Patent Publication Number: US-2011062466-A1

Title: AlxGa(1-x)As Substrate, Epitaxial Wafer for Infrared LEDs, Infrared LED, Method of Manufacturing AlxGa(1-x)As Substrate, Method of Manufacturing Epitaxial Wafer for Infrared LEDs, and Method of Manufacturing Infrared LEDs

Description:
TECHNICAL FIELD 
     The present invention relates to Al x Ga (1-x) As substrates, to epitaxial wafers for infrared LEDs, to infrared LEDs, to methods of manufacturing Al x Ga (1-x) As substrates, to methods of manufacturing epitaxial wafers for infrared LEDs, and to methods of manufacturing infrared LEDs. 
     BACKGROUND ART 
     LEDs (light-emitting diodes) exploiting the compound semiconductor Al x Ga (1-x) As (0≦x≦1)—hereinafter also termed “AlGaAs,” (aluminum gallium arsenide)—are widely employed as infrared light sources. Infrared LEDs are used as infrared light sources in such applications as optical communications and wireless transmission, and in these applications the stepping-up of transmitted data volume and enabling of longer-range transmission distances that are taking place have led to demands for improved output power from infrared LEDs. 
     An example of a method of manufacturing such infrared LEDs is disclosed in Japanese Unexamined Pat. App. Pub. No. 2002-335008 (Patent Reference 1). The implementation of the following process steps is set forth in this patent reference. Namely, to begin with an Al x Ga (1-x) As support substrate is formed onto a GaAs (gallium arsenide) substrate by liquid-phase epitaxy (LPE). At that point, the amount fraction of Al (aluminum) in the Al x Ga (1-x) As support substrate is approximately uniform. Subsequently, epitaxial layers are formed by organometallic vapor-phase epitaxy (OMVPE) or molecular beam epitaxy (MBE). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Pat. App. Pub. No. 2002-335008 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the above-noted Patent Reference 1, the amount fraction of Al in the Al x Ga (1-x) As support substrate is for the most part uniform. As a result of dedicated research efforts, the present inventors discovered a problem with instances in which the Al amount fraction is high, in that the properties of infrared LEDs manufactured employing such Al x Ga (1-x) As support substrates deteriorate. As a further result of their dedicated research efforts, the present inventors also discovered a problem with instances in which the Al amount fraction is low, in that the transmissivity of such Al x Ga (1-x) As support substrates is poor. 
     Therein, an object of the present invention is to make available Al x Ga (1-x) As substrates, epitaxial wafers for infrared LEDs, infrared LEDs, methods of manufacturing Al x Ga (1-x) As substrates, methods of manufacturing epitaxial wafers for infrared LEDs, and methods of manufacturing infrared LEDs, whereby a high level of transmissivity is maintained, and through which, in the fabrication of semiconductor devices, the devices prove to have superior characteristics. 
     Solution to Problem 
     As a result of their especially focused research efforts, the present inventors not only found that the properties of infrared LEDs manufactured employing the Al x Ga (1-x) As support substrates are compromised when the Al amount fraction is high, but they also discovered the cause of the problem. Namely, aluminum has a propensity to oxidize readily, on account of which an oxide layer is liable to form on the surface of an Al x Ga (1-x) As substrate. Since the oxide layer impairs epitaxial layers grown onto the Al x Ga (1-x) As substrate, it proves to be a causative factor whereby defects are introduced into the epitaxial layers. The problem with defects introduced into epitaxial layers is that they are deleterious to the properties infrared LEDs comprising the epitaxial layers. 
     Meanwhile, the present inventor&#39;s research efforts also led them to discover that the transmissivity of Al x Ga (1-x) As substrates worsens the lower is the substrates&#39; amount fraction of Al. 
     Therein, an Al x Ga (1-x) As substrate of the present invention is an Al x Ga (1-x) As substrate furnished with an Al x Ga (1-x) As layer (0≦x≦1) having a major surface and, on the reverse side from the major surface, a rear face, and is characterized in that in the Al x Ga (1-x) As layer, the amount fraction x of Al in the rear face is greater than the amount fraction x of Al in the major surface. 
     In the just-described Al x Ga (1-x) As substrate, the Al x Ga (1-x) As layer preferably contains a plurality of laminae, and the amount fraction x of Al in each of the plural laminae monotonically decreases heading from the plane of the layer&#39;s rear-face side to the plane of its major-surface side. 
     For the just-described Al x Ga (1-x) As substrate, a GaAs substrate preferably is further furnished, contacting the rear face of the Al x Ga (1-x) As layer. 
     An epitaxial wafer of the present invention for infrared LEDs is furnished with an Al x Ga (1-x) As substrate as set forth in any of the foregoing descriptions, and an epitaxial layer formed onto the major surface of the Al x Ga (1-x) As layer, and including an active layer. 
     In the infrared-LED epitaxial wafer just described, preferably the amount fraction x of Al in the plane of epitaxial layer contact with the Al x Ga (1-x) As layer is greater than the amount fraction x of Al in the plane of Al x Ga (1-x) As layer contact with the epitaxial layer. 
     With an aforementioned epitaxial wafer for infrared LEDs, preferably the epitaxial wafer is utilized in an infrared LED whose emission wavelength is 900 nm or greater, and the well layers within the active layer include a material containing indium (In), with the well layers being four or fewer laminae. 
     In the infrared-LED epitaxial wafer described above, preferably the well layers are of InGaAs in which the amount fraction of indium is 0.05 or more. 
     With an aforementioned epitaxial wafer for infrared LEDs, preferably the epitaxial wafer is utilized in an infrared LED whose emission wavelength is 900 nm or greater, and the barrier layers within the active layer include a material containing phosphorous (P), with the barrier layers being three or more laminae. 
     In the infrared-LED epitaxial wafer described above, preferably the barrier layers are of either GaAsP or AlGaAsP in which the amount fraction of phosphorous is 0.05 or more. 
     An infrared LED of the present invention is furnished with: an Al x Ga (1-x) As substrate set forth in any of the foregoing descriptions; an epitaxial layer; a first electrode; and a second electrode. The epitaxial layer is formed onto the major surface of the Al x Ga (1-x) As layer, and includes an active layer. The first electrode is formed on the surface of the epitaxial layer. The second electrode is formed on the rear face of the Al x Ga (1-x) As layer. In Al x Ga (1-x) As substrates of a form furnished with a GaAs substrate, the second electrode may be formed on the rear face of the GaAs substrate. 
     An Al x Ga (1-x) As substrate manufacturing method of the present invention is provided with a step of preparing a GaAs substrate, and a step of growing, by liquid-phase epitaxy, onto the GaAs substrate an Al x Ga (1-x) As layer (0≦x≦1) having a major surface. Then, in the step of growing a Al x Ga (1-x) As layer, the Al x Ga (1-x) As layer is grown with the amount fraction x of Al in the interface between the layer and the GaAs substrate being greater than the amount fraction x of Al in the major surface. 
     With the Al x Ga (1-x) As substrate manufacturing method described above, in the Al x Ga (1-x) As layer growing step, preferably the Al x Ga (1-x) As layer is grown containing a plurality of laminae in which the amount fraction x of Al monotonically decreases heading from the plane along the layer&#39;s interface with the GaAs substrate to the plane of the layer&#39;s major-surface side. 
     With the Al x Ga (1-x) As substrate manufacturing method described above, preferably a step of removing the GaAs substrate is further provided. 
     A method of the present invention of manufacturing an epitaxial wafer for infrared LEDs is provided with: a step of manufacturing an Al x Ga (1-x) As substrate by an Al x Ga (1-x) As substrate manufacturing method set forth in any of the foregoing descriptions; and a step of forming onto the major surface of the Al x Ga (1-x) As layer, by at least organometallic vapor-phase epitaxy or molecular beam epitaxy, or else by a combination of the two techniques, an epitaxial layer containing an active layer. 
     With the infrared-LED epitaxial wafer manufacturing method described above, preferably the amount fraction x of Al in the plane of epitaxial layer contact with the Al x Ga (1-x) As layer is greater than the amount fraction x of Al in the plane of Al x Ga (1-x) As layer contact with the epitaxial layer. 
     With the aforementioned method of manufacturing an epitaxial wafer for infrared LEDs, preferably the method is of manufacturing an epitaxial wafer utilized in infrared LEDs whose emission wavelength is 900 nm or more, and the well layers within the active layer include a material that contains indium (In), with the number of well layers being four laminae or fewer. 
     With the infrared-LED epitaxial wafer manufacturing method described above, preferably the well layers are of InGaAs whose amount fraction of indium is 0.05 or more. 
     With the aforementioned method of manufacturing an epitaxial wafer for infrared LEDs, preferably the method is of manufacturing an epitaxial wafer utilized in infrared LEDs whose emission wavelength is 900 nm or more, and the barrier layers within the active layer include a material that contains phosphorous (P), with the number of barrier layers being three laminae or more. 
     With the infrared-LED epitaxial wafer manufacturing method described above, preferably the barrier layers are of either GaAsP or AlGaAsP whose amount fraction of phosphorous is 0.05 or more. 
     An infrared-LED manufacturing method of the present invention is provided with: a step of manufacturing an Al x Ga (1-x) As substrate by an Al x Ga (1-x) As substrate manufacturing method as set forth in any of the foregoing descriptions; a step of forming onto the major surface of the Al x Ga (1-x) As layer, by either organometallic vapor-phase epitaxy or molecular beam epitaxy, an epitaxial layer containing an active layer; a step of forming a first electrode on the surface of the epitaxial layer; and a step of forming a second electrode on either the rear face of the Al x Ga (1-x) As layer, or the rear face of the GaAs substrate (in Al x Ga (1-x) As substrates of a form furnished with a GaAs substrate). 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     In accordance with Al x Ga (1-x) As substrates, epitaxial wafers for infrared LEDs, infrared LEDs, methods of manufacturing Al x Ga (1-x) As substrates, methods of manufacturing epitaxial wafers for infrared LEDs, and methods of manufacturing infrared LEDs of the present invention, a high level of transmissivity is maintained, and in the fabrication of semiconductor devices, lead to devices having superior characteristics. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional diagram illustratively outlining an Al x Ga (1-x) As substrate in Embodying Mode 1 of the present invention. 
         FIG. 2  is a chart for explaining the amount fraction x of Al in an Al x Ga (1-x) As layer in Embodying Mode 1 of the present invention. 
         FIG. 3  is a chart for explaining the amount fraction x of Al in an Al x Ga (1-x) As layer in Embodying Mode 1 of the present invention. 
         FIG. 4  is a chart for explaining the amount fraction x of Al in an Al x Ga (1-x) As layer in Embodying Mode 1 of the present invention. 
         FIGS. 5  (A) through (G) are charts for explaining the amount fraction x of Al in an Al x Ga (1-x) As layer in Embodying Mode 1 of the present invention. 
         FIG. 6  is a flowchart representing a method of manufacturing an Al x Ga (1-x) As substrate in Embodying Mode 1 of the present invention. 
         FIG. 7  is a sectional diagram illustratively outlining a GaAs substrate in Embodying Mode 1 of the present invention. 
         FIG. 8  is a sectional diagram illustratively outlining an as-grown Al x Ga (1-x) As layer in Embodying Mode 1 of the present invention. 
         FIGS. 9  (A) through (C) are charts for explaining the effect, in Embodying Mode 1 of the present invention, of furnishing an Al x Ga (1-x) As layer with a plurality of lamina in which the amount fraction x of Al monotonically decreases. 
         FIG. 10  is a sectional diagram illustratively outlining an Al x Ga (1-x) As substrate in Embodying Mode 2 of the present invention. 
         FIG. 11  is a flowchart representing a method of manufacturing an Al x Ga (1-x) As substrate in Embodying Mode 2 of the present invention. 
         FIG. 12  is a sectional diagram illustratively outlining an infrared-LED epitaxial wafer in Embodying Mode 3 of the present invention. 
         FIG. 13  is a sectional diagram that is an enlargement of the region XIII in  FIG. 12 . 
         FIG. 14  is a flowchart representing a method of manufacturing an infrared-LED epitaxial wafer in Embodying Mode 3 of the present invention. 
         FIG. 15  is a sectional diagram illustratively outlining an infrared-LED epitaxial wafer in Embodying Mode 4 of the present invention. 
         FIG. 16  is a flowchart representing a method of manufacturing an epitaxial wafer in Embodying Mode 4 of the present invention. 
         FIG. 17  is a sectional diagram illustratively outlining an infrared-LED epitaxial wafer in Embodying Mode 5 of the present invention. 
         FIG. 18  is a sectional diagram illustratively outlining an infrared LED in Embodying Mode 6 of the present invention. 
         FIG. 19  is a flowchart representing a method of manufacturing an infrared LED in Embodying Mode 6 of the present invention. 
         FIG. 20  is a sectional diagram illustratively outlining an infrared LED in Embodying Mode 7 of the present invention. 
         FIG. 21  is a graph plotting transmissivity versus amount fraction x of Al in Al x Ga (1-x) As layers of Embodiment 1. 
         FIG. 22  is a graph plotting surface oxygen quantity versus amount fraction x of Al in Al x Ga (1-x) As layers of Embodiment 1. 
         FIG. 23  is a sectional diagram illustratively outlining an infrared-LED epitaxial wafer in Embodiment 3. 
         FIG. 24  is a chart diagramming light output, in Embodiment 3, from an infrared-LED epitaxial wafer furnished with an active layer having multiquantum-well structures, and from an epitaxial wafer for double-heterostructure infrared LEDs. 
         FIG. 25  is a sectional diagram illustratively outlining an infrared-LED epitaxial wafer in Embodiment 4. 
         FIG. 26  is a chart diagramming the relationship between window-layer thickness and light output power in Embodiment 4. 
         FIG. 27  is a sectional diagram illustratively outlining an infrared LED in a modified example of Embodying Mode 7 of the present invention. 
         FIG. 28  is a chart plotting results of measuring the emission wavelength from an infrared LED in Embodiment 6. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, an explanation based on the drawings will be made of modes of embodying the present invention. 
     Embodying Mode 1 
     To begin with, referring to  FIG. 1 , an explanation of an Al x Ga (1-x) As substrate in the present embodying mode will be made. 
     As represented in  FIG. 1 , an Al x Ga (1-x) As substrate  10   a  is furnished with a GaAs substrate  13 , and an Al x Ga (1-x) As layer  11  formed onto the GaAs substrate  13 . 
     The GaAs substrate  13  has a major surface  13   a , and a rear face  13   b  on the reverse side from the major surface  13   a . The Al x Ga (1-x) As layer  11  has a major surface  11   a , and a rear face  11   b  on the reverse side from the major surface  11   a.    
     The GaAs substrate  13  may or may not be misoriented—for example, the substrate may have a major surface  13   a  that is a {100} plane, or that is tilted more than 0° but 15.8° or less from a {100} plane. It is preferable that the GaAs substrate  13  have a major surface  13   a  that is a {100} plane, or that is tilted more than 0° but 2° or less from a {100} plane. It is further preferable that the GaAs substrate  13  have a surface that is a {100} plane, or that is tilted more than 0° but 0.2° or less from a {100} plane. The GaAs substrate  13  surface may be a specular surface, or may be a rough surface. (It will be understood that the braces “{ }” indicate a family of planes.) 
     The Al x Ga (1-x) As layer  11  has a major surface  11   a  and, on the reverse side from the major surface  11   a , a rear face  11   b . The major surface  11   a  is the surface on the reverse side from the surface that contacts the GaAs substrate  13 . The rear face  11   b  is the surface that contacts the GaAs substrate  13 . 
     The Al x Ga (1-x) As layer  11  is formed so as to contact on the major surface  13   a  of the GaAs substrate  13 . Put differently, the GaAs substrate  13  is formed as to contact on the rear face  11   b  of the Al x Ga (1-x) As layer  11 . 
     In the Al x Ga (1-x) As layer  11 , the amount fraction x of Al in the rear face  11   b  is greater than the amount fraction x of Al in the major surface  11   a . It should be understood that the amount fraction x is the mole fraction of Al, while the amount fraction (1−x) is the mole fraction of Ga. 
     Therein, the mole fraction in the Al x Ga (1-x) As layer  11  will be explained with reference to  FIGS. 2 through 5 . 
     In  FIGS. 2 through 5 , the vertical axis indicates position thickness-wise traversing from the rear face to the major surface of the Al x Ga (1-x) As layer  11 , while the horizontal axis represents the Al amount fraction x in each position. 
     As shown in  FIG. 2 , with the Al x Ga (1-x) As layer  11 , traversing from the rear face  11   b  to the major surface  11   a , the amount fraction x of Al monotonically decreases. “Monotonically decreases” means that heading from the rear face  11   b  to the major surface  11   a  of the Al x Ga (1-x) As layer  11  (heading in the growth direction), the amount fraction x is constantly the same or decreasing, and that, compared with the rear face  11   b , the major surface  11   a  is where the amount fraction x is lower. 
     Put differently, “monotonically decreases” would not include a section in which the amount fraction x increases heading in the growth direction. 
     As indicated in  FIGS. 3 through 5 , the Al x Ga (1-x) As layer  11  may include a plurality of laminae (in  FIGS. 3 through 5 , it includes two laminae). With the Al x Ga (1-x) As layer  11  represented in  FIG. 3 , traversing in each lamina from the rear face  11   b  side to the major surface  11   a  side, the amount fraction x of Al monotonically decreases. Meanwhile, with the Al x Ga (1-x) As layer  11  represented in  FIG. 4 , the amount fraction x of Al is uniform in each lamina, but the amount fraction x of Al in the lamina along the rear face  11   b  is greater than in that along the major surface  11   a . On the other hand, the amount fraction x of Al in the lamina along the rear face  11   b  of the Al x Ga (1-x) As layer  11  represented in  FIG. 5(A)  is uniform, while the amount fraction x of Al in the lamina along the major surface  11   a  monotonically decreases, with the Al amount fraction x in the lamina along the rear face  11   b  being greater than in that along the major surface  11   a . In sum, with the Al x Ga (1-x) As layers  11  represented in  FIGS. 4 and 5(A) , as a whole, the amount fraction x of Al monotonically decreases. 
     It should be understood that the amount fraction x of Al in the Al x Ga (1-x) As layer  11  is not limited to the foregoing, and the composition may be as indicated for example in  FIGS. 5(B)  through (G), or further may be a separate example. Also, the Al x Ga (1-x) As layer  11  is not limited to the above-described implementations containing one lamina or two laminae, but may contain three or more laminae, as long as the amount fraction x of Al in the rear face  11   b  is greater than the amount fraction x of Al in the major surface  11   a.    
     When the Al x Ga (1-x) As substrate  10   a  is utilized in an LED, the Al x Ga (1-x) As layer  11  assumes, for example, the role of a window layer that diffuses current and that transmits light from the active layer. 
     To continue: With reference to  FIG. 6 , an explanation of a method of manufacturing an Al x Ga (1-x) As substrate in the present embodying mode will be made. 
     As indicated in  FIGS. 6 and 7 , initially a GaAs substrate  13  is prepared (Step S 1 ). 
     The GaAs substrate  13  may or may not be misoriented—for example, the substrate may have a major surface  13   a  that is a {100} plane, or that is tilted more than 0° but 15.8° or less from a {100} plane. It is preferable that the GaAs substrate  13  have a major surface  13   a  that is a {100} plane, or that is tilted more than 0° but 2° or less from a {100} plane. It is further preferable that the GaAs substrate  13  have a major surface  13   a  that is a {100} plane, or that is tilted more than 0° but 0.2° or less from a {100} plane. 
     As indicated in  FIGS. 6 and 8 , next an Al x Ga (1-x) As layer (0≦x≦1)  11  having a major surface  11   a  is grown by liquid-phase epitaxy onto the GaAs substrate  13  (Step S 2 ). By Step S 2  of growing the Al x Ga (1-x) As layer  11 , an Al x Ga (1-x) As layer  11  in which the amount fraction x of Al in the layer&#39;s interface with the GaAs substrate  13  (the rear face  11   b ) is greater than the amount fraction x of Al in the major surface  11   a  is grown. 
     The liquid-phase epitaxy is not particularly limited; a slow-cooling or temperature-profile technique can be employed. It should be understood that “liquid-phase epitaxy” refers to a technique of growing the Al x Ga (1-x) As (0≦x≦1) crystal from the liquid phase. A “slow-cooling” technique is a method of gradually lowering the temperature of a source-material solution to grow the Al x Ga (1-x) As crystal. A “temperature-profile” technique refers to a method of setting up a temperature gradient in a source-material solution to grow the Al x Ga (1-x) As crystal. 
     When a lamina in which the amount fraction x of Al in the Al x Ga (1-x) As layer  11  is fixed is to be grown, temperature-profile and slow-cooling techniques are preferably utilized, while when a lamina in which the amount fraction x of Al decreases heading upward (in the growth direction) is to be grown, slow-cooling is preferably utilized. Utilizing slow cooling, owing to its superior volume produciblity and low cost, is particularly preferable. These techniques also may be combined. 
     With LPE, since a chemical equilibrium between the liquid and solid phases is exploited, the growth rate is rapid. On that account, an Al x Ga (1-x) As layer  11  of considerable thickness may be readily formed. Specifically, an Al x Ga (1-x) As layer  11  having a height H 11  preferably of from 10 μm to 1000 μm, more preferably from 20 μm to 140 μm is grown. (In so doing, the height H 11  is the minimum thickness along the Al x Ga (1-x) As layer  11  thickness-wise.) 
     A further preferable condition is that the ratio of the height H 11  of the Al x Ga (1-x) As layer  11  to the height H 13  of the GaAs substrate  13  (H 11 /H 13 ) be from 0.1 to 0.5, more preferably from 0.3 to 0.5. This conditional factor makes it possible to mitigate the incidence of warp in the Al x Ga (1-x) As layer  11  having been grown onto the GaAs substrate  13 . 
     Furthermore, the Al x Ga (1-x) As layer  11  may be grown so as to incorporate p-type dopants such as zinc (Zn), magnesium (Mg) and carbon (C), and n-type dopants such as selenium (Se), sulfur (S) and tellurium (Te), for example. 
     In this way growing an Al x Ga (1-x) As layer  11  by LPE produces a jaggedness in the major surface  11   a  of the Al x Ga (1-x) As layer  11 , as indicated in  FIG. 8 . 
     Next, the major surface  11   a  of the Al x Ga (1-x) As layer  11  is washed (Step S 3 ). In Step S 3 , washing is preferably done using an alkali solution. However, an oxidizing solution such as phosphoric acid or sulfuric acid may also be employed. The alkali solution preferably contains ammonia and hydrogen peroxide. Washing the major surface  11   a  with an alkali solution containing ammonia and hydrogen peroxide etches the surface, which by the surface being in contact with air allows impurities clinging to the major surface  11   a  to be removed. By controlling the process so that, for example, with an etching rate of 0.2 μm/min or less, not more than 0.2 μm is etched from the major surface  11   a  side, impurities on the major surface  11   a  are reduced and at the same time the extent of etching will be slight. It should be noted that Step S 3  of washing the major surface  11   a  may be omitted. 
     Next, the GaAs substrate  13  and the Al x Ga (1-x) As layer  11  are dried with alcohol. This step of drying may be omitted, however. 
     Next, the major surface  11   a  of the Al x Ga (1-x) As layer  11  is polished (Step S 4 ). The method of polishing is not particularly limited; mechanical polishing, chemical-mechanical polishing, electrolytic polishing, or chemical polishing techniques may be employed, while in terms of polishing ease, mechanical polishing or chemical polishing are preferable. 
     The major surface  11   a  is polished so that the RMS roughness of the major surface  11   a  will be, for example, 0.05 nm or less. The RMS surface roughness is preferably minimal. Here, “RMS roughness” signifies a surface&#39;s mean-square roughness, as defined by JIS B 0 601—that is, the square root of the averaged value of the squares of the distance (deviation) from an averaging plane to a measuring plane. It should be noted that this polishing Step S 4  may be omitted. 
     Next, the major surface  11   a  of the Al x Ga (1-x) As layer  11  is washed (Step S 5 ). Inasmuch as this Step  5  of washing the major surface  11   a  is the same as Step  3  of washing the major surface  11   a  prior to implementing polishing Step  4 , explanation of the step will not be repeated. It should be noted that this washing Step S 5  may be omitted. 
     Next, the GaAs substrate  13  and the Al x Ga (1-x) As layer  11  are, by utilizing the Al x Ga (1-x) As substrate  10   a , thermally cleaned in an H 2  (hydrogen) and AsH 3  (arsine) flow prior to epitaxial growth. It should be understood that this thermal cleaning step may be omitted. 
     Implementing the foregoing Steps S 1  through S 5  enables the manufacture of an Al x Ga (1-x) As substrate  10   a  in the present embodying mode, represented in  FIG. 1 . 
     As described in the foregoing, an Al x Ga (1-x) As substrate  10   a  in the present embodying mode is an Al x Ga (1-x) As substrate  10   a  furnished with an Al x Ga (1-x) As layer  11  having a major surface  11   a  and, on the reverse side from the major surface  11   a , a rear face  11   b , and is characterized in that in the Al x Ga (1-x) As layer  11 , the amount fraction x of Al in the rear face  11   b  is greater than the amount fraction x of Al in the major surface  11   a . Then further to this constitution a GaAs substrate  13  is provided, contacting the rear face  11   b  of the Al x Ga (1-x) As layer  11 . 
     In addition, a method of manufacturing an Al x Ga (1-x) As substrate  10   a  in the present embodying mode is provided with a step (Step S 1 ) of preparing a GaAs substrate  13 , and a step (Step S 2 ) of growing, by liquid-phase epitaxy, an Al x Ga (1-x) As layer  11  having a major surface  11   a  onto the GaAs substrate  13 . The method is characterized in that in the step of growing the Al x Ga (1-x) As layer  11  (Step S 2 ), an Al x Ga (1-x) As layer  11  is grown in which the amount fraction x of Al in the interface between the layer and the GaAs substrate  13  (in the rear face  11   b ) is greater than the amount fraction x of Al in the major surface  11   a.    
     According to an Al x Ga (1-x) As substrate  10   a  and a method of manufacturing an Al x Ga (1-x) As substrate  10   a  in the present embodying mode, the amount fraction x of Al in the rear face  11   b  is greater than the amount fraction x of Al in the major surface  11   a . Therefore, the presence of aluminum, which has a propensity to oxidize, on the surface of the major surface  11   a  is kept to a minimum. And therefore, the formation of an oxide layer, which would act as an insulator, on the surface of the Al x Ga (1-x) As substrate  10   a  (the major surface  11   a  of the Al x Ga (1-x) As layer  11  in the present embodying mode) can be restrained. 
     Especially since the Al x Ga (1-x) As layer  11  is grown by LPE, oxygen is unlikely to be taken into the layer-internal region, apart from the major surface  11   a . Accordingly, when epitaxial layers are grown onto the Al x Ga (1-x) As substrate  10   a , defects can be kept from being introduced into the epitaxial layers. The characteristics of an infrared LED furnished with the epitaxial layers can be improved as a result. 
     As has been noted, the Al amount fraction x in the major surface  11   a  is less than the Al amount fraction x in the rear face  11   b . The present inventor&#39;s intensive research efforts led them to discover that the greater the amount fraction x of Al is, the better will the transmissivity of the Al x Ga (1-x) As substrate  10   a  be. And even if the layer contains much aluminum along the rear face  11   b , because the period of time it is exposed on the surface is short, formation of any oxide layer is minimized. Consequently, growing Al x Ga (1-x) As crystal of higher Al amount fraction x onto an area where oxide-layer formation is minimized allows the transmissivity to be improved. 
     In this way, in the Al x Ga (1-x) As layer  11 , the amount fraction x of Al is made lower along the major surface  11   a  so as to improve the device characteristics, while the amount fraction x of Al along the rear face  11   b  is made higher so as to improve the transmissivity. Hence, an Al x Ga (1-x) As substrate  10   a  can be realized whereby a high level of transparency is maintained, and with which, when devices fabricated, the devices prove to have superior characteristics. 
     In the Al x Ga (1-x) As substrate  10   a  described above, preferably, as indicated in  FIG. 3 , the Al x Ga (1-x) As layer  11  contains a plurality of laminae, and the Al amount fraction x in each lamina monotonically decreases heading from the plane of the rear face  11   b  side to the plane of the major surface  11   a  side. 
     In the Al x Ga (1-x) As substrate  10   a  manufacturing method described above, preferably, in the step of growing the Al x Ga (1-x) As layer  11  (Step S 2 ), preferably an Al x Ga (1-x) As layer  11  is grown that contains a plurality of laminae in which the amount fraction x of Al monotonically decreases heading from the plane along the layer&#39;s interface with the GaAs substrate  13  (from the rear face  11   b ) to the plane of the layer&#39;s major-surface  11   a  side. 
     The present inventors discovered that thus constituting an Al x Ga (1-x) As substrate  10   a  makes it possible to mitigate warp occurring in the substrate. Below, with reference to  FIGS. 9(A)  through (C), an explanation will be made of the reasons this is so.  FIG. 9(A)  represents an instance, as indicated in  FIG. 2 , where the laminar section in which the Al amount fraction x in the Al x Ga (1-x) As layer  11  monotonically decreases is a single lamina.  FIG. 9(B)  represents an instance where in the Al x Ga (1-x) As layer  11  the laminar section in which the Al amount fraction x monotonically decreases as indicated in  FIG. 3  is two laminae.  FIG. 9(C)  represents an instance where the laminar section in which the Al amount fraction x monotonically decreases in the Al x Ga (1-x) As layer  11  is three laminae. In  FIGS. 9(A)  through (C) the horizontal axis indicates position thickness-wise traversing from the rear face  11   b  to the major surface  11   a  of the Al x Ga (1-x) As layer  11 , while the vertical axis represents the Al amount fraction x in each position in the Al x Ga (1-x) As layer  11 . With the Al x Ga (1-x) As layers  11  represented in  FIGS. 9(A)  through (C), the amount fraction x of Al in the rear faces  11   b  and in the major surfaces  11   a  are the same. 
     In  FIGS. 9(A)  through (C), imaginary triangles are formed by the highest position (Point A) along the diagonal y representing the amount fraction x of Al, when the point is extended downward, and the lowest position (Point B) along the diagonal y, when the point is extended leftward, and the extensions meet in a point of intersection (Point C). The total surface area of these triangles is the stress that is applied to the Al x Ga (1-x) As layer  11 . Warp occurs in the Al x Ga (1-x) As layer  11  on account of this stress. 
     The present inventors discovered that warp in the Al x Ga (1-x) As layer  11  is more likely to appear the greater is the separation z between the geometric center G of the triangles, and the center along the thickness of the Al x Ga (1-x) As layer  11 . The geometric center G is, in the instance illustrated in  FIG. 9(A) , the geometric center G of the triangle formed based on the diagonal y, while in the instances illustrated in  FIGS. 9(B) and 9(C) , it is the center along a line joining the geometric centers G 1  through G 3  of triangles formed based on the diagonals y. The geometric center G is where the resultant force of the stresses within the Al x Ga (1-x) As layer  11  added together acts. 
     As indicated in  FIGS. 9(A)  through (C), the more the number of laminae in which the amount fraction x of Al monotonically decreases, the shorter becomes the separation z from the center along the thickness to the thickness point where the geometric center G is located, and thus the less warp occurs in the Al x Ga (1-x) As layer  11 . Therefore, forming a plurality of laminae in which the amount fraction x of Al monotonically decreases mitigates warp in a Al x Ga (1-x) As substrate  10   a . Herein, with the several triangles in the figures, the maximum and minimum values of the amount fraction x of Al, and the thickness of the Al x Ga (1-x) As layer  11  are the same, but they do not necessarily have to be the same: They are adjustable depending on such factors as the transmissivity, warp, and state of the interfaces. 
     Embodying Mode 2 
     Referring to  FIG. 10 , an explanation of an Al x Ga (1-x) As substrate  10   b  in the present embodying mode will be made. 
     As represented in  FIG. 10 , an Al x Ga (1-x) As substrate  10   b  in the present embodying mode is basically furnished with the same structural makeup as an Al x Ga (1-x) As substrate  10   a  of Embodying Mode 1, but differs in that it is not furnished with a GaAs substrate  13 . 
     Specifically, the Al x Ga (1-x) As substrate  10   b  is furnished with an Al x Ga (1-x) As layer  11  having a major surface  11   a  and, on the reverse side from the major surface  11   a , a rear face  11   b . Then in the Al x Ga (1-x) As layer  11 , the amount fraction x of Al in the rear face  11   b  is greater than the amount fraction x of Al in the major surface  11   a.    
     It is preferable that the thickness of an Al x Ga (1-x) As layer  11  in the present embodying mode be thick enough for the Al x Ga (1-x) As substrate  10   b  to be a freestanding substrate. Such thickness H 11  is, for example, 70 μm or more. 
     To continue: With reference to  FIG. 11 , an explanation of a method of manufacturing an Al x Ga (1-x) As substrate  10   b  in the present embodying mode will be made. 
     As indicated in  FIG. 11 , initially, in the same manner as in Embodying Mode 1, Step S 1  of preparing a GaAs substrate  13 , Step S 2  of growing an Al x Ga (1-x) As layer  11  by liquid-phase epitaxy, washing Step S 3 , and polishing Step S 4  are implemented. An Al x Ga (1-x) As substrate  10   a  as represented in  FIG. 1  is thereby manufactured. 
     Next, the GaAs substrate  13  is removed (Step S 6 ). For the removal method, a technique such as polishing or etching, for example, can be employed. “Polishing” refers to employing a polishing agent such as alumina, colloidal silica, or diamond in grinding equipment fitted with diamond grinding wheels, to mechanically abrade away the GaAs substrate  13 . “Etching” refers to carrying out GaAs substrate  13  removal employing an etchant selected by optimally mixing, for example, ammonia, hydrogen peroxide, etc. to have a slow etching rate on Al x Ga (1-x) As, but a fast etching rate on GaAs. 
     Next, washing Step S 5  is implemented in the same manner as in Embodying Mode 1. 
     Implementing the foregoing Steps S 1 , S 2 , S 3 , S 4 , S 6 , and S 5  makes it possible to manufacture an Al x Ga (1-x) As substrate  10   b  as represented in  FIG. 10 . 
     It should be understood that apart from the foregoing, the Al x Ga (1-x) As substrate  10   b  and its method of manufacture are otherwise of the same constitution as the Al x Ga (1-x) As substrate  10   a  and its method of manufacture in Embodying Mode 1; thus identical components are labeled with identical reference marks, and their explanation will not be repeated. 
     As described in the foregoing, the Al x Ga (1-x) As substrate  10   b  in the present embodying mode is an Al x Ga (1-x) As substrate  10   b  furnished with an Al x Ga (1-x) As layer  11  having a major surface  11   a  and, on the reverse side from the major surface  11   a , a rear face  11   b , and is characterized in that in the Al x Ga (1-x) As layer  11 , the amount fraction x of Al in the rear face  11   b  is greater than the amount fraction x of Al in the major surface  11   a.    
     In addition, a method of manufacturing an Al x Ga (1-x) As substrate  10   b  in the present embodying mode is provided with a step (Step S 6 ) of removing the GaAs substrate  13 . 
     According to an Al x Ga (1-x) As substrate  10   b  and a method of manufacturing an Al x Ga (1-x) As substrate  10   b  in the present embodying mode, an Al x Ga (1-x) As substrate  10   b  furnished only with an Al x Ga (1-x) As layer  11 , but not furnished with a GaAs substrate  13 , may be realized. Since the GaAs substrate  13  absorbs light of 900 nm or less wavelength, growing epitaxial layers onto an Al x Ga (1-x) As substrate  10   b  from which the GaAs substrate  13  has been removed enables the manufacture of epitaxial wafers for infrared LEDs. Employing such infrared-LED epitaxial wafers to manufacture infrared LEDs enables the realization of infrared LEDs in which a high level of transparency is maintained, and which have superior device characteristics. 
     Embodying Mode 3 
     Referring to  FIG. 12 , an explanation of an epitaxial wafer  20   a  in the present embodying mode will be made. 
     As indicated in  FIG. 12 , the epitaxial wafer  20   a  is furnished with an Al x Ga (1-x) As substrate  10   a , represented in  FIG. 1 , of Embodying Mode 1, and, formed onto the major surface  11   a  of the Al x Ga (1-x) As layer  11 , an epitaxial layer containing an active layer  21 . More specifically, the epitaxial wafer  20   a  is furnished with a GaAs substrate  13 , an Al x Ga (1-x) As layer  11  formed onto the GaAs substrate  13 , and, formed onto the Al x Ga (1-x) As layer  11 , an epitaxial layer containing an active layer  21 . The energy bandgap of the active layer  21  is smaller than that of the Al x Ga (1-x) As layer  11 . 
     It is preferable that the amount fraction x of Al in the plane of contact of the active layer  21  with the Al x Ga (1-x) As layer  11  (in the active layer&#39;s rear face  21   c ) be greater than the amount fraction x of Al in the plane of contact of the Al x Ga (1-x) As layer  11  with the active layer  21  (in the major surface  11   a  in the present embodying mode). It is also preferable that the amount fraction x of Al in the lamina of greatest thickness in the epitaxial layer containing the active layer  21  be greater than the amount fraction x of Al in the plane of contact of the Al x Ga (1-x) As layer  11  with the active layer  21  (in the major surface  11   a  in the present embodying mode). Such an implementation makes it possible to mitigate warp that occurs in the epitaxial wafer  20   a.    
     It should be understood that the region XIII depicted in  FIG. 12  is not limited to an upper portion within the active layer  21 . It is preferable that, as indicated in  FIG. 13 , the active layer  21  have a multiquantum-well structure. The active layer  21  contains two or more well layers  21   a . The well layers  21   a  are each sandwiched between barrier layers  21   b  whose energy bandgap is larger than that of the well layers  21   a . In particular, the plural well layers  21   a  are arranged alternating with plural barrier layers  21   b  whose bandgap is larger than that of the well layers  21   a . With the active layer  21 , all of the plurality of well layers  21   a  may be sandwiched between barrier layers  21   b , or a well layer  21   a  may be arranged on at least one surface of the active layer  21 , and the well layer  21   a  arranged on the one surface of the active layer  21  may be sandwiched between a barrier layer  21   b  and another layer (not illustrated), such as a guide layer or a cladding layer, disposed on the front side of the well layer  21   a.    
     The active layer  21  preferably has between two and one-hundred (both inclusive), more preferably between ten and fifty (both inclusive), well layers  21   a  and barrier layers  21   b , respectively. Implementations having two or more well layers  21   a  and barrier layers  21   b  constitute a multiquantum-well structure. Having ten or more well layers  21   a  and barrier layers  21   b  improves light output by improving the optical emission efficiency. Implementations with no more than one-hundred layers reduce the costs required in order to build the active layer  21 , and implementations with no more than fifty layers further reduce such costs. 
     The thickness H 21  of the active layer  21  preferably is between 6 nm and 2 μm (both inclusive). A thickness H 21  of 6 nm or more allows the optical intensity to be improved. A thickness H 21  of 2 μm or less allows productivity to be improved. 
     The thickness H 21   a  of the well layers  21   a  preferably is between 3 nm and 20 nm (both inclusive). The thickness H 21   b  of the barrier layers  21   b  preferably is between 5 nm and 1 μm (both inclusive). 
     While the material constituting the well layers  21   a  is not particularly limited as long as it has a bandgap that is smaller than that of the barrier layers  21   b , materials such as GaAs, AlGaAs, InGaAs (indium gallium arsenide), and AlInGaAs (aluminum indium gallium arsenide) can be utilized. These materials are infrared light-emitting substances whose lattice match with AlGaAs is quite suitable. 
     In instances where the epitaxial wafer  20   a  is utilized in an infrared LED whose emission wavelength is 900 nm or greater, preferably the material for the well layers  21   a  is InGaAs that contains In, and in which the amount fraction of the In is 0.05 or greater. And in instances in which the well layers  21   a  include a material that contains In, the well layers  21   a  and the barrier layers  21   b  preferably are an active layer  21  having four or fewer laminae of each, more preferably an active layer  21  having three or fewer laminae of each. 
     While the material for the barrier layers  21   b  is not limited as long as the bandgap is larger than that of the well layers  21   a , AlGaAs, InGaP, AlInGaP, InGaAsP and like materials may be utilized. These are materials whose lattice match with AlGaAs is suitable. 
     In instances where an epitaxial wafer  20   a  whose emission wavelength is 900 nm or greater, preferably 940 nm or greater, is utilized in an infrared LED, preferably the material for the barrier layers  21   b  within the active layer  21  contains P, or is either GaAsP or AlGaAsP in which the amount fraction of the P is 0.05 or more. Likewise, in instances in which the barrier layers  21   b  include a material that contains P, preferably the well layers  21   a  and the barrier layers  21   b  are an active layer  21  that has three or more laminae of each. 
     It is preferable that the concentration of atomic elements apart from the atoms within the epitaxial layer containing the active layer  21  (for example, elements such as atoms within the epitaxial growth ambient) be low. 
     It will be appreciated that the active layer  21 , not particularly limited to being a multiquantum-well structure, may be composed of a single layer, or may be a double-heterostructure. 
     Furthermore, although in the present embodying mode an implementation in which only the active layer  21  is included as an epitaxial layer has been explained, other layers such as cladding layers and undoped layers may also be included. 
     To continue: With reference to  FIG. 14 , an explanation of a method of manufacturing an infrared-LED epitaxial wafer  20   a  in the present embodying mode will be made. 
     As indicated in  FIG. 14 , initially an Al x Ga (1-x) As substrate  10   a  is manufactured by a method in Embodying Mode 1 of manufacturing an Al x Ga (1-x) As substrate  10   a  (Steps S 1  through S 5 ). 
     Next, an epitaxial layer containing an active layer  21  is deposited by organometallic vapor-phase epitaxy onto the major surface  11   a  of the Al x Ga (1-x) As layer  11  (Step S 7 ). 
     In Step S 7 , it is preferable that the epitaxial layer (the active layer  21  in the present embodying mode) be formed in such a manner that the amount fraction x of Al in the plane of contact of the epitaxial layer with the Al x Ga (1-x) As layer  11  (in the epitaxial layer&#39;s rear face  21   c ) is greater than the amount fraction x of Al in the plane of contact of the Al x Ga (1-x) As layer with the epitaxial layer (in the major surface  11   a  in the present embodying mode). It is also preferable that the amount fraction x of Al in the lamina of greatest thickness in the epitaxial layer be greater than the amount fraction x of Al in the plane of contact of the Al x Ga (1-x) As layer  11  with the epitaxial layer. 
     Organometallic vapor-phase epitaxy grows an active layer  21  by precursor gases thermal-decomposition reacting above the Al x Ga (1-x) As layer  11 , and molecular-beam epitaxy grows an active layer  21  by a technique that does not mediate the chemical-reaction stages in a non-equilibrium system; thus, the OMVPE and MBE techniques allow the thickness of the active layer  21  to be readily controlled. 
     An active layer  21  having plural well layers  21   a  of two or more laminae may therefore be grown. 
     Furthermore, the thickness H 21  of the epitaxial layer (active layer  21  in the present embodying mode) relative to the thickness H 11  of the Al x Ga (1-x) As layer  11  (the ratio H 21 /H 11 ) is, for example, between 0.05 and 0.25 (both inclusive), more preferably between 0.15 and 0.25 (both inclusive). Such implementations make it possible to mitigate incidence of warp in the state in which an epitaxial layer has been grown onto an Al x Ga (1-x) As layer  11 . 
     In this Step S 7 , an epitaxial layer containing an active layer  21  as described above is grown onto the Al x Ga (1-x) As layer  11 . 
     Specifically, an active layer  21  is formed having between two and one-hundred (both inclusive), more preferably between ten and fifty (both inclusive), well layers  21   a  and barrier layers  21   b , respectively. 
     It is also preferable that the active layer  21  be grown so as to have a thickness H 21  of from 6 nm to 2 μm. Growing well layers  21   a  having a thickness H 21   a  of from 3 nm to 20 nm, and barrier layers  21   b  having a thickness H 21   b  of from 5 nm to 1 μm is likewise preferable. 
     Growing well layers  21   a  made from GaAs, AlGaAs, InGaAs, AlInGaAs, or the like, and barrier layers  21   b  made from AlGaAs, InGaP, AlInGaP, GaAsP, AlGaAsP, InGaAsP, or the like is also preferable. 
     For the active layer  21  it does not matter whether there is lattice misalignment (lattice relaxation) in the GaAs and AlGaAs that constitute the Al x Ga (1-x) As substrate. If there is lattice misalignment in the well layers  21   a , lattice misalignment in the opposite direction may be imparted to the barrier layers  21   b , and for the structure of the epitaxial wafer overall, to balance the crystal warp from compression-extension. Further, the crystal warpage may be may be at or below or at or above the lattice-relaxing limit. If at or above the lattice-relaxing limit, however, because dislocations threading through the crystal are liable to occur, being at or below the limit is desirable. 
     As an example, an instance in which InGaAs is utilized for the well layer  21   a  will be given. Because the lattice constant of InGaAs is large with respect to the GaAs substrate, lattice relaxation occurs if an epitaxial layer of a fixed thickness or greater is grown. Therefore, favorable crystal in which the occurrence of crystal-threading dislocations is controlled to a minimum can be obtained by having the thickness be below the level at which lattice relaxation occurs. 
     Also, if GaAsP is utilized for the barrier layers  21   b , because the lattice constant of GaAsP is small relative to the GaAs substrate, lattice relaxation occurs when epitaxial layer of fixed thickness or greater is grown thereon. Therefore, favorable crystal in which the occurrence of crystal-threading dislocations is controlled to a minimum can be obtained by having the thickness be below the level at which lattice relaxation occurs. 
     Lastly, with respect to the GaAs substrate, taking advantage of the features that the lattice constant of InGaAs is large while the lattice constant of GaAsP is small, by utilizing InGaAs for the well layers  21   a  and utilizing GaAsP for the barrier layers  21   b  to balance out the lattice warp in the crystal as a whole favorable crystal in which the occurrence of crystal-threading dislocations is controlled to a minimum can be obtained without causing lattice relaxation up to or above the aforementioned limit. 
     By implementing the foregoing Steps S 1  through S 5  and S 7 , the epitaxial wafer  20   a  depicted in  FIG. 12  may be manufactured. 
     It will be appreciated that Step S 6  of removing the GaAs substrate  13  may be additionally be implemented. Step S 6  here may be implemented, for example, after Step S 7  of growing an epitaxial layer, but is not particularly limited to that sequence. Step S 6  may be implemented in between polishing Step S 4  and washing Step S 5 , for example. Step S 6  here is the as Step S 6  of Embodying Mode 2 and thus its explanation will not be repeated. In instances in which Step S 6  is implemented, a structure that is the same as that of later-described epitaxial wafer  20   b  of  FIG. 15  results. 
     As described in the foregoing, an infrared-LED epitaxial wafer  20   a  in the present embodying mode is furnished with an Al x Ga (1-x) As substrate  10   a  of Embodying Mode 1, and an epitaxial layer, formed on the major surface  11   a  of the Al x Ga (1-x) As layer  11  and containing an active layer  21 . 
     Furthermore, a method of manufacturing an infrared-LED epitaxial wafer  20   a  in the present embodying mode is provided with a procedure (Steps S 1  through S 6 ) of manufacturing an Al x Ga (1-x) As substrate  10   a  by an Al x Ga (1-x) As substrate  10   a  manufacturing method of Embodying Mode 1, and a step (Step S 7 ) of forming an epitaxial layer containing an active layer  21  onto the major surface  11   a  of the Al x Ga (1-x) As layer  11  by at least either organometallic vapor-phase epitaxy or molecular-beam epitaxy. 
     According to an infrared-LED epitaxial wafer  20   a  and a method of its manufacture in the present embodying mode, an epitaxial layer is formed onto an Al x Ga (1-x) As substrate  10   a  furnished with an Al x Ga (1-x) As layer  11  in which the amount fraction x of Al in the major surface  11   a  is lower than in the rear face  11   b . Consequently, an infrared-LED epitaxial wafer  20   a  can be realized in which a high level of transparency is maintained, and with which, when the epitaxial wafer  20   a  is utilized to fabricate a device, the device proves to have superior characteristics. 
     In the above-described infrared-LED epitaxial wafer  20   a  and method of is manufacture, it is preferable that the amount fraction x of Al in the plane of contact of the epitaxial layer with the Al x Ga (1-x) As layer  11  (the reverse face  21   c  of the epitaxial layer) be greater than the amount fraction x of Al in the plane of contact of the Al x Ga (1-x) As layer  11  with the epitaxial layer (the major surface  11   a ). 
     These conditions, when the Al x Ga (1-x) As layer  11  and the epitaxial layer are seen as a whole, can mitigate warp in the epitaxial wafer  20   a , for the same reasons discussed in Embodying Mode 1. 
     In the above-described method of manufacturing an infrared-LED epitaxial wafer  20   a , preferably provided are: a step of preparing a GaAs substrate  13  (Step S 1 ); a step of growing onto the GaAs substrate  13  by liquid-phase epitaxy an Al x Ga (1-x) As layer  11  as a window layer that diffuses current and that transmits light from the active layer (Step S 2 ); a step of polishing the major surface  11   a  of the Al x Ga (1-x) As layer  11  (Step S 4 ); and a step growing onto the major surface  11   a  of the Al x Ga (1-x) As layer  11  by at least either organometallic vapor-phase epitaxy or molecular-beam epitaxy an active layer  21  having a multiquantum-well structure and whose energy bandgap is smaller than that of the Al x Ga (1-x) As layer  11  (Step S 7 ). 
     Owing to the Al x Ga (1-x) As layer  11  being grown (Step S 2 ) by the LPE technique, the growth rate is rapid. With LPE, moreover, since expensive precursor gases and expensive apparatus need not be employed, the manufacturing costs are low. Therefore, more than with the OMVPE and MBE techniques, costs can be reduced and considerably thick Al x Ga (1-x) As layers  11  formed. Unevenness on the major surface  11   a  of the Al x Ga (1-x) As layer  11  can be reduced by polishing the major surface  11   a  of the Al x Ga (1-x) As layer  11 . Therefore, in forming an epitaxial layer containing an active layer  21  onto the major surface  11   a  of the Al x Ga (1-x) As layer  11 , abnormal growth of the epitaxial layer containing the active layer  21  can be kept under control. Meanwhile, OMVPE, by the thermal-decomposition reaction of the precursor gases, and MBE, which does not mediate the chemical-reaction stages in a non-equilibrium system, allow the film thickness to be optimally controlled. Consequently, forming the epitaxial layer containing the active layer  21  by OMVPE or MBE after Step S 4  of polishing the major surface  11   a  enables abnormal growth to be held in check, and makes it possible to form an active layer having a multiquantum-well structure (MQW structure) in which the film thickness of the active layer  21  has been optimally controlled. 
     Especially since with LEDs, cases where the film thickness is less than with laser diodes (LDs) are numerous, utilizing the OMVPE or MBE technique, whereby film-thickness controllability is excellent, allows an epitaxial layer containing an active layer  21  having a multiquantum-well structure to be formed. 
     Here the active layer  21  is grown by OMVPE or MBE following Step S 2  of growing the Al x Ga (1-x) As layer  11  by LPE. Growing the active layer  21  by OMVPE or MBE following the liquid-phase epitaxy prevents extended-duration, high-temperature heat from being applied to the active layer  21 . Deterioration of crystallinity due to crystalline defects arising in the active layer  21  on account of the high-temperature heat can therefore be prevented, and diffusion into the active layer  21  of dopant introduced by the LPE can held in check. 
     After Step S 7  of growing the active layer  21  in the present embodying mode, the active layer  21  is not exposed to the high-temperature ambients employed in liquid-phase epitaxy. Thus p-type dopants for example, which diffuse readily, introduced into the Al x Ga (1-x) As layer  11  can be prevented from diffusing to inside the active layer  21 . Therefore, the concentration in the active layer  21  of p-type carriers such as Zn, Mg and C can be held low—to, for example, 1×10 18  cm −3  or less. A further resulting advantage is that the formation of impurity bands in the active layer  21  can be prevented, allowing the difference in bandgap between the well layers  21   a  and the barrier layers  21   b  to be sustained. 
     Accordingly, since an active layer  21  having an improved-performance multiquantum-well structure can be formed, when the GaAs substrate  13  is removed (Step S 6 ) and the device electrodes are formed, by the altering of the state density in the active layer  21  efficient recombination of electrons and holes takes place. Epitaxial wafers  20   a  for constituting infrared LEDs in which optical emission efficiency is improved can therefore be grown. 
     It will be appreciated that with the Al x Ga (1-x) As layer  11  as a window layer, since electric current is diffused in a direction (horizontally in  FIG. 1 ) that intersects the direction along which the Al x Ga (1-x) As layer  11  and the active layer  21  are laminated (vertically in  FIG. 1 ), the light-extraction efficiency is improved, thereby allowing the optical emission efficiency to be improved. 
     In the above-described method of manufacturing an infrared-LED epitaxial wafer  20   a , it is preferable that Steps S 3  and S 5  of washing the surface of the Al x Ga (1-x) As layer  11  be provided at least either between Al x Ga (1-x) As layer  11  growth Step S 2  and polishing Step S 4 , or between polishing Step S 4  and epitaxial layer growth Step S 7 . 
     Even should impurities cling to or mix into the Al x Ga (1-x) As layer  11  due to the Al x Ga (1-x) As layer  11  coming into contact with atmospheric air, thus providing the washing steps clears the impurities away. 
     In the above-described method of manufacturing an infrared-LED epitaxial wafer  20   a , it is preferable that in washing Steps S 3  and A 5 , an alkaline solution be employed to wash the major surface  11   a.    
     When impurities have clung to or mixed into the Al x Ga (1-x) As layer  11 , this preferred application of the washing steps allows the impurities to be more effectively removed from the Al x Ga (1-x) As layer  11 . 
     In the above-described infrared-LED epitaxial wafer  20   a  and method of its manufacture, it is preferable that the thickness H 11  of the Al x Ga (1-x) As layer  11  be between 10 μm and 1000 μm (both inclusive), and more preferable that it be between 20 μm and 140 μm (both inclusive). 
     Implementations in which the thickness H 11  is as least 10 μm allow optical emission efficiency to be improved, while implementations in which the thickness H 11  is 20 μm or more enable further improvement of emission efficiency. Keeping the thickness H 11  to 1000 μm or less reduces the costs required to form the Al x Ga (1-x) As layer  11 , while keeping the thickness H 11  to 140 μm or less further allows the costs involved in the deposition of the Al x Ga (1-x) As layer  11  to be held down. 
     In the above-described infrared-LED epitaxial wafer  20   a  and method of its manufacture, it is preferable that in the active layer  21 , the well layers  21   a  be disposed in alternation with the barrier layers  21   b  of bandgap larger than that of the well layers  21   a , and that the active layer  21  has between ten and fifty (both inclusive) well layers  21   a  and between ten and fifty (both inclusive) barrier layers  21   b.    
     Implementations with ten or more layers allow further improvement in emission efficiency, while implementations with no more than fifty layers allow the costs involved in forming the active layer  21  to be held down. 
     With the above-described epitaxial wafer  20   a  for infrared LEDs and the method of its manufacture, preferably the epitaxial wafer, and its method of manufacture, are utilized in an infrared LED whose emission wavelength is 900 nm or greater, and the well layers  21   a  within the active layer  21  include a material containing In, with the number of well layers  21   a  being four or fewer laminae. That the emission wavelength be 940 nm or greater is more preferable. 
     By forming an active layer  21  including a material containing In and having well layers of four or fewer laminae, the present inventors discovered that lattice relaxation was controlled to a minimum. They therefore were able to realize an epitaxial wafer that can be utilized in an infrared LED whose wavelength is 900 nm or greater. 
     In the foregoing infrared-LED epitaxial wafer  20   a  and method of its manufacture, preferably the well layers  21   a  are of InGaAs in which the amount fraction of indium is 0.05 or greater. 
     That makes it possible to realize a useful epitaxial wafer  20   a  that can be utilized in an infrared LED whose wavelength is 900 nm or greater. 
     With the above-described epitaxial wafer  20   a  for infrared LEDs and the method of its manufacture, preferably the epitaxial wafer, and its method of manufacture, are utilized in an infrared LED whose emission wavelength is 900 nm or greater, and the barrier layers  21   b  within the active layer  21  include a material containing P, with the number of barrier layers  21   b  being three or more laminae. 
     By forming an active layer  21  including a material containing P, the present inventors discovered that lattice relaxation was controlled to a minimum. They therefore were able to realize an epitaxial wafer that can be utilized in an infrared LED whose wavelength is 900 nm or greater. 
     In the foregoing infrared-LED epitaxial wafer and method of its manufacture, preferably the barrier layers  21   b  are of either GaAsP or AlGaAsP in which the amount fraction of P is 0.05 or greater. 
     That makes it possible to realize a useful epitaxial wafer  20   a  that can be utilized in an infrared LED whose wavelength is 900 nm or greater. 
     Embodying Mode 4 
     Referring to  FIG. 15 , an explanation of an infrared-LED epitaxial wafer  20   b  in the present embodying mode will be made. 
     As indicated in  FIG. 15 , an epitaxial wafer  20   b  in the present embodying mode is furnished with an Al x Ga (1-x) As substrate  10   b , represented in  FIG. 10 , of Embodying Mode 2, and, formed onto the major surface  11   a  of the Al x Ga (1-x) As layer  11 , an epitaxial layer containing an active layer  21 . 
     An epitaxial wafer  20   b  in the present embodying mode is furnished with basically the same structural makeup as an epitaxial wafer  20   a  of Embodying Mode 3, but differs in that it is not furnished with a GaAs substrate  13 . 
     To continue: With reference to  FIG. 16 , an explanation of a method of manufacturing an epitaxial wafer  20   b  in the present embodying mode will be made. 
     As indicated in  FIG. 16 , initially an Al x Ga (1-x) As substrate  10   b  is manufactured by a method in Embodying Mode 2 of manufacturing an Al x Ga (1-x) As substrate  10   b  (Steps S 1 , S 2 , S 3 , S 4 , S 6  and S 5 ). 
     Next, in the same manner as in Embodying Mode 3, an epitaxial layer containing an active layer  21  is deposited by organometallic vapor-phase epitaxy onto the major surface  11   a  of the Al x Ga (1-x) As layer  11  (Step S 7 ). 
     Implementing the foregoing Steps S 1  through S 7  enables an infrared-LED epitaxial wafer  20   b , represented in  FIG. 15 , to be manufactured. 
     It should be understood that apart from the foregoing, the infrared-LED epitaxial wafer and its method of manufacture are otherwise of the same constitution as the infrared-LED epitaxial wafer  20   a  and its method of manufacture in Embodying Mode 3; thus identical components are labeled with identical reference marks, and their explanation will not be repeated. 
     As described in the foregoing, the infrared-LED epitaxial wafer  20   b  in the present embodying mode is furnished with an Al x Ga (1-x) As layer  11 , and an epitaxial layer formed on the major surface  11   a  of the Al x Ga (1-x) As layer  11  and containing an active layer  21 . 
     In addition, a method of manufacturing an infrared-LED epitaxial wafer  20   b  in the present embodying mode is provided with a step (Step S 6 ) of removing the GaAs substrate  13 . 
     According to an infrared-LED epitaxial wafer  20   b  and its method of manufacture in the present embodying mode, an Al x Ga (1-x) As substrate  10   b  from which the GaAs substrate, which absorbs light in the visible range, has been removed is utilized. Consequently, further forming electrodes on the epitaxial wafer  20   b  enables the realization of an infrared-LED-constituting epitaxial wafer  20   b  in which a high level of transparency is maintained and superior device characteristics are sustained. 
     Embodying Mode 5 
     Referring to  FIG. 17 , an explanation of an infrared-LED epitaxial wafer  20   c  in the present embodying mode will be made. 
     As indicated in  FIG. 17 , an epitaxial wafer  20   c  in the present embodying mode is furnished with a structural makeup that is basically the same as that of an epitaxial wafer  20   b  of Embodying Mode 4, but differs in that the epitaxial layer further includes a contact layer  23 . That is, in the present embodying mode, the epitaxial layer contains an active layer  21  and a contact layer  23 . 
     Specifically, the epitaxial wafer  20   c  is furnished with an Al x Ga (1-x) As layer  11 , an active layer  21  formed on the Al x Ga (1-x) As layer  11 , and a contact layer  23  formed on the active layer  21 . 
     The contact layer  23  consists of, for example, p-type GaAs and has a thickness H 23  of 0.01 μm or more. 
     To continue: A method of manufacturing an infrared-LED epitaxial wafer  20   c  in the present embodying mode will be made. The method of manufacturing an infrared-LED epitaxial wafer  20   c  in the present embodying mode is furnished with the same constitution as the epitaxial wafer  20   b  manufacturing method of Embodying Mode 4, but differs in that Step S 7  of forming an epitaxial layer further includes a substep of forming a contact layer  23 . 
     Specifically, after the active layer  21  is grown, a contact layer  23  is formed onto the surface of the active layer  21 . Although the method whereby the contact layer  23  is formed is not particularly limited, preferably it is grown by at least either organometallic vapor-phase epitaxy or molecular-beam-epitaxy, or else by a combination of OMVPE and MBE, because these deposition techniques enable the formation of thin-film layers. And the contact layer  23  is preferably grown by the same technique as is the active layer  21 , because it can then be grown continuously with growth of the active layer  21 . 
     It should be understood that apart from the foregoing, the infrared-LED epitaxial wafer and its method of manufacture are otherwise of the same constitution as the infrared-LED epitaxial wafer  20   b  and its method of manufacture in Embodying Mode 4; thus identical components are labeled with identical reference marks, and their explanation will not be repeated. 
     It will be appreciated that the infrared-LED epitaxial wafer  20   c  and its method of manufacture in the present embodying mode can find application not only in Embodying Mode 4, but in Embodying Mode 3 as well. 
     Embodying Mode 6 
     Referring to  FIG. 18 , an explanation of an infrared LED  30   a  in the present embodying mode will be made. As indicated in  FIG. 18 , an infrared LED  30   a  in the present embodying mode is furnished with an infrared-LED epitaxial wafer  20   c , represented in  FIG. 17 , of Embodying Mode 5, electrodes  31  and  32 , formed respectively on the front side  20   c   1  and back side  20   c   2  of the epitaxial wafer  20   c , and a stem  33 . 
     The electrode  31  is provided contacting on the front side  20   c   1  of the epitaxial wafer  20   c  (on the contact layer  23  in the present embodying mode), while the electrode  32  is provided contacting on the back side  20   c   2  (on the Al x Ga (1-x) As layer  11  in the present embodying mode). The stem  33  is provided contacting on the electrode  31 , on its reverse side from the epitaxial wafer  20   c.    
     To give specifics of the LED  30   a  makeup: The stem  33  is constituted from, for example, an iron-based material. The electrode  31  is a p-type electrode constituted from, for example, an alloy of gold (Au) and zinc (Zn). The electrode  31  is formed onto the p-type contact layer  23 . The contact layer  23  is formed on the top of the active layer  21 . The active layer  21  is formed on the top of the Al x Ga (1-x) As layer  11 . The electrode  32  formed onto the Al x Ga (1-x) As layer  11  is an n-type electrode constituted from, for example, an alloy of Au and Ge (germanium). 
     To continue: With reference to  FIG. 19 , an explanation of a method of manufacturing an infrared LED  30   a  in the present embodying mode will be made. 
     Initially, an epitaxial wafer  20   a  is manufactured by the procedure of Embodying Mode 3 for manufacturing an infrared-LED epitaxial wafer  20   a  (Step S 1  through S 5 , and S 7 ). In this case, the active layer  21  and the contact layer  23  are formed in epitaxial layer growth Step S 7 . Next, the GaAs substrate is removed (Step S 6 ). It will be appreciated that implementing Step S 6  allows an infrared-LED epitaxial wafer  20   c  as represented in  FIG. 17  to be manufactured. 
     Subsequently, electrodes  31  and  32  are formed on the front side  20   c   1  and back side  20   c   2  of the infrared-LED epitaxial wafer  20   c  (Step S 11 ). Specifically, for example, by a vapor-deposition technique, Au and Zn are vapor-deposited onto the front side  20   c   1 , and further, Au and Ge are alloyed after being vapor-deposited onto the back side  20   c   2 , to form the electrodes  31  and  32 . 
     Next, the LED is surface mounted (Step S 12 ). Specifically, for example the electrode  31  side is turned down, and die attachment is carried out on the stem  33  with a die-attach adhesive such as an Ag paste, or with an eutectic alloy such as AuSn. 
     Implementing the foregoing Steps S 1  through S 12  enables an infrared-LED  30   a , represented in  FIG. 18 , to be manufactured. 
     It should be understood that in the present embodying mode, although an implementation utilizing an infrared-LED epitaxial wafer  20   c  of Embodying Mode 5 has been described, an infrared-LED epitaxial wafer  20   a  or  20   b  of Embodying Mode 3 or 4 is also applicable. Prior to completion of the infrared LED  30   a , however, it is advisable to implement Step S 6  of removing the GaAs substrate  13 . Still, in implementations where the GaAs substrate  13  is not removed, an electrode may be formed on the rear face of the GaAs substrate  13 . 
     As described in the foregoing, an infrared LED  30   a  in the present embodying mode is furnished with: an Al x Ga (1-x) As substrate  10   b  of Embodying Mode 2; an epitaxial layer formed onto the major surface  11   a  of the Al x Ga (1-x) As layer  11  and containing an active layer  21 ; a first electrode  31 , formed on the front side  20   c   1  of the epitaxial layer; and a second electrode  32 , formed on the back side  20   c   2  of the Al x Ga (1-x) As layer  11 . 
     In turn, a method of manufacturing an infrared LED  30   a  in the present embodying mode is furnished with: a procedure of manufacturing an Al x Ga (1-x) As substrate  10   b  by an Al x Ga (1-x) As substrate  10   b  manufacturing method of Embodying Mode 2 (Steps S 1  through S 6 ); a step of forming an epitaxial layer containing an active layer  21  onto the major surface  11   a  of the Al x Ga (1-x) As layer  11  by organometallic vapor-phase epitaxy (Step S 7 ); a step of forming a first electrode  31  onto the front side  20   c   1  of the epitaxial wafer  20   c  (Step S 11 ); and a step of forming a second electrode  32  onto the rear face  11   b  of the Al x Ga (1-x) As layer  11  (Step S 11 ). 
     According to an infrared LED  30   a  and method of its manufacture in the present embodying mode, since an Al x Ga (1-x) As substrate  10   b  in which the amount fraction x of Al in the Al x Ga (1-x) As layer  11  has been controlled is employed, infrared LEDs  30   a  that sustain a high level of transmissivity, and which, in the fabrication of semiconductor devices, have superior characteristics may be realized. 
     Furthermore, the electrode  31  is formed on the wafer&#39;s active layer  21  side, while the electrode  32  is formed on its Al x Ga (1-x) As layer  11  side. This structure enables current from the electrode  32  to be diffused across the entire surface of the infrared LED  30   a  by means of the Al x Ga (1-x) As layer  11 . Infrared LEDs  30   a  of further improved optical emission efficiency can therefore be obtained. 
     Embodying Mode 7 
     Referring to  FIG. 20 , an explanation of an infrared LED  30   b  in the present embodying mode will be made. As indicated in  FIG. 20 , an infrared LED  30   b  in the present embodying mode is furnished with basically the same structural makeup as an infrared LED  30   a  of Embodying Mode 6, but differs in that the stem  33  is disposed on the wafer&#39;s Al x Ga (1-x) As layer  11  side. 
     Specifically, the electrode  31  is provided contacting on the front side  20   c   1  of the epitaxial wafer  20   c  (on the contact layer  23  in the present embodying mode), while the electrode  32  is provided contacting on the back side  20   c   2  (on the Al x Ga (1-x) As layer  11  in the present embodying mode). 
     To serve the purpose of extracting light, the electrode  31  covers a portion of the front side  20   c   1  of the epitaxial wafer  20   c , leaving the remaining area on the front side  20   c   1  of the epitaxial wafer  20   c  exposed. The electrode  32 , meanwhile, covers the entire surface of the back side  20   c   2  of the epitaxial wafer  20   c.    
     A method of manufacturing an infrared LED  30   b  in the present embodying mode is furnished with the basically same constitution as the infrared LED  30   a  manufacturing method of Embodying Mode 6, but as just described differs in Step S 11  of forming the electrodes  31  and  32 . 
     It should be understood that apart from the foregoing, the infrared LED  30   b  and its method of manufacture are otherwise of the same constitution as the infrared LED  30   a  and its method of manufacture in Embodying Mode 6; thus identical components are labeled with identical reference marks, and their explanation will not be repeated. 
     Further, in instances in which the GaAs substrate  13  has not been removed, an electrode may be formed on the reverse face  13   b  of the GaAs substrate  13 . With an epitaxial wafer  20   a  of Embodying Mode 3, in the case where an epitaxial wafer in which the epitaxial layer further contains contact layers is utilized to form an infrared LED, it will have a structure like infrared LED  30   c  illustrated in  FIG. 27  as a representative example. In this case, as indicated in  FIG. 27 , the stem  33  is arranged on the GaAs substrate  13  side of the device. As a modified example of this, the GaAs substrate  13  side may be located on the opposite side from that of the stem  33 . 
     Embodiment 1 
     In the present embodiment, the effect of, in an Al x Ga (1-x) As layer  11 , the amount fraction x of Al in the rear face  11   b  being greater than the amount fraction x of Al in the major surface  11   a  was investigated. Specifically, an Al x Ga (1-x) As substrate  10   a  was manufactured in conformance with the Al x Ga (1-x) As substrate  10   a  manufacturing method of Embodying Mode 1. 
     More particularly, GaAs substrates  13  were prepared (Step S 1 ). Next, a variety of Al x Ga (1-x) As layers  11  whose amount fraction x of Al was 0≦x≦1 were grown on the GaAs substrates  13  by liquid-phase epitaxy (Step S 2 ). 
     The transmissivity and surface oxygen quantity of Al x Ga (1-x) As layers  11  when their emission wavelength was 850 nm, 880 nm and 940 nm were examined. In order to check these characteristics: the Al x Ga (1-x) As layer  11  of  FIG. 1  was created at thicknesses of 80 μm to 100 μm, in such a way that the amount fraction of Al depth-wise would be uniform; the GaAs substrate  13  was removed as in the flow of  FIG. 11 ; and with the layers in the  FIG. 10  state, their transmissivity was measured with a transmittance meter. For the oxygen quantity: the same samples were created, following the flow in  FIG. 14 ; an epitaxial layer was grown by an OMVPE technique; and, before the GaAs substrate  13  was removed, the major surface  11   a  of the Al x Ga (1-x) As layer  11  was measured by secondary ion mass spectrometry (SIMS) characterization. The results are presented in  FIG. 21  and  FIG. 22 . 
     In  FIG. 21 , the vertical axis indicates amount fraction x of Al in the Al x Ga (1-x) As layers  11 , while the horizontal axis indicates transmissivity. The further to the right is the position along the axis in  FIG. 21 , the better is the transmissivity. Also, from looking at the case where the emission wavelength was 880 nm, it was understood that the transmissivity is favorable even with lower Al component levels. Also, in the case where the emission wavelength was 940 nm, it could be confirmed that even with lower Al component levels, deterioration in transmissivity was unlikely to occur. 
     Next, in  FIG. 22 , the vertical axis indicates amount fraction x of Al in the Al x Ga (1-x) As layers  11 , while the horizontal axis indicates surface oxygen quantity. The further to the left is the position along the axis in  FIG. 22 , the better is the oxygen quantity. It will be appreciated that the surface oxygen quantity was the same when the emission wavelength was 850 nm, 880 nm and 940 nm. 
     Herein, in the present embodiment, as described above the Al x Ga (1-x) As layer  11  was created in such a way that the Al amount fraction depth-wise would be uniform, yet it was confirmed by the same experiment described earlier that because the oxygen quantity is determined principally by the amount fraction of Al in the major surface  11   a  of the Al x Ga (1-x) As layer  11 , even in instances in which the layer possesses a gradient in Al amount fraction, as illustrated in  FIG. 2  through  FIG. 5 , the correlation with the Al amount fraction in the major surface is strong. 
     The same tendency holds true with respect to the transmissivity: In instances in which the layer possesses an Al amount fraction gradient as illustrated in  FIG. 2  through  FIG. 5 , the transmissivity is affected by the area where the Al amount fraction is lowest. Specifically, in instances where the layer possesses a gradient as illustrated in  FIG. 2  through  FIG. 5 , if the pattern of the gradient (layer number, gradient in each layer, thickness) and the gradient (ΔAl/distance) are the same, the correlation of the transmissivity to the size of the average Al amount fraction within the layer is strong. 
     It was recognized that, as shown in  FIG. 21 , the greater the amount fraction x of Al in the Al x Ga (1-x) As layer  11  is, the more the transmissivity improves. Likewise, it was recognized that, as shown in  FIG. 22 , the lower the amount fraction x of Al in the Al x Ga (1-x) As layer  11  is, the more the oxygen quantity contained in the major surface can be reduced. 
     From the foregoing, it was understood that according to the present embodiment, in the Al x Ga (1-x) As layers  11 , raising the amount fraction x of Al in the rear face  11   b  maintains a high level of transmissivity, while lowering the amount fraction x of Al in the major surface  11   a  enables the oxygen quantity in the major surface to be reduced. 
     Embodiment 2 
     In the present embodiment, the effect of an Al x Ga (1-x) As layer  11  furnishing a plurality of layers in each of which the amount fraction x of Al heading from the plane of the layer&#39;s rear face  11   b  side to the plane of its major surface  11   a  side monotonically decreases was investigated. Specifically, thirty-two different samples of Al x Ga (1-x) As substrate  10   a  were manufactured in conformance with the  FIG. 1  depicted Al x Ga (1-x) As substrate  10   a  in the manufacturing method of Embodying Mode 1. 
     More particularly, 2-inch and 3-inch GaAs substrates were prepared (Step S 1 ). 
     Next, Al x Ga (1-x) As layers  11  were grown by a slow-cooling technique (Step S 2 ). In Step S 2 , the layers were grown so as to contain one or more laminae in each of which, as diagrammed in  FIG. 2 , the amount fraction x of Al constantly decreased heading in the growth direction. In detail, thirty-two different samples of Al x Ga (1-x) As layer  11  were grown in which the following parameters were as entered in Table I below: the Al amount fraction x in the major surface  11   a  of the Al x Ga (1-x) As layer  11  (minimum value of Al amount fraction x); in each lamina, the difference between the Al amount fraction x in the plane of the layer&#39;s rear face  11   b  side and the Al amount fraction x in the plane of its major surface  11   a  side (difference in Al amount fraction x); and number of laminae in each of which the amount fraction x of Al heading from the plane of the layer&#39;s rear face  11   b  side to the plane of its major surface  11   a  side monotonically decreased (laminae number). Thirty-two different samples of Al x Ga (1-x) As substrate  10   a  were thereby manufactured. 
     With regard to the Al x Ga (1-x) As substrates  10   a  themselves, warp appearing in an Al x Ga (1-x) As substrate  10   a —the gap between the Al x Ga (1-x) As substrate  10   a  with its convexly deviating surface face up, and a planar block—was measured by employing a thickness gauge. The results are tabulated in Table I below. In Table I, instances in which warp occurring in an Al x Ga (1-x) As substrate  10   a  was 200 μm or less when a 2-inch GaAs substrate was used, and was 300 μm or less when a 3-inch GaAs substrate was used are designated “o,” while instances in which warp exceeded 200 μm when a 2-inch GaAs substrate was used, and exceeded 300 μm when a 3-inch GaAs substrate was used are designated “x.” 
     
       
         
           
               
               
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 Al amount 
                 Al amount 
                 Warp for each number of laminae 
               
            
           
           
               
               
               
               
               
               
            
               
                 fraction x 
                 fraction x 
                   
                   
                 3 
                 4 
               
               
                 min. value 
                 difference 
                 1 lamina 
                 2 laminae 
                 laminae 
                 laminae 
               
               
                   
               
               
                 0.1 to 0.3 
                   0 ≦ x &lt; 0.15 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                   
                 0.15 ≦ x &lt; 0.25 
                 X 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                   
                 0.25 ≦ x &lt; 0.35 
                 X 
                 X 
                 ◯ 
                 ◯ 
               
               
                   
                 0.35 ≦ x 
                 X 
                 X 
                 X 
                 X 
               
               
                 0.3 to 0.5 
                   0 ≦ x &lt; 0.15 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                   
                 0.15 ≦ x &lt; 0.25 
                 X 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                   
                 0.25 ≦ x &lt; 0.35 
                 X 
                 X 
                 ◯ 
                 ◯ 
               
               
                   
                 0.35 ≦ x 
                 X 
                 X 
                 X 
                 X 
               
               
                   
               
            
           
         
       
     
     As is evident from Table I, regardless of the Al amount fraction x in the major surface  11   a , the smaller the difference in Al amount fraction x within the laminae in which the amount fraction x of Al monotonically decreases, the less likely warp was to occur in the Al x Ga (1-x) As substrates  10   a . It was understood that in instances in which the difference in Al amount fraction x was 0.15 or greater, but less than 0.35, warp could be mitigated by the Al x Ga (1-x) As layer  11  including many of the laminae in which the amount fraction x of Al monotonically decreases. From this result, it was inferred that in instances in which the difference in Al amount fraction x was a small 0.15 or less, increasing the number of laminae in which the amount fraction x of Al monotonically decreases was efficacious if warp was to be further reduced. It was likewise inferred that in instances in which the difference in Al amount fraction x was 0.35 or greater, increasing the number of laminae in which the amount fraction x of Al monotonically decreases to five or more allowed warp to be mitigated. It should be noted that there were no special differences between using 2-inch and 3-inch GaAs substrates. 
     As described in the foregoing, it was confirmed that according to the present embodiment, warp in the Al x Ga (1-x) As substrates  10   a  can be mitigated by the Al x Ga (1-x) As layer  11  including a plurality of laminae in each of which the amount fraction x of Al heading from the plane of the layer&#39;s rear face  11   b  side to the plane of its major surface  11   a  side monotonically decreases. 
     Embodiment 3 
     In the present embodiment, the effect of an infrared-LED epitaxial wafer providing an active layer with a multiquantum-well structure, and a satisfactory laminae number for the barrier layers and the well layers was investigated. 
     In the present embodiment, four different samples, indicated in  FIG. 23 , of epitaxial wafers  40  were grown in which only the thickness of, and number of laminae in, the active layer  21  having a multiquantum-well structure were varied. 
     Specifically, GaAs substrates  13  were prepared (Step S 1 ). Next, by organometallic vapor-phase epitaxy, an n-type cladding layer  41 , an undoped guide layer  42 , an active layer  21 , an undoped guide layer  43 , a p-type cladding layer  44 , an Al x Ga (1-x) As layer  11 , and a contact layer  23  were grown, in that order. The growth temperature for each layer was 750° C. The n-type cladding layers  41  had a thickness of 0.5 μm and consisted of Al 0.35 Ga 0.65 As; the undoped guide layers  42  had a thickness of 0.02 μm and consisted of Al 0.30 Ga 0.70 As; the undoped guide layers  43  had a thickness of 0.02 μm and consisted of Al 0.30 Ga 0.70 As; the p-type cladding layers  44  had a thickness of 0.5 μm and consisted of Al 0.35 Ga 0.65 As; the Al x Ga (1-x) As layers  11  had a thickness of 2 μm and consisted of p-type Al 0.15 Ga 0.85 As; and the contact layers  23  had a thickness of 0.01 μm and consisted of p-type GaAs. Furthermore, the active layers  21  had a multiplequantum well structure (MQW) in which, respectively, the well layers and the barrier layers had two laminae, ten laminae, twenty laminae and fifty laminae, with the optical emission wavelength being from 840 nm to 860 nm. Each well layer was a lamina having a thickness of 7.5 nm and consisting of GaAs, while each barrier layer was a lamina having a thickness of 5 nm and consisting of Al 0.30 Ga 0.70 As. 
     In addition, in the present embodiment a double-heterostructure epitaxial wafer, differing in being furnished with an active layer composed only of well layers whose emission wavelength was 870 nm and having a thickness of 0.5 μm, was grown as a separate epitaxial wafer for infrared LEDs. 
     As far as the respective grown epitaxial wafers are concerned, the epitaxial wafers were each manufactured without removing the GaAs substrate. Next, onto the contact layer  23 , an electrode consisting of AuZn, and onto the n-type GaAs substrate  13 , an electrode consisting of AuGe were respectively formed by a vapor-deposition technique. Infrared LEDs were thereby obtained. 
     The light output of each infrared LED when a current of 20 mA was passed through it was measured with a constant-current source and a photometric instrument (integrating sphere). The results are diagrammed in  FIG. 24 . It should be noted that “DH” along the horizontal axis in  FIG. 24  signifies an LED having a double heterostructure, “MQW” signifies LEDs furnished with well layers and barrier layers in an active layer, and the number of layers signifies the laminae count of the well layers and of the barrier layers, respectively. 
     It was found that, as indicated in  FIG. 24 , compared with the LED having a double heterostructure, the LEDs furnished with an active layer having a multiquantum-well structure allowed the light output to be improved. In particular, it was understood that the LEDs with between ten and fifty well layers and barrier layers (both inclusive) led to dramatically improved light output. 
     Herein, in the present embodiment, the Al x Ga (1-x) As layers  11  were manufactured by organometallic vapor-phase epitaxy, but OMVPE requires an extraordinary amount of time in order to grow the Al x Ga (1-x) As layers  11  if their thickness is to be as great as in cases such as Embodiment 1. Excluding this point, the characteristics of the formed infrared LEDs are the same as those of infrared LEDs of the present invention for which liquid-phase epitaxy and organometallic vapor-phase epitaxy were utilized, and thus are applicable to infrared LEDs of the present invention. It should be noted that in implementations in which the Al x Ga (1-x) As layer  11  thickness is large, utilizing LPE demonstrates the effect of making it possible to shorten the time needed in order to grow the Al x Ga (1-x) As layer  11 . 
     In addition, in the present embodiment, as still another epitaxial wafer for infrared LEDs, epitaxial wafers of multiquantum-well structure (MQW), differing only in that their emission wavelength was 940 nm and in being furnished with an active layer containing well layers having InGaAs in the well laminae, were grown. With the InGaAs of the well laminae, the thickness was 2 nm to 10 nm and the amount fraction of  1   n  consisted of 0.1 to 0.3. Meanwhile, the barrier layers were consisted of Al 0.30 Ga 0.70 As. 
     Onto these epitaxial wafers also, in the same way as described above, electrodes were formed to create infrared LEDs. As to these infrared LEDs as well, the light output power was characterized in the same way as described above, with the result that light output power whose emission wavelength was 940 nm was obtained. 
     Here, with respect to the barrier layers it has been confirmed by experimentation that even if they are anywhere from GaAs 0.90 P 0.10  to Al 0.30 Ga 0.70 As 0.90 P 0.10  they will have similar results. Further, the fact that the amount fraction of  1   n  and the amount fraction of P are adjustable at will has been confirmed by experimentation. 
     From the foregoing, it could be confirmed that in instances in which the emission wavelength was between 840 nm and 890 nm (both inclusive), the MQW with the well laminae being GaAs was utilized as the active layer, while in instances in which the emission wavelength was between 860 nm and 890 nm (both inclusive), a double heterostructure (DH) constituted by GaAs was applicable. In addition, it could be confirmed that in instances in which the emission wavelength was between 850 nm and 1100 nm (both inclusive), it was possible to create active layers from well layers constituted by InGaAs. 
     Embodiment 4 
     In the present embodiment, the effective range of thickness of the Al x Ga (1-x) As layer  11  in infrared-LED epitaxial wafers was investigated. 
     In the present embodiment, five different samples, indicated in  FIG. 25 , of epitaxial wafers  50  were grown in which only the thickness of the Al x Ga (1-x) As layer  11  was varied. 
     Specifically, GaAs substrates  13  were prepared (Step S 1 ). Next, by liquid-phase epitaxy, Al x Ga (1-x) As layers  11  having thicknesses of 2 μm, 10 μm, 20 μm, 100 μm, and 140 μm, and constituted from p-type Al 0.35 Ga 0.65 As doped with Zn were respectively formed (Step S 2 ). The LPE growth temperature at which the Al x Ga (1-x) As layers  11  were grown was 780° C., and the growth rate was an average 4 μm/h. Next, using hydrochloric acid and sulfuric acid, the major surface  11   a  of the Al x Ga (1-x) As layers  11  was washed (Step S 3 ). Subsequently, the major surface  11   a  of the Al x Ga (1-x) As layers  11  was polished by means of chemical-mechanical planarization (Step S 4 ). The major surface  11   a  of the Al x Ga (1-x) As layers  11  was then washed using ammonia and hydrogen peroxide (Step S 5 ). Next, by organometallic vapor-phase epitaxy, a p-type cladding layer  41 , an undoped guide layer  42 , an active layer  21 , an undoped guide layer  43 , a n-type cladding layer  44 , and an n-type contact layer  23  were grown, in that order (Step S 6 ). The OMVPE growth temperature for growing these layers was 750° C., while the growth rate was 1 to 2 μm/h. It should be noted that the thicknesses and the materials (apart from the dopants) for the p-type cladding layer  41 , the undoped guide layer  42 , the undoped guide layer  43 , the n-type cladding layer  44 , and the n-type contact layer  23  were made the same as in Embodiment 3. Also, active layers  21  having twenty laminae each of well layers and barrier layers were grown. Each well layer was a lamina having a thickness of 7.5 nm and consisting of GaAs, while each barrier layer was a lamina having a thickness of 5 nm and consisting of Al 0.30 Ga 0.70 As. 
     Next, the GaAs substrate  13  was removed (Step S 7 ). Infrared-LED epitaxial wafers furnished with Al x Ga (1-x) As layers having five different thicknesses were thereby manufactured. 
     Next, onto the contact layer  23 , an electrode consisting of AuGe, and onto the rear face  11   b  of the Al x Ga (1-x) As layer  11 , an electrode consisting of AuZn were respectively formed by a vapor-deposition technique. Infrared LEDs were thereby manufactured. 
     The light output of each of the infrared LEDs was measured in the same way as in Embodiment 3. The results are diagrammed in  FIG. 26 . 
     As indicated in  FIG. 26 , infrared LEDs furnished with an Al x Ga (1-x) As layer  11  having a thickness greater than 20 μm but not more than 140 μm made it possible to improve light output significantly, while infrared LEDs furnished with an Al x Ga (1-x) As layer  11  having a thickness between 100 μm and 140 μm (both inclusive) made possible extraordinary improvement in light output. 
     Now, at less than 20 μm, the fact that effectiveness from the GaAs  13  substrate having been removed was not seen is, from luminescent image observations, believed to be because there was hardly any change in the extent of the emission surface area. That is because on account of the low mobility with an Zn-doped p-type Al x Ga (1-x) As layer  11 , current does not diffuse. This can be remedied by having it be a Te-doped n-type Al x Ga (1-x) As layer  11  to raise the mobility. In below described Embodiment 5, making the layers Te-doped was seen to broaden the luminescent image, improving the light output. 
     Embodiment 5 
     In the present embodiment, the effectiveness of dispersion into the active layer being low by means of infrared LEDs of the present invention was investigated. 
     Sample 1 
     A Sample 1 infrared-LED epitaxial wafer was manufactured as follows. Specifically, at first a GaAs substrate  13  was prepared (Step S 1 ). Next, by liquid-phase epitaxy, a Te-doped Al x Ga (1-x) As layer  11  having a thickness of 20 μm and constituted from n-type Al 0.35 Ga (1-x) As was grown (Step S 2 ). Next, using hydrochloric acid and sulfuric acid, the major surface  11   a  of the Al x Ga (1-x) As layer  11  was washed (Step S 3 ). Subsequently, the major surface  11   a  of the Al x Ga (1-x) As layer  11  was polished by means of chemical-mechanical planarization (Step S 4 ). The major surface  11   a  of the Al x Ga (1-x) As layer  11  was then washed using ammonia and hydrogen peroxide (Step S 5 ). Next, by organometallic vapor-phase epitaxy, an Si-doped n-type cladding layer  41 , an undoped guide layer  42 , an active layer  21 , an undoped guide layer  43 , a Zn-doped p-type cladding layer  44 , and a p-type contact layer  23  were grown, in that order (Step S 6 ), as illustrated in  FIG. 25 . It should be noted that the thicknesses and the materials apart from the dopants for the n-type cladding layer  41 , the undoped guide layer  42 , the undoped guide layer  43 , and the p-type cladding layer  44  were made the same as in Embodiment 3. Also, active layers  21  having twenty laminae each of well layers and barrier layers were grown. Each well layer was a lamina having a thickness of 7.5 nm and consisting of GaAs, while each barrier layer was a lamina having a thickness of 5 nm and consisting of Al 0.30 Ga 0.70 As. It should also be noted that the growth temperatures and growth rates in the LPE and OMVPE were made the same as in Embodiment 4. 
     Next, the GaAs substrate  13  was removed (Step S 7 ). Sample 1 infrared-LED epitaxial wafers were thereby manufactured. 
     Next, onto the p-contact layer  23 , an electrode consisting of AuZn, and onto the bottom of the Al x Ga (1-x) As layer  11 , an electrode consisting of AuGe were respectively formed by a vapor-deposition technique (Step S 11 ). Infrared LEDs were thereby manufactured. 
     Sample 2 
     For Sample 2, to begin with a GaAs substrate  13  was prepared (Step S 1 ). Next, by organometallic vapor-phase epitaxy, a p-type cladding layer  44 , an undoped guide layer  43 , an active layer  21 , an undoped guide layer  42 , and an n-type cladding layer  41  were grown, in that order, in the same manner as with Sample 1. Next, an Al x Ga (1-x) As layer  11  was formed by liquid-phase epitaxy. The thickness of and material constituting the Al x Ga (1-x) As layer  11  was made the same as with Sample 1. 
     Next, likewise as with Sample 1, the GaAs substrate  13  was removed, to manufacture Sample 2 infrared-LED epitaxial wafers. 
     Next, electrodes wee formed onto the front and back sides of the epitaxial wafer in the same manner as with Sample 1, whereby Sample 2 infrared LEDs were thereby manufactured 
     Measurement Means 
     The Zn diffusion length in, and the light output from, Samples 1 and 2 were measured. Specifically, the Zn concentration in the interface between the active layer and the guide layers was characterized by secondary ion mass spectrometry, and further, the position in the active layer where the Zn concentration fell to 1/10 or less was measured by SIMS, and the distance into the active layer from the interface between the active layer and the guide layers was taken as the Zn diffusion length. In addition, the light output was measured in the same way as in Embodiment 3. The results are set forth in Table II below. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Zn diffusion 
                 Zn max. conc. within 
                 Light output 
               
               
                   
                 length (μm) 
                 active layer (cm −3 ) 
                 (mW) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Pres. invent. ex. 
                 0 
                 6.0 × 10 15   
                 1.3 
               
               
                 Comp. ex. 
                 0.3 
                 6.0 × 10 17   
                 0.62 
               
               
                   
               
            
           
         
       
     
     Measurement Results 
     As indicated in Table II, with Sample 1, in which the active layer was grown by OMVPE after the Al x Ga (1-x) As layer  11  had been grown by LPE, the Zn doped into the Al x Ga (1-x) As layer  11 , formed ahead of the active layer, could be prevented from diffusing into the active layer, and the Zn concentration within the active layer  21  could be reduced. As a result, the light output from the Sample 1 infrared LED could be dramatically improved over that from Sample 2. 
     From the foregoing, it was confirmed that in accordance with the present invention, forming the active-layer-incorporating epitaxial layer (Step S 7 ) after the Al x Ga (1-x) As layer  11  has been formed by LPE (Step S 2 ) enables the light output to be improved. 
     Embodiment 6 
     In the present embodiment, the effectiveness with which an infrared LED of 900 nm or greater wavelength could be prepared was examined. In the present embodiment, an infrared LED was manufactured in the same way as with the infrared LED manufacturing method of Embodiment 4, while differing only in terms of the active layer  21 . Specifically, in the present embodiment, an active layer  21  having 20 laminae of, respectively, well layers each having a thickness of 6 nm and being made of In 0.12 Ga 0.88 As and barrier layers each having a thickness of 12 nm and being made of GaAs 0.9 P 0.1  was grown. 
     The emission spectrum for this infrared LED was characterized. The result is graphed in  FIG. 28 . As indicated in  FIG. 28 , it was confirmed that an infrared LED of 940 nm emission wavelength could be manufactured. 
     Embodiment 7 
     In the present embodiment, the conditions for an epitaxial wafer to be utilized in an infrared LED of 900 nm or greater emission wavelength were examined. 
     Present Invention Examples 1 through 4 
     The infrared LEDs of Present Invention Examples 1 through 4 were manufactured in the same way as with the infrared LED manufacturing method of Embodiment 6, while differing only in terms of the Al x Ga (1-x) As layer  11  and the active layer  21 . Specifically, the average amount fraction of Al in the Al x Ga (1-x) As layers  11  was made as set forth in Table III below. The Al amount fraction in the major surface and in the rear face of the Al x Ga (1-x) As layers  11  was, to cite single instances in the order (rear face, major surface): for 0.05, (0.10, 0.01); for 0.15, (0.25, 0.05); for 0.25, (0.35, 0.15); and for 0.35, (0.40, 0.30). The average Al amount fraction and the amount fraction in the (rear face, major surface) are, however, adjustable at will. Here, the amount fraction of Al monotonically decreased heading from the rear face to the major surface of the Al x Ga (1-x) As layers  11 . And for the active layer  21  in this case, an active layer  21  having 5 laminae of, respectively, well layers each made of InGaAs and barrier layers each made of GaAs was grown. The infrared LEDs had an emission wavelength of 890 nm. 
     Present Invention Examples 5 through 8 
     The infrared LEDs of Present Invention Examples 5 through 8 were manufactured in the same way as with the infrared LED manufacturing method of Present Invention Examples 1 through 4, while differing in that the emission wavelength was 940 nm. 
     Comparative Examples 1 and 2 
     The infrared LEDs of Comparative Examples 1 and 2 were manufactured in the same way as with the infrared LEDs of Present Invention Examples 1 through 4 and Present Invention Examples 5 through 8, respectively, while differing in not being furnished with an Al x Ga (1-x) As layer  11 . That is, an Al x Ga (1-x) As layer  11  was not formed, nor was the GaAs substrate removed. 
     Measurement Method 
     Lattice relaxation with regard to the infrared LEDs of Present Invention Examples 1 through 8 and Comparative Examples 1 and 2 was determined. The lattice relaxation was carried out by photoluminescence spectroscopy, x-ray diffraction, and visual inspection of the surface. When the lattice-relaxed epitaxial wafers were fabricated into infrared LEDs, they were verified as such by dark lines. Also, the light output power of the infrared LEDs of Present Invention Examples 1 through 8 and Comparative Examples 1 and 2 was measured in the same way as in Embodiment 3. The results are set forth in Table III below. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE III 
               
             
            
               
                   
                   
               
               
                   
                 Substrate 
                 Active layer 
                   
                 Light 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Al amt. 
                   
                 Number 
                 Lattice 
                 Emission 
                 output 
               
               
                   
                 Material 
                 fract. 
                 Composition 
                 laminae 
                 relax. 
                 wvlng. 
                 power 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Pres. Inv. Ex. 1 
                 AlGaAs 
                 0.05 
                 InGaAs/GaAs 
                 5 
                 Absent 
                 890 nm 
                 5 mW 
               
               
                 Pres. Inv. Ex. 2 
                 AlGaAs 
                 0.15 
                 InGaAs/GaAs 
                 5 
                 Absent 
                 890 nm 
                 6 mW 
               
               
                 Pres. Inv. Ex. 3 
                 AlGaAs 
                 0.25 
                 InGaAs/GaAs 
                 5 
                 Absent 
                 890 nm 
                 6 mW 
               
               
                 Pres. Inv. Ex. 4 
                 AlGaAs 
                 0.35 
                 InGaAs/GaAs 
                 5 
                 Absent 
                 890 nm 
                 6 mW 
               
               
                 Comp. Ex. 1 
                 GaAs 
                 — 
                 InGaAs/GaAs 
                 5 
                 Absent 
                 890 nm 
                 1.5 mW   
               
               
                 Pres. Inv. Ex. 5 
                 AlGaAs 
                 0.05 
                 InGaAs/GaAs 
                 5 
                 Pres. 
                 940 nm 
                 2 mW 
               
               
                 Pres. Inv. Ex. 6 
                 AlGaAs 
                 0.15 
                 InGaAs/GaAs 
                 5 
                 Pres. 
                 940 nm 
                 3 mW 
               
               
                 Pres. Inv. Ex. 7 
                 AlGaAs 
                 0.25 
                 InGaAs/GaAs 
                 5 
                 Pres. 
                 940 nm 
                 3.5 mW   
               
               
                 Pres. Inv. Ex. 8 
                 AlGaAs 
                 0.35 
                 InGaAs/GaAs 
                 5 
                 Pres. 
                 940 nm 
                 3.5 mW   
               
               
                 Comp. Ex. 2 
                 GaAs 
                 — 
                 InGaAs/GaAs 
                 5 
                 Absent 
                 940 nm 
                 1.5 mW   
               
               
                   
               
            
           
         
       
     
     As indicated in Table III, in the infrared LEDs whose emission wavelength was 890 nm, there was no lattice relaxation (lattice misalignment), regardless of whether the substrate was a GaAs substrate or an Al x Ga (1-x) As layer. Likewise, in the infrared LED of Comparative Example 2, made from a GaAs substrate alone, there was no lattice relaxation, despite the emission wavelength being 940 nm. In the infrared LEDs of Present Invention Examples 5 through 8, however, which were furnished with an Al x Ga (1-x) As layer  11  as an Al x Ga (1-x) As substrate and which had an emission wavelength of 940 nm, there was lattice relaxation. Accordingly, with infrared LEDs furnished with an Al x Ga (1-x) As layer  11  as an Al x Ga (1-x) As substrate, whereas the output power of the infrared LEDs in which there was no lattice relaxation was 5 mW to 6 mW, the output power of the infrared LEDs in which there was lattice relaxation was a low 2 to 3.5 mW, wherein it was understood that inconsistency within the surface of the same wafer is significant. More particularly, the measurement inconsistency was in wafers having a 2- to 4-inch φ wafer diameter. 
     From this fact it was understood that technology that can be applied on GaAs substrates cannot be applied to epitaxial wafers that are utilized in infrared LEDs whose emission wavelength is 900 nm or greater. 
     Therein, the present inventors devoted research, as discussed below, to investigating the conditions whereby lattice relaxation is curbed in epitaxial wafers that are utilized in infrared LEDs whose emission wavelength is 900 nm or greater. 
     Specifically, in the following way, infrared LEDs of Present Invention Examples 9 through 24 and Comparative Examples 3 through 6, in which the emission wavelength was 940 nm, were manufactured. 
     Present Invention Examples 9 through 12 
     The infrared LEDs of Present Invention Examples 9 through 12 basically were manufactured in the same way as with the infrared LEDs of Present Invention Examples 5 through 8, while differing in that the number respectively of well layers and of barrier layers each was made three laminae. The In amount fraction in the well layers was 0.12. 
     Present Invention Examples 13 through 16 
     The infrared LEDs of Present Invention Examples 13 through 16 basically were manufactured in the same way as with the infrared LEDs of Present Invention Examples 5 through 8, while differing in having the barrier layers be GaAsP, and making the number of well layers and of barrier layers three laminae respectively. The P amount fraction in the barrier layers was 0.10. 
     Present Invention Examples 17 through 20 
     The infrared LEDs of Present Invention Examples 17 through 20 basically were manufactured in the same way as with the infrared LEDs of Present Invention Examples 13 through 16, while differing in that the number of well layers and of barrier layers was made ten laminae respectively. 
     Present Invention Examples 21 through 24 
     The infrared LEDs of Present Invention Examples 21 through 24 basically were manufactured in the same way as with the infrared LEDs of Present Invention Examples 5 through 8, while differing in having the barrier layers be AlGaAsP, and making the number of well layers and of barrier layers twenty laminae respectively. The P amount fraction in the barrier layers was 0.10. 
     Comparative Examples 3 through 6 
     The infrared LEDs of Comparative Example 3 basically were manufactured in the same way as with the infrared LEDs of, respectively, Present Invention Examples 9 through 12, Present Invention Examples 13 through 16, Present Invention Examples 17 through 20, and Present Invention Examples 21 through 24, while differing in that a GaAs substrate not furnished with an Al x Ga (1-x) As layer as an Al x Ga (1-x) As substrate was employed. 
     Measurement Method 
     In the same manner as with the methods explained above, the lattice relaxation and light output power were determined, the results are set forth in Table IV below. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE IV 
               
             
            
               
                   
                   
               
               
                   
                 Substrate 
                 Active layer 
                   
                 Light 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 Al amt. 
                   
                 Number 
                 Lattice 
                 output 
               
               
                   
                 Material 
                 fract. 
                 Composition 
                 laminae 
                 relax. 
                 power 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Pres. Inv. Ex. 9 
                 AlGaAs 
                 0.05 
                 InGaAs/GaAs 
                 3 
                 Absent 
                 6 mW 
               
               
                 Pres. Inv. Ex. 10 
                 AlGaAs 
                 0.15 
                 InGaAs/GaAs 
                 3 
                 Absent 
                 6 mW 
               
               
                 Pres. Inv. Ex. 11 
                 AlGaAs 
                 0.25 
                 InGaAs/GaAs 
                 3 
                 Absent 
                 6 mW 
               
               
                 Pres. Inv. Ex. 12 
                 AlGaAs 
                 0.35 
                 InGaAs/GaAs 
                 3 
                 Absent 
                 6 mW 
               
               
                 Comp. Ex. 3 
                 GaAs 
                 — 
                 InGaAs/GaAs 
                 3 
                 Absent 
                 1.5 mW   
               
               
                 Pres. Inv. Ex. 13 
                 AlGaAs 
                 0.05 
                 InGaAs/GaAsP 
                 3 
                 Absent 
                 6 mW 
               
               
                 Pres. Inv. Ex. 14 
                 AlGaAs 
                 0.15 
                 InGaAs/GaAsP 
                 3 
                 Absent 
                 6 mW 
               
               
                 Pres. Inv. Ex. 15 
                 AlGaAs 
                 0.25 
                 InGaAs/GaAsP 
                 3 
                 Absent 
                 6 mW 
               
               
                 Pres. Inv. Ex. 16 
                 AlGaAs 
                 0.35 
                 InGaAs/GaAsP 
                 3 
                 Absent 
                 6 mW 
               
               
                 Comp. Ex. 4 
                 GaAs 
                 — 
                 InGaAs/GaAsP 
                 3 
                 Absent 
                 1.5 mW   
               
               
                 Pres. Inv. Ex. 17 
                 AlGaAs 
                 0.05 
                 InGaAs/GaAsP 
                 10 
                 Absent 
                 6 mW 
               
               
                 Pres. Inv. Ex. 18 
                 AlGaAs 
                 0.15 
                 InGaAs/GaAsP 
                 10 
                 Absent 
                 6 mW 
               
               
                 Pres. Inv. Ex. 19 
                 AlGaAs 
                 0.25 
                 InGaAs/GaAsP 
                 10 
                 Absent 
                 6 mW 
               
               
                 Pres. Inv. Ex. 20 
                 AlGaAs 
                 0.35 
                 InGaAs/GaAsP 
                 10 
                 Absent 
                 6 mW 
               
               
                 Comp. Ex. 5 
                 GaAs 
                 — 
                 InGaAs/GaAsP 
                 10 
                 Absent 
                 1.5 mW   
               
               
                 Pres. Inv. Ex. 21 
                 AlGaAs 
                 0.05 
                 InGaAs/AlGaAsP 
                 20 
                 Absent 
                 6 mW 
               
               
                 Pres. Inv. Ex. 22 
                 AlGaAs 
                 0.15 
                 InGaAs/AlGaAsP 
                 20 
                 Absent 
                 6 mW 
               
               
                 Pres. Inv. Ex. 23 
                 AlGaAs 
                 0.25 
                 InGaAs/AlGaAsP 
                 20 
                 Absent 
                 6 mW 
               
               
                 Pres. Inv. Ex. 24 
                 AlGaAs 
                 0.35 
                 InGaAs/AlGaAsP 
                 20 
                 Absent 
                 6 mW 
               
               
                 Comp. Ex. 6 
                 GaAs 
                 — 
                 InGaAs/AlGaAsP 
                 20 
                 Absent 
                 1.5 mW   
               
               
                   
               
            
           
         
       
     
     Measurement Results 
     As indicated in Table IV, with Present Invention Examples 9 through 12, which included InGaAs wherein the well layers within the active layer  21  contained In, and whose number of well layers was four laminae or fewer, lattice relaxation did not occur. 
     Likewise, with Present Invention Examples 13 through 24, which included either GaAsP or AlGaAsP wherein the barrier layers within the active layer contained P, and whose number of barrier layers was three laminae or more, lattice relaxation did not occur. 
     From the foregoing, according to the present embodiments, it was discovered that in epitaxial wafers utilized in infrared LEDs whose emission wavelength is 900 nm or greater, lattice misalignment could be controlled to a minimum in instances where the well layers within the active layer include a material containing In and the number of well layers is four or fewer laminae, and in instances where the barrier layers within the active layer include a material containing P, the number of barrier layers is three or more laminae. 
     The presently disclosed embodiments and implementation examples should in all respects be considered to be illustrative and not limiting. The scope of the present invention is set forth not by the foregoing description but by the scope of the patent claims, and is intended to include meanings equivalent to the scope of the patent claims and all modifications within the scope. 
     REFERENCE SIGNS LIST 
     
         
           10   a ,  10   b : Al x Ga (1-x) As substrate;  11 : Al x Ga (1-x) As layer;  11   a ,  13   a : major surface;  11   b ,  13   b ,  20   c   2 ,  21   c : rear face;  13 : GaAs substrate;  20   a ,  20   b ,  20   c ,  40 ,  50 : epitaxial wafer;  20   c   1 : surface;  21 : active layer;  21   a : well layers;  12   b : barrier layers;  23 : contact layer;  30   a ,  30   b ,  30   c : LEDs;  31 ,  32 : electrodes;  33 : stem;  41 ,  44 : cladding layers;  42 ,  43 : undoped guiding layers.