[Introduction]

Transparent conducting oxides (TCOs) have been recognized as one of key-materials which sustain rapidadvance in opto-electronics. Although Sn-doped In2O3 (ITO) is most widely used as a practical TCO, it is still achallenging issue to widen the material variety of TCOs, based on requirements for adding novel electronicproperties to TCOs.

Recently, it has been reported anatase Ti0.94Nb0.06O2 (TNO) epitaxial thin films grown by pulsed laserdeposition (PLD) exhibit excellent transparent conducting properties, i.e., low resistivity (ρ) of 2.1 × 10-4 Ω cmand high internal transmittance of ~95% in the visible region, which are comparable to those ITO1). It is expectedthat Nb doping generates n-type carriers into Ti 3d nature conduction band. However, d-electron-basedcompounds are, in general, disadvantageous for electrical conduction because of anisotropy in chemical bonds.Indeed, conventional TCOs, such as ITO and ZnO, are known to be based on s-electrons. The mechanism of highelectrical conduction in TNO is interesting from both fundamental and practical viewpoints, but suchinvestigations have been scarcely attempted so far.

In this study, I studied the electrical properties and electronic stases of TNO epitaxial films with the aid oftransport and spectroscopic measurements, focusing on the role of excess oxygen, which is anticipated to killcarriers, and thus to significantly affect the electric properties of TNO.

[Experimental Method]

Ti1-xNbxO2 (x = 0, 0.06, 0.1, 0.2, 0.3) thin films were grown on LaAlO3 (100) substrates by PLD at a substratetemperature of 650 °C and an oxygen pressure of 7 x 10-6- 1 x 10-4 Torr.. The as-grown films (x = 0.06) werefurther subjected to post-deposition annealing at 400 °C in anoxygen atmosphere with an oxygen partial pressure (Po2anneal) of3.5 × 10-1 - 3.5 × 102 Torr in a rapid thermal annealing furnace.X-ray diffraction (XRD) measurements confirmed epitaxialgrowth of (001) oriented anatase TiO2 phase, without anyimpurity phases (Fig. 1). Transport properties, includingresistivity (ρ) and carrier concentration (ne), were characterizedby a standard Hall-bar technique. X-ray photoemissionspectroscopy (XPS) measurements were carried out at beam-line2C of the synchrotron radiation source at Photon Factory (PF) inTsukuba.

[Carrier Compensation by Excess Oxygen in Anatase Ti0.94Nb0.06O2+δ Epitaxial Thin Films]

During my master course, I have found that post-deposition annealing gives significant influence on theelectrical transport properties of anatase Ti0.94Nb0.06O2 (TNO) epitaxial thin films. That is, annealing TNO films inpure oxygen drastically reduced the carrier density (ne) and the resistivity, but oxygen annealed films recover highconductivity by annealing in reducing atmosphere (pure H2). Core-level X-ray photoemission spectroscopyrevealed that Ti and Nb respectively exist as tetravalent and pentavalent ions in fully oxidized samples. Theconcentration of Nb5+ relative to that of Nb4+ tends to increase with O2 annealing. These results suggest thatcarriers released by Nb donors are compensated byexcess oxygen atoms incorporated by O2 annealing.

Based on these results, in order to directlyobserve these impurity states associated with excessoxygen, I performed resonant photoemissionspectroscopy.

Figures 2(a) show Ti 2p-3d resonant (hν = 461.2eV) valence band spectra respectively, of as-grownand annealed (Po2 = 3.5 × 102 Torr) films. Figure2(b) compares expansions of resonant andoff-resonant spectra near the band gap. The resonantspectrum of the as-grown film clearly exhibits a peakat the Fermi level, corresponding to the bottom of theTi 3d -nature conduction band. In the O2-annealedsample, the peak intensity is substantially suppressed reflecting the abrupt reduction in ne, and a characteristic shoulder-like in-gap state appears at ~3.0 eV just abovethe top of the valence band. The location of the in-gap state relative to the valence band is consistent with that ofimpurity states originating from interstitial oxygen, as predicted by band calculations.2) As a consequence, Iproposed a carrier compensation model in which p-type acceptor states originating from interstitial excess oxygen kill electron carriers generated by Nb doping.

[Suppression of carrier activation in heavily Nb-doped Ti1-xNbxO2+δ]

Early studies on TNO have shownd that Nb dopants in lightly doped TNO are almost completely ionized withan efficiency of >90% but in heavily Nb doped region the ionized efficiency decreases with increasing Nbconcentration. In order to elucidate the origin of abrupt suppression in carrier activation at higher Nb region, Ihave investigated the electronic properties of TNO thin films as a function of Nb concentration by usingsynchrotron-radiation photoemission spectroscopy.

Figure 3 shows Nb 3d core level spectra of TNO films with different Nb concentration x. Each spectra in Fig. 3exhibits a major Nb5+ peak together with a minor Nb4+ peak at lower binding energy. Notably, the peak intensityof the Nb5+ maintained constant ~80% against increasing x doping.

Figures 4(a) show carrier density ne at room temperatureplotted against Nb concentration x. As seen from the figure, nestarts deviating downward from an ideal ne = nNb line at x>0.06,corresponding to the suppression of ionization efficiency.

Figures 4(b) plot relative intensity ratio of Nb5+ and carrieractivation ratio ne,/nNb as functions of x. The relative intensityratio of Nb5+ maintains constant ~80% up to x = 0.03, whilene,/nNb decreases from 0.9 to 0.45 with increasing x from 0.06to 0.3. These imply that electron carriers generated by Nbdoping were compensated by excess interstitial oxygenincorporated during film deposition, similar to the oxygenannealed samples mentioned above. However, the conductivityof heavily Nb doped films no longer changed by furtherannealing under reducing conditions. This suggests thatthere are two kinds of excess oxygen atoms: one isintroduced into or released from TNO by annealing inoxidizing and reducing conditions, respectively, in areversible way. The other is strongly incorporated intoTNO crystal so that it cannot be released bypost-deposition annealing.

[Summary]

I studied the electrical properties and electronic stases of TNO epitaxial films with different oxygen or Nbcontents. Valence-band photoemission spectroscopic measurements showed that an in-gap state evolved justabove the O 2p valence band accompanied with O2 annealing. The location of the in-gap state relative to thevalence band is consistent with that of impurity states originating from interstitial oxygen, as predicted by bandcalculations. As a consequence, I proposed a model in which excess oxygen atoms located adjacent to Nb behaveas p-type defects, which compensate electron carriers generated by Nb doping. In highly doped TNO, it issuggested that electron carrier released by Nb dopants were compensated by another kind of excess oxygenincorporated during film growth.

Figure 1: Typical XRD patterns ofTNO films(x = 0.06, 0.3)

Figure 2: Valence-band spectra of as-grown(gray line) and oxygen-annealed (black line)TNO(x = 0.06) films. (a) Ti 2p-3d resonant (hν =461.2 eV), and(b) Close-up view of Ti 2p-3d resonant spectranear the Fermi energy.

Figure 3: Nb 3d core-level XPS spectra of TNOfilms. Gray dots indicate experimental data.brack dash curves represent the components ofNb5+ 3d obtained by least-squares fitting. Graydash curves represent the components of Nb4+3d obtained by least-squares fitting. The thincurve represents the total fitting result.

Figure 4: (a) carrier density of TNO films as functions of Nb concentration x.(b) Intensity ratio I(Nb5+) /[I(Nb5+)+I(Nb4+)] and carrier activation as functions of x.

透明導電体は様々な電子デバイスの透明電極として広く利用されており、新たな材料の開発が活発に繰り広げられている。本研究では、新奇透明導電体であるアナターゼ型NbドープTiO2のキャリア生成機構に関する問題を取り扱っている。

本論文は5章からなっている。

第1章は序論であり、透明導電性が生じる機構ならびに既存の透明導電体について概観するとともに、透明導電体に関する最近の研究動向について述べている。特に、本論文で対象としたアナターゼ型Nbドープ野02の研究背景について詳細に記述している。また、本系におけるキャリア生成機構解明の意義と研究の指針について説明しており、キャリア生成機構の解明には、化学ドーピングや酸素欠陥の導入によりキャリア量を精密に制御し、それた伴う物性と電子状態の変化を詳細に調べる必要があると強調している。

第2章は実験手法に関する説明である。薄膜作成法であるパルスレーザー蒸着法(PLD)、薄膜評価方法であるX線回折(XRD)、原子間力顕微鏡(AFM)、4端子抵抗測定法、ホール効果測定、X線光電子分光法(XPS)、X線吸収分光法(XAS)について、それぞれの原理と、どのような情報が得られるかについて概説している。

第3章は、恥ドープTiO2の酸素欠陥がキャリア生成に及ぼす影響について述べている。Nbドープ7MO2薄膜を様々な酸素雰囲気下でアニールすることにより、金属イオンの酸化還元状態を制御した試料を準備し、その輸送特性と電子状態を系統的に評価している。輸送特性からは、酸素雰囲気下でアニールすることにより、抵抗が上昇し、特にキャリア密度が大きく低下するという結果を得ている。一方、TiおよびNbに対する内殻XPS測定からは、NbドープTSO2薄膜にはキャリアを補償する機構が存在することを示唆している。高抵抗試料の価電子帯スペクトル測定からはギャップ内準位を観測し、これが第一原理計算で予想されている格子間酸素の存在と矛盾しないことを示している。以上の結果をもとに、格子間酸素が深い不純物順位を形成し、これがNbの放出するキャリアを補償するため高抵抗化すると結論している。

第4章は、Nbドープ量とキャリア生成との関係について詳細に述べている。先行研究の結果から、Nbのドープ量を増加させていくと、キャリアの活性化率が大きく減少することが分かっているが、その起源について調べている。XPS測定から、Nbはドープ量に依存せず、ほぼ5価として存在することを見出し、Nbを高濃度ドープした領域ではキャリアを補償する機構が存在することを示唆している。一方、Nbを高濃度ドープした薄膜試料の価電子帯スペクトルからは、第3章で示したものと同様の格子間酸素に由来する構造を観測している。低濃度ドープした薄膜や無添加の薄膜では格子間酸素由来の構造はほとんど見られないことから、高濃度ドープ領域でキャリア活性化率が低下する原因としては、製膜時に格子間に酸素が取り込まれることによって、Nbが放出するキャリアが補償されるためと結論している。

第5章は結論と要約である。

以上のように、本論文では、化学ドーピングや酸素欠陥の導入によりNbドープTiO2のキャリア量を精密に制御し、それに伴う物性と電子状態の変化を詳細に調べることにより、格子間酸素に由来する不純物準位が恥によって放出されるキャリアを補償するモデルを提案している。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実験、解析、及び考察を行ったものであり、論文提出者の寄与が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。