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Using infrared transmission and Raman spectroscopy, we have studied the optical phonon modes of GaN layers grown on GaAs(001) substrates by molecular beam epitaxy. The crystal structure of the GaN layers ranges from predominantly wurtzite to predominantly zincblende depending on the growth conditions. The transverse and longitudinal optical phonons in cubic GaN are found to be at 552 and 739 cm−1, respectively. These frequencies are slightly shifted with respect to the corresponding A1 and E1 phonon modes in hexagonal GaN. The frequency shifts, together with Raman scattering selection rules, can be used for identifying the phase composition of GaN. A more distinct fingerprint of the hexagonal phase is provided by the occurrence of the E2 phonon modes that are spectrally separated from optical phonon modes in the cubic phase and thus uniquely related to the hexagonal phase.

We report on the development of a compact, easy-to-use terahertz radiation source, which combines a quantum-cascade laser (QCL) operating at 3.1 THz with a compact, low-input-power Stirling cooler. The QCL, which is based on a two-miniband design, has been developed for high output and low electrical pump power. The amount of generated heat complies with the nominal cooling capacity of the Stirling cooler of 7 W at 65 K with 240 W of electrical input power. Special care has been taken to achieve a good thermal coupling between the QCL and the cold finger of the cooler. The whole system weighs less than 15 kg including the cooler and power supplies. The maximum output power is 8 mW at 3.1 THz. With an appropriate optical beam shaping, the emission profile of the laser is fundamental Gaussian. The applicability of the system is demonstrated by imaging and molecular-spectroscopy experiments.

We report on terahertz quantum-cascade lasers exhibiting discontinuities in the continuous-wave (cw) current-voltage and output power characteristics, which are related to the presence of electric-field domains (EFDs). The investigated lasers are based on a GaAs/Al0.25Ga0.75As heterostructure emitting between 4.2 and 4.4 THz and operate in cw mode up to 50 K. The observed effects related to EFDs are similar to those occurring in weakly coupled superlattices and can be described by the same equations, using an effective drift velocity-field relation. A qualitative agreement between experiments and simulations has been found.

We study the impact of different buffer layers and growth conditions on the properties of InN layers grown on 6H–SiC(0001) by plasma-assisted molecular beam epitaxy. Both GaN and AlN buffer layers result in a significant improvement of the structural quality compared to InN layers grown directly on the SiC substrate. However, to obtain layers exhibiting a high structural integrity, smooth surface morphology, high mobility and strong band-to-band photoluminescence, contradicting growth conditions are found to be required. Furthermore, since InN(0001) dissociates already at temperatures below the onset of In desorption, it is difficult to avoid In accumulation and inclusions of crystalline In in the layer under In-rich conditions.