Interaction of atomic hydrogen with Zn-polar and O-polar ZnO surfaces

Article abstract of DOI:10.1063/1.1870103

The interaction of Zn-polar and O-polar ZnO surfaces with atomic hydrogen produced by a remote rf plasma source is investigated in situ and in real time using spectroscopic ellipsometry. It is found that the reactivity of ZnO with atomic hydrogen depends on polarity. The interaction of O-polar surfaces with atomic hydrogen at 100 °C is suitable for producing clean surfaces, while Zn-polar surfaces strongly react with atomic hydrogen resulting in a disruption of the ZnO lattice with formation of Zn surface clusters.

Full text of DOI:10.1063/1.1870103

Interaction of atomic hydrogen with Zn-polar and O-polar ZnO surfaces
Maria Losurdo and Maria M. Giangregorio
Citation: Applied Physics Letters 86, 091901 (2005); doi: 10.1063/1.1870103
View online: http://dx.doi.org/10.1063/1.1870103
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APPLIED PHYSICS LETTERS 86, 091901 ͑2005͒
Interaction of atomic hydrogen with Zn-polar and O-polar ZnO surfaces
a͒
Maria Losurdo and Maria M. Giangregorio
Institute of Inorganic Methodologies and of Plasmas, IMIP-CNR and INSTM-sec. Bari, via Orabona,
4-70126 Bari, Italy
͑
Received 15 October 2004; accepted 18 January 2005; published online 23 February 2005͒
The interaction of Zn-polar and O-polar ZnO surfaces with atomic hydrogen produced by a remote
rf plasma source is investigated in situ and in real time using spectroscopic ellipsometry. It is found
that the reactivity of ZnO with atomic hydrogen depends on polarity. The interaction of O-polar
surfaces with atomic hydrogen at 100 °C is suitable for producing clean surfaces, while Zn-polar
surfaces strongly react with atomic hydrogen resulting in a disruption of the ZnO lattice with
formation of Zn surface clusters. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1870103͔
Hexagonal zinc oxide, ZnO, is a II-VI semiconductor of
considerable technological interest for optics and optoelec-
tronics due to its wide band gap of 3.4 eV, a strong excitonic
feature even at room temperature and lasing properties a1t
room temperature suitable for ultraviolet laser applications.
Furthermore, a number of other applications of ZnO-based
systems are known in catalysis and for gas-sensor
One-side chemomechanically polished single-crystal
ZnO͑0001͒ and ZnO͑000-1͒ wafers from Cermet, Inc. were
used. The samples were exposed to an atomic hydrogen flux
produced by a remote H rf ͑13.56 MHz͒ plasma source. The
2
H plasma was operated at 60 W, 1 Torr and 800 sccm of H .
2
2
Under these conditions, the atomic hydrogen density at the
substrate position is ϳ5ϫ1015 cm corresponding to an
atomic hydrogen flux impinging on the SiC surface of ϳ4
−3
2
applications. For all the above applications, it is of interest
ϫ1020 atoms cm s . Exposure of the ZnO surface to
atomic hydrogen was performed at 100 °C, since a very high
hydrogen incorporation depth has been reported even at a
temperature as low as 100 °C.
−2 −1
to study the interaction of ZnO with atomic hydrogen.
Atomic hydrogen improves ZnO conductivity ͑up to eight
orders of magnitude͒, passivates the green emission enhanc-
3
ing the band edge luminescence, and modifies the ZnO cata-
4
lytic activity and adsorption of gases. Although a number of
Spectroscopic ellipsometric ͑SE͒ spectra of the
theoretical studies are devoted to the investigation of the
pseudodielectric function, ͗␧͘=͗␧ ͘+i͗␧ ͘, were acquired
1
2
5
,6
interaction of hydrogen with ZnO, very few experimental
investigations reported the real time monitoring of the inter-
action of O-polar and Zn-polar ZnO surfaces with atomic
hydrogen and/or they do not characterize the different reac-
tivity of the O- and Zn-polar ZnO. Mostly, previous
from 1.5 to 5.5 eV ͑UVISEL-Jobin-Yvon͒ before and after
exposure to atomic hydrogen of the ZnO surfaces. Measure-
ments were carried out at angles of incidence of 60° and 65°.
One-side polished substrates, both Zn face and O face, were
used to reduce back reflection. ZnO is an anisotropic hexago-
nal crystal. However, for the present highly c-oriented single
crystals, a mixture of the dielectric functions for the ordinary
and extraordinary rays was measured for the optically aniso-
tropic ZnO, and individual contributions were not separated.
Even with a mixture, spectroscopic ellipsometry can be used
for real-time monitoring during surface processing. Variation
of the pseudodielectric function was monitored in real time
at the photon energy of the excitonic transition during the
atomic hydrogen treatment to monitor the ZnO surface modi-
fications.
7
,8
studies investigated the change of conductivity and photo-
luminescence properties upon hydrogenation of ZnO. Hydro-
gen has been reported to diffuse very rapidly into ZnO even
at temperatures of 100 °C. Hydrogen incorporation depths of
Ͼ15 ␮m at 200 °C by hydrogen plasmas have been
8
reported, although it is not specified which polarity has been
investigated. Specifically, the crystal structure of ZnO is
wurtzite with two inequivalent sequences of atomic planes
along the c axis, with the ͑0001͒ being the Zn-polar face and
the ͑000-1͒ O-polar face. The Zn-polar and O-polar faces of
9
ZnO are structurally and chemically different, and it has
1
0
11
Ex situ x-ray photoelectron spectroscopy ͑XPS͒ analysis
was performed using a Mg K␣ source to evaluate the surface
chemical state upon the hydrogen treatment. Atomic force
microscopy ͑AFM͒ measurements evaluated the surface
morphology.
been reported that optical and electrical properties, ther-
1
2
13
mal stability, impurity incorporation, doping efficiency,
and the adsorption and reactivity of gases are influenced by
polarity.
The present letter is aimed at providing further insight
into the impact of ZnO polarity on the interaction of ZnO
surfaces with atomic hydrogen produced by a remote rf H2.
It is shown that the surface reaction of atomic hydrogen with
ZnO is sensitive to ZnO polarity, being the Zn-polar surface
is more reactive toward atomic hydrogen than the O-polar
ZnO surface. The kinetics of the surface reaction of atomic
hydrogen with ZnO is monitored in real time by spectro-
scopic ellipsometry ͑SE͒.
Figure 1 shows the spectra of the real, ͗␧ ͘, and imagi-
1
nary, ͗␧ ͘, parts of the pseudodielectric function for the Zn-
2
and O-polar ZnO wafers before and after exposure to atomic
hydrogen. Before any treatment, a higher value of the exci-
tonic transition is found for the O-polar ZnO. The effect of a
surface overlayer is to decrease ͗␧1͘ and to increase ͗␧2͘ for
all photon energies in our spectral range. The most abrupt,
clean and ordered ZnO surfaces are those with maximized
and minimized values of ͗␧ ͘ and ͗␧ ͘, respectively. The
1
2
a͒Author to whom correspondence should be addressed; electronic mail:
AFM analysis indicated approximately the same values for
cscpml18@area.ba.cnr.it
the two polar surfaces of the root mean square roughness and
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003-6951/2005/86͑9͒/091901/3/$22.50 86, 091901-1 © 2005 American Institute of Physics
28.42.202.150 On: Tue, 25 Nov 2014 11:53:08
0
1

091901-2
M. Losurdo and M. M. Giangregorio
Appl. Phys. Lett. 86, 091901 ͑2005͒
FIG. 2. ͑a͒ Time variation of the real, ͗␧ ͘, and imaginary, ͗␧ ͘, parts of the
1
2
pseudodielectric function recorded at the photon energy of 3.3 eV ͑corre-
sponding to the ZnO exciton͒ during exposure of the Zn-polar and O-polar
ZnO single crystals to atomic hydrogen. ͑b͒ Zoom of dashed area in ͑a͒.
FIG. 1. ͑a͒ Spectra of the real, ͗␧ ͘, and imaginary, ͗␧ ͘, parts of the
1
2
pseudodielectric function of Zn-polar and O-polar ZnO crystals. ͑b͒ Detail
of the excitonic region in the ͗␧ ͘ spectra for the Zn-polar and O-polar ZnO.
1
A decrease and an increase of the exciton amplitude are found for the Zn-
polar and O-polar surfaces with the increase of time exposure to atomic
hydrogen: line with circles is for the starting surface; thick black line is after
exposure for approximately 1 min; thin black line is after 10 min of expo-
sure and gray line is after approximately 20 min of exposure to atomic
hydrogen.
simply related to cleaning/improvement of the O-polar ZnO
surface. The improvement can also be seen in the increase of
the excitonic transition in the ͗␧ ͘ spectrum of Fig. 1 and in
1
the minimization of the residual absorption below gap in the
͗
␧ ͘ spectrum ͑␧ =2nk where n is the refractive index and k
2 2
peak-to-valley values ͑see Fig. 3͒. From this it is inferred
that a different surface roughness is not responsible for the
different ellipsometric spectra of the Zn- and O-polar ZnO
wafers. The assumption of a surface contaminant overlayer
in the fit analysis of the SE spectra yields a surface overlayer
of approximately 5 and 80 Å for the O-polar and Zn-polar
ZnO surfaces, respectively. This different reactivity to con-
taminant and, hence, the different SE spectra is consistent
with first-principles studies of adsorption of CO on polar
ZnO surfaces reporting that CO ͑and other C contaminants͒
energetically favorably binds to Zn ions present at the Zn-
polar surface with the “C-down” adsorption geometry, while
binding of CO to surface oxygen ions at the O-polar surface
is the extinction coefficient that is proportional to the absorp-
tion coefficient͒. In contrast, for the Zn polar, after a small
͗␧ ͘ increase and ͗␧ ͘ decrease due to surface cleaning, the
trends reverse and a decrease of ͗␧ ͘ and an increase of ͗␧ ͘
without any saturation are found.
The observed different kinetic ellipsometric profiles are
indicative of a different surface reactivity of the Zn-polar and
O-polar ZnO͑0001͒ surface toward atomic hydrogen. The
O-polar surface shows a natural affinity for H atoms, and by
adsorbing an H atom onto the O-polar surface it becomes
1
2
1
2
1
4
more stable, as also predicted by total energy calculations.
This unreactivity of the O-polar surface is consistent with
He-atom scattering experiments that demonstrated that the
O-polar surface exposed to hydrogen shows a hydrogen ter-
mination with a ͑1ϫ1͒ diffraction pattern, which stabilizes
4
is less favorable. Thus, the ellipsometric measurement of the
different air-exposure reactivity of ZnO polar surfaces allows
discerning the Zn-polar and O-polar ZnO surfaces.
15
the O-polar ZnO surface. Formation of hydrogen adlayers
Figure 2 shows the time variation of the real, ͗␧ ͘, and
1
on the polar Zn–ZnO͑0001͒ surface that becomes highly un-
stable for larger exposure to hydrogen and undergoes recon-
structions and structural changes that destroys the lateral or-
der of the ZnO crystal has been reported. Hydrogen
adsorbed on the Zn-polar surface is so reactive that it is
capable of reducing bulk ZnO units to metallic Zn, forming
OH groups according to the exothermic reaction
imaginary, ͗␧ ͘, parts of the pseudodielectric function of
2
ZnO crystals of both polarities during exposure to a flux of
atomic hydrogen at a temperature of 100 °C. It is found that
the Zn- and O-polarity show opposite trends, and a larger
variation is recorded for the Zn-polar surface that shows a
fast modification even after approximately 30 min of treat-
ment while the plasma treatment was terminated after 15 min
for the O-polar surface because of the observed kinetic pla-
teau. In particular, it is found that the O-polar ZnO is almost
1
6
ZnO + H → OH + Zn.
unreactive with atomic hydrogen; a very small ͗␧ ͘ increase
1
The OH- and Zn–H species that are formed at the Zn-polar
surface upon interaction with atomic hydrogen decompose
and ͗␧ ͘ decrease, reaching a plateau value, after a few min-
2
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utes, are observed. It is thought that this observed variation is
and desorb at the relatively low temperatures of 380 and 540
1
28.42.202.150 On: Tue, 25 Nov 2014 11:53:08

091901-3
M. Losurdo and M. M. Giangregorio
Appl. Phys. Lett. 86, 091901 ͑2005͒
i.e., they are Zn cluster formed on the Zn-polar surface upon
interaction with atomic hydrogen.
The chemical modification of the O-polar and Zn-polar
surfaces has been corroborated using the high-resolution
XPS spectra of the Zn 2p3/2 and O 1s core levels of the Zn-
and O-polar surfaces. After exposure to atomic hydrogen, the
O 1s spectrum of the O-polar surface shows oxygen bonded
to zinc in the lattice identified at 531.3±0.1 eV and an in-
crease of the higher binding energy peak at 532.9±0.1 eV
1
8
indicative of OH on the sample surface. For the Zn-polar
surface, the Zn 2p3/2 peak fit components at 1021.7–1022.1
eV due to ZnO and an increase of the component at 1020 eV
due to Zn are seen upon hydrogenation, consistently with the
reactivity highlighted by ellipsometry.
In summary, the interaction of atomic hydrogen with Zn-
polar and O-polar ZnO surfaces has been investigated and
monitored in real time by spectroscopic ellipsometry. It has
been found that the reactivity of ZnO towards atomic hydro-
gen strongly depends on polarity. The interaction with the
O-polar ZnO results in atomically flat and cleaned OH ter-
minated surfaces. In contrast, Zn-polar ZnO strongly reacts
with atomic hydrogen yielding lattice disruption and Zn
clusters.
The authors dedicate this work to Dr. Giovanni Bruno at
IMIP-CNR for his personal efforts in sustaining the authors
during this research activity. The authors acknowledge Dr.
Jeff Nause, President of Cermet, Inc., Atlanta GA, for pro-
viding the ZnO substrates. They also thank the Ministero
dell’Istruzione, dell’Università e della Ricerca ͑MIUR͒ for
financial support.
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