SILICON NANOSTRUCTURES AND THEIR INTERACTIONS WITH ERBIUM IONS

摘要

Silicon, the leading semiconductor in microelectronics industry, has for a long time been considered unsuitable for optoelectronic applications which remained the domain of III-V semiconductors and glass fibers. This is mainly due to the silicon indirect bandgap, which makes it a poor emitter, and to the absence of linear electro-optic effects. The enormous progress in communication technologies in the last years resulted in an increased demand for optoelectronic functions integrated with electronic circuits. This would allow to couple the information processing capabilities of microelectronics with the efficient interconnection properties of optoelectronics. In principle, silicon would be the material of choice, due to its mature processing technology and to its unrivaled domain in microelectronics, the main limiting step being the absence of efficient Si-based light sources. Recently a strong effort has been hence devoted to study all of those processes able to circumvent the physical inability of silicon to emit light. Since the discovery of light emission from porous silicon made in 1990 by Canham a lot of work has been performed in studying silicon nanostructures. These comprehend not only porous silicon but also nanocrystals produced by several techniques, as well as silicon-insulator multilayers, The initial problems related to the instability of the luminescence yield have finally been solved and today reliable, stable structures, compatible with the silicon technology have been fabricated. In particular, the group at the Rochester University has now produced silicon-rich silicon oxide electroluminescent devices integrated with silicon microelectronic circuitry. Alternative approaches comprehend the doping of silicon with rare earths. In this case the luminescence is due to an internal 4f shell transition of the rare earth ion excited through electron-hole recombinations within the silicon matrix. Among rare earths Er ions have been most widely studied since they emit light at 1.54 μm, a wavelength which is strategic in the telecommunication technology matching the window of maximum transmission for the optical fibers. The initial problems related to erbium incorporation and luminescence quenching have been now understood. In particular, it has been shown that Er excitation occurs very efficiently in Si (with a cross section of 3x10~(-15) cm~2, to be compared with the cross section for direct photon absorption of 8xl0~(-21) cm~2) demonstrating that, in principle, Er luminescence in Si can be very efficient. The main limiting step has been recognized in the non-radiative decay channels, back-transfer (with the energy transferred back from excited Er to electron-hole couples) and Auger (with the energy released to free carriers). Nevertheless Er:Si devices operating at room temperature, with efficiencies of 0.1% and modulation speed of 10 MHz, have been fabricated. A particularly interesting field of research concerns the coupling of Er and Si nanostructures. Indeed, erbium doping of Si nanocrystals (nc) has been recently recognized as a quite efficient method of obtaining 1.54 fim luminescence. Indeed, the excited nc preferentially transfer their energy to the Er ions which subsequently de-excite radiatively. Several points of this process are of extreme interest. In particular, the transfer of energy from the nanostructure to Er is much more efficient than direct photon absorption and luminescence intensities 2 orders of magnitude higher than for Er in SiO_2 are observed. Moreover, the non-radiative de-excitation processes typically limiting Er luminescence in Si, namely Auger with free carriers and energy back-transfer, are strongly reduced in this case further improving the luminescence yield. In the present work we will review our recent work on Si nanostructures and their interaction with Er ions.