Lateral subband transitions in the luminescence spectra of a one-dimensional electron-hole plasma in ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/InP quantum wires

${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/InP quantum wires with widths down to about 10 nm have been investigated by high-excitation luminescence spectroscopy. The one-dimensional electron-hole plasma spectra of quantum wires with widths between 50 and 30 nm exhibit shoulders on the high-energy edge of the emission. With decreasing quantum-wire widths the shoulders shift consistently to higher energy. The low- and high-energy features are identified by model calculations with the first, second, and third lateral-subband transitions in the structures. A simple model based on a square-well confinement by the semiconductor vacuum interface and the measured widths of the structures is in quantitative agreement with the experimental data.