Energy transfer from luminescent centers to Er3+ in erbium-doped silicon-rich oxide films
© Jin et al.; licensee Springer. 2013
Received: 2 June 2013
Accepted: 21 August 2013
Published: 28 August 2013
The energy transfer mechanism between luminescent centers (LCs) and Er3+ in erbium-doped silicon-rich oxide (SROEr) films prepared by electron beam evaporation is investigated. Intense photoluminescence of the LCs (weak oxygen bonds, neutral oxygen vacancies, and Si=O states) within the active matrixes is obtained. Fast energy transfer from Si=O states to Er3+ takes advantage in the SROEr film and enhances the light emission from Er3+. The introduction of Si nanoclusters, which induces the Si=O states and facilitates the photon absorption of the Si=O states, is essential to obtain intense photoluminescence from both Si=O states and Er3+.
KeywordsLuminescence centers Silicon nanoclusters Erbium ion Energy transfer Silicon-rich oxide
In the recent years, a great amount of researchers have focused on the luminescent materials of Si-based light sources to obtain efficient light emission, which is a critical step for the achievement of the Si-based optical interconnections. These materials include silicon-rich oxide (SRO)[2–6], silicon-rich nitride[6, 7], Ge-on-Si luminescent materials, and rare-earth-doped Si-based materials[9–14]. Among all these Si-based materials, erbium-doped SRO (SROEr) films have attracted a great research interest in these years as the 1.54-μm luminescence of Er3+ is compatible with both the optical telecommunication and the Si-based microphotonics[11–18]. The excitation mechanism of Er3+ in SROEr has been basically discussed, while three indirect excitation mechanisms of Er3+ have been proposed in the literatures: (1) slow energy transfer process (τr = approximately 4 to 100 μs) from exciton recombination in silicon nanoclusters (Si NCs) followed by internal relaxation to Er3+[11, 16, 18, 19], (2) fast energy transfer process (nanosecond and faster) between hot carriers inside the Si NCs and Er3+[20, 21], (3) fast energy transfer process (very fast, sub-nanosecond) from luminescent centers (LCs) in the SROEr matrixes to Er3+.
The Si NCs acting as the classical sensitizers embedded in the SROEr films can provide large excitation cross-section and efficient energy transfer to Er3+, from which the luminescence of Er3+ can be improved significantly. Both light emitting diodes and optical gain have been achieved from the Si NC-sensitized SROEr systems. However, the luminescence intensity and optical gain of Er3+ are still limited due to the low fraction of Er3+ ions sensitized by the Si NCs. Moreover, the confined carrier absorption (CCA) process that exists in the Si NC-sensitized SROEr systems would be accelerated by the slow energy transfer process between the Si NCs and Er3+, from which the optical properties of Er3+ would be further degenerated[16, 17]. Besides, the introduction of nonradiative decay channels due to the presence of the Si NCs would also degenerate the optical performances of the Si NC-sensitized SROEr systems. Furthermore, the luminescence intensity of Er3+ would be quenched by the Auger process produced during the energy transfer process between hot carriers and Er3+[20, 21].
Compared to the indirect energy transfer process from the Si NCs and hot carriers to the nearby Er3+, the sensitization from the LCs in the SROEr matrixes to Er3+ could effectively overcome the above disadvantages, and the 1.54-μm luminescence of Er3+ might be improved significantly. This improvement partially originated from the “atomic”-size scale of the LCs, where the sensitizer (LCs) with high density could be obtained. Meanwhile, the CCA as well as the Auger process that existed in the Si NC-sensitized SROEr systems could be degenerated obviously since the energy transfer process from the LCs to Er3+ is extremely fast (τr = approximately 100 ns). Furthermore, the LCs could be obtained in the SROEr matrixes with low Si excesses; therefore, the nonradiative decay channels caused by the incorporation of the Si NCs could be significantly suppressed. However, previous research about LC-mediated luminescence of Er3+ in SROEr films has shown that the LCs are unstable during the high-temperature annealing process, which limits the photoluminescence (PL) performance of both LCs and Er3+. Therefore, intense and stable emission of LCs in SROEr film is required in the view of obtaining efficient luminescence of Er3+ by the energy transfer process from LCs to the Er3+.
In this work, SROEr films with stable LCs were prepared by electron beam evaporation (EBE) following a post-annealing process. The evolution of the PL from the SROEr films during the annealing process is investigated. The effect of energy transfer from the LCs to the nearby Er3+ on the luminescent performance of SROEr film is demonstrated, and the optimization of its PL property is expected. Furthermore, the effect of the introduction of Si NCs on the performance of LCs is studied.
The SROEr films were deposited on p-type silicon substrates by EBE using a SiO and Er2O3 mixed target (Er atomic concentration of approximately 20 at%), with the deposition rate of 1 to 3 Å/s controlled by the electron beam current. The base pressure of the deposition chamber was pumped to lower than 5 × 10−3 Pa, and the substrates were maintained at 300°C. The atomic compositions of the as-deposited (A.D.) films were detected by Rutherford back scattering analysis using 2.02-MeV4 He ion beam at a scattering angle of 165°. The Si atomic concentration in the SROEr films was about 36 at%, and the Er concentration was around 3 × 1019 at./cm−3. The Er concentration was low enough to avoid the Er clustering procedure. After the deposition of the SROEr films, a thermally annealing process at 700°C to 1,150°C in a quartz furnace under nitrogen ambient was experienced to form the different sensitizers (LCs and/or Si NCs). The structural characteristics of the films were studied using high-resolution transmission electron microscopy (HRTEM) image. Room temperature PL was detected by charge-coupled device (PIXIS: 100 BR, Princeton Instruments, Trenton, USA) and InGaAs photon multiple tube (PMT, Hamamatsu R5509, Iwata City, Japan) for visible and infrared emission ranges, respectively, where a He-Cd laser with a wavelength of 325 nm was employed as the excitation light source. Time-resolved PL excited by a 405-nm picosecond laser diode was performed by a multichannel photon counting system (Edinburgh Instruments Ltd., Livingston, UK). A xenon lamp with continuous wavelength in the range from 200 to 900 nm was employed for the measurement of the PL excitation (PLE) spectra. The infrared (IR) spectroscopy was performed using a Bruker IFS 66 V/S Fourier transform IR (FTIR, Bruker BioSpin AG Ltd., Beijing, China) spectroscope under the transmission mode and vacuum condition.
Results and discussion
To further determine the energy transfer mechanism in the SROEr films, the PLE spectra of the Si=O states (collected at 2.2 eV) and Er3+ (collected at 0.8 eV) for the SROEr film annealed at 1,150°C for 30 min were measured, as shown in Figure 4b, with the intensities normalized by their correspondingly maximal values. The well overlap between the PLE spectrum for the Si=O states and that for Er3+ indicates that energy transfer from the Si=O states to Er3+ plays a dominant role in this SROEr film. The little discrepancy between these two spectra might have originated from the resonant excitation of Er3+. Besides, the peak around 3.8 eV which appears in the PLE spectra might be related to the optical excitation of the Si NCs since the introduction of the Si NCs would enhance the PL intensity of both Si=O states and Er3+.
In summary, the efficient luminecence of Er3+ in the SROEr film is achieved by the energy transfer process from fast recombination centers (LCs). The SROEr films with abundant LCs (WOBs, NOVs, and Si=O states) and Si NCs are prepared by electron beam evaporation following a post-annealing process. Intense and stable PL of LCs dominated by the Si=O states is obtained in the SROEr matrix. From the investigation of the evolution of the PL properties and microstructures from the SROEr films, we consider the fast energy transfer from the Si=O states to Er3+ as the main transfer mechanism. The introduction of the Si NCs induces the Si=O states and facilitates the photon absorption of the Si=O states, which is essential to obtain intense PL from both Si=O states and Er3+. Further improvement of the PL property of both the Si=O states and Er3+ might be achieved by optimizing the annealing condition of the SROEr films.
DL received his Ph.D. degree in the State Key Laboratory of Silicon Materials and Department of Material Science and Engineering from Zhejiang University, Hangzhou, China, in 2002. He is currently an Associate Professor in the Department of Material Science and Engineering at Zhejiang University. His current research interests include the synthesis of plasmonic microstructure, application of plasmonic microstructure on solar cells, Raman and luminescence, and silicon photonics. LJ, LX, and FW are currently Ph.D. students in the State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou, China. Their current research interests include luminescence from erbium-doped silicon-rich oxide matrix, silicon-rich nitride matrix, and dislocations in silicon, silicon nitride-based light-emitting devices, and localized surface plasmon resonance of metal nanostructures. DY received his B.S. degree from Zhejiang University, Hangzhou, China, in 1985, and Ph.D. degree in Semiconductor Materials from the State Key Laboratory of Silicon Materials in Zhejiang University, Hangzhou, China, in 1991. He has been with the Institute of Metal Materials in Tohoku University, Japan, and worked for Freiberg University, Germany, from 1995 to 1997. He is currently the director of the State Key Laboratory of Silicon Materials. His current research interests include the fabrication of single crystalline silicon materials for ultra-larger-scale integrated circuit and defect engineering, polysilicon materials and compound thin film photo-electric conversion materials for photovoltaic, nano-scale silicon wire/tube and other one-dimensional semiconductor materials, and silicon-based materials for optoelectronics. DQ received his B.S. degree from the Department of Electrical Engineering from Xiamen University, Xiamen, China, in 1951. He has been with the Department of Electrical Engineering, Department of Radio-Based Semiconductor Materials and Devices, Department of Materials Science and Engineering in Zhejiang University, China, since 1953.
Confined carrier absorption
Electron beam evaporation
Fourier transform infrared spectroscope
High-resolution transmission electron microscopy
Neutral oxygen vacancies
- Si NCs:
Erbium-doped silicon-rich oxide
Weak oxygen bonds.
This work is supported by the Program 973 (no. 2013CB632102), the National Natural Science Foundation of China (no. 61176117), and the Innovation Team Project of Zhejiang Province (no. 2009R5005).
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