Reported is the integration of 1D planar silica graded index (GRIN) lenses in a 3D optical interconnect system to reduce free-space propagation loss. The 3D optical interconnect system consists of a pair of facing-up and facing-down 458 micro-mirrors that establish a vertical optical transmission path between stacked dies. Without the silica lens, experimental measurement shows optical power loss of 33.7dB. A pair of GRIN lenses is then integrated into the system. Optical test shows that the system performance is improved by more than 25dB.

This paper develops deposition techniques that are suitable for realizing low stress thick and crack free fluorine doped graded index silica (SixOyFz) films by the hollow cathode plasma enhanced chemical vapour deposition (HC-PECVD) system from a mixture of SiH4/O2/CF4. Silica films are deposited by varying O2, SiH4, CF4 flow rates, and RF powers. The refractive index, stress, deposition rate, chemical bonding state and chemical composition of such films have been investigated. The results indicate that low stress thick graded index silica film can be deposited by either (i) employing RF power below a certain value, which we shall refer it as ‘threshold RF power’ or (ii) increasing O2/SiH4 flow rate ratios. The threshold RF power is found to be 200W using 50sccm of O2, and 20sccm of SiH4. At RF powers below this threshold value, no oxygen deficiency is observed in the film even at a higher CF4 flow rate. On the other hand, stressed silicon rich film results at CF4 flow rates of 10sccm when the RF power is increased to 300W. By increasing O2 flow rate to 100sccm and decreasing SiH4 flow rate to 15sccm, the plasma at 300W RF power remains not oxygen deficient for up to 50sccm of CF4 flow rates. These deposition parameters are employed to deposit 28μm thick 1-D GRIN lens with a net stress of only 30MPa (compressive), and the graded index profile is optically tested. Moreover, the effect of moisture on such films has been investigated and a simple preventative measure is proposed.

This paper investigates various deposition and subsequent processing conditions on UHV e-beam evaporated silicon to obtain low stress film. They include substrate temperature, deposition rate, annealing, thermal oxidation and post-oxidation annealing. Film stress is measured for each condition and cantilever beams made from the films are released for evaluating stress-gradient. Films are also deposited on sloped step structures to observe step and corner coverage. The results indicate that as-deposited evaporated silicon exhibits tensile stress at substrate temperatures below 400 degrees C and compressive stress as substrate temperature is increased above 400 degrees C for a 100nm/min deposition rate. For evaporated amorphous silicon films, performing thermal oxidation at 900 degrees C and annealing at elevated temperatures has been found to be effective in reducing film stress. For fully crystallized poly-silicon films, however, annealing at 1000 degrees C without thermal oxidation seems to be the more effective way of reducing stress in the film. (C) 2012 Elsevier Ltd....Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.

In recent years, increased attention has been focused on the use of lasers in different fabrication steps of solar cells, in particular laser doping to form emitter and/or selective emitter. In this method the laser energy is used to melt silicon, allowing the diffusion of dopant atoms to occur in the liquid phase. The main advantage of this method is the localised nature of the laser beam, which melts and diffuses a limited area without heating the bulk, therefore reducing the possible degradation associated with high temperature processes. At the University of New South Wales a novel laser doping method was developed, which combines the formation of the selective emitter with a self-aligned metallisation pattern. Despite achieving high efficiencies, concerns arose regarding the adhesion of the metal to the shallow laser doped areas. This issue may be alleviated by increasing the roughness of the surface or even more so by creating holes/grooves in the laser doped areas. One simple way of achieving this is by carrying out the laser doping at higher laser energies to deliberately create some ablation. This paper examines the influence of the laser power on the solar cell electrical parameters to ascertain the relationship and the tradeoff between surface roughness and electrical performance. Efficiencies above 18% on a large area commercial grade p-type CZ substrate were achieved despite some ablation, confirming the potential for using this method to improve adhesion. Efficiency of 18.7% on the same substrate, using lower laser power, demonstrates the capability of the laser doping method.

The fabrication and experimental results on the performance of a planar silica lens pair that facilitates low-loss optical beam propagation in free space between opposing planar silica waveguides/singlemode fibres are presented. Each lens uses the parabolic graded refractive index vertically and the convex curvature horizontally to focus the optical beam in free space. The lens pair, designed for 200 mu m of free space propagation distance with ideal zero loss, is fabricated and measured for net loss of 1.6 dB which is 12.4 dB less than the coupling loss without a lens pair at 633 nm wavelength.

Laser doping offers a promising method to define selective emitters for solar cells. Its main advantage is the localised nature of the laser beam, which allows melting of the surface area without heating the bulk. The ability to perform this process over a dielectric film offers further benefits, such as the possibility of creating self-aligned metallisation patterns simultaneously with the selective emitter formation. However, laser induced defects, contaminations and discontinuities in the selective emitter can reduce solar cell performance. In this work the influence of different dielectric films on defect formation is investigated. It was found that a thin oxide beneath the SiNx improves the implied open circuit voltage of the solar cells for a wide range of laser output powers. Fewer defects were observed when using this SiO2/SiNx stack compared to the standard single SiNx anti-reflection coating layer. It was also found that the recrystallised silicon layer grows epitaxially according the substrate orientation. No dislocation or stacking faults were observed in deeper areas using transmission electron microscopy, although some defects were observed near the surface. Electron beam induced current images revealed discontinuities in junctions formed with high laser powers. We conclude that micro-cracks create these discontinuities, which can potentially induce shunts. Finally, laser doped solar cells with a standard SiNx and with a double SiO2/SiNx stack layer as anti-reflection coating were compared. An efficiency of 18.4% on a large area commercial grade p-type CZ substrate was achieved. Copyright (C) 2010 John Wiley & Sons, Ltd.

In this work we report on interlayer spin-polarized transport studies carried out on YBa2Cu3O7-δ/La2/3Sr1/3MnO3/YBa2Cu3O7-δ thin film multilayers. Anisotropic YBa2Cu3O7-δ high-temperature superconductors (YBCO) and the manganite La2/3Sr1/3MnO3 (LSMO) are layered metallic systems where the c-axis interlayer current transport occurs via sequential tunneling of charge carriers. The temperature dependence of the resistance R(T) for different applied fields shows that these heterostructures are clearly influenced by both the insulator-to-metal transition of the LSMO layers at ∼320 K and the stripe fluctuations in the conductivity of the YBCO layers at the onset of electronic inhomogeneity at ∼210 K. Furthermore, the tunneling of spin-polarized charge carriers between the YBCO layers through LSMO showed sharp switching between the high and low resistance states, which we assume corresponds to antiparallel and parallel alignments of the magnetic (spin) moments of these hybrid structures. The investigation of these promising hybrid structures is aimed at understanding the science of the new spin-based devices, laying the foundation for a new generation of ultrafast, nonvolatile electronics.

We investigated the emission of subterahertz frequency electromagnetic radiation from high-TC superconducting c-axis NdBa2Cu3O7−δ∕PrBa2Cu3O7−δ∕NdBa2Cu3O7−δ trilayer thin film tunneling junctions when external electric and magnetic fields are applied. The current-voltage characteristics under applied ab-plane magnetic fields H (up 8T) exhibit well defined steps, Vn, such that eVn≈hωp∕(2nπ), where the plasma frequency ωp≈0.4 THz and n=1,2,3,….. These steps may be interpreted using Josephson plasma dynamics. The applied voltage creates oscillating currents via the Josephson coupling energy EJ (EJ=hIJ∕4πe, where IJ is the Josephson current and h is Planck’s constant) and the charge energy EC (EC=e2∕2C, where C is the junction capacitance). Thus the Josephson plasma becomes excited by the tunneling current, with some of the energy being emitted as subterahertz frequency radiation. Our results provide a new insight into a solid-state quantum system with considerable potential for new solid-state terahertz emission sources.

The room temperature photoluminescence from silicon nitride multilayer structures, grown by plasma enhanced chemical vapour deposition, is monitored for different annealing temperatures and is correlated to structural and molecular changes in the film. Use of various annealing temperatures from 600°C to 1150°C results in films which vary from being completely amorphous to an amorphous matrix containing silicon nanocrystals, and finally to a fully crystallized composite containing silicon, α-Si3N4 and β-Si3N4 nanocrystals. Coupled with the observed trends seen with grazing incidence X-ray diffraction, transmission electron microscopy, and infra-red absorbance with annealing temperature, the photoluminescence from silicon nanocrystals embedded in amorphous silicon nitride is attributed to the presence of the nanocrystals in the film and not to transitions between band tails of the remaining amorphous matrix.

This work reports on the crystallization of α-Si3N4, β-Si3N4, and silicon in plasma enhanced chemical vapour deposition silicon nitride films grown with SiH4 and NH3 at 400∘C and annealed at 1150∘C. Nanometric multilayer structures, composed of alternating layers of silicon nitride and silicon-rich nitride, were used as the starting material. The final product is a thin-film Si–Si3N4 nanocomposite. The formation of this composite is verified using glancing incidence X-ray diffraction, transmission electron microscopy and Fourier transform infra-red spectroscopy. Annealing investigations indicate that the multilayer structure plays a key role in the formation of this composite and for the relatively low temperature formation of α- and β-Si3N4 nanocrystals.

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.

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.

The suitability of using dual-mode PECVD, in conjunction with high temperature annealing, to fabricate arrays of silicon nanocrystals in a nitride matrix over large areas is investigated. The formation of nanocrystals is verified using TEM, XRD, micro-Raman and FTIR. Initial results show reliable growth of a superlattice of Si nanocrystals over an area of 14 by 10.5 cm

A method for removing Si precipitations from the surface of AIC (aluminium-induced crystallisation) poly-Si films is introduced. The basic idea is to remove the aluminium oxide layer that is present between the poly-Si film and the Si precipitates and thus enable a lift-off process. Furthermore, a detailed structural investigation of the resulting smooth AIC poly-Si film prepared on SiN-coated planar glass is performed. Based on Raman, UV reflectance and plan-view TEM, the overall crystal quality is found to be excellent for a poly-Si film on glass. Especially the fact that 10-micron grains without dendritic growth pattern are obtained is encouraging. However, there is still room for improvement as both Raman and plan-view TEM reveal areas (/spl sim/10% of total surface area) of sub-micron grains and/or twinned grains. Work is in progress to understand the mechanism behind these results.

Antennas with rectifiers, "rectennas", have been proposed for high efficiency conversion of solar energy and high efficiencies have been demonstrated for quasi-monochromatic microwaves. Antennas receiving black body radiation may be modelled as a pair of resistors at the black body temperature, each supplying electrical noise power. Extraction of that power from a load as heat is feasible at the same limiting efficiencies as solar thermal converters. Rectennas, relying on rectification of the broadband electrical noise, appear to have lower efficiency limits due to the unavailability of a design for the Carnot conversion of the noise from a hot resistor.

Ion-assisted deposition (IAD) of silicon at a substrate temperature of 630 °C was used for thickening a crystalline silicon seed layer made on glass by aluminum-induced crystallization (AIC) of amorphous silicon. The seed layer consists of randomly oriented crystal grains whose crystallographic orientations with respect to the substrate's normal were determined with cross-sectional transmission electron microscopy (TEM) by analyzing their Kikuchi diffraction patterns. As shown by the TEM analysis, the IAD film grown on the AIC seed layer continues epitaxially the crystallographic structure of the seed layer. The crystallographic orientation of the seed layer does not appear to play the dominant role in determining the crystal quality of the subsequently grown IAD film. The fabrication of continuous AIC seed layers results in the production of island-like silicon protrusions on the surface of the seed layer. The impact of these “islands” on the subsequent IAD growth was investigated by comparison to the gr...

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.

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.