Spectroscopic ellipsometry of metal phthalocyanine thin films
Spectroscopic ellipsometry of metal
phthalocyanine thin films
Aleksandra B. Djurisˇic´, Chung Yin Kwong, Tsz Wai Lau, Zheng Tong Liu,Hoi Sing Kwok, Lillian Sze Man Lam, and Wai Kin Chan
Optical functions of cobalt phthalocyanine, nickel phthalocyanine ͑NiPc͒, and iron phthalocyanine ͑FePc͒have been determined by use of spectroscopic ellipsometry in the spectral range 1.55– 4.1 eV ͑300 – 800nm͒. The samples were prepared by evaporation onto glass and silicon substrates. The optical func-tions were determined by point-to-point fit.
Absorption spectra were also measured.
refraction data for NiPc and FePc are reported for the first time to our knowledge.
the experimental spectra was obtained for all three materials.
CuPc, other metal Pcs have not been extensively
Phthalocyanines are porphyrin derivatives that are
characterized by high degrees of symmetry, planar-
͑CoPc͒ by spectroscopic ellipsometry has been report-
Absorption spectra of various Pc films,15–17 as
͑Pcs͒ have attracted much attention for applications
well as vapor phases and solutions in different sol-
in organic optoelectronic devices such as organic solar
vents,18,19 have also been reported.
cells,1–3 in organic field effect transistors,4 in organic
The growth and morphology of the various Pc films
light-emitting diodes in which Pcs can be used as a
hole transport layers5,6 or emitting layers7; and in gas
ist in several crystalline polymorphs.
polymorphs are metastable ␣ and stable ␤ phases.
tronic devices, the optical function of each layer
The main differences between polymorphs lie in the
tilt angle of the molecules within the columns ͑stacks
usually described in terms of the complex dielectric
of molecules with molecular planes parallel to one
function ε͑͒ ϭ ε ͑͒ ϩ i
ε ͑͒ or the complex index of
another͒ and the mutual arrangement of the col-
͑͒ ϭ n
͑͒ ϩ ik
͑͒. Copper phthalocy-
Transformation from phase ␣ to phase ␤ by
anine ͑CuPc͒ has been the most widely studied metal
exposure to solvent vapors25,29,30 or by annealing or
Dielectric functions and indices of refraction of
deposition at higher temperature26–28,31,32 has been
CuPc were studied experimentally by use of reflec-
The absorption spectra of different poly-
tance and transmission measurements,10 absorption
morphs of the same Pc compound differ significant-
coefficient measurement with Kramers–Kronig anal-
ysis,11 and spectroscopic ellipsometry.12,13
influences the preferred orientation in Pc layers.
The substrate also seems to play a role in the orien-tation of the deposited films.21–24,26
A. B. Djurisˇic´ ͑email@example.com͒, C. Y. Kwong, T. W. Lau,
L. S. M. Lam, and W. K. Chan are with the University of Hong
isomorphism with one another23 and that deposition
Kong, Pokfulam Road, Hong Kong, China.
on mica at temperatures above 400 K results in ori-
C. Y. Kwong are with the Department of Electrical Engineering
Wachtel et al
.24 found that the sta-
and L. S. M. Lam and W. K. Chan are with the Department of
bility of the films may be substrate dependent and
Chemistry; A. B. Djurisˇic´ is also with the Department of Physics.
that lithium Pc films deposited on glass substrates at
Z. T. Liu and H. S. Kwok are with the Department of Electrical
room temperature were stable over several months,
Engineering, Hong Kong University of Science and Technology,
whereas those on NaCl substrates showed marked
changes when they were exposed to ambient condi-
Received 22 May 2003; revised manuscript received 8 July 2003.
epitaxial growth of thin fluorine bridged aluminum
APPLIED OPTICS ͞ Vol. 42, No. 31 ͞ 1 November 2003
Pc polymer by molecular-beam epitaxy was obtained
evaporator to minimize formation of another oxide
on alkali halide substrates, whereas epitaxial growth
Microscope slide glass and quartz substrates
were cleaned in an ultrasonic bath for 30 min, rinsed
with two different b
-axis ͑stacking axis͒ orientations,
with DI water, and dried in flowing nitrogen.
depending on the substrate’s temperature, have been
cleaning, the back side of the glass substrate was
roughened to suppress back side reflections for spec-
maintained at room temperature, the preferred ori-
troscopic ellipsometry ͑SE͒ measurement. For ab-
entation of evaporated CuPc films less than 100 nm
thick is in the ͓200͔ direction, with the ␣ form dom-
deposited on quartz substrates were used.
of the orientation of crystallites occurs; the ␣ form is
holder can hold up as many as four substrates, so the
films fabricated on microscope slides, silicon, and
substrates, the preferential orientation of the crystal-
quartz were deposited in the same deposition process.
lites appeared to depend on the deposition rate and
Pressure during evaporation was of the order 10Ϫ4
on whether the substrate was rotating ͓͑110͔ orien-
The evaporation rate was 0.1– 0.2 nm͞s. The
tation͒ or nonrotating ͓͑100͔ orientation͒.22 Bayliss
distance from source to film was ϳ23 cm to ensure
.32 reported that the ␣ form of the metal-free Pc
uniformity of film thickness, and the substrate holder
H Pc deposited on microscope slides at room temper-
ature consists of small spherical particles that are
controlled by a quartz thickness monitor.
randomly oriented, whereas oriented domains in the
The film thickness was measured by a step profiler.
films have been found only for the ␤ form obtained by
Pc films are absorbing in the spectral region of inter-
annealing the ␣ form. The ␤ form obtained by direct
Absorption is weak in the 450 –550-nm spectral
evaporation onto substrates heated to 330 °C showed
region, but assuming a Cauchy model for the refrac-
elongated crystallites similar to those in the ␤ form
tive index and disregarding the coefficient of extinc-
obtained by annealing, but randomly oriented.32
tion in this spectral region can cause errors in the
Highly oriented CuPc film fabricated by molecular-
thickness determined from fitting of SE data.
beam deposition onto quartz glass substrates at low
film thickness was also verified by AFM.
deposition rates has been demonstrated.34
mode scanning-probe microscope from Digital Instru-
We report spectroscopic ellipsometry measure-
ments was used for AFM measurements.
ments of CoPc, nickel phthalocyanine ͑NiPc͒, and
thickness was measured at 10 points along the step to
iron phthalocyanine ͑FePc͒ thin films deposited on
minimize the influence of possible errors caused by
glass and silicon substrates at room temperature.
variation in thickness around the step.
Because the deposition is performed at room temper-
to revealing film thickness, AFM also yielded a
ature, we expected that the films would be polycrys-
talline and predominantly of the ␣ form. The
roughness in all films was below 3.5 nm; the mean
spectroscopic ellipsometry measurements were per-
formed in the spectral range 1.55– 4.1 eV ͑300 – 800
48 nm for CoPc, 56 nm for FePc, and 45 nm for NiPc.
nm͒. Samples were modeled as isotropic layers, as
Absorption measurements were performed on a
was reported to be adequate for CuPc samples less
Perkin-Elmer Lambda 20 UV͞Vis spectrophotome-
The SE data in the 300 – 800 nm spectral range
cule has an intrinsic anisotropy that is due to its
were measured with a Jobin Yvon UVISEL phase-
planar structure, for random orientation of crystal-
lites in a polycrystalline film an isotropic model is
someters are comparable to the more frequently used
rotating analyzer ellipsometers in terms of attainable
sorption measurements were also performed.
samples for absorption measurements were depos-
The samples exhibited good uniformity, which we
verified by performing measurements at different ar-
eas of the sample on both glass and silicon substrates.
experimental details are described, and Section 3 pre-
Also, the samples did not exhibit any in-plane anisot-
ropy, as we verified by rotating the sample 90° and
of the measured spectra was performed.
simulated annealing algorithm to fit the data.38
Pc powders were purchased from Strem Chemicals.
fitting was performed simultaneously for glass and
First, silicon substrates were cleaned in deionized
͑DI͒ water by a standard four-cycle dump–rinse pro-cedure.
After that, the wafer was cleaned for 10 min
cycle dump–rinse in DI water was performed, fol-
lowed by HF cleaning ͑HF:H O 1:50͒ for 30 s to
drying, the wafers were immediately loaded into an
͒ ϭ ͑X
exp Ϫ X
1 November 2003 ͞ Vol. 42, No. 31 ͞ APPLIED OPTICS
denotes the i
th experimental data point, the
positions of the peaks in the calculated extinction
subscripts glass and Si denote glass and silicon sub-
coefficient spectrum with the measured absorption
strates, respectively, and the superscripts exp and
calc denote experimental and calculated data.
an optional penalty function that can be introduced to
Results and Discussion
ensure the smoothness of the n
Pc films grown upon room-temperature substrates
limiting the allowed difference between values ob-
typically consist of small spherical particles and are
tained in current ͑i
͒ and previous ͑i
Ϫ 1͒ data points.
predominantly of ␣ modification26,32 or possibly of the
We obtained this difference in the following manner:
If the difference between n
values in current
creased, the size of the crystallites increases, some
and previous data points was larger than the speci-
preferential ordering in neighboring domains be-
fied allowed difference, ␥ was set to a large positive
gins,25 and finally at higher temperatures the ␤ form
Otherwise, ␥ was set to zero. Because of the ex-
room temperature results in significantly smoother
istence of multiple solutions, the use of such a penalty
Vincett et al
.35 believed that the films
function to prevent unphysical abrupt changes in n
were in the ␣ form, though at temperatures below
Ϫ50 °C the films may have been partially amorphous.
indices of the substrates from the SE measurements
Brinkmann et al
.,25 however, stated that films grown
of the substrates, instead of taking the data from the
at temperatures of 5 °C–25 °C have disordered, i.e.,
material database, to ensure improved accuracy of
close to amorphous, structures based on diffuse dif-
the determined n
of the Pc films.
Pan et al
.42 have found that vanadyl
strates, fitting with and without a native oxide layer
Pc films deposited at room temperature and exposed
to flowing nitrogen for 50 h are partially amorphous.
between the results obtained in these two cases.
In a previous analysis of the optical functions of CuPc
The result of simultaneous point-to-point fitting for
by variable-angle spectroscopic ellipsometry, Debe12
both substrates shows somewhat inferior agreement
investigated Pc films grown at 0 °C and 70 °C, which
with the experimental data compared with point-to-
he assumed were well oriented, and deduced the ap-
point fitting for one substrate only.
proximate optical functions that correspond to paral-
ability of the data obtained by simultaneous fitting
lel and perpendicular orientation with respect to
for both substrates is considerably improved com-
pared with data obtained by fitting for only one sub-
ited at low temperature may not have been well un-
strate because there are two unknowns and four
iaxially oriented has been acknowledged,12 but the
conditions that need to be satisfied.
possibility of phase change or of amorphousness of
Correction for surface roughness was not made be-
the films grown at low temperature has not been
cause individual points yield different thickness val-
films were grown upon room-temperature substrates,
rough layer to the value averaged over a wavelength
so it is reasonable to assume that the films consist of
polycrystalline, predominantly ␣ phase domains with
model the optical functions of the Pc layer and then to
little or no preferential orientation among the neigh-
fit the thickness of the Pc layer and the thickness of
cases is less than 80 –100 nm, we have considered the
els of the optical functions of inorganic semiconduc-
tors are available, for organic materials the choice is
an isotropic inner layer and an anisotropic outer
typically limited to the Lorentz oscillator model and
layer should be assumed, whereas a film thicker than
its modifications39,40 or the Forouhi–Bloomer mod-
150 nm can be treated as a single homogeneous aniso-
It was shown previously that the conventional
Lorentz model is inadequate for modeling optical
Figures 1–3 show tan͑⌿͒ and cos͑⌬͒ for CoPc, FePc,
and NiPc films, respectively, deposited onto glass and
Bloomer model was used only in a very narrow spec-
tral range ͑365–564 nm͒ for a material that exhibited
a single peak in the spectral range investigated.41
region investigated, two distinct absorption bands
We attempted to use both the conventional Lorentz
can be observed, a band near 2 eV ͑620 nm; Q
model and its modifications, with and without
and one near 3.5 eV ͑350 nm; B
or Soret band͒. Both
bands have been assigned to 3 * transitions of the
ment with the experimentally determined tan͑⌿͒ and
macrocyclic system consisting of C and N at-
cos͑⌬͒ was inferior compared to that from point-to-
sorption in these two bands are shown to be
of optical functions obtained by point-to-point fitting
without surface-roughness correction.
among the visible spectra of various metal Pcs.
the reliability of the data by fitting the data for films
However, metal-dependent absorption bands can be
observed near the infrared spectra of Pcs.18
the quality of the result obtained by comparing the
has been suggested that both Q
bands can be
APPLIED OPTICS ͞ Vol. 42, No. 31 ͞ 1 November 2003
Tan͑͒ and cos͑⌬͒ for CoPc films on glass and silicon
The fit is a point-to-point fit; ͑a͒ glass substrate, ͑b͒
Absorption spectra of CoPc, FePc, and NiPc.
influenced by the metal-to-ligand charge-transfer
of the origin of the observed peaks in the absorption
bands in CoPc ͑Ref. 14͒, and the existence of a charge-
spectrum of a Pc material is rather complex.
transfer band at the red edge of the Q
band has been
calculations of Pc molecules have been per-
However, based on the small difference of
formed, and the calculated results are in good agree-
the shift of absorption peak on the low-energy side of
band of CuPc in solvents of different polarities
spectra of solutions and thin films in which interac-
͑dimethyl sulfoxide and 1-chloronaphtalene͒,18 it is
tions between individual molecules play a role are
reasonable to conclude that the charge transfer does
not play a significant role in transitions on the low-
Figure 5 shows the real and imaginary parts of the
indices of refraction of CoPc, FePc, and NiPc.
agreement between the peak positions in the calcu-lated extinction coefficients and the measured ab-sorption spectra can be observed.
determined the index of refraction of CoPc films onsilicon by spectroscopic ellipsometry in the spectralregion from 550 to 800 nm.
cient and absorption coefficient data show threepeaks in this spectral region, and the value of theextinction coefficient is greater than 0.5 over the en-tire region, although it should be low at 550 and 800nm according to absorption measurements reportedin the literature that consistently have shown onlytwo peaks.15,17
the very thick film ͑greater than 600 nm͒ investi-gated by Chen et al
CuPc an anisotropic model is needed for analysis of
͑͒ and cos͑⌬͒ for FePc films on glass and silicon
Tan͑͒ and cos͑⌬͒ for NiPc films on glass and silicon sub-
Real (n) and imaginary (k) parts of the index of refraction
1 November 2003 ͞ Vol. 42, No. 31 ͞ APPLIED OPTICS
the ellipsometry data of films thicker than 150 nm.
Lucia and Verderame15 identified two peaks in the Q
ropy was detected when the sample was rotated and
band of the absorption spectrum of CoPc, located at
the ellipsometry measurement repeated, but this
1.82 and 2.01 eV ͑680 and 616 nm͒. Davidson17
does not rule out the presence of anisotropy in the
identified peaks at 1.82 and 1.98 eV ͑680 and 626 nm͒
in the Q
band and at 3.79 eV ͑327 nm͒ in the B
are not likely to be ideally ordered, it is not possible
Our results are in good agreement with previously
to determine their anisotropy unambiguously.
reported absorption spectra and with the absorption
ther investigations are needed to resolve this issue.
1.82 eV ͑680 nm͒ and 2.02 eV ͑615 nm͒ in k
from SE and at 1.80 eV ͑688 nm͒ and 2.01 eV ͑618
Optical functions of CoPc, NiPc, and FePc thin films
nm͒ in the absorption spectrum. In the B
have been determined by use of spectroscopic ellip-
observed peaks at 3.81 eV ͑325 nm͒ in k
and of 3.84
sometry in the spectral range 300 – 800 nm.
films are likely to be polycrystalline with randomly
For FePc, no previous data on the index of refrac-
oriented crystallites, so an isotropic model of the sam-
tion were reported to our knowledge.
spectra for FePc have been reported,16,17 and three
determined by simultaneous point-to-point fits of
peaks can be observed in the absorption spectra.
tan͑͒ and cos͑⌬͒ measured for films deposited upon
Davidson17 obtained peaks at 1.73 eV ͑717 nm͒, 1.98
eV ͑626 nm͒, and 2.19 eV ͑566 nm͒ in the Q
between experimental and calculated tan͑͒ and
peaks at 3.51 eV ͑353 nm͒ and 3.83 eV ͑324 nm͒ in the
cos͑⌬͒ curves was obtained for all three materials.
Our results show maxima in the extinction
The agreement between the positions of the maxima
coefficient at 1.74 eV ͑713 nm͒, 1.98 eV ͑625 nm͒, and
in the imaginary part of the index of refraction and
2.18 eV ͑570 nm͒ in the Q
band and wide structure in
the absorption spectra reported in the literature and
band with a maximum at 3.81 eV ͑325 nm͒,
measured by us was good for all materials.
whereas in the absorption spectrum we observepeaks at 1.73 eV ͑717 nm͒, 1.96 eV ͑631 nm͒, 2.18 eV
This study was supported by The Research Grants
͑568 nm͒, and 3.79 eV ͑327 nm͒, is in good agreement
Council ͑RGC͒ of the Hong Kong Special Administra-
with the absorption spectra reported in the litera-
tive Region, China ͑projects HKU 7096͞00P and
HKU 7075͞01P͒, the University of Hong Kong seed
Also for NiPc, we are not aware of any previous
funding research grant, and an RGC Co-operative
study of the complex index of refraction.
Research Centers grant from the Hong Kong Univer-
.44 obtained an absorption spectrum showing a
two-peak structure and identified four transitions in
band of NiPc, located at 1.76 eV ͑704 nm͒, 1.83eV ͑678 nm͒, 1.98 eV ͑626 nm͒, and 2.02 eV ͑614 nm͒.
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1 November 2003 ͞ Vol. 42, No. 31 ͞ APPLIED OPTICS
Volume Sixteen • Number Four • July 2007 © By Eve Greenberg, MA, LPC, CTN, Staff Reporter for EXPLORE! Publications, USA “In recent years, Autism, Asperger’s and other relatedtherapy and various forms of physical therapy, supplements,illnesses on the autism spectrum have become treatable to amedication and “magical formulas”, meaningful laboratorylarge degree, especially wh
PVD Group Newsletter Welcome to the winter edition of the Peripheral Vascular Diseases New protocols published: review group newsletter. In addition to the Editorial base update you will Percutaneous transluminal arterial angioplasty versus find details of our recently published reviews and protocols; information stenting for infrapopliteal arterial lesions in critical limb on C