Transient photoluminescence from a series of asymmetric $\mathrm{InAs}$ quantum-dot bilayers with a $\mathrm{GaAs}$ barrier layer thickness varying from 30 to 60 monolayers between the quantum-dot planes is investigated. The interdot carrier transfer process is analyzed. In the framework of a three-level system, interdot carrier transfer times between 200 and $2500\mathrm{ps}$ are derived and compared with similar data from the literature. Within the semiclassical Wentzel–Kramers–Brillouin approximation, the observed “transfer time-barrier thickness-relation” supports nonresonant tunneling as the microscopic carrier transfer mechanism.

We present a comparative study of the strain relaxation of GaN/AlN short-period superlattices (SLs) grown on two different III-nitride substrates introducing different amounts of compensating strain into the films. We grow by plasma-assisted molecular beam epitaxy (0001)-oriented SLs on a GaN buffer deposited on GaN(thick)-on-sapphire template and on AlN(thin)-on-sapphire template. The ex-situ analysis of strain, crack formation, dislocation density, and microstructure of the SL layers has established that the mechanism of strain relaxation in these structures depends on the residual strain in substrate and is determined mainly by the lattice mismatch between layers. For growth on the AlN film, the compensating strain introduced by this film on the layer prevented cracking; however, the densities of surface pits and dislocations were increased as compared with growth on the GaN template. Three-dimensional growth of the GaN cap layer in samples with pseudomorphly grown SLs on the AlN template is observed. At the same time, two-dimensional step-flow growth of the cap layer was observed for structures with non-pseudomorphly grown SLs on the GaN template with a significant density of large cracks appearing on the surface. The growth mode of the GaN cap layer is predefined by relaxation degree of top SL layers.

Crossed transitions between the wetting layer valence band and the quantum dot (QD) electron states are revealed in the photoluminescence from self-assembled ${\mathrm{In}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ QDs. The strength of these transitions becomes comparable with the excitonic transitions for below-GaAs barrier excitation and decreases significantly with below wetting layer excitation. The observed peculiar QD photoluminescence dependences on temperature and excitation density are due partly to interdot carrier transfer through the continuum states related to the wetting layer morphology and to phonon-assisted processes.

Antimony-incorporated InAsSb quantum dots (QDs) are grown by molecular beam epitaxy on GaAs(001) substrates. The QD density increases ∼7 times while the QD height decreases ∼50% due to the increase of QD nucleation sites after Sb incorporation into the GaAs buffer layer and into the InAs QDs. These Sb-incorporated InAsSb QDs show red-shift in the photoluminescence (PL) spectrum and large energy separation between confined energy levels. More interestingly, besides the typical type-I QD transition, an additional peak from the recombination at wetting layer interface develops as the excitation laser intensity increases. This peak clearly exhibits type-II characteristics from the measurement of a large blue-shift of the PL peak and a long PL decay time. Finally, the mechanism of the coexistence of type-I and type-II band alignments is discussed.

The effects of lattice misfit strain in epitaxial GeSn/Ge/Si(001) heterostructures on Sn incorporation, misfit dislocations (MDs), and the critical thickness were investigated using high-resolution x-ray diffraction. By performing a simulation of the x-ray reciprocal space maps measured in the vicinity of an asymmetrical reflection, we determined the strong correlation between the strain relaxation, density of MDs, and Sn content in the GeSn alloy for a compositional range of 4 to 15 at. %. Herein, we quantitatively describe the phenomenon of strain-suppressed Sn incorporation in GeSn, leading to the formation of top Sn-rich, middle compositionally graded, and bottom Sn-poor regions at fixed growth conditions. It is shown that the thicknesses of the bottom Sn-poor region at which the composition spontaneously changes are correlated with the theoretically predicted critical thickness for the nucleation of MDs. Depending on the degree of strain relaxation in the bottom GeSn layer, the density of MDs at the GeSn/Ge interface varies in the range of $\sim$ 2–5 × 10⁵ cm⁻¹ as the Sn content increases from 4 to 12 at. %. The thickness of the compositionally graded region, 160 ± 25 nm, corresponds to the thickness at which the GeSn layer fully relaxes through the interaction/multiplication of MDs. These results contribute to the fundamental understanding that strain engineering is not only crucial to tune the bandgap of Sn-containing group-IV semiconductors but also a key factor for the growth of Sn-rich metastable GeSn layers for mid-infrared photonic devices.

Strain-free GaAs/Al_0.33Ga_0.67As quantum rings are fabricated by droplet epitaxy. Both photoresponse and photoluminescence spectra confirm optical transitions in quantum rings, suggesting that droplet epitaxial nanomaterials are applicable to intermediate band solar cells. The effects of post-growth annealing on the quantum ring solar cells are investigated, and the optical properties of the solar cells with and without thermal treatment are characterized by photoluminescence technique. Rapid thermal annealing treatment has resulted in the significant improvement of material quality, which can be served as a standard process for quantum structure solar cells grown by droplet epitaxy.