Structural Characterization of GeSn Alloy Nanocrystals Embedded in SiO 2
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Structural Characterization of GeSn Alloy Nanocrystals Embedded in SiO2 Swanee J. Shin,1,2 Julian Guzman,1,2 Chun-Wei Yuan,1,2 Christopher Y. Liao,1,2 Peter R. Stone,1,2 Oscar D. Dubon,1,2 Andrew M. Minor,1,2 Masashi Watanabe,2 Joel W. Ager III,2 Daryl C. Chrzan,1,2 and Eugene E. Haller1,2 1
Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA ABSTRACT GeSn alloy nanocrystals were formed by implantation of Ge and Sn ions into an amorphous SiO2 matrix and subsequent thermal annealing. High resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) with a high angle annular dark field (HAADF) detector were used to show that phase-segregated crystalline bi-lobe nanocrystals were formed. Rapid melting and solidification using a single excimer laser pulse transformed the bi-lobe structure into a homogeneously mixed amorphous structure. Raman spectroscopy was used to monitor the crystalline nature and approximate grain size of the Ge portion of the nanocrystals after each heat treatment, and the Raman spectra were compared with the TEM images. INTRODUCTION Nanocrystals have attracted considerable attention due to their unique size dependent properties and wide range of potential applications in integrated electronic devices, optoelectronic devices, and energy generation systems. The ion beam synthesis (IBS) method has been used to synthesize nanocrystals for many years.[1] Recently, our work has focused on understanding the role of the matrix/nanocrystal interface in determining the properties of nanocrystals made by IBS[2]; for example, we found that the melting point of Ge nanocrystals embedded in silica can be elevated by more than 200°C above the bulk value.[3] Two or more elements can be used to make alloy or compound nanocrystals.[1,4] In some cases, there are multiple phases and geometries possible, which are determined by the significant surface and interface effects at this nanoscale.[5,6] In particular, Ge1-xSnx alloys have been intensively studied as a promising material for light emitting devices due to the reported indirect to direct bandgap transition at x≈0.1[7-9], and at the nanoscale, GeSn quantum dots showed the size dependent quantum confinement effect.[10,11] Several nonequilibrium growth methods have been attempted for films[8,9] and quantum dots[10,11], most of which used epitaxial growth on the underlying substrate. In this study, we used the IBS and laser processing method to synthesize GeSn nanocrystals embedded in an amorphous matrix, and several techniques were used to characterize fully the structure of the nanocrystals, and complementary information each method provided will be highlighted.
EXPERIMENT Isotopically pure 74Ge and 120Sn were implanted at room temperature into 500 nm thick amorphous SiO2 layers grown on Si(100) substrate by wet oxidation. The energy and dose of 74 Ge and 120Sn were
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