JOURNAL OF APPLIED PHYSICS 107, 103720 共2010兲
First-principles study of GaAs„001…-␤2„2 Ã 4… surface oxidation
and passivation with H, Cl, S, F, and GaO
Weichao Wang,1 Geunsik Lee,2 Min Huang,2 Robert M. Wallace,1,2 and
Kyeongjae Cho1,2,a兲
1
Department of Materials Science and EngineeringThe University of Texas at Dallas, Richardson,
Texas 75080, USA
2
Department of Physics, The University of Texas at Dallas, Richardson, Texas 75080, USA
共Received 7 December 2009; accepted 23 February 2010; published online 26 May 2010兲
The interactions of oxygen atoms on the GaAs共001兲-␤2共2 ⫻ 4兲 surface and the passivation of
oxidized GaAs共001兲-␤2共2 ⫻ 4兲 surface were studied by density functional theory. The results
indicate that oxygen atoms adsorbed at back-bond sites satisfy the bond saturation conditions and do
not induce surface gap states. However, due to the oxygen replacement of an As dimer atom at a
trough site or row site, the As–As bond is broken, and gap states are produced leading to the Fermi
level pinning because of unsaturated As dangling bonds. Atomic H, Cl, S, F, and the molecular
species GaO were examined to passivate the unsaturated As dangling bond. The results show that
H, Cl, F, and GaO can remove such gap states. It is also found that the interaction of S with the
unsaturated As dangling bond does not remove the gap states, and new gap states are generated upon
single S adsorption. A higher S coverage forms S–S dimer pairs which passivate two unsaturated As
atoms, and removes the As-induced gap states. © 2010 American Institute of Physics.
关doi:10.1063/1.3369540兴
I. INTRODUCTION
Metal-oxide-semiconductor field effect transistors
共MOSFETs兲 with high mobility channel materials are candidates
for
advanced
complementary
metal-oxidesemiconductor 共CMOS兲 device structures because it is much
easier to enhance device performance through device scaling
compared with traditional Si CMOS. Gallium arsenide, with
five times higher electron mobility compared to silicon, has a
potential to achieve a III-V-based channel device. However,
it is difficult to achieve a practical enhancement-mode GaAsbased MOSFET due to the poor interface quality between
GaAs and gate dielectric films.1 GaAs-based interface is
much more complicated than silicon based interface. For the
silicon based interface, Si interacts with oxygen interfacial
atoms by forming Si–O or O–Si–O bonds. Each Si atom
contributes one electron for each silicon–oxygen bond.
Therefore, interfacial Si dangling bonds could be fully saturated or be fully empty easily by transferring integer number
of electrons. However, in the GaAs-based interface, the interfacial Ga- or As- dangling bonds are partially saturated2
which could hardly be fully saturated to obtain the ideal
high-k/GaAs interface. Furthermore, the partially saturated
bonds could induce midgap states which leads to Fermi level
pinning. This pinning is most likely due to the presence of
As–O and Ga–O bonds since clean GaAs surface does not
pin Fermi level. Therefore, understanding the oxidation and
passivation of GaAs surface are very helpful to gain an important insight for high-k/GaAs interface study.
GaAs共001兲 surface is the most intensively studied system among III-V materials.3–5 It consists of alternating
planes of Ga and As that are separated by 1.41 Å.5 Both
a兲
Electronic mail: [email protected].
0021-8979/2010/107共10兲/103720/10/$30.00
Ga-terminated and As-terminated GaAs共001兲 surfaces were
observed to reconstruct forming As–As dimers or Ga–Ga
dimers on the surface.6 The oxidation and passivation of a
GaAs共001兲 surface are important issues of GaAs-based
MOSFETS, thus of great interest to researchers. Previous
work7–9 studied diverse surface oxidation models, including
SiO adsorption and the replacement of As dimer atoms by
two oxygen atoms on GaAs共001兲 surface. Their results
showed that a possible mechanism of Fermi level pinning is
not due to the intrinsic properties of GaAs共001兲 surface, but
due to the specific bonding geometries resulting from the
oxidation. Recently, based on the scanning tunneling spectroscopy and density functional theory 共DFT兲 studies for different adsorbates bonding to GaAs共001兲 surface, Winn et
al.10 proposed that the Fermi level pinning mechanisms
could be identified to be direct or indirect. The direct Fermi
level pinning is due to the states in the band gap region
directly induced by the adsorbate. On the other hand, the
indirect Fermi level pinning is due to the states in the band
gap region induced by the secondary effects, such as the
generation of undimerized As atoms. Earlier studies11 assumed that the unpinning of surface states for GaAs共001兲
surfaces in air is because of the formation of excess As due
to the oxidation of GaAs. It is known that the oxidation of
GaAs results in the formation of excess As, and oxides such
as Ga2O3 and As2O3 on the surface. The As oxide and As
could be photochemically removed from the surface since
they are highly soluble in oxygenated water. The remaining
As-free surface is passivated by the Ga oxide layer resulting
in the unpinning surface states of GaAs surface. How excess
As and As oxide induce surface gap states is still inaccessible
at atomic understanding. Therefore, clear understanding of
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Wang et al.
the mechanisms of Fermi level pinning is needed and would
be helpful to create unpinned surfaces using passivation
methods.
To passivate GaAs surface and remove the surface states,
several species such as sulfide and gallium oxide were applied to oxidized GaAs共001兲 surfaces. Traub et al.12 used
chlorine to passivate the oxidized surface, and the x-ray photoelectron spectroscopy revealed that GaAs共001兲 surface
with Cl could be effectively passivated with wet chemical
methods. Winn et al.9 proposed that H could be used to passivate the undimerized As atoms induced by the adsorbates.
With H passivation, the As-induced gap states were suppressed into the valence band edge region, yielding a clean
unpinned surface. S interaction with GaAs共001兲 surface had
been studied by Szucs et al.13 with several model geometries
of the GaAs共001兲 surface by DFT. In Ga-rich GaAs surface,
the top layer formed a Ga–S like monolayer, and under certain conditions the S passivated surface can also reconstruct
forming a S–S dimer pair. Guo-Ping and Ruda14 also studied
S passivation of GaAs共100兲 surfaces through ab initio
molecular-orbital calculations. Their results showed that S
pasivation result in the opening of surface Ga dimmers,
which in turn, lowers 共raises兲 the highest occupied 共lowest
unoccupied兲 surface states. In addition to the use of extrinsic
elements to passivate GaAs共001兲 surface, native oxides were
also studied. The origin of Ga2O3 passivation mechanisms
for reconstructed GaAs共001兲 surface was investigated by using Ga7As7O2H20 cluster model.15 The simulation showed
that the reduction in the density of surface states located
within the bulk energy gap derives from initial near-bridgebonded O atoms. However, this cluster models could overestimate energy gap due to the strong quantum size effect of
GaAs material. Although several diverse experimental and
theoretical works were done to investigate the oxidation and
passivation mechanisms for GaAs 共001兲 surface, the systematic theoretical atomic level interpretation of favored oxidation and passivation structures and properties could lead us a
better and complete understanding of the mechanisms thus
very useful for GaAs-based MOSFETs devices
In this paper, we present systematical computational
studies based on DFT to investigate the oxidation and passivation of GaAs 共001兲 surface. The most favored geometries
of oxidation and passivation were predicted, and the Fermi
level pinning mechanisms were studied for GaAs 共001兲 surface. Based on the understanding of Fermi level pinning
mechanism, a series of candidates including H, Cl, F, S, and
GaO are studied for the passivation of oxidized
GaAs共001兲-␤2共2 ⫻ 4兲 surface from the theoretical perspective.
II. MODELS AND COMPUTATIOAL METHOD
The present calculations are based on the DFT with the
generalized gradient approximation PW91 scheme, and have
used a plane wave basis 共Ecutoff = 400 eV兲 and pseudopotentials implemented in the VASP code.16–18 The pseudopotential
we have used is a type of projector augmented wave 共PAW兲
共Refs. 19 and 20兲 and 4s and 4p orbitals are treated as valence shells for Ga and As. For the test of applicability and
J. Appl. Phys. 107, 103720 共2010兲
accuracy of the PAW potentials, we studied the structural
properties of bulk GaAs and compared the obtained results
with available experimental values. The calculated lattice
constant of bulk GaAs is 5.751 Å compared to the experimental value of 5.653 Å and the compressibility modulus is
65.0 GPa compared to experimental value of 75.3 GPa.21
The clean GaAs共001兲 surface was modeled using eightlayer and twelve-layer slabs with the bottom surface passivated with pseudohydrogen 共with 1.25 valence electrons to
mimic bulk As bonding兲. To rigorously keep GaAs bulk behavior and avoid exchange-correlation induced numerical errors, we used GaAs experimental lattice parameter of 5.653
Å and a vacuum thickness of 10 Å separating the slabs for
the GaAs共001兲 surfaces. Our calculated results for surface
reconstruction and stability are in good agreement with the
results obtained from theoretical equilibrium lattice constant
in Refs 22–25. In this specific surface calculation with 2
⫻ 4 surface unit cell, a mesh size of 8 ⫻ 4 ⫻ 1 was used for
the k-point sampling. Several GaAs共001兲 reconstructed surfaces, i.e., 共2 ⫻ 4兲, 共4 ⫻ 2兲, ␣共2 ⫻ 4兲, ␣共4 ⫻ 2兲, ␤共2 ⫻ 4兲,
␤共4 ⫻ 2兲, ␤2共2 ⫻ 4兲, ␤2共4 ⫻ 2兲, ␥共2 ⫻ 4兲, and ␥共4 ⫻ 2兲 were
studied in the present work. The 共2 ⫻ 4兲 reconstruction 关共4
⫻ 2兲 reconstruction兴 corresponds to four As 共Ga兲 dimer pairs
at the top of GaAs共001兲 surface in As 共Ga兲 rich condition.
The ␣共2 ⫻ 4兲 关␣共4 ⫻ 2兲兴 represents two As 共Ga兲 dimers missing compared to 共2 ⫻ 4兲 关共4 ⫻ 2兲 reconstruction兴 in As 共Ga兲
rich condition. The ␤共2 ⫻ 4兲 关␤共4 ⫻ 2兲兴 has one As 共Ga兲
dimer missed at the top surface compared to 共2 ⫻ 4兲 关共4
⫻ 2兲兴. The ␤2共2 ⫻ 4兲 关␤2共4 ⫻ 2兲兴 misses two As 共Ga兲 dimers
at the top surface and one Ga 共As兲 dimmer in the second top
layer compared to the 共2 ⫻ 4兲 关共4 ⫻ 2兲兴. To form ␥共2 ⫻ 4兲
关␥共4 ⫻ 2兲兴, one of 共2 ⫻ 4兲 关共4 ⫻ 2兲兴 As dimer leaves its original position and relocates at the top two As 共Ga兲 dimer pairs
center. And the new As 共Ga兲 dimer pair is perpendicular to
the rest of three As 共Ga兲 dimers however parallel to the
GaAs共001兲 surface. To find the relative stabilities among
these different reconstructed surface structures, formation
energies per unit area were calculated by using EF
tot
tot
= 1 / A共Uslab
− NGa␮Ga − NAs␮As兲, where Uslab
is the total energy of the slab, Ni is the number of atoms of type i, A is the
surface area, and ␮i is the corresponding chemical potential
for species i 共Ga or As兲 in the slab. The Ga and As chemical
potentials are not independent from each other but are constrained by ␮Ga + ␮As = ␮GaAs. Here ␮GaAs is the GaAs chemical potential. The As chemical potential is bounded by
␮GaAs ⬍ ␮As ⬍ 0. In the Ga rich conditions, the Ga chemical
potential keeps the same value as in bulk Ga which is zero,
and the As chemical simply equals to GaAs chemical potential. As a result, EF only depends on one variable, i.e., ␮As,
which varies from ⫺0.78 eV to 0. In As rich limit, EF only
depends on ␮Ga. Figure 1 shows the formation energies of
different GaAs共001兲 surface reconstruction 共using eight
GaAs atomic layer models兲 versus As chemical potentials. It
is clear that GaAs共001兲-␤2共2 ⫻ 4兲 is the most stable surface
over a large arsenic chemical potential range which is consistent with other reports.22
It is important to note that there is a strong quantum
confinement effect for a finite GaAs slab thickness used in
the surface modeling study. To examine this specific effect,
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103720-3
Wang et al.
FIG. 1. Formation energies of different GaAs共001兲 surface reconstructions
共eight GaAs atomic layers兲 vs As chemical potentials. Maximum As chemical potential corresponds to As rich conditions, on the contrary, minimum As
chemical indicates Ga rich conditions.
J. Appl. Phys. 107, 103720 共2010兲
FIG. 3. Top 共a兲 and side 共b兲 view of GaAs共001兲-␤2共2 ⫻ 4兲 surface. Large
共small兲 filled circles indicate top- 共third-兲 layer As atoms, whereas large
共small兲 empty circles represent second- 共fourth-兲 layer Ga atoms.
GaAs slabs with different layers 共4 to 40 atomic layers with
both sides passivated with pseudohydrogens to remove surface states兲 were tested for the energy gap changes. Figure 2
shows that the GaAs共001兲-共1 ⫻ 1兲:H surface energy gap decreases as the slab thickness 共e.g., number of atomic layers兲
increases. In Fig. 2, the green filled squares represent the
calculated energy gaps, and the blue curve is the fitted equation based on quantum confinement effect, i.e., 1 / t2. The red
dash line is the calculated GaAs bulk energy gap 共0.68 eV兲
which is smaller than the experimental band gap of 1.42 eV
due to the well-known DFT band gap underestimation. From
the diagram, strong quantum confinement can be seen for
thin slabs with less than 20 atomic layers 共t = 2.8 nm兲. The
energy gaps decreased from 2.6 eV to 0.68 as atomic layers
increasing from 4 to 40 layers. Furthermore, the electronic
structures indicated that 12-layer slab 共corresponding to Eg
= 1.20 eV兲 was reasonably thick enough to avoid the strong
quantum confinement effect. One should notice that there is a
diverging region between the calculated energy gap values
and the inverse square fit. From the E-k relationship, i.e., E
= 共បk兲2 / 2mⴱ, energy gap values depend on the effective mass
of conducting and valence band edges as well as the size of
the system. However, the fit equation only considers one
variable, i.e., size of the system, which induces the derivation between the calculated gap values from the fit data.
Moreover, one should be careful in the interpretation of the
near edge states close to the valance and conduction band
edges. For the present work, the DFT calculation predicts an
The GaAs共001兲-␤2共2 ⫻ 4兲 surface was first proposed by
Chadi23 in 1987, and is considered as the standard model for
共2 ⫻ 4兲 reconstruction. Previous experimental and theoretical
work24,25 revealed that the GaAs共001兲 ␤ phase, which is
stable over a broad chemical potential range, has a structure
␤2共2 ⫻ 4兲 containing two As dimers at the top-most surface
layer and a third dimer located two layers below shown in
Fig. 3. In this model, the surface is terminated with As atoms, and dangling bonds are easy to buckle together to form
the As dimers.
In Fig. 4 we show the projected band structure of GaAs
bulk presented with dots, together with the bound surface
states for GaAs共001兲-␤2共2 ⫻ 4兲 in the energy region of the
fundamental gap. In the present work, we found that the
FIG. 2. 共Color online兲 GaAs共001兲共1 ⫻ 1兲 surface energy gaps vs atomic
layers used in surface slab.
FIG. 4. 共Color online兲 Left panel represents band structure 共bound states兲
for GaAs共001兲-␤2共2 ⫻ 4兲 surface plotted over the projected bulk band structure 共dot regions兲. Right panel is the corresponding total density of states of
the clean surface.
energy gap of 0.68 eV for the bulk GaAs, which underestimates the experimental value of 1.42 eV by 0.74 eV. However, the existence of midgap states does not change significantly relative to the band edges when the band gap
underestimation is corrected by the GW approximation.26
Therefore, the underestimation of band gap would not
change the overall analysis on the Fermi energy pinning
mechanisms study.
III. RESULTS AND DISCUSSION
A. GaAs„001…-␤2„2 Ã 4… clean surface
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103720-4
Wang et al.
J. Appl. Phys. 107, 103720 共2010兲
rated, thus opening a surface gap shown in Fig. 4. However,
when a high-k oxide material grows on this specific reconstruction structure, Ga–O and As–O bonds could be formed.
As a result, three specific As dimmer pairs are broken so that
this specific surface electron distribution balance is destroyed; surface electronic behavior should change accordingly. Since As–O and Ga–O bonds are partially saturated
due to charge mismatch, the gap states induced by these
partially saturated bonds could generate Fermi level pinning.
B. GaAs„001…-␤2„2 Ã 4… surface oxidation: Atomic
structures and electronic structures
In reality, the modeling of an oxidized GaAs surface is
an important and difficult issue due to the complexity of
possible oxidation structures. In the following, oxygen atoms
interacting with GaAs共001兲-␤2共2 ⫻ 4兲 surface will be systematically studied for this purpose.
FIG. 5. Contour plots of the charge distribution at K point for surface
localized states of the GaAs共001兲-␤2共2 ⫻ 4兲 surface. The contour spacing is
3 ⫻ 10−3 e Å−3. All plots are drawn parallel to the surface normal. C2 and
V5 are plotted along a plane parallel to the 2 ⫻ 4 direction cutting through
the bonds between first- and second-layer anions and through dimer 3 共see
Fig. 3兲. C1, V2 are localized at both dimers 1 and 2 and both dimers have
nearly identical charge distributions. V1 and V3 are localized at dimer 3.
dotted bulk regions in Fig. 4 are contributed by two bottom
Ga and As layers which show GaAs bulk behaviors. For the
clean GaAs共001兲-␤2共2 ⫻ 4兲 surface, the gap is essentially
free of surface states. Five valence bands 共labeled as V1-V5
in Fig. 4兲 appear above the bulk band edge at K point in the
present work rather than four valence bands in Schmidt’s
work.27 Slightly above the bulk valence band edge at K
point, we found the two highest occupied states V1 and V2
lie at 0.41 eV and 0.32 eV, respectively. They correspond to
the combinations of antibonding ␲ⴱ and pz orbitals of As
dimers located at the third-layer 共V1兲 and top-layer 共V2兲 as
shown in Fig. 5. The ␲ bonding of As dimers at the thirdlayer give rise to V3 and V4. V5 is composed of the combinations of bonding ␴ and p orbitals for Ga and top As atoms
located at second top layer center. Compared to other
GaAs共001兲 surface reconstructions, we found that the states
localized at the top-layer dimers show nearly identical charge
distribution due to the symmetry of surface geometry. The
lowest unoccupied state C1 is a combination of antibonding
␴ⴱ and in-plane p orbitals of the top-layer As dimers 共see
Fig. 5兲. C2 共see Fig. 5兲 is related to the threefold-coordinated
Ga atoms located at the second-layer. This state is almost
entirely localized at the Ga atoms on one side of the dimer
block 共close to the third-layer As dimer兲. Based on the analysis, we found that all the As dangling bonds are fully saturated and contribute to valence band edge states. Moreover,
four completely empty Ga dangling bonds 共threefoldcoordinated second-layer Ga atoms兲 contribute conduction
band edge states.
From the charge transfer point of view, the threefoldcoordinated Ga atoms transfer total three electrons to six As
dimer atoms so that all the As dangling bonds are fully satu-
1. Atomic structures for GaAs„001…-␤2„2 ⴛ 4… surface
oxidation
For a single oxygen atom interaction with the surface,
we consider two different oxide structures, viz. adsorption
and replacement. For computational efficiency, slabs with
only eight atom layers passivated by one pseudohydrogen
atom layer were considered for the oxidation discussion.
However, for the passivation work, more accurate electronic
structures are required so that slabs with 12 atomic layers
were used in the passviation part. Experimental work28
showed the presence of native oxides including Ga2O,
Ga2O3, As2O3, etc. in GaAs surface. These native oxides are
critical factors influencing the quality of GaAs surface or
interface. However, the Ga–O and As–O bonds essentially
originate from one or two oxygen atoms interacting with
GaAs pure surface or their combination. Therefore, the interactions between one and two oxygen atoms and GaAs surface would be symmetrically studied in the following.
To examine general stabilities of different oxidized surfaces, formation energy versus arsenic chemical potential
was calculated. Similar to the study of different reconstructed GaAs共001兲 surface stabilities in Fig. 1, formation
tot
− NGa␮Ga − NAs␮As
energies are defined as: EF = 1 / A共Uslab
tot
− NO␮O兲, where Uslab is the total energy of the slab, Ni is the
number of atoms of type i, A is the surface area, and ␮i is the
corresponding chemical potential for species i 共Ga or As兲 in
the slab. O chemical potential is constrained by O2 gas without considering native oxides, i.e., Ga2O3, As2O3, and
As2O5. This is because the initial GaAs surface is a clean one
rather than an oxidized one. So it is reasonable to consider
the ambient O is under rich condition. Consequently, As
chemical potential is the only one variable which varies from
⫺0.78 eV to 0, and the negative slope reflects the As richness.
For one oxygen adsorption cases shown in the first row
of the Fig. 6, oxygen adsorbed on the bridge site of two Ga
atoms 关Fig. 6共1a兲兴 shows its instability compared to the other
three structures 关Figs. 6共1b兲–6共1d兲兴 from the formation energy shown in Fig. 7. Oxygen adsorption on the back-bond
site shown in Fig. 6共1b兲 exhibits a high stability based on its
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Wang et al.
J. Appl. Phys. 107, 103720 共2010兲
FIG. 6. 共Color online兲 Side view of one oxygen atom adsorption 关共1a兲–共1d兲 in the first row兴 and replacement 关共2a兲–共2d兲 in the second row兴 on
GaAs共001兲-␤2共2 ⫻ 4兲 surface. 共3a兲–共3d兲 indicate the two oxygen atom adsorption 共3a兲 and replacement 关共3b兲, 共3c兲, and 共3d兲兴 on GaAs共001兲-␤2共2 ⫻ 4兲 surface.
Big black 共white兲 balls indicate surface As 共Ga兲 atoms and small black 共white兲 balls represent As 共Ga兲 atoms in the sublayer. O is specified in each figure.
low formation energy. The trough site As dimer 关Fig. 1共d兲
site兴 is slightly more stable than the Fig. 6共1c兲 structure. In
the second row of Fig. 6, we show four possible configurations for the replacement of one surface atom by oxygen
atom. In Fig. 6共2a兲, one of the second top layer Ga is substituted by one oxygen atom forming two As–O bonds. From
the high formation energy shown in Fig. 7, we found this is
not an energetically favorable structure. In Fig. 6共2b兲, oxygen replaces one of the row site As dimer atoms and forms
three bonds, i.e., two Ga–O bonds and one O–As 共dimer兲
bond. In Fig. 6共2c兲, row site As atom is replaced by one
oxygen atom, but the O atom only forms two Ga–O bonds
compared to three bonds in Fig. 6共2b兲. The total energies of
structure in Figs. 6共2b兲–6共2d兲 can be directly compared since
they have the same number of Ga, As, and O atoms. Our
calculation exhibits the energy difference of 0.27 eV between
Figs. 6共2b兲 and 6共2c兲, and the structure in Figs. 6共2d兲 is 1.43
eV and 1.16 eV more stable than structure in Figs. 6共2b兲 and
6共2c兲, respectively. Among the four possible replacement
sites, the structure in Fig. 6共2d兲 has the lowest formation
energy. In this case, one of trench site As dimer atoms was
replaced by one oxygen atom. Compared to the first and
second row oxidized surfaces, it is easy to find that one oxygen tends to replace surface As atoms rather than Ga atoms.
For two oxygen atoms interacting with the surface
shown in third row of Fig. 6, a similar study was done to
determine the possible stable structures. Two oxygen atoms
prefer to stay in the two back-bond sites 关see Fig. 6共3a兲兴. We
found that in two oxygen replacement cases, the formation
energy is lower than that of Fig. 6共3a兲, and this lower energy
indicates that two oxygen atoms prefer to replace surface As
atoms rather than adsorb on the surface. This finding is similar to the case of one oxygen atom interacting with the specific surface. In the case of replacement, two oxygen atoms
prefer to replace the trench sites As dimer atoms and form
two Ga–O–Ga bonds 关Fig. 6共3c兲兴. Nevertheless, the structure
with O row shown in Fig. 6共3b兲 indicates the O2 adsorption
site according to the scanning tunneling microscope 共STM兲
study by Hale et al.,7 so it was studied to verify the Fermi
energy pinning mechanism, which is shown in Sec. III B 2.
As shown in Fig. 6共3b兲, two As dimer atoms are replaced by
two oxygen atoms and two dangling bonds associated with
FIG. 7. Different oxidized surface formation energies vs As chemical potentials. Maximum As chemical potential corresponds to As rich conditions,
on the contrary, minimum As chemical indicates Ga rich conditions. The
notation of each line corresponds to that in Fig. 6, respectively.
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103720-6
Wang et al.
J. Appl. Phys. 107, 103720 共2010兲
the top As atoms were kept. Other possible structures for two
O atoms replacing surface As atoms, such as one trench site
As dimer atom and one row site As dimer atom replaced by
O atoms or structure shown in Fig. 6共3d兲, were also studied.
The energies of these structures are only less than 5 meV
lower than that of the structure in Fig. 6共3b兲. Therefore, our
study focuses on Fig. 6共3b兲.
From these data we can make a general statement that
clean surface is easy to be oxidized since most of oxidized
surface formation energies are lower than clean surface. In
addition, replacement cases always show higher stabilities
than adsorption cases. It is also found that the more oxygen
atoms interacting with surface, the more possibilities to be
stabilized on the surface.
2. Electronic structures for GaAs„001…-␤2„2 ⴛ 4…
surface oxidation
To further analyze the surface properties and possible
Fermi level pinning mechanisms of GaAs共001兲 surfaces, the
electronic structures of oxidized GaAs共001兲 surfaces were
investigated. For the electronic structure of an oxygen atom
adsorption on the back-bond site on GaAs-␤2共2 ⫻ 4兲 surface
shown in Fig. 6共1b兲, the corresponding band structure 关Fig.
8共a兲兴 indicates no defect states in the band gap region. In the
case of charge distribution, the specific back-bonds Ga–
O–As are saturated. 0.75 electrons and 1.25 electrons transfer from adjacent Ga atom and As atoms to this oxygen,
respectively. At the K point of this specific oxidized surface
Brillouin zone, surface bands are contributed by the empty
bonds of four three-coordinated Ga atoms at second-layer.
The oxygen replacement of an As dimer atom in the trough
site shown in Fig. 6共2d兲 behaves differently from the oxygen
adsorption on the back-bond site of Fig. 6共1b兲. There is one
band crossing the Fermi level which leads to Fermi energy
pinning according to Tersoff’s pinning model.29 This specific
band contributes one peak in the total density of states gap
region shown in Fig. 8共b兲 right panel. Charge density analysis 关Fig. 9共a兲兴 on the substitution case shows that this specific
band corresponds to the As half-saturated dangling bond in
the remaining undimerized As atom. And this specific As
atom p orbital mainly contributes the gap states as shown
from further local density of states study.
For the two oxygen atoms interacting with the
GaAs共001兲-␤2共2 ⫻ 4兲 surface, two kinds of oxidized surfaces, viz. adsorption and replacement, were also studied.
Figure 6共3a兲 shows that the two oxygen atoms adsorb on the
back-bonds sites forming one Ga–O bond and one As–O
bond. The corresponding band structure shown in Fig. 8共c兲
indicates no gap states like one O atom adsorption case. 0.30
and 0.07 eV above the top of valance band of the clean
surface at K point, there are two valence bands V1 and V2
shown in Fig. 8共c兲. These two bands are mainly contributed
by two As–O bonds similar to one oxygen adsorption at the
back-bonds sites. V1 indicates the combinations of antibonding ␲ⴱ and p orbitals of top As dimmer atoms, and V2 is
related to the oxygen contribution. The corresponding charge
plot is shown in Fig. 9共b兲. Two configurations for the replacement type were studied as shown in Figs. 6共3b兲 and
6共3c兲. The more stable surface 关Fig. 6共c兲兴 exhibits no defect
FIG. 8. 共Color online兲 共a兲, 共b兲, 共c兲, and 共d兲 left panels represent band structures for one oxygen adsorption, one oxygen replacement, two oxygen adsorption, and two oxygen replacement shown as Fig. 6共1b兲, Fig. 6共2c兲, Fig.
6共3a兲, and Fig. 6共3c兲, respectively. The right panel indicates the corresponding oxidized total density of states. The dot region indicates the projected
GaAs bulk bands.
level 关shown in Fig. 8共c兲兴 in the gap region since two As
atoms of the As dimer were replaced by two oxygen atoms
and formed two Ga–O bonds without any partially occupied
As dangling bonds. The structure in Fig. 6共3b兲 shows three
bands in the gap region which is 0.750, 0.82, and 1.57 eV
above the top of valance band clean surface band edge at K
point. These three bands correspond to one main peak and
several shoulder peaks in the total density of state gap region
关Fig. 8共d兲 right panel兴. Figure 9共c兲 reveals that C1 and C2 are
combinations of antibonding ␴ⴱ and p orbitals of O–As
bonds. In case of C3, combination of p orbitals of Ga and O
gives rise to C3.
In the case of the specific oxidized surface 关shown in
Fig. 6共3b兲兴, our result is qualitatively different from the work
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103720-7
Wang et al.
J. Appl. Phys. 107, 103720 共2010兲
reported by Hale et al.7 in 2003. In that work, it was argued
that the Fermi energy pining was due to the Ga atom which
was in the second top layer and bonded to two oxygen atoms. Whereas, more recent and refined work by Winn et al.10
in 2007 revealed that the Fermi energy pinning is due to the
undimerized As atom rather than the specific Ga atom, which
agrees well with the results reported here.
The atomic and electronic structures for multiple oxygen
atoms, i.e., three, four, five, and six oxygen atoms, interactions with GaAs共001兲-␤2共2 ⫻ 4兲 surface were also investigated in the present work. Multiple oxygen atoms oxidized
surfaces are found to be the combination of one and two
oxidized surfaces. The present results again reveal that surface states are from unsaturated surface As atoms rather than
other mechanisms.
C. Atomic structures and electronic structures of
passivation of oxidized GaAs„001…-␤2„2 Ã 4… surface
1. Atomic structures of passivation of oxidized
GaAs„001…-␤2„2 ⴛ 4… surface
FIG. 9. 共Color online兲 共a兲, 共b兲, and 共c兲 present contour plots of the charge
distribution at K point for surface localized states of the oxidized
GaAs共001兲-␤2共2 ⫻ 4兲 surface. The contour spacing is 3 ⫻ 10−3 e Å−3. All
plots are drawn parallel to the surface normal. Red balls and black balls
indicate oxygen and Arsenic atoms. 共a兲 is the charge plot along O and the
undimerzied As atoms. 共b兲 is the charge plot of two oxygen adsorption on
the back-bonds sites shown in Figs. 6共a兲. 共c兲 corresponds to charge plot of
two oxygen replacement of top As dimer atoms shown in Figs. 6共b兲.
In this section, we studied the passivation of oxidized
GaAs共001兲-␤2共2 ⫻ 4兲 by applying several candidates including H, Cl, F, S, and GaO to the oxidized surfaces shown in
Figs. 6共2c兲 and 6共3b兲. Figure 10共a兲 presents the energetically
favorable H position on the Fig. 6共2c兲 structure obtained after examining possible adsorption sites. H prefers to stay
0.28 Å above the top As atomic layer to form a H–As bond
of 1.55 Å with dangling As atom. For F case shown in Fig.
10共b兲, the favorable site is 0.90 Å above the surface top layer
with a F–As bond length of 1.82 Å. Similar result was obtained for a Cl interaction with the specific oxidized surface
关Fig. 10共c兲兴 and the As–Cl bond length is 2.25 Å. To compare the stabilities of these three bonds, a binding energy
analysis was conducted.
The binding energy of passivation is defined as Eb
= Eoxd + Epasv − Etot, where Eb is the binding energy, Etot is the
total energy of H, F, Cl, and S interacting with the oxidized
surface, Eoxd is the energy of oxidized surface. In the case of
H, Cl, F, and S passivation, Epasv could be defined as Epasv
= 共Emol − n ⫻ Eatm兲 / n, where Emol are total energies for H2,
Cl2, F2, and S8 共Ref. 30兲 and Eatm are the energies of the
isolated H, F, Cl, and S atom. n represents the number of
atoms in each molecule. In the case of GaO, Epasv was obtained by calculating the total energy of an isolate GaO molecule. Present calculation results of H2 bond energy is 4.50
eV compared to experimental value of 4.53 eV.31 Based on
the parameters, we found Eb = 2.15 eV for H passivation. F,
Cl, S, 2S, and GaO binding energies were 3.88 eV, 2.22 eV,
0.03 eV, 2.52 eV, and 3.84 eV, respectively.
For S passivation, two different oxidized surface configurations are considered in this work. Figure 10共d兲 presents
two S atoms form two bonds with two dangling As atoms.
And the two S atoms form a S–S dimer pair with a dimer
bond length of 2.11 Å. With the presence of S–S dimer pairs
and S–As bonds, our surface model is consistent with experiment observations.32 Fig. 10共e兲 indicates one S atom interacting with the specific As atom with dangling bond. It sits
0.73 Å above the top layer of the oxidized surface. Figure
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103720-8
Wang et al.
J. Appl. Phys. 107, 103720 共2010兲
FIG. 10. 共Color online兲 Side view of H, Cl, F, S, and GaO adsorption on oxidized GaAs共001兲-␤2共2 ⫻ 4兲 surface 关see Fig. 6共2c兲. For 2S case, it is for Fig.
6共3b兲兴. Big black 共white兲 balls indicate As 共Ga兲 atoms and small black 共white兲 balls represent As 共Ga兲 atoms in the sublayer. O and passivation species are
specified in each figure.
10共f兲 displays the molecular species 共GaO兲 cluster adsorption
on the GaAs共001兲-␤2共2 ⫻ 4兲 oxidized surface. The GaO 关see
Fig. 10共f兲兴 acts as a bridge to connect the dangling As atom
and oxygen atom.
2. Electronic structures of passivation of oxidized
GaAs„001…-␤2„2 ⴛ 4… surface
Hydrogen is known to be very effective at passivating
silicon.33,34 In the case of GaAs, the H atom transfers 0.16
electrons to the specific adjacent As atom based on Bader
charge analysis35 so that the dangling As bond is partially
saturated. The gap bands induced by dangling bond are suppressed into the valence bands region, thus opening a surface
gap like the clean surface. This finding agrees with that of
Winn’s work.9 For the F and Cl cases, F and Cl obtain 0.61
and 0.46 electrons from the adjacent undimerized As atom,
respectively. Therefore, F and Cl help to compress the gap
bands induced by specific As dangling bonds into the conducting region. So F and Cl could unpin the Fermi level. In
these two cases, p orbital surface states move to valence
bands region and form bulklike new p orbital states since the
As half-saturated dangling bond is saturated. Comparing F
passivating Si-Based gate stacks,36 F is also effective to passivate GaAs-based device due to F–As strong bond and
F-induced unpin GaAs oxidized surface.
Sulfur plays dual roles in its interaction with the oxidized GaAs共001兲-␤2共2 ⫻ 4兲 surface. One S atom forms a
S–As bond 关see Fig. 10共e兲兴 with one oxygen atom oxidized
surface. Band structure 关Fig. 11共a兲兴 shows one band crossing
the Fermi level. And this specific S-band contributes one
peak in the total density of states gap region shown in Fig.
11共a兲 right panel. Moreover, 0.57 eV above the bulk band at
K point, there is a second band. Antibonding ␲ⴱ shown in
Fig. 12共a兲 indicates band one is related to the specific undimerized As and S atoms. From charge transfer perspective,
there is only one extra electron in the oxidized surface compared to clean GaAs共001兲-␤2共2 ⫻ 4兲 surface. Nevertheless, S
atom needs two more electrons to form a closed electron
shell when S interacts with oxidized surface. Therefore, the
S–As bond is not fully saturated and leads to high density
surface states trapping Fermi level. When two S atoms inter-
act with two undimerized As surface, charge transfer becomes more complex than one S case. Among three oxidized
gap bands shown in Fig. 8共d兲, i.e., C1, C2, and C3, C1 and
C2 are related to undimerized As bonds and C3 is induced by
O–Ga–O unsaturated bonds. Each of two S atoms gets 0.17
electrons from its own adjacent As atom and form a S–S
dimer pair. Meanwhile, 0.2 more electrons from Ga connected two oxygen atoms compared to corresponding oxidized case transfer to As–S bonds. Finally, C1, C2, and C3
were pushed into conducting band region and open a clean
gap shown in Fig. 11共b兲. In addition, the specific dimer bond
is the antibonding ␴ⴱ combination of p orbitals of two S
atoms shown in Fig. 12共b兲.
In the case of oxide GaO molecule adsorption on an
oxidized GaAs共001兲-␤2共2 ⫻ 4兲 surface, a clean gap was obtained which indicated As dangling bonds are fully saturated
by obtaining a electron from GaO species.
FIG. 11. 共Color online兲 共a兲 and 共b兲 left panel represent band structures for
passivation configurations, i.e., S and 2S, respectively. The right panel indicates the corresponding passivated surface total density of states. The dot
region indicates the projected GaAs bulk bands.
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103720-9
J. Appl. Phys. 107, 103720 共2010兲
Wang et al.
To remove the Fermi level pinning, H, Cl, F, GaO, and S
interaction with oxidized GaAs共001兲-␤2共2 ⫻ 4兲 surface were
investigated. H, Cl, or F could successfully eliminate unsaturated As dangling bonds by forming strong bonds with the
specific undimerized As atom. One S atom forms a bond
with As dangling atom, however, it does not remove As gap
states since S itself generates new surface states due to the
unsaturated dangling S bond. Two S atoms form one S–S
dimer pair and remove two undimerized As gap states. Native oxide cluster, i.e., GaO, could completely clean the gap
region states and therefore become good passivation candidates.
ACKNOWLEDGMENTS
FIG. 12. 共Color online兲 共a兲 and 共b兲 present contour plots of the charge
distribution at K point for surface localized states of the passivated
GaAs共001兲-␤2共2 ⫻ 4兲 surface. The contour spacing is 3 ⫻ 10−3 e Å−3. All
plots are drawn parallel to the surface normal. Solid balls indicate As atoms.
共a兲 indicates charge plot along S and As bonds. 共b兲 is the charge plot along
S-S pair.
Based on the present analysis, H, Cl, F, and GaO could
be used to passivate any system that has undimerized As
atoms. However, once these candidates have been used to
passivate the undimerized As atoms, any states left in the
band gap region should be from the extrinsic adsorbate bonding with the GaAs共001兲-␤2共2 ⫻ 4兲 surface. For the S case, if
S bonds are saturated, it would also help to eliminate the
surface state density. On the other hand, if there remain S
dangling bonds, the generation of new states within the gap
region occurs, leading to Fermi energy pinning.
IV. CONCLUSION
First principles calculations were performed to study
oxygen atoms interaction with the GaAs共001兲-␤2共2 ⫻ 4兲 surface. For one oxygen atom interaction case, we found a preference to absorb in the back-bond sites, and also oxygen
prefers to replace one of the trough site As dimer atoms. For
an interaction of two oxygen atoms with this GaAs surface,
back-bond sites were found to be the preferable absorption
sites and the trench site As dimer were easily replaced by
two oxygen atoms. The present results show that the Fermi
energy pinning is extrinsic rather than intrinsic to the
GaAs共001兲 surface. The present work also predicts that the
additional oxygen atoms adsorption on the back-bond site
satisfies the bond saturation condition leading to no effect on
the surface gap states. However, for the oxygen replacement
of an As dimer atom of a trough site and two oxygen replacement of a row site As dimer, midgap states are produced
leading to the Fermi level pinning due to the unsaturated As
dangling bonds. These results confirm that control of oxygen
on the GaAs surface is critical to the control of states leading
to the Fermi level pinning.
This research is supported by the FUSION/COSAR
project and the MSD Focus Center Research Program. We
thank the III-V materials research groups at UTD for helpful
discussions, in particular Professor Eric Vogel and Dr. Christopher Hinkle. Calculation was done on the Texas Advanced
Computer Center 共TACC兲.
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