Ultraviolet semiconductor light-emitting element and method for manufacturing same

Abstract

Provided is an ultraviolet semiconductor light-emitting element (10) composed of an AlGaN-based semiconductor layer, the ultraviolet semiconductor light-emitting element (10) having n-type semiconductor layers (12A, 12B), an active layer (13), a p-type semiconductor layer (15), and a p-electrode (21) sequentially formed on an AlN substrate (11), wherein the p-type semiconductor layer (15) is co-doped with a donor and an acceptor, has a surface contact layer (SC) in a surface region in contact with the p-electrode (21), and has an emission wavelength of 300 nm or lower.

Description

Ultraviolet semiconductor light emitting device and its manufacturing method

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

[0002] In recent years, AlGaN-based semiconductor light-emitting devices with an emission wavelength band in the deep ultraviolet region have been attracting attention as light sources with the effect of inactivating and sterilizing bacteria and viruses. However, there is a demand for light-emitting devices with even higher output power.

[0003] For example, Patent Document 1 discloses a semiconductor light-emitting device that uses AlGaN or AlInGaN, which have high transparency in the ultraviolet region, as a p-type contact layer. Patent Document 2 discloses a deep ultraviolet light-emitting device in which the p-type contact layer has a superlattice structure and the Al composition of the superlattice structure is optimized.

[0004] Japanese Patent Application Laid-Open No. 2020-064955 Japanese Patent Application Laid-Open No. 6849641

[0005] It has been taught that by making the p-type AlGaN contact layer a compositionally graded layer, the contact resistance can be reduced compared to when the Al content is constant (for example, Patent Document 1), but in AlGaN-based semiconductor light-emitting devices that emit light in the deep ultraviolet region, there is room for further improvement in conductivity such as ohmic characteristics and resistivity.

[0006] The present invention has been made in view of the above-mentioned problems, and aims to provide an ultraviolet semiconductor light-emitting element that 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, and a method for manufacturing the same.

[0007] According to one embodiment of the present invention, there is provided an ultraviolet semiconductor light-emitting device comprising 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 co-doped with donors and acceptors 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.

[0008] 1 is a cross-sectional view schematically showing the structure of an ultraviolet LED according to one embodiment of the present invention; 2 is a diagram schematically showing a band diagram of an ultraviolet LED; 3 is a diagram showing the semiconductor layer structure of an ultraviolet LED; 4 is a diagram schematically showing the cross section of a sample for evaluating Schottky barrier height; Ｙ２ Ga １－Ｙ２ 1 shows the profiles of Al composition, growth temperature TG, Mg concentration, and Si concentration in the N layer. 2 shows the normalized Schottky barrier height versus Si co-doped layer thickness. Ｙ２ Ga １－Ｙ２ The amount of Mg per unit thickness of the surface layer of the N layer (cm －2 1 is a diagram showing the relationship between the Schottky barrier (V) and the barrier Vb. Ｙ２ Ga １－Ｙ２ 1 shows the Mg—Si co-doping profile of the N-layer in Modification Example 1. Ｙ２ Ga １－Ｙ２ FIG. 10 shows the Mg—Si co-doping profile of Modification Example 2 of the N layer.

[0009] In the following, preferred embodiments of the present invention will be described, but these may be modified and combined as appropriate. In the following description and accompanying drawings, substantially the same or equivalent parts are designated by the same reference numerals.

[0010] 1 is a cross-sectional view schematically illustrating 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 an ultraviolet LED 10) formed by epitaxially growing an n-type AlGaN layer 12, an active layer 13, an AlGaN layer 14, and a p-type AlGaN layer 15 serving as a p-type contact layer on a substrate 11, in that order.

[0011] A p-electrode 21 is formed on the p-type contact layer 15, making ohmic contact with the p-type contact layer 15. An n-electrode 23 is formed on the n-type AlGaN layer 12, making ohmic contact with the n-type AlGaN layer 12. While the semiconductor light-emitting device will be described as being composed of an AlN layer and an AlGaN layer, it may also include an AlInGaN layer. In this specification, semiconductors containing AlN and AlInGaN are referred to as AlGaN-based semiconductors. The following description also applies to AlInGaN layers. Figures 2A and 2B show the band diagram and semiconductor layer structure of the ultraviolet LED 10, respectively. The ultraviolet LED 10 will be described in detail below with reference to Figures 1, 2A, and 2B.

[0012] The substrate 11 is preferably a substrate with a low dislocation density so that the dislocation density in the active layer is low, but is not particularly limited. ９ cm －２ Below 10, preferably ８ cm －２ The materials listed below are preferred, and an AlN template substrate in which an AlN film is laminated on a sapphire substrate or a single crystal AlN substrate can be used.

[0013] From the viewpoint of reducing the dislocation density in the active layer, it is preferable to use a single crystal AlN substrate as the substrate 11. The dislocation density of the single crystal AlN substrate is 10 ８ cm －２ It is preferable that the value is 10 or less, and more preferably 10 ６ cm －２ Below, most preferably 10 ４ cm －２ The lower the dislocation density, the lower the ６ cm －２ Below, further 10 ４ cm －２ By using the AlN substrate, it is possible to prevent a decrease in the light emission efficiency in the active layer due to dislocations, and further to prevent problems such as the diffusion of impurities through dislocations that occur when the ultraviolet light emitting element is energized and an increase in leakage current. ４ cm －２An AlN substrate was used.

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

[0015] Furthermore, if the absorption coefficient of the substrate for the ultraviolet light emitted from the active layer is large, the total amount of ultraviolet light that can be extracted to the outside may decrease, which may lead to a decrease in light emission efficiency. Therefore, the absorption coefficient of the AlN substrate and the AlN layer of the AlN template is preferably 20 cm －１ It is preferably 10 cm or less. －１ Less than 10cm －１ By setting the thickness as below, even if the thickness of the AlN substrate 11 is 100 μm, for example, it is possible to ensure an in-line transmittance of 90% or more.

[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 determined appropriately so as to obtain sufficient transparency for the desired ultraviolet light emission wavelength. In the ultraviolet LED 10, 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. Furthermore, as the Al composition of the n-type AlGaN layer increases, the band gap of the n-type AlGaN layer increases, and accordingly, ultraviolet light of shorter wavelengths can be transmitted.

[0017] The n-type AlGaN layer 12 may be formed of a plurality of layers having different Al compositions, and may be a compositionally graded layer in which the Al composition is graded in the stacking direction. For example, the n-type AlGaN layer 12 may be formed of a first n-type Al Ｘ１ Ga １－Ｘ１ N layer 12A and second n-type Al Ｘ２ Ga １－Ｘ２ The first n-type Al layer 12B Ｘ１ Ga １－Ｘ１ The 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 AlＸ２ Ga １－Ｘ２ The N layer 12B may be a compositionally graded layer in which the Al composition X2 decreases from 0.75 to 0.70.

[0018] Furthermore, the thickness of the n-type AlGaN layer 12 is not particularly limited and may be determined appropriately. When a single-crystal AlN substrate is used as the substrate 11, the 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 n-type AlGaN layer is preferable. However, when an AlN substrate is used as the substrate 11, if the n-type AlGaN layer is too thick, lattice relaxation occurs in the n-type AlGaN layer, making it more likely to generate dislocations.

[0019] For example, the n-type AlGaN layer 12 may be formed by the first n-type Al Ｘ１ Ga １－Ｘ１ N layer 12A and second n-type Al Ｘ２ Ga １－Ｘ２ When the layer structure is formed as a stack of the N layer 12B, the first n-type Al Ｘ１ Ga １－Ｘ１ The N layer 12A has a thickness of 200 nm and is made of a second n-type Al Ｘ２ Ga １－Ｘ２ The N layer 12B can have a thickness of 1000 nm. Ｘ１ Ga １－Ｘ１ N layer 12A and second n-type Al Ｘ２ Ga １－Ｘ２ The thickness of the N layer 12B is not limited to the exemplified values, and can be determined appropriately so that the total thickness is 2.0 μm or less.

[0020] The concentration of Si doped into the n-type AlGaN layer 12 may be determined appropriately so as to obtain a desired n-type conductivity. However, from the viewpoint of reducing the resistance value of the n-type AlGaN layer 12, it is preferable to set the concentration of Si to 1×10 １８ ~1 x 10 ２０ cm －３ It is preferable that the １８ ~5 x 10 １９ cm －３The Si doping concentration may be constant in the layer thickness direction within the n-type AlGaN layer, or may be modulated doping in which the Si concentration varies in the layer thickness direction.

[0021] The Si concentration and the Mg concentration, which will be described later, can be measured by known secondary ion mass spectrometry (SIMS) analysis. The Si concentration and Mg concentration measured in the present application are the values ​​of AlN, AlGaN, and GaN layers, respectively. ０．６５ Ga ０．３５ Quantitative values ​​using standard samples of N and GaN are used.

[0022] The active layer (ACT) 13 is made of Al Ａ１ Ga １－Ａ１ A barrier layer consisting of an N layer and an Al Ａ２ Ga １－Ａ２ The active layer 13 has a quantum well structure composed of well layers each made of an N-type layer. The emission peak wavelength of the active layer 13 is in the range of 210 to 300 nm. Because the wavelength of light emitted from the active layer 13 is determined by the Al composition and thickness of the well layer, the Al composition and thickness can be appropriately determined so as to obtain a desired emission wavelength within the above wavelength range. The lower the Al composition of the p-type AlGaN contact layer, the lower the Schottky barrier, making it easier to achieve ohmic contact. Because the present invention demonstrates a method for achieving ohmic contact even with p-type AlGaN with a high Al composition, it can be an effective means of increasing the output of ultraviolet LEDs with emission wavelengths of 300 nm or less, preferably 285 nm or less, and more preferably 270 nm or less.

[0023] For example, the 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. The Al composition and thickness of the barrier layer are not particularly limited, but 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] The well layer and the barrier layer may be n-type layers doped with Si. Both the well layer and the barrier layer may be Si-doped layers, or only the well layer or only the barrier layer may be doped with Si. The doping concentration of Si is not particularly limited, but is preferably 1×10 １７ ~5 x 10 １８ cm －３ The range is preferred.

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

[0026] Al Ｙ１ Ga １－Ｙ１ The N layer 14 is a layer provided adjacent to the active layer 13. Ｙ１ Ga １－Ｙ１ The N layer 14 is formed by absorbing electrons injected into the active layer 13 into the p-type Al Ｙ２ Ga １－Ｙ２ It functions as an electron blocking layer (EBL) for suppressing overflow into the N layer 15. Ｙ１ Ga １－Ｙ１ The N layer 14 is formed between the active layer 13 and the p-type Al Ｙ２ Ga １－Ｙ２ It has a band gap larger than that of the N layer 15, and Ｙ１ Ga １－Ｙ１ The Al composition Y1 of the N layer 14 is determined in 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 becomes higher, and when the emission wavelength is shorter than 270 nm, the Al composition Y1 is preferably 0.9≦Y1≦1.0 in order to fully exhibit the function as an electron blocking layer. Ｙ１ Ga １－Ｙ１ The N layer 14 is made of AlN (Y1=1).

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

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

[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 preferable that the p-type dopant concentration be 5×10 １８ ~1 x 10 ２０ cm －３ From the viewpoint of increasing the efficiency of carrier injection into the active layer 13, it is preferable that １９ ~8 x 10 １９ cm －３ It is particularly preferred that:

[0031] The Al of the present invention Ｙ１ Ga １－Ｙ１ The N layer 14 does not contain an n-type dopant or contains a p-type Al layer, which will be described later. Ｙ２ Ga １－Ｙ２ The n-type dopant may be contained at a concentration lower than that of the n-type dopant contained in the N layer 15. Specifically, Al Ｙ１ Ga １－Ｙ１ The n-type impurity concentration in the N layer 14 is 1×10 １８cm －３ According to the findings of the present inventors, p-type Al Ｙ２ Ga １－Ｙ２ During the growth of the N layer 15, the adjacent Al Ｙ１ Ga １－Ｙ１ It is known that dopant diffusion occurs between the Al layer and the N layer 14. Ｙ１ Ga １－Ｙ１ The concentration of n-type dopant in the N layer 14 is p-type Al Ｙ２ Ga １－Ｙ２ If it is higher than that of the N layer 15, Ｙ１ Ga １－Ｙ１ p-type Al from the N layer 14 Ｙ２ Ga １－Ｙ２ The n-type dopant diffuses into the N layer 15, and the p-type Al Ｙ２ Ga １－Ｙ２ It may be difficult to precisely control the n-type dopant concentration in the N layer 15. Ｙ２ Ga １－Ｙ２ In order to prevent a change in the concentration of the n-type dopant in the N layer 15, at least Al Ｙ１ Ga １－Ｙ１ The n-type dopant in the N layer 14 is p-type Al Ｙ２ Ga １－Ｙ２ The concentration must be less than the concentration of the n-type dopant contained in the N layer 15 .

[0032] Also, Al Ｙ１ Ga １－Ｙ１ The thickness of the N layer 14 is determined based on the function as an electron blocking layer and the p-type Al Ｙ２ Ga １－Ｙ２ The thickness may be determined as appropriate so that holes can be efficiently injected from the N layer 15 into the active layer 13, but is preferably in the range of 1 to 30 nm. If the layer thickness is less than 1 nm, electrons tunnel, which reduces the function as an electron blocking layer. On the other hand, if the layer thickness exceeds 30 nm, the p-type Al Ｙ２ Ga １－Ｙ２ Holes are less likely to be injected from the N layer 15 into the active layer 13. Ｙ１ Ga １－Ｙ１ The thickness of the N layer 14 is preferably 2 to 20 nm, and more preferably 5 to 15 nm.

[0033] As mentioned above, AlＹ１ Ga １－Ｙ１ The Mg doped into the N layer 14 can have a concentration difference in the stacking direction. For example, an undoped AlN layer 14A with a thickness of 1 to 5 nm can be stacked on the side in contact with the active layer 13, and an Mg-doped p-type AlN layer 14B with a thickness of 5 to 15 nm can be stacked on top of that. In this case, the Mg doping concentration is 5×10 １８ ~1 x 10 ２０ cm －３ It is preferable that the １９ ~8 x 10 １９ cm －３ It is particularly preferred that:

[0034] In this embodiment, p-type AlN layer 14B is grown on p-type AlN layer 14B. Ｙ２ Ga １－Ｙ２ It has an N layer 15. p-type Al Ｙ２ Ga １－Ｙ２ The N layer 15 is a compositionally graded layer in which the Al composition Y2 changes in the stacking direction. Ｙ１ Ga １－Ｙ１ It is preferable that the Al composition Y2 decreases in the stacking direction from the side in contact with the N layer 14. Ｙ２ Ga １－Ｙ２ Since a polarization doping effect is obtained in the N layer 15, a higher hole concentration is easily obtained, and as a result, the efficiency of hole injection into the active layer 13 is increased. For example, when the emission wavelength is 270 nm or less, Al Ｙ１ Ga １－Ｙ１ The Al composition on the side in contact with the 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. Ｙ２ Ga １－Ｙ２ The Al composition in the surface layer of the N layer 15 (i.e., the side in contact with the p-electrode 21) is preferably 0.5 to 0.85, and more preferably 0.6 to 0.85. By adopting such a structure, the polarization doping effect described above can be enhanced and transparency to the emission wavelength can be maintained, making it easier to obtain high luminous efficiency.

[0035] In addition, p-type Al Ｙ２ Ga １－Ｙ２The thickness of the N layer 15 is not particularly limited, but may be appropriately determined within the range of 1 to 150 nm. From the viewpoint of polarization doping efficiency, a thin layer or a high gradient is preferable. On the other hand, a thick layer reduces transparency (increases light absorption loss). From this viewpoint and from the viewpoint of practical productivity, a p-type Al Ｙ２ Ga １－Ｙ２ The 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 N layer 15 is grown in a pseudomorphic state with the AlN substrate 11, it has a low dislocation density equivalent to that of the AlN substrate 11. ５ cm －２ The dislocation density is as follows: The relaxation rate of the pseudomorphic semiconductor layer is preferably 20% or less. Ｙ２ Ga １－Ｙ２ The N layer 15 functions as a p-type contact layer. Ｙ２ Ga １－Ｙ２ The N layer 15 is co-doped with p-type impurities that act as acceptors and n-type impurities that act as donors.

[0037] p-type Al Ｙ２ Ga １－Ｙ２ Examples of p-type impurities that can be used to dope the N layer 15 include magnesium (Mg), zinc (Zn), beryllium (Be), and carbon (C). Of these, it is preferable to use Mg, which is commonly used as a p-type dopant material for AlGaN semiconductors. Examples of n-type impurities that can be used include silicon (Si), germanium (Ge), selenium (Se), sulfur (S), and oxygen (O). Of these, it is preferable to use silicon (Si), which is commonly used as an n-type dopant material.

[0038] In addition, p-type Al Ｙ２ Ga １－Ｙ２ The amount of p-type impurities doped into the N layer 15 is 1×10 １７ ~1.2 × 10 ２０ cm －３As theoretically shown in J. Appl. Phys. Vol. 95, No. 8, 15 April (2004), p-type Al Ｙ２ Ga １－Ｙ２ It is believed that the amount of nitrogen defects, which is considered to be a cause of deterioration, increases with the amount of p-type impurities in the N layer 15. ２０ cm －３ If the nitrogen vacancy amount exceeds 100%, the amount of nitrogen vacancies formed in the early stage becomes too large, making it difficult to obtain a high power retention rate.

[0039] Furthermore, when the p-type impurity concentration is low, particularly when the Al composition Y2 is constant, the hole concentration decreases and the minority carrier (electron) mobility increases, and when the Al composition Y2 is inclined, the minority carrier (electron) mobility increases, resulting in a decrease in output and making it difficult to obtain high luminous efficiency. Therefore, the p-type impurity concentration can be appropriately determined within the above range, taking such a trade-off into consideration. However, in order to obtain a higher output maintenance rate and high output, it is necessary to set the p-type impurity concentration within the above range. １９ ~5 x 10 １９ cm －３ It is preferable that the concentration is 1×10 １９ ~4 x 10 １９ cm －３ is.

[0040] In the ultraviolet LED 10 of this embodiment, p-type Al Ｙ２ Ga １－Ｙ２ The N layer 15 is in the stacking direction, i.e., adjacent Al Ｙ１ Ga １－Ｙ１ This is a compositionally graded layer in which the Al composition Y2 decreases with increasing distance from the interface with the N layer 14.

[0041] The Al composition Y2 is Al Ｙ１ Ga １－Ｙ１ It is preferable that the Al composition be a gradient layer in which the Al composition decreases linearly with increasing distance from the interface with the N layer 14, but this is not limiting, and the Al composition may be a gradient layer in which the Al composition decreases with a curved profile. Alternatively, the Al composition may decrease stepwise, or a combination of these may be used.

[0042] p-type Al Ｙ２ Ga１－Ｙ２ The amount of n-type impurities doped into the N layer 15 is 1.1×10 １８ Above 9.0 x 10 １８ cm －３ It is preferably 1.8 × 10 or less, and more preferably 1.8 × 10 １８ Above 8.0 x 10 １８ cm －３ With these amounts of n-type impurities, a light emitting device 10 with high luminous efficiency can be obtained.

[0043] In addition, p-type Al Ｙ２ Ga １－Ｙ２ The concentration of the p-type impurities and n-type impurities doped into the N layer 15 may be constant within the layer, or may vary in the stacking direction. However, in order to drive at a higher output and a lower voltage, it is preferable to use p-type Al Ｙ２ Ga １－Ｙ２ At least the Al Ｙ１ Ga １－Ｙ１ It is preferable that only the interface of the N layer 14 is not doped with n-type impurities or has a low concentration.

[0044] p-type Al Ｙ２ Ga １－Ｙ２ By doping the N layer 15 with n-type impurities only partially, the depletion of the interface is suppressed, thereby suppressing the voltage rise. This also contributes to the reduction of the mobility of minority carriers (electrons), making it possible to obtain a higher output than without n-type impurity doping. Therefore, if a higher wall plug efficiency (WPE) than that without n-type impurity doping is desired, at least Al Ｙ１ Ga １－Ｙ１ N layer 14 and p-type Al Ｙ２ Ga １－Ｙ２ It is preferable that the n-type impurity concentration near the interface of the N layer 15 is low. Ｙ２ Ga １－Ｙ２ Since the N layer 15 is a compositionally graded layer, Al Ｙ１ Ga １－Ｙ１ This is because the width of the depletion layer formed at the interface with the N layer 14 is wider than the interface with the p-type contact layer 16, and therefore the effect of not doping with n-type impurities is significant.

[0045] p-type Al Ｙ２ Ga １－Ｙ２ The Al composition Y2 of the N layer 15 is 0.5 or more and 1.0 or less, and Al Ｙ１ Ga １－Ｙ１ The Al composition of the N layer 14 is equal to or less than Y1.

[0046] p-type Al Ｙ２ Ga １－Ｙ２ In the case of a structure in which Y2 is a constant value in the stacking direction, the Al composition Y2 of the N layer 15 exceeds the Al composition of the barrier layer of the active layer 13 and is Ｙ１ Ga １－Ｙ１ The Al composition of the N layer 14 is preferably Y1 or less. Ｙ２ Ga １－Ｙ２ By setting the Al composition Y2 of the N layer 15 in the above range, a high carrier overflow suppression effect can be obtained even when the amount of injected current of the ultraviolet light emitting device is large. To obtain a higher effect, the Al composition of the well layer of the active layer 13 and the p-type Al Ｙ２ Ga １－Ｙ２ The difference in the Al composition Y2 of the N layer 15 is preferably 0 or more, and more preferably 0.05 or more than the difference in the Al composition Y2 of the barrier layer and the p-type Al Ｙ２ Ga １－Ｙ２ It is preferable that the difference in the Al composition Y2 of the N layer 15 is 0 or more. Ｙ２ Ga １－Ｙ２ The Al composition Y2 of the N layer 15 is preferably larger than the Al composition of the n-type AlGaN layer 12, which enhances the effect of suppressing carrier overflow to the p-type layer and improves the light emission efficiency of the ultraviolet light emitting device. Ｙ２ Ga １－Ｙ２ In the case of a structure in which Y2 is a constant value in the stacking direction, the Al composition Y2 of the N layer 15 is preferably 0.6 or more and 0.9 or less.

[0047] When AlN is used as the substrate 11, all of the AlGaN layers 12, 13, 14, and 15 are grown in a state of pseudo-lattice matching with the AlN substrate 11, and therefore have a low dislocation density equivalent to that of the AlN substrate 11. ５ cm －２The dislocation density is as follows: It is preferable that the relaxation rate of the pseudomorphic semiconductor layer is 20% or less. Although the ultraviolet semiconductor light emitting element 10 has been described as a light emitting diode (LED), it may also be configured as a semiconductor laser element (LD: Laser Diode).

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

[0049] The ultraviolet LED 10 has been described as a semiconductor light emitting device having a substrate 11 on the n-type AlGaN layer 12 side. However, a support substrate other than the substrate 11 used for epitaxial growth is used for p-type AlGaN. Ｙ２ Ga １－Ｙ２ It may also be provided on the N layer 15 side. In this case, the material of the support substrate is not particularly limited, and may be polycrystalline AlN, Si, Al ２ O ３ Any known material used as a support substrate member for a light emitting element, such as Cu, CuW, etc., can be used without any restrictions.

[0050] In the following, the ultraviolet LED 10 will be described as a semiconductor light-emitting element having a substrate 11 as a growth substrate of a semiconductor laminate structure. However, as mentioned above, the semiconductor light-emitting element may also have a substrate 11 that is different from the growth substrate, such as a support substrate.

[0051] [Method for Manufacturing Ultraviolet LED] A method for manufacturing the ultraviolet LED 10 having the structure described above will now be described. The ultraviolet LED 10 of the present invention can be manufactured by the MOCVD method, which has high productivity and is widely adopted industrially. The group III (Al, Ga) source gas and group V (N) source gas used can be any known source gas without any particular restrictions.

[0052] For example, gases such as trimethylaluminum (TMA), triethylaluminum (TEA), trimethylgallium (TMG), and triethylgallium (TEG) can be used as the Group III source gas. Also, ammonia (NH ３ ) is used.

[0053] Furthermore, known materials can be used without limitation as dopant source gases for Mg and Si, such as biscyclopentadienyl magnesium (CpMg) and monosilane (SiH ４ ), tetraethylsilane, etc. can be used. ２ ) and / or nitrogen (N ２ ) onto the substrate 11 to grow the element layers of the ultraviolet LED 10.

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

[0055] Furthermore, the growth temperature of the element layers constituting the UV-LED 10 is not limited unless otherwise specified, and may be determined as appropriate so as to obtain the desired characteristics of each layer and the characteristics of the UV-LED 10. However, growth at a temperature of 1000 to 1200°C is preferred, and a temperature of 1000 to 1150°C is more preferred.

[0056] [Example] In the following, the present invention will be specifically explained using an example in which an ultraviolet LED with an emission wavelength of 265 nm was fabricated, but the present invention is not limited to the example. 1. Fabrication of element The substrate on which the ultraviolet LED element layer is grown is provided with a dislocation density of 10 ４ cm －２The following AlN substrate was used. Referring again to FIG. 2B , a first n-type AlGaN layer 12A (layer thickness: 200 nm) and a second n-type AlGaN layer 12B (1000 nm) were grown on this AlN substrate 11 using an MOCVD apparatus. An AlGaN buffer layer (with an Al composition equal to or greater than the Al composition of the first n-type AlGaN layer 12A, up to 100%) may be present between the substrate 11 and the first n-type AlGaN layer 12A. The first n-type AlGaN layer 12A and the second n-type AlGaN layer 12B are both compositionally graded layers, with the Al composition decreasing from 1.0 to 0.75 from the side in contact with the AlN layer in the first n-type AlGaN layer 12A, and the Al composition decreasing from 0.75 to 0.70 from the side in contact with the first n-type AlGaN layer 12A in the second n-type AlGaN layer 12B. The Si concentration in the n-type AlGaN layer 12 is 1×10 １９ cm －３ It was controlled so that

[0057] Next, n-type Al ０．５９ Ga ０．４１ A barrier layer made of N (layer thickness: 7 nm) and Al ０．５ Ga ０．５ The active layer 13 was grown with a triple quantum well structure consisting of a well layer made of N (4 nm). The Si concentration in the barrier layer was 1×10 １８ cm －３ It was controlled so that

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

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

[0060] 2. p-type Al Ｙ２ Ga １－Ｙ２ Consideration of N layer (1) Fermi level pinning In the present invention, p-type Al Ｙ２ Ga １－Ｙ２ The N layer 15 is co-doped with Si, an anti-type impurity, to improve the device characteristics. Adding anti-type impurities is not usually done because it shifts the Fermi level and impairs ohmic characteristics, but the following verification was conducted based on the idea that moderate co-doping may be more effective in suppressing Fermi level pinning than in shifting the Fermi level.

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

[0062] FIG. 4 shows the p-type Al Ｙ２ Ga １－Ｙ２ 1 is a diagram showing the profiles of the Al composition, the growth temperature TG, the Mg concentration, and the Si concentration in the N layer 15. Specifically, the p-type Al Ｙ２ Ga １－Ｙ２ The N layer 15 was co-doped with Si in addition to Mg. The Mg flow rate was 40 cc (Mg concentration: 2E19 cm ) up to 48 nm from the interface with the AlN substrate. －３ The sample was fabricated with the Mg flow rate set at 400 cc (Mg concentration: 4E19 cm －３ ), and the growth temperature TG was gradually decreased by 40°C from 1030°C (growth temperature decreasing region Rg) (reduced growth temperature = 990°C). Ｙ２ Ga １－Ｙ２ The Mg concentration at the surface of the N layer is 4E19 cm －３ and the Si concentration is 3E18 cm －３The Mg concentration in the layer with increased Mg concentration was 3E19 to 2E20 cm －３ It is preferable that p-type Al Ｙ２ Ga １－Ｙ２ It is preferable that the thickness of the growth temperature reduced region Rg be increased to 1.5 times or more that of the other portions of the N layer 15. It is also preferable that the thickness of the growth temperature reduced region Rg be 20 nm or less.

[0063] (2) Evaluation of Schottky Barrier Figure 5 shows the Schottky barrier height calculated from the temperature dependence of TLM measurements by forming a TLM pattern on the sample shown in Figure 4, versus the thickness of the Si co-doped layer. The barrier height without Si co-doping is normalized to 1. The Schottky barrier height is the energy difference between the Fermi level and the lower edge of the valence electron at the semiconductor surface, and is expressed in eV.

[0064] 5 shows the effect of the Si co-doped layer thickness on the Schottky barrier height compared to the case without Si co-doping, and shows that the Schottky barrier is lowered when Si co-doping is performed up to the surface layer in contact with the p-electrode.

[0065] In other words, this result indicates that codoping with an antitype dopant is more effective in lowering the Schottky barrier by suppressing pinning than in increasing the Schottky barrier by shifting the Fermi level position toward the conduction band, demonstrating that codoping with an antitype dopant is effective in suppressing pinning.

[0066] FIG. 6A shows a p-type Al Ｙ２ Ga １－Ｙ２ The amount of Mg (cm) per unit layer 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 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 portion where the amount of Mg is increased or where Si co-doping is performed will be defined as a "surface contact layer SC" for explanation.

[0069] 6A, it can be seen that by gradually decreasing the growth temperature T 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 is 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. Note that 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 T suppresses the generation of point defects, lowers the Schottky barrier, and improves the ohmic characteristics. The growth temperature T is preferably reduced by 20 to 60°C.

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

[0072] In addition, p-type Al Ｙ２ Ga １－Ｙ２ The N layer 15 also functions as a cladding layer, but the output power maintenance ratio (e.g., the output power ratio after 100 hours from the initial time) increases almost linearly with an increase in the Si / Mg ratio, and it is believed that the Si / Mg ratio is the dominant physical parameter that influences the output power maintenance ratio. The Si / Mg ratio is estimated to be inversely proportional to the concentration of nitrogen defects formed in the undoped state.

[0073] As a result of extensive research by the present inventors, it was found that p-type Al Ｙ２ Ga １－Ｙ２ By co-doping the N layer 15, an ultraviolet LED having high luminous efficiency and a good output maintenance rate (device life) is realized.

[0074] 0.009≦(Si / Mg)<0.185 (1) In this example, p-type Al Ｙ２ Ga １－Ｙ２ The interaction between Mg, which is an acceptor impurity doped into the N layer 15, and Si, which is a donor impurity, produces an effect of reducing nitrogen defects, improving the device life and the ohmic characteristics of the surface layer.

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

[0076] More specifically, the p-type AlN layer 14B is Ｙ２ Ga １－Ｙ２ In addition to Mg, Si is co-doped into the N layer 15. The Mg doping is the same as in the case shown in FIG.

[0077] The Si co-doping is performed by reducing the co-doping amount (for example, 1 / 3 of 1E18 cm ) compared to the case shown in FIG. －３ ), and continues until the growth temperature TG is lowered from the interface with the p-type AlN layer 14B (for example, to a position where the layer thickness is 50 nm). After this, Si co-doping is stopped (co-doping off region Roff), and Si co-doping is again performed in the surface contact layer SC (part with a layer thickness of 4 nm). However, the Si co-doping amount in the surface contact layer SC is 3E18 cm3, the same as in the case shown in FIG. －３ is set to prevent the ohmic contact from being reduced.

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

[0079] [Modification 2] FIG. 8 shows a p-type Al Ｙ２ Ga １－Ｙ２1 is a diagram showing the Mg—Si co-doping profile of the N layer 15 in Modification Example 2. In this Modification Example 2, p-type Al Ｙ２ Ga １－Ｙ２ Only the surface contact layer SC (for example, 8 nm thick) of the N layer 15 is co-doped with Mg—Si, and the other regions are doped only with Mg, which is a p-dopant.

[0080] The donor concentration in at least the surface layer (outermost layer) of the surface contact layer SC is 1×10 １７ ~1 x 10 ２０ cm －３ It is preferable that:

[0081] p-type Al having such a Mg—Si co-doping profile Ｙ２ Ga １－Ｙ２ The N layer 15 has a low contact resistance and provides good ohmic contact. Ｙ２ Ga １－Ｙ２ The N layer 15 has the advantage of having a small specific resistance (series resistance).

[0082] As described above in detail, the present invention can provide an ultraviolet semiconductor light-emitting element that 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, and a method for manufacturing the same.

[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, 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.

2. The ultraviolet semiconductor light-emitting device according to claim 1, further comprising 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, which is a composition gradient layer in which the Al composition decreases as it moves away from the electron blocking layer, 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.

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, wherein the ultraviolet semiconductor light-emitting device is such that the ultraviolet semiconductor light-emitting device is such that the ultraviolet semiconductor light-emitting device is such that the ultraviolet semiconductor light-emitting device is such that the ultraviolet semiconductor light-emitting device is such that the ultraviolet semiconductor light-emitting device is such that the ultraviolet semiconductor light-emitting device is such that the ultraviolet semiconductor light-emitting device is such that the ultraviolet semiconductor light-emitting device is such that the ultraviolet semiconductor light-emitting device is such that the ultraviolet semiconductor light-emitting device is such that the ultraviolet semiconductor light-emitting device is such that the ultraviolet 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light-emitting device is such 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 pseudomorphically aligned with the AlN substrate, and the relaxation rate 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 parts 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 parts 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 in the range of -20°C to -60°C lower than the growth temperature of the p-type semiconductor layer.

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