Decomposition of Metal Alkylamides, Alkyls, and Halides at Reducible Oxide Surfaces: Mechanism of ‘Clean-up’ During Atomic Layer Deposition of Dielectrics onto III–V Substrates

Abstract

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Self-cleaning and surface chemical reactions during hafnium dioxide atomic layer deposition on indium arsenide

“…These results are in contrast to the previous works that reported organic species being incorporated in the HfO 2 films as by-products of the ligand exchange mechanism, even at higher temperature 25,50 . However, the existence of a thermal activation barrier for the complete desorption of organic residuals from the sample surface agrees with density functional theory calculations: while the initial dissociation of N(CH 3 ) 2 ligands from the TDMA-Hf molecule is found to be strongly exothermic, further dissociation and desorption of these ligands are predicted to occur if sufficient thermal activation energy is provided 41 . At deposition temperatures below the thermal activation barrier, we also found an incomplete removal of the native oxide upon TDMA-Hf deposition (Fig.…”

“…During this second step, the XPS binding energy of the Hf atoms is increased toward the typical value corresponding to Hf–O bonds in HfO 2 . A number of theoretical models can be found that describe the chemical reactions involved in this second step, such as ligand dissociation and Hf–O bond formation 24,36,41 This step implies an activation barrier that needs to be overcome, as the bond dissociation energy for breaking the Hf–N bond amounts to about 110 kcal/mol, while the newly formed Hf–O bond has a dissociation energy of about 140 kcal/mol, which results in an exothermic reaction 24 . However, our work shows that the picture of the ALD process remains incomplete as long as it does not also include the first step of precursor molecular adsorption, which has often been neglected until now.…”

“…Until now, it was only known that the self-cleaning happens during the first ALD half-cycle 7,21 and the ligand exchange mechanism was supposed to be the driving force, which would imply that the oxygen atoms of the native oxide are converted into HfO 2 oxygen atoms. Theoretical models can well explain the formation of Hf–O bonds upon this ligand exchange mechanism 24,40 and further decomposition of the ligand 41 , and some also include the molecular adsorption of precursors onto OH-terminated surfaces upon further ALD cycles 36,40 . However, an explanation of how this reaction upon the initial precursor deposition can remove a 1–2 nm thick layer of In and As oxides 15 and at the same time produce less than 1 atomic monolayer of HfO 2 is still lacking.…”

“…However, an explanation of how this reaction upon the initial precursor deposition can remove a 1–2 nm thick layer of In and As oxides 15 and at the same time produce less than 1 atomic monolayer of HfO 2 is still lacking. Some authors reported diffusion of InAs surface oxides through an already deposited TiO 2 42,43 , or Ta 2 O 5 44 layer, followed by oxide removal under continuous ALD precursor supply, enhanced by further ligand decomposition 41,42 . However, this effect cannot explain the complete removal of the native oxide already during the first ALD half-cycle reported elsewhere 7,21 .…”

Comparison of the reactivity of alkyl and alkyl amine precursors with native oxide GaAs(100) and InAs(100) surfaces

“…Recent theoretical studies have indicated that the precursor/surface interaction mechanism is different for these two precursor types [25,26] which may be one of the causes of the varying degrees of removal among the various ALD processes. However, it has been shown recently that the missing step in the reaction mechanism was the transport of the surface oxides through the growing film and its accumulation near the film surface.…”