Chemical Nature and Control of High-k Dielectric/III-V Interfaces

III-V semiconductor surfaces are notoriously difficult to control as their native oxides are not stable. For the growth of high-k dielectric, interface passivation layers have recently been incorporated into InP/high-k dielectric stacks using different approaches, including atomic layer deposition (ALD) to grow thin aluminum silicate layers and deposition of a buffer silicon layer prior to aluminum oxide growth. While beneficial effects have been observed on the interface electrical properties, little is known about the chemical composition and formation mechanisms of these interfacial layers. Futhermore, there is growing evidence that elemental diffusion from the substrate or its oxide may impair electrical properties. There is therefore a need for chemical characterization of surfaces and interfaces under typical processing conditions.

Combined in-situ IR measurements and ab initio calculations of InP(100) surfaces reveal that a mild annealing (~300^oC) typically needed for atomic layer deposition leads to marked chemical changes at the surface, with the formation of surface InPO₄-species (P=O and P-OH species). The initial interaction of trimethylaluminum with this hydrophosphonated surface is characterized by the formation of P-O-Al and dative bonding between Al and the oxygen atom of neighboring P=O moiety, requiring several cycles to complete this interfacial layer. This chemical interface transformation and subsequent impact on the quality of an ALD-grown Al₂O₃ layer can be substantially altered by silicon deposition using trisilane exposure. Comparison of several silicon precursors [SiCl₄, Si(OCH₃)₄ and Si₃H₆] by in-situ spectroscopy reveals that Si₃H₆ best reacts with the hydrophosphonate-terminated surface and reduces the native oxide by forming a SiO_x layer that prevents interaction of TMA with the native oxide, leading to a denser Al₂O₃  layer. Deposition of a thin aluminum silicate barrier layer by ALD is also investigated, and Al from TMA found to play a key role in activating silicon precursors. However, TMA reaction with Si-modified surfaces leads to efficient methyl transfer to Si, a source of interfacial carbon contamination.

Finally, mass transport is examined in HfO₂/In_0.53Ga_0.47As stacks.  Diffusion of indium through HfO₂ is observed after post deposition annealing in N₂ or forming gas environments by low energy ion scattering and X-ray photo electron spectroscopy, and found to be consistent with changes in interface layer thickness observed by transmission electron microscopy. Prior to post processing, elemental arsenic is detected in the form of arsenic oxide at the surface of ALD-grown HfO₂ and is desorbed upon annealing at 350^oC. Indium diffusion occurring upon annealing, resulting in a reduction of the interfacial layer thickness and a potential densification of HfO₂, is confirmed by an increase in capacitance.