Stability and degradation of plasma deposited boron nitride thin films in ambient atmosphere

Boron nitride (BN) thin films were deposited at 296 K, 398 K, 523 K and 623 K by low power radio frequency plasma enhanced chemical vapor deposition with nitrogen (N₂) and hydrogen diluted diborane (15% B₂H₆ in H₂) source gases. Fourier transform infrared and UV–visible spectroscopies were used to investigate the stability and degradation of BN films under ambient air conditions. The action of moisture on the films is reduced with increasing substrate temperature (T_s) to the detriment of the film growth rate. This has been interpreted as related to the decrease in porosity and relative volume fraction of B–O containing disordered tissue at higher T_s. The thickness of the unstable films increases logarithmically with the air exposure time. Parallel to this, although the E₀₄ gap increases logarithmically with time, the Tauc gap remains the same. The increase of subgap absorptions and the decrease of Tauc slope with time indicate reduction of structural order. Crystallites of ammonium borate hydrates, the main product of the chemical reactions, are initially formed within the bulk. At a later time, as a result of increased porosity and disorder, the film thickness decreases while the islands of micro-crystallites rapidly grow above the surface of the film. Stability dependence on other deposition parameters was also studied: it is found that the 1260/1360 cm⁻¹ (O–B–O/B–N) infrared peak area ratio plays an indicator role to reveal the stability of BN films.