The technique of laser annealing has emerged as an essential method for both the creation and improvement of crystalline thin films. It offers numerous advantages when compared to conventional thermal annealing approaches. This article considers the utilization of the laser annealing process for the fabrication of various types of crystalline thin films.
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Crystalline films exhibiting distinct molecular orientations and exceptional thermal stability have gained considerable interest over time. In contrast to amorphous thin films, they hold the potential to substantially enhance device performance by augmenting charge mobility, the efficiency of light out-coupling, and the operational longevity of light-emitting diodes.
The Relationship between Crystalline Thin Films and Laser Annealing
The major factors affecting the performance of crystalline thin films are the presence of grain boundaries and inherent crystal structure. Post-deposition annealing is frequently employed to improve their properties. However, conventional thermal annealing processes lead to undesired effects, which greatly affect the efficiency of thin films.
Consequently, researchers have turned their attention to the efficient method of laser annealing. Laser annealing involves the utilization of a high-energy laser beam to selectively increase the temperature of thin films in localized regions, thereby triggering recrystallization and encouraging the extension of grain sizes.
The core principle pivots on the selective absorption of laser energy by the thin film material, resulting in prompt and controlled elevation of temperature within designated areas. Subsequent controlled cooling procedures serve to avoid the generation of imperfections, resulting in an enhanced quality of the thin film.
Application of Pulsed Laser Annealing for SiC Crystalline Films
Silicon carbide (SiC) is a wide bandgap semiconductor (ranging from 2.3 to 3.2 eV) with superior thermal conductivity and breakdown electric field resilience. Moreover, SiC exhibits increased thermal, chemical, and mechanical stability, rendering it highly valuable within demanding and elevated-temperature settings.
The article published in Materials Today: Proceedings states that Chemical vapor deposition (CVD) is the conventional preparation method for thin films. However, the release of harmful toxic by-products is a major concern. Owing to this, pulsed laser deposition (PLD) followed by laser ablation is the most preferred method.
SiC thin film deposition was executed through Pulsed Laser Deposition (PLD). A YAG laser with a repetition rate of 10 Hz and a pulse width of 10 ns was employed. The resulting SiC film, post-deposition on a Si substrate, was studied utilizing a thermo-temporal model grounded in the heat conduction equation.
Within the simulation, a SiC film measuring 160 nm in thickness (corresponding to the experimental thickness) was envisaged atop a crystalline silicon (c-Si) substrate. The theoretical analysis of these post-deposited SiC films was conducted employing COMSOL Multiphysics.
Upon augmenting the laser fluence, there was an escalation in both the peak surface temperature and the expansion of the heating cycle. The laser fluence requisite for inducing the melting of crystalline silicon (c-Si) was approximated to be approximately 0.6 J cm-2. Notably, at an input laser fluence of 1.1 J cm-2, the temperature of the c-Si substrate exceeded the boiling point of silicon (2915 K).
Furthermore, with the increase in laser pulse duration from 10 ns to 55 ns, an elevation in the requisite laser fluence to attain a surface temperature of 3100 K was observed. When the thickness of the SiC film was heightened to 300 nm, a reduction was noted in the temperature at the SiC/c-Si interface. The film resistance of the SiC thin films produced through laser deposition was of the magnitude of 10^9 ohms. This film resistance exhibited a decline with an amplification of the laser fluence. These results demonstrated the efficiency and accuracy of the laser annealing process for SiC crystalline thin film fabrication.
Laser-Based Fabrication of Gold Thin Films
Gold (Au) holds significant technological importance due to its unique properties. Notably, it exhibits high electrical conductivity, chemical inertness, commendable stability, and notable biocompatibility. Crystalline gold thin films and nanoparticles, characterized by minimal defect occurrences, present promising prospects for various applications.
As per the article published in Nanomaterials, laser annealing is a versatile and efficient technique, providing meticulous and targeted energy dispersion within materials. This method has been documented for gold (Au) films utilizing both continuous wave and pulsed nanosecond lasers. In the context of thin film crystallization, the preference lies in a process that is non-melting and operates at low temperatures. Such an approach facilitates the modification of structural and material properties.
The researchers introduced a method of annealing Au thin films on a quartz substrate achieved through femtosecond (fs) laser pulses of three distinct wavelengths. Au thin films, measuring 18 nm and 39 nm in thickness, were applied using sputter coating. The crystallization of these Au thin films was achieved using a femtosecond laser. The fs laser, operating at a repetition rate of 100 kHz and possessing a pulse duration of 500 fs, was utilized for this purpose.
Notably, it was observed that the threshold fluence required to induce damage increased proportionally with intensifying film thickness. This trend was consistent across all employed wavelengths: infrared (IR), green, and ultraviolet (UV) laser pulses.
Furthermore, a linear correlation between the damage threshold fluence and film thickness was noted for films thinner than a characteristic optical penetration length. Specifically, for 18-nm-thick Au films, the utilization of green, UV, and IR laser treatments resulted in improvements of approximately 40%, 35%, and 33% in average electrical conductivity, respectively. For 39-nm-thick films, enhancements of approximately 29%, 27%, and 15% were achieved for UV, green, and IR laser wavelengths, respectively.
The current method proves to be a promising approach for crystallizing metal (Au) films through an ultra-short laser pulse method conducted at low temperatures.
Nanosecond Laser Annealing for Ferroelectric Thin Films
The field of ferroelectrics research has experienced a revival over the past decade, largely attributed to the revelation of ferroelectric properties in binary or ternary fluorite-based oxide thin films.
Zirconium dioxide (ZrO2), readily available in nature, arises as an enticing selection for extensive industrial implementation. The latest study in Advanced Sciences showcases a novel technique for enhancing the stability of ferroelectric orthorhombic ZrO2 films. This approach is realized via nanosecond laser annealing (NLA), applied to as-deposited Si/SiOx/W(14 nm)/ZrO2(8 nm)/W(22 nm) structures. These films are cultivated through ion beam sputtering at reduced temperatures.
The simulations involving laser heating and the subsequent heat propagation within the multilayer stack were conducted using the COMSOL Multiphysics software. An interesting was that the temperature attained within the ZrO2 layer beneath the upper W electrode surpassed that of the ZrO2 surface directly exposed to the laser beam.
This inconsistency was due to the amplified heat absorption facilitated by the W layer upon laser incidence. Simulation outcomes indicate that a minimum laser fluence of 0.4 J.cm−2 is imperative for achieving ZrO2 layer crystallization in the context of nanosecond laser annealing (NLA) processes.
To validate the crystalline structure of the ZrO2 crystallite, researchers engaged in the simulation of High-Resolution Transmission Electron Microscopy (HRTEM) patterns and atomic structural models. These patterns confirmed the presence of a cubic crystalline structure attributed to metallic W, detected within both the upper and lower layers. The results further unveiled that at a fluence of 0.5 J.cm−2, the ZrO2 layer surface (without the upper W layer) encountered damage. However, the emergence of the orthorhombic phase of ZrO2, synonymous with the ferroelectric phase within the films, was duly verified.
Laser Annealing for Perovskite Crystals
Halide perovskite solar cells (PSCs) have progressed considerably in photovoltaics, primarily owing to their remarkable optoelectronic properties and exceptional absorption coefficients. In this context, lasers present an advantageous approach for manipulating the crystalline structure and electrical characteristics of films. This is attributed to their elevated energy levels, commendable monochromatic properties, and widespread application in the production of perovskite layers.
In a recent article in Crystals, laser annealing demonstrated a noteworthy impact on stress modulation during thin film growth, notably in its ability to mitigate tensile stresses during the growth process. This stress alleviation phenomenon originates from the amplified diffusion facilitated by the laser, operating at ultrafast speeds within perovskite films. Importantly, the benefits of laser annealing extend to flexible polymer substrates as well.
This application safeguards other integral functional layers from potential harm, a crucial consideration within the context of flexible perovskite solar cell (PSC) fabrication.
Researchers have found that the localized temperature gradient stimulated by the laser at perovskite grain boundaries serves to facilitate differential energy absorption and temperature elevation between larger and smaller grains. This intricate mechanism is envisioned to empower larger grains to absorb more energy, resulting in elevated temperatures compared to their smaller counterparts.
As commercial applications continue to progress, the forthcoming exploration of novel laser annealing techniques is anticipated to center around three primary objectives: the manufacturing of superior film quality, exceptionally swift processing intervals, and cost reduction. The fusion of laser annealing methodologies with other applicable techniques, coupled with the integration of state-of-the-art advancements, is necessary for shaping the trajectory of materials science and device technology in the future.
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References and Further Reading
Paneerselvam, E. et. al. (2021). Pulsed laser deposition of SiC thin films and influence of laser-assisted annealing. Materials Today: Proceedings, 35(3), 312-317. Available at: https://doi.org/10.1016/j.matpr.2020.01.535
Crema, A. et. al. (2023). Ferroelectric Orthorhombic ZrO2 Thin Films Achieved Through Nanosecond Laser Annealing. Advanced Science. 10(15). 2207390. Available at: https://doi.org/10.1002/advs.202207390
Sharif A. et. al. (2021). Femtosecond Laser Assisted Crystallization of Gold Thin Films. Nanomaterials.11(5). 1186. Available at: https://doi.org/10.3390/nano11051186
Wang L. et al. (2022). Annealing Engineering in the Growth of Perovskite Grains. Crystals. 12(7):894. Available at: https://doi.org/10.3390/cryst12070894
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