Magnetic resonance studies of GaN based light emitting diodes

We report the application of electrical detection of magnetic resonance (EDMR) and electroluminescence detection of magnetic resonance (ELDMR) to study the recombination processes in InGaN/AlGaN double heterostructure p-n junctions. These techniques are especially well suited to the problems of defects in device structures in that they are much more sensitive than conventional paramagnetic resonance and are responsive to only those defects involved in the electrooptical properties of the structure. One resonance is observed at g≈2.00 and is identified as a Zn-related acceptor trap in the InGaN layer. A second resonance with g≈1.99 is identified as a deep donor.     

A study on full color organic light emitting diodes with blue common layer under the patterned emission layer

Color patterning steps for red, green, and blue emission layers (EMLs) are crucial for the production of full color organic light-emitting diodes (OLEDs). The most common method to form individually patterned EMLs is to use a shadow mask as the key component for patterning. However, most pixel defects are caused by such kinds of patterning steps. Therefore, skipping certain color patterning steps could significantly improve the production yield during the fine metal masking process in the OLED fabrication. A representative example of such approach is the top blue common layer (TBCL) structure with a non-patterned BCL on top of both green and red EMLs. However, this structure could cause blue color mixing in green or red devices. To prevent this effect, we propose a newly devised bottom BCL (BBCL) structure with the BCL that is totally separated from both green and red EMLs. In particular, we utilized 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile interlayer (7 nm) between the underlying BCL and the hole injection layer to completely extinguish blue emission. As a result, both green and red devices with the BBCL structure showed relatively better efficiencies compared to those with the TBCL structure without any color mixing.     

InGaN/GaN light emitting diodes with a p-down structure

Nitride-based p-down blue light emitting diodes (LEDs) were successfully fabricated. It was found that we could improve the crystal quality of these nitride-based p-down LEDs by inserting a codoped interlayer between the p-type cladding layer and MQW active layers. It was also found that the turn-on voltage could be reduced from 15 V to less than 5 V for the p-down LED with codoped layer and tunnel layer. The 20 mA output power was 1 mW for the p-down LED with an Mg+Si codoped interlayer and a rough p-tunnel layer.     

高反射性电流阻挡层用于提高InGaN/GaN发光二极管的光输出功率

由SiO₂/TiO₂分布布拉格反射镜(DBR)和Al镜组成的混合式反射电流阻挡层用于提高InGaN/GaN发光二极管的光输出功率。混合式反射电流阻挡层不仅增强了电流扩展效应而且有效的将射向p金属电极的光子反射防止其对p电极焊点附近光子的吸收。实验结果表明,淀积在p-GaN上1.5个周期的SiO₂/TiO₂DBR和Al镜在455nm垂直入射时的反射率高达97.8%。在20mA的工作电流下,与没有电流阻挡层的发光二极管相比,生长1.5对SiO₂/TiO₂ DBR和Al镜作为电流阻挡层的发光二极管的光输出功率提高了12.5%,且光输出功率的分布更加均匀。     

Atomic layer deposited zirconium oxide electron injection layer for efficient organic light emitting diodes

In this work, we demonstrate efficient polyfluorene-based light emitting diodes on which conformal, thin ZrO₂ layers, formed by atomic layer deposition at a relatively low temperature (175 °C), in order to avoid introducing any damage in the organic under layer, efficiently inject electrons from their high lying conduction band to the polymer’s LUMO. An optimal thickness of 2 nm for ZrO₂ results in a threefold improvement in luminous current efficiency compared to the reference device. The relationship between the thickness of the ZrO₂ layer and the device operational characteristics is further investigated and the possible reasons for the improved device performance are discussed based on the experimental results obtained by a combination of photoemission spectroscopy and electrical/optical measurements.     

Efficient blue emitting organic light emitting diodes based on fluorescent solution processable cyclic phosphazenes

Solution processable blue fluorescent dendrimers based on cyclic phosphazene (CP) cores incorporating amino-pyrene moieties have been prepared and used as emissive layers in organic light emitting diodes (OLEDs). These dendrimers have high glass transition temperatures, are monodisperse, have high purity via common chromatographic techniques, and form defect-free amorphous films via spin/dip coating. The solution processable blue light emitting OLEDs reach current efficiencies of 3.9 cd/A at brightness levels near 1000 cd/m². Depending on the molecular bridge used to attach the fluorescent dendron to the inorganic core, the emission wavelength changes from 470 to 545 nm, corresponding to blue and green light respectively. Via dilution experiments we show that this shift in emission wavelength is likely associated with molecular stacking of the amino-pyrene units.     

半导体GaN基蓝光发光二极管的精确电学特性

对传统的电容-电压(C-V)、电流 -电压(I-V)测量方法和我们自建的表征方法测量得到的蓝光发光二极管(L ED)正向特性的结 果做了详细的对比分析,发现传统测量方法得到的电学参量是不够精确的。但是传统C-V方法得到的表观电容以及我们自建方法得到的结电容在大电压和低 频率下都表现出了明显的负值,该实验结果与经典肖克莱理论相冲突。此外,我们精确 地得到了负的结电容以及结电导随电压和频率变化的经验表达式。这将为半导体二极管的正 向电学特性的理论研究提供实验基础。     

Abnormal blue shift of InGaN micro-size light emitting diodes

Blue InGaN micro-size light emitting diodes (LEDs) with diameters from 3 to 70 μm have been fabricated. An ion implantation technique and a 12 μm electro-ridge were used to simplify the fabrication processes. The 3–70 μm LEDs exhibiting a large emission of photon blue shift (40–240 meV) were observed in electro-luminescence (EL) spectra. The dependence of the blue shift on size is studied. The characteristics of the micro-size LEDs are also investigated numerically with the use of an advanced physical model of semiconductor devices (APSYS). The experimental measurements and simulation results are in close agreement for maximum blue shift. Base on the simulation, the difference between blue shift caused by band filling effect and red shift caused by lateral carrier confinement are approximately the same. Hence, the maximum blue shift increases when the size of micro-size LED increases because of decreased red shift resulted from thermal effect.     

Efficient non-doped blue light emitting diodes based on novel carbazole-substituted anthracene derivatives

In this study, we synthesized three anthracene derivatives featuring carbazole moieties as side groups - 2-tert-butyl-9,10-bis[4-(9-carbazolyl)phenyl]anthracene (Cz⁹PhAnt), 2-tert-butyl-9,10-bis{4-[3,6-di-tert-butyl-(9-carbazolyl)]phenyl}anthracene (tCz⁹PhAnt), and 2-tert-butyl-9,10-bis{4′-[3,6-di-tert-butyl-(9-carbazolyl)]biphenyl-4-yl}anthracene (tCz⁹Ph₂Ant) - for use in blue organic light emitting devices (OLEDs). The anthracene derivatives presenting rigid and bulky tert-butyl-substituted carbazole units possessed high glass-transition temperatures (220 °C). Moreover, the three anthracene derivatives exhibited strong blue emissions in solution, with high quantum efficiencies (91%). We studied the electroluminescence (EL) properties of non-doped OLEDs incorporating these anthracene derivatives, with and without a hole-transporting layer (HTL). OLEDs incorporating an HTL provided superior EL performance than did those lacking the HTL. The highest brightness (6821 cd/m²) was that for the tCz⁹PhAnt-based device; the greatest current efficiency (2.1 cd/A) was that for the tCz⁹Ph₂Ant-based device. The devices based on these carbazole-substituted anthracene derivatives also exhibited high color purity.     

InGaN/GaN light emitting diodes with Ni/Au mesh p-contacts

In order to improve the light transmittance and output efficiency of the InGaN/GaN multi-quantum well (MQW) light emitting diodes (LEDs), we proposed a novel Ni/Au mesh p-contact. As compared with the traditional Ni/Au film p-contact, the proposed Ni/Au mesh p-contact has the less light blocking nature yet still keeps well ohmic contact. Our lab result shows that for 470 nm wavelength the Ni/Au mesh p-contact has 95% light transmittance and 11.3 mW output power at a 20 mA injection current. In contrast, at the same 470 nm wavelength, the traditional Ni/Au film p-contact has 72% light transmittance and 8.12 mW output power at a 20 mA injection current.     

Advantage of InGaN-based light-emitting diodes with trapezoidal electron blocking layer

In this study, a trapezoidal-shaped electron blocking layer is proposed to improve efficiency droop of InGaN/GaN multiple quantum well light-emitting diodes. The energy band diagram, carrier distribution profile, electrostatic field, and electron current leakage are systematically investigated between two light-emitting diodes with different electron blocking layer structures. The simulation results show that, when traditional AlGaN electron blocking layer is replaced by trapezoidal-shaped electron blocking layer, the electron current leakage is dramatically reduced and the hole injection efficiency in markedly enhanced due to the better polarization match, the quantum-confined Stark effect is mitigated and the radiative recombination rate is increased in the active region subsequently, which are responsible for the alleviation of efficiency droop. The optical performance of light-emitting diodes with trapezoidal-shaped electron blocking layer is significantly improved when compared with its counterpart with traditional AlGaN electron blocking layer.     

Origin of the radiative emission in blue-green light emitting diodes based on GaN/InGaN heterostructures

Phase separation effects induced by spinodal decomposition taking place in cubic In_xGa_1−xN epitaxial layers were investigated by means of resonant Raman scattering (RRS) and X-ray diffractometry (XRD) experiments. The alloy epilayers were grown by radio-frequency plasma-assisted molecular beam epitaxy on GaAs (001) substrates. Ab initio theoretical calculation of the alloy phase diagram predicts the formation of In-rich phases in the layers which is confirmed by the RRS and XRD experiments. Photoluminescence observed at room temperature and 30 K from the layers shows light emission in the blue-green region of the spectrum. RRS experiments demonstrated that the observed emission is directly linked to the In-rich separated phases (quantum dots) in the alloy. The results support the model that the origin of light emission in nitride-based light emitting diodes and laser diodes is related to quantum confinement effects taking place in quantum dots formed in the InGaN layers, active media of the devices.     

Tunneling current and electroluminescence in InGaN: Zn,Si/AlGaN/GaN blue light emitting diodes

We investigate electrical and optical characteristics of Nichia NLPB-500 double-heterostructure blue light-emitting diodes (LEDs), measured over a wide temperature range from 10 to 300K. Current-voltage characteristics have complex character and suggest involvement of at least two different tunneling mechanisms. The peak energy of the optical emission follows the applied bias for voltages between 2.3–2.6 V and can be tuned in large spectral range from 2.3 up to 2.8 eV (yellow to blue). This behavior can be understood invoking the photon-assisted tunneling model which was previously successfully applied to highly doped GaAs LEDs. Even at the lowest temperatures, light emission still continues while the increase in the series resistance does not exceed a few tens of kΩ, which indicates absence of complete carrier freeze-out. On leave from P.N. Lebedev Physics Institute, Russian Academy of Sciences, Moscow, Russia On leave from High Pressure Research Center, Warsaw, Poland     

Modification effect of hole injection layer on efficiency performance of wet-processed blue organic light emitting diodes

We examined the performance of solution-processed organic light emitting diodes (OLEDs) by modifying the hole injection layer (HIL), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS). Atomic force microscopy (AFM) showed morphological changes with surface roughness (R_RMS) of 1.47, 1.73, and 1.37 nm for pristine PEDOT: PSS, PEDOT: PSS modified with a 40 v% deionized water and with a 30 v% acetone, respectively. The surface hydrophobicity of the acetone modified PEDOT:PSS HIL layer was decreased by 34% as comparing with the water modified counterpart. Electrical conductivity was increased to two orders of magnitude for the water and acetone modified PEDOT:PSS as compared to pristine. We observed a low refractive index and high transmittance for the modified HILs. We fabricated and explored electroluminescent properties of bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) based sky blue device by utilizing HIL with and without modification. The changes in electrical conductivity, surface roughness, refractive index, and transmittance of the modified HILs strongly influenced the performance of devices. By utilizing a 30% acetone modified HIL, the power efficiency was increased from 14.2 to 24.2 lm/W, an increment of 70% and the EQE from 8.5 to 13.1% at 100 cd/m², an increment of 54%. The maximum luminance also increased from 11,780 to 18,190 cd/m². The findings revealed herein would be helpful in designing and fabricating high efficiency solution processed OLEDs.     

High-brightness blue organic light emitting diodes with different types of guest-host systems

We demonstrate high-brightness blue organic light emitting diodes (OLEDs) using two types of guest-host systems. A series of blue OLEDs were fabricated using three organic emitters of dibenz anthracene (perylene), di(4-fluorophenyl) amino-di (styryl) biphenyl (DSB) and 4,4''-bis[2-(9-ethyl-3-carbazolyl)vinyl]biphenyl (BCzVBi) doped into two hosting materials of 4,4''-bis(9-carbazolyl) biphenyl (CBP) and 2-(4-biphenylyl)-5(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) as blue emitting layers, respectively. We achieve three kinds of devices with colors of deep-blue, pure-blue and sky-blue with the Commission Internationale de L''Eclairage (CIE) coordinates of (0.16, 0.10), (0.15, 0.15) and (0.17, 0.24), respectively, by employing PBD as host material. In addition, we present a microcavity device using the PBD guest-host system and achieve high-purity blue devices with narrowed spectrum.     

A new degradation phenomenon in blue light emitting silicon carbide diodes

In 6H-SiC blue emitting diodes prepared by sawing epitaxial wafers the development of greenish striations in the luminescing layer is observed together with a decrease in external quantum efficiency. It is proposed that this degradation is due to the development of numerous stacking faults leading to an intermediate (cubic?) state of lower band gap than the 6H modification.     

异质结堆叠发光层红色磷光有机发光二极管

本文采用主客体交错结构的发光层,即发光层是 由多组主体材料CBP和客体材料Ir(piq)₂(acac)异质结堆叠构成的。为了改善器件的性能 ,分别优化 了单主体层和单客体层的厚度。研 究表明,单主体层厚度为3~4 nm,单客体层厚度为0.3 nm时,器件能够获得的最大电流效率为3.92 cd/A,色纯度 和发光稳 定性俱佳,1mA工作电流下的CIE色坐标为(0.669,0.308),当工作电流从0.1 mA变化 到1mA,色度坐标的变化值(Δ(x,y)) 仅为(0.004,0.002)。所采用的 主客体交错发光层的制备方法,工艺简单,且因为能分别调整主客体层的厚度而改善因客体 分子聚集或因长程偶极子间相互作用对发光效率的影响,为非掺杂磷光有机发光二极管的制 备提供了思路。     

Reliability of AlGaInP light emitting diodes with an ITO current spreading layer

Three aging experiments were performed for AlGalnP light emitting diodes (LED) with or without indium tin oxide (ITO), which is used as a current spreading layer. It was found that the voltage of the LED with an ITO film increased at a high current stressing, while there was little change for that of the LED without the ITO. The results of the LEDs with different thicknesses of the ITO film show that the LED with a thicker ITO has a higher reliability. The main reason for the voltage increase of the LED with the ITO film might be the current crowding in the ITO film around the P-type electrode.     

ITO作为电流扩展层的AlGaInP 发光二极管可靠性研究

Three aging experiments were performed for AlGaInP light emitting diodes（LED） with or without indium tin oxide（ITO）,which is used as a current spreading layer.It was found that the voltage of the LED with an ITO film increased at a high current stressing,while there was little change for that of the LED without the ITO.The results of the LEDs with different thicknesses of the ITO film show that the LED with a thicker ITO has a higher reliability.The main reason for the voltage increase of the LED with the ITO film might be the current crowding in the ITO film around the P-type electrode.     

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.