Publications: List of publications

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Charge fluctuations at the Si–SiO2 interface and its effect on surface recombination in solar cells

Bonilla, RS, Al-Dhahir, I, Yu, M, Hamer, P, Altermatt, PP
2020
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Journal article
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Solar Energy Materials and Solar Cells
© 2020 Elsevier B.V. The Si–SiO2 interface has and will continue to play a major role in the development of silicon photovoltaic devices. This work presents a detailed examination of how charge at or near this interface influences device performance. New understanding is identified on the effect of charge-induced potential fluctuations at the silicon surface. Such fluctuations have been considered in Si–SiO2 recombination models previously, where a universal value of electrical potential deviation was used to represent the effect. However, the approach disregards that the variation occurs in the charge concentration rather than the potential. We modify the models to accurately reflect fluctuations in external charge, allowing a precise representation of surface recombination velocity, with self-consistent Dit, σp, and σn parameters. Correctly accounting for these parameters can provide insights into the passivation mechanisms which can aid the development of future devices. Using the corrected model, we find that the effect of charge fluctuation at the Si–SiO2 interface is significant for the depletion regime to the weak inversion regime. This indicates that surface passivation dielectrics must operate with charge concentrations in excess of 2x1012 q/cm2 to avoid these effects. TCAD device simulations show that the efficiency of future PERC cells can improve up to 1% absolute when optimally charged dielectric coatings are applied both at the front and rear surfaces.

Special issue: Surface and interface passivation in crystalline silicon solar cells

Bonilla, RS, Hoex, B, Wilshaw, PR
2020
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Journal article
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SOLAR ENERGY MATERIALS AND SOLAR CELLS

Origin of the tunable carrier selectivity of atomic-layer-deposited TiOx nanolayers in crystalline silicon solar cells

Matsui, T, Bivour, M, Ndione, PF, Bonilla, RS, Hermle, M
2020
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Journal article
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SOLAR ENERGY MATERIALS AND SOLAR CELLS
Solar cells, Carrier selective contacts, Titanium oxide, Atomic layer deposition

Understanding and optimizing EBIC pn-junction characterization from modeling insights

Zhou, R, Yu, M, Tweddle, D, Hamer, P, Chen, D, Hallam, B, Ciesla, A, Altermatt, PP, Wilshaw, PR, Bonilla, RS
2020
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Journal article
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JOURNAL OF APPLIED PHYSICS

Exceptional Surface Passivation Arising from Bis(trifluoromethanesulfonyl)-Based Solutions

Pointon, AI, Grant, NE, Bonilla, RS, Wheeler-Jones, EC, Walker, M, Wilshaw, PR, Dancer, CEJ, Murphy, JD
2019
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Journal article
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ACS Applied Electronic Materials

Scalable Techniques for Producing Field-Effect Passivation in High-Efficiency Silicon Solar Cells

Collett, KA, Du, S, Bourret-Sicotte, G, Luo, Z, Hamer, P, Hallam, B, Bonilla, RS, Wilshaw, PR
2019
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Journal article
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IEEE Journal of Photovoltaics
© 2011-2012 IEEE. This paper presents techniques to tailor and optimize the field-effect passivation of silicon surfaces using the deposition and field-Assisted migration of potassium ions. While field-effect passivation of this nature has previously been demonstrated using laboratory scale techniques, in this paper, it is shown how it can be realized using fast and scalable ion deposition and migration methods. Together, the deposition and migration processes are seen to produce excellent improvements to the surface passivation of oxidized 1-Ω·cm n-Type float-zone silicon. Effective lifetimes as high as 2.1 ms are observed, in the best case yielding a surface recombination velocity as low as 3.3 cm/s with a corresponding surface dark saturation current density of 8.4 fA/cm 2

Graphene oxide films for field effect surface passivation of silicon for solar cells

Vaqueiro-Contreras, M, Bartlam, C, Bonilla, RS, Markevich, VP, Halsall, MP, Vijayaraghavan, A, Peaker, AR
2018
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Journal article
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Solar Energy Materials and Solar Cells
© 2018 Elsevier B.V. In recent years it has been shown that graphene oxide (GO) can be used to passivate silicon surfaces resulting in increased photocurrents in metal-insulator-semiconductor (MIS) tunneling diodes, and in improved efficiencies in Schottky-barrier solar cells with either metal or graphene barriers, however, the source of this passivation is still unclear. The suggested mechanisms responsible for the enhanced device performance include the dangling bond saturation at the surface by the diverse functional groups decorating the GO sheets which reduce the recombination sites, or field effect passivation produced by intrinsic negative surface charge of GO. In this work through a series of measurements of minority carrier lifetime with the microwave photo-conductance decay (µPCD) technique, infrared absorption spectra, and surface potential with Kelvin probe force microscopy (KPFM) we show that there is no evidence of significant chemical passivation coming from the GO films but rather negative field effect passivation. We also discuss the stability of GO's passivation and the flexibility of this material for its application as temporary passivation layer for bulk lifetime measurements, or as a potential cheap alternative to current passivation materials used in solar cell fabrication.

The Behavior and Transport of Hydrogen in Silicon Solar Cells Observed through Changes in Contact Resistance

Hamer, P, Li, H, Chan, C, Sen, C, Bonilla, RS, Wilshaw, P, IEEE
2018
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Conference paper
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2018 IEEE 7TH WORLD CONFERENCE ON PHOTOVOLTAIC ENERGY CONVERSION (WCPEC) (A JOINT CONFERENCE OF 45TH IEEE PVSC, 28TH PVSEC & 34TH EU PVSEC)
hydrogen, crystalline silicon, screen-printed contacts, photovoltaics

Hydrogen Related Defects in Float Zone Silicon Investigated Using a Shielded Hydrogen Plasma

Bourret-Sicotte, G, Hamer, PG, Tweddle, D, Bonilla, RS, Wilshaw, PR
2018
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Conference paper
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2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC
© 2018 IEEE. Shielded Hydrogen Passivation (SHP) has been shown to be an effective technique to introduce atomic hydrogen to silicon samples. This allows the study of hydrogen related defects at a range of temperatures without any UV or other process damage. It is found that at 350°C, introduction of atomic hydrogen can cause near surface damage into n-type material that greatly reduces carrier lifetime. In contrast, p-type samples showed no initial degradation, followed by signs of degradation under hot light soaking.

Hydrogen Related Defects in Float Zone Silicon Investigated Using a Shielded Hydrogen Plasma

Bourret-Sicotte, G, Hamer, PG, Tweddle, D, Bonilla, RS, Wilshaw, PR, IEEE
2018
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Conference paper
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2018 IEEE 7TH WORLD CONFERENCE ON PHOTOVOLTAIC ENERGY CONVERSION (WCPEC) (A JOINT CONFERENCE OF 45TH IEEE PVSC, 28TH PVSEC & 34TH EU PVSEC)
hydrogen, bulk defects, n-type float zone, silicon

Taking monocrystalline silicon to the ultimate lifetime limit

Niewelt, T, Richter, A, Kho, TC, Grant, NE, Bonilla, RS, Steinhauser, B, Polzin, J-I, Feldmann, F, Hermle, M, Murphy, JD, Phang, SP, Kwapil, W
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et al
2018
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Journal article
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SOLAR ENERGY MATERIALS AND SOLAR CELLS
Silicon, Float zone silicon, Charge carrier lifetime, Passivation, Al2O3, ONO, TOPCon, Superacid, Record

Hydrogen induced contact resistance in PERC solar cells

Hamer, P, Chan, C, Bonilla, RS, Hallam, B, Bourret-Sicotte, G, Collett, KA, Wenham, S, Wilshaw, PR
2018
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Journal article
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Solar Energy Materials and Solar Cells
© 2018 Elsevier B.V. The origins of an increase in the series resistance of PERC multicrystalline silicon solar cells due to post-firing thermal processes are investigated. This effect has been shown to be capable of reducing the fill factor of finished cells by up to 20%ABS, severely degrading their performance. It is observed that electric currents applied either during or after these thermal processes can greatly alter the series resistance, either causing it to increase by more than an order of magnitude or suppressing the effect entirely. It is demonstrated that this behavior is in good agreement with the expected interactions of hydrogen with dopants and electric fields within silicon wafers. It is therefore speculated that at least part of the observed increase in resistance is due to the motion of hydrogen within the cell itself.

Controlling surface carrier density via a PEDOT:PSS gate: An application to the study of silicon-dielectric interface recombination

Bonilla Osorio, RS
2018
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Journal article
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Solar RRL

Effect of Carrier-Induced Hydrogenation on the Passivation of the poly-Si/SiOx/c-Si Interface

Yang, Y, Altermatt, PP, Cui, Y, Hu, Y, Chen, D, Chen, L, Xu, G, Zhang, X, Chen, Y, Hamer, P, Bonilla, RS, Feng, Z
,
et al
2018
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Conference paper
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SILICONPV 2018: THE 8TH INTERNATIONAL CONFERENCE ON CRYSTALLINE SILICON PHOTOVOLTAICS

Modelling of hydrogen transport in silicon solar cell structures under equilibrium conditions

Hamer, P, Hallam, B, Bonilla, RS, Altermatt, PP, Wilshaw, P, Wenham, S
2018
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Journal article
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Journal of Applied Physics
© 2018 Author(s). This paper presents a model for the introduction and redistribution of hydrogen in silicon solar cells at temperatures between 300 and 700 °C based on a second order backwards difference formula evaluated using a single Newton-Raphson iteration. It includes the transport of hydrogen and interactions with impurities such as ionised dopants. The simulations lead to three primary conclusions: (1) hydrogen transport across an n-type emitter is heavily temperature dependent; (2) under equilibrium conditions, hydrogen is largely driven by its charged species, with the switch from a dominance of negatively charged hydrogen (H-) to positively charged hydrogen (H+) within the emitter region critical to significant transport across the junction; and (3) hydrogen transport across n-type emitters is critically dependent upon the doping profile within the emitter, and, in particular, the peak doping concentration. It is also observed that during thermal processes after an initial high temperature step, hydrogen preferentially migrates to the surface of a phosphorous doped emitter, drawing hydrogen out of the p-type bulk. This may play a role in several effects observed during post-firing anneals in relation to the passivation of recombination active defects and even the elimination of hydrogen-related defects in the bulk of silicon solar cells.

Potassium ions in SiO2: Electrets for silicon surface passivation

Bonilla, RS, Wilshaw, PR
2018
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Journal article
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Journal of Physics D: Applied Physics
© 2017 IOP Publishing Ltd. This manuscript reports an experimental and theoretical study of the transport of potassium ions in thin silicon dioxide films. While alkali contamination was largely researched in the context of MOSFET instability, recent reports indicate that potassium ions can be embedded into oxide films to produce dielectric materials with permanent electric charge, also known as electrets. These electrets are integral to a number of applications, including the passivation of silicon surfaces for optoelectronic devices. In this work, electric field assisted migration of ions is used to rapidly drive K+ into SiO2 and produce effective passivation of silicon surfaces. Charge concentrations of up to ∼5 × 1012 e cm-2 have been achieved. This charge was seen to be stable for over 1500 d, with decay time constants as high as 17 000 d, producing an effectively passivated oxide-silicon interface with SRV < 7 cm s-1, in 1 cm n-type material. This level of charge stability and passivation effectiveness has not been previously reported. Overall, this is a new and promising methodology to enhance surface passivation for the industrial manufacture of silicon optoelectronic devices.

Atom Probe Tomography of Fast-Diffusing Impurities and the Effect of Gettering in Multicrystalline Silicon

Tweddle, D, Shaw, EC, Douglas, JO, Bonilla, RS, Moody, MP, Hamer, P, Wilshaw, PR
2018
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Journal article
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SILICONPV 2018: THE 8TH INTERNATIONAL CONFERENCE ON CRYSTALLINE SILICON PHOTOVOLTAICS

An enhanced alneal process to produce SRV < 1 cm/s in 1 Ω cm n-type Si

Collett, KA, Bonilla, RS, Hamer, P, Bourret-Sicotte, G, Lobo, R, Kho, T, Wilshaw, PR
2017
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Journal article
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Solar Energy Materials and Solar Cells
© 2017 The Authors The alneal is one of the most effective methods of electrically passivating a silicon surface, and has been used by numerous research groups since the 1980s. In this work, we present an enhanced alneal process that substantially improves its effectiveness. Previously, the success afforded by the standard alneal has been attributed to the chemical passivation provided by hydrogenation of the Si-SiO2 interface. However, the work presented here shows that it is possible to enhance the surface passivation by simultaneously introducing a component of Field Effect Passivation (FEP). Where the standard alneal is seen to provide lifetimes of ~2.1 ms, equivalent to a surface recombination velocity (SRV) of 3.3 cm/s, the enhanced alneal can provide a lifetime of 5.6 ms on 1 Ω cm, n-type Si, equivalent to a SRV 0.4 cm/s. The charge required for this enhanced passivation can be introduced in the order of minutes and has the potential to be introduced at the same time as the aluminium is deposited, thus, resulting in no extra processing time. Secondary ion mass spectroscopy showed that the nature of the charge is likely to be K and Na cations residing at the Si-SiO2 interface. The possibility of increasing the surface passivation beyond that of the standard alneal points to the importance of both chemical and field effect components of passivation, and is therefore of significant interest to high efficiency silicon solar cell research.

Effective Antireflection and Surface Passivation of Silicon Using a SiO2/a-T iOx Film Stack

Bonilla, RS, Davis, KO, Schneller, EJ, Schoenfeld, WV, Wilshaw, PR
2017
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Journal article
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IEEE Journal of Photovoltaics
© 2011-2012 IEEE. This paper reports an effective and industrially relevant passivation and antireflection film stack featuring a 10 nm silicon dioxide (SiO2) film followed by a 65 nm amorphous titanium oxide (a-TiOx) film. This film stack has equivalent optical performance to a single-layer silicon nitride (SiN x) antireflection coating (ARC) for unencapsulated cells, and slightly better performance for encapsulated cells (0.2 mA⋅cm-2 increase). The field effect passivation properties of the SiO2/a-TiOx film stack have been modified extrinsically after the film deposition to demonstrate surface recombination velocities below 1.2 cm/s in a \text{1}\;{\rm{\Omega }} \cdot {\text{cm}} n-type silicon wafer. Finally, the TiOx films have been deposited using an inline atmospheric pressure chemical vapor deposition (APCVD) system at temperatures below 350 °C, thus demonstrating this film stack offers both better passivation and optical performance, as well as a potentially lower manufacturing cost compared to SiN x, due to the use of APCVD rather than plasma-enhanced chemical vapor deposition. Simulations indicate further gains in terms of optical performance (0.3 mA⋅cm-2 increase compared to SiNx - encapsulated case) may be possible using a double-layer ARC featuring a polycrystalline TiOx film followed by an aluminum oxide (AlOx). However, potential contamination from the APCVD may pose a risk to maintaining high bulk carrier lifetimes.

Shielded hydrogen passivation - a novel method for introducing hydrogen into silicon

Bourret-Sicotte, G, Hamer, P, Bonilla, RS, Collett, K, Wilshaw, PR
2017
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Conference paper
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7TH INTERNATIONAL CONFERENCE ON SILICON PHOTOVOLTAICS, SILICONPV 2017
Hydrogen, Passivation, Direct Plasma Hydrogenation, Defect Kinetics, Crystalline Silicon

Method of Extracting Solar Cell Parameters From Derivatives of Dark I-V Curves

Hallam, BJ, Hamer, PG, Bonilla, RS, Wenham, SR, Wilshaw, PR
2017
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Journal article
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IEEE JOURNAL OF PHOTOVOLTAICS
Characterization of photovoltaic (PV), curve fitting, PV cells

Long term stability of c-Si surface passivation using corona charged SiO 2

Bonilla, RS, Reichel, C, Hermle, M, Hamer, P, Wilshaw, PR
2017
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Journal article
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Applied Surface Science
© 2017 Elsevier B.V. Recombination at the semiconductor surface continues to be a major limit to optoelectronic device performance, in particular for solar cells. Passivation films reduce surface recombination by a combination of chemical and electric field effect components. Dielectric films used for this purpose, however, must also accomplish optical functions at the cell surface. In this paper, corona charge is seen as a potential method to enhance the passivation properties of a dielectric film while maintaining its optical characteristics. It is observed that corona charge can produce extreme reductions in surface recombination via field effect, in the best case leading to lifetimes exceeding 5 ms at an injection of 10 15  cm −3 . For a 200 μm n-type 1 Ω cm c-Si wafer, this equates to surface recombination velocities below 0.65 cm/s and J 0e values of 0.92 fA/cm 2 . The average improvement in passivation after corona charging gave lifetimes of 1–3 ms. This was stabilised for a period of 3 years by chemically treating the films to prevent water absorption. Surface recombination was kept below 7 cm/s, and J 0e  < 16.28 fA/cm 2 for 3 years, with a decay time constant of 8.7 years. Simulations of back-contacted n-type cells show that front surface recombination represents less than 2% of the total internally generated power in the cell (the loss in power output) when the passivation is kept better than 16 fA/cm 2 , and as high as 10% if front recombination is worse than 100 fA/cm 2 .

Saw Damage Gettering for industrially relevant mc-Si feedstock

Shaw, EC, Hamer, P, Collett, KA, Bourrett-Sicotte, G, Bonilla, RS, Wilshaw, PR
2017
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Journal article
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PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE
feedstock, gettering, saw damage, silicon photovoltaics

Dielectric surface passivation for silicon solar cells: A review

Bonilla, RS, Hoex, B, Hamer, P, Wilshaw, PR
2017
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Journal article
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Physica Status Solidi (A) Applications and Materials Science
© 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Silicon wafer solar cells continue to be the leading photovoltaic technology, and in many places are now providing a substantial portion of electricity generation. Further adoption of this technology will require processing that minimises losses in device performance. A fundamental mechanism for efficiency loss is the recombination of photo-generated charge carriers at the unavoidable cell surfaces. Dielectric coatings have been shown to largely prevent these losses through a combination of different passivation mechanisms. This review aims to provide an overview of the dielectric passivation coatings developed in the past two decades using a standardised methodology to characterise the metrics of surface recombination across all techniques and materials. The efficacy of a large set of materials and methods has been evaluated using such metrics and a discussion on the current state and prospects for further surface passivation improvements is provided.

Shielded hydrogen passivation - A potential in-line passivation process

Bourret-Sicotte, G, Hamer, P, Bonilla, RS, Collett, K, Ciesla, A, Colwell, J, Wilshaw, PR
2017
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Journal article
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PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE
hydrogen, passivation, photovoltaics, silicon, solar cells

A novel source of atomic hydrogen for passivation of defects in silicon

Hamer, P, Bourret-Sicotte, G, Martins, G, Wenham, A, Bonilla, RS, Wilshaw, P
2017
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Journal article
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Physica Status Solidi - Rapid Research Letters
© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Palladium membranes have been used for decades for the separation of hydrogen from other gasses. In this letter the use of thin palladium leaves to act as sources of atomic hydrogen for silicon samples is explored. It has been assumed in the past that although hydrogen diffuses through palladium in atomic form, the atoms recombine to form molecular hydrogen at the surface. In this letter it is shown that hydrogen supplied to one surface of a palladium leaf can result in atomic hydrogen being released from the opposite surface at low pressure. This is demonstrated through the use of a palladium leaf in a direct plasma system which allows for atomic hydrogen to be supplied to a sample while avoiding exposure to UV radiation from the plasma and high energy charged particles. This method is shown to provide significant atomic hydrogen to silicon samples and improve passivation of silicon surfaces. Method of Shielded Hydrogen Passivation: Schematic of direct plasma chamber with a shield inserted between the plasma and the silicon sample.

On the c-Si/SiO2 interface recombination parameters from photo-conductance decay measurements

Bonilla, RS, Wilshaw, PR
2017
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Journal article
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Journal of Applied Physics
© 2017 Author(s). The recombination of electric charge carriers at semiconductor surfaces continues to be a limiting factor in achieving high performance optoelectronic devices, including solar cells, laser diodes, and photodetectors. The theoretical model and a solution algorithm for surface recombination have been previously reported. However, their successful application to experimental data for a wide range of both minority excess carrier concentrations and dielectric fixed charge densities has not previously been shown. Here, a parametrisation for the semiconductor-dielectric interface charge Q i t is used in a Shockley-Read-Hall extended formalism to describe recombination at the c-Si/SiO2 interface, and estimate the physical parameters relating to the interface trap density D i t, and the electron and hole capture cross-sections σ n and σ p. This approach gives an excellent description of the experimental data without the need to invoke a surface damage region in the c-Si/SiO2 system. Band-gap tail states have been observed to limit strongly the effectiveness of field effect passivation. This approach provides a methodology to determine interface recombination parameters in any semiconductor-insulator system using macro scale measuring techniques.

Passivation of all-angle black surfaces for silicon solar cells

Rahman, T, Bonilla, RS, Nawabjan, A, Wilshaw, PR, Boden, SA
2017
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Journal article
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Solar Energy Materials and Solar Cells
© 2016 Optical losses at the front surface of a silicon solar cell have a significant impact on efficiency, and as such, efforts to reduce reflection are necessary. In this work, a method to fabricate and passivate nanowire-pyramid hybrid structures formed on a silicon surface via wet chemical processing is presented. These high surface area structures can be utilised on the front surface of back contact silicon solar cells to maximise light absorption therein. Hemispherical reflectivity under varying incident angles is measured to study the optical enhancement conferred by these structures. The significant reduction in reflectivity (<2%) under low incident angles is maintained at high angles by the hybrid textured surface compared to surfaces textured with nanowires or pyramids alone. Finite Difference Time Domain simulations of these dual micro-nanoscale surfaces under varying angles support the experimental results. In order to translate the optical benefit of these high surface area structures into improvements in device efficiency, they must also be well passivated. To this end, atomic layer deposition of alumina is used to reduce surface recombination velocities of these ultra-black silicon surfaces to below 30 cm/s. A decomposition of the passivation components is performed using capacitance-voltage and Kelvin Probe measurements. Finally, device simulations show power conversion efficiencies exceeding 21% are possible when using these ultra-black Si surfaces for the front surface of back contact silicon solar cells.

Extremely low surface recombination in 1 Ω cm n-type monocrystalline silicon

Bonilla, RS, Reichel, C, Hermle, M, Wilshaw, PR
2017
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Journal article
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Physica Status Solidi - Rapid Research Letters
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim A key requirement in the recent development of highly efficient silicon solar cells is the outstanding passivation of their surfaces. In this work, plasma enhanced chemical vapour deposition of a triple layer dielectric consisting of amorphous silicon, silicon oxide and silicon nitride, charged extrinsically using corona, has been used to demonstrate extremely low surface recombination. Assuming Richter's parametrisation for bulk lifetime, an effective surface recombination velocity Seff = 0.1 cm/s at Δn = 1015 cm–3 has been obtained for planar, float zone, n -type, 1 Ω cm silicon. This equates to a saturation current density J0s = 0.3 fA/cm2, and a 1-sun implied open-circuit voltage of 738 mV. These surface recombination parameters are among the lowest reported for 1 Ω cm c-Si. A combination of impedance spectroscopy and corona-lifetime measurements shows that the outstanding chemical passivation is due to the small hole capture cross section for states at the interface between the Si and a-Si layer which are hydrogenated during nitride deposition. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim).

Lowest surface recombination in n-type oxidised crystalline silicon by means of extrinsic field effect passivation

Bonilla Osorio, RS, wilshaw, Hamer, P
2016
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Conference paper
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http://www.eupvsec-proceedings.com/
Surface recombination remains a major factor limiting the efficiency of silicon solar cells. The post-processing of dielectric films used as surface coatings has been previously demonstrated an effective technique to improve their passivation quality. In this paper extrinsic methods are demonstrated to produce the lowest reported surface recombination velocity in solar relevant n-type silicon. Recombination velocities below 2.8 cm/s at an injection of 1015 cm-3, are achieved using extrinsic field-effect passivation, or < 0.6 cm/s when using combined extrinsic chemical and extrinsic field effect passivation. These are equivalent to emitter saturation current densities J0e<1.4 fA/cm2 and 0.6 fA/cm2.

Corona Charge in SiO2: Kinetics and Surface Passivation for High Efficiency Silicon Solar Cells

Bonilla, RS, Jennison, N, Clayton-Warwick, D, Collett, KA, Rands, L, Wilshaw, PR
2016
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Conference paper
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Energy Procedia
© 2016 The Authors. This manuscript presents a method by which capacitance-voltage measurements can be used in conjunction with Kelvin-probe measurements to calculate the location of charge within a dielectric layer. A first order kinetic model for the transport of charge into SiO2 films after exposure to corona charge deposition is proposed. The rate limiting step for charge migration into oxide films has been observed to be the injection of charge from the surface (air-SiO2 interface). The charge lies preferentially at the air-SiO2 and SiO2-Si interfaces, and its injection into SiO2 has been characterized as having an activation energy of ∼ 50 meV. In this work, corona charge deposition has produced SRV < 2.7 cm/s and J01 < 12 fA/cm2 which is within the requirements of high efficiency silicon solar cells. These results contribute to the understanding of corona charge interaction with SiO2 with a view to its potential application as an extrinsic method of passivation for highly efficient silicon solar cells.

Corona Field Effect Surface Passivation of n-type IBC Cells

Bonilla, RS, Reichel, C, Hermle, M, Wilshaw, PR
2016
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Conference paper
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Energy Procedia
© 2016 The Authors. Passivation of silicon surfaces is an important requirement in achieving high energy conversion efficiencies in interdigitated back contact cells. Surface passivation, commonly achieved by dielectric coatings, can be greatly improved by extrinsic addition of chemical and field effect components. In particular, cell performance is strongly dependent on front surface passivation. In this work device modelling is used to show that 200% relatively better performance can be achieved using charge extrinsically added to a passivation coating without the need for a front surface field. Research scale cells have been tested as a function of front surface corona charge. On average an absolute improvement in efficiency of 0.3% ±0.3 and 0.13% ±0.12 has been observed on cells without and with a front surface field. These results show that extrinsic field effect passivation can produce significant improvements in cell efficiency.

SURFACE PASSIVATION PROVIDED BY AN ALNEAL THROUGH SIO2/TIO2 BILAYER

Collett, KA, Cyrson, M, Bonilla Osorio, RS, Wilshaw, PR
2016
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Conference paper
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EUPVSEC, Munich, Germany, 20 Jun 2016 - 24 Jun 2016. http://www.eupvsec-proceedings.com/

Investigation of Parasitic Edge Recombination in High-Lifetime Oxidized n-Si

Bonilla, RS, Martins, G, Wilshaw, PR
2016
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Conference paper
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Solid State Phenomena
<jats:p>An investigation of the parasitic surface recombination mechanisms in high-lifetime oxidized n-Si is presented. An approximate analytical expression describing recombination at the edge of square shaped specimens is derived. This shows that edge recombination can have a significant effect on the effective lifetime as measured using the transient photo-conductance technique and that for well passivated high quality material edge recombination can be the dominant mechanism in reducing the effective lifetime below the intrinsic level. For 3 x 3 cm<jats:sup>2</jats:sup> pieces of silicon measured using a Sinton photo-conductance lifetime instrument, it is shown that recombination at the edge of the sample results in an additional component to the measured lifetime of around 16 ms at an injection level of 10<jats:sup>15</jats:sup> cm<jats:sup>-3</jats:sup>. When this effect is taken into account measurements of 1 Ωcm FZ-Si show that a SRV as low as 1.5 cm/s is possible when the surface is passivated using a corona charge concentration of +2.2 x 10<jats:sup>12</jats:sup> q/cm<jats:sup>2</jats:sup> deposited on a 100 nm oxide layer.</jats:p>

Minority Carrier Lifetime Improvement of Multicrystalline Silicon Using Combined Saw Damage Gettering and Emitter Formation

Martins, G, Bonilla, RS, Burton, T, MacDonald, P, Wilshaw, PR
2016
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Conference paper
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Solid State Phenomena
<jats:p>In this work we use Saw Damage Gettering (SDG) in combination with emitter formation to improve the minority carrier lifetime of highly contaminated multi-crystalline silicon wafers. This process is applied to wafers from the bottom of ingots, commonly referred to as the “red zone”, which are currently discarded since their high concentration of impurities limits the efficiency of solar cells produced therefrom. SDG is a potentially simple technique designed to upgrade these wafers. In this technique, bulk impurities are dissolved via annealing. The wafers are then cooled which generates a super-saturation of impurities in solution. The system then relaxes through the formation of precipitates in the saw damaged region. SDG is shown to be enhanced when using a temperature dependent cooling rate which maximizes the flux of impurities to the saw damaged regions. In addition, these benefits were observed even after an additional gettering process occurring during an emitter formation procedure. The SDG annealing conditions required to achieve the maximum lifetime were altered by the introduction of the emitter formation process. The enhancement generated by the SDG process may be sufficient to enable red-zone wafers to be processed is the same manner as higher quality no-red zone wafer wafers without adversely affecting the resultant cell efficiency. Due to its simplicity, it is expected that SDG can easily be incorporated into current production methods.</jats:p>

Stable, Extrinsic, Field Effect Passivation for Back Contact Silicon Solar Cells

Bonilla, RS, Collett, K, Rands, L, Martins, G, Lobo, R, Wilshaw, PR
2016
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Conference paper
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Solid State Phenomena
<jats:p>A new technique is described by which ionic species can be rapidly transported into oxide films, and once there provide effective and stable field effect passivation to silicon surfaces. Field effect passivation in thermally grown oxide films has been achieved by embedding potassium ions using a combined drift and diffusion mechanism at high temperature. This process has been shown to be over 10 times faster than a pure diffusion process. The resulting passivation stable for periods exceeding 600 days, with lifetimes reaching 1.4 ms, equivalent to a surface recombination velocity (SRV) ≤ 5.7 cm/s, on 1 Ωcm, n-type, FZ-Si.</jats:p>

Electrical characteristics of flash sintering: thermal runaway of Joule heating

Todd, RI, Zapata-Solvas, E, Bonilla, RS, Sneddon, T, Wilshaw, PR
2015
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Journal article
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JOURNAL OF THE EUROPEAN CERAMIC SOCIETY
Flash sintering, YSZ, Thermal runaway, Electrical conductivity, NTC behaviour

Electrical characteristics of flash sintering: Thermal runaway of Joule heating

Todd, RI, Zapata-Solvas, E, Bonilla, RS, Sneddon, T, Wilshaw, PR
2015
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Journal article
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Journal of the European Ceramic Society
© 2015 Elsevier Ltd. Flash sintering occurs when an electric field is applied to a heated ceramic powder compact. At a critical combination of field and temperature, a power surge occurs (the "flash event") and sintering takes place in a few seconds. This paper investigates the possibility that this surge occurs by runaway Joule heating. The resistivity of 3YSZ was measured under the relevant conditions. To a good approximation, resistivity was found to be history-independent and to have the same temperature dependence before and after the flash event. These data were used to model the thermal and electrical response of 3YSZ to an applied electric field. All electrical characteristics of the flash event observed experimentally were predicted with a high degree of accuracy. It is concluded that the thermal and electric characteristics of flash sintering are a classical consequence of the negative temperature coefficient of resistivity leading to runaway Joule heating at constant voltage.

Erratum: On the location and stability of charge in SiO2/SiNx dielectric double layers used for silicon surface passivation (Journal of Applied Physics (2014) 115 (144105))

Bonilla, RS, Reichel, C, Hermle, M, Wilshaw, PR
2015
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Journal article
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Journal of Applied Physics

Extrinsic Passivation of Silicon Surfaces for Solar Cells

Bonilla, RS, Reichel, C, Hermle, M, Martins, G, Wilshaw, PR
2015
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Conference paper
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Energy Procedia
© 2015 The Authors. In the present work we study the extent to which extrinsic chemical and field effect passivation can improve the overall electrical passivation quality of silicon dioxide on silicon. Here we demonstrate that, when optimally applied, extrinsic passivation can produce surface recombination velocities below 1.2cm/s in planar 1Ωcm n-type Si. This is largely due to the additional field effect passivation component which reduces the recombination velocity below 2.13cm/s. On textured surfaces field effect passivation has a comparable effect, and surface recombination velocities below 32cm/s are achieved when field effect passivation is optimised extrinsically. Modelling of interdigitated back contact cells showed an absolute increase in efficiency of 0.1% when passivation was optimised via extrinsic dielectric charge. These results point to the importance extrinsic passivation will have for future dielectric coatings used in solar cell manufacture.

Saw Damage Gettering for Improved Multicrystalline Silicon

Martins, GF, MacDonald, P, Burton, T, Bonilla, RS, Wilshaw, PR
2015
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Conference paper
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Energy Procedia
© 2015 The Authors. In this work we describe a technique, namely Saw Damage Gettering (SDG),which improves the lifetime of highly contaminated multi-crystal silicon wafers. The wafers to which it is applied are from the top and bottom of ingots, the so called "red zones" of ingots. Such wafers are currently discarded before processing due to high impurity levels which result in low efficiency cells. SDG entails an extra annealing step before the saw damage is removed by etching. The effect of the anneal is to dissolve metallic precipitates present in the material after casting. On subsequent cooling precipitates are preferentially nucleated in the high defect density regions associated with saw damage at the surfaces of the wafer so leaving the concentration of impurities in the bulk of the material significantly reduced. The anneal conditions, typically ∼850oC for ∼20 minutes, are chosen so that the majority of metal precipitates dissolve. The cooling step, taking ∼ 30 minutes, is such that the impurities have sufficient opportunity to diffuse from the center of the wafer to the damaged regions at the edges. This processing is found to improve carrier lifetimes by up to a factor of 4. This enhancement may be sufficient to enable red-zone wafers to be processed in the same manner as other, higher quality, mc-Si wafers without sacrificing cell efficiency. SDG is expected to be cost effective and easily incorporated into current cell processing procedures.

Controlled field effect surface passivation of crystalline n-type silicon and its application to back-contact silicon solar cells

Bonilla, RS, Reichel, C, Hermle, M, Senkader, S, Wilshaw, P
2014
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Conference paper
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2014 IEEE 40th Photovoltaic Specialist Conference, PVSC 2014
© 2014 IEEE. Surface passivation continues to be a significant requirement in achieving high solar-cell efficiency. Single layers of SiO2 and double layers of SiO2/SiN surface passivation have been widely used to reduce surface carrier recombination in silicon solar cells. Passivation films reduce surface recombination by a combination of chemical and electric field effect components. Dielectric films used for this purpose, however, must also accomplish optical functions at the cell surface. In this paper, field effect passivation is seen as a potential method to enhance the passivation properties of a dielectric film while preserving its optical characteristics. It is observed that the field effect can make a large reduction in surface recombination by using corona charged ions deposited on the surface of a dielectric film. The effect is studied for both SiO2 and SiO2/SiN layers, and surface recombination velocities of less than 9 cm/s and 16 cm/s are inferred, respectively, on n-type, 5 cm, Cz-Si. This improvement in passivation was stabilized for period of over a year by chemically treating the films to prevent water absorption. Intense ultraviolet radiation was seen to diminish the surface recombination velocity to its initial value in a time period of up to 7 days. Additionally, external deposition of charge on to the SiO2/SiN passivated front surface of back-contact n-type silicon solar cells provides a 2.5 % relative improvement in conversion efficiency due to enhanced and controlled field effect passivation.

Very low surface recombination velocity in n-type c-Si using extrinsic field effect passivation

Bonilla, RS, Woodcock, F, Wilshaw, PR
2014
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Journal article
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Journal of Applied Physics
In this article, field-effect surface passivation is characterised as either intrinsic or extrinsic, depending on the origin of the charges present in passivation dielectric layers. The surface recombination velocity of float zone, 1 Ω cm, n-type silicon was reduced to 0.15cm/s, the lowest ever observed for a passivating double layer consisting of thermally grown silicon dioxide and plasma enhanced chemical vapour deposited silicon nitride. This result was obtained by enhancing the intrinsic chemical and field-effect passivation of the dielectric layers with uniform, extrinsic field-effect passivation induced by corona discharge. The position and stability of charges, both intrinsic and extrinsic, were characterised and their passivation effect was seen stable for two months with surface recombination velocity <2cm/s. Finally, the intrinsic and extrinsic components of passivation were analysed independently. Hydrogenation occurring during nitride deposition was seen to reduce the density of interfacial defect states from ∼5×10 10cm-2eV-1 to ∼5×109 cm-2eV-1, providing a decrease in surface recombination velocity by a factor of 2.5. The intrinsic charge in the dielectric double layer provided a decrease by a factor of 4, while the corona discharge extrinsic field-effect passivation provided a further decrease by a factor of 3. © 2014 AIP Publishing LLC.

A technique for field effect surface passivation for silicon solar cells

Bonilla, RS, Wilshaw, PR
2014
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Journal article
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Applied Physics Letters
The recombination of electric charge carriers at the surface of semiconductors is a major limiting factor in the efficiency of optoelectronic devices, in particular, solar cells. The reduction of such recombination, commonly referred to as surface passivation, is achieved by the combined effect of a reduction in the trap states present at the surface via a chemical component, and the reduction in the charge carriers available for a recombination process, via a field effect component. Here, we propose a technique to field effect passivate silicon surfaces using the electric field effect provided by alkali ions present in a capping oxide. This technique is shown to reduce surface recombination in a controlled manner, and to be highly stable. Surface recombination velocities in the range of 6-15 cm/s are demonstrated for 1 Ω cm n-type float zone silicon using this technique, and they are observed to be constant for over 300 days, without the use of any additional surface chemical treatment. A model of trapping-mediated ionic injection is used to describe the system, and activation energies of 1.8-2 eV are deduced for de-trapping of sodium and potassium alkali ionic species. © 2014 AIP Publishing LLC.

On the location and stability of charge in SiO2/SiNx dielectric double layers used for silicon surface passivation

Bonilla, RS, Reichel, C, Hermle, M, Wilshaw, PR
2014
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Journal article
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Journal of Applied Physics
Dielectric double layers of thermal silicon dioxide-chemical vapour deposition (CVD) silicon nitride are found to produce excellent passivation of silicon surfaces by combining a chemical reduction of surface defect states, with a field effect reduction of carriers at the surface due to charge in the dielectrics. The charge present in such double-layers has previously been attributed to be characteristic of the interface between the two. However, experimental evidence shows this is indirect and inconclusive. This manuscript reports direct measurements that show the charge lies within 10 nm of the interface between passivating double layers of thermal silicon dioxide-plasma CVD silicon nitride. In addition, the passivation efficiency of oxide-nitride layers, deposited using optimised conditions, was found to be largely unaffected by extra charge subsequently added to the film. The passivation efficiency of textured surfaces or those produced using non-optimised deposition conditions is found to be highly dependent on the field effect component provided by extra deposited charge. Using such extra field effect component, surface recombination velocities <2 cm/s have been obtained on single oxide and oxide/nitride double layers. The extra deposited charge was found to have good long term stability when the dielectric films are submitted to a chemical treatment. By contrast, poor stability of the deposited charge was observed when subjected to ultraviolet radiation. These results point to the importance of the interface between dielectrics when considering how to optimise the charge present in passivating dielectric films. © 2014 AIP Publishing LLC.

Electric field effect surface passivation for silicon solar cells

Bonilla, RS, Reichel, C, Hermle, M, Wilshaw, P
2014
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Conference paper
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Solid State Phenomena
Effective reduction of front surface carrier recombination is essential for high efficiency silicon solar cells. Dielectric films are normally used to achieve such reduction. They provide a) an efficient passivation of surface recombination and b) an effective anti-reflection layer. The conditions that produce an effective anti-reflection coating are not necessarily the same for efficient passivation, hence both functions are difficult to achieve simultaneously and expensive processing steps are normally required. This can be overcome by enhancing the passivation properties of an anti-reflective film using the electric field effect. Here, we demonstrate that thermally grown silicon dioxide is an efficient passivation layer when chemically treated and electrically charged, and it is stable over a period of ten months. Double layers of SiO2 and SiN also provided stable and efficient passivation for a period of a year when the sample is submitted to a post-charge anneal. Surface recombination velocity upper limits of 9 cm/s and 19 cm/s were inferred for single and double layers respectively on n-type, 5 ωcm, Cz-Si. © (2014) Trans Tech Publications, Switzerland.

Preliminary investigation of flash sintering of SiC

Zapata-Solvas, E, Bonilla, S, Wilshaw, PR, Todd, RI
2013
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Journal article
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Journal of the European Ceramic Society
The feasibility of flash sintering a covalent ceramic, SiC, has been investigated for the first time. Flash sintering involves the application of an electrical potential difference across a powder compact during heating, which leads to sintering at low furnace temperatures in a few seconds and has only been demonstrated with ionic ceramics previously. Near-theoretical density was achieved using Al2O3+Y2O3 sintering aids at a furnace temperature of only 1170°C and in a time of 150s. Specimen temperatures were significantly higher than the furnace temperature owing to Joule heating and consequently heat loss limited densification in the near surface region. It was not possible to reach high densities using "ABC" sintering aids (aluminium-boron-carbon) or pure SiC. The mechanisms involved and potential commercial advantages are briefly discussed. © 2013 Elsevier Ltd.

Quantum tunnelling demonstration using a low-cost experimental setup

González, CA, Ruiz, JP, Bonilla, RS, Ávila, AG
2013
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Conference paper
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EAMTA 2013 - Proceedings of the Argentine School of Micro-Nanoelectronics, Technology and Applications 2013
A simple and reliable experimental setup that illustrates one-dimensional quantum tunnelling effect is introduced. The main elements in this setup are the fabrication of sharp tips, and the sensing and approaching mechanism. A technique to fabricate very sharp tips is described, with tips made of 60 μm thick tungsten wire easily obtainable from car light bulbs. The technique produced tips with aspect ratios of 2 and tip diameter of 15 nm ±4.5 nm. A mechanical configuration to scale down semi-lineal displacement is suggested along with an experimental setup, including circuitry and measurement.

Stable field effect surface passivation of n-type Cz silicon

Bonilla, RS, Wilshaw, PR
2013
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Conference paper
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Energy Procedia
Surface recombination of carriers in solar cells can cause a significant reduction in their efficiency and is most commonly minimized by the deposition of surface dielectric layers which simultaneously perform two functions; efficient passivation of surface recombination and the provision of an effective anti-reflection layer. This can be difficult to achieve in practice since the conditions that produce an optimum anti-reflection coating are not necessarily the same as those required for effective passivation. In this work we describe the use of external electrical charging of dielectric layers which serves to improve their passivation properties without affecting their reflection properties. This provides a method by which, to some extent, the electrical and optical properties of the films can be decoupled so allowing better overall performance to be achieved. It is demonstrated that SiO2 and SiO 2/SiN stacks deposited on a silicon surface can provide a stable reduction of surface recombination when chemically treated, electrically charged using a corona discharge and then annealed at low temperature. Surface recombination velocity upper limits of 19 cm/s and 16 cm/s were inferred for single and double layers respectively on n-type, 5 Ωcm, Cz-Si. © 2013 The Authors.

Direct observation of the spatial distribution of charges on a polypropylene fiber via Electrostatic Force Microscopy.

Bonilla, R, Avila, A, Montenegro, C, Hinestroza, J
2012
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Journal article
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J Microsc
The spatial distribution of electrical charges along the longitudinal axes of a polypropylene electret fiber was determined using Electrostatic Force Microscopy (EFM). EFM mapping on highly curved surfaces, such as those of polymeric fibers, is a challenging endeavour and most work reported in the scientific literature has been limited to single line-scan analysis or flat specimens. Charged polymeric fibers, electrets, are extensively used in high performance filtration applications and methods to determine the amount and magnitude of the charges on these fibers remain elusive. Electrical charge maps of individual fibers were obtained by biasing the tip to -10 V and maintaining a constant tip-sample distance of 100 nm. Spatially dependant EFM phase and magnitude gradients were determined and the developed technique may provide a unique understanding into the heterogeneous charge distribution on electrets fibers. Direct mapping of the charge distribution in electrets fibers can offer new insights in the development of antistatic additives, new means to facilitate electrostatic self-assembly of nano-moieties on the surface of fibrous materials and a quantitative metrics capable of determining discharge dynamics and predicting the shelf-life of filtration media.

Local anodic oxidation on silicon (100) substrates using atomic force microscopy

Bernal, AGA, Bonilla, RS
2012
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Journal article
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DYNA (Colombia)
A characterization of local anodic oxidation using scanning probe microscopy is performed on a (100) silicon substrate. The formation of patterns varies as a function of voltage, humidity, and scanning speed. A set of experiments is presented to analyze the voltage and scanning speed dependence under stable environmental conditions (50.5% relative humidity, 22 °C, and 767 mmHg). A finer control of the dimensions of the local oxidation patterns is attained at low voltages and low scanning speeds. Oxide ridges are observed at high voltages independently of the writing speed. Their presence sets up an upper limit for the oxide pattern formation.

Commissioning and performance of the Preshower off-detector readout electronics in the CMS experiment

Antchev, G, Barney, D, Bialas, W, Bonilla Osorio, RS, Chen, KF, Kuo, CM, Lu, RS, Patras, V, Reynaud, S, Rodriguez Estupinan, JS, Vichoudis, P
2009
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Conference paper
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Proceedings of the Topical Workshop on Electronics for Particle Physics, TWEPP 2009
The CMS Preshower is a fine grain detector that comprises 4288 silicon sensors, each containing 32 strips. The data are transferred from the detector to the counting room via 1208 optical fibres producing a total data flow of ∼72GB/s. For their readout, 40 multi-FPGA 9U VME boards are used. This article is focused on the commissioning of the VME readout system using two tools: a custom connectivity test system based on FPGA embedded logic analyzers read out through JTAG and an FPGA-based system that emulates the data-traffic from the detector. Additionally, the performance of the VME readout system in the CMS experiment, including the 2009 Cosmic ray at Four Tesla (CRAFT) run, is discussed.