Modulation‐doped Ga_0.47In_0.53As/Al_0.48In_0.52As single heterojunctions have been investigated by means of optical spectroscopy and magnetotransport measurements. Strong radiative recombination of the two‐dimensional electron gas (2DEG) with photogenerated holes has been observed. The recombination is not k conserving, since the holes are weakly localized at the heterointerface opposite to the 2DEG region. At low temperature strong Fermi edge enhancement occurs in the luminescence of single heterojunctions with, for example, a dense electron sheet concentration of 2.5×10¹² cm⁻². The results show clear evidence for the influence of carrier concentration and temperature on the Fermi edge singularity. The linewidths of the luminescence spectra agree well with those estimated from the electron sheet carrier concentrations, which were determined from the Shubnikov–de Haas oscillations, using an ideal 2DEG model.

The InGaN/GaN multi-quantum wells (MQWs) are prepared at the same condition by metal-organic chemical vapor deposition (MOCVD) except the thickness of cap layers additionally grown on each InGaN well layer. The photoluminescence (PL) intensity of the thin cap layer sample is much stronger than that of thicker cap layer sample. Interestingly, the thick cap layer sample has two photoluminescence peaks under high excitation power, and the PL peak energy-temperature curves show an anomalous transition from reversed V-shaped to regular S-shaped with increasing excitation power. Meanwhile, it exhibits a poorer thermal stability of thick cap layer sample under higher excitation power than that under lower excitation power. Such an untypical phenomenon is attributed to carrier redistribution between the two kinds of localized states which is induced by the inhomogeneous distribution of indium composition in thick cap layer sample. Furthermore, the luminescence of deep localized states has a better thermal stability, and the luminescence of shallow localized states has a poor thermal stability. In fact, such a severer inhomogeneous indium distribution may be caused by the degradation of subsequent epitaxial growth of InGaN/GaN MQWs region due to longer low-temperature GaN cap layer growth time.

We report on the realization of quantum cascade (QC) lasers based on strain-compensated In_xGa_(1-x)As/In_yAl_(1-y)As grown on InP substrates using molecular beam epitaxy. X-ray diffraction and cross section transmission electron microscopy have been used to ascertain the quality of the QC laser materials. Quasi-continuous wave lasing at ë≈3.54-3.7 µm at room temperature was achieved. For a laser with 1.6 mm cavity length and 20 µm ridge-waveguide width, quasi-continuous wave lasing at 34 °C persists for more than 30 min, with a maximum power of 11.4 mW and threshold current density of 1.2 kA cm^-2, both record values for QC lasers of comparable wavelength.

Strong defect-specific low-frequency peaks are detected in low-temperature Raman spectra of hexagonal GaN grown by molecular beam epitaxy on sapphire substrate. The intensity of these peaks is found to be enhanced by excitation in resonance with yellow luminescence transitions. The validity of the assignment to electronic Raman scattering (ERS), as proposed before for their counterparts in cubic GaN on GaAs [M. Ramsteiner, J. Menniger, O. Brandt, H. Yang, and K. H. Ploog, Appl. Phys. Lett. 69, 1276 (1996)], is confirmed. Our results imply that the observed ERS peaks are related to shallow donors which are not necessarily hydrogenic. One Raman peak at very low frequency (11.7 meV) is alternatively explained by a pseudo-localized vibrational mode.