Faculty Profiles - AMANO, Hiroshi

Papers - AMANO, Hiroshi

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Growth and Characterization of High Internal Quantum Efficiency Semipolar (101̅3) GaN-Based Light Emitting Diodes

Hu N., Park J.H., Wang J., Amano H., Pristovsek M.

ACS Applied Electronic Materials Vol. 6 ( 11 ) page： 7960 - 7971 2024.11

Language：English Publishing type：Research paper (scientific journal) Publisher：ACS Applied Electronic Materials

We report on the growth and characterization of semipolar (101̅3) InGaN/GaN light-emitting diodes (LEDs) on m-plane (101̅0) sapphire with high internal quantum efficiencies (IQEs). Based on earlier work on obtaining untwinned (101̅3) GaN templates on m-plane sapphire, we also succeeded in smoothing the relatively unstable (101̅3) surface using very low V/III ratios during metal-organic vapor phase epitaxy and in the growth of n-and p-doped layers to produce LEDs. Fitting the current-dependent emission, we found high IQEs, up to 86% at 460 nm. The IQEs decreased toward longer wavelengths, similar as for conventional Ga-polar (0001) oriented LEDs. However, the semipolar (101̅3) LEDs have their maximum IQE at much higher current densities than the polar (0001) LEDs due to the higher overlap between electron and hole wave function while maintaining a positive polarization. Together, this makes the (101̅3) orientation very attractive for future blue LEDs.

DOI： 10.1021/acsaelm.4c01312

窒化物半導体結晶の魅力と人・社会・地球への貢献 Open Access

天野浩

日本結晶学会誌 Vol. 66 ( Supplement ) page： s2 - s2 2024.11

Study on Degradation of Deep-Ultraviolet Laser Diode

Zhang, ZY; Yoshikawa, A; Kushimoto, M; Sasaoka, C; Amano, H

PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE Vol. 221 ( 21 ) 2024.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Physica Status Solidi (A) Applications and Materials Science

The degradation of an AlGaN-based deep-ultraviolet laser diode during operation is studied. A rapid increase in threshold current without any facet mirror damage under direct current stress below the threshold current is observed, which is considered to limit the lifetime of continuous wave lasing to the level of seconds. The evolution of current–light and current–voltage characteristics suggests that the dominant mechanism of degradation is the increased loss of carriers before they can reach the active layer, i.e., the decrease in carrier injection efficiency. The decrease in emission intensity, which is more pronounced at lower current densities, and subsequent increase in sub-threshold current indicate the increase in defect density under current stress, which is similar to the well-analyzed degradation mechanism found in AlGaN-based light-emitting diodes.

DOI： 10.1002/pssa.202300946

Experimental study of gain characteristics in relation to quantum-well width of deep-ultraviolet laser diodes

Zhang Z., Kushimoto M., Yoshikawa A., Aoto K., Sasaoka C., Amano H.

Applied Physics Letters Vol. 125 ( 18 ) 2024.10

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

The gain characteristics of electrically injected AlGaN-based deep-ultraviolet laser diodes were studied with respect to their quantum-well (QW) width dependence by the analysis of the threshold currents and gains for various cavity lengths. Among the fabricated QWs with widths of d = 9, 4.5, and 3 nm, the QW with the smallest width of 3 nm was found to have the highest material gain and lowest transparency carrier density. In contrast to recent arguments in favor of wider QWs for deep-ultraviolet laser diodes, the strategy of narrowing QWs is still found to be effective in practical terms of reducing the threshold current density.

DOI： 10.1063/5.0240488

Discovering the incorporation limits for wurtzite AlP<inf>y</inf>N<inf>1−y</inf> grown on GaN by metalorganic vapor phase epitaxy Reviewed

Yang X., Furusawa Y., Kano E., Ikarashi N., Amano H., Pristovsek M.

Applied Physics Letters Vol. 125 ( 13 ) 2024.9

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

We report on the growth of AlPN on GaN/sapphire templates by metalorganic vapor phase epitaxy using tertiarybutylphosphine (tBP) and NH3 as group-V precursors. P is easy to incorporate into the group-III lattice site, forming PAl anti-site defects and shrinking lattice constants that are even beyond AlN since Al is larger than P. We found that higher temperatures favor P incorporation on the N-sublattice, forming AlPyN1−y, while growth temperatures below 1000 °C result in dominant P incorporation on the Al-sublattice, forming PAl anisites. Similarly, larger NH3 flows stabilize GaN, leading to flat interfaces, but favor the formation of PAl. Furthermore, the P incorporation into AlPyN1−y is non-linear. At very low tBP flows, it initially increases to reach a maximum. Further increasing the tBP flow increases mostly the incorporation of P on the Al-sublattice, and the c-lattice constant decreases again. This leaves a small window of low V/III ratios below 5 and low P/N ratios of 1% or smaller, leading up to ∼4% P incorporation at typical growth temperatures of GaN. However, at such low V/III ratios, GaN is not stable even with N2 carrier gas and requires optimized switching sequences to minimize its decomposition and preserve flat interfaces. Eventually, a 10 nm coherent layer of AlP0.01N0.99 could be reproducibly grown on top of GaN channels with a smooth surface, an abrupt AlPN/GaN interface, and a two-dimensional electron gas with an electron mobility of ∼675 cm2/V s and a sheet carrier density of 1.5 × 1013 cm−2 at room temperature.

DOI： 10.1063/5.0225115

Hole transport mechanism at high temperatures in p-GaN/AlGaN/GaN heterostructure Reviewed

Sikder, B; Hossain, T; Xie, QY; Niroula, J; Rajput, NS; Teo, KH; Amano, H; Palacios, T; Chowdhury, N

APPLIED PHYSICS LETTERS Vol. 124 ( 24 ) 2024.6

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

This Letter reports an investigation of hole transport in p-GaN/AlGaN/GaN heterostructures through experimental and theoretical analyses under varied conditions. Highly non-linear current-voltage (I-V) characteristics, obtained via the linear transmission line method measurements, are utilized for this study. At low bias voltage, the transport can be ascribed to the Schottky nature of the contact, while at high bias, the conduction is observed to be governed by space-charge limited current (SCLC). The Schottky characteristics (Schottky barrier height and non-ideality factor) and the SCLC exponent were analyzed for devices with varying contact spacings and at different high temperatures. The SCLC exponent, m, is in the range of 2 ≤ m ≤ 4 depending on the applied voltage range, revealing the existence of the trap states in the channel region. The findings of this work indicate that the charge injection, field-induced ionization, and trap states in the p-GaN channel are critical factors in the current transport of p-GaN/AlGaN/GaN heterostructure.

DOI： 10.1063/5.0203344

Recent advances in micro-pixel light emitting diode technology Reviewed

Park, JH; Pristovsek, M; Amano, H; Seong, TY

APPLIED PHYSICS REVIEWS Vol. 11 ( 2 ) 2024.6

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

Display technology has developed rapidly in recent years, with III-V system-based micro-light-emitting diodes (μLEDs) attracting attention as a means to overcome the physical limitations of current display systems related to their lifetime, brightness, contrast ratio, response time, and pixel size. However, for μLED displays to be successfully commercialized, their technical shortcomings need to be addressed. This review comprehensively discusses important issues associated with μLEDs, including the use of the ABC model for interpreting their behavior, size-dependent degradation mechanisms, methods for improving their efficiency, novel epitaxial structures, the development of red μLEDs, advanced transfer techniques for production, and the detection and repair of defects. Finally, industrial efforts to commercialize μLED displays are summarized. This review thus provides important insights into the potential realization of next-generation display systems based on μLEDs.

DOI： 10.1063/5.0177550

Impacts of vacancy complexes on the room-temperature photoluminescence lifetimes of state-of-the-art GaN substrates, epitaxial layers, and Mg-implanted layers Reviewed

Chichibu, SF; Shima, K; Uedono, A; Ishibashi, S; Iguchi, H; Narita, T; Kataoka, K; Tanaka, R; Takashima, S; Ueno, K; Edo, M; Watanabe, H; Tanaka, A; Honda, Y; Suda, J; Amano, H; Kachi, T; Nabatame, T; Irokawa, Y; Koide, Y

JOURNAL OF APPLIED PHYSICS Vol. 135 ( 18 ) 2024.5

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

For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission ( τ PL RT ) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τ PL RT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3-4, in ammonothermal GaN. The values of τ PL RT in n-GaN samples are compared with those of p-GaN, in which τ PL RT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively.

DOI： 10.1063/5.0201931

Metastable atomic-ordered configurations for Al<sub>1/2</sub>Ga<sub>1/2</sub>N predicted by Monte-Carlo method based on first-principles calculations Reviewed

Gueriba, JS; Mizuseki, H; Cadatal-Raduban, M; Sarukura, N; Kawazoe, Y; Nagasawa, Y; Hirano, A; Amano, H

JOURNAL OF PHYSICS-CONDENSED MATTER Vol. 36 ( 13 ) 2024.4

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Physics Condensed Matter

Metastability of Aln/12Ga1−n/12N (n = 2–10: integer) with the 1–2 monolayer (ML) in-plane configuration towards the c [0001] direction has been demonstrated recently. To theoretically explain the existence of these metastable structures, relatively large calculation cells are needed. However, previous calculations were limited to the use of small calculation cell sizes to estimate the local potential depth (∆σ) of ordered Al1/2Ga1/2N models. In this work, we were able to evaluate large calculation cells based on the interaction energies between proximate Al atoms (δEAl–Al) in AlGaN alloys. To do this, δEAl–Al values were estimated by first-principles calculations (FPCs) using a (5a1 × 5a2 × 5c) cell. Next, a survey of the possible ordered configurations using various large calculation cell models was performed using the estimated δEAl–Al values and the Monte-Carlo method. Then, various ∆σ values were estimated by FPCs and compared with the configurations previously reported by other research groups. We found that the ordered configuration obtained from the (4a1 × 2a2 × 1c) calculation cell (C42) has the lowest ∆σ of −9.3 meV/cation and exhibited an in-plane configuration at the c(0001) plane having (–Al–Al–Ga–Ga–) and (–Al–Ga–) sequence arrangements observed along the m{11̄00} planes. Hence, we found consistencies between the morphology obtained from experiment and the shape of the primitive cell based on our numerical calculations.

DOI： 10.1088/1361-648X/ad1137

Investigation of carbon-related complexes in highly C-doped GaN grown by metalorganic vapor phase epitaxy Reviewed

Honda, A; Watanabe, H; Takeuchi, W; Honda, Y; Amano, H; Kato, T

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

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

We investigated the C-related complexes in highly C-doped GaN by electron spin resonance (ESR) spectroscopy, Fourier transform IR spectroscopy (FTIR), and minority carrier transient spectroscopy (MCTS) measurements. In the ESR spectra, two resonances with g values of 2.02 and 2.04 were found to be assigned by (0/−) deep acceptor and (+/0) charge transition levels of carbon substituting for nitrogen site (CN). In the FTIR spectra, two local vibrational modes positioned at 1679 and 1718 cm−1 were confirmed to be associated with tri-carbon complexes of CN-CGa-CN (basal) and CN-CGa-CN (axial), respectively. In the MCTS spectra, we observed the hole trap level of E v + 0.25 ± 0.1 eV associated with the tri-carbon complexes, which are the dominant C-related defects, suggesting that these complexes affect the electronic properties in the highly C-doped GaN.

DOI： 10.35848/1347-4065/ad3b54

Improving the Barrier Height of N-Polar GaN Schottky Diodes Using Mg-Diffusion Process Reviewed

Sarkar, B; Wang, J; Watanabe, H; Amano, H

IEEE TRANSACTIONS ON ELECTRON DEVICES Vol. 71 ( 3 ) page： 1416 - 1420 2024.3

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Transactions on Electron Devices

In this work, we report a low-cost methodology to increase the barrier height of N-polar GaN Schottky diodes. Physical vapor deposition (PVD) of Mg followed by a thermal diffusion anneal at 800 °C leads to the formation of N-polar GaN Camel diode offering a larger barrier height than the conventional N-polar GaN Schottky diodes. The increase in barrier height after the Mg diffusion process is validated using current-voltage ({I} - {V}) and capacitance-voltage (CV) measurements. A barrier height of 0.7 eV and a near-unity ideality factor observed in the N-polar GaN Camel diode confirms that the proposed Mg diffusion process is an alternative method for improving the performance of future N-polar GaN diodes.

DOI： 10.1109/TED.2023.3341831

Impacts of off-angle and off-direction on surface morphology of GaN grown by metalorganic vapor phase epitaxy on (0001) GaN substrate Reviewed

Watanabe, H; Nitta, S; Ando, Y; Ohnishi, K; Honda, Y; Amano, H

JOURNAL OF CRYSTAL GROWTH Vol. 628 2024.2

Sn-doped n-type GaN layer with high electron density of 1020 cm-3 grown by halide vapor phase epitaxy Reviewed

Hamasaki, K; Ohnishi, K; Nitta, S; Fujimoto, N; Watanabe, H; Honda, Y; Amano, H

JOURNAL OF CRYSTAL GROWTH Vol. 628 2024.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Crystal Growth

A Sn-doped n-type GaN layer with a high electron density of 2 × 1020 cm−3 and a low resistivity of 8.7 × 10−4 Ω∙cm was grown by halide vapor phase epitaxy (HVPE). Sn doping was performed through the reaction between Sn metal and HCl gas. The Sn concentration markedly increased with decreasing growth temperature and the activation energy of Sn desorption from the GaN surface was found to be 4.1 eV. Smooth surfaces were obtained by introducing the Sn precursor even though the samples were grown at a low temperature of 905 °C, suggesting that Sn atoms act as surfactants and promote the migration of Ga adatoms. Almost all the Sn atoms act as donors in GaN below the Sn concentration of 2 × 1020 cm−3. These results indicate that using the Sn donor is promising for fabricating low-resistivity n-type GaN substrates by HVPE.

DOI： 10.1016/j.jcrysgro.2023.127529

Impact of unintentionally formed compositionally graded layer on carrier injection efficiency in AlGaN-based deep-ultraviolet laser diodes Reviewed

Zhang Z., Yoshikawa A., Kushimoto M., Aoto K., Sasaoka C., Amano H.

Applied Physics Letters Vol. 124 ( 6 ) 2024.2

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

Increasing the injection efficiency, a critical factor constraining the reduction in threshold current in AlGaN-based deep-ultraviolet laser diodes, represents one of the paramount remaining technical challenges. In this study, the impact of compositionally graded layers that were unintentionally formed at the interface between the p-cladding and the core layer on carrier injection efficiency was analyzed. Experimental evaluations using laser diodes have shown that the elimination of an unintentionally formed layer increases the injection efficiency above the threshold current, from the conventional 3% to 13%. It has been postulated that the electron overflow toward the p-side exerts a substantial deleterious effect on the injection efficiency. An improvement in this aspect is achieved by increasing the electron-blocking capability due to the improved interface abruptness between the p-cladding layer and the core layer. The lasing threshold was strongly reduced, and characteristic temperature increased from 76 to 107 K for the improved devices.

DOI： 10.1063/5.0184543

Optical activation of praseodymium ions implanted in gallium nitride after ultra-high pressure annealing Reviewed

Ito, S; Sato, S; Bockowski, M; Deki, M; Watanabe, H; Nitta, S; Honda, Y; Amano, H; Yoshida, K; Minagawa, H; Hagura, N

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 547 2024.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms

Lanthanide (Ln)-implanted gallium nitride (GaN) semiconductors have a variety of potential applications as light-emitting devices and quantum light sources, but their optical properties are not well understood. In this study, we investigate the room temperature optical properties of praseodymium (Pr) ion implanted single-crystal GaN and their changes upon ultrahigh pressure annealing (UHPA) up to 1480 °C. Photoluminescence (PL) spectra, luminescence transition lifetime, and excitation cross section of the implanted Pr ions are analyzed. In addition, the recovery of implantation-induced damage and the thermal diffusion of implanted Pr ions by UHPA are investigated by X-ray diffraction, Raman spectroscopy, and secondary ion mass spectrometry. The results show that the implantation damage is recovered by annealing at temperatures above 1200 °C, but Pr ions thermally diffuse to the surface as the annealing temperature increases. The annealing temperature at which a maximum PL intensity is obtained increases with increasing the implantation dose. However, the PL intensity decreases in all cases after annealing at 1480 °C, indicating that a quenching factor is dominant in this temperature range.

DOI： 10.1016/j.nimb.2023.165181

Impurity reduction in lightly doped <i>n</i>-type gallium nitride layer grown via halogen-free vapor-phase epitaxy Reviewed

Kimura, T; Shimazu, H; Kataoka, K; Itoh, K; Narita, T; Uedono, A; Tokuda, Y; Tanaka, D; Nitta, S; Amano, H; Nakamura, D

APPLIED PHYSICS LETTERS Vol. 124 ( 5 ) 2024.1

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

The development of gallium nitride (GaN) vertical-type metal-oxide-semiconductor field-effect transistors and p-i-n diode devices has gathered increasing attention. These devices require an n-type drift layer with a low doping level of 1016 cm−3 or less, minimized point defects inhibiting electron conduction, and a layer approximately 10 μm thick. Therefore, a practical method with a growth rate of at least several tens of μm/h and impurity concentrations of less than 1015 cm−3, except for that of dopants, is necessary. Halogen-free vapor-phase epitaxy (HF-VPE) has a high growth rate suitable for fabricating thick drift layers and utilizes a simple reaction between Ga vapor and ammonia gas (without a corrosive halogen gas), resulting in lower impurity levels. Herein, we eliminated the quartz content from the high-temperature zone to reduce the excess unintentional Si doping and identified that the nitrile gloves used for the growth preparation are other impurity contamination sources. We obtained a lightly n-type ([Si]=∼1016 cm−3) GaN layer, in which C, O, B, Fe, Mg, Al, Ca, Cr, Zn, Ni, Mn, and Ti impurity contents were below the detection limits of secondary ion mass spectrometry. Deep-level transient spectroscopy revealed that electron traps at EC − 0.26 and at EC − 0.59 eV were 2.7 × 1013 and 5.2 × 1014 cm−3, respectively. Moreover, the Hall effect analysis showed the acceptor-type defect-compensating donor content as approximately 2.7 × 1015 cm−3, resulting in a high electron mobility of HF-VPE GaN in the 30-710 K temperature range. Furthermore, we identified the Ca impurity as a deep acceptor, another killer defect leading to mobility collapse.

DOI： 10.1063/5.0191774

Droop and light extraction of InGaN-based red micro-light-emitting diodes Reviewed

Park, JH; Pristovsek, M; Cai, WT; Kumabe, T; Choi, SY; Lee, DS; Seong, TY; Amano, H

SEMICONDUCTOR SCIENCE AND TECHNOLOGY Vol. 39 ( 1 ) 2024.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Semiconductor Science and Technology

In this letter, we investigate the impact of periphery, width, length and area on the external quantum efficiency (EQE) of stripe-type InGaN-based red micro-light-emitting diodes (µLEDs). A longer periphery resulted in a higher light extraction efficiency ( η e ) via the sidewall regardless of the area of the µLEDs. However, as the injection current increased a somewhat larger efficiency droop was observed at the longer periphery due to current crowding. Additionally, larger µLEDs experienced more self-heating than smaller ones, resulting in a red shift of wavelengths and a larger efficiency droop. When the current density exceeded 100 A cm−2, the EQE ratio of smaller-area μLEDs to larger-area ones increased significantly due to the difference in efficiency droop. Besides, a short light propagation length and a long emission width yielded a higher η e . Hence, the periphery, width, length and area of the µLEDs determine EQE, which provides insight into the pixel design of µLED displays.

DOI： 10.1088/1361-6641/ad0b88

Temperature Dependence of α-Particle Detection Performance of GaN PIN Diode Detector Reviewed

Nakagawa, H; Hayashi, K; Miyazawa, A; Honda, Y; Amano, H; Aoki, T; Nakano, T

SENSORS AND MATERIALS Vol. 36 ( 1 ) page： 169 - 176 2024

Language：English Publishing type：Research paper (scientific journal) Publisher：Sensors and Materials

Group-III nitride semiconductors, such as gallium nitride (GaN), have been proposed as novel materials for radiation detection owing to their wide bandgap and their ability to operate at high temperatures. In this study, the radiation detection properties of GaN PIN diode detectors were evaluated at high temperatures (~573 K). The energy spectrum peak profiles of241Am α-particles were obtained at different temperatures, confirming the operation of GaN PIN diodes up to 573 K. The peak positions shifted toward the lower-energy side and the full width at half maximum (FWHM) of the detection energy peak improved with increasing temperature. Furthermore, the variation in electron carrier mobility–lifetime product (μeτe) between 293 and 573 K was not significant. These results indicate the potential high-temperature operation of group-III nitride semiconductors. Additionally, the variation in each detection characteristic was caused by increasing the atmospheric temperature, which affected the mobility, lattice scattering, bandgap, and built-in potential differently.

DOI： 10.18494/SAM4647

Light-Stimulated Artificial Synapses with Accelerating Photopic Adaption Based on III-Nitride Heterojunction Transistor Reviewed

Yan J., Sun Z., Fang L., Yan Y., Shi Z., Shi F., Jiang C., Choi H.W., Amano H., Liu Y., Wang Y.

ACS Photonics 2024

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

Neuromorphic computing, inspired by biological synapses, has emerged as a pivotal approach to overcome the limitations of von Neumann architecture. In this work, we employ a three-terminal III-nitride metal-oxide-semiconductor field-effect transistor (MOSFET) structure based on the GaN-on-silicon platform, characterized by ultralow power consumption, to emulate artificial synapses through light stimulation. Remarkably, our device exhibits a minimal power consumption of 7 fJ per synaptic event, surpassing that of biological synapses (∼10 fJ). Notably, we observe that the paired pulse facilitation (PPF) index can be modulated by the external light intensity, pulse width, and light pulse interval. By manipulating the light intensity and pulse width, we achieve a PPF index exceeding 300% in our device. Furthermore, our devices demonstrate gate-tunable synaptic plasticity, enabling electric/light cooperative control and increasing the tuning freedom. Finally, acceleration of photopic adaption and a 91.28% handwritten digit recognition accuracy are achieved, which provide strong support in potential visual sensory applications.

DOI： 10.1021/acsphotonics.4c01038

SEM imaging of high aspect ratio trench by selectively controlling the electron beam irradiation using photocathode Reviewed

Arakawa, Y; Niimi, K; Otsuka, Y; Sato, D; Koizumi, A; Shikano, H; Iijima, H; Nishitani, T; Honda, Y; Amano, H

METROLOGY, INSPECTION, AND PROCESS CONTROL XXXVIII Vol. 12955 2024

Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of SPIE - The International Society for Optical Engineering

Observation of high aspect ratio (HAR) structures is a difficult challenge in metrology and inspection in semiconductor manufacturing. In imaging HAR trenches using a scanning electron microscope (SEM), obtaining SEM images without information loss due to whiteouts and blackouts is challenging. One reason for the difficulty is that the probe current is constant in conventional SEM imaging. Suppose the probe current is increased to detect more secondary electrons from the bottom of the trench. In that case, excessive secondary electron emission from the top of the trench will result in a whiteout. The SEM equipped with a photocathode electron gun (PC-SEM) can change the probe current on a pixel-by-pixel basis by applying a pulsed electron beam. In this study, we propose two methods of SEM observation for HAR trenches. The first method uses a lower probe current at the top of the trench and a higher probe current at the bottom. With this method, the top and bottom of the trench could be observed simultaneously without any whiteout or blackout. Another method is to adjust the probe current so that the SEM image is in a constant grayscale. In this case, information about the sample appears in the probe current. The image of the probe current captured the trench bottom more clearly than the conventional SEM image under equivalent conditions.

DOI： 10.1117/12.3010733

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