JP6330604B2 - Semiconductor light emitting device - Google Patents

Description

  The present invention relates to a semiconductor light emitting device.

  In general, semiconductor light emitting elements such as light emitting diodes (LEDs) and laser diodes (LDs) are widely used in various light sources used for backlights, lighting, traffic lights, large displays, and the like. In particular, development of deep ultraviolet light-emitting elements using aluminum gallium nitride, which can be expected to be used for sterilization, has been actively conducted.

  However, aluminum gallium nitride has a very large band gap, and the level of doped impurities is formed at a deep energy level in the band gap, causing an increase in the activation energy of carriers. For this reason, even if an impurity is doped, the electrical conductivity of the semiconductor layer is difficult to increase (in other words, it is difficult to make it n-type or p-type). As a result, there has been a problem that the contact resistance between the electrode and the semiconductor layer in contact with the electrode and the surface resistance of the semiconductor layer in contact with the electrode are increased, leading to an increase in the forward drive voltage Vf. In addition, the efficiency of carrier injection into the active layer has also decreased, making it difficult to develop high-power and high-efficiency semiconductor elements.

Japanese Patent Laying-Open No. 2005-235908 JP 2007-149713 A JP 2012-049337 A

  In view of such a problem, the present applicant has developed a structure in which the Al composition ratio in the n-type contact layer is inclined. A schematic diagram of the semiconductor multilayer structure of such a semiconductor light emitting device is shown in FIG. 12 as a reference example. In this structure, the Al composition ratio in the n-type contact layer 410 gradually decreases in the stacking direction so that the Al composition of the n-type contact layer 410, which is Si-doped AlGaN, decreases as it approaches the contact surface with the n-side electrode 440. I am letting. By tilting the Al composition ratio in this way, it is possible to reduce the increase in the surface resistance of the n-type contact layer 410 on the higher Al composition ratio side, while the n-type contact layer on the lower Al composition ratio side. The contact resistance between 410 and the n-side electrode 440 can be effectively reduced.

  However, as shown in FIG. 12, in order to provide the p-side electrode 450 and the n-side electrode 440 on the same surface side of the light emitting element, it is necessary to form a junction surface with the n-side electrode 440 in the n-type contact layer 410. However, in order to expose the n-type contact layer 410, it is necessary to dig a part of the n-type contact layer 410 by etching or the like. In other words, the surface A where the n-type contact layer 410 and the n-side electrode 440 are actually bonded is not the final surface B on which the n-type contact layer 410 has been grown, but is in front of it. For this reason, in the configuration in which the Al composition ratio is gradually reduced in the n-type contact layer 410, the final Al composition ratio is higher than the Al composition ratio in the contact surface A between the n-type contact layer and the n-side electrode. It needs to be set lower by the minute. On the other hand, in order to set the emission wavelength of the light emitting element to a deep ultraviolet region of, for example, 300 nm or less, it is necessary to further increase the Al composition ratio in order to suppress light absorption.

  Therefore, the present invention has been made to solve the conventional problems. An object of the present invention is to provide a semiconductor light emitting device in which light absorption from an active layer by an n-type semiconductor layer (for example, an n-type contact layer) is reduced to improve light extraction efficiency.

In order to achieve the above object, according to a semiconductor light emitting device according to one aspect of the present invention, an n-type semiconductor layer, an active layer provided in a part on the n-type semiconductor layer, and the active layer A p-type semiconductor layer provided on the n-type semiconductor layer, an n-side electrode provided on another part of the n-type semiconductor layer, and a p-side electrode provided on the p-type semiconductor layer. The emission peak wavelength of the light-emitting element is 240 nm or more and 280 nm or less, and the n-type semiconductor layer is formed of Al _x Ga _1-X N (0.60 <X ≦ 0.65) and a high Al composition layer made of Al _Y Ga _1-Y N (0.70 ≦ Y <1) in contact with the low Al composition layer, and the low Al composition layer The thickness of the composition layer is made thinner than the thickness of the high Al composition layer.

  According to the semiconductor light emitting device, the light extraction efficiency can be improved by reducing the absorption of light from the active layer by the n-type semiconductor layer. Furthermore, by ensuring a wide band gap with a high Al composition layer, while suppressing an increase in the surface resistance of the n-type semiconductor layer, a low Al composition layer is interposed on the connection surface with the n-side electrode, thereby making contact resistance Can be lowered.

FIG. 1 is a schematic cross-sectional view showing a semiconductor light emitting device according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing a process of exposing the n-type semiconductor layer of the semiconductor light emitting device shown in FIG. FIG. 3 is a schematic cross-sectional view showing the semiconductor multilayer structure of the semiconductor light emitting device according to Example 1. FIG. 4 is a schematic cross-sectional view showing a semiconductor multilayer structure of a semiconductor light emitting device according to Example 2. FIG. 5 is a schematic cross-sectional view showing a semiconductor multilayer structure of a semiconductor light emitting device according to Example 3. FIG. 6 is a schematic cross-sectional view showing a semiconductor laminated structure of a semiconductor light emitting device according to a reference example. FIG. 7 is a graph showing current-voltage characteristics (IV characteristics) of the semiconductor light emitting devices according to Examples 1 to 3 and the reference example. FIG. 8 is a graph showing the results of measuring the forward voltage Vf (V) and the light intensity VL (au) of the semiconductor light emitting devices according to Examples 1 to 3 and the reference example. FIG. 9 is a schematic cross-sectional view showing a semiconductor multilayer structure of a semiconductor light emitting element according to Comparative Example 1. FIG. 10 is a schematic cross-sectional view showing a semiconductor multilayer structure of a semiconductor light emitting element according to Comparative Example 2. FIG. 11 is a graph showing the results of measuring the forward voltage Vf (V) and the light output Po (mW) of the semiconductor light emitting devices according to Example 1, Comparative Examples 1 and 2, and Reference Example. FIG. 12 is a schematic cross-sectional view showing a semiconductor light emitting device according to Reference Example 2. FIG. 13 is a graph showing the relationship between the semiconductor lattice constant and the band gap.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments and examples according to the present invention will be described with reference to the drawings. However, the following embodiments and examples exemplify semiconductor light emitting elements for embodying the technical idea of the present invention, and the present invention does not specify the semiconductor light emitting elements as follows. Note that the size, positional relationship, and the like of the members shown in each drawing may be exaggerated for clarity of explanation. Furthermore, in the following description, the same name and symbol indicate the same or the same members, and detailed description thereof will be omitted as appropriate. Furthermore, each element constituting the embodiment and the example according to the present invention may be configured such that a plurality of elements are configured by the same member and the plurality of elements are shared by one member. This function can also be realized by sharing a plurality of members. In addition, the contents described in some examples and embodiments may be used in other examples and embodiments. Further, in this specification, the term “upper” as used on the layer or the like is not necessarily limited to the case where the upper surface is formed in contact with the upper surface, but includes the case where the upper surface is formed apart. It is used to include the case where there is an intervening layer between them.

  The present applicant has previously developed a structure in which the composition ratio of Al in the n-type contact layer 410 is inclined so as to gradually decrease toward the active layer 430 as shown in FIG. This configuration is intended to reduce the contact resistance between the n-type contact layer 410 and the n-side electrode 440 to achieve ohmic contact with the n-side electrode 440. However, such a composition gradient technique is premised on that the absorption of light from the active layer does not increase even if the Al composition is lowered below a composition that provides good contact resistance with the n-side electrode. If this technique is applied to a semiconductor laminated body having an emission wavelength in the ultrashort wavelength region, the Al composition cannot be lowered sufficiently from the viewpoint of light absorption, and good contact resistance with the n-side electrode cannot be obtained.

  Therefore, in order to improve the contact resistance with the n-side electrode without reducing the light extraction efficiency in the ultra-short wavelength region of the emission wavelength, the configuration of the semiconductor light emitting device according to this embodiment was examined. In the present specification, the term “ultrashort wave region” refers to a wavelength of about 280 nm or less. Deep ultraviolet light (Deep UV) refers to ultraviolet light having a wavelength of 300 nm or less.

  In order to obtain good contact resistance with the n-side electrode, the n-type contact layer preferably promotes impurity activation by lowering the Al composition. Accordingly, the present inventors have configured a semiconductor having a small band gap only in a portion in contact with the n-side electrode while constituting the majority of the n-type semiconductor layer with a semiconductor layer having a band gap sufficient to transmit the emission wavelength. By inserting a layer, it is possible to promote the activation of impurities at the portion in contact with the n-side electrode. Note that although the light absorption occurs in the semiconductor layer with a small band gap inserted here, the amount of absorption is reduced by reducing the thickness of the insertion layer.

With such a configuration, absorption of light from the active layer by the n-type semiconductor layer can be reduced. Further, by using a semiconductor layer having a large band gap as the n-type semiconductor layer, a semiconductor layer having a small band gap is interposed on the connection surface with the n-side electrode while suppressing an increase in the surface resistance of the n-type semiconductor layer. So we succeeded in reducing the contact resistance.
(Embodiment)

A semiconductor light emitting device 100 according to an embodiment shown in FIG. 1 includes an n-type semiconductor layer 10, an active layer 30 provided in a part on the n-type semiconductor layer 10, and a p-type semiconductor provided on the active layer 30. The layer 20 includes an n-side electrode 40 provided on another part of the n-type semiconductor layer 10, and a p-side electrode 50 provided on the p-type semiconductor layer 20. Note that the n-side electrode 40 does not have to be provided in all regions other than the active layer 30 on the n-type semiconductor layer 10 and may be a region where current can be supplied. For example, Al _a Ga _1-a N (0.35 ≦ a ≦ 0.60) is used for the active layer 30 of this semiconductor light emitting device, and the emission peak wavelength is 240 nm or more and 280 nm or less. The n-type semiconductor layer 10 includes, in order from the active layer 30 side, a low Al composition layer 11 made of Al _x Ga _1-X N (0.60 <X ≦ 0.65) on which the n-side electrode 40 is provided, and a low Al A high Al composition layer 12 made of Al _Y Ga _1-Y N (0.70 ≦ Y <1) in contact with the composition layer 11. The thickness of the low Al composition layer 11 is made thinner than the thickness of the high Al composition layer 12. With such a configuration, while maintaining a wide band gap in the high Al composition layer 12, the low Al composition layer 11 is interposed on the connection surface with the n-side electrode 40, thereby suppressing an increase in surface resistance and contact. The resistance is reduced. Further, by reducing the thickness of the low Al composition layer 11, it is possible to further suppress the absorption of light at this portion and to avoid a decrease in light extraction efficiency. In addition, Al _X Ga _1-X N of the low Al composition layer 11 can significantly reduce the light absorption by making the composition ratio X larger than 0.60, and the composition ratio X should be 0.65 or less. As a result, the contact resistance is lowered and the ohmic contact can be improved. On the other hand, Al _Y Ga _1-Y N of the high Al composition layer 12 has almost no light absorption by the high Al composition layer 12 by setting the composition ratio Y to 0.70 or more and further making the composition ratio Y smaller than 1. The surface resistance can be lowered without increasing.

The light emission peak wavelength of the light emitting element can be set to about 250 nm to 260 nm by using, for example, Al _a Ga _1-a N (0.47 ≦ a ≦ 0.54) for the active layer 30. At this time, the low Al composition layer 11 takes Al _X Ga _1-X N (0.63 ≦ X ≦ 0.65) into consideration, as will be described later, in consideration of light absorption reduction to an emission peak wavelength of about ± 15 nm. It is preferable to do. Furthermore, the thickness of the low Al composition layer 11 is preferably 0.5 μm or more and 1.0 μm or less. Furthermore, it is desirable that the low Al composition layer 11 is located closest to the active layer 30 in the n-type semiconductor layer 10. Thereby, the area | region where light is absorbed can further be reduced compared with the structure which reduces Al composition ratio gradually.

On the other hand, the thickness of the high Al composition layer 12 is preferably 2.0 μm or more and 5.0 μm or less. Thereby, an increase in the surface resistance of the n-type semiconductor layer 10 can be effectively suppressed. Moreover, it is more preferable to set it as 2.0 micrometers or more and 2.5 micrometers or less, and light absorption is further reduced by this. The high Al composition layer 12 preferably has an undoped region 12b that is not doped with an n-type impurity in a region opposite to the side on which the low Al composition layer 11 is provided. In addition, the low Al composition layer 11 and the high Al composition layer 12 preferably have an Al composition ratio that is substantially constant in the stacking direction. In other words, by not changing the Al composition ratio in an inclined manner in the stacking direction, there is an advantage that the semiconductor stacking process can be simplified and the composition ratio can be accurately controlled. In addition, by providing a difference in the Al content between the low Al composition layer 11 and the high Al composition layer 12, in other words, it is possible to lower the contact resistance at once by changing discretely instead of gradually decreasing it. As a result, as in the conventional configuration in which the Al composition ratio is inclined, a certain amount of distance is required until the Al composition ratio is reduced, and the situation where the film thickness increases and the light absorption increases can be avoided. It becomes possible.
(N-side electrode 40)

  The n-side electrode 40 preferably contains Al in a layer connected to the low Al composition layer 11. By containing Al, the contact resistance with the low Al composition layer 11 can be further reduced. At the time of electrode formation, the n-side electrode 40 has a Ti / Al laminated structure in order from the low Al composition layer 11 side, but becomes a layer in which Ti and Al are mixed by annealing. Further, the n-side electrode 40 may be Ti / Al / Ti / Au, Ti / Al / W / Au, or the like.

In the semiconductor light emitting device 100, since most of the constituent elements are highly mixed crystal AlGaN, the buffer layer 2 made of AlN having a certain thickness or more in contact with the upper surface of the specific base substrate 3 (sapphire substrate). The template substrate 1 is provided, and the semiconductor light emitting device 100 is configured in contact with the template substrate 1. If a thick AlGaN layer is grown directly on the template substrate 1, a very strong stress is generated, which induces cracks and the like. For this reason, first, the superlattice layer 14 made of AlN and AlGaN is provided to disperse the stress. The n-type semiconductor layer 10 is formed in contact with the superlattice layer 14, and the layer necessary for deep ultraviolet light emission is formed in contact with the n-type semiconductor layer 10, so that a nitride-free semiconductor layer having good crystallinity and crack-free properties is formed. The body is obtained.
(Absorption wavelength)

Here, the absorption wavelength λp of Al _x Ga _1-x N constituting the n-type semiconductor layer 10 can be calculated from the following equation. First, the relational expression between the band gap and the wavelength is expressed by the following expression.

  E = hc / λ (E: band gap; h: Planck constant; c: speed of light; λ: wavelength)

As shown in FIG. 13, the band gap of AlN is about 6.3 eV, while the band gap of GaN is about 3.4 eV, and during that time (Al _x Ga _1-x N) is considered to change almost linearly. Therefore, here we approximate with a linear function. Therefore, when an arbitrary Al composition is x and a band gap with respect to the Al composition x is y [eV], it is expressed by the following equation.

  Y = 2.9X + 3.4

  Λ can be calculated by substituting the band gap obtained from this linear function into the above equation E = hc / λ. When this linear function is put together into one equation, the emission wavelength (absorption wavelength) λp for an arbitrary composition x is expressed by the following equation.

  λp = (hc) / (2.9X + 3.4)

  In addition, hc in the above equation is approximately 1240, and therefore is represented by the following equation.

  λp = 1240 / (2.9X + 3.4)

  In this equation, for example, when λp = 240 nm, X = 0.6. In other words, for light having a wavelength of 240 nm, absorption is suppressed if the Al composition layer has an Al composition ratio larger than 0.6. it is conceivable that. That is, in the present embodiment where the emission peak wavelength is 240 nm or more and 280 nm or less, the light absorption by the low Al composition layer 11 is greatly reduced by setting the Al composition ratio of the low Al composition layer 11 to be larger than 0.6. This is preferable.

On the other hand, the emission peak has a certain width, for example, about ± 15 nm, and in the case of 240 nm which is the lower limit value of the emission peak wavelength in this embodiment, it is considered to have a wavelength range of 225 nm to 255 nm. . Although the amount of light occupying the entire light having a wavelength of around 225 nm is small, the high Al composition layer 12 is more film than the low Al composition layer 11 in order to effectively suppress an increase in surface resistance of the n-type semiconductor layer 10. The thickness is set to be thick, and it is preferable to consider the possibility of the influence of light absorption. For this reason, it is preferable to set the Al composition ratio of the high Al composition layer 12 to 0.7 or more as in this embodiment, since absorption of light having a wavelength of 225 nm or more can be almost suppressed.
(Example)

  In order to form an n-side electrode in the nitride semiconductor multilayer body, an etching depth ED for exposing the n-type semiconductor layer 10 is set to 1.2 μm (Examples 1 and 2), and an etching depth ED is set to 0. An example (Example 3) having a thickness of 8 μm was produced.

A method for manufacturing the semiconductor light emitting device according to Example 1 will be described below with reference to FIG.
[Step of stacking nitride semiconductor laminate on template substrate]

  First, a template substrate in which a buffer layer made of aluminum nitride having a thickness of 3.5 μm is formed on a base substrate 3 made of sapphire having a diameter of 7.62 cm (3 inches) and having a c-plane on the upper surface is placed in a reaction vessel. A buffer layer made of single crystal aluminum nitride having a thickness of about 0.1 μm is additionally formed using ammonia and trimethylaluminum (TMA) as a source gas. Thus, the thickness of the buffer layer 2 in the template substrate 1 is set to about 3.6 μm.

Subsequently, a layer (layer a) made of Al _0.7 Ga _0.3 N having a thickness of about 27.0 nm is formed using ammonia, TMA, and trimethyl gallium (TMG). Next, the introduction of TMG is stopped, and a layer (layer b) made of AlN having a thickness of about 10.2 nm is formed using ammonia and TMA. Layer a and layer b are alternately and repeatedly formed 30 times to form superlattice layer 14.

Next, the high Al composition layer 12 is formed. First, an undoped Al _0.75 Ga _0.25 N layer is subsequently formed as an undoped region 12b using ammonia, TMA, and TMG. The undoped region 12b has a thickness of 500 nm. Thus, in this specification, the n-type semiconductor layer may include not only a layer that clearly shows the n-type, but also a layer such as an undoped AlGaN layer.

Subsequently, an Si-doped Al _0.75 Ga _0.25 N layer is formed as the Si-doped region 12a using ammonia, TMA, TMG, and monosilane. The thickness of the Si doped region 12a is 1500 nm.

Next, the low Al composition layer 11 is formed. First, a Si-doped Al _0.65 Ga _0.35 N layer is formed using ammonia, TMA, TMG, and monosilane. The thickness of the low Al composition layer 11 is 1000 nm.

Next, the active layer 30 is formed. First, after the low Al composition layer 11 is formed, all gases are temporarily stopped, and the inside of the reaction vessel is adjusted to 970 ° C. and 26.7 kPa (200 Torr). After the adjustment, a layer (layer c) made of Si-doped Al _0.63 Ga _0.37 N having a thickness of about 50.0 nm is formed using ammonia, TMA, triethylgallium (TEG), and monosilane.

Next, the introduction of monosilane is stopped, and a layer (layer d1) made of Al _0.45 Ga _0.55 N having a thickness of about 4.4 nm is formed using ammonia, TMA, and TEG. Subsequently, a layer (layer d2) made of Si-doped Al _0.56 Ga _0.44 N having a thickness of about 2.5 nm is formed using ammonia, TMA, TEG, and monosilane. Layer d1 and layer d2 are alternately and repeatedly formed twice to form layer d. The layer c and the layer d are combined to form an active layer 30. The layer d1 serves as a light emitting layer (emission peak wavelength of about 260 nm).

  Next, the p-type semiconductor layer 20 is formed. After the active layer 30 is formed, all gases are temporarily stopped, and the inside of the reaction vessel is adjusted to 870 ° C. and 13.3 kPa. After the adjustment, a layer (layer e) made of p-type impurity-doped aluminum nitride having a thickness of about 1.0 nm is formed using ammonia and TMA.

Subsequently, a layer (layer f) made of p-type impurity doped Al _0.78 Ga _0.22 N having a thickness of about 4.0 nm is formed using ammonia, TMA, and TMG.

Subsequently, a layer (layer g) made of magnesium-doped Al _0.63 Ga _0.37 N having a thickness of about 78.0 nm is formed using ammonia, TMA, TMG, and biscyclopentadienyl magnesium (Cp ₂ Mg; magnesocene).

Subsequently, ammonia, TMA, TMG, and Cp ₂ Mg are used to form a composition-graded layer (layer h) that is magnesium-doped and in which the Al composition ratio sequentially decreases in the stacking direction from Al _0.6 Ga _0.4 N to GaN. The thickness of the composition gradient layer is 23 nm.

Subsequently, a layer (layer i) made of Mg-doped gallium nitride having a thickness of about 309.0 nm is formed using ammonia, TMG, and Cp ₂ Mg.

Subsequently, a layer (layer j) made of Mg-doped gallium nitride having a thickness of about 14.6 nm is formed using ammonia, TMG, and Cp ₂ Mg.

The layers e to j are collectively referred to as a p-type semiconductor layer 20. In this way, the target nitride semiconductor laminated body was obtained. The layers f to h also serve as an electron barrier layer.
[Step of exposing n-type semiconductor layer 10]

Here, a so-called n extraction process for exposing the n-type semiconductor layer 10 will be described. First, a mask is formed on the semiconductor stacked body so that only a predetermined region corresponding to the n-type semiconductor layer 10 to be exposed is etched. After forming the mask, the semiconductor laminate is put into a dry etching apparatus and the inside of the apparatus is adjusted to 10.5 Pa. After the adjustment, the bias output is set to 250 W, and chlorine and silicon tetrachloride are used to etch about 1.2 μm from the p-type semiconductor layer 20 side as shown in FIG. 2 to expose the n-type semiconductor layer 10. After the etching, the semiconductor stacked body is taken out from the dry etching apparatus and the mask is removed.
[Step of forming n-side electrode 40]

Further, the n-side electrode 40 is formed on the exposed n-type semiconductor layer 10. First, a mask is formed so that only the exposed n-type semiconductor layer 10 is sputtered. After forming the mask, the semiconductor laminate is put into a sputtering apparatus, and the inside of the apparatus is adjusted to 1.0 × 10 ⁻⁴ Pa. After the adjustment, an alloy of titanium and aluminum is sputtered on the exposed n-type semiconductor layer 10 in an argon atmosphere. The output of the high frequency voltage applied to the titanium target is 300 W, and the output of the high frequency voltage applied to the aluminum target is 500 W. After sputtering, the semiconductor stacked body on which the n-side electrode 40 is formed is taken out from the sputtering apparatus. At this time, a plurality of semiconductor stacked bodies are formed on a single wafer, and the individual semiconductor stacked bodies share the same n-type semiconductor layer 10.

  The structure of the semiconductor stacked body stacked as described above can change the film thickness and Al composition ratio of the low Al composition layer 11 and the high Al composition layer 12 in the n-type semiconductor layer. Here, the schematic diagram of the semiconductor laminated structure of the semiconductor light-emitting device according to Example 1 is shown in FIG. In the semiconductor light emitting device shown in this figure, the etching depth ED is 1.2 μm, the thickness of the inserted low Al composition layer 11 is 1 μm, the thickness of the high Al composition layer 12 is 2 μm, and the n-type semiconductor layer 10 is formed. A two-layer structure is adopted.

  Here, as the film thickness of the n-type semiconductor layer 10 increases, the surface resistance can be expected to decrease. However, if the film thickness is excessively increased due to stress, the crystallinity may be deteriorated. Therefore, the total film thickness was set to 3 μm of the conventional structure, and the film thickness of the low Al composition layer 11 to be inserted was changed.

In the example of FIG. 3, the low Al composition layer 11 is Al _0.65 Ga _0.35 N with an Al composition ratio of 65%, and the high Al composition layer 12 is Al _0.75 Ga _0.25 N with an Al composition ratio of 75%. is there. In the following Examples 2, 3 and Reference Examples, the Al composition ratio is the same as that of Example 1 unless otherwise specified.

  FIG. 4 shows a schematic diagram of the semiconductor multilayer structure of the semiconductor light emitting device according to Example 2. As shown in FIG. The semiconductor light emitting device shown in this figure is formed by inserting an n-type semiconductor layer 10 made of AlGaN and inserting a low Al composition layer 11 between two high Al composition layers, a Si doped region 12a and an undoped region 12b. It has a layer structure. In this semiconductor light emitting device, the etching depth ED is 1.2 μm, the thickness of the low Al composition layer 11 to be inserted is 0.8 μm, and the thickness of the undoped region 12 b and the Si doped region 12 a which are the high Al composition layer 12 is set. The n-type semiconductor layer 10 has a three-layer structure with a thickness of 2 μm and 0.2 μm, respectively. Here, the Al composition ratio of each layer was 75% in the Si doped region 12a, 65% in the low Al composition layer 11, and 75% in the undoped region 12b.

FIG. 5 shows a schematic diagram of a semiconductor multilayer structure of a semiconductor light emitting device according to Example 3. As shown in FIG. The semiconductor light emitting device shown in this figure has an etching depth ED = 0.8 μm, the thickness of the inserted low Al composition layer 11 is 0.8 μm, the thickness of the high Al composition layer 12 is 2.2 μm, and the n-type. The semiconductor layer 10 has a two-layer structure. Also in this example, the low Al composition layer 11 is Al _0.65 Ga _0.35 N with an Al composition ratio of 65%, and the high Al composition layer 12 is Al _0.75 Ga _0.25 N with an Al composition ratio of 75%.

Further, as a reference example, a semiconductor light emitting device having a low Al composition layer and having a Si-doped AlGaN layer of 3 μm stacked as the n-type semiconductor layer 10 was fabricated as shown in FIG. In this example, the Al composition ratio of the Si-doped AlGaN layer is 75%.
[Voltage-current characteristics]

  Electricity was passed between the n-side electrodes 40 of adjacent semiconductor stacked bodies, and voltage-current characteristics (n-n curves) were obtained for the semiconductor light emitting devices according to Examples 1 to 3 and the reference example. The result is shown in the graph of FIG.

  In this example, Vf of the semiconductor light emitting device according to the reference example shown in FIG. 7 is higher than usual, which may be an irregular lot. Usually it is between 9-10V.

  Moreover, as shown in FIG. 7, in the voltage-current characteristic (n-n curve), any of Examples 1 to 3 in which the low Al composition layer is inserted is compared with the reference example having no low Al composition layer. Thus, the improvement of the nn curve was confirmed.

  Example 1 has a structure in which the layer ratio of the lowest Al composition is the highest among Examples 1 to 3. Therefore, light absorption is large, resulting in a decrease in output. Further, as shown in FIG. 7, the Vn of the nn curve changed from Schottky contact to ohmic contact, and the Vf value decreased.

  In Example 2, the proportion of the low Al composition layer is the same as in Example 3 below. In this structure, the phenomenon in which light absorption is increased as in Example 1 did not occur. Further, as shown in FIG. 7, the nn curve is also in ohmic contact, and Vf is reduced.

  Further, the structure of Example 3 is very similar to that of Example 2, and the insertion position of the low Al composition layer is moved upward by changing the depth of n-out. As in Example 2, an increase in light absorption did not occur, and a decrease in Vf could be confirmed.

In the inspection of the deep ultraviolet light emitting device according to the present embodiment, the light is taken from the back surface of the wafer. Therefore, this value is highly dependent on the absorption of the n-type semiconductor layer.
[Measurement of forward voltage and light intensity]

  Next, the semiconductor light emitting devices according to Examples 1 to 3 and the reference example are energized with a forward drive current If = 20 mA, the forward voltage Vf is measured, and the light intensity VL (au) is measured. The results are shown in FIG. Here, a prototype is made with a □ 320 μm square FD die, mounted on a general stem and shell type TO-18 package, and light intensity VL (au) and forward voltage Vf (V ) Was measured. As shown in this figure, in the structure in which the film thickness of the low Al composition layer of Examples 2 and 3 is reduced as much as possible, the VL value remains as it is, and only Vf is greatly reduced. Even considering that the reference example is high, a reduction effect of about 2 V can be expected. On the other hand, in the structure having a large proportion of the low Al composition layer of Example 1, Vf is also decreased, but VL is also decreased. This is presumed to be due to an increase in absorption in the n-type semiconductor layer.

Comparative Example 1

  Next, FIG. 9 shows a schematic diagram of a semiconductor stacked structure of a semiconductor light emitting element according to Comparative Example 1. FIG. In the semiconductor light emitting device shown in this figure, the n-type semiconductor layer 10 has a composition gradient structure composed of two layers of an undoped AlGaN composition gradient layer 16 and a Si-doped AlGaN composition gradient layer 17, and the Al composition ratio is in the range of 75% to 65%. Then, the composition was successively inclined so that the composition ratio decreased from the top of the AlGaN / AlN superlattice layer 14 toward the active layer 30.

  The semiconductor light emitting device according to Comparative Example 1 has an emission peak wavelength of 255 nm as in Examples 1 to 3 and the reference example. In this manufacturing method, the gradient of the Al composition ratio starts from the composition of the underlying superlattice layer AlGaN, and the Al composition is gradually decreased so that the final value of the gradient is about 65%. This is because the light absorption at 255 nm is considered to be relatively light if the Al composition ratio is about 65%. When the emission peak wavelength is in the ultrashort wavelength region, it is difficult to sufficiently reduce the Al ratio in order to prevent light absorption by the n-type semiconductor layer, and it becomes difficult to obtain the gradient width of the composition necessary to reduce Vf.

Comparative Example 2

FIG. 10 shows a schematic diagram of a semiconductor laminated structure of a semiconductor light emitting device according to Comparative Example 2. Also in the semiconductor light emitting device shown in this figure, the n-type semiconductor layer has a composition gradient structure composed of two layers of an undoped AlGaN composition gradient layer 16 and a Si-doped AlGaN composition gradient layer 17 in which the Al composition ratio is gradient. Here, the Al composition ratio is in the range of 95% to 65%, and the composition is sequentially and continuously inclined so that the composition decreases from the AlGaN / AlN superlattice layer 14 toward the active layer 30. In Comparative Example 1 described above, the inclination width could not be obtained, and there was a concern that the contact state would change greatly due to fluctuations in the amount of n projection. Therefore, Comparative Example 2 adopted a structure in which the inclination start was 95%. In this structure, an inclination width can be obtained, but since the Al composition as a whole increases, it cannot be expected to reduce the surface resistance.
[Measurement of forward voltage and light output]

  The above sample is prototyped with a □ 320 μm square FD die, mounted on a general stem and shell type TO-18 package, and optical output Po (mW) and forward voltage Vf (V) are measured as inspection items. The results are shown in the graph of FIG. In the structures of Comparative Examples 1 and 2, no decrease in Vf could be confirmed. In this sample, as shown in FIG. 7, the nn curve was not ohmic contact but Schottky contact. From this fact, the composition of the n-type AlGaN layer is inclined so that the Al composition is high in the n-out portion even if the resistance value of the contact surface is improved to about 65% at the upper part of the n-type semiconductor layer. I can guess.

  In order to make the Al composition 65% at the reaching point of n, it is necessary to further reduce the Al composition above the n-type semiconductor layer, and light absorption cannot be ignored. The structure in which the composition is continuously inclined is effective in reducing Vf for a light emitting element having a wavelength of about 280 nm or more, but cannot be applied when the wavelength is shorter than that.

  On the other hand, the n-type contact layer has a two-layer structure, so that light absorption can be reliably suppressed, and improvement in contact property can be realized. From the above, it was confirmed that the use of a plurality of layers is more useful in the ultra-high frequency region than the configuration in which the Al composition ratio is inclined.

  The semiconductor light emitting device of the present invention is suitable for backlight sensors, illumination light sources, headlights, various sensors such as displays, traffic lights, illumination switches, and image scanners, and various indicators that are arranged in a dot matrix using the light emitting elements as light sources. Available to:

DESCRIPTION OF SYMBOLS 100 ... Semiconductor light-emitting device 1 ... Template substrate 2 ... Buffer layer 3 ... Base substrate 10 ... N-type semiconductor layer 11 ... Low Al composition layer 12 ... High Al composition layer 12a ... Si doped region 12b ... Undoped region 14 ... AlGaN / AlN super Lattice layer 16 ... undoped AlGaN composition graded layer 17 ... Si doped AlGaN composition graded layer 20 ... p-type semiconductor layer 30 ... active layer 40 ... n-side electrode 50 ... p-side electrode 410 ... n-type contact layer 420 ... p-type semiconductor layer 430 ... active layer 440 ... n-side electrode 450 ... p-side electrode ED ... etching depth

Claims (8)

an n-type semiconductor layer;
An active layer provided in a part on the n-type semiconductor layer;
A p-type semiconductor layer provided on the active layer;
An n-side electrode provided on another part of the n-type semiconductor layer;
A semiconductor light emitting device having a p-side electrode provided on the p-type semiconductor layer,
The emission peak wavelength of the light emitting element is 240 nm or more and 280 nm or less,
The n-type semiconductor layer is sequentially from the active layer side.
A low Al composition layer made of Al _x Ga _1-X N (0.60 <X ≦ 0.65) provided with the n-side electrode;
A high Al composition layer made of Al _Y Ga _1-Y N (0.70 ≦ Y <1) in contact with the low Al composition layer,
A thickness of the low Al composition layer is made thinner than a thickness of the high Al composition layer. The semiconductor light emitting device according to claim 1,
The emission peak wavelength of the light emitting element is 250 nm or more and 260 nm or less,
The low Al composition layer is Al _X Ga _1-X N (0.63 ≦ X ≦ 0.65). The semiconductor light-emitting device according to claim 1 or 2,
A thickness of the low Al composition layer is not less than 0.5 μm and not more than 1.0 μm. The semiconductor light-emitting device according to claim 1,
A thickness of the high Al composition layer is 2.0 μm or more and 5.0 μm or less. The semiconductor light-emitting device according to claim 1,
The n-side electrode contains Al in a layer connected to the low Al composition layer. A semiconductor light emitting device according to claim 1,
The low Al composition layer and the high Al composition layer have a composition ratio of Al in each layer substantially constant in the stacking direction. The semiconductor light-emitting device according to claim 1,
The low Al composition layer is located closest to the active layer among the n-type semiconductor layers. The semiconductor light emitting device according to claim 1,
The high Al composition layer has an undoped region which is not doped with an n-type impurity in a region opposite to the side where the low Al composition layer is provided.

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