Nanometer-Scale Oxide Particles in Gesi Films Grown by Wet Oxidation
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INTRODUCTION Interfacial contamination is a very important consideration for the formation of epitaxial Si and Ge by SPE (solid phase epitaxy)l. Prokes et al. 2 reported the formation of epitaxial GeSi layers from amorphous GeSi films by wet oxidation at 900 (C. In contrast to the SPE growth of Ge and Si films, it was found that interfacial oxide contamination did not prevent epitaxy by this method. The formation of epitaxy was explained in terms of the motion of both Si and Ge during the oxidation process. 3 In the present work, GeSi films with various thicknesses and oxygen
contents were deposited by electron beam evaporation. These samples were then wet oxidized at 900 oC. A VG HB501A STEM equipped with a windowless x-ray detector and an electron energy loss spectrometer (EELS) was employed to study the GeSi films grown by wet oxidation. Both EELS and the windowless x-ray detector are capable of detecting light elements such as oxygen. The probe size of the STEM call be 2 A to 10 A. High spatial resolution either in chemical analysis (e.g. local compositional or phase change) or imaging (e.g. lattice fringes) can thus be attained. EXPERIMENTAL Si (1(00)) wafers were RCA cleaned 4 and then dipped in HF (10 %) before being loaded into an electron evaporator. GeSi films with different thicknesses and oxygen contents were deposited onto Si wafers by electron beam evaporation. The oxygen content was uncontrolled and might have resulted from the degassing of the inner wall of evaporator. The samples used in this study are listed in Table I.
615 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
Fig. 1. STEM images of sample AI after oxidation. (a) BF image shows strain contrast in the GeSi film. The GeSi/Si interface is sharp and well defined. (b) Higher mag. image of la) shows the stacking faults and twins. Mismatch dislocations are found at the GeSi/Si interface. The density of dislocations is higher than that in a totally relaxed GeSi film with the same Ge concentration. (c) High resolution BF image of the GeSi/Si interface. We speculate that the dark regions in the interface are dislocation cores. (d) High resolution ADF image of the same area as l(c) displays both Z contrast and strain contrast from the lattice mismatch. Fig. 2. STEM energy selected and ADF images of a GeSi film near the GeSi/oxide interface in sample A2 after wet oxidation. (a) Zero loss image is similar to BF image. It is difficult to tell oxide particles from the GeSi matrix. (b) ADF image shows dark particles in the GeSi film. (c) Mass thickness mapping shows a possible atomic mass difference between the dark particles and the matrix. (d) Oxide plasmon mapping of the same area reveals the existence of oxides.
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Table I sample Al A2 B1 B2
Ge concentration
film thickness
(atomic %)
(A)
16 20 20 18
1800 1800 600 600
oxygen content
(atomic %) 0 16 0 18
The concentrations of Si, Ge and oxygen in A l and B I were measured by EDS (energy dispersive spectrometry) and those of A2 and B2 were measured by RBS (Ruther
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