Effects of Excimer Laser Ablation on the Surface of Hexagonal Boron Nitride
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EFFECTS OF EXCIMER LASER ABLATION ON THE SURFACE OF HEXAGONAL BORON NITRIDE G. L. DOLL, T. A. PERRY, AND J. A. SELL General Motors Research Laboratories, 30500 Mound Road, Warren, MI 48090 ABSTRACT The laser-induced changes in the hexagonal boron nitride targets that are used in the deposition of pulsed laser-deposited cubic boron nitride films are examined. Although the structure of the hexagonal boron nitride in the irradiated areas is unaltered, the surface morphology changes considerably. Boron-rich spheroids were found to cover the irradiated area. Their origin is consistent with the hydrodynamic sputtering of the hexagonal boron nitride targets, and a liberation of nitrogen atoms to the gas phase. INTRODUCTION In this study, we examine the structural, compositional, and morphological changes incurred in hexagonal boron nitride targets as a result of the ablative interaction with a KrF pulsed excimer laser. The growth technique of pulsed laser deposition (PLD) has made significant contributions over the past several years in the field of epitaxial film growth [1]. Most notably, PLD has been used extensively [21 in the growth of epitaxial superconducting copper oxide films. Rockwell scientists [3,41 refined the art of PLD such that the quality of their PLDgrown semiconductor films rivals or exceeds those grown by MBE. PLD has been applied successfully to the growths of metal [5], ferroelectric [6], and polymeric films [7]. Recently, some of us [8] have found that PLD can be used to grow cubic boron nitride (cBN) films epitaxially on silicon. cBN is a material that has physical properties similar to those of diamond, including a large band gap and an extremely high thermal conductivity. Contrary to diamond, cBN has a favorable thermal expansion coefficient with silicon, and can be doped easily both n- and p-type. Therefore, PLD is a growth technique that has made significant contributions in the most attentive research areas in materials science and condensed matter physics. Yet, for all its attributes, PLD is a deposition process that is materials sensitive and not well understood. In hopes of elucidating the PLD process relevant to our research in epitaxial cBN films, we have undertaken the study of the laser-target interaction employed in our growth process. EXPERIMENTAL CONSIDERATIONS The excimer laser used in this study was KrF producing a 248 nm (,- 5 eV) laser line.
2 The laser pulse width is 22 ns, the fluence at the target was 3.9 J/cm , and the
repetition rate was 10 Hz. A MgF2 lens (f/4) was used to focus the laser radiation to a 3 mm diameter spot 650 from normal on a hexagonal boron nitride (hBN) target, which continually rotated to reduce the effects of cratering of the target surface. In our PLD growths of cBN films, we have used four types of commercially availMat. Res. Soc. Symp. Proc. Vol. 201.
1991 Materials Research Society
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able [9] hBN targets: HBR, HBN, HBC, and pyrolytic boron nitride (pBN). The first three target types are different grades of machinable sintered ceramics that co
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