Experimental determination of equilibrium constants for excitonic systems in stressed, ultrapure silicon

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Abstract
A basic question regarding excitonic states in semiconductors is whether the relative numbers of these particles, in spite of their finite lifetime, can be described by equilibrium thermodynamics. To answer this question we have made absolute measurements of the equilibrium constants for several excitonic systems in silicon by studying the recombination luminescence. This was accomplished by using a strain-confinement technique to determine the particle densities and by measuring the lifetimes. We find that the small mobile particles—free carriers, free excitons, and excitonic molecules—behave as an ideal gas of particles with classical equilibrium constants. The bound excitonic complexes, on the other hand, do not exhibit a classical equilibrium with free excitons in our experiments. In addition, we find that the phase transition from excitonic gas to electron-hole liquid displays a classical saturated gas density over a wide range of densities.