UV Raman Spectroscopy of Boron Nitrides

Boron nitride is a remarkable material both for its fundamental properties and for applications [1]. It exists in different polytypes, the sp2 bonded hexagonal and rhombohedral structure and the sp3 cubic and wurzite phases. All phases are high band-gap semiconductors (>5 eV). Cubic boron nitride, c-BN, is of particular interest for its properties similar to diamond [1,2]. Physical or chemical vapor deposition of boron nitride films can often result in the growth of the less dense hexagonal BN (h-BN). BN nanotubes are also very interesting [3]. Their properties, as for carbon nanotubes, can be derived by studying the wrapping of a single layer of h-BN [3]. Boron Carbon Nitrides (BCN) thin films are also useful for hard coating applications and as semiconductors with variable gap [4]. Raman and Infrared spectroscopy are quick and non-destructive tools to assess the properties of materials. Infrared spectroscopy is the most widely used for boron nitrides. It is very sensitive to polar BN bonds. The spectra for c-BN and h-BN show distinct IR features [1]. Raman spectroscopy is rarely used because of its inferior sensitivity, especially in the case of thin films. This is opposite to diamond-like carbons, where Raman spectroscopy is a standard characterization tool [5]. However, Infrared spectroscopy has limitations. The substrate has to be either transparent or not highly reflective. Raman spectroscopy has no requirement on the substrate properties and can simultaneously give informantion on phase composition, crystallinity, size and stress. In order to successfully apply Raman scattering in boron nitrides its low sensitivity has to be overcome. This can be done by measuring the samples with an excitation energy closer to their band gap. Here we measure BN samples by UV Raman spectroscopy at 5.1 and 5.4 eV. This enhances the BN Raman cross-section and paves the way to making UV Raman scattering a standard tool for boron nitrides characterization. We studied the UV Raman spectra of a variety of boron nitride samples ranging from h-BN (bulk and thin film), c-BN (single crystals, thin and thick films), mixed h-BN and c-BN phases and BCN films with varying B, C and N content. The UV Raman spectra of c-BN and h-BN are so intense that a clear measurement of the second order Raman signal is possible. This allows us to probe for the first time non-zone center phonons and validate the predictions of first-principles calculations of BN properties [3,6]. For h-BN, whilst the high-energy optical phonons are in excellent agreement with theory, the ionic interaction was strongly underestimated for lower energies [6]. The first order UV Raman spectra of thin and nano-crystalline films also show the activation of non-zone center phonons and IR modes, such as the 800 cm-1 mode of h-BN. In BCN films the direct signature of BN bonds can be seen, being no more shadowed by the carbon phase.

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