GB2155689A - Electroluminescent devices - Google Patents

Abstract

An electroluminescent device has the structure first electrode-insulator-semiconductor-second electrode. The insulator is formed of 2 to 11 (preferably 5 to 9) monolayers of phthalocyanine e.g. Cooper phthalocyanine formed by the Langmuir-Blodgett deposition method. The semiconductor material may be a single material forming the electroluminescent layer or an electroluminescent layer on a supporting semiconductor substrate. The electroluminescent material may be ZnSe, ZnS or alloys of these two, GaP, CdS, GaN, SiC.

Description

SPECIFICATION Electroluminescent devices The invention concerns electroluminescent (E.L.) devices which emit light when a unidirectional electric current is applied to a luminescent material.

The introduction of minority carriers into the luminescent region of a semiconductor is a necessary prerequisite for radiative recombination in most electroluminescent devices. In the majority of cases this is achieved using a p-n homojunction where the minority carrier injection ratio y is inherently very high. However, an alternative method is needed for semiconductors which do not conveniently exhibit both p- and n-type conductivity. One such method uses a metal-semiconductor (Schottky barrier) structure. In such a device y is known to be small, typically of the order of 10-4.

However this can be substantially increased by the incorporation of a thin insulator between the semiconductor and the metal electrode. Such a thin insulator may be deposited by the Langmuir-Blodgett (L.B.) technique.

Langmuir-Blodgett films are prepared by depositing a small quantity of a solution of a suitable organic material onto a liquid surface and waiting for the solvent to evaporate; the floating molecules are then compressed until a quasi-solid one molecule thick is formed. A suitably prepared substrate is dipped and raised through the surface of the sub-phase (usually highly purified water). The film thickness deposited depends on the number of monolayers deposited and molecular size of the material used. Pick up may be on each traversal of the liquid surface or during alternate dips.

Construction of Electroluminescent devices and Langmuir-Blodgett techniques are described in Sensors and Actuators, 4 (1983) 131-145; Thin Solid Films, 99 (1983) 283-290; together with associated references.

Unfortunately existing Langmuir Blodgett films are not suitable for commercially viable Electroluminescent Devices. For example the electrical breakdown strength or lifetime is poor. In one test using 11 monolayers of CdSt on GaP the relative electroluminescent intensity dropped below a quarter of its original value in under 1 6 hours and eventually dropped to zero.

The present invention overcomes the lifetime problems encountered with previous devices.

According to this invention an electroluminescence device comprises the structure first electrode-insulator-semiconductor-second electrode, characterised by an insulator formed of 2 to 11 monolayers of porphyria or phthalocyanine (about 1.6 to 8.8 nanometers total thickness).

Preferably the insulator is formed of between 5 and 9 mono-layers of phthalocyanine (about 4 to 7.2 nm total thick).

The semiconductor may be a single material or a layer of one semiconductor, forming the electroluminescent material, supported on another semiconductor.

The electroluminescent material may be GaP, CdS, GaN, SiC, ZnSe, ZnS or alloys of these two i.e. ZnS,Se1(0 < x < 1) e.g.

x = 0.06. The ZnSe may be supported on GaAs.

The electroluminescent material may be crystalline or polycrystalline and may be formed by metal organic vapour growth, molecular beam epitaxial growth, evaporation or sputtering, and supported on a semiconductor or an electrode bearing glass e.g.

NESA glass. The second electrode is preferably matched (work functions) to the electroluminescent material.

The insulator material phthalocyanine is a known material described for example in "The Chemistry of Synthetic Dyes", edited by K.

Venkataraman, published by Academic Press Inc. in Volume II pages 1118-1142 and Volume V pages 241-282. Phthalocyanine compounds which may be used as the insulator include the parent metal-free compound, the various metal complexes and the nuclear substituted derivates of such compounds with or without axial ligands co-ordinated to the metal atom. The metal atom may be Cu, Ni, Co, Fe, Mn, Cr, V, Pd, Pt, Zn or a lanthanide metal.

One method of depositing Langmuir-Blodgett layers of phthalocyanine is described in European Patent Application 0,076,060 A.

The invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a sectional view of a GaP light emitting diode; Figure 2 is a sectional view of a ZnSe light emitting diode; Figure 3 is a graph of D.C power conversion efficiency against thickness of insulator for the diode of Fig. 1; Figure 4 is a graph of relative electroluminescent intensity against time for the diode of Fig. 1.

The diode of Fig. 1 comprises a serial order a top gold electrode 1 1 5 u thick, a Langmuir Blodgett layer 2, 5.6 nm thick of phthalocyanine, an electroluminescent layer 3 of n-type GaP 10 um thick, an n + GaP substrate 4 2mm thick, and an In bottom electrode 5.

Example 1 The GaP substrate 4 was S doped to give a carrier concentration of about 1 8 cm - 3. An epi-layer of GaP 40 um thick was grown on the substrate and doped with S to give a carrier concentration in the range 10'5 to 10'7cm-3. Additionally the top 10 um was doped with N to a carrier concentration of 1018 to 1019 cm-3. This top 10 um formed the electroluminescent layer 3.

The insulating layer 2 of copper phthalocyanine [Cu Pc tris (CH2NH3C3H7-iso)] was deposited on the electroluminescent layer by Langmuir-Blodgett deposition. Details of Langmuir-Blodgett deposition are given in Proc. of the 2nd Internal Conference on Insulating Films on Semiconductors, Erlangen, (Springer Verlag 1981) p 56. In essence the Langmuir Blodgett layers were grown as follows:- a container held a liquid sub phase, e.g. water which supported a monolayer of phthalocyanine (Pc). This monolayer was contained within a constant perimeter plastic coated fibre barrier. A motor varied the area within the barrier under the control of a microbalance which measured surface pressure.

The subphase was high purity water with a pH of 5.4 (* 0.2) maintained at a temperature of 18( + 2)"C. Careful cleaning of the water surface was necessary, for example by depositing an amount of phthalocyanine material on the surface and then removing it by a vacuum nozzle.

The phthalocyanine material was dissolved in Arister grade chloroform (concentration about 1 mg cm) and added to the surface of the water one drop at a time. The solvent was allowed to evaporate before the phthalocyanine layer was compressed to a uniform monolayer thickness. Deposition then commenced.

The GaP electroluminescent layer 3 was surface cleaned as follows:- dipping in ferricyanide, or BrJCH3OH followed by dipping in HF.

Deposition of phthalocyanine was effected as follows: the barrier was opened and the cleaned substrate dipped into the water at a typical rate of 1-32 cm/min; no phthalocyanine was deposited on dipping. The barrier was adjusted to give the required monolayer of phthalocyanine on the surface of the water.

The substrate and electroluminescent layer was withdrawn through the monolayer of phthalocyanine at a controlled rate e.g.1-2 mm/min. On withdrawing, a monolayer of phthalocyanine was formed on the electroluminescent surface. Such deposition is known as Z-deposition; other forms of phthalocyanine may be deposited both on dipping and withdrawing. This dipping and withdrawing was repeated until the required number of monolayer had been deposited. As seen in Fig. 3 maximum D.C. electroluminescent efficiency was obtained for about 7 monolayers of phthalocyanine, i.e. about 5.6 nm thick. After deposition of the phthalocyanine the layer 2 was allowed to dry, e.g. by holding it under a low pressure of dry nitrogen for about 2 days.

The top electrode 1 was then formed by evaporating Au onto the Langmuir-Blodgett layer 2 under pressure of less than 10-8 torr.

Deposition rate was about 0.5 nm/min in stages of about 1 nm at a time until a layer about 1 5 nm had been formed.

Fig. 4 shows the variation of relative electroluminescent intensity against time for a device having 10 mono-layers of phthalocyanine. Electric current was about 5A/cm2. After an initial rise the intensity falls with time and settles to a steady value lo. This is in contrast to devices using a fatty acid, e.g. co-tricose- noic acid, type of Langmuir-Blodgett layer whose electroluminescent intensity gradually reduces to zero.

Fig. 2 shows an alternative diode. It comprises, in serial order, an Au top electrode 1 a Langmuir-Blodgett layer 2, an ZnSe electroluminescent layer 6, a GaAs substrate 7, and an 8 In/Sn bottom electrode.

Example 2 The GaAs substrate 7 was insulating or heavily doped material with an epi-layer 6 of n-type ZnSe grown by organometallic chemical vapour deposition to a thickness of 3 um.

Annealing at 275"C for 20 minutes was used on some samples. This deposition technique is described by P J Wright and B Cockayne in J Crystal Growth 59 (1982) 148. Carrier concentration of the ZnSe was in the range 1 018 to 1018 cm-3 A preformed ln/5% Sn alloy contact was made to the GaAs and annealed in an inert atmosphere for 10 minutes at 275"C to form the bottom electrode 8.

Prior to depositing the Langmuir-Blodgett layer 2 the ZnSe 6 was cleaned as follows: reflux in isopropyl alcohol vapour for several hours and then polish, at room temperature, using a mixture of bromine and methanol (0.5% bromine by volume) for 1 minute; rinse in carbon disulphide.

The Langmuir-Blodgett layers were deposited as in Example 1 to the required thickness 2. Gold was then evaporated, as before, for the top electrode 1.

When stimulated by a voltage about 290 volts and a current density of about 5 A/cm2 light was emitted, peaking at 469 nm wavelength. Different samples of device emitted light at much lower voltage. Increased light output may be obtained by using higher current. For example up to 30A/cm2 and more may be used. This contrast with e.g. CdSt Langmuir-Blodgett layers which are incapable of handling such large current without failure.

Claims (1)

1. An electroluminescent device comprising the structure first electrode-insulator-semiconductor-second electrode, the insulator being formed of 2 to 11 monolayers of phthalocyanine, the arrangement being such that light is emitted from the semiconductor when an electric current flows between the two electrodes.

2. The device of claim 1 wherein the insulator is formed of 5 to 9 monolayers of phthalocyanine.

3. The device of claim 1 wherein the insulator is formed of a suitably substituted copper phthalocyanine derivative.

4. The device of any one of claims 1 to 4 wherein the semiconductor is a single material forming the electroluminescent layer.

5. The device of any one of claims 1 to 3 wherein the semiconductor is a layer of an electroluminescent material formed on a semiconductor substrate.

6. The device of any one of claims 1 to 5 wherein the electroluminescent material is GaP, ZnSe, ZnS, alloys of ZnSe and ZnS, CdS, GaN, SiC.

7. The device of claim 4 or 5 wherein the semiconductor material is single crystalline.

8. The device of claim 4 or 5 wherein the semiconductor material is polycrystalline.

8. The device of claim 8 wherein the semiconductor is supported on a glass substrate.

10. The device of claim 1 constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.