GaN-based laser diodes with misfit dislocations displaced from the active region

1. A GaN-based edge emitting laser comprising a semi-polar GaN substrate, an active region, an N-side waveguiding layer, a P-side waveguiding layer, an N-type cladding layer, and a P-type cladding layer, wherein:

the GaN substrate is characterized by a threading dislocation density on the order of approximately 1×10 6 /cm 2 ;

the active region is interposed between and extends substantially parallel to the N-side waveguiding layer and the P-side waveguiding layer;

the N-type cladding layer is interposed between the N-side waveguiding layer and the GaN substrate;

the P-type cladding layer is formed over the P-side waveguiding layer;

a strain-thickness product of the N-side waveguiding layer exceeds its strain relaxation critical value; a cumulative strain-thickness product of the active region calculated for growth on a relaxed N-side waveguiding layer is less than its strain relaxation critical value;

an N-side interface between the N-type cladding layer and the N-side waveguiding layer comprises a set of N-side misfit dislocations; and

a P-side interface between the P-type cladding layer and the P-side waveguiding layer comprises a set of P-side misfit dislocations.

2. A GaN edge emitting laser as claimed in claim 1 wherein the GaN substrate defines a 20 2 1 crystal growth plane and a glide plane and the strain relaxation is mono-directional along the glide plane of the GaN substrate.

3. A GaN edge emitting laser as claimed in claim 1 wherein the GaN substrate defines a semipolar crystal growth plane and a glide plane and the strain relaxation is mono-directional along the glide plane of the GaN substrate.

4. A GaN edge emitting laser as claimed in claim 1 wherein the threading dislocation density of the GaN substrate is sufficient to catalyze formation of the misfit dislocations at the interface between the N-type cladding layer and the N-side waveguiding layer.

5. A GaN edge emitting laser as claimed in claim 1 wherein the GaN substrate is characterized by a threading dislocation density less than approximately 1×10 6 /cm 2 .

6. A GaN edge emitting laser as claimed in claim 1 wherein:

the active region comprises single or multi-period quantum wells comprising compressively strained quantum wells and tensile barrier layers; and

the strain-thickness product of individual ones of the compressively strained quantum wells and tensile barrier layers is less than the strain relaxation critical value of the layer.

7. A GaN edge emitting laser as claimed in claim 1 wherein:

the active region comprises single or multi-period quantum wells comprising compressively strained quantum wells characterized by a compressive strain-thickness product and tensile barrier layers characterized by a tensile strain-thickness product; and

the compressive strain-thickness product of the quantum wells is approximately equivalent to the tensile strain-thickness product of the barrier layers.

8. A GaN edge emitting laser as claimed in claim 1 wherein the active region, the N-side waveguiding layer, and the P-side waveguiding layer define a substantially coherent lattice constant region that is characterized by an enhanced degree of lattice matching, relative to remaining portions of the laser structure.

9. A GaN edge emitting laser as claimed in claim 1 wherein:

the active region comprises a single or multi-period quantum wells comprising GaInN quantum wells and AlGaInN barrier layers; and

the N-side waveguiding layer comprises a GaInN waveguiding layer.

10. A GaN edge emitting laser as claimed in claim 9 wherein:

the In content of the GaInN quantum wells is greater than the In content of the N-side GaInN waveguiding layer; and

the In content of the AlGaInN barrier layers is less than the In content of the N-side GaInN waveguiding layer.

11. A GaN edge emitting laser as claimed in claim 1 wherein the P-side waveguiding layer and the N-side waveguiding layer comprise GaInN bulk or superlattice waveguiding layers characterized by approximately equal average In concentrations.

12. A GaN edge emitting laser as claimed in claim 1 wherein the N-type cladding layer and the P-type cladding layer comprise a bulk crystal of GaN, AlGaN, or AlGaInN or a superlattice of AlGaN/AlGaN or AlGaN/GaN.

13. A GaN edge emitting laser as claimed in claim 1 wherein:

the P-type cladding layer and the N-type cladding layer comprise Al;

the strain-thickness product of the P-type cladding layer exceeds its strain relaxation critical value; and

the strain-thickness product of the N-type cladding layer exceeds its strain relaxation critical value.

14. A GaN edge emitting laser as claimed in claim 13 wherein the strain-thickness product of the N-type cladding layer exceeds its strain relaxation critical value by at least approximately 10%.

15. A GaN edge emitting laser as claimed in claim 1 wherein the N-side interface comprises an N-side interfacial layer.

16. A GaN edge emitting laser as claimed in claim 15 wherein:

the N-side interfacial layer comprises an N-type GaN transition layer that is under compression; and

the strain-thickness product of the N-side interfacial layer is less than its strain relaxation critical value.

17. A GaN edge emitting laser as claimed in claim 16 wherein the N-side misfit dislocations are located on a side of the N-type GaN transition layer that is proximate the N-type cladding layer.

18. A GaN edge emitting laser as claimed in claim 1 wherein the P-side interface comprises a P-side interfacial layer.

19. A GaN edge emitting laser as claimed in claim 18 wherein:

the P-side interfacial layer comprises a P-type GaN transition layer; and

the P-type GaN transition layer is sufficiently thin to yield P-side misfit dislocations located on a side of the P-type GaN transition layer that interfaces with the P-type cladding layer.

20. A GaN edge emitting laser as claimed in claim 1 wherein the cumulative strain-thickness product of both the P-side waveguiding layer and the active region calculated for growth on a relaxed N-side waveguiding layer is less than its strain relaxation critical value.

21. A GaN edge emitting laser as claimed in claim 1 wherein:

the active region comprises one or more current blocking layers; and

the strain-thickness product of the blocking layer calculated for growth on a relaxed N-side waveguiding layer is less than its strain relaxation critical value.

22. A method of fabricating a GaN edge emitting laser comprising a semi-polar GaN substrate, an active region, an N-side waveguiding layer, a P-side waveguiding layer, an N-type cladding layer, and a P-type cladding layer, wherein:

the GaN substrate is characterized by a threading dislocation density on the order of approximately 1×10 6 /cm 2 ;

the active region is interposed between and extends substantially parallel to the N-side waveguiding layer and the P-side waveguiding layer;

the N-type cladding layer is interposed between the N-side waveguiding layer and the GaN substrate;

the P-type cladding layer is formed over the P-side waveguiding layer;

the active layer is fabricated such that a strain-thickness product of the active region is less than its strain relaxation critical value during fabrication; the N-side waveguiding layer is fabricated such that a strain-thickness product of the N-side waveguiding layer exceeds its strain relaxation critical value during fabrication;

an N-side interface between the N-type cladding layer and the N-side waveguiding layer comprises a set of N-side misfit dislocations; and

a P-side interface between the P-type cladding layer and the P-side waveguiding layer comprises a set of P-side misfit dislocations.