“Strained silicon” technology is a technique where a silicon thin film is deposited onto a silicon-germanium (SiGe) film. Both Si and SiGe have the same crystal structure, but the lattice parameter of SiGe is larger than that of pure silicon. Therefore, when a film of silicon is deposited on SiGe, the bonds in silicon are stretched apart (lattice parameters increase) and bonds in SiGe are compressed together (lattice parameters decrease). Based on what you know about how electronic bands form, how do you think the band gap of SiGe is changed when SiGe is deposited onto a silicon film compared to a bulk piece of SiGe? How do you think that the electrical conductivity of a compressed SiGe film compares to that of bulk SiGe? Please explain your reasoning.
To understand how the band gap of SiGe is changed when deposited onto a silicon film compared to bulk SiGe, we need to look at the effects of strain on electronic bands. Strain refers to the deformation or change in the lattice structure of a material. In this case, the strain arises from the difference in lattice parameters between Si and SiGe.
When a film of silicon is deposited onto SiGe, the silicon bonds get stretched due to the larger lattice parameter of SiGe. This stretching leads to changes in the energy levels of the electronic bands and can impact the band gap of SiGe.
The band gap of a material refers to the energy difference between the valence band (VB) and the conduction band (CB). In bulk SiGe, the band gap is determined by the properties of the SiGe crystal, and the lattice structure remains uniform throughout.
When a silicon film is deposited on SiGe, the stretching of silicon bonds increases their energy. This increase in energy raises the valence band of SiGe. As a result, the energy difference between the valence band and the conduction band decreases, reducing the band gap compared to bulk SiGe. In other words, the band gap of SiGe is smaller when SiGe is deposited onto a silicon film.
Regarding the electrical conductivity of a compressed SiGe film compared to bulk SiGe, we need to consider the impact of strain on carrier mobility. Carrier mobility refers to the ability of charge carriers (electrons or holes) to move through a material.
When SiGe is compressed by the deposition of a silicon film, the bonds in SiGe are brought closer together. As a result, the lattice is more ordered and less prone to scattering, leading to increased carrier mobility. This higher mobility enhances the electrical conductivity of the compressed SiGe film compared to bulk SiGe.
To summarize, when SiGe is deposited onto a silicon film, the band gap of SiGe decreases due to the stretching of silicon bonds. Simultaneously, the electrical conductivity of the compressed SiGe film increases because the compression of SiGe improves carrier mobility.