D. G. Zhao

Improved performance of electroluminescent devices based on an europium complex

Electroluminescent (EL) devices using an europium complex Eu(DBM)3 bath as the electron-transport emitting layer were fabricated. The quenching effect of the metal cathode and the unstable nature of the Eu complex under EL operation markedly influence the EL efficiency. By keeping the emitting area far from the metal cathode and partly doping the Eu(DBM)3 bath layer with a hole-transport material, the EL performance was significantly improved. Sharp-band red emissions with turn-on voltage of 3 V, brightness of 820 cd/m2, and external quantum efficiency of 1% were achieved.

Carriers capturing of V-defect and its effect on leakage current and electroluminescence in InGaN-based light-emitting diodes

The effect of quantum well (QW) number on performances of InGaN/GaN multiple-quantum-well light-emitting diodes has been investigated. It is observed that V-defects, originated from various InGaN well layers intercepted by threading dislocations (TDs), increase in density and averaged size with more periods of QWs, resulting in larger reverse-bias leakage current and lower emission efficiency of light-emitting diodes. Conductive atomic force microscopy measurements demonstrate that V-defects may preferentially capture carriers, subsequently enhance local current and nonradiative recombinations at associated TD lines, which suggest that TD lines with V-defects at vertex have larger influence on emission efficiency than those without V-defects.

Exciplex emissions in bilayer and doped thin films containing a non-fluorescent gadolinium complex

Abstract Bilayer and doped thin films of a non-fluorescent gadolinium complex gadolinium(dibenzoylmethanato) 3 (bathophenanthroline) [Gd(DBM) 3 bath] and N,N ′-diphenyl- N,N ′bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (TPD) are prepared. The exciplex formation between TPD and Gd(DBM) 3 bath in doped film is confirmed through the measurement of the emission, excitation and absorption spectra. Exciplex emissions in electroluminescent process and the different electroluminescent spectra in the bilayer and the doped films are observed.

Infrared electroluminescence of ytterbium complexes in organic light emitting diodes

Abstract A comparison of near infrared (IR) emitting properties among Yb 3+ complexes containing devices was made. Ytterbium (dibenzoylmethanato) 3 (bathophenanthroline) [Yb(DBM) 3 bath] was used as electron-transport and emitting material. N , N ′-diphenyl- N , N ′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) acted as a hole-transport layer. Utilizing a device of trilayer structure ITO/TPD (40 nm)/Yb(DBM) 3 bath:TPD (weight ratio approx. 1:1, 40 nm)/Yb(DBM) 3 bath (80 nm)/Ag/Mg (150 nm), an intense emission corresponding to the 2 F 5/2 → 2 F 7/2 electronic transition of the trivalent Yb ion, Yb 3+ , was observed from the organic electroluminescent device. Current density dependence on drive voltage revealed good electron injection and transport properties of the Yb(DBM) 3 bath. The tendency of the efficiencies of IR and visible emissions under different current densities was discussed. A broad band at approximately 580 nm was observed in the EL spectrum, which could be assigned to exciplex emission.

Investigation on the compensation effect of residual carbon impurities in low temperature grown Mg doped GaN films

The influence of unintentionally doped carbon impurities on electrical resistivity and yellow luminescence (YL) of low-temperature (LT) grown Mg doped GaN films is investigated. It is found that the resistivity of Mg doped GaN films are closely related to the residual carbon impurity concentration, which may be attributed to the compensation effect of carbon impurities. The carbon impurity may preferentially form deep donor complex CN-ON resulting from its relatively low formation energy. This complex is an effective compensate center for MgGa acceptors as well as inducing YL in photoluminescence spectra. Thus, the low resistivity LT grown p-type GaN films can be obtained only when the residual carbon impurity concentration is sufficiently low, which can explain why LT P-GaN films with lower resistivity were obtained more easily when relatively higher pressure, temperature, or NH3/TMGa flow rate ratio were used in the LT grown Mg doped GaN films reported in earlier reports.

The investigation on carrier distribution in InGaN/GaN multiple quantum well layers

The carrier distribution and recombination dynamics of InGaN/GaN multiple quantum well (MQW) light-emitting diode structure are investigated. Two emission peaks were observed in the low temperature photoluminescence spectra of an InGaN/GaN MQW structure, but only one peak was observed in the electroluminescence (EL) spectra. Combined with the spatially resolved cathodoluminescence (CL) measurements, it is found that the electrically injected carrier distribution is governed by hole transport and diffusion in InGaN/GaN MQW structure due to the much lower mobility of hole. And the electron and hole recombination of EL occurs predominantly in the QWs that are located closer to the p-GaN layer.

Suppression of electron leakage by inserting a thin undoped InGaN layer prior to electron blocking layer in InGaN-based blue-violet laser diodes

InGaN-based blue-violet laser diodes (LDs) suffer from electron leakage into the p-type regions, which could be only partially alleviated by employing the electron blocking layer (EBL). Here, a thin undoped InGaN interlayer prior to EBL is proposed to create an additional forbidden energy range above the natural conduction band edge, which further suppresses the electron leakage and thus improve the characteristics of LDs. Numerical device simulations reveal that when the proper composition and thickness of InGaN interlayer are chosen, the electron leakage could be efficiently eliminated without inducing any severe accumulation of electrons at the interlayer, resulting in a maximum output power of the device.

Effect of V-defects on the performance deterioration of InGaN/GaN multiple-quantum-well light-emitting diodes with varying barrier layer thickness

The effect of quantum barrier (QB) thickness on performances of InGaN/GaN multiple-quantum-well light-emitting diodes (MQW LEDs) with relatively large barrier layer thicknesses has been investigated. It is found that the density and averaged size of V-defects increases with QB thickness, resulting in larger reverse- and forward-bias current in LEDs. Electroluminescence measurement shows that LED with thinner QB has higher internal quantum efficiency but lower efficiency droop-onset current density, which should be ascribed to the faster saturation of carrier leakage into V-defects. Correspondingly, above the droop-onset current density, severer Auger recombination and carrier overflow are induced by higher carrier density due to the less V-defect related carrier leakage, leading to the more serious droop phenomenon in LEDs with thinner QB.

Optical and structural characteristics of high indium content InGaN/GaN multi-quantum wells with varying GaN cap layer thickness

The optical and structural properties of InGaN/GaN multi-quantum wells (MQWs) with different thicknesses of low temperature grown GaN cap layers are investigated. It is found that the MQW emission energy red-shifts and the peak intensity decreases with increasing GaN cap layer thickness, which may be partly caused by increased floating indium atoms accumulated at quantum well (QW) surface. They will result in the increased interface roughness, higher defect density, and even lead to a thermal degradation of QW layers. An extra growth interruption introduced before the growth of GaN cap layer can help with evaporating the floating indium atoms, and therefore is an effective method to improve the optical properties of high indium content InGaN/GaN MQWs.

High efficient GaN-based laser diodes with tunnel junction

High-efficient GaN-based laser diodes (LDs) with tunnel junction are designed by replacing conventional p-type AlGaN cladding layers and p-type GaN contact with lower-resistant n-type AlGaN cladding layers and n-type GaN contact. In addition, the characteristics of the LDs with tunnel junction are numerically investigated by using the commercial software lastip. It is found that the performance of these LDs is greatly improved. As a comparison, the absorption loss and non-radiative recombination are greatly reduced. The threshold current and series resistance are decreased by 12% and 59%, respectively, and the slope efficiency is raised up by 22.3%. At an injection current of 120u2009mA, the output power and wall-plug-efficiency are increased by 34% and 79%, respectively.