JP2015103768A - n-TYPE NEGATIVE ELECTRODE FORMATION METHOD AND GROUP III NITRIDE SEMICONDUCTOR LIGHT EMITTING ELEMENT - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light emitting element using a nitride semiconductor, which achieves excellent luminous efficiency by concurrently causing a negative electrode to serve functions of low contact resistance and light reflection.SOLUTION: An n-type negative electrode formation method comprises the steps of: sectioning an n-type layer surface into a first region closer to a mesa structure and a second region other than the first region, which has a wider area than the first region and forming a first metal electrode layer of Ti and the like in the first region; subsequently forming a second metal electrode layer of Ti and the like in the second region and on the first metal electrode layer; subsequently forming a reflection electrode layer and a noble metal layer on the second metal electrode layer; performing a heat treatment at a temperature of not less than 400°C and not more than 1000°C after forming the noble metal layer; and performing a heat treatment at a temperature of not less than 800°C and not more than 1200°C after forming the first metal electrode layer and before forming the reflection electrode layer.

Description

  The present invention relates to a novel method for forming an n-type negative electrode in a light emitting device made of a nitride semiconductor, and to a novel nitride semiconductor light emitting device obtained by the method. Among them, a novel method suitable for producing an n-type negative electrode of a deep ultraviolet light emitting device having a light emission wavelength of 200 to 350 nm (a deep ultraviolet light emitting device having a light emission wavelength peak at 200 to 350 nm), and a novel deep ultraviolet light The present invention relates to a light emitting element.

  Currently, gas light sources such as deuterium and mercury are used for deep ultraviolet light sources of 350 nm or less. However, the gas light source has disadvantages such as short life, harmfulness, and large size. Therefore, it is awaited to realize a light-emitting element using a semiconductor that can solve the disadvantages and can be easily handled.

  Current light-emitting elements using nitride semiconductors have problems that light output is weaker and light emission efficiency is lower than that of deuterium gas lamps or mercury gas lamps. In order to improve the light emission efficiency, it is necessary to increase the internal quantum efficiency indicating the light conversion efficiency in the active layer in the light emitting element and the light extraction efficiency for extracting the light emitted from the active layer to the outside of the light emitting element. Necessary.

  In order to increase the light extraction efficiency, it is known to introduce a light extraction structure (see Non-Patent Document 1). In addition, introduction of n and p-type reflective electrodes is effective (see Non-Patent Document 2). According to the method described in Non-Patent Document 2, an n-type reflective electrode is formed by installing a small ohmic electrode layer in a region close to a mesa structure and an aluminum (Al) reflective electrode layer so as to cover it. The negative electrode is formed by separating the ohmic contact portion and the reflection portion. By doing so, the light extraction efficiency by reflection can be improved.

  However, according to studies by the present inventors, it has been found that although this method can surely achieve a high reflectance, there is room for improvement in terms of ohmic properties. In particular, it has been found that ohmic characteristics tend to decrease when a light-emitting element made of a group III nitride single crystal containing aluminum (Al) is manufactured. Among these, when manufacturing a deep ultraviolet light emitting device having an emission wavelength of 350 nm or less, the n-type layer must be an AlGaN layer having a high Al composition. In this case, however, the tendency for the ohmic property to decrease is prominent.

  Usually, in a light emitting element using a nitride semiconductor, an element structure is formed of AlGaN having a high Al composition on an insulating substrate. In order to obtain ohmic contact from the n-type layer, a part of the p-type layer is etched to expose the n-type layer, and a negative electrode is formed on the exposed surface. This n-type layer was difficult to obtain ohmic contact due to damage caused by etching.

  As a method of obtaining ohmic contact of the n-type layer, the present inventors performed a heat treatment after forming a Ti layer as the first electrode layer, and then formed a reflective electrode layer containing Al, and further performed the heat treatment. The method to perform was found (refer patent document 1). According to this method, ohmic contact with an n-type AlGaN layer having a high Al composition is possible, and since the reflective electrode layer is also provided, the light extraction efficiency can be improved.

Appl. Phys. Lett. 89, 24113 (2006) Jpn. J. et al. Appl. Phys. 50, 122101 (2011)

International Publication WO2011 / 078252 Pamphlet

  The present inventors consider that if the method described in Patent Document 1 is applied to the ohmic contact portion in Non-Patent Document 2, the light extraction efficiency by the reflective electrode layer can be improved while maintaining high ohmic characteristics. Went. However, it has been found that the effect of improving the ohmic characteristics is not exhibited so much by simply applying the method described in Patent Document 1 as a method of manufacturing the ohmic contact portion.

  Accordingly, an object of the present invention is to provide a deep ultraviolet light emitting device using a nitride semiconductor having a low resistance (ohmic contact) and high luminous efficiency, by solving the above-described problems.

  In order to solve the above-mentioned problems, the present inventors have conducted intensive studies. Then, the method described in Patent Document 1 was applied to Non-Patent Document 2 to examine the point that ohmic contact could not be improved so much. Then, it was thought that there was a cause not in the ohmic contact part but in the reflection part. As a result of further investigation, in the n-type layer composed of a group III nitride single crystal containing Al, particularly in the n-type AlGaN layer having a high Al composition, it is more noticeable that the ohmic characteristics are not improved and the reflection is improved. It has been found that the ohmic characteristics of the part must also be improved. In order to improve the reflection efficiency while improving the ohmic characteristics of the reflective portion, it is necessary to form a thin second electrode metal layer on the reflective portion, and a specific heat treatment is required. The present invention has been completed.

That is, the first aspect of the present invention provides a group III nitride semiconductor light-emitting device having a stacked structure in which an n-type layer, an active layer, and a p-type layer are stacked in this order, and one of the active layer and the p-type layer. Forming a mesa structure by removing the region of the n-type layer to expose the n-type layer, and forming a negative electrode layer on the exposed surface of the n-type layer,
Dividing the region where the negative electrode layer on the n-type layer surface is formed into at least a first region close to a mesa structure and a second region having a larger area than the first region;
Forming a first metal electrode layer made of at least one metal selected from the group consisting of Ti, V and Ta in the first region;
A second metal electrode layer made of at least one metal selected from the group consisting of Ti, V and Ta and having a thickness smaller than that of the first metal electrode layer is formed on the second region and the first metal electrode layer. Forming step,
A reflective electrode layer made of a metal having a work function of 4.0 to 4.8 eV and a specific resistance of 1.5 × 10 ^{−6 to} 4.0 × 10 ⁻⁶ Ω · cm is formed as the second metal electrode layer. Forming on the top,
Forming a noble metal layer comprising at least one noble metal selected from the group consisting of Au and Pt on the reflective electrode layer;
After forming the noble metal layer, including a step of heat treatment at a temperature of 400 ° C. or more and 1000 ° C. or less,
A method for forming an n-type negative electrode, comprising a step of performing a heat treatment at a temperature of 800 ° C. or higher and 1200 ° C. or lower after forming the first metal electrode layer and before forming the reflective electrode layer. .

In the first aspect of the present invention, the n-type layer is Al _x In _y Ga _z N (x, y, z are 0 <x ≦ 1.0, 0 ≦ y ≦ 0.1, 0 ≦ z <1). When the film is made of a group III nitride single crystal satisfying a composition represented by a rational number satisfying .0 and x + y + z = 1.0), an excellent effect is exhibited.

  In the first aspect of the present invention, it is particularly preferable to perform the heat treatment at a temperature of 800 ° C. or higher and 1200 ° C. or lower before forming the second metal electrode layer. By doing so, the process can be simplified and the productivity can be increased.

  In the first invention, the thickness of the second metal electrode layer is preferably 0.1 to 5 nm. Furthermore, in the first aspect of the present invention, the area of the second region is preferably more than 1 time and 40 times or less with respect to the area of the first region.

  The second aspect of the present invention is a group III nitride semiconductor light emitting device having an n-type negative electrode manufactured by the above method.

Further, in the group III nitride semiconductor light emitting device of the present invention, the first metal electrode layer and the second metal electrode layer in the first region are combined to form a first junction metal layer, and the first junction metal layer And a first electrode layer in which a reflective electrode layer and a noble metal layer are laminated in this order,
The second metal electrode layer in the second region becomes a second bonding metal layer having a thickness smaller than that of the first bonding metal layer, and the second bonding metal layer, the reflective electrode layer, and the noble metal layer are laminated in this order. The form which has a 2nd ohmic electrode layer may be sufficient.

  According to the present invention, it is possible to suppress light absorption while maintaining low contact resistance, and to realize both low power consumption and high efficiency light emission. In particular, the method of the present invention can be suitably employed as a method for forming a negative electrode in a deep ultraviolet light emitting device made of a nitride semiconductor having an emission wavelength of 200 to 350 nm.

It is the schematic which showed the structure of the negative electrode of this invention. 6 is a simplified diagram of a process when forming a negative electrode in Example 1. FIG. It is the figure which calculated | required the relationship between the film thickness of Ti in a preliminary experiment, and a reflectance.

  In the group III nitride semiconductor light emitting device having a laminated structure in which an n-type layer, an active layer, and a p-type layer are laminated in this order, a method of the present invention removes a partial region of the p-type layer. In this method, a mesa structure is formed by exposing the n-type layer, and a negative electrode layer is formed on the exposed surface of the n-type layer.

  In the following, description will be given in order. FIG. 2 shows a process diagram which is a preferred embodiment of the present invention. First, a group III nitride semiconductor light emitting device having a stacked structure in which an n-type layer, an active layer, and a p-type layer are stacked in this order will be described.

(N-type layer consisting of group III nitride single crystal layer (preparation of n-type layer))
In the present invention, the n-type layer made of a group III nitride single crystal can be produced by a known method. In the present invention, the group III nitride means a general formula Al _A In _B Ga _1- ABN (where A, B, and C are 0 ≦ A ≦ 1.0, 0 ≦ B ≦ 1.0). , 0 ≦ A + B ≦ 1.0 is satisfied).

  What is necessary is just to determine a composition and a structure suitably for the said n-type layer according to the use. For example, as shown in FIG. 1 (as shown in the figure, the n-type layer 20 is formed on a single crystal substrate 10 such as an aluminum nitride (AlN) substrate, or a group III nitride semiconductor layer (III A group nitride single crystal layer) may be formed on a laminate in which one or more layers are formed, and the n-type layer 20 may contain Si as a dopant. What is necessary is just to determine suitably the thickness of the n-type layer 20 according to the use to be used, and the thickness of the n-type layer 20 is 0.5-5.0 micrometers normally.

  Such an n-type layer can be formed by, for example, a metal organic chemical vapor deposition method (MOCVD method). Specifically, using a commercially available apparatus, a group III source gas, for example, an organic metal gas such as trimethylaluminum, and a nitrogen source gas, for example, on the single crystal substrate or the laminate. The n-type layer can be formed by supplying a source gas such as ammonia gas onto the substrate. As a condition for forming the n-type layer by the MOCVD method, a known method can be adopted.

In the present invention, an n-type layer can be formed according to the above method. The n-type layer is not particularly limited, and may be composed of a group III nitride single crystal having the above composition. Therefore, the n-type layer may be a GaN layer. However, in the method of the present invention, particularly when the n-type layer is composed of a group III nitride single crystal containing Al, Al _x In _y Ga _z N (x, y, z is 0 <x ≦ A rational group satisfying 1.0, 0 ≦ y ≦ 0.1, 0 ≦ z <1.0, and x + y + z = 1.0). It has an excellent effect. In particular, the present invention is particularly preferably applied to an n-type layer made of a group III nitride single crystal having a high Al content. Specifically, among the group III nitride single crystals having a high Al content, x is preferably 0.5 or more, and particularly preferably from a group III nitride single crystal having x of 0.6 or more. In the case of an n-type layer, the method of the present invention is particularly preferably applied. In this case, y may be 0 or more and 0.1 or less, but y is particularly preferably 0.

(Mesa structure formation and damage layer removal)
In the present invention, the obtained nitride semiconductor light emitting device includes a portion in which a contact electrode portion (first metal electrode layer 61 in FIG. 1) is formed on the n-type layer made of a group III nitride single crystal, a reflective electrode Part (refers to a layer made of the second metal electrode layer 62a / reflecting electrode layer 62b / noble metal layer 62c). Normally, when manufacturing a group III nitride semiconductor, an active layer 30 and a p-type layer 40 for forming a p-type contact electrode are stacked on an n-type layer 20 for forming an n-type contact electrode. The active layer has a quantum well structure in which a well layer and a barrier layer are stacked. The p-type layer 40 may have a multilayer structure with different compositions. FIG. 1 illustrates a p-type layer 40 including a p-type electron block layer 40a, a p-type cladding layer 40b, and a p-type contact layer 40c. Although not limited to this structure, in particular, the p-type contact layer 40 in contact with the p-type contact electrode is preferably formed of a GaN layer among group III nitride single crystals.

  Then, the mesa structure is formed by removing a part of the active layer and the p-type layer. As a method of deleting, an etching process (for example, a dry etching process using a halogen-based gas, for example, a chlorine-based gas containing chlorine atoms or a fluorine-based gas containing fluorine atoms) may be performed. Such a etching process is performed to form a mesa structure.

  Furthermore, the method of the present invention can be effectively applied to the case where a contact electrode is formed on an n-type layer that has been subjected to a surface treatment with an acid solution or an alkali solution after dry etching. As a matter of course, the method of the present invention can be effectively applied to a case where a contact electrode is formed on an n-type layer which is not subjected to a dry etching process but only subjected to a surface treatment with an acid solution or an alkali solution. . By this surface treatment, the oxide layer, the hydroxide film on the surface of the n-type layered body, or the damaged layer of the n-type nitride semiconductor layer received by the dry etching treatment can be removed. For the surface treatment with an acid solution and the surface treatment with an alkali solution, the method described in Patent Document 1 can be referred to.

  In the present invention, the region where the negative electrode layer 60 is formed on the surface of the n-type layer surface-treated by the method as necessary is divided into a first region closer to the mesa structure, and other second regions. Divide into Next, this division process will be described. First, it divides into the 1st area | region 60a and the 2nd area | region 60b in FIG.

(Division process of 1st area and 2nd area)
In the present invention, the exposed surface of the n-type layer includes a first region 60a on which the first metal electrode layer 61 is formed and a second region in which the second metal electrode layer 62a is in direct contact with the n-type layer 20. It is divided at least into 60b. In order to classify, the second region 60b may be masked (protected) with a photoresist or the like in order to form the first metal electrode layer 61 in the first region 60a close to the mesa structure. At this time, in addition to the second region 60b, a region (first region) where the mesa structure (side surface) and the first metal electrode layer 61 are formed thereon so that the first metal electrode layer 61 does not contact the mesa structure. 60a) is preferably masked (protected) with a photoresist or the like. Naturally, the region between the side surface of the mesa structure and the first region 60a does not correspond to the region where the negative electrode layer is formed. The distance between the side surface of the mesa structure and the first region 60a is not particularly limited. However, in order to prevent a short circuit and improve the yield, the distance is preferably 1 to 10 μm, and more preferably 2 to 8 μm. It is preferable. In addition, as a mask to be used, for example, a photoresist made by Zeon Corporation: ZPN1150-90 can be mentioned. This mask can be removed with acetone.

  After the masking, the first metal electrode layer 61 may be formed in the first region 60a on the n-type layer by a known method.

  In the present invention, the first region 60a needs to be on the side close to the mesa structure and further to have an area smaller than that of the second region 60b. By providing the first metal electrode layer 61 on the first region 60a on the side close to the mesa structure, the resistance can be reduced. Further, the reflection efficiency can be increased by making the area smaller than that of the second region 60b. The area ratio between the first region 60a and the second region 60b is not particularly limited, but considering the ohmic characteristics and the reflection efficiency, the area of the second region 60b is set to the area of the first region 60a. It is preferably more than 1 time and not more than 40 times, more preferably more than 1 time and not more than 20 times, still more preferably not less than 2 times and not more than 10 times. Therefore, the first region 60a (first metal electrode layer 61) is preferably formed within 120 μm, more preferably within 60 μm, and even more preferably within 40 μm from the side surface of the mesa structure. However, the first region 60a (the first metal electrode layer 61) does not necessarily need to surround the entire mesa structure, and may be partially missing. As described above, it is preferable to provide an interval (distance) of 1 to 10 μm from the side surface of the mesa structure to the first region 60a.

  After applying (masking) a photoresist or the like between the second region 60b and the mesa structure and the first region 60a so as to satisfy the above-described requirements, next, on the first region 60a, A first metal electrode layer 61 is formed. Next, the first metal electrode layer will be described.

(Method for forming first metal electrode layer)
In the present invention, a known method can be adopted as a method of forming the first metal electrode layer 61 on the n-type layer 20. However, the metal forming the first metal electrode layer 61 is at least one metal selected from the group consisting of Ti, V, and Ta.

A specific method for forming the first metal electrode layer 61 includes a method of forming a metal film on the surface of the n-type layer 20 (on the first region 60a) by an electron beam vacuum deposition method. it can. The pressure in the chamber when depositing the metal film is preferably 1.0 × 10 ⁻³ Pa or less in order to reduce the influence of impurities and the like.

  The metal forming the first metal electrode layer 61 is at least one metal selected from the group consisting of Ti, V, and Ta. These metals are active against group III nitrides and have the common property of reacting at high temperatures to form nitrides. Therefore, titanium nitride (TiN), vanadium nitride (VN), nitridation is performed at the interface between the first metal electrode layer 61 and the first region 61a by heat treatment at a temperature of 800 ° C. or higher and 1200 ° C. or lower as described in detail below. A layer (reaction layer) composed of a nitride of the metal such as tantalum (TaN) or a composite nitride of the metal and a group III atom is formed, the electron depletion layer is thinned (the Schottky barrier width is narrowed), and the tunnel effect It is considered that the interface state is such that the contact resistance value can be reduced. The first metal electrode layer 61 is most preferably composed of Ti considering the balance between the second metal electrode layer 62a and the reflective electrode layer 62b described later.

  In the examples of the present invention to be described later, an example in which Ti is used for the first metal electrode layer 61 is illustrated, but “JOURNAL OF ELECTRONIC MATERIALS, Vol. 37, No. 5, 2008”, “JOURNAL”. OF APPLIED PHYSICS 100, 046106 (2006) "etc., it is considered that the same effect can be obtained when V or Ta is used.

  In the present invention, the thickness of the first metal electrode layer 61 is not particularly limited, but is preferably 10 nm or more. When the thickness of the first metal electrode layer 61 satisfies the above range, even if the metal diffuses through the process of heat treatment at a temperature of 400 ° C. or more and 1000 ° C. or less after forming the noble metal layer described in detail below, The first region 60a of the n-type layer 20 can be covered with the first metal electrode layer 61, and a good contact resistance value can be obtained. The upper limit of the thickness of the first metal electrode layer 61 is not particularly limited, but is 50 nm in consideration of productivity and economy.

  In the present invention, after forming the first metal electrode layer 61 on the first region of the n-type layer, before forming the reflective metal layer 62b described in detail below, a temperature of 800 ° C. or more and 1200 ° C. or less. And heat treatment. Next, this heat treatment (hereinafter sometimes referred to as a first heat treatment) will be described.

(First heat treatment)
This first heat treatment is preferably performed before forming the second metal electrode layer 62a described in detail below. That is, after forming the first metal electrode layer 61, it is preferable to perform this first heat treatment. The reason is as follows. When the electrode is formed by vapor deposition, it takes time to deposit the electrode. In addition, as will be described in detail below, since the second metal electrode layer 62a may be a thin film, the second electrode metal layer 62a, the reflective electrode layer 62b, and the noble metal layer 62c are continuously formed by vapor deposition. It leads to time reduction. Furthermore, since the second metal electrode layer 62a is a thin film, it is considered that the contact resistance value can be sufficiently lowered only by the heat treatment after the formation of the noble metal layer 62c, and the reflection efficiency is only the heat treatment after the formation of the noble metal layer 62c. Is considered to be more advantageous.

  When the first heat treatment is performed after the first metal electrode layer 61 is formed, the second region 60b and the mask between the mesa structure and the first region 60a are removed with a resist stripping solution or the like. A known method can be used as a method of removing the mask of the second region 60b. Specifically, for example, when a photo resist ZPN1150-90 manufactured by ZEON is used as a mask, it may be removed with acetone.

  After removing the mask, a first heat treatment is performed. By performing this heat treatment, the adhesion between the first metal electrode layer 61 and the n-type layer 20 is increased, and a good contact resistance value is obtained.

  In the present invention, the first heat treatment temperature must be 800 ° C. or higher and 1200 ° C. or lower. When the temperature is less than 800 ° C., the adhesion between the first metal electrode layer 61 and the n-type layer 20 is not preferable. On the other hand, if it exceeds 1200 ° C., the n-type layer 20 may be thermally decomposed, which is not preferable. Considering the adhesion strength between the n-type layer 20 and the first metal electrode layer 61 and the thermal decomposition of the n-type layer 20, the temperature of the first heat treatment is preferably 800 ° C. or higher and 1100 ° C. or lower. The first heat treatment may be a constant temperature as long as the temperature is in the above range, or may vary within the above range. Further, in the case of fluctuation, the average value may be compared with the heat treatment temperature after the noble metal layer 62c is formed.

  In the present invention, the time for the first heat treatment may be appropriately determined according to the composition of the n-type layer 20, the type of the first metal electrode layer 61, the thickness, etc., but the time is 30 seconds or more and 90 seconds or less. It is preferable. The time for the heat treatment does not include the time for the temperature raising process. Although the temperature raising time is preferably as short as possible, it is usually preferably 120 seconds or shorter, more preferably 60 seconds or shorter because it is affected by the volume, performance, heat treatment temperature, etc. of the apparatus. The shortest heating time is greatly influenced by the performance of the apparatus and cannot be generally limited, but is usually 10 seconds.

  In the present invention, the first heat treatment is not particularly limited, but is preferably performed in an inert gas atmosphere, for example, a nitrogen atmosphere from the viewpoint of preventing reaction with the n-type layer 20.

  Such a first heat treatment can be performed using a RTA (Rapid Thermal Annealing) apparatus that is usually used when forming an n-type contact electrode.

  Next, the second metal electrode layer 62 a is formed on the second region 60 b and the first metal electrode layer 61. A method for forming the second metal electrode layer will be described.

(Method for forming second metal electrode layer)
In the present invention, the first metal electrode layer 61 and the second region 60 b are made of at least one metal selected from the group consisting of Ti, V, and Ta, and are thinner than the first metal electrode layer 61. The second metal electrode layer 62a is formed. Since the second metal electrode layer 62a is present on the second region 60b and the first metal electrode layer 61, the light emitting device obtained by the method of the present invention exhibits excellent effects.

In the present invention, the method for forming the second metal electrode layer 62a may be a known method, similar to the method for forming the first metal electrode layer 61. Specifically, a method of forming a metal film on the surface of the second region 60b and the first metal electrode layer 61 by an electron beam vacuum deposition method can be exemplified. The pressure in the chamber when depositing the metal film is preferably 1.0 × 10 ⁻³ Pa or less in order to reduce the influence of impurities and the like. By such a method, as shown in FIG. 1, the second metal electrode layer 62a can be formed on the first metal electrode layer 61 and the second region 60b. Before the second metal electrode layer 62a is formed, a mask (protection) is performed between the mesa structure and the first region 60a with a photoresist or the like in order to prevent a short circuit. As described above, the distance between the first region 60a and the side surface of the mesa structure is preferably 1 to 10 μm.

  The second metal electrode layer 62 a is made of at least one metal selected from the group consisting of Ti, V, and Ta, and must be thinner than the first metal electrode layer 61. The metal constituting the second metal electrode layer 62 a is preferably the same type as the metal constituting the first metal electrode layer 61. For this reason, when the first metal electrode layer 61 is made of Ti, the second metal electrode layer 62a is preferably made of Ti.

  The thickness of the second metal electrode layer 62 a must be smaller than the thickness of the first metal electrode layer 61. If the thickness is the same as or thicker than the first metal electrode layer 61, the light reflection effect by the reflective electrode layer 62b is reduced, which is not preferable. The specific thickness is preferably 0.1 nm or more and 5 nm or less.

  In the present invention, the first heat treatment can also be performed after the second metal electrode layer 62a is formed. Furthermore, after forming the first metal electrode layer 61, the first heat treatment can be performed, and then, after forming the second metal electrode layer 62a, the first heat treatment can be further performed. As a matter of course, when the first heat treatment is performed, it is necessary to remove the photoresist between the first region 60a and the mesa structure before performing the heat treatment. According to these methods, a light emitting element with low contact resistance can be obtained. However, in order to simplify the process and increase the light reflection efficiency, as described above, after forming the second metal electrode layer 62a, Then, it is preferable to form the reflective electrode layer 62b. In order to simplify the process most, it is preferable that the second metal electrode layer 62a, the reflective electrode layer 62b, and the noble metal layer 62c are continuously formed by the same vapor deposition apparatus.

(Method of forming a reflective electrode layer)
In the present invention, the work function is 4.0 to 4.8 eV and the specific resistance is 1.5 × 10 ^{−6 to} 4.0 × 10 ⁻⁶ Ω · cm on the second metal electrode layer 62a. A reflective electrode layer 62b made of a metal is formed.

Reflective electrode layer 62b is, the work function is 4.0EV～4.8EV, and metal resistivity is ^{1.5 × 10 -6 Ω · cm~4.0 ×} 10 -6 Ω · cm ( specific height It is a highly conductive metal layer made of a conductive metal. In general, the work function of a metal may vary slightly depending on the measurement method and the source, but in the present invention, it refers to the work function described in JAP_48_4729 (1977). After the first heat treatment, a metal layer made of a specific highly conductive metal is formed as the reflective metal layer 62b, and the heat treatment described in detail below is further performed to maintain the interface state obtained by the first heat treatment, A highly conductive metal can be joined without increasing the Schottky barrier, and the contact resistance can be lowered. Also from this, the first heat treatment needs to be performed before the reflective electrode layer 62b is formed.

Specific high conductivity metals include Al (specific resistance 2.65 × 10 ⁻⁶ Ω · cm, work function: 4.28 eV), Ag (specific resistance 1.59 × 10 ⁻⁶ Ω · cm, work function: 4 .26 eV), Cu (specific resistance 1.92 × 10 ⁻⁶ Ω · cm, work function: 4.65 eV), and the like. preferable.

  The reflective electrode layer 62b can be formed by a known method. The thickness of the reflective electrode layer 62b is not particularly limited, but is preferably 100 to 500 nm in consideration of the effects of reflection efficiency and low contact resistance. More preferably, it is 250-400 nm.

(Method of forming a noble metal layer)
In the present invention, a noble metal layer made of at least one kind of noble metal selected from the group consisting of Au and Pt is formed on the reflective metal layer 62b. By forming the noble metal layer 62c, the contact resistance can be stably lowered, and an ohmic junction can be realized. Further, a Ni layer can be provided between the reflective metal layer 62b and the noble metal layer 62c. By providing the Ni layer, the noble metal is prevented from diffusing by the heat treatment described in detail below, and the noble metal diffuses and contacts the n-type layer 20 without impairing the function of preventing void generation (filling the void). Can be more reliably prevented.

  The noble metal layer 62c can be formed by a known method. The thickness of the noble metal layer 62c is not particularly limited, but is preferably 5 to 60 nm in consideration of maintaining the reflection efficiency and reducing the contact resistance. More preferably, the thickness is 5 to 30 nm.

  In the present invention, after the noble metal layer 62c is formed, a heat treatment process is performed at a temperature of 400 ° C. or higher and 1000 ° C. or lower. Hereinafter, this heat treatment may be a second heat treatment. Next, the second heat treatment will be described. In addition, before performing this 2nd heat processing, the photoresist which exists between the 1st area | region 60a and a mesa structure is removed by a well-known method.

(Second heat treatment)
In the present invention, an excellent effect is exhibited by performing the second heat treatment. When this heat treatment is not performed, the adhesion between the first metal electrode layer 61 and the second metal electrode layer 62a and between the n-type layer 20 and the second metal electrode layer 62a is insufficient. Further, when the second metal electrode layer 62a is a multilayer film, it cannot be alloyed, so that a good contact resistance value cannot be obtained.

  In the present invention, the temperature of the second heat treatment is 400 ° C. or higher and 1000 ° C. or lower. If the temperature is out of this temperature range, the adhesiveness is lowered or a good contact resistance value cannot be obtained. In addition, the reason why the heat treatment temperature after the formation of the noble metal layer 62c may be lower than that of the method described in Patent Document 1 is considered to be due to having the reflective electrode portion on the second region 60b. That is, since it is considered that the reflective electrode portion on the second region 60b also contributes to ohmic contact, it is considered that an excellent effect is exhibited even if heat treatment is performed at a relatively low temperature. In addition, the reflectance can be improved by carrying out at a low temperature. Considering maintenance of high light reflectance between the second metal electrode layer 62a and the n-type layer 20 and ohmic contact, the temperature of the second heat treatment is more preferably 400 ° C. or more and 950 ° C. or less. In particular, the temperature is preferably 700 ° C. or higher and 900 ° C. or lower. In addition, when the contact electrode is formed on the surface-treated n-type layer, the second heat treatment temperature is preferably changed depending on the surface treatment mode. The reason for this is not clear, but is considered to be caused by the difference in the surface state of the n-type layer due to the difference in the surface treatment mode, and the difference in the surface state affects the second heat treatment temperature. It is thought that gives.

  Explaining specific temperature conditions, when the n-type layer is surface-treated with an acid solution, the temperature of the second heat treatment is preferably 400 ° C. or higher and 950 ° C. or lower, and more preferably 430 ° C. or higher and 940 ° C. In order to improve ohmic contact, the temperature is preferably set to 750 ° C. or higher and 910 ° C. or lower.

  On the other hand, when the surface treatment is performed with an alkaline solution, the temperature of the second heat treatment is preferably 400 ° C. or more and 950 ° C. or less, more preferably 425 ° C. or more and 930 ° C. or less, particularly ohmic contact. In order to improve this, it is preferable to set it as 750 to 900 degreeC.

  In order to maintain the reflectance of the second metal electrode layer 62a, the temperature of the second heat treatment is preferably lower than the temperature of the first heat treatment. Specifically, the temperature is preferably lower by 100 ° C. or more than the first heat treatment.

  In addition, this 2nd heat processing may be fixed temperature, if it is the said range, and may be fluctuated within the said range. Moreover, when it fluctuates, the comparison with the first heat treatment temperature may be performed by comparing the average values.

  In the present invention, the second heat treatment is not particularly limited, but it is preferably performed in a nitrogen atmosphere from the viewpoint of preventing reaction with the n-type layer as in the first heat treatment. The heat treatment time is preferably 30 seconds or longer and 90 seconds or shorter in consideration of the time for alloying and the reduction of damage to the n-type layer. The time for the heat treatment does not include the time for the temperature raising process. Although the temperature raising time is preferably as short as possible, it is usually preferably 120 seconds or shorter, more preferably 60 seconds or shorter because it is affected by the volume, performance, heat treatment temperature, etc. of the apparatus. The shortest heating time is greatly influenced by the performance of the apparatus and cannot be generally limited, but is usually 10 seconds.

  Similar to the first heat treatment, the second heat treatment can be performed using an RTA (Rapid Thermal Annealing) apparatus used when forming an n-type contact electrode.

  As described above, when the n-type contact electrode formation is divided into multiple stages, for example, when a metal containing Ti is used when forming the first metal electrode layer 61, the entire surface of the n-type layer is made of TiN. It is thought that it can be covered with good adhesion. As a result, voids and diffused metal formed by the second heat treatment can be prevented from entering the interface between the n-type layer and the n-type contact electrode layer, so that a good contact resistance value can be obtained. Conceivable. Furthermore, it is considered that a reflective film having a high reflectance can be formed by the same process for obtaining a good contact resistance value. The first metal electrode layer 61, the second metal electrode layer 62a, the reflective electrode layer 62b, and the noble metal layer 62c formed by this series of methods are all the negative electrode layer 60.

(Method for forming positive electrode layer on p-type layer)
The positive electrode layer 50 on the p-type layer is not particularly limited, and can be manufactured by a known method. Specifically, the Ni / Au electrode can be formed by a vacuum deposition method. The thickness of the Ni layer is not particularly limited and is preferably 5 to 50 nm. Also, the Au layer is not particularly limited and is preferably 10 to 100 nm.

(Group III nitride semiconductor light emitting device)
According to the above method, a negative electrode layer having good ohmic characteristics and high reflectance can be formed on the n-type layer. The group III nitride semiconductor thus obtained can improve the light extraction efficiency by the n-type reflective film and can be driven at a low voltage. Therefore, it can be used for an LED device or other device in which energy saving is indispensable. it can.

In the group III nitride semiconductor light emitting device of the present invention, the first metal electrode layer and the second metal electrode layer in the first region are combined to form a first bonding metal layer, and the first bonding metal layer, A first ohmic electrode layer in which a reflective electrode layer and a noble metal layer are laminated in this order,
The second metal electrode layer in the second region becomes a second bonding metal layer having a thickness smaller than that of the first bonding metal layer, and the second bonding metal layer, the reflective electrode layer, and the noble metal layer are laminated in this order. The form which has a 2nd ohmic electrode layer may be sufficient.

  At this time, the first metal electrode layer and the second metal electrode layer are preferably made of the same metal. And in order to set it as a 1st, 2nd ohmic electrode layer, it is preferable to set it as the electrode layer of the layer structure demonstrated above, and to implement 1st heat processing and 2nd heat processing.

Therefore, the group III nitride semiconductor light-emitting device of the present invention is a group III nitride semiconductor light-emitting device having a stacked structure in which an n-type layer, an active layer, and a p-type layer are stacked in this order. A group III nitride semiconductor light emitting device having a mesa structure formed by removing a part of a p-type layer to expose the n-type layer, and having a negative electrode layer on the exposed surface of the n-type layer,
The region where the negative electrode layer on the surface of the n-type layer is formed has a first region close to a mesa structure and a second region having an area larger than the first region,
A first bonding metal layer made of at least one metal selected from the group consisting of Ti, V and Ta, a work function of 4.0 to 4.8 eV, and a specific resistance of 1.5 × 10 ⁻⁶ to A first ohmic in which a reflective electrode layer made of a metal having 4.0 × 10 ⁻⁶ Ω · cm and a noble metal layer made of at least one kind of noble metal selected from the group consisting of Au and Pt are laminated in this order. Having an electrode layer on the first region;
The second bonding metal layer, which is made of at least one metal selected from the group consisting of Ti, V, and Ta and is thinner than the first bonding metal layer, the reflective electrode layer, and the noble metal layer in this order. Can be regarded as having a second ohmic electrode layer laminated on the second region.

  Specific examples and comparative examples of the present invention will be described below with reference to the drawings. However, the present invention is not limited to these examples.

Preliminary experiment (examination of thickness of second metal electrode layer)
Before producing the nitride semiconductor light emitting device, first, the reflection efficiency due to the change in the thickness of the second metal electrode layer was examined. Specifically, an aluminum nitride single crystal layer (AlN single crystal layer) is grown on the sapphire substrate by MOCVD, and then Ti / Al (thickness 200 nm) / Au (thickness 5 nm) in this order on the surface of the AlN single crystal layer. The sample for reflectance measurement was produced by vapor deposition. From the sapphire substrate side, ultraviolet light having an emission wavelength of 265 nm was incident, and the reflectance of the Ti / Al (200 nm) / Au (5 nm) film was examined. In this preliminary experiment, only the thickness of Ti was changed, and the relationship between the reflectance and the thickness of Ti was examined. The relationship is shown in FIG. As is clear from FIG. 3, it was found that the thinner the Ti film, the higher the reflectivity, which is advantageous for extracting light. When the film thickness of Ti is 20 nm or less, the reflectivity becomes higher. In particular, when the film thickness of Ti is 5 nm or less, the reflectivity can be a value exceeding 60%, and the effect is more effective. It was confirmed that this was demonstrated.

Example 1
(Preparation of n-type layer 20)
By MOCVD, an Al _0.75 Ga _0.25 N layer (Si concentration 1 × 10 ¹⁹ [cm ⁻³ ]) doped with Si is formed as an n-type layer 20 on the sapphire substrate 1 with a layer thickness of 2.0 μm. did.

On the n-type layer 20, the active layer 30 has a quantum well structure, the well layer (composition Al _0.5 Ga _0.5 N) has a layer thickness of 2 nm, and the barrier layer (composition Al _0.75 Ga _0.25 N). The layer thickness was 7 nm. Three well layers and four barrier layers were formed. (Each barrier layer has the same composition and thickness. Each well layer has the same composition and thickness.)

Next, an MgN-doped AlN layer (band gap 6.00 eV, Mg concentration 5 × 10 ¹⁹ [cm ⁻³ ]) was formed as the electron blocking layer 40 a on the active layer 30 with a layer thickness of 15 nm. On the electron block layer 40a, an Al _0.80 Ga _0.20 N layer (Mg concentration 5 × 10 ¹⁹ [cm ⁻³ ]) doped with Mg was formed as a p-type cladding layer 40b with a layer thickness of 50 nm. Thereafter, a GaN layer doped with Mg (band gap 3.40 eV, Mg concentration 2 × 10 ¹⁹ [cm ⁻³ ]) was formed as a p-type contact layer 40 c with a layer thickness of 240 nm ((0) in FIG. 2).

Next, heat treatment was performed at 900 ° C. for 20 minutes in a nitrogen atmosphere. Thereafter, a predetermined resist pattern was formed on the surface of the p-type contact layer 51 by photolithography, and a window portion where no resist pattern was formed was etched by reactive ion etching until the surface of the n-type layer 20 was exposed. Thereafter, the substrate was immersed in hydrochloric acid having a concentration of 37 wt% at a temperature of 40 ° C. for 15 minutes to perform a surface treatment of the Al _0.75 Ga _0.25 N layer (n-type layer 20) ((a of FIG. 2). )).

(Formation of the first region 60a)
A predetermined resist pattern was formed by photolithography on the exposed n-type layer 20 so as to surround the mesa structure formed by reactive ion etching, and the first region 60a was secured in the window portion. The first region 60a was formed so as to surround the mesa structure with an interval of 5 μm from the mesa structure.

(Formation of first metal electrode layer 61 and first heat treatment)
The first metal electrode layer is formed on the Al _0.75 Ga _0.25 N layer (n-type layer 20) that has been subjected to surface treatment and masked between the second region 60 b and the side surface of the mesa structure and the first region 60 a. As Ti, 61 nm of Ti was deposited by electron beam evaporation ((b) of FIG. 2). Thereby, the 1st metal electrode layer 61 was able to be formed in the 1st field 60a. Next, the second region 60b and the mask between the side surface of the mesa structure and the first region 60a were removed with acetone.

  Next, the first heat treatment was performed in a nitrogen gas atmosphere, and the heat treatment was performed by instantaneous heat treatment using an RTA (Rapid Thermal Annealing) apparatus. The heat treatment time was 1 minute, and the first heat treatment temperature was 1000 ° C. The temperature raising time until reaching 1000 ° C. was 60 seconds.

(Formation of second region 60b, formation of second metal electrode layer, reflective electrode layer, noble metal layer, and second heat treatment)
Next, a predetermined resist pattern was formed on the first region 60a (first metal electrode layer 61) and the exposed n-type layer 20 so as to surround the mesa structure again by photolithography (the first region 60a and the mesa structure). The first metal electrode layer 61 and the second region 60b are not masked. At this time, the resist pattern was designed so that the area ratio between the first region 60a and the second region 60b was 1: 6. On the Ti layer formed as the first metal electrode layer 61 and on the second region 60b, the Ti layer as the second metal electrode layer 62a, the Al layer as the reflective electrode layer 62b, and the Au layer as the noble metal layer 62c. The layers were sequentially formed by electron beam evaporation, and the film thicknesses were 1 nm, 300 nm, and 5 nm, respectively ((c) in FIG. 2). Next, the resist pattern was peeled off with acetone.

  Thereafter, the second heat treatment was performed in a nitrogen gas atmosphere in the same manner as the first heat treatment, and the heat treatment was performed by instantaneous heat treatment using an RTA (Rapid Thermal Annealing) apparatus. The heat treatment time was 1 minute, and the heat treatment temperature was 810 ° C. The temperature rising time until reaching 810 ° C. was 60 seconds.

(Formation of positive electrode layer on p-type layer)
Thereafter, a Ni (20 nm) / Au (50 nm) electrode (positive electrode layer 50) is formed on the p-type contact layer 40c by vacuum deposition, and then heat treatment is performed in an oxygen atmosphere for 3 minutes at 550 ° C. A wafer having the above layer structure was manufactured. Subsequently, it cut out to 700 micrometers square, and produced the group III nitride semiconductor light-emitting device 1 ((d) of FIG. 2).

  The current-voltage characteristics of the negative electrode layer 60 (the first metal electrode layer 61, the second metal electrode layer 62a, the reflective electrode layer 62b, and the noble metal layer 62c) thus manufactured were measured. When the voltage value of Reference Example 1 shown below at 20 mA was normalized as 1, the voltage value was 1.1 (1.1 times the voltage value of Reference Example 1). Further, the optical output-current characteristics of the obtained group III nitride semiconductor light emitting device were measured. When the luminous efficiency value of Reference Example 1 shown below at 60 mA was normalized as 1, the luminous efficiency was 1.1 (1.1 times the luminous efficiency of Reference Example 1). . The results are summarized in Table 1.

Reference example 1
In Example 1, the second region 60 b is not formed, and the negative electrode 60 is formed on the n-type layer 20.

  That is, first, after forming the first metal electrode layer 61 with a thickness of 10 nm on the n-type layer 20 by the same method as in Example 1, the first heat treatment was performed by the same method as in Example 1. The portion where the first metal electrode layer 61 is formed is the sum of the first region 60a and the second region 60b of the first embodiment. Next, a reflective electrode layer 62b and a noble metal layer 62c of 5 nm are formed on the entire surface of the first metal electrode layer 61 in the same manner as in the first embodiment, and then a second heat treatment is performed in the same manner as in the first embodiment. went. The formation of the positive electrode layer, the measurement of the current-voltage characteristics of the obtained group III nitride semiconductor light-emitting device, and the measurement of the light output-current characteristics were carried out in the same manner as in Example 1. In the current-voltage characteristics of the group III nitride semiconductor light-emitting device obtained in Reference Example 1, the voltage at 20 mA was normalized as 1, and compared with the voltages of the examples and comparative examples. Further, in the light output-current characteristics of the obtained group III nitride semiconductor light emitting device, the value of the light emission efficiency at 60 mA was normalized as 1, and compared with the light emission efficiency values of Examples and Comparative Examples.

Comparative Example 1
In Example 1, except that the second metal electrode layer 62a and the noble metal layer 62c were not manufactured, the reflective electrode layer 62b was made of Al alone, the film thickness was 300 nm, and the second heat treatment was not performed. (The first heat treatment was performed after the first metal electrode layer 61 was formed, and then only the reflective electrode layer 62b was formed.). The voltage value at 20 mA of the Group III nitride semiconductor light-emitting device 1 manufactured in this way was 2.5 times that of Reference Example 1. In addition, the value of the luminous efficiency at 60 mA could not be evaluated because the light emitting element deteriorated because the voltage was too high. The results are summarized in Table 1.

Comparative Example 2
In Comparative Example 1, after the reflective electrode layer 62b (Al layer thickness 300 nm) was formed, the second heat treatment was performed in the nitrogen gas atmosphere except that an instantaneous heat treatment using an RTA (Rapid Thermal Annealing) apparatus was performed. The same operation as in Example 1 was performed. The heat treatment time was 1 minute, and the heat treatment temperature was 810 ° C. The temperature rising time until reaching 810 ° C. was 60 seconds. The voltage value at 20 mA of the group III nitride semiconductor light-emitting device 1 manufactured in this way was 1.3 times that of Reference Example 1. Further, the value of the luminous efficiency at 60 mA was 1.1 times the value of the luminous efficiency of Reference Example 1. The results are summarized in Table 1.

Example 2
In Example 1, the same operation as in Example 1 was performed except that the heat treatment temperature in the second heat treatment was 860 ° C. The temperature rising time until reaching 860 ° C. was 60 seconds. The voltage value at 20 mA of the group III nitride semiconductor light-emitting device 1 manufactured in this way was 1.0 times that of Reference Example 1. Further, the value of the luminous efficiency at 60 mA was 1.1 times the value of the luminous efficiency of Reference Example 1. The results are summarized in Table 1.

Example 3
In Example 1, the same operation as in Example 1 was performed except that the heat treatment temperature in the second heat treatment was 760 ° C. The temperature rising time until reaching 760 ° C. was 60 seconds. The voltage value at 20 mA of the group III nitride semiconductor light-emitting device 1 manufactured in this way was 1.3 times that of Reference Example 1. Further, the value of the luminous efficiency at 60 mA was 1.3 times the luminous efficiency value of Reference Example 1. The results are summarized in Table 1.

  The Group III nitride semiconductor light-emitting device of the present invention obtained in Example 1 was found to have an improved luminous efficiency, although the voltage value was higher than that of Reference Example 1. As is apparent from comparison with Comparative Examples 1 and 2, the group III nitride semiconductor light-emitting device of the present invention was able to increase the light emission efficiency while suppressing an increase in voltage value. That is, the Group III nitride semiconductor light-emitting device of the present invention obtained by forming the second metal electrode layer and performing the first heat treatment and the second heat treatment increases luminous efficiency while maintaining ohmic contact. I was able to. Further, if the second heat treatment temperature is adjusted, the luminous efficiency can be increased with the same voltage value (Example 2), or the luminous efficiency can be further increased although the voltage value is somewhat higher. (Example 3).

DESCRIPTION OF SYMBOLS 1 Light emitting element 10 Substrate 20 N type layer 30 Active layer area | region 40 P type layer 40a Electron block layer 40b P type cladding layer 40c P type contact layer 50 Positive electrode layer 60 Negative electrode layer 60a First area 60b Second area 61 1st Metal electrode layer 62a Second metal electrode layer 62b Reflective metal layer 62c Noble metal layer

Claims (7)

In the group III nitride semiconductor light-emitting device having a stacked structure in which an n-type layer, an active layer, and a p-type layer are stacked in this order, a part of the active layer and the p-type layer is removed to remove the n A method of forming a mesa structure by exposing a mold layer and forming a negative electrode layer on a surface of the exposed n-type layer,
Dividing the region where the negative electrode layer on the n-type layer surface is formed into at least a first region close to a mesa structure and a second region having a larger area than the first region;
Forming a first metal electrode layer made of at least one metal selected from the group consisting of Ti, V and Ta in the first region;
A second metal electrode layer made of at least one metal selected from the group consisting of Ti, V and Ta and having a thickness smaller than that of the first metal electrode layer is formed on the second region and the first metal electrode layer. Forming step,
A reflective electrode layer made of a metal having a work function of 4.0 to 4.8 eV and a specific resistance of 1.5 × 10 ^{−6 to} 4.0 × 10 ⁻⁶ Ω · cm is formed as the second metal electrode layer. Forming on the top,
Forming a noble metal layer comprising at least one noble metal selected from the group consisting of Au and Pt on the reflective electrode layer;
After forming the noble metal layer, including a step of heat treatment at a temperature of 400 ° C. or more and 1000 ° C. or less,
A method for forming an n-type negative electrode, comprising a step of performing a heat treatment at a temperature of 800 ° C. or higher and 1200 ° C. or lower after forming the first metal electrode layer and before forming the reflective electrode layer. The n-type layer is Al _x In _y Ga _z N (x, y, z are rational numbers satisfying 0 <x ≦ 1.0, 0 ≦ y ≦ 0.1, 0 ≦ z <1.0, and x + y + z 2. The method for forming an n-type negative electrode according to claim 1, comprising a group III nitride single crystal satisfying a composition represented by:   The method for forming an n-type negative electrode according to claim 1 or 2, wherein the step of heat-treating at a temperature of 800 ° C or higher and 1200 ° C or lower is performed before forming the second metal electrode layer.   The method for forming an n-type negative electrode according to claim 1, wherein the thickness of the second metal electrode layer is 0.1 to 5 nm.   The method for forming an n-type negative electrode according to any one of claims 1 to 4, wherein the area of the second region is more than 1 time and not more than 40 times the area of the first region.   A group III nitride semiconductor light-emitting device having an n-type negative electrode formed by the method according to claim 1. In the group III nitride semiconductor light-emitting device having a stacked structure in which an n-type layer, an active layer, and a p-type layer are stacked in this order, a part of the active layer and the p-type layer is removed to remove the n A group III nitride semiconductor light-emitting device in which a mesa structure is formed by exposing a mold layer, and a negative electrode layer is provided on the surface of the exposed n-type layer,
The region where the negative electrode layer on the surface of the n-type layer is formed has a first region close to a mesa structure and a second region having an area larger than the first region,
A first bonding metal layer made of at least one metal selected from the group consisting of Ti, V and Ta, a work function of 4.0 to 4.8 eV, and a specific resistance of 1.5 × 10 ⁻⁶ to A first ohmic in which a reflective electrode layer made of a metal having 4.0 × 10 ⁻⁶ Ω · cm and a noble metal layer made of at least one kind of noble metal selected from the group consisting of Au and Pt are laminated in this order. Having an electrode layer on the first region;
The second bonding metal layer, which is made of at least one metal selected from the group consisting of Ti, V, and Ta and is thinner than the first bonding metal layer, the reflective electrode layer, and the noble metal layer in this order. A group III nitride semiconductor light-emitting device comprising a second ohmic electrode layer laminated on the second region.

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