JP2011146573A - Epitaxial wafer for infrared led and infrared led - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an epitaxial wafer for an infrared LED and an infrared LED, capable of improving reliability. <P>SOLUTION: An epitaxial wafer 20b for an infrared LED includes: an Al<SB>y</SB>Ga<SB>(1-y)</SB>As substrate (0≤y≤1) 10 including an Al<SB>x</SB>Ga<SB>(1-x)</SB>As layer 11 (0≤x≤1) having a major surface 11a and a rear face 11b on the reverse side of the major surface 11a; and an epitaxial layer 21 formed on the major surface 11a of the Al<SB>x</SB>Ga<SB>(1-x)</SB>As layer 11 and including an active layer. In the Al<SB>x</SB>Ga<SB>(1-x)</SB>As layer 11, a composition ratio x of Al of the rear face 11b is greater than a composition ratio x of Al of the major surface 11a, and a carrier concentration changes from the rear face 11b side toward the major surface 11a side. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an infrared wafer for an infrared LED and an infrared LED.

LED (Light Emitting Diode) using Al _a Ga _(1-a) As (0 ≦ a ≦ 1) (hereinafter also referred to as AlGaAs (aluminum gallium arsenide)) and a GaAs (gallium arsenide) compound semiconductor material Is widely used as an infrared light source. Infrared LEDs as an infrared light source are used in optical communications, spatial transmission, projectors, etc., and output increases with the increase in data transmission capacity, transmission distance, and illumination distance. Improvement is demanded.

  Semiconductor light emitting devices using such AlGaAs compound semiconductors are disclosed in, for example, Japanese Patent Application Laid-Open No. 2002-335008 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2002-335007 (Patent Document 2).

JP 2002-335008 A JP 2002-335007 A

  In Patent Documents 1 and 2, the Al composition ratio of the AlGaAs support substrate is made substantially uniform. As a result of intensive studies, the present inventor has found that when the Al composition ratio is high, defects are generated in the epitaxial layer when an epitaxial layer is formed on the AlGaAs support substrate. Further, as a result of intensive studies, the present inventor has found that when the Al composition ratio is low, there is a problem that the transmission characteristics of the AlGaAs support substrate are poor. Due to this, the present inventors have found that there is a problem that the reliability of the semiconductor light emitting devices of Patent Documents 1 and 2 is not sufficient.

  Then, the objective of this invention is providing the epitaxial wafer and infrared LED for infrared LED which can improve reliability.

Infrared-LED epitaxial wafer of the present invention, Al _y Ga ₍₁ including a main _{surface, Al x Ga (1-x} ) As layer having an opposed rear surface and a main surface (0 ≦ x ≦ 1) _{-y) an} As substrate (0 ≦ y ≦ 1) and an epitaxial layer formed on the main surface of the Al _x Ga _(1-x) As layer and including an active layer, Al _x Ga _{(1-x ) In the} As layer, the Al composition ratio x on the back surface is higher than the Al composition ratio x on the main surface, and in the Al _x Ga _(1-x) As layer, the carrier concentration from the back surface side toward the main surface side. Has changed.

In the infrared LED epitaxial wafer, preferably, the Al _x Ga _(1-x) As layer includes a layer in which the carrier concentration monotonously increases or monotonously decreases from the back surface side to the main surface side.

In the infrared LED epitaxial wafer, the carrier concentration of the Al _x Ga _(1-x) As layer is preferably 1 × 10 ¹⁷ cm ⁻³ or more and 3 × 10 ¹⁸ cm ⁻³ or less.

In the infrared LED epitaxial wafer, the absolute value of the carrier concentration gradient of the Al _x Ga _(1-x) As layer is preferably 3 × 10 ¹⁵ cm ⁻³ / μm or more and 5 × 10 ¹⁷ cm ⁻³ / It is below μm.

Preferably, in the infrared LED epitaxial wafer, the dopant of the Al _x Ga _(1-x) As layer is selected from the group consisting of silicon (Si), zinc (Zn), selenium (Se), and tellurium (Te). At least one kind of substance.

In the infrared LED epitaxial wafer, the Al _y Ga _(1-y) As substrate preferably includes a GaAs substrate in contact with the back surface of the Al _x Ga _(1-x) As layer.

  Preferably, the infrared LED epitaxial wafer further includes a transparent conductive film formed on the epitaxial layer.

  The infrared LED of the present invention comprises the above infrared LED epitaxial wafer and an electrode formed on the epitaxial wafer.

  The infrared LED preferably has a relative output of 70% or more with respect to the initial output when 1000 hours have passed with 100 mA energization at 85 ° C. or lower.

  The infrared LED preferably has a relative output of 70% or more with respect to the initial output when 3000 hours elapses with 100 mA energization at 85 ° C. or lower.

  According to the infrared-LED epitaxial wafer and the infrared LED of the present invention, the reliability can be improved.

It is sectional drawing which shows schematically the epitaxial wafer for infrared LED in Embodiment 1 of this invention. It is a figure for demonstrating the Al composition ratio x of the AlGaAs layer in Embodiment 1 of this invention. It is a figure for demonstrating the Al composition ratio x of the AlGaAs layer in Embodiment 1 of this invention. It is a figure for demonstrating the Al composition ratio x of the AlGaAs layer in Embodiment 1 of this invention. It is a figure for demonstrating the carrier concentration of the AlGaAs layer in Embodiment 1 of this invention. It is a figure for demonstrating the carrier concentration of the AlGaAs layer in Embodiment 1 of this invention. It is a figure for demonstrating the carrier concentration of the AlGaAs layer in Embodiment 1 of this invention. It is sectional drawing which shows schematically the epitaxial wafer for infrared LED in Embodiment 2 of this invention. It is sectional drawing which shows roughly infrared LED in Embodiment 3 of this invention. It is sectional drawing which shows roughly the infrared LED in Embodiment 4 of this invention. It is a figure which shows the carrier concentration of the AlGaAs layer in the example 1 of this invention. It is a figure which shows the gradient of the carrier concentration of the AlGaAs layer in the example 1 of this invention. It is a figure which shows the carrier concentration of the AlGaAs layer in the example 2 of this invention. It is a figure which shows the gradient of the carrier concentration of the AlGaAs layer in the example 2 of this invention. It is a figure which shows the carrier concentration of the AlGaAs layer in the example 3 of this invention. It is a figure which shows the gradient of the carrier concentration of the AlGaAs layer in the example 3 of this invention. It is a figure which shows the reliability of infrared LED of the example 1 of this invention. It is a figure which shows the reliability of infrared LED of the example 2 of this invention. It is a figure which shows the reliability of infrared LED of the example 3 of this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(Embodiment 1)
First, an infrared LED epitaxial wafer 20a in the present embodiment will be described with reference to FIG.

With reference to FIG. 1, epitaxial wafer 20a in the present embodiment will be described. As shown in FIG. 1, the epitaxial wafer 20a includes the AlGaAs substrate _{(Al y Ga (1-y} ) As substrate (0 ≦ y ≦ 1)) 10, an epitaxial layer 21 formed on the AlGaAs substrate 10, an epitaxial And a transparent conductive film 26 formed on the layer 21.

  The AlGaAs substrate 10 includes a GaAs substrate 13 and an AlGaAs layer 11 formed on the GaAs substrate 13.

  The GaAs substrate 13 may or may not have an off angle. For example, the GaAs substrate 13 has a {100} plane or a main surface inclined from 0 ° to 2 ° or less from {100}. It is preferable. The surface of the GaAs substrate 13 may be a mirror surface or a rough surface. In addition, {} indicates a collective surface.

  The AlGaAs layer 11 has a main surface 11a and a back surface 11b opposite to the main surface 11a. The main surface 11 a is a surface opposite to the surface in contact with the GaAs substrate 13. The back surface 11 b is a surface in contact with the GaAs substrate 13.

The AlGaAs layer 11 is represented by Al _x Ga _(1-x) As (0 ≦ x ≦ 1). In the AlGaAs layer 11, the Al composition ratio x of the back surface 11b is higher than the Al composition ratio x of the main surface 11a. The composition ratio x is the molar ratio of Al. The composition ratio (1-x) is a molar ratio of Ga.

  Here, the molar ratio of the AlGaAs layer 11 will be described with reference to FIGS. 2 to 4, the vertical axis indicates the position in the thickness direction from the back surface to the main surface of the AlGaAs layer 11, and the horizontal axis indicates the Al composition ratio x at each position.

  2 to 4, in the AlGaAs layer 11, the Al composition ratio x of the back surface 11b is higher than the Al composition ratio x of the main surface 11a. As shown in FIG. 2, in the AlGaAs layer 11, the Al composition ratio x preferably monotonously decreases from the back surface 11b to the main surface 11a. The monotonic decrease means that the composition ratio x is always the same or decreasing from the back surface 11b of the AlGaAs layer 11 toward the main surface 11a (in the growth direction), and the main surface 11a is more composing than the back surface 11b. It means that the ratio x is low. That is, the monotonic decrease does not include a portion where the composition ratio x increases toward this growth direction.

  As shown in FIG. 3 or 4, the AlGaAs layer 11 may include a plurality of layers (two layers in FIGS. 3 and 4). In this case, as shown in FIG. 3, the AlGaAs layer 11 preferably includes a layer in which the Al composition ratio x monotonously decreases from the back surface 11b toward the main surface 11a. Thereby, the curvature which arises in the AlGaAs substrate 10 can be relieved.

  The Al composition ratio x of the AlGaAs layer 11 is not limited to the above. For example, as shown in FIG. 4, the Al composition ratio x of each layer of the AlGaAs layer 11 may be uniform, and the layer on the back surface 11b side may be higher than the Al composition ratio x on the main surface 11a side.

  In the AlGaAs layer 11, the Al composition ratio x is preferably 0 or more and 0.45 or less. In this case, formation of an oxide layer on the AlGaAs layer 11 can be effectively suppressed and high transmission characteristics can be maintained.

  Here, the carrier concentration of the AlGaAs layer 11 will be described with reference to FIGS. 5 to 7, the vertical axis indicates the position in the thickness direction from the back surface 11b to the main surface 11a of the AlGaAs layer 11, and the horizontal axis indicates the carrier concentration at each position.

  As shown in FIGS. 5 to 7, in the AlGaAs layer 11, the carrier concentration changes from the back surface 11 b side to the main surface 11 a side.

  Note that “in the AlGaAs layer 11, the carrier concentration changes from the back surface 11 b side to the main surface 11 a side” means a layer in which the carrier concentration changes from the back surface 11 b side to the main surface 11 a side. This means that the carrier concentration on the main surface 11a and the back surface 11b is not particularly limited.

  As shown in FIGS. 5 and 6, it is preferable to include a layer in which the carrier concentration monotonously increases or monotonously decreases from the back surface 11b side to the main surface 11a side. The layer in which the carrier concentration is monotonously increased is that the carrier concentration is always the same or increased from the back surface 11b side of the AlGaAs layer 11 to the main surface 11a side (in the growth direction). This means that the surface of the main surface 11a has a higher carrier concentration than the surface on the back surface 11b side. That is, the layer in which the carrier concentration monotonously increases does not include a portion in which the carrier concentration decreases in the growth direction. The layer in which the carrier concentration is monotonously decreased is that the carrier concentration is always the same or decreased from the back surface 11b side of the AlGaAs layer 11 to the main surface 11a side (in the growth direction). This means that the main surface 11a side surface has a lower carrier concentration than the back surface 11b side surface. That is, the layer in which the carrier concentration monotonously decreases does not include a portion in which the carrier concentration increases in the growth direction in this layer.

  As shown in FIGS. 6 and 7, the AlGaAs layer 11 may include a plurality of layers (two layers in FIGS. 6 and 7). In this case, the AlGaAs layer 11 may include a layer that does not monotonously increase (or a layer that does not monotonously decrease), as shown in FIG.

  The AlGaAs layer 11 includes a layer in which the carrier concentration monotonously increases from the back surface 11b side to the main surface 11a side, and a layer in which the carrier concentration monotonously decreases from the back surface 11b side to the main surface 11a side. It is preferable not to include both. That is, the AlGaAs layer 11 includes only a layer whose carrier concentration monotonously increases from the back surface 11b side to the main surface 11a side, or the carrier concentration monotonously increases from the back surface 11b side to the main surface 11a side. Or a layer having a uniform carrier concentration, or a layer in which the carrier concentration monotonously decreases from the back surface 11b toward the main surface 11a, or a carrier concentration monotonously decreases from the back surface 11b toward the main surface 11a. And a layer having a uniform carrier concentration. In this case, since the AlGaAs layer 11 can be easily manufactured, the cost can be reduced.

  Note that “the AlGaAs layer 11 includes a layer whose carrier concentration is monotonously increasing or monotonically decreasing from the back surface 11b toward the main surface 11a” means that at least a part of the AlGaAs layer 11 is on the back surface 11b side. Means that the carrier concentration is monotonously increased or monotonously decreased from the main surface 11a toward the main surface 11a, and the carrier concentration on the main surface 11a and the back surface 11b is not particularly limited.

The absolute value of the gradient of the carrier concentration in the AlGaAs layer 11 is preferably 3 × 10 ¹⁵ cm ⁻³ / μm or more and 5 × 10 ¹⁷ cm ⁻³ / μm or less, and preferably 5 × 10 ¹⁵ cm ⁻³ / μm or more. × and more preferably 10 ¹⁷ cm ^-3 / μm or less. In other words, it is preferable that the absolute value of the gradient of the carrier concentration of at least a partial region of the AlGaAs layer 11 is not more than 3 × 10 ¹⁵ cm ^-3 / μm or more ^{5 × 10 17 cm -3 / μm} , 5 × 10 15 More preferably, it is not less than cm ⁻³ / μm and not more than 5 × 10 ¹⁷ cm ⁻³ / μm. The carrier concentration gradient is a value obtained by measuring Δcc / Δd by measuring a carrier concentration difference Δcc at an arbitrary thickness Δd of the AlGaAs layer 11.

Range of the carrier concentration of the AlGaAs layer 11, it is preferably, 1 × 10 ¹⁷ cm ^-3 or more 2 × 10 ¹⁸ cm ^-3 or less than 1 × 10 ¹⁷ cm ^-3 3 × 10 ¹⁸ cm ^-3 or less It is more preferable.

  The dopant of the AlGaAs layer 11 is not particularly limited. For example, p-type dopants such as zinc, magnesium (Mg), and carbon (C), and n-type dopants such as selenium, sulfur (S), and tellurium can be used. The dopant of the AlGaAs layer 11 is preferably at least one substance selected from the group consisting of silicon, zinc, selenium, and tellurium. However, when the AlGaAs layer 11 has a plurality of layers, it may be changed for each layer.

  Epitaxial layer 21 includes an active layer. The active layer desirably has a smaller band gap than the AlGaAs layer 11. However, since the Al composition ratio has a gradient, the magnitude relationship may be partially reversed.

  The active layer preferably has a multiple quantum well structure (MQW structure) in which well layers and barrier layers having a larger band gap than the well layers are alternately stacked. In the case where the active layer has a multiple quantum well structure, the active layer preferably has 3 to 20 well layers. In this case, when an infrared LED is manufactured, the rise time and fall time in pulse application can be shortened.

  The material of the well layer is not particularly limited as long as the band gap is smaller than that of the barrier layer. be able to. These materials are materials that emit infrared light that are compatible with the degree of lattice matching with the AlGaAs layer 11. However, even if the lattice matching is deviated, it is sufficient that the distortion is not relaxed completely or partially.

  The material of the barrier layer is not particularly limited as long as the band gap is larger than that of the well layer. For example, AlGaAs, GaAsP (gallium arsenide phosphorus), AlGaAsP (aluminum gallium arsenide phosphorus), InGaP, AlInGaP, InGaAsP, or the like can be used. These materials are materials that are compatible with the degree of lattice matching with the AlGaAs layer 11. Further, as in the case of the well layer, even if the lattice matching is deviated, it is sufficient that the strain is not relaxed completely or partially.

  The active layer preferably has a thickness of 1.5 μm or less, and more preferably has a thickness of 300 nm to 1300 nm. In this case, when an infrared LED is manufactured, the rise time and fall time in pulse application can be shortened.

  The active layer is not particularly limited to a multiple quantum well structure, and may be composed of one layer or may have a double hetero structure.

  Moreover, although the case where the epitaxial layer 21 contained the active layer was demonstrated in this Embodiment, the epitaxial layer 21 may contain another layer. The epitaxial layer 21 including other layers includes, for example, an n-type buffer layer, an n-type cladding layer formed on the n-type buffer layer, an active layer formed on the n-type cladding layer, and an active layer. A p-type cladding layer formed, a p-type window layer formed on the p-type cladding layer, and a p-type contact layer formed on the p-type window layer are included. In the above configuration, at least one of the other layers may be omitted, and the p-type and n-type may be reversed.

The uppermost layer of the epitaxial layer 21 (in this embodiment, a layer in contact with the transparent conductive film 26) is preferably p-type. In this case, the epitaxial layer 21 includes a p-type cladding layer formed on the active layer, and the p-type cladding layer has a carrier concentration of 1 × 10 ¹⁷ cm ⁻³ or more and 2 × 10 ¹⁸ cm ⁻³ or less. Is preferred. The epitaxial layer 21 includes a p-type cladding layer formed on the active layer and a p-type window layer formed on the p-type cladding layer, and at least one of the p-type cladding layer and the p-type window layer is More preferably, it has a carrier concentration of 1 × 10 ¹⁷ cm ⁻³ or more and 2 × 10 ¹⁸ cm ⁻³ or less.

  The transparent conductive film 26 has an effect of spreading the current over the entire surface (lateral direction in FIG. 1) on the top surface of the chip when an infrared LED is manufactured using the epitaxial wafer 20a. As a result, current is injected into the active layer over the entire surface of the chip, light emission occurs, and high output can be achieved. Further, when there is no transparent conductive film 26, the series resistance on the upper surface of the chip becomes high, which causes the high-speed response to deteriorate.

  The transparent conductive film 26 has high permeability and low resistivity. Such a transparent conductive film 26 has a transmittance of 85% or more at a wavelength of 850 nm or more and 1000 nm or less, for example, and a resistivity of 5 mΩcm or less at a thickness of 300 nm, for example.

As the transparent conductive film, for example, ITO which is indium oxide (In ₂ O ₃ ) doped with tin (Sn), indium oxide (In ₂ O ₃ ), IFO which is In ₂ O ₃ doped with fluorine (F) , Tin oxide (SnO ₂ ), antimony (Sb) -doped SnO ₂ ATO, F-doped SnO ₂ FTO, cadmium (Cd) -doped SnO ₂ CTO, aluminum (Al ) Doped with zinc oxide (ZnO), IZO as Zn doped with In, GZO as ZnO doped with Ga, and the like. In particular, it is preferable that the transparent conductive film 26 is ITO because transparency and conductivity can be maintained in a well-balanced manner.

  Note that the transparent conductive film 26 may be omitted. Moreover, when the epitaxial wafer 20a is provided with the transparent conductive film 26, the transparent conductive film 26 is not specifically limited to said material, For example, the metal film which made thickness 50 nm or less so that light may permeate | transmit is used. May be. As such a metal film, for example, gold (Au) or Al having a thickness of 15 nm or less can be used.

  The thickness of the epitaxial layer 21 is preferably 50 μm or more and 300 μm or less. In the case of 50 μm or more, handling can be improved. In the case of 300 μm or less, the output can be improved and the cost can be reduced.

  Then, the manufacturing method of the epitaxial wafer 20a in this Embodiment is demonstrated.

  First, a GaAs substrate 13 is prepared. Next, the AlGaAs layer 11 having the main surface 11a is grown on the GaAs substrate 13 by, for example, an LPE (Liquid Phase Epitaxy) method. In the step of growing the AlGaAs layer 11, the Al composition ratio x at the interface (back surface 11b) with the GaAs substrate 13 is higher than the Al composition ratio x of the main surface 11a, and the main surface 11a from the back surface 11b side. The AlGaAs layer 11 whose carrier concentration is changed toward the side is grown.

  The LPE method is not particularly limited, and a slow cooling method, a temperature difference method, or the like can be used. The LPE method is a method for growing an AlGaAs crystal from a liquid phase. The slow cooling method is a method of growing AlGaAs crystals by gradually lowering the temperature of the raw material solution. The temperature difference method is a method in which a temperature gradient is created in a raw material solution to grow an AlGaAs crystal.

  When growing a layer having a constant Al composition ratio x and / or carrier concentration in the AlGaAs layer 11, it is preferable to use a temperature difference method or a slow cooling method. When growing a layer in which the Al composition ratio x decreases upward (growth direction) and / or when growing a layer whose carrier concentration changes in the growth direction, it is preferable to use a slow cooling method. In particular, since it is excellent in mass productivity and low cost, it is preferable to use a slow cooling method. Moreover, you may combine them.

In this step, it is preferable to grow an AlGaAs layer including a layer in which the carrier concentration monotonously increases or monotonously decreases from the back surface 11b side to the main surface 11a side. In this case, it is preferable to grow the AlGaAs layer 11 so that the absolute value of the gradient of the carrier concentration is 3 × 10 ¹⁵ cm ⁻³ / μm to 5 × 10 ¹⁷ cm ⁻³ / μm. Further, it is preferable to grow the AlGaAs layer 11 so that the carrier concentration is 1 × 10 ¹⁷ cm ⁻³ or more and 3 × 10 ¹⁸ cm ⁻³ or less.

  Next, the main surface 11a of the AlGaAs layer 11 is polished. When the AlGaAs layer 11 is grown by the LPE method, the main surface 11a is uneven, but the main surface 11a can be flattened by this process.

  Thereby, the AlGaAs substrate 10 including the AlGaAs layer 11 and the GaAs substrate 13 can be manufactured.

  Next, an active layer is included on the main surface 11a of the AlGaAs layer 11 of the AlGaAs substrate 10 by an OMVPE (Organo Metallic Vapor Phase Epitaxy) method or an MBE (Molecular Beam Epitaxy) method. Epitaxial layer 21 is formed. In this step, an epitaxial layer including the active layer as described above is grown on the AlGaAs layer 11.

  In the OMVPE method, the active gas is grown by the thermal decomposition reaction of the source gas on the AlGaAs layer 11, and in the MBE method, the active layer is grown by a method that does not involve a chemical reaction process in a non-equilibrium system. Can easily control the thickness of the active layer. Therefore, an active layer having a plurality of well layers of two or more layers can be easily grown.

  Further, since the main surface 11a of the AlGaAs layer 11 of the AlGaAs substrate 10 is flat, the abnormal growth of the epitaxial layer 21 can be suppressed when the epitaxial layer 21 including the active layer is formed on the main surface 11a of the AlGaAs layer 11. it can.

  Next, a transparent conductive film 26 is formed on the epitaxial layer 21. A method for forming the transparent conductive film 26 is not particularly limited, and for example, an electron beam evaporation method, a sputtering method, or the like can be employed.

  By performing the above steps, the epitaxial wafer 20a shown in FIG. 1 can be manufactured. A step of removing a part of the GaAs substrate 13 may be further performed. In this case, an epitaxial wafer 20a having a GaAs substrate having a smaller thickness than the prepared GaAs substrate is manufactured.

  As described above, epitaxial wafer 20a in the present embodiment is formed of GaAs substrate 13, AlGaAs layer 11 formed on GaAs substrate 13 and having main surface 11a and back surface 11b opposite to main surface 11a. And an epitaxial layer 21 formed on the main surface 11a of the AlGaAs layer 11 and including an active layer. In the AlGaAs layer 11, the Al composition ratio x of the back surface 11b is equal to that of the main surface 11a. It is higher than the Al composition ratio x, and in the AlGaAs layer 11, the carrier concentration changes from the back surface 11b side to the main surface 11a side.

  According to epitaxial wafer 20a in the present embodiment, Al composition ratio x of back surface 11b is higher than Al composition ratio x of main surface 11a. For this reason, it can suppress that Al which has the property which is easy to be oxidized exists in the main surface 11a. Thus, it is possible to suppress the formation of an insulating oxide layer on the surface of the AlGaAs substrate 10 (in this embodiment, the main surface 11a of the AlGaAs layer 11). For this reason, it is possible to suppress the introduction of defects into the epitaxial layer formed on the AlGaAs substrate 10.

  The Al composition ratio x of the main surface 11a is lower than the Al composition ratio x of the back surface 11b. As a result of diligent research, the present inventor has found that the higher the Al composition ratio x, the better the transmission characteristics of the AlGaAs substrate 10. Even if a large amount of Al is contained on the back surface 11b side, since the time exposed on the surface is short, the formation of an oxide layer on the main surface 11a can be reduced. For this reason, the transmission characteristics can be improved by growing an AlGaAs crystal having a high Al composition ratio x in a portion where the formation of the oxide layer can be suppressed.

  In this way, in the AlGaAs layer 11, the Al composition ratio x is lowered so as to suppress the introduction of defects on the main surface 11a side, and the Al composition ratio x is improved so as to improve the transmission characteristics on the back surface 11b side. Is high. Therefore, when an infrared LED is manufactured using epitaxial wafer 20a in the present embodiment, heat generation due to defects can be suppressed, and heat generation due to light absorption due to improved transmission characteristics can be suppressed.

  Further, in the AlGaAs layer 11, the carrier concentration changes from the back surface 11b side to the main surface 11a side. In a region where the carrier concentration is high, heat generation can be suppressed by improving the conductivity. In a region where the carrier concentration is low, the conductivity is low but the transmittance can be improved, so that heat generation due to light absorption can be suppressed. For this reason, the carrier concentration can be controlled in the entire AlGaAs layer 11 by changing the carrier concentration from the back surface 11b toward the main surface 11a so that the balance between conductivity and transmittance is good. Therefore, the influence of heat generation due to conductivity and transmittance can be reduced.

  As described above, since the heat generation of the infrared LED can be suppressed when the infrared LED is manufactured using the epitaxial wafer 20a, the reliability can be improved.

  In addition, since the resistivity and transmission characteristics are controlled by the entire AlGaAs layer 11, the epitaxial wafer 20a can reduce the cost and improve the reliability.

  In the epitaxial wafer 20a of the present embodiment, the AlGaAs substrate 10 includes a GaAs substrate 13 in contact with the back surface 11b of the AlGaAs layer 11.

  When the AlGaAs substrate 10 includes the GaAs substrate 13, not only the AlGaAs layer 11 but also the entire epitaxial wafer 20a can be designed with the GaAs substrate 13 to suppress the occurrence of wafer cracking in the chip manufacturing process. be able to.

  In addition, since it is not necessary to remove the GaAs substrate 13 in the present embodiment, it is possible to shorten the time for the process of removing the GaAs substrate 13 as compared with the second embodiment in which the GaAs substrate 13 described later is removed. For this reason, cost can be reduced more.

  Further, when an electrode is formed on the GaAs substrate 13 in order to fabricate an infrared LED using the epitaxial wafer 20a in the present embodiment, a contact electrode having a lower resistance than the case where an electrode is formed on the AlGaAs layer 11. Can be produced. For this reason, an infrared LED having higher reliability and lower operating temperature can be manufactured.

  Preferably, in the epitaxial wafer 20a, the AlGaAs layer 11 includes a layer whose carrier concentration monotonously increases or decreases monotonously from the back surface 11b side toward the main surface 11a side.

  Thereby, in order to improve the balance between conductivity and transmittance, the carrier concentration can be controlled more easily in the entire AlGaAs layer 11, so that an epitaxial wafer 20a with improved reliability by suppressing an increase in cost is realized. can do.

In the epitaxial wafer 20a, the carrier concentration of the AlGaAs layer 11 is preferably 1 × 10 ¹⁷ cm ⁻³ or more and 3 × 10 ¹⁸ cm ⁻³ or less. Thereby, since high conductivity and high transmittance can be compatible in the AlGaAs layer 11, the reliability can be further improved.

In the epitaxial wafer 20a, the absolute value of the carrier concentration gradient of the AlGaAs layer 11 is preferably 3 × 10 ¹⁵ cm ⁻³ / μm or more and 5 × 10 ¹⁷ cm ⁻³ / μm or less. As a result, the high conductivity and high transmittance of the AlGaAs layer 11 can be compatible, and the reliability can be further improved.

  Preferably, in the epitaxial wafer 20a, the dopant of the AlGaAs layer 11 is at least one substance selected from the group consisting of silicon, zinc, selenium, and tellurium. Thereby, since the carrier concentration can be controlled well, the reliability can be further improved.

  The epitaxial wafer 20 a preferably further includes a transparent conductive film 26 formed on the epitaxial layer 21.

  The transparent conductive film 26 can promote current diffusion even when the epitaxial layer 21 is thin. Also, the lateral resistance can be reduced. For this reason, the high output and high-speed responsiveness of infrared LED produced using this epitaxial wafer 20a are realizable.

(Embodiment 2)
With reference to FIG. 8, infrared LED epitaxial wafer 20b in the present embodiment will be described.

  Referring to FIG. 8, epitaxial wafer 20b in the present embodiment basically has the same configuration as epitaxial wafer 20a in the first embodiment, but differs in that GaAs substrate 13 is not provided.

  The thickness of the AlGaAs layer 11 in the present embodiment is preferably so thick that the AlGaAs substrate 10 becomes a free-standing substrate. Such a thickness is, for example, 70 μm or more.

  Then, the manufacturing method of the epitaxial wafer 20b in this Embodiment is demonstrated. The method for manufacturing epitaxial wafer 20b in the present embodiment has basically the same configuration as the method for manufacturing epitaxial wafer 20a in the first embodiment, but further includes a step of removing GaAs substrate 13. It is different in point.

  The method for removing the GaAs substrate 13 is not particularly limited, and for example, a method such as polishing or etching can be used. Polishing refers to mechanically scraping off the GaAs substrate 13 using a polishing agent such as alumina, colloidal silica, diamond, or the like in a grinding facility having a diamond grindstone. Etching means that the GaAs substrate 13 is removed using a selective etching solution having a slow etching rate with AlGaAs and a fast etching rate with GaAs by optimally preparing ammonia, hydrogen peroxide, or the like.

  As described above, epitaxial wafer 20b in the present embodiment includes AlGaAs substrate 10 including AlGaAs layer 11 having main surface 11a and back surface 11b opposite to main surface 11a, and main surface 11a of AlGaAs layer 11. And an epitaxial layer 21 including an active layer formed thereon. In the AlGaAs layer 11, the Al composition ratio x of the back surface 11b is higher than the Al composition ratio x of the main surface 11a, and the AlGaAs layer 11 The carrier concentration changes from the back surface 11b side to the main surface 11a side.

  The epitaxial wafer 20b according to the present embodiment includes the AlGaAs substrate 10 having only the AlGaAs layer 11 without including the GaAs substrate 13. Since the GaAs substrate 13 absorbs light having a wavelength of 870 nm or less, if an infrared LED is manufactured using the epitaxial wafer 20b including the AlGaAs substrate 10 from which the GaAs substrate 13 has been removed, the transmittance can be further improved. For this reason, since heat_generation | fever of the infrared LED produced using this epitaxial wafer 20b can be suppressed more, reliability can be improved more.

In the epitaxial wafer 20b, the absolute value of the carrier concentration gradient of the AlGaAs layer 11 is preferably 3 × 10 ¹⁵ cm ⁻³ / μm or more and 5 × 10 ¹⁷ cm ⁻³ / μm or less. Thereby, the reliability can be further improved and the connection with the electrode when the electrode is formed on the AlGaAs layer 11 can be maintained well.

(Embodiment 3)
With reference to FIG. 9, the infrared LED 30a in the present embodiment will be described. As shown in FIG. 9, infrared LED 30a in the present embodiment is formed on epitaxial wafer 20a for infrared LED shown in FIG. 1 in the first embodiment, and on the main surface and the back surface of this epitaxial wafer 20a. Electrodes 31 and 32. In order to extract light, the electrode 31 covers only a part of the surface of the epitaxial wafer 20a and exposes the remaining part. The electrode 32 covers the back surface of the epitaxial wafer 20a. The electrode 32 may cover the entire back surface of the epitaxial wafer 20a or a part thereof. When a part is covered, it can be formed in a dot shape or a lattice shape, for example.

  When the layer in contact with the transparent conductive film 26 in the epitaxial layer 21 is p-type, the electrode 31 is an alloy of, for example, Au (gold) and Zn (zinc) when the transparent conductive film 26 is present. If there is no p-type electrode, for example, it is a p-type electrode made of Au. The electrode 32 formed under the AlGaAs substrate 10 is an n-type electrode made of an alloy of, for example, Au and Ge (germanium).

  The emission wavelength of the infrared LED 30a is, for example, not less than 850 nm and not more than 1 μm. In the infrared LED 30a, the relative output with respect to the initial output when 1000 hours have passed with 100 mA energization at 85 ° C. or lower is 70% or more, preferably 90% or more. Moreover, it is more preferable that the relative output with respect to the initial output when lapse of 3000 hours with 100 mA energization at 85 ° C. or less is 70% or more. Thus, the reliability of the infrared LED 30a is very high.

  The “relative output with respect to the initial output” means that the initial output and the output when 1000 hours have passed are measured with, for example, an integrating sphere at 85 ° C., and 1000 hours have elapsed when the initial output is taken as 100%. It means the output ratio of hour.

  Unlike the present embodiment, the preceding examples (Japanese Patent Laid-Open No. 2002-335008 (Patent Document 1) and Japanese Patent Laid-Open No. 2002-335007 (Patent Document 2)) describe LEDs having no gradient in Al composition. It has been. However, there are many problems in industrial use when it is put to practical use. The outline is summarized below.

  In Patent Document 1, it is described that an InAlGaAs layer is used as a material of the buffer layer. When this layer is not used, the problem that the light output decreases with the energization time is described as a comparative example. Specifically, there is a problem that the current is reduced to about 71% of the initial output by energization at 100 mA for 1000 hours.

  The method used here is characterized in that an InAlGaAs layer is used as the buffer layer and the lattice constant is made larger than that of the base substrate, or a single layer and a plurality of layers are used. In the case of this method, there are three problems, which are problematic in terms of manufacturing cost, yield, and reliability.

  First, it is necessary to sufficiently control the composition and thickness of the InAlGaAs layer. In particular, in the present embodiment, there are two to three constituent elements, whereas in Patent Document 1, there are four constituent elements, which makes controllability more difficult and causes an increase in cost. In addition, since there is no lattice matching, if there is a deviation between the lattice constant and the thickness, crystal dislocation occurs easily, which causes a decrease in reliability. Further, due to lattice mismatch, the wafer is warped and not only causes wafer breakage, but also causes a decrease in reliability and a decrease in yield.

  In the present embodiment, binary or ternary materials such as AlGaAs or GaAs are used, so that these manufacturing cost, yield, and reliability problems can be solved.

  Patent Document 2 describes the light output retention rate when the natural oxide film removal process of the AlGaAs substrate is performed before the active layer growth. When the natural oxide film removal treatment is not performed, the current is reduced to about 60% of the initial output by energization at 100 mA for 1000 hours, and even if the natural oxide film removal treatment is performed in the atmosphere, the initial output is reduced to about 65%. It has been.

In the method used here, in order to prevent oxidation of the AlGaAs substrate surface, a glove box in an N ₂ atmosphere is used. In this method, the substrate is washed with a halogen-based solution and then dried without washing with water. Moreover, finally, it conveys to OM equipment, without exposing to air | atmosphere.

  Even in this method, there are three problems, which are problematic in terms of manufacturing cost, yield, and reliability.

  It is important not to be exposed to the atmosphere between the two types of equipment, etching and OM furnace growth, but in general, preventing air contamination requires considerable equipment management labor, and wastes time and money. Become. In the mass production stage using a large number of wafers, there are many restrictions in terms of work management and expandability. Although etching is performed with a halogen-based solution, alcohol cleaning is used instead of water cleaning. In general, halogen-based ions (fluorine, chlorine, etc.) are likely to remain on the wafer surface, and if left, cause long-term reliability deterioration and also decrease yield.

  In the present embodiment, since the main surface Al composition is lowered to prevent oxidation, the above-described large-scale equipment is not necessary, and the problems of manufacturing cost, yield, and reliability are solved. it can.

  In this embodiment, 100 mA is energized at 85 ° C., and the initial output and the rate of decrease in output when 1000 hours have elapsed are verified. From this, when the reliability time of energization at 25 ° C. and 100 mA is calculated, 10000 It has been estimated that the relative output with respect to the initial output is 70% or more until the time or more. As a result, it has been confirmed that the LED is not affected by the lamp shape, the lamp mounting form, and the usage environment of the lamp, that is, industrially sufficient characteristics can be obtained in a wide range from room temperature to 85 ° C.

  Then, the manufacturing method of infrared LED30a in this Embodiment is demonstrated. First, the epitaxial wafer 20a is manufactured by the manufacturing method of the infrared LED epitaxial wafer 20a in the first embodiment.

  Next, the electrodes 31 and 32 are formed on the main surface and the back surface of the infrared-LED epitaxial wafer 20a. Specifically, for example, by vapor deposition, Au and Zn are vapor-deposited on the main surface, and Au and Ge are vapor-deposited on the back surface, and then alloyed to form the electrodes 31 and 32. .

  By performing the above steps, the infrared LED 30a shown in FIG. 9 can be manufactured.

  As described above, the infrared LED 30 a in the present embodiment includes the epitaxial wafer 20 a in the first embodiment and the electrodes 31 and 32 formed on the epitaxial layer 21.

  According to the infrared LED 30a in the present embodiment, since the AlGaAs substrate 10 in which the Al composition ratio x and the carrier concentration of the AlGaAs layer 11 are controlled is used, the infrared LED 30a with improved reliability can be realized.

  In the infrared LED 30a, the relative output with respect to the initial output when 1000 hours have passed with 100 mA energization at 85 ° C. or less is preferably 70% or more. More preferably, the infrared LED 30a has a relative output of 70% or more with respect to the initial output when 3000 hours have passed with 100 mA energization at 85 ° C. or lower. By using the epitaxial wafer 20a of the first embodiment, the infrared LED 30a having improved reliability can be realized.

(Embodiment 4)
With reference to FIG. 10, infrared LED 30b in the present embodiment will be described. As shown in FIG. 10, the infrared LED 30 b in the present embodiment basically has the same configuration as the infrared LED 30 a in the third embodiment, but differs in that the GaAs substrate 13 is not provided.

  Specifically, the infrared LED 30b of the present embodiment includes the epitaxial wafer 20b of the second embodiment. The electrode 31 is provided in contact with the surface of the epitaxial wafer 20b, and the electrode 32 is provided in contact with the back surface (in this embodiment, the back surface 11b of the AlGaAs layer 11).

  The manufacturing method of the infrared LED 30b in the present embodiment basically has the same configuration as the manufacturing method of the infrared LED 30a in the third embodiment, but is implemented instead of the epitaxial wafer 20a in the first embodiment. This is different in that the epitaxial wafer 20b of the form is manufactured.

  As described above, the infrared LED 30 b in the present embodiment includes the epitaxial wafer 20 b in the second embodiment and the electrodes 31 and 32 formed on the epitaxial layer 21.

  According to the infrared LED 30b in the present embodiment, the AlGaAs substrate 10 including only the AlGaAs layer 11 without including the GaAs substrate 13 is provided. Since the GaAs substrate 13 absorbs light having a wavelength of 870 nm or less, the infrared LED 30b including the AlGaAs substrate 10 from which the GaAs substrate 13 has been removed can further improve the transmittance. Therefore, since heat generation can be further suppressed, the reliability of the infrared LED 30b can be further improved.

  In this embodiment, in the AlGaAs layer 11, the Al composition ratio x of the back surface 11b is higher than the Al composition ratio x of the main surface 11a, and the carrier concentration changes from the back surface 11b side to the main surface 11a side. We investigated the effect of being.

(Invention Example 1)
Invention Example 1 followed the manufacturing method of the infrared LED 30b in the fourth embodiment. Specifically, first, a GaAs substrate 13 manufactured by the VB method and having a diameter of 2 inches and a thickness of 270 μm was prepared.

Next, three AlGaAs layers 11 were grown on the GaAs substrate 13 by a slow cooling method under a temperature condition between 920 ° C. and room temperature. In each layer, the Al composition ratio on the back surface 11b side is 0.3, the Al composition ratio on the main surface 11a side is 0.1, and the Al composition ratio always decreases from the back surface 11b side to the main surface 11a side. Was. Further, as shown in FIG. 11, in each layer, the back surface 11b has a carrier concentration of 1.0 × 10 ¹⁷ cm ⁻³ and the main surface 11a has a carrier concentration of 2.0 × 10 ¹⁸ cm ⁻³ . The carrier concentration always increased from the 11b side toward the main surface 11a side. The position on the horizontal axis in FIG. 11 means that the position of the back surface 11b of the AlGaAs layer 11 is 0, and the position from the back surface 11b to the main surface 11a (the thickness direction from the back surface 11b of the AlGaAs layer 11 to the main surface 11a Position) (the same applies to FIGS. 12 to 16). The thickness of each layer was 50 μm, and the thickness of the AlGaAs layer was 150 μm. Further, tellurium was doped as a dopant.

The absolute value of the carrier concentration gradient of the AlGaAs layer 11 was not less than 5 × 10 ¹⁵ cm ⁻³ / μm and not more than 5 × 10 ¹⁷ cm ⁻³ / μm, as shown in FIG. As the gradient of the carrier concentration shown in FIG. 12, the gradient at each position shown in FIG. 11 (carrier concentration difference Δcc / position difference Δd) was calculated. In this embodiment, the gradient of the tangent line at each position is defined as a gradient.

Next, an epitaxial layer including an active layer was formed by OMVPE. Specifically, an n-type buffer layer, an n-type cladding layer, an active layer, a p-type cladding layer, a p-type window layer, and a p-type contact layer were grown in this order on the main surface 11a of the AlGaAs layer 11. The growth temperature of each layer was 760 ° C. The n-type buffer layer had a thickness of 0.5 μm, was made of Al _0.15 Ga _0.85 As doped with Si, and had a carrier concentration of 5 × 10 ¹⁷ cm ⁻³ . The n-type cladding layer had a thickness of 1.0 μm, and was made of Al _0.35 Ga _0.65 As doped with Si, and had a carrier concentration of 5 × 10 ¹⁷ cm ⁻³ . The p-type cladding layer had a thickness of 1.0 μm, was made of Zn-doped Al _0.35 Ga _0.65 As, and had a carrier concentration of 5 × 10 ¹⁷ cm ⁻³ . The p-type window layer had a thickness of 3.5 μm, was made of Zn-doped Al _0.20 Ga _0.80 As, and had a carrier concentration of 5 × 10 ¹⁷ cm ⁻³ . The p-type contact layer had a thickness of 0.2 μm, was made of Zn-doped GaAs, and had a carrier concentration of 4 × 10 ¹⁹ cm ⁻³ . The active layer has an emission wavelength of 940 nm, a thickness of 5 nm, a well layer made of In _0.25 Ga _0.75 As, and a barrier layer made of Al _0.30 Ga _0.70 As, each having a thickness of 15 nm. The layer had a multiple quantum well structure (MQW). The thickness of the active layer was 897 nm.

  Next, a transparent conductive film was formed on the surface of the epitaxial layer opposite to the surface in contact with the AlGaAs substrate by electron beam evaporation at a temperature of 300 ° C. in an oxygen atmosphere in a vacuum furnace. The transparent conductive film was ITO having a thickness of 300 nm.

  Next, a p-type electrode was formed as the electrode 31 on the transparent conductive film 26. The pad diameter of the electrode 31 was 120 μm, the electrode material was Au, and the total thickness was 1 μm.

  Next, the GaAs substrate 13 was removed, and an n-type electrode was formed as the electrode 32 on the back surface 11 b of the AlGaAs layer 11. The electrode 32 was in the form of dots, the material was an AuGe alloy, and the thickness was 1 μm.

By carrying out the above steps, an infrared LED 30b of Example 1 of the present invention was manufactured.
(Invention Example 2)
Infrared LED 30b of Invention Example 2 was basically manufactured in the same manner as Infrared LED 30b of Invention Example 1, but provided with a thickness of the AlGaAs layer, an Al composition ratio and a carrier concentration gradient, and a transparent conductive film. It was different in that it was not. The emission wavelength of Example 2 of the present invention was 850 nm.

  Specifically, the AlGaAs layer 11 includes three layers, each having a thickness of 60 μm to 70 μm, and the entire thickness of the AlGaAs layer 11 is 200 μm.

  In each layer, the Al composition ratio on the back surface 11b side is 0.45, the Al composition ratio on the main surface 11a side is 0.2, and the Al composition ratio from the back surface 11b side to the main surface 11a side is It was constantly decreasing.

Further, the carrier concentration at each position of the AlGaAs layer 11 and the gradient of the carrier concentration were as shown in FIGS. That is, as shown in FIG. 13, in each layer, the back surface 11b has a carrier concentration of 1.0 × 10 ¹⁷ cm ⁻³ and the main surface 11a has a carrier concentration of 2.0 × 10 ¹⁸ cm ⁻³ . The carrier concentration always increased from the 11b side toward the main surface 11a side. As shown in FIG. 14, in each layer, the absolute value of the gradient of the carrier concentration was 3.0 × 10 ¹⁵ cm ⁻³ / μm or more and 2.5 × 10 ¹⁷ cm ⁻³ / μm or less.

  A p-type electrode was formed as the electrode 31 so as to be in contact with the contact layer of the epitaxial layer 21. The pad diameter of the electrode 31 was 120 μm, and the electrode material was an alloy of Au and Zn. The total thickness of the electrodes 31 was 1 μm.

(Invention Example 3)
Infrared LED 30a of Invention Example 3 followed the manufacturing method of infrared LED 30a of Embodiment 3. Specifically, the infrared LED 30b according to Invention Example 3 is basically manufactured in the same manner as the infrared LED 30b according to Invention Example 1, but after the epitaxial layer 21 is grown on the GaAs substrate 13, GaAs is polished. The substrate 13 is partly removed from the back side, and the thickness of the GaAs substrate 13 is 195 μm, the AlGaAs layer is only one layer having a thickness of 25 μm, and the transparent conductive film is not formed. It was. The emission wavelength of Invention Example 3 was 940 nm.

  Specifically, in the AlGaAs layer 11, the Al composition ratio of the back surface 11 b is 0.25, the Al composition ratio of the main surface 11 a is 0.05, and Al is formed from the back surface 11 b side to the main surface 11 a side. The composition ratio was constantly increasing.

The carrier concentration at each position of the AlGaAs layer 11 and the gradient of the carrier concentration are as shown in FIGS. That is, as shown in FIG. 15, in the AlGaAs layer 11, the carrier concentration on the back surface 11b side is 3.0 × 10 ¹⁷ cm ⁻³ and the carrier concentration on the main surface 11a side is 1.1 × 10 ¹⁸ cm ⁻³ . The carrier concentration always increased from the back surface 11b side to the main surface 11a side. Further, as shown in FIG. 16, in the AlGaAs layer 11, the absolute value of the gradient of the carrier concentration was 1.0 × 10 ¹⁶ cm ⁻³ / μm or more and 7.0 × 10 ¹⁶ cm ⁻³ / μm or less. .

  A p-type electrode was formed as the electrode 31 so as to be in contact with the contact layer of the epitaxial layer 21. The method and material for forming the p-type electrode were the same as those of Example 2 of the present invention.

(Evaluation methods)
Ten or more infrared LEDs of Invention Example 1 were produced, and 20 or more infrared LEDs of Invention Examples 2 and 3 were produced. Each infrared LED was energized at a high temperature at an ambient temperature of 85 ° C. and a forward current of 100 mA. After a predetermined time, the output when a direct current (DC) current was passed at 25 ° C. and a forward current of 20 mA was measured with an integrating sphere. And the relative output with respect to the initial output before high temperature electricity supply was calculated | required. The average values of the test samples are shown in FIGS. In addition, the vertical axis | shaft in FIGS. 17-19 shows the average value (unit:%) of the ratio of the relative output with respect to an initial output in each measurement time.

(Evaluation results)
As shown in FIGS. 17-19, the Al composition ratio x of the back surface 11b is higher than the Al composition ratio x of the main surface 11a, and the carrier concentration changes from the back surface 11b side to the main surface 11a side. In the infrared LEDs of Invention Examples 1 to 3 having the AlGaAs layer, the time during which the relative output with respect to the initial output can be maintained at 70% or more under the condition of the forward current of 100 mA at 85 ° C. or lower is 3000%. It was over time.

  As described above, according to the present embodiment, in the AlGaAs layer 11, the Al composition ratio x of the back surface 11b is higher than the Al composition ratio x of the main surface 11a, and the carrier is directed from the back surface 11b side to the main surface 11a side. It was confirmed that an infrared LED having a relative output of 70% or more with respect to the initial output when 1000 hours passed with a forward current of 100 mA energized at 85 ° C. or lower can be realized by changing the concentration.

  Although the embodiments and examples of the present invention have been described above, it is also planned from the beginning to appropriately combine the features of the embodiments and examples. The embodiments and examples disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the embodiments and examples described above but by the scope of claims for patent, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims for patent.

  10 AlGaAs substrate, 11 AlGaAs layer, 11a main surface, 11b back surface, 13 GaAs substrate, 20a, 20b epitaxial wafer, 21 epitaxial layer, 26 transparent conductive film, 30a, 30b LED, 31, 32 electrodes.

Claims (10)

An Al _y Ga _(1-y) As substrate (0 ≦ y ≦ 1) including an Al _x Ga _(1-x) As layer (0 ≦ x ≦ 1) having a main surface and a back surface opposite to the main surface. )When,
An epitaxial layer formed on the main surface of the Al _x Ga _(1-x) As layer and including an active layer;
In the Al _x Ga _(1-x) As layer, the Al composition ratio x on the back surface is higher than the Al composition ratio x on the main surface,
In addition, an epitaxial wafer for an infrared LED, wherein in the Al _x Ga _(1-x) As layer, a carrier concentration changes from the back side toward the main surface side. 2. The infrared LED according to claim 1, wherein the Al _x Ga _(1-x) As layer includes a layer in which a carrier concentration monotonously increases or monotonously decreases from the back surface side toward the main surface side. Epitaxial wafer. 3. The infrared-LED epitaxial wafer according to claim 1, wherein a carrier concentration of the Al _x Ga _(1-x) As layer is 1 × 10 ¹⁷ cm ⁻³ or more and 3 × 10 ¹⁸ cm ⁻³ or less. . The absolute value of the gradient of the carrier concentration of the Al _x Ga _(1-x) As layer is 3 × 10 ¹⁵ cm ⁻³ / μm or more and 5 × 10 ¹⁷ cm ⁻³ / μm or less. The epitaxial wafer for infrared LEDs of any one of Claims 1. 5. The red according to claim 1, wherein a dopant of the Al _x Ga _(1-x) As layer is at least one substance selected from the group consisting of silicon, zinc, selenium, and tellurium. Epitaxial wafer for outer LED. 6. The infrared LED according to claim 1, wherein the Al _y Ga _(1-y) As substrate includes a GaAs substrate in contact with the back surface of the Al _x Ga _(1-x) As layer. Epitaxial wafer.   The infrared-LED epitaxial wafer according to claim 1, further comprising a transparent conductive film formed on the epitaxial layer. An epitaxial wafer for infrared LEDs according to any one of claims 1 to 7,
An infrared LED comprising an electrode formed on the epitaxial wafer.   The infrared LED according to claim 8, wherein a relative output with respect to an initial output when 1000 hours have passed with a current of 100 mA at 85 ° C. or less is 70% or more.   The infrared LED according to claim 9, wherein a relative output with respect to an initial output when lapse of 3000 hours with 100 mA energization at 85 ° C. or less is 70% or more.

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