Qing-Yuan Cai

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.

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.

Application of image spectrometer to in situ infrared broadband optical monitoring for thin film deposition

A path-folded infrared image spectrometer with five sub-gratings and five linear-array detectors was applied to a broadband optical monitoring (BOM) system for thin film deposition. Through in situ BOM, we can simultaneously acquire the thickness and refractive index of each layer in real time by fitting the measured spectra, and modify the deposition parameters during deposition process according to the fitting results. An effective data processing method was proposed and applied in the BOM process, and it shortened the data processing time and improved the monitoring efficiency greatly. For demonstration, a narrow band-pass filter (NBF) at 1540 nm with ~10 nm full width at half-maximum (FWHM) had been manufactured using the developed BOM system, and the results showed that this BOM method was satisfying for monitoring deposition of thin film devices.

Path-folded infrared spectrometer consisting of 10 sub-gratings and a two-dimensional InGaAs detector

A new compact infrared spectrometer without any mechanical moving elements has been designed and constructed using a two-dimensional InGaAs array detector and 10 sub-gratings. The instrument is compact, with a double-folded optical path configuration. The spectra are densely 10-folded to achieve 0.07-nm spectral resolution and a 2-ms data acquisition time in the 1450- to 1650-nm wavelength region, making the instrument useful for real-time spectroscopic data analyses in optical communication and many other fields.

Optical constants of e-beam evaporated and annealed Nb2O5 thin films with varying thickness

Niobium pentoxide (Nb2O5) films with different thicknesses were prepared using the electron beam evaporation method and then annealed at temperatures from 300 °C to 800 °C before being characterized by variable-angle spectroscopic ellipsometry, x-ray diffraction, etc. The results showed that the optical constants and microstructures of Nb2O5 films exhibit a strong dependence on the annealing temperature. In the visible light region, the refractive indices show a positive correlation with the annealing temperature from 300 °C to 600 °C, but a negative correlation from 600 °C to 800 °C. The amorphous Nb2O5 film converts into TT- Nb2O5 (pseudo-hexagonal) after annealing at 500–600 °C, and into T-Nb2O5 (orthorhombic) after annealing at 700–800 °C.

Tunable optical properties of co-sputtered Ti-SiO 2 nanocomposite thin films

In this work, optical constants of direct current (DC) and radio frequency (RF) co-sputtered Ti-SiO2 nanocomposite thin films were investigated and tuned by controlling the deposition power of Ti in the SiO2 host matrix. X-ray photoelectron spectroscopy (XPS) results confirmed that the metallic Ti was completely oxidized into different titanium oxide states while the Si4+ state was reduced to the Si2+ or Si0 state by observing the Ti 2p, O 1s and Si 2p line shapes changing under different deposition conditions. The optical constants of the composites were characterized with a spectroscopic ellipsometer (SE) and reduced by using the modified harmonic oscillator approximation (HOA) model. The results show that the metal-dielectric nanocomposite will have an advantage over natural materials because its optical properties of n and k can be properly tuned by adjusting the concentration of Ti in the Ti-SiO2 nanocomposites, thus satisfying the requirements of photonic device design and applications in broad spectral regions.

Design of non-polarizing cut-off filters based on dielectric-metal-dielectric stacks

Cut-off filters are usually operating at oblique incidence and exhibit polarization dependence properties. We propose a simple approach to design cut-off filters with low linear polarization sensitivity (LPS) based on dielectric-metal-dielectric (DMD) stacks. The designing method is derived from the theory of optical film characteristic matrix. The admittance loci of the film are adjusted to achieve similar spectral properties of s- and p-polarized light at oblique incidence. Different film structures are designed non-polarizing at different angles of incidence with the method. The results show that the designing method is efficient for designing non-polarizing cut-off filters, which are widely used in non-polarizing optical system.

Study of the new ellipsometric measurement method using integrated analyzer in parallel mode

A new type of ellipsometer using an integrated analyzer composed of 12 sub-analyzers with different azimuth angles was constructed and studied. By using a two-dimensional CCD array camera to measure the light intensity emerging in parallel from each sub-analyzer with the azimuth angles uniformly distributed in the range of about 180 degrees , the ellipsometric parameters were extracted within the data acquisition time less than 1 second. The ellipsometric parameters for the polished bulk Si sample were measured to show good agreement with the results measured by using another two ellipsometric methods. The new method having the merits of high speed and reliability in the optical data measurement can be potentially used in the fields where the in situ data acquisition with high precision is the key issue as required.

Edge Filter Coating With Direct Optical Monitoring Using Reciprocal of Transmittance

A method of direct optical monitoring using reciprocal of transmittance is proposed with thickness accuracy investigated. Using this method, a long-pass filter consisting of four different quarter-wave stacks of dielectric layers was fabricated.

An optical monitoring method for depositing dielectric layers of arbitrary thickness using reciprocal of transmittance

An approach extracting information of both optical monitoring signal and phase thickness of deposited layer on a trace diagram is proposed. Realtime fitting and calculation are performed to get both practical thickness and refractive index of deposited layer with the assist of quartz crystal monitoring for keeping steady rate of deposition. Monitoring error of thickness using this approach is analyzed. It was used to obtain the refractive indices and thickness of Ge layer and SiO layer in in situ measurement mode, and the results were compared with those of ex-situ spectral measurement using infrared spectrometer. The effectiveness of the proposed monitoring method was verified by fabricating narrow bandpass filter consisting of quarter-wave and non-quarter-wave layers.