Light-emitting element and method of making the same

1. A light-emitting element, comprising: a semiconductor substrate; a light emitting portion comprising an active layer sandwiched between a first cladding layer of a first conductivity type and a second cladding layer of a second conductivity type different from the first conductivity type;

a reflective portion provided between the semiconductor substrate and the light emitting portion for reflecting light emitted from the active layer; and

a current spreading layer provided on the light emitting portion opposite to the reflective portion and comprising a concavo-convex portion on a surface thereof,

wherein the reflective portion comprises a plurality of pair layers each comprising a first semiconductor layer and a second semiconductor layer different from the first semiconductor layer, and the first semiconductor layer has a thickness T A1 defined by formula (1), and the second semiconductor layer has a thickness T B1 defined by formula (2),

T A1 =λ p /(4 n A √(1−(n In sin θ/ n A ) 2 ))  Formula (1)

T B1 =λ p /(4 n B √(1−(n In sin θ/ n B ) 2 ))  Formula (2)

wherein T A1 ≧λ p /4n A , T B1 ≧λ p /4n B , is a peak wavelength of the light emitted from the active layer, n A is a refractive index of the first semiconductor layer, n B is a refractive index of the second semiconductor layer, n In is a refractive index of the first cladding layer, and θ is an incident angle which is defined as an angle with respect to a normal line of an incident plane from the first cladding layer to the first semiconductor layer, wherein the reflective portion comprises at least three pair layers, thicknesses of a plurality of the pair layers are different from each other due to a θ value in the formulas (1) and (2) being different in each of a plurality of the pair layers, and at least one of a plurality of the pair layers comprises the first and second semiconductor layers with a θ value of not less than 50°.

2. The light-emitting element according to claim 1 , further comprising:

an intermediate layer provided between the second cladding layer and the current spreading layer,

wherein the intermediate layer comprises a semiconductor having a bandgap energy between bandgap energies of a semiconductor composing the second cladding layer and a semiconductor composing the current spreading layer.

3. The light-emitting element according to claim 2 , wherein

a plurality of the pair layers comprise a pair layer comprising the first semiconductor layer having a thickness T A1 not less than 1.5 times λ p1 /4 n A , and the second semiconductor layer having a thickness T B1 not less than 1.5 times λ p1 /4 n B .

4. The light-emitting element according to claim 3 , wherein

the reflective portion comprises a semiconductor material having a bandgap energy greater than that of a semiconductor composing the active layer, the semiconductor material being transparent to the light emitted from the active layer.

5. The light-emitting element according to claim 4 , wherein

the first semiconductor layer comprises Al X Ga 1-X As (0≦X≦1) or Al 0.5 In 0.5 P, and

the second semiconductor layer is formed of Al Y Ga 1-Y As (0≦Y≦1) and has a refractive index different from that of the first semiconductor layer.

6. The light-emitting element according to claim 5 , wherein in a first pair layer, or first and second pair layers of the reflective portion counting from a side of the semiconductor substrate, the first semiconductor layer comprises AlAs or Al 0.5 In 0.5 P, and the second semiconductor layer comprises a semiconductor having a bandgap smaller than that of a semiconductor composing the active layer or GaAs which is not transparent to the light emitted from the active layer.

7. The light-emitting element according to claim 1 , wherein the concavo-convex portion has arithmetic mean roughness Ra of not less than 0.04 μm and not more than 0.25 μm.

8. The light-emitting element according to claim 1 , wherein

the concavo-convex portion has a root mean square roughness RMS of not less than 0.05 μm and not more than 0.35 μm.

9. The light-emitting element according to claim 1 , wherein

the reflective portion comprises at least six pairs or more of the pair layers.

10. The light-emitting element according to claim 1 , wherein

the semiconductor substrate comprises GaAs.

11. The light-emitting element according to claim 2 , wherein

the intermediate layer comprises Ga z In 1-z P (0.6≦Z≦0.9), and

the current spreading layer comprises GaP.

12. The light-emitting element according to claim 1 , further comprising:

a front surface electrode provided at a predetermined position in a region of the current spreading layer except a region where the concavo-convex portion is provided; and

a light extracting layer formed at a portion except the front surface electrode, and comprising a material transparent to the light emitted from the active layer and having a refractive index smaller than that of the semiconductor composing the current spreading layer and larger than that of the air.

13. The light-emitting element according to claim 12 , wherein

the light extracting layer has a thickness d in a range of ±30% of a value defined by A+λ p /(4×n), where λ p is a wavelength of the light emitted from the active layer, n is a refractive index of a material composing the light extracting layer, and A (which is an odd number) is a constant.

14. A method of making a light-emitting element, comprising:

forming on a semiconductor substrate a reflective portion comprising a plurality of pair layers each comprising a first semiconductor layer and a second semiconductor layer different from the first semiconductor layer, a light emitting portion comprising an active layer sandwiched between a first cladding layer of a first conductivity type and a second cladding layer of a second conductivity type different from the first conductivity type, and a current spreading layer formed on the light emitting portion;

forming a front surface electrode at a predetermined position of the current spreading layer; and

forming a concavo-convex portion in a portion except a region where the front surface electrode is formed,

wherein the first semiconductor layer has a thickness T A1 defined by formula (1), and the second semiconductor layer has a thickness T B1 defined by formula (2),

T A1 =λ p /(4 n A √(1−(n In sin θ/ n A ) 2 ))  Formula (1)

T B1 =λ p /(4 n B √(1−(n In sin θ/ n B ) 2 ))  Formula (2)

wherein T A1 ≧λ p /4n A , T B1 ≧λ p /4n B , λ p is a peak wavelength of the light emitted from the active layer, n A is a refractive index of the first semiconductor layer, n B is a refractive index of the second semiconductor layer, n In is a refractive index of the first cladding layer, and θ is an incident angle which is defined as an angle with respect to a normal line of an incident plane from the first cladding layer to the first semiconductor layer, wherein the reflective portion comprises at least three pair layers, thicknesses of a plurality of the pair layers are different from each other due to a θ value in the formulas (1) and (2) being different in each of a plurality of the pair layers, and at least one of a plurality of the pair layers comprises the first and second semiconductor layers with a θ value of not less than 50°.

15. The method of making a light-emitting element according to claim 14 , wherein

the forming of the concavo-convex portion is carried out after the forming of the front surface electrode.