Properties of the strain-confined electron-hole liquid in Ge

Abstract

A large volume of electron-hole liquid is formed by optically exciting a suitably stressed crystal of Ge. A contact stress produces a maximum shear region inside the crystal which acts as an attractive potential well for photoexcited carriers. Properties of the electron-hole liquid confined to this strain well are determined from spectral and spatial measurements of the recombination luminescence under wide variations in stress, temperature, and excitation level. Both electron-hole liquid and free-exciton phases are observed near 4 K, confirming the interpreta ion of a first-order liquid-gas phase transition and giving the exciton condensation energy $\phi \simeq 1$ meV. The liquid pair density at intermediate $〈111〉$ stress is determined to be (0.50±0.05) × ${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$ from a luminescence line-shape analysis which takes into account the reduced electron band degeneracy and the strain-dependent hole mass. Magneto-oscillations in the luminesence intensity are observed which yield a similar density. A tenfold enhancement of the liquid lifetime is observed for stresses above 5 kgf/${\mathrm{mm}}^2$ and $1.8\le T\le 4.2$ K, consistent with the reduced pair density and inhibited liquid evaporation in the strain well. Compression of the liquid at high excitation level is reflected in the line-shape, lifetime, and spatial imaging measurements. Time-resolved imaging of the liquid luminescence provides a striking contrast between the strain-confined liquid and the usual cloud of droplets in unstressed Ge.