Exciton-plasma crossover with electron-hole density in T-shaped quantum wires studied by the photoluminescence spectrograph method

We investigated the evolution of photoluminescence (PL) spectra with the electron-hole (e-h) pair density in a single T-shaped quantum wire of high quality grown by a cleaved-edge overgrowth method with molecular-beam epitaxy. By using a spectrograph imaging method for the PL measurements, we obtained PL spectra free from carrier migration effects for the one-dimensional (1D) e-h system in the T wire for a wide range of e-h pair densities from 5× 101 to 1.2× 106 cm-1. In the low e-h pair density region below 4× 103 cm-1, PL only from 1D excitons in the wire was observed. At a pair density of 4× 103 cm-1, a new peak characteristic of biexcitons with a binding energy of 2.8 meV appeared below the exciton peak. At a pair density of 1× 105 cm-1, the biexciton peak started broadening without a peak energy shift and completely changed to an e-h plasma at 6× 105 cm-1. The transition from the dilute exciton gas to the e-h plasma is, therefore, a gradual crossover via biexcitons, which indicates the importance of biexcitonic effects in 1D e-h systems. Moreover, we found that during the continuous exciton-plasma crossover, the band edge of 1D excitons showed no significant energy shift at an e-h pair density as high as 2× 105 cm-1, where the biexciton PL peak had already started broadening in linewidth and changing to the degenerate e-h plasma peak. The level crossing between the band edge and the exciton was not observed experimentally.