Radiation induced modification in nanoscale hardness of ZnO cone
Rupali Nagar,1R. Teki,2N. Koratkar,3V. G. Sathe,4D. Kanjilal,5B. R. Mehta,1and
J. P. Singh1,a?
1Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
2Department of Chemical & Biological Engineering, Rensselaer Polytechnic Institute, 110 8th St., Troy,
New York 12180, USA
3Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute,
110 8th St., Troy, New York 12180, USA
4UGC-DAE, Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India
5Inter-University Accelerator Centre, Aruna Asaf Ali Marg, Post Box 10502, New Delhi 110067,
?Received 23 June 2010; accepted 23 July 2010; published online 21 September 2010?
In this paper, the effect of ion irradiation on nanoscale hardness of ZnO microcones is reported. The
hardness of ZnO cones determined by nanoindentation using atomic force microscope initially
increases from 4.7?1.4 to 9.5?1.6 GPa after irradiation with 1.2 MeV Ar+8ions at an ion fluence
of 1015ions cm−2and then decreases with increasing ion fluence. This change in mechanical
hardness has been correlated with the residual stress of the sample revealed by Raman peak shift in
the E2?H? mode. These results show that the generally reported radiation-hard nature of ZnO
depends critically on irradiation conditions, especially the irradiation temperature. © 2010
American Institute of Physics. ?doi:10.1063/1.3482026?
The unique properties of zinc oxide ?ZnO? in nanowire,
nanorod, or nanobelt form make this material a potential can-
didate for piezoelectric, spintronic, and optoelectronic
applications.1–3ZnO is a relatively soft semiconducting ma-
terial as compared to conventional semiconductors like Si
and GaN. For practical realization of ZnO-based devices, it
is important that the mechanical properties of ZnO be tested,
particularly at nanoscale, for reliable performance and long
device life times. Mechanical properties of ZnO nanostruc-
nanoscale.4–8For instance, in situ transmission electron mi-
croscopy has been used to study the mechanical resonance of
a single ZnO nanobelt induced by an alternating electric field
and the bending modulus was measured to be ?52 GPa.7
The deformation behavior of bulk ZnO single crystals has
been studied by a combination of spherical nanoindentation
and atomic force microscopy ?AFM? techniques.9,10There
are few reports on the modification of mechanical properties
of ZnO thin films, achieved primarily by doping and control-
ling growth temperature.11,12Besides the aforementioned ap-
plications of ZnO-based devices, use of ZnO in radiation and
accelerator environments have also been cited following the
electrical and optical “radiation hardness” under high-energy
electron and ion beam irradiation.13,14The room-temperature
bombardment by electrons,13,15protons,16and heavy ions17
causes relatively less damage in ZnO than in other semicon-
ductors, e.g., Si, GaAs, or GaN.18This behavior in case of
ZnO has been attributed to the recombination of the vacancy-
interstitial pairs as soon as they were created, and it was
understood that permanent damage required a high enough
bombardment energy to move the interstitials far from the
vacancies.18Though there exist reports of the study of irra-
diation on the electrical and optical properties of ZnO, the
effect of radiation on the mechanical properties of ZnO
structures has not been explored so far. In this paper, the
effect of ion irradiation on mechanical properties of ZnO
ZnO microcones were grown by oblique angle dc mag-
netron sputter deposition on Si?100? substrates prepatterned
by silica spheres of diameters of about 500 nm. The angle of
incidence of the incident vapor flux with the substrate normal
was ?85°. The details of the growth can be found
elsewhere.19These ZnO cones were subjected to ion irradia-
tion by the Low Energy Ion Beam Facility at the Inter-
University Accelerator Centre, New Delhi, India.20The
samples were irradiated by 1.2 MeV Ar+8ion beam at differ-
ent ion fluences of 1015, 1016, and 1017ions cm−2under
10−6mbar vacuum. The samples were mounted on liquid
nitrogen cooled ??80 K? hollow copper block ladder. The
hardness of ZnO microcones was tested by a diamond
Berkovich-type tip ?DNISP, Veeco Probes, force constant
220 N m−1? attached in an AFM ?NanoScope IIIa, Veeco-DI,
USA?. The loading force was increased in steps and opti-
mized for performing the indentations until the indent marks
were visible. The samples were characterized by scanning
electron microscopy ?SEM? ?Zeiss Supra 55 FESEM? before
and after irradiation for morphological studies. Micro-Raman
?LABRAM HR-800 Jobin Yvon Horiba, Olympus BX-41
microscope with a charge-coupled device detector? with an
Ar ion laser of 488 nm wavelength was used at 5 mW power
a?Electronic mail: email@example.com.
JOURNAL OF APPLIED PHYSICS 108, 063519 ?2010?
0021-8979/2010/108?6?/063519/6/$30.00© 2010 American Institute of Physics
to record the Raman spectra for all the samples in a back-
scattering geometry. The spectra were recorded for 40 s du-
ration each. Transmission electron microscope ?TEM? and
energy dispersive x-ray analysis ?EDX? studies were carried
out in a high-resolution TEM ?FEI’s Tecnai G2S-TWIN
model? by examining the samples prepared on carbon-coated
III. RESULTS AND DISCUSSION
A. Growth of ZnO microcones
Figure 1?a? shows the top-view SEM micrograph of the
as-deposited ZnO cone structures. The hexagonal symmetry
of the microcones is due to the use of a patterned substrate.
The elemental composition of the cones was confirmed by
EDX studies as shown in Fig. 1?b?. The atomic percentage of
Zn:O in ZnO microcones was found to be 1:0.9 suggesting
that the as-deposited ZnO cones are slightly oxygen defi-
cient. The atomic percentage concentration of Al was found
to be ?2% which is same as that of the sputtering target. The
as-deposited samples exhibit a granular morphology. The mi-
crocones have a broad base which narrows toward the top as
shown in the Fig. 1?a? inset. These structures that gradually
grow into a conical shape defy the phenomenon of lateral
broadening normally observed during oblique angle deposi-
tion. It is noteworthy that two distinct features of oblique
angle deposition are ?a? tilting of columnar structures toward
the substrate normal, and ?b? lateral broadening.21–26Tilting
occurs because the adatoms from the angular incident flux
tend to move preferentially in the same direction as the inci-
dent flux to conserve atomic momentum parallel to the film
surface resulting in the slanted columnar growth. Lateral
broadening of the columns occurs as a result of anisotropic
incident flux ?due to instabilities in deposition rates or angu-
lar spread of the incident flux? and anisotropic shadowing
effect deposition plane ?plane containing the substrate nor-
mal and the vapor source?. Shadowing is anisotropic because
it exists only in the direction parallel to the deposition plane
?plane containing the substrate normal and the incident flux?.
In the perpendicular direction, however, a uniform relative
flux is received by the substrate which may eventually result
in the formation of tilted wall-like structures aligned perpen-
dicular to the incident flux. This phenomenon of anisotropic
shadowing has been discussed by Kennedy et al.25Patterned
substrates reduce column broadening to some extent but do
not nullify it. Thus, column broadening is an inherent feature
of oblique angle deposition. In view of this, the conical mor-
phology of ZnO columns grown by oblique angle sputter
deposition is quite unusual. This unusual conical shape has
also been reported in case of Cr “nanohalf-moons,”24which
are structures similar to the conical ZnO columns observed
in the present study. The authors, however, attribute the
unique curved shape to the “the combination of deposition
and imaging angles.” Transmission electron microscopy
?TEM? was carried out in case of ZnO microcones to inves-
tigate this aspect. Figure 2 shows the TEM micrograph of
ZnO columns which reveals an alternate view of the ZnO
columns. As seen from the figure, the ZnO columns are ini-
tially broad near their base before they start converging. On
the basis of the scanning and transmission electron micros-
copy techniques, a growth model comprising of three stages
influenced by the properties of the seeding layer as well as
the growth habits of ZnO is proposed. In the first stage, the
ZnO clusters nucleate uniformly on the silica sphere ?see
Fig. 3?a?, top-panel?. This is due to limited diffusion of ZnO
on silica which promotes the Volmer–Weber type of growth
on the silica sphere. Xu et al.27have studied the diffusion
FIG. 1. Top-view SEM micrograph of ZnO microcones at 35 kX magnifi-
cation. The hexagonal symmetry of SiO2spheres patterned on Si?100? sub-
strate is replicated in the as-deposited ZnO microcones. Inset: a single ZnO
cone structure captured at 70 kX magnification. The length and width of the
structure are marked. Scale bar in inset corresponds to 1 ?m. ?b? EDX
spectrum of ZnO cones. Pt is present due to Pt deposition on masked portion
of substrate for electrical contacts. The signatures of C and Cu appear due to
use of carbon-coated copper grids for TEM sample preparation.
FIG. 2. ?Color online? TEM micrograph of ZnO columns grown on SiO2
spheres. The micrograph presents different views of the columns. An initial
straight column growth has been indicated by dashed ?yellow? arrows.
FIG. 3. ?Color online? Schematic depicting the proposed three-stage growth
model for conical microstructures of ZnO grown by oblique angle deposi-
tion on SiO2spheres.
063519-2 Nagar et al. J. Appl. Phys. 108, 063519 ?2010?
behavior of ZnO on SiO2thin films and found that even after
heating at 773 K for 24 h, ZnO hardly diffused onto the
surface of SiO2film. Thus, limited mobility of adatoms gives
rise to the Volmer–Weber type of growth ?see Fig. 3?a?,
bottom-panel?. In the second stage, due to incidence of an
angular flux on the substrate, the clusters on the top of the
sphere intercept the particle flux and start growing in the
form of nanocolumns in the direction of the incoming vapor
flux. During this stage, the columns grow independently. The
amount of flux received by individual columns decides the
rate at which they grow. As the growth proceeds, strong
growth competition between neighboring columns initiates.
Longer columns start intercepting significantly larger portion
of the incident flux resulting in the extinction of shorter col-
umns. The diameter of the seeding sphere plays a critical role
beyond this stage, i.e., the third stage. It is noteworthy that
curvature of the silica sphere introduces a disparity between
the heights of the columns. For instance, the columns labeled
A, B, and C ?Fig. 3?a?, top-panel? would have different
heights even if the flux captured by them was same at all
times. The distribution of flux, nonetheless, itself is a sto-
chastic process. In addition, larger spatial extent of the inci-
dent flux is captured by large diameter spheres as compared
to smaller diameter spheres. Fluctuations in the incident flux,
therefore, result in greater instabilities on a large diameter
sphere. After some time, the column at P end ?Fig. 3?b??
would start to grow at a faster rate than that at P?due to its
close proximity to the vapor flux and start shadowing its
immediate neighboring column. The shadowing ?due to an-
gular flux? would be anisotropic occurring only in the plane
parallel to the deposition plane. In the direction perpendicu-
lar to it, broadening with increase in column length may lead
to their coalescence closing the voids between the simulta-
neously growing nanocolumns. Closure of voids may in turn
promote diffusion of the sputtered adatoms by reducing the
step-edge barriers at the nanocolumn boundaries. A dense
morphology visible in the SEM micrograph ?see inset Fig.
1?a?? supports this hypothesis. Ion peening from energetic O
and Ar+ion species during deposition may also contribute to
denser morphologies.28Comparing the surface diffusion en-
ergies of Zn and O in ZnO, it is easier for Zn adatoms to
diffuse faster as compared to O atoms. The surface diffusion
energies in ZnO are reported to be 1.0–3.3 eV for Zn and
1.5–7.5 eV for O. Also, first principle studies by Kim et al.28
show that the adsorption energies for Zn and Zn–O mon-
odimers are 0.679, 7.578 eV for ?101¯0? and 3.486 and 11.397
eV for ?112¯0?, respectively. Knuyt et al.29have developed a
model for the evolution of texture based on the assumption
that the film tends to lower its surface energy during deposi-
tion. A particular orientation during deposition may be
achieved by diffusion of adatoms within a thin surface layer
which forms the crystallites having a lower surface energy.
The adsorbed adatoms may move on the surface and by dif-
fusion or hopping reside on those planes which have lower
surface potential. Consequently, the lower-surface-energy
grain will become larger as the film thickness increases. The
orientation distribution at the surface then evolves toward
that crystallographic direction which also has lowest surface
energy. Deng et al. reported that the surface free-energy den-
sities of ?0001?, ?112¯0?, and ?101¯0? planes in ZnO are
0.14 J m−2, 1.97 J m−2, and 3.34 J m−2, respectively, indi-
cating that preferential growth of ?0002? plane is expected.30
The cumulative effect of growth habits of ZnO coupled with
oblique angle deposition seem to be the possible cause of
tapered edges of ZnO columns in the final stage.
B. Nanoscale hardness of ion irradiated ZnO
Figure 4 shows the SEM micrographs of the pristine
ZnO microcones ?a? along with the cone irradiated at
1017ions cm−2fluence ?b?, respectively. The granular mor-
phology of the as-deposited cones is clearly visible. This
morphology tends to smoothen as the microstructures are
irradiated. The height of the cones measured from the base
was observed to be ?1.7 ?m, while the diameter at base
was ?1.5 ?m. No significant deformation of ZnO micro-
cones is observed on their physical dimensions after their
irradiation. Interestingly, tremendous plastic deformation
was observed in case of Si nanosprings irradiated under iden-
tical conditions in our earlier work.31Figure 5?a? shows a
5?5 ?m2AFM image of ZnO cones before indenting the
sample. The hexagonal symmetry along with a pattern defect
?depicted by a white dotted line? can also be seen. Figure
5?b? shows indent marks on two neighboring pristine ZnO
cones indented in succession. Inset shows. Figures 6?a? and
FIG. 4. ?a?–?b? Magnified view showing SEM micrographs of individual
1017ions cm−2. Scale bars correspond to 500 nm.
FIG. 5. ?Color online? ?a? AFM micrograph of 5?5 ?m2area before in-
denting the sample shows the hexagonal symmetry along with a pattern
defect depicted by white dotted line. ?b? AFM micrograph of pristine ZnO
microcones showing two successive indent marks marked “i1” and “i2.”
063519-3Nagar et al. J. Appl. Phys. 108, 063519 ?2010?
6?b? show representative load-penetration ?P–h? curves cor-
responding to pristine sample and sample irradiated at
1015ions cm−2, respectively. Since the indent marks were
not always visible after indentation, the nanoscale hardness
was calculated from the unloading part of the P–h curves
following the Oliver–Pharr method.32It was first established
that the value of hardness evaluated by the P–h curves and
that by determining the projected area of contact from indent
marks agreed well within the experimental error. Therefore,
in the event of invisible indent marks, the hardness of the
cones was evaluated from the P–h curves. The Oliver–Pharr
method involves extrapolating the slope of the unloading
curve at the maximum depth to meet the penetration axis
from where contact depth ?hc? is determined as hc
=hmax–??Pmax/S?, S being the slope of the unloading curve
at maximum load and ? is a constant equal to 0.72 for Berk-
ovich indenters.32Meyer hardness ?H? defined as the ratio of
maximum indentation load Pmaxto the projected area of con-
tact Acwas determined for samples irradiated at different ion
fluences. The projected contact area of the indenter depends
upon the indenter geometry and changes with the penetration
depth h.33For the Berkovich-type indenter, the hardness can
be calculated using the relation Ac=3?3hc
indent marks were visible, hardness was also calculated us-
ing the median method.34In this method, the projected area
is calculated in terms of the medians ?m1,m2,m3? of the tri-
angular indent mark. The projected area of contact is then
given by Ac=4??s?s−m1??s−m2??s−m3?/3, where s=?m1
+m2+m3?/2. Figure 7?a? summarizes the nanoscale hardness
of the irradiated ZnO microcones. The hardness values have
been averaged for more than five indents for each sample. It
was found that the hardness of the cones initially increased
1015ions cm−2as compared to the pristine sample. How-
ever, it decreased with increasing ion fluence. Interestingly,
the hardness of ZnO cones ??9.5 GPa? becomes compa-
rable to the hardness of other semiconductors like Si ?12
2/8. In case the
at ion fluenceof
1015ions cm−2.11For the maximum ion fluence, the micro-
cones exhibit a hardness value even lower than the as-
C. Simulating irradiation of ZnO microcones
During irradiation an energetic ion loses its energy via
electronic energy losses ?Se? occurring due to inelastic colli-
sion of incident ions with the electrons of the target or the
nuclear energy losses ?Sn? occurring due to the elastic colli-
sion of incident ions with the target atoms.35A Monte Carlo
simulation program Stopping and Range of Ions in Matter
?SRIM? was employed to understand the interaction of 1.2
MeV Ar ions in a ZnO matrix.36The Ar+8ions transfer en-
ergy to the ZnO matrix through Se
1.45 keV nm−1and at 0.268 keV nm−1via Sn. When Seis
greater than a threshold value ?Seth?, permanent damage
zones known as latent tracks can be created within the target
material. Following Szenes,37the value of Sethfor ZnO was
estimated to be 11.44 keV nm−1. Therefore, for the values of
ion irradiation in the present study, Seis well below Sethto
result in the formation of permanent damage zones in ZnO.
The Ar+8ions have a range of about 740 nm in ZnO matrix
with about 180 nm of longitudinal straggling. Therefore, the
ions get implanted in the 1.7 ?m long ZnO cones. However,
during indentation experiments, the maximum penetration
depths of the nanoindenter tip were less than 120 nm ?ob-
served for 1017ions cm−2sample?, which is about one-sixth
of the Ar+ion range in ZnO. Thus, the observed hardness
change in the ZnO microcones is attributed to Selosses dur-
ing Ar+8ion irradiation rather than implantation. The results
of SRIM simulations performed on ZnO target irradiated with
1.2 MeV Ar+8ions show that the Zn atoms absorb more
energy from the recoil collisions owing to their larger atomic
radius as compared to oxygen ions in ZnO lattice. Corre-
spondingly, a higher number of zinc vacancies are created as
compared to oxygen vacancies. This is because the displace-
ment energy for Zn atoms ?18.5 eV? is quite low as com-
pared to the O atoms ?41.4 eV? in ZnO.38
at a rate of
D. Micro-Raman spectroscopy of ZnO microcones
To probe the effect of Ar+ion irradiation on the Zn and
O sublattices separately, micro-Raman studies were carried
out. The maximum intense peak at 101 cm−1corresponding
to E2?L? mode ?due to vibration of Zn sublattice? is observed
followed by 580 cm−1?due to defects, oxygen vacancies, or
zinc interstitials?39and 434 cm−1?due to vibration of oxygen
sublattice?. The position of E2?H? mode is very sensitive to
any stress ?internal/external? within the sample and results in
its shift.2Figure 7?b? shows the E2?H? mode Raman spectra
for all the samples. It was observed that as ion fluence in-
creases, the E2?H? mode shifted its position indicating that
the residual stress state of the samples change with ion irra-
diation. For stress free bulk ZnO, the position of E2?H? mode
has been reported at 437 cm−1.39For our pristine ZnO cones,
the E2?H? mode appears at 434 cm−1and shifts to 436 cm−1,
432 cm−1, and 430 cm−1at ion fluences of 1015ions cm−2,
1016ions cm−2, and 1017ions cm−2, respectively. The shift
FIG. 6. ?Color online? ?a? and ?b? present the load-penetration depth curves
obtained for indent mark labeled “i2” for pristine sample and sample irradi-
ated at 1015ions cm−2fluence.
FIG. 7. ?Color online? ?a? Variation in nanoscale hardness ?H? of ZnO cones
and relative Raman shift in E2?H? mode from stress-free value of 437 cm−1
plotted as a function of Ar+8ion fluence ???. ?b? Micro-Raman spectra of
E2?H? mode of ZnO microcones.
063519-4Nagar et al.J. Appl. Phys. 108, 063519 ?2010?
toward lower wave numbers from 437 cm−1indicates a ten-
sile stress within the samples.39Therefore, the pristine
sample is under residual tensile stress. Irradiation at
1015ions cm−2relaxes this residual stress due to irradiation
by the energetic ions in the ZnO matrix. This energy may
lead to a rearrangement of Zn and O atoms in the lattice
relieving some stress at an ion fluence of 1015ions cm−2.
Damage due to irradiation can be quantified by calculating
the displacements per atom ?dpa? caused by irradiation and is
directly proportional to the ion fluence. Irradiation at
1015ions cm−2corresponds to 0.1 dpa, whereas higher ion
fluences of 1016ions cm−2and 1017ions cm−2lead to 1.2
dpa and 11.6 dpa, respectively. The higher dpa values possi-
bly lead to displacement of Zn and O atoms from their re-
spective lattice positions resulting in tensile stress. Thus,
dpa?1 can be regarded as a critical dose beyond which ZnO
lattice starts getting damaged. Therefore, cones irradiated at
1015ions cm−2are the least stressed and exhibit a higher
hardness value as compared to pristine sample. Figure 7?a?
shows the relative shift in the Raman peak position from the
stress-free state of 437 cm−1?designated as RRS? with ion
fluence and brings out the correlation between the stress and
nanoscale hardness of ZnO cones. The residual stresses are
known to affect the indentation loads and penetration depths
during indentation. For an equibiaxial stress, the residual ten-
sile stress at the indented surface can be considered as a
combination of a tensile hydrostatic stress plus a uniaxial
compressive stress acting in the direction of the indented
load.40The hydrostatic stress does not give rise to plastic
deformation, and therefore does not affect the extent to
which the indenter tip penetrates the specimen. However, the
compressive stress induces an indentation load in addition to
the applied load P, and acts in the same direction as P.
Therefore, the residual stress influences the measured hard-
ness by affecting the onset of plastic deformation.40,41This
effect is observed for the pristine and samples irradiated at
1016and 1017ions cm−2. The ZnO cones irradiated at
1015ions cm−2fluence, which have least tensile stress ex-
hibit the maximum hardness.
ZnO is generally considered to be radiation hard in com-
parison to other semiconductors like Si and GaN.11The sta-
bility in electrical and optical properties on electron and ion
beam irradiation observed in ZnO has been explained due to
the defect annihilation taking place at higher temperatures
??110 K or above? or rapidly in time. However, in our
study ion irradiation carried out at ?80 K causes structural
changes in ZnO microcones and reveals their radiation-
susceptible nature. It appears that the irradiation temperature
plays an important role in determining the changes in me-
chanical hardness of ZnO. Therefore, the changes in different
properties of ZnO depend critically upon the irradiation con-
ditions, especially the irradiation temperature, which controls
the creation and annihilation of defects that strongly affect
the material properties.
The ZnO microcones were grown by dc magnetron ob-
lique angle deposition technique. This resulted in the growth
of conical structures defying lateral broadening which is a
characteristic feature observed in oblique angle deposition
due to anisotropic shadowing. The growth is understood to
take place in three stages: ?a? the nucleation stage which
promotes Volmer–Weber type of growth, ?b? competitive
growth between neighboring columns, and ?c? competition
between the growths at opposite ends of the patterning
sphere. It is shown that ion irradiation affects the nanoscale
mechanical hardness of ZnO microcones with initial increase
of about 100% at 1015ions cm−2. On increasing the fluence
from 1015to 1017ions cm−2the hardness reduces. The ob-
served change in the hardness is correlated with the residual
stress measured from the Raman shift of the E2?H? mode.
The final change in properties of ZnO is governed by the
creation and annihilation of defects during irradiation and
irradiation temperature is one of the crucial parameters.
Since ion-surface interaction zone is limited to few hundreds
of nanometer, this technique is well suited for carrying out
selective area modification of the surface properties. This
study is important due to the potential applications of ZnO
thin films and nanostructures in optical, electronic, and pi-
ezoelectric sensors in harsh radiation environment condi-
R.N. acknowledges CSIR, India for providing SRF. We
are thankful to Dr. P. Kumar, LEIBF, IUAC, New Delhi,
India for ion irradiation.
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