Bevel Structure Based XPS Analysis as a Non-Destructive Chemical Probe for Complex Interfacial Structures of Organic Semiconductors.

The bevel structure of organic multilayers produced by finely controlled Ar gas cluster ion beam sputtering preserves both the molecular distribution and chemical states. Nevertheless, there is still an important question of whether this method can be applicable to organic multilayer structures composed of complex or ambiguous interfaces used in real organic optoelectronic devices. Herein, various bevel structures are fabricated from different types of organic semiconductors using a solution-based deposition technique: complicatedly intermixed electron-donor and electron-acceptor bulk heterojunction structure, thin film structure with an internal donor-acceptor concentration gradient, and multi-layered structure with more than three layers. For these organic material combinations listed above, the bevel structure is fabricated with finely tuned Ar gas cluster ion beam sputtering. The location-dependent X-ray photoelectron spectroscopy (XPS) results obtained for each bevel structure exactly correspond to the XPS depth profiles. This result demonstrates that the bevel structure analysis is a powerful method to distinguish subtle differences in chemical component distributions and chemical states of organic semiconductors even with complex or ambiguous interfaces. Ultimately, due to its reliability as verified by this study, the proposed bevel structure analysis is expected to greatly expand other analytical techniques with a limited spatial or depth resolution.

Degradation by water vapor of hydrogenated amorphous silicon oxynitride films grown at low temperature.

We report on the degradation process by water vapor of hydrogenated amorphous silicon oxynitride (SiON:H) films deposited by plasma-enhanced chemical vapor deposition at low temperature. The stability of the films was investigated as a function of the oxygen content and deposition temperature. Degradation by defects such as pinholes was not observed with transmission electron microscopy. However, we observed that SiON:H film degrades by reacting with water vapor through only interstitial paths and nano-defects. To monitor the degradation process, the atomic composition, mass density, and fully oxidized thickness were measured by using high-resolution Rutherford backscattering spectroscopy and X-ray reflectometry. The film rapidly degraded above an oxygen composition of ~27 at%, below a deposition temperature of ~150 °C, and below an mass density of ~2.15 g/cm3. This trend can be explained by the extents of porosity and percolation channel based on the ring model of the network structure. In the case of a high oxygen composition or low temperature, the SiON:H film becomes more porous because the film consists of network channels of rings with a low energy barrier.

Direct characterization of the energy level alignments and molecular components in an organic hetero-junction by integrated photoemission spectroscopy and reflection electron energy loss spectroscopy analysis.

A novel, direct method for the characterization of the energy level alignments at bulk-heterojunction (BHJ)/electrode interfaces on the basis of electronic spectroscopy measurements is proposed. The home-made in situ photoemission system is used to perform x-ray/ultraviolet photoemission spectroscopy (XPS/UPS), reflection electron energy loss spectroscopy (REELS) and inverse photoemission spectroscopy of organic-semiconductors (OSCs) deposited onto a Au substrate. Through this analysis system, we are able to obtain the electronic structures of a boron subphthalocyanine chloride:fullerene (SubPC:C60) BHJ and those of the separate OSC/electrode structures (SubPC/Au and C60/Au). Morphology and chemical composition analyses confirm that the original SubPC and C60 electronic structures remain unchanged in the electrodes prepared. Using this technique, we ascertain that the position and area of the nearest peak to the Fermi energy (EF = 0 eV) in the UPS (REELS) spectra of SubPC:C60 BHJ provide information on the highest occupied molecular orbital level (optical band gap) and combination ratio of the materials, respectively. Thus, extracting the adjusted spectrum from the corresponding SubPC:C60 BHJ UPS (REELS) spectrum reveals its electronic structure, equivalent to that of the C60 materials. This novel analytical approach allows complete energy-level determination for each combination ratio by separating its electronic structure information from the BHJ spectrum.

Anion control as a strategy to achieve high-mobility and high-stability oxide thin-film transistors.

Ultra-definition, large-area displays with three-dimensional visual effects represent megatrend in the current/future display industry. On the hardware level, such a "dream" display requires faster pixel switching and higher driving current, which in turn necessitate thin-film transistors (TFTs) with high mobility. Amorphous oxide semiconductors (AOS) such as In-Ga-Zn-O are poised to enable such TFTs, but the trade-off between device performance and stability under illumination critically limits their usability, which is related to the hampered electron-hole recombination caused by the oxygen vacancies. Here we have improved the illumination stability by substituting oxygen with nitrogen in ZnO, which may deactivate oxygen vacancies by raising valence bands above the defect levels. Indeed, the stability under illumination and electrical bias is superior to that of previous AOS-based TFTs. By achieving both mobility and stability, it is highly expected that the present ZnON TFTs will be extensively deployed in next-generation flat-panel displays.

Density of states-based design of metal oxide thin-film transistors for high mobility and superior photostability.

A novel method to design metal oxide thin-film transistor (TFT) devices with high performance and high photostability for next-generation flat-panel displays is reported. Here, we developed bilayer metal oxide TFTs, where the front channel consists of indium-zinc-oxide (IZO) and the back channel material on top of it is hafnium-indium-zinc-oxide (HIZO). Density-of-states (DOS)-based modeling and device simulation were performed in order to determine the optimum thickness ratio within the IZO/HIZO stack that results in the best balance between device performance and stability. As a result, respective values of 5 and 40 nm for the IZO and HIZO layers were determined. The TFT devices that were fabricated accordingly exhibited mobility values up to 48 cm(2)/(V s), which is much elevated compared to pure HIZO TFTs (∼13 cm(2)/(V s)) but comparable to pure IZO TFTs (∼59 cm(2)/(V s)). Also, the stability of the bilayer device (-1.18 V) was significantly enhanced compared to the pure IZO device (-9.08 V). Our methodology based on the subgap DOS model and simulation provides an effective way to enhance the device stability while retaining a relatively high mobility, which makes the corresponding devices suitable for ultradefinition, large-area, and high-frame-rate display applications.

Bandgap measurement of thin dielectric films using monochromated STEM-EELS.

High-resolution electron energy-loss spectroscopy (HR-EELS), achieved by attaching electron monochromators to transmission electron microscopes (TEM), has proved to be a powerful tool for measuring bandgaps. However, the method itself is still uncertain, due to Cerenkov loss and surface effects that can potentially influence the quality of EELS data. In the present study, we achieved an energy resolution of about 0.13 eV at 0.1s, with a spatial resolution of a few nanometers, using a monochromated STEM-EELS technique. We also assessed various methods of bandgap measurement for a-SiNx and SiO2 thin dielectric films. It was found that the linear fit method was more reliable than the onset reading method in avoiding the effects of Cerenkov loss and specimen thickness. The bandgap of the SiO2 was estimated to be 8.95 eV, and those of a-SiNx with N/Si ratios of 1.46, 1.20 and 0.92 were measured as 5.3, 4.1 and 2.9 eV, respectively. These bandgap-measurement results using monochromated STEM-EELS were compared with those using Auger electron spectroscopy (AES)-reflective EELS (REELS).

A high-density array of size-controlled silicon nanodots in a silicon oxide nanowire by electron-stimulated oxygen expulsion.

Methods of producing Si nanodots embedded in films of silicon oxide and silicon nitride abound, but fabrication of Si nanodots in a nanowire of these materials is very rare despite the fact that nanowire architecture enhances the charge collection and transport efficiencies for solar cells and field-effect transistors. We report a novel fabrication method for a high-density array of size-controlled sillicon nanodots from a silicon oxide nanowire using electron-beam irradiation. Our results demonstrate that a highly dense phase of Si nanodots with a narrow size distribution can be made from a silicon oxide nanowire with a core-shell structure of crystalline silicon-rich oxide (c-SRO)/amorphous silicon oxide (a-SiO(2)). This new nanomaterial shows the carrier transport characteristics of a semiconductor. The initially produced amorphous Si nanodots can be readily turned into crystalline Si (c-Si) nanodots by thermal annealing. Key characteristics of c-Si nanodots such as their size, number density, and rate of nucleation and growth are easily controlled by varying the electron radiation dose and annealing temperature. Nanodot formation is mechanistically initiated by electron trapping at the c-SRO core as well as at the core-shell interface, which leads to out-diffusion of the negatively charged oxygen through Coulomb repulsion, fostering the aggregation of Si atoms.