A METHOD FOR GAN VERTICAL MICROCAVITY SURFACE EMITTING LASER (VCSEL)

• US 20170237234A1
• Filed: 09/30/2015
• Published: 08/17/2017
• Est. Priority Date: 09/30/2014
• Status: Active Grant

• Current Assignee: Yale University
• Original Assignee: Yale University
• Inventors: Zhang, Cheng, Han, Jung
• Granted Patent: US 11,043,792 B2
• Time in Patent Office: Days
• CPC Class Codes: H01L 21/30635 of AIIIBV compounds
• CPC Class Codes: H01L 33/10 with a light reflecting structure, e.g. semiconductor Bragg reflector
• CPC Class Codes: H01L 33/16 with a particular crystal structure or orientation, e.g. polycrystalline, amorphous or porous
• CPC Class Codes: H01L 33/32 containing nitrogen
• CPC Class Codes: H01S 5/1042 Optical microcavities, e.g. cavity dimensions comparable to the wavelength
• CPC Class Codes: H01S 5/183 having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
• CPC Class Codes: H01S 5/18308 having a special structure for lateral current or light confinement
• CPC Class Codes: H01S 5/18361 Structure of the reflectors, e.g. hybrid mirrors
• CPC Class Codes: H01S 5/18369 based on dielectric materials
• CPC Class Codes: H01S 5/187 using Bragg reflection
• CPC Class Codes: H01S 5/305 characterised by the doping materials used in the laser structure
• CPC Class Codes: H01S 5/34333 with a well layer based on Ga(In)N or Ga(In)P, e.g. blue laser

Abstract

Structures and methods for forming highly uniform and high-porosity gallium-nitride layers with sub-100-nm pore sizes are described. Electrochemical etching of heavily-doped gallium nitride at low bias voltages in concentrated nitric acid is used to form the porous gallium nitride. The porous layers may be used in reflective structures for integrated optical devices such as VCSELs and LEDs.

Claims

1. A porous gallium-nitride layer having a majority of its pores with a maximum transverse width less than approximately 100 nm and having a volumetric porosity greater than 30%.

2. The porous gallium-nitride layer of claim 1, wherein more than 90% of the pores have a maximum transverse width less than approximately 100 nm.

3. The porous gallium-nitride layer of claim 1, wherein over half of the pores of the gallium-nitride layer have a maximum transverse width between approximately 30 nm and approximately 90 nm.

4. The porous gallium-nitride layer of claim 1, wherein more than 70% of the pores of the gallium-nitride layer have a maximum transverse width between approximately 30 nm and approximately 90 nm.

5. The porous gallium-nitride layer of claim 1, wherein the pores have walls with a root-mean-square surface roughness less than approximately 10 nm.

6. The porous gallium-nitride layer of claim 1, wherein an n-type doping density of the porous gallium-nitride layer is between approximately 5×

10¹⁹ cm<sup>−

3</sup> and approximately 2×

10²⁰ cm<sup>−

3</sup>.

7. The porous gallium-nitride layer of any one of claims 1-6, wherein a dopant in the porous gallium-nitride layer for the n-type doping is germanium.

8. The porous gallium-nitride layer of any one of claims 1-6, wherein the volumetric porosity is greater than 60%.

9. The porous gallium-nitride layer of claim 7 included in a distributed Bragg reflector.

10. The porous gallium-nitride layer of claim 9 included in a vertical-cavity surface-emitting laser.

11. The porous gallium-nitride layer of any one of claims 1-8 included in a light-emitting diode.

12. The porous gallium-nitride layer of any one of claims 1-8 included in an electrode.

13. A semiconductor light emitting device comprising:

at least one buried porous gallium-nitride layer wherein a majority of the pores of the at least one buried porous gallium-nitride layer have a maximum transverse width less than approximately 100 nm and the at least one buried porous gallium-nitride layer has a volumetric porosity greater than 30%.

14. The semiconductor light emitting device of claim 13, wherein over 70% of the pores of the at least one buried porous gallium-nitride layer have a maximum transverse width between approximately 30 nm and approximately 90 nm.

15. The semiconductor light emitting device of claim 13 or 14, wherein the at least one buried porous gallium-nitride layer comprises a plurality of porous gallium-nitride layers separated by non-porous gallium-nitride layers arranged in a first distributed Bragg reflector (DBR).

16. The semiconductor light emitting device of claim 15, wherein the plurality of porous gallium-nitride layers include non-porous regions located centrally within the DBR that form a pillar of non-porous gallium nitride.

17. The semiconductor light emitting device of claim 15 or claim 16, wherein the first DBR is arranged as an n-side reflector for a vertical-cavity surface-emitting laser (VCSEL).

18. The semiconductor light emitting device of claim 17, wherein the first DBR has a reflectance greater than 99% for a lasing wavelength of the VCSEL.

19. The semiconductor light emitting device of claim 18, wherein the first DBR has reflectance values greater than 98% over a bandwidth greater than approximately 20 nm.

20. The semiconductor light emitting device of claim 17, further including a cavity region having a length L and a second DBR, wherein the cavity region is located between the first DBR and the second DBR.

21. The semiconductor light emitting device of claim 20, wherein the cavity region includes multiple quantum wells or a superlattice.

22. The semiconductor light emitting device of claim 20 or claim 21, wherein the length L of the cavity region is between approximately one and five optical wavelengths of an lasing wavelength for the VCSEL.

23. The semiconductor light emitting device of claim 15, further comprising a current-spreading layer having a doping density greater than 1×

10¹⁸ cm<sup>−

3</sup> located adjacent to the distributed Bragg reflector.

24. The semiconductor light emitting device of claim 13 or claim 15, wherein the pores of the at least one buried porous gallium-nitride layer have walls with a root-mean-square surface roughness less than approximately 10 nm.

25. The semiconductor light emitting device of claim 13 or claim 15, wherein the at least one buried porous gallium-nitride layer has an n-type doping density between approximately 5×

10¹⁹ cm<sup>−

3</sup> and approximately 2×

10²⁰ cm<sup>−

3</sup>.

26. The semiconductor light emitting device of claim 25, wherein a dopant for the n-type doping in the at least one buried porous gallium-nitride layer is germanium.

27. A method for forming porous gallium nitride, the method comprising:

exposing heavily-doped gallium nitride to an etchant, wherein the heavily-doped gallium nitride has an n-type doping density between approximately 5×

10¹⁹ cm<sup>−

3</sup> and approximately 2×

10²⁰ cm<sup>−

3</sup>;

applying an electrical bias between the etchant and the heavily-doped gallium nitride, wherein the electrical bias has a value between approximately 1.3 volts and 3 volts; and

electrochemically etching the heavily-doped gallium nitride to produce porous gallium nitride having a volumetric porosity greater than approximately 30% and a majority of pores with a maximum transverse width less than approximately 100 nm.

28. The method of claim 27, wherein over 70% of the pores of the etched gallium nitride have a maximum transverse width between approximately 30 nm and approximately 90 nm.

29. The method of claim 27, wherein the electrochemical etching does not require illumination of the heavily-doped gallium nitride.

30. The method of claim 27, wherein a dopant for the heavily-doped gallium nitride is germanium.

31. The method of any one of claims 27-30, wherein the etchant is nitric acid having a concentration between 60% and approximately 80% by weight.

32. The method of any one of claims 27-30, wherein the etchant is nitric acid having a concentration of approximately 70% by weight.

33. The method of claim 32, wherein the heavily-doped gallium nitride is arranged in a plurality of layers that are separated by undoped or moderately-doped gallium-nitride layers.

34. The method of claim 33, further comprising spreading etching current during the electrochemical etching with a current-spreading layer of doped gallium nitride located adjacent to the DBR.

35. The method of claim 33, further comprising etching vias into the plurality of layers and the undoped or moderately-doped gallium-nitride layers to expose edges of the plurality of layers.

36. The method of claim 35, wherein the electrochemical etching comprises lateral etching of the plurality of layers.

37. The method of claim 33, further comprising depositing the plurality of layers and the undoped or moderately-doped gallium-nitride layers to form a first distributed Bragg reflector (DBR) for a vertical-cavity surface-emitting laser (VCSEL).

38. The method of claim 37, further comprising stopping the electrochemical etching to leave a pillar of unetched gallium nitride centrally within the first DBR.

39. The method of claim 37, further comprising forming a cavity region having multiple quantum wells or a superlattice adjacent to the first DBR.

40. The method of claim 39, further comprising forming a second DBR on an opposite side of the cavity region from the first DBR.