Rasit Turan - Academia.edu (original) (raw)

Papers by Rasit Turan

Research paper thumbnail of Effect of laser parameters and post-texturing treatments on the optical and electrical properties of laser textured c-Si wafers

Surface plays a crucial role in the performance of crystalline silicon (cSi) based solar cells as... more Surface plays a crucial role in the performance of crystalline silicon (cSi) based solar cells as it affects both electrical and optical properties. To minimize reflection from the flat surface and thus improve light trapping, the cSi wafers must be textured. For mono-cSi cells, anisotropic alkaline etchants are commonly utilized to create pyramids on the surface. However, this method is not viable for multi-crystalline silicon (mc-Si) wafers due to the presence of different and random crystallographic orientations. In this work, we employed laser texturing, which is an isotropic texturing process, as an alternative texturing method for mc-Si wafers. This approach utilizes a laser process to create pits on the cSi surface. The laser's processing parameters were justified by performing a series of experiments. After texturing, physical (ultrasonic bath with deionized water) and chemical (in KOH with two different concentrations of 1 and 20%) cleanings with different durations were performed which were essential to remove laser-induced damages and other residues from the surface. In order to evaluate the optical response of the textured surfaces, weighted reflection values were measured and correlated with scanning electron microscopy (SEM) images of the textured features before and after post-texturing cleaning step. An impressive low weighted reflection of only 4.2% was measured from laser textured mc-Si with anti-reflection coating after optimizing the laser and post-texturing processes. Moreover, an implied open-circuit voltage (iVoc) of up to 692 mV was achieved by passivating the laser-textured surfaces by Al2O3.

Research paper thumbnail of Advanced plasmonic interfaces for optimized light trapping in photovoltaics

Journal of Physics: Conference Series, 2017

Plasmonic interfaces are integrated to photovoltaic devices to enhance light trapping and improve... more Plasmonic interfaces are integrated to photovoltaic devices to enhance light trapping and improve efficiency. The optimum thickness of the spacer layer used to passivate the absorber layer and adjust its distance from the metal nanoparticles remains unclear. We integrate plasmonic interfaces consisting of Ag nanoparticles and silicon nitride spacers of different thicknesses to the back of a-Si:H absorber to investigate the optimum thickness of the spacer layer and use the photocurrent in a-Si:H to indicate the enhancement in light-trapping. For integration to the back or front of the device, the localized surface plasmon resonance (LSPR) is shifted and broadened into the red with increased spacer layer thickness and the effect is more pronounced for integration to the back. An appreciable enhancement of photocurrent in a-Si:H is consistent with this broadening of LSPR and has a critical dependence on spacer layer thickness.

Research paper thumbnail of Reactively Sputtered Cu2ZnTiS4 Thin Film as Low-Cost Earth-Abundant Absorber

Journal of Electronic Materials, 2017

Cu 2 ZnTiS 4 thin films have been deposited on glass by the reactive cosputtering technique with ... more Cu 2 ZnTiS 4 thin films have been deposited on glass by the reactive cosputtering technique with high-purity ZnS and Cu and Ti metals as targets and H 2 S as reactive gas. Cu 2 ZnTiS 4 thin films were obtained at various temperatures and H 2 S flows and were annealed in H 2 S atmosphere. The structural, morphological, and optical properties of the Cu 2 ZnTiS 4 thin films were examined by scanning electron microscopy, energy-dispersive spectroscopy, x-ray diffraction (XRD) analysis, and ultraviolet-visible (UV-Vis) spectroscopy. Agglomeration was found to increase with increasing temperature. The XRD peaks of the Cu 2 ZnTiS 4 thin films were consistent with those of Cu 2 ZnSnS 4. Furthermore, the optical bandgaps of the Cu 2 ZnTiS 4 films were lower than those of conventional Cu 2 ZnSnS 4 thin films.

Research paper thumbnail of All Solution-Based Fabrication of Copper Oxide Thin Film/Cobalt-Doped Zinc Oxide Nanowire Heterojunctions

Journal of the American Ceramic Society, 2016

Research paper thumbnail of Low temperature aluminum doped zinc oxide thin film deposition on ultra-thin flexible glass and PET substrates by RF magnetron sputtering

physica status solidi (c), 2015

Aluminum doped zinc oxide (AZO) thin films are prepared on ultra-thin flexible glass and flexible... more Aluminum doped zinc oxide (AZO) thin films are prepared on ultra-thin flexible glass and flexible polyethylene terephthalate (PET) substrates at room temperature by radio frequency (RF) magnetron sputtering. Optimization of films has been achieved by varying process parameters to reach the demands of proper conductive layer for thin silicon solar cell applications. Structural analysis of the films was done by X-ray diffraction spectroscopy. Optical and electrical properties of the films were carried out by means of UV-Visible spectroscopy, and four point probe measurements. Thickness of the films was obtained by spectroscopic ellipsometry. Transmission measurements clearly show that by decreasing pressure transmission of the AZO film is improved in the UV region. Our results show that by varying the deposition parameters, low resistivity films of 1.1×10-3 Ωcm and 1×10-3 Ωcm were obtained on PET and on ultra-thin flexible substrates, respectively.

Research paper thumbnail of N-structure based on InAs/AlSb/GaSb superlattice photodetectors

Superlattices and Microstructures, 2015

Research paper thumbnail of High haze nature of textured Al:ZnO with Ag nanoparticles for light management in thin film solar cells

Optical Materials Express, 2015

ABSTRACT We report on fabrication of plasmonic interfaces consisting of Ag nanoparticles on flat ... more ABSTRACT We report on fabrication of plasmonic interfaces consisting of Ag nanoparticles on flat and textured Al:ZnO for use at the front surface of thin film solar cells to enhance light trapping and photo-conversion efficiencies. We show that outstandingly high transmittance haze is achieved from single step HCl surface textured Al:ZnO and demonstrate Ag dewetting on textured and flat Al:ZnO surfaces upon annealing at moderate temperatures. Optical response of these plasmonic interfaces clearly display plasmonic resonances in the visible and near infrared, which is crucial for enhancement of photovoltaic conversion efficiency in thin film solar cells.

Research paper thumbnail of Improved diode properties in zinc telluride thin film-silicon nanowire heterojunctions

Philosophical Magazine, 2015

In this study, structural and optoelectronic properties and photodedection characteristics of dio... more In this study, structural and optoelectronic properties and photodedection characteristics of diodes constructed from p-zinc telluride (ZnTe) thin film/n-silicon (Si) nanowire heterojunctions are reported. Dense arrays of vertically aligned Si nanowires were successfully synthesized on (1 1 0)-oriented n-type single crystalline Si wafer using simple and inexpensive metal-assisted etching (MAE) process. Following the nanowire synthesis, p-type ZnTe thin films were deposited onto vertically oriented Si nanowires via radio frequency magnetron sputtering to form three-dimensional heterojunctions. A comparative study of the structural results obtained from X-ray diffraction and Raman spectroscopy measurements showed the improved crystallinity of the ZnTe thin films deposited onto the Si nanowire arrays. The fabricated nanowire-based heterojunction devices exhibited remarkable diode characteristics and enhanced optoelectronic properties and photosensitivity in comparison to the planar reference. The electrical measurements revealed that the diodes with nanowires had a well-defined rectifying behaviour with a rectification ratio of 104 at ±2 V and a relatively small ideality factor of n = 1.8 with lower reverse leakage current and series resistance at room temperature in dark condition. Moreover, an open-circuit voltage of 100 mV was also observed under illumination. Based on spectral photoresponsivity measurements, the nanowire-based device exhibited a distinct responsivity and high detectivity in visible and near-infrared (NIR) wavelength regions. The device characteristics observed here offer that the fabricated ZnTe thin film/Si nanowire-based p–n heterojunction structures will find important applications in future and will be a promising candidate for high-performance and low-cost optoelectronic device applications, NIR photodedectors in particular.

Research paper thumbnail of Characterization of CdZnTe thin films prepared by magnetron sputtering from a single CdZnTe target

2011 37th IEEE Photovoltaic Specialists Conference, 2011

ABSTRACT

Research paper thumbnail of Low dark current N structure superlattice MWIR photodetectors

SPIE Proceedings, 2014

Commercially available read out integrated circuits (ROICs) require the FPA to have high dynamic ... more Commercially available read out integrated circuits (ROICs) require the FPA to have high dynamic resistance area product at zero bias (R 0 A) which is directly related to dark current of the detector. Dark current arises from bulk and surface contributions. Recent band structure engineering studies significantly suppressed the bulk contribution of the type-II superlattice infrared photodetectors (N structure, M structure, W structure). In this letter, we will present improved dark current results for unipolar barrier complex supercell superlattice system which is called as "N structure". The unique electronic band structure of the N structure increases electron-hole overlap under bias, significantly. N structure aims to improve absorption by manipulating electron and hole wavefunctions that are spatially separated in T2SLs, increasing the absorption while decreasing the dark current. In order to engineer the wavefunctions, we introduce a thin AlSb layer between InAs and GaSb layers in the growth direction which also acts as a unipolar electron barrier. Despite the difficulty of perfect lattice matching of InAs and AlSb, such a design is expected to reduce dark current. Experiments were carried out on Single pixel with mesa sizes of 100 × 100-700 × 700 µm photodiodes. Temperature dependent dark current with corresponding R 0 A resistance values are reported.

Research paper thumbnail of Light management on industrial size c-Si solar cells by Si nanowires fabricated by metal-assisted etching

Research paper thumbnail of Exploration of the horizontally staggered light guides for high concentration CPV applications

Optics Express, 2012

The material and processing costs are still the major drawbacks of the c-Si based photovoltaic (P... more The material and processing costs are still the major drawbacks of the c-Si based photovoltaic (PV) technology. The wafer cost comprises up to 35-40% of the total module cost. New approaches and system designs are needed in order to reduce the share of the wafer cost in photovoltaic energy systems. Here we explore the horizontally staggered light guide solar optics for use in Concentrated Photovoltaic (CPV) applications. This optical system comprises a lens array system coupled to a horizontal light guide which directs the incoming light beam to its edge. We have designed and simulated this system using a commercial ray tracing software (Zemax). The system is more compact, thinner and more robust compared to the conventional CPV systems. Concentration levels as high as 1000x can easily be reached when the system is properly designed. With such a high concentration level, a good acceptance angle of + −1 degree is still be conserved. The analysis of the system reveals that the total optical efficiency of the system could be as high as %94.4 without any antireflection (AR) coating. Optical losses can be reduced by just accommodating a single layer AR coating on the initial lens array leading to a %96.5 optical efficiency. Thermal behavior of high concentration linear concentrator is also discussed and compared with a conventional point focus CPV system.

Research paper thumbnail of Metal Nanoparticles for Plasmonic Solar Cell Applications

Advanced Photonics & Renewable Energy, 2010

ABSTRACT

Research paper thumbnail of Developing a trilayer processing technique for superconducting Y Ba 2 Cu 3 O 7−δ thin films by using Ge ion implantation

Superconductor Science and Technology, 2005

For making trilayer superconducting devices based on YBa 2 Cu 3 O 7−δ (YBCO) thin film processing... more For making trilayer superconducting devices based on YBa 2 Cu 3 O 7−δ (YBCO) thin film processing, we developed a new technique by employing Ge ion implantation. A YBCO thin film of 150 nm thickness having high c-axis orientation and a transition temperature, T c , of 90 K was implanted with 80 keV, 1 × 10 16 Ge ions cm −2 at room temperature. By the result of TRIM calculation, Ge ions were found to penetrate into the YBCO thin film approximately 60 nm below the surface of the film, thus leaving the lower part of the film as a superconductor. Upon implantation with Ge ions, the implanted upper part of the sample lost its electrical conductivity and diamagnetism while its original crystalline structure was preserved. The implanted ions we found did not alter the overall crystal structure of the YBCO thin film; this allowed us to grow an epitaxial superconducting upper layer of YBCO on top of the implanted area, leaving no need to use any buffer layer. The superconducting properties of the upper layer were similar to those of the pure YBCO base layer with an increased room temperature resistivity and a lowered T c (88 K). This process provides an effective method for fabrication of a trilayer HTS device structure.

Research paper thumbnail of Performance of nanowire decorated mono- and multi-crystalline Si solarcells

Physica E: Low-dimensional Systems and Nanostructures, 2013

Research paper thumbnail of Charge retention in quantized energy levels of nanocrystals

Physica E: Low-dimensional Systems and Nanostructures, 2007

Understanding charging mechanisms and charge retention dynamics of nanocrystal memory devices is ... more Understanding charging mechanisms and charge retention dynamics of nanocrystal memory devices is important in optimization of device design. Capacitance spectroscopy on PECVD grown germanium nanocrystals embedded in a silicon oxide matrix was performed. Dynamic measurements of discharge dynamics are carried out. Charge decay is modelled by assuming storage of carriers in the ground states of nanocrystals and that the decay is dominated by direct tunnelling. Discharge rates are calculated using the theoretical model for different nanocrystal sizes and densities and are compared with experimental data. Experimental results agree well with the proposed model and suggest that charge is indeed stored in the quantized energy levels of the nanocrystals.

Research paper thumbnail of Mechanisms of light emission from terbium ions (Tb3+) embedded in a Si rich silicon oxide matrix

Optical Materials, 2012

ABSTRACT a b s t r a c t Mechanisms of light emission in Tb doped Si rich SiO x matrix prepared b... more ABSTRACT a b s t r a c t Mechanisms of light emission in Tb doped Si rich SiO x matrix prepared by magnetron sputtering are stud-ied by photoluminescence spectroscopy (PL). Characteristic PL peaks of Tb 3+ ions and Si nanocrystals are simultaneously observed with an inverse relationship between their intensity. With a prolonged heat treatment at high temperatures, light emission from Tb 3+ ions enhances at the expense of total quenching of the PL signal from the nanocrystals. It is suggested from the annealing studies as a function of process conditions and structural characterization that the light emission from Tb ions is mediated by trap states formed in the band gap of the oxide matrix by Tb x Si y O z complexes or excess Si states. Ó 2012 Elsevier B.V. All rights reserved.

Research paper thumbnail of Effect of γ-radiation on HfO2 based MOS capacitor

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2010

Radiation effects on Metal Oxide Semiconductor (MOS) capacitors with a HfO2 gate insulator have b... more Radiation effects on Metal Oxide Semiconductor (MOS) capacitors with a HfO2 gate insulator have been studied. Because HfO2 is a promising high-k dielectric material for microelectronic applications, radiation effects on its performance in MOS devices is of interest. New results on radiation effects on HfO2, particularly at low gamma radiation doses, are presented. The results are compared with other systems

Research paper thumbnail of The quantum confined Stark effect in silicon nanocrystals

Nanotechnology, 2008

The quantum confined Stark effect (QCSE) in Si nanocrystals embedded in a SiO(2) matrix is demons... more The quantum confined Stark effect (QCSE) in Si nanocrystals embedded in a SiO(2) matrix is demonstrated by photoluminescence (PL) spectroscopy at room and cryogenic temperatures. It is shown that the PL peak position shifts to higher wavelengths with increasing applied electric field, which is expected from carrier polarization within the quantum dots. It is observed that the effect is more pronounced at lower temperatures due to the improved carrier localization at the lowest energy states of the quantum dots. Experimental results are shown to be in good agreement with phenomenological model developed for the QCSE model.

Research paper thumbnail of Skin-like self-assembled monolayers on InAs/GaSb superlattice photodetectors

Journal of Physics D: Applied Physics, 2012

We report on the effects of monolayer (ML) thick skin-like octadecanethiol (ODT, CH3[CH2]17SH) on... more We report on the effects of monolayer (ML) thick skin-like octadecanethiol (ODT, CH3[CH2]17SH) on type-II InAs/GaSb MWIR photodetectors. Circumventing the ageing effects of conventional sulfur compounds, we use ODT, a self-assembling, long molecular chain headed with a sulfur atom. Photodiodes coated with and without the self-assembled monolayer (SAM) ODT were compared for their electrical and optical performances. For ODT-coated diodes, the dark current density was improved by two orders of magnitude at 77K under −100mV bias. The zero bias responsivity and detectivity were 1.04AW −1 and 2.15 × 10 13 Jones, respectively, at 4µm and 77K. The quantum efficiency was determined to be 37% for a cutoff wavelength of 5.1µm.

Research paper thumbnail of Effect of laser parameters and post-texturing treatments on the optical and electrical properties of laser textured c-Si wafers

Surface plays a crucial role in the performance of crystalline silicon (cSi) based solar cells as... more Surface plays a crucial role in the performance of crystalline silicon (cSi) based solar cells as it affects both electrical and optical properties. To minimize reflection from the flat surface and thus improve light trapping, the cSi wafers must be textured. For mono-cSi cells, anisotropic alkaline etchants are commonly utilized to create pyramids on the surface. However, this method is not viable for multi-crystalline silicon (mc-Si) wafers due to the presence of different and random crystallographic orientations. In this work, we employed laser texturing, which is an isotropic texturing process, as an alternative texturing method for mc-Si wafers. This approach utilizes a laser process to create pits on the cSi surface. The laser's processing parameters were justified by performing a series of experiments. After texturing, physical (ultrasonic bath with deionized water) and chemical (in KOH with two different concentrations of 1 and 20%) cleanings with different durations were performed which were essential to remove laser-induced damages and other residues from the surface. In order to evaluate the optical response of the textured surfaces, weighted reflection values were measured and correlated with scanning electron microscopy (SEM) images of the textured features before and after post-texturing cleaning step. An impressive low weighted reflection of only 4.2% was measured from laser textured mc-Si with anti-reflection coating after optimizing the laser and post-texturing processes. Moreover, an implied open-circuit voltage (iVoc) of up to 692 mV was achieved by passivating the laser-textured surfaces by Al2O3.

Research paper thumbnail of Advanced plasmonic interfaces for optimized light trapping in photovoltaics

Journal of Physics: Conference Series, 2017

Plasmonic interfaces are integrated to photovoltaic devices to enhance light trapping and improve... more Plasmonic interfaces are integrated to photovoltaic devices to enhance light trapping and improve efficiency. The optimum thickness of the spacer layer used to passivate the absorber layer and adjust its distance from the metal nanoparticles remains unclear. We integrate plasmonic interfaces consisting of Ag nanoparticles and silicon nitride spacers of different thicknesses to the back of a-Si:H absorber to investigate the optimum thickness of the spacer layer and use the photocurrent in a-Si:H to indicate the enhancement in light-trapping. For integration to the back or front of the device, the localized surface plasmon resonance (LSPR) is shifted and broadened into the red with increased spacer layer thickness and the effect is more pronounced for integration to the back. An appreciable enhancement of photocurrent in a-Si:H is consistent with this broadening of LSPR and has a critical dependence on spacer layer thickness.

Research paper thumbnail of Reactively Sputtered Cu2ZnTiS4 Thin Film as Low-Cost Earth-Abundant Absorber

Journal of Electronic Materials, 2017

Cu 2 ZnTiS 4 thin films have been deposited on glass by the reactive cosputtering technique with ... more Cu 2 ZnTiS 4 thin films have been deposited on glass by the reactive cosputtering technique with high-purity ZnS and Cu and Ti metals as targets and H 2 S as reactive gas. Cu 2 ZnTiS 4 thin films were obtained at various temperatures and H 2 S flows and were annealed in H 2 S atmosphere. The structural, morphological, and optical properties of the Cu 2 ZnTiS 4 thin films were examined by scanning electron microscopy, energy-dispersive spectroscopy, x-ray diffraction (XRD) analysis, and ultraviolet-visible (UV-Vis) spectroscopy. Agglomeration was found to increase with increasing temperature. The XRD peaks of the Cu 2 ZnTiS 4 thin films were consistent with those of Cu 2 ZnSnS 4. Furthermore, the optical bandgaps of the Cu 2 ZnTiS 4 films were lower than those of conventional Cu 2 ZnSnS 4 thin films.

Research paper thumbnail of All Solution-Based Fabrication of Copper Oxide Thin Film/Cobalt-Doped Zinc Oxide Nanowire Heterojunctions

Journal of the American Ceramic Society, 2016

Research paper thumbnail of Low temperature aluminum doped zinc oxide thin film deposition on ultra-thin flexible glass and PET substrates by RF magnetron sputtering

physica status solidi (c), 2015

Aluminum doped zinc oxide (AZO) thin films are prepared on ultra-thin flexible glass and flexible... more Aluminum doped zinc oxide (AZO) thin films are prepared on ultra-thin flexible glass and flexible polyethylene terephthalate (PET) substrates at room temperature by radio frequency (RF) magnetron sputtering. Optimization of films has been achieved by varying process parameters to reach the demands of proper conductive layer for thin silicon solar cell applications. Structural analysis of the films was done by X-ray diffraction spectroscopy. Optical and electrical properties of the films were carried out by means of UV-Visible spectroscopy, and four point probe measurements. Thickness of the films was obtained by spectroscopic ellipsometry. Transmission measurements clearly show that by decreasing pressure transmission of the AZO film is improved in the UV region. Our results show that by varying the deposition parameters, low resistivity films of 1.1×10-3 Ωcm and 1×10-3 Ωcm were obtained on PET and on ultra-thin flexible substrates, respectively.

Research paper thumbnail of N-structure based on InAs/AlSb/GaSb superlattice photodetectors

Superlattices and Microstructures, 2015

Research paper thumbnail of High haze nature of textured Al:ZnO with Ag nanoparticles for light management in thin film solar cells

Optical Materials Express, 2015

ABSTRACT We report on fabrication of plasmonic interfaces consisting of Ag nanoparticles on flat ... more ABSTRACT We report on fabrication of plasmonic interfaces consisting of Ag nanoparticles on flat and textured Al:ZnO for use at the front surface of thin film solar cells to enhance light trapping and photo-conversion efficiencies. We show that outstandingly high transmittance haze is achieved from single step HCl surface textured Al:ZnO and demonstrate Ag dewetting on textured and flat Al:ZnO surfaces upon annealing at moderate temperatures. Optical response of these plasmonic interfaces clearly display plasmonic resonances in the visible and near infrared, which is crucial for enhancement of photovoltaic conversion efficiency in thin film solar cells.

Research paper thumbnail of Improved diode properties in zinc telluride thin film-silicon nanowire heterojunctions

Philosophical Magazine, 2015

In this study, structural and optoelectronic properties and photodedection characteristics of dio... more In this study, structural and optoelectronic properties and photodedection characteristics of diodes constructed from p-zinc telluride (ZnTe) thin film/n-silicon (Si) nanowire heterojunctions are reported. Dense arrays of vertically aligned Si nanowires were successfully synthesized on (1 1 0)-oriented n-type single crystalline Si wafer using simple and inexpensive metal-assisted etching (MAE) process. Following the nanowire synthesis, p-type ZnTe thin films were deposited onto vertically oriented Si nanowires via radio frequency magnetron sputtering to form three-dimensional heterojunctions. A comparative study of the structural results obtained from X-ray diffraction and Raman spectroscopy measurements showed the improved crystallinity of the ZnTe thin films deposited onto the Si nanowire arrays. The fabricated nanowire-based heterojunction devices exhibited remarkable diode characteristics and enhanced optoelectronic properties and photosensitivity in comparison to the planar reference. The electrical measurements revealed that the diodes with nanowires had a well-defined rectifying behaviour with a rectification ratio of 104 at ±2 V and a relatively small ideality factor of n = 1.8 with lower reverse leakage current and series resistance at room temperature in dark condition. Moreover, an open-circuit voltage of 100 mV was also observed under illumination. Based on spectral photoresponsivity measurements, the nanowire-based device exhibited a distinct responsivity and high detectivity in visible and near-infrared (NIR) wavelength regions. The device characteristics observed here offer that the fabricated ZnTe thin film/Si nanowire-based p–n heterojunction structures will find important applications in future and will be a promising candidate for high-performance and low-cost optoelectronic device applications, NIR photodedectors in particular.

Research paper thumbnail of Characterization of CdZnTe thin films prepared by magnetron sputtering from a single CdZnTe target

2011 37th IEEE Photovoltaic Specialists Conference, 2011

ABSTRACT

Research paper thumbnail of Low dark current N structure superlattice MWIR photodetectors

SPIE Proceedings, 2014

Commercially available read out integrated circuits (ROICs) require the FPA to have high dynamic ... more Commercially available read out integrated circuits (ROICs) require the FPA to have high dynamic resistance area product at zero bias (R 0 A) which is directly related to dark current of the detector. Dark current arises from bulk and surface contributions. Recent band structure engineering studies significantly suppressed the bulk contribution of the type-II superlattice infrared photodetectors (N structure, M structure, W structure). In this letter, we will present improved dark current results for unipolar barrier complex supercell superlattice system which is called as "N structure". The unique electronic band structure of the N structure increases electron-hole overlap under bias, significantly. N structure aims to improve absorption by manipulating electron and hole wavefunctions that are spatially separated in T2SLs, increasing the absorption while decreasing the dark current. In order to engineer the wavefunctions, we introduce a thin AlSb layer between InAs and GaSb layers in the growth direction which also acts as a unipolar electron barrier. Despite the difficulty of perfect lattice matching of InAs and AlSb, such a design is expected to reduce dark current. Experiments were carried out on Single pixel with mesa sizes of 100 × 100-700 × 700 µm photodiodes. Temperature dependent dark current with corresponding R 0 A resistance values are reported.

Research paper thumbnail of Light management on industrial size c-Si solar cells by Si nanowires fabricated by metal-assisted etching

Research paper thumbnail of Exploration of the horizontally staggered light guides for high concentration CPV applications

Optics Express, 2012

The material and processing costs are still the major drawbacks of the c-Si based photovoltaic (P... more The material and processing costs are still the major drawbacks of the c-Si based photovoltaic (PV) technology. The wafer cost comprises up to 35-40% of the total module cost. New approaches and system designs are needed in order to reduce the share of the wafer cost in photovoltaic energy systems. Here we explore the horizontally staggered light guide solar optics for use in Concentrated Photovoltaic (CPV) applications. This optical system comprises a lens array system coupled to a horizontal light guide which directs the incoming light beam to its edge. We have designed and simulated this system using a commercial ray tracing software (Zemax). The system is more compact, thinner and more robust compared to the conventional CPV systems. Concentration levels as high as 1000x can easily be reached when the system is properly designed. With such a high concentration level, a good acceptance angle of + −1 degree is still be conserved. The analysis of the system reveals that the total optical efficiency of the system could be as high as %94.4 without any antireflection (AR) coating. Optical losses can be reduced by just accommodating a single layer AR coating on the initial lens array leading to a %96.5 optical efficiency. Thermal behavior of high concentration linear concentrator is also discussed and compared with a conventional point focus CPV system.

Research paper thumbnail of Metal Nanoparticles for Plasmonic Solar Cell Applications

Advanced Photonics & Renewable Energy, 2010

ABSTRACT

Research paper thumbnail of Developing a trilayer processing technique for superconducting Y Ba 2 Cu 3 O 7−δ thin films by using Ge ion implantation

Superconductor Science and Technology, 2005

For making trilayer superconducting devices based on YBa 2 Cu 3 O 7−δ (YBCO) thin film processing... more For making trilayer superconducting devices based on YBa 2 Cu 3 O 7−δ (YBCO) thin film processing, we developed a new technique by employing Ge ion implantation. A YBCO thin film of 150 nm thickness having high c-axis orientation and a transition temperature, T c , of 90 K was implanted with 80 keV, 1 × 10 16 Ge ions cm −2 at room temperature. By the result of TRIM calculation, Ge ions were found to penetrate into the YBCO thin film approximately 60 nm below the surface of the film, thus leaving the lower part of the film as a superconductor. Upon implantation with Ge ions, the implanted upper part of the sample lost its electrical conductivity and diamagnetism while its original crystalline structure was preserved. The implanted ions we found did not alter the overall crystal structure of the YBCO thin film; this allowed us to grow an epitaxial superconducting upper layer of YBCO on top of the implanted area, leaving no need to use any buffer layer. The superconducting properties of the upper layer were similar to those of the pure YBCO base layer with an increased room temperature resistivity and a lowered T c (88 K). This process provides an effective method for fabrication of a trilayer HTS device structure.

Research paper thumbnail of Performance of nanowire decorated mono- and multi-crystalline Si solarcells

Physica E: Low-dimensional Systems and Nanostructures, 2013

Research paper thumbnail of Charge retention in quantized energy levels of nanocrystals

Physica E: Low-dimensional Systems and Nanostructures, 2007

Understanding charging mechanisms and charge retention dynamics of nanocrystal memory devices is ... more Understanding charging mechanisms and charge retention dynamics of nanocrystal memory devices is important in optimization of device design. Capacitance spectroscopy on PECVD grown germanium nanocrystals embedded in a silicon oxide matrix was performed. Dynamic measurements of discharge dynamics are carried out. Charge decay is modelled by assuming storage of carriers in the ground states of nanocrystals and that the decay is dominated by direct tunnelling. Discharge rates are calculated using the theoretical model for different nanocrystal sizes and densities and are compared with experimental data. Experimental results agree well with the proposed model and suggest that charge is indeed stored in the quantized energy levels of the nanocrystals.

Research paper thumbnail of Mechanisms of light emission from terbium ions (Tb3+) embedded in a Si rich silicon oxide matrix

Optical Materials, 2012

ABSTRACT a b s t r a c t Mechanisms of light emission in Tb doped Si rich SiO x matrix prepared b... more ABSTRACT a b s t r a c t Mechanisms of light emission in Tb doped Si rich SiO x matrix prepared by magnetron sputtering are stud-ied by photoluminescence spectroscopy (PL). Characteristic PL peaks of Tb 3+ ions and Si nanocrystals are simultaneously observed with an inverse relationship between their intensity. With a prolonged heat treatment at high temperatures, light emission from Tb 3+ ions enhances at the expense of total quenching of the PL signal from the nanocrystals. It is suggested from the annealing studies as a function of process conditions and structural characterization that the light emission from Tb ions is mediated by trap states formed in the band gap of the oxide matrix by Tb x Si y O z complexes or excess Si states. Ó 2012 Elsevier B.V. All rights reserved.

Research paper thumbnail of Effect of γ-radiation on HfO2 based MOS capacitor

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2010

Radiation effects on Metal Oxide Semiconductor (MOS) capacitors with a HfO2 gate insulator have b... more Radiation effects on Metal Oxide Semiconductor (MOS) capacitors with a HfO2 gate insulator have been studied. Because HfO2 is a promising high-k dielectric material for microelectronic applications, radiation effects on its performance in MOS devices is of interest. New results on radiation effects on HfO2, particularly at low gamma radiation doses, are presented. The results are compared with other systems

Research paper thumbnail of The quantum confined Stark effect in silicon nanocrystals

Nanotechnology, 2008

The quantum confined Stark effect (QCSE) in Si nanocrystals embedded in a SiO(2) matrix is demons... more The quantum confined Stark effect (QCSE) in Si nanocrystals embedded in a SiO(2) matrix is demonstrated by photoluminescence (PL) spectroscopy at room and cryogenic temperatures. It is shown that the PL peak position shifts to higher wavelengths with increasing applied electric field, which is expected from carrier polarization within the quantum dots. It is observed that the effect is more pronounced at lower temperatures due to the improved carrier localization at the lowest energy states of the quantum dots. Experimental results are shown to be in good agreement with phenomenological model developed for the QCSE model.

Research paper thumbnail of Skin-like self-assembled monolayers on InAs/GaSb superlattice photodetectors

Journal of Physics D: Applied Physics, 2012

We report on the effects of monolayer (ML) thick skin-like octadecanethiol (ODT, CH3[CH2]17SH) on... more We report on the effects of monolayer (ML) thick skin-like octadecanethiol (ODT, CH3[CH2]17SH) on type-II InAs/GaSb MWIR photodetectors. Circumventing the ageing effects of conventional sulfur compounds, we use ODT, a self-assembling, long molecular chain headed with a sulfur atom. Photodiodes coated with and without the self-assembled monolayer (SAM) ODT were compared for their electrical and optical performances. For ODT-coated diodes, the dark current density was improved by two orders of magnitude at 77K under −100mV bias. The zero bias responsivity and detectivity were 1.04AW −1 and 2.15 × 10 13 Jones, respectively, at 4µm and 77K. The quantum efficiency was determined to be 37% for a cutoff wavelength of 5.1µm.