Surface Science 540 (2003) L577–L582
www.elsevier.com/locate/susc

Surface Science Letters

Surface reconstruction of GaAs(0 0 1) nitrided under
the controlled As partial pressure
Takahiro Imayoshi a,b,1, Haruhiro Oigawa c,*, Hidemi Shigekawa c,
Hiroshi Tokumoto b,2a
Department of Applied Physics, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
Joint Research Center for Atom Technology, National Institute for Advanced Interdisciplinary Research,
Tsukuba, Ibaraki 305-8562, Japanc
Institute of Applied Physics, 21st Century COE, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japanb

Received 19 March 2003; accepted for publication 5 June 2003

Abstract
Under the controlled As partial pressure, the nitridation process of GaAs(0 0 1)-(2 · 4) surface was studied using ascanning tunneling microscope (STM) combined with an electron cyclotron resonance plasma-assisted molecular beam
epitaxy system. With either prolonging the nitridation time or decreasing the As partial pressure, the previously reported (3 · 3) structure with two dimers per surface cell ((3 · 3)-2D) was found to progressively convert into a new (3 · 3)
structure characterized by one dimer per surface cell ((3 · 3)-1D). Reversely the exposure to arsenic transformed thestructure from (3 · 3)-1D to (3 · 3)-2D, suggesting that the topmost layer is composed of As2 -dimers. Based on these
STM images together with the X-ray photoelectron spectroscopy data, we propose the new As2 -dimer coverage models
to explain both (3 · 3)-1D and -2D structures involving the exchange reaction of arsenic with nitrogen in the subsurface
region of GaAs.
 2003 Elsevier B.V. All rights reserved.
Keywords: Gallium arsenide; Gallium nitride; Nitrogen atom; Surface relaxation and reconstruction; Scanning tunneling microscopy;
Reﬂection high-energy electron diﬀraction (RHEED); X-ray photoelectron spectroscopy

The nitridation of GaAs(0 0 1) surface wasstudied initially for the subsequent epitaxial
growth of cubic GaN (b-GaN) on a large lattice-*Corresponding author. Tel.: +81-29-853-5127; fax: +81-29853-5205.
E-mail address: oigawa@ims.tsukuba.ac.jp (H. Oigawa).
1
Present address: Optical Component Company, Oki Electric Industry, Hachioji, Tokyo 193-8550, Japan.
2
Present address: Nanotechnology Research Center, Hokkaido University, Sapporo, Hokkaido 060-0812, Japan.

mismatched GaAs substrate (20%) [1,2]. At thesame time, atomic scale analyses of the nitridation
process were performed by either reﬂection highenergy electron diﬀraction (RHEED) or scanningtunneling microscopy (STM) combined with molecular beam epitaxy (MBE) [3–7]. For the nitrided
GaAs(0 0 1)-(2 · 4) surface, we have found that theinitial (2 · 4) As2 -dimer phase reconstructs ﬁrst to
the intermediate (3 · 4) As2 -dimer phase and thenis rearranged to the (3 · 3) with two dimers per
surface cell, hereafter denoted as (3 · 3)-2D.

0039-6028/$ - see front matter  2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0039-6028(03)00821-5

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However, the origin of (3 · 3)-2D dimers remains
controversial, because the existence of N2 -dimers
is energetically unfavorable due to the large distortion caused by a sizable mismatch of atomic
radii and the high cohesive energy of N–N bondsin N2 molecules. Total energy calculations also
indicate that the chemisorption of a submonolayer
arsenic signiﬁcantly lowers the surface energy of
the Ga-terminated GaN [8,9]. Such As atoms as anunintentional adsorbate come from the GaAs
substrate itself and a residual atmosphere in the
As-related MBE chamber.
To ascertain the atomic species of surface
dimer, we investigated the inﬂuence of trace arsenic on the atomic arrangement of fully nitrided
GaAs(0 0 1)-(2 · 4), i.e., GaAs:N(0 0 1)-(3 · 3) surfaces by varying either exposure time to active
nitrogen (tN ) or As partial pressure during nitridation (qAs ). The experiments were performed in a
multiple-chamber ultra-high vacuum system composed of MBE, STM and X-ray photoelectron
spectroscopy (XPS). The change in surface reconstruction during the GaAs growth, surface
nitridation and the arsenic exposure of nitrided
surface was monitored by RHEED equippedto the MBE chamber. Especially the electron
cyclotron resonance plasma-assisted MBE forGa(As1x Nx ) alloys was used to obtain the damage-less atomically smooth surface, since theplasma source is eﬀective to produce the neutral N
radicals. Except for the nitridation conditions of tN
and qAs , the preparation of the initial As-terminated GaAs(0 0 1)-(2 · 4) surfaces and the nitridation process inclusive of quenching procedures
were the same as those in our previous paper [6].
The qAs throughout the experiments was precisely
controlled in the range from 1 · 109 to 1 · 107
Torr by adjusting a heater current of As eﬀusion
cell. Filled-state STM images of the samples weretaken at room temperature and background pressure below 1 · 1010 Torr in a constant current
mode with the sample bias voltage of )3.0 V andtunneling current of 0.7 nA. The surface concentrations and bonding states were analyzed fromthe photoelectron spectra of N 1s, Ga 3d and As 3dcore levels using XPS with MgKa characteristic Xray. The N 1s peak chosen from a viewpoint ofphotoionization cross section was deconvoluted

from the energetically coincident Ga LMM Auger
peaks.
Fig. 1 shows the ﬁlled-state STM images of
the GaAs:N(0 0 1)-(3 · 3) surfaces nitrided for (a)tN ¼ 240 s and (b) tN ¼ 330 s under the As background pressure below 3 · 109 Torr. Despite of
the similarities between their (3 · 3) RHEED patterns, the STM images reveal changes in theatomic arrangement caused by the further nitridation under As-deﬁcient conditions. In Fig. 1(a),small domains with a high degree of disorder can
be classiﬁed into three kinds labeled I–III. Compared with the STM images of starting (2 · 4)
surface not shown here, straight rows labeled I,

Fig. 1. STM images of the GaAs:N(0 0 1)-(3 · 3) surfaces nitrided under the As background pressure below 3 · 109 Torr;(a) tN ¼ 240 s and (b) tN ¼ 330 s. Arrows in (a) represent threecharacteristic structures: (I) remaining (2 · 4), (II) (3 · 3)-2Dand (III) (3 · 3)-1D. An inset of (b) is the magniﬁed image.

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run along [)1 1 0] azimuth, correspond to the 2·ordered rows of As-terminated (2 · 4) unit cells.
Adopting the b2-phase of (2 · 4) reconstruction
composed of two As2 -dimers and two missing dimers, this structure is identiﬁed with the remainedrows of two As2 -dimers even after tN ¼ 240 s. The
square-like protrusions labeled II contain a groupof four maxima being ascribable to the adjacent
two dimers parallel to the [)1 1 0] direction. Such
units aligned along the [)1 1 0] azimuth with a 1.2
nm periodicity constitute the 3·-ordered rows of(3 · 3)-2D observed by Gwo et al. [5].
In contrast to (3 · 3)-2D, the structure labeled
III consists of a pair of equivalent protrusions,suggesting a dimerization between the same atomic
species along [)1 1 0] azimuth. Each dimer arranges with threefold periodicity of 1.2 nm along
both [1 1 0] and [)1 1 0], and forms domains exhibiting a new (3 · 3) reconstruction with one
dimer per surface cell, which is called (3 · 3)-1D
hereafter. We note that these reconstructions occur
on the same terrace, since the diﬀerences in height
among them are much smaller than the interlayer
atomic distances for GaAs(0 0 1) and GaN(0 0 1).
Either local electronic diﬀerences or internal stressrather than the topographic eﬀect may be responsible for the depression feature of both (3 · 3).
In particular, the [)1 1 0] dimerization direction of(3 · 3)-1D/-2D indicates that a likely candidate fordimer species is either anion of arsenic or nitrogen,though the N 1s XPS signal was not measurable
due to very weak intensity below our detection
limit. The removal of either dimer from (3 · 3)-2D
could establish the (3 · 3)-1D accompanied with aconsiderable number of kinks being recognizable
in a series of short rows along [)1 1 0]. It should be
mentioned that the actually observed (3 · 3)
RHEED pattern is attributable to a statisticallateral distribution of both (3 · 3) small domains.
This is conﬁrmed by the fast Fourier transformation (FFT) of the STM images.
Furthermore we found that the prolonged exposure to active nitrogen led to a well-ordered
array of (3 · 3)-1D, and simultaneously to a complete disappearance of (3 · 3)-2D and As-terminated (2 · 4) rows, as shown in Fig. 1(b). The
single domain of (3 · 3)-1D structure clearly seenin an inset of magniﬁed image impresses us as an

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advanced stage in nitridation of the GaAs(0 0 1)
surface. The etching traces pointed out by Xue
et al. [7] might be caused by the evaporation of As
atoms decomposed from the GaAs surface, because the nitridation conditions of low qAs andhigh sample temperature (Ts ¼ 525 C) promote
such behaviors. The vacancy islands partly retrieved the (3 · 3)-1D are associated with a precursor of surface portions etched by exposing the(1 1 1) crystal planes with the slowest nitridation
rate described by Okumura et al. [10]. We add a
few words that no appreciable change in the STM
images was observed against the exposure to dry
O2 gas of 1 · 107 Langmuir, demonstrating thesurface passivation by nitridation.
Taking into account that (3 · 3)-1D with lower
dimer coverage is a further nitrided phase, thesurface dimers in (3 · 3)-2D are assigned not to N–
N but to As–As. However, the STM image obtained for (3 · 3)-1D displays no evidence for the
existence of surface As2 -dimers, though the predominant formation of (3 · 3)-1D in consequenceof the further nitridation is focused on the transition from (3 · 3)-2D to (3 · 3)-1D. With the purpose of observing the opposite transformationfrom (3 · 3)-1D to (3 · 3)-2D, we adsorbed the As
atoms on the (3 · 3)-1D surface as follows. Afterchecking the (3 · 3)-1D structure by STM, thesample was transferred back to the MBE chamberand heated up to 450 C. While keeping thebackground pressure at around 1 · 109 Torr, theAs cell heater was raised up to 200 C and its
shutter was opened for 300 s to expose the (3 · 3)1D surface to As4 ﬂux. Fig. 2 shows the STM
image of this surface on which two kinds of atomic
arrangements composed of (3 · 3)-1D and (3 · 3)2D are observed. More precisely, the (3 · 3)-1D
surface labeled A is partly converted to (3 · 3)-2D
surface labeled B which was absent before exposure to arsenic. The domain area of (3 · 3)-2D wasfound to increase with the exposure time, and oneof two dimers in (3 · 3)-2D seems to be that in(3 · 3)-1D, indicating that exposure to arsenic induces the transformation from (3 · 3)-1D to(3 · 3)-2D. It should be noticed here that no nitrogen atoms are present in the environment
during exposure to arsenic, and two dimers in(3 · 3)-2D exhibit the same contrasts and the same

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Fig. 2. STM image of the GaAs:N(0 0 1)-(3 · 3)-1D surface
after exposure to As4 ﬂux at qAs  1  109 Torr. The exposure
time and substrate temperature were 300 s and 450 C, respectively. Addition of another adjacent dimer to the (3 · 3)-1D
structure labeled A gives rise to the (3 · 3)-2D structure labeledB.

separations to each other. Therefore we can conclude that both one dimer in (3 · 3)-1D and two
dimers in (3 · 3)-2D consist of the same atomic
species which could be not N atoms but As atoms.
Based on these STM data, the (3 · 3)-2D
structure is formed by adding another As2 -dimer
to the adjacent to the As2 -dimer in (3 · 3)-1D
structure. Fig. 3 illustrates such atomic arrangements of (a) (3 · 3)-1D and (b) (3 · 3)-2D structures. Each As atom forms bonds with two
underlying Ga atoms and dimerizes along [)1 1 0]
azimuth. In this ﬁgure, only As2 -dimers in the
topmost layer and Ga atoms in the second layer
are depicted, but the existence of N atoms is still
uncertain. To grapple with this problem, we analyzed the (3 · 3)-1D surface using XPS and detected the N 1s signal. As for (3 · 3)-1D shown in
Fig. 1(b), the signal intensity ratios of N 1s/Ga 3d
and As 3d/Ga 3d were obtained 0.02 and 0.88, respectively. By assuming a submonolayer N coverage to avoid a mathematical complexity [11,12],
the surface N content of (3 · 3)-1D was roughly
estimated to be 0.2 ± 0.1 monolayer (ML). Though
the noticeable chemical shifts in the XPS spectra
were unobservable due to a small amount of nitrogen, a slight increase (6%) in the full width at
half maximum for Ga 3d peak and no change forFig. 3. Structural models proposed for the GaAs:N(0 0 1) surfaces of (a) (3 · 3)-1D and (b) (3 · 3)-2D, where the respective
topmost layers consist of one and two As2 -dimers per surface
cell. Open, ﬁlled and gray circles represent the topmost As
atoms, the underlying Ga atoms and the exposed Ga atoms,
respectively. The incorporated N atoms not shown here partly
replace the As atoms in the subsurface region.

As 3d peak suggest the formation of Ga–N bonds.
The N atoms substituted for As atoms in thesubsurface region should bond with the nearestneighbor Ga atoms. The reason is that heat offormation of GaN (6.81 eV) is much larger thanthat of GaAs (5.55 eV) [13]. Most of XPS andAuger electron spectroscopy studies support theformation of Ga–N bonds through the product ofa crystalline GaN layer [10,14,15]. In either case,
the N content in the third or deeper As layer iscomparable to the above estimated submonolayerone. If the N atoms of (3 · 3)-1D uniformly distribute only in the third As layer of GaAs and theintensity of N 1s XPS signal is diminished by amonomolecular overlayer of GaAs, the incorporated N content is calculable from the surface N
coverage of 0.2 ML using an attenuation factor ofexpð2d=kN1s Þ. Here, d is the plane distance for
GaAs(0 0 1) and kN 1s the escape depth of N 1s
photoelectron. By introducing d ¼ 0:14 nm andkN1s ¼ 1:96 nm, the N content of 0.23 ML in thethird As layer was obtained, which turns out to bewithin the numerical errors of ±0.1 ML.
The proposed structural models for (3 · 3)-1D
and -2D are consistent with the recent ﬁrst-principles total-energy calculations for the clean andAs-covered GaN(0 0 1) surfaces [8]. In contrast to
the N-terminated surface with energetically unfavorable N2 -dimers, the Ga-terminated one can bestabilized by the Ga metallic bonds, since the Ga

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atoms on GaN have approximately the same distance as in bulk Ga due to a small radius of N
atom. That surface is further stabilized by thechemisorption of a submonolayer As atoms. In
fact, both topmost layers shown in Fig. 3 consistof only arsenic and the As coverage decreases from
0.44 ML for (3 · 3)-2D to 0.22 ML for (3 · 3)-1D
with prolonging tN or increasing the nitrogencontent. Because of a comparatively high vapor
pressure of arsenic at the nitridation temperatureof 525 C, even the surface As2 -dimers on thestarting (2 · 4) and the free As atoms segregatedfrom the bulk are regarded as an arsenic source.
The dimerization and atom missing to minimize
the surface energy are commonly observed at thereconstructed surfaces of GaAs(0 0 1). However,neither model in Fig. 3 satisﬁes the electron
counting rule [16] which requires the autocompensation involving the charge transfer from cation to anion dangling bonds. This is probably
related to a rise of electrostatic energy and strainenergy contributions to the surface energy, since
there is a large diﬀerence in the electron negativity
and atomic radius between As and N atoms.
To further elucidate the eﬀect of trace As, we
compared the STM images of sample surfaces
nitrided under various qAs in a constant time(tN ¼ 330 s). Fig. 4 shows the domain area ratios
among three structures of (2 · 4), (3 · 3)-2D and(3 · 3)-1D against qAs in the range of 3 · 109 –1 · 107 Torr. The depressed area under low qAs ,which is similarly observed in Fig. 1(b), was excluded statistically. Obviously, with decreasing qAs
the area of (3 · 3)-1D as an advanced nitridation
phase increases, while those of (3 · 3)-2D and(2 · 4) decrease. The fact that low qAs encourages
the growth of (3 · 3)-1D phase at the sacriﬁce of(3 · 3)-2D agrees with our models. In other words,
high qAs suppresses the formation of (3 · 3)-1D,
because the atmospheric As atoms arriving at thesurface necessarily hinder the superﬁcial reaction
between impinging N radicals and exposed Ga atoms, and also the penetration of N atoms leading
to the nitridation in the subsurface region [4,10].
In summary, the surface reconstruction of
GaAs(0 0 1) nitrided under the controlled As partial pressure has been studied using high-resolution
STM in combination with RHEED and XPS. By

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Fig. 4. Area ratios of three reconstructed domains against theqAs : (I) (2 · 4), (II) (3 · 3)-2D and (III) (3 · 3)-1D. The
GaAs(0 0 1)-b2(2 · 4) surfaces were nitrided under the various
qAs in the range of 3 · 109 –1 · 107 Torr in a constant time(tN ¼ 330 s).

varying either tN or qAs , the initial As-terminated(2 · 4) surface was found to change to two kinds of(3 · 3). One is the (3 · 3)-1D consisted of one dimer
per surface cell, and the other (3 · 3)-2D of two
dimers. The former (3 · 3)-1D surface predominantly formed during the further nitridation in an
As-free atmosphere converted to the latter (3 · 3)2D after exposure to As atoms. From these results,
we reasoned that both surface dimers in (3 · 3)1D/-2D are not N–N but As–As. Considering theincorporated N atoms conﬁrmed by XPS, theproposed models for As2 -dimer coverage of (3 · 3)1D/-2D are explicable as follows. The incoming N
atoms partly substitute the As atoms in the subsurface region, and then bond with Ga atoms. Thisphenomenon toward the formation of b-GaN
should energetically prefer the Ga-terminatedsurface to the N-terminated one. According to thetheoretical calculations, such Ga-terminated surface is further stabilized by a submonolayer As
coverage, resulting in the formation of As2 -dimerson (3 · 3)-1D/-2D.

Acknowledgements
We would like to thank Dr. Y. Morita and Mr.
T. Shimizu of NAIR for their technical support,

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and Prof. S. Gwo of Taiwan National Tsing-hua
University for useful discussion. This work waspartly supported by NEDO.

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