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Large area Germanium Tin nanometer optical film coatings on highly flexible aluminum substrates

View Article: PubMed Central - PubMed

ABSTRACT

Germanium Tin (GeSn) films have drawn great interest for their visible and near-infrared optoelectronics properties. Here, we demonstrate large area Germanium Tin nanometer thin films grown on highly flexible aluminum foil substrates using low-temperature molecular beam epitaxy (MBE). Ultra-thin (10–180 nm) GeSn film-coated aluminum foils display a wide color spectra with an absorption wavelength ranging from 400–1800 nm due to its strong optical interference effect. The light absorption ratio for nanometer GeSn/Al foil heterostructures can be enhanced up to 85%. Moreover, the structure exhibits excellent mechanical flexibility and can be cut or bent into many shapes, which facilitates a wide range of flexible photonics. Micro-Raman studies reveal a large tensile strain change with GeSn thickness, which arises from lattice deformations. In particular, nano-sized Sn-enriched GeSn dots appeared in the GeSn coatings that had a thickness greater than 50 nm, which induced an additional light absorption depression around 13.89 μm wavelength. These findings are promising for practical flexible photovoltaic and photodetector applications ranging from the visible to near-infrared wavelengths.

No MeSH data available.


(a) Schematic of the GeSn nano films with thickness of t coating on Al foils. (b–g) Widely color spectra of the nanometer GeSn coating on Al foils, t = 0, 20, 40, 60, 100 and 180 nm (Photos are taken under illumination from conventional fluorescent ceiling lights).
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f1: (a) Schematic of the GeSn nano films with thickness of t coating on Al foils. (b–g) Widely color spectra of the nanometer GeSn coating on Al foils, t = 0, 20, 40, 60, 100 and 180 nm (Photos are taken under illumination from conventional fluorescent ceiling lights).

Mentions: Figure 1(a) shows the schematic of the GeSn film with thickness t on flexible Al foil. Inset graph describes the behavior of light (red arrow) incident from air into the highly absorptive GeSn nanometer thin film as an optical coatings, which was deposited on a metallic Al foil substrate. We assume that there is no transmission through the Al foil substrate, the absorption of the structure can be written as A = 1 − R, where A is light absorption and R is the light reflection. For a metal Al foil substrate in the perfect electric conductor limit, its’ complex refractive index Al = nAl + ikAl, nAl → ∞ and kAl → ∞. The incident light is completely reflected at the Al-GeSn interface with a phase shift of π, which makes the GeSn thickness much lower than the wavelength with h ≈ λ/4nGeSn (nGeSn is the refractive index of the GeSn). Figure 1(b–g) presents a photograph of samples of Al foil coated GeSn from 0 to 180 nm in thickness, which creates a spectrum of colors including silver, golden, dark blue and light blue. Although the surface of the Al foil substrates are unpolished, the various colors still clearly appear. It should be mentioned that the Al foil is polycrystalline with a thickness of ~200 μm. The grain distribution changes from 102 nm to 238 nm, which has been checked using an atomic force microscopy. The findings agree well with a previous report18. The wide optical absorption band comes from the remarkable reflectivity change of the aluminum foil by coating it with nanometer thick GeSn films. GeSn was selected because it is highly absorbing at visible/near-infrared wavelengths. Moreover, its indirect band gap can be tuned to a direct band gap with the application of tensile strain, which is very important for its photovoltaic and photodetector applications. The samples exhibit excellent mechanical flexibility and do not crack even after repeated bending. It can be cut into any shapes by shears, which facilitate flexible photonics fabrication.


Large area Germanium Tin nanometer optical film coatings on highly flexible aluminum substrates
(a) Schematic of the GeSn nano films with thickness of t coating on Al foils. (b–g) Widely color spectra of the nanometer GeSn coating on Al foils, t = 0, 20, 40, 60, 100 and 180 nm (Photos are taken under illumination from conventional fluorescent ceiling lights).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5036029&req=5

f1: (a) Schematic of the GeSn nano films with thickness of t coating on Al foils. (b–g) Widely color spectra of the nanometer GeSn coating on Al foils, t = 0, 20, 40, 60, 100 and 180 nm (Photos are taken under illumination from conventional fluorescent ceiling lights).
Mentions: Figure 1(a) shows the schematic of the GeSn film with thickness t on flexible Al foil. Inset graph describes the behavior of light (red arrow) incident from air into the highly absorptive GeSn nanometer thin film as an optical coatings, which was deposited on a metallic Al foil substrate. We assume that there is no transmission through the Al foil substrate, the absorption of the structure can be written as A = 1 − R, where A is light absorption and R is the light reflection. For a metal Al foil substrate in the perfect electric conductor limit, its’ complex refractive index Al = nAl + ikAl, nAl → ∞ and kAl → ∞. The incident light is completely reflected at the Al-GeSn interface with a phase shift of π, which makes the GeSn thickness much lower than the wavelength with h ≈ λ/4nGeSn (nGeSn is the refractive index of the GeSn). Figure 1(b–g) presents a photograph of samples of Al foil coated GeSn from 0 to 180 nm in thickness, which creates a spectrum of colors including silver, golden, dark blue and light blue. Although the surface of the Al foil substrates are unpolished, the various colors still clearly appear. It should be mentioned that the Al foil is polycrystalline with a thickness of ~200 μm. The grain distribution changes from 102 nm to 238 nm, which has been checked using an atomic force microscopy. The findings agree well with a previous report18. The wide optical absorption band comes from the remarkable reflectivity change of the aluminum foil by coating it with nanometer thick GeSn films. GeSn was selected because it is highly absorbing at visible/near-infrared wavelengths. Moreover, its indirect band gap can be tuned to a direct band gap with the application of tensile strain, which is very important for its photovoltaic and photodetector applications. The samples exhibit excellent mechanical flexibility and do not crack even after repeated bending. It can be cut into any shapes by shears, which facilitate flexible photonics fabrication.

View Article: PubMed Central - PubMed

ABSTRACT

Germanium Tin (GeSn) films have drawn great interest for their visible and near-infrared optoelectronics properties. Here, we demonstrate large area Germanium Tin nanometer thin films grown on highly flexible aluminum foil substrates using low-temperature molecular beam epitaxy (MBE). Ultra-thin (10–180 nm) GeSn film-coated aluminum foils display a wide color spectra with an absorption wavelength ranging from 400–1800 nm due to its strong optical interference effect. The light absorption ratio for nanometer GeSn/Al foil heterostructures can be enhanced up to 85%. Moreover, the structure exhibits excellent mechanical flexibility and can be cut or bent into many shapes, which facilitates a wide range of flexible photonics. Micro-Raman studies reveal a large tensile strain change with GeSn thickness, which arises from lattice deformations. In particular, nano-sized Sn-enriched GeSn dots appeared in the GeSn coatings that had a thickness greater than 50 nm, which induced an additional light absorption depression around 13.89 μm wavelength. These findings are promising for practical flexible photovoltaic and photodetector applications ranging from the visible to near-infrared wavelengths.

No MeSH data available.