ICMCTF2013 Session C3-1: Optical Characterization Of Thin Films, Surfaces and Devices

Spectroscopic ellipsometry is a well known and established tool for various types of thin film analysis. It is used to determine for example the dielectric properties in the visible spectral range and film thicknesses directly (reference free) since many years. However, polarization optical methods in general can provide information far beyond these “conventional applications”. The very high accuracy in detecting polarization changes qualifies it for ultra thin film, sub-monolayer and even atomic surface reconstruction sensitivity. On the other hand the optical approach in general permits an application in different environments namely in ultra high vacuum (UHV), gases and in liquid environments. The combination of both capabilities gives us a very powerful tool to explore electronic properties as well as microscopic structure information of surfaces in application relevant surroundings. Together with well known complementary surface science methods like scanning tunnelling microscopy, low energy electron diffraction or photo electron spectroscopy and with the help of ab-initio model-calculations it was thus possible to elucidate the surface structures/reconstruction down to the atomic level of several semiconductors and metals under UHV conditions. Our achievements will be exemplarily demonstrated for various arsenides, copper and small organic molecules on Si. In case of these highly symmetric cubic substrates the results are mainly obtained by reflectance anisotropy spectroscopy (RAS/RDS) - a special ellipsometer/polarimeter configuration which yields a surface selective response. In case of the hexagonal III-Nitrides, which are typically grown by metal organic vapour phase epitaxy, surface terminations and reconstructions could be identified by ellipsometry even in gas-phase. Most recently polarization optical methods and scanning tunnelling microscopy were also successively applied to study metal surfaces in electrolytes. This promising approach will be demonstrated by current measurements on copper.

New refining technology of soda – calcium – silicon glassy surfaces with inorganic compounds nano-molecules has been presented in the present study. In order to determine modification of the glass surface there were carried out SEM observation and EDAX analysis. The glass samples were subjected to the UV-VIS, photo-elasticity and ellipsometry examination. From obtained results it follows, that refining glass surface by nanopowder inorganic compounds provide to form very thin surface layer. This type of modification improves physical and chemical glass properties.

The optical constants of uranium oxides are important for many applications including monitoring nuclear proliferation and domestic security. Uranium forms many different oxides in many phases, making the study of surfaces and thin films challenging. We used reactive DC magnetron sputtering to prepare uranium oxide thin films on silicon wafer over three thickness ranges, namely about 44nm, 114nm, and 413nm. The partial pressure of oxygen (in argon) was chosen to be high, 40 to 70%, to prepare totally oxidized, that is, UO₃ films. (A planetary system was used to improve sample thickness uniformity.) The x-ray diffraction pattern of the thickest film showed evidence of the presence of UO₃. We characterized the films’ thickness and optical constants via multiangle spectroscopic ellipsometry (Woollam M-2000). We were able to obtain good fits for all samples. The constants of all three films had similar shapes, showing the typical dispersive behavior of a moderate band gap metal oxide. The n of the material started at about 1.95 in the IR (1.25 eV), rose to 2.2 at 3.1 eV – the direct band of all films studied – and fell thereafter. The imaginary index, k, was much lower in the IR and over most of the visible, began rising slowly at about 2.5 eV and steeply at 3.0 eV reaching a maximum of 0.55 at about 5.5 eV. The 114 nm and 413 nm films were found to have higher apparent optical constants than those of the 44nm film suggesting it was rough or contained voids. Varying the void fraction of the 44nm film as a function of depth within the film allowed its ellipsometric data to be fit reasonably well by constants obtained the 114nm sample. Subsequent AFM showed the 44nm sample to be considerably rougher than the other two (16nm rms roughness versus 7nm for the 413 nm film.

Ellipsometry is a sensitive and non-contact method for characterization of thin films. Depending on the spectral range from far infrared (FIR) to ultraviolet (UV), ellipsometry can probe conducting, structural, molecular and electronic properties, and thickness. [1,2] Ellipsometry in the mid infrared (MIR) spectral range is meanwhile established for structural analysis of thin films. Examples range from the studies of hydrogen monolayers, silicon oxides, metamaterials, metallic nanowires, metallic island films, polymer films, protein and other ultrathin organic films. [3-11] Recently, IR ellipsometry was introduced as versatile tool for in situ studies during growth or modification of smart surfaces and films in aqueous solutions, thereby used for identification, structural analysis and quantitative determination of optical properties. [8-10]

In the linear optics approach the material-related dielectric function can be described as a sum of the fundamental excitations. Interpretation of the optical response in this approach and the description of the layered sample in an optical model can reversely deliver information on the sample. Using optical models and best-fit simulations, the measured ellipsometric spectra can be interpreted with respect to structure and thickness. By exploring the anisotropic optical properties of vibrational bands, also molecular orientations [11] can be determined.

[1] Handbook of Ellipsometry; Tompkins, H. G., Irene, E. A., Eds.; Springer: Berlin, 2005; [2] Spectroscopic Ellipsometry: Principles and Applications, Fujiwara H. Ed., Wiley (2007); [3] Röseler A. In Handbook of Ellipsometry; Tompkins, H. G., Irene, E. A., Eds.; Springer: Berlin, 2005; pp 763-797. [4] Hinrichs K. et al; Appl. Spectrosc. 59, 272A (2005); [5] Chandola S. et al, PRL 102, 226805 (2009); [6] Oates et al, Optics Express 20, 11166-11177 (2012); [7] Hinrichs K., Silaghi S. et al, Phys. Stat. Sol. 24, 2681-2687 (2005); [8 ] Mikhaylova Y. et al, Anal. Chem. 79, 7676-7682 (2007); [9] Hoy O. et al, Adv. Funct. Mat. 20, 2240 (2010); [10] Furchner A. et al, Thin Solid Films, accepted; [11] Rosu D.M. et al, Langmuir 25, 919-923 (2009).

Micro-arc oxidation (MAO) technique, with feature of the low-cost manufacturing, low environmental impact and feasibility for tailoring microstructure, is a relatively novel anodic technique for metal titanium in recent years. As has been revealed that microstructure and properties of the obtained MAO layer are strongly governed with two aspects, i.e. electrical power and electrolyte bath, it was however rarely been discovered to correlate with the discharge being created over the metal surface. Optical emission spectroscopy (OES) technique was thus employed in this study to reveal systematically how the optical-active species in the discharge rule the microstructure and properties of the obtained MAO layer that grown on metal titanium. During MAO process, pH value and power delivery mode were adjusted in a phosphate electrolyte bath, respectively and the discharge over the anodically charged titanium was monitored by an OES system.

Experimental results revealed that the anodic current is much higher in an alkaline bath than in an acidic one, indicating much abundance of the OH^‑ groups (and the associated sodium ions) participated into anode region. As a result, much higher intensity of the emission lines corresponding to the OH^‑ group and sodium was detected. Interestingly, rutile phase and Na₂Ti(PO₄)₂ phase were both detected in the obtained MAO layer, in addition to the anatase phase that formed when in an acidic bath. Moreover, only OH^‑ was detected at low discharge voltage and low anodic current (regardless of the pH value), apparently corresponding to an essential anodic current that was contributed by the TiO₂ formation through typical electron tunneling mechanism. Based on these OES findings, a contour map has been constructed to describe the most effective region to form the designated TiO₂ MAO layer.

Keywords: micro-arc oxidation (MAO), optical emission spectroscopy (OES), titanium,

Transition Metal Chalcogenides e.g. Tungsten diselenide, Molybdenum diselenide, tin selenide were found applications in optoelectronic devices like photoelectrochemical(PEC)Solar Cell, Schottky barrier and P-N junction diode etc. The present paper reports the preparation of Nanocrystalline semiconductor Tin Selenide (Nc p-SnSe) thin films for its electronic device applications. For this purpose, n-MoSe₂ crystals grown by Direct Vapour Transport (DVT) technique were used as a substrate and Nc p-SnSe thin films are deposited on to them by economical and simple “spin coating” technique in order to form n-MoSe₂/Nc p-SnSe hetrojunction devices. The effect of annealing and thereby particle size of deposited Nc p-SnSe thin films(with glass substrates) on optical band gap are studied by absorption spectroscopy in the wavelength range 200-2200 nm. The current voltage (I-V) measurements of the prepared devices are made in the temperature range 300K-420K under dark condition. The I-V characteristics of n-MoSe₂/Nc p-SnSe junction exhibits rectifying behavior that confirms the formation of diode. The diode parameters such as rectification ratio, reverse saturation current (I₀), ideality factor (η), barrier height (Φ_b0) and series resistance (R_s) are determined from I-V curves using thermionic emission diode equation. In addition to this Cheung’s function and Norde method are also applied to estimate more realistic values of the device parameters. The conduction mechanisms are explained on the basis of the forward bias I-V characteristics using the power law.