Zheng Shuwen

Enhancement of blue InGaN light-emitting diodes by using AlGaN increased composition-graded barriers*

The characteristics of nitride-based blue light-emitting diodes (LEDs) with AlGaN composition-graded barriers are analyzed numerically. The carrier concentrations in the quantum wells (QWs), the energy band diagrams, the electrostatic fields, and the light output power are investigated by APSYS software. The simulation results show that the LED with AlGaN composition-graded barriers has a better performance than its AlGaN/InGaN counterpart owing to the increase of hole injection and the enhancement of electron confinement. The simulation results also suggest that the output power is enhanced significantly and the efficiency droop is markedly improved when the AlGaN barriers are replaced by AlGaN composition-graded barriers.

Electronic structures and energy band properties of Be- and S-doped wurtzite ZnO

The energy band properties, density of states, and band alignment of the BexZn1−xO1−ySy alloy (Be- and S-doped wurtzite ZnO) are investigated by the first-principles method. BexZn1−xO1−ySy alloy is a direct band gap semiconductor, the valence band maximum (VBM) and the conduction band minimum (CBM) of BexZn1−xO1−ySy are dominated by S 3p and Zn 4s states, respectively. The band gap and lattice constant of BexZn1−xO1−ySy alloy can be modulated by changing the doped content values x and y. With the increase in Be content value x in the BexZn1−xO1−ySy alloy, the band gap increases and the lattice constant reduces, but the situation is just the opposite when increasing the S content value y in the BexZn1−xO1−ySy alloy. Because the lattice constant of Be0.375Zn0.625O0.75S0.25 alloy is well matched with that of ZnO and its energy gap is large compared with that of ZnO, so the Be0.375Zn0.625O0.75S0.25 alloy is suitable for serving as the blocking material for a high-quality ZnO-based device.

Performance improvement of blue InGaN light-emitting diodes with a specially designed n-AlGaN hole blocking layer

Blue InGaN light-emitting diodes (LEDs) with a conventional electron blocking layer (EBL), a common n-AlGaN hole blocking layer (HBL), and an n-AlGaN HBL with gradual Al composition are investigated numerically, which involves analyses of the carrier concentration in the active region, energy band diagram, electrostatic field, and internal quantum efficiency (IQE). The results indicate that LEDs with an n-AlGaN HBL with gradual Al composition exhibit better hole injection efficiency, lower electron leakage, and a smaller electrostatic field in the active region than LEDs with a conventional p-AlGaN EBL or a common n-AlGaN HBL. Meanwhile, the efficiency droop is alleviated when an n-AlGaN HBL with gradual Al composition is used.

The influence of AlGaN/GaN superlattices as electron blocking layers on the performance of blue InGaN light-emitting diodes

P-AlGaN/P-GaN superlattices are investigated in blue InGaN light-emitting diodes as electron blocking layers. The simulation results show that efficiency droop is markedly improved due to two reasons: (i) enhanced hole concentration and hole carrier transport efficiency in AlGaN/GaN superlattices, and (ii) enhanced blocking of electron overflow between multiple quantum-wells and AlGaN/GaN superlattices.

Comparison of GaN-Based Light-Emitting Diodes by Using the AlGaN Electron-Blocking Layer and InAlN Electron-Blocking Layer

Optical properties of GaN-based light-emitting diodes (LEDs) are studied numerically by using AlGaN and InAlN electron-blocking layers (EBLs). Through the simulations of emission spectra, carrier concentration distribution, energy band, electrostatic field, internal quantum efficiency and output power, the results show that the LEDs with design of the InAlN EBL structure have a better performance over the original LEDs using an AlGaN EBL. The spectrum intensity and output power are enhanced significantly, and the efficiency droop of internal quantum efficiency is improved effectively with this design of InAlN EBL structure. It is proved that the strengths of carrier confinement and electron leakage current play a critical role in the performance of luminescence in LEDs.

Simulation study of blue InGaN multiple quantum well light-emitting diodes with different hole injection layers

InGaN-based light-emitting diodes with p-GaN and p-AlGaN hole injection layers are numerically studied using the APSYS simulation software. The simulation results indicate that light-emitting diodes with p-AlGaN hole injection layers show superior optical and electrical performance, such as an increase in light output power, a reduction in current leakage and alleviation of efficiency droop. These improvements can be attributed to the p-AlGaN serving as hole injection layers, which can alleviate the band bending induced by the polarization field, thereby improving both the hole injection efficiency and the electron blocking efficiency.

Performance enhancement of an InGaN light-emitting diode with an AlGaN/InGaN superlattice electron-blocking layer

The efficiency enhancement of an InGaN light-emitting diode (LED) with an AlGaN/InGaN superlattice (SL) electron-blocking layer (EBL) is studied numerically, which involves the light-current performance curve, internal quantum efficiency electrostatic field band wavefunction, energy band diagram carrier concentration, electron current density, and radiative recombination rate. The simulation results indicate that the LED with an AlGaN/InGaN SL EBL has better optical performance than the LED with a conventional rectangular AlGaN EBL or a normal AlGaN/GaN SL EBL because of the appropriately modified energy band diagram, which is favorable for the injection of holes and confinement of electrons. Additionally, the efficiency droop of the LED with an AlGaN/InGaN SL EBL is markedly improved by reducing the polarization field in the active region.

Electronic structures and optical properties of IIIA-doped wurtzite Mg0.25Zn0.75O

The energy band structures, density of states, and optical properties of IIIA-doped wurtzite Mg0.25Zn0.75O (IIIA = Al, Ga, In) are investigated by a first-principles method based on the density functional theory. The calculated results show that the optical bandgaps of Mg0.25Zn0.75O:IIIA are larger than those of Mg0.25Zn0.75O because of the Burstein—Moss effect and the bandgap renormalization effect. The electron effective mass values of Mg0.25Zn0.75O:IIIA are heavier than those of Mg0.25Zn0.75O, which is in agreement with the previous experimental result. The formation energies of MgZnO:Al and MgZnO:Ga are smaller than that of MgZnO:In, while their optical bandgaps are larger, so MgZnO:Al and MgZnO:Ga are suitable to be fabricated and used as transparent conductive oxide films in the ultra-violet (UV) and deep UV optoelectronic devices.

Performance improvement of GaN-based light-emitting diode with a p-InAlGaN hole injection layer

The characteristics of a blue light-emitting diode (LED) with a p-InAlGaN hole injection layer (HIL) is analyzed numerically. The simulation results indicate that the newly designed structure presents superior optical and electrical performance such as an increase in light output power, a reduction in current leakage and alleviation of efficiency droop. These improvements can be attributed to the p-InAlGaN serving as hole injection layers, which can alleviate the band bending induced by the polarization field, thereby improving both the hole injection efficiency and the electron blocking efficiency.

Performance improvement of blue light-emitting diodes with an AlInN/GaN superlattice electron-blocking layer

The characteristics of a blue light-emitting diode (LED) with an AlInN/GaN superlattice (SL) electron-blocking layer (EBL) are analyzed numerically. The carrier concentrations in the quantum wells, energy band diagrams, electrostatic fields, and internal quantum efficiency are investigated. The results suggest that the LED with an AlInN/GaN SL EBL has better hole injection efficiency, lower electron leakage, and smaller electrostatic fields in the active region than the LED with a conventional rectangular AlGaN EBL or a AlGaN/ GaN SL EBL. The results also indicate that the efficiency droop is markedly improved when an AlInN/GaN SL EBL is used.