Yonghui Zhang

On the hole accelerator for III-nitride light-emitting diodes

In this work, we systematically conduct parametric studies revealing the sensitivity of the hole injection on the hole accelerator (a hole accelerator is made of the polarization mismatched p-electron blocking layer (EBL)/p-GaN/p-AlxGa1−xN heterojunction) with different designs, including the AlN composition in the p-AlxGa1−xN layer, and the thickness for the p-GaN layer and the p-AlxGa1−xN layer. According to our findings, the energy that the holes obtain does not monotonically increase as the AlN incorporation in the p-AlxGa1−xN layer increases. Meanwhile, with p-GaN layer or p-AlxGa1−xN layer thickening, the energy that the holes gain increases and then reaches a saturation level. Thus, the hole injection efficiency and the device efficiency are very sensitive to the p-EBL/p-GaN/p-AlxGa1−xN design, and the hole accelerator can effectively increase the hole injection if properly designed.

On the electric-field reservoir for III-nitride based deep ultraviolet light-emitting diodes

The drift velocity for holes is strongly influenced by the electric field in the p-type hole injection layer for III-nitride based deep ultraviolet light-emitting diodes (DUV LEDs). In this work, we propose an electric-field reservoir (EFR) consisting of a p-AlxGa1-xN/p-GaN architecture to facilitate the hole injection and improve the internal quantum efficiency (IQE). The p-AlxGa1-xN layer in the EFR can well reserve the electric field that can moderately adjust the drift velocity and the kinetic energy for holes. As a result, we are able to enhance the thermionic emission for holes to cross over the p-EBL with a high Al composition provided that the composition in the p-AlxGa1-xN layer is properly optimized to avoid a complete hole depletion therein.

On the Hole Injection for III-Nitride Based Deep Ultraviolet Light-Emitting Diodes

The hole injection is one of the bottlenecks that strongly hinder the quantum efficiency and the optical power for deep ultraviolet light-emitting diodes (DUV LEDs) with the emission wavelength smaller than 360 nm. The hole injection efficiency for DUV LEDs is co-affected by the p-type ohmic contact, the p-type hole injection layer, the p-type electron blocking layer and the multiple quantum wells. In this report, we review a large diversity of advances that are currently adopted to increase the hole injection efficiency for DUV LEDs. Moreover, by disclosing the underlying device physics, the design strategies that we can follow have also been suggested to improve the hole injection for DUV LEDs.

A charge inverter for III-nitride light-emitting diodes

In this work, we propose a charge inverter that substantially increases the hole injection efficiency for InGaN/GaN light-emitting diodes (LEDs). The charge inverter consists of a metal/electrode, an insulator, and a semiconductor, making an Electrode-Insulator-Semiconductor (EIS) structure, which is formed by depositing an extremely thin SiO2 insulator layer on the p+-GaN surface of a LED structure before growing the p-electrode. When the LED is forward-biased, a weak inversion layer can be obtained at the interface between the p+-GaN and SiO2 insulator. The weak inversion region can shorten the carrier tunnel distance. Meanwhile, the smaller dielectric constant of the thin SiO2 layer increases the local electric field within the tunnel region, and this is effective in promoting the hole transport from the p-electrode into the p+-GaN layer. Due to the improved hole injection, the external quantum efficiency is increased by 20% at 20 mA for the 350 × 350 μm2 LED chip. Thus, the proposed EIS holds great pr...

UVA light-emitting diode grown on Si substrate with enhanced electron and hole injections

In this work, III-nitride based ∼370  nm UVA light-emitting diodes (LEDs) grown on Si substrates are demonstrated. We also reveal the impact of the AlN composition in the AlGaN quantum barrier on the carrier injection for the studied LEDs. We find that, by properly increasing the AlN composition, both the electron and hole concentrations in the multiple quantum wells (MQWs) are enhanced. We attribute the increased electron concentration to the better electron confinement within the MQW region when increasing the AlN composition for the AlGaN barrier. The improved hole concentration in the MQW region is ascribed to the reduced hole blocking effect by the p-type electron blocking layer (p-EBL). This is enabled by the reduced density of the polarization-induced positive charges at the AlGaN last quantum barrier (LB)/p-EBL interface, which correspondingly suppresses the hole depletion at the AlGaN LB/p-EBL interface and decreases the valence band barrier height for the p-EBL. As a result, the optical power is improved.

On the Importance of the Polarity for GaN/InGaN Last Quantum Barriers in III-Nitride-Based Light-Emitting Diodes

In this paper, we investigate the electron injection efficiency in terms of different polarities and different polarization charge densities at the GaN/InGaN interface. We find that the growth orientation for the GaN/InGaN-type last quantum barrier is essentially vital, i.e., the GaN/InGaN-type last quantum barrier is not able to effectively reduce the electron leakage and will degrade the light-emitting diode (LED) performance when the GaN/InGaN interface is [000-1] polarized. However, a suppressed electron leakage and enhanced optical power can be obtained for III-nitride LEDs grown along the [0001] orientation when the GaN/InGaN interface possesses polarization-induced negative charges. We conclude that the polarization-induced negative charges at the [0001] oriented GaN/InGaN interface facilitate the surface depletion in the GaN region, i.e., the conduction band of the GaN region is bent in the way of favoring electron depletion and contributes to an enhanced conduction band barrier height for electrons.

High-performance photodetectors based on two-dimensional tin(II) sulfide (SnS) nanoflakes

As a kind of two-dimensional (2D) materials, tin sulfides including tin(IV) sulfide (SnS2) and tin(II) sulfide (SnS) have been attracting wide attention because of its earth abundance, low cost, and environment-friendly characteristics as promising photovoltaic and photocatalytic materials. Among them, only SnS is a p-type semiconductor with appealing physical properties. However, its further investigation and functionalization have been hampered by the absence of highly crystallized nanostructures relative to other intensively studied tin sulfides, such as tin(IV) sulfide (SnS2). Herein, high quality 2D SnS is synthesized via the chemical vapor deposition (CVD) method, with a new precursor of stannous oxide (SnO). Its morphology and properties are characterized by optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive spectrometry (EDS), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Furthermore, field-effect transistors (FETs) and photodetectors based on such SnS are fabricated using Photolithographic-Pattern-Transfer (PPT) technology. The results show that the as-grown SnS is indeed a p-type semiconductor and exhibits high responsivity (156.0 A W−1), normalized detectivity (2.94 × 1010 jones) and external quantum efficiency (4.77 × 104%) with fast response time (5.1 ms) under illumination of a 405 nm laser. This work demonstrates that 2D SnS holds great promise for next-generation optoelectronic applications.

Synthesis of Quantum Dot-ZnS Nanosheet Inorganic Assembly with Low Thermal Fluorescent Quenching for LED Application

In this report, to tackle the thermal fluorescent quenching issue of II-VI semiconductor quantum dots (QDs), which hinders their on-chip packaging application to light-emitting diodes (LEDs), a QD-ZnS nanosheet inorganic assembly monolith (QD-ZnS NIAM) is developed through chemisorption of QDs on the surface of two-dimensional (2D) ZnS nanosheets and subsequent assembly of the nanosheets into a compact inorganic monolith. The QD-ZnS NIAM could reduce the thermal fluorescent quenching of QDs effectively, possibly due to fewer thermally induced permanent trap states and decreased Förster resonance energy transfer (FRET) among QDs when compared with those in a reference QD composite thin film. We have demonstrated that the QD-ZnS NIAM enables QDs to be directly packaged on-chip in LEDs with over 90% of their initial luminance being retained at above 85 °C, showing advantage in LED application in comparison with conventional QD composite film.

Effects of Inclined Sidewall Structure With Bottom Metal Air Cavity on the Light Extraction Efficiency for AlGaN-Based Deep Ultraviolet Light-Emitting Diodes

An inclined sidewall scattering structure with air cavity characterized by a metal bottom and flat parallel top (Bottom_metal) is proposed to enhance the light extraction efficiency (LEE) for AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). Compared to the reported sidewall metal inclined sidewall (Sidewall_metal) structure, the Bottom_metal structure can greatly enhance the LEE of DUV LEDs based on three-dimensional finite difference time domain simulations. Further analysis indicates that the existence of the air cavity promotes the Bottom_metal DUV LEDs to mainly utilize the total internal reflection and the Fresnel scattering to scatter the light into the escape cone, which avoids the light absorption from the sidewall metal mirror in the Sidewall_metal structure. Moreover, the unique air cavity having a bottom metal also enhances the scattering ability of the Bottom_metal DUV LEDs because any light within the cavity directing downward will be reflected back, and the parallel top interface of air cavity/AlGaN functions as additional out-light planes not limited by total internal reflection.