ICMCTF2010 Session C3: Optical Characterization of Thin Films
Wednesday, April 28, 2010 8:00 AM in Royal Palm 1-3
C3-1 Photoluminescence, Photoacoustic and Raman Spectra of Zinc Oxide Films Grown by LP-MOCVD Using Diethylzinc and Water as Precursors
Tomoaki Terasako, Takahiro Yamanaka, Shinichiro Yura (Ehime University, Japan); Masakazu Yagi (Kagawa National College of Technology, Japan); Sho Shirakata (Ehime University, Japan)
One of the most conceivable candidates for replacing widely used transparent conducting oxide In2O3:Sn (ITO) is zinc oxide (ZnO). Among various growth methods, low pressure metalorganic chemical vapor deposition (LP-MOCVD) is known to be an effective method for obtaining high-quality films under good controllability of growth rate and composition. In this paper, we will discuss photoluminescence (PL), photoacoustic (PA) and Raman spectra of polycrystalline ZnO films grown on alkali-free glass substrates by LP-MOCVD in terms of substrate temperature (TS).
In our LP-MOCVD apparatus, diethylzinc (DEZn) and water (H2O) were used as precursors and transported to the vertical reactor by the N2 carrier gas. To obtain films with good surface homogeneity, the LP-MOCVD reactor has a rotatable substrate stage and a shower head for feeding DEZn.
PL spectra of all the films were composed of a near-band-edge (NBE) emission at ～3.3 eV and an orange band (OB) emission at ～2.0 eV. For the films grown at lower temperatures than 200℃, the NBE emission extended to higher energies than the bandgap energy of ZnO (3.37 eV), implying the existence of the secondary phase. In fact, Raman spectrum of the film grown at TS=125℃ showed a broad Raman peak ranging from 750 to 850 cm-1 peculiar to Zn(OH)2 with a larger bandgap than that of ZnO. The relative intensity of the OB emission to the NBE emission (IOB/INBE) decreased with increasing TS. Since interstitial oxygen atoms (Ois) are responsible for the OB emission, the decrease in IOB/INBE can be interpreted as the decrease in OI concentration with increasing TS.PA measurements were done for investigating optical absorption processes. For the films grown at TS=125-200℃, the PA edge (which is characteristic “knee” on each PA spectrum) shifted towards lower energies with increasing TS. This tendency is probably related to the decrease in Zn(OH)2 secondary phase. On the contrary, for the films grown at TS=200-300℃, the PA edge shifted towards higher energies with increasing TS, resulting from the decrease in concentration of Oi and/or zinc vacancy (VZn) related defects accompanied with nonradiative transitions.
C3-2 ZnO Preferred Orientation Control by Homo- and Hetro-Structure Buffer Layer Growth on Si (111) Using Atomic Layer Deposition with Flow-Rate Interruption Method
Ching-Shun Ku (National Synchrotron Radiation Research Center, Taiwan); Jheng-Ming Huang, Chih-Ming Lin (National Hsinchu University of Education, Taiwan); Hsin-Yi Lee (National Synchrotron Radiation Research Center, Taiwan)
ZnO thin film growth on Si (111) substrate using atomic layer deposition (ALD) at 25-260oC without buffer layer result indicated polycrystalline structure verified by x-ray diffraction (XRD). Both homo- and hetro-structure buffer layer growth by ALD or RF sputtering showed orientation change to (002) or (101) direct preferred. The photoluminescence (PL) results showed near-band-edge narrowing and blue-shift as preferred Bragg peak intensity increased with raised ZnO growth temperature from room temperature. Both PL and XRD intensity were increased with grain size grow up as increased the buffer layer thickness. The buffer layer also reduced the surface roughness respect to no buffer layer ZnO thin films. The resistivity measure from hall measurement showed will change with different orientation and ZnO growth temperature. All results showed that buffer layer will control the preferred orientation not only enhanced the XRD and PL intensity due to large grain size reduced defect come from the grain boundary but also influence the hall mobility and resistivity.
C3-3 Study of Magnetic Interfaces with Polarized Soft X-Rays
Cecilla Sánchez-Hanke (Brookhaven National Laboratory)
The definition of interface “a surface forming a common boundary of two bodies, spaces”, is so generic that almost every system has an interface to study. The reality is that in most of the cases research focus more on the characteristics of the bulk, layers of the systems, meanwhile interfaces are almost not considered but as elements that perturb their characteristics. Fortunately there is a growing interest in understanding the contribution and influence that interfaces provide to the properties of the materials. Synchrotron sources are an excellent tool for materials characterization. The wide energy range, from Infrared to hard x-rays, together with the high intensity of their photon beams, the tunability of the incident photon energy and the possibility to perform polarization sensitive experiments provides with a large range of possibilities for materials characterization. In the special case of interfaces the question remains what is an interface and how deep into the system goes. An important limiting factor to answer these questions is the spatial resolution as well as the wavelength to be used at the characterization experiment. In the presentation we will provide a brief introduction of common surface and interface sensitive experimental techniques that can be performed at synchrotron sources. Additionally we will show the efforts to characterize magnetic interfaces of magnetic interfaces characterized by means of elliptically polarized soft x-rays.
C3-5 Vectorial Scattering Spectroscopic Ellipsometry Analysis of Dielectric Thin Films on Textured Solar Cells
Mario F. Saenger (University of Nebraska-Lincoln); Craig M. Herzinger (J.A. Woollam Co., Inc.); Martin Schadel (Q-Cells A.G., Germany); James Hilfiker, Jianing Sun (J.A. Woollam Co., Inc.); Tino Hofmann, Mathias Schubert (University of Nebraska-Lincoln); John A. Woollam (University of Nebraska-Lincoln and J.A. Woollam Co., Inc.)
State of the art silicon solar cells commonly use surface texturing and antireflective coatings to optimize the device efficiency. Optical characterization of thin films on textured substrates has been difficult due to scattering caused by substrate texture. Recently spectroscopic ellipsometry with non-traditional measurement geometry has been demonstrated capable of collecting specular reflections from orientated surfaces on textured silicon. Previous attempts for data analysis used an effective medium approximation to determine the thickness and refractive index of antireflection coatings. However, discrepancies exist depending on the measurement geometry . Here we present a new method where scattering effects can be quantified in the data analysis. With our method it is possible to obtain consistent results, regardless the measurement geometry, for anti-reflection coatings on textured multi- and mono-crystalline silicon based solar cells.
 M.F. Saenger, J. Sun, M. Schädel, J. Hilfiker, M. Schubert, and J.A. Woollam, Thin Solid Films, (2009). (in press)
C3-6 In-Situ Studies on Thin Functional Polymer Films by Ellipsometry and Vibrational Spectroscopy
Klaus-Jochen Eichhorn (Leibniz-Institut für Polymerforschung Dresden e.V., Germany)
The control and understanding of biomolecule-surface interactions are very important for biosensor, biomedical and antibiofouling application. Specific desired properties can be incorporated into material surfaces by their modification using thin functional polymer films. So, “smart” surfaces can be developed for controlled adsorption and release of biomolecules.
Two surface engineering approaches will be presented:
- Coatings with thin layers of hyperbranched polyesters having tuned molecular architecture and functionalities.
- Fabrication of stimuli-responsive surfaces using immobilized thermo-responsive hydrogel films (PNIPAAM-PEG copolymers) or pH-responsive binary polyelectrolyte brushes (PAA and P2VP).
The properties of the surfaces and films in appropriate aqueous media (swelling/deswelling), their switching with temperature or pH as well as the resulting adsorption and release of biomolecules (model proteins such as Human Serum Albumin, Lysozym, Chymotrypsin, and cells) were studied in real time using optical in-situ methods: Spectroscopic Vis- and IR-Ellipsometry, Imaging Ellipsometry, FTIR-ATR Spectroscopy, and Microscopy.
C3-8 Monitoring Protein Deposition of Self-Assembled Monolayers of Alkanethiols on Gold In Situ with Combined Quartz Crystal Microbalance and Spectroscopic Ellipsometry
Keith B. Rodenhausen, Beth A. Duensing, Angela K. Pannier, Mathias Schubert (University of Nebraska-Lincoln)
We report a new combinatorial approach to study organic thin films. This novel technique consists of in-situ spectroscopic ellipsometry and quartz crystal microbalance methods. In contrast to the quartz crystal microbalance, which is sensitive to the total mass attached to the surface, including the trapped solvent, spectroscopic ellipsometry only measures the amount of adsorbant on the surface. By using these two techniques in tandem, we are able to determine the thickness and water fraction of visco-elastic thin films.
We investigate cetyltrimethylammonium bromide (CTAB) thin films deposited onto a gold-coated quartz crystal as a model system. CTAB grown from a 2.5 mM solution demonstrates several phases in porosity evolution, including a temporary hold in water fraction as the film is rinsed off the substrate with water; these effects may be related to the structure of a CTAB bi-layer.
In addition, a variety of self-assembled monolayers (SAMs) of alkanethiols on gold-coated quartz crystals were used as model biomaterials to determine the water fraction of an adsorbed fibronectin layer. The porosity information was used to distinguish the proteins’ conformation, dictated by the defined surface chemistries of the SAMs. Two protein concentrations in PBS buffer were studied (0.1 mg/mL and 0.01 mg/mL) to isolate how protein concentration affects the above variables.
C3-9 Evolution of Crystalline Phase Formation of Titanium Oxide Thin Films using Reactive Magnetron Sputtering and Annealing
Chen-Kuei Chung, Ming-Wei Liao (National Cheng Kung University, Taiwan)
Titanium oxide (TiOx) thin films were deposited on the Si(100) substrates by direct-current reactive magnetron sputtering at 100~200 W Ti-target power and 3~10 % oxygen flow ratios (FO2% = FO2 / (FO2+FAr) × 100%), and then annealed by rapid thermal annealing (RTA) in atmosphere at 350, 550 and 750 °C for 2 min . The morphology, chemical composition, phase formation, and bonding behavior of the as-deposited and annealed TiOx thin films were analyzed by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), grazing incidence X-ray diffraction (GIXRD), and Raman spectroscopy, respectively. The as-deposited and 350 °C annealed TiOx films were amorphous from GIXRD and showed weak Raman intensity. In contrast, the distinct crystalline peaks of anatase or rutile phases were detected after RTA at 550~ 750 °C from both GIXRD and Raman spectra. The peak intensity of rutile phase increases with oxygen flow ratio and anatase phase increases with Ti-target power . Contrast with EDS results, a mixture of anatase and rutile phases was obtained by RTA at 750 °C when the stoichiometry x of as-deposited TiOx films is between 1.57 and 1.97. The pure anatase and pure rutile phase were detected in x>1.97 and x<1.57 specimens after RTA, respectively.
C3-10 Pulsed Laser Deposition of (MoO3)1-x(V2O5)x Thin Films: Preparation, Characterization and Gasochromic Studies
Chung-Chieh Chang, Ta-Kun Chen, Jiu-Yong Luo, Tzu-Wen Huang, Kuo-Wei Yeh, Chung-Ting Ke, Po-Chun Hsu, Ming-Jye Wang, Mau-Kuen Wu (Academia Sinica, Taiwan)
In this study (MoO3)1-x(V2O5)x mixed oxide films were fabricated by KrF (l = 248 nm) pulsed laser deposition (PLD), from mixed pressed powders of (MoO3)1-x(V2O5)x, x = 0, 0.1, 0.2, and 0.3, at 300 mtorr oxygen partial pressure and 25 0℃ temperature on glass substrates. The hydrogen gas sensing performance of platinum (Pt) catalyst activated modified (MoO3)1-x(V2O5)x thin films were investigated. The behavior of (MoO3)1-x(V2O5)x thin films exhibited a gasochromic effect; i.e., a reversibly change in color from transparency when in air to blue when in H2. The all processes proceeded rapidly at room temperature. A layer of platinum (Pt) was then evaporated onto the surface of (MoO3)1-x(V2O5)x film. The cycling of the coloration was obtained from UV-Vis spectra and the mechanism deduced from both visible and Fourier transform infrared (FTIR) spectra.. Therefore, we could detect the existence of H2 by the coloration of (MoO3)1-x(V2O5)x thin film. Sensor properties of (MoO3)1-x(V2O5)x (Pt) films were investigated at room temperature in H2–N2 mixtures containing 0-50 mole% of H2. The results show that the transmittance change(ΔT) of the (MoO3)1-x(V2O5)x hydrogen sensor.
C3-11 Numerical Ellipsometry: Advanced Analysis of Thin Absorbing Films in the n-k Plane
Frank Urban, D. Barton (Florida International University)
A major challenge for those utilizing ellipsometry is numerical processing of the measured data. The transcendental, multivalued equations arising from the physics of simple reflection are problematic for the least-squares numerical methods in common use. Previously we have applied Complex Analysis in the n-k plane and this has led to a growing array of new numerical methods which achieve computational accuracy at the limit of the computer (~10-14 decimal digits) rather than functioning to a mean square value of a large number of measurements. The work presented here applies new numerical methods for use with absorbing films deposited on transparent and on absorbing substrates. The method finds intersections of projections of three “twisted curves” resulting from three (or more) light incidence angle measurements at three film thicknesses. The method is employed here to determine the thickness and optical properties of chromium films (between 10 and 25 nm nominal thickness) deposited onto two different substrates, silicon and silica oxide, employing measurements made at three different angles of incidence for light wavelengths between 280 and 2500 nm. Treatment of an oxide overlayer and alloy or compound underlayer is included.