  Planar light-emitting diodes (LEDs) fabricated within a single high-mobility
quantum well are demonstrated. Our approach leads to a dramatic reduction of
radiative lifetime and junction area with respect to conventional vertical
LEDs, promising very high-frequency device operation. Devices were fabricated
by UV lithography and wet chemical etching starting from p-type
modulation-doped AlGaAs/GaAs heterostructures grown by molecular beam epitaxy.
Electrical and optical measurements from room temperature down to 1.8 K show
high spectral purity and high external efficiency. Time-resolved measurements
yielded extremely short recombination times of the order of 50 ps,
demonstrating the relevance of the present scheme for high-frequency device
applications in the GHz range.
