A. K. Shukla

Spectroscopic investigations of porous silicon prepared by laser-induced etching of silicon

Porous silicon was fabricated by using a Nd:YAG laser in a laser-induced etching process. Scanning electron microscopy was used to monitor changes in surface morphology produced during the etching process. Porous silicon samples were subjected to spectroscopic investigations using an argon-ion laser. The first-order Raman line asymmetry was found to decrease with decrease of the incident argon-ion laser excitation photon energy, while the peak position remained unchanged for a given etching time and power density. The photoluminescence spectra exhibits a red shift in peak position and a dramatic decrease in intensity as the incident photon energy was decreased. Both Raman and photoluminescence data were explained using appropriate quantum confinement models involving two-dimensional confinement and Gaussian size distributions of nanocrystallites constituting porous silicon samples. There is reasonable agreement between the results obtained from Raman and photoluminescence spectroscopic investigations of the PS samples.

Tailoring of structural and electron emission properties of CNT walls and graphene layers using high-energy irradiation

Structural and electron emission properties of carbon nanotubes (CNTs) and multilayer graphene (MLG) are tailored using high-energy irradiation by controlling the wall thickness and number of layers. Ion irradiation by 100 MeV Ag+ ions at different fluences is used as an effective tool for optimizing defect formation in CNTs and MLGs, as analysed by micro-Raman spectroscopy. It is found that the cross section for defect formation (η) is 3.5 × 10−11 for thin-walled CNTs, 2.8 × 10−11 for thick-walled CNTs and 3.1 × 10−11 for MLGs. High-resolution transmission electron microscopy results also show that thin-walled CNTs and MLGs are more defective in comparison with thick-walled CNTs. Carbon atoms rearrange at a fluence of 1 × 1012 ions cm−2 in thick-walled CNTs to heal up the damage, which aggravates at higher fluences. The observed electron emission parameters of the modified thin-walled CNTs and MLGs are confirmed with the changes in the structures and are optimized at a fluence of 1 × 1011 ions cm−2. However, the electron emission properties of thick-walled CNTs are modified at a fluence of 1 × 1012 ions cm−2. The enhancement in the electron emission properties is due to the rearrangement of bonds and hence modified tips due to irradiation.

Effect of titanium interlayer on the microstructure and electron emission characteristics of multiwalled carbon nanotubes

The influence of the titanium (Ti) interlayer thickness on the growth and electron emission characteristics of carbon nanotubes (CNTs) deposited on silicon (Si) coated with an iron (Fe) catalyst layer was investigated. Ti films 5 nm, 10 nm, and 15 nm in thickness were deposited beneath the Fe catalyst layers. Multiwalled carbon nanotubes (MWCNTs) were deposited via microwave plasma enhanced chemical vapor deposition. The Ti interlayer hinders the diffusion of Fe into the silicon substrate and thus helps in the growth of MWCNTs. In addition, the role of Ti as a sacrificial layer on the catalytic diffusion, surface morphology, microstructure, and, thus, the growth of MWCNTs was probed through scanning and high resolution transmission electron microscope studies. The enhanced electron emission mechanism as a result of the introduction of a Ti interlayer is explained on the basis of a double barrier model and the formation of a conducting channel between the substrate and the CNTs. The intensified micro-Raman...

High Performance of Midwave Infrared HgCdTe e-Avalanche Photodiode Detector

This letter reports on the design and fabrication of HgCdTe electron-avalanche photodiode (e-APD) for low dark current and high gain for imaging applications. HgCdTe e-APD photodiodes were fabricated in the n+-ν-p+ configuration for FPA at 30 μm × 30 μm pitch. Process for creating the required carrier profile and compositional grading in the absorption and multiplication regions was developed. Graded bandgap profile in the absorption region has been introduced. Shallow mesa etch isolation and effective passivation of side walls were introduced to control lateral currents. High quantum efficiency of 65% makes these APDs suitable for applications like quantum encryption. These measures helped in achieving high avalanche gain of 5550 at 8 V reverse bias in HgCdTe MWIR e-APD for the first time.

Low-frequency Raman scattering from silicon nanostructures

Low-frequency Raman scattering due to acoustic phonons is studied for silicon nanostructures. The lineshapes of the first-order Raman active modes exhibit asymmetry. A tail is observed toward low frequency and high frequency for the optic mode and acoustic mode, respectively. The Raman lineshapes of these modes are determined by a Gaussian envelope function convoluted with the vibrational density of states. The observed blueshift of the acoustic mode with reducing size of the nanostructures can be explained by the relaxation of the wavevector selection rule (q = 0), which is used in the phonon confinement model for positive-slope (dω/dq > 0) phonon dispersion. Because the acoustic and optical phonon branches have high positive and moderate negative slopes, respectively, around the “Gamma”-point in the phonon dispersion, a larger Raman shift of the acoustic mode to a higher frequency is observed in comparison with the shift to a lower frequency of the optic mode for a given nanostructure size.

Structure-Modified Stress Dynamics and Wetting Characteristics of Carbon Nanotubes and Multilayer Graphene for Electron Field Emission Investigations

In the present work, feasibility of achieving enhanced electron field-emission properties of stress-induced carbon nanotubes (CNTs) and multilayer graphene (MLGs) by ion modification is studied. Micro-Raman spectroscopy is used as a potent technique for in-depth investigations of stress-induced CNTs and MLGs. It is found that iron used as a catalyst, compresses at particular fluence and induces stresses in CNTs and MLGs to modify these structures, supported by high-resolution transmission electron microscopy (HRTEM) studies. The stresses are explained by the buckling wavelength (λ ∝ e((r/t)0.5)). Furthermore, the stresses induced in exotic nanostructures are studied for investigating wetting properties, which are well-corroborated with electron emission characteristics. It is found out that less-wetted CNTs and MLGs display enhanced emission properties with turn-on voltages (Eon) of 1.5 and 2.1 V/μm, respectively, in comparison to hydrophilic CNTs and MLGs with Eon of 2.6 and 4 V/μm, respectively.

Field emission studies of carbon nanostructures synthesised over Ni–Cr catalyst layer

Current research on the carbon-based nanotechnology needs progressive methods to control the shape, location and size of the nanostructures. Here, we report significant progress by synthesising the density controlled carbon nanostructures (CNSs) using acetylene and hydrogen in microwave plasma enhanced chemical vapour deposition system. Thin films of Ni–Cr (80 : 20, 60 : 40 and 50 : 50) sputtered over silicon (1 0 0) were used as catalysts. Scanning electron microscopy images of plasma annealed Ni–Cr coated silicon substrates show distributed nanoparticles of varying compositions over plasma annealed substrates. Morphologically and structurally different CNS were obtained when plasma annealed substrates were exposed to carbon vapours present in plasma. Transmission electron microscopy images suggested that the length and tip of CNS were in the range 50–100 nm and 4–6 nm, respectively. High resolution transmission electron microscopy images of the samples confirmed the presence of graphite (0 0 2) and nickel (2 0 0) planes in CNS. The field emission studies and Kelvin probe measurements of CNS grown over 80 : 20 Ni–Cr substrate show turn-on field and corresponding work function as 1.4 V µm−1 and 4.4 eV, respectively. Preliminary results show that these nanostructures could act as stable field emitters.

Performance of Graded Bandgap HgCdTe Avalanche Photodiode

This paper reports the performance of midwave infrared (MWIR) electron-injection avalanche photodiode (e-APD) fabricated using graded bandgap HgCdTe epilayers. Carrier transport in the e-APD is dominated by drift transport due to the built-in electric field associated with gradient in the bandgap. Carriers encounter fewer collision events before entering the multiplication region as the dead space effect is reduced. On-set of generation and multiplication processes is controlledmore effectively. High gain indicates a reduced in-elastic scattering by phonon-emission due to gradient. Quantum efficiency above 80% is achieved in merely 2–3-

Optical properties of lonsdaleite silicon nanowires: A promising material for optoelectronic applications

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Tuning of Microwave Absorption Properties and Electromagnetic Interference (EMI) Shielding Effectiveness of Nanosize Conducting Black-Silicone Rubber Composites Over 8-18 GHz

-thick absorbing layer because of more efficient collection of the photogenerated carriers. Lower generation volume is beneficial in terms of low dark current. The generation is confined in the vicinity of themultiplication region. Generated carriers are readily evacuated from the absorber region under the built-in electric field. An order of magnitude improvement over the state-of-the art performance in MWIR e-APD is achieved by introducing a controlled energy bandgap gradient in the HgCdTe epilayers.