Sandeep Chandril - Academia.edu (original) (raw)
Papers by Sandeep Chandril
Academia letters, Aug 17, 2021
APS, Mar 1, 2010
ABSTRACT One major challenge in the stoichiometric growth of complex oxides, such as YMnO3, is th... more ABSTRACT One major challenge in the stoichiometric growth of complex oxides, such as YMnO3, is the control of the relative compositions of the constituent materials. Desirable properties of oxide materials, such as ferroelectricity, are highly dependent upon material stoichiometry, making stoichiometry control an important issue. While RHEED (Reflection High Energy Electron Diffraction) analysis is typically used as a qualitative tool, RHEED generated x-rays can be used to give quantitative compositional information. The relative compositions of Y and Mn in MBE grown YMnO3 samples were studied using the grazing exit x-rays generated by RHEED electrons. Comparing the results with RBS characterization suggested that the technique has the potential for real-time compositional analysis.
Bulletin of the American Physical Society, Mar 13, 2008
high-temperature (850 • C) molecular beam epitaxy deposition of YMnO 3 on HVPE GaN(0001) resulted... more high-temperature (850 • C) molecular beam epitaxy deposition of YMnO 3 on HVPE GaN(0001) resulted in the spontaneous formation of YMnO 3 nanoprism structures due to Stranski-Krastanow growth mode. Their dimensions ranged from 20 nm to 60 nm in thickness and 50 nm to 500 nm in lateral size. The local dielectric properties of the sample were investigated using scanning surface potential microscopy (SSPM). Remanent hysteresis loops were used to compare the switching properties of the nanoprisms and underlying film. A larger remanence was observed for the prisms, most probably due to the lack of surrounding film areas that would clamp the ferroelectric response. As a result, the remanent polarization increased roughly linearly with increasing prism surface area.
Foremost, I would like to thank my advisor, Dr. David Lederman, for his constant guidance and sup... more Foremost, I would like to thank my advisor, Dr. David Lederman, for his constant guidance and support. His role has been instrumental in every aspect of the project. He has pulled me out of many tight situations and has maintained a positive attitude throughout. I would also like to thank Dr. Tom Myers for his guidance for the thesis project and, especially, during the early graduate years. Though a lot of work done with Dr. Myers couldn't be a part of this thesis because of totally disjoint topics, it helped me immensely as a researcher. I would also like to thank the other members of my committee for their support and taking the time out of their busy schedules to serve on my committee. I would also like to thank my colleagues, Randy Tompkins, Cameron Keenan and Eric Schires for helping me on countless occasions. Randy, Cameron and Eric helped install the RHEED-TRAXS arm on the oxide system. Randy and Cameron helped perform the initial experiments and Cameron has assisted me with every aspect of the research from growth to analysis. I extend my sincerest thanks to Randy and Cameron for numerous discussions, valuable feedbacks and moral support. I would also like to thank Eric Schires, Felio Perez, Carl Weber, and Doug Matthes for their technical assistance that made the experiments possible. Also Sherry Puskar, Sandy Jones and Siobhan Byrne for doing all the paperwork for us. Thanks to Lillyan Dylla and Phillip Staib from Staib Instruments Inc. for their patience, cooperation and technical support that was vital for the success of the experiments. Most of all, I am grateful to my family and my parents for all the love and support, and believing in me. For that, I am forever indebted to them.
MRS Proceedings, 2005
GaAs 1-x N x layers and quantum dot-like structures were grown on (100) GaAs substrates by molecu... more GaAs 1-x N x layers and quantum dot-like structures were grown on (100) GaAs substrates by molecular beam epitaxy. The dependence of photoluminescence emission spectra on annealing temperature is consistent with literature at lower temperatures but after annealing at 750 º C a net red-shift is consistently observed. X-ray photoelectron spectroscopy measurements indicate that for different annealing times and temperatures, the nitrogen and arsenic surface concentrations changed compared to that of as-grown samples, specifically arsenic is lost from the material. Raman measurements are consistent with the trends in photoluminescence and also suggest the loss of arsenic occurs at higher annealing temperatures in both samples capped with GaAs and uncapped samples.
Journal of Applied Physics, Jun 1, 2011
A major challenge in the stoichiometric growth of complex oxide compounds is the control of the r... more A major challenge in the stoichiometric growth of complex oxide compounds is the control of the relative compositions of the constituent materials. A potential avenue for compositional analysis during growth is the use of x-ray fluorescence generated during reflection high energy electron diffraction measurements. Using this technique, relative compositions of Y and Mn in molecular beam epitaxy grown YMnO3 samples were studied. Comparing the results with Rutherford back scattering spectroscopy suggests that the technique has the potential for real-time analysis of elemental fluxes and stoichiometry control during sample growth.
In the present work large area 156.75 mm by 156.75 mm crystalline silicon passivated emitter and ... more In the present work large area 156.75 mm by 156.75 mm crystalline silicon passivated emitter and rear contact (PERC) solar cells are examined at the cell level and at the module level as well. 21% efficient mono crystalline PERC cells are cross sectioned and observed under microscope for local back surface field (LBSF) formation and laser contact opened regions. Quantum efficiency testing was done to see the photo current generated for different wavelengths. Finally the moduling of 72 cells were done and the IV measurements of PERC module before sun soaking and after sun soaking was done to see the effect of light induced degradation (LID) in PERC modules.
Journal of Applied Physics, Jul 15, 2009
Porous, high-surface-area electrode architectures are described that allow structural characteriz... more Porous, high-surface-area electrode architectures are described that allow structural characterization of interfacial amorphous thin films with high spatial resolution under device-relevant functional electrochemical conditions using high-energy X-ray (>50 keV) scattering and pair distribution function (PDF) analysis. Porous electrodes were fabricated from glass-capillary array membranes coated with conformal transparent conductive oxide layers, consisting of either a 40 nm-50 nm crystalline indium tin oxide or a 100 nm-150 nm-thick amorphous indium zinc oxide deposited by atomic layer deposition. These porous electrodes solve the problem of insufficient interaction volumes for catalyst thin films in two-dimensional working electrode designs and provide sufficiently low scattering backgrounds to enable high-resolution signal collection from interfacial thin-film catalysts. For example, PDF measurements were readily obtained with 0.2 Å spatial resolution for amorphous cobalt oxide films with thicknesses down to 60 nm when deposited on a porous electrode with 40 mm-diameter pores. This level of resolution resolves the cobaltate domain size and structure, the presence of defect sites assigned to the domain edges, and the changes in fine structure upon redox state change that are relevant to quantitative structure-function modeling. The results suggest the opportunity to leverage the porous, electrode architectures for PDF analysis of nanometrescale surface-supported molecular catalysts. In addition, a compact 3D-printed electrochemical cell in a three-electrode configuration is described which is designed to allow for simultaneous X-ray transmission and electrolyte flow through the porous working electrode.
Physica status solidi, Jan 23, 2008
Determining fundamental properties of InN is beset with difficulties. Since a conducting surface ... more Determining fundamental properties of InN is beset with difficulties. Since a conducting surface accumulation layer creates difficulties when assessing the impact of dopants, variable magnetic field Hall effect measurements were performed to separate various conducting layers. In addition to verifying p‐type doping with Mg, variable magnetic field Hall effect was used to show that growth of InN on YSZ will greatly reduce the background n‐type concentration for thinner layers, (with n ∼ 2×1017 cm−3, the lowest achieved on half micron thick material), making microns‐thick growth less important than the typical method of growth on sapphire with GaN buffer layers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Bulletin of the American Physical Society, Mar 16, 2009
Submitted for the MAR09 Meeting of The American Physical Society In-situ chemical and structural ... more Submitted for the MAR09 Meeting of The American Physical Society In-situ chemical and structural characterization via RHEED-total reflection analysis of x-rays (TRAXS). 1 SANDEEP CHANDRIL, CAMERON KEENAN, THOMAS MYERS, DAVID LEDERMAN, West Virginia University-The use of x-ray fluorescence produced inside a molecular beam epitaxy chamber by the RHEED electron gun to simultaneously characterize the thin films for thickness, roughness and the chemical composition is described. This technique requires only slight modifications to the chamber and can be a powerful tool for beam flux calibration and in-situ analysis, especially where surfaces have to be protected under vacuum and for the stoichiometry control during growth. The angular dependence of the x-ray fluorescence signal from the thin film over the substrate is analyzed using Parratt's approach and simulating electrons' trajectories inside the film to account for grazing angle electron beam as a source for x-rays. We have found good agreement between the experiment and the theory for the thickness and roughness estimates. Experiments for chemical composition determination are currently underway.
Applied Physics Letters, 2008
Growth of YMnO3 on GaN (0001) using molecular beam epitaxy at temperatures greater than 850°C res... more Growth of YMnO3 on GaN (0001) using molecular beam epitaxy at temperatures greater than 850°C resulted in the spontaneous formation of crystalline prisms, ranging from 20to60nm in height and 50to500nm in lateral size, surrounded by a 6nm thick continuous YMnO3 film. The local dielectric properties were measured using scanning surface probe microscopy. The prisms were ferroelectric at room temperature and their ferroelectric properties were enhanced for taller prisms. This is consistent with these structures being less constrained than the continuous layer, which is clamped by the surrounding unpolarized film.
Journal of Electronic Materials, Jun 1, 2006
Variable magnetic field Hall and transient photoconductance lifetime measurements were performed ... more Variable magnetic field Hall and transient photoconductance lifetime measurements were performed on a series of undoped, In-doped, and As-doped HgCdTe samples grown by molecular beam epitaxy and metalorganic chemical vapor deposition. Temperature variation and, in the case of Hall, magnetic-field variation are needed to give a more complete picture of the mechanisms that control lifetimes in HgCdTe samples. Recent predictions of recombination lifetimes from full band structure calculations were compared to experimental lifetimes at various doping levels at long-wave infrared (LWIR) and mid-wave infrared (MWIR) compositions. For n-type material, lifetimes from low doping levels fall well below the predictions, implying that Shockley-Read-Hall (SRH) recombination is still dominant. MWIR samples have a lifetime that increases somewhat with carrier concentration, suggesting that In doping passivates the SRH defects for that composition. Lifetimes in p-type MWIR material appear to be well-explained by recent theoretical calculations. In p-type material, trapping states may be introduced during the incorporation and activation of As, since some samples with unusually long lifetimes had a distinctly different type of temperature dependence.
Applied Physics Letters, Oct 30, 2006
Variable magnetic field Hall effect, photoluminescence, and capacitance-voltage ͑CV͒ analysis hav... more Variable magnetic field Hall effect, photoluminescence, and capacitance-voltage ͑CV͒ analysis have been used to study InN layers grown by plasma assisted molecular beam epitaxy. All three techniques reveal evidence of a buried p-type layer beneath a surface electron accumulation layer in heavily Mg-doped samples. Early indications suggest the Mg acceptor level in InN may lie near 110 meV above the valence band maximum. The development of p-type doping techniques offers great promise for future InN based devices.
Journal of Crystal Growth, Jun 1, 2007
Bi was investigated as a possible surfactant for growth of GaAs 1Àx N x layers on (1 0 0) GaAs su... more Bi was investigated as a possible surfactant for growth of GaAs 1Àx N x layers on (1 0 0) GaAs substrates by molecular beam epitaxy (MBE) using a radio frequency (RF) plasma nitrogen source. Importantly, Bi extends the useable growth conditions producing smoother surfaces to a significantly higher group V fractional N content than without Bi, enhancing possibilities for growth of structures requiring a larger nitrogen content. The conductivity of Be-doped GaAsN and GaInAsN decreased significantly with increasing N concentration. Temperature-dependent Hall measurement suggests possible compensation and increased activation energy. SIMS and Raman measurements indicate that the N composition increased with introducing Be, and for low [N], with the presence of Bi. The addition of Bi during growth of Be-doped GaAsN only produced semi-insulating layers at all concentrations investigated suggesting it enhances the formation of compensating defects.
IEEE Journal of Photovoltaics, Jul 1, 2020
Light trapping and current losses are critical factors for the performance of a solar cell. The a... more Light trapping and current losses are critical factors for the performance of a solar cell. The analysis of the light trapping is generally carried out by the optical path length and back reflectance estimations using the inverse internal quantum efficiency (IQE) approach or the simulation methods. Simulation tools, such as PC1D or wafer ray tracers, provide an improved estimation as compared with the inverse IQE method but are time-consuming. Another approach for light trapping based on the statistical ray optics has been reported in earlier literature, providing a practical limit against the 4n2 limit. This approach has been studied and compared with the well-studied inverse IQE approach. It is further modified for application to the widely available silicon solar cell structures. Also, the current loss analysis has been carried out using the proposed method that separates the optical losses from the electrical losses in the long-wavelength range without the use of simulation. Further, the accuracy of back reflectance values determined using analytical methods, including the inverse IQE method, is examined using PC1D simulations and the current loss analysis.
Bulletin of the American Physical Society, Mar 10, 2008
International Journal of Renewable Energy Research, 2013
The effect of process parameters such as saw damage removal, gas dilution and silane depletion ar... more The effect of process parameters such as saw damage removal, gas dilution and silane depletion are studied to maximize the efficiency of silicon a-Si/c-Si heterojuction solar cells. Furthermore, the effect of chemical treatment and annealing are also studied to understand their effect on performance of silicon heterojunction solar cells. A smooth and defect free junction is required to exploit the heterojunction properties of the junction and produce high open circuit voltage. Hence a smooth c-Si surface and the incorporation and migration of hydrogen in the amorphous silicon layers, especially the intrinsic amorphous layer, are very important. The process parameters affecting these properties have been studied and direct relation between the process parameters and the cell performance has been found.
ABSTRACT One major challenge in the stoichiometric growth of complex oxides, such as YMnO3, is th... more ABSTRACT One major challenge in the stoichiometric growth of complex oxides, such as YMnO3, is the control of the relative compositions of the constituent materials. Desirable properties of oxide materials, such as ferroelectricity, are highly dependent upon material stoichiometry, making stoichiometry control an important issue. While RHEED (Reflection High Energy Electron Diffraction) analysis is typically used as a qualitative tool, RHEED generated x-rays can be used to give quantitative compositional information. The relative compositions of Y and Mn in MBE grown YMnO3 samples were studied using the grazing exit x-rays generated by RHEED electrons. Comparing the results with RBS characterization suggested that the technique has the potential for real-time compositional analysis.
ABSTRACT RHEED-total reflection x-ray spectroscopy (-TRAXS) is an in-situ chemical and structural... more ABSTRACT RHEED-total reflection x-ray spectroscopy (-TRAXS) is an in-situ chemical and structural characterization technique which is highly surface sensitive. This consists of a grazing-angle electron beam from which characteristic x-rays from the sample are measured also at grazing angles. We have demonstrated that monolayer sensitivity in Y and Mn films on GaN can be achieved. We have also developed a theoretical model for the angular dependence of the x-ray Kalpha peaks for the thin films, based on Parratt's formalism for x-ray reflectivity and the electron trajectory simulation software CASINO, to correct for grazing angle electron beam as a source for x-rays. As the angular dependence is highly dependent upon the film thickness and the smoothness of the film, it can be used to determine the deposition rate of individual elements as well as the interface chemical roughness
Academia letters, Aug 17, 2021
APS, Mar 1, 2010
ABSTRACT One major challenge in the stoichiometric growth of complex oxides, such as YMnO3, is th... more ABSTRACT One major challenge in the stoichiometric growth of complex oxides, such as YMnO3, is the control of the relative compositions of the constituent materials. Desirable properties of oxide materials, such as ferroelectricity, are highly dependent upon material stoichiometry, making stoichiometry control an important issue. While RHEED (Reflection High Energy Electron Diffraction) analysis is typically used as a qualitative tool, RHEED generated x-rays can be used to give quantitative compositional information. The relative compositions of Y and Mn in MBE grown YMnO3 samples were studied using the grazing exit x-rays generated by RHEED electrons. Comparing the results with RBS characterization suggested that the technique has the potential for real-time compositional analysis.
Bulletin of the American Physical Society, Mar 13, 2008
high-temperature (850 • C) molecular beam epitaxy deposition of YMnO 3 on HVPE GaN(0001) resulted... more high-temperature (850 • C) molecular beam epitaxy deposition of YMnO 3 on HVPE GaN(0001) resulted in the spontaneous formation of YMnO 3 nanoprism structures due to Stranski-Krastanow growth mode. Their dimensions ranged from 20 nm to 60 nm in thickness and 50 nm to 500 nm in lateral size. The local dielectric properties of the sample were investigated using scanning surface potential microscopy (SSPM). Remanent hysteresis loops were used to compare the switching properties of the nanoprisms and underlying film. A larger remanence was observed for the prisms, most probably due to the lack of surrounding film areas that would clamp the ferroelectric response. As a result, the remanent polarization increased roughly linearly with increasing prism surface area.
Foremost, I would like to thank my advisor, Dr. David Lederman, for his constant guidance and sup... more Foremost, I would like to thank my advisor, Dr. David Lederman, for his constant guidance and support. His role has been instrumental in every aspect of the project. He has pulled me out of many tight situations and has maintained a positive attitude throughout. I would also like to thank Dr. Tom Myers for his guidance for the thesis project and, especially, during the early graduate years. Though a lot of work done with Dr. Myers couldn't be a part of this thesis because of totally disjoint topics, it helped me immensely as a researcher. I would also like to thank the other members of my committee for their support and taking the time out of their busy schedules to serve on my committee. I would also like to thank my colleagues, Randy Tompkins, Cameron Keenan and Eric Schires for helping me on countless occasions. Randy, Cameron and Eric helped install the RHEED-TRAXS arm on the oxide system. Randy and Cameron helped perform the initial experiments and Cameron has assisted me with every aspect of the research from growth to analysis. I extend my sincerest thanks to Randy and Cameron for numerous discussions, valuable feedbacks and moral support. I would also like to thank Eric Schires, Felio Perez, Carl Weber, and Doug Matthes for their technical assistance that made the experiments possible. Also Sherry Puskar, Sandy Jones and Siobhan Byrne for doing all the paperwork for us. Thanks to Lillyan Dylla and Phillip Staib from Staib Instruments Inc. for their patience, cooperation and technical support that was vital for the success of the experiments. Most of all, I am grateful to my family and my parents for all the love and support, and believing in me. For that, I am forever indebted to them.
MRS Proceedings, 2005
GaAs 1-x N x layers and quantum dot-like structures were grown on (100) GaAs substrates by molecu... more GaAs 1-x N x layers and quantum dot-like structures were grown on (100) GaAs substrates by molecular beam epitaxy. The dependence of photoluminescence emission spectra on annealing temperature is consistent with literature at lower temperatures but after annealing at 750 º C a net red-shift is consistently observed. X-ray photoelectron spectroscopy measurements indicate that for different annealing times and temperatures, the nitrogen and arsenic surface concentrations changed compared to that of as-grown samples, specifically arsenic is lost from the material. Raman measurements are consistent with the trends in photoluminescence and also suggest the loss of arsenic occurs at higher annealing temperatures in both samples capped with GaAs and uncapped samples.
Journal of Applied Physics, Jun 1, 2011
A major challenge in the stoichiometric growth of complex oxide compounds is the control of the r... more A major challenge in the stoichiometric growth of complex oxide compounds is the control of the relative compositions of the constituent materials. A potential avenue for compositional analysis during growth is the use of x-ray fluorescence generated during reflection high energy electron diffraction measurements. Using this technique, relative compositions of Y and Mn in molecular beam epitaxy grown YMnO3 samples were studied. Comparing the results with Rutherford back scattering spectroscopy suggests that the technique has the potential for real-time analysis of elemental fluxes and stoichiometry control during sample growth.
In the present work large area 156.75 mm by 156.75 mm crystalline silicon passivated emitter and ... more In the present work large area 156.75 mm by 156.75 mm crystalline silicon passivated emitter and rear contact (PERC) solar cells are examined at the cell level and at the module level as well. 21% efficient mono crystalline PERC cells are cross sectioned and observed under microscope for local back surface field (LBSF) formation and laser contact opened regions. Quantum efficiency testing was done to see the photo current generated for different wavelengths. Finally the moduling of 72 cells were done and the IV measurements of PERC module before sun soaking and after sun soaking was done to see the effect of light induced degradation (LID) in PERC modules.
Journal of Applied Physics, Jul 15, 2009
Porous, high-surface-area electrode architectures are described that allow structural characteriz... more Porous, high-surface-area electrode architectures are described that allow structural characterization of interfacial amorphous thin films with high spatial resolution under device-relevant functional electrochemical conditions using high-energy X-ray (>50 keV) scattering and pair distribution function (PDF) analysis. Porous electrodes were fabricated from glass-capillary array membranes coated with conformal transparent conductive oxide layers, consisting of either a 40 nm-50 nm crystalline indium tin oxide or a 100 nm-150 nm-thick amorphous indium zinc oxide deposited by atomic layer deposition. These porous electrodes solve the problem of insufficient interaction volumes for catalyst thin films in two-dimensional working electrode designs and provide sufficiently low scattering backgrounds to enable high-resolution signal collection from interfacial thin-film catalysts. For example, PDF measurements were readily obtained with 0.2 Å spatial resolution for amorphous cobalt oxide films with thicknesses down to 60 nm when deposited on a porous electrode with 40 mm-diameter pores. This level of resolution resolves the cobaltate domain size and structure, the presence of defect sites assigned to the domain edges, and the changes in fine structure upon redox state change that are relevant to quantitative structure-function modeling. The results suggest the opportunity to leverage the porous, electrode architectures for PDF analysis of nanometrescale surface-supported molecular catalysts. In addition, a compact 3D-printed electrochemical cell in a three-electrode configuration is described which is designed to allow for simultaneous X-ray transmission and electrolyte flow through the porous working electrode.
Physica status solidi, Jan 23, 2008
Determining fundamental properties of InN is beset with difficulties. Since a conducting surface ... more Determining fundamental properties of InN is beset with difficulties. Since a conducting surface accumulation layer creates difficulties when assessing the impact of dopants, variable magnetic field Hall effect measurements were performed to separate various conducting layers. In addition to verifying p‐type doping with Mg, variable magnetic field Hall effect was used to show that growth of InN on YSZ will greatly reduce the background n‐type concentration for thinner layers, (with n ∼ 2×1017 cm−3, the lowest achieved on half micron thick material), making microns‐thick growth less important than the typical method of growth on sapphire with GaN buffer layers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Bulletin of the American Physical Society, Mar 16, 2009
Submitted for the MAR09 Meeting of The American Physical Society In-situ chemical and structural ... more Submitted for the MAR09 Meeting of The American Physical Society In-situ chemical and structural characterization via RHEED-total reflection analysis of x-rays (TRAXS). 1 SANDEEP CHANDRIL, CAMERON KEENAN, THOMAS MYERS, DAVID LEDERMAN, West Virginia University-The use of x-ray fluorescence produced inside a molecular beam epitaxy chamber by the RHEED electron gun to simultaneously characterize the thin films for thickness, roughness and the chemical composition is described. This technique requires only slight modifications to the chamber and can be a powerful tool for beam flux calibration and in-situ analysis, especially where surfaces have to be protected under vacuum and for the stoichiometry control during growth. The angular dependence of the x-ray fluorescence signal from the thin film over the substrate is analyzed using Parratt's approach and simulating electrons' trajectories inside the film to account for grazing angle electron beam as a source for x-rays. We have found good agreement between the experiment and the theory for the thickness and roughness estimates. Experiments for chemical composition determination are currently underway.
Applied Physics Letters, 2008
Growth of YMnO3 on GaN (0001) using molecular beam epitaxy at temperatures greater than 850°C res... more Growth of YMnO3 on GaN (0001) using molecular beam epitaxy at temperatures greater than 850°C resulted in the spontaneous formation of crystalline prisms, ranging from 20to60nm in height and 50to500nm in lateral size, surrounded by a 6nm thick continuous YMnO3 film. The local dielectric properties were measured using scanning surface probe microscopy. The prisms were ferroelectric at room temperature and their ferroelectric properties were enhanced for taller prisms. This is consistent with these structures being less constrained than the continuous layer, which is clamped by the surrounding unpolarized film.
Journal of Electronic Materials, Jun 1, 2006
Variable magnetic field Hall and transient photoconductance lifetime measurements were performed ... more Variable magnetic field Hall and transient photoconductance lifetime measurements were performed on a series of undoped, In-doped, and As-doped HgCdTe samples grown by molecular beam epitaxy and metalorganic chemical vapor deposition. Temperature variation and, in the case of Hall, magnetic-field variation are needed to give a more complete picture of the mechanisms that control lifetimes in HgCdTe samples. Recent predictions of recombination lifetimes from full band structure calculations were compared to experimental lifetimes at various doping levels at long-wave infrared (LWIR) and mid-wave infrared (MWIR) compositions. For n-type material, lifetimes from low doping levels fall well below the predictions, implying that Shockley-Read-Hall (SRH) recombination is still dominant. MWIR samples have a lifetime that increases somewhat with carrier concentration, suggesting that In doping passivates the SRH defects for that composition. Lifetimes in p-type MWIR material appear to be well-explained by recent theoretical calculations. In p-type material, trapping states may be introduced during the incorporation and activation of As, since some samples with unusually long lifetimes had a distinctly different type of temperature dependence.
Applied Physics Letters, Oct 30, 2006
Variable magnetic field Hall effect, photoluminescence, and capacitance-voltage ͑CV͒ analysis hav... more Variable magnetic field Hall effect, photoluminescence, and capacitance-voltage ͑CV͒ analysis have been used to study InN layers grown by plasma assisted molecular beam epitaxy. All three techniques reveal evidence of a buried p-type layer beneath a surface electron accumulation layer in heavily Mg-doped samples. Early indications suggest the Mg acceptor level in InN may lie near 110 meV above the valence band maximum. The development of p-type doping techniques offers great promise for future InN based devices.
Journal of Crystal Growth, Jun 1, 2007
Bi was investigated as a possible surfactant for growth of GaAs 1Àx N x layers on (1 0 0) GaAs su... more Bi was investigated as a possible surfactant for growth of GaAs 1Àx N x layers on (1 0 0) GaAs substrates by molecular beam epitaxy (MBE) using a radio frequency (RF) plasma nitrogen source. Importantly, Bi extends the useable growth conditions producing smoother surfaces to a significantly higher group V fractional N content than without Bi, enhancing possibilities for growth of structures requiring a larger nitrogen content. The conductivity of Be-doped GaAsN and GaInAsN decreased significantly with increasing N concentration. Temperature-dependent Hall measurement suggests possible compensation and increased activation energy. SIMS and Raman measurements indicate that the N composition increased with introducing Be, and for low [N], with the presence of Bi. The addition of Bi during growth of Be-doped GaAsN only produced semi-insulating layers at all concentrations investigated suggesting it enhances the formation of compensating defects.
IEEE Journal of Photovoltaics, Jul 1, 2020
Light trapping and current losses are critical factors for the performance of a solar cell. The a... more Light trapping and current losses are critical factors for the performance of a solar cell. The analysis of the light trapping is generally carried out by the optical path length and back reflectance estimations using the inverse internal quantum efficiency (IQE) approach or the simulation methods. Simulation tools, such as PC1D or wafer ray tracers, provide an improved estimation as compared with the inverse IQE method but are time-consuming. Another approach for light trapping based on the statistical ray optics has been reported in earlier literature, providing a practical limit against the 4n2 limit. This approach has been studied and compared with the well-studied inverse IQE approach. It is further modified for application to the widely available silicon solar cell structures. Also, the current loss analysis has been carried out using the proposed method that separates the optical losses from the electrical losses in the long-wavelength range without the use of simulation. Further, the accuracy of back reflectance values determined using analytical methods, including the inverse IQE method, is examined using PC1D simulations and the current loss analysis.
Bulletin of the American Physical Society, Mar 10, 2008
International Journal of Renewable Energy Research, 2013
The effect of process parameters such as saw damage removal, gas dilution and silane depletion ar... more The effect of process parameters such as saw damage removal, gas dilution and silane depletion are studied to maximize the efficiency of silicon a-Si/c-Si heterojuction solar cells. Furthermore, the effect of chemical treatment and annealing are also studied to understand their effect on performance of silicon heterojunction solar cells. A smooth and defect free junction is required to exploit the heterojunction properties of the junction and produce high open circuit voltage. Hence a smooth c-Si surface and the incorporation and migration of hydrogen in the amorphous silicon layers, especially the intrinsic amorphous layer, are very important. The process parameters affecting these properties have been studied and direct relation between the process parameters and the cell performance has been found.
ABSTRACT One major challenge in the stoichiometric growth of complex oxides, such as YMnO3, is th... more ABSTRACT One major challenge in the stoichiometric growth of complex oxides, such as YMnO3, is the control of the relative compositions of the constituent materials. Desirable properties of oxide materials, such as ferroelectricity, are highly dependent upon material stoichiometry, making stoichiometry control an important issue. While RHEED (Reflection High Energy Electron Diffraction) analysis is typically used as a qualitative tool, RHEED generated x-rays can be used to give quantitative compositional information. The relative compositions of Y and Mn in MBE grown YMnO3 samples were studied using the grazing exit x-rays generated by RHEED electrons. Comparing the results with RBS characterization suggested that the technique has the potential for real-time compositional analysis.
ABSTRACT RHEED-total reflection x-ray spectroscopy (-TRAXS) is an in-situ chemical and structural... more ABSTRACT RHEED-total reflection x-ray spectroscopy (-TRAXS) is an in-situ chemical and structural characterization technique which is highly surface sensitive. This consists of a grazing-angle electron beam from which characteristic x-rays from the sample are measured also at grazing angles. We have demonstrated that monolayer sensitivity in Y and Mn films on GaN can be achieved. We have also developed a theoretical model for the angular dependence of the x-ray Kalpha peaks for the thin films, based on Parratt's formalism for x-ray reflectivity and the electron trajectory simulation software CASINO, to correct for grazing angle electron beam as a source for x-rays. As the angular dependence is highly dependent upon the film thickness and the smoothness of the film, it can be used to determine the deposition rate of individual elements as well as the interface chemical roughness