堀田 育志

The interface properties of strongly correlated electron system/band semiconductor junctions were investigated in the La_1−xSr_xVO₃/p-Si(100) structure. Spectroscopic observations show that the electronic structure of the interface is typical of insulator/semiconductor junctions for x ≤ 0.2 and Schottky junctions for x ≥ 0.25. For the forward current density–bias voltage (J–V) characteristics, energy barriers that were otherwise unexplainable by spectroscopy were estimated from the thermionic emission model fitting of the J–V characteristics, indicating that the injected carriers from Si to La_1−xSr_xVO₃ can also feel the correlation force of Coulomb repulsion at the interface.

Artificially fabricated 3$d$/5$d$ superlattices (SLs) involve both strong electron correlation and spin-orbit coupling in one material by means of interfacial 3$d$-5$d$ coupling, whose mechanism remains mostly unexplored. In this work we investigated the mechanism of interfacial coupling in LaMnO$_3$/SrIrO$_3$ SLs by several spectroscopic approaches. Hard x-ray absorption, magnetic circular dichroism and photoemission spectra evidence the systematic change of the Ir ferromagnetism and the electronic structure with the change of the SL repetition period. First-principles calculations further reveal the mechanism of the SL-period dependence of the interfacial electronic structure and the local properties of the Ir moments, confirming that the formation of Ir-Mn molecular orbital is responsible for the interfacial coupling effects. The SL-period dependence of the ratio between spin and orbital components of the Ir magnetic moments can be attributed to the realignment of electron spin during the formation of the interfacial molecular orbital. Our results clarify the nature of interfacial coupling in this prototypical 3$d$/5$d$ SL system and the conclusion will shed light on the study of other strongly correlated and spin-orbit coupled oxide hetero-interfaces.

We studied the effect of post-deposition annealing (PDA) on the electrical property and the structure of aluminum oxide (AlOx) passivation films prepared by atomic layer deposition with various temperatures. Surface recombination velocity (S-max), interface trap density (D-it), and fixed charge density (Q(eff)/q) before and after the PDA were evaluated employing lifetime and capacitance?voltage measurements. The structural change by the PDA was investigated by a Stokes ellipsometer and X-ray reflectivity (XRR) measurements at SPring-8. The S-max of all samples was improved by the PDA. The improvement of S-max in the samples deposited at the temperature of 200 C and 300 C was due to the D-it decrease, but that in the sample deposited at the temperature of 25 C was due to both the D-it decrease and the Q(eff)/q increase. From the XRR measurements, it revealed that the formation of the interfacial thin layer is essential for the appearance of negative fixed charges.

Perovskite rhodates are characterized by intermediate strengths of both electronic correlation as well as spin-orbit coupling (SOC) and usually behave as moderately correlated metals. A recent publication (Phys. Rev. B 95, 245121(2017)) on epitaxial SrRhO$_3$ thin films unexpectedly reported a bad-metallic behavior and suggested the occurrence of antiferromagnetism below 100 K. We studied this SrRhO$_3$ thin film by hard x-ray photoemission spectroscopy and found a very small density of states (DOS) at Fermi level, which is consistent with the reported bad-metallic behavior. However, this negligible DOS persists up to room temperature, which contradicts with the explanation of antiferromagnetic transition at around 100 K. We also employed electronic structure calculations within the framework of density functional theory and dynamical mean-field theory. In contrast to the experimental results, our calculations indicate metallic behavior of both bulk SrRhO$_3$ and the SrRhO$_3$ thin film. The thin film exhibits stronger correlation effects than the bulk, but the correlation effects are not sufficient to drive a transition to an insulating state. The calculated uniform magnetic susceptibility is substantially larger in the thin film than that in the bulk. The role of SOC was also investigated and only a moderate modulation of the electronic structure was observed. Hence SOC is not expected to play an important role for electronic correlation in SrRhO$_3$.

Aluminum acetylacetonate-based AlOx thin films were introduced as a low-cost, high-quality passivation layers for crystalline silicon solar cells. Films were formed by a spin coating method on p-type silicon substrates at 450 degrees C in ambient air, O-2, or water vapor (H2O/O-2) for 15 or 120 min. XPS analysis confirms the AlO2 formation and reveals a high intensity of interfacial SiOx at the AlO2/Si interface of processed wafers. Ambient H2O/O-2 was found to be more beneficial for the activation of introduced AlO2 passivation films which offers high lifetime improvements with a low thermal budget. Carrier lifetime measurements provides that symmetrically coated wafers reach 119.3 mu s and 248.3 mu s after annealing in ambient H2O/O-2 for 15 min and 120 min, respectively.

The concept of the tricolor superstructure (TCS), which is a triple-layer stack structure containing two types of high dielectric constant (high-k) layers (designated HK1 and HK2) and a SiO2 layer, is proposed to control the moment and the polarity of the interface dipole layer that are induced at the high-k/SiO2 interfaces. The interface dipole layer is formed by oxygen ion migration from the layer with higher oxygen areal density (sigma) to that with lower r. When the two high-k materials are selected with the order of sigma(HK1)> sigma(SiO2)> sigma(HK2) in a SiO2/HK2/HK1/SiO2 TCS, the dipole directions of the interface dipole layers at the SiO2/HK2 and the HK1/SiO2 interfaces are aligned. Additionally, in the transposed SiO2/HK1/HK2/SiO2 TCS, the total polarity is reversed. The concept is demonstrated using Al2O3 and Y2O3 layers because they offer the order of sigma(Al2O3)> sigma(SiO2)> sigma(Y2O3). The two stacking sequence samples composed of SiO2/Y2O(3)/Al2O3/SiO2 and SiO2/Al2O3/Y2O3/SiO2 that were fabricated using superlattice technique by pulsed laser deposition obviously show opposite dipole polarities. Increasing repetition of the deposited TCS unit also causes the dipole moments to increase systematically. The TCS technique enables control of the properties of the interface dipole layer at high-k/SiO2 interfaces in amorphous systems. Published by AIP Publishing.

We studied the effective net charge density (Q(eff)) of strontium silicate (SrxSiOx+2, x = 1, 2, 3) films grown on silicon (Si) (100) substrates. The SrxSiOx+2 layers were deposited from a Sr2SiO4 polycrystalline target by pulsed laser deposition, and then annealed at 400-600 degrees C in an oxygen atmosphere with a tube furnace. The Qeff values of the SrxSiOx+2/Si (100) samples were obtained from the shift in the voltage of the flat band state in their capacitance-voltage curves. The SrxSiOx+2/Si (100) samples with a thickness of 15 nm annealed at 400 degrees C showed the maximum Q(eff)/q value of 1.03 X 10(13) cm(-2), where q is the elementary charge. With increasing annealing temperature, the SrxSiOx+2 layer penetrated into the Si (100) substrate. This penetration may degrade the interfacial properties and decrease the Qeff value of the layers. For all the samples, the charges concentrated near the SrxSiOx+2/Si (100) interface. Our results suggest that anion and cation migration plays an important role in charge generation at the SrxSiOx+2/Si (100) interface, consistent with the findings of a previously reported molecular dynamics calculation. Published by AIP Publishing.

The authors studied the correlation between the chemical bonding (CB) states and fixed charge (FC) states of Sr-silicate films grown on Si(100) substrates [Sr-silicate/Si(100)]. The Sr-silicate/Si(100) samples were synthesized by silicate reaction of SrO layers on the Si substrates through the diffusion of Si atoms from the substrates by thermal annealing in oxygen atmosphere. The CB states and the FC states of the Sr-silicate/Si(100) samples were obtained from their O 1s core-level x-ray photoemission spectra and the shift in voltage from the flat band state in their capacitance-voltage curves, respectively. Peak fittings of the O 1s core-level spectra for each sample were carried out with the three components of Si-O, Si-O-Sr, and Sr-O bonds to determine the CB state. The thin Sr-silicate layers were mainly constructed of the Si-O-Sr component. With increasing thickness, the amount of Si-O-Sr component decreased while that of Sr-O increased. The thickness dependency of the FC density showed a good agreement with that of the Si-O-Sr component, revealing a clear correlation between FC and Sr-O-Si bonding. Our results suggest that silicate bonding plays an important role in FC generation in Sr-silicate systems. (C) 2016 American Vacuum Society.

We investigated the room temperature growth of HfO2 layers on Si substrates by pulsed laser deposition under ultra-high vacuum conditions. The laser fluence (LF) during HfO2 layer growth was varied as a growth parameter in the experiments. X-ray photoemission spectroscopy (XPS) was used to observe the interface chemical states of the HfO2/Si samples produced by various LFs. The XPS results indicated that an interface Hf-silicate layer formed, even at room temperature, and that the thickness of this layer increased with increasing pulsed LF. Additionally, Hf-Si bonds were increasingly formed at the interface when the LF was more than 2 J/cm(2). This bond formation process was related to decomposition of HfO2 to its atomic states of Hf and O by multiphoton photochemical processes for bandgap excitation of the HfO2 polycrystalline target. However, the Hf-Si bond content of the interface Hf-silicate layer is controllable under high LF conditions. The results presented here represent a practical contribution to the development of room temperature processing of Hf-compound based devices. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Aluminum oxide (AlOx) films were deposited by mist chemical vapor deposition (MCVD) in air for p-type crystalline silicon, and the effects of the deposition temperature (T-dep) and AlOx film thickness on the maximum surface recombination velocities (S-max) were evaluated. It was found that S-max was improved with increasing T-dep. The AlOx film deposited at 400 degrees C exhibited the best S-max value of 2.8cm/s, and the passivation quality was comparable to that of AlOx deposited by other vacuum-based techniques. S-max was also improved with increasing film thickness. When the film thickness was above 10 nm, S-max was approximately 10 cm/s. From the Fourier transform infrared spectra, it was found that the AlOx films deposited by MCVD consisted of an AlOx layer and a Si-diffused AlOx layer. In addition, it is important for the layers to be thick enough to obtain high-quality passivation. (C) 2015 The Japan Society of Applied Physics

The authors fabricated 2 x 1 Sr-reconstructed Si(100) substrates using thin SrO layers, and used them to direct growth of crystalline perovskite oxide on Si. The SrO layers used to reconstruct the Si(100) substrates were grown by pulsed laser deposition from a SrO single crystal target, followed by postdeposition-annealing (PDA) of the SrO/Si(100) structure. In situ observations of reflective high-energy electron diffraction during PDA confirmed a 2 x 1 reconstruction of the Si surface and x-ray photoemission spectroscopy of the annealed samples confirmed the existence of Sr atoms in a silicate phase, which indicated that a 2 x 1 Sr-reconstructed Si surface was achieved. The optimal fabrication conditions were annealing at 720 degrees C for 1 min and an equivalent SrO layer thickness (MLeq) of 2.5 MLeq. The temperature condition was very narrow, at 720 +/- 20 degrees C, for an acceptable product. Subsequently, the authors demonstrated the growth of crystalline SrTiO3 films on the 2 x 1 Sr-reconstructed Si(100) surfaces. (C) 2014 American Vacuum Society.

Eu doped rare-earth orthovanadates are known to be good red phosphor materials. In particular, LaVO4:Eu is a promising candidate due to the low Eu-site point symmetry, and thus high dipole transition probability within Judd-Ofelt theory. However, the low solubility limit (<3 mol %) of Eu in LaVO4 prevents its efficient use as a phosphor. We present optical evidence of enhanced Eu solubility as high as 10 mol % in LaVO4:Eu thin films grown by pulsed laser deposition and postannealing. The photoluminescent intensity exceeded that of YVO4:Eu thin films when excited below the host bandgap, indicating stronger direct emission of Eu in LaVO4. (C) 2011 American Institute of Physics. [doi:10.1063/1.3554749]

We demonstrated signal transmission using the nonlinearity of VO2 in the manner of stochastic resonance (SR). A correlated insulator state of VO2 changes nonlinearly to a metallic state when an applied bias voltage increases beyond a threshold. The transition of the states between insulator and metal is adaptable to the SR theory. In this study, the response to a weak pulse signal was optimized by a particular level of noise via SR. Numerical SR simulations suggests the existence of multiple threshold channels in the insulating state, spontaneously enhancing the reliability of signal transmission. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3455335]

Perovskite oxides show a rich variety of electronic phases in bulk. The prospect of combining these phases at the atomic scale has fueled the interest in oxide heterostructures. The existence of interfaces can, however, greatly affect the macroscopic properties of these structures, leading on the one hand to exotic new phases stabilized at the interface or on the other hand to the degradation of bulk-like properties. With recent advances in electron microscopy the composition and bonding at such buried interfaces can now be imaged with atomic resolution. The bonding information is obtained from the near-edge fine-structure of core-level electron energy loss spectra. Here, we discuss the near-edge fine-structure focusing on the O-K and transition metal L2,3 edges which are of particular importance for the family of perovskite oxides. Spectroscopic imaging of a vanadate/titanate heterostructure demonstrates the capability of the technique to characterize atomic-scale interdiffusion at interfaces. Resolution limits in spectroscopic imaging due to inelastic delocalization effects are discussed.

We present an x-ray absorption study of the dependence of the V oxidation state on the thickness of LaVO(3) (LVO) and capping LaAlO(3) (LAO) layers in the multilayer structure of LVO sandwiched between LAO. We found that the change of the valence of V as a function of LAO layer thickness can be qualitatively explained by a transition between electronically reconstructed interfaces and a chemical reconstruction. The change as a function of LVO layer thickness is complicated by the presence of a considerable amount of V(4+) in the bulk of the thicker LVO layers.

We have studied the valence redistribution of V in LaAlO3/LaVO3/LaAlO3 trilayers, which are composed of only polar layers grown on SrTiO3 (001) substrates, by core-level photoemission spectroscopy. We have found that the V valence is intermediate between V3+ and V4+ for thin LaAlO3 cap layers, decreases with increasing cap-layer thickness, and finally recovers the bulk value of V3+ at similar to 10 unit-cell thickness. In order to interpret these results, we propose that the atomic reconstruction of the polar LaAlO3 surface competes with the purely electronic V valence change so that the polar catastrophe is avoided at the cost of minimum energy.

不揮発性メモリ等への応用を含め、酸化物の強誘電体薄膜の研究が活発に行われている。理想的な構造MFS-FET構造を得るためには、酸化物強誘電体薄膜を化学反応活性なSi素子上へ直接形成する必要がある。しかし酸化物を直接成膜した場合、界面にSiO_xが形成するという深刻な問題が避けられない。この問題を解決する方法として、非酸化物系のカルコゲナイド(II-VI族半導体)薄膜をSi基板上へ作製し、強誘電性を示すことを明らかにした。(Zn,Cd)Se、(Zn,Cd)TeおよびLiドープZnO薄膜に於いて、各々0.25V, 0.2V, O.18Vのメモリウィンドウ特性を得た。

We present evidence for hole injection into LaAlO(3)/LaVO(3)/LaAlO(3) quantum wells near a polar surface of LaAlO(3) (001). As the surface is brought in proximity to the LaVO(3) layer, an exponential drop in resistance and a decreasing positive Seebeck coefficient are observed below a characteristic coupling length of 10-15 unit cells. We attribute this behavior to a crossover from an atomic reconstruction of the AlO(2)-terminated LaAlO(3) surface to an electronic reconstruction of the vanadium valence. These results suggest a general approach to tunable hole doping in oxide thin-film heterostructures.

Problems with artificial neural networks originate from their deterministic nature and inevitable prior learnings, resulting in inadequate adaptability against unpredictable, abrupt environmental change. Here we show that a stochastically excitable threshold unit can be utilized by these systems to partially overcome the environmental change. Using an excitable threshold system, attractors were created that represent quasi-equilibrium states into which a system settles until disrupted by environmental change. Furthermore, noise-driven attractor stabilization and switching were embodied by inhibitory connections. Noise works as a power source to stabilize and switch attractors, and endows the system with hysteresis behavior that resembles that of stereopsis and binocular rivalry in the human visual cortex. A canonical model of the ring network with inhibitory connections composed of class 1 neurons also shows properties that are similar to the simple threshold system.

We have investigated cooperative dynamics of an artificial stochastic resonant system, which is a recurrent ring connection of neuron-like signal transducers (NST) based on stochastic resonance (SR), using electronic circuit experiments. The ring showed quasi-periodic, tunable oscillation driven by only noise. An oscillation coherently amplified by noise demonstrated that SR may lead to unusual oscillation features. Furthermore, we found that the ring showed synchronized oscillation in a chain network composed of multiple rings. Our results suggest that basic functions (oscillation and synchronization) that may be used in the central pattern generator of biological system are induced by collective integration of the NST element.

Room-temperature-photoinduced magnetization (PIM) was observed in spinel ferrite Al(0.2)Ru(0.8)Fe(2)O(4) thin films with a spin-cluster-glass structure. Additionally, the films exhibited significant properties as spintronic materials, showing a low saturation magnetization under 0.6 mu(B)/unit cell and good conductivity with a high spin polarized electron level of over 75%. A combination of high-temperature PIM and the electronic properties associated with spintronics would generate an area of research and development that utilize the degrees of freedom offered by optical systems in the field of spintronics. (c) 2008 American Institute of Physics.

Spinel ferrite oxides doping non-magnetic ions show the photo-induced magnetization (PIM) effect at high temperature [1-3]. Such a magnetization enhancement by light irradiation is a unique property in this material. In order effectively to use the PIM effect and precisely to control the magnetism, direct light-emission from light-emitting element substrates would be a useful technique. In this study, spinel ferrite Al0.2Ru0.8Fe 204 (ARFO) thin films, with the high temperature PIM effect, were prepared on GaAs(001) substrates by a pulsed laser deposition technique to aim integration with light-emitting devices based on GaAs lattice-matched materials in the future. Results showed that (001)-oriented ARFO thin films were successfully grown by using MgO buffer layers. The magnetic properties were approximately the same as ARFO films using other substrates such as Al2O3(0001) or MgO(001). © 2008 Materials Research Society.

We have studied the electronic structure of multilayers composed of a band insulator LaAlO(3) (LAO) and a Mott insulator LaVO(3) (LVO) by means of hard x-ray photoemission spectroscopy, which has a probing depth as large as similar to 60 A. The Mott-Hubbard gap of LVO remained open at the interface, indicating that the interface is insulating unlike the LaTiO(3)/SrTiO(3) multilayers. We found that the valence of V in LVO was partially converted from V(3+) to V(4+) only at the interface on the top side of the LVO layer and that the amount of V(4+) increased with LVO layer thickness. We suggest that the electronic reconstruction to eliminate the polarity catastrophe inherent in the polar heterostructure is the origin of the highly asymmetric valence change at the LAO/LVO interfaces.

We have investigated the transport properties of LaVO(3)/SrTiO(3) Mott-insulator-band-insulator heterointerfaces for various configurations. The (001)-oriented n-type VO(2)/LaO/TiO(2) polar discontinuity is conducting, exhibiting a LaVO(3) thickness-dependent metal-insulator transition and low temperature anomalous Hall effect. The (001) p-type VO(2)/SrO/TiO(2) interface, formed by inserting a single layer of bulk metallic SrVO(3) or SrO, drives the interface insulating. The (110) heterointerface is also insulating, indicating interface conduction arising from electronic reconstructions.

Surface segregation effects play an important role in the growth of traditional III-V semiconductor heterointerfaces. Here we show that segregation processes can also set an upper limit to the obtainable interface sharpness in perovskite oxide heterostructures. In particular, the structure of LaVO3/SrTiO3 superlattices was studied on the atomic scale by electron microscopy and spectroscopy. The vanadate layers exhibit a growth asymmetry, with diffuse lower and atomically abrupt upper interfaces, caused by preferential Sr surface segregation. Switching the SrTiO3 termination layer does not change the interface abruptness, which excludes the interfacial polar discontinuity as the driving force for the observed growth asymmetry. (C) 2007 American Institute of Physics.

We characterized the electronic structure of LaVOx thin films by x-ray photoemission spectroscopy. We could distinguish the photoemission spectra among the x=3 phase (LaV3+O3), the x=4 phase (LaV5+O4), and the mixed phase regime. In the mixed phase, the surface was found to be mostly LaVO4. We could observe the evolution of the electronic structure in the growth phase diagram by systematically measuring core-level photoemission spectra. The V4+ component present in the x=3 phase may be due to charge transfer to avoid the polar catastrophe of the polar LaVO3 thin-film surface and/or to the oxidation of the surface. (C) 2007 American Institute of Physics.

We used x-ray photoemission spectroscopy to investigate the electronic structure of one to five unit cell thick layers of the Mott insulator LaVO3 embedded in LaAlO3. By limiting the upper layer of LaAlO3 to three unit cells, the underlying LaVO3 could be probed. The V 2p core-level spectra had both V3+ and V4+ components, and above two unit cell thick LaVO3, the structures exhibited spectra similar to bulk samples. The atomically flat surfaces enabled the study of the emission angle dependence, which indicates that the V4+ is localized to the topmost layer. These results demonstrate the potential for probing interface electronic structure in oxide ultrathin films by surface spectroscopy. (c) 2006 American Institute of Physics.

Electrons at interfaces, driven to minimize their free energy, are distributed differently than in bulk. This can be dramatic at interfaces involving heterovalent compounds. Here we profile an abrupt interface between V 3d(2) LaVO3 and V 3d(0) LaVO4 using electron energy loss spectroscopy. Although no bulk phase of LaVOx with a V 3d(1) configuration exists, we find a nanometer-wide region of V 3d(1) at the LaVO3/LaVO4 interface, rather than a mixture of V 3d(0) and V 3d(2). The two-dimensional sheet of 3d(1) electrons is a prototypical electronic reconstruction at an interface between competing ground states.

We have studied the phase diagram of LaVOx films grown on (001) SrTiO3 substrates by pulsed laser deposition. With increasing oxygen partial pressure, the growth phase diagram varies between epitaxial perovskite LaV3+O3 single crystal films and polycrystalline monoclinic LaV5+O4. An interesting feature is the lack of an accessible phase corresponding to V4+, resulting in an extended region of phase coexistence of LaVO3 and LaVO4. Atomically flat LaVO3 could be grown in both layer-by-layer and step-flow growth modes, making this a promising candidate for incorporating strongly correlated electrons in atomic-scale perovskite heterostructures. (c) 2006 American Institute of Physics.

The in-plane dielectric and ferroelectric properties of c-axis oriented epitaxial strontium bismuth tantalate (SBT) thin films were investigated by interdigital surface electrode measurement. The four types of SBT thin films, which have various Sr/Bi atomic ratios, were prepared on MgO substrates by pulsed laser deposition. The dielectric properties at kilohertz and megahertz frequencies were studied from 20 to 870 K. Above room temperature, all the films show the phase transition. Sr0.99Bi1.61Ta2O9, Sr0.97Bi2.10Ta2O9, and Sr0.88Bi1.79Ta2O9 thin films show frequency dispersions near phase transition temperature. At low temperatures, dielectric and ferroelectric properties of Sr0.97Bi2.10Ta2O9 and Sr0.83Bi2.08Ta2O9 thin films were investigated. Both dielectric constant and remanent polarization show a tendency to decrease as temperature decreases, and the rapid change, which may indicate a phase transition, was observed in Sr0.97Bi2.10Ta2O9 near 80 K.

Artificial control for arranging the B-site ion has been demonstrated by superlattices to elucidate relaxor behavior. The relaxor behavior of ferroelectric material depends on order state of the constructing ion in the crystal. To clarify the mechanism of this relaxor behavior, Ba(Zr,Ti)O-3. is chosen as the target material because its B-site ions are not naturally ordered. The superlattice samples were prepared by using a pulsed laser deposition (PLD) technique. Observed X-ray diffraction (XRD) patterns of the superlattices and their theoretical calculation revealed that the degree of order state was well controlled by our technique. The relaxor behavior occurs in the order degree (OD) ratio below -25% indicating that the OD is a key function for determining the origin of the relaxor behavior.

Relaxer type ferroelectrics are treasure boxes both for basic physics and practical applications. Among them, Ba(Zr,Ti)O-3 is one of the candidate materials for elucidating the mechanism of relaxor phenomena. It has homo-valent B-site ions of Zr4+ and Ti4+. Therefore, there is no driving force to push the ions into ordered structure. On the other hand, it has a possibility of co-existence of multi phases (so-called pinching effect). In case of the bulk state, we have found that the relaxer behavior is strongly affected by the annealing conditions. To make clear the intrinsic parameter for the relaxer, we have demonstrated the artificial control the positioning of the B-site ions by the superlattice technique. The stacking periodicity and the compositional combination of BaTiO3 and BaZrO3 are changed systematically with this method and the sign of the relaxer is detected.

Thin films of ferroelectric binary mixed II-VI compounds such as (ZnxCd1-x)S, as well as (ZnyCd1-y)Te and (ZnzCd1-z)Se (0less than or equal tox,y,zless than or equal to1), were examined from the standpoint of the application to Si-based nonvolatile memories. Electronic-band discontinuities at the ferroelectric-Si interface decreased significantly with increase in the atomic number of the constituent chalcogenide atoms, which favored (ZnxCd1-x)S as the most potential gate ferroelectrics among the three compounds. Polarization-field (P-E) characteristics of the (ZnxCd1-x)S films were found to largely depend on the cation composition. No hysteretic behaviors in the P-E curves were observed for high-Zn concentrations above x=0.5, while the P-E curves traced hysteretic loops due to the ferroelectricity for x<0.5. The remnant polarization was greatly dependent on the Zn concentration, and yielded a maximum of 0.03 muC/cm(2) for x=0.3. On the other hand, the coercive field was not composition dependent, and was approximately 12 kV/cm. (C) 2002 American Institute of Physics.

Using pulsed-laser deposition, ferroelectric B4Ti3O12 (BiT) films were grown on Si(100) with and without an ultrathin buffer layer of silicon oxynitride (SiON), and the interface states were investigated using X-ray photoelectron spectroscopy. For both as-grown specimens, the additional oxidation of the interface Si was observed, and their thicknesses were almost identical. Due to the postannealing at 700 degreesC in an oxygen ambient, on the other hand, a large difference in the Si oxidation was observed between the two specimens. The BiT films on Si(100) without the SiON buffer layer failed in preventing the significant development of the interfacial Si oxidation. On the contrary it was clarified that the 1.3-nm-thick SiON buffer layers suppressed the additional oxidation to less than 3.5 nm.

Electrical properties of Bi4Ti3O12 (BiT) films on Si(100) were improved due to insertion of silicon oxynitride (SiON) buffer layers with thicknesses of 1-2 nm. Capacitance-voltage measurements indicated that the improvement was largely attributable to better Si interface properties rather than to the difference of the BiT film quality. By means of x-ray photoelectron spectroscopy and high-resolution transmission microscopy, the Si interfaces of the specimens with and without the SiON buffer layers were investigated. Consequently, we found that a postannealing treatment at 680 degreesC inevitably resulted in nonuniform growth of Si oxide layers at the Si interface of the specimen without the SiON buffer layer, and that the layer thickness mounted to approximately 10 rim. In contrast, 1-2-nm-thick. SiON buffer layers terminated the growth of the additional oxide layer of less than about 3 nm, and the resulting Si oxide layers were quite uniform. (C) 2001 American Institute of Physics.

Electrical properties of ferroelectric Bi4Ti3O12 (BiT) films on Si(100) using a 1 nm thick silicon oxynitride (SiON) buffer were investigated. The capacitance-voltage (C-V) characteristics of Au/BiT/SiON/Si(100) exhibited hysteresis loops with a memory window of 2 V due to the ferroelectricity, and did not show large carrier injections. The effects of the SiON buffer were demonstrated in current-voltage characteristics. In the reverse bias region, a leakage current density of the specimen without the SiON buffer was much larger than that of the specimen with the buffer. Apart from these electrical measurements, anomalous features appeared in C-V characteristics of the illuminated specimen, which were likely to be due to the ac response of the optically generated electrons in some trap states at the interface. (C) 2001 American Institute of Physics.

We have obtained ferroelectric characteristics from nonoxide (ZnxCd1-x)S (x=0.1-0.3) thin films. On the basis of x-ray photoelectron and visible-ultraviolet light absorption spectroscopy measurements, the conduction-band discontinuity at the (ZnxCd1-x)S/Si(100) interfaces is found to vary between 0.4 and 1.3 eV with a change in composition x between 0.1 and 0.9. The leakage current density, which strongly depends on the conduction-band discontinuity, is reduced to less than 10(-6) A/cm(2) at a gate voltage of 4 V. (C) 2001 American Institute of Physics.

We have grown YMnO3 (YMO) thin films on Si(111) using silicon oxynitride (SiON) as a buffer layer. Thickness of SiON buffer layer was well controlled within 2 nm. High resistance of ultrathin SiON layer (d(SiON)similar to0.7 nm) to Si oxidation was confirmed by x-ray photoelectron spectroscopy (XPS). Using the ultrathin SiON layer, we obtained c-axis oriented ferroelectric phase of YMO. Although capacitance-voltage curves of Al/YMO/SiON/Si(111) showed hystereses attributed to ferroelectricity of the YMO films, the memory window was not sufficient (0.2 V), seemingly due to poor crystallinity of the YMO films. On the other hand, leakage current characteristic was good enough for application. The typical value of leakage current density was 10(-8) A/cm(2) at a drive voltage of +/-5 V. In this article, the details of the characterization elucidated by using x-ray diffraction, atomic force microscopy, and XPS will be shown as well. (C) 2000 American Institute of Physics. [S0021-8979(00)03722-1].

We demonstrate that non-oxide (II-VI type semiconductors) ferroelectric thin films on Si(100), such as (Zn,Cd)Te, (Zn,Cd)Se and (Zn,Cd)S, have ferroelectric properties. Furthermore, thin films of (LixZn1-x)O, which is also a II-VI type semiconductor, display ferroelectric hysteretic features with memory windows of between 0.2 V and 0.5 V corresponding to the value of x. The material design for producing the ferroelectric nature is done by replacing some of the host atoms with other smaller atoms. The substituted atoms can occupy off-center positions, thus locally inducing electric dipoles, thereby leading to ferroelectric behavior. These II-VI wide gap semiconducting ferroelectric films will open the door to new memory devices.