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We examined the transistor properties of an N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8)/quaterrylene (QT) heteromolecular layer, in which the highly ordered molecular layers were stacked on the monolayer level. Ambipolar behavior was clearly observed when behaving as a field effect transistor (FET). The field effect mobilities and threshold voltages were evaluated to gain an insight into the injection and transport of the carriers. The obtained carrier mobilities for holes and electrons were calculated to be 3.6 × 10−3 and 2.7 × 10−2 cm2 V−1 s−1, respectively. These values are comparable to those of the respective FET behaviors of QT (p-channel) and PTCDI-C8 layers (n-channel), indicating that a well-defined heteromolecular interface is an essential factor for improving carrier transport in ambipolar FETs. On the other hand, in the PTCDI-C8 layer, a marked shift from 5 to 61.6 V was observed in the threshold voltage for electron transport. We concluded this drastic change in threshold voltage to be due to the presence of hole carriers in the underlying QT layers. This finding demonstrates the potential to manipulate threshold voltage according to the electronic states of the underling layer without disturbing carrier transport.

The temperature dependences of the photoluminescence (PL) spectra of N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8)/Quaterrylene (QT) heteromolecular thin film and neat-PTCDI-C8 thin film were investigated to elucidate the effects of a heteromolecular interface on exciton relaxation and recombination processes. The PL spectra were deconvoluted into six components related to Frenkel, two charge-transfer (CT), and excimer exciton states. With the heteromolecular thin film, we found the behavior of the CT excitons to be intriguingly different in terms of luminous efficiency and temperature dependence from that of the Frenkel exciton. In this Letter, we provide detailed analyses of the exciton relaxation and recombination processes at a heteromolecular interface.

A sigle-electron tunneling (SET) in a metal-insulator-semiconductor (MIS) structure is demonstrated, in which C60 and copper phthalocyanine (CuPc) molecules are embedded as quantum dots in the insulator layer. The SET is found to originate from resonant tunneling via the energy levels of the embedded molecules, (e.g., the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)). These findings show that the threshold voltages for SET are tunable according to the energy levels of the molecules. Furthermore, SET is observable even near room temperature. The results suggest, together with the fact that these properties are demonstrated in a practical device configuration, that the integration of molecular dots into the Si-MIS structure has considerable potential for achieving novel SET devices. Moreover, the attempt allows large-scale integration of individual molecular functionalities.

We investigate the difference of flatband voltage (Vfb) behavior in high-k/SiO2/Si stack structure due to oxygen vacancy (Vo) and additional oxygen generated by the reduction and oxidation annelaing process, respectively. The Vfb of Mg and La-incorporated Hf-based high-k dielectrics is also influenced by Vo generation in high-k layer. We found that the non-linear relationships of Vfb behavior appears in HfSiOx, Mg2+- and La3+-HfSiOx dielectrics as a function of the oxidation annealing temperature, while the HfO2, N3+-HfSiOx, Mg2+- HfO2, and La3+-HfO2 dielectrics show the linear relationships of Vfb shift by introducing additional oxygen. Furthermore, it is clear that the Vfb shift of all high-k materials satisfies the diffusion equation; which indicates that the oxygen transfer in high-k layer is a dominant factor in determining Vfb. We found that the oxygen diffusion in high-k materials can be ordered as follow: Mg2+- and La3+-HfO2 > HfO2 >> N3+-HfSiOx and La3+-HfSiOx > HfSiOx and Mg2+-HfSiOx.

We reveal the effect of UV–ozone treatment on the electrical properties of poly (styrenesulfonate) doped poly (3,4-ethylenedioxythiophene) (PEDOT:PSS) film by clarifying the respective roles of UV light irradiation and exposure to ozone gas. The UV–ozone treatment induced increases in both work function and resistivity. Furthermore, the film thickness was reduced at a rate of 0.13 nm/min. The ozone-exposed films also exhibited a marked increase in the work function. However, such variations were not observed in the resistivity and film etching. Angle resolved X-ray photoelectron spectroscopy revealed that the main role of the UV light was to decompose the chemical bonds in the PEDOT:PSS film, resulting in a resistivity increase and film etching. In contrast, the ozone and atomic oxygen were absorbed and oxidized the surface, which was responsible for the increase in the work function. Due to these different functions, UV–ozone treatment is capable of controlling the work function and resistivity of PEDOT:PSS film thus allowing them be adjusted to the device application.

We have elucidated the electronic states for Ru/SiON/Si(100) and Pt/HfO2/Pt structures under device operation by means of bias-application in hard x-ray photoelectron spectroscopy (BA-HXPES). For the Ru/SiON/Si(100) structure, interface electronic states are increased and new states near the conduction band minimum and valence band maximum are formed compared to the case of the Ru/SiO2/Si(100) structure. This is because nitrogen atoms at the SiO2/Si interface cause bond breaking and weakened the Si-O and Si-Si bonds at the interface. For the Pt/HfO2/Pt structure, oxygen atoms migrate to the top electrode under forward bias, forming Pt-O bond at the Pt/HfO2 interface. Under reverse bias, on the other hand, oxygen and Hf atoms are migrated to the top electrode, resulting in the formation of Pt-O and Pt-Hf bonds at the interface. The migration and bond formation at the interface should form vacancy in the HfO2 film, which might induce resistance switching behavior in this system. Here, it is emphasized that we present our approach using BA-HXPES, which successfully detects the electronic states under device operation. Therefore BA-HXPES could be applicable to various kinds of advanced materials and will be indispensable for evaluating their physical properties in detail.

In this study, a co-sputtering method with In2O3 and SiC targets was used to fabricate carbon-doped In-Si-O (In1-xSixO1-yCy) as the channel material for oxide thin-film transistors (TFTs). Three types of In1-xSixO1-yCy channels, namely, In0.88Si0.12O0.99C0.01 (Si0.12C0.01), In0.76Si0.24O0.99C0.01 (Si0.24C0.01), and In0.60Si0.40O0.98C0.02 (Si0.40C0.02), were prepared. After annealing at 300 °C, the Si0.24C0.01 and Si0.40C0.02 films retained an amorphous structure, while the Si0.12C0.01 films exhibited a body-centered-cubic structure. However, all the In1-xSixO1-yCy films maintained a smooth surface with a root-mean-square roughness of approximately 0.28 nm, despite structural differences. Results showed that the conductivities of all the In1-xSixO1-yCy films were not sensitive to the O2 partial pressure during sputtering, indicating that In1-xSixO1-yCy films exhibit more stable electrical conductivity than other InOx-based oxides. The field-effect mobility (μFE) with respect to the Si concentration of In1-xSixO1-yCy and In1-xSixO TFTs showed very similar behavior. In contrast, the threshold voltage (Vth) behavior of the two types varied dramatically, with the In1-xSixO TFTs Vth value increasing drastically from −57.7 to 9.7 V with increasing Si concentration, and the Vth of In1-xSixO1-yCy TFTs increasing only gradually from −9.2 to 2.4 V. This indicates that incorporated carbon has a significant effect on Vth at a low Si concentration due to strong C—O bond formation. The highest bond dissociative energy occurs between O and C atoms in the In1-xSixO1-yCy channel. The amount of oxygen vacancy in Si0.12C0.01, Si0.24C0.01, and Si0.40C0.02 was 18.9%, 13.3%, and 12.9%, respectively. As a result, the Si0.12C0.01 TFT exhibited superior transistor properties of Vth = −9.2 V while maintaining a μFE of 32.4 cm2/Vs. Therefore, an In1-xSixO1-yCy film is significantly advantageous as a channel material for oxide TFTs given that it can result in mobility exceeding 30 cm2/Vs.In this study, a co-sputtering method with In2O3 and SiC targets was used to fabricate carbon-doped In-Si-O (In1-xSixO1-yCy) as the channel material for oxide thin-film transistors (TFTs). Three types of In1-xSixO1-yCy channels, namely, In0.88Si0.12O0.99C0.01 (Si0.12C0.01), In0.76Si0.24O0.99C0.01 (Si0.24C0.01), and In0.60Si0.40O0.98C0.02 (Si0.40C0.02), were prepared. After annealing at 300 °C, the Si0.24C0.01 and Si0.40C0.02 films retained an amorphous structure, while the Si0.12C0.01 films exhibited a body-centered-cubic structure. However, all the In1-xSixO1-yCy films maintained a smooth surface with a root-mean-square roughness of approximately 0.28 nm, despite structural differences. Results showed that the conductivities of all the In1-xSixO1-yCy films were not sensitive to the O2 partial pressure during sputtering, indicating that In1-xSixO1-yCy films exhibit more stable electrical conductivity than other InOx-based oxides. The field-effect mobility (μFE) with respect to the Si concentration of In1-...

We have investigated the role of hetero interface of ionic high-k / covalent SiO2 for Pt gate MOS capacitors with strong and weak ionic high-k dielectrics on Vfb shift. The HfO2-based high-k and HfSiON materials of the strong ionic oxide caused the positive Vfb shifts due to dipole, while the Vfb of the HfSiOx-based high-k materials of the weak ionic oxide was almost constant. These suggest that the ionicity of high-k materials is an important because the oxygen diffusion which is related to the dipole formation strongly depends on the ionicity.

We investigated ZrO2-based new dielectric materials for random access memory by pulsed laser deposition (combi-PLD). We found that substitution of yttrium (Y) to ZrO2 system suppressed the leakage current effectively. MIM capacitor property of the promising composition candidates found in this system showed leakage current density less than 5×10-7 A/cm2 and the dielectric constant was 20. In addition, substitution of titanium (Ti) and tantalum (Ta) caused the dielectric constant up to ~ 22 within the allowed leakage level as well as 5×10-7 A/cm2 . Zr-Y-Ti-O and Zr-Y-Ta-O system have a potential for high dielectric materials as a new material of DRAM capacitors instead of SiO2.

The band alignment at the interface between Pt and O-terminated ZnO(0001) was investigated by depositing Pt films on ZnO using X-ray photoelectron spectroscopy and ultra violet photoelectron spectroscopy in an ultrahigh vacuum system. Angle-resolved X-ray photoelectron spectroscopy measurement showed the band was bent down by 0.06 eV at the ZnO(0001) surface. The binding energy of Zn 2p doublet shifted toward higher values by 0.37 eV when Pt was deposited on ZnO(0001). The work function of ZnO(0001) was 4.08 eV and the valence band maximum measured by UPS on the clean ZnO(0001) surface was 2.82 eV. As a result, the Schottky barrier height of Pt/ZnO(0001) was 0.72 eV in this experiment.

The dielectric degradation under electrical stress application was attributed to generation of oxygen vacancy Vo, and structural transformation" in HfSiON films, in addition to widely known charge trapping. The former two result from ion drift under electrical field due to its ionic character of Hf-based dielectrics. While Vo generation and charge trapping are atomic-range phenomena, it is surprising that long-range structural transformation can also occur under electrical filed before dielectric breakdown even at room temperature. The transformation is derived by intensive mass transport in the dielectric film, thereby can not be annealed out.

We succeeded in applying electron-beam-induced current (EBIC) technique to visualize the current leakage sites in the MOS devices with HfSiON gate dielectrics. The occurrence of initial leakage sites in fresh devices has been investigated with consideration of the effects of well type, gate size and gate electrode. Stress-induced leakage and breakdown sites were studied by in situ voltage stress and EBIC characterization. Finally, the micro-structure and physical damages at various leakage sites were characterized by TEM. Based on the EBIC and TEM results, the leakage and breakdown mechanisms of HfSiON gate stacks were discussed.

X-ray photoelectron spectroscopy (XPS) measurements of a Pt/HfO2(SiO2)/Si metal-oxide-semiconductor (MOS) structure under a bias voltage applied between the gate metal and the silicon substrate were studied. The binding energy shifts of Pt 4f, Hf 4f, O 1s and Si 2p according to the applied voltage were investigated using the MOS structure. After the influence of measurements on the results was carefully examined under various conditions, the amount of the shifts was analyzed from a viewpoint of band alignment. Based on the experimental results, a new way of interpreting the deviation of the electric properties from the ideal ones in a band diagram was proposed. It was demonstrated that the biased XPS is a very powerful method to understand the origin of the electric properties of MOS. Copyright © 2010 John Wiley & Sons, Ltd.

The authors have investigated the effects of Al doping and annealing on the photoemission spectra and thermal stability of Y2O3∕Si gate stacks by photoemission spectroscopy and x-ray absorption spectroscopy. They have found that the SiO2 components diffuse into the Y2O3 layer by annealing, resulting in the formation of Y silicate; however, the formation of metallic Y silicide is not observed. The changes in valence- and conduction-band offsets by doping Y2O3 with Al with respect to both Al concentration and annealing temperature have been systematically investigated. With an increase in the Al concentration, the band offsets and band gaps increase and the conduction-band edges change nonlinearly.

Quaterrylene field-effect transistors (FETs) were formed on a silicon oxide (SiO2) layer and on an octadecyltrichlorosilane self-assembled monolayer (OTS-SAM). To elucidate the transport mechanisms in the respective devices, we examined the dependence of carrier mobility on film thickness and temperature. On the OTS surface, a marked increase in the carrier mobility was observed in the initial layers, indicating that the accumulated carriers were distributed closer to the interface than were those on the SiO2 surface. Moreover, the carrier transport in the respective devices exhibited distinct behaviors in the low temperature range, particularly in the initial layers. On the SiO2 surface the carrier mobility depended strongly on temperature; the value drastically declined with the decreasing temperature from 300 K down to 60 K. On the OTS surface, the carrier mobility showed temperature-independent transport below 210 K. This maintenance of the carrier transport at low temperatures was caused by the termination of the trap-state density near the interface. These results clearly reveal that the OTS treatment effectively helped improve the interface properties because of a reduction in the density of the carrier traps, dramatically facilitating the carrier transport in the initial layers.

The adsorption structure of phenylphosphonic acid (PPOA) on an alumina surface was investigated as a function of exposure and temperature using infrared reflected absorption spectroscopy (IRAS) and a Kelvin probe. The alumina surface was held at room temperature during deposition. At monolayer adsorption, P O and Phenyl-P bands are observed, which indicate the creation of P O Al bonds. The aromatic ring plane is positioned perpendicular to the surface, i.e. deprotonated PPOA stands vertically on the surface. At multilayer adsorption, P O and P OH bands appear in the spectra. The multilayer PPOA film starts to desorb at 400 K. From 400 to 700 K, the IRAS spectra are similar to the monolayer spectrum, indicating that the multilayer structure reverses to the monolayer-like PPOA adlayer by heating. The acid molecules start to desorb at 700 K.

The authors examined the use of focused Ar ion beam sputtering (FAIS) for combinatorial synthesis. A Langmuir probe revealed that the electron temperature and density for FAIS of metal film deposition was lower than that of other major combinatorial thin film growth techniques such as pulsed laser deposition. Combining FAIS with the combinatorial method allowed the compositional fraction of the Pt–Ru binary alloy to be systematically controlled. Pt–Ru alloy metal film grew epitaxially on ZnO substrates, and crystal structures changed from the Pt phase (cubic structure) to the Ru phase (hexagonal structure) in the Pt–Ru alloy phase diagram. The alloy film has a smooth surface, with the Ru phase, in particular, showing a clear step-and-terrace structure. The combination of FAIS and the combinatorial method has major potential for the fabrication of high quality composition-spread metal film.

We have fabricated epitaxial La 1.2 Sr 1.8 Mn 1.7 Ru 0.3 O 7 thin films with n =2 Ruddlesden–Popper (RP) structure on TiO 2 -terminated and SrO-buffered SrTiO 3 (0 0 1) single-crystal substrates. It is shown that even small deviation from the narrow growth window results in the nucleation of (AO) 2 double layers in random directions and the loss of the desired structure. The atomic layer ordering at the interface between the thin films and SrTiO 3 substrates was studied by high-pressure, in situ reflection high-energy electron diffraction and transmission electron microscopy. It is demonstrated that growth on TiO 2 -terminated SrTiO 3 commences with the nucleation of two perovskite sub-units before the first (AO) 2 double-layer forms. To obtain a sharp RP/SrTiO 3 interface structure, it is necessary to buffer the TiO 2 -terminated SrTiO 3 substrate with a sub-unit cell SrO layer before the deposition of the RP film.

Highly uniform and c-axis-aligned ZnO nanorod arrays were fabricated in predefined patterns by a low temperature homoepitaxial aqueous chemical method. The nucleation seed patterns were realized in polymer and in metal thin films, resulting in, all-ZnO and bottom-contacted structures, respectively. Both of them show excellent geometrical uniformity: the cross-sectional uniformity according to the scanning electron micrographs across the array is lower than 2%. The diameter of the hexagonal prism-shaped nanorods can be set in the range of 90–170 nm while their typical length achievable is 0.5–2.3 μm. The effect of the surface polarity was also examined, however, no significant difference was found between the arrays grown on Zn-terminated and on O-terminated face of the ZnO single crystal. The transmission electron microscopy observation revealed the single crystalline nature of the nanorods. The current–voltage characteristics taken on an individual nanorod contacted by a Au-coated atomic force microscope tip reflected Schottky-type behavior. The geometrical uniformity, the designable pattern, and the electrical properties make the presented nanorod arrays ideal candidates to be used in ZnO-based DC nanogenerator and in next-generation integrated piezoelectric nano-electromechanical systems (NEMS).

The conduction mechanism of HfSiON stacked-gate dielectric films after a spike annealing is investigated. FETs fabricated using the Damascene process is used to evaluate the electronic characteristics of a stacked-gate dielectric film. The effect of an additional spike annealing after the TiN gate electrode fabrication on the conductivity is analyzed. The electron current over the entire range of gate voltage is increased, while the hole current remains unchanged. The TAT and Schottky analyses derive a consistent energy level of the intermediate trap centers contributing to the electron conduction. The average lifetime is improved by annealing. However, the spike annealing does not change the slope of the TDDB Weibull plots. The average lifetime is improved by annealing. From NBTI analyses, the threshold voltages displayed linearly shifts with increasing injection charge concentration, both with and without the spike annealing. The slope of the threshold voltage shift is defined as a degradation rate.

Electrical and structural properties of Ni silicide films formed at various temperatures ranged from 200^oC to 950^oC on both heavily doped n^+ and p^+ Si substrates were studied. It was found that surface morphology as well as the sheet resistance properties of the Ni silicide films formed on n^+ and p^+ Si substrates at the temperatures higher than 600^oC were very different. Agglomerations of Ni silicide films on n^+ Si substrates begin to occur at around 600^oC while there is no agglomeration observed in Ni silicide films on p^+ Si substrates up to a forming temperature of 700^oC. It was also found that the phase transition temperature from NiSi phase to NiSi"2 phase depend on substrate types; 900^oC for NiSi film on n^+ Si substrate and 750^oC for NiSi film on p^+ Si substrate, respectively. Our results show that the agglomeration is, especially, important factor in the process temperature dependency of the sheet resistance of Ni silicides formed on n^+ Si substrates.

In this paper deposited materials using focused ion beam-induced chemical vapor deposition (FIB-CVD) in an AuSi alloyed metal source was investigated. Results using Au and Si FIB-CVD were almost the same as those using Ga FIB-CVD. It was suggested that differences in deposited materials among the ions used should be investigated in more detail.

We propose a novel approach to control the effective work function by taking advantage of crystal structures in metal gate electrodes. The crystal structures determine the predominant material elements at the metal/high-k interface. We have found that, in a Ru-Mo alloy system, a randomly-oriented Ru (fcc) structure promotes the segregation of Mo at the interface, enabling us to achieve a wide controllability of flatband voltage by the amount of 0.6-0.8 eV. In addition, the segregation of Mo within a Ru-rich electrode is a key to reducing Fermi level pinning at the metal/HfSiON interfaces.

HfO2-based oxide is a promising gate insulator for the next generation of the field effect transistors. Yet it is accompanied by a problematic effect, that is, the Fermi level of a metal electrode deposited on HfO2 is pinned at a certain position regardless of the type of the metal. On the other hand, this effect, the so-called Fremi level pinning, does not occur in a metal/La2O3/Si gate stack. This is puzzling because both oxides have similar band gaps and dielectric constants. We suggest that the high solubility of Si in La2O3, and thereby the formation of a silicate layer LaxSiyOz between La2O3 and the Si substrate plays important role in the suppression of electron transfers from La2O3 to the metal electrode: The charge neutrality is satisfied at the ionic/ionic interface La2O3/LaxSiyOz, and thus there is no need to exchange electrons with outside. At the ionic/covalent interface HfO2/SiO2, however, the charge neutrality is insufficient and electrons are readily transferred into the metal electrode causing the Fermi level pinning.

We filled one-dimensional alumina pores with a molten derivative of vanadyl-phthalocyanine (VOPcHt) and evaluated the effect of the surface of pore wall on the molecular packing structure of VOPcHt. We also evaluated the effect of a magnetic field of 5.0 T on the packing structure of VOPcHt that was solidified on a flat substrate. X-ray diffraction measurements revealed that both the surface effect as well as the magnetic field controlled the packing structure of VOPcHt. When both the pore wall and the magnetic field existed, the surface effect was dominant in the control of the packing structure. These results provide practical methods to fabricate nanostructures of organic molecules with a controlled molecular packing structure.

Synthesis of continuous composition-spread (CCS) thin films is widely recognized in combinatorial material science as a powerful technique for rapidly investigating the properties of new functional materials. However, there are very few reports of CCS organic thin films due to the fact that the thermal evaporation method with Knudsen cells, which is commonly used to fabricate organic thin films in vacuum, does not offer the necessary level of deposition rate control as, for example, does pulsed laser deposition (PLD). We have successfully fabricated organic CCS thin films of pentacene and 6,13-pentacenequinone by continuous-wave laser molecular beam epitaxy (CWL-MBE), which we developed as a new fabrication method for organic thin films. The composition-spread films were characterized systematically by ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, atomic force microscopy, and two-probe conductivity measurements. The present work brings the advantages of high-throughput parallel synthesis and systematic characterization to the field of organic electronics, allowing for quick exploration and rapid optimization of organic functional materials and devices.

Quaterrylene thin films were grown on a SiO2 surface at an ultralow flux rate using a vacuum deposition technique with a hot wall cell, and their detailed growth process was investigated. We discuss the influence of growth parameters such as substrate temperature, flux rate of molecules, and film thickness. Atomic force microscopy (AFM) observations revealed the presence of two different phases: one with a layered structure and the other with a fibrous structure. X-ray diffractometry (XRD) clarified the orientation of the molecules in each phase, which were lying down in the fibrous structure and standing up in the layered one. The fibrous structure appeared on the surface of the underlying layered structure at low substrate temperature and high flux rate, showing that the phase was formed under nonequilibrium conditions. On the other hand, the layered structure with an upright orientation grew consistently under equilibrium conditions of high substrate temperature and low flux rate. Next, the initial gr...

We have succeeded in imaging the leakage sites of hafnium silicate gate dielectrics of metal-oxide-semiconductor field-effect transistors (MOSFETs) by using electron-beam-induced current (EBIC) method. Leakage sites of p-channel MOSFETs were identified as bright spots under appropriate reverse bias condition when the electron beam energy is high enough to generate carriers in the silicon substrate. Most of the leakage sites were observed in the peripheries of shallow trench isolation. These results suggest that some process induced defects are the cause of leakage in these MOSFETs. Our observation demonstrates the advantage of EBIC characterization for failure analysis of high-k MOSFETs.

Quaterrylene molecules, which have a planar and highly π-conjugated chemical structure, were deposited on a SiO2 surface, and their thin film structures, including surface morphology and molecular orientation, were examined by atomic force microscopy (AFM) and X-ray diffractometry (XRD). AFM observations revealed the grain size and surface roughness to be closely dependent on the substrate temperature in the range from 27 °C to 200 °C. Particularly at a substrate temperature of 140 °C, grain sizes of up to 6 μm and low surface roughness of 1.67 nm were successfully obtained in the 8 ML-thick film. XRD measurements of the quaterrylene thin film revealed (0 0 l) Bragg reflections, corresponding to a spacing of 1.89 nm. This value coincides with the average height of the terraces of the stepped structure observed in the AFM images. These results clearly demonstrate the quaterrylene molecules to have an upright orientation and that thin films grow as layered structures on the surface. From the full width of half maximum (FWHM) of the XRD rocking curve, the degree of alignment of the molecular planes (mosaicity) was estimated to be 0.09°, which shows that the film has a highly ordered structure.

Methods of improving Fermi-level pinning of pure metal gate electrodes on Hf-based high-k dielectrics have been investigated. The pinning phenomenon is a crucial problem, resulting in an unintentional threshold voltage increase in p-MOSFETs when applying pure metal gate electrodes such as Ru and TiN in Hf-based high-k CMOS. After systematic investigation of the relation between oxygen vacancies in Hf-based high-k dielectrics and electrical characteristics, we concluded that the Fermi-level pinning is unavoidable in principle with a thin EOT, but is a stable phenomenon that should be intentionally utilized. It is necessary to modulate the stable pinning energy position of Hf-based high-k dielectrics in p-MOSFETs in order to obtain thin EOTs in the gate-first process.

Vacancy-type defects in MOSFET structures fabricated with high-k (HfSiON) gate dielectrics were studied by monogenetic positron beams. An expansion of open volumes in HfSiON fabricated on Si substrates using atomic layer deposition technique was observed with increasing nitrogen concentration. This fact was discussed in terms of a role of nitrogen in Hf-related oxide using results obtained by first-principles calculation and XPS. MOSFETs fabricated by F-channel implantation technique were also characterized. The major defect species which causes the Vth shift of MOSFETs was identified as vacancy-fluorine complexes (such as V3F2) locate in channel regions of Si substrates. The preset work suggests that positron annihilation can be used to detect microscopic defects in MOSFETs, and it is a useful tool for determining process parameters for MOSFET fabrications with high-k gate dielectrics.

Ionicity has been raised as an important factor in discussing defect formation in Hf-based oxides. It has been elucidated from our first-principles calculations, that the stability of defects is dominated by Coulomb interactions between charged defects and surrounding ions. For instance, the formation energy of positively charged oxygen vacancies (VO+2) is markedly decreased when they are coupled with substitutional N atoms at O sites, as the nominal charges of N3- are negatively greater than those of the oxygen ions O2-. Our computational results have further revealed that the ionic character of Hf causes generation of a low-lying Si dangling bond level in Si doped HfSiOx. These results suggest that one must take into account the ionicity of the Hf-based compounds, which possess totally different properties from the conventional gate insulator SiO2 in terms of defect formation.

Leakage current characteristics with significantly reduced dispersion were achieved for HfSiON gate dielectric film by selecting TiN gate electrode with low reactivity, compared to the conventional poly-Si gate. The breakdown lifetime and its characteristic homogeneity were also enormously improved. With intensive study on these electrical characteristics, it was concluded that the concentration of oxygen vacancy was significantly reduced, giving rise to the homogeneity improvement. Moreover, we have shown an explanation for the dielectric breakdown in terms of oxygen transport in the Hf-based dielectric film.

High-dielectric constant (high-k) gate materials, such as HfSiOx and HfAlOx, fabricated by atomic-layer-deposition techniques were characterized using monoenergetic positron beams. Measurements of the Doppler broadening spectra of annihilation radiation and the lifetime spectra of positrons indicated that positrons annihilated from the trapped state by open volumes that exist intrinsically in amorphous structures of the films. The size distributions of the open volumes and the local atomic configurations around such volumes can be discussed using positron annihilation parameters, and they were found to correlate with the electrical properties of the films. We confirmed that the positron annihilation is useful technique to characterize the matrix structure of amorphous high-k materials, and can be used to determine process parameters for the fabrication of high-k gate dielectrics. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The impact of TiN deposition on thin high-k (HfO2 and HfSiON) films formed on Si substrates was studied using monoenergetic positron beams. For the predeposition sample, the positrons implanted into Si diffuse toward the high-k/Si interface under the influence of the electric field, suggesting the presence of negative charges in the high-k films. After TiN was deposited on HfO2, positive charges were introduced at the TiN/HfO2 interface, which were associated with the incorporation of nitride into HfO2, resulting in the formation of positively charged oxygen vacancies (VOs). From the isochronal annealing experiments for TiN/HfSiON/Si, it was found that positively charged defects (such as VO) were introduced into HfSiON after annealing at 700–900 °C. These defects were introduced by the interaction between TiN and HfSiON, resulting in the formation of polycrystalline TiO2 at the interface. The positively charged defects were annealed out at 1100 °C, but the dielectric properties of HfSiON degraded.

We consider the cause of different behavior of effective work function of gate metals after high and low temperature treatments. After high temperature treatment, reaction between Hf-based high-k dielectrics and a Si substrate generates oxygen vacancies (Vo) in high-k dielectrics, which leads to the subsequent electron transfer from Vo to gate metals. As a result, effective work function of gate metals becomes small and Fermi level pinning of gate metals occurs. Thus, Fermi level pinning is unavoidable phenomena for gate-first- processes. On the other hand, intrinsic hybridization between metal and high-k wave function at the interface is crucial factor to determine effective work function of gate metals after low temperature treatment. Accordingly, interface structures and band structures of metals govern the intrinsic effective work functions of metals.

Silicon dangling bonds (Si-DBs) in HfSiOx have been studied using first-principles calculations. Interestingly, our computational result revealed that the Si-DB-related gap state in HfSiOx locates in a much lower energy region than that in SiOx. This is because Hf atoms enhance the ionic character of the HfSiOx film, which in turn induces a positive charge at the Si site. We consider that the low-lying Si-DB level, which is now very near the N 2p state, contributes to the formation of strong Si–N bonds in HfSiON. The lower shift of the Si-DB level upon cation metal inclusion can be useful information not only for improving the electric properties of high-k gate stacks but also for developing prominent silicon–oxide–nitride–oxide–silicon (SONOS) nonvolatile memories where controllability of the charge trap level is a crucial issue.

We fabricated Y2O3-ZrO2 composition film (YZO) on Al2O3 for the field effect passivation with high negative fixed charge densities on p-type Si. The surface recombination velocity was improved down to 30 cm/s after annealing at 400 °C. High thermal tolerance was confirmed over 600 °C by inserting 2-nm-thick ZrO2 layer between YZO and Al2O3 interface. This result showed ZrO2 layer work as protecting barrier of Al and Y interdiffusions.

International audience; Photochromic diarylethene (DAE) molecules were employed as the channel layer of a field-effect transistor, where the drain current was effectively modulated by the reversible phase transition between a semiconductor (closed-ring) and an insulator (open-ring) under ultraviolet or visible light irradiation. Our goal was the further improvement of optical switching properties by interface engineering. First, we reduced the hole injection barrier by introducing an a -sexithiophene (6T) thin film at the interface between the source–drain electrodes and the DAE channel layer. As a result, the threshold voltage of the DAE-FETs was greatly reduced from -64 to -4 V. Second, we improved the optical switching performance by the surface treatment of a SiO 2 gate insulator with poly(methyl methacrylate) (PMMA). The drain current was unchanged even after 10 cycles of optical switching in contrast to the rapid degradation found with untreated DAE transistors. The combination of these improvements and interdigitated source–drain electrodes eventually resulted in a light irradiation driven on/off ratio of over 10^3. Significantly, the light-induced on/off ratio was comparable to that driven by an electrical field, which satisfies the requirement for industrial optical applications. Our findings will provide useful ways of realizing high-performance optical switching transistors.

Typical p-metals show similar effective work functions close to p+ polycrystalline silicon (poly-Si) pinning position irrespective of materials after high-temperature process. We found that this phenomenon can be explained by the modified Vo model taking into account the effect of Si substrate. Oxygen absorption by Si substrate and subsequent electron transfer to metal electrode clearly explain the p-metal Fermi level pinning as well as p+ poly-Si pinning. In addition, unsuppressed Fermi level pinning by insertion of barrier layer at p+ poly-Si/barrier layer/high-k gate stack, which is one of the open issues concerning p+ poly-Si pinning, has the same overall reaction scheme. The modified model also consistently explains this phenomenon.

The conventional theory of work functions (Schottky barriers) does not work at pure-metal/high-k-dielectrics interfaces. This occurs due to the selective interface atom bonding reflecting the large ionicity of high-k materials and the characteristic density of p-metal electronic states. Taking into accounts these features, we have constructed a new theory of work functions based on a concept of generalized charge neutrality levels. This theory systematically explains work functions of various gate materials on high-k dielectrics, in particular the unusual behavior of p-metal work functions, and naturally reproduces band offsets at various semiconductor/semiconductor interfaces.

Combinatorial methodology is a rapid technique for surveying new gate dielectrics and gate metal electrodes for the very complex advanced CMOS gate stack. Here, we report on a typical metal gate electrode alloy system, the Ni-Ti-Pt ternary. We have fabricated this metal gate thin film library on HfO2 using magnetron co-sputtering, to investigate flat- band voltage shift (DeltaVfb) and leakage current density (JL) variations. Wavelength dispersive spectroscopy (WDS) results show that over 90% pm 1% of Ni and Ti, and 75% pm 1% of Pt were attained in the library. A more negative DeltaVfb is observed close to the Ti-rich corner than close to the Ni- and Pt-rich corners, implying smaller Phi m near the Ti-rich corners and higher Phi m near Ni- and Pt-rich corners. In addition, measured JL values are consistent with the observed DeltaVfb variations.

We have theoretically investigated poly-Si and metal gates on Hf-related high-k gate dielectrics. First, we have investigated the cause of the substantial threshold voltage ( V th ) shifts observed in Hf-related high- k gate stacks with p+poly-Si gates. The oxygen vacancy ( Vo ) level in ionic HfO 2 is located in a relatively higher part of the band gap. If the p+poly-Si-gate is in contact with HfO 2 , Vo formation in the HfO 2 induces a subsequent electron transfer across the interface because of the higher energy level position of Vo , causing a substantial V th shifts in p+poly-Si gate MISFETs. Next, we also investigate the microscopic electronic structures at metal gates/HfO 2 interfaces. We have found that the wave functions of metal induced gap states (MIGS) have large amplitudes both around Hf and O atoms, which may be the cause of unusual work function behaviors of p-like metals. INTRODUCTION The considerable downscaling of Si devises has brought about the requirement for a continuing need for significant reduction in gate dielectric film thickness, and even current commercial based large scale integrated (LSI) circuits have SiO

We have investigated theoretically the cause of the substantial threshold voltage (Vth) shifts observed in Hf-related high-k gate stacks with p+poly-Si gates, by focusing on the ionic nature of HfO2. The oxygen vacancy (Vo) level in ionic HfO2 is located in a relatively higher part of the band gap. This high position of the Vo level results in a significant elevation of the Fermi level for p+poly-Si gates, if the p+poly-Si gate is in contact with the high-k HfO2. Vo formation in the HfO2 induces a subsequent electron transfer across the interface, causing a substantial Vth shifts in p+poly-Si gate MISFETs. Moreover, our theory also systematically reproduces other recent experimental results.

Epitaxial SrTiO3 (STO) thin films with high electrical properties were grown on Si using ZnS single- and SrS/MnS hetero-buffer layers. STO films on both ZnS-buffered and SrS/MnS-buffered Si showed two growth orientations, (100) and (110). The temperature dependence of the growth orientation for STO films was different for the ZnS single-buffer layer in comparison with the SrS/MnS heterobuffer layers. (100) growth of STO films on SrS/MnS-buffered Si became dominant at high temperatures about 700 ?C, while (100) growth of STO films on ZnS-buffered Si became dominant at a relatively low growth temperature of 550 ?C. STO(100) films on ZnS-buffered and SrS/MnS-buffered Si showed lattice and domain matches for epitaxial relationships with [001]ZnS?[011]STO and SrS[001]?[011]STO, respectively via 45? in-plane rotation of STO films relative to both ZnS and SrS layers. The ZnS buffer layer contained many stacking faults because of the mismatch between ZnS and Si, however, those defects were terminated at the ZnS/STO interface. In contrast, the MnS buffer was very stable against stacking defect formation. Transmission electron microscopy measurements revealed the presence of a disordered region at the ZnS/Si and MnS/Si interfaces. Auger electron spectroscopy and transmission electron microscopy results showed that a good MnS/Si interface at the initial growth stage degraded to a SiS2-x-rich phase during MnS deposition and again into a SiO2-x-rich phase during STO deposition at the high growth temperature of 700 ?C. It was also observed that STO on SrS/MnS-buffered Si showed a markedly high dielectric constant compared with that of STO on ZnS-buffered Si.