Exploration into novel properties of ultra-high concentration hydrogen doped rutile-TiO₂

概要

Titanium(IV)oxide (TiO2) is one of the most studied metal oxide semiconductors for p otential applications such as photocatalysis, gas sensing, and transparent conductors. Esp ecially bandgap engineering of rutile-TiO2 by hydrogen doping has attracted much atten tion since a report in 2011 on bandgap narrowing by hydrogenation. However, there is a limitation of hydrogen doping amounts in equilibrium condition. In this thesis, the au thor investigated non-equilibrium hydrogen doping into rutile-TiO2 by the low-temperatu re H2+ beam irradiation, and studied its effects on electrical properties.

As the first step, the author performed the fabrication of the rutile-TiO2 thin film bec ause the penetration depth of hydrogen is limited for the ion beam experiment. To obta in the proper rutile-TiO2 thin films with nanoscale roughness, the author optimized the fabrication condition. The amorphous TiO2 thin film were deposited for 10, 20, 30 or
40 min using a RF magnetron sputtering system and subsequently annealed in air at 900 or 1200 ℃ for 10 hours for the crystallization. As a result, the author optimized the fabrication condition and obtained 120 nm thickness polycrystalline rutile-TiO2 thin fil m with the (100) preferred orientation.

The author studied the effects of excess hydrogen-doping on the rutile-TiO2 thin film by the low-temperature H2+ beam irradiation with in situ transport measurements. The H2+ beam irradiation at 300 K with an acceleration voltage at 5 kV drastically decreas ed the resistivity of the rutile-TiO2 film. The transport properties were well described b y 3D Mott variable range hopping (VRH) model after the irradiation. The author perfor med further irradiations at 50 K for excess hydrogen doping. The resistivity of the fil m increased after the subsequent low-temperature H2+ irradiation. The in situ transport measurement revealed that the 3D Mott VRH changed to Efros–Shklovskii (ES) VRH a fter the irradiation at 50 K. Remarkably, the resistivity returned back to the initial valu e by heating to 300 K with a kink around 210 K. Importantly, such a behavior was w ell reproduced many times by the low-temperature irradiation and heating to 300 K. Th e results suggested that excess hydrogen doping by the low-temperature irradiation and partial desorption around 210 K induce a reversible change in resistance between 3D Mott VRH and ES VRH behaviors.

To evaluate the hydrogen doping amounts, the author performed in situ nuclear reactio n analysis (NRA) using a tandem accelerator at the University of Tokyo. The total am ount of the hydrogen in rutile-TiO2 (100) single crystal was limited as 3.0 × 1016 ions/ cm2 with the maximum concentration of x = 0.3 for HxTiO2 by the room temperature H2+ irradiation with an acceleration voltage at 2.5 kV. By the subsequent low-temperatu re H2+ beam irradiation at 50 K, the total amount of the hydrogen drastically increased to 1.1 × 1017 ions/cm2 with the maximum concentration of x = 1.2 for HxTiO2. After the excess hydrogen doping by the low-temperature irradiation, temperature dependent N RA up to 300 K was performed. Surprisingly, the distribution of hydrogen concentratio n along the depth direction did not change up to 300 K. This result indicates that the hydrogen desorption is negligible up to 300 K and is not the origin of the reversible r esistance switching in hydrogen-doped rutile-TiO2.

To understand the stability and dynamics of the excessively doped hydrogen in rutile- TiO2 (100) single crystal, in situ NRA was performed from 50 K to 600 K. No signifi cant change of hydrogen depth profile was observed up to 380 K, but the hydrogen st arted to desorb above 400 K. It was also found that the vacuum annealing up to 580 K resulted in an irreversible change in charge transport properties, implying that oxyge n vacancies were generated by the vacuum annealing. Therefore, it is considered that t he doped-hydrogen is desorbed not only as H2 but also as H2O.

In this study, the author first demonstrated the ultra-high concentration hydrogen dopi ng into rutile-TiO2 using the low-temperature H2+ beam irradiation, which was confirme d by in situ NRA. The unforeseen transport properties such as reversible resistance swi tching were discovered in hydrogen doped rutile-TiO2 by the in situ transport measure ments. Furthermore, the thermal stability and dynamics of the excessively doped hydrog en in rutile-TiO2 was studied up to 600 K by in situ NRA and transport measurement up to 580 K.