Rodrigo Martins

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.

A series of hydrogenated amorphous silicon carbide films were prepared by plasma enhanced chemical vapor deposition (PECVD) using a gas mixture of silane, methane, and hydrogen as the reactive source and an excitation frequency of 27.12 MHz. Compared to the typical radio frequency deposition technique, the very high plasma excitation frequency increases the density of the electrons and decreases the electron temperature, which helps the dissociation of the SiH4 and CH4, and reduces the energetic ion impact on the growth surface of the thin film. Thus, dense-films with lower bulk density of states and higher growth rate are expected, as confirmed by spectroscopic ellipsometry data. Apart from that, a substantial reduction of bulk defects is achieved, allowing an improvement of the valence controllability (widening of the optical gap from about 1.9 to 3.6 eV). In this work results concerning the microstuctural and photoelectronic properties of the silicon carbide films will be discussed in detail, correlating them with the deposition process conditions used as well as with the gas phase composition of the mixtures used. © 2004 Elsevier B.V. All rights reserved.

Polymorphous silicon (pm-Si:H) films have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the pm-Si:H has higher photoconductivity (σph), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. There are a blue shift for the stretching mode of IR. spectra and a red shift for the wagging mode. The shifts are attributed to the variation of the microstructure. By using pm-Si:H film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100mw/cm2) at room temperature (T R).

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (σph), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100 mw/cm2) at room temperature. © 2006.

A series of hydrogenated amorphous silicon carbide (a-Si 1- xC x:H) films were prepared by plasma-enhanced chemical vapour deposition (PECVD) using a gas mixture of silane, methane, and hydrogen as the reactive source. The previous results show that a high excitation frequency, together with a high hydrogen dilution ratio of the reactive gases, allow an easier incorporation of the carbon atoms into the silicon-rich a-Si 1-xC x:H film, widen the valence controllability. The data show that films with optical gaps ranging from about 1.9 to 3.6 eV could be produced. In this work the influence of the hydrogen dilution ratio of the reactive gases on the a-Si 1-xC x:H film properties was investigated. The microstuctural and photoelectronic properties of the silicon carbide films were characterized by Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERDA), and FT-IR spectrometry. The results show that a higher hydrogen dilution ratio enhances the incorporation of silicon atoms in the amorphous carbon matrix for carbon-rich a-Si 1-xC x:H films. One pin structure was prepared by using the a-Si 1-xC x:H film as the intrinsic layer. The light spectral response shows that this structure fits the requirement for the top junction of colour sensor. © 2004 Elsevier B.V. All rights reserved.

A series of amorphous silicon carbide films were prepared by plasma enhanced chemical vapor deposition technique on (100) silicon wafers by using methane, silane, and hydrogen as reactive resources. A very thin (around 15 Å) gold film was evaporated on the half area of the a-SiC:H films to investigate the metal induced crystallization effect. Then the a-SiC:H films were annealed at 1100°C for 1 hour in the nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to analyze the microstructure, composition and surface morphology of the films. The influences of the high temperature annealing on the microstructure of a-SiC:H film and the metal induced metallization were investigated.

A series of silicon film samples were prepared by plasma enhanced chemical vapor deposition (PECVD) near the threshold from amorphous to nanocrystalline state by adjusting the plasma parameters and properly increasing the reactions between the hydrogen plasma and the growing surface. The microstucture of the films was studied by micro-Raman and Fourier transform infrared (FTIR) spectroscopy. The influences of the hydrogen dilution ratio of silane (R H = [H2]/[SiH4]) and the substrate temperature (Ts) on the microstructural and photoelectronic properties of silicon films were investigated in detail. With the increase of RH from 10 to 100, a notable improvement in the medium-range order (MRO) of the films was observed, and then the phase transition from amorphous to nanocrystalline phase occurred, which lead to the formation of diatomic hydrogen complex, H 2 * and their congeries. With the increase of T s from 150 to 275 °C, both the short-range order and the medium range order of the silicon films are obviously improved. The photoconductivity spectra and the light induced changes of the films show that the diphasic nc-Si/a-Si:H films with fine medium-range order present a broader light spectral response range in the longer wavelength and a lower degradation upon illumination than conventional a-Si:H films. © 2004 Elsevier B.V. All rights reserved.

Amorphous silicon carbide films were obtained by plasma enhanced chemical vapor deposition (PECVD) technique using a gas mixture of silane, methane, and hydrogen with a high excitation frequency and a high hydrogen dilution ratio. The high temperature annealing behavior of the amorphous silicon carbide films was studied by annealing at 1373 K for 1 h in nitrogen atmosphere. A very thin Au film was deposited on part of the films to investigate the metal induced crystallization effect. Well aligned nanotubes were found on the silicon carbide films covered by a thin gold layer after the high temperature annealing by atomic force microscopy. Further study is necessary to identify the nature of the nanotubes and elucidate their growth mechanism. © 2006 Elsevier B.V. All rights reserved.

This work deals with the study of the role of the film thickness and composition on the color selectivity of the collecting spectrum of glass/ZnO:Ga/p-a-Si1-xCx:H/ a-Si1-x C x:H /a-Si:H/n-a-Si:H/Al photoelectronic detectors produced in a single chamber plasma enhanced chemical vapor deposition (PECVD) system. The cross contaminations were minimized by a rotate-cover substrate holder system. The devices can detect the blue illumination at small reverse bias and detect red illumination at large reverse bias. The role of the process parameters, especially the thickness of the p-type and intrinsic a-Si1-x C x:H, and the intrinsic a-Si:H layers on the device performances were studied in detail aiming to achieve a better detectivity. © 2005 Materials Research Society.

A series of hydrogenated amorphous silicon carbide (a-SiC:H) films was prepared by plasma enhanced chemical vapor deposition (PECVD) technology. The microstructure and photoelectronic properties of the film are investigated by absorption spectra (in the ultraviolet to near-infrared range) and Fourier transform infrared (FTIR) spectra. The results show that good band gap controllability (1.83-3.64 eV) was achieved by adjusting the plasma parameters. In the energy range around 2.1 eV, the a-Si1-xCx:H films exhibit good photosensitivity, opening the possibility to use this wide band gap material for device application, especially when blue color detectors are concerned. A multilayer device with a stack of glass/TCO(ZnO:Ga)/P(a-SiC:H)/I(a- SiC:H)/N(a-Si:H)/I(a-Si:H)/P(a-Si:H)/Al has been prepared. The devices can detect blue and red colors under different bias voltages. The optimization of the device, especially the film thickness and the band gap offset used to achieve better detectivity, is also done in this work. © 2005 Elsevier B.V. All rights reserved.

Hydrogenated nanocrystalline silicon (nc-Si:H) n-layers have been used to prepare heterojunction solar cells on flat p-type crystalline silicon (c-Si) wafers. The nc-Si:H n-layers were deposited by radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD), and characterized using Raman spectroscopy, optical transmittance and activation energy of dark-conductivity. The nc-Si:H n-layers obtained comprise fine grained nanocrystallites embedded in amorphous matrix, which have a wider bandgap and a smaller activation energy. Heterojunction solar cells incorporated with the nc-Si n-layer were fabricated using configuration of Ag (100 nm)/lT0 (80 nm)/n-nc-Si:H (15 nm)/buffer a-Si:H/p-c-Si (300 μm)/Al (200 nm), where a very thin intrinsic a-Si:H buffer layer was used to passivate the p-c-Si surface, followed by a hydrogen plasma treatment prior to the deposition of the thin nanocrystalline layer. The results show that heterojunction solar cells subjected to these surface treatments exhibit a remarkable increase in the efficiency, up to 14.1% on an area of 2.43 cm2. © 2006 Elsevier B.V. All rights reserved.

The microstructural aspects related to crystalline or amorphous structure of as-deposited and annealed films of sol-gel-derived WO 3 are shown in the literature to be critical for electrochromic (EC) performance. In consideration of ion insertion materials, there is a need for developing light and at the same time nanocrystalline structures to improve both coloration efficiency and switching kinetics. By controlling microstructure and morphology, one could design a material with optimal EC performance. This report compares the microstructural and morphological characteristics of standard WO 3 wet deposition techniques versus inkjet printing technology (IPT), correlating these features with their optical and electrochemical performances, emphasizing the importance of the dual-phase a-WO 3/TiO 2/WO X film composition proposed in this work for high-performance EC applications. The effect of the type and content of metal oxide nanoparticles in the precursor sols formulated in various peroxopolytungstic acid (PTA) and oxalic acid (OAD) proportions on film properties is comprehensively studied using multi-factorial design of experiment (DOE). To the authors' knowledge, no other report on sol-gel deposition of inorganic EC materials via the inkjet printing technique exists, in which furthermore the film crystallinity can be controlled under low-temperature process conditions. The proposed method enables development of EC films which irrespective of their composition (a-WO 3, a-WO 3/TiO 2 or a-WO 3/TiO 2/WO X) outperform their amorphous or nanocrystalline analogues presented as the state-of-the-art due to their superior chemical and physical properties. © 2012 The Royal Society of Chemistry.

Engineering procedures governing the selection or development of printable nanostructured metal oxide nanoparticles for chromic, photovoltaic, photocatalytic, sensing, electrolyte-gated TFTs, and power storage applications are established in this chapter. The main focus is given on how to perform the material selection and formulation of printable dispersion in order to develop functional films for electrochemical applications. This chapter is divided into four main parts. Firstly, a brief introduction on electrochemically active nanocrystalline metal oxide films developed via printing techniques is given. This is followed by the description of the film morphology, structure, and required functionality. A theoretical approach to understand the impact of size and shape of nanoparticles on an ink formulation and electrochemical performance being the subject of the third section provides a greater control over the material selection. We attempt to describe these properties and show that for a given material, geometry and size of the nanoparticles have a major influence on the electrochemical reactivity and response time. This gives the ability to tune the performance of the film simply by varying the morphology of incorporated nanostructures. This section is completed by the recommendations on each major step of an ink formulation, together with imposed critical constraints concerning the fluid control. Finally, the performance of the ink-jetprinted dual-phase electrochromic films is discussed as a case study. By providing such a rather systematic survey, we aim to stress the importance of proper design strategy that would result in both improved physicochemical properties of nanoparticle-loaded inks and enhanced electrochemical performance of printed functional films. © Springer International Publishing Switzerland 2016.

This paper focuses on the engineering procedures governing the synthesis of tungsten oxide nanocrystals and the formulation of printable dispersions for electrochromic applications. By that means, we aim to stress the relevancy of a proper design strategy that results in improved physicochemical properties of nanoparticle loaded inks. In the present study inkjet printable nanostructured tungsten oxide particles were successfully synthesized via hydrothermal processes using pure or acidified aqueous sol-gel precursors. Based on the proposed scheme, the structure and morphology of the nanoparticles were tailored to ensure the desired printability and electrochromic performance. The developed nanomaterials with specified structures effectively improved the electrochemical response of printed films, resulting in 2.5 times higher optical modulation and 2 times faster coloration time when compared with pure amorphous films. © The Royal Society of Chemistry 2015.

P- and n-type weakly absorbing highly conductive (σ>0·1Ω-1 cm-1) SiC thin films with similar structural and optoelectronic properties have been prepared in a two-consecutive-decomposition-deposition-chamber reactor. These films are composed of Si microcrystals (δ = 50-100 Å) embedded in an amorphous Si:0:C:H matrix, with concentrations up to 25at.%O and 20at.%C. From diffraction studies there is no evidence for the presence of SiC crystallites. Electrical conduction appears to be in extended states via percolation channels through Si crystallites of sufficient volume fraction. © 1991 Taylor & Francis Ltd.

Diffraction and other structural measurements on n-type SiC thin films prepared in a TCDDC (two consecutive decomposition and deposition chamber) system indicate the presence of Si microcrystals (without evidence for SiC crystallites). Weakly absorbing, highly conductive layers (σ ≥ 10-1 (Ω-cm)-1) contain up to 20 at.% C and 25 at.% O. The optoelectronic properties of these films can be explained in terms of a sufficient volume fraction (above the percolation threshold) of Si microcrystals surrounded by an a-Si:C:O:H matrix.

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.

Nitrogen and Phosphorus co-doped (N+P)- zinc oxide (ZnO) films were RF sputtered on corning glass substrates at 350 °C and comparatively studied with undoped, N-, and P- doped ZnO. X-ray diffraction spectra confirmed that the ZnO structure with a preferred orientation along <002> direction. Scanning electron microscope analysis showed different microstructure for the N+P co-doping, and thus probably confirming the co-existence of both the dopants. X-ray photoelectron spectroscopy spectra revealed that the chemical composition in N+P co-doped ZnO are different from that found in undoped, N-, and P- doped ZnO. The atomic ratio of N and P in N+P co-doped ZnO is higher than that in single N or P doped ZnO. One broad ZnO emission peak around 420 nm is observed in photoluminescence spectra. The relative intensity of the strongest peak obtained from co-doped ZnO films is about twice than the P- doped and thrice than the pure and N- doped films. © (2013) Trans Tech Publications, Switzerland.

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.

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.

N, (N+Ga) and (N+Al) doped ZnO films were deposited on c-plane sapphire substrates by RF magnetron sputtering at room temperature. The samples were characterized by their structural, surface morphological, compositional and optical properties. The x-ray diffraction studies confirmed the co-doping of (N+Ga) and (N+Al) besides showing improvement in the crystallinity when compared with the single Ndoping. The surface of the films becomes rougher after co-doping. The x-ray photoelectron spectroscopy and Rutherford back-scattering analysis indicate that the co-doping changes the chemical states and varies the amount of nitrogen (N) in ZnO. The amount of 'N' has been greatly increased for (N+Ga) co-doping, indicating that it is the best co-doping pair for p-type ZnO. Additionally, co-doping has increased the average visible transmittance (40-650nm) and the optical band gap is shifted towards shorter wavelength. In the case of (N+Al) co-doping, the band gap becomes wider than that of undoped ZnO. © IOP Publishing Ltd.

In order to study the properties of P-doped ZnO films deposited at low temperature substrates, P-doped ZnO films were RF-sputtered on sapphire substrates in the range from RT to 350°C. XRD spectra indicated the growth of the crystallites along the strongest <001> orientation. Further ZnO (002) peak became the weakest when the film was sputtered at 250°C. AFM pictures showed that the surface morphology varied with the deposition temperature. The sample RMS increased with the increase of substrate temperature. XPS spectra showed a clear broad P 2p peak at about 134 eV. Further the film composition varied with the substrate temperature. The average visible transmittance calculated in the wavelength ranging 400-600 nm was more than 60%. The optical band gap calculated from the absorption coefficient was about 3.2 eV. The Hall measurements confirm the n-type conductivity of the films. The carrier concentration in the films decreases with the increase of substrate temperature. The study is helpful for understanding the properties of P-doped ZnO films sputtered at lower substrate temperature and achieving p type ZnO films at lower temperature.

N-doped ZnO films were deposited by RF magnetron sputtering in N2/Ar gas mixture and were post-annealed at different temperatures (Ta) ranging from 400 to 800 °C in O2 gas at atmospheric pressure. The as-deposited and post-annealed films were characterized by their structural (XRD), compositional (SIMS, XPS), optical (UV-vis-NIR spectrometry), electrical (Hall measurements), and optoelectronic properties (PL spectra). The XRD results authenticate the improvement of crystallinity following post-annealing. The weak intensity of the (0 0 2) reflection obtained for the as-deposited N-doped ZnO films was increased with the increasing Ta to become the preferred orientation at higher Ta (800 °C). The amount of N-concentration and the chemical states of N element in ZnO films were changed with the Ta, especially above 400 °C. The average visible transmittance (400-800 nm) of the as-deposited films (26%) was increased with the increasing Ta to reach a maximum of 75% at 600 °C but then decreased. In the PL spectra, A0X emission at 3.321 eV was observed for Ta = 400 °C besides the main D0X emission. The intensity of the A0X emission was decreased with the increasing Ta whereas D0X emission became sharper and more optical emission centers were observed when Ta is increased above 400 °C. © 2008 Elsevier B.V. All rights reserved.

Two self-buffer layers were grown on c-plane sapphire substrates by atmospheric MOCVD method using DEZn, tert-butanol as precursors and H 2 as carrier gas. Then, they were respectively annealed in growth process and oxygen environment. After that, ZnO films were respectively grown on them. XRD spectra show that all the films were grown in [002] orientation. Furthermore, the film with a buffer layer annealed in oxygen exhibits much higher crystal quality. Its FWHM of the rocking curve is only 567arcsec. Furthermore, its Raman scattering spectrum appears a much stronger E2 mode peak at 436cm-1 and its PL spectrum appears a shoulder at 3.367eV on the higher energy side. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA,.