Ultraviolet semiconductor light emitting element

Abstract

To provide a violet semiconductor light emitting element capable of enhancing external quantum efficiency and having high efficiency and high power.SOLUTION: A violet semiconductor light emitting element includes: a substrate consisting of a single crystal AlN; and a semiconductor structure layer having an n type AlGaN layer, an active layer, an Mg doped p type AlY1Ga1-Y1N layer (0.5≤Y1≤1.0) and an Mg doped p type AlY2Ga1-Y2N layer (0.5≤Y2≤1.0 and Y2≤Y1) epitaxially grown in this order on the substrate. The emission peak wavelength of the active layer is in a range of 210-300 nm; a main peak and a sub peak derived from Mg are exhibited in an emission spectrum by current application to the active layer; and the intensity of the sub peak to that of the main peak in a drive current density of 20 mA / mm2 is 3-15%.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an ultraviolet semiconductor light-emitting device, and more particularly to a nitride semiconductor light-emitting device that emits ultraviolet light. [Background technology]

[0002] In recent years, semiconductor light-emitting elements that emit light in the deep ultraviolet region have been attracting attention as light sources that have the effect of inactivating and sterilizing bacteria and viruses.

[0003] For example, Patent Document 1 discloses a nitride semiconductor light-emitting device in which the external quantum efficiency is improved by controlling the dopant concentration at the interface between the light-emitting layer and another semiconductor layer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2016-098632 Summary of the Invention [Problem to be solved by the invention]

[0005] In conventional ultraviolet semiconductor light-emitting devices, the stacking structure, composition, impurity concentration, layer thickness, and other aspects of each semiconductor layer have been studied, but it has been difficult to realize a device with sufficiently high efficiency and high output.

[0006] In particular, as described in Patent Document 1, when the main peak emission intensity decreases as the sub-peak emission intensity increases in the emission wavelength spectrum, it has been suggested that the emission efficiency decreases due to an increase in non-radiative transitions.

[0007] The present inventors have found that it is difficult to sufficiently increase the luminous efficiency by simply reducing the sub-peak luminous intensity in the emission wavelength spectrum. The present invention was made based on this finding, and aims to provide an ultraviolet semiconductor light-emitting element that can increase the external quantum efficiency and has high luminous efficiency and high output. [Means for solving the problem]

[0008] An ultraviolet semiconductor light emitting device according to one embodiment of the present invention comprises: a substrate made of single-crystal AlN; On the substrate, an n-type AlGaN layer, an active layer, an Mg (magnesium) doped p-type Al Y1 Ga 1-Y1 N layer (0.5≦Y1≦1.0) and Mg-doped p-type Al Y2 Ga 1-Y2 a semiconductor structure layer in which N layers (0.5≦Y2≦1.0, Y2≦Y1) are epitaxially grown in this order; the active layer has an emission peak wavelength in the range of 210 to 300 nm; When a current was applied to the active layer, a main peak and a sub-peak derived from Mg appeared in the emission spectrum, and the driving current density was 20 mA / mm 2 The peak intensity (S / M) of the sub-peak intensity (S) relative to the main peak intensity (M) is 3 to 15%.

[0009] Furthermore, an ultraviolet semiconductor light emitting device according to another embodiment of the present invention comprises: a substrate made of single-crystal AlN; On the substrate, an n-type AlGaN layer, an active layer, an Mg (magnesium) doped p-type Al Y1 Ga 1-Y1 N layer (0.5≦Y1≦1.0) and Mg-doped p-type Al Y2 Ga 1-Y2 a semiconductor structure layer in which N layers (0.5≦Y2≦1.0, Y2≦Y1) are epitaxially grown in this order; the active layer has an emission peak wavelength in the range of 210 to 300 nm; The p-type Al Y1 Ga1-Y1 The Mg concentration in the N layer is 3.0×10 19 cm -3 ~5.0×10 19 cm -3 The p-type Al Y2 Ga 1-Y2 The Mg concentration in the N layer is 2.0×10 19 cm -3 ~1.0×10 20 cm -3 is within the range of The p-type Al Y1 Ga 1-Y1 The thickness of the N layer is in the range of 4 to 10 nm. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view schematically showing the structure of an ultraviolet semiconductor light-emitting element 10 according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram schematically showing a band diagram of an ultraviolet LED 10. [Figure 3A] 10 is a table summarizing the characteristics of ultraviolet LEDs obtained from wafers having different Mg concentrations in the spacer layer 13S and the p-type AlGaN layer 15. [Figure 3B] 1 is a graph plotting the luminous efficiency (EQE: external quantum efficiency) of the ultraviolet LED obtained from each wafer and the concentration of the spacer layer 13S. [Figure 4] FIG. 1 is a diagram showing an example of the emission spectrum of an ultraviolet LED element 10 according to one embodiment (Example: EMB) and an ultraviolet LED of a comparative example (CMP). [Figure 5] 1 is a graph plotting the luminous efficiency (EQE: external quantum efficiency) and S / M ratio (%) of ultraviolet LEDs 10 obtained from the wafer (EX1) of Example 1 and the wafer (EX2) of Example 2. [Figure 6] FIG. 1 is a diagram showing an example of SIMS profiles of the LED elements of an example (EMB) and a comparative example (CMP). [Figure 7] FIG. 1 is a conceptual diagram for explaining the relationship between the S / M ratio and the luminous efficiency or the amount of injected carriers in Examples (EMB1, EMB2) and Comparative Example (CMP). DETAILED DESCRIPTION OF THE INVENTION

[0011] 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. [Structure of ultraviolet semiconductor light-emitting element] 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 also referred to as LED element 10), and is manufactured by, for example, metal-organic chemical vapor deposition (MOCVD).

[0012] The LED element 10 is formed by epitaxially growing an n-type AlGaN layer 12, an active layer 13, a p-type AlGaN layer 14, a p-type AlGaN layer 15, and a p-type GaN layer 16 on a substrate 11 in this order.

[0013] 2 is a schematic diagram showing a band diagram of the LED element 10. A more detailed description will be given with reference to FIGS.

[0014] The substrate 11 has a dislocation density of 10 8 cm -2 The AlGaN-based semiconductor material constituting the ultraviolet light-emitting device of the present invention has a dislocation density of 10 to 100 nm, as described in OPTICS EXPRESS Vol. 25 No. 16 A639 (2017). 7 ~10 8 cm -2 It is known that the luminous efficiency drops sharply when the dislocation density exceeds 10. Therefore, the lower the dislocation density of the single crystal AlN substrate, the better. 6 cm -2 More preferably, 10 4 cm -2By using the AlN substrate 11 with such a low dislocation density, the dislocation density in the active layer 13 (described later) can be reduced to 10% without reducing the light emission efficiency. 7 cm -2 It can be the following:

[0015] The growth plane (surface) of the AlN substrate 11 of the present invention is not particularly limited and can be a C-plane, M-plane, or other growth plane. However, the C-plane is preferred, as it is commonly used as a growth plane for AlGaN-based materials. Furthermore, when the C-plane is used as the crystal growth plane, the AlN substrate 11 is preferably an off-axis substrate that is slightly inclined from the C-plane, for purposes such as improving the smoothness of the AlGaN layer grown on the substrate. The inclination angle from the C-plane is not particularly limited and may be determined as appropriate to obtain a smooth AlGaN layer, but is typically selected in the range of 0.1 to 1.0°. The inclination direction from the C-plane is also not particularly limited and may be selected as appropriate, such as the A-axis or M-axis. However, the M-axis direction is preferred, as it increases the linearity of the step edges.

[0016] Furthermore, since a large surface roughness of the AlN substrate 11 can cause abnormal growth of the AlGaN layer on the substrate, the surface roughness (RSM) is preferably 1.0 nm or less, and more preferably 0.5 nm or less. In order to obtain such a smooth surface or to remove damaged layers formed on the substrate surface during the substrate manufacturing process, the substrate surface is preferably subjected to chemical mechanical polishing (CMP).

[0017] 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 less than 10 cm, and more preferably -1 Less than 10cm -1 By setting it as follows, 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.

[0018] The n-type AlGaN layer 12 is an n-type conductive layer doped with Si (silicon). In an ultraviolet semiconductor light-emitting device, ultraviolet light emitted from the light-emitting layer is usually emitted to the outside after passing through the n-type AlGaN layer 12 and the substrate 11. 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. Therefore, the Al composition of the n-type AlGaN layer can be appropriately determined so as to obtain sufficient transmittance for the desired ultraviolet light emission wavelength. Furthermore, the n-type AlGaN layer may be formed from multiple layers with different Al compositions, and may further be a compositionally graded layer in which the Al composition is graded in the stacking direction. For example, the 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 is a laminated structure. 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 (growth direction), for example, and the second n-type Al X2 Ga 1-X2 The N layer 12B is a composition gradient layer in which the Al composition X2 decreases from 0.75 to 0.70. X1 Ga 1-X1 N layer 12A and second n-type Al X2 Ga 1-X2 It is preferable that the Al composition at the interface of the N layer 12B is uniform.

[0019] The thickness of the n-type AlGaN layer is not particularly limited and may be determined appropriately. However, if the thickness of the n-type AlGaN layer is too thick, lattice relaxation occurs between the AlN substrate 11 and the n-type AlGaN layer 12, and dislocations are likely to occur. Therefore, it is preferable that the total thickness of the n-type AlGaN layer 12 is set in the range of 0.5 to 2.0 μm. For example, when the n-type AlGaN layer 12 is made of the first n-type Al X1 Ga 1-X1 N layer 12A and second n-type Al X2 Ga 1-X2 In the case of a laminated structure consisting of the N layer 12B, the first n-type Al X1Ga 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 may have a layered structure having a thickness of 1000 nm. Naturally, the thicknesses of the first and second n-type AlGaN layers are not limited to the values shown in the example, and may be appropriately determined so that the total thickness is 2.0 μm or less.

[0020] The doping concentration of Si may be appropriately determined so as to obtain the desired n-type conductivity. However, from the viewpoint of reducing the resistance value of the n-type AlGaN layer, it is preferable to dope the Si layer with a concentration of 1×10 18 ~1×10 20 cm -3 It is preferable that the 18 ~5×10 19 cm -3 Preferably, the Si doping concentration is constant in the thickness direction of the n-type AlGaN layer, or it can be modulated doping in which the Si concentration varies in the thickness direction. 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 in this application are measured by AlN, AlGaN, and GaN layers, respectively. 0.65 Ga 0.35 Quantitative values using standard samples of N and GaN are used.

[0021] The active layer (ACT) 13 is Al A1 Ga 1-A1 The barrier layer 13B is made of an N layer and Al A2 Ga 1-A2 The active layer 13 has a quantum well structure formed of well layers 13W each made of an N layer. The active layer 13 also has a spacer layer 13S, which will be described later. The emission peak wavelength of the active layer 13 is in the range of 210 to 300 nm. The wavelength of light emitted from the active layer 13 is determined by the Al composition and film thickness of the well layer, so the Al composition and film thickness can be appropriately determined so as to obtain a desired emission wavelength within the above wavelength range.

[0022] For example, the film thickness of the well layer can be set within the range of 2 to 10 nm, and the Al composition can be determined so as to obtain a desired emission wavelength. Also, regarding the Al composition and film thickness of the barrier layer, although not particularly limited, for example, the Al composition can be set within the range of A2 < A1 ≤ 1.0, and the film thickness can be set within the range of 2 to 15 nm. Further, the well layer and the barrier layer can also be n-type layers doped with Si. Both the well layer and the barrier layer may be Si-doping layers, or the structure may be such that only the well layer or only the barrier layer is doped with Si. The Si concentration to be doped is not particularly limited, but is preferably in the range of 1×10 17 ~5×10 18 cm -3 . Also, the number of quantum wells is not particularly limited, and it may be a multiple quantum well (MQW: Multi Quantum Well) structure in which a plurality of well layers are formed, or it may be a single quantum well (SQW: Single Quantum Well). The number of well layers is preferably appropriately determined within the range of 1 to 5.

[0023] As shown in FIG. 2, in the active layer 13, when explaining the portion from the end of the n-type AlGaN layer 12 to the end of the uppermost quantum well layer 13W of the quantum well layer 13W (that is, the quantum well layer 13W closest to the p-type Al Y1 Ga 1-Y1 N layer 14 to be described later) as the quantum well structure layer 13Q, the spacer layer 13S is provided between the uppermost quantum well layer 13W and the p-type Al Y1 Ga 1-Y1 N layer 14.

[0024] The Al composition of the spacer layer 13S can be optimally selected as follows according to the emission wavelength or in the process of improving the emission efficiency. It may be the same composition as the barrier layer 13B of the quantum well structure, or the barrier layer 13B and the p-type Al Y1 Ga 1-Y1The spacer layer 13S may have an intermediate composition between the N layer 14. Alternatively, the spacer layer 13S may be composed of a layer having the same composition as the barrier layer 13B and a layer having the intermediate composition formed thereon. Furthermore, the spacer layer 13S may be a graded composition layer or a stepped composition layer whose composition changes stepwise.

[0025] To specifically illustrate an embodiment of the present invention, Si-doped Al 0.6 Ga 0.4 The N layer is a barrier layer 13B and the undoped Al 0.5 Ga 0.5 In a three-layer multi-quantum well (MQW) structure in which an N layer serves as a quantum well layer (well layer) 13W, the spacer layer 13S has the same composition as the barrier layer 13B. However, in order to increase the efficiency of carrier injection into the active layer 13W, the Al composition (Y) of the spacer layer 13S is preferably Y=0.55 to 0.65, and more preferably Y=0.57 to 0.62.

[0026] Although the thickness of the spacer layer 13S is not particularly limited, a larger thickness increases the physical distance between the p-type layer and the well layer 13W, which tends to reduce the efficiency of carrier injection into the active layer 13W. Therefore, the thickness of the spacer layer 13S is preferably in the range of 2 to 15 nm, and more preferably in the range of 4 to 10 nm.

[0027] The spacer layer 13S may have a structure doped with Si, similar to the barrier layer described above. The doping concentration of Si is not particularly limited, but is preferably 1×10 17 ~5×10 18 cm -3 The range is preferred.

[0028] The spacer layer 13S also contains Mg. The Mg contained in the spacer layer 13S is a p-type Al Y1 Ga 1-Y1This may be due to diffusion doping from the N layer 14, or may be intentional doping with Mg. The inventors have found that the light emission efficiency tends to improve as the Mg concentration in the spacer layer 13 increases. This effect is thought to be due to the fact that the spacer layer 13S contains Mg, a p-type dopant, which effectively shortens the distance between the p-type layer and the active layer, thereby improving the carrier injection efficiency. Specifically, the Mg concentration in the spacer layer 13S is 5.0×10 17 ~5.0×10 18 cm -3 is preferably 7.0 × 10 17 ~3.0×10 18 cm -3 , most preferably 1.0 × 10 18 ~2.5×10 18 cm -3 is.

[0029] In the SIMS analysis performed to determine the Mg concentration described above, the sample is irradiated with primary ions (oxygen), and a quantitative evaluation is performed by determining the amount of secondary ions (Mg) that are ejected from the sample. Generally, in SIMS analysis, the impurity concentration tends to have a tail in the depth direction due to factors such as the primary ions pushing impurities into the sample and the surface roughness of the sample making it difficult to irradiate the primary ions uniformly. In addition, since the etching rate differs depending on the material of the sample (in the example of this application, the Al composition of the AlGaN layer), it is difficult to obtain accuracy in the depth direction on the order of several nanometers. Therefore, the Mg concentration of the spacer layer described above is determined by measuring the amount of p-type Al Y1 Ga 1-Y1 The Mg concentration was defined as the Mg concentration at a position (evaluation position) 20 nm from the peak position of the Mg concentration in the N layer 14 toward the active layer 13 side.

[0030] p-type Al on 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) to suppress overflow to the N layer 15. Therefore, Al Y1 Ga 1-Y1 The N layer 14 has a larger bandgap than the active layer 13 and the p-type Al Y2 Ga 1-Y2 N layer 15, and the Al composition Y1 of the Al Y1 Ga 1-Y1 N layer 14 is determined within the range of 0.8 < Y1 ≤ 1.0. As the emission wavelength shortens, the Al composition of the AlGaN layer epitaxially grown on the substrate 11 increases. When the emission wavelength is shorter than 270 nm, in order to fully exhibit the function as an electron blocking layer, it is preferable that the Al composition Y1 is 0.9 ≤ Y1 ≤ 1.0. In this embodiment, Al Y1 Ga 1-Y1 N layer 14 uses AlN (Y1 = 1).

[0031] Also, as long as the Al Y1 Ga 1-Y1 N layer 14 can exhibit the function as an electron blocking layer, it may be an undoped layer or may be doped with a p-type dopant. As the p-type dopant material in the Al Y1 Ga 1-Y1 N layer 14, Mg (magnesium), Zn (zinc), Be (beryllium), C (carbon), etc. can be used. In particular, it is preferable to use Mg, which is generally used as the p-type dopant material for the AlGaN layer, and Mg is also used in the embodiments of the present invention described later. The p-type dopant material may be uniformly doped in the stacking direction of the Al Y1 Ga 1-Y1 N layer 14, or the concentration of the dopant material can be changed in the stacking direction. For example, it can also be a stacked structure composed of an undoped AlN layer 14A (Y1 = 1) and a p-type AlN layer 14B doped with Mg (magnesium) from the side in contact with the active layer. Al Y1 Ga 1-Y1The concentration of the p-type dopant in the N layer 14 is set to 1.0×10 19 ~8.0×10 19 cm -3 is preferably 3.0 × 10 19 ~5.0×10 19 cm -3 , particularly preferably 3.0 × 10 19 ~4.0×10 19 cm -3 is.

[0032] In addition, p-type Al Y1 Ga 1-Y1 The N layer 14 is a p-type Al Y1 Ga 1-Y1 The N layer 14 preferably has a thickness in the range of 4 to 10 nm, because if it is less than 4 nm, the effect as an electron blocking layer is small due to the tunneling effect, and if it is 10 nm or more, the hole injection efficiency decreases.

[0033] p-type Al Y2 Ga 1-Y2 The N layer 15 is a p-type Al Y1 Ga 1-Y1 It is formed on the N layer 14 and functions as a p-type cladding layer doped with Mg. The above-mentioned materials can be used as the p-type dopant material without any restrictions, but Al Y1 Ga 1-Y1 It is preferable to use Mg as in the N layer 14. In the ultraviolet light emitting device of the present invention, p-type Al Y2 Ga 1-Y2 The Mg concentration in the N layer 15 is 2.0×10 19 ~1.0×10 20 cm -3 is preferably 2.0 × 10 19 ~5.0×10 20 cm -3 p-type Al Y2 Ga 1-Y2 By setting the Mg concentration in the N layer 15 within the above range, the S / M ratio becomes 3 to 15%, and high luminous efficiency can be obtained.

[0034] 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 barrier 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 0.5 to 1.0. 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, which enhances the effect of suppressing carrier overflow to the p-type layer and can increase the luminous efficiency of the ultraviolet light emitting device.

[0035] 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. 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 0.95 to 1.0, and the p-type Al Y2 Ga 1-Y2The Al composition in the surface layer of the N layer 15 is preferably 0.60 to 0.85. By adopting such a structure, the above-mentioned polarization doping effect can be enhanced and transparency to the emission wavelength can be maintained, making it easier to obtain high luminous efficiency.

[0036] 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-Y2 If 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. Y1 Ga 1-Y1 The Al composition of the N layer 14 is reduced in the growth direction (Al composition Y2 is reduced from 1.0 to 0.8). Y2 Ga 1-Y2 The thickness of the N layer 15 is 60 nm.

[0037] p-type Al Y2 Ga 1-Y2 On the N layer 15, a p-type GaN layer 16 doped with a p-type dopant may be formed 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. The Mg doping concentration in the p-type GaN layer 16 is not particularly limited, but in order to reduce the resistance value in the p-type GaN layer and the contact resistance, it is preferable to use a concentration of Mg of 1×10 18 ~2×10 20 cm -3Furthermore, the thickness of the p-type GaN layer 16 is not particularly limited, and may be appropriately determined within the range of 5 to 500 nm.

[0038] All of the AlGaN layers 12, 13, 14, and 15, except for the p-type GaN layer 16, are grown in a state of lattice matching 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:

[0039] Although the LED element 10 is a light emitting diode (LED) in the following description, it may be configured as a semiconductor laser element (LD: Laser Diode). Next, a method for manufacturing an ultraviolet LED having the structure described above will be described. The ultraviolet LED 10 of the present invention can be manufactured by known crystal growth methods such as metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Among these, MOCVD is preferred because it has high productivity and is widely used industrially. The Group III (Al, Ga) source gases and Group V (N) source gases used in the present invention can be any known source gas without any particular restrictions.

[0040] For example, the Group III source gas may be trimethylaluminum, triethylaluminum, trimethylgallium, triethylgallium, etc. The Group V source gas is usually ammonia.

[0041] Furthermore, known materials can be used without any restrictions as dopant source gases for Mg and Si, and examples that can be used include biscyclopentadienyl magnesium, monosilane, and tetraethylsilane.

[0042] The above-mentioned source gases are supplied onto the substrate 11 together with a carrier gas such as hydrogen and / or nitrogen, thereby growing the element layers of the ultraviolet LED 10.

[0043] 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.

[0044] The growth temperature of the element layers constituting the ultraviolet 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 ultraviolet LED 10. However, growth at a temperature of 1000 to 1200°C is preferable, and 1000 to 1150°C is more preferable. [Example]

[0045] The present invention will be specifically explained below 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.

[0046] [Fabrication of LED elements] The substrate on which the UV LED element layers are grown is an AlN single crystal substrate fabricated by the method described in Applied Physics Express 5 (2012) 122101. Specifically, it is a laminated substrate comprising a C-plane AlN seed substrate fabricated by physical vapor transport (PVT) and an AlN thick film grown by hydride vapor phase epitaxy (HVPE). The dislocation density of this AlN substrate is 10 5 cm -2 5 x 5 μm or less 2 The surface roughness (RMS) in the range was 0.1 nm.

[0047] On this AlN substrate, an AlN layer (100 nm), a first n-type AlGaN layer (200 nm), and a second n-type AlGaN layer (1000 nm) were grown using an MOCVD apparatus. Both the first and second n-type AlGaN layers were 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, and the Al composition decreasing from 0.75 to 0.70 from the side in contact with the first n-type AlGaN layer in the second n-type AlGaN layer. The Si concentration in the n-type AlGaN layer was 1×10 19 cm -3It was controlled so that

[0048] Next, n-type Al 0.6 Ga 0.4 A barrier layer made of N (7 nm) and Al 0.5 Ga 0.5 The triple quantum well layer was grown with a Si concentration of 1×10 18 cm -3 It was controlled so that

[0049] Next, the electron blocking layer 14 made of AlN (10 nm) was grown. The electron blocking layer on the side in contact with the barrier layer was an undoped layer 14A (2 nm), and the remaining electron blocking layer 14B was grown with a 4±1×10 19 cm -3 It is doped with Mg.

[0050] Next, a p-type AlGaN layer 15 (60 nm) was grown. The p-type AlGaN layer 15 was a compositionally graded layer in which the Al composition decreased from 1.0 to 0.8 from the side in contact with the electron blocking layer. The Mg concentration of the p-type AlGaN layer 15 was 2×1.0×10 19 cm -3 ~1.0×10 20 cm -3 Several types of wafers were fabricated with the temperature varied within the range of

[0051] Next, a p-type GaN layer 16 (270 nm) was grown to complete the growth of the deep ultraviolet LED element layer. The Mg concentration in the p-type GaN layer was 5×10 19 cm -3 It was decided.

[0052] Next, the second n-type AlGaN layer was exposed by ICP dry etching, after which an n-type electrode made of Ti / Au was formed and heat-treated in a nitrogen atmosphere at 900°C. Next, a p-type electrode made of Ni / Au was formed on the p-type GaN layer and heat-treated in an oxygen atmosphere at 500°C.

[0053] Next, the backside of the AlN substrate (the side opposite to the UV LED element layer) was mechanically polished until the HVPE AlN thick protective film was exposed, completing the UV LED. Next, the substrate was cut into chips measuring 0.75 mm x 0.95 mm by dicing, and then flip-chip bonded onto a ceramic submount to complete the UV LED element, and its spectrum and characteristics were evaluated.

[0054] 3A is a table showing the Mg concentrations in the spacer layer 13S and p-type AlGaN layer 15 of the wafers fabricated and the device characteristics of the LEDs obtained from those wafers. Note that in the following, exponents, for example, 1.0×10 19 may be written as 1.0E19.

[0055] The upper and lower rows of each column in the figure show the device characteristics according to different wafer growth temperatures. Specifically, the upper row shows the device characteristics when the growth temperatures of the p-type AlN layer 14 and the p-type AlGaN layer 15 are 1115°C and 1090°C, respectively (growth temperature condition: GT1), while the lower row shows the device characteristics when the growth temperatures are 1095°C and 1070°C, respectively (growth temperature condition: GT2), which are slightly lower (by 20°C) than the growth temperature condition GT1.

[0056] In each column, the maximum EQE (external quantum efficiency) among the devices obtained from each wafer and the S / M ratio of the device from which the maximum EQE was obtained are shown in parentheses.

[0057] FIG. 3B is a graph in which the maximum EQE (external quantum efficiency) of the devices obtained from each wafer shown in the upper part of FIG. 3A is plotted against the concentration of the spacer layer 13S on the horizontal axis.

[0058] [Element evaluation (emission spectrum)] 4 is a graph showing an example of the emission spectrum of the ultraviolet LED element 10 according to one embodiment of the present invention (Example: EMB) and the ultraviolet LED of the comparative example (CMP). 2 1 shows the emission spectrum at

[0059] In the emission spectrum of the example (EMB), a main peak MAIN with an intensity of M and a sub-peak SUB with an intensity of S, which are emission peaks from the active layer 13, were observed. Hereinafter, for ease of explanation and understanding, the same symbols are used for the intensity and the peak, and they are also referred to as the main peak M or the sub-peak S.

[0060] Sub-peak S was observed approximately 30 nm longer than the main peak M. On the other hand, no clear sub-peak S was observed in the emission spectrum of the comparative example (CMP).

[0061] The sub-peak S observed in the emission spectrum of the example (EMB) is interpreted from its wavelength as emission via a p-type impurity level, i.e., emission derived from Mg. Furthermore, as the current density increases, emission from the active layer becomes dominant and the emission derived from Mg decreases, so the ratio of the emission intensity S of the sub-peak to the emission intensity M of the main peak (hereinafter referred to as the S / M ratio) decreases.

[0062] Therefore, from the viewpoint of detection sensitivity, the S / M ratio at low current densities was used as the evaluation standard. Specifically, when the driving current density J was 20 mA / mm 2 The S / M ratio (%) was used as the evaluation standard.

[0063] [Element evaluation (light emitting characteristics and S / M ratio)] FIG. 5 shows the Mg concentration of the spacer layer 13S at 2.0E18 cm -3 The Mg concentration of the p-type AlGaN layer 15 is 4.0E19 cm -3 The wafer (Example 1: EX1) and the spacer layer 13S have an Mg concentration of 1.0E18 cm -3 The Mg concentration of the p-type AlGaN layer 15 is 1.0E20 cm -3 1 is a graph plotting the luminous efficiency (EQE: external quantum efficiency) and S / M ratio (%) of the ultraviolet LED 10 obtained from the wafer (Example 2: EX2).

[0064] In both the wafers (EX1, EX2) of Example 1 and Example 2, the luminous efficiency increases as the S / M ratio increases.

[0065] In the case of Example 1 (EX1), the luminous efficiency (EQE) reaches a maximum value of 3.4% when the S / M ratio is 4.0%, and then the luminous efficiency gradually decreases as the S / M ratio increases.

[0066] In the case of Example 2 (EX2), the luminous efficiency (EQE) reaches a maximum value of 2.8% when the S / M ratio is 10.9%, and then decreases as the S / M ratio increases.

[0067] The results of Examples 1 and 2 (EX1, EX2) show that when the S / M ratio is 3 to 15%, higher luminous efficiency can be obtained than when no subpeak is observed. Also, the results of Example 1 (EX1) show that when the S / M ratio is 3 to 6%, extremely high luminous efficiency can be obtained.

[0068] Referring again to FIGS. 3A and 3B, the maximum EQE (Equation Efficiency) values for the LED elements 10 of the wafers with different Mg concentrations in the spacer layer 13S and the p-type AlGaN layer 15 are shown in Table 1. max (%) and S / M ratio (%) are shown.

[0069] From these results, the Mg concentration of the spacer layer 13S is 5.0E17 to 5.0E18 cm -3 The Mg concentration in the p-type AlGaN layer 15 is in the range of 2.0E19 cm -3 ~1.0E20cm -3 It can be seen that a high luminous efficiency (EQE) can be obtained within this range.

[0070] In particular, the Mg concentration of the spacer layer 13S is 7.0E17 to 3.0E18 cm -3 The Mg concentration in the p-type AlGaN layer 15 is in the range of 2.0E19 cm -3 ~5.0E19cm -3 It is preferable that the range is within the range of

[0071] [Relationship between S / M ratio and luminous characteristics] Regarding the emission intensity, for example, Patent Document 1 describes that the increase in non-radiative transitions in the active layer is due to an increase in the sub-peak intensity, and that the increase in the sub-peak intensity leads to a decrease in the main peak emission intensity, resulting in a decrease in the external quantum efficiency.

[0072] It has also been shown that the subpeak intensity decreases with a decrease in the Mg concentration in the spacer layer, which is the layer between the quantum well layer and the EBL layer (AlN layer), and the preferred range of the Mg concentration in the spacer layer is 1E17 cm -3 The following is stated:

[0073] The luminous efficiency (internal quantum efficiency) of an LED semiconductor layer is determined by the following formula: The above-mentioned EQE (external quantum efficiency) is expressed as the product of the internal quantum efficiency and the light extraction efficiency from the LED semiconductor layer.

[0074] Internal quantum efficiency = recombination probability in the active layer × carrier injection efficiency into the active layer As mentioned above, it was known that in conventional structures, the luminous efficiency decreases as the Mg concentration in the p-layer near the active layer increases. This was understood to be because the Mg impurity level becomes physically close to the active layer, and carriers are consumed in the transition process between Mg impurity levels, resulting in a decrease in the carriers injected into the active layer.

[0075] In order to improve the light-emitting efficiency of ultraviolet LEDs, the inventors of the present application have examined the structure and growth conditions (Mg supply flow rate, growth temperature) of the p-AlN layer 14 and the p-AlGaN layer 15, as well as the Mg concentration of the spacer layer 13S, which is determined based on the results of these growth conditions.

[0076] Specifically, as a result of examining the Mg concentration in the p-AlN layer 14 and the p-AlGaN layer 15, it was found that it is difficult to sufficiently increase the luminous efficiency by simply reducing the sub-peak intensity, and that the luminous efficiency tends to decrease as the sub-peak intensity decreases.

[0077] Then, by focusing on the structure of the p-AlN layer 14 and the p-AlGaN layer 15, particularly the thickness of the p-AlN layer 14, and the relationship between the Mg concentration in the p-type AlGaN layer 15 and the diffusion doping concentration in the spacer layer 13S, we have gained insight into improving the light-emitting efficiency of ultraviolet LEDs.

[0078] (Mg concentration in the spacer layer) FIG. 6 shows an example of the results of SIMS (Secondary Ion Mass Spectrometry) measurement of the LED element 10 of the embodiment (EMB) of the present invention and the LED element of the comparative example (CMP). Specifically, it shows the Mg concentration in the depth direction (horizontal axis) of the LED. The upper column of the SIMS profile also shows the position of each layer in the semiconductor layer. As mentioned above, the depth on the horizontal axis cannot accurately reflect the film thickness of each layer, so the Al 0.65 Ga 0.35 The etching depth of the N standard sample is shown.

[0079] The profile (solid line) of the LED element 10 of the example (EMB) shows that Mg is diffused into the spacer layer 13S. On the other hand, the profile (dashed line) of the LED element of the comparative example (CMP) shows that the Mg concentration in the spacer layer 13S drops sharply, indicating that there is little Mg diffusion.

[0080] More specifically, LED elements were fabricated in which the p-AlN layer 14 and the p-AlGaN layer 15 had the same thickness, but the growth temperature and Mg concentration were varied. The Mg concentration of each LED element was measured at a position (evaluation position) 20 nm from the peak position of the Mg concentration in the p-AlN layer 14 toward the active layer 13. The luminous efficiency of each LED element was also measured, and the correlation with the Mg concentration at the evaluation position was investigated.

[0081] As a result, it was found that there is a correlation between the Mg concentration in the spacer layer 13S and the luminous efficiency. Specifically, when the Mg concentration at the evaluation position in the spacer layer 13S is 5.0E17 to 5.0E18 cm -3 High luminous efficiency is obtained at 7.0E17~3.0E18cm -3On the other hand, a higher luminous efficiency was obtained when the Mg concentration was 8.0E16 cm -3 In the case of ZnO, the S / M ratio was less than 1%, and high luminous efficiency was not obtained. -3 In this case, the S / M ratio was 80% or more, and high luminous efficiency was not obtained.

[0082] That is, the Mg concentration diffused in the spacer layer 13 is at least 7.0E17 cm -3 High luminous efficiency is obtained with a value of at least 1.0E18cm -3 As a result, higher luminous efficiency was obtained.

[0083] It was also found that the luminous efficiency could be improved by making the thickness of the p-AlN layer 14 10 nm or less, increasing the Mg flow rate in the p-AlN layer 14, and raising the growth temperature. (Relationship between luminous efficiency and S / M ratio) As shown in the evaluation results of the Mg concentration and luminous efficiency in the spacer layer 13 of the above LED element and the evaluation results of the S / M ratio and luminous efficiency in Figure 5, the diffusion of Mg into the spacer layer 13 is considered to be equivalent to the appearance of a subpeak in the emission spectrum.

[0084] In other words, it was found that a sub-peak appears in the emission spectrum as a result of Mg diffusion into the spacer layer 13, and that high emission efficiency can be obtained when Mg diffusion is within a certain range, i.e., when a sub-peak of a certain intensity appears. This is thought to be because the carrier injection efficiency into the active layer 13 increases as the Mg concentration in the spacer layer 13 near the active layer 13 increases.

[0085] FIG. 7 is a conceptual diagram for explaining the relationship between the S / M ratio and the luminous efficiency or the amount of injected carriers in the examples (EMB1, EMB2) of the present invention and the comparative example (CMP) in which the Mg concentration in the spacer layer is low.

[0086] As described above, in examining the Mg concentration in the p-AlN layer 14 and the p-AlGaN layer 15, it was found that there is an optimal range of the S / M ratio for increasing the luminous efficiency, based on the relationship between the S / M ratio and the luminous efficiency.

[0087] From this, it was inferred that there is a trade-off between the amount of radiative transitions and the amount of carrier injection. First, because an increase in the subpeak intensity means carrier consumption (a decrease in the radiative transition of the main peak in the active layer), the amount of radiative transitions decreases as the S / M ratio increases (solid line). On the other hand, an increase in the S / M ratio (diffusion of Mg into the spacer layer 13) increases the amount of carrier injection (EMB1, EMB2: dashed-dotted lines, and CMP: dashed line).

[0088] Since the luminescence efficiency is the product of the radiative transition probability and the carrier injection efficiency, the S / M ratio has an optimum value (the intersection of the radiative transition probability and the carrier injection efficiency). Furthermore, in the case of Example EMB2, in which the Mg flow rate and growth temperature are increased compared to Example EMB1, the S / M ratio has a lower optimum value. On the other hand, in the case of the comparative example (CMP), the Mg concentration in the spacer layer is low and the amount of carrier injection is small, so the luminescence efficiency improves as the subpeak intensity decreases, but the luminescence efficiency is lower than in the examples (EMB1 and EMB2).

[0089] As described above, the relationship between the S / M ratio and the luminous efficiency was examined by examining the Mg concentrations in the spacer layer 13S, the p-AlN layer 14, and the p-AlGaN layer 15. It was found that the luminous efficiency was maximized when the S / M ratio was 3 to 15%. It was also found that an even higher luminous efficiency was obtained when the S / M ratio was 3 to 6%.

[0090] As explained above in detail, according to the present invention, it is possible to provide an ultraviolet semiconductor light emitting device that has high efficiency in injecting carriers into the active layer and is highly efficient and has high output. [Explanation of symbols]

[0091] 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, 13Q: quantum well structure layer, 13S: spacer layer, 14: p-type Al Y1Ga 1-Y1 N layer, 15: p-type Al Y2 Ga 1-Y2 N layer, 16: p-type GaN layer

Claims

1. a substrate made of single crystal AlN; On the substrate, an n-type AlGaN layer, an active layer, an Mg (magnesium) doped p-type Al Y1 Ga 1-Y1 N layer (0.5≦Y1≦1.0) and Mg-doped p-type Al Y2 Ga 1-Y2 a semiconductor structure layer in which an N layer (0.5≦Y2≦1.0, Y2≦Y1) is epitaxially grown in this order; the emission peak wavelength of the active layer is in the range of 210 to 300 nm; When a current is applied to the active layer, a main peak and a sub-peak derived from Mg appear in the emission spectrum, and the driving current density is 20 mA / mm 2 a peak intensity (S / M) of the sub-peak (S) relative to the main peak (M) is 6 to 15%; the active layer has a well layer and a barrier layer, The barrier layer further comprises p-type Al Y1 Ga 1-Y1 a spacer layer in contact with the N layer; The spacer layer is 5.0×10 17 ~5.0 x 10 18 cm -3 containing Mg, Ultraviolet semiconductor light-emitting element.

2. The p-type Al Y1 Ga 1-Y1 The Mg concentration in the N layer is 3.0 × 10 19 cm -3 ~5.0 x 10 19 cm -3 and the p-type Al Y2 Ga 1-Y2 The Mg concentration in the N layer is 2.0 × 10 19 cm -3 ~1.0 x 10 20 cm -3 The ultraviolet semiconductor light-emitting device according to claim 1 , wherein the wavelength of the ultraviolet semiconductor light-emitting device is in the range of

3. The p-type Al Y2 Ga 1-Y2 The Mg concentration in the N layer is 2.0 × 10 19 cm -3 ~5.0 x 10 19 cm -3 The ultraviolet semiconductor light-emitting device according to claim 1 , wherein the wavelength of the ultraviolet semiconductor light-emitting device is in the range of

4. The p-type Al Y1 Ga 1-Y1 4. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the N layer has a thickness in the range of 4 to 10 nm.

5. The p-type Al Y1 Ga 1-Y1 5. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the N layer is a p-type AlN layer.

6. The Mg concentration of the spacer layer is 7.0×10 17 ~3.0 x 10 18 cm -3 6. The ultraviolet semiconductor light-emitting element according to claim 1, wherein

7. The Mg concentration of the spacer layer is 1.0×10 18 ~2.5 x 10 18 cm -3 7. The ultraviolet semiconductor light-emitting element according to claim 6, wherein

8. the active layer has a multiple quantum well structure consisting of a plurality of the well layers and a plurality of the barrier layers, 8. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the spacer layer has a larger Mg content than the other barrier layers.

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