Intrinsic ferromagnetism in the high-quality wurtzite ${\mathrm{Ga}}_{0.937}{\mathrm{Mn}}_{0.063}\mathrm{N}$ semiconductor is unambiguously demonstrated by both macroscopic magnetization measurements and x-ray magnetic circular dichroism. The structural quality of the samples grown by plasma-assisted molecular beam epitaxy is confirmed by x-ray diffraction and x-ray linear dichroism. The Curie temperature of a (Ga,Mn)N sample with 6.3% Mn is $\approx 8\mathrm{K}$ with a spontaneous magnetic moment of $2.4{\mu }_B$ per Mn at $2\mathrm{K}$.

We demonstrate terahertz intersubband absorptions at frequencies of 2.1 THz (λ≈143 μm) and 4.2 THz (λ≈70 μm) in nitride-based semiconductor quantum wells. The structures consist of a 3 nm thick GaN well, an Al0.05Ga0.95N step barrier, and a 3 nm thick Al0.1Ga0.9N barrier. The absorption is detected at 4.7 K. The structure design has been optimized to approach a flat-band potential in the wells to allow for an intersubband absorption in the terahertz frequency range and to maximize the optical dipole moments.

Thick layers of GaN on AlN, AlN on GaN, and InN on GaN were grown by plasma-assisted molecular beam epitaxy. Their plastic strain relaxation was studied by reflection high-energy electron diffraction (RHEED) and high resolution x-ray diffraction (HRXRD). The results are consistent with a mechanism of progressive introduction of misfit dislocations based on the coalescence of dynamically formed platelets. Due to the lack of proper gliding planes in the wurtzite structure, such dislocations are not mobile, leading to inhomogeneity of the strain state along the growth axis. The agreement between in situ RHEED and ex situ HRXRD measurements provides evidence that the strain state is frozen in during growth.