Semiconductor device manufacturing: process – Having organic semiconductive component
Reexamination Certificate
2002-01-24
2004-03-16
Fahmy, Wael M (Department: 2814)
Semiconductor device manufacturing: process
Having organic semiconductive component
C438S024000, C438S029000
Reexamination Certificate
active
06706551
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a particular type of a Thin Film Inorganic Light Emitting Diode.
BACKGROUND OF THE INVENTION
At present several thin film electroluminescent devices (ELDs) exist or are still in development. They are all characterized by one (or more) electroluminescent active layer(s) sandwiched between two electrodes. Optionally a dielectric layer may also occur. The substrate can be plastic or glass.
The thin film ELDs can be subdivided into the organic and the inorganic based ELDs. The organic based thin film ELDs
(1)
are further subdivided into Organic molecular or Oligomeric Light Emitting Diodes (OLEDs) and Polymer Light Emitting Diodes (PLEDs). The inorganic ELDs on the other hand can be further subdivided into the High Voltage Alternating Current (HV-AC) ELDs and the Low Voltage Direct Current (LV-DC) ELDs. Amongst the HV-AC ELDs, one can distinguish Thin Film ElectroLuminescent Devices (TFEL devices or TFELDs) and Powder ELDs (PEL Devices or PELDs). Amongst LV-DC ELDs one can distinguish Powder ELDs (DC-PEL Devices or DC-PELDs) and thin film DC-ELDs, hereinafter called Inorganic Light Emitting Diodes (ILEDs).
The basic construction of organic ELDs (PLED and OLED) comprises following layer arrangement: a transparent substrate (glass or flexible plastic), a transparent conductor, e.g. Indium Tin Oxide (ITO), a hole transporting layer, a luminescent layer, and a second electrode, e.g. a Ca, Mg/Ag or Al/Li electrode. For OLEDs the hole transporting layer and the luminescent layer are 10-50 nm thick and applied by vacuum deposition; for PLEDs the hole transporting layer is 40 nm thick and the luminescent layer is 100 nm and applied by spin coating. Between both electrodes a direct voltage of 5-10 V is applied.
For OLEDs the hole transporting layer and electroluminescent layer consist of low molecular organic compounds, including oligomers. E.g. N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (TPD) can be used as hole transporter, and as electroluminescent compounds the aluminium (III) 8-hydroxyquinoline complex (Alq
3
), polyaromatics (anthracene derivatives, perylene derivatives and stilbene derivatives) and polyhetero-aromatics (oxazoles, oxadiazoles, thiazoles etc.) can be used. The main advantages of these low molecular OLEDs include high fluorescent quantum yield, high purification and crystal growth capability and a wide selection of material design. Crystallisation of initially amorphous samples, production of exciplexes with other materials, and often high chemical reactivity create serious problems in their application to stable EL systems. Moreover, the layers are applied by using vacuum-vapour deposition techniques, which are often cumbersome and expensive. However, recently monochromic displays based on this principle are launched on the market by Pioneer
(3)
.
On the other hand the electroluminescent compounds that can be used in PLEDs are polymers like the non-conjugated polyvinylcarbazole derivatives (PVK) or the conjugated polymers like poly(p-phenylene vinylenes) (PPV), poly(3-alkylothiophene, poly(p-phenylene ethynylenes) etc. These high molecular weight materials allow for the easy preparation of thin films by casting, and show a high resistance to crystallization. The difficulties in purifiation procedures (and hence reproducability) and, mostly, a low fluorescent quantum yield, in addition to a complex structure and high sensitivity towards oxygen and moisture are severe drawbacks in their application to organic ELDs.
As mentioned before, today, two kinds of HV-AC inorganic based ELDs exist
(2)
: PELDs and TFELDs. PEL Devices are used mainly for backlighting and low information content displays and can be manufactured on flexible substrates. TFEL devices are used for high information content matrix displays and can only be produced on glass substrates, due to the high processing temperature needed. Their basic structure is essentially the same. A PELD consists of a transparent substrate, a transparent conductor (ITO), a luminescent layer of doped ZnS (50 &mgr;m), an insulator/reflector layer (50 &mgr;m), and a top electrode, e.g. Ag-paste. Between both electrodes an alternating voltage of 110 V and 400 Hz is applied. A TFEL device consists of a transparent substrate, a transparent conductor, and a luminescent phosphor layer (0.6 &mgr;m) between two moisture protective insulator layers (0.6 &mgr;m), and a top electrode, e.g. evaporated metal. Between both electrodes an alternating voltage of 200 V and 400 Hz is applied.
For both devices, doped ZnS is used as the active luminescent layer. The doping centers (e.g., Mn
2+
, Cu
+
) can be excited by the impact of injected electrons by means of a high electric field or by transfer of recombination energy towards an electroluminescent centre. These excited centers relax to the ground state by a luminescence process. Several mechanisms for the electroluminescence process have been proposed
(4)
. The insulating layer for the PELD (screen printed BaTiO
3
−powder+binder) fulfils three functions: (1) it prevents catastrophic breakdowns, (2) due to the high dielectric constant, the electric field will predominantly be focussed over the ZnS layer and (3) it will serve as diffuse light reflector, which increases the light output. For the TFEL device the vacuum deposited Al
2
O
3
prevents also catastrophic breakdowns and it functions as tunnelling barrier for the electron injection. The structures are completed by two electrodes, one of which should be transparent.
For the PEL device, the ZnS layer comprises ZnS powder doped with copper ions and possibly other elements like chlorine and manganese mixed in an organic binder with a high dielectric constant like polyvinylidene fluoride or some kind of cyanoresin. The powders are prepared by high temperature sintering processes (>700° C.) in order to allow diffusion of the dopants into the ZnS crystal matrix and integration of Cu
x
S-rich needles in- and outside the ZnS latice after cooling. For the HV-AC PELDs, the Cu
x
S-needles at the surface are chemically removed by etching with cyanide ions. This to ensure electrical isolating particles. After that the particles are provided with a moisture protective layer like Al
2
O
3
or NC (carbon-nitride) in order to increase the lifetime of the phosphor. The particle size of these powders varies between 2-20 &mgr;m. The dispersion of this powders can be used in a screen printing process in order to apply them in an electroluminescent layer with a thickness of 50-100 &mgr;m.
Upon applying an electric field on the phosphor particles, charges (e.g. electrons) are generated inside the particles at stacking faults in the ZnS latice and/or in the Cu
x
S needles. These charges can move according to the direction of the applied field (AC). By this way recombination processes can occur whereby the recombination energy can be transferred to an electroluminescent centre (e.g. Cu
+
or Mn
2+
). Also direct impact excitation of the luminescence centre can occur if the kinetic energy of the electron is high enough (ca 4-5 eV). These processes can give rise to electroluminescence.
Electroluminescence occurs normally at field strengths in the order of 1-2 MV/cm. For a layer thickness of 100 &mgr;m, applying 110 V results in a mean value of the field strength of 50 kV/cm. The Cu
x
S needles at the interior of the ZnS-particles increase locally the electric field strength by a factor of 50 to 100 resulting in field strength values of up to 1 MV/cm, thereby making electroluminescence possible. The thick layer causes a slow luminance-voltage response, making PEL devices unsuitable for display applications.
General drawbacks of these PELDs are price (large amounts of products needed (about 150 g phosphor per square meter), the expensive BaTiO
3
layer, the high driving voltages (around 110 V) and their inherent instability. The latter is caused by the slow decrease in number of crystal stacking faults and Cu
X
S needles by applying an electric field (recrystal
Agfa-Gevaert
Fahmy Wael M
Guy Joseph T.
Le Thao X.
Nexsen Pruet Jacobs & Pollard LLC
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