Fabrication technology of SiGe hetero-structures and their properties

SiGe hetero-structures have a high potential to improve the state-of-art Si devices particularly VLSIs and add such new functions as optics and also provide a new scientific field of materials growth and characterization. This comes from their unique properties of hetero-structures where the strain modifies the band structures. The band modification brings the increase of the mobility both of electrons and holes and the hetero-structures such as quantum wells and dots make it possible to realize light emitting devices even with indirect band-gap materials. After the success of development of hetero-structure bipolar transistors (HBTs), many people are now engaged in the research and development of FET-type devices that have much wider applications. Optical devices including light emitters and micro-cavities will provide a new possibility to enhance the performances and functions of Si VLSIs by realizing optical interconnection and opto-electronic ICs. It is obvious that these fascinating applications largely depend on the material growth, particularly control of surface reaction and formation of dislocations and surface roughness that strongly affect device performances. Here we review the fabrication technology of SiGe hetero-structures aiming at growth of high quality materials. The relaxation of strain of SiGe buffer layers grown on Si substrates is discussed in details, since many devices are formed on the strain-relaxed buffer layers that are sometimes called as 'virtual substrates'. Carbon incorporation and dot formation that are now studied to extend the possibility of SiGe are discussed in this article too. Some device applications are finally reviewed to show their high potential in the real world.