High Energy Excitation Transfer from Silicon Nanocrystals to Neodymium Ions in Silicon-Rich Oxide Film

Abstract

In this work, direct experimental evidence of the excitation energy transfer from silicon quantum dots Si-QDs to Nd ions has been given based on photoluminescence PL and PL excitation measurements in a wide spectral range. The indirect excitation of Nd ions by transfer from excited Si-QDs is possible and even more efficient at higher energy levels 4 I 9/2 → 4 D 3/2 , 4 D 5/2 than the indirect excitation reported up to now at lower energy 4 I 9/2 → 4 F 3/2 – 4 F 7/2 levels. The recent discovery of efficient luminescent emission from silicon nanoclusters 1 and their sensitizing action toward rare-earth RE ions 2 has opened the field of important applications for integrated optoelectronic devices using the complementary metal oxide semiconductor technology. For example, in the fields of optical telecommunications , the erbium ion that emits at 1.54 ␮m has become attractive because this wavelength corresponds to the minimum absorption of the optic silica fiber. For this reason, silica-based matrices co-doped with erbium ions and silicon quantum dots Si-QDs have been extensively investigated. 3-9 However, up to now, among all the teams working on that domain, only one group has reported net gain in an Er-doped structure. 10 One of the main reasons lies in the nature of the three-level electronic 4f structure of the Er 3+ ions so that the de-excitation from 4 I 13/2 to 4 I 15/2 may lead to a reabsorp-tion of the photon emitted by the neighboring Er 3+ ions. Among the other RE ions that can benefit from the sensitizing effect of Si-QDs described by Kenyon et al., 2 Nd 3+ offers the 4 F 13/2 – 4 I 11/2 emission at about 1060 nm, which corresponds to an operating mode based on four levels. The relaxation toward the fundamental state 4 I 9/2 takes place by nonradiative de-excitation. Consequently, the signal emitted by the Nd 3+ ions is not reabsorbed by another Nd 3+ ion. This confers to the Nd-doped material the possibility of reaching a net gain. Therefore, such a Nd-doped system appears to be more favorable than its Er-doped counterpart for the achievement of a significant population inversion. It would consequently be the preferred material to obtain a net gain in structures containing Si nanoclus-ters even if the fiber absorption is higher at this emission wavelength than at 1.54 ␮m. The possible sensitizing role of Si-QDs toward Nd 3+ in such a dielectric matrix has been reported so far by only a few studies. 11-14 However, Seo et al. 11 claimed that only films with Si content lower than 44 atom % show significant Nd 3+ lumines-cence because low Si excess is needed to precipitate small Si nano-clusters. Similar results were also reported by Franzo et al., 3 who investigated Nd-implanted Si-rich silicon oxide SRSO. In this article, we discuss the indirect excitation of Nd 3+ ions embedded in SRSO thin films produced by reactive magnetron cosputtering. We propose an excitation channel describing the energy transfer from the high energy levels of Si-QDs to the high energy levels of the Nd 3+ ions. Thin RE-doped layers were deposited by reactive magnetron cosputtering of a pure SiO 2 target topped with Nd 2 O 3 chips. The silicon excess in the layer was obtained through the monitoring of the hydrogen partial pressure mixed to the Ar plasma in the chamber. This reactive deposition approach was based on the ability of hydrogen to reduce the oxygen species originating from the sputtered SiO 2 target. This allowed the control of Si incorporation in the growing thin films. 15 The RE content was controlled through the number of the Nd 2 O 3 chips placed on the SiO 2 target. For this study, Si excess was kept constant at about 7 atom %, while Nd concentration was fixed at 0.08 atom %. These parameters have been chosen based on our previous investigation 16 and have led to an optimal Nd emission intensity. The Nd-doped SRSO Nd-SRSO layers were deposited at a power density of 0.76 W cm −2 on 1 0 0 p-type Si wafers and were annealed at 1100°C for 1 h under a N 2 flow. Photoluminescence PL experiments have been obtained using a 266 nm excitation wavelength. The UV-visible spectral range has been detected by an HR4000 Ocean Optics spectrophotometer, while the near-IR visible emission has been collected by an InGaAs charge-coupled device camera after dispersing through a Triax 550 Jobin Yvon monochromator. For the total photoluminescence exci-tation TPLE measurements, a 450 W xenon lamp has been used as the excitation source. The configuration Triax 180, Jobin Yvon monochromator and HR4000 Ocean Optics spectrophotometer gives an averaged excitation flux lower than ϳ10 19 photons/s cm