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A flexible hazy substrate (FHS) with embedded air bubbles to increase light extraction efficiency of organic light-emitting diodes (OLEDs) is reported. In order to embed the air bubbles in the flexible substrate, micropatterned substrates are fabricated by plasma treatment, and then coated with a planarization layer. During the planarization layer coating, air bubbles are trapped between the substrate and the planarization layer. The haze of the FHS can be controlled from 1.7% to 68.4% by changing the size of micropatterns by adjusting the plasma treatment time. The FHS shows average haze of 68.4%, average total transmittance of 90.3%, and extremely flat surface with average roughness (R a) of 1.2 nm. Rigorous coupled-wave analysis and finite-difference time-domain simulations are conducted to demonstrate that the air bubbles in the substrate can effectively extract photons that are trapped in the substrate. The FHS increases the power efficiency of OLEDs by 22% and further increases by 91% combined with an external extraction layer. Moreover, the FHS has excellent mechanical flexibility. No defect has been observed after 10 000 bending cycles at bending radius of 4 mm.

Parallel aligned mesopore arrays in pyramidal-shaped GaN are fabricated by using an electrochemical anodic etching technique, followed by inductively coupled plasma etching assisted by SiO2 nanosphere lithography, and used as a promising photoelectrode for solar water oxidation. The parallel alignment of the pores of several tens of micrometers scale in length is achieved by the low applied voltage and prepattern guided anodization. The dry etching of single-layer SiO2 nanosphere-coated GaN produces a pyramidal shape of the GaN, making the pores open at both sides and shortening the escape path of evolved gas bubbles produced inside pores during the water oxidation. The absorption spectra show that the light absorption in the UV range is ∼93% and that there is a red shift in the absorption edge by 30 nm, compared with the flat GaN. It also shows a remarkable enhancement in the photocurrent density by 5.3 times, compared with flat GaN. Further enhancement (∼40%) by the deposition of Ni was observed due to the generation of an electric field, which increases the charge separation ratio.

Three primary colors, cyan, yellow, and green, are obtained from Ag nano-dot embedded organic light emitting diodes (OLEDs) by localized surface plasmon resonance (LSPR). By changing the thickness of the Ag film, the size and spacing of Ag nano-dots are controlled. The generated light from the emissive layer in the OLEDs interacts with the free electrons near the surface of the Ag nano-dots, which leads to LSPR absorption and scattering. The UV-visible absorption spectra of glass/ITO/Ag nano-dot samples show intense peaks from 430 nm to 520 nm with an increase of Ag nano-dot size. And also, the Rayleigh scattering spectra results show the plasmon resonance wavelength in the range of 470–550 nm. The effect of the LSPR of Ag nano-dots on the change of emission color in OLEDs is demonstrated using 2 dimensional finite-difference time-domain simulations. The intensity of the electro-magnetic field in the sample with 5 nm-thick Ag is low at the incident wavelength of 500 nm, but it increases with the incident wavelength. This provides evidence that the emission color change in OLEDs originates from LSPR at the Ag nano-dots. As a result, the emission peak wavelength of OLEDs shifted toward longer wavelengths, from cyan to yellow-green, with the increase of Ag nano-dot size.

A chemical approach was applied to decrease the work function of few-layer graphene. Li2CO3, K2CO3, Rb2CO3, and Cs2CO3 were used as n-doping materials. The sheet resistance of graphene doped with carbonate salt slightly increased from 1100 to 1700–2500 Ω/sq, and the transmittance of doped graphene with 0.1 M alkali metal at 550 nm decreased from 96.7 to 96.1–94% due to the formation of metal particles on the surface of graphene. A higher sheet resistance and lower transmittance were obtained at a higher concentration of alkali metal carbonate. The G peak in the Raman spectra was shifted to a lower wavenumber after alkali metal carbonate doping and the intensity ratio of the carbon double bond to the carbon single bond decreased with doping in the X-ray photoemission spectroscopy spectra, suggesting the charge transfer from metal ions to graphene sheets. Ultraviolet photoemission spectroscopy data showed that the work function of the graphene sheets decreased from 4.25 eV to 3.8, 3.7, 3.5, and 3.4 eV for g...

We investigated the electronic structures of alkali metal doped Alq3 molecules prepared by codeposition of Na and tris(8-hydroxyquinolinato)aluminum (Alq3) (Na:Alq3) using in situ synchrotron radiation photoelectron spectroscopy. The doping ratio of Na to Alq3 (RNa) was 0, 0.3, 0.6, 0.8, 2.7, 3.4, or 3.9. The work function, calculated from photoemission spectra, remained at 3.6 eV ± 0.03 eV for all samples, while the energy of the highest occupied molecular orbital increased from 2.2 to 2.85 eV as the doping ratio increased from RNa = 0 to RNa = 2.7. The work function and valence band spectra indicated that there is no band bending or surface work function change due to an alkali doping effect, in contrast to the findings of previous reports. The doped layer was composed of an n-type organometallic complex according to the analysis of O 1s and N 1s spectra. The N-type doping effects shown in the N 1s spectra of coevaporation samples were reflected in the schematic band diagram, so the energy difference be...

Cone-shaped nanostructures with controllable side-wall angle are success- fully fabricated with a SiO(2) nanosphere lithography (NSL) etching mask. Vertical LEDs with cone-shaped nanostructures with a 24.1° side-wall angle provide 6% more light output power compared to those using hexagonal pyramids formed by photochemical etching. This achievement is attributed to effective elimination of total internal reflection by angle-controlled nanostructures.

The mechanism of charge generation in metal oxide-based charge generation layers (CGLs) in tandem organic light emitting diodes (OLEDs) was studied via in situ synchrotron radiation photoelectron spectroscopy (SRPES) and in situ ultraviolet photoemission spectroscopy (UPS). The energy band structure and interface dipole energy of a CGL architecture comprising Ca doped tris(8-hydroxyquinoline) aluminum (Alq3), 4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (α-NPD), and various kinds of metal oxides are studied. The charge generation property is contributed to the amount of work function and interface dipole energy of metal oxide CGLs. The hole injection barrier at the metal oxide/α-NPD interface decreased as a function of the work function of the metal oxide. However, contrary to common belief, the large interface dipole resulted in a small hole injection barrier and low operation voltage of the device. Using data on interface energetics measured by in situ SRPES and UPS, it is shown that the work function ...

We investigated phase-controllable copper oxide (CuOx) as a hole injection layer for improving the electrical and optical properties of organic light emitting diodes (OLEDs). The phase of CuOx is changed from CuO to Cu2O by elevating the deposition rate during thermal evaporation, and a non-stoichiometric film mixing with two phases occurs in the intermediate condition. In non-stoichiometric films, unbonded oxygen atoms with a large density of gap states are induced near the Fermi level, acting as a hole transport path. Thus, holes can be more easily injected into the highest occupied molecular orbital states of α-NPD, lowering the hole injection barrier significantly. Films of non-stoichiometric CuOx are inserted at the interface of α-NPD with an Ag electrode, serving as the hole injection layer. Significantly improved current densities and luminance were achieved, reducing the operation voltage at 1 mA cm−2 from 6.1 V to 3.2 V.

Nanostructured vertical light-emitting diodes (V-LEDs) with a very dense forest of vertically aligned ZnO nanowires on the surface of N-face n-type GaN are reported with a dramatic improvement in light extraction efficiency (∼3.0×). The structural transformation (i.e., dissociation of the surface nitrogen atoms) at the nanolevel by the UV radiation and Ozone treatments contributes significantly to the initial nucleation for the nanowires growth due to the interdiffusion of Zn into GaN, evident by the scanning photoemission microscopy (SPEM), high-resolution transmission electron microscopy (HR-TEM), and ultraviolet photoelectron spectroscopy (UPS) measurements. This enables the growth of densely aligned ZnO nanowires on N-face n-type GaN. This approach shows an extreme enhancement in light extraction efficiency (>2.8×) compared to flat V-LEDs, in good agreement with the simulation expectations (∼3.01×) obtained from 3D finite-difference time-domain (FDTD) tools, explained by the wave-guiding effect. The further increase (∼30%) in light extraction efficiency is also observed by optimized design of nanogeometry (i.e., MgO layer on ZnO nanorods).

The effects of metal oxide diffusion on the interface dipole (ID) energy at a metal oxide (SnO2)/organic semiconductor (copper phthalocyanine, CuPc) interface were studied. In situ synchrotron radiation photoelectron spectroscopy and ultraviolet photoemission spectroscopy studies showed that the ID energy for SnO2-on-CuPc (−0.65 eV) was higher by 0.15 eV than that of CuPc-on-SnO2 (−0.50 eV). When SnO2 deposited on a CuPc layer, hot Sn atoms release enough condensation energy to disrupt the weakly bonded CuPc and diffuse through the surface. The diffused Sn atoms made a chemical reaction with nitrogen atoms in CuPc molecules and made organo-metallic compounds, Sn2CuPc, resulting in the generation of gap states at the former lowest unoccupied molecular orbital. This observation explains why the ID and hole injection barrier at SnO2-on-CuPc are larger than those at the CuPc-on-SnO2 interface. Organic light-emitting diodes with a SnO2-on-CuPc interface showed a lower luminous efficiency (2.63 cd/A) than that ...

We fabricated a self-connected and habitually tilted ZnO nanorod (NR) array, which is free of any patterning process for the connection of the NRs and is easily bent by a normal force. The vertically well-aligned ZnO NRs were grown by a strain relaxation process on MgO-buffered C-plane sapphire, and the remaining epitaxial ZnO wetting layer acted as a self-connecting layer of NRs. The epitaxial ZnO film on the step-terrace structured substrate caused the tilting angle from the surface normal direction (~0.2°) to match the step between the ZnO film and MgO-buffered C-plane sapphire, resulting in easy bending of the ZnO NRs by normal force. The unsymmetrical strain between the tensile and compressive stressed region in the habitually tilted ZnO NRs caused a gradient in the piezoelectric potential, resulting in an electrical field along the lateral direction of NR growth, resulting in the control of the current direction and level to be about 0.1 μA/cm(2) at 2 kgf normal force.

We demonstrated a novel contact structure of a silver (Ag) reflector that had an enhanced thermal stability at an elevated temperature. The Ag film was epitaxially grown on GaN by the insertion of an ultrathin Ni contact layer (∼10 A). The Ag film in the Ni/Ag structure, which was tensilely strained due to the expansion of the lattice constant by oxidation of Ni (3.532 A) to NiO (4.176 A), could compensate for the thermal compressive stress of 0.83 GPa that developed at the interface of Ag with GaN at 500 °C. The tensile stress, as determined by high resolution X-ray diffraction, was 0.24 GPa for a single period of the Ni/Ag contact, but it increased to 0.65 GPa for triple periods of the Ni/Ag contact, resulting in further suppression of Ag agglomeration and improved long-term thermal stability.

A number of oxygen vacancies were found in the MgO film when a nanoscale-metal embedded MgO film (MgO/nanoscale metal film/MgO) was exposed to a pulsed KrF excimer laser. This was due to the interfacial reaction between embedding nanoscale metal and MgO film. In a case of Nb embedding metal, the Nb2O5 crystallites were evidenced to proceed the reaction at the MgO/Nb interface; 5MgO + 2Nb → Nb2O5 + 5Mg + 5VO··. The oxygen vacancies formed the F centers in the MgO bandgap, resulting in high cathodoluminescence intensity at the 431 and 526 nm region, corresponding to F+ and F centers. The F centers in MgO played a role in increasing secondary electron emission coefficient (γ). As several kinds of nanoscale metals for creating F centers (Cr, Ti, V, In, Ta, and Nb) were embedded into MgO film, the γ values were found to be inversely linear with the ΔGf for the formation of metal oxides. The Nb embedded MgO film showed the highest γ value of ∼0.232 compared with normal MgO film (0.034).

Organic light emitting diodes (OLEDs) have rapidly progressed in recent years due to their potential applications in flat panel displays and solid-state lighting. In spite of the commercialization of OLEDs, they still have a low out-coupling efficiency of about 20% due to factors such as the total internal reflection, absorption, and surface plasmon coupling. This light out-coupling efficiency is a major limitation on the high efficiency levels of OLEDs. Hence, enhancing the light out-coupling efficiency of OLEDs offers the greatest potential for achieving a substantial increase in the external quantum efficiency and power efficiency of OLEDs. Accordingly, significant advancements in OLEDs have driven the development of light extraction technologies as well as highly transparent conducting electrode materials. Recent efforts to combine light extraction structures with the improved out-coupling efficiency of OLEDs have produced OLEDs with an efficiency level that matches the efficiency of a fluorescent tube (>100 lm/W). This paper reviews the technical issues and recent progress in light extraction technologies and discusses ways of enhancing the out-coupling efficiency of OLEDs.

Epitaxial growth of ZnO film on Si(111) substrates using an MgO interfacial layer has been investigated. The MgO layer acts as a thermodynamically stable buffer layer to suppress interfacial reaction between ZnO and Si, producing a clear interface. A domain-matched structure with mixed 4/3 and 5/4 domains was formed at the interface of Si and MgO to achieve the lowest domain mismatch. Moreover, the epitaxial ZnO film was grown on a MgO buffer layer with a domain matched structure of 11/12 domains, resulting in an (0001)[1210]ZnO∥(111)[110]MgO∥(111)[110]Si epitaxial relation. The high crystallinity of ZnO film grown on MgO/Si shows good optical performance with strong photoluminescence and improved Hall mobility.

We present the epitaxial growth of Mg-containing Ag film on p-type GaN using domain matching epitaxy. No epitaxial growth was found in the as-deposited Ag film, but it changed to epitaxial growth as Mg atoms were added into Ag film. This is due to the fact that Mg atoms were preferentially bonded to O ones, which played a role in shrinking the surrounded Ag lattice from 4.0871 to 4.0858. In addition to that, the volume expansion induced by Mg–O chemical bonding moves the Ag atoms to the site where they are energetically most stable at the Ag/GaN interface, resulting in strong adhesion between Ag films and GaN substrate. As a result, nine domains of the (111)-oriented Ag layer (2.558 nm) are matched with eight domains of the (0001) GaN layer (2.551 nm). This arrangement has the lowest domain mismatch of 8.9%, resulting in the epitaxial growth of the (111) Ag film with the lowest surface energy. Consequently, the Mg-containing Ag film shows better thermal stability, resulting in the suppression of Ag agglom...

The effects of derivatives in unsymmetrical distyrylbiphenyl compounds on the performance of blue organic light emitting diodes (OLEDs) were investigated. Organic compounds based on unsymmetrical distyrylbiphenyl were synthesized with respect to the numbers and locations of methyl and tert-butyl groups. The main peak wavelength of photoluminescence (PL) spectra in a solid state and the difference in the PL main peak between the solid and solution states decreased as the number of tert-butyl groups in unsymmetrical distyrylbiphenyl decreased. Furthermore, discernible diffraction peaks and roll-like grain were not observed as the number of tert-butyl groups increased, indicating that the tert-butyl group prohibits the formation of a planar structure and thus reduces chromophoric π–π interaction and fluorescence quenching. The luminance value of OLEDs used as an active layer in the tert-butyl–tert-butyl functional group was as high as 1860 cd/m2 at a current density of 150 mA/cm2 due to the reduction in fluo...

A uniform high-aspect-ratio nanotextured surface with a very dense forest of vertically aligned nanowires that was produced by surface chemical postfabrication of ZnO was induced by a structural transformation at the nanolevel. X-ray photoelectron spectroscopy verified the formation of the heterogeneous surface structures (GaOx on GaN and AuOx on Au) at the nanolevel as well as the modification of surface stoichiometry by UV/ozone treatment. The structural transformation contributes to generating a surface charge, acting as nucleation sites for nanotip growth. This led to a significant increase in density and improvement in diameter of the nanotips for various substrates, such as ZnO, Au, GaN, and graphene. However, the surface oxygen by O2 plasma treatment reduced the activity as activation sites of the exposed surface, resulting in the decrease of the density of the nanotips. This rational engineering of substrates by a structural transformation at the nanolevel may provide a promising method for produc...

An ultrathin iridium layer was treated with O2-plasma to form an iridium oxide (IrOx), employed as a hole extraction layer in order to replace poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) in organic photovoltaic (OPV) cells with poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The IrOx layer affects the self-organization of the P3HT:PCBM photo-active layer due to its hydrophobic nature, inducing a well-organized intraplane structure with lamellae oriented normal to the substrate. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.57 eV as the Ir layer on ITO changed to IrOx by the O2-plasma treatment. The OPV cell with IrOx (2.0 nm) exhibits increased power conversion efficiency as high as 3.5% under 100 mW cm−2 illumination with an air mass (AM 1.5G) condition, higher than that of 3.3% with PEDOT:PSS.

We investigate the crystallographic orientation and strain states of the Ni/Ag ohmic contacts on p-type GaN. The Ag film in the Ni/Ag contact was severely agglomerated during high temperature annealing in air ambient. As a results, after annealing for 24 h, the Ni/Ag contact shows non-linear I-V curve and low light reflectance of ∼21% at 460 ㎚ wavelength. High-resolution X-ray diffraction results show that the interplanar spacing of Ag (111) planes is almost same to that of bilk Ag after annealing for 24 hrs, indicating that the in-plane tensile strain in the Ag film was fully relaxed due to the Ag agglomeration.

We report a simple strategy to synthesize highly crystalline ruthenium dioxide (RuO2) nanowires by atmospheric pressure chemical vapor deposition in supercooled liquid nanodroplets without the use of catalyst particles. The nanowires are single crystals with no discernible amorphous layers or defects. The pretreatment of the substrate is found to be effective for the position controlled growth of the nanowires as small as 10 nm. The RuO2 nanowires have extremely low electrical resistivity of 62.5 ± 8.8 μΩ·cm and uniform electrical materials characteristics, regardless of size. By passing high current (>107 A/cm2) through the nanowires, very small nanogaps of less than 5 nm are produced, which can be explained by a heuristic model, in which electromigrative and surface diffusional effects are included.

We investigated the optical properties of a dielec-tric−metal−dielectric multilayer for the transparent top cathode in top-emitting organic light emitting diodes (TOLEDs). The optical transmittance of the metal layer was enhanced by depositing a dielectric material which had a high refraction index n below and above the metal (Ag) layer. Due to multiple reflections and interferences, the Ag layer sandwiched between dielectric materials with a high value of n can show improved transmittance. Because the WO3 had a high value of n (>2.0), a thin WO3 layer could fulfill the optimum zero-reflection condition with an Ag metal layer. Thus, a WO3/Ag/WO3 multilayer should have high transmittance with a low sheet resistance. The optimum thicknesses of both Ag and WO3 to obtain the best transmittance value were determined by theoretical calculation, and they agreed well with the experimental results. The best results were obtained for the thermally evaporated WO3 (300 A)/Ag (120 A)/WO3 (300 A) structure, a high tran...

We present the growth mechanism of MgO film on Si (100), specifically, epitaxial growth at the interface, strain relaxation at critical thickness, and preferred orientation formation after strain r...

We report how treatment of nickel (Ni) with O(2) plasma affects the polarity of Ni surface, crystallinity of pentacene film on the Ni, and electrical properties of pentacene organic thin-film transistors (OTFTs) that use Ni as source-drain electrodes. The polar component of surface energy in Ni surface increased from 8.1 to 43.3 mJ/m(2) after O(2)-plasma treatment for 10 s. From X-ray photoelectron spectra and secondary electron emission spectra, we found that NiO(x) was formed on the O(2)-plasma-treated Ni surface and the work function of O(2)-plasma-treated Ni was 0.85 eV higher than that of untreated Ni. X-ray diffraction and atomic force microscopy measurements showed that pentacene molecules are well aligned as a thin-film and grains grow much larger on O(2)-plasma-treated Ni than on untreated Ni. This change in the growth mode is attributed to the reduction of interaction energy between pentacene and Ni due to formation of oxide at the Ni/pentacene interface. Thus, O(2)-plasma treatment promoted the growth of well-ordered pentacene film and lowered both the hole injection barrier and the contact resistance between Ni and pentacene by forming NiO(x), enhancing the electrical property of bottom-contact OTFTs.

The enhancement of quantum efficiency using a suface modified Ag anode in top-emitting organic light emitting diodes (TEOLEDs) is reported. The operation voltage at the current density of 1 mA/cm 2 of TEOLEDs decreased from 9.3 V to 4.3 V as the surface of anode coated with CuOx layer. The work function of these structures were quantitatively determined using synchrotron radiation photoemission spectroscopy. Secondary electron emission spectra revealed that the work function of the Ag/CuOx structure is higher by 0.6 eV than that of Ag. Thus, the CuOx structure acts as a role in reducing the hole injection barrier by about 0.6 eV, resulting in a decrease of the turn-on voltage of top-emitting light emitting diodes.

We determined the interface dipole energies between interfacial layers with different thicknesses coated on indium tin oxides (ITOs) and tris(8-hydroxyquinoline) aluminum using ultraviolet and synchrotron radiation photoemission spectroscopy. After O 2 plasma treatment on 20-nm thick metal coated ITO, the work function and interface dipole energy increased. In 2-nm thick metal coated ITO, no change in the interface dipole energy was found though the work function increased. According to the valence band spectra, 2-nm thick metals are fully oxidized, but 20-nm thick metals are partially oxidized after O 2 plasma treatment. Therefore, it is considered that the contribution of the surface dipole by the deposition of Alq 3 on 2-nm thick metal is lower, resulting in a lower interface dipole. Thus, the thickness of interfacial layer has a great impact on the formation of interface dipole.

The effects of O2 plasma treatment on both the chemical composition and work function of an indium–tin-oxide (ITO) film were investigated. ITO films were deposited on non-alkali glass substrate by super density arc plasma ion plating for application in active-matrix organic light-emitting diodes (OLEDs). The water contact angle decreased from 38 to 11° as the ITO films were treated with O2 plasma for 60 s at a plasma power of 150 W, indicating an increase in the hydrophilicity of the surface. It was found that there were no distinct changes in the microstructure or electrical properties of the ITO films with O2 plasma treatment. Synchrotron radiation photoemission spectroscopy data revealed that O2 plasma treatment decreased the amount of carbon contamination and increased the number of unscreened states of In3+ and (O2)2- peroxo species. This played the role of increasing the work function of the ITO films by 1.7 eV. As a result, the turn-on voltage of the OLED decreased markedly from 24 to 8 V and the maximum luminance value of the OLED increased to 2500 cd/m2.

Low-resistance and high-transparency cobalt-doped indium–tin oxides (ITO) showing ferromagnetic behavior were obtained by magnetron cosputtering technique. The majority of Co atoms occupied In (or Sn) sites of ITO at 8% doping, leading to hole generation and a concomitant ferromagnetic response with a maximum magnetization of approximately 0.26 µB/Co. As the Co concentration increased to 14%, oxide precipitates were formed. The resistivity at 8% doping was as low as 4.28×10-3 Ωcm. The optical transmissions at 460 nm and 520 nm reached maximums of 72.7% and 86.6%, respectively.

Indium tin oxide (ITO) films were deposited on glass by super density arc plasma ion plating (SDAP-IP) method. In X-ray diffraction profiles, no obvious changes were found after O 2 plasma treatment. The transmittance, roughness, X-ray diffraction pattern, and sheet resistance also negligibly changed with O 2 plasma treatment. However, the water contact angle decreased with the O 2 plasma treatment, suggesting the increase of cohesive force between SDAP-IP ITO and organic materials. Synchrotron radiation photoemission spectroscopy showed that O 2 plasma treatment resulted in an increase of SDAP-IP ITO work function. Incorporation of oxygen atoms near the SDAP-IP ITO surface during the O 2 plasma treatment induced a peroxidic ITO surface, increasing the work function.

The samples with the Mn concentrations of 0.290% and 0.062% revealed that both in the calculated diffraction pattern and experimental pattern of transmission electron microscopy (TEM), the FCC lattice was still maintained after the Mn doping. The regularly spaced spots of ordered Mn produce the anomalous Hall effect (AHE) showing the characteristics of diluted magnetic semiconductor, which is caused by hole-mediated ferromagnetism due to the increase of hole concentration in tetrahedrally coordinated semiconductor. Ferromagnetic semiconductor of T c between 100 and 200 K demonstrated by TEM and AHE can be formed in diluted magnetic semiconductor based on InMnP:Zn epilayers additionally co-doped with Zn.

We report enhanced hole injection using an RuOx layer between indium tin oxide anodes and 4,4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl in organic light emitting diodes (OLEDs). The operation voltage of OLEDs at a current density of 100 mA/cm2 decreased from 17 V to 14 V and the maximum luminance value increased from 120 cd/m2 to 2500 cd/m2 upon transformation of the Ru layer to RuOx by surface treatment using O2 plasma. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.4 eV as the Ru layer was transformed to RuOx. Thus, the hole injection energy barrier was lowered, reducing the turn-on voltage and increasing the quantum efficiency of the OLEDs.

The effects of implanted N ions on the magnetic properties of Mn-implanted GaN were studied. The ferromagnetic signal increased when N ions were implanted into GaN prior to the implantation of Mn ions and annealed at 900°C. Synchrotron radiation photoemission spectroscopy revealed that the Ga−Mn magnetic phases contributing to ferromagnetic properties increased. Mn−N binary phases such as Mn6N258 and Mn3N2 decreased and sheet resistivity significantly increased, indicating a reduction of N-vacancies. Consequently, it is suggested that the enhancement of the ferromagnetic properties in (Mn+N)-implanted GaN originated from the decrease of N-vacancies and the increase of Ga−Mn magnetic phases.

We investigated the reaction chemistry of metal contacts to GaN during annealing using X-ray photoelectron spectroscopy (XPS). GaN decomposition was estimated, using XPS, to occur in N-2 annealed Ni-alloy contacts at 550 degreesC. The reaction was greatly accelerated by the catalytic effect of Au and Pt. The decomposition was correlated with the rapid degradation of electrical properties during annealing. The results suggest that high-temperature applications may be critically limited by the degradation of metal contacts especially due to activated Ni reactivity in Ni-alloyed contacts. Meanwhile, the thermal stability of Ni/Au contact greatly improves by suppressing the activated Ni reactivity, which is able to be obtained by forming preferential Ni-O bonding through annealing in air.

The effects of O2 inductively coupled plasma (ICP) treatment on the chemical composition and work function of indium-tin-oxide (ITO) surface were investigated. Synchrotron radiation photoemission spectroscopy showed that the O2 ICP treatment resulted in the increase of the ITO work function by 0.8 eV. Incorporation of oxygen atoms near the ITO surface during the ICP treatment induced a peroxidic ITO surface, increasing the work function. The enhanced oxidation of a thin Ni overlayer on the O2-ICP-treated sample suggests that preventing the migration of oxygen atoms into the active region of organic light-emitting diodes is important for improving device lifetime.

Electrical properties of rare-earth metal contacts on AlGaN/GaN heterostructure were interpreted in terms of the changes in microstructure and chemical bonding state. When the contacts were annealed under oxygen ambient at 500 °C, the Schottky barrier height increased from 0.56 to 1.10 eV for the Ru and from 0.68 to 1.07 eV for the Ir contact. Moreover, the reverse leakage current at −10 V dramatically reduced by 4 orders of magnitude by oxidation annealing. Such an improvement originated from the formation of RuO2 and IrO2, playing a key role in increasing the solubility of group-III atoms, namely, Ga and Al atoms. As a result, the surface Fermi level shifted toward the energy levels of group-III vacancies, resulting in the increase of Schottky barrier height. The electrical properties of heterostructure field effect transistor (HFET) applying the Ru gate contact significantly degraded when the device was annealed at 500 °C under O2 ambient. This was due to the indiffusion of the Ru atoms into the AlGaN ...