| Conference | 19th International Conference on Atomic Layer Deposition (ALD 2019), the 6th International Atomic Layer Etching Workshop (ALE 2019), ALD-ALE 2019, July 21-24, 2019, Bellevue, WA, USA |
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| Abstract | The harvesting of ambient heat and light could potentially lead to useful power for driving off-grid, internet-of-things sensors. Recent theoretical predictions suggest that ultra-thin Ni films can show thickness dependent optical properties that can lead to enhanced solar absorptance. The maximum solar absorptance is predicted to occur at a very thin layer thickness of ~12 nm and is attributed to an impedance matching effect [N. Ahmad, et. al, Nano Energy 1, 777 (2012)]. Light absorptance in ultra-thin Ni films can be further enhanced by patterning the Ni films and depositing multi-layer structures. Atomic layer deposition (ALD) produces conformal films with a high level of control and uniformity with respect to the film thickness. While ALD is most widely known for depositing ultra-thin dielectric materials for advanced electronics applications, we have successfully used the technique to deposit ultra-thin Ni films [P. Motamedi, et al., ACS Appl. Mater. Interfaces 9(29), 24722 (2017); P. Motamedi, et al., Adv. Mater. Interfaces 5(24), 1800957 (2018)]. In this study, ALD Ni films are deposited with varying thicknesses onto fused silica and sapphire substrates. Structural characterization indicates that the films are continuous, polycrystalline Ni films with a relatively low degree of roughness and are suitable for comparison with the theoretical predictions. Optical absorptance is measured using an integrating sphere as a function of film thickness and compared with theory. An optimum thickness is found but the experimentally determined absorptance maximum is less significant than theoretically predicted. Possible explanations include surface roughness or deviations from the refractive index of bulk Ni. Nevertheless, the thickness for optimum absorptance is found to be around 10 nm for planar ALD Ni films. Glancing angle deposition (GLAD) is used to produce quasi-periodic silica pillars on fused silica wafers and ALD Ni films are deposited conformally onto the GLAD pillars. It is hypothesized that the GLAD films with ultra-thin ALD Ni coatings will act similarly to lithographically patterned gratings and lead to enhanced optical absorptance. Optical absorptance measurements reveal a low reflectance at all wavelengths, but an extremely high absorptance in the UV / visible range that decreases to very low values in the NIR. The spectra are seen to have a strong dependence on the ALD Ni thickness but to be less sensitive to changes in the GLAD deposition parameters. Structural characterization, further control experiments, and theoretical modeling are planned to understand these optical results. |
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