arXiv subjects

Takuma Kobayashi

Publications and source records attributed to Takuma Kobayashi.

8 recordsLinked to original sources

arXiv2019

Energetics and electronic structure of native point defects in $α$-Ga2O3

We report first-principles calculations that clarify the formation energies and charge transition levels of native point defects (Ga and O vacancies, interstitials, and a Ga vacancy-O vacancy pair) in corundum structured $α$-Ga2O3. Either under a Ga- or O-rich growth condition, the negatively-charged Ga vacancy and the positively-charged Ga interstitial on a site surrounded by six O atoms are dominant when the Fermi level approaches the conduction and valence band edges, respectively. These defects would compensate carrier electrons and holes, respectively. Ga-rich conditions relatively suppress the formation of the Ga vacancy and, therefore, are suited for extrinsic n-type doping of $α$-Ga2O3.

cond-mat.mtrl-sci↗

arXiv2019

Anomalous conduction band fluctuation in silicon carbide metal-oxide-semiconductor structures revealed by electrical characterization of field effect transistors combined with self-consistent numerical calculations

We determined the interface state density ($D_{\rm it}$) distributions in the vicinity of the conduction band edge in silicon carbide (SiC) metal-oxide-semiconductor (MOS) structures by reproducing the experimental current-voltage characteristics of MOS field effect transistors (MOSFETs) with numerical calculations. In the calculation, potential distributions and energy sub-bands in the inversion layer were calculated by solving Poisson and Schrödinger equations, respectively. We demonstrate that gate characteristics of the MOSFETs are well described by considering that the interface states are caused by fluctuation of free-electron density of states in a two-dimensional system. The $D_{\rm it}$ distributions are almost uniquely determined by the oxide formation process (as oxidation or interface nitridation) and independent of the acceptor concentration ($3\times10^{15}-1\times10^{18}\ {\rm cm^{-3}}$).

cond-mat.mtrl-sci↗

arXiv2019

Influence of vacuum annealing on interface properties of SiC (0001) MOS structures

We investigated the influence of vacuum annealing on interface properties of silicon carbide (SiC) metal-oxide-semiconductor (MOS) structures. For as-oxidized and nitric oxide (NO)-annealed samples, the interface state density ($D_{\rm it}$) near the conduction band edge ($E_{\rm C}$) of SiC did not increase by subsequent vacuum annealing. For phosphoryl chloride (POCl$_3$)-annealed samples, in contrast, $D_{\rm it}$ at $E_{\rm C}-0.2$ eV increased from $1.3\times10^{10}$ to $2.2\times10^{12}$ cm$^{-2}$eV$^{-1}$ by the vacuum annealing, and the channel mobility of MOS field effect transistors (MOSFETs) decreased from 109 to 44 cm$^2$V$^{-1}$s$^{-1}$. Mechanism of the observed increase in $D_{\rm it}$ was discussed based on the results of secondary ion mass spectrometry measurement.

cond-mat.mtrl-sci↗

arXiv2019

Structure and energetics of carbon-related defects in SiC (0001)/SiO$_{\rm 2}$ systems revealed by first-principles calculations: Defects in SiC, SiO$_{\rm 2}$, and just at their interface

We report first-principles calculations that reveal the atomic forms, stability, and energy levels of carbon-related defects in SiC (0001)/SiO$_{\rm 2}$ systems. We clarify the stable position (SiC side, SiO$_{\rm 2}$ side, or just at the SiC/SiO$_{\rm 2}$ interface) of defects depending on the oxidation environment. Under an O-rich condition, the di-carbon antisite ((C$_{\rm 2}$)$_{\rm Si}$) in the SiC side is stable and critical for $n$-channel MOSFETs, whereas the di-carbon defect (Si-C-C-Si) at the interface becomes critical under an O-poor condition. Our results suggest that the oxidation of SiC under a high-temperature O-poor condition is favorable in reducing the defects, in consistent with recent experimental reports.

cond-mat.mtrl-sci↗

arXiv2019

Native point defects and carbon clusters in 4H-SiC: A hybrid functional study

We report first-principles calculations that clarify the formation energies and charge transition levels of native point defects and carbon clusters in the 4H polytype of silicon carbide (4H-SiC) under a carbon-rich condition. We applied a hybrid functional that reproduces the experimental bandgap of SiC well and offers reliable defect properties. For point defects, we investigated single vacancies, antisites, and interstitials of Si and C on relevant sites. For carbon clusters, we systematically introduced two additional C atoms into the perfect 4H-SiC lattice with and without removing Si atoms and performed structural optimization to identify stable defect configurations. We found that neutral Si antisites are energetically favorable among Si-point defects in a wide range of the Fermi level position around the intrinsic regime, whereas negatively-charged Si vacancies and a positively-charged Si interstitial on a site surrounded by three Si and three C atoms become favorable under n- and p-type conditions, respectively. For C-point defects, neutral C antisites are favorable under intrinsic and n-type conditions, whereas positively-charged C vacancies become favorable under p-type conditions. We also found that a di-carbon antisite is more favorable than a C-split interstitial, which is the most stable form of single C interstitials.

cond-mat.mtrl-sci↗

arXiv2019

Reduction of interface state density in SiC (0001) MOS structures by low-oxygen-partial-pressure annealing

We report that annealing in low-oxygen-partial-pressure (low-p$_{\rm O2}$) ambient is effective in reducing the interface state density (D$_{\rm IT}$) at a SiC (0001)/SiO$_{\rm 2}$ interface near the conduction band edge (E$_{\rm C}$) of SiC. The D$_{\rm IT}$ value at E$_{\rm C}$$-$0.2 eV estimated by a high (1 MHz)-low method is 6.2$\times$10$^{12}$ eV$^{-1}$cm$^{-2}$ in as-oxidized sample, which is reduced to 2.4${\times}$10$^{12}$ eV$^{-1}$cm$^{-2}$ by subsequent annealing in O$_{\rm 2}$ (0.001%) at 1500${}^\circ$C, without interface nitridation. Although annealing in pure Ar induces leakage current in the oxide, low-p$_{\rm O2}$ annealing (p$_{\rm O2}$ = 0.001 - 0.1 %) does not degrade the oxide dielectric property (breakdown field ~ 10.4 MVcm$^{-1}$).

cond-mat.mtrl-sci↗

arXiv2017

Carbon Ejection from a SiO2/SiC(0001) Interface by Annealing in High-Purity Ar

We found that carbon-associated byproducts formed at the dry-oxidized SiO2/SiC(0001) interface could be decomposed and be taken out to the SiO2 side by high-purity Ar annealing. We evaluated the concentration of the ejected carbon atoms in the SiO2 by secondary ion mass spectrometry, and discovered that it clearly depends on the condition of oxide formation (dry-oxidation, nitridation treatment, and phosphorus treatment). This work provides an indirect but unambiguous evidence for the carbon-byproducts existing at the SiO2/SiC interfaces, and also indicates that the phosphorus treatment removes the carbon-byproducts, leading to significant reduction of interface defects.

cond-mat.mtrl-sci↗

arXiv2017

Microscopic Mechanism of Carbon Annihilation upon SiC Oxidation due to Phosphorous Treatment: Density-Functional Calculations Combined with Ion Mass Spectrometry

We report first-principles static and dynamic calculations that clarify the microscopic mechanism of carbon annihilation due to phosphorous treatment upon oxidation of silicon carbide (SiC). We identify the most stable form of the phosphorus (P) in the oxide as the four-fold coordinated with the dangling PO unit and find that the unit attracts carbon ejected from the interface, thus operating as a carbon absorber. This finding provides a microscopic reasoning for the first time for the promotion of the oxidation reaction on one hand and the annihilation of the C-related defects at the interface on the other. Secondary ion mass spectrometry measurements are also performed and the obtained carbon profile corroborates the theoretical finding above.

cond-mat.mtrl-sci↗