Observation of atomic ordering of triple-period-A and

Observation of atomic ordering of triple-period-A and

Observation of atomic ordering of triple-period-A and -B type in GaAsBi
Mingjian Wu, Esperanza Luna, Janne Puustinen, Mircea Guina, and Achim Trampert
Citation: Applied Physics Letters 105, 041602 (2014); doi: 10.1063/1.4891854
View online: http://dx.doi.org/10.1063/1.4891854
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APPLIED PHYSICS LETTERS 105, 041602 (2014)
Observation of atomic ordering of triple-period-A and -B type in GaAsBi
Mingjian Wu,1,a) Esperanza Luna,1 Janne Puustinen,2 Mircea Guina,2 and Achim Trampert1
1
Paul-Drude-Institut f€
ur Festk€
orperelektronik, Hausvogteiplatz 5–7, D-10117 Berlin, Germany
Optoelectronics Research Centre, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere,
Finland
2
(Received 23 June 2014; accepted 20 July 2014; published online 30 July 2014)
We report the observation of atomic ordering of triple-period (TP)-A and -B type in low temperature
(LT) grown GaAsBi alloy using transmission electron microscopy (TEM). In addition to previous
reports, where only TP-A ordering was identified in III-V alloys, here, we confirm by electron
diffraction, high-resolution (HR) TEM, and HR Z-contrast scanning TEM that two ordering variants
coexists for LT-GaAsBi. We find that the TP-A ordering variant dominates over the TP-B variant.
TP-A domains extend over 50–100 nm (projected lateral width) and are of higher perfection
compared to TP-B domains. HR Z-contrast scanning TEM on different domains reveals a variation in
the Bi occupancy in the {111} planes with triple period sequence. Since the formation of ordered
phases has been directly linked to the occurrence of specific surface reconstructions, our results
suggest a correlation between the TP-A and B type domains and the multiple stability of n 3
C 2014
and 3 n reconstructions on the (001) surface of GaAsBi under low temperature growth. V
AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4891854]
Spontaneous atomic ordering in semiconductor ternary
or quaternary alloys has been in focus since mid-80s.1,2
Because of the Brillouin zone folding effect, ordering causes
a dramatic change in the electron band structure and, therefore, changes in the electronic and optical properties, making
it of great interest both from fundamental and technological
point of view. CuPt-B type, CuPt-A type, and triple-period
(TP)-A ordering are the most widely observed and most
intensively studied ordered phases.2 The A- and B-variants
refer to ordering in the four equivalent h111i directions, i.e.,
[111], ½
1
11; ½
111, and ½111 directions, where the first two
are the A-variant and the later two the B-variant.
While CuPt-type ordering was found in both A- and
B-variants, the TP ordering, in contrast, has been observed
only in the A-variant. The first experimental evidence of
TP-A ordering was found in the InAlAs alloy system by
Gomyo et al.3 Indeed, theoretical studies have established a
connection between the surface thermodynamics and the experimental observation of the occurrence of ordering.4 The
explanation is that the periodic arrangement of surface
dimers, which is linked to a specific surface reconstruction,
produces stress modulations at the atomic level and which
induce an atomic site selectivity and determine whether the
ordering is of CuPt-A, CuPt-B, or TP-A type. Hence, there
exists a one-to-one correspondence between a particular surface reconstruction and a particular atomic ordering type.5
Consequently, there are some proposals regarding the use of
surfactant atoms of large covalent radius, like Sb or Bi,
which can lead to certain stable surface reconstructions, in
order to selectively induce ordered structures in III-V
alloys.6 In general, the observed occurrence of ordered
phases is in accordance to the surface reconstruction, which
is well explained by the above mentioned theories. In all
cases, however, no TP-B ordering has been reported so far.
Furthermore, despite cross-section scanning tunnelling
a)
[email protected]
0003-6951/2014/105(4)/041602/4/$30.00
microscopy studies provide information on the atomic
arrangements in the ordered planes at very localized
regions,7 in general, there is a lack of experimental data
regarding the chemical composition within the TP ordered
domain(s).
In this Letter, we report on the observation by (scanning) transmission electron microscopy [(S)TEM] of both
TP-A and TP-B type ordering in GaAsBi thin films grown at
low temperature (LT) (<300 C). Using high-resolution
Z-contrast imaging, we provide concrete evidences of the
variation of Bi occupancy in the lattice among the different
ordered domains.
The samples were grown by solid-source molecular
beam epitaxy on semi-insulating GaAs(001) substrates. After
depositing a 120–140 nm GaAs buffer layer at 580 C, the
240–270 nm thick GaAsBi layers were grown at a temperature of 220 C. Samples were grown with growth rates of
0.4–0.5 lm/h and with varying Bi fluxes. The Bi contents
were between 1.5% and 2.9% and were determined from the
lattice mismatch between GaAsBi and GaAs layers measured
by X-ray diffraction8 after assuming Vegard’s law and a
GaBi lattice parameter of 6.33 Å.9
Cross-section TEM specimens were prepared using
standard procedures, i.e., by mechanical lapping and dimpling, followed by broad beam Argon ion milling. The samples were investigated either with a JEOL 2100F microscope
operating at 200 kV or on a JEOL 3010 microscope operating at 300 kV. Selected area electron diffraction (SAED)
patterns were acquired within a section area of about 120 nm
in diameter. The high-angle annular dark-field (HAADF)
STEM were acquired on the JEOL 2100F system with a
probe semi-angle of about 14 mrad, and using a detector collection semi-angle of 80–210 mrad, ensuring the incoherent
Z-contrast imaging condition.10
In this Letter, we adopt the previously introduced definition for A- and B-variant of the triple-period ordering
(TPO),2 i.e., the TP-A and TP-B type ordering in zincblende
105, 041602-1
C 2014 AIP Publishing LLC
V
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Wu et al.
structure, as illustrated in Figs. 1(a) and 1(b). In addition, we
use the general term TPO to refer to TP-A and TP-B, in general. An important issue for the determination of the A- or
B-variant of the TPO phase is to identify the projected specimen orientation, because the TP-A (TP-B) type is best
observable in the ½110 ([110]) zone axis. In this respect, special attention is paid to label the sample orientation before
the TEM specimen preparation. In addition, the specimen
orientation is afterwards determined and confirmed in the
electron microscope using convergent beam electron diffraction (CBED) techniques.11
Figure 2 shows representative images of the TP-A and
TP-B type domains observed in the samples under investigation. Figures 2(a) and 2(d) are g004 dark-field TEM overview
images of the specimen near the ½110 zone axis and the
[110] zone axis, respectively. The pseudomorphic GaAsBi
epilayer (called alloy phase hereafter) is free of extended
defects. Some V-shaped or “semi” V-shaped domain structures are observed however in both zone axes. They have a
depth (along the growth direction) of about 50–120 nm and
are located about 150 nm apart from the GaAs buffer/
GaAsBi interface. As can be clearly seen from the image
contrast, these domains are better defined (cf. the large black
inclined bands) in the ½110 zone axis compared to the [110]
zone axis. In order to investigate in more detail the microstructure of the V-shaped domains we performed SAED and
high-resolution TEM (HRTEM). SAED on these near surface features evidences the presence of superlattice spots at
about ðh6 13 ; k6 13 ; l6 13Þ and ðh6 23 ; k6 23 ; l6 23Þ positions
from the main zincblende GaAsBi spots, as shown in the
inset of Figs. 2(a) and 2(d). Similar SAED patterns have
been reported in the literature and have been attributed to the
presence of TPO.3 In particular, Gomyo et al. found that in
the InAlAs system, the features correspond to TPO in the
group III element sub-lattice. Contrary to our observation,
they only observed the superlattice structure in the SAED
recorded along the ½110 zone axis, i.e., they only observed
the TP-A variant, while similar results were reported by
other groups as well.12 Interestingly, we observe the superlattice structure in SAED from both zone axes, suggesting
that here we deal with the TP-A variant but also with the
TP-B one. We notice that the TP-A domains show clear
superlattice spots, suggesting uniform lattice plane distances
in the superstructure within the selected region; whereas the
TP-B domains show streaky superlattice spots, indicating a
wider distribution of the lattice plane distances within the
FIG. 1. Schematic representation of the zincblende structure projected near
the (a) ½110 and (b) [110] directions. The A- and B-variants of the h111i
directions are indicated by the red and blue arrows, respectively, so that the
TP-A (TP-B) phase are best observed along the ½
110 ([110]) zone axis.
Appl. Phys. Lett. 105, 041602 (2014)
selected region. Furthermore, based on the SAED patterns of
TP-A type domains, we find that the intensity maximum of
the superlattice spots is domain-dependent, i.e., neither
exactly located at the above mentioned positions (i.e.,
6 13 ; 6 23) nor fixed at the same position for different TPO
1
domains: the spots are shifted at positions ðh6 1d
3 ;
1d1
1d1
2þd2
2þd2
2þd2
k6 3 ; l6 3 Þ and ðh6 3 ; k6 3 ; l6 3 Þ, with d1
and d2 in the range between 0.06 and 0.15 depending on the
selected domain(s). This suggests that the superstructure lattice distances are slightly different from domain to domain,
as will be further confirmed by other techniques and discussed later.
With HRTEM imaging, the triple-period can be clearly
resolved. Examples are shown in Figs. 2(b) and 2(e), where
the triple period is marked by the alternating color arrows in
the direct space images. In the Fourier transformed image
[cf. insets in Figs. 2(b) and 2(e)], the presence of the superlattice spots further confirm the triple-period in the contrast
modulation. TP-A domains are of good crystalline quality
with domain widths of about 20–50 nm in the ½1 1 1 or ½111
direction. In contrast, the TP-B domains look tangled and
each individual domain has a width of at most about 10 nm
in ½111 or ½111 direction. By direct measurement of the
plane distances, we find that the modulation periodicity for
different domains varies between 9.97 and 10.05 Å in both
TP-A and TP-B type domains, which is slightly larger than
three times the distance of {111} planes in GaAs. The fact
that the lattice plane distances are domain dependent is consistent with the observation that the superlattice spots in the
SAED pattern (i) are not at fixed positions in the TP-A type
domains, and (ii) are streaky-like in the TP-B type domains.
Moreover, it is possible to obtain information on the
chemical composition of the TPO domains using HAADF;
since in this imaging technique, the signal intensity is proportional to the total projected atomic number Z.10 Due to
the large difference in Z between the different elements in
GaAsBi (ZBi ¼ 83, ZGa ¼ 31, and ZAs ¼ 33), a qualitative
insight into the Bi distribution of the ordered phase can be
directly obtained from the HAADF image contrast.
Representative high-resolution HAADF images from TP-A
and TP-B type domains are shown in Figs. 2(c) and 2(f),
respectively. As is clearly seen, the ordered domains appear
bright in the image, indicating a higher Bi content in the
TPO region compared to the GaAsBi alloy phase. Moreover,
the HAADF intensity is modulated by a triple-period, as is
clearly evidenced by the presence of extra superlattice spots
in the Fourier transform of the HAADF images [cf. insets in
Figs. 2(c) and 2(f)]. Note that, despite the different physical
mechanisms (i.e., different electron interaction processes)
operating in HAADF, SAED, and HRTEM, in all cases the
superlattice spots are located at similar positions, at about
6 13 ; 6 23 from the main zincblende GaAs reflections. Hence,
the triple period modulation manifests itself both in the structure [Figs. 2(b) and 2(e)] and in the chemistry [Figs. 2(c) and
2(f)].
In order to get a detailed insight into the chemical composition of the TPO areas, we perform line profiles from the
high-resolution Z-contrast images perpendicular to the ordering planes as illustrated by the blue box and the arrow in Fig.
2(c). The line profile integrates over 100–150 pixels which
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Wu et al.
Appl. Phys. Lett. 105, 041602 (2014)
FIG. 2. Observation of TP-A and TP-B type ordering variants. The micrographs in (a)–(c) and (d)–(f) are recorded near or along the ½
110 and [110] zone axes,
respectively. (a) and (d) are g004 dark-field TEM images, revealing the V-shaped or “semi” V-shaped domains corresponding to TPO near the surface region.
The insets in (a) and (d) are the SAED patterns corresponding to the regions marked by the red circles. The superlattice spots marked by arrow heads evidence
the presence of TPO. HRTEM micrographs of TP-A and TP-B ordered domains are shown in (b) and (e), respectively. (c) and (f) are high-resolution HAADF
images of the TP-A and TP-B ordered domains, respectively. The bright contrast in the TPO domains in the HAADF images indicates the higher Bi content in
the TPO area. The triple-period is resolvable by the contrast modulation in the {111} planes, as marked by arrows. The triple-period modulation in both the
structure [(b) and (e)] and the chemical composition [(c) and (f)] is confirmed. The blue box in (c) illustrates how the line profiles in Fig. 3 are obtained.
correspond to about 10 III-V dumb-bells. In the STEM
images, it is not possible to distinguish between the group III
and group V sites. Nonetheless, it should be reasonable to
assume that Ga atoms occupy only the III sites and As and
Bi atoms take only the V sites. As can be clearly seen in Fig.
3, depending on the TPO domain, there are one or two planes
(marked by red arrows) displaying a higher intensity within
the triple-period. Furthermore, the higher-intensity contrast
follows a three-fold periodicity (cf. Fig. 3). Also note that
the line profiles performed in the area outside the TPO domain do not exhibit any preferential distribution in the intensity contrast (gray arrows in Fig. 3), suggesting that all
planes have a similar composition as expected for a random
GaAsBi alloy. The peak intensity sequence in three-fold periodicity in the intensity profile suggests that: (i) the TPO
domains contain at least one Bi-rich {111} plane and (ii) the
chemical composition of different TPO domains is slightly
different. Taking into consideration that Bi has a larger covalent radius compared to Ga and As, the increased (local)
accumulation of Bi atoms at TPO domains will result in a
larger lattice plane distance compared to the random GaAsBi
phase. This, in turn, would lead to a shift of the superlattice
reflections in SAED from the exact 6 13 ; 6 23 positions of the
main zincblende reflections and to larger plane distances in
the TPO area (cf. HRTEM results), as experimentally
observed. Therefore, HAADF results are consistent with
those extracted from SAED and HRTEM. Based on the
FIG. 3. Line profiles across TP-A and TP-B type domains obtained from
high-resolution HAADF (Z-contrast) images. Red arrows indicate the
Bi-rich planes. As observed, there are one or two Bi-rich planes within the
triple-period sequence. Grey arrows mark the intensity distribution in the
line profiles from the GaAsBi alloy phase. A proposed stick-and-ball model
for the TP-A and TP-B type ordered domains is presented in the bottom
panel. The red, green, and blue balls represent different Bi occupation sites
in the group V site sub-lattice; grey atoms are Ga.
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Wu et al.
above information, we propose an atomistic model for the
TP-A and TP-B type domains, as schematically depicted in
the bottom panel of Fig. 3. In the models, the Ga atoms take
the group III sites and As and Bi atoms share the group V
sites, in which the Bi occupation is ordered in triple-period
sequence along the ½1 1 1 and ½111 directions (TP-A) and
½
111 and ½1
11 (TP-B) directions, respectively.
The occurrence of TP-A has been directly linked to the
presence of surface reconstructions during growth with n 3
period. Our experimental results (of detecting both TP-A
and TP-B type variants) suggest the simultaneous presence
of domains with mixed (n 3) and (3 n) surface reconstructions. Note that (3 n) has not been reported in
GaAsBi(001) so far. We have experimentally observed,
using reflection high-energy electron diffraction (RHEED)
during growth of GaAsBi epilayers at a substrate temperature of 380 C, the presence of faint features indicating a
n 3 (or 3 n) reconstruction during growth. However, due
to the setup limitations in the MBE chamber, we could not
clearly identify similar features at the lower growth temperature of 220 C. Interestingly, recent theoretical work on the
stability and configuration disorder of surface reconstructions on Bi-terminated GaAs(001) surfaces predict a multiple
stability of surface reconstruction in this system.13 In particular, Duzik et al. predict that at chemical potentials with
nearly equal number of Bi and As atoms, there are many
(n 3) configurations which are close in energy, whereas the
dimers disorder among the individual (n 3) surface units
may promote the formation of Bi clusters and/or ordering.
Further work is nevertheless needed to clarify the actual origin of the TPO observed in LT-GaAsBi.
In conclusion, we report the intriguing observation of
TPO domains of both TP-A and TP-B variants in
LT-GaAsBi layers, which is an amendment to previous studies on atomic ordering in III-V alloys where only the TP-A
variant was observed. TP-A domains dominate, are bigger in
size and show a clearer ordering in the group V sub-lattice
compared to the TP-B type, which are smaller in size.
Z-contrast images clearly reveal the existence of atomic
ordering for both TP-A and TP-B variants and evidence the
Appl. Phys. Lett. 105, 041602 (2014)
presence of alternating sequences of Bi-rich {111} planes
with a three-fold periodicity.
The authors acknowledge financial support from the
COST Action MP0805. J.P. and M.G. acknowledge the
financial support from the Academy of Finland via the
HIGHMAT Project (Ref. 259111). The authors thank Joonas
Hilska for help in sample growth, Doreen Stefen for the help
in preparing TEM samples, and Professor Thomas F. Kuech
for helpful discussions. The authors thank Dr. Xiang Kong
for critical reading of the manuscript.
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