Shivesh Yadav - Academia.edu (original) (raw)

Papers by Shivesh Yadav

Research paper thumbnail of Controlling the persistence of photoconductivity through additional sub-bandgap photoexcitation in individual m-axial GaN nanowires

Journal of Applied Physics, 2019

The persistence of photoconductivity after switching off the photoexcitation is investigated in i... more The persistence of photoconductivity after switching off the photoexcitation is investigated in individual m-axial n-GaN nanowires as a function of temperature. At room temperature, photoconductivity is found to decay with a time scale of several hours. The capture barrier height is estimated to be 450 meV from the stretched exponential fitting of the decay characteristics recorded at different temperatures. This energy value is found to be much less than the surface band-bending energy of 770 meV, which is believed to act as the capture barrier in this system. This finding indicates the tunneling of electrons through the top part of the band-bending barrier. Interestingly, the decay rate of photoconductivity is observed to reduce significantly when the photoconductivity in these wires is quenched by an additional sub-bandgap illumination prior to the switching off the photoexcitation. A rate equation model is proposed to explain the upward band bending at the surface as well as the persistent photoconductivity effect in terms of the transfer of holes between the valence band and acceptor-type surface states of the nanowires. Photoconductivity decay profiles simulated from the model are found to match very well with the experimental data recorded at different temperatures in both quenched and unquenched cases.

Research paper thumbnail of Photocurrent modulation under dual excitation in individual GaN nanowires

Nanoscale, Jan 9, 2018

The photo-response properties of vapor-liquid-solid (VLS) grown [101[combining macron]0] oriented... more The photo-response properties of vapor-liquid-solid (VLS) grown [101[combining macron]0] oriented individual GaN nanowires of the diameter ranging from 30 to 100 nm are investigated under the joint illumination of above and sub-bandgap lights. When illuminated with above-bandgap light, these wires show persistent photoconductivity (PPC) effects with long build-up and decay times. The study reveals the quenching of photoconductivity (PC) upon illumination with an additional sub-bandgap light. PC recovers when the sub-bandgap illumination is withdrawn. A rate equation model attributing the PPC effect to the entrapment of photo-generated holes in the surface states and the PC quenching effect on the sub-bandgap light driven release of the holes from the trapped states has been proposed. The average height of the capture barrier has been found to be about 400 meV. The study also suggests that the capture barrier has a broad distribution with an upper cut-off energy of ∼2 eV.

Research paper thumbnail of One step growth of GaN/SiO 2 core/shell nanowire in vapor-liquid-solid route by chemical vapor deposition technique

Journal of Crystal Growth, 2017

GaN/SiO2 core/shell nanowires are grown by cobalt phthalocyanine catalyst assisted vapor-liquid-s... more GaN/SiO2 core/shell nanowires are grown by cobalt phthalocyanine catalyst assisted vapor-liquid-solid route, in which Si wafer coated with a mixture of gallium and indium is used as the source for Ga and Si and ammonia is used as the precursor for nitrogen and hydrogen. Gallium in the presence of indium and hydrogen, which results from the dissociation of ammonia, forms Si-GaIn alloy at the growth temperature 910 C . This alloy acts as the source of Si, Ga and In. A detailed study using a variety of characterization tools reveals that these wires, which are several tens of micron long, has a diameter distribution of the core ranging from 20 to 50 nm, while the thickness of the amorphous SiO2 shell layer is about 10 nm. These wires grow along ] 0 1 10 [ direction. It has also been observed that the average width of these wires decreases, while their density increases as the gallium proportion in the GaIn mixture is increased.

Research paper thumbnail of Structural and luminescence properties of GaN nanowires grown using cobalt phthalocyanine as catalyst

Journal of Applied Physics, 2015

Catalyst free methods have usually been employed to avoid any catalyst induced contamination for ... more Catalyst free methods have usually been employed to avoid any catalyst induced contamination for the synthesis of GaN nanowires with better transport and optical properties. Here, we have used a catalytic route to grow GaN nanowires, which show good optical quality. Structural and luminescence properties of GaN nanowires grown by vapor-liquid-solid technique using cobalt phthalocyanine as catalyst are systematically investigated as a function of various growth parameters such as the growth temperature and III/V ratio. The study reveals that most of the nanowires, which are several tens of microns long, grow along ½10 10 direction. Interestingly, the average wire diameter has been found to decrease with the increase in III/V ratio. It has also been observed that in these samples, defect related broad luminescence features, which are often present in GaN, are completely suppressed. At all temperatures, photoluminescence spectrum is found to be dominated only by a band edge feature, which comprises of free and bound excitonic transitions. Our study furthermore reveals that the bound excitonic feature is associated with excitons trapped in certain deep level defects, which result from the deficiency of nitrogen during growth. This transition has a strong coupling with the localized vibrational modes of the defects.

Research paper thumbnail of Controlling the persistence of photoconductivity through additional sub-bandgap photoexcitation in individual m-axial GaN nanowires

Journal of Applied Physics, 2019

The persistence of photoconductivity after switching off the photoexcitation is investigated in i... more The persistence of photoconductivity after switching off the photoexcitation is investigated in individual m-axial n-GaN nanowires as a function of temperature. At room temperature, photoconductivity is found to decay with a time scale of several hours. The capture barrier height is estimated to be 450 meV from the stretched exponential fitting of the decay characteristics recorded at different temperatures. This energy value is found to be much less than the surface band-bending energy of 770 meV, which is believed to act as the capture barrier in this system. This finding indicates the tunneling of electrons through the top part of the band-bending barrier. Interestingly, the decay rate of photoconductivity is observed to reduce significantly when the photoconductivity in these wires is quenched by an additional sub-bandgap illumination prior to the switching off the photoexcitation. A rate equation model is proposed to explain the upward band bending at the surface as well as the persistent photoconductivity effect in terms of the transfer of holes between the valence band and acceptor-type surface states of the nanowires. Photoconductivity decay profiles simulated from the model are found to match very well with the experimental data recorded at different temperatures in both quenched and unquenched cases.

Research paper thumbnail of Photocurrent modulation under dual excitation in individual GaN nanowires

Nanoscale, Jan 9, 2018

The photo-response properties of vapor-liquid-solid (VLS) grown [101[combining macron]0] oriented... more The photo-response properties of vapor-liquid-solid (VLS) grown [101[combining macron]0] oriented individual GaN nanowires of the diameter ranging from 30 to 100 nm are investigated under the joint illumination of above and sub-bandgap lights. When illuminated with above-bandgap light, these wires show persistent photoconductivity (PPC) effects with long build-up and decay times. The study reveals the quenching of photoconductivity (PC) upon illumination with an additional sub-bandgap light. PC recovers when the sub-bandgap illumination is withdrawn. A rate equation model attributing the PPC effect to the entrapment of photo-generated holes in the surface states and the PC quenching effect on the sub-bandgap light driven release of the holes from the trapped states has been proposed. The average height of the capture barrier has been found to be about 400 meV. The study also suggests that the capture barrier has a broad distribution with an upper cut-off energy of ∼2 eV.

Research paper thumbnail of One step growth of GaN/SiO 2 core/shell nanowire in vapor-liquid-solid route by chemical vapor deposition technique

Journal of Crystal Growth, 2017

GaN/SiO2 core/shell nanowires are grown by cobalt phthalocyanine catalyst assisted vapor-liquid-s... more GaN/SiO2 core/shell nanowires are grown by cobalt phthalocyanine catalyst assisted vapor-liquid-solid route, in which Si wafer coated with a mixture of gallium and indium is used as the source for Ga and Si and ammonia is used as the precursor for nitrogen and hydrogen. Gallium in the presence of indium and hydrogen, which results from the dissociation of ammonia, forms Si-GaIn alloy at the growth temperature 910 C . This alloy acts as the source of Si, Ga and In. A detailed study using a variety of characterization tools reveals that these wires, which are several tens of micron long, has a diameter distribution of the core ranging from 20 to 50 nm, while the thickness of the amorphous SiO2 shell layer is about 10 nm. These wires grow along ] 0 1 10 [ direction. It has also been observed that the average width of these wires decreases, while their density increases as the gallium proportion in the GaIn mixture is increased.

Research paper thumbnail of Structural and luminescence properties of GaN nanowires grown using cobalt phthalocyanine as catalyst

Journal of Applied Physics, 2015

Catalyst free methods have usually been employed to avoid any catalyst induced contamination for ... more Catalyst free methods have usually been employed to avoid any catalyst induced contamination for the synthesis of GaN nanowires with better transport and optical properties. Here, we have used a catalytic route to grow GaN nanowires, which show good optical quality. Structural and luminescence properties of GaN nanowires grown by vapor-liquid-solid technique using cobalt phthalocyanine as catalyst are systematically investigated as a function of various growth parameters such as the growth temperature and III/V ratio. The study reveals that most of the nanowires, which are several tens of microns long, grow along ½10 10 direction. Interestingly, the average wire diameter has been found to decrease with the increase in III/V ratio. It has also been observed that in these samples, defect related broad luminescence features, which are often present in GaN, are completely suppressed. At all temperatures, photoluminescence spectrum is found to be dominated only by a band edge feature, which comprises of free and bound excitonic transitions. Our study furthermore reveals that the bound excitonic feature is associated with excitons trapped in certain deep level defects, which result from the deficiency of nitrogen during growth. This transition has a strong coupling with the localized vibrational modes of the defects.