Effect of AlGaN interlayer on luminous efficiency and reliability of GaN-based green LEDs on silicon substrate

The effect of AlGaN interlayer in quantum barrier on the electroluminescence characteristics of GaN-based green light emitting diodes(LEDs)grown on silicon substrate was investigated.The results show that AlGaN interlayer is beneficial to improve the luminous efficiency of LED devices and restrain the phase separation of In GaN.The former is ascribed to the inserted AlGaN layers can play a key role in determining the carrier distribution and screening dislocations in the active region,and the latter is attributed to the increased compressive stress in the quantum well.However,when the electrical stress aging tests were performed at a current density of 100 A/cm^2,LED devices with AlGaN interlayers are more likely to induce the generation/proliferation of defects in the active region under the effect of electrical stress,resulting in the reduced light output power at low current density.

Dependence of limited radiative recombination rate of InGaN-based light-emitting diode on lattice temperature with high injection

It is observed that the radiative recombination rate in InGaN-based light-emitting diode decreases with lattice temperature increasing.The effect of lattice temperature on the radiative recombination rate tends to be stable at high injection.Thus,there should be an upper limit for the radiative recombination rate in the quantum well with the carrier concentration increasing,even under the same lattice temperature.A modified and easily used ABC-model is proposed.It describes that the slope of the radiative recombination rate gradually decreases to zero,and further reaches a negative value in a small range of lattice temperature increasing.These provide a new insight into understanding the dependence of the radiative recombination rate on lattice temperature and carrier concentration in InGaN-based light-emitting diode.

Electroluminescence from the InGaN/GaN Superlattices Interlayer of Yellow LEDs with Large V-Pits Grown on Si(111)

A blue emission originated from In GaN/GaN superlattice（SL） interlayer is observed in the yellow LEDs with V-pits embedded in the quantum wells（QWs）, revealing that sufficient holes have penetrated through the QWs into SLs far away from the p-type layer. In the V-pits embedded LEDs, hole transport has two paths： via the flat c-plane region or via the sidewalls of V-pits. It is proved that the holes in SLs are injected from the sidewalls of V-pits, and the transportation process is significantly affected by working temperature, current density, and the size of V-pits. Four motion possibilities are discussed when the holes flow via the sidewalls. All these may contribute to a better understanding of hole transport and device design.

Carrier Dynamics Determined by Carrier-Phonon Coupling in InGaN/GaN Multiple Quantum Well Blue Light Emitting Diodes

Phonon sidebands in the electrolumiescence(EL) spectra of InGaN/GaN multiple quantum well blue light emitting diodes are investigated. S-shaped injection current dependence of the energy spacing(ES) between the zero-phonon and first-order phonon-assisted luminescence lines is observed in a temperature range of 100–150 K.The S-shape is suppressed with increasing temperature from 100 to 150 K, and vanishes at temperature above200 K. The S-shaped injection dependence of ES at low temperatures could be explained by the three stages of carrier dynamics related to localization states:(i) carrier relaxation from shallow into deep localization states,(ii) band filling of shallow and deep localization states, and(iii) carrier overflow from deep to shallow localization states and to higher energy states. The three stages show strong temperature dependence. It is proposed that the fast change of the carrier lifetime with temperature is responsible for the suppression of S-shaped feature.The proposed mechanisms reveal carrier recombination dynamics in the EL of InGaN/GaN MQWs at various injection current densities and temperatures.

Aging mechanism of GaN-based yellow LEDs with V-pits

GaN-based yellow light-emitting diodes(LEDs) on Si substrates are aged at a direct current density of 50 A/cm^2 for500 h. After the aging process, it can be found that the LEDs have a stable electrical property but their light output power is decayed by 4.01% at 35 A/cm^2. Additionally, the aging mechanism of GaN-based yellow LED is analyzed. It is found that the decay of light output power may be attributed to the following two reasons: one is the increase of Shockley–Rrad–Hall recombination and the other is the change of the transport path of holes via V-pits after aging, which may induce the radiative recombination current to decrease. In this paper, not only the aging mechanism of GaN-based yellow LED is investigated, but also a new possible research direction in LED aging is given.

Effect of Barrier Temperature on Photoelectric Properties of Ga N-Based Yellow LEDs

The effect of growth temperature of barriers on photoelectric properties of Ga N-based yellow light emitting diodes(LEDs)is investigated.It is found that as the barrier temperature increases,the crystal quality of multiquantum wells(MQWs)and the quality of well/barrier interface are improved,and the quantum well is thermally annealed,so that the indium atoms in the quantum well migrate to the equilibrium position,reducing the phase separation of the quantum well and improving the crystal quality of quantum wells(QWs).However,the external quantum efficiency(EQE)of the samples begins to decrease when raising the barrier temperature even further.One explanation may be that the higher barrier temperature destroys the local state in the quantum well and reduces the well/barrier interface quality.Therefore,a suitable barrier temperature is proposed,contributing to the improvement of the luminous efficiency of the yellow LEDs.

Effect of Mg-Preflow for p-AlGaN Electron Blocking Layer on the Electroluminescence of Green LEDs with V-Shaped Pits

In GaN-based green light-emitting diodes（LEDs） with and without Mg-preflow before the growth of p-Al GaN electron blocking layer（EBL） are investigated experimentally.A higher Mg doping concentration is achieved in the EBL after Mg-preflow treatment,effectively alleviating the commonly observed efficiency collapse and electrons overflowing at cryogenic temperatures.However,unexpected decline in quantum efficiency is observed after Mg-preflow treatment at room temperature.Our conclusions are drawn such that the efficiency decline is probably the result of different emission positions.Higher Mg doping concentration in the EBL after Mg-preflow treatment will make it easier for a hole to be injected into multiple quantum wells with emission closer to pGaN side through the（8-plane rather than the V-shape pits,which is not favorable to luminous efficiency due to the preferred occurrence of accumulated strain relaxation and structural defects in upper QWs closer to p-GaN.Within this framework,apparently disparate experimental observations regarding electroluminescence properties,in this work,are well reconciled.

Effect of Green Quantum Well Number on Properties of Green GaN-Based Light-Emitting Diodes

In GaN-based green light-emitting diodes（LEDs） with different green quantum well numbers grown on Si（111）substrates by metal organic chemical vapor deposition are investigated. It is observed that V-shaped pits appear in the AFM images with the green quantum well number increasing from 5 to 9, and results in larger reverse-bias leakage current. Meanwhile, in the case of the sample with the number from 5 to 7 then to 9, the external quantum efficiency increases firstly, and then decreases. These phenomena may be related to the size of V-shaped pits in the active area and the distribution of electrons and holes in the active area caused by V-shaped pits. The optimal number of green quantum wells is determined to be 7.

Effects of Hydrogen Treatment in Barrier on the Electroluminescence of Green InGaN/GaN Single-Quantum-Well Light-Emitting Diodes with V-Shaped Pits Grown on Si Substrates

Effect of hydrogen （142） treatment during the GaN barrier growth on the electroluminescence performance of green InGaN/GaN single-quantum-well light-emitting diodes （LEDs） grown on Si substrates is experimentally investigated. We prepare two LED samples with different carrier gas compositions during the growth of GaN barrier. In the H2 free LED, the GaN barrier is grown in full nitrogen （N2） atmosphere. For the other H2 treated LED, a mixture of N2 and H2 was used as the carrier gas. It is observed that V-shaped pits decrease in size after H2 treatment by means of the scanning electron microscope. Due to the fact that the p n junction interface would be closer to the p-GaN as a result of smaller V-shaped pits, the tunneling barrier for holes to inject into the InGaN quantum well would become thicker after tt2 treatment. Hence, the external quantum efficiency of the H2 treated LED is lower compared to the H2 free LED. However, LEDs would exhibit a better leakage behavior after H2 treatment during the GaN barrier growth because of more effective blocking of the threading dislocations as a result of the H2 etching at V-shaped pits.

智能物质的创制与应用

人工智能的崛起正深刻地影响着我们的社会,也催生出智能物质这一新兴的交叉研究领域.智能物质是由分子组装而成的复杂物质体系,包含传感、存储和驱动基元及其之间的相互作用网络.复杂相互作用导致“涌现”现象的产生,使智能物质能够胜任特定的功能,并可实现自主的学习、适应和进化.目前,智能物质的创制主要有“自下而上”和“自上而下”两种策略,前者通过设计分子基元及其相互作用,构造全新的智能体系;后者通过改造生命体等已有体系,赋予全新的功能角色.智能物质具有独特的应用场景,本文以柔性器件与软体机器人、生物医学应用为例,阐述了智能物质的优势和广阔前景.智能物质的研究是物质科学的机遇与挑战,也是人工智能领域的新方向.