KR102300718B1 - Ultraviolet light-emitting diode and electric apparatus having the same - Google Patents

Abstract

[Task] Increase the light extraction efficiency of deep ultraviolet light emitting diodes.
[Solutions] A typical LED device 100A of one embodiment of the present invention includes a single crystal sapphire or AlN crystal substrate 110, an n-type conductive layer 132, a recombination layer 134, and a p-type conductive layer ( 136) is provided with an ultraviolet light emitting layer 130 stacked in this order on the side of the substrate layer. A p-type contact layer 150 and a second electrode 160 (reflective electrode) are further laminated on the p-type conductive layer 136 . Both the ultraviolet light emitting layer and the p-type contact layer are made of a mixed crystal of AlN and GaN. As the transmittance of the p-type contact layer with respect to the emission wavelength increases, the light extraction efficiency is improved. The present invention also provides an electric device equipped with the LED element 100A.

Description

Ultraviolet light emitting diode and electric device having same

The present invention relates to an ultraviolet light emitting diode and an electric device having the same. More particularly, the present invention relates to an ultraviolet light emitting diode having improved light extraction efficiency and an electric device having the same.

A blue light emitting diode is widely used as a solid light emitting device using a nitrogen compound semiconductor. A solid-state light source is required even in the shorter wavelength ultraviolet band, so ultraviolet light emitting diodes (UVLEDs) using similar materials are being developed. Deep ultraviolet LED (DUVLED) is being developed because it is expected that a wide range of uses ranging from sterilization and water purification to medical applications is expected for the deep ultraviolet band of 350 nm or less, especially the ultraviolet in the wavelength band of about 260 to 280 nm. A typical DUVLED has a stacked structure made of a gallium aluminum nitride-based semiconductor containing aluminum (Al), gallium (Ga) and nitrogen (N) as main components using a sapphire substrate or an AlN single crystal substrate. DUVLED output can also be improved, and DUVLEDs that operate at an ultraviolet output of about 10 mW are also being put to practical use.

One of the technical problems of DUVLED is improvement of luminous efficiency. One of the luminous efficiency indicators is the external quantum efficiency η _{EQE, which} is defined by dividing the number of photons per unit time emitted to the outside of the LED device by the number of electrons per unit time input as a driving current. This external quantum efficiency η _EQE is expressed in the form of a product of three factors: internal quantum efficiency η _IQE , electron injection efficiency η _EIE and light extraction efficiency η _{LEE .} That is, the relationship of _{η EQE} = η _IQE × η _EIE × η _{LEE holds.}

As a result of the development so far, a dramatic improvement has been achieved with respect to the _{internal quantum efficiency η IQE} and the electron injection efficiency η _EIE among the three factors in the DUVLED. A specific technique that brought about _{the improvement is reduction of crystalline dislocations of the ultraviolet light emitting layer for improvement of internal quantum efficiency η IQE} (for example, Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2). In addition, in order to improve the electron injection efficiency η _EIE , a technique for reinforcing electron blocking properties in the p-type semiconductor layer is effective by adopting a structure called MQB (Multiple Quantum Barrier) structure using a superlattice structure in the p-type semiconductor layer. For example, Patent Document 2). At the present time, _{the value of internal quantum efficiency η IQE} × electron injection efficiency η _EIE can be expected to be 50% to 80%.

However, the internal quantum efficiency η _IQE × electron injection efficiency η gateuna will be expected to have a high value even if the value of the _EIE actual DUVLED cattle growing external quantum efficiency η _EQE is staying in by up to 4%. Obviously, the low light extraction efficiency η _LEE is the cause. The light extraction efficiency η _LEE of the conventional DUVLED is only about 10%.

[Patent Document 1] Japanese Patent Laid-Open No. 2009-54780 [Patent Document 2] International Publication No. 2011/104969

[Non-Patent Document 1] H. Hirayama et al., "231-261 nm AlGaN deep-ultraviolet light-emitting diodes fabricated on AlN multilayer buffers grown by ammonia pulse-flow method on sapphire, "Appl. Phys. Lett. 91, 071901 (2007) [Non-Patent Document 2] H. Hirayama et al., "222-282 nm AlGaN and InAlGaN-based deep-UV LEDs fabricated on high-quality AlN on sapphire, "Phys. Stat. Solidi(a), 206, 1176, (2009) [Non-Patent Document 3] S, Fujikawa, T. Takano, Y. Kondo and H. Hirayama, “Realization of 340-nm-band high-output-power (7mW) InAlGaN quantum well ultraviolet light-emitting diode with p-type InAlGaN", Jap. J. Appl. Phys. 47, 2941 (2008).

The present inventors examined the method of improving the _{light extraction efficiency η LEE} in an ultraviolet light emitting diode from various viewpoints. Further, attention was paid to the fact that in the conventional DUVLED device, the p-type contact layer disposed to secure the electrical connection to the p-type conductive layer exhibits absorption with respect to the wavelength of ultraviolet light (radiation UV) emitted from the ultraviolet light emitting layer. That is, in a typical DUVLED, an LED element is manufactured by arranging a substrate, a buffer layer, an ultraviolet light emitting layer, a p-type conductive layer, a p-type contact layer, and a metal electrode in this order. And the radiation UV from the ultraviolet light emitting layer is radiated outward from both sides of the layer structure. Among the radiation UVs, those directed to one side of the substrate are extracted to the outside, or part or all of them (components in the direction of total reflection) return to the inside. Radiation UV directed to the p-type contact layer on the other side passes through the p-type contact layer, is reflected from the metal electrode, and passes through the p-type contact layer again. Radiation UV directed to the p-type contact layer leaves the possibility of being extracted to the outside by redirecting to the substrate side only for components that are not absorbed until that stage. The absorption of the p-type contact layer significantly deteriorates the _{light extraction efficiency η LEE} because the radiated UV absorbed by the p-type contact layer is lost. A typical conventional p-type contact layer is made of p-type GaN, which is easy to increase the hole concentration for the purpose of ensuring good electrical connection. Since this p-type GaN exhibits strong absorption especially in the deep ultraviolet wavelength band, when it is employed in the p-type contact layer, it absorbs radiated UV.

In particular, it is necessary to pay attention to the absorption of the p-type contact layer to attenuate the effect of other technical studies. For example, even if the reflectance of the reflective electrode is increased, if the absorption in the p-type contact layer is strong, the effect is small. Similarly, despite the implementation of various studies for extracting the radiation UV propagating inside the waveguide mode, if the absorption in the p-type contact layer is strong, the UV of the waveguide mode itself is weakened, so the effect is limited. In order to increase the light extraction efficiency η _LEE , the absorption of the p-type contact layer should be suppressed.

The present invention provides _{a DUVLED for increasing light extraction efficiency η LEE} , and contributes to realization of a high-efficiency DUVLED.

The present inventors noted that absorption is weak when the composition containing Al (aluminum) is used in the p-type contact layer as in the ultraviolet light emitting layer, and that the operation of the DUVLED is possible even if a p-type contact layer of such a composition is employed. It was confirmed experimentally and led to the creation of the present invention.

In one embodiment of the present invention, a single crystalline substrate of sapphire or AlN crystal and a single crystalline substrate of sapphire or AlN crystal are disposed in contact with the substrate or in contact with an additional buffer layer provided on the substrate to be at least n-type A conductive layer, a recombination layer and a p-type conductive layer are stacked in this order from the side of the substrate, and an ultraviolet light emitting layer of a mixed crystal of AlN and GaN, and a mixed crystal of AlN and GaN electrically connected to the p-type conductive layer. There is provided an ultraviolet light emitting diode having a p-type contact layer and a reflective electrode that exhibits reflectivity to radiation UV, which is ultraviolet light emitted from the above-described ultraviolet light emitting layer, and is disposed in contact with the p-type contact layer.

In the present invention, a composition of a mixed crystal of AlN and GaN is employed for the p-type contact layer. Also, in the present application, a mixed crystal of AlN and GaN is described as AlGaN, and a mixed crystal of (AlN) _x and (GaN) _1-x is described as Al _x Ga _1-x N, respectively. In addition, it should be noted that the mixed crystal of AlN and GaN may contain components other than AlN and GaN.

The band gap of AlGaN is larger than that of GaN. In addition, the bandgap can adjust and change the Al ratio (the ratio of AlN in the AlN and GaN mixed crystal, the AlN mixed crystal composition ratio). By using this property, the transmittance for radiated UV can be adjusted by adjusting the absorption edge of the p-type contact layer employing AlGaN. In particular, in a configuration in which a mixed crystal of AlN and GaN is also employed in the ultraviolet light emitting layer, when AlGaN is employed in the p-type contact layer, it is easy to adjust the transmittance to correspond to the wavelength of UV emitted from the ultraviolet light emitting layer. The present invention actively utilizes this principle.

What may be a problem here is the carrier concentration of the p-type contact layer. In the conventional p-type GaN employed for the p-type contact layer, a sufficient carrier concentration can be obtained by doping with Mg, and electrical connection with the electrode has been established. However, in AlGaN, it is difficult to increase the carrier concentration. It has been thought that if the AlN mixed crystal composition ratio is increased according to the wavelength of the UV emitted from the ultraviolet light emitting layer, the p-type acceptor level during Mg doping will deepen, and the lack of carriers (lack of hole concentration) in the p-type conductive layer is likely to occur. According to this conventional idea, if AlGaN is employed as the p-type contact layer as in the present invention, there is a concern that a sufficient carrier concentration will not be obtained and electrical connection with the electrode will be insufficient. However, as a result of fabricating a DUVLED device by employing AlGaN in the p-type contact layer, the present inventors have confirmed that the light extraction efficiency η _LEE is significantly improved, and the external quantum efficiency η _EQE is rather increased. Accordingly, a configuration employing AlGaN for the p-type contact layer of the DUV LED device is sufficiently practical.

In addition, in the description of the entire application, a problem in the transmittance of the p-type contact layer is the value of the wavelength of radiation UV emitted from the ultraviolet light emitting layer. However, in order to explain the invention, wavelengths other than radiation UV will be appropriately described.

In the above-described embodiment of the present invention, it is preferable that the transmittance of the p-type contact layer is 90% or more. More preferably, the transmittance is 95% or more. In addition, the transmittance of the p-type contact layer of the present application refers to a value that does not include the interfacial reflection effect, that is, the internal transmittance. Transmittance is a value for one pass in the depth direction, unless otherwise specified. Since the radiating UV travels through the p-type contact layer, absorption of the p-type contact layer can occur at least twice, until it is incident on the reflective electrode and after it is emitted (reflected) from there. Therefore, it is important to sufficiently suppress the absorption of the p-type contact layer, that is, to realize a sufficiently high transmittance _{to increase the light extraction efficiency η LEE .} In order to increase the transmittance, it can be realized by increasing the Al composition ratio contained in the p-type contact layer, that is, the AlN mixed crystal composition ratio.

In the above-described embodiment of the present invention, it is preferable if the p-type conductive layer is provided with an electron block layer, and it is more preferable that the electron block layer is a multiple quantum barrier (MQB). In the DUVLED device, when the hole concentration is insufficient, electrons leak (or overflow) from the recombination layer, so that the electron injection efficiency η _EIE tends to decrease. In preparation for such a case, if the electron block layer is disposed at a position downstream of the recombination layer when viewed from the electrons, the ratio of electrons involved in recombination can be increased _{to increase the electron injection efficiency η EIE .} A typical example of this electron blocking layer is a single barrier, that is, a single layer in which the energy level of the conduction band is increased by increasing the AlN mixed crystal composition ratio. Another more preferred example for an electron block layer is MQB. In MQB, a plurality of layers with different AlN mixed crystal composition ratios are stacked to provide unevenness in the conduction band energy level in the thickness direction, and again, if necessary, the period is changed according to the position in the thickness direction. Multiple reflections of the MQB help to more effectively suppress the leakage of electrons. Furthermore, MQB does not require an excessively high AlN mixed-crystal composition ratio, which is advantageous in suppressing a decrease in hole concentration.

In the above-described embodiment of the present invention, it is preferred that the material of the p-type contact layer be such that the compositional wavelength is shorter than the main wavelength of the radiation UV. Here, the composition wavelength is one of the indicators for the material of the p-type contact layer or its growth conditions, and is defined by the peak wavelength of photo luminescence of the layer. In order to experimentally determine the composition wavelength, a thick film may be prepared under the same composition and growth conditions as the p-type contact layer to be characterized, and the photoluminescence may be measured. In general, since the composition wavelength is an indicator of the absorption edge, the transmittance at a long wavelength with the composition wavelength as a boundary is high and decreases at a short wavelength. The main wavelength of radiation UV contrasting with this composition wavelength may be, for example, a peak wavelength of radiation UV with a certain wavelength width. If the material of the p-type contact layer is selected and growth conditions are determined so that the composition wavelength is shorter than the main wavelength of the radiation UV, the transmittance of the p-type contact layer is increased at the main wavelength. Therefore, paying attention to the composition wavelength to characterize the p-type contact layer and correlating it with the dominant wavelength of the radiated UV can provide a good indicator for the above-described embodiment. In addition, the main wavelength of the radiation UV is simply referred to as the emission wavelength in the present application.

The above-described embodiment of the present invention, when _{the AlN mixed crystal composition ratio x of the composition Al x} Ga _{1- x} N of the p-type contact layer is the value of the main wavelength of the radiation UV, W (nm)

x _min = -0.0060 W + 2.26

Obtained by the lower limit is preferred if it is more than the value x _min. The AlN mixed crystal composition ratio x can realize the transmittance described above if the lower limit thereof is determined in relation to the value of the main wavelength of radiation UV. The AlN mixed crystal composition ratio x is a numerical value such that x = 0 when there is no AlN and only GaN, and x = 1 when there is only AlN, and when described together with AlGaN, it is also expressed as _{Al x} Ga _{1- x N. The lower limit x min} of the AlN mixed crystal composition ratio x is, for example, 0.7 (70%) at 260 nm and 0.58 (58%) at 280 nm in relation to a specific wavelength. Even at wavelengths other than these, if the _{AlN mixed crystal composition ratio x of the p-type contact layer is determined with x min} determined by the above formula as the lower limit, the effect of the present invention can be exhibited by the p-type contact layer with increased transmittance.

The present inventors also noted that the effect of increasing the reflectance of the reflective electrode is limited in a state where the transmittance of the p-type contact layer is low. Conversely, in the above-described embodiment of the present invention, when the transmittance of the p-type contact layer is ensured, the UV reflection effect of the reflective electrode significantly improves the _{light extraction efficiency η LEE .} In the above-described embodiment of the present invention, it is advantageous to increase the reflectance of the reflective electrode.

That is, in the preferred configuration of the above-described embodiment of the present invention, the reflective electrode is a metal film containing Al as a main component, and an interposed metal layer for ohmic contact is sandwiched in contact with the p-type contact layer and the metal film. Au (gold) is widely used in the prior art for the reflective electrode. In contrast, Al exhibits high reflectivity for ultraviolet wavelengths. Paying attention to this point, it is advantageous to employ a metal film mainly composed of Al in order to increase the reflectance of the reflective electrode.

It is preferable that the insertion metal layer of the above-described embodiment is made of a Ni film and is thinner than 5 nm, and more preferably has a substantial thickness of 1 nm. For the insertion metal layer, a material is selected to realize ohmic contact, and palladium (Pd), platinum (Pt), silver (Ag), titanium (Ti), etc. may be employed other than Ni. Suitable as an intercalation metal layer are generally metals having a large work function. It is also useful to make an alloy by adding each metal or other elements to each other. However, in the case of a Ni film, it is preferable that the thickness is thinner than 5 nm in order to maintain a high reflectance. In particular, when the actual thickness becomes 1 nm, ohmic contact can be realized and high reflectance can be obtained, making it possible to achieve both optical and electrical aspects for DUVLED. In addition, the 'substantial' thickness means that an error in film thickness that may occur in actual film formation or thickness distribution due to non-uniform film formation is allowed. Also, in the case of other than the Ni film, by adjusting the film thickness similarly, the electrical connection can be established while increasing the reflectance.

In addition, in the above-described embodiment of the present invention in which the transmittance of the p-type contact layer is high, various configurations for extracting radiation UV propagating in the waveguide mode to the outside are also useful. In the LED element of the above-described embodiment, a void (void) may be formed in the buffer layer. For example, the cross section of the buffer layer in the in-plane direction at the time of crystal growth is formed into a columnar structure by growing crystals only in each graphic part of a plurality of isolated graphic patterns or island-shaped parts of a sea-island pattern. can grow The space between the pillars at that time becomes a void. Conversely, in the cross section in the in-plane direction, an open portion is formed by growing crystals only between a portion between each figure of a plurality of isolated figure patterns or a sea portion of the sea island structure pattern. In this case, this opening becomes a void. In addition to these, when grown to form a stripe pattern, multiple walls are formed and voids are formed between the walls. By studying the crystal growth conditions, it is possible to obtain again a single crystal continuous layer of the crystal quality required for the buffer layer by closing the voids with growth. Among various techniques for forming voids, it is particularly useful to form pillars and extend the pillars in the device thickness direction to form a waveguide. When a columnar structure with a high aspect ratio is made to allow vertical propagation of light, light can easily propagate the column. In this case, only about 2 to 3 round trips are required before the light source in waveguide mode is extracted to the outside. Therefore, it is very advantageous when the reflectance of the second electrode is not necessarily sufficient. In addition, in a simple void structure, since light is extracted to the outside through internal reflection of about 10 times, it is easy to attenuate in the middle of the reciprocation. Moreover, it is also preferable to remove a board|substrate in the structure in which the void is formed in the buffer layer. For example, when a pillar is formed, it is advantageous because, when the substrate is removed, a large number of vertical waveguides connected in the direction of extracting light are arranged and they are located on the surface as they are.

Moreover, in the LED element of the above-mentioned embodiment, the main wavelength of radiation|emission UV may just be one wavelength of 260 nm or more and 280 nm or less. By fabricating a moth-eye structure on a surface or interface involving a refractive index step, it is possible to reduce the surface reflection or interfacial reflection normally caused by the refractive index step. Since this level of refractive index is a problem in the propagation path of radiation UV and the surface when it is extracted to the outside, a moth-eye structure is formed on the surface opposite to the reflective electrode or on one of the surfaces where the refractive index level increases among other interfaces. It is preferable to have Examples of such surfaces and interfaces include, for example, the surface that becomes the outermost surface of the LED element and the outer world, and when sealing the LED element with a UV-transparent medium (resin, glassy transparent material, etc.), the interface between the LED element and the UV-transmissive medium can be heard

Furthermore, in the above-described embodiment of the present invention, it is also preferable that either of the p-type conductive layer and the p-type contact layer contains In in addition to the material of the mixed crystal of AlN and GaN. When In is included in the p-type conductive layer or the p-type contact layer including the electron barrier layer, the effect of increasing the career (hole) concentration can be expected.

Furthermore, the ultraviolet light emitting diode of any one of the above-described embodiments of the present invention is useful when equipped as an ultraviolet emitting source and applied to an electric device. This is because the usefulness of these electric devices increases when the efficiency of the ultraviolet emitting source is improved.

Ultraviolet light emitting diodes (hereinafter referred to as 'UVLEDs') are light emitting diodes that emit electromagnetic waves (ultraviolet rays) in the ultraviolet band. In the present application, UVLEDs (DUVLEDs) in a wavelength range centered on deep ultraviolet rays, which are mainly referred to as a wavelength band of 220 to 350 nm, can be manufactured. In addition, the present invention also provides a DUVLED that emits light in the deep ultraviolet band of the wavelength range of the so-called sterilization wavelength (260 to 280 nm). In addition, in the present application, for example, for electromagnetic radiation other than visible light, which is referred to as an ultraviolet band, according to convention, expressions in the optical field such as 'light', 'light source', 'light emission', 'light extraction' are used.

In addition, the ultraviolet light emitting layer is typically a multilayer body of AlGaN layers, that is, the composition of each layer constituting the multilayer body is Al _y Ga _{1- y} N (y is any value of 0 ≤ y ≤ 1). It is a multilayer body further doped with a small amount of an element (dopant) for positive or negative conductivity type. The ultraviolet light emitting layer is approximately produced in a configuration in which an n-type conductive layer, a recombination layer, and a p-type conductive layer are laminated in this order. Incidentally, in some cases, each of the n-type conductive layer, the recombination layer, and the p-type conductive layer itself is a multilayer film for a quantum well structure or the like.

In any one of the embodiments of the present invention, an ultraviolet light emitting diode with _{reduced absorption by the p-type contact layer and increased light extraction efficiency η LEE is realized.}

BRIEF DESCRIPTION OF THE DRAWINGS It is explanatory drawing which shows the concept of embodiment of this invention.
Fig. 2 is a perspective view showing a schematic configuration common to both the conventional LED element and the LED element according to the embodiment of the present invention.
Fig. 3 is a schematic cross-sectional view showing a schematic configuration common to both the conventional LED element and the LED element according to the embodiment of the present invention.
Fig. 4 is a flowchart showing a manufacturing method according to an embodiment of the present invention.
[FIG. 5] The dependence of _{external quantum efficiency η EQE} on the current density of a sample in which the AlN mixed crystal composition ratio of the p-type contact layer was changed in an example in which AlGaN was employed for the p-type contact layer of the embodiment of the present invention It is one graph.
[Fig. 6] A graph showing the dependence of _{external quantum efficiency η EQE} on the current density of an example in which AlGaN is employed for the p-type contact layer in the embodiment of the present invention and the AlN mixed crystal composition ratio of the p-type contact layer is changed.
[Fig. 7] Fig. 7 is a graph obtained by measuring the transmission spectrum of a film formed under the same material and film formation conditions as those of the p-type contact layer used in Examples of the present invention.
[Fig. 8] Fig. 8 is an explanatory diagram showing the dependence of the AlN mixed crystal composition ratio on the wavelength of radiation UV for the p-type contact layer employed in the embodiment of the present invention.
[Fig. 9] A reflection spectrum of the metal film for the intercalated metal layer and the second electrode (reflecting electrode) of the present invention.
[FIG. 10] It is an actual measurement graph showing a state in which characteristics are improved by making only the second electrode (reflecting electrode) different from each other in the embodiment of the present invention.
_{[FIG. 11] shows a change in external quantum efficiency η EQE} when the second electrode (reflecting electrode) is changed to one having high reflectivity in the embodiment of the present invention. (a) is an explanatory diagram schematically showing the state of a comparison target sample for which a change is calculated, and (b) is a graph thereof.
[Fig. 12] Fig. 12 is a graph showing the measured characteristics in the case where ultraviolet light emitting layers having different emission wavelengths are introduced to the p-type contact layer of a common composition in the embodiment of the present invention. (a) shows the dependence of the external quantum efficiency η _EQE on the current density, and (b) shows the emission spectrum when the _{maximum external quantum efficiency η EQE is obtained.}
[Fig. 13] Fig. 13 is a cross-sectional view showing an element configuration of an LED obtained in a modified example of the embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of an ultraviolet LED element (hereinafter, 'LED element') according to the present invention and a manufacturing method thereof will be described with reference to the drawings. In the description, common reference numerals are assigned to common parts or elements throughout the drawings, unless otherwise specified. In addition, in each figure, each part is not shown to maintain the scale of each other.

1. Embodiment

BRIEF DESCRIPTION OF THE DRAWINGS It is explanatory drawing which shows the concept of this embodiment. In FIG. 1, the schematic cross-sectional structure of the conventional LED element 100 and the LED element 100A of this embodiment is shown to each left and right. In both LED devices, light is emitted in all directions from the light emitting layer formed through several layers on the substrate. Conventionally, since the p-type contact layer is p-type GaN, the radiated UV of deep ultraviolet is absorbed when passing through the p-type contact layer. Moreover, the refractive index of the material which transmits UV in common to both LED elements is about 2 or more. Therefore, most of the radiated UV is not emitted from the substrate, but becomes a waveguide mode and propagates inside. Specifically, the light directly extracted from the UV light emitting layer without going through reflection or refraction is limited to those directed toward the substrate among the emitted light (radiated UV), and has an apex angle of about 20 degrees in the normal direction of the substrate. is limited to light in the range of solid angles produced by the cone of . This solid angle is only about 8% of the total. If the conventional p-type contact layer is employed, there is no possibility that the remaining components are extracted to the outside.

On the other hand, in the present invention, AlGaN, that is, a mixed crystal of AlN and GaN, which is a material having high UV transmittance, is employed for the p-type contact layer. Accordingly, absorption of light directed to the electrode (second electrode, reflective electrode) is suppressed. The light passing through the p-type contact layer includes not only the radiation UV directed directly from the ultraviolet light emitting layer, but also the light propagating inside the waveguide mode. If the UV transmittance of the p-type contact layer is increased, the radiation UV emitted from the reflective electrode side, which has not been used before, can also be directed toward the substrate, so that the light in the waveguide mode can be used without being absorbed unnecessarily. This is the object of this embodiment.

1-1. Configuration of LED element

Fig. 2 is a perspective view showing a schematic configuration common to both the conventional LED element 100 and the LED element 100A of the present embodiment. 3 is a schematic cross-sectional view thereof. In the LED devices 100 and 100A, a buffer layer 120 is epitaxially grown according to a material such as AlN crystal on one surface 104 of the sapphire substrate 110, which _{is generally a flat α-Al 2} O _{3 single crystal.} The ultraviolet light emitting layer 130 is disposed so as to be in contact with the buffer layer 120 . The crystallinity of the ultraviolet light emitting layer 130 is also epitaxially grown with respect to the buffer layer 120 . The configuration of the ultraviolet light emitting layer 130 is common regardless of whether it is the conventional LED element 100 or the LED element 100A of the present embodiment. Specifically, in the configuration of the ultraviolet light emitting layer 130 , the n-type conductive layer 132 , the recombination layer 134 , and the p-type conductive layer 136 are stacked in this order next to the buffer layer 120 . The material of the ultraviolet light emitting layer 130 is typically AlGaN or a composition doped with a trace element (Si for n-type, Mg for p-type, etc.). The first electrode 140 is electrically connected to the n-type conductive layer 132 . In contrast, the p-type conductive layer 136 has a p-type contact layer 150 in the case of the conventional LED device 100 , and the second electrode 160 is disposed thereon. In the case of the LED element 100A of this embodiment, the p-type contact layer 150A is formed on the p-type conductive layer 136, and the second electrode 160A is disposed thereon. The second electrode 160 or 160A establishes electrical connection with the p-type conductive layer 136 via the p-type contact layer 150 or 150A, respectively. And the light output L is emitted from the light output surface 102 on the other side of the sapphire substrate 110 .

The configuration of each layer will be described in more detail. The buffer layer 120 is introduced to epitaxially grow AlGaN crystals on the sapphire substrate 110, and is manufactured to a thickness of, for example, about 2 μm. That is, the buffer layer 120 is employed to satisfy the crystal growth request of forming a high-quality AlGaN layer crystal on the sapphire substrate 110 _{to increase the internal luminous efficiency η IQE.} In the LED element 100A of the present embodiment, the buffer layer 120 can be the same as this.

With respect to the ultraviolet light emitting layer 130 , the n-type conductive layer 132 is, for example, a layer _{of Al 0.60} Ga _{0.40 N doped with Si to be n-type, that is, an Al 0.60} Ga _0.40 N;Si layer. The recombination layer 134 is _{an MQW (multiple quantum well) stack in which thin films having a composition of Al 0.60} Ga _0.40 N and Al _0.53 Ga _0.47 N are stacked to form a superlattice structure, and the number of quantum wells in the recombination layer 134 is For example, about 3. The p-type conductive layer 136 is an AlGaN; Mg layer, that is, a layer of AlGaN doped with Mg to be p-type. In the LED element 100A of the present embodiment, the p-type conductive layer 136 may be optionally provided with the electron blocking layer 138 . In that case, the electron block layer 138 is made of, for example, MQB.

In the conventional LED element 100, the p-type contact layer 150 is, for example, a layer of GaN; Mg (also called p-type GaN), which is gallium nitride doped with magnesium. On the other hand, in the p-type contact layer 150A employed in the LED element 100A of the present embodiment, a material containing Al, that is, a material of AlN, GaN and mixed crystal (AlGaN) is doped with Mg (hereinafter referred to as "AlGaN"). The material is also called p-type AlGaN).

The reason that p-type GaN is employed in the conventional p-type contact layer 150 is to establish an electrical connection with an electrode to perform a current injection operation. However, as described above, GaN;Mg causes deterioration of light extraction efficiency because it absorbs radiated UV. On the other hand, the p-type contact layer 150A of the present embodiment is made of p-type AlGaN to make it highly permeable, and absorption of UV is suppressed. Even in this case, the present inventors confirmed that the LED element 100A actually works. This experimental confirmation is detailed in the Examples.

The first electrode 140 is a metal electrode having a stacked structure of Ni/Au from the base side. This Ni is a thin, for example, 25 nm thick layer interposed between Au and the underlying semiconductor layer to realize an ohmic contact.

In the case of the conventional LED device 100 , the second electrode 160 is also the same as the first electrode 140 . On the other hand, in the LED element 100A, high-purity Al or an alloy containing Al as a main component is employed for the second electrode 160A. This is because, in the LED element 100A of the present embodiment, the transmittance of the p-type contact layer 150A is high, and when the second electrode 160A has reflectivity, the radiation UV can be reflected and extracted efficiently. That is, the second electrode 160A also functions as a reflective film that returns UV to the sapphire substrate 110 side. Therefore, a metal film mainly composed of Al having a higher reflectance than gold is employed for the second electrode 160A (also referred to as a 'reflecting electrode') so as to have a high reflectance in the UV wavelength band. Also in the second electrode 160A of this configuration, Ni, which becomes the insertion metal layer 162, is inserted into the base side from an electrical point of view. However, in the case of Ni, when the thickness exceeds 5 nm, the reflectance to UV decreases. Accordingly, in the present embodiment, the thickness of Ni used as the intercalating metal layer 162 of the second electrode 160 is thinner than 5 nm, and preferably, the actual thickness is about 1 nm. This point will also be described in detail in the Examples. In addition, Al employed in the second electrode in the embodiment of the present application includes not only high-purity Al but also an Al alloy, and a film containing both is also called a 'metal film containing Al as a main component'.

It supplements the shape of the actual LED elements 100 and 100A. These are generally flat-panel shapes with a thickness from the buffer layer 120 to the second electrode 160 or 160A of at most 100 μm, and an in-plane size of the LED element 100 or 100A of about 500 μm × 500 μm or larger. Here, the refractive index at the wavelength of the radiation UV of the material constituting the LED element 100 or 100A is significantly larger than that of the surroundings. In fact, the refractive index of the sealing member (if employed) by an ambient medium (such as air or vacuum) or a UV-transmissive medium is about 1 and 1.5, respectively. On the other hand, the refractive index of the material constituting the LED element 100 or 100A is 2 or more than that. Due to this shape and refractive index, most of the light emitted as a result of recombination of conductive carriers in the recombination layer 134 becomes light in the waveguide mode. That is, light directed in a direction exceeding the critical angle from the normal to the plane of the LED element 100 or 100A is not emitted from the LED element 100 or 100A due to total reflection. In the conventional LED device 100 , since the light in the waveguide mode is absorbed by the p-type contact layer 150 until it is emitted to the outside, it does not contribute to the light output extracted from the light output surface 102 . In the conventional LED device 100 , among the lights generated by the recombination layer 134 , the only effective light output is the light directly emitted from the light output surface 102 opposite to the one surface 104 . The solid angle that can be directly emitted is at most about 8% of the omnidirectional range, so that most of the rest is affected by the absorption of the p-type contact layer 150 .

In contrast, in the LED device 100A of the present embodiment, in addition to the light generated in the recombination layer 134 and directed to the light output surface 102 , light directed to the p-type contact layer 150A is also extracted from the light output surface 102 . ingredients become more Also, if the light entering the waveguide mode is not absorbed by the p-type contact layer 150A, it is reflected by the second electrode 160A and extracted from the light output surface 102 . That is, in the LED element 100A of the present embodiment, a p-type contact layer 150A with low absorption is employed and a reflective electrode with high reflectance is employed for the second electrode 160A, and the ultraviolet light emitting layer 130 is double-sided. Both the directed light and the light serving as the waveguide mode can be utilized. _{This is why the light extraction efficiency η LEE} is improved by increasing the total amount of light output from the light output surface 102 in the LED element 100A.

From this point of view, improvements for more efficiently extracting light in the waveguide mode are also useful in the present embodiment. _{Specifically, in the LED element 100A of the embodiment, the light extraction efficiency η LEE} of the LED element 100A by forming a void in the buffer layer 120 to change the direction of light, such as scattering or refracting radiation UV. can be improved The buffer layer is, for example, manufactured in a structure called a bonding pillar AlN buffer (FIG. 13, buffer layer 120B). This structure is a structure that fulfills the function of generating a direction-changing action while satisfying the request for crystal growth. The present inventors confirmed that even if the buffer layer has a structure such as a bonding pillar AlN buffer, more generally voids, the internal luminous efficiency η _IQE comparable to that of the buffer layer 120 ( FIG. 2 ) in which these are not formed can be maintained. are doing Accordingly, voids or pillars that change the direction of propagating light lead to an improvement in _{light extraction efficiency η LEE of the waveguide mode.} This point will be described later as a modified example.

1-2. LED device manufacturing method

Next, the manufacturing method of the LED element of this embodiment is demonstrated. 4 is a flowchart showing a manufacturing method of the LED element 100A of the present embodiment. In this embodiment, the manufacturing process of the LED device 100A includes a substrate preparation process (S110), a buffer layer growth process (S120), an ultraviolet light emitting layer formation process (S130), a p-type contact layer formation process (S140), an electrode formation process ( S150) is included.

First, in the substrate preparation process ( S110 ), the sapphire substrate 110 is prepared. The sapphire substrate 110 is preferably a single-crystal α-Al ₂ O ₃ (0001)-plane-oriented substrate having a small lattice dislocation. The sapphire substrate 110 is disposed in a reactor of a MOVPE device having a temperature control function and a gas supply function.

Next, the buffer layer 120 is formed in the buffer layer growth process ( S120 ). This buffer layer growth process (S120) is implemented in this embodiment as an example in two processes: a nucleation|crystallization formation process (S122) and an embedding process (S124). In the nucleus crystal forming process ( S122 ) and the embedding process ( S124 ), it is advantageous to apply the technique for high-quality crystal growth developed by the present inventors in order to realize a crystal of as high quality as possible in the buffer layer 120 .

Specifically, in the nucleation crystal forming step ( S122 ), minute AlN crystals are grown under conditions in which minute AlN crystals are easily formed. The produced crystal nuclei function as seed crystals for subsequent crystal growth. For this method, the MOVPE process in which ammonia is supplied intermittently in time series (called 'ammonia pulse flow growth method') is very suitable.

In the subsequent embedding process (S124), a base crystal layer (not shown) is grown so as to fill between the crystal nuclei. In this process, it is preferable to carry out the ammonia pulse flow growth method and the simultaneous supply growth method at least once. In addition, the ammonia pulse flow growth method employed here adopts a growth condition different from the growth condition of the nucleation crystal forming step ( S122 ), so that new crystal nuclei are difficult to be generated, but coherence is applied to the crystals of the already formed crystal nuclei. A condition called enhanced lateral growth is adopted, which is easy to grow crystals in a runt lateral direction (in-plane direction, non-polar direction). In addition, the simultaneous feed growth method is a method of growing at a high speed mainly in the film thickness direction while continuing to supply both TMAl (tri-methyl-aluminum) and ammonia. The AlN at this point seems to affect the later crystal quality. Therefore, in the present embodiment, for example, it is preferable to alternately repeat the above-described ammonia pulse flow growth method and simultaneous feed growth method several times (ammonia pulse flow multi-stage growth method). Through this repetition, the penetration dislocation extending in the film thickness direction is reduced. In a specific growth process, the conditions for crystal growth by the MOVPE method are precisely controlled. Among the crystal growth conditions, the condition that the present inventors consider particularly important is the supply ratio of the gas (ammonia) for the group V element and the gas (TMAl) for the group III element in the source gas.

In the light emitting layer forming process (S130), using the crystal lattice of the buffer layer 120 as a template, an _{ultraviolet light emitting layer whose main component is a composition of Al x} Ga _1-x N (x is any value of 0 < x ≤ 1) on the surface. 130 is formed. The ultraviolet light emitting layer 130 is a laminate in which the n-type conductive layer 132 , the recombination layer 134 , and the p-type conductive layer 136 are stacked in the buffer layer 120 in this order, as shown in FIG. 1 . In order to form the n-type conductive layer 132 , the recombination layer 134 , and the p-type conductive layer 136 , tetraethyl silane (TESi) for Si and TMGa for Ga as a source gas in addition to TMAl gas and ammonia gas as a source gas (tri-methyl-gallium) or TEGa (tri-ethyl-gallium), and for Mg, _{various gases such as Cp 2} Mg (bis-cyclopentadienyl magnesium) may be employed. When the electron blocking layer is provided on the p-type conductive layer 136 , the growth is performed by adjusting the gas ratio according to the configuration of the electron blocking layer.

A p-type contact layer 150A for electrical connection is formed thereon in the p-type contact layer forming process (S140). Even at this time, the same source gas used for manufacturing the p-type conductive layer 136 may be employed. Also, in the case of the conventional p-type contact layer 150 , TMAl gas is not used in the p-type contact layer forming process ( S140 ) in order to produce p-type GaN.

Thereafter, an electrode is formed by an electrode forming process ( S150 ). This is a process of forming the first electrode 140 and the second electrode 160 shown in FIG. 1 . For example, the first electrode 140 is formed on the surface of the n-type conductive layer 132 . In addition, the insertion electrode layer 162 is disposed on the surface of the p-type contact layer 150A, and the second electrode 160A is formed thereon.

2. Examples of this embodiment

Next, the present embodiment will be described in more detail with reference to Examples. Materials, amounts used, ratios, treatment details, treatment procedures, directions, specific arrangements, and the like of elements or members presented in the following examples may be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples below. In the following description, reference is also made to the drawings described so far, and the reference numerals of the elements already described are used as they are.

2-1. Example 1: Confirmation of operability of the p-type contact layer 150A

First, in Example 1 of this embodiment, it was confirmed whether operation|movement from an electrical aspect was possible. In addition, in Example 1, p-type AlGaN is generally used for the p-type contact layer 150A, but for the second electrode 160, a configuration having a low reflectance as in the prior art is adopted.

Fig. 5 shows the dependence _{of the external quantum efficiency η EQE} on the current density of a sample in which the AlN mixed crystal composition ratio of the p-type contact layer is changed in the example (Example 1) in which AlGaN is employed for the p-type contact layer of this embodiment. is a graph showing Measurements were carried out with continuous light emission at room temperature. Hereinafter, the sample prepared for the Example is called Example Sample E1 etc., and the sample prepared for the Comparative Example is called Comparative Example Sample C1. The preparation conditions of each sample are extracted and shown in Table 1. In addition, the p-type conductive layer 136 was composed of only the electron block layer 138 of MQB.

[Table 1]

In each sample, an LED device was manufactured according to the above-described manufacturing method. In Example 1, Comparative Example Sample C1 and Example Samples E1 to E3 were produced. All of these samples were manufactured so that the emission wavelength (the main wavelength of radiation UV) was 265 nm, and the electron blocking layer 138 of the multiple quantum barrier (MQB) was provided on the p-type conductive layer 136 . In addition, all of the second electrodes 160 were made of Ni/Au as in the prior art (described above). In each sample, only the p-type contact layer was made different. Specifically, in Comparative Example Sample C1, p-type GaN was employed for the conventional p-type contact layer 150 . In p-type GaN, the AlN mixed crystal composition ratio is 0%. On the other hand, in Example Samples E1 to E3, p-type AlGaN was employed for the p-type contact layer 150A. The AlN mixed crystal composition ratio is 47%, 56%, and 60%, respectively. The composition wavelength obtained by measuring photoluminescence at room temperature by forming a thick film according to the same composition and the same film formation conditions as each p-type contact layer is also shown in FIG. 1 . The measurements of the external quantum efficiency η _{EQE versus} the current density of these samples are also shown in the graph of FIG. 5 .

As shown in FIG. 5 , it was confirmed that Example Samples E1 to E3 were sufficiently operated as LED devices. Comparative Example Sample C1 and Example Samples E1 to E3 tended to have the highest electrical potential in Comparative Example Sample C1, and Example Samples E1, E2, and E3, in which the AlN mixed crystal composition ratio increased, were inferior in that order. That is, Comparative Example Sample C1 showed the highest external quantum efficiency η _EQE . However, it is important to note that none of the LED elements of Example Samples E1, E2, and E3 have a problem in operation.

Next, the same kind of trend was investigated by targeting an LED element with an emission wavelength of 277 nm, which was slightly shifted to a longer wavelength in the wavelength range for sterilization (260 to 280 nm). 6 is a graph showing the dependence _{of the external quantum efficiency η EQE} on the current density in an example in which AlGaN is employed for the p-type contact layer in the present embodiment and the AlN mixed crystal composition ratio of the p-type contact layer is precisely changed. 5 and 6, there is a difference in that the composition wavelength of the p-type contact layer is longer (FIG. 5) and shorter (FIG. 6) than the emission wavelength. That is, Example samples E4 and E5 are samples in which the AlN mixed crystal composition ratio x of the p-type contact layer 150A is changed in a range where the composition wavelength becomes shorter than the emission wavelength. The conditions for these samples are also listed in Table 1. The measurement was performed with continuous light emission at room temperature in the same manner as in FIG. 5 . The composition wavelengths of the p-type contact layers 150A of Example Samples E4 and E5 having AlN mixed crystal composition ratio x of 60% and 63%, respectively, were found to be 270 and 265 nm, respectively.

The LED elements of Examples Samples E4 and E5 in which a wavelength shorter than the emission wavelength as the composition wavelength of the p-type contact layer 150A operated without any problem. As far as the present inventor knows, the AlN mixed crystal composition ratio x of the p-type contact layer 150A is the largest in the DUV LED device whose operation has been actually confirmed. In addition, it can be said that the external quantum efficiency η _EQE of about 3% is sufficiently high even for an LED device with an emission wavelength of 277 nm. Also, in these samples, it was confirmed that _{the external quantum efficiency η EQE} decreases when the AlN mixed crystal composition ratio x increases for the same wavelength of radiation UV.

Both of the p-type contact layers 150A of Example samples E4 and E5 exhibit a certain degree of transmittance to a wavelength of 277 nm, but as the AlN mixed crystal composition ratio x increases (as the composition wavelength becomes shorter), the transmittance becomes high. that is expected However, excessively increasing the AlN mixed crystal composition ratio x does not necessarily give good results. A negative effect due to electrical connection due to an increase in the AlN mixed crystal composition ratio x, specifically, a lack of hole concentration in the p-type contact layer 150A starts to become a problem. That is, it can be said that the AlN mixed crystal composition ratio x of the p-type contact layer 150A should be appropriately adjusted according to the wavelength of the radiation UV. It should be noted that, although the second electrode 160 has a Ni/Au configuration in which reflection cannot be expected, sufficient operability as shown in FIG. 6 is confirmed. The problem of the electrical aspect of the p-type contact layer can be dealt with.

As described above, it was confirmed that the operation was sufficiently possible even with the p-type contact layer 150A including Al in the composition. In addition, knowledge was also obtained about the relationship between the emission wavelength at this time and the Al composition of the p-type contact layer 150A defined by the composition wavelength.

2-2. Example 2: Transmission characteristics of the p-type contact layer 150A

Next, as Example 2, the transmittance of the material for the p-type contact layer 150A was measured in order to confirm the optical properties of the material employed for the p-type contact layer 150A. 7 is a graph in which the transmission spectrum of the same film as that of the p-type contact layer 150A is measured. The film of the measurement sample was formed from AlGaN under the film formation conditions for the p-type contact layer 150A with the AlN mixed crystal composition ratio x 0.6 (60%, composition wavelength 270 nm) with respect to the buffer layer of the sapphire substrate on which the AlN buffer layer for epitaxial growth was formed. it will be formed In the sample for transmittance measurement, the thickness of AlGaN was doubled as the thickness actually employed for the p-type contact layer 150A in order to increase the accuracy of the measurement. In addition, in order to calculate the internal transmittance value which does not include the influence of interfacial reflection, the reflectance was also measured simultaneously. As a result, it was confirmed that the transmittance measurement sample had a high transmittance of 94% at 279 nm when the AlN mixed crystal composition ratio x was 0.6 (in the case of a composition having a composition wavelength of 270 nm). This value for a double-thick film corresponds to an internal transmittance of 97% at 279 nm of the p-type contact layer 150A when the AlN mixed crystal composition ratio x is 0.6. Incidentally, the wavelength of the radiation UV of 279 nm can be realized as shown in other examples. In addition, the AlN mixed crystal composition ratio x of 0.6 is a value that does not cause any disruption in the operation in the electrical aspect as described in Example 1. As such, the internal transmittance of the p-type contact layer 150A exceeded 90% at the wavelength of the radiation UV, and a high value exceeding 95% was confirmed. In addition, although the measured value is not shown, it was confirmed that the wavelength (absorption edge) at which the transmittance rapidly decreased by changing the AlN mixed crystal composition ratio x (composition wavelength) shifted.

Next, the relationship between the emission wavelength and the AlN mixed crystal composition ratio of the p-type contact layer 150A appropriate thereto was revealed. As described above, from an electrical point of view, the AlN mixed crystal composition ratio is preferably as small as possible. On the other hand, in order to realize the p-type contact layer 150A having high transmittance, the AlN mixed crystal composition ratio has a lower limit. The inventors of the present application have decided to make the range of the preferable value as the AlN mixed crystal composition ratio correspond to the LED emission wavelength. 8 is an explanatory diagram showing the dependence of the AlN mixed crystal composition ratio on the emission wavelength for the p-type contact layer employed in the present embodiment.

In the sterilization use, which can be assumed as one of the typical uses of the LED device of the present application, it is preferable that the emission wavelength of the LED is 260 nm to 280 nm. In each of the LEDs having emission wavelengths of 260 nm and 280 nm, the AlGaN mixed crystal composition ratio of AlGaN employed for the p-type contact layer 150A is preferably 70% or more and 58% or more, respectively. However, this idea can also be applied to LEDs of emission wavelengths other than 260 nm to 280 nm, which is shown in FIG. 8 . Specifically, the AlN mixed crystal composition ratio x is the value of the emission wavelength (the main wavelength of radiation UV) as W (nm).

x _min = -0.0060 W + 2.26

The lower limit value obtained by the preferable if it is more than the value x _min. It is preferable to employ the AlN mixed crystal composition ratio x in the region hatched in FIG. 8 at each wavelength, and x _min in the above equation is shown by a straight line.

2-3. Example 3: Reflection characteristics of the second electrode 160A

Next, as Example 3, enhancement of reflection by the second electrode 160A (reflecting electrode) was tested. 9 is a reflection spectrum of various metal films for the insertion metal layer and the second electrode (reflection electrode). The measurement is performed by forming a metal film on a sapphire substrate to obtain measurement values for incidence and reflection from the p-type contact layer 150A side of the second electrode formed on the p-type contact layer 150A, and incident on the metal film from the sapphire substrate side. The reflection spectrum for light was measured. When only the Al plate was formed with a sufficient thickness on the sapphire substrate, for example, a reflectance of about 91% at 270 nm was measured. On the other hand, in the case of Ni (25 nm)/Au (150 nm), which has been conventionally employed as the second electrode 160, the reflectance was about 30% at 270 nm. In addition, Ni (25 nm)/Au (150 nm) is a metal film in which Ni is first formed to a thickness of 25 nm as an insertion metal layer, and Au is formed to a thickness of 150 nm in contact with the Ni layer. That is, there is room for improvement in the conventional second electrode 160 . The present inventors examined the effect of Ni used as an intercalation metal layer while maintaining the high reflectance of Al as much as possible.

First, Ni was 5 nm and 100 nm of Al was coated thereon. As a result, the reflectance was only about 34% at 270 nm, not significantly different from Ni/Au. So, I guess the problem is the low reflectance of Ni, and as a result of making Ni thinly 1 nm thick, it was combined with 100 nm Al to obtain a reflectance of about 64%. Further increasing the thickness of Al did not change substantially. Conversely, when aluminum was thinned, for example, to 50 nm, the reflectance dropped to about 47%. From this, it was confirmed that good reflectance was realized when Al was adopted for the second electrode 160A with a thickness of substantially about 1 nm of nickel as the insertion metal layer 162 . It is advantageous in terms of electrical and reflectance to form thick Al of the second electrode 160A.

Although the graph of FIG. 9 is of a sample for measuring reflectance, the present inventors believe that the same trend is observed in an actual LED device. That is, even in an actual LED device, the second electrode of the Ni/Au configuration shows a low reflectance of about 10 to 30%, so it is considered that about 64% can be secured if the Ni/Al configuration is used. In addition, by selecting a contact metal such as Pd, Pt, Ag or Ti and controlling the film thickness thereof, a high reflectance of 70% or more can be expected.

2-4. Example 4: Demonstration in LED element 100A

Next, as Example 4, the practicality of the LED element 100A combining the p-type contact layer 150A with the increased transmittance and the second electrode 160A with the increased reflectance was demonstrated. Specifically, it was confirmed that the light extraction efficiency η _LEE actually increased in the LED element 100A and that the external quantum efficiency η _EQE was increased. In addition, in the sample of the LED element 100A of Example 4, MQB was optimized in the samples of the previous Example. This is the to increase the transmittance of the p-type contact layer (150A), that is AlN mixed crystal ratio x is turned in accordance with the increased enough hole concentration likely to occur in the electronic leakage (overflow) reduction in electron injection efficiency η _EIE concerns It is for this reason, and it is also for drawing out the possibility which LED element 100A of this embodiment has. The specific details of the optimization are to increase the barrier height of MQBs disposed on the p-type conductive layer 136 to enhance the electron block effect. The p-type contact layer 150A with increased transmittance can be dealt with by an electron blocking layer such as MQB, even if there is a disadvantage in electrical terms such as a decrease in _{electron injection efficiency η EIE due to it.}

10 is an actual measurement graph illustrating a state in which characteristics are improved by making only the second electrode (reflecting electrode) different from each other in the present embodiment. Moreover, when the 2nd electrode (reflecting electrode) is changed to the thing of high reflectance in FIG. 11, it shows a mode that external quantum efficiency η _EQE changes. Specifically, Fig. 11(a) is an explanatory diagram schematically showing the state of a comparison target sample for which a change is calculated, and Fig. 11(b) is a graph thereof. The conditions for each sample are listed in Table 1. _{The present inventors have demonstrated that the light extraction efficiency η LEE} is increased by the combination of the p-type contact layer 150A with increased transmittance and the second electrode 160A with increased reflectance, as schematically shown in FIG. 11(a). A sample in which only the material of the second electrode was changed was prepared, and the change in light extraction efficiency η _LEE before and after the change was investigated.

_{Specifically, the relationship between the current density and the external quantum efficiency η EQE} was investigated for the second electrode of Ni/Au (Example Sample E6) and Ni/Al (Example Sample E7) ( FIG. 10 ). . For all of these samples, the ultraviolet light emitting layer 130 was fabricated so that the light emission wavelength was 279 nm, and a p-type contact 150A having a composition wavelength of 270 nm (AlN mixed crystal composition ratio x was 60%) was adopted, and measurement was performed by continuous operation at room temperature. are doing As shown in Fig. 10, by changing the second electrode from Ni/Au (E6) to Ni/Al (E7), it was _{confirmed that the maximum value of external quantum efficiency η EQE} was about 1.67 times from 4.1% to 7.0%. .

This is directly important for two reasons. First, it is important in that _{an external quantum efficiency η EQE} of 7.0% is actually realized for the emission wavelength 279 nm, which is a wavelength for sterilization applications. Another important point is that by contrasting the LED element 100 and the LED element 100A, it is actually confirmed that the difference in reflectivity of the second electrode is reflected in the actual LED element because of the high transmittance of the p-type contact 150A. do. According to this embodiment, it has been confirmed _{that the light extraction efficiency η LEE} is increased by combining the second electrode with the increased reflectivity in the p-type contact 150A with the AlN mixed crystal composition ratio of 60% to increase the transmittance according to the present embodiment.

The result of Fig. 10 is also important in that it suggests a detailed relationship between the composition of the p-type contact 150A and the emission wavelength in the operation of the LED element. From Fig. 10, it can be seen that the AlGaN p-type contact 150A having a composition wavelength of 270 nm (AlN mixed crystal composition ratio x is 60%) has a high transmittance with respect to the emission wavelength of 279 nm, and the high reflectance of the second electrode 160A. It shows that it is actually happening. Here, as described with reference to Fig. 7, the wavelength dependence of the transmittance of the p-type contact 150A depends on the AlN mixed crystal composition ratio x (composition wavelength). Therefore, the improvement effect of the external quantum efficiency η _EQE by the second electrode depends on the combination of the composition of the p-type contact 150A and the emission wavelength. _{Paying attention to this point, the improvement effect of the external quantum efficiency η EQE} by the reflectance of the second electrode for wavelengths other than the emission wavelength (279 nm) adopted for FIG. 10 was investigated in FIG. 11( b ). A value representing this improvement effect is called a 'LEE Enhancement Factor'. _{The light extraction ratio is the external quantum efficiency η EQE} in the case of the second electrode with high reflectance (Ni/Al, Ni being 1 nm), and the external quantum efficiency η EQE in the case of the conventional second electrode having low reflectivity (Ni/Au). Quantum efficiency η divided by _EQE. Here, for the sample to be irradiated, the material of the p-type contact layer 150A was fixed (composition wavelength 270 nm, AlN mixed crystal composition ratio x 60%), and samples having different emission wavelengths were prepared. The results are shown in Fig. 11(b). 11(b) shows the light extraction ratios for the 6 wavelengths of emission wavelengths (279, 277, 275, 273, 271, and 265 nm). In addition, the value of the emission wavelength of 279 nm was obtained in the combination of Example samples E6 and E7 which are the same as that of FIG.

As shown in FIG. 11( b ), in the configuration in which the emission wavelength is short with respect to the composition wavelength (270 nm) of the p-type contact layer 150A, the light extraction ratio was a value of about 1.3 to 1.4. In contrast, when the emission wavelength is longer than the composition wavelength, the light extraction ratio increases, resulting in a maximum light extraction ratio of about 1.7.

This result supports the understanding that the contribution of the reflectance of the second electrode to the external quantum efficiency η _EQE becomes small when the transmittance of the p-type contact layer 150A is low and, conversely, becomes large when the transmittance is high. In particular, this point becomes easy to understand when the transmission spectrum shown in FIG. 7 is applied. That is, while the reflection spectrum of the metal thin film of the second electrode shows only a gentle change with respect to the wavelength as shown in FIG. 9 , the transmission spectrum of the p-type contact layer 150A is sharp with respect to the wavelength as shown in FIG. 7 . is changing drastically As shown in Fig. 11(b), the emission wavelength dependence of the light extraction ratio also changes rapidly around the composition wavelength, so that the transmittance of the p-type contact layer 150A contributes greatly to the _{light extraction efficiency η LEE.} show It was confirmed that the p-type contact layer 150A having such an increased transmittance greatly contributed to the improvement of the _{light extraction efficiency η LEE .}

In addition, confirmation was also performed by fixing the material of the p-type contact layer 150A and changing the emission wavelength. Fig. 12 is a graph showing measured characteristics in the case where ultraviolet light emitting layers having different emission wavelengths are employed for p-type contact layers of a common composition. 12 (a) is an external quantum efficiency η _EQ showing the dependence of the E, and Figure 12 (b) is substantially maximum external quantum efficiency η luminescence spectra when the _EQE is the current density of the obtained 20mA / cm ² for the current density shows Specifically, FIG. 12 shows samples having emission wavelengths of 277 and 275 nm for FIG. 11 (Example Samples E8 and E9) in addition to Example Sample E7 in which Ni/Al was used for the second electrode and the emission wavelength was 279 nm. also shows the current density dependence of the external quantum efficiency η _{EQE .} It has been confirmed that, even when the emission wavelength is changed by fixing the composition wavelength of the p-type contact 150A (both 270 nm for these samples) to one, high external quantum efficiency η _{EQE for the emission wavelength of a longer wavelength than the composition wavelength} that you can get This was confirmed according to the value of the actual external quantum efficiency η _{EQE .} Another thing that has been confirmed is the quantitative relationship. _{In the case of the emission wavelength 5 nm longer than the composition wavelength (270 nm) (Example sample E9), the maximum value of external quantum efficiency η EQE} becomes significantly higher at the emission wavelength E8 7 nm longer than in the case of the emission wavelength E9 of the example sample E9, and the external quantum efficiency at the emission wavelength E7 9 nm long It was also confirmed that the maximum value of the efficiency η _{EQE was higher.} The value obtained by subtracting the composition wavelength from the emission wavelength is preferably about 7 nm, and more preferably about 9 nm.

3. Variant

The above-described embodiment of the present invention may be implemented by a form accompanied by various modifications. In particular, when the transmittance of the p-type contact 150A increases and the reflectivity of the second electrode 160 increases, all methods for extracting the light propagating inside the LED element 100A by the waveguide mode to the outside work effectively. . In addition, it is possible to modify the countermeasures in the electrical aspect.

3-1. Variant 1: AlN buffer layer with voids

One of the variants is to provide voids (voids) in the buffer layer. Voids exhibit a direction-changing action that changes the direction of propagating light by inducing phenomena such as refraction and scattering. 13 is a cross-sectional view showing the configuration of an LED element obtained in a modification of the present embodiment, and is a modification in which the void V is formed. The void V is formed in the crystal growth process of the buffer layer 120B. As a typical method for its formation, a height difference is formed on the surface of the sapphire substrate 110A by patterning. When forming the buffer layer 120B, the crystal growth rate is high at the high (凸) portion of the sapphire substrate 110A and slows down at the low (凹) portion of the sapphire substrate 110A. When this phenomenon is used, crystal growth can be promoted or suppressed in a position-selective manner in the process of growing the buffer layer 120B. In addition, if a condition for promoting growth in the in-plane direction is applied in the crystal growth conditions of the buffer layer 120B, the buffer layer 120B that becomes a continuous crystal by blocking the voids V along with the growth can be formed. In the buffer layer 120B to include voids in this way, when the UV radiation from the LED element 100B is emitted, it is inclined to a waveguide mode by inclining to a critical angle or more, and the direction of propagating light can be changed without energy loss. When the p-type contact 150A strongly absorbs light, the waveguide mode incident on it also absorbs light. In the configuration of the LED element 100A of the present embodiment, since the p-type contact 150A is difficult to absorb, it is engaged with the reflection function of the second electrode 160 and contributes to extracting the light that has entered the waveguide mode again. The direction-changing action of the void can effectively improve the _{light extraction efficiency η LEE due} to the small absorption of the p-type contact 150A.

Several methods can be used to form voids. In the elevation difference pattern of the surface of the above-described sapphire substrate 110A, when the island portion of the sea island structure pattern structure increases, the pillars 124 are formed, and the gap between the pillars 124 becomes a void (V) ( FIG. 13 ). Conversely, when the sea portion rises, an open portion corresponding to the island portion is formed and becomes a void. When the buffer layer 120B is grown, the growth reflecting the symmetry of the crystal lattice is performed, so that a side or an angle due to the symmetry of the crystal lattice appears in the cross-sectional shape of the pillars and the voids in the plane. In addition, it can be formed in a stripe pattern or an arbitrary pattern. Furthermore, it is possible to make a mask by patterning and applying a film having a composition that inhibits crystal growth on the surface of the sapphire substrate 110A, irrespective of the height difference pattern.

In the case of the buffer layer 120B in which the void V is formed, it is also preferable to remove the sapphire substrate 110A. 13 shows a position where the sapphire substrate 110A is removed along a dotted line on the sapphire substrate 110A side of the pillar 124 . When the void V of the buffer layer 120B is exposed on the surface by removing the sapphire substrate 110A, the structure of the void reduces the total amount of light returned to the interior by total reflection or surface reflection. When the substrate is removed when the buffer layer 120B is in the form of a coupling pillar, each pillar becomes a waveguide for UV and is emitted to the outside while passing through each pillar when the radiation UV is extracted to the outside again. The light due to reflection is reduced and the light extraction efficiency η _LEE is increased. Depending on the size or propagation direction of the pillar, the impedance at the interface of radiated UV emitted to the outside may decrease, like a moth-eye structure to be described later, but light due to total reflection or surface reflection is reduced.

3-2. Variant 2: Moth eye structure

Increasing the transmittance of the p-type contact layer in the present embodiment may be compatible with other methods _{of increasing the light extraction efficiency η LEE.} In order to increase the light extraction efficiency η _LEE , the light emitted from the ultraviolet light emitting layer 130 toward the second electrode or the light propagated in the waveguide mode are finally emitted to the outside world. If a moth-eye structure is formed on a surface located opposite to the electrode of the LED element, or on any one of the surfaces or interfaces that involve a refractive index step among other interfaces, it is preferable because it is possible to reduce the light returning to the interior when the light is emitted. When exemplifying the side in which the moth-eye structure is formed, the surface (for example, the light output surface 102 of the substrate 110) that is the outermost surface between the LED element located opposite the reflective electrode and the outside world, and the LED element This is the interface between the LED element and the UV-transmissive medium when sealing with a transmissive medium (resin, glassy transparent body, etc.). It is preferable if a moth-eye structure is formed on any one of the side surfaces in which the refractive index step becomes large. The moth-eye structure refers to an arbitrary surface shape with irregularities in a size within a smaller plane based on the wavelength, and the reflection by the refractive index step is suppressed because of the surface shape on the face with the refractive index step. In this structure, the surface reflection is reduced because the impedance of the radiated UV emitted to the outside at the interface with the refractive index step is lowered.

3-3. Modification 3: Configuration of the electronic block layer

An electron block layer will be described as another typical example of the present embodiment described above. The method for increasing the _{electron injection efficiency η EIE} in the present embodiment for increasing the light extraction efficiency η _LEE is important in terms of increasing the _{external quantum efficiency η EQE .} In particular, since the present embodiment sacrifices the hole concentration in order to increase the transmittance of the p-type contact 150A, the practicality is affected whether or not a method of compensating for the decrease in _{the electron injection efficiency η EIE caused by it can be used in combination.} As a means for increasing the electron injection efficiency η _EIE , as already described above with respect to MQB, the electron block layer 138 provided on the p-type conductive layer 136 is an effective means.

In general, when the emission wavelength is a long wavelength in the range of the ultraviolet band, the composition wavelength of the p-type contact 150A also becomes a large value accordingly. That is, the AlN mixed crystal composition ratio x of the p-type conductive layer 136 is at least good. In this case, the problem of hole concentration is also not serious. Even in the case of this embodiment, the electron blocking layer is not necessarily required in the case of a long wavelength even in UV. As the wavelength becomes shorter, the AlN mixed crystal composition ratio x of the p-type conductive layer 136 or the p-type contact layer 150A must be increased, and the hole concentration is sacrificed that much, so the effect on the _{electron injection efficiency η EIE becomes severe.} Accordingly, the need to use the electron block layer 138 increases. Depending on the wavelength, a relatively simple single barrier structure provided in a single layer region with a high Al composition ratio may be sufficient. In the case of a polarization crystal orientation, an effective electron blocking action is realized even in a single barrier structure. If the emission wavelength becomes shorter, employing MQB, which can expect higher electron-blocking action than a single barrier structure, or performing its optimization precisely is an appropriate solution. In particular, in MQB using the multiple reflection effect, it is possible to reduce the increase in the Al mixed crystal composition ratio for the electron block layer of the p-type conductive layer, the electrical connection is maintained, and the above-described embodiment as a method for compensating for the decrease in _{electron injection efficiency η EIE} is suitable for In addition, in the wavelength range (260 to 280 nm) for sterilization, it is useful both to have a single barrier structure and to employ MQB as shown in the examples.

Importantly, in the present embodiment, a solution has already been proposed for side effects that may be caused by the p-type contact layer having an increased transmittance so that the _{light extraction efficiency η LEE can be increased, and its effectiveness has been confirmed.} In addition, in order to increase the effect of MQB, it is also useful to change the period in the thickness direction, which causes a change in the Al composition ratio. The details of MQB including this point are described in detail in Patent Document 2 of the present inventor.

3-4. Modification 4: Introduction of In into the p-type layer

As a modified example of this embodiment, among the p-type conductive layer 136, the electron block layer 138 and the p-type contact layer 150A provided as an option, a p-type layer in which the carrier (hole) concentration is lowered. It is also useful to introduce indium (In) into either one. The present inventors have confirmed that the carrier concentration increases by introducing In into p-type InAlGaN (Non-Patent Document 3). Specifically, it is preferable to use p-type InAlGaN prepared by containing In in a concentration range from a low doping level (1×10 ¹⁸ cm ^{-3 ) to about 2%.} In addition, in the p-type contact layer 150A having such a composition, the composition wavelength is largely determined by the AlN mixed crystal composition ratio, and the transmittance in a region having a wavelength longer than the composition wavelength is hardly affected. For this reason, the above description of the present embodiment can be applied to the p-type conductive layer 136, the electron block layer 138, and the p-type contact layer 150A with little change. In order to introduce In, an appropriate amount of a raw material gas such as trimethyl indium diisopropylamine adduct (TMIn adduct) is added in a timely manner.

3-5. Modification 5: Incorporation of In into the ultraviolet light emitting layer

In this embodiment, it is useful to introduce In also into the ultraviolet light emitting layer, particularly the recombination layer 134 , in order to improve the light emission output. The present inventors confirmed that the quantum efficiency inside the quantum well emitting light at around 280 nm is enhanced by incorporating In, for example, about 0.3% in the composition of the mixed crystal of AlN and GaN of the ultraviolet light emitting layer 130 or the recombination layer 134, There is (non-patent document 2). As for the mechanism, fluctuations (modulation) of the composition of In in AlGaN cause localization of carriers, and as a result, the ratio of recombination electrons and holes contributing to light emission increases, from crystal defects of penetration dislocations to non-luminescence. The present inventors believe that the loss due to recombination is suppressable. Also in this modified example, it is useful to set the composition ratio of In to about 2% at the maximum at the ^{doping level (1×10 18} cm ^{-3 ).}

3-6. Variant 6: Substrate of AlN crystal

As a modification of the present embodiment, an AlN crystal substrate may be employed instead of the above-described sapphire substrate 110 . In this modified example, the buffer layer 120 may be omitted by AlN crystals. The ultraviolet light emitting layer 130 is epitaxially grown by being disposed in contact with the AlN crystal or buffer layer (if used). Even when an AlN crystal substrate is used, increasing the transmittance of the p-type contact layer or increasing the reflectance of the second electrode (reflecting electrode) improves light extraction efficiency and also helps to improve external quantum efficiency. All descriptions in the case of employing the sapphire substrate 110 also apply to the modified example employing the substrate of AlN crystal.

3-7. Variant 7: Application to electrical equipment

The ultraviolet emission source with increased efficiency obtained by the LED element 100A of this embodiment improves the usefulness of the electric device using it. Such electric devices are optional and are not particularly limited. Non-limiting examples of such electrical equipment include a sterilization device, a water purification device, a chemical decomposition device (including an exhaust gas purification device, etc.), an information recording/reproducing device, and the like. When a high-efficiency ultraviolet emitting source is obtained when operating these electric devices, the power for operation can be suppressed, thereby reducing the environmental load and reducing the running cost. In addition, when the efficiency of the emission sources is increased, not only the number of emission sources itself is suppressed in the configuration of these electric devices, but also the heat dissipation structure and the configuration of the driving power supply are simplified. These contribute to miniaturization and light weight of electric equipment, and also reduce equipment cost.

As described above, when various modifications or other modifications for increasing the transmittance of the p-type contact layer are applied to the present embodiment, the practicality of the DUVLED can be enhanced by increasing _{the external quantum efficiency η EQE of the DUVLED.}

The embodiment of the present invention has been specifically described above. Each of the above-described embodiments and structural examples has been described for explaining the invention, and the scope of the invention of the present application should be defined by the description of the claims. In addition, modifications within the scope of the present invention including other combinations of the respective embodiments are also included in the claims.

The ultraviolet light emitting diode of the present invention can be used in any electric device that generates ultraviolet rays.

100, 100A, 100B LED element (deep ultraviolet LED element)
102 light output side
104 One side of the board
110, 110A board
120, 120B buffer layer
130 UV light emitting layer
132 n-type conductive layer
134 recombination layer
136 p-type conductive layer
140 first electrode
150 p-type contact layer
150A p-type contact layer (high permeability)
160 second electrode
160A second electrode (high reflectivity)
162 insert metal layer

Claims (15)

a single crystal sapphire or AlN crystal substrate;
A mixed crystal of AlN and GaN disposed in contact with the substrate or in contact with an additional buffer layer provided on the substrate, and in which at least an n-type conductive layer, a recombination layer and a p-type conductive layer are stacked in this order on the side of the substrate an ultraviolet light emitting layer;
a p-type contact layer of a mixed crystal of AlN and GaN electrically connected to the p-type conductive layer;
and a reflective electrode disposed in contact with the p-type contact layer that exhibits reflectivity with respect to radiation UV, which is an ultraviolet light emitted from the ultraviolet light emitting layer,
The material of the p-type contact layer has a shorter wavelength than the main wavelength of the UV radiation as a composition wavelength,
_{In the composition Al x} Ga _1-x N of the p-type contact layer, the AlN mixed crystal composition ratio x is the value of the main wavelength of the radiation UV as W (unit: nm),
X _min = -0.0060 W + 2.26
An ultraviolet light emitting diode having a value greater than or equal to the lower limit X _{min obtained by}
The method according to claim 1,
The p-type contact layer is an ultraviolet light emitting diode having a transmittance of 90% or more with respect to the radiation UV when the radiation UV passes once in the thickness direction.
3. The method according to claim 2,
An ultraviolet light emitting diode having the transmittance of 95% or more.
4. The method according to any one of claims 1 to 3,
An ultraviolet light emitting diode in which an electron block layer is disposed on the p-type conductive layer.
5. The method according to claim 4,
An ultraviolet light emitting diode wherein the electron block layer is a multiple quantum barrier.
The method according to claim 1,
The reflective electrode is a metal film containing Al,
An ultraviolet light emitting diode in which an insertion metal layer for ohmic contact is sandwiched in contact with the p-type contact layer and the reflective electrode.
7. The method of claim 6,
An ultraviolet light emitting diode wherein the interposed metal layer is a Ni film and the thickness is thinner than 5 nm.
7. The method of claim 6,
An ultraviolet light emitting diode wherein the interposed metal layer is a Ni film and has a substantial thickness of 1 nm.
The method according to claim 1,
An ultraviolet light emitting diode having the additional buffer layer, wherein a void is formed in the additional buffer layer.
10. The method of claim 9,
An ultraviolet light emitting diode from which the substrate is removed.
The method according to claim 1,
An ultraviolet light emitting diode in which the main wavelength of the radiation UV is any one wavelength of 260 nm or more and 280 nm or less.
The method according to claim 1,
An ultraviolet light emitting diode in which the main wavelength of the radiation UV is any wavelength of 220 nm or more and 260 nm or less.
The method according to claim 1,
An ultraviolet light emitting diode in which In is additionally included in the material of the AlN and GaN mixed crystal of any one of the p-type conductive layer and the p-type contact layer.
An electric device provided with the ultraviolet light emitting diode according to any one of claims 1 to 3 and 6 to 13. delete

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