Electron affinity engineered VCSELs
View Patent ↗A VCSEL having an N-type Bragg mirror with alternating layers of high bandgap (low index) and low bandgap (high index) layers of AlGaAs. The layers may be separated by a step change of Al composition followed by a graded region, and vice versa for the next layer, in the N-type mirror to result in a lower and more linear series resistance. Also, an N-type spacer layer may be adjacent to an active region of quantum wells. There may be a similar step in a change of Al composition from the nearest layer of the N-type mirror to the N-type spacer formed from a lower bandgap direct AlGaAs layer to provide lower free carrier absorption. With electron affinity engineering, a minority carrier hole barrier may be inserted adjacent to the quantum wells to improve hole confinement at high current density and high temperature.
1. A VCSEL system comprising:
a substrate;
a first mirror stack situated on the substrate;
an active region situated on the first mirror stack;
a second mirror stack situated on the active region;
wherein:
the first mirror stack comprises a plurality of pairs of AlAs and GaAs layers;
at least one interface of first and second interfaces, is situated between each AlAs layer and GaAs layer;
the first interface comprises:
a ramp increase of Al from GaAs to Al x Ga 1−x As; and
a step increase of Al from Al x Ga 1−x As to Al y Ga 1−y As; and
the second interface comprises:
a step decrease of Al from Al y Ga 1−y A to Al x Ga 1−x As; and
a ramp decrease of Al from Al x Ga 1−x As to GaAs; and
wherein x ramps to a final value that is ≦0.4 for the first and second interfaces.
2. The system of claim 1 , wherein 0.3≦x≦0.4; and 0.9≦y≦1.0.
3. The system of claim 2 , wherein:
we first mirror stack is a distributed Bragg reflector;
and the first mirror stack has an N-type doping.
4. The system of claim 3 , further comprising a first spacer layer situated between the first mirror stack and the active region.
5. The system of claim 4 , wherein:
the spacer layer comprises Al z Ga 1−x As;
0.3≦z≦0.4;
and the spacer has an N-type doping.
6. The system of claim 5 , further comprising a first hole confinement layer situated between the first spacer layer and the active region.
7. The system of claim 6 , wherein the first hole confinement barrier comprises AlAs.
8. The system of claim 7 , further comprising a transition barrier situated between the hole confinement barrier and the active region.
9. The system of claim 8 , wherein the transition barrier comprises:
a composition of Al that has a ramp variation from Al w Ga 1−w As to Al v Ga 1−v As;
wherein:
the Al v Ga 1−v As is adjacent to the active region;
0.3≦w≦0.4; and 0.2≦v≦0.3.
10. The system of claim 9 , wherein;
the active region comprises:
at least one well; and at least one well barrier proximate to the well;
and the well barrier comprises:
Al u Ga 1−u As; and 0.2≦u≦0.3.
11. The system of claim 1 , wherein:
x≈0.33
y≈1.0.
12. The system of claim 11 , further comprising:
a first spacer layer situated between the first mirror stack and the active region;
and wherein the spacer layer comprises:
Al z Ga 1−z As; and z≈0.38.
13. The system of claim 12 , further comprising:
a first hole confinement layer situated between the first spacer layer and the active region; and
wherein the first hole confinement layer comprises AlAs.
14. The system of claim 13 , further comprising:
a transition barrier situated between the first hole confinement layer and the active region; and wherein:
the transition barrier comprises a composition of Al that has a ramp variation from Al w Ga 1−w AS to Al v Ga 1−v As;
the Al v Ga 1−v As is adjacent to the active region;
w≈0.38; and v≈0.25.
15. The system of claim 14 , wherein the active region comprises:
at least one well; and at least two well barriers adjacent to the at least one well;
and wherein:
the well barrier comprises Al u Ga 1−u As; and
u≈0.25.
16. A VCSEL system comprising:
a first mirror; and wherein:
the first mirror comprises a plurality of pairs of layers;
each pair of layers of the plurality of pairs has a first layer and a second layer;
the first layer comprises A x B 1−x C;
the second layer comprises A y B 1−y C;
x+y=1;
a transition layer is situated between each first layer and each second layer;
the transition layer, in a direction of a first layer to a second layer, has a ramp change in content of A from x=a first value of x to a second value of x=P, wherein P is less than 0.4, and a step change in content of A from x=P to y;
the transition layer in a direction of a second layer to a first layer, has a step change in content of A from y to y=P and a ramp change from P to the first value of x.
17. The system of claim 16 , further comprising:
a spacer layer situated proximate to the first mirror;
and wherein:
the spacer layer comprises A z B 1−z C; and 0≦z≦1.
18. The system of claim 17 , further comprising:
a barrier layer proximate to the spacer layer; and wherein the barrier layer is a hole confinement barrier.
19. The system of claim 18 , further comprising:
a transition layer proximate to the barrier layer;
and wherein:
the transition layer comprises A v B 1−v C;
0≦v≦1; and
v is equal to a value relative to a distance from the barrier layer.
20. The system of claim 19 , further comprising:
an active region; and wherein:
the active region comprises at least one quantum well bound by active region barrier layers;
the at least one quantum well comprises A w B 1−w C;
the active region barrier layers comprises A u B 1−u C;
0≦w≦1; and 0≦u≦1.
21. The system of claim 16 , wherein:
A is Al;
B is Ga;
and C is As.
22. The system of claim 21 , wherein:
x≈0;
and y≈1.
23. The system of claim 22 , wherein 0.3≦P≦0.4.
24. The system of claim 23 , wherein:
the ramp change from x to P is approximately linear over a distance between 15 and 25 nm; and the ramp change from P to x is approximately linear over a distance between 15 and 25 nm.
25. The system of claim 24 , wherein z<0.45.
26. The system of claim 25 , wherein the barrier layer comprises AlAs.
27. The system of claim 26 , wherein 0.25≦v≦0.4.
28. The system of claim 27 , wherein w≈1.
29. The system of claim 28 , wherein 0.2<u<0.3.
30. The system of claim 29 , wherein u≈0.25.
31. The system of claim 30 , wherein the first mirror is an N-doped distributed Bragg reflector.
32. The system of claim 31 , further comprising a second mirror proximate to the active region.
33. A VCSEL system comprising:
a first mirror; and wherein:
the first mirror comprises a plurality of pairs of layers;
each pair of layers has a first layer and a second layer;
the first layer comprises A x C;
the second layer comprises A y C;
x+y=1;
a transition layer is situated between each first layer and a second layer;
the transition layer, in a direction of a first layer to a second layer, has a first change from x to P, and a second change from P to y, wherein P≦0.4 for the direction of a first layer to a second layer:
the transition layer in a direction of a second layer to a first layer, has a third change from y to P, and fourth change from P to x; and P≦1 for the direction of a second layer to the first layer.
34. The system of claim 33 , wherein:
the first and fourth changes are ramp changes; and the second and third changes are step changes.
35. The system of claim 33 , wherein:
the second and third changes are ramp changes; and the first and fourth changes are step changes.
36. The system of claim 34 , further comprising:
a spacer layer situated proximate to the first mirror; and the active region; and wherein:
the spacer layer comprises A z E;
the spacer layer is adjacent to the first mirror; and 0≦z≦1.
37. A VCSEL system comprising:
a substrate;
a first mirror stack situated on the substrate;
an active region situated on the first mirror stack;
a second mirror stack situated on the active region;
wherein:
the first mirror stack comprises a plurality of pairs of Al x Ga 1−x As and Al y Ga 1−y As layers;
an interface between each Al x Ga 1−x As layer and Al y Ga 1−y As layer;
the interface comprises:
at least one linear ramp change of Al composition from x=first value to x≦a second value, the second value less than 0.4; and at least one step change of Al composition from the second value to a third value.
38. A VCSEL system comprising:
a substrate;
a first mirror stack formed over the substrate, the first mirror having pairs of layers, each pair having a first layer and a second layer;
an active region including a plurality of quantum wells formed over the first mirror stack;
a second mirror stack formed over the active region, the second mirror stack having pairs of layers, each pair including a third layer and a fourth layer;
a first spacer layer between the active region and the first mirror stack and a second spacer layer between the active region and the second mirror stack; and
an interface formed between the first layer and second layer of each pair in the first mirror stack, the interface comprising:
a first portion where a composition of a first element ramps from a first value to a second value, and a step in the composition from the second value to a third value, the second value selected such that electron affinity on a first side of the step is substantially the same as an electron affinity on a second side of the step.