Atomic Layer Deposition Of Localized Boron- And Hydrogen-Doped Aluminum Oxide Using Trimethyl Borate As A Dopant Precursor

Atomic layer deposition (ALD) of boron-containing films has been mainly studied for use in two-dimensional 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) in combination with O-2 plasma as a precursor for ALD of B-containing films, targeting the growth of B2O3. It is found that after initial growth on a SiO2 or Al2O3 surface, a rapid decrease in the rate of growth during subsequent ALD cycles occurs, indicating surface inhibition during continued growth. Density functional theory (DFT) cluster calculations in combination with in situ Fourier transform infrared spectroscopy (FTIR) demonstrated that the growth is governed by two different mechanisms depending on the Lewis acidity of the surface: chemisorption on an Al-OH- and Si-OH-terminated surface and physisorption on more acidic B-OH surface sites. The growth could be maintained in a mixed process, by reactivating the surface through single exposures to trimethyl aluminum (TMA) and O-2 plasma and thus resetting the surface to Al-OH, on which TMB chemisorption is energetically more favorable. 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 a uniform boron distribution, the Al2O3 films grown from this process contain a large amount of hydrogen, up to 17 at. % 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 on the basis of in situ FTIR and ellipsometry measurements and DFT calculations and was attributed to subsurface reactions of the TMA with the B-OH films grown by TMB-O-2 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.