Kunio Okimura

Low temperature growth of rutile TiO2 films in modified rf magnetron sputtering

Abstract Low temperature (≤300°C) growth of rutile TiO2 films with high refractive index which is equal to bulk TiO2 crystal was achieved by using a modified sputtering method. Depositions were carried out in rf magnetron sputtering apparatus equipped with an auxiliary permanent magnet just under the grounded electrode. The as-deposited films showed rutile polycrystalline structure and fine surface morphology indicating higher densification. Remarkable changes in composition, total current and energy of incident ions were presented at a Ar–O2 total pressure of 2.7 Pa. The rutile phase grows in a modified sputtering method in contrast with anatase phase growth in conventional sputtering apparatus.

Preparation of Rutile TiO2 Films by RF Magnetron Sputtering.

TiO2 films were prepared by rf (13.56 MHz) magnetron sputtering using a mixture of Ar and O2 gases. At a total pressure of 2 mTorr, 100% rutile TiO2 films were successfully obtained on non heated substrates with rf power of 200 W, while 100% anatase TiO2 films were deposited at a pressure of 20 mTorr. Spatial profiles of both emission of excited species and plasma parameters were measured by optical emission spectroscopy (OES) and the Langmuir probe method. At a pressure of 2 mTorr, it was found that high-energy electrons are generated at a certain radial position near the cathode surface where the transverse magnetic field is maximum, and the strong localization of plasma was observed. It was proven that the energetic species impinging on the growing film are responsible for the formation of the rutile phase even if the substrate is at room temperature.

Characteristics of rutile TiO2 films prepared by r.f. magnetron sputtering at a low temperature

Abstract TiO2 films were prepared by r.f. (13.56 MHz) magnetron sputtering using a mixture of Ar and O2 gases. It was found that the crystalline structure strongly depends on the radial position of the substrate and on the total pressure of the sputtering gas. At a total pressure of 0.27 Pa, rutile TiO2 films were successfully obtained on a non-heated substrate located at a radial position of 33 mm with r.f. power of 200 W. While anatase TiO2 films were deposited at a pressure of 2.7 Pa. The deficiency of oxygen and the incorporation of Ar were observed in rutile films prepared on a Si substrate. In addition, rutile films grown on Si substrate were composed of crystal grains which are larger than 100 nm. The optical analysis showed that rutile films on a quartz substrate have a high refractive index of 2.67 at a wavelength of 370 nm.

Electric-Field-Induced Multistep Resistance Switching in Planar VO2/c-Al2O3 Structure

The electric-field-induced resistance switching of vanadium dioxide (VO2) films grown on c-Al2O3(001) substrates was studied in planar devices with two terminal electrodes. We demonstrated multistep resistance switching in a device with dimensions of 5/1500 µm (electrode gap/electrode length), while only a single-step resistance switching was observed in a device with dimensions of 10/10 µm. Each step in the multistep resistance switching occurred within 100 ns and exhibited a time-independent constant value. Optical microscope observations of a filamentary current path in a VO2 layer imply that initial filamentary current path formation and its development are responsible for the multistep resistance switching. Temperature-controlled X-ray diffraction (XRD) measurements suggested the coexistence of regions with a variety of transition temperatures, which is considered to be closely related to the appearance of the multistep resistance switching. The multistep resistance switching in the VO2-based planar device is promising not only for functional electronic device applications but also as a fundamental research for switching mechanism in oxide materials.

Preparation of VO2 films with metal-insulator transition on sapphire and silicon substrates by inductively coupled plasma-assisted sputtering

We have achieved single phase crystalline VO2 film growth on sapphire (0001) and Si(100) substrates by the introduction of inductively coupled plasma (ICP)-assisted sputtering with an internal coil. The VO2 film on Si substrate showed a metal–insulater (M–I) transition at a temperature of 60°C with three orders of change in resistivity, with a small hysteresis width of 2.2°C for temperature. On the other hand, we could not eliminate vanadium oxides with excess oxygen such as V2O5 and V3O7 from stoichiometric VO2 in conventional reactive magnetron sputtering, regardless of the control maintained over the deposition conditions. The growth of VO2 film under a relatively wide range of deposition conditions in ICP-assisted sputtering is expected to aid in the development of integrated devices based on the M–I transition that is due to strong electron correlation.

Effect of heating probe on reactively sputtered TiO2 film growth

In this paper we propose a simple method to ionize sputtered neutral Ti and added oxygen gas, by inserting a heating probe into the plasma space. The effect of bias voltage applied to the heating probe on the crystalline structure and optical properties was investigated for the TiO2 films deposited by reactive sputtering using this method. In the films deposited with the heating probe applying positive bias voltage to the substrate, the rutile phase was grown on the Si(111) substrate at a low temperature, and this film showed better optical properties than the films prepared without bias voltage. These changes in crystalline and optical properties were attributed to the ionization and acceleration of Ti and oxygen particles.

Stress-induced VO2 films with M2 monoclinic phase stable at room temperature grown by inductively coupled plasma-assisted reactive sputtering

We report on growth of VO2 films with M2 monoclinic phase stable at room temperature under atmospheric pressure. The films were grown on quartz glass and Si substrates by using an inductively coupled plasma-assisted reactive sputtering method. XRD-sin2Ψ measurements revealed that the films with M2 phase are under compressive stress in contrast to tensile stress of films with M1 phase. Scanning electron microscopy observations revealed characteristic crystal grain aspects with formation of periodical twin structure of M2 phase. Structural phase transition from M2 to tetragonal phases, accompanied by a resistance change, was confirmed to occur as the temperature rises. Growth of VO2 films composed of M2 phase crystalline is of strong interest for clarifying nature of Mott transition of strongly correlated materials.

Radio frequency substrate biasing effects on the insulator-metal transition behavior of reactively sputtered VO2 films on sapphire (001)

Comprehensive studies have been performed on the effects of radio frequency (rf) substrate biasing on insulator–metal transition (IMT) properties of VO2 thin films grown on Al2O3 (001) substrates. As the rf substrate bias power increased, the ion energy during deposition increased, inducing a strong in-plane stress conversion and a modification of the rutile c-axis (cR-axis) length in the VO2 films. The reduced cR-axis length significantly lowered the IMT temperature. Increased IMT sharpness via improved crystallinity was obtained with rf substrate biasing at an appropriate power. Temperature coefficient of resistance at room temperature was comparable with reported values for impurity-doped VO2 films. Thus, the rf substrate biasing during reactive sputtering has great potential to control stress in thin films, which could finally control the IMT in oriented VO2 films.

Pulsed laser-deposited VO2 thin films on Pt layers

VO2 films were deposited on Pt (111)/TiO2/SiO2/Si (001) substrates by means of a pulsed laser deposition technique. An x-ray diffraction peak at 2θ = 39.9° was deconvoluted into two pseudo-Voigt profiles of Pt (111) and VOx-originated components. The VOx diffraction peak was more obvious in a VOx/Pt (111)/Al2O3 (0001) sample, having a narrower width compared with a VO2/Al2O3 (0001) sample. Temperature-controlled Raman spectroscopy for the VOx/Pt/TiO2/SiO2/Si sample has revealed the monoclinic VO2 phase at low temperature and the structural phase transition at about 72 °C in a heating process. The electronic conductive nature at the high temperature phase was confirmed by near normal incidence infrared reflectivity measurements. Out-of-plane current-voltage characteristics showed an electric field-induced resistance switching at a voltage as low as 0.2 V for a 50 nm-thick film. A survey of present and previous results suggests an experimental law that the transition voltage of VO2 is proportional to the sq...

Mass and Energy Analyses of Substrate-incident Ions in TiO 2 Deposition by RF Magnetron Sputtering

Mass-resolved measurements of substrate-incident ion (SII) current and ion kinetic energy distributions (IEDs) were carried out for TiO2 deposition by rf (13.56 MHz) magnetron sputtering. Substrate-incident ion current was mainly composed of Ar+, O2+ and O+. Several minor ions, such as Ti+,TiO+, ArO+ and Ar2+ were also detected. The dominant species of substrate-incident ion was O2+ at a total pressure higher than 20 mTorr, while Ar+ was dominant one at a total pressure of 2 mTorr with which rutile TiO2 grew on a non-heated substrate. Variations in IEDs for Ar+ and O2+ against total gas pressure, oxygen flow rate and applied rf power were presented. The energy with peak intensity in IED and the mean ion energy increased with both decreasing pressure and increasing rf power. Strong correlation between IEDs and the growth of crystallized TiO2 were observed. Finally we discussed the growth condition of rutile TiO2 films based on the results including the dc self-bias voltage.