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Simultaneous ultraviolet and thermal treatments (SUTs) enhanced the stability of tungsten-doped indium zinc oxide (W-IZO) thin-film transistor (TFT) compared to the furnace-annealed W-IZO TFT. To understand the change in electrical device instability, we used the in-situ measurement under the gate bias stress to investigate the electronic structure of active layer in actual TFT device. The physical properties of the active layer in the actual TFT structure, such as the unoccupied states in the conduction band and the band edge states below the conduction band, are related to the defect states of the oxide material and show the behavior of charge trapping. Improvement of the device stability mainly comes about from dynamic changes in the electron trap site in the channel region. The in-situ analysis, the relative conduction band area, and the band edge states for SUT-treated W-IZO active layer are much less changed under the gate bias stress than the furnace annealed W-IZO active layer.

Although solid-state phase transformations through chemical reaction with the surrounding environment are important in the field of materials science, the atomic-level dynamics at reacting surfaces have been difficult to observe directly. Herein, we found highly ordered arrays of 1D intermediate crystals with a unique atomic configuration during the thermal sulfidation of 3D-structured MoO2 to 2D layer-structured MoS2. These arrays reveal a dimension-breaking reconstruction process (3D → 1D → 2D) as well as a unique electronic structure evolution. Theoretical calculations show that the 1D crystals have a cross-sectional structure of four transition-metal atoms arranged in a diamond shape; these are critical to the atomic layer-by-layer formation of 2D transition-metal dichalcogenides. Furthermore, electronic structure analyses reveal that the 1D intermediate crystals alter the MoO2/MoS2 contact structure from p- to n-type with increases in the number of formed MoS2 layers.

Owing to their remarkable spin-charge conversion (SCC) efficiency, topological insulators (TIs) are the most attractive candidates for spin-orbit torque generators. The simple method of enhancing SCC efficiency is to reduce the thickness of TI films to minimize the trivial bulk contribution. However, when the thickness reaches the ultrathin regime, the SCC efficiency decreases owing to intersurface hybridization. To overcome these contrary effects, we induced dehybridization of the ultrathin TI film by breaking the inversion symmetry between surfaces. For the TI film grown on an oxygen-deficient transition-metal oxide, the unbonded transition-metal d-orbitals affected only the bottom surface, resulting in asymmetric surface band structures. Spintronic terahertz emission spectroscopy, an emerging tool for investigating the SCC characteristics, revealed that the resulting SCC efficiency in symmetry-broken ultrathin Bi2Se3 was enhanced by up to ∼2.4 times.

2D multiferroics with combined ferroic orders have gained attention owing to their novel functionality and underlying science. Intrinsic ferroelastic-ferroelectric multiferroicity in single-crystalline van der Waals rhenium dichalcogenides, whose symmetries are broken by the Peierls distortion and layer-stacking order, is demonstrated. Ferroelastic switching of the domain orientation and accompanying anisotropic properties is achieved with 1% uniaxial strain using the polymer encapsulation method. Based on the electron localization function and bond dissociation energy of the Re-Re bonds, the change in bond configuration during the evolution of the domain wall and the preferred switching between the two specific orientation states are explained. Furthermore, the ferroelastic switching of ferroelectric polarization is confirmed using the photovoltaic effect. The study provides insights into the reversible bond-switching process and potential applications based on 2D multiferroicity.

In this study, local structural changes induced by electrical SET/RESET operations in single crystalline Ge2Sb2Te5 (GST) nanowires fabricated via the vapor-liquid-solid method have been investigate...

Ge-Sb-Te pseudo-binary compounds are known to be phase-change materials (PCM). Most of these chalcogenide compounds are candidate for future phase-change random access memory (PCRAM) applications since they show abrupt change on crystalline-amorphous phase-change process. For the use in next-generation applications, increase of retention properties and decrease of power needed for phase-change process are required. These phase-change properties depend on various material characteristics, and thermal conductivity is one of them. In this study, to introduce an easier method for evaluating the local structural anharmonicity of phase-change materials, optical pump terahertz (THz) probe experiments were performed. By investigating the phonon behaviours in PCM by this method and comparing them with local structural information extracted from extended X-ray absorption fine structure (EXAFS) on Ge1Sb2Te4 and Ge2Sb2Te5 films, the effects of resonant bonding on lattice anharmonicity and thermal conductivity were determined. As resonant bonding in the local structure get enhanced, local distortion of the system decrease which cause the decrease in anharmonicity. The quantitatively-measured anharmonicity obtained from the optical pump THz probe experiments can be closely related to the structural and electrical properties, thus reflecting well the difference of phase-change properties between Ge1Sb2Te4 and Ge2Sb2Te5 films.

Topological insulators (TIs) have become popular in the field of optoelectronic devices because of their broadband and high-sensitivity properties, which are attributed to the narrow band gap of the bulk state and high mobility of the Dirac surface state. Although perfectly grown TIs are known to exhibit strong stability against oxidation, in most cases, the existence of vacancy defects in TIs reacts to air and the characteristics of TIs is affected by oxidation. Therefore, changes in the band structure and electrical characteristics by oxidation should be considered. A significant change occurs because of the oxidation; however, the dependence of the photoresponse of TIs on oxidation has not been studied in detail. In this study, the photoresponsivity of oxidized Bi2Se3 films is enhanced, rather than degraded, after oxidation in air for 24 h, resulting in a maximum responsivity of 140 mA W-1. This responsivity is substantially higher than previously reported values for Bi2Se3. Furthermore, a change in the photoresponse time of Bi2Se3 due to air exposure is systematically observed. Based on variations in the Fermi level and work function, using photoelectron spectroscopy, it is confirmed that the responsivity is improved from the junction effect of the Bi-based surface oxidized layer.

The direct control of topological surface states in topological insulators is an important prerequisite for the application of these materials. Conventional attempts to utilize magnetic doping, mechanical tuning, structural engineering, external bias, and external magnetic fields suffer from a lack of reversible switching and have limited tunability. We demonstrate the direct control of topological phases in a bismuth selenide (Bi2Se3) topological insulator in 3 nm molecular beam epitaxy-grown films through the hybridization of the topological surface states with the hafnium (Hf) d-orbitals in the topmost layer of an underlying oxygen-deficient hafnium oxide (HfO2) substrate. The higher angular momentum of the d-orbitals of Hf is hybridized strongly by topological insulators, thereby enhancing the spin-orbit coupling and perturbing the topological surface states asymmetry in Bi2Se3. As the oxygen defect is cured or generated reversibly by external electric fields, our research facilitates the complete electrical control of the topological phases of topological insulators by controlling the defect density in the adjacent transition metal oxide. In addition, this mechanism can be applied in other related topological materials such as Weyl and Dirac semimetals in future endeavors to facilitate practical applications in unit-element devices for quantum computing and quantum communication.

The transition metal dichalcogenides (TMDs) have great optical absorption and mobility in a subnanometer thickness. The MoSe2 among TMDs is significantly interested as an active layer in optical devices because it is superior to other TMDs in light absorption. Herein, we investigated the mechanism and characteristics of phototransistors using bi-layer MoSe2 film grown by molecular beam epitaxy. The interaction between the Al2O3 and MoSe2 was explored by X-ray and ultraviolet photoelectron spectroscopy, which showed that the chemical bonding and energy-band bending in the MoSe2 due to the electron doping. The optical devices exhibited well-controlled performance of the phototransistors with gate bias and illumination power-density, resulting in superior photoresponsivity (R) of 242 A/W. The mechanism of the R was fully interpreted by the change in quasi Fermi levels, defect states, mobility, and metal contact. Especially, capturing the electrons (holes) generated in the defect states increases the probability that the untrapped holes (electrons) reach the source and drain electrode, resulting in the high R. In addition, the decay mechanism was described by lowering the temperature, which persistent photocurrent was not observed at the low temperature. Finally, the performance of logic gates composed of the MoSe2 demonstrates its applicability to future photo-sensing systems.

Owing to their remarkable spin-charge conversion (SCC) efficiency, topological insulators (TIs) are the most attractive candidates for spin-orbit torque generators. The simple method of enhancing SCC efficiency is to reduce the thickness of TI films to minimize the trivial bulk contribution. However, when the thickness reaches the ultrathin regime, the SCC efficiency decreases owing to intersurface hybridization. To overcome these contrary effects, we induced dehybridization of the ultrathin TI film by breaking the inversion symmetry between surfaces. For the TI film grown on an oxygen-deficient transition-metal oxide, the unbonded transition-metal d-orbitals affected only the bottom surface, resulting in asymmetric surface band structures. Spintronic terahertz emission spectroscopy, an emerging tool for investigating the SCC characteristics, revealed that the resulting SCC efficiency in symmetry-broken ultrathin Bi

Topological insulator (TI), a band insulator with topologically protected edge states, is one of the most interesting materials in the field of condensed matter. Bismuth selenide (Bi2Se3) is the mo...

Defects in HfO2 dielectric film caused by indium and arsenide diffusion from InAs were investigated. To investigate the dissociation of InAs during post-deposition annealing (PDA) at 600 °C, we analyzed the ratio of the elements on the surface of the oxide layer and the chemical states by using time-of-flight secondary-ion mass spectroscopy and X-ray photoelectron spectroscopy, respectively. In–As bonding was dissociated and In and As atoms were diffused through the HfO2 layer from InAs. Fortunately, the diffusion and trap density could be controlled by using a 1-nm-thick Al2O3 passivation layer. In addition, we used the nitridation process to control the trap density. We evaluated the thermal and electrical stability of three samples—HfO2/InAs, HfO2/Al2O3/InAs, and nitrided HfO2/Al2O3/InAs—by analyzing the change in trap density before and after PDA at 600 °C and the stress-induced leakage current. In conclusion, the passivation layer effectively improved the thermal and electrical stability, whereas the nitridation process using NH3 gas did not. Moreover, although nitridation could reduce the interfacial defect states, due to structure distortion, it induced the degradation of the device.

The thermal conduction by the difference in the substrate of InAs nanowires (NWs) was investigated by determining the local temperature using Raman spectroscopy as an optical method. The optoelectric characteristics of the supported and suspended InAs nanowires were also investigated. The change in the current with the laser power produced a characteristic profile depending on the InAs nanowire configuration. The persistent photoconductivity (PPC) effect in the InAs nanowires has not been previously studied in detail despite its importance for the material photoresponse. This report presents a study of the PPC effect and the underlying mechanism. The correlation between the PPC effect and the thermal conduction by the substrate-induced carrier redistribution in the InAs nanowires is highlighted. In addition, a unique PPC effect associated with a specific device structure is studied. Our results can enable novel functionalities for InAs-based optoelectric applications in the future.

We report the growth mechanism and optical characteristics of type-II band-aligned GaSb quantum dots (QDs) grown on GaAs using a droplet epitaxy-driven nanowire formation mechanism with molecular beam epitaxy. Using transmission electron microscopy and scanning electron microscopy images, we confirmed that the QDs, which comprised zinc-blende crystal structures with hexagonal shapes, were successfully grown through the formation of a nanowire from a Ga droplet, with reduced strain between GaAs and GaSb. Photoluminescence (PL) peaks of GaSb capped by a GaAs layer were observed at 1.11 eV, 1.26 eV, and 1.47 eV, assigned to the QDs, a wetting-like layer (WLL), and bulk GaAs, respectively, at the measurement temperature of 14 K and excitation laser power of 30 mW. The integrated PL intensity of the QDs was significantly stronger than that of the WLL, which indicated well-grown GaSb QDs on GaAs and the generation of an interlayer exciton, as shown in the power- and temperature-dependent PL spectra, respectively. In addition, time-resolved PL data showed that the GaSb QD and GaAs layers formed a self-aligned type-II band alignment; the temperature-dependent PL data exhibited a high equivalent internal quantum efficiency of 15 ± 0.2%.

Topological insulators (TIs) have become popular in the field of optoelectronic devices because of their broadband and high-sensitivity properties, which are attributed to the narrow band gap of the bulk state and high mobility of the Dirac surface state. Although perfectly grown TIs are known to exhibit strong stability against oxidation, in most cases, the existence of vacancy defects in TIs reacts to air and the characteristics of TIs is affected by oxidation. Therefore, changes in the band structure and electrical characteristics by oxidation should be considered. A significant change occurs because of the oxidation; however, the dependence of the photoresponse of TIs on oxidation has not been studied in detail. In this study, the photoresponsivity of oxidized Bi

The direct control of topological surface states in topological insulators is an important prerequisite for the application of these materials. Conventional attempts to utilize magnetic doping, mechanical tuning, structural engineering, external bias, and external magnetic fields suffer from a lack of reversible switching and have limited tunability. We demonstrate the direct control of topological phases in a bismuth selenide (Bi

A comprehensive understanding of the nano-structural effects that cause reduction in thermal conductivity represents important challenges for the development of thermoelectric materials with an improved figure of merit ZT. Bismuth telluride (Bi2Te3)-based thermoelectric materials exhibit very low levels of thermal conductivity. In this study, a Te crystal-embedded Bi2Te3 (Te–Bi2Te3) thin film was formed by establishing a specific annealing temperature for a Te-rich Bi/Te multilayered structure. Modulations in structure and composition were observed at the boundaries between the two phases of Te and Bi2Te3. Furthermore, the samples contained regularly shaped nanometer-scale Bi2Te3 single grains. Therefore, we obtained a dramatic ZT value of 2.27 (+ 0.04, − 0.08) at 375 K from the Te–Bi2Te3 thin film. Finally, we confirmed that interface phonon scattering between the Te–Bi2Te3 boundaries plays an important role in inter-grain phonon transport, which results in a reduction in the lattice thermal conductivity.

The thermal stabilities and interfacial properties of HfO2 films created on conditioned i-InP surfaces using atomic layer deposition were investigated. When HfO2 was deposited on sulfur passivation...

We evaluated the change in the chemical structure between dielectrics (AlOx and HfOx) grown by atomic layer deposition (ALD) and oxidized black phosphorus (BP), as a function of air exposure time. Chemical and structural analyses of the oxidized phosphorus species (PxOy) were performed using atomic force microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, first-principles density functional theory calculations, and the electrical characteristics of field-effect transistors (FETs). Based on the combined experiments and theoretical investigations, we clearly show that oxidized phosphorus species (PxOy, until exposed for 24 h) are significantly decreased (self-reduction) during the ALD of AlOx. In particular, the field effect characteristics of a FET device based on Al2O3/AlOx/oxidized BP improved significantly with enhanced electrical properties, a mobility of ∼253 cm2 V-1 s-1 and an on-off ratio of ∼105, compared to those of HfO2/HfOx/oxidized BP with a mobility of ∼97 cm2 V-1 s-1 and an on-off ratio of ∼103-104. These distinct differences result from a significantly decreased interface trap density (Dit ∼ 1011 cm-2 eV-1) and subthreshold gate swing (SS ∼ 270 mV dec-1) in the BP device caused by the formation of stable energy states at the AlOx/oxidized BP interface, even with BP oxidized by air exposure.

Platinum diselenide (PtSe2) is an emerging transition-metal dichalcogenide showing both strong interlayer coupling and remarkable electrical properties. Because field-effect transistors (FETs) recently fabricated using PtSe2 films grown by various methods have not shown the high mobilities and on–off ratios compared with exfoliated PtSe2 and predicted by theoretical calculations, the PtSe2 growth mechanism must be better understood to prevent the degradation of electrical properties and improve the electrical performances of PtSe2-based devices. Therefore, molecular beam epitaxy is used in the current study to grow PtSe2 films, and a correlation is demonstrated between the initial PtSe2 growth kinetics and the electrical performances of PtSe2-based devices. X-ray photoelectron spectroscopy and annular dark field–scanning transmission electron microscopy show that metallic Platinum (Pt) is present in the thin PtSe2 films, as explained by the density functional theory calculations comparing Pt–Pt bond with Pt–Se bond. Electrical measurements of the PtSe2-based FETs show that the electrical performances are closely correlated with the initial growth kinetics of the PtSe2 films, indicating that removing Pt (which degrades device performance) is the most important aspect of growing thin PtSe2 films to optimize device performance.

Germanium telluride (GeTe), which has long been used in phase-change devices, has very unique structural characteristics. While atomic movement and related bond-order switching have been believed to determine the ferroelectric characteristics of this compound, structural changes accompanying ferroelectric switching have not been examined. In this study, using transmission electron microscopy and Raman spectroscopy, we directly observed the atomic movement and the related change in the local structure of GeTe during ferroelectric switching under an applied E-field. The crystal symmetry of GeTe could be alternated between two phases (R3m (rhombohedral) and Cm (monoclinic)) by changing the compound's polarization direction, indicating that the ferroelectric switching behavior of GeTe can be controlled. This observation suggests that ferroelectric switching of GeTe may originate from the phase transition between R3m and Cm through the formation of a morphotropic phase boundary (MPB). Since these two phases can generate MPBs with similar energies, the rotation of lone-pair direction during the phase transition can effectively change the polarization direction. The exotic phenomenon of ferroelectric switching, which involves changes in the chemical bonds, can be predicted on the basis of the role of several dopants through DFT calculations.

The device and defect characteristics of amorphous indium–gallium–zinc oxide (In:Ga:Zn = 1:1:1 at.%) thin-film transistors (TFTs) as a function of the oxygen partial pressure were investigated. It was found that as the oxygen partial pressure increased, the field effect mobility decreased, the threshold voltage saw a positive shift, and this shift of threshold voltage increased under a negative gate bias stress. From our qualitative analysis of defect states below the conduction band, it was found that as the oxygen partial pressure increased, defect states in the shallow levels decreased, while defect states in the deep levels increased. A quantitative analysis of the defect states in the TFT structures was conducted using photoinduced current transient spectroscopy. It was found that as the oxygen partial pressure used during fabrication of the TFTs increased from 0% to 10% to 60%, the defect states in the shallow levels decreased from 2.74 × 1018 to 2.93 × 1017 to 3.55 × 1016 cm−3, while the defect states in the deep levels increased from non-availability to 1.86 × 1016 to 3.25 × 1016 cm−3. As the oxygen partial pressure increased, the decrease in shallow level defect density is strongly related to a decrease in carrier concentration; the increase in deep level defect density affects the mobility and causes device instability.

Topological insulators (TIs) have been investigated as attractive candidates for the channels and contact metals of high-performance photodetectors over a broad spectral range. To achieve excellent photoresponse performances, several suggestions regarding material selection, various heterostructures, and proper device structure have been proposed. Here, we investigated the photoresponse of Bi2Se3/graphene heterostructures in lateral photodetectors. Graphene, with similar lattice symmetry, can improve the crystallinity of grown Bi2Se3 films, as well as induce large internal band bending with Bi2Se3, thereby causing electron–hole (e–h) separation. By comparing the photoresponses in vertical Bi2Se3 stack structures grown on various substrates, we conclude that the significantly improved photoresponse in the Bi2Se3/graphene stack structure results from graphene-induced internal band bending. Moreover, by investigating the thickness dependence of the Bi2Se3/graphene heterostructures on photoresponse, the maximum photoresponse is determined to be the result of an enhanced e–h separation caused by internal band bending and limiting of the optical penetration depth. As a well-matched stack structure based on band bending and the crystal growth system, TI/graphene heterostructures with stable and excellent photoresponse abilities can be considered as attractive candidates to attain outstanding performances for optoelectronic applications.

In this study, a native oxide layer was transformed from TeO2 into Bi2O3 on Bi2Te3 by changing the annealing temperature, and the change in the effects of the interface between the oxide layers and Bi2Te3 nanowire (NW) was studied. Caused by the change in the surface oxide layer, the change in the interface between the Bi2Te3 NW and oxide layer depending on the annealing temperature alters the band-alignment and charge states of defects, which significantly modulates the carrier dynamics (optical excitation and recombination processes) at the interface. Because of the trap/de-trap processes at a defect of the Bi2Te3/TeO2 interface, the photoresponse of the Bi2Te3/TeO2 system shows the defect-induced photogating effect, whereas the photoresponse of the Bi2Te3/Bi2O3 system shows a capacitive response without trap/de-trap processes. The systematic analysis of the effects of the surface oxidation and related defects on the photoresponse of TIs gives insight into TI-related photodetector devices and their properties under various conditions. Finally, a simple method for controlling the oxidation of the surface and accompanied interface properties such as the valance band offset (VBO), conduction band offset (CBO), and trap/de-trap processes, which can directly affect the operation of TI-based photodetector devices, is proposed.

Si1–xGex nanowires (NWs) (0.22 ≤ x ≤ 0.78) were synthesized using a vapor–liquid–solid procedure with a Au catalyst. We measured the intrinsic physical, chemical, and electrical properties of the oxidized Si1–xGex NWs using several techniques, including transmission electron microscopy, X-ray photoemission spectroscopy, and optical pump-THz probe spectroscopy. We suggest two distinct oxidation mechanisms depending on the Ge content in the Si1–xGex NWs: (i) when the Ge content is around 0.22, a Au catalytic effect brings about oxidation in both the axial and lateral directions; and (ii) when the Ge content is greater than 0.22, the Au tip is detached from the NW body and does not act as a catalyst, which is a result of the high degree of Ge-atom participation in the oxidation process. Additionally, we measured the photoconductivity decay time distribution for the Si1–xGex NWs before and after oxidation process; the decay time is significantly shortened in oxidized Si1–xGex NWs (0.22 < x), whereas it is mai...

Transition metal dichalcogenides (TMDCs) are promising next-generation materials for optoelectronic devices because, at subnanometer thicknesses, they have a transparency, flexibility, and band gap in the near-infrared to visible light range. In this study, we examined continuous, large-area MoSe2 film, grown by molecular beam epitaxy on an amorphous SiO2/Si substrate, which facilitated direct device fabrication without exfoliation. Spectroscopic measurements were implemented to verify the formation of a homogeneous MoSe2 film by performing mapping on the micrometer scale and measurements at multiple positions. The crystalline structure of the film showed hexagonal (2H) rotationally stacked layers. The local strain at the grain boundaries was mapped using a geometric phase analysis, which showed a higher strain for a 30° twist angle compared to a 13° angle. Furthermore, the photon–matter interaction for the rotational stacking structures was investigated as a function of the number of layers using spectro...

Ag-Incorporated Ge2Sb2Te5 (AGST) crystallizes faster and at a lower temperature than Ge2Sb2Te5 (GST). Many researchers have reported that these improvements could be attributed to a minute structural change because of the incorporated Ag. In a detailed study, we investigated this small structural change and its relationship with the phase change characteristics. Using various structural measurement methods and chemical bonding measurement tools, we confirmed the fine structural change, electron density distribution, and bond strength in GST thin films incorporated with 0%, 1%, and 10% Ag. From the experimental results, we identified that as the amount of incorporated Ag increases, the Ge–Te short bond length decreases and a homo-polar Ge–Ge bond is induced. When Ag is incorporated into the Ge site, the nearest neighbor bond strength between Ge and Te becomes much weaker. At the atomic level, the incorporated Ag induces one strong bond with the nearest Te, while the other bonds with Te are weakened. Therefore, the change in bond strength modulates the bond length between Ge and Te. Specifically, the Ge–Te short bond becomes shorter, and the long bond becomes relatively longer. This structural change indicates that the AGST has stronger Peierls distortion characteristics than GST. Finally, the modulated Peierls distortion changes the phase transition characteristics, such as the increase in the transition speed and the decrease in the phase change temperature.

The phase-change characteristics of Sn2Sb2Se5 (SSS), a pseudo-binary chalcogenide material, were investigated for use in phase-change random access memory applications. Although an analysis of the power and phase-change speed using laser static test equipment showed superior phase-change properties, several instabilities existed during the phase-change process. It was also found that the difference in resistivity between the crystalline and amorphous structures was high, as compared to conventional Ge2Sb2Te5 (GST). The SSS material also required a higher set/reset switching power than GST in electrical pulse switching tests. Based on extended X-ray absorption fine structure measurements and ab initio calculations of the charge distribution, short and long bonds were not found to co-exist around the Sn atoms, unlike the Ge atoms in GST. This evidence leads to enhanced resonant bonding in SSS, which prevents the Sn atoms from participating in the Ge-like phase-change mechanism. While the Ge atoms in crystalline GST tend to occupy defective octahedral sites, the Sn atoms in SSS prefer a tightly bonded resonant bonding state with a six-fold geometry. This strong resonant bonding results in a lack of Peierls-like distortion in the SSS structure. As a result, the competition between Peierls-like distortion and resonant bonding significantly affects the phase-change characteristics such as the SSS instability and switching process.

Slightly tapered Si1−xGex nanowires (NWs) (x = 0.29–0.84) were synthesized via a vapor–liquid–solid procedure using Au as a catalyst. We measured the optically excited carrier dynamics of Si1−xGex NWs as a function of Ge content using optical pump–THz probe spectroscopy. The measured −ΔT/T0 signals of Si1−xGex NWs were converted into conductivity in the THz region. We developed a fitting formula to apply to indirect semiconductors such as Si1−xGex, which explains the temporal population of photo-excited carriers in the band structure and the relationship between the trapping time and the defect states on an ultrafast time scale. From the fitting results, we extracted intra- and inter-valley transition times and trapping times of electrons and holes of Si1−xGex NWs as a function of Ge content. On the basis of theoretical reports, we suggest a physical model to interpret the trapping times related to the species of interface defect states located at the oxide/NW: substoichiometric oxide states of Si(Ge)0+,1+,2+, but not Si(Ge)3+, could function as defect states capturing photo-excited electrons or holes and could determine the different trapping times of electrons and holes depending on negatively or neutrally charged states.

Topological insulator (TI), a band insulator with topologically protected edge states, is one of the most interesting materials in the field of condensed matter. Bismuth selenide (Bi

We report on changes in the structural, interfacial, and electrical characteristics of sub-1 nm equivalent oxide thickness (EOT) HfO2 grown on InAs by atomic layer deposition. When the HfO2 film was deposited on an InAs substrate at a temperature of 300 °C, the HfO2 was in an amorphous phase with an sharp interface, an EOT of 0.9 nm, and low preexisting interfacial defect states. During post deposition annealing (PDA) at 600 °C, the HfO2 was transformed from an amorphous to a single crystalline orthorhombic phase, which minimizes the interfacial lattice mismatch below 0.8%. Accordingly, the HfO2 dielectric after the PDA had a dielectric constant of ∼24 because of the permittivity of the well-ordered orthorhombic HfO2 structure. Moreover, border traps were reduced by half than the as-grown sample due to a reduction in bulk defects in HfO2 dielectric during the PDA. However, in terms of other electrical properties, the characteristics of the PDA-treated sample were degraded compared to the as-grown sample, with EOT values of 1.0 nm and larger interfacial defect states (Dit) above 1 × 10(14) cm(-2) eV(-1). X-ray photoelectron spectroscopy data indicated that the diffusion of In atoms from the InAs substrate into the HfO2 dielectric during the PDA at 600 °C resulted in the development of substantial midgap states.

In this study, we evaluated the change of electronic structure during redox process in cerium-doped ZrO₂ grown by sol gel method. By sol-gel method, we could obtain cerium-doped ZrO₂ in high oxygen partial pressure and low temperature. After post annealing process in nitrogen ambient, the film is deoxidized. We used spectroscopic and theoretical methods to analysis change of electronic structure. X-ray absorption spectroscopy (XAS) for O K1-edge and Density Functional Theory (DFT) calculation using VASP code were performed to verify the electronic structure of the film. Also, high resolution x-ray photoelectron spectroscopy (HRXPS) for Ce 3d was carried out to confirm chemical bond of cerium doped ZrO₂. Through the investigation of the electronic structure, we verified as followings. (1) During reduction process, binding energy of oxygen is increase. Simultaneously, oxidation state of cerium was change to 4+ to 3+. (2) Cerium 4+ and cerium 3+ states were generated at different energy level. (3) Absorption states in O K edge were mainly originated by Ce 4+ f 0 and Ce 3+, while occupied states in valance band were mainly originated from Ce 4+ f₂.

Measuring defect density through non-contact, non-destructive methods without any additional sample processing has been of great interest in both academia and industry. In this study, we propose a new method to quantify the point and line defect densities of Si0.6Ge0.4 films by using the recombination time of the photoexcited carrier, as well as the optical pump THz probe method (OPTP). The change in the crystallinity of Si0.6Ge0.4 obtained from various measurements was consistent with the recombination time of the point- and line-defect states in OPTP, which changed from 107 ps to 172 ps and from 3961 ps to 870 ps, respectively. The actual defect density of each sample was extracted from photoinduced current transient spectroscopy (PICTS) for comparison with the recombination time. In addition, the non-Drude behavior of the photoexcited carrier was analyzed using two-dimensional terahertz time-domain spectroscopy (2D-TDS), which corresponds with previous measurement tools. The quantification methodology proposed in this study is expected to be advantageous to both academia and industry, as it will enable fast and accurate analysis of defects without requiring further sample processing.

Nonideal topological insulator (TI) films in which the bulk states are not insulating due to unintentional doping exhibit strong surface--bulk coupling. Such surface--bulk coupling can further induce intersurface coupling that affects the electrical conductivity of the TI films through a quantum interference effect known as weak antilocalization. Increased understanding and control of intersurface coupling is therefore crucial for the use of TI-based quantum devices. In this report on the transport properties of doped ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ films under perpendicular and parallel magnetic fields, we observe a crossover between coupled and decoupled surface channels that is mediated by intentional disorder controlled by a post-annealing process. The intentional disorder causes the surface state carriers to rapidly lose their quantum phase and coherence, and as a result, more disordered ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ films exhibit a shorter penetration depth of the surface state into the bulk states and weaker intersurface coupling, even though stronger surface--bulk coupling is expected. In previous studies, the role of disorder has generally been considered by determining its effect on surface--bulk scattering, but our results indicate that the role of disorder must be considered as a source of decoherence.

The thermal conduction characteristics of GeTe and Ge2Sb2Te5(GST) nanowires were investigated using an optical method to determine the local temperature by Raman spectroscopy. Since the localization of surface charge in a single-crystalline nanostructure can enhance charge-phonon scattering, the thermal conductivity value (κ) of single crystalline GeTe and GST nanowires was decreased significantly to 1.44 Wm(-1) K(-1) for GeTe and 1.13 Wm(-1) K(-1) for GST, compared to reported values for polycrystalline structures. The SET-to-RESET state in single-crystalline GeTe and GST nanowires are characteristic of a memory device. Unlike previous reports using GeTe and GST nanowires, the SET-to-RESET characteristics showed a bipolar switching shape and no unipolar switching. In addition, after multiple cycles of operation, a significant change in morphology and composition was observed without any structural phase transition, indicating that atoms migrate toward the cathode or anode, depending on their electronegativities. This change caused by a field effect indicates that the structural phase transition does not occur in the case of GeTe and GST nanowires with a significantly lowered thermal conductivity and stable crystalline structure. Finally, the formation of voids and hillocks as the result of the electromigration critically degrades device reliability.

For three-dimensional (3D) topological insulators that have a layered structure, strain was used to control critical physical properties. Here, we show that tensile strain decreases bulk carrier density while accentuating transport of topological surface state using temperature-dependent resistance and magneto-resistance measurements, terahertz-time domain spectroscopy and density functional theory calculations. The induced strain was confirmed by transmittance X-ray scattering measurements. The results show the possibility of reversible topological surface state device control using structural deformation.

BN-incorporated amorphous Ge2Sb2Te5 (GST) films were deposited by an ion beam sputtering deposition (IBSD) method using GST and BN targets. Based on in situ sheet resistance measurements, we confirmed that as the amount of BN increased, the crystallization temperature (Tc) increased from 150 °C to 260 °C. It was demonstrated that the phase change speed of BN-incorporated GST is ten times faster than that of GST using a nanosecond laser. By evaluating Johnson–Mehl–Avrami (JMA) plots and scanning electron microscopy (SEM) images, it was confirmed that the one-dimensional grain growth is dominant during the fast phase change of BN-incorporated GST because BN impurities can act like nuclei during the initial stage of crystal growth. After the 100 iteration test under rigorous acceleration conditions of SET/RESET switching using the pulsed laser system, it was confirmed that the void formation and thickness variation are very limited in the BN-incorporated GST, as compared to GST. This result originates from the low phase change stress of the BN-incorporated GST films during one-dimensional growth. The electrical SET speed and cyclability of the BN-incorporated GST device also improved significantly compared to GST.

Change in defect states in the nitrided 4H-SiC(0001) channel formed by a plasma nitridation (PN) process was investigated as a function of rapid processing time (180 s) at room temperature. The electronic structure of the interface between the nitride layer and the SiC substrate was investigated using X-ray photoelectron spectroscopy (XPS), medium-energy ion scattering (MEIS) and first-principles DFT calculations. The findings clearly showed that N adsorption occurred through the spontaneous incorporation at the SiC subsurface, resulting in the formation of an N-laminated structure at the interface. The results are consistent with an enhancement in the interface electrical characteristics because the energetically most stable gap state of SiC could only be observed in the N-laminate structure. In particular, the stress induced leakage current (SILC) characteristics showed that the generation of the defect state was significantly suppressed in a metal oxide semiconductor (MOS) structure with a nitride layer. These results provide an in-depth understanding of the process involved in the incorporation of N into the SiC subsurface and for the enhanced electrical characteristics of the interfacial nitride layer on SiC in the PN system.

To investigate the reproducibility and I–V non-linearity characteristics in resistive-switching random-access memory (RRAM), we studied the switching characteristics through Pt/TiO2 interface control using a non-stoichiometric TiO2−x/TiN interface formation in a resistive switching Pt/TiO2/TiN stack. Using the TiO2−x/TiN interface instead of the TiO2/TiN interface induced nearly forming-free switching, decreased the reset current, suppressed the gradual reset process, and resulted in faster switching by electric pulse. These results indicate that the Pt/TiO2 interface experienced reduced oxygen-vacancy-mediated switching. The discrepancy between the reduced oxygen-vacancy-mediated switching and the initially large number of oxygen vacancies can be resolved via the oxygen vacancy distribution dependent field effect. To clarify this process, we performed reaction-diffusion-drift model simulations. The drift velocity, which was calculated using the vacancy distribution, described the dynamic movement, and the simulation results supported the experimentally observed faster switching response. The field effect, which provided successive feedback between the drift velocity and vacancy distribution, can potentially be exploited to generate vacancy-designed devices.

Films with a newly observed monoclinic phase of Ge2Sb2Te5 (GST) were analyzed by high-resolution transmission electron microscopy (HRTEM) analysis and ab initio calculations. After an annealing treatment at 220 °C, the amorphous GST films were partly crystallized to an unknown monoclinic crystal structure. The transformation of the face-centered cubic (FCC) phase to monoclinic phases resulted from crystallization-induced stress caused by volume change during FCC formation. The crystallization-induced stress at the amorphous-FCC boundary was estimated to be 1.18 GPa. The volume per atom in the monoclinic phase was about 7.3% greater than that in the FCC phase. The stress value measured in situ was much smaller than the zx stress tensor (shear stress) calculated ab initio because the stress in the actual film was minimized by plastic deformation of the GST itself. Moreover, there is an activation stress barrier to deformation; this barrier corresponds to a deformation angle (γ) of approximately 78°. Slip of the (111) plane along the [110] direction also occurs in the FCC phase during annealing treatment. Based on the calculated total energy difference per atom in GST, the martensitic deformation as well as the slip system can occur at deformation angles as low as 70°.