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Pentacene-based thin film transistors with ultrathin (6 nm) (HfO2)x(SiO2)1-x gate dielectric layers (x=0.25 and 0.75) were fabricated for low-voltage operation. The devices with ultrathin (HfO2)x(SiO2)1-x as the gate dielectric layer were operated at a gate voltage lower than -4.0 eV. However, the threshold voltage and drain current have different values depending on the composition of the (HfO2)x(SiO2)1-x gate dielectric layer. The device with (HfO2)0.75(SiO2)0.25 gate dielectrics, having larger capacitance, shows a higher drain current than that with (HfO2)0.25(SiO2)0.75 gate dielectrics. On the other hand, the device with (HfO2)0.25(SiO2)0.75 gate dielectrics, which has a larger work function, shows a lower threshold voltage. The in situ ultraviolet photoelectron spectroscopy shows that this is caused by the difference in electronic structures and by changes in band alignment of the interface between the pentacene and dielectric layers. [ABSTRACT FROM AUTHOR]

For this study, the phase-change materials Ge2Sb2Te5 and N-doped Ge2Sb2Te5 films were investigated using x-ray absorption near-edge structure and extended x-ray absorption fine structure. During the phase transition, change in electronic structure is observed by the shift of the absorption edge energy, i.e., structural coordination of Ge–Te changes from tetrahedral to octahedral coordination, of which the interatomic distances are 3.12 and 2.83 Å, respectively. In addition, nitrogen incorporation into the film led to a p-p orbital hybridization and a different crystallization behavior. The hybridization caused by the formation of a Ge–N bond was related to suppression of the phase transition. [ABSTRACT FROM AUTHOR]

The electronic structures of tris-(8-hydroquinoline) aluminum (Alq3)/Li2O/Al interfaces were studied using in situ ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS). The UPS and XPS spectra allowed us to evaluate the complete energy level diagrams and to analyze the chemical interactions at the interfaces. Inserting Li2O between Al and Alq3 led to the highest occupied molecular orbital (HOMO) of Alq3 shifting to a higher binding energy compared to that without Li2O, which resulted in an improved electron injection. We also observed that the magnitude of the secondary cutoff shift was almost identical to that of the HOMO shift with the insertion of Li2O. This implies that the energy level alignment depends on the interface dipole and ionization energy of the adsorbate. Additionally, a gap state was observed in the gap of Alq3, which is related to the interfacial reaction. The N 1s spectra revealed that there were destructive chemical reactions between Alq3 and Al, which could be prevented by inserting Li2O between them. [ABSTRACT FROM AUTHOR]

Changes in the electronic structures of Ge2Sb2Te5 (GST) and N-doped Ge2Sb2Te5 film during the phase transition from an amorphous to a crystalline phase were studied using synchrotron radiation high-resolution x-ray photoemission spectroscopy. The changes in tetrahedral and octahedral coordinated Ge 3d peaks are closely related to the changes in the chemical bonding state of GST films. The metallic Sb peak in the Sb 4d spectra of annealed GST films demonstrates that the metallic Sb atoms become segregated during thermal treatment resulting in phase separation. The incorporation of nitrogen into the GST film affects its structure and chemical bonding state, resulting in the suppression of crystallization. The incorporation of nitrogen also increases the optical band gap of the film due to the formation of a nitride. [ABSTRACT FROM AUTHOR]

The origin of the lowered electron injection barrier height of C60/Cu was investigated by in situ ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy (XPS). The onset of the highest occupied molecular orbital level was shifted by 0.2 eV toward high binding energy upon the heat treatment, resulting in the improved injection characteristics of the device. Moreover, an unexpected gap state has been observed at 1.2 eV below the Fermi level. The XPS core-level spectra revealed that the chemical reaction between C60 and Cu at the interface induced the gap state after heat treatment. The gap state pinned the Fermi level close to the lowest unoccupied molecular orbital of C60. We obtained the complete energy level diagram of C60/Cu before and after the heat treatment. [ABSTRACT FROM AUTHOR]

Structural, electrical resistivity, and magnetization properties of 200-MeV Ag+15-ion-irradiated Co-implanted ZnO thin films are presented. The structural studies show the presence of Co clusters whose size is found to increase with increase of Co implantation. The implanted films were irradiated with 200-MeV Ag+15 ions to fluence of 1×1012 ions/cm2. The Co clusters on irradiation dissolve in the ZnO matrix. The electrical resistivity of the irradiated samples is lowered to half. The magnetization hysteresis measurements show ferromagnetic behavior at 300 K, and the coercive field increases with the Co implantation. The ferromagnetism at room temperature is confirmed by magnetic force microscopy measurements. The results are explained on the basis of the close interplay between the electrical and the magnetic properties. [ABSTRACT FROM AUTHOR]

We fabricated ambipolar organic thin-film transistors (OTFTs) using C60 and pentacene. The electronic structure of the interface was investigated by using ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy. The magnitude of the interface dipole and the band bendings at the interface was determined, and the complete energy level diagram for C60 on pentacene (C60/pentacene) was obtained. The lowered band offsets, due to the enhanced charge redistribution in C60/pentacene relative to pentacene on C60, are favorable for the ambipolar OTFTs. The measured field-effect mobilities were 0.017 cm2/V s and 0.007 cm2/V s for the p-channel and the n-channel operations, respectively. The threshold voltages were -2 V for the p channel and 15.6 V for the n channel, comparable to those of unipolar OTFTs using C60 or pentacene. [ABSTRACT FROM AUTHOR]

Interfacial reactions as a function of the stack structure of Al2O3 and HfO2 grown on Si by atomic-layer deposition were examined by various physical and electrical measurements. In the case of an Al2O3 film with a buffer layer of HfO2, reactions between the Al2O3 and Si layers were suppressed, while a HfO2 film with an Al2O3 buffer layer on the Si readily interacted with Si, forming a Hf–Al–Si–O compound. The thickness of the interfacial layer increased dramatically after an annealing treatment in which a buffer layer of Al2O3 was used, while no change in thickness was observed in the film in which a HfO2 buffer layer was used. Moreover, the stoichiometric change caused by a different reaction process altered the chemical state of the films, which affected charge trapping and the interfacial trap density. [ABSTRACT FROM AUTHOR]

Optical properties of ZnS0.78Te0.22/ZnTe single quantum wells grown on GaAs (100) substrates by hot wall epitaxy technique with varying the ZnS0.78Te0.22 well width from 0.3 to 1.8 nm were investigated by photoluminescence (PL) measurements at low temperature and by temperature-dependent PL measurements. PL results show the evidence of type-II transition and their peak energy shifts to higher energies as the ZnS0.78Te0.22 well width decreases. In addition, temperature-dependent PL measurements show the increase of the activation energy as the well thickness decreases, indicating the increase of confinement effect. This study makes it possible to introduce proper band diagram for this structure, and can give very useful information on their device applications. [ABSTRACT FROM AUTHOR]

The thermal stability of Gd[sub 2]O[sub 3] films, containing added Zr, was investigated using various techniques. The structural characteristics of epitaxial Gd[sub 2]O[sub 3] are maintained on the Si(111) substrate when the Zr is codeposited along with Gd. The incorporation of ZrO[sub 2] into Gd[sub 2]O[sub 3] improves the crystallinity of the film and no interfacial layers were observed. In particular, the structural stability with no deformation is greatly enhanced, and silicate formation is drastically suppressed, up to an annealing temperature of 800 °C. The interfacial defects caused by extensive interactions between Gd and Si are also minimized. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

High-quality epitaxial Pt films were grown by a sputtering deposition method using epitaxial Y[sub 2]O[sub 3] as a dielectric buffer layer. A high degree of crystallinity was achieved with an ion-beam minimum channeling yield (χ[sub min]) of ∼11%, and a high degree of coherence between the film surface and interface was obtained for the Pt(111)/Y[sub 2]O[sub 3](111)/Si, with a large lattice mismatch. High-resolution transmission electron microscopy results showed that the atomic arrangement at the interface between the Pt and the oxide was well ordered, and no perceptible interdiffusion was observed, even at an annealing temperature of up to 700 °C under an oxygen atmosphere. The atomic arrangement at the Pt/Y[sub 2]O[sub 3] interface was drastically degraded after a high-temperature annealing at 900 °C due to the deformation of Y[sub 2]O[sub 3]. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structural characteristics and the chemical state of a HfO[sub 2]-A1[sub 2]O[sub 3] 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 AL[sub 2]O[sub 3] and HfO[sub 2]. [ABSTRACT FROM AUTHOR]

The thermal stability and structural characteristics for gate stack structure of HfO[sub 2] dielectrics deposited by atomic-layer deposition (ALD) were investigated. The structural characteristics and chemical state of the HfO[sub 2] 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 HfO[sub 2] films are closely affected by the interfacial layer and are depended on the thickness of the films. The 45 Å, thick HfO[sub 2] 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 and changes into the silicide layers. [ABSTRACT FROM AUTHOR]