Photoluminescence oscillations in porous alumina films

Article abstract of DOI:10.1063/1.2390645

The dominant oscillation shown in the laser-excited photoluminescence spectrum of the porous alumina film can be ascribed to the interference within a Fabry-Prot optical cavity where the separation between two neighboring oscillations is highly sensitive to the film's thickness and refractive index. The method, designated as photoluminescence oscillation, endows photoluminescence spectrum with another dimension and results in a nondestructive method for the measurement of the thickness and refractive index with a high resolution.

Full text of DOI:10.1063/1.2390645

Photoluminescence oscillations in porous alumina films
K. Huang, L. Pu, Y. Shi, P. Han, R. Zhang, and Y. D. Zheng
Citation: Applied Physics Letters 89, 201118 (2006); doi: 10.1063/1.2390645
View online: http://dx.doi.org/10.1063/1.2390645
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APPLIED PHYSICS LETTERS 89, 201118 ͑2006͒
Photoluminescence oscillations in porous alumina films
K. Huang, L. Pu, Y. Shi,a͒ P. Han, R. Zhang, and Y. D. Zheng
Department of Physics, Nanjing University, Nanjing 210093, China
͑Received 20 July 2006; accepted 5 October 2006; published online 17 November 2006͒
The dominant oscillation shown in the laser-excited photoluminescence spectrum of the porous
alumina film can be ascribed to the interference within a Fabry-Pérot optical cavity where the
separation between two neighboring oscillations is highly sensitive to the film’s thickness and
refractive index. The method, designated as photoluminescence oscillation, endows
photoluminescence spectrum with another dimension and results in a nondestructive method for the
measurement of the thickness and refractive index with a high resolution. © 2006 American
Institute of Physics. ͓DOI: 10.1063/1.2390645͔
Thickness monitoring is an indispensable growth control
in chemical vapor deposition. Moreover, thickness is also an
important parameter in material characterization. There are
many methods for thickness measuring. Atomic force micro-
scope ͑AFM͒, scanning electron microscope ͑SEM͒, and
transmission electron microscope ͑TEM͒ are common de-
vices to obtain thickness data, but making a TEM sample is
somewhat a tedious work. Other measuring methods include
spectral reflectance or transmittance, x-ray diffraction, and
ellipsometry. However, the above methods have restrictions.
For example, spectral transmittance can only be used to mea-
sure transparent substrate, and ellipsometry is not suitable for
measureing porous films because the effective refractive in-
dex varies with the porosity of porous films.
We report a photoluminescence ͑PL͒ study on the porous
alumina films.1–6 The study reveals that the evident oscilla-
tions exposed on the laser-excited PL spectrum, designated
as PL oscillation, contains information related to the film
thickness and effective refractive index. Detailed analysis in-
dicated that this structure sensitivity could be used for mea-
suring thickness and refractive index. It should be noted that
a similar explanation on the PL oscillation in porous Si has
been reported previously.7
ing a SEM ͑LEO 1530VP͒. The PL was excited by a He–Cd
laser ͑325 nm͒.
We investigated two typical samples of porous alumina,
which were anodized respectively from polished and unpol-
ished Al foils. Figure 1 shows the laser-excited PL spectra of
these two samples, which the spectra feature with two sub-
bands of 2.66 ͑F center͒ and 3.06 eV ͑F+ center͒.13,14 How-
ever, the spectrum b of the polished sample is surprisingly
different from that of the unpolished sample. The emission
intensity presented evident oscillations in a period of about
0.095 eV. Note that polishing Al foil or not before anodizing
is the only difference between the two samples. Therefore
the surface roughness should be the key factor to the PL
oscillation. On the other hand, using a laser to excite PL is
also important in obtaining the PL oscillation.
Figure 2 shows the PL spectra of the polished samples
with different thicknesses. The related parameters are listed
in Table I which show two trends: ͑1͒ the oscillating period is
inversely proportional to the film thickness; ͑2͒ the product
of thickness and oscillation period is nearly constant, 3.8
ϫ105 nm meV, with an average deviation of 5.2% for all
samples. This is the typical character of a plane-parallel cav-
Porous alumina film with periodic nanopore lattice is
widely used as a template or mask in the fabrication of
mesostructures.8–12 It also acts as an insulator host to sustain
the functional material on many occasions. Moreover, porous
alumina film can emit strong blue light.13,14 Thus the study is
valuable in optical applications, and it is helpful in elucidat-
ing some confused explanations on luminescence spectra.
In our experiment, porous alumina film was made by
anodizing Al foils ͑99.999%, 0.5 mm thick͒. After annealing
under N2 ambient at 450 °C for 4 h, Al foil was ultrasoni-
cally degreased in acetone for 5 min, and then it was elec-
trochemically polished in a mixture solution of HClO4 and
C2H5OH with a volume ratio of 20:80. The polished Al foil
was placed on the anode ͑Cu plate͒ and anodized under con-
stant dc 40 V in 0.3M oxalic acid at 5 °C. A Pt wire served
as the cathode. Serial samples were made with different an-
odizing durations ranging from 10 min to 8 h. For compari-
son, the unpolished Al foils were also used in anodization.
The mean surface roughness was measured using AFM
͑NanoScope IIIa͒. The sample morphology was observed us-
FIG. 1. PL spectra of the porous alumina films. The samples were anodized
for 5 h from the unpolished Al foils ͓sample ͑a͔͒ and 1 h from the polished
Al foil ͓sample ͑b͔͒, respectively. The surface mean roughness of the unpol-
ished and polished Al foils were 38 and 9 nm, respectively, measured by
AFM over a scan area of 3 ␮m2.
a͒
Electronic mail: yshi@nju.edu.cn
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129.105.215.146 On: Thu, 18 Dec 2014 13:01:41

201118-2
Huang et al.
Appl. Phys. Lett. 89, 201118 ͑2006͒
FIG. 3. Sketch of the PL oscillations due to the interference between emit-
ting light 1 and 1 under the approximation of plane-parallel ͑or Fabry-
Ј
Pérot͒ resonator. Note that the size of the round cell tip is much less than the
wavelength of emitting light.
measure region ͑2.25–3.54 eV͒, and the absorption coeffi-
cient is very small. With these facts, the mode spacing ⌬E
can be deduced from formula ͑1͒ as ⌬E=hc/2nL, or nL
=hc/2⌬E, where h is Planck’s constant ͑6.626ϫ10−34 J s͒, c
is the speed of light ͑2.998ϫ108 m/s͒, and nL is generally
defined as the optical length of the material.
FIG. 2. ͑Color online͒ PL spectra of the porous alumina films with the same
pore diameter but different thicknesses ͑samples 1–7, from the lower to the
upper; note Table I͒. All samples were anodized from polished Al foils. The
PL spectrum of sample 4 is also highlighted in Fig. 1 ͓sample ͑b͔͒. The
inserted SEM image of porous alumina film indicates a porosity of 8%. The
scale bar is 200 nm.
Figure 4 shows the linear dependence between the thick-
ness of the porous alumina films ͑L͒ and the optical lengths
͑nL͒ calculated from the data in Table I. It is valuable be-
cause the slope of the fitted line represents the refractive
index of the porous alumina film, which was 1.66 in our
experiment. For comparison, the Bruggeman equation16 also
gives an access to the effective refractive index neff of the
complex material,
ity where the oscillation period corresponds to the difference
between two longitudinal modes. Note that the pore diameter
of the porous alumina is about 30 nm ͑insert, Fig. 2͒. This
diameter corresponds to a cell tip ͑barrier layer͒ of about
100 nm. Therefore, the plane approximation is reasonable;
i.e., the porous alumina film is a Fabry-Pérot cavity since the
size of the barrier layer is much less than the wavelength of
the emitting light.
The origin of the PL oscillation related to the Fabry-
Pérot cavity is schematically shown in Fig. 3. We deduced
that the oscillation should result from the interference be-
tween the emitting wave ͑1͒ and its round-trip reflected wave
n2alumina − n2eff
n2alumina + 2n2eff
1 − n2eff
1 + 2n2eff
͑1 − p͒
+ p
= 0,
͑2͒
where nalumina is the refractive index of the matrix phase of
alumina ͑1.67͒,17 and the porosity p is the percentage of the
minor phase of the air columns ͑pores͒. Considering the
structure geometry of the porous alumina film, the porosity p
can be obtained from the statistic of the pore area using a
top-view SEM image; in our experiment, it is typically 8%
͑insert, Fig. 2͒. These data result in an effective refractive
index neff of 1.62 from the Bruggeman equation. This result
matches the coefficient of average deviation of 2% to 1.66
extracted from the PL oscillation.
In conclusion, the dominant oscillations shown in the
laser-excited PL spectrum of the porous alumina film can be
ascribed to the interference within a Fabry-Pérot optical cav-
ity, where the separation between two neighboring oscilla-
tions is highly sensitive to the film thickness and refractive
index. Because the material dispersion ͑dn/d␭͒ is zero under
͑1 ͒. In general, the separation of longitudinal modes of a
Ј
Fabry-Pérot cavity of length L reads as15
2
␭
⌬␭ Ϸ
,
͑1͒
2L͓n − ␭͑dn/d␭͔͒
where n is the refractive index of the sample of porous alu-
mina, dn/d␭ is the material dispersion and L is the sample
thickness. Note that the separation ⌬E of two neighboring
oscillating peaks is almost constant. This phenomenon hints
that the effective refractive index of the sample should
change a little for the light with photon energy ranging from
3.54 to 2.25 eV ͑350–550 nm͒; thus the material dispersion
͑dn/d␭͒ can be treated as zero. The reliability of this suppo-
sition was also strengthened by the following facts: the band
gap of alumina is much bigger than the photon energy of the
TABLE I. Parameter dependence of the PL oscillation ͑the thickness was measured using SEM͒.
Porous alumina film
Anodized time ͓t ͑min͔͒
1
2
3
4
5
6
7
10
20
30
60
120
300
480
Film thickness ͓L ͑nm͔͒
8.200ϫ102 1.520ϫ103 2.09ϫ103 3.84ϫ103 6.45ϫ103 1.3ϫ104 1.9ϫ104
Oscillation period ͓⌬E ͑meV͔͒ 5.0ϫ102
L·⌬E ͑nm meV͒
2.4ϫ102
1.9ϫ102 9.6ϫ10
5.8ϫ10
2.6ϫ10 2.1ϫ10
3.8ϫ105 nm meV
4.1ϫ105
3.6ϫ105
3.9ϫ105 3.7ϫ105 3.7ϫ105 3.4ϫ105 4.0ϫ105
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201118-3
Huang et al.
Appl. Phys. Lett. 89, 201118 ͑2006͒
The authors acknowledge J. Chen and Y. X. Fan for
helpful discussions. They are also grateful to the NSFC funds
of 60225014, 60206001, and 60390074 and the 973 project
of G2001CB3095.
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