Thin Solid Films 571 (2014) 194–197
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Thin Solid Films
journal homepage: www.elsevier.com/locate/tsf
Al–Ni–Y–X (X = Cu, Ta, Zr) metallic glass composite thin films for
broad-band uniform reflectivity
C.M. Chang, C.H. Wang, J.H. Hsu, J.C. Huang ⁎
Department of Materials and Optoelectronic Science, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung, Taiwan, ROC
a r t i c l e
i n f o
Article history:
Received 26 May 2014
Received in revised form 6 October 2014
Accepted 10 October 2014
Available online 16 October 2014
Keywords:
Aluminum alloy
Thin films
Metallic glass
Optical reflectivity
Optical reflector
Sputtering
a b s t r a c t
The Al–Ni–Y–X (X = Cu, Ta, Zr) thin film metallic glasses are manufactured by sputtering, and their optical reflectivity characteristics are explored. The relationship among composition, atomic structure and reflectivity performance is established. Compared with pure Al films, the Al–Ni–Y film surface roughness is much lower and
hardness is much higher, more suitable for optical reflector applications. For composite Al–Ni–Y films, the reflectance varies within 80–91%. For fully amorphous films, the reflectivity exhibits unusual uniform reflection at
~70%, perfect for broad-band reflector.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
It has been well known that aluminum (Al), silver (Ag), gold (Au),
copper (Cu), and rhodium (Rh) are the metals widely used for highreflection purpose [1]. Among them, Al covers a wide light spectral
range. It provides high reflection from ultraviolet (UV) ~ 200–400 nm,
visible ~400–700 nm, to infrared (IR) over 1000 nm regions, with a shallow dip at around 800 nm. Hence Al mirrors are widely used in various
applications. In parallel, gold and copper exhibit high reflection in the IR
region but they would show yellow or red color, a result from the absorption of the blue and UV wavelengths by the d-band electronic transitions [2]. Similar band-electronic absorption also occurs in silver, but it
shifts to the UV region. The reflectivity of rhodium is relatively low in
the visible region, but with dielectric overcoats, enhanced rhodium mirror can provide high reflection in specific UV wavelength range.
With the face-centered cubic structure, Al exhibits easy processing
capability into thin sheet and foil shapes. In addition, Al has strong adhesion to glass and other substrates. The dense oxide layer on the Al surface has reasonable capability to protect the metal from corrosion or
oxidation under normal condition [3], but the surface oxide layer
would affect the optical reflectivity. Under more severe acid or alkaline
conditions, Al can be corroded appreciably. Moreover, the relatively
lower hardness would result in weak wear resistance.
⁎ Corresponding author. Tel.: +886 7 5254070; fax: +886 7 5254099.
E-mail address: [email protected] (J.C. Huang).
http://dx.doi.org/10.1016/j.tsf.2014.10.048
0040-6090/© 2014 Elsevier B.V. All rights reserved.
Metallic glasses (MGs) have attracted continuous attention for decades, owing to their outstanding properties, such as high strength
and hardness, high elastic strain limit and good corrosion resistance
[4–11]. It can form a smooth surface without the influence of crystalline
facets and grain boundaries, more promising for optical reflection purposes. In our previous papers, the optical reflection of the Ag–Mg–Al
thin film metallic glasses (TFMGs) has been reported [3,12]. However,
the high price of Ag might limit wide applications. Recently, it has
been found that the metallic glass structure can also be formed in the
Al-based systems with the composition of Al–RE (rare-earth)–TM (transition metal) [13]. The optical reflection behavior of such Al-based MGs
has never been explored. The reflection shallow dip of crystalline Al at
~800 nm light wavelength has been concerned [14,15]. Therefore, it is
inspired to develop if the cheaper Al-based MGs could avoid such a reflection dip and to provide uniform reflectivity over a wide light spectral
range.
In this work, we select nickel (Ni) for TM and yttrium (Y) for RE for
the developing of suitable Al–Ni–Y–X (X = Cu, Ta, Zr) MGs, either being
monolithic fully amorphous or forming a nanocomposite structure with
nanocrystalline particles embedded in the glassy matrix. A previous
study has reported that such a material can contain Al nanocrystals
about 30% in volume fraction and about 5–20 nm in size dispersed within the amorphous matrix [16]. In this paper, Al–Ni–Y is prepared into
thin films by sputtering for reflection purposes. The results are compared with our previous studies on the Ag–Mg–Al TFMG [3,12]. The relationship between composition, atomic structure and reflectivity
performance is aimed.
C.M. Chang et al. / Thin Solid Films 571 (2014) 194–197
195
four-point probe, and the electrical resistivity ρ was subsequently calculated by the multiplication of Rs and thin film thickness d.
2. Experimental details
A series of Al based metallic thin films were deposited on Si substrate
by co-sputtering the Al90Ni5Y5 (in at.%) alloy target with pure Cu, Zr or
Ta targets, respectively. The alloy target was placed on the directcurrent (DC) cathode and the pure metal targets were placed on the
radio frequency (RF) cathode. The sputtering yield rate for the RF cathode is lower [17], so that the pure element target is placed on the RF
cathode in order to control more precisely the minor addition of Cu, Zr
or Ta in the resulting films. The alloy targets were prepared by arc melting 99.99% Al, 99.99% Ni and 99.9% Y, into the target composition of
Al90Ni5Y5. The purity of the other elemental targets in use, Cu, Zr and
Ta, is 99.999%, 99.7% and 99.99%, respectively. The size of all targets
was 50.8 mm in diameter and 6 mm in thickness. The chamber was initially evacuated to the pressure of 2.6 × 10−5 Pa before being operated
with highly pure Ar gas. During deposition process, the holder was set at
a rotation speed of 15 rpm and it could result in uniform distribution of
the film thickness. At the same time, the Ar gas flows at a fixed rate of 25
standard cubic centimeters per minute (sccm) and working pressure
was rigorously maintained at 0.4 Pa, the optimum condition found previously [12]. The compositions of Al-based alloy thin films were controlled by changing the power values of the Al90Ni5Y5 and Cu/Zr/Ta
cathodes. The applied power and sputtering time were adjusted based
on the calibrated coating rates of each target, and the final film thickness
was around 200 nm. In this paper, the focus is on the composition,
atomic structure and reflection performance. The sputter conditions
are all fixed at the optimum parameters [3,12].
Basic phase characterization was performed by the X-ray powder
diffractometer (XRD, Bruker D8) with a monochromatic Cu-Kα radiation (λ = 1.5406 A), operated at 40 kV and 40 mA, and equipped
with a 0.01 mm graphite monochromator. The ranges of the diffraction
angle 2θ were set from 20° to 60°. The quantitative composition analyses of films were done by the energy dispersive spectroscopy (EDS),
with an operative voltage of 5 kV, equipped in the JEOL JSM 6330 field
emission scanning electron microscope. The microstructure and local
amorphous or nanocrystalline phase characterization were performed
by JEOL 3010 analytical transmission electron microscope with operating voltage of 200 kV. The transmission electron microscopy (TEM)
foils Al89Ni8Y3 thin films were fabricated using dual-beam focusedion-beam system (Seiko, SMI3050) with operating voltage of 30 kV
and 1 pA ion beam current. The optical reflection was measured using
n & κ Analyzer 1280, calibrated with a standard silicon wafer. Data
were collected in the wavelength range from 190 to 1000 nm under
the reflection mode. The average surface roughness (Ra) was measured
by atomic force microscopy (AFM) under the tapping mode, and the
elastic modulus and hardness were measured by MTS nanoindenter
XP equipped with a standard Berkovich tip, under the continuous stiffness measurement mode. The sheet resistance (Rs) was measured by
3. Results and discussion
In this study, numerous Al based thin films were prepared by
sputtering, and their compositions measured by SEM/EDS are listed in
Table 1. Fig. 1 shows the XRD results of the Al-based thin films.
Among them, some are not fully amorphous, including the Al92Ni5Y3,
Al89Ni8Y3, Al85Ni11Y4 and Al74Ni4Y2Cu20 thin films, possessing a composite structure with the nanocrystalline face-centered cubic Al phase
embedded in the amorphous matrix. In comparison, with the addition
of Zr or Ta, the films become fully amorphous, including the
Al83Ni4Y2Zr11, Al83Ni3Y1Ta13 and Al77Ni2Y1Ta20 thin films.
In the Al85Ni11Y4 film sample, the diffraction maximum occurred at
37.7° with a XRD peak width at half intensity of 1.3°. When the Al content is increased in Al92Ni5Y3, the diffraction peak is shifted to 38.3°, and
the width is reduced to 0.4°. The diffraction peak widths of Al92Ni5Y3,
Al89Ni8Y3 and Al85Ni11Y4 are much narrower than the diffraction diffuse
hump for typical metallic glasses, but still wider than those of typical
crystalline structures, implying a composite structure with the nanocrystalline Al-rich phase dispersed in the amorphous matrix. TEM characterization was performed, and the typical lattice image and its
associated diffraction pattern are shown in Fig. 2. The typical Al-rich
crystalline grains are seen to be about 5 nm, embedded in the amorphous matrix. With the addition of Zr and Ta, the microstructure is basically featureless, characteristic of the fully amorphous nature. The
XRD and TEM results are consistent.
Since the optical reflection is strongly influenced by the surface
smoothness. The surface morphology was examined by SEM and AFM.
The measured Ra values are included in Table 1, where it can be seen
that the crystalline and composite films with grain and interface boundaries are typically inherent with higher surface roughness, and the fully
amorphous films tend to possess much flatter surface.
The film modulus and hardness are measured by using nanoindentation. The pure Al films show an elastic modulus of 70 GPa and a hardness of about 1.0 GPa. For the Al-based metallic glasses or composites,
the modulus increases to about 120 ± 10 GPa, and the hardness reaches
about 6 ± 1 GPa, both much higher than the pure Al films. The significantly harder surface will result in much higher surface scratch wear resistance, more favorable for long-term exposure in general living
environment.
The optical reflectance of all Al based films with 200 nm film thickness was systematically measured for multiple times, and the representative results are presented in Fig. 3 and Table 1 over the light
wavelength from 200 to 1000 nm. The pure Al films are prone to humidity so that the optical reflection spectrum can vary appreciably, depending on the film surface oxidation condition. Note that the reflectance in
Table 1
The surface roughness (Ra), film resistivity (ρ), and optical reflection, R%, of the as-sputtered Al-based films at various light wavelengths. The maximum datum scattering for Ra and ρ is 3%
and 1%, respectively. Composite is referred to the nanocrystalline particles embedded in the amorphous matrix. The Al films are prone to humidity so that the light reflection depends on
the film surface oxidation condition, particularly over the UV regime. Pure Al (good) and pure Al (bad) are referred to the as-sputtered Al films without or with surface oxidation. Note that
the three amorphous TFMGs (lower three) exhibit highly uniform reflectance near 70% over the entire spectrum from 200 to 1000 nm.
Sample
Pure Al (good)
Pure Al (bad)
Al92Ni5Y3
Al89Ni8Y3
Al85Ni11Y4
Al74Ni4Y2Cu20
Al83Ni4Y2Zr11
Al83Ni3Y1Ta13
Al77Ni2Y1Ta20
Structure
Nanocrystalline
Nanocrystalline
Composite
Composite
Composite
Composite
Amorphous
Amorphous
Amorphous
Ra
(nm)
ρ
(Ωnm)
UV
200 nm
300 nm
400 nm
700 nm
850 nm
1000 nm
2.9
3.0
2.6
2.3
1.2
2.8
0.3
0.8
0.3
41
50
177
204
468
409
1005
985
1278
75
25
73
72
75
60
70
62
69
92
45
87
81
78
76
69
66
67
92
70
87
82
79
79
68
66
65
90
83
88
86
82
82
69
69
67
88
80
90
87
83
82
71
70
69
95
91
91
88
84
83
72
72
70
Visible
IR
196
C.M. Chang et al. / Thin Solid Films 571 (2014) 194–197
a
b
Al92Ni5Y3
Al77Ni21Y2Ta20
Al74Ni4Y2Cu20
Al83Ni3Y1Ta13
Al89Ni8Y3
Al83Ni4Y2Zr11
Intensity
Intensity
Al85Ni11Y4
20
30
40
50
60
20
30
2 (degree)
40
50
60
2 (degree)
Fig. 1. XRD patterns for the Al-based films: (a) the group showing the composite structure with the Al nanocrystallites embedded in the amorphous matrix, and (b) the other group
possessing the fully amorphous structure.
the UV region can vary from 25–45% for the films with more severe oxidation to 75–92% for the films protected from humidity and oxidation.
The sensitive nature of UV light reflection, coupled with the low surface
hardness, low wear resistance and low resistance to environmental oxidation and corrosion, appears to be the weak aspects of pure Al films for
the optical reflection purpose in living environment.
Also, the pure Al films exhibit a shallow reflection dip at ~800 nm,
due to the inter-band absorption of the Al crystalline structure in the
film [18]. As expected, this reflection dip could disappear in the Albased alloy films. The light reflectance of the Al–Ni–Y–X films shown
in Fig. 3 and Table 1 is in fact not really higher than the high-quality
(good) Al films, but can be much superior to the low-quality (bad) Al
films when their surfaces are oxidized. The terms high-quality (good)
and low-quality (bad) are mainly a reflection of surface oxidation degree. When the deposition is conducted in high vacuum and the pure
Al films are carefully protected from surface oxidation, the pure Al
film would exhibit high optical reflectance (good). In contrast, when
the sputtering vacuum is low and the pure Al films would be inherited
with oxidation in the as-sputtered condition. When such pure Al films
are further exposed in humid and salty environment, the films could
be oxidized more severely by the Cl ions showing low optical reflectance (bad). With the addition of Ni and Y (which are both more oxidation resistant), the Al-based metallic glass or composite films are
basically much more stable upon exposure in air. The reflectance
would not vary much as did for the pure Al films. For visible and IR
reflection concern, the high Al containing Al92Ni5Y3 and Al89Ni8Y3 composite films, containing the metallic-glass/nanocrystal (MG/NC) composite structure, still exhibits sufficient reflection around 85–91%, a
level feasible for commercial application judging the facts that these
films are much harder and wear/environmental resistant.
On the other hand, for the fully amorphous films, such as
Al83Ni4Y2Zr11 and Al77Ni2Y1Ta20 thin films, the reflectance is consistently about 70% over the entire spectrum from UV, visible to IR region,
which is a rather unusual phenomenon. A reflectance of 70% might
not be sufficient for high light reflection purpose, but the high uniformity of reflectivity from 200 to 1000 nm might shine light on other applications to deliver light with less spectroscopic distortion. For most
commercial films, reflectance in the UV region would be much lower
than the IR region. The current Al-based metallic glass and composite
films exhibit unusual highly uniform reflectance.
Combining the optical reflection performances of the MG/NC composite structure films and the fully amorphous TFMG films, it can be
seen that the NC amount (depending on the film composition) can
serve as a tunable parameter to result in various reflectivity behaviors,
either highly reflective over 90% for the MG/NC composite films or the
extraordinarily uniform reflectivity around 70% for the fully amorphous
TFMG films. The light reflection performance can be easily adjusted to
tune the required optical reflectivity based on the sputtered film atomic
100
90
Reflectance (%)
80
70
60
50
40
30
20
10
0
200
Pure Al (good)
Al92Ni5Y3
Al83Ni4Y2Zr11
Al89Ni8Y3
Al83Ni3Y1Ta13
Al85Ni11Y4
Al77Ni2Y1Ta20
Al76Ni4Y2Cu20
Pure Al (bad)
400
600
800
1000
Wavelength (nm)
Fig. 2. The representative high-resolution TEM lattice image, with an inserted electron diffraction pattern, taken from the Al89Ni8Y3 thin film, showing the typical Al crystalline
grains about 5 nm embedded in the amorphous matrix.
Fig. 3. The reflectance of the pure Al and the various Al-based amorphous or composite
thin films.
C.M. Chang et al. / Thin Solid Films 571 (2014) 194–197
Reflectance, %
100 Ag
4. Conclusions
Ag76Mg17Al7
Ag73Mg17Al10
Al Mg
Ag60Mg27Al13
90
Al89Ni8Y3
Al85Ni11Y4
Al76Ni4Y2Cu20
Ag30Mg45Al25
80
Cu
Al84Ni4Y2Zr10
Al83Ni3Y1Ta13
70
Ag45Mg37Al18
Al77Ni2Y1Ta20
Zr50Cu50
60
0
10
20
30
40
50
197
60
Resistivity1/2 (nm)1/2
Fig. 4. The dependence of light reflectance as a function of electric resistivity. The previous
data reported in Ref. [3] are presented in black color and fit by the solid lines. The new data
obtained in this paper are indicated in red color. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.).
structures, which can be adjusted by the film composition. The relationship among the composition, atomic structure and performance is
established.
In comparison with pure metals, TFMGs can suppress the optical
transition between the crystalline bands, hence, it has the potential for
broadband optical reflector applications. Pure metals are generally too
soft for such applications. In contrast, the mechanical, chemical and
electrochemical (such as corrosion or pitting) properties of MGs are
much better than those of pure metals. In addition, TFMG exhibits superior surface flatness, which is also an advantage for the optical reflection
applications. However, there is one disadvantage for TFMGs, the high
electric resistivity due to the nature of the disordered structure of the
MGs and the high ratio of free volume or vacancy formation during
the process. Optical reflection of pure metal at IR wavelength is related
to the electrical resistance. Such vacancy-typed or free-volume-typed
defects in amorphous materials are so well defined as those individual
vacancies or di-vacancies in the crystalline materials. But it can be easily
visualized when we measure the density of a fully crystalline metal and
a metallic glass of the same composition. The density of the metallic
glass is always lower than that of the crystalline metal by a few percent,
as a result of the presence of free volumes and atomic defects in the randomly packed amorphous metallic glasses. Our previous results indicate
that the light reflection R is inversely related to the electrical resistance
ρ (R ∝ 1 − Aρ1/2, where A is a proportional constant) [3]. The higher
electric resistivity of the fully amorphous films (Table 1) would lead to
lower but extremely uniform reflectivity at ~ 70%. The high freevolume and vacancy densities in the fully amorphous TFMGs can be improved by the post-annealing process (below the glass transition temperature) [12]. This is because that the highly random atomic packing
formed during the sputtering can be relaxed to lower the free-volume
and vacancy concentration. In the latter case, the optical reflection can
be upgraded [12] to 75% or slightly higher. The atomic packing structure
will influence the electric resistivity, and in-turn the light reflection. The
current data, independent of the composite and amorphous films, can
be well fit into our previous model [3], as depicted in Fig. 4 for the relation between R and ρ for all the Ag-based, Zr–Cu, and Al–Ni–Y-based
MGs.
In summary, no matter fully amorphous or composite, the film surface roughness is much lower, and the surface hardness is much harder
(by six times) than those of the pure Al films. The high-quality featureless flat surface is quite suitable for the optical reflector applications. The
optical reflectance of the Al–Ni–Y based films, with the MG/NC composite structure, is nearly constant in the optical wavelength range from UV
to IR with reflectance slightly varying within 80–91%. And the reflectivity of the fully amorphous Al–Ni–Y based films exhibits the unusual
highly uniform characteristic at ~ 70% over the entire wavelength
range from 200 nm to 1000 nm, shining light on other applications for
uniform light reflection merit over a very wide range from UV to IR regime to deliver light with less spectroscopic distortion. The MG/NC
composite atomic structure, which depends on the film composition,
can serve as a simple controllable parameter for various optical reflector
requirements.
Acknowledgment
The authors gratefully acknowledge the support from the National
Science Council of Taiwan, ROC, under grant No. NSC 102-2120-M110-006.
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