Claims
- 1. A pure green light emitting diode, comprising:
- an n-type gallium phosphide substrate;
- an n-type gallium phosphide epitaxial layer, said n-type layer being formed overlying said substrate as a liquid phase epitaxial layer; and
- a p-type gallium phosphide epitaxial layer grown by a liquid phase epitaxial process on said n-type layer, said p-type layer containing donor impurities in an average donor concentration less than or equal to 5.times.10.sup.16 cm.sup.-3, such that brightness of said pure green light emitting diode attains a predetermined high brightness value with an emission peak wavelength of 555 nm.
- 2. A method for manufacturing a pure-green light emitting diode, comprising the steps of:
- preparing an n-type gallium phosphide substrate;
- bringing a melt for epitaxial growth containing donor impurities into contact with said substrate and decreasing a temperature of said melt at a predetermined cooling rate to cause an n-type gallium phosphide layer to grown epitaxially in liquid phase on said substrate, said donor impurities being sulfur;
- maintaining the temperature of said melt at a temperature at which growth of said n-type gallium phosphide layer becomes complete and maintaining an ambient atmosphere at a reduced pressure for a predetermined period of time thereby to volatilize donor impurities from said melt; and
- terminating maintenance of said ambient atmosphere at said reduced pressure and decreasing the temperature of said melt at a predetermined cooling rate while adding receptor impurities to said melt in vapor phase thereby to form a p-type gallium phosphide epitaxial layer having an average donor concentration of less than or equal to 5.times.10.sup.16 cm.sup.-3 on said n-type gallium phosphide layer and thereby to form a pn junction, said pure-green average donor concentration of 5.times.10.sup.16 cm.sup.-3 being selected such that brightness of said light emitting diode attains a predetermined high brightness value with an emission peak wavelength of 555 nm, said acceptor impurities being zinc.
- 3. A method for manufacturing a pure green light emitting diode, comprising the steps of:
- preparing an n-type gallium phosphide substrate;
- bringing a melt for epitaxial growth containing donor impurities into contact with said substrate and decreasing a temperature of said melt at a predetermined cooling rate to cause an n-type gallium phosphide layer to grow epitaxially in liquid phase on said substrate;
- maintaining the temperature of said melt at a temperature at which growth of said n-type gallium phosphide layer becomes complete and maintaining an ambient atmosphere at a reduced pressure for a predetermined period of time thereby to volatilize donor impurities from said melt; and
- terminating maintenance of said ambient atmosphere at said reduced pressure and decreasing the temperature of said melt at a predetermined cooling rate while adding acceptor impurities to said melt in vapor phase thereby to form a p-type gallium phosphide epitaxial layer having an average donor concentration of less than or equal to 5.times.10.sup.16 cm.sup.-3 on said n-type gallium phosphide layer and thereby to form a pn junction, said average donor concentration of 5.times.10.sup.16 cm.sup.-3 being selected such that brightness of said light emitting diode attains a predetermined high brightness value with an emission peak wavelength of 555 nm.
- 4. A method for manufacturing a pure-green light emitting diode according to claim 3, wherein said predetermined period during which said ambient atmosphere is maintained at said reduced pressure is longer than or equal to 3 hours.
- 5. A method for manufacturing a pure-green light emitting diode according to claim 3, wherein said predetermined cooling rate of said melt is 1.degree. C./min.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 57-132735 |
Jul 1982 |
JPX |
|
Parent Case Info
This application is a continuation of application Ser. No. 516,443, filed July 22, 1983 now abandoned.
This invention relates to pure green light emitting diodes with high brightness which emit pure-green light clearly distinguishable from yellow-green light, and a method of manufacturing the same.
Visible light emitting diodes (LED's) are now being widely used as display devices. Of them, gallium phosphide (GaP) LED's can emit light with either of three kinds of color--red, yellow-green and pure green--by using a GaP substrate as a starting material. These LED's are widely used because of good characteristics and low cost. However, the pure green GaP LED's have lower brightness and inferior hue as compared with those of red and yellow-green GaP LED's. It is important to improve the above-mentioned characteristics in order to use GaP LED's as display devices.
The peak emission wavelength of GaP pure-green LED's is about 555 nm. The conventional process of forming the LED's is through a growth of n-type and p-type epitaxial layers on an n-type GaP substrate by a liquid phase method. The method of liquid phase epitaxial growth is roughly classified into the temperature difference method and the gradual cooling method. The former method is superior to the latter on the point of the quality of crystallization, but the control of the temperature difference is difficult. The gradual cooling method is more favorable in view of mass production, and is applied in the production of cheap LED's. In the formation of an epitaxial layer by the gradual cooling method, if the n-type layer and p-type layer are grown from different melts by using a slide boat, mechanical damage occurs on the junction interface. This leads to a lowering in brightness. Therefore, after the n-type layer is grown from a melt containing donor impurities, acceptor impurities with a concentration higher than that of the donor impurities are added and subsequently the p-type layer is grown. Application of the so-called overcompensation method can prevent increase in brightness.
GaP pure green LED's emit light either from the n-type or p-type layer. However, the emission of pure green light occurs in the vicinity of the band gap of GaP and hence self- absorption is negligible. Therefore, light emission from the p-type layer which is nearer to the LED surface than the n-type layer can have a higher efficiency of external light emission. However, if GaP pure green LED's are formed by the usual overcompensation method, donor impurities existing in high concentrations in the p-type layer disturb the light emission. Thus, most of the GaP pure green LED's tend to be low-brightness emission LED's. Furthermore, if donor impurities in high concentrations exist in the p-type layer, emission of yellow light appears at 570 nm due to donor-accepter pair emission or donor-free hole pair emission. The hue of GaP pure green LED's is thus a tincture of yellow. If such GaP LED's are combined with yellow-green LED's to be used as displays, the contrast of hue against yellow-green is obscured.
The first objective of this invention is to realize GaP pure green LED's with high brightness and excellent hue in spite of using an epitaxial layer grown by the overcompensation method.
The second objective of this invention is to provide a method of manufacturing the above-mentioned GaP pure-green LED's.
The third objective of this invention is to provide GaP pure-green LED's with high brightness and excellent hue, which enable the use of the overcompensation method suitable for mass production.
A further objective of this invention is to realize GaP pure-green LED's with clear contrast of hue with the yellow-green LED's thereby to extend the application range of GaP LED's for displays, etc.
The structure of pure-green LED's of this invention is as follows. On an n-type GaP substrate, another GaP epitaxial layer is formed with the same conduction type as that of the substrate. A p-type GaP epitaxial layer is formed on the above GaP epitaxial layer by an overcompensation liquid phase epitaxial method.
The average donor concentration in the p-type epitaxial layer is set at a value less than or equal to 5.times.10.sup.16 cm.sup.-3. This structure ensures emission of pure-green light with high brightness, in which the component of yellow light emission is suppressed. Furthermore, according to the method of the invention for obtaining such pure-green LED's, the n-type GaP epitaxial layer is formed, and subsequently acceptor impurities are supplied to this melt to compensate the donor impurities by using one melt. Prior to this subsequent process, however, the ambient atmosphere of the melt is kept at a reduced pressure for a prescribed period, where the volatilization process of part of the donor impurities from the melt is undertaken. This process ensures reduction of the donor concnetration in the p-type GaP epitaxial layer to a magnitude less than or equal to 5.times.10.sup.16 cm.sup.-3.
The present invention will be made more apparent from the following description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a cross-sectional view showing the structure of GaP pure-green LED's of this invention;
FIG. 2 is an explanatory view on the method of manufacturing the GaP pure-green LED's shown in FIG. 1 according to this invention;
FIG. 3 is a diagram showing the relation between the brightness of GaP pure-green LED's and the average donor concentration in the p-type epitaxial layer of the above GaP LED's;
FIG. 4 is a diagram showing the donor and acceptor concentration of GaP pure-green LED's formed by undergoing the process of the reduced pressure for 3 hours in the manufactuirng processes; and
FIG. 5 is a diagram showing the dependence of the spectra of emitted light upon the period of the reduced pressure process.
US Referenced Citations (6)
Non-Patent Literature Citations (5)
| Entry |
| Japanese magazine "Electronics Parts and Materials", Feb. 1980, pp. 73-77, Pure Green (555 nm) LED written by Kaoru Takahashi (English translation of pp. 73 to 75 being also attached herewith.). |
| J. Appl. Phys., 53(5), May 1982, pp. 2878-3883 "Photocapacitance Study of Deep Levels Due to Nonstoichiometry in Nitrogen Free CAP Light-Emitting Diodes". |
| Proceedings of the 9th Conference on Solid State Devices, Tokyo 1977, pp. 87-92, "Effects of Vapor Pressure on GaP LED's". |
| Nishizawa, Jun-ichi, et al., "Effects of Vapor Pressure on GaP LED's", Proceedings of the 9th Conference on Solid State Devices, Tokyo, 1977; Japanese Journal of Applied Physics, vol. 17, (1988), Supplement 17-1, pp. 87-92. |
| J. Nishizawa, et al., "Photocapacitance Study of Deep Levels Due to Nonstoichiometry in Nitrogen-free GaP Light-Emitting Diodes," Journal of Applied Physics, 53(5), May 1982. |
Continuations (1)
|
Number |
Date |
Country |
| Parent |
516443 |
Jul 1983 |
|
Patent Grant
4965644
