Ram Kishore

Screen printed titanium oxide and PECVD silicon nitride as antireflection coating on silicon solar cells

Silicon nitride and titanium oxide coatings have been used to reduce the reflection losses from silicon solar cells. Both 100-mm-diameter circular and 100 × 100 mm pseudo-square single crystalline silicon solar cells have been used in the present studies. More than 27% enhancement in the short circuit current has been demonstrated in polished cells using screen printed titanium oxide antireflection coating. Solar cells made from textured silicon wafers were used for plasma enhanced CVD grown silicon nitride antireflection coating on them. In these cells more than 23% enhancement in short-circuit current has been observed after silicon nitride antireflection coating.

PECVD grown silicon nitride AR coatings on polycrystalline silicon solar cells

Abstract Silicon nitride films produced by plasma enhanced chemical vapor deposition (PECVD) have been studied as antireflection (AR) coating on polycrystalline silicon solar cells. A substantial enhancement (28%) in the short circuit current ( I sc ) has been obtained. The open circuit voltage ( V oc ) of these cells has also been found to improve after silicon nitride deposition. The deposition conditions to optimise the improvement in the cell performance have been discussed.

Controlled growth of ZnS:Mn nanophosphor in porous silica matrix

The development of nanophosphors of desired sizes and properties for various practical applications and its growth in quantitative amounts inside the pores of an inorganic matrix is presented. By doing so, nanophosphors get surface passivated and are stabilized against environmental attacks. Accordingly, in the present study, the growth parameters for ZnS:Mn nanophosphors were systematically studied inside a SiO2 gel matrix, which can act as a capping agent as well. The samples were prepared using the sol-gel technique, followed by annealing at different temperatures to remove the trapped fluid inside the amorphous silica cage. Two categories of samples with lower (3.11×10−4) and higher (1.5×10−1) ZnS∕SiO2 molar ratios were studied. The x-ray diffraction and scanning electron microscopy observations show that upon annealing, the nanocrystals grow in size and undergo a phase transition from cubic to hexagonal at temperatures between 700 and 900°C. This is one of the very few known reports published on nano...

Thermionic emission diffusion model of current conduction in polycrystalline silicon and temperature dependence of mobility

In this paper a comprehensive model of current conduction in polycrystalline silicon (polysilicon) based on the thermionic‐emission‐diffusion (TED) theory is developed, and on the basis of this model an expression for the effective majority carrier mobility μeff is derived. This expression is quite general in nature and some thermionic emission (TE) theory based expressions for μeff can be obtained from it straightaway under certain simplifying assumptions. In addition, it helps in understanding the physical significance of the scaling factor used by earlier workers to explain their experimental results. Also, the experimental data on Hall mobility, which we obtained under an ohmic conduction regime in the 300–440 K temperature range for dark and illuminated conditions in lightly doped n‐type polysilicon samples of different grain sizes, are presented and are interpreted on the basis of the TED model. Under strong illumination, the Hall mobility μHL was observed to vary with temperature T according to the...

Modeling and observation of photoconductivity in polycrystalline silicon

We have developed a phenomenological model for the photoconductivity in polycrystalline semiconductors which takes into account the contributions from the majority carriers due to mobility enhancement in addition to the contributions due to photogenerated excess (minority) carriers. The model is applicable for the cases of both partially and completely depleted grains at all intensities of illumination. The model is compared with the experimental data of photoconductivity as a function of intensity of illumination obtained in polycrystalline silicon at different temperatures. The data are in good agreement with the theoretical predictions. The results clearly demonstrate that in polycrystalline materials, photoconductivity is a majority carrier phenomenon at low optical illuminations irrespective of whether the suppression of grain boundary potential barriers is partial or complete. However, at high level conditions, the photoconductivity behavior becomes an excess carrier effect as observed in single cry...

SEM, STM/STS and heavy ion irradiation studies on magnesium diboride superconductor

Abstract We have presented here the results of SEM and STM/STS investigations carried out on MgB 2 superconductor. The SEM pictures show porous surface and well-linked granular structure in which bigger grains (few micrometer size) seem to be agglomeration of smaller, nearly hexagonal grains (size nearly 100 nm). Hexagonal structure of Mg and B layers have been directly observed in atomically resolved STM images. The lattice constants have been determined to be a Mg =3.1 A, a B =1.7 A and c =3.5 A. Grain boundaries (GB) of width ranging from 50 to 200 A have been observed. Absence of weak link effects despite wide GBs has been attributed to the metallic nature of the amorphous region of the GB interior as inferred from STS analysis. Irradiation with 200 MeV 107 Ag ions gives tracks of about 65 A in the bulk of the sample. This is expected to give higher value of critical current density owing to the flux pinning mechanism, which is related to possibility of wide practical application of this material.

Interfacial Diffusion Effect on Metal Induced Crystallization of an Amorphous Silicon A Microstructural Pathway

In situ annealing experiments on hydrogenated amorphous-Si thin films coated with a thin layer of Al were performed under a scanning electron microscope equipped with a heating stage and an energy dispersive spectroscope. A sequential change in microstructural features due to interfacial diffusion at the apex of the boundary between Al and amorphous Si has been delineated. Transmission electron microscopy studies affirmed the evolution of polycrystalline Si as a result of the phase transition from amorphous to randomly oriented fine grained Si. A possible mechanism has been postulated to explore the metallurgical aspects of crystallization phenomena that persists during phase transformation.

Hard antireflecting PECVD silicon nitride coatings on polycrystalline germanium

Abstract In thermal imaging, germanium has found its important application in the form of windows, lenses and detectors. Roughening on the external surface of the exterior optical elements made from germanium is the most common form of damage. The surface roughening causes reduced transmission through the external optical components. In the present work, we have developed a thin film coating that protects the surface of germanium and also increases its transmission in the lower wavelength range, i.e.2.5 to 7 μm. The polycrystalline germanium ingots grown by a directional solidification technique and the silicon nitride films used in the present studies have been grown using the plasma enhanced chemical vapor deposition (PECVD) system. Infrared transmission measurements were carried out in the 2.5–25 μm wavelength range. It has been found that this coating enhances the transmission in the 2.5–7 μm wavelength range and also provides a good hard protective coating on the germanium surface.

Temperature dependence of resistivity and Hall mobility in floating zone grown bulk silicon-germanium alloys

Silicon germanium (Si1−xGex) alloys have been grown by the floating zone method with x varying from 0.5 to 20 wt %. Resistivity and Hall effect measurements have been carried out in the temperature range 80–410 K. The Hall mobility has been found to decrease with Ge concentration varying between 0.5 and 4.5 wt % and thereafter an increase has been observed. The Hall mobility has also been found to decrease with an increase in temperature in these alloys following the AT−y relation, where A is a constant and T is the temperature in K.

Transmission Electron Microscopy and X-Ray Diffraction Analysis of Aluminum-Induced Crystallization of Amorphous Silicon in [alpha]-Si:H/Al and Al/[alpha]-Si:H Structures

Solid phase crystallization of plasma-enhanced chemical-vapor-deposited (PECVD) amorphous silicon (alpha-Si:H) in alpha-Si:H/Al and Al/alpha-Si:H structures has been investigated using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Radiative heating has been used to anneal films deposited on carbon-coated nickel (Ni) grids at temperatures between 200 and 400 degrees C for TEM studies. alpha-Si:H films were deposited on c-Si substrates using high vacuum (HV) PECVD for the XRD studies. TEM studies show that crystallization of alpha-Si:H occurs at 200 degrees C when Al film is deposited on top of the alpha-Si:H film. Similar behavior was observed in the XRD studies. In the case of alpha-Si:H deposited on top of Al films, the crystallization could not be observed at 400 degrees C by TEM and even up to 500 degrees C as seen by XRD.