Atomic-level Interfacial Broadening in Ultra-Short Superlattices

Artificially fabricated superlattices (SL) offers the possibility to render optically inactive materials into strong light emitters and absorbers by melding two indirect-gap materials into one strongly dipole-allowed direct-gap material. The artificial periodicity in these low-dimensional systems provides an additional degree of freedom to engineer their band structure and thus improve their electronic and optical properties. Using SiGe/Si SLs as a model system, recent experiments indicated that the interfacial abruptness and uniformity are of key importance to control the electronic and optical properties. Herein, based on a recent method to map in 3-D the roughness and uniformity of buried epitaxial interfaces in Si/SiGe SLs with a layer thickness in the 1.5-7.5 nm range  [1], we address the optical properties of these SLs using room temperature spectroscopic ellipsometry. Our systematic studies revealed a new SL-related optical transition between 2.1 and 2.9 eV. A theoretical framework has been developed based on the 14-band k·p formalism, where the interfacial roughness specificities were included, to explain the origin of the transition. Additionally, the k·p Hamiltonian was modified to consider the effect of microscopic interface asymmetry (MIA). To validate the developed framework, experimental optical characterization of four different Si_m/(Si_1-xGe_x)_m  SLs (the mean Ge concentration of the Si_1-xGe_x  layers within the SLs is in the ~25 to ~30 at. % range and m is the periodicity of the SLs) will be presented and discussed.

References

[1]        S. Mukherjee, et al, ACS Appl. Mater. Interfaces 12, 1728 (2020).