Papers

ICMCTF2008 Session D1: Boron Nitride, Carbon Nitride and Group-III (Al, Ga, In) Nitride Materials

Friday, May 2, 2008 8:00 AM in Room Royal Palm 4-6

Friday Morning

Start	Invited?	Item
8:00 AM		D1-1 Plasma Enhanced Deposition of Cubic Boron Nitride Films Under Ultralow-Energy Ion Impact: Structural Evolution and Electrical Properties K. Teii (Kyushu University, Japan); S. Matsumoto (National Institute for Materials Science, Japan) Since the development of vapor-phase deposition techniques for cubic boron nitride (cBN) films in the early 90's, ion-bombardment is believed to be the essential factor. The threshold ion energies tend to decrease with increasing ion to depositing boron flux ratio and are typically 50 to several hundreds of eV for low-pressure plasmas and sputtering. A break-through has been made by using BF₃ as the source gas in a plasma jet¹. High quality, faceted cBN films have been deposited due mainly to specific chemical effects of fluorine on the growing surface. However, the ion energy at a substrate bias of -85 V cannot be measured due to collisional high-pressure conditions. Very recently, we demonstrated that cBN films can be deposited on Si with mean ion impact energies from 45 to a few eV or less in an inductively coupled plasma (ICP) by using the fluorine chemistry². Here, we present the phase and morphology evolution of cBN films by regulating the ion impact energy down to a few eV or less. We also present the electrical properties of the films such as field emission characteristics to explore the potential of cBN films for electronic devices. ¹S. Matsumoto and W. J. Zhang, Jpn. J. Appl. Phys. Part 2 39 (2000) L442. ²K. Teii, R. Yamao, T. Yamamura, and S. Matsumoto, J. Appl. Phys. 101 (2007) 033301.
8:40 AM		D1-3 c-BN Coating Systems for Cemented Carbide Cutting Inserts M. Keunecke, S.T. Park (Fraunhofer IST, Germany); J. Fuentes (Technical University Berlin, Germany); K. Weigel, K. Bewilogua (Fraunhofer IST, Germany) The requirements to tools continuously increase in production engineering and this leads to a strong demand for new super hard tool coatings. c-BN coatings could be one possibility. Cubic boron nitride (c-BN) has a high application potential especially as a super hard cutting tool coating. c-BN is the second hardest of all known materials, has a high wear resistance and a high thermal stability. Today sintered cutting inserts with c-BN grains, which were synthesized in an expensive high-pressure, high-temperature process, are in industrial use. But up to now no c-BN tool coatings are available. Unfortunately the preparation of c-BN coatings in µm scale is difficult, due to some serious drawbacks and succeeded only in the last years by different research groups. The technique of the Fraunhofer IST is a PVD sputtering technique of a boron carbide target in an argon/nitrogen atmosphere. This PVD process allows the preparation of c-BN films between 1 and 2 µm c-BN on TiAlN respectively TiN pre-coated cemented carbide cutting inserts. Measurements of mechanical properties like hardness and Young´s modulus reveal that the properties of the c-BN coatings, with hardness of about 60 Gpa, are nearly identical to those of c-BN bulk material. Furthermore properties like adhesion, growth defects and dependencies of the coatings system to different cemented carbide grades, different cutting inserts and deposition parameter variation will be presented as well as results of machining tests of the developed c-BN coating systems for cutting inserts. These new results confirm the application potential of c-BN coatings for tools.
9:00 AM		D1-5 Water Adsorption on Fullerene-Like Carbon Nitride Overcoats E. Broitman (Carnegie Mellon University); G.K. Gueorguiev, A. Furlan (Linköping University, Sweden); V.V. Pushkarev (Carnegie Mellon University); N.T. Son (Linköping University, Sweden); A.J. Gellman (Carnegie Mellon University); S. Stafström, L. Hultman (Linköping University, Sweden) Humidity influences the tribological performance of the head-disk interface in magnetic data storage devices. In this work we compare the uptake of water of amorphous carbon nitride (a-CN_x) films, widely used as protective overcoats in computer disk drive systems, with fullerene-like carbon nitride (FL-CN_x) and amorphous carbon (a-C) films. Carbon-based films with thickness in the range 10-300 nm were deposited on quartz crystal substrates by reactive DC magnetron sputtering. A quartz crystal microbalance placed in a vacuum chamber was used to measure the water adsorption. Electron microscopy and electron paramagnetic resonance (EPR) have been used to correlate water adsorption to film microstructure and surface defects (dangling bonds). Measurements indicate the amount of water adsorbed by the FL-CNx films is significantly lower than the amorphous ones, being the higher for pure a-C. EPR data correlate the lower water adsorption on FL-CNx films with a possible lack of dangling bonds on the film surface. To provide additional insight into the atomic structure of defects in the FL-CN_x, a-CN_x and a-C compounds, we performed first principles calculations within the framework of Density Functional Theory. Emphasis was put on the energy cost formation for vacancy defects and dangling bonds of relaxed systems. Cohesive energy comparison reveals the energy cost formation for different types of dangling bonds in different configurations is considerable higher in FL-CN_x than for a-CN_x and a-C films, being the lower to pure amorphous C. The theoretical calculations thus confirm the experimental results that dangling bonds are much less likely in FL-CN_x than in a-CN_x and a-C films.
9:20 AM		D1-6 Chemical and Mechanical Characterizations of Silicon Carbon Nitride Thin Films Synthesized by Magnetron Reactive Physical Vapour Deposition P. Kouakou, V. Hody - Le Caër, P. Choquet (Centre de Recherche Public Gabriel Lippmann, Luxembourg); M. Belmahi (Université de Nancy, France); H.-N. Migeon (Centre de Recherche Public Gabriel Lippmann, Luxembourg) Carbon nitride (preferably β-C₃N₄) is considered as a very promising material for mechanical applications, due to its predicted high hardness and low friction coefficient. Thus the production of CN_x has been studied by several techniques. However, the obtained carbon nitride films are mainly amorphous. Recently, some works reported that small amounts of silicon could promote the crystallization of CN_x films. Moreover, silicon carbon nitride materials show excellent mechanical properties, low friction coefficient, low chemical reactivity and high oxidation resistance. The preparation of SiCN thin films has then been investigated by various techniques including reactive magnetron sputtering of a SiC target¹. In this work, SiCN has been deposited by magnetron sputtering in N₂/Ar gas mixture with carbon and silicon targets. The use of two targets allows to synthesize material containing very small percentages of Si. Films with various compositions were deposited on Si and WC/Co substrates and were observed by Scanning Electron Microscopy and Transmission Electron Microscopy. The films have also been analyzed by high-temperature X-ray diffraction in order to study their thermal stability. Their hardness has been determined by nanoindentation measurements. The discussion has been carried out on the effect of Si percentage into the films on their morphology and crystallinity. The Si content has also been related to the mechanical properties as well as to the thermal stability of the SiCN films. ¹ K.B. Sundaram, Z. Alizadeh, R.M. Todi, V.H. Desai, Materials Science and Engineering A368 (2004) 103-108.
9:40 AM		D1-7 A Comparative Study of Carbon Nitride Coatings Deposited by DC-Magnetron Sputtering in Laboratory and Industrial-Scale Deposition Systems E. Broitman (Carnegie Mellon University); G. Greczynski, J. Böhlmark (Chemfilt Ionsputtering AB, Sweden); M. Alunovic, R. Cremer (CemeCon AG, Germany); L. Hultman (Linköping University, Sweden) Due to their superior wear resistance, high hardness, and low friction coefficient, carbon nitride (CN_x) coatings have been proposed as the best candidates to replace diamond-like carbon (DLC) films. The first successful industrial application of this material has been the use of very thin (~2 nm) films for the protection of hard disks. However, the scalability of CN_x coatings produced in laboratories to industrial scale has been difficult in applications where thicker films (1-5 µm) on steel substrates were required. In this paper we compare the deposition of thick CNx films using a commercial CemeCon CC800/9 and a laboratory-scale systems. A HIPIMS power supply (Chemfilt Ionsputtering) was used to treat 304 stainless steel and M2 high speed steel surfaces before deposition. During the pretreatment, the substrates were biased to Ubias=-450 V in the environment of a HIPIMS of Cr plasma in order to sputter clean the surface and to implant Cr metal ions. Subsequently, carbon nitride films were prepared by DC unbalanced magnetron sputtering from a high purity graphite target in a N2/Ar discharge at 3 mTorr. A series of depositions were obtained with samples at different rotation speeds and substrate temperatures. Cross sectional scanning electron microscopy studies revealed a columnar structure in rotating samples and a featureless structure in stationary ones. High resolution transmission electron microscopy (HRTEM) has shown in all cases an amorphous structure. The amount of nitrogen in the coatings, analyzed by X-ray electron spectroscopy (XPS), varied between 15 and 25 %. Identification of coating layer failures was done by the Daimler-Benz Rockwell-C adhesion test.
10:00 AM		D1-8 Defects and Doping in In-Rich Group III-Nitrides R.E. Jones, K.M. Yu, J.W. Ager III (Lawrence Berkeley National Laboratory); L. Hsu (University of Minnesota); E.E. Haller (University of California and Lawrence Berkeley National Laboratory); W. Walukiewicz (Lawrence Berkeley National Laboratory) The past six years have witnessed a rapid growth of interest in In-rich InGaN and InAlN as the ability to grow high quality, single crystal films has developed. These materials show promise for application in high efficiency solar cells, high speed transistors and other optoelectronic devices. However, the realization of such devices hinges on the ability to understand and control the behavior of the native donor defects that unintentionally dope the materials n-type and create heavily n-type layers on the surfaces of all films. The n-type doping causes a Burstein-Moss shift of the absorption edge, complicating measurements of the band gaps. We have used energetic particle irradiation to study the native defects in these materials. We show that the electron concentration and electron transport properties can be reproducibly controlled by the intentional introduction of native donor defects. The electron mobility can be explained by scattering by singly charged donor impurities and triply charged native defects, in addition to alloy disorder scattering. Further, thermal annealing of irradiated films is used to create highly doped films with high electron mobilities and improved photoluminescence. Due to the presence of the donor defects, p-type doping of In-rich, group III-nitrides has been elusive. We show evidence for p-type doping of these materials with Mg, which is hidden by the n-type surface layers. Achieving electrical contact to the p-type films is the major challenge for the realization of In-rich, group III-nitride-based devices.
10:40 AM		D1-11 Growth and Characterization of Er-Doped GaN Nano-Crystals J.H. Ahn, H.R. Kim, J. Kim (Korea University, Korea); M.A. Mastro, J.A. Freitas, R.T. Holm, C.R. Eddy, R.L. Henry (US Naval Research Laboratory) Erbium-doped semiconductors have been intensively researched for applications in optical communications. An intriguing host semiconductor is gallium nitride (GaN), which has a high solubility for incorporating erbium ions on the gallium cation sublattice. Additionally, the wide bandgap (3.39eV at room temperature) of GaN is advantageous for rare-earth based optoelectronics. A novel ErCl₃ seed was employed to selectively grown Er-doped GaN nano-crystals on sapphire substrates in a metal organic chemical vapor deposition (MOCVD) reactor at a temperature of 900°C. The vapor-liquid-solid (VLS) growth mechanism requires a liquid seed at growth temperature, thus, a chloride compound was employed to decrease the melting point of the seed. In contrast to a standard VLS mechanism, the seed was intentionally consumed during the growth process. This process is advantageous as rare-earth precursors are normally difficult to introduce into a MOCVD system and rare-earth compounds have low solubility into GaN at standard film MOCVD conditions (Tgrowth >1000°C). The length of the GaN nano- and micro-crystals was proportional to the growth time and their orientation was dependent on the orientation of the underlying c-, a- and r-plane sapphire substrate. X-ray diffraction and photoluminescence imaging confirmed the high-quality of the Er-doped GaN nanocrystals. Room temperature photoluminescence showed strong, distinctive green emission by erbium ions. Cross-sections of Er-doped GaN nano-crystals were created by focused ion beam cutting and, subsequently, analyzed by micro-Raman spectroscopy and Energy dispersive X-Ray spectroscopy. Micro-Raman spectroscopy of the A₁(TO), E₂(High) and E₁(TO) phonons confirmed the high-quality of the hexagonal GaN.