Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2002-10-11
2004-05-04
Pert, Evan (Department: 2829)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S079000, C257S089000, C257S094000, C257S102000, C438S035000, C438S046000, C438S478000
Reexamination Certificate
active
06730943
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a thin, semi conducting layer of GaInN and its preparation method.
The invention also deals with a light emitting diode (LED), and particularly a light emitting diode emitting white light, comprising such a thin layer in its active zone and finally a lighting device comprising such a diode.
BACKGROUND OF THE INVENTION
The technical field of the invention may be defined as being the generation of white light. The white light satisfies the criteria set down by the IEC in 1964.
A large amount of research is now being carried out on the generation of white light in order to develop devices particularly with low cost, little energy consumption, long life and good energy efficiency.
White light has been generated for many years essentially using incandescent lamps which have a very low efficiency and short life since the efficiency is close to 5% and the life is about 1000 hours. Fluorescent tubes have a better efficiency and a longer life, with an efficiency of about 25% and a life of about 1000 hours for fluorescent lamps, but they have a number of disadvantages; they are vacuum tubes and are difficult and expensive to manufacture, and also contain several milligrams of highly toxic mercury, and therefore their destruction at the end of their life causes a serious environmental problem.
Furthermore, fluorescent tubes produce light that is not pleasant, and this often limits their use.
In terms of efficiency, the best existing industrial lamps are low pressure sodium lamps which have an efficiency of the order of 35%. In the same way as for fluorescent tubes, these lamps are not pleasant, or may even be unacceptable for standard lighting, due to their colour. Therefore, research was carried out on other devices that are safe, reliable, robust and have a long life that can produce white light at low cost with an energy efficiency greater than or equal to the energy efficiency of the devices mentioned above.
For example, these and other sources of white light, including phosphors, light emitting polymers and semiconductors, have been envisaged.
Light emitting polymers such as PPV are very inexpensive and the technology for their use that consists of simply inserting the polymer between semi-transparent electrodes, is very simple. The entire range of visible colors can be obtained and white is emitted either by a combination of colors or by the use of a single compound with a fairly wide spectrum. However, these compounds are not used at the moment except to illuminate liquid crystal screens with orange light. The short life of green and blue emitters of this type makes it impossible to use light emitting polymers to generate white light.
The generation of white light by semiconductors is based essentially on nitrides, and particularly nitrides of group m elements which alone emit green or blue with very high efficiency and long life. The most frequently used nitride type compound is GaInN which emits blue to red light.
Thus a thin layer of nitrides, for example GaInN, inserted in a material such as Ga(Al)N and for which the prohibited band fixes the emission wave length and therefore the colour, forms the elementary building block of the active zone of extra bright light emitting diodes (LEDs).
The GaInN layer is usually less than or equal to 100 Å thick and in this case we frequently refer to GalnN/Ga(Al)N quantum well LEDs, which have an emission centred on a particular colour, for example blue or green.
It is frequently accepted that the indium composition and/or the thickness of the GaInN layer fixes the transition energy of the quantum well, and consequently the emission wave length of the LED. However, for indium compositions greater than 10%, GaInN/Ga(Al)N quantum wells have optical properties that are very specific, including an abnormally long radiation life of the excitons and the very small variation of the energy of the prohibited band as a function of the pressure.
Bright blue LEDs, which already have a very high efficiency (more than 10%) are used for the generation of white light using a hybrid technology in which the blue LED is used to pump phosphorus or polymers. A combination of the yellow light from these compounds with the light from the LED produces white light by the combination of colors. This technology is currently widely used both by NICHIA® and by HEWLETT-PACKARD®, GELCORE® or SIEMENS-OSRAM®, and its application to domestic lighting is quite promising.
However, coupling a LED and another constituent such as phosphor or polymer in order to obtain white light, due to its,hybrid nature, is an expensive and complex process including several technological steps, for example involving a subsequent deposit of a phosphor or polymer type compound before encapsulation, on a blue LED.
It is not easy to balance colors with a hybrid LED-polymer/phosphorus device, and it is not easy to obtain a white qualified as being “pleasant” for domestic lighting. Furthermore, like the method for its preparation, the device is complex and comprises a large number of elements and is therefore not as reliable as the basic nitride LED that has an intrinsic life of about 100 000 hours.
Finally, the inherent efficiency of a hybrid system in which losses are inevitable is less than the efficiency of a pump nitride LED.
Therefore, it would be interesting to be able to use light emitting diodes (LEDs) that emit white light directly, to overcome the disadvantages of hybrid devices involving coupling of a blue (or green) LED with a phosphor or polymer.
The article by B. DAMILANO, N. GRANDJEAN, F. SEMOND, J. MASSIES and M. LEROUX, AppI. Phys. Lett. 75, 962 (1999) recently showed that it is possible to produce white light under optical excitation by means of a laser, in a monolithic structure based on semiconductors consisting of nitrides of elements in group III in the periodic classification. It uses a stack of four planes of GaN quantum boxes separated by AIN layers. The size of the quantum boxes fixes the emission wave length and therefore it is sufficient to adjust the wave length in each plane such that the resulting emission from the four planes is white by the simple principle of combining colors. Unfortunately, this structure cannot be used as an active zone in a light emitting diode (LED) type optoelectronic device for the simple reason that the AIN material cannot have a p type doping.
Furthermore, electric injection is difficult to control in this type of structure and it may cause a colour balancing problem which eventually makes it difficult to generate white.
Therefore, there is a need for a semi conducting layer that emits white light (or any other light resulting from the combination of several colors, and particularly primary colors) directly and that can be inserted directly as an active zone in a LED, like the GaInN/Ga(Al)N quantum wells now used for these blue and green commercial LEDs; this single layer may be manufactured easily at low cost using a safe, reliable, and tested method with a limited number of steps.
One of the purposes of this invention is to satisfy these needs.
BRIEF DESCRIPTION OF THE INVENTION
This purpose and other purposes are achieved according to this invention by a single, thin, semi conducting layer of GaInN that emits at least two visible lights with determined colors which can be added particularly to obtain white light.
A GaInN layer means a layer of GaInN, or GaInN that may, for example, contain a small percentage of arsenic, phosphorus or antimony.
A small percentage usually means a percentage less than 5%.
More precisely, the thin layer according to the invention is formed by putting at least two deposits of GaInN (with the same composition or a different composition), each of which emits visible light with a determined colour, adjacent to each other or superposed with each other.
Advantageously, the combination of said at least two visible lights with determined colors produces white light.
A completely unexpected result of the invention is that several lights with determined colors are
Damilano Benjamin Gérard Pierre
Grandjean Nicolas Pierre
Massies Jean Claude
Centre National de la Recherche Scientifique
Pert Evan
Sarkar Asok Kumar
Thelen Reid & Priest LLP
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