First-principles Calculations of ZrO2/Silicene/ZrO2 Sandwich Structures
|關鍵字:||矽烯;第一原理;能隙;三明治結構;Silicene;first principles;GWA;energy gap;sandwich structure|
As the state-of-the-art technology in semiconductor industry drives devices toward remarkably tiny dimension, the traditional Si-based MOSFETs will soon approach their scaling limits. An emerging development in this field is to find a replacement for the Si channel. A two-dimensional material consists of only a single atomic layer, being a good candidate of the MOSFET channel upon the ongoing scaling down. A two-dimensional silicon allotrope, silicene has its great advantage in being significantly compatible with the Si-based process. Silicene has a mobility two order of magnitude higher than bulk semiconductors and is expected to maintain a satisfactory device on-current. Yet there are also two major challenges to make it a feasible device. A stress-free stand-alone silicene, like graphene, has a gapless Dirac-cone band dispersion and needs additional treatments to open its gap. Besides, the silicene π bonds are chemically active, making it unstable in the air. In this thesis, I perform first-principles calculations to look for the strain-induced energy gap of silicene and how its electronic structures, especially the mobility, are affected when being sandwiched between dielectric materials. I find the energy gap is opened up to 0.17eV among the strains I have applied. We also calculate the electron and hole mobility of silicene under a particular strain to be 16.6 and 14.4m2V-1s-1, respectively, preserving the same order of magnitude of the free-standing mobility. I choose ZrO2/silicene/ZrO2 to be my model sandwich structure, where ZrO2 is a widely used high-k material in conventional semiconductor devices. I have considered both unpassivated and H-passivated interfaces of the above sandwich structure, and find that the former has no Dirac cone while the latter contains a Dirac cone and have indirect gap closing. I further apply a strain to the silicene in the H-passivated case such that the silicene restores its stand-alone symmetry, and find that the gap is re-opened. The symmetry-breaking mechanism of the unstrained (relaxed) silicene in a H-passivated sandwich is very likely due to the electrostatic charge that is transferred from ZrO2 to silicene, as calculated by the Bader analysis. In summary, my first-principles calculations show that silicene can have strain-induced gaps, and the ZrO2/silicene/ZrO2 sandwich structure needs both the H passivation (preserving the π bonds) and the silicene-symmetry restoration to open the gap. The mobility is also calculated from first principles and are found to maintain a high value in the order 10m2V-1s-1. These results provide preliminary guidance to develop silicene-based transistors.
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