Published papers

© 2019 The Japan Society of Applied Physics. We systematically investigated the core-level X-ray photoelectron spectroscopy (XPS) binding energy shifts of B 1s and Al 2p and formation energies for defects including boron and aluminum in 3C-silicon carbide (SiC) by first-principles calculation. We analyzed the relation between the XPS binding energy shift and defect states and found that the defects with localized electrons in the band gap or energy gap in the valence band have larger XPS relaxation energies (XPSREs) than those without localized electrons. In contrast, spread electrons and electrons localized away from the core hole hardly affect the XPSREs. Further, we analyzed the dependence on crystal matrices, that is, elemental and compound semiconductors, on XPS spectra by comparing the XPS spectra of Si and 3C-SiC. Although the variation of the local potential in 3C-SiC is larger than that in Si, the variations of their relaxation energies are comparable.

© 2019 The Japan Society of Applied Physics. We systematically investigated the formation energies and the core-level X-ray photoelectron spectroscopy binding energy (XPSBE) shifts of nitrogen (N) 1s and phosphorus (P) 2p for defects including N and P in 3C-SiC by a first-principles calculation using the generalized gradient approximation, whose reliability for n-type defects was confirmed by some tests using the HSE06 hybrid functional. XPSBEs were separated into the local potential average around the impurity and the relaxation energy of the wave function to analyze the relationship between the XPSBE shift and the defect structures. It is difficult to understand the relaxation energy intuitively. The electrons localized around the impurity atom, which have energy levels in energy gaps, make a large contribution to the relaxation energies. Considering the formation energies, we predicted some XPS peaks expected to be found.

We systematically investigated the arsenic (As) 3d core-level x-ray photoelectron spectroscopy (XPS) binding energy and formation energy for As defects in silicon by first-principles calculation with a high accuracy of 0.1 eV by careful evaluation of the supercell size. For As, we adopt a pseudopotential with 3d states as the valence and the spherical hole approximation to ensure the convergence of self-consistent calculation for the XPS binding energy with large size systems. Some of the examined model defects have threefold coordinated As atoms. The XPS binding energies of these As atoms are distributed in the narrow region from −0.66 eV to −0.73 eV in neutral charge states. Such defects in negative charge states have a lower XPS binding energy by about 0.1 eV. From the XPS binding energy and electrical activity, negatively charged defects of a vacancy and two adjacent substitutional As atoms (As2V) are the most probable candidates for the experimentally observed peak at −0.8 eV called BEM from the reference substitutional As peak. Under the experimental condition, we find that As2V#U−,2−$UR do not deeply trap electrons and are electrically inactive. We also demonstrate the surface effect that surface states near the bandgap decrease the XPS binding energy, which may generate defects with low binding energies similarly to the experimental peak at −1.2 eV called BEL.

We carried out a comprehensive study on the B 1s core-level X-ray photoelectron spectroscopy (XPS) binding energies and formation energies for boron defects in crystalline silicon by first-principles calculation with careful evaluation of the local potential boundary condition for the model system using the supercell corresponding to 1000 Si atoms. It is reconfirmed that the cubo-octahedral B12 cluster in silicon crystal is unstable and exists at the saddle point decaying to the icosahedral and S4 B12 clusters. The electrically active clusters without any postannealing of ion-implanted Si are identified as icosahedral B12 clusters. The experimentally proposed threefold coordinated B is also identified as a h001iB-Si defect. For an as-doped sample prepared by plasma doping, the calculated XPS spectra for complexes consisting of vacancies and substitutional B atoms are consistent with the experimental spectra. It is proposed that, assuming that the XPS peak at 187.1 eV is due to substitutional B (Bs), the experimental XPS peaks at 187.9 and 186.7 eV correspond to interstitial B at the H-site and h001iB-Si defects, respectively. In the annealed samples, the complex of Bs and interstitial Si near the T-site is proposed as a candidate for the experimental XPS peak at 188.3 eV.

密度汎関数法の基礎に関する約６時間の講義

A procedure is proposed to predict electronic structures in thin films from bulk properties using density functional k · p perturbation method. This procedure is based on the phenomenological expression of zero-point energy using effective mass approximation. It is found that the results by this procedure and those calculated using slab models agrees very well. A prediction is also demonstrated for the electronic structures of < 110 > uniaxially strained silicon thin films in the < 001 > confinement.

シリコンナノ構造における輸送特性をきめる物理量としての有効質量に着目し、板状チャネルにおける膜厚並びに二軸歪みをパラメタとして第一原理計算を用いて検討したところ、あるパラメタ領域で異常を呈することを予測した。この異常の物理的原因を閉じ込め、歪みの両側面から解明した。

本稿では最初に第一原理計算をナノ構造に適用する場合に生じる基本的な問題点（原子数と計算量のスケーリング）並びに現状における解決方法の方向性に関して簡単に紹介する．その後，ナノ構造第一原理計算の一例として，Ｓｉナノスケール薄膜構造に二軸歪を印加した系において，電子伝導の重要なパラメタである有効質量の縦方向成分が発散するという理論的予測について解説する．

USLIの微細化にともない、高性能なMOSFETチャネルとして、歪みを印加した半導体薄膜の研究が盛んに行われている。本論文では、nmスケールのSi薄膜においては、薄膜膜面方位、歪み、膜厚に依存して、素子特性に大きな影響を与える物理量である電子の有効質量が特異的に変化するパラメタ領域が存在することを第一原理計算によって初めて示した。特に＜１１１＞方向に閉じ込めた場合について詳細に報告している。