Patent Publication Number: US-6984850-B2

Title: Semiconductor light-emitting diode

Description:
This is a continuation of application Ser. No. 09/373,544, filed Aug. 13, 1999. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a semiconductor light-emitting diode, and more specifically, to a semiconductor light-emitting diode having a current diffusion layer. 
   2. Description of the Related Art 
   An AlGaInP type material has drawn attention as a material to-be used for a light-emitting element which emits light having a wavelength in a range of 550 to 650 nm, since the AlGaInP type material has the largest bandgap of a direct transition type among III–V group compound semiconductor materials excluding a nitride. In particular, a pn-junction type light-emitting diode, in which a light-emitting structure (a layered structure including an active layer) made of an AlGaInP type material lattice-matching with GaAs is grown on a GaAs substrate, is capable of emitting light with higher luminance in a wavelength region corresponding to red to green light, as compared with a light-emitting diode provided with a light-emitting structure made of a material such as GaP or AlGaAs. 
   In order to form a light-emitting diode with high luminance, it is important to enhance a light-emission efficiency as well as a current injection efficiency into a light-emitting structure, and to allow light to efficiently emit from a device. 
   A conventional light-emitting diode having a light-emitting structure made of an AlGaInP type material will be described with reference to the drawings.  FIG. 8  is a cross-sectional view of such a light-emitting diode  200 . 
   As shown in  FIG. 8 , a light-emitting diode  200  has a structure in which on an n-type GaAs substrate  61 , an n-type GaAs buffer layer  62 , a light-emitting structure  69  made of an AlGaInP type material, and a p-type Al x Ga 1-x As current diffusion layer  66  are successively layered. The light-emitting structure  69  includes an n-type AlGaInP cladding layer  63 , a p-type AlGaInP cladding layer  65 , and an AlGaInP active layer  64  interposed between the cladding layers  63  and  65 . A p-type electrode  68  is provided on the top surface of the Al x Ga 1-x As current diffusion layer  66 , and an n-type electrode  67  is provided on the bottom surface of the substrate  61 . 
   A p-type Al x Ga 1-x As layer is often used as the current diffusion layer  66  in such a light-emitting diode  200  as described above for the following reason. 
   The p-type Al x Ga 1-x As layer is transparent to light having a wavelength of 550 to 650 nm which can be emitted by the light-emitting structure  69  made of a (Al x Ga 1-x ) y In 1-y P type semiconductor material, and therefore advantageous for obtaining a higher light-emission efficiency. Furthermore, the p-type Al x Ga 1-x As layer has a low resistivity, which makes it easy to obtain an ohmic contact with the p-type electrode  68  when employed as the current diffusion layer  66 . In addition, it is easy to grow a p-type Al x Ga 1-x As layer including crystal of a higher quality, as compared with an (Al x Ga 1-x ) y In 1-y P type semiconductor material. Thus, the p-type Al x Ga 1-x As layer can be relatively easily grown after the growth of a double hetero layer (the “DH layer”) made of an (Al x Ga 1-x ) y In 1-y P type, i.e., the light-emitting structure  69 . 
   Regarding the material to be used for the current diffusion layer  66 , comparisons between a conventional Al x Ga 1-x As type material and an (Al x Ga 1-x ) y In 1-y P type material will be explained below. Throughout the present specification, the term “Al mole fraction” refers to a mole fraction x of Al with respect to Ga (i.e., x=Al/(Al+Ga)). The term “In mole fraction” refers to a mole fraction 1−y of In with respect to Al and Ga (i.e., 1−y=In/(Al+Ga+In)). Moreover, the compositions of “(Al x Ga 1-x ) y In 1-y P” and “Al x Ga 1-x As” may be simply referred to as “AlGaInP” and “AlGaAs”, respectively. 
     FIG. 9  is a graph showing the relationship between the resistivity of an Al x Ga 1-x As current diffusion layer lattice-matching with a GaAs substrate and the Al mole fraction x thereof, and between the resistivity of an (Al x Ga 1-x ) 0.51 In 0.49 P current diffusion layer (i.e., 1−y=0.49) lattice-matching with the GaAs substrate and the Al mole fraction x thereof. 
   It is understood from  FIG. 9  that the Al x Ga 1-x As current diffusion layer exhibits a resistivity of about 0.06 Ω cm, for example, at an Al mole fraction x of 0.8. Thus, a low resistivity can be obtained even at a high Al mole fraction x. 
   In contrast, the (Al x Ga 1-x ) 0.05 In 0.49 P current diffusion layer exhibits a resistivity of about 0.15 to about 3 Ω cm at an Al mole fraction x in the range of 0 to 0.8. These values of resistivity are larger by one order of magnitude than those obtainable with the Al x Ga 1-x As layer. Even if the Al mole fraction is decreased, the resistivity is still higher by 50 times than that of the Al x Ga 1-x As layer. Accordingly, the (Al x Ga 1-x ) 0.51 In 0.49 P current diffusion layer is inferior to the Al x Ga 1-x As current diffusion layer, since a low resistivity cannot be obtained. 
   Furthermore, in order for the (Al x Ga 1-x ) 0.51 In 0.49 P current diffusion layer to allow light having a wavelength of 550 to 650 nm emitted from the light-emitting structure  69  to transmit therethrough, it is required to prescribe the Al mole fraction x to be 0.50 or more. In this case, the resistivity of the (Al x Ga 1-x ) 0.51 In 0.49 P current diffusion layer becomes higher by two orders of magnitude, as compared with that of the Al x Ga 1-x As current diffusion layer. 
   If the resistivity is high, the current diffusion ability of the current diffusion layer is decreased, and a current does not spread over the entire chip. As a result, light-emission from a portion of the light-emitting structure right below the electrode becomes dominant. The light emitted from such a portion is likely to be blocked by the electrode, whereby the emitted light is unlikely to be output. Accordingly, the increase in resistivity of the current diffusion layer causes a light-emission efficiency to decrease. Furthermore, the increase in resistivity of the current diffusion layer causes an operating voltage to increase. 
   Thus, the (Al x Ga 1-x ) 0.51 In 0.49 P current diffusion layer which is lattice-matched with GaAs has a higher resistivity than that of the Al x Ga 1-x As current diffusion layer, and consequently has adverse effects on the operational characteristics of a resultant light-emitting diode. Therefore, the Al x Ga 1-x As layer is typically employed as the current diffusion layer in the conventional art, instead of the AlGaInP type layer. 
   As described above, the Al x Ga 1-x As layer suffices as the current diffusion layer of a light-emitting diode, as far as the resistivity is concerned. In order for the Al x Ga 1-x As current diffusion layer to be transparent with respect to light having a wavelength of 550 to 650 nm, it is required to prescribe an Al mole fraction x thereof to be 0.65 or more. However, when the Al mole fraction x becomes high, the Al x Ga 1-x As layer will exhibit a deliquescence. Thus, in the case where a light-emitting diode having an Al x Ga 1-x As layer with a high Al mole fraction x is operated under the conditions of high temperature and high humidity, light intensity is likely to be remarkably decreased. 
     FIG. 10  shows changes in a chip light intensity (i.e., an intensity of light obtained from the semiconductor light-emitting diode chip) with a passage of time, in the case where a light-emitting diode chip having the Al x Ga 1-x As current diffusion layer is operated under the conditions of a temperature of 60° C. and a humidity of 95%. In  FIG. 10 , data for the chip light intensities are indicated as relative values. 
   As seen from  FIG. 10 , as an operating time becomes longer, a chip light intensity is decreased. Furthermore, as an Al mole fraction becomes larger, a chip light intensity is more remarkably decreased. 
   Such a deterioration of a light-emitting diode will be described with reference to  FIG. 11 .  FIG. 11  shows the light-emitting diode  200  previously described with reference to  FIG. 8 , but in a deteriorated condition. Since like components are designated with like reference numerals, the explanations thereof are omitted here. 
   As shown in  FIG. 11 , while operating the light-emitting diode  200  under the conditions of high temperature and high humidity, the surface of the AlGaAs current diffusion layer  66  with a high Al mole fraction tends to absorb moisture so as to be deliquescent, thereby resulting in black-colored portions  66   a  on the surface thereof. Such black-colored portions  66   a  on the surface of the current diffusion layer  66  absorb the light (represented by arrows in  FIG. 11 ) emitted from the inside of the light-emitting diode  200 . Thus, in the case where the AlGaAs layer with a high Al mole fraction is employed as the current diffusion layer, it is difficult to provide a light-emitting diode exhibiting stable luminance over a long period of time. 
   As described above, although the AlGaAs layer has been typically used as the current diffusion layer in the conventional semiconductor light-emitting diode for the reason that a low resistivity can be obtained, the AlGaAs layer is not reliable under the conditions of high temperature and high humidity. On the other hand, when the (Al x Ga 1-x ) 0.51 In 0.49 P layer capable of lattice-matching with the GaAs substrate typically used is employed as the current diffusion layer in place of the AlGaAs layer, the resultant current diffusion layer will then have a higher resistivity, so that sufficient luminance cannot be obtained. 
   SUMMARY OF THE INVENTION 
   A light-emitting diode of the present invention includes: a semiconductor substrate; and a layered structure, made of an AlGaInP type compound semiconductor material and provided on the semiconductor substrate. The layered structure includes: a light-emitting structure composed of a pair of cladding layers and an active layer for emitting light provided between the pair of cladding layers; and a current diffusion layer which is lattice-mismatched with the light-emitting structure, wherein a lattice mismatch Δ a/a of the current diffusion layer with respect to the light-emitting structure defined by the following expression is −1% or smaller:
 
Δ a/a= ( a   d   −a   e )/ a   e 
 
where a d  is a lattice constant of the current diffusion layer, and a e  is a lattice constant of the light-emitting structure.
 
   Crystal of the semiconductor substrate may be inclined by 8° (8 degrees) to 20° (20 degrees) in a [011] direction with respect to a (100) plane thereof. 
   Preferably, a composition of the current diffusion layer is selected in such a manner that the current diffusion layer becomes transparent with respect to a wavelength of light emitted from the light-emitting structure. 
   In one embodiment, a composition of the current diffusion layer is expressed as (Al x Ga 1-x ) y In 1-y P, and x is set in the range of 0.01 to 0.05 and 1×y is set in the range of 0.01 to 0.30 in the composition. 
   In one embodiment, a composition of the current diffusion layer is expressed as (Al x Ga 1-x ) y In 1-y P and at least one of a value of x and a value of 1−y in the composition varies along a thickness direction of the layered structure. 
   Both the values of x and 1−y in the composition of the current diffusion layer may vary, independent of each other. 
   In one embodiment, a composition of the current diffusion layer is expressed as (Al x Ga 1-x ) y In 1-y P and at least one of a value of x and a value of 1−y in the composition decreases in a step-like manner along a thickness direction of the layered structure from an interface with the light-emitting structure toward an opposite end of the current diffusion layer. 
   Both the values of x and 1−y in the composition of the current diffusion layer may decrease, independent of each other. 
   In one embodiment, a composition of the current diffusion layer is expressed as (Al x Ga 1-x ) y In 1-y P and at least one of a value of x and a value of 1−y in the composition varies in a step-like manner along a thickness direction of the layered structure from an interface with the light-emitting structure toward an opposite end of the current diffusion layer, thereby controlling a resistivity of the current diffusion layer in the thickness direction. 
   Both the values of x and 1−y in the composition of the current diffusion layer may vary, independent of each other. 
   Thus, the invention described herein makes possible the advantage of providing a semiconductor light-emitting diode which has a high luminance and a low resistivity with being transparent to light having a wavelength of 550 to 650 nm emitted from a light-emitting structure in the light-emitting diode, and which does not cause deterioration of light-emitting characteristics even under the conditions of high temperature and high humidity. 
   This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a diagram illustrating the relationship between the lattice constant a and the bandgap Eg for various AlGaInP type materials. 
       FIG. 2  shows graphs illustrating the relationship between the resistivity and the Al mole fraction x of AlGaInP type materials. 
       FIG. 3  shows graphs illustrating the relationship between the resistivity and the lattice mismatch of AlGaInP type materials. 
       FIG. 4  is a schematic cross-sectional view of a light-emitting diode in Embodiment 1 of the present invention. 
       FIG. 5  shows a graph illustrating changes in a chip light intensity of a light-emitting diode according to the present invention with a passage of time. 
       FIG. 6  shows a schematic cross-sectional view of a light-emitting diode in Embodiment 2 of the present invention. 
     FIG.  7 A(a) shows a graph illustrating values of the Al mole fraction x at various thickness positions of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer in Embodiment 2. 
     FIG.  7 A(b) shows a graph illustrating values of the In mole fraction 1−y at various thickness positions of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer in Embodiment 2. 
       FIG. 7B  shows a graph illustrating values of the resistivity at the various thickness positions of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer in Embodiment 2. 
       FIG. 8  shows a schematic cross-sectional view of a conventional light-emitting diode. 
       FIG. 9  shows a graph illustrating the relationship between the resistivity of AlGaAs type and AlGaInP type current diffusion layers, which are both lattice-matched with a GaAs substrate, and the Al mole fraction thereof. 
       FIG. 10  shows graphs illustrating changes in a chip light intensity of conventional light-emitting diodes with a passage of time. 
       FIG. 11  shows a schematic cross-sectional view of the conventional light-emitting diode of  FIG. 8  but in a deteriorated condition. 
       FIG. 12A  shows the [011] direction and the (100) plane relative to a substrate unit cell. 
       FIG. 12B  is a top view of the crystal substrate showing [011] direction and the (100) plane. 
       FIG. 12C  is a side view of a substrate having an 8° to 20° incline in the [011] direction. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The (Al x Ga 1-x ) y In 1-y P layer (x=0.50, 1−y=0.49), which lattice-matches with the substrate and is transparent with respect to a light-emission wavelength, has a higher resistivity than that of the Al x Ga 1-x As layer. One reason for this is that a P (phosphorus) group material contained in the AlGaInP layer has a lower mobility than that of an As (arsenic) group material contained in the AlGaAs layer, and hence, an effective mass of the AlGaInP layer is large. However, this does not have a significant effect. The more important reason is that the Al mole fraction x and the In mole fraction 1−y in the (Al x Ga 1-x ) y In 1-y P layer lattice-matching with the substrate are high (i.e., x=0.50, 1−y=0.49). 
   More specifically, if the mole fractions of Al and In of the (Al x Ga 1-x ) y In 1-y P layer become high, the AlGaInP layer becomes more likely to take in oxygen since Al and In are more likely to be oxidized than Ga. Furthermore, it is difficult to obtain an enhanced purity for Al and In, as compared with Ga. Therefore, when the Al and In mole fractions become high, the (Al x Ga 1-x ) y In 1-y P layer will contain a larger amount of impurities such as oxygen and silicon. As a result, the resistivity of the AlGaInP layer is likely to increase. 
   Thus, in order to decrease the resistivity of the AlGaInP layer, the Al and In mole fractions therein should be decreased. 
   However, when the In mole fraction of the AlGaInP layer (to be employed as the current diffusion layer) is decreased, the lattice constant thereof is varied, whereby the AlGaInP current diffusion layer becomes lattice-mismatched with the underlying light-emitting structure. Hereinafter, the relationship between the In mole fraction and the lattice mismatch will be described. 
   Herein, a lattice mismatch (a lattice mismatch ratio) Δ a/a of the current diffusion layer with respect to the light-emitting structure is defined by the following expression:
 
Δ a/a =( a   d   −a   e )/ a   e 
 
where a d  denotes a lattice constant of the current diffusion layer, and a e  denotes a lattice constant of the light-emitting structure.
 
   As an example, the case where a GaAs substrate is employed is described below. 
   The lattice constant of GaAs is about 5.65 Å. Since a pair of cladding layers and an active layer provided therebetween in the light-emitting structure are successively formed on the GaAs substrate, these layers lattice-match with the GaAs substrate as well as with each other. Thus, the lattice constant of the light-emitting structure is equal to that of the GaAs substrate. 
     FIG. 1  shows the relationship between the bandgap Eg and the lattice constant a for various AlGaInP type materials. 
   As shown in  FIG. 1 , when the In mole fraction is decreased in the AlGaInP type materials (so as to approach to a line connecting AlP and GaP in  FIG. 1 ), the lattice constant of the AlGaInP type materials gradually becomes smaller than the lattice constant of a GaAs substrate (i.e., that of the light-emitting structure). 
   The lattice constant of the (Al x Ga 1-x ) y In 1-y P current diffusion layer is determined by the In mole fraction 1−y thereof. The lattice mismatch between the (Al x Ga 1-x ) y In 1-y P current diffusion layer and the GaAs substrate becomes maximum in the case where the current diffusion layer contains substantially no In, which exhibits the lattice mismatch of about −4%. It is found that such a level of the lattice mismatching will not have a significant effect on a resistivity of a bulk material. 
   Although the above descriptions are related to the case where the GaAs substrate is employed, the similar effects can be obtained with any other appropriate substrates, such as a GaP substrate, an InP substrate and the like. In the case where there is no limit to a material for the substrate, the lattice mismatch between the (Al x Ga 1-x ) y In 1-y P current diffusion layer and the substrate may become about 8% at most with the variation of the In mole fraction. However, such a lattice mismatch will not have a significant effect on a resistivity of a bulk material. 
   As described above, although the lattice mismatching is generated between the (Al x Ga 1-x ) y In 1-y P current diffusion layer and the underlying light-emitting structure by decreasing the In mole fraction of the constituting material for the current diffusion layer, this will not have significant disadvantages on the characteristics of a resultant light-emitting diode. Thus, by decreasing the In mole fraction of the (Al x Ga 1-x ) y In 1-y P current diffusion layer to increase an absolute value of a lattice mismatch in the negative phase, the resistivity of the (Al x Ga 1-x ) y In 1-y P current diffusion layer can be reduced. Accordingly, by decreasing the In mole fraction 1−y as well as the Al mole fraction x of the (Al x Ga 1-x ) y In 1-y P current diffusion layer so that the current diffusion layer becomes intentionally lattice-mismatch with the light-emitting structure, the resistivity of the (Al x Ga 1-x ) y In 1-y P current diffusion layer can be prescribed at the same level as that of the conventional AlGaAs current diffusion layer. Thus, it becomes possible to form the satisfactory current diffusion layer even by using the (Al x Ga 1-x ) y In 1-y P layer. 
     FIG. 2  shows data of a resistivity in the case where the Al and In mole fractions in various (Al x Ga 1-x ) y In 1-y P type materials are decreased. 
   As is understood from  FIG. 2 , the resistivity of the (Al x Ga 1-x ) y In 1-y P layer can be remarkably decreased by decreasing the Al and In mole fractions thereof. In particular, the (Al x Ga 1-x ) y In 1-y P layer with the Al mole fraction x of 0.05 and the In mole fraction 1−y of 0.05 shows substantially the same resistivity as that of the AlGaAs current diffusion layer. 
   The relationship shown in  FIG. 2  will be further described in terms of the lattice mismatches of the current diffusion layer. In  FIG. 3 , the horizontal axis indicates the lattice mismatch Δ a/a of the (Al x Ga 1-x ) y In 1-y P current diffusion layer with respect to the substrate and the light-emitting structure, and the vertical axis indicates the resistivity of the (Al x Ga 1-x ) y In 1-y P current diffusion layer. 
   It is understood from  FIG. 3  that as the absolute value of the lattice mismatch becomes larger in the negative phase by decreasing the In mole fraction of the (Al x Ga 1-x ) y In 1-y P current diffusion layer, the resistivity of the (Al x Ga 1-x ) y In 1-y P current diffusion layer is also decreased. Since a practical level of resistivity of the current diffusion layer of a light-emitting diode is desirably about 0.1 Ω cm or less, it is understood from  FIG. 3  that the lattice mismatch between the (Al x Ga 1-x ) y In 1-y P current diffusion layer and the underlying light-emitting structure is preferably set to be about −1% or less. In order to realize the lattice mismatch in the above range, the In mole fraction 1−y of the (Al x Ga 1-x ) y In 1-y P layer is required to be about 0.35 or less. Furthermore, it is preferable that the Al mole fraction x of the (Al x Gal) y In 1-y P current diffusion layer is as low as possible. Accordingly, referring again to  FIG. 3 , it is more effective that the Al mole fraction x of the (Al x Ga 1-x ) y In 1-y P current diffusion layer is prescribed to be about 0.05 or less. 
   In the light-emitting diode of the present invention, the lattice mismatch of the AlGaInP type current diffusion layer is about −1% or less. Therefore, the AlGaInP type current diffusion layer has substantially the same resistivity as that of the conventional AlGaAs type current diffusion layer. Thus, an operating voltage and power consumption of the resultant light-emitting diode are not increased, and a high light output efficiency from the light-emitting structure is obtained. Furthermore, the light-emitting diode of the present invention has high luminance and is highly reliable. 
   In a preferred embodiment of the present invention, crystal of the substrate is inclined by about 8° (8 degrees) to about 20° (20 degrees) in a [011] direction with respect to a (100) plane (see  FIGS. 12A–12C ). Therefore, when an (Al x Ga 1-x ) y In 1-y P current diffusion layer with a thickness of about 5 to 10 μm which lattice-mismatches with a light-emitting structure is to be grown on a light-emitting structure, a hillock is not generated (which is otherwise generated due to lattice mismatching), so that a flat current diffusion layer can be obtained. 
   In a preferred embodiment of the present invention, the mole fraction of the (Al x Ga 1-x ) y In 1-y P current diffusion layer is selected in such a manner that the current diffusion layer becomes transparent with respect to a light-emission wavelength of the light-emitting structure. It is important that the current diffusion layer is transparent to a light-emission wavelength of the light-emitting structure and has a sufficiently low resistivity. Even in the case where there is lattice mismatching, as long as the above-mentioned two characteristics are satisfied, no disadvantages will arise. 
   Referring back to  FIG. 1 , a bandgap of AlGaInP type materials is increased by decreasing the In mole fraction thereof. Thus, a current diffusion layer which is transparent to light having a wavelength of about 550 nm to about 650 nm can be formed even without increasing the Al mole fraction thereof. 
   In a preferred embodiment of the present invention, the Al mole fraction x of the (Al x Ga 1-x ) y In 1-y P current diffusion layer is preferably set to be about 0.01 to about 0.05, while the In mole fraction 1−y thereof is preferably set to be about 0.01 to about 0.30. The AlGaInP current diffusion layer with such a small Al fraction is unlikely to be deliquescent. Thus, the light-emission characteristics do not deteriorate even when operated under the conditions of high temperature and high humidity, unlike the case where the conventional Al 0.65 Ga 0.35 As current diffusion layer is employed. 
   Thus, according to the present invention, a current diffusion layer capable of being practically used can be produced. 
   The present invention will be now described by way of illustrative embodiments with reference to the accompanying drawings. However, the present invention is not limited thereto. 
   Embodiment 1 
   A semiconductor light-emitting diode exemplified in Embodiment 1 of the present invention will be described below with reference to  FIG. 4 .  FIG. 4  is a cross-sectional view showing a structure of a light-emitting diode  100  in Embodiment 1. 
   As shown in  FIG. 4 , the light-emitting diode  100  includes an n-type GaAs substrate  1 , a layered structure  12 , an n-type electrode  7 , and a p-type electrode  8 . The layered structure  12  includes an n-type GaAs buffer layer  2 , an (Al x Ga 1-x ) 0.51 In 0.49 P light-emitting structure  11 , and a p-type (Al x Ga 1-x ) y In 1-y P current diffusion layer  10 . The (Al x Ga 1-x ) 0.51 In 0.49 P light-emitting structure  11  includes an n-type (Al x Ga 1-x ) 0.51 In 0.49 P lower cladding layer  3 , an (Al x Ga 1-x ) 0.51 In 0.49 P active layer  4 , and a p-type (Al x Ga 1-x ) 0.51 In 0.49 P upper cladding layer  5 . The p-type electrode  8  is provided on the top surface of the current diffusion layer  10 , and the n-type electrode  7  is provided on the bottom surface of the substrate  1 . 
   In the (Al x Ga 1-x ) 0.51 In 0.49 P light-emitting structure  11 , the mole fractions x in the lower cladding layer  3 , the active layer  4 , and the upper cladding layer  5  are about 1.0, about 0.3, and about 1.0, respectively. However, the mole fractions x are not limited to these values, and can independently have any value in a range of 0≦x≦1. 
   In the p-type (Al x Ga 1-x ) y In 1-y P current diffusion layer  10 , an Al mole fraction x is about 0.05, and an In mole fraction 1−y is about 0.05. 
   A method for producing the light-emitting diode  100  in Embodiment 1 will be described below. The mole fraction of each layer is as described above. 
   The buffer layer  2 , the lower cladding layer  3  (thickness: about 1.0 μm), the active layer  4  (thickness: about 0.5 μm), and the upper cladding layer  5  (thickness: about 1.0 μm) are successively formed on the substrate  1  by any known method in the art. The current diffusion layer  10  (thickness: about 7.0 μm) is then formed on the upper cladding layer  5  by any known method in the art. Then, an Au—Be film is formed by vapor deposition on the current diffusion layer  10 , and patterned to, for example, a circular shape to form the p-type electrode  8 . On the bottom surface of the GaAs substrate  1 , then-type electrode  7  (e.g., made of an Au—Zn film) is formed by vapor deposition. Thus, the light-emitting diode  100  is produced. 
   Any appropriate method known in the art can be used for forming each layer. The electrodes  7  and  8  can have any other appropriate shape and can be formed by any other appropriate method. 
   In the light-emitting diode  100  thus produced, the GaAs substrate  1 , the buffer layer  2 , the lower cladding layer  3 , the active layer  4 , and the upper cladding layer  5  are lattice-matched with each other. However, the current diffusion layer  10  lattice-mismatches with these layers. This is because the Al and In contents of the current diffusion layer  10  as described above are smaller than the values required for realizing the lattice-matching. The lattice mismatch of the current diffusion layer  10  with respect to the underlying light-emitting structure  11  and the substrate  1  is about −4% in the above-mentioned structure. Alternatively, the current diffusion layer  10  preferably has a lattice mismatch with respect to the underlying light-emitting structure  11  and the substrate  1  of about −1% or less, more preferably in the range of about −4% to about −3%. 
   Furthermore, the AlGaInP current diffusion layer  10  has a resistivity of about 0.1 Ω cm, which is similar to that of a conventional current diffusion layer made of AlGaAs. 
   As described above, the light-emitting diode  100  of the present invention is different from the conventional light-emitting diode  200  described with reference to  FIG. 8  in terms of the material to be used for forming the current diffusion layer  10 . 
   Specifically, in the conventional light-emitting diode  200 , the current diffusion layer  66  is made of an AlGaAs material. Therefore, when the light-emitting diode  200  is operated under the conditions of high temperature and high humidity, the surface of the current diffusion layer  66  becomes deliquescent to form the black-colored portions  66   a  (see  FIG. 8 ). As a result, a chip light intensity is deteriorated, whereby the reliability of the conventional light-emitting diode  200  is likely to decrease. 
   In contrast, in the light-emitting diode  100  of the present invention, the current diffusion layer  10  is made of a p-type (Al x Ga 1-x ) y In 1-y P material (e.g., x=0.05, 1−y=0.05). Thus, the Al mole fraction x of the current diffusion layer  10  is small. Accordingly, when the light-emitting diode  100  is operated under the conditions of high temperature and high humidity, the current diffusion layer  10  does not have deliquescence and does not become black. As a result, the light-emitting diode  100  of the present invention can be stably operated even with high reliability. 
     FIG. 5  shows, as reliability data of the light-emitting diode  100  of the present invention which has the above-mentioned structure, the chip light intensity with a passage of time. In  FIG. 5 , data for the chip light intensities are indicated as relative values. 
   It is understood from  FIG. 5  that a chip light intensity shows only a slight change (deterioration) during a relatively long period of time up to about 1000 hours under the conditions of a temperature of about 60° C., a humidity of about 95%, and an operating current of about 50 mA. 
   As described above, in Embodiment 1, the current diffusion layer  10  is made of an AlGaInP type material. Therefore, a light-emitting diode  100  is provided, which has high reliability over a long period of time even when operated under the conditions of high temperature and high humidity. 
   In Embodiment 1, by using the current diffusion layer  10  made of AlGaInP containing a small amount of Al and In as described above, the resistivity of the AlGaInP current diffusion layer  10  can be prescribed to be as low as that of the conventional current diffusion layer made of AlGaAs. Thus, a light-emitting diode  100  is provided, which has an AlGaInP current diffusion layer  10  provided with a long current diffusion distance from the upper electrode  8  and therefore with a high current diffusion ability. As a result, even when the current diffusion layer  10  is made of an AlGaInP type material, a light-emitting diode  100  having the similar luminance characteristics to those of a conventional light-emitting diode using an AlGaAs type material can be provided. 
   Furthermore, as the substrate  1  of the light-emitting diode  100  of the present invention, a substrate which is inclined, preferably, by about 8° (8 degrees) to about 20° (20 degrees) in a [011] direction with respect to a (100) plane may be used. Thus, even when the (Al x Ga 1-x ) y In 1-y P current diffusion layer  10  is a lattice-mismatching layer as described above, growth of the (Al x Ga 1-x ) y In 1-y P current diffusion layer  10  is prevented from starting from a step which functions as a growth nucleus in a certain orientation. Accordingly, even when a lattice-mismatching (Al x Ga 1-x ) y In 1-y P current diffusion layer  10  has a thickness of about 5 μm to about 10 μm, the flat layer can be grown. Therefore, a p-type electrode  8  of a high quality can be formed on the (Al x Ga 1-x ) y In 1-y P current diffusion layer  10  with good controllability. 
   Furthermore, the (Al x Ga 1-x ) y In 1-y P current diffusion layer  10  which is transparent to light having a wavelength of about 550 nm to 650 nm emitted from the light-emitting structure  11  can be obtained due to its small amount of In therein. 
   Embodiment 2 
   A semiconductor light-emitting diode exemplified in Embodiment 2 of the present invention will be described below with reference to  FIG. 6 .  FIG. 6  is a cross-sectional view showing a structure of a light-emitting diode  150  in Embodiment 2. 
   As shown in  FIG. 6 , the light-emitting diode  150  in Embodiment 2 includes an n-type GaAs substrate  1 , a layered structure  12 , an n-type electrode  7 , and a p-type electrode  8 . The layered structure  12  includes an n-type GaAs buffer layer  2 , an (Al x Ga 1-x ) 0.51 In 0.49 P light-emitting structure  11 , an n-type (Al x Ga 1-x ) y In 1-y P current blocking layer  9 , and a p-type (Al x Ga 1-x ) y In 1-y P current diffusion layer  106 . The light-emitting structure  11  includes an n-type (Al x Ga 1-x ) 0.51 In 0.49 P lower cladding layer  3 , an (Al x Ga 1-x ) 0.51 In 0.49 P active layer  4 , and a p-type (Al x Ga 1-x )  0.51 In 0.49 P upper cladding layer  5 . The p-type electrode  8  is provided on the top surface of the current diffusion layer  106 , and the n-type electrode  7  is provided on the bottom surface of the substrate  1 . 
   In the (Al x Ga 1-x ) 0.51 In 0.49 P light-emitting structure  11 , the mole fractions x in the lower cladding layer  3 , the active layer  4 , and the upper cladding layer  5  are about 1.0, about 0.3, and about 1.0, respectively. However, the mole fractions x are not limited to these values, and can independently have any value in a range of 0≦x≦1. 
   In the n-type (Al x Ga 1-x ) y In 1-y P current blocking layer  9 , an Al mole fraction x is about 0.30, and an In mole fraction 1−y is about 0.49. 
   Furthermore, in the p-type (Al x Gal) y In 1-y P current diffusion layer  106  of Embodiment 2, the mole fractions are varied along the thickness direction. Accordingly, the p-type (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  in the light-emitting diode  150  of the present embodiment is a graded layer. 
   FIGS.  7 A(a) and  7 A(b) show values of the Al mole fraction x and the In mole fraction 1−y at various positions along the thickness direction (i.e., at the various thickness positions) of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106 , respectively.  FIG. 7B  shows values of the resistivity at the various thickness positions of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106 . Herein, the thickness direction of the current diffusion layer  106  is shown in  FIG. 6 . Thus, in the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  in Embodiment 2, the In and Al mole fractions are gradually varied from the lower portion (i.e., from the interface with the light-emitting structure  11 ) toward the upper portion (i.e., toward the top surface thereof). 
   A method for producing the light-emitting diode  150  in Embodiment 2 will be described below. The mole fraction of each layer is as described above. 
   The buffer layer  2 , the lower cladding layer  3  (thickness: about 1.0 μm), the active layer  4  (thickness: about 0.5 μm), and the upper cladding layer  5  (thickness: about 1.0 μm) are successively formed on the substrate  1 , and the current blocking layer  9  (thickness: about 1.0 μm) is further formed on the upper cladding layer  5 , in a growth furnace by any known method in the art. After the substrate  1  having the grown layers thereon is taken out of the growth furnace, a part of the current blocking layer  9  is removed by etching so as to be patterned into a prescribed shape. Then, the substrate  1  with the resultant layered structure is again set in the growth furnace, and the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  (thickness: about 6 μm) is regrown while gradually varying the Al and In mole fractions thereof along its thickness direction (i.e., x=0.20 to 0.01, and 1−y=0.49 to 0.01, as illustrated in FIGS.  7 A(a) and  7 A(b), respectively). Thereafter, the n-type electrode  7  is formed on the bottom surface of the substrate  1 , and the p-type electrode  8  is formed on the graded current diffusion layer  106 . The p-type electrode  8  is then selectively etched away in such a manner that only the portion thereof right above the current blocking layer  9  remains. Thus, the light-emitting diode  150  is produced. 
   As described above, the light-emitting diode  150  in Embodiment 2 has the current blocking layer  9 . This is advantageous for the following reason. 
   Light emitted from a portion of the light-emitting structure  11  right below the p-type electrode  8  cannot be taken out since it is blocked by the electrode  8 . Thus, by providing the current blocking layer  9  in the lower portion of the graded current diffusion layer  106  so as to be positioned right below the p-type electrode  8 , a current to be injected into the light-emitting structure  11  is allowed to be effectively spread so as to not flow in the portion right below the p-type electrode  8 . Thus, the light-emission from the portion right below the p-type electrode  8  is prevented. Accordingly, an invalid current which otherwise flows into the portion right below the p-type electrode  8  is reduced, and a light-emission efficiency can be improved. 
   The graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  in Embodiment 2 will be further described with reference to  FIGS. 6 ,  7 A(a),  7 A(b), and  7 B. 
   In an initial growth stage of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106 , i.e., at a portion of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  in the vicinity of the light-emitting structure  11  and the current blocking layer  9  (in  FIG. 6 , in the vicinity of a position of about 0 in a thickness direction, i.e., in the vicinity of a thickness position of about 0), the Al mole fraction x and the In mole fraction 1−y are set to be about 0.20 and about 0.49, respectively as illustrated in FIGS.  7 A(a) and  7 A(b). On the other hand, referring again to  FIG. 1 , a lattice constant of (Al x Ga 1-x ) y In 1-y P is mainly influenced by an In mole fraction 1−y, not by an Al mole fraction x (or a Ga mole fraction complementary to Al mole fraction x). As described above, the In mole fraction 1−y of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  in the vicinity of the thickness position of 0 is substantially equal to the In mole fraction (about 0.49) of the light-emitting structure  11  and the current blocking layer  9 . Accordingly, the lattice constant of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  in an initial growth stage thereof is substantially equal to that of the light-emitting structure  11 , and thus, the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  can be grown to be flat. Furthermore, the Al mole fraction x of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  in the initial growth stage is set at a relatively high value, the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  in the vicinity of the thickness position of 0 (i.e., in the vicinity of the interface with the underlying light-emitting structure  11 ) is likely to match the composition of the underlying upper cladding layer  5 ; thus, satisfactory crystallinity can be obtained. 
   The Al and In mole fractions of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  are gradually decreased along the thickness direction toward the upper position thereof, whereby both of the Al and In mole fractions in the uppermost portion (at a thickness position  1  in  FIG. 6 ) of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  are prescribed to be 0.01. 
     FIG. 7B  shows the corresponding changes in resistivity of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106 . 
   It is understood from  FIG. 7B  that since the Al and In mole fractions are high in an initial growth stage, the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  shows a resistivity substantially the same as that of the light-emitting structure  11 . However, the Al and In mole fractions are decreased with the increase in the thickness, so that a resistivity is also decreased. It should be noted that in the light-emitting diode  150  provided with the current blocking layer  9 , a current is more likely to be spread throughout the whole chip through a portion of the current diffusion layer  106  which has a lower resistivity positioned closer to the p-type electrode  8 , whereby an operating voltage is less likely to be increased. On the other hand, even when a resistivity of the current diffusion layer  106  in a portion closer to the light-emitting structure  11  is relatively high there is no significant influence on, current diffusion capability and operating voltage. 
   As set forth above, in Embodiment 2, the portion of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  in the vicinity of the underlying light-emitting structure  11  has substantially the same In mole fraction as those of the light-emitting structure  11  and the current blocking layer  9 . Accordingly, the lattice constant of the portion of the graded current diffusion layer  106  in the vicinity of the light-emitting structure  11  becomes substantially equal to those of the light-emitting structure  11  and the current blocking layer  9 . This enhances crystallinity and flatness of a portion of the graded current diffusion layer  106  to be grown in an initial growth stage. 
   Furthermore, the interface of the graded current diffusion layer  106  with the light-emitting structure  11  and the current blocking layer  9  is in a satisfactory condition. Moreover, the lattice constant of the graded current diffusion layer  106  is not rapidly changed since the mole fraction thereof changes gradually. Therefore, the improved crystallinity and flatness can be obtained throughout the entire current diffusion layer  106 . 
   Furthermore, the Al and In mole fractions of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  are decreased as described above along the thickness direction toward the upper portion thereof. Therefore, the resistivity of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  is allowed to gradually decrease in the thickness direction while being kept uniform in a plane parallel to the top surface of the current diffusion layer  106 . Thus, the injected current can be uniformly spread between the electrodes  7  and  8 . 
   Accordingly, in Embodiment 2, the improved crystallinity and flatness of the (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  can be obtained without decreasing a light output efficiency or increasing an operating voltage. 
   Furthermore, in the case where the Al and In mole fractions are small (i.e., about 1% to about 5%) in an initial growth stage of the (Al x Ga 1-x ) y In 1-y P current diffusion layer, a notch (a discontinuous portion) in the energy band structure is likely to be generated at the interface between the current diffusion layer and the upper cladding layer due to a difference in the bandgap energy or the interface level. Such a notch causes an operating voltage and a drive voltage to increase. On the other hand, in Embodiment 2, both of the graded current diffusion layer  106  and the upper cladding layer  5  in the vicinity of the interface therebetween have substantially the same In mole fractions as each other. Therefore, no notch in the energy band structure is generated. Thus, an operating voltage and power consumption are not increased. 
   Due to the above-mentioned advantage, the light-emitting diode  150  in Embodiment 2 can realize light-emission luminance which is about 1.2 times that of the conventional light-emitting diode. 
   In Embodiment 2, the light-emitting diode  150  has been described, in which the Al mole fraction x and the In mole fraction 1−y in an initial growth stage of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  are about 0.20 and about 0.49, respectively. However, the present invention is not limited thereto. Even with the different values of the Al mole fraction x and the In mole fraction 1−y, the same effect as set forth above can be obtained as long as the mole fractions of the graded current diffusion layer  106  are varied in the thickness direction. 
   Moreover, in Embodiment 2, the Al and In mole fractions of the graded (Al x Ga 1-x ) y In 1-y P current diffusion layer  106  are gradually varied along the thickness direction. However, even when the mole fractions may be varied in a step-like manner into two, three, or more steps, the same effect as set forth above can be obtained. 
   As described above, the light-emitting diode of the present invention is provided with the current diffusion layer made of an (Al x Ga 1-x ) y In 1-y P material, which is intentionally lattice-mismatched with the underlying light-emitting structure by appropriately selecting the Al mole fraction x and the In mole fraction 1−y in the composition thereof. Thus, a light-emitting diode whose light intensity is not deteriorated even under the conditions of high temperature and high humidity can be provided without decreasing a resistivity of the current diffusion layer. Furthermore, a light output efficiency from the light-emitting structure is enhanced, whereby a highly reliable light-emitting diode can be provided. 
   Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.