1. Introduction

Erbium doped silicon-based materials provide a valuable approach for optoelectronic devices that can be monolithically integrated atop the widespread silicon electronics platform. Erbium, in its trivalent Er³⁺ state, could emit sharp luminescence at the standard telecommunication wavelength of 1.5 μm, and has hence attracted much attention [1]. Particularly, the observation of efficient energy transfer from Si nanoclusters (Si-NCs) embedded in SiO₂ matrix to Er³⁺ ions has made a major breakthrough in this subject, increasing the Er³⁺ excitation cross section by 3~4 orders of magnitude as well as broadening the excitation band [2–4]. Recently, intense 1.54 μm luminescence of Er³⁺ ions has been observed in Er doped silicon rich oxynitride (Er:SRON), and non-resonant Er excitation via Si-NCs has been demonstrated [5,6]. Compared with silicon rich oxide (SRO), the widely studied host matrix for Er³⁺ ions, the narrower band gap of silicon rich oxynitride (SRON) favors the carrier injection, and the higher refractive index is beneficial for optical mode confinement. Additionally, the band structure of SiO_xN_y can be easily modulated by changing the oxygen/nitrogen ratio, and equivalent carrier injections could be achieved in Si/SiO_xN_y system [7], shedding light on the potential applications in bipolar devices for SRON. Generally, the Er excitation in the electroluminescence process in unipolar devices is quite different from that in bipolar devices. In unipolar devices [8–10], e.g. most of the reported SiO₂ based light emitting devices under direct current conditions where electron conduction is dominant, the excitation probability of sensitizers via capture of electrons and holes is low and the efficient route to excite Er³⁺ ions is via impact excitation by hot carriers when the applied electric field is large enough. While in bipolar devices [11,12], injected electrons and holes can be efficiently captured by sensitizers and create excited states, which upon recombination transfer their energy to Er³⁺ ions and significantly increase the Er³⁺ excitation cross section, in a low electric field which is not sufficient to generate hot carriers. The onset voltage of Er³⁺ EL for the unipolar devices is usually high and thus incompatible with current CMOS technology. It would be reduced by bipolar injection, increasing consequently the reliability of devices. SRON containing Si-NCs is thus an ideal host matrix for Er³⁺ doping, taking into account both the optical and electrical pumping.

On the other hand, recent studies have shown that the presence of Si-NCs is not necessary to obtain intense Er-related luminescence. In parallel to the sensitization mechanism via Si-NCs, competing models based on luminescent centers (LCs) in SRO [13,14] and localized states in silicon rich nitride (SRN) [15] have been invoked. Due to the atomic size scale of LCs and localized states, sensitizers with much higher densities can be obtained, as opposed to Si-NCs. Indeed, there is now consensus in the literature, that the principal limitation for achievement of optical gain in Er-doped Si-NCs is the small fraction of Er³⁺ ions sensitized by Si-NCs [16,17], mainly due to the short interaction distance between them and the finite density of Si-NCs [18,19]. Hence, it is believed that the use of sensitizers with high densities, e.g. LCs or localized states, would increase the fraction of sensitized Er³⁺ ions and improve the photoluminescence (PL) yield, as demonstrated in [13]. However, up to now, there is little information regarding the sensitization mechanism of Er³⁺ ions in SRON. Obviously, this issue deserves intensive investigation to engineer efficient light emitting devices based on Er:SRON.

In this letter, we study the optical properties of Er:SRON films prepared by reactive co-sputtering. Post-deposition annealing is performed in a wide range of temperatures (T_a), from 600 °C where the films are amorphous, to 1100 °C where phase separation occurs. The evolution of sensitization mechanism with T_a has been particularly examined.

2. Experiment

Er:SRON films with thicknesses of approximately 150 nm were deposited onto p-type silicon substrates by reactive co-sputtering from Er, Si and Si₃N₄ targets. The deposition was conducted under the plasma of Ar-diluted 1% O₂ atmosphere and the Si substrates were maintained at 100 °C. The concentrations of Si, O, N and Er in the films were obtained by Rutherford backscattering analysis, and the composition of the samples was found to be Si_0.479O_0.366N_0.151Er_0.004, with the Er concentration ~1.8 × 10²⁰ at./cm³. The refractive index of the as-deposited sample is about 1.74 at 1540 nm measured by spectroscopic ellispometry. We have also measured the absorption coefficient of the as-deposited sample and derived the optical bandgap (2.3 eV). After deposition, the films were annealed under N₂ flux for 1 h at temperatures in the range of 600-1100 °C. The microstructures of the annealed films were analyzed by transmission electron microscopy (TEM, Tecnai F30G2, FEI Company, USA). Matrix-related luminescence measurements in the visible range were carried out by pumping with a He-Cd laser at 325 nm and detecting with a charge coupled device (PIXIS: 100BR, Princeton Instruments, USA). For Er-related luminescence around 1540 nm, the samples were excited resonantly by a laser diode at 980 nm or non-resonantly by a Xe lamp at 325 nm. The PL signal in the near infrared range was measured by a liquid nitrogen cooled photomultiplier tube (PMT, Hamamatsu R5509, Japan). Time-resolved PL was excited by a microsecond lamp at 325 nm and recorded with a multichannel photon counting system (Edinburg Photonics, UK). A Xe lamp was used as light source in the PL excitation (PLE) spectroscopy measurements. All the PL measurements were taken at room temperature unless otherwise specified.

3. Results and discussion

Figure 1 shows the matrix-related PL spectra from Er:SRON films annealed at different temperatures. When T_a is below 700 °C, the PL intensity of samples is nearly indistinguishable. Though intense PL from LCs has been observed in low temperature annealed silicon rich oxide [13,14], cases seem to be different in our films, which indicate the absence of LCs in SRON. When T_a exceeds 800 °C, a broad PL band peaking between 700 and 800 nm can be clearly observed. The PL peaks redshift with the increase of T_a. Figure 1(b) shows the typical time-resolved PL spectra recorded at 700 nm from the specimen annealed at 800 °C. By fitting the spectra with a stretched exponential function, we can obtain that the decay time is about 42 μs. Such slow PL at these wavelengths is commonly attributed to radiative recombination of excitons confined in Si-NCs. These observations reveal the formation of Si-NCs after annealing at 800 °C. Since no lattice fringes can be detected in the cross sectional TEM image of 800 °C annealed specimen (figures not shown), Si-NCs formed at this temperature should be amorphous rather than crystalline. Si-NCs will continue to grow and gradually crystallize with the increase of T_a. In the TEM image of 1100 °C annealed specimen, lattice fringes of Si-NCs can be observed, as shown in the inset of Fig. 1(a). Redshift of the PL peaks is due to an increase of Si-NC diameter with T_a, and a resulting reduction of the band gap.

 

Fig. 1 (a) Matrix-related PL spectra from Er:SRON films annealed at temperatures in the range of 600-1100 °C. The inset shows the cross sectional TEM image from the specimen annealed at 1100 °C. (b) Time resolved PL spectra recorded at 700 nm from the specimen annealed at 800 °C. The excitation wavelength is 325 nm.

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Figure 2(a) shows the PL excitation (PLE) spectra of Er³⁺ measured at 1540 nm. Interestingly, all the PLE spectra show a broad band in the range of 250-500 nm, with no direct Er³⁺ absorption peaks superimposed on. These results clearly show non-resonant excitation of Er³⁺ in all the samples. In addition, the PLE spectra shift to shorter wavelengths by increasing T_a from 600 °C to 900 °C, while they show an identical shape, except for an increase in the intensity, when further increasing T_a to 1100 °C. For annealing temperatures lower than 700 °C, there seems to be “dark sensitizers” of Er³⁺ ions in our samples. Indeed, sensitization of Er³⁺ via “dark” triplet Si-related oxygen deficient centers (ODCs) has been demonstrated in Er doped SRO, with a reversed temperature dependence in which the integrated Er³⁺ PL intensity increases upon heating [20]. However, we have measured the temperature dependence of integrated Er³⁺ PL intensity in the 600 °C annealed specimen and find it quenches upon heating (shown in the inset of Fig. 2(c)), indicating that ODCs may not be the main sensitizers in our samples. Nevertheless, taking the PLE peak shift into consideration, we suggest that the excess Si related localized states in the band gap of silicon oxynitride may play the dominant role in sensitizing Er³⁺ in low temperature annealed samples [15]. Generally, the annealing process would lead to the microstructure reconfiguration of SRON:Er films. The excess Si would concentrate to form Si-rich regions upon annealing, and phase separation would start when the annealing temperature exceeds 800 °C. Since the solubility of Er in SiO₂ is much higher than that in Si, it is reasonable to expect that Er³⁺ ions tend to stay in Si-poor regions. The blue shift of PLE peaks is then attributed to the degenerate absorption of low-energy photons in Si-poor regions. Note that no noticeable PLE peak shifts can be observed when T_a exceeds 900 °C, indicating that the phase separation has completed or another sensitization mechanism takes place.

 

Fig. 2 (a) PL excitation spectra of Er³⁺ measured at 1540 nm. The PLE spectra measured at 750 nm for the 1100 °C annealed specimen is denoted by empty squares.(b) Er-related PL spectra of samples annealed at different temperatures for excitation at 325 nm. (c) Er³⁺ PL decay time τ_dec as a function of the annealing temperatures. The inset shows the evolution of integrated Er³⁺ PL intensity with temperatures in the 600 °C annealed specimen. (d) I_PL/τ_dec as a function of the annealing temperatures for excitation at 325 nm and 980 nm (right hand scale).

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The dependence of Er-related PL with T_a is shown in Fig. 2(b) for excitation at 325 nm. The spectra show the typical features of Er³⁺ emission, originated from the intra-4f ⁴I_13/2→⁴I_15/2 transition, with a main peak at 1540 nm. The thermal treatments improve gradually the Er³⁺ emission by a factor of ~2 as T_a is increased from 600 to 1100 °C. Simultaneously, the full width at half maximum (FWHM) of Er³⁺ related PL peaks around 1540 nm decreases monotonically by ~12%, suggesting the modulation of Er³⁺ local environment, resulting from the continuous microstructure evolvement such as phase separation and lattice relaxation during thermal treatments. The decay traces of Er³⁺ PL at 1540 nm are fitted with a stretched exponential function and the extracted PL decay times τ_dec are plotted in Fig. 2(c) as a function of the annealing temperatures. τ_dec shows a monotonic increase with T_a, from 0.11 ms to 0.47 ms, as T_a is increased from 600 to 1100 °C. Meanwhile, the extracted exponent beta value increases initially from 0.69 in the 600 °C annealed specimen to 0.75 in the 800 °C annealed specimen, and then decreases eventually to 0.63 in the 1100 °C annealed specimen. The relatively short decay time is probably due to the abundant nonradiative channels, i.e. Si dangling bonds in our samples [21]. During the thermal treatments, the gradual removal of non-radiative channels leads to the increase of Er³⁺ decay time.

In the linear excitation regime, we have [22]

Note that the ratio of I_PL/τ_dec excited at 325 nm in low temperature (700 °C ≤) annealed samples is more than twice of that in high temperature (≥ 800 °C) annealed samples, indicating that higher N_Er,sen can be acquired via sensitization by excess Si related localized states, as opposed to Si-NCs. This is due to an increase in the density of sensitizers, taking into account the degradation of optically active Er³⁺ ions by decreasing T_a. The main limitation of Er³⁺ PL in low temperature (700 °C ≤) annealed samples seems to lie in the relatively short lifetime, which is due to the unpassivated nonradiative channels. Different from SRO [25], high density of Si dangling bond, an efficient nonradiative channel for Er³⁺, would be obtained in SRN as well as SRON [21]. By further optimization such as hydrogen passivation, it is believed that intense Er³⁺ PL via energy transfer from localized states, comparable to that sensitized by Si-NCs, could be achieved when reducing the annealing temperature to 600-700 °C, a temperature range which is more compatible with the CMOS technology.

4. Conclusions

In conclusion, sensitization of Er³⁺ luminescence was studied in Er doped silicon rich oxynitride films fabricated by reactive co-sputtering with different annealing temperatures. It is demonstrated that the sensitization mechanisms evolve with T_a as well as the microstructures of the films. In the low temperature (700 °C ≤) annealed samples, the films are amorphous and the “dark” excess Si related localized states would dominate in sensitizing Er³⁺. However, such states are unstable during thermal treatments and their density degrades significantly when annealing temperature exceeds 800 °C, where phase separation starts and Si-NCs play an essential role in sensitizing Er³⁺ ions. Though encountered by a reduction in the density of optically active Er³⁺ ions in low temperature annealed samples, higher density of sensitized Er³⁺ ions is acquired via sensitization by excess Si related localized states, due to an increase in the density of sensitizers, as opposed to Si-NCs.