Atomic layer deposition of localised boron- and hydrogen-doped aluminium oxide using trimethyl borate as a dopant precursor

Abstract

Atomic layer deposition (ALD) of boron-containing films has been mainly studied for use in 2D materials and for B-doping of Si. Furthermore, lithium-containing borates show great promise as solid electrolyte coatings for enhanced energy storage. In this work, we examine trimethyl borate (TMB) and triethyl borate (TEB) in combination with O2 plasma as precursors for ALD of B-containing films, targeting the growth of B2O3. It is found that films grown from TEB contain no boron. Further work with TMB as a boron-containing precursor showed promising initial growth on a SiO2 or Al2O3 surface, but a rapid decrease of the growth rate during subsequent ALD cycles indicating surface inhibition during continued growth. DFT cluster calculations in combination with in-situ FTIR demonstrated that because of its weak Lewis acidity, the TMB molecule is found to adsorb via hydrogen-bonding to B-OH covered surfaces, without elimination of ligands, so that it is ubsequently removed in the plasmapulse and does not contribute to growth. The growth could be maintained in a mixedprocess, by reactivating the surface through single exposures to trimethyl aluminum(TMA) and oxygen plasma and thus resetting the surface to Al-OH, on which TMB chemisorption is energetically more favourable. Surprisingly, this process did not result in B2O3 (or Al-doped B2O3) films, but instead in B- and H-doped Al2O3 films. Moreover, rather than uniform boron doping, the Al2O3 films grown from this process contain a large amount of hydrogen, up to 17At% under certain processing conditions, and displayed non-uniform depth distributions of boron and hydrogen with a degree of control over the doping distribution based on the deposition conditions. Finally, the mechanism for the atypical growth mode is proposed based on in-situ FTIR and ellipsometry measurements and density functional theory calculations, and was attributed to sub-surface reactions of the TMA with the B-OH films grown by TMB-O2 plasma.This makes the process an interesting, albeit atypical, ALD process that allows for a quasi-continuous tuning of the B-concentration in the top region of high-purity Al2O3 films.