Femtosecond Laser Annealing Research Articles

Fabrication of silicon carbide color center nanoparticles by femtosecond laser ablation in liquid

Combined with the fabrication of low-dimensional materials, certain silicon carbide color centers can become excellent platforms for nano-optics, and be applied to quantum and biomedical fields. In this work, color center nanoparticles were prepared based on high purity semi-insulating silicon carbide substrate using liquid-assisted femtosecond laser machining. Effects of multi-pulse array ablation and line scanning fabrication were investigated, and parameters such as repetition frequency and pulse number were also optimized. We employed photoluminescence spectroscopy, field emission scanning electron microscopy and transient fluorescence spectroscopy to characterize optical properties and micromorphology of nanoparticles. The broadband photoluminescence within the range of 850–950 nm should be attributed to silicon vacancy color centers, and V1/V1’ zero-phonon lines were confirmed at low temperature. It is noted that the femtosecond laser annealing is critical for the luminescence enhancement of color center nanoparticles. The evolution of particles and cavitation bubbles during processing was elucidated. After optimization, silicon vacancy color center nanoparticles were prepared by multi-pulse (107) array ablation and 200 kHz pulse repetition frequency, with an average diameter of approximately 15 nm and an average density reaching 273.94 counts/μm2. The sample can be preserved stably in the form of nanoparticle solutions. Processing methods and corresponding conclusions of this study would be applicable to the femtosecond laser fabrication and micromorphology control of color center nanoparticles of various hard-brittle semiconductors.

Femtosecond laser controllable annealing for color centers based on ion-implanted silicon carbide substrate

As a representative of third-generation semiconductors, silicon carbide serves as a new platform for quantum nano-optics with readable paramagnetic color centers such as silicon vacancies. The potential applications include biomedical imaging, quantum sensing and micro/nano environment detection. Color centers are usually prepared based on ion-irradiated substrates, then annealed at high temperature to increase the yield. Currently, annealing furnaces have been the main method to heat the substrate and achieve annealing of the whole sample. However, further exploration is still necessary for fine processing requirements such as specialized annealing regions with certain shapes. In this work, we applied femtosecond laser to achieve controllable annealing at micron scale, based on silicon carbide substrate after ion injection. The whole process of point, line and pattern annealing was realized. The annealing area demonstrates enhanced photoluminescence, attributed to silicon vacancy color centers. Raman and photoluminescence spectra reveal that the annealing effect is primarily influenced by the number of pulses per unit area. The mechanism of femtosecond laser annealing was introduced. Finally, the controllable annealing areas with various shapes were prepared based on the comprehensive optimal parameters (100 kHz pulse repetition frequency, 105 point annealing pulses, 10 μm/s line scanning speed). This annealing strategy demonstrates excellent stability and uniformity, as well as micron scale accuracy. Additionally, the excited state lifetime of silicon vacancy was extracted and quantified. The result means that femtosecond laser annealing enhances the optical properties and quantum application potential of color centers. This work can be further extended to other semiconductors as well as patterned modification of local surface/planes at some depth, and helpful to explore the mechanism and applications of femtosecond laser annealing.

Refining silicon nitride waveguide quality through femtosecond laser annealing

We present a method for modification of silicon nitride (Si3N4) waveguide resonators using femtosecond laser annealing. The quality (Q) factor of the waveguide resonators can be improved by approximately 1.3 times after annealing. Notably, waveguides that originally had a high Q value maintained their quality after the annealing process. However, those with a lower initial Q value experienced a noticeable improvement post-annealing. To characterize the annealing effect, the surface morphologies of Si3N4 films, both pre- and post-annealing, were analyzed using atomic force microscopy. The findings suggest a potential enhancement in surface refinement. Furthermore, Raman spectroscopy confirmed that the Si3N4 film's composition remains largely consistent with its original state within the annealing power range of 0.6–1.6 W. This research underscores the potential of femtosecond laser annealing as an efficient, cost-effective, and localized technique for fabricating low-loss integrated photonics.

Femtosecond laser annealing of fluorine-doped tin oxide films towards high-performance perovskite photovoltaics

Femtosecond laser annealing preparation of a superhydrophilic FTO surface helps promote the photoelectric conversion efficiency of perovskite solar cells to 22.33%.

Femtosecond laser annealing of 4H-SiC interfaces and optimization of their electrical performance

Femtosecond laser annealing of 4H-SiC interfaces and optimization of their electrical performance

Ultrafast Infrared Laser Crystallization of Amorphous Ge Films on Glass Substrates.

Amorphous germanium films on nonrefractory glass substrates were annealed by ultrashort near-infrared (1030 nm, 1.4 ps) and mid-infrared (1500 nm, 70 fs) laser pulses. Crystallization of germanium irradiated at a laser energy density (fluence) range from 25 to 400 mJ/cm2 under single-shot and multishot conditions was investigated using Raman spectroscopy. The dependence of the fraction of the crystalline phase on the fluence was obtained for picosecond and femtosecond laser annealing. The regimes of almost complete crystallization of germanium films over the entire thickness were obtained (from the analysis of Raman spectra with excitation of 785 nm laser). The possibility of scanning laser processing is shown, which can be used to create films of micro- and nanocrystalline germanium on flexible substrates.

Single-shot selective femtosecond and picosecond infrared laser crystallization of an amorphous Ge/Si multilayer stack

Pulsed laser crystallization is an efficient annealing technique to obtain polycrystalline silicon or germanium films on non-refractory substrates. This is important for creating “flexible electronics” and can also be used to fabricate thin-film solar cells. In this work, near- and mid-infrared femtosecond and picosecond laser pulses were used to crystallize a Ge/Si multilayer stack consisting of alternating amorphous thin films of silicon and germanium. The use of infrared radiation at wavelengths of 1030 and 1500 nm with photon energies lower than the optical absorption edge in amorphous silicon allowed obtaining selective crystallization of germanium layers with a single laser shot. The phase composition of the irradiated stack was investigated by the Raman scattering technique. Several non-ablative regimes of ultrashort-pulse laser crystallization were found, from partial crystallization of germanium without intermixing the Ge/Si layers to complete intermixing of the layers with formation of GexSi1-x solid alloys. The roles of single- and two-photon absorption, thermal and non-thermal (ultrafast) melting processes, and laser-induced stresses in selective pico- and femtosecond laser annealing are discussed. It is concluded that, due to a mismatch of the thermal expansion coefficients between the adjacent stack layers, efficient explosive solid-phase crystallization of the Ge layers is possible at relatively low temperatures, well below the melting point.

Graphitisation of Waste Carbon Powder with Femtosecond Laser Annealing.

Graphitisation of structural characteristics and improvement in electrical conductivity was reported onto waste carbon powder through femtosecond laser annealing. Raman spectroscopy on the carbon powder pre- and post-annealing showed a shift from amorphous-like carbon to graphitic-like carbon, which can be explained by the three-stage model. Electrical I-V probing of the samples revealed an increase in conductivity by up to 90%. An increase in incident laser power was found to be correlated to an increase in conductivity. An average incident laser power of 0.104 W or less showed little to no change in electrical characteristics, while an average incident laser power of greater than 1.626 W had a destructive effect on the carbon powder, shown through the reduction in powder. The most significant improvement in electrical conductivity has been observed at laser powers ranging from 0.526 to 1.286 W. To conclude, the graphitisation of waste carbon powder is possible using post-process femtosecond laser annealing to alter its electrical conductivity for future applications.

Selective ultrashort laser annealing of amorphous Ge/Si multilayer stacks

We report on single-short laser crystallization of Ge/Si multilayer stacks consisting of alternating amorphous nanosized films of silicon and germanium using near- and mid-infrared femtosecond and picosecond laser pulses. The phase composition of the irradiated stacks was investigated by the Raman scattering technique. Several non-ablative regimes of crystallization were found, from partial crystallization of germanium without intermixing the Ge/Si layers to complete intermixing of the layers with formation of GexSi1-x solid alloys. The roles of one- and two-photon absorption, thermal and non-thermal (ultrafast) melting processes, and laser-induced stresses in selective pico- and femtosecond laser annealing are analysed based on theoretical estimations and comparison with experimental data. It is concluded that, due to a mismatch of the thermal expansion coefficients between the adjacent stack layers, efficient explosive solid-phase crystallization of the Ge layers is possible at relatively low temperatures, well below the melting point. The possibility of ultrafast non-thermal phase transition in germanium in the studied regimes is also discussed.

Enhancement in the extinction ratio of a wire grid polarizer in the mid-wavelength infrared range via hot electron diffused cold-annealing

Theoretical and experimental investigations were performed on the fabrication and polarization property enhancement of a single-layer Au grating serving as a wire grid polarizer (WGP) in the mid-wavelength infrared (MWIR) range. Femtosecond laser (FSL) annealing was performed to overcome the performance limitations of the WGP fabricated by nanotransfer printing (NTP), and the results were compared with those of the conventional thermal annealing methods using scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. Consequently, the extinction ratio of the WGP annealed with FSL reached up to 1378 (∼31 dB) in the 3–5 μm wavelength range, which corresponds to a 2.18-fold improvement over the non-annealed WGP. This result suggests that it is possible to manufacture MWIR WGPs at a low cost and over large areas through a simple NTP and FSL annealing process, ultimately yielding WGPs with improved performances.

Femtosecond Laser Annealing of Multilayer Thin-Film Structures Based on Amorphous Germanium and Silicon

The processes of femtosecond laser annealing of thin-film multilayer structures based on amorphous silicon and germanium produced by plasma-chemical deposition on a glass substrate have been studied. The formation of periodic structures on the surface of irradiated films has been detected by the scanning electron microscopy method. Analysis of Raman spectra has shown that amorphous germanium crystallizes and a mixing of germanium and silicon layers depending on the pulse energy density occurs in the absence of amorphous silicon layers crystallization as a result of exposure to femtosecond laser pulses.

Room-temperature crystallization of amorphous silicon by near-UV femtosecond pulses

Near ultraviolet (λ ≈ 400 nm) femtosecond laser annealing (400 nm-FLA) in a scanning mode was employed to crystallize amorphous silicon (a-Si) films at room temperature. The average grain size of polycrystalline silicon annealed was studied as a function of the incident laser fluence and beam overlap or the number of laser shots irradiated. In general, the grain size can be enlarged by either increasing the beam overlap at a fixed laser fluence or increasing the laser fluence for a fixed number of laser shots. An apparent threshold for the onset of rapid enlargement of grain size was observed for processing at ∼90% overlap and fluences above 25 mJ/cm2. A maximum grain size of ∼280 nm was attained at a laser fluence of 30 mJ/cm2 and overlap of 93.75%, beyond which the grain size attained was smaller, and eventually, ablation was observed at an overlap of 97.5% and higher. These trends and observed surface morphology of annealed samples suggest that the crystallization mechanism is like sequential lateral solidification, similar to 800 nm-FLA and excimer laser annealing. Raman spectroscopic studies show that the degree of crystallization achieved with 400 nm-FLA is even higher than that of 800 nm-FLA. Cross-sectional scanning electron microscopic images indicate that the 100 nm-thick a-Si film is not fully crystallized. This can be explained by the much shorter penetration depth of 400 nm light than that of 800 nm light in a-Si.

Фемтосекундный лазерный отжиг многослойных тонкопленочных структур на основе аморфных германия и кремния

Femtosecond laser annealing of thin-film multilayered structures based on amorphous silicon and germanium were studied. The original samples were synthesized via plasma-enhanced deposition on glass substrate. Scanning electron microscopy revealed formation of periodic surface structures in the irradiated films. Raman spectra analysis revealed crystallization of amorphous germanium as a result of femtosecond laser pulses action, as well as fluence-dependent mixture of the germanium and silicon layers at absence of crystallization of the amorphous silicon layers.

Phosphorous-Doped $\alpha$ -Si Film Crystallization Using Heat-Assisted Femtosecond Laser Annealing

Heat effects on femtosecond laser annealing to crystallize doped amorphous Si films are studied. The structural, optical and electronic properties of phosphorus-doped amorphous Si films before and after femtosecond laser treatment are characterized. As the temperature increases from room temperature to 200 °C controlled by a hot-stage, the grain size and number of crystalline Si on the films are gradually enhanced, which is confirmed by comparing the surface morphologies and analyzing the Raman spectrum. It is demonstrated that heating the substrate can promote the phase transformation of amorphous Si and the activation of phosphorus dopants, yielding a significant improvement in the light-trapping capability and carrier conductivity of the laser-annealed films. By using the proposed heat-assisted femtosecond laser annealing technique, polycrystallized phosphorus-doped amorphous Si films are produced showing highly absorptive and conductive, which might be further applied in photovoltaic and microelectronic devices.

Real time monitoring of fs laser annealing on indium tin oxide

In this study, a set of time-resolved pump-probe systems for the real-time monitoring of femtosecond (fs) laser annealing of ITO substrates is established. A pump beam is used as the light source for thermal annealing, and a probe beam is used to monitor the transmittance variation of ITO in real time. Measurement results show that with increasing machining energy, the transmittance variation detected by the probe beam gradually increases. When the laser energy is small, the transmittance changes arithmetically. When the laser energy is sufficient to complete the modification of ITO, the transmittance variation rises suddenly and sharply, indicating that the thermal annealing modification has been completed in the irradiated area. Then, the laser source is shut down to achieve real-time monitoring. Degenerate and non-degenerate pump-probe experimental results indicate that the 400-nm probe beam is more sensitive than the 800-nm probe beam in measuring curve variation because the variation in the ITO transmission spectrum in the 400-nm wave band is greater than that in the 800-nm wave band. Electron backscatter diffraction analysis indicates that the crystallographic directions of ITO after laser annealing are consistent. This shows that the ITO changed from non-crystallized to crystallized after being irradiated by the pump beam, confirming the thermal annealing modification.

Hot-Electron Intraband Luminescence from GaAs Nanospheres Mediated by Magnetic Dipole Resonances.

Significantly enhanced electric field in plasmonic hot spots can dramatically increase the linear and nonlinear absorption of light, leading to a high-temperature electron gas which radiates, through mainly intraband transition, a broadband luminescence quite similar to blackbody radiation. Here, we demonstrate that such hot-electron intraband luminescence (HEIL) can also be achieved by exploiting the significantly enhanced electric field at the magnetic dipole resonances of gallium arsenide (GaAs) nanospheres (NSs). We show that monocrystalline GaAs NSs with distinct electric and magnetic dipole (ED and MD) resonances can be obtained by using femtosecond laser ablation and annealing. Significantly enhanced second harmonic generation and broadband HEIL are observed when the MD resonances of such GaAs NSs are resonantly excited. The lifetime of the HEIL is found to be as short as ∼82 ps, indicating a significant enhancement in radiative intraband transition rate. We reveal that the slope extracted from the dependence of the HEIL intensity on the irradiance is linearly proportional to the energy of the emitted photon. The existence of distinct ED and MD resonances in combination with a direct bandgap makes GaAs NSs an attractive candidate for constructing novel all-dielectric metamaterials and active photonic devices.

Sulfur-Doped Silicon Photodiode by Ion Implantation and Femtosecond Laser Annealing

Femtosecond (fs) laser annealing has been applied to improve the crystalline quality and absorptance below bandgap of ion implanted silicon (Si) with sulfur (S). The doping concentration of S is up to $10^{20}$ atoms/cm3, which is at least four orders of magnitude higher than the solid solubility of S in crystalline Si. According to the Raman spectra, after fs laser irradiation with appropriate laser energy, the crystal quality is evidently better than primary ion implanted sample. Moreover, the optical absorption coefficient at wavelengths of 1100~2400 nm is up to $1.54 \times 10^{4}$ cm $^{\mathrm {-1}}$ , which is much larger than the contribution of free carriers. Excessive laser energy will induce serious ablation effect and cause the removing of doping layer. We consider that the high absorption is a combination of sub-bandgap transition of S-related localized states, free carrier absorption, ion implantation induced defect absorption and fs laser irradiation induced defect absorption. In the end, a photo-response of 8.4 mA/W is obtained for the photodiode for 1310 nm detection light.

Crystalline Engineering Toward Large-Scale High-Efficiency Printable Cu(In,Ga)Se2 Thin Film Solar Cells on Flexible Substrate by Femtosecond Laser Annealing Process

Ink-printing method emerges as a viable way for manufacturing large-scale flexible Cu(In,Ga)Se2 (CIGS) thin film photovoltaic (TFPV) devices owing to its potential for the rapid process, mass production, and low-cost nonvacuum device fabrication. Here, we brought the femtosecond laser annealing (fs-LA) process into the ink-printing CIGS thin film preparation. The effects of fs-LA treatment on the structural and optoelectronic properties of the ink-printing CIGS thin films were systematically investigated. It was observed that, while the film surface morphology remained essentially unchanged under superheating, the quality of crystallinity was significantly enhanced after the fs-LA treatment. Moreover, a better stoichiometric composition was achieved with an optimized laser scanning rate of the laser beam, presumably due to the much reduced indium segregation phenomena, which is believed to be beneficial in decreasing the defect states of InSe, VSe, and InCu. Consequently, the shunt leakage current and recombination centers were both greatly decreased, resulting in a near 20% enhancement in photovoltaic conversion efficiency.

Dependence of the optimum parameters of femtosecond laser annealing of lead zirconate titanate films on their thickness

The optimum parameters of laser annealing (crystallization) induced by repetitive pulses with a pulse duration of 100 fs and a wavelength of 800 nm, which falls in the transparency region of the film and, simultaneously, in the absorption region of the substrate, have been investigated experimentally as a function of the thickness of the ferroelectric film. It has been shown that, with an increase in the thickness of the ferroelectric film by 100 nm (in the range from 300 to 600 nm), the required power density of the laser beam increases, on the average, by 0.1 MW/cm2. The optimum exposure time of the laser beam with the desired power increases nonlinearly with an increase in the thickness of the film.

Hyperdoping silicon with selenium: solid vs. liquid phase epitaxy

Chalcogen-hyperdoped silicon shows potential applications in silicon-based infrared photodetectors and intermediate band solar cells. Due to the low solid solubility limits of chalcogen elements in silicon, these materials were previously realized by femtosecond or nanosecond laser annealing of implanted silicon or bare silicon in certain background gases. The high energy density deposited on the silicon surface leads to a liquid phase and the fast recrystallization velocity allows trapping of chalcogen into the silicon matrix. However, this method encounters the problem of surface segregation. In this paper, we propose a solid phase processing by flash-lamp annealing in the millisecond range, which is in between the conventional rapid thermal annealing and pulsed laser annealing. Flash lamp annealed selenium-implanted silicon shows a substitutional fraction of ~ 70% with an implanted concentration up to 2.3%. The resistivity is lower and the carrier mobility is higher than those of nanosecond pulsed laser annealed samples. Our results show that flash-lamp annealing is superior to laser annealing in preventing surface segregation and in allowing scalability.