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

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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) Refection spectra (from 400 to 2400 nm in wavelength) of Al foil coated with 10, 20, 30, 40, 50, 60, 100 and 180 nm of GeSn thin films. (b) Refection spectra (from 5 to 18 μm in wavelength) of Al foil coated with various thickness of GeSn thin films.
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f6: (a) Refection spectra (from 400 to 2400 nm in wavelength) of Al foil coated with 10, 20, 30, 40, 50, 60, 100 and 180 nm of GeSn thin films. (b) Refection spectra (from 5 to 18 μm in wavelength) of Al foil coated with various thickness of GeSn thin films.

Mentions: The visible/infrared reflection spectra were obtained using a Vis/NIR spectrophotometer (Lambda750). The incident light was unpolarized with an incident angle of ~5° with respect to the Al foil’s normal. An integrating sphere is used to collect the light back scattered in all directions. The reflectivity of a bare Al foil was employed as a reference. Figure 6(a) shows the measured reflection spectra of Al foil coated with various thicknesses of GeSn thin films (where t = 10, 20, 30, 40, 50, 60, 100, and 180 nm) over a wavelength range of 400–2400 nm. It is noticed that the change in reflectivity for different t samples is remarkable. With an increase of GeSn coating thickness, the response wavelength (the light reflectivity <50%) can be tailored for a range of wavelengths from 400 nm to 1800 nm. From the discussion above, without considering the Al transmission, the optical absorption of GeSn films can be obtained as A = 1 − R. The maximum light absorption ratio is as high as 85%, while a dip in reflectance is around 15% at t = 100 nm. The reflectivity spectra have the same shape and tend to a red shift with increasing GeSn coating thickness on Al foil substrate. The results agree well with the evolution of the GeSn/Al foil color as presented in Fig. 1(b). The enhanced light absorption comes from the light interference effect in GeSn/Al foil structure, which has great potential to be employed in highly efficient photovoltaic and photodetector applications. However, the epitaxial breakdown for thick GeSn films will reduce the efficiency of photovoltaics due to the Sn segregation induced poor layer quality. Especially, the epitaxial breakdown will cause a large surface roughness and a crystalline structure degradation. In order to suppress the Sn segregation, a minimum growth rate is preferred27. Figure 6(b) shows the reflection spectra in the range of 5–18 μm measured by FTIR. The reflection spectrum of bare Al foil has a uniform reflectance of ~60% (as shown as the black curve). The reflectance is boosted ~20 to 35% with coatings of GeSn nanometer films. In addition, there is an additional absorption dip around 13.89 μm. The relation between the wavelength of absorption light and the energy band gap Eg is given by λabs = 1.24/Eg 25. So, Eg is extracted as 0.089 eV. This agrees well with the band gap of Tin (~0.08 eV).


Large area Germanium Tin nanometer optical film coatings on highly flexible aluminum substrates
(a) Refection spectra (from 400 to 2400 nm in wavelength) of Al foil coated with 10, 20, 30, 40, 50, 60, 100 and 180 nm of GeSn thin films. (b) Refection spectra (from 5 to 18 μm in wavelength) of Al foil coated with various thickness of GeSn thin films.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5036029&req=5

f6: (a) Refection spectra (from 400 to 2400 nm in wavelength) of Al foil coated with 10, 20, 30, 40, 50, 60, 100 and 180 nm of GeSn thin films. (b) Refection spectra (from 5 to 18 μm in wavelength) of Al foil coated with various thickness of GeSn thin films.
Mentions: The visible/infrared reflection spectra were obtained using a Vis/NIR spectrophotometer (Lambda750). The incident light was unpolarized with an incident angle of ~5° with respect to the Al foil’s normal. An integrating sphere is used to collect the light back scattered in all directions. The reflectivity of a bare Al foil was employed as a reference. Figure 6(a) shows the measured reflection spectra of Al foil coated with various thicknesses of GeSn thin films (where t = 10, 20, 30, 40, 50, 60, 100, and 180 nm) over a wavelength range of 400–2400 nm. It is noticed that the change in reflectivity for different t samples is remarkable. With an increase of GeSn coating thickness, the response wavelength (the light reflectivity <50%) can be tailored for a range of wavelengths from 400 nm to 1800 nm. From the discussion above, without considering the Al transmission, the optical absorption of GeSn films can be obtained as A = 1 − R. The maximum light absorption ratio is as high as 85%, while a dip in reflectance is around 15% at t = 100 nm. The reflectivity spectra have the same shape and tend to a red shift with increasing GeSn coating thickness on Al foil substrate. The results agree well with the evolution of the GeSn/Al foil color as presented in Fig. 1(b). The enhanced light absorption comes from the light interference effect in GeSn/Al foil structure, which has great potential to be employed in highly efficient photovoltaic and photodetector applications. However, the epitaxial breakdown for thick GeSn films will reduce the efficiency of photovoltaics due to the Sn segregation induced poor layer quality. Especially, the epitaxial breakdown will cause a large surface roughness and a crystalline structure degradation. In order to suppress the Sn segregation, a minimum growth rate is preferred27. Figure 6(b) shows the reflection spectra in the range of 5–18 μm measured by FTIR. The reflection spectrum of bare Al foil has a uniform reflectance of ~60% (as shown as the black curve). The reflectance is boosted ~20 to 35% with coatings of GeSn nanometer films. In addition, there is an additional absorption dip around 13.89 μm. The relation between the wavelength of absorption light and the energy band gap Eg is given by λabs = 1.24/Eg 25. So, Eg is extracted as 0.089 eV. This agrees well with the band gap of Tin (~0.08 eV).

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&ndash;180&thinsp;nm) GeSn film-coated aluminum foils display a wide color spectra with an absorption wavelength ranging from 400&ndash;1800&thinsp;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&thinsp;nm, which induced an additional light absorption depression around 13.89&thinsp;&mu;m wavelength. These findings are promising for practical flexible photovoltaic and photodetector applications ranging from the visible to near-infrared wavelengths.

No MeSH data available.