Transmittance, Reflectivity and Band gap

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Discussion Overview

The discussion centers around the relationship between transmittance, reflectivity, and the band gap of materials, particularly focusing on how these optical properties can inform us about the electronic structure of metals and semiconductors. Participants explore the differences in behavior between these two types of materials and seek methods for extracting optical constants from reflectance data.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions how transmittance and reflectance curves can provide insights into the band gap and electronic structure of metals, and how these curves differ for semiconductors.
  • Another participant presents an explanation regarding semiconductor transmission, noting that the presence of a band gap leads to opacity in the high energy range, with a significant change at the onset of absorption.
  • This participant also describes the behavior of metals in relation to transmission and suggests that energy absorption near the surface may contribute to reflection, which complicates the relationship between transmission and reflectivity.
  • A question is raised about calculating the refractive index (n) and extinction coefficient (k) from reflectance curves, with concerns about the accuracy of using the formula for k values.
  • References to solid state physics texts are provided, with differing opinions on the best sources for understanding optical properties.

Areas of Agreement / Disagreement

Participants express differing views on the best resources for studying optical properties, and there is no consensus on the methods for extracting n and k values from reflectance data. The discussion remains unresolved regarding the accuracy of the proposed methods and explanations.

Contextual Notes

Some assumptions about the nature of the material (metallic vs. semiconducting) are not clarified, and there are unresolved mathematical steps in the proposed methods for calculating optical constants. The discussion also highlights the complexity of interpreting optical properties without additional calculations.

Ravian
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I was wondering that how the knowledge o transmittance and reflectance curves can give us information about the band gap and electronic structure of a metal? How these curve differ for semiconductors? I desperately tried to extract n, k values from the reflectance curve of a nitride material about which I have no information whether it is metallic or semiconducting. Is there any way to do that?
 
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No one seems to be interested so I present my own explanation to see if some one agrees or not. consider the case of semiconductor transmission, since there is a band gap (see schematic fig, energy values increase towards right and transmission upwards) so material is fairly opaque in highe energy range but there is abrupt change at the onset of absorption and in IR range it becomes transmitting. Transmission stays flat there because there are only interband transitions. On the other hand for metal, we almost similar behavior by the metal in the high energy range and at the band gap (band gap does not necessarily mean a material is semiconducting until and unless we have several other calculations to support SC state) but in low frequency region we again fall in the transmission. Possible explanation for this fall might that electrons very close to surface absorb EM waves, vibrate and loose energy. This energy can be seen as reflection (which will be less than transmission but still can be significant). Finally not to mention that there is a reasonably good amount of absorption as well. What does this explanation sound like?
Secondly let us say we have reflectance curve instead of transmission then how can I calculate n, k vaules from it? I know the reflectivity formula and I am afraid to use (alpha= 4pi*k/lambda) to find k values. It gives wron answer.
 

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Almost every book on solid state physics should have a chapter on optical properties.
I recommend Ashcroft Mermin, solid state physics. An interesting source is also Landolt Boernstein.
 
Yu and Cardona's text is much better than Ashcroft and Mermin for optical properties materials.

The Landolt-Borstein books are reference tables of properties and not a text per say.
 

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