071901-3
Hinkle et al.
Appl. Phys. Lett. 92, 071901 ͑2008͒
bonding arrangement. For HfO2, the Hf is in a 4+ oxidation
state which is not an exact complement of either As oxida-
tion states. With no direct ligand exchange chemically favor-
able, the reduction of As oxidation states using TEMA-Hf
must be due to other reasons.
Ms. Rocio Contreras-Guerrero and Professor Alberto
Herrera-Gomez on the surface preparation work. This work
is supported by the National Institute of Standards and Tech-
nology, Semiconductor Electronics Division, the MARCO
Focus Center on Materials, Structures, and Devices, Science
Foundation Ireland, COSAR ͑Consortium Of Semiconductor
Advanced Research͒ in Korea, and the Texas Enterprise
Fund.
The removal of As2O5 before As2O3 seems contradic-
tory when compared with published equilibrium thermody-
namic data for room temperature.12 A possible explanation is
the coordination of the Hf atom versus the As in the native
oxide. The coordination of the Hf atom can be 6, 7, or 8 but
remains in approximately a 4+ oxidation state regardless of
coordination due to bond ionicity.17 First-principles calcula-
tions have shown that higher oxygen coordination of the Hf
in HfO2 is more stable than lower coordination Hf.18 This
suggests that despite the bulk thermodynamic values of
As–O, it may be energetically favorable for the Hf to replace
the higher coordinated As of As2O5 rather than force the Hf
into a lower coordination state to remove As2O3. There are
also studies that propose As2O5 is transformed into As2O3
with excess O2 given off as a byproduct.19,20 We also note
that steric hindrance effects may be expected to play a sig-
nificant role in the high-k deposition process not removing
all of the native oxides as would be predicted based on equi-
librium reactions. Studies are ongoing to interpret the reac-
tion pathways in more detail for comparison to those ob-
served on Si surfaces.21 The “clean-up effect” using ALD,
therefore, appears to be governed by simple charge balance,
energy considerations due to oxygen coordination, and ther-
modynamics.
In summary, we have shown the reduction and in some
cases the removal of As–O and Ga–O bonds on GaAs
through high-k ALD deposition using TMA and TEMA-Hf
as the metal precursors. This interfacial self-cleaning is
shown to be precursor dependent as well as oxidation state
dependent. Thermodynamics and compatibility of oxidation
states explain the specific oxides that are removed and why
some remain. These conclusions also suggest that interfacial
bonding can be discriminately chosen with the proper selec-
tion of ALD precursor with particular attention paid to the
oxygen coordination and valence charge state of the metal
atom, and the Gibbs free energies of formation of its oxides.
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The authors thank Professor K. J. Cho for his valuable
discussions. The authors acknowledge the contributions of
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