Compound Semiconductor Nanocrystals formed by Sequential Ion Implantation
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EXPERIMENTAL DETAILS
The individual constituents of the desired compound semiconductor were implanted sequentially at the proper stoichiometric ratio. Substrates used were fused silica, thermally grown Si0 2 films (-8000 A thick) on (001) Si, and (0001) A120 3 crystals. Energies were chosen for each ion species to ensure an overlap of the profiles. Doses were in the range of 3 x 1016/cm2 to 3 x 1017/cm 2 for each species. Substrate temperatures ranged from room temperature to 650*C. Following implantation, annealing was carried out in a reducing atmosphere (Ar + 4% H2) for 1 h at temperatures in the range of 600*C to 1100'C. Sample characterization was carried out using x-ray diffraction (CuKal radiation), cross-section transmission electron microscopy (TEM), Raman spectroscopy (488 nm excitation) as well as Rutherford backscattering-ion channeling to monitor the impurity depth profile and crystallinity in the near-surface region. Optical properties of selected nanocrystal composites were determined using visible, UV, and infrared absorption as well as photoluminescence measurements. RESULTS AND DISCUSSION Figure 1 shows a cross-section TEM micrograph and particle size distribution measured for SiGe nanocrystals in Si0 2 . In this case, Si ions (215 keV) and Ge ions (500 keV) were implanted into Si0 2 to give an impurity profile peaked at a depth of -3100 A with a full width at half maximum of -2500 A for each constituent. Following implantation, the sample was annealed at 1000'C to induce precipitation and nanocrystal formation. X-ray diffraction showed strong diffraction lines characteristic of randomly oriented, diamond cubic SiGe, and the Raman spectrum from this sample was similar to that measured from bulk SiGe. These techniques identify the nanoparticles as a SiGe alloy, but Si and Ge are completely soluble in each other, and there is almost certainly a range of compositions in the alloy. The lack of x-ray diffraction lines and Raman lines characteristic of pure Si or pure Ge shows that there are few, if any, precipitates of the pure phase of each constituent. The TEM results show the individual SiGe nanocrystals. The size distribution is peaked at -50 A with a few precipitates having diameters as large as 200 A. It should be possible to produce smaller nanocrystals and a narrower 15
distribution by using lower doses or lower annealing temperatures as demonstrated in our work on Ge nanocrystals in Si0 2. SiGe nanocrystals have also been produced in A120 3 by the implantation of Si and Ge, followed by annealing. In that case, the nanocrystals are oriented preferentially with their (111) planes parallel to (0001) A120 3 planes. These nanocrystals also exhibit strong in-plane alignment. Details will be published separately. 35 [...,, "'"
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Particle Size (A) Fig. 1. (left) SiGe nanocrystals in Si0 2. Equal doses (3 x 1017/cm2) of each constituent were implanted at energies chosenr to give an overlap of the profile. (right) Measured size
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