C. N. Whang

Thermal stability and structural characteristics of HfO2 films on Si (100) grown by atomic-layer deposition

The thermal stability and structural characteristics for gate stack structure of HfO2 dielectrics deposited by atomic-layer deposition (ALD) were investigated. The structural characteristics and chemical state of the HfO2 films in relation to the film thickness and postannealing temperature were examined by x-ray diffraction and x-ray photoelectron spectroscopy. An interfacial layer of hafnium silicate with an amorphous structure was grown on the oxidized Si substrate at an initial growth stage. The structural characteristics of the HfO2 films are closely affected by the interfacial layer and are depended on the thickness of the films. The 45 A thick HfO2 film with an amorphous structure was changed into a polycrystalline structure after rapid temperature annealing of 750 °C for 5 min, while thicker films were grown into a polycrystalline structure of monoclinic or tetragonal crystal structure. The silicate layer grown at the interfacial region is not stable even at 700 °C under ultrahigh vacuum condition...

The origin of the hole injection improvements at indium tin oxide/molybdenum trioxide/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl- 4,4′-diamine interfaces

We investigated the interfacial electronic structures of indium tin oxide (ITO)/molybdenum trioxide (MoO3)/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) using in situ ultraviolet and x-ray photoemission spectroscopy to understand the origin of hole injection improvements in organic light-emitting devices (OLEDs). Inserting a MoO3 layer between ITO and NPB, the hole injection barrier was remarkably reduced. Moreover, a gap state in the band gap of NPB was found which assisted the Ohmic hole injection at the interface. The hole injection barrier lowering and Ohmic injection explain why the OLED in combination with MoO3 showed improved performance.

Dielectric characteristics of Al2O3–HfO2 nanolaminates on Si(100)

The structural characteristics and the chemical state of a HfO2–Al2O3 nanolaminate structure, depending on the postannealing temperature, were examined by x-ray diffraction and x-ray photoelectron spectroscopy. The structural stability is significantly enhanced up to 870 °C and so is able to sustain its amorphous and laminate structure. However, the laminate structure is drastically broken at the annealing temperature of 920 °C and the crystallization is locally generated. In particular, the formation of the interfacial layer during the postannealing treatment is effectively suppressed in the nanolaminated structure. The dielectric constant of the nanolaminate structure calculated from the accumulation capacitance increases from ∼10 to ∼17 as the annealing temperature increases. This change is closely related to the degree of the mixture composed by Al2O3 and HfO2.

Influence of postannealing on polycrystalline pentacene thin film transistor

We studied systematically the influence of in situ postannealing treatment of ultrahigh vacuum grown polycrystalline pentacene thin film transistor. The gradual grain growth with the elimination of defects and misoriented crystallites is confirmed in x-ray diffraction (XRD) data and the atomic force microscopy image as the annealing temperature increases. The XRD data reveal that the pentacene molecules are packed parallel to each other in an upright position with a tilting angle of 15.5°. The postannealing results in the enhanced field effect mobility of pentacene organic thin film transistors increases from 0.19±0.04 to 0.49±0.05 cm2/V s after annealing at 90 °C. We suggest that the abnormally small on/off current ratio (∼103) due to the large leakage current is attributed to the conduction via impurity levels originated from the structural isomers of pentacene.

Epitaxial growth of Y2O3 films on Si(100) without an interfacial oxide layer

Heteroepitaxial Y2O3 films were grown on Si(100) substrates by the technique of reactive ionized cluster beam deposition. The crystallinity of the films was investigated with reflection high energy electron diffraction (RHEED), glancing angle x-ray diffraction (GXRD), and the interface was examined by high resolution transmission electron microscopy (HRTEM). Under the condition of 5 kV acceleration voltage at the substrate temperature of 800 °C, the Y2O3 film grows epitaxially on the Si(100) substrate. RHEED and GXRD results revealed that the epitaxial relationship between Y2O3 and Si(100) is Y2O3(110)//Si(100), and HRTEM observation showed a sharp interface without an amorphous layer.

Optical and luminescence characteristics of thermally evaporated pentacene films on Si

We report on the optical and luminescence properties of pentacene films deposited on n-Si by thermal evaporation at room temperature, 40, 60, 80, and 120 °C. Ellipsometric spectra of the films deposited at room temperature and 60 °C exhibited a main absorption peak at 1.82 eV and additional weak features at higher energy levels. Photoluminescence spectra taken at room temperature revealed band-to-exciton and band-to-acceptor transitions at 1.7 and 1.76 eV, respectively. An electroluminescence band was also observed near 1.65 eV under a forward bias from p-pentacene/n-Si diodes which show strong rectifying behavior.

Enhancing the electroluminescent properties of organic light-emitting devices using a thin NaCl layer

We report on the fabrication of organic light-emitting devices (OLEDs) using a thin NaCl interlayer as an electron-injection medium. The results show that the device containing the NaCl layer has a higher brightness and electroluminescent efficiency than the device without this layer. We also fabricated similar-structured comparable devices, which were prepared with a LiF layer as a different electron-injection medium. The maximum electroluminescent efficiency of the NaCl (1 nm)/Al cathode device was 2.85 cd/A, which is higher than the 2.25 cd/A of the LiF (1 nm)/Al cathode device. The ultrathin NaCl layer modified the carrier injection properties. In conclusion, the NaCl layer between a cathode and an emitting layer of OLEDs can be used as the carrier injection layer to improve the EL properties.

Room-temperature ferromagnetism of Cu-implanted GaN

1MeV Cu2+ ion was implanted into GaN with a dose of 1×1017cm−2 at room temperature. After implantation, the samples were subsequently performed by rapid thermal annealing at 700, 800, and 900°C for 5min. Both nonmagnetic Cu ion implanted samples annealed at 700 and 800°C exhibit the ferromagnetism at room temperature, and the saturation magnetization of these samples is estimated to be 0.057μB and 0.27μB per Cu atom from M-H curve, respectively. However, the sample annealed at 900°C does not show ferromagnetism due to clustering of Cu during the annealing process.

Biocompatibility and charge injection property of iridium film formed by ion beam assisted deposition

Iridium thin film formed by electron-beam evaporation with simultaneous bombardment of Ar ion beam was evaluated for a stimulating neural electrode. The electrochemical behavior of as-deposited Ir film on Ni-Ti sample was almost identical to bulk Ir by producing much higher open-circuit corrosion potential and much lower anodic current density than the uncoated Ni-Ti in both 1N sulfuric acid and saline solution. The charge injection capability of Ir film was compared with that of Pt electrode currently used mostly as a stimulating neural electrode. The charge density of Pt was small and unchanged with increasing number of activating cycles in 0.1M H(2)SO(4), whilst the Ir film continuously produced increases in charge density. The charge injection density of Ir film in physiological solution was higher for the more activated sample under the identical stimulating condition. Attachment and proliferation with PC12 cells on Ir-coated CP Ti without applying electrical stimulation was similar to the polished CP Ti. A network of neurons and extending axons were formed on Ir film.

Characterization of iridium film as a stimulating neural electrode

Iridium films having near-bulk properties were formed by electron-beam evaporation with simultaneous bombardment of Ar ion beam. The charge-injection capabilities of Ir film were investigated, and the detrusor pressure induced by S2 stimulation with Ir-coated Pt electrode was measured and compared with the uncoated Pt electrode. The charge densities of Ir film were continuously increased with increase in the number of cycles in 0.1 M H2SO4 due to the accumulation of the iridium oxide phase. The iridium oxide formed contained nano-pores, and oxides had different dielectric properties. The Ir film could inject various amounts of charge in physiological solution under the identical stimulating condition depending on the degree of activation in 0.1 M H2SO4. S2 stimulation by Ir-coated Pt electrode caused more efficient bladder contraction of the male dog than the uncoated Pt electrode under the identical stimulus condition.