Comparative study of thermal and radical-enhanced methods for growing boron nitride films from diborane and ammonia

Abstract

This work studies the deposition of boron/boron nitride (B/BN) composite films at low substrate temperature (275–375 °C) by alternating pulses of diborane (B2H6) and ammonia (NH3) with argon purging in between to avoid gas-phase reactions of the precursors. This process is similar to atomic layer deposition in which the dominance of surface reactions simplifies the growth mechanism. However, non-self-limiting decomposition of B2H6 and incomplete nitridation lead to the incorporation of pure boron (pure-B), causing deviation from the desired 1:1 B:N stoichiometry. Using the pure-B fraction as a measure of incomplete nitridation, this article describes consecutive experiments to control this effect and ultimately understand it in the context of a surface reaction model. First, it is demonstrated that, in a purely thermal mode, the growth of the layers and their composition strongly depend on the total gas pressure. The pure-B content (not to be confused with the total boron content) could thus be varied in the range of ∼6–70 vol. %. Next, enhancement of nitridation by the dissociation of NH3 into reactive radicals using a hot-wire was found to be insufficient to produce stoichiometric BN. Finally, plasma-assisted deposition at 310 °C resulted in nearly stoichiometric polycrystalline BN with an interplane distance matching that of hexagonal BN; the material was stable in air for at least six months. The pressure dependence in the purely thermal mode is consistent with a growth model of BN from B2H6 and NH3 via the so-called surface-adduct mechanism. The effects of the radical-enhanced methods on nitridation are explained using this model.