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

Abstract

We investigated the evolution of photoluminescence (PL) spectra with the electron-hole $(e\text{\ensuremath{-}}h)$ 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) $e\text{\ensuremath{-}}h$ system in the T wire for a wide range of $e\text{\ensuremath{-}}h$ pair densities from $5\ifmmode\times\else\texttimes\fi{}{10}^{1}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.2\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. In the low $e\text{\ensuremath{-}}h$ pair density region below $4\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, PL only from 1D excitons in the wire was observed. At a pair density of $4\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, a new peak characteristic of biexcitons with a binding energy of $2.8\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ appeared below the exciton peak. At a pair density of $1\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, the biexciton peak started broadening without a peak energy shift and completely changed to an $e\text{\ensuremath{-}}h$ plasma at $6\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The transition from the dilute exciton gas to the $e\text{\ensuremath{-}}h$ plasma is, therefore, a gradual crossover via biexcitons, which indicates the importance of biexcitonic effects in 1D $e\text{\ensuremath{-}}h$ systems. Moreover, we found that during the continuous exciton-plasma crossover, the band edge of 1D excitons showed no significant energy shift at an $e\text{\ensuremath{-}}h$ pair density as high as $2\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, where the biexciton PL peak had already started broadening in linewidth and changing to the degenerate $e\text{\ensuremath{-}}h$ plasma peak. The level crossing between the band edge and the exciton was not observed experimentally.