Effects of oxygen and hydrogen adsorption on the electron energy loss
features of diamond surfaces
T. E. Beerling and C. R. Helms
Department of Electrical Engineering, Stanford University, Stanford, California 94305
͑Received 2 May 1994; accepted for publication 8 August 1994͒
Electron energy loss spectroscopy has been used to study the surface of diamond and other states of
carbon. Other authors have suggested that a loss feature observed at approximately 9 eV may be due
to oxygen adsorption on the diamond surface. We report electron energy loss spectroscopy data, in
combination with Auger electron spectroscopy data, that shows a correlation of a loss feature at 8.5
eV with adsorbed oxygen on diamond surfaces. Surfaces were oxygenated by low-energy oxygen
ion bombardment and oxygen removed by annealing in vacuum and atomic hydrogen. The atomic
hydrogen was used to suppress the
loss feature by etching nondiamond carbon and passivating the
diamond surface. © 1994 American Institute of Physics.
Characterization of diamond surfaces is critical to better
understanding diamond chemical vapor deposition ͑CVD͒
growth and the effects of subsequent processing steps used in
electronic device fabrication. Electron energy loss spectros-
copy ͑EELS͒ is an effective carbon surface characterization
tool as diamond, graphite, and amorphous carbon have dis-
tinctive EELS spectrum. This is demonstrated in Fig. 1. Dia-
mond has bulk and surface plasmons at approximately 34
and 22 eV, respectively. Highly ordered pyrolitic graphite has
a dominant
plasmon loss feature at 6.5 eV, with weaker
features at 13, 19.5, and 26 eV. Amorphous carbon, formed
by bombarding diamond surfaces with 1 keV argon ions, has
a
plasmon at approximately 5.5 eV, and a broad
ϩ
amorphous plasmon at approximately 25 eV.
1
For 1 keV ar-
gon ion bombarded diamond, there are also weaker, purely
graphitic loss features. EELS possesses a surface sensitivity
͑on the order of the incident electron scattering length͒ that is
not found with Raman spectroscopy.
At energies lower than the bulk and surface plasmon
losses, diamond has loss features attributable to electronic
transitions. Lurie et al. observed a very weak loss feature at
9 eV for hydrogenated diamond.
2
Loss spectra we have ob-
tained for oxygenated diamond have a stronger loss feature
at approximately this energy ͑8.5 eV͒. Mori et al. have sug-
gested that oxygen adsorption on diamond may explain the
loss feature they observed at approximately 9 eV.
3
We
present EELS data, in coordination with Auger electron spec-
troscopy ͑AES͒ data, which correlates oxygen coverage with
the loss feature observed at 8.5 eV.
The diamond studies were performed in an ultrahigh
vacuum chamber with a base pressure in the 10
Ϫ10
Torr
range, achieved using an 160
l
/s ion pump. To oxygenate
diamond surfaces, low-energy oxygen ion bombardment was
employed, using a differentially pumped ion gun. Base pres-
sures during oxygen ion bombardment were in the 10
Ϫ6
Torr
range, achieved using a 170
l
/s turbomolecular pump.
Sample annealing was performed using a tungsten filament
contained within a molybdenum can, with the diamond
sample clipped to a molybdenum disk over the filament.
Sample temperatures were determined by measuring the mo-
lybdenum disk temperature with an optical pyrometer. This
is believed to overestimate the diamond temperature by 50–
100 °C.
͑100͒ 2b diamond samples ͑from Doubledee Harris͒, 1.5
mmϫ1.5 mmϫ0.25 mm in size, were used for this study. 2b
samples were used as they have a lower resistivity ͑ϳ10
4
⍀ cm range͒ than 2a diamond. The lower resistivity elimi-
nated charging problems when collecting EELS and AES
spectra. Atomic hydrogen was created by cracking H
2
with a
tungsten filament placed approximately 7 in. from the dia-
mond sample. The filament temperature during atomic hy-
drogen exposure was 2200 °C, with a H
2
pressure during
exposure of 2.2ϫ10
Ϫ5
Torr. Due to the low background pres-
sure during atomic hydrogen exposure, atomic hydrogen re-
combination was minimal. Although not detectable with
AES, atomic hydrogen is known to terminate ͑111͒
4
and
͑100͒
5
diamond surfaces.
The spectroscopies were performed using a single pass
cylindrical mirror analyzer of 0.3% resolution, with a con-
centric electron gun. Incident electron energies were 3 keV
for AES, and either 200 or 500 eV for EELS, where EELS
data was collected in reflection mode. Derivative spectra
were obtained for EELS and AES using a lock-in amplifier.
In performing electron spectroscopies, low electron current
FIG. 1. Electron energy loss spectra for varying states of carbon. ͑a͒ Highly
ordered pyrolitic graphite, ͑b͒ amorphous carbon ͑by bombarding diamond
with 1 keV argon ions͒, ͑c͒ oxygenated ͑100͒ 2b diamond by 200 eV oxygen
ion bombardment. EELS primary beam energyϭ500 eV.
1912 Appl. Phys. Lett. 65 (15), 10 October 1994 0003-6951/94/65(15)/1912/3/$6.00 © 1994 American Institute of Physics