Effect of thermal treatment on the growth, structure and luminescence of nitride-passivated silicon nanoclusters
© Wilson et al; licensee Springer. 2011
Received: 24 September 2010
Accepted: 23 February 2011
Published: 23 February 2011
Silicon nanoclusters (Si-ncs) embedded in silicon nitride films have been studied to determine the effects that deposition and processing parameters have on their growth, luminescent properties, and electronic structure. Luminescence was observed from Si-ncs formed in silicon-rich silicon nitride films with a broad range of compositions and grown using three different types of chemical vapour deposition systems. Photoluminescence (PL) experiments revealed broad, tunable emissions with peaks ranging from the near-infrared across the full visible spectrum. The emission energy was highly dependent on the film composition and changed only slightly with annealing temperature and time, which primarily affected the emission intensity. The PL spectra from films annealed for duration of times ranging from 2 s to 2 h at 600 and 800°C indicated a fast initial formation and growth of nanoclusters in the first few seconds of annealing followed by a slow, but steady growth as annealing time was further increased. X-ray absorption near edge structure at the Si K- and L3,2-edges exhibited composition-dependent phase separation and structural re-ordering of the Si-ncs and silicon nitride host matrix under different post-deposition annealing conditions and generally supported the trends observed in the PL spectra.
Quantum confinement effects have been found to improve the efficiency of radiative recombination in silicon . In accordance with Heisenberg's uncertainty principle, the spatial confinement of the charge carriers induces a spread in their momenta, allowing for quasi-direct radiative transitions to occur in an indirect bandgap semiconductor. Utilizing these quantum confinement effects, efficient light emission has been achieved from silicon nanoclusters (Si-ncs) formed in a dielectric host matrix. While the properties of this luminescence have been observed to depend on the size of the Si-ncs, difficulties arise in the understanding of these materials from the effects related to the Si-nc/dielectric interface, as well as from the specific physical properties of the dielectric matrix. This situation is further compounded by fabrication-specific issues, where the use of different deposition systems or source gases for the fabrication of Si-nc-containing thin films can alter the observed optical behaviour of the materials, requiring continued research to gain a better understanding of this materials system [2, 3].
Forming Si-ncs in a silicon nitride host matrix offers several key advantages over silicon oxide, which was the focus of many early studies [4–9]. Silicon nitride is a promising host matrix candidate since it is a structurally stable dielectric commonly used in microelectronic fabrication processes. Favourable electrical properties resulting from the lower tunnelling barriers allow for better transport of electrons and holes into Si-ncs formed in silicon nitride, making these films better suited for electroluminescent device applications . In addition, Si-ncs coordinated with oxygen atoms are subject to charge trapping related to double-bonds between silicon and oxygen at the interface, which effectively limits the emission from such Si-ncs to energies less than approximately 2 eV, regardless of Si-nc dimensions . Since Si-ncs coordinated with nitrogen atoms do not exhibit the same limitation, emission has been demonstrated to occur at energies across the entire visible spectrum [10, 12, 13]. The process of forming Si-ncs in silicon nitride is also more favourable due to much lower annealing temperature requirements for bright luminescence compared to silicon oxide films where temperatures must typically exceed 1000°C . In fact, even before annealing, silicon-rich silicon nitride (SRSN) films grown by plasma-enhanced chemical vapour deposition (PECVD) can exhibit efficient luminescence. However, the formation of Si-ncs in SRSN films has been found to occur in a more complex fashion, with formation of both amorphous and crystalline clusters being reported and a strong dependence on both deposition and processing conditions [10, 15–17].
In this article, Si-ncs formed in SRSN films deposited with varied compositions using three different chemical vapour deposition (CVD)-based systems are compared and discussed: plasma-enhanced CVD (PECVD), inductively coupled plasma CVD (ICP CVD), and electron cyclotron resonance PECVD (ECR PECVD). Results from these studies have been previously reported in two conference proceedings [18, 19]. Most studies to date have employed isochronal annealing steps after deposition to induce diffusion of excess silicon to nucleation sites. Conventionally, this has been done using a quartz tube furnace with an ambient gas of N2 or N2 + 5% H2 (i.e. forming gas) over 60 min. For consistency, this approach has been taken to provide a good comparison amongst the three deposition systems studied. However, whilst this provides for good comparison amongst the results of various studies, to date there has not been an in-depth isothermal study wherein the annealing is performed over a large time scale ranging from seconds to hours. To address this gap in reported data, in this study, SRSN thin films have been annealed for times ranging from 2 s to 2 h using rapid thermal annealing to provide a basis for investigating the growth process and thermal evolution of these films as well as determining the flexibility of the processing conditions over which such a film could be incorporated into a larger device design.
System specific details for SRSN thin film depositions
Si source gas
N source gas
RF/MW power (W)
Film thickness (Å)
Deposition rate (Å/min)
Results and discussion
The films produced by each of the three deposition systems for the isochronal annealing experiments covered a broad range of compositions from stoichiometric Si3N4 to 14 at.% excess silicon content (Siex) relative to stoichiometry. Here, the excess silicon content for substoichiometric silicon nitride films with composition SiNx has been defined as:
Siex = Siat.%/(Siat.% + Nat.%) - 3/7 = (1 + x)-1 - 3/7.
Film compositions were determined by fitting experimental RBS data from the as-deposited (AD) films with simulated spectra using the SIMNRA software package  and all quoted percentages in this study refer to atomic percentages derived from these measurements. Owing to the inherently poor sensitivity of RBS in measuring lower atomic number elements such as nitrogen, the values obtained from the fits have been rounded to the nearest percent, and values measured below 0.5% have been labelled as <1% to account for the uncertainty in the data. The films used in the isothermal annealing experiments were measured to be moderately silicon-rich, having excess silicon contents of 2-3%. Of these films, the one used to study the Si K-edge XANES was deposited at a slightly lower substrate temperature of 300°C, which could have a minor effect on the film's properties. However, for the purposes of this study, the compositions of these films were similar enough to draw qualitative comparisons between the trends observed in the PL and XANES spectra obtained from the different samples.
Isochronal comparison of deposition systems
Hydrogen passivation of dangling bonds at the Si-nc interface is also observed to play a significant role in improving the PL efficiency. The use of N2 + 5% H2 rather than pure N2 as an ambient gas in the annealing process significantly improves the emission intensity in the ICP CVD- and ECR PECVD-deposited films. This enhancement is not observed in the PECVD-deposited films, which may be because this system uses NH3 as a nitrogen source. Higher concentrations of hydrogen may remain in the film after dissociating from the NH3 gas molecules during the CVD reaction process. Having increased levels of hydrogen in the AD PECVD films could be very beneficial when considering incorporating these types of luminescent films into a larger scale design process, such as for electroluminescent and integrated circuit device processing, provided it does not reduce the quality of the film through increased porosity or the effects of out-gassing. Low temperature rapid thermal annealing is preferable in such cases due to the shorter timescale and reduced thermal budget, providing better compatibility with other materials, structures, or processes. Lower temperatures with shorter anneals become particularly important for avoiding the diffusion of metals from contacts, and potentially reducing the number of design steps required compared to the typically longer quartz tube furnace annealing. The effects of the annealing time on the growth, structure and luminescence of SRSN films are addressed in 'Isothermal anneals at 600°C' and 'Isothermal anneals at 800°C' below.
Isothermal anneals at 600°C
Isothermal anneals at 800°C
It is probable that the decay in PL intensity observed for longer anneals at 800°C will occur after annealing for a minimum time at higher temperatures as well. If this assumption is true and the onset of decay occurs at earlier times as the temperature is increased, then this phenomenon may be linked to the decrease in PL intensity observed in SRSN films annealed for 60 min in a quartz tube furnace at temperatures above 700 or 800°C. Incidentally, as shown in Figure 9b, the 60 min mark resides in the time interval where the 800°C annealed films became less intense than the 600°C annealed films.
We have demonstrated that bright luminescence can be attained from Si-ncs formed in SRSN thin films deposited by PECVD, ICP CVD and ECR PECVD using different combinations of source gases. Each system produced films with highly tunable luminescence through adjustment of the process gas flow rates. Post-deposition annealing only had a minor impact on the peak PL energy, but the annealing temperature and ambient gas strongly affected the PL intensity. For 60 min anneals in a quartz tube furnace, the best results were achieved at low temperatures under flowing N2 + 5% H2 gas. Hydrogen appeared to play an important role in enhancing luminescence from SRSN films. Much of this may be attributed to hydrogen passivation of dangling bonds at the Si-nc surfaces, but XANES spectra at the Si K- and L3,2-edge also indicated that hydrogen incorporated within the AD film may increase the number of nucleation sites for Si-nc formation. In addition, the XANES spectra provided evidence of composition-dependent phase separation and structural re-ordering of both the Si-ncs and the nitride host matrix upon annealing. Unfortunately, self-absorption or photon scattering from void formation in the film obscures the Si-Si and Si-N resonance peaks at the Si L3,2-edge, and a full account of this effect has yet to be realized. This obstacle must be addressed before realistic information about the Si-nc and nitride host matrix structures could be derived from such spectra.
Expanding upon the results obtained from the isochronal annealing experiments, an extended series of time-varied anneals of SRSN films was performed at 600 and 800°C using a rapid thermal processor. Based on these experiments, it has been shown that the luminescent and structural properties were in accordance with those expected from theory if emission occurs through quantum confinement effects. The PL peak steadily shifted to lower energy as the annealing time was increased at both temperatures correspondingly with increasing diameter of Si-ncs. Further, the peak shifting occurred more slowly as it became lower in energy, which could be expected since a greater number of additional Si atoms must be added to further increase the Si-nc diameter and make the nanoclusters grow larger in size. Remarkably, the Si-ncs appeared to form and grow very rapidly, with large, abrupt shifts in peak PL intensities of 0.45 and 0.57 eV relative to the AD film after only 2 s of annealing at 600 and 800°C, respectively. The apparent fast growth was indicative of a fast transient diffusion mechanism for excess silicon within SRSN films. The intensity of the annealed films was also an interesting fact in that the total power density showed an increasing trend with longer annealing times over the time period studied for the lower temperature anneals. However, the higher temperature anneals peaked in total power density after 6-30 s of annealing before steadily decaying with longer times. The decay in total power density observed in the higher temperature data was attributed to the Si-ncs undergoing Ostwald ripening and restructuring in the silicon nitride host matrix. XANES spectra at the Si K- and L3,2-edges, which revealed a steady increase in the Si-Si bonding resonance in the 600°C films following an abrupt increase after 2 s of annealing, support the proposed growth model. These spectra also exhibited large changes in the Si-N resonance as the films were annealed. At 800°C, a much larger increase in the Si-Si resonance was observed after 2 s of annealing, but this peak did not grow noticeably larger as the annealing time was further increased, which supports the possibility of Ostwald ripening. There was also a large change in the Si-N resonance between 10 and 60 s of annealing, which suggested that the decay in luminescence intensity observed at longer annealing times could also be related to restructuring of the silicon nitride matrix.
chemical vapour deposition
- ECR PECVD:
electron cyclotron resonance PECVD
total fluorescence yield
- I0 :
incident X-ray intensity
- ICP CVD:
inductively coupled plasma CVD
Rutherford backscattering spectrometry
rapid thermal processor
spherical grating monochromator
silicon-rich silicon nitride
silicon-rich silicon oxide
total electron yield
- VLS PGM:
variable line spacing plane grating monochromator
X-ray absorption near edge structure.
Thanks to Tom Regier (SGM), Robert Blyth (SGM), Lucia Zuin (VLS PGM), and Yongfeng Hu (VLS PGM) for their assistance in conducting the X-ray absorption near edge structure experiments at the Canadian Light Source synchrotron facility. We also wish to thank T. K. Sham from University of Western Ontario for valuable discussions on X-ray absorption near edge structure experiments, Jack Hendriks and Willy Lennard for their help in performing Rutherford backscattering spectroscopy experiments at the University of Western Ontario, Jim Garrett at McMaster University for his help with annealing of samples, and Matthew Betti at McMaster for providing support in the XANES data analysis. This work has been supported by the Centre for Photonics, a division of Ontario Centres of Excellence Inc, by the Canadian Institute for Photonics Innovations (CIPI), and by the Natural Sciences and Engineering Research Council of Canada (NSERC). Part of this work was performed at the Canadian Light Source facility, which is supported by NSERC, CIHR, NRC, and other government agencies.
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