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Vacuum 82 (2008) 561–565

www.elsevier.com/locate/vacuum

Iodization of antimony thin films: XRD, SEM and optical studiesof nanostructured SbI3

D. Bharathi Mohan, Anu Philip1, C.S. Sunandana�

School of Physics, University of Hyderabad, Gachibowli, Hyderabad 500046, Andhra Pradesh, India

Received 6 May 2007; accepted 11 August 2007

Abstract

Antimony films of thicknesses of 25, 40 and 80 nm deposited using thermal evaporation technique were annealed at 200 1C for 6 h.

Programmed iodization was carried out at room temperature for periods ranging from 5min to 9 h on both as-deposited and annealed

films. X-ray diffraction studies on iodized films reveal that antimony tri-iodide nanoparticles grow only on annealed antimony films.

Surface morphology as revealed through SEM consists of antimony and antimony tri-iodide nanoparticles are 25 nm and 1 mm,

respectively. Optical absorption of Sb and SbI3 nanoparticles carried out at room temperature. As-deposited antimony film of thickness

25 nm exhibits a sharp rise in the absorption near ultraviolet region while post-deposition annealed films were characterized by red

shifted absorption. Interestingly 45 nm thick Sb films exhibit a broad volume plasmon resonance peak around 500 nm with a width of

200 nm. Progressive iodization of 25 nm thick film reveals two absorption bands at 381.7 nm (A2) and 458.3 nm (B3) with photon energies

3.25 and 2.70 eV, respectively, due to the development of SbI3 valence band structure. Last members of hydrogen-like series of

absorption levels A2 and B3 due to halogen doublet (3/2, 1/2) splitting have been observed at room temperature.

r 2007 Published by Elsevier Ltd.

Keywords: Thermal evaporated antimony thin films; Antimony tri-iodide; Crystal structure; Surface morphology; Optical absorption

1. Introduction

Antimony tri-iodide with hexagonal structure [1] is animportant precursor in the synthesis of ferroelectric SbSI[2]. Crystalline SbI3 exhibits second-harmonic generation[3]. This photosensitive film has found applications insolid-state batteries as cathode [4], high-resolution imagemicrorecording and information storage [5–7]. SbI3 belongsto the trigonal system with a space group C2

3i. Unlikealkali halides antimony tri-iodide has a rhombohedralcrystal structure in which the halogen atoms are in almostperfect hexagonal packing. In the present work, we reporton: (1) structural, microscopic and optical characterizationof Sb films deposited on glass substrates by thermalevaporation; (2) short- and long-duration iodizationof Sb films as functions of (a) iodization time and (b) filmthickness; (3) effect of thermal annealing on the (micro)-

ee front matter r 2007 Published by Elsevier Ltd.

cuum.2007.08.014

ing author. Tel.: +9140 23134324; fax: +91 40 23010227.

ess: csssp@yahoo.com (C.S. Sunandana).

e M.Sc. project of Anu Philip.

structural and optical properties from the point of view ofnanostructure development. The main focus of thisinvestigation is to look into the effect of film thickness on(a) iodization kinetics, (b) crystal structure and (c) aspectsof valence band structure of SbI3. This study forms a partof our continuing studies on iodization of metal halide(doped AgI mainly) films which have potential applicationsas sensor and optoelectronic device elements [8–10].Certain interesting features of structure and opticalcharacteristics of precursor Sb thin films are also high-lighted in this paper. In an earlier investigation [11,12] wehave used Sb as an alloying element to Ag to retardiodization kinetics of Ag1�xSbx (0oxo0.13) thin films.

2. Experimental procedure

Portions of 6N pure antimony (Sb) (Nuclear FuelComplex, Hyderabad, India) placed in molybdenum (Mo)boat in HINDHIVAC 20A Vacuum Evaporator and apressure of 5� 10�6mm was established. Antimony thinfilms with average thicknesses 25, 45 and 80nm were

ARTICLE IN PRESSD. Bharathi Mohan et al. / Vacuum 82 (2008) 561–565562

fabricated on commercially available microglass slides bythermal evaporation. Prior to the deposition, these sub-strates were cleaned as follows: substrates were firstimmersed boiling 10% soap solution with 90% water,rubbed with cotton in cold water to remove weathering, keptin chromic acid up to boiling point for removing organiccontaminates, washed in cold water to remove surfacecontaminants followed by ultra-sonification in iso-propylalcohol for 3–5min duration, and finally these substrateswere dried in air before loading in to the system fordeposition. Thickness of coated films was determined by adigital thickness monitor (DTM model no. DTM101) usinga quartz crystal. To iodize Ag films, a figure of 8-shapediodization glass chamber was fabricated with dimensions20 cm height� 4 cm diameter and 1mm hole made at thecenter of the chamber to control iodization rate which playsan important role for nanoparticles (structure) production.Iodine kept at the bottom of the lower half of the chambersublimates at room temperature and slowly deposits on theAg films kept at the top of the chamber. The small orifice atthe center serves as ‘flux limiter’ of I2 vapor molecules. Thus,iodization was carried for selected durations in the range5min to 9 h. Thin films were characterized by XRD using aPHILLIPS X-ray powder diffractometer with Cu Ka

(l ¼ 1.54056 A) radiation. To analyze the surface morphol-ogy, films were examined by SPA 400 atomic forcemicroscope (AFM) using non-contact dynamic force mode(DFM). SHIMADZU UV-3101 spectrophotometer hasbeen used for absorption studies at 300K in the UV–visrange from 300 to 800 nm with scanning rates of 4 nm/s.

3. Results and discussion

Fig. 1(a) shows X-ray diffraction patterns of thickness25 nm as-deposited antimony, post-annealed and iodized

10 20 30 40 50 60

(a1)

#

(006

)

(003

)

# (a2)

2θ in deg.

(a3)

(124

) *

(119

)

(a4)

(a5)

Inte

nsity

(a.

u.) (1

23)

*

(a6)##

(a7)*

(300

)

*

Fig. 1. X-ray diffraction patterns of thermally evaporated Sb films of thicknes

5min, a4: iodized for 10min, a5: iodized for 30min, a6: iodized for 1 h and a7:

and iodized for periods ranging from 5min to 6 h (b1: as-deposited, b2: anneale

for 1 h and b6 and b7: iodized after annealing for 6 and 9 h, respectively).

for periods ranging from 5min to 6 h. Unannealed(as-deposited) films are X-ray amorphous indicating the‘quenched disorder’ effect with much strain stabilizedthereby. Annealing at 200 1C relieves strain and promotescrystallization of antimony as shown in Fig. 1(b1).Iodization of these films favors facile growth of antimonytri-iodide as reflected from (3 0 0), (1 2 3) and (1 2 4) crystalplanes while iodization of unannealed films reveals onlyretarded growth of SbI3 nanoparticles.Fig. 1(b) depicts the typical XRD of an as-deposited

antimony film of thickness 80 nm which shows two weakBragg peaks at 2y values of 23.61 and 40.191 identified as(0 0 3) and (1 0 4) reflections characteristic of hexagonal Sb(a ¼ 0.7485 nm, c ¼ 2.0932 nm). The broad hump in thelower angle region is due to the amorphous nature ofthe film possibly involving intrinsic stress and strain at thelattice sites. Stress and strain are apparently released uponannealing at 200 1C for 6 h as seen from non-amorphousnature (Fig. 1(b1)). Though the broad hump became ahorizontal straight line, the intensities of the XRD peaksare still poor which could be due to a special nature ofantimony. Brief iodization (5–10min) of unannealedantimony films does not change the structure appreciably.After 1 h iodization, however, the film exhibits five Braggpeaks which corresponds to hexagonal structure of SbI3confirmed after indexing from the JCPDS values. Iodiza-tion was carried out on annealed films as well, the XRD ofwhich shows facile growth of SbI3 nano/micro particles asclearly seen from the Fig. 1(b). Thermal annealingcompletely removes intrinsic strain in an as-depositedantimony film there by promotes crystallization concur-rently. Hence the growth of SbI3 structure is tremendous inthis case.We used AFM to investigate the surface topography of

SbI3 nanoparticles growth upon iodization at room

10 20 30 40 50 60

(b1)##

(104

)

(003

)

Inte

nsity

(a.

u.)

2θ in deg

(b2)

(b3)

(b4)

(b5)

(b6)

(b7)*

*

*

**

** (1

19)

(300

)

(116

)

(113

)

(006

)

(012

)(0

03)

s (a) 25 nm (a1: as-deposited, a2: annealed at 200 1C for 6 h, a3: iodized for

iodized for 6 h after annealing) and (b) 80 nm as-deposited, post-annealed

d at 200 1C for 6 h, b3: iodized for 5min, b4: iodized for 30min, b5: iodized

ARTICLE IN PRESSD. Bharathi Mohan et al. / Vacuum 82 (2008) 561–565 563

temperature. A set of AFM images with 2 mm� 2 mm fieldof view are shown in Fig. 2 for (a) 25 nm thick as-depositedSb film and (b) for an Sb films iodized for 6 h. Nearlyhomogeneous distribution of antimony nanoparticles withmesoporous structure has been observed on as-depositedantimony film (Fig. 2(a)). The particle size is 2572 nm, asmeasured using DFM analysis software. After a 6 hiodization, antimony particles are converted in to anti-mony tri-iodide nanoparticles; the corresponding morphol-ogy often shows bigger size of particles of about 1 mmapproximately. The shape of the SbI3 particles is like smalltiles packed rather loosely.

Antimony tri-iodide is photosensitive in the visibleregion of the electromagnetic spectrum. Since it is an ionicmaterial possessing hexagonal structure it would beinteresting and important to investigate the opticalabsorption in order to unravel aspects of its electronicband structure. Fig. 3 shows the optical absorption spectraof as-deposited, post-annealed and iodized antimony films,recorded at ambient temperature.

As-deposited antimony film of thickness 25 nm exhibits asharp rise in the absorption near ultraviolet region which isoften possible in partially or fully transparent films(thickness could be p50 nm) with smaller particle size(nanocrystals) and large surface roughness when depositedon glass. On increasing film thickness to 45 nm, a verysharp rise in the absorption is still observed, in addition toa very broad strong and inhomogeneous broadenedresonance in the visible region. It possibly arises from thecollective excitation of valence electrons in bulky Sbnanoparticles, i.e., the bulk or volume plasmons insteadof surface plasmons. Surface plasmons cannot be excitedon a bulk metal sample by direct photon irradiationbecause it is not possible to conserve energy andmomentum simultaneously in a photon–surface plasmoninteraction. Also surface plasmons propagate a speed lessthan the speed of light in air/vacuum and therefore havehigher momentum than a photon of the same energy. The

Fig. 2. Atomic force micrographs of 25 nm film thickness of thermally

line width of the plasmon resonance is approximately200 nm which is very large and is controlled by processessuch as crystallite/cluster shape fluctuations. In addition,cluster matrix interaction can also contribute to thelinewidth. In fact the ‘composite’ linewidth in such a caseis the sum of several individual plasmon absorption lines,belonging to clusters of different sizes. At the microscopiclevel the range of wavelengths for which a plasmon isexcited depends on the following effects: (a) electronconfinement in a particle with size much smaller thanelectronic mean free path in bulk; (b) elastic electronscattering by defects and impurities inside the particle;(c) surface morphological features such as roughness andshape of the particles and, finally, (d) the surroundingenvironment [13–16]. Upon controlled iodization, iodinevapor was deposited on the Sb surface and by the action ofseveral processes initiated by surface diffusion at thegas–solid interface, antimony surface was eventuallyiodized. Antimony tri-iodide nanoparticles develop on thesurface at the early stage of iodization (15, 30min), whereasthe bottom of the film remains as antimony nanostruc-ture—the two constituting a Sb–SbI3 composite film.Formation of SbI3 nanostructure upon controlled iodiza-tion chiefly depends on the rate of iodization and the filmthickness. SbI3/Sb interface is probably formed duringshort-time iodization where SbI3 acts as a dielectric matrixin which Sb particles embedded or aggregated on thesurface shift the plasmon resonance to lower wavelengthsand become close enough to interact electromagnetically.During intermediate-term iodization, a portion of the Sbfilm probably iodizes first and becomes SbI3 which possiblyacts as a surrounding medium for Sb nanoparticles thatwere left on the surface and thus increasing the surfaceroughness which eventually leads to the shift and broad-ening of volume plasmon resonance. In our previous work,a volume plasmon–exciton (metal to semiconductor)transition was studied upon controlled iodization on rfsputtered Ag films at room temperature [17]. Ag/AgI

evaporated (a) as-deposited antimony and (b) after 3 h iodization.

ARTICLE IN PRESS

300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

(a2)

(a1)

Abs

orba

nce

(a. u

.)

Wavelength (nm)

Thickness: 250 A0

(a1) As deposited Sb

(a2) Annealed for 6 hrs at 200°C

300 400 500 600 700

0.0

0.2

0.4

0.6

0.8

1.0

559.

6 nm45

8.3

nm

381.

7 nm

(b4)

(b3)

(b2)(b1)

Abs

orba

nce

(a. u

.)

Wavelength (nm)

Thickness: 250 A0

(b1) As deposited Sb

(b2) 5 min iodized

(b3) 1 hr iodized

(b4) 8 hrs iodized

300 400 500 600 700

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

448.

7 nm

380

nm

547

nm

(c5)(c4)

(c3)

(c2)

(c1)

Abs

orba

nce

(a. u

.)

Wavelength (nm)

Thickness: Sb 450 A0

(c1) As deposited Sb(c2) Annealed for 6 hrs

(c3) Iodized for 5 min

(c4) Iodized for 30 min

(c5) Iodized for 1 hr

300 400 500 600 700-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

383.

7 nm

460.

25 n

m

Thickness: 800 A0

(d1) As deposited Sb

(d2) Annealed for 6 hrs

(d3) Iodized for 6 hrs (afterannealing)

(d4) Iodized for 9 hrs (afterannealing)(d4)

(d3)

(d2)

(d1)

Abs

orba

nce

(a. u

.)

Wavelength (nm)

Fig. 3. Optical absorption spectra of antimony with the thicknesses of (a and b) 250 A, (c) 450 A and (d) 800 A antimony films recorded at room

temperature. Note: A step at 360 nm is due to change in the lamp from visible to ultraviolet region.

D. Bharathi Mohan et al. / Vacuum 82 (2008) 561–565564

interface was studied during intermediate-term iodization.The range of wavelengths for which a plasmon excited ishighly sensitive to the refractive index of the medium thatsurrounds the metallic particles and surface roughness iswell known [18].

Further increase in the thickness of the film completelywipes out the steep absorption and volume plasmonresonance. One then obtains a negative absorption at300 nm (which is very common in all opaque metal films)due to antimony particles reflecting the electromagneticradiation in that region. Annealing reduces the defects likeshort-range order thus enhancing the growth of antimonyclusters with defect less structure probably resulting in redshifted absorption (Fig. 3(d)). Progressive iodization of25 nm thick film reveals two absorption bands at 381.7 and458.3 nm perhaps due to the development of SbI3 valenceband structure. Band gap energy is determined from theexpression of the semiconductor with a direct band gap(ahn)2 ¼ B(hn�Eg), where a is the absorbance coefficient,

hn the energy of incident light, B the parameter thatdepends on the interband transition probability, and Eg isthe energy gap. The plot of (ahn)2 vs. incident photonenergy hn is found to be a straight line. The intercepts ofthe linear plots on the energy axis at (ahn)2 ¼ 0 gave bandgap value. The corresponding photon energies of twoabsorption bands of 381.7 and 458.3 nm are 3.25 and2.70 eV, respectively. These band structures are developedat room temperature itself after 8 h iodization. Evansobserved five absorption peaks (corresponding photonenergies are 2.59, 2.69, 3.05, 3.17 and 3.23 eV labeled as A1,A2, B1, B2 and B3, respectively) at 125K, which are absentin room temperature measurements [5]. Among them onlyA2 and B3 due to halogen doublet (3/2, 1/2) splitting havebeen observed in the present work and attributed to lastmembers of two hydrogen-like series of absorption levels.The energy separation between A2 and B3 absorptions atroom temperature is calculated as 0.55 eV, whereas Evansobserved 0.53 eV in low temperature measurements.

ARTICLE IN PRESSD. Bharathi Mohan et al. / Vacuum 82 (2008) 561–565 565

Thicker films (80 nm) of antimony were initially annealedand soon after iodized for periods ranging from 6 to 9 hreveal blue shifted absorption by a few nanometers withincreasing absorption intensity as shown in Fig. 3(d).Volume plasmon is not observed on evaporated antimonyfilms with increasing film thickness to 80 nm due toincreased size of antimony particles.

4. Conclusions

Antimony films of thicknesses of 25 and 80 nm weredeposited using thermal evaporation technique and filmswere annealed at 200 1C for 6 h. Controlled iodization wascarried at room temperature for periods ranging from5min to 9 h on both unannealed and post-annealed films.Antimony tri-iodide nanoparticles growth has beenobserved on annealed antimony films as investigated fromX-ray diffraction. Particle size of antimony and antimonytri-iodide nanoparticles are 25 nm and 1 mm, respectively,realized through AFM. Optical absorption of Sb and SbI3nanoparticles is carried out at room temperature. As-deposited antimony film of thickness 25 nm exhibits asharp rise in the absorption near ultraviolet region whilepost-annealed films resulting in red shifted absorption.Progressive iodization of 25 nm thick film reveals twoabsorption bands at 381.7 nm (A2) and 458.3 nm (B3), andto corresponding photon energies are 3.25 and 2.70 eV,respectively, perhaps due to the development of SbI3valence band structure. A2 and B3 absorptions due tohalogen doublet (3/2, 1/2) splitting have been observed andattributed to last members of two hydrogen-like series ofabsorption levels.

Acknowledgment

The authors acknowledge the University of Hyderabadfor the award of a research fellowship to D. BharathiMohan under the UPE programme.

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