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  • 1
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 59 (1991), S. 312-314 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The conductivity of a hydrogenated amorphous silicon n+-intrinsic-n+ (n-i-n) structure is reported as a function of temperature. The space-charge-limited conductivity σ is shown to follow the Meyer–Neldel rule (MNR) [W. Meyer and H. Neldel, Z. Tech. Phys. 18, 588 (1937)]: σ =σ00 exp(Ea/kT0) exp(−Ea/kT), where Ea is the conductivity activation energy, k is Boltzmann's constant, and T is the absolute temperature. The characteristic MNR parameters found are σ00=10−2.4±0.1(Ω cm)−1 and T0=590±10 K. These values are practically equal to those previously found for the MNR in the ohmic conductivity in a series of hydrogenated amorphous silicon n-i-n structures with varying i-layer thicknesses. It is argued that the MNR can be quantitatively explained by the statistical shift of the Fermi energy and that a single set of parameters corresponding to σ00 =10−3±1(Ω cm)−1 and T0=550±100 K is applicable for both the space-charge limited and the ohmic conductivity of i-type hydrogenated amorphous silicon. The MNR parameters are rather insensitive to density of states details.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A study of the zero-bias barrier heights of hydrogenated amorphous-silicon-based Schottky diodes and the prevailing current transport mechanisms in these structures is made using electrical and electro-optical techniques. Several series of devices were made using Cr, Mo, W, and Pt as Schottky metals. The current-voltage characteristics of the devices were obtained with their temperature dependence. The barrier heights were determined independently using internal photoemission experiments at three temperatures between 270 and 380 K. In devices where the Schottky barrier is deposited on top of the semiconductor material, the saturation current density is found to be most likely determined by combined drift and diffusion of the carriers. In devices where the Schottky barrier is formed at the bottom of the diode, the transport mechanism tends towards thermionic (field) emission, but only slight effects of the prevailing transport mechanism on the electrical performance of these diodes were observed. Also, in these devices a relatively defect-rich a-Si:H layer at the bottom (Schottky) contact could be detected opto-electronically, which prohibited barrier height determination using internal photoemission. For the influence of the metal work function φm on the electron-barrier height φb we obtain φb=(0.10±0.02)φm+(0.48±0.12), which is in good agreement with a previous determination of the dependence of the barrier for holes on φm.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The physical mechanisms that determine the current transport in reverse-biased Schottky diodes on undoped "device-grade'' hydrogenated amorphous silicon (a-Si:H) are elucidated. The current-voltage (J-V) curves for several Schottky diodes up to reverse-biases of 40 V have been measured at temperatures between 40 and 180 °C. The reverse currents generally increase approximately exponentially with reverse bias. The decrease of the apparent barrier height as obtained from internal photoemission experiments is in good agreement with the decrease of the thermal activation energy with reverse bias. Extra information on the current transport mechanism can be obtained from the bias dependence of the prefactor in the Arrhenius plot. A theoretical model is presented which gives a semiquantitative fit to all the features observed in the experimental data. The model involves quantum-mechanical tunnelling of a thermal distribution of carriers through an image-force lowered triangular potential shape. At low reverse bias, the apparent barrier height decreases due to image-force lowering alone and the prevailing carrier transport mechanism is drift/diffusion or thermionic emission over the barrier, which can be determined from the bias dependence of the conduction prefactor in the Arrhenius plots. At higher fields, the apparent barrier height decreases faster than the image-force lowering. This is due to tunnelling of carriers through (the top of) the potential barrier and the apparent barrier becomes approximately equal to the mean energy at which the carriers move through the barrier. This energy is lowered with increasing reverse bias. The conduction prefactor from the Arrhenius plot now decreases with increasing applied bias and gives an indication of the effective tunnel probability.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Optical transitions between the 5D4 excited state of the Tb3+ (4f)8 configuration and the crystal-field split components of the (4f)7(5d) configuration are observed using high-resolution laser excitation of Tb-doped powder samples at room temperature. Excited-state absorption spectra of Tb3+ in YAG, YAGaG, Y2SiO5, and LaOBr are presented. Superimposed on the broadband excitation spectrum of these transitions we find the relatively narrow 4f→4f lines. We present a theoretical model for interpreting the 4f→5d transitions, which predicts a strict proportionality between the (4f)8 5D4→(4f)7(5d) and the (4f)8 7F6→(4f)7(5d) transitions. This model is used to estimate the optical-absorption cross section for the former transitions. Although the terminating 4f levels lie at the same energy as the 5d bands, the 4f→4f transitions can be interpreted on the basis of the Judd-Ofelt theory. We derived a method for calibration of the Tb3+ 4f→5d fluorescence-excitation spectra from powder samples. For YAG:Tb, the resulting optical cross sections were confirmed by direct-absorption measurements on a Tb3+ -doped epitaxial YAG layer. To our knowledge, this is the first time that the probabilities for excited-state absorption to highly excited 5d and 4f states in Tb3+ have been obtained theoretically and experimentally from Tb-doped powder materials.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Photon-emission experiments on silicon-rich hydrogenated amorphous silicon-nitride metal–semiconductor–metal diodes, have shown the existence of hot electrons under applied field strengths of approximately 106 V/cm. The effective temperatures and mean free path between collision for the electrons were estimated from the spectra. It is shown that, in general, asymmetrical changes in the electrical characteristics of the devices occur after prolonged dc stressing at high fields. Two drift mechanisms can be distinguished. The first is called "cathodic'' drift and is driven by recombination between band-tail carriers in the semiconductor. The other is called "anodic'' drift, and results from the effects of hot electrons at the anode. The spatial and time dependence of these drift mechanism is explained using a simple model. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 4399-4404 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A detailed study of hydrogenated amorphous silicon (a-Si:H) surfaces before and after thermal and plasma oxidation treatments was carried out using x-ray photoelectron spectroscopy. The thickness of the surface oxides is correlated with the electrical properties of corresponding Mo Schottky barrier structures. Oxide layers up to 1.5 nm in thickness cause a decrease of the reverse current of nearly two orders in magnitude, while the forward current is hardly affected. For oxide thicknesses above 2.0 nm a large reduction in the forward current is observed. Surprisingly, the associated tunneling probabilities of the oxide interface layers in the a-Si:H Schottky diodes are the same as those previously reported for c-Si-based tunnel diodes. Tunneling in the a-Si:H devices cannot be simply described by the properties of a rectangular barrier, which is adopted most frequently in these studies. A potential form where the barrier height increases quadratically with thickness fits the observed tunneling characteristics more quantitatively, both in absolute magnitude and oxide-thickness dependence of the tunnel current.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A physical model based on resonant electric dipole interaction between excited activators is presented that may be used for gaining insight into the second-order energy-loss process in cathode-ray-irradiated Tb phosphors. It is shown that resonant up-conversion of the 5DJ excitons (J=3,4) to highly excited 4f and 5d states may be of equal importance. The derivation of the rate constants for this second-order energy-loss process is given and experimental values for the 5D4 -5D4 and 5D3 -5D4 interaction are presented for Tb-doped YAG, YAGaG, LaOBr, and Y2SiO5. The values depend on previously determined optical cross sections for excited-state absorption. Based on this model we explain well-known differences in saturation behavior between these Tb-doped phosphors under cathode-ray excitation.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 62 (1993), S. 1815-1817 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The tunneling effective mass of electrons in undoped a-Si:H has been determined from measurements on Schottky diodes operating with high reverse fields. Under these conditions, the change of current with electric field is a sensitive function of effective mass. The tunneling effective mass was measured to be 0.09±0.02 me for a range of different samples giving a tunneling constant of ≈40 A(ring).
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A novel manifestation of carrier induced degradation of α-Si:H is presented. Amorphous silicon n-i-n devices, which have been subjected to severe constant-current bias stress, show a strongly decreased ohmic conductivity. At the same time, the J–V characteristics in the space-charge limited regime show pronounced asymmetry between the "forward'' and "reverse'' directions. Annealing the devices, without bias, restores the initial symmetrical characteristics. The decrease in the ohmic conductivity is interpreted as due to an increased density of deep (dangling bond) states in the lower part of the band gap within the i layer, resulting from the thermal equilibration with an increased density of trapped electrons in the conduction band tails. The asymmetry in the J–V characteristics is due to the formation of a spatially inhomogeneous distribution of dangling bond states in the upper half of the gap, induced by the thermal re-equilibration to an inhomogeneous distribution of holes in the valence-band tail states during the bias stress. The density of states is increased in the region next to the electron injecting contact and decreased in the region next to the opposite contact. Model calculations are presented which show how the asymmetric J–V characteristics arise from such an inhomogeneous density of states and indicate the importance of holes for the state-creation process at the electron injecting contact.
    Type of Medium: Electronic Resource
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