Organic thin film devices are of interest for a variety of forthcoming ubiquitous electronics applications. In order to build ubiquitous high-performance devices, it is necessary to fabricate crystalline thin films of various organic materials onto “ubiquitous substrates” that are dictated by applications. However, many organic thin films crystallize only on a limited selection of substrates. Unfortunately, promising organic molecules, which have a large overlap of pi-orbitals between molecules, cannot migrate freely on a substrate because of stronger cohesion between molecules than interaction between the molecule and the substrate. Therefore, enhancement of the molecule-substrate interaction, i.e. ‘molecular wettability’ should promote crystallization. We found that an ultrasmooth monolayer of pentacene (C22H14), which can be grown on many general dielectric substrates, changes the molecular wettability of a substrate for other poorly wettable organic materials. We also demonstrate that a field effect transistor (FET) using a crystalline C60 thin film on a pentacene-buffered substrate can have a mobility of 4.9 cm2/Vs, which is 5-fold higher than that of C60 FETs without the buffer. Molecular wetting-controlled substrates can thus offer a general solution to the fabrication of high-performance crystalline plastic and molecular electronics.

We have fabricated C60 thin films on various substrates (mica, MoS2, HOPG, LiF, NaCl, KBr, KCl and CaF2) by using continuous-wave laser molecular beam epitaxy (CWL-MBE), which is very suitable technique to grow epitaxial organic thin films because of good controllability of evaporation as compared with Knudsen-cell method. The films were evaluated by reflection high-energy electron diffraction with micro channel imaging plate (MCP-RHEED) and atomic force microscopy (AFM). AFM images of the C60 films on mica, MoS2 and HOPG substrates show flat and homogeneous, morphology, and epitaxial growth of the films on mica and MoS2 substrates were observed by RHEED. This result shows mica, MoS2, HOPG substrates are good candidates for epitaxial growth of C60 thin films.

Aluminum nitride (AlN), a wide band gap semiconductor (Eg = 6.2eV), has potential applications in microelectronics due to its excellent insulating properties and compatibility with silicon [1,2]. More recently, the use of AlN thin films in high electron mobility transistors, light emitting diodes and UV sources is explored by altering the band gap of the material [3]. The present work describes the combinatorial synthesis of (Al,Ti)N thin films via pulsed laser deposition (PLD) technique to obtain desirable compositional spreads and corresponding variations in the electrical properties. Films of AlN, TiN and (Al,Ti)N were deposited on 6H-SiC (0001) substrates held at a temperature of 680°C. The surface quality of the films examined using an AFM revealed island growth of SiO2 and other growth patterns possibly related to substrate defects.

X-ray diffraction studies indicated that the growth of AlN and TiN films occurred with corresponding habit planes of (0002) and (111) parallel to the substrate surface. Compositional investigations conducted using energy dispersive spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS) showed systematic changes in the Al and Ti composition across the thickness of the compositional spread film. Cross-sectional analysis of (Al,Ti)N films conducted in a high-resolution transmission electron microscope revealed that the films were multi-layered. Several orders of magnitude decrease in the measured resistivity across a 15 mm length (Al,Ti)N film was noted corresponding to a systematic increase in the Ti content. Further optimization of deposition conditions is essential for producing thicker films.

We discuss visualization and data dependency issues related to managing sample characterization data from high-throughput experiments. We are developing software for on-line storage, sharing, and visualization of data from combinatorial or high-throughput experiments. The system is based on a centralized database and a Web interface that can be accessed from any networked computer. The greatest challenge facing us is the variety of data types and formats that such a system has to support so that data from many different sources, i.e. from synthesis and characterization instruments, could be combined. In order to minimize the software development effort, we have developed generalized extensible markup language (XML) schema for storing and processing experimental data. The use of a standard data format like XML, together with the relevant schema definitions can be used to implement storage, processing, visualization and other tools that are generic and thus greatly simplify the task of managing high-throughput experiments.

By means of insertion of thin pentacene buffer layer, we have succeeded in the fabrication of highly c-axis oriented rubrene (5,6,11,12-tetraphenylnaphthacene) thin films and their field effect transistors (FETs). In the case without pentacene buffers, only amorphous rubrene films were obtained and their FETs did not show operation. After optimization of pentacene buffer by using combinatorial thickness-gradient method, we obtained the crystalline rubrene thin films and their FETs showed p-type operation with a mobility of 0.05 cm2/V·s and an on-off ratio of 106.

Combinatorial pulsed laser deposition approach is suitable to study complex effects such as composition, thickness and surface structure on thin film-based photocatalysis in high-throughput manner. We fabricated composition spreads and thickness gradient films of transition metal-doped SrTiO3(001) on various oxide single crystal substrates and evaluated their photocatalytic activity by the use of photoreduction of Ag+ in AgNO3 solution. We found anomalous film thickness and substrate effects on the Ag-photodeposition in the V-doped SrTiO3 epitaxial thin films grown on the Nb:SrTiO3(001) substrate.

We developed a variable-temperature scanning microwave microscope (VT-SμM) that can perform high-throughput materials characterization in the temperature range between 4K and room temperature. As a sensor probe we used a high-Q coaxial cavity resonator, which was mounted on the low-temperature stage to allow variable-temperature measurements. We carried out systematic studies on the thermal degradation of the conducting polymers using the combinatorial libraries of polyaniline and polythiophene thin films, which showed rapid decrease of conductivity above 300C and 250C, respectively. The low-temperature performance of the VT-SμM was demonstrated by the measurement of composition-spread Nd1-xSrxMnO3 thin film, for which we succeeded in detecting the clear metal-insulator transition at 100K. We also propose a simple and easy method for the quantitative analysis of conductive thin films, by using the standard composition-spread thin films of Ti1-xNbxO2.

We explored a new fluid flux stabilized even in the vacuum condition at high temperature by combinatorial pulsed laser deposition (PLD) approach. A small amount of CuO doped in BiOx was found to effectively stabilize BiOx even in 6 Torr O2 at 800°C. The application of this novel Bi-Cu-O flux to the vacuum process lead to the successful fabrication of a single crystal Bi4Ti3O12 film and the establishment of a more reliable process for preparation of an atomically flat LaAlO3(001) substrate.

The past impossibility to create optical diffraction fringe image out of transparent TiO2(100) substrate was overcome by pumping light pulse of 266-nm wavelength. Thermal diffusivities (D) of TiO2(100) substrate and (Ca1–xSrx)3Co4O9 (0 ≤ x ≤ 0.125) thin films can successfully be determined with a newly proposed transient grating configuration method arising from a generic nanoscale measurement technique. The D values obtained at room temperature were 0.04 cm2/s for the former and 0.07–0.12 cm2/s for the latter, respectively.

We developed a scanning microwave microscope (SμM) designed for characterizing local electric properties at low temperatures. A high-Q λ/4coaxial cavity was used as a sensor probe, which can detect the change of quality factor due to the tip-sample interaction with enough accuracy. From the measurements of combinatorial samples, it was demonstrated that this SμM system has enough performance for high-throughput characterization of sample conductance under variable temperature conditions.

Novel phases of functional inorganic chemical systems can be efficiently explored using high-throughput thin-film fabrication techniques coupled with rapid characterization schemes. High-throughput investigation of thin-film materials has already led to the discovery of new dielectric and magnetic materials. In this article, we review various high-throughput thin-film synthesis/evaluation techniques and discuss examples of exciting discoveries and new applications of combinatorial techniques.

Composition spreads of La1-xSrxCoO3 (LSCO) were synthesized as thermo-electric transducer material by means of combinatorial material synthesis, and information on their thermal diffusivity throughout the specimens was obtained. Meanwhile, it is anticipated that the composition spreads of LSCO have a variety of characteristic properties of light absorption corresponding to their composition, namely their compositional variable, x. Hence, we used transient optical pump-and-probe techniques in a reflection geometry to measure thermal diffusion times on LSCO. The results of signal analysis indicate that LSCO is a peculiar synthesized substance whose apparent thermal diffusivity, 1/ι, changes abruptly at the critical point, where the compositional variable x is equal to 0.3.

A systematical growth temperature and oxygen ambient dependency of SrTiO3/Si interface structures were investigated using a growth temperature gradient pulse laser deposition (PLD) system and cross sectional high resolution transmission electron microscopy (HRTEM). A SiO2 interfacial layer and an amorphized SrTiO3 layer were observed at the interface for the thin films grown on Si (100) at growth temperatures above 600°C. Our results show that at growth temperatures higher than 600°C, the formation of the amorphized SrTiO3 layer is strongly growth temperature and also oxygen partial pressure dependent.

Impurity effects were investigated in (Ba,Sr)TiO3 (BST) systems in order to suppress leakage currents under relatively low oxygen pressure conditions by Pulsed Laser Deposition (PLD). We tried to dope transition metals, such as Mo, Mn, Cr, W and Fe into the BST target and used the targets to fabricate the films. By measuring electrical properties, we found Fe-doping had a significant effect on suppressing leakage current. Subsequently, we changed the amount of Fe doping from 0.1mol% to 6%. As a result, with post annealing, the sample with Fe:4% showed the lowest leakage current among those analyzed. Even without post annealing, the sample with Fe:6% showed the lowest leakage current. As for the dielectric constants, they decreased as the doping increased. At most, a 30% reduction was observed, compared with non-doped BST. XANES (X-ray Absorption Near Edge Structures) results indicated that the valency of the Fe ion was 3+ and located at the B-site of BST.

A scanning microwave microscope (Sm M) for high-throughput characterization of combinatorial dielectric materials has been developed using a lumped constant resonator probe. The probe consists of a microwave oscillator module equipped with a thin conducting needle and an outer conductor ring, which detects the dielectric constant of the sample just beneath the needle as a frequency shift of the resonator. The quantitative analysis of the dielectric constant for the bulk and the thin-film samples was carried out based on the measurement of gap-length dependence of the frequency shift. The analysis method was successfully applied to the characterization of composition-spread BaxSr1-xTiO3 thin film sample. The evaluation of far-field contribution to the frequency shift was found to be crucial for the accurate determination of dielectric constant especially in the characterization of combinatorial thin films.

The thermal diffusivities throughout a composition-spread film comprising La1- xSrxCoO3 series were determined by Transient Reflecting Grating (TRG) measurements. Thermal diffusivities of transparent SrCoO3 film and SrTiO3 substrate were determined by using third harmonics having wavelength of 266 nm as an excitation light with only a certain length of grating spacing. The thermal diffusivities of them were 0.02 and 0.05 cm2/s, respectively. With regard to untransparent thin films, the experiment was performed by setting three kinds of grating spacing for the excitation light having the fundamental wavelength, 800 nm in order to obtain high precision. The distribution profile of obtained values of the thermal diffusivity throughout the sample was traced as 0.06 - 0.18 - 0.01 cm2/s corresponding to the variation of in La1-xSrxCoO3, thus showing a convex profile, however, this anomalous profile was verified by comparing with the data obtained by the X-ray diffractometer.

Metal oxides exhibit a wide variety of properties originating mainly from strong electron correlation. Electronic properties of oxides had been utilized mostly in the bulk form till recently and people frequently observed significant changes in such properties when these materials were converted into thin films. For the electronic application in recent years, hybridization and integration of materials in thin film forms are becoming more and more important. In view of our recent studies on high-Tc superconducting junctions and ZnO light emitting devices, this paper is devoted to stimulate the exploration of oxides as new innovative electronic materials by discussing the control of epitaxial thin film growth.

The growth mode of a-Si:H plasma CVD film was observed by AFM to range from homogeneous deposition to heterogeneous island formation depending on the substrate material and deposition temperature. The correlation between the grain-like structure and electric property of the film deposited on transparent conductive SnO2 substrates has been elucidated by using an AFM with a conductive cantilever. The local distribution of photo-current in the a-Si:H films was found to agree well with the topographic image; the photo-current around the grain boundaries was lower than that on the grains. The effect of surface treatment with an aqueous HF solution indicates that the imhomogeneity in the photo-current originates from the preferential oxidation around the grain boundaries rather than intrinsic electric property.

The growth mode of a-Si:H plasma CVD film was observed by AFM to range from homogeneous deposition to heterogeneous island formation depending on the substrate material and deposition temperature. The correlation between the grain-like structure and electric property of the film deposited on transparent conductive SnO2, substrates has been elucidated by using an AFM with a conductive cantilever. The local distribution of photo-current in the a-Si:H films was found to agree well with the topographic image; the photo-current around the grain boundaries was lower than that on the grains. The effect of surface treatment with an aqueous HF solution indicates that the imhomogeneity in the photo-current originates from the preferential oxidation around the grain boundaries rather than intrinsic electric property.

This issue of the MRS Bulletin, on the crystal engineering of high Tc and related oxide films, essentially responds to four questions. The first three are

∎ Why has a high Tc superconducting tunnel junction not yet been fabricated?

∎ Is it possible to fabricate a high Tc tunnel junction and if so, how?

∎ How can thin-film technology contribute to the progress of high Tc superconductor research beyond the fabrication of electronic devices?

Experimental efforts to answer these questions and solve associated problems must focus on atomic-scale control and characterization of surfaces and interfaces, which we designate here as crystal engineering of high Tc and related oxide films. Meanwhile, we have started to realize that the crystal engineering technology of oxides need not be limited to high Tc super-conductor research; it can be extended to electronic materials research, where it may have a somewhat more profound significance. In this light, I would like to pose the fourth question, one that may be especially relevant to this particular topic in the near future:

∎ Will we find or develop an electronic material superior to silicon?

Ever since the first success in superconducting La-Sr-Cu-O film deposition in early 1987, extensive studies have been conducted on the fabrication of high Tc oxide films and devices. Success has been achieved to some extent in relatively simple devices using plain superconducting films or grain boundary and step edge junctions. Nobody, however, has succeeded in fabricating an SIS (superconductor-insulator-superconductor) runnel junction that shows a clear hysteresis in I-V characteristics at a temperature above 77 K. Such a junction is presumably the key to the wide electronic device application of high Tc superconductors, for example, for switching devices expected to work much faster than conventional semiconductor transistors.