Zhongping Wang

Investigation of TiO2-doped HfO2 thin films deposited by photo-CVD

Abstract TiO 2 -doped HfO 2 thin films, as potential replacements for SiO 2 as high- k gate dielectric material, have been grown by photo-induced CVD using 222 nm excimer lamps at a temperature of 400 °C. Vaporised titanium isopropoxide and hafnium (IV) tetra- t -butoxide were used as the precursors. Films from approximately 45–70 nm in thickness with refractive indices from 1.850 to 2.424 were grown with various Ti:Hf ratios. The as-deposited films were found to be amorphous by X-ray diffraction when the Ti/(Ti+Hf) value was up to 33%, while the crystalline TiO 2 anatase phase formed when the Ti/(Ti+Hf) was 41%. We also found that the refractive index increased sharply when the Ti/(Ti+Hf) was over 25%. Fourier transform infrared spectroscopy, XPS and TEM were also used to monitor as well as the presence of Ti, interface and microstructure of the films on Si-substrate. The effect of UV-annealing on the electrical properties of these films will also be discussed.

Characterisation of HfO2 deposited by photo-induced chemical vapour deposition

Abstract The deposition of thin Hafnia (HfO2) films on crystalline Si by photo-induced chemical vapour deposition (CVD) using 222 nm excimer lamps at temperatures between 300 and 450 °C is presented. Hafnium (IV) tetra-t-butoxide was used as the precursor, which was carried into the photochemical reaction chamber by flowing Ar carrier gas at a pressure of several mbar using a special precursor injection system. Films from approximately 6–70 nm in thickness with refractive indices from 1.60 to 1.85 were grown. The deposition rate measured by ellipsometry was found to be 15 nmxa0min−1 at a temperature of 400 °C. The deposited HfO2 films were characterised by various techniques. XRD showed that as-deposited HfO2 films were basically amorphous. Fourier transform infrared spectroscopy, revealed that Hf-O absorption in the photo-CVD deposited HfO2 films is quite different at various deposition parameters. The substrate temperature, system pressure and UV-annealing dependence of the physical and interfacial properties of these films will also be presented.

Thickness Dispersion of Surface Plasmon of Ag Nano-thin Films: Determination by Ellipsometry Iterated with Transmittance Method

Effective optical constants of Ag thin films are precisely determined with effective thickness simultaneously by using an ellipsometry iterated with transmittance method. Unlike the bulk optical constants in Paliks database the effective optical constants of ultrathin Ag films are found to strongly depend on the thickness. According to the optical data two branches of thickness dispersion of surface plasmon energy are derived and agreed with theoretical predication. The thickness dispersion of bulk plasmon is also observed. The influence of substrate on surface plasmon is verified for the first time by using ellipsometry. The thickness dependent effective energy loss function is thus obtained based on this optical method for Ag ultrathin films. This method is also applicable to other ultrathin films and can be used to establish an effective optical database for ultrathin films.

Hafnium oxide layers derived by photo-assisted sol–gel processing

Abstract UV irradiation of sol–gel derived HfO 2 thin films in N 2 O using excimer lamps was found to effectively remove OH groups, dissociate unreacted Hf–OH and further oxidize sol–gel layers, forming HfO 2 thin films with a high stoichiometry of 1.97. No interface oxidation and silicate formation has been observed when irradiated at 400xa0°C for 20xa0min while the films retain a desired amorphous structure. UV-irradiated sol–gel layers exhibited significantly improved optical properties, with high optical transmittances of 81–97% in the visible region of the spectrum, a high refractive index of up to 1.90 being achieved. A leakage current density as low as 5.8×10 −6 xa0Axa0cm −2 at 1xa0MVxa0cm −2 and breakdown field of larger than 5xa0cm −2 were obtained on these sol–gel layers with radiation for 20xa0min.

Photoemission study of the interaction of a Pr 2 O 3 overlayer with Si(100) as a function of annealing temperature

A layer of Pr2O3 with a thickness of about 20 A was grown on Si(100) at room temperature by evaporating Pr in an oxygen ambiance. The interaction of the Pr2O3 overlayer with Si(100) has been investigated in situ after annealing at different temperatures by means of X-ray photoelectron spectroscopy (XPS). The Pr2O3 deposition enhances the oxidation rate of the substrate. The silicon atoms can easily diffuse from the substrate into the Pr2O3 layer, forming a Pr-O-Si silicate as well as SiO2. Annealing facilitates the diffusion of the silicon atoms, leading to the continual growth of the silicate. The intensity of Pr2O3 decreases at temperatures over 590 K and the silicate becomes dominant after annealing at 790 K. Angle-dependent XPS taken after annealing at 1090 K indicates that the silicate moves to the top surface. The amount of SiO2 remains almost constant during annealing.

Upconversion luminescence of NaYF4:Yb,Er nanocrystals with high uniformity

Abstract The NaYF4:Yb,Er nanocrystals were synthesized via the thermal decomposition of metal oleate precursors. The nanocrystals in hexagonal structure were highly uniform and in size of 25 nm. The bright upconversion luminescence was observed under the excitation of 980 nm laser and the upconversion emission spectra were investigated at different pump powers. The emission intensity ratio of red light to green light linearly increased with pump power increasing. This result indicated that there existed a large threshold power of saturation pump for the first excitation state in NaYF4:Yb,Er nanocrystals comparing to that in bulk material.

High- k dielectrics by UV photo-assisted chemical vapour deposition

An overview of our recent work on thin films of metal oxides deposited on silicon by a novel excimer lamp-assisted ultraviolet injection liquid source CVD (UVILS-CVD) process for advanced high-k gate dielectrics applications will be presented. Recent results on TiO2, Ta2O5, ZrO2, HfO2, and TiO2-doped Ta2O5 will be demonstrated. The physical, structural, surface and interfacial properties and electrical characterisation of the as-deposited and UV-annealed new high dielectric constant (high-k) materials, determined using ellipsometry, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV spectrophotometry, SEM, TEM and C-V, I-V measurements, showed that good quality layers could be produced. The investigation of high-k dielectrics grown by the UVILS-CVD process clearly demonstrates that low cost, high power density excimer lamp systems can provide an interesting alternative to conventional UV lamps and excimer lasers for industrial large-scale low temperature materials processing. UVILS-CVD is a promising technique for the controlled deposition of ultra-thin high-k metal-oxide dielectrics for deep sub-micron CMOS devices at temperatures as low as 350 °C.

Structural transition of VO2 (A) nanorods studied by vibrational spectroscopies

Single crystalline metastable phase VO2 (A) nanorods were successfully synthesized by using a hydrothermal method, and the evolution of vibrational modes of VO2 (A) nanorods has been observed in the vicinity of the structural phase transition. Fourier transform infrared (FT-IR) spectroscopy was employed to study the structural phase transition, and the abrupt changes of vibrational absorption bands at different temperatures showed a reversible phase transition. Raman spectrum measurement was performed to observe firstly the temperature-induced phase transition of VO2 (A) nanorods. The Raman modes near the phase transition temperature display the phonon softening behaviour.

Photoluminescence study of SiO2 coated Eu3+:Y2O3 core-shells under high pressure

Abstract Uniform core-shell Eu 3+ :Y 2 O 3 /SiO 2 spheres were synthesized via precipitation and the Stober method. The structural transition of core-shell Eu 3+ :Y 2 O 3 /SiO 2 was studied by using high pressure photoluminescence spectra. With pressure increasing, the emission intensities of 5 D 0 → 7 F 0,1,2 transitions of Eu 3+ ions decreased and the transition lines showed a red shift. The relative luminescence intensity ratio of 5 D 0 → 7 F 2 to 5 D 0 → 7 F 1 transitions decreased with increasing pressure, indicating lowering asymmetry around Eu 3+ ions. During compression, structural transformation for cores in the present core-shell Eu 3+ :Y 2 O 3 /SiO 2 sample from cubic to monoclinic took place at 7.5 GPa, and then the monoclinic structure turned into hexagonal above 15.2 GPa. After the pressure was released, the hexagonal structure transformed back to monoclinic and the monoclinic structure was kept stable to ambient pressure.

Pressure and temperature dependent up-conversion properties of Yb3+-Er3+ co-doped BaBi4Ti4O15 ferroelectric ceramics

Abstract Yb 3+ and Er 3+ co-doped BaBi 4 Ti 4 O 15 (BBT) ceramic samples showed brighter up-conversion photoluminescence (UC-PL) under excitation of 980 nm. The monotonous increase of fluorescence intensity ratio (FIR) from 525 to 550 nm with temperature showed that this material could be used for temperature sensing with the maximum sensitivity to be 0.0046 K −1 and the energy difference was 700 cm −1 . Moreover, the sudden change of red and green emissions around 400 °C might imply a phase transition. With increasing pressure up to 4 GPa, the PL intensity decreased but was still strong enough. These results illustrated the wide applications of BBT in high temperature and high pressure conditions.