Flip-chip light emitting diode with resonant optical microcavity

• US 6,969,874 B1
• Filed: 06/12/2003
• Issued: 11/29/2005
• Est. Priority Date: 06/12/2003
• Status: Active Grant

• Current Assignee: National Technology & Engineering Solutions Of Sandia LLC (Honeywell International Inc.)
• Original Assignee: Sandia Corporation (Martin Marietta Corporation)
• Inventors: Fischer, Arthur J., Bogart, Katherine H. A., Gee, James M.
• Primary Examiner(s): Hu, Shouxiang
• Application Number:
  US10/459,965
• Time in Patent Office: 901 Days
• Field of Search: 257 79-103, 438/27, 438/29
• US Class Current: 257/98
• CPC Class Codes: H01L 2224/05124 Aluminium [Al] as principal constituent
• CPC Class Codes: H01L 2224/05166 Titanium [Ti] as principal constituent
• CPC Class Codes: H01L 2224/05568 the whole external layer protruding from the surface
• CPC Class Codes: H01L 2224/05573 Single external layer
• CPC Class Codes: H01L 2224/05624 Aluminium [Al] as principal constituent
• CPC Class Codes: H01L 2224/05639 Silver [Ag] as principal constituent
• CPC Class Codes: H01L 2224/05644 Gold [Au] as principal constituent
• CPC Class Codes: H01L 2224/05647 Copper [Cu] as principal constituent
• CPC Class Codes: H01L 2224/05655 Nickel [Ni] as principal constituent
• CPC Class Codes: H01L 2224/05664 Palladium [Pd] as principal constituent
• CPC Class Codes: H01L 2224/05666 Titanium [Ti] as principal constituent
• CPC Class Codes: H01L 2224/05669 Platinum [Pt] as principal constituent
• CPC Class Codes: H01L 2224/06102 the bonding areas being at different heights
• CPC Class Codes: H01L 2224/13 of an individual bump connector
• CPC Class Codes: H01L 2224/1703 Bump connectors having different sizes, e.g. different diameters, heights or widths
• CPC Class Codes: H01L 24/05 of an individual bonding area
• CPC Class Codes: H01L 24/06 of a plurality of bonding areas
• CPC Class Codes: H01L 2924/00014 the subject-matter covered by the group, the symbol of which is combined with the symbol of this group, being disclosed without further technical details
• CPC Class Codes: H01L 33/105 with a resonant cavity structure
• CPC Class Codes: H01L 33/405 Reflective materials
• CPC Class Codes: H01L 33/465 : with a resonant cavity structure

Abstract

A flip-chip light emitting diode with enhanced efficiency. The device structure employs a microcavity structure in a flip-chip configuration. The microcavity enhances the light emission in vertical modes, which are readily extracted from the device. Most of the rest of the light is emitted into waveguided lateral modes. Flip-chip configuration is advantageous for light emitting diodes (LEDs) grown on dielectric substrates (e.g., gallium nitride LEDs grown on sapphire substrates) in general due to better thermal dissipation and lower series resistance. Flip-chip configuration is advantageous for microcavity LEDs in particular because (a) one of the reflectors is a high-reflectivity metal ohmic contact that is already part of the flip-chip configuration, and (b) current conduction is only required through a single distributed Bragg reflector. Some of the waveguided lateral modes can also be extracted with angled sidewalls used for the interdigitated contacts in the flip-chip configuration.

Claims

1. A flip-chip light emitting diode with enhanced light extraction efficiency, comprising:

a transparent growth substrate;

a first n-type buffer layer overlaying the transparent growth substrate;

a partially reflective mirror layer overlaying the first n-type buffer layer;

a second n-type barrier layer overlaying the partially reflective mirror layer;

a light emitting active layer overlaying the second n-type barrier layer;

a p-type barrier layer overlaying the light emitting active layer;

a highly reflective p-contact pad overlaying the p-type barrier layer; and

an n-contact pad overlaying the first n-type buffer layer;

wherein the n-contact pad and the highly reflective p-contact pad are disposed on the same side of the light emitting diode;

wherein a resonant optical microcavity is disposed in-between the partially reflective mirror layer and the highly reflective p-contact pad;

wherein the highly reflective p-contact pad comprises a reflective metal stack comprising;

an adhesion layer, in direct contact with the p-type barrier layer, consisting of Pd or Ti or a combination of Pd and Ti;

a reflective layer, in direct contact with the adhesion layer, comprising at least one metal selected from the group consisting of Al and Ag; and

a protective coating layer, covering said reflective layer, comprising Au;

wherein the P-doped barrier layer comprises p-doped GaN or p-doped AlGaN;

wherein the partially reflective mirror layer comprises a distributed Bragg reflector;

wherein the distributed Bragg reflector comprises alternating layers of AlGaN and GaN; and

wherein the distributed Bragg reflector has a reflectivity equal to approximately 25% to 40%.

2. The flip-chip light emitting diode of claim 1, wherein the transparent growth substrate comprises one or more materials selected from the group consisting of sapphire, glass, GaP, GaAs, InP, GaAsP, ZnSe, ZnS, ZnSSe, SiC, GaN, AlN, ZnO, a spinel material, and combinations thereof.

3. The flip-chip light emitting diode of claim 1, wherein the first n-type buffer layer and the second n-type barrier layer comprise n-doped GaN.

4. The flip-chip light emitting diode of claim 1, wherein the light emitting active layer comprises one or more structures selected from the group consisting of a homostructure, single heterostructure (SH), double heterostructure (DH), a quantum well (QW) and a multiple quantum well (MQW).

5. The flip-chip light emitting diode of claim 1, wherein the n-contact pad is disposed within a trench, and further wherein a sidewall of the trench is sloped at an angle, wherein light emitted as waveguided lateral modes is internally reflected off the sloped sidewall and redirected towards the transparent growth substrate.

6. The flip-chip light emitting diode of claim 5, wherein the angle of the sloped sidewall is about 45 degrees.

7. The flip-chip light emitting diode of claim 1, wherein the highly reflective p-contact pad has a reflectivity greater than or equal to about 70%.

8. The flip-chip light emitting diode of claim 1, wherein the optical length, L, of the resonant optical microcavity is a low integer multiple of λ

/2, so that L=mλ

/2;

wherein m is an integer from 1 to 4, and λ

is the wavelength of the spontaneous emission from the light emitting active layer.

9. The flip-chip light emitting diode of claim 1, wherein the reflective metal stack comprises about 2.5 nm Pd/200 nm Ag/300 nm Au.

10. The flip-chip light emitting diode of claim 1, wherein the light emitting active layer is disposed at an anti-node of the resonant optical microcavity.

11. The flip-chip light emitting diode of claim 1, wherein the light emitting active layer comprises one or more of the group consisting of (Al,In)GaN MQW, InGaN MQW, AlGaInP DH, InGaAs MQW, InGaAsP MQW, and InGaAsP DH.

12. The flip-chip light emitting diode of claim 1, wherein the light emitting active layer is disposed at an anti-node of said microcavity.

13. The flip-chip light emitting diode of claim 1, wherein the the highly reflective p-contact pad further comprises a diffusion barrier layer disposed in-between the reflective layer and the protective coating layer;

and wherein the diffusion barrier layer comprises at least one metal selected from the group consisting of Pt, Mo, Rh and Ni.

14. The flip-chip light emitting diode of claim 1, wherein the distributed Bragg reflector comprising less than 20 alternating layers of AlGaN and GaN.