We have investigated the effect of thermal annealing on the electrical characteristics of Pd/Au contact layer to p-GaN and on its crystalline ordering. While an as-deposited Pd/Au layer on p-GaN showed Schottkycontact characteristics, a thermally annealed Pd/Au layer yielded Ohmic characteristics, accompanying singlecrystalline ordering with an abrupt interface with GaN. The Ohmic contact characteristics and crystalline ordering of PdAu layer were attributed to the substantial elimination of oxidation layers at the interface during thermal annealing.Recently, the thin films of transition metals have drawn much attention due to increasing demands on various engineering applications such as heat-and corrosion-resistant coatings or metallization layers of microelectronic devices. 1,2 In particular, their roles in III-nitrides research have become undeniably significant in relation to the fact that a large work function of transition metals can improve Ohmic-contact characteristics to wide band-gap p-GaN.III-nitrides and related ternary alloys have been extensively investigated owing to their wide applicability to highpower electronic devices and short-wavelength optical devices. 3-5 Currently, one of the main concerns in this area pertains to accomplishing a reliable Ohmic contact to p-GaN with a low resistance and thermal stability. So far, many experiments on the electrical characteristics of metal contacts to p-GaN have been carried out, mainly focusing on the methodology of surface treatments. Various surface-cleaning treatments are known to effectively reduce a barrier height between a contact layer and p-GaN. 6,7 In this work, we have investigated the contact characteristics of the Pd/Au system to p-GaN, focusing on the effect of thermal annealing on the elimination of supposedly buried oxide layers at the interface. The removal of oxidation layers had a significant influence on the formation of an abrupt interface, accompanying a subsequent ordering of an alloyed Pd/Au layer and yielding an Ohmic-contact formation.Two-micrometers-thick p-GaN was grown by metalorganic chemical-vapor deposition on ͑0001͒ sapphire substrate. The hole concentration and resistivity were 3.0 ϫ10 17 cm Ϫ3 and 1.67 ⍀ cm, respectively. Prior to metal deposition, surface cleaning with acetone and methanol followed by etching in HF was performed to remove contaminants. Upon surface cleaning and rinsing, samples were placed in an e-beam evaporation chamber. The vacuum level and the deposition rate were maintained constant during operation at 7ϫ10 Ϫ7 Torr and 1 Å/sec, respectively. Two samples were prepared by depositing Pd and Au in sequence and the nominal thickness for each layer was 50 Å. One of the samples was thermally annealed at 550°C for 20 min in nitrogen ambience.Current-voltage characteristics of our samples are plotted in Fig. 1. For the as-deposited and the annealed sample, Schottky-and Ohmic-contact characteristics were observed, respectively. While some other groups reported that a nonalloyed Ohmic contact could be obtaine...
We examined the electro-optical characteristics of organic light emitting diodes according to the N,N'-diphenyl-N,N'-di(m-tolyl)-benzidine (TPD) thicknesses. The thicknesses of TPD were varied from 5 nm to 50 nm. The current density of the device with a TPD thickness of 5 nm was 8.94 times higher than that with a thickness of 50 nm at a driving voltage of 10 V. According to the conduction–current characteristics of conductors, the current densities improved with a decreasing TPD thickness. Different from the current density–voltage characteristics, the current efficiency–current density characteristics showed an improved efficiency with a 50 nm TPD thickness. The current efficiencies of a device with a 5 nm TPD thickness at a driving voltage of 10 V was 0.148 and at a 50 nm TPD thickness 0.993 cd/A, which was 6.7 times higher than the 5 nm TPD thickness. These results indicated that hole transport in Organic Light-Emitting Diode (OLED) devices were more efficient with thin 5 nm TPD than with thick 50 nm TPD, while electron transport was more efficient with thick 50 nm TPD, which caused conflicting results in the current efficiency-current density and current density-voltage characteristics according to TPD thicknesses.
Herein, we confirm the performance difference according to the structure of self-assembling monolayer (SAM) and investigate the characteristics of the indium tin oxide (ITO) surface when ITO substrates are deposited by (3,3,3-trifluoropropyl)trimethoxysilane (F-3SAM) and (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxysilane (F-10SAM) having different chain lengths with trifluoromethyl group as terminal functional group, as well as SAM benzoic acid (BA) and 2-naphthoic acid (NA) with benzene ring forms. Through these, it is possible to control the wetting properties, surface roughness, and work function of the ITO surface. Wetting characteristics, average roughness, and changes in work function of the ITO surface were characterized by contact angle measurement, atomic force microscopy (AFM), and UV photoelectron spectroscopy (UPS). The measured contact angles were 41.1°, 82.25°, and 118° for the bare ITO, NA, and F-10SAM, respectively, the average roughnesses of the SAM-modified surfaces were 1.377, 1.033, and 0.838 nm for the bare ITO, NA, and F-10SAM, respectively. The work function of the ITO surface modified with NA and F-10SAM increased from 0.21 and 0.36 eV to 5.01 and 5.16 eV, respectively. As a result, the surface properties of ITO were better for aliphatic SAM than for aromatic ring SAM.
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