Structural and morphological behavior of bismuth thin films grown through DC-magnetron sputtering

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Ingeniare. Revista chilena de ingeniería, vol. 23 Nº 1, 2015, pp. 92-97

Structural and morphological behavior of bismuth thin films

grown through DC-magnetron sputtering

_Comportamiento estructural y morfológico de películas delgadas_

_de bismuto obtenidas a través de sputtering con magnetrón DC_

Claudia Milena Bedoya-Hincapié 1 Jhonattan de la Roche 1 Elisabeth Restrepo-Parra 1

José Edgar Alfonso 2 Jhon Jairo Olaya-Florez 2

Recibido 28 de abril de 2014, aceptado 28 de agosto de 2014

_Received: April 28, 2014 Accepted: August 28, 2014_

ABSTRACT

Bismuth thin films were grown onto glass substrates through the DC magnetron sputtering technique.
The effects of substrate temperature on the microstructure of the films were evaluated. The structural
behavior was analyzed via X-ray diffraction, and it showed a marked influence of the substrate temperature
on the crystallite size and the micro-stress. The morphologies evaluated through Electron Probe Microanalyzer images indicated a slight difference in the grain size with an increase in temperature as well as
a significant increase in surface roughness, according to profilometer measurements.

Keywords: Bismuth, thin film, thermal effects, sputtering, microstructure.

_RESUMEN_

_En este trabajo se depositaron películas delgadas de bismuto sobre sustratos de vidrio mediante la_
_técnica de sputtering con magnetrón DC y se evaluaron los efectos de la temperatura del sustrato en_
_la microestructura de las películas. El comportamiento estructural se analizó mediante difracción de_
_rayos X, mostrando una notable influencia de la temperatura del sustrato en el tamaño de cristalito y_
_microestrés. La morfología evaluada por medio de imágenes de la microsonda electrónica, presentó una_
_leve diferencia en el tamaño de grano con el incremento de la temperatura así como un significativo_
_incremento de la rugosidad superficial, según medidas de perfilometría._

_Palabras clave: Bismuto, película delgada, efectos térmicos, sputtering, microestructura._

INTRODUCTION

Bismuth (Bi), a typical semimetal, has a unique set of
properties, such as large Fermi wavelengths [1], strong
diamagnetism, and very high magnetoresistance [2].
Recently, bismuth nanostructures have attracted
much attention because of their quantum transport
and finite-size effects and their potential application
for magnetic field sensors, optical devices, catalysts
and thermoelectric coolers or power generators [3-7].

At the same time, with increasing environmental
concerns and the need for green reagents, interest
in bismuth and its compounds has increased
tremendously during the last decade [8]. Up to
now, many papers on bismuth nanostructures
synthesized by means of various techniques have
been reported, such as thin films [9], nanowires

[10], nanotubes [11], nanoplates [12], and nanobelts

[13]. In particular, bismuth thin film deposition is
complicated due to its melting point, which leads to

1 PCM Computational Applications. Laboratorio Física del Plasma. Universidad Nacional de Colombia-Sede Manizales.
E-mail: cmbedoyahi@unal.edu.co; jdey@unal.edu.co; erestrepopa@unal.edu.co
2 Grupo de Ciencia de Materiales y Superficies. Departamento de Física. AFIS Análisis de Fallas, Integridad y Superficies.
Universidad Nacional de Colombia-Sede Bogotá. Bogotá, Colombia. E-mail: jealfonsoo@unal.edu.co; jjolayaf@unal.edu.co

_Bedoya-Hincapié, De la Roche, Restrepo-Parra, Alfonso and Olaya-Florez: Structural and morphological behavior of bismuth…_

higher crystallization rates and low vapor pressure.
They have been deposited using several deposition
techniques, including pulsed laser deposition (PLD)

[14-15], molecular beam deposition method [16],
rf sputtering [17], thermal evaporation [18] and
electro-deposition [19]. Some of the advantages of
using sputtering techniques are the possibility of
evaporating materials with different natures and the
good adherence of the thin film. Recently, Dong-Ho
Kim and co-workers [17] have successfully prepared
bismuth thin films through rf-magnetron sputtering,
demonstrating the effects of deposition temperature
on surface morphology and their electrical transport
properties. They found that homogenous thin films
cannot be obtained above 175 °C, and the onset of
the coalescence of grains is around 100 °C, where
movement of grains and surface diffusion of atoms
are enhanced. Also, Latha Kumari, S.-J. Lin, J.-H.
Lin, Ma, Lee and Liou [18] deposited bismuth thin
films onto (100) Si through thermal evaporation at
various substrate temperatures (30, 100 and 200 °C),
observing the formation of coalescence grains
and highly textured crystallographic thin films at
low temperatures, while for a high temperature
(200 °C) there is no uniform growth, due to grain
segregation, island formation, and random-oriented
polycrystalline structure. However, there have been
few reports up to now about the effect of substrate
temperature on the microstructure of Bi thin films
deposited via dc magnetron sputtering.

In this paper, we study the effect of the substrate
temperature on the structural and morphological
behavior of bismuth thin films deposited via dcmagnetron sputtering onto a glass substrate.

EXPERIMENTAL DETAILS

Bismuth monolayers were deposited via the dc
magnetron sputtering technique using a bismuth
target (99.99%) 2 inches of diameter. Coatings were
deposited on glass substrates, varying the substrate
temperature from 100 to 160 °C. The substrates
were ultrasonically cleaned in acetone solution for
15 minutes and underwent a pretreatment through
a plasma cleaning process that was carried out for
15 minutes before the deposition. The deposition
parameters used for the growth of the thin films
were: base pressure of 10 -4 Pa, work pressure of
6x10 -3 Pa, 1 W/cm 2 for power density, argon gas
flow of 20 sccm and a deposition time of 20 minutes.

Structural characterization was performed with a
Bruker AXS D8 advanced X-Ray difractometer
(XRD), using parallel beam geometry and a grazing
incidence angle φ = 1.5° with Cu-kα radiation
(λ = 1.5406 Å) ranging between 20-80° with steps
of 0.02°. Cross section and morphological surface
analyses were carried out using the JEOL JXA8530F Field Emission Electron Probe Microanalyzer
(EPMA) with 10 keV of energy and a magnification
of 50.000X. Also, thickness and roughness
measurements of the thin films were determined
with a Veeco Dektak 150 surface profilometer, using
a 2.3 mm length and a measuring range of 6.5 µm.

RESULTS AND DISCUSSION

XRD patterns of bismuth monolayers deposited
at different substrate temperatures are shown in
Figure 1. The identified and indexed XRD peaks
for all the samples belong to a hexagonal structure
( _a_ = _b_ = 4.547, _c_ = 11.8616 Å, α = β = 90° and γ = 120°)
matching PDF 44-1246, being consistent with a
hexagonal primitive cell, which is regularly used
to describe a bismuth lattice [20-21]. Furthermore,
Bi can take the form of a rhombohedral cell and
can also be assimilated into a slightly deformed
pseudo-cubic cell [15]. XRD patterns exhibited
no presence of preferred orientation, indicating
randomly oriented polycrystalline thin films. The
width of the peaks related to crystallite size and
micro-stress revealed the influence of the substrate

Figure 1. XRD patterns of Bi thin films grown at

various substrate temperatures: a) 100,
b) 120, c) 140 and d) 160 °C.

Ingeniare. Revista chilena de ingeniería, vol. 23 Nº 1, 2015

temperatures. Average crystallite size and micro-stress
were estimated using the Williamson-Hall method,
fitting the profiles with a Lorenzian function, by
employing the following equation, which includes
the main contributions (crystallite size, micro-stress
and instrumental error) to the width of the peak:

0.9λ
β= (1)
_D_ cosθ + 4ε tanθ + β0

where β is the Full Width at Half Maximum, λ is
the X-ray wavelength, β _0_ is the instrumental error,
_D_ is the crystallite size and ε is the micro-stress.

Figure 2 shows the calculated crystallite grain size
and the micro-stress as a function of the substrate
temperature. Crystallite size exhibited a linear
behavior with an increase in substrate temperature,
indicating an enhanced crystallite quantity, seen by
the increase in the intensity of the peaks, while the
micro-stress diminished at higher temperatures,
helping to relax and release the stress in the thin film.

The average thickness values of the thin films
measured with profilometry equipment were in
the range 600-900nm. No significant differences
in thickness between the films were observed with
variation of substrate temperature.

In order to establish the temperature effect in
bismuth thin films over a wide range, an XRD
analysis of temperature in the range from 100 to
300 °C in situ was carried out on the film grown at
120 °C in air atmosphere, with temperature steps of
50°C. These results are shown in Figure 3. Bismuth

Figure 2. Crystallite grain size and micro-stress

as a function of substrate temperature
in Bi thin films.

Figure 3. XRD patterns of bismuth thin film

annealed in air atmosphere at different
temperatures: a) 100, b) 150, c) 200, d)
250 and e) 300 °C.

diffraction peaks appear at low temperatures up
to approx. 200 °C, and its decrease is associated
with the bismuth oxide β-Bi2O3 (PDF 27-0050), a
formation that is clearer above 250 °C. This indicates
the change from hexagonal to tetragonal structure,
and also a shift to a yellow-plated color surface. An
example of this is the research developed by DongHo Kim, S.-H. Lee, J.-K. Kim and G.-H. Lee [17],
who deposited Bi thin films at room temperature
which were then annealed for 1 h at over 250 °C,
showing XRD patterns of bismuth oxide.

An EPMA cross-section image of the bismuth thin
films is shown in Figure 4. Columnar growth with
large irregular particles occurs in the films, which

Figure 4. EPMA Cross-section image of Bi thin

films’ growth morphology.

_Bedoya-Hincapié, De la Roche, Restrepo-Parra, Alfonso and Olaya-Florez: Structural and morphological behavior of bismuth…_

has been also reported for Bi films produced through
dc sputtering [22, 23], rf sputtering [17] and PLD
techniques [14]. The typical columnar growth in the
thin films, as well as the low melting point of Bi
and a rapid solidification process, can be observed,
due to the high mobility of the adsorbed species on
the heated surface [18]. According to the Thornton
diagram [24], the structural model for thin films
obtained through PVD techniques depends on the
pressure and the relation between the substrate
temperature and melting temperature ( _T/Tm_ ). Bi
has a low _Tm_, equal to 271 °C [14], so _T/Tm_ takes
values of 0.68, 0.72, 076 and 0.79, clearly locating
the thin film morphology in zone 2, ascribed to a
dense columnar growth.

Figure 5 illustrates surface EPMA images of thin
films for various substrate temperatures. The
distribution of small grains with some large ones
appears at low temperatures in the range 100140 °C, while at 160 °C the grains tend to be slightly
bigger on average. This indicates the increase of
grain size due to the high growth rate at raised
substrate temperatures, caused by the contribution
of the thermal energy to the effects of the diffusion
and the coalescence of the grains. In the range of
temperatures considered in this study, segregation
effects did not appear, because they become visible
at higher temperatures, according reports by DongHo Kim, S.-H. Lee, J.-K. Kim and G.-H. Lee [17],
who suggest segregation can occur above 175 °C,
or even above 200 °C [18].

Some reports indicate that the grain geometry is
strongly determined by the deposition technique.
The grain geometry obtained with dc- magnetron
sputtering was polyhedral-like structures. Also,
similar surface morphology has been found in thin
films grown via the thermal evaporation method and
spherical grains with pulsed laser deposition [14].
These results are in agreement with profilometer
results, which suggest a strong dependence of
surface morphologies on the substrate temperature,
with the increases of the average roughness with
temperature, as shown in Figure 6.

CONCLUSIONS

The effect of substrate temperature on the structural
and morphological behavior was studied for
bismuth thin films grown on glass substrates

Figure 5. EPMA morphological images of Bi

thin films growth at different substrate
temperatures a) 100, b) 120, c) 140,
d) 160 °C.

Ingeniare. Revista chilena de ingeniería, vol. 23 Nº 1, 2015

Figure 6. Roughness of bismuth thin films deposited

at different substrate temperatures.

via the dc-magnetron sputtering technique. A
considerable influence of the substrate temperature
on the microstructure was found. The increase
of crystallite size with temperature and a drop in
micro-stress were observed through the WilliamsonHall method. Morphological images showed the
columnar growth of the thin films and grains with
a polyhedral-like geometry. Segregation in Bi thin
films was not observed in the range of substrate
temperatures studied, but it may appear at higher
temperatures, over 170 °C.

ACKNOWLEDGEMENT

The authors gratefully acknowledge the financial
support of the Dirección Nacional de Investigaciones
of the Universidad National de Colombia during
the course of this research, under project 12920
“Desarrollo teórico-experimental de nanoestructuras
basadas en Bismuto y materiales similares” and
“Bisnano project”, and to “Universidad Nacional
de Colombia” and “Cinvestav” Querétaro-México
for the assistance in the measurements.

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