JP2001308452A - Semiconductor laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser device capable of increasing a laser edge face reflectivity even in a blue or ultraviolet ray area in which the refractivity of materials is small. SOLUTION: In a semiconductor laser device having a laser active area and a cyclic structure in the direction of a laser oscillator, the cyclic structure of the semiconductor laser device is formed of air and a semiconductor so that the optical length of a unit cycle in the cyclic structure, that is, a value obtained by adding length da of the air part to the product of length ds of the semiconductor part and effective refractivity neff of the waveguide of the semiconductor laser can be constituted of a value which is odd times of a value obtained by dividing oscillation wavelength by 2 as shown in a following formula (1); dsneff+da=mλ/2. In this case, AlXGa(1-X)N(0<=X<=1) or InXGa(1-X)N (0<=X<=1) is used for the semiconductor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a semiconductor laser device.

[0002]

2. Description of the Related Art A semiconductor laser which oscillates in a blue or ultraviolet light region is indispensable for increasing the density of an optical disk. Until now, G lasers that oscillate in the blue or ultraviolet region
Semiconductor lasers made of aN-based materials have been developed.

[0003]

However, the refractive index of a GaAs-based material used as a light source of the current optical disk or an InP-based material used for optical communication is 3.5.
On the other hand, since the refractive index of the GaN-based material is as small as about 2.8, the reflectivity of the end face is not sufficient in the ordinary semiconductor laser structure using the cleaved end face as a laser resonator mirror, and the oscillation as it is. There is a problem that the threshold value becomes large.

[0004] In order to solve this problem, a method has heretofore been adopted in which a dielectric multilayer film is formed by a method such as electron beam evaporation after cleaving a laser wafer to increase the end face reflectivity. Since such a method is not a monolithic manufacturing process, the manufacturing cost is high and this is an obstacle to cost reduction. In order to solve this problem, the present invention provides a semiconductor laser device that can increase the laser facet reflectance even if the material has a small refractive index, such as a nitride semiconductor used as a blue or ultraviolet light region material. It is.

[0005]

According to a first aspect of the present invention, there is provided a semiconductor laser device having a periodic structure in a laser active region and a laser resonator direction. The periodic structure of the device is composed of air and a semiconductor, and as shown in the following equation (1), the optical length of a unit period in the periodic structure, that is, the length d _{s of the} semiconductor portion and the effective refractive index n _{eff of the} waveguide of the semiconductor laser.
Is a periodic structure in which a value obtained by adding the length d _a of the air portion to the product of the air portion is an odd multiple of a value obtained by dividing the oscillation wavelength by 2. As the semiconductor, Al _x Ga _(1-x) N (where 0 ≤ x ≤ 1) or In _x Ga _(1-x) N (where 0 ≤ x ≤ 1). _{d s n eff + d a =} mλ / 2 ... (1) where, m is an odd number, lambda is the wavelength.

According to a second aspect of the present invention, there is provided a semiconductor laser device having a laser active region and a laser active region surrounding the laser active region.
In a semiconductor laser device having a periodic structure in a three-dimensional plane, the periodic structure of the semiconductor laser device is made of air and semiconductor, is two-dimensional around a resonator region, and is four- or six-fold symmetric in the plane, the length d _a of the optical length or length d _s and the semiconductor laser product to the air portion of the effective refractive index n _eff of the waveguide of the semiconductor portion of the unit period of the periodic structure, as shown in the above equation (1) A periodic structure in which the added value is an odd multiple of a value obtained by dividing the oscillation wavelength by 2;
As the semiconductor, Al _x Ga _(1-x) N (where 0 ≦ x ≦
1) or In _x Ga _(1-x) N (where 0 ≦ x ≦ 1).

[0007]

[Embodiment 1] FIGS. 1 and 2 show a first embodiment of the present invention. On the n-type GaN substrate 11, an Al _x Ga _(1-x) N cladding layer 22 doped with n-type is formed.
On this cladding layer 22, In _x Ga _(1-x) N or Al _x
An active layer 23 of Ga _(1-x) N is grown. The thickness of the cladding layer 22 needs to be sufficiently large so that the skirt of the light confined in the active layer 23 does not reach the substrate 11, and is, for example, 500 nm.

The oscillation wavelength of the active layer 23 can be controlled by the composition x of In and Al. The thickness of the active layer 23 is, for example, 50 nm. Further, a p-type doped Al _x Ga _(1-x) N cladding layer 24 is formed on the active layer 23. The p-cladding layer 24 has the same thickness as the n-cladding layer 22. A p-electrode 25 is formed on the upper surface of the p-cladding layer 24, and an n-electrode 21 is formed on the lower surface of the n-type GaN substrate 11.
Are formed. To improve the efficiency of the semiconductor laser, a double hetero structure is suitable. For this reason, the Al composition x of the cladding layers 22 and 24 needs to be smaller than the refractive index of the active layer 23 and the band gap needs to be large.

In this embodiment, the active region 13 has a ridge structure so that light can be guided. The width of the active region 13 is, for example, about 1 μm. In the present embodiment, the active region 13 is of a ridge type, but it is also effective to form a buried structure using a material having a higher refractive index than the material of the active layer. Further, grooves are periodically formed at both output ends of the active region 13 to form the one-dimensional photonic crystals 12 and 14. On the upper surfaces of the one-dimensional photonic crystals 12 and 14,
Refractive index control electrodes 26 and 27 are formed. In general, a structure having a large refractive index difference and a period of about a wavelength is called a photonic crystal structure, and a structure having a periodic structure in one direction as in this embodiment is called a one-dimensional photonic crystal.

In this embodiment, the semiconductor laser has a photonic crystal structure because the effective refractive index of the semiconductor laser is about 2.6, which is a sufficiently large difference from the refractive index 1 of air. In the one-dimensional photonic crystal 12, the width d _a width d _s and the groove portion of the air of the semiconductor light output direction forming a periodic structure, the oscillation wavelength lambda, when the effective refractive index n _eff And the following equations (2) and (3). _{d s = mλ / 4n eff ...} (2) d a = mλ / 4 ... (3) Here, m is an odd number. The following equations are derived from the equations (2) and (3). _{d s n eff + d a =} mλ / 2 ... (1)

In the present embodiment, m = 1 and λ = 400 nm are designed, and d _s = 38 nm and d _a = 100 nm. When the end face is formed by cleavage or etching, the laser end face reflectivity R is expressed by the Fresnel equation R = (n
_The approximate end face reflectance can be estimated by _eff −1) ² / (n _eff +1) ² . In this embodiment, the effective refractive index n _eff
Is about 2.6, and the reflectance of the simply cleaved end face is R = 19.8%.

In a GaAs or InP semiconductor laser,
It can be seen that 30% or more as the end face reflectivity can be obtained only by cleavage, while only about 20% can be obtained in the GaN system. In the case of this embodiment, the effective refractive index is 2.6.
FIG. 3 and FIG. 4 show the results of calculating the reflectance with respect to the plane wave using. FIG. 3 shows the case where m = 1, and FIG. 4 shows the case where m = 3. In this calculation, when the number of cycles (= the number of semiconductor / air pairs) is 1, reference numerals 31 and 41 indicate, when the number of cycles is 2, reference numerals 32 and 42, and when the number of cycles is 3, reference numerals 33 and 43. And when the number of cycles is 5, reference numerals 34 and 44
Indicated by The design wavelength was 400 nm.

As shown in the plan view, it can be seen that the end face reflectivity close to 100% can be obtained in the calculation for the plane wave. m =
1 can obtain high reflectivity over a wider wavelength range, but 38 nm and m = 3 when the width of the semiconductor portion is m = 1.
Is 115 nm, so that m = 3 is easier to fabricate. Actually, since the laser light output from the active region spreads by diffraction, the amount of the laser light reflected by the photonic crystal region and coupled to the active region again becomes smaller than the calculated value.

In this embodiment, since the same material as that of the active layer is present in the photonic crystal portion, absorption loss occurs in this portion. By biasing the refractive index control electrodes 26 and 27 in the forward direction, the absorption loss can be reduced. In any case, the effective end face reflectivity when the photonic crystal structure as in the present embodiment is used is sufficiently larger than the reflectivity of a simple cleavage end face, so that the oscillation threshold value is reduced and the output is increased. It becomes possible.

In this embodiment, n-type Ga is used as the substrate.
Although the N substrate 11 is used, an n-type SiC or sapphire substrate that can achieve relatively good lattice matching with the GaN-based material may be used. However, when a sapphire substrate is used, it is not possible to make electrical contact from the substrate, so it is necessary to adopt a structure in which electrical contact is made from the n-cladding layer 22 on the upper surface. Further, although the active layer of this embodiment uses the bulk of In _x Ga _(1-x) N or Al _x Ga _(1-x) N, it is more effective to use these quantum well structures.

Further, in this embodiment, the cladding layers 22 and 2
A bulk AlGaN was used as No. 4, but this part was applied for a patent by Kazuhide Kumakura et al.
It is effective to form a good p-type or n-type cladding layer by using a doped superlattice structure described in US Pat. However, also in this case, it is necessary to design so that the average refractive index of the cladding layer formed by the superlattice is smaller than the refractive index of the active layer.

[Embodiment 2] FIGS. 5, 6 and 7 show a second embodiment of the present invention. On the n-type GaN substrate 51, an Al _x Ga _(1-x) N cladding layer 64 doped to the n-type is formed.
On this cladding layer 64, In _x Ga _(1-x) N or Al _x
An active layer 63 of Ga _(1-x) N is grown. The thickness of the cladding layer 64 needs to be sufficiently large so that the foot of the light confined in the active layer 63 does not reach the substrate 51, and is, for example, 500 nm.

The oscillation wavelength of the active layer 63 can be controlled by the composition x of In and Al. The thickness of the active layer 63 is, for example, 50 nm. Further, a p-type doped Al _x Ga _(1-x) N cladding layer 62 is formed on the active layer 63. The p-cladding layer 62 has the same thickness as the n-cladding layer 64. Here, similarly to the first embodiment, the cladding layer 6 is formed.
The Al composition x of 2, 64 is smaller than the refractive index of the active layer 63, and the band gap needs to be large. A p-electrode 61 is formed on the upper surface of the p-cladding layer 62, and n-type GaN
On the lower surface of the substrate 51, an n-electrode 65 is formed.

The width of the active region 53 is, for example, about 1 μm. In the present embodiment, the active region 53 is of a ridge type, but it is also effective to form a buried structure using a material having a higher refractive index than the material of the active layer. A two-dimensional photonic crystal 52 is formed around the active region 53 as a two-dimensional periodic structure. In the present embodiment, a two-dimensional photonic crystal structure of triangular lattice having six-fold symmetry is used, but a two-dimensional photonic crystal structure of square lattice having four-fold symmetry may be used.

In the first embodiment, the reflectivity in one axis direction can be increased because of the one-dimensional photonic structure. However, when the active region 53 is surrounded by the two-dimensional photonic crystal structure as in this embodiment, the two-dimensional The Q value as an in-plane resonator increases,
Further, the oscillation threshold can be reduced. In order to extract a laser beam, it is only necessary to form the fluctuation of the periodic structure in the extraction direction to reduce the reflectance. In this embodiment, the vertical confinement is performed only by the cladding layers 62 and 64, but a three-dimensional photonic crystal structure in which a periodic structure is introduced in this direction is also possible.

When a three-dimensional photonic crystal structure is used, photons can be completely confined three-dimensionally, so that the life of photons in the resonator can be further extended, and a semiconductor laser with a lower oscillation threshold can be realized. can do.
In the present embodiment, an n-type GaN substrate is used as the substrate. However, it is also possible to use an n-type SiC or sapphire substrate which can perform relatively good lattice matching with a GaN-based material. Is the same as

However, when a sapphire substrate is used,
Since electrical contact cannot be made from the substrate, it is necessary to have a structure in which electrical contact is made from the portion 64 on the upper surface. Further, although the active layer of this embodiment uses the bulk of In _x Ga _(1-x) N or Al _x Ga _(1-x) N, it is also possible to use these quantum well structures.

Further, in this embodiment, bulk AlGaN is used as the cladding layer, but this portion is formed into a doped superlattice structure which was filed by Kazuhide Kumakura et al. (Japanese Patent Application No. 212195). By doing so, a good p-type or n-type clad layer can be used. However, also in this case, it is necessary to design so that the average refractive index of the cladding layer formed by the superlattice is smaller than the refractive index of the active layer.

Embodiment 3 In Embodiments 1 and 2 described above,
As a result of satisfying the above expressions (2) and (3), the above expression (1) is satisfied. However, the present invention is not limited to this. In the present embodiment, the above expression (1) is eventually satisfied. The case also applies. In the present embodiment, since the above equations (2) and (3) are not satisfied, the effect is limited as compared with the first and second embodiments.

[0025]

As described above, according to the present invention, it is possible to increase the Q value of the resonator by increasing the effective reflectivity of the end face even in the blue or ultraviolet region. Accordingly, an efficient semiconductor laser having a small oscillation threshold can be obtained, and a high-density optical disk system can be realized.

[Brief description of the drawings]

FIG. 1 is a plan view of a structure in which a one-dimensional photonic crystal structure is incorporated in a semiconductor laser according to a first embodiment of the present invention.

FIG. 2 is a sectional view taken along line A-A 'of FIG.

FIG. 3 is a graph showing a calculation result of a reflectance for a plane wave when m = 1.

FIG. 4 is a graph showing a calculation result of a reflectance for a plane wave when m = 3.

FIG. 5 is a plan view of a structure in which a two-dimensional photonic crystal structure is incorporated in a semiconductor laser according to a second embodiment of the present invention.

FIG. 6 is a sectional view taken along line A-A ′ of FIG. 5;

FIG. 7 is a sectional view taken along line B-B 'of FIG.

[Explanation of symbols]

 11, 51 GaN substrate 23, 63 active layer 22, 64 n-type cladding layer 24, 62 p-type cladding layer 12, 14 one-dimensional photonic crystal 52 two-dimensional photonic crystal 13, 53 laser active region 21, 65 n electrode 25 , 61 p electrode 26, 27 refractive index control electrode 31 m = 1, reflectance 32 m = 1 when the number of semiconductor / air pairs is 1, reflectance 33 m = 1 when the number of semiconductor / air pairs is 2, 33 m = 1, The reflectance 34 m = 1 when the number of semiconductor / air pairs is 3, the reflectance 41 m = 3 when the number of semiconductor / air pairs is 5, and the reflectance 42 m = 3 when the number of semiconductor / air pairs is 1. The reflectance 43 m = 3 when the number of semiconductor / air pairs is 2, the reflectance 44 m = 3 when the number of semiconductor / air pairs is 3, and the reflectance when the number of semiconductor / air pairs is 5

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroaki Ando 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Naoki Kobayashi 2-3-1, Otemachi, Chiyoda-ku, Tokyo No. 1 F-term in Nippon Telegraph and Telephone Corporation (reference) 5F073 AA62 AA74 BA06 CA07 CB02 CB05 DA31 EA05

Claims (2)

[Claims] 1. A semiconductor laser device having a periodic structure in a laser active region and a laser resonator direction, wherein the periodic structure of the semiconductor laser device is made of air and a semiconductor, and an optical length of a unit period in the periodic structure, that is, a semiconductor portion. of a periodic structure formed by the odd multiple of the length and value of a value obtained by adding a length of the semiconductor laser air portion to the product of the effective refractive index of the waveguide is divided oscillation wavelength 2, as the semiconductor, Al _x Ga _(1-x) N (where 0 ≦ x ≦ 1) or In _x
A semiconductor laser device using Ga _(1-x) N (where 0 ≦ x ≦ 1). 2. A semiconductor laser device having a laser active region and a periodic structure in a two-dimensional plane surrounding the laser active region, wherein the periodic structure of the semiconductor laser device is made of air and a semiconductor and is two-dimensionally formed around a resonator region. It is four-fold or six-fold symmetric in the plane, and is a value obtained by adding the length of the air portion to the product of the optical length of the unit period in the periodic structure, that is, the length of the semiconductor portion and the effective refractive index of the waveguide of the semiconductor laser. Is a periodic structure composed of an odd multiple of a value obtained by dividing the oscillation wavelength by 2 and Al _x Ga _(1-x) N (where 0
≦ x ≦ 1) or In _x Ga _(1-x) N (where 0 ≦ x ≦
A semiconductor laser device characterized by using 1).