We demonstrate the growth of pure wurtzite phase ScxAl1−xN with a Sc composition as high as x = 0.34 on GaN and AlN templates using plasma-assisted molecular beam epitaxy. The wurtzite structure is well maintained even at high growth temperatures up to 900 °C for Sc0.2Al0.8N. Smooth surface morphology (root mean square roughness less than 1 nm) and excellent crystal quality [(002) plane rocking curve full-width at half maximum below 450 arc sec] are achieved over the range of x ≤ 0.34. Optical absorption studies indicate a decreasing bandgap with increasing Sc with a linear relationship of Eg(x) = 6.1 − 3.39x, which is in good agreement with the theoretical prediction. A monotonically tunable refractive index between AlN and GaN is further measured for ScxAl1−xN with various Sc compositions. This work provides a viable path for the epitaxy of wurtzite ScxAl1−xN with high Sc compositions. The distinct effect of substitutional Sc on bandgap and refractive index could be used in designing high-performance optoelectronic, electronic, and piezoelectric devices, and III-nitride integrated photonics and optical cavities.
Simultaneously achieving efficient and stable operation is a major challenge for developing sustainable and economical solar water-splitting systems. In this work, we demonstrate, for the first time, a monolithically integrated InGaN/Si double-junction photocathode, which can enable relatively efficient and stable unassisted solar water splitting. The device consists of a p-type InGaN top junction, which is monolithically integrated on a bottom Si p−n junction through a dislocation-free n ++ /p ++ InGaN nanowire tunnel junction. With the incorporation of Pt catalysts and a thin Al 2 O 3 surface passivation layer, a solar-to-hydrogen efficiency of ∼10.3% and stable operation of 100 h was measured in 0.5 M H 2 SO 4 in a two-electrode configuration for unbiased photoelectrochemical water splitting. Significantly, such an efficient and stable water-splitting device is achieved using the two most produced semiconductors, i.e., Si and Ga(In)N, promising large-scale implementation of efficient, stable, and low-cost solar hydrogen production systems.
AlGaN is the material of choice for high-efficiency deep UV light sources, which is the only alternative technology to replace mercury lamps for water purification and disinfection. At present, however, AlGaN-based mid- and deep UV LEDs exhibit very low efficiency. Here, we report a detailed investigation of the epitaxy and characterization of LEDs utilizing an AlGaN/GaN/AlGaN tunnel junction structure, operating at ∼ 265 nm , which have the potential to break the efficiency bottleneck of deep UV photonics. A thin GaN layer was incorporated between p + and n + -AlGaN to reduce the tunneling barrier. By optimizing the thickness of the GaN layer and thickness of the top n -AlGaN contact layer, we demonstrate AlGaN deep UV LEDs with a maximum external quantum efficiency of 11% and wall-plug efficiency of 7.6% for direct on-wafer measurement. It is also observed that the devices exhibit severe efficiency droop under low current densities, which is explained by the low hole mobility, due to the hole hopping conduction in the Mg impurity band and the resulting electron overflow.
Tantalum nitride is a promising photoanode material for solar water splitting, but further study and practical use are constrained by the harsh conditions of the synthesis from Ta metal. Here, we report the direct deposition of crystalline Ta3N5 on fluorine-doped tin oxide (FTO) substrate via a custom-built atomic layer deposition (ALD) system. A combination of TaCl5 (Ta precursor) and ammonia (N source) was sequentially pulsed into the ALD reactor with the substrate heated to 550 °C to deposit compact and thin films of Ta3N5 with controllable thicknesses on FTO substrates. Importantly, it is shown that the FTO is chemically and structurally stable under the reducing conditions of ammonia at 550 °C. These electrodes produced an exceptional photocurrent onset potential of ∼0.3 V versus reversible hydrogen electrode (RHE) with a maximum photocurrent of ∼2.4 mA cm–2 at 1.23 V versus RHE. Results of photoelectrochemical investigations as a function of film thickness and illumination direction reveal that the performance of Ta3N5 is controlled by a hole diffusion length of ∼50 nm. These results are crucial for the successful integration of Ta3N5 in efficient unassisted water-splitting applications.
We present experimental results confirming extreme quantum confinement in GaN/Al x Ga1–x N (x = 0.65 and 1.0) nanowire and planar heterostructures, where the GaN layer thickness is of the order of a monolayer. The results were obtained from temperature- and excitation-dependent and time-resolved photoluminescence measurements. In the GaN/AlN nanowire heterostructure array sample, the measured emission peak at 300 K is ∼5.18–5.28 eV. This is in excellent agreement with the calculated optical gap of 5.23 eV and 160–260 meV below the calculated electronic gap of 5.44 eV, suggesting that the observed emission is excitonic in nature with an exciton binding energy of ∼160–260 meV. Similarly, in the monolayer GaN/Al0.65Ga0.35N planar heterostructure, the measured emission peak at 300 K is 4.785 eV and in good agreement with the calculated optical gap of 4.68 eV and 95 meV below the calculated electronic gap of 4.88 eV. The estimated exciton binding energy is 95 meV and in close agreement with our theoretical calculations. Excitation-dependent and time-resolved photoluminescence data support the presence of excitonic transitions. Our results indicate that deep-ultraviolet excitonic light sources and microcavity devices can be realized with heterostructures incorporating monolayer-thick GaN.
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