Multiple release layer study of the intrinsic lateral etch rate of the epitaxial lift-off process

The lateral etch rate of AlGaAs in HF in the Epitaxial Lift-Off (ELO) process consists of two parts, an intrinsic and a radius-induced part. The intrinsic part is studied with a new approach in which multiple release layers are introduced in one sample. By letting an essential ELO process parameter vary over the different release layers, this parameter is examined, using only samples from one wafer. In this study, the influence of thickness, aluminium fraction, and doping concentration of the release layer on the lateral etch rate is investigated. For release layers with thicknesses below 10 nm, a positive correlation between thickness and intrinsic etch rate is found. Thicker release layers do not result in higher etch rates. Increasing aluminium fractions in the Al_xGa_1-xAs release layers result in higher etch rates. For aluminium fractions between 0.3 and 1, this effect covers almost six orders of magnitude. From the width of the V-shaped etch slits in samples that have been etched for 12 hours or more, the selectivity, i.e., the ratio of the etch rate of Al_xGa_1-xAs to GaAs, is determined. Selectivities between 4.3 and 8.6×10⁵ are found for x=0.3 and x=1, respectively. A variation in silicon doping is found to have no effect on the lateral etch rate, while increased zinc doping raises the etch rate significantly. PACS 81.05.Ea; 68.37.Hk; 81.15.Gh     

Improved Bend Loss Formula Verified for Optical Fiber by Simulation and Experiment

This paper presents an improved curvature loss formula for optical waveguides, which is shown to accurately predict the bend loss of both single-mode and multimode fibers. The formula expands upon a previous formula derived by Marcuse, greatly improving its accuracy for the case of multimode fiber. Also presented are the results of bent fiber simulations using the beam propagation method (BPM), and experimental measurements of bend loss. Agreement among simulation, formula and measurement support the validity of both theoretical methods. BPM simulations showed that the lowest order modes of the bent fiber were reduced to their linearly polarized constituents prior to the onset of significant bend loss. This implies that certain LP mode orientations should propagate with much lower loss than previously expected, and should impact the mode stripping ability of bent large mode area fibers, as employed in fiber lasers and amplifiers.     

Zwei neue Santoninreactionen

Ohne Zusammenfassung     

On a computationally efficient approach to boron-interstitial clustering

The physical concepts developed to describe the transient activation of boron during post-implantation annealing are based on the concurrent formation of complexes comprising boron atoms and self-interstitials. A complete implementation into TCAD software leads to a high number of equations to be solved which is often inadmissible for multi-dimensional simulations. In this work, a minimum number of such complexes is taken into considerations. We show that such a model is nevertheless able to reproduce profile shape and dopant activation for a large variety of implant and annealing conditions.     

Susceptibility of lithium-niobate modulator to high-power microwave pulses

The first measurement of the susceptibility of an off-the-shelf lithiumniobate intensity modulator to damage and disruption from high-power microwave pulses is reported. The device tested survived 1 kHz repetition rate pulses at 2.5 GHz centre frequency and 40 μs width up to 200 W peak power. The results are discussed in terms of material parameters and device characteristics.     

Ultra‐thin,high performance tunnel junctions for III–V multijunction cells

Tunnel junctions (TJ) made of p‐Al_0.1 Ga_0.9As/n‐GaAs are used because of their high peak current and low series resistance, but are not fully transparent. The influence of reducing the thickness of these tunnel junctions on the characteristics of InGaP/GaAs tandem cells was investigated. It was found that ultra‐thin TJs with excellent performance can be realized. Even for a 7.5/6‐nm thick TJ, which is the thinnest possible in our growth reactor, the peak current density is at least 600 A/cm². The series resistance of the TJs was found to be at a constant level of 0.6 ± 0.2 mΩ cm² for all total thicknesses of the TJ in the 13.5–40 nm range. Because of a lower absorption in the TJ, a tandem cell with a 7.5/6‐nm thick TJ, compared with a cell with a 20/20‐nm thick TJ, gained 0.53 ± 0.05 mA/cm² in short circuit current to a value of 14.8 mA/cm². Copyright © 2012 John Wiley & Sons, Ltd.     

Homo-epitaxial GaN growth on exact and misoriented single crystals: suppression of hillock formation

GaN single crystals were used as substrates for MOCVD growth. The ( ) plane of the substrate crystals was polished to obtain off-angle orientations of 0, 2, and 4° towards the [ ] direction. The highest misorientation resulted in a reduction of the hexagonal hillock density by nearly two orders of magnitude as compared with homo-epitaxial films grown on the exact ( ) surface. The features that are still found on the 4° off-angle sample after growth can be explained by a model involving the interaction of steps, introduced by the misorientation, and the hexagonal hillocks during the growth process. Following from this explanation it could be concluded that surface diffusion is found to be not important during growth on the N-side. The material quality of the N-side was examined by photoluminescence (PL) measurements. The PL spectrum measured at 5 K shows dominant donor bound excitons with a FWHM of 1.4 meV as well as free excitonic transitions.     

Wasser,Wäsche,Umwelt: Alles sauber?

The time‐consuming and tiring laundry job at the river has developed into a highly sophisticated technical task in washing machines with micro‐chip processors. The basic parameters time, chemistry, mechanics and temperature, however, remained unchanged, while their importance has shifted. When optimizing laundry detergents and process efficacy, the possible impact on waste water and environment has to be considered. The authors describe approaches to balancing the product efficacy and the ecological impact of the most important laundry detergent ingredients.     

Inversion of convolution equations on a finite interval and realization triples

This paper develops further the connections between convolution equations and realization triples. Here the emphasis is on equations on a finite interval. For each system of equations an operator (called indicator) is introduced which can be used to describe the inversion properties of the system. This indicator may be of simpler form than the convolution operator defined by the equations even for the case when the corresponding symbol is non-rational     

A genuine circular contact grid pattern for solar cells

A model was developed for a genuine circular contact grid, based on the same principles that led to previously reported models for other contact grid types. The circular grid differs from the regular (radial) grid often applied to round solar cells, in that it has considerably less radial lines and many more concentric rings, to the effect that the rings now take on the role of fingers, while the lines serve as busbars. For a 16 mm² n‐on‐p GaAs cell and irradiances ranging from 1 to 1000 suns, optimised grids of this new circular design were compared to equivalent radial, square and inverted square grids. Furthermore, in order to allow for a fair comparison of the different grid types, the previously reported radial grid model was modified to obtain additional freedom in design that is comparable to that of the other grid configurations. The circular grid turned out to suffer less power loss than a comparable radial grid up to a concentration ratio of 30. At higher concentration ratios, cells with a radial grid performed better, while comparable square and inverted square grids perform better over the entire range of concentration ratios. For GaAs cells serving as the bottom cell in a mechanical tandem, the effects of secondary shadowing were calculated for a range of translational and rotational misalignments, for the four patterns optimised for the case where the GaAs cell is effectively subjected to a concentration ratio of 500. The effects of translational misalignment varied little between the grids, but the rotational misalignment effects demonstrated the strength of the circular grid, since its shadowing loss hardly increases with increasing rotational misalignment angle, while those of the other three grid types quickly approach a doubling. When these secondary shadowing effects are taken into account, the circular grid can be the preferred pattern for the constituent cells of a mechanical stack, depending on the accuracy with which those cells can be rotationally aligned. Copyright © 2011 John Wiley & Sons, Ltd.