Preparation of Ge (100) Substrates for High-Quality Epitaxial Growth of Group IV Materials
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Preparation of Ge (100) Substrates for High-Quality Epitaxial Growth of Group IV Materials Mark Nowakowski1, Jordana Bandaru, L.D. Bell, and Shouleh Nikzad Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, 91125, USA 1 Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801, USA Abstract We compare various wet chemical treatments, in preparing high-quality Ge (100) surfaces suitable for molecular beam epitaxy (MBE). Various surface treatments are explored such as UV-ozone treatment followed by exposure to chemical solutions such as de-ionized (DI) water, hydrofluoric acid (HF), or hydrochloric acid (HCl). Chemical treatments to remove the oxide are performed in a nitrogen environment to prevent further formation of surface oxide prior to surface analysis. Following chemical treatments, in situ reflection high-energy electron diffraction (RHEED) analysis is performed to observe the surface evolution as a function of temperature. In a separate chamber, we analyze each sample, before and after chemical treatment by x-ray photoelectron spectroscopy (XPS) to directly determine the oxide desorption following each chemical treatment. Our results of this comparative study, the effectiveness of each chemical treatment, and the stability of the passivated surface suggest that UV ozone cleaning, followed by 10% HCl is the best choice for removing most of the oxide. Furthermore, we present evidence of high quality epitaxial growth of SnxGe1-x on wafers prepared by our method. Introduction Group IV quantum dots and thin films using Sn and Ge are relevant to a wide range of applications, including infrared detectors and thermophotovoltaic (TPV) converters for missions to the outer planets. The high efficiency and light absorption of direct bandgap materials make them the ideal choice for such applications. The primary advantage of the Sn-Ge system is its wide range of bandgap tunability. The direct bandgap of α-Sn is zero, but can be increased due to quantum confinement in Sn quantum dots smaller than ~ 30 nm radius [1]. SnxGe1-x alloys are of interest as well; with the addition of Sn to Ge, the indirect bandgap becomes direct [2]. By varying the Sn concentration in both thin films and dots one can tailor the direct bandgap to each individual application. Current quantum dot research has focused on group III-V materials because of their rich abundance of alloys with direct bandgaps. Group IV elements are still the economically desirable choice and can be engineered with direct bandgaps. We plan to explore SnxGe1-x thin films and self assembled quantum dot structures using molecular beam epitaxy (MBE) on Ge substrates. It is therefore important that a surface preparation process for growth on Ge wafers be developed. Ge has significantly different oxidation characteristics from Si. Where SiO2 is a necessary component layer in silicon devices, germanium’s oxides are unstable making germanium’s use less common in today’s industry [3]. Germanium forms
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