Formation of microcones accompanied with ripple patterns in laser-activated plasma CVD of sp 3 -bonded BN films
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Masaharu Shiratani Kyushu University, Motooka, Fukuoka 819-0395, Japan (Received 13 September 2013; accepted 9 December 2013)
Scanning electron microscopic (SEM) observations revealed that individual and independent microcones of sp3-bonded boron nitride grown by laser-activated plasma chemical vapor deposition were accompanied by ripple patterns spreading around them in the dimension of micrometers or sub-micrometers. The ripples were expanding equidistantly from each other and diminishing as they depart from a cone. The origin of the ripples was attributed to the interference of a direct laser cast on the plane surface and that reflected from the side of a cone; this model was satisfactorily in agreement with the SEM measurement, in which the side surface of a cone was mapped onto the plane surface surrounding the cone in the mathematical meaning of “bijection.” This micro-optical effect due to the wave nature of laser was considered to indicate and support the photochemically activated growth reactions in this process.
I. INTRODUCTION
We reported previously the formation of cones with micrometer or below in their sizes prevailing over the sp3-bonded boron nitride (BN) films prepared by laseractivated plasma chemical vapor deposition (CVD).1–4 Utilizing some of the BN films with semiconducting properties, we successfully fabricated heterojunctions of p-type BN on n-type silicon wafers and which proved to work as photovoltaic cells.5 In that case, the surface morphology occupied by the microcones was considered as effective as anti-reflection, which should contribute to improving the efficiency of the solar cell. On the other hand, these BN films proved to show electron field emission with very excellent achievements.2,6 The surface morphology covered with cones plays an essential role as a geometrical enhancement factor.7 However, many unknown things about the cones remain to be studied, including their growth mechanism. It is at least known that an (ArF at 193 nm) excimer laser irradiation upon the film surface renders the sp2-bonded structure of BN films prepared by mere plasma CVD into those with sp3-bonded structure.8 This laser-induced phase transition is considered to proceed during the growth of the sp3-bonded BN in our plasma CVD activated with the excimer laser irradiation, which was pulsated at 2 to 40 Hz with a pulse duration of about 20 ns. As for the effects induced by the laser irradiation in our plasma-CVD, we also previously mentioned photochemical a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.7 J. Mater. Res., Vol. 29, No. 4, Feb 28, 2014
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effects4 to activate surface chemical (growth and/or etching) reactions and migration of precursor species on the surface,9 in addition to the laser-induced phase (structural) transformation cited above. This reaction-diffusion scheme10 was formulated as a master equation to spatio-temporally govern the surface density of precursor radicals, previously.4 Th
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