Guanghan Fan

Performance enhancement of blue light-emitting diodes without an electron-blocking layer by using special designed p-type doped InGaN barriers

In this study, the characteristics of the nitride-based blue light-emitting diode (LED) without an electron-blocking layer (EBL) are analyzed numerically. The emission spectra, carrier concentrations in the quantum wells (QWs), energy band diagrams, electrostatic fields, and internal quantum efficiency (IQE) are investigated. The simulation results indicate that the LED without an EBL has a better hole-injection efficiency and smaller electrostatic fields in its active region over the conventional LED with an AlGaN EBL. The simulation results also show that the LED without an EBL has severe efficiency droop. However, when the special designed p-type doped InGaN QW barriers are used, the efficiency droop is markedly improved and the electroluminescence (EL) emission intensity is greatly enhanced which is due to the improvement of the hole uniformity in the active region and small electron leakage.

Improvement of Efficiency Droop in Blue InGaN Light-Emitting Diodes With p-InGaN/GaN Superlattice Last Quantum Barrier

The blue InGaN light-emitting diodes (LEDs) with specific designs of p-InGaN/GaN superlattice (SL) last quantum barrier are investigated numerically and experimentally. The proposed SL with a graded indium mole fraction from 0% to 5% shows improved efficiency droop and superior optical characteristics in comparison with the conventional LEDs. As indicated by the simulation results, the promotion of hole injection and the reduction of electron leakage play important roles in these improvements. Fabricated LEDs with this specific design exhibit stronger emission intensity, smaller forward voltage, and larger light output power compared to its counterparts.

Advantages of GaN-Based Light-Emitting Diodes With Polarization-Reduced Chirped Multiquantum Barrier

The advantages of GaN-based light-emitting diodes with polarization-reduced chirped multiquantum barrier (PR-CMQB) as electron blocking layer (EBL) are investigated numerically and experimentally. Both simulation and experiment results indicate that the LED with PR-CMQB possesses higher internal quantum efficiency and light output power as compared with its counterparts with either conventional single AlGaN EBL or CMQB. These improvements are mainly attributed to the suppression of electron leakage and the enhancement of hole injection efficiency. Furthermore, the efficiency droop is markedly reduced when the PR-CMQB is employed.

Performance of Blue LEDs With N-AlGaN/N-GaN Superlattice as Electron-Blocking Layer

The characteristics of InGaN-based blue light-emitting diodes (LEDs) with an AlGaN/GaN superlattice (SL) electron-blocking layer (EBL) of gradual Al molar fraction below the active region are analyzed numerically. The output power, internal quantum efficiency, electrostatic fields, energy band diagrams, carrier concentrations, radiative recombination, and electron leakage current are investigated. The results indicate that the LED with n-type AlGaN/GaN SL EBL of gradual Al molar fraction has smaller electrostatic fields and lower electron leakage in its active region than the LED with a rectangular p-AlGaN EBL or n-AlGaN EBL. These result in a markedly reduced efficiency droop.

Investigation of blue InGaN light-emitting diodes with AlGaN barriers of the increasing Al composition

The characteristics of blue InGaN light-emitting diodes with AlGaN barriers of different step-like growth range Al composition and gradually increasing Al composition are investigated numerically. The simulation results indicate that the enhanced electron confinement and hole injection efficiency are mainly attributed to the mitigated downward band bending of the last barrier induced by polarization field, and the improved carrier distribution is owing to the increasing blocking for electrons as well as the decreasing blocking for holes. Whats more, the output power, the distribution and rate of radiative recombination and the efficiency droop are markedly improved.

Efficiency enhancement of an InGaN light-emitting diode with a p-AlGaN/GaN superlattice last quantum barrier

In this study, a new design on last quantum barrier (LQB) is investigated numerically with the purpose of improving the optical performance of InGaN light-emitting diodes (LEDs). Through the analysis of the energy band diagrams, electrostatic fields, carrier concentrations, carrier current densities, and radiative recombination rates, we have got the simulation results that the proposed undoped-InGaN/AlInGaN superlattice (SL) LQB can significantly improve the output power and internal quantum efficiency, which is mainly attributed to the successful improvement in hole injection efficiency and suppression of electron current leakage. Moreover, the efficiency droop of the LEDs is improved slightly by using u-InGaN/AlInGaN SL as last barrier.

Numerical simulation of blue InGaN light-emitting diode with gradual Al and In composition p-AlInGaN electron-blocking layer

In this study, three kinds of electron-blocking layer (EBL) in blue InGaN light-emitting diodes (LEDs) are investigated numerically. They are conventional AlGaN EBL, AlInGaN EBL and gradual Al and In composition p-AlInGaN EBL. Through the analysis of the output power, internal quantum efficiency, turn-on voltage, energy band diagrams, carrier current densities and radiative recombination rates, we have got the simulation results that the LED with gradual Al and In composition p-AlInGaN EBL exhibits a higher output power, a lower electron leakage, a better hole injection efficiency and a more peaceable efficiency droop than the LED with a conventional AlGaN EBL or with a AlInGaN EBL.

First-principles prediction of a new class of photovoltaic materials: I-III-IV2-V4 phosphides

A new class of quaternary I-III-IV2-V4, including CuAlGe2P4, CuGaGe2P4, CuAlSn2P4, and CuGaSn2P4 are studied by density functional theory and beyond for potential photovoltaic application. We found that CuAlGe2P4 and CuGaGe2P4 have a ground state of kesterite (KS) structure, while CuAlSn2P4 and CuGaSn2P4 are nearly energetically degenerated for KS and stannite structures. Interestingly, the band gaps of all the studied quaternary compounds are predicted to be in the range of 1.1–1.7 eV by the hybrid functional calculation and Δ-sol approach [M. K.Y. Chan and G. Ceder, Phys. Rev. Lett. 105, 196403 (2010)]. In particular, CuAlSn2P4 in KS structure is predicted to be a potential high-efficiency photovoltaic material since it contains no rare or toxic elements with a direct gap around 1.52 eV.

Advantages of Blue InGaN Light-Emitting Diodes with a Mix of AlGaN and InGaN Quantum Barriers

In this study, a new design of quantum barriers has been investigated numerically with the purpose of improving the optical performance of blue InGaN light-emitting diodes (LEDs). Through analysis of energy band diagrams, carrier concentrations, carrier current densities, Auger recombination rates, and radiative recombination rates, we obtain simulation results showing that the proposed structure with a mix of AlGaN and InGaN quantum barriers can significantly improve the light output power and internal quantum efficiency (IQE), being mainly attributed to successful enhancement of the hole injection efficiency and suppression of the electron leakage current. Moreover, the efficiency droop of the LEDs is markedly improved by using the newly designed structure.

Performance improvement of InGaN blue light-emitting diodes with several kinds of electron-blocking layers

The performance of InGaN blue light-emitting diodes (LEDs) with different kinds of electron-blocking layers is investigated numerically. We compare the simulated emission spectra, electron and hole concentrations, energy band diagrams, electrostatic fields, and internal quantum efficiencies of the LEDs. The LED using AlGaN with gradually increasing Al content from 0% to 20% as the electron-blocking layer (EBL) has a strong spectrum intensity, mitigates efficiency droop, and possesses higher output power compared with the LEDs with the other three types of EBLs. These advantages could be because of the lower electron leakage current and more effective hole injection. The optical performance of the specifically designed LED is also improved in the case of large injection current.