Ion Beam Synthesis of Luminescent Si and Ge Nanocrystals in a Silicon Dioxide Matrix
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363 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
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Depth (rim) Figure 1: Concentration depth profiles for Ge-rich samples as-implanted and annealed at 700'C for 30 min, derived from RBS spectra. The respective peak concentrations for the three implant doses are 3, 6, and 13 at. % Ge. Upon annealing, a slight steepening of the concentration profile was observed. luminescence reported in this work. Since then, several groups have reported the formation of nanocrystals in Si0 2 by annealing supersaturated solid solutions of group IV semiconductors prepared by rf co-sputtering of Si0 2 and different group IV semiconductorsf7,9,10,11]. They observed visible photoluminescence from samples containing these nanocrystals, the spectra appearing quite similar to those of porous silicon. However, the specific relationship between nanocrystal formation and luminescence is not yet clear. Ion implantation of Si into SiO 2 also yielded visible photoluminescence, although detailed structural characterization was not carried out[13]. It has been suggested that this luminescence is also due to either transitions between discrete energy levels of a quantum dot, or recombination of molecular-like excitons trapped at these nanocrystals. II. ION BEAM SYNTHESIS OF LUMINESCENT GROUP IV NANOCRYSTALS In this work, ion implantation was chosen to create supersaturated Si and Ge solid solutions because of the inherent high purity of the process, and the ease of controlling the concentration profiles of implanted material. In addition, vacuum annealing of ion implanted samples permits the structural and optical properties of nanocrystals to be characterized in the absence of hydrogen, thus enabling luminescence related to hydrogenated amorphous silicon and siloxene to be neglected. Silicon dioxide grown by thermal oxidation of Si is also an extremely pure and well-characterized material, and the Si-Si0 2 interface has a very low density of interface states. Films prepared in this manner also lend themselves to straightforward optoelectronic device fabrication via processes readily integrable with existing ultralarge-scale integrated circuit technology. Ion implanted structures may also enable planar waveguides, since a few atomic percent excess of Ge increases the refractive index of glass enough to produce a weak guide, as routinely practiced in optical fiber synthesis. Thermally-grown silicon dioxide films 100 nm thick on P-doped (100) silicon wafers were 74 29 ion implanted with Si at 50 keV and with Ge at 70 keV, with implant energies cho-
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AY (cm-') Figure 2: Raman spectra of Ge- and Si-implanted samples for doses of 5x10' 6 /cm 2, before and after annealing at 7000C for 30 min (Ge and Si spectra are offset relative to each other for clarity). The implanted, unannealed Ge spectru
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