Crystallization of β-FeSi2 droplets on silicon substrates by room-temperature pulsed laser deposition
Introduction
Research interests on semiconducting iron disilicide β-FeSi2 have been motivated recently because of its excellent properties such as direct optical band gap of 0.8–0.85 eV [1], a large optical absorption coefficient [2], good physical–chemical stability at high temperature and high resistance to oxidation, and the possibility of growing epitaxially on Si substrates [3], [4]. This facilitates the integration of infrared devices with iron disilicide and silicon electronic devices [5], [6], [7]. Recently, Suemasu et al. have successful fabricated light emitting devices out of growing material on silicon wafers by molecular beam epitaxy (MBE) [8]. So far, many attempts have been performed to achieve high quality β-FeSi2. However, high thermal treatments were necessary in all cases, which lead to possible drawbacks when integrated with silicon electronic circuits. Therefore, it is highly desired to develop a new technique that provides good quality β-FeSi2 with a low thermal process. In this context, we have sought for possible low temperature scheme for crystal growth of β-FeSi2 on the basis of pulsed laser deposition (PLD). Special emphases were placed on β-FeSi2 droplets with diameters of a few micrometers [9]. In our previous report, droplets of β-FeSi2 polycrystalline were formed by low temperature PLD followed by thermal treatment at 400 °C for 20 min. In this paper, we showed that the thermal treatments after PLD could be replaced by chemical treatments of substrates before PLD.
Section snippets
Materials and procedure
CZ-grown p-type silicon (100) substrates were applied to the treatment with hydrofluoric acid for natural oxide removal. NH4F solution (27 ml of H2O and 14 g of NH4F) treatment was performed at 50 °C for 20 h successionally.
After the substrate preparation, droplets were deposited by PLD method at room temperature, using FeSi2 (99.99%) alloy target. The substrates were set parallel with the target at a distance of 18 mm. An ArF excimer laser operating at 193 nm was used as the PLD light source,
Results and discussion
Result from the energy dispersive X-ray spectroscopy (EDX) is shown in Fig. 1. The inset of Fig. 1 displays the atomic ratio of Si to Fe. It clearly indicates that the chemical treatment had no effect on the compositional ratio of the droplets.
By scanning electron microscope (SEM) observation, the droplet on the substrate with chemical treatment has a smooth surface as shown in Fig. 2. The surface morphology of the chemically treated substrate shows some pits with diameter less than 100 nm.
Conclusion
Control of the cooling rate is very important for the beta-iron disilicide formation at room temperature. Chemical treatment of the substrate is applicable for this purpose because of the thermal resistance of porous surface. The β-FeSi2 droplet formed at room temperature includes strain but the strain relaxes by annealing processes.
Acknowledgements
This work is supported partly by the selective research fund from Tokyo Metropolitan University during 2005 and 2006 fiscal years.
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