Bandgap Engineering of 2D Materials and its Electric and Optical Properties

Kumar Vishal, Wright State University


Since their invention in 1958, Integrated Circuits (ICs) have become increasingly more complex, sophisticated, and useful. As a result, they have worked their way into every aspect of our lives, for example: personal electronic devices, wearable electronics, biomedical sensors, autonomous driving cars, military and defense applications, and artificial intelligence, to name some areas of applications. These examples represent both collectively, and sometimes individually, multi-trillion-dollar markets. However, further development of ICs has been predicted to encounter a performance bottleneck as the mainstream silicon industry, approaches its physical limits. The state-of-the-art of today’s ICs technology will be soon below 3nm. At such a scale, the short channel effect and power consumption become the dominant factors impeding further development. To tackle the challenge, projected by the ITRS (International Technology Roadmap for Semiconductors) a thinner channel layer seems to be the most viable solution. This dissertation will discuss the feasibility of using 2D (two-dimensional) materials as the channel layer. The success of this work will lead to revolutionary breakthroughs by pushing silicon technology to the extreme physical limit. Starting from graphene in 2004, 2D materials have received a lot of attention associated with their distinct optical, electrical, magnetic, thermal, and mechanical properties. In the year 2010, IBM demonstrated a graphene-based field effect transistor with a cut-off frequency above 100 GHz. The major challenge of applying graphene in large-scale digital circuits is its lack of energy bandgap. Other than carbon, a variety of graphene-like 2D materials have been found in various material systems, like silicene, germanene, phosphorene, MoS2, WS2, MoSe2, HfS2, HfSe2, GaS, and InS, etc. Among all the 2D materials, silicene appears to be the most favored option due to its excellent compatibility with standard silicon technology. Similar to its counterpart graphene, silicene does not exhibit an opened energy band gap, which prevents its immediate applications in the mainstream semiconductor industry. In-plane tensile strain leads to the formation of a bandgap in bilayer silicene with low buckling. This dissertation conducts a study on energy bandgap opening in silicene and ultra-thin silicon films by engineering the strain and stress. Density functional theory has been employed to optimize the atomic structures and to calculate the energy band diagrams. Bandgap opening up to 0.17 eV has been found in the AA-stacked bilayer silicon film under in-plane strain 10.7%~15.4%. A number of common semiconductor substrates have been investigated to examine the possibility of growing bilayer silicon with excessive in-plane strain i.e., CdSe, InAs, GaSb, and AlSb. In addition, by taking into the account of magnetic spin polarization, energy bandgap 0.25 eV in AB-stacked bilayer silicon films have been observed. Without applying strain, the spin up and down state are degenerated. By applying the strain 7.56 %, the spin degeneration has been removed both in the conduction band and valence band. This opens the opportunity to build spin-based field effect transistor.