Source: http://www.google.com/patents/US7842966?dq=6011510
Timestamp: 2016-12-05 02:50:15
Document Index: 29945399

Matched Legal Cases: ['§111', '§119', 'Application No. 60', 'Application No. 2005', '§111', 'art 12']

A compound semiconductor light-emitting diode includes a light-emitting layer (133) formed of aluminum-gallium-indium phosphide, a light-emitting part (13) having component layers individually formed of a Group III-V compound semiconductor, a transparent supporting layer (14) bonded to one of the outermost surface layers (135) of the light-emitting part (13) and transparent to the light emitted from the light-emitting layer (133), and a bonding layer (141) formed between the supporting layer (14) and the one of the outermost surface layers (135)of the light-emitting part (13) containing oxygen atoms at a concentration of 1×1020 cm−3 or less.
a bonding layer formed between the supporting layer and the outermost surface layer of the light-emitting part and containing oxygen atoms at a concentration of 1 ×1020 cm −3 or less,
2. A compound semiconductor light-emitting diode according to claim 1, wherein the bonding layer formed between the supporting layer and the outermost surface layer of the light-emitting part contains carbon atoms at a concentration of 1 ×1020 cm−3 or less.
a bonding layer formed between the supporting layer and the outermost surface layer of the light-emitting part and containing oxygen atoms at a concentration of 1 ×1020 cm−3 or less,
This application is an application filed under 35 U.S.C. §111(a) claiming the benefit pursuant to 35 U.S.C. §119(e)(1) of the filing dates of Provisional Application No. 60/700,346 filed Jul. 19, 2005 and Japanese Patent Application No. 2005-197009 filed Jul. 6, 2005 pursuant to 35 U.S.C. §111(b).
(1) A method for directly bonding the supporting layer to a semiconductor layer while applying pressure thereto at an elevated temperature of several hundred degrees (refer to Japanese Patent No. 3230638). (2) A method for effecting bonding by a means called wafer bonding (refer to JP-A HEI 6-302857). (3) A method utilizing a transparent adhesive substance, such as epoxy resin (refer to JP-A 2002-246640). (4) A method for bonding a semiconductor layer and the transparent supporting layer through a transparent electrically conductive thin film, such as of an indium-tin complex oxide (ITO) (refer to Japanese Patent No. 2588849). (5) A method comprising the steps of mirror-polishing both a semiconductor layer and a supporting layer, bonding the two layers after removal of defiling matter and heat-treating the bonded layers (refer to JP-A 2001-57441). The technical means of (1) which attempts to bond a transparent supporting layer directly to the surface of a semiconductor, however, necessitates elevation of temperature to a high level of 600° C. or more and application of pressure as well (“Semiconductors and Semimetals,” Vol. 48, edited and written by G. B. Stringfellow and M. George Craford (published in 1997 by Academic Press (U.S.A.)), refer to pp. 196-206). An attempt to bond a transparent supporting layer under such conditions of high temperature and high pressure leads to inducing a disadvantage in easily inflicting a crystal defect on the supporting layer because stress is exerted, for example, on a Group III-V compound semiconductor layer which possesses brittleness. When the surface of the Group III-V compound semiconductor layer to be bonded, for example, is not flat, the pressure is unevenly applied to the layer, with the result that the bonded layers will frequently form such union as is inferior in quality and deficient in strength. Further, the disadvantage of the conventional means of bonding under the conditions of high temperature and high pressure consists in the fact that an effort to bond a supporting layer which reveals difference in thermal expansion coefficient results in inducing a warp due to mechanical stress and eventually entailing occurrence of crystal defects in a large amount in the interface of union.
With a view to accomplishing the object mentioned above, the first aspect of the invention is directed to a compound semiconductor light-emitting diode comprising a light-emitting layer formed of aluminum-gallium-indium phosphide ((AlXGa1-X)YIn1-YP wherein 0≦X≦1 and 0 <Y≦1), component layers individually having a light-emitting part formed of a Group III-V compound semiconductor, a transparent supporting layer bonded to one of the outermost surface layers of the light-emitting part and transparent to the light emitted from the light-emitting layer, and a bonding layer formed between the supporting layer and the one of the outermost surface layers of the light-emitting part and containing oxygen atoms at a concentration of 1×1020 cm−3 or less.
In the second aspect of the invention, besides fulfilling the configuration of the first aspect of the invention, the bonding layer formed between the supporting layer and one of the outermost surface layers of the light-emitting part contains carbon atoms at a concentration of 1×1020 cm−3 or less.
In the twelfth aspect of the invention, besides fulfilling the configuration of any one of the eighth to eleventh aspects of the invention, the bonding step is performed at room temperature or more and 100° C. or less.
According to this invention, since the concentration of oxygen atoms in the bonding layer formed between the supporting layer and one of the outermost surface layers of the light-emitting part is set at 1×1020 cm−3 or less, the supporting layer and the one outermost surface layer of the light-emitting part can be bonded fast. Further, the introduction of a crystal defect into the light-emitting part can be suppressed and consequently the uncalled-for addition to the electric resistance in the direction of flow of the device operating current can be avoided. As a result, a compound semiconductor light-emitting diode that shows a low forward voltage (Vf) and a small leak current via crystal defect as well and abounds in reverse voltage, for example, can be configured.
According to this invention, since the bonding in the bonding step is carried out at room temperature or more and 100° C. or less, the transparent supporting layer can be directly bonded to the light-emitting part without entailing uncalled-for impartation of distortion and a compound semiconductor light-emitting diode of high luminance can be fabricated stably.
In the compound semiconductor light-emitting diode comprising a light-emitting layer formed of aluminum-gallium-indium phosphide ((AlXGa1-X)YIn1-YP wherein 0≦X≦1, 0≦Y≦1), component layers individually having a light-emitting part formed of a Group III-V compound semiconductor and a transparent supporting layer bonded to one of the outermost surface layers of the light-emitting part and transparent to the light emitted from the light-emitting layer, this invention contemplates setting the concentration of oxygen atoms in a bonding layer formed between the supporting layer and one of the outermost surface layers of the light-emitting part at 1×1020 cm−3 or less and the concentration of carbon atoms in the bonding layer at 1×1020 cm−3 or less.
The bonding strength between the outermost surface layer of the light-emitting part and the transparent supporting layer destined to be bonded to the surface thereof depends very conspicuously on the concentration of oxygen in the bonding layer. The adhesive strength decreases in accordance as the concentration of oxygen atoms in the bonding layer increases. In order that the individual devices being separated by cutting may withstand the impact of chipping without inducing separation from the bonding layer, the concentration of oxygen atoms in the bonding layer is preferably set at 1×1020 atoms/cm3 or less. The treatment of the surface of the outermost surface layer of the light-emitting part that is performed by the method described above manifests the effect of stably lowering the concentration of oxygen in the bonding layer to below the concentration of atoms. If the oxygen atoms are present in the bonding layer in a large amount exceeding the concentration of 1×1020 atoms/cm3, the bonding strength will decrease conspicuously. For this reason, the separation of individual devices by cutting, for example, will be at a disadvantage in suffering the transparent supporting layer to peel off the outermost surface layer of the light-emitting part, preventing the devices from being normally fabricated, and so forth.
Further, when the transparent supporting layer of GaP crystal material, for example, which has undergone the wet or dry surface treatment for the removal of an oxide film is to be bonded in a high degree of vacuum to the surface of the outermost surface layer of the light-emitting part which has undergone the same surface treatment for the removal of an oxide film, the atomic concentrations of oxygen in their bonding layers can be decreased to below 1×1020 atoms/cm3 with still better reproducibility. Alternatively mentioned, the outermost surface layer of the light-emitting part and the supporting layer together form a bonding layer of great bonding strength. Further, by performing the bonding in a vacuum, it becomes possible for the necessity of an environment of very high cleanliness needed in effecting the bonding in air to be obviated and the fabrication to be accomplished stably and inexpensively.
Particularly, it is enabled to form a strong bond by having the concentration of carbon atoms in the bonding layer set at 1×1020 atoms/cm3 or less. Further, when the outermost surface layer of the light-emitting part and the transparent supporting layer are bonded in a vacuum of high degree of 1×10−4 Pa or less, for example, a bond possessing high bonding strength and having the concentration of carbon atoms in the bonding layer set at 1×1019 atoms/cm3 or less is obtained. By decreasing the atomic concentrations of oxygen and carbon in the bonding layer, it is made possible not only to form a strong bond but also to suppress the introduction of a crystal defect in the bonding layer and consequently avoid uncalled-for aggravation of electric resistance in the direction of flow of the electric current for operating the device (device operating current). As a result, a compound semiconductor light-emitting diode (LED) exhibiting a low forward voltage (Vf) and a small leak current via a crystal defect and abounding in reverse voltage can be configured. The atomic concentrations of oxygen and carbon can be determined by an analytic means, such as the Secondary Ion Mass Spectrometry (SIMS) or the Auger Electron Spectroscopy (AES).
The transparent supporting layer or the outermost surface layer of the light-emitting part is subjected to the bonding in vacuum of 1×10−2 Pa or less and preferably 1×10−3 Pa or less in pressure. A particularly strong bond can be formed by mutually bonding the two flat surfaces that have been polished as described above. It is important that the two surfaces, prior to being mutually bonded, be activated by being individually irradiated with an atom beam or ion beam possessing energy of 50 eV or more. The term “activation” refers to the creation of surfaces in a clean state by depriving the two surfaces being bonded of an impurity layer or a polluting layer containing an oxide film, carbon, etc. and existing on the surfaces. By performing this irradiation on the surface of either the transparent supporting layer or the component layers of the light-emitting part, the two surfaces are strongly bonded infallibly. By performing the irradiation on both the surfaces, they can be bonded with still greater strength.
When the surfaces of the transparent supporting layer and the outermost surface layer of the light-emitting part are stacked as opposed to each other and are consequently readied for bonding, the measure adopted for enabling applied mechanical pressure to cover wholly the surfaces being bonded proves convenient for strongly bonding both the surfaces. To be specific, pressure of 5 g·cm−2 or more and 100 g·cm−2 or less is applied in a perpendicular direction to the surfaces being bonded. This method manifests the effect of fulfilling the bonding with uniform strength even when the transparent supporting layer or the outermost surface layer of the light-emitting part or both happen to be warped.
The transparent supporting layer and the light-emitting part are bonded in a vacuum of the preferred degree mentioned above while the temperature of the surface of the supporting layer or the light-emitting part or both is set at 100° C. or less, preferably at 50° C. or less, and more preferably at room temperature. When the bonding is performed in an environment of high temperature exceeding about 500° C., the light-emitting layer formed of (AlXGa1-X)YIn1-YP (0≦X≦1, 0<Y≦1) and provided for the light-emitting part is thermally denatured. This proves to be inconvenient for stably obtaining a compound semiconductor LED capable of emitting a light of an expected wavelength.
An LED (LED chip) 10 shown in FIG. 1 and FIG. 2 was fabricated by using an epitaxially stacked structure 101 provided with semiconductor layers 13 sequentially stacked on a semiconductor substrate 11 formed of an Si-doped n-type GaAs single crystal possessing a surface inclined by 15° from the (100) plane and a p-type GaP substrate (transparent supporting layer) 14 bonded thereto as shown in FIG. 4.
In the present example, the individual semiconductor layers 130 to 135 constituted an epitaxial wafer formed as stacked on the GaAs substrate 11 by the low-pressure MetalOrganic Chemical Vapor Deposition method (MOCVD method) using trimethyl aluminum ((CH3)3Al), trimethyl gallium ((CH3)3Ga) and trimethyl indium ((CH3)3In) as raw materials for Group III component elements. As the raw material for the Mg doping, biscyclopentadiethyl magnesium (bis-(C5H5)2Mg) was used. As the raw material for the Te doping, dimethyl tellurium ((CH3)2Te) was used. Then, as the raw material for a Group V component element, phosphine (PH3) or arsine (AsH3) was used. The GaP layer 135 was grown at 750° C. and the other semiconductor layers 130 to 134 forming the semiconductor layer 13 were grown at 730° C.
The GaAs buffer layer 130 had a carrier concentration of about 5×1018 cm−3 and a layer thickness of about 0.2 μm. The contact layer 131 was formed of (Al0.5GaO.5)0.5In0.5P and had a carrier concentration of about 2×1018 cm−3 and a layer thickness of about 1.5 μm. The n-clad layer 132 had a carrier concentration of about 8×1017 cm−3 and a layer thickness of about 1 μm. The light-emitting layer 133 was not doped and had a layer thickness of 0.8 μm. The p-clad layer 134 had a carrier concentration of about 2×1017 cm3 and a layer thickness of 1 μm. The GaP layer 135 had a carrier concentration of about 3×1018 cm−3 and a layer thickness of 9 μm. The p-type GaP layer 135 on the outermost surface layer of the light-emitting part 12 had a region reaching a depth of about 1 μm from the first surface polished and subjected to mirror finishing. By the mirror finishing, the surface of the p-type GaP layer 135 was given roughness of 0.18 nm.
The p-type GaP substrate 14 (FIG. 4) was prepared as a transparent supporting layer to be affixed to the mirror polished surface of the p-type GaP 135. The GaP substrate 14 ready to be affixed had Zn added thereto so as to acquire a carrier concentration of about 2×1018 cm−3. A single crystal having a plane direction of 15° off (100) was used. The GaP substrate 14 to be affixed had a diameter of 50 mm and a thickness of 250 μm. This GaP substrate 14, prior to being bonded to the p-type GaP layer 135, had the surface thereof mirror-polished and finished till 0.12 mn in the rms value.
The GaP substrate 14 and the epitaxially stacked structure 100 were carried into an ordinary semiconductor material affixing device, and the semiconductor material affixing device was then evacuated to a vacuum of 3×10−5 Pa. Thereafter, in the semiconductor material affixing device which had excluded members made of carbon materials with a view to avoiding pollution with carbon, the GaP substrate 14 and the epitaxially stacked structure 100 mounted therein were kept heated to a temperature of about 800° C. in the vacuum while the surface of the GaP substrate 14 was irradiated with Ar ions accelerated to an energy of 800 eV. Consequently, a bonding layer 141 having a nonstoichiometric composition was formed on the surfaces of the GaP substrate 14 and the epitaxially stacked structure 100. Subsequent to the formation of the bonding layer, the radiation of Ar ions was discontinued and the temperature of the GaP substrate 14 was lowered to room temperature.
Then, the surfaces of both the GaP substrate 14 possessing in the surface region the bonding layer 141 made of a nonstoichiometric composition and the GaP layer 135 were irradiated over a period of 3 minutes with an Ar beam neutralized by bombardment with electrons. Thereafter, in the semiconductor material-affixing device maintained at a vacuum, the surfaces of both the layers 135 and 14 were overlapped as illustrated in FIG. 4 and severally loaded with a pressure till 20 g/cm2, and mutually bonded at room temperature. When a wafer resulting from the bonding was removed from the vacuum chamber of the semiconductor material affixing device and the interface of bonding in the wafer was analyzed, the bonding layer 141 formed of a nonstoichiometric composition of Ga0.6P0.4 was detected in the bonded parts. The bonding layer 141 had a thickness of about 3 nm, the concentration of oxygen atoms in the bonding layer 141 determined by the ordinary method of SIMS analysis was 7×1018 cm−3, and the concentration of carbon atoms was 9×1018 cm−3.
The back surface of the affixed GaP substrate 14 that lay opposite the surface of bonding was clad by the ordinary method of vacuum deposition with a multi-layered film composed of a gold-beryllium (AuBe) alloy film having a thickness of 0.2 μm and an Au film having a thickness of 0.8 μm. Then, the multi-layered film was processed by the ordinary means of photolithography to form patterns, and the patterns of multi-layered film measuring 50 μm in diameter and assuming a circular shape in the plan view were regularly arrayed like lattice points separated by a distance of 150 μm. Subsequently, these circular patterns of multi-layered film were subjected to a heat treatment performed at 450° C. for 10 minutes so as to be alloyed and transformed into a p-type ohmic electrode 16 of low contact resistance.
An AlGaInP LED chip was fabricated by affixing a p-type GaP substrate having no bonding layer formed on the surface thereof to a p-type GaP layer under the conditions different from those of the example cited above. In Comparative Example 1, while the p-type GaP substrate was maintained in an atmosphere of nitrogen at a temperature of 800° C. and kept continuously over one hour under such a load as to produce pressure of 200 g/cm2 on the bonding surface, the surfaces of the p-type GaP substrate and the p-type GaP layer were bonded at an elevated temperature under the atmospheric pressure. In the bonding performed under the conditions described in Comparative Example 1, the presence of a large amount of crystal defects was detected in the bonding interface between the p-type GaP substrate and the p-type GaP layer.
The concentration of oxygen atoms in the interface region was 2.0×1020 cm−3 and the concentration of carbon atoms therein was 1.1×1020 cm−3, both being high concentrations. Further, the shear strength was as low as 180 g. Consequently, separation of bond occurred in the region equivalent to about 10% of the surface area of the bonding surface during the dicing step performed for the formation of chips.
An AlGaInP LED chip was fabricated by affixing a p-type GaP substrate having no bonding layer formed on the surface thereof to a p-type GaP layer under the conditions different from those of the example and Comparative Example 1 cited above. In Comparative Example 2, the p-type GaP layer bonded to the p-type GaP substrate had a layer thickness of 0.3 μm. This p-type GaP layer, after having the surface thereof rinsed with hydrofluoric acid (HF), was bonded to the p-type GaP substrate and they were together subjected to a heat treatment at 500° C. so as to finish the bonding. The concentration of oxygen atoms in the bonding interface was 3×1020 cm−3 and the concentration of carbon atoms therein was 2×1020 cm−3, both being high concentrations. The shear strength was as low as 100 g. Consequently, separation of the bonding surface occurred in the region equivalent to about 40% of the surface area of the bonding surface during the dicing step performed for the formation of chips.
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