JP3582021B2 - Optical semiconductor device - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to an optical semiconductor element, and particularly, to an active layer having high luminous efficiency and having lattice distortion with excellent temperature characteristics in which an initial value such as an oscillation threshold is maintained over a wide temperature range. The present invention relates to a semiconductor laser used for 1 μm band optical communication.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, an optical semiconductor device in the 1 μm band has been mainly used for optical communication, and a semiconductor laser which operates with a narrow spectral line width and a low threshold has been obtained by using an active layer having lattice distortion. .
[0003]
In recent years, semiconductor lasers having higher luminous efficiency and capable of following large temperature changes and requiring no management of temperature conditions, that is, a temperature characteristic in which an initial value such as an oscillation threshold value is maintained in a wide temperature range. An excellent semiconductor laser is required.
[0004]
It is necessary to improve the crystallinity of the active layer in order to increase the luminous efficiency of the semiconductor laser. In order to improve the temperature characteristics of the semiconductor laser, a leakage current that does not contribute to light emission that increases with an increase in temperature is required. In order to achieve this, it has been proposed to produce a 1 μm band semiconductor laser using GaAs as a semiconductor substrate.
[0005]
GaAs has a smaller lattice constant than InP which has been used as a substrate of a conventional 1 μm semiconductor laser, and a semiconductor material having a small lattice constant generally has a larger forbidden band width. Therefore, by using GaAs as a substrate, a semiconductor material having a larger forbidden band width can be used as a cladding layer. This cladding layer serves as a barrier to carriers leaking from the active layer without contributing to light emission. The current is reduced.
[0006]
Another proposal is to use InGaAs instead of GaAs as the substrate. In this case, since the lattice constant of InGaAs is larger than that of GaAs, the amount of strain applied to the active layer is reduced. Crystallinity is improved.
[0007]
FIG. 8 shows a conventionally proposed 1 μm band semiconductor laser using a GaAs substrate or an InGaAs substrate. An n-type AlGaAs cladding layer 3d and an n-type InGaAsP optical guide layer 4c are formed on a GaAs or InGaAs substrate 1d. , An InGaAs active layer 6d, a p-type InGaAsP light guide layer 8c, and a p-type AlGaAs cladding layer 9d.
[0008]
In this case, since the AlGaAs cladding layers 3d and 9d have a sufficiently large forbidden band width as compared with the InGaAs active layer 6d in the 1 μm band, the leakage current is reduced, and excellent temperature characteristics that can follow a relatively large temperature change are expected. You.
[0009]
Another proposal for using GaAs as a substrate is to use a short-period superlattice growth layer in which several monolayers of InAs and GaAs are alternately grown as an active layer. Is improved by using a short-period superlattice structure to improve crystallinity and realize a laser oscillation of 1.3 μm which cannot be obtained by the InGaAs mixed crystal.
[0010]
[Problems to be solved by the invention]
However, when GaAs is used for the substrate, an extremely large stress is applied to the active layer based on the difference in lattice constant between the substrate and the active layer, and the active layer becomes a high-strain active layer. However, crystal growth of the high-strain layer is difficult. Therefore, an active layer having excellent crystallinity could not be obtained, and expected characteristics could not be obtained.
[0011]
Further, even if the strain is reduced by using InGaAs as a substrate, a substrate having a sufficiently large lattice constant (that is, an InGaAs substrate having an In ratio of 0.2 or more) having excellent crystallinity has not been prepared. Thus, an active layer having excellent crystallinity could not be obtained.
[0012]
Furthermore, when an InAs / GaAs short-period superlattice growth layer is used for the active layer, the light emission intensity required for laser oscillation has not yet been obtained because of insufficient crystallinity around the active layer.
[0013]
A drawback common to these cases is that when a semiconductor having a large band gap is used as the cladding layer to improve the temperature characteristics, the composition in the obtained active layer, in particular, the distribution of In is not uniform and the whole is not uniform. As a result, an InGaAs active layer having a uniform composition cannot be obtained, which has been an obstacle to realizing a 1 μm band semiconductor laser using a GaAs substrate or an InGaAs substrate.
[0014]
Therefore, the present invention prevents the diffusion of the In component in the active layer, which causes the composition of the active layer to be non-uniform, so that the cladding layer having a small lattice constant and a large forbidden band width can be used. An object of the present invention is to grow an active layer having a uniform composition having excellent crystallinity, thereby obtaining a semiconductor laser having a high luminous efficiency and excellent temperature characteristics in a 1 μm band.
[0015]
[Means for Solving the Problems]
The present invention provides an optical semiconductor device provided with III-V group compound semiconductor mixed crystal III-V compound semiconductor substrate, it shall be the constituent element In and has a large in-plane lattice constant than the lattice constant of the substrate so as to be in contact with at least one main surface of the active layer, characterized in that a GaAs layer of 20Å or more thickness as a spacer layer for preventing the diffusion of in.
[0016]
Further, the present invention is characterized in that an InGaAs mixed crystal is used as a semiconductor substrate for an InGaAs active layer, or a short-period superlattice growth layer active layer in which an InAs layer and a GaAs layer are alternately deposited. Things.
[0017]
[Action]
The function of the spacer layer and the function of the InGaAs mixed crystal semiconductor substrate of the present invention will be described with reference to FIGS. FIGS. 1 and 2 (A) to 2 (D) are cross-sectional views of Samples 1 to 5 prepared to confirm the effect of the GaAs spacer layer and the effect of the InGaAs substrate. In each sample, the active layers are an InAs layer and a GaAs. The layer monolayer is a short-period superlattice growth layer in which crystals are alternately grown 12 times.
[0018]
2A and 2B correspond to a conventional semiconductor laser using a GaAs substrate. FIG. 2A shows a GaAs cladding layer 3b, an active layer 6a, and a GaAs cladding layer 9b provided on a GaAs substrate 1b. (Sample 1), and (B) uses In _0.05 Ga _0.95 As layers 3a and 9a having a buffer effect as cladding layers instead of the GaAs cladding layers 3b and 9b (sample 2). .
[0019]
2C and 2D correspond to a conventional semiconductor laser using an InGaAs substrate, and FIG. 2C shows a GaAs cladding layer 3b, an active layer 6a, and a GaAs cladding layer 9b provided on an InGaAs substrate 1a. (Sample 3), and (D) uses In _0.05 Ga _0.95 As layers 3a and 9a as cladding layers instead of GaAs cladding layers 3b and 9b (sample 4).
[0020]
FIG. 1 shows a principle configuration corresponding to the semiconductor laser of the present invention, in which 40 ° is provided between the In _0.05 Ga _0.95 As clad layers 3 a and 9 a provided on the InGaAs substrate 1 a and the active layer 6 a. GaAs spacer layers 5a and 7a are provided (sample 5). In the figure, spacers are provided above and below the active layer, but providing them on only one of them has a sufficient effect.
[0021]
In Samples 1 to 5, the thicknesses of the cladding layer on the substrate side, the active layer, and the cladding layer on the side opposite to the substrate were 0.5 μm, 75 °, and 500 °, respectively, and the photoluminescence intensity was In order to investigate, the thickness of the upper clad layer on the side opposite to the substrate is made smaller than that of the actual semiconductor laser.
[0022]
FIG. 3 shows reciprocal lattice mapping on the (224) plane revealed by X-ray diffraction measurement for the samples 1 to 5 shown in FIGS. 1 and 2, and the results of five separately measured samples are displayed in an overlapping manner. I have.
[0023]
In FIG. 3, the horizontal axis represents the reciprocal of the lattice constant in the XY direction, that is, the (hh0) direction, and the vertical axis represents the reciprocal of the lattice constant in the Z-axis direction, that is, the (00h) direction. In the case of (1), since the lattice constant increases as the In content increases, the region with a small vertical axis, that is, the region with a large lattice constant in the Z-axis direction indicates a high In content.
[0024]
On the other hand, in Samples 1 to 3, the substrate or the cladding layer is made of GaAs or In _0.05 Ga _0.95 As, and the In content is small, so that the lower region of FIG. It will represent.
[0025]
Comparing Sample 1 and Sample 2, or Sample 3 and Sample 4, Sample 1 and Sample 3 are on the smaller side of the Z-axis respectively. Therefore, when the same substrate is used, GaAs is used as the cladding layer. It was found that the use of the active layer, that is, the contact between the active layer and the GaAs layer, resulted in a higher content of In in the active layer. It can be seen that when there is, the In content of the active layer is higher.
[0026]
FIG. 4 is a plot of the correlation between the in-plane lattice constant of the active layer in the XY plane and the photoluminescence intensity at room temperature, which were revealed from the reciprocal lattice mapping of FIG. From the comparison between Sample 4 and Sample 5, it is clear that the use of the GaAs spacer layer has the effect of improving the intensity of photoluminescence.
[0027]
Further, FIG. 5 shows the effect of the composition of the cladding layer when the InGaAs cladding layer is used for the GaAs substrate, that is, the relationship between the amount of relaxation based on the amount of lattice mismatch between the substrate and the cladding layer and the photoluminescence intensity, Also, it shows the correlation between the thickness of the GaAs spacer layer and the photoluminescence intensity. For any of the buffer layers, the photoluminescence intensity is improved when the thickness of the spacer layer is set to 20 ° or more.
[0028]
In this case, if the thickness of the spacer layer is too large, the photoluminescence intensity decreases. This is considered to be because the spacer layer acts as a barrier for carriers excited by light in the cladding layer or the like, so that the carriers do not reach the active layer, but there is no particular hindrance to the laser operation. is there.
[0029]
Therefore, from the results of the above experiments, it can be seen that the diffusion of In is prevented by the action of the spacer layer provided so as to be in contact with the active layer, and a high-quality active layer having a high In content and excellent optical characteristics is obtained. It can be seen that it can be obtained.
[0030]
Next, the operation of the InGaAs mixed crystal semiconductor substrate will be described. In FIG. 3, since the GaAs cladding layer provided on the InGaAs mixed crystal semiconductor substrate of Sample 3 has a smaller horizontal axis direction than the illustrated GaAs substrate. , XY plane is large, that is, relaxation is occurring.
[0031]
Referring to FIG. 4, it can be seen from the comparison between Sample 1 and Sample 3 that the use of an InGaAs substrate improves the intensity of photoluminescence. Therefore, by using the InGaAs mixed crystal semiconductor substrate, even when a cladding layer having a large band gap is used, an active layer having excellent crystallinity, that is, an active layer having excellent optical characteristics can be obtained by the relaxation effect. It is.
[0032]
【Example】
FIG. 6 shows a first embodiment of the present invention in which the principle configuration shown in FIG. 1 is applied to a semiconductor laser having an optical guide layer, and an n-type InGaAs buffer layer 2 and an n-type InGaP clad layer 3c, n-type InGaAsP light guide layer 4c, non-doped InGaAsP light guide layer 4d, GaAs spacer layer 5a, short-period superlattice active layer 6a, GaAs spacer layer 7a, non-doped InGaAsP light guide layer 8d, and , A p-type InGaAsP light guide layer 8c and a p-type InGaP cladding layer 9c.
[0033]
In the first embodiment shown in FIG. 6, although the substrate is InGaAs, a GaAs substrate may be used, and a short-period superlattice growth in which a plurality of InAs layers and GaAs layers are alternately deposited as active layers. Although a layer is used, an InGaAs mixed crystal may be used.
[0034]
FIG. 7 shows a second embodiment of the present invention, in which an AlGaAs cladding layer 3d, an n-type InGaAsP optical guide layer 4c, a GaAs spacer layer 5a, an InGaAs active layer 6d, a GaAs spacer layer 7a, p InGaAsP light guide layer 8c and p-type AlGaAs cladding layer 9d are sequentially deposited.
[0035]
Although the InGaAs mixed crystal is used as the active layer in the second embodiment shown in FIG. 7, a short-period superlattice growth layer in which a plurality of InAs layers and GaAs layers are alternately deposited may be used.
[0036]
In the first and second embodiments, the GaAs layer, which is the most suitable as the spacer layer, has been described. However, any material may be used as long as it prevents diffusion of In, which is a component of the active layer. The reason why the GaAs spacer layer prevents the diffusion of In is that, considering that interdiffusion occurs between homologous atoms, Ga, which is the same Group III atom as In, has a smaller atomic radius than In. This is because GaAs has a smaller lattice constant than InAs.
[0037]
In other words, in order for In to pass through the GaAs spacer layer, it is necessary for In having a large atomic radius to spread the small group III site occupied by Ga so far, and therefore, when diffusing, excessive energy of the lattice strain is required. It is thought that diffusion is suppressed.
[0039]
Further, although not shown in the first and second embodiments, the present invention also includes a semiconductor laser in which a cap layer for improving the ohmic property of an electrode is provided on a clad layer opposite to a substrate. Things.
[0040]
【The invention's effect】
According to the present invention, when the temperature characteristics are improved by using a clad having a large forbidden band width, the diffusion of the In component in the active layer is prevented by inserting a spacer layer, and a uniform crystal having excellent crystallinity is obtained. Obtaining an active layer having a composition greatly contributes to the practical use of a 1 μm band semiconductor laser having high luminous efficiency and excellent temperature characteristics.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of an optical semiconductor device having a spacer layer showing a basic configuration of the present invention.
FIG. 2 is an element cross-sectional view of a comparative reference sample for confirming the operation and effect of the present invention.
FIG. 3 is a correlation diagram showing the dependence of the in-plane lattice constant of the active layer on the composition of the substrate, the presence of a buffer layer, and the presence of a spacer layer.
FIG. 4 is a correlation diagram showing the dependence of photoluminescence intensity on the lattice constant in the crystal plane of the active layer and the presence of a spacer layer.
FIG. 5 is a correlation diagram showing the dependence of the photoluminescence intensity on the substrate composition and the thickness of the spacer layer.
FIG. 6 is a sectional view of a 1 μm band semiconductor laser using an InGaP layer as a cladding layer according to a first embodiment of the present invention.
FIG. 7 is a sectional view of a 1 μm band semiconductor laser using an AlGaAs layer as a cladding layer according to a second embodiment of the present invention.
FIG. 8 is a cross-sectional view of a conventional 1 μm band semiconductor laser using a GaAs substrate or an InGaAs substrate.
[Explanation of symbols]
1a In _0.05 Ga _0.95 As substrate 1b GaAs substrate 1d InGaAs (GaAs) substrate 2 n-type InGaAs buffer layer 3a In _0.05 Ga _0.95 As cladding layer 3b GaAs cladding layer 3cn n-type InGaP cladding layer 3dn AlGaAs cladding layer 4c n-type InGaAsP optical guide layer 4di i-type InGaAsP optical guide layer 5a GaAs spacer layer 6a InAs / GaAs short-period superlattice active layer 6d InGaAs active layer 7a GaAs spacer layer 8c p-type InGaAsP optical guide layer 8di InGaAsP optical guide layer 9a In _0.05 Ga _0.95 As clad layer 9b GaAs clad layer 9cp p-type InGaP clad layer 9d p-type AlGaAs clad layer

Claims (8)

In the optical semiconductor device provided with group III-V compound semiconductor substrate III-V group compound semiconductor mixed crystal, the active layer shall be the constituent element In and has a large in-plane lattice constant than the lattice constant of the substrate An optical semiconductor device comprising a GaAs layer having a thickness of 20 ° or more provided as a spacer layer for preventing diffusion of In so as to be in contact with at least one main surface. 2. The optical semiconductor device according to claim 1, wherein said spacer layer is provided in contact with both main surfaces of said active layer. 3. The optical semiconductor device according to claim 1, wherein an optical guide layer is provided between the spacer layer and the clad layer. The semiconductor substrate is _{In x Ga 1-x As (} 0 ≦ x <1) The optical semiconductor device according to any one of claims 1 to 3, characterized in that a. The optical semiconductor device according to any one of claims 1 to 4, characterized in that the emission wavelength of the active layer is 1μm band. The optical semiconductor device according to any one of claims 1 to 5 the active layer is characterized in that it is a In _y Ga _1-y As mixed crystal (0 <y <1). The optical semiconductor device according to any one of claims 1 to 5 , wherein the active layer is a short-period superlattice growth layer in which InAs layers and GaAs layers are alternately deposited. 8. The optical semiconductor device according to claim 7 , wherein each of the alternately provided InAs layer and GaAs layer has a thickness of one monolayer.

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