Pratima Agarwal

Persistent photocurrent and decay studies in CdS nanorods thin films

Structural and electrical transport properties of thin films of CdS nanorods synthesized by solvothermal process are studied through x-ray diffraction, scanning electron microscopy, transmission electron microscopy, activation energy measurements, and decay of photoconductivity. The films show thermally activated conductivity, high photosensitivity, and persistent photoconductivity, which decays initially fast followed by slow decay. The decay time constant for fast decay are of the order of a few seconds, whereas for slow decay it is in the range of about 100 s. The electrical transport is similar to that in the polycrystalline material. The slow decay of excess conductivity has been attributed to the presence of defects and local potential fluctuations within the nanorods.

Changes in surface topography of amorphous silicon germanium films after light soaking

Light-induced metastable degradation of hydrogenated amorphous silicon and silicon germanium thin films (a-SiGe:H) is conjectured to be accompanied by structural changes but there has not been a direct measurement of the same. We measure the surface topography of these films in the annealed and the light soaked state using atomic force microscopy. We quantified the surface topography in terms of surface roughness and find that the surface roughness increases after light soaking. Our results provide direct evidence of the light-induced structural changes in these films.

Determination of thickness, refractive index, and spectral scattering of an inhomogeneous thin film with rough interfaces

The magnitude of spectral transmittance and reflectance is affected by the presence of inhomogeneity and interfacial roughness. Therefore, the methods, based on the magnitude of spectral transmittance and reflectance, are not adequate for the determination of thickness and optical constants of films with inhomogeneity and interfacial roughness. The present article proposes a method for the determination of thickness and refractive index using only the positions of the interference fringes in spectral transmittance and reflectance at two different angles of incidence. The proposed method is verified through numerical simulations, which result in <1% error for the film thickness. The complete parametrical dependence of spectral transmittance and reflectance of inhomogeneous film with rough interfaces on a substrate have been worked out for the film on transparent and opaque substrates, respectively. The spectrum envelopes have been solved simultaneously and the mathematical formulae are given for the determ...

Evolution of nanostructure in hydrogenated amorphous silicon thin films with substrate temperature studied by Raman mapping, Raman scattering and spectroscopic ellipsometry

Raman mapping and Raman scattering studies have been performed on hydrogenated silicon thin films deposited at different substrate temperature by rf-PECVD technique. In Raman mapping a clear difference in contrast, corresponding to different phases, was observed for films deposited at substrate temperature (Ts) ≥ 130 °C. The Raman spectra recorded at bright and dark spots in Raman image correspond to nanocrystalline and amorphous phase respectively. The fractional area of bright spots in Raman mapping was found to increase with increasing Ts. Also, the nanocrystalline fraction and size of nanocrystallites is found to increase with increasing Ts. These studies suggest that nucleation sites for the growth of nc-Si:H are formed at substrate temperature as low as 130 °C. With further increase in Ts more and more nucleation sites are formed which subsequently enhance crystallinity in these films. Films deposited at Ts ≥ 150 °C have larger band gap, higher roughness and higher photosensitivity and are of excellent device quality.

An alternative approach to understand the photoluminescence and the photoluminescence peak shift with excitation in porous silicon

There have been many different models proposed for the luminescence in porous silicon (PS), yet it is believed that the quantum confinement effect persists at the absorption. However, from our investigation on both constant and pulsed electrochemically etched silicon (PS), the absence of quantum confinement effect at the absorption has been identified from the close correspondence of photoluminescence excitation (PLE) spectra of PS to the simulated absorption spectrum of an ultrathin silicon film with the bulk optical constants. In the simulation of absorption spectrum, the spectral dependence of reflectivity of the solid, which had been omitted in the traditional analysis of PLE, is considered. Further, although nanocrystallites of silicon are present in the PS matrix, the absence of quantum confinement is explained on the basis of structural characteristics of PS. Following that, many common observations in the luminescence of PS are attributed to the surface states. The blueshift of the PL peak with th...

Nanocrystalline cubic Silicon Carbide thin films for the window layer of solar cells deposited by Hot Wire CVD

Nanocrystalline cubic silicon carbide (nc-3C-SiC) films are deposited using hot wire chemical vapour deposition technique at ~350 °C on glass substrates using SiH4 /CH4/H2 as precursor gases. We investigated the influence of total gas pressure on the structural, optical and transport properties of nc-3C-SiC films. Raman scattering spectra and X-ray diffraction patterns revealed that the film prepared below 2 mbar is nanocrustalline silicon (nc-Si), while at ≥ 2 mbar films are nc-3C-SiC. We achieved high deposition rate (≥ 14-20 nm/min), high optical band gap (3.2-3.4 eV) and high conductivity (~ 10-4 -10-2 Ω-1cm-1) suitable for window layer for Solar cells.

Structural and stability studies of SnS nanoflakes synthesized by solvothermal process for solar photovoltaic applications

SnS nano-flakes are synthesized by solvothermal process and characterized well by X-ray diffraction (XRD) and scanning electron microscope (SEM). Widths of the as-synthesized SnS nano-flakes vary from 100 nm to several micrometers whereas thicknesses of these SnS nano-flakes vary from 70-80 nm. Chemical purity and stoichiometric ratio of the SnS nano-flakes are observed by energy dispersive analysis of X-rays (EDAX). Thermal stability of the SnS nano-flakes is investigated by thermo gravimetric analysis (TGA). Different kinetic parameters such as activation energy (E), Arrhenius parameter (A) and entropy change (DeltaS) are calculated by Redfern-Coats and Satava model.

Origin of lateral photovoltage in hydrogenated amorphous silicon and silicon germanium thin films

Large lateral photovoltages (LPVs) have been measured in hydrogenated amorphous silicon and silicon germanium thin films. We find that LPV increases upon light soaking (LS) the samples. The diffusion lengths of photocarriers have been measured by the steady state photocarrier grating (SSPG) technique in annealed and light soaked states. Our study shows that the knowledge of carrier diffusion lengths and photoconductivity is not sufficient in understanding LPV and that it can be explained in terms of potential fluctuations present in the sample.

FEM Based Device Simulator for High Voltage Devices

TCAD simulation of electronic device has always been the basic approach to understand solid state electronics and to frame road-map for the evolution of future technology. Design of devices on these materials require better understanding of the physical insights to the internals of the device structure. In such a scenario, TCAD tool can help to visualize internal dynamics of carriers and fields in the device structure, thus helping to improve them further. Device structures are evolving continuously leading to an increase in complexity of computation of simulation. There is an increasing challenge to these simulators to improvise compact device models, whereby generating precise results. The responsibility of TCAD designers is ever increasing to develop improved solvers featuring better predictive capabilities. In this work, an effort has been made to compare the performance of an FEM based proposed simulator with conventional available device simulator. A simple pn junction diode is designed in both the simulators and a comparison of different electrical properties has been done by incorporating similar models and exactly same material parameters.

Third order optical nonlinear studies on highly conducting vertically aligned carbon nanoflakes

Third order optical nonlinearity of carbon nanoflakes were studied by modified single beam closed aperture Z-scan technique using a continuous wave He–Ne laser at 632.8 nm. Thin films of vertically aligned carbon nanoflakes were synthesized on corning glass substrate at substrate temperature of 400 °C by hot filament chemical vapor deposition. Films were characterized by scanning electron microscope and atomic force microscopy which confirmed that carbon nanoflakes were vertically aligned on the substrate. Temperature dependent electrical conductivity measurements in temperature range of 300–480 K under high vacuum (~10−5 mbar) showed that conductivity of the films was increased almost linearly with increasing temperature with a weak temperature dependence. The negative temperature coefficient of resistance indicates semiconducting behavior of the films. Nonlinear refractive index coefficient (n 2) of the films was found to be of the order of 10−5 cm2 W−1, which can be important for the applications in the field of nonlinear photonics.