T. Puzzer

Aluminum-induced crystallization of amorphous silicon on glass substrates above and below the eutectic temperature

The achievement of high-quality continuous polycrystalline silicon (poly-Si) layers onto glass substrates by using aluminum-induced crystallization is reported. The crystallization behavior of dc sputtered amorphous silicon on glass induced by an Al interface layer has been investigated above and below the eutectic temperature of 577 °C. Secondary electron micrographs in combination with energy-dispersive x-ray microanalysis show that annealing below this temperature leads to the juxtaposed Al and Si layers exchanging places. The newly formed poly-Si layer is fully crystallized and of good crystalline quality, according to Raman spectroscopy and transmission electron microscopy investigations. At 500 °C, the time needed to crystallize a 500-nm-thick Si layer is as short as 30 min. By annealing above the eutectic temperatures, layer exchange is not as pronounced and the newly formed Al layer is found to contain a network of crystallized Si.

Diffusion lengths of silicon solar cells from luminescence images

A method for spatially resolved measurement of the minority carrier diffusion length in silicon wafers and in silicon solar cells is introduced. The method, which is based on measuring the ratio of two luminescence images taken with two different spectral filters, is applicable, in principle, to both photoluminescence and electroluminescence measurements and is demonstrated experimentally by electroluminescence measurements on a multicrystalline silicon solar cell. Good agreement is observed with the diffusion length distribution obtained from a spectrally resolved light beam induced current map. In contrast to the determination of diffusion lengths from one single luminescence image, the method proposed here gives absolute values of the diffusion length and, in comparison, it is much less sensitive to lateral voltage variations across the cell area as caused by local variations of the series resistance. It is also shown that measuring the ratio of two luminescence images allows distinguishing shunts or surface defects from bulk defects.

Solar energy collection by antennas

The idea of collecting solar electromagnetic radiation with antenna-rectifier (rectenna) structures was proposed three decades ago but has not yet been achieved. The idea has been promoted as having potential to achieve efficiency approaching 100% but thermodynamic considerations imply a lower limit of 85.4% for a non-frequency-selective rectenna and 86.8% for one with infinite selectivity, assuming maximal concentration in each case. This paper reviews the history and technical context of solar rectennas and discusses the major issues: thermodynamic efficiency limits, rectifier operation at optical frequencies, harmonics production and electrical noise.

Fourier transform infrared spectroscopy of annealed silicon-rich silicon nitride thin films

A correlation between bonding changes in silicon-rich silicon nitride films, subjected to high temperature annealing under N2 ambient, and the formation of silicon nanocrystals is presented. The postannealing appearance of a shoulder between 1000 and 1100 cm−1 in the Fourier transform infrared (FTIR) spectra of silicon-rich silicon nitride films is attributed to a reordering in the films toward an increased SiN4 bonding configuration resulting from the precipitation of silicon nanocrystals. The FTIR monitoring of bonding changes in these films allows for the indirect verification of silicon nanocrystal formation.

Quantitative interpretation of electron-beam-induced current grain boundary contrast profiles with application to silicon

An improved method is described for extracting material parameters from an experimental electron-beam-induced current (EBIC) contrast profile across a vertical grain boundary by directly fitting an analytical expression. This allows the least-squares values of the grain boundary recombination velocity and the diffusion length in each grain to be determined without the need for the reduction of the experimental profile to a few integral parameters, as is required in a previously reported method. Greater accuracy of the extracted values is expected since none of the information contained in the experimental contrast data is discarded and a less extensive spatial range of measured data is required than in the commonly used method. Different models of the carrier generation volume are used in the fitting and the effect of the choice of generation model on extracted values is investigated. In common with other EBIC approaches, this method is insensitive to changes in the diffusion length when the collection ef...

Josephson behaviour for high critical current density YBCO+Ag thick films on YSZ substrates

Abstract High critical current density thick films of yttrium barium copper oxide plus 10 wt.% silver have been fabricated on yttria stablised zirconia substrates. The films are approximately 10 μm thick, superconducting with T c ( R = 0) in the range 90.5 to 91.0 K and are found to have j c s close to 3000 A cm -2 at 77 K in zero applied magnetic field. The thermal cycle for the production of these films requires the use of temperatures in excess of the peritectic and therefore involves melt processing. This has a number of advantages, including film texturing, which is significantly improved with the addition of Ag, and the formation of an inert BaZrO 3 barrier layer which has fine CuO needles dispersed throughout. This layer prevents atomic diffusion and film poisoning. At 77 K, the j c values decreased markedly for applied magnetic fields increasing from 0–20 Oe, and then decreased slowly for increasing fields, being almost constant for field values ∥#62; 150 Oe. The temperature variation, near T c , of the critical current i c was best described by i c £ (1 − T / T c ) 1.52 , the Ginzburg-Landau 3 2 power law characteristic, and indicates that the granular films may be modelled as an array of Josephson-coupled grains which have a short coherence length. Variable thickness bridges patterned into the thick films showed very clear DC and AC characteristics that were very similar to those expected from a single Josephson junction.

Effects of helium absorption on the superconducting mechanism of YBa2Cu3Oy

Abstract During the course of a series of measurements of the electrical conductivity and Hall effect in the YBa2Cu3Oy superconductors, it was observed that the presence of an helium atmosphere during the measurements led to reversible changes in the electrical properties including increases in the critical temperature. The results of a series of measurements to investigate this effect are described and possible mechanisms considered. It is suggested that helium enters the lattice and leads to different internal pressure effects in different parts of the unit cell.

Surface morphology of flux-grown single crystals of YBa2Cu3O7−δ

Abstract Single crystals of YBa 2 Cu 3 O 7−δ have been grown in a CuO-BaCuO 2 flux at a temperature of 930°C and examined using SEM, XRD and electron diffraction techniques. The observed crystal habit is predominantly square planar with the c -axis normal to the plane. Surface growth spirals are observed in approximately 25% of the crystallites and in some cases extensive step decoration is exhibited. The growth surface usually involves two such spirals with Burgers vectors of the same sign. Opposed spirals have been observed in only two cases.

Conductivity of self-organized silicon quantum dots embedded in silicon dioxide

Silicon quantum dots (SiQDs) embedded in silicon dioxide are being investigated as a means of engineering a wide band gap semiconductor for potential application in silicon based tandem solar cells. The conductivity of the self-organized silicon dots embedded in the oxide is an important parameter in characterizing the electronic transport mechanisms. We present in this paper our initial results on measurement of the resistivity as a function of temperature. In order to reduce contact resistance aluminium contacts are annealed to induce spiking through upper layers of oxide and thus producing a large contact surface area. Samples with various initial silicon rich concentrations are compared. Activation energies for various tentative conduction mechanisms are calculated from this data and possible conduction models presented.

Epitaxially grown GaAsN random laser

We have studied the photoluminescence properties of as-grown GaAs1−xNx epitaxial layers grown on GaAs containing 0.6%, 1.77%, and 2.8% nitrogen. We found laser emission from thick (d>200 nm) GaAs0.972N0.028 layers exhibiting the characteristic lasing properties of random lasers. This is unusual because random lasers have so far only been associated with highly disordered or random media. We believe that high gain in combination with structural inhomogeneities that are evident in these GaAs0.972N0.028 layers, can explain the random lasing in such epitaxial layers.