Some of my old research interests!
I am not actively continuing these topics at present, however, they are still alive for me.
Anharmonic effects in semiconductors:
Anharmonicity in a material can be activated easily by raising the lattice temperature. The anharmonicity in the vibrational potential leads to the decay of strongly interacting optical phonons into weakly interacting acoustic phonons, which can take place through various decay channels governed by the dispersion relations, provided the decay obeys energy and momentum conservation. This decay results in a shift and broadening of observed phonons in Raman spectrum. The anharmonic constants and the relaxation times can be estimated from the temperature dependent Raman experiments. Disordered sample are usually found to be much more sensitive to temperature, depicting a larger value of anharmonicity and lower life times. This is because such a sample has considerable topological variations and defects, which act as scattering points promoting phonons to decay anharmonically. Small samples have an additional contribution from the boundary effects, which is temperature independent. This study was mainly concentrated to GaAs, ZnSe and related materials, including nanoparticle systems.
Ion-implantation and laser annealing of semiconductor:
Ion implantation followed by pulse laser annealing (PLA) is a method of introducing dopant in a controlled way into semiconductors. This method is of some interest for the manufacture and development of fast electronic devices and their associated systems. GaAs samples were implanted with various ions, like P, Si, Zn and Se at implantation doses ranging from 1012 to 1016 ions/cm2 and these samples were studied using Raman spectroscopy. Various kind of defects created by ion implantation lead to a topological disorder and the sample became amorphous at higher implantation doses. This radiation damage was then removed by subsequent PLA using a Q switched Nd:YAG laser. The crystalline to amorphous transition in the implantation process and amorphous to crystalline transition in the PLA process were monitored by Raman scattering experiments to estimate the threshold values of fluence and annealing energy for complete amorphization and re crystallization, respectively. The degree of disorder and crystallization were quantitatively estimated using the intensity ratio of allowed to forbidden phonon modes. The size of crystalline structure was estimated from the position, width and asymmetry in the LO phonon as the size of crystalline structure changes from microcrystalline to almost single crystal structure during the annealing process. The impurity localization was studied by the local phonon modes in the Raman spectra.
Charge carrier activation after laser annealing:
Apart from the lattice healing, PLA process also activates charge carriers in an implanted sample. This can be very well studied by Raman scattering experiments. Owing to the presence of free carriers in an implanted and annealed sample, a strong interaction takes place between the charge carriers and the optical phonons. The TO phonon couples with single particle excitations through the deformation potential interaction, whereas the LO phonon couples strongly with plasmon through the Fröhlich interaction, which gives rise to coupled plasmon phonon modes. The measured peak positions, linewidths and intensities of coupled modes in Raman spectra were used to estimate the activated charge carrier concentration, their distribution and the depletion width. The temperature dependence of depletion layer was also studied using Raman scattering. The wavelength dependent Raman experiments, which allows one to change the penetration depth, were used to profile the activated charge carrier distribution in the samples.
Fano effects in Semiconductors:
(text will be added soon!)
Generation of high-Tc superconducting thin film by laser ablation:
For a short period, I was involved in the generation of Y-Ba-Cu-O superconducting thin films by laser ablation and characterization by Raman and infrared spectroscopy. Laser ablation is one of the best techniques to deposit a c -axis oriented twin free 1-2-3 superconducting thin films with good stoichiometry. Growth of 1-2-3 films on metallic substrate has been a subject of great interest to achieve perfect c -oriented films. We have shown that the addition of an underlayer of Pt metal between the buffer layer of yttria stabilized zirconia (YSZ) and hastelloy (HA) improves the growth of well-textured films. The Pt layer introduces a PtCl2 layer between YSZ and HA which acts as a barrier suppressing the interdiffusion between YSZ and HA. Drastic improvement in stoichiometry was observed for the Y-Ba-Cu-O thin films grown by laser ablation on HA/Pt/YSZ and HA/Pt/MgO substrates. The critical temperatures for these films were found to be about 90K. Room temperature Raman spectrum from such a film shows two prominent modes at 337 and 502 cm-1. The latter, which is due to the bond stretching of oxygen atom along c -axis in the primitive lattice, is easily observed in a c -axis oriented 1-2-3 sample (bulk and thin film), whereas the former, which is due to the bond bending of oxygen atom in a-b plane of the primitive lattice, is generally weak and is often found to be missing, which is due to the presence of twining in the sample. Our experiments show greater oscillator strength for the 337 cm-1 mode than for the 502 cm-1 mode, indicating that the laser ablated films have negligible twining.