Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 3875–3878
Part 1, No. 6B, June 1999c°1999
Publication Board, Japanese Journal of Applied Physics

Initial Stage of Nitridation of GaAs(001): Atomic Scale View
Takahiro I MAYOSHI1 , Haruhiro O IGAWA2 , Hidemi S HIGEKAWA2 and Hiroshi TOKUMOTO
Joint Research Center for Atom Technology, National Institute for Advanced Interdisciplinary Research,Higashi 1-1-4, Tsukuba, Ibaraki 305-0046, Japan
1 Department of Applied Physics, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan
2 Institute of Materials Science, University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8573, Japan(Received March 15, 1999; accepted for publication March 29, 1999)

The nitridation process of an As-terminated GaAs(001)-β2(2 × 4) surface was studied by time-resolved reflection highenergy electron diffraction (RHEED) and in situ scanning tunneling microscopy (STM). On the intermediate nitrided surface,a (3 × 4)-ordered phase was formed by the rearrangement of the As-dimer pairs at 520◦ C. This phase reverted back to the(2 × 4)-ordered phase below 400◦ C (reverse reconstruction by the re-adsorption of As atoms). We propose a model to explain
this behavior.
KEYWORDS: N-for-As exchange reaction, nitridation of GaAs(001) surface, GaAs(001)-β2(2 × 4)As, GaAs(001)-(3 × 4)As,GaAs(001)-(3 × 3)N, RHEED, STM

1. Introduction
Special attention has been paid to GaN because it has enormous potential for applications in optoelectronic devices inthe short-wavelength region, high-frequency electronic devices and for various other uses.1) For these applications,
it is essential to grow high-quality GaN and related materials. However, we cannot find good substrates that are latticematched with GaN. For example, in the case of GaAs(001)substrates, the lattice mismatch is as high as 20% (0.565 nmfor GaAs; 0.51 nm for GaN). Despite this, we can utilize thislattice-mismatch effect to grow self-organized nanometerscale structures such as quantum dots.2) Unfortunately, few
studies have been conducted on the atomistic process of GaN
growth.
In the case of nitridation on compound semiconductor surfaces, there are several experimental findings that suggestthat the exchange reaction plays an important role. Gemmeren et al.3) studied the nitridation of the InAs(110) surface at room temperature (RT) by X-ray photoelectron spectroscopy (XPS) and found that the N-for-As anion exchange
reaction took place. Gwo et al.4) also observed the N-forAs exchange reaction by the reflection high-energy electron
diffraction (RHEED) measurement during the nitridation ofGaAs(001) surface at 565◦ C and obtained scanning tunnelingmicroscopy (STM) images showing that N atoms replace allAs atoms at the topmost layer and form dimers at the finalstage. These dimers most likely form the 3 × 3 reconstruction
at the first layer of the nitrided GaAs(001) surface. Kasu etal.5) also studied the nitridation of the InAs(001) surface andfound that the nitrogen radicals replace As atoms at the topmost layer of As-stabilized surface at 100◦ C, whereas theybond mainly with the topmost In atoms on an In-stabilizedsurface at 350◦ C. Aksenov et al.6) measured the temporalchange of Auger electron spectroscopy (AES) spectra duringnitridation of the GaAs(001)-(2 × 4) surface at 300–400◦ Cand found the clear chemical shifts of the Auger transitionsof Ga, As and N. However their exposure was quite large so
it is very difficult to discuss the initial stage. Collins et al.7)
also observed a similar temporal change in the RHEED intensity due to the Sb-for-As anion exchange reaction on theInAs(001) surface.
Here we extend our former work4) by examining the intermediate nitridation stage of GaAs(001)-β2(2 × 4) surface

using in situ RHEED and STM techniques. We attempted toreplicate the same experimental conditions as before. Unfortunately, we modified several parts in our plasma source andtherefore could not realize the exact same conditions. This
results in slightly different temporal dependence, especially,
the absolute value of time. The overall behavior was the same
as before and we discuss the atomistic view of the nitridationof the GaAs(001)-β2(2 × 4) surface by taking account of bothexperimental results in this paper.
2. Experimental
Experiments were performed in a multiple chamber ultrahigh vacuum system composed of an MBE chamber for nitridation of GaAs and an STM chamber for observing theatomic arrangement.4) The MBE system used in our study isequipped with a compact ECR microwave (2.45 GHz) plasma
source to provide the active nitrogen flux and a valved cracker
for introducing As4 flux. After degassing at 300◦ C and removing the surface oxide layer at 600◦ C, an Si-doped GaAs
buffer layer (∼100 nm) was grown at 565◦ C. This surface
exhibited an atomically flat and well-ordered As-terminatedβ2(2 × 4)-reconstructed surface.8) Prior to nitridation, highpurity (99.9998%) N2 gas was introduced through a variableleak valve at a pressure of around 2.0 × 10−3 Pa into theECR plasma source. During nitridation, the input power ofthe ECR plasma source was kept at 265 W, the substrate temperature was 520◦ C, and both the Ga cell shutter and the As
cracking cell valve were closed. Under these conditions, theECR plasma mainly consists of neutral nitrogen radicals withless ionized species and is very suitable for nitriding the surface without roughening.4) In situ RHEED observation wasperformed with two incident beam directions, one along the[1̄10] direction ([1̄10] RHEED) of GaAs(001) substrate andthe other along the [110] direction ([110] RHEED).
After nitridation, we quickly shut off the valve used for
introducing N2 gas and quenched the sample to 400◦ C. The
sample temperature was kept at 400◦ C until backgroundpressure below 1.3 × 10−7 Pa (about 30 min) to minimize
the As adsorption on the nitrided surface. Then the sample was quenched to room temperature (RT) and transferred to the STM chamber while keeping the pressure at
1.3 × 10−8 Pa. All STM images shown here were obtained
under 1.3×10−8 Pa at RT in constant tunneling current mode.

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T. I MAYOSHI et al.

Figure 1 shows the temporal change in the [1̄10] RHEED
intensity during nitridation. We can see 1/4-order peaks at theinitial surface as expected and the intensity of the 1/4-order
peaks decreases with respect to the nitrogen exposure time.
At around 135 s of nitridation, 1/3-order peaks begin to grow.
At 180 s of nitridation∗ , only 1/3-order peaks are clearly seen.
Figure 2 shows the temporal change in the [110] RHEED
intensity during nitridation under the same conditions as inFig. 1. We can see 1/2-order peaks at the initial surface andthe intensity of the 1/2-order peaks decreases until around 60 sof nitridation and then 1/3-order peaks begin to grow. After
120 s of nitridation, only 1/3-order peaks exist.
Figures 1 and 2 provide several findings on the surface
structural change during nitridation: (1) At 120 s of nitridation, the 3×-reconstruction along the [1̄10] direction isformed whereas the ×3-reconstruction along the [110] direction is not formed yet. In other words, the (3 × 4) reconstruction is appeared. (2) After 120 s of nitridation, the×4-reconstruction along the [110] direction changes to the×3-one, while the 3×-reconstruction along the [1̄10] direction lasts. Then, the fully nitrided surface structure changes
into the (3 × 3) phase. These results indicate an anisotropic

RHEED Intensity (arb. units)

nitridation time[110] RHEED

nitridation process.
Figure 3 shows a typical STM image of nearly fully nitrided GaAs(001) surfaces, i.e., 150 s of nitridation. We
can see several characteristic features: a (3 × 3)-orderedphase (the area marked by A), a bright 2×-ordered row along
the [1̄10] direction (B), a bright 3×-ordered row along the[1̄10] direction (C), a bright phase consisting of neighboring
2×-ordered or 3×-ordered rows (D and E). The first twophases A and B are the same as those seen in the previous study.4) It should be noted here that there are many
2×-ordered rows along the [1̄10] direction in the STM image, although the [110] RHEED exhibits no 1/2-order peaks
after 120 s of nitridation, as shown in Fig. 2. This may become clear when we take the fast Fourier transform (FFT) of
the STM image. In fact, the FFT image showed no clear 2×spots along the [1̄10] direction only clear 3× spots. This feature can be explained by the fact that these 2×-ordered rows
are too small to exhibit RHEED spots.
In the present study, we find the 3×-ordered rows (C)with almost the same height and the same direction as the2×-ordered rows (B). This fact suggests that the origin of
these 3×-ordered rows could be the same as that of the2×-ordered rows. In Fig. 3, we cannot find the clear longrange order for these bright rows (B and C) along the [110]direction, but in other areas we can observe several neighboring bright rows whose distances are 1.6 nm, i.e., they form a(2 × 4)-ordered phase (D) and a (3 × 4)-ordered phase (E).
Here, we assume that the 2×-ordered rows are formed by theremaining As-dimer rows on the GaAs(001)-β2(2 × 4) sur-180s
135s120s60s0s-101Surface wave number (units of 2 3/2π/a 0 )Fig. 1. Temporal change in the [1̄10] RHEED intensity during nitridationof the As-terminated GaAs(001)-β2(2 × 4) surface. Dotted lines represent
how the 1/4-order and 1/3-order peaks behave with respect to the nitrogen
exposure time.
∗ The duration time (180 s) of intensity change differs from that of the pre-vious study (35 s).4) As described in the introduction, this may be due todifferent experimental conditions in the plasma source and a substrate temprature of 520◦ C.

[110] RHEED
nitridation time

3. Results and Discussion

RHEED Intensity (arb. units)

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120s
90s
60s
30s0s-101Surface wave number (units of 2 3/2π/a 0 )Fig. 2. Temporal change in the [110] RHEED intensity during nitridationof the As-terminated GaAs(001)-β2(2 × 4) surface. Dotted lines represent
how the 1/2-order and 1/3-order peaks behave with respect to the nitrogen
exposure time.

Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6B

T. I MAYOSHI et al.

trenchCDB3877

trenchAE

2×4

3×4

[110]

3×3

As atom at the top layer
N atom at the top layer
Ga atom at the 2nd layer (if existent)
As atom at the 3rd layer[110]

Fig. 3. A typical STM image of GaAs(001)-β2(2 × 4) surfaces nitrided
for 150 s. The sample bias voltage is −3.0 V and the tunneling current is0.2 nA (filled-state imaging). Dashed circles represent the characteristicphases: (3 × 3) phase (A), bright 2×-ordered phase (B), bright 3×-orderedphase (C), neighboring bright 2×- (D) and 3×- (E) ordered rows. The
crystallographic directions and the imaging size are shown in the inset.

face and the 3×-ordered rows are reconstructed from these2×-ordered rows. According to the temporal change ofRHEED and this assumption, we assume that the reconstruction process of GaAs(001)-β2(2 × 4) surface during nitridation is composed of two successive processes. In the firstprocess, the initial As-terminated β2(2 × 4) surface reconstructs from (2×4)-ordered As-dimer phase to (3×4)-orderedAs-dimer phase. In the second process, this intermediate(3 × 4)-ordered As-dimer phase is rearranged so as to form
the (3 × 3)-ordered N-dimer phase. In Fig. 4, we draw anatomistic model of these three phases. The (2 × 4) phasestands for the GaAs(001)-β2(2 × 4) surface and the (3 × 3)
phase is the same as that proposed previously.4) In the intermediate (3 × 4) phase, the As-dimer pairs, whose origin isthat of the (2 × 4) phase, exist at the topmost layer and rearrange with 3×-ordering along the [1̄10] direction. When theN atoms form bonds with the Ga atoms of the second layer,large tensile strains are induced. These large strains may stabilize this (3 × 4) phase. However, we cannot determine theadsorption site of the N atoms and the atomic arrangement on
the “trench” (missing dimer row region in the (2 × 4) phase)with only our present data.
To examine the stability of the bright 2×-ordered rows in
the phase B, we measured their length and frequency in theSTM image. The results are summarized in Fig. 5, together
with the data taken from the former work.4) In this plot, wecan see a peak at 4 units of As-dimer pairs. This indicatesthat the bright rows most probably consist of four As-dimerpairs. The existence of characteristic length suggests that theFrequency

Fig. 4. An atomistic model for (2×4), (3×4) and (3×3) phases in nitrided
GaAs(001)-β2(2 × 4) surface. The various circles represent atom sites, asindicated in the lower half. In the (3 × 4) phase, the trenches might beoccupied by the Ga atoms at the second layer.

2468101214Length (units of As-dimer pairs)Fig. 5. Histogram of the distribution of the bright 2×-ordered row length
calculated from the STM image shown in Fig. 3 (black) and the previouswork (white).4)

stabilized mechanism is related to surface atomic configuration. In Fig. 6, we draw three possible types of surface atomic
arrangements of the 2×-ordered As-dimer pair rows (X, Y, Z)
surrounded by the (3 × 3) phase which is composed of the Ndimer pairs. In the case of X, both sides of the 2×-ordered Asdimer pairs match with the neighboring 3×-ordered N-dimerpairs as marked by α and β with leaving three Ga atoms ofthe second layer at both ends. In the cases of Y and Z, the2×-ordered As-dimer pairs do not match at one end as shownby γ and δ with leaving extra exposed Ga atoms just below
these pairs. These Ga atoms may be unstable due to the existence of dangling bonds. This could be a reason why thepeak positions appeared at 4 units of As-dimer pairs in Fig. 5.
The peak is broader for the present data than for the previousdata. This may be due to the lower density of surrounding N-3878

T. I MAYOSHI et al.

Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6B(X)(Y)(Z)αδβγ

N-dimer atom of 3×-ordered region[110]

As-dimer atom of 2×-ordered row
Ga atom at 2nd layer (if existent)[110]

As atom at 3rd layer

Fig. 6. Three possible types of surface atomic arrangement of the2×-ordered As-dimer pair rows surrounded by the (3 × 3) N-dimer region.
In case X, the As-dimer pairs match with the N-dimer pairs at positions αand β and in cases Y and Z, they do not, as shown by γ and δ.

dimers which is caused by the less nitrided condition of thissurface.
Figure 7 shows a typical STM image of partially nitrided GaAs(001)-β2(2 × 4) surfaces, i.e., 60 s of nitridation. Experimental conditions except for nitridation time are
the same as that in Fig. 3. Although the [110] RHEED
exhibited weak 1/3-order peaks as shown in Fig. 2 just after nitridation, they changed to diffuse 1/2-order peaks during sample cooling and, in fact, we did not observe a clear
3×-ordered structure in the STM image at RT, as seen inFig. 7. This suggests that the reverse process of the reconstruction from the nitrided GaAs(001)-(3 × 4) surface to Asterminated GaAs(001)-β2(2 × 4) surface occurs due to the readsorption of As atoms on the surface. This process is characteristic of the partially nitrided surface. On the other hand, thefully nitrided surface remains unchanged clear 3 × 3 RHEED
patterns for several minutes even at 520◦ C and with As4 flux.
In other words, once the 3 × 3 phases are formed, the reverse
process rarely occurs. This means that the existence of the intermediate 3 × 4 phase is essential for the reverse process. In
our model for the 3 × 4 phase (Fig. 4), the As-dimer pairs remain at the topmost layer and keep the ×4-ordering along the[110] direction, which could make the reverse process easier.
As mentioned above, the strains induced by the N incorporation can stabilize this 3 × 4 phase. Therefore, the desorptionof N atoms after stopping N irradiation directly makes the Asdimer pairs on the (2 × 4) surface unstable. As atoms fromthe MBE chamber can easily adsorb on the surface. In thisway, the reverse process may proceed and an As-terminatedβ2(2 × 4) surface is formed.

Fig. 7. A typical STM image of partially nitrided GaAs(001)-β2(2×4) surfaces nitrided for 60 s. The sample bias voltage is −3.0 V and the tunnelingcurrent is 0.2 nA (filled-state imaging). The surface is uniformly coveredby the 2×4 reconstruction without 3×-ordered phase. The crystalographic
directions and the image size are shown in the inset.

4. Summary
We studied the intermediate stage of the nitridation process of the As-terminated GaAs(001)-β2(2 × 4) surface using time-resolved RHEED analysis and in situ STM observation. We found characteristic structures on the intermediate
nitrided surface: 2×-ordered As-dimer pair rows, 3×-ordered
As-dimer pair rows and (3×4) regions. All of these structures
were determined by the rearrangement of the As-dimer pairs.
We also found the reverse reconstruction from the (3 × 4)
phase to the (2×4) phase on the intermediate nitrided surface.
We proposed a model to explain these findings. However, toconfirm this model, we need more systematic experimental
data as well as theoretical calculations.
Acknowledgments
The authors would like to thank Professor S. Gwo ofTaiwan National Tsing-Hua University for his valuable advice. This work was partly supported by NEDO.

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