Patent Application
20060043398
1. Field of the Invention
The present invention relates to a method for fabricating a light emitting diodes (LED). More particularly, the invention relates to a method of fabricating LED with pure colour and enhanced light extraction efficiency.
2. Description of the Prior Art
Generally light extraction efficiency of LEDs is limited by high refractive index of the LED chip material which prevents the light escape from the LED chip when its incident angles is higher than the angle of total internal reflection
To enhance the light extraction efficiency various methods had been proposed.
These are pyramidal-like shaped LED chip taught by M. R. Krames et. al. Applied Physics Letters, 75, pp. 2365, (1999), a random surface texture taught by Schnitzer, et al in Applied Physics Letters 63, 2174 (1993), an ordered interface texturing taught by M. R Krames et al. U.S. Pat. No. 5,779,924.
All above methods allow to suppress the light reflection at the surface of the LED chip and change the angular bandwidth of light which may transmit power into the ambient, but they are not very sensitive to the emitted wavelength. This does not allow a precise fitting the light extraction properties to a given wavelength and filtering of the light spectrum emitted by the LED.
The present invention allows to overcome this disadvantage by the using of special hexagonal diffraction lattice with precisely determined parameters that allow to convert the laterally propagating light into the vertically propagating light and simultaneously filter the light spectrum emitted by the LED.
This invention states LED with a colour purifying diffraction lattice (CPDL).
The essence of the invention is in the use of the coherent scattering of the light by the CPDL for colour purifying of the light emitted by the LED and enhancement its extraction efficiency.
Use of CPDL allows to convert the laterally propagating light into the vertically propagating light with high efficiency and, simultaneously filter the light spectrum emitted by the LED.
The LED spectrum filtering by the diffraction lattice allows to purify the colour of the light emitted by LED. Also, the LED spectrum filtering allows to reduce the difference in the wavelengths of the LED chips produced from different part of the wafer and from different wafers.
A method of obtaining the two-dimensional CPDL as a self organized ordered porous pattern of Al2O3 amorphous films developed on Al film by an anodic oxidation. The period and depth of the pores in Al2O3 films are controlled by applied voltage, content of electrolyte and time of oxidation.
In the accompanying drawings:
The invention will be more fully understood by reference to the following examples:
The principal scheme of the LED embodied in Example 1 is shown in
On the gallium-nitride-based LED structure a two-dimensional CPDL 3 is formed by dry surface etching. The light scattering by CPDL convert the laterally propagating light 4 into the vertically propagating light 5 and, thus, enhance the light extraction efficiency.
The CPDL structure is shown in
The period d of the CPDL should satisfy the equation d=mλ/n, where m=1, 2, 3 . . . and λ is the wavelength of the light generated by LED, and n is the refraction index of GaN. To make the scattering with m=1, 2, 3 . . . most effective the zero order of diffraction with m=0 should be suppressed. This happens when height of the hexagonal islands forming CPDL is h=λ(2l+1)/2n, l=0, 1, 2, 3 . . . , and total areas of islands and trenches in CPDL are equal. To make these areas equal the side s hexagon islands should satisfy the equation s=d/2√2. Thus, for LED with λ=0.42 μm the parameters of the CPDL with m=1, l=0 are d=0.17 μm, h=0.085 μm, s=0.06 μm. Use of CPDL allows to convert the laterally propagating light into the vertically propagating light with high efficiency and, simultaneously filter of the light spectrum emitted by the LED.
The LED spectrum filtering by the diffraction lattice allows to purify the colour of the light emitted by LED. Also, the LED spectrum filtering allows to reduce the difference in the wavelengths of the LED chips produced from different part of the wafer and from different wafers.
The principal scheme of the LED embodied in Example 2 is shown in
The CPDL structure is shown in
The light scattering by CPDL convert the laterally propagating light 4 into the vertically propagating light 5 and, thus, enhance the light extraction efficiency.
The period d of the CPDL should satisfy the equation d=mλ/n, where m=1, 2, 3 . . . and λ is the wavelength of the light generated by LED, and n is the refraction index of GaN. To make the scattering with m=1, 2, 3 . . . most effective the zero order of diffraction with m=0 should be suppressed. This happens when heights of the hexagonal islands forming CPDL is h=λ(2l+1)2n, 1=0, 1, 2, 3 . . . , and total areas of islands and trenches in CPDL are equal. To make these areas equal the side s hexagon islands should satisfy the equation s=d/2√2.
For LED with λ=0.5 μm the parameters of the CPDL with m=2, l=0 are d=0.4 μm, h=0.1 μm, s=0.14 μm.
Use of CPDL allows to convert the laterally propagating light into the vertically propagating light with high efficiency and, simultaneously filter of the light spectrum emitted by the LED.
The LED spectrum filtering by the diffraction lattice allows to purify the colour of the light emitted by LED. Also, the LED spectrum filtering allows to reduce the difference in the wavelengths of the LED chips produced from different part of the wafer and from different wafers.
The principal scheme of the LED embodied in Example 3 is shown in
On the gallium-nitride-based LED structure a two-dimensional Al2O3 CPDL 3 is deposited.
The Al2O3 CPDL 3 is formed by an anodic oxidation of Al film.
The CPDL structure is shown in
The period d of the CPDL should satisfy the equation d=mλ/n, where m=1, 2, 3 . . . and λ is the wavelength of the light generated by LED, and n is the refraction index of GaN. To make the scattering with m=1, 2, 3 . . . most effective the zero order of diffraction with m=0 should be suppressed. This happens when depths of the cylindrical holes forming CPDL is h=λ(2l+1)/2n, l is a positive integer number or zero, and their radii r satisfy the equation r=d(√¾π)1/2.
For LED with λ=0.5 μm the parameters of the CPDL with m=1, 1=0 are d=0.21 μm, h=0.1 μm, r=0.08 μm.
Use of CPDL allows to convert the laterally propagating light into the vertically propagating light with high efficiency and, simultaneously filter of the light spectrum emitted by the LED.
The LED spectrum filtering by the diffraction lattice allows to purify the colour of the light emitted by LED. Also, the LED spectrum filtering allows to reduce the difference in the wavelengths of the LED chips produced from different part of the wafer and from different wafers.
The principal scheme of the LED embodied in Example 4 is shown in
On the AlGaInP-based LED structure a two-dimensional Al2O3 CPDL 3 is deposited.
The Al2O3 CPDL 3 is formed by an anodic oxidation of Al film.
The CPDL structure is shown in
The period d of the CPDL should satisfy the equation d=mλ/n, where m=1, 2, 3 . . . and λ is the wavelength of the light generated by LED, and n is the refraction index of AlGaInP. To make the scattering with m=1, 2, 3 . . . most effective the zero order of diffraction with m=0 should be suppressed. This happens when depths of the cylindrical holes forming CPDL is h=λ(2l+1)/2n, and l is a positive integer number or zero, and their radii r satisfy the equation r=d(√¾π)1/2.
For LED with λ=0.6 μm the parameters of the CPDL with m=1 are d=0.18 μm, h=0.09 m (1=10), r=0.066 μm.
Use of CPDL allows to convert the laterally propagating light into the vertically propagating light with high efficiency and, simultaneously filter of the light spectrum emitted by the LED.
The LED spectrum filtering by the diffraction lattice allows to purify the colour of the light emitted by LED. Also, the LED spectrum filtering allows to reduce the difference in the wavelengths of the LED chips produced from different part of the wafer and from different wafers.
Many changes and modifications in the above-described embodiments of the invention can, of course, be carried out without departing from the scope thereof. Accordingly, to promote the progress in science and the useful arts, the invention is disclosed and is intended to be limited only by the scope of the appended claims.
