Visible Luminescence in Si/SiO 2 Superlattices
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ness required. It is thus now possible to prepare nanometer thick Si layers intercalated with a wide band gap material to produce the required electron confinement. Such semiconductor/insulator superlattices employing hydrogenated amorphous Si (a-Si:H) and a-SiN,:H were first demonstrated in 1983 by Abeles and Tiedje [16]. Since then, there has been considerable work aimed at understanding the optical properties of these amorphous semiconductor layered structures (e.g., see Refs. 17-25). More recently, room temperature luminescence has been reported from a-Si/Si0 2 [26], a-Ge:H/a-SiN.:H [27], c-Si:H/a-Si:H [28], polycrystalline Si/CaF 2 [29,30], recrystallized aSi:I-I/a-SiNx:H [31] and recrystallized a-Ge:H/a-SiNX:H [32] superlattices, but no convincing evidence for quantum confinement induced emission was obtained with the possible exception of Refs. 30 and 31. Here we report onthe visible light emitting properties of Si/SiO 2 superlattices grown on Si(001). These regular periodic structures are free of the size distribution and contamination effects that complicate the optical properties of Si nanoparticles and i-Si. In the earlier work of Zayats et al. [26], superlattices were prepared by magnetron sputtering of Si0 2 and Si in an argon atmosphere. Some evidence was obtained of confinement shifted luminescence from very thin a-Si layers, but only under pulsed laser excitation. The analysis of the optical emission was complicated by the presence of additional features due to recombination across the a-Si/SiO 2 interfaces and within the Si0 2 layers. More recently, we have prepared Si/SiO 2 superlattices under quite different growth conditions using MBE, magnetron sputtering (MS), and plasma enhanced chemical vapor deposition (PECVD) techniques [33,34]. For these superlattices, the Si layer thickness dependence of the optical emission when correlated with the conduction and valence band shifts provides the first direct evidence of light emission due to quantum confinement in Si nanostructures [35-38]. EXPERIMENT MBEgro3Ah
The a-Si/SiO 2 superlattices were grown at room temperature on lightly phosphorous-doped ntype (001) Si wafers. The Si wafer was first submitted to a 600 s exposure to ultraviolet (UV) ozone before introduction into the vacuum chamber. This UV-ozone treatment is a rate-limited oxidation process that produces a - I nm thick carbon-free oxide layer [39]. A thin Si layer was next deposited at a 0.1 nm/s rate on the oxidized wafer by MBE in a VG Semicon V80 system. The wafer was then taken out of the ultra-high vacuum chamber and submitted to another 600 s exposure to UV ozone (the wafer was typically exposed to air for - 100 s before and after the oxidation treatment). This procedure was repeated to create a six period a-Si/SiO2 superlattice.
The a-Si/SiO 2 superlattices were fabricated using an automated radio-frequency MS deposition system developed at the National Research Council of Canada [40]. The layers were deposited as follows: after reaching a base pressure of 4-6 x 10-7 Torr, argon gas
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