Symposium C Poster Session

Wednesday, May 2, 2001 5:00 PM in Room Atlas Foyer

CP-2-1 Electrical and Optical Properties of Co/ITO Contacts to p-GaN
D.W. Kim, Y.J. Sung, G.Y. Yeom (Sungkyunkwan University, Korea); Y.J. Park, J.S. Kwak (Samsung Advanced Institute of Technology, Korea)
In the fabrication of GaN-based light emitting diodes(LEDs), ohmic contacts are generally made by depositing metal layers on the top of GaN-based LEDs. Among the metal contacts to GaN-based LEDs, highly conductive ohmic contacts to p-GaN are known to be difficult to be fabricated due to the difficulty in the doping of p-GaN. Currently, using appropriate metal layers such as Ni/Au, low resistive ohmic contacts to p-GaN were successfully obtained with the sacrifice of optical transmittance lower than 40@%@. However, if the transmittance through the contact could be increased by using optically transparent material layers as the ohmic contact to p-GaN instead of partially transparent metal layers, improved light emitting properties of the GaN-based LEDs could be obtainable when the resistance of the contact is acceptable as the ohmic contact. In this study, we attempted to form more transparent and low resistance ohmic contacts to p-GaN using Co/ITO and Co oxide/ITO thin films as contact materials. Electrical, physical, and optical properties of these material combinations were investigated and, using circular TLM patterns, contact properties of these materials to p-GaN were also investigated. I-V curves of as-deposited Co/ITO and Co oxide/ITO contacts showed a schottky contact characteristics but these were changed to ohmic-like contact with annealing conditions having less than 10@super -1@ @ohm@-cm@super 2@ of contact resistivity. Also, these contacts showed the transmittance higher than 80@%@ at 420 nm (blue light wavelength region). The electrical and light emitting properties of GaN LEDs fabricated using these contacts were also investigated.
CP-2-2 Optical, Structural and Mechanical Properties of Tungsten Oxide Thin Films
A. Monteiro, V. Teixeira, B. Almeida, H.N. Cui, M.F. Costa (Universidade do Minho, Portugal)
Electrochromic devices present the prospective to arbitrary control the level of light transition in response to the change in the environment. Electrochromism is defined as a reversible optical change of a material to a bleached, colored or removal under an applied voltage. The interests in these materials have gained the attention of a great number of researchers due their promising applications as flat panel displays and smart windows. Reactive DC magnetron sputtering of tungsten was performed in plasma of Ar+Oxygen in microscopy glass and Indium-Tin-Oxide (ITO) coated glass, at different temperatures (RT, 200 C and 350C) and Bias voltage. The working pressure was 1.3 Pa and 4.4 Pa for different experiences, for a constant current and substrate-target distance (60mm). Thin Film thickness was determined from Swanepoel, interferometer of Michelson measurements and SEM analysis. X-ray diffraction analysis was used in order to characterise the structure film. Spectral transmittance in the visible and near infrared of the deposited oxide films was measured using a double-beam spectrophotometer. Residual stress was estimated by parabolic profile of PVD films using a micro-displacement laser transducer. PRAXIS/P/CTM/11235/1998” Research Project in part supported this work. A. Monteiro is gratefully for the support of Portuguese Foundation for Science and Technology (“PRAXIS/CTM/11235/BIC/99”).
CP-2-3 Stress Effect on the Binding Energies of Noble Gases
R.G. Lacerda (Instituto de Fisica, Unicamp, Brazil); P. Hammer, F. Alvarez (Universidade Estadual de Campinas, Brazil); F.C. Marques (UNICAMP, BRAZIL)
Deposition systems such as ion beam sputtering and rf sputtering are very common in industry and usually use noble gases for film deposition as well as for sputter-cleaning surfaces and depth profiling process. Usually some small concentration of noble gas is unintentionally implanted into the host structure. In this work, we report the influence of the structure of an amorphous carbon matrix on the core level electrons of implanted noble gases (He, Ne, Ar, Kr and Xe) used in the sputtering deposition process. The films were prepared in an ion-beam-assisted deposition chamber (IBAD) including two Kauffman ion sources. Some fractional noble gas is trapped in the film during the assisted deposition process and is subjected to the highly strained environment of the carbon matrix. The stress of a-C:H films can be as high as 10 GPa, which is an extremely high stress. Thus, the carbon matrix provides an interesting environment for observing the effect of pressure on the electron wave function of noble gases. X-ray photoelectron spectroscopy shows that the noble gas core level energies shift linearly to lower binding energies as a function of compressive stress. It is suggested that these shifts are caused by the compression of the outer valence wave function of the implanted gas and by an extra-screening effect from valence electrons of the host atoms. Since XPS is a surface technique, we propose the use of the noble gas core level energy (obtained by XPS) as a stress probe of very thin amorphous carbon films. This new technique can be a very important tool for measuring stress of very thin films, since conventional techniques require thick films.
CP-2-4 Optical, Structural, and Photoelectric Properties of Thin n-ZnO Films on p-Si
H.Y. Kim, J.H. Kim, Y.J. Kim, M.O. Park, S. Im (Yonsei University, Korea)
ZnO has been extensively studied because it can be used in many applications such as transparent conductive films, optical sensors, solar cell windows, and surface acoustic wave transducers. There are several techniques to produce ZnO thin films; metal organic chemical vapor deposition(MOCVD), molecular beam epitaxy(MBE), RF-sputter, and pulsed laser deposition(PLD). In the present paper, we introduce the RF-sputter deposited ZnO as a transparent conductive window on Si photodetector. N-ZnO/p-Si heterojunction photodiodes were fabricated with varying the substrate temperature and the Ar:O@sub 2@ ratio. Current-voltage measurements were performed on n-ZnO/p-Si diode with or without a red laser beam delivering 56 mW/cm@super 2@ power density onto the ZnO surface at a dark room. Under a reverse bias of 5 V, we clearly observed significant amount of photocurrent when the red laser beam was applied to the ZnO surface. Infrared reflectivity and transmission measurements show the presence of symmetry-forbidden c-axis ZnO phonon mode at non-optimal temperatures. Free carrier density was insensitive to growth temperature. Besides, other characterizations such as XRD, Hall measurement were performed and the results will be discussed to study the correlations among the physical properties of ZnO thin films.
CP-2-5 Study of the Properties of Sol-Gel Prepared ZnO:Al Thin Films, as a Function of the Al Concentration
O. Jiménez-Sandoval, R. Castanedo-Pérez, D.C. Altamirano-Juárez, J. Márquez-Marín, G. Torres-Delgado, S. Jiménez-Sandoval (CINVESTAV-Querétaro, Mexico)
In this work, we present a study of the properties of Al-doped ZnO thin films, obtained by the sol-gel technique, as a function of the aluminum concentration. The precursor solution was prepared by using zinc acetate dihydrate as the metal source, and aluminum(III) nitrate nonahydrate as the doping agent. The films were deposited on slide-glass substrates and annealed at 400°C, in open air. The aluminum concentration was varied in a wide range, from 0.001 at.% to values as high as 20 at.% in solution. The X-ray diffraction patterns show an increase in the full-width at half-maximum of the peaks, with the Al concentration. The grain size, calculated from such values, decreases from approximately 350 (0.001 at.%) to 110 Å (20 at.%). The Atomic Force Microscopy images, for films with concentrations lower than 0.5 at.%, are in keeping with this tendency, whereas for higher concentrations conglomerates of microcrystals are likely to be present. The UV-Vis spectra of the films show a shift of the absorption edge towards shorter wavelengths, as the Al concentration increases, without modification of their high transmission (> 90%, at @lambda@ @>=@ 450 nm). The resistivity of the films in the range studied, decreased from 10@super 2@ to 10@super 1@ @ohm@ cm, with the Al concentration.
CP-2-6 Transparent Conducting (CdO)@sub y@(ZnO)@sub 1-y@ Thin Films, Obtained by the Sol-Gel Technique
S. Jiménez-Sandoval, G. Torres-Delgado, C.I. Zúñiga-Romero., R. Castanedo-Pérez, O. Jiménez-Sandoval (CINVESTAV-Querétaro, Mexico); B. Chao (Energy Conversion Devices)
(CdO)@sub y@(ZnO)@sub 1-y@ thin films were prepared by the sol-gel method, with Cd content in solution in the 0 @<=@ x @<=@ 1 range, at @DELTA@x intervals of approximately 0.17. The precursor solution had a basic pH, which is a novel feature for this system and allows a good substrate wetting. The films, on glass substrates, were annealed at 400 °C, in open atmosphere.The X-ray diffraction data clearly show that the films are a mixture of both oxides in the 0 @<@ x @<@ 1 range. Electron Dispersion Spectroscopy results indicated that the Cd content in the films, is lower than that in solution. The UV-Vis spectra also show the presence of both oxides, with transmission values higher than 90 percent at @lambda@ @>=@ 500 nm. The resistivity decreases from a value of 10@super 2@ @ohm@-cm to 1-2 x 10@super -3@ @ohm@-cm, i. e. five orders of magnitude, as x (and y) increases. On this basis, the (CdO)@sub y@(ZnO)@sub 1-y@ thin films herein reported may be considered as an example of a system where the optical and electrical properties of the films can be varied, according to the application sought.
CP-2-7 Application of FTIR Phase-Modulated Ellipsometry to the Characterisation of Thin Films on Surface-Enhanced IR Absorption Active substrates
E. Garcia-Caurel, E. Bertran, A. Canillas (Universitat de Barcelona, Spain)
FTIR phase-modulated ellipsometry has been used to characterise amorphous hydrogenated silicon (a-Si:H) thin films, deposited by PECVD, on substrates covered by an Au discontinuous film. The Au metallic islands generates surface-enhanced infrared absorption (SEIRA) in the deposited a-Si:H material. The ellipsometric infrared spectra of the substrate/(Au+void) and substrate/(Au+a-Si:H)/a-Si:H structures were simulated assuming a multilayer model, and using the Bruggeman effective medium approximation for the heterogeneous layer. The dielectric function of the Au islands was calculated taking into account their shape distribution. TEM results were used to assits the fitting procedure avoiding parameter correlation. The a-Si:H material shows two clear bands at 2130 and 2250 cm@super -1@ that can be associated to the OSi@sub 2@-SiH and O@sub 3@-SiH stretching vibration modes respectively.
CP-2-8 Superconducting Coating on Policrystalline Nickel Substrate
A Verdyan, I Lapsker, J Azoulay (Holon Academic Institute of Technology, Israel); V Beilin (The Hebrew University of Jerusalem, Israel)
Superconducting YBCO thin films were grown by resistive evaporation on multicrystalline Ni film which was evaporated on MgO substrate and on weak textured Ni-tape without any buffer layer. Evaporation was fabricated in inexpensive conventional vacuum system consist of a turbomolecular pump and featuring a base pressure better than P=1mPa. Ni film was first evaporated on MgO substrate kept at 400°C during the evaporation after which, with no further annealing, a precursor mixture of yttrium small grains and Cu and BaF@sub 2@ in powder form weighed in the atomic proportion to yield stoichiometric YBCO was evaporated. Upon moderate rolling deformation (about 35% relative thickness reduction) and recrystalisation annealing in air at 900°C a texture is built in Ni-tape and after than YBCO film was evaporated under the same conditions. X-ray diffraction, scanning electron and atom force microscopy techniques were used for texture and surface analysis. Electrical properties were determined using a standard dc four-probe method for electrical measurements.
CP-2-9 Initial Studies Into Dielectric Films Produced by the Dual Bipolar, Pulsed, Unbalanced, Magnetron Deposition Process in the Coplanar Configuartion.
J. O'Brien, P.J. Kelly (University of Salford, United Kingdom); J.W. Bradley (UMIST, United Kingdom)
The use of pulsing to promote arc-free, stable deposition is well known and a number of studies have been, and are presently, being carried out relating the pulsing parameters to plasma and film properties for the single magnetron case. For long-term deposition of dielectrics the process has the inherent problem of anode loss due to the build up of insulating layers. Dual bipolar deposition overcomes this problem by maintaining a clean anode surface in the form of alternative target faces. In this system the two magnetrons used are powered by the same power supply and pulsing unit and as such each target can be controlled to act alternatively as an anode and a cathode. This technique has been applied in industry. However, the interrelation of pulsing parameters and film and plasma properties is less well documented for this process. In this study we consider the effect of pulsing parameters of the dual bipolar system on dielectric films deposited from coplanar targets in a small production-scale system. Film properties investigated include structural, optical and tribological characteristics. Moreover, variations in these properties are discussed in terms of the variation in current-voltage waveforms, power supplied and plasma characteristics with varying pulse parameters.
CP-2-10 An Increased Reaction Rate for Benzospiropyran Photochromics
J.P. Hagen, D.A. Drakulich, I. Becerra, R.O. Dillon (University of Nebraska)
Photochromic materials have optical properties that change reversibly in response to irradiation. These changes in the visible and infrared spectra were reported earlier for the benzospiropyran used in this work@footnote 1@. A reasonable reaction rate for both the forward and reverse reaction is necessary for applications. These applications include modulating visible light for smart windows in buildings and changing infrared emissivity for thermal control of satellites. The time constant for colorless spiropyrans to darken to the merocyanine form under ultraviolet radiation is on the order of ten minutes, but the reverse reaction is very much slower. The objective of this work was to increase the reaction rate for the reverse reaction. This was achieved by modifying the benzospiropyran with particular porphyrins. Benzospiropyran in a polymethylmethacrylate matrix was modified by the addition of one of eight porphyrins. We report second order rate constants obtained from the slope of the reciprocal of the maximum absorption amplitude vs. time. Data taken with an FTIR spectrometer showed that every porphyrin increased the reaction rate, with phthalocyanine showing the fastest reversion rate, twenty one times that of the unmodified benzospiropyran. The effect on the visible reaction rate was determined using vanadyl porphyrins. A Perkin Elmer UV-VIS spectrometer showed that a particular vanadyl porphyrin gave an increased reaction time by a factor of about three. This work has been supported by NASA under Grant No. NCC5-169.@footnote 1@ Yeon-Gon Mo, R.O. Dillon and P.G. Snyder, J. Vac. Sci. Technol. A17, 170-175 (1999).