Preparation and Characterization of Multifilms Based on

 Preparation and Characterization of Multifilms Based on Phthalocyanina Bolsista Paola Andrea Benavides Universidad del Valle Orientador Prof. Dr. Carlo César Bof Bufon/ LNNano Preparation and characterization of
multifilmsbased on phthalocyanines
Bolsista
Paola Andrea Benavides
Estudiante de Química
Universidad del Valle
Relatório técnico-científico apresentado como
requisito parcial exigido no
22º Programa Bolsas de Verão do CNPEM Centro Nacional de Pesquisa em Energia e Materiais.
Orientador
Prof. Dr. Carlos César Bof Bufón/ LNNano
Campinas, SP, 2013
RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM Agradecimientos
“Caminando en línea recta no se logra llegar muy lejos”… por ello inicialmente doy
gracias a la vida, al universo y a la casualidad por permitirme ser hija de mi madre.
Gracias a toda mi amplia familia, mi abuela, mi mamá Elena, mi hermana, por ese
apoyo incondicional durante estos pocos años de vida. A Michael, por darme un
ejemplo y una meta por conquistar. Majo, Juan, Chami, Vivian, Gisella y Myleidi,
gracias por darme la mano y mostrarme lo lejos que puedo llegar durante mi nuevo
comienzo, por todas las madrugadas que ayudaron a construir el momento que ahora
estoy viviendo. Libo, Alejandra y Nathaly sin esa gran experiencia investigadora
que me han compartido, el desarrollo de este trabajo no habría sido tan bueno. Mi
total gratitud hacia los profesores Manuel N. Chaur, Alberto Bolaños, Fabio
Zuluaga, Nelson Porras y Rodrigo Abonía por haber confiado en mis capacidades y
brindarme el apoyo necesario para poder participar en este programa.
Listo!, a mi orientador Carlos Cesar, gracias por su entusiasmo y total disponibilidad
al momento de resolver mis dudas y por incentivar mi curiosidad hacia nuevos
campos de la ciencia. A Sthephany por sus clases de portuñol para extranjeros y
haber logrado que entendiera cómo funcionaba el evaporador, a Maria Elena,
Angelo y David por el soporte técnico durante el desarrollo de mi trabajo y al grupo
de MTA por el préstamo de la estación de prueba para la caracterización eléctrica.
Gracias al CNPEM por dar la oportunidad a estudiantes en el nivel intermedio de su
vida universitaria, la oportunidad de desenvolverse durante dos meses en un
ambiente de investigación científica.
Fiona, Maghalenha, Thiago y Patricio, gracias por todos esos momentos en que sus
oídos sirvieron para escuchar mis canciones y porque sus palabras y gestos
permitieron que desarrollara una sonrisa, al parecer, eterna. A todos los bolsistas, fue
grato este tiempo con ustedes y creo que Augusto Mestre Profundo Conhecedor
espera nuestro reencuentro…“Yo no sabía que sería difícil”.
RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM “It's never too late to start all over again”
RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM Abstract
In this work we investigate the electrical transport across thin molecular films with
thicknesses close to the charge depletion limit. For that we adopted a model
molecular system based on phtalocyanines (Pc) consisting of pure-Pc, CuPc and the
bilayer Pc/CuPc with 40 ± 2 nm thicknesses. The molecular layers were grown using
the thermal evaporation method with vacuum of ~10-6 mbar. Due to structural
differences between the molecules used to prepare the films, it was necessary first to
calibrate the deposition rate and the distance source-substrate to ensure the lowest
dispersion in the thickness. In order to allow charge injection, Au contacts were
deposited on Pc’s films by electron beam evaporation. The patterns were defined by
a set of shadow masks. Finally, we realize current-voltage measurements to compare
the electrical behavior between Pc and CuPc films as well as the bilayer Pc/CuPc.
Keywords: organic semiconductors, phthalocyanines, tooling, Au deposition,
surfaces states, hopping, Frenkel-Poole emission, Depletion Width.
RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM Table of Contents
1. Introduction
1
1.1.
Phthalocyanines, gas sensor
2
1.2.
Deposition Method
5
1.3.
Overview of operation of working equipment
6
2. Objectives
9
3. Methodology
10
3.1.
3.2.
Devices fabrication based on thin films of Pc, CuPc and
Pc/CuPc with ~ 40nm
10
I-V Characteristics of the thin films of Pcs fabricated
12
4. Results
14
4.1.
Fabrication of the thin films
14
4.2.
Electrical Characterization
18
5. Conclusions
23
References
24
Appendage
26
RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM 1. Introduction
During the second half of the twentieth century, a lot of research focused on
understanding the charge transport properties of organic semiconductors was done to
achieve the knowledge about the fabrication of organic electronic devices such as
light emitting diodes and field-effect transistor. Nowadays organic-based devices
have emerged in the market, beginning to replace/complement application up to now
dominated by silicon technology. The motivations behind using organic active
materials come from their low-cost manufactured methods; compatibility with
flexible substrate and the possibility of tuned the electrical properties of the material
modifying its chemical structure, making them very versatile.
Inorganic semiconductors made from materials such as silicon, germanium,
gallium arsenide and gallium nitride have been widely used in the manufacture of
electronic devices.[4]However, they present certain processing properties like high
temperature manufacturing steps that are not convenient or compatible with a wide
range of modern applications. In contrast, the organic semiconductors are flexible,
lightweight and offers low cost processing based on their low sublimation point.
The performance of devices like organic light-emitting diodes (OLEDs), organic
field-effect transistors (OFETs), solar cells and sensors, is mainly determined by the
charge carrier mobility (µ). This property reflects the speed of the hole/electrons
under an electric field according to the relation:1
ν= µE (eq. 1)
Therefore, the advances in the organic electronic devices have been connected
to the continuous increase in the charge mobility in semiconducting materials.
1
GÓMEZ-­‐LOR, Berta et al. Organic semiconductors toward electronic devices / High mobility and easy proessability. J. Phys. Chem. Lett, p. 1428-­‐1436. 2012. 1 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM From the table 1, we observe that some specific organic materials have reached
mobility values surpassing the amorphous silicon. It is clear that organic materials
aren’t suitable for the fabrication of high-speed electronics, but these mobility values
are already acceptable for many low-frequency applications, as listed above.
Table 1. Mobility range for inorganic and inorganic materials2
!
Mobility (𝑐𝑚 𝑉𝑠)
Type of material
Poorly conducting organic semiconductors
≤10!!
Good quality organic materials
~10!!
Well-ordered organic materials
~1-10
Amorphous silicon
~1
Cristalline silicon
~10!
Excepcionally clean systems of inorganic semiconductor
10!
In general, the functional act of the all type of molecular devices could be
explained by charge transfer and energy transfer process. The phenomena could be
complex in the supramolecular system because the high number of molecular
components. In the present work we investigate the electrical properties of
phthalocyanines based devices, because these organic semiconductors have a
number of characteristic properties that contribute in a mayor way to extraordinary
versatility for future applications on gas sensor devices.
1.1.
Phthalocyanines, gas sensors.
Currently, one of the organic semiconductors most studying are the
phthalocyanine-based compounds. Their structural similarity with biological
molecules such as chlorophyll and hemoglobin has made this one of the most used
2
MAS-­‐TORRENT, Marta; ROVIRA, Concepció. Role of molecular order and solid-­‐state structure in organic field-­‐effect transistors. Chem. Rev., p. 4833-­‐4856. 2011. 2 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM materials. Between 1965 and 2001 more than 5,000 works related to their synthesis,
characterization and application were published.3
The metal-phthalocyanines show a number of special properties which account
for the great interest they have always aroused. They are easily crystallized and
sublimed, and show an exceptional thermal and chemical stability; there are no signs
of degradation up to several hundred degrees Celsius in a vacuum and they exhibit a
good resistance against strong acids or bases. Over 70 different phthalocyanine
complexes can be obtained by substituting the central hydrogen atoms by metal or
metalloid atoms.
Like organic semiconductor, the atoms in the Pcs molecules are bonded by
conjugated π-bonds (see Figure 1) and the molecules in the solid held together by
Van der Waals interactions. The π-conjugated system formed by sp2 hybrid orbitals
can have different bonding configuration according to the electron wave function
overlap of neighboring atoms, resulting in bonding and antibonding states like the
HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied
Molecular Orbital) respectively. On account of the weak interaction between the
molecules, although the HOMO and LUMO levels in organic semiconductors may
take shape of an energy bands, this bands are rather narrow compared with the
valence and conduction bands in the inorganic semiconductors which are much
larger because of the nature of the crystal lattice.
Because of the Van der Waals interactions are weaker than the covalent
interactions in the inorganic semiconductors, the transport models in organic
semiconductors, considers the degree of the order of the materials. For single
crystals, a band-like transport is proposed and for amorphous and polymers
materials, at least at room temperature, the charge mobility is determined by
hopping regime, which can be depicted as electron (or hole) transfer reaction in
which one electron (hole) is transferred from one molecule to the neighboring one.
3
SMITH, Kevin M.; GUILLARD, Roger.Phthalocyanines: Properties and Materials. San Diego: Elsevier Science, 2003. 3 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM This is highly different for inorganic semiconductors where the charge transport
occurs via delocalized states following a band transport regime in which, at room
temperature, the conductivity is limited by the availability of carries.
Figura 1. (a) The structure of the phthalocyanine. (b) Core structure of porfinrines;
chlorophyll, hemoglobin and phthalocyanine.
Since the electrical conductivity of metal-phthalocyanines varies substantially in
the presence of oxidizing or reducing gases, thin films of these materials have been
extensively studied as chemiresistive gas sensors for the detection of halogens such
as chlorine (Cl2), and nitrogen dioxide (NO2) as well as organic vapors.4 In this kind
of devices the variations in the electrical conductivity occurs due to the change in
charge carrier concentration upon adsorption of gas molecules. So, the thickness is
one of the main factors that influences the gas sensing properties, like sensitivity,
response and recovery times and reproducibility. For that, through the development
of this work, we investigate the electrical properties of ultra-thin (< 50nm)
phthalocyanines based devices. In order to improve the electrical transport we
investigate not only pure Pc and CuPc but also the bilayer Pc/CuPc.
4
SALCEDO, Walter J. et al. Electrical Transport Mechanisms in Mono-­‐Layer Phthalocyanine Device. Ecs Trans, [s. L.], p. 597-­‐606. 2008.
4 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM 1.2.
Deposition Method
The deposition by sublimation have been widely used to obtain films of materials
based on unsubstituted and substituted derivates of phthalocyanines because of their
extraordinary thermal stability. A typical vacuum chamber for deposition provides a
vapour pressure of between 10-4 and10-6 mbar. Under this range of pressures, the
density of CuPc thin films is ~1.64 g/cm-3 and usually form polycrystalline films of
the α type (Figure 3) onto a substrate maintained at room temperature.5
Figure 3. Schematic representation of the three main molecular stacking found for PcZn,
which is representative of metallophthalocyanines
Then, like the molecular ordering, crystallinity degree, grain size, film
morphology, or orientation of crystalline domains are critical to determine mobility
and have to be tuned by optimizing different parameters such as the deposition rate,
substrate temperature. Here, we defined specific parameters to realize the fabrication
of thin films of 40 nm of Pc and CuPc.
1.3.
Overview of operation of working equipment
5
SIMON, J.; GUILLAUD, G.; GERMAIN, J. P.. Metallophtalocyanines / gas sensors, resistors and field effect transistors. Coordination Chemistry Reviews,Cali, p. 1433-­‐1484. 20 mar. 1998. 5 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM • Thermal deposition system
The thermal deposition system used has an oil mechanical pump to make a prevacuum (~10-2 mbar), and another turbomolecular pump to obtain high vacuum
(~10-6 mbar). To get the vacuum inside the chamber, first one has to turn the pump
for establish a pre-vacuum until ~10 mbar. Then the high vacuum pump can be
turned on. Like show the figure 2, the material to be deposited is loaded into a boatsource that is heated by a tungsten resistance connected to a power supply. As the
material in the boat source heats, the molecules sublimate. Since the pressure in the
chamber is smaller than 10-4 mbar, the atoms travel across the chamber towards the
substrate. The vacuum chamber contain two boats to allow the deposition of
multiple layers without breaking vacuum, and a quartz crystal microbalance (QCM)
for monitoring the deposition rate and deposited thickness, according to the change
in frequency of a quartz crystal resonator.
Figure 2. Schematic diagram of an thermal vacuum sublimator
The deposition rate computation is based on thickness changes over time.
However, instrumental calibration is affected by three different parameters, material
6 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM density, material, Z-factor, and tooling factor. The sensor and the sample generally
cannot be in the same position with respect the material source. Consequently, the
thickness measured on the sensor does not always agree with the value found on the
sample and the match between these values is done by the tooling factor. For careful
work, the tooling factor has to be checked by measuring the amount of material
deposited on some samples after the process and comparing it to what the thickness
monitor has actually measured:6
𝐹! = 𝐹!
𝑇𝑚
(𝑒𝑞. 2)
𝑇𝑖
where Fm is the measured tooling factor, Fi is the indicated tooling factor, Tm is the
measured film thickness and Ti is the indicated film thickness.
• Electron Beam Evaporator
This is another evaporation technique used for the gold deposition. The figure 2
is an illustration of the principle of an electron-beam source, which consists of a
copper holder or crucible with a center cavity which contains the metal material. A
beam electrons is generated and bend by a magnet flux so that it strikes the center of
the charge cavity. In addition, the solid metal within the crucible is heated to its
melting point such that it presents a smooth and uniform surface where the electron
beam hit, thus ensuring that the deposition on the wafer is uniform. The record of
the thickness of the film and the deposition rate, too is done with an quartz crystal
microbalance.
6
TECHNISCHE FACULTT DER CHRISTIAN-­‐ALBRECHTS UNIVERSITAT (Org.). Evaporation Deposition.Disponível em: <http://www.tf.uni-­‐kiel.de/cma/m101_evaporation_methods.pdf>. Acesso em: 19 feb. 2013. 7 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM Figure 3. Schematic diagram of an electron-beam evaporator source
• Surface profiler: Dektak 150
To make the measurements of the surface profile, the system uses the
technique of contact profilometry. This method consists in the measuring the
mechanical movement of a cantilever as it shifts across the substrate. A diamond
needle connected to a tip with radius of ~10 µm serves as the electromagnetic
pickup. The cantilever force is adjustable from 1 to 15mg, and vertical
magnifications of a few thousand up to a million times are possible. The film
thickness is directly read out as the height of the resulting step-contour trace. The
leveling and measurement functions are computer-controlled. The vertical cantilever
movement is digitalized, and the data processed to magnify areas of interest and
yield best profile fits.
8 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM 2. Objetives
2.1.
General
In this work, we aim the fabrication of ultra-thin films based on metal-free
phthalocyanine and Cu-phthalocyanine, and obtain information about their electrical
transport properties.
2.1.
Specifics
• The fabrication of thin films of ~ 40 nm of Pc, CuPc and a bilayer of Pc/CuPc
of 14/16 nm are targeted.
• The I-V characteristics of the thin films are used to probe the electrical
properties of the films.
• The changes on the transport properties of thin films of CuPc are evaluated
when it is prepared on Pc/Glass.
9 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM 3. Metodology
The procedure in this work consists in the preparation of samples containing
devices with Au/Pcs/Glass layers, as shown by the in the figure 1, and subsequent IV characterization.
3.1.
Devices fabrication based on thin films of Pc, CuPc and Pc/CuPc with
~ 40nm.
For the preparation of the devices, we used glass substrates that were first
submitted to a cleaning process. Initially the glass substrate is placed on one Teflon
support into a beaker with detergent solution (Extran 5%) and sonicated for ten
minutes. Then, in the clean room, the glasses were cleaned with clear room sponge
on both faces and washed on DI water. Next the substrates were immersed in a
piranha solution (H2SO4-H2O2, 2:1) to remove the organic impurities at the surface,
and rinsed with DI water again.Finally, the plates were put into a becker with boiling
isopropyl alcohol to remove the water residues. The drying process was make using
dry nitrogen and a hot plate.
After the cleaning process, thin films of phthalocyanines (Pc, CuPc and
Pc/CuPc) were deposited on the glass substrates using the vacuum sublimation
process at room temperature, and under a pressure range between 5×10-6and 2×10-4
mbar. The material was sublimate from a source located at 150 mm from de
substrate. The material parameters were fixed and registered on the quartz crystal
thickness monitor (Edwards FTM5) and are show in the table 2. The deposition time
was of approximately three minutes and to define the patterns in sample was used
the shadow mask, shown in the figure 4a.
10 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM Table 2. Parameters programed and registered on the thickness monitor to obtain thin films of 40 ±
2 nm.
Material
Pc
CuPc
Pc/CuPc
Density
1.6
1.6
1.6
Z-Value
16.0
16.0
16.0
Tooling
4.0
0.6
4.0/0.6
Data/Rate (Å/s)
0.3-0.6
0.6-1.0
Not Registered*
Thickness (Å) QMC
220
400
74.5/253.3
Thickness (Å)
Surface Profiler
380 ± 20
380 ± 33
Not Registered*
*This parameters was not registered because of equipment limitations.
In the table 2, the thickness registered in the QMC differs from the desired
thickness (~ 40 nm). So, this parameter was tuned according to the thickness
measured in the surface profiler (Dektak® 150).
a) b) Figure 4. Kapton shadow maks to defined the patterns for the: a) Pcs films and b) used for
the Au contacts.
To realize the electrical characterization, Au contacts with 55 ± 3 nm was
deposited by electron beam method with a start pressure of 4×10-6and a final one of
2×10-4 mbar. In this case, the parameters for the deposition were: deposition rate of 4
Å/s, Au density 13.6 g/cm3, film thickness of 55 nm, and filament current 80 mA.
The figure 5 shows the steps to obtain the Pcs devices and the schematic structure of
the devices for the electrical characterization. The last shadow mask using for the
11 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM gold deposition (figure 4b), was carefully aligned with the patterns over Pcs layer,
since these contacts were critically useful for the suitable electrical characterization
of the thin films.
a) b) Figure 5. a) Schematic structure of the device and b) Patterns in the shadow mask for the
Au deposition and devices used for electrical characterization.
3.2.
I-V Characteristics of the thin films of Pcs.
The electrical characterization was realizedat room temperature. The voltage
bias was applied to the devices by a source-meter (Keithley 2636A) with interface
software (LabTracer™). We used a probe station (Alessi REL-4100A) connected to
the source-meter to do the measurements of the devices in sample (see Figure 2).
The measurements were realizeddone 3x by sweeping the voltage bias between 1
and 50 V. Finally, to calculate of current density and electric field applied to the
12 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM devices, the geometrical parameters, namely the injection area and the distance
between the Au contacts were measured by an optical microscope.
a) b) Figure 2. a) Station probe and the electrometer. b) Image of the positioning of the
microprobes above the devices in the sample.
13 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM 4. Results
4.1.
Fabrication of the thin films
To realize the deposition of films of Pc, the thickness monitor was programmed
with the parameters of density, Z-factor, and tooling that was showed in the
methodology.
Initially the tooling value calculated using the eq. 2. for CuPc was found 0.6
under the conditions I (see table 3). It was observed that modifications on the current
applied to the source-boat from ~45 A to 75 A resulted in a variation Tm/Ti of 5%,
and modifications in the deposition rate from 0.2 Å/s to 3.3 Å/s. The variation of the
deposition rate for one fixed distance has one important effect in the quantity of
molecules deposited on the substrate: at very low rates and great source-substrate
distance the difference between Tm and Ti was larger than for higher rates.
From the table 3, we observed that the deposition of CuPc under the conditions
I, results in low thickness dispersion (± 2). Because of limitation on the equipment,
we don’t have certainty about the maximum current applied to the CuPc, but the
material didn’t decompose for values above 75A to obtain deposition rates up to 3.3
Å/s.
The tooling for Pc films was first calibrated based on the tooling factor of the
CuPc and following eq. 2 with Fi= 0.6 and Fm= 2.2. In this first attempt, the
variation Tm/Ti was found to be 140%. As shown in table 4, it was not possible to
calibrate the equipment by adjusting the tooling in the QCM. In a second round, the
calibration is carry out by depositing Pc and calculating again the factor Tm/Ti.
Them this value is multiplied by the desired thickness(𝑇𝑚! ) and this will be the
thickness to be recorded ( 𝑇𝑖! ) on the monitor to close the shutter. The equation
below summarizes it:
14 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM 𝑇𝑚
𝑇𝑖! =
𝑇𝑖
!!
𝑇𝑚! (𝑒𝑞. 3)
Table 3. Conditions for the deposition of CuPc thin films.
Conditions
I
II
III
4.0
4.0
4.0
0.3-0.5
0.2
Variation
150
150
150
Current (mA)
42.5 - 45
42.5
>75
Pressure (10-6 mbar)
1.7 – 2.2
3
3
0
40
5
Tooling
Deposition Rate (Å/s)
Distance Source-Substrate (mm)
% Relative Error
With this alternative method, the variation percentage between 𝑇𝑚! /𝑇𝑖! is just
of 5 %, although the dispersion (± 7.2) in the thickness of the films was greater than
for CuPc films.
Table 4. Conditions for the deposition of CuPc thin films.
Conditions
I
II
III
IV
V
VI
Tooling
2.0
3.0
3.8
4.0
4.0
4.0
Deposition Rate (Å/s)
3.5
0.7
0.3
1.0
0.6-1
3.3
Distance Source-Substrate (mm)
150
150
150
150
150
235
Current (mA)
>75
>75
>75
>75
>75
>75
Pressure (10-6 mbar)
2
4
5
4
4
8.8
% Relative Error
140
27
20
70
5
12
To guarantee higher thickness uniformity on one or more substrates, rotating
holders are employed. The rotation moves the substrates from positions near the
source to locations far away from the source. For the Pc, this option was not
implemented and the change on the source-substrate distance was used instead.
15 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM During the modification of the distance source-substrate, we kept the parameters in
the QMC for the CuPc and Pc fixed, including the tooling.
During the process of calibration with the height of 235 mm was necessary
enhancing the deposition rate because the percentage variation Tm/Ti increased
47.5% for CuPc and 42.5% for Pc. In the process of calibration, to find the
appropriate parameters for the deposition of Pc films, we worked with current values
greater than 75A and deposition rates of 3.3 Å/s. Under these conditions, we got
better uniformity of the Pc films (± 3.3) although the % 𝑇𝑚! /𝑇𝑖! was increased
progressively from 12% until 22.5 %, and in some cases, the QMC registered values
for the rate and thickness but with no material was effectively deposited on the
susbtrate due to a decomposition of the Pc at high temperatures. To obtain large
deposition rates for the Pc and CuPc the boat should be operated with very high
temperatures. Insomuch, as in the CuPc the presences of the metallic atom make the
bond length to be lower than in the Pc, the thermal stability of both materials differ.
While we could work with high rates for the CuPc, it was found that it is not
possible to increase the deposition rate for Pc.
Currently we are using 175 mm source-substrate separation to achieve a
optimum deposition rate and uniformity.
For the deposition of the Au contacts, the configuration in the electron beam
was previously calibrated to obtain any desired thickness. The Au contact pattern on
top of the Pcs is also defined by a shadow mask. Figure 5a showed the fabrication
scheme to generate the patterns on the samples. The location of the patterns in the
mask for Au deposition does not fully coincide with the patterns in samples of Pc.
Therefore, some devices present alignment problems (Figure 5a) and this generates
variations in the current through the devices since one of the factors that controlled
the I-V characteristics is the transport of charge carriers in the area between the
electrodes.
The first deposition of Au was realized above one thin film of CuPc with a
deposition rate of 5 Å/s and thickness of 55 ± 2 nm. The distance between the Au
16 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM contacts is 24 µm and was measured by an optical microscope. We observed that the
Au penetrated under the shadow-mask generating a decrease in the distance between
the contacts of the 28 %. We tried rotating the sample during the contact preparation
but this resulted in devices with short circuit between the two electrodes due to poor
contact between masks and substrates (Figure 5b). To improve the contact patterning
and reduce the Au penetration, we use permanent magnets (see the appendage) to
hold the shadow-mask tight to the substrate and the result was a sample with devices
whose average distance between the electrodes of 24 µm (Figure 5c).
a)
b)
c)
d)
Figure 5. a) Aligment problems. b) Short circuit between the two electrodes due to poor
contact between masks and substrates. c) Good device. d) Dust after Au deposition.
The image shown in the figure 5d are a devices based on of Pc films where the
gold contacts were damaged at bias voltages above 20V due to Joule heating. Under
charge injection from the metal into the organic semiconductor, there is a transfer
length (TL) within which the current transfers from the metal to the semiconductor.
The transmission line model applied to electrical contacts in semiconductors allows
extracting the specific contact resistance of a contact with lateral current flow. In
addition, it shows that the potential distribution under the contact is such that the
17 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM voltage is highest near the contact edge and drops nearly exponentially with distance
bellow the metal contact. The transfer length is given by the equation:7
𝐿! =
𝜌!
𝑅!!!!" (𝑒𝑞. 4)
As higher the resistence of the material, lower the transfer distance. In this case,
the charge will accumulate in a small space close to the interface contactsemiconductor (Figure 6). In our case, when the potential is above of 20 V, the
movement of charge accumulates in the interface metal/Pc generating thermal
energy sufficient to melt the gold contact. This happens when gold thickness was
similar to the thickness of Pc films so that the electrode does not possess sufficient
thickness to resist the current passing by.
Figure 6. Horizontal or lateral current flow geometry, the current flows non-uniformly
through the contact.
4.2.
Electrical Characterization
Figure 7 show the current density as a function of the electric field for the
Au/Pc, CuPc or Pc-CuPc/Glass heterojunction. At room temperature and low
electric field, the hopping conduction is considered the main process of charge
7
SCHUBERT, F.. Specific contact resistence. Disponível em: <www.ecse.rpi.edu%2F~schubert%2FCourse-­‐
Teaching-­‐modules%2FA007-­‐Specific-­‐contact-­‐ resistance.pdf>. Acesso em: 25 feb. 2013. 18 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM transport. This means that the carriers can move from one molecule to another by
jumping over the potential barrier via an excited state.
0.1
Pc/CuPc
CuPc
Pc
2
J / (A/cm )
Hopping
0.01
0.001
Frenkel-Poole
emision
0.0001
4
1000
10
E / (V/cm)
Figure 7. Different transports regimes observed for the heterojunction Au/Pc, CuPc or PcCuPc/Glass. The solid line represents the transition electric field from the hopping
conduction to Frenkel-Poole emission.
The current increase because the height of the barriers drops with the
enhancement of the electric field (E). At high electric fields the transport mechanism
suffers a transition to a field-enhanced emission of thermally excited trapped
charges, also known as Frenkel-Poole emission. The change in the mechanism of
carrier transport from hopping to Frenkel-Poole is generated by the trapped charges
at the interfaces between metal-semiconductor as well as glass-semiconductor.
Figure7 displays an energy-band diagram of metal-semiconductor contacts where a
large density of surface states is present on the semiconductor. In this case the
barrier height (𝑞𝜙!" ) is determined by the property of the semiconductor surface,so
19 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM ill be one point where the barrier height will be independent of the field applied and
the carriers will be trapped in surface states. 8
Figure 7. Energy-band diagram of metal-semiconductor contact, with a large density of
surface states on the semiconductor surface.
The increasing of the electric field induces a lowering of the potential barrier
between localized states. However, this decrease is affected by the surface states that
pins the Fermi level. These states are place in band between the HOMO and LUMO
in the molecule.9 In this way, the charge injected can be trapped in depth surface
states until a high electric field enhances the conduction of the trapped charges
electrons into the conduction band. As shown in the graphic 8, the transition voltage
is correlated with the layer composition: 𝑉!!" < 𝑉!!"#$ < 𝑉!!"/!"#$
In the Pc films, no metal atoms are present; the only states present in the band
gap are those from the surface states. In this case, the hopping across shallow states
is limited to very low applied bias and the Frenkel-Poole emission is expected to
take place earlier than for the other metallic-Pcs. As shown in Figure 8, the barrier
lowering starts at 3.0 V and the complete transition takes place at 7.5 V. Although in
the CuPc films surface states also play a role, the hopping mechanism are extended
to higher bias (VT = 12.1 V) because of the hybrid level generated by the Cu in the
center of the molecule.
8
SZE, S.M. Physics of Semiconductor Devices: Carrier transport in insulating films. 2. ed.: Wiley-­‐interscience, John Wiley & Sons, Inc, 1981. 9
HEINZEL, T.. Mesoscopic electronics in solid state nanostructures: Surfaces, interfaces, and layered devices. 2. ed. S.l: Weinheim: Wiley-­‐vch, 2010. 20 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM -5.5
Hopping
-6
-6.5
V = 7.8 V
T
Log (I)
-7
V = 12.1 V
T
-7.5
-8
-8.5
V =3.0 V
T
CuPc
Pc/CuPc
-9
Pc
-9.5
0
0.4
0.8
1.2
1.6
Log (V)
-17
Frenkel-Poole emission
-17.5
V = 25.5 V
T
-18
Ln (I/V)
-18.5
V = 28.8 V
-19
T
-19.5
V = 7.5 V
-20
T
Pc/CuPc
-20.5
CuPc
Pc
-21
1
2
3
4
V^(1/2)
5
6
7
8
Figura 8. a) At low bias (V < ~12 V) hopping conduction between localized site and at b)
high bias the Frenkel Poole emission.3 According with the IV characteristics, the Pc layer
passivates the substrate surface and allow the transport across the top CuPc layer thin with
thickness bellow the depletion regime.
21 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM The depletion width for CuPc can be estimated by the Poisson equation:
𝑊=
2 ∈!
𝑘𝑇
𝑉!" − 𝑉 −
(𝑒𝑞. 5)
𝑞𝑁!
𝑞
Assuming the table values for the effective dielectric constant and a density of
diamantes between 1016 to 1018 cm-3, we estimate a depletion layer at the air-Pc and
glass-Pc of ~30 nm. Therefore, both pure Pc and CuPc films are close to the
depletion regime and for thicknesses bellow W no current can pass across the film.
The thickness of the CuPc in the bilayer was setto ~ 26 nm. This value is also lower
than𝑊!"#$ , but the curves in the figures shows that this devices has better properties
for the transport current than those formed by a single materials. Here we suggest
that the Pc layer deposited on the glass surface in the bilayer suppress the surface
states, leading a better transport of the carriers in the CuPc layer.
22 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM Conclusions
The uniformity in the thickness of metal-free Pc films is difficult since the material
needs high deposition rates (> 1.2 nm/s). Nevertheless, after adjusting the samplessource distance the layers were deposited and we successfully fabricate the ultra-thin
Pcs devices using vacuum sublimation.
According to the electrical characterization, at low applied bias all devices exhibit a
transport dominated by hopping, while above a transition voltage (VT) the charge
transport it explained by Frankel-Poole emission. There is a strong indication that
metal-free phtalocyanine passivates the glass substrate by suppressing the surface
states. Therefore, it was possible to reduce the CuPc layer thickness to a value
bellow the depletion limit (~30nm). This achievement is an important step towards
sensors based on thin organic layers.
23 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM References
BREDAS, J. L. et al. Charge transport in organic semiconductors. Chem. Rev, p.
926-952. 2007.
BREDAS, J. L.; ORTIZ, E.. Eletronic structure of metal-free phthaloyanine: A
valence effective Hamiltonin / Theoretical study. J. Chem. Phys, p. 1009-1015.
1988.
CHOY, W. C.. Organic Solar Cells: Overview of organic semiconductors:
Springerlinker, 2013. 2-3 p.
EISENMERGER-SITTNER,
Technology.
C.. Technology
of
Viena.
Thin
Films. Universtiy
Avaible
of
in:
<http://sxs.ifp.tuwien.ac.at/forschung/duenne_schichten/english/teaching.htm>.
Acesso em: 16 Feb. 2013.
GÓMEZ-LOR, Berta et al. Organic semiconductors toward electronic devices / High
mobility and easy proessability. J. Phys. Chem. Lett, p. 1428-1436. 2012.
GUPTA, D.. Organic Electronics II. Indian Institute of Technology. (Research
Symposium).
Avaible
in:
<http://www.iitk.ac.in/directions/directions_dec07/3jan~DEEPAK.pdf.>.
Acesso
em: 16 Feb. 2013.
HEINZEL, T.. Mesoscopic electronics in solid state nanostructures: Surfaces,
interfaces, and layered devices. 2. ed. S.l: Weinheim: Wiley-vch, 2010.
KOLTOVER,
I.. Organic
Electronics. Sigma
Aldrich.
Avaible
<http://www.sigmaaldrich.com/content/dam/sigma-aldrich/materialsscience/material-matters/material_matters_v2n3.pdf.>. Acesso em: 16 Feb. 2013.
24 in:
RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM MADHAVAN, N.. Small-molecule organic semiconductors. Illinois University .
Avaible
in:
<http://www.chemistry.illinois.edu/research/organic/seminar_extracts/2001_2002/s0
2_Madhavan.pdf>. Acesso em: 16 Feb. 2013.
MAS-TORRENT, Marta; ROVIRA, Concepció. Role of molecular order and solidstate structure in organic field-effect transistors. Chem. Rev., p. 4833-4856. 2011.
OHRING, M.. Materials Science of Thin Films:Characterization of thin films and
surfaces. 2. ed. San Diego: Academic Press, 2002. 261-264 p.
PADMA, N. et al. NO2 sensors with room temperature operation and long term
stability using copper phthalocyanine thin films. Sensors And Actuators, p. 246252. 2009.
RAZEGHI, Manijeh. Technology of Quantum Devices: Vacuum Evaporation.
New York: Springer Science, 2010. 2-3 p.
SALCEDO, Walter J. et al. Electrical Transport Mechanisms in Mono-Layer
Phthalocyanine Device. Ecs Trans, [s. L.], p. 597-606. 2008.
SCHEINER, S.; LIAO, M-s.. Electronic structure and bonding in metal
phthalocyanines, metal = Fe, Co, Ni, Cu, Zn, Mg. J. Chem. Phys, Cali, p. 97509791. 12 mar. 2001.
SCHUBERT,
F.
Specific
contact
resistence. Avaible
in:
<www.ecse.rpi.edu%2F~schubert%2FCourse-Teaching-modules%2FA007Specific-contact- resistance.pdf>. Acesso em: 25 feb. 2013.
SIMON, J.; GUILLAUD, G.; GERMAIN, J. P.. Metallophtalocyanines / gas
sensors,
resistors
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field
effect
Reviews,Cali, p. 1433-1484. 20 mar. 1998.
25 transistors. Coordination
Chemistry
RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM SMITH, Kevin M.; GUILLARD, Roger. Phthalocyanines: Properties and
Materials. San Diego: Elsevier Science, 2003. 9 p.
SZE, S.m.. Physics of Semiconductor Devices:Carrier transport in insulating films.
2. ed. S.l: Wiley-interscience, John Wiley & Sons, Inc, 1981.
TECHNISCHE FACULTT DER CHRISTIAN-ALBRECHTS UNIVERSITAT
(Org.). Evaporation
Deposition.
Avaible
in:
<http://www.tf.uni-
kiel.de/cma/m101_evaporation_methods.pdf>. Acesso em: 19 feb. 2013.
26 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM Appendage
A. Conditions for the operation of surface profiler
Scan Duration
• Length : 400.0 um
• Resolution: 0.044
• Duration: 30 secs
• Scan type: Standard Scan
• Stylus force: 1.00 mg
• Meas range: 6.5 um
• Profile: Hill & Valleys
B. Programation in the software for the electrical characterization. Lab Tracer
2.0.
Sweep
• Star voltage: 1
• Stop voltage: 50
• Number point: 100
• Sweep type: Logarithmic
• Compilance (A): 0.01
• Delays: 300
27 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM C. Equipment used
Thermal vacuum sublimator
Electron Beam Evaporator
28 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM Surface Profiler
Station Probe
29 RELATÓRIO FINAL DE BOLSISTA – 22º PROGRAMA BOLSAS DE VERÃO DO CNPEM D. Magnets position of the Au shadow mask
30