Electron Microscopy Study of Cubic Boron Nitride Thin Films Grown by Ion-Assisted Pulsed Laser Deposition
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ELECTRON MICROSCOPY STUDY OF CUBIC BORON NITRIDE THIN FILMS GROWN BY ION -ASSISTED PULSED LASER DEPOSITION D.L. MEDLIN, T.A. FRIEDMANN, P.B. MIRKARIMI, K.F. McCARTY, AND MJ. MILLS Sandia National Laboratories, Livermore, California 94551 USA ABSTRACT We present a microstructural study of boron nitride films grown by ion-assisted pulsed Fourier transform infra-red spectroscopy, electron energy loss laser deposition. spectroscopy, and electron diffraction measurements indicate that within the irradiated region of the substrate, the film consists of high fraction of cBN with a small amount of the turbostratic phase; outside of the irradiated region, only the turbostratic phase is detected. Conventional and high resolution electron microscopic observations of the boron nitride microstructure indicate that the cBN is in the form of twinned crystallites, up to 30 nm in diameter. We also observe particulates, formed by the laser pulse, that reduce the yield of cBN in the irradiated regions by shadowing local areas from the ion beam. INTRODUCTION We are synthesizing thin films of cubic boron nitride (cBN) using ion-assisted pulsed laser deposition. Cubic boron nitride has a number of useful applications as a hard coating and as an optical and semiconductor material [1]. However, several phases of boron nitride conditions. In many exist, and the phase that forms depends critically upon3 the deposition ways analogous to3 carbon, boron nitride forms both sp - and sp2-bonded phases. The cubic which is sp -bonded, has the zinc blende structure. In addition, BN can also form an phase, sp 3 -bonded, hexagonal phase with the wurtzite structure (wBN) [2]. An sp 2-bonded hexagonal structure (hBN) is the stable room-temperature phase, but like its analog, graphite, the sp2-bonded material can also exist in a highly disordered state, often described as turbostratic (tBN), in which the basal planes form out of registry and are severely bent. It has been found by several groups that the yield of cubic material is controlled in part by the simultaneous irradiation of the growing film with a low energy beam of ions, which is commonly a mixture of N2 and Ar [e.g. see references 3-6]. In order to understand how this transformation process occurs, we are using transmission electron microscopy (TEM) to investigate how the microstructure and phase distribution varies with film deposition conditions. EXPERIMENTAL Our laser ablation deposition apparatus has been described previously [7], and the details of the system are only briefly summarized here. We are using a KrF (248 nm) laser to ablate from a rotating target of pressed hBN (Union Carbide). The plume of ablated boron and nitrogen species deposits material onto a resistively heated Si (100) substrate (p-doped with B to 0.01-0.03 Q cm) that is positioned 4 cm away from the target. The substrate temperature is measured with a thermocouple pressed to the backside of the sample. During deposition, a Kaufmann ion source, positioned at an angle of 300 from the plane of the substrate, irradiates
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