Growth and Optical Studies of Boron Nitride Films on Silicon Substrates.
Abstract
This dissertation details significant advances in the thin film growth of the wide bandgap semiconductor cubic boron nitride. We have developed a novel high temperature growth process which combines RF-magnetron sputtering, a high density nitrogen plasma produced by an electron cyclotron resonance (ECR) ion source, and negative substrate bias potentials to precisely control the ion energy at the substrate surface. We find that under high ion flux conditions, the growth of cubic BN is possible only within narrow, ~30 eV, ranges of ion energy. Through a better understanding of the growth kinetics, we have significantly reduced the ion energy needed to form the metastable cubic phase, with values now substantially less than 100 eV. The reduction in ion energy has led to improvements in film crystallinity and orientation, with an associated reduction of the high film stress which has severely limited film thickness in the past. The BN films exhibit a layered morphology: a thin amorphous region at the silicon substrate interface, followed by an oriented hexagonal layer upon which the cubic phase nucleates. Scanning force microscopy images show that cubic BN nucleates as well aligned triangular crystalites, indicative of (111) cubic growth. More specifically, the cubic BN appears to nucleate on the edges of perpendicularly oriented hexagonal BN planes, such that the cubic BN (111) is normal to the hexagonal BN (0002) (c-axis). The crystallites coalesce to a smooth film (RMS roughness ~ 20 A) with an average grain size of ~ 700 A. Selected area electron diffraction and infrared spectroscopy confirm that the cubic layer is single phase cubic BN. The results suggest a path to "compliant" oriented growth on a variety of substrates in which hexagonal BN planes serve to both absorb strain and provide a nucleation mechanism for cubic BN. New Raman and cathodoluminescence (CL) spectroscopy results are presented for both hexagonal and cubic BN films. Raman scattering from cubic BN samples, in which the cubic LO and TO modes are clearly resolved, are the first reported well resolved Raman features from cubic BN films. Significant results from CL spectroscopy include the first reported observation of near bandgap emission from hexagonal BN films in the range 5.5 eV to 4.9 eV. The emission features are attributed to recombination processes involving shallow donors and deep acceptors. Deep level defect-related luminescence is observed from cubic BN films centered near 3.75 eV. The deep level luminescence is strongly correlated with growth conditions, decreasing in intensity and shifting to higher energy as parameters are optimized.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1996
- Bibcode:
- 1996PhDT........35T
- Keywords:
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- Engineering: Materials Science; Physics: Condensed Matter