Weidong Song

The Advantages of AlGaN-Based Ultraviolet Light-Emitting Diodes With Al Content Graded AlGaN Barriers

A novel five-period AlGaN/AlGaN multiple quantum wells light-emitting diodes (LEDs) structure with Al content graded AlGaN barriers is designed in order to improve the electrical and optical performance of ultraviolet LEDs (UV-LEDs), and the effects are analyzed by using the APSYS simulation programs. The results show that the effective potential height for electrons is increased, and simultaneously the effective potential height for holes is decreased with the increased number of graded AlGaN barriers, which contributes to less electron leakage and better hole injection efficiency. Thus, the internal quantum efficiency and light output power are significantly improved, and the efficiency droop is also mitigated effectively as the number of graded AlGaN barriers increases. However, there is an undesired peak emission in the last barrier when it is replaced by the graded AlGaN barrier.

High-performance self-powered UV-Vis-NIR photodetectors based on horizontally aligned GaN microwire array/Si heterojunctions

Considering their reduced size and weight, low cost and portability, self-powered photodetectors that can be functioned independently of an external power supply are extremely important for developing future sensor networks and Internets of Things. Here, we present a self-powered photodetector with a broadband wavelength photoresponse from the ultraviolet to near-infrared region based on GaN microwire array/Si heterojunctions. Due to the highly efficient separation of photogenerated charge carriers, the photodetector shows ultrahigh EQE, responsivity and detectivity, reaching up to 71%, 4.7 × 102 mA W−1 and 9.5 × 1012 Jones, respectively, at zero bias in the 320–850 nm range. A fast response is observed with rise/decay times as low as 2/2 ms. The LDR values are over 80 dB. The performance characteristics are competitive with commercially available biased photodetectors and other reported heterojunction self-powered broadband photodetectors in the literature. In addition, a binary photoresponse is observed under small bias voltages. The heterostructures are fabricated by directly heteroepitaxially growing GaN microwires on patterned Si, enabling large-area device applications, and stand out from low-dimensional semiconductor photodetection systems. The overall high performance coupled with large-scale production makes GaN microwire array/Si heterojunctions promising for practical self-powered broadband photodetectors.

Alleviating hysteresis and improving device stability of perovskite solar cells via alternate voltage sweeps

The anomalous hysteresis in a perovskite solar cell induced by an asymmetric field is confirmed by a capacitance–voltage measurement. By applying several cycles of alternating reverse and forward scans, this hysteresis phenomenon is obviously alleviated, resulting in a hysteresis-less state in the perovskite solar cell. Meanwhile, the open-circuit voltage and power conversion efficiency of the perovskite solar cell are enhanced by 55.74% and 61.30%, respectively, while the current density and fill factor keep almost invariable. The operation of alleviating hysteresis is essential for further research and is likely to bring in performance gains.

a-Axis GaN/AlN/AlGaN Core–Shell Heterojunction Microwires as Normally Off High Electron Mobility Transistors

Micro/nanowire-based devices have been envisioned as a promising new route toward improved electronic and optoelectronic applications, which attracts considerable research interests. However, suffering from applicable strategies to synthesize uniform core-shell structures to meet the requirement for the investigations of electrical transport behaviors along the length direction or high electron mobility transistor (HEMT) devices, heterojunction wire-based electronics have been explored limitedly. In the present work, GaN/AlN/AlGaN core-shell heterojunction microwires on patterned Si substrates were synthesized without any catalyst via metalorganic chemical vapor deposition. The as-synthesized microwires had low dislocation, sharp, and uniform heterojunction interfaces. Electrical transport performances were evaluated by fabricating HEMTs on the heterojunction microwire channels. Results demonstrated that a normally off operation was achieved with a threshold voltage of 1.4 V, a high on/off current ratio of 108, a transconductance of 165 mS/mm, and a low subthreshold swing of 81 mV/dec. The normally off operation may attribute to the weak polarization along semipolar facets of the heterojunction, which leads to weak constrain of 2DEG.

Improved performance of near UV light-emitting diodes with a composition-graded p-AlGaN irregular sawtooth electron-blocking layer*

We investigate the performances of the near-ultraviolet (about 350 nm–360 nm) light-emitting diodes (LEDs) each with specifically designed irregular sawtooth electron blocking layer (EBL) by using the APSYS simulation program. The internal quantum efficiencies (IQEs), light output powers, carrier concentrations in the quantum wells, energy-band diagrams, and electrostatic fields are analyzed carefully. The results indicate that the LEDs with composition-graded p-Al x Ga1−x N irregular sawtooth EBLs have better performances than their counterparts with stationary component p-AlGaN EBLs. The improvements can be attributed to the improved polarization field in EBL and active region as well as the alleviation of band bending in the EBL/p-AlGaN interface, which results in less electron leakage and better hole injection efficiency, thus reducing efficiency droop and enhancing the radiative recombination rate.