Light emitting diode and method of making the same

[0001] The present invention relates to a structure and a method of making a light emitting diode (LED) chip, and more particularly to a structure and a method of making an AlGaInP LED chip.

[0002] The conventional AlGaInP LED, as shown in FIG. 1, has a double heterostructure (DH), which is consisted of a N-type GaAs substrate 3 and a plurality of layers sequentially formed thereon, wherein the layers are: an N-type (Al_xGa_1-x) _0.5In_0.5P lower cladding layer 4 with an Al composition of about 70%˜100%, an (Al_xGa_1-x)_0.5In_0.5P active layer 5, a P-type (Al_xGa_1-x)_0.5In_0.5P upper cladding layer 6 with an Al composition of about 70%˜100% and a P-type high energy gap GaAsP, InGaP, AlGaP, GaP, or AlGaAs current spreading layer 7. The emitting wavelength of the conventional LED structure can be changed by adjusting the composition of the active layer 5 to a wavelength from 650 nm red to 555 nm pure green. One disadvantage of the conventional LED is that, when the light generated by the active layer 5 is emitted downward to the GaAs substrate 3, the light is absorbed by the GaAs substrate 3 due to a smaller energy gap of the GaAs substrate 3. Accordingly, the light-output performance of the LED is greatly reduced.

[0003] Some conventional LED technologies have been disclosed to prevent the light from being absorbed by the substrate. However, these conventional technologies still have some disadvantages and limitations. For example, Sugawara et al. disclosed a method, which has been published in Appl. Phys. Lett. Vol. 61, 1775–1777 (1992), that adding a distributed Bragg reflector (DBR) layer onto the GaAs substrate so as to reflect the light emitted downward to the GaAs substrate for decreasing the light absorbed by the GaAs substrate. However, because the DBR layer only reflects the light almost normal to the GaAs substrate, its efficiency is not very great.

[0004] Kish et al. disclosed a wafer-bonded transparent-substrate (TS) (Al_xGa_1-x)_0.5In_0.5P/GaP light emitting diode [Appl. Phys. Lett. Vol. 64, No. 21, 2839 (1994); Very high-efficiency semiconductor wafer-bonded transparent-substrate (Al_xGa_1-x)_0.5In_0.5P/GaP]. This TS AlGaInP LED is fabricated by growing a very thick (about 50 μm) P-type GaP window layer with the use of hydride vapor phase epitaxy (HVPE). Before bonding, the P-type GaAs substrate is selectively removed by using chemical mechanical polishing and etching techniques. The exposed N-type (Al_xGa_1-x)_0.5In_0.5P cladding layers are subsequently wafer-bonded to 8 mil-10 mil thick N-type GaP substrate. The resulting TS AlGaInP LED exhibits the improvement in light output twice as much as the absorbing substrate (AS) AlGaInP LED. However, the fabrication process of TS AlGaInP LED is too complicated. Therefore, it is difficult to manufacture these TS AlGaInP LEDs in high yield and low cost.

[0005] Horng et al. reported a mirror-substrate (MS) AlGaInP/metal/SiO₂/Si LED fabricated by wafer-fused technology [Appl. Phys. Lett. Vol. 75, No. 20, 3054 (1999); AlGaInP light-emitting diodes with mirror substrates fabricated by wafer bonding]. They used AuBe/Au as the adhesive to bond the Si substrate and LED epilayers. However, the luminous intensity of these MS AlGaInP LEDs is about 90 mcd with 20 mA injection current, and is still 50% lower than the luminous intensity of TS AlGaInP LED.

[0006] As described above, the conventional LED has many disadvantages. Therefore, the present invention provides a LED structure and method of making the same to overcome the conventional disadvantages.

[0007] The present invention provides a light emitting diode. The light emitting diode comprises a multi-layered epitaxial structure with a light-emitting layer, a high-reflectivity metal layer formed on the multi-layered epitaxial structure, a dielectric adhesive layer, and a substrate having high thermal conductivity coefficient, wherein the multi-layered epitaxial structure is bonded to the substrate having high thermal conductivity coefficient by the dielectric adhesive layer. The light-emitting layer of the LED can be the structure of homostructure, single heterostructure (SH), double heterostructure (DH), or multi quantum wells (MQWs). Meanwhile, first and second ohmic contact metal layers are deposited to the first and second conductive epitaxial layers respectively. Besides, both the first and second ohmic contact metal layers are located on the same side.

[0008] The present invention provides a method for manufacturing a light emitting diode, which comprises the steps of: providing a LED epitaxial structure having a multi-layered AlGaInP epitaxial structure formed on a light-absorbing substrate; providing a substrate having high thermal conductivity coefficient (such as Si, Cu, Al, etc.); and using a dielectric adhesive layer, for example, BCB (B-staged bisbenzocyclobutene) resin, epoxy resin, polyimide, SOG (spin-on glass) or silicone to bond the substrate having high thermal conductivity coefficient and the multi-layered AlGaInP epitaxial structure. The light-absorbing substrate is then removed to expose the first conductive etching stop layer, so that for example a first ohmic contact metal layer is formed. The etching step also exposes the second conductive epitaxial layer to form a second ohmic contact layer. In addition, both the first and second ohmic contact metal layers are located on the same side.

[0009] An advantage of the present invention is to provide a simple LED structure, wherein the adhesion process for forming the LED structure can be performed at a lower temperature to prevent the evaporation problem of V group elements. Moreover, because the light is not absorbed by the substrate, the light emitting efficiency of the LED can be significantly improved.

[0010] Another advantage of the present invention is the use of the elastic dielectric adhesive layer to bond the LED and the substrate having high thermal conductivity coefficient. Therefore, an excellent bonding result can be obtained by using the elastic dielectric adhesive layer even if the epitaxial structure has a rough surface.

[0015] The present invention discloses a LED structure and a method of making the same, and will be described in details as follows.

[0016] Referring to FIG. 2, the epitaxial structure of light emitting diode of the present invention is composed of an N-type GaAs substrate 26, an etching stop layer 24, an N-type (Al_xGa_1-x)_0.5In_0.5P lower cladding layer 22, an (Al_xGa_1-x)_0.5In_0.5P active layer 20, a P-type (Al_xGa_1-x)_0.5In_0.5P upper cladding layer 18, and a P-type ohmic contact epitaxial layer 16.

[0017] Thereafter, a mirror surface protection layer 14 is deposited over the P-type ohmic contact epitaxial layer 16, wherein the material of the mirror surface protection layer 14 is selected from a group consisting of SiN_x, SiO₂, Al₂O₃, magnesium oxide, zinc oxide, tin oxide, indium oxide, and indium tin oxide.

[0018] Thereafter, a metal mirror surface layer 12 is deposited over the mirror surface protection layer 14, wherein the material of the metal mirror surface layer 12 is selected from a group consisting of Ag, Al, and Au. Then, a mirror surface protection layer 11 is deposited over the metal surface mirror layer 12, wherein the material of the mirror surface protection layer 11 is selected from a group consisting of SiN_x, SiO₂, Al₂O₃, magnesium oxide, zinc oxide, tin oxide, indium oxide, and indium tin oxide.

[0019] In the above description, the material of the P-type ohmic contact epitaxial layer 16 can be AlGaAs, AlGaInP, or GaAsP, as along as the energy gap of the material is larger than that of the active layer 20, and no light emitted from the active layer 20 is absorbed.

[0020] Moreover, the active layer 20 has an Al composition of about 0≦x≦0.45, the lower cladding layer 22 has an Al composition of about 0.5≦x≦1, and the upper cladding layer 18 has an Al composition of about 0.5≦x≦1. If x=0, then the composition of the active layer 20 is Ga_0.5In_0.5P, and the wavelength λd of the LED is 635 nm.

[0021] In the above description, the compound ratio in (Al_xGa_1-x)_0.5In_0.5P is a preferred embodiment, and the present invention is not limited thereto. Additionally, the structure of the AlGaInP active layer 20 of the present invention can be a homostructure, a single heterostructure, a double heterostructure, or a multiple quantum wells structure. The so-called double heterostructure comprises the N-type (Al_xGa_1-x)_0.5In_0.5P lower cladding layer 22, the (Al_xGa_1-x)_0.5In_0.5P active layer 20 an the P-type (Al_xGa_1-x)_0.5In_0.5P upper cladding layer 18, such as shown in FIG. 2, wherein the preferred thickness of the lower cladding layer 22, that of the active layer 20, and that of the upper cladding layer 18 are about 0.5˜3.0 μm, 0.5˜2.0 μm, and 0.5˜3.0 μm, respectively.

[0022] The preferred material of the etching stop layer 24 of the invention can be any III–V compound semiconductor material, provided that the lattice thereof is matched with that of the GaAs substrate 26, and the etching rate thereof is much smaller than that of the GaAs substrate 26. For example, InGaP or AlGaAs are suitable for forming the etching stop layer 24. In addition, the etching rate of the N-type AlGaInP lower cladding layer 22 is also far smaller than that of the GaAs substrate 26. Therefore, as long as the thickness of the lower cladding layer 22 is sufficient, it is not necessary to form an optional epitaxial layer of different composition as the etching stop layer 24.

[0023] The structure shown in FIG. 3 comprises a dielectric adhesive layer 10, for example, BCB (B-staged bisbenzocyclobutene) resin and a substrate having high thermal conductivity coefficient 8. The material of the dielectric adhesive layer 10 is not limited to BCB. Any adhesive material with similar property, such as epoxy resin, polyimide, SOG, or silicone, etc. is also applicable to the present invention. The substrate having high thermal conductivity coefficient 8 can be composed of Si wafer, Cu wafer, or Al wafer, etc. One advantage of the present invention is that the substrate having high thermal conductivity coefficient 8 does not have to be a single crystal wafer. The substrate having high thermal conductivity coefficient 8 is used for mechanically supporting the LED epitaxial layer to prevent the epitaxial layer from breaking during the manufacturing process, and meanwhile is used as a heat sink, and the current does not flow through the substrate having high thermal conductivity coefficient 8 when the LED emits the light. In other words, the polycrystal or amorphous crystal can be used as the carrier substrate. Accordingly, the manufacture cost is significantly decreased.

[0024] Thereafter, the epitaxial layer structure shown in FIG. 2 is bonded together with the substrate having high thermal conductivity coefficient 8 shown in FIG. 3 by the dielectric adhesive layer 10. The adhesion step is performed under high pressure and temperature, such as 250° C., according to the method of the present invention. A layer of adhesion promoter can be coated on the surface of the LED epitaxial structure and the surface of the substrate having high thermal conductivity coefficient 8 to improve the adhesion feature between the LED epitaxial structure and the substrate having high thermal conductivity coefficient 8. After that, a BCB layer is coated, and then the adhesion between the LED epitaxial structure and the substrate having high thermal conductivity coefficient 8 is completed under high pressure and temperature, such as 250° C. In order to provide better adhesion, the LED epitaxial structure, which is bonded with the substrate having high thermal conductivity coefficient 8 by the dielectric adhesive layer 10, can be first heated at a lower temperature, for example, 60° C. to 100° C., thereby removing the organic solvent in BCB, and then be heated at the temperature raised to a range between 200° C. and 600° C., so that the LED epitaxial structure, the substrate having high thermal conductivity coefficient 8, and the dielectric adhesive layer 10 can be tightly bonded. Thereafter, the opaque N-type GaAs substrate 26 is then removed by an etchant, for example, 1NH₄OH:1 H₂O₂. However, since the etching stop layer 24, InGaP or AlGaAs, still absorbs the light emitted from the active layer 20, it is necessary to remove the etching stop layer 24 completely or with only remaining a portion of the etching stop layer 24 contacting the N-type ohmic contact metal layer 30, such as shown in FIG. 4. A dry etching method, for example, RIE (reactive ion etching), is then applied to remove portion of the N-type AlGaInP lower cladding layer 22, that of the AlGaInP active layer 20, and that of the P-type AlGaInP upper cladding layer 18 to further expose the P-type ohmic contact epitaxial layer 16. A P-type ohmic contact metal layer 28 is then formed on the P-type ohmic contact epitaxial layer 16, and a N-type ohmic contact metal layer 30 is formed on the N-type AlGaInP lower cladding layer 22, so that a LED structure is formed, wherein the P-type and N-type ohmic contact metal layers formed on the same side, such as shown in FIG. 4.

[0025] According to the present invention, the light output power of the AlGaInP LED having wavelength of 635 nm is more than 4 mW (at 20 mA injection current), and is twice as much as that of the conventional absorbing substrate AlGaInP LED.

[0026] The present invention is not limited to the AlGaInP LED having high brightness, and is also suitable for other LED materials, for example, red and infrared-red AlGaAs LED. The epitaxial structure shown on FIG. 5 is a cross-sectional view of the second embodiment of the present invention. The AlGaAs red LED (650 nm) includes a stacked structure of an N-type GaAs substrate 51, an N-type AlGaAs lower cladding layer 52 with Al composition of about 70˜80% and thickness of 0.5 μm˜3 μm, an AlGaAs active layer 53 with Al composition of about 35% and thickness of 0.5 μm˜2 μm, and a P-type AlGaAs upper cladding layer 54 with Al composition of about 70˜80% and thickness of 0.5 μm˜3 μm, wherein the AlGaAs active layer 53 can be a homostructure, a single heterostructure, a double heterostructure, or a quantum well structure. Thereafter, a mirror surface protection layer 55, a metal mirror surface layer 56, and a mirror surface protection layer 59 are formed on the P-type AlGaAs upper cladding layer 54 in sequence, wherein the material of the mirror surface protection layer 55 and that of the mirror surface protection layer 59 can be selected from a group consisting of SiN_x, SiO₂, Al₂O₃, magnesium oxide, zinc oxide, tin oxide, indium oxide and indium tin oxide, and the material of the metal mirror surface layer 56 could be Ag, Al, or Au.

[0027] Referring to FIG. 6, the structure shown in FIG. 6 comprises a dielectric adhesive layer 50 and a substrate having high thermal conductivity coefficient 60, wherein the material of the dielectric adhesive layer 50 is BCB resin, for example. The material of the dielectric adhesive layer 50 in the present invention is not limited to BCB. Any adhesive material with similar property, such as epoxy resin, polyimide, SOG, or silicone, etc. is also applicable to the present invention. The substrate having high thermal conductivity coefficient 60 can be composed of Si wafer, Cu wafer, or Al wafer, etc. One advantage of the present invention is that the substrate having high thermal conductivity coefficient 60 does not have to be a single crystal wafer. The substrate having high thermal conductivity coefficient 60 is used for mechanically supporting the LED epitaxial layer to prevent the epitaxial layer from breaking, and is also used as a heat sink, and the current does not flow through the substrate having high thermal conductivity coefficient 60 when the LED emits the light. In other words, the polycrystal or amorphous crystal can be used as the carrier substrate. Accordingly, the manufacture cost is significant decreased.

[0028] Referring to FIG. 7, the above-mentioned AlGaAs red LED structure in FIG. 5 is then bonded to the substrate having high thermal conductivity coefficient 60 by the dielectric adhesive layer 50. The epitaxial structure is then etched by an etchant, such as 1NH₄OH:1H₂O₂ to remove the opaque N-type GaAs substrate 51. Thereafter, a wet etching or a dry etching is applied to remove portion of the N-type AlGaAs lower cladding layer 52 and that of the AlGaAs active layer 53 to further expose the P-type AlGaAs upper cladding layer 54. Then, a P-type ohmic contact metal layer 57 is formed on the P-type AlGaAs upper cladding layer 54, and a N-type ohmic contact metal layer 58 is then formed on the N-type AlGaAs lower cladding layer 52, so that a LED structure is formed, wherein the P-type and N-type ohmic contact metal layers are formed on the same side.

[0029] The light output power of the present invention AlGaAs LED with wavelength of about 650 nm is twice as much as that of the conventional absorbing substrate AlGaAs LED under the 20 mA injection current.

[0030] The present invention uses the substrate having high thermal conductivity coefficient to enhance the heat dissipation of the chip, thereby increasing the performance stability of the LED, and making the LED applicable at higher currents.

[0031] Moreover, the LED of the present invention uses of the elastic property of dielectric adhesive material to bond the substrate having high thermal conductivity coefficient and the multi-layered AlGaInP epitaxial structure. Therefore, an excellent bonding result can be obtained by the use of the elastic property of dielectric adhesive material, even if the epitaxial structure has a rough surface.

[0032] As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrations of the present invention rather than limitations of the present invention. It is intended to cover various modifications and similar arrangements comprised within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.