Rodrigo Martins

This paper presents results of the role of the oxygen partial pressure used during the deposition process on the transport properties exhibited by doped microcrystalline silicon oxycarbide films produced by a Two Consecutive Decomposition and Deposition Chamber system, where a spatial separation between the plasma and the growth regions is achieved. This paper also presents the interpretative models of the optoelectronic behaviour observed in these films (highly conductive and transparent with suitable properties for optoelectronic applications) as well as the interpretation of the growth process that leads to film's microcrystallization.

The aim of this work is to interpret the role of the lateral leakage current on the a-Si:H solar cell performances (J-V characteristics, responsivity and the apparent device degradation behaviour), under low illumination conditions.

This paper discusses the effect of order and disorder on the electrical and optical performance of ionic oxide semiconductors based on zinc oxide. These materials are used as active thin films in electronic devices such as pn heterojunction solar cells and thin-film transistors. Considering the expected conduction mechanism in ordered and disordered semiconductors the role of the spherical symmetry of the s electron conduction bands will be analyzed and compared to covalent semiconductors. The obtained results show p-type c-Si/a-IZO/poly-ZGO solar cells exhibiting efficiencies above 14% in device areas of about 2.34 cm2. Amorphous oxide TFTs based on the Ga-Zn-Sn-0 system demonstrate superior performance than the polycrystalline TFTs based on ZnO, translated by ION/IOFF ratio exceeding 107, turn-on voltage below 1-2 V and saturation mobility above 25 cm2/Vs. Apart from that, preliminary data on p-type oxide TFT based on the Zn-Cu-O system will also be presented. © 2009 SPIE.

This paper discusses the electron transport in the n-type amorphous indium-zinc-oxygen system produced at room temperature by rf magnetron sputtering, under different oxygen partial pressures. The data show that the transport is not band tail limited, as it happens in conventional disordered semiconductors, but highly dependent on its ionicity, which explains the very high mobilities (≥ 60 cm 2 V -1 s -1) achieved. The room temperature dependence of the Hall mobility on the carrier concentration presents a reverse behaviour than the one observed in conventional crystalline/polycrystalline semiconductors, explained mainly by the presence of charged structural defects in excess of 4 × 10 10 cm -2 that scatter the electrons that pass through them. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The aim of this work is to present experimental data concerning the role of the oxygen partial pressure during the production process on the properties (structure, morphology, composition, and transport properties) exhibited by doped microcrystalline silicon oxycarbide films. The films were produced by a two consecutive decomposition and deposition chamber system, where a spatial separation between the plasma and the growth regions is achieved. The films produced by this technique are highly conductive and highly transparent with suitable properties for optoelectronic applications requiring wide band-gap and low-conductivity materials. © 1995, American Vacuum Society. All rights reserved.

This paper presents the performances of an in-line plasma enhanced chemical vapor deposition system constituted by 5 chambers and one external unloaded chamber used in the simultaneous manufacturing of 4 large (30 cm × 40 cm) solar cells deposited on glass substrates. The system is fully automatically controlled by a Programmable Logic Controller using a specific developed software that allows devices mass production without losing the flexibility to perform process innovations according to the industrial requests, i.e. fast and secure changes and optimizations. Overall, the process shift is of about 15 min per each set of 4 solar cells. Without a buffer layer, solar cells with efficiencies of about 9% were produced by the proper tuning of the i-layer production conditions. © 2005 Elsevier B.V. All rights reserved.

The objective of this work is to present an analytical model able to interpret the experimental dependence of the uniformity of films produced by the plasma-enhanced chemical vapour deposition technique on the deposition parameters (discharge pressure, gas flow temperature and rf power density). The model proposed is based on the Navier-Stokes equations applied to a gas flow considered to be quasi-incompressible and quasi-inviscous, whenever the Mach number is below 0·3. This condition leads to the establishment of the proper quasisteady-state gas flow equations, and the corresponding equations of energy and momentum balance ascribed to the mass profile of the species formed, under the presence of a low-rf-power plasma density, are able to predict the uniformity distribution of the film over the entire deposited substrate area.

The aim of this work is to provide the basis for the interpretation, under steady state, of the lateral photoeffect in p-i-n a-Si:H 1D Thin Film Position Sensitive Detectors (1D TFPSD) through an analytical model. The experimental data recorded in 1D TFPSD devices with different performances are compared with the predicted curves and the obtained correlation's discussed.

This paper deals with the interpretation of transport properties of amorphous silicon hydrogenated films (a-Si:H) through dark conductivity, photoconductivity and pulse controlled capacitance-voltage measurements. a-Si:H films were produced by rf glow discharge coupled either inductively or capacitively to a 3% SiH4/Ar mixture at different crossed electromagnetic static fields. The data concerned with the dark activation energy, photoactivation energy, variation of the density of localized states and photosensitivity, (σph/σd)25°C, of a-Si:H films can account for their optoelectronic properties which are strongly dependent on the deposition parameters. We also observed that crossed electromagnetic static fields applied during film formation influences hydrogen incorporation in a different manner than previously proposed. © 1983.

Today there is a strong interest in the scientific and industrial community concerning the use of biopolymers for electronic applications, mainly driven by low-cost and disposable applications. Adding to this interest, we must recognize the importance of the wireless auto sustained and low energy consumption electronics dream. This dream can be fulfilled by cellulose paper, the lightest and the cheapest known substrate material, as well as the Earth's major biopolymer and of tremendous global economic importance. The recent developments of oxide thin film transistors and in particular the production of paper transistors at room temperature had contributed, as a first step, for the development of disposable, low cost and flexible electronic devices. To fulfil the wireless demand, it is necessary to prove the concept of self powered devices. In the case of paper electronics, this implies demonstrating the idea of self regenerated thin film paper batteries and its integration with other electronic components. Here we demonstrate this possibility by actuating the gate of paper transistors by paper batteries. We found that when a sheet of cellulose paper is covered in both faces with thin layers of opposite electrochemical potential materials, a voltage appears between both electrodes - paper battery, which is also self-regenerated. The value of the potential depends upon the materials used for anode and cathode. An open circuit voltage of 0.5V and a short-circuit current density of 1μA/cm2 were obtained in the simplest structure produced (Cu/paper/Al). For actuating the gate of the paper transistor, seven paper batteries were integrated in the same substrate in series, supplying a voltage of 3.4V. This allows proper ON/OFF control of the paper transistor. Apart from that transparent conductive oxides can be also used as cathode/anode materials allowing so the production of thin film batteries with transparent electrodes compatible with flexible, invisible, self powered and wireless electronics. © 2011 SPIE.

This work aims to report results of the spatial and frequency optical detection limits of integrated arrays of 32 one-dimensional amorphous silicon thin film position sensitive detectors with nip or MIS structure, under continuous and pulsed laser operation conditions. The arrays occupy a total active area of 45 mm2 and have a plane image resolution better than 15 μm with a cut-off frequency of about 6.8 kHz. The non-linearity of the array components varies with the frequency, being about 1.6% for 200 Hz and about 4% for the cut-off frequency (6.8 kHz).

In this paper the influence of the DC grid bias on the plasma impedance and the I-V behaviour of silane plasmas used to grow undoped amorphous silicon films by plasma enhanced chemical vapour deposition technique using a triode configuration at or close to the powder regime is studied. The aim is to determine the correlation between the r.f. power and the DC grid voltage with the plasma parameters, under isothermal gas conditions. The results should lead to the production of nanostructured films, with the required optoelectronic characteristics for photovoltaic applications. The results achieved show the existence of a boundary region close to the γ-regime (powder formed) where nanoparticles can be formed by moderated ion bombardment of the growing surface. This is characterised by the plasma resistance of the same order of magnitude of the plasma reactance. Under this condition, it is possible to grow amorphous silicon films that can incorporate nanoparticles, exhibiting photosensitivities of about 107 (two orders of magnitude larger than the one exhibited by films grown under conventional conditions) with densities of states determined by the constant photocurrent method below 3 × 1015 cm3. Apart from that, the growth of the films is less affected by light soaking than the conventional films grown by standard techniques. © 2001 Elsevier Science Ltd. All rights reserved.

Amorphous Silicon solar cells have been produced by a two consecutive decomposition and deposition chamber system, using polished S. S. substrates. Through a systematic investigation of the electrical and optical properties of doped and undoped amorphous silicon layers (1) we observe that the deposition conditions (gas partial pressure, density of r. f. power, substrate temperature, electromagnetic static fields applied to the substrate, and gas flow rate) influence films properties. In the course of this investigation we have been studying the role of the sheet resistance, R//s, of the I. T. O. layer on the short circuit current, I//s//c, and on the open circuit voltage, V//o//c, of p. i. n. structures of 16cm**2 in area. The obtained results indicate that V//o//c is almost independent on R//s, while I//s//c variation approaches a square root dependence on R//s.

P-type thin-film transistors (TFTs) using room temperature sputtered tin and copper oxide as a transparent oxide semiconductor have been produced on rigid and paper substrates. The SnO x films shows p-type conduction presenting a polycrystalline structure composed with a mixture of tetragonal β-Sn and α-SnO x phases, after annealing at 200°C. These films exhibit a hole carrier concentration in the range of ≈ 10 16-10 18 cm -3, electrical resistivity between 101-102 Ωcm, Hall mobility of 4.8 cm 2/Vs, optical band gap of 2.8 eV and average transmittance ≈ 85 % (400 to 2000 nm). Concerning copper oxide Cu xO thin films they exhibit a polycrystalline structure with a strongest orientation along (111) plane. The Cu xO films produced between an oxygen partial pressure of 9 to 75% showed p-type behavior, as it was measured by Hall effect and Seebeck measurements. The bottom gate p-type SnO x TFTs present field-effect mobility above 1.24 cm 2/Vs (including the paper p-type oxide TFT) and an on/off modulation ratio of 10 3 while the Cu xO TFTs exhibit a field-effect mobility of 1.3×10 -3 cm 2/Vs and an on/off ratio of 2×10 2. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

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 deals with a new design method for the interfaces of a-Si:H pin solar cells that improves the stability and performances of devices deposited in a single batch chamber process. The method consists in removing a deposited sacrificial layer placed between the p/i and/or i/n interfaces by etching. This layer is an absorber of defects and impurities that are introduced in the interfaces, mainly from the chamber walls cross-contamination and the substrate surface. The results achieved increase the device fill factor and short circuit current density, respectively towards 75% and 16.3 mA cm-2, with a final efficiency of about 10%, before light soaking experiments. © 2000 Elsevier Science B.V. All rights reserved.

The aim of this work is to provide the basis for the interpretation, under steady state and in the low-voltage regime of the dark current-density-voltage (J-V) characteristics of transverse asymmetric amorphous silicon (a-Si:H) p-i-n and n-i-p diodes. The transverse asymmetric a-Si:H diodes present ratios between the metal contact and the underneath doped layer areas larger than five, leading to the inclusion, in the diode equation, of a lateral leakage current, responsible for the high saturation current density and the forward shape of the J-V curves recorded. The leakage current depends on the lateral spatial potential developed with which varies following a power-law dependence. The experimental J-V curves in diodes with the doped layer around the metal contact unetched and etched prove the role and origin of this lateral leakage current and, thus, the proposed model. © 1995 American Institute of Physics.

One of the main problems in producing large area amorphous silicon devices concerns films uniformity. In this paper we present data concerning the role of reactor geometry and design and on the film performances as well as the problems related to mechanical mismatches in scaling up the reactor size. © 1991 Elsevier Science Publishers B.V. All rights reserved.

The ability to process and dimensionally scale field-effect transistors with and on paper and to integrate them as a core component for low-power-consumption analog and digital circuits is demonstrated. Low-temperature-processed p- and n-channel integrated oxide thin-film transistors in the complementary metal oxide semiconductor (CMOS) inverter architecture are seamlessly layered on mechanically flexible, low-cost, recyclable paper substrates. The possibility of building these circuits using low-temperature processes opens the door to new applications ranging from smart labels and sensors on clothing and packaging to electronic displays printed on paper pages for use in newspapers, magazines, books, signs, and advertising billboards. Because the CMOS circuits reported constitute fundamental building blocks for analog and digital electronics, this development creates the potential to have flexible form factor computers seamlessly layered onto paper. The holistic approach of merging low-power circuitry with a recyclable substrate is an important step towards greener electronics. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

This paper aims to discuss the effect of order and disorder on the electrical performances of covalent silicon semiconductors and ZnO based ionic oxide semiconductors used as active channel layers in thin film transistors. The effect of disorder on covalent semiconductors directly affects their electrical transport properties due to the asymmetric behaviour of sp states, while in ionic oxide semiconductors it is found that this effect is small due to the fact that angular disorder has no effect on the spherical symmetry of s states. To this we must add that the mobility of carriers in both systems is quite different, being also affected by electron-phonon interactions (weak in silicon and strong in ionic oxides leading to formation of polarons). Besides, the impurity doping effect and the presence of vacancies in disordered silicon and in ionic oxides behave differently, which will influence the thin film properties and so, the performances of the devices produced. © 2007 Springer-Verlag.

This paper presents data concerning the composition structure and optical characteristics of polymorphous silicon films produced by plasma enhanced chemical vapour deposition at 27.12 MHz and determined respectively by infrared spectrometry, micro Raman, exodiffusion and spectroscopic ellipsometry measurements. When compared to the pm-Si:H films produced at 13.56 MHz, the films produced at 27.12 MHz present hydrogen contents in the range of 21 at%, the sharp peak ascribed to the exodifusion measurements is shifted towards high temperatures and the imaginary part of the dielectric function 〈ε2〉 is larger and shifted to high energies. Apart from that the peaks of the infrared spectra ascribed to the stretching modes shift towards high wave numbers and the half width of the micro Raman peaks shrinks, meaning that the films produced at 27.12 MHz are more compact and dense.

The aim of this paper was to present results of the role of the d.c. grid bias on the silane plasma impedance and its I-V characteristics to grow undoped amorphous silicon (a-Si:H) films by plasma enhanced chemical vapour deposition (PECVD) in conditions where some nanoparticles can be formed in the growth region of the deposition process, under proper ion bombardment. The results achieved show that the performances of the films produced are dependent on the self bias voltage that can present photosensitivities of approximately 107 (two orders of magnitude larger than the one exhibited by films grown under conventional conditions) with density of states determined by the constant photocurrent method below 4×1015 cm-3. Apart from that, the films grown are less affected by light soaking than the conventional films.

The aim of this work is to present a model to interpret the static and the dynamic detection and resolution limits of 1D thin film position sensitive detectors based on p-i-n a-Si:H devices. The model can determine the device characteristics that influence the spatial limits and the response time of the device.

This paper presents results of the role of the oxygen partial pressure (pO2) used on the properties exhibited by doped μc silicon oxycarbide films produced by a Two Consecutive Decomposition and Deposition Chamber (TCDDC) system [1], where a spatial separation between the plasma and the growth regions is achieved. The films produced are highly conductive and transparent with suitable properties for optoelectronic applications.