The tuning of structural, optical, and electrical properties of Al-doped ZnO films deposited by atomic layer deposition technique is reported in this work. With the increasing Al doping level, the evolution from (002) to (100) diffraction peaks indicates the change in growth mode of ZnO films. Spectroscopic ellipsometry has been applied to study the thickness, optical constants, and band gap of AZO films. Due to the increasing carrier concentration after Al doping, a blue shift of band gap and absorption edge can be observed, which can be interpreted by Burstein-Moss effect. The carrier concentration and resistivity are found to vary significantly among different doping concentration, and the optimum value is also discussed. The modulations and improvements of properties are important for Al-doped ZnO films to apply as transparent conductor in various applications.

The aluminum oxide (Al₂O₃) thin films with various thicknesses under 50 nm were deposited by atomic layer deposition (ALD) on silicon substrate. The surface topography investigated by atomic force microscopy (AFM) revealed that the samples were smooth and crack-free. The ellipsometric spectra of Al₂O₃ thin films were measured and analyzed before and after annealing in nitrogen condition in the wavelength range from 250 to 1,000 nm, respectively. The refractive index of Al₂O₃ thin films was described by Cauchy model and the ellipsometric spectra data were fitted to a five-medium model consisting of Si substrate/SiO₂ layer/Al₂O₃ layer/surface roughness/air ambient structure. It is found that the refractive index of Al₂O₃ thin films decrease with increasing film thickness and the changing trend revised after annealing. The phenomenon is believed to arise from the mechanical stress in ALD-Al₂O₃ thin films. A thickness transition is also found by transmission electron microscopy (TEM) and SE after 900°C annealing.

We report a structure with 4 thin film layers composed of pure metal and dielectric materials and prepared by sputtering. The reflectance and transmittance are lower than 5% with the absorption to be achieved higher than 95% in the 400–1000nm wavelength region as match to the solar radiance spectrum. The thermal emittance of the structure is in the range of 0.063–0.10 through data analysis. The good reproducibility and stability of spectral data associated with the deposition process imply the advantage of the solar energy absorber which is cost-effective in application.

Optical properties and thermal stability of the solar selective absorber based on the metal/dielectric four-layer film structure were investigated in the variable temperature region. Numerical calculations were performed to simulate the spectral properties of multilayer stacks with different metal materials and film thickness. The typical four-layer film structure using the transition metal Cr as the thin solar absorbing layer [SiO₂(90nm)/Cr(10nm)/SiO₂(80nm)/Al (≥100nm)] was fabricated on the Si or K9 glass substrate by using the magnetron sputtering method. The results indicate that the metal/dielectric film structure has a good spectral selective property suitable for solar thermal applications with solar absorption efficiency higher than 95% in the 400-1200nm wavelength range and a very low thermal emittance in the infrared region. The solar selective absorber with the thin Cr layer has shown a good thermal stability up to the temperature of 873K under vacuum atmosphere. The experimental results are in good agreement with the calculated spectral results.

Titanium dioxide (TiO2) ultrathin films with different thicknesses below 20 nm were grown by atomic layer deposition (ALD) on silicon substrates at 300 °C. Spectroscopic ellipsometry (SE) measurements were operated to investigate the effect of thickness on the optical properties of ultrathin films in the spectra range from 200 to 1000 nm with Forouhi–Bloomer (F-B) dispersion relation. It has been found that the refractive index and extinction coefficient of the investigated TiO2 ultrathin film increase while the band gap of TiO2 ultrathin film decreases monotonically with an increase in film thickness. Furthermore, with the purpose of studying the temperature dependence of optical properties of TiO2 ultrathin film, the samples were annealed at temperature from 400 to 900 °C in N2 atmosphere. The crystalline structure of deposited and annealed films was deduced by SE and supported by X-ray diffraction (XRD). It was revealed that the anatase TiO2 film started to transform into rutile phase when the annealing temperature was up to 800 °C. In this paper, a constructive and effective method of monitoring the phase transition in ultrathin films by SE has been proposed when the phase transition is not so obvious analyzed by XRD.

The BiFeO₃ (BFO) thin film was deposited by pulsed-laser deposition on SrRuO₃ (SRO)-buffered (111) SrTiO₃ (STO) substrate. X-ray diffraction pattern reveals a well-grown epitaxial BFO thin film. Atomic force microscopy study indicates that the BFO film is rather dense with a smooth surface. The ellipsometric spectra of the STO substrate, the SRO buffer layer, and the BFO thin film were measured, respectively, in the photon energy range 1.55 to 5.40 eV. Following the dielectric functions of STO and SRO, the ones of BFO described by the Lorentz model are received by fitting the spectra data to a five-medium optical model consisting of a semi-infinite STO substrate/SRO layer/BFO film/surface roughness/air ambient structure. The thickness and the optical constants of the BFO film are obtained. Then a direct bandgap is calculated at 2.68 eV, which is believed to be influenced by near-bandgap transitions. Compared to BFO films on other substrates, the dependence of the bandgap for the BFO thin film on in-plane compressive strain from epitaxial structure is received. Moreover, the bandgap and the transition revealed by the Lorentz model also provide a ground for the assessment of the bandgap for BFO single crystals. PACS codes: 78.67.-n, 78.20.-e, 07.60.Fs