Electroluminescence from GaAs/AlGaAs Heterostructures in Strong in-Plane Electric Fields: Evidence for k- and Real-Space Charge Transfer

In the Gunn effect, which occurs in certain semiconductors in strong electric fields, electrons are driven out of a low-mass central valley into a heavy-mass side valley in k (momentum) space, ultimately resulting in negative differential resistance (NDR). Recently, there has been interest in the possibility of exploiting this phenomenon in heterostructured semiconductors, as a means to realize novel terahertz sources. Here, we demonstrate that a GaAs/AlGaAs heterostructure planar Gunn diode exhibits both NDR and electroluminescence (EL), the characteristics of which are consistent with the combined action of simultaneous k-space and real-space electron transfer. By making simultaneous electrical-transport and EL measurements, we reveal the following scenario as a function of the applied voltage. At sufficiently large bias, the onset of k-space transfer gives rise to NDR and results in the formation of traveling high-field domains. As each domain travels toward the anode, impact ionization taking place within it results in electron-hole pair generation and concomitant recombination. While some of this recombination takes place within the GaAs, it also occurs within the AlGaAs barrier after electrons undergo real-space tunneling into this layer from the GaAs. The real-space transfer is identified through the presence of a broad spectral peak in the EL consistent with the AlGaAs gap. Both the GaAs and AlGaAs EL peaks increase in intensity superlinearly with increasing bias, reflecting the nonlinear current-voltage characteristic of the device. Furthermore, we map out the spatial distribution of EL intensity in a long-channel device and observe the strongest EL near the anode, consistent with the notion that the high-field domain grows as it propagates from the cathode toward the anode. This planar Gunn diode provides a good platform for high-electric-field effect investigations in semiconductors under low bias and possible applications for terahertz technology.