Supporting Data
Lattice strains and polarized luminescence in homoepitaxial growth of a-plane ZnO
Hiroaki Matsui 1, 2 and Hitoshi Tabata 1, 2
1
Department of Bioengineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
2
Department of Electronics Engineering, The University of Tokyo, Bunkyo-ku,
Tokyo 113-8656, Japan
1
1. Surface and structural properties of ZnO substrate annealed at the high temperature.
(a)
500 nm
(b)
[1-100]
[0001]
<0001>
[11-20]
18.2 nm
(c)
50 nm
5 nm
r-plane
[11-20]
[0001]
[1-100]
Figure S1. (a) AFM image (2 × 2 mm2) and (b) RHEED pattern with the [0001] azimuth of the
Crystec ZnO substrate. (c) Cross-section [1-100] TEM image of the Crystec ZnO substrate. The
inset figure represents a high-magnification TEM image.
Figure 1 shows the atomic force microscopy (AFM) image of the Crystec ZnO substrate annealed at
the high temperature. The V-grooved structure was aligned along the [1-100] direction with a high
density of 106 cm-1, resulting in a reflection high energy electron diffraction (RHEED) pattern with
three-dimensional (3D) spots [Fig. 1(b)]. From the cross-section transmittance electron microscopy
(TEM) image with the [10-10] azimuth, the V-groove was triangle-shaped in cross section with a step
and terrace structure [Fig. 1(c)]. The plane angle between the side facets and (11-20) plane was 32o,
which was equivalent with the r-planes (1-102) of ZnO. This is due to the fact that the a-plane is the
roughest surface of the four low-index planes because of the high surface cleavage energy 1.
2
2. Surface morphologies of a-plane homoepitaxial ZnO layers grown at different temperatures
(a)
500 nm
(b)
500 nm
(c)
500 nm
[0001]
[0001]
[0001]
4.28 nm
3.40 nm
2.51 nm
Figure S1. Surface morphologies of a-planes homoepitaxial ZnO layers grown at (a) 400, (b) 550 and (c)
700oC.
The a-plane ZnO layers were grown at different temperatures on the (11-20) plane of Crystec ZnO
substrates by a pulse laser deposition (PLD) method. All layers showed smooth surface morphologies
at growth temperatures of 400, 550 and 700oC. However, slight roughening was found at a low
temperature of 400oC, which may be related to surface migrations of adatoms during layer growth. For
all ZnO layers, the elongated layer surfaces were observed along the c-axis direction. Therefore, the
nanostripe structure obtained on the m-plane homoepitaxial ZnO layers was not observed on the
a-plane ZnO layers at temperatures from 400 to 700oC. In contrast, the m-plane homoepitaxial ZnO
layers showed surface morphologies with nanostripes along the c-axis direction, which were observed
at temperatures from 400 to 600oC 2, 3. Therefore, the AFM results are associated with the difference in
growth evolution between a- and m-plane homoepitaxial ZnO layers. A flat layer surface can be
realized with homoepitaxial growth of a-plane ZnO when using a PLD method.
3
3. Structural characteristics of basal stacking faults (BSFs) and partial dislocations (PDs)
Table SI Summary of BSFs and PDs in a wurtzite structure along with their extinction rules
Stacking
I1-type
I2-type
E-type
ABABCBCBC
ABABCACAC
ABABCABAB
1/6<20-23>
1/3<10-10>
1/2<0001>
sequence
Burger Vector
BSF
PD
BSF
PD
BSF
PD
g = [0002]
Out-of
Contrast
Out-of
Out-of
Out-of
Contrast
contrast
contrast
contrast
g = [10-10]
Contrast
Contrast
Contrast
Contrast
Out-of
Out-of
Contrast
Contrast
contrast
Dislocation-type
Partial
Shockley partial
Frank partial
(Frank-Shockley)
Basal-plane stacking faults (BSFs) in a wurtzite structure are treated as planar defects that locally
generate the ABC cubic structure within the usual …ABABAB… stacking sequence
4, 5
. BSFs exist
two intrinsic (I1 and I2)-types and one extrinsic (E)-type. The I1-type BSF is produced by removal or
insertion of a basal plane. This fault is changed from the perfect stacking sequence of …ABABAB…
to …ABABCBCBC…, which is bound by a sessile Frank-Shockley dislocation with a Burgers vector
b = 1/6<20-23>. In contrast, the I2-type BSF having the …ABABCACAC… stacking sequence is
formed by dissociation of a perfect dislocation (b = 1/3<11-20>) into two Shockley partial dislocations
with b = 1/3<10-10>. Finally, the E-type BSF with a stacking sequence …ABABCABAB… appears
to be due to insertion of an extra basal plane. This type of fault is bound by Frank partial dislocations
with b = 1/2<0001>. The energy of BSFs is proportional to the number of cubic bilayers in the
particular fault. The I1, I2 and E-type defects consist of one, two, and three cubic bilayers, respectively.
The I1-type BSF has the lowest formation energy. The highest formation energy is required to form the
E-type BSF. The abovementioned observations can be applied to ZnO with a wurtzite structure.
4
4. Structural properties of different kinds of ZnO substrates
Table SII. Lattice parameters and crystallinity of Crystec and Goodwill ZnO substrates
Structural properties
Crystec ZnO
Goodwill ZnO
a-axis length
3.249 Å
3.250 Å
c-axis length
5.206 Å
5.208 Å
x // (11-20) plane
34 arcsec
101 arcsec
z // (0002) plane
36 arcsec
143 arcsec
y // (10-10) plane
36 arcsec
113 arcsec
Lattice parameters
FWHM of -rocking curve
Table S III. Residual impurities of Crystec and Goodwill ZnO substrates
Substrate type
Residual impurities
Crystec ZnO
Si < 8 ppm, Fe < 4 ppm, Mg < 3 ppm, Ca < 1 ppm
Goodwill ZnO
Mg < 5 ppm, Al < 20 ppm, Si < 2 ppm, Ti < 10 ppm
Cr < 15 ppm, Ca < 1 ppm, Ag < 2 ppm
Homoepitaxial layer growth is affected by the presence of defects in the substrate such as strain
variation, grain boundaries, etc. The low substrate quality of a substrate results in the formation of the
unusual strain, which is correlated with the lattice constant 6. The difference in lattice constants (a- and
c-axis lengths) between Crystec and Goodwill ZnO substrates was as small as 0.03%. The strains of
ZnO layers on the Goodwill ZnO substrate are not attributed to lattice mismatch between the layer and
substrate. The full-width half-maximum (FWHM) of -rocking curves of the (11-20) plane for the
Crystec ZnO substrate was as narrow as 34 arcsec. The FWHM values of the (0002) and (10-10)
planes along the in-plane directions also included 36 arcsec, which revealed that the Crystec ZnO
substrate was a uniform crystal. On the other hand, the FWHM values of the (11-20), (0002) and
(10-10) planes for the Goodwill ZnO substrate were three times larger than those for the Crystec ZnO
5
substrate, which indicated the presence of a degraded surface layer. Moreover, residual impurities in a
substrate as shown in Table SIV were higher on the Crystec ZnO substrate than the Goodwill ZnO
substrate. Therefore, it is concluded that the structural quality was better on the Crystec ZnO substrate
than the Goodwill ZnO substrate.
5. Optical properties of Crystec and Goodwill ZnO substrates
T = 300 K
60
40
20
0
1.5
Crystec ZnO
(b)
T = 10 K, H = 1T
Crystec ZnO
Goodwill ZnO
0
-200
-400
Goodwill ZnO
2
2.5
3
Photon energy (eV)
200
MCD (mdeg/cm)
(a)
80
MCD (mdeg/cm)
Transmittance (%)
100
400
200
0
-200
-400
-1.5 -1 -0.5
0
0.5
1
1.5
Magnetic field (Oe)
3.5
1.5
2
2.5
Photon energy (eV)
3
Fig. S2. (a) Transmittance spectra and (b) MCD spectra of Crystec and Goodwill ZnO substrates.
Transmittance spectra were measured at 300 K. MCD spectra were taken at 10 K under a magnetic field
of 1 T. Inset of Fig. S2(b) shows the MCD response at 2.82 eV under magnetic fields.
Transmittance spectra taken at 300 K of Crystec and Goodwill ZnO substrates are shown in Fig.
S2(a). A decrease of transmittance was observed on the Goodwill ZnO substrate with a photon energy
from 2.5 to 3.1 eV, indicating the existence of color centers in the Goodwill ZnO substrate. Figure
S2(b) shows magnetic circular dichroism (MCD) spectra of Crystec and Goodwill ZnO substrates. The
MCD is the differential absorption of the sample for right and left circularly polarized light that
propagates along the direction of the applied magnetic field. It is experimentally determined from the
intensities of the transmitted light of polarization ±, namely,
MCD = const (I+ - I-) / (I+ - I-)
(2)
6
The MCD spectra of optical absorption show a very specific fingerprint of the paramagnetic charge
state in a host
7, 8
. This is in contrast to electron paramagnetic resonance, which indicates the presence
of an unpaired electron in a point defect. The MCD at 2.83 eV showed a linear magnetic dependence
because an unpaired electron behaves as s magnetic spin. The transmittance and MCD spectra revealed
that point defects with unpaired electrons were formed in the Goodwill ZnO substrates.
PL intensity (a.u.)
T = 10 K
DoX
DoX
Goodwill ZnO
FAX
FBX
Fig. S3. PL spectra at 10 K for an E⊥c configuration
Crystec ZnO
of the Crystec and Goodwill ZnO substrates.
3.1
3.2
3.3
3.4
Photon energy (eV)
3.5
Photoluminescence (PL) spectra taken at 10 K of Crystec and Goodwill ZnO substrates are shown
in Fig. S3. A- and B-excitonic light emissions (FAX and FBX) were observed at 3.377 and 3.384 eV in
the Crystec ZnO substrate, respectively, which was consistent with a previous report 9. Moreover, a
sharp donor-bound exciton (DoX) peak was obtained with a linewidth of 4.3 meV. On the other hand,
the Goodwill ZnO substrate showed a broad DoX peak with a linewidth of 17 meV. We could not
clearly observe free-related emissions. Figures S2 and S3 show that the optical properties of the
Crystec ZnO substrate were of a higher quality than those of the Goodwill ZnO substrate, which was
consistent with the results regarding structural quality.
7
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