Measuring Wide Spectral Absorption of Laser Textured Germanium

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Sähkötekniikan korkeakoulu | Bachelor's thesis
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Date
2024-05-07
Department
Major/Subject
Bioinformaatioteknologia
Mcode
ELEC3016
Degree programme
Sähkötekniikan kandidaattiohjelma
Language
en
Pages
37
Series
Abstract
Germanium shows high potential as a new material in optoelectronic applications, which are applications that detect and absorb light. Its affordability and nontoxicity enhance its appeal compared to currently used materials. The materials are textured using a femtosecond laser. By varying the laser power and scan speed the aim is to discover the optimal laser parameters to maximize the absorptance. During laser irradiation, dopants can be introduced on the surface of the material and their effect is studied with different coating thicknesses. Additionally, some samples undergo thermal annealing to test their thermal stability. The irradiation process induces structural changes on the surface of the germanium, resulting in a higher absorption. This new surface texture scatters light in multiple directions, requiring a spectrophotometer with an integrating sphere setup to capture and measure all the scattered light. The device effectively minimizes measurement errors and ensures normalized results. The experimental results revealed that laser processing can increase both aboveand below-bandgap absorption. Furthermore, the coating has a minimal effect on the absorptance if the laser parameters are more intense. Moreover, the absorptance is significantly increased for higher laser power and lower laser scan speed. Such laser treatment shows a remarkable increase in absorptance for longer wavelengths, notably below the bandgap of germanium. While thermal annealing causes a small decrease in absorptance, it has other benefits such as structural stability, useful in optoelectronic applications.
Description
Supervisor
Turunen, Markus
Thesis advisor
Liu, Xiaolong
Keywords
laser texturing, germanium, absorptance, spectrophotometry
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