High-quality MoS2 Research Articles

Controlled growth of MoS2 via surface-energy alterations

Monolayer MoS2 in triangular configurations with rich edges or high-quality uniform films are either catalytically active for the hydrogen evolution reaction or flexible for functional electronic and optoelectronic devices. Here, we have experimentally discovered that these two types of MoS2 products can be selectively synthesized on graphene or sapphire substrates, which are associated with both different adsorption energy and diffusion-energy barrier for vapor precursors during growth. Our study not only provides insights into the on-surface synthesis of high-quality MoS2 monolayers, but also can be applied to the growth of vertically-stacked and large-scale in-plane lateral MoS2–graphene heterostructures.

Inward growth of monolayer MoS2 single crystals from molten Na2MoO4 droplets

Two dimensional molybdenum disulfide (MoS2) holds great application prospects for next generation electronics. In order for controllable production of large-scale and high-quality MoS2 monolayer, a full picture of the growth process in all aspects is essential which remains to be developed. Recent study [Nat. Mater., 17, 535–542 (2018)] has shown that chemical vapor deposition (CVD) growth of MoS2 can involve a crawling movement of Na–Mo–O droplet in a vapor-liquid-solid (VLS) growth mechanism mediated by surface diffusion. Here, we report the inward growth of monolayer MoS2 can occur by re-evaporation of the Na–Mo–O droplets when surface diffusion is inhibited. During this regasification process, the nuclei formed at droplet surfaces are pushed away by underneath vapor until the vapor pressure drops to zero at the last atomic layer, which ensures the easy formation of large quantities of monolayers. The as-prepared MoS2 monolayers exhibit same crystal quality as those obtained by traditional CVD methods. The advancement of understanding this VLS growth process on different substrates enables us to better control the preparation of atomic thin MoS2 films on various nanoelectronic devices.

Annealing effects on sulfur vacancies and electronic transport of MoS2 films grown by pulsed-laser deposition

We have synthesized high quality and large area MoS2 films on flexible fluorophlogopite substrates using the pulsed-laser deposition (PLD) technique. Annealing in a sufficient sulfur atmosphere was adopted to eliminate oxide molybdenum and sulfur vacancies introduced during the growth in the vacuum chamber. X-ray photoelectron spectroscopy results demonstrate the advantages benefitted from the annealing process. The S/Mo ratio of the annealed MoS2 film was 1.98:1, which was much closer to the theoretical value. Raman spectroscopy, Photoluminescence spectroscopy, and X-ray diffraction spectroscopy provided direct evidence for the crystallinity improvement. Due to the elimination of molybdenum oxide, the Fermi level was shifted by 0.175 eV, and the conductive type changes from the Ohmic contact to the Schottky contact. The optimized method in this paper makes the PLD-derived MoS2 films promising candidates for microelectronic device application.

Direct synthesis of two-dimensional MoS2 on p-type Si and application to solar hydrogen production

Transition metal dichalcogenides (TMDs) are promising two-dimensional (2D) materials, and MoS2 has been specifically utilized in electronic devices and integrated circuits. However, the direct synthesis of MoS2 on traditional semiconductors, such as silicon, remains challenging due to the hydrophobic surface of nonoxide wafers (e.g., Si, GaAs, and InP). Herein, a novel, facile, reliable, and one-step method for the direct synthesis of single-crystal MoS2 on a p-Si wafer via hybrid thermolysis is proposed. To demonstrate the applicability of the proposed method, a MoS2/p-Si heterojunction was fabricated and used for solar-driven hydrogen production. The as-fabricated n-MoS2/p-Si heterojunction exhibited a benchmark current density of −13.5 ± 1 mA/cm2 at 0 V and an onset potential of +0.02 V. This method reliably and efficiently produced high-quality MoS2 crystals on a wafer scale and is sufficiently simple to overcome the challenges associated with previous approaches. The method developed herein represents a tremendous advancement in the fabrication of 2D electronic devices.

Growth of Single-Layer MoS2 by Chemical Vapor Deposition on sapphire substrate

Single-layer molybdenum disulfide (MoS2) has attracted a significant amount of interest owing to its excellent electrical, optical, and mechanical properties. In this paper, we study that the effects of the distance between the molybdenum source and substrate as well as the substrate angle on the morphology, size, and structure of MoS2 films grown from molybdenum trioxide (MoO3) sulfide on sapphire substrates via the Chemical Vapor Deposition (CVD) by using Scanning electron microscopy (SEM), Raman spectra and Photoluminescence spectra (PL). On the results show that the distance between the substrate and molybdenum source affects the controllable growth of MoS2 films. When the substrate is too close to the molybdenum source, it results in increasing amount of non-reduced MoO3 particles which were deposited on the substrate. When the distance between the substrate and molybdenum source is too large, only a small amount of MoS2 is deposited on the substrate. High-quality MoS2 films can be prepared when the molybdenum source and substrate are 9.5 cm apart. When the substrate is inclined 30° and placed downstream of the molybdenum source with a distance of 9.5 cm, the size of the prepared single-layer MoS2 is approximately 100 μm, which is greater than that of MoS2 prepared on the horizontal face-up substrate.

Mask-free patterning and selective CVD-growth of 2D-TMDCs semiconductors

This work reports on a novel mask-free approach that enables controlled selective growth of molybdenum disulfide (MoS2) and tungsten disulfide (WS2) crystalline islands on Si/SiO2 substrates. In particular, the direct-write patterning technique and chemical vapor deposition method are employed to produce arrays of 2D-TMDCs nanostructures at pre-defined locations on the substrates. It is shown that by adjusting the patterning parameters, composition and concentrations of ink-precursors, and the growth conditions, patterns of MoS2 and WS2 nanostructures with controlled geometry can be produced. The prepared 2D-TMDCs materials were analyzed by atomic force microscopy, Raman spectroscopy, transmission electron microscopy, and x-ray photoelectron spectroscopy, all of which indicated to high-quality double-layer MoS2 and WS2 nanostructures. The back-gated field effect transistors were fabricated, and their charge carrier mobility values of 11 cm2 V−1 s−1 for MoS2 and 4 cm2 V−1 s−1 for WS2 were extracted from the device measurements.

Analysis of photoluminescence behavior of high-quality single-layer MoS2

The ability to tailor and enhance photoluminescence (PL) behavior in two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2) is significant for pursuing optoelectronic applications. To achieve this, it has been essential to obtain high-quality single-layer MoS2 and fully explore its intrinsic PL performance. Here, we fabricate single-layer MoS2 by a thermal vapor sulfurization method in which a pre-deposited molybdenum trioxide (MoO3) thin film is sulfurized over a short period (for several minutes) to turn into MoS2. These as-grown MoS2 crystals show quite strong PL, which is about one order of magnitude higher than that of chemical-vapor-deposited MoS2. Temperature- and power-dependent spectroscopy measurements disclose the apparent influence of sulfur (S) vacancies on the PL behavior and the noticeable free-to-bound exciton recombinations in the luminescence process. The fact that PL intensity of the sample in vacuum sharply lowered down relative to in air reveals that the high PL is facilitated by molecular adsorption on S vacancies in air. And multi-channel decay processes coupled with S vacancies are revealed in the time-resolved PL spectroscopy. In our work, single-layer MoS2 with high PL is synthesized and its defect-induced PL features are analyzed, which is of great importance for developing advanced nano-electronics and optoelectronics based on 2D structures.

Employing a Bifunctional Molybdate Precursor To Grow the Highly Crystalline MoS2 for High-Performance Field-Effect Transistors.

Growth of the large-sized and high-quality MoS2 single crystals for high-performance low-power electronic applications is an important step to pursue. Despite the significant improvement made in minimizing extrinsic MoS2 contact resistance based on interfacial engineering of the devices, the electron mobility of field-effect transistors (FETs) made of a synthetic monolayer MoS2 is yet far below the expected theoretical values, implying that the MoS2 crystal quality needs to be further improved. Here, we demonstrate the high-performance two-terminal MoS2 FETs with room-temperature electron mobility up to ∼90 cm2 V-1 s-1 based on the sulfurization growth of the bifunctional precursor, sodium molybdate dihydrate. This unique transition-metal precursor, serving as both the crystalline Mo source and seed promotor (sodium), could facilitate the lateral growth of the highly crystalline monolayer MoS2 crystals (edge length up to ∼260 μm). Substrate surface treatment with oxygen plasma prior to the deposition of the Mo precursor is fundamental to increase the wettability between the Mo source and the substrate, promoting the thinning and coalescence of the source clusters during the growth of large-sized MoS2 single crystals. The control of growth temperature is also an essential step to grow a strictly monolayer MoS2 crystal. A proof-of-concept for thermoelectric device integration utilizing monolayer MoS2 sheds light on its potential in low-voltage and self-powered electronics.

Thermal annealing effects on the electrophysical characteristics of sputtered MoS2 thin films by Hall effect measurements

Transition-metal dichalcogenide (MoS2) is gaining increasing attention as a promising high-performance material in practical applications because of its wide-range electrical properties. However, the effect of thermal annealing on the electrophysical characteristics of MoS2 remains unclear. In this study, the physical and electrophysical properties of MoS2 thin films were characterized by Hall effect measurements. Large-area and high-quality MoS2 thin films were obtained by RF sputtering and then subjected to ex situ thermal annealing. As the thermal annealing temperature was increased, the crystallinity of the MoS2 thin films improved and the corresponding Fermi level shifted toward the valence band. At thermal annealing temperatures above 900 °C, the conductivity type of the MoS2 thin films changed from the intrinsic n-type to p-type due to the emergence of MoO3. In addition, the orientation of the Hall coefficient was indefinite under thermal annealing, implying the electrical transport anisotropy of the MoS2 thin films, which can be attributed to the spin–orbit coupling of the exposed Mo atoms in the films with sulfur vacancies. Furthermore, the Hall mobility and carrier concentration of the annealed MoS2 thin films were up to 4.40 cm2 V−1 s−1 and 12.5 × 1016 cm−3, respectively, at a Hall testing temperature of 305 K and an external magnetic field of 0.5 T.

MoS2-OH Bilayer-Mediated Growth of Inch-Sized Monolayer MoS2 on Arbitrary Substrates.

Due to remarkable electronic property, optical transparency, and mechanical flexibility, monolayer molybdenum disulfide (MoS2) has been demonstrated to be promising for electronic and optoelectronic devices. To date, the growth of high-quality and large-scale monolayer MoS2 has been one of the main challenges for practical applications. Here we present a MoS2-OH bilayer-mediated method that can fabricate inch-sized monolayer MoS2 on arbitrary substrates. This approach relies on a layer of hydroxide groups (-OH) that are preferentially attached to the (001) surface of MoS2 to form a MoS2-OH bilayer structure for growth of large-area monolayer MoS2 during the growth process. Specifically, the hydroxide layer impedes vertical growth of MoS2 layers along the [001] zone axis, promoting the monolayer growth of MoS2, constrains growth of the MoS2 monolayer only in the lateral direction into larger area, and effectively reduces sulfur vacancies and defects according to density functional theory calculations. Finally, the hydroxide groups advantageously prevent the MoS2 from interface oxidation in air, rendering high-quality MoS2 monolayers with carrier mobility up to ∼30 cm2 V-1 s-1. Using this approach, inch-sized uniform monolayer MoS2 has been fabricated on the sapphire and mica and high-quality monolayer MoS2 of single-crystalline domains exceeding 200 μm has been grown on various substrates including amorphous SiO2 and quartz and crystalline Si, SiC, Si3N4, and graphene This method provides a new opportunity for the monolayer growth of other two-dimensional transition metal dichalcogenides such as WS2 and MoSe2.

Ion-Intercalation Assisted Solvothermal Synthesis and Optical Characterization of MoS2 Quantum Dots

Many of the previously reported preparation methods for MoS2 quantum dots (QDs) are limited by production rate, time consumption, tedious processes or the final quality of the as-prepared QDs. Therefore, a simple and productive method for large-scale production of high-quality MoS2 QDs is still a challenge. We report a facile, low cost and environmentally friendly ion-intercalation assisted solvothermal route for the preparation of MoS2 QDs. In the reported method, NaOH is used as the Na+ ion source to intercalate and exfoliate the commercial MoS2 powder into nanosheets and then QDs. The reaction is carried out at a certain temperature in a Teflon-lined autoclave reactor. The UV-Vis absorption spectra of the as synthesized QDs show a peak in the near UV region (λ < 300 nm) instead of the characteristic peaks for the nanosheets. Characterization by X-ray diffraction, atomic force microscopy and photoluminescence spectroscopy also confirmed that the as-synthesized QDs had a uniform size distribution in the range of a few nanometers with mostly a monolayer structure and showed good photoluminescence (PL) properties. The proposed method has much potential for further enhancing the yield of MoS2 QDs by taking advantage of the nature of solution-based processes.

Research on the Factors Affecting the Growth of Large-Size Monolayer MoS₂ by APCVD.

The transition-metal chalcogenides (TMDs) are gaining increased attention from many scientists recently. Monolayer MoS2 is an emerging layered TMD material with many excellent physical and electrical properties. It can be widely used in catalysis, transistors, optoelectronics and integrated circuits. Here, the large-sized monolayer MoS2 is grown on the silicon substrate with a 285-nm-thick oxide layer by atmospheric pressure chemical vapor deposition (APCVD) of sulfurized molybdenum trioxide. This method is simple and it does not require vacuum treatment. In addition, the effects of growth conditions, such as sulfur source, molybdenum source, growth temperature, and argon flow rate on the quality and area of MoS2 are further studied systematically. These analysis results help to master the morphology and optical properties of monolayer MoS2. The high quality, excellent performance, and large-size monolayer MoS2 under the optimal growth condition is characterized by optical microscopy, AFM, XPS, photoluminescence, and Raman spectroscopy. The Raman spectrum and PL mapping show that the grown MoS2 is a uniform triangular monolayer with a side length of 100 μm, which can pave the way for the applications of photodetectors and transistors.

Effects of Strategically Placed Water Droplets on Monolayer Growth of Molybdenum Disulfide

Two-dimensional (2D) molybdenum disulfide (MoS2) films with a tunable bandgap hold great promise for next-generation electronic and optoelectronic devices. Synthesis of large areas of high-quality MoS2 monolayers lacks experimental reproducibility. Moreover, the outcome of MoS2 growth by chemical vapor deposition is dependent on several interconnected growth parameters. In this study, we present results of MoS2 monolayer growth by strategically placing water droplets on the growth substrate and/or in the source prior to its loading in the growth chamber. The volume and distribution of water on the growth substrate and in the source had a direct impact on the morphology of the as-grown MoS2. Characterized by scanning electron microscopy (SEM), Raman microscopy, and atomic force microscopy (AFM), the number and size of MoS2 layers as well as its distribution on the growth substrate were found to have a strong dependence on the positioning of the water droplet. This study on MoS2 monolayer growth using water droplets as a promoter provides a simple and reproducible experimental technique enabling growth with high reliability.

Biological interactions of biocompatible and water-dispersed MoS2 nanosheets with bacteria and human cells

Two dimensional materials beyond graphene such as MoS2 and WS2 are novel and interesting class of materials whose unique physico-chemical properties can be exploited in applications ranging from leading edge nanoelectronics to the frontiers between biomedicine and biotechnology. To unravel the potential of TMD crystals in biomedicine, control over their production through green and scalable routes in biocompatible solvents is critically important. Furthermore, considering multiple applications of eco-friendly 2D dispersions and their potential impact onto live matter, their toxicity and antimicrobial activity still remain an open issue. Herein, we focus on the current demands of 2D TMDs and produce high-quality, few-layered and defect-free MoS2 nanosheets, exfoliated and dispersed in pure water, stabilized up to three weeks. Hence, we studied the impact of this material on human cells by investigating its interactions with three cell lines: two tumoral, MCF7 (breast cancer) and U937 (leukemia), and one normal, HaCaT (epithelium). We observed novel and intriguing results, exhibiting evident cytotoxic effect induced in the tumor cell lines, absent in the normal cells in the tested conditions. The antibacterial action of MoS2 nanosheets is then investigated against a very dangerous gram negative bacterium, such as two types of Salmonellas: ATCC 14028 and wild-type Salmonella typhimurium. Additionally, concentration and layer-dependent modulation of cytotoxic effect is found both on human cells and Salmonellas.

Versatile and Scalable Strategy To Grow Sol-Gel Derived 2H-MoS2 Thin Films with Superior Electronic Properties: A Memristive Case.

Transition metal dichalcogenides, such as molybdenum disulfide (MoS2), show peculiar chemical/physical properties that enable their use in applications ranging from micro- and nano-optoelectronics to surface catalysis, gas and light detection, and energy harvesting/production. One main limitation to fully harness the potential of MoS2 is given by the lack of scalable and low environmental impact synthesis of MoS2 films with high uniformity, hence setting a significant challenge for industrial applications. In this work, we develop a versatile and scalable sol-gel-derived MoS2 film fabrication by spin coating deposition of an aqueous sol on different technologically relevant, flexible substrates with annealing at low temperatures (300 °C) and without the need of sulfurization and/or supply of hydrogen as compared to cutting-edge techniques. The electronic and physical properties of the MoS2 thin films were extensively investigated by means of surface spectroscopy and structural characterization techniques. Spatially homogenous nanocrystalline 2H-MoS2 thin films were obtained exhibiting high chemical purity and excellent electronic properties such as an energy band gap of 1.35 eV in agreement with the 2H phase of the MoS2, and a density of states that corresponds to the n-type character expected for high-quality 2H-MoS2. The potential use of sol-gel-grown MoS2 as the candidate material for electronic applications was tested via electrical characterization and demonstrated via the reversible switching in resistivity typical for memristors with a measured ON-OFF ratio ≥102. The obtained results highlight that the novel low-cost fabrication method has a great potential to promote the use of high-quality MoS2 in technological and industrial-relevant scalable applications.

The morphological control of MoS2 films using a simple model under chemical vapor deposition

Chemical vapor deposition (CVD) is one of the important methods for the synthesis of high quality MoS2 films at present. Herein, a facile model described the concentration distribution of MoO3 vapor is advocated for preparing MoS2 films with different morphology. Due to the gradual reduction in the concentration of MoO3 vapor from center to edge of the substrate surface calculated using this model, the morphology of as-grown MoS2 films changes from triangle to truncated triangle and then hexagon, respectively. The optimum concentration of MoO3 vapor for the growth of different morphology of MoS2 films is obtained by this model, and a direction for synthesis of MoS2 films with controllable morphology can been given. It is a simple way to control the morphology of objective thin films by tuning the concentration of precursor in CVD composition of 2D materials including MoS2 films.

Coupling Interface Constructions of MoS2 /Fe5 Ni4 S8 Heterostructures for Efficient Electrochemical Water Splitting.

Water splitting is considered as a pollution-free and efficient solution to produce hydrogen energy. Low-cost and efficient electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are needed. Recently, chemical vapor deposition is used as an effective approach to gain high-quality MoS2 nanosheets (NSs), which possess excellent performance for water splitting comparable to platinum. Herein, MoS2 NSs grown vertically on FeNi substrates are obtained with in situ growth of Fe5 Ni4 S8 (FNS) at the interface during the synthesis of MoS2 . The synthesized MoS2 /FNS/FeNi foam exhibits only 120 mV at 10 mA cm-2 for HER and exceptionally low overpotential of 204 mV to attain the same current density for OER. Density functional theory calculations further reveal that the constructed coupling interface between MoS2 and FNS facilitates the absorption of H atoms and OH groups, consequently enhancing the performances of HER and OER. Such impressive performances herald that the unique structure provides an approach for designing advanced electrocatalysts.

Intrinsic excitonic emission and valley Zeeman splitting in epitaxial MS2 (M = Mo and W) monolayers on hexagonal boron nitride

Two-dimensional (2D) semiconductors, represented by 2D transition metal dichalcogenides (TMDs), exhibit rich valley physics due to strong spin-orbit/spin-valley coupling. The most common way to probe such 2D systems is to utilize optical methods, which can monitor light emissions from various excitonic states and further help in understanding the physics behind such phenomena. Therefore, 2D TMDs with good optical quality are in great demand. Here, we report a method to directly grow epitaxial WS2 and MoS2 monolayers on hexagonal boron nitride (hBN) flakes with a high yield and high optical quality; these monolayers show better intrinsic light emission features than exfoliated monolayers from natural crystals. For the first time, the valley Zeeman splitting of WS2 and MoS2 monolayers on hBN has been visualized and systematically investigated. This study paves a new way to produce high optical quality WS2 and MoS2 monolayers and to exploit their intrinsic properties in a multitude of applications.

Epitaxial growth of single-orientation high-quality MoS2 monolayers

We present a study on the growth and characterization of high-quality single-layer MoS2 with a single orientation, i.e. without the presence of mirror domains. This single orientation of the MoS2 layer is established by means of x-ray photoelectron diffraction. The high quality is evidenced by combining scanning tunneling microscopy with x-ray photoelectron spectroscopy measurements. Spin- and angle-resolved photoemission experiments performed on the sample revealed complete spin-polarization of the valence band states near the K and -K points of the Brillouin zone. These findings open up the possibility to exploit the spin and valley degrees of freedom for encoding and processing information in devices that are based on epitaxially grown materials.

Suppression of Sulfur Desorption of High-Temperature Sputtered MoS2 Film By Applying DC Bias

As the research field of 2D layered materials expands and progresses, especially for the transition metal dichalcogenaides (TMD), the demand for high quality thin films is also rising. There have been many techniques proposed to fabricate TMD films [1-2]. Among the fabrication techniques, RF magnetron sputtering is expected to realize MoS2 device operating in normally-off mode due to large-area uniformity and suppression of impurity [3]. On the other hand, during sputtering deposition using compound MoS2 target, it is known that the film suffers from sulfur desorption especially when the substrate is heated to obtain sufficient crystallization of the film. We have reported the 2-step technique with the post-deposition sulfurization (PDS) to achieve the S implementation [4-5], however the crystallization is still insufficient due to the lack of migration of Mo atoms. Therefore prevention of sulfur desorption with 1-step method is desired in order to fabricate high quality MoS2 thin film. In this study, we proposed applying positive DC bias to the substrate during high-temperature sputter deposition in order to prevent sulfur desorption and obtain superior crystallinity by the enhanced Mo migration. MoS2 was deposited on SiO2 substrate by RF sputtering. The substrate temperature was varied from 250°C to 600°C. Ar partial pressure was also varied from 0.09 to 1.0 Pa with RF power of 50 to 225 W. The sputtering duration was fixed to 600 s. In order to investigate the effect of the sputtering parameters, the sputter-deposited MoS2 flms were evaluated with X-ray electron spectroscopy (XPS) to confirm Mo-S bond formation. The S/Mo ratio of the film was calculated from the ratio of Mo 3d and S 2p spectra. The samples with elevated deposition temperature were evaluated with Raman spectroscopy and compared to the as-depo. sample and the PDS sample [4]. As a result of applying bias voltage, higher positive bias voltage yielded films with higher S/Mo. Stoichiometric S/Mo ratio of 2.0 was achieved by applying positive bias voltage even with high-temperature sputtering. This is considered to be the attracting effect of S2- to the substrate due to the positive bias. When the Ar partial pressure was increased, the S/Mo ratio decreased. This result shows that the mean free path of the sputtered particles have large effect on the amount of S2- reaching the substrate determining S/Mo ratio of the film. As the sputter RF power was increased S/Mo also increased, which may also provide larger Mo migration energy, therefore result in the better crystallinity. By optimizing each sputtering parameters, S/Mo of 2.0 was achieved at 400-600°C. This work was partly supported by JST CREST Number JPMJCR16F4, Japan. This work was also partly supported by JSPS KAKENHI Grant Number 16J11377. REFERENCE D. Sarkar, X. Xie, W. Liu, W. Cao, J. Kang, Y. Gong, S. Kraemer, P. M. Ajayan, K. Banerjee, Nature 526, 91 (2015)S. Larentis, B. Fallahazad, E. Tutuc, Appl. Phys. Lett. 101, 223104 (2012)T. Ohashi, K. Suda, S. Ishihara, N. Sawamoto, S. Yamaguchi, K. Matsuura, K. Kakushima, N. Sugii, A. Nishiyama, Y. Kataoka, K. Natroi, K. Tsutsui, H. Iwai, A. Ogura, and H. Wakabayashi, Jpn. J. Appl. Phys. 54, 04DN08 (2015)S. Ishihara, Y. Hibino, N. Sawamoto, K. Suda, T. Ohashi, K. Matsuura, H. Machida, M. Ishikawa, H. Sudoh, H. Wakabayashi and A. Ogura, Jpn. J. Appl. Phys. 55,04EJ07 (2016)S. Ishihara, Y. Hibino, N. Sawamoto, K. Suda, T. Ohashi, K. Matsuura, H. Machida, M. Ishikawa, H. Sudoh, H. Wakabayashi and A. Ogura, Jpn. J. Appl. Phys. 55,06GF01 (2016) Figure 1