GaN based LED lighting extraction efficiency using digital diffractive phase grating

• US 20030213969A1
• Filed: 11/12/2002
• Published: 11/20/2003
• Est. Priority Date: 11/16/2001
• Status: Active Grant

• Current Assignee: Gelcore (General Electric Company)
• Original Assignee: Emcore Corporation
• Inventors: Wang, Michael, Venugopalan, Hari
• Granted Patent: US 6,903,379 B2
• Time in Patent Office: Days
• US Class Current: 257/98
• CPC Class Codes: H01L 2933/0083 Periodic patterns for optical field-shaping in or on the semiconductor body or semiconductor body package, e.g. photonic bandgap structures
• CPC Class Codes: H01L 33/46 Reflective coating, e.g. dielectric Bragg reflector

Abstract

A light emitting diode incorporating an active emitting layer (14) overlying a transparent substrate (10) is provided with a reflective diffraction grating (30) on the bottom surface of the substrate. Emitted light passing downwardly through the substrate is diffracted outwardly toward edges (21) of the substrate and passes out of the die through the edges. This effect enhances the external quantum efficiency of the diode.

Claims

1. A light emitting diode comprising:

(a) a stacked structure having a top surface, a bottom surface, edges extending between said top and bottom surfaces and an emitting junction extending generally in a horizontal plane above the bottom surface, said emitting junction having an emission wavelength;

said stacked structure having vertical directions normal to the emitting junction, and

(b) a reflective diffraction grating disposed below said emitting junction on or in said stacked structure, said grating including a plurality of elements spaced apart from one another in a repeat direction, said grating being operative to diffract light at said emission wavelength directed vertically downwardly from said emission layer so that a substantial portion of such light is diffracted upwardly into the stacked structure in major diffraction directions oblique to the vertical and toward the edges of the structure.

2. A light emitting diode as claimed in claim 1 wherein said structure includes a substrate disposed between said junction and said grating, said substrate being transparent to light at said emission wavelength, said substrate defining portions of said edges.

3. A light emitting diode as claimed in claim 2 wherein said grating is arranged so that at least some of the upwardly-diffracted light passes through portions of said edges defined by said substrate.

4. A light emitting diode as claimed in claim 3 wherein said substrate has a thickness t in the vertical direction such that:

t>

w/tan θ

where;

w is the horizontal dimension of the substrate in the repeat direction of the grating; and

θ

is the angle between one said major diffraction direction and the vertical.

5. A light emitting diode as claimed in claim 1 wherein said grating is in the form of a plurality of elongated strips extending substantially parallel to one another, each said strip extending in a lengthwise direction perpendicular to said repeat direction, and wherein said stack structure has a dimension x in the lengthwise direction greater than said dimension w in the repeat direction.

6. A light emitting diode as claimed in claim 1 wherein said grating is a binary phase grating having alternating regions arranged to reflect light at phases differing by π

.

7. A light emitting diode as claimed in claim 1 wherein said grating is disposed at the bottom of said substrate.

8. A light emitting diode as claimed in claim 7 wherein includes a set of first regions and second regions having different vertical elevations, such that said first regions project below said second region by a predetermined distance.

9. A light emitting diode as claimed in claim 1 wherein said stacked structure has a critical angle for total internal reflection at said edges of said stacked structure, and said major diffraction directions intercept the edges of the stacked structure at angles to the normal of the edge surface less than said critical angle.

10. A light emitting diode as claimed in claim 2 wherein said stack structure includes one or more nitride-based semiconductors defining said emitting junction.

11. A light emitting diode as claimed in claim 10 wherein said substrate consists essentially of sapphire.