], whereby the solar wind is the source of the observed protons.

Observations of low-energy (0.3- to 1.8-Mev) Differential spectrums of trapped protons

Measurements of differential energy spectrums of trapped protons obtained from several passes during the period January to November 1969 using the polar-orbiting (inclination, 80.7°), low-altitude (2525 km by 644 km) satellite Injun 5 of the University of Iowa are presented. The solid-state detector proton-electron telescope on Injun 5 consists of a 24-μ front element and two back elements of 862 and 905 μ, respectively, and provides several differential energy channels in the range 0.3 to ∼25 Mev. The study reveals the existence of a quasi-persistent peak in the differential energy spectrum in the L range 2 to 2.6 and the energy range ∼0.38 to 0.72 Mev. The average position of this peak is at L≃2.3 and E≃0.54 Mev, but both the position in L and the peak energy appear to vary with geomagnetic activity in an as yet undetermined manner. The fact that the shape of the spectrum is stable for several days or can change with time scales as small as 4 hours suggests an impulsive acceleration mechanism deep in the radiation belts like that suggested by other authors for energetic electrons. Other features of the spectrum show that the spectral parameter E0 in the form dj/dE=K exp (−E/E0) varies as ∼L−3 in the L range 2 to 4, in general agreement with previous observations. The last result adds to the general body of evidence favoring diffusion from the solar wind as the primary mechanism for populating the radiation belts.

Acceleration of trapped electrons and protons through bimodal diffusion in the Earth's radiation belts

Trapped particles can be accelerated or decelerated by diffusing in L by means of either of two competing diffusion modes, of which one conserves the first adiabatic invariant μ and the other conserves the particle energy E. Acceleration through such ‘bimodal diffusion’ is most efficient when the two diffusion modes are about equally probable. A computer model is used to study the effect of bimodal diffusion on particle intensity profiles and energy spectra under a variety of conditions. Through the action of bimodal diffusion, an outer belt of energetic electrons can be created, with detailed spatial features governed by the L dependence of the particle transition probabilities for undergoing one or the other diffusion mode. If we simulate magnetic storms by enhancing constant-μ diffusion over constant-E diffusion and displacing the trapping boundary inward, then low-energy electron intensities in the outer belt rise, while high-energy intensities decrease at the start of the storm but subsequently increase. At infrequent intervals, unusually strong magnetic storms may push electrons into the inner-belt region. Once there, these electrons may remain trapped for a long time owing to the absence of strong perturbing mechanisms. Acceleration of trapped protons through bimodal diffusion can account for high-energy trapped protons in the inner-belt region. The effects of bimodal diffusion are less obvious for low-energy proton intensity profiles, and are of consequence mostly at higher L shells; generally speaking, low-energy protons can be accounted for by constant-μ diffusion acting alone. The widely different spatial patterns and temporal behavior displayed by the proton and electron belts can be understood as resulting primarily from different ratios of constant-E to constant-μ diffusion rates for the two particle species. A typical L-dependence pattern of trapped-particle energy spectra in the presence of bimodal diffusion consists of a domain at low L's where spectra become softer with increasing L and a domain at higher L's where spectra are independent of L. The point of transition between the two domains and the absolute values of the spectral parameter E0 (approximate e-folding energy of the integral particle intensity) depend primarily upon the energy spectrum and the location of the particle source and the ratio of constant-E to constant-μ diffusion rates. Sequences simulating geomagnetic events produce various spatial and temporal patterns of trapped-particle energy spectra very similar to those actually observed in the radiation belts.

The Energy of Hexagonal Dislocation Arrays Forming Twist Boundaries

The strain energy of low-angle twist boundaries has been computed using isotropic elasticity. The boundaries considered are hexagonal networks of dislocations whose Burgers vectors form an equiliateral triangle in the boundary plane. The Read-Shockley relationship for the dependence of grain-boundary energy on misorientation, θ, across the boundary, E=E0θ(A-lnθ), is valid for such boundaries. The parameter E0 is proportional to the sum of the self-energy contribution from each dislocation in the array; the parameter A was found by detailed calculation for the hexagonal networks of dislocations. A flat energy minimum was computed for the boundary consisting of an array of regular hexagons. For shapes not very different from this, the A parameter remained nearly constant. The energy increase over the minimum energy can be effectively estimated using a ``line-tension'' model. Observations of twist boundaries in twisted and annealed zinc single crystals show that some nearly regular hexagonal networks are present.