Comparative study of physical properties of vapor chopped and nonchopped Al2O3 thin films

Article abstract of DOI:10.1016/j.materresbull.2011.02.037

Aluminium oxide being environmentally stable and having high transmittance is an interesting material for optoelectronics devices. Aluminium oxide thin films have been successfully deposited by hot water oxidation of vacuum evaporated aluminium thin films. The surface morphology, surface roughness, optical transmission, band gap, refractive index and intrinsic stress of Al ₂O₃ thin films were studied. The cost effective vapor chopping technique was used. It was observed that, optical transmittance of vapor chopped Al₂O₃ thin film showed higher transmittance than the nonchopped film. The optical band gap of vapor chopped thin film was higher than the nonchopped Al₂O₃, whereas surface roughness and refractive index were lower due to vapor chopping.

Full text of DOI:10.1016/j.materresbull.2011.02.037

Materials Research Bulletin 46 (2011) 815–819
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Comparative study of physical properties of vapor chopped and nonchopped
Al2O3 thin films
c
c
Sikandar H. Tamboli a, Vijaya Puri b, R.K. Puri a, , R.B. Patil , Meng Fan Luo
*
a Vacuum Techniques & Thin Film Laboratory, USIC, Shivaji University, Kolhapur 416004, India
b Department of Physics, Shivaji University, Kolhapur 416004, India
c Department of Physics, National Central University, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history:
Aluminium oxide being environmentally stable and having high transmittance is an interesting material
for optoelectronics devices. Aluminium oxide thin films have been successfully deposited by hot water
oxidation of vacuum evaporated aluminium thin films. The surface morphology, surface roughness,
optical transmission, band gap, refractive index and intrinsic stress of Al2O3 thin films were studied. The
cost effective vapor chopping technique was used. It was observed that, optical transmittance of vapor
chopped Al2O3 thin film showed higher transmittance than the nonchopped film. The optical band gap of
vapor chopped thin film was higher than the nonchopped Al2O3, whereas surface roughness and
refractive index were lower due to vapor chopping.
Received 13 October 2010
Received in revised form 12 February 2011
Accepted 17 February 2011
Available online 24 February 2011
Keywords:
A. Oxides
A. Thin films
ß 2011 Elsevier Ltd. All rights reserved.
D. Optical properties
D. Mechanical properties
D. Surface properties
1. Introduction
In this paper, we report the physical properties of Al2O3 thin
films that have been prepared by using hot water oxidation of
A
combination of high dielectric constant, high thermal
vacuum evaporated vapor chopped and nonchopped aluminium
thin films. The effects of vapor chopping on optical transmittance,
optical band gap, refractive index, surface morphology, surface
roughness and optical signal transmission loss properties were
investigated.
conductivity, wear resistance, mechanical strength, chemical
inertness, good adhesion to glass substrate and transparency over
wide wavelength range [1–5] makes Al2O3 thin film a leading
candidate for optoelectronics and microelectronics devices. The
refractive index (1.76) of these films is in the range suitable for
optical waveguide purpose also [4,5]. The properties of deposited
Al2O3 thin films depend on the deposition process and optimized
parameters. Reduction in defects and void formation and improved
surface morphology are the basic requirements for minimum
optical signal loss in optical waveguides.
Number of methods have been reported for deposition of Al2O3
thin films such as reactive magnetron sputtering [3], ion beam
assisted deposition [6], pulsed laser deposition [4], spray pyrolysis
[2], chemical vapor deposition, electron beam evaporation [7],
cathodic vacuum arc [1], etc. The crystal structure, refractive index
and adhesion properties of electron beam deposited vapor
chopped Al2O3 thin films have been reported by our group
previously [7].
2. Experimental
Aluminium oxide thin films have been prepared by using hot
water oxidation of vacuum evaporated vapor chopped and
nonchopped aluminium thin films, deposited on glass substrates
under a vacuum of 10ꢀ5 mbar. Pure aluminium wire (Balzers
99.99%) was used as the evaporation source. After the deposition of
vapor chopped and nonchopped aluminium thin films by resistive
heating, these aluminium thin films were oxidized by using hot
water oxidation method [7]. Three thicknesses of the Al2O3 thin
film 100, 200 and 300 nm were investigated. During hot water
oxidation the formation of Al2O3 could be observed from the
disappearance of mirror reflecting aluminium metal thin film and
becoming transparent. The vapor chopping technique (VCT)
consists of a chopper which is a circular aluminium metal sheet
of 10 cm diameter having a V-cut (1558) shape. This thin circular
vane was fixed to a light aluminium rod attached to a motor. The
rate of chopping in this study was 6 rot/s. As the chopper rotated,
the filaments were exposed to the substrates. The vane was at a
height of 10.5 cm from the source. The distance between source
* Corresponding author. Tel.: +91 0231 2609245; fax: +91 0231 2691533.
E-mail addresses: sikandar.physics@gmail.com (S.H. Tamboli),
vrp_phy@unishivaji.ac.in (V. Puri), rkp_usic@unishivaji.ac.in (R.K. Puri),
rrahulpatil@gmail.com
(R.B. Patil), mfl28@phy.ncu.edu.tw (M.F. Luo).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.02.037

816
S.H. Tamboli et al. / Materials Research Bulletin 46 (2011) 815–819
40
35
30
25
20
15
10
5
and substrate was 12.5 cm. As the chopper rotated, the filaments
were exposed to the substrates [7–9].
The film thickness was as measured by Tolansky interferomet-
ric method. The structural analysis was carried out with a Philips
VC
˚
diffractometer (Philips PW 3710) at 1.54056 A. Surface morpholo-
gy study was done by SEM (JSM-6360 JEOL, JAPAN). The surface
roughness was measured by atomic force microscope (AFM). The
optical transmittance spectra were measured in the wavelength
range from 200 to 800 nm by using a spectrophotometer (U 2800,
Hitachi, Japan). The refractive index of Al2O3 thin films was
measured by spectrophotometric method. Optical signal trans-
mission loss property was measured by using prism coupling
method. The optical transmission loss measurement setup details
have been given elsewhere [10]. He–Ne laser has been used in this.
α
α
Laser beam consists spot size 150
mm and 458 angle of incident.
The intrinsic stress of the thin films was measured by
interferometric method [11] which works on Newton’s ring
principle. In this method the substrates were soda lime cover
slips of diameter 1.9 cm and 0.022 cm thickness. The stress was
calculated by measuring the variation in diameter of Newton’s ring
before and after deposition. The intrinsic stress was measured by
equation
0
30
40
50
60
70
80
90
100
2θ (degrees)
40
35
30
25
20
15
10
5
NC
Yh2ðKx ꢀ KyÞ
S ¼
6tð1 ꢀ Þ
y
where Young’s modulus (Y) = 3.44 ꢁ 1011
, Poisson’s ration
(y) = 0.22, thin film thickness (t), h = 0.022 cm (substrate thick-
ness), Kx, Ky are the slope difference of plot nl/2 vs. radius
Newton’s ring of before and after deposition.
α
α
3. Results and discussion
3.1. Structural
0
30
40
50
60
70
80
90
100
Fig. 1 shows the X-ray diffraction patterns of the nonchopped
and vapour chopped Al2O3 thin films. The dominant cubic (2 2 0),
2θ (degrees)
a
-Al2O3 and (4 2 2) phase was observed (reference code: 00-079-
Fig. 1. XRD spectra of vapour chopped (VC) and nonchopped (NC) Al2O3 thin films.
1557). (6 2 0) plane orientation was observed in only VC Al2O3 thin
films, it was absent in NC. It was observed that the peaks observed
in vapour chopped films were more intense than those in
nonchopped films. The increased intensity can be attributed to
the improved crystallinity, i.e. vapour chopped films were more
crystalline than the nonchopped thin films. The improved
structural order can be attributed to the increases in film density.
No characteristic peaks of impurity and other phases were
observed. The XRD patterns did not show evidence of the presence
of aluminium metal, indicating complete oxidation of aluminium
metal thin films to aluminium oxide thin films.
3.3. Optical transmittance
Fig. 4 shows the optical transmittance of vapor chopped and
nonchopped Al2O3 thin films of different thicknesses. The optical
transmittance of vapor chopped and nonchopped thin films
increased with the wavelength from 200 to 800 nm. It was
observed that, optical transmittance decreased with increase in
thin film thickness. Vapor chopped Al2O3 thin film showed higher
optical transmittance than the nonchopped thin film. Vapor
chopping produces better stoichiometry of the films, lesser defects
and voids formation. The smaller grains in the vapor chopped thin
film reduces the scattering losses at the grain boundaries whereby
increasing the optical transmittance.
3.2. Surface morphology
Fig. 2 shows the surface morphology of vapor chopped and
nonchopped Al2O3 thin films. It is seen that, nonchopped Al2O3
thin films show highly porous fibril morphology, whereas vapor
chopped Al2O3 thin film gives smooth, dense continuous
morphology.
Fig. 3 shows the 2-dimensional and 3-dimensional atomic force
micrographs of vapor chopped and nonchopped Al2O3 thin films.
The surface roughness of vapor chopped thin film was lesser than
the nonchopped Al2O3 thin film. The granular structured grains
were observed in vapor chopped as well as nonchopped Al2O3 thin
films. The grain size of nonchopped thin film was higher than the
vapor chopped Al2O3. Vapor chopped thin film was found to be
denser than the nonchopped Al2O3 thin film.
3.4. Optical band gap
Fig. 5 gives the graph of (athy y of Al2O3 thin
)2 as a function of h
films of 300 nm. From the absorption data, the band gap energy
was calculated using formula:
n
a0ðh ꢀ EgÞ
y
ðh
a
¼
yÞ
where ‘Eg’ is the separation between bottom of the conduction
band and top of the valence band, = absorption of thin film and
a

S.H. Tamboli et al. / Materials Research Bulletin 46 (2011) 815–819
817
Fig. 2. Surface morphology of vapor chopped and nonchopped Al2O3 thin films.
a
0 = absorption coefficient, ‘h
y
’ is the photon energy and ‘n’ is a
observed that, band gap increases with increase in thin film
thickness. The band gap of nonchopped film was lesser than
those of vapor chopped thin film. The lower observed band gap
energies of nonchopped Al2O3 film might be due to varied extent
of non-stoichiometry of the deposited layers, which may be due
to the various lattice associated atomic interaction phenomena
constant. The value of n depends on the probability of transition; it
takes values as 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden,
indirect allowed and indirect forbidden transition respectively.
Table 1 shows the optical band gap of vapor chopped and
nonchopped Al2O3 thin films for different thicknesses. It was
Fig. 3. 2-dimensional and 3-dimensional atomic force micrographs of vapor chopped (VC) and nonchopped (NC) Al2O3 thin films.

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S.H. Tamboli et al. / Materials Research Bulletin 46 (2011) 815–819
100
90
80
70
60
50
40
30
20
10
0
Table 1
Optical band gap and refractive index of vapor chopped and nonchopped Al2O3 thin
films for different thin film thickness.
Thin film thickness (nm)
Optical
band
Refractive index
gap (eV)
NC
VC
NC
VC
100
200
300
4.5
5
4.7
5.3
5.8
1.63
1.66
1.69
1.59
1.62
1.66
5.6
VC
coming into play due to the ionic crystalline nature. The band
gap values obtained by us are in the reported range [12,13] and
lesser than band gap of bulk aluminium oxide [14].
The lesser values of band gap of nonchopped film than vapor
chopped film might also be due to the presence of more oxygen
vacancies or due to defect related absorption or mobility gap of the
more amorphous films. This induces change in the electronic
structure of surfaces leading to a reduction in the ionic gap and in
turn its total band gap [9].
100 nm
200 nm
600
300 nm
200
300
400
500
λ (nm)
700
800
During the evaporation process, the solid material turns to
vapour form. The evaporated atoms (adatoms) acquires the kinetic
energy, these adatoms may or may not be completely thermally
equilibrated. When adatoms reaches nearer to the substrate, due to
their different diffusion coefficient, adatoms moves horizontally
over the substrate surface by jumping from one potential well to
the other [15,16], it depends on adsorption residence time,
adatoms nucleation rate and thermal activation energy of adatoms.
In this adsorption residence time, the adatoms may interact with
other adatoms to form a stable cluster. This adsorption residence
time depends on various parameters such as binding energy
between two adatoms, nucleation formation and their growth,
material evaporation rate, density of adatoms, vapor pressure, etc.
[15,16].
VCT interruptsthe vapour flow during the adatoms condensation
process before the condensation on the substrate with constant rate
which give more residence time to the previously evaporated
adatoms to settle completely at minimum energy potential well.
This helps to create new nucleation centers and helps to minimize
the columnar growth and form a uniform dense thin film. Adatoms
nucleation density and nucleation rate become lesser which
provides lesser defects and higher mobility of small cluster which
reduces the defect formation. It is seen that if all the stable adatom
pairs move quickly to join pre-existing larger clusters, then there
will be a major suppression of the nucleation rate [16].
100
90
80
70
60
50
40
30
20
10
0
NC
100 nm
200 nm
300 nm
200
300
400
500
λ (nm)
600
700
800
Fig. 4. Optical transmittance spectra of vapor chopped and nonchopped Al2O3 thin
films.
In vapour chopping technique, vapour chopper interrupt with
6 rot/s speed the evaporated material’s vapour flow, block some
evaporated adatoms and give more residential time for adatoms
condensation on substrate. Same thickness of VC and NC Al2O3 thin
film was maintained by providing more time for vapor chopped
thin film deposition than nonchopped film.
7
Band gap
6
5
4
3.5. Refractive index
The analytical method have been used for the calculation of
refractive index n using the following formula [17]:
NC
VC
3
2
1
0
"
#
1=2
pffiffiffiffiffiffi
2
ns2T f þ nsð1 þ R f Þ
T f þ nsð1 ꢀ R f Þ
n ¼
pffiffiffiffiffiffi
2
where n = refractive index of the film, ns = refractive index of the
substrate, Tf = transmittance of the film, Rf = reflectance of the film.
The refractive index values of nonchopped and vapour chopped
aluminium oxide thin films for different thicknesses calculated at
1.5
2.5
3.5
4.5
5.5
6.5
hυ (eV)
2
Fig. 5. (athy) against hy for vapor chopped and nonchopped Al2O3 thin films.

S.H. Tamboli et al. / Materials Research Bulletin 46 (2011) 815–819
819
80
60
40
20
0
grow as columnar structure with voids in between which causes
density fluctuations. These columnar internal structures strongly
scatter light. Even the grain boundaries and inhomogeneity in the
grain boundaries also scatter light. In most of these films the grain
size was of the order of magnitude of wavelength. Due to the vapor
chopping technique films grow with smaller grain size, lesser
defects, lesser voids, etc. causing lower scattering in these films.
The improved surface morphology and reduced columnar struc-
ture, cracks and voids formation of MgO thin films by using vapor
chopping technique observed by taking cross section SEM were
reported in our earlier report [20]. The optical transmission loss of
the vapor chopped aluminium oxide thin film waveguide being
lesser than that of nonchopped waveguide is due to the combined
effect of all the above processes. The more homogeneous
nucleation occurring in the vapor chopped films might be reducing
the reflection losses in the waveguide. The scattering losses due to
the irregularities at the film surface or interface and also due to
density fluctuations in the film are reduced due to vapor chopping.
Intrinsic stress
VC
NC
100
200
300
Thin film thickness (nm)
Fig. 6. Intrinsic stress of vapor chopped and nonchopped Al2O3 thin film for
different thin film thickness.
4. Conclusion
590 nm are tabulated in Table 1. It was observed that, refractive
index of VC and NC increased with increase in thin film thickness
whereas it decreased due to vapor chopping. The refractive indices
for vapor chopped thin films were in between 1.59 and 1.66 range
whereas it was 1.63–1.69 for nonchopped thin films. These values
are in the range reported [14,12].
Highly transparent aluminium oxide thin films were success-
fully deposited by hot water oxidized vacuum evaporated vapor
chopped and nonchopped Al thin films. The cost effective vapor
chopping technique has been found to modify the surface
morphology and increase the optical transmittance, band gap
with decrease in surface roughness and optical transmission loss of
the Al2O3 thin films. Thicknesses variation also affects the optical
transmittance, band gap, stress and refractive index whereas it has
very negligible effect on optical transmission loss. Vapor chopping
technique gives better quality and low scattering losses in optical
waveguiding applications which may open a new window in the
cutting edge research in this field.
3.6. Intrinsic stress
Fig.
6 shows the intrinsic stress of vapor chopped and
nonchopped thin films for various thin film thickness. It was
observed that, the intrinsic stress of vapor chopped and
nonchopped Al2O3 thin films decreased with increase in thin film
thickness. The vapor chopped thin films showed lesser intrinsic
stress than the nonchopped Al2O3 thin film for all thicknesses. The
intrinsic stress was tensile in nature.
Acknowledgements
Stress is related to crystal disorder, dislocations, voids [18] and
density of thin films [19]. Vapor chopping technique helps to
improve crystallinity and density of film and decrease in crystallite
size, lattice disorder and voids in thin films resulting in decrease in
intrinsic stress. At lower densities the stress distribution at the
interface is sufficient to deform individual particles and affect the
increment in intrinsic stress [18]. The dense vapor chopped Al2O3
thin film with less voids showed lesser intrinsic stress than
nonchopped thin films. The voids and pores in nonchopped thin
film is higher than in the vapor chopped thin film. Increase in
porosity of the thin film causes an increase in stress.
Mr. Sikandar H. Tamboli is thankful to Shivaji University,
Kolhapur for Departmental Research Fellowship (DRF). Vijaya Puri
acknowledges Research Scientists (C) award from University
Grants Commission (U.G.C.), India.
References
[1] Z.W. Zhao, B.K. Tay, G.Q. Yu, D.H.C. Chua, S.P. Lau, L.K. Cheah, Thin Solid Films 447–
448 (2004) 14.
[2] M. Aguilar-Frutis, M. Garcia, C. Falcony, G. Plesch, S. Jimenez-Sandoval, Thin Solid
Films 389 (2001) 200.
[3] K. Koski, J. Holsa, P. Juliet, Thin Solid Films 339 (1999) 240.
[4] A. Pillonnet, C. Champeaux, C. Bovier, R. Brenier, H. Jaffrezic, J. Mugnier, Appl. Phys.
A 69 (1999) 735.
3.7. Optical transmission loss
[5] R.T. Kersten, H.F. Mahlein, W. Rauscher, Thin Solid Films 28 (1975) 369.
[6] Q.Y. Zhang, W.J. Zhao, P.S. Wang, L. Wang, J.J. Xu, P.K. Chu, Nucl. Instrum. Meth.
Res. B 206 (2003) 357.
[7] P.V. Patil, D.M. Bendale, R.K. Puri, Vijaya Puri, Thin Solid Films 288 (1996) 120.
[8] R.B. Patil, R.K. Puri, Vijaya Puri, Mater. Lett. 62 (2008) 198.
[9] S.H. Tamboli, R.B. Patil, S.V. Kamat, R.K. Puri, Vijaya Puri, J. Alloys Compd. 477
(2009) 855.
[10] P.V. Patil, R.K. Puri, Vijaya Puri, J. Opt. 27 (1998) 33.
[11] R.B. Patil, R.K. Puri, Vijaya Puri, J. Phys. Conf. Ser. 114 (2008) 012036.
[12] K.S. Shamala, L.C.S. Murthy, K.N. Rao, Mater. Sci. Eng. B 106 (2004) 269.
[13] S.D. Mo, W.Y. Ching, Phys. Rev. B 57 (1997) 15297.
[14] M. Schmid, M. Shishkin, G. Kresse, E. Napetschnig, P. Varga, M. Kulawik, N. Nilius,
H.P. Rust, H.J. Freund, Phys. Rev. Lett. 97 (2006) 046101.
[15] K.L. Chopra, Thin film Phenomena, McGraw-Hill, New York, 1969.
[16] J.A. Venables, Surf. Sci. 299/300 (1994) 798.
The optical transmission loss study of Al2O3 thin film
waveguide of thickness 300 nm showed that, optical transmission
loss of vapor chopped thin film (3.73 dB/cm) was lesser than the
nonchopped Al2O3 (6.01 dB/cm). The optical transmission loss
values of both VC and NC thin films were lesser than those reported
(10 dB/cm) by Kersten et al. [5]. The effect of thin film thickness
variations, i.e. 100, 200 and 300 nm was negligible ꢂ0.5–0.8 dB/
cm. The input optical beam scattering from the surface of thin film
as it passes through the guiding structure is a root cause of optical
signal transmission loss. Scattering of the beam occurs during the
interaction of beam and thin film surface. AFM data clearly shows
that, the vapor chopping technique gives smoother Al2O3 films
than the nonchopped films. Surface roughness might be a reason of
scattering of optical signal. Vacuum evaporated dielectric films
[17] Y. Zheng, K. Kikuchi, Appl. Opt. 36 (1997) 25.
[18] R. Koch, D. Winau, K.H. Rieder, Phys. Scripta T49 (1993) 539.
[19] A.K. Sinha, H.J. Levinstrin, T.E. Smith, J. Appl. Phys. 49 (4) (1978) 2423.
[20] S.H. Tamboli, Vijaya Puri, R.K. Puri, J. Alloys Compd. 503 (2010) 224.