JP2912624B2 - Semiconductor laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to a semiconductor laser device having high output and low noise characteristics suitable as a light source for a rewritable optical disk.

[Prior art]

Conventionally, a single quantum well structure of a semiconductor laser is mainly used to reduce a threshold current. Further, superlattice optical waveguide layers are provided above and below the single quantum well active layer in order to improve the flatness of the quantum well interface and improve the crystallinity.
This type of semiconductor laser is described in, for example, the 46th Annual Conference of the Japan Society of Applied Physics, Proceedings 1P-N-8, P196.

[Problems to be solved by the invention]

In the above prior art, the lateral mode control of the laser beam and the longitudinal mode control based on the refractive index difference in the lateral direction of the active layer are not considered in the semiconductor laser device structure, and the desired high output characteristics and low noise characteristics are not obtained. There was a problem that it could not be obtained. An object of the present invention is to realize a semiconductor laser device that satisfies high output and low noise characteristics.

[Means for Solving the Problems]

In order to achieve the above object, the active layer has a structure in which a superlattice multiple quantum well layer is provided above and below a single quantum well layer, and the structure, film thickness, and impurity concentration of each layer are set to predetermined values. An element was produced. That is, in the present invention, an active layer having a small band gap (difference in energy band between a conductor and a valence band) and an optical waveguide layer having a large band gap (for example, a cladding layer) sandwiching the active layer are formed on a semiconductor substrate. In the semiconductor laser device, the active layer is formed of a single quantum well layer having a de Broglie wavelength of electrons or less, and multiple quantum well layers formed on both surfaces thereof. Further, in order to obtain a self-pulsation laser element, a ridge waveguide structure is formed, and an effective refractive index difference in a lateral direction of the active layer is controlled so as to have a predetermined value.

[Operation]

The fact that the present invention can provide low noise and high output characteristics required for a write / erase light source and a read light source of an optical disk will be described below. Conventionally, it is known that a threshold current of laser oscillation is reduced by introducing a single quantum well structure into an active layer. Furthermore, as described in Applied Physics Letters, Appl. Phys. Lett. 45 (1984) p836, a laser device having a single quantum well active layer is more likely to have a conventional double hetero active layer or multiple quantum well active layer. The decrease in the refractive index due to the carrier injection is smaller than that of the laser device of the above.
Thus, a laser device having a single quantum well active layer has
It is suggested that a high output characteristic with a large kink generation light output can be obtained without making the effective refractive index difference built in the lateral direction of the active layer unstable to a high carrier injection level. However, since the single quantum well active layer has a small thickness, the laser light largely spreads to the cladding layer. Therefore, in a laser device in which the light absorption layer is provided and the transverse mode is controlled, there is a problem that the effective refractive index difference in the lateral direction of the active layer becomes large. There is also a problem that it is difficult to control the longitudinal mode by controlling the difference in refractive index. On the other hand, a self-sustained pulsation laser that achieves excellent low noise characteristics with relative noise intensity of 10 ^-14 to 10 ^-13 Hz even when return light is generated has an effective refractive index difference of 1 × 10 in the lateral direction of the active layer. ^{-3 to} 5 × 10 ^-3 . Therefore, it is important to control the effective refractive index difference in the lateral direction of the active layer in order to realize a self-pulsation laser. The effective refractive index difference in the lateral direction of the active layer can be controlled by the active layer thickness and the clad layer thickness of the device. In the present invention, since the superlattice multiple quantum well layers are provided above and below the single quantum well layer, the wave functions of electrons and holes as carriers are not confined within the single quantum well layer, but are Large seepage on both sides of the well layer. As a result, the spread of the laser beam distribution in the active layer is smaller than that in the conventional single quantum well active layer. The width of the single quantum well layer can take a relatively large value within a range where the quantum size effect occurs, and is preferably in the range of 10 to 30 nm.
As described above, by controlling the laser beam distribution and providing the light absorbing layer at a predetermined position from the active layer, the effective refractive index difference in the lateral direction of the active layer can be realized to have a desired value. By controlling the refractive index difference, a self-pulsation laser element can be manufactured, and in a single quantum well layer, the refractive index decrease with respect to carrier injection is small, so that the built-in refractive index difference is not lost up to a high injection level and is stable. In the basic transverse mode, a high output characteristic with a large kink generated light output can be obtained.

【Example】

Embodiment 1. Embodiment 1 of the present invention will be described with reference to FIG. First,
On an n-type GaAs (001) substrate (thickness 100 μm), an n-type GaAs buffer layer 2 (thickness 0.5 μm) and an n-type Al _x Ga _1-x As cladding layer 3 (thickness 1.0 to 1.5 μm, x = 0.45 to 0.55), the undoped or n-type or p-type multiple quantum well layer 4 and the quantum barrier layer 4 ″ are Al _y G
a _1-y As layer, the width 2 to 5 nm, and y = 0.20 to 0.45, the quantum well layer 4 'is Al _z Ga _1-z As layer, the width 3 to 10 nm, and z = 0-0.20 and these 3-6 By repeatedly forming the layer, the width of the Al _y Ga _{1 -y} As superlattice barrier layer (corresponding to the quantum barrier layer width) is 0.5 to ₁ nm, and the width of the Al _z Ga ₁ -z As superlattice well layer (quantum well layer). By repeatedly forming 0.5 to 2 nm (corresponding to the width) 10 to 15 times, an energy band structure of a conduction band and a valence band in the active layer as shown in FIG. 2 is formed. ), An undoped single quantum well layer _{5 (Al z 'Ga 1-} z' As layer, 10 to 30 nm, z '
= 0 to 0.15), undoped or n-type or p-type multiple quantum well layer 6 (quantum barrier layer 6 'is an Al _y Ga _1-y As layer, width 2 to 5 nm,
y = 0.20 to 0.45, the quantum well layer 6 ′ is an Al _z Ga ₁ -z As layer,
These are repeatedly formed 3 to 6 times with a width of 3 to 10 nm and z = 0 to 0.20. Or Al _y Ga _1-y As superlattice barrier layer and width respectively
0.5-1 nm, Al _z Ga _1-z As superlattice well layer, width 0.5-2 nm 10 ~
Formed 15 times repeatedly. ), A p-type Al _x Ga _{1 -x} As cladding layer 7 (thickness: 1.0 to 1.6 μm, x = 0.45 to 0.55), and a p-GaAs layer (0.2 to 0.3 μm) in sequence by molecular beam epitaxy (MBE) or Epitaxial growth is performed by metal organic chemical vapor deposition (MOCVD). Next, an SiO ₂ film is formed, and a stripe mask pattern is formed by photolithography. Using the striped SiO ₂ film as a mask, the layers 8 and 7 are etched with a phosphoric acid solution to form a ridge-shaped optical waveguide. Thereafter, the n-GaAs current block layer 9 (0.7 to 1.0 μm in thickness) is selectively grown while leaving the SiO ₂ film. next,
After the SiO ₂ film is removed by etching with a mask hydrofluoric acid solution, a p-GaAs cap layer 10 (1.0 to 2.0 μm in thickness) is buried and grown. Thereafter, a p-type layer-side electrode 11 and an N-type layer-side electrode 12 are formed, cleaved, scribed, and cut into an element shape. In this embodiment, it is appropriate that the stripe width S at the bottom of the ridge is 4 to 6 μm in order to perform laser oscillation at the basic lateral mode and a low threshold current. Further, the thickness d of the cladding layer after forming the ridge waveguide is 0.3 to 0.6 μm.
A self-sustained pulsation laser element was obtained. This device oscillates with a threshold current of 10 to 20 mA and emits light with a power of 2 to 30 mA.
Self-excited oscillation in the range of mW. Kink generation light output is 50-60
mW. Further, by doping the multiple quantum well layer in the active layer with an n-type or p-type impurity at 1 × 10 ¹⁸ to 1 × 10 ¹⁹ cm ⁻³ , laser oscillation is performed at a threshold current of 10 to ²⁰ mA, and self-excited oscillation is performed. The frequency could be controlled by the polarity of the dopant and the doping concentration. Further, by asymmetrically coating the end face of the resonator of the laser element, an element having self-excited oscillation in an optical output range of 2 to 50 mW and a kink generation optical output of 80 to 90 mW could be obtained. By controlling the thickness of the entire active layer and the thickness of the cladding layer to predetermined values, (the thickness of the active layer: 0.04 to 0.08 μm, the thickness of the cladding layer: 0.
An element having self-excited oscillation in the range of 3 to 0.6 μm) and an optical output of 2 to 10 mW and having a kink generation optical output of 110 to 120 mW was also realized. As a result, the low-noise and high-output characteristics required as the light source for writing and erasing and the light source for reading of the optical disk could be satisfied by one element. Embodiment 2 Embodiment 2 of the present invention will be described with reference to FIG. FIG. 3 schematically shows the energy band structure of the conduction band in the active layer. In the multiple quantum well layers 4 and 6 provided above and below the single quantum well layer in the active layer, the quantum well layers Al _z Ga _1-z
A laser device in which the Al composition z of the As layer is larger than the single quantum well layer 5 and the Al composition z 'of the Al _z ' Ga ₁ -z'As layer (z> z '). Here, the multiple quantum well layers 4 and 6 are set so that the quantum level formed in the multiple quantum well layer is equal to or higher than the quantum level formed in the single quantum well layer. The structure was designed. According to this embodiment, the threshold current can be further reduced, and laser oscillation was possible at 5 to 10 mA. Other characteristics similar to those of the first embodiment are obtained. Embodiment 3 Embodiment 3 of the present invention will be described with reference to FIG. FIG. 4 schematically shows the energy band structure of the conduction band in the active layer. In the multiple quantum well layers 4 and 6 provided above and below the single quantum well layer in the active layer, the quantum well layers Al _z Ga _1-z
The width and the Al composition of the As layers 4 'and 6'
A laser device having a structure in which the thickness was gradually increased and gradually increased toward the claddings 3 and 7 from that of the laser device was fabricated. The multiple quantum well layers 4 and 6 are formed so that the quantum levels formed in the multiple quantum well layer are at the same level or higher than the quantum levels formed in the single quantum well layer.
The structure was designed. This embodiment also has the same effect as the second embodiment. Embodiment 4 Embodiment 4 of the present invention will be described with reference to FIG.
FIG. 5 schematically shows the energy band structure of the conduction band in the active layer. In the multiple quantum well layers 4 and 6 provided above and below the single quantum well layer in the active layer, the quantum barrier layers Al _y G
A laser device was fabricated as a graded layer in which the Al composition y of the a _1-y As layer was gradually increased from the single quantum well layer toward the cladding layer. In this case, the quantum levels formed in the quantum well layers of the multiple quantum well layers are graded energy levels that gradually increase from the single quantum well layer toward the cladding layer. You. This embodiment also has the same effects as those of the second and third embodiments.

【The invention's effect】

According to the present invention, it was difficult to realize with the conventional technology,
Since the effective refractive index difference in the lateral direction of the single quantum well structure active layer can be controlled, the transverse mode and the longitudinal mode can be easily controlled, and the desired laser characteristics can be obtained. In the ridge waveguide structure of the present invention, when the thickness of the cladding layer after forming the ridge is 0.3 to 0.6 μm and the total thickness of the active layer is in the range of 0.05 to 0.08 μm, the light output is 2 to 30 μm.
A laser device which self-oscillates in the range of mW and has a kink generation light output of 50 to 60 mW was obtained. Devices having a threshold current of 5 to 10 mA were obtained. Further, by applying the asymmetric coating, self-oscillation was performed in the range of light output of 2 to 50 mW, and a device having kink generation light output of 80 to 90 mW was obtained. In addition, by controlling the effective refractive index difference in the lateral direction of the active layer, self-excited oscillation in the optical output range of 2 to 10 mW and kink-generated optical output 11
An element having a power of 0 to 120 mW was obtained. Therefore, the high output characteristics required for the write / erase light source of the optical disk and the low noise characteristics required for the read light source could be satisfied. The present invention has been described using an AlGaAs-based material.
It goes without saying that the same can be applied to the InP / GaAs system and the InGaAsP / InP system.

[Brief description of the drawings]

FIG. 1 is a sectional view of a semiconductor laser according to Examples 1 to 4 of the present invention, and FIGS. 2 to 5 are schematic views of the energy band structure of an active layer in Examples 1 to 4, respectively. 1... N-GaAs substrate, 2... N-GaAs buffer layer, 3.
_{... n-Al x Ga 1-} x As cladding layer, 4 ...... undoped or n
-Type or p-type impurity-doped multiple quantum well layer, 5 ... undoped single quantum well layer, 6 ... undoped or n-type or p-type
-Type impurity-doped multiple quantum well layer, 7... P-Al _x Ga _1-x As
Cladding layer, 8 p-GaAs layer, 9 n-GaAs current blocking layer, 10 p-GaAs cap layer, 11 p-layer side electrode, 12 n-layer side electrode.

Continuation of the front page (56) References JP-A-61-101089 (JP, A) JP-A-60-189983 (JP, A) Proceedings of the Fall Reactive Society of Japan in 1985 1P-N-8 p. 196 1988 (Showa 63) Proceedings of the Japan Society for Autumn Science 5P-ZC-21 p. 864 IEICE Technical Report 88 [4] (1988) p. 33-38 (58) Field surveyed (Int. Cl. ⁶ , DB name) H01S 3/18

Claims (5)

(57) [Claims] 1. A semiconductor device comprising: a quantum well layer; and first and second semiconductor regions provided above and below the quantum well layer and alternately stacking well layers and barrier layers having a larger band gap than the well layer. An active layer, an optical waveguide layer bonded to the active layer and having a stripe-shaped ridge formed on the opposite side of the bonding surface and having a band gap larger than that of the active layer, and formed so as to narrow both sides of the stripe-shaped ridge. A thickness of the optical waveguide layer on both sides of the stripe-shaped ridge and a thickness of the active layer are set so that laser light can self-oscillate in the active layer; The well layer has a band gap smaller than that of the barrier layer, and barrier layers of the first and second semiconductor regions are respectively joined above and below the well layer, and the well layer and the barrier layer are thinner than the quantum well layer. The semiconductor laser device characterized by. 2. The quantum well layer has a thickness of 10 to 30 nm, the well layer has a thickness of 3 to 10 nm, and the barrier layer has a thickness of 2 to 5 nm. 2. The semiconductor laser device according to claim 1, wherein: 3. The thickness of the quantum well layer is set in the range of 10 to 30 nm, the thickness of the well layer is set in the range of 0.5 to 2 nm, and the thickness of the barrier layer is set in the range of 0.5 to 1 nm. 2. The semiconductor laser device according to claim 1, wherein: 4. The method according to claim 1, wherein the thickness of the active layer is set in a range of 0.04 to 0.08 μm, and the thickness of the optical waveguide layer is set in a range of 0.3 to 0.6 μm. Item 4. The semiconductor laser device according to any one of Items 3 to 3. 5. The stripe of the active layer, comprising: an active layer; and an optical waveguide layer joined to the active layer and having a stripe-shaped ridge formed on a side opposite to the junction surface and having a band gap larger than that of the active layer. In a semiconductor laser device in which an effective refractive index difference between a portion facing the ridge and another portion is set in a range of 1 × 10 ^{−3 to} 5 × 10 ⁻³ , the active layer is a quantum well layer and a quantum well layer. First and second semiconductor regions provided above and below the quantum well layer and alternately stacking well layers and barrier layers having a larger band gap than the well layer, wherein the quantum well layer is formed of the barrier layer A semiconductor having a smaller bandgap, and barrier layers of the first and second semiconductor regions are respectively joined above and below the semiconductor layer, and the well layer and the barrier layer are thinner than the quantum well layer. Laser element.