Geeta Mutta | Heriot-Watt University (original) (raw)
Papers by Geeta Mutta
In pursuit of an abundant, inexpensive and stable counter electrode as an alternative to platinum... more In pursuit of an abundant, inexpensive and stable counter electrode as an alternative to platinum for dye-sensitized solar cells (DSSCs), we report a new, low-cost substitute material. Here for the first time, we demonstrate that V 2 O 5 can be used as a counter electrode material in DSSCs. We note that the efficiency of DSSCs with commercial V 2 O 5 and hydrothermal treated V 2 O 5 are upto 1.2% and 1.6%, respectively. The results indicate that, with optimization, V 2 O 5 can be a promising choice to replace platinum from a cost perspective. The innovation of new economical counter electrodes offers a potential way to cut down the industrial costs which is crucial for large-scale production and commercial applications of DSSCs.
Due to its small band-gap and its high mobility, InN is a promising material for a large number o... more Due to its small band-gap and its high mobility, InN is a promising material for a large number of key applications like band-gap engineering for high efficiency solar cells, light emitting diodes, and high speed devices. Unfortunately, it has been reported that this material exhibits strong surface charge accumulation which may depend on the type of surface. Current investigations are conducted in order to explain the mechanisms which govern such a behavior and to look for ways of avoiding it and/or finding applications that may use such an effect. In this framework, low frequency noise measurements have been performed at different temperatures on patterned MBE grown InN layers. The evolution of the 1/f noise level with temperature in the 77 K-300 K range is consistent with carrier number fluctuations thus indicating surface mechanisms: the surface charge accumulation is confirmed by the noise measurements.
Semiconductor Science and Technology, 2014
This paper reports on a case study of the impact of fabrication steps on InN material properties.... more This paper reports on a case study of the impact of fabrication steps on InN material properties. We discuss the influence of annealing time and sequence of device processing steps. Photoluminescence (PL), surface morphology and electrical transport (electrical resistivity and low frequency noise) properties have been studied as responses to the adopted fabrication steps. Surface morphology has a strong correlation with annealing times, while sequences of fabrication steps do not appear to be influential. In contrast, the optical and electrical properties demonstrate correlation with both etching and thermal annealing. For all the studied samples PL peaks were in the vicinity of 0.7 eV, but the intensity and full width at half maximum (FWHM) demonstrate a dependence on the technological steps followed. Sheet resistance and electrical resistivity seem to be lower in the case of high defect introduction due to both etching and thermal treatments. The same effect is revealed through 1/f noise level measurements. A reduction of electrical resistivity is connected to an increase in 1/f noise level.
Electrical and structural properties of extended defects including threading dislocations/V-defec... more Electrical and structural properties of extended defects including threading dislocations/V-defects and nanopipes in unintentionally doped
GaN, InGaN (50 nm)/GaN and AlInN (33 nm)/AlN(1 nm)/GaN heterostructures have been investigated by means of various scanning probe (Kelvin
probe and conductive-Atomic Force Microscopy) and electron beam (electron beam induced current and transmission electron microscopy) microscopy
methods. Due to low energy measurements of Kelvin probe force microscopy, charge state of the dislocations have been correctly identified
where threading dislocations (TDs) with screw-component are negatively charged, while pure-edge type TDs are neutral in InGaN/GaN. It is
explained how various factors such as indium segregation, surface termination, presence of vacancies and/or impurities affect the electrical charge,
conductivity and recombination properties of the extended defects. They are found to be strongly correlated to the type of dislocations as identified
from TEM
We report here on the micro structural, electronic and optical properties of a GaN-based InGaN/Ga... more We report here on the micro structural, electronic and optical properties of a GaN-based InGaN/GaN MQW LED grown by the MOVPE method. The present study shows that the threading dislocations present in these LED structures are terminated as V pits at the surface and have an impact on the electrical and optical activity of these devices. It has been pointed that these dislocations were of edge, screw and mixed types. EBIC maps suggest that the electrically active defects are screw and mixed dislocations and behave as nonradiative recombinant centres.
Semiconductor Science and Technology
This paper reports on a case study of the impact of fabrication steps on InN material properties.... more This paper reports on a case study of the impact of fabrication steps on InN material properties. We discuss the influence of annealing time and sequence of device processing steps. Photoluminescence (PL), surface morphology and electrical transport (electrical resistivity and low frequency noise) properties have been studied as responses to the adopted fabrication steps. Surface morphology has a strong correlation with annealing times, while sequences of fabrication steps do not appear to be influential. In contrast, the optical and electrical properties demonstrate correlation with both etching and thermal annealing. For all the studied samples PL peaks were in the vicinity of 0.7 eV, but the intensity and full width at half maximum (FWHM) demonstrate a dependence on the technological steps followed. Sheet resistance and electrical resistivity seem to be lower in the case of high defect introduction due to both etching and thermal treatments. The same effect is revealed through 1/f noise level measurements. A reduction of electrical resistivity is connected to an increase in 1/f noise level.
Physica Status Solidi A-applications and Materials Science, 2010
An investigation of InN layers grown on GaN templates by molecular beam epitaxy (MBE) has been ca... more An investigation of InN layers grown on GaN templates by molecular beam epitaxy (MBE) has been carried out by X-ray diffraction (XRD), Raman spectroscopy (RS) and photoluminescence (PL). A good correlation is noticed between their crystalline quality and optical properties. The best samples exhibit a PL emission between 0.6 and 0.7 eV. The surface structure was quite different from one sample to the other, pointing out to a critical role of the growth conditions, which probably need to be tightly optimized for a good reproducibility.
Physica Status Solidi B-basic Solid State Physics, 2011
Phone: þ00 332 31 45 26 53, Fax: þ00 332 31 45 25 57
Applied Physics Letters, 2011
We report on the nonlinear optical absorption of InN/InxGa1-xN (x=0.8,0.9) multiple-quantum-well ... more We report on the nonlinear optical absorption of InN/InxGa1-xN (x=0.8,0.9) multiple-quantum-well structures characterized at 1.55 μm by the Z-scan method in order to obtain the effective nonlinear absorption coefficient (α2) of the samples at high repetition rate. Saturable absorption is observed for the sample with x=0.9, with an effective α2~-9×103 cm/GW for the studied optical regime. For lower In content in the barrier, reverse saturable absorption is observed, which is attributed to two-photon absorption.
Applied Physics Letters, 2011
Bulk conduction in molecular beam epitaxial InN layers has been confirmed using low frequency noi... more Bulk conduction in molecular beam epitaxial InN layers has been confirmed using low frequency noise measurements versus temperature. A generation-recombination process has been identified at low temperatures Յ100 K and attributed to a trap with a discrete energy level in the band gap. The energy position of this trap has been determined to be around 52 meV below the conduction band minimum.
Due to its small band-gap and its high mobility, InN is a promising material for a large number o... more Due to its small band-gap and its high mobility, InN is a promising material for a large number of key applications like band-gap engineering for high efficiency solar cells, light emitting diodes, and high speed devices. Unfortunately, it has been reported that this material exhibits strong surface charge accumulation which may depend on the type of surface. Current investigations are conducted in order to explain the mechanisms which govern such a behavior and to look for ways of avoiding it and/or finding applications that may use such an effect. In this framework, low frequency noise measurements have been performed at different temperatures on patterned MBE grown InN layers. The evolution of the 1/f noise level with temperature in the 77 K-300 K range is consistent with carrier number fluctuations thus indicating surface mechanisms: the surface charge accumulation is confirmed by the noise measurements.
In pursuit of an abundant, inexpensive and stable counter electrode as an alternative to platinum... more In pursuit of an abundant, inexpensive and stable counter electrode as an alternative to platinum for dye-sensitized solar cells (DSSCs), we report a new, low-cost substitute material. Here for the first time, we demonstrate that V 2 O 5 can be used as a counter electrode material in DSSCs. We note that the efficiency of DSSCs with commercial V 2 O 5 and hydrothermal treated V 2 O 5 are upto 1.2% and 1.6%, respectively. The results indicate that, with optimization, V 2 O 5 can be a promising choice to replace platinum from a cost perspective. The innovation of new economical counter electrodes offers a potential way to cut down the industrial costs which is crucial for large-scale production and commercial applications of DSSCs.
Due to its small band-gap and its high mobility, InN is a promising material for a large number o... more Due to its small band-gap and its high mobility, InN is a promising material for a large number of key applications like band-gap engineering for high efficiency solar cells, light emitting diodes, and high speed devices. Unfortunately, it has been reported that this material exhibits strong surface charge accumulation which may depend on the type of surface. Current investigations are conducted in order to explain the mechanisms which govern such a behavior and to look for ways of avoiding it and/or finding applications that may use such an effect. In this framework, low frequency noise measurements have been performed at different temperatures on patterned MBE grown InN layers. The evolution of the 1/f noise level with temperature in the 77 K-300 K range is consistent with carrier number fluctuations thus indicating surface mechanisms: the surface charge accumulation is confirmed by the noise measurements.
Semiconductor Science and Technology, 2014
This paper reports on a case study of the impact of fabrication steps on InN material properties.... more This paper reports on a case study of the impact of fabrication steps on InN material properties. We discuss the influence of annealing time and sequence of device processing steps. Photoluminescence (PL), surface morphology and electrical transport (electrical resistivity and low frequency noise) properties have been studied as responses to the adopted fabrication steps. Surface morphology has a strong correlation with annealing times, while sequences of fabrication steps do not appear to be influential. In contrast, the optical and electrical properties demonstrate correlation with both etching and thermal annealing. For all the studied samples PL peaks were in the vicinity of 0.7 eV, but the intensity and full width at half maximum (FWHM) demonstrate a dependence on the technological steps followed. Sheet resistance and electrical resistivity seem to be lower in the case of high defect introduction due to both etching and thermal treatments. The same effect is revealed through 1/f noise level measurements. A reduction of electrical resistivity is connected to an increase in 1/f noise level.
Electrical and structural properties of extended defects including threading dislocations/V-defec... more Electrical and structural properties of extended defects including threading dislocations/V-defects and nanopipes in unintentionally doped
GaN, InGaN (50 nm)/GaN and AlInN (33 nm)/AlN(1 nm)/GaN heterostructures have been investigated by means of various scanning probe (Kelvin
probe and conductive-Atomic Force Microscopy) and electron beam (electron beam induced current and transmission electron microscopy) microscopy
methods. Due to low energy measurements of Kelvin probe force microscopy, charge state of the dislocations have been correctly identified
where threading dislocations (TDs) with screw-component are negatively charged, while pure-edge type TDs are neutral in InGaN/GaN. It is
explained how various factors such as indium segregation, surface termination, presence of vacancies and/or impurities affect the electrical charge,
conductivity and recombination properties of the extended defects. They are found to be strongly correlated to the type of dislocations as identified
from TEM
We report here on the micro structural, electronic and optical properties of a GaN-based InGaN/Ga... more We report here on the micro structural, electronic and optical properties of a GaN-based InGaN/GaN MQW LED grown by the MOVPE method. The present study shows that the threading dislocations present in these LED structures are terminated as V pits at the surface and have an impact on the electrical and optical activity of these devices. It has been pointed that these dislocations were of edge, screw and mixed types. EBIC maps suggest that the electrically active defects are screw and mixed dislocations and behave as nonradiative recombinant centres.
Semiconductor Science and Technology
This paper reports on a case study of the impact of fabrication steps on InN material properties.... more This paper reports on a case study of the impact of fabrication steps on InN material properties. We discuss the influence of annealing time and sequence of device processing steps. Photoluminescence (PL), surface morphology and electrical transport (electrical resistivity and low frequency noise) properties have been studied as responses to the adopted fabrication steps. Surface morphology has a strong correlation with annealing times, while sequences of fabrication steps do not appear to be influential. In contrast, the optical and electrical properties demonstrate correlation with both etching and thermal annealing. For all the studied samples PL peaks were in the vicinity of 0.7 eV, but the intensity and full width at half maximum (FWHM) demonstrate a dependence on the technological steps followed. Sheet resistance and electrical resistivity seem to be lower in the case of high defect introduction due to both etching and thermal treatments. The same effect is revealed through 1/f noise level measurements. A reduction of electrical resistivity is connected to an increase in 1/f noise level.
Physica Status Solidi A-applications and Materials Science, 2010
An investigation of InN layers grown on GaN templates by molecular beam epitaxy (MBE) has been ca... more An investigation of InN layers grown on GaN templates by molecular beam epitaxy (MBE) has been carried out by X-ray diffraction (XRD), Raman spectroscopy (RS) and photoluminescence (PL). A good correlation is noticed between their crystalline quality and optical properties. The best samples exhibit a PL emission between 0.6 and 0.7 eV. The surface structure was quite different from one sample to the other, pointing out to a critical role of the growth conditions, which probably need to be tightly optimized for a good reproducibility.
Physica Status Solidi B-basic Solid State Physics, 2011
Phone: þ00 332 31 45 26 53, Fax: þ00 332 31 45 25 57
Applied Physics Letters, 2011
We report on the nonlinear optical absorption of InN/InxGa1-xN (x=0.8,0.9) multiple-quantum-well ... more We report on the nonlinear optical absorption of InN/InxGa1-xN (x=0.8,0.9) multiple-quantum-well structures characterized at 1.55 μm by the Z-scan method in order to obtain the effective nonlinear absorption coefficient (α2) of the samples at high repetition rate. Saturable absorption is observed for the sample with x=0.9, with an effective α2~-9×103 cm/GW for the studied optical regime. For lower In content in the barrier, reverse saturable absorption is observed, which is attributed to two-photon absorption.
Applied Physics Letters, 2011
Bulk conduction in molecular beam epitaxial InN layers has been confirmed using low frequency noi... more Bulk conduction in molecular beam epitaxial InN layers has been confirmed using low frequency noise measurements versus temperature. A generation-recombination process has been identified at low temperatures Յ100 K and attributed to a trap with a discrete energy level in the band gap. The energy position of this trap has been determined to be around 52 meV below the conduction band minimum.
Due to its small band-gap and its high mobility, InN is a promising material for a large number o... more Due to its small band-gap and its high mobility, InN is a promising material for a large number of key applications like band-gap engineering for high efficiency solar cells, light emitting diodes, and high speed devices. Unfortunately, it has been reported that this material exhibits strong surface charge accumulation which may depend on the type of surface. Current investigations are conducted in order to explain the mechanisms which govern such a behavior and to look for ways of avoiding it and/or finding applications that may use such an effect. In this framework, low frequency noise measurements have been performed at different temperatures on patterned MBE grown InN layers. The evolution of the 1/f noise level with temperature in the 77 K-300 K range is consistent with carrier number fluctuations thus indicating surface mechanisms: the surface charge accumulation is confirmed by the noise measurements.