Hi,
Does the bend in absorption curve of a semiconductor thin films depicts the presence of both direct and indirect phase transition?
Thankyou
Dear Muhammad,
welcome
The absorption coefficient of the a semiconductor depends on its energy band structure. In case of indirect band gap the absorption coefficient increases relatively slowly with the the increase of the photon energy till the photon energy crosses the value of the direct energy gap, the absorption coefficient increases relatively rapidly with the further increase in the incident photons. You can get such fact out of the published photo absorption curves for the common semiconductors including direct and indirect bandgap semiconductors. In case of direct band gap semiconductors the absorption curves increase rapidly with the photon enrgy like GsAS.
in case of thin film materials with polycrystalline structure, the energy band structure will change appreciably with the grain size such that the energy gap increases and the material moves towards the direct bandgap energy band structure. This phenomena is observed for silicon and other materials.
From the conceptual point of view if the material is of indirect ban gap then the photo absorption is dominated by indirect band gap transitions until the photon energy becomes equal or greater than the direct band gap the the transitions will be dominated by direct band transition and the absorption coefficient begins to increase rapidly relative to the indirect bandgap case.
Best wishes
Yes, a material can have both direct and indirect band gaps; the related transitions occur at different energies (as already explained by Abdelhalim Zekry).
Abdelhalim Zekry and Jan-Martin Wagner , Thanks for your valuable answers. Can I confirm, whether the material is direct or indirect band gap material from transmission or absorption curves?
Yes, a material can have both direct and indirect band gap. If the absorption coefficient of a material rapidly increase with increase photon energy, the band energy, is called direct band gap energy. Again, if the absorption coefficient of a material slowly increase with increase photon energy, the band gap is called indirect band gap.
Best of luck
Yeah, some semiconductors reveal both indirect and direct band gap. Their observation depends of the experimental conditions (Depostion condition). For the observation of a direct band gap is better very thin samples whereas for indirect gap is better thick samples.
In general, semiconducting materials may have direct or indirect band gap (allowed or forbidden) depending upon their crystal structures which resulting from the preparative conditions. Usually and according to Tauc, its direct if the material is crystalline and indirect if the material is non-crystalline. But the same material may has the both if it has a partial crystalline nature.
Hope, following attched article may help you:
https://pubs.rsc.org/-/content/articlepdf/2016/ee/c6ee03474h
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