Ion-Assisted Pulsed Laser Deposition of Cubic Boron Nitride on Si (100) Substrates
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Ion-Assisted Pulsed Laser Deposition of Cubic Boron Nitride on Si (100) Substrates
T. A. Friedmann, K. F. McCarty, E. J. Klaus, H. A. Johnsen, D. L. Medlin, M. J. Mills, D. K. Ottesen, and R. H. Stulen Sandia National Laboratories, Livermore CA, 94551 ABSTRACT We are studying the boron nitride system by using a pulsed excimer laser to ablate from hexagonal BN (hBN) targets to form cubic BN (cBN) films. We are depositing BN films on heated (400'C) Si (100) surfaces and are using a broad beam ion source operated with Ar and N2 source gasses to produce BN films with a high percentage of sp 3 -bonded cBN. The best films to date show -85% sp 3-bonded BN as determined from infrared (IR) reflection spectroscopy. High resolution transmission electron microscopy (TEM) and selected area electron diffraction (SAD) confirm the presence of cBN in these samples. The films are polycrystalline and show grain sizes up to 500 A. INTRODUCTION Pulsed laser deposition (PLD) is an emerging technique for synthesis of thin film materials of many different types[l. In particular, thin films of high temperature superconductors with very high quality have been synthesized by PLD and studied extensively. One of the claimed advantages of PLD is its ability to deposit stoichiometric films of many different elements from a single target. However, the stoichiometry of the target is not always preserved in the deposited films. A prime example of this is found in the high temperature superconductors where the deposited films must be further oxidized, usually by providing a supplemental background of ambient 02 gas. In studies of unassisted PLD of BN films, it has generally been observed that the films deposited from stoichiometric hBN targets are highly nitrogen deficient and consist mostly of sp 2 -bonded hBN[2,3]. There have been two reports of sp 3 -bonded cBN grown by unassisted PLD. We report[4] films grown at room temperature that are mixed phase sp 2- and sp 3 -bonded material and are nitrogen deficient. Doll et al.[5,6] report films grown at 600'C that are polycrystalline yet epitaxial to the Si (100) substrate. The 2 epitaxial cBN layer was found to be -200 A thick with amorphous, nitrogen-deficient sp bonded hBN deposited above it. Despite extensive work, we have never produced cBN at 600'C by unassisted PLD[2]. There are several possible explanations for the nitrogen deficiency in these films. One is that partial melting of the target occurs leading to the loss of nitrogen from the target[7,8]. Subsequent ablation from the same spot leads to nitrogen deficiencies in the deposited sample. More likely is that higher energy nitrogen species are needed than those generated in the laser ablation plume (up to -50 eV). In either case, it is necessary to provide a supplemental supply of reactive nitrogen species in order to grow thick stoichiometric films. Indeed, a common component of all the successful methods used to grow films with a high fraction of cBN is the presence of ionized nitrogen species (N 2* and N+) with energies ranging from 0.
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