NITRIDE SEMICONDUCTOR LIGHT EMITTING ELEMENT AND METHOD FOR MANUFACTURING NITRIDE SEMICONDUCTOR LIGHT EMITTING ELEMENT

Abstract

To provide nitride semiconductor light emitting elements of good quality and a method for manufacturing nitride semiconductor light emitting elements of good quality.SOLUTION: A nitride semiconductor light emitting element 1 has an n-AlInN layer 12 including Al and In in composition, a GaN cap layer 13 stacked on the surface of the n-AlInN layer 12 and including Ga in composition, and an n-AlGaN composition graded layer 14 stacked on the surface of the GaN cap layer 13 and including Al and Ga in composition, and the mole fraction of AlN at the interface of the n-AlGaN composition graded layer 14 stacked on the surface of the GAN cap layer 13 is more than 0.36 and less than 0.44.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a nitride semiconductor light-emitting device and a method for manufacturing the nitride semiconductor light-emitting device. [Background technology]

[0002] When manufacturing a semiconductor multilayer film by stacking nitride semiconductor thin films with different bandgaps and refractive indices, the presence of high density defects such as threading dislocations has been a problem, particularly in multilayer reflectors made of nitride semiconductor thin films containing In. Specifically, there is a concern that defects may occur at the heterojunction where a nitride semiconductor thin film that does not contain In, such as GaN, is formed on a nitride semiconductor thin film that contains In, such as AlInN.

[0003] Patent Document 1 discloses that the rate of decrease in the In composition at the interface between the upper AlInN layer and the lower GaN layer is greater than the rate of decrease in the Al composition, i.e., a structure in which a layer with substantially low In content exists at the interface between the AlInN layer and the GaN layer, suppressing the occurrence of defects such as threading dislocations. Specifically, it discloses that immediately after forming the AlInN layer, a cap layer containing more than 0 nm but not more than 1 nm of In is formed, and then a heating process is performed to increase the substrate temperature, thereby removing In clusters present on the surface of the AlInN layer and suppressing the occurrence of defects. Here, the cap layer includes a state in which it uniformly covers the surface of the AlInN layer, as well as a state in which it is formed in an island-like shape on the surface of the AlInN layer. Patent Document 2 also discloses that the effective refractive index step is increased by providing an AlGaN layer with an Al composition (AlN mole fraction) of less than 20% (0.2) on the cap layer, which is several nanometers thick. This increases the reflectivity. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] : JP 2018-98347 A [Patent Document 2] : JP 2018-98340 A [Non-Patent Document 1] :Schulz, et al., "Composition dependent band gap and band edge bowing in AlInN: A combined theoretical and experimental study"Appl. Phys. Express 6, 121001 (2013) [Non-patent document 2] :Coughlan, et al., "Band gap bowing and optical polarization switching in Al1-xGaxN alloys"Phys. Stat. Sol. B 252,879 (2015) [Non-patent document 3] :Bernardini, et al., "Spontaneous polarization and piezoelectric constants of III-V nitrides"Phys. Rev. B 56, R10024 (1997) Summary of the Invention [Problem to be solved by the invention]

[0005] Under these circumstances, we investigated the development of a nitride semiconductor thin-film multilayer reflector structure with conductivity, which would be advantageous in terms of current injection, with the aim of forming a device structure. As a result, we discovered the following new problem. Specifically, we considered the formation of a conductive multilayer reflector structure by providing an n-AlGaInN compositionally graded layer doped with Si on an n-AlInN layer. First, a 5 nm n-AlGaInN compositionally graded layer, i.e., a layer whose composition gradually changes from n-AlInN to n-GaN, was formed on the n-AlInN layer at the same growth temperature as the n-AlInN layer to ensure the necessary In content. Next, a 0.3 nm GaN cap layer was formed according to Patent Document 1, followed by a heating process in which the substrate temperature was increased.

[0006] Although the multilayer mirror fabricated using this process exhibited good electrical conductivity, it was found that it had more pits (holes) on its surface than non-conductive multilayer mirrors without the n-AlGaInN compositional gradient. This is thought to be because the n-AlGaInN compositional gradient layer was formed at the same temperature (i.e., a lower temperature) as the n-AlInN layer to a thickness of 5 nm, resulting in the substantial presence of a GaN layer equivalent to a thickness of 1 nm or more on the surface side, which weakened the effect of reducing pits and threading dislocations. On the other hand, even when an n-AlGaN layer with an AlN mole fraction of 20% (0.2%) or less was formed and n-type impurity Si was added according to Patent Document 2, good electrical conductivity was not obtained. Thus, it became clear that the techniques disclosed in Patent Documents 1 and 2 alone cannot produce a multilayer mirror that suppresses dislocation generation and has good electrical conductivity.

[0007] The present invention has been made in view of the above-mentioned conventional circumstances, and aims to provide a nitride semiconductor light-emitting device with high output and long life, and a method for manufacturing the nitride semiconductor light-emitting device, by obtaining a multilayer film reflector having a low surface pit density, i.e., few defects, and good electrical conductivity. [Means for solving the problem]

[0008] The nitride semiconductor light-emitting device of the first invention is a first layer containing Al and In in its composition; a cap layer stacked on the surface of the first layer and containing Ga; a second layer stacked on the surface of the cap layer and containing Al and Ga; Equipped with The mole fraction of AlN at the interface of the second layer laminated on the surface of the cap layer is 0.36 or more and 0.44 or less.

[0009] According to this configuration, the energy barrier between the first layer and the second layer can be reduced, and electrons can be exchanged well between the first layer and the second layer.

[0010] The method for manufacturing a nitride semiconductor light-emitting device according to the second invention includes the steps of: A method for manufacturing a nitride semiconductor light-emitting device using metal organic chemical vapor deposition, comprising: a first layer lamination step of growing a crystal of a first layer containing Al and In; a cap layer lamination step of growing a cap layer containing Ga as a composition on the surface of the first layer after the first layer lamination step is performed; a second layer lamination step of growing a second layer containing Al and Ga as a composition on the surface of the cap layer after the cap layer lamination step is performed; a third layer lamination step of growing a third layer containing Ga as a composition on a surface of the second layer after the second layer lamination step is performed; Equipped with The temperature at which the second layer is crystal-grown in the second layer laminating step is equal to or higher than the temperature at which the third layer is crystal-grown in the third layer laminating step.

[0011] According to this configuration, Al migration can be activated in the second layer lamination step, which makes it easier to flatten the surface of the second layer, leading to improved quality in the third layer, which is subsequently grown as a crystal. [Brief explanation of the drawings]

[0012] [Figure 1] 1A and 1B are schematic diagrams showing an example of the structure of a nitride semiconductor light-emitting device manufactured by a method for manufacturing a nitride semiconductor light-emitting device. [Figure 2] 1 is a graph showing the change in the energy barrier when the composition is changed from GaN to AlN and from AlN to InN. [Figure 3] 10 is a graph showing the relationship between the polarization charge density at the interface between the n-AlInN layer and the n-AlGaN compositionally graded layer and the starting AlN mole fraction in the n-AlGaN compositionally graded layer. [Figure 4] 1 is a graph showing the relationship between the energy barrier occurring at the interface between the n-AlInN layer and the n-AlGaN compositionally graded layer and the starting AlN mole fraction. [Figure 5]1A is a schematic diagram showing the structure of the samples of Examples 1 to 4 and Comparative Examples 1 and 2, and FIG. 1B is a table showing the starting AlN mole fraction, growth temperature, and differential resistance value in Examples 1 to 4 and Comparative Examples 1 and 2. [Figure 6] 1 is a graph showing the relationship between voltage and current in the samples of Examples 1 to 4 and Comparative Examples 1 and 2. [Figure 7] 1 is a graph plotting the relationship between the differential resistivity and the starting AlN mole fraction for the samples of Examples 1 to 4 and Comparative Example 1. [Figure 8] 1 is a graph in which differential resistance values ​​at multiple points are plotted for each of the samples of Examples 1 to 4 and Comparative Example 1. [Figure 9] 1 is a graph showing the relationship between the maximum reflectance and the thickness of an n-AlGaN compositionally graded layer. [Figure 10] 10 is a graph showing the relationship between the energy barrier occurring at the interface between the n-AlInN layer and the n-AlGaN compositionally graded layer and the thickness of the n-AlGaN compositionally graded layer. [Figure 11] 1 shows AFM images of the surfaces of Example 5 and Comparative Examples 3 and 4 measured over an area of ​​10 μm×10 μm. DETAILED DESCRIPTION OF THE INVENTION

[0013] A preferred embodiment of the present invention will now be described.

[0014] The first invention may further include a third layer laminated on the surface of the second layer, having a smaller AlN molar fraction than the first layer and containing Ga in its composition, and the AlN molar fraction in the second layer may be graded so as to gradually decrease toward the interface with the third layer. This configuration reduces the energy barrier at the interface between the second layer and the third layer, thereby improving the conductivity between the second layer and the third layer and thereby improving the exchange of electrons between the first layer and the third layer (i.e., improving the conductivity).

[0015] In the first aspect of the invention, the thickness of the cap layer in the stacking direction may be greater than 0 and equal to or less than 1 nm. This configuration allows the surface of the first layer to be well protected.

[0016] In the first aspect of the present invention, the thickness of the second layer in the stacking direction may be 4 nm or more and 15 nm or less. This configuration makes it possible to achieve both a low differential resistance and high reflectivity.

[0017] <Examples 1 to 4, Comparative Examples 1 and 2> Next, an example of a nitride semiconductor light-emitting device according to the present invention will be described with reference to the drawings. Specifically, this is an example of a method for forming an n-AlInN / GaN multilayer reflector structure having an n-AlGaN compositionally graded layer 14 on the surface side of an n-AlInN layer 12, and a method for manufacturing a surface-emitting laser (hereinafter also referred to as nitride semiconductor light-emitting device 1) using this structure. The n-AlInN / GaN multilayer reflector structure can be used not only in surface-emitting lasers, but also in various optical device structures such as light-emitting diodes and solar cells. A cross-sectional schematic diagram of an example of the structure of a surface-emitting laser (nitride semiconductor light-emitting device 1) using this structure is shown in FIG. 1.

[0018] 1, the nitride semiconductor light-emitting element 1 includes an n-GaN substrate 10, an n-GaN buffer layer 11, an n-AlInN / GaN multilayer reflector M, a second n-GaN layer 16, a GaInN quantum well active layer 17, a p-AlGaN layer 18, a p-GaN layer 19, a p-GaN contact layer 20, a p-side electrode 22A, and an n-side electrode 22B. The n-AlInN / GaN multilayer reflector M is formed by repeating four layers 40 times: a first n-AlInN layer 12, a GaN cap layer 13, a second n-AlGaN compositionally graded layer 14, and a third first n-GaN layer 15. The n-GaN substrate 10 is a single-crystal GaN substrate having n-type characteristics. The nitride semiconductor light emitting device 1 can be manufactured by epitaxial growth on the surface of the n-GaN substrate 10, that is, the Ga face (upper face in FIG. 1), using metal organic vapor phase epitaxy (MOVPE).

[0019] When manufacturing the nitride semiconductor light-emitting device 1, TMA (trimethylaluminum), TMGa (trimethylgallium), TEGa (triethylgallium), and TMI (trimethylindium) are used as group III materials (MO raw materials). Ammonia (NH3) is used as the raw material gas for group V materials. SiH4 (silane) is used as the raw material gas for donor impurities. CP2Mg (cyclopentadienylmagnesium) is used as the MO raw material for acceptor impurities.

[0020] First, an n-GaN buffer layer 11 is grown as a buffer layer on the surface of an n-GaN substrate 10 by crystal growth. The thickness of the n-GaN buffer layer 11 in the stacking direction is 500 nm. Here, the stacking direction is the up-and-down direction in FIG. 1. Hereinafter, the thickness in the stacking direction will be simply referred to as the thickness. Specifically, the n-GaN substrate 10 is placed in a reactor of an MOVPE apparatus (hereinafter, simply referred to as the reactor). Then, the temperature in the reactor is adjusted so that the temperature (growth temperature) of the n-GaN substrate 10 becomes 1050°C, and H2 (hydrogen) is supplied into the reactor as a carrier gas. The concentration of Si (silicon) in the n-GaN buffer layer 11 is 5×10 18 cm -3 The amount of SiH4 supplied to the reactor is adjusted so that

[0021] [First layer lamination process] Next, a first layer deposition step is performed. Specifically, an n-AlInN layer 12 containing Al and In as its composition is epitaxially grown on the surface of the n-GaN buffer layer 11. Specifically, the temperature inside the reactor is adjusted so that the temperature (growth temperature) of the n-GaN substrate 10 becomes 840°C, and N2 (nitrogen) is supplied into the reactor as a carrier gas. Furthermore, TMA, TMI, SiH4, and NH3 are supplied into the reactor, and the n-AlInN layer 12 is crystal-grown to a thickness of 38 nm in the deposition direction. The molar fraction of AlN in the n-AlInN layer 12 is approximately 81% (0.81), and the molar fraction of InN is approximately 19% (0.19).

[0022] Basically, it is preferable to match the lattice constant of the n-AlGaN compositionally graded layer 14 (described later) with the lattice constant of the n-GaN substrate 10, which is the substrate. However, in this embodiment, the lattice constant of the n-AlGaN compositionally graded layer 14 is set to be smaller than the lattice constant of the n-GaN substrate 10. To compensate for this, the mole fraction of InN in the n-AlInN layer 12 is increased slightly (less than 1% (0.01)) to approximately 19% (0.19). This cancels out the accumulated lattice strain in the entire multilayer mirror structure. Note that the mole fractions of AlN and InN in the n-AlInN layer 12 are not limited to the values ​​disclosed in this embodiment, and may be any value that can cancel out the accumulated lattice strain in the entire multilayer mirror structure. The Si concentration in the n-AlInN layer 12 is 1.5×10 19 cm -3 The amount of SiH4 supplied into the reactor is adjusted so that the carrier gas is equal to or greater than N2. In addition to N2, the carrier gas may be an inert gas such as Ar or Ne (neon), or a mixed gas of these.

[0023] [Cap layer lamination process] Next, after the first layer deposition step, the cap layer deposition step is performed. Specifically, while maintaining the temperature (growth temperature) of the n-GaN substrate 10 and the supply of carrier gas (N2) and NH3 into the reactor, the supply of TMA, TMI, and SiH4 is stopped, and TEGa is supplied into the reactor to grow a GaN cap layer 13 on the surface of the n-AlInN layer 12. The GaN cap layer 13 contains Ga in its composition. The thickness of the GaN cap layer 13 in the deposition direction is 0.6 nm. If the thickness of the GaN cap layer 13 is less than 1 nm, there is no significant effect if SiH4 is not added, but SiH4 may be supplied. When SiH4 is supplied, the Si concentration should be 5×10 18 cm -3 Adjust it to a certain extent.

[0024] Furthermore, as described below, it is preferable to adjust the thickness of the GaN cap layer 13 depending on the substrate temperature during crystal growth in the step following the cap layer deposition step. For example, if the substrate temperature during crystal growth in the step following the cap layer deposition step is set to a temperature higher than the substrate temperature during the cap layer deposition step, the thickness of the GaN cap layer 13 is increased. This reduces damage to the surface of the n-AlInN layer 12 due to the high substrate temperature in the next step, and enables the surface of the n-AlInN layer 12 to be formed more flatly. Specifically, if the substrate temperature in the next step is approximately 1050°C, the thickness of the GaN cap layer 13 is preferably 0.3 nm to 0.6 nm. Furthermore, if the substrate temperature in the next step is 1100°C or higher, the thickness of the GaN cap layer 13 is preferably 0.6 nm to 1 nm. That is, regardless of the substrate temperature, the thickness of the GaN cap layer 13 in the deposition direction should be greater than 0 and less than or equal to 1 nm.

[0025] [Temperature rising process] Next, a temperature-raising step is performed. Specifically, after stopping the supply of TEGa, the carrier gas supplied into the reactor is gradually switched from N to H while maintaining the supply of NH into the reactor, and the temperature (growth temperature) of the n-GaN substrate 10 is raised to 1100°C, the growth temperature of the n-AlGaN compositionally graded layer 14. At this time, the supply of TMA, TMGa, TEGa, TMI, and SiH into the reactor is stopped, so crystal growth is interrupted. After forming the thin GaN cap layer 13 that does not contain In, a temperature-raising step is performed to raise the temperature of the n-GaN substrate 10 at the surface of the GaN cap layer 13. As disclosed in Patent Document 1, this removes In clusters present on the surface of the n-AlInN layer 12 and suppresses the generation of defects. However, since the next step involves crystal growth of the n-AlGaN compositionally graded layer 14, which contains Al, which has a high binding energy, it is necessary to activate Al migration on the substrate surface. Therefore, by setting the growth temperature in the second layer stacking step to be equal to or higher than the growth temperature for the first n-GaN layer 15 stacked on the surface of the n-AlGaN composition gradient layer 14, a flatter surface of the n-AlGaN composition gradient layer 14 can be obtained.

[0026] [Second layer lamination process] Next, the second layer deposition process is carried out. Specifically, after the temperature of the n-GaN substrate 10 reaches 1100°C, the supply of TMGa, TMAl, and SiH4 into the reactor is started, and an n-AlGaN compositionally graded layer 14 containing Al and Ga but not In is crystal-grown. The thickness of the n-AlGaN compositionally graded layer 14 in the deposition direction is 5 nm. The n-AlGaN compositionally graded layer 14 is deposited on the surface of the GaN cap layer 13. The n-AlGaN compositionally graded layer 14 is crystal-grown as follows.

[0027] First, crystal growth is initiated with predetermined supply rates of TMA and TMGa. The AlN mole fraction at the start of crystal growth of the n-AlGaN compositionally graded layer 14 (hereinafter simply referred to as the starting AlN mole fraction) can be adjusted by varying the ratio of the TMA and TMGa supply rates. Immediately after the start of crystal growth of the n-AlGaN compositionally graded layer 14, the supply rate of TMA into the reactor is gradually decreased, while the supply rate of TMGa is gradually increased. Finally, the supply of TMA into the reactor is completely stopped, and the supply rate of TMGa is increased until the growth rate of the n-AlGaN compositionally graded layer 14 is approximately the same as the growth rate at the start of growth. This results in the growth of the n-AlGaN compositionally graded layer 14, which is n-AlGaN at the start of crystal growth in the stacking direction and gradually changes to n-GaN at the end of crystal growth. In other words, the AlN mole fraction in the n-AlGaN compositionally graded layer 14 is compositionally graded so that it gradually decreases toward the interface with the first n-GaN layer 15 (described later). The thickness of the n-AlGaN compositionally graded layer 14 is 5 nm. The n-AlGaN compositionally graded layer 14 has a preferred thickness, which will be described later. In addition, in order to obtain n-type characteristics, the Si concentration in the n-AlGaN compositionally graded layer 14 is set to 6×10 19 cm -3 The amount of SiH4 supplied to the reactor is adjusted so that

[0028] After the supply of TMGa and SiH4 is stopped and the growth of the n-AlGaN compositionally graded layer 14 is completed, the temperature of the n-GaN substrate 10 is lowered to 1050°C, which is an appropriate temperature for the subsequent crystal growth of the first n-GaN layer 15. In other words, the temperature (1100°C) for the crystal growth of the n-AlGaN compositionally graded layer 14 in the second layer deposition step is higher than the temperature (1050°C) for the crystal growth of the first n-GaN layer 15 in the subsequent third layer deposition step.

[0029] [Third layer lamination process] Next, after the second layer lamination step is performed, the third layer lamination step is performed. Specifically, after the temperature of the n-GaN substrate 10 reaches 1050°C, the supply of TMGa into the reactor is started, and the first n-GaN layer 15, which has a thickness of 43 nm in the lamination direction and contains Ga as its composition, is crystal-grown. The mole fraction of AlN in the first n-GaN layer 15 is 0 (i.e., smaller than that of the n-AlInN layer 12). The Si concentration in the first n-GaN layer 15 is 5×10 18 cm -3 The amount of SiH4 supplied into the reactor is adjusted so that the n-AlInN layer 12, the GaN cap layer 13, the n-AlGaN compositionally graded layer 14, and the first n-GaN layer 15 correspond to one pair P of the multilayer mirror structure.

[0030] After the first n-GaN layer 15 is grown, the supply of TMGa to the reactor is stopped to interrupt the crystal growth so that the n-AlInN layer 12 can be grown again on the surface of the first n-GaN layer 15 (i.e., the pair P can be repeatedly grown). While the crystal growth is interrupted, the temperature of the n-GaN substrate 10 is lowered from 1050°C to 840°C. At this time, the carrier gas is gradually switched from H2 to N2. In this way, one pair P (n-AlInN layer 12, GaN cap layer 13, n-AlGaN compositionally graded layer 14, and first n-GaN layer 15) of the multilayer mirror structure is repeatedly grown 40 times, thereby forming an n-AlInN / GaN multilayer mirror M in which 40 pairs P are stacked.

[0031] [Verification of the starting AlN mole fraction of the AlGaN compositionally graded layer] Here, the nitride semiconductor light emitting device 1 uses an n-AlInN layer 12 and an n-AlGaN compositionally graded layer 14. Therefore, the connection of the band structure at the heterointerface formed by the upper part of the n-AlInN layer 12 and the lower part of the n-AlGaN compositionally graded layer 14 (i.e., the n-AlGaN compositionally graded layer 14 at the start of growth) via the GaN cap layer 13 has a significant effect on the conductivity of the multilayer film structure.

[0032] That is, because electrons move in layers with n-type conductivity, such as the n-AlInN layer 12 and the n-AlGaN compositionally graded layer 14, it is extremely important to select a starting AlN mole fraction that does not create an unnecessary barrier in the energy level at the heterointerface at the bottom of the conduction band. Incidentally, if an AlGaInN compositionally graded layer is used instead of the n-AlGaN compositionally graded layer 14, in which the mole fractions of AlN, GaN, and InN are continuously changed during crystal growth to match the composition of the n-AlInN layer 12 with that of the first n-GaN layer 15, the band gap changes continuously from the n-AlInN layer 12 to the first n-GaN layer 15, and the above-mentioned problem does not arise.

[0033] To determine the starting AlN mole fraction that avoids the generation of an unnecessary barrier in the energy level at the heterointerface of the conduction band minimum, it is important to consider both the conduction band offset due to the difference in band gap between the AlInN and AlGaN layers and the polarization charge offset due to the difference in polarization charge. Specifically, it is believed that there exist mole fractions of AlN at which both the conduction band offset due to the difference in band gap between the AlInN and AlGaN layers and the polarization charge offset due to the difference in polarization charge become zero.

[0034] Here, using the physical property values ​​disclosed in Non-Patent Documents 1 and 2, we calculated the AlN molar fraction of AlGaN whose conduction band minimum coincides with that of the n-AlInN layer 12 (AlN molar fraction is 0.81) used in the n-AlInN / GaN multilayer film reflector M, and found that this value was 0.25 (see FIG. 2). On the other hand, using the physical property values ​​disclosed in Non-Patent Document 3, we calculated the AlN molar fraction of AlGaN whose conduction band minimum coincides with the polarization charge in the upper part of the n-AlInN layer 12 and makes the offset zero, and found that this value was 0.45 (see FIG. 3).

[0035] Based on these two physical property values, we simulated the potential barrier at the conduction band minimum. As shown in Figure 4, the potential barrier at the conduction band minimum between the n-AlInN layer 12 and the AlGaN layer was minimized (approximately 0.18 eV) when the starting AlN mole fraction of the AlGaN layer stacked on the n-AlInN layer 12 was 0.27. Furthermore, the potential barrier increased when the starting AlN mole fraction was less than 0.27 and increased when the starting AlN mole fraction was greater than 0.27. That is, Figure 4 suggests that the device resistance can be minimized by setting the AlN mole fraction at the start of crystal growth in the n-AlGaN compositionally graded layer 14 to 0.27, and that the device resistance can increase regardless of whether the AlN mole fraction is greater or less than 0.27. Furthermore, Figure 4 shows that when the starting AlN mole fraction is 0.36 or greater, the potential barrier increases monotonically, resulting in an increase in the device resistance.

[0036] In order to understand the relationship between the starting AlN mole fraction and the resistance of the n-AlInN / GaN multilayer reflector M, five types of samples (Examples 1 to 4 and Comparative Example 1) were prepared, each having a multilayer reflector Mt formed by repeating pairs P 10 times, as shown in Fig. 5. In these samples, a u-GaN layer 30 and an n-GaN layer 31 were stacked in this order on the surface of a sapphire substrate S having a c-plane (0001) surface, and a multilayer reflector Mt was stacked on the surface of the n-GaN layer 31. In each pair P in the multilayer reflector Mt of these samples, an n-AlInN layer 12 (45 nm thick), a GaN cap layer 13 (0.6 nm thick), an n-AlGaN compositionally graded layer 14 (5 nm thick), and a first n-GaN layer 15 (35 nm thick) were stacked in this order. The starting AlN mole fractions of these samples were varied as follows: 0.36, 0.39, 0.42, 0.44, and 0.47. The starting AlN mole fractions for each sample were determined from the X-ray diffraction curves of GaN / AlGaN superlattices separately grown under the same growth conditions.

[0037] As Comparative Example 2, a sample was also fabricated in which an n-AlGaInN compositionally graded layer 114 (5 nm thick) containing many dislocations was used instead of the GaN cap layer 13 and the n-AlGaN compositionally graded layer 14. The n-AlGaInN compositionally graded layer 114 has good conductivity because, in principle, the interface with the upper part of the n-AlInN layer 12 and the interface with the lower part of the first n-GaN layer 15 do not form heterointerfaces. In order to measure the differential resistance of the multilayer reflector Mt in the vertical direction of each sample, the multilayer reflector Mt was etched into a mesa shape, and n-electrodes E were formed on the surface of the multilayer reflector Mt and on the etched surface around the multilayer reflector Mt, and the vertical current-voltage characteristics were measured.

[0038] FIG. 6 shows a graph plotting the best results (smallest differential resistance values) of the current-voltage characteristics of each fabricated sample. To illustrate the dependency of differential resistance on the starting AlN mole fraction, FIG. 7 shows a graph showing the dependency of differential resistance on the starting AlN mole fraction. Unlike the values ​​(0.27 being the minimum) obtained by simulation using theoretical values, the minimum differential resistance was obtained for the sample with a starting AlN mole fraction of 0.39 (sample of Example 2). The differential resistance increased whether the starting AlN mole fraction was greater or less than 0.39. Furthermore, the differential resistance of the sample with a starting AlN mole fraction of 0.39 (sample of Example 2) was almost the same as that of the sample of Comparative Example 2, which used the n-AlInN layer 12 and the n-AlGaInN graded composition layer 114, which exhibited good conductivity and did not form a potential barrier due to band offset at the interface between the n-AlInN layer 12 and the n-AlGaInN graded composition layer 114.

[0039] Specifically, as shown in FIG. 5 , the differential resistance value of the sample of Example 1 (starting AlN molar fraction 0.36) was 30.6Ω. The differential resistance value of the sample of Example 2 (starting AlN molar fraction 0.39) was 22.3Ω. The differential resistance value of the sample of Example 3 (starting AlN molar fraction 0.42) was 25.6Ω. The differential resistance value of the sample of Example 4 (starting AlN molar fraction 0.44) was 33.7Ω. The differential resistance value of the sample of Comparative Example 1 (starting AlN molar fraction 0.47) was 36.1Ω. Furthermore, the differential resistance value of Comparative Example 2 (sample using the n-AlGaInN compositionally graded layer 114) was 21.5Ω. This indicates that when the starting AlN molar fraction was 0.39 (Example 2), the effects of the band offset and polarization charge offset were minimized, resulting in a low differential resistance value almost equivalent to that of an ideal n-AlGaInN compositionally graded layer 114. In other words, it was found that when the starting AlN molar fraction was 0.39 (Example 2), the potential barrier of the energy level at the bottom of the conduction band was minimized.

[0040] Furthermore, Fig. 8 shows not only the minimum differential resistance value, but also the results of measuring the differential resistance 32 times at different measurement positions for each of the five types of samples (Examples 1 to 4 and Comparative Example 1), as well as a graph plotting the average values ​​of these measurement results. That is, the measurement results shown in Fig. 8 include the distribution within the wafer for each sample. Looking at the average differential resistance value for each sample, it was found that the minimum differential resistance value was obtained when the starting AlN mole fraction was 0.39.

[0041] The simulation results shown in Figure 4 suggest that when the starting AlN mole fraction is 0.27 or greater, i.e., 0.36 or greater, the differential resistance increases monotonically with increasing starting AlN mole fraction. On the other hand, the study results shown in Figure 8 indicate that the differential resistance decreases when the starting AlN mole fraction is greater than 0.36, reaching a minimum value at a starting AlN mole fraction of 0.39. Furthermore, from a starting AlN mole fraction of greater than 0.39 to approximately 0.44, the differential resistance remains low, comparable to that at 0.36. Furthermore, it was found that at starting AlN mole fractions of 0.47 or greater, the differential resistance is greater than that at 0.36.

[0042] From the above verification results, it was found that when the starting AlN mole fraction is in the range of 0.36 to 0.44, a low-resistivity region exists where the differential resistance is low, which is unpredictable from the simulation results (theoretical values). It was also found that by setting the starting AlN mole fraction in this range, a low-resistivity n-AlInN / GaN multilayer reflector M can be fabricated. It was also found that, more preferably, an n-AlInN / GaN multilayer reflector M with even lower resistance can be fabricated by setting the starting AlN mole fraction to 0.39 to 0.42. That is, it was found that an appropriate range of the starting AlN mole fraction (0.36 to 0.44) exists to reduce the vertical differential resistance of the n-AlInN / GaN multilayer reflector M in which multiple pairs P are stacked. In other words, the AlN mole fraction at the interface of the n-AlGaN compositionally graded layer 14 stacked on the surface of the GaN cap layer 13 is preferably 0.36 to 0.44.

[0043] In this verification, a sample grown by repeating 10 pairs P was used, but it is believed that the effect of reducing the vertical differential resistance can be achieved by setting the starting AlN mole fraction in this range (0.36 or more and 0.44 or less) even when the number of stacked pairs P is 11 or more. In other words, in an n-AlInN / GaN multilayer reflector M using an n-AlGaN compositionally graded layer 14, by setting the starting AlN mole fraction of the n-AlGaN compositionally graded layer 14 to a value between 0.36 or more and 0.44 or less, the vertical differential resistance of the n-AlInN / GaN multilayer reflector M in which multiple pairs P are stacked can be effectively reduced.

[0044] [Preferable thickness of n-AlGaN compositionally graded layer] Increasing the thickness of the n-AlGaN composition gradient layer 14 in the stacking direction reduces the potential barrier and results in low resistance, while the refractive index step is substantially reduced, resulting in a decrease in reflectivity. Reducing the thickness of the n-AlGaN composition gradient layer 14 produces the opposite result. Figure 9 shows the calculation results for the dependence of the maximum reflectivity on the thickness of the n-AlGaN composition gradient layer 14. Figure 10 shows the calculation results for the dependence of the energy barrier (hereinafter simply referred to as the energy barrier) at the interface between the n-AlInN layer 12 and the n-AlGaN composition gradient layer 14 on the thickness of the n-AlGaN composition gradient layer 14. As shown in Figure 9, the thickness of the n-AlGaN composition gradient layer 14 is preferably 15 nm or less, which can achieve a reflectivity of 99.9% or more, which is essential for a surface-emitting laser, and at which the decrease in reflectivity is gradual.

[0045] On the other hand, as shown in FIG. 10 , the rate of decrease in the energy barrier is rapid as the thickness of the n-AlGaN compositionally graded layer 14 increases from 0 nm to 4 nm. However, the rate of decrease in the energy barrier becomes gradual when the thickness of the n-AlGaN compositionally graded layer 14 increases beyond 4 nm. Therefore, to obtain an n-AlInN / GaN multilayer reflector M with well-controlled differential resistance, it is preferable to set the thickness of the n-AlGaN compositionally graded layer 14 to 4 nm or more. Therefore, to obtain an n-AlInN / GaN multilayer reflector M that achieves both low differential resistance and high reflectivity, it is preferable to set the thickness of the n-AlGaN compositionally graded layer 14 in the stacking direction to 4 nm or more and 15 nm or less. Note that this range of values ​​is considered to be a range in which both low differential resistance and high reflectivity can be achieved in the n-AlInN / GaN multilayer reflector M, regardless of the value of the starting AlN mole fraction.

[0046] <Example 5, Comparative Examples 3 and 4> [Verification of the surface condition of n-AlInN / GaN multilayer mirrors using n-AlGaN compositionally graded layers] Next, we will explain the results of examining the surface condition of an n-AlInN / GaN multilayer reflector M using an n-AlGaN compositionally graded layer 14. In Example 5, a sample was fabricated in which 40 pairs of n-AlGaN compositionally graded layers 14 were formed on a GaN substrate, with the starting AlN mole fraction of the n-AlGaN compositionally graded layer 14 set to 0.39, which yields the lowest differential resistance. In Comparative Example 3, a sample was fabricated in which 40 pairs of non-conductive multilayer reflectors, which were not provided with an n-AlGaN compositionally graded layer 14 and did not contain Si, were formed on a GaN substrate. In Comparative Example 4, a sample was fabricated in which 40 pairs of conductive multilayer reflectors, which had n-AlGaInN compositionally graded layers and were doped with Si, were formed on a GaN substrate. AFM images of the surfaces of the samples of Example 5 and Comparative Examples 3 and 4, measured over a 10 μm × 10 μm area, are shown in FIG. 11 .

[0047] On the surface of the sample of Comparative Example 3, no pits were observed within a measurement range of 10 μm×10 μm, and the pit density was 1×10 6 cm -2 On the other hand, the surface of the sample of Comparative Example 4 is estimated to be less than 1 × 10 7 cm -2 The pit density on the surface of the sample of Example 5 was 3×10 6 cm -2 This result is almost as low as the pit density of the sample of Comparative Example 3. This means that by using an n-AlGaN compositionally graded layer 14 with a starting AlN mole fraction of 0.36 or more and 0.44 or less, an n-AlInN / GaN multilayer reflector M with good electrical conductivity and crystallinity, i.e., surface flatness, can be fabricated. In other words, by setting the starting AlN mole fraction of the n-AlGaN compositionally graded layer 14 to 0.36 or more and 0.44 or less, an n-AlInN / GaN multilayer reflector M with good surface flatness can be fabricated.

[0048] Next, we return to the explanation of the method for fabricating the nitride semiconductor light-emitting device 1 in Fig. 1. After forming the n-AlInN / GaN multilayer reflector M by repeating crystal growth of the pair P 40 times, a second n-GaN layer 16 having a thickness of 400 nm is grown on the surface of the n-AlInN / GaN multilayer reflector M at a growth temperature of 1050°C. The Si concentration in the second n-GaN layer 16 is 5 x 10 18 cm -3 The amount of SiH4 supplied to the reactor is adjusted so that

[0049] Next, a GaInN quantum well active layer 17 is grown on the surface of the second n-GaN layer 16. The GaInN quantum well active layer 17 is composed of five pairs of GaInN quantum well layers and GaN barrier layers. Then, a p-AlGaN layer 18 having a thickness of 20 nm is grown by crystal growth on the surface of the GaInN quantum well active layer 17. The molar fraction of AlN in the p-AlGaN layer 18 is 0.2, and the molar fraction of GaN is 0.8. The Mg concentration in the p-AlGaN layer 18 is 2×10 19 cm -3 The amount of Cp2Mg supplied is adjusted so that

[0050] Next, a p-GaN layer 19 having a thickness of 70 nm is grown as a p-type cladding layer on the surface of the p-AlGaN layer 18. The Mg concentration in the p-GaN layer 19 is 2×10 19 cm -3 The supply amount of Cp2Mg is adjusted so that the concentration of Mg in the p-GaN contact layer 20 is 2×10 20 cm -3 The amount of Cp2Mg supplied is adjusted so that

[0051] In this way, a resonator structure is formed, which includes an n-AlInN / GaN multilayer reflector M having an n-AlGaN compositionally graded layer 14 with an initial AlN mole fraction of 0.39, and a GaInN quantum well active layer 17 sandwiched between pn junctions and emitting light in the purple region, with the resonator length being an integer multiple of the emission wavelength, and corresponding to four wavelengths.

[0052] 1, a surface-emitting laser is completed by crystal growth of an insulating film 21, a p-side electrode 22A, an n-side electrode 22B, and an SiO2 / Nb2O5 dielectric multilayer reflector D on a wafer having an n-AlInN / GaN multilayer reflector M and a resonator structure. Below, we will explain the process of fabricating a surface-emitting laser (nitride semiconductor light-emitting element) by crystal growth of the insulating film 21, the p-side electrode 22A, the n-side electrode 22B, and the SiO2 / Nb2O5 dielectric multilayer reflector D.

[0053] First, after crystal growth of the p-GaN contact layer 20, H is desorbed from the p-type semiconductor layers of the semiconductor wafer (p-AlGaN layer 18, p-GaN layer 19, p-GaN contact layer 20) to activate the p-type dopant Mg added to the p-type semiconductor layers. Next, a photoresist pattern is formed on the surface of the semiconductor wafer, and the surface is partially etched to form a circular mesa structure with a diameter of 40 μm that will become the element.

[0054] A 20-nm-thick SiO2 film is then deposited on the surface of the p-GaN contact layer 20, and an insulating film 21 is formed by photolithography and sputtering, with a circular opening H having a diameter of 10 μm. A transparent p-side electrode 22A made of ITO is then formed by sputtering so as to contact the surface of the p-GaN contact layer 20 exposed through the opening H. At the same time, an n-side electrode 22B is provided on the back surface of the n-GaN substrate 10. A pad electrode made of Cr / Ni / Au is then formed on the outer periphery of the surface of the p-side electrode 22A (not shown).

[0055] Finally, photolithography and sputtering are used to grow a SiO2 / Nb2O5 dielectric multilayer reflector D on the surface of the p-side electrode 22A. This completes a nitride semiconductor light-emitting device that has a SiO2 / Nb2O5 dielectric multilayer reflector D above the GaInN quantum well active layer 17 and an n-AlInN / GaN multilayer reflector M below it, and that functions as a vertical-cavity surface-emitting laser with a cavity length corresponding to an integral multiple of the wavelength.

[0056] This element has a multilayer reflector with good conductivity, crystallinity, and surface flatness, which allows for high-efficiency, high-power operation due to high uniformity of the injected current across the surface and low element resistance.In addition, the element has few pits and defects, resulting in a long element life and high reliability.

[0057] Next, the operation of the above embodiment will be described.

[0058] The nitride semiconductor light-emitting element 1 includes an n-AlInN layer 12 containing Al and In in its composition, a GaN cap layer 13 containing Ga in its composition and stacked on the surface of the n-AlInN layer 12, and an n-AlGaN compositionally graded layer 14 containing Al and Ga in its composition and stacked on the surface of the GaN cap layer 13, wherein the mole fraction of AlN at the interface of the n-AlGaN compositionally graded layer 14 stacked on the surface of the GaN cap layer 13 is 0.36 or more and 0.44 or less. This configuration reduces the difference in energy barrier between the n-AlInN layer 12 and the n-AlGaN compositionally graded layer 14, thereby improving electron exchange between the n-AlInN layer 12 and the n-AlGaN compositionally graded layer 14.

[0059] The n-AlGaN compositionally graded layer 14 further includes a first n-GaN layer 15 stacked on the surface of the n-AlGaN compositionally graded layer 14, the first n-GaN layer 15 having a smaller AlN mole fraction than the n-AlInN layer 12 and containing Ga in its composition, and the n-AlGaN compositionally graded layer 14 has a compositionally graded AlN mole fraction that gradually decreases toward the interface with the first n-GaN layer 15. With this configuration, the energy barrier at the interface between the n-AlGaN compositionally graded layer 14 and the first n-GaN layer 15 can be kept small, thereby improving the conductivity between the n-AlGaN compositionally graded layer 14 and the first n-GaN layer 15, thereby improving the exchange of electrons between the n-AlInN layer 12 and the first n-GaN layer 15 (i.e., improving the conductivity).

[0060] The thickness of the cap layer in the stacking direction is greater than 0 and equal to or less than 1 nm. With this configuration, the surface of the n-AlInN layer 12 can be protected well.

[0061] The thickness of the n-AlGaN compositionally graded layer 14 in the stacking direction is 4 nm or more and 15 nm or less. This configuration makes it possible to achieve both a low differential resistance and high reflectivity.

[0062] The method for manufacturing a nitride semiconductor light-emitting device is a method for manufacturing a nitride semiconductor light-emitting device using metal-organic chemical vapor deposition, and includes the following steps: a first layer lamination step of growing an n-AlGaN compositionally graded layer (14) containing Al and In as its composition; a cap layer lamination step of growing a GaN cap layer (13) containing Ga as its composition on the surface of the n-AlGaN compositionally graded layer (14) after the first layer lamination step; a second layer lamination step of growing an n-AlGaN compositionally graded layer (14) containing Al and Ga as its composition on the surface of the GaN cap layer (13) after the cap layer lamination step; and a third layer lamination step of growing a first n-GaN layer (15) containing Ga as its composition on the surface of the n-AlGaN compositionally graded layer (14) after the second layer lamination step. The temperature for growing the n-AlGaN compositionally graded layer (14) in the second layer lamination step is higher than the temperature for growing the first n-GaN layer (15) in the third layer lamination step. This configuration can activate Al migration in the second layer stacking step, making it easier to flatten the surface of the n-AlGaN composition gradient layer 14, leading to improved quality in the first n-GaN layer 15 that is subsequently grown as a crystal.

[0063] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is not limited to the embodiments disclosed herein, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. (1) Unlike the above embodiment, Ge, Te, etc. may be used as the n-type impurity. (2) Unlike the above embodiment, crystal growth may be performed using other substrates such as a sapphire substrate. (3) Unlike the above-described embodiment, the GaN cap layer may contain elements other than GaN. (4) The crystal growth temperatures in the second layer lamination process and the third layer lamination process are not limited to those disclosed in the above examples. Furthermore, the crystal growth temperature in the second layer lamination process may be the same as the crystal growth temperature in the third layer lamination process. In other words, the crystal growth temperature in the second layer lamination process may be equal to or higher than the crystal growth temperature in the third layer lamination process. (5) Unlike the above examples, the first n-GaN layer (third layer) may contain AlN at a molar fraction lower than that of the n-AlInN layer (first layer). In other words, the third layer may be a layer stacked on the surface of the n-AlGaN compositionally graded layer (second layer), with a lower AlN molar fraction than that of the first layer, and containing Ga in its composition. [Explanation of symbols]

[0064] 1...Nitride semiconductor light emitting device 12...n-AlInN layer (1st layer) 13...GaN cap layer (cap layer) 14...n-AlGaN composition gradient layer (second layer) 15...1st n-GaN layer (3rd layer)

Claims

1. a first layer containing Al and In in its composition; a cap layer stacked on a surface of the first layer and containing Ga; a second layer stacked on the surface of the cap layer and containing Al and Ga; Equipped with a mole fraction of AlN at the interface of the second layer stacked on the surface of the cap layer is 0.36 or more and 0.44 or less; The nitride semiconductor light emitting device, wherein the thickness of the cap layer in the stacking direction is greater than 0 and equal to or less than 1 nm.

2. A first layer containing Al and In in its composition; a cap layer stacked on a surface of the first layer and containing Ga; a second layer stacked on the surface of the cap layer and containing Al and Ga; Equipped with a mole fraction of AlN at the interface of the second layer stacked on the surface of the cap layer is 0.36 or more and 0.44 or less; the first layer, the cap layer, and the second layer are nitride semiconductors; The cap layer is a nitride semiconductor light emitting device made of Ga and N.

3. a third layer stacked on a surface of the second layer, the third layer having a mole fraction of AlN smaller than that of the first layer and containing Ga in its composition; 3. The nitride semiconductor light-emitting device according to claim 1, wherein the mole fraction of AlN in the second layer has a composition gradient that gradually decreases toward the interface with the third layer.

4. 3. The nitride semiconductor light-emitting device according to claim 1, wherein the second layer has a thickness in the stacking direction of 4 nm or more and 15 nm or less.

5. A method for manufacturing a nitride semiconductor light-emitting device using metalorganic chemical vapor deposition, comprising: a first layer lamination step of growing a crystal of a first layer containing Al and In; a cap layer lamination step of growing a cap layer containing Ga as a composition on the surface of the first layer after the first layer lamination step is performed; a second layer lamination step of growing a second layer containing Al and Ga as a composition on the surface of the cap layer after the cap layer lamination step is performed; a third layer lamination step of growing a third layer containing Ga as a composition on a surface of the second layer after the second layer lamination step is performed; Equipped with a temperature at which the second layer is crystal-grown in the second layer laminating step is equal to or higher than a temperature at which the third layer is crystal-grown in the third layer laminating step; the first layer, the cap layer, and the second layer are nitride semiconductors; The cap layer is made of Ga and N.

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