Improvement of near-ultraviolet InGaN-GaN light-emitting diodes with an AlGaN electron-blocking layer grown at low temperature

The 400-nm near-ultraviolet InGaN-GaN multiple quantum well light-emitting diodes (LEDs) with Mg-doped AlGaN electron-blocking (EB) layers of various configurations and grown under various conditions, were grown on sapphire substrates by metal-organic vapor phase epitaxy system. LEDs with AlGaN EB layers grown at low temperature (LT) were found more effectively to prevent electron overflow than conventional LEDs with an AlGaN one grown at high temperature (HT). The electroluminescent intensity of LEDs with an LT-grown AlGaN layer was nearly three times greater than that of LEDs with an HT-grown AlGaN. Additionally, the LEDs with an LT-grown AlGaN layer in H/sub 2/ ambient were found to increase the leakage current by three orders of magnitude and reduce the efficiency of emission.     

Nitride-based near-ultraviolet multiple-quantum well light-emitting diodes with AlGaN barrier layers

The In_0.05Ga_0.95N/GaN, In_0.05Ga_0.95N/Al_0.1Ga_0.9N, and In_0.05Ga_0.95N/Al_0.18Ga_0.82N multiple-quantum well (MQW) light-emitting diodes (LEDs) were prepared by metal-organic chemical-vapor deposition. (MOCVD). It was found that the 20-mA electroluminescence (EL) intensity of the InGaN/Al_0.1Ga_0.9N MQW LED was two times larger than that of the InGaN/GaN MQW LED. The larger maximum-output intensity and the fact that maximum-output intensity occurred at a larger injection current suggest that Al_0.1Ga_0.9N-barrier layers can provide a better carrier confinement and effectively reduce leakage current. In contrast, the EL intensity of the InGaN/Al_0.18Ga_0.82N MQW LED was smaller because of the relaxation that occurred in the MQW active region of the sample.     

Improvement of InGaN-GaN light-emitting diodes with surface-textured indium-tin-oxide transparent ohmic contacts

Presents a surface-textured indium-tin-oxide (ITO) transparent ohmic contact layer on p-GaN to increase the optical output of nitride-based light-emitting diodes (LED) without destroying the p-GaN. The surface-textured ITO layer was prepared by lithography and dry etching, and dimensions of the regular pattern were approximately 3 /spl times/ 3 /spl mu/m. The operating voltage of the surface-textured LED was almost the same as that of the typical planar LED since the ITO layer was in ohmic contact with the p-GaN. The experimental results indicate that the surface-textured ITO layer is suitable for fabricating high-brightness GaN-based light emitting devices.     

Nitride-based green light-emitting diodes with high temperature GaN barrier layers

High-quality InGaN-GaN multiquantum well (MQW) light-emitting diode (LED) structures were prepared by temperature ramping method during metalorganic chemical vapor deposition (MOCVD) growth. It was found that we could reduce the 20-mA forward voltage and increase the output intensity of the nitride-based green LEDs by increasing the growth temperature of GaN barrier layers from 700/spl deg/C to 950/spl deg/C. The 20-mA output power and maximum output power of the nitride-based green LEDs with high temperature GaN barrier layers was found to be 2.2 and 8.9 mW, respectively, which were more than 65% larger than those observed from conventional InGaN-GaN green LEDs. Such an observation could be attributed to the improved crystal quality of GaN barrier layers. The reliability of these LEDs was also found to be reasonably good.     

Enhanced output power of near-ultraviolet InGaN-GaN LEDs grown on patterned sapphire substrates

Near-ultraviolet nitride-based light-emitting diodes (LEDs) with peak emission wavelengths around 410 nm were fabricated onto c-face patterned sapphire substrates (PSS). It was found that the electroluminescence intensity of the PSS LED shown 63% larger than that of the conventional LED. For a typical lamp-form PSS LED operating at a forward current of 20 mA, the output power and external quantum efficiency were estimated to be 10.4 mW and 14.1%, respectively. The improvement in the light intensity could be attributed to the decrease of threading dislocations and the increase of light extraction efficiency in the horizontal direction using a PSS.     

High-brightness inverted InGaN-GaN multiple-quantum-well light-emitting diodes without a transparent conductive layer

Unlike the conventional layer structure of an InGaN-GaN multiple-quantum-well light-emitting diode (LED), an LED with reversed p-type and n-type layer sequence, and an n+/p+ tunnel junction has been investigated. When operated at 20 mA, the output power of the inverted LED is almost twice that of the conventional LED. Since the structures of these two LEDs are alike when analyzed by X-ray diffraction, the improvement in the light intensity could be attributed to the elimination of the absorption/reflection by the transparent conductive layer and/or some quality improvement of p-type GaN in the inverted LED.     

Enhanced output power of InGaN-GaN light-emitting diodes with high-transparency nickel-oxide-indium-tin-oxide Ohmic contacts

This study develops a highly transparent nickel-oxide (NiO/sub x/)-indium-tin-oxide (ITO) transparent Ohmic contact with excellent current spreading for p-GaN to increase the optical output power of nitride-based light-emitting diodes (LEDs). The NiO/sub x/-ITO transparent Ohmic contact layer was prepared by electron beam in-situ evaporation without postdeposition annealing. Notably, the transmittance of the NiO/sub x/-ITO exceeds 90% throughout the visible region of the spectrum and approaches 98% at 470 nm. Moreover, GaN LED chips with dimensions of 300 /spl times/ 300 /spl mu/m fabricated with the NiO/sub x/-ITO transparent Ohmic contact were developed and produced a low forward voltage of 3.4 V under a nominal forward current of 20 mA and a high optical output power of 6.6 mW. The experimental results indicate that NiO/sub x/-ITO bilayer Ohmic contact with excellent current spreading and high transparency is suitable for fabricating high-brightness GaN-based light-emitting devices.     

Improvement of InGaN-GaN light-emitting diode performance with a nano-roughened p-GaN surface

This investigation describes the development of InGaN-GaN light-emitting diode (LED) with a nano-roughened top p-GaN surface which uses Ni nano-mask and wet etching. The light output of the InGaN-GaN LED with a nano-roughened top p-GaN surface is 1.4 times that of a conventional LED, and wall-plug efficiency is 45% higher. The operating voltage of InGaN-GaN LED was reduced from 3.65 to 3.5 V at 20 mA and the series resistance was reduced by 20%. The light output is increased by the nano-roughening of the top p-GaN surface. The reduction in the series resistance can be attributed to the increase in the contact area of nano-roughened surface.     

Improved ESD protection by combining InGaN-GaN MQW LEDs with GaN Schottky diodes

GaN Schottky diodes were built internally inside the GaN green LEDs by using etching and redeposition techniques. By properly selecting the etching areas underneath the bonding pads, one can minimize the optical loss due to the etching process. Although the reverse current and the forward turn-on voltage were both higher for the GaN LED with a Schottky diode, it was found that the internal Schottky diode could significantly increase the electrostatic discharge threshold from 450 to 1300 V.     

Temperature-dependent light-emitting characteristics of InGaN/GaN diodes

Temperature-dependent light-emitting and current-voltage characteristics of multiple-quantum well (MQW) InGaN/GaN blue LEDs were measured for temperature ranging from 100 to 500 K. The measurement results revealed two kinds of defects that have pronounced impact on the electroluminescent (EL) intensity and device reliability of the LEDs. At low-temperature (<150 K), in addition to the carrier freezing effect, shallow defects such as nitrogen vacancies or oxygen in nitrogen sites can trap the injected carriers and reduces the EL intensity. At high temperature (>300 K), deep traps due to the structure dislocations at the interfaces significantly reduce the efficiency for radiative recombination though they can enhance both forward and reverse currents significantly. In addition, the significant enhancement of trap-assisted tunneling current causes a large heat dissipation and results in a large redshift of the emission peak at high temperature.     

Superficial fluoropolymer layers for efficient light-emitting diodes

Fluoropolymers are characterized by high chemical inertness and, when in solid state, by superficial dipoles due to the C–F bond where the charge density is strongly displaced. These two characteristics are exploited here for fine control of charge balance in organic light-emitting devices and for preventing electrochemical interaction between heterogeneous layers. The insertion of a thin layer of polytetrafluoroethylene, PTFE, at the interface between poly(ethylene dioxythiophene):poly(styrene sulfonic acid), PEDOT:PSS, and an electroluminescent polymer leads to improved device efficiency and longevity. The presence of the superficial dipole increases the effective work function of the anode and improves the charge balance which enhances the external quantum efficiency, EQE, of the devices by up to a factor of two without significant effects on the luminance levels. The insertion of the PTFE layer reduces the photoluminescence quenching at the PEDOT:PSS/polymer interface, however we show that the EQE enhancement is mainly due to a better confinement of minority carrier electrons in the active layer. The lifetime of the devices shows a remarkable increase correlated with the insertion of the PTFE layer. Such improvements are ascribed to the reduced electrochemical interaction between the electroluminescent polymer and PEDOT:PSS due to the chemically inert nature of PTFE. The PTFE acts as a chemical zipper of two heterogeneous media with the added functionality of control over the charge balance.     

Low-operation voltage of InGaN-GaN light-emitting diodes withSi-doped In0.3Ga0.7N/GaN short-period superlatticetunneling contact layer

InGaN/GaN multiple-quantum-well light-emitting diode (LED) structures including a Si-doped In_0.23Ga_0.77N/GaN short-period superlattice (SPS) tunneling contact were grown by metalorganic vapor phase epitaxy. In_0.23Ga_0.77N/GaN(n⁺)-GaN(p) tunneling junction, the low-resistivity n⁺-In_0.3Ga_0.77 N/GaN SPS instead of high-resistivity p-type GaN as a top contact layer, allows the reverse-biased tunnel junction to form an “ohmic” contact. In this structure, the sheet electron concentration of Si-doped In_0.23Ga_0.77N/GaN SPS is around 1×10¹⁴/cm², leading to an averaged electron concentration of around 1×10²⁰/cm³. This high-conductivity SPS would lead to a low-resistivity ohmic contact (Au/Ni/SPS) of LED. Experimental results indicate that the LEDs can achieve a lower operation voltage of around 2.95 V, i.e., smaller than conventional devices which have an operation voltage of about 3.8 V     

High-quality AlGaN layers over pulsed atomic-layer epitaxially grown AlN templates for deep ultraviolet light-emitting diodes

In this paper, we report the pulsed atomic-layer epitaxy (PALE) of ultrahigh-quality AlN epilayers over basal-plane sapphire substrates and their use as templates to grow high-quality AlGaN layers with Al content ranging from 0.3 to 1. Symmetric/asymmetric x-ray diffraction (XRD) and room-temperature (RT) photoluminescence (PL) measurements were used to establish the high-structural and optical quality. The XRD (002) and (114) rocking-curve full-width at half-maximum (FWHM) values of the PALE-grown AlN epilayers were less than 60 arcsec and 250 arcsec, respectively. Using these ultrahigh-quality layers as templates, Si-doped AlGaN layers with a large Al content from 30% to 100% were grown and used for milliwatt power sub-280-nm, deepultraviolet (UV) light-emitting diodes (LEDs).     

具有相分离InGaN有源层的白光发光二极管

韩国Kwangju理工学院光电子材料中心和材料科学工程系利用相分离InGaN有源层，无需添加荧光材料，制造出了白光发光二极管。这种二极管的白光发射归因于分离相InGaN三元合金中铟组分和类量子点富铟区域尺寸的宽分布。     

Temperature-dependent electroluminescence in InGaN/GaN multiple-quantum-well light-emitting diodes

We present a comparative study on temperature dependence of electroluminescence (EL) of InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with identical structure but different indium contents in the active region. For the ultraviolet (UV) and blue LEDs, the EL intensity decreases dramatically with decreasing temperature after reaching a maximum at 150 K. The peak energy exhibits a large redshift in the range of 20–50 meV with a decrease of temperature from 200 K to 70 K, accompanying the appearance of longitudinal-optical (LO) phonon replicas broadening the low energy side of the EL spectra. This redshift is explained by carrier relaxation into lower energy states, leading to dominant radiative recombination at localized states. In contrast, the peak energy of the green LED exhibits a minimal temperature-induced shift, and the emission intensity increases monotonically with decreasing temperature down to 5 K. We attribute the different temperature dependences of the EL to different degrees of the localization effects in the MQW regions of the LEDs.     

P 型氮化镓层生长压力对InGaN/GaN基发光二极管性能的影响

The advantages of In Ga N/Ga N light emitting diodes(LEDs) with p-Ga N grown under high pressures are studied.It is shown that the high growth pressure could lead to better electronic properties of p-Ga N layers due to the eliminated compensation effect.The contact resistivity of p-Ga N layers are decreased due to the reduced donor-like defects on the p-Ga N surface.The leakage current is also reduced,which may be induced by the better filling of V-defects with p-Ga N layers grown under high pressures.The LED efficiency thus could be enhanced with high pressure grown p-Ga N layers.     

Improvement in the characteristics of GaN-based light-emitting diodes by inserting AlGaN-GaN short-period superlattices in GaN underlayers

We report the influence of short-period superlattice (SPSL)-inserted structures in the underlying undoped GaN on the characteristics of GaN-based light-emitting diodes (LEDs). The measurements of current-voltage (I-V) curves indicate that GaN-based LEDs having pseudomorphic Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL-inserted structures exhibit improvements in device characteristics with the best LED being inserted with two sets of five-pair Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL structure. Based upon the results of etch pit counts, double-crystal X-ray diffraction measurements and transmission electron microscopic observations of the GaN-based LEDs, it was found that the Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL-inserted structures tended to serve as threading dislocation filters in the LEDs so that the improved I-V characteristics were achieved.     

Properties of GaP light-emitting diodes grown on spinel substrates

Red and green GaP electroluminescent diodes have been successfully fabricated from GaP grown heteroepitaxially on spinel substrates by a vapor phase/liquid phase two-stage process. Current-voltage and light emission characteristics of the diodes are compared with those grown on bulk substrates. Quantum efficiencies up to 0·1 per cent in the red and 0·01 per cent in the green have been obtained.     

Changes in properties of GaN blue light-emitting diodes over time

Electrical conduction tests of metal-insulator-semiconductor(MIS)-structure GaN blue light-emitting diodes have been performed. Samples in which the emission intensity improved over time and samples in which the emission intensity degraded with time were observed. The pattern of change in both types was studied by measuring the device properties. Degradation of emission intensity was seen to be caused by changes in the emission and electron injection mechanism. Reference was made to the action of Zn in the I-S region.     

Mechanism of light production in metal-insulator-semiconductor diodes; GaN:Mg violet light-emitting diodes

The mechanism of electroluminescence in MIN GaN:Mg violet light-emitting diodes was analyzed by considering observed structural features of the diodes as well as their electrical and optical characteristics. Comparison of the observed I–V characteristic, including dependence upon temperature and upon film thickness, with all possible mechanisms for conduction in insulators lead to a proposed conduction mechanism of quantum mechanical tunneling, with the I–V characteristic being well represented by the Fowler-Nordheim equation. The proposed mechanism of light production involved impact ionization of luminescent centers near the i-n junction, with subsequent radiative recombination. This proposed mechanism was supported by measurements of carrier multiplication in the device and a steep voltage gradient at the i-n junction. The impact ionization process occurs in discrete regions coincident with sub-grain boundaries in the GaN film.