ACTFEL - Alternating Current Thin Film Electroluminescence
Electroluminescence is the
phenomenon in which electrical energy is converted to luminous energy
without thermal energy generation. Electron interaction with
luminescent atoms results in the excitation of the luminescent atom,
which subsequently relaxes emitting a photon of light. The thin film
structure required to achieve electroluminescence is shown here. 
The
structure simply consists of a bottom electrode layer of aluminum which
is reflective to light at about 200nm thick with the phosphor layer of
Zinc Sulfide Erbium (ZnS:Er) containing the luminescent atoms on top at
about 600nm followed by an insulating transparent layer consisting of
aluminum titanium oxide (ATO) then a conducting transparent layer of
indium tin oxide (ITO) all below a glass substrate through which the
light is emitted. ZnS:Er has strong emissions in the near infrared at
1550nm and in the visible wavelength of 550nm green light.
Physics
Electrons enter into the device through the aluminum or ITO
conducting layers depending upon the polarity of the alternating
current. 
The following processes are portrayed visually in the figure. 1)
Electrons that have sufficient energy due to the high field of the
alternating current tunnel through the insulating layer (ATO) and are
injected into the phosphor layer (ZnS:Er). 2) Once electrons are in the phosphor layer they experience an electric field causing there acceleration through the device. 3)
High energy electrons impact excite luminescent centers causing
electrons in the luminescent centers to excite to higher levels which
is followed by a relaxation event which results in the creation of a
photon of equivalent energy to the difference in electron energy
states. 4) The impacting electron continues its path through the device where it is trapped at the other end. 5)
Upon the switching of polarity from the imposed alternating current
this process occurs in the reverse direction through the device. 6)
Photons created in the phosphor layer must pass through the device and
all the layers of different material therein in order to escape to air
and be seen by the eye. The process of light transversing from one
media to another is called optical outcoupling.
Total Internal Reflection
A major limitation to the efficiency of this process is due to the
phenomena of total internal reflection. When light crosses from one
medium to another of differing refractive index it experiences
refraction according to Snell’s law: n1sinθ1 = n2sinθ2.
Light going from air at a refractive index of 1 to water at a
refractive index of 1.5 bends away from the surface of the water. 
If we reverse this and have light going from water to air light
impinging the interface at an angle shallower then 41.8 degrees or the
critical angle will be totally reflected. The refractive index of a
material is the difference between the speed of light traveling in a
vacuum and the speed of light traveling through the material. This
phenomenon occurs in the thin layer stack of materials in an ACTFEL
device. The figure shows some light paths in green for reflected and
transmitted light. Theoretically only 10 percent of all the light
produced escapes the device. Most of the light is totally internally
reflected and is guided out the side of the layer that the light is
reflected in. These are called guided modes of internal reflection and
are in lateral directions. In order to increase light modes or paths
that are normal to the surface and actually escape the device and
become useful the internal reflection guided modes must be somehow
affected in order to transfer to emitted modes. This is accomplished
through a photonic crystal structure.
Photonic Crystals
Photonic crystals are used to control the propagation of light. A
periodic structure of alternating dielectric (or refractive index)
media causes the electric and magnetic field energy of electromagnetic
radiation or light to be confined in either the high or low refractive
index material. This forced separation of energies causes light
propagation to be prohibited in certain directions. A 2 dimensional
alternating structure of high and low refractive index material can be
constructed to prohibit light from propagating in any direction in the
plane of the structure. 
The structure shown is such a structure and with tuning the dimensions
of the spacing between columns and the diameter of the columns specific
wavelengths of light can be effected. The rule of thumb is the larger
the spacing the larger the wavelengths of light that are effected and
vice versa. Extensive information regarding the physics of photonic
crystals can be found through a tutorial by MIT’s Steven G. Johnson at ab-initio.mit.edu/photons/tutorial.
Also an excellent book covering this topic is J. D. Joannopoulos, R. D.
Meade, and J. N. Winn, Photonic Crystals: Molding the Flow of Light
(Princeton Univ. Press, 1995), with a greatly expanded second edition
currently in press, for availability in late 2007 or early 2008.
Photonic Crystals in an ACTFEL Device
The basic idea is by putting a photonic crystal structure into the
ZnS:Er layer of an ACTFEL device light will be prohibited from lateral
propagation forcing that energy to be transferred to light modes that
emit from the device as pictorially shown. 
For my research the goal is to increase the near infrared 1550nm
emission from the ZnS:Er, therefore the photonic crystal structure
necessary will require a column spacing on the order of 660nm with a
diameter of 264nm in order to only effect 1550nm radiation taking into
account the refractive index of the ZnS:Er layer is about 2.3 and the
refractive index of the silicon oxide used to make the columns is about
1.44. It is important to note that this structure should have no effect
to the green 550nm light that is also emitted from ZnS:Er.
Fabrication
This design flow chart describes the basic fabrication process. E-beam
lithography is used to achieve nanometer scale features. An e-beam is
used to expose in specific locations a photoresist such as
polymethylmethacrylate (PMMA), once exposed the PMMA is broken apart by
the high energy electron beam and is ready for development. A chemical
treatment is used to dissolve the exposed PMMA leaving behind the PMMA
that has not been exposed. Spin coating is the simple process of
dropping a liquid onto a sample and spinning the sample at high rpm
typically around 3000 rpm for PMMA to create a uniform layer. ZnS:Er is
deposited onto the substrate by a physical vapor deposition process. In
a vacuum chamber at 10-6 Torr an argon gas is introduced
into a high energy radio frequency field which ionizes the argon and
accelerates it at a target consisting of a bulk of the material desired
to be deposited. The argon ions cause the target material atoms to be
ejected and once ejected they hit the substrate and are deposited
creating a uniform layer.
Measurement and Results
Applying an alternating electric current to the ACTFEL device 
causes the emission of electromagnetic radiation. Quantifying these
emissions is accomplished by using an optical bench which is set up to
collect the light in a parabolic mirror aimed at a monochromator which
splits the light into its component wavelength which is then focused
into a detector. This graph shows the data collected from an ACTFEL
device with and without the photonic crystal structure. Clearly it is
shown that the visible green light emissions at 550nm are equivalent
for the two devices while the emission of near infrared light at 1550nm
is much greater for the photonic crystal structure device. Clearly the
photonic crystal structure increases the near infrared emission.