Faculty Profiles - Katsunori Makihara

Papers - Katsunori Makihara

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Crystalline phase transition of ultra-thin Ni-silicide film during SiH<sub>4</sub> exposure

Tanida, S; Taoka, N; Makihara, K

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 64 ( 2 ) 2025.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers

Formation of an ultrathin Ni-silicide on SiO2 by exposing a Ni ultrathin film to pure SiH4 gas at 280 °C was tried. During the formation, impacts of Ni-film thickness and SiH4 exposure time on silicidation reaction were investigated. It was found that surface roughness depends on the Ni-film thickness, and that, in the case of the 3 nm thick Ni-film, a smooth surface was obtained even after the SiH4 exposure. Also, it was found that the silicidation reaction is limited between 3 and 5 min, and that, after the SiH4 exposure time more than 5 min, crystalline phase transition from the Si-rich Ni-silicide to the Ni-rich Ni-silicide and formation of Si-rich Ni-silicide with poor crystallinity were confirmed. Consequently, we successfully formed the ultra-thin Ni-silicide film with the smooth surface even at the low temperature.

DOI： 10.35848/1347-4065/adaafc

Study of dot size effect on electron emission from Si-QDs multiple-stacked structures

Baek, J; Makihara, K; Obayashi, S; Imai, Y; Taoka, N; Miyazaki, S

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 63 ( 9 ) 2024.9

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers

We have fabricated diodes with different sized Si quantum dots (QDs) by precisely controlled low-pressure chemical vapor deposition using a pure SiH4 gas and studied the effect of dot size on field electron emission properties of their multiple‒stacked structures. At an applied bias of ∼9 V, the emission current of ∼4.0 nm height dot‒stacks is two orders of magnitude higher than that of ∼5.9 nm height dot‒stacks. These results can be interpreted in terms of an increase in the number of electrons with higher kinetic energy due to the increase in discrete energy levels associated with the reduction in the dot size, which suppresses electron scattering within the dot, and the electric field concentration resulting from the decrease in the curvature of the dot.

DOI： 10.35848/1347-4065/ad759b

Electron emission from alignment-controlled multiple stacks of SiGe nanodots embedded in Si structures

Makihara, K; Yamamoto, Y; Yagi, H; Li, LR; Taoka, N; Tillack, B; Miyazaki, S

MATERIALS SCIENCE IN SEMICONDUCTOR PROCESSING Vol. 174 2024.5

Language：English Publishing type：Research paper (scientific journal) Publisher：Materials Science in Semiconductor Processing

We fabricated a vertically aligned and staggered structure comprising 20–stacking layers of SiGe–nanodots (NDs) embedded in Si via reduced–pressure chemical vapor deposition and investigated their electron emission properties. The SiGe–NDs with a 35% Ge content were deposited using SiH4–GeH4, while Si spacers were deposited using SiH4 or SiH2Cl2 to control a 3D–alignment of staggered or dot–on–dot structure, respectively. Top Au electrodes with 5–nm–thick SiO2 and bottom Al contact were fabricated for electron emission measurements. After applying a bias of −3.8 V to the bottom Al–electrode with respect to the grounded top Au–electrode, electron emission was observed from the staggered SiGe–ND stack, which was slightly lower than that of the vertically–aligned NDs. In addition, we also observed a reduction in sample current with the formation of the staggered SiGe–ND stack. These results indicate that aligning SiGe–NDs in a staggered configuration suppresses leakage current and improves electron emission efficiency.

DOI： 10.1016/j.mssp.2024.108227

Self-assembling mechanism of Si-QDs on thermally grown SiO<sub>2</sub>

Baek, J; Imai, Y; Tsuji, R; Makihara, K; Miyazaki, S

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 63 ( 4 ) 2024.4

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

The self-assembling formation of Si quantum dots (Si-QDs) on as-grown SiO2 layers was shown by controlling the early stages of low-pressure chemical vapor deposition of SiH4. The QD height and radius distributions assessed by atomic force microscopy and scanning electron microscopy images revealed that the Si-QDs become hemispherical due to them being rate-limited by aggregation, which reduces the surface energy at substrate temperatures above ∼580 °C. Moreover, at temperatures below ∼580 °C, semi-ellipsoidal shaped Si-QDs are formed because the precursor supply is a dominant factor.

DOI： 10.35848/1347-4065/ad2fe1

Formation of one-dimensionally self-aligned Si-QDs and their local electron discharging properties

Imai, Y; Makihara, K; Yamamoto, Y; Wen, WC; Schubert, MA; Baek, J; Tsuji, R; Taoka, N; Ohta, A; Miyazaki, S

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 63 ( 4 ) 2024.4

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

Self-aligned Si-quantum-dots (Si-QDs) with an areal density as high as ∼1011 cm−2 have been fabricated on ultrathin SiO2 by using a ∼4.5 nm thick poly-Si on insulator (SOI) substrate, and controlling low-pressure CVD using monosilane (SiH4), and followed by thermal oxidation. By controlling the thermal oxidation processes of Si-QDs and the poly-Si layer, we have successfully demonstrated the vertical alignment of Si-QDs, where the Si-QDs are also used as a shadow mask of the underlying poly-Si layer. We also demonstrated in-plane alignment of the one-dimensionally self-aligned Si-QDs on line-patterned SiO2. In addition, from surface potential measurements by using atomic force microscopy/Kelvin probe force microscopy, we confirmed that the initial surface potential change caused by valence electron extraction from the dots to the tip was stably maintained until ∼120 min, implying the quantum confinement effect at discrete energy levels of the upper and lower-QDs.

DOI： 10.35848/1347-4065/ad38f7

Self-assembling formation of Si-QDs on SiO<sub>2</sub> line patterns

Tsuji, R; Imai, Y; Baek, J; Makihara, K; Miyazaki, S

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 63 ( 3 ) 2024.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

The lateral growth of Si-quantum dots (QDs) on line-patterned SiO2 from the thermal decomposition of pure monosilane (SiH4) has been systematically examined. We confirmed that the Si-QDs diameter in the line direction of the SiO2 patterns has the same growth rate as the SiO2 thin film surface without the line patterns. Moreover, it has been found that in the growth of Si-QDs in the width direction, a surface migration of Si precursors adsorbed from space regions could contribute to dots growth on line-patterned SiO2, which results in an elliptical growth. Furthermore, we have demonstrated a one-dimensional arrangement of highly dense-elliptical shaped Si-QDs with high uniformity in size by controlling line width, CVD time, and temperature.

DOI： 10.35848/1347-4065/ad1ca0

Formation of ultra-thin nickel silicide on SiO<sub>2</sub> using Si/Ni/Si structures for oxidation control

Kimura, K; Taoka, N; Ohta, A; Makihara, K; Miyazaki, S

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 63 ( 2 ) 2024.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

We have demonstrated the formation of ultrathin Ni silicide on SiO2 by annealing Si/Ni/Si structures and have systematically evaluated the impacts of the Si layer thickness on oxidation, surface roughening and the silicidation reaction. X-ray photoelectron spectroscopy analyses revealed that suppression of Ni oxidation due to the top Si layer makes it possible to form an ultrathin Ni silicide layer with a thickness of around 2 nm. Then, it turned out that the composition ratio of Ni and Si depends on not only the annealing temperature but also the initial thickness ratio of the top and bottom Si layers. Furthermore, this work clarified that the ultra-thin top Si layer has a large impact on the surface morphology during Ni silicide formation with diffusion and preferential oxidation.

DOI： 10.35848/1347-4065/ad1777

Formation of β-FeSi<sub>2</sub> nanodots by SiH<sub>4</sub> exposure to Fe nanodots

Saito, H; Makihara, K; Taoka, N; Miyazaki, S

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 63 ( 2 ) 2024.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

We have demonstrated the formation of β-FeSi2 nanodots (NDs) with an areal density as high as ∼1011 cm−2 on SiO2 by exposing Fe NDs to SiH4 at 400 °C and characterized their RT light-emission properties. After SiH4 exposure, even at 1.0 Pa for 60 s, stable photoluminescence (PL) signals, being characteristic of the semiconducting phase of β-FeSi2, were observed in the energy region from 0.7 to 0.85 eV. With an increase in the amount of SiH4 exposure from 60 to 600 Pa·sec, PL intensity increased by a factor of ∼13. Note that, with a further increase in the amount of SiH4 over 600 Pa·sec, the PL intensity is weakened slightly. The observed decrease in the PL intensity is attributable to the selective growth of Si onto the NDs after the formation of the β-FeSi2 phase.

DOI： 10.35848/1347-4065/ad1898

Alignment control of self-assembling Si quantum dots

Yuki Imai, Ryoya Tsuji, Katsunori Makihara, Noriyuki Taoka, Akio Ohta, Seiichi Miyazaki

MATERIALS SCIENCE IN SEMICONDUCTOR PROCESSING Vol. 162 2023.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

We have demonstrated the formation of one-dimensionally aligned self-assembling silicon quantum dots (Si-QDs) by low-pressure (LP) CVD using pure monosilane (SiH4) gas. Formation of Si-QDs on line-patterned SiO2 formed on p-Si(100) after low-temperature oxidation was carried out. From AFM images taken after LPCVD, we confirmed high-density Si-QDs formation with an areal density as high as-1011 cm-2 on the OH-terminated SiO2-line while dot density as low as-1010 cm-2 on as-grown SiO2. We also confirmed that, with a decreasing the SiO2 line width, the number of Si-QDs on the SiO2-line was decreased evidently. Based on the results, we have succeeded in the one-dimensional arrangement of Si-QDs on the-30 nm-width SiO2 line-patterns.

DOI： 10.1016/j.mssp.2023.107526

Layer transfer of ultrathin Ge crystal segregated on Al/Ge(111) structure

Matsushita Keigo, Ohta Akio, Shibayama Shigehisa, Tokunaga Tomoharu, Taoka Noriyuki, Makihara Katsunori, Miyazaki Seiichi

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 62 ( SG ) 2023.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

Effects of the surface modification by O2 plasma exposure on the Al/Ge(111) structure have been investigated in order to get an insight into a layer transfer technique of the ultrathin Ge layer segregated on the Al/Ge(111) structure toward the device fabrication, and then the wafer bonding of the Al/Ge(111) structure to the thermally-grown SiO2/Si structure has been demonstrated. The O2 plasma treatment and the subsequent pure water rinse were found to be effective to form the hydrophilic surface of the Al/Ge(111) structure with a suppression of the segregated Ge layer oxidation. The Al/Ge(111) structure with the hydrophilic surface was then bonded to the SiO2/Si substrate, and its bonding strength was enough to perform Ge thinning by the chemical mechanical polishing and the wet-chemical etching using H2O2-based solutions. Ohmic contact of the ring-type device pattern with the segregated Ge/Al stack was achieved by using the remaining p-type Ge substrate as the contact pads.

DOI： 10.35848/1347-4065/acb65c

Room Temperature Light Emission from Superatom-like Ge-Core/Si-Shell Quantum Dots

Katsunori Makihara, Yuji Yamamoto, Yuki Imai, Noriyuki Taoka, Markus Andreas Schubert, Bernd Tillack, Seiichi Miyazaki

NANOMATERIALS Vol. 13 ( 9 ) 2023.4

Language：English Publishing type：Research paper (scientific journal) Publisher：MDPI

We have demonstrated the high-density formation of super-atom-like Si quantum dots with Ge-core on ultrathin SiO2 with control of high-selective chemical-vapor deposition and applied them to an active layer of light-emitting diodes (LEDs). Through luminescence measurements, we have reported characteristics carrier confinement and recombination properties in the Ge-core, reflecting the type II energy band discontinuity between the Si-clad and Ge-core. Additionally, under forward bias conditions over a threshold bias for LEDs, electroluminescence becomes observable at room temperature in the near-infrared region and is attributed to radiative recombination between quantized states in the Ge-core with a deep potential well for holes caused by electron/hole simultaneous injection from the gate and substrate, respectively. The results will lead to the development of Si-based light-emitting devices that are highly compatible with Si-ultra-large-scale integration processing, which has been believed to have extreme difficulty in realizing silicon photonics.

DOI： 10.3390/nano13091475

Characterization of magnesium channeled implantation layers in GaN(0001)

Suyama Atsushi, Kawanowa Hitoshi, Minagawa Hideaki, Maekawa Junko, Nagamachi Shinji, Aoki Masahiko, Ohta Akio, Makihara Katsunori, Miyazaki Seiichi

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 62 ( SC ) 2023.4

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

The effect of Mg channeled implantation into epitaxially grown gallium nitride (GaN) was studied using Hall-effect measurements, photoluminescence (PL), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and Rutherford backscattering spectroscopy (RBS). In the channeled implantation, deeper profiles were obtained with lower implantation energy and less damage compared to random implantation. The donor-acceptor pair signal at ∼3.28 eV, suggesting that Mg is activated, was confirmed by PL measurement when the ion dose and implantation energy are 1 × 1014 cm−2 and 20 keV, respectively. However, even with channeled implantation, several types of defects including point defects and oblong defects as seen in the random implantation were observed by TEM/STEM analysis. RBS analysis showed slightly worse crystal qualities in channeled implantation compared to non-implanted samples. Mg channeled implantation is useful to achieve deeper profiles (>1 μm), but further condition tuning of process will be necessary for practical application.

DOI： 10.35848/1347-4065/acb951

Formation of ultra-thin NiGe film with single crystalline phase and smooth surface

Nishimura Shunsuke, Taoka Noriyuki, Ohta Akio, Makihara Katsunori, Miyazaki Seiichi

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 62 ( SC ) 2023.4

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

Formation of an ultra-thin nickel-germanide (Ni-germanide) film on a SiO2 film has been attempted with stacking structures of Ni with various thicknesses formed on Ge films with thicknesses of 4 nm or 20 nm and annealing in an N2 ambient condition. Physical analyses revealed that the ultra-thin Ni-germanide films with smooth surfaces could be formed on the SiO2 film after annealing at 400 °C without depending on the Ni thickness on the 4 nm thick Ge films. In the formation, reductive and oxidative reactions occurred in the films, which are quite important for determining a composition of the Ni-germanide.

DOI： 10.35848/1347-4065/acac6f

Evaluation of chemical structure and Si segregation of Al/Si(111)

Sakai Taiki, Ohta Akio, Matsushita Keigo, Taoka Noriyuki, Makihara Katsunori, Miyazaki Seiichi

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 62 ( SC ) 2023.4

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

Control of diffusion and segregation of Si atoms through a thin metal layer from a stacked Si structure is one of the effective techniques to grow two-dimensional (2D) or ultrathin Si crystals. We have studied the formation of the Al layer with a flat surface and high crystallinity on a wet-cleaned Si(111) wafer by thermal evaporation in order to use it as a growth template for ultrathin Si crystals. Then, the impacts of the annealing in N2 ambient on the surface flatness, the Al crystallinity, and the chemical bonding features for the Al/Si(111) structure were investigated. A formation of a sub-nanometer Si layer on the Al(111) surface using Si segregation with keeping surface flatness was demonstrated by the control of annealing temperature.

DOI： 10.35848/1347-4065/acb1fd

Effects of Cl passivation on Al2O3/GaN interface properties

Nagai Taisei, Taoka Noriyuki, Ohta Akio, Makihara Katsunori, Miyazaki Seiichi

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 62 ( SA ) 2023.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

We investigated the effects of a HCl-based cleaning (SC2) and post-deposition annealing (PDA) on an Al2O3/GaN interface and electrical properties. X-ray photoelectron spectroscopy revealed the existence of the Cl atoms near the interface after the Al2O3 deposition and subsequent PDA, resulting in a band bending at the GaN surface. A C-V curve of a MOS capacitor with the Al2O3/GaN interface with SC2 was shifted toward the positive bias direction compared with that without SC2. It was found that PDA induced negative shifts of the C-V curves, and that the SC2 treatment increases interface trap density at the Al2O3/GaN interface. These results indicate that the Cl termination of the GaN surface has clear impacts on the interface and electrical properties.

DOI： 10.35848/1347-4065/ac73d9

Study on Electron Emission from Phosphorus ?-Doped Si-QDs/Undoped Si-QDs Multiple-Stacked Structures

Katsunori Makihara, Tatsuya Takemoto, Shuji Obayashi, Akio Ohta, Noriyuki Taoka, Seiichi Miyazaki

IEICE TRANSACTIONS ON ELECTRONICS Vol. E105C ( 10 ) page： 610 - 615 2022.10

Language：English Publishing type：Research paper (scientific journal) Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATION ENGINEERS

We have fabricated two-tiered heterostructures consisting of phosphorus delta-doped Si quantum dots (Si-QDs) and undoped Si-QDs and studied their electron field emission properties. Electron emission was observed from the P-doped Si-QDs stack formed on the undoped Si-QDs stack by applying a forward bias of similar to 6 V, which was lower than that for pure Si-QDs stack. This result is attributed to electric field concentration on the upper P-doped Si-QD layers beneath the layers of the undoped Si-QDs stack due to the introduction of phosphorus atom into the Si-QDs, which was positively charged due to the ionized P donor. The results lead to the development of planar-type electron emission devices with a low-voltage operation.

DOI： 10.1587/transele.2021FUP0006

Impact of substrate heating during Al deposition and post annealing on surface morphology, Al crystallinity, and Ge segregation in Al/Ge(111) structure

Matsushita Keigo, Ohta Akio, Taoka Noriyuki, Hayashi Shohei, Makihara Katsunori, Miyazaki Seiichi

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 61 ( SH ) 2022.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

We have studied the impact of Ge substrate heating during ∼25 nm thick Al deposition and post annealing in N2 ambient on the surface flatness of an Al/Ge(111) structure, the crystallographic structure of the deposited Al layer, and formation of a Ge segregated layer. Surface segregation of Ge atoms on a flat metal surface is an effective means of growing two-dimensional Ge crystals as well as an ultrathin Ge crystalline layer. The surface morphology of the Al/Ge(111) structure becomes flat by substrate heating during Al deposition. The crystallinity of the Al layer on Ge(111) can be improved by both substrate heating and post annealing. Ge segregation on a flat Al(111) surface also occurred by post annealing.

DOI： 10.35848/1347-4065/ac5fbc

Study on silicidation reaction of Fe nanodots with SiH<sub>4</sub>

Furuhata, H; Makihara, K; Shimura, Y; Fujimori, S; Imai, Y; Ohta, A; Taoka, N; Miyazaki, S

APPLIED PHYSICS EXPRESS Vol. 15 ( 5 ) 2022.5

Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Physics Express

We have demonstrated the formation of Fe-silicide nanodots (NDs) on SiO2 by exposing Fe NDs to SiH4. The Fe NDs were formed by exposing ultrathin Fe film deposited on SiO2 to remote H2-plasma. After SiH4 exposure at 400 °C, formation of Fe-silicide NDs with an areal dot density over 1011 cm-2 was confirmed. Photoluminescence from the Fe-silicide NDs was observable at room temperature in the near-infrared, being attributed to radiative recombination between quantized states in the NDs. The results will lead to the development of Si-based light-emitting devices that are highly compatible with Si ultralarge-scale-integration processing.

DOI： 10.35848/1882-0786/ac6727

Effect of substrate temperature on plasma-enhanced self-assembling formation of high-density FePt nanodots

Honda Shunsuke, Makihara Katsunori, Taoka Noriyuki, Furuhata Hiroshi, Ohta Akio, Oshima Daiki, Kato Takeshi, Miyazaki Seiichi

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 61 ( SA ) 2022.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

We formed FePt magnetic nanodots (NDs) by exposing an ultrathin bilayer metal stack on ∼3.0 nm SiO2/Si(100) substrates to a remote H2 plasma (H2-RP) and studied the effect of external heating during the exposure to H2-RP on the formation and magnetic properties of NDs. The ultrathin bilayer with a uniform surface coverage drastically changed to NDs with an areal density as high as ∼3.5 × 1011 cm-2 by exposing to H2-RP with external heating. We also found that NDs formed by the exposure to H2-RP at 400 °C exhibited a perpendicular anisotropy with a perpendicular coercivity of ∼1.5 kOe, reflecting the magneto-crystalline anisotropy of (001)-oriented L10 phase FePt.

DOI： 10.35848/1347-4065/ac2036

Segregation control for ultrathin Ge layer in Al/Ge(111) system

Akio Ohta, Masato Kobayashi, Noriyuki Taoka, Mistuhisa Ikeda, Katsunori Makihara, Seiichi Miyazaki

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 61 ( SA ) 2022.1

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP Publishing Ltd

An impact of the vacuum anneal of Al/Ge(111) structure on the Ge segregation has been investigated to get an insight into the precise control of ultrathin Ge crystalline growth. The Al/Ge(111) structure was prepared by thermal evaporation of Al on wet-cleaned Ge(111) and then vacuum annealed without air exposure to promote Ge formation on the Al surface. The Ge formation and its chemical bonding features were evaluated by X-ray photoelectron spectroscopy analysis. In addition, changes in the average Ge thickness depending on annealing temperature and time were crudely estimated. We found that the annealing temperature had a greater effect than time on the control of sub-nanometer scale Ge growth.

DOI： 10.35848/1347-4065/ac19ff

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