Heterostructure and light-emitting device employing the same

1. A multiple quantum well structure for a light emitting device or a photodetector, comprising alternately stacked AlGaN barriers and AlGaN wells, wherein a thickness of each of the AlGaN barriers and the AlGaN wells respectively satisfies:

h

i

≤

-

σ

i

2

⁢

ρ

0

⁢

i

,

where h i is the thickness of i th AlGaN barrier or well; σ i is sheet charge density of a sheet of charge on a surface of the i th AlGaN barrier or well, the surface being oppositely charged in regard to net activated dopant in the i th AlGaN barrier or well; and

ρ 0i =eN Di −eN Ai is maximal bulk charge density, allowed by applied doping concentration, in a depletion zone of the i th AlGaN barrier or well generated by the sheet of charge, N Di and N Ai are donor and acceptor concentrations, respectively, in the i th AlGaN barrier or well, e is electric elementary charge.

2. The multiple quantum well structure of claim 1 , wherein one or more of the AlGaN wells comprises an n-type doped AlGaN prior-well spacer, an n-type doped AlGaN post-well spacer, and an AlGaN main well sandwiched between the n-type doped AlGaN prior-well spacer and the n-type doped AlGaN post-well spacer, wherein an Al-composition of the n-type doped AlGaN prior-well spacer and an Al-composition of the n-type doped AlGaN post-well spacer are different than an Al-composition of the AlGaN main well, and a thickness of the n-type doped AlGaN prior-well spacer and a thickness of the n-type doped AlGaN post-well spacer are smaller than a thickness of the AlGaN main well and a thickness of adjacent AlGaN barrier.

3. The multiple quantum well structure of claim 2 , wherein the n-type doped AlGaN prior-well spacer and the n-type doped AlGaN post-well spacer are Si-doped with a doping concentration of 1.0×10 18 -8.0×10 18 cm −3 , respectively, the AlGaN main well is undoped or Si-doped with a doping concentration less than 5.0×10 17 cm −3 , at least one of the AlGaN barriers is Si-doped with a doping concentration of 1.0×10 18 -8.0×10 18 cm −3 .

4. The multiple quantum well structure of claim 2 , wherein a thickness of the n-type doped AlGaN prior-well spacer and a thickness of the n-type doped AlGaN post-well spacer are in the range of 0.1 nm to 0.52 nm, respectively.

5. The multiple quantum well structure of claim 2 , wherein the Al-composition of the n-type doped AlGaN prior-well spacer and the Al-composition of the n-type doped AlGaN post-well spacer are higher than an Al-composition of adjacent AlGaN barrier.

6. The multiple quantum well structure of claim 5 , wherein the n-type doped AlGaN prior-well spacer and the n-type doped AlGaN post-well spacer are made of AlN and have a thickness in the range of 0.1-0.52 nm, respectively.

7. The multiple quantum well structure of claim 2 , wherein the Al-composition of the n-type doped AlGaN prior-well spacer and the Al-composition of the n-type doped AlGaN post-well spacer are lower the Al-composition of the AlGaN main well.

8. The multiple quantum well structure of claim 7 , wherein the n-type doped AlGaN prior-well spacer and the n-type doped AlGaN post-well spacer are made of GaN and have a thickness in the range of 0.1-0.52 nm, respectively.

9. The multiple quantum well structure of claim 2 , wherein the Al-composition of the n-type doped prior-well spacer is higher than an Al-composition of adjacent AlGaN barrier and the Al-composition of the n-type doped AlGaN post-well spacer is lower the Al-composition of the AlGaN mail well; or the Al-composition of the n-type doped AlGaN post-well spacer is higher than an Al-composition of the AlGaN barrier and the Al-composition of the AlGaN prior-well spacer is lower the Al-composition of the AlGaN main well.

10. The multiple quantum well structure of claim 9 , wherein one of the n-type doped AlGaN prior-well spacer and the n-type doped AlGaN post-well spacer is made of AlN and the other is made of GaN, and have a thickness in the range of 0.1-0.52 nm, respectively.

11. The multiple quantum well structure of claim 2 , further comprising an undoped AlGaN barrier formed on one of the AlGaN wells on one side and to be in contact with a p-type structure of the light emitting device or photodetector on the other side.

12. The multiple quantum well structure of claim 1 , wherein one or more of the AlGaN barriers contains one or more sheet of positive charge, and a distance between two adjacent sheets of positive charge is larger than a depletion depth of a depletion zone generated by any one of the two adjacent sheets of positive charge.

13. The multiple quantum well structure of claim 12 , wherein the sheets of positive charge are formed via Si-delta doping with a sheet doping density equal to or greater than 10 12 cm −2 .

14. The multiple quantum well structure of claim 12 , wherein each of the AlGaN barriers that contain the sheet of positive charge comprises a Si-doped layer with a doping concentration of 1.0-8.0×10 18 cm −3 and an undoped layer separated by the sheet of positive charge.

15. The multiple quantum well structure of claim 14 , wherein a thickness of the Si-doped layer of each of the AlGaN barriers that contain the sheet of positive charge is in the range of 6-10 nm, respectively, and a thickness of the undoped layer of each of the AlGaN barriers that contain the sheet of positive charge is in the range of 2-4 nm, respectively.

16. The multiple quantum well structure of claim 12 , further comprising an undoped AlGaN barrier formed on one of the AlGaN wells on one side and to be in contact with a p-type structure of the light emitting device or photodetector on the other side.

17. The multiple quantum well structure of claim 12 , wherein one or more of the AlGaN wells comprises an n-type doped AlGaN prior-well spacer, an n-type doped AlGaN post-well spacer, and an AlGaN main well sandwiched between the n-type doped AlGaN prior-well spacer and the n-type doped AlGaN post-well spacer, wherein an Al-composition of the n-type doped AlGaN prior-well spacer and an Al-composition of the n-type doped AlGaN post-well spacer are different than an Al-composition of the AlGaN main well, and a thickness of the n-type doped AlGaN prior-well spacer and a thickness of the n-type doped AlGaN post-well spacer are smaller than a thickness of the AlGaN main well and a thickness of adjacent AlGaN barrier.

18. The multiple quantum well structure of claim 17 , wherein the n-type doped AlGaN prior-well spacer and the n-type doped AlGaN post-well spacer are Si-doped with a doping concentration of 1.0×10 18 -8.0×10 18 cm −3 , respectively, the AlGaN main well is undoped or Si-doped with a doping concentration less than 5.0×10 17 cm −3 , at least one of the AlGaN barriers is Si-doped with a doping concentration of 1.0×10 18 -8.0×10 18 cm −3 .

19. The multiple quantum well structure of claim 17 , wherein a thickness of the n-type doped AlGaN prior-well spacer and a thickness of the n-type doped AlGaN post-well spacer are in the range of 0.1 nm to 0.52 nm, respectively.

20. The multiple quantum well structure of claim 17 , wherein the Al-composition of the n-type doped AlGaN prior-well spacer and the Al-composition of the n-type doped AlGaN post-well spacer are higher than an Al-composition of adjacent AlGaN barrier.

21. The multiple quantum well structure of claim 17 , wherein the Al-composition of the n-type doped AlGaN prior-well spacer and the Al-composition of the n-type doped AlGaN post-well spacer are lower the Al-composition of the AlGaN main well.

22. The multiple quantum well structure of claim 17 , wherein the Al-composition of the n-type doped prior-well spacer is higher than an Al-composition of adjacent AlGaN barrier and the Al-composition of the n-type doped AlGaN post-well spacer is lower the Al-composition of the AlGaN mail well; or the Al-composition of the n-type doped AlGaN post-well spacer is higher than an Al-composition of the AlGaN barrier and the Al-composition of the AlGaN prior-well spacer is lower the Al-composition of the AlGaN main well.

23. A light emitting diode comprising:

an n-type AlGaN structure;

a p-type AlGaN structure; and

an active-region sandwiched between the n-type AlGaN structure and the p-type AlGaN structure,

wherein the active-region comprises the multiple quantum well structure of claim 1 .