Search

The semiconducting metal silicides and Mg2Si in particular have been extensively studied in the last few years because of the following reasons: the materials have the potential to be successfully integrated in the optoelectronic silicon technology, the materials are nontoxic and the constituting elements are found in a large amount in nature, and lastly the formation of nanocrystals in a perticular matrix offers the possibility of semiconducting structures with new properties. The Mg-Si phase diagram [1] shows that Mg2Si is the only silicide in this system and it melts congruently at 1085 ◦C. Therefore, crystal growth from the melt should be possible, but one has to be careful about the considerable evaporation of the Mg component as the boiling point of pure Mg is very close to the compound melting temperature. Growth by chemical vapor transport is hardly possible because of the lack of stable magnesium halides and the growth of Mg2Si bulk material has been accomplished exclusively from a melt [2–5]. The compound Mg2Si has, similar to Si, a cubic structure with a larger lattice parameter. The lattice mismatch for the Mg2Si (111) face on the Si (111) face is 1.9%. Due to the comparatively small lattice mismatch high quality epitaxial growth of Mg2Si onto monocrystalline silicon appears to be possible [6]. However thin film formation is difficult because of the low condensation and high vapor pressure of Mg and that is why thin film preparation and studies are scarce. The first reports on Mg2Si thin film formation [7, 8] note that the silicidation proceeds rather rapidly in Mg/Si thin film substrates at relatively low temperatures and silicide columns matching the monocrystalline silicon substrate are formed. There has been an attempt to fabricate Mg2Si by laser melting of magnesium film deposited on a Si substrate [9]. The synthesized layers contain both silicides and Si crystallites. Mahan et al. [10] have prepared Mg2Si films, textured in the (111) direction by molecular-beam epitaxy. They find that intended reactive deposition of magnesium onto silicon substrate at temperatures from 200 to 500 ◦C results in no accumulation of magnesium. However, codeposition of magnesium and silicon at 200 ◦C, using a magnesium-rich flux ratio, gives stoichiometric Mg2Si films. A solidphase growth technique with preliminary formation of templates was used by Galkin et al. [11] to prepare continuous Mg2Si films with a thickness of the order of 20 nm on Si (111) substrates. The very low condensation coefficient of magnesium as well as the limited Mg2Si films thickness due to the barrier behavior of Mg2Si can be overcome using the ion-beam synthesis (IBS) method, reported in this paper. The samples were prepared by IBS, followed by rapid thermal annealing (RTA). The implantation of 24Mg+ ions was performed using an ion accelerator type ILU-4, allowing a high current density. As an ion plasma source 4N pure Mg, heated at 500 ◦C, was used. The vapor pressure of Mg at this temperature is about 10−2–10−1 Torr. During the implantation, in order to avoid amorphization [12], the substrates were heated to a temperature of about 230 ◦C by means of the incident ion beam (current density 10–12 μA · cm−2). The mass separated 24Mg+ ions were implantated into n-type Si wafers with (100) orientation and resistivity 4.3–4.5 ·cm. Four types of samples were prepared. The first two types were fabricated by two-step ion implantation with two different doses—5 × 1016 and 1 × 1017 cm−2, with energies 15 and 40 keV respectively. The other two types of samples were prepared by one-step implantation of two different higher doses—2 × 1017 and 4 × 017 cm−2 with energy 40 keV. The Mg implantation profiles were simulated by SRIM (Stopping and Range of Ions in Matter) and the initial profiles of the implanted Mg ions were estimated as

Continuous layers and precipitates of the β-FeSi2 phase were formed by ion-beam synthesis followed by rapid thermal annealing. Two steps of Fe+ ion implantation in Si, at energies of 20 and 60 keV, were performed, using three different doses – 5×1015, 5×1016, and 1017 cm−2. The infrared spectra were used as a criterion for the formation of the β-FeSi2 phase. The refractive-index-energy dependences were calculated from the transmittance and reflectance spectra, using the general matrix method. From these dependences, the energy band gap and the energies of the higher-energy transitions in the vicinity of the gap were determined.