B. Lalevic

Dielectric properties of amorphous Nb2O5 thin films

The dielectric constant, a.c. resistance and loss tangent tan δ of amorphous Nb2O5 films with Nb electrodes have been investigated in the frequency range 3 Hz to 5 × 106 Hz, as a function of temperature and applied field. n nThe capacitance decreases with frequency ƒ according to a C(ƒ)∝ƒ−0.3 relation in the range 3 Hz 2 × 10su5 Hz. The activation energy of a dielectric oscillator is 0.032 eV; C−V, G−V and tan δ−V data are presented. n nThe real and imaginary parts of the complex dielectric constant e∗ = e′ − je″ and polarizability α∗ = α′ − jα″ were calculated from the data. The Cole-Cole (e″versus e′), Scaife (α″versus α′) and tan δ versus log ƒ plots indicate the existence of two relaxation times below 8 × 103 Hz. n nThe films are in a thickness range where the principal conduction mechanism is hopping conduction, as shown by the σ(ƒ)∝ƒn relation, with 0.75

RF relaxation oscillations in polycrystalline TiO2 thin films

Abstract Negative resistance oscillations were studied in polycrystalline TiO2 thin films in the Tiue5f8TiO2ue5f8Cr device structure. Stable oscillations were readily produced in the 1–3 MHz frequency range and showed lifetimes of more than 8 × 1011 cycles. A simple equivalent circuit model for current controlled negative resistance (CCNR) devices was used to interpret the bias voltage and temperature dependence of amplitude and frequency of negative resistance oscillations. Bias voltage dependence was found not to involve changes in the device parameters. Temperature variations produced changes in the device threshold and minimum (holding) voltages and the “on” and “off” state resistances. The “on” state and “off” state resistances showed thermal activation energies of 0.006 and 0.052 eV respectively. I-V characteristics for these devices are shown to be in agreement with the theory of filamentary double injection space charge limited currents.

Transport and dielectric properties of thin polycrystalline CoO films

Transport and dielectric properties of polycrystalline CoO films were studied as functions of the applied field, frequency and temperature. TheI–V plots showed that the Poole-Frenkel field emission mechanism is responsible for conduction at fields>105 V/cm. The ac conductivity σ(ω), the imaginary part of the dielectric constantε2, and tan δ plots as functions of frequency revealed three dispersion regions. The σ(ω) andε2 frequency dependence indicates a non-adiabatic hopping of charge carriers at low frequencies and adiabatic hopping at high frequencies. The activation energy of a dielectric oscillator is 0.15 eV.

Threshold switching in polycrystalline TiO2 thin films

I‐V characteristics, Balberg’s double‐pulse test, and rf relaxation oscillation were used to analyze the switching mechanism in Ti‐TiO2‐Cr devices. Experimental results show an overall agreement with Lampert’s double‐injection model; however, alternate electronic mechanisms should not be ruled out.

Hard transparent dielectric coatings

Abstract Hard transparent corrosion-resistant dielectric coatings were investigated (a) for application to the direct protection of glass surfaces, (b) for application to the protection of thin film metallic window coatings used for reflection or transmission of solar radiation and (c) for use in codeposited metal-ceramic coatings. The dielectric coatings were Al 2 O 3 , SiO 2 and SiC in the thickness range 100–100 000 A. The coatings were deposited under varying conditions of substrate temperature, gas composition, r.f. power and substrate preparation. The results of the following measurements on the dielectric coatings are presented: diamond pyramid hardness tests, optical reflectance and transmittance in the region 0.6–4.0 eV and corrosion resistance tests under cyclic conditions. The results showed a considerable improvement in the scratch resistance and hardness of glass by layering of hard dielectric coatings 2–5 μm thick.

Hopping conduction in amorphous Nb2O5 thin films

Abstract The low field conduction mechanism in amorphous Nb 2 O 5 doped with Nb is investigated by measurements of the ac conductivity as a function of frequency (3 Hz−6 × 10 6 Hz), dc conductivity as a function of temperature (100–400 K), capacitance as a function of frequency (3 Hz−6 × 10 6 Hz) and conductance G as a function of voltage at 10 3 Hz. Loss tangent and quality factor data are also given because of their technical and scientific relevance. Evidence for hopping conduction at low applied fields is presented by the following results: (1) a monotonic increase in ac conductivity σ ( ω ) αω n where 0.5 n 6 Hz; (2) a linear dependence of current on voltage at low fields; and (3) low activation energy for dc conduction with a transition at 210 K to a still lower activation energy; and (4) a decrease in polarizability with frequency. At high fields, E > 10 5 V/cm, dc conductivity is dominated by the field emission mechanism of the Poole-Frenkel or Poole type.

Photoconductive impulse generation and radiation

The generation of extremely narrow, high peak power pulses using an optically activated impulse generator is described. Radiative measurements at 1 Hz PRF have been conducted at pulse bias levels up to 15 kV, using an optical pulse from a Nd:YAG laser to trigger the device. The measured pulse from a wideband antenna has a pulsewidth of 1.5 ns with a risetime of 900 ps. The frequency spectrum of this radiated waveform ranged from 50 MHz to 1 GHz. >

Dependence of structural and compositional characteristics of chromium metal films as a function of deposition rate during the fabrication of metal/insulator/semiconductor solar cells

Abstract Metal/insulator/semiconductor solar cells on single-crystal and polycrystalline silicon substrates have been fabricated under various electrode metal (chromium) deposition rates. The effects of deposition rate on the electrical, structural and compositional characteristics of the resulting films have been studied. Auger electron spectroscopy and transmission electron microscopy results correlate well with the electrical characterization of the resulting devices. It is found that the oxygen content, which is inversely proportional to the rate of metal deposition, determines the structure of the resulting metal films which can be amorphous or polycrystalline. The oxygen content also has a beneficial effect on the performance of the solar cells by inducing a reduction in the electrode metal (chromium) work function.

Electrical conduction and dielectric breakdown in crystalline NiO and NiO(Li) films

Charge transport, including dielectic breakdown in NiO films (2−10 μ thick), is studied as a function of doping (0−0.4 at.% Li), temperature (−150−+150 °C), frequency (0−108 Hz), and voltage using sputtered nickel contact electrodes. The current I is found to vary as V1 at low and as V3 at higher fields until a breakdown field Es is reached which varies with Li concentration N as Es∝T exp(−N). The field E1 at which I−V nonlinearity begins is given by E1 ?0.2(2kT/eae′) and E1∝T exp(1/T). The ac conductivity σ (ω) ∝ω2τ2/(1+ω2τ2) and the dc conductivity σ=σ0 exp(−Eth/kT). These transport properties are shown to be consistent with the hopping−carrier model. Under current−controlled conditions, reversible dielectric breakdown (i.e., repetitive switching) was associated with the field−induced delocalization of charge carriers. The results are consistent with the view that the charge carriers prior to reversible dielectric breakdown are small polarons.

Dielectric properties of NiO and NiO(Li)

Abstract The dielectric constant of NiO produced by the oxidation of Ni has been studied as a function of frequency up to 108 Hz and of temperature from 300° to 550°K. The effect of Li doping on the dielectric constant has also been investigated. In addition, the behavior of the a.c. conductivity σ(ω) as a function of frequency ω has been determined up to 108Hz and the loss tangent tan δ has been calculated. The results are interpreted on the basis of a non-adiabatic hopping mechanism at lower frequencies and an adiabatic hopping mechanism at higher frequencies. The dielectric breakdown field as a function of Li concentration has also been determined and is found to decrease exponentially with increasing Li concentration. In the Niue5f8NiOue5f8Ni structure, the capacitance is independent of the applied field up to fields of 10 5 V cm , and the resistivity is high up to fields of 10 5 V cm .