Semiconductor laser device

Abstract

To provide a semiconductor laser device having a quantum dot structure capable of improving high-temperature operating characteristics.SOLUTION: A semiconductor laser device has an active layer structure including a single or multiple active layers. The active layer 20 includes a quantum dot structure 40. The quantum dot structure 40 includes an island crystal 52, a lateral barrier layer 53 that at least partially embeds the periphery of the island crystal 52, and an upper crystal layer 30 that covers an upper end 52t of the island crystal 52 and the lateral barrier layer 53. A first band gap of the lateral barrier layer 53 is larger than a second band gap of the upper crystal layer 30.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a semiconductor laser device having a quantum dot structure and related techniques. [Background technology]

[0002] In recent years, light sources having a quantum dot structure have been attracting attention as light sources that can be applied to optical devices such as optical communication devices, optical integrated circuits, lighting elements, and high-efficiency displays. By using a quantum dot structure in the active layer of a semiconductor laser, it is expected that characteristics such as high-temperature operation characteristics and modulation characteristics will be improved. Prior art related to semiconductor lasers having such quantum dot structures is disclosed, for example, in JP 2022-166543 A (Patent Document 1) and the following Non-Patent Documents 1 and 2. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2022-078795 [Non-patent literature]

[0004] [Non-Patent Document 1] L. Jarvis, B. Maglio, CP Allford, S. Gillgrass, A. Enderson, S. Shutts, H. Deng, M. Tang, H. Liu, and PM Smowton, "1.3-μm InAs Quantum Dot Lasers with P-type modulation and direct N-type co-doping," in 28th International Semiconductor Laser Conference (ISLC) (2022), paper WA-02. [Non-Patent Document 2] Zun-Ren Lv, S. Wang, H. Wang, H.-M. Wang, H.-Y. Chai, X.-G. Yang, L. Meng, C. Ji, and T. Yang, "Significantly improved performances of 1.3 μm InAs / GaAs QD laser by spatially separated dual-doping," Appl. Phys. Lett. 121(2), 021105 (2022). Summary of the Invention [Problem to be solved by the invention]

[0005] When the energy band of the conduction band of a quantum dot structure is split into multiple levels (subbands), the conduction band has an excited level in addition to the ground level that can contribute to laser oscillation. The energy difference between the ground level and the excited level is the thermal energy (= k B T;k B If θ is not sufficiently large compared to the Boltzmann constant, the probability that electrons will exist in the excited level increases with increasing temperature, which poses the problem of increasing the threshold current required for laser oscillation.

[0006] In view of the above, an object of the present disclosure is to provide a semiconductor laser device having a quantum dot structure that can improve high-temperature operating characteristics with respect to threshold current. [Means for solving the problem]

[0007] A semiconductor laser device according to an embodiment of the present disclosure includes an active layer structure including a single or multiple active layers, each of the active layers being configured to include one or more quantum dot structures, each of the quantum dot structures including an island crystal, a lateral barrier layer having a first bandgap and at least partially burying the periphery of the island crystal, and an upper crystal layer having a second bandgap and covering an upper end of the island crystal and the lateral barrier layer, the first bandgap being larger than the second bandgap. Effect of the Invention

[0008] According to one aspect of the present disclosure, the quantum dot structure has a side barrier layer that at least partially buries the periphery of the island crystal, and the first band gap of the side barrier layer is larger than the second band gap of the upper crystal layer. By combining the island crystal, the side barrier layer, and the upper crystal layer, it is possible to enhance the quantum confinement effect and increase the energy difference between the subbands (between the ground level and the excited level) of the conduction band in the island crystal while ensuring a desired current injection efficiency (carrier injection efficiency) to the active layer. This suppresses the increase in threshold current at high temperatures, and therefore it is possible to provide a semiconductor laser device with excellent high-temperature operating characteristics. [Brief description of the drawings]

[0009] [Figure 1] 1A and 1B are diagrams illustrating a schematic configuration of an active layer including a quantum dot structure according to one embodiment. [Diagram 2] FIG. 2 is a diagram showing a schematic band structure of an island crystal (quantum dot). [Diagram 3] 3A and 3B are diagrams illustrating schematic configurations of active layers including quantum dot structures according to other embodiments. [Figure 4] FIG. 4 is a diagram showing a schematic configuration of a quantum dot structure which is a modified example of the quantum dot structure of FIGS. 3A and 3B. [Diagram 5] Fig. 5A is a diagram showing a STEM image of a cross section of an actually fabricated quantum dot structure, and Fig. 5B is a diagram for explaining the STEM image of Fig. 5A. [Figure 6] 5B is a graph showing the composition analysis results of the quantum dot structure shown in FIG. 5A. [Figure 7] FIG. 4 is a diagram showing a schematic configuration of a quantum dot structure which is another modified example of the quantum dot structure of FIGS. 3A and 3B. [Figure 8] 13 is a graph showing the calculation results of the energy level difference for various thicknesses of the lateral barrier layer. [Figure 9]13 is a graph showing the calculation results of the energy level difference depending on the thickness of the lateral barrier layer of two types of quantum dot structures. [Figure 10] FIG. 10A is a graph showing the calculation results of the energy level difference ΔE depending on the thickness of the lateral barrier layer of a quantum dot structure, and FIG. 10B is a diagram showing various dimensions of the quantum dot structure used in obtaining the calculation results of FIG. 10A. [Figure 11] 1 is a schematic cross-sectional view of a configuration of a semiconductor laser device having an active layer structure. [Figure 12] 1 is a schematic cross-sectional view of a configuration of a semiconductor laser device having an active layer structure. [Figure 13] 13 is a schematic cross-sectional view of an active layer structure of the semiconductor laser device shown in FIGS. 11 and 12. [Figure 14] FIG. 11 is a cross-sectional view illustrating another example of an active layer structure of a semiconductor laser device. [Figure 15] 1 is a schematic cross-sectional view of a configuration of a semiconductor laser device having a diffraction grating structure. [Figure 16] 13 is a graph showing calculation results of high-temperature operation characteristics (temperature dependence of threshold current density) of a semiconductor laser device. [Figure 17] FIG. 17A is a schematic cross-sectional view of a configuration example of a semiconductor laser device according to this embodiment, and FIG. 17B is a schematic cross-sectional view of an active layer structure of the semiconductor laser device shown in FIG. 17A. [Figure 18] FIG. 18A is a graph showing the results of measuring the PL spectra at room temperature of the semiconductor laser devices of Examples 1 and 2, and FIG. 18B is a schematic diagram for explaining the structure of the semiconductor laser device of Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Next, various embodiments and their modifications will be described in detail with reference to the drawings. Note that components with the same reference numerals throughout the drawings have the same configurations and functions.

[0011] The semiconductor laser device according to the present embodiment includes an active layer structure including a single or multiple active layers, and each active layer is configured to include one or more quantum dot structures. A quantum dot (QD) structure refers to a quantum confinement structure that can three-dimensionally confine carriers such as electrons with nanometer-scale minute dimensions. Each quantum dot structure according to the present embodiment includes an island crystal (quantum dot) formed on an underlying crystal layer, a lateral potential barrier layer (LPBL) that at least partially embeds the periphery of the island crystal, and an upper crystal layer that covers both the upper end of the island crystal and the lateral barrier layer. The band gap of the island crystal is smaller (i.e., narrower) than the band gap (first band gap) of the lateral barrier layer and smaller (i.e., narrower) than the band gap (second band gap) of the upper crystal layer. One feature is that the band gap of the lateral barrier layer is larger (i.e., wider) than the band gap of the upper crystal layer. This feature allows us to calculate the energy difference between multiple levels (subbands) in the conduction band of an island crystal (quantum dot) in terms of thermal energy (=k B T;k B Since the θ is sufficiently large compared to the Boltzmann constant, the probability that electrons exist in the excited level is low. Therefore, the temperature stability of the threshold current required for laser oscillation can be improved.

[0012] 1A and 1B are diagrams that show a schematic configuration of an active layer 20 including three quantum dot structures 40 according to an embodiment. FIG. 1A is a diagram that shows a schematic cross-sectional structure of the active layer 20 in the XZ plane, and FIG. 1B is a plan view showing a part of the active layer 20 (a part excluding the upper crystal layer 30 in FIG. 1A) when viewed from the positive direction of the Z axis, that is, the height direction. Here, the X axis, the Y axis, and the Z axis form an orthogonal coordinate system. The drawing in FIG. 1B is parallel to the XY plane. In the example of FIG. 1A and FIG. 1B, the number of quantum dot structures 40 is three, but this is not limited thereto.

[0013] As shown in Figures 1A and 1B, the quantum dot structure 40 includes a wetting layer 51 formed over the entire surface of the base crystal layer 30B, an island-shaped crystal (quantum dot) 52 formed on the base crystal layer 30B via the wetting layer 51, a side barrier layer 53 partially embedding the lateral periphery of the island-shaped crystal 52 (i.e., the side surface 52s of the island-shaped crystal 52) on the base crystal layer 30B, and an upper crystal layer 30 covering the upper end 52t of the island-shaped crystal 52 and the side barrier layer 53. The island-shaped crystal 52 has an outer shape of a truncated quadrangular pyramid (a shape obtained by chamfering the apex of a quadrangular pyramid) and has one upper end 52t and four side surfaces 52s. The upper end 52t of the island-shaped crystal 52 is heterojunctioned with the upper crystal layer 30, and the side surface 52s of the island-shaped crystal 52 is heterojunctioned with the side barrier layer 53 over the entire periphery. 1A and 1B, three island crystals 52 are arranged along the X-axis, but the present invention is not limited to this. The illustrated island crystals 52 and other island crystals 52 (not shown) may be distributed along the XY plane to form a single quantum dot layer.

[0014] 1A and 1B are merely examples. The shape of the quantum dot is not limited to a square pyramid, since it depends on the crystal growth conditions and crystal orientation. It should be noted that the shape of the island crystal 52 may be other shapes, such as a polygonal pyramid or a cone (a shape obtained by chamfering the apex of a polygonal pyramid or a cone), other than a square pyramid, depending on the respective constituent materials of the base crystal layer 30B and the island crystal 52, the formation method of the island crystal 52, and the growth process conditions of the island crystal 52 (for example, the plane orientation, temperature conditions, and pressure conditions of the base crystal layer 30B).

[0015] The lateral barrier layer 53 is formed so as to fill the periphery of the island-shaped crystal 52 to a position lower than the height position of the upper end 52t of the island-shaped crystal 52. In this case, the uppermost end portion of the lateral barrier layer 53 that joins with the side surface 52s of the island-shaped crystal 52 has a substantially rectangular shape in a top view in the example of Fig. 1B. The height position of the uppermost end portion of the lateral barrier layer 53 is controlled to be lower than the height position of the upper end 52t of the island-shaped crystal 52, and to be a height position midway between the lower end (base end) 52b and the upper end 52t of the island-shaped crystal 52, as shown in Fig. 1A.

[0016] The band gap of the island crystal 52 is smaller than the band gap of each of the underlayer crystal layer 30B, the upper crystal layer 30, and the lateral barrier layer 53, which three-dimensionally surround the island crystal 52. The band gap of the lateral barrier layer 53 is larger (i.e., wider) than the band gap of the upper crystal layer 30. The underlayer crystal layer 30B, the island crystal 52, the upper crystal layer 30, and the lateral barrier layer 53 can each be made of a III-V compound semiconductor material. For example, the island crystal 52 and the wetting layer 51 can be made of an InAs (indium arsenide)-based single crystal material, and the upper crystal layer 30 and the underlayer crystal layer 30B can be made of a single crystal material selected from the group consisting of GaAs (gallium arsenide)-based materials, InP (indium phosphide)-based materials, and InGaAs-based materials. The lateral barrier layer 53 may be made of a single crystal material selected from the group consisting of an AlGaAs (aluminum gallium arsenide)-based material, an AlAs-based material, an InAlAs-based material, and an InGaAlAs-based material.

[0017] For example, as a combination of materials constituting the island crystals 52, the upper crystal layer 30, and the side barrier layer 53, any of the following examples 1 to 4 can be used. Example 1: Island crystal / Upper crystal layer / Side barrier layer = InAs / GaAs / AlGaAs Example 2: Island crystal / Upper crystal layer / Side barrier layer = InAs / InP / AlAs Example 3: Island crystal / Upper crystal layer / Side barrier layer = InAs / InGaAs / InAlAs Example 4: Island crystal / Upper crystal layer / Side barrier layer = InAs / InGaAs / InGaAlAs Example 5: Island crystal / Upper crystal layer / Side barrier layer = InAs / InP / InAlAs Example 6: Island crystal / Upper crystal layer / Side barrier layer = InAs / InP / InGaAlAs

[0018] The quantum dot structure 40 can be fabricated by a crystal growth method such as molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD). The island-like crystals 52 can be formed by a method using a microfabrication method, a self-forming method, or a combination of the microfabrication method and the self-forming method. As the self-forming method, a self-forming method based on the so-called VW (Volmer-Weber) mode growth or SK (Stranski-Krastanov) mode growth may be used. According to the self-forming method based on the SK mode growth, when a compound semiconductor is grown on the base crystal layer 30B, a thin wetting layer 51 of about several atoms is formed, and then a large number of three-dimensional island-like crystals 52 can be grown on the wetting layer 51. On the other hand, instead of the self-forming method based on the VW mode growth or the SK mode growth, a droplet epitaxy method may be adopted. If the droplet epitaxy method is used, the formation of the wetting layer 51 can be avoided.

[0019] The quantum dot structure 40 described above is characterized in that the band gap of the side barrier layer 53 is larger than the band gap of the upper crystal layer 30. Fig. 2 is a diagram showing a schematic band structure of an island-like crystal (quantum dot) 52. In the band structure of Fig. 2, the vertical direction indicates energy E, and the horizontal direction indicates height position in the Z-axis direction. The energy band of the conduction band is divided into multiple subband levels E 1 ,E 2 ,E 3 The valence band is split into subband levels E h The subband level E 1 is the ground level, and E is the subband level 2 ,E 3are the first and second excited states, respectively. Light emission occurs when the quantum dot structure 40 is excited to the ground level E 1 Electrons and subband levels of the valence band E h In the quantum dot structure 40 of this embodiment, the upper end 52t of the island crystal 52 is joined to the upper crystal layer 30. In addition, since the band gap of the side barrier layer 53 is larger than the band gap of the upper crystal layer 30, a high quantum confinement effect can be obtained. In other words, the ground level E 1 and the first excited level E 2 The energy difference ΔE between the upper end 52t of the island crystal 52 and the side barrier layer 53 is larger than the energy difference between the ground level and the excited level in the conduction band of the island crystal 52 when the side 52s of the island crystal 52 is not joined to the side barrier layer 53 but joined to the upper crystal layer 30, and therefore a high quantum confinement effect is obtained. If a material with a large band gap is used for both the upper crystal layer 30 and the side barrier layer 53 covering the upper end 52t of the island crystal (quantum dot) 52, the quantum confinement effect increases, and the energy difference ΔE can be further increased. However, in that case, there is a risk that the current injection efficiency (carrier injection efficiency in the vertical direction) into the island crystal 52 will decrease. In contrast, in this embodiment, the side barrier layer 53 is made of a material different from that of the upper crystal layer 30, and the material of this side barrier layer 53 is selected to have a band gap larger than the band gap of the upper crystal layer 30, so that a large energy difference ΔE can be secured while maintaining the current injection efficiency. Therefore, the energy difference ΔE of the conduction band can be increased while securing the desired current injection efficiency. This suppresses an increase in the threshold current at high temperatures. By using such a quantum dot structure 40, it is possible to provide a semiconductor laser device with excellent high-temperature operating characteristics.

[0020] 1A and 1B, the lateral barrier layer 53 is formed so as to partially bury the periphery of the island-shaped crystal 52. From the viewpoint of further improving the quantum confinement effect, it is desirable that the lateral barrier layer completely bury the periphery of the island-shaped crystal 52 up to the height position of the upper end 52t of the island-shaped crystal 52.

[0021] 3A and 3B are diagrams that show a schematic configuration of an active layer 21 including three quantum dot structures 41 according to another embodiment. FIG. 3A is a diagram that shows a schematic cross-sectional structure of the active layer 21 in the XZ plane, and FIG. 3B is a top view showing a part of the active layer 21 (a part excluding the upper crystal layer 30 in FIG. 3A) when viewed from the positive direction of the Z axis, i.e., the height direction. The top surface of FIG. 3B is parallel to the XY plane. Note that, in the example of FIG. 3A and FIG. 3B, the number of quantum dot structures 40 is three, but is not limited to this.

[0022] 3A and 3B includes an island-shaped crystal (quantum dot) 52 formed on an underlying crystal layer 30B via a wetting layer 51, a lateral barrier layer 54 that completely embeds the lateral periphery of the island-shaped crystal 52 (i.e., the side surface 52s of the island-shaped crystal 52) on the underlying crystal layer 30B, and an upper crystal layer 30 that covers an upper end 52t of the island-shaped crystal 52 and the lateral barrier layer 54. The upper end 52t of the island-shaped crystal 52 is heterojunctioned with the upper crystal layer 30, and the side surface 52s of the island-shaped crystal 52 is heterojunctioned with the lateral barrier layer 54 over the entire periphery.

[0023] The lateral barrier layer 54 is formed so as to completely bury the periphery of the island-shaped crystal 52 up to the height position of the upper end 52t of the island-shaped crystal 52. At this time, the height position of the uppermost end portion of the lateral barrier layer 54 that joins with the side surface 52s of the island-shaped crystal 52 is controlled so as to coincide with the height position of the upper end 52t of the island-shaped crystal 52. Such a quantum dot structure 41 can be fabricated by the same method and using the same constituent materials as the quantum dot structure 40 shown in Figures 1A and 1B.

[0024] The quantum dot structure 41 described above has the characteristic that the band gap of the lateral barrier layer 54 is larger than the band gap of the upper crystal layer 30, and the lateral barrier layer 54 completely buries the side surface 52s of the island-like crystal 52. Therefore, a higher quantum confinement effect can be obtained than in the quantum dot structure 40 shown in Figures 1A and 1B. By using such a quantum dot structure 41, a semiconductor laser device with even better high-temperature operating characteristics can be provided.

[0025] 3A and 3B, the upper surface of the lateral barrier layer 54 is substantially flat over the entire lateral direction of the active layer 21, and the height position of the upper surface of the lateral barrier layer 54 is substantially the same over the entire lateral direction of the active layer 21, but is not limited to this. FIG. 4 is a diagram showing a schematic configuration of a quantum dot structure 42 which is a modified example of the quantum dot structure 41.

[0026] The quantum dot structure 42 shown in Fig. 4 includes an island-shaped crystal (quantum dot) 52 formed on the underlying crystal layer 30B via a wetting layer 51, a side barrier layer 55 completely burying the lateral periphery of the island-shaped crystal 52 (i.e., the side surface 52s of the island-shaped crystal 52) on the underlying crystal layer 30B, and an upper crystal layer 30 covering the upper end 52t of the island-shaped crystal 52 and the side barrier layer 55. The upper end 52t of the island-shaped crystal 52 is heterojunctioned with the upper crystal layer 30, and the side surface 52s of the island-shaped crystal 52 is heterojunctioned with the side barrier layer 55 over the entire periphery. In the vicinity of the island-shaped crystal 52, the upper surface of the side barrier layer 55 is inclined along the side surface 52s, and in a region laterally distant from the upper end 52t of the island-shaped crystal 52, the upper surface of the side barrier layer 55 is flattened. This quantum dot structure 42 also has the characteristic that the band gap of the lateral barrier layer 55 is larger than the band gap of the upper crystal layer 30, and the lateral barrier layer 55 completely embeds the side surface 52s of the island-shaped crystal 52. Therefore, similar to the case of the quantum dot structure 41, by using the quantum dot structure 42, it is possible to provide a semiconductor laser device with excellent high-temperature operating characteristics.

[0027] FIG. 5A is a diagram showing a STEM image of a cross section of a quantum dot structure actually fabricated. A STEM image is an image observed by scanning transmission electron microscopy (STEM). The STEM image of FIG. 5A was obtained by high-angle annular dark-field STEM (HAADF-STEM). FIG. 5B is a diagram for explaining the STEM image of FIG. 5A. As shown in FIG. 5B, this quantum dot structure has InAs quantum dots (InAs QDs) formed on an underlayer crystal layer made of GaAs, an upper crystal layer made of GaAs, and a lateral barrier layer (AlGaAs (LPBL)) made of AlGaAs. As with the structure shown in FIG. 4, in the vicinity of the InAs quantum dots, the lateral barrier layer is formed on the side of the InAs quantum dot, and the upper surface of the lateral barrier layer is inclined along the side of the InAs quantum dot. It can also be seen that the upper surface of the lateral barrier layer is flattened in a region laterally distant from the upper end of the InAs quantum dot. No lateral barrier layer material (AlGaAs) was observed on top of the InAs quantum dots.

[0028] FIG. 6 is a graph showing the composition analysis results of the quantum dot structure shown in FIG. 5A. This graph was obtained by line scan analysis using energy dispersive X-ray spectroscopy (EDX). The line scan was performed along the thickness direction (vertically from top to bottom) of the InAs quantum dot region. In the graph of FIG. 6, the horizontal axis indicates distance (unit: nm), and the vertical axis indicates the ratio of the constituent elements (unit: atomic percentage (atomic %)). The ratios of Al atoms, Ga atoms, As atoms, and In atoms correspond to the intensities of Al-K line, Ga-K line, As-K line, and In-L line, respectively. According to FIG. 6, it can be seen that an AlGaAs layer is formed on the side surface of the InAs quantum dot from the ratio of the region from the top to the bottom of the InAs quantum dot.

[0029] In the quantum dot structure 42 shown in FIG. 4, the upper end 52t of the island crystal 52 is heterojunctioned with the upper crystal layer 30. Alternatively, a part of the lateral barrier layer may be formed as a thin film on the upper end 52t of the island crystal 52 within a range that does not substantially impair the current injection efficiency. In this case, the thin film is interposed between the upper end 52t and the upper crystal layer 30. FIG. 7 is a diagram showing a schematic configuration of a quantum dot structure 43, which is another modified example of the quantum dot structure 41. The quantum dot structure 43 shown in FIG. 7 includes a lateral barrier layer 56 that completely embeds the lateral periphery of the island crystal 52 (i.e., the side surface 52s of the island crystal 52). Since a part of the lateral barrier layer 56 is formed as a thin film on the upper end 52t of the island crystal 52, the upper end 52t is not joined to the upper crystal layer 30, but the formation process of the lateral barrier layer 56 can be controlled so that the thickness of the thin film is within a range that does not substantially impair the current injection efficiency. As a result, even if the quantum dot structure 43 is used, a semiconductor laser device having excellent high-temperature operating characteristics can be provided.

[0030] 8 is a graph showing the calculation results of the energy level difference ΔE for various thicknesses of the lateral barrier layer. The calculation was performed based on the quantum dot structure 40 of FIG. 1A and FIG. 1B. The calculation conditions are as follows:

[0031] <Calculation conditions> ·Temperature: 300K (27℃), -Materials of the underlayer and upper layer: GaAs, -Materials of island crystals (quantum dots): InAs, Height of island crystals: 6 nm, -Width of island crystal: 20nm, -Material of the side barrier layer: Al x Ga 1-x As, · Lateral barrier layer thickness: 1nm, 2nm, 3nm, 4nm, 5nm, 6nm.

[0032] In the graph of FIG. 8, the horizontal axis indicates the Al composition x in the range of 0.0 to 1.0, and the vertical axis indicates the energy level difference ΔE (unit: meV) between the ground level and the first excited level in the conduction band of the island crystal. When the thickness of the side barrier layer is 6 nm, the height position of the side barrier layer coincides with the height position of the island crystal, so that the quantum dot structure 41 of FIG. 3A and FIG. 3B is formed. As the value of the Al composition x approaches 1.0, the Al composition x decreases. x Ga 1-x It is known that the band gap of As is wider. The graph in Figure 8 shows that as the Al composition x value increases, the band gap of the side barrier layer expands and the energy level difference ΔE increases. It also shows that as the thickness of the side barrier layer increases, the energy level difference ΔE increases from 95 meV to a maximum of 132 meV.

[0033] Furthermore, Fig. 9 is a graph showing the calculation results of the energy level difference ΔE according to the thickness of the side barrier layer of two types of quantum dot structures. This calculation was performed based on two types of quantum dot structures, namely, the quantum dot structure 40 (hereinafter also referred to as "flat type") of Fig. 1A and Fig. 1B and the quantum dot structure 42 (hereinafter also referred to as "mound type") of Fig. 4. However, when the thickness of the side barrier layer is 6 nm, the height position of the side barrier layer coincides with the height position of the island crystal, so that the quantum dot structure 41 of Fig. 3A and Fig. 3B is formed. The value of the Al composition x is a constant value (=0.2).

[0034] In the graph of Fig. 9, the horizontal axis indicates the thickness (unit: nm) of the lateral barrier layer in the numerical range of 0 to 6 nm, and the vertical axis indicates the energy level difference ΔE (unit: meV). According to the graph of Fig. 9, it can be seen that in the early stage of the thickness increase process, the energy level difference ΔE of the mound type increases sharply compared to the flat type.

[0035] 10A is a graph showing the calculation results of the energy level difference ΔE depending on the thickness of the side barrier layer of the mound-type quantum dot structure. FIG. 10B is a diagram showing various dimensions of the quantum dot structure 42 used in obtaining the calculation results of FIG. 10A. In the graph of FIG. 10A, the horizontal axis is the thickness of the Al of the side barrier layer 55. x Ga 1-x The Al composition x of the As layer is shown, and the vertical axis indicates the energy level difference ΔE. As shown in Fig. 10B, the width of the lower end of the island-like crystal (quantum dot) 52 is Δb = 20 nm, the height H of the island-like crystal 52 is 6 nm, the width of the upper end of the island-like crystal 52 is Δu = 8 nm, and the thickness t of the side barrier layer 55 is 0 to 6 nm. The shape of the island-like crystal 52 is a quadrangular pyramid (pyramid shape), the constituent material of the island-like crystal 52 is InAs, the constituent material of the upper crystal layer 30 is GaAs, the constituent material of the base crystal layer 30B is GaAs, and the temperature is 300K (= 27°C). As shown in FIG. 10A, for each of the Al compositions x=0.2, x=0.4, x=0.6, x=0.8, and x=1.0, when the thickness t of the lateral barrier layer 55 is approximately equal to half the height of the island crystals (quantum dots) 52 (t=3 nm), the energy level difference ΔE reaches approximately 90% of its maximum value.

[0036] 11 to 13 are diagrams that generally show the configuration of a semiconductor laser device 1 including an active layer structure 13. Fig. 11 is a schematic cross-sectional view of the configuration of the semiconductor laser device 1 when viewed from the positive direction of the Y-axis, and the cross section of Fig. 11 is parallel to the XZ plane. Fig. 12 is a schematic cross-sectional view of the configuration of the semiconductor laser device 1 when viewed from the positive direction of the X-axis, and the cross section of Fig. 12 is parallel to the YZ plane.

[0037] As shown in FIG. 11 and FIG. 12, the semiconductor laser device 1 has a structure in which a single crystal substrate 11, a lower cladding layer 12, an active layer structure 13, an upper cladding layer 14, and a contact layer 15 are laminated in this order. A lower electrode 10 is provided on the back surface of the single crystal substrate 11. A part of the contact layer 15 is etched into a mesa stripe shape, and an upper electrode 16 is provided on the contact layer 15. For example, the single crystal substrate 11 can be formed as an n-type GaAs substrate, the lower electrode 10 as an n-type electrode, the lower cladding layer 12 as an n-type AlGaAs layer, the upper cladding layer 14 as a p-type AlGaAs layer, the contact layer 15 as a p-type GaAs layer, and the upper electrode 16 as a p-type electrode. This semiconductor laser device 1 can have a Fabry-Perot type resonator structure. Both end faces 17 and 18 of the semiconductor laser device 1 shown in FIG. 12 form resonator end faces for a Fabry-Perot type laser.

[0038] Fig. 13 is a cross-sectional view showing a schematic example of an active layer structure 13 of a semiconductor laser device 1, and the cross section of Fig. 13 is parallel to the XZ plane. As shown in Fig. 13, the active layer structure 13 has a structure in which a plurality of active layers 20 are stacked on a base crystal layer 30C. This active layer structure 13 has a structure similar to the quantum dot structure 40 of the active layer 20 shown in Figs. 1A and 1B, but is not limited thereto. Each of the active layers 20 includes a plurality of island-like crystals (quantum dots) 52 distributed along the XY plane, and these island-like crystals 52 constitute a quantum dot layer.

[0039] The upper crystal layer 30 may include a layered region into which a dopant is introduced. Specifically, the structure of the upper crystal layer 30 may be modified to include a layered region into which either a p-type dopant or an n-type dopant (first conductive type dopant) is introduced by modulation doping. Modulation doping is a process of locally introducing a dopant during the growth of the upper crystal layer 30. Modulation doping that introduces a p-type dopant is called p-type modulation doping or modulated p-type doping, and modulation doping that introduces an n-type dopant is called n-type modulation doping or modulated n-type doping. For example, when the upper crystal layer 30 is made of GaAs, Be (beryllium) can be used as the p-type dopant. It is known that the temperature dependence of the threshold current can be suppressed by such a layered region into which a p-type dopant is introduced (for example, see Non-Patent Document 1). FIG. 14 is a cross-sectional view that shows a schematic diagram of another example of the active layer structure 13 of the semiconductor laser device 1. The cross section of FIG. 14 is parallel to the XZ plane. In the active layer structure 13 shown in Fig. 14, in each of the active layers 20, the upper crystal layer 30 includes i-type crystal layers 30u and 30m to which no dopant has been introduced (undoped), and a p-type layer region 30p as an intermediate layer sandwiched between these crystal layers 30u and 30m. In the example of Fig. 14, the layer region 30p is formed at a position spatially separated from the island-like crystals (quantum dots) 52 so as not to substantially affect the potential of the island-like crystals 52.

[0040] The configuration of the semiconductor laser device 1 of Fig. 11 and Fig. 12 may be modified to have a diffraction grating structure for a distributed feedback (DFB) type laser or a diffraction grating structure for a distributed Bragg reflector (DBR) type laser instead of the resonator structure of a Fabry-Perot type laser. The diffraction grating structure for a DFB type laser may be formed at the junction interface between the upper cladding layer and the active layer structure, or at the junction interface between the lower cladding layer and the active layer structure, by processing any one of the upper cladding layer, the active layer structure, and the lower cladding layer. Alternatively, the diffraction grating structure for a DFB type laser may be formed at the junction interface between the optical guide layer and the active layer structure by processing an optical guide layer provided between the upper cladding layer and the active layer structure or between the lower cladding layer and the active layer structure.

[0041] On the other hand, the diffraction grating structure for the DBR type laser may be formed by processing one of the upper cladding layer or the lower cladding layer in a waveguide region (i.e., a non-active region through which light generated in the active layer structure propagates) that is spatially separated from the active layer structure and optically coupled to the active layer structure. Alternatively, the diffraction grating structure for the DBR type laser may be formed by processing an optical guide layer formed on the upper cladding layer or the lower cladding layer in the waveguide region (non-active region). A semiconductor laser device having a diffraction grating structure for a DFB type laser or a diffraction grating structure for a DBR type laser can operate as a light source that outputs laser light in a single longitudinal mode.

[0042] Fig. 15 is a schematic cross-sectional view of a semiconductor laser device 1B having a diffraction grating structure as an example. This semiconductor laser device 1B has a structure in which a single crystal substrate 11, a lower cladding layer 12, an active layer structure 13B, an upper cladding layer 14B, and a contact layer 15 are laminated in this order. A lower electrode 10 is provided on the back surface of the single crystal substrate 11, and an upper electrode 16 is provided on the contact layer 15. The basic structure of this semiconductor laser device 1B is the same as that of the semiconductor laser device 1 shown in Figs. 11 and 12, except that a diffraction grating structure for a DFB type laser is formed at the bonding interface between the active layer structure 13B and the upper cladding layer 14B.

[0043] 16 is a graph showing the results of calculation of the high-temperature operating characteristics (temperature dependency of threshold current density) of a semiconductor laser device. In the graph of FIG. 16, the horizontal axis represents temperature (unit: °C) and the vertical axis represents threshold current density (unit: A / cm 2 ) This calculation was performed based on the structure of the semiconductor laser device 1 shown in FIGS. 11, 12, and 14. The calculation conditions were as follows:

[0044] <Calculation conditions> -Materials of the underlayer and upper layer: GaAs, -Materials of island crystals (quantum dots): InAs, -Materials of the lateral barrier layers: AlGaAs, ·Temperature: 30~210℃, Quantum dot density (in-plane density): 5×10 10 cm -2 , Number of quantum dot layers: 10 layers, Quantum dot layer spacing: 40nm, Ground level transition energy: 0.954 eV (corresponding to a wavelength of 1300 nm), p-type modulation doping density of dopant (Be) in layered region: 1×10 17 cm -3 , Threshold gain: 16cm -1 (equivalent to mirror loss for a cavity length of 750 μm), Ratio of transition energy difference between conduction band and valence band = 8:1, Degeneracy of levels (including spin degree of freedom): 2, 4, 8, 12, -Transition energy difference between adjacent levels ΔE: 95~115meV.

[0045] It was assumed that the threshold current density at an energy level difference of ΔE=95 meV and 90°C was the upper limit at which laser oscillation was possible. This upper limit is shown by a dotted line in the graph of Figure 16. According to the graph of Figure 16, it can be seen that laser oscillation is possible up to 120°C (=90°C + 30°C) when ΔE=105 meV, and laser oscillation is possible even at 150°C (=90°C + 60°C) when ΔE=115 meV. This confirmed that laser oscillation is possible up to higher temperatures.

[0046] Next, a semiconductor laser device according to still another embodiment will be described.

[0047] Conventionally, in order to improve the gain of quantum dot lasers, a method called direct doping has been adopted in which a dopant is introduced into an island crystal (quantum dot) during the formation process of the island crystal. Direct doping in which an n-type dopant is introduced is called direct n-type doping, and direct doping in which a p-type dopant is introduced is called direct p-type doping. Direct doping is thought to improve the carrier recombination efficiency by changing the band structure in each quantum dot to enhance the effect of confining carriers such as electrons and holes, thereby lowering the threshold current required for laser oscillation. However, in quantum dot lasers subjected to direct doping, there is a problem that the temperature dependence of the threshold current may be relatively high. On the other hand, although the above-mentioned modulation doping makes it possible to suppress the temperature dependence of the threshold current, there is a problem that the threshold current may be increased due to internal losses. The above-mentioned non-patent documents 1 and 2 propose a combination of direct n-type doping and modulation p-type doping in order to solve both of these problems. The combination of direct doping and modulation doping is called co-doping or dual-doping.

[0048] According to Non-Patent Documents 1 and 2, the above problems related to direct n-type doping in the temperature range of 27°C to 97°C can be solved to some extent by co-doping consisting of direct n-type doping and modulated p-type doping (for example, see FIG. 3 and FIG. 4 of Non-Patent Document 1 and FIG. 5(a) of Non-Patent Document 2). However, it cannot be said that the temperature dependence of the threshold current in the high temperature range of 100°C or higher is suppressed by simply applying modulated p-type doping to a quantum dot laser to which direct n-type doping has been applied. It is believed that direct n-type doping improves the effect of confining electrons in the conduction band in each quantum dot, thereby increasing the probability of radiative recombination. As described above, if the energy level difference ΔE in the conduction band of the quantum dot is not sufficiently large compared to the thermal energy at the ambient temperature T, the probability of electrons existing in the ground level decreases and the probability of electrons existing in the excited level increases in the high temperature range, resulting in a problem of an increase in the threshold current. Even if the effect of confining electrons is improved by direct n-type doping, if the probability that the confined electrons exist in an excited level increases in a high temperature range, the threshold current may increase.

[0049] The inventors of the present application have found that by applying the above-mentioned lateral barrier layer (LPBL) to a directly doped quantum dot laser, the temperature dependence of the threshold current in the high temperature range can be suppressed.

[0050] Each of the quantum dot structures of the semiconductor laser device according to this embodiment includes island-shaped crystals (quantum dots) formed on an underlayer crystal layer, and either an n-type dopant or a p-type dopant (a second conductive type dopant) is introduced into these island-shaped crystals by direct doping. Such direct doping is expected to improve the carrier recombination efficiency and lower the threshold current by controlling the band structure in each quantum dot to enhance the effect of confining carriers. Furthermore, each of the quantum dot structures includes a lateral barrier layer that at least partially buries the periphery of the island-shaped crystal into which the dopant is introduced, and an upper crystal layer that covers both the upper end of the island-shaped crystal and the lateral barrier layer. The band gap of the island-shaped crystal is smaller (i.e., narrower) than the band gap (first band gap) of the lateral barrier layer, and smaller (i.e., narrower) than the band gap (second band gap) of the upper crystal layer. The band gap of the lateral barrier layer is larger (i.e., wider) than the band gap of the upper crystal layer. This allows us to calculate the energy difference ΔE between the subband levels (between the ground level and the excited level) in the conduction band of the island crystal (quantum dot) as a function of the thermal energy (= k B T;k B Since the valence constant (V) can be made larger than the Boltzmann constant, the probability of electrons existing in the ground state increases and the probability of electrons existing in the excited state decreases. This makes it possible to reduce the threshold current by direct doping while improving the temperature stability of the threshold current even in high temperature regions.

[0051] In the direct doping process, in order to avoid deterioration of the optical properties of the semiconductor laser device, it is preferable not to introduce the dopant in the wetting layer formation process, but to selectively introduce the dopant only in the island crystal formation process. The dopant concentration is preferably controlled so that each quantum dot contains 1 to 2 dopant atoms (in other words, the number of dopants per quantum dot is in the range of approximately 1.0 to 2.0).

[0052] In order to further suppress the temperature dependence of the threshold current, the upper crystal layer preferably includes a layer region into which a dopant (a first conductive type dopant) is introduced by modulation doping. This layer region may be formed at a position spatially separated from the island crystal (quantum dot) so as not to substantially affect the potential of the island crystal.

[0053] In some embodiments, either a p-type dopant or an n-type dopant is introduced into the lateral barrier layer (LPBL) by modulation doping.

[0054] FIG. 17A is a schematic cross-sectional view of a configuration example of the semiconductor laser device 2 according to this embodiment, and FIG. 17B is a cross-sectional view of a schematic example of an active layer structure 13C of the semiconductor laser device 2 shown in FIG. 17A. FIG. 17A is a cross-sectional view when viewed from the positive direction of the X-axis, and the cross section of FIG. 17A is parallel to the YZ plane. The semiconductor laser device 2 has a configuration in which a single crystal substrate 11, a lower cladding layer 12, an active layer structure 13C, an upper cladding layer 14, and a contact layer 15 are laminated in this order. Both end faces 17 and 18 of the semiconductor laser device 2 form resonator end faces for a Fabry-Perot type laser. The configuration of the semiconductor laser device 2 is the same as that of the semiconductor laser device 1 of FIG. 12, except that the semiconductor laser device 2 has an active layer structure 13C instead of the active layer structure 13 of FIG. 12. The configuration of the semiconductor laser device 2 in FIG. 17A may be modified so as to have a diffraction grating structure for a distributed feedback laser or a diffraction grating structure for a distributed Bragg reflector laser, instead of the resonator structure of a Fabry-Perot laser.

[0055] Referring to FIG. 17B, the active layer structure 13C is a structure in which a plurality of active layers 22 are stacked on the base crystal layer 30C. The quantum dot structure of each active layer 22 includes a wetting layer 51 formed over the entire surface of the base crystal layer (base crystal layer 30C or upper crystal layer 30), an island-shaped crystal (quantum dot) 52N formed on the base crystal layer via the wetting layer 51, a side barrier layer 55 that embeds the lateral periphery of the island-shaped crystal 52N, and an upper crystal layer 30 that covers the upper end of the island-shaped crystal 52N and the side barrier layer 55. The upper end of the island-shaped crystal 52N is heterojunctioned with the upper crystal layer 30, and the side of the island-shaped crystal 52N is heterojunctioned with the side barrier layer 55 over the entire periphery. The band gap of the side barrier layer 55 is larger than the band gap of the upper crystal layer 30 and is larger than the band gap of the island-shaped crystal 52N. The quantum dot structure can basically be formed using the same constituent materials and methods as the quantum dot structure 40 described above.

[0056] An n-type dopant is introduced into the island crystals 52N by direct n-type doping. The concentration of the n-type dopant is preferably controlled so that each island crystal (each quantum dot) 52N contains 1 to 2 dopant atoms (in other words, the number of dopants per quantum dot is in the range of about 1.0 to 2.0). For example, when the island crystals 52N are made of InAs, Si (silicon) can be used as the n-type dopant. From the viewpoint of avoiding deterioration of the optical characteristics of the semiconductor laser device 2, it is preferable not to introduce the n-type dopant during the process of forming the wetting layer 51, but to selectively introduce the n-type dopant only during the process of forming the island crystals 52N.

[0057] The upper crystal layer 30 includes crystal layers 30u and 30m and a p-type layer region 30p, and the layer region 30p is a region into which a p-type dopant is introduced by modulated p-type doping. In the example of FIG. 17B, the layer region 30p is formed at a position spatially separated from the island crystal (quantum dot) 52N so as not to substantially affect the potential of the island crystal 52N, and is configured as an intermediate layer sandwiched between the crystal layers 30u and 30m. The crystal layers 30u and 30m are i-type layers into which no dopant is introduced (doped). The p-type dopant may be locally introduced during the growth process of the upper crystal layer 30. When the upper crystal layer 30 is made of GaAs, Be (beryllium) can be used as the p-type dopant.

[0058] In the example of FIG. 17B, the quantum dot structure has a mound-type structure. That is, in the vicinity of the island crystal 52N, the lateral barrier layer 55 is formed on the side surface of the island crystal 52N, and the upper surface of the lateral barrier layer 55 is inclined along the side surface of the island crystal 52N. In the region laterally away from the upper end of the island crystal 52N, the upper surface of the lateral barrier layer 55 is flattened. However, the quantum dot structure is not limited to the mound type. The configuration of the lateral barrier layer 55 may be changed so as to have a structure other than the mound type as shown in FIG. 1A to 1B, FIG. 3A to 3B, or FIG. 7. In addition, the outer shape of the island crystal 52N can be, for example, a quadrangular pyramid when viewed from above (when viewed from the positive direction of the Z axis), but is not limited thereto.

[0059] In the quantum dot structure of the semiconductor laser device 2 described above, the lateral barrier layer 55 is formed so as to bury the periphery of the island-like crystal (quantum dot) 52N into which the n-type dopant is directly introduced. In addition, the upper crystal layer 30 is formed so as to cover the upper end of the island-like crystal 52N and the lateral barrier layer 55. The band gap of the lateral barrier layer 55 is selected so that the material of the lateral barrier layer 55 has a band gap larger than that of the upper crystal layer 30, so that a large quantum confinement effect can be obtained while maintaining the current injection efficiency (carrier injection efficiency in the vertical direction) to the island-like crystal 52N. In addition, since the band gap of the lateral barrier layer 55 is larger than that of the island-like crystal 52N, the energy difference ΔE between the subband levels (between the ground level and the excited level) in the conduction band of the island-like crystal 52N can be increased. This allows the threshold current to be reduced by direct n-type doping while improving the temperature stability of the threshold current in the high temperature range. Furthermore, the upper crystal layer 30 has a layer region 30p into which a p-type dopant is introduced by modulated p-type doping, which can further suppress the temperature dependence of the threshold current.

[0060] FIG. 18A is a graph showing the measurement results of the PL (photoluminescence) spectrum at room temperature of two types of semiconductor laser devices actually fabricated. In the graph of FIG. 18A, the horizontal axis represents photon energy (unit: eV) corresponding to the wavelength, and the vertical axis represents PL intensity (unit: ×10 cps). Here, cps means counts per second. One of the two types of semiconductor laser devices is Example 1 having a structure basically the same as that of the semiconductor laser device 2 shown in FIG. 17A and FIG. 17B. In Example 1, direct n-type doping and modulated p-type doping (co-doping) are performed. The other is Example 2 having the same structure as Example 1, except that direct n-type doping is not performed. In Example 2, modulated p-type doping is performed.

[0061] Fig. 18B is a schematic diagram for explaining the structure of the semiconductor laser device of Example 1. As shown in Fig. 18B, the structure of Example 1 includes a single crystal substrate 11 made of an n-type GaAs substrate, a lower cladding layer 12 made of an n-type AlGaAs layer, a base crystal layer 30C made of an i-type GaAs layer, a structure in which ten active layers 22 are laminated, an i-type GaAs layer 14I, an upper cladding layer 14 made of a p-type AlGaAs layer, and a contact layer 15 made of a p-type GaAs layer. Each of the active layers 22 includes a large number of island-like crystals (quantum dots) 52N each made of n-type InAs, and Al 0.2 Ga 0.8 The island crystal 52N includes a lateral barrier layer (LPBL) 55 made of an As layer, and an upper crystal layer 30 made of a GaAs layer covering both the upper end of the island crystal 52N and the lateral barrier layer 55. The upper crystal layer 30 includes a layer region 30p made of a p-type GaAs layer into which a p-type dopant (Be) has been introduced by modulated p-type doping, and crystal layers 30u and 30m made of i-type GaAs layers sandwiching the layer region 30p. An n-type dopant (Si) has been introduced into the island crystal (quantum dot) 52N by direct n-type doping. The concentration of the n-type dopant is 0.5×10 11 cm -2 This corresponds to one dopant atom per quantum dot. In addition, Be is introduced into the layer region 30p as a p-type dopant. The concentration of the p-type dopant is 2.0×10 11 cm -2 which corresponds to four dopant atoms per quantum dot.

[0062] As shown in FIG. 18A, it was confirmed that Example 1, in which co-doping (modulated p-type doping and direct n-type doping) was performed, had a light emission intensity about 1.2 times higher than Example 2, in which direct n-type doping was not performed.

[0063] Although various embodiments and their modifications have been described above, the above embodiments and their modifications are merely illustrative and do not limit the scope of the present invention. It should be understood that modifications, additions, and improvements to the above embodiments can be made as appropriate without departing from the spirit and scope of the present invention. The scope of the present invention should be interpreted based on the description of the claims, and should be understood to include equivalents thereof. [Industrial Applicability]

[0064] A semiconductor laser device according to the present disclosure can be used in optical devices such as optical communication devices and optical integrated circuits. [Explanation of symbols]

[0065] 1, 1B, 2: semiconductor laser device, 10: lower electrode, 11: single crystal substrate, 12: lower cladding layer, 13, 13B: active layer structure, 14: upper cladding layer, 15: contact layer, 16: upper electrode, 17, 18: resonator end facets, 20, 21, 22: active layer, 30: upper crystal layer, 30B, 30C: underlying crystal layer, 40, 41, 42, 43: quantum dot structure, 51: wetting layer, 52: island-shaped crystal, 52t: upper end, 52s: side, 52b: lower end (base end), 53, 54, 55, 56: lateral barrier layers.

Claims

1. 1. A semiconductor laser device, comprising: An active layer structure including a single or multiple active layers, each of the active layers includes one or more quantum dot structures; Each of the quantum dot structures comprises: Island crystals; a lateral barrier layer having a first bandgap and at least partially embedded around the island; an upper crystal layer having a second band gap and covering the upper end of the island and the side barrier layer; Including, the first band gap is larger than the second band gap; 1. A semiconductor laser device comprising:

2. 2. A semiconductor laser device according to claim 1, wherein an energy difference between a ground level and an excitation level in the conduction band of the island crystal is larger than an energy difference between a ground level and an excitation level that would occur in the conduction band of the island crystal if a side surface of the island crystal were to be bonded to the upper crystal layer without being bonded to the lateral barrier layer.

3. 2. The semiconductor laser device according to claim 1, the lateral barrier layer is embedded around the island-like crystal to a position lower than the height position of the upper end of the island-like crystal, The upper crystal layer is joined to the upper end of the island-shaped crystal.

4. 2. The semiconductor laser device according to claim 1, wherein the lateral barrier layer is embedded around the island-shaped crystal up to a height position of the upper end of the island-shaped crystal.

5. 5. The semiconductor laser device according to claim 4, wherein the upper crystal layer is joined to the upper end of the island-shaped crystal.

6. 2. The semiconductor laser device according to claim 1, wherein the upper crystal layer includes a layer region into which a first conductivity type dopant is introduced by modulation doping.

7. 7. The semiconductor laser device according to claim 6, wherein the first conductivity type dopant is a p-type dopant.

8. 2. The semiconductor laser device according to claim 1, wherein a second conductive type dopant is introduced into said island crystal by direct doping.

9. 9. The semiconductor laser device according to claim 8, wherein the upper crystal layer includes a layer region into which a first conductivity type dopant is introduced by modulation doping.

10. 10. The semiconductor laser device according to claim 9, wherein the first conductivity type dopant is a p-type dopant, and the second conductivity type dopant is an n-type dopant.

11. 11. The semiconductor laser device according to claim 1, wherein the island crystal, the upper crystal layer and the lateral barrier layer are each made of a III-V compound semiconductor material.

12. 12. The semiconductor laser device according to claim 11, the island crystals are made of an InAs-based material; The upper crystal layer is made of one material selected from the group consisting of GaAs-based materials, InP-based materials, and InGaAs-based materials.

13. 12. The semiconductor laser device according to claim 11, wherein the lateral barrier layer is made of one material selected from the group consisting of an AlGaAs-based material, an AlAs-based material, an InAlAs-based material, and an InGaAlAs-based material.

14. 11. The semiconductor laser device according to claim 1, comprising a resonator structure for a Fabry-Perot type laser.

15. 11. The semiconductor laser device according to claim 1, comprising a diffraction grating structure for a distributed feedback laser or a diffraction grating structure for a distributed Bragg reflector laser.

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