ICMCTF2009 Session D1: Boron Nitride and Carbon Nitride and Group-III (Al, Ga, In) Nitride Materials

Tuesday, April 28, 2009 1:30 PM in Room Royal Palm 4-6

Tuesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	D1-1 Cubic Boron Nitride Based Metastable Coatings and Nanocomposites J. Ye, S. Ulrich, M. Stüber, C. Ziebert (Forschungszentrum Karlsruhe, Germany) Various PVD and plasma-assisted CVD methods presently used for the deposition of cubic boron nitride (c-BN) thin films demand adequate conditions relating to ion bombardment of growing films, growth temperature, film stoichiometry etc. The ion-bombardment conditions, often appearing rather apparatus-specific, can be however well categorized according to the fundamental parameters of bombarding ions as well as condensing neutral particles, namely their energy and flux density. In terms of these fundamental parameters, the mechanisms for the c-BN formation are briefly discussed, along with an evaluation of the available growth models. Due to intensive ion bombardment during deposition, c-BN films are known for their extremely high compressive stress and poor adhesion. The present study focuses then on the magnetron-sputtered, c-BN-based metastable films and nanocomposite films. Under suitable and well-controlled growth parameters, the deposited films exhibit microstructur es dominated by c-BN phase, while on the other side show considerably reduced internal stresses in comparison to conventional c-BN films. Some examples will be shown, particularly c-BN/a-C nanocomposite and c-BN:O metastable films, including their deposition details, structure and composition characterization (HRTEM, SEM, XRD, XPS, AES, FTIR, etc.), and corresponding mechanical properties (hardness, E-modulus, stress). A film design concept will also be demonstrated as well as its successful realization for well-adherent films consisting of an adhesion-promoting base layer, a graded cubic-phase nucleation, and a low-stress, superhard, above 2 µm thick, c-BN-dominated top-layer.
2:10 PM		D1-3 Machining of High Performance Workpiece Materials with cBN Coated Cutting Tools E. Uhlmann, J.A. Oyanedel Fuentes (Institute for Machine Tools and Factory Management, Germany); M. Keunecke (Fraunhofer Institute for Surface Engineering and Thin Films, Germany) The machining of high performance workpiece materials requires significantly harder cutting materials. In hard machining, the early tool wear occurs due to high process forces and temperatures. The hardest known material is the diamond, but steel materials cannot be machined with diamond tools because of the reactivity of iron with carbon. Cubic boron nitride (cBN) is the second hardest of all known materials. The supply of such PcBN indexable inserts, which are only geometrically simple available, requires several work procedures and is cost-intensive. The development of a cBN coating for cutting tools, combine the advantages of a thin film system and of cBN. Flexible cemented carbide tools, in respect to the geometry can be coated. The cBN films with a thickness of up to 2 µm on cemented carbide substrates show excellent mechanical and physical properties. This paper describes the results of the machining of various workpiece materials in turning and milling operatio ns regarding the tool life, resultant cutting force components and workpiece surface roughness. In turning tests of Inconel 718 and milling tests of chrome steel the high potential of cBN coatings for dry machining was proven. The results of the experiments were compared with common used tool coatings for the hard machining. Additionally, the wear mechanisms adhesion, abrasion, surface fatigue and tribooxidation were researched in model wear experiments.
2:30 PM		D1-4 Magnetron Sputter Deposition of Low-Stress Cubic Boron Nitride Films using Ar-N₂-CH₄ Gas Mixtures S. Ulrich, J. Ye, M. Stüber, C. Ziebert (Forschungszentrum Karlsruhe, Germany) Cubic boron nitride (c-BN) films produced by PVD and plasma-assisted CVD techniques typically exhibit undesired high compressive stresses. One of the effective and feasible methods to reduce stress and hence improve film adhesion has been a controlled addition of a third element into the film during deposition. In the present study, BN films were grown on to silicon substrates using reactive magnetron sputtering with a hexagonal BN target. An auxiliary flow of methane was mixed into argon and nitrogen as the working gas. The deposition was conducted at various methane flow rates at 400°C substrate temperature, 0.3 Pa total working pressure, and -250 V substrate bias. The structure and mechanical characteristics of the deposited films were then examined in dependence of the methane flow rate. With increasing methane flow rate from 0 to approx. 1.0 sccm, the fraction of the cubic BN phase in the deposited films decreased gradually down to approx. 70%, whereas the film stress was reduced much rapidly and almost linearly in relation to the methane flow rate. At 1.0 sccm methane, the stress became approx. 3 times reduced. Owing to the significantly decreased film stress, adherent, micrometer thick, c-BN dominant films can be allowed to form on silicon substrate. The microstructure and mechanical properties of the films will be illustrated through FTIR, SEM, AFM, nanoindentaion and so on.
2:50 PM		D1-5 Texture Development and Microstructure Characterization of AlN Thin Films Fabricated by Pulsed Closed Field Unbalanced Magnetron Sputtering F. Wang, A. Feldman, J. Lin, J.J. Moore, B. Mishra, M. Hasheminiasari (Colorado School of Mines) Aluminum nitride (AlN) thin films were synthesized by reactively sputtering Al metal target in pure nitrogen atmosphere using a pulsed closed field unbalanced magnetron sputtering system on different substrates with a thin Cr interlayer. The texture orientation and the microstructure of AlN films were characterized by means of X-ray diffraction and scanning electron microscopy. The mechanical properties of the coatings were studied using the nanoindentation. It was found that both the pulsing frequency and the working pressure significantly affected the (002) orientation in AlN films. Strong (002) orientation has been achieved in the AlN thin films under various pulsing conditions, which can be correlated to the high ion energies associated with the pulsed magnetron sputtering. In addition, the effects of the pulsing parameters and working pressures on the microstructure, crystalline size, and residual stress of the AlN films were investigated in detail.
3:10 PM		D1-6 Theoretical and Spectroscopic Investigations on the Structure and Bonding in B-C-N Thin Films E. Bengu, M.F. Genisel, O. Gulseren, R. Ovali (Bilkent University, Turkey) In this study, we have synthesized boron, carbon, and nitrogen containing films through rf magnetron sputtering. We investigated the effects of deposition parameters on the chemical environment of B, C, and N atoms inside the films deposited. Techniques used for this purpose were GIR-FTIR, XPS, TEM and EELS. A hydrogen terminated surface was generated on the substrates before deposition. A 20nm Ti buffer-layer was deposited on the hydrogen terminated Si substrate just before the B-C-N films were deposited on the substrate. As for the boron and carbon source a 2 inch diameter B₄C target was used. During the deposition the pressure of the chamber was kept at 2.5 x 10^-3 torrs during the deposition while the magnetron power on the B₄C target was adjusted to be 200 watts during period of deposition (~ 90 mins). A separate r.f. power supply attached to the substrate holder was used to apply and independently adjust the d.c. bias on the substrate between gr ound and -500 V. Meanwhile, gas flow into the reaction chamber was controlled by two mass flow controllers which helped manage the Ar to N₂ gas ratio for the deposition. GIR-FTIR experiments on the B-C-N films deposited indicated presence of multiple features between 600 cm^-1 to 1700 cm^-1 range in the infrared spectra. Analysis of the IR spectra and the corresponding XPS data from the films has been done in a collective manner. The results from this study suggested under nitrogen rich synthesis conditions (higher N₂ ratio in Ar/N₂ mixture) carbon in the B-C-N films prefers to be surrounded by other carbon atoms rather than boron and/or nitrogen. Furthermore, we have observed a similar behavior in the chemistry of B-C-N films deposited with increasing substrate bias using again IR and XPS data together. In order to better understand these results, we have compared and evaluated the energetic for various nearest-neighbor and structur al configurations of carbon atoms on a single BN sheet environment using DFT calculations. These theoretical calculations also indicated that structures and configurations that increase the relative amount of C-C bonding with respect to C-B and/or C-N were energetically favorable than otherwise. As a conclusion, carbon tends to phase-segregate in to carbon clusters rather than displaying a homogeneous distribution in the films deposited in this study.
3:30 PM	Invited	D1-7 Atomic and Electronic Structure of GaN Surfaces J.E. Northrup (Palo Alto Research Center) The InAlGaN materials system is employed in several important technologies including light emitting diodes, blue/green lasers, and electronic devices. Realization of the technological promise of the nitride materials system has required growth of high quality epitaxial material. This requirement has motivated many experimental and theoretical studies of GaN surfaces, with attempts to understand epitaxial growth in terms of the surface structure. Nitride surfaces exhibit considerable complexity in their atomic structure, the nature of which is dependent on the relative abundance of the group-III and group-V species and the crystallographic orientation of the surface. In addition surface states are the source of electrons in the two-dimensional electron gas at the AlGaN/GaN interface in high-electron mobility transistors, and this has motivated studies of surface electronic structure. In this talk I will present the results of first-principles calculations employing pseudopotential density functional theory. The calculations provide insight into the atomic and electronic structure of nitride surfaces. Some emphasis will be placed on the group-III metallic adlayers which are stabilized under group-III rich conditions, and how these structures impact surface diffusion and impurity incorporation. This work was supported in part by the US-DARPA-VIGIL program (Dr. H. Temkin).
4:10 PM		D1-9 Highly Reflective and Thermally Stable Ni/Ag Multi-Layer Ohmic Contact to p-Type GaN J.H. Son, Y.H. Song, J.-L. Lee (Pohang University of Science and Technology (POSTECH), Korea) Vertical-structure have been exploited in GaN-based light emitting diodes to improve light extraction efficiency and thermal stability. In VLEDs, emitted light from active regions is reflected-up from reflective ohmic contacts on p-type GaN. Ag is very suitable for reflective ohmic contacts due to its high reflectance (>95%)In addition, low contact resistivity have been obtained from Ag-based ohmic contacts annealed in oxygen ambient. However, annealing in oxygen ambient causes Ag to be oxidized and/or agglomerated, leading to degradation in both reflectance and adhesion to GaN and overlaid metals. Therefore, preventing Ag from oxidation and/or agglomeration is a key aspect in obtaining high quality Ag-based ohmic contacts suitable for application to high-power LEDs of solid-state lighting. Mg-doped p-type GaN films used in this work were grown on (0001) sapphire substrate using MOCVD. For measurements of contact resistivity using the TLM, active regions were defined by ICP. Ni/Ag single-layer and multi-layer contacts were deposited by electron beam evaporation. Both samples were annealed at temperatures ranging from 300 to 500°C in air ambient. Current-voltage characteristics of the contacts were examined using a semiconductor parameter analyzer. Light reflectance of the contacts was measured using a monochromator and tungsten-halogen light source. In this paper, new metallization scheme with high reflectance and excellent thermal stabiltiy has been developed for obtaining low resistance ohmic contacts on p-type GaN. Ni/Ag multi-layer contact showed lower contact resistivity as low as 8.2×10^-6 ωcm², higher reflectance of 84.3% at 460 nm than Ni/Ag single-layer contact after annealing at 450°C in air ambient. To evaluate the thermal stability of the contacts, the changes of contact resistivity and light reflectance as a function of annealing time at 500°C were measured. The contact resistivity increased from 4.6×10-5 to 6.1×10^-3 ωcm² for the Ni/Ag single-layer contact after annealing for 24 hrs, but it increased from 2.1×10-5 to 8.5×10^-4 Ωcm2 for the Ni/Ag multi-layer contact. The light reflectance of the Ni/Ag single-layer contact decreased to 72% after annealing for 24 hrs, but the Ni/Ag multi-layer contact showed higher reflectance of 81%. High resolution x-ray diffraction scans were to evaluate the microstructural changes of the contacts. We propose the origin on the enhanced thermal stability of Ni/Ag multi-layer ohmic contact to p-type GaN.
4:30 PM		D1-10 Influence of rf Power on the Electrical and Mechanical Properties of CN Thin Films Deposited by Reactive RF Magnetron Sputtering I. Banerjee, N. Kumari, M. Kumar, S.K. Mahapatra (Birla Institute of Technology, India); S.K. Pabi (Indian Institute of Technology Kharagpur, India); P.K. Barhai (Birla Institute of Technology, India) Nano structured Carbon nitride thin films were deposited at different rf power in the range of 150 to 225 W at a constant pressure and Ar/N2 ratio of 1:1 by reactive rf sputtering. The films were characterized by SEM, AFM for surface morphology and roughness respectively. Atomic weight percentage ratio of C/N in the films was determined using EDX and Raman Spectroscopy. The N/C ratio decreased from 0.75 to 0.59 with increase in rf power from 150 W to 225 W. The electrical properties were measured by LCR meter. The surface resistivity decreased with increase in rf power and C/N ratio in the films. The hardness of the films were measured using nano indenter. The hardness of the film increased with increase in rf power due to increase in sp3 hybridized C-N sites in the deposited films. The chemical bonding between Carbon and Nitrogen was characterized by FTIR spectroscopy and Raman spectroscopy. FTIR studies showed absorption band positioned at 1000-1700 cm-1 due to C–N, C=N, C=C, and at 2200 cm-1due to C≡N in each film.