High barrier properties of transparent thin-film encapsulations for top emission organic light-emitting diodes

This paper reported a low-temperature thin film encapsulation (TFE) process based on atomic layer deposition Al₂O₃ layer for top-emission organic light-emitting devices (TE-OLEDs). The barrier characteristics of both H₂O-based and O₃-based Al₂O₃ films were investigated. O₃-based Al₂O₃ TFE showed lower water vapor transmission rate (WVTR) of 8.7 × 10⁻⁶ g/m² day and longer continuous operation lifetime of 5 folds compared to the device with H₂O-based Al₂O₃ TFE under identical environmental and driving conditions. Furthermore, the extraction of emitting light of the devices with barrier layer was enhanced compared to the bared one. The theory simulation data were consistent with our experimental results and showed the potential for the design of TFE structures optimized for enhancing light transmission.     

High-contrast top-emitting organic light-emitting diodes with AlO1.086 dark-and-conductive electrodes

To improve the poor contrast of conventional organic light-emitting diodes (OLEDs) resulting from highly reflective metal electrode, a dark-and-conductive electrode with an average reflectance of 28.1% and a resistivity of 4.6 × 10⁻⁴ Ω/cm was fabricated by fine-tuning O₂/Ar flow ratio on aluminum electrode sputtering. X-ray photoelectron spectroscopy analysis indicates pure aluminum and aluminum oxide coexist in the fabricated dark-and-conductive electrodes. With the proposed dark-and-conductive AlO_1.086 electrodes, top-emitting OLEDs exhibit significantly improved contrast, whereas maintain moderate luminous efficiency. The demonstrated AlO_1.086 dark-and-conductive electrodes can potentially replace the circular polarizers for high-contrast OLED display applications.     

原子层沉积方法制备低温多层Al2O3/TiO2复合封装薄膜的研究

原子层沉积(ALD)方法可以制备出高质量薄膜,被认为是可应用于柔性有机电致发光器件(OLED)最有发展前景的薄膜封装技术之一。本文采用原子层沉积(ALD)技术,在低温(80℃)下,研究了Al₂O₃及TiO₂薄膜的生长规律,通过钙膜水汽透过率(WVTR)、薄膜接触角测试等手段,研究了不同堆叠结构的多层Al₂O₃/TiO₂复合封装薄膜的水汽阻隔特性,其中5 nm/5 nm×8 dyads(重复堆叠次数)的Al₂O₃/TiO₂叠层结构薄膜的WVTR达到2.1×10^-5 g/m²/day。采用优化后的Al₂O₃/TiO₂叠层结构薄膜对OLED器件进行封装,实验发现封装后的OLED器件在高温高湿条件下展现了较好的寿命特性。     

Electrical properties and interfacial chemical environments of in situ atomic layer deposited Al2O3 on freshly molecular beam epitaxy grown GaAs

Interfacial chemical analyses and electrical characterization of in situ atomic layer deposited (ALD) Al₂O₃ on freshly molecular beam epitaxy (MBE) grown n- and p- GaAs (001) with a (4 × 6) surface reconstruction are performed. The capacitance-voltage (C-V) characteristics of as-deposited and 550 °C N₂ annealed samples are correlated with their corresponding X-ray photoelectron spectroscopy (XPS) interfacial analyses. The chemical bonding for the as-deposited ALD-Al₂O₃/n- and p-GaAs interface is similar, consisting of Ga₂O (Ga¹⁺) and As-As bonding (As⁰) without any detectable arsenic oxides or Ga₂O₃; the interfacial chemical environments remained unchanged after 550 °C N₂ annealing for 1hr. Both as-deposited and annealed p-GaAs metal-oxide-semiconductor capacitors (MOSCAPs) exhibit C-V characteristics with small frequency dispersion (<5%). In comparison, n-GaAs MOSCAPs shows much pronounced frequency dispersion than their p-counterparts.     

Si/Al2O3/ZnO:Al capacitor arrays formed in electrochemically etched porous Si by atomic layer deposition

High surface area Si/Al₂O₃/ZnO:Al capacitors were formed in electrochemically etched porous silicon. The Al₂O₃ dielectric and the ZnO:Al top electrode were deposited by atomic layer deposition in high aspect ratio porous Si. A single capacitor with a typical area of about 1 mm² consisted of about 10⁵ pores. Effective capacitance densities were between 2.0 and 2.5 μF/cm², i.e., approximately 30 times higher than for a planar capacitor prepared under identical conditions, illustrating the effect of the enhanced surface area in the porous structure.     

Optimization of Al2O3/ZrO2 nanolaminate structure for thin-film encapsulation of OLEDs

We report thin-film moisture barriers based on Al₂O₃/ZrO₂ nanolaminates grown by ALD for an encapsulation of OLEDs. In order to optimize the moisture-barrier performance of the nanolaminates, the most important factors affecting the performance were sought by measuring WVTR of the nanolaminates via an electrical Ca test. We found out that both the number of interfaces in the nanolaminates and the thickness of ZrO₂ in a unit layer were responsible for the performance. By optimizing the nanolaminate structure, the moisture-barrier performance was enhanced up to 350% from a single layer of the same thickness. The WVTR of 30-nm-thick optimized nanolaminate barrier was 2 × 10⁻⁴ g/(m² day) or less at ambient condition. A storage-lifetime measurement of an OLED with a 100-nm-thick encapsulation layer showed that it could exceed 70,000 h if stored at ambient condition.     

Atomic layer deposition of high capacitance density Ta2O5-ZrO2 based dielectrics for metal-insulator-metal structures

We have investigated electrical properties of laminated atomic layer deposited films: ZrO₂-Ta₂O₅, ZrO₂-Nb₂O₅-Ta₂O₅, ZrO₂-Ta_xNb_1−xO₅ and Ta₂O₅-Zr_xNb_yO_z. Even though the capacitances of laminates were often higher compared to films of constituent materials with similar thickness, considerably higher charge storage factors, Q, were achieved only when tetragonal ZrO₂ was stabilized in ZrO₂-Ta₂O₅ laminate and when the laminate thickness exceeded 50 nm. The decreased Q values in the case of most laminates were the result of increased leakage currents. In the case of thinner films only Ta₂O₅-Zr_xNb_yO_z stack possessed capacitance density and Q value higher than reference HfO₂. Concerning the conduction mechanisms, in the case of thinner films, the Ta₂O₅ or Ta_xNb_1−xO₅ apparently controlled the leakage either by Richardson-Schottky emission or Poole-Frenkel effect.     

Influence of Al2O3 barrier on the interfacial electronic structure of Au/Ti/n-GaAs structures

The Au/Ti/n-GaAs structures with and without Al₂O₃ interfacial layer have been fabricated.The Al₂O₃ interfacial layer has been formed on the GaAs substrate by atomic layer deposition.The effects of the interfacial layer on the current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the devices have been investigated in the temperature range of 60-300 K.It has been seen that the carrier concentration from C-V characteristics for the MIS (metal/insulating layer/semiconductor) diode with Al₂O₃ interfacial layer has a higher value than that for the reference diode without the Al₂O₃ interfacial layer (MS).Such a difference in the doping concentration has been attributed not to doping variation in the semiconductor bulk but to the presence of the Al₂O₃ interfacial layer because both diodes have been made on the pieces cut from the same n-type GaAs wafer.The temperaturedependent I-V characteristics of the MIS diode do not obey the thermionic emission current theory because of the presence of the Al₂O₃ layer.An electron tunneling factor,aδ(χ)^1/2,value of 20.64 has been found from the I-V-T data of the MIS diode.An average value of 0.627 eV for the mean tunneling barrier height,χ,presented by the Al₂O₃ layer has been obtained.     

Diffusion barrier properties of TaNx films prepared by plasma enhanced atomic layer deposition from PDMAT with N2 or NH3 plasma

Carbon free TaN_x films were deposited by plasma enhanced atomic layer deposition (PEALD) using a combination of pentakis(dimethylamino)Ta (PDMAT) and either N₂ or NH₃ plasma. Good linearity and saturation behavior were observed for the TaN_x films grown with NH₃ plasma while non-ideal saturation features were observed for the films grown with N₂ plasma. The thermal stability of the TaN_x films could be improved by reducing the pressure of the reactants and by increasing the plasma exposure time. The TaN_x films deposited using N₂ plasma exhibit better diffusion barrier properties than the films deposited using NH₃ plasma.     

Interface composition of atomic layer deposited HfO2 and Al2O3 thin films on InAs studied by X-ray photoemission spectroscopy

We present a synchrotron-based XPS investigation on the interface between InAs and Al₂O₃ or HfO₂ layers, deposited by ALD at different temperatures, for InAs substrates with different surface orientations as well as for InAs nanowires. We reveal the composition of the native Oxide and how the high-κ layer deposition reduces Oxide components. We demonstrate some of the advantages in using synchrotron radiation revealing the variation in Oxide composition as a function of depth into the subsurface region and how we can indentify Oxides even on nanowires covering only a small fraction of the surface.     

A very high-contrast top-emitting organic light-emitting diode with a Ni/ZnS/MgF2/Ni contrast-enhancing stack and a CuPc/C60 anti-reflection bilayer

A top-emitting organic light-emitting diode (TEOLED) with a very low luminous reflectance of 1.66% and very high contrast ratios of 405.9:1 and 1350.6:1 under on-state luminance of 300 and 1000 cd/m² is fabricated by using a Ni/ZnS/MgF₂/Ni contrast-enhancing stack (CES) and a copper-phthalocyanine (CuPc)/C60 anti-reflection (AR) bilayer. Although the introduction of the CES structure, it almost does not influence electrical performances of devices and a high luminance is obtained in TEOLED by using light output CuPc/C60 bilayer. The working mechanism on the reduced reflectance in high contrast TEOLED is further explored. Research shows that very low reflectance is attributed to both the effective absorption of C60 and CuPc and a destructive interference to the ambient light.     

基于PEDOT:PSS掺杂MoO3修饰有机电致发光器件性能的研究

有机电致发光器件因具有质量轻、亮度高、柔性 、宽视角和响应速度快等优点已经成 为下一代平板显示和照明领域的潜在主流技术。本文证实了MoO₃掺杂于PEDOT:PSS作为空 穴注入 层时,可以改善器件性能。与未掺杂器件相比,掺杂器件的亮度和效率显著提高,启亮电压 降低0.5 V。AFM,透光性和单空穴器件实验表明,当在ITO和空穴传 输层之间插入PEDOT:PSS :MoO₃后,由于修饰了ITO表面膜形貌,增加了绿光区透光性以及降低了空穴注入层电阻 从而提高了器件的性能。     

In situ atomic layer deposition and synchrotron-radiation photoemission study of Al2O3 on pristine n-GaAs(0 0 1)-4 × 6 surface

This work presents the in situ reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and synchrotron-radiation photoemission studies for the morphological and interfacial chemical characterization of in situ atomic layer deposited (ALD) Al₂O₃ on pristine molecular beam epitaxy (MBE) grown Ga-rich n-GaAs (1 0 0)-4 × 6 surface. Both the RHEED pattern and STM image demonstrated that the first cycle of ALD-Al₂O₃ process reacted immediately with the GaAs surface. As revealed by in situ synchrotron-radiation photoemission studies, two types of surface As atoms that have excess in charge in the clean surface served as reaction sites with TMA. Two oxidized states were then induced in the As 3d core-level spectra with chemical shifts of +660 meV and +1.03 eV, respectively.     

Fabrication of flexible organic light-emitting diodes with Alq3 as emitting layer

Fabrication of flexible organic light-emitting diodes（FOLEDs） with ITO/PVK ：TPD/Alq3/Al configuration prepared on PET substrates is reported. Alq3 is used as the light-emitting material. The curves of the current density vs. voltage,optical current vs. voltage and quantum efficiency vs. current density of the devices are investigated. Compared the devices with the ones that have the same configuration and are fabricated under the same conditions but on glass substrates,the characteristics of the two kinds of devices are very similar except that the threshold voltage of the flexible FOLEDs is a little higher. Under the driving voltage of 20V,the corresponding brightness and the external quantum efficiency are 1000 cd/m^2 and 0. 27%, respectively. In addition, the anti-bend ability of the devices is tested and the reasons of failure of the devices are analyzed.     

Thin-film encapsulation of top-emission organic light-emitting devices with polyurea/Al2O3 hybrid multi-layers

We developed a room-temperature encapsulation process based on multi-stack of ultra thin Al₂O₃ and polyurea layers for top-emission organic light-emitting devices (TEOLEDs). Device structure, including a capping layer for refractive-index matching and a thick polyurea buffer layer, was optimized to enhance light extraction without distorting electroluminescence spectrum. The efficiency of a TEOLED encapsulated with 5 pairs of Al₂O₃(50 nm)/polyurea(20 nm) layers was better than that of a glass-encapsulated TEOLED, whereas their color coordinates were almost identical. Moreover, the half-decay lifetime of a TEOLED encapsulated with 5 pairs of Al₂O₃/polyurea layers was 86% of that of a glass-encapsulated TEOLED. Water vapor transition rate of 5 pairs of Al₂O₃(50 nm)/polyurea(20 nm) layers on PET film was measured as low as 5 × 10⁻⁴ g/m² day.     

Resistive Switching Characteristics of Al2O3/ZnO Bilayer Thin Films for Flexible Memory Applications

Unipolar resistive switching behaviors of the ZnO and Al2O3/ZnO films fabricated on flexible substrates by pulse laser deposition were studied in this paper. The films were deposited at room temperature without post-annealing treatment during the process. X-ray diffraction results indicated that ZnO film has a dominant peak at (002). Scanning electron microscopy observation showed a columnar grain structure of the ZnO film to the substrate. The bilayer device of Al2O3/ZnO films had stable resistive switching behaviors with a good endurance performance of more than 200 cycles, high resistive switching ratio of over 103 at a read voltage of 0.1 V, which is better than that of the single oxide layer device of ZnO film. A possible resistive switching filamentary mode was demonstrated in this paper. The conduction mechanisms of high and low resistance states can be explained by space charge limited conduction and Ohmics behaviors. The endurance of the bilayer (BL) device was not degraded upon bending cycles, which indicates the potential of the flexible resistive switching random access memory applications.     

石墨烯掺杂Cs2CO3作为高效电子注入层的OLEDs性能研究

研制了石墨烯掺杂Cs₂CO₃(Cs₂CO₃:Graphe ne )作为高效电子注入层、结构为ITO/N,N′-bis-(1-naphthyl) -N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)(50 nm)/tris-(8-hydroxy quinoline)-aluminum Alq₃(80 nm)/Cs₂CO₃:Gra phene (mss 20% 1nm)/Al(120 nm)的OLEDs。将其与标准器件ITO/NPB(50 nm)/Alq₃(80 n m)/LiF(0.5 nm)/Al(120 nm)作性能比较,研究石墨烯掺杂在Cs₂CO₃中作为电子注入层 对 OLEDs性能的影响。结果表明,基于Cs₂CO₃:Graphene结构作为电子注入层的器 件效率要高于LiF作为电子注入层的器件,其最大电流效率达到2.02 cd/A, 是标准器件的2.59倍;亮度也高于LiF作为电子注入层的器件,在10 V时达 到最大值7690cd/m²,是标准器件最大亮度 的2.07倍。性能得到提高的主要机理是由于Cs₂CO₃:Graphene的引入提高了电子注入效率。     

Al2O3 stacks on In0.53Ga0.47As substrates: In situ investigation of the interface

In this work we present an in situ investigation of the interface composition between an In_0.53Ga_0.47As substrate and an Al₂O₃ oxide grown by molecular beam deposition in ultra high vacuum conditions. In the effort to improve the chemical quality of the interface, reduction of semiconductor-oxygen bonding at the interface can be obtained by growing a few Å thick pure Al layer before starting exposure of the surface to the atomic oxygen flux. Conversely, when a Ge interface passivation layer is intercalated between the semiconductor and the oxide stack, the interface chemistry is governed by Ge reaction with other species (Al, O), leading only to a partial suppression of the interface oxides.     

Improved retention characteristic of charge-trapped flash device with sealing layer/Al2O3 or Al2O3/high-k stacked blocking layers

Although programming and erase speeds of charge trapping (CT) flash memory device are improved by using Al₂O₃ as blocking layer, its retention characteristic is still a main issue. CT flash memory device with Al₂O₃/high-k stacked blocking layer is proposed in this work to enhance data retention. Moreover, programming and erase speeds are slightly improved. In addition, sealing layer (SL), which is formed by an advanced clustered horizontal furnace between charge trapping layer and Al₂O₃ as one of the blocking layers is also studied. The retention characteristic is enhanced by SL approach due to lower gate leakage current with less defect. With the combination of SL and Al₂O₃/high-k stacked blocking layer approaches, retention property can be further improved.     

Optical properties and tunable laser action of Verneuil-grownsingle crystals of Al2O3:Ti3+

Using the Verneuil technique, the authors have grown large single crystals of Al₂O₃:Ti³⁺ having concentrations up to 0.15%. Laser action was observed in this material, tunable over the range 700-810 nm. Losses in the 800-nm region are less than 0.03 cm^-1 (below the detection limit in the measurements)