Light emitting diode and method for producing the same

[0001] 1. Field of the Invention

[0002] The present invention relates to a light emitting diode (LED) and a method for producing the same, and particularly to a light emitting diode including a transparent window and a transparent conductive film and a method for producing the same.

[0003] 2. Description of the Related Prior Art

[0004] General processes for fabricating LEDs can be divided into up stream, middle stream and down stream. In the up stream, LPE (Liquid Phase Epitaxy), MBE (Molecular Beam Epitaxy) and MOVPE (Metal Organic Vapor Phase Epitaxy) are usually applied to producing single chips or epitaxial chips. In the middle stream, mask, dry or wet etching, vacuum vapor deposition and cutting are applied to forming dice. In the down stream, dice are placed on lead frames, and finally packaged as lamps, digit displays, dot matrix or surface mount, optionally.

[0005] In the up stream, GaAs and GaP are frequently served as substrates for depositing the light emitting material thereon. However, GaAs can absorb visible light emitted from double hetero-junction structure and GaP can absorb yellow-green wavelength, which significantly limit the application thereof. Theoretically, the external quantum efficiency (η_ext) is a product of the internal quantum efficiency (η_int) and the LED emitting efficiency (η_optical), that is, η_ext=η_intη_optical. The emitting efficiency is further influenced by absorption of the material itself (η_A), Fresnel loss (η_Fr), and total reflection critical angle (η_cr); that is, η_optical=η_Aη_Frη_cr. Among these factors, η_Fr is caused by the large difference between n_LED (refractive index of the LED illuminant media; ≈3.4) and n_air (refractive index of the air; =1); that is, η_Fr=1.84 (by η_Fr=4/[2+(η_air/n_LED)+(n_LED/n_air)]. Therefore, if one material is provided for the LED and has a refractivity ranging between those of the air and the LED, η_Fr will be greatly improved. A method for solving this problem is to coat a resin film (n=1.5), whereby the light transmission can be thus promoted 16.2%. However, the absorption of light by the substrates still exists.

[0006] R.O.C. Pat. No. 466,784 and 474,034 mentioned methods for producing the AlGaInP LED and the AlGaAs LED. In these methods, the GaAs substrate can be removed after a transparent substrate, such as glass, is attached to the LED epitaxial layer by thermal pressing or adhesives, such as spin on glass, polyimide, silicon resin, etc. Unfortunately, the adhesives easily result in bubbles at the interface. Additionally, current spreading is not satisfying by such methods, unless the cladding layer adjacent to the transparent substrate is thick enough, or a GaP layer used for oxidation barrier is thicker than 3 μm, which certainly increases the cost.

[0007] Therefore, it is desirable to provide an improved LED structure to mitigate and/or obviate the aforementioned problems.

[0008] An object of the present invention is to provide a light emitting diode (LED), which exhibits higher emitting efficiency, lower resistance and better current spreading than a conventional LED.

[0009] Another object of the present invention is to provide a method for producing the above LED, which is suitable for bonding a transparent substrate to an LED epitaxial layer.

[0010] The LED of the present invention primarily includes a transparent window, an LED epitaxial layer including at least an active layer and defining a front surface and a back surface, and a transparent conductive film formed between the transparent window and the front surface of the LED epitaxial layer.

[0011] The method for producing the LED is to first provide a transparent window and an LED epitaxial layer, wherein the LED epitaxial layer includes an active layer and defines a front surface and a back surface. Next, a transparent conductive film is formed between the transparent window and the front surface of the LED epitaxial layer. When the wafers are bonded by thermal-pressure, atoms of the transparent conductive film can well diffuse to the window and the epitaxial layer, which greatly enhance the association process.

[0012] The transparent window can be made of glass, sapphire, Al₂O₃, ZnSe, ZnS, ZnSSe, SiC, GaP, GaAsP, etc.

[0013] The transparent conductive film can be made of metal oxides, nitrides or fluorides, for example, material containing ITO, In₂O₃, Sn₂O₃ or ZnO. The transparent conductive film can be formed on the transparent window and/or the LED epitaxial layer, and then the transparent window is bonded to the LED epitaxial layer by thermal-pressure. Since the transparent conductive film can easily form ohmic contact without energy barrier with an ohmic contact layer of the LED, this film can serve as current spreading layer of an electrode.

[0014] The active layer of the LED epitaxial layer is not restricted, for example, AlGaInP-, AlGaAs- and GaN-based materials. A first cladding layer and a second cladding layer are usually respectively formed on the back surface and the front surface of the active layer. Additionally, an ohmic contact layer and/or an oxidation barrier for preventing the second cladding layer from oxidizing can be further formed between the active layer and the transparent conductive film. The first cladding layer, the second cladding layer, the oxidation barrier and the ohmic contact layer can be made from suitable material and depend on the active layer.

[0015] A reflective layer and/or a heat-transferring set can also be provided on the back surface of the LED epitaxial layer. The LED epitaxial layer can be flipped and then bonded to the heat-transferring set.

[0016] More detailed description about the present invention will be present in the following embodiments accompanying with drawings.

[0025] Referring to FIG. 1, the first embodiment in accordance with the present invention includes a transparent window 10 made from glass, a transparent conductive film 20 made from ITO, a p-type ohmic contact layer 30, an LED epitaxial layer 40 formed on partial surface of the p-type ohmic contact layer 30, and a positive electrode 51 formed on other area of the p-type ohmic contact layer 30. The LED epitaxial layer 40 primarily includes an active layer 43 of undoped AlGaInP, wherein a front surface and a back surface are defined. Between the front surface and the transparent conductive film 20, a second cladding layer 42 of p-type AlGaInP and an oxidation barrier 41 of GaP for preventing the cladding layer 42 from oxidization are formed. On the back surface, a first cladding layer 44 of n-type AlGaInP is formed, on partial area of which the negative electrode 52 is formed.

[0026] The transparent conductive film 20 can be made from other materials than ITO. A thin film made from an oxide, a nitride or a fluoride, and having light transmission more than 80% within visible light range (wavelength 380–760 nm), and resistance lower than 1×10⁻³ Ω·cm can be applied. Examples of the oxides include In₂O₃ doping Sn (90–95% In₂O₃ and 10–5% SnO₂, represented as In₂O₃:Sn or ITO), SnO₂ doping Sb or F, and ZnO doping In, Ga (GZO) or Al(AZO). In this embodiment, ITO is used in view of its high transmission particularly in the middle range of visible light to which the human eyes are most sensitive, and low resistance (about 1.55×10⁻⁴ Ω·cm). Moreover, ITO possesses strong adhesion to the glass substrate, and good stability in electricity and chemistry.

[0027] The active layer 42 of this embodiment is undoped AlGaInP, and represented by (Al_x,Ga_1−x) _yIn_1−yP, wherein 0≦x≦1, 0≦y≦1, and preferably 0≦x≦0.5, 0.3≦y≦0.7. AlGaInP can be applied to high-brightness red, orange, yellow and yellow-green LED with double hetero-junction (DH) and quantum well (QW) structure. Mass production can be achieved by MOVPE. In general, AlGaAs is also suitable for the active layer 42 in the present invention. However, the red AlGaInP LED exhibits longer life in high temperature and high moisture than the red AlGaAs LED, so that the AlGaAs may be gradually replaced with AlGaInP.

[0028] FIG. 2 shows a cross section view of the second embodiment, in which a reflective layer 60 is coated on the first cladding layer 44. Therefore, the LED in this embodiment can exhibit higher brightness than that shown in FIG. 1, because all light beams emit from the front side of the LED.

[0029] FIG. 3 shows the third embodiment, which is similar to the second embodiment but the positive electrode 51 is moved to the GaP oxidation barrier 41 from the ohmic contact layer 30.

[0030] FIG. 4 shows the forth embodiment, which is similar to the second embodiment but a heat-transferring set 70 is adhered to the electrodes 51, 52 with wire-bonding glue 71 during wire binding. Accordingly, heat caused by electric power can be brought away, and therefore life of the device is elongated.

[0031] FIGS. 5 and 6 show the manufacturing process of the forth embodiment. First, on a GaAs substrate 80, the first cladding layer 44, the active layer 43, the second cladding layer 42, the oxidation barrier 41 and the ohmic contact layer 30 are sequentially deposited by epitaxy. Meanwhile, on the glass substrate 10, the ITO transparent conductive film 20 is deposited by sputtering, as shown in FIG. 5. Next, the transparent conductive film 20 is bonded to the ohmic contact layer 30 by thermal-pressure, and then the GaAs substrate 80 is removed by etching, as shown in FIG. 6. A part of the first cladding layer 44, the active layer 43, the second cladding layer 42 and the oxidation barrier 41 are then etched, whereby the positive and negative electrodes 51, 52 can be respectively formed on the ohmic contact layer 30 and the first cladding layer 44. The reflective layer 60 is coated on the first cladding layer 44, as shown in FIG. 2. At last, wires (not shown in figures) are bonded to the electrodes 51, 52 after flipping the chip, and the heat-transferring set 70 is adhered to the electrodes 51, 52 with the wire-bonding glue 71, and the LED as shown in FIG. 4 is eventually completed.

[0032] FIG. 7 shows a different way of the manufacturing process, in which the transparent conductive film 20′ is first formed on the p-type ohmic contact layer 30, and then bonded to the glass substrate 10. As for FIG. 8, the transparent conductive films 20, 20′ are respectively coated and deposited on the glass substrate 10 and the p-type ohmic contact layer 30, and then the transparent conductive films 20, 20′ are bonded.

[0033] FIG. 9 shows a vertical LED structure according to the present invention. In such structure, a p-type electrode 51′ is formed on an edge of the bonded transparent conductive substrate 10′, and the reflective layer 60 is added on the first cladding layer 44.

[0034] FIG. 10 shows another vertical LED structure according to the present invention. In such structure, a p-type electrode 51″ is formed on the central part of the bonded transparent conductive substrate 10′. An n-type electrode 52′ is formed on an edge of the first cladding layer 44. The reflective layer 60 is added on the conductive substrate 10′, so that light can be emitted from different direction.

[0035] According to the present invention, the transparent conductive film, such as ITO, takes the place of the known spin on glass, polyimide and silicon resin, whereby the transparent substrate, such as glass, can be more easily associated with the epitaxial layer. Consequently, the light-emitting efficiency can be easily improved by removing the opaque substrate, such as GaAs. Moreover, resistance of the LED is reduced since the ohmic contact layer and the transparent conductive film provide larger cross area for current flowing therethrough, which is quite important for the LED requiring larger current. In addition, a SiO₂ film can be coated on the glass substrate before the ITO film is formed, which may enhance adhesion of the transparent conductive film.

[0036] In the present invention, sputtering deposition is used to form the transparent conductive film because the uniform film with a large area and a precise thickness can be more easily obtained by controlling input current of target material and sputtering time. Furthermore, attractive strength between the substrate and the film is ten times larger than films formed by general vapor deposition. The substrate also can be remained at lower operation temperature since the sputtering particles possess high energy which drives the particles diffuse at the interface to build a rigid and dense film. The method for producing the LED of the present invention can utilize the present technologies of semiconductor manufacturing, such as epitaxy, vapor deposition, sputtering deposition, thermal pressing, etc., without developing a new skill.