Nonpolar m-plane GaN/AlGaN heterostructures with intersubband transitions in the 5-10 THz band

Dominant Influence of Interface Roughness Scattering on the Performance of GaN Terahertz Quantum Cascade Lasers

Effect of interface roughness of quantum wells, non-intentional doping, and alloy disorder on performance of GaN-based terahertz quantum cascade lasers (QCL) has been investigated by the formalism of nonequilibrium Green's functions. It was found that influence of alloy disorder on optical gain is negligible and non-intentional doping should stay below a reasonable concentration of 10¹⁷ cm^-3 in order to prevent electron-impurities scattering degradation and free carrier absorption. More importantly, interface roughness scattering is found the dominating factor in optical gain degradation. Therefore, its precise control during the fabrication is critical. Finally, a gain of 60 cm^-1 can be obtained at 300 K, showing the possibility of fabricating room temperature GaN Terahertz QCL.

The Peculiarities of Strain Relaxation in GaN/AlN Superlattices Grown on Vicinal GaN (0001) Substrate: Comparative XRD and AFM Study

Superlattices (SLs) consisting of symmetric layers of GaN and AlN have been investigated. Detailed X-ray diffraction and reflectivity measurements demonstrate that the relaxation of built-up strain in the films generally increases with an increasing number of repetitions; however, an apparent relaxation for subcritical thickness SLs is explained through the accumulation of Nagai tilt at each interface of the SL. Additional atomic force microscopy measurements reveal surface pit densities which appear to correlate with the amount of residual strain in the films along with the appearance of cracks for SLs which have exceeded the critical thickness for plastic relaxation. These results indicate a total SL thickness beyond which growth may be limited for the formation of high-quality coherent crystal structures; however, they may indicate a growth window for the reduction of threading dislocations by controlled relaxation of the epilayers.