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The luminescence efficiency of AlxGa1−xN quantum dots (QDs) and quantum wells (QWs), buried in AlN cladding layers and emitting in the ultraviolet range between 234 and 310 nm, has been investigated. The growth and optical properties have been done using similar aluminum composition (varying from 0.4 to 0.75) for both QDs and QWs. In order to compare as much as possible the optical properties, the QWs were fabricated with a growth time tuned such that the QW width is similar to the average height of the QDs. The photoluminescence (PL) showed emission ranging from 4 to 5.4 eV, putting into evidence differences in terms of full width at half maximum, PL intensity, and asymmetry of the line shape between QDs and QWs. The results show shorter wavelengths and a slightly narrower PL linewidth for QWs. To determine the light emission dependence with the electric field direction in the crystal, the evolutions of the emission diagrams for all samples were recorded along two orthogonal directions, namely, the "in-plane" (growth) and the "on-side" directions, from which the light emission was collected. For the whole QDs and QWs samples' series, the shapes of the emission diagram indicate emission in both in-plane and on-side directions, as evidenced by intra-valence band mixings caused by strain effects combined with the anisotropic Coulomb interactions that are particularly contributing to the polarization at wavelengths below 260 nm. Furthermore, the degree of polarization, determined for each sample, showed good agreement with results from the literature. [ABSTRACT FROM AUTHOR]

A significant enhancement in the output power of a GaAs-based terahertz quantum cascade laser (THz QCL) was achieved by implementing a relatively high impurity doping concentration. The QC structure was precisely designed using the nonequilibrium green function method by considering the band bending effect caused by a higher doping concentration. This enabled us to avoid the subband misalignment induced by the strong band bending effect, to provide efficient carrier transport even at high doping concentrations, and to improve the output power. The maximum output power of a GaAs/Al0.16Ga0.84As 3.5 THz-band QCL at 10 K was enhanced from 390 to 600 mW by increasing the doping concentration at the depopulation layers from 6 × 1016 to 1.2 × 1017 cm−3. The average power of 21 mW was achieved by implementing the appropriate pulse drive conditions. [ABSTRACT FROM AUTHOR]

The correlation between the recombination dynamics of excitons and the internal quantum efficiency (IQE) of AlGaN-based UV-A multiple quantum wells (MQWs) was studied via photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy. The probability ratio of the capture of excitons by nonradiative recombination centers (NRCs) and the radiative recombination of excitons was evaluated individually via two different experimental analyses. The IQE was evaluated via temperature- and excitation power density-dependent PL measurements and its dependence on excitation density was analyzed using a rate equation model based on the radiative and nonradiative recombination of excitons. Moreover, the radiative and nonradiative recombination lifetimes were evaluated via temperature-dependent TRPL measurements; furthermore, they were analyzed as functions of temperature and excitation energy density. The probability ratios obtained from the two individual analyses were in agreement. This quantitative agreement indicated that the analysis based on the radiative and nonradiative recombination processes of excitons, which included the process of filling NRCs, was valid for AlGaN-based UV-A MQWs. [ABSTRACT FROM AUTHOR]