Two-Dimensional Electron-Hole Liquid in Systems of Spatially Direct and Indirect Excitons in Si/SiGe Heterostructures

Abstract

The two-dimensional electron-hole liquid (EHL) in 2- and 4-nm-thick SiGe layers of Si/Si0.91Ge0.09/Si heterostructures is discovered and its properties are studied by photoluminescence (PL) spectroscopy in the near-infrared and visible spectral ranges at low temperatures. It is shown that the PL in the visible range observed at high excitation levels originates from two-electron recombination transitions in the EHL. For the SiGe layer thickness d = 2 nm, the barrier formed by this layer for electrons in the conduction band is tunnel-transparent, and the EHL is spatially direct. For d = 4 nm, this barrier is nontransparent, and the EHL has dipolar character, with holes being confined in the SiGe layer and electrons occupying Si layers. It is found that the binding energy and the critical temperature of the dipolar EHL is substantially less than the spatially direct. The binding energy of free biexcitons in the tunnel-transparent SiGe layer is determined.