Wet Chemical Functionalization
of III–V Semiconductor
Surfaces: Alkylation of Gallium Arsenide and Gallium Nitride by a
Grignard Reaction Sequence
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Abstract
Crystalline gallium arsenide (GaAs) (111)A and gallium
nitride
(GaN) (0001) surfaces have been functionalized with alkyl groups via
a sequential wet chemical chlorine activation, Grignard reaction process.
For GaAs(111)A, etching in HCl in diethyl ether effected both oxide
removal and surface-bound Cl. X-ray photoelectron (XP) spectra demonstrated
selective surface chlorination after exposure to 2 M HCl in diethyl
ether for freshly etched GaAs(111)A but not GaAs(111)B surfaces. GaN(0001)
surfaces exposed to PCl<sub>5</sub> in chlorobenzene showed reproducible
XP spectroscopic evidence for Cl-termination. The Cl-activated GaAs(111)A
and GaN(0001) surfaces were both reactive toward alkyl Grignard reagents,
with pronounced decreases in detectable Cl signal as measured by XP
spectroscopy. Sessile contact angle measurements between water and
GaAs(111)A interfaces after various levels of treatment showed that
GaAs(111)A surfaces became significantly more hydrophobic following
reaction with C<sub><i>n</i></sub>H<sub>2<i>n</i>–1</sub>MgCl (<i>n</i> = 1, 2, 4, 8, 14, 18). High-resolution
As 3d XP spectra taken at various times during prolonged direct exposure
to ambient lab air indicated that the resistance of GaAs(111)A to
surface oxidation was greatly enhanced after reaction with Grignard
reagents. GaAs(111)A surfaces terminated with C<sub>18</sub>H<sub>37</sub> groups were also used in Schottky heterojunctions with Hg.
These heterojunctions exhibited better stability over repeated cycling
than heterojunctions based on GaAs(111)A modified with C<sub>18</sub>H<sub>37</sub>S groups. Raman spectra were separately collected that
suggested electronic passivation by surficial Ga–C bonds at
GaAs(111)A. Specifically, GaAs(111)A surfaces reacted with alkyl Grignard
reagents exhibited Raman signatures comparable to those of samples
treated with 10% Na<sub>2</sub>S in <i>tert</i>-butanol.
For GaN(0001), high-resolution C 1s spectra exhibited the characteristic
low binding energy shoulder demonstrative of surface Ga–C bonds
following reaction with CH<sub>3</sub>MgCl. In addition, 4-fluorophenyl
groups were attached and detected after reaction with C<sub>6</sub>H<sub>4</sub>FMgBr, further confirming the susceptibility of Cl-terminated
GaN(0001) to surface alkylation. However, the measured hydrophobicities
of alkyl-terminated GaAs(111)A and GaN(0001) were markedly distinct,
indicating differences in the resultant surface layers. The results
presented here, in conjunction with previous studies on GaP, show
that atop Ga atoms at these crystallographically related surfaces
can be deliberately functionalized and protected through Ga–C
surface bonds that do not involve thiol/sulfide chemistry or gas-phase
pretreatments