Advancing Optical Technologies with Laser Annealed Films

By Nidhi DhullApr 29 2024Reviewed by Lexie Corner

The importance of optical technologies is well established in various high-tech applications, including telecommunications, healthcare, and consumer electronics. Optoelectronic devices, known for their low-loss and quick data transfer, thin and lightweight components, high security, and low power consumption, continue to expand their scope of application.

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However, the development of new-age devices necessitates optical components with specific properties to ensure high functionality. Thus, tools like laser annealing are crucial for selectively modifying material properties and further advancing optical technologies.¹

The present article explores the concept of laser annealing and its role in developing advanced optical films.

Principles of Laser Annealing 

Laser annealing uses the principle of light absorption to impart energy to a material. The intensity of incident light diminishes upon penetrating the material, thereby confining the energy to thin surface layers. The material’s absorption coefficient, laser wavelength, and temperature govern the laser penetration depth.

The intense laser absorption on a thin film surface—only a few nanometers deep—generates exceptionally high temperatures, which is required to alter surface properties locally without modifying the bulk material. This helps avoid unwanted changes in the film properties.¹

Laser annealing helps treat lattice damage from ion implantation, diffuse surface-deposited dopants in thin films, and crystallize amorphous films. It is one of the most effective alternatives to conventional thermal annealing, which involves heating films to temperatures above 1000 °C in furnaces.

However, subjecting the whole sample to such high temperatures can cause undesirable side effects like impurity redistribution, damage to implanted ions, and melting of prior-deposited metals.¹ The substrates also require careful selection to tolerate such high temperatures.²

Laser annealing offers many control parameters, such as laser wavelength, fluence, annealing time, and ambient atmosphere, which can be tuned as per the desired properties of the film. These can be optimized according to the sample properties like film thickness, microstructure, and optical behavior.²

With the correct laser parameters and ultrafast annealing restricted to specific regions, high-performance optical films can be produced at scale.¹ This versatile technique applies to various optical materials, including semiconductors, lanthanides, chalcogenides, and perovskites.^1-3

Applications in Optical Technologies

Laser-annealed films exhibit enhanced functionality and efficiency in multiple optical devices.

For example, Eu³⁺-doped oxide thin films show high potential for use in waveguides, imaging detectors, display luminophores, solar cells, and scintillators because of the strong emission of Eu³⁺ in the visible spectral range. However, these films require post-deposition annealing to activate Eu ions and obtain optimal photoluminescence.

Using pulsed laser annealing provides efficient vertical thermal profile control of Eu³⁺-doped oxide thin films, such as those made from Eu-doped ZnO, TiO₂, and Lu₂O₃.²

Chalcogenide thin films, like As-Se-Ge systems, exhibit unique properties such as linear and nonlinear refractive index, transparency in a broad spectral range, low phonon energy, and 3^rd-order optical susceptibility. With proper tuning of these properties, chalcogenide thin films can be integrated into various optical devices, such as optical switching fibers, acousto-optic devices, infrared transmitters, and supercontinuum-generating waveguides.

However, chalcogenide materials exhibit aging effects in their structural and optical properties, which change their exploitation parameters with time. Laser annealing can help detect the aging-induced changes in such thin films and tune their properties for further applications.³

A recent article in Optical Materials demonstrated the use of vacuum laser annealing of cerium-substituted yttrium iron garnet (Ce:YIG) films. These magneto-optical films stabilize optical communication in optical isolators. The researchers used laser annealing to selectively crystallize Ce-YIG films. The magnetization of laser-annealed Ce:YIG was similar to that of the film prepared by thermal annealing.⁴

In another recent article published in Applied Physics Letter, the researchers used laser-micro-annealing for conditioning nano-light emitting diodes (LEDs) in arrays. Nano-LEDs are based on InGaN/GaN multi-quantum well structures and offer significant advantages in Li-Fi communication, displays, and other optoelectronic applications.

However, the production of stable and reliable nano-LEDs is challenging, which can be overcome by localized laser annealing. The photoluminescence and electroluminescence measurements used in this study revealed enhanced performance of the nano-LEDs. The laser-micro-annealing process also increased the long-term stability of the electroluminescence intensity of the LEDs.⁵

Challenges and Technological Barriers

Despite the promising prospects of laser-annealed thin films in advancing optical technologies, laser annealing faces several challenges, such as costs, scalability, and material compatibility, which hinder its large-scale adoption.

Alternatively, furnace annealing, while lacking selective material treatment and offering slower heating, is often preferred over laser annealing in many commercial applications due to a simpler setup, higher scalability, and simultaneous treatment of multiple wafers.¹

While some properties of the laser-annealed films may show improvement, others may remain inferior to those of the thermal-annealed films.⁴ Thus, careful control over the processing parameters is required when using highly energetic laser sources to avoid damaging the film constituents and properties.

Finding the optimal laser annealing parameters for a sample is often challenging and requires multiple tests. This can be overcome by in situ monitoring of the annealing process to reach the optimal parameters quickly and accurately.²

Environmental conditions can also influence laser annealing outcomes, as they increase the local temperature of a film. The heated material can react with atmospheric gases and adversely impact film properties. As a result, a vacuum or an inert atmosphere is required, which increases the equipment and process expenses.¹

Laser annealing technology thus requires further improvements, such as overlapping laser exposures to eliminate unexposed gaps and refining the laser spot shape using masks, lenses, and spatial light modulators.⁴ 

Future Outlooks

The use of lasers to process and modify materials gained popularity in the material sciences in the 1970s. Since then, lasers have evolved significantly, finding novel applications today, such as the fabrication of nano-optoelectronic devices.⁵

To fully exploit the potential of laser annealing and control its outcome, advanced in situ characterization tools like scanning electron microscope-electron backscatter diffraction and micro-focus X-Ray diffraction should be integrated into the process. This will allow precise real-time monitoring of the sample conditions in the laser-exposed region and adjacent areas.⁴

Overall, laser annealing is a powerful tool to enhance material properties. The rapidly developing optical fields of optoelectronics and photocatalysis are revolutionizing health care, green energy, communications, and robotics.

To further advance these areas and explore new applications, such as flexible optical devices, novel materials and improved laser annealing technologies are required. These innovations must be capable of producing high-performing, customizable optical materials at speed and scale.⁶

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References and Further Reading

1. Arduino, D., Stassi, S., Chiara Elfi Spano, Scaltrito, L., Ferrero, S., Bertana, V. (2023). Silicon and Silicon Carbide Recrystallization by Laser Annealing: A Review. Materials. doi.org/10.3390/ma16247674

2. Novotný, M., Remsa, J., Havlová, Š., More-Chevalier, J., Irimiciuc, SA., Chertopalov, S., Písařík, P., Volfová, L., Fitl, P., Kmječ, T., Vrňata, M., Lančok, J. (2021). In Situ Monitoring of Pulsed Laser Annealing of Eu-Doped Oxide Thin Films. Materials. doi.org/10.3390/ma14247576

3. Sahoo, D., Priyadarshini, P., Aparimita, A., Alagarasan, D., Ganesan, R., Varadharajaperumal, S., Naik, R. (2020). Role of annealing temperature on optimizing the linear and nonlinear optical properties of As₄₀Se₅₀Ge₁₀ films. RSC Advances. doi.org/10.1039/d0ra04763e

4. Miyashita, H., Yoshihara, Y., Mori, K., Koguchi, T., Lim, P.B., Inoue, M., Ishiyama, K., Goto, T. (2023). Vacuum laser annealing of magnetooptical cerium-substituted yttrium iron garnet films. Optical Materials. doi.org/10.1016/j.optmat.2023.114530

5. Mikulics, M., Kordoš, P., Gregušová, D., Sofer, Z., Winden, A., Trellenkamp, ST., Moers, J., Mayer, J., Hardtdegen, H. (2021). Conditioning nano-LEDs in arrays by laser-micro-annealing: The key to their performance improvement. Applied Physics Letters, 118(4). https://doi.org/10.1063/5.0038070

6. Lipovka, A., Garcia, A., Abyzova, E., Maxim Fatkullin, Song, Z., Li, Y., Wang, R., Rodriguez, RD., Sheremet, E. (2024). Laser Processing of Emerging Nanomaterials for Optoelectronics and Photocatalysis. Advanced Optical Materials. doi.org/10.1002/adom.20230319

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