Search

We investigated the effect of Mg doping on the electron affinity of ZnO thin films. MgxZn1−xO (x = 0–0.29) composition-gradient films were deposited on an α-Al2O3(001) substrate using combinatorial pulsed laser deposition. The combinatorial high-throughput analysis of the optical transmittance systematically revealed that the bandgap of ZnO films was tunable between 3.3 and 4.0 eV by doping the ZnO thin films with Mg2+ ions. To investigate the electronic structure, photoelectron yield spectroscopy measurements were performed on the MgxZn1−xO composition-gradient films. The ionization potential, which denotes the distance between the valence band maximum and vacuum level, was independent of the Mg content in the ZnO films. By comparing with the optical bandgap results, the electron affinity was tunable from 4.1 to 3.5 eV by the Mg content in the MgxZn1−xO films.

The chemical properties and electronic structures of atomically flat SrTiO3(001) substrates with various surface treatments have been investigated by high-resolution synchrotron-radiation photoemission spectroscopy. We have found that an annealing process causes the segregation of Sr oxides at the TiO2-terminated surface and that the TiO2-terminated surface is more stable than the SrO terminated one. We demonstrate that the use of chemical etching process is effective as a removal technique for Sr oxides accumulating on TiO2-terminated surfaces and that the adding of a reetching process after annealing in an O2 environment is one of the best methods to obtain well defined and nearly perfect substrate surface.

There has been considerable debate regarding the origin of ferromagnetism in the ferromagnetic semiconductor anatase phase TiO2 doped with Co, ranging from carrier induced ferromagnetism in a framework of diluted magnetic semiconductor to merely the precipitation of ferromagnetic and metallic Co precipitates in the TiO2 matrix. This paper reports on a systematic investigation of the structures, together with the electronic and magneto-optical properties, of thin films of anatase phase TiO2, doped with Co. Films of anatase Ti1−xCoxO2 (x=0–0.10) were grown epitaxially on LaSrAlO4 (001) substrates, using pulsed laser deposition method. In order to control n-type carrier concentration, the oxygen pressure during the growth was systematically varied, yielding films with insulative, semiconductive (carrier concentration n∼1×1017 cm−3), and metallic (n∼2×1019 cm−3) properties. Reflection high energy electron diffraction patterns, atomic force microscopy, and transmission electron microscopy show no evidence of C...

Co doped rutile thin films were fabricated on α-Al2O3 (10-12) substrates by laser molecular beam epitaxy. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy indicated that the rutile thin films are (101) oriented and have smooth surfaces with no impurity phases. Co K-edge x-ray absorption near-edge structure, extended x-ray absorption fine structure, and x-ray photoelectron spectroscopy revealed the coexistence of different valence states of Co in the film. Magnetic circular dichroism studies suggest that the observed ferromagnetism is uniform and is related to the electron band structure of TiO2 rutile. Contribution of oxidized (Co2+) and metallic (Co0) cobalt to the ferromagnetism is discussed.

We have constructed a high-resolution synchrotron-radiation angle-resolved photoemission (ARPES) spectrometer combined with a combinatorial laser molecular-beam epitaxy (laser MBE) thin film growth system in order to investigate the electronic structure of transition metal oxide thin films. An ARPES spectrometer GAMMADATA SCIENTA SES-100 was selected for the high-throughput and high-energy and angular-resolution ARPES measurements. A total energy resolution of 6.3 meV and a momentum (an angular) resolution of 0.02 A−1 (0.2°) were obtained at a photon energy of 40 eV. The system is installed at the high-resolution vacuum-ultraviolet beamline BL-1C or the soft-x-ray undulator beamline BL-2C at the Photon Factory as an end-station. Another distinctive feature of this system is the direct connection from the spectrometer to a laser MBE chamber. Thin film samples can be transferred quickly into the photoemission chamber without breaking ultrahigh vacuum. Laser MBE is one of the best methods to grow thin films ...

Zinc oxide (ZnO) thin films grown on ScAlMgO4 substrates were characterized by means of positron annihilation. We measured Doppler broadening spectra of annihilation radiation and photoluminescence spectra for the ZnO films deposited by laser molecular-beam epitaxy and single-crystal ZnO. Although the lifetime of positrons in single-crystal ZnO was close to the lifetime of positrons annihilated from the free state, the diffusion length of positrons was shorter than that for typical defect-free materials. We attribute this to the scattering of positrons by native defects. For the ZnO films, we observed a correlation between the defects and the lifetime of bound exciton emissions τEx; the main defect species detected by positron annihilation was Zn vacancies or other related defects. Isochronal annealing at 750–850 °C was found to introduce additional vacancy-type defects into the film, although the value of τEx was scarcely changed by the annealing.

The effect of confinement on the exciton binding energies has been systematically investigated for two series of ZnO/ZnMgO multiquantum wells with various well widths and barrier heights. The exciton binding energies were extracted from the energy difference between the stimulated emission band induced by inelastic exciton–exciton scattering and the free exciton absorption band. The binding energies of excitons are found to be sensitively dependent on the well widths. The experimental results of the well width dependence of binding energies are in good agreement with Coli and Bajaj’s theoretical calculations for these structures [G. Coli and K. K. Bajaj, Appl. Phys. Lett. 78, 2861 (2001)]. The remarkable reduction in coupling strength between excitons and longitudinal optical phonons is closely correlated with the enhancement of the exciton binding energy, indicating that the stability of excitons is greatly increased by the enhancement of exciton binding energy in quantum wells. Published version

Co-doped ZnO(Zn1−xCoxO) and Co, Al codoped ZnO(Zn1−xCoxO:Al) films were grown on c-plane sapphire (0001) substrates by pulsed laser deposition using a KrF excimer laser. In order to enhance homogeneous substitution of Co2+ for Zn2+ in the ZnO film, an alternating deposition (AD) method was employed for doping. ZnO films doped with the same elements were grown without employing the AD method under the same fabrication conditions to compare with the corresponding AD films. Despite Co incorporation, the Zn1−xCoxO film by AD methods showed better crystallinity than pure ZnO film. Also, the crystallinity of Zn1−xCoxO:Al films by AD methods was better than those of Zn1−xCoxO and ZnO:Al films by conventional doping methods. Root mean square roughnesses of the films by AD methods were less than 1.6 nm. Absorption peaks caused by d–d transitions were observed at 1.88, 2.01, and 2.19 eV in Zn1−xCoxO films. The electric conductivity of Zn1−xCoxO:Al film by AD methods was comparable to that of ZnO:Al film by conventi...

A high-temperature, oxygen compatible, and compact laser molecular beam epitaxy (laser MBE) system has been developed. The 1.06 μm infrared light from a continuous wave neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was used to achieve a wide range and rapid control of substrate temperature in ultrahigh vacuum and at up to 1 atm oxygen pressure. The maximum usable temperature was limited to 1453 °C by the melting point of the nickel sample holder. To our knowledge, this is the highest temperature reported for pulsed laser deposition of oxide films. The efficient laser heating combined with temperature monitoring by a pyrometer and feedback control of the Nd:YAG laser power by a personal computer made it possible to regulate the substrate temperature accurately and to achieve high sample heating and cooling rates. The oxygen pressure and ablation laser triggering were also controlled by the computer. The accurate growth parameter control was combined with real-time in situ surface structure monitor...

Layer-by-layer heteroepitaxial growth processes of a BaO layer on SrTiO3(001) were simulated in order to predict an appropriate buffer layer for a YBa2Cu3O7−x(YBCO)/SrTiO3(001) heterojunction by using our crystal growth molecular dynamics (MD) simulation code. The SrTiO3(001) terminated by a TiO2 atomic plane was employed as the substrate in the present simulations. BaO molecules were continuously deposited on the SrTiO3(001) one by one, and finally a two-dimensional and epitaxial growth of a BaO layer was observed at 700 K. Moreover, the constructed BaO layer was atomically flat and smooth without defects, retaining a NaCl-type structure and (001) oriented configuration. However, the stress of the BaO/SrTiO3(001) heterojunction gradually increased and finally reached to approximately 1.2 GPa during the epitaxial growth process. It is expected that the large stress disturbs the subsequent fabrication of the uniform YBCO/SrTiO3(001) heterojunction. We also simulated the epitaxial growth process of a BaO la...

Energy profiles have been evaluated by an ab initio molecular-orbital method for hydrogen-abstraction reactions from surface model compounds of growing hydrogenated amorphous silicon (a-Si:H) by a SiH3 radical, a presumed main precursor to a-Si:H, as well as by a hydrogen radical which should coexist in the silane plasma chemical vapor deposition. The activation energies calculated for these two reactions decrease as the cluster size of the film surface model SinH2n+2 increases from n=1 to n=4 to converge for n⩾4. This trend is in parallel with the variation of atomic charge delocalization. Both activation energies (0.22 and 0.28 eV, respectively) for the largest model, Si7H16, were low enough to induce the hydrogen abstractions from the surface to form dangling bonds, which spontaneously react with SiH3 radicals to form Si–Si bond. From thus produced H3Si–Si≡surface, hydrogen can be eliminated with SiH3 (or H) to reproduce a dangling bond. The initial step of the a-Si:H film growth is deduced by the calc...

Nonradiative photoluminescence (PL) lifetime (τnr) and point defect density in the (0001) ZnO epilayer grown on (111) Si substrates by laser-assisted molecular-beam epitaxy (L-MBE) using a (0001) ZnS epitaxial buffer layer were compared with those in the ZnO films on (111) and (001) Si substrates prepared by direct transformation of ZnS epilayers on Si by thermal oxidation [Yoo et al., Appl. Phys. Lett. 78, 616 (2001)]. Both the ZnO films exhibited excitonic reflectance anomalies and corresponding PL peaks at low temperature, and the density or size of vacancy-type point defects (Zn vacancies), which were measured by the monoenergetic positron annihilation measurement, in the L-MBE epilayer was lower than that in the films prepared by the oxidation transformation. The ZnO epilayer grown on a (0001) ZnS epitaxial buffer on (111) Si exhibited longer τnr of 105ps at room temperature.

We have examined the thermal stability of TiO2-terminated SrTiO3(100) surfaces obtained by buffered HF etching and widely used as substrates for oxide film growth. In situ coaxial impact-collision ion scattering spectroscopy was used to measure the composition of the terminating atomic layer at temperatures up to 1000°C, simulating a broad range of thin-film growth conditions. The TiO2 termination of a nonannealed but HF-etched surface was found to start collapsing at temperatures as low as 300°C regardless of atmosphere, showing thermal instability of the chemically cleaved surface. Here, we introduce an alternative way to prepare a stabilized SrTiO3 surface, which maintains a perfect TiO2 termination up to 700°C, ideal for the growth of atomically sharp oxide heterointerfaces.

We have investigated the Mn3d spectral function in La0.6Sr0.4FeO3(LSFO)∕La0.6Sr0.4MnO3(LSMO) heterointerfaces as well as of La1−xSrxMnO3 films using Mn2p−3d resonant photoemission spectroscopy. The strong enhancement of the Mn3d spectra at the Mn2p−3d threshold enables us to extract the Mn3d spectra of LSMO layers in the vicinity of the interface with the LSFO overlayers. We have found that the spectral intensity of eg↑ states near the Fermi level is gradually reduced with increasing LSFO overlayer thickness and is finally saturated at 5–7ML. The close similarity in reduction of the intensity of the eg↑ states between the LSFO∕LSMO interface and hole-doped LSMO films indicates the occurrence of charge transfer at the interface between the LSMO and LSFO layers.

ZnO tubes were epitaxially grown on sapphire (0001) substrates by metalorganic chemical vapor deposition. The tubes grew along the substrate normal and were characterized by hexagon-shaped cross sections. All of the tubes possessed the same epitaxial relationships with respect to the substrate. Both reactor pressure and growth temperature were found to play an important role in the formation of ZnO tubes. Spiral column growth mode was found to be responsible for the formation of ZnO tubes.

Epitaxial ZnO thin films were grown by laser molecular-beam epitaxy on lattice-matched ScAlMgO4 substrates following the deposition and annealing of suitable buffer layers. The samples were characterized by low-temperature photoluminescence (PL), absorption, and reflectivity measurements. PL from higher order (n=2) excitons (A exciton) was observed at temperatures lower than 40 K. The absorption spectrum contained lines and the reflection spectrum exhibited anomalies that were assigned to the excited-states (n=2,3) of A and B excitons. The optical quality could be improved dramatically by using annealed ZnO or MgZnO buffer layers.

We have investigated the electronic structure of Zn1−xMxO (M: 3d transition metal) by x-ray absorption spectroscopy. Using configuration–interaction cluster-model analyses, electronic structure parameters have been deduced and their chemical trend is discussed. Results show that the p–d exchange constant Nβ is negative and large in cases of Mn, Fe, and Co, which is consistent with the enhancement of magnetic circular dichroism.

Thin CaHfO3 films were grown on (100) and (110)-oriented SrTiO3 surfaces with the aim of obtaining an insulator film for epitaxial oxide device design. We show that films grown on the (100) surface of SrTiO3 have a multidomain structure, which increases film roughness and decreases the maximum breakdown field of the insulator. Single-domain films were obtained on the SrTiO3 (110) surface. These films had a breakdown field of 5 MV/cm and a dielectric constant of er=16 to 17 at room temperature.

V defects were observed in the ZnO films epitaxially grown on the ZnS-buffered Si. Although the defects were located on the surface, strong near-bandedge emission confined to the {1011} facets of V defects was observed at room temperature. The near-bandedge emission spreads out over the whole film centering at V defects at 30 K. The detailed optical characterization shows that activation of excitonic absorption is responsible for this unique optical behavior.

The growth mode of ZnO thin films can be well regulated in a molecular layer-by-layer growth by employing a ZnO buffer layer deposited on a lattice-matched ScAlMgO4 substrate and annealed at high temperature. The annealed buffer layer has atomically flat surface and relaxed (strain-free) crystal structure. The intensity oscillation of reflection high-energy electron diffraction persisted for more than a 100-nm film deposition under optimized conditions on such a buffer layer. Thus prepared thin films show free exciton emissions in a 5 K photoluminescence spectrum and excited-state exciton resonance structures in a reflection spectrum, both indicating very high optical quality.

Combinatorial laser molecular-beam epitaxy method was employed to fabricate epitaxial Zn1−xMnxO thin films in a high throughput fashion. Local structures around Mn were investigated for these c-axis-oriented epitaxial films by fluorescence x-ray absorption fine structure measurements. It was shown that Mn substitutionally replaces Zn in Zn1−xMnxO (x

Sulfide was employed as a buffer layer for the growth of SrTiO3 (STO) thin films on Si. In order to utilize a relationship of a 45° rotational lattice match between Si and STO, ZnS, with almost the same lattice constant as Si, was used as the buffer. The buffer layer showed a partially disordered region at the ZnS/Si interface, owing to steady interdiffusion between ZnS and Si. STO film on ZnS buffered Si showed the rotational epitaxy with respect to Si and sharp STO/ZnS interface. Propagation of stacking faults from the ZnS/Si interface was observed, but those plane defects were terminated at the rotational STO/ZnS interface, resulting in high-quality STO films. The dielectric constant of the STO/ZnS film was 34.

We report on the fabrication of terminating-layer-controlled La0.6Sr0.4MnO3 (LSMO) thin films and in situ characterization of the films using photoemission spectroscopy. The terminating layer of the LSMO films was changed from B to A site by inserting one atomic layer of SrO between the LSMO film and a TiO2-terminated SrTiO3(001) substrate. The successful control of the terminating layer was confirmed by measuring the angular dependence of core-level spectra. Detailed analysis of the Sr 3d core levels revealed considerable differences in chemical bonding states of Sr atoms in the surface regions of films with different terminating layers.

Epitaxial thin films of pyrochlore-type ferromagnetic molybdates Sm2Mo2O7, as a geometrically spin-frustrated system, were fabricated by a pulsed-laser deposition. The temperature gradient method combined with the concurrent x-ray diffraction method was exploited for high throughput optimization of the film quality. The excess supply of Mo for compensating the volatile Mo-related species improved the crystallinity. The resistivity and magnetization of the optimized film were 3 mΩ cm and 0.8 μB/Mo at 10 K, respectively, being similar to those of a single crystal. The observed finite anomalous Hall term that persists down to the lowest temperature reflects an appearance of spin chirality in this spin-frustrated system.

We screened bright blue phosphor materials for a ZnO–WO3 binary system. The film was deposited by combinatorial pulsed laser deposition at room temperature and then annealed. The film composition changes continuously in a full-range from ZnO to WO3 on a single substrate. We obtained two results from the distribution of blue emission-intensity observed by cathodoluminescence of the film: (1) not only did the film have a W composition of 50 mol % (stoichiometric ZnWO4 as the well-known blue phosphor material) but also the W composition region from 45 to 60 mol % had the most emissive phase, and (2) two emissive phases other than the ZnWO4, which have not been described in the phase diagram, were found in the W composition region from 10 to 20 mol % and from 65 to 75 mol %.

Influences of point defects on the nonradiative processes in ZnO were studied using steady-state and time-resolved photoluminescence (PL) spectroscopy making a connection with the results of positron annihilation measurement. Free excitonic PL intensity naturally increased with the increase in the nonradiative PL lifetime (τnr). Density or size of Zn vacancies (VZn) decreased and τnr increased with increasing growth temperature in heteroepitaxial films grown on a ScAlMgO4 substrate. Use of homoepitaxial substrate further decreased the VZn density. However, τnr was the shortest for the homoepitaxial film; i.e., no clear dependence was found between τnr and density / size of VZn or positron scattering centers. The results indicated that nonradiative recombination processes are not solely governed by single point defects, but by certain defect species introduced by the presence of VZn such as vacancy complexes.

As‐grown surfaces of c‐axis‐oriented laser‐ablated YBa2Cu3Oy (YBCO) films have been investigated by scanning tunneling microscopy and spectroscopy (STM/STS) at 4.2 K. The square atomic arrangement with an average spacing of 0.4 nm was observed on the specimens even after exposure to ambient atmosphere, indicating that the (001) surface of the YBCO thin film was stable and free from severe contamination or chemical reaction. The STS observations revealed the semiconductive nature of the surface with a band gap of 0.1 eV. However, the tunneling spectrum varied its shape depending on the tip‐to‐sample distance and a clear superconducting gap structure with 2Δ∼40 meV appeared when the STM tip was closer to the surface than the usual position of the scanning mode, suggesting that the semiconductive layer is confined in the surface region of atomic size thickness.

Structural and electrical properties of c-axis oriented epitaxial Sr2Rh1−xRuxO4 (0⩽x⩽1) films were systematically investigated. The composition-spread films were fabricated on a temperature gradient (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate to quickly optimize the growth temperature for a high-quality crystalline film having each composition (x). An anomalous c-axis length dependence on x was observed, and it was accompanied by a distinct change in electric property, that is, a metal–insulator–metal transition.

S-doped ZnO (ZnO:S) film was fabricated by supplying ZnS species from laser ablation of a ZnS target during ZnO growth. Variations of lattice constants and band gaps with respect to S content did not follow Vegard’s law. The ZnO:S film showed semiconducting behavior with lower activation energy and resistivity than those of ZnO owing to higher carrier concentration. Despite the absence of magnetic elements, the large magnetoresistance amount of 26% was observed at 3 K from ZnO:S film.

High-quality interface between an insulator and ZnO as a wide-band-gap semiconductor should realize devices based on field-effect carrier modulation or superlattices having large band offset over 1 eV. We demonstrate that LiGaO2 could be a possible candidate for this purpose. Heteroepitaxy of LiGaO2 is demonstrated on ZnO films, giving atomically sharp interface and fairly good exciton-related optical properties in the ZnO under layer. Although slight distortion of a basal-plane hexagon with a lattice mismatch of about 3% in LiGaO2 gives multidomain epitaxial structure with relaxed lattices, a possible solution is proposed to realize coherent heterointerface.

An electron biprism for a 1 million-volt field-emission electron microscope was developed. This biprism is controlled similarly as a specimen holder so that it can be driven and rotated precisely and is tough against mechanical vibration and stray magnetic field. We recorded the maximum number of interference fringes by using this biprism in order to confirm the overall performance as a holography electron microscope, and obtained a world record of 11 000 interference fringes.

We have grown high-crystallinity ZnO thin films on lattice-matched ScAlMgO4 substrates by pulsed-laser deposition with doping donor (Ga) and acceptor (N) simultaneously. Alternating ablation of ceramics with concentrated Ga addition and highly pure single crystal targets yielded in a controlled Ga concentration (CGa) in a wide range of 1018–1020 cm−3 with minimal contamination of undesired impurities such as Al and Si. The use of the originally developed temperature-gradient method, where controlled and continuous gradient of the growth temperature is given to the single substrate with a range of about 50–200 °C, results in a continuous spread of N concentration (CN) in a controlled fashion. Therefore, the ratio of CN/CGa can be varied continuously in a wide range for each film, assuring that a region satisfying p-type codoping condition predicted by T. Yamamoto and H. K. Yoshida [Jpn. J. Appl. Phys., Part 2 38, L166 (1999)] is included in the sample. The electrical properties were measured for over thous...

Photoreflectance spectra of a high-quality ZnO epilayer on the ScAlMgO4 (0001) substrate grown by laser molecular-beam epitaxy exhibited clear excitonic resonances due to three excitons associated with uppermost valence bands (A, B, and C excitons). The oscillator strengths of the B and C excitons are larger than that of the A exciton. Their broadening was interpreted to be due to the contribution by exciton–polaritons in terms of large longitudinal–transverse splitting of respective excitons. Dependence of the exciton energy on temperature was well fitted assuming the Bose–Einstein statistics giving the Einstein characteristic temperature ΘE of 380 K (33 meV).

We have developed a way to map out concurrently the in-plane lattice constant of a heteroepitaxial thin film with a spatial resolution of 0.1 mm by using concurrent x-ray diffractometer equipped with a convergent x-ray source and two-dimensional detector. Spatial distribution of heteroepitaxial strain is analyzed for a (BaxSr1−x)TiO3 composition-spread thin film grown on a SrTiO3 substrate. As x increases, elastic deformation caused by compressive stress due to the lattice mismatch forces the lattice of the film to be coherent with that of the substrate until a critical point of x=0.6, above which the film lattice relaxes. By just taking three snap shots of x-ray diffraction image at a symmetric and two asymmetric diffraction configurations, such useful information inherent in heteroepitaxy can be revealed.

Persistent ferromagnetic magnetization (M) as large as 0.6 μB per Mn site can be induced by irradiating visible laser pulses on a thin film of 1% Cr-doped Pr0.5Ca0.5MnO3. The enhancement of M upon the photoexcitation was also confirmed by the scanning superconducting quantum interference device microscope as an increase of magnetic field from ferromagnetic domains. The observed photoinduced transition from a metastable charge/orbital ordered state into a ferromagnetic one is likely to be assisted by the microscopic phase separation characteristic of such a relaxor ferromagnet as the present Cr-doped manganite.

We have developed a parallel film growth method on a temperature-gradient substrate to quickly control and optimize the film growth mode. A continuous-wave neodymium-doped yttrium–aluminum–garnet laser heating was used to achieve a stable temperature gradient covering a 300 °C range of temperatures over a distance of 11 mm. The growth mode was determined by time-resolved scanning reflection high-energy electron diffraction. Transition from layer-by-layer to step-flow growth by the deposition temperature was observed during La0.5Sr0.5MnO3 film growth on a single SrTiO3 substrate, proving a powerful tool not only for investigating the growth dynamics but also for seeking the optimized deposition conditions in one run of experiment.

Epitaxial films of an oxide-diluted magnetic semiconductor with rutile structure, Mn-doped SnO2, have been fabricated by pulsed-laser deposition. As the Mn content increases, systematic changes in lattice constants and in-gap absorption are observed. Magnetization measurements show almost paramagnetic behavior. The injection of n-type carrier over 1020 cm−3 is achieved by Sb doping. A Sn0.95Mn0.05O2:Sb film shows giant positive magnetoresistance as large as 60% at 5 K.

We report on the fabrication of single-crystalline thin films of RBa2Cu3O7−y (R: rare-earth element, R123) using an approach of vapor–liquid–solid tri-phase epitaxy. This method is based on application of pulsed-laser deposition under appropriate compositions and conditions predetermined from the relevant thermodynamic phase diagram. The laser-ablated gases of R, Ba, and Cu, and their oxides dissolve into a liquid Ba3Cu5Ox (3BaCuO2+2CuO) layer placed on the film/substrate surface, penetrate to reach the liquid–solid interface with a seed R123, and are condensed into the solid R123 phase under a quasiequilibrium state. The uniform single-crystalline nature of the film was verified by x-ray diffraction, atomic-force microscopy, and transmission electron microscopy by the observation of giant grain size and atomic-scale surface smoothness.

Reactions of SiHn(n=1–4) with a fluorine atom were studied by using an ab initio molecular orbital method [fourth‐order Mo/ller–Plesset (MP4/6‐21G**//second‐order Mo/ller–Plesset MP2/6‐21G**)]. Structures of the transition states were determined. In the cases of SiH3+F and SiH+F, fluorine atom addition to a silicon atom was a favorable reaction at the singlet state and abstraction of a hydrogen by a fluorine atom was a favorable route at the triplet state. Abstraction of a hydrogen was also favorable at the doublet state in the case of SiH2+F. A selection rule of these reactions on the basis of the atomic spin density on each atom was proposed. The selection rule proposed was proved to also be applicable to the reaction CH3+F.

We studied the atomic-scale growth control and second-harmonic generation (SHG) properties of GdxY1−xCa4O(BO3)3 thin films. Combinatorial laser molecular-beam epitaxy was employed to optimize the growth conditions of the film. Ultrasmooth GdxY1−xCa4O(BO3)3 thin films with step-and-terrace structure were obtained by using atomically controlled GdCa4O(BO3)3 substrates. The dependence of SHG intensity on the fundamental wave polarization angle was studied for the epitaxial film on (010) GdCa4O(BO3)3 substrate in comparison for the randomly oriented film on (0001) sapphire substrate. It was found that only the epitaxially grown film clearly exhibited a fourfold SHG intensity showing type-II phase-matching direction.

Combinatorial laser molecular-beam epitaxy method was employed to fabricate epitaxial ZnO thin films doped with all the 3d transition metal (TM) ions in a high throughput fashion. The solubility behavior of TM ions was discussed from the viewpoints of the ionic radius and valence state. The magneto-optical responses coincident with absorption spectra were observed for Mn- and Co-doped samples. Cathodoluminescence spectra were studied for Cr-, Mn-, Fe-, and Co-doped samples, among which Cr-doped ZnO showed two sharp peaks at 2.97 eV and 3.71 eV, respectively, at the expense of the exciton emission peak of pure ZnO at 3.25 eV. Different magnetoresistance behavior was observed for the samples codoped with n-type carriers. Ferromagnetism was not observed for Cr- to Cu-doped samples down to 3 K.

Magnetic circular dichroism (MCD) spectra of Zn1−xTMxO (TM=Sc, Ti, V, Cr, Mn, Co, Ni, and Cu) films were measured to clarify their magneto-optical properties. Noticeable magneto-optical effect was not observed for Zn1−xScxO, Zn1−xTixO, Zn1−xVxO, and Zn1−xCrxO. On the contrary, Zn1−xMnxO, Zn1−xCoxO, Zn1−xNixO, and Zn1−xCuxO showed pronounced negative MCD peaks near 3.4 eV which correspond to the band gap of the host ZnO semiconductor. These results show that ZnO alloyed with Mn, Co, Ni, and Cu is a diluted magnetic semiconductor with strong exchange interaction between sp-band carriers and localized d electrons.

Epitaxial anatase thin films were fabricated on lattice-matched (−0.2%) LaAlO3 (001) substrates in the layer-by-layer fashion by laser molecular-beam epitaxy. X-ray diffraction and transmission electron microscope show the films to exhibit high crystallinity and atomically defined interfaces. By virtue of the adoption of LaAlO3 substrate, which is transparent to photoexcitation of TiO2, optical band gaps could be determined to be 3.3 eV at room temperature. A photoluminescence band due to recombination of self-trapped excitons was observed at 5 K to give the peak maximum at 2.2 eV. As a result of the high degree of orientation of the epitaxial films, anisotropic optical absorption was clearly observed.

We have examined the impact of a simple, wet chemical postgrowth treatment of “immersing in KCN solution” on the performance of inverted staggered amorphous silicon n-channel thin film transistors. Results show that the cyanide treatment significantly improves the overall transistor performance by the elimination of defect states.

ZnS films were grown on Si (100) at high temperatures by pulsed laser deposition using a KrF excimer laser. The growth temperature was varied from 200 to 700 °C and all films were found to have a specific preferential orientation. With increasing Ts, growth rate decreased but the quality of the film improved. The highest quality ZnS film was obtained at 700 °C. The presence of ZnS+ ions among the ablation products of a ZnS target was verified by laser desorption time of flight mass spectroscopy measurements. ZnO was formed by thermal oxidation of ZnS and the films showed strong near band-edge emission at 3.26 eV.

(110) oriented YBa2Cu3O7−δ thin films have been epitaxially grown on vicinal (110)SrTiO3 substrates by pulsed-laser deposition. Films with different oxygen deficiencies were obtained by annealing under various oxygen ambient. These films have very smooth surface and with their c axis aligned in one direction in the substrate plane. Electronic transport and polarized far-infrared reflectivity measurements reveal the unique features of the c-axis charge dynamics. A c-axis far-infrared reflectivity edge associated with the Josephson plasma in the superconducting state is observed. Our results demonstrate the potential usage of (110) thin films for the investigation of intrinsic interlayer charge dynamics as well as device application of high Tc superconductors.

We propose a method of rapid construction of a structural–magnetic–electronic phase diagram of doped Mott insulators. The composition-spread method is utilized for fabricating a film whose doping concentration varies from 0 to 1 continuously. The concurrent x-ray diffractometer that measures x-ray diffraction spectra of all the composition simultaneously, the scanning superconducting quantum interference device microscope, and the infrared optical spectroscopy are employed for characterizing the film. A demonstration is given for a colossal magnetoresistive material, La1−xSrxMnO3.

Thin films of La1/2Na1/2TiO3 fabricated by pulsed laser deposition show interesting dielectric properties exhibiting quantum paraelectric-type temperature-insensitive dielectric constant below 50 K. The dielectric constant (er∼180) is also very stable under electric bias up to 4×104 V/cm. These properties indicate that these films coupled with high temperature superconductors have a great potential for microwave applications.

We have mapped the growth mode of homoepitaxial SrTiO3 thin films as a function of deposition rate and substrate temperature during pulsed laser deposition. The transition from layer by layer growth to step flow growth was mapped by making 260 depositions, 3 monolayers each, on a single substrate. The growth mode was determined by time-resolved reflection high-energy electron diffraction. An atomically smooth surface was regenerated after each deposition by annealing the sample at temperatures above 1200 °C. The depositions were performed at an oxygen pressure of 10−6 Torr and covered a temperature range from 900 to 1380 °C. The effective activation energies of surface migration on Ti- and Sr-terminated surfaces were determined from the mapping results.

We have examined the thermal stability of wurtzite-phase MgxZn1-xO alloy films and ZnO/MgxZn1-xO bilayer films with x exceeding the reported solubility limit of 0.04. When a Mg0.23Zn0.78O film was annealed, the segregation of MgO started at 850 degrees C and the band gap was reduced to the value of that for an x = 0.15 film after annealing at 1000 degrees C. Mg0.15Zn0.85O films showed no change of the band gap even after annealing at 1000 degrees C. Therefore, we conclude that the thermodynamic solubility limit of MgO in MgxZn1-xO epitaxial film is about x = 0.15. The thermal diffusion of Mg across the MgxZn1-xO/ZnO interface was observed only after annealing above 700 degrees C. Unlike other II-VI semiconductors, ZnO-based alloy films and heterointerfaces are stable enough for the fabrication of high-crystallinity heterostructures.