Reduction of the electron density in GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As single heterojunctions by continuous photoexcitation

Using magnetotransport and magneto-optics we demonstrate that under continuous photoexcitation by laser light the concentration of two-dimensional (2D) electrons in a GaAs-${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As single heterojunction can be strongly decreased as compared to the value which is realized in the dark after illumination. The magnitude of the effect depends on the wavelength and on the intensity of the light as well as on the donor concentration in the ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As. The reduction of the 2D concentration is greatest if the photon energy is higher than the band gap of ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As. The observed decrease of concentration is not accompanied by a reduction of the mobility of the 2D electrons and is important for optical investigations of the fractional quantum Hall effect and Wigner crystallization.