Yu-Hsuan Lu

Enhanced efficiency of GaN-based light-emitting diodes with periodic textured Ga-doped ZnO transparent contact layer

GaN-based light-emitting diodes (LEDs) with indium tin oxide (ITO)/Ga-doped ZnO (GZO) composite oxide films serving as a transparent contact layer (TCL) were demonstrated. In this study, the wall-plug efficiency of LEDs (LED-III) with textured ITO/GZO composite TCL can be markedly improved by 200% and 45% of magnitude as compared to conventional LEDs with Ni∕Au TCL(LED-II) and planar ITO/GZO TCL(LED-I), respectively. Compared to LED-II, this enhancement is due to the enhanced light extraction efficiency of ITO/GZO composite TCL with high transparency. Compared to LED-I, ZnO-based TCL with a higher refractive index (n∼2.0) allows further enhancement of light extraction through the creation of a textured structure on transparent conductive oxide TCL deposited on the top surface of LEDs. In addition, the ITO/GZO composite TCL with a thickness of 550nm is far larger than that of Ni∕Au TCL with a thickness of approximately 15nm. Therefore, in addition to the effect of high transparency, the thicker ITO/GZO TCL...

Ga-Doped ZnO Transparent Conductive Oxide Films Applied to GaN-Based Light-Emitting Diodes for Improving Light Extraction Efficiency

In this study, Ga-doped ZnO (GZO) thin films were deposited on a sapphire substrate utilizing a magnetron sputtering approach. ZnO and Ga2O3 targets were employed as the sputtering sources during a cosputtering deposition. After thermal annealing in nitrogen ambient conditions, the electrical resistivity and optical transparency of the GZO films were analyzed in detail. The GZO films exhibited high transparency (~90%) in visible light and low resistivity (~5.3 x 10-4Omega-cm) when they were annealed at a temperature of 600-800deg C. Although the utilization of indium tin oxide (ITO) serving as the transparent contact layer (TCL) in conventional GaN-based light-emitting diodes (LEDs) is a well accepted technology, ZnO-based TCLs with a high refractive index of around 2.0 would render another advantage when a roughening process is performed on the surface. In other words, since packaged LEDs are generally encapsulated using epoxy with a refractive index of around 1.5, surface roughening performed on ITO TCL would thus result in only a minor improvement in light extraction because the typical refractive index of an ITO film prepared by our e-beam evaporator is around 1.7. In this study, GaN-based LEDs that utilized ITO/GZO composite oxide films as a TCL were also demonstrated. The light output power of an LED (LED-C) with a textured ITO/GZO composite TCL is markedly improved by 42 % and 48 % of magnitude as compared to LEDs with a planar GZO TCL (LED-A) and a ITO/GZO composite TCL (LED-B), respectively. This enhancement is due to the fact that a ZnO-based TCL with a higher refractive index (n~2.0) allows further enhancement of light extraction through the creation of a textured structure on the TCL that is deposited on the top surface of LEDs.

Efficiency enhancement in ultraviolet light-emitting diodes by manipulating polarization effect in electron blocking layer

The characteristics of the ultraviolet light-emitting diode (LED) with conventional and specifically designed electron blocking layers (EBLs) are investigated numerically and experimentally in this work. Simulation results show that delicately designed EBLs can not only capably perform the electron blocking function but also eliminate the incidental drawback of obstruction of hole injection caused by the nature of the large polarization field at the c-plane nitride heterojunction. It is shown that the polarization induced downward band bending can be mitigated when the portion of conventional EBL lying adjacent to the active region is replaced by a graduated AlGaN layer. The conduction band profile indicates that this replacement structure could have the capability of electron confinement similar to the conventional structure, and the valence band profile indicates that the spike induced by the polarization field is simultaneously eliminated, assisting the process of hole injection and distribution in the...

Study of InGaN-Based Light-Emitting Diodes on a Roughened Backside GaN Substrate by a Chemical Wet-Etching Process

The InGaN-based light-emitting diodes (LEDs) with a roughened backside on the N-face surface of GaN substrate were fabricated through a chemical wet-etching process to increase light-extraction efficiency. The stable crystallographic etching planes were formed as the GaN {101̅1̅} planes. When the near-ultraviolet and blue LED were operated as a forward current of 20 mA, the output power of LEDs was improved from 13.2 and 19.9 mW to 25.6 and 24.0 mW, respectively. The different enhanced ratio is attributed to the different transmittance as a function of wavelength is caused from hexagonal pyramid on N-face GaN substrate after wet-etching process.

Tailoring of polarization in electron blocking layer for electron confinement and hole injection in ultraviolet light-emitting diodes

The influence of the AlGaN electron blocking layer (EBL) with graded aluminum composition on electron confinement and hole injection in AlGaN-based ultraviolet light-emitting diodes (LEDs) are investigated. The light output power of LED with graded AlGaN EBL was markedly improved, comparing to LED with conventional EBL. In experimental results, a high increment of 86.7% can be obtained in light output power. Simulation analysis shows that via proper modification of the barrier profile from the last barrier of the active region to EBL, not only the elimination of electron overflow to p-type layer can be achieved but also the hole injection into the active region can be enhanced, compared to a conventional LED structure. The dominant factor to the performance improvement is shown to be the modulation of polarization field by the graded Al composition in EBL.

Hole Injection and Electron Overflow Improvement in 365 nm Light-Emitting Diodes by Band-Engineering Electron Blocking Layer

The work reports a theoretical and experimental study on the device performance of near ultraviolet light-emitting diodes (LEDs) with specific design on the electron blocking layer (EBL) by employing the band-engineering. The simulation results show the polarization-induced downward band bending is mitigated in the specific EBL design and, hence, the capability of hole transportation increases and the behavior of electron overflow decreases. The experimental results show the LEDs with specific EBL design exhibited a reduction of forward voltage from 4.40 to 4.07 V and a much enhancement of light output power from 30.6 to 51.9 mW, compared with conventional LED.

Investigation of lattice-modulated AlInGaN as a barrier layer in near-ultraviolet light-emitting diodes by numerical analysis and fabrication

The effects of using lattice-modulated AlInGaN as barriers in the active region were investigated in near-ultraviolet light-emitting diodes (LEDs). Both a stronger localization effect with wider barriers and a higher energy band gap existed in AlInGaN/InGaN LEDs, compared with GaN/InGaN LEDs. An increase in the carrier concentration in the active layer, a reduction in lattice mismatch that induced polarization mismatch in the active layer, and suppression of electron overflow can be found by numerical simulation. By 100 mA current injection, the AlInGaN/InGaN LED output power can be increased by 33.1%, compared with that of GaN/InGaN LED.

Improved Performance of InGaN/GaN Light-Emitting Diodes With Thin Intermediate Barriers

In this work, the performance of blue InGaN/GaN light-emitting diodes (LEDs) with thin intermediate barriers at high injection current is investigated. From the experimental results, it is found that the performance of LEDs with intermediate 5-nm-thick barriers is improved about 15% at 200 mA, compared with the sample with unique 9-nm-thick barriers. A numerical study is executed to analyze the hole distributions in the quantum wells. From the simulated results, it is found that the hole injection efficiency can be improved at high injection current. Hence, the effective recombination of electron and hole is also enhanced at high injection current.

Suppression of Nonradiation Recombination by Selected Si Doping in AlGaN Barriers for Ultraviolet Light-Emitting Diodes

The effect of selective Si doping on the emission efficiency in ultraviolet (UV) light-emitting diodes (LEDs) is investigated both experimentally and theoretically. The results show that the light output power increases with the number of Si-doped barriers (QBs). Experimental results indicate that compared with an all-undoped-QB LED, a factor of 3.17 can be achieved for the output power of an all-doped-QB LED at 350 mA. Detailed analysis on this phenomenon shows that the Si-doped QB is beneficial to suppress the nonradiative recombination rate by excess electrons in doped barriers.

Effect of AlGaN Si-Doped Barrier Layer on Optical Properties of Ultraviolet Light-Emitting Diodes

InGaN/AlGaN ultraviolet light-emitting diodes (UV LEDs) with AlGaN barriers having various Si doping concentrations are grown by metal–organic chemical vapor deposition. The light output power of UV LEDs was obviously improved as a result of Si doping of the AlGaN barriers. Detail analysis of this improvement by simulation modeling showed that the increase in Si doping concentration in AlGaN barrier is beneficial for increasing electron injection efficiency and simultaneously the radiative recombination distribution.