Laser-Induced Redistribution of Sn atoms in GeSn Solid solutionsFriday (26.06.2020) 11:30 - 11:50 Room 2
The GeSn/Ge heterostructure is promising for application for short-wave IR photodetectors (Gassenq et al. 2012). GeSn exhibits a direct bandgap for Sn atomic concentration of 6-8% (Polak, Scharoch, and Kudrawiec 2017). Epitaxial growth of uniform GeSn relaxed layers is problematic due to: the large lattice mismatch of 14.7% (19.5%) between α-Sn and Ge (Si) (Liu et al. 2016); the low equilibrium solid solubility of Sn in Ge (less than 1%) (Kasper 2016) and the extremely high surface segregation of tin at higher Sn content (Kormoš et al. 2017). The limited solubility of Sn in Ge and the large lattice mismatch between GeSn and Si leads to compositional fluctuations, Sn segregation, and significant roughening (Kormoš et al. 2017). The possible solution to these problems could be Sn-rich GeSn growth under non-equilibrium conditions (Biswas et al. 2016).
Here we propose a new approach – providing of non-equilibrium conditions by powerful pulsed laser radiation with the aim to redistribute impurity atoms - Sn in the host material Ge of epitaxial solid solutions and overcome equilibrium solid solubility. GeSn layers were grown by molecular-beam epitaxy (MBE) method with Sn atomic concentration 4% and irradiated by Q-switched pulsed Nd:YAG laser with 1064 nm in the range of intensities from 107.8 to 463.5 MW/cm2. The TEM/EDS cross-section analysis and X-ray photoelectron spectroscopy (XPS) confirmed the increase of Sn atomic content at the surface by order of magnitude.
The mechanism of redistribution of Sn atoms at the surface of GeSn solid solution is explained by thermogradient effect induced by laser radiation. According to this effect, Sn atoms with larger covalent radius drift towards the surface. Analysis of TEM/EDS, AFM, XRD/reciprocal space map (RSM) results provided evident microstructural and compositional changes as Sn drift to surface and its more homogeneous lateral redistribution. The obtained structure had a graded band gap, which was confirmed by cross-section EDS/TEM analysis and time-resolved differential reflectivity studies. A graded bandgap structure can significantly improve the performance of infrared detectors. This study can be applied for epitaxial solid solutions based on group IV elements, such as: GeSn, SiGe and SiGeSn. The originality and main innovative idea are based on the application of the temperature gradient effect for epitaxial solid solutions.
This work has been supported by the European Regional Development Fund within the Activity 126.96.36.199 “Post-doctoral Research Aid” of the Specific Aid Objective 1.1.1 “To increase the research and innovative capacity of scientific institutions of Latvia and the ability to attract external financing, investing in human resources and infrastructure” of the Operational Programme “Growth and Employment” (No.188.8.131.52/VIAA/3/19/409).