Usha Philipose | University of North Texas (original) (raw)
Papers by Usha Philipose
Bulletin of the American Physical Society, Mar 16, 2017
Applied Physics Letters, Dec 10, 2007
Ultrafast carrier dynamics of ZnSe nanowires grown under different growth conditions have been st... more Ultrafast carrier dynamics of ZnSe nanowires grown under different growth conditions have been studied. Transient absorption measurements reveal the dependence of the competing effects of state filling and photoinduced absorption on the probed energy states. The relaxation of the photogenerated carriers occupying defect states in the stoichiometric and Se-rich samples are single exponentials with time constants of 3–4ps. State filling is the main contribution for probe energies below 1.85eV in the Zn-rich grown sample. This ultrafast carrier dynamics study provides an important insight into the role that intrinsic point defects play in the observed photoluminescence from ZnSe nanowires.
Proceedings of SPIE, Sep 28, 2005
Arrays of free-standing ZnSe nanowires of length 8-10 μm and diameter 60-150 nm were fabricated b... more Arrays of free-standing ZnSe nanowires of length 8-10 μm and diameter 60-150 nm were fabricated by Au-catalyzed vapor-liquid-solid growth. Electron microscopy showed that these were high quality single crystal nanowires. Photoluminescence (PL) measurements of the as-grown ...
Bulletin of the American Physical Society, 2019
Journal of Applied Physics, 2015
Capacitance measurements were made on an array of parallel ZnO nanowires embedded in a polymer ma... more Capacitance measurements were made on an array of parallel ZnO nanowires embedded in a polymer matrix and provided with two electrodes perpendicular to the nanowires. The capacitance monotonically increased, and saturated at large negative (depleting) and large positive (accumulating) voltages. A qualitative explanation for this behavior is presented, taking into account specific features of quasi-one-dimensional screening. The increasing or decreasing character of the capacitance-voltage characteristics were determined by the conductivity type of the nanowires, which in our case was n-type. A dispersion of the experimental capacitance was observed over the entire frequency range of 1 kHz to 5 MHz. This phenomenon is explained by the slow discharge of the nanowires through the thin dielectric layer that separates them from the top electrode. Separate measurements on individual identical nanowires in a field effect transistor configuration yielded an electron concentration and mobili...
Journal of Alloys and Compounds, Aug 1, 2009
In recent years, ZnSe nanowires have been widely investigated for their potential applications in... more In recent years, ZnSe nanowires have been widely investigated for their potential applications in optoelectronics. A typical room temperature photoluminescence spectrum of ZnSe nanowires grown by vapor-liquid-solid growth under different growth conditions shows that the spectrum is dominated by two characteristic emission peaks. The first peak is attributed to the band edge emission peak at 2.68 eV whereas the second to the broad deep defect-related emission peak in the region of 1.8-2.4 eV. In this work, we present a study of ultrafast time-resolved spectroscopy of defect states of ZnSe nanowires grown under Se-rich growth conditions. We investigate in detail the carrier dynamics of these nanostructure materials using selective optical excitation femtosecond pulses from a wavelength tunable optical parametric amplifier system. The effects of intrinsic point defects inherent in the manufacturing of these materials and in particular the relaxations of the photogenerated carriers occupying these defect states are examined. Temporal dynamics on a few picoseconds timescale provided information on effects such as state filling and secondary excitation and their contribution to the overall induced absorption. Long timescale probing of induced absorption provided information on the defect states associated with the observed photoluminescence in this material.
The objective of this program is to investigate the mechanisms of electrical conduction in amorph... more The objective of this program is to investigate the mechanisms of electrical conduction in amorphous silicon and identify factors affecting the performance of uncooled imaging focal plane array systems based on amorphous silicon microbolometer thermal detector structures. Measurements of the temperature dependence of conductivity and noise show that the dominant conduction mechanism in p-type a-Si:H is that of Mott variable range hopping. The hopping parameters are controlled by the film deposition conditions such as hydrogen dilution of the silane precursor and boron dopant level. The low frequency noise has a weak temperature dependence and is dominated
Bulletin of the American Physical Society, Mar 23, 2011
Texas, A. J. SYLLAIOS, L-3 Communications EOS-We report on progress in the characterization of am... more Texas, A. J. SYLLAIOS, L-3 Communications EOS-We report on progress in the characterization of amorphous silicon thin-films utilized in infrared detectors. Specifically, we have observed changes in the Raman spectra, resistivity, and activation energy in protocrystalline Si films grown by PECVD as substrate temperature, dopant type and concentration, and hydrogen dilution of the reactants are varied. Both n-and p-type films exhibit four Raman spectral peaks [1]. The TO Raman peak becomes better defined and shifts towards the crystalline TO energy for increasing substrate temperature or H dilution, or for decreasing dopant concentration. Hall and resistivity measurements as a function of both magnetic field and temperature on the same material have been conducted to better understand the relationships between specific growth parameters and key electrical properties.
Journal of Non-crystalline Solids, Jul 1, 2021
Abstract Temperature dependent conductivity measurements were performed on phosphine doped hydrog... more Abstract Temperature dependent conductivity measurements were performed on phosphine doped hydrogenated amorphous silicon grown by plasma enhanced chemical vapor deposition over a range of argon dilution and phosphine [PH3] doping. Resistance curve derivative analysis was used along with hopping conductivity modeling to determine the nature of the conduction mechanism. The results of this analysis show that anomalous hopping conduction, where ln σ ∝ T − p with p ≈ 0.75 , describes the temperature dependent conductivity of most of the samples studied, and represents the first observation of anomalous conduction in phosphine doped amorphous silicon. Argon dilution modifies the efficiency of dopant incorporation into electrically active sites, and the resultant defects within the silicon network appear to sufficiently modify the density of states, leading to anomalous conduction. Raman spectroscopy was used as additional evidence of these structural changes.
Materials, Feb 12, 2023
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Nanotechnology, Mar 25, 2021
Through the positive role of alkali halides in realizing large area growth of transition metal-di... more Through the positive role of alkali halides in realizing large area growth of transition metal-di-chalcogenide layers has been validated, the film-growth kinematics has not yet been fully established. This work presents a systematic analysis of the MoS2 morphology for films grown under various pre-treatment conditions of the substrate with sodium chloride (NaCl). At an optimum NaCl concentration, the domain size of the monolayer increased by almost two orders of magnitude compared to alkali-free growth of MoS2. The results show an inverse relationship between fractal dimension and areal coverage of the substrate with monolayers and multi-layers, respectively. Using the Fact-Sage software, the role of NaCl in determining the partial pressures of Mo-and S-based compounds in gaseous phase at the growth temperature is elucidated. The presence of alkali salts is shown to affect the domain size and film morphology by affecting the Mo and S partial pressures. Compared to alkali-free synthesis under the same growth conditions, MoS2 film growth assisted by NaCl results in ≈ 81% of the substrate covered by monolayers. Under ideal growth conditions, at an optimum NaCl concentration, nucleation was suppressed, and domains enlarged, resulting in large area growth of MoS2 monolayers. The monolayers were found to be free of unintentional doping with alkali
Materials, Dec 7, 2021
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Journal of Nanoparticle Research, 2016
We have developed an effective strategy for synthesizing p-type indium antimonide (InSb) nanowire... more We have developed an effective strategy for synthesizing p-type indium antimonide (InSb) nanowires on a thin film of InSb grown on glass substrate. The InSb films were grown by a chemical reaction between Sb 2 S 3 and I n and were characterized by structural, compositional, and optical studies. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies reveal that the surface of the substrate is covered with a polycrystalline InSb film comprised of sub-micron sized InSb islands. Energy dispersive X-ray (EDX) results show that the film is stoichiometric InSb. The optical constants of the InSb film, characterized using a variable-angle spectroscopic ellipsometer (VASE) shows a maximum value for refractive index at 3.7 near 1.8 eV, and the extinction coefficient (k) shows a maximum value 3.3 near 4.1 eV. InSb nanowires were subsequently grown on the InSb film with 20 nm sized Au nanoparticles functioning as the metal catalyst initiating nanowire growth. The InSb nanowires with diameters in the range of 40-60 nm exhibit good crystallinity and were found to be rich in Sb. High concentrations of anions in binary semiconductors are known to
Materials
The influence of carbon nanotube (CNT) dispersions on the electrical properties and noise signal ... more The influence of carbon nanotube (CNT) dispersions on the electrical properties and noise signal amplitude of VOx films is investigated. For a critical range of the CNT dispersion density on VOx films, the intrinsic properties of the VOx films are modified by the CNTs. The CNT concentrations reported in this work are about 0.3 μg/cm2 and 1.6 μg/cm2, allowing for low density and high density dispersions on the VOx film surface to be investigated. These values are higher than the percolation threshold of about 0.12 μg/cm2 for these films. The composite film exhibits a significant reduction in the temperature coefficient of resistance (TCR) (from ≈3.8% K−1 to ≈0.3% K−1) for high density dispersions. In contrast, while VOx–CNT composites with low density single wall CNT dispersions exhibit no significant change in TCR values, an approximate two orders of magnitude reduction in the low frequency 1/f noise is measured. The noise signal amplitude measured at 0.1 V and at 1.0 Hz reduces fro...
Infrared Sensors, Devices, and Applications XII
APS March Meeting Abstracts, Mar 1, 2011
Bulletin of the American Physical Society, Mar 6, 2018
Journal of Non-Crystalline Solids, 2021
Abstract Temperature dependent conductivity measurements were performed on phosphine doped hydrog... more Abstract Temperature dependent conductivity measurements were performed on phosphine doped hydrogenated amorphous silicon grown by plasma enhanced chemical vapor deposition over a range of argon dilution and phosphine [PH3] doping. Resistance curve derivative analysis was used along with hopping conductivity modeling to determine the nature of the conduction mechanism. The results of this analysis show that anomalous hopping conduction, where ln σ ∝ T − p with p ≈ 0.75 , describes the temperature dependent conductivity of most of the samples studied, and represents the first observation of anomalous conduction in phosphine doped amorphous silicon. Argon dilution modifies the efficiency of dopant incorporation into electrically active sites, and the resultant defects within the silicon network appear to sufficiently modify the density of states, leading to anomalous conduction. Raman spectroscopy was used as additional evidence of these structural changes.
Semiconductors - Growth and Characterization, 2018
Template-assisted synthesis of nanowires is a simple electrochemical technique commonly used in t... more Template-assisted synthesis of nanowires is a simple electrochemical technique commonly used in the fabrication of semiconducting nanowires. It is an easy and cost-effective approach compared to conventional lithography, which requires expensive equipment. The focus of this chapter is on the various mechanisms involving mass transport of ions during successive stages of the template-assisted electrochemical growth of indium antimonide (InSb) nanowires. The nanowires were grown in two different templates such as commercially available anodic aluminum oxide (AAO) templates and polycarbonate membranes. The chapter also presents the results of characterizing the InSb nanowires connected in a field effect transistor (FET) configuration. The Sb-rich InSb nanowires that were fabricated by DC electrodeposition in nanoporous AAO exhibited hole-dominated transport (p-type conduction). Temperature-dependent transport measurement shows the semiconducting nature of these nanowires.
Bulletin of the American Physical Society, Mar 16, 2017
Applied Physics Letters, Dec 10, 2007
Ultrafast carrier dynamics of ZnSe nanowires grown under different growth conditions have been st... more Ultrafast carrier dynamics of ZnSe nanowires grown under different growth conditions have been studied. Transient absorption measurements reveal the dependence of the competing effects of state filling and photoinduced absorption on the probed energy states. The relaxation of the photogenerated carriers occupying defect states in the stoichiometric and Se-rich samples are single exponentials with time constants of 3–4ps. State filling is the main contribution for probe energies below 1.85eV in the Zn-rich grown sample. This ultrafast carrier dynamics study provides an important insight into the role that intrinsic point defects play in the observed photoluminescence from ZnSe nanowires.
Proceedings of SPIE, Sep 28, 2005
Arrays of free-standing ZnSe nanowires of length 8-10 μm and diameter 60-150 nm were fabricated b... more Arrays of free-standing ZnSe nanowires of length 8-10 μm and diameter 60-150 nm were fabricated by Au-catalyzed vapor-liquid-solid growth. Electron microscopy showed that these were high quality single crystal nanowires. Photoluminescence (PL) measurements of the as-grown ...
Bulletin of the American Physical Society, 2019
Journal of Applied Physics, 2015
Capacitance measurements were made on an array of parallel ZnO nanowires embedded in a polymer ma... more Capacitance measurements were made on an array of parallel ZnO nanowires embedded in a polymer matrix and provided with two electrodes perpendicular to the nanowires. The capacitance monotonically increased, and saturated at large negative (depleting) and large positive (accumulating) voltages. A qualitative explanation for this behavior is presented, taking into account specific features of quasi-one-dimensional screening. The increasing or decreasing character of the capacitance-voltage characteristics were determined by the conductivity type of the nanowires, which in our case was n-type. A dispersion of the experimental capacitance was observed over the entire frequency range of 1 kHz to 5 MHz. This phenomenon is explained by the slow discharge of the nanowires through the thin dielectric layer that separates them from the top electrode. Separate measurements on individual identical nanowires in a field effect transistor configuration yielded an electron concentration and mobili...
Journal of Alloys and Compounds, Aug 1, 2009
In recent years, ZnSe nanowires have been widely investigated for their potential applications in... more In recent years, ZnSe nanowires have been widely investigated for their potential applications in optoelectronics. A typical room temperature photoluminescence spectrum of ZnSe nanowires grown by vapor-liquid-solid growth under different growth conditions shows that the spectrum is dominated by two characteristic emission peaks. The first peak is attributed to the band edge emission peak at 2.68 eV whereas the second to the broad deep defect-related emission peak in the region of 1.8-2.4 eV. In this work, we present a study of ultrafast time-resolved spectroscopy of defect states of ZnSe nanowires grown under Se-rich growth conditions. We investigate in detail the carrier dynamics of these nanostructure materials using selective optical excitation femtosecond pulses from a wavelength tunable optical parametric amplifier system. The effects of intrinsic point defects inherent in the manufacturing of these materials and in particular the relaxations of the photogenerated carriers occupying these defect states are examined. Temporal dynamics on a few picoseconds timescale provided information on effects such as state filling and secondary excitation and their contribution to the overall induced absorption. Long timescale probing of induced absorption provided information on the defect states associated with the observed photoluminescence in this material.
The objective of this program is to investigate the mechanisms of electrical conduction in amorph... more The objective of this program is to investigate the mechanisms of electrical conduction in amorphous silicon and identify factors affecting the performance of uncooled imaging focal plane array systems based on amorphous silicon microbolometer thermal detector structures. Measurements of the temperature dependence of conductivity and noise show that the dominant conduction mechanism in p-type a-Si:H is that of Mott variable range hopping. The hopping parameters are controlled by the film deposition conditions such as hydrogen dilution of the silane precursor and boron dopant level. The low frequency noise has a weak temperature dependence and is dominated
Bulletin of the American Physical Society, Mar 23, 2011
Texas, A. J. SYLLAIOS, L-3 Communications EOS-We report on progress in the characterization of am... more Texas, A. J. SYLLAIOS, L-3 Communications EOS-We report on progress in the characterization of amorphous silicon thin-films utilized in infrared detectors. Specifically, we have observed changes in the Raman spectra, resistivity, and activation energy in protocrystalline Si films grown by PECVD as substrate temperature, dopant type and concentration, and hydrogen dilution of the reactants are varied. Both n-and p-type films exhibit four Raman spectral peaks [1]. The TO Raman peak becomes better defined and shifts towards the crystalline TO energy for increasing substrate temperature or H dilution, or for decreasing dopant concentration. Hall and resistivity measurements as a function of both magnetic field and temperature on the same material have been conducted to better understand the relationships between specific growth parameters and key electrical properties.
Journal of Non-crystalline Solids, Jul 1, 2021
Abstract Temperature dependent conductivity measurements were performed on phosphine doped hydrog... more Abstract Temperature dependent conductivity measurements were performed on phosphine doped hydrogenated amorphous silicon grown by plasma enhanced chemical vapor deposition over a range of argon dilution and phosphine [PH3] doping. Resistance curve derivative analysis was used along with hopping conductivity modeling to determine the nature of the conduction mechanism. The results of this analysis show that anomalous hopping conduction, where ln σ ∝ T − p with p ≈ 0.75 , describes the temperature dependent conductivity of most of the samples studied, and represents the first observation of anomalous conduction in phosphine doped amorphous silicon. Argon dilution modifies the efficiency of dopant incorporation into electrically active sites, and the resultant defects within the silicon network appear to sufficiently modify the density of states, leading to anomalous conduction. Raman spectroscopy was used as additional evidence of these structural changes.
Materials, Feb 12, 2023
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Nanotechnology, Mar 25, 2021
Through the positive role of alkali halides in realizing large area growth of transition metal-di... more Through the positive role of alkali halides in realizing large area growth of transition metal-di-chalcogenide layers has been validated, the film-growth kinematics has not yet been fully established. This work presents a systematic analysis of the MoS2 morphology for films grown under various pre-treatment conditions of the substrate with sodium chloride (NaCl). At an optimum NaCl concentration, the domain size of the monolayer increased by almost two orders of magnitude compared to alkali-free growth of MoS2. The results show an inverse relationship between fractal dimension and areal coverage of the substrate with monolayers and multi-layers, respectively. Using the Fact-Sage software, the role of NaCl in determining the partial pressures of Mo-and S-based compounds in gaseous phase at the growth temperature is elucidated. The presence of alkali salts is shown to affect the domain size and film morphology by affecting the Mo and S partial pressures. Compared to alkali-free synthesis under the same growth conditions, MoS2 film growth assisted by NaCl results in ≈ 81% of the substrate covered by monolayers. Under ideal growth conditions, at an optimum NaCl concentration, nucleation was suppressed, and domains enlarged, resulting in large area growth of MoS2 monolayers. The monolayers were found to be free of unintentional doping with alkali
Materials, Dec 7, 2021
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Journal of Nanoparticle Research, 2016
We have developed an effective strategy for synthesizing p-type indium antimonide (InSb) nanowire... more We have developed an effective strategy for synthesizing p-type indium antimonide (InSb) nanowires on a thin film of InSb grown on glass substrate. The InSb films were grown by a chemical reaction between Sb 2 S 3 and I n and were characterized by structural, compositional, and optical studies. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies reveal that the surface of the substrate is covered with a polycrystalline InSb film comprised of sub-micron sized InSb islands. Energy dispersive X-ray (EDX) results show that the film is stoichiometric InSb. The optical constants of the InSb film, characterized using a variable-angle spectroscopic ellipsometer (VASE) shows a maximum value for refractive index at 3.7 near 1.8 eV, and the extinction coefficient (k) shows a maximum value 3.3 near 4.1 eV. InSb nanowires were subsequently grown on the InSb film with 20 nm sized Au nanoparticles functioning as the metal catalyst initiating nanowire growth. The InSb nanowires with diameters in the range of 40-60 nm exhibit good crystallinity and were found to be rich in Sb. High concentrations of anions in binary semiconductors are known to
Materials
The influence of carbon nanotube (CNT) dispersions on the electrical properties and noise signal ... more The influence of carbon nanotube (CNT) dispersions on the electrical properties and noise signal amplitude of VOx films is investigated. For a critical range of the CNT dispersion density on VOx films, the intrinsic properties of the VOx films are modified by the CNTs. The CNT concentrations reported in this work are about 0.3 μg/cm2 and 1.6 μg/cm2, allowing for low density and high density dispersions on the VOx film surface to be investigated. These values are higher than the percolation threshold of about 0.12 μg/cm2 for these films. The composite film exhibits a significant reduction in the temperature coefficient of resistance (TCR) (from ≈3.8% K−1 to ≈0.3% K−1) for high density dispersions. In contrast, while VOx–CNT composites with low density single wall CNT dispersions exhibit no significant change in TCR values, an approximate two orders of magnitude reduction in the low frequency 1/f noise is measured. The noise signal amplitude measured at 0.1 V and at 1.0 Hz reduces fro...
Infrared Sensors, Devices, and Applications XII
APS March Meeting Abstracts, Mar 1, 2011
Bulletin of the American Physical Society, Mar 6, 2018
Journal of Non-Crystalline Solids, 2021
Abstract Temperature dependent conductivity measurements were performed on phosphine doped hydrog... more Abstract Temperature dependent conductivity measurements were performed on phosphine doped hydrogenated amorphous silicon grown by plasma enhanced chemical vapor deposition over a range of argon dilution and phosphine [PH3] doping. Resistance curve derivative analysis was used along with hopping conductivity modeling to determine the nature of the conduction mechanism. The results of this analysis show that anomalous hopping conduction, where ln σ ∝ T − p with p ≈ 0.75 , describes the temperature dependent conductivity of most of the samples studied, and represents the first observation of anomalous conduction in phosphine doped amorphous silicon. Argon dilution modifies the efficiency of dopant incorporation into electrically active sites, and the resultant defects within the silicon network appear to sufficiently modify the density of states, leading to anomalous conduction. Raman spectroscopy was used as additional evidence of these structural changes.
Semiconductors - Growth and Characterization, 2018
Template-assisted synthesis of nanowires is a simple electrochemical technique commonly used in t... more Template-assisted synthesis of nanowires is a simple electrochemical technique commonly used in the fabrication of semiconducting nanowires. It is an easy and cost-effective approach compared to conventional lithography, which requires expensive equipment. The focus of this chapter is on the various mechanisms involving mass transport of ions during successive stages of the template-assisted electrochemical growth of indium antimonide (InSb) nanowires. The nanowires were grown in two different templates such as commercially available anodic aluminum oxide (AAO) templates and polycarbonate membranes. The chapter also presents the results of characterizing the InSb nanowires connected in a field effect transistor (FET) configuration. The Sb-rich InSb nanowires that were fabricated by DC electrodeposition in nanoporous AAO exhibited hole-dominated transport (p-type conduction). Temperature-dependent transport measurement shows the semiconducting nature of these nanowires.