Ultraviolet semiconductor light emitting element and method for manufacturing the same

Abstract

To provide an ultraviolet semiconductor light emitting element and a method for manufacturing the same, which has high transparency even in the deep ultraviolet region, excellent ohmic contact, and excellent element characteristics such as high efficiency and high output.SOLUTION: In an ultraviolet semiconductor light emitting element consisting of an AlGaN-based semiconductor layer, in which an n-type semiconductor layer, an active layer, a p-type semiconductor layer, and a p-electrode are sequentially formed on an AlN substrate, the p-type semiconductor layer is codoped with donors and acceptors, has a surface contact layer in the surface area in contact with the p-electrode, and has an emission wavelength of 300 nm or less.SELECTED DRAWING: Figure 2B

Description

Technical Field

[0001] The present invention relates to an ultraviolet semiconductor light-emitting device and a method for manufacturing the same, and particularly to a nitride semiconductor light-emitting device that emits deep ultraviolet light and a method for manufacturing the same.

Background Art

[0002] In recent years, AlGaN-based semiconductor light-emitting devices having a deep ultraviolet region as an emission wavelength band have attracted attention as light sources having an inactivating action and a bactericidal effect on bacteria and viruses. However, further increase in the output of the light-emitting device is required.

[0003] For example, Patent Document 1 discloses a semiconductor light-emitting device using AlGaN or AlInGaN having high transparency in the ultraviolet region as a p-type contact layer. Further, Patent Document 2 discloses a deep ultraviolet light-emitting device having a superlattice structure as a p-type contact layer and optimizing the Al composition of the superlattice structure.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] Although it is taught that by making the p-type AlGaN contact layer a composition gradient layer, the contact resistance can be reduced compared to the case where Al is constant (for example, Patent Document 1), in an AlGaN-based semiconductor light-emitting device having a deep ultraviolet region as an emission wavelength band, there is still room for further improvement in conductivity such as ohmic characteristics and specific resistance.

[0006] The present invention has been made in view of the above problems, and provides an ultraviolet semiconductor light-emitting device having a p-contact layer with high transparency and excellent ohmic characteristics even in the deep ultraviolet region, and having excellent device characteristics such as high efficiency and high output, and a method for manufacturing the same.

Means for Solving the Problems

[0007] An ultraviolet semiconductor light-emitting device according to one embodiment of the present invention is an ultraviolet semiconductor light-emitting device composed of an AlGaN-based semiconductor layer, in which an n-type semiconductor layer, an active layer, a p-type semiconductor layer, and a p-electrode are sequentially formed on an AlN substrate, wherein the p-type semiconductor layer is codoped with a donor and an acceptor, and has a surface contact layer in a surface region in contact with the p-electrode, and the emission wavelength of the ultraviolet semiconductor light-emitting device is 300 nm or less.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2A

Figure 2B

Figure 3

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Figure 6A

Figure 6B

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Figure 8

Embodiments for Carrying Out the Invention

[0009] Hereinafter, preferred embodiments of the present invention will be described, but these may be appropriately modified and combined. Also, in the following description and the accompanying drawings, substantially the same or equivalent parts will be described with the same reference numerals.

[0010] [Structure of Ultraviolet Semiconductor Light - Emitting Element] FIG. 1 is a cross - sectional view schematically showing the structure of an ultraviolet semiconductor light - emitting element 10 according to one embodiment of the present invention. The ultraviolet semiconductor light - emitting element 10 is an ultraviolet light - emitting diode (hereinafter referred to as ultraviolet LED 10), and on a substrate 11, an n - type AlGaN layer 12, an active layer 13, an AlGaN layer 14, and a p - type AlGaN layer 15 which is a p - type contact layer are sequentially formed by epitaxial growth.

[0011] Also, a p - electrode 21 having an ohmic contact with the p - type contact layer 15 is formed on the p - type contact layer 15. Also, an n - electrode 23 having an ohmic contact with the n - type AlGaN layer 12 is formed. Note that a semiconductor light - emitting element composed of an AlN layer and an AlGaN layer will be described, but it may have an AlInGaN layer. In this specification, a semiconductor including AlN and AlInGaN is described as an AlGaN - based semiconductor. That is, the following description can also be applied to an AlInGaN layer. FIGS. 2A and 2B show the band diagram and the semiconductor layer structure of the ultraviolet LED 10, respectively. Hereinafter, the ultraviolet LED 10 will be described in detail with reference to FIGS. 1, 2A, and 2B.

[0012] The substrate 11 is preferably a substrate with a low dislocation density such that the dislocation density in the active layer is low, but it is not particularly limited. The dislocation density in the active layer is 10 9 cm -2 or less, preferably 10 8 cm -2 or less. Materials that can be lowered as such are preferable, and an AlN template substrate in which an AlN film is laminated on a sapphire substrate or a single-crystalline AlN substrate can be used.

[0013] Also, from the viewpoint of reducing the dislocation density in the active layer, it is preferable to use a single-crystalline AlN substrate as the substrate 11. The dislocation density of the single-crystalline AlN substrate is preferably 10 8 cm -2 or less, more preferably 10 6 cm -2 or less, and most preferably 10 4 cm -2 or less. By using an AlN substrate with an even lower dislocation density, 10 6 cm -2 or less, and further 10 4 cm -2 it is possible to prevent a reduction in the light emission efficiency in the active layer due to dislocations, and furthermore, it is possible to prevent problems such as diffusion of impurities through dislocations generated when the ultraviolet light emitting element is energized and an increase in leakage current. In the ultraviolet LED 10 of the present embodiment, an AlN substrate with a dislocation density of 10 4 cm -2 was used.

[0014] Also, the surface roughness (RMS) of the single-crystalline AlN substrate 11 is preferably 1.0 nm or less, more preferably 0.5 nm or less, for the same reason as the above-described AlN template substrate. Naturally, also in the case of the AlN substrate, the surface may be polished by a known polishing method such as chemical mechanical polishing.

[0015] In addition, if the absorption coefficient of the substrate for the ultraviolet light emitted from the active layer is large, there is a concern that the total amount of ultraviolet light that can be extracted to the outside will decrease, leading to a reduction in luminous efficiency. Therefore, the absorption coefficient of the AlN layer of the AlN substrate and the AlN template is preferably 20 cm -1 or less, and more preferably 10 cm -1 or less. By setting it to 10 cm -1 or less, for example, even if the thickness of the AlN substrate 11 is 100 μm, a linear transmittance of 90% or more can be ensured.

[0016] The n-type AlGaN layer 12 is an n-type conductive layer doped with Si (silicon). The Al composition of the n-type AlGaN layer can be appropriately determined so as to obtain sufficient transmittance for the desired emission wavelength of ultraviolet light. In the ultraviolet LED 10, the ultraviolet light emitted from the active layer 13 passes through the n-type AlGaN layer 12 and the substrate 11 and is emitted to the outside. This is because as the Al composition of the n-type AlGaN layer increases, the bandgap of the n-type AlGaN layer increases, and accordingly, ultraviolet light with a shorter wavelength can be transmitted.

[0017] In addition, the n-type AlGaN layer 12 may be formed of a plurality of layers having different Al compositions, and may further be a composition gradient layer in which the Al composition is inclined in the stacking direction. For example, the n-type AlGaN layer 12 includes a first n-type Al X1 Ga 1-X1 N layer 12A and a second n-type Al X2 Ga 1-X2 N layer 12B. The first n-type Al X1 Ga 1-X1 N layer 12A is a composition gradient layer in which the Al composition X1 decreases from 1.0 to 0.75 in the stacking direction, and the second n-type Al X2 Ga 1-X2 N layer 12B can be a composition gradient layer in which the Al composition X2 decreases from 0.75 to 0.70.

[0018] Further, the layer thickness of the n-type AlGaN layer 12 is not particularly limited and is appropriately determined. When a single-crystalline AlN substrate is used for the substrate 11, the layer thickness of the n-type AlGaN layer 12 is preferably 0.5 μm or more and 2 μm or less. From the viewpoint of reducing the resistance value of the n-type AlGaN layer, a thicker layer thickness of the n-type AlGaN layer is preferable. However, when an AlN substrate is used for the substrate 11, if the layer thickness of the n-type AlGaN layer becomes too thick, the n-type AlGaN layer is likely to undergo lattice relaxation and generate dislocations.

[0019] For example, when the n-type AlGaN layer 12 is formed as a laminated structure including the above-described first n-type Al X1 Ga 1-X1 N layer 12A and the second n-type Al X2 Ga 1-X2 N layer 12B, the first n-type Al X1 Ga 1-X1 N layer 12A may have a layer thickness of 200 nm, and the second n-type Al X2 Ga 1-X2 N layer 12B may have a layer thickness of 1000 nm. Further, the layer thicknesses of the first n-type Al X1 Ga 1-X1 N layer 12A and the second n-type Al X2 Ga 1-X2 N layer 12B are not limited to the exemplified numbers and can be appropriately determined so that the total layer thickness is 2.0 μm or less.

[0020] Also, the Si concentration doped into the n-type AlGaN layer 12 may be appropriately determined so as to obtain a desired n-type conductivity. From the viewpoint of reducing the resistance value of the n-type AlGaN layer, it is preferably 1×10 18 ~1×10 20 cm -3 , and more preferably 5×10 18 ~5×10 19 cm -3 . Also, the Si doping concentration may be constant in the layer thickness direction within the n-type AlGaN layer, or modulation doping with different Si concentrations in the layer thickness direction can also be performed.

[0021] Note that the Si concentration and the Mg concentration described below can be measured by known Secondary Ion Mass Spectrometry (SIMS) analysis. Also, the measured values of the Si concentration and the Mg concentration in the present application are the quantitative values using standard samples of AlN, Al 0.65 Ga 0.35 N, and GaN for the AlN layer, the AlGaN layer, and the GaN layer, respectively.

[0022] The active layer (ACT) 13 is a quantum well structure composed of a barrier layer made of an Al A1 Ga 1-A1 N layer and a well layer made of an Al A2 Ga 1-A2 N layer. The emission peak wavelength of the active layer 13 is in the range of 210 to 300 nm. Since the wavelength of the light emitted from the active layer 13 is determined by the Al composition and the layer thickness of the well layer, the Al composition and the layer thickness can be appropriately determined so as to obtain a desired emission wavelength in the above wavelength range. The lower the Al composition of the p-type AlGaN contact layer, the lower the Schottky barrier, and thus it is easier to realize an ohmic contact. Since the present invention shows a method for realizing an ohmic contact even with p-type AlGaN having a high Al composition, it can be an effective means for increasing the output power of an ultraviolet LED having an emission wavelength of 300 nm or less, more preferably 285 nm or less, and even more preferably 270 nm or less.

[0023] For example, the layer thickness of the well layer can be set in the range of 2 to 10 nm, and the Al composition (aluminum composition) can be determined so as to obtain a desired emission wavelength. Also, although the Al composition and the layer thickness of the barrier layer are not particularly limited, for example, the Al composition can be set in the range of A2 < A1 ≤ 1.0 and the layer thickness can be set in the range of 2 to 15 nm.

[0024] Also, the well layer and the barrier layer can be n-type layers doped with Si. Both the well layer and the barrier layer may be Si-doping layers, or a structure in which only the well layer or only the barrier layer is doped with Si may be used. The Si concentration to be doped is not particularly limited, but is 1×10 17 ~5×1018 cm -3 is preferably in the range of.

[0025] Also, the number of quantum well layers is not particularly limited, and it may be a multi - quantum well (MQW) structure in which a plurality of well layers are formed, or it may be a single - quantum well (SQW). The number of well layers is preferably determined appropriately within the range of 1 to 5.

[0026] Al Y1 Ga 1-Y1 The AlGaN layer 14 is a layer provided adjacent to the active layer 13. Al Y1 Ga 1-Y1 The AlGaN layer 14 functions as an electron blocking layer (EBL) to suppress the overflow of electrons injected into the active layer 13 into the p - type Al Y2 Ga 1-Y2 GaN layer 15. Therefore, the AlGaN layer 14 has a larger bandgap than the active layer 13 and the p - type Al Y1 Ga 1-Y1 GaN layer 15 to be described later. The Al composition Y1 of the AlGaN layer 14 is determined within the range of 0.8 < Y1 ≦ 1.0. Y2 Ga 1-Y2 N layer 14 has a larger bandgap than the active layer 13 and the p - type Al Y1 Ga 1-Y1 N layer 15, and the Al composition Y1 of the AlGaN layer 14 is determined within the range of 0.8 < Y1 ≦ 1.0.

[0027] As the emission wavelength becomes shorter, the Al composition of the AlGaN layer epitaxially grown on the substrate 11 increases. When the emission wavelength is shorter than 270 nm, in order to fully exhibit the function as an electron blocking layer, the Al composition Y1 is preferably 0.9 ≦ Y1 ≦ 1.0. In this embodiment, Al Y1 Ga 1-Y1 N layer 14 uses AlN (Y1 = 1).

[0028] Also, Al Y1 Ga 1-Y1As long as the N layer 14 can exhibit the function as an electron blocking layer, it may be an undoped layer or may be doped with a p-type dopant. Al Y1 Ga 1-Y1 As the p-type dopant material in the N layer 14, Mg (magnesium), Zn (zinc), Be (beryllium), C (carbon), etc. can be used. In particular, it is preferable to use Mg which is generally used as the p-type dopant material for the AlGaN layer, and Mg is also used in the examples of the present invention described later.

[0029] The p-type dopant material is Al Y1 Ga 1-Y1 In the stacking direction of the N layer 14, it may be uniformly doped, or the concentration of the dopant material can be changed in the stacking direction. In the present embodiment, it has a stacked structure composed of an undoped AlN layer 14A (layer thickness: 1 nm) and a p-type AlN layer 14B (layer thickness: 8 nm) doped with Mg (magnesium) from the side in contact with the active layer 13.

[0030] Al Y1 Ga 1-Y1 The p-type dopant concentration in the N layer 14 is not particularly limited, but in order to obtain the function as an electron blocking layer, it is preferably 5×10 18 ~1×10 20 cm -3 , and from the viewpoint of enhancing the carrier injection efficiency into the active layer, it is particularly preferably 1×10 19 ~8×10 19 cm -3 .

[0031] The Al Y1 Ga 1-Y1 N layer 14 of the present invention may not contain an n-type dopant or may contain an n-type dopant at a concentration lower than the n-type dopant contained in the p-type Al Y2 Ga 1-Y2 N layer 15 described later. Specifically, the n-type impurity concentration in the Al Y1 Ga 1-Y1 N layer 14 is 1×10 18 cm -3The following are preferred. According to the findings of the present inventors, during the growth of the p-type Al Y2 Ga 1-Y2 N layer 15, it has been found that dopant diffusion occurs between the adjacent Al Y1 Ga 1-Y1 N layer 14. Therefore, when the n-type dopant concentration in the Al Y1 Ga 1-Y1 N layer 14 is higher than that in the p-type Al Y2 Ga 1-Y2 N layer 15, n-type dopants diffuse from the Al Y1 Ga 1-Y1 N layer 14 into the p-type Al Y2 Ga 1-Y2 N layer 15, and precise control of the n-type dopant concentration in the p-type Al Y2 Ga 1-Y2 N layer 15 may become difficult. In order to prevent the change in the n-type dopant concentration in the p-type Al Y2 Ga 1-Y2 N layer 15 due to this diffusion, at least the n-type dopants in the Al Y1 Ga 1-Y1 N layer 14 need to be less than the concentration of the n-type dopants contained in the p-type Al Y2 Ga 1-Y2 N layer 15.

[0032] Also, the layer thickness of the Al Y1 Ga 1-Y1 N layer 14 may be appropriately determined so that it can function as an electron blocking layer and holes can be efficiently injected from the p-type Al Y2 Ga 1-Y2 N layer 15 into the active layer, but the range of 1 to 30 nm is preferred. If the layer thickness is less than 1 nm, electrons will tunnel, resulting in a decrease in the function as an electron blocking layer. On the other hand, if the layer thickness exceeds 30 nm, it will be difficult for holes to be injected from the p-type Al Y2 Ga 1-Y2 N layer 15 into the active layer. Considering these factors, the layer thickness of the Al Y1 Ga 1-Y1 N layer 14 is preferably 2 to 20 nm, more preferably 5 to 15 nm.

[0033] Also, as described above, AlY1 Ga 1-Y1 Mg doped in the GaN layer 14 can also have a concentration difference in the stacking direction. For example, an undoped AlN layer 14A with a layer thickness of 1 to 5 nm can be stacked on the side in contact with the active layer 13, and further, a Mg-doped p-type AlN layer 14B with a thickness of 5 to 15 nm can be stacked. The Mg doping concentration at this time is, as described above, 5×10 18 ~1×10 20 cm -3 is preferably, and 1×10 19 ~8×10 19 cm -3 is particularly preferably.

[0034] In this embodiment, it has a p-type Al Y2 Ga 1-Y2 N layer 15 grown on the p-type AlN layer 14B. The p-type Al Y2 Ga 1-Y2 N layer 15 is a composition gradient layer in which the Al composition Y2 changes in the stacking direction. In particular, it is preferably structured such that the Al composition Y2 decreases in the stacking direction from the side in contact with the p-type Al Y1 Ga 1-Y1 N layer 14. Thereby, a polarization doping effect can be obtained within the p-type Al Y2 Ga 1-Y2 N layer 15, so that a higher hole concentration is more easily obtained, and as a result, the injection efficiency of holes into the active layer is increased. For example, when the emission wavelength is 270 nm or less, the Al composition on the side in contact with the p-type Al Y1 Ga 1-Y1 N layer 14 is preferably 0.85 to 1.0, more preferably 0.9 to 1.0. At this time, the relationship Y1≧Y2 is satisfied. On the opposite side, the Al composition on the surface layer of the p-type Al Y2 Ga 1-Y2 N layer 15 (that is, the side in contact with the p-electrode 21) is preferably 0.5 to 0.85, more preferably 0.6 to 0.85. By adopting such a structure, the above-described polarization doping effect can be enhanced and transparency with respect to the emission wavelength can be maintained, so that a high emission efficiency is more easily obtained.

[0035] Also, the p-type Al Y2Ga 1-Y2 The layer thickness of the p-type AlGaN layer 15 is not particularly limited, and may be appropriately determined within the range of 1 to 150 nm. From the viewpoint of the efficiency of polarization doping, a thinner layer or a higher gradient is preferable. On the other hand, when the layer thickness is thick, the transparency decreases (the optical absorption loss increases). From such viewpoints and the viewpoint of practical productivity, the p-type Al Y2 Ga 1-Y2 The layer thickness of the N layer 15 is preferably 2 to 120 nm, and particularly preferably 5 to 100 nm.

[0036] p-type Al Y2 Ga 1-Y2 Since the p-type AlGaN layer 15 is grown in a state of pseudomorphic alignment with the AlN substrate 11, it has a low dislocation density equivalent to that of the AlN substrate 11. Specifically, it has a dislocation density of 10 5 cm -2 or less. The relaxation rate of the pseudomorphic semiconductor layer is preferably within 20%. p-type Al Y2 Ga 1-Y2 The p-type AlGaN layer 15 functions as a p-type contact layer. In the p-type Al Y2 Ga 1-Y2 N layer 15, p-type impurities serving as acceptors and n-type impurities serving as donors are codoped.

[0037] p-type Al Y2 Ga 1-Y2 As the p-type impurities doped into the p-type AlGaN layer 15, Mg (magnesium), Zn (zinc), Be (beryllium), C (carbon), etc. can be used. Among them, it is preferable to use Mg, which is generally used as a p-type dopant material for AlGaN semiconductors. As the n-type impurities, Si, Ge (germanium), Se (selenium), S (sulfur), O (oxygen), etc. can be used. Among them, it is preferable to use Si, which is generally used as an n-type dopant material.

[0038] Also, the amount of p-type impurities doped into the p-type Al Y2 Ga 1-Y2 N layer 15 is 1×10 17 ~1.2×10 20 cm-3 is preferable. Also, as theoretically shown in J. Applmaru Physmaru, Volmaru 95, No. 8, 15 April (2004), as the amount of p-type impurities in the p-type Al Y2 Ga 1-Y2 N layer 15 increases, the amount of nitrogen defects considered to be a cause of degradation is also thought to increase. Therefore, when the amount of p-type impurities exceeds 1.2×10 20 cm -3 , the amount of nitrogen defects formed initially becomes too large, making it difficult to obtain a high output maintenance rate.

[0039] Also, when the p-type impurity concentration decreases, especially when the Al composition Y2 is constant, due to the decrease in hole concentration and the increase in minority carrier (electron) mobility, or when the Al composition Y2 is inclined, due to the increase in minority carrier (electron) mobility, the output decreases and it becomes difficult to obtain a high luminous efficiency. Therefore, the p-type impurity concentration can be appropriately determined within the above range in consideration of such a trade-off. However, in order to obtain a higher output maintenance rate and a higher output, it is preferably 1×10 19 ~5×10 19 cm -3 , and more preferably 1×10 19 ~4×10 19 cm -3 .

[0040] In the ultraviolet LED 10 of the present embodiment, the p-type Al Y2 Ga 1-Y2 N layer 15 is a composition gradient layer in which the Al composition Y2 decreases as it moves away from the lamination direction, that is, the interface with the adjacent Al Y1 Ga 1-Y1 N layer 14.

[0041] The Al composition Y2 is Al Y1 Ga 1-Y1It is preferably a composition gradient layer that linearly decreases as it moves away from the interface with the N layer 14, but it is not limited to this, and it may be a composition gradient layer with a curved profile in which the Al composition decreases. Alternatively, the Al composition may decrease stepwise, or these may be combined.

[0042] p-type Al Y2 Ga 1-Y2 The amount of n-type impurity doped into the N layer 15 is 1.1×10 18 or more and 9.0×10 18 cm -3 or less, and more preferably 1.8×10 18 or more and 8.0×10 18 cm -3 or less. With these amounts of n-type impurities, a light-emitting device 10 with high luminous efficiency can be obtained.

[0043] Also, p-type Al Y2 Ga 1-Y2 The p-type impurities and n-type impurities doped into the N layer 15 may have a constant concentration within the layer or may have a concentration difference in the stacking direction. However, in order to drive at a higher output and a lower voltage, p-type Al Y2 Ga 1-Y2 It is preferable that no n-type impurities are added or the concentration is low at least at both interface portions of the N layer 15 or at least at the interface with the Al Y1 Ga 1-Y1 N layer 14.

[0044] p-type Al Y2 Ga 1-Y2 By suppressing the depletion of the interface portion by partially limited n-type impurity doping into the N layer 15, the increase in voltage is suppressed. And since it also contributes to the reduction of the mobility of minority carriers (electrons), a higher output can be obtained than without n-type impurity doping. Therefore, when a higher power conversion efficiency (WPE: wall plug efficiency) is desired than in the case without n-type impurity doping, at least Al Y1 Ga 1-Y1 N layer 14 and p-type Al Y2 Ga1-Y2 It is preferable that the n-type impurity concentration near the interface of the N layer 15 is low. This is because p-type Al Y2 Ga 1-Y2 Since the N layer 15 is a composition gradient layer, the depletion layer width formed at the interface with the Al Y1 Ga 1-Y1 N layer 14 is wider than that at the interface with the p-type contact layer 16, so the effect of no n-type impurity doping is large.

[0045] p-type Al Y2 Ga 1-Y2 The Al composition Y2 of the p-type Al Y1 Ga 1-Y1 N layer 15 is 0.5 or more and 1.0 or less, and is equal to or less than the Al composition Y1 of the Al

[0046] p-type Al Y2 Ga 1-Y2 In the case where the Al composition Y2 of the p-type Al Y1 Ga 1-Y1 N layer 15 is a constant value in the stacking direction, it is preferable that it exceeds the Al composition of the barrier layer of the active layer and is equal to or less than the Al composition Y1 of the Al Y2 Ga 1-Y2 N layer 14. By setting the Al composition Y2 of the p-type Al Y2 Ga 1-Y2 N layer 15 within the above range, even when the injection current amount of the ultraviolet light emitting device is large, a high carrier overflow suppression effect can be obtained. To obtain a higher effect, it is preferable that the difference between the Al composition of the well layer of the active layer and the Al composition Y2 of the p-type Al Y2 Ga 1-Y2 N layer 15 is 0 or more, and more preferably the difference between the Al composition of the barrier layer and the Al composition Y2 of the p-type Al Y2 Ga 1-Y2 N layer 15 is 0 or more. Also, the Al composition Y2 of the p-type Al Y2 Ga 1-Y2When the Al composition Y2 of the N layer 15 has a structure in which Y2 is a constant value in the stacking direction, it is preferably 0.6 or more and 0.9 or less.

[0047] In addition, when using AlN as the substrate 11, since all the AlGaN layers 12, 13, 14, and 15 are crystal-grown in a state of being pseudomorphically aligned with the AlN substrate 11, they have a low dislocation density equivalent to that of the AlN substrate 11. Specifically, 5 cm -2 It has the following dislocation density. Note that the relaxation rate of the pseudomorphically aligned semiconductor layer is preferably within 20%. In addition, although the case where the ultraviolet semiconductor light-emitting element 10 is a light-emitting diode (LED) has been described, it may be configured as a semiconductor laser element (LD: Laser Diode).

[0048] Although the semiconductor light-emitting element having the substrate 11 has been described as the ultraviolet LED 10, the substrate 11 can be removed as necessary or provided arbitrarily in view of the properties of the substrate to be used.

[0049] Although the semiconductor light-emitting element having the substrate 11 on the n-type AlGaN layer 12 side has been described as the ultraviolet LED 10, a support substrate different from the substrate 11 used for epitaxial growth can be provided on the p-type Al Y2 Ga 1-Y2 N layer 15 side. In this case, the material of the support substrate is not particularly limited, and known materials used as support substrate members of light-emitting elements, such as polycrystalline AlN, Si, Al2O3, Cu, and CuW, can be used without limitation.

[0050] Hereinafter, the semiconductor light-emitting element having the substrate 11 as a growth substrate of the semiconductor stack structure will be described as the ultraviolet LED 10. However, as described above, a semiconductor light-emitting element having a substrate 11 different from the growth substrate such as a support substrate can also be used.

[0051] [Manufacturing method of ultraviolet LED] A method for manufacturing the ultraviolet LED 10 having the structure described above will be described. The ultraviolet LED 10 of the present invention can be manufactured by the MOCVD method, which is highly productive and widely adopted industrially. As the group III (Al, Ga) source gas and the group V (N) source gas to be used, known source gases can be used without particular limitation.

[0052] For example, as the group III source gas, gases such as trimethylaluminum (TMA), triethylaluminum (TEA), trimethylgallium (TMG), and triethylgallium (TEG) can be used. As the group V source gas, ammonia (NH3) is usually used.

[0053] In addition, as the dopant source gas for Mg and Si, known materials can be used without limitation. For example, biscyclopentadienylmagnesium (Cp2Mg), monosilane (SiH4), tetraethylsilane, etc. can be used. By supplying the above source gases onto the substrate 11 together with a carrier gas such as hydrogen (H2) and / or nitrogen (N2), the element layer of the ultraviolet LED 10 is grown.

[0054] The supply ratio of the group III source gas to the group V source gas (V / III ratio) may be appropriately determined so as to obtain desired characteristics, but it is preferably set within the range of 500 to 10,000.

[0055] Regarding the growth temperature of the element layer constituting the ultraviolet LED 10, except for the points specifically specified, it is not limited and may be appropriately determined so as to obtain the desired characteristics of each layer and the characteristics of the ultraviolet LED 10. However, it is preferably grown at 1000 to 1200 °C, more preferably 1000 to 1150 °C.

[0056] [Examples] Hereinafter, the present invention will be specifically described using an example in which an ultraviolet LED having an emission wavelength of 265 nm is fabricated, but the present invention is not limited to the examples. 1. Fabrication of the element As the substrate for growing the ultraviolet LED element layer, the dislocation density is 104 cm -2 The following AlN substrate was used. Referring again to FIG. 2B, on this AlN substrate 11, a first n-type AlGaN layer 12A (layer thickness: 200 nm) and a second n-type AlGaN layer 12B (1000 nm) were grown by an MOCVD apparatus. An AlGaN buffer layer may be provided between the substrate 11 and the first n-type AlGaN layer 12A (the Al composition is equal to or higher than the Al composition of the first n-type AlGaN layer 12A and up to 100%). Both the first n-type AlGaN layer 12A and the second n-type AlGaN layer 12B are compositionally graded layers. In the first n-type AlGaN layer 12A, the Al composition decreases from 1.0 to 0.75 from the side in contact with the AlN layer. In the second n-type AlGaN layer 12B, the Al composition decreases from 0.75 to 0.70 from the side in contact with the first n-type AlGaN layer 12A. Also, the Si concentration in the n-type AlGaN layer 12 was controlled to be 1×10 19 cm -3 to this value.

[0057] Next, an active layer 13 having a three quantum well layer structure composed of a barrier layer made of n-type Al 0.59 Ga 0.41 N (layer thickness: 7 nm) and a well layer made of Al 0.5 Ga 0.5 N (4 nm) was grown. The Si concentration of the barrier layer was controlled to be 1×10 18 cm -3 to this value.

[0058] Next, an electron blocking layer 14 made of AlN (9 nm) was grown. The electron blocking layer on the side in contact with the barrier layer of the active layer 13 was an undoped AlN layer 14A (1 nm), and the remaining electron blocking layer was a p-type AlN layer 14B with Mg doping of 4×10 19 cm -3 . Note that Si is not doped in the undoped AlN layer 14A and the Mg-doped p-type AlN layer 14B.

[0059] Next, the p-type AlGaN layer 15 was grown. The p-type AlGaN layer 15 is a composition gradient layer with an Al composition decreasing from 1.0 to 0.6 starting from the side in contact with the p-type AlN layer 14B which is an electron blocking layer. Also, the layer thickness of the p-type AlGaN layer 15 was 60 nm. Below, the p-type AlGaN layer 15 will be described in detail.

[0060] 2. Investigation of p-type Al Y2 Ga 1-Y2 N Layer (1) Fermi Level Pinning In the present invention, the p-type Al Y2 Ga 1-Y2 N layer 15 is codoped with Si which is an anti-type impurity to obtain an improvement in device characteristics. Adding an anti-type impurity usually is not done because it shifts the Fermi level and impairs ohmic characteristics, but based on the consideration that appropriate codoping can be effective in suppressing Fermi level pinning more than the shift of the Fermi level, the following verification was conducted.

[0061] FIG. 3 is a diagram schematically showing a cross-section of a sample for evaluating the Schottky barrier height. More specifically, the sample has the same structure as the p-type Al Y2 Ga 1-Y2 N layer 15, that is, a p-type Al Y2 Ga 1-Y2 N layer with the same gradient composition (Al composition: 100% to 66%) formed on an AlN substrate, and the Si codoping layer thickness of the p-type Al Y2 Ga 1-Y2 N layer (layer thickness: 60 nm) was fabricated as a parameter.

[0062] FIG. 4 is a diagram showing the profiles of the Al composition, growth temperature TG, Mg concentration, and Si concentration in the p-type Al Y2 Ga 1-Y2 N layer 15. Specifically, in addition to Mg, Si was codoped into the p-type Al Y2 Ga 1-Y2 N layer 15 from the interface with the AlN substrate. Up to 48 nm from the interface with the AlN substrate, the Mg flow rate was 40 cc (Mg concentration: 2E19 cm -3) Samples were fabricated. When the Mg flow rate after 48 nm from the interface was increased to 400 cc (Mg concentration: 4E19 cm -3 ), the growth temperature TG was gradually decreased by 40 °C from 1030 °C until then (in the growth temperature reduction region Rg) and growth was carried out (reduced growth temperature = 990 °C). Note that the Mg concentration on the surface of the p-type Al Y2 Ga 1-Y2 N layer was 4E19 cm -3 , and the Si concentration was 3E18 cm -3 . Note that the Mg concentration in the layer where the Mg concentration was increased is preferably 3E19~2E20 cm -3 . Also, it is preferably increased to 1.5 times or more of other parts of the p-type Al Y2 Ga 1-Y2 N layer 15. Further, the layer thickness of the growth temperature reduction region Rg is preferably 20 nm or less.

[0063] (2) Evaluation of Schottky barrier FIG. 5 shows the height of the Schottky barrier calculated from the temperature dependence of the TLM measurement with the TLM pattern formed on the sample shown in FIG. 4, plotted against the Si-doped layer thickness. It is shown normalized with the barrier height in the case of no Si doping set to 1. Note that the height of the Schottky barrier is the energy difference between the Fermi level and the lower end of the valence electrons at the semiconductor surface, and the unit is eV.

[0064] That is, FIG. 5 shows the influence of the Si-doped layer thickness on the height of the Schottky barrier compared to the case of no Si doping. This figure shows the result that the Schottky barrier becomes lower when Si doping is performed up to the surface layer in contact with the p-electrode.

[0065] That is, this result shows that doping with an anti-type dopant is more effective in reducing the Schottky barrier by suppressing pinning than in increasing the Schottky barrier due to the shift of the Fermi level position to the conduction band side, indicating that doping with an anti-type dopant is effective in suppressing pinning.

[0066] Figure 6A shows the p-type Al Y2 Ga 1-Y2 The amount of Mg (cm) per unit thickness (1 nm) in the surface layer of the N layer -3 / nm) and the barrier Vb (V). That is, the horizontal axis represents the increased Mg content (cm -3 ) divided by the thickness of the region where the Mg content is increased (or the average value).

[0067] In addition, in the figure, "no Si co-doping" indicates that Si co-doping was not performed only in the surface layer portion (layer thickness 12 nm).

[0068] Hereinafter, the surface layer region where the amount of Mg is increased or where Si co-doping is performed will be defined as a "surface contact layer SC" and explained.

[0069] As shown in Figure 6A, by gradually decreasing the growth temperature T G by 40°C in the growth temperature reduction region Rg and by co-doping the surface contact layer SC with Si, a good ohmic contact can be achieved even with a relatively low Mg content. More specifically, when the Mg content (acceptor content) of the surface contact layer SC is 8×10 18 cm -3 / nm or more, the barrier Vb (V) is 0.1 (V) or less, and good contact performance is obtained. Here, the barrier (or p-barrier) is defined as the voltage value Vb at the intersection of the tangent to the IV (current-voltage) curve and the V axis (X axis), as shown schematically in Figure 6B.

[0070] This is thought to be because lowering the growth temperature TG suppresses the generation of point defects, lowers the Schottky barrier, and improves the ohmic characteristics. The growth temperature TG is preferably lowered by 20 to 60°C.

[0071] The appropriate Si co-doping amount is 1E17≦co-doping amount (Si)≦2E19. If the Si co-doped layer thickness is 1 nm or more, the Schottky barrier height can be reduced.

[0072] Also, p-type Al Y2 Ga 1-Y2 The N layer 15 also functions as a cladding layer, but the output maintenance rate (for example, the output ratio after 100 hours from the initial stage) increases substantially linearly with the increase in the Si / Mg ratio, and it was considered that the Si / Mg ratio is a dominant physical parameter that affects the output maintenance rate. Note that the Si / Mg ratio is estimated to be inversely proportional to the nitrogen defect concentration formed during undoping.

[0073] As a result of the intensive studies by the present inventors, by codoping the p-type Al Y2 Ga 1-Y2 N layer 15 so as to satisfy the following formula, an ultraviolet LED having high luminous efficiency and good output maintenance rate (element lifetime) is realized.

[0074] 0.009 ≦ (Si / Mg) < 0.185 ··· Formula (1) In this embodiment, due to the interaction between Mg, which is an acceptor impurity doped into the p-type Al Y2 Ga 1-Y2 N layer 15, and Si, which is a donor impurity, the effect of reducing nitrogen defects is manifested, the element lifetime is improved, and the ohmic characteristics of the surface layer are improved.

[0075] [Modification Example 1] FIG. 7 is a diagram showing the Mg-Si codoping profile of Modification Example 1 of the p-type Al Y2 Ga 1-Y2 N layer 15. In this Modification Example 1, Si codoping is not performed in a part of the region of the p-type Al Y2 Ga 1-Y2 N layer 15, that is, a codoping-off region Roff is provided.

[0076] More specifically, in addition to Mg, Si is codoped into the p-type Al Y2 Ga 1-Y2 N layer 15 from the interface with the p-type AlN layer 14B. The Mg doping is the same as the case shown in FIG. 4.

[0077] Si codoping reduces the amount of codoping compared to the case shown in FIG. 4 (e.g., 1 / 3 of 1E18 cm -3 ), and is performed until the growth temperature TG is lowered from the interface with the p-type AlN layer 14B (e.g., at a position with a layer thickness of 50 nm). After that, Si codoping is stopped (codoping off region Roff), and Si codoping is performed again in the surface contact layer SC (portion with a layer thickness of 4 nm). However, the amount of Si codoping in the surface contact layer SC is set to the same 3E18 cm as in the case shown in FIG. 4 -3 to prevent the ohmic contact property from degrading.

[0078] In the case of this modification example, good ohmic contact is achieved even with a relatively low Mg amount, and a p-type Al Y2 Ga 1-Y2 N layer 15 with a smaller specific resistance (series resistance) can be realized.

[0079] [Modification Example 2] FIG. 8 is a diagram showing the Mg-Si codoping profile of modification example 2 of the p-type Al Y2 Ga 1-Y2 N layer 15. In this modification example 2, Mg-Si codoping is performed only in the surface contact layer SC (e.g., with a layer thickness of 8 nm) of the p-type Al Y2 Ga 1-Y2 N layer 15, and only Mg, which is a p-dopant, is doped in other regions.

[0080] Note that it is preferable that the donor concentration in at least the surface layer (the outermost surface layer) 1 nm of the surface contact layer SC is 1×10 17 ~1×10 20 cm -3 .

[0081] The p-type Al Y2 Ga 1-Y2 N layer 15 having such an Mg-Si codoping profile has a small contact resistance, realizes good ohmic contact, and also has the advantage that the specific resistance (series resistance) of the p-type Al Y2 Ga 1-Y2 N layer 15 is small.

[0082] As described above in detail, the present invention can provide an ultraviolet semiconductor light-emitting element and a method for manufacturing the same, which has high transparency even in the deep ultraviolet region, has a p-contact layer with excellent ohmic characteristics, and has excellent element characteristics such as high efficiency and high output. [Explanation of symbols]

[0083] 10: ultraviolet semiconductor light emitting element, 11: substrate, 12: n-type AlGaN layer, 12A: first n-type Al X1 Ga 1-X1 N layer, 12B: second n-type Al X2 Ga 1-X2 N layer, 13: active layer, 14: Al Y1 Ga 1-Y1 N layer, 14A: AlN layer, 14B: p-type AlN layer, 15: p-type AlGaN layer, 21: p-electrode, Roff: co-doping off region, Rg: growth temperature reduction region, SC: surface contact layer

Claims

1. An ultraviolet semiconductor light-emitting device comprising an AlGaN-based semiconductor layer, on which an n-type semiconductor layer, an active layer, a p-type semiconductor layer, and a p electrode are sequentially formed on an AlN substrate, wherein the p-type semiconductor layer is codoped with a donor and an acceptor, and has a surface contact layer in a surface region in contact with the p electrode, and the ultraviolet semiconductor light-emitting device has an emission wavelength of 300 nm or less.

2. having an electron blocking layer provided between the active layer and the p-type semiconductor layer, wherein the p-type semiconductor layer is a p-AlGaN layer, and is a composition gradient layer in which the Al composition decreases as the distance from the electron blocking layer increases, the Al composition on the side in contact with the electron blocking layer is 0.8 to 1.0, and the Al composition on the opposite side is 0.5 to 0.

85. The ultraviolet semiconductor light-emitting device according to Claim 1.

3. The acceptor amount per unit layer thickness (1 nm) in the surface contact layer where the p-type semiconductor layer contacts the p-electrode is 8 × 10 １９ cm －３ / nm or more, and the ultraviolet semiconductor light-emitting device according to claim 1.

4. The donor concentration in at least the surface layer of 1 nm of the surface contact layer is 1×10 １７ to 1×10 ２０ cm －３ The ultraviolet semiconductor light-emitting device according to claim 1.

5. The ultraviolet semiconductor light-emitting device according to Claim 1, wherein the donor and the acceptor in the p-type semiconductor layer are silicon (Si) and magnesium (Mg), respectively.

6. The ultraviolet semiconductor light-emitting device according to Claim 1, wherein the n-type semiconductor layer, the active layer, and the p-type semiconductor layer are pseudomorphic to the AlN substrate, and the relaxation rate thereof is within 20%.

7. The ultraviolet semiconductor light-emitting device according to any one of Claims 2 to 6, wherein the concentration of magnesium (Mg) in the surface contact layer is 1.5 times or more that of the other portions of the p-type semiconductor layer.

8. A method for manufacturing the ultraviolet semiconductor light-emitting device according to any one of Claims 1 to 6, wherein the p-type semiconductor layer is grown by gradually reducing the growth temperature to a reduced growth temperature, and has a growth temperature reduction region on the surface side in which the Mg concentration is increased to 1.5 times or more that of the other portions of the p-type semiconductor layer, the layer thickness of the growth temperature reduction region is 20 nm or less, and the reduced growth temperature is a low growth temperature within a range of -20°C to -60°C with respect to the growth temperature of the p-type semiconductor layer.

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