Electron-Hole Plasma and Liquid

The presence of a gas of free electrons and holes is essential for many opto-electronic devices. Electrons and holes are generated pair-wise in solar cells or photo-diodes. Radiative recombination of electron-hole pairs is the basis of light-emitting diodes or laser diodes. Of importance is here that the electrons and holes have strong interactions which also determine details of the device operation. The interactions are due to the Coulomb interaction of the carriers and their fermionic nature. The results are primarily density-dependent effects like screening, correlation, band-gap renormalization and state filling. The consequences of these effects are manifold and prominently reflected in optical non-linearities like optical gain or optical bistability. But also different phases of the carrier gas like the electron-hole plasma or the electron-hole liquid evolve. We will treat in this chapter the named many-body interactions and how they result in the Mott transition. We discuss the phase diagram of the electron-hole gas and which phases are realized in prominent semiconductors and their nanostructures. Since the electron-hole plasma is characterized by an inverted electronic population optical gain arises which is the basis of applications like the laser diode and random lasers. We detail here the physics of degenerate electron-hole plasmas in various materials and dimensions required for the understanding of such lasers. The experimental technique optical gain spectroscopy which is an excellent tool to study electron-hole plasmas is presented in this chapter.