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.

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Supervisor

Turunen, Markus

Thesis advisor

Liu, Xiaolong

Keywords

laser texturing, germanium, absorptance, spectrophotometry

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https://urn.fi/URN:NBN:fi:aalto-202406044019

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[kand] Sähkötekniikan korkeakoulu / ELEC