Indirect excitation of Er3+ ions in silicon nitride films prepared by reactive evaporation
Résumé
Er-doped silicon nitride films were obtained by reactive evaporation of silicon under a flow of nitrogen ions and were annealed at temperatures up to 1300°C. Samples were studied by infrared absorption and Raman spectrometries and by transmission electron microscopy. The 1.54 m Er-related photoluminescence ͑PL͒ was studied in relation with the structure with pump excitation at 488 and 325 nm. Steady-state PL, PL excitation spectroscopy, and time-resolved PL were performed. The results demonstrate that Er 3+ ions are indirectly excited both via silicon nanocrystals and via localized states in the silicon nitride matrix. Er-doped silicon-based materials have attracted much attention in the scientific community because of their potential use for optoelectronics. 1 Indeed, Er 3+ ions can emit sharp luminescence at 1.54 m, which is the commonly used wavelength for optical communications. The Er sensitization has been widely studied in Si rich SiO 2 layers. In silica containing silicon nanocrystals ͑Si-nc͒, the Er-related photolu-minescence is strongly improved due to a strong energy transfer from Si-nc to Er 3+ ions. 2-4 The Er 3+ ions can then be indirectly excited by Si-nc which have an absorption cross section several orders of magnitude higher than that of direct Er excitation. While SiN x is a particularly interesting host matrix for electrically pumped light-emitting devices, the Er excitation mechanism in silicon nitride films is still not clear. Similarly to the SiO x based samples, the sensitization of Er 3+ ions by Si nanoparticules has been reported in SiN x samples prepared by plasma enhanced chemical vapour deposition ͑PECVD͒ 5 or by magnetron sputtering. 6 However, some works have also demonstrated that indirect excitation of Er 3+ ions could occur via electronic states localized in the SiN x band tail states. 7,8 In this letter, we study the Er-related PL at 1.54 m in Er-doped silicon nitride thin films prepared by an ion-beam-assisted evaporation technique. The evolutions of the structure and of the PL properties with the annealing treatments are studied. It is demonstrated that the Er excitation is indirect and that Si-nc is able to improve the PL intensity. It is also shown that another indirect excitation path presumably exists in the amorphous SiN x matrix. Silicon was evaporated from an electron beam gun with a deposition rate equal to 0.1 nm/s. The 200 nm thick films were deposited on silicon substrates maintained at 100°C. The nitrogen ions were provided by an electron cyclotron resonance microwave plasma source. The nitrogen flow was regulated by maintaining the total pressure in the evaporation chamber at 2 ϫ 10 −5 Torr. The Er doping was performed from an effusion cell. Rutherford backscattering spectrom-etry was used to analyze the chemical content of the film. The Si, N, O, and Er atomic concentrations are equal to 47%, 48%, 5%, and 0.3%, respectively. The oxygen content is due to the low density of the layer and to exposure to the air. This concentration corresponds to a 12 at. % Si excess compared to the Si 3 N 4 equilibrium stoichiometry. The Fourier transform infrared ͑FTIR͒ experiments were carried out with a spectrometer with a resolution of 2 cm −1. Raman measurements were carried out with a mutichannel spectrometer equipped with a 1800 grooves mm −1 grating. The samples were excited by the 514 nm line from an argon laser. Transmission electron microscopy was performed with a 200 keV microscope. For the steady-state PL experiments, the samples were excited by a 30 mW He-Cd laser using the 325 nm line or by a 60 mW laser diode emitting at 488 nm. For the PL excitation ͑PLE͒ experiments, the samples were excited by an optical parametric oscillator laser. The PL signal was measured by a photomultiplier tube cooled at 190 K. For the time-resolved PL experiments, the samples were pumped by the 355 nm line of a frequency-tripled YAG:Nd laser. The laser pulse frequency, energy, and duration were typically equal to 10 Hz, 50 J, and 20 ns, respectively. The time response of the detection system was better than 1 s. Figure 1͑a͒ shows the FTIR spectra of the films for as-deposited sample and samples annealed at 1000 and 1100°C. The spectrum shows a very intense band at around 850 cm −1 , characteristic of the asymmetric stretching vibration of the SiN bonds. 9 The spectra are not significantly modified for annealing temperatures lower than 1000°C since only a 6 cm −1 shift occurs to higher wavenumbers. For higher annealing temperature, the peaks shift again a few cm −1 and a shoulder appears at high wavenumbers, demonstrating a modification of the SiN bonds, which could be correlated to the precipitation of Si-nc. 10
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