In the progress of nano-technology, manipulation of atoms and molecules in the scale of really nanometers or even atomic sizes has become an important issue. For such atomic manipulations, the use of electronic excitations may provide an efficient approach. That desorption of atoms and molecules from solid surfaces by electron beam irradiation (electron-stimulated desorption =ESD) and that by light illumination (photo-stimulated desorption =PSD) are well known phenomena. Such manipulation is interpretable in terms of adiabatic potential surfaces which generally differ in different electron states: The instability of atoms in the excited states, cause displacements towards new stable positions along the adiabatic potential surface in order to lower the total potential energy. In some cases, the atom configuration, even though it is structurally stable in the electron ground state, disrupts or substantially alters while the electron (electrons) is (are) in the excited state.

Usually in solids, however, the lifetime of electronic excitation states is generally short, because before inducing atomic displacements, the electronic excitation states are rapidly dissipated to other freedoms, such as photons (fluorescence), other electrons (Auger process), another electron excitation state (resonant transfer), etc, through many paths that are absent in isolated molecules. For this reason, electron-stimulated atomic displacements are usually uneasy in solids comparing with that in isolated molecules. In this respect, carbon is an exception: Carbon is a light element and therefore its movement can be accelerated more quickly than heavier elements. Also carbon has high potential to condense to various thermally stable structures. Hence, for carbon materials, it has been expected that the solid structure can be changed as a result of electronic excitations.

In fact, our group found that a restructuring of ta-C (tetrahedral (sp3 bonding atoms being more than 80%) amorphous carbon) films is induced by irradiation of 200keV electron beam in transmission electron microscope (TEM). There are newly formed short-range ordered structures by examining the TEM images and by analysis of diffraction patterns. This phenomenon was proved to be a structural transformation from sp3 to sp2 configurations by EELS (electron energy loss spectrum) measurement.

The salient feature of carbon solids, the quite structure-sensitive physical properties, arouses our interest in the applications of the electronic excitations to fabricate some solid structures, peculiarly with the properties that could not be achieved otherwise, such as high efficient negative emission effect. This motivated us to study the microscopic mechanism of such effects of electronic excitations, especially how electronic excitation states are responsible for the restructuring in carbon solids. In the case of electron irradiation, however, different excitations are induced depending on the electron beam energy, phonon excitation, valence electron excitations (plasma excitations), core electron excitations, displacement damage, secondary electron emissions, and so on. Therefore, controlled excitations are highly necessary for the microscopic mechanism to be elucidated.

This dissertation consists of five sections. The section 1 is for the introduction of the present study. The section 2 is devoted to provide the readers with basic knowledge about the concept often used in the literature and the theories related to the effects investigated in the present study. The sections 3 and 4 present the main originality of the present dissertation. Section 3 concerns a systematic study of the sp3 to sp2 structural conversion in ta-C films induced by TEM electron beam irradiation, and Section 4 describes a newly found similar effect induced by soft X-ray illumination that excites carbon core electrons. The summarization of present study in Section 5 added with some comments for problems that remain for future study.

For the electron-irradiation-induced effect, using JEM-2010F (JEOL) equipped with a filed emission gun and a parallel EELS spectrometer (Gatan), it's been systematical measurements of the temporal evolution of the π* peak in EELS core loss spectra, which representing graphite structure. The kinetics of the sp3 to sp2 structural conversion in ta-C films induced by TEM electron beam irradiation, was analyzed in the framework of the Johnson-Mehl-Avrami model, with the carefully consideration of the non-uniform profile of the electron beam used for intense irradiation. As shown in Fig. 1, the theoretical curves(lines) are of fitting with the experimental data(marks), calculated for various combinations of adjustable parameters, which denoting the growth dimension and the beam intensity dependence. It shows that the expanding velocity of the new phase front increases in proportion to the irradiation beam current density, proving an experimental support for the notion that the effect is possibly caused by electronic excitation effect. It is also to be concluded that the growth proceeds is in three dimensions from preexisting nuclei.

Generally, for such atomic rearrangement to occur in an extended scale, atomic diffusion must take place to some extent. Here arose a question whether or not the enhanced process is the atomic diffusion. Then, the measurement of the increase of π* peak in EELS spectra with elapsed time t of annealing at various temperatures for the sample films, has been conducted under a condition that the irradiation with the same electron beam current density. In this measurement, the samples had been first subject to heating at the maximum temperature so that the annealing should start isostructurally. By assuming the expanding velocity v of the phase front to be proportional to exp(-Q/kBT), and considering that the experimental data should be scaled in one unit of v×t, which should be unified to a master curve, there is an Arrhenius plot of v(T) vs 1/ T. Based on the measurement result that activation energy Q is as small as ~0.03eV, it indicates a dissociative diffusion mechanism that is triggered by electron irradiation: Carbon atoms in the stable positions are displaced by electronic excitations or electron knock-on impacts to meta-stable sites, from which the atoms diffuse thermally with the very low migration energy.

In order to avoid possible effects of electron knock-on impacts, which would not occur by soft X-ray illumination, the experiment of soft X-ray illumination in ta-C films has be performed, with selective incident photon energies. The effects of soft X-ray illumination in ta-C films was expected from previous experiments by Ma et al. on diamond crystals and those by Harada et al on HOPG (highly oriented pyrolytic graphite) which both have shown a large atomic displacement during the very short lifetime of core excitons generated by resonant excitations of carbon 1s core electrons. Several theoretical calculations also showed that the sp3 carbon-carbon bonding becomes unstable in the core exciton state.

Intense soft X-ray illumination experiments have been done in BL27SU, undulator beam line at Spring-8 equipped with a flat field soft X-ray spectrometer. We illuminated the ta-C film samples with nearly monochromatic soft X-rays (FWHM=7 eV) in various photon energies ranging near the C(1s) core edge. The illumination power, of the order of 10(18) photons/s・cm2 in the intensity, was adjusted to equalize the total electron yield of the sample, so that possible effects of secondary electrons were made even. It was measured in-situ spectra of X-ray absorption (XAS), to detect a structural change that occurred between before and after the illumination. The XAS spectra were acquired in total electron yield (TEY) mode and fluorescence (FL) mode for surface-sensitive and bulk-sensitive measurements, respectively.

As shown in Fig. 2, after soft X-ray illumination, graphitization took place with the increase of π* peak in XAS-TEY spectra. By varying the soft X-ray photon energy for illumination, it is able to measure the excitation spectrum of the total structural change effect vs photon energy. The result indicated that the increase of π* peak intensity has two components, a resonant component peaking around the characteristic core excitation absorption peak of ta-C films (288eV), and a non-resonant increase component independent of the photon energy used for the illumination. A careful examination of the change in the XAS-spectra for various illumination energies revealed that the structural change induced by the resonant excitation is characterized by the increase of 284.5eV XAS peak representing graphitic order, while the change induced by the non-resonant excitation is by the increase of spectral intensity around the 283.5eV and 285.5eV XAS peaks. In bulk-sensitive XAS-FL spectra, a more dramatic increase in 283.5eV XAS peak was evident and a blue shift of the characteristic core absorption peak of ta-C films was induced. Since 283.5eV peak is attributed to disordered structures or dangling bonds in graphite, these facts suggest disordering effects of non-resonant excitations.

Fig.1

Fig.2

More detailed measurements of the excitation spectrum of the energies around the characteristic core excitation peak of ta-C films, showed a resonant increase of XAS intensity around π* peak in XAS-TEY and XAS-FL when the sample was illuminated with 289.2eV soft X-ray (Fig. 3). The fact that the excitation spectrum has a peak near the 288 eV, which is characteristic core excitation absorption peak of ta-C films, clearly indicates that the resonant structural change is induced by the core excitation.

Fig.3

Although the origin of the 288 eV peak in XAS is not known at present, some literature claims that it originates in core excitons as we had expected. However, the resonant soft X-ray emission spectra excited by 289.2eV photons did not exhibit hot-luminescence, which would be direct evidence for the core exciton mechanism. Other literature claims a similar XAS peak induced by the interlayer states in graphite, possibly originates that the characteristic core excitation peak of ta-C films. If this interpretation is correct, one may have to devise a different mechanism not considered before. Two-hole-states induced by a resonant Auger process might instabilize the structures, however the resonant Auger emission spectra excited by photon energies around the characteristic core excitation absorption peak of ta-C films shows no resonant increase of Auger signal. One more possibility from the resonant Auger emission spectra is the characteristic core excitation peak of ta-C films is explanatively compounded with sp3-sp2, which is not well understood.

In summary, It has been studied the structural transformation from sp3 to sp2 configuration in ta-C films induced by electron beam irradiation and soft X-ray illumination. Kinetic analysis of phase growth clearly showed that the phase front velocity increases in proportion to the irradiation beam current density. There is a proposed model that once carbon atoms are displaced by electrons (by electron excitation or by knock-on impacts) to meta-stable positions, they diffuse very rapidly but thermally with low migration energy until they reach graphitic phase boundaries. The similar effects of soft X-ray illumination was found, too, which should not cause knock-on damage at the same time. The effect is a dramatic resonant increase around π* peak in XAS spectra by intense illumination of the sample with 289.2eV soft X-rays, close to a characteristic core excitation absorption peak of ta-C films. It is also found non-resonant increase of unknown structures independent of the soft X-ray photon energies for illumination.

　本論文は，四面体結合性アモルファスカーボン膜に高エネルギーの電子線や軟X線を照射するとsp3的結合からsp2的結合へと構造変化が起こる現象を明らかにし，その特徴を系統的に調べ，これら現象が電子励起効果によって起こるものであることを実験的に示したものである。

　本論文は英文で書かれ，6章よりなる。

　第1章は緒言であり，本論文の背景となっているナノスケールの分解能を持った新しい方式のナノリソグラフィ技術のアイデアについて紹介し，その開発にはナノスケールでパターニングされた電子エミッターパネルが必要であること，またその材料としてアモルファスカーボン膜が有望であることが述べられている。さらに，そのような工学的応用の基礎としての本研究の目的が述べられ，論文全体の構成が示されている。

　第2章では，本研究にあたって必要となる基礎的知識がまとめられている。まず，非金属物質においてしばしば観測される電子励起で誘起される原子移動現象を理解するのに頻繁に用いられる配位座標モデルが説明され，引き続き，電子励起原子移動の機構として本論文に関係する可能性のある3つの機構―オージェ崩壊を含む種々のイオン化励起による結合不安定化機構，ヤンテラー・擬ヤンテラー効果，フォノンキック機構―についてやや詳しい解説がなされている。さらに，カーボン物質に特有な現象として，ダイアモンドやグラファイトの結晶では内殻励起子の生成に伴いカーボン原子が格子位置から大きく変位する現象を示唆する実験と，その機構に関するヤンテラー・擬ヤンテラー効果にもとづく理論が紹介されている。

　第3章は，電子線照射によって誘起される構造変化現象に関するものである。試料はイオン堆積法によってSi基板上に堆積したsp3成分の非常に多いアモルファスカーボン(tetrahedral amorphous carbon = ta-C)膜で，基板を除去したta-C薄膜(50nm厚)に透過電子顕微鏡で用いられる80keV-200keVの電子線を照射すると，マルチウォールカーボンナノチューブ様のsp2結合的な秩序構造が発生する。この現象は，電子エネルギー損失分光(Electron Energy Loss Spectroscopy = EELS)スペクトルの炭素内殻吸収端の微細構造に最初ほとんど存在しなかったsp2的結合に由来するπ*ピークの増大として観測され，構造変化量を定量的に評価できる。これに注目し，一般に電子線照射で考えられる3つの効果―電子線照射による加熱効果，高エネルギー電子線の衝突によるはじき出し効果，電子励起効果―のいずれが原因であるかを明らかにするため，照射強度依存性を系統的に調べた。構造変化量の時間依存性は照射時間に比例しない特異なS字型カーブを描くが，これを核生成・成長に関するJohnson-Mehl-Avramiモデルを用いて解析した結果，sp2的秩序構造の'相境界'の進展速度は照射強度に比例すること，sp2的'相'は既存の核から3次元的に成長することを明らかにしている。さらに'相境界'進展速度は熱活性化型の温度依存性を示し，その活性化エネルギーは約0.03eVと非常に小さいことから，電子線励起によって解離拡散的機構が誘起されたと結論づけている。

　第4章は，軟X線照射によって誘起される構造変化現象に関するものである。試料は電子線照射効果の実験に用いたと同じta-C膜で，これに対し，SPring-8にあるBL27SUにおいて高強度の軟X線を照射し，その効果を照射前後のX線吸収スペクトル(XAS)を比較して調べている。表面敏感な全電子収量(TEY)検出―XASスペクトルでは，電子線照射効果と同様のsp2的構造の増加を示す変化が認められる。炭素1s内殻吸収端付近の比較的広いフォトンエネルギー範囲で，構造変化の励起スペクトルを測定したところ，照射光のエネルギーに依存しない非共鳴的成分と，ある特定のエネルギーで照射効果が増大する共鳴的成分が存在することが明らかになった。さらにこの共鳴成分の励起スペクトルを詳細に測定した結果，XAS-TEYスペクトルでもバルク敏感な蛍光X線(FL)検出―XASスペクトルでも，ta-C膜に特有の288eVに存在するXAS吸収ピークに近い289eVの照射光で，構造変化が共鳴的に増大することが分かった。本論文では，288eVのXASピークの起源として，炭素内殻励起子の生成と，グラファイト的構造に付随するInterlayer状態への励起，のふたつの可能性を論じたのち，共鳴励起による電子移動の機構として，結晶と本質的に同じ炭素内殻励起子様状態におけるヤンテラー・擬ヤンテラー効果による原子移動モデルにもとづく議論を展開している。

　第5章では，電子線照射効果と軟X線照射効果の統一的モデルとして，電子励起効果によって直接引き起こされる現象の本質は，いずれの場合もアモルファス中で安定位置にあった炭素原子が隣接する準安定位置に変位するという，一種のDisorderingであることが提案されている。実際にグラファイト的秩序構造ができるか否かが，表面に存在する既存核の有無，試料温度，照射強度によって異なるという本研究で明らかにされた諸事実は，準安定状態に移った炭素原子が小さなバリアを熱的に越えてグラファイト様の核まで拡散し安定化するための条件が整っているか否かで決まると考えると，よく説明できることが示されている。

　第6章は本論文のまとめである。本論文では，sp3からsp2への構造変化には長距離の熱活性化拡散が関与するとの結論から，最終的に落ち着く構造は熱力学的に安定なグラファイト的構造であり，出発状態としてta-C膜を用いればナノサイズで電子放出特性に大きな差がある電子エミッターパネルを実現できる見通しが得られたとしている。

　以上を要するに，本研究は，sp3的結合性の強いアモルファスカーボンに透過電子顕微鏡の電子線を照射すると起こるグラファイト様秩序構造の発生は，電子線による加熱効果でもはじき出し効果でもなく，電子励起効果によるものであることを示すために，原理上はじき出しの起こらない軟X線照射でも同様の現象が起こることを初めて系統的な実験により明らかにしたものである。これらの業績は，学術的な意義のみでなく，新しい超微細加工技術の開発に寄与するものと評価できる。よって本論文は博士(工学)の学位請求論文として合格と認められる。