AVS2002 Session TF-WeP: Poster Session

Wednesday, November 6, 2002 11:00 AM in Room Exhibit Hall B2

Wednesday Morning

Time Period WeP Sessions | Topic TF Sessions | Time Periods | Topics | AVS2002 Schedule

TF-WeP-1 Effects of Starting Material of Aluminum Doped Zinc Oxide Under-layer on the Electric Properties of Palladium Doped Silver Film
T. Oyama, M. Maekawa, T. Yanagisawa (Asahi Glass Co., Ltd., Japan)
Silver-based multilayer has been widely utilized as heat mirror for Low-E coating or transparent conductor for electromagnetic shielding. In this study, deposition conditions of the AZO(Aluminum doped zinc oxide) film were investigated to show how they affected the crystallographic property of the film and the electric property of palladium doped silver layer formed on the film in the layer system of Glass/AZO(42nm)/AgPd(1%Pd:10.5nm) deposited by DC magnetron sputtering. Starting material for AZO was aluminum doped zinc metal or aluminum oxide doped zinc oxide. Metal target was sputtered in oxygen atmosphere or carbon dioxide atmosphere whereas oxide target was sputtered in argon or argon/oxygen atmosphere. AgPd layer was deposited under the same condition. The effects of the over-layer were also investigated. Electrical resistivity was changed by up to 17% according to the deposition process of the under-layer. AgPd film deposited on AZO showed the lowest resistivity of 7.1x10E-6 Ωcm in the case of metal target with oxygen. On the contrary, the highest resistivity of 8.3x10E-6 Ωcm was observed in the case of oxide target with argon/oxygen. XRD patterns showed the most developed Ag(111) peak coupled with ZnO(002) peak in the case of metal target with oxygen and AFM observation revealed the smallest surface roughness in this case. Deposition of thin metal(Zn-Al) over-layer increased the resistivity implying the diffusion of zinc atom into silver-based layer, however the resistivity decreased with storage time. Optical constants of the films determined by spectroscopic ellipsometry(SE) and surface roughness of the films estimated by SE will be discussed.
TF-WeP-2 The Synthesis and Characterization of Indium Tin Oxide Films by Cesium Assisted Sputtering System at Low Temperature
D.Y. Lee, H.K. Baik (Yonsei University, Korea); S.J. Lee (Kyungsung University, Korea); K.M. Song (Konkuk University, Korea)
In this study, we investigated the properties of indium tin oxide (ITO) thin films synthesized by using a cesium assisted sputtering method. Cesium assisted sputtering enables to supply a cesium vapors on the target surface of sputter and generate the negative ions of target materials. The flux and energy of negative ions are independently controlled by the changing target voltage and cesium contents. ITO target (SnO2: In2O3 = 1: 9) was sputtered by mixture gas of argon and oxygen. Oxygen partial pressure, working pressure and other deposition parameters were optimized. RF coupled DC was used as a power source and its voltage was changed from 45 V(only RF) to 350 V(only DC). The temperature of cesium reservoir was varied from 80 to 200 °C. Negative sputtered particles including In, Sn, O and electron were generated from the target surface and accelerated to the substrate by target voltage. We applied additional substrate bias to control deposition energy and flux. The main object of our study is the fabrication of ITO films with low resistivity and high transparency at room temperature by negative ion beam process. The effects of cesium addition not only enable to generate the negative ions by lowering the target work-function but increase the plasma density. From the result of XRD, we concluded that the ion beam effects induced by positive and negative ion enhance the crystallinity of ITO films. In the case of positive bias, negative ions and electrons play a main role in improvement of ITO crystallinity. For the negative bias, argon ions, which fluxes were increased by cesium, promote ITO crystallinity by enhancing the mobility of adatoms on the substrate surface. Also we found that the preferred orientation of ITO films was changed with applied bias. The characterization of deposited ITO films was performed by XRD, four point probe, AFM.
TF-WeP-3 Deposition of ZnO Films by Reactive Sputtering in CO2 Atmosphere
N. Aomine, K. Sato, E. Shidouji, J. Ebisawa (Asahi Glass Co., Ltd., Japan)
Transparent oxide films are often deposited by reactive sputtering from metal target in Ar/O2 atmosphere because of cheep target cost. It is well known that deposition rate and absorbance of obtained films change drastically with a ratio of O2 flow to total gas flow, and a transparent film can be obtained with a maximum deposition rate in the transition region from metallic to oxide mode. However, it is difficult to keep sputtering process in the transition region without monitoring plasma state e.g., emission, impedance, gas fraction, feed-backing to the process. In this work, reactive sputtering using CO2 gas instead of Ar/O2 gas was studied. ZnO films were deposited by reactive sputtering of a 10 cm-diameter Zn metal target. Process-film property relationship such as discharge voltage, deposition rate, film absorbance, and carbon content was investigated. Furthermore, gas fraction of CO2 sputtering atmosphere was analyzed by quadrupole mass spectrometer (Q-mass). It was found that CO2 sputtering has advantage to enable reactive sputtering in the transition region by only simply adjusting CO2 flow rate. Films obtained with small CO2 flow rate were absorptive, while films, with increasing CO2 flow rate, became transparent and deposition rate finally began to decrease. Result from Q-mass spectra, showing CO2 was partially decomposed into O2 and CO by glow discharge, was analyzed based on gas supply and consumption balance model. It was concluded that CO2 sputtering was advantageous owing not only to low O2 partial pressure but also to deoxidation effect by CO. The advantage of CO2 sputtering will be detailed.
TF-WeP-4 Fabrication of ZnO Thin Films by Pulsed Laser Ablation with Remote Radical Source
T. Suzuki, M. Hiramatsu, N. Shimizu (Meijo University, Japan)
Zinc oxide (ZnO) is a II-VI compound semiconductor with wide direct bandgap of 3.3 eV at room temperature. ZnO exhibits good piezoelectric, photoelectric and optical properties, and might be a good candidate for an electroluminescence device such as ultra-violet (UV) laser diode. ZnO films also have potential applications for surface acoustic wave devices and low loss wave-guides. In this study, ZnO thin films have been fabricated on Si (100) substrate at temperatures of 300 - 400°C by pulsed laser ablation using a KrF excimer laser with the wavelength of 248 nm. During the deposition, a remote microwave O2/N2 plasma as a radical source was used in order to improve the optical quality of the film. We have investigated the effect of the radical source on the optical and structural properties of ZnO films using photoluminescence (PL). From the PL measurement, all the films fabricated with and without using radical source showed a typical luminescence behavior with the two emissions of a narrow UV and a broad visible band. It is reported that the visible luminescence of ZnO was caused by the defects, such as oxygen vacancies and zinc interstitials in the films. As the partial pressure of N2 in the microwave O2/N2 plasma increased, the intensity of UV luminescence increased and that of visible luminescence decreased. From this result, it was suggested that the injection of the nitrogen radicals into plasma plume might be effective to reduce the visible luminescence of ZnO and to improve the optical quality of the film.
TF-WeP-5 XPS Study of First Stages of ZnO Growth and Nano-structure Dependence of the Polarisation Effects at ZnO/SiO2 and ZnO/Al2O3 Interfaces
A.I. Martin-Concepcion, F. Yubero, J.P. Espinos, A.R. Gonzalez-Elipe (CSIC-Univ. Sevilla, Spain); S. Tougaard (University of Southern Denmark)
A critical characteristic of nanoparticles and, in general, nanomaterials, is the overweighing importance of the surface and interface layers in respect to the bulk because of the small size of the aggregates (in 3D) or thickness of the layers (in the case of 2D) that constitute the nanomaterial. The present paper reports about the characterization of these interface layers in the particular case of ZnO/MO (MO: Al2O3 and SiO2) interfaces by using X-ray photoelectron spectroscopy (XPS). Careful experiments have been performed consisting in the deposition of ZnO material on SiO2 and Al2O3 substrates. Several samples were produced and characterized in situ. The nanostructure of the first stages of growth of the ZnO deposited was determined by Tougaard -peak shape analysis of several photoelectron peaks from both substrate and overlayer to check the consistency of the determined growth mechanisms. Thus the actual nanostructure of the growing ZnO films were carefully determined. In addition, the chemical interaction at the ZnO/MO interface was monitored by following the variation of the Auger parameter of the Zn atoms, as the amount of ZnO deposited was increased. Thus, the changes of the Auger parameter of the ZnO atoms have been correlated with the actual nanostructures formed by the ZnO deposits. From this information, a model is presented that accounts for the changes in the electronic parameters determined by XPS as a result of bonding and polarization interactions at the interface.
TF-WeP-6 Fabrication of CuIn1-xGaxSe2 Thin Film Solar Cells by Two-step Process
H.K. Song (Seoul National University, Korea); S.G. Kim (Kyungwon University, Korea); H.J. Kim (Seoul National University, Korea); S.K. Kim, K.H. Yoon (Korea Institute of Energy Research, Korea)
Photovoltaic (PV) modules based on CuIn1-xGaxSe2 polycrystalline thin films have been promising candidates as an effective absorber material for solar cells. It has been reported that solar efficiency over 17% by co-evaporation of elemental sources (Cu, In, Ga, and Se) through three-stage process. But this process is difficult to scale-up for a large-manufacturing system. So we prepared CuIn1-xGaxSe2 thin films by 2-step process that can be easily scaled-up to industrial process. Metallic precursors were deposited on Mo coated soda-lime-silicate glass by DC magnetron sputtering followed by selenization process using Se vapor in an evaporation system. Ga was incorporated by using a Cu-Ga(23 at.%) alloy target. CdS buffer layer, intrinsic and n-type ZnO layer and Al top contact electrode were deposited by CBD (chemical bath deposition), RF sputtering system, and E-beam evaporator system, respectively. Fabricated CuIn1-xGaxSe2 thin film solar cells have relatively smooth surface morphology and mono-crystalline chalcopyrite phase. The efficiency of solar cell are measured by solar simulator and the characteristics of CuIn1-xGaxSe2 films are analyzed by X-ray diffraction (XRD), Auger Electron Spectroscopy (AES), Electron Probe Micro Analysis (EPMA), and Field Emission Scanning Electron Microscope (FESEM).
TF-WeP-8 Structural and Optical Properties of the Novel Semiconducting Alloy Films CuxCd1-xTe:O Grown by rf Sputtering
J. Santos-Cruz, G. Torres-Delgado, R. Castanedo-Pérez, O. Jiménez-Sandoval (CINVESTAV, Mexico); B.S. Chao (Energy Conversion Devices); S. Jiménez-Sandoval (CINVESTAV, Mexico)
Current efforts towards understanding the degradation processes in CdS/CdTe heterojunction solar cells are strongly focused on the role of copper and oxygen in these two materials. We have studied the effects of incorporating both Cu and O in a controlled manner, during growth, on the structural and optical properties of CdTe films. When only Cu is introduced up to a concentration of around 3 at.%, the obtained material has a composition of the form CuxCd1-xTe which does not present significant changes in its physical properties as compared to those of CdTe. However, when oxygen is incorporated in the chamber during the growth of the CuxCd1-xTe films, the structural and optical properties may be varied significantly: depending upon the oxygen partial pressure, the structure of the films may be changed from polycrystalline to amorphous, and the optical band gap may present values between 1.4 and 2.8 eV. These results are addressed in terms of the chemical composition, structural properties and on the known behavior of copper and oxygen in the CdTe host.
TF-WeP-9 Properties of Co-deposited ITO and ZnO Films Using a Bi-polar Pulse Power Supply and a Dual Magnetron Sputter Source
M.S. Hwang, H.S. Jeong, Y.W. Seo (ITM, Inc., Korea)
Multilayer coatings consisting of metal layers sandwiched between transparent conducting oxide(TCO) layers are widely used for flat panel display electrodes and electromagnetic shield coatings for plasma displays, due to their high electrical conductivity and light transmittance. The electrical and optical properties of these multilayer films depend largely on the surface characteristics of the TCO thin films. A smoother surface on the TCO thins films makes it easier for the metal layer to form a continuous film, thus resulting in a higher conductivity and visible light transmittance. ITO and ZnO films were co-deposited, using a dual magnetron sputter and a bi-polar pulse power supply, to decrease the surface roughness of the TCO films. The symmetric pulse mode of the power supply was used to simultaneously sputter an In2O3 (90wt%) : SnO2 (10wt%) target and a ZnO target. We varied the duty of the pulses to control the ratio of ITO : ZnO in the thin films. The electrical and optical properties of the films were studied, and special attention was paid to the surface roughness and the crystallinity of the films.
TF-WeP-10 The Effects of Substrate Temperature and Ion Flux on the Opto-electronic Properties of dc Magnetron Sputtered Aluminum-doped Zinc Oxide
N.W. Schmidt, T.S. Totushek, W. Kimes, J.R. Doyle (Macalester College)
We present a study of the effects of substrate temperature and ion flux on the opto-electronics properties of reactive dc magnetron sputtered aluminum-doped zinc oxide (AZO). Near-substrate plasma density is varied using an unbalanced magnetron with external coils which allow variation of the ion-to-neutral ratio from 0.2 to 2. The substrates are electrically floating with a floating voltage of about -10 V, implying that the ions have 10 eV of energy upon arrival at the growing film. The ion flux is measured using cylindrical and flat plasma probes. Film quality is characterized by conductivity, visible transmission, x-ray diffraction, scanning electron microscopy, and Hall effect measurements. Film quality is found to be very sensitive to substrate temperature in the range of room temperature to 120 C, with transparency increasing and resistivity decreasing with increasing temperature. Film quality is also improved with increasing near-substrate plasma density up to 120 C. The best films have resistivities less than 0.0004 Ohm-cm with average transmission above 85% in the visible. Film quality maximizes in the range 120 - 150 C and then becomes independent of ion flux. The improvement in film quality observed with increasing ion flux at lower temperatures is similar to that found with increasing temperature. However, careful calibration of the substrate temperature indicates that the beneficial effect of increased ion flux is not simply due to substrate heating, implying that kinetic or momentum transfer effects are also important during ion bombardment.
TF-WeP-11 Electronic Properties of Cu(In,Ga)Se2 (CIGS)-Based Solar Cells
J. AbuShama (Colorado School of Mines)
We examined the electronic properties of CIGS-based polycrystalline thin-film solar cells by Deep Level Transient Spectroscopy (DLTS) and Capacitance-Voltage (C-V) measurements. We prepared four CIGS thin films (by Physical Vapor Deposition) in the region where the film transitions from Cu-rich to In(Ga)-rich. Cu-rich sample exhibits a shallow majority carrier trap with an activation energy of 0.12 eV and another deeper trap with an activation energy of 0.27 eV. The shallow trap with a concentration of about 2.1x10^14 cm^-3 yields the dominant emission in Cu-rich CIGS in the observed temperature and frequency range. The In-rich sample has a shallow minority carrier trap with an activation energy of 0.12 eV. This trap with a concentration of 6x10^14 cm^-3 yields the dominant emission in In(Ga)-rich CIGS in the observed temperature and frequency range. The two samples show evidence of a deeper trap at higher temperature. C-V measurements showed that the carrier concentration around the junction of the cell changed as the film transitions from Cu-rich to In-rich. We see that the acceptor (donor)-like traps are dominant in the Cu(In)-rich samples. The transition from the Cu-rich to the In(Ga)-rich compositions causes an extensive transformation of intrinsic defects from acceptor-like traps (e.g. Cu on In(Ga) sites) to donor-like traps (e.g. In(Ga) on Cu sites).
TF-WeP-12 Low Temperature Indium Tin Oxide Films Using Dual Magnetron Sputtering
H.S. Jeong, M.S. Hwang, H.J. Lee, Y.W. Seo (ITM Inc., Korea); S.J. Kwon (Kyungwon University, Korea)
Recently, the needs of low temperature deposition processes for transparent conducting oxide(TCO) films on a plastic substrate are increasing. Flexible plastic substrates have been used in liquid crystal displays, organic lighting-emitting diodes, and organic EL displays. Indium tin oxide films were deposited in a horizontal in-line sputter system equipped with a pulsed dual magnetron sputter source[DC ~50kHz]. An ITO target of In2O3(90 wt%):SnO2(10 wt%) has been used. The substrates(glass and flexible plastic) were cleaned by an O2 plasma processes. The properties of ITO films such as electrical resistivity, optical transmission, and surface roughness were investigated with respect to O2 partial pressure, pulse frequencies, and substrate temperature(room temperature ~ 150°C). Also, we will investigate the film structure and plasma contamination difference between the pulsed single magnetron and the pulsed dual magnetron sources by XRD and RGA, respectively.
TF-WeP-13 High Rate Deposition of ZnO Thin Films by Vacuum Arc Plasma Evaporation
T. Miyata, S. Ida, T. Minami (Kanazawa Institute of Technology, Japan)
Present requirements for zinc oxide (ZnO) thin films in various applications call for achieving further preparation cost reductions and higher deposition rates on larger area substrates. Recently, a newly developed vacuum arc plasma evaporation (VAPE) method providing high-rate film depositions on large area substrates has attracted much attention. In this paper, we describe the preparation of undoped and impurity-doped ZnO thin films on large area substrates by a VAPE method using oxide fragments as a low-cost source material. For example, the difficulty of preparing FZO thin films by sputtering deposition is well known. However, fluorine-doped ZnO (FZO) thin films suitable for transparent and conductive thin-film applications could be prepared. The film depositions were carried out under the following conditions: substrate, large area glass; substrate temperature, RT to 450°C; oxide fragments, sintered mixture of ZnO and ZnF2 powders; pressure, 0.08 to 1 Pa; Ar and O2 gas flow rates, 10 to 50 and 0 to 10 ccm; and cathode plasma power, 2.5 to 10 kW. A deposition rate of 150 nm/min and a resistivity on the order of 10-4Ωcm as well as a uniform distribution of resistivity and thickness on the substrate surface were obtained for FZO films deposited at a pressure of 0.15 Pa, an Ar gas flow rate of 20 ccm, a cathode plasma power of 4.5 kW and a substrate temperature of 250°C. In film depositions by VAPE, the rate was easily controlled by varying the cathode plasma power. It should be noted that deposition rates from 55 to 375 nm/min were obtained in ZnO films deposited at a deposition pressure of 0.25 Pa with a cathode plasma power that ranged from 2.5 to 10kW.
TF-WeP-14 Photocatalytic Properties of TiO2/WO3 Bilayers Deposited by Reactive Sputtering
T. Takahashi, H. Nakabayashi, J. Tanabe, N. Yamada (Toyama University, Japan)
Titanium dioxide (TiO2) has attracted considerable attention because it is known to have strong activity as a photocatalyst under irradiation of ultraviolet rays. However, TiO2 photocatalyst can only react slightly to solar energy because of its optical band gap EG smaller than 3.2 eV (λ of 380 nm). So, it is desirable for most solar energy applications to have new materials that can highly react strongly to visible-light. In this study, the bilayers composed of TiO2 and WO3 films have been prepared by reactive sputtering at room temperature. At first, TiO2 films with a thickness of about 1.4 µm have been deposited on glass-slide substrates at working gas pressure PW of 1 mTorr and a mixed atmosphere of Ar and O2, using the facing targets sputtering with dc power supply. Then WO3 films have been deposited onto as-deposited TiO2 ones at various PW ranging from 1 to 8 mTorr in an atmosphere of mixture gas of 80%Ar and 20%O2, using RF magnetron sputtering. The gaseous methyl alcohol by using the bilayers have been decomposed under irradiation with artificial sunrays of wavelength λ ranging from 330 to 750 nm for 2 h at room temperature. The bilayers were successful in carrying out a photocatalytic reaction of the decomposition of CH3OH into CO2 and H2O from measurement of infrared transmittance spectra. Its decomposition rate using the bilayers significantly increases, as compared with that using the monolayers of TiO2 films. It is found that the bilayers can decompose CH3OH under irradiation of only slight visible-light. Consequently, the bilayers deposited in this study have a good photocatalytic reaction and activate the decomposition of gaseous methyl alcohol.
TF-WeP-15 Influence of Preparation Conditions on Structure and Properties of WO3Films Reactively Deposited by RF Magnetron Sputtering
T. Takahashi, J. Tanabe, H. Nakabayashi, N. Yamada (Toyama University, Japan)
TiO2 photocatalyst can only react slightly to solar energy because of its optical band gap EG smaller than 3.2 eV (λ of 380 nm). So, it is desirable for most solar energy applications to have new materials that can highly react strongly to visible-light . Therefore, WO3 films with EG of 2.5 eV are very useful for the underlayer of TiO2 photocatalyst. In this study, WO3 films with thickness of 0.9-6.7 µm have been deposited on glass-slide substrates, using RF magnetron sputtering in an atmosphere of mixture gas of 80%Ar and 20%O2. The crystallographic and the surface structures and the optical properties of WO3 films deposited at the working gas pressure PW ranging from 1 to 8 mTorr have been investigated in detail. The as-deposited films showed dark metallic color as like as target at PW of 1 mTorr. The as-deposited films were yellow at PW of 3 mTorr. With further increase of PW, the color of the films changed to pale yellow. From the X-ray diffraction patterns, the as-deposited films were polycrystalline crystallizing in the monoclinic or the triclinic crystal structure with high c-axis orientation perpendicular to the film plane. The optical transmittance of the films deposited at PW of 1 mTorr is nearly zero. However, the transmittance of the films deposited at other PW are larger than 70 % in the wavelength λ ranging from 500 to 900 nm. With decreasing λ to 400 nm, the transmittance steeply become zero. The λ at this absorption edge is longer than that in TiO2 and comes in the visible region. The surface morphology of the films depends on PW. As PW increased, the surfaces of the films become rougher and grain sizes of the films also become larger. The WO3 films deposited in this study may be available for the underlayer of TiO2 photocatalyst.
TF-WeP-16 Ti Target Characteristics of Medium Frequency Reactive Sputtering: Process Modeling Improvement and Experimental Verification
Lai Zhao (Tsinghua University, China); S. Xu, C. Fan, W. Gao (HIVAC Technology (Group) Co. Ltd., China); X. Hou, Liangzhen Cha (Tsinghua University, China)
Deposition of TiO2 with reactive dual magnetron sputtering (DMS) is difficult to control due to its critical transition region of hysteresis curve caused by the high metallicity of Ti target. To stabilize the sputtering process, electrical characteristics of the Ti target is investigated in an on-line experiment. An improvement to the sputtering process modeling developed by S.Berg is proposed based on these experimental data. To better explain the hysteresis curve, it is found that the accumulation of positive argon ion charges on the oxidized target surface affects the electrical field and argon ion incident current density. Hence, the coverage of oxide component on the target will change correspondently. The changing argon ion current density due to charge accumulation is introduced to the equations. The simulated result fits well with the measured hysteresis curve.
TF-WeP-17 Effects of Deposition Conditions on Step Coverage Quality in Metal-organic Chemical Vapor Deposition of TiO2
S.Y. No, J.H. Oh, C.S. Hwang, H.J. Kim (Seoul National University, Korea)
Dielectric thin films including Ti element such as (Ba,Sr)TiO3(BST), SrTiO3(STO) have been investigated intensively by many researchers for next generation DRAM capacitor. Recently, the problem of cationic composition non-uniformity was reported when these multi-component films were deposited on patterned structure by metal-organic chemical vapor deposition(MOCVD) method. To understand the problem of composition non-uniformity in multi-component films, it needs to know the step-coverage variation of each component film. In this study, the step coverage of TiO2 thin films were investigated, which were grown on patterned Si substrates by liquid-injection MOCVD using Ti(O-i-Pr)2(thd)2 at substrate temperature ranging from 410°C to 500°C. The effects of various deposition parameters such as the chamber-wall and gas-line temperature, the source injection rate, the substrate temperature, solvents and deposition atmosphere on the step coverage quality were investigated and activation energy of deposition was also evaluated in each case. As the chamber-wall temperature increased from 230°C to 400°C, the step-coverage improved while degraded when gas line temperature was changed from 230°C to 400°C. With increasing the partial pressure of Ti source, the step-coverage improved slightly. When the substrate temperature was changed from 470°C to 410°C, the step-coverage quality enhanced, and the degree of variation in step-coverage was larger in oxygen-rich atmosphere than in Ar-rich atmosphere. Tetrahydrofuran(THF) and ethyl alcohol were used as solvents for Ti precursor dissolution and step-coverage is better when THF is used.
TF-WeP-18 Raman Spectroscopy Measurement of TiO2 Sputtered Films Changing Degree of Plasma Exposure
T. Takahashi, H. Nakabayashi, J. Tanabe, N. Yamada (Toyama University, Japan); W. Mizuno (Toyama Industrial Technology Center, Japan)
The relationship between the Raman spectra and the crystallographic orientation of the TiO2 films deposited at different working gas pressures have been investigated in detail. The TiO2 films with thickness of about 800-2300 nm have been reactively deposited on glass-slide substrates at an atmosphere of Ar and O2 mixture, using the facing targets sputtering. The maximum deposition rate of the film was 12.8 nm/min. The Raman spectra, and the crystallographic orientation of the films were measured using Raman spectroscopy, and X-ray diffractometry, respectively. Most of films deposited in this study were transparent and had anatase crystal structure, while some of them were semitransparent. The anatase phase of as-deposited TiO2 films has revealed in the Raman spectra with a Raman shift of 145 cm-1 and the X-ray diffraction patterns. When the TiO2 films have been deposited at the working gas pressures of 3 mTorr with increasing plasma exposure, the highest peak intensity with a Raman shift of 145 cm-1 steeply increased, while the X-ray peak intensity of A(220) with preferential crystallographic orientation also steeply increased, where A shows the anatase phase of TiO2. On the other hand, in the films deposited at the working gas pressures of 1 mTorr with decreasing plasma exposure, the Raman peak intensity at 145 cm-1 gradually decreased at the condition that the preferential crystal orientation changed from A(220) to A(101). This implies that the change in the Raman peak intensity at 145 cm-1 may be closely related to the change in the A(220) peak intensity. The A(220) peak was significantly higher than other peaks when the TiO2 crystallites became larger regardless of the working gas pressure. Consequently, it was found that the Raman peak intensity at 145 cm-1 in as-deposited TiO2 films were strongly affected by the plasma exposure related to the substrate position during film deposition.
TF-WeP-19 Growth Morphology of Sputter Deposited Vitreous Titanium Dioxide Films
J.D. DeLoach (Texas Instruments); R.S. Sorbello (University of Wisconsin-Milwaukee); G. Scarel (Laboratorio MDM-INFM, Italy); C.R. Aita (University of Wisconsin-Milwaukee)
Room temperature growth of a ceramic film usually occurs by coalescence of three-dimensional nuclei. This initial morphology leads to column formation. The boundaries between columns can be open and can have a different physiochemistry than the column interior. In this study, we use high resolution transmission electron microscopy and electron diffraction to study the growth structure of TiO2 films. The films were sputter deposited at room temperature on <111> Si substrates from which the nascent SiO2 layer had not been removed. The film thickness ranged from 250 to 700 nm. The films' atomic structure was vitreous, that is, had no long range order. Most of the films’ volume was amorphous with short-range order characteristic of anatase Ti-O nearest neighbor coordination. The remaining volume consisted of both anatase and rutile nanocrystallites. The most striking morphological feature observed by HREM was a network structure encompassing both amorphous and nanocrystalline regions in all films. Isolated regions within this network were on the order of tens of nanometers, increasing in size with increasing film thickness. Suboxide rings on electron diffraction patterns indicate that the network may be conducting. We suggest that this network has a profound effect on the properties of as-grown TiO2 films, in particular on their ultraviolet and infrared optical absorption behavior. A recent1 infrared absorption-reflection study showed that the model dielectric function which best fit the experimental data was obtained by averaging the dielectric functions of weakly interacting TiO2 regions of different orientations. This result was surprising because of the large polarizability of TiO2. Further analysis in the present study using a Maxwell-Garnett approach shows that this dielectric function is consistent with TiO2 units embedded in a conducting network.


1G.S. Scarel et al. J. Appl. Phys. 91 1118 (2002).
Time Period WeP Sessions | Topic TF Sessions | Time Periods | Topics | AVS2002 Schedule