Papers by Ravindra Nuggehalli
MRS Proceedings, 1997
ABSTRACTWe describe a new technique for producing large-grain, poly-Si thin films on low-cost gla... more ABSTRACTWe describe a new technique for producing large-grain, poly-Si thin films on low-cost glass substrates for solar cell applications. A layer of fine-grain poly-Si is deposited on metal-coated substrate followed by a grain enhancement using optical/thermal annealing at low temperatures (∼ 500 °C). The results show that in thin-layer silicon, less than 3 microns, grains can be formed in a short time (few minutes) with grain sizes larger than the film thickness. The possible mechanisms involved in this process are also presented.
Solar Energy Materials, 1979
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MRS Proceedings, 1994
ABSTRACTThe effect of furnace grown SiO2 layers on the optical properties of p- on p+ (100) Si su... more ABSTRACTThe effect of furnace grown SiO2 layers on the optical properties of p- on p+ (100) Si substrates are investigated. The real part, n, of the complex refractive index n* = n + ik is calculated for radiation measured in the infra-red (IR) region between 3000 and 8000 cm−1 where the extinction coefficient, k, is negligible. The expression for n is obtained using the Fresnel coefficients for a three medium air-oxide-Si model. Strain in the silicon, which affects n, and caused by the stress in the SiO2 layer, increases with oxide thickness. X-ray diffraction (XRD) was used to measure the strain in Si for oxides layers ranging from native to 5124Å. The data showed a monotonically increasing normal compressive strain, εN (up to 0.47%) with oxide thickness, however, the corresponding change in n due to strain was not well defined. The effect of strain on the direct optical gap, Ed, at 3.46 eV when determined from results of other investigators by electroreflectance, suggests an aver...
Il Nuovo Cimento D, 1985
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Journal of Electronic Materials
MRS Proceedings, 1996
A method of estimation of wavelength-dependent emissivity of silicon wafers using thermocouple me... more A method of estimation of wavelength-dependent emissivity of silicon wafers using thermocouple measurements is presented here. A dynamic observer and a persistent excitation of the lamps are required for convergence of the observer's estimates to the actual values of parameters for emissivity approximation. A TI designed RTP equipment and adaptive close-loop temperature control have been used to generate the required persistent excitation. Theory of emissivity estimation by this method has been developed and illustrated by experiment with this 3- zone RTP system.
MRS Proceedings, 1998
Multicrystalline Si (mc-Si) wafers, used for the commercial solar cell fabrication, have spatial ... more Multicrystalline Si (mc-Si) wafers, used for the commercial solar cell fabrication, have spatial nonuniformities in the material properties that cause strong variations in the minority carrier lifetime, τ. We present the results of two-dimensional modeling to show carrier generation, recombination and transport in such a material. These results are used to infer measurement conditions that can yield meaningful spatially weighted average value of τ.
MRS Proceedings, 1997
ABSTRACTExperimental studies of the room temperature emissivity of polysilicon are reported here.... more ABSTRACTExperimental studies of the room temperature emissivity of polysilicon are reported here. These measurements have been performed using a spectral emissometer operating in the wavelength range of 0.8 – 20 μm. The measured optical properties are deconvolved to yield the wavelength dependent refractive indices and extinction coefficient of polysilicon. An in house developed computer program, OPCalc, is deployed to perform these calculations. Experimental results of the temperature dependent emissivity of a-Si / SiO2 / Si / SiO2 / a-Si, fabricated on single side polished silicon substrates, in the temperature range of 300 to 1100 K have been reported here. These measurements are performed for a-Si thickness of 2100A. Comparisons of the temperature dependent radiative properties of these structures between the front and the back show that the contribution of surface roughness to emissivity is negligible- The exposure of these a-Si coated structures to high temperatures in open en...
Materials Processing and Energy Materials, 2011
ABSTRACT Optically active Er+3 in silica films, which are suitable for silicon-based optical appl... more ABSTRACT Optically active Er+3 in silica films, which are suitable for silicon-based optical applications, have been prepared by a sol-gel deposition process with high Er atomic concentrations of 6 and 12 %, followed by furnace annealing. Films were characterized for their thickness, index of refraction, and photoluminescence at room temperature. Photoluminescence yield is strongly enhanced for vacuum annealing in the temperature range 500 - 750 degrees C. Reasonable room-temperature photoluminescence is also observed for annealing at 1000 degrees C. These results demonstrate the viability of incorporating Er at high concentrations.
MRS Proceedings, 1991
ABSTRACTExperimental analysis and simulation of the formation and electrical characterization of ... more ABSTRACTExperimental analysis and simulation of the formation and electrical characterization of TiSi2/+/p-Si shallow junctions are presented here. The formation of shallow n+-p junction, by ion implantation of As through Ti films evaporated on p-Si substrates followed by Rapid Thermal Annealing (RTA) and conventional furnace annealing has been performed in these experiments. Structural techniques such as Secondary Ion Mass Spec-troscopy (SIMS) and Rutherford Backscattering (RBS) experiments have been employed to characterize these devices. RUMP simulations were used to analyze and interpret the RBS data. Current-voltage characteristics have been simulated using PISCES simulator.
Ceramic Transactions, 2012
ABSTRACT Cobalt aluminate-Fe2O3 p-n nanocomposite electrodes were deposited on the silver coated ... more ABSTRACT Cobalt aluminate-Fe2O3 p-n nanocomposite electrodes were deposited on the silver coated stainless steels and annealed at 800 °C. Their photoresponses were investigated and compared with that of p-type Cobalt aluminate. We found that nanocomposite electrodes exhibited much improved photoresponses as compared to p-type Cobalt aluminate. We attribute the improvement to the band offset at the formed threedimensional p-n junction, which promote photo-generated carrier separation and reduce carrier recombination.
Materials Processing and Interfaces, 2012
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Ceramic Transactions, 2012
In current manufacturing regimes, silicon solar cells are made from low-cost silicon wafers, whic... more In current manufacturing regimes, silicon solar cells are made from low-cost silicon wafers, which are produced in one of three ways: low-grade, Czochralski-type single-crystal wafers; multicrystalline silicon wafers grown by casting; or shaped ribbons produced by directional solidification. These materials contain high concentrations of impurities and defects, which lead to spatial variations in the material properties. A detailed characterization of the substrates is needed to understand the nature of defects and impurities and their influence on solar cell performance. Although many techniques can be applied to map material properties, the influence of the various materials on cell performance is difficult to determine. We have developed a technique to fabricate arrays of edge-passivated diodes (small-area solar cells), which can be probed using an automatic prober to generate dark and illuminated I-V characteristics of each device. These devices can be readily used for other measurements, such as measurement of minority-carrier diffusion length, light or electron beam induced current, C-V analyses, and DLTS. The data from diode arrays are used to study influence of defects and impurities on cell performance and to predict the highest large-area cell performance.
MRS Proceedings, 1992
THERMID, made of acetylene-terminated polyimide oligomers, was developed by National Starch & Che... more THERMID, made of acetylene-terminated polyimide oligomers, was developed by National Starch & Chemical Co. It is capable of forming thicker deposits than conventional polyimides. It is a candidate for a broad array of electronic and optoelectronic applications, including passivation coating, alpha-particle barrier, interlayer dielectric, encapsulation of discrete devices and IC, optical waveguide, and multilayer dielectric for thin hybrids. The study and evaluation of electrical and optical properties of THERMID polyimide give more support to their application in the semiconductor industry.
MRS Proceedings, 1998
We have investigated the microstructure and the IR range optical properties(emittance, transmitta... more We have investigated the microstructure and the IR range optical properties(emittance, transmittance and reflectance) of diamond-like carbon (DLC) films with the incorporation of dopants. The DLC films were deposited by pulsed laser deposition with a novel target configuration allowing incorporation of dopants, such as silicon, titanium and copper, into the films. Raman spectroscopy, radial distribution function (RDF) and coordination defect analysis of the electron diffraction pattern of the films showed typical features of DLC with a structure dictated by sp3 bonded carbon, indicating that the overall DLC characteristics did not change upon doping. The IR range optical measurements showed that in addition to the general band-toband transitions, free carriers and phonon contributions, localized states also contribute to the emissivity in DLC and they smooth out the sharp features of the emissivity spectra. The effect of dopants is to enhance the contribution from the free carriers ...
MRS Proceedings, 1988
Studies of the electrical and structural properties of TiSi2 films formed during the fabrication ... more Studies of the electrical and structural properties of TiSi2 films formed during the fabrication of shallow junctions by ion implanting As+ into p–Si through Ti films are reported here. Electrical measurements of the temperature dependent specific contact resistance and structural measurements such as transmission electron microscopy have been performed on these device structures. The specific contact resistance is seen to decrease with increasing measurement temperature. These studies have been carried out on furnace and rapid thermally annealed films.
MRS Proceedings, 1987
Single crystal (111) and (100) Ge wafers were implanted with 16O (180 keV, 2.0 x 1018/cm2, 14–28 ... more Single crystal (111) and (100) Ge wafers were implanted with 16O (180 keV, 2.0 x 1018/cm2, 14–28 ¼A/cm2 ) at substrate temperatures of 250, 330, and 500°C. Implanted samples were annealed at 350, 450, 550, and 650°C for 30–90 minutes in an Ar ambient. Rutherford backscattering channeling analysis and cross-sectional transmission electron microscopy indicate that an amorphous buried layer is formed by implantation and that the overlayer contains a dense network of precipitates. Electron spin resonance measurements indicate that the layer does not contain GeO2, but rather oxygen deficient GeO2. Annealing of samples up to 550°C showed no change in the morphology, however, after annealing at 650°C the buried layer was gone and all that remained was a damaged Ge substrate with little or no oxygen. Further annealing for 60 min left nearly virgin Ge.
Supplemental Proceedings, 2012
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Papers by Ravindra Nuggehalli