Quantum state engineering with ultra-short-period (AlN)m/(GaN)n superlattices for narrowband deep-ultraviolet detection

Enhancing deep-UV emission at 234 nm by introducing a truncated pyramid AlN/GaN nanostructure with fine-tuned multiple facets

The external quantum efficiency of a high-Al content (>0.6) AlGaN deep-ultraviolet (DUV) light-emitting diode is typically below 1% in the sub-250 nm wavelength range. One of the main reasons for this low efficiency is the fundamental properties of high-Al content AlGaN comprising the transverse-magnetic (TM)-dominant emission and low light extraction due to the total internal reflection (TIR). This work demonstrates a truncated pyramid nanostructure with fine-tuned multiple facets in an (AlN)₈/(GaN)₂ digital alloy to achieve highly efficient DUV emission at 234 nm. By applying nanoimprint lithography, dry and wet etching, a hexagonal truncated pyramid nanohole structure is fabricated featuring multiple crystal facets of (0001), (10-13), and (20-21) planes. These fine-tuned multiple facets act as reflecting mirrors that can effectively modulate the light propagation and extraction patterns to overcome the TIR via multiple reflections and enhanced scattering. Consequently, significant light extraction enhancements of 5.6 times and 1.1 times for TM and transverse-electric emissions are achieved in the truncated pyramid nanohole structure, respectively. The total luminous intensity of this unique nanostructure is greatly increased by 191% compared to that of a conventional planar structure. The truncated pyramid AlN/GaN nanostructure with fine-tuned multiple facets used in this work provides a promising approach for realizing highly efficient sub-250 nm DUV light-emitting devices.

Multiple fields manipulation on nitride material structures in ultraviolet light-emitting diodes

As demonstrated during the COVID-19 pandemic, advanced deep ultraviolet (DUV) light sources (200-280 nm), such as AlGaN-based light-emitting diodes (LEDs) show excellence in preventing virus transmission, which further reveals their wide applications from biological, environmental, industrial to medical. However, the relatively low external quantum efficiencies (mostly lower than 10%) strongly restrict their wider or even potential applications, which have been known related to the intrinsic properties of high Al-content AlGaN semiconductor materials and especially their quantum structures. Here, we review recent progress in the development of novel concepts and techniques in AlGaN-based LEDs and summarize the multiple physical fields as a toolkit for effectively controlling and tailoring the crucial properties of nitride quantum structures. In addition, we describe the key challenges for further increasing the efficiency of DUV LEDs and provide an outlook for future developments.

Reversing abnormal hole localization in high-Al-content AlGaN quantum well to enhance deep ultraviolet emission by regulating the orbital state coupling

AlGaN has attracted considerable interest for ultraviolet (UV) applications. With the development of UV optoelectronic devices, abnormal carrier confinement behaviour has been observed for c-plane-oriented AlGaN quantum wells (QWs) with high Al content. Because of the dispersive crystal field split-off hole band (CH band) composed of p z orbitals, the abnormal confinement becomes the limiting factor for efficient UV light emission. This observation differs from the widely accepted concept that confinement of carriers at the lowest quantum level is more pronounced than that at higher quantum levels, which has been an established conclusion for conventional continuous potential wells. In particular, orientational p z orbitals are sensitive to the confinement direction in line with the conducting direction, which affects the orbital intercoupling. In this work, models of Al0.75Ga0.25N/AlN QWs constructed with variable lattice orientations were used to investigate the orbital intercoupling among atoms between the well and barrier regions. Orbital engineering of QWs was implemented by changing the orbital state confinement, with the well plane inclined from 0° to 90° at a step of 30° (referred to the c plane). The barrier potential and transition rate at the band edge were enhanced through this orbital engineering. The concept of orbital engineering was also demonstrated through the construction of inclined QW planes on semi- and nonpolar planes implemented in microrods with pyramid-shaped tops. The higher emission intensity from the QWs on the nonpolar plane compared with those on the polar plane was confirmed via localized cathodoluminescence (CL) maps.

Ferroelectricity and Large Piezoelectric Response of AlN/ScN Superlattice

Based on density functional theory, we investigate the ferroelectric and piezoelectric properties of the AlN/ScN superlattice, consisting of ScN and AlN buckled monolayers alternating along the crystallographic c-direction. We find that the polar wurtzite (w-ScAlN) structure is mechanically and dynamically stable and is more stable than the nonpolar hexagonal flat configuration. We show that ferroelectric polarization switching can be possible for an epitaxially tensile-strained superlattice. Because of the elastic constant C₃₃ softening, together with an increase in e₃₃, the piezoelectric coefficient d₃₃ of the superlattice is doubled compared to that of pure w-AlN. The combined enhancement of Born effective charges ( Z₃₃) and sensitivity of the atomic coordinates to the external strain (∂u3∂η3) is the origin of the large piezoelectric constant e₃₃. Moreover, we show that the epitaxial biaxial tensile strain significantly enhances the piezo-response, so that d₃₃ becomes 7 times larger than that of w-AlN at 4% strain. The tensile strain results in a huge enhancement in e₃₃ by increasing Z₃₃ and ∂u3∂η3 , which boost the piezoelectric.

Narrowband Perovskite Photodetector-Based Image Array for Potential Application in Artificial Vision

Image sensor arrays are widely used in digital cameras, smartphones, and biorobots. However, most commercial image arrays rely on the dichroic prisms or a set of interference filters to distinguish characteristic color spectrum, which significantly increases the cost and fabrication processing complexity. In this work, an ultranarrow response photodetector with full-width at half-maximum being ∼12 nm and specific detectivity over 10¹¹ Jones at 545 nm are successfully achieved in CsPbBr₃ polycrystalline films using freeze-drying casting method to adjust the surface-charge recombination. To our best knowledge, this is the narrowest spectrum response for perovskite photodetectors in the visible light waveband. More importantly, a series of narrowband photodetectors are developed to enhance diverse selectivity for target signals covering from blue light to red light via bandgap tuning in CsPbX₃ by tailoring the halide component. Finally, an integrated sensing array with CsPbX₃ (X = Cl, Br, I) narrowband photodetectors acting as color recognition cones is constructed, which presents clear color and shape recognition paving the way for commercialization of perovskite photodetector in artificial vision.

Effect of electrical injection-induced stress on interband transitions in high Al content AlGaN MQWs

The light extraction from AlGaN deep ultraviolet light-emitting diodes (UV LEDs) is known to be limited by the fundamental valence band crossover issue.

A combinatorial approach to solution-processed InGaO3(ZnO)m superlattice films: growth mechanisms and their thermoelectric properties

The strong single-phase InGaO₃(ZnO)_m films with superlattice structures showed superior thermoelectric power factors. Their microstructural growth evolution was investigated using transmission electron microscopy in detail.

Direct Observation of the Biaxial Stress Effect on Efficiency Droop in GaN-based Light-emitting Diode under Electrical Injection

Light-emitting diode (LED) efficiency has attracted considerable interest because of the extended use of solid-state lighting. Owing to lack of direct measurement, identification of the reasons for efficiency droop has been restricted. A direct measurement technique is developed in this work for characterization of biaxial stress in GaN-based blue LEDs under electrical injection. The Raman shift of the GaN E₂ mode evidently decreases by 4.4 cm⁻¹ as the driving current on GaN-based LEDs increases to 700 mA. Biaxial compressive stress is released initially and biaxial tensile stress builds up as the current increases with respect to the value of stress-free GaN. First-principles calculations reveal that electron accumulation is responsible for the stress variation in In_xGa_1−xN/GaN quantum wells, and then reduces the transition probability among quantum levels. This behavior is consistent with the measured current-dependent external quantum efficiency. The rule of biaxial stress-dependent efficiency is further validated by controlling the biaxial stress of GaN-based LEDs with different sapphire substrate thicknesses. This work provides a method for direct observation of the biaxial stress effect on efficiency droop in LEDs under electrical injection.