Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
2001-08-03
2004-09-14
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S499000, C313S501000, C313S506000, C257S078000, C257S101000, C438S045000, C438S095000, C427S066000, C445S023000, C445S024000
Reexamination Certificate
active
06791257
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a useful technique applied to an electro luminescence device and a method for producing the same, such as a LED (light emitting diode) or a LD (laser diode) produced by employing a compound semiconductor crystal substrate comprising a Group 12 (2B) element and a Group 16 (6B) element in the periodic table.
BACKGROUND ART
With compound semiconductors that comprise a Group 12 (2B) element and a Group 16 (6B) element in the periodic table (hereinafter, that are referred to Group II-VI compound semiconductors), generally, free control of conduction types of p-type and n-type is difficult except CdTe (cadmium telluride). Thus, extremely a few electro luminescence devices provided with these materials and methods for producing the same are made practicable, and the ranges thereof remain limited.
For example, with a method for fabricating a light emitting diode as an electro luminescence device by using a ZnSe system material, a large number of mixed crystal thin films of ZnSe system are formed on a GaAs substrate by a molecular beam epitaxial growth method, thereafter electrodes are formed, and then a pn junction type light emitting diode is fabricated.
When fabricating the light emitting diode, with the ZnSe system material, since the control for a p-type semiconductor is difficult in a thermal equilibrium state, the epitaxial growth method which is not in the thermal equilibrium state was applied to formation of the mixed crystal thin films by using a particular apparatus which is referred to as a radical gas source.
As an electro luminescence device provided with such a ZnSe system material, for example, 480 nm blue LEDs are manufactured by way of trial. Furthermore, fabrication of blue LDs in quantum well structure of CdZnSe—ZnSe is reported, and it draws attention as a blue light emitting device.
However, as above-described, with the electro luminescence device provided with the Group II-VI compound semiconductor, the material system is extremely limited because the physical property that the control of conduction types in the Group II-VI compound semiconductor is difficult. Thus, the electro luminescence device having the Group II-VI compound semiconductor has not bean put to practical use except for the ZnSe system materials.
When the electro luminescence device provided with the ZnSe system material was fabricated, the epitaxial growth method was required to be applied to the fabrication, to make control of the conduction types possible. Thus, there were problems that the productivity was low, and that the production cost increased because an expensive apparatus such as the radical gas source or the like was required.
Then, the inventors or the like proposed a method for forming an electro luminescence device by using the Group II-VI compound semiconductor single crystal substrate, and forming a pn junction by thermally diffusing a diffusion source including an element converting the substrate of a first conduction type into one of a second conduction type from a front surface of the substrate.
However, there was a problem that the characteristics of the electro luminescence device fabricated by the method depended heavily on quality of the used substrate, and thus the electro luminescence device having superior light emission efficiency was not stably fabricated.
The present invention was developed to solve the above-described problems. A main object of the present invention is to provide a method that is capable of stably producing an electro luminescence device having superior light emission efficiency by using a Group II-VI compound semiconductor crystal substrate.
DISCLOSURE OF THE INVENTION
At first, the inventors investigated depositing diffusion sources over ZnTe substrates of compound semiconductors (Group II-VI compound semiconductors) comprising Group 12 (2B) elements and Group 16 (6B) elements in the periodic table and being produced by some producing methods, and then formed pn junctions by thermally diffusing the diffusion sources. Thereafter, the inventors investigated the correlation between the light emission characteristics and the qualities of the substrate (particularly, crystal dislocation).
As a result, green light emission was able to be recognized from light emitting diodes produced by using substrates on which density of pits (hereinafter, it referred to as etch pits), which were obtained by etching with high temperature sodium hydroxide aqueous solution, was not more than 20,000/cm
2
, preferably not more than 10,000/cm
2
, more preferably not more than 5,000/cm
2
, furthermore, not more than 2,000/cm
2
On the other hand, with light emitting diodes produced by using substrates on which the density of the etch pits exceeded 20,000/cm
2
, no light emission was able to be recognized.
It was verified that the etch pits formed using sodium hydroxide occurred due to the dislocation in the crystal by another experiment. Therefore, with the ZnTe substrate, the dislocation density and etch pit density can be treated equally.
From the result of the above-described researches, it was ascertained that the light emitting phenomenon of the light emitting diode depends largely on the dislocation density or the etch pit density of the front surface of the substrate.
It has been known that a large number of inclusions exist inside of crystals of the Group II-VI compound semiconductor depending upon growth methods or growth conditions. For example, the Group II-VI compound semiconductor, which is applied to a substrate for a visible light emitting diode, has wide forbidden band width and is transparent. Thus, the inclusions inside the substrate can be observed by an optical microscope.
Thus p-type ZnTe substrates that were different from each other in densities of inclusions were prepared. Then, as a diffusion source, for example, Al or In was deposited over the front surfaces of the substrates, and pn junctions were formed by the thermal diffusion. The characteristics of the light emitting diodes formed by such a method were compared with one another. When the density of the inclusions having grain diameters of 0.3 to 10 &mgr;m on the pn junction interfaces, which were observed in a focal field of the optical microscope of ×100 to ×200 magnification, was not more than 100,000/cm
2
, preferably not more than 50,000/cm
2
, it was possible to obtain the light emitting diodes having a little leakage current due to recombination and superior light emission efficiency.
On the other hand, when the density of the inclusions exceeded 100,000/cm
2
, the light emission efficiency decreased. In particular, in the substrate having inclusions which were larger than 5 &mgr;m, even if the density of the inclusions was in single figure smaller 10,000 to 50,000/cm
2
, it was found that the leakage current increased and the light emission efficiency lowered.
Consequently, it is considered that the leakage current occurs because the inclusions in the pn junction interface form current passages.
Therefore, it is supposed that suppression of the inclusions in the pn junction interfaces plays a role for decreasing the leakage current and thus improving light emission efficiency.
From a result of observation by a scanning electron microscope, the number of the inclusions existed in the interface is generally smaller than the number of the inclusions observed by the optical microscope.
This depends on the sizes of the inclusions. When the sizes of the inclusions are about 1 &mgr;m, the density of the inclusions in the interface and the density of the inclusions observed by the optical microscope are at the same level, while when the sizes of the inclusions are small, the density of the inclusions in the interface is about in single figure small compared with the density of the inclusions observed by the optical microscope.
Then, as a result of the researches, when the number of the inclusions existed in the junction interface was not more than 50,000/cm
2
, it was possible to obtain the electro luminescence device in which the l
Arakawa Atsutoshi
Hanafusa Mikio
Noda Akira
Sato Kenji
Birch & Stewart Kolasch & Birch, LLP
Japan Energy Corporation
Quarterman Kevin
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