21-09-2012, 04:00 PM
Graphene Thin Film Transistors
[attachment=34282]
TFT Background
A transistor is the most important element in electronics. It responsible for performing
crucial tasks in both digital and analog circuits such as signal processing and logic functions. In
LCDs, it serves a critical role of controlling individual pixels, as discussed earlier. Most
conventional integrated circuits (ICs) are produced from single crystal silicon transistors.
However, applications that demand substrates other than standard silicon often require
deposition of all the materials necessary to form transistors, usually in form of thin films. An
LCD is a perfect example of this, where a glass substrate is used to allow light transmission. A
thin film transistor (TFT) is then “grown” on top by sequentially depositing patterned thin films.
Currently this field is dominated by amorphous silicon (a-Si) TFTs; consequently, this is the
technology against which all novel approaches, including printed transistors, are compared.
TFT Modeling and Parameter Extraction
In order to properly compare different TFT technologies, it is important to have a
meaningful metrics of evaluation. Generally, device characteristics such as mobility, drive
current, on/off ratio, threshold voltage (VT), and subthreshold slope (SS) are used for this
purpose. Thin film transistors often operate in accumulation mode due to a large number of
trap sites associated with impurities and defects in the semiconductor. These imperfections in
the active layer cause additional energy states, in particular in the band gap of the material,
which act like traps. Consequently, it is often hard or impossible to invert the semiconductor,
which limits the operation range to accumulation. However, overall operation is quite similar to
ideal transistors, where cut-off, linear, and saturation regimes are clearly present. This enables
simple modeling of TFTs with basic square law equations and extraction of operational metrics,
such as mobility, threshold voltage, and subthreshold slope.
Active Materials
The most actively researched aspect of printed TFTs has been the active material. As
previously mentioned, there are multiple material systems that have shown promise as
candidates for solution-processed semiconductors.
Organic SCs
Organic semiconductors were some of the earliest materials widely investigated for
printed applications, and are still heavily researched. Original work was done on polythiophene[
21], and a good portion of the exploration since dealt with thiophene[22] or
pentacene[23] derivatives. Figure 2.3 shows some of the more commonly used semiconductors
for organic thin film transistors (OTFT). The basic building block of an organic semiconductor is
a conjugated system with sp2 hybridized carbon-carbon bonds. This bond scheme allows delocalization of electrons which can effectively become (partially) free carriers[24]. Each
molecule or unit of a polymer acts as a site with associated delocalized electrons. An organic
film can then be represented as an assembly of such sites and individual delocalized carriers can
hop from one site to another, thereby resulting in electrical conductivity.
Graphene
History
Graphene has become one of the most researched materials since its initial recent
demonstration in 2004 by Geim and Novoselov[32]. Graphene is a truly two-dimensional form
of carbon where individual atoms are arranged in a hexagonal, “honeycomb” pattern (Figure
2.4). When stacked together these sheets form graphite. The theoretical concept of a 2-D
material has been around for over 60 years[33]; however, it was widely assumed that such
structure could not exist physically due to thermodynamic instability. It was believed that
thermal energy at any appreciable temperature would induce enough fluctuation of the lattice
to preclude sustainability of the film and cause it to segregate into islands or decompose[34]. It
was not until 2004 that the first demonstration of graphene was achieved with a rather
unconventional method. The method involved separating layers from bulk graphite by
physically peeling them away with adhesive tape and transferring onto a silicon dioxide (SiO2)
substrate[32,35]. This has since been referred to as the “scotch tape method”, or more formally
as micro-mechanical cleavage. The great advantage of this particular technique as compared to
previous similar attempt is the ability to distinguish the thickness of a resulting film based on
the color it forms when deposited on SiO2. Utilizing interference effects, the color of the
graphene film can be carefully matched to the thickness, down to single layer, by using an
optical microscope. Thus, films could be rapidly examined without the need of vacuum or
electron microscopy and areas of interest (i.e. few layer graphene) could be identified for
further testing. With significant fine-tuning and careful implementation, the “scotch tape”
method can provide single layer films tens of micrometers in size. While sufficient for materials
research and preliminary investigations, this method is not usable for real scale manufacturing.
Other deposition methods will be discussed later in this chapter.
[attachment=34282]
TFT Background
A transistor is the most important element in electronics. It responsible for performing
crucial tasks in both digital and analog circuits such as signal processing and logic functions. In
LCDs, it serves a critical role of controlling individual pixels, as discussed earlier. Most
conventional integrated circuits (ICs) are produced from single crystal silicon transistors.
However, applications that demand substrates other than standard silicon often require
deposition of all the materials necessary to form transistors, usually in form of thin films. An
LCD is a perfect example of this, where a glass substrate is used to allow light transmission. A
thin film transistor (TFT) is then “grown” on top by sequentially depositing patterned thin films.
Currently this field is dominated by amorphous silicon (a-Si) TFTs; consequently, this is the
technology against which all novel approaches, including printed transistors, are compared.
TFT Modeling and Parameter Extraction
In order to properly compare different TFT technologies, it is important to have a
meaningful metrics of evaluation. Generally, device characteristics such as mobility, drive
current, on/off ratio, threshold voltage (VT), and subthreshold slope (SS) are used for this
purpose. Thin film transistors often operate in accumulation mode due to a large number of
trap sites associated with impurities and defects in the semiconductor. These imperfections in
the active layer cause additional energy states, in particular in the band gap of the material,
which act like traps. Consequently, it is often hard or impossible to invert the semiconductor,
which limits the operation range to accumulation. However, overall operation is quite similar to
ideal transistors, where cut-off, linear, and saturation regimes are clearly present. This enables
simple modeling of TFTs with basic square law equations and extraction of operational metrics,
such as mobility, threshold voltage, and subthreshold slope.
Active Materials
The most actively researched aspect of printed TFTs has been the active material. As
previously mentioned, there are multiple material systems that have shown promise as
candidates for solution-processed semiconductors.
Organic SCs
Organic semiconductors were some of the earliest materials widely investigated for
printed applications, and are still heavily researched. Original work was done on polythiophene[
21], and a good portion of the exploration since dealt with thiophene[22] or
pentacene[23] derivatives. Figure 2.3 shows some of the more commonly used semiconductors
for organic thin film transistors (OTFT). The basic building block of an organic semiconductor is
a conjugated system with sp2 hybridized carbon-carbon bonds. This bond scheme allows delocalization of electrons which can effectively become (partially) free carriers[24]. Each
molecule or unit of a polymer acts as a site with associated delocalized electrons. An organic
film can then be represented as an assembly of such sites and individual delocalized carriers can
hop from one site to another, thereby resulting in electrical conductivity.
Graphene
History
Graphene has become one of the most researched materials since its initial recent
demonstration in 2004 by Geim and Novoselov[32]. Graphene is a truly two-dimensional form
of carbon where individual atoms are arranged in a hexagonal, “honeycomb” pattern (Figure
2.4). When stacked together these sheets form graphite. The theoretical concept of a 2-D
material has been around for over 60 years[33]; however, it was widely assumed that such
structure could not exist physically due to thermodynamic instability. It was believed that
thermal energy at any appreciable temperature would induce enough fluctuation of the lattice
to preclude sustainability of the film and cause it to segregate into islands or decompose[34]. It
was not until 2004 that the first demonstration of graphene was achieved with a rather
unconventional method. The method involved separating layers from bulk graphite by
physically peeling them away with adhesive tape and transferring onto a silicon dioxide (SiO2)
substrate[32,35]. This has since been referred to as the “scotch tape method”, or more formally
as micro-mechanical cleavage. The great advantage of this particular technique as compared to
previous similar attempt is the ability to distinguish the thickness of a resulting film based on
the color it forms when deposited on SiO2. Utilizing interference effects, the color of the
graphene film can be carefully matched to the thickness, down to single layer, by using an
optical microscope. Thus, films could be rapidly examined without the need of vacuum or
electron microscopy and areas of interest (i.e. few layer graphene) could be identified for
further testing. With significant fine-tuning and careful implementation, the “scotch tape”
method can provide single layer films tens of micrometers in size. While sufficient for materials
research and preliminary investigations, this method is not usable for real scale manufacturing.
Other deposition methods will be discussed later in this chapter.