Electronic and Optical Properties of Short-Period Si/Sige Superlattices: Effects of Interfacial Atomic-Scale Roughness

SiGe/Si superlattice (SL) are currently a central building block of nanosheet transistors proposed for the 7 nm technology nodes and beyond [1], where elective wet-etching of the SiGe layers has been used to release the Si layers and form a vertically stacked channels architecture [2]. Consequently, the interfacial abruptness and uniformity in heterostructures are critical to control their electronic and optical properties. Recently, we demonstrated a 3-D atomistic-level mapping of the roughness and uniformity of buried epitaxial interfaces in Si/SiGe SLs with a layer thickness in the 1.5-7.5 nm range [3]. This direct quantification of the abruptness of buried interfaces enabled a direct evaluation of interfacial effects on electronic and optical properties of epitaxial heterostructures. For instance, spectroscopic ellipsometry indicated the first observation of a new superlattice-related optical transition between 2.2 and 2.7 eV. However, the interpretation of this new transition can only be achieved using a theoretical framework considering atomic-level details of the interfacial roughness. To that end, we have been carrying out theoretical investigations to build a correct quantum mechanical model that incorporates the effect of the interface, directly measured from atom probe tomography (APT), to interpret the possible SL-related optical transition. Thus, an 8-band k⋅p formalism was developed and validated for group IV semiconductors [4], where a quantum mechanical incorporation of the interface width is highly coveted because the microscopic interface asymmetry (MIA) effect can greatly influence the electronic and optical properties of short-period SLs, induce strong interactions between different SL subbands, and enhance the absorption strength considerably [5]. It has been shown that the nature of the SL interfaces can have a significative impact on the optical confinement properties of other group IV heterostructures [6]. To test this model, experimental optical characterization of four different SLs (the mean Ge concentration of the layers within the SLs is in the 25 to 30 at. % range and is the periodicity of the SLs) will be presented and discussed. Then, by simulating the SL optical properties with the developed 8-band k⋅p method, the effect of the interface will be investigated to try and explain the observed spectroscopic transition. [1] G. Hellings, et al, in 2018 IEEE Symp. VLSI Technol. (IEEE, 2018), pp. 85–86. [2] K. Komori, et al, Solid State Phenom. 282, 107 (2018). [3] S. Mukherjee, et al, ArXiv: Cond-Mat 1908.00874, 1 (2019). [4] T. B. Bahder, Phys. Rev. B 41, 11992 (1990). [5] H. M. Dong, et al, Thin Solid Films 589, 388 (2015). [6] F. Szmulowicz, et al, Phys. Rev. B - Condens. Matter Mater. Phys. 69, 155321 (2004).