DFT investigation into alumina ALD growth inhibition on hydroxylated amorphous silica surface

Alumina (Al2O3), a suitable replacement for silica (SiO2) as gate oxide in metal oxide semiconductor field effect transistors (MOSFET), is deposited on the amorphous silica layer of the semiconductor substrate by atomic layer deposition (ALD) using trimethylaluminum (TMA) and water as precursors. A computationally efficient model for the hydroxylated amorphous silica surface is obtained by means of molecular dynamics and is used to investigate the reason behind the observed growth inhibition during alumina ALD. The reactions of TMA are investigated by periodic DFT calculations on surfaces with hydroxyl coverage of 3.38 OH nm(-2) and 5.07 OH nm(-2). The formation of SiCH3 surface species is found to be possible only on the less hydroxylated surface during the first TMA half-cycle, while the subsequent reaction of water with the SiCH3 surface species is found to be highly activated (E-a = 196 kJ mol(-1)). Since these SiCH3 surface species are rather unreactive toward water, fewer hydroxyls are regenerated during this first water half-cycle, resulting in the observed initial growth inhibition. Moreover, alumina growth can continue over the aluminum surface species, trapping the unreactive SiCH3 species at the interface between deposited alumina and silica. Such carbon impurities at the interface should be avoided nonetheless, since they can create undesirable tunneling currents in MOSFETs