SUPPLEMENTARY INFORMATION
Surface energy-mediated construction of anisotropic semiconductor wires
with selective crystallographic polarity
Jung Inn Sohn1§, Woong-Ki Hong2§, Sunghoon Lee3, Sanghyo Lee4, JiYeon Ku3, Young Jun
Park3, Jinpyo Hong4, Sungwoo Hwang3, Kyung Ho Park5, Jamie H. Warner6, SeungNam
Cha1*, Jong Min Kim1*
1
Department of Engineering Science, University of Oxford, Oxford OX1 3PJ, UK
2
Jeonju Center, Korea Basic Science Institute, Jeonju, Jeollabuk-do 561-180, South Korea
3
Frontier Research Lab., Samsung Advanced Institute of Technology, Yongin, South Korea
4
Department of Physics, Hanyang University, Seoul, South Korea
5
Korea Advanced Nano Fab Centre, Suwon, South Korea
6
Department of Materials, University of Oxford, Oxford OX1 3PH, UK
Figure S1 Structural models of the polar
and the non-polar
and
surfaces without and with Ga atoms replacing surface Zn atoms, respectively.
The red, blue, and yellow-brown spheres indicate O, Zn, and Ga atoms, respectively. As
expected, our first-principles calculations based on the density functional theory show that for
pure ZnO the surface energy of the polar
polar
and
plane is much larger than that of the non-
planes, leading to a polar growth direction with equivalent non-
polar side facets, which are low-energy surfaces. Interesting, in contrast with surface energies
of pure ZnO, it is found that the polar
surfaces such as
and
surface can be more stable than the non-polar
when surface Zn atoms are replaced by Ga atoms.
This implies that a Ga atom could be an attractive foreign species to allow the electrostatic
energy gain at the polar surface, resulting in lowering of surface energy of the polar plane.
Figure S2 ZnO wires grown by using ZnO powder mixed with a GaAs wafer as a source
material. a, An optical microscopy image of ultra-long ZnO wires as-grown in an alumina
boat, which was loaded into a quartz reaction tube located in a high-temperature tube furnace.
It is noted that ZnO wires with lengths of several millimeters were formed. b, XRD patterns
of ZnO wires confirming that wires are wurtzite structured ZnO. All peak position in XRD
spectra obtained at room temperature are indexed unambiguously to hexagonal phase
crystalline ZnO (JCPDS file 36-1451).
Figure S3 Statistical analysis of ZnO wires with polar and nonpolar growth directions.
It is observed that the geometrical morphology of ZnO wires is significantly affected by the
relative amount of GaAs source materials. As the relative amount of GaAs sources increase,
ZnO wires with non-polar growth directions become dominant. On the other hand, ZnO wires
grow preferentially along the polar growth direction with decreasing the relative amount of
GaAs sources. These results suggest that the deterministic control of crystallographic
orientations of ZnO wires can be achieved by modifying surface energy through the
introduction of the foreign atoms.
Figure S4 SIMS and XPS measurements. a, The SIMS depth profiles of ZnO wires grown
along the non-polar direction, confirming the presence of Ga atoms. The inset shows the
SIMS depth profile of ZnO wires with the polar growth direction for comparison. Here, note
that the profile shows the almost constant concentration of Ga atoms, which is the average
information of Ga atoms favored at the surface, because we carried out measurements of
ensembles of ZnO wires mechanically transferred to Si substrates. b, SEM images of
representative ZnO wires grown by using pure Ga source materials. The insets clearly show
non-hexagonal cross-sections of wires. c, XPS spectra of ZnO wires with the non-polar
growth direction. The survey spectrum (left) shows typical photoelectron peaks of ZnO
assigned to Zn- and O-derived bands. The XPS spectrum of Zn 2p regions (middle) shows a
doublet at 1022 and 1045.1 eV corresponding to the Zn 2p3/2 and 2p1/2 core levels, which are
in agreement with previous reported results obtained from pure ZnO. The O 1s band (right)
exhibits a large asymmetry, corresponding to Gaussian components centred at 530.1 and 532
eV. The lower binding energy spectrum of the O 1s might is associated to the O2- ions in the
wurtzite structure surrounded by the Zn atoms with the full supplement of nearest-neighbor
O2- ions. The shift of O 1s to a higher binding energy is attributed to the presence either of
point defects or chemisorbed oxygen or OH species. Therefore, it is clear from the SIMS,
SEM, and XPS results that the presence of Ga atoms incorporated into ZnO wires with a nonhexagonal cross-section has been proven, whereas Ga atoms have not been detected from
ZnO wires with the polar growth direction.
Figure S5 TEM images of another type of diamond ZnO wires. a, A TEM image obtained
from a cross-section of a diamond wire. b, The corresponding SAED pattern indexed to
wurtzite ZnO. These results reveal that the ZnO wire was also grown along a non-polar
direction, but exhibits a diamond cross-section consisting of four equivalent
side planes. It is noteworthy that the vertical direction of the diamond cross section of the
ZnO wire is perpendicular to the
non-polar plane. In addition, the measured angle
between equivalent side planes is 121°, which is in good agreement with the structural
orientation between two adjacent facets. c, A high-magnification SEM image, showing the
morphology of a ZnO wire with well-defined surface facets, which are consistent with TEM
results.
Table S1 Summary of Δγ by As or Ga incorporation at a 1/2 ML coverage at
,
, and
,
surfaces. Experimental growth conditions for rectangular-
shaped ZnO wires correspond to Zn-rich and Ga-rich conditions, because the growth
temperature of 910 oC is much higher than the boiling temperatures of oxygen molecular
liquids and metallic As. When tha Ga atoms replace surface Zn atoms with a half ML
coverage at the O-terminated
energy by 1.85 J/m2, whereas at
surface, the Ga incorporation lowers the surface
and
surfaces it increases the surface
energy by ~ 0.8 and 0.9 J/m2, respectively. The As incorporation replacing surface O atoms
also reduces the surface energy at the
surface, but the energy gain is too small (0.28
J/m2) to affect the energetic ordering among the surfaces. Thus, we believe that Ga atoms
play an important role in modifying surface energy.
Table S2 Summary of Δγ by As or Ga incorporation at 1 ML calculated at
,
, and
surfaces. The
,
surface with 1 ML coverage of Ga
incorporation is less stable than that with 1/2 ML of Ga, because the additional electrons from
Ga atoms could not fill low-energy O dangling-bond states which are fully filled at 1/2 ML
coverage. The As incorporation at
surface further lowers the surface energy at 1 ML
compared to 1/2 ML, but the surface energy gain is yet too small to affect the growth modes.
At
surface.
,
, and
surfaces, the Ga or As incorporation destabilizes the
Figure S6 Surface energy changes by Ga incorporation at the
surface. a,
Dependence of  on the depth of Ga incorporation. The surface energies are compared for
Ga incorporation at a 1/2 ML coverage at the
surface with varying position of Ga.
The Ga incorporation is most favored at the surface layer. The subsurface incorporation of Ga
is significantly less stable, suggesting out-diffusion of Ga atoms during growth. It is
important to note that substitutional Ga atoms favor surface sites over bulk sites by the
amount of energy changes, which is about ~ 1.0 J/m2 = ~ 1.2 eV/atom. This indicates that the
probability for Ga atoms to occupy surface sites is 105 times higher than that to occupy bulk
sites. Given this low probability for bulk incorporation of Ga atoms, it was anticipated that
the limited incorporation would be the basis of modest increase/decrease in defect sites. In
fact, the amount of Ga atoms was not a readily detectable level by XPS, EDS, and FE-AES
measurements. b, Surface energy lowering mechanism by Ga substitution. An additional
electron from high-energy Ga 4p states transfers to low-energy dangling bond states derived
from surface oxygen atoms, attaining an electrostatic energy gain. The stable surfaces of IIIV or II-VI compound semiconductors have all the anion dangling bonds filled and all the
cation dangling bonds empty. However, at the bare
surface, each surface O atom has
a single dangling bond with one and half electron occupancy as depicted in Figure 3b. The
partial vacancy of oxygen dangling bonds, which is absent in bulk by charge transfer from Zn
4s states, is responsible for the high surface cleavage energy of the surface. ECR assures that
the electrostatic energy gain is maximized by the charge transfer from high-energy electronic
states of cations to low-energy states of anions. That is, additional electrons at high-energy
Ga 4p states fully fill the low-energy dangling bond states of surface O atoms by replacing
surface Zn atoms at half ML coverage. This allows the electrostatic energy gain and thereby
results in lowering the surface energy.
Figure S7 Effects of Ga or As incorporation in the O-terminated
Atomic structures of clean, Ga-incorporated, and As-incorporated
surface. a-c,
surfaces,
respectively. The red, blue, yellow-brown, and violet spheres represent O, Zn, Ga, and As
atoms, respectively. d-f, Respective surface band structures of the atomic structures shown in
a-c. Gray regions indicate electronic states derived from bulk atoms. Square symbols indicate
the electronic states that have a substantial contribution from surface atoms. Larger symbols
reflect higher contribution. The clean surface exhibits a partially-empty (1/2 per bond)
surface band originating from surface O atoms. When surface Zn atoms are replaced by Ga at
a 1/2 ML coverage, the surface band is fully filled and stabilized by ~1 eV, leaving no surface
states in the band gap. On the other hand, when surface O atoms are replaced by As, a
number of surface states located below the valence band edge at the clean surface become
destabilized because of the structural distortion by the large atomic size of As and the much
reduced electronegativity of As compared to O.
Figure S8 Effects of Ga or As incorporation in the Zn-terminated
Atomic structure of clean, Ga-incorporated, and As-incorporated
surface. a-c,
surfaces,
respectively. The red, blue, yellow-brown, and violet spheres represent O, Zn, Ga, and As
atoms, respectively. d-f, Respective surface band structures of the atomic structures shown in
a-c. Gray regions indicate electronic states derived from bulk atoms. Square symbols indicate
the electronic states that have a substantial contribution from surface atoms. Larger symbols
reflect higher contribution. The clean surface exhibits a partially-filled (1/2 per bond) surface
band originating from surface Zn atoms. When surface Zn atoms are replaced by Ga at a 1/2
ML coverage, the Ga-derived surface band is almost fully filled. However, similar to the Asincorporated
surfaces, when surface O atoms are replaced by As atoms, the band
structure of As-incorporated (0001) surfaces becomes destabilized because of the reduced
electronegativity and the bigger atomic size of As compared to O atoms.
Figure S9 Surface energy change calculations. a, Summary of surface energy changes at
various surfaces before and after the introduction of substitutional group III and II atoms. It is
found that the incorporation of group III elements, such as In, Ga, and Al atoms, can give a
much higher energy gain at the polar
surface compared with other surfaces. In
incorporation shows relatively small surface energy lowering compared to Ga or Al atoms
due to the steric effect resulting from the large ionic size of In. We also further examined the
effect of group II elements (Cd, Ca, and Mg) incorporation and found that group II elements
change overall surface energies, that is, increasing or decreasing at all the considered surfaces,
not altering the order of surface stability. This implies that the incorporation of group II
elements could not change the growth direction of ZnO wires but the morphology such as the
surface to volume ratio. b, SEM images of representative ZnO wires grown by using pure Al
source materials. The inset shows non-hexagonal cross-sections of a ZnO wire. Similar
results to Ga atoms achieved for the growth of ZnO wires along non-polar directions were
expect. In fact, we have achieved similar control over the growth orientation of ZnO wires by
modulating surface energy with Al elements.
Figure S10 The effects of the polarization field on optical characteristics of ZnO wires. a,
Temperature-dependent CL emission energy of a single ZnO wire grown in different polar
(filled blue circles) and non-polar (filled red circles) directions, respectively. The inset shows
representative room-temperature CL spectra obtained from two different types of ZnO wires,
that is, polar and non-polar. It is clearly observed that the optical emission from the polar
plane exhibits the red shift of ~ 100 meV as compared with the non-polar plane. This is
consistent with PL results shown in Figure 4a. b, Representative cross-sectional CL
spectroscopy obtained from a non-polar surface plane of a ZnO wire. The inset shows a
cross-sectional SEM image of a ZnO wire with a diamond cross section, which is a non-polar
surface, prepared using a focused ion beam. This result clearly reveals that transition energy
of a polar surface is red-shifted compared to that of a non-polar surface.