JP2011187591A - Nitride semiconductor ultraviolet light-emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor ultraviolet light-emitting element that suppresses a degradation in light emission efficiency by relaxing an internal electric field generated in an active layer without depending upon In composition modulation effect. <P>SOLUTION: At least an n-type clad layer 6 made of an n-type AlGaN-based semiconductor, the active layer 7 of an AlGaN-based semiconductor having a quantum well structure of one or more layers, and a p-type clad layer 9 made of a p-type AlGaN-base semiconductor are arranged in order on a surface of a substrate or on a template 5 formed of one or more AlGaN-based semiconductor layers formed on a surface of a substrate, and composition modulation is applied to an Al composition ratio at least in one well layer 7b of the active layer 7 so as to decrease band gap energy from the side of the p-type clad layer 9 toward the n-type clad layer 6. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a nitride semiconductor light emitting device used for a light emitting diode, a laser diode or the like, and more particularly to a nitride semiconductor ultraviolet light emitting device having an emission center wavelength of 400 nm or less.

  Conventionally, a GaN-based nitride semiconductor is based on GaN or an AlGaN layer having a relatively low Al composition ratio (AlN molar fraction), and a light-emitting element and a light-receiving element having a multilayer structure have been produced thereon (for example, Non-patent document 1). FIG. 14 shows a crystal layer structure of a typical conventional GaN-based light emitting diode. In the light-emitting diode shown in FIG. 14, an underlying layer 102 made of AlN is formed on a sapphire substrate 101, a periodic groove structure is formed by photolithography and reactive ion etching, and then ELO (Epitaxial Lateral Overgrowth) -AlN. The layer 103 is formed as a template. On the ELO-AlN template 103, an n-type AlGaN n-type cladding layer 104 having a film thickness of 2 μm, an AlGaN / GaN multiple quantum well active layer 105, and an Al composition ratio having multiple quantum well activity. A p-type AlGaN electron blocking layer 106 having a thickness of 20 nm higher than the layer 105, a p-type AlGaN p-type cladding layer 107 having a thickness of 50 nm, and a p-type GaN contact layer 108 having a thickness of 20 nm are sequentially stacked. It has a laminated structure. The multiple quantum well active layer 105 has a structure in which five layers of a structure in which a GaN well layer having a thickness of 2 nm is sandwiched between AlGaN barrier layers having a thickness of 8 nm are stacked. After the crystal growth, the multi-quantum well active layer 105, the electron blocking layer 106, the p-type cladding layer 107, and the contact layer 108 are removed by etching until a partial surface of the n-type cladding layer 104 is exposed. For example, a Ni / Au p-electrode 109 is formed on the surface of the layer 108, and a Ti / Al / Ti / Au n-electrode 110 is formed on the exposed n-type cladding layer 104. Using a GaN well layer as an AlGaN well layer, the emission wavelength is shortened by changing the Al composition ratio and film thickness, or the emission wavelength is lengthened by adding In, and the wavelength is from 200 nm to 400 nm. A light emitting diode in the ultraviolet region can be manufactured. A semiconductor laser can be fabricated with a similar configuration.

  By the way, the nitride semiconductor has a wurtzite type crystal structure and is asymmetric in the c-axis direction, and thus has a strong polarity, and an electric field due to spontaneous polarization is generated in the c-axis direction. A nitride semiconductor is a material having a large piezoelectric effect. For example, in a GaN-based semiconductor grown on a sapphire substrate in the c-axis direction, the outermost surface of the crystal is a nitrogen surface, and compressive strain is applied in a direction parallel to the substrate surface. And an electric field (piezoelectric field) due to piezoelectric polarization is generated in the normal direction of the interface. Here, when considering the case where a light emitting diode having the above-described stacked structure is manufactured by performing crystal growth in the c-axis direction, the well layer of the quantum well active layer includes both sides of the hetero interface between the well layer and the barrier layer. An internal electric field is generated in which the electric field due to the difference in spontaneous polarization and the piezoelectric field due to compressive strain are combined along the same c-axis direction. In the GaN-based nitride semiconductor, due to this internal electric field, the potential of both the valence band and the conduction band decreases from the n-type cladding layer side to the p-type cladding layer side in the well layer of the active layer, as shown in FIG. To do. As a result, in the well layer, electrons are unevenly distributed to the p-type cladding layer side, and holes (holes) are unevenly distributed to the n-type cladding layer side. Since it is separated and recombination is inhibited, the light emission efficiency (internal quantum efficiency) decreases.

  In order to alleviate the decrease in light emission efficiency due to the internal electric field generated in the well layer of the quantum well active layer, about 4% or more of In (indium) is added to the AlGaN-based nitride semiconductor to form a quaternary mixed crystal. In the crystal growth process, there is a method of utilizing an effect (In composition modulation effect) in which fluctuation of the composition in which the In composition is non-uniformly distributed in the order of nm occurs (see Non-Patent Document 2 below). By forming the quantum well active layer as an InAlGaN quaternary mixed crystal, the above-mentioned In composition modulation effect results in non-uniformly dispersed regions with low energy potential (sites with high In concentration) where electrons and holes are easily trapped, In spite of the presence of the internal electric field, it is known that the luminous efficiency does not significantly decrease.

Kentaro Nagamatsu, et al. "High-efficiency AlGaN-based UV light-emitting diode on laterally overgrown AlN", Journal of Crystal Growth, 2008, 310, pp. 199 2326-2329 “Appearance of high-power light-emitting diode that emits deep ultraviolet light at 10 mW, suitable for sterilization applications”, [online], RIKEN, Matsushita Electric Works, Ltd. [searched September 5, 2008], Internet <URL : Http:https://www.riken.jp/r-world/info/release/press/2008/080704/detail.html>

  In the case of a nitride semiconductor ultraviolet light emitting device having an emission center wavelength of 400 nm or less, if In is added to the quantum well active layer by about several percent or more, it is necessary to increase the Al composition ratio to form an InAlGaN quaternary mixed crystal. In general, it is known that a quaternary mixed crystal has difficulty in stable crystal growth compared to a ternary mixed crystal. This is because the crystal growth temperature of InN is 800 ° C. or lower, whereas the crystal growth temperature of GaN is 1000 to 1100 ° C., and the crystal growth temperature of AlGaN is 1050 to 1200 ° C. This is because crystal growth becomes unstable.

  Further, as described above, the presence of In makes it necessary to lower the crystal growth temperature of InAlGaN. However, since the growth temperature of the electron blocking layer formed on the active layer is high, in the quantum well active layer, In decomposition occurs, and as a result, the effect of adding In is not sufficiently exhibited. In order to prevent this, it is necessary to provide a GaN or AlGaN layer (called a cap layer) for preventing the decomposition of In between the quantum well active layer and the electron blocking layer. Since this cap layer has a high resistance, the applied voltage is increased to compensate for the voltage drop in the cap layer, and the light emission efficiency is lowered, which is not preferable.

  From the above, when the quantum well active layer is an InAlGaN quaternary mixed crystal, there is a problem of unstable crystal growth and a problem of decomposition of In. It is necessary to solve the decrease in luminous efficiency due to the electric field without depending on the In composition modulation effect.

  The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to nitride semiconductor ultraviolet light that suppresses a decrease in luminous efficiency by relaxing an internal electric field generated in an active layer without relying on an In composition modulation effect. The object is to provide a light emitting element.

  In order to achieve the above object, a nitride semiconductor ultraviolet light emitting device according to the present invention includes at least an n-type AlGaN-based material on a substrate surface or a template formed of one or more AlGaN-based semiconductor layers formed on the substrate surface. An n-type cladding layer made of a semiconductor, an active layer of an AlGaN semiconductor having a single or multiple quantum well structure, and a p-type cladding layer made of a p-type AlGaN semiconductor are arranged in order, and at least of the active layers A composition modulation with respect to the Al composition ratio is provided in one well layer so that the band gap energy decreases from the p-type cladding layer side toward the n-type cladding layer.

  Furthermore, in the nitride semiconductor ultraviolet light-emitting device having the first feature, it is preferable that the active layer has a quantum well structure having three or less layers.

  Furthermore, in the nitride semiconductor ultraviolet light emitting device having the above characteristics, when the active layer has a multiple quantum well structure of two or more layers, the composition modulation is provided at least in the well layer closest to the p-type cladding layer. It is preferable.

  Furthermore, in the nitride semiconductor ultraviolet light emitting device having the above characteristics, an electronic block made of a p-type AlGaN-based semiconductor having an Al composition ratio higher than that of the active layer and the p-type cladding layer between the active layer and the p-type cladding layer. It is preferred that the layers are arranged.

  Furthermore, in the nitride semiconductor ultraviolet light emitting device having the above characteristics, composition modulation with respect to the Al composition ratio is provided in the electron blocking layer so that band gap energy increases from the p-type cladding layer side toward the n-type cladding layer. It is preferable that

  According to the nitride semiconductor ultraviolet light-emitting device having the above characteristics, the potential gradient descending toward the p-type cladding layer on both sides of the valence band and the conduction band in the well layer of the active layer is on the conduction band side. Relaxed and steeper on the valence band side. As a result, electrons injected from the n-type clad layer side into the active layer are relaxed and dispersed in the well layer of the active layer and localized in the p-type clad layer side. , Recombination with holes localized on the n-type cladding layer side is likely to occur, and the internal quantum efficiency is improved. As a result, it is possible to suppress a decrease in light emission efficiency due to the internal electric field without making the quantum well active layer an InAlGaN quaternary mixed crystal.

  The effect of the present invention is also exhibited in the case of an InAlGaN quaternary mixed crystal, and is therefore effective in an InAlGaN quaternary mixed crystal. For example, when the In composition ratio is small and the above In composition modulation effect is small, it is an effective means for relaxing the internal electric field.

  By the way, since electrons are higher in mobility of electrons and holes, electrons injected from the n-type cladding layer side are injected from the p-type cladding layer side in the well layer of the active layer. It exists more abundantly than holes. In other words, abundantly dispersed electrons promote efficient recombination with holes.

  On the other hand, contrary to the above feature, when compositional modulation with respect to the Al composition ratio is provided so that the band gap energy increases from the p-type cladding layer side toward the n-type cladding layer, in the well layer of the active layer The slope of the potential falling toward the p-type cladding layer on both sides of the valence band and the conduction band becomes steeper on the conduction band side and relaxed on the valence band side. Then, abundant electrons are localized and a small number of holes are dispersed, so that the number of holes used for recombination is reduced and the internal quantum efficiency is decreased.

  In addition, as described above, electrons are more abundant in the well layer of the active layer and have higher mobility, so that recombination of electrons and holes is actively generated in the active layer close to the p-type cladding layer. Thus, even if the active layer has a quantum well structure of four or more layers, the number of well layers involved in light emission is about three layers close to the p-type cladding layer. Therefore, when the active layer has a quantum well structure of three or less layers, the effect of providing compositional modulation of the Al composition ratio in the well layer is sufficiently exhibited. In that case, the effect of suppressing the internal electric field is better exhibited by providing compositional modulation of the Al composition ratio in the well layer closest to the p-type cladding layer.

  In the nitride semiconductor ultraviolet light emitting device having the above characteristics, an electron block layer made of a p-type AlGaN-based semiconductor having an Al composition ratio higher than that of the active layer and the p-type cladding layer is disposed between the active layer and the p-type cladding layer. Thus, the electron blocking layer becomes an energy barrier against electrons injected from the n-type cladding layer side, and the electrons injected into the active layer are prevented from overflowing to the p-type cladding layer side, As a result, it is possible to prevent the light emission efficiency from being lowered.

  Here, the electron blocking layer effectively suppresses the electrons injected into the active layer from overflowing to the p-type cladding layer side. On the other hand, in order to suppress the electron overflow, the electron blocking layer is more Al than the active layer. It is necessary to increase the band gap energy by increasing the composition ratio. However, since the Al composition ratio of the electron block layer increases as the emission center wavelength becomes shorter, it becomes difficult to activate the p-type impurity. Since the resistance is increased without obtaining a high acceptor concentration, the threshold voltage increases due to a voltage drop in the electron block layer, leading to a decrease in light emission efficiency.

  Therefore, by providing composition modulation to the Al composition ratio in the electron blocking layer so that the band gap energy increases from the p-type cladding layer side to the n-type cladding layer, The efficiency of injection into the active layer can be increased, the effect of increasing the resistance of the electron blocking layer can be reduced, and the light emission efficiency can be increased.

  DESCRIPTION OF EMBODIMENTS Embodiments of a nitride semiconductor ultraviolet light emitting device according to the present invention (hereinafter referred to as “the present device” as appropriate) will be described with reference to the drawings.

<First Embodiment>
An example of the structure and manufacturing method of the element of the present invention will be described assuming that the element of the present invention is a light emitting diode.

  As shown in FIG. 1, the element 1 of the present invention has an ELO-AlN layer 4 grown on a sapphire substrate 2 after a periodic groove structure is formed with an underlayer 3 made of AlN by photolithography and reactive ion etching. Using the prepared substrate as the template 5, an n-type cladding layer 6 made of n-type AlGaN having a thickness of 2 μm, a multiple quantum well active layer 7, and a film having an Al composition ratio higher than that of the multiple quantum well active layer 105 on the template 5. A p-type AlGaN electron blocking layer 8 having a thickness of 20 nm, a p-type cladding layer 9 having a thickness of 50 nm, and a contact layer 10 having a thickness of 20 nm are sequentially stacked. Reactive ions until a part of the surface of the n-type cladding layer 6 is exposed in a part of the multiple quantum well active layer 7, the electron blocking layer 8, the p-type cladding layer 9, and the contact layer 10 above the n-type cladding layer 6. For example, a Ni / Au p-electrode 11 (anode electrode) is removed on the surface of the contact layer 10 by etching or the like, and an exposed n-type cladding layer 6 surface is formed of, for example, Ti / Al / Ti / Au. An n-electrode 12 (cathode electrode) is formed. The element structure shown in FIG. 1 is the same as that of the conventional light emitting diode shown in FIG. 14 except for the internal structure of the multiple quantum well active layer 7. Therefore, the element 1 of the present invention is characterized by the internal structure of the multiple quantum well active layer 7.

  As shown in FIG. 2, the multiple quantum well active layer 7 of the present embodiment is laminated with a barrier layer 7a made of AlGaN so that the Al composition ratio (AlN molar fraction) does not exceed the Al composition ratio of the barrier layer 7a. The well layers 7b made of AlGaN modulated along the direction are alternately stacked, and the well layer 7b has a multiple quantum well structure sandwiched between the barrier layers 7a. More specifically, the barrier layer 7a is made of AlGaN with a film thickness of 8.5 nm and an Al composition ratio of 35%, and the well layer 7b has a film thickness of 3 nm, as shown in FIG. Composition modulation is provided for the Al composition ratio so that the band gap energy monotonously decreases from the end face on the p-type cladding layer 9 side to the end face on the n-type cladding layer 6 side. For example, the Al composition ratio of the well layer 7b is 0 to 5% (GaN in the case of 0%) at the end surface on the n-type cladding layer 6 side, and monotonically increases from there to 5 to 5 at the end surface on the electron blocking layer 8 side. It reaches about 15%. As a result, the potential gradient (see FIG. 15) that decreases from the n-type cladding layer side to the p-type cladding layer side in the valence band and the conduction band in the well layer 7b is relaxed in the conduction band, and the valence band. The side becomes steeper. However, by reducing the potential gradient in the conduction band, the spatial distribution of electrons in the well layer 7b becomes broad, and compared with the case where there is no composition modulation with respect to the Al composition ratio (see FIG. 15), Hole recombination becomes difficult to be inhibited, and luminous efficiency (internal quantum efficiency) is improved. The number of well layers 7b is not particularly limited, but is preferably 3 or less as will be described later. Further, the thickness and Al composition ratio of the barrier layer 7a and the modulation range (upper limit and lower limit) of the thickness and Al composition ratio of the well layer 7b are appropriately set according to the emission wavelength (emission center wavelength). Change it.

  The n-type AlGaN of the n-type clad layer 6 and the p-type AlGaN of the p-type clad layer 9 are set such that each Al composition ratio is higher than the band gap energy of the multiple quantum well active layer 7. , About 15% to 20%.

  The electron block layer 8 is provided to suppress carrier overflow from the multiple quantum well active layer 7 to the p-type cladding layer 9, and the band gap energy thereof is determined by the multiple quantum well active layer 7 and the p-type cladding layer 9. The Al composition ratio is set so as to be higher than the band gap energy of, for example, about 35%.

  As described above, the element 1 of the present invention is characterized in that the internal structure of the multiple quantum well active layer 7, that is, the AlGaN constituting the well layer 7 b is subjected to composition modulation of the Al composition ratio. The manufacturing method of the element 1 of the present invention is the same as the manufacturing method of the conventional AlGaN-based light emitting diode except for the step of forming the well layer 7b of the multiple quantum well active layer 7, and the manufacturing method of the known template 5, It can be manufactured using an AlGaN film forming method.

  Hereinafter, the manufacturing method of the element 1 of the present invention will be described. First, for example, on a template 5 produced by a known manufacturing method disclosed in Non-Patent Document 1 above, by a reduced pressure metal organic compound vapor phase growth (MOVPE) method or a molecular beam epitaxy (MBE) method, n The type cladding layer 6, the multiple quantum well active layer 7, the electron block layer 8, the p-type cladding layer 9, and the contact layer 10 are continuously grown. In Non-Patent Document 1, the barrier layer of the multiple quantum well active layer 7 is doped with Si to reduce the internal electric field in the quantum well. In the present embodiment, however, the inside of the well layer 7b Since this is not necessary due to the compositional modulation of the Al composition ratio, Si doping is not performed.

  In the MOVPE method, the compositional modulation of the Al composition ratio in the well layer 7b of the multiple quantum well active layer 7 is performed by controlling the supply flow ratio of the source gas of each composition constituting the AlGaN. Controllability can be improved by suppressing the growth rate to 0.1 μm / h or less. For example, when a well layer 7b having a thickness of 3 nm is grown at a growth rate of 90 nm / h, the growth time of one well layer 7b is 2 minutes. While the response time of the mass control flow meter used for flow rate control is about 3 seconds, the growth time is sufficiently long as 2 minutes, so that composition modulation can be controlled. Note that the compositional modulation of the Al composition ratio in the well layer 7b is performed by controlling the flux intensity ratio in the case of the MBE method.

  Next, light emission characteristics of the element 1 of the present invention in which the composition modulation is applied to the well layer 7b of the multiple quantum well active layer 7 and a conventional light emitting diode in which the well layer 7b of the multiple quantum well active layer 7 is not subjected to composition modulation. Are compared based on the simulation result of the light emission characteristics.

4 and 5, a conventional light emitting diode in which the well layer 7b is GaN and the Al composition ratio of the well layer 7b are 5% from the end face on the n-type cladding layer 6 side to the end face on the electron block layer 8 side. The light emission intensity (unit: 1 / (cm ² · s) of the three types of the element 1 of the present invention that linearly and monotonously increases in three ways of 10%, 2.5% to 12.5%, and 0% to 15%. (Nm))) and frequency characteristics between the internal quantum efficiency and the forward applied voltage (unit: V). In the figure, the characteristic curve of the conventional light emitting diode (Al composition ratio of the well layer 7b is 7.5%) is indicated by a broken line, and the characteristic curves of the three types of the element 1 of the present invention correspond to the increments of the Al composition ratio, respectively. And 5%, 10%, and 15% labels. The three types of the present invention element 1 and the conventional light emitting diodes all have a common average value of 7.5% of the Al composition ratio of the well layer 7b in order to facilitate mutual comparison.

Further, in FIGS. 6 and 7, as a comparative example, the compositional modulation of the Al composition ratio of the well layer 7 b was performed in the opposite direction, that is, the Al composition ratio of the well layer 7 b was n from the end face on the electron blocking layer 8 side. Three types of comparative samples that linearly and monotonously increase in three ways of 5% to 10%, 2.5% to 12.5%, and 0% to 15% toward the end surface on the mold cladding layer 6 side, Frequency characteristics of conventional light-emitting diodes (Al composition ratio of well layer 7b: 7.5%) (unit: 1 / (cm ² · s · nm)), internal quantum efficiency and forward applied voltage (units) : V). In the figure, the characteristic curve of the conventional light emitting diode is indicated by a broken line, and the signs of Δ5%, Δ10%, and Δ15% are respectively added to the characteristic curves of the three types of comparative samples according to the increment of the Al composition ratio. It distinguishes by attaching.

In each of the above simulations, the thickness and AlN composition ratio of each layer other than the well layer 7b are the values described for the element structure shown in FIGS. The same compositional modulation is applied to the well layer 7b. In the simulation of light emission intensity, the current density J of the current flowing through the multiple quantum well active layer 7 was set to a constant value (50 A / cm ² ).

  As is clear from the simulation results of FIGS. 4 and 5, both the light emission intensity and the internal quantum efficiency are improved as compared with the conventional light emitting diodes when the AlN composition ratio increment is 5% to 15%. I understand that. It can also be seen that the improvement effect is increased by increasing the increment of the AlN composition ratio due to the composition modulation. In addition, FIG. 4 shows that since the average value of the AlN composition ratio is unified to 7.5%, the emission center wavelength is the same regardless of the increment of the AlN composition ratio.

  On the other hand, as is apparent from the simulation results of FIGS. 6 and 7, in the comparative sample in which the compositional modulation of the Al composition ratio of the well layer 7b is performed in the reverse direction, the larger the increment of the AlN composition ratio, the more the emission intensity becomes the conventional one. Although it is lower than the light emitting diode and the internal quantum efficiency is improved in a part of the forward applied voltage range (3 V to 4 V), the internal quantum efficiency is decreased on the high voltage side of 4 V or higher. The voltage dependency on the forward applied voltage is high. Therefore, it can be seen that the same improvement effect as that of the element 1 of the present invention cannot be obtained with the comparative sample subjected to the composition modulation in the reverse direction. In addition, in the comparative sample, although the average value of the AlN composition ratio was unified to 7.5%, when the increment of the AlN composition ratio was increased, the emission center wavelength was shifted to the higher wavelength side, and the emission wavelength Lowering the wavelength is hindered.

  Next, the results of studying the number and position of the well layers 7b subjected to compositional modulation of the Al composition ratio among the plurality of well layers 7b of the multiple quantum well active layer 7 will be described with reference to FIGS. . FIG. 8 shows the characteristics between the internal quantum efficiency and the forward applied voltage when the number of well layers 7b is 3 when the number of well layers 7b is 0 to 3. The number of well layers 7b subjected to compositional modulation is 0, which represents a conventional light emitting diode. The number of well layers 7b subjected to compositional modulation is sequentially increased by one from the well layer 7b on the electron block layer 8 side. FIG. 9 shows the internal quantum efficiency in the case where the number of well layers 7b is 3, and the position of one well layer 7b subjected to composition modulation is the first, second, and third positions from the electron block layer 8 side. The characteristic between forward applied voltages is shown. Further, FIG. 9 shows a characteristic curve (shown by a broken line) of a conventional light emitting diode as a reference example. FIG. 10 shows the characteristics between the internal quantum efficiency and the forward applied voltage when the composition modulation is applied to all the well layers 7b when the number of the well layers 7b is 1 to 4. FIG. 11 shows that each of the first, second, third, and fourth well layers 7b from the electron block layer 8 side when the number of well layers 7b is four and the composition modulation is applied to all the well layers 7b. And the frequency characteristics of the emission intensity from the entire multiple quantum well active layer 7 are shown. Numbers 1 to 4 in FIG. 11 indicate the order of each well layer 7b from the electron block layer 8 side. 8 to 11, the compositional modulation of the Al composition ratio is linearly changed from 0% to 15% from the end surface on the n-type cladding layer 6 side to the end surface on the electron blocking layer 8 side. The case where it increases monotonously is assumed. Other conditions are the same as those of the simulation having the same characteristics shown in FIG. 5, and a duplicate description is omitted.

  From the simulation results of FIG. 8, the larger the number of well layers 7b subjected to compositional modulation, the higher the internal quantum efficiency, and the effect of applying compositional modulation even when the number of well layers 7b subjected to compositional modulation is 1. I understand that there is. From the simulation results of FIG. 9, it can be seen that the effect of improving the internal quantum efficiency is greater when the position of the well layer 7b subjected to composition modulation is closer to the electron block layer 8 side, that is, the p-type cladding layer 9 side. 8 and 9, when there are a plurality of well layers 7b and all the well layers 7b are subjected to compositional modulation, the degree of compositional modulation is increased toward the p-type cladding layer 9 side. Further, it may be changed according to the position of the well layer 7b. In this case, it is preferable to set the average Al composition ratio of the plurality of well layers 7b subjected to composition modulation to be equal between the well layers 7b.

  Further, from the simulation result of FIG. 10, it is understood that the number of the well layers 7b subjected to the composition modulation is sufficient to be 3 or less. Furthermore, the simulation result of FIG. 10 shows that the number of well layers 7b subjected to composition modulation is preferably 2 or more. However, in comparison with the case where the number of well layers 7b in the simulation results of FIG. It can be seen that the internal quantum efficiency is higher in the case. From this, it can be seen that even if the number of well layers 7b is 1, that is, a single quantum well structure, the composition modulation is effective.

  Furthermore, from the simulation results of FIG. 11, when the well layer 7b has four layers, the emission intensity distribution of each layer is concentrated in the three layers closer to the p-type cladding layer 9 side, and from the p-type cladding layer 9 side. It can be seen that the emission intensity distribution of the fourth well layer 7b is very small compared to the other three layers. This result also shows that the number of well layers 7b does not need to increase beyond four.

  As described above, from the simulation results of FIGS. 8 to 11, the active layer structure is preferably a multiple quantum well structure, but may be a single quantum well structure, and the number of well layers 7b is increased to four or more. However, the effect of applying the composition modulation does not increase greatly, and in the case of the multiple quantum well structure, even the single well layer 7b to which the composition modulation is performed is effective, and the side closer to the p-type cladding layer 9 is most effective. It became clear that the effect was great.

Second Embodiment
Next, a second embodiment of the element of the present invention will be described on the assumption that the element of the present invention is a light emitting diode. In the first embodiment, the Al composition ratio of the electron blocking layer 8 is within the film thickness in order to modulate the Al composition ratio with respect to the well layer 7b of the multiple quantum well active layer 7 and suppress the carrier overflow. It was constant. In contrast, in the second embodiment, the electronic block layer 8 is also subjected to compositional modulation of the Al composition ratio. Specifically, compositional modulation is provided for the Al composition ratio so that the band gap energy increases from the end face on the p-type cladding layer 9 side to the end face on the multiple quantum well active layer 7 side. For example, the Al composition ratio of the electron blocking layer 8 is 20% at the end face on the p-type cladding layer 9 side, and monotonically increases from there to reach about 50% at the end face on the multiple quantum well active layer 7 side. Note that the average value of the Al composition ratio is set to be the same value as when no composition modulation is performed. Since the Al composition ratio of the electron block layer 8 is the same as that of the first embodiment except that the composition is modulated, the redundant description of the other layers and the manufacturing method is omitted. The composition modulation of the electron block layer 8 may be performed in the same manner as the composition modulation of the well layer 7b of the multiple quantum well active layer 7.

Next, after the composition modulation of the Al composition ratio is performed on the well layer 7b of the multiple quantum well active layer 7, the effect of the composition modulation of the Al composition ratio on the electron block layer 8 is further examined. The results obtained will be described with reference to FIGS. 12 and 13, when the number of well layers 7 b is 3 and the composition modulation is applied to all the well layers 7 b, the composition ratio of the Al composition ratio is applied to the electron block layer 8 (first order). 2 embodiment) and the frequency characteristics of the light emission intensity (unit: 1 / (cm ² · s · nm)) of the element 1 of the present invention when not performed (first embodiment), the internal quantum efficiency and the forward applied voltage ( The characteristic between unit: V) is shown respectively. 12 and 13, the compositional modulation of the Al composition ratio of the well layer 7b is 0% to 15% from the end surface on the n-type cladding layer 6 side to the end surface on the electron block layer 8 side. It is assumed that the compositional modulation of the Al composition ratio of the electron block layer 8 increases linearly monotonically from 20% to 50%. Other conditions are the same as those of the simulation having the same characteristics shown in FIG. 5, and a duplicate description is omitted.

  From the simulation results of FIGS. 12 and 13, in addition to the compositional modulation of the Al composition ratio for the well layer 7 b of the multiple quantum well active layer 7, the compositional modulation of the Al composition ratio is performed for the electron blocking layer 8. It can be seen that the internal luminous efficiency is further improved.

Hereinafter, another embodiment will be described.
<1> In the first and second embodiments described above, it is assumed that the element of the present invention is a light emitting diode. However, a semiconductor laser (laser diode) also has a similar stacked structure, and has a p-electrode and By applying a voltage between the n-electrodes, the principle until electrons and holes are injected into the active layer and recombined to emit light is the same. Therefore, a multiple quantum well structure or a single quantum well is used. It is clear that the effect of applying compositional modulation to the Al composition ratio of the well layer of the active layer of the structure is the same. Therefore, the element of the present invention is not limited to a light emitting diode, but is also applicable to a semiconductor laser.

  <2> In the first and second embodiments, the ELO-AlN template shown in FIG. 1 is taken as an example of the template constituting the element of the present invention, but is limited to the template ELO-AlN used for the element of the present invention. It is not a thing. Furthermore, the film thickness and Al composition ratio of each layer of AlGaN or GaN constituting the element of the present invention exemplified in the first and second embodiments are examples, and can be appropriately changed according to the specifications of the element.

  <3> In the first embodiment, it is assumed that the electron block layer 8 is provided. However, the electron block layer 8 is not necessarily provided. Regardless of the presence or absence of the electron block layer 8, the effect of applying compositional modulation to the Al composition ratio of the well layer of the active layer having the multiple quantum well structure or the single quantum well structure is exhibited.

  <4> The element of the present invention is intended to solve the problem when the quantum well active layer is an InAlGaN quaternary mixed crystal containing In. However, a small amount of In that does not sufficiently exhibit the In composition modulation effect is present. Even if it is included, the effect of applying the composition modulation to the Al composition ratio of the well layer of the active layer of the multiple quantum well structure or the single quantum well structure is similarly exhibited, so that the element of the present invention is configured. It is not excluded that a trace amount of In is contained in the AlGaN-based semiconductor layer.

  The nitride semiconductor ultraviolet light-emitting device according to the present invention can be used for a light-emitting diode, a laser diode, or the like having an emission center wavelength of 400 nm or less.

Sectional drawing which shows typically the general | schematic laminated structure of the nitride semiconductor ultraviolet light emitting element concerning this invention Sectional drawing which shows typically the cross-section of the multiple quantum well active layer of the nitride semiconductor ultraviolet light emitting element shown in FIG. Energy band diagram of multiple quantum well active layer of nitride semiconductor ultraviolet light emitting device shown in FIG. The characteristic diagram which compares and compares the frequency characteristic of the emitted light intensity of the nitride semiconductor ultraviolet light emitting element which concerns on this invention, and the conventional light emitting diode The characteristic view which compares the characteristic between the internal quantum efficiency of the nitride semiconductor ultraviolet light emitting element which concerns on this invention, and the conventional light emitting diode, and the forward applied voltage The characteristic view which compares and compares the frequency characteristic of the light emission intensity of the comparative sample which gave the composition modulation different from the nitride semiconductor ultraviolet light emitting element which concerns on this invention, and the conventional light emitting diode The characteristic diagram which compares and compares the characteristic between the internal quantum efficiency of the comparative sample which gave the composition modulation different from the nitride semiconductor ultraviolet light emitting element which concerns on this invention, and the conventional light emitting diode, and a forward applied voltage The figure which shows the relationship between the number of well layers which perform a composition modulation | alteration, the internal quantum efficiency, and the characteristic between forward applied voltages in the case where the number of well layers in the nitride semiconductor ultraviolet light emitting element which concerns on this invention is 3 The figure which shows the relationship between the position of the well layer which performs compositional modulation | alteration, the internal quantum efficiency, and the characteristic between forward applied voltages in the nitride semiconductor ultraviolet light emitting element which concerns on this invention The figure which shows the relationship between the number of layers of a well layer at the time of performing composition modulation | alteration in all the well layers in the nitride semiconductor ultraviolet light emitting element which concerns on this invention, an internal quantum efficiency, and the characteristic between forward applied voltages. The figure which shows the frequency characteristic of the emitted light intensity from each well layer in the multiple quantum well active layer in the nitride semiconductor ultraviolet light emitting element which concerns on this invention. FIG. 7 is a characteristic diagram comparing the frequency characteristics of the emission intensity with and without the compositional modulation of the Al composition ratio for the electron blocking layer in the nitride semiconductor ultraviolet light emitting device according to the present invention. The characteristic view which compares the characteristic between internal quantum efficiency and a forward applied voltage in the case where it does not perform the composition modulation | alteration of Al composition ratio with respect to the electronic block layer in the nitride semiconductor ultraviolet light emitting element concerning this invention Sectional view schematically showing the crystal layer structure of a conventional GaN-based light emitting diode Energy band diagram showing the effect of internal electric field in the well layer of the active layer of a conventional GaN-based light emitting diode

1: Nitride semiconductor ultraviolet light emitting element 2,101: Sapphire substrate 3,102: Underlayer (AlN)
4, 103: ELO-AlN layer 5: Template 6, 104: n-type cladding layer (n-type AlGaN)
7, 105: Multiple quantum well active layer 7a: Barrier layer 7b: Well layer 8, 106: Electron block layer (p-type AlGaN)
9, 107: p-type cladding layer (p-type AlGaN)
10, 108: Contact layer (p-type GaN)
11, 109: p-electrode 12, 110: n-electrode

Claims (5)

At least an n-type cladding layer made of an n-type AlGaN semiconductor, an AlGaN semiconductor having a single or multiple quantum well structure on a substrate surface or a template made of one or more AlGaN semiconductor layers formed on the substrate surface. Active layer and a p-type cladding layer made of a p-type AlGaN-based semiconductor are arranged in order,
A compositional modulation with respect to the Al composition ratio is provided in at least one well layer of the active layer so that the band gap energy decreases from the p-type cladding layer side toward the n-type cladding layer. Nitride semiconductor ultraviolet light emitting device.   The nitride semiconductor ultraviolet light-emitting device according to claim 1, wherein the active layer has a quantum well structure of three or less layers.   3. The composition modulation according to claim 1, wherein when the active layer has a multiple quantum well structure of two or more layers, the composition modulation is provided at least in a well layer closest to the p-type cladding layer. Nitride semiconductor ultraviolet light emitting device.   The electron block layer made of a p-type AlGaN-based semiconductor having an Al composition ratio higher than that of the active layer and the p-type cladding layer is disposed between the active layer and the p-type cladding layer. The nitride semiconductor ultraviolet light-emitting device according to any one of 1 to 3.   5. The compositional modulation with respect to the Al composition ratio is provided in the electron block layer so that a band gap energy increases from the p-type cladding layer side toward the n-type cladding layer. Nitride semiconductor ultraviolet light emitting device.

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