JP2015119002A - Semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such a problem that although a laser having an active layer composed only of an InGaAsP-based material has good long term stability, a carrier block layer capable of significantly suppressing leakage of electrons from an active layer to a p-type layer has not yet proposed, and degradation of the laser element characteristics is severe during high temperature operation compared with a laser in which the active layer is composed of an AlGaInAs material.SOLUTION: In a semiconductor laser manufactured on an InP substrate, AlInPSb or AlGaInPSb is used as the material of a carrier block layer. When compared with prior art where the carrier block layer is composed of such a material as AlInAs, the band discontinuity can be increased in the conduction band. Furthermore, the degree of freedom of design can be widened, by reducing the constraints of layer structure including the carrier block layer.

Description

  The present invention relates to a semiconductor laser used in an optical communication system.

  In recent years, with the progress of multimedia technologies such as the Internet, the traffic volume of communication networks is rapidly increasing. As a result, both the subscriber access network and the metro access network that provides connectivity between corporate buildings and between data centers, and campus campus / campus connectivity, can respond to the increase in traffic volume. It is desired.

  In order to cope with this rapid increase in traffic volume, optical modules used particularly in metro access networks are required to have low power consumption. In order to satisfy this requirement, it is desirable that the semiconductor laser serving as the light source of the optical module does not require temperature adjustment. This is because most of the power consumption in the semiconductor laser is due to the temperature adjusting Peltier element or the like. Therefore, a semiconductor laser with good so-called temperature characteristics is desired in which even if the operating temperature rises, the increase in oscillation threshold current is small and the decrease in light output is small.

  The wavelength range currently used in metro access networks and which may be used in the future is a wavelength from 1.260 μm to 1.625 μm. This is a wavelength band (O band, E band, S band, C band, L band, recommended for optical communication in the International Telecommunication Union Telecommunication Standardization Sector (ITU-T)). Corresponds to the wavelength range excluding the U band used for optical fiber monitoring light.

  In order to improve the temperature characteristics of a semiconductor laser, it is necessary to efficiently confine electrons and holes in the active layer. Conventionally, as an active layer of a semiconductor laser on an InP substrate, a multiple quantum well structure using a material composed of In, Ga, As, and P (hereinafter referred to as an InGaAsP-based material) excluding Al has been used. . This is because an InGaAsP-based material that does not contain Al is not easily affected by oxidation, so that there is little deterioration over time in laser characteristics. However, in the multi-quantum well structure of InGaAsP-based material, electron confinement is often insufficient due to small band discontinuity in the conduction band. In such a case, electrons injected from the n-type semiconductor layer side during high-temperature operation pass through the multiple quantum well structure as the active layer, reach the opposite p-side cladding layer, and recombine with holes. To do. Due to this recombination, there is a problem that the confinement of electrons and holes becomes insufficient, and the temperature characteristics of the laser characteristics deteriorate.

  In order to overcome this problem, an active layer (MQW active layer) structure using AlGaInAs that can increase the band discontinuity in the conduction band more than InGaAsP-based materials has recently been studied. Even if this active layer using AlGaInAs is employed, leakage of electrons from the active layer to the p-type semiconductor layer cannot be sufficiently suppressed during high-temperature operation or high injection operation. For this reason, a structure has been proposed in which an InAlAs layer is further provided as an energy barrier against electrons between the active layer and the p-side InP cladding layer. In this structure, the InAlAs layer operates to repel electrons that have passed through the active layer and inject them again into the active layer. (Patent Document 1 and Non-Patent Document 1). Since this InAlAs layer is installed for the purpose of blocking carriers (electrons), it is called a carrier block layer.

  Due to the structural contrivance for efficiently confining electrons and holes as described above, even a semiconductor laser fabricated on an InP substrate can now be operated at an ambient temperature exceeding 100 ° C. It has become.

JP 2012-15134 A Specification

K. Nakahara et al., “12.5−Gb / s Direct Modulation Up to 115 C in 1.3−m InGaAlAs−MQW RWG DFB Lasers With Notch−Free Grating Structure,” IEEE Journal of Lightwave Technol, Vol. 22, No. 1, 2004, 159-165. H. Yokoyama et al., “Reduction of In Composition in Heavily Zn-Doped InAlGaAs Layers Grown at Low Temperature by Metalorganic Chemical Vapor Deposition,” Japanese Journal of Applied Physics, Vol. 51, 2012, 025601. C. G. Van de Walle, “Band lineups and deformation potentials in the model-solid theory,” Physical Review B, Vol. 39, No. 3, 1989, 1871-1883. C. Wang, “OMVPE growth of GaInAsSb in the 2-2.4 μm range,” Journal of Crystal Growth, Vol. 191, 1998, 631-640.

  However, even in such a laser using an AlGaInAs material as an active layer on an InP substrate, a laser at a high temperature operation is compared with a multiple quantum well laser or a quantum dot laser using an InGaAs well layer on a GaAs substrate. Degradation of characteristics is large. One of the reasons for the low degradation of laser characteristics during high-temperature operation with a laser constructed on a GaAs substrate is to use AlGaAs with a large Al composition ratio as the carrier block layer, which can provide a large band discontinuity in the conduction band. This is because it can. Furthermore, this AlGaAs has a lattice mismatch of less than 0.15% with respect to GaAs, no matter how much the Al composition ratio is changed. Therefore, since the lattice matching with GaAs related to the Al composition ratio can be almost lattice matched, the degree of freedom in designing the layer structure is large. On the other hand, in the AlInAs carrier block layer used for the laser configured on the InP substrate, there is a restriction on the degree of freedom in designing the layer configuration as described below.

  FIG. 8 is a diagram schematically showing a laser band lineup on an InP substrate using an AlGaInAs / AlGaInAs multiple quantum well (MQW) structure as an active layer. The vertical direction corresponds to the energy level of electrons, and the horizontal direction corresponds to the position in the stacking direction of the laser stack structure. In a normal manufacturing method, the layers are sequentially stacked from the right side (AlGaInAs light confinement layer 801) to the left side (InGaAsP light confinement layer 803a) in FIG. In FIG. 8, electrons are injected into the AlGaInAs / AlGaInAs MQW 802, which is an active layer, from the right AlGaInAs light confinement layer 801 side. On the other hand, holes are injected into the AlGaInAs / AlGaInAs MQW 802 from the left InGaAsP optical confinement layer 803a side. The AlInAs carrier blocking layer 804 is inserted between the AlGaInAs light confinement layer 803b and the InGaAsP light confinement layer 803a on the left side of the active layer 802.

In the laser structure shown in FIG. 8, electrons that have passed through the active layer 802 during high-temperature operation leak into the left AlGaInAs light confinement layer 803b. Here, the AlInAs carrier block layer 804 functions as an energy barrier against electrons for returning the leaked electrons to the active layer 802 again. This energy barrier corresponds to a band discontinuity of the AlInAs carrier block layer 804 with respect to the AlGaInAs optical confinement layer 803b in the conduction band, and is indicated by ΔE _C in FIG. AlInAs carrier block layer 804, as this Delta] E _C is large, effectively acts as a carrier blocking layer for electrons. Performance of the carrier blocking layer is also the layer thickness to another Delta] E _C becomes important. This is because the passage of electrons due to the tunnel effect cannot be ignored if the thickness of the carrier block layer is too thin. In the case of an AlInAs carrier block layer, a thickness of 10 nm or more is generally required. However, new problems arise as the film thickness increases.

In a layer having a large lattice strain and a large film thickness, crystal defects due to lattice relaxation occur. Therefore, in order to avoid crystal defects, the AlInAs carrier block layer 804 needs to have a composition close to lattice matching with InP. The composition that can be used specifically is limited to the composition in the vicinity of In _0.52 Al _0.48 As. Since the AlInAs carrier block layer 804 is limited to the composition in the vicinity of In _0.52 Al _0.48 As and there are restrictions on the degree of freedom in designing the layer configuration, the AlGaInAs optical confinement layer 803b is used in the laser layer structure shown in FIG. If the composition of is determined, ΔE _C is almost determined.

For example, ΔE _C when an AlGaInAs optical confinement layer having a band gap wavelength of 1.1 μm (with a composition of Al _0.29 Ga _0.18 In _0.53 As) is about 220 meV. The value of this Delta] E _C, albeit with a larger value as the lasers on InP substrates, small compared to the operation at a high temperature of the laser constructed on a GaAs substrate mentioned above. To further reduce the deterioration of the device characteristics in the structure shown in FIG. 8 at a high temperature operation, further Delta] E _C is the energy difference between the bottom of each of the conduction band of the AlInAs carrier block layer 804 and the AlGaInAs light confinement layer 803a It needs to be bigger.

To further increase the Delta] E _C in the structure shown in FIG. 8, it is effective to the AlInAs carrier block layer 804 in the p-type doped layer of the relatively high concentration of an undoped layer. However, AlInAs is difficult to carry out high-concentration p-type doping compared to materials such as GaInAs and AlGaInAs, and also has a low carrier mobility (see Non-Patent Document 2). For this reason, it is difficult to increase ΔE _C by doping the AlInAs carrier block layer 804 into p-type. Other means to increase the Delta] E _C, it is conceivable to increase the composition ratio of Ga to Al in the AlGaInAs light confinement layer 803b. However, increasing the Ga composition ratio increases the leakage of electrons from the MQW active layer 802 to the AlGaInAs optical confinement layer 804, which is not very effective.

  As described above, when the AlInAs carrier block layer 804 is used in the laser structure in which AlGaInAs is used for the active layer 802 and the optical confinement layer 803a, it is difficult to suppress the leakage of electrons by changing the layer structure, and the layer related to the carrier block. The design freedom of the configuration was limited.

  As described above, when an active layer is formed only from an InGaAsP-based material that does not contain Al, device characteristics are generally less likely to deteriorate over time than when an active layer is formed from a material containing Al. It is known. When an InGaAsP-based material is used as a laser active layer and AlInAs is used as a carrier block layer material, a higher barrier against electrons can be realized than when an AlGaInAs material is used as a laser active layer.

  FIG. 9 is a diagram schematically showing a laser band lineup on an InP substrate using an InGaAsP / InGaAsP MQW structure as an active layer. As in FIG. 8, the vertical direction corresponds to the energy level of electrons, and the horizontal direction corresponds to the position of the laser stack structure in the stacking direction. In a normal manufacturing method, the layers are sequentially stacked from the right side (InGaAsP light confinement layer 901) to the left side (InGaAsP light confinement layer 903a) in FIG. In FIG. 9, electrons are injected into the InGaAsP / InGaAsP MQW layer 902 which is an active layer from the right InGaAsP optical confinement layer 901 side. On the other hand, holes are injected into the InGaAsP / InGaAsP MQW layer 902 from the left InGaAsP optical confinement layer 903a side. The AlInAs carrier block layer 904 is disposed between the InGaAsP light confinement layer 903a and the InGaAsP light confinement layer 903b on the left side of the active layer 902.

In the layer structure shown in FIG. 9, for example, when an InGaAsP optical confinement layer 904 having a band gap wavelength of 1.1 μm (composition is In _0.86 Ga _0.40 As _0.32 P _0.68 ), In the band, the energy difference (ΔE _C ) of the AlInAs carrier blocking layer 904 relative to the InGaAsP optical confinement layer 903 is about 390 meV. The AlGaInAs material on the aforementioned InP substrate as compared to 220meV in the case of the structure with the active layer, it is possible to achieve greater Delta] E _C.

  However, when attention is paid to the valence band side in the band structure shown in FIG. 9, the position of the valence band of the AlInAs carrier block layer 904 is above the InGaAsP optical confinement layers 903a and 903b. Therefore, the injected holes are trapped in the AlInAs carrier block layer 904. After all, the layer structure having the band structure shown in FIG. 9 is effective in preventing electrons from leaking into the p-type semiconductor layer, but the hole injection efficiency is low, so that the actual laser layer structure It is not used as. As in this example, a laser in which an active layer is composed of only an InGaAsP-based material has good long-term stability, but a carrier block layer that can significantly suppress the leakage of electrons from the active layer to the p-type layer is Not yet proposed. Compared to a laser having an active layer made of an AlGaInAs material, the problem that the degradation of laser element characteristics during high-temperature operation is large has not been solved.

  The present invention has been made in view of the above-described problems, and a main object thereof is to realize a semiconductor laser having a small deterioration in laser characteristics even at high temperature operation as a light source of an optical module used in a communication network. .

  In order to solve such a problem, the present invention provides a semiconductor laser formed on an InP substrate, wherein an active layer and two active layers are sandwiched between the active layer and the active layer. An optical confinement layer and at least one of the two optical confinement layers, disposed in the vicinity of the active layer, including at least In and Al as a group III element and at least P and Sb as a group V element A semiconductor laser comprising a group III-V compound semiconductor layer.

  According to a second aspect of the present invention, in the semiconductor laser according to the first aspect, the ratio of Sb in the group V composition in the group III-V compound semiconductor layer is 0.5 or less.

  The invention according to claim 3 is the semiconductor laser according to claim 1 or 2, wherein the group III-V compound semiconductor layer further contains Ga as a group III element, and a proportion of Ga in the group III element Is 0.3 or less.

  According to a fourth aspect of the present invention, in the semiconductor laser of the first or second aspect, the III-V compound semiconductor layer has a lattice mismatch with respect to InP set in a range of -0.3% to + 0.3%. It is characterized by that.

  According to a fifth aspect of the present invention, in the semiconductor laser according to the first to fourth aspects, the III-V compound semiconductor layer functions as a block layer for carriers injected into the active layer. .

  According to a sixth aspect of the present invention, in the semiconductor laser according to any one of the first to fifth aspects, each of the two optical confinement layers has a III-V group compound semiconductor layer.

  A seventh aspect of the present invention is the semiconductor laser according to any one of the first to fifth aspects, wherein the III-V group compound semiconductor layer is made of AlInPSb or AlGaInPSb.

  According to the present invention, in the semiconductor laser fabricated on the InP substrate, the band discontinuity in the conduction band is made larger than when the carrier block layer is made of AlInAs or the like used in the prior art. be able to. The degree of freedom in designing the layer structure can be increased. In addition, it is possible to provide a good electron block in a laser structure using an MQW active layer made of an InGaAsP-based material for which a layer that can be a sufficient barrier against electrons has not been found in the prior art. The carrier block layer of the present invention realizes a semiconductor laser with little deterioration in laser characteristics even during high temperature operation. Even in a high temperature environment, an increase in oscillation threshold current and a decrease in optical output can be suppressed, temperature adjustment is not required, and an increase in power consumption of the optical module can be suppressed.

FIG. 1 is a diagram showing the energy positional relationship between the bottom of the conduction band and the top of the valence band, comparing AlInAs and Al (Ga) InPSb. FIG. 2 is a diagram showing changes in band discontinuities in the conduction band and valence band of AlInPSb with respect to AlInAs under the condition of lattice matching with InP. FIG. 3 is a diagram showing changes in band discontinuities in the conduction band and valence band of AlGaInPSb with respect to AlInAs under the condition of lattice matching with InP. FIG. 4 is a diagram showing a change in group III composition with respect to a change in Sb composition of AlGaInPSb under a condition in which a GaInPSb ratio in AlGaInPSb is fixed. FIG. 5 is a diagram showing a band discontinuity change in the conduction band and valence band of AlGaInPSb with respect to InGaAsP under the condition of lattice matching with InP. FIG. 6 is a diagram showing a layer configuration of the semiconductor laser according to Example 1 of the present invention. FIG. 7 is a diagram showing a layer configuration of a semiconductor laser according to Example 2 of the present invention. FIG. 8 is a diagram schematically showing a laser band lineup on an InP substrate using an AlGaInAs / AlGaInAs multiple quantum well structure as an active layer. FIG. 9 is a diagram schematically showing a laser band lineup on an InP substrate using an InGaAsP / InGaAsP MQW structure as an active layer.

  In the semiconductor laser of the present invention, a new characteristic of AlInPSb or AlGaInPSb, which is a material that has not been used in a laser formed on an InP substrate, is found, and by utilizing this characteristic, the laser characteristics at high temperature operation can be improved. Reduce deterioration. AlInPSb or AlGaInPSb has the characteristic that the energy at the bottom of the conduction band and the top of the valence band can be changed greatly only by changing the composition, even in the vicinity of the lattice matching condition with InP. By using AlInPSb or AlGaInPSb as a carrier block layer for electrons and changing its composition in accordance with the material used for the active layer and the oscillation wavelength, it is possible to suppress degradation of laser characteristics during high-temperature operation than before.

  The energy arrangement of the bottom of the conduction band and the top of the valence band of AlInPSb or AlGaInPSb is compared with AlInAs lattice-matched to InP, which is a typical carrier block layer in the prior art, depending on the Sb composition. How it changes. In the following description, for the sake of simplicity, when AlInPSb or AlGaInPSb is meant, it is expressed as Al (Ga) InPSb. As shown in two examples described later, the two materials exhibit the same function and effect as the carrier block layer.

FIG. 1 is a diagram schematically showing the energy positional relationship between the bottom of the conduction band and the top of the valence band, comparing AlInAs and Al (Ga) InPSb. FIG. 1 is a diagram for explaining a band change when two materials are virtually joined. In FIG. 1, the band discontinuity ΔE _C that is the energy difference of the conduction band of Al (Ga) InPSb with respect to AlInAs can be expressed by the following equation.
ΔE _C = [energy at the top of valence band of Al (Ga) InPSb + band gap of Al (Ga) InPSb] −
[AlinAs top energy of valence band + AlInAs band gap]
Formula (1)

As for the Al (Ga) InPSb band discontinuity Delta] E _V is the energy difference between the valence for AlInAs of can be expressed by the following equation.
ΔE _V = [the energy at the top of the valence band of AlInAs] −
[Energy of the top of valence band of Al (Ga) InPSb] Formula (2)

Here, for electrons, when ΔE _C is positive (a gap is present above the conduction band in the band diagram), it means that Al (Ga) InPSb becomes a barrier higher in energy than AlInAs. Further, for the hole, Delta] E _V is (downward there is a gap from the valence band in the band diagram) If positive, Al (Ga) InPSb means that the energetically higher barrier than AlInAs. If the energy 101, 102 at the top of the valence band and the band gaps 103, 104 of each material are known, the band band of the conduction band of Al (Ga) InPSb with respect to AlInAs can be calculated based on the equations (1) and (2). Continuous (ΔE _C ) and band discontinuity (ΔE _V ) of valence electrons can be obtained. Regarding ternary and quaternary mixed crystals of group III-V compound semiconductors, it is known that the energy and band gap at the top of the valence band can be estimated by calculation and agree well with experiments ( For example, see Non-Patent Document 3.) Therefore, for AlInAs, AlInPSb, and GaInPSb, it is relatively easy to obtain the energy at the top of the valence band and the band gap from the calculation. First, regarding AlInPSb, which is one of the materials employed in the semiconductor laser of the present invention, band discontinuity is obtained based on the formulas (1) and (2), and the operation as a carrier block layer is examined.

FIG. 2 is a diagram showing changes in the band discontinuity in the conduction band of AlInPSb and the band discontinuity in the valence band with respect to AlInAs under the condition of lattice matching with InP. Specifically, when it is assumed that AlInAs and AlInPSb are joined, when the Sb composition of AlInPSb is changed under the condition of lattice matching with InP (horizontal axis), the band discontinuity (ΔE _C ) in the conduction band and The calculated value of the band discontinuity (ΔE _V ) in the valence band is shown (vertical axis).

In FIG. 2, the position where the Sb composition ratio on the horizontal axis is 0 corresponds to InP. In this composition, ΔE _C of the conduction band is a negative value, and the energy position of the conduction band of AlInPSb is lower than the energy position of the conduction band of AlInAs. However, as the Sb composition increases, the energy position of the conduction band of AlInPSb increases. When the Sb composition of AlInPSb exceeds 0.3, ΔE _C becomes a positive value, and the energy position of the conduction band of AlInPSb is higher than the energy position of the conduction band of AlInAs. At this time, it can be seen that AlInPSb becomes a higher barrier against electrons. That is, AlInPSb can act as a higher carrier block layer for electrons compared to AlInAs.

  The effect of AlInPSb as a carrier block layer on electrons is further increased by increasing the Sb composition above 0.5 in the calculation, but when considering the crystal growth of AlInPSb, the proportion of Sb in the V group composition is Less is desirable. This is because in the crystal growth of a material containing Sb, as the Sb composition increases, the surface segregation of Sb becomes remarkable, and the influence of the film quality due to the surface segregated Sb increases. In general, a group III-V compound semiconductor containing no Sb is relatively stable if the ratio of the supply amount of the group V source to the group III source (generally referred to as the V / III ratio) is larger than 1 during crystal growth. It is known that crystal growth is possible under certain conditions. However, for a material having a large Sb composition, the V / III ratio needs to be close to 1, and a slight change in the V / III ratio greatly affects the film quality (Non-Patent Document 4). For this reason, in the case of the semiconductor laser of the present invention in which a layer containing Sb exists only in a part of the layers constituting the laser, it is desirable that the Sn composition in AlInPSb and AlGaInPSb described later be 0.5 or less.

  Therefore, in the semiconductor laser of the present invention, the carrier block layer is disposed in at least one of the two optical confinement layers and in the vicinity of the active layer, and includes at least In and Al as group III elements, and group V It is a group III-V compound semiconductor layer containing at least P and Sb as elements, and the proportion of Sb in the group V composition in the group III-V compound semiconductor layer is desirably 0.5 or less.

Referring again to FIG. 2, in contrast to the band discontinuity in the conduction band, the band discontinuity (ΔE _V ) in the valence band is always a positive value regardless of the change in the Sb composition of AlInPSb. This means that AlInPSb is a higher barrier with respect to holes than AlInAs. However, as can be seen from FIG. 2, for InP, AlInPSb is not a barrier for holes. For this reason, when an AlInPSb carrier block layer is used for a semiconductor laser formed on an InP substrate, the hole injection efficiency is not hindered if the upper and lower layer configurations of the carrier block layer are devised. For example, in the structure of the semiconductor laser having the band configuration shown in FIG. 8, when the material of the AlInAs carrier block layer 804 is changed to AlInPSb, the AlGaInAs optical confinement layer and the InGaAsP optical confinement are shown in detail in Example 1 described later. If the composition of the layer is adjusted, a decrease in hole injection efficiency can be avoided.

  As described above, AlInPSb can change its conduction band and valence band positions relatively freely by changing its Sb composition even under the constraints near the condition of lattice matching with InP. . Therefore, the carrier block layer can be designed according to the composition of the other layers constituting the semiconductor laser, and each layer configuration is compared with the carrier block layer using the AlInAs of the prior art in which the composition is almost fixed. Design flexibility can be increased.

  Next, the operation as a carrier block layer of AlInGaPSb, which is another material employed in the semiconductor laser of the present invention, will be examined. In the AlGaInPSb described below, the degree of design freedom of each layer configuration can be further increased as compared with the AlInPSb.

Since AlInGaPSb is a quaternary mixed crystal containing five elements, it is generally difficult to obtain the energy and band gap of the top of the valence band necessary for the above formulas (1) and (2). However, AlInGaPSb is almost lattice matched to InP, and the band gap can be easily estimated under the condition that the composition ratio of Al to Ga is large. In this case, the material parameter of AlGaInPSb can be easily obtained by the following equation as a mixed crystal of AlInPSb and GaInPSb lattice-matched to InP.
[Material parameters of AlGaInPSb] = A × [Material parameters of AlInPSb]
+ (1-A) × [Material Parameters of GaInPSb] Formula (3)

  Here, A is the ratio of AlInPSb to AlGaInPSb which is a ternary mixed crystal. Equation (3) gives a good approximation because the influence of bowing factor is small if the ratio of AlInPSb to AlGaInPSb is sufficiently large compared to GaInPSb.

FIG. 3 is a diagram showing a change in band discontinuity in the conduction band of AlGaInPSb relative to AlInAs and band discontinuity in the valence band under the condition of lattice matching with InP. When it is assumed that AlInAs and AlGaInPSb are joined, the band discontinuity (ΔE _C ) in the conduction band and the band in the valence band when the Sb composition of AlGaInPSb is changed under the condition of lattice matching with InP (horizontal axis). The calculated value of discontinuity (ΔE _V ) is shown (vertical axis). FIG. 3A shows the calculated value of the band discontinuity (ΔE _C ) in the conduction band, and FIG. 3B shows the calculated value of the band discontinuity (ΔE _V ) in the valence band. In any case, the cases of 0, 0.1, 0.2, 0.3, 0.4, and 0.5 are shown using the ratio of GaInPSb in AlGaInPSb as a parameter.

FIG. 3A shows that the band discontinuity (ΔE _C ) in the conduction band decreases as the proportion of GaInPSb in AlGaInPSb increases, and the effect as a barrier to electrons decreases. In particular, when the ratio of GaInPSb is 0.5, the increase in band discontinuity in the conduction band is small. Therefore, in order to utilize the effect as a barrier against electrons, it is preferable to use a carrier block layer in which the proportion of GaInPSb in AlGaInPSb is 0.4 or less.

  FIG. 4 is a diagram showing a change in the group III composition with respect to a change in the Sb composition of AlGaInPSb under the condition that the ratio of GaInPSb to AlGaInPSb is fixed at 0.4. The horizontal axis shows the Sb composition of AlGaInPSb, and the vertical axis shows the group III composition. From FIG. 4, when the Sb composition (horizontal axis) is 0.5, the Ga composition in AlGaInPSb is 0.3. Therefore, the Ga composition in the AlGaInPSb carrier block layer is desirably 0.3 or less.

  Therefore, the semiconductor laser of the present invention includes an active layer, two optical confinement layers configured to sandwich the active layer, and at least one of the two optical confinement layers, in the vicinity of the active layer And a group III-V compound semiconductor layer containing at least In and Al as group III elements and at least P and Sb as group V elements. The group III-V compound semiconductor layer includes a group III element Further, Ga is included, and the proportion of Ga in the group III element is 0.3 or less.

Returning to FIG. 3 again, as is clear from FIG. 3B, AlGaInPSb can change the Sb composition of AlGaInPSb by changing the Sb composition of AlGaInPSb (horizontal axis) even under the constraint of the lattice matching with InP. The position of the top of the belt can be changed greatly. Under the condition that the ratio of GaInPSb to AlGaInPSb is 0.4 or less, the band discontinuity (ΔE _V ) in the valence band of AlGaInPSb with respect to AlInAs is positive in most compositions. For this reason, between AlGaInPSb and AlInAs, AlGaInPSb becomes a barrier for holes. However, when InGaAsP or the like having a lower valence band energy position than AlInAs is used as the optical confinement layer, the band discontinuity (ΔE _V ) can be made substantially zero. This will be described using InGaAsP having a band gap wavelength of 1.05 μm as an example.

FIG. 5 is a diagram showing a band discontinuity change in the conduction band and the valence band of AlGaInPSb with respect to InGaAsP under the condition of lattice matching with InP. More specifically, the ratio of GaInPSb occupying in AlGaInPSb under the condition of lattice matching with InP is fixed to 0.1, and InGaAsP (In _0.89 Ga ₀ where the band gap wavelength for lattice matching with InP is about 1.05 μm. _.11 As _0.24 P _0.76 ), the change in band discontinuity (ΔE _C ) in the conduction band and band discontinuity (ΔE _V ) in the valence band of AlGaInPSb was obtained. Here, “the ratio of GaInPSb to AlGaInPSb” corresponds to the coefficient (1-A) of the equation (3). The horizontal axis is the Sb composition of AlGaInPSb, and the vertical axis is each band discontinuity (ΔE _C , ΔE _V ).

  From FIG. 5, when the Sb composition (horizontal axis) is about 0.3, the band discontinuity in the valence band becomes almost zero. This means that when holes are moved between InGaAsP and AlGaInPSb, there is almost no energy barrier to the holes. On the other hand, the band discontinuity in the conduction band when the Sb composition is 0.3 is about 300 meV. This value is a high value that was difficult to obtain with the prior art as a blocking layer for laser electrons on an InP substrate, and was combined with an active layer of an InGaAsP material system and an AlGaInPSb carrier blocking layer during high-temperature operation. Improvement of laser characteristics can be expected.

  As described above, the thickness of the carrier block layer is required to be 10 nm or more in order to suppress the passage of electrons due to the tunnel effect. On the other hand, if the film thickness is too large, it causes an increase in element resistance, so that a film thickness of 50 nm or less is generally used. When the film thickness is about 10 to 50 nm, if the lattice mismatch with respect to InP is in the range of -0.3% to + 0.3%, the generation of crystal defects due to the lattice mismatch can be avoided. . Here, the lattice mismatch is defined as ([lattice constant of carrier block layer] − [lattice constant of InP]) / [lattice constant of InP]).

  In the above two examples, even when AlInPSb and AlGaInPSb are used for the carrier block layer under the condition of lattice matching with InP, if the lattice mismatch with respect to InP is in the range of −0.3% to + 0.3%, the carrier As the block layer, AlAlPSb and AlGaInPSb carrier block layers have almost the same effect.

  In the above-described configuration example of the carrier block layer, AlInPSb and AlGaInPSb containing Al, Ga, In, etc. as Group III elements and P, Sb etc. as Group V elements have been described. However, even if a small amount of As enters these carrier block layers, the effect as a carrier block layer does not change greatly. For this reason, the composition of the carrier block layer does not need to be strictly AlInPSb or AlGaInPSb, and if the As composition is small, even if it is AlInAsPSb or AlInGaAsPSb, almost the same effect can be obtained as the carrier block layer.

  Next, more specific embodiments of the semiconductor laser of the present invention will be described with reference to the drawings.

  FIG. 6 is a diagram showing a layer configuration of the semiconductor laser according to Example 1 of the present invention. In this example, an example in which an AlInPSb carrier block layer is applied to a structure including an AlGaInAs material in an active layer will be described.

For the production of the layer structure shown in FIG. 6, trimethylindium (TMIn), triethylgallium (TEGa), trimethylaluminum (TMAl) as group III materials, phosphine (PH ₃ ), arsine (AsH ₃ ) as group V source gases. An organic metal vapor phase epitaxy method using trimethylantimony (TMSb) is used. The semiconductor laser of this example is manufactured by the following process.

First, an n-type InP buffer layer 2 having a thickness of 0.5 μm is grown on the n-type InP substrate 1. Subsequently, after growing an AlGaInAs optical confinement layer 3 having a film thickness of 0.1 μm and a band gap wavelength of 1.1 μm, a strained multiple quantum well structure 4 composed of six AlGaInAs well layers and seven AlGaInAs barrier layers is grown. . Further, an AlGaInAs light confinement layer 5 having a film thickness of 0.05 μm and a band gap wavelength of 1.1 μm is grown thereon. The AlGaInAs light confinement layer 3 and the AlGaInAs light confinement layer 5 are composed of the same composition and are substantially lattice-matched to InP, and a specific composition is Al _0.29 Ga _0.18 In _0.53 As.

  In the strained multiple quantum well structure 4, the composition and film thickness of each of the well layer and the barrier layer are adjusted so that the oscillation wavelength is 1.3 μm. On the AlGaInAs light confinement layer 5, an AlInPSb layer 6 having a thickness of 0.02 μm is grown as a carrier block layer for electrons. On the carrier block layer, an InGaAsP optical confinement layer 7 having a film thickness of 0.05 μm and a band gap wavelength of 1.0 μm, a p-type InP cladding layer 8 having a film thickness of 1.8 μm, and a film having a film thickness of 0.25 μm are further provided. A p-type InGaAs contact layer 9 is grown sequentially.

The AlInPSb carrier block layer 6 and the InGaAsP optical confinement layer 7 are both lattice-matched to almost InP, and the specific compositions are Al _0.51 In _0.49 P _0.66 Sb _0.34 and In _{0. It is 93} Ga _0.07 As _0.16 P _0.84 . The band discontinuity in the valence band between the two materials is almost zero, and the band discontinuity in the conduction band is about 350 meV.

  Therefore, the semiconductor laser of the present invention is a semiconductor laser formed on an InP substrate, and includes at least one of an active layer, two optical confinement layers configured to sandwich the active layer, and the two optical confinement layers. And a group III-V compound semiconductor layer including at least In and Al as group III elements and at least P and Sb as group V elements.

  In order to evaluate the characteristics of the semiconductor laser of this example, the layer structure of the semiconductor laser of the present invention described above [Test body 1] and a layer in which only the AlInPSb layer 6 does not exist in the layer structure shown in FIG. A comparative [test body 2] having a configuration was prepared.

  The characteristics of the semiconductor laser are evaluated using a Fabry-Perot laser. A specific method for manufacturing a Fabry-Perot laser is as follows. After a silicon oxide film is deposited on the front surface of the wafer, the silicon oxide film in a region having a width of 40 μm is removed in a stripe shape. AuZn is vapor-deposited in this region where the p-type InGaAs layer 9 is exposed on the surface, followed by heat treatment to form the p-type electrode 10. The n-type electrode 11 is formed by thinly polishing the n-type InP substrate 1 and then depositing AuGeNi and heat-treating it. The resonator is formed by cleavage, and the resonator length is 600 μm.

For the two test specimens, the oscillation threshold current density at 25 ° C. is 1.0 kA / cm ² for [Test specimen 1] provided with an AlInPSb carrier block layer, and does not include the AlInPSb carrier block layer [test specimen]. 2], it was 0.9 kA / cm ² . The oscillation threshold current density was lower when the AlInPSb carrier block layer was not included. However, the threshold currents of [Test body 1] and [Test body 2] at 65 ° C. were both 1.7 kA / cm ² . Further, when operating at a temperature higher than 65 ° C., the threshold current density was lower in [Test body 1] having the AlInPSb carrier block layer.

In order to evaluate temperature characteristics in a semiconductor laser, a parameter generally called characteristic temperature is used. The characteristic temperature can be obtained from the following equation by measuring the temperature change of the threshold current.
_{J th = J 0 exp (T} / T 0) (4)
Here, J _th is the threshold current density at the operating temperature T, J ₀ is a constant, and T ₀ is the characteristic temperature. From equation (4), the larger the T _{0, the} smaller the increase in threshold current density with respect to the increase in operating temperature, and the better the temperature characteristics of the semiconductor laser.

  The characteristic temperature of the threshold current density in the range from 25 ° C. to 65 ° C. was 75K for [Test body 1] and 65K for [Test body 2]. This is because the threshold current increases when the operating temperature of the laser with the structure [test body 1] including the AlInPSb carrier block layer is increased as compared with the laser with the structure [test body 2] not including this. It means less. The maximum oscillation temperature of the laser of the structure [test body 1] including the AlInPSb carrier block layer was 135 ° C., which was higher than the maximum oscillation temperature of 125 ° C. of the laser of the structure [test body 1] not including this. In a laser structure including an AlInPSb carrier block layer, the maximum operating temperature and characteristic temperature are high because electrons leaked from the active layer due to temperature rise are rebounded by the AlInPSb carrier block layer and injected into the active layer again. It is reflected.

  In this embodiment, the case where the active layer has an AlGaInAs / AlGaInAs strained multiple quantum well structure, the optical confinement layers on both sides of the active layer are made of AlGaInAs layers, and the laser has an oscillation wavelength of 1.3 μm has been described as an example. However, the semiconductor laser of the present invention is not limited to the above-described configuration, and various modifications can be made. For example, in order to increase the oscillation wavelength, the active layer can be changed to a structure including an InGaAs well layer. Moreover, even when the material of the AlGaInAs light confinement layer on both sides of the active layer is changed to AlGaInPSb for adjusting the band discontinuity and the optical confinement coefficient, AlInPSb can be used as an effective electron blocking layer. In this embodiment, the AlInPSb carrier block layer is undoped, but doping may be performed. Furthermore, in the present embodiment, the case where the AlInPSb carrier block layer is provided only on one side of the active layer is shown, but it may be provided on both sides of the active layer.

  In this embodiment, an example in which a metal organic vapor phase epitaxy method is used as a method for manufacturing a semiconductor laser has been described. However, the present invention is characterized by gas source molecular beam epitaxy (organometallic vapor phase epitaxy), molecular beam epitaxy, etc. as long as it can grow a quaternary or quaternary III-V group semiconductor mixed crystal. Needless to say, the present invention is also effective when manufacturing using other growth methods.

  FIG. 7 is a diagram showing a layer configuration of a semiconductor laser according to Example 2 of the present invention. In this embodiment, an example in which an AlGaInPSb carrier block layer is applied to a structure including an InGaAsP-based material in an active layer will be described.

For the production of the layer structure of FIG. 7, a gas source molecular beam epitaxy method using metal In, metal Ga, metal Al as a group III material, phosphine (PH ₃ ), arsine (AsH ₃ ), and metal Sb as a group V material. Is used. The semiconductor laser of this example is manufactured by the following process.

  First, an n-type InP buffer layer 13 having a thickness of 0.5 μm is grown on the n-type InP substrate 12. Subsequently, after growing an InGaAsP optical confinement layer 14 having a film thickness of 0.1 μm and a band gap wavelength of 1.1 μm, a strained multiple quantum well structure 15 comprising eight InAsP well layers and nine InGaAsP barrier layers is grown. . In the strained multiple quantum well structure 15, the composition and film thickness of each of the well layer and the barrier layer are adjusted so that the oscillation wavelength is 1.3 μm. An InGaAsP optical confinement layer 16 having a film thickness of 0.1 μm and a band gap wavelength of 1.1 μm is grown on the strained multiple quantum well structure 15 with an AlGaInPSb carrier block layer 17 having a film thickness of 0.03 μm interposed therebetween.

The AlGaInPSb carrier block layer 17 and the InGaAsP optical confinement layer 16 are both lattice-matched to almost InP, and the specific composition is Al _0.36 Ga _0.09 In _0.55 P _0.70 Sb _0.30. And In _0.86 Ga _0.14 As _0.32 P _0.69 . The band discontinuity in the valence band between the two materials is almost zero, and the band discontinuity in the conduction band is about 300 meV. A p-type InP cladding layer 18 having a thickness of 1.8 μm and a p-type InGaAs contact layer 19 having a thickness of 0.25 μm are sequentially grown on the InGaAsP optical confinement layer 16.

  In order to evaluate the characteristics of the semiconductor laser of this example, the layer structure of the above-described semiconductor laser of the present invention [Test body 3] and the layer structure in which only the AlGaInPSb layer 17 does not exist in the layer structure shown in FIG. [Test body 4] for comparison was prepared. The characteristics of the semiconductor laser are evaluated using a Fabry-Perot laser. The fabrication method of the Fabry-Perot laser was the same as in Example 1.

The oscillation threshold current density at 25 ° C. is 0.9 kA / cm ² in [Test body 3] provided with an AlGaInPSb carrier block layer, and is 0. 0 in [Test body 4] not including an AlGaInPSb carrier block layer. It was 8 kA / cm ² . The laser having a configuration not including the AlGaInPSb carrier block layer had a lower oscillation threshold current density. However, the threshold currents of [Test body 3] and [Test body 4] at 65 ° C. were 1.6 kA / cm ² and 1.7 kA / cm ² , respectively. The threshold current of the laser including the AlGaInPSb carrier block layer was lower. The characteristic temperature of the threshold current density in the range from 25 ° C. to 65 ° C. was 69K for [Test body 3] and 60K for [Test body 4]. By using a laser having an AlGaInPSb carrier block layer, an increase in threshold current can be suppressed when the operating temperature is raised.

  Further, the laser with the structure including the AlGaInPSb carrier block layer [test body 3] has a maximum oscillation temperature of 125 ° C., which is higher than the maximum oscillation temperature of 115 ° C. with a laser having a structure not including this [test body 1]. It was.

  As described above in detail, even in a laser structure using an InGaAsP-based material as an active layer, which has been a problem of degradation of laser characteristics during high temperature operation in the prior art, this high temperature can be achieved by using an AlGaInPSb carrier block layer. It is possible to suppress deterioration of characteristics during operation.

  The semiconductor laser of this embodiment can be modified in various ways as described in the first embodiment, and can be modified with respect to the composition of each layer and the presence or absence of doping. Moreover, although the case where the AlGaInPSb carrier block layer is provided only on one side of the active layer is shown, it may be provided on both sides of the active layer. Furthermore, as long as the quaternary or quaternary III-V group semiconductor mixed crystal can be grown, it is also effective when other growth methods such as a metal organic vapor phase epitaxy method or a molecular beam epitaxy method are used.

  As described above in detail, in the semiconductor laser of the present invention fabricated on the InP substrate, AlInPSb or AlGaInPSb is used as the material of the carrier block layer. As a result, the band discontinuity in the conduction band can be further increased as compared with the case where the carrier block layer is made of AlInAs or the like used in the prior art. Furthermore, the degree of freedom in designing the layer configuration can be further increased.

  In addition, it is possible to provide a good electron block in a laser structure using an MQW active layer made of an InGaAsP-based material, for which a layer that can be a sufficient barrier against electrons has not been found in the prior art. The carrier block layer in the present invention can more efficiently confine electrons in the vicinity of the active layer even during high-temperature operation. Therefore, it is possible to realize a semiconductor laser with little deterioration in laser characteristics even during high-temperature operation. Even in a high temperature environment, an increase in oscillation threshold current and a decrease in light output can be suppressed. Temperature adjustment of the semiconductor laser becomes unnecessary, and an increase in power consumption of the optical module using the semiconductor laser of the present invention as a light source can be suppressed.

  The present invention is generally applicable to communication systems. In particular, it can be used as a light source for an optical communication system.

1, 12 n-type InP substrate 2, 13 n-type InP buffer layer 3, 5, 7, 14, 16, 801, 803a, 803b, 901, 903a, 903b Optical confinement layer 4, 15, 802, 902 Multiple quantum well structure 6, 17, 804, 904 Carrier block layer 8, 18 p-type InP cladding layer 9, 19 p-type InGaAs contact layer 10, 20 p-type electrode 11, 21 n-type electrode 101, 102 energy 103 at the top of the valence band 103, 104 band gap

Claims (7)

In a semiconductor laser formed on an InP substrate,
An active layer,
Two optical confinement layers configured to sandwich the active layer;
A III-V group compound that is disposed in the vicinity of the active layer in at least one of the two optical confinement layers and includes at least In and Al as group III elements and at least P and Sb as group V elements A semiconductor laser comprising: a semiconductor layer.   2. The semiconductor laser according to claim 1, wherein a ratio of Sb to a group V composition in the group III-V compound semiconductor layer is 0.5 or less.   The group III-V compound semiconductor layer further contains Ga as a group III element, and the proportion of Ga in the group III element is 0.3 or less. Semiconductor laser.   3. The semiconductor laser according to claim 1, wherein the III-V group compound semiconductor layer has a lattice mismatch with respect to InP in a range of −0.3% to + 0.3%.   5. The semiconductor laser according to claim 1, wherein the III-V compound semiconductor layer functions as a blocking layer for carriers injected into the active layer. 6.   6. The semiconductor laser according to claim 1, wherein each of the two optical confinement layers has a III-V compound semiconductor layer.   6. The semiconductor laser according to claim 1, wherein the III-V compound semiconductor layer is made of AlInPSb or AlGaInPSb.

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