JPH0777283B2 - Semiconductor laser - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser having a layer structure for reducing the oscillation threshold current value.

[0002]

2. Description of the Related Art In recent years, semiconductor lasers have been used for a molecular beam growth method (hereinafter abbreviated as MBE method) or a metal organic vapor phase crystal growth method (hereinafter M
Formed by the OVPE method). Due to the characteristics of this growth method, a semiconductor laser having a quantum well structure active layer made of a very thin semiconductor layer is easily manufactured. AlGa
Taking an InP-based visible light semiconductor laser as an example, the semiconductor laser having this quantum well structure active layer has basically the same device structure as that of an embodiment of the present invention described later in FIG. 1, but the n-type cladding layer 2 is A single n-type (Al _0.7 G
The layered structure is made of a _0.3 ) _0.5 In _0.5 P (thickness 1 μm). By the way, the oscillation threshold current value of the quantum well laser largely depends on the quality of the hetero-interface forming the quantum well. That is, when there are many non-radiative centers near the interface,
The ratio of non-radiative recombination in the current injected into the active layer of the laser increases, and the oscillation threshold current value increases. Further, when the interface is uneven, the energy levels due to the quantum effect depending on the thickness of the quantum well become non-uniform in the plane, so that the gain spectrum width of the semiconductor laser is widened. As a result, the gain peak value decreases and the oscillation threshold current value increases. If the oscillation threshold current value increases, power consumption increases and reliability decreases, which is not preferable. Even in the active layer having no quantum well structure, the loss of the optical waveguide increases due to the unevenness of the interface, which is not preferable.
In addition, a semiconductor laser in which a superlattice structure for reflecting electrons is provided near the active layer in the p-cladding layer (Japanese Patent Laid-Open No. Sho 63-63)
However, even if the thickness of each layer of the superlattice is slightly different from the design value, the electron reflection effect of the superlattice structure is lost, so it is important to reduce the unevenness of the interface.

FIG. 3 shows a conventional quantum well laser devised to eliminate this problem (Japanese Patent Laid-Open No. 60-202).
981). Crystal growth is performed by the MBE method, and Ga
On the As substrate 1, the n-type GaAs buffer layer 11 and the n-type A
1 GaAs clad layer 12, continuously changing Al
GaAs layer 20, active layer 13, continuously changing composition A
lGaAs layer 21, p-type AlGaAs cladding layer 14,
In a structure including a p-type GaAs cap layer 16 and a p-type GaAs contact layer 17, a 15 nm thick AlGaAs layer and a 15 nm thick AlGaAs layer are provided in a part of the n-type AlGaAs cladding layer 12.
A thick GaAs superlattice structure 19 is provided.

[0004]

However, as shown in FIG. 4, the crystal growth is performed by the MOVPE method, and the GaA
On the s substrate 1, an n-type AlGaInP clad layer 2, an active layer 3, a p-type AlGaInP clad layer 4, a p-type GaIn
In the structure including the P layer 5, the p-type GaAs cap layer 6, and the p-type GaAs contact layer 7, the n-type AlGaInP
15 nm thick AlGaInP and 1 are formed on a part of the cladding layer 2.
When a semiconductor laser of a conventional invention (Japanese Unexamined Patent Publication No. 60-202981) provided with a GaInP superlattice structure 10 having a thickness of 5 nm was prototyped, unevenness of the hetero interface of the active layer was found to be greater than that in the case without the superlattice structure 10. It became larger and the oscillation threshold current value increased. An object of the present invention is to provide a semiconductor laser having a reduced oscillation threshold current value by reducing such unevenness of the hetero interface of the active layer.

[0005]

In order to solve the above-mentioned problems, the means provided by the present invention is to form a double hetero structure in which an active layer is sandwiched between an n-type cladding layer and a p-type cladding layer on a substrate. And a clad layer existing between the substrate and the active layer has a thickness of 10 nm or more and 30 nm or more.
A multilayer structure including a first thin film layer containing Al having a thickness of 0 nm or less and a second thin film layer having an Al composition smaller than that of the first thin film layer or containing no Al and having a thickness of 3 nm or less is formed. It is characterized by having.

[0006]

When the crystal of the semiconductor material is grown on the substrate, fine irregularities are formed on the surface of the semiconductor crystal. The height difference of the unevenness tends to increase as the thickness increases. But,
The film thickness dependence of the height difference of the unevenness is slightly different depending on the composition, especially the Al composition. In a crystal having a large Al composition, as the film thickness increases due to crystal growth, the unevenness formed on the crystal surface gradually deepens, and the height difference of the unevenness is hardly reduced. On the other hand, a crystal having a small Al composition or containing no Al exhibits the effect of reducing the height difference of the unevenness when the film thickness is 3 nm or less at the initial stage of growth, the crystal easily grows in the recess. As the crystal grows further, the difference in height of the unevenness increases rapidly, and for the same thickness, the difference in height of the unevenness is larger than that of a crystal having a large Al composition. Therefore, after growing a layer having a large Al composition to a thickness of 10 nm or more and 300 nm or less to cause some irregularities on the crystal surface, a small Al composition or Al
If a crystal not containing is grown to a thickness of 3 nm or less, the crystal is concentrated and grown in the recessed portion, so that the height difference of the unevenness can be relaxed. The thickness of the layer having a large Al composition is 10
The reason why the thickness is not less than nm is that the Al composition is small on the surface having almost no unevenness, or the height difference of the unevenness is increased if the thickness of the layer not containing Al is 3 nm. The thickness of the layer having a large Al composition was set to 300 nm or less because
This is because if the thickness is 0 nm or more, the unevenness is too large and the Al composition is small, or the flattening effect does not appear in the layer not containing Al. As described above, a layer having a large Al composition and a layer having a small Al composition or a layer not containing Al are combined to form a multi-layer structure, so that unevenness can be obtained even on a surface laminated to a thickness of about 1 μm required as a clad layer of a semiconductor laser. Can be made smaller.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.

FIG. 1 is a cross-sectional view of a semiconductor laser showing one embodiment of the present invention (hatching showing a cut surface is omitted).
FIG. 2 is a manufacturing process diagram of this semiconductor laser. Figure 1
The semiconductor laser of the example is shown by being cut along a plane perpendicular to the cavity axis. Hereinafter, the AlGaInP-based visible light semiconductor laser will be described as an example.

In the fabrication of this embodiment, first, the n-type (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P (thickness 2 was formed on the n-type GaAs substrate 1 by the first growth by the low pressure MOVPE method.
0 nm) and n-type Ga _0.5 In _0.5 P (thickness 1 nm) are repeated to form an n-type cladding layer 2 (thickness 1 μm), Ga
_0.5 In _0.5 P (10 nm thick) well and (Al _0.5 G
a _0.5 ) _0.5 In _0.5 P (thickness 5 nm) Barrier quantum well structure active layer 3 with 3 wells, p-type (Al _0.7 G
a _0.3 ) _0.5 In _0.5 P clad layer 4 (thickness 1 μm),
A p-type Ga _0.5 In _0.5 P layer 5 and a p-type GaAs cap layer 6 were sequentially formed (FIG. 2A). The growth conditions are a temperature of 660 ° C., a pressure of 70 torr, and V / III = 200. As raw materials, trimethylaluminum (TMAl), triethylgallium (TEGa), trimethylindium (TMIn), phosphine (PH ₃ ), arsine (As)
H ₃ ) and disilane (Si ₂ H
₆ ), dimethyl zinc (DMZ as p-type dopant)
n) was used. On the thus grown wafer, a stripe-shaped SiO ₂ mask 9 having a width of 5 μm was formed by photolithography (FIG. 2B). Next, using this SiO ₂ mask 9, the p-type (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P cladding layer 4 was etched into a mesa shape up to the middle (FIG. 2).
(C)). Second MOV using the same mask 9
The Si-doped GaAs layer 8 was selectively embedded on both sides of the mesa portion by PE growth (FIG. 2D). Then, the SiO ₂ mask 9 is removed (FIG. 2E), and the p-type GaAs contact layer 7 is formed by the third MOVPE growth (FIG. 2).
(F)). After that, the electrodes 31 and 32 were formed and cleaved to form a laser light emitting end face, and the semiconductor laser shown in FIG. 1 was obtained.

When the unevenness of the active layer was observed by a transmission electron microscope with respect to the cross sections of the semiconductor laser of the present invention thus manufactured and the conventional laser of FIG. 4, the height difference of the unevenness was 1 nm or less and 6 nm, respectively. The oscillation threshold currents were 40 mA and 55 mA, respectively. As described above, in the semiconductor laser of the present invention, the hetero interface was flattened and the oscillation threshold current value was lower than that of the conventional semiconductor laser.
Further, the semiconductor laser of the present invention showed high reliability of continuous light emission for 10000 hours or more at an ambient temperature of 50 ° C. and an optical output of 5 mW.

In the above embodiment, AlGaIn
Although the case of the P-based visible light semiconductor laser has been described, the present invention is not limited to AlGaAs, AlGaInPAs, and Al.
Needless to say, it can be applied to a semiconductor laser using a cladding layer containing Al such as GaSb system.

[0012]

As described above in detail with reference to the embodiments, the present invention provides a semiconductor laser having a low oscillation threshold current value and excellent reliability.

[Brief description of drawings]

FIG. 1 is a sectional view of a semiconductor laser of the present invention.

FIG. 2 is a manufacturing process diagram of the semiconductor laser of FIG.

FIG. 3 is a sectional view of a conventional semiconductor laser devised to improve the quality of a hetero interface.

FIG. 4 is a cross-sectional view of a semiconductor laser manufactured by attempting to improve the quality of a hetero interface according to the conventional method.

[Explanation of symbols]

1 n-type GaAs substrate 2 n-type GaInP / AlGaInP multilayer structure clad layer 3 GaInP / AlGaInP quantum well structure active layer 4 p-type AlGaInP clad layer 5 p-type GaInP layer 6 p-type GaAs cap layer 7 p-type GaAs contact layer 8 n-type GaAs current confinement layer 9 SiO ₂ mask

Claims (2)

[Claims] 1. A semiconductor laser comprising a substrate and a double hetero structure in which an active layer is sandwiched between an n-type clad layer and a p-type clad layer, the clad layer being present between the substrate and the active layer. Consists of a first thin film layer containing Al having a thickness of 10 nm or more and 300 nm or less and a second thin film layer having an Al composition smaller than that of the first thin film layer or containing no Al and having a thickness of 3 nm or less. A semiconductor laser having a multilayer structure. 2. The multi-layer structure is AlGaInP-based, Al
GaAs type, AlGaInPAs type or AlGaSb
The semiconductor laser according to claim 1, which is made of a system material.