Nanometer Scale Composition Variations in Ge Quantum Dots on Si(100)
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Nanometer Scale Composition Variations in Ge Quantum Dots on Si(100) Yangting Zhang, Margaret Floyd, Jeff Drucker, P.A. Crozier, David J. Smith Arizona State University, Department of Physics and Astronomy and Center for Solid State Science, Tempe, AZ 85287-1504, U.S.A. K.P. Driver Department of Physics and Astronomy, University of Louisville, Louisville, KY 40292 U.S.A. ABSTRACT Electron-energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) was used to measure nm-scale composition variations in Ge/Si(100) islands grown by molecular beam epitaxy (MBE) at substrate temperatures 400 °C ≤ T ≤ 700 °C and a growth rate of 1.4 ML/min (1 monolayer, ML=6.78×1014 atoms / cm2). These measurements were correlated with island ensemble morphology determined by atomic force microscopy (AFM). The average Si concentration of the islands and Si/Ge interface width increased monotonically with growth temperature. Integrated island volumes measured by AFM were proportional to the equivalent Ge coverage, θGe, with slopes greater than one for the higher deposition temperatures. This result confirms that the islands grow faster than the Ge deposition rate. Linear behavior of the island volume vs. θGe curves implies that the average Ge composition is independent of island size. The volume at which islands change shape from pyramids to domes correlates well with the average Ge content of the islands in the context of simple strain-scaling arguments. For T=700°C, rapid Si interdiffusion precludes formation of pure Ge pyramids for growth at 1.4 ML/min. Growth at 4.8 ML/min kinetically stabilizes pure Ge pyramid clusters, allowing their formation prior to Si interdiffusion. INTRODUCTION Spontaneously formed Ge quantum dots have attracted much recent attention for both possible applications and fundamental growth issues. Si interdiffusion plays an important role in the strain relief process at high growth temperatures (T≥600°C) [1-6]. Ge concentration also determines the optical and electronic properties of individual dots. Quantitative x-ray photoelectron spectroscopy measurements of the average Ge concentration have been reported [5] indicating an increase in Si concentration as the growth temperature increases above 550°C. Here, we use EELS to measure nm-scale composition variations in individual islands. In the Ge / Si(100) system, 3-D nanoscale coherent islands form in order to relieve mismatch-induced strain energy at the cost of increased surface energy. The layer-to-island transition occurs at a kinetically determined critical thickness near 3 ML. The smallest islands formed are square-based pyramid or rectangular-based hut clusters bound by {105} facets [7, 8]. Continued growth of the clusters initiates a transformation into approximately octagonal-based dome clusters bound by steeper {102} and {113} facets [9, 10]. Further growth results in island dislocation at a size analogous to the critical thickness for dislocation in strained-layer epitaxy. Our measurements indicate that the Si concentrat
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