This is an observation that has been made from first priciples studies and i have the same observation from my TCAD modeling results.
This is because for low thicknesses bulk mass assumption is no longer valid. For example you can not use bulk mass for germanium channel lower than 8nm. You have to consider the relation between channel thickness and effective mass. Quantum confinement also modifies charge carrier distribution.
Confinement in nanowires leads to quantum size effects, causing the energy levels to become quantized. As the diameter of the nanowire decreases, the available energy states become quantized, leading to an increase in the bandgap. This is because the confinement restricts the motion of electrons and holes, causing their energy levels to become discrete. The increase in bandgap is a result of the quantization of energy levels, which in turn affects the electronic properties and optical characteristics of nanowires.
Confinement in nanowires causes quantum size effects, leading to changes in the electronic structure. As the diameter decreases, the spatial confinement of electrons increases, resulting in quantization of energy levels. This leads to an increase in the bandgap due to reduced overlap of electron wavefunctions. The confinement-induced increase in bandgap is a result of the quantum mechanical confinement of charge carriers within the nanowire structure.
This effect is also known from quantum wells. In the following paper you can find an example for silicon wells bounded by silicon dioxide: Article Electronic band gap of Si/SiO quantum wells: Comparison of a...
The most simple explanation is to look at the eigenvalues of a "particle in a box" system in dependence on the size of the box: The smaller the box, the higher in energy the lowest level.
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