Deep ultraviolet LED

Abstract

The present invention provides a deep ultraviolet LED capable of improving light extraction efficiency (LEE). A deep ultraviolet LED having an emission wavelength of [lambda] comprises, in order from the opposite side of a substrate, a p-type electrode layer comprising indium tin oxide (ITO), a p-type GaN contact layer, a p-type AlGaN layer transparent to the wavelength [lambda], an electron barrier layer transparent to the wavelength [lambda], a multi-quantum well layer transparent to the wavelength [lambda], and an n-type AlGaN layer transparent to the wavelength [lambda]. The deep ultraviolet LED has a photonic crystal periodic structure, the photonic crystal periodic structure has a plurality of pillars formed in a thickness direction range from an upper portion of the p-type electrode layer to an inside of the n-type AlGaN layer in the direction of the substrate, and the photonic crystal periodic structure has a photonic band gap open with respect to a TM polarization component. The period a of the photonic crystal periodic structure satisfies the Bragg condition (m * lambda / neff = 2a, where m is the order and neff is the effective refractive index of the photonic crystal periodic structure) with respect to light having a wavelength lambda, the order m satisfies 3 < = m < = 7, and 0.25 < = R / a < = 0.35 with the radius of the pillar as R.

Description

Technical Field

[0001] The present invention relates to deep ultraviolet LEDs, and particularly to deep ultraviolet LEDs of the AlGaN system. Background Art

[0002] In deep ultraviolet LEDs (UVC-LEDs) with a light emission wavelength of 220 nm to 280 nm, the inactivation of the COVID-19 virus has attracted attention. However, the electro-optical conversion efficiency (WPE) of the LED is about 3%, which is significantly lower than 20% of the mercury lamp. The main reason is that since most of the emitted light is absorbed by the p-GaN contact layer, the light extraction efficiency (LEE) is about 6%, which is low.

[0003] According to Patent Document 1, a photonic crystal is formed in the thickness direction of the interface including the p-type GaN contact layer and the p-type AlGaN layer to reflect light and suppress the above absorption. Thus, at a light emission wavelength of 280 nm, the maximum magnification of the increase in LEE is 2.76 times. When the LEE in the structure without a photonic crystal is 6%, in the structure with a photonic crystal, 16.6% is achieved. It should be noted that the ratio of TE light to TM light at 280 nm is 7:3.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent No. 6156898 Gazette Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] However, the photonic crystal of the above Patent Document 1 achieves almost 100% reflection effect for TE light, but cannot achieve a reflection effect for TM light. Further, as the light emission wavelength is shortened to 220 nm, the ratio of TM light increases and the LEE decreases.

[0009] An object of the present invention is to provide a new technology for improving the LEE in UVC-LEDs.

[0010] Means for Solving the Problems

[0011] An example of the deep ultraviolet LED according to the present invention is a deep ultraviolet LED with a light emission wavelength of λ, wherein,

[0012] the deep ultraviolet LED sequentially has a p-type electrode layer made of indium tin oxide (ITO), a p-type GaN contact layer, a p-type AlGaN layer transparent to the wavelength λ, an electron barrier layer transparent to the wavelength λ, a multi-quantum well layer transparent to the wavelength λ, and an n-type AlGaN layer transparent to the wavelength λ from the opposite side of the substrate.

[0013] The deep ultraviolet LED has a photonic crystal periodic structure, and the photonic crystal periodic structure has a plurality of columns formed in a range in the thickness direction from the upper part of the p-type electrode layer to the inside of the n-type AlGaN layer in the substrate direction.

[0014] The photonic crystal periodic structure has a photonic band gap open to the TM polarization component.

[0015] For light with a wavelength of λ, the period a of the photonic crystal periodic structure satisfies Bragg's condition (m×λ / n eff = 2a, where m is the order and n eff : the effective refractive index of the photonic crystal periodic structure).

[0016] The order m satisfies 3 ≤ m ≤ 7.

[0017] With the radius of the column being R, it satisfies 0.25 ≤ R / a ≤ 0.35.

[0018] In one example, the space part of the photonic crystal periodic structure is filled with an insulating layer made of SiO2.

[0019] This specification includes the disclosure of Japanese Patent Application No. 2022-169829, which is the basis of the priority of this application.

[0020] Advantages of the Invention

[0021] According to the present invention, by forming a columnar photonic crystal periodic structure, the LEE of the UVC-LED can be dramatically improved. Description of the Drawings

[0022] Figure 1 It is a structure (cross-sectional view and top view) of a UVC-LED with a light emission wavelength λ of 220 nm according to the first embodiment.

[0023] Figure 2 It is the photon band structure of TE light and TM light analyzed by the plane wave expansion method according to a light emission wavelength of 220 nm, a refractive index n1 of the p-type GaN contact layer of 2.814, a refractive index n2 of air of 1, a radius R of the column, and R / a of 0.30 for the period a.

[0024] Figure 3 It is a graph with the horizontal axis being R / a and the vertical axis being the magnitude (ΔPBG) of the PBG.

[0025] Figure 4A It is a calculation model of the presence of a photonic crystal periodic structure analyzed by the FDTD method.

[0026] Figure 4B It is a calculation model without a photonic crystal periodic structure analyzed by the FDTD method.

[0027] Figure 5A It is a graph related to each R / a with the horizontal axis being the order m and the vertical axis being the magnification at a luminous wavelength of 220 nm.

[0028] Figure 5B It is a graph related to each R / a with the horizontal axis being the order m and the vertical axis being the magnification at a luminous wavelength of 280 nm.

[0029] Figure 6A It is an electric field diagram showing the electric field strength in the presence of a photonic crystal periodic structure.

[0030] Figure 6B It is an electric field diagram showing the electric field strength in the absence of a photonic crystal periodic structure.

[0031] Figure 7 It is a graph with the horizontal axis being the emission angle and the vertical axis being the output.

[0032] Figure 8 It is the structure (cross-sectional view and top view) of a UVC-LED with a luminous wavelength λ of 220 nm according to the second embodiment.

[0033] Figure 9A It is a graph related to each R / a with the horizontal axis being the order m and the vertical axis being the magnification at a luminous wavelength of 220 nm in the second embodiment.

[0034] Figure 9B It is a graph related to each R / a with the horizontal axis being the order m and the vertical axis being the magnification at a luminous wavelength of 280 nm in the second embodiment. Detailed implementation mode

[0035] Hereinafter, a deep ultraviolet LED (hereinafter, sometimes referred to as "UVC-LED") according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

[0036] As the UVC-LED according to the first embodiment of the present invention, Figure 1 It represents the structure (cross-sectional view and top view) of a UVC-LED with a luminous wavelength λ of 220 nm. Figure 1 In (a) is a cross-sectional view, Figure 1 In (b) is a top view observed from below the plane of (a) in Figure 1 In.

[0037] The xyz orthogonal coordinate system is defined as the coordinate system for explanation. Figure 1In (a), the stacking direction, i.e., the vertical direction of the paper surface, is the z direction. In particular, the upper side of the paper surface is the +z direction, and the lower side of the paper surface is the -z direction. The cross-section is parallel to the xy plane.

[0038] In Figure 1 In the cross-sectional view of (a), from the +z side toward the -z side in sequence, the sapphire substrate 1, the AlN layer 2, the n-type AlGaN layer 3, and the air 9 between the columns 101 together form a photonic crystal periodic structure 100. The column 101 sequentially constitutes a part of the n-type AlGaN layer 3, the multiple quantum well layer 4, the electron barrier layer 5, the p-type AlGaN layer 6, the p-type GaN contact layer 7, and the p-type electrode layer 8 (ITO) from the +z side toward the -z side. The photonic crystal periodic structure 100 reflects light of wavelength λ by having a photonic band gap.

[0039] Thus, the UVC-LED has, in sequence from the opposite side of the sapphire substrate 1 (substrate), a p-type electrode layer 8 made of indium tin oxide (ITO), a p-type GaN contact layer 7, a p-type AlGaN layer 6 transparent to the wavelength λ, an electron barrier layer 5 transparent to the wavelength λ, a multiple quantum well layer 4 transparent to the wavelength λ, and an n-type AlGaN layer 3 transparent to the wavelength λ. Further, as shown in the figure, an AlN layer 2 may also be provided between the n-type AlGaN layer 3 and the sapphire substrate 1.

[0040] The photonic crystal periodic structure 100 has a plurality of columns 101 formed in the thickness direction range from the upper part (i.e., the -z direction side end) of the p-type electrode layer 8 to the inside of the n-type AlGaN layer 3 in the direction of the sapphire substrate 1 (thickness direction, i.e., the direction orthogonal to the stacking plane of the sapphire substrate 1). In the n-type AlGaN layer 3, only a part is included in the column 101.

[0041] As Figure 1 As shown in (b) as a view of the xy plane, in the photonic crystal periodic structure 100, the columns 101 having a circular cross-section with a radius R larger than that of the air 9 in the space part are formed in a regular triangular lattice pattern with a period a in the xy plane. Here, the "space part" refers to, for example, the region in the photonic crystal periodic structure 100 where the columns 101 do not exist.

[0042] It should be noted that, in Figure 1 In (b), for the simplicity of the figure, only 3 columns 101 are shown, but more columns 101 may also be provided.

[0043] In the above structure, TE light and TM light in the light with wavelength λ emitted from the multiple quantum well layer 4 radiate omnidirectionally and propagate in the medium while being elliptically polarized. According to the radius R of the pillars and the period a constituting the photonic crystal periodic structure 100, the filling factor f of the photonic crystal is calculated by Equation 1.

[0044] f = 2π / 3 0.5 ×(R / a) 2 …(Equation 1)

[0045] Moreover, when the emission wavelength is λ, the period a of the photonic crystal periodic structure 100 satisfies the Bragg condition with respect to the light with wavelength λ. That is, Equation 2 is satisfied.

[0046] m×λ / n eff = 2a…(Equation 2)

[0047] Wherein, n eff : the effective refractive index of the photonic crystal periodic structure 100, a: the period of the photonic crystal periodic structure 100, m: the order.

[0048] Here, when the refractive indices of the two media constituting the photonic crystal periodic structure 100 are n1 and n2, it is as shown in Equation 3.

[0049] n eff =(n2 2 +(n1 2 -n2 2 )×f) 0.5 …(Equation 3)

[0050] Therefore, if λ = 220 nm, the refractive index n1 of the p-type GaN contact layer 7 that absorbs light is 2.814, the refractive index n2 of air is 1, and the ratio R / a of the radius R of the pillar 101 and the period a is 0.30 are substituted into the above three equations and the photonic band structures of TE light and TM light are obtained by the plane wave expansion method, then Figure 2 .

[0051] Regarding TM light, the photonic band gap PBG1 opens between the first photonic band and the second photonic band (2ndPBTM). In addition, the photonic band gap PBG2 opens between the third photonic band (3rdPBTM) and the fourth photonic band (4thPBTM). Thus, the photonic crystal periodic structure 100 has a photonic band gap that is open with respect to the TM polarization component. It should be noted that regarding TE light, the photonic band gap does not open.

[0052] Further, for R / a = 0.20, 0.25, 0.30, 0.35, and 0.40, the sizes of PBG1 and PBG2 are obtained by analyzing the photonic band structure using the above-mentioned plane wave expansion method, and a graph with the horizontal axis being R / a and the vertical axis being the size of the PBG (ΔPBG) becomes Figure 3 . Thus, if the value of R / a falls within the range of 0.25 ≤ R / a ≤ 0.35, it is preferable because a relatively large ΔPBG (e.g., about 0.06 or more) can be ensured for both the photonic band gaps PBG1 and PBG2.

[0053] Here, in order to investigate the relationship between the above-mentioned R / a, ΔPBG, and the order m and LEE, calculation models were respectively made for the structure with the photonic crystal periodic structure ( Figure 4A ) and the structure without the photonic crystal periodic structure ( Figure 4B ). An Al reflector 10 was provided at the end face of the p-type electrode layer 8 (the end face on the opposite side of the p-type GaN contact layer 7).

[0054] In each structure, the output (W) was obtained by the FDTD method (finite difference time domain method), and the LEE increase ratio was calculated based on the ratio of the output with the photonic crystal to the output without the photonic crystal. The structures and optical parameters at the emission wavelengths of 220 nm and 280 nm are shown in Table 1.

[0055] Table 1

[0056]

[0057] Note that the top row "220 nm" and "280 nm" represent the emission wavelengths (the same applies to Tables 2 to 4 below). In Figure 3 , the values of R / a = 0.25, 0.30, 0.35 with a relatively large ΔPBG and the orders m = 3, 4, 5, 6, 7 were respectively substituted into the above three equations to obtain the diameter and period of the columns. The same was done for the emission wavelength of 280 nm and is shown in Table 2.

[0058] Table 2

[0059]

[0060] Then, the results of calculating the LEE increase ratio and LEE (%) by analyzing the columnar photonic crystal periodic structure shown in Table 2 using the FDTD method are shown in Table 3.

[0061] Table 3

[0062]

[0063] Note that the calculation method of LEE(%) in Table 3 is described below. The LEE of the structure without the photonic crystal periodic structure at a luminescence wavelength of 280 nm is 6%. Since the output ratio of 220 nm / 280 nm is 0.253, the LEE of the structure without the photonic crystal periodic structure at 220 nm is calculated to be 1.5%. Then, it can be calculated by multiplying these values by the magnification factor obtained through each structure.

[0064] In addition, regarding the luminescence wavelengths of 220 nm and 280 nm, the curves related to each R / a with the order m on the horizontal axis and the magnification factor on the vertical axis are respectively shown in Figure 5A 、 Figure 5B . Here, for the analysis region, spatial resolution, number of steps, and analysis time, they are set to be the same in the two calculation models with luminescence wavelengths of 220 nm and 280 nm. Among them, regarding the polarization photometric value: (TE light intensity - TM light intensity) / (TE light intensity + TM light intensity), it is -0.13 at 220 nm and 0.40 at 280 nm.

[0065] As shown in Figure 5A and Figure 5B , if the order m satisfies the range of 3 ≤ m ≤ 7, a large magnification factor can be ensured, which is preferable. In addition, according to the results of Table 3, Figure 5A and Figure 5B , at a luminescence wavelength of 220 nm, the LEE in the case without the photonic crystal periodic structure 100 is 1.5%, which is reduced to about 1 / 4 compared with the LEE of about 6% in the case without the photonic crystal periodic structure at 280 nm. The reason is that due to the increase in the extinction coefficient of the p-type GaN contact layer 7 and the decrease in the polarization photometric value, the light emission in the lateral direction (the direction parallel to the xy plane) increases, so the light disappearance further increases.

[0066] However, by forming the columnar photonic crystal periodic structure 100, the light is reflected and emitted from the direction of the sapphire substrate 1, thereby suppressing the light disappearance, and the LEE increases significantly from 1.5% to 7.7% - 27.6%. The above results can be easily understood based on Figure 6A and Figure 6B after comparing the electric field strengths with and without the photonic crystal periodic structure, and Figure 7 which is a curve graph with the emission angle on the horizontal axis and the output on the vertical axis. Note that in the results of Figure 6A and Figure 7 , "column m6_Ra0.25" refers to the case where m = 6 and R / a = 0.25. For this reflection, it can be considered Figure 3The magnitudes ΔPBG of the photon band gaps PBG1 and PBG2 shown have an effect when R / a is between 0.25 and 0.35. Additionally, for a luminescence wavelength of 280 nm, similarly, the LEE is significantly improved from 6% to 16.8% - 41.1%.

[0067] As described above, for the UVC-LED according to the first embodiment of the present invention, a columnar photonic crystal periodic structure 100 is formed, enabling a dramatic improvement in LEE.

[0068] As the UVC-LED according to the second embodiment of the present invention, the structure (cross-sectional view and top view) of a UVC-LED with a luminescence wavelength λ of 220 nm is shown Figure 8 as Figure 8 (a) in Figure 8 is a cross-sectional view, Figure 8 and (b) in

[0069] Specifically, in Figure 8 (a), from the +z side towards the -z side in sequence, a sapphire substrate 1, an AlN layer 2, an n-type AlGaN layer 3, and the insulating layer 9a (SiO2) between the columns 101 form a photonic crystal periodic structure 100 together with the columns 101. Similar to the first embodiment, each column 101 is composed of, from the +z side towards the -z side in sequence, a part of the n-type AlGaN layer 3, a multi-quantum well layer 4, an electron barrier layer 5, a p-type AlGaN layer 6, a p-type GaN contact layer 7, and a p-type electrode layer 8 (ITO).

[0070] It should be noted that it has the same structure as the first embodiment except that the air 9 is replaced with the insulating layer 9a. Thus, in the second embodiment, the insulating layer 9a made of SiO2 fills the space part of the photonic crystal periodic structure 100. Figure 1

[0071] Then, the results of calculating the LEE increase ratio and LEE (%) by analyzing the columnar photonic crystal periodic structure recorded in Table 2 by the FDTD method are respectively recorded in Table 4, Figure 9A and Figure 9B . The calculation method is the same as that of the first embodiment. It should be noted that the refractive indices of the insulating layer 9a at 220 nm and 280 nm are 1.529 and 1.494 respectively.

[0072] Table 4

[0073]

[0074] According to Table 4, Figure 9A and Figure 9B ​As a result, at a light emission wavelength of 220 nm, the LEE is 8.9% to 16.7%, showing a significant increase. Additionally, at a light emission wavelength of 280 nm, the LEE is 10.2% to 31.2%, also showing a significant improvement.

[0075] In each of the above-described embodiments, the configurations shown in the drawings and the like are not limited to the above content and can be appropriately changed within the range where the effects of the present invention can be exhibited. In addition, as long as it does not deviate from the scope of the object of the present invention, it can be appropriately changed and implemented. Further, each component of the present invention can be arbitrarily selected within the appended technical solutions, and the invention having such a selection also falls within the scope of the present invention.

[0076] Industrial Applicability

[0077] The present invention can be used for deep ultraviolet LEDs.

[0078] Description of Reference Numerals

[0079] 1: Sapphire substrate (substrate); 2: AlN layer; 3: n-type AlGaN layer; 4: Multiple quantum well layer; 5: Electron barrier layer; 6: p-type AlGaN layer; 7: p-type GaN contact layer; 8: p-type electrode layer; 9: Air; 9a: Insulating layer; 10: Al reflector; 100: Photonic crystal periodic structure; 101: Post; R: Radius of the post; a: Period of the photonic crystal periodic structure.

[0080] All publications, patents, and patent applications cited in this specification are incorporated herein by reference in their entirety.

Claims

1. A deep ultraviolet LED, which is a deep ultraviolet LED with a luminous wavelength of λ, characterized in that, The deep ultraviolet LED sequentially has, from the opposite side of the substrate: a p-type electrode layer made of indium tin oxide (ITO); a p-type GaN contact layer; a p-type AlGaN layer transparent with respect to the wavelength λ; an electron barrier layer transparent with respect to the wavelength λ; a multi-quantum well layer transparent with respect to the wavelength λ; and an n-type AlGaN layer transparent with respect to the wavelength λ, the deep ultraviolet LED has a photonic crystal periodic structure, and the photonic crystal periodic structure has a plurality of columns formed in a range in the thickness direction from the upper part of the p-type electrode layer to the inside of the n-type AlGaN layer in the substrate direction, the photonic crystal periodic structure has a photonic band gap open with respect to the TM polarization component, For light with a wavelength of λ, the period a of the photonic crystal periodic structure satisfies Bragg's condition (m×λ / n eff = 2a, where m is the order number and n eff : the effective refractive index of the photonic crystal periodic structure), the order m satisfies 3 ≤ m ≤ 7, and with the radius of the column being R, 0.25 ≤ R / a ≤ 0.35 is satisfied.

2. The deep ultraviolet LED according to claim 1, characterized in that, An insulating layer made of SiO2 fills the space part of the photonic crystal periodic structure.

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