Cross sectional HRTEM image of sp²-BN grown on 6H-SiC. After a thickness of 4 nm, the h-BN growth is followed by r-BN growth as a result of stress relaxation.

Boron nitride (BN) is a promising semiconductor material for application as microelectronic components such as optoelectronic, radiation detection, spintronic, biosensing, high-temperature, and radiation resistant devices. Boron nitride (BN) is a wide band gap semiconductor with a direct band gap of around 6 eV, high thermal and chemical stability, high thermal conductivity, and a low density. BN can exist in sp²- or sp³-hybridized form where the former possesses two crystal structures: hexagonal (h-BN) and rhombohedral (r-BN). The epitaxial BN films were deposited at Link&oumlping University in a hot-wall CVD reactor at 1500ºC using triethyl boron (TEB, B(C₂H₅)₃) and ammonia (NH₃) as boron and nitrogen precursors and hydrogen as a carrier gas. We compared the growth of sp²-BN by chemical vapor deposition on (0001) 6H-SiC and on (0001) α-Al₂O₃ substrates with an AlN buffer layer. Polytype-pure rhombohedral BN (r-BN) with a thickness of 200 nm is observed on SiC whereas hexagonal BN (h-BN) nucleates and grows on the AlN buffer layer. For the latter case after a thickness of 4 nm, the h-BN growth is followed by r-BN growth (see figure). We found that the polytype of the sp²-BN films is determined by the ordering of Si−C or Al−N atomic pairs in the underlying crystalline structure (SiC or AlN). In the latter case the change from h-BN to r-BN is triggered by stress relaxation.

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