Rodrigo Martins

The impact of a parylene top-coating layer on the illumination and bias stress instabilities of indium-gallium-zinc oxide thin-film transistors (TFTs) is presented and discussed. The parylene coating substantially reduces the threshold voltage shift caused by continuous application of a gate bias and light exposure. The operational stability improves by 75%, and the light induced instability is reduced by 35%. The operational stability is quantified by fitting the threshold voltage shift with a stretched exponential model. Storage time as long as 7 months does not cause any measurable degradation on the electrical performance. It is proposed that parylene plays not only the role of an encapsulation layer but also of a defect passivation on the top semiconductor surface. It is also reported that depletion-mode TFTs are less sensitive to light induced instabilities. This is attributed to a defect neutralization process in the presence of free electrons. © 2016 Author(s).

This paper presents a study concerning the role of channel length scaling on IGZO TFT technology benchmark parameters, which are fabricated at temperatures not exceeding 180\, ^{\circ}C. The parameters under investigation are unity current-gain cutoff frequency, intrinsic voltage-gain, and on-resistance of the bottom-gate IGZO TFTs. As the channel length varies from 160 to 3 μm, the measured cutoff frequency increases from 163 {\rm kHz} to 111.5 {\rm MHz}, which is a superior value compared to the other competing low-temperature thin-film technologies, such as organic TFTs. On the other hand, for the same transistor dimensions, the measured intrinsic voltage-gain is changing from 165 to 5.3 and the on-resistance is decreasing from 1135.6 to 26.1 kØmega. TFTs with smaller channel length (3 μ m) have shown a highly negative turn-on voltage and hump in the subthreshold region, which can be attributed to short channel effects. The results obtained here, together with their interpretation based on device physics, provide crucial information for accurate IC design, enabling an adequate selection of device dimensions to maximize the performance of different circuit building blocks assuring the multifunctionality demanded by system-on-panel concepts. © 2005-2012 IEEE.

The post-deposition modification of ZnO-based transparent conductive oxides (TCOs) can be the key to produce thin films with optoelectronic properties similar to indium tin oxide (ITO), but at a much lower cost. Here, we present electro-optical results achieved for post-deposition annealing of Al–Zn–O (AZO), AZO:H, Ga–Zn–O:H (GZO:H), and Zn–O:H (ZNO:H) thin films deposited by RF sputtering at room temperature. These studies comprise results of thermal annealing at atmospheric pressure, vacuum, forming gas, H2 and Ar atmospheres, and H2 and Ar plasmas, which lead to significant enhancement of their electro-optical properties, which are correlated to morphological and structural improvements. The post-deposition annealing leads to an enhancement in resistivity above 40% for AZO, AZO:H, and GZO:H, reaching ρ ≈ 2.6–3.5 × 10−4 Ωcm, while ZnO:H showed a lower improvement of 13%. The averaged optical transmittance in the visible region is about 89% for the investigated TCOs. Such results match the properties of state-of-art ITO (ρ ≈ 10−4 Ωcm and transmittance in VIS range of 90%) employing much more earth-abundant materials. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Aluminum-doped zinc oxide (AZO) transparent conductor coating has emerged as promising substitute to tin-doped indium oxide (ITO) as electrode in optoelectronic applications such as photovoltaics or light emitting diodes (LEDs). Besides its high transmission in the visible spectral region and low resistivity, AZO presents a main advantage over other candidates such as graphene, carbon nanotubes or silver nanowires; it can be deposited using the technology industrially implemented to manufacture ITO layers, the magnetron sputtering (MS). This is a productive, reliable and green manufacturing technique. But to guarantee the robustness, reproducibility and reliability of the process there are still some issues to be addressed, such as the effect and control of the target state. In this paper a thorough study of the influence of the target erosion grade in developed coatings has been performed. AZO films have been deposited from a ceramic target by RF MS. Structure, optical transmittance and electrical properties of the produced coatings have been analyzed as function of the target erosion grade. No noticeable differences have been found neither in optoelectronic properties nor in the structure of the coatings, indicating that the RF MS is a stable and consistent process through the whole life of the target. © 2016 Elsevier B.V.

Metal nanoparticles are of great interest for light trapping in photovoltaics. They are usually incorporated in the rear electrode of solar cells, providing strong light scattering at their surface plasmon resonances. In most cases, the nanoparticles are self-assembled by solid-state dewetting over a transparent conductive oxide (TCO) layer incorporated in the cell's rear electrode. Up to now, this process has been optimized mainly by tuning the thermal annealing parameters responsible for dewetting, or the thickness of the precursor metallic layer; but little attention has been paid to the influence of the underlying TCO layer properties on the morphology of the nanoparticles formed, which is the focus of the present article. This work investigates Ag nanoparticles structures produced on distinct surfaces by a simple, fast and highly reproducible method employing rapid thermal annealing. The results indicate that both the thermal conductivity and surface roughness of the TCO layer play a determinant role on the morphology of the nanostructures formed. This is of particular relevance, since we show in the study performed that the parasitic absorption of these Ag nanostructures is reduced, while the scattering is enhanced when the Ag nanostructures are formed on TCO layers with the highest conductivity and the lowest surface roughness (∼1 nm). These results unveil novel possibilities for the improvement of plasmonic nanostructures fabricated by thermal dewetting, via the careful adjustment of the physical properties of the underlying surface. © 2016 American Chemical Society.

This letter presents a novel high-gain four-quadrant analog multiplier using only n-type enhancement indium- gallium-zinc-oxide thin-film-transistors. The proposed circuit improves the gain by using an active load with positive feedback. A Gilbert cell with a diode-connected load is also presented for comparison purposes. Both circuits were fabricated on glass at low temperature (200 °C) and were successfully characterized at room temperature under normal ambient conditions, with a power supply of 15 V and 4-pF capacitive load. The novel circuit has shown a gain improvement of 7.2 dB over the Gilbert cell with the diode-connected load. Static linearity response, total harmonic distortion, frequency response, and power consumption are reported. This circuit is an important signal processing building block in large-area sensing and readout systems, specially if data communication is involved. © 2016 IEEE.

The aim of presented work is to show the improvements obtained in the properties of TiO2 films for dye sensitized solar cells fabricated by inkjet printing using an innovative methodology. We describe the development and properties of TiO2-based inks used in a lab-scale printer, testing various commercial TiO2 pastes. The porosity of the deposited inkjet printed TiO2 films is much higher than using the conventional "doctor blade" deposition technique, as the ink solvent evaporates during the droplet fly from the nozzle to the substrate due to its picoliter volume and the applied heating of a printing stage (70°C). Thanks to higher surface area, the dye sensitized solar cells incorporating inkjet printed TiO2 film gave higher efficiencies (ηmax≈3.06%) than the more compact films obtained by the "doctor blade" method (ηmax≈2.56%). Furthermore, electrochemical analysis indicates that for whole tested thickness range, the inkjet printed layers have higher effective electron diffusion length indicating their better transport properties. © 2015 Elsevier Inc..

Oxide semiconductor thin-film transistors can show anomalous behavior under bias stress. Two types of anomalies are discussed in this paper. The first is the shift in threshold voltage (VTH) in a direction opposite to the applied bias stress, and highly dependent on gate dielectric material. We attribute this to charge trapping/detrapping and charge migration within the gate dielectric. We emphasize the fundamental difference between trapping/detrapping events occurring at the semiconductor/dielectric interface and those occurring at gate/dielectric interface, and show that charge migration is essential to explain the first anomaly. We model charge migration in terms of the non-instantaneous polarization density. The second type of anomaly is negative VTH shift under high positive bias stress, with logarithmic evolution in time. This can be argued as electron-donating reactions involving H2O molecules or derived species, with a reaction rate exponentially accelerated by positive gate bias and exponentially decreased by the number of reactions already occurred. © 2016 Author(s).

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.

Titanium oxide (TiO 2) finds potential applications in various fields such as solar cells, optical coatings due to its high refractive index and it also has been widely used in memory devices owing to its high dielectric constant. TiO 2 films have been deposited on p-type silicon (100) substrates by RF magnetron sputtering technique. Thickness, structure and surface morphology of the films were analyzed by using α-step profilometer, Raman spectroscopy and atomic force microscope respectively. Thin film capacitors of the type Al/TiO 2/Si were fabricated by evaporation of Aluminium on to the TiO 2 films. The current - voltage and capacitance - voltage characteristics were carried out to understand the electrical conduction and dielectric properties of the deposited films with a stack of Al/TiO 2/Si. The leakage current density was decreased and capacitance was increased with increase of substrate bias voltage.

Molybdenum (0.5at%) doped indium oxide thin films deposited by spray pyrolysis technique were irradiated by 100MeV O 7+ ions with different fluences of 5×10 11, 1×10 12 and 1×10 13ions/cm 2. Intensity of (222) peak of the pristine film was decreased with increase in the ion fluence. Films irradiated with the maximum ion fluence of 1×10 13ions/cm 2 showed a fraction of amorphous nature. The surface microstructures on the surface of the film showed that increase in ion fluence decreases the grain size. Mobility of the pristine molybdenum doped indium oxide films was decreased from  122 to 48cm 2/Vs with increasing ion fluence. Among the irradiated films the film irradiated with the ion fluence of 5×10 11ions/cm 2 showed relatively low resistivity of 6.7×10 -4Ωcm with the mobility of 75cm 2/Vs. The average transmittance of the as-deposited IMO film is decreased from 89% to 81% due to irradiation with the fluence of 5×10 11ions/cm 2. © 2012 Elsevier Ltd.

N-doped ZnO films were deposited on glass substrates by RF magnetron sputtering with different deposition pressures. The samples were characterized by X-ray diffraction (XRD), atomic force morphology (AFM), X-ray photoelectron spectroscopy (XPS), Hall measurements and optical spectrophotometer. The XRD patterns confirmed that the films are polycrystalline and the influence of deposition pressure on the structural properties. AFM microstructures also authenticated the change in the size and shape of the grains as a function of deposition pressure; the root mean square (RMS) roughness has reached a maximum (10.65 nm) at 1.5 x 10 -2 mbar. XPS spectra revealed the change in the chemical composition. The amount of adsorbed oxygen and nitrogen at oxide sites has reached the maximum at 9.0 x 10 -3 mbar, where the film showed p-type conductivity. The optical transmittance spectra have indicated that the absorption edge is shifted towards the shorter wavelength at higher deposition pressure. Correspondingly, the optical band gap is increased from 2.17 to 2.80 eV. The average visible transmittance in the wavelength ranging 500-800 nm has been increased from 49% to 82%. Copyright © 2010 American Scientific Publishers All rights reserved.

N-doped ZnO films were radio frequency (RF) sputtered on glass substrates and studied as a function of oxygen partial pressure (OPP) ranging from 3.0×10-4 to 9.5×10-3 Pa. X-ray diffraction patters confirmed the polycrystalline nature of the deposited films. The crystalline structure is influenced by the variation of OPP. Atomic force microscopy analysis confirmed the agglomeration of the neighboring spherical grains with a sharp increase of root mean square (RMS) roughness when the OPP is increased above 1.4×10-3 Pa. X-ray photoelectron spectroscopy analysis revealed that the incorporation of N content into the film is decreased with the increase of OPP, noticeably N 1s XPS peaks are hardly identified at 9.5×10-3 Pa. The average visible transmittance (380-700 nm) is increased with the increase of OPP (from ∼17 to 70), and the optical absorption edge shifts towards the shorter wavelength. The films deposited with low OPP (≤ 3.0×10-4 Pa) show n-type conductivity and those deposited with high OPP (≥ 9.0×10-4 Pa) are highly resistive (>105 ·cm) © 2010 The Nonferrous Metals Society of China.

A dye sensitized TiO2 photodetector has been integrated with a DNA detection method based on non-cross-linking hybridization of DNA-functionalized gold nanoparticles, resulting in a disposable colorimetric biosensor. We present a new approach for the fabrication of dye sensitized TiO2 photodetectors by an inkjet printing technique-a non-contact digital, additive, no mask and no vacuum patterning method, ideal for cost efficient mass production. The developed biosensor was compared against a dye sensitized photodetector fabricated by the traditional "doctor blade" method. Detection of gold nanoparticle aggregation was possible for concentrations as low as 1.0 nM for the "doctor blade" system, and 1.5 nM for the inkjet printed photodetector. The demonstrated sensitivity limits of developed biosensors are comparable to those of spectrophotometric techniques (1.0 nM). Our results show that a difference higher than 17% by traditional photodetector and 6% by inkjet printed in the photoresponses for the complementary and non-complementary gold nanoprobe assays could be attained for a specific DNA sequence from Mycobacterium tuberculosis, the etiologic agent of human tuberculosis. The decrease of costs associated with molecular diagnostic provided by a platform such as the one presented here may prove of paramount importance in developing countries. © 2009 Elsevier B.V. All rights reserved.

{A nonvacuum and low temperature process for passivating transparent metal oxides based thin-film transistors is presented. This process uses the epoxy-based SU-8 resist which prevents device degradation against environmental conditions, vacuum or sputtering surface damage. The incorporation of SU-8 as a passivation layer is based on the ability of this polymer to provide features with high mechanical and chemical stability. With this approach, lithography is performed to pattern the resist over the active area of the device in order to form the passivation layer. The resulting transistors demonstrate very good electrical characteristics, such as μFE =61 cm2 /V s

High visible to near infrared (NIR) transparent Mo (0-1 at%) doped In2O3 (IMO) thin films with high carrier mobility were deposited on Corning-1737 glass substrates at 400 °C by spray pyrolysis experimental technique. The films were annealed in vacuum (∼1×10-4 mbar) at 550 °C for 45 min. XRD analysis confirmed that indium oxide belongs to cubic bixbyite structure. The preferred growth orientation along (2 2 2) plane for low Mo doping level shifts to (4 0 0) for higher Mo doping levels. Crystallite sizes extracted from the XRD data corroborate the changes in full-width at half-maximum due to the variation in Mo doping. Scanning electron microscopy study illustrates the evolution in surface microstructures as a function of Mo doping. The negative sign of Hall coefficient confirmed n-type conductivity. Films with high mobility of ∼149 cm2/(V s), carrier concentration of ∼1.0×1020 cm-3, resistivity of ∼4.0×10-4 Ω cm and high figure of merit of ∼1.02×10-2 Ω-1 were observed for post-annealed films (0.5 at% Mo). The obtained high average transparency of ∼83% in the wavelength range 400-2500 nm confirms that transmittance is well extended into the NIR region. © 2009 Elsevier B.V. All rights reserved.

Indium molybdenum oxide thin films were deposited using different radio-frequency sputtering units on glass substrates at room temperature from an In 2O 3 (95 wt.%): Mo (5 wt.%) target. The film thickness ranges between 160 and 275 nm. The chamber volume of Unit-1 was ̃2.4 times larger than that of Unit-2. Apart from the chamber volume, a significant difference between the two units was the sputtering pressure. The films were characterized by their structural, morphological, optical, and electrical properties. A strong reflection from (222) plane was obtained for the ̃275 nm thick films deposited in Unit-1. The films deposited with <275 nm thickness and those deposited in Unit-2 are close to amorphous with a small crystalline fraction. The surface of the films deposited in Unit-1 is comprised of randomly arranged crystallites, which is restructured with the increasing film thickness to become a well defined "rice field" like structure (275 nm thick). The films deposited in Unit-2 are comprised of many holes on the surface that is presumably due to back sputtering. The average visible transmittance calculated in the wavelength between 400 and 800 nm ranges from 70 to 82%. The optical band gap is found to vary between 3.80 and 3.86 eV. The lowest bulk resistivity of the films deposited in Unit-1 was increased from ̃4.06×10 -3 to 4.07×10 -1ωcm when deposited in Unit-2. The carrier concentration was decreased from 1.31×10 20 to 1.03×10 18 cm -3 but the Hall mobility increased from 11.7 to 15.0 cm2 V -1 s -1. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

ZnO films were deposited on c-plane sapphire substrates in Ar atmosphere by ii magnetron sputtering and were post-annealed at 400°C in green gas (95% N2 + 5% H2). The properties of the as-grown and annealed films have been characterized by X-ray diffraction (XRD), Rutherford backscattering (RBS), elastic recoil detection analysis (ERDA), Hall measurement and photoluminescence spectra. XRD studies confirmed the variation in strain and an improvement in crystallinity. From RBS and ERDA analysis, the presence of H atoms on the surface of the as-grown ZnO films was evidenced. Annealing in green gas increased the amount of H in the film. Compared with the as-grown films, the ultra exciting intensity obviously decreases in the annealed films and new optical active centres in the blue/violet ( 3.0 eV) and red ( 1.9) regions are emerged in the PL spectrum. The positive sign of Hall coefficient confirmed the low (-type conductivity in the as grown films, which was improved after annealing. However, the (-type conductivity was not stable, especially for the annealed sample it changes from ( type to n type after 9 days. Copyright © 2009 American Scientific Publishers All rights reserved.

This paper describes a novel colorimetric method for detection of nucleic acid targets in a homogeneous format with improved sensitivity by means of a system based on the combination of a tunable monochromatic light source and an amorphous/nanocrystalline silicon photodetector that detects color and light intensity changes undergone by samples/assays containing tailored gold nanoparticles probes. This new low cost, portable, fast and simple optoelectronic platform, with the possibility to be re-used, permits detection of at least 400 fentomole of specific DNA sequences without target or signal amplification and was applied to the rapid detection of human pathogens in large variety of clinical samples such as Mycobacterium tuberculosis. © 2008 Elsevier B.V. All rights reserved.

ZnO films were grown on sapphire (001) substrate by atmospheric MOCVD using diethyl zinc and tertiary butanol precursors. The influence of different carrier gases (H2 and He) on the properties was analyzed by their structural (XRD), microstructural (SEM) and compositional (SIMS) characterization. The intensity of the strongest diffraction peak from ZnO (002) plane was increased by about 2 orders of magnitude when He is used as carrier gas, indicating the significant enhancement in crystallinity. The surface of the samples grown using H2 and He carrier gases was composed of leaf-like and spherical grains respectively. Hydrogen [H] content in the film grown using H2 is higher than that using He, indicating that the [H] was influenced by the H2 carrier gas. Ultraviolet emission dominates the low temperature PL spectra. The emission from ZnO films grown using He show higher optical quality and more emission centers.

Transparent conductive oxides (TCOs) such as indium tin oxide (ITO) thin films onto glass substrates are widely used as transparent and conductive electrodes for a variety of technological applications including flat panel displays, solar cells, smart windows, touch screens, etc. ITO films on glass and polycarbonate (PC) substrates were prepared at room temperature (RT) and at different PO2. The films were characterized in terms of the surface roughness (δ), sheet resistance, the refractive index (n) and extinction coefficient (k). The free carrier density (nc) and the carrier mobility (μ) of the ITO (In2O3:Sn) films were measured and studied. The nc and μ values vary in different ratio of oxygen partial pressure (PO2) of ITO deposition. The observed changes in the ITO film resistivity are due to the combined effect of different parameter values for nc and μ. From AFM analysis and spectra calculations, the surface roughness values of the ITO films were studied and it was observed that the δ values were lower than 15 nm. The energy band gap Eg ranges from 3.26 eV to 3.66 eV as determined from the absorption spectrum. It was observed an increase on the energy band gap as the PO2 decrease in the range of 20-2% PO2. The Lorentz oscillator classical model has also been used to fit the ellipsometric spectra in order to obtain both refractive index n and extinction coefficient κ values. © 2008 Elsevier Ltd. All rights reserved.

In the present work, we investigate the influence of hydrocarbon (CHx) thin film coating on porous silicon (PS) by plasma enhanced chemical vapor deposition (PECVD) technique to detect CO2 gas. The fabricated CHx/PS heterojunction device shows up to one and two orders of magnitude enhancement in current under CO2 gas exposure. FTIR spectroscopy measurements reveal a remarkable structural modification of the CHx/PS device during CO2 gas exposure. Further, the enhancement of CHx related absorbance bands by a factor 6.2 for the CHx/PS specimen in comparison with PS confirm the good quality of the deposited CHx thin films. © 2007 Elsevier B.V. All rights reserved.

In this work we present a study on the effect of annealing temperatures on the structural, morphological, electrical and optical characteristics of gallium doped zinc oxide (GZO), indium zinc oxide (IZO) and indium-tin-oxide (ITO) films. GZO and IZO films were deposited at room temperature by r.f. magnetron sputtering, whereas the ITO films were commercial ones purchased from Balzers. All films were annealed at temperatures of 250 and 500 °C in open air for 1 h. The GZO and ITO films were polycrystalline. The amorphous structure of as-deposited IZO films becomes crystalline on high temperature annealing (500 °C). The sheet resistivity increased with increase in annealing temperature. GZO films showed an increase of 6 orders of magnitude. The optical transmittance and band gap of as-deposited films varied with annealing. The highest transmittance (over 95 %) and maximum band gap (3.93 eV) have been obtained for ITO films. © 2007 Elsevier B.V. All rights reserved.

Nitrogen-doped ZnO films were deposited by RF magnetron sputtering in 75% of N2 / (Ar + N2) gas atmosphere. The influence of substrate temperature ranging from room temperature (RT) to 300 °C was analyzed by X-ray diffractometry (XRD), spectrophotometry, X-ray photoelectron spectroscopy (XPS), secondary-ion mass spectrometry (SIMS) and Hall measurements setup. The XRD studies confirmed the hexagonal ZnO structure and showed that the crystallinity of these films increased with increasing substrate temperature (Ts). The optical studies indicate the average visible transmittance in the wavelength ranging 500-800 nm increases with increasing Ts. A minimum transmittance (9.84%) obtained for the films deposited at RT increased with increasing Ts to a maximum of 88.59% at 300 °C (500-800 nm). Furthermore, it was understood that the band gap widens with increasing Ts from 1.99 eV (RT) to 3.30 eV (250 °C). Compositional analyses (XPS and SIMS) confirmed the nitrogen (N) incorporation into the ZnO films and its decreasing concentration with increasing Ts. The negative sign of Hall coefficients confirmed the n-type conducting. © 2007 Elsevier B.V. All rights reserved.

Nitrogen (N)-doped ZnO thin films were RF sputtered with different N2 volume (ranging from 10% to 100%) on sapphire (001) substrates. The influence of N2 vol.% on the properties of ZnO films was analyzed by various characterization techniques. The X-ray diffraction studies showed that the films grow along the preferential (002) crystallographic plane and the crystallinity varied with varying N2 vol.%. The films sputtered with 25 vol.% N2 showed better crystallinity. The transmittance was decreased with increasing N2 volume until 25% and was almost constant above 25%. A maximum optical band gap (2.08 eV) obtained for 10 vol.% N2 decreased with increasing N2 volume to reach a minimum of 1.53 eV at 100%. The compositional analysis confirmed the incorporation of N into ZnO films, and its concentration increased with increasing N2 volume to reach a maximum of ∼ 3.7 × 1021 atom/cm3 at 75% but then decreased slightly to 3.42 × 1021 atoms/cm3. The sign of Hall coefficient confirmed that the films sputtered with ≤ 25 vol.% N2 possess p-type conductivity which changes to n-type for > 25 vol.% N2. © 2007 Elsevier B.V. All rights reserved.