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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

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journal contribution
posted on 08.04.2014, 00:00 by Sylwia Klejna, Simon D. Elliott
The pairing of high-k dielectric materials with high electron mobility semiconductors for transistors is facilitated when atomic layer deposition (ALD) is used to deposit the dielectric film. An interfacial cleaning mechanism (‘clean-up’) that results in consumption of semiconductor native oxides and in practically sharp dielectric/semiconductor interfaces has been observed during ALD of Al2O3, HfO2, TiO2, and Ta2O5 with various degrees of success. We undertake a comprehensive study using density functional theory (DFT) to explain differences in the performance of various classes of precursor chemicals in removing native oxide from III–V substrates. The study covers the metals Ta­(V), Ti­(IV), Zr­(IV), Hf­(IV), Al­(III), Mg­(II) combined with methyl, amide, and chloride ligands. Of these, we show that clean-up is most effective when depositing MgO. Clean-up with metal alkylamides has a similar mechanism to clean-up with metal methyls insofar as oxygen is scavenged by the metal. The difference in operation of alkylamide and methyl ligands lies in the affinity of the ligand to the substrate. Alkylamide is shown to be prone to decomposition rather than the migration of the entire ligand evinced by methyl. We investigate the multistep chemical processes associated with decomposition of alkylamide. These processes can also occur during later cycles of high-k ALD and give a chemical vapor deposition (CVD) component to the ALD process. These transformations lead to formation of clean-up products such as aziridine, ethene, N-methyl methyleneimine, hydrogen cyanide, and methane. Somebut not allof the reactions lead to reduction of surface As2O3 (i.e., clean-up). These results explain the experimentally observed accumulation of metallic arsenic and arsenic suboxide at the interface. Such understanding can help achieve control of oxide-semiconductor interfaces through the appropriate choice of chemical precursor.