I. Ferreira

Zinc oxide as an ozone sensor

This work presents a study of intrinsic zinc oxide thin film as ozone sensor based on the ultraviolet (UV) photoreduction and subsequent ozone re oxidation of zinc oxide as a fully reversible process performed at room temperature. The films analyzed were produced by spray pyrolysis, dc and rf magnetron sputtering. The dc resistivity of the films produced by rf magnetron sputtering and constituted by nanocrystallites changes more than eight orders of magnitude when exposed to an UV dose of 4mW∕cm2. On the other hand, porous and textured zinc oxide films produced by spray pyrolysis at low substrate temperature exhibit an excellent ac impedance response where the reactance changes by more than seven orders of magnitude when exposed to the same UV dose, with a response frequency above 15kHz, thus showing improved ozone ac sensing discrimination.

Role of order and disorder on the electronic performances of oxide semiconductor thin film transistors

The role of order and disorder on the electronic performances of n-type ionic oxides such as zinc oxide, gallium zinc oxide, and indium zinc oxide used as active (channel) or passive (drain/source) layers in thin film transistors (TFTs) processed at room temperature are discussed, taking as reference the known behavior observed in conventional covalent semiconductors such as silicon. The work performed shows that while in the oxide semiconductors the Fermi level can be pinned up within the conduction band, independent of the state of order, the same does not happen with silicon. Besides, in the oxide semiconductors the carrier mobility is not bandtail limited and so disorder does not affect so strongly the mobility as it happens in covalent semiconductors. The electrical properties of the oxide films (resistivity, carrier concentration, and mobility) are highly dependent on the oxygen vacancies (source of free carriers), which can be controlled by changing the oxygen partial pressure during the deposition...

Gate-bias stress in amorphous oxide semiconductors thin-film transistors

A quantitative study of the dynamics of threshold-voltage shifts with time in gallium-indium zinc oxide amorphous thin-film transistors is presented using standard analysis based on the stretched exponential relaxation. For devices using thermal silicon oxide as gate dielectric, the relaxation time is 3×105 s at room temperature with activation energy of 0.68 eV. These transistors approach the stability of the amorphous silicon transistors. The threshold voltage shift is faster after water vapor exposure suggesting that the origin of this instability is charge trapping at residual-water-related trap sites.

Complementary Metal Oxide Semiconductor Technology With and On Paper

One of today’s challenges in electronics is to produce portable, fl exible, low cost, and easily recyclable products, [ 1 ] such as paper [ 2 ] since they do not require the high process temperatures used in crystalline silicon (c-Si) technologies. In addition, the devices should have low power energy consumption to allow densely packed integrated circuits for a plethora of applications such as computer memory chips, digital logic and microprocessors, to (linear) analogue circuits, among others, to fuel the next-generation microelectronics revolution for information and communication technologies. [ 3 ] For illustrative purposes, we consider a temporary register as an example. In a static circuit the contents of the register remain fi xed until new information arrives to be stored and remains active unless the power goes out or the computer is turned off. In a dynamic circuit, the contents of the register leak away and must be periodically refreshed. The advantage of dynamic circuits is that they do not draw current between refreshing; the disadvantage is that refreshing requires additional circuitry including clocks to synchronize the refresh cycle with the operation of the register. [ 3 , 4 ]

Growth of ZnO:Ga thin films at room temperature on polymeric substrates: thickness dependence

In this paper, we present results concerning the thickness dependence (from 70 to 890 nm) of electrical, structural, morphological and optical properties presented by gallium-doped zinc oxide (GZO) deposited on polyethylene naphthalate (PEN) substrates by r.f. magnetron sputtering at room temperature. For thicknesses higher than 300 nm an independent correlation between the electrical, morphological, structural and optical properties are observed. The lowest resistivity obtained was 5 =10 V cm with y4 a sheet resistance of 15 Vyh and an average optical transmittance in the visible part of the spectra of 80%. It is also shown that by passivating the surface of the polymer by depositing a thin silicon dioxide layer the electrical and structural properties of the films are improved nearly by a factor of two. 2003 Elsevier B.V. All rights reserved.

Write-erase and read paper memory transistor

We report the architecture and the performances of a memory based on a single field-effect transistor built on paper able to write-erase and read. The device is composed of natural multilayer cellulose fibers that simultaneously act as structural support and gate dielectric; active and passive multicomponent amorphous oxides that work as the channel and gate electrode layers, respectively, complemented by the use of patterned metal layers as source/drain electrodes. The devices exhibit a large counterclockwise hysteresis associated with the memory effect, with a turn-on voltage shift between 1 and −14.5V, on/off ratio and saturation mobilities of about 104 and 40cm2V−1s−1, respectively, and estimated charge retention times above 14000h.

New challenges on gallium-doped zinc oxide films prepared by r.f. magnetron sputtering

Abstract Gallium-doped zinc oxide films were prepared by r.f. magnetron sputtering at room temperature as a function of the substrate–target distance. The best results were obtained for a distance of 10 cm, where a resistivity as low as 2.7×10 −4 Ω cm, a Hall mobility of 18 cm 2 /Vs and a carrier concentration of 1.3×10 21 cm −3 were achieved. The films are polycrystalline presenting a strong crystallographic c -axis orientation (002) perpendicular to the substrate. The films present an overall transmittance in the visible part of the spectra of approximately 85%, on average.

Flexible a-Si:H Position-Sensitive Detectors

Flexible and large area (5 mm /spl times/80 mm with an active length of 70 mm) position-sensitive detectors (PSDs) deposited onto polymeric substrates (polyimide-Kapton VN) have been fabricated. The optimized structure presented is based on a heterojunction of amorphous silicon (a-Si:H)/ZnO:Al. The sensors were characterized by spectral response, photocurrent dependence as a function of light intensity, and position detection measurements. The set of data obtained on one-dimensional PSDs based on the heterojunction show excellent performances with a maximum spectral response of 0.12 A/W at 500 nm and a nonlinearity of /spl plusmn/10% over 70-mm length. The produced sensors present a nonlinearity higher than those ones produced on glass substrates, due to the different thermal coefficients exhibited by the polymer and the a-Si:H film. In order to prove this behavior, it was measured the defect density obtained by the constant photocurrent method on a-Si:H thin films deposited on polymeric substrates and bent with different radii of curvature.

Transparent, conductive ZnO:Al thin film deposited on polymer substrates by RF magnetron sputtering

In this paper, we present the optical, electrical, structural and mechanical properties exhibited by aluminum-doped zinc oxide (ZnO:Al) thin films produced by RF magnetron sputtering on polymeric substrates (polyethylene terephthalate, PET; Mylar type D from Dupont®) with a standard thickness of 100 μm. The influence of the uniaxial tensile strain on the electrical resistance of these films was evaluated in situ for the first time during tensile elongation. In addition, the role of the thickness on the mechanical behavior of the films was also evaluated. The preliminary results reveal that the increase in electrical resistance is related to the number of cracks, as well as the crack width, which also depends on the film thickness.

Influence of the layer thickness in plasmonic gold nanoparticles produced by thermal evaporation

Metallic nanoparticles (NPs) have received recently considerable interest of photonic and photovoltaic communities. In this work, we report the optoelectronic properties of gold NPs (Au-NPs) obtained by depositing very thin gold layers on glass substrates through thermal evaporation electron-beam assisted process. The effect of mass thickness of the layer was evaluated. The polycrystalline Au-NPs, with grain sizes of 14 and 19 nm tend to be elongated in one direction as the mass thickness increase. A 2 nm layer deposited at 250°C led to the formation of Au-NPs with 10-20 nm average size, obtained by SEM images, while for a 5 nm layer the wide size elongates from 25 to 150 nm with a mean at 75 nm. In the near infrared region was observed an absorption enhancement of amorphous silicon films deposited onto the Au-NPs layers with a corresponding increase in the PL peak for the same wavelength region.