Explaining the Unexpected Spikes in a Quantum Well Absorption Spectrum

In summary, The discussion of a semiconductor quantum well being irradiated and an absorption spectrum produced reveals that electrons and holes are being created and promoted across the band gap. This spectrum shows heavy hole and light hole transitions and the threshold energy for each step is given by the equation \hbar\omega = E_g + \frac{\hbar^2n^2\pi^2}{2m_e^*d^2} + \frac{\hbar^2n'^2\pi^2}{2m_h^*d^2}. While following the derivation, it becomes clear why these steps occur at certain threshold energies. However, there is a noticeable difference in the absorption vs. photon energy plot, with a large spike
  • #1
jeebs
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I'm looking in a textbook at a discussion of a semiconductor quantum well which is being irradiated and an absorption spectrum produced. Electrons are being promoted across the band gap and holes are being created. There are heavy hole and light hole transitions, and the spectrum shown rises in steps (corresponding to the energy levels formed from the 1-dimensional confinement characteristic of the quantum well). The threshold energy for each step is given by [tex] \hbar\omega = E_g + \frac{\hbar^2n^2\pi^2}{2m_e^*d^2} + \frac{\hbar^2n'^2\pi^2}{2m_h^*d^2} [/tex] where d is the well width, mh* can be the heavy hole or light hole mass, and n/n' = 0,1,2,3...

Having followed the derivation through I am happy about why these steps at certain threshold energies are predicted and observed. However, I notice that, on the absorption vs. photon energy plot, rather than being a perfectly square step, there will be a large spike at the heavy hole n = n' = 1, followed immediately by a smaller spike for the light hole, then the flat top of the step appears until the next threshold. The next threshold (n=n'=2) has the spikes separated a bit more, and the heavy hole peak is only slightly higher than the light hole peak. I cannot think of a reason for this.

Can anyone explain this behaviour?
 
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  • #2


Are you looking at room temperature or low temperature data?

The step-like absorption spectrum is the prediction for free electrons and holes in a quantum well. At low temperatures the Coulomb interaction between electrons and holes can become strong enough to form excitons, bound hydrogen-like complexes of electrons and holes. Energetically those are located at or to be more precise slightly below the band gap. That causes a peak in the absorption seen near the step edges.

At higher temperatures [tex]k_B T[/tex] becomes large compared to the eciton binding energy and the peaks should vanish.
 

1. What is a quantum well absorption spectrum?

A quantum well absorption spectrum is a graphical representation of the absorption of light by a material with a quantum well structure. It shows the amount of light absorbed at different wavelengths, which can provide valuable information about the electronic and optical properties of the material.

2. What causes unexpected spikes in a quantum well absorption spectrum?

Unexpected spikes in a quantum well absorption spectrum can be caused by a variety of factors, including defects or impurities in the material, changes in the surrounding environment, or interactions between the material and other substances.

3. Why is it important to explain these unexpected spikes?

Understanding the underlying causes of unexpected spikes in a quantum well absorption spectrum is crucial for accurately interpreting the data and making informed conclusions about the material's properties. It can also help identify and address any issues or limitations in the experimental setup.

4. How can scientists explain these unexpected spikes?

Scientists can use a variety of techniques and methods to investigate and explain unexpected spikes in a quantum well absorption spectrum. This can include theoretical calculations, experimental measurements, and comparisons with known materials or models.

5. Are there any practical applications for explaining these unexpected spikes?

Yes, understanding the unexpected spikes in a quantum well absorption spectrum can have important practical applications in fields such as material science, electronics, and photonics. It can also aid in the development of new materials and technologies with improved properties and performance.

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