Deep ultraviolet LED and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-9929317-B2
Application number	US-201515540510-A
Country	US
Kind code	B2
Filing date	Jul 29, 2015
Priority date	Jan 16, 2015
Publication date	Mar 27, 2018
Grant date	Mar 27, 2018

Abstract

Provided is a deep ultraviolet LED with a design wavelength λ, including an Al reflecting electrode layer, an ultrathin metal layer, and a transparent p-AlGaN contact layer that are sequentially arranged from a side opposite to a substrate, and a photonic crystal periodic structure provided in the range of the thickness direction of the transparent p-AlGaN contact layer. The photonic crystal periodic structure has a photonic band gap.

First claim

The invention claimed is:

1. A deep ultraviolet LED with a design wavelength λ, comprising:
a reflecting electrode layer, an ultrathin metal layer, and a transparent p-AlGaN contact layer that are sequentially arranged from a side opposite to a growth substrate; and
a first reflecting photonic crystal periodic structure including a plurality of voids provided in a range of a thickness direction of the transparent p-AlGaN contact layer,
wherein:
the first reflecting photonic crystal periodic structure has a photonic band gap that opens for traversal electric (TE) polarized components,
the design wavelength λ and a period a and an average refractive index n av of the first reflecting photonic crystal periodic structure satisfy a formula of a Bragg condition (m×λ/n av =2a),
an order m of the Bragg condition is in a range of 2<m<5, and
provided that a radius of each void is R, R/a at which a maximum photonic band gap is obtained is selected, and
provided that a depth of the void is h, h≥2/3a.

2. The deep ultraviolet LED according to claim 1 , wherein the first reflecting photonic crystal periodic structure is provided such that it further extends in the thickness direction to a range of the reflecting electrode layer.

3. A method for producing the deep ultraviolet LED according to claim 1 , wherein the first reflecting photonic crystal periodic structure is formed using an imprinting technique based on a nanoimprint lithography method.

4. The method for producing the deep ultraviolet LED according to claim 3 , wherein the first reflecting photonic crystal periodic structure is formed using dry etching based on a bi-layer resist method that uses resist with high fluidity and resist with high etch selectivity.

5. A method for producing the deep ultraviolet LED according to claim 1 , wherein parameters of the first reflecting photonic crystal periodic structure are determined using a parameter computation method including:
a step of tentatively determining a ratio (R/a) between the period a and a radius R of each void that are parameters of the periodic structure;
a step of computing the average refractive index n av from refractive indices n 1 and n 2 of the void and the R/a, and substituting the average refractive index n av into a formula of the Bragg condition, thereby obtaining the period a and the radius R for each order m;
a step of analyzing a photonic band structure for TE light using a plane wave expansion method that uses the R/a, the wavelength λ, and dielectric constants ∈ 1 and ∈ 2 of the structure obtained from the refractive indices n 1 and n 2 ;
a step of determining R/a at which a PBG between a first photonic band and a second photonic band for TE light becomes maximum by repeatedly changing a value of the tentatively determined R/a;
a step of determining, for R/a at which the PBG becomes maximum, light extraction efficiency for light with the wavelength λ through a simulation analysis using a finite-difference time-domain method (FDTD method) that is performed by using as variables the individual period a and radius R corresponding to each order m of the Bragg condition and a given depth h of the periodic structure; and
a step of determining the order m of the Bragg condition at which the light extraction efficiency for light with the wavelength λ becomes maximum, and the radius R and the depth h that are the parameters of the periodic structure corresponding to the order m by repeatedly performing simulation analysis using the FDTD method.

6. The deep ultraviolet LED according to claim 1 , further comprising a second photonic crystal periodic structure on a rear surface (side) of the growth substrate, the second photonic crystal periodic structure including two structures with different refractive indices,
wherein:
the second photonic crystal periodic structure includes a second structure having a periodic structure of the air and a medium of the growth substrate.

7. The deep ultraviolet LED according to claim 6 ,
wherein:
in the second photonic crystal periodic structure, the design wavelength λ in a vacuum and the period a and the radius R, which are the parameters of the periodic structure, satisfy the Bragg condition, and a photonic band structure for TM light includes two photonic band gaps within first to fourth photonic bands when R/a is in a range of 0.20 to 0.40,
the photonic band gap is open for TM light and thus has a high transmission effect,
the R/a is a value corresponding to a maximum value of each photonic band gap at the order m=3 or 4, or
the R/a is a value that is, at the order m=3 or 4, in point contact with or proximate to, when an ordinate axis (ωa/2πc) of the photonic band structure is converted into the wavelength λ v in a vacuum, the wavelength λ v in a vacuum ×m at one of points Γ, M, and K that are points of symmetry of a second photonic band (2 nd PB), or
the R/a is a value that is, at the order m=3, in point contact with or proximate to, when the ordinate axis (ωa/2πc) of the photonic band structure is converted into the wavelength λ v in a vacuum ×3 (λ v ×3), one of points of symmetry of the fourth photonic band (4 th PB) obtained through multiplication of the length of the period of the fourth photonic band (4 th PB) by 5 and 6, or
the R/a is a value that is, at the order m=4, in point contact with or proximate to, when the ordinate axis (ωa/2πc) of the photonic band structure is converted into the wavelength λ v in a vacuum ×4 (λ v ×4), one of points of symmetry of the fourth photonic band (4 th PB) obtained through multiplication of the length of the period of the fourth photonic band (4 th PB) by 6, 7, and 8, and
parameters of each periodic structure are parameters obtained by computing photonic crystals having the selected R/a and the depth h that is greater than or equal to 0.5a, using the FDTD method, and are finally determined so as to optimize a rate of change of the light extraction efficiency and a light distribution property.

8. A method for producing the deep ultraviolet LED according to claim 7 , wherein parameters of the second photonic crystal periodic structure are determined using a parameter computation method including:
a first step of changing a ratio (R/a) between a period a and a radius R of the second structure that are parameters of the periodic structure;
a second step of computing an average refractive index n av from refractive indices n 1 and n 2 of the second structure and the R/a, and substituting the average refractive index n av into the formula of the Bragg condition, thereby obtaining the period a and the radius R for each of the order m=3 and m=4;
a third step of analyzing a photonic band structure for TM light using a plane wave expansion method that uses the R/a, the wavelength λ, and dielectric constant ∈ 1 and ∈ 2 of the structure obtained from the refractive indices n 1 and n 2 ;
a fourth step of converting the ordinate axis (ωa/2πc) of each of the second photonic band (2 nd PB) and the fourth photonic band (4 th PB) for TM light into the wavelength λ v in a vacuum and obtaining a photonic band structure for λ v and ka/2π at the order m=1;
a fifth step of determining R/a that is, at the order m=3 and m=4, in point contact with or proximate to the wavelength λ v in a vacuum ×m at each of points of symmetry of the second photonic band (2 nd PB) and the fourth photonic band (4 th PB) for TM light, and selecting the determined R/a as a candidate for optimization; and
a sixth step of computing the rate of change of the light extraction efficiency and the light distribution property of the photonic crystals corresponding to the R/a selected in the fifth step and selecting, as the depth, a given value that is greater than

Assignees

• Marubun Co Ltd
• Toshiba Machine Co Ltd
• Riken
• Ulvac Inc
• Tokyo Ohka Kogyo Co Ltd

Inventors

• Kashima Yukio
• Matsuura Eriko
• Kokubo Mitsunori
• Tashiro Takaharu
• Ookawa Takafumi
• Hirayama Hideki
• Maeda Noritoshi
• Jo Masafumi
• Kamimura Ryuichiro
• Osada Yamato
• Shimatani Satoshi

Classifications

• H01L33/32

  Electricity · mapped topic

• H01L33/405 Primary

  Electricity · mapped topic

• H01L2933/0016

  Electricity · mapped topic

• H01L33/52

  Electricity · mapped topic

• H01L2933/005

  Electricity · mapped topic