Electron-Hole Liquids in Semiconductors

Abstract

In this paper the energetics of the formation of electron-hole metallic liquids in semiconductors is examined. The ground-state energies of electron-hole metals are calculated using Hubbard's approximate treatment of the electron gas for the following cases: (a) germanium, (b) germanium with a large (111) strain, (c) silicon, and (d) GaAs. The simple case of a single isotropic maximum for the valence band and a single minimum for the conduction band is also treated. It is shown that for both Si and Ge, the binding energy of the metallic state relative to free excitons is 5.7 and 1.7 meV, respectively. These values and the values of the equilibrium density are in good agreement with experiment. In the isotropic model the metallic state is not bound while for GaAs and strained Ge the metallic-state energy per electron is essentially equal to that for a gas of free excitons. The low-density limit of the isotropic band model is examined and the ground state for this system is predicted to be a dilute gas of molecules. It is argued that the forces between molecules are repulsive and will cause this state to break up at relatively low densities. If the density is increased, the system will undergo a first-order transition to the metallic state. The relevance of these calculations to the metal-insulator transition problem is discussed. It is pointed out that the fact that anisotropic and many-valleyed bands favor the metallic state means that the metal-insulator transition must ultimately be first order.