Ultraviolet semiconductor light emitting element and method for manufacturing same

Abstract

According to the present invention, a p-type GaN contact layer has a first p-type GaN layer that is grown on a p-type AlGaN layer, and a second p-type GaN layer that is grown on the first p-type GaN layer. The first p-type GaN layer and the second p-type GaN layer are grown using magnesium (Mg) as a p-dopant. The growth of the first p-type GaN layer is performed using hydrogen (H2) and nitrogen (N2) as a carrier gas. The N2 proportion in the carrier gas is 50% or more. The growth of the second p-type GaN layer is performed using a carrier gas in which the hydrogen (H2) proportion is 100%. The first p-type GaN layer is grown under such conditions that the ratio of the Mg material to the Ga material is not less than 5 times the ratio of the Mg material to the Ga material in the growth of the second p-type GaN layer.

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 attracted attention as light sources with bacteria and virus inactivation and sterilization effects. However, in AlGaN-based ultraviolet light-emitting devices, further reductions in contact resistance and driving voltage are important for achieving higher output and longer life of the light-emitting devices.

[0003] Previously, studies have been conducted on group III nitride semiconductor light-emitting devices with reduced p-electrode contact resistance and low driving voltage. For example, Patent Document 1 discloses a method of gradually increasing the Mg concentration in the p-GaN contact layer from the cladding layer side toward the surface side. This is intended to improve the crystallinity of the initial growth of the p-GaN layer and reduce the contact resistance with the electrode metal.

[0004] Furthermore, Patent Document 2 discloses that a first p-type GaN layer and a second p-type GaN layer on the first p-type GaN layer are formed by supplying at least nitrogen gas as a carrier gas, and that no p-type dopant gas is supplied when the first p-type GaN layer is formed.

[0005] Generally, after the growth of the p-GaN contact layer, nitrogen (N ２ Activation is performed in a fluorine-containing gas atmosphere at high temperatures (for example, 600 to 800° C.) (for example, Patent Document 1).

[0006] Furthermore, in AlN-based semiconductor light-emitting elements, it is disclosed that the thickness of the p-type semiconductor layer cannot be increased (for example, to 100 nm or less) in order to maintain good contact with the electrodes and suppress an increase in driving voltage and electrical defects (for example, Patent Document 3).

[0007] Japanese Patent No. 5423026 Japanese Patent No. 6229609 Japanese Patent Application Laid-Open No. 2021-97193

[0008] AlN and GaN, which make up ultraviolet light-emitting devices, are known to have very large bandgaps and high forward voltages. In addition, lattice relaxation occurs during GaN growth on an AlN-based semiconductor layer, resulting in threading dislocations. The more threading dislocations there are, the more carriers are trapped by these defects, increasing the proportion of thermal energy and other components that do not contribute to light emission, resulting in poor conductivity and an increase in forward voltage. Furthermore, activation can easily cause alloy spikes during electrode formation, posing a problem of accelerated device degradation.

[0009] Therefore, it has been difficult to realize conventional ultraviolet semiconductor light-emitting devices that are not only highly efficient and high-power, but also highly reliable. In particular, the shorter the emission wavelength, the faster the device deteriorates when driven with a large current to obtain high optical output. In other words, it has been difficult to achieve both high-power output characteristics and high reliability (long life).

[0010] The present invention has been made in view of the above-mentioned problems, and has as its object to provide an ultraviolet semiconductor light-emitting element that is highly efficient, has high output, is minimally deteriorated, and has an excellent element life, and a method for manufacturing the same.

[0011] A manufacturing method according to one embodiment of the present invention is a method for manufacturing an ultraviolet semiconductor light-emitting device by sequentially growing an n-type AlGaN layer, an active layer, a p-type AlGaN layer, and a p-type GaN contact layer on an AlN substrate by MOCVD, wherein the p-type GaN contact layer has a first p-type GaN layer grown on the p-type AlGaN layer and a second p-type GaN layer grown on the first p-type GaN layer, the first p-type GaN layer and the second p-type GaN layer are grown using magnesium (Mg) as a p-dopant, and the growth of the first p-type GaN layer is performed using hydrogen (H ２ ) and nitrogen (N ２ ) as a carrier gas, and N in the carrier gas ２ The ratio is 50% or more, and the second p-type GaN layer is grown using hydrogen (H ２) 100%, and the first p-type GaN layer is grown under conditions in which the Mg source material / Ga source material ratio is 5 times or more the Mg source material / Ga source material ratio in the growth of the second p-type GaN layer.

[0012] According to another embodiment of the present invention, there is provided an ultraviolet semiconductor light-emitting device in which an n-type AlGaN layer, an active layer, a p-type AlGaN layer, and a p-type GaN contact layer are sequentially grown on an AlN substrate, the p-type GaN contact layer having a thickness of 200 nm or more, and a p-electrode formed on the as-grown p-type GaN contact layer.

[0013] 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 showing the evaluation results of samples (Comparative Example: CX1, Examples: EX1 to EX3) formed by changing the Mg flow rate of each of the first to third p-type GaN layers; 5 is a diagram showing the definition of p-barrier Vb; 6 is a diagram showing reflectance oscillations measured by a reflectance monitor on the crystal surface during growth; 7 is a graph plotting the enhancement factor MF (=MR1 / MR2), which is the ratio of the Mg / Ga ratio (MR1) of the first p-type GaN layer to the Mg / Ga ratio (MR2) of the second p-type GaN layer, on the horizontal axis and the maximum value Rmax of the reflectance R measured in situ during growth on the vertical axis; 8 is a diagram showing the evaluation results of samples (Comparative Example: CX1, Examples: EX1 to EX3) formed by changing the Mg flow rate of each of the first to third p-type GaN layers; ２ 1 is a graph plotting the maximum value Rmax of the reflectance R measured in-situ when the ratio of ２ 10 shows the growth mechanism (left) of the second p-type GaN layer in a comparative example (CMP, N ２ The growth mechanism of the p-type GaN layer (left) and the p-type GaN layer after growth (CMP, N ２ 1 shows a cross-sectional TEM image (right side) of the third p-type GaN layer grown under the same carrier gas conditions (H ２1 shows a surface AFM image of a p-type contact layer grown under the condition of SiO 2 (SiO 2 = 100%). 2 shows a comparison of the growth conditions of the p-type contact layer of the example (EMB) and the comparative example (CMP). 3 shows a comparison of the contact characteristics and crystallinity of the p-type contact layer of the example (EMB) and the comparative example (CMP).

[0014] Preferred embodiments of the present invention will be described below, but these may be modified and combined as appropriate. Furthermore, in the following description and accompanying drawings, substantially identical or equivalent parts will be described with the same reference numerals. [Structure of Ultraviolet Semiconductor Light-Emitting Device] FIG. 1 is a cross-sectional view schematically illustrating the structure of an ultraviolet semiconductor light-emitting device 10 according to one embodiment of the present invention. The ultraviolet semiconductor light-emitting device 10 is an ultraviolet light-emitting diode (hereinafter also referred to as an ultraviolet LED 10), and is formed by epitaxially growing an n-type AlGaN layer 12, an active layer 13, an AlGaN layer 14, a p-type AlGaN layer 15, and a p-type contact layer 16, which is a p-type GaN layer, on a substrate 11, in that order.

[0015] Furthermore, a p-electrode 21 that makes ohmic contact with the p-type contact layer 16 is formed on the p-type contact layer 16, and an n-electrode 23 that makes ohmic contact with the substrate 11 is formed on the substrate 11. Figures 2A and 2B respectively show a band diagram and a semiconductor layer structure of the ultraviolet LED 10. The ultraviolet LED 10 will be described in more detail with reference to Figures 1, 2A, and 2B.

[0016] 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.

[0017] 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 via dislocations that occur when a current is applied to the ultraviolet light emitting element and an increase in leakage current.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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 Al Ｘ２ 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.

[0022] 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.

[0023] 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.

[0024] On the other hand, when an AlN template substrate is used, the thickness of the n-type AlGaN layer 12 is preferably 1.5 μm or more and 10 μm or less. This is because a thicker n-type AlGaN layer 12 is expected to have the effect of reducing dislocations in addition to reducing the resistance value, and therefore a thickness of 1.5 μm or more is considered preferable. Furthermore, when an AlN template substrate is used, the upper limit of the thickness of the n-type AlGaN layer 12 is considered to be approximately 10.0 μm, taking into account industrial viewpoints such as manufacturing time.

[0025] 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 －３ The 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.

[0026] 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.

[0027] 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－Ａ２The active layer 13 has a quantum well structure composed of well layers each consisting of an N layer. The emission peak wavelength of the active layer 13 is in the range of 210 to 300 nm. Since the wavelength of light emitted from the active layer 13 is determined by the Al composition and layer thickness of the well layer, the Al composition and layer thickness can be appropriately determined so as to obtain a desired emission wavelength within the above wavelength range. The reliability of ultraviolet light-emitting devices is wavelength-dependent, and the shorter the emission wavelength, the shorter the device life is generally. Therefore, in the present invention, the shorter the emission wavelength, the more effective it is, and the preferable wavelength range is considered to be 210 to 270 nm.

[0028] For example, the thickness of the well layer can be set in the range of 2 to 10 nm, and the Al 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.

[0029] 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.

[0030] 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.

[0031] 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－Ｙ２It 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.

[0032] 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).

[0033] 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.

[0034] 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).

[0035] AlＹ１ 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, it is preferable that １９ ~8 x 10 １９ cm －３ It is particularly preferred that:

[0036] 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 １８ cm －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 Ga１－Ｙ２ The concentration must be less than the concentration of the n-type dopant contained in the N layer 15 .

[0037] 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, 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 into the active layer from the N layer 15. Ｙ１ Ga １－Ｙ１ The thickness of the N layer 14 is preferably 2 to 20 nm, and more preferably 5 to 15 nm.

[0038] As mentioned above, Al Ｙ１ 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:

[0039] p-type Al of this embodiment Ｙ２ Ga １－Ｙ２ The N layer 15 is made of the Al Ｙ１ Ga １－Ｙ１ It is formed on the N layer 14 and functions as a p-type cladding 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.

[0040] p-type Al Ｙ２ Ga１－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.

[0041] 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 －３ As 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.

[0042] 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.

[0043] In the ultraviolet LED 10 of this embodiment, p-type AlＹ２ 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.

[0044] The Al composition Y2 is Al Ｙ１ Ga １－Ｙ１ Although 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, the present invention is not limited to this, and the Al composition may be a gradient layer in which the Al composition decreases in a curved manner, or the Al composition may decrease stepwise, or a combination of these may be used.

[0045] p-type Al Ｙ２ Ga １－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.

[0046] 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 It is preferable that both interface portions of the N layer 15 are not doped with n-type impurities or have a low concentration.

[0047] p-type Al Ｙ２ Ga １－Ｙ２By 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.

[0048] 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.

[0049] 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 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 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－Ｙ２It 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.

[0050] In addition, p-type Al Ｙ２ Ga １－Ｙ２ The N layer 15 may be 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 the 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 is increased. 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-type contact layer 16 (p-type GaN layer)) 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.

[0051] In addition, p-type Al Ｙ２ Ga １－Ｙ２ The thickness of the N layer 15 is not particularly limited, but may be determined appropriately within the range of 10 to 150 nm. Ｙ２ Ga １－Ｙ２If the thickness of the N layer 15 is less than 10 nm, it becomes difficult to obtain the above-mentioned effect of suppressing carrier overflow. On the other hand, if the thickness is too thick and exceeds 150 nm, the p-type Al Ｙ２ Ga １－Ｙ２ The resistance value of the N layer 15 increases, resulting in an increase in the operating voltage of the ultraviolet light emitting device. Ｙ２ Ga １－Ｙ２ The thickness of the N layer 15 is preferably 40 to 120 nm, and particularly preferably 50 to 100 nm.

[0052] p-type Al Ｙ２ Ga １－Ｙ２ A p-type contact layer 16 made of a p-type GaN layer doped with a p-type dopant is formed on the N layer 15 in order to reduce the contact resistance with the electrode. The above-mentioned known p-type dopant materials can be used as the p-type dopant material, but for the same reason, it is preferable to use Mg.

[0053] As shown in FIGS. 1 and 2, the p-type contact layer 16 (p-type GaN layer) is formed by growing a first p-type GaN layer 16A, a second p-type GaN layer 16B, and a third p-type GaN layer 16C in this order.

[0054] When AlN is used as the substrate 11, all of the AlGaN layers 12, 13, 14, and 15, except for the p-type GaN layer (p-type contact layer) 16, are grown in a state of lattice bonding with the AlN substrate 11, and therefore have a low dislocation density equivalent to that of the AlN substrate 11. ５ cm －２ The dislocation density is as follows:

[0055] 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).

[0056] 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.

[0057] Although 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, a support substrate other than the substrate 11 used for epitaxial growth can also be provided on the p-type GaN layer (p-type contact layer) 16 side. In this case, the material of the support substrate is not particularly limited, and can be polycrystalline AlN, Si, Al ２ O ３ Any known material, such as Cu or CuW, that is used as a support substrate member for a light emitting element can be used without any restrictions.

[0058] 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.

[0059] [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.

[0060] 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.

[0061] 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.

[0062] The above raw material gases are mixed with hydrogen (H ２ ) and / or nitrogen (N ２ ) onto the substrate 11 to grow the element layers of the ultraviolet LED 10.

[0063] 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.

[0064] The growth temperature of the element layers constituting the UV-LED 10 is not particularly limited and may be determined appropriately 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 1000 to 1150°C is more preferred.

[0065] [Example] In the following, the present invention will be specifically described 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. [Fabrication of element] The substrate on which the ultraviolet LED element layer is grown is provided with a dislocation density of 10 ４ cm －２ The following AlN substrate was used. Referring again to FIG. 2B , a first n-type AlGaN layer 12A (layer thickness: 200 nm, first n-clad layer) and a second n-type AlGaN layer 12B (1000 nm, second n-clad layer) were grown on this AlN substrate 11 using an MOCVD apparatus. The first n-type AlGaN layer 12A and the second n-type AlGaN layer 12B were both compositionally graded layers, with the Al composition of the first n-type AlGaN layer 12A decreasing from 1.0 to 0.75 from the side in contact with the AlN layer, and the Al composition of the second n-type AlGaN layer 12B decreasing from 0.75 to 0.70 from the side in contact with the first n-type AlGaN layer 12A. The Si concentration in the n-type AlGaN layer 12 was 1×10 １９ cm －３ It was controlled so that

[0066] Next, n-type Al ０．５９ Ga ０．４１ A barrier layer made of N (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

[0067] 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.

[0068] Next, a p-type AlGaN layer 15 (p-cladding layer) was grown. The p-type AlGaN layer 15 was a compositionally graded layer in which the Al composition decreased from 0.98 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.

[0069] Next, a p-type contact layer 16 (p-type GaN layer) was grown on the p-type AlGaN layer 15. The p-type contact layer 16 will be described in detail below.

[0070] [Growth of p-Type Contact Layer (p-Type GaN Layer)] The p-type contact layer 16 (p-type GaN layer) is formed by growing a first p-type GaN layer 16A, a second p-type GaN layer 16B, and a third p-type GaN layer 16C in this order on the p-type AlGaN layer 15. The growth mechanisms and growth conditions for each of the first to third p-type GaN layers 16A to 16C will be described in detail below.

[0071] In the ultraviolet LED 10 of this embodiment, the carrier gas and Mg flow rates for each of the first to third p-type GaN layers 16A to 16C were investigated based on the knowledge gained about the growth mechanism and crystallinity of p-type GaN.

[0072] In this specification, powers of 10 are sometimes expressed using E. For example, 2.0×10 １９ may be written as 2.0E19 or 2.0E+19.

[0073] 1. Growth Conditions for First to Third P-Type GaN Layers 1-1 Growth Conditions for First P-Type GaN Layer It has been found that the following growth conditions (i) to (iii) are suitable for growing the first p-type GaN layer 16A. (i) The Mg concentration is 2.0E+19 or more. (ii) The Mg source / Ga source ratio (hereinafter simply referred to as the Mg / Ga ratio), i.e., the ratio of Cp2Mg flow rate / TMG flow rate, is 5 times or more that of the second p-type GaN layer 16B. (iii) Hydrogen (H ２ ) and nitrogen (N ２ ) in the carrier gas ２ The ratio should be 50% or more. ２ The ratio is preferably 60 to 80%. 1-2 Growth Conditions for the Second P-Type GaN Layer (iv) The growth of the second p-type GaN layer 16B is performed using hydrogen (H ２ ) 100%. That is, the carrier gas is nitrogen (N ２ 1-3 Growth Conditions of the Third P-Type GaN Layer (v) In the growth of the third p-type GaN layer 16C, hydrogen (H ２ ) and nitrogen (N ２ ) in the carrier gas ２ The ratio of N is 50% or more. ２ The ratio is preferably 60 to 80%.

[0074] 2. Study on the Growth Mechanism of the First to Third P-Type GaN Layers The following provides a detailed explanation of the study on the growth mechanism of each of the first to third p-type GaN layers 16A to 16C and the growth conditions derived from the results of the study.

[0075] FIG. 3 shows the evaluation results of samples (comparison example: CX1, examples: EX1 to EX3) formed by changing the Mg flow rate (cc / min) for each of the first to third p-type GaN layers 16A to 16C. Specifically, the results show the fluorescence microscope images of the growth surfaces, the root-mean-square roughness (RMS), the number of hillocks and the number of anchorless structures, as well as the p-barrier Vb, contact resistance and resistivity of the growth layers measured using the TLM (Transmission Line Method).

[0076] 2-1 First p-type GaN layer First, a p-type cladding layer (p-type AlGaN layer) Ｙ２ Ga １－Ｙ２ In the initial stage of growth of p-GaN on the N layer 15), the flow rate of the Mg source material is increased to generate nuclei originating from Mg on the AlGaN surface, in addition to normal GaN nuclei, thereby increasing the surface density of nuclei and increasing the N in the carrier gas. ２ In order to obtain a uniform GaN layer, it is effective to increase the temperature to prevent re-evaporation of GaN and grow GaN over the entire AlGaN surface.

[0077] 3 shows the cases where the Mg flow rate of the first p-type GaN layer 16A is 200 cc (CX1), 300 cc (EX1), 400 cc (EX2), and 600 cc (EX3). Note that the Mg concentration in these cases is 1.4E+19 cm －３ (CX1), 2.0E+19cm －３ (EX1), 2.7E+19cm －３ (EX2), 4.0E+19cm －３ (EX3).

[0078] Mg flow rate is 200cc (CX1, Mg concentration 1.4E+19cm －３ ), hillocks (shown by dashed lines in the figure) were observed in the fluorescence microscope image. On the other hand, when the Mg flow rate was 300 cc or more, that is, when the Mg concentration was 2.0E+19 cm －３ It was found that the number of hillocks and anchorless structures was reduced.

[0079] Furthermore, measurements using the TLM method also showed that the Schottky barrier component was small, as seen in the p-barrier Vb, which is an index (or degree of nonlinearity) of the Schottky barrier component, and good ohmic characteristics were obtained. In addition, the contact resistance and resistivity were also small, and the lateral conductivity characteristics were also good.

[0080] The p-barrier Vb (V) shown in Fig. 3 is defined as the voltage value at the intersection of the tangent to the IV (current-voltage) curve and the V axis (X axis), as shown schematically in Fig. 4. The Schottky barrier height is the energy difference between the Fermi level and the lower end of the valence electron on the semiconductor surface, and is expressed in eV.

[0081] Therefore, it was found that the Mg concentration is preferably set to 2.0E+19 or more (growth condition (i)).

[0082] 5 is a graph showing the reflectance fluctuations observed by a reflectance monitor on the crystal surface during growth in an MOCVD apparatus, where the horizontal axis represents growth time and the vertical axis represents reflectance R.

[0083] Specifically, H is used as a carrier gas. ２ Only H was used as a carrier gas. ２ (30%) and N ２ In the cases where the SiO 2 film was used (70%) and where the SiO 2 film was used (70%), light of a predetermined wavelength was irradiated onto the crystal surface during growth, and the reflectance was measured.

[0084] For example, when growing p-type GaN on AlGaN, the reflectance R oscillates as the p-type GaN grows (increases in layer thickness) due to interference of reflected light from two surfaces: the AlGaN / GaN interface and the p-type GaN surface. If the reflectance R does not oscillate, it indicates that no crystal has grown, and if the reflectance R is low, it indicates that the crystal surface is rough and uneven. Furthermore, if the maximum value Rmax of the reflectance R is high, it indicates that the crystal growth is progressing while the surface remains flat.

[0085] FIG. 6 is a graph plotting the enhancement factor MF (=MR1 / MR2), which is the Mg enhancement ratio of the Mg / Ga ratio (MR1) during growth of the first p-type GaN layer 16A to the Mg / Ga ratio (MR2) during growth of the second p-type GaN layer 16B, on the horizontal axis and the maximum value Rmax of the reflectance R measured in situ during growth on the vertical axis.

[0086] As shown in FIG. 6, as the enhancement factor MF increases, the maximum value Rmax of the reflectance R increases, and the flatness of the first p-type GaN layer 16A improves.

[0087] As described above, in the initial stage of p-GaN growth, the flow rate of the Mg source material is increased to increase the surface density of nuclei originating from Mg on the AlGaN surface, and the N in the carrier gas is ２ This is because by increasing the temperature to prevent re-evaporation of GaN, GaN can be grown evenly over the entire AlGaN surface.

[0088] In the case of the above-described Example EX3 (FIG. 3), the Mg flow rate for the first p-type GaN layer 16A was 600 sccm, the TMG flow rate was 9.0 sccm, and the Mg flow rate for the second p-type GaN layer 16B was 200 sccm, and the TMG flow rate was 31.0 sccm. Therefore, the Mg / Ga ratios of the first p-type GaN layer 16A and the second p-type GaN layer 16B were MR1=66.7 and MR2=6.5, respectively, and therefore the MF of the first p-type GaN layer 16A and the second p-type GaN layer 16B in EX3 was MR1 / MR2=10.3.

[0089] In Example EX1, the Mg flow rate for the first p-type GaN layer 16A was 300 sccm, the TMG flow rate was 9.0 sccm, and the Mg flow rate for the second p-type GaN layer 16B was 200 sccm, and the TMG flow rate was 31.0 sccm. Therefore, the Mg / Ga ratios of the first p-type GaN layer 16A and the second p-type GaN layer 16B were MR1=33.3 and MR2=6.5, respectively, and therefore the MF of the first p-type GaN layer 16A and the second p-type GaN layer 16B in EX3 was MR1 / MR2=5.17.

[0090] 3, in the case of comparative example CX1, the Mg flow rate for the first p-type GaN layer 16A was 200 sccm and the TMG flow rate was 9.0 sccm, so MF = MR1 / MR2 = 3.44. In the case of comparative example CX1, hillocks were observed and the anchorless number was large. In addition, from the graph of the maximum value Rmax of reflectance R shown in FIG. 6, it is found that MF ≥ 5 is an appropriate condition for achieving flat growth of the first p-type GaN layer 16A.

[0091] Therefore, it was found that in the growth of the first p-type GaN layer 16A, it is preferable to set the Mg / Ga raw material ratio to be 5 times or more (enhancement factor MF≧5) the Mg / Ga raw material ratio of the second p-type GaN layer 16B (growth condition (ii)).

[0092] FIG. 7 shows the N ２ The ratio (N ２ Ratio=N ２ / (N ２ +H ２1 is a graph plotting the maximum value Rmax of the reflectance R measured in-situ when the temperature (N) is changed. ２ The maximum reflectance Rmax changes with the ratio, and N ２ The ratio is about 70% and is the maximum. ２ In this case, as shown in FIG. 5, the reflectance R is very small and p-type GaN hardly grows.

[0093] Therefore, from this data, it can be seen that in the growth of the first p-type GaN layer 16A, the N ２ It is appropriate to set the ratio at 50% or more, and N ２ It was found that a ratio of 60 to 80% was more preferable (growth condition (iii)).

[0094] 2-2 Second p-type GaN layer FIG. 8A shows the present embodiment (EMB), that is, the case where the carrier gas is hydrogen (H ２ 10 shows the growth mechanism (left) of the second p-type GaN layer 16B under growth condition (iv) when the growth rate is 100% (growth condition (iv)), and a cross-sectional TEM (Transmission Electron Microscope) image (right) of the second p-type GaN layer 16B after growth.

[0095] FIG. 8B shows a comparative example (CMP), that is, a case in which hydrogen (H ２ ) and nitrogen (N ２ ) in the carrier gas ２ The growth mechanism of the p-type GaN layer when the ratio of N in the carrier gas is 50% or more (left) and ２ 10 is a diagram showing a cross-sectional TEM image (right side) of a p-type GaN layer after growth when the ratio of

[0096] First, as shown in FIG. 8B, the carrier gas is H ２ / N ２ Conventional technology using mixed gas (N ２ In the case of CMP (a non-volatile atmosphere), vapor phase etching is not performed, so the adhesion rate (sticking coefficient) of GaN is high and GaN grows randomly even on surfaces with high surface energy.

[0097] Therefore, GaN grows flat on the substrate, but there are many crystal defects, resulting in a high threading dislocation density. In fact, as shown in the cross-sectional TEM image (right), a high density of threading dislocations was observed. The threading dislocation density was 5.4E+9 (cm －２ ) was calculated.

[0098] On the other hand, as shown in FIG. 8A, the carrier gas is H ２ In the case of this embodiment (EMB) where H ２ In this atmosphere, there is a high probability that the deposited GaN will be re-evaporated by vapor-phase etching, that is, the deposition rate (sticking coefficient) of GaN is low.

[0099] Therefore, the attached atoms diffuse through the surface and attach from areas with low surface energy, resulting in facets that grow slowly relative to the substrate, but improving crystallinity. In fact, as shown in the cross-sectional TEM image (right), it was found that the threading dislocation density was significantly improved. Furthermore, the threading dislocation density was 6.2E+8 (cm －２ ) That is, the threading dislocation density was calculated to be 1.0E+9 cm －２ It was found that a p-type contact layer 16 of less than 100 nm could be obtained. From the cross-sectional TEM image, it was found that the thickness of the p-type contact layer 16 was 270 nm, and that according to the present invention, a very thick p-type GaN contact layer with a low threading dislocation density could be realized.

[0100] Therefore, it was found that a p-type contact layer that is unlikely to cause alloy spikes even during electrode formation and that can suppress device degradation can be provided. The thickness of the p-type contact layer 16 is preferably 200 nm or more.

[0101] As explained above, the growth method differs depending on the carrier gas. ２ By growing the film in an atmosphere, the adhesion coefficient is lowered and the film grows from areas with low surface energy, which is thought to have improved the crystallinity. The improved crystallinity is thought to have resulted in low-resistance conductivity without activation (activation annealing).

[0102] 2-3 Third p-type GaN layer FIG. 9A shows the present embodiment (EMB), that is, the third p-type GaN layer formed using H as a carrier gas. ２ / N ２ Mixed gas (H ２ / N ２ FIG. 9B shows an AFM (Atomic Force Microscope) image of the surface of the third p-type GaN layer 16C when the same carrier gas conditions (H = 30% / 70%) were used (growth condition (v)). Also, FIG. 9B shows an AFM (Atomic Force Microscope) image of the surface of the third p-type GaN layer 16C when the same carrier gas conditions (H = 30% / 70%) were used (growth condition (v)). ２ 100%).

[0103] 9A and 9B are AFM images of the as-grown crystal surface that has not been subjected to activation (activation annealing).

[0104] First, as shown in FIG. 9B, H ２ (H ２ When the third p-type GaN layer was grown using SiO 2 (SiO 2 = 100%), the surface roughness (RMS) was 14.3 nm.

[0105] On the other hand, as shown in FIG. 9A, H ２ / N ２ Mixed gas (H ２ / N ２ When the third p-type GaN layer 16C was grown using a SiO 2 / SiO 3 ratio of SiO 2 =30% / 70%), the surface roughness (RMS) was 6.7 nm, and it was found that the surface flatness was improved.

[0106] In addition, H ２ / N ２ When the mixed gas was used as the carrier gas (FIG. 9A), the p-barrier Vb was 0.0091 (V), and the contact resistance was 1.25E-2 (Ωcm ２ ) and H as a carrier gas. ２ Only (H ２ = 100%) (FIG. 9B), the p-barrier Vb was 0.0624 (V), and the contact resistance was 1.29E-3 (Ωcm ２ ) was.

[0107] H ２ / N２ When the third p-type GaN layer 16C was grown using the mixed gas as a carrier gas, the p-barrier (or Schottky barrier component) was lowered, improving the contact properties.

[0108] As explained in the growth conditions (growth condition (iii)) for the first p-type GaN layer 16A, the growth of the third p-type GaN layer 16C also requires N in the carrier gas. ２ The ratio is preferably 50% or more, and N ２ It is more preferable that the ratio is 60 to 80%.

[0109] [Contact Characteristics of Ultraviolet LEDs] Fig. 10 shows a comparison of the growth conditions of the p-type contact layer 16 (EMB) of the example of the present invention and the p-type contact layer (CMP) of the comparative example, and also shows a comparison of the characteristics of the p-type contact layer 16 (EMB) of the example and the p-type contact layer (CMP) of the comparative example.

[0110] 10, the p-type contact layer 16 (EMB) of the embodiment was formed under the same growth conditions as the first to third p-type GaN layers 16A to 16C. Specifically, the second p-type GaN layer 16B was grown under the conditions of H ２ The growth factor MF (=MR1 / MR2) of the Mg / Ga ratio was 10.3.

[0111] The thicknesses of the first to third p-type GaN layers 16A to 16C were 5 to 10 nm, 240 to 260 nm, and 5 to 10 nm, respectively, for a total thickness of 270 nm. Note that, because it is difficult to distinguish the boundaries between these layers even by analysis such as TEM, the thicknesses of the first and second p-type GaN layers 16A and 16B are estimated values.

[0112] On the other hand, in the comparative example, the p-type contact layer (CMP) is formed by using H as a carrier gas in the second p-type GaN layer. ２ / N ２ Mixed gas (H ２ : 30%, N ２ The p-type contact layer 16 (EMB) of the example differs from that of the example in that the Mg / Ga ratio was 70%. The enhancement factor MF (=MR1 / MR2) of the Mg / Ga ratio was 4.5.

[0113] Furthermore, the p-type contact layer (CMP) of the comparative example was subjected to activation at 700° C. for 30 minutes in a nitrogen atmosphere, but the p-type contact layer 16 (EMB) of the present invention was not subjected to activation.

[0114] 11 compares the contact characteristics and crystallinity of the p-type contact layer 16 (EMB) of the example and the p-type contact layer (CMP) of the comparative example. In the comparative example (CMP), a TLM-patterned electrode was formed after activation of the p-type contact layer, and measurements were performed. On the other hand, in the example (EMB), a TLM-patterned electrode was formed on the as-grown p-type contact layer 16, and measurements were performed.

[0115] It was confirmed that the forward voltage Vf of the p-type contact layer 16 (EMB) of the example was 7.1 V, which was improved over the forward voltage Vf of 8.2 V of the p-type contact layer (CMP) of the comparative example.

[0116] Furthermore, the p-type contact layer 16 (EMB) of the example has a small p-barrier Vb and high ohmic characteristics. It can also be seen that the contact resistance and resistivity are significantly improved. In other words, good contact characteristics and conductivity were obtained without activation (activation annealing).

[0117] 1, an ultraviolet LED 10 according to this embodiment is formed by sequentially growing an n-type AlGaN layer 12, an active layer 13, a p-type AlGaN layer 15, and a p-type contact layer 16 on an AlN substrate 11. As described above, an AlGaN layer 14 (including an AlN layer) serving as an electron blocking layer may be provided between the active layer 13 and the p-type AlGaN layer 15.

[0118] The n-type AlGaN layer 12 may be composed of multiple layers instead of a single layer, and in that case, all layers do not need to be n-layers (n-doped layers) and may include an undoped layer (i-layer). The same applies to the p-type AlGaN layer 15.

[0119] The p-electrode 21 is formed on the as-grown p-type contact layer 16 that has not been subjected to activation (activation annealing).

[0120] More specifically, the p-electrode 21 is formed by forming a metal layer, for example, a stack of Ni and Au layers, on the p-type contact layer 16, and then performing heat treatment at a temperature of 500°C, for example, to make ohmic contact with the p-type contact layer 16.

[0121] Although the p-electrode 21 is exemplified as a metal layer in which a Ni layer and an Au layer are stacked, the present invention is not limited to this.

[0122] In this specification, the "as-grown" p-type contact layer 16 includes a case where a process has been performed that does not affect the crystallinity (e.g., dislocation density) of the p-type contact layer 16. Examples of such a process include cleaning a wafer on which the semiconductor layers of the UV-LED 10 are grown, and etching a portion of the surface layer.

[0123] Therefore, it is possible to provide an ultraviolet semiconductor light-emitting element that does not require activation, has high efficiency and high output, and has a long element life with little deterioration.

[0124] (Co-doping impurities) In the above embodiment, the p-type Al Ｙ２ Ga １－Ｙ２ Although the case where Mg is used as the p-type impurity for the N layer 15 and Si is used as the co-doped impurity has been described, the present invention is not limited to this.

[0125] p-type Al Ｙ２ Ga １－Ｙ２ By co-doping the N layer 15, an ultraviolet semiconductor light-emitting device with high luminous efficiency and good output maintenance rate (device life) is realized. The mechanism behind this is as follows: Ｙ２ Ga １－Ｙ２ It is believed that the effect of reducing nitrogen defects is achieved by the interaction between Mg, which is an acceptor impurity doped into the N layer 15, and Si, which is a donor impurity. Ｙ２ Ga １－Ｙ２Any material that functions as an acceptor and a donor for the N layer 15 can be used without any restrictions.

[0126] Specifically, in addition to Mg, Zn (zinc), Be (beryllium), C (carbon), etc. can be used as p-type impurities that serve as acceptors. In addition to Si, Ge (germanium), Se (selenium), S (sulfur), O (oxygen), etc. can be used as n-type impurities (co-doped impurities) that serve as donors.

[0127] As described above in detail, according to the present invention, it is possible to provide an ultraviolet semiconductor light-emitting element and a method for manufacturing the same, which can thicken the p-type GaN contact layer, does not require activation, has high efficiency and high output, and has a long device life with little degradation.

[0128] 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, 16: p-type contact layer, 16A: first p-type GaN layer, 16B: second p-type GaN layer, third p-type GaN layer, 21: p-electrode

Claims

1. A method for manufacturing an ultraviolet semiconductor light-emitting device by sequentially growing an n-type AlGaN layer, an active layer, a p-type AlGaN layer, and a p-type GaN contact layer on an AlN substrate by MOCVD, the p-type GaN contact layer having a first p-type GaN layer grown on the p-type AlGaN layer and a second p-type GaN layer grown on the first p-type GaN layer, the first p-type GaN layer and the second p-type GaN layer being grown using magnesium (Mg) as a p-dopant, and the growth of the first p-type GaN layer being performed using hydrogen (H ２ ) and nitrogen (N ２ ) as a carrier gas, and N in the carrier gas ２ The ratio is 50% or more, and the second p-type GaN layer is grown using hydrogen (H ２ ) 100%, and the first p-type GaN layer is grown under conditions that result in a Mg source / Ga source ratio that is 5 times or more the Mg source / Ga source ratio used in the growth of the second p-type GaN layer.

2. The method for producing an ultraviolet semiconductor light-emitting element according to claim 1, wherein the Mg concentration of the first p-type GaN layer is 2.0E+19 or more.

3. Growing a third p-type GaN layer on the second p-type GaN layer, the growth of the third p-type GaN layer being carried out under hydrogen (H ２ ) and nitrogen (N ２ ) as a carrier gas, and N in the carrier gas ２ The method for producing an ultraviolet semiconductor light-emitting element according to claim 1 or 2, wherein the ratio is 50% or more.

4. The method for manufacturing an ultraviolet semiconductor light emitting device according to claim 1, wherein the p-type GaN contact layer is grown to a thickness of 200 nm or more.

5. The method for producing an ultraviolet semiconductor light emitting device according to claim 3, further comprising the step of forming a p-electrode on the as-grown third p-type GaN layer.

6. An ultraviolet semiconductor light emitting device in which an n-type AlGaN layer, an active layer, a p-type AlGaN layer, and a p-type GaN contact layer are sequentially grown on an AlN substrate, the p-type GaN contact layer having a layer thickness of 200 nm or more, and a p-electrode formed on the as-grown p-type GaN contact layer.

7. The threading dislocation density of the p-type GaN contact layer is 1.0E+9 cm －２ The ultraviolet semiconductor light-emitting device according to claim 6 , wherein the optical density of the ultraviolet semiconductor light-emitting device is less than 100 nm.

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