S. M. Mominuzzaman

Characterization of phosphorus-doped amorphous carbon and construction of n-carbon/p-silicon heterojunction solar cells

Phosphorus (P)-doped carbon (n-C) films are deposited by pulsed laser deposition technique using a camphoric C target. The activation energy increased to approximately 0.23 eV for the film deposited using a 1% P target compared to undoped C film (0.17 eV), after which it decreases with further increase in P content to approximately 0.12 eV for the film deposited from a 7% P target. Study of activation energy reveals that the Fermi level of the n-C film moves from the valence band to near the conduction band edge through the midgap. The quantum efficiency (QE) of the n-C/p-Si cells is observed to improve with P content. The contribution of QE in the lower wavelength region (below 750 nm) may be due to photon absorption by C film and in the higher wavelength region is due to Si substrates. The current-voltage photovoltaic characteristics of n-C/p-Si cells under 1 sun air-mass 1.5 (AM 1.5) illumination condition (100 mW/cm2, 25°C) are improved up to 5% P and deteriorate thereupon. The open circuit voltage (Voc) and short circuit current density (Jsc) vary from 220 to 270 mV and 9 to 12 mA/cm2, respectively. The cell with 5% P yields the highest electrical conversion efficiency, η=1.25% and fill factor, FF = 53%. The structural, Tauc gap, conductivity and activation energy (together with electron spin resonance spectroscopy) studies reveal successful doping of P in the films deposited from target containing up to 5% P upon modifications in the gap states.

An experimental approach of DLC film deposition on metal substrates

In this paper an experimental approach of depositing diamond like carbon (DLC) on metal substrates has been described. By electrolysis of 10% (by mass) camphoric solution in methanol, attempts were made to deposit DLC films on Copper (Cu) and Aluminum (Al) substrates at room temperature. Solution is prepared using camphor (C10H16O), a natural source in methanol solvent. At first we applied this approach on Cu substrate and then on Al substrate. The surface morphologies of deposited films were examined by scanning electron microscopy (SEM). A comparison between Cu and Al substrates has been also presented under this approach.

A comparative performance analysis of 10 nm Si nanowire and carbon nanotube field effect transistors

Both Carbon Nanotube and Silicon Nanowire are emerging as promising materials for the development of next generation electronic devices. Our focus is mainly on the comparative study of Silicon Nanowire Field Effect Transistor (SiNW-FET) and Carbon Nanotube Field Effect Transistor (CNT-FET). In this work, we have simulated an n-type single walled CNT-FET and a SiNW-FET. A brief comparison between the transconductances of both types of devices due to the applied strain has been studied where SiNW-FET shows incremental change in transconductance which happens to decrease for CNT-FET for lower input voltage range. Afterwards, we have observed the velocity vs applied electric field curves for both CNT-FET and SiNW-FET. It has been shown that although SiNW-FET has lower saturation velocity than CNT-FET, it can be improved by applying tensile strain. Finally, the direct tunneling gate leakage currents for CNT-FET and SiNW-FET have been investigated, where CNT-FET has been proved to be a better choice for gate leakage reduction.

Many body corrections for higher optical transitions in semiconducting SWCNTs

In this work, Coulomb effect on determining optical transition energies of semiconducting single wall carbon nanotubes is discussed. Due to their quasi-one-dimensional structure, electron-electron and electron-hole interaction and corresponding self energy and exciton binding energy are important in SWCNTs and the difference between these two energies gives many body effect. Here, at first, a brief review of nature of correction in the conventional single particle electronic picture of SWCNTs to include many body effect in first four optical transitions is presented. Then, many body corrections needed for next three higher optical transitions i.e. 5th, 6th and 7th transitions of semiconducting SWCNTs are investigated. Experimental values of these higher transitions are collected from recent experimental reports so as to explain those data by extending single-particle picture corrected for nanotube curvature and chirality effect along with many body corrections. Our result shows that these three transitions excellently follow the proposed corrected picture with less than 0.5% average absolute error which proves that excitonic behaviour is strong in 5th, 6th and 7th optical transitions of semiconducting SWCNTs unlike 3rd and 4th transitions as reported in some recent works.

Universal empirical formula for optical transition energies of semiconducting single-walled carbon nanotubes

A general empirical relation for calculating first seven optical transition energies of semiconducting single wall carbon nanotubes (SWCNTs) is proposed here for the first time. The proposed formula effectively relates first seven optical transition energies of semiconducting SWCNTs with their chiral indices (n, m) through exponential form containing two specific terms (n+2m) and (2n-m). Both mod 1 and mod 2 types of semiconducting tubes are considered here over a wide diameter range from 0.4 nm to 4.75 nm. It was observed that the proposed empirical relations can predict the recent experimental data of those optical transitions with high accuracy.

Empirical prediction of optical transitions in metallic armchair SWCNTs

In this work, a quick and effective method to calculate the second and third optical transition energies of metallic armchair single wall carbon nanotubes (SWCNT) is presented. In this proposed method, the transition energy of any armchair SWCNT can be predicted directly just by knowing one of its chiral index. The predicted results are compared with recent experimental data and found to be accurate over a wide diameter range. The empirical equation proposed here is also compared with that proposed in earlier works. The proposed way may help the research works or applications where information of optical transitions of armchair metallic nanotubes is needed.

Empirical prediction of bandgap in semiconducting single-wall carbon nanotubes

Necessity for improved calculation of bandgap energies of semiconducting single wall carbon nanotubes is discussed. An effective empirical equation for nearest neighbor hopping parameter (γ0) in tight binding model of carbon nanotube is proposed in terms of nanotube diameter and chiral index combination. Bandgap energies of all semiconducting single-wall carbon nanotubes in between theoretical minimum and maximum diameter range are calculated from simplest tight binding model using this empirical γ0. Calculated bandgap values excellently match with experimental results over the full diameter range. The proposed hopping parameter greatly improves tight binding model, and remove its quantitative failure in predicting bandgap energy of nanotube.

Raman spectral analysis of metallic Single Wall Carbon Nanotube

This paper includes a detailed study of Raman G peak analysis for metallic Single Wall Carbon Nanotubes. The higher frequency Lorentzian line shape and the lower frequency Breit-Wigner-Fano (BWF) line shape are two most intense features of Raman G peaks. BWF line shape originated from the Phonon-Plasmon coupling to an electronic continuum is an intrinsic property for metallic SWNTs. From different Raman samples it is observed that BWF line shape has a strong relation with the diameter distributions of SWNTs. We established an equation form for both line width and interaction parameter (1/q) components of BWF line shape which is consistent with the theoretical prediction.

Raman Spectra of the carbon films by pulsed laser deposition using C 60 target

Carbon films were grown on single crystal silicon substrate by XeCl excimer pulsed laser deposition (PLD) and fullerene (C<inf>60</inf>) is used as a target. A detailed Raman analysis is presented here to investigate the structural change of the film and hence the result is compared with those of the films produced from graphite and camphor (C<inf>10</inf>H<inf>16</inf>O) targets deposited in the similar conditions. Raman analysis of the films obtained from C<inf>60</inf> target indicates 3 distinct peaks located at 1357.4 cm⁻¹, 1529.3 cm⁻¹, 1592.1 cm⁻¹ named D peak, F (fullerene) peak and G peak respectively. High energetic pulsed laser causes polymeric semiconducting film which retains crystalline graphite structure. Such improved quality of film proves that C<inf>60</inf> can be used as a better target material instead of graphite or camphor for semiconducting/optoelectronic applications.