Terahertz Intersubband Electroluminescence From Nonpolar M-Plane Zno Quantum Cascade Structures

The ZnO-based heterostructures are predicted to be promising candidates for optoelectronic devices in the infrared and terahertz (THz) spectral domains owing to their intrinsic material properties. Specifically, the large ZnO LO-phonon energy reduces the thermally activated LO-phonon scattering, which is predicted to greatly improve the temperature performance of THz quantum cascade lasers. However, to date, no experimental observation of intersubband emission from ZnO optoelectronic devices has been reported. Here, we report the observation of THz intersubband electroluminescence from ZnO/MgxZn1-xO quantum cascade structures grown on a nonpolar m-plane ZnO substrate up to room temperature. The electroluminescence peak shows a line width of similar to 20 meV at a center frequency of similar to 8.5 THz at 110 K, which is not accessible for GaAs-based quantum cascade structures because of the reststrahlen band absorption from 8 to 9 THz. This result is an important step toward the realization of ZnO-based THz quantum cascade lasers.