BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for producing a light emitting diode (LED) and, more particularly, to a method for producing a high-brightness light emitting diode by bonding a reflective layer formed on an LED epitaxial layer to an adhesive layer formed on an Si substrate.
2. Description of Related Art
Currently, a trend of developing the light emitting diodes is to promote the brightness. In order to achieve this object, one or more reflective metal layers are combined therein. However, this metal layer has to also possess properties of adhesion and ohmic contact.
For example, R.O.C. Patent No. 369731 disclosed an LED in which the GaAs substrate is replaced with a Si substrate having a reflective metal layer thereon by wafer bonding technology. Unfortunately, such design cannot improve the lighting effect of short wavelengths.
Additionally, in U.S. Pat. No. 5,376,580, the GaAs substrate is a temporary substrate for epitaxying and then removed after being bonded to a transparent substrate. Though this method prevents absorption of the GaAs substrate, the processes have to be carried out at high temperature, which might damage the structure and thus decrease the lighting effect. Additionally, this transparent substrate is made by GaP, which can absorb the short-wavelength light.
R.O.C. Patent No. 415116 mentioned an LED 10 as shown in FIG. 1, in which two reflective adhesive metal layers 16, 12 are respectively attached on the bottom surface of the LED epitaxial layer 15 and the top surface of the substrate 11. By bonding the two reflective adhesive metal layers 16, 12, the light beams can be propagated from the front side of the LED 10 and the brightness can be promoted.
FIG. 2 shows another conventional LED 20, in which a metal adhesive layer 22 is formed on a top surface of the SiO2 substrate 21, and an LED epitaxial layer 25 is bonded to the top surface of the metal adhesive layer 22. Brightness of the LED 20 can be improved due to ohmic contact between the substrate 21 and the metal adhesive layer 22.
For the above LED structures, all of the reflective layers have combining functions of adhesion and ohmic contact, therefore only metal material is suitable. A disadvantage of such material is that atomic diffusion occurs at the interface of the metal layer and the LED epitaxial layer. When a light source more than 600 nm is applied, the lighting effect will be reduced since total reflection in the metal layer is not available. FIG. 3 shows the reflectivity varied with wavelengths, in which the reflectivity is 0.9 at 600 nm. Further, the reflectivity rapidly reduces at wavelength less than 600 nm, for example, 590 nm or 570 nm of yellow-green light. Consequently, the reflective layer can hardly perform expected effect. Both the reflective metal layers as shown in FIGS. 1 and 2 exist such problem.
Therefore, it is desirable to provide a method for producing an improved LED structure to promote the brightness, particularly at short wavelength.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for producing a high-brightness light emitting diode, which can exhibit superior lighting effect at long wavelength.
Another object of the present invention is to provide a method for producing a high-brightness light emitting diode, which can exhibit much better lighting effect than the conventional at short wavelength.
In order to achieve the above object, the method primarily includes steps of: A) forming a reflective layer on an LED epitaxial layer and a first adhesive layer on an Si substrate, wherein the LED epitaxial layer is grown on a temporary substrate and has a pn junction structure to define a first-type layer adjacent to the temporary substrate and a second-type layer; B) bonding the reflective layer to the first adhesive layer by thermal pressing and then removing the temporary substrate; C) forming a first-type ohmic contact electrode on partial top surface of the LED eptaxitial layer by physical vapor deposition; D) etching at least partial remained LED epitaxial layer from top surface thereof and stopping at the second-type layer to form a metal contact layer; and E) forming a second-type ohmic contact electrode on the metal contact layer by physical vapor deposition.
The LED eptaxitial layer aforementioned can further include a p-type confining layer and a n-type confining layer. The first ohmic contact electrode can further include a transparent electrode thereon to enhance electric conduction. The reflective layer preferably includes at least two materials, for example, a metal and an insulator, a high-dielectric material and a low-dielectric material, etc, wherein the insulator and the low-dielectric material are preferably adjacent to the LED epitaxial layer.
The metal can be Al, Ag, Au, Pt, Pd, etc. The insulator can be Al2O3, MgF2, SiO2, TiO2, Si3N4, etc. The high-dielectric material preferably has a refractive index larger than 2.1, and the low-dielectric material has a refractive index less than 1.56. The high-dielectric material can be TiO2, CeO2, Si, etc. The low-dielectric material can be Al2O3, MgF2, SiO2, Si3N4, etc.
The etching process of step D) is preferably chemical wet etching or dry etching. The physical vapor deposition of steps C) and E) can be e-gun evaporation deposition, thermal evaporation deposition, or sputter deposition.
The reflective layer can further include a second adhesive layer therebelow to reinforce attachment with the first adhesive layer. The first or second adhesive layer is preferably made from metal, for example, Au, Au/Be alloy, Au/Zn alloy, Pt, Pd, Cu, Ni, In and Al. The first adhesive layer can be also a polymer nonconductor.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.
Referring to FIG. 4, the flow diagram for producing the high-brightness LED 50 of the present invention is shown. In step A, also with refer to FIG. 5, an Si substrate 51 having a first adhesive layer 52 and a LED epitaxial layer 55 having a nonconductive reflective layer 56 or a mirror layer formed on a bottom surface thereof are provided. The LED epitaxial layer 55 is an active structure and fabricated by II-VI or III-V alloys, for example, direct-bandgap LEDs, AlGaInP. The LED epitaxial layer 55 previously grows on a temporary GaAs substrate 59 and has a pn junction structure, wherein the upper one is a p-type layer 55A and the lower one is an n-type layer 55B. In this embodiment, the p-type layer 55A is arranged to be adjacent to the reflective layer 56 and the n-type layer 55B to the temporary substrate 59. The reflective layer 56 is a composite material including two materials. In this embodiment, the reflective layer 56 includes a metal 561 and a insulator 562, wherein the metal 561 is not provided for omhic contact, and the insulator 562 is adjacent to the LED epitaxial layer 55. The composite reflective layer 56 is not restricted, and can be Al/Al2O3, Al/SiO2, Al/MgF2, Pt/Al2O3, Pt/SiO2, Pt/MgF2, Ag/Al2O3, Ag/SiO2, Ag/MgF2, etc.