Excitonic Effects at the Direct Bandgap of Germanium
![Experimental and standard-theoretical indirect absorption in Ge [J. Menéndez et al., PRB 101, 195294 (2020)], showing the contribution from different phonons. The absorption increases resonantly as the energy approaches the direct band gap at 0.80 eV. The theoretical expression diverges at this energy. At the same time, measurements become very difficult because samples with a thickness of a few micrometers are necessary.](https://imagedelivery.net/qXYabcYg0JPsn0bBVWE4SA/cfb38832-4ef7-4949-8bc9-17b3d0650d45/menendez_2.png/raw)
Indirect gap light absorption in semiconductors is mediated by direct gap transitions. The energy difference between the incident photon and the direct gap appears in the denominator of the theoretical expressions for the indirect gap absorption strength. In quasi-direct semiconductors, the direct band gap is only slightly above the indirect gap, and the denominator approaches zero. Examples of quasi-direct semiconductors include germanium and many 2D materials.
The divergence of the indirect absorption (see figure) leads to the paradoxical conclusion that in the immediate vicinity of the direct gap, indirect absorption could become stronger than direct absorption. Eliminating this inconsistency requires a new theory of indirect absorption, which is one of the primary goals of this DMREF project.
We started the experimental part of the project by attempting to quantify the accuracy of the direct gap expressions for predicting the dielectric function of Ge near its direct gap. This work was done in collaboration with the group of Stefan Zollner at NMSU.
