Reconstruction of an InP (001) surface grown by metalorganic vapor phase epitaxy in atmospheric hydrogen environment

Article abstract of DOI:10.1063/1.1289034

A reconstructed surface of InP (001) substrate, grown by metalorganic vapor phase epitaxy under atmospheric hydrogen environment, is investigated by using grazing incident x-ray diffraction. Fractional-order diffractions of (n/2 m) were observed, showing the existence of a (2 X 1) domain on the surface. Calculations based on the P-dimer model suggest that there are P dimers whose bonding is parallel to the [110] direction and indium displacement in the second layer.

Full text of DOI:10.1063/1.1289034

Reconstruction of an InP(001) surface grown by metalorganic vapor phase epitaxy in
atmospheric hydrogen environment
T. Kawamura, Y. Watanabe, Y. Utsumi, K. Uwai, J. Matsui, Y. Kagoshima, Y. Tsusaka, and S. Fujikawa
Citation: Applied Physics Letters 77, 996 (2000); doi: 10.1063/1.1289034
View online: http://dx.doi.org/10.1063/1.1289034
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APPLIED PHYSICS LETTERS
VOLUME 77, NUMBER 7
14 AUGUST 2000
Reconstruction of an InP„001… surface grown by metalorganic
vapor phase epitaxy in atmospheric hydrogen environment
T. Kawamura,a) Y. Watanabe, Y. Utsumi,b) and K. Uwaic)
NTT Basic Research Laboratories, Wakamiya, Morinosato, Atsugi 243-0198, Japan
J. Matsui, Y. Kagoshima, Y. Tsusaka, and S. Fujikawa
Himeji Institute of Technology, Kouto, Kamigori, Ako 678-1297, Japan
͑Received 15 March 2000; accepted for publication 20 June 2000͒
A reconstructed surface of InP ͑001͒ substrate, grown by metalorganic vapor phase epitaxy under
atmospheric hydrogen environment, is investigated by using grazing incident x-ray diffraction.
Fractional-order diffractions of (n/2 m) were observed, showing the existence of a (2ϫ1) domain
on the surface. Calculations based on the P-dimer model suggest that there are P dimers whose
¯
bonding is parallel to the 110 direction and indium displacement in the second layer. © 2000
͓
͔
American Institute of Physics. ͓S0003-6951͑00͒02733-9͔
Owing to the importance of InP-based optical devices,
the surface structures of InP͑001͒ substrates have been inves-
tigated by several techniques, including electron probes, and
photon probes in an ultrahigh vacuum environment. The re-
sults revealed the presence of several dimer structures, in-
cluding (2ϫ4), (4ϫ2), (4ϫ1), and c(8ϫ2) structures, on
In-rich surfaces.1–3
combined goniometer/reactor system.10 A z-axis arrange-
ment is a good solution for satisfying both the x-ray mea-
surement and growth conditions.9 To obtain an x-ray source
of sufficient brightness, all instruments were set up at the
BL24XU experimental station of the SPring-8 synchrotron
radiation facility.
The InP epitaxial growth, using trimethylindium ͑TMI͒
and tertiarybutyl phosphine ͑TBP͒ as precursors, was carried
out in a reactor chamber set on the goniometer. Exactly ori-
ented InP ͑001͒ Fe-doped semi-insulating, superclean wafers
were used as substrates. The substrate was placed on the
sample stage. Then, after desorption of the oxide at 550 °C
under TBP overpressure, a 0.12 ␮m InP layer was formed on
the InP substrate in 15 min. The substrate was then annealed
at 350 °C for 1 h in a 76 Torr H2 flow for desorption of
excess phosphines and alkyl species. Figure 1 shows a sche-
matic diagram of the growth process. The flow rates of TMI
and TBP were 3.2ϫ10Ϫ6 and 2.9ϫ10Ϫ4 mol/min, respec-
tively. The ͓V͔/͓III͔ ratio was 90.6. Film quality was checked
using atomic force microscopy ͑AFM͒ after growth. The
AFM images showed large terrace structures with 0.29 nm
step heights, which corresponded to a monolayer height of
InP ͑001͒ crystal, which suggested that the grown surface
was of high quality.
Optical techniques, such as reflection anisotropy spec-
troscopy and surface photoabsorption, which detects the
dimer bond configuration at the surface, have been used for
characterizing surfaces during metalorganic vapor phase ep-
itaxy ͑MOVPE͒. P-rich surfaces formed by MOVPE and
chemical beam epitaxy have been investigated through a
combination of optical and electron-based probes. These sur-
faces show similar reconstructed structures, including (2
ϫ1) and (2ϫ2).4–6 More recently, a combination of
MOVPE and scanning tunnel microscopy revealed (2ϫ2)
and c(4ϫ2) structures on P-rich surfaces.7,8 However, be-
cause of the difficulties inherent in using electron-based
techniques in the growth environment and the lower spatial
resolution of optical techniques, atomic structures on recon-
structed surfaces in the gas environment are still uncertain.
The grazing incident x-ray diffraction technique has re-
cently been adapted for use in analyzing surface structures in
the gas-phase environment.9 The penetrating nature of x ray
allows one to observe the diffractions from superstructures
on the surface under an atmospheric environment. Addition-
ally, the small amount of interference between x rays and
materials makes it easy to interpret the measured data. These
features offer the possibility of direct determination of
atomic structures on surfaces. In this letter, we report on a
grazing x-ray diffraction experiment on a reconstructed
InP͑001͒ surface grown using MOVPE under an atmospheric
hydrogen environment.
To meet the requirements of both x-ray diffraction mea-
surement and MOVPE epitaxial growth, we developed a
a͒
Author to whom correspondence should be addressed; electronic mail:
kawamura@will.brl.ntt.co.jp
FIG. 1. Schematic diagram of epitaxial growth before the x-ray measure-
ment. The x-ray measurement was performed at room temperature under a
31 Torr H2 flow.
b͒
Present address: Himeji Institute of Technology.
c͒
Present address: NTT Communications.
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0003-6951/2000/77(7)/996/3/$17.00 996 © 2000 American Institute of Physics
129.120.242.61 On: Thu, 27 Nov 2014 11:06:00

Appl. Phys. Lett., Vol. 77, No. 7, 14 August 2000
Kawamura et al.
997
FIG. 2. Intensity of fractional Bragg reflection for each reciprocal lattice
point.
After annealing, the surface structure was characterized
by using grazing incidence x-ray diffraction in a 31 Torr H2
1
flow. Here we note that 2(220) and 12(220) in reciprocal
¯
space were used as the ͑10͒ and ͑01͒ unit vectors of surface
structure, respectively. To decrease the background noise,
which arises from the bulk crystal, the incident angle of x
rays was set to 0.2°, which corresponds to the critical angle
of InP crystal. During measurement, we used 0.06 nm x rays,
which correspond to the second peak of the undulator source
at the BL24XU beamline. Upstream of the goniometer, the x
rays were monochromatized by ͑220͒ reflection of a set of
diamond crystals and collimated as 0.1 mm ͑V͒ and 1.0 mm
͑H͒ with x– y slits. To eliminate the first-order x rays and
scattering from the paths, a 0.5 mm aluminum sheet was set
at the entrance port of the experimental hutch. A NaI detec-
tor and a solar slit with 0.2° divergence was used to detect
Bragg diffractions from the surface structures. Typical mea-
surement time was 15 min for each diffraction point, and the
measured area in reciprocal space ranged from ͑0 0͒ to ͑2 2͒,
owing to the limited beam time.
FIG. 3. Measured and calculated structure factors for fractional-order dif-
fractions along ͑3/2x͒. The inset in ͑a͒ represents the model used in the
calculations. Numerically calculated atomic scattering factors of phosphine
and indium atoms were used in the calculations. Dispersion parts of the
scattering factor were omitted from the calculation.
on the intensity variation, we calculated the structure factors
of the P-dimer model. Figures 3͑a͒, 3͑b͒, and 3͑c͒ show mea-
sured and calculated structure factors of a (2ϫ1) domain.
The inset of Fig. 3͑a͒ shows the model used in the calcula-
tions. Variables x and y represent the atomic displacement of
¯
phosphine and indium along 110 direction in a fundamen-
͓
͔
tal cell unit. The dependence of sin ␪/␭ on atomic scattering
factors was included with an approximate numerical
expression.11 The dispersion parts of the atomic scattering
factors were omitted because the wavelength was far from
the absorption edge of phosphine and indium atoms.
Figure 2 shows the relative x-ray intensity for each dif-
fraction point. Open circles represent points that were omit-
ted during the measurement because of the overlapping of
bulk diffractions. Solid circles represent observed diffraction
points, and the diameter of each circle is proportional to the
measured intensity. The X’s represent the lattice points
where no diffraction was observed.
As shown in Fig. 3͑b͒, diffractions from the phosphine
atoms composing the buckled dimer only produce monotoni-
cally decreasing structure factors along ͓110͔. A similar ten-
dency was observed after changing the value of x to 0.4, and
it is difficult to explain the lack of diffraction at ͑3/2 1͒.
When the displacement of indium atoms at the second layer
was added into the calculation, the structure factors changed
dramatically ͓Fig. 3͑c͔͒. Variation in the structure factors
¯
Half-order diffractions along the 110 direction were
͓
͔
¯
along 110 qualitatively matched that in the measured ones,
͓ ͔
observed, showing 2ϫ periodicity. Additionally, no frac-
tional orders except ͑3/2 1/2͒ were observed along the ͓110͔
direction. These results suggest the existence of (2ϫ) do-
mains on the reconstructed surface. Considering the surface
preparation process, we believe the 2ϫ periodicity was
with only a 2% change in the indium atoms positions. Note
that these values were not optimized to match the measured
data, and further analysis is required for determining the sur-
face structure.
In conclusion, the properties of an InP ͑001͒ surface pre-
pared by MOVPE and annealed at 350 °C in an H2 environ-
ment were investigated by using grazing x-ray diffraction.
The results revealed (2ϫ1) domains on the reconstructed
InP ͑001͒ surface in a hydrogen atmospheric environment. A
comparison of the observed and calculated structure factors
suggests the existence of P dimers and indium atom displace-
¯
caused by P dimers whose bonding is parallel to the 110
͓
͔
direction.
Another feature is that some diffractions of half orders,
such as ͑1/2 1͒, ͑3/2 1͒, are missing except ͑3/2 1/2͒. ͑We
concluded that this peak resulted from the scattering of bulk
diffractions because the diffraction angle and the peak profile
were different than expected from the surface structure.͒ To
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estimate the effects of local arrangement in a (2ϫ1) domain
ment in the second layer.
129.120.242.61 On: Thu, 27 Nov 2014 11:06:00

998
Appl. Phys. Lett., Vol. 77, No. 7, 14 August 2000
Kawamura et al.
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