The Fine-Structure Constant as a Ruler for the Band-Edge Light Absorption Strength of Bulk and Quantum-Confined Semiconductors.

Abstract

Low-dimensional semiconductors have found numerous applications in optoelectronics. However, a quantitative comparison of the absorption strength of low-dimensional versus bulk semiconductors has remained elusive. Here, we report generality in the band-edge light absorptance of semiconductors, independent of their dimensions. First, we provide atomistic tight-binding calculations that show that the absorptance of semiconductor quantum wells equals <i>m</i>πα (<i>m</i> = 1 or 2 with α as the fine-structure constant), in agreement with reported experimental results. Then, we show experimentally that a monolayer (superlattice) of quantum dots has similar absorptance, suggesting an absorptance quantum of <i>m</i>πα per (confined) exciton diameter. Extending this idea to bulk semiconductors, we experimentally demonstrate that an absorptance quantum equal to <i>m</i>πα per exciton Bohr diameter explains their widely varying absorption coefficients. We thus provided compelling evidence that the absorptance quantum πα per exciton diameter rules the band-edge absorption of all direct semiconductors, regardless of their dimension.