1. Introduction

Hydrogenated amorphous silicon (a-Si:H) is one of the most widely used material for photovoltaics application due to its low fabrication cost compared to their crystalline counterpart because of its high deposition rate, low deposition temperature, which can enable the usage of inexpensive substrates such as glass, plastic and metal foils. However, the metastable structure of amorphous silicon films needs to be continuously improved to address the concerns on low carrier mobility, high reflectivity across the electromagnetic spectrum and light-induced degradation (Staebler-Wronski (S-W) effect) [1], for PV applications. In order to improve the efficiency and sensitivity of a-Si based devices, a post processing is usually needed. Crystallization of a-Si:H has been studied as one of the potential solutions to this problem [2–5]. Excimer laser crystallization has been a preferred technique for crystallization of a-Si:H deposited on cheap substrates such as glass. Upon irradiation, melting and solidifying of the a-Si:H film occur during tens of nanoseconds, with the melt depth mainly determined by the laser energy density, often without affecting the underlying substrate. Recently, ultrafast laser-induced crystallization of amorphous silicon film has been investigated as a new approach for crystallization purpose [6–8]. Unlike excimer laser annealing process, ultrafast pulsed laser interaction with a-Si thin film material involves nonlinear photoenergy absorption and nonequilibrium thermodynamics that are expected to dominate the interaction [9–11]. Such a nonlinear process provides precise and low-threshold fluence associated with femtosecond laser ablation [6,11,12]. In this paper, we demonstrated that infrared femtosecond laser is able to induce crystallization of amorphous silicon and simultaneous surface nanostructuring on a-Si:H surface in one step process. Upon fs laser treatment, the absorbance of the a-Si:H thin film is increased due to the crystallization and surface nanotexturing. The developed process has potential application for fabrication of high efficiency Si thin film solar cells, thin film transistor (TFT) for AMLCD application, and other novel optoelectronic devices.

2. Experimental

The amorphous silicon films of thickness of 80 nm were deposited onto Corning 1737 glass substrates, using a low temperature plasma enhanced chemical vapor deposition (PECVD) technique. The samples were treated with a femtosecond laser beam. The fs laser system used is based on a regenerative Ti:Sapphire amplifier using chirped pulse amplification technique (Clark-MXR, CPA 2001) which provides high-intensity fs laser pulses for the processes. The pulse duration of the output beam from amplifier is 150 fs with nominal wavelength at 775 nm. The repetition rate is 1000 Hz and the beam profile emitted from the regenerative amplifier is approximately Gaussian shape. The average output power can reach 800 mW at repetition rate of 1000 Hz. The sample was placed on a stationary stage and the laser beam was deflected by a Scanlab galvanometer scanner for scanning treatment of the sample surface. The focused beam spot size was measured to be around 30 μm of the diameter on the sample surface. In order to find optimum conditions, the laser scanning treatments were conducted at various laser fluencies from 1 mJ/cm² to 50 mJ/cm² and scanning speeds from 1 mm/s to 100 mm/s. The morphology of the laser treated samples was analyzed using optical microscope, scanning electron microscopy (SEM), and atom force microscope (AFM, Digital Instruments, Nanoscope III). Micro-Raman spectra are recorded at room temperature with Renishaw’s inVia Raman microscope in backscattering geometry using 514 nm line of argon ion laser. The transmittance and reflectance were measured with UV-VIS-NIR scanning spectrophotometer (UV3101PC).

3. Results and discussion

Figure 1 shows the optical microscope image of laser treated and untreated surfaces of a-Si:H film deposited on a Corning 1737 glass substrate where the laser fluence was 6.9 mJ/cm² and scanning speed was 20 mm/s. It can be clearly seen that after laser irradiation the a-Si film was turned into dark brown from original shinny light yellow color, which indicates the surface property such as surface finish, and phase state might change after laser treatment.

 

Fig. 1 Optical microscope image of fs laser treated a-Si:H thin film.

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Figure 2 shows the scanning electron microscope (SEM) images of original and laser treated Si surfaces. It is obvious from the image that surface nano-structures have been formed upon laser treatment. Further enlarged by AFM images as shown in Fig. 3 , it was observed that a regular nano-spike patterned texture was formed after fs laser irradiation. The distance between the nano-spikes is about 200 nm and the diameter of the formed nanobump is about 90 nm at full width at half maximum (FWHM) and its height is about 20 nm.

 

Fig. 2 SEM images of (a) original a-Si:H thin film surface and (b) fs laser treated a-Si:H thin film surface.

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Fig. 3 AFM images of (a) original a-Si:H thin film surface and (b) fs laser treated a-Si:H thin film surface.

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The produced nano-spike pattern is expected to be beneficial to reducing the light reflectance, so as to increase light trapping. Actually, femtosecond laser induced nano-spike patterned ‘black silicon’ has been achieved and extensively studied by Mazur’s group on crystalline bulk silicon substrate [13,14], where the crystalline Si substrate was processed in the presence of a sulfur containing gas such as SF6. The nano-spike patterned black silicon surface was demonstrated to be strongly light-absorbing and the surface of silicon, normally gray and shiny, turned deep black. Also, the surface textured black silicon has been realized with reactive ion etching (RIE) on mono-crystalline and multi-crystalline silicon wafers [15,16]. The etched silicon surface showed a significant reduced reflectance in the visible region as well as in near-IR region.

To study the crystalline property of the laser treated area, micro-Raman analysis was conducted. Raman spectroscopy is a sensitive probe to local atomic arrangement and vibrations (phonons) in solids [17] and this technique has been used to characterize nanostructures that provide information about the nature of crystalline structure or amorphous disorder structure.

Figure 4 showed micro-Raman spectra for as-grown a-Si thin film and fs laser treated a-Si thin film, where the Gaussian line profiles fitting has been conducted to the Raman spectra of untreated and fs laser treated a-Si:H film as the dashed lines. As shown in Fig. 4, it can be seen that for as grown a-Si:H, the Raman spectrum consists mainly of two broad peaks, one peak centered at 465.55 cm⁻¹ which is characteristic of amorphous silicon corresponding to the TO zone-edge phonon, and another broad peak centered at 332.20 cm⁻¹ which is attributed to LO amorphous phonon mode [18].

 

Fig. 4 Raman spectra of original and laser treated a-Si:H thin film, the dashed lines are Gaussian profile fittings.

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After fs laser treatment, interestingly, besides two broad-bands at 477.97 and 347.78 cm⁻¹, the Raman spectra shows a sharp Raman peak centered at 511.7 cm⁻¹, which is an evidence of a crystalline phase. However, the sharp peak is shifted by an amount of 8.3 cm⁻¹ from the peak at 520 cm⁻¹ that corresponds to bulk crystalline silicon. This shift might be attributed to phonon confinement [19], possibly due to the presence of nanocrystals embedded in a-Si:H environment. Stress-induced effects are also reported to cause this behavior. As a result, in Fig. 4, the two distinct peaks at 511.7 and 477.97 cm⁻¹ for the fs laser treated a-Si:H thin film are believed to be corresponding to an mixed phase silicon consisting of an amorphous phase and crystalline phase. In mixed phase silicon, the momentum selection rule of the Raman process is more relaxed compared to crystalline silicon. With increasing momentum the TO photon energy lowers, leading to broadening of the Raman peak towards the lower energies [18,20]. The crystalline volume fraction of the fs laser treated sample can be calculated from the integrated intensities of the Raman peaks with Gaussian fits for the amorphous peak (Ia) and the crystalline peak (Ic) as shown in Fig. 4. The calculation was done as proposed by Tsu et al. with crystalline volume fraction (Χc) given by Eq. (1), where γ is the ratio of the backscattering cross-section of amorphous and crystalline phases [21],

The selection of a value of γ is complex due to its dependency on absorption coefficient of amorphous and crystalline silicon [22]. γ has been calculated to be between 0.8 and 0.9, the most widely used value being 0.8 for mixed phase silicon [20,22]. Here, γ was taken to be 0.8. According to Eq. (1), based on the integrated intensities of crystalline part and amorphous part in Fig. 4, the crystalline volume fraction of fs laser treated a-Si:H thin film was determined to be approximately 34.7%.

In order to study the optical characteristics we measured the reflectance and transmittance of as-grown and laser treated a-Si thin film with an area of 20 x 20 mm² using a spectrophotometer. The reflectance (R in %) and transmittance (T in %) were then used to obtain the absorbance (α in %) of the samples: α = 1-R-T. Figure 5 shows the absorbance of laser treated and original a-Si films. It is clear from Fig. 5 that there is a significant enhancement in the optical absorption below the a-Si:H band gap (1.7 eV) in the case of laser treated films.

 

Fig. 5 Absorptance of original and laser treated a-Si:H thin film.

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It is suggested that the increase of below band edge absorption might be due to the textured surface resulting from laser treatment, by helping to trap the light due to multiple reflection. Furthermore, the increase in the absorption is most probably caused by the reduction in the reflection which could be due to the anti-reflective properties of the fs laser formed nano spikes which may have gradient index structure [23]. Also, structural defects induced during the laser surface texturing process may likely produce bands of defect and impurity states in the band gap and thus enhances the overall absorption further [8].

Unlike continuous-wave laser annealing and excimer laser annealing, the fact that very low laser fluence of 6.9 mJ/cm² is required for fs laser annealing suggests that ultrafast or nonthermal melting of silicon is the dominant mechanism [9,10,12] involved in the process. Femtosecond optical pulses can excite significant number of the valence electrons in the semiconductor through nonlinear absorption. This high level of electronic excitation can severely weaken interatomic bonds so that “cold” atomic motion can lead to disordering of the lattice and then cause structural transformation such as re-crystallization. This can occur while the electronic system and lattice are not in a thermal equilibrium [9]. The ultrafast transition caused by the high-density electron hole plasma inherently carries no conventional molten phase. The studies with femtosecond time-resolved optical microscopy and reflectivity measurements [24–26] suggested existence of a liquid layer whose properties and transient behavior differ from normal thermal melting. Also the distortion of the silicon diamond lattice structure was shown to be related to lattice instability in the time scale of hundreds of femtoseconds following the ultrashort pulsed laser irradiation [11]. Electronic excitation effects were suggested to the likely cause for the crystallization of amorphous semiconductor thin films by laser pulses shorter than 800 fs [27].

4. Conclusions

In conclusion, one step fs laser processing to produce surface nano-structuring and simultaneous crystallization of amorphous silicon thin film was developed. Fs laser irradiation on 80 nm-thick a-Si deposited on Corning 1737 glass substrate led to a color change from light yellow to dark brown on the sample surface. The a-Si:H thin film was converted to nanocrystalline silicon and at the same time, nano-spike texturing was fabricated on the surface of amorphous-Si:H film. The crystalline volume fraction of fs laser treated a-Si:H thin film was determinted to be about 34.7%. The absorptance of the fs laser treated Si thin film was found to increase compared to original untreated a-Si. The developed process has potential applications for high efficiency Si thin film solar cells, thin film transistors, large area sensors, and other novel optoelectronic devices.