Characterization of wafer-scale MoS2 and WSe2 2D films by spectroscopic ellipsometry

Here, we present the spectroscopic ellipsometry investigation of synthetically grown wafer-scale two-dimensional (2D) MoS₂ and WSe₂ films to access quality and thickness uniformity. MoS₂ and WSe₂ samples were grown by chemical vapor deposition and atomic layer deposition, respectively. Complex dielectric function ($\epsilon ={\epsilon }_1+i{\epsilon }_2$) and thickness information of these 2D films were extracted from the measured data using multilayer optical calculations. Broad spectral range (1.2–6 eV) and multiple angles of incidence were used to reduce correlations among fitting parameter. Lineshape of ε of MoS₂ and WSe₂ monolayer films are consistent with literature but shows higher values, suggests better quality of our samples. Eight-inch wafer size MoS₂ monolayer sample shows ～ 70% uniformity with an average thickness of 0.65 ± 0.2 nm, and three-layer WSe₂ sample of 8 × 1 cm² area shows ～ 80% uniformity with an average thickness of 2.5 ± 0.4 nm. Our results will be helpful to accelerate commercialization process of 2D devices.     

AlN:Cr thin films synthesized by pulsed laser deposition: Studies by X-ray diffraction and spectroscopic ellipsometry

The structure and optical properties of AlN thin films doped with Cr atoms were studied by X-ray diffractometry, Fourier transform infrared spectroscopy and spectroscopic ellipsometry analyses. The films were synthesized by pulsed laser deposition from an AlN:Cr (10% Cr) target onto Si(1 0 0) wafers in vacuum at residual pressure of 10⁻³ Pa or in nitrogen at a dynamic pressure of 0.1 Pa. The study of the XRD patterns revealed that both phases co-existed in the synthesized films and that the amorphous one was prevalent. Two different amorphous matrices, i.e. two types of chemical bond arrangements, were found in films deposited at 0.1 Pa N₂. By difference, deposition in vacuum resulted in the coexistence of hexagonal and cubic crystallites embedded into an amorphous matrix. The introduction of Cr atoms into the AlN lattice causes a broadening of the IR spectrum along with the shift toward higher wavenumbers of the characteristic Al-N bands at 2351 cm⁻¹ and 665 cm⁻¹, respectively. This was related to the generation of a compressive stress inside films. In comparison to the optical constants of pure AlN films, the synthesized AlN:Cr films exhibited a smaller refractive index and showed a weak absorption throughout the 300-800 nm spectral region, characteristic to amorphous AlN structure.     

Thermally tunable optical constants of vanadium dioxide thin films measured by spectroscopic ellipsometry

Smart materials with reversible tunable optical constants from visible to near-infrared wavelengths could enable excellent control over the resonant response in metamaterials, tunable plasmonic nanostructures, optical memory based on phase transition and thermally tunable optical devices. Vanadium dioxide (VO₂) is a promising candidate that exhibits a dramatic change in its complex refraction index or complex dielectric function arising from a structural phase transition from semiconductor to metal at a critical temperature of 70 °C. We demonstrated the thermal controllable reversible tunability of optical constants of VO₂ thin films. The optical/dielectric constants showed an abrupt thermal hysteresis which confirms clearly the electronic structural changes. Temperature dependence of dielectric constants as well as optical conductivity of sputtered VO₂ thin films was also reported and compared to previous theoretical and experimental reports.     

FTIR and spectroscopic ellipsometry investigations of the electron beam evaporated silicon oxynitride thin films

FTIR and variable angle spectroscopic ellipsometer in conjunction with computer simulation were employed to investigate the electron beam evaporated SiO_xN_y thin films. FTIR showed a large absorption band located between 600 and 1250 cm⁻¹, which indicates that Si-O and Si-N bands are overlap in SiO_xN_y films. A three layers model was used to fit the calculated data to the experimental ellipsometric spectra. The main layer was described by Cauchy model while the interface layer and the surface layer were described using Tauc-Lorenz oscillator and Bruggeman effective medium approximation, respectively. The thickness, the refractive index and the extinction coefficient were accurately determined. The refractive index at 630 nm was found to increase from 1.74 to 1.85 with increasing the film thickness from 191.6 to 502.2 nm. The absorption coefficient was calculated from the obtained extinction coefficient values and it has been used to calculate the Tauc and Urbach energies.     

Observation of strong light-matter coupling by spectroscopic ellipsometry

We report on the observation of strong coupling between excitons and cavity photons in a ZnO-based microresonator up to room temperature. The ZnO-based resonator was grown by means of pulsed laser deposition (PLD) on c-sapphire substrate and consists of a half-wavelength ZnO-cavity between two Bragg reflectors, each made of 10.5 layer pairs of yttria stabilized zirconia and Al₂O₃. Angle-resolved spectroscopic ellipsometry revealed the resonator to be in the strong coupling regime at room temperature. This was confirmed for temperatures between 10 K and 290 K by means of angle-resolved photoluminescence and reflection experiments. Prior studies on a Fabry–Perot resonator containing a half-wavelength YSZ-cavity (empty resonator) demonstrate the ability of ellipsometry to gain comprehensive information on mode-structure properties of resonators without coupling effects.     

Quantifying protein adsorption on combinatorially sputtered Al-, Nb-, Ta- and Ti-containing films with electron microprobe and spectroscopic ellipsometry

Although metallic biomaterials are widely used, systematic studies of protein adsorption onto such materials are generally lacking. Combinatorial binary libraries of Al_1−xNb_x, Al_1−xTa_x, Al_1−xTi_x, Nb_1−xTa_x, Nb_1−xTi_x, and Ta_1−xTi_x (0 ? x ? 1) and a ternary library of Al_1−xTi_xTa_y (0 ? x ? 1 and 0 ? y ? 0.7), along with their corresponding pure element films were sputtered onto glass substrates using a unique magnetron sputtering technique. Films were characterized with wavelength-dispersive spectroscopy (WDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Fibrinogen and albumin adsorption amounts were measured by wavelength-dispersive spectroscopy (WDS) and spectroscopic ellipsometry (SE) equipment, both high throughput techniques with automated motion stage capabilities. Protein adsorption onto these films was found to be closely correlated to the alumina surface fraction, with high alumina content at the surface leading to low amounts of adsorbed fibrinogen and albumin. Protein adsorption amounts obtained with WDS and SE were in good agreement for all films.     

Optical arrangement and proof of concept prototype for mid infrared variable angle spectroscopic ellipsometry

We present an optical setup for variable angle mid infra red spectroscopic ellipsometry. The arrangement can be placed into the sample compartment of a Bruker ifs66v/s vacuum Fourier transform infrared spectrometer. A first prototype of the setup has been tested in the spectral range from 650 cm⁻¹ to 4000 cm⁻¹ and can measure incidence angles between 8° and 87°. We compare the measured data to reference measurements with a commercial variable angle infrared spectroscopic ellipsometer. The comparison gives a proof of concept for the discussed optical arrangement.     

Nanostructure characterization of high k materials by spectroscopic ellipsometry

In this work, the optical and structural properties of high k materials such as tantalum oxide and titanium oxide were studied by spectroscopic ellipsometry, where a Tauc-Lorentz dispersion model based in one (amorphous films) or two oscillators (microcrystalline films) was used. The samples were deposited at room temperature by radio frequency magnetron sputtering and then annealed at temperatures from 100 to 500 °C. Concerning the tantalum oxide films, the increase of the annealing temperature, up to 500 °C does not change the amorphous nature of the films, increasing, however, their density. The same does not happen with the titanium oxide films that are microcrystalline, even when deposited at room temperature. Data concerning the use of a four-layer model based on one and two Tauc-Lorentz dispersions is also discussed, emphasizing its use for the detection of an amorphous incubation layer, normally present on microcrystalline films grown by sputtering.     

Effect of oxygen partial pressure on the structural and optical properties of ZnO film deposited by reactive sputtering

Effects of variation of the oxygen partial pressure on the structural and optical properties of zinc oxide (ZnO) thin films prepared by reactive radio-frequency sputtering were investigated. Measurements by X-ray diffraction (XRD) and atomic force microscopy (AFM) indicated that the crystallinity and the surface morphology were sensitive to the oxygen partial pressure. The interfacial and optical properties of the targeted films were investigated by spectroscopic ellipsometry (SE) characterization. Based on Tauc-Lorentz (TL) model, the optical constants of ZnO films were tentatively extracted in the photon energy ranging from 1.5 to 6.0 eV. Analyses by XRD and SE revealed that the oxygen partial pressure had effect on the orientation of the ZnO films, the surface morphology, the packing density, and the interfacial layers. And the relationship between crystallinity and interfacial layer, as well as the relationship between surface roughness and packing density was discussed. All these had a significant impact on the optical properties illustrated by SE analysis.     

金刚石薄膜的红外椭圆偏振光谱研究

采用红外椭圆偏振光谱对微波等离子体化学气相沉积法（MPCVD）和热丝化学气相沉积法（H-FCVD）制备的金刚石薄膜在红外波长范围（2.5—12.5μm）的光学参数进行了测量.建立了不同的光学模型，且在模型中采用Bruggeman有效介质近似方法综合考虑了薄膜表面和界面的椭偏效应.结果表明，MPCVD金刚石膜的椭偏数据在模型引入了厚度为77.5nm的硅表面氧化层、HFCVD金刚石膜引入879nm粗糙层之后能得到很好的拟合.最后对两种模型下金刚石薄膜的折射率和消光系数进行了计算，表明MPCVD金刚石薄膜的红外 关键词： 金刚石薄膜 红外椭圆偏振光谱 光学参数 有效介质近似     

Investigation of the optical anisotropy of PET and PEN films by VIS-FUV to IR spectroscopic ellipsometry

In the last years, a significant amount of research is being performed in the field of polymer research for novel applications, such as flexible electronic devices, photovoltaic cells, high performance optics, data storage, etc. Toward this direction, in this work, the optical anisotropy of biaxially stretched poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) films has been extensively investigated. The optical properties of the films have been studied in terms of their optical, electronic and vibrational response, by Fourier transform IR spectroscopic ellipsometry (FTIRSE) (900-3500 cm⁻¹) and Vis-fUV variable angle SE (1.5-6.5 eV) techniques. The films optical anisotropy is the result of the stretching procedure during their fabrication, which results to the structural rearrangement of the macromolecular chains parallel to the stretching direction and to a higher structural symmetry. During the SE spectra analysis, the films have been approximated as uniaxial materials with their optic axis parallel to the sample/ambient interface leading to the accurate determination of the principal components ?_||(ω) and ?_⊥(ω) of the dielectric function ?(ω). The detailed study of the electronic transitions has been performed in the Vis-fUV region, where the characteristic features corresponding to the n → π* electronic transitions of the carbonyl -CO group and the ¹A_1g → ¹B_1u transition due to the π → π* excitation of the π-electron structures have been identified and analysed. Furthermore, the FTIRSE spectra allowed the accurate identification and assignment of the features of ?(ω) to the vibrational modes of the various bonding structures characteristic of the PET and PEN macromolecular chains.     

Structural and optical characterization of DC magnetron sputtered molybdenum oxide films

Thin films of molybdenum trioxide were deposited on glass substrates employing direct current (DC) magnetron sputtering by sputtering of molybdenum at different oxygen partial pressures in the range 8 × 10⁻⁵–1 × 10⁻³ mbar and at a substrate temperature of 473 K. The glow discharge characteristics of magnetron cathode target of molybdenum were studied. The influence of oxygen partial pressure on the structural and optical properties of molybdenum trioxide films was investigated. The films formed at an optimum oxygen partial pressure of 2 × 10⁻⁴ mbar were polycrystalline in nature with orthorhombic α- phase and an optical band gap of 3.16 eV. The refractive index of the films formed at an oxygen partial pressure of 2 × 10⁻⁴ mbar decreased from 2.08 to 1.89 with increase of wavelength from 450 to 1,000 nm, respectively. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, Dec. 7–9, 2006.     

Spetroscopic ellipsometry study on electrical and elemental properties of Sb-doped ZnO thin films

Room-temperature spectroscopic ellipsometry data has been analyzed to determine the complex dielectric functions, ɛ(E) = ɛ₁(E) + iɛ₂(E) of as-deposited Sb-doped ZnO (SZO) thin films grown on n-Si(100) substrates by dual ion beam sputtering deposition system for different growth temperatures (T_g). The dielectric functions have been obtained from ellipsometry data analyses using Cody-Lorentz oscillator in the GenOsc model. A gradual reduction in the value of electron concentration and finally the conversion of doping characteristics from donor type to acceptor type was observed with the rise in T_g. This, in turn, resulted in the decline of broadening of ɛ₁ peaks, and hence in the increase of excitonic lifetime. Optical band-gap energy was observed to decrease with increase in T_g from 200 to 300 °C, and then rise continuously with further increase in T_g. X-ray diffraction measurements showed that all SZO films had (002) preferred crystal orientation. Hall measurement and X-ray photoelectron spectroscopy analysis confirmed that the change in the electrical conduction from n-to p-type was due to the enhancement in the value of Sb⁵⁺/Sb³⁺ ratio and Sb_Zn–2V_Zn complex formation in SZO films.     

Cu-In-O composite thin films deposited by reactive DC magnetron sputtering

Cu-In-O composite thin films were deposited by reactive DC magnetron sputtering at room temperature. The samples were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), UV/vis spectrophotometer, four-probe measurement and Seebeck effect measurement, etc. The samples contain Cu, In and O. The ratios of Cu to In and O to In increase with increase in O₂ flow rates. The ratio of Cu to In is over 1 and this suggests that Cu is in excess. The obtained Cu-In-O thin films are very possibly made of rhombohedral In₂O₃ and monoclinic CuO. Transmittance of the films decreases with increase in O₂ flow rate. The decrease in transmittance results from increase in Cu content in the films. The optical band gap of all the samples is estimated to be 4.1-4.4 eV, which is larger than those of In₂O₃ and CuO. The sheet resistance of the films decreases with increase in O₂ flow rate. Conductivity of the films is a little low, due to the addition of Cu and the poor crystalline quality of the film. The conduction behavior of the films is similar to that of In₂O₃ and the conduction mechanism of Cu-In-O thin films is through O vacancy.     

Vacuum ultra-violet spectroscopic ellipsometry study of sputtered BeZnO thin films

This paper reports on a systematic investigation of the optical properties of BeZnO thin films fabricated by radio frequency reactive magnetron sputtering technique using vacuum ultraviolet spectroscopic ellipsometry (VUV-SE). The thicknesses and optical constants of the thin films were determined in the wavelength range 138–1650 nm, using VUV-SE through the Tauc–Lorentz and Gaussian models. Refractive indices and extinction coefficients of the thin films were determined to be in the range n = 1.58–1.99 and κ = 1.0 × 10⁻²⁷–0.37, respectively. The absorption coefficient and the optical bandgap energy were then calculated. Measurement of the polarized optical properties reveals a high transmissivity (>90%) and very low absorptivity (<4%) for BeZnO films in the visible and near infrared regions at different angles of incidence. From the angle dependence of the p-polarized reflectivity we deduced a Brewster angle of about 58.5°.     

离子注入硅片快速退火后的红外椭偏光谱研究

离子注入硅片经高温退火后晶体结构缺陷会被修复,其在可见光波段下的光学性质趋于单晶硅,常规的可见光椭偏光谱法对掺杂影响的测量不再有效.本研究将测量波段扩展到红外区域(2—20μm),报道了利用红外椭偏光谱法测量经离子注入掺杂并高温退火的硅片掺杂层光学和电学性质的方法和结果.通过建立基于Drude自由载流子吸收的等效光学模型,得到了杂质激活后掺杂层的杂质浓度分布、电阻率和载流子迁移率等电学参数,以及掺杂层的红外光学常数色散关系,分析了这些参数随注入剂量的关系并对其物理机理给予了解释.研究表明,中远红外椭偏测量是表征退火硅片的有效方法,且测量波长越长,所能分辨的掺杂浓度越低.     

Real time ellipsometry for monitoring plasma-assisted epitaxial growth of GaN

GaN is grown on Si-face 4H-SiC(0 0 0 1) substrates using remote plasma-assisted methods including metalorganic chemical vapour deposition (RP-MOCVD) and molecular beam epitaxy (MBE). Real time spectroscopic ellipsometry is used for monitoring all the steps of substrate pre-treatments and the heteroepitaxial growth of GaN on SiC. Our characterization emphasis is on understanding the nucleation mechanism and the GaN growth mode, which depend on the SiC surface preparation.     

Optimization of parameters for deposition of Ga-doped ZnO films by DC reactive magnetron sputtering using Taguchi method

Ga-doped ZnO (ZnO:Ga) transparent conductive films were deposited on glass substrates by DC reactive magnetron sputtering. Taguchi method was used to find the optimal deposition parameters including oxygen partial pressure, argon partial pressure, substrate temperature, and sputtering power. By employing the analysis of variance, we found that the oxygen and argon partial pressures were the most influencing parameters on the electrical properties of ZnO:Ga films. Under the optimized deposition conditions, the ZnO:Ga films showed acceptable crystal quality, lowest electrical resistivity of 2.61 × 10⁻⁴ Ω cm, and high transmittance of 90% in the visible region.