In Ge and Si, and also in Ge-Si alloys (74), there is extensive evidence for the stable binding of electrons and holes into a cold plasma of constant density, which undergoes a phase separation. Liquid metallic drops 1 to 300 microm in size are formed, with lifetimes ranging from 0.1 to 600 microsec. For Ge a surprising amount is known: the phase diagram, the surface energy, the work function, the decay kinetics. Much less is known for Si. There is good agreement between theoretical and experimental values of the liquid density, the critical density, the critical temperature, and the binding energy. The stability of the liquid phase is strikingly dependent on band structure. The multivalley structure and mass anisotropy of Si, Ge, and Ge-Si, together with their indirect band gap, are no doubt responsible for the observed stability in these crystals. In the similar semiconductor gallium phosphide, drops have not yet been observed, most likely because the high impurity content traps the excitons. In gallium arsenide the existence of drops is controversial (75). Undoubtedly drops will be found to exist in other semiconductors, perhaps at even higher temperatures. This is an exciting field for the experimentalist; new phenomena are being rapidly discovered, usually before they are predicted. For the theorist, the electron-hole drop is of high intrinsic interest. It represents the first example of a quantum liquid of constant density in a periodic crystal lattice. A number of challenging experimental and theoretical problems remain.

https://science.sciencemag.org/content/sci/189/4207/955.full.pdf

Electron-Hole Condensation in Semiconductors: Electrons and holes condense into freely moving liquid metallic droplets, a plasma phase with novel properties

Publisher Summary The chapter presents a discussion on the experimental aspects of the electron-hole liquid in semiconductors The chapter presents an analysis on the experimental aspects of the condensation of e-h droplets and their properties The chapter presents a discussion on the macroscopic properties of the condensate, its “external” properties The formation of drops, the form they take (for example, their size, their spatial distribution, and their concentration), the surface properties of drops, and finally, the motion of drops from diffusion and from transport because of external force fields are discussed The chapter discusses the decay properties of e-h drops as determined from pulsed kinetic experiments The decay modes and lifetimes of drops and, surprisingly enough, the condensate “work function” ϕ derived by a thermodynamic approach are also discussed The chapter discusses the nature of the condensate and inquires about the characteristic physical parameters of the condensate, that is, the chemical potential, binding energy, Fermi energies, and the e-h pair density Mainly, the spectroscopic method is used in these investigations Examination of a large class of experiments in which the condensate has been strongly perturbed by uniaxial stress, large magnetic fields, or the effects of impurities or alloying Ge with Si is presented The effects of finite temperature on the e–h condensate are presented A possible Mott transition in the exciton gas and its consequences as regards the gas-liquid phase diagram is discussed

The Electron-Hole Liquid in Semiconductors: Experimental Aspects

Spectroscopic determination of condensation energy and density of electron-hole droplets in pure Ge

Resonance techniques at 337 mu m (890 GHz) are used to study the performance of modified H guide as a low-loss waveguide and of open resonators using spherical mirrors for media characteristic measurement.

Waveguide and Open-Resonator Techniques for Submillimeter Waves

The far infrared absorption spectra of bound excitons in silicon

Spectra of Photoexcitation of Free Excitons by Submillimeter Radiation in "Ultrapure" Germanium

The results are presented of the detailed studies on the exciton absorption spectra in ultrapure Ge in the spectrum regions 0.67-1.1 and 1.7-3.7 meV with a backward-wave tube radiation. It is shown that the measured spectrum corresponds to the exciton's transitions from the ground to excited states. As a result of temperature measurements at 1.6-4.2 K the phase transition of excitons into the condensed state has been observed without the formation of excitonic molecules.

Submillimeter Absorption Spectra amd Phase Transition of Indirect Excitons in Germanium

The results are presented of the studies on the exciton and electron-hole drops absorption spectra in ultrapure Ge in the spectral region 0.67–1.1 and 1.7–3.7 mev using backwardwave tube radiation and in the region 1.8–20 mev using diffraction spectrometer under temperatures 1.6–4.2°K.

V. A. Zayats

Papers

Spectra of Photoexcitation of Free Excitons by Submillimeter Radiation in "Ultrapure" Germanium

Submillimeter Absorption Spectra amd Phase Transition of Indirect Excitons in Germanium

Excitation Spectra and Phase Transition of Excitons in Ultrapure Germanium