Advanced defect engineering and surface passivation for improving the carrier lifetime in femtosecond laser sulfur hyperdoped silicon

• Applicants: Professor Dr. Stefan Kontermann; Professor Dr. Michael Seibt
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429413061

In general, 'hyperdoping' describes the intentional doping of semiconductors with deep-level impurities at concentrations that considerably exceed their solubility. The goal of hyperdoping is to achieve overlapping impurity wave functions, level broadening and eventually the formation of deep intermediate bands in the band gap of the host semiconductor material. The resulting broadband infrared (sub band gap) absorption promises applications for infrared detectors and photovoltaics. Hyperdoping processing schemes inevitably involve (ultra)short high temperature annealing and high cooling rates. These requirements are met by femtosecond (fs) laser processing with the additional effect of producing a surface texture which further reduces reflection losses. The main obstacle for a widespread application of fs-laser hyperdoping schemes is the formation of process-induced defects. For silicon, these are mainly dislocations and inclusions of high-pressure phases in addition to point defects. Sulfur-hyperdoped silicon is a well-established system that shows the required insulator-to-metal transition at concentrations exceeding 1019 cm-3. Nevertheless, the problem of strong excess carrier recombination has not been solved up to now and even its origin has not been clearly identified. Possible candidates are process-induced extended defects, which are most likely decorated by sulfur impurities, in addition to obvious recombination via deep sulfur states that still exist as isolated defects. Therefore, it is of high importance to understand defect formation and defect reactions in fs-laser sulfur hyperdoped silicon and develop control strategies in order to tune the performance of the material. The overarching goal of our project is to understand the correlation between macroscopic and microscopic material characteristics and their relation to fs-laser sulfur hyperdoping process conditions and subsequent annealing treatments. For this purpose, we join macroscopic measurements of the spectral photovoltaic response (external and internal quantum efficiency, excess carrier lifetime) with microscopic investigations of excess carrier recombination (electron beam induced current, EBIC), defect levels (deep level transient spectroscopy, DLTS) and structural characteristics of surfaces (atomic force and Kelvin probe force microscopy, AFM and KFM) and of the bulk beneath (scanning and transmission electron microscopy, SEM and (S)TEM) in order to identify and model device-relevant defect formation and reactions. The project will develop defect engineering and passivation strategies tailored to sulfur hyperdoped silicon. More specifically, we will focus on (i) surface passivation using dielectric layers, (ii) post hyperdoping surface modifications, (iii) post hyperdoping annealing schemes, and (iv) defect engineering by pulse shaping in order to explore non-thermal effects of defect formation.

• DFG Programme: Research Grants
• Major Instrumentation: Atomlagen-Abscheide-Anlage (ALD-Anlage)
• Instrumentation Group: 0920 Atom- und Molekularstrahl-Apparaturen