ICMCTF2011 Session C3: Optical Characterization of Thin Films, Surfaces, and Devices

Atomic force microscopy is a powerful tool for the characterisation of surfaces down to the molecular and even atomic level. It is based on a microscopically small oscillating tip which fixed to a cantilever. The tip is moved across the sample to obtain topographical information.

Recently, AFM and Raman microscopy were combined to allow morphological/topographical as well as chemical imaging of the same sample area. Raman microscopy ideally complements AFM imaging since it is adding molecular spectroscopic information. In this paper typical examples for analyzing surfaces by FTIR imaging and AFM/Raman microscopy are presented. Data from polymer structures on metallic substrates as well as from polymer structures on different semiconductors are shown. To avoid any modification of these samples during the optical inspection, all experiments were performed contact-free in transmission or reflection mode.

X-ray fluorescence (XRF) is one of the most well-established optical, non-destructive techniques for elemental analysis of bulk materials. Unlike X-Ray Photoelectron Spectroscopy, which is considered the most sensitive technique for chemical analysis of surfaces and thin films, XRF was considered too vague for thin film studies. However, new advances in XRF instrumentation enable today its implementation in studies of thin films and coatings, as well.

In this work we present a case study of XRF analysis of AlN and AlN-based nanostructured thin films. We have selected AlN as an extreme case of a ceramic material, which consists exclusively of light elements where Auger emission and not fluorescent X-Rays is more probable for the recombination of the 1s vacancy and a high-energy level electron.

We have considered AlN and AlN_x films grown by reactive magnetron sputtering and pulsed laser deposition and we observed the chemical shifts of the Ka and Kb lines and the emergence of the Kb’ line of Al due to the chemical environment around it (Al or N first neighbors). We compared the chemical shifts observed in AlN thin films to those observed for Al₂O₃ (useful for studying oxidized Al and AlN) and AlCl₃ (the most popular solid precursor for AlN growth by chemical vapor deposition) and we show that there is a rational variation according to the electro-negativity of the ligand. We note that XRF has been able to identify the formation of stoichiometric, yet amorphous, AlN where X-Ray Diffraction was useless due to the absence of crystalline periodicity. Finally, we identified the film thickness limits for which XRF can provide reliable results.

XRF has been also applied in investigating the oxidation of AlN by recording the evolution of oxygen’s Ka line. The strength variation of this line for various AlN thicknesses indicates whether an oxide layer is located at the surface or the interface of the film.

In order to investigate the potential of XRF to study more complex materials, we have employed it for the study of AlN-based films with Ag inclusions in various forms. Thus, we have studied films where Ag was: 1) atomically dispersed into the AlN lattice, 2) nanospheres (~3.5 nm) embedded into an AlN matrix, and 3) Ag nano-sheets into AlN (i.e. AlN/Ag nano-multilayers). Comparing the Ka, Kb and Kb’ chemical shifts in these films to those observed for Al, AlN and AlAg intermetallics, we were able to evaluate the nature of AlN-Ag bonds for the various considered cases.

In the present study, MoO₃/V₂O₅ composite thin films were prepared by sol-gel spin-coating technique. The spin-coated MoO₃/V₂O₅ thin films were amorphous before calcining, nanocrystalline MoO₃/V₂O₅ thin films were afterward. The surface morphology of crystallized MoO₃/V₂O₅ thin films was examined by scanning electron microscope ( SEM ). X-ray diffraction for phase identification was performed by an X-ray diffractometry . Electrochromic properties of these thin films were evaluated by cyclic voltammetry, using a standard three-electrode cell system. All CV measurements were performed from -2 to 2 V at a scan rate 30 mV/sec at room temperature. Transmittance spectra of the films were examined by a spectrophotometer.

The effect of annealing temperature ranged from room temperature to 400 ^oC on film morphology was investigated. Experimental results showed that after heat-treated MoO₃/V₂O₅ composite thin films exhibited batter electrochromic properties among the films examined in the present study.

Tungsten oxide (WO₃) is a wide band gap (E_g) semiconductor (E_g~3.2 eV), which exhibits excellent properties suitable for the development of integrated chemical sensors, electrochromics, and photoelectrochemical cells. Recently, band gap modification with anoion doping has received a significant attention in view the materials’ application as photo-electrode for hydrogen generation by photo catalytic water splitting. The present work was performed to understand the effect of nitrogen-doping induced changes in the microstructure and optical properties of WO₃ films grown by reactive magnetron sputter-deposition. The objective of this work is to optimize the conditions to produce materials suitable for photo-electrodes for hydrogen generation by water splitting. The characterizations performed using grazing incidence X-ray diffraction and optical absorption measurements indicate that the effect of reactive nitrogen pressure is significant on the microstructure evolution and optical properties. The films grown at various reactive gas pressures (0–5.6 mTorr) while keeping the deposition temperature fixed at 400^oC shown the degradation of crystal quality for the initial nitrogen-doping while higher end nitrogen-pressure leads to the formation of morphological disorders. Optical spectroscopy analysis of the films indicates that Eg becomes narrowed upon nitrogen indicating the electronic structure changes. Eg decreases from 2.98 eV to 2.12 eV with increasing nitrogen pressure. The results will be presented and discussed.

Significant research efforts have been directed in recent years on the growth of Y₂O₃ thin films because of their interesting physical, electronic, and optical properties. The diverse range of potential applications of Y₂O₃ films includes storage capacitors, random access memory (RAM) and metal−insulator−semiconductor (MIS) devices, protective and antireflective coatings for IR detectors, and optical filters. In the present work, Y₂O₃ films have been produced by the reactive magnetron sputter-deposition. The effect of growth temperature (T_s) on the microstructure, electrical and optical characteristics Y₂O₃ films has been investigated. The results indicate that the films grown at room temperature (RT) are amorphous while the films grown at the substrate of T_s=300-500ºC are nanocrystalline crystallizing in cubic phase. Scanning electron microscopy analysis of Y₂O₃ films indicates the fine microstructure and uniform distribution of dense particles. Grain-size increases from ~5 to 50 nm with increasing T_s. Frequency variation of the electrical conductivity measurements in the range 20 Hz - 1 MHz indicate that the conductivity increases with increasing frequency. The conductivity data fits to a modified Debye’s function, which considers multiple ions contributing to the relaxation. Spectroscopic ellipsometry measurements indicate that the refractive index (at 300 nm) of Y₂O₃ films increase from 2.03 to 2.25 with increasing T_s. This is attributed to the increased packing density and crystallinity of the films with increasing T_s. The results will be discussed and presented to derive a correlation between microstructure, electrical and optical properties in Y₂O₃ films.

Physical properties of nanometer films are decisive for many optical and electronic devices. Characteristics like thickness, roughness, density and reflectance have a strong impact on the performance of functional coatings. Currently, one of the main driving forces for the development of optical thin films is the extreme ultraviolet (EUV) lithography. EUV lithography will be used for the production of next generation integrated circuits of the 22 nm node. In this technology nanometer multilayers are applied as reflection coatings on every optical element of the beam path (source collector, illumination and projection optics, photomask).

In order to measure the necessary properties of the films X-ray reflectometry is a very powerful tool. Due to the short wavelength the interaction of the radiation with the films results in interference associated with distinctive features in the reflectance spectra.

Initially a short overview about the theoretical background, commonly used experimental equipment and available software algorithms of the reflectometry method will be presented. The influence of specific thin film parameters on the reflectance spectra will be discussed.

Zinc oxide has been used as transparent and conducting oxide film in solar cells and electrochromic devices. Also, ZnO thin films have great potential for applications in flexible display technology. In this work, surface morphology evolution, and optical and electrical properties of ZnO thin films deposited by RF sputtering technique onto polyethylene terephthalate and glass substrates have been investigated. Surface morphology was measured with atomic force microscopy (XE-100, Park Instruments) operating in air. Both 5 um x 5um and 1um x1um AFM images were acquired and the surface roughness was characterized in terms of root mean square roughness. Optical Transmittance was performed using a Uv-Vis-NIR spectrometer (Lambda 750, Perkin Elmer) ranging from 190 nm to 3300 nm. The crystalline degree of ZnO thin films was measured using a X-ray diffractometer system (D/MAX-2100/PC, Rigaku). X-ray diffraction patterns confirm the proper phase formation of the material. Optical transmittance data show high transparency (more than 85%) of the films in the visible range. Electrical characterizations show the room-temperature conductivity of the films deposited onto polyethylene terephthalate substrates. For films deposited on glass substrates the surface resistivity measures are less than 40 Ω/sq. The authors would like to thank Brazilian agencies CNPq and FAPESP for financial support.

We hereby report the synthesis of Zn_0.55Cd_0.45S:Mn/ZnS core shell structured QDs with varying shell thickness by a two step co-precipitation technique at 280K. With increase in shell thickness the intensity of the XRD peaks of core decreases while that of shell increases which is an indication of the formation of core shell nanoparticles. UV-Vis measurements showed a red shift with increase in the shell thickness. The TEM micrograph confirms the formation of core/shell structures. The particle size obtained from TEM and UV-Vis measurements are in well agreement with each other. Photoluminescence spectra reveal that the maximum intensity of the band at ~ 590 nm (which is a characteristic emission of Mn²⁺ ion) is found for a shell thickness of 0.2 nm and is ~ 35 times higher than the Core QD’s.

We investigated the effect of post-annealing temperature with non passivation a-InGaZnO TFT. As increasing the annealing temperature from 25°C to 45°C, the electrical parameter such as threshold voltage、subthreshold swing and mobility improve apparently. The bonding energy of oxygen in a-InGaZnO film became stronger with higher annealing temperature shows in XPS data. In addition to that, the storage of 45°C annealed device was excellent with ±0.5V of V_th variation under the environment after 9 days and the anti-UV ability was also better with 1.5V threshold voltage shift under UV light illumination. After being gate bias stress, the Vth shift decrease with higher post-annealing temperaure and that mean the improvement of film quality were actually better in 45°C. The passivation-free a-InGaZnO TFT annealed in higher temperature performs a better electrical characteristic and the binding energy of oxygen increases in a-InGaZnO film. According to that, the environment storage is stable after 9 days because the film is stable which avoiding the oxygen desorption in the ambient atmosphere. Besides, the anti-UV ability also improves with higher annealing temperature. The V_th shift is also smaller with less interstitial oxygen. The positive gate bias stress (PGBS) also shows a good performance caused by less absorption of oxygen in the ambient atmosphere with high annealing temperature. The thermal annealing could actually improve the a-InGaZnO TFT to be more stable and insensitive to UV light.

Amorphous hydrogenated chlorinated carbon (a-C:H:Cl) films were produced by the plasma polymerization of chloroform/acetylene/argon mixtures in an rf PECVD system. The main parameter of interest was the ratio of chloroform to acetylene in the chamber feed. Films were deposited onto aluminum covered glass slides and aluminum plates, respectively, for subsequent characterization by infrared reflection-absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS). IRRAS spectra revealed the presence of C-Cl groups and XPS confirmed the presence of chlorine. Several optical properties were obtained via film thickness data and ultraviolet-visible-near infrared spectra. The latter were obtained using a Perkin Elmer Lambda 750 spectrophotometer. Thus dependencies of the refractive index, n, absorption coefficient, α(E), and optical gap, E₀₄, on the degree of chlorination were obtained.