Transmission electron microscopy study of Ge implanted into SiC
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Hexagonal 6H– and 4H–SiC wafers were implanted with (1–1.5) × 1016 cm−2 germanium ions at room temperature and at 700 °C with subsequent annealing between 1000 and 1600 °C. Structural changes in the SiC matrix were studied in detail by means of transmission electron microscopy (TEM). After implantation at room temperature the hexagonal SiC matrix becomes amorphous and, after annealing, recrystallizes into cubic SiC. The latter process was accompanied by the creation of voids and cracks. In case of high-temperature (700 °C) implantation, where amorphization was avoided, no polytype change in as-implanted and annealed SiC wafers was observed. In annealed samples nanocrystalline precipitates with high Ge content were observed in high-resolution TEM images.
I. INTRODUCTION
The observation of strong visible light emission from group IV nanocrystals opened a new class of material, indirect-gap semiconductors, for the use in optoelectronic device fabrication.1,2 The band-gap energies of bulk Si and Ge are 1.11 and 0.67 eV,3 respectively. Small clusters of Si or Ge embedded inside a matrix with comparatively high bandgap energy should be able to trap a single electron. For optoelectronic application it is necessary that the matrix can be electrically excited; e.g., a conductive matrix has to be used. Silicon carbide (SiC) satisfies these demands. The cubic polytype (3C–SiC) has a band-gap energy of 2.3 eV, and hexagonal 6H–SiC, a band-gap energy of 3.0 eV.4 The material forms ohmic contacts with a large number of metals and metal alloys. Furthermore, it has high-temperature stability, high thermal conductivity, and good chemical resistance making it very useful in the fabrication of devices operating at high temperatures, high frequencies, and high power. It has been shown theoretically that Si and Ge clusters smaller then 1.5–2 nm should show the quantum confinement effect.5–7 Size-dependent photoluminescence has been already experimentally confirmed for Si and Ge quantum dots embedded in a SiO2 matrix.8,9 One method to grow nanoclusters embedded in a matrix is the molecular beam epitaxy (MBE). The method approved itself in III–V quantum dot fabrication. The situation is more difficult in the Ge–SiC system, since, at
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 2, Feb 2002
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the high temperature needed for SiC growth, evaporation of previously deposited Ge dots occurs, leading to difficulties in lateral overgrowth. Ion implantation allows distribution of doping material inside a bulk matrix. Subsequent annealing is needed, both for enhancement of the precipitate formation and recombination of defects in the matrix which were introduced by implantation. The defect elimination and annealing behavior of the matrix is very critical for further applications of the device. The concentration of point and planar defects in the matrix has to be strongly decreased, because the excited electrons can recombine on def
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