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:

jpsingh@physics.iitd.ac.in.

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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