AVS1998 Session EM-ThP: Electronic Materials and Processing Poster Session
Thursday, November 5, 1998 5:30 PM in Room Hall A
Thursday Afternoon
Time Period ThP Sessions | Topic EM Sessions | Time Periods | Topics | AVS1998 Schedule
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EM-ThP-1 Synthesis and I-V Characterization of Tin Dioxide Varistors
A. Kale (University of Central Florida); R.N. Barve (College of Engineering, India); S.K. Date (National Chemical Laboratories, India); P.N. Santhosh (Indian Institute of Science, India); S. Seal (University of Central Florida) Tin dioxide varistors are novel semiconducting ceramics having interesting non-ohmic current-voltage characteristics. They are widely used in consumer and military electronics, industrial protection, communications, transportation, data processing and other applications like medical devices. An overview of the steady state time dependent electrical properties of Tin dioxide varistors is presented. For a quantitative agreement with the experimental data the electronic defect states in the bulk of the Tin dioxide grains and at the interfaces between the grains were studied. The varistors were fabricated in two ways. The normal physical process (ceramic route) of mixing and compaction of Tin dioxide powder with various dopants and the second method being that of sol-gel processing in which dopants were taken in their aqueous solutions. Samples prepared by both the processes were studied for the dopant densification using Scanning Electron Microscope and X-ray diffraction. The effect of various sintering temperatures on their non-ohmic or non-linear characteristics was also studied. The varistors prepared by the sol-gel method showed high non-linearity, high density and high threshold voltage properties than the ceramic route owing to their finer grain structure and better densification of dopants around the grain boundaries. It was shown that the Tin dioxide non-linear properties are a function of grain size and depend upon the factors that influence the course of events at the sintering temperature. It is hoped that in coming years, the research efforts are likely to have path breaking impacts in design and development of better varistors |
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EM-ThP-2 First Principles Calculations of Ge (100) Covered by Up to One Monolayer of Pb
N. Takeuchi (Universidad Nacional Autonoma de Mexico) We have performed first principles total energy calculations to determine the atomic structure of the Ge(100) surface covered with different amounts of Pb. For low coverages, Pb starts growing parallel to the underlying Ge dimers (without breaking them), and it forms asymmetric dimers. This situation continues up to half a monolayer. At this coverage, several structures are possible: (2x2), p(4x2), etc. They are also formed by asymmetric Pb dimers on top of the Ge(100) surface. They differ from each other by the orientation of the buckled dimers. When the coverage is larger, the Ge dimer bonds start to break. >From this coverage,and up to one monolayer, the stable surface shows a c(4x8) reconstruction, similar to the one found in Sn on Si(100) and Pb on Si(100).It consists of rows of Pb ad-dimers, with one missing row out of every four, yielding a coverage of 0.75 of a monolayer.The dimers are also asymetric. Calculated local density of states are in excellent agreement with scanning tunneling microscope images. Surface formation energies show that this configuration is more stable than the full monolayer (2x1) structure. |
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EM-ThP-3 TEM Study of TiAl3 Formation
C.C. Pace (University of North Carolina, Chapel Hill and MCNC); M.K. Lamvik, M.A. Ray (Microelectronics Center of North Carolina); A. McTeer (Micron Technology) Metallizations consisting of Al-Cu alloys and Ti layers are widely used in microelectronic device fabrication, because of their low contact resistance and resistance to electromigration and hillock formation. As device dimensions are reduced, reaction mechanisms between layers must be understood at the atomic level to develop device structures that minimize failures due to resistive heating and stress induced voiding. Structures consisting of 350nm Al-0.5% Cu on 20nm Ti sputter deposited onto SiO2/Si substrates have been studied using a Philips EM 430 transmission electron microscope (TEM) equipped with a heating stage. TEM specimens of as-deposited samples were prepared and annealed in situ in the TEM. The temperature was varied up to 450 °C and times up to several hours. Within this temperature range the reaction rate for the formation of TiAl3 in the in-situ experiments agree with the published literature, although the range in published values is quite broad. Cross-sectional TEM micrographs reveal a non-uniform TiAl3/AlCu interface. The roughness revealed by TEM contradicts interpretations of Rutherford backscattering spectra (RBS). RBS suggest a layer-by-layer growth mechanism, which would produce a TiAl3 layer of uniform thickness. To show the equivalence of an in situ anneal and a furnace anneal, TEM samples were prepared from a wafer annealed in a furnace at 400 °C for 50 minutes. Cross-sectional micrographs revealed similar interface roughness. For both anneals, less than 10nm of Ti was consumed. An unreacted Ti layer remained even after extended annealing. In-depth profiling with Auger electron spectroscopy (AES) was utilized to confirm the presence of unreacted Ti. A model of the reaction mechanism that results in a non-uniform TiAl3 layer in contact with unreacted Ti will be proposed. It is critical that this mechanism be understood and controlled as device dimensions are scaled to ever smaller sizes. |
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EM-ThP-4 Side-Wall Damage in a Transmission Electron Microscopy Specimen of Crystalline Silicon Prepared by a Focused-Ion-Beam
N. Kato (IBM, Japan); H. Saka (Nagoya University, Japan); Y. Kohno (IBM, Japan) Focused ion beam (FIB) milling has recently been widely used for the preparation of cross-sectional transmission electron microscopy (TEM) specimens. Inevitably, however, such specimens are caused a certain amount of damage by highly accelerated ion beams. The damage to the side-wall of a specimen is visible under TEM as amorphization, and the structure of the undamaged intermediate layer can be observed through the damaged layers. Nevertheless, these damaged layers do pose a serious problem for TEM observation, especially when high-resolution observation is required. Many techniques have been developed for reducing the damage, such as gas-assisted etching. In this study, we experimentally investigated the depth of the side-wall damage in silicon, and the effect of the damaged layer on TEM observation. The depth of damage caused by a 30-keV FIB without gas-assisted etching was 20 nm, which was reduced to 10 nm by lowering the acceleration voltage to 10 kV. Gas-assisted etching with iodine does not markedly reduce the damage, as would be expected from the enhanced etching ratio. We also investigated several methods for reducing the damage after FIB fabrication. Broad-argon-ion-beam milling reduced the damaged layer to 12 nm, and wet-etching the specimen with a mixture of nitric and hydrofluoric acid removed most of the damage. Removal of the amorphous silicon layer significantly improved TEM observation. No amorphous ring such as that observed in the diffraction pattern (DP) of the specimen immediately after FIB fabrication was seen in the DP of the wet-etched specimen, in which a silicon lattice fringe was clearly observed. |
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EM-ThP-5 Heat-Transfer in UHV-Scanning Thermal Microscopy
W. Müller-Hirsch, J.P. Parisi, M.T. Hirsch, A. Kittel (University of Oldenburg, Germany); L.V. Govor, A.Yu. Olevanov (The State University of Belarus) Scanning thermal microscopy (SThM) offers the capability to map temperature distributions of samples with subµm resolution. A wide variety of microscope configurations and sensor designs have been described in literature. However, most experiments have been performed under ambient conditions and the heat-transfer between sensor and sample was attributed to be mostly due to a liquid film-bridge between sensortip and sample.1 In this study we use a Scanning Thermal Microscope to investigate the heat transfer under ultra-high vacuum (UHV) conditions. A needle-shaped Au/Ni-thermocouple-sensor is brought into close proximity of a liquid nitrogen cooled sample. We find an almost linear increase of the heat-transfer with decreasing tip-sample distance on a length-scale in the range of 10nm. Since our experiments are performed at a base-pressure of 10-10 mbar, the heat conduction path via a liquid film-bridge can be excluded. We discuss other mechansims like electron-, phonon- or photon-coupling between tip and sample as an origin of the heat-transfer in the near-field regime of the sample. Additionally, the observed near-field heat-transfer is sensitive with respect to the sample topography. Measurements of the plateau structure of Au-samples with a homogeneous temperature distribution will be presented and exlained by a simple geometric model. Further measurements investigate the joule-heating of a MIS-diode array with diode-dimensions of 2µm. The inhomogeneous temperature distribution due to the joule-heating of the diodes is clearly resolved in the thermal image of the sample.
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EM-ThP-6 Boron Phosphide Films Grown by Solid-Source Molecular Beam Epitaxy
D. Buchenauer, D. Dibble, K.F. McCarty, J.C. Lund, R.J. Anderson, M. Clift, D.L. Medlin, J.A. Schneider (Sandia National Laboratories) A significant improvement in the detection of thermal neutrons could be made through the use of zinc blende boron phosphide (BP) as the detection medium in a solid-state neutron detector. Earlier work on the growth of BP using vapor transport,1 flux growth,2 and chemical vapor deposition2 has produced crystalline material of the required thickness, however, autodoping of the BP has been too high to allow their use in neutron detection. Recent progress on the growth of amorphous films using Molecular Beam Epitaxy (MBE) has led to nearly stoichiometric films with improved electrical properties.3 Here we report on the first growth of BP films using electron beam evaporation of boron and thermal cracking of phosphorus vapor by a three-cell EPI cracker. Stoichiometric films have been grown at substrate temperatures as low as 300°C. The relationship between the electronic and microstructural properties of the BP films and their performance as solid-state neutron detectors will be examined.
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EM-ThP-7 Effects of Variously Configured Magnets on the Characteristics Of Inductively Coupled Plasmas
S.W. Hwang, Y.J. Lee (SungKyunKwan University, Korea); S.W. Joe (Kyoungki University, Korea); K.H. Kim (Hanyang University, Korea); G.Y. Yeom (SungKyunKwan University, Korea) Although high density plasma(HDP) sources have been employed for dry etching in microelectronics, their scale-up to process larger wafer size such as the substrates used in flat panel display(FPD) technology is not that easy due to the uniformity problem over a large area wafer size. In this study, to enhance plasma uniformity and density of an inductively coupled plasma source, the effects of variously configured magnets on the characteristics of the plasmas were investigated. As the magnets, Helmholtz type axial electromagnets and multi-dipole magnets composed of 8sets of permanent magnets around the chamber wall were used. The chamber was designed as a square mainly for the FPD application such as liquid-crystal display(LCD), plasma display panel(PDP), etc. To characterize the plasmas as a function of magnetic field strengths and the combination of the magnets, electrostatic probe(Hiden Analytical Ltd.), optical emission spectroscopy(OES: SC Tech. PCM402), and quadrupole mass spectrometer(QMS: Hiden Analytical Ltd. PSM) were used. Ion density, plasma potential, and electron temperature were measured along the chamber diameter and axial direction for Ar and Cl2/HBr gas combinations using the electrostatic probe. The results showed that high density plasma(1011-1012/cm3) with excellent uniformity(≤3%) near the wafer surface could be achieved along the chamber diameter by the combination of the axial magnets and multi-dipole magnets. Optical emission spectra and mass spectra (positive and negative ions, radicals, and neutrals) were also studied as a function of axial magnetic strengths and with/without multi-dipole magnets for Cl2/HBr gas combinations, and showed enhanced ionization and dissociation with the combined magnets. We believe that a suitable combination of axial magnets and multi-dipole magnets would also improve etch uniformities and etch rates of the large size wafers used in FPD. |
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EM-ThP-8 Atomic Order and Electron Affinity at AlN(0001) Surfaces
C.I. Wu, A. Kahn (Princeton University); E.S. Hellman, D.N.E. Buchanan (Bell Laboratories, Lucent Technologies) We have used Auger electron spectroscopy (AES), low energy electron diffraction (LEED), x-ray and ultraviolet photoemission (XPS and UPS) and inverse photoemission spectroscopy (IPES) to investigate the preparation, atomic order and electron affinity of AlN(0001) surfaces. AlN films 0.2µm thick were grown by molecular beam epitaxy on Si(111) substrates. Following ambient transfer to the surface analysis chamber, repeated cycles of nitrogen sputtering (1 keV) and annealing (10 min., 1050 °C) were necessary to obtain ordered surfaces with only a few percent of a monolayer of O contaminant. The resulting surfaces exhibited sharp 1x1 LEED patterns. The UPS (HeI and HeII) spectra exhibited sharp features allowing a clear identification of the valence band maximum (Ev). The position of Ev was confirmed by XPS measurements of the Al 2p core level and using the known binding energy of this level with respect to Ev.1 The UPS-IPES combination showed the Fermi level at 4.8 eV above EV and 1.6 eV below the conduction band minimum at the surface of our AlN. Given these numbers and in spite of the concomitant observation of a sharp feature at the onset of photoemission, generally associated with the occurrence of negative electron affinity, the electron affinity was calculated to be +2.3 eV. Finally, the deposition of 1-2 monolayer of Al on the (1x1) surface followed by a 5 min. 1250°C anneal led to a structure characterized by a (√3 x √3)-R30° diffraction pattern. This LEED pattern is consistent with RHEED observations for increasing Al surface concentration during growth.
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EM-ThP-9 Enhanced Electrical Performance of Au/n-GaN Schottky Diodes by Novel Processing
L. He (Northern Illinois University); X.J. Wang (University of Maryland at Baltimore County) GaN has attracted great attention recently for its application in electronic and optoelectronic devices. Applications including in blue lasers, visible light emitting diodes (LED), metal-semiconductor field-effect transistor (MESFET), high electron mobility transistors(HEMT), ultraviolet photodetectors, have been demonstrated. Metal/GaN contacts, both ohmic and Schottky, are of important for device applications. Studies of Schottky contacts on GaN were especially interested. Schottky barrier height is expected to be dependent on the metal work function due to the ionic nature of GaN, though the work function of the contact metal is not the exclusive factor determining the Schottky barrier height. In this study, the low temperature (LT = 77K) metal deposition technique was used to improve the electrical characteristics of Au/n-GaN Schottky diode. The LT deposition technique has been successfully used to fabricate high quality Schottky contacts on InP, GaAs and InGaAs. A comparison of the barrier heights is conducted with previously reported results. The same chip of GaN epitaxial layer was used for room temperature (RT) and LT Schottky diodes. The LT Schottky diodes exhibit excellent performance. The leakage current density as low as 2.55x10-11A×cm-2 was obtained from the LT diodes. The linear region in the I-V curve of the LT diodes at forward bias could extend more than eight orders in the magnitude of the forward current. Current-voltage-temperature (I-V-T) measurements were carried out to study the characteristics of the LT diodes. A typical barrier height of 1.32eV for the LT diode was obtained, which is the highest value ever reported. The obvious enhancement in electrical performance makes the LT processing a high promising technique for GaN device application. Analysis through photoluminescence (PL) and x-ray diffraction measurements were conducted to collaborate with the electrical characteristics. |
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EM-ThP-10 W-Based Ohmic Contacts on p- and n-Type GaN
X. Cao, F. Ren, S.J. Pearton (University of Florida, Gainesville); A. Zeitouny, M. Eizenberg (Technion-Israel Institute of Technology); J.C. Zolper (Office of Naval Research); C.R. Abernathy (University of Florida, Gainesville); R.J. Shul (Sandia National Laboratories); J.R. Lothian (Bell Laboratories, Lucent Technologies) W and WSi ohmic contacts on both p- and n-type GaN have been annealed at temperatures from 300-1000°C. There is minimal reaction (≤100Å broadening of the metal/GaN interface) even at 1000°C. Specific contact resistances in the 10-5 ohm cm2 range are obtained for WSix on Si-implanted GaN with a peak doping concentration of ~5x1020cm-3, after annealing at ~750°C. On p-GaN, leaky Schottky diode behavior is observed for W, WSix and Ni/Au contacts at room temperature, but true ohmic characteristics are obtained at 250-300°C, where the specific contact resistances are typically in the 10-2 ohm cm2 range. The best contacts for W and WSix are obtained after 700°C annealing for periods of 30-120 secs. The formation of Β-W2N interfacial phases appear to be important in determining the contact quality. |
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EM-ThP-11 Redistribution and Activation of Implanted S, Se, Te, Be, Zn and C in GaN
R.G. Wilson (Consultant); J.M. Zavada (U.S. Army Research Office); X. Cao, S.J. Pearton, R.K. Singh (University of Florida, Gainesville); M. Fu, J.A. Sekhar, V. Sarvepalli (Mycropyretics Heaters International); R.J. Shul, J. Han, D.J. Rieger (Sandia National Laboratories); C.R. Abernathy (University of Florida, Gainesville) We have previously found that implanted Si in GaN shows minimal redistribution after annealing at 1500°C, with an effective diffusion coefficient of ≤2x10-13cm2.s-1 at this temperature. In this experiment, common donor (S, Se and Te) and acceptor (Be, Zn and C) dopants were implanted at a typical dose of ~5x1014cm-2, with a projected range of ~1500Å. Annealing was performed with AlN encapsulation on the GaN, at temperatures up to 1500°C in a novel rapid thermal furnace utilizing intermetallic heating elements. After selective removal of the AlN cap in hot KOH solution, Secondary Ion Mass Spectrometry profiling was performed to measure the redistribution of the different dopant species. Effective diffusion coefficients were obtained from the broadening at full-width-half-maximum of the implanted profile. |
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EM-ThP-12 Etch Characteristics of GaN using Chemically Assisted Ion Beam Etching(CAIBE) and Its Effects on Ohmic Contact Formation to n-type GaN
W.J. Lee, G.Y. Yeom (Sungkyunkwan University, Korea); J.W. Lee, Y.J. Park, T.I. Kim (Samsung Advanced Institute of Technology, Korea) Currently, GaN facets required for GaN laser devices are fabricated using dry etching due to the difference in the cleavage planes of sapphire substrates and GaN epitaxial layers grown on them. Fabrication of GaN facets using dry etching not only requires high GaN etch rates, high selectivity over mask layers, and vertical etch profile with a smooth sidewall, but also requires damage-free etch surface which could be induced due to the energetic ion bombardment during the etching. This induced damage could degrade the electrical performance of the device. Therefore, in this study, we investigated the etch properties, etch-induced damage, and resistances of contacts formed on etched n-type GaN. The GaN samples used in the experiment were grown by metalorganic chemical vapor deposition on sapphire substrates and GaN etching was performed using CAIBE system(RF-350 Etching System, Veeco) having a 210mm diameter ion source, a Meissner trap, and a load-lock chamber. Ar was introduced into the ion source while Cl2, BCl3, and HCl were distributed around the substrate through the nozzle. GaN samples patterned with PR or SiO2 were loaded on the rotational fixture which could be tilted between 0° and 60° and also could be heated up to 300 C or cooled down to 0 C. To determine the etch characteristics, GaN samples were etched as a function of Ar ion beam parameter, gas chemistry, tilt angle, and substrate temperature. Etch characteristics such as etch rates, selectivities, and etch profiles were estimated using a profilometer and scanning electron microscopy(SEM). Variation of surface composition of the etched GaN samples was investigated using X-ray photoelectron spectroscopy(XPS). Contact resistances of the etched n-type GaN samples were measured by transmission line measured(TLM) and physical damage on the etched GaN surface was observed using HRTEM. The relations of surface composition and physical damage on GaN surface to the ohmic contact resistances were also studied. |
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EM-ThP-13 Growth of ß-SiC Thin Films on Si (100) at Low Temperature using Ultra-high Vacuum Electron Cyclotron Resonance Chemical Vapor Deposition
J.H. Pyo, K.W. Whang (Seoul National University, Korea) ß-SiC thin films were grown on Si (100) at low temperature using ultra-high vacuum electron cyclotron resonance chemical vapor deposition with gas mixtures of H2 / CH4 / SiH4 and their properties such as crystallinity and stoichiometry were investigated. Care was taken to prepare the clean, damageless Si surface prior to the growth. In situ H2 plasma cleaning, as well as conventional wet cleaning, were performed to confirm Si (2X1) reconstruction structure, which seemed to be an essential process to grow the ß-SiC thin films at low temperature. In addition, Si surface was Carburized with H2 / CH4 plasma at 700 °C before the growth of the film. After the in situ cleaning and the carburization, SiC thin films were grown on the carburized surface with the various ranges of microwave input power, SiH4 to CH4 flow ratio, and substrate temperature. Reflection high energy electron diffraction (RHEED) patterns of the films which were deposited at the substrate temperature 600 °C showed the changes from textured to ring pattern as the microwave power increased or the flow ratio decreased. Analyses of the films using XPS and spectroscopic ellipsometry showed that the ring and textured patterns were polycrystalline ß-SiC and Si-rich films, respectively. These results implies that stoichiometry of the film is affected by the microwave power and the flow ratio which presumably cause modification of the chemistry in plasmas. On the other hand, the crystallinity was affected by the substrate temperature. As the temperature increased up to 750 °C, the crystallinity was improved. |
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EM-ThP-14 Study of Pulsed versus Continuous Wave Plasma Deposition of Amorphous, Hydrogenated Silicon Carbide (a-Si1-xCx:H) from Silane/Methane Mixtures
P.M. McCurdy, J.M. Truitt, E.R. Fisher (Colorado State University) Hydrogenated amorphous silicon carbide (a-Si1-xCx:H) is an important material because of its current use as a window coating for amorphous Si solar cells and its potential applications in photoelectronics and as a hard coating. Equival ently powered, pulsed and continuous wave (cw) radio-frequency discharges (13.56 MHz) were used to deposit a-Si1-xCx:H films from silane and methane. Deposited films were studied using FTIR, XPS, Raman scattering spectroscopy, scanning electr on microscopy and profillometery. Deposition parameters investigated included pulsed plasma power, duty cycle (d.c.) substrate temperature, substrate bias, and addition of a carrier gas (H2, or He). Films deposited from pulsed plasmas show a large decrease in hydrogen incorporation compared to films deposited in equivalently powered CW systems. Notably, there is a significant decrease in modes associated CH3 moities. CH3 groups have been associated with electrical and mechanical inst abilities of a-Si1-xCx:H films. Grounding the silicon substrate is shown to have a significant effect on both the CW and pulsed plasma deposited films. |