New Ge-Sn materials with adjustable bandgaps and lattice constants
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New Ge-Sn materials with adjustable bandgaps and lattice constants Matthew R. Bauer,1 John Tolle,1 A. V. G. Chizmeshya,2 S. Zollner,3 J. Menendez,4 and J. Kouvetakis 1 1
Department of Chemistry and Biochemistry, Arizona State University, Tempe AZ 85287 Center for Solid State Science, Arizona State University, Tempe AZ 85287 3 Motorola Inc., MD EL622, 2100 E. Elliot Road, Tempe, AZ 85287 4 Department of Physics and Astronomy, Arizona State University, Tempe AZ 85287 2
ABSTRACT The synthesis and optical properties of a new class of Si-based infrared semiconductors in the Ge1-x Snx system are described. Chemical methods based on deuterium-stabilized Sn hydrides and UHV-CVD were used to prepare a wide range of metastable compositions and structures directly on silicon. These materials exhibit high thermal stability, superior crystallinity, and unique crystallographic and optical properties, such as adjustable band gaps and lattice constants. These properties are characterized by Rutherford backscattering, low-energy secondary ion mass spectrometry, high-resolution transmission electron microscopy, x-ray diffraction as well as infrared and Raman spectroscopies and spectroscopic ellipsometry. The films grow essentially strain free and display a strong compositional dependence of the band structure. INTRODUCTION Synthesis of Ge1-xSnx alloys has attracted considerable attention because of predictions that Ge-Sn heterostructures might exhibit tunable direct bandgaps [1,2]. The major problem with Sn incorporation into the Ge lattice is the large (15%) lattice mismatch between the elements and the instability of the diamond-cubic structure at room temperature [3]. Accordingly, Ge1-xSnx alloys are thus metastable and their synthesis requires conditions that are far from equilibrium. Nevertheless, significant efforts have been devoted to growing Ge1-xSnx alloys by molecular beam epitaxy (MBE) [1,3,5]. A major problem encountered with MBE is the propensity of Sn to segregate toward the film surface [3,6]. To counteract this effect, MBE growth of Ge-Sn must be conducted at very low temperatures, i. e. ~100oC, placing a severe limit on the film critical thickness. Under these conditions Ge-Sn alloys have been reported to grow on Ge as thin and highly strained layers [3]. However, only strain-free materials are predicted to exhibit direct band gaps. Our work in this area has focused on development of chemical methods to synthesize device quality Ge1-xSnx structures on Si with a wide range of single-phase compositions that may not be accessible by conventional MBE [7]. Here we describe growth and characterization of unstrained Ge1-xSnx alloys with concentrations of 2-19% Sn prepared directly on Si(100). The crystal properties are superior to those of pure Ge films grown on Si, indicating that Sn incorporation in tetrahedral Ge sites, even at modest concentrations, leads to novel structural and strain behavior compared to Ge, Si-Ge, and
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related systems. The key to the successful synthesis is the development of a lo
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