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Revisiting the structure zone model for sculptured silver thin filmsdeposited at low substrate temperaturesDhruv P. Singh, Pratibha Goel, and J. P. Singh Citation: J. Appl. Phys. 112, 104324 (2012); doi: 10.1063/1.4767634 View online: http://dx.doi.org/10.1063/1.4767634 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i10 Published by the American Institute of Physics. Related ArticlesTransition from laminar to three-dimensional growth mode in pulsed laser deposited BiFeO3 film on (001) SrTiO3 Appl. Phys. Lett. 101, 201602 (2012) Modeling and numerical simulations of growth and morphologies of three dimensional aggregated silver films J. Appl. Phys. 112, 094310 (2012) Impact of oxygen bonding on the atomic structure and photoluminescence properties of Si-rich silicon nitride thinfilms J. Appl. Phys. 112, 073514 (2012) Growth and characterization of LuAs films and nanostructures Appl. Phys. Lett. 101, 141910 (2012) Structure determination of thin CoFe films by anomalous x-ray diffraction J. Appl. Phys. 112, 074903 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors
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Revisiting the structure zone model for sculptured silver thin films depositedat low substrate temperatures
Dhruv P. Singh, Pratibha Goel, and J. P. Singha)
Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
(Received 19 July 2012; accepted 31 October 2012; published online 29 November 2012)
In this study, we examined the low substrate temperature (Ts) growth mechanism of Ag thin films in the
atomic shadowing regime (Ts � melting point Tm). The Ag thin films were deposited using glancing
angle deposition (GLAD) at different substrate temperatures varying from 320 K to 100 K. Interestingly,
it is observed that on lowering the substrate temperature instead of showing a monotonic variation, the
Ag film morphology changes from the ordered nanocolumns to random and distorted columns, and then
to the columnar bunches of nanowires. These growth results suggest that this temperature regime of
effective adatom shadowing does not hold a unique growth mechanism for the GLAD within the low
temperature range from 320 K to 100 K and depending on the observed temperature dependent variation
in morphological and structural properties of the Ag film, it can be sub-divided into three characteristic
zones. The observed growth mechanism of the Ag film is explained in terms of the temperature
dependent change in terrace diffusion and the interlayer diffusion of Ag adatoms which finally controls
the formation of nucleation centers in initial stage and their evolution during the final growth. The
understanding of low temperature growth mechanism along with the identification of appropriate
temperature range for the growth of nanocolumnar metallic films during GLAD is the novelty in this
work. VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767634]
I. INTRODUCTION
The substrate temperature is an important growth pa-
rameter during the deposition of thin films and nanostruc-
tures which plays a crucial role in deciding their physical
and structural properties. The effect of substrate temperature
has been explained in detail using the structure zone model
(SZM), which relates the film morphology to the homolo-
gous temperature Ts/Tm (where, Ts is the substrate tempera-
ture and Tm is the melting point of the film material). SZM
identify three primary characteristic structure zones, namely,
zone 1, zone 2, and zone 3 as an increasing value of Ts/Tm.1,2
Zone 1 covers the low temperature range in which adatom
mobility remains very low and hence, the adatom shadowing
effect controls the morphology of the grown film which con-
sists of the tapered columnar grains separated by voids. Zone
2 covers the next higher temperature range in which the ada-
tom mobility plays an effective role. It causes the adatoms to
move into the voids and leads to the film growth having
dense columnar grains. On still higher temperature, the zone
3 consists of the temperature range in which the effective
bulk diffusion and the possibility of recrystallization make
the film fully dense with the growth of equiaxed grains.3–5
In case of glancing angle deposition (GLAD), shadow-
ing effect is a crucial parameter which, during deposition at
very high oblique angle a (� 85�) with respect to the sub-
strate normal, results in growth of a columnar film.6–14
Therefore, according to the SZM, GLAD should be done
preferentially in the atomic shadowing regime or zone 1,
which for metallic films lies at Ts/Tm� 0.3.5,15 This tempera-
ture range shows a good agreement with the experimental
results of GLAD and it was observed that almost none or
very poor columnar growth occurs for metallic films above
this temperature range.16–21 It is really interesting to probe
that whether the columnar growth persists throughout this
zone 1 even when the substrate temperatures goes down to
liquid nitrogen. In an interesting research by Hara et al., they
observed a non-monotonic variation in the inclination angle
of columnar grains when the homologous temperature get
reduced to a value less than 0.1 during deposition of Fe at an
oblique angle of 60�.22 However, in their study, the value of
a¼ 60� was comparatively smaller and was insufficient for
the growth of well separated nanocolumnar films. In an
another study, Mukherjee and Gall have observed a transi-
tion from two-dimensional (2D) to three-dimensional (3D)
islands growth during GLAD on a rotating substrate at the
homologous temperature of 0.2.17 The understanding of
the growth behavior for higher value of a� 85� on reducing
the homologous temperature within zone 1 is still unknown.
In the present research work, we have investigated the mor-
phological evolution of GLAD grown Ag thin films with change
in the substrate temperature within zone 1 of SZM. Interestingly,
it is observed that on reducing substrate temperature, the growth
behavior for the formation of nanocolumnar films does not
remain the same or shows any monotonic variation but to our
surprise, it changes dramatically after certain intervals of sub-
strate temperature. Our results suggest that on the basis of physi-
cal and structural properties of the GLAD film, zone 1 of
SZM can be sub-divided into three structure zones, namely,
zone C (0.2� Ts/Tm� 0.3), zone B (0.1�Ts/Tm� 0.2), and
zone A (Ts/Tm� 0.1). The understanding of low temperature
growth mechanism along with the identification of suitable tem-
perature range for the growth of nanocolumnar metallic films
during GLAD is the novelty of this research work.
a)Author to whom correspondence should be addressed. Electronic mail:
0021-8979/2012/112(10)/104324/6/$30.00 VC 2012 American Institute of Physics112, 104324-1
JOURNAL OF APPLIED PHYSICS 112, 104324 (2012)
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II. EXPERIMENTAL DETAILS
Silver films were grown over Si(100) substrates by ther-
mal evaporation of silver powder (99.9%) using GLAD
method.6–14 For growth of silver films, Si substrates were
inclined in the polar direction such that the substrate normal
made a very high angle (a¼ 85�) with the direction of inci-
dent vapor flux. To study the effect of substrate temperature
(Ts) on the morphology of GLAD grown films, the substrates
were kept at different temperatures varying from 320 K to
100 K. The Ts was adjusted with a customized substrate
heater and a controlled supply of the liquid nitrogen to the
sample holder. The temperature was measured with an accu-
racy of 62 K using a PT100 temperature sensor placed
closed to the substrates. During deposition, pressure in
GLAD chamber was better than 2� 10�6 Torr. The deposi-
tion rate (normal incidence) of about 12 A s�1 and deposition
time of about 8 min were kept constant for all the samples.
Film morphology and structural analysis were performed
using scanning electron microscope (SEM, ZEISS EVO 50)
and glancing angle X-ray diffraction (GAXRD, Cu Ka radia-
tion of wavelength 1.54 A, Phillips X’pert PRO-PW 3040).
III. RESULTS AND DISCUSSION
SEM micrographs of the Ag films grown at different
substrate temperatures (Ts) varying from 320 K to 100 K are
shown in Fig. 1(a). During deposition, the vapor incidence
FIG. 1. (a) SEM images of Ag films
grown at different substrate tempera-
tures. Depending on the film morphol-
ogy, the results are divided into three
structural zones (zone C, zone B, and
zone A). (b) Schematic of film morphol-
ogy in the three zones: (I) zone C: per-
fect columnar growth, (II) zone B:
distorted columnar growth, and (III)
zone A: columnar bunches of nanowires.
104324-2 Singh, Goel, and Singh J. Appl. Phys. 112, 104324 (2012)
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angle a was fixed to 85� and this value of a was sufficient to
initiate the columnar growth in GLAD. However, it appears
from SEM images that the decrease in the Ts affects the co-
lumnar growth morphology of Ag film. It is important to
notice that initially the growth starts with the formation of
inclined nanocolumns towards the incoming vapor flux
direction at 320 K but this well defined morphology of
inclined nanocolumns starts changing after the Ts value of
245 K. Although, the silver film undergoes some morpholog-
ical changes like the increase in surface column density and
a reduction in the column width upon lowering the Ts down
to 245 K, but throughout this temperature range film can be
considered as consisting of the arrays of inclined Ag nano-
columns. As the Ts is decreased further, a clear deviation
from the nanocolumnar growth morphology is evident. In
between the temperature range of 220 K to 145 K, the shad-
owing effect appears to be less effective and as a result
instead of well-ordered and inclined Ag nanocolumns; 3D
clusters of Ag nanoparticles or distorted columns with bead-
like structures grow in a random fashion. On lowering the Ts
further during growth, the observed film morphology
changes itself and surprisingly, below the temperature value
of 135 K, the columnar growth reappears. The film morphol-
ogy in this temperature range (135 K–100 K) appears to be
similar to the growth obtained in the temperature range
between 320 K–245 K. However, a close observation shows
that these individual columns are basically grown as a bunch
of multiple nanowires, all growing in the direction of inci-
dent vapor flux. We observed this type of growth up to the
lowest value of Ts (100 K) that could be attained in our sys-
tem. This observed Ts dependent variation in the film mor-
phology suggests that the silver film growth in low
temperature range can be divided into three different struc-
ture zones, zone C, zone B, and zone A. Where, zone C cov-
ers the range of Ts in which the perfect nanocolumnar
growth can be obtained and it lies from 320 K to 245 k, zone
B covers the temperature range of 220 K to 145 K which is
the region of random and distorted columnar growth, and
zone A is the low temperature region lying from 135 K to
100 K in which the bunches of nanowires grow following the
direction of vapor flux. The schematic of film morphology in
these three zones, zone C, zone B, and zone A is shown in
Fig. 1(b). It is important to notice that if we consider the
observed changes in film morphology to be associated with
the homologous temperature (Ts/Tm), then these structure
zones can be redefined in the range of 0.2� zone C� 0.3,
0.1� zone B� 0.2, and zone A� 0.1. These homologous
temperature ranges of the proposed structure zones are
marked in Fig. 1(a).
The GAXRD spectra of the two Ag samples grown at
320 K and 100 K are shown in Fig. 2. The patterns represent
a polycrystalline growth. However, the intensity and FWHM
value of the peaks appear to be different for the two samples
suggesting that not only the morphology but also the crystal-
linity of Ag columnar films got affected with the decrease in
the substrate temperature. The average crystallite size d of
all the Ag film samples was calculated using Scherrer for-
mula for the most intense peak corresponding to the (111)
plane and is plotted in Fig. 3.23–27 The d was found to vary
with the substrate temperature. The variation of d with sub-
strate temperature Ts shows a unique behavior and this varia-
tion of d with Ts can be divided into the same three different
structure zones as described above. Specifically, in the tem-
perature range of zone C, d remains almost constant with a
value of about 24 nm, and then it undergoes a slow decre-
ment (0.02 nm K�1) as temperature is reduced further
roughly in the range of zone B, and finally it falls down with
a steep slope (0.27 nm K�1) for lowering the substrate tem-
perature down to 100 K, i.e., in the range of zone A. Previ-
ously, Hara et al. have also observed variation in
morphology of the inclined grains of Fe film when deposited
at the vapor incidence angle of 60� and in the homologous
temperature range of 0.26 to 0.04.22 The researchers found
that the morphology of inclined Fe grains deviates from the
ordered growth in the homologous temperature range of 0.17
to 0.13. It is interesting to notice that this temperature range
is almost similar to what identified in the present case as
zone B of the distorted columnar growth. The observed simi-
larity in the temperature range of the distorted growth
reflects that the observations made in the present study are in
FIG. 2. GAXRD pattern of the Ag films grown at Ts of (a) 300 K and (b)
100 K.
FIG. 3. The variation of average crystallite size d (for 111 plane) with Ts.
104324-3 Singh, Goel, and Singh J. Appl. Phys. 112, 104324 (2012)
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agreement with the previous studies made on other materials
during the oblique angle deposition.
During GLAD, the initial stage evolution of a rough sur-
face consisting of 3D mounds is very crucial for the start of
nanocolumnar growth. If an appropriate high value of inci-
dent vapor flux angle a (>80�) is provided with a sufficient
low adatom surface diffusion to maintain the shadowing
effect, these mounds can act as nucleation centers and attract
more and more vapor flux to grow up in the form of colum-
nar morphology following the direction of incoming vapor
flux. Therefore, in GLAD to understand the observed effect
of Ts on the resulted thin film morphology, it is necessary to
examine the behavior of initial stage growth including the
3D mounds formation with a decrease in Ts. The homoepi-
taxial growth kinetics of Ag film has been explored by
researchers in past.28–30 It was clearly observed that during
the growth of Ag on Ag film, the surface roughness associ-
ated with the 3D mounds formation depends on the Ts. They
observed a non-monotonic variation of surface roughness
with the decrease in the Ts from 300 K to 100 K and depend-
ing on the variation and roughness pattern this temperature
range was divided into three different regions.28–30 Specifi-
cally, the rms roughness of 25 monolayers of Ag on Ag film
was increased by 94% as Ts was reduced from 300 K to
220 K, and then it decreased down to 43% as Ts was further
reduced to 140 K resulting in a smoother surface. Further
reduction in Ts values down to 100 K increased the rms
roughness again up to 52% of the 300 K value. The observed
increase in the rms surface roughness with Ts down to 220 K
was attributed to an increased effect of Ehrlich-Schwoebel or
step-edge barrier to the interlayer diffusion. This inhibition
of interlayer diffusion simply reduces the probability of
atoms to reach the lower layers and results in the evolution
of 3D mounds as shown in a schematic in Fig. 4(a). Since, it
is experimentally shown that the diffusion constant follows
an Arrhenius type behavior that is D¼ (a02/2)� exp(�Ed/
kTs), where a0 is the jump distance between the adjacent
adsorption sites and is equal to the lattice constant of surface
material, Ed is the diffusion energy, � is the vibration fre-
quency of adatom, and k is the Boltzmann constant.31 There-
fore, reduction in Ts simply inhibits the terrace diffusion of
atoms which eventually for this temperature range between
300 K and 220 K results in a continuous size reduction of the
3D mounds (Fig. 4(b)). The evolution of a large number of
small size mounds increases the step-edge density on the sur-
face. On reducing the temperature further, i.e., below 220 K,
a critical enhancement in the step-edge density is considered
as the determining effect which leads to the increment in
role of downward funneling (DF) over the effect of step-
edge barrier. DF is simply the deflection of deposited atoms
from step-edge to the lower fourfold hollow adsorption
sites.28 Hence, by increasing the approach of atoms to the
lower layers, DF discourages the mound formation and with
reduction in Ts it eventually leads to the evolution of a rela-
tively flat but irregular surface as illustrated in Fig. 4(c). This
continuous inhibition of mound formation along with a
downfall in the step-edge density in turns successively
diminishes the active DF mechanism with lowering the Ts in
this range. On reaching down to 140 K, the mound formation
disappears significantly with a critical downfall in the step-
edge density which finally leads to the breakdown of DF
mechanism. In addition to this, the adatom mobility in the
lower temperature range below 140 K also becomes suffi-
ciently low, making the incident atoms adhere where they
impinge, with a sticking coefficient of Ag adatoms close to
unity. In this case, a random sticking of atoms with almost
no probability of downfall to the lower layers promotes the
stacking of atoms in the upper region and the process ini-
tiates the increase in surface roughness on lowering the tem-
perature down to 100 K (Fig. 4(d)).
It is important to notice that in case of GLAD, we
observed the transition in columnar growth behavior for
almost the same temperature ranges in which the change in
surface roughness was observed during the initial stage
growth as discussed above. During GLAD, the main differ-
ence in growth behavior from the normal deposition occurs
only when in the initial stage the nucleation centers grow up
sufficiently in size to create the necessary shadow inline for
the incoming vapor flux. Therefore, a similar mechanism as
discussed above for the normal deposition of thin film can be
assumed to be happening for GLAD during the initial stage
growth of Ag film in the temperature range from 320 K to
100 K. The increased rate of 3D Ag mound formation in the
first temperature range at initial growth stage simply
FIG. 4. Schematics of initial stage nucleation at low substrate temperature: (a) effective step-edge reflection (SER) over DF promotes the formation and evolu-
tion of 3D mounds; colored atoms represent the incident adatoms, (b) zone C: formation of small size mounds with steep edges which encourage the DF of
atoms, (c) zone B: increase in DF flattens the mounds and makes the surface irregular, (d) zone A: following the DF breakdown and low adatom mobility, the
random sticking of adatoms in upper layers increases the surface roughness.
104324-4 Singh, Goel, and Singh J. Appl. Phys. 112, 104324 (2012)
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increases the availability of the nucleation centers on the sur-
face, leading to the growth of inclined Ag nanocolumns.
This temperature range of increased mound formation turned
Ag nanocolumnar growth lies from 320 K to 245 K. Below
this range, the DF dominates and terrace diffusion decreases
continuously, reducing the probability of formation and size
evolution of the nucleation centers. The DF of atoms also
diminishes the necessary shadow effect for columnar growth.
Hence, this DF dominated temperature range, observed to be
extending from 220 K to 145 K results in the random and dis-
torted columnar growth of Ag film. It is discussed earlier
that the DF mechanism diminishes itself with flattening the
mounds over the surface and finally breaks down for lower
temperature values. In this temperature range below 135 K,
the sticking of atoms at over layers and the resulting atomic
stacking lead to the evolution of a large number of small
mounds on the surface during initial stage growth. These
mounds act as the nucleation centers and with reduced effect
of the interlayer diffusion of atoms exhibit the crucial shad-
owing effect. However, the terrace diffusion becomes almost
zero in this temperature range, leading to the formation of
very small size nanocolumns with very low probability for
them to grow up in lateral size by self broadening or coales-
cence with the surrounding nanocolumns. Hence, the bunch
of independent wire like columns with very small size
evolves over each nucleation centers during the GLAD
growth. This type of nanocolumnar growth was observed
down to the temperature 100 K. The average crystallite size
d of Ag film also follows a similar non-monotonic trend with
the decrease in the Ts, i.e., a constant in zone C, slight decre-
ment with Ts (0.02 nm K�1) in zone B, and steep fall
(0.27 nm K�1) in zone A. This can be explained in terms of
decrease in terrace diffusion of the Ag adatoms. The
decrease in terrace diffusion reduces the probability of ada-
toms to move over the surface to arrange themselves in the
minimum energy configuration and in turn reduces the size
of crystallites. The value of terrace diffusion can be realized
by calculating the diffusion constant (D) for different tem-
perature values using the parameters a0¼ 4.08 A,
�¼ 1013 s�1, and Ed¼ 0.4 eV in the relation discussed ear-
lier.28 Since, the diffusion constant follows the Arrheniuspattern and therefore, its value decreases sharply with the
decrease in Ts and finally it reduces to almost zero value for
the Ts lower than 135 K. The calculations show that for the
temperature range of 320 K to 245 K within zone C, the dif-
fusion constant changes from 4.2� 105 nm2 s�1 to 5� 103
nm2 s�1. The decrement in diffusion constant suggests for a
steep fall in the terrace diffusion, however, it can be noticed
that in this zone the value remains sufficiently high to allow
the adatoms to form the best possible crystalline configura-
tion for the present conditions, and therefore, resulting in a
constant crystallite size within the zone C. As the tempera-
ture was reduced further down to 145 K, i.e., in zone B, the
terrace diffusion observed a strong inhibition and the value
of D reduced to 10�2 nm2 s�1. This small terrace diffusion
with a continuous fall in its values with the decrease in the
temperature is in agreement with the observed reduction in
crystallite size with the decrease in Ts within the zone B.
Below this temperature, the terrace diffusion becomes almost
negligible, and therefore, as observed for the temperature
range of 135 K to 100 K of zone A, it results in random
growth with a steep fall in the crystallite size of Ag nanocol-
umns. In addition to the reduction in crystallite size, the
decreasing terrace diffusion also results in the decrease of
the column width and the inter-column separation when low-
ering the substrate temperature. Since, the columnar growth
was not observed for zone B (220 K–145 K), hence, it was
not possible to calculate the column width and the column
separation for this zone. However, the average column width
decreases by 33% with lowering the temperature within zone
A. In zone C, the variation in column width is rather small
(21%). This is in accordance with the observed small varia-
tion in crystallite size. But, in zone A, the change in column
width is not as prominent as that observed in the crystallite
size. This may be due to the fact that coalescence of two or
more closely spaced nanocolumns can result in the broaden-
ing of the columns. Similarly, the average inter-columnar
separation at 320 K is found to be 195 nm 6 35 nm which
decreased to 127 nm 6 29 nm at 245 K substrate temperature
within the zone C. In temperature range of zone A, the col-
umns growth appears as bunches of closely spaced nano-
wires. For this case, these nanowires possess a negligible
intermediate separation which cannot be calculated from the
SEM images, however, in agreement with the calculated val-
ues, it clearly indicates that a very low atomic diffusion acts
in this temperature range.
The proposed zone model for the low temperature
GLAD growth is found to be in agreement with the observed
variation in the morphology of nanocolumnar Ag film. In the
present study, the final growth morphology is considered to
be decided by the limited adatom diffusion and the pattern of
3D mound formation during the initial stage growth at
reduced substrate temperature. This type of non-monotonic
variation in the initial stage growth under normal deposition
has been observed experimentally as well as theoretically for
the growth of low melting point materials particularly Ag
and Cu.28,32 However, in previous studies, researchers have
observed the non-monotonic variation in structural properties
of columnar film of high melting point materials in almost
the same homologous temperature range as discussed in the
present study.16,17,22 Although, their findings were not as
prominent and moreover, were related either to the deposi-
tion at rotating substrate or to the very low vapor incidence
angle (a) which do not represent our case. But, their results
signify the approach of the proposed zone model for the
other materials also. However, a detail investigation of the
low temperature growth behavior of different materials is
required to check the material independence of the proposed
zone model.
IV. CONCLUSION
In summary, the present work investigates the effect of
substrate temperature on GLAD grown Ag films within the
limits of atomic shadowing regime or zone 1 (Ts/Tm� 0.3),
identified under the classic SZM. It is observed that the
growth mechanism changes itself after particular intervals of
the temperature, and therefore, depending on the structure
104324-5 Singh, Goel, and Singh J. Appl. Phys. 112, 104324 (2012)
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and morphology of the film the zone 1 can be further classi-
fied into three characteristic zones, lying roughly in the
homologous temperature range of 0.2� zone C� 0.3,
0.1� zone B� 0.2, and zone A� 0.1. The observed growth
behavior is explained in terms of the temperature dependent
change in adatom terrace diffusion and the adatom interlayer
diffusion which finally controls the formation of nucleation
centers in the initial stage and their evolution during the final
growth.
ACKNOWLEDGMENTS
The authors D.P.S. and P.G. kindly acknowledge CSIR,
India for the research fellowship. This research was sup-
ported by the financial Grant No. SR/S2/CMP-13/2010 from
Department of Science and Technology, India.
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