SiGe and Ge Epitaxy on Si

Silicon and germanium can be mixed in any ratio, allowing us to tailor their electronic properties by adapting the mixing ratio. Despite having the same crystal structure as bulk materials, germanium's lattice constant is 4.2% larger than silicon's. This difference of two structurally identical lattices leads to the fact that, as a layer grows on a substrate, it must adapt to the substrate's lattice, causing strain with repect to its bulk structure. This growing layer, which replicates the lattice planes parallel to the substrate surface (while different bond lengths occur perpendicular to it, see figure on the right), is referred to as pseudomorphic growth. In addition to affecting the solid solution effect, this internal layer stress also influences the material's properties, which are crucial for electronic applications. In recent years, various devices utilizing silicon-germanium layer systems have been successfully developed, including heterobipolar transistors, MODFETs, tunnel diodes and -transistors, hetero-MOSFETs, waveguides, IR detectors, and photodetectors.

Bipolar transistors typically operate in a vertical configuration, meaning that the thickness of the individual active layers plays a crucial role in determining their electronic properties. This is different from the more commonly known field-effect transistors, where the electronic properties are largely determined by lateral dimensions. One notable advancement in bipolar transistor technology is the heterobipolar transistor (HBT). In HBTs, the base layer is crafted using a heteroepitaxial SiGe layer that is less than 25 nanometers thick and possesses a smaller energy gap than the emitter and collector materials. As a result, SiGe HBTs have achieved impressive frequencies exceeding 700 gigahertz, with the successful realization of circuits operating at 200 gigahertz as a notable milestone.

The base layer of an HBT is thin enough for the epitaxial SiGe layer to remain pseudomorphic. When SiGe layers become thicker, they can relax their strain and approach their bulk structure. Consequently, they serve as "virtual substrates," allowing the growth of further heteroepitaxial layers under renewed strain, as illustrated in the figure on the right. For achieving optimal high mobility channels, strained pure layers of either silicon or germanium are optimal. A notable example is the use of strained silicon layers on virtual SiGe substrates. For strained silicon, a remarkable increase in charge carrier mobilities, exceeding a factor of three, has already been demonstrated . Germanium exhibits significantly higher charge carrier mobility than silicon, a characteristic that has gained renewed attention in research and development efforts in recent times.