bluuebunny/crossref_metadata_2025_split huggingface dataset:
Threshold Switching of ALD-NbOx Films for Neuromorphic Applications
Authors: Moon Jaehyun , Lee Ju-Hun , Kim Kitae , Kim Junho , Park Soohyung , Yi Yeonjin , Kang Seung-Youl 
Published: February 2024

Neuromorphic architecture has been suggested as an alternative to the existing von Neumann counterpart. The neuromorphic approach allows massively parallel processing and asynchronous timing schemes with low power consumption. This work presents ALD- NbOx thin films as a potential material for neuromorphic computation. NbO2 shows metal-insulator transition which is the desired property for threshold switching (TS). However, direct forming of NbO2 is rather difficult and deposited NbOx tends predominantly to result in Nb2O5. To obtain NbO2 we used an oxygen scavenger layer of Ti to alter Nb2O5 to NbO2. After a proper electroforming process, the device with a Ti insert showed metal-insulator transition above a specific voltage, i.e., a threshold voltage, and a stable threshold switching characteristics with wide operation range, characteristics not observed in the absence of a Ti insert. The well-defined TS characteristics clearly indicated the Ti role in controlling the oxidation state of NbOx.To verify the oxidation state of NbOx, XPS depth profile analyses were used. The presence of a Ti insertion layer resulted in an increase of 10~30% in Nb4+ states, depending on the total thickness of the NbOx films, while Nb5+ states decreased. Bearing in mind practical applications, we used only CMOS-compatible materials and processes to fabricate TS devices. Our approach suggests a reliable method to fabricate NbO2 neurons.
Acknowledgment: This work was supported by Electronics and Telecommunications Research Institute (ETRI) grant funded by the Korean government under grant " Non-CMOS Neuromorphic Device Basic Technology." (22BB1110).
Multi-Bit Self-Rectifying Synaptic Memristor Having Tri-Layer Structure for Quantization Aware Training of Quantized Neural Network
Authors: Woo Dae-Seong , Choi Hyun-Do , Jin Hong-Uk , Kim Jae-Kyeong , Shim Tae-Hun , Park Jea-Gun 
Published: February 2024

Recently, a cross-point synaptic memristor arrays have been employed to implement synaptic cores of various ANNs, i.e., deep neural networks (DNNs), spiking neural networks (SNNs), and convolutional neural networks (CNNs). The most of reported studies conducted training and inference of ANNs using analog synaptic weight modulation of memristors such as resistive random access memory (ReRAM), phase change random access memory (PCRAM), and ferroelectric random access memory (FeRAM). In other words, no case has been reported so far in which a synaptic memristor with quantized multi-bit is utilized onto a synaptic core of a quantized neural network (QNN).
In this study, for the first time, we introduced the multi-bit self-rectifying synaptic memristor having tri-layer structure being composed of oxygen-rich AlOx rectifying layer, oxygen-deficient HfOx top switching layer, and oxygen-rich HfOy bottom switching layer for quantization aware training of QNN. The resistive switching and self-rectifying mechanism of the multi-bit self-rectifying synaptic memristor was evidently proven by precisely investigating migration of oxygen ions and vacancies in resistive switching layers via ToF-SIMS and XPS depth profiles depending on the resistance states (i.e., pristine, set, and reset). The designed multi-bit self-rectifying synaptic memristor presented linear, discrete, and quantized 4-bit (i.e., 16-level) conductance level depending on incremental write pulse number, which was 4-bit self-rectifying synaptic memristor for the first time. In addtion, the quantization aware training (QAT) was conducted using 4-bit quantized conductance level of the desgiend multi-bit self-rectifying synaptic memristor via stright through estimator (STE). Finally, three different iris datasets were successfully classified using a quantized neural network designed via SPICE circuit simulation. The conductance mechanism of the self-rectifying synaptic memristor and its application of QNN will be presented in detail.

Acknowledgement

This research was supported by National R&D Program through the National Research Foundation of Korea(NRF) funded by Ministry of Science and ICT(2021M3F3A2A01037733)

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(Digital Presentation) Improving Field-Effect Transistor Performance through Pulsed Laser Irradiation-Mediated MoS2-Metal Contact Engineering
Authors: Wani Sumayah Shakil , Kuo Yao-Zen , Kimbulapitiya K.M.M.D.K , Chueh Yu-Lun 
Published: February 2024

Two-dimensional (2D) layered materials, such as MoS2, with their atomically thin characteristics, offer promising novel solutions for the field of nanoelectronics. MoS2 has been identified as a prospective building block for flexible and transparent nanoelectronics in the future, and has the potential to contribute to the continuation of Moore's law by enabling high integration levels with low power consumption. Although the use of 2D semiconductor materials as contact interfaces presents advantages over conventional Si-based metal-semiconductor contacts practical applications still face several obstacles. One of the biggest concerns being is the presence of the large contact resistance which can caps the available field effect carrier mobility making intrinsic quantum transport phenomena in the material inaccessible. In this work, we demonstrate the use of 1064 nm pulsed-laser to selectively anneal the metal contacts of the CVD synthesized MoS2 based back gated FET. The improved metal–semiconductor interface after laser annealing resulted in higher mobility and improved On-Off ratio (10^6~10^7). This selective contact annealing enhanced back gated MoS2 FET overall performance metrics by increasing peak field effect mobility, improving current saturation due to decreased contact resistance.
Our findings suggest that Pulsed laser annealing of metal contacts in TMD-based FETs can significantly reduce contact resistivity without causing channel or substrate distortion, which is not possible with conventional thermal annealing. This approach can provide most promising application in the field of high-performance two-dimensional semiconductor in flexible/ wearable electronics.

Keywords: TMDCs, CVD, MoS2 FET, laser annealing, contact resistance.
(Invited) Applications of Electrochemistry Toward Blue/Green and SWIR-Wavelength VCSELs
Authors: Li Bingjun , Mi Chenziyi , Elafandy Rami , Kang Jin-ho , Han Jung 
Published: February 2024

Unlike the more established edge-emitting lasers (EELs) with a horizontal Fabry-Perot (FB) cavity, VCSELs have a vertical cavity with highly reflective top and bottom mirrors sandwiching the active region. This design enables wafer-level testing, one- and two-dimensional arrayed integration, and scalability for low- and high-power applications. VCSELs are poised to surpass EELs as a more versatile and intelligent option for semiconductor lasers in the future.
Currently commercially available VCSELs only cover the red to near-infrared (NIR) range of 700-1100nm, whereas EELs offer products across a wide spectral range. The optimal active regions for blue and short-wavelength infrared (SWIR) spectra are only supported on GaN and InP substrates, respectively. Lattice-matched, epitaxial mirrors compatible with these two substrates remain major challenges. As a solution, we propose the use of electrochemistry to engineer the refractive index of the material. The innovation is based on the recognition that nanoporous (NP) semiconductor layers can be considered a nanocomposite with nanoscale voids of air uniformly dispersed in a single-crystalline semiconductor, thus rendering a unique way to control the effective optical index with a tunability that is typically un-attainable through epitaxy.
We have studied the electrochemistry of semiconductors for applications in photonics, beginning with GaN and most recently with InP. In both material systems, we have discovered a selective porosification process based on the doping concentration. Due to the doping selectivity in EC etching, layers with high doping concentration will be converted to low index nanoporous material with an unprecedented tunability in the refractive index depending on the porosity. Meanwhile, the lightly doped layers remain intact. As a result, highly reflective mirrors were fabricated on GaN and InP substrates with wide stopbands. In this talk, we will provide latest updates in the VCSEL performances in the blue/green and long-wavelength region.
(Invited) Towards GaN-on-GaN High-Power Electronic Devices
Authors: Bockowski Michal 
Published: February 2024

Application of gallium nitride (GaN) substrates in electronic and optoelectronic industries is constantly increasing. In order to fabricate wafers, GaN crystals of the highest structural quality and desired electrical (and sometimes optical) properties must be grown. Today, there are three main GaN crystallization methods: i/ halide vapor phase epitaxy (HVPE) with its derivatives: halide-free VPE and oxide VPE; ii/ sodium-flux; and iii/ ammonothermal. The last approach can be basic or acidic depending on what mineralizer is used to increase the solubility of GaN in the feedstock zone. In this paper we will focus on HVPE and basic ammonothermal growth of GaN [1]. Not only bulk growth will be presented. The HVPE method will also be discussed as the best method to crystallize the drift layers necessary for high-power vertical electronic devices (FET transistors, Schottky diodes).
One of the best methods for introducing dopants into semiconductors is ion implantation. The introduced structural damage can be removed by a proper annealing process. The high-temperature treatment enables also electrical and/or optical activation of the implanted dopants. In the case of GaN, annealing at high temperature (~1300°C - 1400°C) seems difficult. This compound loses its thermodynamic stability slightly above 800°C at atmospheric pressure. At higher temperature the crystal will decompose. One of the solutions is to anneal GaN at high nitrogen (N2) pressure. Such technology is called ultra-high-pressure annealing (UHPA) [2]. In this paper, application of UHPA for GaN crystals and layers implanted by different ions (acceptors and donors) will be presented. The latest results of the implantation with magnesium (Mg), beryllium (Be), zinc (Zn), and calcium (Ca) ions into GaN in order to obtain p-type conductivity will be discussed [3,4]. Silicon (Si) implantation into GaN for n-type doping will also be analyzed. Structural, electrical and optical properties of implanted GaN after UHPA will be discussed in terms of application for GaN-based devices.
A kV class, low ON-resistance, vertical GaN junction barrier Schottky (JBS) diode with selective-area p-regions formed via Mg implantation followed by high-temperature, ultra-high pressure post-implantation activation anneal without a capping layer was already demonstrated [5]. The forward characteristics of the JBS diode showed ideality factor (n) 1.03, turn-on voltage (VON) 0.75 V, current ON/OFF ratio (at ±3 V) ~1011, and specific differential ON-resistance (RON) 0.6 mΩ·cm2. The breakdown voltage of the JBS diode was 915 V.
Delamination Behavior Characterization during Bond Strength Measurement
Authors: Iwata Tomoya , Fuse Junya , Sano Marie , Tanaka Fabiana Lie , Inoue Fumihiro 
Published: February 2024

Direct bonding is a key enabler for future node 3D semiconductor devices [1]. It is crucial to have sufficient bonding strength to withstand thinning and subsequent processes. However, there is no standardized method for bonding strength measurement. The most frequently used bonding strength measurement technique is the double cantilever beam (DCB) method. However, it is known that delamination length propagates with time by water stress corrosion. Nevertheless, the reported bonding strength is not considered these phenomena. Therefore, interfacial reactions occurred by blade insertion have been deeply investigated in this study. We unveiled the influence of annealing and plasma conditions on the bonding strength measurement using the DCB method in a controlled environment. In addition, we discuss the behavior of delamination during blade insertion.
Our DCB system is compatible with 300 mm wafers and the insertion of the blade is fully controlled by actuator. The blade is inserted at the bonding interface as shown in Fig.a, and the delamination length is measured from a captured IR image.
Fig.b shows the results of bonding strength measurements on 300 mm bonded wafers with different annealing conditions. It can be seen that the bonding strength increases with the annealing temperature. When the annealing temperature is lower than 150℃, the delamination length is propagated by the time even when the measurement is performed in anhydrous environments. Therefore, the calculated results of bonding energy decrease due to this phenomenon.
Fig.c shows the bonding energy reduction rate for each annealing temperature with time. These values are calculated based on the value immediately after insertion and after 180 seconds. The results show that water stress corrosion significantly affects the delamination length when annealed at 150℃, however, the samples annealed above 250℃ show almost no propagation of delamination length with time and the values are almost saturated.
In this paper, we present the experimental results on the effects of annealing temperatures on bond strength as well as the propagation of delamination length caused by water stress corrosion. Additionally, we will discuss other factors that can affect bond strength measurements during the presentation.

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A Comprehensive Study on Chemical Bonding and Open Spaces with Silicon Carbon Nitride Films for Direct Bonding
Authors: Ebiko Sodai , Onishi Koki , Uedono Akira , Iacovo Serena , Inoue Fumihiro 
Published: February 2024

Wafer-to-wafer (W2W) direct bonding has been extensively developed as a key unit process for BS-PDN, CFET and advanced chiplet. As a bonding dielectric layer, CVD-SiO2 has been comprehensively studied for direct/hybrid bonding. SiCN is emerging dielectric bonding layer due to its unique properties such as high bonding strength at low annealing temperatures, suppressing void formation after Post Bond Annealing (PBA) and barrier properties against Cu diffusion [1][2]. However, the bonding mechanism for SiCN has not yet been clearly described. In this study, we investigated the mechanism from several kinds of perspectives by analyzing the properties of different films under various conditions. From those results, SiCN is characterized and compared to SiO2.
SiO2 and SiCN films were deposited on 300mm Si wafers using plasma-enhanced chemical vapor deposition (PECVD). Subsequently, the wafers were polished and bonded.
Atomic level open spaces in dielectric films were analyzed by positron annihilation spectroscopy (PAS). For the SiO2 samples after plasma activation, the S parameter did not show significant changes. This indicates the open spaces in SiO2 were not significantly influenced by plasma. On the contrary, the S parameter of SiCN samples increased after post deposition annealing (PDA) and decreased after plasma activation. This atomic level open spaces might be contributed the unique properties of SiCN for direct bonding.
Since the analysis by PAS is one of the examples, we will report the overall results of the analysis on SiCN, e.g. TDS, XPS, AFM, and so on, then discuss the bonding mechanism for SiCN-SiCN.
[1] F. Inoue, et. al., ECS Journal of Solid State Science and Technology 8(6), P346(2019)
[2] F. Nagano, et. al., ECS Journal of Solid State Science and Technology 9 123011(2020)

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Crystal Lattice Rearrangement Occurred at Au/Ag Bonded Interface in Atomic Diffusion Bonding in Vacuum
Authors: Goto Fuki , Iemura Hikaru , Uomoto Miyuki , Shimatsu Takehito 
Published: February 2024

Atomic diffusion bonding (ADB) processing of wafers using thin metal films is a promising route to achieving room-temperature wafer bonding [1,2]. When two film surfaces mutually contact at room temperature during ADB, crystal lattice rearrangement occurs, thereby enhancing the bonding performance. Generally, films of the same material used for bonding are fabricated on two flat wafer surfaces. Therefore, methods for examining the crystal lattice rearrangement mechanism are few. Two representative material films used for bonding, those of the metals Au and Ag, have fcc crystal structure and show a solid solution in the whole Au–Ag composition range. Moreover, lattice mismatch of these materials is very slight: less than 0.2%. For this study, Au film and Ag film were bonded in vacuum using ADB. The crystallographic structure of Au/Ag bonded films and depth profiles of Au and Ag atoms were examined.
Au and Ag films were fabricated respectively on two flat wafer surfaces using sputter deposition, followed by bonding of the two films on the wafers in vacuum. Figure 1 portrays cross-section STEM images of Au(20 nm)/Ag(20 nm) bonded films: (A) a bright field image, and (B) an HAADF image corresponding to (A). Films were deposited on Si wafers, with 5-nm-thick Ta films underneath. Figure 1(A) shows that no interface corresponding to the original Au/Ag interface is visible; no vacancy is observed. Grains were formed continuously across the original interface over the entire thickness of the bonded films, indicating that crystal lattice rearrangement occurred at room temperature when the films mutually contacted in vacuum. Although the crystallographic structures of Au and Ag layers are identical because of the crystal lattice rearrangement, the contrast between the upper and lower half areas in Figure 1(B) is expected to be attributable mainly to the different atomic weights of Au and Ag. Figure 2 presents the depth profiles of Au, Ag, and Ta evaluated using EELS analysis along line (a) in Figure 1(B). Results supported that interdiffusion at the Au/Ag bonded interface is not remarkable. This report presents discussion of the relation between crystal lattice rearrangement and interdiffusion of atoms based on results of analyses of Au/Ag bonded films with various layer structures.

T. Shimatsu and M. Uomoto, J. Vac. Sci. Technol., B 28, 706 (2010).

T. Shimatsu and M. Uomoto, ECS Trans., 33 (4), 61 (2010).

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(Invited) Exciton Transport in Strained 2D Semiconductors
Authors: Michaelis de Vasconcellos Steffen , Schmidt Robert , Rosati Roberto , Brem Samuel , Perea-Causín Raul , Niehues Iris , Kern Johannes , Preuß Johann , Schneider Robert , Malic Ermin , Bratschitsch Rudolf 
Published: February 2024

Being atomically thin, flexible, and exhibiting considerable light emission and ultrafast non-equilibrium dynamics, semiconducting transition metal dichalcogenides (TMDs) have been considered as promising candidates for next-generation optoelectronic devices. The optical and electronic properties of TMDs are governed by a rich landscape of tightly bound excitons, including regular bright excitons, as well as optically inaccessible dark exciton states. Recently, strain engineering of monolayer TMDs has been introduced to tune their optical properties, such as the exciton transition energy, exciton-phonon coupling, or the Stokes shift [1-3].
Transport of charge carriers is crucial for nanoelectronics. In conventional materials, electronic transport can be conveniently controlled by external electric fields. However, the tightly bound excitons, being neutral particles, are only weakly affected by electrical fields. We demonstrate that mechanical strain can also be used to manipulate the transport of excitons in TMDs. To this end, we apply homogeneous tensile strain to a WS2 monolayer by bending the substrate [1], which causes a redshift of the X0 exciton photoluminescence. By measuring the spatiotemporal photoluminescence after near-resonant excitation with femtosecond laser pulses, we map the spread of excitons and extract the strain-dependent diffusion coefficient [4].
Furthermore, we demonstrate the propagation of excitons in an inhomogeneous strain landscape. We create inhomogeneous tensile strain in TMD monolayers by transferring them onto patterned substrates with nanopillars or by a nanoimprint technique [5]. Due to the redshift of the exciton resonances with applied strain, excitons are expected to move towards high-strain regions in an inhomogeneous strain field - the so-called "funneling" effect. We verify this behavior for the "bright" TMD material monolayer MoSe2. In the case of the "dark" monolayer WS2, we observe exactly the opposite effect. Here, the excitons are expelled from the high-strain regions ("anti-funneling") [6]. By comparing our experimental results with a microscopic theory, we explain this observation by the drift of momentum-dark KΛ excitons, which, in contrast to bright excitons, shift to higher energies with strain.
Our joint experiment-theory study highlights the dominant role of momentum-dark excitons for the dynamics in monolayer TMDs and provides crucial design guidelines for TMD devices based on exciton transport. References [1] J. Roldan et al., Journal of Physics: Condensed Matter 27, 313201 (2015) [2] I. Niehues et al., Nano Letters 18, 1751 (2018) [3] I. Niehues et al., Nanoscale 12, 20786 (2020) [4] R. Rosati et al., 2D Materials 8, 015030 (2021) [5] J. Bensmann et al., arXiv:2212.11873 (2022) [6] R. Rosati et al., Nature Communications 12, 7221 (2021)