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Reversible conversion over multi-million-times in bond types between metavalent and covalent bonds becomes one of the most promising bases for universal memory. As the conversions have been found in metastable states, extended category of crystal structures from stable states via redistribution of vacancies, researches on kinetic behavior of the vacancies are highly on demand. However, they remain lacking due to difficulties with experimental analysis. Herein, the direct observation of the evolution of chemical states of vacancies clarifies the behavior by combining analysis on charge density distribution, electrical conductivity, and crystal structures. Site-switching of vacancies gradually occurs with diverged energy barriers owing to a unique activation code-the accumulation of vacancies triggers spontaneous gliding along atomic planes to relieve electrostatic repulsion. Study on the behavior can be further applied to multi-phase superlattices composed of Sb2Te3 (2D) and GeTe (3D) sublayers, which represent the superior memory performances but their operating mechanisms were still under debates due to their complexity. The site-switching is favorable (suppressed) when Te-Te bonds are formed as physisorption (chemisorption) over the interface between Sb2Te3 (2D) and GeTe (3D) sublayers driven by configurational entropic gain (electrostatic enthalpic loss). Depending on the type of interfaces between sublayers, phases of the superlattices are classified into metastable and stable states, where the conversion could be only achieved in the metastable state. From this comprehensive understanding on the operating mechanism via kinetic behaviors of vacancies and the metastability, further studies towards vacancy engineering are expected in versatile materials., Comment: 32 pages, 4 figures

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.

The photovoltaic characteristics of high-quality Bi2Se3 grown on black phosphorus (BP) heterostructure via modulated elemental reactants (MER, annealed in situ at 220 °C for 20 min in a vacuum of 10−9 torr) growth of a Bi2Se3 topological insulator (TI) film on an exfoliated BP two-dimensional (2D) nanosheet was investigated. The Bi2Se3/BP heterostructure showed high-efficiency photocharacteristics (responsivity: 12.1 mA W−1, rise time: 1.3 ms, and broadband detection range: λ = 350–950 nm), which are attributed to the narrow band gap of the bulk state and mobility of the Dirac surface state. Moreover, the Bi2Se3/BP heterostructure can produce heterojunctions in the sharp interface region, thereby resulting in the contribution of a strong built-in electric field and photoexcited carrier tunnel through the interactive interfacial region at the TI/2D heterostructure based on the photovoltaic device. This work demonstrates practical photovoltaic applications, as well as the integration of such TIs on 2D materials, which can offer great opportunities for the realization of next-generation optoelectronic devices.

In this study, we investigated the effect of phase-change characteristics on the device performance of carbon-incorporated Ge2Sb2Te5 (CGST) to understand the origin of the enhanced reliability and stabilization of the device. Macroscopic and microscopic measurements confirmed that the structural stability significantly increased with the incorporation of as much as 10% carbon. After the completion of bond formation between C and Ge, the excess C (>5 atomic%) engages in bonding with Sb in localized regions because of the difference in formation energy. These bonds of C with Ge and Sb induce non-uniform local charge density of the short-range order. Finally, because the strong bonds between Ge and C shorten the short Ge–Te bonds, the high thermal stability of CGST relative to that of GST can be attributed to intensified Peierls distortion. The formation of strong bonds successfully underpins the local structures and reduces the stochastic effect. Moreover, extension of the C bonding to Sb enhances the structural reliability, resulting in highly stable CGST in the amorphous phase. Finally, the device stability of CGST in the reset state of the amorphous structure during the device switching process was significantly improved.

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.

Metavalent bonding is crucial for the determination of phase transition and improvement of device performance in phase-change materials, which are attracting interest for use in memory devices. Although monitoring dielectric and phononic parameters provides a direct measure of the metavalent bonding, the control of phase-change phenomena and metavalent bonding in the dynamical regime has yet to be demonstrated. This study reports the photoenhanced metavalent bonding and resulting hidden metallic crystalline state of Ti-doped Sb

Phase-change memory utilizing amorphous-to-crystalline phase-change processes for reset-to-set operation as a nonvolatile memory has been recently commercialized as a storage class memory. Unfortunately, designing new phase-change materials (PCMs) with low phase-change energy and sufficient thermal stability is difficult because phase-change energy and thermal stability decrease simultaneously as the amorphous phase destabilizes. This issue arising from the trade-off relationship between stability and energy consumption can be solved by reducing the entropic loss of phase-change energy as apparent in crystalline-to-crystalline phase-change process of a GeTe/Sb2Te3 superlattice structure. A paradigm shift in atomic crystallography has been recently produced using a quasi-crystal, which is a new type of atomic ordering symmetry without any linear translational symmetry. This paper introduces a novel class of PCMs based on a quasicrystalline-to-approximant crystalline phase-change process, whose phase-change energy and thermal stability are simultaneously enhanced compared to those of the GeTe/Sb2Te3 superlattice structure. This report includes a new concept that reduces entropic loss using a quasicrystalline state and takes the first step in the development of new PCMs with significantly low phase-change energy and considerably high thermal stability.

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...

Although Sb2Te3, as a candidate material for next-generation memory devices, has attractive properties such as higher operation speed and lower power consumption than Ge2Sb2Te5, its poor stability ...

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.

Time-dependent resistance drift in chalcogenide phase change materials is known to cause instabilities in phase-change random access memories (PRAMs). In this study, the resistance drift of the RESET state of GeTe and Ge2Sb2Te5 (GST) nanowires during memory switching was investigated using an optical method to determine the relationship between variation in resistance and time-dependent thermal properties. Changes in morphology and crystal structure were caused by the formation of voids and hillocks in nanowires in the RESET state, which affected the phonon scattering in GeTe and GST nanowires. Consequently, the thermal conduction of the GeTe and GST nanowires in the RESET state was observed to be significantly lower than that in the initial single-crystalline state. Further, it was observed that as the annealing duration of the retention test was increased, the resistance decreased and thermal conduction increased, even though the morphologies of the nanowires were restored to the initial state. These changes were induced by thermally assisted atomic migration, indicating the occurrence of a local structural phase transition in the GeTe and GST nanowires via the formation of voids and hillocks. Our results explain the cause behind the resistance drift observed in GeTe and GST nanowires in the RESET state with bi-polar memory switching characteristics.

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

Although some methods to improve phase-change memory efficiency have been proposed, an effective experimental approach to induce a phase-change like process without external heat energy has not yet been reported. Herein we have shown that GeTe is a prototype phase-change material, which can exhibit a non-thermal phase-change-like process under uniaxial stress. Due to its structural characteristics like directional structural instability and resonance bonding under 1% uniaxial stress, we observed that bond switching in the GeTe film between short and long bonds is possible. Due to this phase change, GeTe displays the same phase-change as crystal layer rotation. Crystal layer rotation has not been observed in the conventional phase change process using intermediate states, but it is related to the structural characteristics required for maintaining local coordination. Moreover, since the resonance bonding characteristics are effectively turned off upon applying uniaxial stress, the high-frequency dielectric constant can be significantly decreased. Our results also show that the most significant process in the non-thermal phase transition of phase-change materials is the modulation of the lattice relaxation process after the initial perturbation, rather than the method inducing the perturbation itself. Finally, these consequences suggest that a new type of phase-change memory is possible through changes in the optical properties under stress.

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.

Vertically aligned and dense InGaAs nanowires were grown on Si (111) substrates by Au-assisted molecular beam epitaxy, and their antireflection characteristics were studied. The bandgap of InGaAs nanowires was tuned to be about 1.0 eV by adjusting the In to Ga ratio. The grown nanowires were vertically aligned with a diameter of ~ 20 nm near the top and ~ 44 nm at the bottom, with a slightly tapered structure. This tapered nanostructure was formed due to the different surface diffusivities and affinities of In and Ga to the Au catalyst. The grown InGaAs nanowires have no significant stacking, kinking, and bending defects. High-resolution transmission electron microscopy study showed that the grown InGaAs nanowires have a pure wurtzite single crystalline structure with the maximum length of ~ 18 µm. Photo-reflectometry measurement showed a significant reduction in the reflectance less than ~ 5% at normal incidence in the wavelength range of 200–1700 nm. In addition, spectroscopic ellipsometry study showed a reduced reflectance at various incident angles of 30–70° in the wavelength range of 200–1100 nm. These optical investigations demonstrate the antireflection characteristics of the InGaAs nanowires. Furthermore, piezoelectric responses were collected from the top of the vertically standing InGaAs nanowires at five different points using piezoelectric force microscopy. The measured area for one point was about 50 nm × 50 nm, and the piezoelectric responses of one or two InGaAs nanowires per point were expected to be measured, as the growth direction was along with the polar c -axis [0001] direction.

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%.

In this study, we investigated the effect of phase-change characteristics on the device performance of carbon-incorporated Ge

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

Reversible crystallization from the amorphous phase is one of the most promising bases on which to store data for universal electronic memory. However, crystallization mechanism at atomic scale and role of element in Ag-In-Sb-Te are still unsolved, whose memory properties are outstanding. To elucidate them, we studied the bonding topologies through spectroscopic techniques and density functional simulations. It is shown by in-situ X-ray photoelectron spectroscopy that element In dramatically changes its bonding topology on crystallization from Sb-bonds to Te-bonds, which are characterized as that of InSb and AgInTe2, respectively by density functional simulations. Further, the degree of atomic ordering is identified to highly dependent on local environment of In by spectroscopic ellipsometry. The overall results suggest that crystallization of Ag-In-Sb-Te is triggered by the local environment transition of In from InSb-like to AgInTe2-like site. Thereby, Ag provides structural flexibility for local environment transition of In, In disrupts crystallization, and Te assists the other elements to play their respective roles. The present work accounts for unsolved memory properties of Ag-In-Sb-Te and opens the gate to develop enhanced materials for the memory.

Robust massless Dirac states with helical spin textures were realized at the boundaries of topological insulators such as van der Waals (vdW) layered Bi2Se3 family compounds. Topological properties of massless Dirac states can be controlled by varying the film thickness, external stimuli, or environmental factors. Here, we report single-crystal-quality growth of ultrathin Bi2Se3 films on flexible polyimide sheets and manipulation of the Dirac states by varying the vdW gap. X-ray diffraction unambiguously demonstrates that under uniaxial bending stress the vdW gap substantially changes with interatomic-layer distances unaltered. Terahertz and photoelectron spectroscopy indicate tuning of the number of quantum conducting channels and of work function, by the stress, respectively. Surprisingly, under compressive strain, transport measurements reveal dimensional crossover and suppressed weak antilocalization. First-principles calculations support the observation. Our findings suggest that variation of vdW gap...

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.

The changes of the defect states below the conduction band in atomic-layered HfO 2 film grown on SiC substrate were examined as a function of the post-nitridation annealing temperature in an NH 3 ambient. As the post-nitridation annealing temperature increased up to 600 °C, the incorporated nitrogen into the HfO 2 /SiC interface was gradually increased. The band gap and valence band offset were mostly increased as a function of the post-nitridation annealing temperature and the band alignment of HfO 2 films changed. O K-edge absorption features revealed two distinct band edge states below the conduction band edge in HfO 2 films, and these defect states were dramatically reduced with increasing of the post-nitridation annealing temperature. The reduction of defect states in HfO 2 /SiC improved the electrical properties such as the leakage current density, breakdown voltage, and trap charge density in the HfO 2 film and interface of HfO 2 /SiC.