Synthesis and Characterization of Atomically Flat Methyl-Terminated Ge(111) Surfaces

Article abstract of DOI:10.1021/jacs.5b03339

Atomically flat, terraced H-Ge(111) was prepared by annealing in H₂(g) at 850 °C. The formation of monohydride Ge-H bonds oriented normal to the surface was indicated by angle-dependent Fourier-transform infrared (FTIR) spectroscopy. Subsequent

Full text of DOI:10.1021/jacs.5b03339

Article
pubs.acs.org/JACS
Synthesis and Characterization of Atomically Flat Methyl-Terminated
Ge(111) Surfaces
Keith T. Wong,† Youn-Geun Kim,‡ Manuel P. Soriaga,‡ Bruce S. Brunschwig,‡,§
and Nathan S. Lewis*,†,‡,§,∥
‡
§
∥
†Division of Chemistry and Chemical Engineering, Joint Center for Artificial Photosynthesis, Beckman Institute, and Kavli
Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: Atomically flat, terraced H−Ge(111) was
prepared by annealing in H2(g) at 850 °C. The formation of
monohydride Ge−H bonds oriented normal to the surface was
indicated by angle-dependent Fourier-transform infrared
(FTIR) spectroscopy. Subsequent reaction in CCl3Br(l)
formed Br-terminated Ge(111), as shown by the disappear-
ance of the Ge−H absorption in the FTIR spectra
concomitant with the appearance of Br photoelectron peaks
in X-ray photoelectron (XP) spectra. The Br−Ge(111) surface
was methylated by reaction with (CH3)2Mg. These surfaces
exhibited a peak at 568 cm−1 in the high-resolution electron
energy loss spectrum, consistent with the formation of a Ge−C bond. The absorption peaks in the FTIR spectra assigned to
methyl “umbrella” and rocking modes were dependent on the angle of the incident light, indicating that the methyl groups were
bonded directly atop surface Ge atoms. Atomic-force micrographs of CH3−Ge(111) surfaces indicated that the surface remained
atomically flat after methylation. Electrochemical scanning−tunneling microscopy showed well-ordered methyl groups that
covered nearly all of the surface. Low-energy electron diffraction images showed sharp, bright diffraction spots with a 3-fold
symmetry, indicating a high degree of order with no evidence of surface reconstruction. A C 1s peak at 284.1 eV was observed in
the XP spectra, consistent with the formation of a C−Ge bond. Annealing in ultrahigh vacuum revealed a thermal stability limit
of ∼400 °C of the surficial CH3−Ge(111) groups. CH3−Ge(111) surfaces showed significantly greater resistance to oxidation in
air than H−Ge(111) surfaces.
infrared (FTIR) spectroscopy, and surface recombination
velocities of <100 cm s−1 were observed. However, unlike
Si(111), which can be anisotropically etched in NH4F(aq), no
anisotropic etch of the Ge(111) surface has been identified to
date. Thus, CH3−Ge(111) surfaces prepared by wet etching
and halogenation/alkylation are expected to be atomically
rough. Improvement in the chemical and electrical passivation
of Ge(111) may be obtained by the preparation of highly
ordered, atomically flat CH3−Ge(111) surfaces. We describe
herein a combination of annealing and a two-step halogen-
ation/alkylation procedure that results in atomically flat CH3−
Ge(111) surfaces. The chemical composition, thermal stability,
resistance to oxidation in air, surface topography, and ordering
of such surfaces have been characterized by FTIR spectroscopy,
high-resolution electron-energy loss spectroscopy (HREELS),
XPS, atomic-force microscopy (AFM), electrochemical scan-
ning-tunneling microscopy (EC-STM), and low-energy elec-
tron diffraction (LEED).
INTRODUCTION
■
Although the first transistor was made on a Ge crystal, the
dominant substrate for semiconductor electronics eventually
became Si, owing largely to the ease with which a Si/SiO2
interface of high electronic quality can be formed. However, as
device sizes continually shrink, high leakage current has
required the SiO2 gate dielectric to be replaced by high-κ
dielectrics. The move to high-κ dielectrics removes one of the
primary reasonsthe high defect density at the Ge/GeOx
interfacethat Ge has not been frequently used in semi-
conductor devices, and thus, its use opens the door to take
advantage of the high hole mobility of Ge.1−3 Moreover, the
0.67 eV band gap of Ge is well suited to the absorption of
infrared radiation, and Ge or SiGe alloys are frequently used as
rear absorbers in multijunction solar cells.4,5 Applications for
Ge in solar cells or in semiconductor electronics require a
stable, low defect-density surface, which cannot be achieved by
oxidation.6
Methyl-terminated Ge(111) surfaces7,8 have been prepared
in a similar manner to that used to alkylate Si(111),9 specifically
through the use of a two-step halogen/alkylation process.
Methyl termination of Ge(111) was demonstrated by X-ray
photoelectron spectroscopy (XPS) and Fourier-transform
Received: March 31, 2015
Published: July 8, 2015
© 2015 American Chemical Society
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Article
temperature was measured by an E-type (chromel−constantan)
thermocouple that was spot-welded to the back side of the Mo disc.
Feedback control of the temperature was provided by a Eurotherm
controller. Temperature ramp rates no greater than 1 °C s−1 were
used. The temperature was held constant for 60 min once the desired
temperature had been reached and the sample was then cooled to
ambient temperature for analysis by XPS.
3. High-Resolution Electron-Energy Loss Spectroscopy. HREELS
data were obtained using an LK Technologies ELS5000MCA electron-
energy loss spectrometer equipped with a multichannel electron-
energy analyzer. The HREELS and LEED spectrometers were attached
via a transfer arm to the Kratos XPS system described above, enabling
transfer under ultrahigh vacuum (UHV) among all three systems.
HREELS spectra were collected with a 5 eV electron-beam energy in a
specular geometry, with the monochromator exit and analyzer
entrance both positioned 55° from the surface plane. Collection
times of at least 300 s were used, and the pressure in the analysis
chamber was <1 × 10−9 Torr while data were collected.
EXPERIMENTAL SECTION
■
A. Materials. Unless otherwise noted, chemicals were obtained
from Sigma-Aldrich or Fisher Scientific and were used as received.
Water was obtained from a Barnstead E-Pure water purification system
and had a resistivity of 18.2 MΩ cm. Solutions of 10% HF(aq) were
prepared by diluting 48 wt % HF(aq) (Transene). Dimethylmagne-
sium ((CH3)2Mg) was prepared by addition of small amounts of 1,4-
dioxane to 1.0 M methylmagnesium chloride in tetrahydrofuran
(THF) (diluted from 3.0 M CH3MgCl) until the MgCl2·dioxane
complex ceased to precipitate. The precipitate was then removed by
passing the solution several times through baked glass wool.
Undoped, n-type Ge wafers (MTI Corp.) that were polished on
both sides and oriented within 0.5° of the (111) crystal plane were
cut into the desired size using a diamond scribe. The samples had a
resistivity of >50 Ω cm. Ge(111) wafers with a miscut angle of 0.1°
(El-Cat Inc., undoped, double-side polished, ρ > 30 Ω cm) were used
for AFM imaging.
B. Surface Modification. Immediately prior to surface mod-
ification, samples were cleaned by rinsing sequentially with water,
methanol, acetone, methanol, and water, followed by 5 min of
sonication in acetone and then by 5 min of sonication in methanol.
The samples were subsequently repeatedly dipped in 10% HF(aq) for
1 min and in 30% H2O2(aq) for 1 min, with a water rinse between the
steps. After three cycles of etching and oxidation, the samples were
immersed in 10% HF (aq) for 1 min, rinsed with water, and dried
under a stream of Ar(g).
Cleaned Ge samples were loaded into a tube furnace that was
repeatedly purged with He(g) and pumped out before being pumped
down to <0.5 mTorr. Hydrogen-termination of the Ge samples was
achieved by annealing the cleaned Ge surfaces at 850 °C for 15 min
under 1 atm of H2(g) at a flow rate of 500 SCCM (standard cubic
centimeters at STP). Samples were cooled to <100 °C under H2(g)
and were immediately transferred into an N2(g)-purged flushbox upon
removal from the tube furnace.
H-terminated Ge(111) surfaces were rinsed with anhydrous THF,
and they were brominated in neat CCl3Br for 1 min at 50 °C, with
several grains of benzoyl peroxide added as a radical initiator. The
samples were rinsed with anhydrous THF following bromination. The
Br−Ge(111) surfaces were methylated for 12−16.5 h at 50 °C in
(CH3)2Mg/THF/1,4-dioxane. Methylated surfaces were then rinsed
thoroughly with anhydrous THF; removed from the N2(g)-purged
flushbox; sequentially sonicated for 10 min in THF, methanol, and
water; and dried under a stream of Ar(g).
C. Characterization. 1. FTIR Spectroscopy. IR spectra were
collected using a Thermo Scientific Nicolet 6700 FTIR spectrometer
equipped with a thermoelectrically cooled deuterated triglycine sulfate
(DTGS) detector and a dry N2(g) purge. Spectra were collected in
transmission mode with the wafer oriented such that the angle
between the surface normal and incident light beam was either 74° or
30°. The reported spectra represent averages of 1000 scans with 4
cm−1 resolution, collected after at least 25 min of purging the chamber
following loading of the sample.
4. Low-Energy Electron Diffraction. LEED patterns were recorded
using an LK technologies RVL2000 reverse-view LEED system.
Patterns were collected using a 3.05 A filament current, a 3.0 kV screen
voltage, and a 100 V retarding voltage at the specified electron-beam
energies. The analysis chamber was maintained below 2 × 10−9 Torr
during operation of the LEED instrumentation. A Canon EOS Rebel
Tli (f/8, 10 s exposure, ISO400) camera was used to record the LEED
patterns.
5. Atomic-Force Microscopy. Atomic-force micrographs were
recorded using a Bruker MultiMode 8 AFM with a Bruker Nanoscope
V controller. Bruker TESPA and TESPA-V2 probes were used in
tapping mode. Data were recorded and processed using Bruker
Nanoscope 8.15 and Nanoscope Analysis 1.50 software, respectively.
Postcollection data processing was limited to image flattening to
remove the background tilt and bow. A diamond scribe was used to cut
double-side polished, undoped Ge wafers (El-Cat Inc.) oriented within
0.1° of the (111) crystal plane into the desired size for AFM. The
samples had a resistivity of >30 Ω cm.
6. Electrochemical Scanning-Tunneling Microscopy. EC-STM
images were obtained using a Nanoscope E (Digital Instruments,
Veeco) STM with a three-electrode potentiostat. An electrochemical
cell was custom-crafted from Kel-F (Emco Industrial Plastics, Inc.) and
fitted with a Pt counter and a Pt pseudoreference electrode calibrated
against a Ag/AgCl reference cell. W tips were electrochemically etched
(15 V AC in 1.0 M KOH(aq)) from 0.25 mm diameter W wires.
Experiments were performed in constant-current mode under
potential control at the measured at open-circuit potential (−0.2 V
vs Ag/AgCl) with the Ge surface immersed in 0.10 M HClO4(aq).
D. Data Analysis. XPS data were analyzed using CasaXPS v2.3.16
software. High-resolution scans were fit using a Shirley (Ge 3d, C 1s)
or offset Shirley (Br 3d) background. Gaussian/Lorentzian product
lineshapes were used with a 30% Lorentzian component except for the
bulk Ge 3d5/2 and 3d3/2 peaks, which were fit with an asymmetric
Gaussian/Lorentzian convolution that accounted for the inherent
asymmetry due to surface states.
2. X-ray Photoelectron Spectroscopy. XP spectra were collected
using a Kratos AXIS Ultra DLD spectrometer equipped with a
magnetic immersion lens that consisted of a spherical mirror and
concentric hemispherical analyzers with a delay-line detector (DLD).
A monochromatic Al Kα source (1486 eV) was used for X-ray
excitation, and ejected electrons were collected normal to the surface.
The analysis chamber was maintained at <5 × 10−9 Torr. All XPS
energies are reported as binding energies in eV.
To identify the elements present on the surface, survey scans from 0
to 1200 eV were collected at a pass energy of 80 eV. High-resolution
scans were collected at a pass energy of 10 eV for the Ge 3d (25−37
eV), C 1s (280−292 eV), and Br 3d (64−76 eV) photoelectron
regions.
The methods used for quantitative analysis of the XPS data are
based on those described previously.10 The fractional monolayer
coverage of Br or hydrocarbon overlayers, Φov, was obtained using the
relationship:
⎛
⎜
⎝
⎞⎛
⎞
⎟
⎠
N
SFGe Iov
(λsin θ)
ov
Φ
=
=
ρ
Ge
⎟⎜
ov
N
N
SF
I
Ge
ov ⎠⎝
(1)
sites
sites
where λ is the photoelectron escape depth (assumed to be
approximately equal for Ge and the overlayer), θ is the photoelectron
takeoff angle measured from the plane of the surface, Nsites is the areal
density of surface sites on the Ge(111) surface, SF is the modified
sensitivity factor, ρGe is the atomic density of Ge, and I is the peak
intensity. The photoelectron escape depth was taken to be 3.5 nm,11
the atomic density of Ge was taken to be 4.42 × 1022 cm−3,7 and the
areal density of the Ge(111) surface was taken to be 7.22 × 1014 cm−2.
As necessary, sample heating was performed in the XPS analysis
chamber. The sample was held to a Mo disc (5 mm diameter ×1 mm)
by a stainless steel clip. The disc was suspended between two W wires
through which current was passed to heat the sample. The sample
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Modified sensitivity factors, SFx, accounting for the energy dependence
of the electron escape depth, were calculated using
Sexp
⎛
⎜
⎞
⎟
1486 − BE
1486 − 284
SF = SF
x
⎝
⎠
(2)
where SF is the transmission-corrected sensitivity factor obtained from
the appropriate Kratos relative sensitivity factor library, BE is the
electron binding energy in eV, and Sexp is the sensitivity exponent,
taken to be 0.69 for our system.
For comparison, the coverage of methyl groups on CH3−Ge(111)
surfaces was also calculated by comparison to the XPS data obtained
on CH3−Si(111) surfaces. Proper preparation of CH3−Si(111)
surfaces has been shown by synchrotron XPS,12 low-temperature
scanning-tunneling microscopy,13,14 and helium-atom scattering15 to
produce surfaces with near 100% coverage of surface Si atoms, a high
degree of long-range ordering, and a low defect density. Therefore, the
CH3−Si(111) surface served as an excellent standard for estimating
the overlayer coverage of the CH3−Ge(111) surfaces. The coverage of
a CH3−Ge(111) surface was accordingly also calculated using the
relationship:
Figure 1. Transmission FTIR spectra of (a) an H−Ge(111) surface at
a 30° incident angle, (b) an H−Ge(111) surface at a 74° incident
angle, and (c) a Br−Ge(111) surface at a 74° incident angle. The
background sample for all of these spectra was an HF-etched Ge(111)
surface that had been exposed to ambient air. The incident angle is
given as the angle between the surface normal of the sample and the
excitation beam.
⎡
⎤
SFGe
IC
⎢
⎣
⎥
( I )⎦
Ge
CH −Ge(111) ⎛ σSi ⎞
3
⎜
⎝
⎟
⎠
Φ
=
ov
⎡
⎤
SFSi
Ge
IC
⎢
⎣
⎥
⎦
( )
ISi
Table 1. Position, Primary Polarization Direction, and
Assignment of H−Ge(111) and CH3−Ge(111) Vibrational
Modes Observed by FTIR Spectroscopy and HREELS
CH3−Si(111)
(3)
where σSi and σGe represent the surface atomic densities of the Si(111)
and Ge(111) surfaces7.83 × 1014 and 7.22 × 1014 cm−2
,
FTIR spectroscopy
primary
HREELS
respectively. Although this method relies on the relative accuracy of
the sensitivity factors of SFSi and SFGe, the substrate-overlayer model
of eq 1 also relies on the accuracy of λ, SFGe, and SFC.
position
polarization
direction
position
(cm−1
)
(cm−1
)
assignment
The methods used to calculate the thickness and coverage of
oxidized Ge have also been described previously.11 The thickness, d, of
oxidized Ge was calculated using the formula:
H−Ge(111)
1975
563
z
n/a
Ge−H stretch mode
Ge−H bend mode
∥
n/a
⎛
⎜
⎞
⎠
⎡
⎤
⎥
⎛
⎜
⎝
⎞
⎟
⎠
°
IGe
I
CH3−Ge(111)
⎟
⎟
⎢
d = λ sin θ ln 1 +
ov
⎜
⎢
⎣
°
⎥
⎦
2956
2928
2906
2860
-
∥
z
z
∥
-
2910
2910
2910
2910
1411
C−H stretching mode
⎝
ov
(4)
C−H stretching mode
C−H stretching mode
C−H stretching mode
where λov is the photoelectron escape depth through the oxide
overlayer, taken to be 3.0 nm for Ge 3d photoelectrons,16 and IGe°/Iov°
is an experimentally determined normalization factor based on the
intensity ratio of the signal for pure Ge relative to the signal for pure
Ge oxide, and was taken to have a value of 1.51.11 The coverage was
then calculated assuming a value of 0.32 nm for the thickness of a
monolayer of GeO2.17
asymmetric C−H deformation
mode
1234
z
1234
symmetric C−H deformation
mode (CH3 umbrella mode)
762
-
∥
-
780
568
CH3 rocking mode
Ge−C stretching mode
RESULTS
■
A. Vibrational Spectroscopy. Figure 1 shows transmission
FTIR spectra at two different angles of incident light for a H−
Ge(111) surface that was formed by annealing at 850 °C under
H2(g). Table 1 presents the observed peaks, their primary
polarization directions, and the peak assignments for such
surfaces. The sharp vibrational modes at 1975 and 563 cm−1
were assigned to the monohydride Ge−H stretching (ν(Ge−
H)) and bending (δ(Ge−H)) modes, respectively.18,19 Fitting
the ν(Ge−H) and δ(Ge−H) peaks with Gaussian lineshapes
yielded a full width at half-maximum of 3.1 and 3.2 cm−1,
respectively, establishing an upper limit to the actual line width
due to the instrument-limited 4 cm−1 resolution at which the
spectra were collected. The ν(Ge−H) and δ(Ge−H) modes
both showed strong signals at an incident light angle of 74° off
the surface normal, whereas at an angle of 30° the ν(Ge−H)
mode was very weak while the δ(Ge−H) mode absorbed
strongly. At a 74° incident angle, significantly more light
polarized perpendicular to the surface is transmitted then light
polarized parallel to the surface, while at 30° the opposite
situation occurs. Accordingly, the relative intensities of
vibrational modes perpendicular to the surface should increase
in intensity as the incident angle is increased, whereas modes
oriented parallel to the surface should exhibit a decrease in
intensity. Thus, the data were consistent with the ν(Ge−H)
mode being polarized perpendicular to the surface and with the
δ(Ge−H) mode being polarized parallel to the surface. No
absorbance characteristic of Ge−H stretching associated with
GeH2 and GeH3 species was observed in the 2000−2100 cm−1
region.18,19
Figure 1 also displays the transmission FTIR spectrum of a
Br−Ge(111) surface that was formed by reaction of a H−
Ge(111) surface with CCl3Br(l). No absorbance was observed
at the frequencies that corresponded to ν(Ge−H) and δ(Ge−
H) vibrational modes, indicating complete removal of Ge−H
species from the surface. The Ge−Br stretching and
deformation modes were expected to occur at frequencies too
low to be detected by our experimental apparatus.
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Figure 2 presents the transmission FTIR spectra of a CH3−
Ge(111) surface formed by the reaction of Br−Ge(111) with
Figure 3. HREELS data for a CH3−Ge(111) surface (a) as-prepared
and (b) after a 60 min anneal at 350 °C. The region above 150 cm−1
energy loss has been magnified 20-fold for clarity.
Figure 2. Transmission FTIR spectra of a CH3−Ge(111) collected
with the surface normal oriented at an angle of (a) 74° or (b) 30°
relative to the incident excitation beam. Both spectra used a H−
Ge(111) surface as a background, and the absorbance in the 2700−
3100 cm−1 region was magnified 8-fold for clarity of presentation.
cm−1, and a shoulder on the elastic peak at 261 cm−1, were also
observed in the HREELS data of as-prepared CH3−Ge(111).
These peaks were not readily assigned, however, and annealing
the surface to 350 °C for 60 min in UHV (Figure 3b) led to the
disappearance of the peaks at 921 and 1045 cm−1, suggesting
that these signals resulted from physisorbed adventitious
species that desorbed upon annealing. No significant change
in the peaks assigned to the vibrational modes of the CH3−
Ge(111) surface was observed upon annealing of these samples.
B. X-ray Photoelectron Spectroscopy. Figure 4 displays
a representative XP survey spectrum of a CH3−Ge(111)
surface. The spectrum was dominated by Ge photoelectron and
Auger lines from the bulk crystal, at Ge 3d (30 eV), Ge 3p (122
eV), Ge 3s (181 eV), Ge LMM (300−600 eV), and by
associated plasmon loss peaks. C 1s and O 1s photoelectron
peaks were also observed at 285 and 533 eV, respectively,
consistent with the presence of surface-bound methyl groups,
adventitious hydrocarbons, and/or oxygen-containing adventi-
tious hydrocarbons. No other elements were detected in the
survey spectrum, and no Mg from the (CH3)2Mg was detected
in high-resolution XPS scans (see Supporting Information).
Survey spectra of H−Ge(111) and Br−Ge(111) surfaces also
contained no detectable contaminants (see Supporting
Information).
(CH3)2Mg/THF/1,4-dioxane and collected using light incident
at two different angles. The peak positions, polarization
directions, and assignments are also shown in Table 1. At a
74° incident light angle, positive peaks were observed at 2956,
2928, 2906, and 2860 cm−1, and were assigned to the C−H
stretching modes (ν(C−H)) of both the methyl surface
functionalization as well as to adventitious hydrocarbon species.
The positive peaks at 1234 and 762 cm−1 were assigned to the
symmetric CH3 deformation (methyl “umbrella” mode,
δs(CH3)) and the CH3 rocking mode (ρ(CH3)), respectively,
as previously reported.8 The negative peaks observed at 1975
and 563 cm−1 corresponded to ν(Ge−H) and δ(Ge−H) signals
exhibited by the background H−Ge(111) surface. At a 30°
incident light angle, the δs(CH3) mode was not visible above
the noise, whereas the ρ(CH3) mode absorbed strongly. The
incident angle dependence indicated that the methyl umbrella
and rocking modes were polarized perpendicular and parallel to
the surface, respectively, and was consistent with the presence
of a Ge−C bond oriented perpendicular to the surface.
Figure 3 shows the HREELS data of a CH3−Ge(111) surface
as prepared and after a 60 min anneal at 350 °C in UHV,
respectively. Peak positions and assignments are shown in
Table 1. The peaks at 2910, 1234, and 780 cm−1 were assigned
to the ν(C−H), δs(CH3), and ρ(CH3) vibrational modes,
respectively, and were consistent with the transmission FTIR
spectra shown in Figure 2. However, the individual C−H
stretching modes were not resolved by HREELS. In addition,
the HREELS peaks observed at 1411 and 568 cm−1 were
assigned to the asymmetric CH3 deformation (δa(CH3)) and to
the Ge−C stretching (ν(Ge−C)) modes, respectively. The
δa(CH3) mode was not observed by FTIR spectroscopy, as this
mode does not have a significant change in dipole moment and
therefore should be IR inactive, whereas IR inactive modes can
be observed in HREELS even in a specular geometry.20 The
ν(Ge−C) mode is expected to be IR active but was not
observed by transmission FTIR spectroscopy due to the low
absorption cross-section of this mode and the high noise level
that was present in the low-wavenumber region for the
particular IR instrument used. Small peaks at 921 and 1045
Figure 4 also shows high-resolution XP spectra of the Ge 3d
and Br 3d photoelectron regions taken at three points during
the synthesis procedure: the hydrogen-terminated, bromine-
terminated, and methyl-terminated Ge(111) surfaces. No
oxidation of the underlying Ge, which would produce peaks
shifted to higher binding energy (31−33 eV) than the bulk Ge
3d3/2 and 3d5/2 peaks, was detected in any of the Ge 3d spectra.
The Br 3d XP spectra showed the appearance of Br 3d3/2 and
3d5/2 peaks upon 1 min reaction of H−Ge(111) in CCl3Br(l).
No Br was detectable after subsequent methylation of the Br−
Ge(111) surface in (CH3)2Mg/THF/1,4-dioxane. The bromine
coverage for the Br−Ge(111) surface was estimated to be 0.74
0.13 monolayers (ML; 1 ML = 1 adsorbed Br per surface Ge
atom), and longer bromination times did not lead to an
increase in coverage.
Figure 5 shows a representative high-resolution XP spectrum
of the C 1s photoelectron region of a CH3−Ge(111) surface,
with the spectrum fit to five peaks. The largest peak was
centered at 285.2 eV and is attributed to the C−C species
associated with adventitious hydrocarbons. A small peak at
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Figure 4. Left panel: XP survey spectrum of a CH3−Ge(111) surface. Right panels: high-resolution XP spectra of the Br 3d (top) and Ge 3d
(bottom) photoelectron regions of (a) an H−Ge(111) surface, (b) a Br−Ge(111) surface, and (c) a CH3−Ge(111) surface.
Figure 5. High-resolution XP spectrum of the C 1s photoelectron
region of a CH3−Ge(111) surface. As described in the text, the fitted
peaks are assigned to C−O (purple), C−C (blue), C−Ge (solid
orange), and C−Ge with vibrational losses (dotted orange).
higher binding energy (286.7 eV) can be attributed to C−O
species in oxygen-containing adventitious hydrocarbons. The
peak at 284.1 eV can be attributed to C−Ge from methyl
groups covalently bonded to the Ge surface, similar to previous
reports for CH3−Ge(111) and CH3−Si(111) surfaces.7,21 In
Figure 6. Representative atomic-force micrographs of (a) an HF-
addition, to account for the vibrational fine structure, peaks
etched Ge(111) surface, (b) an H−Ge(111) surface prepared by
were fit at fixed energy (+0.38, +0.76 eV) and relative area
annealing under H2(g), and (c) a CH3−Ge(111) surface.
(49%, 11%) compared to the main C−Ge peak; these
vibrational loss peaks are shown by the dotted peaks in Figure
5. The vibrational fine structure has been shown to produce an
asymmetric peak shape for CH3−Si(111) surfaces,21 and the
same peak shape was found to provide the best fit for as-
prepared CH3−Ge(111) surfaces as well as for CH3−Ge(111)
surfaces that had been annealed to remove adventitious
hydrocarbons (vide infra). The methyl surface coverage was
estimated to be 0.80 0.02 ML (1 ML = 1 methyl group per
surface Ge atom) using the substrate-overlayer model (eq 1) or
exhibited nanometer-scale roughness. Annealing under H2(g)
removed this roughness, and similar to previous reports, a
terraced surface with terrace widths on the order of hundreds of
microns was produced.22−24 The terraces were atomically flat,
aside from small raised triangular protrusions that were
attributed to raised (111) facets. The step height between
terraces, as determined by averaging several measurements
from Figure 6b, was 318 12 pm, in close agreement with the
327 pm (111) interplanar spacing calculated on the basis of the
lattice parameter of Ge (565.8 pm25). The surface structure was
mostly unchanged by methylation in (CH3)2Mg/THF/1,4-
dioxane (Figure 6c), aside from the appearance of small
particles and etch pits. The terraced structure remained intact,
0.83
0.03 ML by comparison to reference CH3−Si(111)
surfaces (eq 3). No Br was detected by XPS (Figure 4).
C. Surface Imaging. Figure 6 presents atomic-force
micrographs of an HF-etched Ge(111) surface, an H−
Ge(111) surface after annealing under H2(g), and a CH3−
Ge(111) surface. After etching in HF (Figure 6a), the surface
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with similar step heights to those observed for the H−Ge(111)
surface.
not necessarily provide direct information regarding the
ordering and symmetry of the methyl overlayer, as the thin
organic layer may have a low cross section for scattering low-
energy electrons. The energy dependence of the LEED pattern
was analogous to LEED data on CH3− and C2H5−Si(111)
surfaces.21,26
Figure 7 shows representative EC-STM images of a CH3−
Ge(111) surface. The 500 × 500 nm2 image (Figure 7a)
E. Thermal Stability in UHV. Figure 9 shows the high-
resolution C 1s XP spectra of a CH3−Ge(111) surface as-
Figure 7. EC-STM images of a CH3−Ge(111) surface: (a) 500 × 500
nm2 area collected at the open-circuit potential with −500 mV bias
voltage and 2 nA tunneling current, (b) 5 × 5 nm2 area collected at the
open-circuit potential with −300 mV bias voltage and 5 nA tunneling
current.
showed a terraced surface, similar to the AFM image in Figure
6c. The terrace step height was measured to be 320 20 pm, in
agreement with the AFM data. Figure 7b shows a high-
resolution EC-STM image of a 5 × 5 nm2 area of a CH3−
Ge(111) surface. The data showed an array of bright
protrusions, which are assigned to individual surface-bound
methyl groups. The distance between neighboring (methyl
group) bright spots was measured to be 380 20 pm, which is
in agreement with the distance between neighboring Ge atoms
on an unreconstructed Ge(111) surface (400 pm based on a
lattice parameter of 565.8 pm) considering that some drift was
observed during collection of EC-STM images. Thus, the
pattern of EC-STM bright spots associated with methyl groups
is consistent with methylation of the atop Ge(111)-(1 × 1)
sites. The dark spots in both EC-STM images likely represent
surface sites that had not been terminated with methyl groups,
and these sites may be bare Ge, oxidized Ge, H-terminated Ge,
or Ge atoms terminated by other species. The methyl surface
coverage in the area imaged in Figure 7b was 0.85−0.90 ML.
D. Low-Energy Electron Diffraction. Figure 8 shows the
LEED patterns, at three different primary electron energies,
Figure 9. High-resolution XP spectra of the C 1s photoelectron region
of a CH3−Ge(111) surface (bottom to top) as-prepared and after
sequential 60 min anneals in UHV at 300, 350, 400, and 450 °C,
respectively. As described in the text, the fitted peaks are assigned to
C−O (purple), C−C (blue), C−Ge (solid orange), and C−Ge with
vibrational losses (dotted orange).
prepared and after annealing in UHV for 60 min at 300, 350,
400, and 450 °C, respectively. Annealing up to 400 °C resulted
in a reduction in the peaks attributed to adventitious C−C and
C−O species, with negligible change in the position or area of
the C−Ge peak. Upon annealing to 450 °C, several
experiments showed that the area of the C−Ge peak had
been reduced, indicating the loss of methyl carbon from the
surface. In addition, the C−Ge peak broadened and shifted
∼0.2 eV higher in binding energy after annealing to 450 °C,
suggesting a chemical change of some or all of the surface-
bound methyl groups. Even when little or no adventitious
carbon remained after annealing, the C 1s data were best fit by
including the vibrational fine structure peaks, supporting the
use of this fitting method. No change was observed in the Ge
3d XP spectrum throughout the annealing process (see
Supporting Information).
F. Surface Oxidation. Figure 10 shows high-resolution Ge
3d XP spectra of an H−Ge(111) surface and of a CH3−
Ge(111) surface, upon exposure to ambient laboratory air.
Under such conditions, the H−Ge(111) surface oxidized on a
time scale of hours, as indicated by the appearance of a peak at
32.5−33.3 eV, corresponding to GeO2. The CH3−Ge(111)
surface also oxidized in air, but at a much slower rate than the
H−Ge(111) surface. The difference in oxidation rates between
the H−Ge(111) and CH3−Ge(111) surfaces is depicted in
Figure 10c, which shows the oxide coverage in ML calculated
using eq 4. After 134 h in air, the H−Ge(111) surface
contained 1.15 ML of oxide, whereas the CH3−Ge(111)
surface contained 0.12 ML of oxide after 432 h in air.
Figure 8. LEED patterns of a CH3−Ge(111) surface recorded at
primary electron energies of 37, 42, and 48 eV, respectively.
obtained from a CH3−Ge(111) surface. Relatively low primary
electron energies (37−48 eV) were used to minimize the
electron escape depth. Patterns with a 3-fold symmetry were
observed, consistent with a 1 × 1 structure. The sharp, bright
diffraction spots and low background intensity indicated that
the surface was well-ordered, and were consistent with the
large, atomically flat terraces observed by AFM. The observed
LEED patterns indicate that the Ge(111) surface is
unreconstructed and well-ordered, but the LEED patterns do
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because the CCl3Br reagent did not result in significant etching
of the Ge(111) surface.
C. Methyl Termination. The formation of a methyl-
functionalized Ge(111) surface was demonstrated by FTIR
spectroscopy, HREELS, and XPS. The FTIR and HREEL
spectra of CH3−Ge(111) exhibited ν(C−H), δs(CH3),
δa(CH3), and ρ(CH3) modes at similar frequencies to those
reported for methyl-terminated Ge(111) and Si(111) surfa-
ces8,30 as well as at frequencies similar to those typically
observed for Ge−CH3-containing organometallic com-
pounds.31 The angular dependence of the δs(CH3) and
ρ(CH3) modes in the FTIR spectra (Figure 2) is consistent
with the Ge−C bond being oriented perpendicular to the
surface. The HREELS peak at 568 cm−1 (Figure 3) provides
direct evidence for the formation of a covalent Ge−C bond.
For comparison, the Si−C stretching mode of CH3−Si(111)
surfaces occurs at 678−683 cm−1.32−35 Assuming a harmonic
oscillator model and considering the observed ν(Ge−H)
frequency in conjunction with the reduced masses of Ge−H
versus Ge−CH3, the ν(Ge−C) mode is thus predicted to
appear at 556 cm−1, in close agreement with the observed peak
at 568 cm−1. In addition, the observed 568 cm−1 peak lies
within the 535−641 cm−1 range typical of Ge−C stretching
modes in organometallic compounds that contain Ge−CH3
groups.31 Future work will compare the experimentally
observed vibrational frequencies to frequencies predicted by
theoretical calculations of a CH3−Ge(111) surface.36 Together,
these data from vibrational spectroscopy provide evidence for
covalent functionalization of the Ge(111) surface with methyl
groups situated directly atop surface Ge atoms, to yield a Ge−C
bond perpendicular to the surface.
The presence of a peak at 284.1 eV in the C 1s XP spectrum
(Figure 5) of the surface reacted with (CH3)2Mg is also
consistent with covalent functionalization of the surface with
methyl groups. Due to the lower electronegativity of Ge than C,
C covalently bonded to Ge is expected to have a lower C 1s
binding energy than C bonded to C in adventitious carbon.
Consistently, a C 1s peak at ∼284 eV has been attributed to the
C−Ge and C−Si species of alkylated Ge(111) and Si(111)
surfaces, respectively.7,21,26,37,38 Additional evidence that the C
1s photoelectron peak at 284.1 eV corresponds to covalently
bonded C−Ge is provided by the XPS data on annealed
surfaces (Figure 9). Annealing to 400 °C led to no significant
change in the area of the C−Ge peak. Annealing to 450 °C led
to a reduction in the C−Ge peak area along with a small
broadening and shift (+0.2 eV) of the C−Ge peak (Figure 9)
and no significant change of the Ge 3d peaks (Supporting
Information Figure S3), suggesting that the methyl group
desorbed from the surface or otherwise underwent a chemical
transformation on the surface. These data indicate a thermal
stability limit of the CH3−Si(111) surface of ∼400 °C, similar
to the ∼440 °C measured for a CH3−Si(111) surface.26 The
450 °C anneal led to a reduction in the C−Ge peak area
without a significant change in either the C 1s (Figure 9) or Ge
3d peaks (Supporting Information Figure S3), suggesting that
the methyl group desorbed from the surface at this temperature
rather than incorporating into the bulk. Annealing to 400 °C
removed essentially all physisorbed adventitious hydrocarbons,
as evidenced by the disappearance of the C 1s peaks at 285.2
and 286.7 eV, respectively. These data indicate that the C 1s
peak at 284.1 eV corresponds to a carbon species strongly
bound to the surface, consistent with the assignment of this
Figure 10. High-resolution XP spectra of the Ge 3d photoelectron
region of (a) an H−Ge(111) surface and (b) a CH3−Ge(111) surface
exposed to ambient laboratory air for varying amounts of time and (c)
the calculated oxide coverages of both surfaces as a function of time
exposed to air.
DISCUSSION
■
A. Hydrogen Termination. The presence of Ge−H
stretching and deformation modes in the FTIR spectra of
Ge(111) annealed under H2 at 850 °C (Figure 1a,b) is
consistent with the formation of a H-terminated Ge surface.
The angular dependence of these vibrational modes indicates
that the ν(Ge−H) and δ(Ge−H) modes are primarily polarized
perpendicular and parallel to the surface, respectively, which is
consistent with the Ge−H bond being oriented perpendicular
to the surface. The sharpness and polarization of the ν(Ge−H)
and δ(Ge−H) peaks, the absence of vibrations associated with
GeH2 or GeH3, and the absence of detectable contaminants by
XPS, all suggest that the H−Ge(111) surface formed by this
procedure is chemically analogous to that formed by NH4F
etching of Si(111),27 and that such surfaces consist of H atoms
bonded directly on atop surface Ge atoms. In addition, AFM
micrographs of the H−Ge(111) surface (Figure 6b) showed
atomically flat terraces separated by monatomic steps, similar to
what has been observed for Si(111) surfaces etched by
NH4F(aq).28 Building on previous work demonstrating
formation of atomically flat Ge(111) by annealing in H2,22−24
the data presented here demonstrates that the resulting surface
is characterized by a highly homogeneous Ge−H surface
termination.
B. Bromine Termination. Bromination of the H−Ge(111)
surface was achieved by reaction in CCl3Br, as has been done
for Si(111) surfaces.29 The appearance of Br 3d photoelectron
peaks in the XP spectrum of Br−Ge(111) (Figure 4) provides
evidence for bromination of the Ge by this reaction sequence.
The Br coverage estimated from the XPS data (0.74
0.13
ML), and the lack of observable ν(Ge−H) and δ(Ge−H) peaks
in the FTIR spectrum of the Br−Ge(111) surface (Figure 1c),
suggest that, upon bromination, Ge−H surface species were
essentially completely replaced by Ge−Br species. Bromination
in CCl3Br was chosen over other halogenation methods
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signal to C−Ge associated with covalently bound methyl
groups.
fashion to what has been observed for CH3−Si(111) by
scanning-tunneling microscopy.13
Two of the defining features of methyl-terminated Si(111)
surfaces are the essentially 100% coverage of surface Si atoms
achieved and the chemical passivation afforded by the methyl
overlayer.39 Using the area of the C 1s photoelectron peak at
284.1 eV assigned to C−Ge, the coverage of CH3−Ge(111)
CONCLUSIONS
■
Atomically flat Ge(111) surfaces were prepared by annealing in
H2(g), and FTIR spectroscopy confirmed that these surfaces
were Ge−H terminated with the Ge−H bond oriented normal
to the surface, with no detectable GeH2 or GeH3 surface
species. Bromination of H−Ge(111) surfaces was achieved by
reaction of H−Ge(111) in neat CCl3Br. FTIR spectroscopy
showed the complete removal of Ge−H species, and XPS data
showed 0.74 0.13 ML of Br on the surface. Atomically flat
CH3−Ge(111) surfaces were prepared by methylation of Br−
Ge(111) surfaces in (CH3)2Mg. HREELS data provided
evidence for the formation of a covalent Ge−C bond, and
the dependence of the methyl umbrella and rocking IR
absorptions on the incident light angle indicated that methyl
groups were oriented such that the Ge−C bond was normal to
the surface. AFM and EC-STM images of the CH3−Ge(111)
surface showed that the surface remained mostly terraced and
atomically flat, and LEED images showed sharp, bright
diffraction spots with 3-fold symmetry indicating a highly
ordered unreconstructed surface. EC-STM also confirmed that
methyl groups form an ordered array with spacing equal to that
of a Ge(111)-(1 × 1) surface. XP spectra contained a low
binding-energy C 1s peak that remained unchanged upon
annealing to 400 °C, consistent with formation of a C−Ge
covalent bond. The surface coverage calculated based on the
area of this peak was estimated from XPS data to be 0.80−0.83
ML, but EC-STM data indicated that the surface coverage is
closer to a complete monolayer with few defects. No surface
oxidation was detected immediately after preparation of the
CH3−Ge(111) surfaces, and the methyl-terminated surface
exhibited significantly greater resistance to oxidation in air than
a hydrogen-terminated Ge(111) surface.
surfaces was estimated to be 0.80
0.02 ML (substrate-
overlayer model) or 0.83 0.03 ML (comparison to CH3−
Si(111) surface). It should be noted that both models rely on
the accuracy of a number of parameters that are not easily
measured. However, XPS data showed no residual Br, no
measurable oxidation immediately after synthesis, and no other
contaminant elements; and FTIR spectroscopy showed no
measurable Ge−H stretching or bending. By ruling out other
likely surface species, these data suggest that the surface
coverage of methyl groups may, in fact, be closer to 1 ML than
indicated by XPS coverage estimates. In fact, the EC-STM
images (Figure 7) provided direct, compelling evidence that the
methyl surface coverage is closer to 100% than estimated from
XPS data. Specifically, the high-resolution EC-STM image of
Figure 7b shows that the methyl groups formed an ordered
array with spacing that matched that of surface atoms on an
unreconstructed Ge(111) surface, with the large area image
(Figure 7a) showing relatively few defects over a 500 × 500
nm2 area. In addition, relatively high count rates have been
observed for helium-atom scattering off of CH3−Ge(111)
surfaces prepared by the method described here, suggesting
that these surfaces have relatively low defect densities.36
Methyl termination of Ge(111) also produced a significantly
improved resistance to oxidation in air as compared to an H−
Ge(111) surface (Figure 10). A H−Ge(111) surface contained
0.41 ML of oxide after 1 h of air exposure and contained 1.15
ML of oxide after 134 h of air exposure. In contrast, a CH3−
Ge(111) surface contained only 0.12 ML of oxide after 432 h of
air exposure. In fact, the CH3−Ge(111) surface exhibited even
greater oxidation resistance than CH3−Si(111) surfaces. A
CH3−Si(111) surface was reported to contain 0.8 0.4 ML of
oxide after 216 h in air40 compared to 0.03 ML oxide after 288
h in air measured for CH3−Ge(111).
ASSOCIATED CONTENT
■
S
* Supporting Information
Mg 2s XP spectrum of a CH3−Ge(111) surface, XP survey
spectra of H−Ge(111) and Br−Ge(111) surfaces, and Ge 3d
XP spectra of a CH3−Ge(111) surface after annealing. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/jacs.5b03339.
Previous studies of Ge methylation used wet-chemical
functionalization methods, which resulted in atomically rough
surfaces.7,8,11 In contrast, the method described herein enabled
the formation of H−Ge(111) surfaces with atomically flat
terraces (Figure 6b). Moreover, annealing in H2(g) yielded a
very homogeneous and well-defined monohydride surface
without detectable di- or trihydride surface species. The AFM
images after methylation (Figure 6c), and the sharp, bright 1 ×
1 diffraction spots observed by LEED of CH3−Ge(111)
surfaces (Figure 8), indicated that the surface structure
remained atomically flat and a highly homogeneous (111)-(1
× 1) structure was preserved upon methylation. This
conclusion is further supported by the high-resolution EC-
STM image in Figure 7b, which shows bright protrusions
assigned to methyl groups in a (111)-(1 × 1) array with a
spacing commensurate with that of atop Ge atoms on an
unreconstructed Ge(111) surface. The appearance of pits in the
atomic-force and EC-STM micrographs of the CH3−Ge(111)
surface suggests that the methylation step slowly etched the
surface; however, most of the surface remained terraced and
atomically flat. Given that the Ge lattice constant is only ∼4%
larger than that of Si, the atomically flat Ge(111) surface can be
100% terminated by closely packed methyl groups, in a similar
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: nslewis@caltech.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
grant CHE-1214152 and by the Gordon and Betty Moore
Foundation (GBMF1225). The research was in part carried out
in the Molecular Materials Research Center of the Beckman
Institute of the California Institute of Technology and in part
through the Joint Center for Artificial Photosynthesis, a DOE
Energy Innovation Hub, supported through the Office of
Science of the U.S. Department of Energy under Award
Number DE-SC0004993, which provided support for Y.-G.K.
and M.P.S. to perform the EC-STM experiments.
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(32) Yamada, T.; Kawai, M.; Wawro, A.; Suto, S.; Kasuya, A. J. Chem.
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