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Vacuum 85 (2010) 307e311
Contents lists available at ScienceDirect
Vacuum
journal homepage: www.elsevier.com/locate/vacuum
Grain growth and structural transformation in ZnS nanocrystalline thin films
Shiv P. Patel a, *, J.C. Pivin b, V.V. Siva Kumar c, A. Tripathi c, D. Kanjilal c, Lokendra Kumar a
a
Physics Department, University of Allahabad, Allahabad-211002, India
CSNSM, IN2P3-CNRS, Batiment 108, F-91405 Orsay Campus, France
c
Inter-University Accelerator Centre (IUAC), Aruna Asaf Ali Marg, New Delhi 110067, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 April 2010
Received in revised form
28 June 2010
Accepted 30 June 2010
Fabrication of ZnS thin films having similar stoichiometry at different substrate temperatures (TS)
e.g. 200 C, 300 C and 400 C by means of RF magnetron sputtering method is presented. The films
grown at TS of 200 C are in cubic zinc-blende phase and textured along (111) plane. The films deposited
at TS of 300 C and 400 C are in hexagonal wurtzite phase. The surface roughness and grain size of the
films increase with increasing TS. The ultra-violet and visible absorption studies show that the bandgap
of films can be tailored by varying TS, taking advantage of the structural transformation.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Thin films
Surface roughness
Structural transformation
X-ray diffraction
Atomic force microscopy
Bandgap
1. Introduction
Zinc sulfide (ZnS), a IIeVI compound semiconductor, is an
important material having various potential applications [1e5]
particularly as laser [6] and light emitting diode [7]. ZnS is a suitable material as infrared (IR) window and solar dome of missiles
[8]. An emerging field of applications of ZnS are data storage
devices, data transfer, optical interfacing because electronics
require semiconducting nanocrystalline thin film, which are
sensitive in ultra-violet part of electromagnetic spectrum [9].
The exploration of properties of thin films as function of growth
conditions like substrate temperature (TS), working pressure and
fabrication method is a topic attracting intensive activities in
materials science. Among these, substrate temperature greatly
influences the atomic motion and grain growth kinetics of films,
and is also an essential parameter for the development of crystalline films [10]. The substrate temperature also determines the
crystal structure and stoichiometry of compound semiconductors
thin films which affects quite a few properties [11e14]. Several
techniques like sputtering [1,2], pulsed laser deposition (PLD)
[12,13], thermal evaporation [15,16], chemical bath deposition
(CBD) [17,18] and molecular beam epitaxy (MBE) [19,20] have been
* Corresponding author. Tel./fax: þ91 532 2460993.
E-mail addresses: shivpoojanbhola@gmail.com (S.P. Patel), lkumarau@gmail.
com (L. Kumar).
0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.vacuum.2010.06.011
used by various researchers for the development of ZnS thin film.
Each fabrication method offers some advantages as well as drawbacks from the viewpoint of adhesion, film uniformity, stoichiometry and effective cost. However, RF magnetron sputtering method
overcomes above drawbacks and is widely used in numerous
applications [1,2,11]. The magnetic field applied behind the cathode
improves the films quality because the probability of ionization of
gases in the sputtering chamber increases due to increase in path
length of electrons in the presence of magnetic and electric field.
ZnS has two polytypes with zinc-blende (ZB) cubic and wurtzite
(W) hexagonal crystal structures, out of which the ZB phase is more
stable at ambient temperature and pressure. The bandgap of bulk
ZnS is 3.67 eV for ZB phase and 3.90 eV for W phase, and the ZB
phase transforms into W phase at 1020 C [21]. In the case of
nanoparticles, the transition temperature depends on their size and
is for instance of 400 C for 3 nm [22]. There are other experimental
and theoretical studies which deal with stability of these phases
[23,24], but only one at our knowledge which discussed the
transformation in ZnS thin films with substrate temperature and
the increase in bandgap was correlated with the volume fraction
of W phase [25]. The structural transformation has been observed
between 300 C and 400 C. As for any IIeVI compound semiconductor, the bandgap of ZnS is a direct consequence of the crystal
structure and stoichiometry [12e14,26]. Therefore, it is interesting
to study variation in the bandgap and crystal structure of ZnS thin
films with substrate temperature. In the present paper, the RF
magnetron sputtering is used to grow crystalline and textured ZnS
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S.P. Patel et al. / Vacuum 85 (2010) 307e311
thin films at different substrate temperatures (TS). The correlation
between the grain growth, surface roughness, structural transformation from ZB to W phase, stoichiometry and bandgap of films
with TS will be discussed in detail.
2. Experimental
RF magnetron sputtering (13.56 MHz) method was utilized
to fabricate ZnS thin films on (100)-oriented silicon (Si) and fused
silica substrates. The films were deposited at 200 C, 300 C and
400 C substrate temperatures. The vacuum chamber was evacuated up to 1.2 10 5 mbar base pressure using a turbo molecular
pump. A commercially available high purity (99.99%) ZnS target of
51 mm in diameter was used for the sputtering. The sputtering was
carried out with Argon-plasma at gas pressure of the order of
2.0 10 2 mbar using 150 W RF power. A 200-Gauss magnet was
kept behind the target to apply the magnetic field in the sputtering
chamber. The distance from the ZnS target to substrates was fixed
to about 40 mm for the deposition. The Rutherford backscattering
spectrometry (RBS) measurements were performed to determine
the composition and thickness of films on Si substrates. The RBS
analyses were carried out using 1.8 MeV Heþ1 ions provided by the
ARAMIS accelerator of CSNSM at Orsay campus, France, at normal
incidence to the sample surface with collection of the backscattered
ions at an angle of 165 . The crystal structures and grain size of the
film were analyzed by X-ray diffraction (XRD). The XRD patterns
were recorded with Bruker D8 Advance diffractometer at glancing
A) source. The surface topogangle of 1, using Cu Ka (l ¼ 1.5406
raphy, roughness and grain size of films were analyzed by means of
atomic force microscopy (AFM) in taping mode, using a Digital
Instruments Nanoscope IIIa. Ultra-Violet and Visible (UVeVis)
absorption spectra were recorded on dual beam Hitachi U-3300
spectrophotometer for electronic bandgap studies of films.
3. Results and discussion
Fig. 1 displays the RBS spectrum of ZnS nanocrystalline thin film
deposited on Si at substrate temperature 400 C. There is no
detectable contamination of carbon, oxygen or any other impurity
Fig. 1. RBS spectrum of ZnS thin film deposited at substrate temperature (TS) 400 C.
The star symbols show the experimental data and solid line shows the simulated result
carried out by using XRUMP code. The film thickness is around 800 nm. The content of
Zn is 50.5 atomic % and S is 49.5 atomic % in the film.
in the film. The contents of Zn and S in the film are found to be 50.5
at % and 49.5 at %, respectively as analyzed by XRUMP simulations
code with an accuracy better than 1 atomic%. The thickness of the
film is around 800 nm as determined by simulation using RBS data.
The RBS analyses of films deposited at 200 and 300 C gave similar
results of stoichiometry and film thickness. The XRD patterns of
films deposited on silicon at different TS are shown in Fig. 2. The
film deposited at 200 C shows an intense diffraction peak centered
at 28.54 corresponding to the (111) lattice plane of zinc-blende
phase (Joint Committee on Powder Diffraction Standard, JCPDS
Card No. 77-2100), marked by dotted line and labeled ZB in the
pattern. No other diffraction peaks is observed in the pattern which
confirms that the film deposited at 200 C TS is textured along (111)
plane. The films deposited at 300 C TS contain a mixture of W and
ZB phases. The films deposited at 400 C exhibit diffraction peaks at
26.84 , 28.34 , 47.36 and 56.18 which are characteristic positions
of (001) (002), (110) and (112) lattice planes for the wurtzite-ZnS
(JCPDS Card No. 75-1547), as labeled W in the pattern. Inset of Fig. 2
shows more clearly the shifting of (111) peak of ZB phase to (002)
peak of W phase containing structural transformation of ZnS from
ZB to W phase. Similar XRD pattern and structural transformation
have been observed on fused silica substrate as well. It is found that
the growth of textured ZnS film is independent of underlying
periodicity of the substrate. The texturing of films has been
reported in some of the oxides materials [27,28] and ZnS [29] with
different film deposition method, in which films orientation is
independent of periodicity of substrates. The fused silica has no
long range periodicity that can help the oriented/epitaxial growth
of crystalline films as in case of single crystal (100)-oriented Si
substrate but we have observed texturing on fused silica as well
[29]. The minimization of surface energy, which is driving force for
the film growth, is responsible for the texturing of ZnS film on fused
silica and silicon. In growth process, the orientation of films
preferred in such a crystallographic plan which have highest atomic
density lies parallel to the substrate surface [27,28]. The DebyeScherrer formula (D ¼ kl/bCosq) is used to determine the grain size,
where D is the mean grain size, k is a geometric factor (¼0.9), l is
the wavelength of the X-ray wavelength, b is the full width at half
Fig. 2. XRD patterns of ZnS thin films grown at different TS. The film deposited at TS of
200 C is extremely textured along (111) plane of ZB-ZnS while those deposited at TS
of 300 and 400 C are in W-ZnS and lose their textured nature. The inset shows the
extended part of Fig. 2 which shows the shifting of (111) plane of ZB phase to (002)
plane of W phase.
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S.P. Patel et al. / Vacuum 85 (2010) 307e311
maximum (FWHM) of diffraction peaks and q is the diffraction
angle [30]. The grain size of films at different TS of 200 C, 300 C,
and 400 C are found to be 19 nm, 23 nm, and 25 nm, respectively.
Fig. 3(a), (b) and (c) shows the AFM images of ZnS thin films
deposited on Si at TS of 200 C, 300 C, and 400 C, respectively. The
surface roughness and grain size of the films at various TS are
shown in Fig. 3(d). As it is clear from the topographic images that
the higher TS lead to bigger grains at the surface which are in
agreement with XRD results. It should be pointed out here that the
XRD gives the in plane grains size while AFM gives the out of plane
grain size. The lower surface roughness and grain size at TS of
200 C is due to the extremely textured nature of the films. On the
contrary, the film which is grown at TS of 300 and 400 C grains has
more random orientation instead of columnar shape. At higher TS,
the enhancement of surface mobility of adatom also promotes the
grain growth at the expense of smaller grains [31,32]. Both factors,
loss of texture and grain growth explain the higher surface
roughness. The final grain size in the film is a function of the
respective crystal nucleation and growth rates, and the latter is
affected by the stress developed in the film [31].
Fig. 4 shows the UVeVisible absorption spectra of ZnS thin film
deposited at different TS. The bandgap of films is calculated by using
309
Fig. 4. UVeVis absorption spectra of ZnS thin films deposited at different TS. The inset
shows the plot of (ahn)2 versus hn for determination of bandgap. The bandgap of films
grown at TS of 200, 300 and 400 C are found to be 3.38, 3.44 and 3.49 eV, respectively.
Fig. 3. AFM images of ZnS thin films deposited at (a) TS ¼ 200 C (b) TS ¼ 300 C (c) TS ¼ 400 C and (d) variation in roughness and grain size with TS.
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Tauc’s plots [12e14] as shown in inset of Fig. 4. The bandgaps are
found to be 3.38 eV, 3.44 eV, and 3.49 eV at TS of 200 C, 300 C, and
400 C respectively. The observed bandgap in present study are less
as compared to that of bulk ZnS of 3.67 eV for ZB and 3.90 eV for W
phase [21]. A smaller value of bandgap for all the films compared to
bulk compounds as observed here is usually due to defect states
which widen the tail in band edge. Thin film may contain a few
hundreds ppm of S vacancies or Zn interstitials as supported by RBS
results, explaining the observation of lower values of bandgap than
that for bulk stoichiometric phases [12]. One interesting observation is that the bandgap of films increases with increasing TS.
Theoretically, the bandgap of nanocrystalline ZnS thin films can be
affected by following factors: i) the small grain size which tends to
increase the gap of any semiconductor, ii) the fraction of W and ZB
phases [25], iii) the Zn to S stoichiometry ratio. Recently, Luo et al.
have reported an increase in bandgap of ZnS thin films with
increasing deposition temperature and explained their results on
the basis of Zn and S stoichiometry [13]. They proposed that at low
temperatures, the films are deficient in S, which causes the
reduction in ZnS bandgap. With the increase in deposition
temperature, the composition ratio of Zn and S become close to 1,
so that the bandgap increases up to 3.7 eV. In the present work, RBS
analyses indicates that the films deposited at different TS have
similar composition, and the stoichiometry does not play any role
in increasing the bandgap. The observed growth of grains in the
films with increasing TS should lead to a narrowing of the gap by
suppression of the confinement effect, so that the actual increase in
bandgap of films with TS is surprising. Therefore remain only the
structural phase change as origin of increase of the bandgap with
TS. The similar explanation about increase in bandgap of ZnS thin
film with increasing TS has been proposed by Yeung et al. [25].
In their study the bandgaps were 3.28 eV, 3.44 eV, 3.47 eV and
3.5 eV for the film deposited at 150 C, 250 C, 350 C and 450 C,
respectively. Qadri et al. [22] have reported that ZnS nanoparticles
of sizes 2.7 nm change its structure at 400 C during annealing.
They observed no W phase up to 350 C but above this annealing
temperature, W phase started to appear and the proportion of W
phase was of 28% at 400 C. The proportion of W phase is already
preponderant in our samples deposited at 300 C and films
deposited at 400 C are made of W only, probably because of the
different preparation technique than in Qadri et al. study [22]. As
far as size dependent properties of low-dimensional materials are
concerned, the present study relates to nanostructured thin films,
whereas Qadri et al. [22] studied structural transformations in
nanoparticles. Nanostructured materials (NSs) and nanoparticles
(NPs) may have different properties, such as melting temperature,
lattice expansion, and cohesive energy [33,34]. The differences of
cohesive energy of NSs in comparison with NPs reflect that the
thermal properties of NSs are more stable than the NPs. Clearly, the
differences in properties is caused by the coordination number
imperfection at the surface related to the presence of broken bonds
in NPs while in NSs is attributed to the bond elongation at the grain
boundaries [33].
Thermodynamically, a chemical process always develops
towards the lower Gibbs free energy. The ZnS films minimize their
Gibbs free energy by taking the W structure at higher TS of 300 and
400 C [35,36]. The coefficient of volume thermal expansion,
defined by aV ¼ (dV(T)/dT)/V(T), of W-ZnS is lower than that of
ZB-ZnS [36]. The Gibbs free energy variation DG ¼ DF þ PDV, and
the smaller volume expansion coefficient of W phase make it stable
at higher TS [35,36]. The change in Helmholtz free energy DF is
always negative as temperature increases, and W phase of ZnS has
larger DF than the ZB phase up to 1200 K [36]. It may be noted here
that the temperature is not the only factor which decides the
structural phase stability and phase transition. There are several
other parameters like, pressure and thickness of the film. Experimentally, it was observed that when ZnS nanoparticles of 3 nm
sizes were heated in vacuum, transformed into W phase from ZB
phase, slightly at lower temperature [23]. Le et al. have developed
a model and included many parameters in variation of Gibbs free
energy to explain the phase selection phenomena of ZnS thin films
and nanoparticles. The variation of Gibbs free energy is given as DG
(T,P,D) ¼ DGv (T,0,N) þ DGs (D) þ DGe (P,D), where DGv (T,0,N) is
variation of Gibbs free energy in bulk crystal, DGs (D) is a Gibbs
surface free energy as function of size, DGe (P,D) is Gibbs elastic
energy dependent on size and pressure [35]. In the present case, the
nanocrystalline nature of film, substrate temperature and very low
deposition pressure makes the formation of stable W phase of ZnS
at 400 C.
4. Conclusion
Textured and well crystallized ZnS thin films have been
synthesized at substrate temperature (TS) of 200 C. The films
grown at TS of 300 C and 400 C lose their textured nature. The
films deposited at different TS have similar stoichiometry as shown
by RBS analyses. Grain growth in the film is observed with
increasing TS which leads to the increase in surface roughness.
A structural transition of ZnS from ZB to W phase is observed which
causes the increase in bandgap of films with increasing TS. In
conclusion, by controlling the substrate temperature (TS), one can
engineer several properties like grain size, surface roughness,
crystal structure and bandgap of the ZnS films.
Acknowledgment
The author (SPP) would like to express his sincere thanks to
Council of Scientific and Industrial Research (CSIR), New Delhi for
providing financial assistance through junior research fellowship
(JRF) to carry out this work. The authors are thankful to Mr. P. K.
Kulariya for XRD measurements. We are also thankful to Mr. S. A.
Khan for kind help. The help received from Mr. Jai Prakash is greatly
acknowledged.
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