JPH0732285B2 - Semiconductor laser device - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a semiconductor laser having a low operating voltage and high reliability.

[Technical background of the invention and its problems]

A semiconductor laser having a double hetero structure has attracted attention as a light source for optical communication and a light source for optical information processing, and is composed of an InGaAsP-based III-V compound semiconductor mixture of InGaAsP and GaAs-based InGaAsP and GaAs. Semiconductor lasers made of crystal have been put to practical use. In recent years GaAs
InGaAlP-based semiconductor lasers are being developed that are lattice-matched to the substrate.

InGaAlP lattice-matched to GaAs has a band gap energy of about 1.9 to 2.35 eV, which is a relatively large value as a III-V group compound semiconductor mixed crystal. Therefore, by using this as a cladding layer material, a band gap energy is increased as an active layer material. Sufficient carrier and light confinement can be achieved even with a large gap, which enables oscillation at short wavelengths that could not be realized with the material systems that have been practically used so far. There is. Such a semiconductor laser has many advantages over conventional ones, and one example is that it enables higher density recording than when it is used as a light source for an optical disk / audio / video disk.

In addition, II-VI such as ZnSSe and CuGaS ₂ created on GaAs substrate
Group. By using a compound semiconductor of I-III-VI group,
It is possible to further shorten the wavelength of the semiconductor laser.

However, this type of semiconductor laser has the following problems. Here, a semiconductor laser having InGaAlP as a cladding layer will be described as an example.

In order to shorten the oscillation wavelength, it is necessary to make the band gap of the active layer and the band gap of the cladding layer sufficiently large. As an example from a practical viewpoint of low threshold current and high reliability, the bandgap energy of the active layer is 2.0 eV in the active layer in order to set the oscillation wavelength to 620 nm.
Band gap energy of 2.35 eV in nGeAlP cladding layer
Although InGaAlP was used, it is desirable to have a double hetero structure.

Such double heterostructures metallic thermal decomposition method (M etalor
ganicc C hemical V apor Deposition: less MOCVD) or molecular beam epitaxy (M olecular B eam E pitaxi
y: MBE) to obtain epitaxial growth on a GaAs substrate. In addition, when current is injected into such a double hetero structure, one is made by making the substrate conductive and providing an ohmic electrode to use the substrate as an electrode contact layer. Electrode injection from the side opposite to the substrate of the active layer is performed by providing a GaAs gap layer having conductivity opposite to that of the substrate on the clad layer and forming an ohmic electrode on the clad layer and using this as an electrode contact layer. . Here, the advantages of using GaAs as the electrode contact layer are as follows. That is, since GaAs is crystalline, it is possible to obtain a substrate crystal at a low cost, it is possible to dope at a high concentration, and it is possible to obtain a layer with low specific resistance. It is easy to form an electrode contact with low ohmic resistance. That is, the heat resistance is small, and the heat generated near the active layer can be quickly dissipated.

However, the band gap energy of GaAs is 1.42 eV.
There is a large difference from the band gap energy of the cladding layer.

In general, a potential barrier is formed at the interface between two semiconductor layers having the same conductivity type and different band gaps due to band discontinuity. The height of such a barrier increases as the band discontinuity increases. It is generally considered that the larger the difference between the band gaps, the larger the discontinuity between the bands of the two semiconductors. Therefore, a large potential barrier is formed at the interface between the two semiconductor layers having a large band gap difference. Further, it is known that the thickness of such a barrier is mainly determined by the carrier concentration of a layer having a large band gap, and the barrier having a higher carrier concentration becomes thinner. In order to inject a current through the interface where the barrier exists, a high voltage is applied to reduce the height of the barrier or the doping is performed at a high concentration to make the barrier thin, and the current due to the tunnel effect is dominant. It is necessary to

When the inventors of the present invention repeatedly conducted experiments, the bandgap energy of the cladding layer in a conventional semiconductor laser using InGaP as an active layer, InGaAlP as a cladding layer and GaAs as a substrate and a cap layer as shown in FIG. Is 2.
If it was about 15 eV or less, there was no great problem in current injection, and the operating voltage was as low as about 2.0 V. However, InGaAl
When the band gap of the P clad layer is set to 2.15 eV or more, the operating voltage gradually increases, and InGaAlP with a band gap of 2.35 eV
When was used as the cladding layer, the operating voltage was as high as 3.5 V or higher, and no good characteristics were obtained. It is considered that this is because the above-mentioned barrier was formed at the GaAs InGaAlP interface because the band gap difference with GaAs is large and it is difficult to obtain a layer with a high carrier concentration in InGaAlP having a large band gap.

[Object of the Invention]

The present invention has been made in consideration of the above circumstances, and its object is to achieve high reliability without increasing the operating voltage even in a DH laser having a large band gap difference between the cladding layer and the electrode contact layer. To provide a semiconductor laser device having a property.

[Outline of Invention]

The essence of the present invention is to reduce the influence of the barrier generated at the interface by providing an intermediate bandgap layer having an intermediate value of the bandgap between the clad layer having a large bandgap and the electrode contact layer having a small bandgap. The purpose is to obtain a highly reliable semiconductor laser device by reducing the voltage.

〔The invention's effect〕

According to the present invention, even in a semiconductor laser device having a large band gap difference between the cladding layer and the electrode contact layer, it is possible to realize a semiconductor laser device having a low operating voltage and high reliability, and its usefulness is improved. It is tremendous.

Example of Invention

 Details of the present invention will be described below with reference to illustrated embodiments.

FIG. 1 is a sectional view showing a schematic structure of a semiconductor laser according to an embodiment of the present invention. In the figure, 11 is an N-GaAs substrate,
On this substrate 11, N-In _0.5 (Ga _1-x Al _x ) _0.5 PN-intermediate bandgap layer 12, N-In _0.5 (Ga _1-y Al _y ) _0.5 PN-cladding layer 13, In _0.5 (Ga _1-z Al _z ) _0.5 P active layer 14, P-In _{0.5
(Ga 1-y Al y)} 0.5 PP- cladding layer _{15, P-In 0.5 (Ga} 1-x A
l _x ) _0.5 PP-intermediate bandgap layer 16 and P-GaAs P-electrode contact layer 17 are sequentially grown. An insulating film 18 such as SiO ₂ is formed on the P-electrode contact layer 17,
This insulating film 18 is removed in a stripe shape. Then, P is formed on the upper surfaces of the insulating film 18 and the P-electrode contact layer 17.
The side electrode 19 is formed. The substrate 11 also serves as an n-electrode contact layer, and the N-side electrode is formed on the lower surface of the substrate 11.
20 are formed.

The band gap thickness carrier concentration of each layer in the above structure is set as follows. N-GaAs substrate 11 (1.
42eV.70μm.3 × 10 ¹⁸ cm ^-3 ), N-intermediate band gap layer
12 (2.1 eV.0.2 μm.3 × 10 ¹⁸ cm ^-3 ), N-clad layer 13
(2.35eV.1.5μm.1 × 18 ¹⁸ cm ^-3 ), Active layer 14 (2.00eV, 0.
1 μm, undoped), P-cladding layer 15 (2.35 eV, 1.5 μm, 5
× 10 ¹⁷ cm ^-3 ), P− intermediate bandgap layer 16 (2.1 eV, 0.2
μm, 3 × 10 ¹⁸ cm ^-3 ), P-GaAs electrode contact layer 17 (1.42e
V, 0.2 μm, 3 × 10 ¹⁸ cm ^-3 ).

The above structure is different from the conventional structure in that an N-intermediate band gap layer 12 is provided between the N-clad layer 13 and the N-GaAs substrate 11, and the P-clad layer 15 and the P-GaAs electrode contact layer 17 are provided. The P-intermediate band gap layer 16 is provided between them. Further, the carrier concentration of the intermediate band gap layers 12 and 16 is made higher than that of the cladding layers 13 and 15.

Since the superiority of such a structure is considered to be the same on the n-side and the P-side, the potential distributions of the P-clad layer 15, P-intermediate band gap layer 16 and P-electrode contact layer are shown in FIG. 2 here. This will be described in detail below using this as an example.

P-clad layer 15 and P having a large band gap difference
-When the electrode contact layer is directly joined, a large barrier as shown in FIG. 4 is formed, and it is extremely difficult to dope the P-InGaAlP having a wide band gap, which is the P-clad layer, at a high concentration. Therefore, the width of the barrier is wide, and it is difficult for all the carriers injected to oscillate the laser to pass through the barrier due to the tunnel effect.

On the other hand, as shown in FIG. 2, P-clad layer 15 and P-
By forming the P-intermediate band gap layer between the electrode contact layers 17, the height of the barrier at one interface can be reduced. Further, since the P-electrode contact layer 16 can be highly doped, the thickness of the barrier can be reduced.

As described above, according to this embodiment, it is possible to realize a semiconductor laser device having high reliability without increasing the operating voltage even in the DH laser having a large band gap difference between the cladding layer and the electrode contact layer. And its usefulness is immense.

The present invention is not limited to the laser using the materials described in the above embodiments. For example, the usefulness is extremely great also in a semiconductor laser in which GaAs is used as the substrate, ZnS _1-x SexCuGaS ₂ or the like is used as the cladding layer, and GaAs is used as the electrode contact layer. In addition, various modifications can be made without departing from the scope of the present invention.

[Brief description of drawings]

FIG. 1 is a sectional view showing a schematic structure of a semiconductor laser according to an embodiment of the present invention, FIG. 2 is a characteristic view showing a potentiation distribution of the semiconductor laser shown in FIG. 1, and FIG. 3 is a conventional semiconductor laser. 4 is a sectional view showing the schematic structure of FIG. 4, and FIG. 4 is a characteristic diagram showing the potential distribution of a conventional laser. 11 ... N-GaAs substrate, 12 ... N-InGaAlP intermediate bandgap layer, 13 ... N-InGaAlP cladding layer, 14 ... P-InGaAlP active layer, 15 ... P-InGaAlP cladding layer, 16 ... P-InGaAlP intermediate bandgap layer , 17 ... P-GaAs electrode contact layer,
18 ... Insulating film, 19, 20 ... Electrode layer.

Claims (5)

[Claims] 1. An active layer, a clad layer sandwiching the active layer from above and below and having a band gap and a refractive index smaller than that of the active layer, and the clad layer on the opposite side of the at least one clad layer from the active layer. An electrode contact layer having a smaller band gap and the same conductivity type as that of the clad layer, and a band gap between the clad layer and the electrode contact layer that is less than or equal to the band gap of the clad layer and that of the electrode contact layer. A semiconductor laser device comprising at least one intermediate bandgap layer having a bandgap or more. 2. The bandgap of the bandgap layer of the intermediate bandgap layer is large in a portion close to the cladding layer and small in a portion close to the electrode contact layer, and gradually changes or is constant in the meantime. The semiconductor laser device according to claim 1, which is characterized. 3. A clad layer sandwiching the active layer from above and below,
The semiconductor laser device according to claim 1, wherein the semiconductor laser device is a P clad layer and an n clad layer. 4. The carrier concentration of a layer adjacent to the electrode contact layer in the intermediate band gap layer is higher than the carrier concentration of the cladding layer, according to claim 1 or 2. Semiconductor laser device. 5. The semiconductor laser device according to claim 1, wherein the cladding layer and the intermediate bandgap layer are InGaAlP and the electrode contact layer is GaAs.