Far-infrared radiation from $n$-InGaAs/GaAs quantum-well heterostructures in high lateral electric fields under injection conditions

The infrared radiation emitted by hot electrons in $n$-InGaAs/GaAs quantum-well heterostructures subjected to a lateral electric field is investigated under conditions of carrier injection from the current contacts. In structures with double tunneling-coupled wells one of which is $\delta$-doped, a pronounced increase in the intensity of far-infrared radiation upon the onset of carrier injection is observed. At the same time, this effect is lacking in single-quantum-well structures with doped wells or barriers. The observed increase in the radiation intensity is associated with the direct intersubband transitions of electrons which contribute to emission upon the real-space transfer of charge carriers between wells. The intensity of these transitions increases due to compensation of the space charge existing between the wells by the injected holes.