Reaction of diborane and ammonia gas mixtures in a chemical vapor deposition hot-wall reactor

Abstract

Boron nitride has been deposited'by chemical vapor deposition from diborane and ammonia gas mixtures, in the form of either films or powders depending on the pressure in the reactor. IR analysis of the boron nitride deposits has been used to obtain the composition of the films. In the temperature range studied, 600-850 OC, the effect of the deposition temperature on the reaction kinetics has been interpreted by assuming a change in the relative concentration of activated species arising from diborane and ammonia molecules. This change modifies the reaction path followed in the boron nitride synthesis through the formation of different intermediate compounds: aminodiborane in the low-temperature range (<700 "C) and borazine at the higher temperatures (1775 "C). The presence of either of these intermediate compounds determines the deposition rate and the final composition of the boron nitride films.

Influence of Diborane Flow Rate on the Structure and Stability of CVD Boron Nitride Films

“…We have studied the effect of diborane flow rate on the composition, structure, and stability of CVD boron nitride films deposited at 800 °C and 267 Pa from ammonia and diborane gas mixtures, keeping the ammonia flow rate constant at 100 sccm. This work confirms our previous view that the reaction mechanisms for the formation of the boron nitride films for a given deposition temperature are controlled by the relative composition of the gas mixture . For low [B 2 H 6 ]/[NH 3 ] ratios, turbostratic boron nitride films were preferentially obtained at low deposition rates (≈40 nm/min), whereas amorphous films were formed at high deposition rates (≈160 nm/min) when high [B 2 H 6 ]/[NH 3 ] ratios are used.…”

“…Two regions, with low ( r ≤ 0.25) and high deposition rates ( r > 0.25), can be distinguished in the curve, and these can be associated with two different growth mechanisms for the boron nitride films. In a previous work concerning the temperature effect on the boron nitride deposition (600−850 °C range) we considered borazine and aminodiborane as the two most probable intermediates for the BN synthesis . In the temperature range of the study and for a [B 2 H 6 ]/[NH 3 ] ratio of 0.056, we concluded that an increase in the temperature produces a change from aminodiborane to borazine as an intermediate compound.…”

“…Moreover, the activation energy determined for the formation of boron nitride from borazine as the intermediate compound was 35 kcal mol -1 , which corresponds to a mechanism controlled by a surface reaction. Likewise, when aminodiborane is considered as the precursor in boron nitride deposition ( T ≤ 700 °C), 5 kcal mol -1 was obtained for the apparent activation energy . This low activation energy could be related to a diffusion process, which would be the limiting step in the total reaction.…”

“…At temperatures below 700 °C and low [B 2 H 6 ]/[NH 3 ] flow ratios (0.055), boron-rich BN films were obtained through the formation of aminodiborane. On the other hand, at higher temperatures (≥775 °C) the formation of borazine, as an intermediate compound, led to stoichiometric partially ordered BN films at lower deposition rates …”

Structure and Stability of (HBNH)n Clusters

“…Note that at low temperature, the most thermodynamically favorable product in the gas phase reaction is the needle-shaped cluster with n＝16, which dominates the gas phase up to 800-900 K. As the temperature was increased further, low-mass clusters became more pronounced due to the entropy factors. The trimeric imido compound was predicted to be the most stable gas phase molecule at temperature higher than 1100 K. It was supported by available experimental evidence, 25 from which (HBNH) 3 has been obtained as the intermediate compound at 775 ℃. In addition, the octamer has considerable abundance at higher temperature.…”