1. Influence of film thickness on the structural, electrical and photoluminescence
properties of vacuum deposited Alq3 thin films on c-silicon substrate
J.Y. Koay ⁎, Khairul Anuar M. Sharif, Saadah A. Rahman
Solid State Research Laboratory, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
a b s t r a c ta r t i c l e i n f o
Available online 24 March 2009
Keywords:
Photoluminescence
Current–voltage
Capacitance–voltage
Transition voltage
In this work, the influence of film thickness on the structural, electrical and photoluminescence (PL)
properties of tris (8-hydroxyquinoline) aluminum (Alq3) films prepared by vacuum evaporation technique
on crystal silicon (c-Si) substrate were studied. Fourier transform infrared (FTIR) spectroscopy, current–
voltage (I–V), capacitance–voltage (C–V) and PL spectroscopy measurements were done to investigate these
properties. The results demonstrated that the molecular organization of the deposited film was not affected
by the film thickness. The PL emission energy and intensity increased with increase in film thickness. The
results showed that the turn-on voltage (Von) and transition voltage (VT) were also dependent on the film
thickness.
© 2009 Published by Elsevier B.V.
1. Introduction
Low molecular weightorganic material (small molecules) like Alq3 is
widely studied and used as the active layer in organic light-emitting
device (OLED) since Tang and Van Slyke used Alq3 as an electron-
transport layer and light-emitting layer in their work in the 1980s [1].
Alq3 is a stable metal chelate that can be fabricated by using vacuum
deposition method and stands as one of the most successful materials
used in organic electroluminescence (EL) applications. Generally, the
device performance is influenced by the organic layer thickness [2].
Most studies to date are concentrated on the optimization of the device
characteristics, improvement of the morphological stability, under-
standing the mechanism of charge transport and the specifics of EL of
Alq3 based OLEDs [3]. However, understanding of the intrinsic proper-
ties of Alq3 films is usually limited to applications of Alq3 based devices
[4]. Inthis work, the PL, structural and electrical properties of single layer
devices based on Alq3 films deposited by thermal evaporation on c-Si
substrates as a function of organic layer thickness are studied. FTIR and
PL spectroscopy techniques are used to study the molecular structure
and luminescent properties of the Alq3 films respectively. The influence
of film thickness on the FTIR absorbance peak intensity and PL intensity
as well as the peak energy of Alq3 emission are studied. The
performances of the devices are measured from the current–voltage
(I–V) and capacitance (C–V) characteristics of these films on c-Si
substrates. The dependence of theorganic layer thickness ontheturn-on
voltage, Von and the transition voltage, VT are studied from I–V and C–V
curves obtained. Von is taken as the minimum voltage for the current to
increase exponentially while VT is taken as the inflection point between
accumulation and inversion region.
2. Experimental
Alq3 thin films were deposited by thermal evaporation on (111)
p-type c-Si substrates. The silicon pieces of dimension 2×2.5 cm2
and resistivity of 1–10 Ω cm were used as the substrates. The silicon
pieces were cleaned using the wet chemical treatment process under
atmospheric conditions in the following order: degreasing of organic
contaminants by dipping into acetone followed by de-ionised water
in an ultrasonic bath for at least 2 min each, removal of surface
oxides by etching in concentrated HF for 3 min for hydrogen-pas-
sivation of the substrates [5] followed by a final rinse in de-ionized
water in ultrasonic bath for at least 2 min. The substrates were
immediately installed in the deposition chamber after this cleaning
process. The Alq3 source material (purchased from Aldrich) was
vacuum evaporated at a growth rate of ~0.4 Å/s at deposition
pressure of ~8×10− 6
mbar. The film thickness was monitored using a
digital quartz crystal thickness monitor. The PL spectra of the Alq3
thin films were recorded at room temperature using a Perkin-Elmer
luminescence spectrometer LS50B. The excitation source was a
Xenon flash tube coupled to a monochromator interfaced with a
personal computer. The FTIR absorption measurements were per-
formed using a Perkin-Elmer FTIR spectrometer. Aluminium electro-
des of 400 nm thick were deposited in transverse configuration onto
the back of the c-Si substrates and the surface (as gate electrode) of
the Alq3 films for I–V and C–V measurements (see inset of Fig. 4). The
I–V and C–V characteristics were measured using a computer driven
Thin Solid Films 517 (2009) 5298–5300
⁎ Corresponding author.
E-mail addresses: janyeong@um.edu.my (J.Y. Koay), saadah@um.edu.my
(S.A. Rahman).
0040-6090/$ – see front matter © 2009 Published by Elsevier B.V.
doi:10.1016/j.tsf.2009.03.145
Contents lists available at ScienceDirect
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journal homepage: www.elsevier.com/locate/tsf

2. Keithley 236 Source Measure Unit and a 590 CV analyser respectively
at room temperature.
3. Result and discussion
Fig. 1 shows the FTIR spectra obtained from the Alq3 thin films of
different thicknesses. The FTIR spectrum of Alq3 powder prepared on
KBr is also included as a reference. The region shown in the spectra
corresponds to the “fingerprint region” of the vibrational modes
present in Alq3 films. The FTIR spectra of all the films irrespective of
film thickness are quite similar to the spectra of Alq3 powder prepared
on KBr confirming that the same functional groups are present in all
the films and the Alq3 powder. Going down the spectral range, from
1700–1000 cm−1
, two bands centered at 1606 and 1578 cm−1
as-
signed to a CC stretching vibration involving the quinoline group
of ligands are first observed followed by the bands at 1498 and
1468 cm−1
which correspond to a CC/CN stretching + CH bending
vibration associated with both the pyridyl and phenyl groups in Alq3.
Then, vibrations at 1385 and 1328 cm−1
involving CC/CN stretching +
CH bending of the quinoline fragments of Alq3 are observed. The bands
recorded at the end of this spectral range with peak positions at 1282,
1230 and 1115 cm−1
are assigned to a CH/CCN bending + CN stretch-
ing vibrations. In the region below 1000 cm−1
, the most intense ab-
sorption is observed at 789 cm−1
which is assigned to the out-of-plane
CH wagging vibrations of the quinoline groups. No significant change
in the chemical bonding structures of Alq3 films is observed within the
detection of the FTIR spectra with an increase in film thickness. The
increase in the FTIR absorbance peak intensity with increase in film
thickness of the Alq3 film is due to the increase in the number of
molecules with increase in film thickness [6]. The PL spectra of the Alq3
films of different thicknesses are shown in Fig. 2. The measurement
was made at room temperature with an excitation wavelength of
224 nm. All films showed strong PL peaks at a wavelength of ~519 nm
in all films. The PL intensity increases and the PL emission peak energy
is red-shifted with increase in film thickness. The thickness depen-
dence of the PL emission intensity and peak energy are also displayed
in Fig. 3. With the increase in the Alq3 film thickness from 70 to 280 nm,
the PL intensity increases by a factor of 2 and the peak energy is shifted
from about 514 to 525 nm. The increase in the PL intensity with film
thickness showed that PL intensity is dependent on the number of Alq3
molecules in the film structure. The Alq3 emission peak also showed a
significant red-shift with increase in film thickness. For thinner Alq3
film, the surface component is dominant while for thicker film the
contribution from the bulk (the 3D exciton) is more dominant [2]. This
explains the red-shift of Alq3 emission peak with increase in film
thickness. Fig. 4 shows the I–V characteristics Al/Alq3/c-Si/Al devices
with Alq3 film of different thicknesses with the inset showing the
device configuration. The current increases exponentially with in-
crease in applied bias voltage. It is also evident that the organic layer
thickness has an influence on the I–V characteristics of the films. The
electron injection barrier increases as the Alq3 film thickness increases.
This may due to the different positions of the HOMO and LUMO levels
as the Alq3 film thickness increases, leading to unbalanced electron
Fig. 2. PL spectra (excited at 224 nm) of evaporated Alq3 thin film on silicon substrates.
Fig. 3. Variation of PL emission intensity and energy with film thickness of evaporated
Alq3 films.
Fig. 4. I–V characteristic of Al/Alq3/c-Si/Al structure thin films.
Table 1
Turn-on and operating voltage of Al/Alq3/c-Si/Al structure for different Alq3 layer
thickness.
Alq3 layer thickness
(±5 nm)
Turn-on voltage (Von)
(±2 V)
Operating voltage (Vop)
(±2 V)
70 8 8.0–11.5
120 9 9.0–12.0
180 10.5 10.5–13.0
230 11.5 11.5–15.0
Fig. 1. FTIR spectra of evaporated Alq3 thin films and powder prepared on KBr.
5299J.Y. Koay et al. / Thin Solid Films 517 (2009) 5298–5300

3. and hole current. Therefore, in order to achieve the same current
density in the devices, the applied bias voltage has to be increased with
increase in Alq3 film thickness. The results given in Table 1 shows that
the turn-on voltage, Von and operating voltage, Vop increase when the
Alq3 layer thickness increases. This is because the Alq3 layer thickness
plays a role in improving the balance of electron and hole current [7], as
the cathode contact for electron injection in all the devices was the
same. The Von for the device is controlled by the majority carrier
injection from the electrode to gate. Since the electron is the majority
carrier, the turn-on voltage, Von is mainly determined by the organic–
electrode interface barrier. The higher Vop for the thicker films shows
that the contribution of the minority carrier current to the total current
becomes more significant [8,9] as the film thickness increases. Fig. 5
shows the Fowler–Nordheim (FN) plots obtained from the I–V curves
of the Alq3 films. According to the FN model, assuming that the current
is limited by high injection barriers at the electrodes, implying that no
space charges are present, for a given electric field the current should
be independent of the film thickness. However, the current for a given
field decreases when the film thickness increases from 70 to 230 nm.
This clearly contradicts the theoretical models based on pure injection
limitation [10]. They show a linear variation at high field which
deviates to a power law evolution at lower fields. This behavior shows
that either holes or electrons can be injected to the organic layer by
tunneling through the existing barriers existing at the organic–
electrode interfaces. The deviation from the linear portion of the
curve is supposed to be due to the thermionic emission (Richardson–
Schottky) contribution. The linear part of the curves plotted in Fig. 5
has gradually shifted to lower fields with increase in Alq3 layer
thickness. This behavior can be related to the development of a space
charge region near the Alq3/Al interface [11]. Fig. 6 shows the C–V
characteristic of a single Alq3 layer of thickness ranging from 70 to
230 nm. The transition voltage, VT is strongly dependent on the
thickness of the Alq3 layer. Fig. 7 shows the dependence of the VT
(taken as the inflection point of the C–V curves) [12] on the Alq3
thickness. As the Alq3 film thickness increases, VT obviously increases
linearly to a more positive value within the error limits.
4. Conclusion
The effects of film thickness on the chemical bonding, PL and
electrical properties of Alq3 films deposited by thermal evaporation on
c-Si substrates have been studied. The FTIR results have shown that
increasing film thickness produces no significant change in the
chemical bonding structures. However, increasing film thickness
increases the number of Alq3 molecules in the film structure and
this is represented by the increase in the absorption band intensity of
the vibrational modes present in the film. The PL intensity also
increases with increase in film thickness showing dependence of PL
intensity on the number of Alq3 molecules. Decrease in film thickness
produces a blue shift in PL emission peak energy as a result in
dominance of the surface component for thinner films. I–V measure-
ment results show that thicker films result in higher Von and higher
Vop indicating that the total current is mainly contributed by minority
carriers in thinner films. Thinner films are more favorable for current
injection into the organic layer by tunneling through the barriers as
these films can easily produce higher fields at small applied voltage.
Shifting of the linear part of the FN plots to lower fields for thicker
films shows indication of development of space charge region near the
Alq3/Al interface. An increase in capacitance is produced by thinner
film and VT increases with increase in film thickness.
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Fig. 6. C–V characteristic of Al/Alq3/c-Si/Al structure thin films.
Fig. 7. Variation of transition voltage with film thickness for the Al/Alq3/c-Si/Al
structure.
Fig. 5. Fowler–Nordheim equation plots for the Al/Alq3/c-Si/Al structure for different
Alq3 layer thickness.
5300 J.Y. Koay et al. / Thin Solid Films 517 (2009) 5298–5300