Emission spectrum and stability of two types of electron–hole liquid in shallow Si/Si$_{1-x}$Ge$_{x}$Si quantum wells

Based on calculations within the density functional theory and an analysis of low-temperature photoluminescence spectra, the structure of electron–hole liquid in shallow Si/Si$_{1-x}$Ge$_{x}$Si (100) quantum wells 5 nm wide with germanium content $x$ = 3–5.5% is studied. It is shown that the energy of quasi-two-dimensional electron–hole liquid localized in quantum wells for this composition range as a function of carrier concentration exhibits two local minima. The position of the deeper (major) minimum depends on the quantum well design and controls properties of quasi-two-dimensional electron–hole liquid at low temperatures. For the series of Si/Si$_{1-x}$Ge$_{x}$Si quantum wells, modification of properties of electron–hole liquid was experimentally shown, which can be interpreted as a change of the major minimum due to an increases in the germanium concentration in the Si$_{1-x}$Ge$_{x}$Si layer. The effect of the multicomponent composition (electrons, light and heavy holes) of the electron–hole liquid on low-temperature photoluminescence spectra of Si/Si$_{1-x}$Ge$_{x}$Si quantum wells is discussed.