Yu-Xiang Zheng

Optical properties of cubic Ti3N4, Zr3N4, and Hf3N4

A systematic theoretical study is presented for the electronic, mechanical, and optical properties of cubic Ti3N4, Zr3N4, and Hf3N4 with the Th3P4 structure in the framework of density functional theory. The calculated band structures of Ti3N4, Zr3N4, and Hf3N4 show the indirect band gaps of 0.268, 0.909, and 1.00eV, respectively. Furthermore, the optical properties for all three materials were calculated and analyzed in detail. The calculated results are well consistent with available experimental data. Also, it is shown that all these materials have relatively large static dielectric constants at zero frequency, rendering them potential applications in microelectronic devices.

High solar absorption of a multilayered thin film structure

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.

Influence of nanocrystal size on dielectric functions of Si nanocrystals embedded in SiO2 matrix

The complex dielectric functions of Si-nanocrystals (nc-Si) with different sizes embedded in SiO2 matrix synthesized by SiOx/SiO2 superlattice approach is obtained by spectroscopic ellipsometry. The Maxwell–Garnett effective medium approximation and the Lorentz oscillator model are employed in the spectra fitting. The dependence of the dielectric functions on the nc-Si size is observed. A significant suppression in amplitude of the dielectric functions with respect to bulk crystalline silicon, and a large influence of the nc-Si size on the E1 and E2 critical points are observed and discussed.

Effects of Al Doping on the Properties of ZnO Thin Films Deposited by Atomic Layer Deposition

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.

Nano-Cr-film-based solar selective absorber with high photo-thermal conversion efficiency and good thermal stability

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 [SiO(2)(90nm)/Cr(10nm)/SiO(2)(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.

Evolution of optical constants of silicon dioxide on silicon from ultrathin films to thick films

A series of SiO2 films with thickness range 1–600 nm have been deposited on crystal silicon (c-Si) substrates by electron beam evaporation (EBE) method. Variable-angle spectroscopic ellipsometry (VASE) in combination with a two-film model (ambient-oxide-interlayer substrate) was used to determine the optical constants and thicknesses of the investigated films. The refractive indices of SiO2 films thicker than 60 nm are close to those of bulk SiO2. For the thin films deposited at the rate of ~1.0 nm s−1, the refractive indices increase with decreasing thickness from ~60 to ~10 nm and then drop sharply with decreasing thickness below ~10 nm. However, for thin films deposited at the rates of ~0.4 and ~0.2 nm s−1, the refractive indices monotonically increase with decreasing thickness below 60 nm. The optical constants of the ultrathin film depend on the morphology of the film, the stress exerted on the film, as well as the stoichiometry of the oxide film.

Optical transition properties of β‐FeSi2 film

Optical transition of properties of β‐FeSi2 film have been investigated by optical transmittance absorption measurement, spectroscopic ellipsometry and reflectivity. Optical transmittance absorption measurement ranging from 0.5 to 1.1 eV revealed the direct transition at E0=0.84 eV, while absorption curve obtained from the spectroscopic measurement in the range of 1.5–4.5 eV implied additional transition at E’=1.05 eV, it is suggested that such additional transition originates from spin–orbit splitting at Γ (center of Brillouin’s zone). Reflectivity goes to the maximum value near Eg, indicating that the joint density of states in the transition at Eg is very high. Another peak which is related with the transition at E’ was also observed in the reflectivity spectra.

Densely folded spectral images of the CCD spectrometer working in the full 200-1000nm wavelength range with high resolution.

A new charge-coupled device (CCD) spectrometer has been studied and constructed by using a two-dimensional CCD detector and an integrated grating consisting of 10 subgratings. Effective spectral images of 268 mm along the dispersion direction have been densely folded 10 times to cover the full 200-1000 nm working wavelength range without any mechanical moving elements. The results show that the system has a spectral resolution and acquisition time of better than 0.07 nm and less than 100 ms, respectively, in the entire spectral range after system calibration.

New design of the variable angle infrared spectroscopic ellipsometer using double Fourier transforms

A new type of incidence-angle-variable infrared spectroscopicellipsometer working in the 2100–10 000 wave number range has been designed and constructed. For the system, the analyzer and polarizer were driven by two microstepping motors having hollow shafts and rotating synchronously with a speed ratio of 2:1, i.e., A=2P. The incidence angle can be varied from 30° to 90° with an accuracy of 0.01°. The doubled Fourier transforms as both functions of the wave number and the azimuthal angles of the polarizer and analyzer were carried out and integrated in the system. Two sets of ellipsometric parameters measured in the experiment have been used to test the data self-consistency of the system. In data reduction, the error arising from the slight anisotropy of the stray light was corrected. In application, the complex dielectric function of the Au film was measured with a data accuracy better that 1% in the entire spectral range.

Phase change characteristics of aluminum doped Ge2Sb2Te5 films prepared by magnetron sputtering

Aluminum-doped Ge(2)Sb(2)Te(5) (Al(x)GST) films were deposited on Si(100) substrates by co-magnetron sputtering system. The Aluminum concentrations in these films are determined by X-ray photoelectron spectroscopy (XPS). The influence of Al doping upon phase change characteristics of these Al(x)GST alloy films has been investigated by X-ray diffraction (XRD) and a temperature-regulable UVISEL(TM) typed spectroscopic ellipsometry (TRSE). With the augment of Al doping concentration, the crystalline temperatures of Al(x)GST films went up while annealing, and the face-centered-cubic (fcc) phase had high thermal stability. The reflectivity contrast of the films increases obviously, which is effective to improve the signal to noise ratio (SNR) for optical phase-change storage.