Dynamics of carrier tunneling and recombination in asymmetric coupled InGaN multiple quantum wells.

In this work, dynamics of carrier tunneling and recombination in InGaN-based asymmetric coupled multiple quantum wells (AC-MQWs) are systematically studied by excitation power-dependent and temperature-dependent photoluminescence (PL) measurements. With different pumping wavelengths of 405 and 325 nm, distinctly different PL spectral evolutions are observed, which could be well explained by the proposed anomalous carrier "reverse tunneling" based on the forbidden 1h→2e transitions in the AC-MQWs. The forbidden transitions are identified through the well agreement between the measured photo-modulated reflectance (PR) spectrum and the calculated interband transition energies. Our results indicate that, by ingeniously designing the MQW structure of the InGaN-based optoelectronic devices, it is possible to realize a specific interband optical transition which is even not allowed by the selection rule, and thereby effectively improve the carrier distribution across the QWs through the conventional and/or anomalous "reverse" carrier tunneling.

Nanometer scale fabrication and optical response of InGaN/GaN quantum disks.

In this work, we demonstrate homogeneously distributed In0.3Ga0.7N/GaN quantum disks (QDs), with an average diameter below 10 nm and a high density of 2.1 × 10(11) cm(-2), embedded in 20 nm tall nanopillars. The scalable top-down fabrication process involves the use of self-assembled ferritin bio-templates as the etch mask, spin coated on top of a strained In0.3Ga0.7N/GaN single quantum well (SQW) structure, followed by a neutral beam etch (NBE) method. The small dimensions of the iron cores inside ferritin and nearly damage-free process enabled by the NBE jointly contribute to the observation of photoluminescence (PL) from strain-relaxed In0.3Ga0.7N/GaN QDs at 6 K. The large blueshift of the peak wavelength by over 70 nm manifests a strong reduction of the quantum-confined Stark effect (QCSE) within the QD structure, which also agrees well with the theoretical prediction using a 3D Schrödinger equation solver. The current results hence pave the way towards the realization of large-scale III-N quantum structures using the combination of bio-templates and NBE, which is vital for the development of next-generation lighting and communication devices.

Well-width dependence of the emission linewidth in ZnO/MgZnO quantum wells.

Photoluminescence (PL) spectra were measured as a function of well width (LW) and temperature in ZnO/Mg0.1Zn0.9O single quantum wells (QWs) with graded thickness. The emission linewidth (full width at half maximum) was extracted from the emission spectra, and its variation as a function of LW was studied. The inhomogeneous linewidth obtained at 5 K was found to decrease with increasing LW from 1.8 to 3.3 nm due to the reduced potential variation caused by the LW fluctuation. Above 3.3 nm, however, the linewidth became larger with increasing LW, which was explained by the effect related with defect generation due to strain relaxation and exciton expansion in the QW. For the homogenous linewidth broadening, longitudinal optical (LO) phonon scattering and impurity scattering were taken into account. The LO phonon scattering coefficient ΓLO and impurity scattering coefficient Γimp were deduced from the temperature dependence of the linewidth of the PL spectra. Evident reduction of ΓLO with decreasing LW was observed, which was ascribed to the confinement-induced enhancement of the exciton binding energy. Different from ΓLO, a monotonic increase in Γimp was observed with decreasing LW, which was attributed to the enhanced penetration of the exciton wave function into the barrier layers.