Vrije Universiteit Brussel

Optical reflectance from anodized Al-0.5 wt % Cu thin films: Porosity and refractive indexcalculationsAbd-Elnaiem, Alaa; Asafa, Tesleem B.; Trivinho-Strixino, Francisco; Delgado-Silva, Adriana;Callewaert, Manly; De Malsche, WimPublished in:Journal of Alloys and Compounds

DOI:10.1016/j.jallcom.2017.06.082

Publication date:2017

License:Unspecified

Document Version:Accepted author manuscript

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Citation for published version (APA):Abd-Elnaiem, A., Asafa, T. B., Trivinho-Strixino, F., Delgado-Silva, A., Callewaert, M., & De Malsche, W. (2017).Optical reflectance from anodized Al-0.5 wt % Cu thin films: Porosity and refractive index calculations. Journal ofAlloys and Compounds, 721, 741-749. https://doi.org/10.1016/j.jallcom.2017.06.082

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Optical reflectance from anodized Al-0.5 wt % Cu thin films:

porosity and refractive index calculations

Alaa M. Abd-Elnaiem1, 2 *, T. B. Asafa3, Francisco Trivinho-Strixino4, Adriana de O.

Delgado-Silva4, Manly Callewaert2, Wim De Malsche2

1Physics Department, Faculty of Science, Assiut University, Assiut 71516, Egypt; 2Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel,

Belgium;

3Mechanical Engineering Department, Ladoke Akintola University of Technology, PMB 400,

Ogbomoso, Oyo State, Nigeria;

4Departamento de Física, Química e Matemática, Universidade Federal de São Carlos -

Campus Sorocaba, 18052-780 Sorocaba - SP, Brasil.

Abstract

Porous anodic alumina (PAA) films with periodically arranged horizontal and vertical

nanopores have been applied as templates or platforms upon which optical sensors can be based.

While the porosity and the refractive index of these 3D PAA can be tuned by varying the

anodization parameters, estimating them from reflectance data has not been explored. In this

study, we estimated the porosity and refractive index of thin anodized Al-0.5 wt% Cu from the

alumina film thickness and the interference pattern composed of the incident and reflected light

beam in the alumina film. The 3D PAA films were prepared by anodization of Al-0.5 wt% Cu

thin film deposited on TiN/Si substrate. Anodization was performed in three electrolytes at

different concentrations (1 M sulfuric acid, 0.3 M oxalic acid and 0.75 M phosphoric acid)

within the voltage range of 10-90 V. The structural characteristics showed that both pore size

and inter-pore distance increased with increased anodization voltage and time while the pore

density decreases exponentially. The results from reflectance spectra clearly showed that the

effective refractive index of the samples decreased with voltage and anodization time, while

porosity (total, vertical and horizontal) increased. Also, the density of PAA samples is estimated

and shows an inverse proportionality with the porosity.

Keywords: Anodization; optical interference; porous anodic alumina; inter-pore distance,

horizontal pores.

*E-mail: alaa.abd-elnaiem@science.au.edu.eg

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1. Introduction

Anodic aluminum oxide (AAO) can be of a porous or nonporous type. The porous type

has received much attention owing to its unique properties such as providing a surface area for

many applications in nanoscience and nanotechnology. Examples of devices for applications

are chemical sensors, biosensors, optical waveguide sensors, distributed-Bragg reflectors or

rugate filters, etc. [1-3]. The porous anodic alumina (PAA) is formed when aluminum (Al)

samples are electrochemically anodized in an acidic medium under specified anodization

conditions [1-20]. The characteristics of the PAA strongly depend on the anodization

parameters. Based on the pore shape, PAA pores can be classified into highly straight, branched,

meshed and three-dimensional (3D) [5-11].

Highly ordered PAA with a close-packed array of columnar hexagonal pores can be

obtained by anodization of pure Al at low temperatures (close to 0oC) using the well-known

two-step method [5]. In this procedure, a cooling system is required during the anodization

process to prevent the surface of Al from burning and pores from branching [12, 13]. With

respect to the electrolytes, anodization is usually performed in H2SO4 at low voltage (20-25 V),

while H2C2O4 is suitable for voltages at around 40 V, and H3PO4 for higher voltages (160-195

V), with a considerable influence of the concentration of the electrolyte. H2SO4 can be used as

electrolyte for the formation of the smallest pore, while the use of H3PO4 results in the largest

pore size [13-18]. Alternatively, the costly process by the textured pattern of the surface of

aluminum samples can be made by molding before the anodization process [6].

While mesh PAA type can be fabricated by anodization at low anodizing

voltages/currents for long durations [8], the branched pores can be obtained by decreasing the

anodizing voltage to a specified value depending on required number of branched pores [19].

It can also be made by using doped Al with some impurities like silicon [9, 10]. The 3D

hierarchical nanostructures of PAA can be prepared by various methods including pulse

anodization process followed by chemical etching [20] or anodization of raw Al-containing

copper impurities [7, 11]. These methods provide a controllable 3D porous structure through

which the required structure can be modulated by changing the anodizing current/voltage.

There is an increasing attention towards optical characterization of different kinds of PAA

[1-4]. The optical reflectance of the thin film is dependent on its refractive index, porosity, and

the film thickness. For most optical film structures, reflected/transmitted light are combined as

a total reflection/transmission with the spectra showing a few number of peaks and valleys. For

instance, by using the film thickness and the subsequent maxima of the reflectance spectra, we

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can obtain the porosity and refractive index based on the interference pattern [4, 21-22]. While

anodization parameters can be tuned, their influence on the 3D PAA film porosity and the

refractive index has not been well explored. In this study, we extracted porosity and refractive

index from the film thickness and reflectance spectra, and then parametrically studied the

influence of the anodization parameters on these characteristics of anodized Al-0.5 wt% Cu

thin films.

2. Experimental procedures

2.6 µm-thick Al-0.5wt% Cu (Al-0.5Cu) films were deposited by physical vapor

deposition on 200 mm Si wafers coated with 100 nm TiN. A Si/TiN/Al-0.5Cu coupon was then

clamped onto a glass cell with an O-ring exposing a 5 cm2 circular area of Al surface to the

anodization process. Anodization was performed under constant voltage (10, 15, 20, 22, 60 and

90 V) in a two-electrode electrochemical cell. A Ti sheet (5 cm × 3.5 cm) was placed as a

counter electrode (CE) opposite to the Al substrate at a distance of 5 cm. Three different

electrolyte solutions (sulfuric, oxalic, and phosphoric acids) and of different concentrations (1,

0.3 and 0.75 mol/L, respectively) were used. The anodization conditions are summarized in

Table 1. The cell voltage and current were supplied and measured by an AUTOLAB

PGSTAT100, controlled by Gpes electrochemical software. An AUTOLAB voltage multiplier

allowed for a voltage range of 0-100 V. The anodization was carried out at 22 °C for all samples.

After the anodization process, the samples were rinsed with deionized water and dried

under nitrogen flow. For samples anodized at 15 and 22 V, chemical etching was done in 0.75

M H3PO4 for 22 min at 30 ˚C. This was meant to clearly reveal the interconnected (horizontal)

pores in the film. The structures of anodized porous alumina were then characterized using a

scanning electron microscope (Model Philips XL30). A Java image processing software

(ImageJ, Version 1.37) was used to estimate inter-pore distances as well as the number and size

of the pores.

The optical reflectance of 3D PAA was characterized using a double-beam

spectrophotometer (Shimadzu UV-2101) equipped with an integrating device. Reflectance

spectra of the samples were obtained at room temperature (RT) between 200 nm and 2500 nm

using a spectral bandwidth of 2 nm and a scan rate of 500 nm/min. All measurements were

performed in double beam mode, using reduced slit height and zero/baseline correction. Prior

to the measurements, each sample was positioned using the sample clip while baseline

correction was performed before the acquisition of sample spectra in order to set 0 and 100 %

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transmission (T) values. From the reflectance spectra, the porosity and refractive index were

calculated based on the Fabry-Perot and the Bruggeman equations (see results section 3.3) [4,

22, 23].

3. Results and Discussion

3.1 Current-time characteristics

The current density against anodization time (j-t) curves obtained for Al-0.5Cu films

anodized in 1 M H2SO4 at 10-22 V (Fig. 1a), 0.3 mol/L H2C2O4 at 60 V (Fig. 1b) and 0.75

mol/L H3PO4 at 90 V (Fig. 1c) are typical for anodized Al-0.5Cu [11]. The different stages

observe in the curves can be attributed to various processes occurring during the anodization

process. These observations have been extensively discussed in our previous publications [9,

11, 24]. While the curves have similar features, the minimum current density, rate of change of

steady-state current density and oscillation are influenced by the anodizing voltage and

electrolyte. For anodization performed in 1 mol/L H2SO4, the steady-state current density

changes from 2.56 at anodization voltage of 10 V to 16.36 mA cm-2 at 22 V. However, similar

values of steady-state current density, 155 and 3315 mAcm2, were obtained for anodization

performed in 0.3 mol/L H2C2O4 at 60 V and 0.75 mol/L H3PO4 at 90 V, respectively (Fig 1b &

c). The oscillation of current in the plateau region is observed for samples anodized in oxalic

or phosphoric acids, and not for samples anodized in sulfuric acid within the voltage range

examined. The steady state current density for sample P90C increased from 16 to 60 mA cm-2

due to increased electrolyte temperature occasioned by joule heating of the oxide-covered

aluminum electrode. The measured temperature after the anodization was about 40 ◦C which

agrees with estimated value as per Ref. [24].

In the initial stage of anodization (stage-I), the current density drops from a high to a

minimum value. This is related to the formation of a thin, continuous and dense oxide layer

which blocks the current flow (passivation stage). At the minimum point in the j-t curve, the

insulating layer reached its maximum thickness which is limited to a few to several tens of

nanometers depending on the applied voltage. The thickness of the oxide layer achieved at this

point and thereafter is thought to remain about the same and it is equal to the barrier layer

thickness at the bottom of the pores [8, 24]. Hence, the chemical dissolution rate and electro-

oxidation rate become comparable. Therefore, no net change in the oxide thickness would be

observed (steady-state condition) if the acidic medium does not lead to pitting of the surface.

As the electric field lines concentrate in concave areas, the field-enhanced Al3+ dissolution leads

to pore formation and increased current density (pore nucleation stage-II). From this point on,

5

the pores grow further under steady-state conditions of Al electro-oxidation and Al3+ dissolution

at the concave pore bottom. The current density remains constant and forms the current plateau

in the j-t curve (steady-state growth stage-III).

When the pores reach the TiN layer, the current abruptly drops as Al is no longer available

to sustain the process (termination stage). The average charge consumed for the transformation

of 2.6 µm thick Al-Cu films to porous anodic alumina in sulfuric or oxalic acid is 8.5 C/cm2

while for a phosphoric acid this value was 12.7 C/cm2. The higher charge (and instability of

steady state current) observed for anodization in phosphoric acid could be attributed to low

current efficiency and hydrogen evolution during the anodization process [11]. The plateau

region (region-III) constitutes the greater part of the transient time. The termination time is

inversely proportional to the current density since anodization at higher steady-state current

density terminates within shorter period compared to samples with lower current density.

3.2 Characterization of the 3D porous alumina structure

Irrespective of the anodization conditions, vertically pores with horizontal

interconnections were obtained in all samples of anodized alumina layers (Figs 2-4). In our

previous study [11], it was shown that the presence of 0.5 wt% Cu in Al initiates the formation

of horizontal pores which interconnect the vertical pores. The critical dimensions of both pore

types follow similar trends with respect to the anodization conditions. Consequently, the

spacing between the horizontal and the vertical pores was linked with a model based on plastic

flow of the alumina barrier layer. The results of this model have been considered significant for

an in-depth understanding of the formation of 3D AAO templates [11].

At low anodization voltages (< 22 V) in H2SO4 acid, the horizontal connection could not

be clearly observed until the samples were chemically etched in 0.75 M H3PO4 for 22 min at

30 C (Fig. 2 d & h). This might be due to small pores formed at such a low anodization voltage.

The pore size (17 to 22 nm) and inter-pore distance (26 to 55 nm) increased with increasing

anodization voltage (10 to 22 V) while pore density decreased exponentially (82 to 46 ×109

/cm2) (Table 2). The anodizing voltage (V) and pore density (Np) can be related via the

following model:

𝑁𝑝 = 1.15𝑥1010𝑒−0.044𝑉 (1)

At a higher anodization voltage (for C2H2O4 and H3PO4 media), both pore size and inter-

pore distance are significantly increased at the expense of pore density. In addition, the

anodization time increased both the pore diameter and porosity due to pore widening. By

6

increasing the anodization time from 3 to 9 min for H3PO4 electrolyte, the pores widen from 58

to 82 mm while the pore density decreased from 4.2 to 2.8 ×109/cm2 (Table 2). This indicates

that, irrespective of the anodization media, the anodization voltage and time can be used to

modulate the parameters of porous aluminum. The relationship between porosity and effective

refractive index are discussed in the following section.

3.3 Reflectance from the 3D porous alumina

The total reflectance spectra of all the samples (Figs 5-7) clearly indicate the influence of

anodization voltage, electrolyte type and anodization time on the interference pattern. The

occurrence of interference fringes obtained for each sample is due to partial reflection on the

top surface (alumina-air interface) and on the bottom surface (alumina-TiN interface). For a

determined wavelength, if the reflected rays on the both surfaces are in phase, they interfere

constructively and originate a maximum on the reflectance spectrum. Whereas, if they have

opposite phases, a destructive interference occurs and causes a minimum on the reflectance

spectrum [23]. The number of interference maxima and minima is directly related to the PAA

layer thickness and the refractive index, and the last depends on the porosity. For example,

sample P90A has the minimum number of maxima and minima (Fig. 7a), and so is the oxide

thickness (Table 2) compared to the other samples. This is because for thinner films the possible

wavelengths that can promote constructive interference is reduced when compared with thicker

films. Similarly, more peaks are generally observed for samples anodized in H2SO4 compared

to those anodized in H3PO4 (Table 2). This may be related to different morphologies observed

for both cases. In H2SO4 electrolyte, the films have higher thicknesses and in the same order,

whereas for H3PO4, the films present lower thickness, that drastically increased with

anodization time. The porosity also changed for both electrolytes and different anodization

conditions. This parameter might alter the effective refractive index, modifying the effective

optical length and hence the interference pattern.

While the porosity of PAA with hexagonal pores array is around 10% [25], its precise

value can be obtained based on Eq. (2).

𝑃 =2𝜋

√3(

𝑑𝑝𝑜𝑟𝑒

𝑑𝑖𝑛𝑡𝑒𝑟𝑝𝑜𝑟𝑒)2 × 100 (2)

with P the porosity in %, 𝑑𝑝𝑜𝑟𝑒 the pore size and 𝑑𝑖𝑛𝑡𝑒𝑟𝑝𝑜𝑟𝑒 the inter-pore distance. The porosity

can be largely modulated by changing the anodizing parameters [26], and P can be greater than

40% at extreme anodization conditions. The higher porosity (>10%) can be obtained by partial

7

etching of the formed anodized alumina or by anodization at non-conventional conditions (25V

in sulfuric acid, 40V in oxalic acid and 160 V in phosphoric acid) similar to the conditions used

in the current study. However, due to the presence of inter-connected vertical and horizontal

pores in the samples, Eq. (2) cannot be used to estimate the bulk porosity, since it is based on

top view images and do not represent the bulk film in this case. Consequently, the porosity was

determined based on the effective refractive index

The effective refractive index for each sample was calculated from the Fabry-Perot

equation (Eq. 3) [23]:

{𝑚𝜆𝑖 = 2𝑛𝜆𝑖

𝐿

(𝑚 + 1)𝜆𝑖+1 = 2𝑛𝜆𝑖+1𝐿

(3)

where m is the interference order of the fringe maxima on reflectance spectra, which contains

information about the optical path length difference and reflection phase shift, and 𝑛𝜆 is the

effective refractive index at wavelength (λ). The symbols 𝜆𝑖 and 𝜆𝑖+1 are the wavelengths of

two neighboring fringe maxima at interference order of m and m+1, respectively and L is the

PAA thickness (can be obtained from SEM images). Because the band gap of PAA is large (6

- 7 eV) [21], 𝑛𝜆 can be considered constant at the long-wavelength region. Therefore:

𝜆𝑖

𝜆𝑖+1=

𝑚+1

𝑚 (4)

Using Eqs. (3) and (4), the interference order of the fringe maxima and the effective

refractive index of the samples were estimated (Table 3). Based on the refractive index, the

total porosity was estimated using the Bruggeman equation (Eq. 5) [22]:

(1 − 𝑝)𝑛𝑎𝑙𝑢𝑚𝑖𝑛𝑎

2 −𝑛𝑒𝑓𝑓2

𝑛𝑎𝑙𝑢𝑚𝑖𝑛𝑎2 +2𝑛𝑒𝑓𝑓

2 + 𝑝1−𝑛𝑒𝑓𝑓

2

1+2𝑛𝑒𝑓𝑓2 = 0 (5)

where 𝑛𝑎𝑙𝑢𝑚𝑖𝑛𝑎 (=1.8) is the refractive index of the matrix phase of amorphous alumina [27].

The estimated values are summarized in Table 3. While the vertical porosity was estimated with

ImageJ, the horizontal porosity is the difference between the total and vertical porosity.

Thereafter, the density of porous alumina 𝜌𝑝 was also calculated from the theoretical density of

alumina 𝜌𝑎 and porosity p using Eq. (6).

𝜌𝑝 = 𝜌𝑎(1 − 𝑝) (6)

While the effective refractive indices and density of the samples decrease with voltage

and anodization time, both interference order and porosity increase (Table 3). The slight

reduction in the refractive index with anodization voltage and time indicates that the samples

8

become less dense allowing for light to travel at a higher speed compared to the non-porous

alumina. Regardless of the anodization voltage, medium or anodization time, a value of m

should be an integer number if we consider that for reflectance spectra the alumina film acts

only as a propagation medium. Otherwise one must consider a phase shift due to the absorption

of waves on the substrate surface (alumina-TiN interface) [28]. In this case, the non integer

values could be regarded to differences on the refrective index of TiN as a function of

wavelenght [29, 30].. Therefore particularly for sample P90B the value of m indicates a phase

shift of in the reflection at the bottom surface, since m value was a half-integer (Table 3).

The porosity and effective refractive index are inversely proportional (Figure 8). For all

the samples, the vertical porosity is higher than horizontal porosity because the horizontal

porosity is initiated by Cu impurity which was kept to a very low value (0.5 wt %) for all the

samples. For a chosen electrolyte, an increased anodization voltage slightly lowers the index of

refraction (Table 3). This presupposes that as the film grows thicker, its index of refraction

decreases marginaly irrespective of current density. Similarly, a longer anodization time results

in an increased porosity and thickness, and subsequently it lowers index of refraction. These

results point to an important fact especially for thick films; to grow a film of uniform index of

refraction across the thickness, the applied current density must be constantly reduced

throughout the growth period.

4. Conclusion

Three dimensionally structured (3D) porous anodic alumina (PAA) films were fabricated and

characterized structurally and optically. The structural characteristics showed that both pore

size and inter-pore distance increased with increasing anodization voltage and time while pore

density decreased. From the optical spectra, the porosity/order of interference and refractive

index/density are inversely proportional. This indicates that irrespective of the anodization

media, anodization voltage and time can be used to modulate the parameters of porous

aluminum.

Acknowledgement

Alaa M. Abd-Elnaiem would like to thank Prof. P.M. Vereecken (imec-Belgium) for his

assistance on the anodization experiments and SEM images.

9

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films: thickness and preparation effects." Applied optics 25, no. 20 (1986): 3624-3630.

List of Figures:

I II II

I

I II II

I

I II II

I

12

Figure 1: Current density–time curves for Al-0.5Cu films anodized in (a) 1 M H2SO4 at 10 V

(Sample S10), 15 V (Sample S15), 20 (Sample S20) and 22 V (Sample S22), (b) 0.3 M H2C2O4

at 60 V (Sample C60) and (c) 0.75 M H3PO4 at 90 V for anodizing durations of 3 (Sample

P90A), 5.87 (Sample P90B) and 9.8 mins (Sample P90C). Stages are denoted as I, II and III.

Figure 2: SEM images (top, cross-section view) of Al-0.5Cu thin films fully anodized in 1 M

H2SO4 at (a, b) 10 V (c, d) 15 V (e, f) 20 V and (g, h) 22 V. Only samples anodized at 15 and

22 V were chemically etched in 0.75 M H3PO4 for 22 min at 30 ˚C.

13

Figure 3: SEM images showing (a) top view and (b) cross-section view of fully anodized Al-

0.5Cu thin films in 0.3 M H2C2O4 at 60 V.

3.00 min.

(a) (b)

14

Figure 4: SEM images (top, cross-section view) of Al-0.5Cu thin films fully anodized in 0.75

M H3PO4 at 90 V for (a, b) 3.00 min (c, d) 5.87 min and (e, f) 9.80 min.

5.87 min.

9.80 min.

(e)

(c) (d)

(f)

15

Figure 5: Total reflection spectra of the PAA films for aluminum samples anodized in 1 M

H2SO4 at (a) 10, (b) 15, (c) 20 and (d) 22 V.

(d)

(c)

(b)

(a)

16

Figure 6: Total reflection spectra of the PAA films for aluminum samples which 0.3 M H2C2O4

at 60 V for 46 min.

17

Figure 7: Total reflection spectra of the PAA films for aluminum samples anodized in 0.75 M

H3PO4 at 90 V for (a) 3, (b) 5.87 and (c) 9.8 min.

Fig. 8:

(a)

(c)

(b)

18

Figure 8: Porosity is inversely proportional the effective refractive index.

1.2 1.4 1.6 1.8

0

10

20

30

40

50

60

70

Total porosity

Vertical porosity

Horizontal porosity

Po

rosit

y (

%)

Effective refractive index