1. Introduction

This dissertation deals with new materials composed of superlattices. Superlattice is a periodic structure of layers of two or more different materials. Figure 1 shows the schematic illustration of superlattice composed of two components. In the early stage of the research on superlattice, epitaxial grown III-V compound semiconductors such as GaAs/AlAs have been mainly investigated. However, it is difficult to deposit more than several hundred layers because deposition rate is very small. In next stage, inorganic amorphous semiconductor, especially Si-based superlattices, have been investigated due to the large deposition rate. However, there are still disadvantages such as high defect density near interfaces and low electric performance.

In recent studies, many kinds of organic amorphous materials have been developed in the research of organic light emitting diodes, solar cells, polymer actuators, and so on. There is no disadvantage that inorganic materials have for organic materials. Therefore, it has been considered that these materials open up a new opportunity for fabricating macroscopic materials with the nano-scale multilayer as designed.

A final goal in this study is the development of photo-switching piezoelectric material. Photo-switching piezoelectricity is piezoelectricity that can be induced by light irradiation, not by electric field application. To develop the photo-switching piezoelectric materials, it is necessary to combine the concepts of the photovoltaic effect and the piezoelectric effect. Tri-color superlattice (three-component superlattice) (TCS) is one of the most realistic candidates to fabricate the photo-switching piezoelectric materials because the photovoltaic and piezoelectric effect can be achieved by selection of appropriate materials. p-type and n-type semiconductor (p-n junction) are needed for photovoltaic effect. Insulator is needed for piezoelectric effect.

2. Experimental

A new fully-automated apparatus has been developed for fabricating amorphous superlattices (Fig. 2). It utilized an optical heating for the rapid deposition and a QCM for precise thickness control. The sample was rotated to guarantee the in-plane uniformity of the superlattice films. The deposition sequence and thickness of each layer could be set by only inputting parameters to fabricate superlattices as designed. In order to complete one layer, 20 sec was typically required. Therefore, a superlattice with more than hundreds layers could be prepared in almost one day.

Amorphous materials used in this study were N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (NPB) tris(8-hydroxyquinolinato) aluminum (Alq3), molybdenum oxide (MoO3), and polyurea (PU). NPB is a p-type semiconductor and Alq3 is an n-type semiconductor. NPB and Alq3 are used in organic light emitting diodes as a hole transport layer and a electron transport layer, respectively. MoO3 is a wide gap semiconductor (mostly insulator) and used for a charge separation layer in mechanically flexible solar cells. PU is a piezoelectric insulator and can be fabricated by vacuum deposition polymerization without by-products.

First, Alq3/NPB superlattice was fabricated in order to confirm the machine performance. X-ray diffraction (XRD) pattern derived from superlattice was observed and the surface morphology was smooth by AFM observations. These results indicated the developed apparatus worked as designed.

Next, PU films were prepared by alternating deposition of DAF (diamine) and MBCI (diisocyanate), and then I investigated its physical properties. Indium tin oxide (ITO) substrates and Si substrates with thermal oxide layer (700 run) and aluminum layer (200 nm) (Al/Si02/Si) were used for suitable characterization technique. IR spectroscopy was carried out to investigate the degree of the polymerization and piezoelectric measurement was done by mechanical stress application method and capacitance method.

TCS films composed of Alq3, MoO3 and NPB (Alq3/MoO3/NPB) were prepared on the Si substrates. XRD and AFM measurements were carried out to investigate the smoothness of the interfaces and the uniformity of thickness. Photovoltage measurements were also carried out, which is very important to develop the photo-switching piezoelectric materials.

PU/NPB/Alq3 films were prepared by the sequence of ((DAF -MBCI)m - NPB - Alq3)n. The morphology of the surface was estimated by AFM observations. Photovoltaic measurement was carried out, and then the photo-switching piezoelectricity was measured by capacitance method.

3. Results and discussion

3-1 Physical properties of PU films

It was found that PU can be synthesized by alternating deposition although the conditions of the supply ratio (DAF:MBCI) to achieve particle-free and flat PU films was very severe. Judging from the reaction scheme, the ratio of DAF to MBCI should be exactly 1:1. If the ratio is appropriate, the surface of PU films is free from particles and the film is optically transparent. Otherwise the surface of PU films becomes white with particles. Furthermore, the surface of PU films fabricated by alternating deposition was smooth by AFM observations.

It was found that PU can be synthesized by alternating deposition although the conditions of the supply ratio (DAF:MBCI) to achieve particle-free and flat PU films was very severe. Judging from the reaction scheme, the ratio of DAF to MBCI should be exactly 1:1. If the ratio is appropriate, the surface of PU films is free from particles and the film is optically transparent. Otherwise the surface of PU films becomes white with particles. Furthermore, the surface of PU films fabricated by alternating deposition was smooth by AFM observations. cm-1 formed by stretching vibration of isocyanate group was observed for (a) as-deposited sample although the peak disappeared (b) after thermal treatment. This result indicates that remained isocyanate group further copolymerize with amino group by thermal treatment.

Figure 4 shows the piezoelectric response obtained as current signals when mechanical stresses were applied and released to the poled PU films. It indicates that PU films fabricated by alternating deposition show the piezoelectric property by the poling treatment and that PU could be used as a component of TCSs for fabricating the photo-switching piezoelectric materials. Capacitance method also indicated the poled PU film has the piezoelectricity and piezoelectric constant calculated from this method was 2.6 pC/N. This value is smaller than the reported value of aromatic polyurea, which may be attributed to larger molecular weight.

3-2 Alq3/MoO3/NPB superlattice

X-ray diffraction (XRD) measurement was carried out and diffraction peak originating from TCS of Alq3/MoO3/NPB were observed. This result indicates interfaces of Alq3/MoO3/NPB were smooth and thickness of each layer could be controlled. Root-mean-square roughness obtained by AFM observation was around 0.50 nm, which also indicates the interfaces of Alq3/MoO3/NPB3 were smooth. A 100-multilayer sample showed a photovoltage of more than 10 V.

3-3 Photo-switching piezoelectricity of PU/NPB/Alq3

The surface morphology of the PU/NPB/Alq3 films was examined by AFM as a function of repetition time. The surface of PU/NPB/Alq3 films was relatively smooth despite the films were relatively thick. Furthermore, large photovoltage was observed if PU layer in PU/NPB/Alq3 superlattice was thick. Figure 5 shows the results of photovoltaic measurement on three kinds of PU/NPB/Alq3. Maximum voltage was more than 14.1V that is the limitation of the measurement when (PU(2.4 nm)/NPB(15 nm)/Alq3(15 nm))240. Figure 6(a) shows the photo-switching piezoelectric measurement by capacitance method. The effect of thermal expansion was comparable to the piezoelectric effect. Then, a difference between TCSs poled by forward and reverse bias was calculated (Fig. 6(b)). This capacitance change induced by light irradiation clearly shows the photo-switching piezoelectric effect.

4. Conclusions

An apparatus specially designed to fabricate amorphous superlattices with more than hundred layers was developed. Piezoelectric PU was prepared with nm-thickness control by alternating deposition. Fabrication of TCSs of Alq3/MoO3/NPB and PU/NPB/Alq3 was succeeded and large photovoltage was observed. The photo-switching piezoelectricity was observed for PU/NPB/Al3 superlattice by capacitance method.

Fig. 1: Superlattice composed of two component

Fig. 2: Automated deposition apparatus for fabricating amorphous superlattices

Fig. 3: IR spectra (a) as-deposited, (b) after thermal treatment

Fig. 4: Current signals when mechanical stresses were applied

Fig. 5: Photovoltage of PU/NPB/Alq3

Fig. 6: Photo-switching piezoelectric measurement by capacitance method

三色超格子は反転対称性が無く、ユニークな物性を発現することが期待される。また近年、有機エレクトロニクスの発展により、多くの有機アモルファス材料が開拓され、利用できる状況にある。アモルファス材料は高速で積層することが可能なため、多積層の超格子作製に適している。本研究では、三色超格子とアモルファス材料の特徴を組み合わせることにより、光スイッチング可能な圧電材料の開発について検討している。

本論文は、以下の序章および6章より構成されている。

序章では本論文を概観している。以降の1章から6章の内容の理解を助けるとともに、本論文の構成についても言及している。

第1章では本論文の背景および目的が述べられている。この章では超格子の歴史と近年の有機エレクトロニクスによる有機アモルファス材料の開発・発展について概観している。その上で、これら二つのコンセプトを組み合わせることにより、光スイッチング可能な圧電材料の作製を提案している。また、考えられるメカニズムを提示し、提案した材料の実現が可能であることを明示している。

第2章では実験装置・解析装置の説明およびそれらの原理の説明である。本研究で用いたX線回折(XRD)、原子問力顕微鏡(AFM)、金属蒸着機、ポーリング処理、圧電測定、光起電力測定、光スイッチング圧電測定、赤外分光法の原理について述べている。

第3章ではアモルファス超格子を作製するための装置に関して述べている。アモルファス超格子を作製する上で重要となる高速蒸着を達成するための工夫について説明し、実際の超格子作製装置の性能について述べている。また自動蒸着の際に利用できるシークエンスの具体例を示し、作製可能な超格子の構造について言及している。試作した装置を用いてAlq3/NPB超格子を作製し、AFM、XRDによる評価した結果、必要な性能が得られたと結論づけている。

第4章は交互蒸着法によるポリ尿素の作製とその評価について説明している。従来、ポリ尿素膜は共蒸着法により作製されていたが、三色超格子にポリ尿素を利用するには共蒸着は不向きである。そこで、ポリ尿素膜が交互蒸着法により作製出来るかを検証している。蒸着比を変化させながらポリ尿素膜を作製し、その表面を光学顕微鏡で観察した結果、蒸着比が1:1付近であれば非常に平坦なポリ尿素膜が得られることが明らかにしている。さらに、交互蒸着法により作製したポリ尿素膜について圧電性を確認している。これらの結果から、交互蒸着法を用いることで、ポリ尿素を三色超格子へ応用することが可能であると結論している。

第5章では光スイッチング可能な圧電材料の作製について報告している。まず、酸化モリブデンを絶縁層に用いた三色超格子(Alq3/MoO3/NPB)を作製し、それぞれの層の膜厚が薄い時は光起電力が小さいが、7-8nmの膜厚があれば10V以上の非常に大きな光起電力が得られることを見出している。このAlq3/MoO3/NPB超格子では圧電性は確認できなかったが、ポリ尿素を絶縁層に用いた三色超格子(ポリ尿素/NPB/Alq3)では、大きな光起電力を観測している。繰り返し回数を増やすことで光起電力が増大することを見出し、測定限界の14.1V以上にまで達すると述べている。さらに、キャパシタンス法を用いて圧電特性を評価し、ポリ尿素膜由来の圧電性を観測し、0.3nmと微小な変位ながら光スイッチング特性も確認している。これにより、当初の目的である光スイッチング圧電材料の開発に成功したと結論づけている。

第6章は結論と総括である。

以上のように、本研究は、光スイッチング可能な圧電材料の開発に成功し、超格子構造を用いた新しい物性の探索に道を開くものである。これらの研究は理学の発展に大きく寄与する成果であり、博士(理学)に値する。なお本論文は複数の研究者との共同研究であるが、論文提出者が主体となって行ったものであり、論文提出者の寄与は十分であると判断する。

したがって、博士(理学)の学位を授与できると認める。