Rodrigo Martins

In this letter we compare the room temperature ozone sensing properties of intrinsic zinc oxide (ZnO) thin films deposited by spray pyrolysis, dc and r.f. magnetron sputtering. Their sensor response exceeds 8 orders of magnitude when the film structure is constituted by nanocrystallites. These preliminary results clearly demonstrate that the films could be potentially used for ozone detection at room temperature.

Highly conducting and transparent gallium doped zinc oxide thin films have been deposited at high growth rates by r.f. magnetron sputtering at room temperature on inexpensive soda lime glass substrates. The argon sputtering pressure was varied between 0.15 and 2.1 Pa. The lowest resistivity was 2.6 × 10-4 Ω cm (sheet resistance ≈6 Ω/sq. for a thickness ≈600 nm) and was obtained at an argon sputtering pressure of 0.15 Pa and a r.f. power of 175 W. The films present an overall transmittance in the visible spectra of approximately 90%. The increase on the resistivity for higher sputtering pressures is due to a decrease of both, mobility and carrier concentration, and is associated to a change on the surface morphology. The low resistivity, accomplished with a high growth rate (290 Å/min) and with a room temperature deposition enables these films deposition onto polymeric substrates for flexible optoelectronic devices. © 2002 Elsevier Science B.V. All rights reserved.

This work presents for the first time a study on the deposition of polymorphous silicon at an excitation frequency of 27.12 MHz in a large-area plasma enhanced chemical vapor deposition (PECVD) reactor. Moreover, the films produced at 13.56 MHz were also investigated to compare their performance with that of the films produced at 27.12 MHz. The SiH4/H2 plasma was characterized by impedance probe measurements, aiming to identify the plasma conditions that lead to produce polymorphous films, under quasi-isothermal conditions. The films were characterized by spectroscopic ellipsometry, infrared absorption, Raman spectroscopy, and hydrogen exodiffusion experiments. These techniques enable a detailed structural characterization of the polymorphous films and a study of the differences between the films deposited at 27.12 MHz and 13.56 MHz. Conductivity measurements were also performed to determine the transport properties of the films. The results show that by using a 27.12 MHz frequency, the growth rate increased by 70% and a more stable, relaxed and denser structure was obtained.

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 cm2/Vs and a carrier concentration of 1.3 × 1021 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. © 2003 Elsevier B.V. All rights reserved.

Despite of a growing interest in this material, until now the studies on polymorphous silicon (pm-Si:H) have been performed on small laboratory reactors working at 13.56 MHz. Envisaging an industrial application of pm-Si:H, the technology was transferred to a large area plasma enhanced chemical vapour deposition reactor (25 × 40 cm2) working at excitation frequencies of 13.56 and 27.12 MHz. The plasma was characterized by impedance probe measurements and the films were characterized by spectroscopic ellipsometry, infrared spectroscopy and hydrogen evolution experiments, which are techniques that allow a rapid and reliable identification of pm-Si:H structure. Conductivity measurements were also performed to determine their transport properties. The results show that scaling up using the 13.56 MHz was successfully done and pm-Si:H films were deposited at a growth rate of ≈ 12 nm/min. Moreover, by using the 27.12 MHz excitation frequency the growth rate was even further increased to above 18 nm/min, as desired for industrial production. © 2002 Elsevier Science B.V. All rights reserved.

This paper describes, for the first time, a method of producing polymorphous silicon (pm-Si:H) films by plasma-enhanced (PE) CVD, using an excitation frequency of 27.12 MHz. The aim is to produce, at high growth rates, nanostructured films that are more stable than the conventional amorphous or polymorphous silicon films grown by PECVD at 13.56 MHz. The processing data show that, at 27.12 MHz, the pm-Si:H films are produced close to the transition region from amorphous to microcrystalline silicon films, at a growth rate of about 0.3 nm s-1, using pressures above 160 Pa. Apart from that, the analysis of the exodiffusion, spectroscopic ellipsometry (SE), and micro Raman data reveal that these films are more dense and compact than the polymorphous films grown at 13.56 MHz.

This paper describes, for the first time, a method of producing polymorphous silicon (pm-Si:H) films by plasma-enhanced (PE) CVD, using an excitation frequency of 27.12 MHz. The aim is to produce, at high growth rates, nanostructured films that are more stable than the conventional amorphous or polymorphous silicon films grown by PECVD at 13.56 MHz. The processing data show that, at 27.12 MHz, the pm-Si:H films are produced close to the transition region from amorphous to microcrystalline silicon films, at a growth rate of about 0.3 nms-1, using pressures above 160 Pa. Apart from that, the analysis of the exodiffusion, spectroscopic ellipsometry (SE), and micro Raman data reveal that these films are more dense and compact than the polymorphous films grown at 13.56 MHz.

This work refers to a study performed on polymorphous silicon (pm-Si:H) at excitation frequencies of 13.56 and 27.12 MHz in a large area PECVD reactor. The plasma was characterised by impedance probe measurements, aiming to identify the plasma conditions that lead to produce pm-Si:H films. The films produced were characterised by spectroscopic ellipsometry, infrared and Raman spectroscopy and hydrogen exodiffusion experiments, which are techniques that permit the structural characterisation of the pm-Si films and to study the possible differences between the films deposited at 13.56 and 27.12 MHz. Conductivity measurements were also performed to determine the transport properties of the films produced. The set of data obtained show that the 27.12 MHz pm-Si:H can be grown at higher rates with less hydrogen dilution and power density, being the resulting films denser, chemically more stable and with improved performances than the pm-Si:H films grown at 13.56 MHz.

In this work, spectroscopic ellipsometry was used to characterise oxide films produced by anodic oxidation of amorphous silicon using an ethylene glycol (0.04 M KNO3) solution. The data obtained show that the growth of the oxide is not only a function of the voltage applied, but also of the current density and of the time process. An empiric model based on a power law is proposed for the growth of the oxide using, as parameters, the voltages and the time process. The oxide produced shows porosity of approximately 12%, which can be reduced down to 6% under well-controlled growth conditions. © 2002 Elsevier Science B.V. All rights reserved.

In this paper, we present the preliminary results concerning the deposition of highly transparent and conductive gallium-doped zinc oxide (GZO) deposited on transparent flexible substrate based on cellulose derivatives. Prior to the deposition of the GZO film, the surface of the polymer have been coated with a thin silicon dioxide (SiO2) layer deposited by thermal evaporation assisted by an electron gun. By doing this surface treatment, we succeeded in depositing highly conductive and transparent GZO with an electrical resistivity of 2.0 × 10-3 Ω cm and an average optical transmittance in the visible part of the spectrum (400-700 nm) of 70% by r.f. magnetron sputtering at room temperature. Besides the optoelectronic properties, the films are mechanically stable with a polycrystalline structure with a strong preferred (002) orientation, parallel to the substrate. © 2003 Elsevier B.V. All rights reserved.

In this paper we present results concerning the performance exhibited by an integrated array of 32 one-dimensional amorphous silicon thin film position sensitive detectors based on nip and hetero amorphous silicon structures, with a total active area size below 1 cm2 linearity, its spatial resolution and response time, that make it one of the most interesting analog detector to be used in unmanned optical inspection control systems where a continuous detection process is required. This opens a wide range of applications for amorphous silicon devices in the area of image processing. © 2002 Elsevier Science B.V. All rights reserved.

In this paper we present results concerning the optimisation of the electronic and mechanical properties presented by amorphous silicon (a-Si:H) thin films produced on polyimide (Kapton® VN) substrates with different thicknesses (25, 50 and 75 μm) by the plasma enhanced chemical vapour deposition (PECVD) technique. The purpose of this study is to obtain a low defect density as well as low residual stresses (specially at the interface) in order to provide good performances for large area (10 mm wide by 80 mm long) flexible position sensitive detectors. The electrical and optical properties presented by the films will be correlated to the sensor characteristics. The properties of samples have been measured by dark/photoconductivity, constant photocurrent measurements (CPM) and the results have been compared with films deposited on Corning 7059 glass substrates during the same run deposition. The residual stresses were measured using an active optical triangulation and angle resolved scattering. The preliminary results indicate that the thinner polymeric substrate with 25 μm presents the highest density of states, which is associated to the residual stresses and strains associated within the film. © 2002 Elsevier Science B.V. All rights reserved.

Indium-Tin-Oxide (ITO) thin films were deposited on glass substrates using DC magnetron reactive sputtering at different bias voltages and substrate temperatures. Some improvements were obtained on film properties, microstructure and other physical characteristics for different conditions. Amorphous and polycrystalline films can be obtained for various deposition conditions. The transmission, absorption, spectral and diffuse reflection of ITO films were measured in some ranges of UV-Vis-NIR. The refractive index (n), Energy band gap Eg and the surface roughness of the film were derived from the measured spectra data. The carrier density (nc) and the carrier mobility (μ) of the film micro conductive properties were discussed. The films exhibited suitable optical transmittance and conductivity for electrochromic applications. © 2002 Elsevier Science B.V. All rights reserved.

We report, for the first time, results on transparent ZnO:Al thin films deposited on polyester (Mylar type D, 100 μm thickness) substrates at room temperature by magnetron sputtering. The structural, optical and electrical properties of the deposited films have been studied. The samples are polycrystalline with a hexagonal wurtzite structure and a strong crystallographic c-axis orientation (0 0 2) perpendicular to the substrate surface. The ZnO:Al thin films with 83% transmittance in the visible region and a resistivity as low as 3.6 × 10-2 Ωcm have been obtained, as deposited. The obtained results are comparable to those obtained on glass substrates, opening a new field of low cost, light weight, small volume, flexible and unbreakable large area optoelectronic devices. © 2002 Elsevier Science Ltd. All rights reserved.

In this work, we study the electro-optical behaviour of cellulose/liquid crystal-based composite systems, in particular the influence of the flexible substrates and its conductive layers in the electro-optical behaviour of these kind of cells. Four cells were made using, respectively, two different substrates (a flexible polymer (poly(ethylene terephthalate) (PET)) and a soda lime glass) and two different conductive layers (indium tin oxide (ITO) and aluminium zinc oxide (AZO)). The conductive layer (AZO) was deposited in both, flexible and rigid substrates, for the same conditions, and the same substrates coated with ITO are commercially available. The cells were prepared from solid films of hydroxypropylcellulose (HPC) (30 μm thick) cross linked with 1,4-diisocyanatobutane (BDI) (7% w/w) and the nematic liquid crystal E7 (Merck, UK). The four different substrates were electrically and morphologically characterised. We have analysed all samples by light transmission and determined the maximum transmission, contrast and Von. We show a comparison of the results obtained for both flexible and rigid cells and discuss them in terms of the proposed working mechanism for these systems. © 2002 Elsevier Science B.V. All rights reserved.

In this work we present results concerning the composition and structure of intrinsic thin film silicon carbide alloys obtained by hot wire and hot wire plasma assisted techniques using ethylene as carbon gas source. The data show that by increasing the percentage of ethylene in the gas mixture from 14% to 60% the optical band gap is enhanced from 1.8 eV to 2.3 eV, for films produced by hot wire technique at a filament temperature of 2123K (1850°C). This is attributed to the increase of carbon incorporation, which was confirmed by the infrared spectra data where an increase is observed in the SiC stretching vibration mode ascribed to the peak located at around 750cm-1. On the other hand, the films produced by combining hot wire and rf plasma show a more efficient carbon incorporation. The SEM photographs of samples produced with hot wire technique reveal an amorphous structure, confirmed by micro-Raman spectroscopy data, while the samples produced with plasma assisting the process show a granular structure with grain sizes in the range of 100-200nm. © 2002 Elsevier Science Ltd. All rights reserved.

The influence of residual stresses on the performances of large area position sensitive detectors produced on flexible substrates are presented here. For evaluating the residual stresses, two main techniques were used: An active optical triangulation and angle resolved scattering and the constant photocurrent method (CPM). From the results it was possible to correlate the stresses and the density of defects present in the films.

In this work we studied the influence of the dopant elements and concentration on the properties of ZnO thin film deposited by spray pyrolysis. The results show that the doping affects the thin films properties mainly the electrical ones, function of dopant concentration and nature. The most important changes were observed for films doped with 1at% of indium which exhibit a resistivity of 1.9 × 10-1 Ωcm associated with a transmitance of 90%. After the annealing treatment, the resistivity of the film decreases to 5.9 × 10-3 Ωcm without significative changes in the optical properties. The films were also used to produce amorphous silicon solar cells where the best results were obtained for ZnO:In. © 2002 Elsevier Science Ltd. All rights reserved.

Where a-Si:H pin devices are concerned, one of the main obstacles regarding improved performance is device stability, usually attributed to adverse behaviour at various interfaces within the device. Several attempts have been made to overcome this problem, such as the use of blocking layers at the interfaces. Although these have led to some improvements in device performance, most of the problems associated with device stability remain. This is mainly due to the defects at the interfaces, since the blocking layers (silicon alloys with carbon, nitrogen or oxygen) usually have a high density of bulk states, in comparison to intrinsic a-Si:H films. In this paper, we present a method that seems to be capable of improving device stability. It consists of performing a controlled removal of oxide interlayers at the interfaces, by an appropriate etching process. This enables the production of highly smoothed interfaces, and reduces possible cross-contamination of the i-layer from the adjacent doped layers. This amounts to a new design of typical pin devices, in which thin absorber layers are placed at the p/i and i/n interfaces. Their purpose is to trap most of the impurity atoms diffused from the doped layers, after which they are removed by appropriate etching. The fabrication of the absorbers (sacrificial layers), the nature of the etching and the tailoring of the defect profile at the interfaces will be discussed, including the performance exhibited by the resulting devices. © 2002 Elsevier Science B.V. All rights reserved.

The model presented is based on the heat transfer and energy balance equations that rule the set of physical and chemical interactions that take place on the gas phase of a growth process, assuming that the deposition process occurs under laminar dynamic flow conditions (Knudsen number below 1). In these conditions, the chemistry and physics of the process involved in the growth mechanism of silicon thin films produced by the hot wire or the hot-wire plasma assisted technique can be proper derived by balance equations that supply information about how the plasma density, the gas dilution and the gas temperature influence the growth mechanism and the equilibrium of the concentration of species presented on the growth surface. The model developed establishes a relation between the abundance species formed and the parameters initiators of the process such as the filament temperature and the rf power density used.

This work presents the I-V results of a-Si:H/SiOx/Pd MIS (metal-insulator-semiconductor) structures. The a-Si:H was deposited by non-conventional modified triode PECVD. This new configuration allows the deposition of high quality a-Si:H with a photosensitivity of 106, indicating the presence of low density of defects. Spectroscopic ellipsometry measurements revealed that these films are highly dense and present a very smooth surface so allowing a low defect interface between the Pd and the a-Si:H. As a result, we could make MIS photodiodes with barrier heights of 1.17 eV, which give a high reduction of the reverse dark current, an increase of the signal to noise ratio of 106 and an open circuit voltage VOC = 0.5 V. © 2002 Elsevier Science B.V. All rights reserved.

Transparent conducting ZnO:Al thin films have been deposited on polyester (Mylar type D, 100 μm thickness) substrates at room temperature by r.f. magnetron sputtering. The structural, optical and electrical properties of the deposited films have been studied. The samples are polycrystalline with a hexagonal wurtzite structure and a strong crystallographic c-axis orientation (002) perpendicular to the substrate surface. As deposited ZnO:Al thin films have an 85% transmittance in the visible and infra-red region and a resistivity as low as 3.6×10-2 Ωcm. The obtained results are comparable to those ones obtained on glass substrates, opening a new field for low cost, light weight, small volume, flexible and unbreakable large area optoelectronic devices.

We have produced amorphous intrinsic silicon thin films by hot-wire plasma assisted chemical vapor deposition, a process that combines the traditional rf plasma and the recent hot-wire techniques. In this work we have studied the influence of hydrogen gas dilution and rf power on the surface morphology, composition, structure and electro-optical properties of these films. The results show that by using this deposition technique it is possible to obtain at moderate rf power and filament temperature, compact i-type silicon films with ημτ of the order of 10 -5cm 2V -1, without hydrogen dilution. © 2002 American Institute of Physics.

In this work we show that it is possible to control the plasma regime in the region close to the substrate in r.f. silane discharges. The PECVD reactor works in a modified triode configuration, where the control over the plasma regime is performed by polarising a grid electrode, placed close to the r.f. electrode, with a DC power source. Besides that, the DC grid allows also to control the energy of the ion bombardment, because the plasma potential will be a function of the voltage (Vpol) applied to the DC grid. The silane plasma was characterised with a Langmuir probe and an impedance probe. We were able to identify three plasma regimes in the region close to the substrate: γ′ regime for Vpol<0 V; γ′-α regime for 0 V<Vpol<40 V; and α regime for Vpol40 V. The γ′ regime is associated with a high concentration of dust particles in plasma and high electron energy (≈8eV), while the α regime is associated with a free dust plasma and low electron energy (≈2eV). The intermediate regime, γ′-α, is characterised by the presence of smaller particles (≈2-5nm) that can be beneficial for the film's properties. © 2002 Elsevier Science Ltd. All rights reserved.

P- and n-type silicon thin films have been produced using a new hot wire plasma assisted deposition process that combines the conventional plasma enhanced chemical vapor deposition and the hot wire techniques. The films were produced in the presence of different hydrogen gas flow and their optoelectronic, structural and compositional properties have been studied. The optimized optoelectronic results achieved for n-type Si:H films are conductivity at room temperature of 9.4(Ωcm)-1 and optical band gap of 2eV while for p-type SiC:H films these values are 1 × 10-2(Ωcm)-1 and 1.6eV, respectively. The films exhibit the required optoelectronic characteristics and compactness for device applications such as solar cells.