Rodrigo Martins

In today's main crystalline silicon (c-Si) applications in MOS (metal-oxide-silicon), MIS (metal-insulator-semiconductor) or SIS (Semiconductor-Insulator-Semiconductor), the growing of the oxide layer plays the main role, dictating the device performances, in particular if it has to be grown by a low temperature process. Of fundamental importance is the SiO2 interface with the c-Si. A very low defect density interface is desirable so that the number of trapping states can be reduced and the devices performance optimised. A two step low temperature oxidation process is proposed. The process consists of growing first a layer of oxide by a wet process and then treating the grown oxide with an oxygen plasma. The oxygen ions from the plasma bombard the oxide causing compaction of the oxide and a decrease in the interface roughness and defect density. Infrared spectroscopy and spectroscopic ellipsometry measurements were performed on the samples to determine the oxide thickness, optical and structural properties. SIS structures were built and capacitance measurements were performed under dark and illuminated conditions from which were inferred the interface defect density and correlated with the oxide growth process.

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.

This work presents a study performed on several Au contacts deposited by evaporation on oxide free and oxidised (5-20Å of oxide) a-Si:H surfaces. The characterisation of the films was performed on as deposited, aged and annealed at 150°C structures. SIMS and RBS measurements show that the Au diffuses very easily on oxide free a-Si:H surfaces, even at room temperature, resulting in the formation of an oxide at the device surface that acquires a blue colour instead of the gold colour of the contacts. This was also visible in the SEM pictures of the cross section of the structures produced and on the changes of the surface morphology observed by AFM measurements. On the other hand, when the Au is deposited on oxidised a-Si:H surfaces, the results show that the oxide prevents the Au from diffusing and the nature of the contact is preserved. That is, better rectifying and stability performances are obtained in MIS like structures than in Schottky structures.

This work presents a study performed on the deposition of pm-Si:H by plasma enhanced chemical vapor deposition using excitation frequencies of 13.56 and 27.12 MHz, where the interest of increasing the excitation frequency relies on higher plasma dissociation and reduced energy of ion bombardment, thus allowing the deposition of superior grade material at higher growth rates. The plasma impedance, which allows the monitoring of particle formation in the plasma, was correlated to the film properties, characterized by spectroscopic ellipsometry and hydrogen exodiffusion experiments. The set of data obtained show that by using the 27.12-MHz excitation frequency the hydrogen dilution and the r.f. power density needed to produce pm-Si:H can be reduced. Growth rates above 3.1 Å/s were obtained, the films being more dense and chemically more stable than those obtained with the standard 13.56 MHz. © 2003 Elsevier B.V. All rights reserved.

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.

The present development of non-wafer-based photovoltaics (PV) allows supporting thin film solar cells on a wide variety of low-cost recyclable and flexible substrates such as paper, thereby extending PV to a broad range of consumer-oriented disposable applications where autonomous energy harvesting is a bottleneck issue. However, their fibrous structure makes it challenging to fabricate good-performing inorganic PV devices on such substrates. The advances presented here demonstrate the viability of fabricating thin film silicon PV cells on paper coated with a hydrophilic mesoporous layer. Such layer can not only withstand the cells production temperature (150 C), but also provide adequate paper sealing and surface finishing for the cell's layers deposition. The substances released from the paper substrate are continuously monitored during the cell deposition by mass spectrometry, which allows adapting the procedures to mitigate any contamination from the substrate. In this way, a proof-of-concept solar cell with 3.4% cell efficiency (41% fill factor, 0.82 V open-circuit voltage and 10.2 mA cm-2 short-circuit current density) is attained, opening the door to the use of paper as a reliable substrate to fabricate inorganic PV cells for a plethora of indoor applications with tremendous impact in multi-sectorial fields such as food, pharmacy and security. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this work we present results of studies performed on Schottky and metal-insulator-semiconductor (MIS) position sensitive detectors (PSD) structures: substrate (glass)/ Cr (300 nm) / a-Si:H [n] (37 nm) / a-Si:H [i] (600 nm) / SiO2 (1.5 nm - for the MIS) / Au (7 nm). The effect of the interfacial oxide layer between Au and a-Si:H, for the MIS structures, was studied and compared with the Schottky, in order to determine how beneficial it could be for device performances and time degradation. For doing so, the Au thickness of 70Å was deposited by thermal evaporation on an oxide free (Schottky) and oxidized (≈20Å) (MIS) a-Si:H surfaces. These structures were characterized by SIMS, RBS, SEM and AFM in order to correlate the obtained diffusion profile of Au at the interface and the topography with the presence of the oxide at the interface. The results show that the Au inter-diffuses very easily in the oxide free a-Si:H surface, even at room temperature, degrading the devices performance. On the other hand, the MIS structures, with their interfacial oxide present no structural changes after annealing and the PSD produced are stable. We believe that this effect is associated with the barrier effect of the interfacial oxide that prevents the Au diffusion. The optimized 1D MIS sensors are stable and exhibit a linearity error as low as 0.8 % and sensitivities of 33 mV/cm for a 5 mW spot beam intensity at a wavelength of 532 nm, while the Schottky sensors showed a time degradation of their characteristics. © 2006 Materials Research Society.

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.

This work aims to study the role of the r.f. electrode configuration on the plasma characteristics of a PECVD asymmetric reactor. The configurations used are the usual diode configuration, the triode configuration and a new configuration that we named short-circuited grid electrode (SGE). The plasma generated was characterized with the use of a Langmuir probe and an impedance probe. We demonstrate that the plasma parameters are highly dependent on the reactor geometry. The results achieved show that by changing the r.f. electrode configuration the DC self-bias varies from about 100 to close to 0 V. This variation causes changes in the ion bombardment of the reactor surfaces, which can affect the growing of the films deposited. We also demonstrate that for the SGE configuration the area seen by the plasma does not correspond to the exposed physical area of the electrode, and we suggest a model to explain this phenomenon.

The present development of non-wafer-based photovoltaics (PV) allows supporting thin film solar cells on a wide variety of low-cost recyclable and flexible substrates such as paper, thereby extending PV to a broad range of consumer-oriented disposable applications where autonomous energy harvesting is a bottleneck issue. However, their fibrous structure makes it challenging to fabricate good-performing inorganic PV devices on such substrates. The advances presented here demonstrate the viability of fabricating thin film silicon PV cells on paper coated with a hydrophilic mesoporous layer. Such layer can not only withstand the cells production temperature (150 °C), but also provide adequate paper sealing and surface finishing for the cell's layers deposition. The substances released from the paper substrate are continuously monitored during the cell deposition by mass spectrometry, which allows adapting the procedures to mitigate any contamination from the substrate. In this way, a proof-of-concept solar cell with 3.4% cell efficiency (41% fill factor, 0.82 V open-circuit voltage and 10.2 mA cm-2 short-circuit current density) is attained, opening the door to the use of paper as a reliable substrate to fabricate inorganic PV cells for a plethora of indoor applications with tremendous impact in multi-sectorial fields such as food, pharmacy and security. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

This work presents a study on the effect of an interfacial silicon oxide layer placed between Au and a-Si:H MIS (metal-insulator-semiconductor) photodiodes in their performances, by stopping the Au diffusion towards the a-Si:H. The results show that the Au diffuses very easily to the oxide free a-Si:H surface, even at room temperature, degrading the photodiode performance. On the other hand, the MIS photodiodes with the interfacial oxide show an improvement of their characteristics after annealing, function of its thickness, and degree of film's compactness. This effect is associated with the presence of oxide of thicknesses ≥5 Å at the Au/a-Si:H interface that prevents the Au diffusion and improves the photodiode characteristics, which does not happen when the interfacial oxide is absent. © 2004 Elsevier B.V. All rights reserved.

In this work we present a study performed in a plasma-enhanced chemical vapour deposition reactor, where different rf electrode configurations were used with the aim of achieving conditions that lead to growth of highly uniform amorphous silicon films, with the required electronic quality, at high growth rates. This study consists in determining the plasma characteristics under different electrode configurations, in an argon plasma, using as diagnostic tools a Langmuir probe and impedance probe. These results were correlated with the hydrogenated amorphous silicon films produced, thereby allowing us to establish the best electrode configuration to grow electronic-grade-quality amorphous silicon films.

Trimethylboron (TMB) has been receiving attention as a valid alternative to diborane and methane mixtures for the deposition of p-type silicon films for applications in optoelectronic devices such as solar cells. In this paper we report on p-type hydrogenated nanocrystalline silicon carbide (nc-Si:C:H) films produced by standard 13.56 MHz plasma enhanced chemical vapour deposition technique, using TMB as gas source, under high hydrogen dilution (98%) and using high deposition pressures (3 Torr). The films obtained were characterized by spectroscopic ellipsometry (SE), Raman spectroscopy (RS), and electrical measurements to determine their optical, structural and electrical properties. We achieved conductivities as high as 8.3 (Ω cm) -1, one of the highest values of conductivity published to date using TMB with standard rf-PECVD. Spectroscopic ellipsometry modeling revealed that the films growth mechanism proceeds through a sub-surface layer mechanism that leads to the formation of nanocrystalline silicon. Copyright © 2010 American Scientific Publishers All rights reserved.

In this work metal-insulator-semiconductor (MIS) photodiodes with a structure: Cr/a-Si:H(n+)/a-Si:H(i)/oxide/Au were studied, where the main objective was to determine the influence of the oxide layer on the performance of the devices. The results achieved show that their performance is a function of both oxide thickness and oxide density. The a-Si:H oxidation method used was the immersion in H2O2 solution. By knowledge of the oxide growth process it was possible to fabricate photodiodes exhibiting an open circuit voltage of 0.65V and short circuit current density under AM1.5 illumination of 11mA/cm2, with a response times less than 1μs for load resistance <400Ω, and a signal to noise ratio of 1×107. © 2003 Elsevier B.V. All rights reserved.

In this work we show that it is possible to control the plasma species present near the substrate surface, from what is usually associated with an α regime (a plasma free of particles) to a γ' regime (a plasma where particles are present) and simultaneously control the energy of the ions striking the substrate during a-Si:H deposition from a silane glow discharge in a modified triode (MT) type PECVD reactor, where a DC mesh electrode biased with Vpol is located in front of the r.f electrode. The presence of large particles in the plasma leads to the deposition of the films with the poorest optoelectronic properties. When the particle size in the plasma decrease the film properties improve, but, when particles are no longer present in the plasma region close to the substrate, like in a α like regime, the properties of the films deteriorate again. The results show that the best transport properties are achieved for the films deposited in the α-γ' transition regime corresponding to 0V<Vpol<51V. Under this condition the films present a dark conductivity, σ d ≈ 10-11 (Ωcm)-1, photosensitivity, S ≈ 107, activation energy, ΔE ≈ 0.9 eV, hydrogen content, CH ≈ 10%, factor of microstructure, R ≈ 0.085 and an optical gap, Eop ≈ 1.77 eV.

This paper presents the results achieved in optimized 1D position sensitive detectors (PSD) using a metal-insulator-semiconductor (MIS) structure and different length to width ratios, in order to determine the optimal geometrical factor for the desired 3D integration. The results show that the optimized MIS PSD produced, exhibited linearity errors as low as 0.8% and sensitivities of 32 mV/cm, for a 5 mW spot beam intensity at a wavelength of 532 nm. The sensors can achieve a longitudinal spatial resolution of 1.25 μm (estimated by modulation transfer function calculation), while the transverse resolution depends on the minimum width used for each sensor. The calculated Jones parameter of the sensors is higher than 1011 J, with a fall-off parameter of 0.012 cm-1, indicating a high signal to noise detection ratio. © 2006 Elsevier B.V. All rights reserved.

Co-doped TiO 2 films were characterized by spectroscopic ellipsometry to determine their thickness, deposition rate and optical properties as function of substance temperature and background gas composition. To fit the data we used a combination of a single Tauc-Lorentz oscillator with the Drude free electron model to take in account the free electrons present in the film. The Co doping and addition of H 2 to the gas phase during film growth cause the formation of a titanium oxide which containsfree electrons that absorb the energy of the red part of the spectrum, causing k to increase. The n of the film at 1.5 eV is about 2.3 eV. The fittings also show that the n of films decreases and k increase at the surface. This can be related to a segregation of Co to the surface, which in some cases, of high substrate temperature and high H 2 flow during deposition, can lead to and even higher concentration of free electrons at the surface. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.

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.

It is experimentally known that the linearity and sensitivity of the position sensitive detectors (PSD) are dependent on the resistance of the collecting layer and of the load resistance, mainly if the detection is based on the measurement of the photo-lateral voltage. To determine the value of the load resistance to be used in metal - insulator - semiconductor (MIS) PSDs structures that lead to the maximum value of sensitivity and linearity, we propose an electrical model through which it is able to simulate the proper sensor response and how the load resistance influence the results obtained. This model is valid for PSDs where the resistance of the collecting resistive layer is quite low (≤ 500 Ω), leading to a low output impedance. Under these conditions we conclude that the value of the load resistance should be of about 1 kΩ in order to achieve a good compromise between the linearity and the sensitivity of the PSD. This result is in agreement with the set of experiments performed. © 2005 Materials Research Society.

In this work we present a study of influence of ion bombardment on the optical, electrical and compositional properties of the intrinsic amorphous silicon films deposited in a modified triode plasma-enhanced chemical vapour deposition (PECVD) reactor. The application of a DC voltage to a grid placed in front of the r.f. electrode allowed us to control the energy and polarity of the ions striking the substrate during film growth. The results show a variation by two orders of magnitude in the dark conductivity from 10-10 to 6.2 × 10-12 (Ω cm)-1, while the photosensitivity varied from 2 × 105 to 2 × 107. A process plasma that takes place in the γ-type regime was associated with the use of a negative bias, while a process plasma like the one of the α-type regime was associated with the use of a positive bias. The films deposited with a bias ≈ 38 V are highly intrinsic and the abrupt change in the conductivity properties observed at this bias is attributed to a change in the density of the states ascribed to the position of the Fermi level. That is, a precise control of the energy of the ions striking the substrate during the film growth leads to improved film optoelectronic properties, very important for device applications. © 2001 Elsevier Science Ltd. All rights reserved.

In this work we investigate the properties of a polymorphous silicon (pm-Si:H) metal-insulator-semiconductor (MIS) structure used in 3D position sensitive detectors (PSD). For the first time a 3D sensor made-up by pm-Si:H/SiO2/Au layers is presented. MIS structures present several advantages over p-i-n structures, such as easier fabrication, fast response time and higher resolution. The 1D MIS PSD that constitute the array were extensively studied aiming its application in 3D pattern recognition. The results obtained show that MIS PSD can achieve non-linearities below 2% and sensitivities of 3.2 μA/cm over 6 mm length sensors. The miniaturization of the sensors length to arrays of 6 and 16 mm, respectively showed average non-linearities of about 1.9% for the 16 mm sensor which proved to be the best solution for this MIS structure. © 2006 Elsevier B.V. All rights reserved.

{In this work we report how it is possible to control the plasma regime near the substrate surface, from predominantly α to predominantly γ', passing trough and intermediate α-γ' regime, and simultaneously control the energy of the ions striking the substrate during a-Si:H deposition from a silane glow discharge in a modified triode type PECVD reactor. To do so, we apply a DC voltage (Vpol to a set of grids placed in front of the r.f. electrode and by doing this, we control the energy of the ions striking the substrate during the film's growth and the plasma regime near the substrate. Under a plasma of the γ' regime, the surface roughness is high and the films are poorly compact. In the α-γ' regime, the ion bombardment is moderate and the films are highly smooth and compact. In the α regime the ion bombardment is higher and so the films can become more compact but the surface roughness increases and the electrical properties deteriorate. The results achieved show that the best transport properties are achieved for the films deposited in the α-γ' regime corresponding to a Vpol of 38 V. Under this condition the films presented a dark conductivity, σd = 6.2×10-12 (Ωcm)-1, activation energy, ΔE ≈ 0.9 eV, hydrogen content

In this work we present results of a study performed on MIS diodes with the following structure: substrate (glass) / Cr (2000Å) / a-Si:H n + (400Å) / a-Si:H i (5500Å) / oxide (0-40Å) / Au (100Å) to determine the influence of the oxide passivation layer grown by different techniques on the electrical performance of MIS devices. The results achieved show that the diodes with oxides grown using hydrogen peroxide present higher rectification factor (2×106) and signal to noise (S/N) ratio (1×107 at -1V) than the diodes with oxides obtained by the evaporation of SiO2, or by the chemical deposition of SiO 2 by plasma of HMDSO (hexamethyldisiloxane), but in the case of deposited oxides, the breakdown voltage is higher, 30V instead of 3-10 V for grown oxides. The ideal oxide thickness, determined by spectroscopic ellipsometry, is dependent on the method used to grow the oxide layer and is in the range between 6 and 20 Å. The reason for this variation is related to the degree of compactation of the oxide produced, which is not relevant for applications of the diodes in the range of ± 1V, but is relevant when high breakdown voltages are required.

This work reports on the fabrication process and performances presented by metal insulator semiconductor (MIS) linear position sensitive detectors (PSD) with an active length of 6 cm. The use of long sensitive areas allows the PSD to achieve higher resolution without the need of a highly accurate light spot integration mechanism. The PSD is built in a multilayered structure consisting of Cr/a-Si:H (n+ doped)/a-Si:H (intrinsic)/SiOx (passivation layer)/Au, where the active a-Si:H layers were deposited by a modified triode plasma enhanced chemical vapour deposition (MTPECVD), which allows the deposition of highly electronic grade material with a low (≈ × 1015 cm-3) defect density inferred by CPM. The sensor linearity and sensitivity shows dependence on the sensor width to length ratio and on the value of load resistance. Sensitivities of more than 30 mV/cm were achieved with linearity near 99%. Besides that, this type of MIS structure allows an improved spectral response near the UV region and has the maximum response at 540 nm. © 2004 Published by Elsevier B.V.

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.