High‐quality epitaxy consisting of Al1−xGaxN/Al1−yGayN multiple quantum wells (MQWs) with sharp interfaces and emitting at ≈280 nm is successfully grown on sapphire with a misorientation angle as large as 4°. Wavy MQWs are observed due to step bunching formed at the step edges. A thicker QW width accompanied by a greater accumulation of gallium near the macrostep edge than that on the flat‐terrace is observed on 4° misoriented sapphire, leading to the generation of potential minima with respect to their neighboring QWs. Consequently, a significantly enhanced photoluminescence intensity (at least ten times higher), improved internal quantum efficiency (six times higher at low excitation laser power), and a much longer carrier lifetime are achieved. Importantly, the wafer‐level output‐power of the ultraviolet light emitting diodes on 4° misoriented substrate is nearly increased by 2–3 times. This gain is attributed to the introduction of compositional inhomogeneities in AlGaN alloys induced by gallium accumulation at the step‐bunched region thus forming a lateral potential well for carrier localization. The experimental results are further confirmed by a numerical modeling in which a 3D carrier confinement mechanism is proposed. Herein, the compositional modulation in active region arising from the substrate misorientation provides a promising approach in the pursuit of high‐efficient ultraviolet emitters.

Heterojunction solar cells (HSCs) featuring half and full contact of poly (3,4-ethylenedioxythiophene):polystyrene (PE-DOT:PSS) with pyramid-textured silicon (Si) were thoroughly compared via simulations and experiments, and the following conclusions have been reached: (1) The insufficient electrical passivation inherent to the half contact results in enormous decline in short-circuit current density (J sc ) and open-circuit voltage (V oc ). ( 2) For the full-contact HSCs, J sc is mainly dependent on the recombination at the rear interface. With tuning of the contact properties from both sides, calculated (experimental) efficiencies of 14.46%/16.89% (13.94%/16.21%) for the half-/full-contact HSCs were finally obtained. A superior power conversion efficiency (PCE) over 21% is further predicted by considering more optimal contact resistance as well as doping concentration of Si. Our findings clarify why textured-Si/PEDOT:PSS HSCs show V oc and PCE that are inferior to those of planar counterparts in previous reports and further suggest a pathway to fully explore the efficiency potential of Si/PEDOT:PSS hybrid solar cells.

We present a combined theoretical and experimental effort to enable strong light absorption (>70%) in atomically thin MoS2 films (≤4 layers) for either narrowband incidence with arbitrarily prespecified wavelengths or broadband incidence like solar radiation. This is achieved by integrating the films with resonant photonic structures that are deterministically designed using a unique reverse design approach based on leaky mode coupling. The design starts with identifying the properties of leaky modes necessary for the targeted strong absorption, followed by searching for the geometrical features of nanostructures to support the desired modes. This process is very intuitive and only involves a minimal amount of computation, thanks to the straightforward correlations between optical functionality and leaky modes as well as between leaky modes and the geometrical feature of nanostructures. The result may provide useful guidance for the development of high-performance atomic-scale photonic devices, such as solar cells, modulators, photodetectors, and photocatalysts.

Surface states and their influence on the Fermi level at the surface of GaN and AlN are studied using x-ray photoelectron spectroscopy (XPS). The effect of polarity on surface electronic properties was studied. Accurate modeling of the valence band edge and comparison with XPS data revealed the presence of donor surface states at 1.4 eV and acceptor states at energies >2.7 eV from the valence band in GaN. Al polar AlN showed acceptor states at energies >3.3 eV. Density of acceptor surface states was estimated to be between 1013 and 1014 eV−1 cm−2 in both GaN and AlN. The shift in charge neutrality levels and barrier heights due to polarity and the density of surface states on AlN and GaN were estimated from XPS measurements. Theoretical modeling and comparison with XPS data implied full compensation of spontaneous polarization charge by charged surface states. Barrier height measurements also reveal a dependence on polarity with ϕmetal-polar > ϕnon-polar > ϕnitrogen-polar suggesting that the N-polar surface is the most suitable for Ohmic contacts.

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The control of the polarity of metalorganic chemical vapor deposition grown AlN on sapphire is demonstrated. Al-polar and N-polar AlN is grown side-by-side yielding a lateral polarity structure. Scanning electron microscopy measurements reveal a smooth surface for the Al-polar and a relatively rough surface for the N-polar AlN domains. Transmission electron microscopy shows mixed edge-screw type dislocations with polarity-dependent dislocation bending. Raman spectroscopy reveals compressively strained Al-polar and relaxed N-polar domains. The near band edge luminescence consists of free and bound excitons which are broadened for the Al-polar AlN. Relaxation, better optical quality, and dislocation bending in the N-polar domains are explained by the columnar growth mode.

Aluminum-gallium-nitride alloys (Al x Ga 1-x N, 0 ≤ x ≤ 1) can emit light covering the ultraviolet spectrum from 210 to 360 nm. However, these emitters have not fulfilled their full promise to replace the toxic and fragile mercury UV lamps due to their low efficiencies. This study demonstrates a promising approach to enhancing the luminescence efficiency of AlGaN multiple quantum wells (MQWs) via the introduction of a lateral-polarity structure (LPS) comprising both III and N-polar domains. The enhanced luminescence in LPS is attributed to the surface roughening, and compositional inhomogeneities in the N-polar domain. The space-resolved internal quantum efficiency (IQE) mapping shows a higher relative IQE in N-polar domains and near inversion domain boundaries, providing strong evidence of enhanced radiative recombination efficiency in the LPS. These experimental observations are in good agreement with the theoretical calculations, where both lateral and vertical band diagrams are investigated. This work suggests that the introduction of the LPS in AlGaN-based MQWs can provide unprecedented tunability in achieving higher luminescence performance in the development of solid state light sources.