Optical spectroscopy of two-dimensional electrons in $\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}-{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ single heterojunctions

The radiative recombination of two-dimensional electrons with nonequilibrium photoexcited holes in the magnetic field has been investigated in $\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}-{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ single heterojunctions for the first time. Two different channels of radiative recombination are observed—a recombination with free holes and with the holes bound to acceptor atoms. In a magnetic field perpendicular to the two-dimensional (2D) layer, both luminescence lines are split into Landau levels, and these splittings depend only on the normal component of the magnetic field. It is shown that when the concentration of two-dimensional electrons is increased, the first excited two-dimensional subband is populated and an additional intense line appears in both components of the spectrum. The high intensity of this line originates from a considerable overlap of corresponding wave functions of 2D electrons and of the holes and enables one to investigate independently the properties of 2D electrons from the first excited subband.