ICMCTF2014 Session CP: Symposium C Poster Session

A series of molecular dynamic simulations have been performed to elucidate the adatom mobility contribution to stress evolution during thin film growth. The simulations have explored intrinsic segregation of atoms to and from the surface and grain boundaries. The movement of these adatoms was found to assist in regulating either the tensile or compressive modes of growth. The computational results have been compared to experimental stress findings measured as the thin film grew. Case study metals, including Cu, Nb, Ni, and Fe have been investigated. The results revealed a preferential enrichment of specific atoms to the grain boundaries during growth. The preferential segregation of specific atoms to the boundaries appears as a means to tailor the stress within the film. The computational simulation results of the atomic placement of specific atoms are compared to analytical microscopy results of grain boundary structure and composition.

Keywords: Thermal Decomposition、Screen Printing、Gas sensor、Tin oxide、Pt ​

Sc-based III-nitride semiconductors are of potential interest as new materials for short-wavelength optoelectronic devices and high-electron mobility transistors. In this work we employed density functional theory together with special quasi-random structure methodology to study their structural, elastic and electronic properties .

Sc_xAl_1-xN and Sc_xGa_1-xN alloys are found to be stable in hexagonal phases up to x≈0.56 and x≈0.66, respectively, above which their rock-salt polymorphs are more stable. The elastic constants were calculated for hexagonal Sc_xGa_1-xN and Sc_xAl_1-xN up to x=0.375 using a stress-strain approach. The elastic constants C₁₁, C₃₃, C₄₄ and C₆₆ decreased while C₁₂ and C₁₃ increased slightly with increasing x for both alloys. The reduction in the bulk moduli and in most elastic constants of both alloys with increasing x is linked to a corresponding increas ing structural deviation from ideal tetrahedral bonding towards five-coordinate d local environments, and an increase in the average metal-nitrogen bond ionicity. The increase in Sc content expands the in-plane lattice parameter of Sc_xAl_1-xN and Sc_xGa_1-xN alloys, and leads to composition- and strain-tunable band gaps and polarization, and ultimately introduces ferroelectric functionality in Sc_xGa_1-xN at x≈0.625. Finally, we estimated critical thicknesses for stress relaxation by misfit dislocation s for several heterostructures using an energy balance model. These are greater than for the In_xGa_1-xN/GaN system at corresponding lattice mismatches.

Overall, our results indicate that Sc-based III-nitrides may be suitable for practical application in devices.

Nickel oxide is a wide band gap semiconductor with an energy gap in the range of 3.6-4.0 eV. Due to its good chemical stability, electrical and optical properties, the NiO film is a promising material for applications in electrochromic display devices, gas sensors, p-type conductive films etc. In the past, physical vapor deposition with ion beam assistance has been employed to deposit Transparent Conductive Oxide (TCO) films. Liu et. al. [1] reported that the electrical properties of indium tin oxide (ITO) films can be improved at ambient temperature by oxygen ion beam assisted electron-beam evaporation. Wang et al. have also reported that the electrical resistivity of NiO films by electron-beam evaporation is much lower than that of stoichiometric NiO films when oxygen ion source assistance is introduced [2]. However, the influence of oxygen ion beam assistance on the optoelectronic properties of the NiO films deposited by sputtering has not been explored yet.

In this work, the non-stoichiometric NiO films are deposited on glass corning 1737F substrates at ambient temperature through radio frequency (rf) sputtering of NiO targets with oxygen ion source by ion gun at various ion beam currents. An electrical resistivity that is too high and cannot be measured by four-point probe results when the NiO film is deposited without oxygen ion beam assistance. However, it drops significantly to 0.49 Ω-cm when an oxygen ion source is introduced from an ion gun set at a discharge current of 0.22 A. The electrical resistivity of the NiO films decreases continuously from 0.18 to 0.13 Ω-cm as the current is further increased from 0.28 to 0.42 A. The Hall measurements for all NiO films deposited with oxygen ion source assistance show p-type conduction. It is found that the crystallinity of the NiO films degrades when an oxygen ion beam is added during deposition. On the other hand, the transmittance of NiO films deposited without ion source assistance is around 69 %. It decreases significantly to 35 % when the discharge current of the oxygen ion gun is set at 0.22 A. Upon further increasing the current to 0.28 A, 0.33 A, and 0.42 A, the transmittance of the films drops further to 33 %, 28 %, and 22 %, respectively.

[1] C. Liu, T. Matsutani, T. Asanuma, K. Murai, M. Kiuchi, E. Alves, M. Reis, J. Appl. Phys. 93 (2003) 2262-2266.

In the present work, we report the fabrication of obliquely deposited hydrophobic coating of HfO₂ thin film on glass insulators as a function of substrate temperature. The main objective of this work was to prevent the outdoor glass insulators installed in a power system network from degradation through sunlight radiation and atmospheric contamination. The structural, optical, electrical and wettability characteristics of the coated thin film was investigated using XRD, AFM, FE-SEM/EDS, impedance analyzer, contact angle goniometry and four probe method.The XRD data reveals that all the deposited film exhibits a monoclinic structure with a change in the dominant peak orientation beyond 300 °C substrate temperature. A correlation was established between crystallite size and the deposition rate. The hydrophobicity of the deposited samples follows a linear trend not only with the roughness but also with the stoichiometric ratio. The packing density of the films evaluated from the transmission data dictated the electrical resistivity. The change in the bandgap with the deposition temperature was attributed to the defects in the samples. The dielectric constant evaluated by measuring capacitance was found out to be thickness dependent.

In this study, the crystalline structure of the ZnO film was observed using cross-sectional transmission electron microscopy (TEM). The dislocation density in the films was also evaluated under two-beam condition. The characterized ZnO epitaxial film was approximately 5 μm thick and the electron mobility at room temperature was 187 cm²/Vs. Although contrast was observed in the image due to the interference of thickness fluctuation from focused ion beam (FIB) processing, no clear grain boundaries were evident, which indicates the ZnO film is a single crystal. The dislocation densities estimated using Ham’s method [3] revealed lower density at the film surface than at the boundary between the film and substrate. The total dislocation density at the film surface was approximately 1.1×10⁹ cm^-2, of which the edge, screw, and mixed dislocation densities were estimated to be 4.7×10⁸, 5.0×10⁸, and 1.6×10⁸ cm^-2, respectively. On the other hand, the total dislocation density near the film-substrate interface was approximately 3.0×10⁹ cm^-2, of which the edge, screw, and mixed dislocation densities were estimated to be 1.4×10⁹, 8.2×10⁸, and 8.2×10⁸ cm^-2, respectively.

Acknowledgement: This work was supported in part by a Grant-in-Aid for Scientific Research (No. 24360014) from the Japan Society for the Promotion of Science.

[1] K. Yasui et al., MRS Symp. Proc. 1315 (2011) 21.

[2] N. Yamaguchi et al., Thin Solid Films (2013) in press.

[3] R. K. Ham, Philos. Mag. 6 (1961) 1183.

Reactive annealing of metallic Cu-Ga-In precursors under H₂Se and/or H₂S ambient has been employed as a promising method for high-performance Cu(InGa)Se₂ thin film solar cell absorber formation. It is, however, often observed that Ga is accumulated near the Mo side of glass/Mo/CIGS structure during or after the selenization of Cu-Ga-In precursors yielding the lower energy band gap at the junction of CIGS/CdS and thus low open-circuit voltages of device. One way to compensate for the loss of energy band gap near surface region is the sulfurization of film using H₂S gas. It is reported that the Ga depth homogeneity was improved by adopting 2-step selenization/sulfurization process.

In this paper, the effect of selenization step with different temperatures on sulfurization process of preformed CIGS by H₂S was investigated with a particular emphasis on the phase evolution and compositional depth profile. The CuGaIn metal precursors were prepared by sequential sputtering of CuGa and element In target, followed by thermal evaporation of Se. The reactive annealing of metal precursors was performed in a rapid thermal process system consisting of a quartz tube reactor with an inner diameter of 62 mm, quartz sample tray and infrared heater. Selenization of precursors was carried out by Se layer at various temperatures of 250~570°C, followed by the sulfurization under flowing H₂S gas at higher temperature of 600°C. In-situ phase evolution during sulfurization was observed by high-temperature X-ray diffraction scan. Compositional depth profile was measured by transmission electron microscope-energy dispersive X-ray spectroscopy. The results showed that sulfurization could drive Ga to move toward the surface region of CIGS layer, and increase the band gap. Low temperature selenization at 250~350°C resulted in the formation of Cu_xSe secondary phase, but better incorporation of sulfur and Ga into chalcopyrite structure.

Indium tin oxide (ITO)-based transparent conductive films (TCFs) have been widely used as transparent electrodes for electronic devices including liquid crystal displays, solar cells, and touch screen panels. ITO-based TCFs, however, have several limitations in applications of next-generation flexible electric devices because they are easy to crack and their sheet resistances are significantly increased under bending or other stresses due to their brittle nature. Recently, several materials, which include metal meshes, conducting polymers, and nano-structured carbon materials such as carbon nanotube (CNT) and graphene, and other metal-oxide films, have been introduced to replace the ITO-based TCFs. Among those, CNTs have much attraction for flexible TCFs because of their superior properties such as chemical stability, thermal conductivity, mechanical strength, and flexibility. Furthermore, in order to improve the electrical properties of CNT-based TCFs, composite materials which are made by mixing CNTs with poly-ethylenedioythiophene:poly-styrenesulfonate (PEDOT:PSS) have also been suggested . However, the CNTs/PEDOT:PSS composite may cause the visibility problem because PEDOT:PSS naturally shows blue color.

In this study, we fabricated the CNT-based TCFs, which have low sheet resistance (< 100 Ω/sq), high transmittance (> 80 %) in the visible wavelength range, and low yellowness (b* < 2), by coating PEDOT:PSS on CNTs. The CNTs were deposited on the polyethylene terephthalate ( PET) substrates by spray coating and then PEDOT:PSS layers were coated by spin coating. The morphologies and sheet resistances of the three kinds of materials, such as CNTs, PEDOT:PSS layers, and PEDOT:PSS-coated CNTs, were measured via field-emission scanning electron microscopy (FESEM) and four point probe methods respectively. The transmittances and color properties of PEDOT:PSS-coated CNTs were measured using a UV-VIS spectrometer. Also, the flexibilities of PEDOT:PSS-coated CNTs were tested using a bending machine (more than 10,000 times) with the angle and distance fixed. The experimental results confirmed that the fabricated CNTs/PEDOT:PSS TCFs would satisfy the requirements for flexible transparent electrodes of touch screen panels.

Despite the exceptional optoelectronic properties of indium-tin-oxide (ITO) thin films as transparent conductive (TC) electrodes, they suffer from considerable drawbacks under bending or other stresses due to their brittle nature. This has motivated the researches for alternative flexible TC materials. Carbon nanotubes (CNTs) have been considered to be one of the promising flexible electrodes. For flexible applications, plastic substrates, such as polyethylene terephthalate (PET), polyethylene (PE), and polypropylene (PP), can be used for the deposition of CNTs. Also, the indirect methods such as spray coating, dip coating, bar coating, and inkjet printing are favored when the CNTs are deposited on the plastic substrates. These methods, however, have common problems regarding the weak adhesion between the substrates and the CNTs. For obtaining the enhanced CNT-substrate adhesion, surface modification techniques, by which plastic substrates are pre-treated using plasma, corona-discharge, and ultraviolet light prior to the deposition of CNTs, have been suggested recently. Among those, corona-discharge treatment may be commercially favorable because it can be carried out at atmospheric pressure and room temperature.

Amorphous oxide materials have been potentially applied to thin film transistors (TFTs) for flat panel displays due to their superior electrical characteristics compared to hydrogenated amorphous silicon (a-Si:H) as well as the transparency in the visible wavelength range. Most of the currently-developing oxide-TFTs adopt indium (In)-incorporated zinc-oxide (ZnO)-based thin films such as indium-zinc-oxide (IZO) and indium-gallium-zinc-oxide (IGZO) because they exhibit high electron mobilities even when they are deposited at room temperature. However, the In element is expensive and relatively rare on Earth. Recently, silicon (Si)-incorporated zinc-oxide (SZO) films have been suggested as one of the candidates for substituting the In-incorporated oxide films because Si can act as a donor in ZnO. Also, post-annealing process is generally used to improve electrical properties of oxide films. However, post-annealing may increase the surface roughness of oxide films, deteriorating the device characteristics of oxide-TFTs.

Amorphous oxide semiconductor (AOS)-based thin film transistors (TFTs) have been intensively investigated for various electronic applications including active-matrix organic light emitting diode (AMOLED) and active-matrix liquid crystal display (AMLCD) due to their high field-effect mobility than that of conventional hydrogenated amorphous silicon (a-Si:H) TFT. AOS materials also have good optical transparency in the visible light region. Most of the successful AOS-based TFTs incorporate indium (In), which is relatively rare on Earth, such as indium-zinc oxide (IZO) and indium-gallium-zinc oxide (IGZO). This makes those technologies easily subject to a material shortage. Recently, the experimental results, which demonstrate new oxide semiconductors free of In, have been reported. Also, several studies have suggested that silicon (Si) atoms may act as effective donors in zinc oxide (ZnO) lattices and silicon-incorporated zinc oxide (SZO)-based TFTs can be one of alternative candidates replacing the In-incorporated oxide-TFTs. Until now, however, there has been a scarcity of comprehensive studies on the SZO-based TFTs.

In this study, we present experimental results regarding the effects of air exposure and thermal treatment on the material properties of SZO thin films and the device characteristics of SZO-based TFTs. The SZO films, which were used as the channel layers of TFTs, were deposited using an RF magnetron co-sputtering method by varying the power of Si target. The SZO-TFT was fabricated with a bottom gate structure. As a gate, a highly doped n-type Si substrate with a low resistance (≤ 2×10^-3 Ωcm) was used, and a gate insulator (300 nm) was formed by thermally oxidizing the Si substrate, and then the SZO-channel layer was deposited. Photolithography was performed to form the pattern, and Al electrode (source/drain) was deposited via RF sputtering. Then, lift-off was carried out to remove photo-resist (PR). The various methods, including four-point probe, X-ray diffraction (XRD), UV/visible spectrophotometer, secondary ion-mass spectrometer (SIMS), were used to investigate the effects of air exposure and thermal treatment on the electrical, structural, and optical properties of the SZO films. The device characteristics of the fabricated SZO-TFTs were measured in a dark environment using a semiconductor parameter analyzer to monitor the variations of device parameters, such as threshold-voltage and field-effect mobility, due to air exposure and thermal treatment. Also, the roles of Si in determining the device characteristics of the SZO-TFTs were discussed in detail.

Antimony selenide (Sb₂Se₃) has received considerable attention in recent years due to its excellent Seebeck coefficient (~1800 μVK^-1) which is at least 6-fold higher than that of the optimized bismuth telluride (Bi₂Te₃). However, the relatively low electrical conductivity (10^-6~10^-2 Sm^-1) seriously deteriorates the thermoelectric figure of merits and possible applications. Here, we fabricated a series of unusual Sb₂Se₃ nanoassembles including nanorods (400 ^oC), nanodecks (450 ^oC), nanotwizers (550 ^oC), and nanotubes (600 ^oC) with a distinct c-axis preferred orientation by pulsed laser deposition. The optimized electrical conductivity is 10⁴ times higher than that of the Sb₂Se₃ bulks and films, indicating that controlling the orientation and alignment of the nanostructured films is an effective solution for fundamentally resolving high resistance of Sb₂Se₃.

This study investigates the influence of pre-metal / post-metal annealing on reliability with High-k/metal gate metal-oxide-semiconductor feild effect transistors. After positive and negative bias temperature instability (P/NBTI), the amount of high-k bulk and interface traps increases, leading to electric characteristic changed. To f urther analysis in fast I-V measurement , the threshold voltage shift in positive direction after PBTI with post-metal annealing device is smaller than that with pre-metal annealing device. In addition, the transconductance and subthreshold swing are slightly decayed. This is because that the n itrogen of TiN metal gate diffuses to high-k layer to passivate the shallow traps. On the other hand , the interface traps measured by the charge pumping technique with pre-metal annealing device is almost similar to that with post-metal annealing device under initial. This phenomenon indicates that the n itrogen do not diffuse from metal gate to interface layer.

The resistive switching behaviors of hafnium doped silicon oxide by co-sputtering technology at room temperature were investigated in this study. Although non-doped SiO₂ based device has no switching phenomenon, silicon oxide with a few Hf dopants can successfully be used as switching layer in resistive random access memory (RRAM). The reliability were exhibited by good endurance and retention in the Hf-doped silicon oxide (Hf:SiO_x) RRAM.

On the other hand, we also presented the special role of hydrogen ions in Hf:SiO_x RRAM. In addition to the more typical oxygen ion-dominated resistive switching, hydrogen ions were also observed to trigger a resistance transformation phenomenon. Unlike a normal RRAM device, a hydrogen plasma-treated RRAM device is operated with a reversed voltage polarity, and the direction of hydrogen ion migration results in the chemical bonds breaking and repairing. This particular hydrogen-induced switching behavior suggests a different RRAM switching mechanism and is finally explained by our model.

This work investigated the contribution of dynamic gate-induced-drain-leakage stress (GIDLS) associated dynamic hot carrier stress (HCS) in HfO₂/TiN n-channel metal-oxide-semiconductor field-effect transistors. In this work, the individual dynamic GIDLS and dynamic HCS show band-to-band hot hole injection at the drain side and interface states generation, respectively. However, there is no change in Sub-threshold swing (S.S.) after dynamic GIDLS associated HCS due to band-to-band hot hole injection at the drain side which acts to diminish the lateral electric field. Moreover, t he impaired lateral electric field causes most of interface states are mainly concentrated on shallow states. This result in ON state current and transconductance decreases but no significant S.S. degradation after dynamic GIDLS associated HCS. The proposed model is confirmed by one- side charge pumping measurement and gate-to-drain capacitance at varying frequencies.

This letter investigates the impact of gate bias stress with and without light illumination in a-Si:H thin film transistors. It can be observed that the I-V curve shifts toward the positive direction after negative and positive gate bias stress due to interface state creation at the gate dielectric. However, this study found that threshold voltages shift negatively and the transconductance curves maxima are anomalously degraded under illuminated positive gate bias stress. In additional, threshold voltages shift positively under illuminated negative gate bias stress. These degradation behaviors can be ascribed to charge trapping in the passivation layer dominates degradation instability and are verified by dual gate a-Si:H device.

Comparing the devices with and without ultraviolet (UV) light treatment, the threshold voltage shift of devices with UV light treatment under negative bias illumination stress (NBIS) in a-InGaZnO thin film transistor is less than that without UV light treatment. Under NBIS, the origin of degradation contains oxygen vacancy generation and hole trapping in gate insulator or the interface between gate insulator and a-InGaZnO layer. Excluding the oxygen vacancy generation, the degradation behavior of devices with UV light treatment is the same as that without UV treatment. The result indicates that the vacancy-induced degradation is constant in the devices with and without UV treatment and part of oxygen vacancy was generated by UV light treatment. Therefore, during NBIS, the less vacancy-induced degradation is generated in the devices with UV light treatment. In another word, oxygen vacancy generated by previous UV light treatment in a-InGaZnO layer causes the threshold voltage instability.

Nanocrystalline cobalt ferrites (CoFe₂O₄) were synthesized in one step by hydrothermal process, using sodium hydroxide as precipitating agent and polysaccharides like cellulose and starch. The samples were labeled as HT-1a (blank), HT-1b (cellulose) and HT-1c (starch), respectively, where the molar ratios of Co : Fe was 1:2. The effect of NaOH, cellulose and starch on structural, morphological and optical properties of nanocrystalline cobalt ferrites were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR) and ultraviolet visible spectroscopy (UV-vis). The infrared spectroscopy studies confirm the presence of metal oxide. By means of the above mentioned investigations, it is found that the using of cellulose decrease the crystallite size but starch increases the crystallite size of cobalt ferrites therefore, band gap increases with cellulose. We hope that the procedure mentioned can be suitable for the high–grade synthesis of CoFe₂O₄ nanoparticles and may have potential applications in waste water treatment, electrode, sensors, catalysts etc. The objective of this work is to compare the structure, size and optical properties of CoFe₂O₄ nanoparticles obtained by the hydrothermal method.

Transparent and electrically conductive niobium-doped TiO₂ thin films have been deposited on glass surfaces by d.c.-pulsed reactive magnetron sputtering from a composite Ti:Nb target, using oxygen as reactive gas. A rapid 1 min annealing at 500 ºC in an atomic hydrogen rich atmosphere, obtained by flowing H₂ on a Ta filament resistively heated to 1750ºC in vacuum (hot-wire), proved to be very efficient in enhancing the electrical properties of these ~100 nm thick TiO₂:Nb (2at.%) thin films. Dark conductivity (σ_d) and its activation energy was measured as a function of (inverse) temperature and the value of σ_d at room temperature was used to assess the effect of the H₂ annealing on the transport properties. A 5-order of magnitude increase in electrical conductivity was observed for optimised treatment conditions at a hydrogen pressure of 10 Pa. A maximum value of σ_d in the range of ~1.4´10³ S/cm was attained for optimised conditions, where a level of ~6 at. % of H doping was measured close to the film surface. X-ray photoelectron spectroscopy, elastic recoil detection analysis, Rutherford backscattering and Raman spectroscopies were used to access information of composition and film structure for the explanation of the strong enhancement of the film’s electrical conductivity and band-gap widening to 3.45 eV following hot-wire treatments. These thin films can be used as transparent conductive oxide contact layers for photovoltaic applications.

Keywords: Nb doping; titanium dioxide; hot-wire; TCO; electrical properties; reactive sputtering.

Recently, we reported a substantial improvement in conversion efficiency in Cu2O-based p-n heterojunction solar cells fabricated by forming an n-type oxide semiconductor thin film on a thermally oxidized p-type cuprous oxide (Cu2O) sheet [1]. In this paper, we describe the optimization of formation conditions for various n-oxide thin films, including multicomponent metal oxide semiconductors, in order to achieve further improvement in obtainable photovoltaic properties in heterojunction solar cells using the p-Cu2O sheet. Thermally oxidized polycrystalline p-Cu2O sheets were prepared with a low resistivity, on the order of 100-103 Ωcm (Hall mobility above roughly 100 cm2/Vs), for use in this work. The evaluation of photovoltaic properties was conducted under simulated AM1.5G solar illumination. All of the n-oxide thin films were prepared with a thickness of 30-70 nm on non-intentionally heated Cu2O sheets by a magnetron sputtering deposition or pulsed laser deposition (PLD). It was found that the obtained photovoltaic properties in Al-doped ZnO (AZO) transparent electrode/Ga2O3-based multicomponent oxide/Cu2O heterojunction solar cells tended to improve as the Ga content was increased in the multicomponent oxide thin films prepared by PLD. For example, the values of open circuit voltage, short-circuit current density, fill factor and conversion efficiency obtained in AZO/(GaXIn1-X)2O3/Cu2O heterojunction solar cells all decreased as the Ga content (X) was decreased. However, in heterojunction solar cells with either Ga2O3-ZnO or Ga2O3-Al2O3 thin films, the highest efficiency was obtained by adding an amount of ZnO or Al2O3 that was small relative to that of Ga2O3. In AZO/(GaXAl1-X)2O3/Cu2O heterojunction solar cells with (GaXAl1-X)2O3 thin films prepared by varying X with a thickness of 50 nm, an AZO/(Ga0.975Al0.025)2O3/Cu2O heterojunction solar cell exhibited the highest efficiency of 5.72%.

References