Anita R. Warrier

Transverse photothermal beam deflection technique for determining the transport properties of semiconductor thin films

Nonradiative transitions occurring in semiconductors result in thermal emissions carrying information on the material’s thermal and electronic properties. A simple one-dimensional theoretical model is devised which accounts for the photothermal signal variations due to nonradiative transitions occurring in semiconductor thinfilms. The theory was verified by determining the transport properties of p-type silicon wafer. We could get the thermal diffusivity, minority carrier lifetime, surface recombination velocity, and minority carrier mobility of CuInS2 thin films, thereby proving the efficiency and simplicity of photothermal beam deflection technique for real time characterization of semiconductor thin films. The film fabrication history, composition, and post deposition treatments play crucial role in determining the transport properties and the effect of these conditions on transport properties of the film as well as on the solar cell parameters is discussed.

Raman mode random lasing in ZnS-β-carotene random gain media

Raman mode random lasing is demonstrated in ZnS-β-carotene random gain media at room temperature. A self assembled random medium is prepared with ZnS sub micron spheres synthesized by homogeneous precipitation method. β-Carotene extracted from pale green leaves is embedded in this random medium. The emission band of ZnS random medium (on excitation at 488 nm) overlaps considerably with that of β-carotene, which functions as a gain medium. Here, random medium works as a cavity, leading to Raman mode lasing at 517 nm and 527 nm triggered by stimulated resonance Raman scattering.

Structural and Optical Properties of Indium Sulfide Thin Films Prepared by Silar Technique

Indium sulfide thin films were prepared using a relatively new, simple and inexpensive technique called Suc- cessive Ionic Layer Adsorption and Reaction (SILAR). SILAR deposition conditions for obtaining good quality � -Indium sulfide (In2S3) films were optimized. The films were structurally and optically characterized using X-ray diffraction (XRD), photosensitivity measurements and optical absorption studies. Effects of using different precursor solutions, in- dium chloride (InCl3) and indium nitrate (In(NO3)3) and post deposition annealing were also studied. Films fabricated with In(NO3)3 showed good crystallinity without any post deposition annealing while films prepared using InCl3 were crystalline only when annealed at 400 0 C. The band gap of the films varied from 2.32 to 2.92 eV depending on the deposi- tion conditions.

Enhancement of photoluminescence from defect states in ZnS random photonic crystal: An effect of electronic and photonic mode coupling

This paper reports on the enhanced defect state emission from ZnS in the form of a random photonic crystal (RPC) medium. ZnS photonic crystals with varied randomness are fabricated by colloidal self assembly of ZnS nanospheres (215 ± 10 nm). Reflection and transmission studies reveal mid band gap wavelength at ∼435 nm. The band structure calculated for BCC lattice with reduced packing fraction (53%) is in good agreement with experimental results. The reflection due to the photonic band gap diminishes with increased randomness in the nanosphere arrangement. The features of fluorescence from ZnS are modified in the RPC medium, resulting in suppression at wavelengths in the photonic band gap region and an enhancement at band edge wavelengths of 415 and 468 nm. This enhancement becomes less prominent with increasing randomness in the structure. Interestingly these two modes correspond to the electronic defect states of ZnS. Emission enhancement is shown to be due to the strong coupling of electronic defect st...

FRET controlled photoluminescence in β-In2S3 microflower—Au nanoparticle ensemble

We report on the exciton–plasmon interaction and fluorescence resonance energy transfer controlled photoluminescence quenching and switching in β-In2S3 microflowers dispersed in Au nanoparticle colloid. The strong resonant interaction of excited β-In2S3 microflowers with the surface plasmons of Au nanoparticles (~520 nm) lead to shift in the excitonic binding energy (2.4 eV) with a magnitude of ~50 meV. In the proximity of Au nanoparticles, the broad emission spectrum of β-In2S3 microflowers with prominent peak at wavelength of ~540 nm is quenched and the peak switches to wavelength of ~600 nm. We demonstrate that the quenching and switching of emission band depends on the rate of fluorescence resonance energy transfer, extent of spectral overlap and β-In2S3 microflowers (donor)- Au nanoparticles (acceptor) distance. This study opens the wide possibility of fabricating sensors and photonic devices with tunable optical properties.

Surface plasmon polariton assisted red shift in excitonic emission of semiconductor microflowers

We report on the study of metal nanoparticle-semiconductor hybrid system composed of β-indium sulfide (β-In2S3) and gold (Au) nanoparticles. β-In2S3 micron sized flower like structures (∼1 μm) and Au nanoparticles (∼10 nm) were synthesized by chemical route. These Au nanoparticles have surface plasmon resonance at ∼ 520 nm. We study the influence of Au surface plasmon polaritons on the radiative properties of the β-In2S3 microflowers. As a result of the coupling between the surface plasmon polaritons and the excitons there is a red shift ∼ 50 nm in emission spectrum of hybrid β-In2S3-Au system. Such hybrid systems provide scope for a control on the optical properties of semiconductor microstructures, thus rendering them suitable for specific device applications in optoelectronics and photovoltaics.