Reduction of metal film thickness down to the electron wavelength induces energy quantization by the quantum size effect, resulting in the formation of quantum well states. Recently, there has been growing interest in such quantum films on solid surfaces. In contrast to a freestanding metal film, these quantum well states (resonances) have been reported to show additionally intriguing physical properties such as spin polarization, anomalous in-plane dispersion, and oscillation of the superconducting transition temperature with thickness.

In the present work, transport properties of quantum well states confined in Ag films prepared on the Si(111)4×1-In are investigated by a newly developed system for conductivity measurements, comparing with the results of Angle resolved photoemission spectroscopy (ARPES) measurements and the tightbinding calculations. This thesis mainly consists of two parts.

In the first part, we have developed the variabletemperature independently-driven four-tip scanning tunneling microscopy (VT4tipSTM) [Fig.1], and I improved to perform stable measurements of my target system, Ag/Si(111)4×1-In, and other various kinds of nanodevices.

At first, the efficient cooling system and the units for vibration isolation are introduced to the main chamber. Secondly, an micro channel plate (MCP) for scanning electron microscopy (SEM) detector, new cables, and a test kit are installed for the stable measurements. Thirdly, the flow type cooling system and Reflection high energy electron diffraction (RHEED) spot intensity monitoring system are added to the preparation chamber. Finally, the voltage differential amplifier is designed and installed in the preamplifier for the measurements of a low-resistance sample.

The STM tips have great advantages as elec-tronic probes because they allow arbitrary config-urations and have high spatial resolution. More-over, as the four tips are positioned in arbitrary ar-rangements at aimed areas on the sample surface, the 4-tip STM can measure anisotropic conductiv-ity by a square arrangement without the effect of contact resistance. We can obtain transport prop-erties of low imensional systems (ultra-thin films, nanowires, quatum dots, etc.) and several nano-scale devices by in-situ measurements without any processes to make electrodes on the sample. Besides, the cooling capability allows us not only to suppress thermal perturbation but also to perform more advanced study of physical phenomena, such as superconductivity, localization, and various kinds of phase transitions.

The second part of this thesis deals with the transport properties of the Ag films on the Si(111)4x1-In surface.

Recently, it has been reported that uniform Ag(111) thin films which have stacking-fault planes with x4 period (~13.3A) [Fig.2] can be grown epitaxially on atomic-chain like surface superstruc-ture, Si(111)4x1-In surface, by the two step growth method. In advance, I have reported that the quantum well states (QWSs) in the Ag/Si(111)4 x 1-In have highly =isotropic electronic states revealed by ARPES measurements. They show a free-electron like parabolic dispersion along the In-chain direction but almost a flat one in the perpendicular to In-chain direction.

But in general, stacking faults are ex-pected to give only small perturbation to the electronic states in fcc noble metals, so this change of the QWS from isotropic Ag film are considered to be curious results and the conductivity behavior of QWSs in the Ag/Si (111)4 x 1-In is very interesting to measure.

Therefore I have measured transport properties of this quasi-1D quantized states using the VT4tipSTM mentioned above by the square micro 4-point probe (4PP) method.

As a result, anisotropic conductivity was clearly detected at 3-16 MLAg, although the measured conductivity was isotropic at 26MLAg on Si(111)4×1-In [Fig.3]. Here 1MLAg means 1 atomic layer of Ag(111) and 1.4×10(15) [atoms/cm2], which is the surface atomic density of Ag(111)1×1.1MLAg thickness corresponds to 2.36A.

The origin of the anisotropic conductivity is discussed in the view points of (i) anisotropic electronic states and (ii) anisotropic interface scatterings. I estimated the contributions of the electronic states and the interface scatterings to the measured anisotropic conductivity by solving the Boltzmann transport equation in the diffusive transport region.

The contribution of each mechanism to the anisotropic conductivity is revealed to be classified on thickness.

The anisotropic conductivity is mainly due to the anisotropic surface scatterings at 3-5MLAg. At 6-7MLAg, both anisotropic electronic states and anisotropic surface scatterings are equally contribute to the measured anisotropic conduc-tivity in square-4PP measurements. According to the STM observations by Uchihashi, the clitical thickness of the peculiar stripe struc-tures with periodic stacking-fault planes is 6MLAg and this stripe structures are grown uniformly in large areas. At 11-16MLAg, effects of anisotropic surface scatterings are reduced by appearance of the flat regions at the film surface. These flat regions are caused by structural relaxation. The stacking-fault planes are considered to remain under the flat surface regions in the film at only near the film/4 x 1-In interface. The contribution of the anisotropic electronic states to 4PP-measured anisotropic conductivity is so small compared with that of the anisotropic surface scatterings, that the total anisotropic ratio A (A x 100 [%]) decrease as films become thicker. As the thickness get larger to 26MLAg, the conductivity turns into isotropic because of the growth of flat regions without stacking-fault stripes and the reduction of the contribution from the surface scatterings compared to the bulk region.

Here we define x-direction as [110] perpendicular to the stacking-fault planes in the Ag films and In-chains at the interface, and y-direction as [112] parallel to the stacking-fault planes in the Ag films and In-chains at the interface.

In the discussion above, the electron scatterings due to stacking-fault planes are taken into account by the deformation of the quantized Fermi surfaces and quantized band structures. When the probe spacing is reduced down to tens of nm by using carbon nanotube tips, Ag films on Si substrates are no longer the ideal 2D systems. Then I have estimated the resistance due to stacking-fault planes in the film by treating stacking-fault planes as tunnel barriers in ballistic region for more minitualized measurements in the near future. The square-4PP resistance is considered to be larger than the present results. When this tunnel resistance is dominant compared to the diffusive transport which I have adopted in case of pm-scale probe spacing, the difference of film resistivity between thickness, py-px, may depend on probe spacing.

The square micro-4PP temperature-dependent conductivity measurements showed the almost constant value at 6MLAg thickness irrespective of temperature, while the semiconducting behavior at 3MLAg were observed in which the conductivity decreased with decreasing temperature [Fig.5].

The constant conductivity which does not depend on temperature at 6MLAg can be explained by the dom-inant mechanism of the carrier flow in the film, the interface scatterings which are independent of temperature.

The semiconducting behavior of the temperature dependence at 3MLAg cannot be explained by the activation tunneling current which is the popular effect for discontinuous films, because the continuous film has been already completed at 3MLAg in the case of the Si(111)4x 1-In substrate. The effect of the weak localization is conceivable, but it has to be examined by magnetoresistance measurements in the further study.

The QWS confined in the Ag ultra-thin film on the Si(111)4x1-In has anisotropic electronic properties due to the modulation induced by one-dimensional surface superstructure 4 x 1-In, at the film/substrate interface. This indicates that the electronic/transport properties of QWSs can be controIled by interface atomic layer.

By synthesizing the results from different viewpoints such as the square-4PP conductivity measurements, ARPES measurements, and theoretical calculations, I have clarified the important factor in electronic states and atomic structures to determine the behavior of electronic conductivity.

My studies show an example to understand the transport properties of atomically controlled epitaxial quantum metal films beyond the existent mesoscopic systems realized by lithography techniques

Figure1.Schematic drawing of the VT4tipSTM whole system.

Figure2 Schematic image of Ag(111) ultra-thin film prepared on Si(111)4x1-In based on the stacking-fault model.

Figure3 The relation between the film thickness and the square-4PP resistance. An SEM image during mea-surements and the measurement schematics are indi-cated.

Figure4 Anisotropic ratio of 2D conductivity in Ag/Si(111)4x1-In with changing the film thickness.

Figure5 Temperature dependence of 2D conductivity of 3MLAg and 6MLAg Ag films on Si(111)4x1-In.

半導体超格子構造や金属多層膜においては電子の一方向の運動が制約されることによる量子サイズ効果が現れる。これらは新奇かつ多様な電子物性の舞台として基礎から応用にいたる研究が盛んに行われてきた。半導体結晶表面に形成される金属薄膜は第3の舞台として表面物理学の新しい研究対象になっている。本論文は、典型的半導体基板であるシリコン(Si)(111)表面に、金属インジウム(In)を緩衝層として成長させた銀(Ag)薄膜が自発的に形成するストライプ状の特異な構造とその異方的電子輸送現象を探求したものである。論文は全6章からなる。

第1章では、金属薄膜や半導体界面研究の歴史、半導体シリコン上の金属薄膜研究の背景と現状を要約し、その上に立った本研究の目的が述べられている。

第2章は、本研究でえられた実験結果の解析と考察の基礎となる、表面電子構造や電気伝導の理論が紹介され、第3章では本研究の主要な実験手段である表面構造解析法、表面電子構造を調べるための角度分解光電子分光(ARPES)と走査型トンネル顕微鏡(STM)の原理、そのSTM探針を4本用いた表面電気抵抗測定手法が説明されている。

第4章は本論文の主要部の一つである。STM探針を4本正方形状に並べることにより薄膜表面の電気抵抗率の異方性をマイクロメーター領域で測定するための装置の改良が詳しく述べられている。この装置の従来の問題点は、電気抵抗測定の不安定性と低温域での温度制御の難しさであった。本研究では、機械的振動の除去、電気信号回線の改良、差動増幅器の導入により第一の問題を克服し、半導体から金属までの広い電気抵抗領域での安定な電気抵抗率測定と電気抵抗率の異方性の測定を可能にした。さらに、熱遮蔽を強化し、STM装置と寒剤との間の熱伝導を向上させることで液体ヘリウム温度の極低温でも安定的に測定ができる装置と測定システムの開発に成功した。

続く第5章では、もう一つの主要部である半導体Si上に作製したAg薄膜の構造評価と異方的電気抵抗率測定結果が示され、これらの結果に対する考察が記述されている。Si(111)表面上にInを1原子層蒸着させるとIn原子が1次元的に並び、そのIn鎖が4本づつ周期的に配列する表面構造が実現する。この上にAg薄膜を成長させると、下地のIn鎖周期構造を反映してAg薄膜内部に積層欠陥が周期的に入る。その結果、Ag薄膜表面にストライプ状の凹凸ができることになる。これが本研究の異方的電気伝導測定の対象である。研究では、反射高速電子線回折(RHEED)、STM直接観察により所期の構造ができていることを検証するとともに、 Ag薄膜を16原子層積んでもストライプ構造が保持されることを見出した。このようなAg薄膜に対して正方形状に並べたSTM4探針を用い電気抵抗測定を行った結果、ストライプ方向の電気抵抗率がそれに直交する方向よりも低くなるという予想通りの異方的電気伝導の観測に成功した。電気抵抗率の異方性は膜厚の増大とともに減少するが、表面ストライプ構造が保持されるAg16原子層までは観測されることから、この構造が電気的異方性をもたらしていることが実証された。

第5章後半部では、薄膜構造と照らし合わせ、異方的電気伝導がどのようなメカニズムによって生じているかを議論している。周期的な積層欠陥あるいはストライプ状表面の凹凸による電子の散乱が異方性の原因と考えられ、膜厚が5原子層までは表面の凹凸による散乱が支配し、6原子層以上では凹凸と積層欠陥の両者が寄与していると推論している。

第6章は全体のまとめであり、将来の研究の方向が述べられている。金属多層膜、半導体超格子構造の研究が基礎、応用両面で成熟をむかえた現在、半導体表面上の金属薄膜は新しい研究対象として発展途上にある。本研究は、STM4探針を用いた金属薄膜の電気伝導測定の手法を発展させ様々な形態の金属薄膜の電気抵抗率測定を可能にしたとともに、積層欠陥を利用することで電気的異方性をもった薄膜が作製できることを実証したことで、この分野の研究の新しい方向を示したといえる。学位論文として相応しい水準に達しているということで審査委員全員の意見の一致をみた。

なお、本論文は、指導教員である長谷川修司教授ほか10名との共同研究に基づいているが、論文提出者が主体的に実験および考察を行ったもので、論文提出者の寄与が充分であると判断できる。

したがって、博士(理学)の学位を授与できると認める。