The optical constants of optical thin films, like refractive index and extinction coefficient, are important factors that directly affect the design and preparation of optical thin films. In the Fabry-Perot （F-P） filters, at a given membrane structure, increasing the ratio of the high and low refractive index, can narrow the passband width and expand the cut-off band width^[1,2]. The shortwave infrared region （0.75~2.5 μm） is the transition region between the visible and infrared regions. High refractive index materials commonly used in the visible region, such as Ta₂O₅, Nb₂O₅ and TiO₂, can be applied to this band. Nevertheless, the number of film layers in the F–P filter is high on account of the low refractive index, and consequently coating time greatly enhances. At the same time, the out-of-band rejection of filter is difficult to deal with, which makes the optical coating design more difficult. Silicon thin film materials can overcome the shortcomings of the oxide materials in the shortwave infrared band. Silicon films have been widely used in the field of infrared optical thin films for its many advantages, such as high refractive index, good transparency, high stability and so on^[3,4]. Although electron beam evaporation has been extensively applied to prepare silicon thin films, the absorption of silicon film is relatively high, especially in the near infrared region with wavelengths less than 1.5µm. Annealing can be taken as an effective method to improve film packed density and reduce defects^[5,6,7], so as to improve the optical performance of thin films.

Some papers concerned the influence of annealing on optical properties and structure of a-Si:H films^{[8,9,10,11,12]}. Wang et al.^[10] reported the small vacancy-type defects coalesced in a-Si thin film around 450°C and the a-Si crystallized near 650°C. Goh Boon Tong et al.^[11] studied the effect of annealing on optical properties of Si:H thin film deposited by layer-by-layer （LBL）. When annealed at the transition temperature of 600 °C, the films with the highest refractive index had the most disordered structure. Therefore, it was speculated that annealing could further reduce optical absorption, yielding to improve the optical performance of silicon films. However, few papers researched the influence of annealing on the structure and optical properties of silicon films deposited by electron beam evaporation. Moreover, seldom studies further investigated of the relationship between structure of silicon films and the optical properties in the near infrared region, which were very critical for the application of silicon film in optical thin film coating devices.

In this paper, the influence of annealing temperature on the structure and optical properties of silicon films was systemically investigated. Silicon films were deposited by electron beam evaporation and then annealed in N₂ ambience within 200 ~ 500 °C. The films were characterized by XRD, Raman spectroscopy, ESR and optical transmittance measurement, respectively.

Silicon films were deposited by electron beam evaporation on sapphire substrates under identical conditions. Before deposition, the specimens with dimensions of Φ10×1 mm were cleaned with ethyl ether alcohol solution. During silicon films growth, the temperature of substrates was keeping 250°C. The silicon films were deposited by the evaporation rate of 0.4 nm/s at a base pressure of 1×10^-3Pa. The film deposition time was approximately 45 minutes. After deposition, the samples were annealed for 1h in N₂ atmosphere. Annealing temperatures were 200°C, 300°C, 400°C and 500°C, respectively. The temperatures were heightened to desired level gradually.

The X-ray diffractometer （XRD） was wielded to investigate the structure of silicon films by Cu-target Kα radiation at 40 kV in the scanning angular from 10° to 80° at 2°/min. The Raman spectra were acquired to estimate the evolution of network structure through Micro Raman Spectrometer with the HeCd laser beam at 532 nm. The ESR measurement was performed by a double-cavities BRUKER ESP4105 spectrometer operated by X-band microwave radiation with a power of 20 mW at 80 K. Via a Lambda 900 spectrophotometer, the transmission spectrum of silicon films were measured and the measurement error was within 0.08%. From transmittance spectra, the refractive index （n） and extinction coefficient （k） of silicon films were calculated by using the Cauchy Exponential Model. In the dispersion model, the absorption coefficient varied exponentially with frequency. This allowed the Cauchy Exponential to model a large variation in the value of k versus wavelength. The following formulas were used to define the optical constants^[13,14]:

n(λ)=An+Bnλ2+Cnλ4

k(λ)=AkExpBk1.2398λ-Ck

where λ is wavelength, A_n, B_n, and C_n are material coefficients, A_k is amplitude, B_k is the exponent factor and C_k is the band edge.

As shown in Fig. 1, XRD measurement was utilized to investigate the structural transformation of as-deposited silicon film and those upon annealing at different temperatures. It was clearly identified that all silicon films were amorphous in structure, because no obvious diffraction peak was found, which indicated that silicon film was not crystallized at 500°C. These results very closely corresponded to the report of Goh Boon Tong et al.^[11], which showed that the structure of silicon film changed from amorphous to crystalline phase, when hydrogenated silicon films annealed at 800°C. An intensive research to the evolution of network structure could be achieved by the analysis of Raman spectra.

The widespread usage of Raman scattering into the research of microstructure materials lied in the fact that its intensity was susceptible to the order degree of solid structure. The Raman spectra of as-deposited and annealed silicon films were shown in Fig. 2（a）. The Gauss-deconvolution result of silicon film annealed at 400°C was shown in Fig. 2（b）. It could be discovered that all Raman spectrum contained several vibration modes, including the transverse optical mode （TO）, transverse acoustic mode （TA）, longitudinal optical mode （LO） and longitudinal acoustic mode （LA）^[15]. Table 1 stated the corresponding characteristic parameters. As we could see, the broad TO peaks were approximately at 480 cm^-1, which was sensitive to the amorphous network order on short-range scales^[16]. This further confirmed the amorphous structure of silicon films after annealing. Meanwhile, the TO peak width Г_TO decreased from 68.4 to 65.9 cm^-1, which showed the enhancement of short-range order in silicon films with annealing temperature increased. The TA mode at about 150 cm^-1 associated with the amorphous network order on medium-range scales^[17]. In addition, while the annealing temperature went up, the intensity ratio I_TA/I_TO decreased from 0.670 to 0.478, which indicated the improvement of medium-range order. The LO mode at about 410 cm^-1 along with the LA mode at 300 cm^-1 were connected to the coordination defects in silicon films. Moreover, where there were more defects, the values of I_LA/I_TO as well as I_LO/I_TO became larger^[18]. As shown in Table 1, before and after annealing, I_LA/I_TO decreased from 1.02 to 0.678 and I_LO/I_TO from 0.358 to 0.331, reflecting the decrease of defects. These results indicated that the structure of silicon films maintained amorphous, but, with rise of temperature, the amorphous network order was improved in the short and medium range.

ESR was among few experiments offered defects information in structure^[19]. Fig.3 illustrated the derivative ESR spectrum of silicon films, g factor and density of dangling bonds （Ns） at different annealing temperatures. It could be discovered that g factor values of all silicon films were around 2.0055, which originated from the dangling bonds in amorphous structure^[20]. It was worth noting that when the annealing temperature increased, the strength of resonance signals decreased obviously. As shown in Fig. 3, Ns decreased from 1.1×10¹⁷ cm^-3 for as-deposited sample to a minimum of 3.4×10¹⁶ for sample annealed at 400°C. It showed the rise of annealing temperature could reduce, to a large extent, the dangling bonds in silicon films. It almost accorded with the result obtained by Raman scattering.

Fig. 4 described the transmittance spectra of as-deposited and annealed silicon films from 800 to 2 000 nm. The transmittance spectrum of silicon films exhibited peaks and valleys that were associated with interference effects. Along with annealing temperature heightened, the optical transmittance spectra shifted to a shorter wavelength due to the reduction of film optical thicknesses. When annealed at 400°C, the sample showed higher peak transmittance than those of films annealed at other temperatures, especially from 800nm to 1 200 nm band. This result indicated a notable decrease of optical absorption with the rise of annealing temperature. Fig. 5 showed the plots of the films optical and physical thickness against annealing temperature calculated from transmittance spectra. As could be seen, when annealed at 200~300°C, the variation on physical thickness of silicon film was not obvious. But when annealed at a higher temperature （300~500 °C）, the physical thickness rapidly decreased from 1104.4 nm to 1 071.4 nm, indicating that the film packing density was greatly improved. This might be because, as annealing temperature increased, the heated atoms were filled into the voids and diffused each other, which made the film compact and thus reduced the thickness.

The optical constants of silicon films were fitted by using the FilmWizard software from SCI. During fitting process, the Cauchy Exponential Model was used as the dispersion model of material to fit the transmission spectrum. Table 2 stated the dispersion model parameters of the as-deposited and annealed silicon films, which could be used to describe the variation of optical constants. Fig. 6（a） showed the refractive index n of the as-deposited and annealed silicon films. From the figure, it could be evidently found that the refractive index decreased with adding wavelength of the incident light. Along with annealing temperature increased, the refractive index n of silicon films initially declined and then rose at a specific wavelength. For example, the refractive index n decreased from 3.52 for as-deposited sample to 3.48 for sample annealed at 300°C and went up to 3.58 at 500°C （at 1 000 nm）. The decrease of n was probably associated with the decrease in the number of dangling bonds. When annealed at a higher temperature （300~500°C）, it was the packing density, though still along with changes in dangling bond density, that became the main factor affecting the variation in refractive index. Thermal annealing could remarkably improve the packing density of silicon films, which had been confirmed by the decrease of physical thickness.

Fig. 6（b） presented the extinction coefficients k of the as-deposited and annealed silicon films. From the figure, it could be obviously found that after annealing extinction coefficient k decreased apparently, especially in the near infrared region with wavelengths less than 1.5µm. For instance, the extinction coefficient k dropped from 6.14×10^-3 for as-deposited sample to 1.02×10^-3 for sample annealed at 400°C （at 1 000 nm）. It was interesting to notice that the extinction coefficient and film defect density possessed similar variation tendency. The extinction coefficient k also decreased initially to its minimum value at 400 °C and increased by rising annealing temperature further. Therefore, the decrease of extinction coefficient might be attributed to the decline in the number of dangling bonds in silicon films.

The silicon films were deposited on sapphire wafer by electron beam evaporation and then annealed in N₂ atmosphere in a temperature range from 200 to 500 °C. It was concluded that all silicon films maintained amorphous in microstructure, but with annealing temperature increased, the amorphous network order was improved on the short and medium range. Meanwhile, the defect concentration from ESR measurement decreased distinctly. When sample being annealed at 400°C, the defect density declined to the minimum, about one fifth of the as-deposited sample. With annealing temperature increased, transmittance spectra of silicon films shifted to the shorter wavelength direction, owing to the decrease of film thickness and the change of refractive index. When sample being annealed at 400°C, extinction coefficient also cut down to the minimum, and then mounted up by rising annealing temperature further. According to these results, annealing at an appropriate temperature could effectively reduce the optical absorption of silicon films in the near infrared region, which were very critical for the application in optical thin film coating devices.