We report on the lattice location of Er in Si using the emission channeling technique. The angular distribution of conversion electrons emitted by the decay chain 167 Tm ͑t 1͞2 9.25 d͒ ! 167m Er ͑2.27 s͒ was monitored with a position-sensitive detector following room temperature implantation and annealing up to 950 ± C. Our experiments give direct evidence that Er is stable on tetrahedral interstitial sites in float-zone Si. We also confirm that rare earth atoms strongly interact with oxygen, which finally leads to their incorporation on low-symmetry lattice sites in Czochralski Si. [S0031-9007(97) Rare earth doping of Si is known to result in the formation of luminescent centers and is considered as a possible way to manufacture Si-based optoelectronic devices [1]. Among the various rare earth elements, Er is of special interest since its atomic transition at 1.54 mm matches the absorption minimum of SiO 2 , a highly desirable feature both for signal transmission through glass fiber cables and optical on-chip communication. Luminescence at this wavelength from Er-implanted Si was already established several years ago [2]. Meanwhile Er-based light-emitting diodes operating at room temperature have been reported [3]. The basic understanding of Er luminescence in Si, however, is far from complete. This concerns both the lattice sites of Er and the role of codopants such as O, N, or F, which were found to have a beneficial influence on luminescence yield. Photoluminescence (PL) spectroscopy studies have identified a number of Er-related centers with different crystal surroundings in Si [4]. The most intense PL yield was due to two centers having cubic and axial symmetry, respectively. While the cubic center occurred in both float-zone (FZ) and Czochralski (CZ) Si and was attributed to tetrahedral ͑T ͒ interstitial Er, the center with axial symmetry was observed only in CZ Si and ascribed to Er-O complexes. The existence of tetrahedral interstitial Er would be also in agreement with theoretical studies, which predict that T sites are the most stable sites for all oxidation states of isolated Er atoms in Si [5]. Direct lattice location using the Rutherford backscattering (RBS) channeling technique only suggested substitutional [6] or hexagonal ͑H͒ interstitial Er [7,8]. The reasons for these discrepancies, however, are unclear.To study the lattice sites and damage recovery after rare earth implantation, we have applied conversion electron emission channeling [9] combined with position sensitive detection. Emission channeling makes use of the fact that charged particles emitted from radioactive isotopes in single crystals experience channeling or blocking effects along low-index crystal directions. This leads to an anisotropic particle emission yield from the crystal surface which depends in a characteristic way on the lattice sites occupied by the emitter atoms. While this technique as such is not new and, in case of rare earths, was already used once for the lattice location of 175 Yb in Si [10], we have for the fi...

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Mg 2 Si is a semiconductor with a band gap previously reported to be in the range 0.6-0.8 eV. In spite of potential optoelectronic applications in an important infrared range, the growth of Mg 2 Si thin films on silicon substrates has received scant attention. This may be due to the difficulty of preparing Mg 2 Si in thin-film form. We find that intended reactive deposition of magnesium onto a silicon substrate, at temperatures from 200 to 500°C, results in no accumulation of magnesium. However, codeposition of magnesium with silicon at 200°C , using a magnesium-rich flux ratio, gives a stoichiometric Mg 2 Si film. The amount of magnesium which accumulates is determined by the total amount of silicon which was codeposited; the excess magnesium in the flux does not condense. Measurements of the optical transmittance of thin films thus obtained reveal an absorption edge. Extraction of the absorption coefficient from the data, and analysis of its energy dependence, suggest an indirect band gap of ϳ0.74 eV, plus direct transitions at ϳ0.83 and ϳ0.99 eV.

We report on the lattice location of Pr in thin film, single-crystalline hexagonal GaN using the emission channeling technique. The angular distribution of β − particles emitted by the radioactive isotope 143 Pr was monitored by a position-sensitive electron detector following 60 keV room temperature implantation of the precursor isotope 143 Cs at a dose of 1×10 13 cm −2 and annealing up to 900°C. Our experiments provide direct evidence that Pr is thermally stable at substitutional Ga sites.

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Slow (low-rate) reactive deposition of a metal onto a Si substrate can result in direct formation of a metal disilicide, thereby skipping the metal-rich phases in the formation sequence. These observations have been explained thermodynamically by using the effective heat of formation model. As a result of this concentration-controlled phase selection, it is possible to form disilicides, such as CoSi2, NiSi2, or β-FeSi2 at much lower growth temperatures than possible in conventional solid-phase reaction of a metal layer deposited onto Si at room temperature (i.e., lower than the nucleation temperature). Moreover, epitaxial growth of CoSi2/Si(100), which is not possible by solid-phase reaction, becomes achievable when depositing Co atoms sufficiently slowly onto a heated Si substrate.

Electron emission channeling allows direct lattice location studies of low doses of radioactive atoms implanted in single crystals. For that purpose the anisotropic emission yield of conversion electrons from the crystal surface is measured, most conveniently by use of position-sensitive detectors. We discuss characteristic features of this method, including quantitative data analysis procedures, which are achieved by fitting simulated two-dimensional emission distributions for different lattice sites to the experimental patterns. The capabilities of this approach are illustrated by the case of rare earth atoms (Er, Tm, Yb) in Si, where we were able to do lattice location experiments down to implanted doses which are 150 times lower compared to previous RBS studies.

We report on the lattice location of ion implanted Cu in Si using the emission channeling technique. The angular distribution of b 2 particles emitted by the radioactive isotope 67 Cu was monitored following room temperature implantation into Si single crystals and annealing up to 600 ± C. The majority of Cu was found close to substitutional sites, however, with a significant displacement, most likely 0.50(8) Å along the ͗111͘ directions towards the bond center position. The activation energy for the dissociation of near-substitutional Cu is estimated to be 1.8 -2.2 eV.