Oxide materials have rich history and also widely used by early humans in various forms such as paint medicinal ointment etc. In mid of the 19th century, it was started to use as oxide electronics. Especially in the 1980s, many researches were concentrated on oxide semiconductors; due to the dramatically advanced to produce oxides. Thus far, many studies have concentrated on oxide semiconductors because of various functionalities such as magnetism, optics and electronics. Many researchers have directed attention to many types of oxide semiconductors (ZnO, In2O3, TiO2, VO2 and WO3), which possess several electronic states such as a metal, semiconductor and insulator. In particular, impurity-doped ZnO and In2O3 provide new insight for optoelectronics in the near-infrared region, which have been applied for photonics and medical engineering. Rare-earth ion-doped ZnO is regarded as promising for integration of a light source because light emissions are produced in the wavelength region from ultra-violet to NIR rehion. Furthermore, surface plasmon polaritons of ZnO:Ga and In2O3:Sn are observed in the NIR region, which plays an important role in enhancing NIR light emissions due to an increased electromagnetic field by surface plasmons.

ZnO is a semiconductor of wurtzite structure with wide band gap of 3.4eV and large exciton binding energy of 60meV at room temperature, which is much higher than that of GaN (21 meV). This makes ZnO an ideal material to realize room temperature excitonic devices. Another big advantage of ZnO is ability to grow on single crystal or even on glass substrates since its c-axis preferred orientation. Because of this c-axis orientation, higher electron mobility can be expected even on glass substrate. Thus, ZnO can be a suitable candidate for applications like SPR sensors, which uses glass prism. ZnO is available in large single crystals, therefore the epitaxy of ZnO film on native substrate leads to low concentration of extended defects. Another big advantage over standard semiconductor is that, ZnO is amenable to wet chemical etching and low temperature growth, which is particularly important in the device design and fabrication. Impurities such as Ga and Al doping into ZnO can be easily incorporated to very high carrier concentrations. Since wide band-gap energy, ZnO is the suitable host materials for the doping of luminescence centers. Moreover, ZnO is biocompatible and can be used for biomedical applications without coating.

In2O3 is also a transparent semiconductor with a wide band gap of 3.7 eV. Sn and F doped In2O3 are an excellent materials for solar cells, flat-panel displays etc, because of ability to control thickness and carrier concentration. Doped In2O3 has the highest conductivity metal oxides, because of its high electron mobility. This is one of the most important parameter in surface plasmon resonance. Thus optical, electrical properties of In2O3 are well studies as a conductive oxide in electronic industry.

Even though, oxide semiconductors have long history, commercialization of ZnO based products have been started recent years due to development of amorphous oxide semiconductors (AOSs). The first AOS thin-film transistor (TFT) was reported in 2004 and have created a new area of electronics known as 'giant microelectronics' typified by devices such as solar cells and flat-panel displays. Since poly-ZnO is known to act as an active layer in a semiconductor device even fabricated at low temperatures below 300 which is expected to replace hydrogenated amorphous silicon (a-Si:H). This thin film fabrication technique advancement gives driving force for even for consideration of surface plasmons excitation in oxide semiconductors, since it is needed to consider the fabricated thin film in wide range of substrates such as glasses. However poly-ZnO TFTs also still have many issues such as It is recognized that poly-ZnO TFTs still have many issues to be addressed, such as low mobility of charge carriers and unstable electrical properties, which are largely due to grain boundaries. Current researches in TFTs will affect the development exciation of surface plasmon in oxide semiconductors.

Recently Plasmonics is the one of the hot research topics, because it can be merged the fields of optics and nanoelectronics by confining light with relatively large free-space wavelength to the nanometer scale enabling novel devices. Thereby researchers in the field of high speed information processing are paid attention to overcome the difficulties faced in RC-delay of electronic devices and diffraction limitation in optical devices by utilizing plasmonic properties. However, the operation frequency is shifted to longer wavelength from visible frequency plasmon electric magnetic field spatial expansion of noble metal cannot be controlled. Even though, plasmonic devices are famous on subwavelength confinement, this property no longer exists for large operating wavelengths. This is the major drawback in when applying to nano-scale devices such as waveguides. Furthermore, plasmonic devices at NIR frequencies are attracted due to telecommunication and optical frequencies. However researches have to face significant difficulties due to loss encountered with noble metals. Therefore, researches on new plasmonic materials which can be alternated by noble metals are become essential. Which will useful to development of novel devices with unprecedented functionalities such as optical antennas, subwavelength waveguides, etc.

In this thesis plasmonic excitation of oxide semiconductors in optical and telecommunication frequencies are discussed as an alternative for noble metals at NIR frequencies. Since oxide semiconductors are capable to confine the plasmon electromagnetic field.

In chapter 2, the dielectric functions of ZnO:Ga and ITO are discussed, as it plays a main role in surface plasmon physics. I then extend the discussion to loss involved with surface plasmons, and compare low carrier density materials with noble metals. For a surface plasmon to exist in air, ε1 should be less than (-1). However energy loss reaches a maximum around this dielectric value. When the operating frequency differs significantly from the plasma frequency, ε2 becomes large, resulting in loss and a small propagation length. Therefore, a balance must be struck between maximizing the propagation length and reducing the entire losses. In conclusion, it is expected that the SPR curves in the NIR for any material will have larger FWHMs when compared with the visible frequencies.

The basic principle of a current injected emitter operated in NIR and their fabrication method was analyzed in Chap. 3. First optical properties of Zn1-xEuxO epitaxial layers were discussed, and were systematically investigated in correlation with structural analysis. Future more, emission spectra were analysed by exciting the band gap of ZnO and discussed about energy transferring process. Excitonic emissions were remarkably suppressed with increasing Eu content by the formation of a band tail at the band edge, which played an important role in activation energy transferring from the ZnO host to the Eu3+ ion. Strong Eu3+ emission was only observed at low temperatures, and the recombination process was explained by two types of nonradiative activation energies. Considering the recombination process in Zn1-xEuxO layers, excited carriers were trapped at the shallow states near the band edge, forming an electron-hole pair at low temperatures. This process produced nonradiative energy transfer to the Eu3+ ion through the high-lying excited levels of Eu3+ ions.

Surface plasmon excitation of oxide semiconductors were introduced in Chap.4.Ga doped ZnO and Sn doped In2O3 were selected as the plasmon materials in NIR region, since their low dielectric loss and higher carrier density. SP modes guided by ZnO: Ga layers are theoretically and experimentally studied. Clear SPR reflectivity is observed above the cutoff thickness of the s-mode propagated at the air-ZnO interface. In contrast, the QB mode associated with a hybrid surface mode was found in the SPR reflectivity of layers with thicknesses below the cutoff thickness. Sensor ability was also investigated in oxide semiconductors using water and glucose solutions as examples. An obvious enhancement of the absorption band due to water was confirmed when the absorption peak of water and the SPR peak overlapped. The acquired sensitivity on In2O3:Sn was close to that of the detection ability of Au metal-SPR, which was further discussed in regard to the spatial coherence of an SP wave.

In this thesis, the emission spectra of Er3+ in 1540nm exciting ZnO band gap and their coupling with localized surface plasmon of In2O3:Sn nano-particles were successfully demonstrated. Rare-earth (RE) doped materials have been investigated intensively as one of the promising optoelectronic materials, since sharp, photostable and long lived emission spectrum due to intra-4f transition. The luminescent centers in RE doped materials can act as an atomic emitter by pumping excitation energy. Plasma coupling emission enhancement by modifying the dielectric environment was discussed to overcome several drawbacks in these atomic emitters. The LSPR was observed in In2O3:Sn nano particles in NIR frequency who's diameter was 15 nm and we theoretically supported their LSPR using Mie theory.

There is a so-called transmission window of optical fiber communication at 1.5μm with a low loss allowing long distance transmission. It is a highly desired monolithic integration with complex circuits. The surface plasmon nanophotonic could address several existing challenges due to its nature involving hybrid technology at the interface of optics and electronics

Utilizing transparency in visible and strong plasmonic absorption of Indium oxide nano crystals as discussed in Chap. 5 in NIR than noble metals, oxide semiconductors can be applied to absorb the NIR heat from sun in solar cells to reduce the heating. One the other hand there is a recent trend energy harvesting in NIR by solar cells. As nearly half of the Sun's energy arrives at the earth in the near-infrared frequency range, scientists are investigated how to increase the absorbing amount of sunlight that can be harvested by solar panels for power generation. Furthermore NIR radiation is the only solar radiation that can be collected in sunny/cloudy day or at night. Organic material-based solar cells have been extensively studied due to their low-cost, simple and cost-effective processes. Plasmon-assisted light trapping in the active layer of a solar cell is employed to enhance weak absorbance of the organic photoactive layers.

Up-conversion luminescence based on multi-photon excitations is one of the important photonic research areas due to potential applications in laser technology, optical communications, storage, displays, imaging techniques, optical sensing and biological probing. Among these applications, up-conversion luminescence is extensively documented in rare-earth compounds for volumetric 3D displays. Surface plasmons enhanced NIR emission will be the light source to excite rare-earth ions to produce visible emission for display monitors. Indeed, surface plasmons in transparent oxides are the only candidates for display applications.

長い研究の歴史を有し、幅広い分野で研究・応用されている酸化物材料の中で、近年注目されている酸化物半導体を対象とした研究を実施した。その中でもZnO, In2O3, TiO2, VO2 とWO3などの材料は、ドーピングにより広範囲にキャリア濃度を制御できると共に、各種イオン置換により発光特性、磁性など様々な機能の付与が可能である興味深い材料群である。本論文では酸化物半導体中でも、ZnOとIn2O3に注目し、これらの二つの材料に新たなに光学特性の視点から二つの機能の付与を実現した。

第1章では、表面・局在プラズモン現象に関する現状を概観すると共に、酸化物半導体を用いることによる近赤外波長領域への拡張性に関して説明した。

第2章では、酸化物半導体の中でも酸化亜鉛(ZnO)および酸化インジウム(In2O3)において、不純物ドーピングによるキャリア密度制御と可視から近赤外領域に亘るプラズマ周波数制御について議論した。

第3章では、可視および近赤外領域での発光光源の実現を目指し、ZnOおよびIn2O3各々において希土類ドーピングによる結晶構造の変化や発光スペクトルについて議論した。希土類の発光は4f間の電子遷移に起因する為、温度変化に対して極めて安定しており、光通信分野では広範囲な応用が期待されている。そこで、汎用性の観点から可視光を、また生体透過機能と光通信分野での応用の観点から近赤外光を研究対象として、各々の波長域に発光特性を有するErとEuをドーピング希土類(発光体)として選定した。325nmレーザにより励起された価電子帯の電子は伝導体により低いエネルギー帯でトラップされ、このエキシトンエネルギーがEu3+イオンにエネルギーを受け渡していることを確認した。このように可視領域ではEuをドーピングすることによって615nmに、近赤外領域ではErドーピングによって1540nmに発光特性を観測した。

第4章では、酸化物半導体の表面プラズモンに関して議論した。非平衡結晶成長法である紫外光パルスレーザ用いて、ZnOにGaを、In2O3にはSnをドーピングすることでキャリア密度を制御し、近赤外波長域で表面プラズモンを励起させることに成功した。さらに、表面プラズモン膜厚依存性についても議論し、酸化物の膜厚の減少に伴い、対称モードが存在できなくなる事を明らかにし、酸化物(電子密度10-19cm3)におけるその遮断膜厚は約100nmということを見出した。一方既存の貴金属では、酸化物の遮断膜厚の1/100(1~2nm)にまで極薄膜化することが要求されるため、実用の観点からも酸化物半導体薄膜による表面プラズモン応用が有効である事を示した。また従来の研究では、遮断膜厚について理論(計算)のみの報告に留まっていたが、本論文では実際のデバイスを作製し、実証実験によって遮断膜厚について初めて議論した。これまで貴金属を用いた実験では観察出来なかった非対称モードを含め、非対称モードと対称モードが同時存在することを確認した。この技術により、水の結合状態の指標となる水酸基(OH基)の合成振動モードに(近赤外)にプラズマ周波数を設計した酸化物半導体薄膜の近赤外域での表面プラズモンにより、従来の赤外分光法に比べて約150倍の検出感度を実現した。またグルコース検出においても3桁の線形応答性があり、糖尿病の指標となる血糖値100mg/dlの感度をほぼ達成した。

第5章では、酸化物半導体ナノ結晶による局在プラズモン増強について議論した。In2O3にSnを10%ドーピングした直径15nmのナノ結晶は局在型プラズモン吸収を近赤外領域に示した。Mie散乱計算を用いて、ナノ結晶の直径依存性、分散されている媒体の誘電率の依存性について議論した。直径を200nmまで、散乱スペクトルは小さい半値幅を維持可能であるが、それ以下の直径では半値幅が大きくなり、高次モードが出現した。さらにZnO:Er3+2%薄膜(70nm)上にIn2O3:Snナノ結晶を分散させた系を作製し、Er由来の1540nmの発光スペクトル増強を初めて確認した。この増強の原因として、最初にEr3+イオンとナノ結晶のダイポール相互作用による局在プラズモン励起、これに引き続くナノ結晶周りの散乱光増強が考えられる。

以上要するに、本研究では酸化物半導体(ZnOやITO)における不純物ドーピング量を制御することで、プラズマ周波数を可視から赤外域へ任意に制御できることを実験、理論両面から検証した。特に光情報通信への適用性、非侵襲検査技術としての生体透過性に優れた近赤外波長域において、希土類ドーピングした酸化物半導体(Er-ZnO)により、発光および表面・局在プラズモン増強を実験的に初めて観測した。これらの技術は、今後の酸化物エレクトロニクス、フォトニクス工学に於ける貢献が少なくないものと考えられる。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。