Lujia Xu

Secondary Electron Microscopy Dopant Contrast Image (SEMDCI) for Laser Doping

Laser doping has been the subject of intense research over the past decade, due to its potential to enable high-efficiency, low-cost silicon solar cell fabrication. Information about the doping profile that is created by the process is critical for process optimization but is generally difficult to obtain. We apply the technique of secondary electron image (SEI) contrast to the characterization of the cross sections of laser-doped lines. We demonstrate that this technique can be used for a large range of different dopant sources and different laser doping methods and that good dopant contrast can be obtained under a relatively wide range of microscope parameters. Comparison of dopant contrast and doping density profiles shows that the substrate doping is an important parameter that can significantly influence the dopant contrast, particularly at low (~1018 cm-3) and high (~10 20 cm-3 ) dopant densities. When suitable calibration samples are used, the technique can be employed to obtain quantitative dopant density images for p-type laser-doped regions, albeit currently over a limited range of dopant densities and with relatively large error. Furthermore, the technique can be used to evaluate the risk of metallization shunts near the edges of dielectric film windows that are opened by the laser.

The Impact of SiO

Laser doping of semiconductors has been the subject of intense research over the past decades. Previous work indicates that the use of SiO2/SiNx stacks instead of a single dielectric film as the anti-reflection coating and passivation layer results in laser doped lines with superior properties. In this paper, the impact of the SiNx layer thickness in the SiO2/SiNx stacks on the properties of laser doped lines is investigated through resistance measurements of the laser doped line and the silicon-metal contact and the doping profile near the edge of the dielectric window, the latter being an important factor in determining the likelihood of high recombination or even shunting from the subsequent metallization process. Fundamentally, a problem of exposed and undoped silicon near the dielectric window is identified for most of the investigated parameter range. However, optimization of the laser parameters and dielectric film conditions is shown to be capable of preventing or at least minimizing this problem. The results indicate that for the used laser system, samples with thick dielectric stack processed using a low pulse energy and pulse distance yield the most favorable properties, such as low line resistance and low contact resistivity. Under these conditions, the laser doped regions laterally extend underneath the dielectric films, thus reducing the likelihood of high surface recombination.

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The authors acknowledge financial support from the Australian Solar Institute (ASI)/Australian Renewable Energy Agency (ARENA) under the ANU PV Core project, Postdoctoral Fellowship and Australia-Germany Collaborative Solar Research and Development projects. The authors also acknowledge support from the Australian Government’s NCRIS/EIF funding programs for access to Heavy Ion Accelerator Facilities at the Australian National University.

/SiN

The main challenge for interdigitated back-contact (IBC) solar cells is to reduce the fabrication complexity, which consists of multiple high-temperature processing and patterning steps. Patterned ion implantation has been proposed to simplify the manufacture of IBC solar cells, and the annealing of boron and phosphorus implanted areas is still a problem for the application. In this study, a new method consisting of laser annealing and a subsequent low-temperature oxidation (LA&OX) has been developed to co-anneal boron implanted p+ and phosphorus implanted n+ regions by a single step. We found that an additional laser annealing before oxidation could improve the electrical properties of boron-implanted p+ regions effectively; however, it has almost no effect on the phosphorus-implanted n+ regions. An industrially feasible IBC solar cell fabrication technology has been proposed based on the patterned ion implantation and LA&OX processing. The main fabrication steps of the IBC solar cell could be reduced to ten steps, and only one high-temperature oxidation step is required. As-designed IBC cell shows a potential efficiency higher than 23% according to simulations with the experimental parameters.

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Laser doping of silicon is a complex process involving thermal effects and interactions between different materials far from equilibrium and over a short period. In this paper, diffused samples capped with different dielectric films (including bare surfaces) are processed using laser pulses of 20-400 ns duration and characterized by photoluminescence (PL) imaging to study the degradation of the electronic properties of the processed regions. This way, without the interference of a dopant precursor, the thermal and dielectric effects are separately investigated. It is found that the thermal effects (melting and recrystallization of the silicon) do not lead to significant damage and additional recombination, provided no severe silicon evaporation occurs. However, when a dielectric film is present, a considerable increase in recombination is observed, irrespective of laser parameters, indicating the formation of additional defects. The magnitude of the increase in recombination varies substantially, depending on the dielectric used. Repeated pulses appear to repair silicon damage introduced by the first pulse or pulses for long pulse durations but result in a slight degradation for short pulse durations. Combining the PL results and four-point probe measurement of laser-doped samples, it is demonstrated that both high dopant incorporation (sufficient silicon melting) and low recombination can, in principle, be achieved, particularly when samples are processed using long pulse durations and small pulse distances.