Kinetics Approach to the Growth of Cubic Boron Nitride
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Figure 1. Percent of sp3-bonding (cubic BN) calculated from the FTIR absorptance spectra of two series of BN films grown with different nitrogen ion currents. The solid line is a polynomial fit to the data. In each case, formation of the cubic phase is achieved in a narrow (-30 eV) window of ion-energy and optimization occurs in an even narrower range - only several eV in width.
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Electron diffraction and high resolution imaging have revealed that the well crystallized BN films grow with a layered morphology; a thin amorphous layer (-50A) at the substrate interface, followed by a region of hexagonal/turbostratic BN oriented with the (0002) planes perpendicular to the substrate, upon which a polycrystalline cubic BN layer forms. This has been well documented for BN films grown by ion-assisted processes on silicon[9-1 1] as well as other[4,12] substrates. An example of the film morphology can be seen in the high resolution cross-sectional TEM micrograph shown in Figure 2. The film is near the initial nucleation stage of the cubic BN layer and shows a cubic crystallite, - 500 A in length, on top of perpendicularly oriented hexagonal planes. The amorphous and hexagonal layer thicknesses are - 50 and 300 A, respectively.
Figure 2. High resolution cross-sectional TEM micrograph of a BN film near the initial nucleation stage of the cubic BN layer. The micrograph depicts the layered film morphology; an amorphous layer (-50 A) at the substrate interface, followed by a region of hexagonal/turbostratic BN oriented with the (0002) planes perpendicular to the substrate, upon which a polycrystalline cubic BN layer forms.
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Although the interface between the hexagonal and cubic layers is quite rough, once a continuous layer of cubic BN is formed, further deposition leads to essentially single phase cubic material. This is supported by the IR absorptance spectra shown in Figure 3. The sp2 -bonding features associated with the amorphous and hexagonal interface layers remain of similar intensity, while the sp 3 -bonding feature of the cubic BN TO phonon increases with film thickness. This demonstrates that for well crystallized films, the cubic BN fraction (or sp 3 -bonding percentage) is simply a function of film thickness.
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Wavenumber (cm 1) Figure 3. Infrared absorptance spectra from a series of cubic BN films of different thickness. The cubic TO phonon absorptance at 1078 cm- 1 increases with film thickness while the hexagonal BN features, near 770 cm- 1 and 1380 cm- 1 , are of similar intensity in each film. This supports TEM diffraction data
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