People usually ask direct and indirect band difference. Most of the answers are in terms of election transition. Here my question is different in the sense that OPTICAL direct and indirect band gaps?
Characteristic of semiconducting and insulating materials is a band gap between valence and conduction bands. The band gap can be classified as direct or indirect: a band gap is said to be ``direct'' when the energy minimum (the bottom) of the conduction band lies directly above the energy maximum (the top) of the valence band in reciprocal k-space, otherwise, we call it an ``indirect'' band gap.The crystal orbitals at the top of the valence band and at the bottom of the conduction band have the same wave vector in a direct band gap solid, but different wave vectors in an indirect band gap material.
You can get details in this link:
http://www.doitpoms.ac.uk/tlplib/semiconductors/direct.php
I am not sure if I understand your question correctly, so please let me know if I am answering a question you did not ask. I assume you know how different bands originate in solids and what a band structure is.
If you have a look at the band structure of a semiconductor, the separation of the minimum of the energetically lowest conduction band and the maximum of the highest valence band is the optical band gap. If these two extrema occur at the same momentum, then you have a direct band gap, such as GaAs. If they appear at different momenta, you'll have an indirect band gap such as Ge (have a look at the link I provided with a slide show of the band structure of different semiconductors). Before carriers recombine radiatively, they usually scatter to these points. Considering optical transitions, one difference is that you have to provide the missing momentum (e.g. via phonons) if you want them to radiatively recombine in an indirect semiconductor. This is not very efficient which is the reason you cannot (or at least should not) build lasers using indirect semiconductors. I recommend you to read the Wikipedia article about band structures (https://en.wikipedia.org/wiki/Electronic_band_structure). And I also found a post on this page that digs a little deeper into the difference between optical and electrical band gaps (https://www.researchgate.net/post/What_is_the_basic_difference_between_optical_band_gap_and_electrical_band_gap).
https://en.wikipedia.org/wiki/Electronic_band_structure#/media/File:Bulkbandstructure.gif
Hello ,
This book is useful for you , see the link :
http://people.phys.ethz.ch/~mesoqc/lectures/optical_properties.pdf
Hello ,
This book is useful for you , see the link :
http://people.phys.ethz.ch/~mesoqc/lectures/optical_properties.pdf
Dear Dr. venkatesha babu kr rathnaiah setty
Regards
Direct and indirect band gaps:
In semiconductor physics, the band gap of a semiconductor is always one of two types, a direct band gap or an indirect band gap.
The minimal-energy state in the conduction band and the maximal-energy state in the valence band are each characterized by a certain crystal momentum (k-vector) in the Brillouin zone. If the k-vectors are the same, it is called a "direct gap".
If they are different, it is called an "indirect gap".
The band gap is called "direct" if the momentum of electrons and holes is the same in both the conduction band and the valence band; an electron can directly emit a photon. In an "indirect" gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice.
From the free encyclopedia
A direct band-gap (DBG) semiconductor is one in which the maximum energy level of the valence band aligns with the minimum energy level of the conduction band with respect to momentum.
In a DBG semiconductor, a direct recombination takes place with the release of the energy equal to the energy difference between the recombining particles.
The probability of a radiative recombination is high.
The efficiency factor of a DBG semiconductor is higher. Thus, DBG semiconductors are always preferred over IBG for making optical sources.
Example, Gallium Arsenide (GaAs).
An Indirect band-gap (IBG) semiconductor is one in which the maximum energy level of the valence band and the minimum energy level of the conduction band are misaligned with respect to momentum.
In case of a IBG semiconductor, due to a relative difference in the momentum, first, the momentum is conserved by release of energy and only after the both the momenta align themselves, a recombination occurs accompanied with the release of energy.
The probability of a radiative recombination is comparatively low.
The efficiency factor of a IBG semiconductor is lower.
Example, Silicon and Germanium
In direct bandgap semiconductor the electron “rising” from valence band to the conduction band will only change it’s potential (energy).
In indirect bandgap semiconductor the electron “rising” from valence band to the conduction band will change it’s potential (energy) and momentum.
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