The femtosecond laser-induced grating (FLIG) formation and crystallization were investigated in amorphous silicon (a-Si) films, prepared on glass by plasma-enhanced chemical-vapor deposition. Probe-beam diffraction, micro-Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy were employed to characterize the diffraction properties and the microstructures of FLIGs. It was found that i) the FLIG can be regarded as a pattern of alternating a-Si and microcrystalline-silicon (μc-Si) lines with a period of about 2 μm, and ii) efficient grating formation and crystallization were achieved by high-intensity recording with a short writing period.
©2005 Optical Society of America
Amorphous semiconductors have attracted considerable interests due to their various applications to solar cell, xerography, and flat-panel display. Much of this attention has focused on amorphous silicon (a-Si) and its crystallization for the use in active-matrix-based flat-panel displays such as active-matrix liquid-crystal displays and active-matrix organic-light-emitting diodes [1–3]. In comparison with a-Si, crystallized silicon reveals larger carrier mobility, faster switching, and higher stability, all of which are key for the design of thin-film transistors. The crystallization of a-Si is typically achieved by excimer-laser annealing [4, 5] or solid-phase crystallization (SPC) [6, 7]. The excimer-laser crystallization has been improved through lateral laser-induced crystallization [8, 9], and SPC has been enhanced by performing the crystallization process in the presence of a metal catalyst [10, 11]. Although several studies have been conducted on the excimer-laser-based crystallization of a-Si [8, 9], only a few experiments have been reported on the femtosecond-laser crystallization . Femtosecond laser can provide an extremely high heating rate in a particular material region, thus causing a rapid accumulation of energy and making it possible to induce a phase change in the amorphous or the crystalline structure of material. Femtosecond laser also allows the precise structuring, since the electron-lattice coupling by the transfer of the absorbed laser energy is ignored . Recently, the use of femtosecond laser in fabricating particular patterns inside a bulk or on a surface has been increased because applications such as gratings, waveguides, splitters, directional couplers, optical memory and photonic crystals can be accelerated utilizing such a patterning [14–16]. By combining the laser crystallization with the laser patterning, crystallization can be spatially controlled. Laser interference pattern is employed to obtain a spatially-selected crystallization in a-Si. Therefore, it is worthwhile to investigate the femtosecond laser-induced grating (FLIG) formation and crystallization in a-Si film.
In this work, the FLIG formation and crystallization were studied by femtosecond holography, in which the gratings were recorded in the a-Si film using a two-beam interference of near-infrared femtosecond laser pulses. The diffraction behaviors of the probe beam from the FLIGs were monitored to find the grating formation and relaxation profiles. The laser-induced surface deformations were also investigated by optical microscopy and atomic force microscopy (AFM). The structural characterization of laser-crystallized silicon was performed by micro-Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).
Hydrogenated amorphous silicon films were deposited on Corning 1737 glass by plasma-enhanced chemical-vapor deposition (PECVD) using a gas mixture of silane (SiH4) and hydrogen (H2) at a substrate temperature of 270°C. The other deposition parameters were fixed as follows: Silane and hydrogen gas flow rates were fixed at 300 and 100 sccm (standard cubic centimeters), respectively. Chamber working pressure and RF power were 2 torr and 30 W, and the resulting film was 100 nm thick.
The femtosecond laser system used in this experiment was a regeneratively amplified Ti:Sapphire laser (Spectra-Physics, Hurricane) with an 800-nm output wavelength, 130-fs pulse duration, 1.0-mJ maximum pulse energy, and 1-kHz repetition rate. In order to record the gratings in a-Si film, we used the two-beam interference of femtosecond laser pulses. The pulse energies of the two interfering beams were equal to each other, and their intersection angle was 24°. Synchronization of the two writing pulses was achieved by adjusting the optical delay of the two pulses. The grating formation and relaxation profiles were recorded using a photomultiplier tube and a digitizing oscilloscope, where the probe beam was from a He-Ne laser. The surface profiles of the FLIGs were investigated with an optical microscope and an atomic force microscope (PSIA, XE-100). In order to study the crystallization of the femtosecond-laser-modified region, we measured the Raman spectra of all the samples at room temperature using the 514.5 nm line of a stabilized argon ion laser (Coherent, Innova 307) as the excitation source. At the sample, the laser power was measured at 3 mW. In order to obtain the micro-Raman spectra for the microcrystalline silicon (μc-Si) film formed by femtosecond holography, the laser was focused onto the sample using a microscope objective (x60). The scattered light was collected by the same objective lens, dispersed by a 55 cm monochromator and then detected by a liquid-nitrogen-cooled back-thinned charge-coupled-device array detector. TEM (JEOL, JEM-2010) was used to characterize the laser-treated sample. TEM samples were prepared by mechanical grinding and an ion mill. SEM (JEOL, JSM-6630F) was also employed to characterize the surface morphology.
3. Results and discussion
Figures 1(a) and 1(b) show the diffraction behaviors of the probe beam from two different gratings, one using the interference pattern of two 68-μJ beams and the other using two 365-μJ beams, while Fig. 1(c) shows the diffraction pattern of the probe beam from the FLIG. Under our experimental conditions, the low and high pulse energies correspond to the laser fluences per pulse of 11 mJ/cm2 and 57 mJ/cm2. For the lower energy writing, the grating formation dynamics shows two peculiar characteristics. In the initial stage, the diffraction intensity did not show an appreciable increase until over 10600 laser shots. For the laser-shot number larger than this threshold, the diffraction intensity slowly increased with the laser shot count until it reached a maximum intensity around 46000 laser shots. Figure 2(a) is the AFM images of the FLIGs with 51720 laser shots under the lower pulse energy condition. The groove spacing confirms that the FLIG was formed on the surface of a-Si film by the two-beam interference of femtosecond laser pulses. Interestingly, there exist one-third period fringes between the adjacent grooves. It is thought this might be due to the self-phase-matching process based on the third-order nonlinear optical effects between two photons from one pump beam and one photon from the other pump beam. As shown in Fig. 1(a) and 1(b), the diffraction behavior for the higher energy writing is different from that for the lower energy writing. The higher energy writing shows a steeper rise and the higher diffraction efficiency when compared with the lower energy writing. Upon irradiation of two interfering femtosecond laser pulses, the diffraction intensity increased a maximum value within 0.2 s. Figure 2(b) shows the AFM images of the FLIGs with 200 laser shots under the higher pulse energy condition. These AFM observations show that the high-intensity recording during a short writing period produces a large surface modulation.
The FLIGs are generated from the femtosecond laser-induced modification of a-Si. In order to determine the nature of the laser-induced modification in a-Si film, it is necessary to conduct additional experiments. The possibility of the amorphous-to-crystalline phase transformation can be checked by measuring Raman spectrum for the characteristic vibration mode of the expected crystal. Figure 3 shows typical micro-Raman spectra for femtosecond-laser-modified region as well as that for an unexposed area. From the previous reports by Iqbal et al. and Houben et al. [17, 18], it is believed that the 520 cm-1 band is associated with the transverse optic phonon mode of crystallites formed by femtosecond holography, whereas the broad and weak signal centered approximately at 480 cm-1 is associated with the a-Si phase. When the Raman spectrum of the femtosecond-laser-modified region is compared with that of the unexposed area, the intensity of the crystalline Raman peak is increased by irradiation of femtosecond laser. These results indicate that the femtosecond laser pulses transform the sample from the amorphous phase to the crystalline phase. Generally, it is believed that the laser crystallization occurs through a rapid solidification following the pulsed-laser melting of a-Si film . If laser crystallization is performed with femtosecond pulses shorter than the thermal diffusion time necessary for the energy transport of the absorbed laser energy into the amorphous matrix, the electron-network coupling can be neglected . Thus, a rapid quenching can assist the formation of the crystalline structure whose ordering is different from the surrounding random network. Moreover, if the laser crystallization occurs from the multiphoton absorption-induced melting and subsequent solidification, the crystallization reaches deeper into the sample film due to a negligible amount of linear absorption in spite of an appreciable amount of multi-photon absorption in an incident wavelength region.
In order to obtain direct evidence for laser crystallization and to estimate the grain size in the crystallized silicon film, we measured SEM and TEM images for the femtosecond-laser-treated samples. Figure 4 shows the plan-view SEM images for the femtosecond-laser-modified region. The SEM micrograph in Fig. 4(a) shows regularly spaced bright regions which are thought to be the microcrystallized regions with irradiation of the femtosecond laser. The crystallites can be clearly seen in the field-emission scanning electron microscopy (FESEM) image [Fig. 4(b)] for the laser-crystallized region. Meanwhile, Fig. 4(c) indicates that the surface morphology of the amorphous film in the unexposed region has a uniform and featureless topology, and surface is dark unlike Fig. 4(a). Figure 5 shows the bright-field TEM images and the corresponding indexed electron diffraction (ED) patterns for the μc-Si film formed by femtosecond holography. TEM micrographs [Fig. 5(a) and 5(b)] clearly reveal that the FLIG can be regarded as a pattern of alternating a-Si and μc-Si lines with a period of about 2 μm. The selected area ED patterns [Fig. 5(c) and 5(d)] indicate that polycrystalline silicon (Poly-Si) is formed by femtosecond holography and that the crystals have a diamond cubic structure, as can be seen from the indexed diffraction pattern.
To determine the optimum condition for laser crystallization, we measured the dependence of crystallization on laser-shot-number. Figure 6(a) shows a plot of the average grain size as a function of laser-shot number. Figure 6(b) shows a plot of the diffraction efficiency and the crystalline Raman peak height as a function of laser-shot number. In the femtosecond laser-induced grating formation and crystallization experiment, the diffraction of the probe beam is generated from a periodic modulation of the optical constants of a-Si by the light interference pattern. Therefore, the optimum laser fluence for efficient laser crystallization can be estimated from the diffraction efficiency dependence on the laser-shot-number. As shown in Fig. 6(a) and 6(b), both the grain size and the Raman peak intensity plots showed a maximum near the laser-shot number at which the diffraction intensity also exhibited its peak value. These results indicated that an optimum fluence condition existed for efficient laser crystallization. When the laser beam has a lower fluence than the melting threshold, the crystallization of amorphous film cannot proceed. When the laser fluence is too high, it might induce reamorphization, ablation, or damage to the crystallized film. It is thought that these processes account for the decrease in diffraction efficiency, Raman intensity, and grain size in the high fluence regime [Fig. 6]. TEM analysis indicated that the average grain size of 170 nm was obtained through the laser interference crystallization using 200 shots of two 365-μJ laser beams. This average grain size was larger than that obtained with single femtosecond-laser beam with similar beam parameters . One possible explanation for the grain-size enhancement is that the light interference pattern facilitated the laser crystallization. In other words, the two-beam interference will initiate the lateral growth along the intensity gradient of interference pattern with a fringe spacing of about 2 μm [20, 21]. The lateral growth usually results in Poly-Si with relatively large grains [8, 9, 20, 21]. In conventional homogeneous crystallization, it is difficult to achieve the lateral growth necessary for large grain formation. The structural characteristics of laser-crystallized silicon can be related with groove spacing as well as beam parameters (pulse duration, pulse energy, laser-shot number, and beam profile). If they are optimized, it is expected that the obtained Poly-Si will have a larger grain. Further improvement in the grain size might be pursued by the proper selection of film thickness, substrate, underlayer and capping layer.
By femtosecond holography, the FLIG formation and crystallization were studied in a-Si films prepared on a glass plate by the PECVD method. Probe-beam diffraction, micro-Raman spectroscopy, AFM, SEM, and TEM analyses reveal that i) the FLIG can be regarded as a pattern of alternating a-Si and μc-Si lines with a period of about 2 μm, and ii) efficient laser crystallization is achieved by high-intensity recording with a short writing period. The TEM analysis indicated that the average grain size of 170 nm was obtained through laser-interference crystallization using 200 shots of two 365-μJ laser beams.
This work was supported by the KOSEF through Quantum Photonic Science Research Center at Hanyang University, Seoul, Korea.
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