Fractal growth of sp³-bonded 5H-BN microcones by plasma-assisted laser chemical vapor deposition

It was reported previously that sp³-bonded 5H-BN films grown by plasma-assisted laser chemical vapor deposition (PAL-CVD) exhibited cone-shaped units with dimensions on the order of ∼10 μm prevailing over the surface, and which contributed to excellent electron field emission properties. Here we show that these cone-shaped electron emitters form fractal distribution patterns on Si(100) substrates. The fractal samples exhibited nested cellular patterns with irregular polyhedral shapes scaling from 10 mm to 100 μm, where the fractal dimensions estimated by box-counting method ranged from 1.064 to 1.623. Contrary to the Arrhenius law, the growth rates of the films in PAL-CVD decreased with increased substrate temperature, and which was consistently explained by a simple photochemically activated growth model. The growth rates of the films in PAL-CVD were 60 times faster than that by mere plasma-assisted chemical vapor deposition at the T_s of 300°C, which also supports the photochemical growth mechanism. A clear transition from uniform to fractal growth was found at a critical substrate temperature around 700°C, suggesting a strong nonlinearity inherent in this "forced-oscillatory photochemical vapor growth phenomenon."