Micron-scale characterization of laser processed silicon via low temperature micro-photoluminescence spectroscopy

Laser processing is now regarded as a promising tool to reduce the cost and complexity of fabricating the formation of localized contacts between heavily doped silicon and metal, features which have become an important element in high efficiency silicon solar cells, such as a passivated emitter and rear cell (PERC) and an interdigitated back contact cell (IBC). However, characterization of localized features with conventional PV characterization tools is challenging, mainly due to the limitations of spatial resolution. This thesis develops and applies novel characterization methods to these localized features using low temperature micro-photoluminescence spectroscopy (μ-PLS). This technique demonstrates that localized features, even single laser pulse processed regions typically tens of micrometres in scale, can be investigated directly without the need for specific sample structures and their electronic properties can be mapped spatially in the sub-micrometre regime. Utilizing the sub-micron precision of these measurements, the laser-induced crystallographic damages were investigated at various positions within the laser-processed region, particularly at specific points such as the boundary/edge of processed and unprocessed regions. It was found that the edge, or pulse overlapped regions, were significantly more defective than the centre region. The impact of laser parameters, such as laser pulse fluence and number of repeat pulses, on laser-induced damage was also analysed. Significantly different levels of defect-related PL signals were observed after laser processing of the two different substrate surface conditions. This suggests that wafer surface preparation can be an important factor impacting on the quality of laser-processed silicon. The doping profiles of thermally boron-diffused silicon samples, which have Gaussian function type doping profiles, can be estimated from the measured PL spectra alone. The wavelength of the doping-related PL peak (doping peak) has a reliable and simple linear relationship with the surface dopant density on a semi-log plot. The PL intensity of the doping peak also shows a linear relationship with the doping depth metric (depth factor), but only after considering the reduction of PL intensity due to enhanced incomplete dopant ionization at low temperature. Doping profiles can be easily reconstructed based on these two linear relationships and their vi accuracy was verified by comparisons with existing doping profiles (via ECV profiling). Mapping of the surface dopant density and the depth factor of micron-scale locally diffused features was undertaken using 2-D mapping with μ-PLS measurements at 2 μm spatial resolution. This method was also applied to 532 nm laser-doped silicon to show its effectiveness on locally laser-doped features. The doping profiles of laser-doped silicon were also successfully estimated from PL spectra measurements alone, along with 2-D maps of the surface dopant density and the depth factor of the laser-doped silicon. In addition, the impact of temporal pulse parameters, such as pulse duration and temporal pulse shapes, on the doping profiles and recombination properties of laser-doped silicon were investigated. By correlating defect-related PL band counts with the quantified recombination parameters determined by the luminescence-coupled numerical device simulations, it was shown that μ-PLS measurements are able to perform quantitative measurements of recombination properties. The last chapter of this thesis demonstrates an application of an advanced laser doping process using a stack of intrinsic amorphous silicon (Si:H(i)) and boron-doped amorphous silicon (a-Si:B). The results showed that this stack is able to provide excellent surface passivation as well as a sufficient amount of dopant source for laser doping. The method presented in this thesis is a very effective, simple and rapid characterization for analysing localized features, in particular spatially inhomogeneous laser-processed features on the micron-scale. This method enables the observation of the variation in properties within localized features which is not possible using conventional methods. It allows for a more in-depth study of laser processing and promotes further development of laser technologies for high efficiency cell fabrication

Evaluation of the Oscillatory Interference Model of Grid Cell Firing through Analysis and Measured Period Variance of Some Biological Oscillators

Models of the hexagonally arrayed spatial activity pattern of grid cell firing in the literature generally fall into two main categories: continuous attractor models or oscillatory interference models. Burak and Fiete (2009, PLoS Comput Biol) recently examined noise in two continuous attractor models, but did not consider oscillatory interference models in detail. Here we analyze an oscillatory interference model to examine the effects of noise on its stability and spatial firing properties. We show analytically that the square of the drift in encoded position due to noise is proportional to time and inversely proportional to the number of oscillators. We also show there is a relatively fixed breakdown point, independent of many parameters of the model, past which noise overwhelms the spatial signal. Based on this result, we show that a pair of oscillators are expected to maintain a stable grid for approximately t = 5µ3/(4πσ)2 seconds where µ is the mean period of an oscillator in seconds and σ2 its variance in seconds2. We apply this criterion to recordings of individual persistent spiking neurons in postsubiculum (dorsal presubiculum) and layers III and V of entorhinal cortex, to subthreshold membrane potential oscillation recordings in layer II stellate cells of medial entorhinal cortex and to values from the literature regarding medial septum theta bursting cells. All oscillators examined have expected stability times far below those seen in experimental recordings of grid cells, suggesting the examined biological oscillators are unfit as a substrate for current implementations of oscillatory interference models. However, oscillatory interference models can tolerate small amounts of noise, suggesting the utility of circuit level effects which might reduce oscillator variability. Further implications for grid cell models are discussed

Reduction in Recombination Current Density in Boron Doped Silicon Using Atomic Hydrogen

The solar industry has grown immensely in recent years and has reached a point where solar energy has now become inexpensive enough that it is starting to emerge as a mainstream electrical generation source. However, recent economic analysis has suggested that for solar to become a truly wide spread source of electricity, the costs still need to plummet by a factor of 8x. This demands new and innovative concepts to help lower such cost. In pursuit of this goal, this dissertation examines the use of atomic hydrogen to lessen the recombination current density in the boron doped region of n-type silicon solar cells. This required the development of a boron diffusion process that maintained the bulk lifetime of n-type silicon such that the recombination current density could be extracted by photoconductance spectroscopy. It is demonstrated that by hydrogenating boron diffusions, the majority carrier concentration can be controlled. By using symmetrically diffused test structures with quinhydrone-methanol surface passivation the recombination current density of a hydrogenated boron profile is shown to be less than that of a standard boron profile, by as much as 30%. This is then applied to a modified industrial silicon solar cell process to demonstrate an efficiency enhancement of 0.4%

One-Dimensional Spacecraft Formation Flight Testbed for Terrestrial Charged Relative Motion Experiments

Spacecraft operating in a desired formation offers an abundance of attractive mission capabilities. One proposed method of controlling a close formation of spacecraft is with Coulomb (electrostatic) forces. The Coulomb formation flight idea utilizes charge emission to drive the spacecraft to kilovolt-level potentials and generate adjustable, micronewton- to millinewton-level Coulomb forces for relative position control. In order to advance the prospects of the Coulomb formation flight concept, this dissertation presents the design and implementation of a unique one-dimensional testbed. The disturbances of the testbed are identified and reduced below 1 mN. This noise level offers a near-frictionless platform that is used to perform relative motion actuation with electrostatics in a terrestrial atmospheric environment. Potentials up to 30 kV are used to actuate a cart over a translational range of motion of 40 cm. A challenge to both theoretical and hardware implemented electrostatic actuation developments is correctly modeling the forces between finite charged bodies, outside a vacuum. To remedy this, studies of Earth orbit plasmas and Coulomb force theory is used to derive and propose a model of the Coulomb force between finite spheres in close proximity, in a plasma. This plasma force model is then used as a basis for a candidate terrestrial force model. The plasma-like parameters of this terrestrial model are estimated using charged motion data from fixed-potential, single-direction experiments on the testbed. The testbed is advanced to the level of autonomous feedback position control using solely Coulomb force actuation. This allows relative motion repositioning on a flat and level track as well as an inclined track that mimics the dynamics of two charged spacecraft that are aligned with the principal orbit axis. This controlled motion is accurately predicted with simulations using the terrestrial force model. This demonstrates similarities between the partial charge shielding of space-based plasmas to the electrostatic screening in the laboratory atmosphere

Boussinesq-equation and rans hybrid wave model

This dissertation presents the development of a novel hybrid wave model, comprised of the irrotational, 1-D horizontal Boussinesq and 2-D vertical turbulence-closed Reynolds Averaged Navier-Stokes (RANS) wave models. The two constituents are two-way coupled with the interface placed at a location where turbulence is relatively small. Boundary conditions on the interfacing side of each model is provided by its counterpart model through data exchange, requiring certain transformation due to the difference in physical variables employed in both models. The model is intended for large-scale wave simulation, accurate in both the nonbreaking and breaking zones with relatively coarser grid in the former and finer in latter, and yet efficient. Hybrid model tests against idealized solitary and standing wave motions and wave-overtopping on structure exhibit satisfactory to very good agreement. Compared with pure RANS simulations, the hybrid model saves computational time by a factor proportional to the reduction in the size of the RANS domain. Also, a large-scale tsunami simulation is provided for a numerical setup that is practically unapproachable using RANS alone; not only does the hybrid model offer more rapid simulation of relatively small-scale problems, it provides an opportunity to examine very large total domains with the fine resolution typical of RANS simulations. To allow for implementation on even larger domain with affordable CPU time, the hybrid model is parallelized to run on distributed memory machine. This is done by parallelizing the RANS model while leaving the Boussinesq model serial. One of the processors is responsible for both the sub-RANS and Boussinesq calculations. ICCG(0) for solving the pressure equation is parallelized using the nonoverlappingdecomposition technique, requiring more iterations than the serial one. Standing wave and hypothetical tsunami simulations with 960×66 and 1000×100 grids, and using 8 processors confirm model validity and computational efficiency of 82% and 65%. Finally, the 2-D Boussinesq model is parallelized using domain decomposition technique. The solution to the tridiagonal system arising in the model is calculated as the sum of the homogeneous and particular solutions. Parallel model tests using up to 32 processors exhibit model accuracy and efficiency of 80% for simulation with 500×500–2000×2000 grids

Aerospace medicine and biology: A continuing bibliography with indexes (supplement 349)

This bibliography lists 149 reports, articles and other documents introduced into the NASA Scientific and Technical Information System during April, 1991. Subject coverage includes: aerospace medicine and psychology, life support systems and controlled environments, safety equipment, exobiology and extraterrestrial life, and flight crew behavior and performance

Application of poly-Si on oxide junctions as one or both polarities of high-efficiency IBC solar cells

This work deals with the application of passivating contacts, specifically POLO contacts, which can increase the selectivity of the charge carriers at the metal contact and thus the efficiency due to their excellent passivating effect. These poly-Si based contacts, also called TOPCon, are just establishing themselves in industrial solar cell manufacturing. However, there are still some open questions regarding their operation and optimal fabrication. The first part of the paper is focused on the so-called POLO2-IBC cell, with which Felix Hasse’s team, with the collaboration of the author of this thesis, was able to achieve a record efficiency of 26.1 % for p-type material in 2018. With an area of 4 cm² and very complex patterning processes, this back-contacted record cell is not an industrially relevant cell, but it demonstrates the great potential of POLO contacts. A distinctive feature of this cell is the continuous layers of thin oxide and overlying poly-Si that form the so-called „poly-Si on oxide“ (POLO) junctions. The electron-collecting and hole-collecting contacts, fabricated by doping via ion implantation, are separated only by narrow intrinsic POLO regions. Extensive monitoring of the fabrication process and a simulation study show that the potential of this cell type with 26.1 % has not yet been fully exploited. Furthermore, the comparison with a high resistivity base material shows the significantly higher susceptibility of the passivation quality with decreasing doping concentration, so that despite higher intrinsic recombination, the used 1.3 Ohm cm material is identified as most suitable. Separating the p+ and n+ contact through the undoped region proves to be an elegant solution, which, however, only works under certain conditions. Diffusion of dopants from the n+ and p+ regions into the intrinsic region improves the otherwise poor passivation quality there. At the same time, however, this increases the unwanted recombination current between the contacts, so the choice of the appropriate width is of immense importance. With the understanding of the working principle gained, less complex structuring is conceivable in the future. However, in order to make the leap from the laboratory cell to industrial application, stability against the firing step for contact formation with common screen printing pastes is essential. Although the POLO contacts prove by their manufacturing process alone that they can withstand high temperatures with very good passivation quality, firing without a capping layer leads to deterioration of the passivation quality. In the second experimental part of this work, it is shown that this behavior is probably due to the two orders of magnitude higher heating and cooling rates in combination with thermal stresses. However, the degradation can be counteracted with the help of hydrogen rich dielectric layers. Nevertheless, it is shown, that too much hydrogen can also have a negative effect here. For typical firing temperatures of around 800 °C, a stack of Al2O3/SiNy layers yields the best results. In the last section of this work, the use of a n+POLO contact in a screen-printed POLO-IBC cell can be successfully demonstrated with an efficiency of 23.92 %. The excellent passivation quality of 0.2 fA/cm², which can be obtained using Al2O3/SiNy stack to cap the n+POLO junction, plays an important role here. Overall, this work has helped to transfer passivating contacts from high-efficiency POLO2-IBC laboratory cells to the promising POLO-IBC cell concept suitable for industrial applications

Electron accumulation and doping in InN and InGaN alloys

InN and group III nitride materials have attracted great interest due to their potential applications for optoelectronic devices, as the range of band gaps cover the ultra-violet to the near infrared. InN and all In-rich InxGa1−xN alloys exhibit a surface electron accumulation layer. This is due to the unusually low conduction band minimum (CBM) at the Brouillon zone centre (Γ-point) with respect to the charge- neutrality level. Electron accumulation has been observed at the surface of almost all n-type and p-type InN, making proof of p-type doping of this material very difficult. Routine characterization of p-type conductivity of Mg-doped samples using single-field Hall effect is prevented by the presence of a surface inversion space-charge layer, and hence the surface electron-rich region dominates the measurements. In this thesis, the results of investigations on non-polar InN surfaces, Mg-doped InN surfaces and a range of InxGa1−xN alloys across the composition entire range are presented. Considerable improvement of the quality of a- and m-plane InN thin films has been achieved using free standing GaN substrates in conjunction with a GaN buffer layer and grown by PAMBE. Using a combination of infrared reflectivity (FTIR), x-ray photoemission spectroscopy (XPS) and electrochemical capacitance voltage (ECV) measurements, the surface space charge properties of these samples have been investigated. The surface Fermi level has been determined to be lower than previously observed on non- cleaved InN samples. Additionally a high carrier concentration has been found on the non-polar InN, close to the interface with the GaN buffer layer, associated with unintentionally incorporated oxygen impurities. The increased concentration of oxygen impurities near the InN/GaN interface, confirmed by secondary ion mass spectrometry (SIMS), is due to the relatively low growth temperature (380 - 450 ◦C) used to produce the non-polar InN films. XPS has been also used in the investigations of Mg-doped InN. A significant lowering of the surface Fermi level has been observed with increasing Mg-doping for the highest Mg concentration (> 1 × 1019 cm−3) indicating a highly desirable reduction in the degree of surface electron accumulation. While for moderate Mg concentrations the surface Fermi level is at the previously determined ‘universal’ value of ~ 1.4 eV above the valence band maximum, for [Mg]=1.2×1020 cm−3, a value of 0.83 eV is found. As a consequence, for [Mg]> 1 × 1019 cm−3 the donor surface state density increases while the surface electron density decreases enormously, resulting in a transition from electron accumulation to almost just hole depletion layer. This reduction of electron accumulation in high Mg-doped InN can be improved by additional surface treatment, therefore results of a series of sulfur treated Mg-doped InN sample are also reported in this thesis. Finally, the electronic properties of InxGa1−xN alloys with a composition range of 0.20 >= x >= 1.00 have been investigated, using XPS and FTIR. The transition from electron accumulation to electron depletion has been observed at a composition of x = 0.20, while for x >= 0.20 an increasing electron accumulation with decreasing Ga fraction has been observed