Transmission Electron Microscopy Characterizations of Local Amorphization of Single Crystal Silicon by Nanosecond Pulsed Laser Direct Writing

The concept for fabrication of waveguides by an in‐volume laser direct writing in single‐crystal silicon is explored using a nanosecond pulse laser. The key innovation of this technology relies on the generation of amorphous silicon, which has a higher refractive index than that of crystalline silicon. Herein, transmission electron microscopy (TEM) together with selected area electron diffraction (SAED) and high‐resolution TEM (HRTEM) characterizations are used to better understand the microstructural evolutions. TEM images reveal the core‐shell structures, while SAED patterns and HRTEM directly observe the presence of amorphous silicon in the core surrounded by a crystalline silicon shell. With a lower laser scanning speed, a higher density of defects yet less amorphous silicon is formed by laser direct writing.


Introduction
[3] For the latter application, waveguides made of Si may serve as optical interconnects on integrated circuits or transport optical clock signals on a microprocessor.One of the keys to the fabrication of waveguides in Si is a core with a higher refractive index than the crystalline silicon (c-Si).The crystal structures of c-Si include diamond cubic silicon (i.e., Si-I, space group Fd3 m, which is the most common phase), Si-II (space group I41/amd), Si-III (space group Ia3), Si-IV (space group P6 3 /mmc), etc. [4,5] .Given the higher refractive index of amorphous Si (a-Si) compared to the one of crystalline Si (c-Si) , [6] a-Si is a promising candidate for the core material of waveguides.
The fabrication of a-Si for optical waveguides has been studied extensively through the years.Methods for fabrication include chemical vapor deposition, [7][8][9] ion irradiation, [10,11] and laser irradiation. [12,13]Laser direct writing employs a pulsed laser source to modify the structure of Si resulting in the recrystallization or amorphization of c-Si, [14,15] which has attracted great attention from manufacturers and researchers due to its high precision and efficiency. [15]This technique relies on a wide variety of material changes induced by a high-intensity laser focused inside the bulk sample. [16,17]Direct laser writing in c-Si using femtosecond (fs) laser pulses was reported to successfully generate a large thickness of the amorphous layer of 128 nm inside the wafer. [14]Kiani et al. [18] also reported the feasibility of amorphization of silicon induced by fs laser irradiation.Micro-Raman spectroscopy was used to confirm the formation of a-Si in the irradiated region.Despite the possibility of amorphization of c-Si using fs laser irradiation, fs laser direct writing in bulk Si is a challenge due to the wavelength dependence of fs laser-Si interaction, which causes issues for the design of in-volume laser direct writing.Therefore, the use of nanosecond (ns) laser pulses was introduced to mitigate the aforementioned technical issues.
Verburg et al. showed the transformation of c-Si to a-Si using a pulsed laser beam with a wavelength of 1.55 μm and a pulse duration of 3.5 ns, which was confirmed by the Raman spectroscopy. [16]This led a new direction for the fabrication of DOI: 10.1002/adem.202301377 The concept for fabrication of waveguides by an in-volume laser direct writing in single-crystal silicon is explored using a nanosecond pulse laser.The key innovation of this technology relies on the generation of amorphous silicon, which has a higher refractive index than that of crystalline silicon.Herein, transmission electron microscopy (TEM) together with selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) characterizations are used to better understand the microstructural evolutions.TEM images reveal the coreshell structures, while SAED patterns and HRTEM directly observe the presence of amorphous silicon in the core surrounded by a crystalline silicon shell.With a lower laser scanning speed, a higher density of defects yet less amorphous silicon is formed by laser direct writing.
waveguides using in-volume ns laser direct writing.Tokel et al. [19] used a custom-developed ns-pulse fiber laser at a wavelength of 1.55 μm to generate three-dimensional (3D) structures inside Si samples that resulted in permanent modification and crystal structure changes.Wang et al. [20] are one of the first researchers to report the transversely direct written lines in a bulk c-Si using ns laser regime.Zhao et al. [17] demonstrated that laserdriven shock compression can generate two distinct a-Si regions comprised of a bulk layer close to the surface and an amorphous band within the sample, using a pulsed neodymium glass laser with a wavelength of 351 nm, a pulse duration of 1 ns, and a high laser energy of 20-450 J.However, there is a lack of fundamental understanding of the micro/nanostructure changes that lead to waveguide formation in these studies.Most of the a-Si generation in these studies were characterized using Raman spectroscopy.Transmission electron microscopy (TEM) imaging together with selected area electron diffraction (SAED) patterns can allow the direct observation of structural modifications formed by laser irradiation and reliable detection of α-Si in the local areas.
In this study, TEM is employed to directly observe the structural changes inside c-Si samples under nanosecond (ns) pulsed laser irradiation leading to the waveguide formation.SAED helps spotlight the localized electron diffraction patterns, while highresolution transmission electron microscopy (HRTEM) images allow direct observations of α-Si.

Experimental Section
Single-crystal Si plates (30 Â 10 Â 1 mm 3 ), obtained from Sil'tronix Silicon Technologies (Archamps, France), with surfaces on the (1 0 0) plane while the edges on the (0 0 1) and (0 1 0) planes were used for experiments.The experimental setup for direct laser writing inside a c-Si single-crystal plate is shown in Figure 1a.The laser source was a fiber master oscillator power amplifier laser (MWTechnologies, Model PFL-1550) with a center wavelength of 1.55 μm and 3.5 ns pulse duration at the full-width-at-half-maximum and Gaussian beam profile.The laser irradiates near the [1 0 0] crystallographic direction (i.e., along z axis) to generate writing lines inside the singlecrystal silicon plate.The writing (i.e., laser scanning) direction is perpendicular to [0 1 0] direction (along y axis).The pulse repetition rate is 150 kHz resulting in a pulse energy of 20 μJ.During direct laser writing, a spherically corrected objective lens (Olympus, LCPLN100XIR, NA = 0.85) is used to focus the beam into the silicon sample.At the focal point in the x-z plane, the focal spot size is 2w 0 ¼ 1.22λ=NA ¼ 2.2 μm, and the Rayleigh length is z R = 2.6 μm in air and z R ¼ 9.2 μm in Si.Along the y and z directions, the spot radii are both 9.2 μm.The relative position between the sample and the objective lens was controlled using an XYZ translation stage (ILS100PP, Newport, USA).For the current study, two laser modification lines, line 1 and line 2 (Figure 1b), were written with a scanning speed of 10 and 1 mm s À1 , respectively, while keeping all other parameters constant.After laser direct writing, the samples were cleaved for analysis of the cross-section of the waveguide.
The observation of the cross-section of the modification lines with a SEM and preparation of the TEM sample using the focused ion beam (FIB) technique were performed by a SEM/ FIB dual-beam workstation (FEI, Helios 660 NanoLab).A thin lamella was extracted from the cross-section of the inscription line perpendicular to the laser optical axis and parallel to the laser writing direction for TEM analysis, as shown in Figure 1b.The lamella was first thinned to about 100 nm-thick by a Ga ion beam operating at 30 keV and 2.5 mA and then final cleaning at 2 keV and 0.24 mA to minimize the ion beam damage in the sample.TEM characterizations including bright field imaging, SAED, and HRTEM were carried out on a S/TEM system (FEI, Technai Osiris) operating at 200 kV.SAED patterns were taken with the objective aperture of about 120 nm in diameter.The Burgers vector of dislocations was identified by the classic g • b analysis.

Result
Figure 2a shows microstructural features in the laser modification line "1" (Figure 1a).There is no clear boundary between the laser-modified and pristine regions.A strong strain field is associated with these microstructural features, indicated by bending contours surrounding them.These features were formed and remained after laser irradiation as permanent structural modifications.The formation of the permanent structural modifications indicates that the laser energy within the nanosecond duration may have provided sufficient heat to locally melt c-Si.Afterward, rapid resolidification of the molten pool may occur due to the short pulse duration, during which certain areas were unable to return to the original single-crystal structure. [21]eanwhile, a few pores (white arrows in Figure 2a) are also observed, which were formed during the thinning of the TEM lamella using FIB.The enlarged views of the randomly selected regions in the direct laser writing track are shown in Figure 2b-d, respectively.A core-shell structure is observed in Figure 2b, while line defects are present in Figure 2c,d.The diffractioncontrast TEM images of these line defects suggest that they may be dislocations.Two-beam condition under the [1 0 0] zone axis was used to analyze the dislocations in Figure 2c.Burgers vector for dislocations in diamond cubic silicon is a/2 < 1 1 0>, [22] where a is the lattice parameter.The g • b analysis determined that that Burgers vectors of these dislocations were possibly a/2[0 1 1] and a/2[0 1 1].
To further analyze the core-shell structure, SAED patterns were taken at different regions (labeled as A, B, and C in Figure 2b).All the diffraction patterns were taken along the [1 0 0] zone axis.Figure 3a shows the electron diffraction pattern of single-crystal c-Si, which was taken outside of the defect zone.Meanwhile, electron diffraction of the shell region (B in Figure 2b) and the core (C in Figure 2b) indicate the transition from the single crystal to polycrystalline.In the SAED patterns in Figure 3b,c, the indexed diffraction rings represent those from the diamond cubic silicon (or Si-I).However, besides the indexed diffraction spots that belong to Si-I, there are a number of additional diffraction spots off the dash-blue circles.They may be attributed to lattice distortion, or even the presence of highpressure phases of Si such as Si-II. [23]For instance, Sun et al. [24] studied the phase transformation in single crystal silicon induced by nanosecond laser irradiation and reported that the lattice distortion induced by a high dislocation density can generate extra diffraction spots.Therefore, further studies will be needed to investigate the origin of those additional spots in the electron diffraction patterns, particularly the hypothesis of high-pressure phases.
To directly observe the unique core-shell structure, HRTEM images were taken from the local regions.Figure 4b-d shows HRTEM images of the unaffected area ("A"), shell ("B"), core  ("C") of the laser modification area in Figure 2b, respectively.A fast Fourier transform (FFT) image was added to the top right corner of individual HRTEM images.It is clear that the unaffected (Figure 4b) and shell (Figure 4c) areas exhibit lattice fringes of crystalline phases, although there is a slight difference in the orientation based on the FFT images.Moreover, different local areas within shell region B were captured with HRTEM images, and most of them still showed well-defined lattice fringes of crystalline phases.In contrast, HRTEM and FFT images taken from the core region C show significant disorder and even amorphous features.Therefore, based on the electron diffraction patterns (Figure 3) and HRTEM images (Figure 4), one can assume that the core-shell structure is associated with the structural transition from the single-crystal, polycrystalline, to the amorphous phase of Si.The unaffected area is a single crystal, the shell region becomes polycrystalline, while the core region is a mixture of polycrystalline and amorphous phases.
TEM analysis of the laser modification line "2" is shown in Figure 5. Overall, line 2 shows a much higher density of permanent modifications than line 1, but it is difficult to find a core-shell structure.A small, bright area surrounded by a dark contrast is shown in Figure 5b.The amorphous phase was captured using HRTEM, as shown in Figure 5c,d, in which the dashed yellow lines indicate the boundaries between crystalline and amorphous phases.

Discussion
In a separate study by our team, Raman spectroscopic characterizations of the laser-modified zone exhibit two peaks, one for amorphous Si (489.2 cm À1 ), and the other for c-Si (518.59cm À1 ).The Raman spectroscopic characterizations are consistent with the phase transformation (single crystal-topolycrystalline, amorphous) observed from SAED and HRTEM.
The formation mechanism of a-Si during laser direct writing of silicon was discussed in the literature.The formation mechanism of a-Si may rely on the process of thermal melting, rapid cooling, and resolidification. [21]To generate permanent structural modifications in c-Si, the laser pulse energy should be sufficient to melt Si.After melting, rapid cooling and resolidification can occur during the ultrashort laser pulse (femtoseconds to nanoseconds) that prevents the amorphous silicon from developing the crystalline state. [25]A time-resolved study on melting of c-Si under nanosecond laser irradiation suggests that the laser influence threshold for melting c-Si is 650 mJ cm À2 . [26]Sun et al. [24] studied the behavior of single-crystal c-Si under a nanosecond laser irradiation with a wavelength of 1.064 μm, which showed that the phase transformation including polycrystalline, amorphous, and amorphous-to-polycrystalline phase formation can occur at the laser fluence of 1.47 J cm À2 .Furthermore, melting of the silicon under the same laser condition can happen at a laser fluence of 1.28 J cm À2 . [27]In this study, the laser fluence was calculated as 7.53 J cm À2 , which is sufficient to melt the c-Si and induce thermal accumulation.In the core-shell structure observed above, while the core contains a-Si and multiple crystals, the shell is composed of a number of crystallites that are indicated by the additional diffraction spots lines on different planes from the indexed planes.The formation of multiple crystals in the shell also indicates the recrystallization of the sample due to nanosecond laser irradiation while keeping the diamond cubic structure (or Si-I).However, further study is needed to elaborate the formation mechanism of the core-shell structure.Compared to line "1", a lower scanning speed of laser writing was used for line "2", which resulted in a higher laser energy deposition in the local area, leading to a larger number of defects and permanent structural modifications.However, it is believed that the lower scanning speed allowed more time for the molten silicon to recrystallize the structure during the cooling process by reducing the temperature gradient between the adjacent laser pulses.Thus, there is a more limited amount of amorphous phase remaining and a core-shell structure is less likely to form.
The presence of α-Si is expected to increase the refractive index of waveguides, and laser direct writing conditions have a significant effect on the formation of α-Si, as indicated by the two samples investigated in this study.In this research, an estimation of the volume percentage of laser-induced a-Si ranges from near zero to 0.11%.By treating the modified zone as a composite of a-Si and c-Si and using the rules of the mixture, a calculation for the change in the refractive index of the lasermodified zone can be conducted.Given the refractive index of a-Si (n a-Si = 3.73) and c-Si (n c-Si = 3.45) at λ = 1.55 μm, [24] a positive refractive index change in the laser-modified zone is found to be from near zero to about 10 À4 for these samples.The increase of refractive index due to the generation of a-Si should contribute to the waveguiding ability of the laser-modified Si wafers.Future research will optimize the laser direct writing conditions to increase the volume of phase transformation from c-Si into a-Si, which will contribute to the waveguiding ability.

Conclusion
Permanent structural modifications of single-crystal silicon using laser direct writing were achieved using a nanosecond pulse laser.The laser-modified zones contain polycrystalline and amorphous phases, which were characterized using TEM images along with SAED and HRTEM analysis.The core-shell structures were associated with the structural transition from single-crystal, polycrystalline, to amorphous phase of Si, which were formed through melting, rapid cooling, and resolidification processes during the ultrafast laser pulse of nanoseconds.The lower laser scanning speed resulted in a higher density of defects with less a-Si formation, likely due to the lower temperature gradient between laser pulses allowing the molten silicon to recrystallize.Given the higher refractive index of a-Si, the generation of a-Si by pulsed laser direct writing will allow promising waveguide applications.

Figure 1 .
Figure 1.a) Experimental set-up of direct laser writing of c-Si using nanosecond laser; b) orientation of TEM sample preparations; c) scanning electron microscopy (SEM) image of the cross-section of the modification lines ("1" and "2").

Figure 2 .
Figure 2. TEM images of a) overview of the laser modification line "1" in c-Si and the magnified images of area; b) I, c) II, and d) III in a).

Figure 3 .
Figure 3. Electron diffraction pattern at region: a) A, b) B, and c) C in Figure 2b.

Figure 4 .
Figure 4. a) TEM image of the core-shell structure in the laser modification line "1" along with HRTEM images of b) A, c) B, and d) C areas at the zone axis of [0,0,1] with the FFT insets at the top-right corner of the images.

Figure 5 .
Figure 5. a) TEM image of the laser modification line "2", b) enlarged view of the yellow rectangle frame, c,d) HRTEM images of the blue circle in (b).