Oxygen-doped amorphous silicon nitride thin films were fabricated in a plasma enhanced chemical vapor deposition system by using H2 diluted gases under a substrate temperature of 250 °C. An intense photoluminescence was achieved under 325 nm laser excitation and the luminescence is reduced to about 70% of the initial emission intensity after 1hr laser irradiation, which is improved compared with that deposited at low substrate temperature. Time-resolved photoluminescence behaviors were characterized and two recombination processes were observed, one is a “fast” nanosecond PL decay component which was explained in terms of the relaxation process to the localized O-Si-N related states, and the other is a “slow” microsecond radiative recombination component via the localized states associated with O-Si-N bands.
© 2014 Optical Society of America
Exploration on high efficiency Si-based emitters has received considerable attention in recent years due to their potential applications in monolithic optoelectronic integrated circuits. However, bulk silicon is an inefficient light emitter due to its indirect transition characteristic. In order to get efficient light emission from Si-based materials, many kinds of approaches have been proposed, such as low-dimensional Si nanostructures and Si-based luminescent films [1–10]. As one of the interesting Si-based luminescent materials, amorphous silicon nitride (a-SiN) has been widely studied and both intense photo and electroluminescence have been observed at room temperature. For example, amorphous SiN thin films were fabricated at low temperature (50 °C) and intense blue light emission with external quantum efficiency over 3.0% was realized . Moreover, tunable photoluminescence (PL) from 1.90 to 2.90 eV was obtained in a-SiN thin films through controlling the ratio of NH3/SiH4 in a plasma-enhanced chemical vapor deposition (PECVD) process . The electroluminescence based on the material showed a yellowish emission with a power efficiency of 10−6 .
In our previous work, the oxygen-doped amorphous SiN (a-SiN:O) films were fabricated at room temperature, and very strong light emission characteristics were obtained . Consequently, light emitting devices were prepared on which green light emission was achieved . More recently, both the internal and external quantum efficiency of a-SiN:O thin films were measured and the internal quantum efficiency of photoluminescence can be as high as 60% . We also designed and fabricated waveguide structures based on a-SiN:O thin films on which optical gain was successfully obtained . However, the physical origin and mechanism of light emission and optical gain in a-SiN:O thin films remains an open question. Another challenge is that the luminescence stability of a-SiN:O at low substrate temperature is quite poor, which impedes their device applications. In the present work we prepared a-SiN:O thin films at a substrate temperature of 250 °C. The microstructures and bonding configuration were investigated, and the luminescence stability was obviously improved. Time-resolved photoluminescence (TRPL) measurements were performed in order to demonstrate the dynamical process of photo-generated carriers in a-SiN:O films. Two PL decay processes were observed, one is a “fast” nanosecond carrier relaxation process, the other is a “slow” microsecond radiative recombination component.
The a-SiN:H films with thickness of 120 nm were prepared by using silane (SiH4), ammonia (NH3) and mixed hydrogen (H2) in a PECVD system. The substrate temperature and plasma power were kept at 250 °C and 20 W, respectively. The SiH4 and NH3 were kept at 5 and 40 SCCM (cubic centimeters per minute at standard temperature and pressure), respectively. The hydrogen was the diluent gas, which was used to keep the total pressure at 450 mTorr. The hydrogenated amorphous silicon oxynitride (a-SiN:O) films were obtained by in situ oxidizing the a-SiN:H with 27 SCCM pure O2 gas for 10 min oxygen plasma treatment.
The microstructures were characterized by cross-sectional transmission electron microscopy (X-TEM). The optical absorption spectrum was measured in the spectral range of 300-2000 nm using a Shimazu UV-3600 spectrophotometer. The elemental compositions of Si, N and O were obtained through x-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250). The steady-state PL spectra were measured using a 30 mW continuous wave He-Cd laser (325 nm) with a photomultiplier detector (Hamamatsu R928), of which the beam diameter and spectral resolution were 1.2 mm and 1 nm, respectively. Time-resolved photoluminescence spectra were measured by using an Edinburgh instruments FLS980 fluorescence spectrophotometer equipped with a EPL-375 picosecond (ps) pulse diode laser (λexc = 375 nm, pulse duration 75 ps and repetition frequency 5MHz) and a frequency-tripled Nd:YAG nanosecond (ns) laser (λexc = 355 nm, 6 ns pulse width and repetition rate 10 Hz), the time resolution of this system is about 100 ps. The beam diameter/average power for ps and ns laser were about 0.05 mm/0.1 mW and 1 mm/10 mW, respectively. No polarizer was used in the luminescence measurements. The PL decay was measured from nanosecond to microsecond time scales.
3. Results and discussion
Figure 1(a) shows the optical absorption spectrum of as-prepared a-SiN:O thin films, it is found that the absorption becomes stronger when the wavelength is shorter than 400 nm. The bandgap can be deduced to be 4.0 eV according to the Tauc plots . Figure 1(b) is the room temperature PL spectrum of a-SiN:O films under the excitation of the He-Cd laser with wavelength of 325 nm. It is shown that the PL peak is centered at 525 nm (2.4 eV), and the band width is quite broad (1.0 eV). The luminescence intensity is strong enough and the green-yellow light can be clearly identified by the naked eye in a dark room.
The origin of light emission in a-SiN and a-SiN:O is still controversial. Several groups reported a strong photoluminescence from a-SiN films [6,16,17]. They attributed it to the radiative recombination of photo-generated carriers within amorphous Si nanoparticles existing in an a-SiN matrix. They found the amorphous Si nanoclusters in the cross-section TEM images due to the inhomogenous microstructures of prepared a-SiN films. However, in our case, the homogenous microstructures are shown in Fig. 2.No trace of amorphous Si quantum dots or nanoclusters can be identified. The inset of Fig. 2 shows the corresponding electron diffraction image which also represents the amorphous nature of the prepared sample. Another possibility is that the luminescence is associated with the band tail states or defect states [7–9]. It has been reported that the Si dangling bond center may responsible for the PL emission centered at 520 nm which is insensitive to the Si/N ratio in amorphous SiN films . However, in our previous work, we observed a strong light emission through introducing oxygen in amorphous silicon nitride films and we also found that the PL peak could be tuned in a wide range from 450 to 600 nm by controlling the Si/N ratios [11,18]. Therefore, we attributed the emission observed in our case to the localized states related to the O-Si-N bonds though we can’t completely rule out of the contribution of Si-Si bonds to the emission observed in our samples. The large stokes shift between optical absorption and photoluminescence spectrum as shown in Fig. 1 supports the idea that deep-level states may act as the luminescence centers to emit the green-yellow light in a-SiN:O films.
In order to investigate the chemical bonding configurations of prepared a-SiN:O films in the present case, the XPS spectra were measured. The Si 2p and N 1s signals are displayed in Fig. 3(a) and 3(b), respectively.From the XPS results, the N/Si ratio of a-SiN:O can be estimated as 0.81. The oxygen content is about 5.0%. It is interesting to find that the intermediate phases exist between the Si3N4 (101.9 eV) and SiO2 (103.4 eV) signals as revealed in Fig. 3(a). The signal centered at 120.65 eV can be attributed to the O-Si-N bonding configurations and the O-Si-N bonds can form the deep-level states in the gap of a-SiN:O films as suggested previously [13,18,19].
In our previous work, we found that the film quality could be improved through hydrogen plasma annealing . The inset of Fig. 1(b) gives the change of integrated PL intensity as a function of 325 nm laser irradiation time. It is found that the PL intensity is gradually decreased with the light irradiation. After 30 minutes, the PL intensity is reduced to 75% of the initial emission intensity. It is obviously improved compared with that of a-SiN:O films prepared at 25 °C substrate temperature (say low temperature), which is reduced to 60% after 20 min (not shown here). A similar phenomenon was also observed in a-SiN films prepared by using hot filament CVD. They attributed the PL fatigue to the generation of K centers (N3≡Si∙) as the SiNx films were irradiated under UV Xe lamp .
The improvement of luminescence stability can be ascribed to the high substrate temperature and the usage of hydrogen diluent gas during the film deposition process. It is well known that increasing the substrate temperature and using hydrogen diluent gas are helpful for enhancing absorbed precursors diffusion length on the surface and results in the more dense microstructures of amorphous semiconductors, which can be against the formation of light induced defects and improve the luminescence stability [22,23]. However, the films’ preparation parameters need optimizing to further improve the luminescence stability.
In order to further understand the dynamic process of photo-generated carriers in a-SiN:O films, TRPL measurements were performed. Figure 4(a) shows the TRPL spectra of a-SiN:O films under ps laser (λexc = 375 nm, pulse width = 75 ps) excitation by changing the detected wavelength. The measurement data were fitted by a multi-exponential function, which was usually used to fit the PL decay curves of amorphous Si-based alloys by deconvoluting the original PL decay data with the instrument response [10,21]. The equation can be expressed by :7]. Figure 4(b) displays the PL decay dynamics of a-SiN:O films under ns laser excitation (λexc = 355 nm, 6 ns). The multi-exponential function could also fit the experimental data very well. It is interesting to find that the lifetime is energy-independent, which is different from the results excited by ps laser. The obtained τ is 183.6 μs, which is comparable to the porous and nc-Si embedded in SiO2 materials [25,26]. We denoted this lifetime in the nanosecond regime as the “fast” decay time and the microseconds radiative time as the “slow” decay time. Our results are similar with that reported by Kato et al. . They observed a broad lifetime distribution ranging from 10−8 to 10−4 s in the a-SiNx:H and a-SiOxNy:H systems. They considered that the nanosecond decay time consisted of two recombination processes, one is an exciton-like recombination process through the band-tail states, while the other is the thermalization process, from which the photo-generated carriers relaxes to the deeper localized states . They ascribed the longer decay time to radiative recombination of the electrons and holes trapped in the deeper localized states . In our case we proposed the excitation and recombination process in our a-SiN:O films as shown in Fig. 5.First, the carriers were excited by incident laser from the valence band to the conduction band, and then rapidly relaxed to the localized O-Si-N related states. The O-Si-N states act as the luminescent centers to emit the visible light observed in the present samples. The nanosecond PL decay mainly originated from the carriers relaxing to the localized O-Si-N related states via a non-radiative process. The “slow” microsecond PL decay was due to the recombination process of the trapped carriers in the localized states.
The existence of two distinct recombination lifetimes consisting of “fast” and “slow” processes was also reported previously. Pavesi et al. observed optical gain in the nc-Si embedded in SiOx systems. They proposed a three level model to explain it, which includes a “fast” photo-excited carriers relaxation process from the highest occupied molecular orbital to the interface state via a non-radiative process, and the radiative recombination process between the lowest unoccupied molecular orbital and the interface states. The radiative recombination rate is on the microsecond scale, which is relatively “slow” compared to the non-radiative relax process . Moreover, Monroy et al. reported positive optical gain in nc-Si embedded in SiNx films. They found “fast” and “slow” processes through TRPL measurements, and attributed the nanosecond PL decay to excitons trapped at the nitrogen-related localized states through a non-radiative process and the longer μs PL decay to the recombination of the trapped carriers . These results suggest that the existence of fast and slow recombination processes play an important role in obtaining optical gain in Si-based materials. Chen et al. observed the distinct optical properties in the nc-Si embedded in SiOx films and nc-Si embedded in SiNx systems. No optical gain was measured in nc-Si embedded in SiNx films since only the fast emission process on the nanosecond scale was observed .
In summary, oxygen doped amorphous SiN films were fabricated in a PECVD system. The stability of photoluminescence is improved and the emission is attributed to the radiative recombination of photo-generated carriers via deep level states associated with O-Si-N bonds. Two PL decay processes were observed based on time-resolved measurements. We attributed the “fast” nanosecond lifetime to the process in which the carriers relaxed from the band tail states to the localized O-Si-N related states. The “slow” microsecond lifetime was attributed to the radiative recombination of carriers at localized O-Si-N states. The large time difference between the two processes can result in the emergence of the population inversion, which can cause optical gain in a-SiN:O films. Our results indicate that the a-SiN:O films have potential application in optical amplification devices.
This work is supported by “973 project (2013CB632101), NSFC (No. 11274155 and 61036001).
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