Theoretical Studies On The Reaction Mechanism Of Atomic Layer Deposition Of Silica

Atomic layer deposition(ALD)is a powerful nanofabrication technique of thin films with precise control of the thickness and composition at the atomic level.It has attracted both experimental and theoretical interests.Over the past several decades,theoretical and computational chemistry is widely used to explore the mechanisms of chemical reactions.In this thesis,density functional theory(DFT)and Born-Oppenheimer molecular dynamics(BOMD)methods are utilized to carry out systematic studies on the ALD mechanisms of SiO2,including Lewis-base catalyzed ALD and rapid ALD(RALD)reactions.In addition,in order to study ALD on the large-scale surface,many attempts have been done to parameterize the force fields for Berry pseudorotation.The theoretical results are summarized as follows:(1)The ALD of SiO2 undergoes successive self-terminating SiC4(A)and H2O(B)half-reactions,respectively.Most of the previous works show a concerted pathway for the uncatalyzed reaction.There may exist an alternative stepwise pathway in the uncatalyzed SiO2 ALD reaction,including the formation of Si-O bond and the elimination of HC1.In stepwise pathway of the uncatalyzed SiO2 ALD reaction,the rate-determining step is the Si-O bond formation accompanied by the rotation of SiCl4.The introduction of the Lewis-base catalyst,pyridine and ammonia,can significantly reduce the activation free energy of the Si-O bond formation in the catalyzed ALD reactions.However,the desorption energy in the second H2O(B)half-reaction is larger than the activation energy,making the removal of byproduct a rate-limiting step.In experimental condition,the reaction temperature is over 300 K,it may be easy for the removal of byproduct by N2 purge.(2)Static transition state searches and BOMD simulations have been further used to investigate the reaction pathways for the pyridine-and ammonia-catalyzed ALD of SiO2.The low energy barrier and flexible pentacoordinated intermediate facilitate the surface pseudorotation(SPR)pathway.The catalyzed SiO2 ALD reaction may undergo the multi-step pathways,including adsorption of precursor,axial addition,surface pseudorotation,axial elimination,and desorption of byproduct steps.Surface pseudorotation is similar to Berry pseudorotation(BPR)pathway of the trigonal bipyramid(TBP)molecules,which occurs in pentacoordinated compounds with Si,P,and some metal complexes of Fe,Co,and Rh as central atoms.With one ligand pivot linked to the surface,the catalyzed reaction possesses three possible rotation modes.Through the low-barrier pseudorotation transition states,the axial angle changes from near 180° to 120 0 and the equatorial angle changes from 120° to near 180 °,indicating the pair-wise exchange of axial and equatorial ligands,called pseudorotation.The generality of Berry and surface pseudorotations with the characterized TBP topology exhibits the common fluxional behavior in pentacoordinated compounds containing main-group and metal elements.Different from Berry pseudorotation,surface pseudorotation is reactive.In order to extend the present work to the mesoscopic scale,several attempts have been made to produce a reasonable force field for Berry pseudorotation.Although the potential of rotation reaction based on PCFF(Polymer Consistent Force Field)force field is similar to that obtained from ab initio methods,the results of molecular dynamics simulation are not optimistic.ESFF(Extensible and Systematic Force Field)force field may describe the difference of various angles of TBP geometry and be applied in future molecule simulations at large scale.(3)Rapid atomic layer deposition(RALD)has been applied to prepare various nanolaminates with the multilayer repeated structures.In RALD,the gas-phase precursor molecules are introduced into the reaction chamber alternately.RALD still maintains the complementary and self-limiting characteristics of the conventional ALD.By the introduction of the Al and silanol precursors,silica nanolaminates were rapidly deposited at 100 times rates than the growth rate of the conventional ALD of silica.The possible reaction pathways for RALD of the Al2O3/SiO2 nanolaminate using tri-methylaluminum(TMA)and tris(tert-butoxy)silanol(TBS)are investigated by using DFT calculations.The rate-determining step of whole RALD is the elimination of isobutene.The introduction of Lewis-acid catalyst,TMA,can result in the formation of the catalytic site,which accelerates the propagation of the siloxane polymer.The Bronsted acid site of[A1O4]can catalyze the elimination of isobutene.At the same time,the interfacial interactions,such as hydrogen bonding interactions between tert-butoxy groups and the surface,further catalyze the elimination of isobutene and accelerate SiO2 RALD reactions.The synergy between the covalent and noncovalent catalysis may be applied in the rational design and fabrication of other nanolaminates and effective catalysts.