Vertical-cavity surface-emitting laser

Abstract

To provide a vertical cavity surface-emitting laser comprising an active layer having high photoluminescence intensity.SOLUTION: A vertical cavity surface-emitting laser for emitting laser light having a wavelength of 830 nm to 910 nm includes a substrate having a main surface including GaAs, a first distributed Bragg reflector, an active layer, and a second distributed Bragg reflector. The substrate, the first distributed Bragg reflector, the active layer, and the second distributed Bragg reflector are arranged along a first axis direction intersecting the main surface. The main surface has an off angle of 6° or more with respect to the (100) plane. The active layer includes InxAlyGa1-x-yAs (0<x<1, 0≤y<1). The active layer has a strain. The absolute value of the strain is 0.5% or more and 1.4% or less.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a vertical cavity surface emitting laser.

Background Art

[0002] Patent Documents 1 and 2 disclose a vertical cavity surface emitting laser having a lower reflector, an active layer, and an upper reflector on a substrate. In Patent Document 1, the active layer has a GaInP / AlGaInP multiple quantum well structure. In Patent Document 2, the active layer has a triple quantum well structure composed of GaInAsP / Al ,

[0004] , , y ,

[0006] , , , ,<00001​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​The present disclosure provides a vertical cavity surface emitting laser having an active layer with high photoluminescence intensity.

Means for Solving the Problems

[0007] A vertical cavity surface emitting laser according to one aspect of the present disclosure is a vertical cavity surface emitting laser for emitting laser light having a wavelength of 830 nm or more and 910 nm or less, and includes a substrate having a main surface containing GaAs, a first distributed Bragg reflector, an active layer, and a second distributed Bragg reflector. The substrate, the first distributed Bragg reflector, the active layer, and the second distributed Bragg reflector are arranged along a direction of a first axis intersecting the main surface. The main surface has an off-angle of 6° or more with respect to the (100) plane, and the active layer contains In x Al y Ga 1-x-y As (0 < x < 1, 0 ≤ y < 1), the active layer has strain, and an absolute value of the strain is 0.5% or more and 1.4% or less.

Effects of the Invention

[0008] According to the present disclosure, a vertical cavity surface emitting laser having an active layer with high photoluminescence intensity is provided.

Brief Description of the Drawings

[0009] [Figure 1] FIG. 1 is a plan view schematically showing a vertical cavity surface emitting laser according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1. [Figure 3] FIG. 3 is a cross-sectional view of a current confinement layer of a vertical cavity surface emitting laser according to an embodiment. [Figure 4] FIG. 4 is a cross-sectional view of an active layer of a vertical cavity surface emitting laser according to an embodiment. [Figure 5] FIG. 5 is a graph showing an example of the relationship between the In composition and the absolute value of strain. [Figure 6] FIG. 6 is a graph showing an example of the relationship between the off-angle and the PL intensity. [Figure 7] FIG. 7 is a graph showing an example of the relationship between the off-angle and the RMS value of the surface roughness. [Figure 8] FIG. 8 is a diagram showing an example of a transmission electron microscope image of the active layer when the off-angle is 2°. [Figure 9] FIG. 9 is a diagram showing an example of a transmission electron microscope image of the active layer when the off-angle is 15°. [Figure 10] FIG. 10 is a graph showing an example of the relationship between the absolute value of the strain and the relaxation oscillation frequency. [Figure 11] FIG. 11 is a graph showing an example of the relationship between the off-angle and the film thickness fluctuation. BEST MODE FOR CARRYING OUT THE INVENTION

[0010] [Description of Embodiments of the Present Disclosure] A vertical resonance surface emitting laser according to an embodiment is a vertical resonance surface emitting laser for emitting laser light having a wavelength of 830 nm or more and 910 nm or less, and includes a substrate having a main surface containing GaAs, a first distributed Bragg reflector, an active layer, and a second distributed Bragg reflector. The substrate, the first distributed Bragg reflector, the active layer, and the second distributed Bragg reflector are arranged along the direction of a first axis intersecting the main surface. The main surface has an off-angle of 6° or more with respect to the (100) plane, and the active layer contains In x Al y Ga 1-x-y As (0 < x < 1, 0 ≤ y < 1), the active layer has strain, and the absolute value of the strain is 0.5% or more and 1.4% or less.

[0011] According to the above vertical resonance surface emitting laser, even if the absolute value of the strain in the active layer increases, since the off-angle is large, the active layer has a high photoluminescence intensity.

[0012] The off-angle may be 10° or more. In this case, the surface roughness of the interface in the active layer becomes small.

[0013] The vertical-cavity surface-emitting laser may further include a current confinement layer disposed between the active layer and the second distributed Bragg reflector, the current confinement layer including an aperture portion and an oxide portion surrounding the aperture portion, and the current confinement layer may have a uniaxially symmetric shape or an asymmetric shape in a cross section perpendicular to the first axis. In this case, the aperture portion may have an asymmetric shape in the cross section.

[0014] The active layer has a thickness of 3×10 16 cm -3 The carbon concentration may be as follows: In an active layer provided on a substrate with a large off-angle, defects due to As vacancy are less likely to occur. In this case, a high carbon concentration in the active layer to fill defects due to As vacancy is not required. Good laser characteristics can be obtained while using an active layer with a low carbon concentration.

[0015] The absolute value of the strain may be 1.1% or more and 1.3% or less, in which case a higher relaxation oscillation frequency can be obtained.

[0016] The off-angle may be 26° or less, which can improve the crystallinity.

[0017] [Details of the embodiments of the present disclosure] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same or equivalent elements are designated by the same reference numerals, and duplicate explanations will be omitted. X, Y, and Z coordinate axes are shown in the drawings as necessary. The X-axis direction, Y-axis direction, and Z-axis direction intersect (for example, are perpendicular to) each other.

[0018] FIG. 1 is a plan view schematically illustrating a vertical-cavity surface-emitting laser according to one embodiment. FIG. 2 is a cross-sectional view taken along line II-II in FIG. 1. A vertical-cavity surface-emitting laser (VCSEL) 10 illustrated in FIGS. 1 and 2 can emit laser light L. The laser light L has a wavelength of 830 nm or more and 910 nm or less. The vertical-cavity surface-emitting laser 10 includes a substrate 12 having a primary surface 12a made of GaAs, a first distributed Bragg reflector 14, an active layer 20, and a second distributed Bragg reflector 24. The substrate 12, the first distributed Bragg reflector 14, the active layer 20, and the second distributed Bragg reflector 24 are arranged along a first axis Ax1 (e.g., the Z-axis direction) that intersects (e.g., is perpendicular to) the primary surface 12a.

[0019] The substrate 12 may be a GaAs substrate. The substrate 12 may include a base substrate and a GaAs layer provided on the base substrate. In this case, the upper surface of the GaAs layer is the primary surface 12a.

[0020] The main surface 12a of the substrate 12 has an off angle of 6° or more with respect to the (100) plane. The off angle may be 10° or more, or 15° or more. The off angle may be 26° or less, 25.2° or less, or 25° or less. The normal vector (positive direction of the Z-axis) of the main surface 12a is, for example, the normal vector of the (100) plane. <100> From the direction <111> Alternatively, the direction may be inclined by an off angle. <111> The direction projected onto the XY plane may be the negative direction of the Y axis.

[0021] The first distributed Bragg reflector 14 has a semiconductor laminate structure. The semiconductor laminate structure may have a first conductivity type (e.g., n-type). The semiconductor laminate structure includes first and second semiconductor layers alternately arranged in the Z-axis direction. The first and second semiconductor layers have different refractive indices. The first semiconductor layer may be a GaAs layer. The second semiconductor layer may be an AlGaAs layer. The first distributed Bragg reflector 14 may include a first lower distributed Bragg reflector 14a and a second lower distributed Bragg reflector 14b. The first lower distributed Bragg reflector 14a is disposed between the substrate 12 and the second lower distributed Bragg reflector 14b. The first lower distributed Bragg reflector 14a may have, for example, an i-type semiconductor laminate structure. In this case, the second lower distributed Bragg reflector 14b has a first conductivity type (e.g., n-type) semiconductor laminate structure. A semiconductor layer of a first conductivity type (for example, n-type) may be provided between the first lower distributed Bragg reflector 14a and the second lower distributed Bragg reflector 14b.

[0022] The second distributed Bragg reflector 24 has a semiconductor laminate structure of a second conductivity type (e.g., p-type). The semiconductor laminate structure includes third and fourth semiconductor layers alternately arranged in the Z-axis direction. The third and fourth semiconductor layers have refractive indices different from each other. The third semiconductor layer may be a GaAs layer. The fourth semiconductor layer may be an AlGaAs layer. The dopant for imparting p-type conductivity is, for example, carbon. The carbon concentration doped into the layers of the second distributed Bragg reflector 24 is 1×10 18 cm -3 From 1×10 19 cm -3 may be in the range of

[0023] The vertical-cavity surface-emitting laser 10 may further include a current confinement layer 26 disposed between the active layer 20 and the second distributed Bragg reflector 24. The current confinement layer 26 includes an aperture portion 26a and an oxide portion 26b surrounding the aperture portion 26a. The first axis Ax1 passes through the aperture portion 26a. The aperture portion 26a may include a III-V compound semiconductor containing aluminum as a group III element. The aperture portion 26a includes, for example, AlGaAs. The aperture portion 26a has a second conductivity type (p-type) and includes, for example, carbon as a p-type dopant. The carbon concentration in the aperture portion 26a is, for example, 5×10 18 cm -3 The oxidized portion 26b contains, for example, aluminum oxide. In this case, the oxidized portion 26b can be formed by oxidizing a III-V compound semiconductor containing aluminum using, for example, water. A semiconductor layer 24a of a second conductivity type (for example, p-type) may be disposed between the current confinement layer 26 and the active layer 20. The semiconductor layer 24a may be a GaAs layer. The carbon concentration of the semiconductor layer 24a is, for example, 1×10 18 cm -3 is.

[0024] A spacer layer 18 may be disposed between the active layer 20 and the first distributed Bragg reflector 14. The spacer layer 18 may include GaAs or AlGaAs. A spacer layer 22 may be disposed between the active layer 20 and the current confinement layer 26. The spacer layer 22 may be disposed between the active layer 20 and the semiconductor layer 24a. The spacer layer 22 may include GaAs or AlGaAs. The spacer layer 22 may have a second conductivity type (p-type) or may have no conductivity. When the spacer layer 22 has p-type conductivity, the carbon concentration in the spacer layer 22 may be, for example, 1×10 18 cm -3 is.

[0025] The vertical-cavity surface-emitting laser 10 may have a post PS provided on the primary surface 12a of the substrate 12. The first axis Ax1 passes through the post PS. The post PS includes a second lower distributed Bragg reflector 14b, a spacer layer 18, an active layer 20, a spacer layer 22, a semiconductor layer 24a, a current-confining layer 26, and a second distributed Bragg reflector 24. For example, a trench TR is provided around the post PS. The bottom of the trench TR reaches the first lower distributed Bragg reflector 14a.

[0026] An insulating layer 50 may be provided on the top surface and side surfaces of the post PS. The insulating layer 50 has an opening 50a on the top surface of the post PS. A first electrode 30 electrically connected to the second distributed Bragg reflector 24 may be provided within the opening 50a. The first electrode 30 has a ring shape surrounding the first axis Ax1. The first electrode 30 is connected to the pad electrode 34 by a wiring conductor 32. A second electrode 40 electrically connected to the first distributed Bragg reflector 14 may be provided at the bottom of the trench TR. The second electrode 40 is connected to the pad electrode 44 by the wiring conductor 42. A bias power supply 60 may be electrically connected to the first electrode 30 and the second electrode 40. Specifically, the bias power supply 60 is connected to the pad electrode 34 and the pad electrode 44 by wiring. The bias power supply 60 can apply a voltage between the first electrode 30 and the second electrode 40. When a voltage is applied between the first electrode 30 and the second electrode 40, a bias current is supplied to the active layer 20 through the current confinement layer 26. As a result, laser light L is emitted in the Z-axis direction.

[0027] FIG. 3 is a cross-sectional view of a current confinement layer of a vertical-cavity surface-emitting laser according to one embodiment. As shown in FIG. 3, in this embodiment, the current confinement layer 26 has a uniaxially symmetric shape in a cross section (XY cross section) perpendicular to the Z-axis direction. A uniaxially symmetric shape is a shape that is symmetric with respect to a single axis. The current confinement layer 26 has a shape that is symmetric only with respect to a second axis Ax2 that is perpendicular to the first axis Ax1. The second axis Ax2 extends along the X-axis direction. In the XY cross section, the current confinement layer 26 has a shape that is asymmetric with respect to all axes other than the second axis Ax2. On the second axis Ax2, the distance from the first axis Ax1 to one edge is greater than the distance from the first axis Ax1 to the other edge. In the XY cross section, the current confinement layer 26 has a shape that includes, for example, an arc portion centered on the first axis Ax1 and a straight portion extending in the Y-axis direction. The central angle of the arc portion is, for example, greater than 180°. The length of the straight portion is, for example, shorter than the diameter of the arc portion and longer than the radius of the arc portion. The post PS also has the same shape as the current confinement layer 26 in the XY cross section.

[0028] The aperture portion 26a has an asymmetric shape in the XY cross section. The aperture portion 26a has an asymmetric shape with respect to all axes in the XY cross section. The maximum length of the aperture portion 26a in the Y-axis direction is greater than the maximum length in the X-axis direction. The aperture portion 26a has, for example, a triangular shape with three sides of different lengths. The longest side extends along the Y-axis direction. Each vertex of the triangle may be rounded. Because the major surface 12a of the substrate 12 has an off-angle, the rate of progression of the oxidation process from the edge of the current confinement layer 26 toward the first axis Ax1 in the XY cross section varies depending on the progression direction. Therefore, the aperture portion 26a has an asymmetric shape in the XY cross section. For example, the rate of progression of the oxidation process in the positive Y-axis direction is greater than the rate of progression of the oxidation process in the negative Y-axis direction.

[0029] The shape and area of the aperture portion 26a can be adjusted by, for example, the area of the current constriction layer 26 in the XY cross-section, the oxidation time when forming the oxidation portion 26b, the off-angle of the main surface 12a of the substrate 12, and the like. When the area of the current constriction layer 26 in the XY cross-section is small, the shape of the aperture portion 26a approaches the triangular shape in FIG. 3. When the area of the current constriction layer 26 in the XY cross-section is large, the shape of the aperture portion 26a approaches the pentagonal shape. When the area of the current constriction layer 26 in the XY cross-section is even larger, the shape of the aperture portion 26a approaches the hexagonal shape. The shape of the aperture portion 26a may be other polygonal shapes or may be a shape including a curve. The area of the aperture portion 26a becomes smaller, for example, when the oxidation time when forming the oxidation portion 26b is shortened. The area of the aperture portion 26a in the XY cross-section is, for example, 7 μm 2 or more and 100 μm 2 or less or 12 μm 2 or more and 100 μm 2 or less.

[0030] The current constriction layer 26 may have an asymmetric shape in the XY cross-section. In this case, the current constriction layer 26 has a shape that is asymmetric with respect to all axes in the XY cross-section. The current constriction layer 26 may have a biaxially symmetric shape in the XY cross-section. In this case, the current constriction layer 26 has, for example, a circular shape or a rectangular shape in the XY cross-section.

[0031] FIG. 4 is a cross-sectional view of an active layer of a vertical resonance surface-emitting laser according to an embodiment. The active layer 20 may have a quantum well structure including a well layer 20a and a barrier layer 2Ob. The well layer 20a and the barrier layer 20b are alternately arranged along the direction of the first axis Ax1. The active layer 20 may have a multiple quantum well structure including a plurality of well layers 20a.

[0032] The active layer 20 contains In x Al y Ga 1-x-y As (0 <x <1, 0 ≤ y <1). The active layer 20 may not contain nitrogen. In this embodiment, the well layer 20a is In x Al y Ga 1-x-yIt includes As(0 < x < 1, 0 ≤ y < 1). By changing the Al composition y, the wavelength of the laser beam L can be adjusted. The barrier layer 20b may include GaAs or AlGaAs.

[0033] The active layer 20 is 0 cm -3 super 3×10 16 cm -3 It may have the following carbon concentration. The carbon concentration can be measured by secondary ion mass spectrometry (SIMS). The carbon concentration may be an average value over the thickness of the active layer 20.

[0034] The active layer 20 has strain. In this embodiment, the well layer 20a has strain. The absolute value of the strain is 0.5% or more and 1.4% or less. The absolute value of the strain may be 1.1% or more and 1.3% or less. The absolute value of the strain may be 1.0% or more. The absolute value of the strain increases with an increase in the In composition x in the well layer 20a. Let the lattice constant of the material of the main surface 12a of the substrate 12 be a0, and the lattice constant of the material of the well layer 20a be a w When this is done, the strain ε w of the well layer 20a with respect to the main surface 12a of the substrate 12 is calculated by the following formula (1). a w is larger than a0, so the strain ε w becomes compressive strain in the XY plane. ε w =(a0 - a w ) / a w ×100 (%)… (1)

[0035] For example, the lattice constant a0 of GaAs is 5.65 angstroms. In x Ga 1-x As (x = 0.07) has a lattice constant a w of 5.68 angstroms. In this case, the strain ε w is -0.5%. Also, In x Ga 1-x As (x = 0.14) has a lattice constant a w of 5.71 angstroms. In this case, the strain ε w is -1.0%. Also, Inx Ga 1-x The lattice constant a of GaAs(x = 0.154) w is 5.72 angstroms. In this case, the strain ε w is -1.1%.

[0036] FIG. 5 is a graph showing an example of the relationship between the In composition and the absolute value of the strain. As shown in FIG. 5, for In x Ga 1-x As layers (0 < x < 1) formed on a GaAs substrate, the absolute value of the strain linearly increases with an increase in the In composition x. When the In composition x is 0.07 or more and 0.14 or less, the absolute value of the strain is 0.5% or more and 1.0% or less. When the In composition x is 0.07 or more and 0.196 or less, the absolute value of the strain is 0.5% or more and 1.4% or less. When the In composition x is 0.154 or more and 0.182 or less, the absolute value of the strain is 1.1% or more and 1.3% or less.

[0037] Generally, when the absolute value of the strain in the active layer increases, defects for relaxing the strain are formed in the active layer. Due to the defects, the photoluminescence intensity of the active layer decreases. On the other hand, according to the above-described vertical resonance surface-emitting laser 10, even when the absolute value of the strain in the active layer 20 increases, the active layer 20 has a high photoluminescence (PL) intensity. The PL intensity of the active layer 20 increases as the off-angle of the main surface 12a of the substrate 12 increases. The reason is considered as follows. When the off-angle increases, the As site density on the main surface 12a of the substrate 12 decreases. For example, when the off-angle is increased from 2° to 15°, theoretically the As site density decreases to about 1 / 3. Theoretically, when the off-angle is 25.2° (i.e., the main surface 12a is the (311) plane), the As site density is minimized. Since As causes defects due to As vacancies, when the As site density decreases, the defects in the semiconductor layer formed on the main surface 12a of the substrate 12 decrease. As a result, the defects in the active layer 20 also decrease. Also, when the off-angle increases, the trap level density of the semiconductor layer formed on the main surface 12a of the substrate 12 decreases. As a result, the trap level density of the active layer 20 also decreases. For example, when the off-angle is increased from 2° to 15°, the trap level density of the AlGaAs layer formed on the GaAs substrate decreases to about 1 / 10. The trap level density can be measured by the DLTS (Deep Level Transient Spectroscopy) method.

[0038] FIG. 6 is a graph showing an example of the relationship between the off-angle and the PL intensity. As shown in FIG. 6, when the off-angle of the main surface of the GaAs substrate is 6° or more, the PL intensity of the In x Ga 1-x As layer (0 < x < 1) becomes remarkably high at 4.0 or more. The GaAs substrate is an example of the substrate 12. In x Ga 1-x As layer (0 < x < 1) is an example of the well layer 20a. In this example, the absolute value of the strain of the In x Ga 1-x As layer (0 < x < 1) formed on the GaAs substrate is 0.7%. The PL intensity is measured for light emission having a wavelength from 830 nm to 840 nm.

[0039] When carbon is contained in a III-V compound semiconductor, carbon atoms are coordinated to defects due to As vacancies, reducing the defect density. Carbon doped at a high concentration into a III-V compound semiconductor is effective in filling As vacancy defects. On the other hand, if the carbon concentration is high in the active layer, light absorption by carbon deteriorates the laser characteristics of the surface-emitting laser, such as threshold or slope efficiency. In an active layer 20 provided on a substrate 12 with a large off-angle, As vacancy defects are less likely to occur. Therefore, there is no need to fill As vacancy defects with carbon, and the active layer 20 can be formed with a 3×10 16 cm -3 The carbon concentration can be as follows: Therefore, the vertical-cavity surface-emitting laser 10 including the substrate 12 with a large off-angle maintains a high PL intensity of the active layer 20 and has good laser characteristics.

[0040] Furthermore, when the off-angle of the primary surface 12a of the substrate 12 is 6° or more, the surface roughness at the interface between the well layer 20a and the barrier layer 20b is smaller than when the off-angle is 2°. The surface roughness may be evaluated by the root mean square (RMS) value. The RSM value of the surface roughness can be measured using an atomic force microscope (AFM). When the surface roughness at the interface between the well layer 20a and the barrier layer 20b is small, the half-width of the photoluminescence spectrum of the well layer 20a can be reduced.

[0041] 7 is a graph showing an example of the relationship between the off-angle and the RMS value of surface roughness. As shown in FIG. 7, when the off-angle of the primary surface of the GaAs substrate is 6° or more, the RMS value of the surface roughness of the GaAs layer is small, at 0.4 nm or less. When the off-angle of the primary surface of the GaAs substrate is 10° or more, the RMS value of the surface roughness is significantly small, at 0.3 nm or less or 0.2 nm or less. The GaAs substrate is an example of the substrate 12. The GaAs layer is an example of the barrier layer 20b.

[0042] In addition, when the off-angle of the main surface of the GaAs substrate is 6° or more, the step density on the surface of the GaAs layer becomes as large as 30 steps / μm or more. When the off-angle of the main surface of the GaAs substrate is 10° or more, the step density on the surface of the GaAs layer becomes as large as 60 steps / μm or more. The step density can be measured by AFM.

[0043] FIG. 8 is a diagram showing an example of a transmission electron microscope image of an active layer when the off-angle is 2°. FIG. 9 is a diagram showing an example of a transmission electron microscope image of an active layer when the off-angle is 15°. FIGS. 8 and 9 show examples of cross-sections of well layers 20a and barrier layers 20b alternately arranged along the first axis Ax1. In FIG. 8, a region R1 including the interface between the In x Ga 1-x As layer (0 < x < 1) and the GaAs layer is shown. In FIG. 9, a region R2 including the interface between the In x Ga 1-x As layer (0 < x < 1) and the GaAs layer is shown. As shown in FIGS. 8 and 9, the interface of region R1 is rougher than the interface of region R2. Therefore, it can be seen that as the off-angle of the main surface of the GaAs substrate increases, the interface between the In x Ga 1-x As layer (0 < x < 1) and the GaAs layer becomes smoother.

[0044] Furthermore, the following experiment was conducted. The present disclosure is not limited to the following experiment.

[0045] Six vertical resonant cavity surface emitting lasers having the same structure except that the absolute value of the strain of the active layer is different were prepared. The well layer is an In x Ga 1-x As layer (0 < x < 1) formed on a GaAs substrate. The barrier layer is an Al y Ga 1-y As layer (0 < y < 1). The off-angle of the main surface of the GaAs substrate is 15°. The absolute values of the strain are 0.6%, 0.9%, 1.1%, 1.2%, 1.4% and 1.7% respectively. By measuring the frequency response of the optical output from each vertical resonant cavity surface emitting laser, the relaxation oscillation frequency (fr) was obtained. The relaxation oscillation frequency is the measured value at a temperature of 25°C and a bias current of 9 mA.

[0046] FIG. 10 is a graph showing an example of the relationship between the absolute value of strain and the relaxation oscillation frequency. As shown in FIG. 10, when the absolute value of strain is 0.5% or more and 1.4% or less, the relaxation oscillation frequency becomes large, for example, 16 GHz or more. When the absolute value of strain is 1.1% or more and 1.3% or less, the relaxation oscillation frequency becomes large, for example, 18 GHz or more. When the absolute value of strain exceeds 1.4%, the relaxation oscillation frequency becomes small. This is considered to be because the quality of the active layer deteriorates due to an increase in the absolute value of strain. The deterioration of the quality of the active layer causes an increase in the laser threshold and a decrease in the slope efficiency, so the relaxation oscillation frequency becomes small.

[0047] Next, a plurality of multiple quantum well structures having the same structure except that the width w of the well layer is different were prepared. The well layer is an In x Ga 1-x As layer (0 <x <1). The width w of the well layer was changed within the range of 2 nm to 10 nm. The barrier layer is an Al y Ga 1-y As layer (0 <y <1). Photoluminescence measurement was performed for each multiple quantum well structure. The full width at half maximum FWHM(w) of the photoluminescence spectrum is a function of the width w of the well layer. The peak wavelength λ(w) of the photoluminescence spectrum is a function of the width w of the well layer. Using the following formula (1), the film thickness fluctuation Δw (nm) and the composition fluctuation Δc (nm) can be obtained from the full width at half maximum FWHM(w) and the peak wavelength λ(w).

Equation

[0048] The decrease in the film thickness fluctuation Δw means a decrease in the surface roughness at the interface between the well layer and the barrier layer. Such an interface quality evaluation method using the film thickness fluctuation Δw is described in, for example, Non-Patent Documents 1, 2, etc. below. Non-patent document 1: Hiroyuki SAKAKI et al., "One Atomic Layer HeterointerfaceFluctuations in GaAs-AlAs Quantum Well Structures and Their Suppression byInsertion of Smoothing Period in Molecular Beam Epitaxy", Japanese Journal of Applied Physics, Vol.24, No.6, pp. L417-L420, June 1985 Non-patent document 2: Satoshi SHIMOMURA et al., "Extremely Flat Interfaces inGaAs / AlGaAs Quantum Wells Grown on GaAs (411)A Substrate by Molecular Beam Epitaxy", Japanese Journal of Applied Physics, Vol.32, Part 2, No.12A, pp.L1728-L1731, 1 December 1993

[0049] Furthermore, five identical multiple quantum well structures were prepared, each with a different off-angle. The off-angles of the GaAs substrate primary surface were 2°, 15°, 19.5°, 25.2°, and 35.3°, respectively. The film thickness fluctuation Δw was calculated for each multiple quantum well structure using the method described above.

[0050] Fig. 11 is a graph showing an example of the relationship between the off-angle and the film thickness fluctuation. As shown in Fig. 11, the film thickness fluctuation Δw decreases as the off-angle increases. Therefore, as the off-angle increases, the surface roughness at the interface between the well layer and the barrier layer decreases.

[0051] On the other hand, if the off-angle exceeds 26°, Ga sites become more dominant than As sites on the primary surface of the GaAs substrate. As a result, surface segregation of In, which substitutes for Ga sites on the surface of the growing well layer, becomes significant. Furthermore, if the off-angle exceeds 26°, hillock-like surface defects increase. Therefore, an off-angle of 26° or less can improve the crystallinity of the semiconductor layer formed on the GaAs substrate.

[0052] Although the preferred embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the above embodiments.

[0053] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above meaning, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0054] 10...Vertical-cavity surface-emitting laser 12... Circuit board 12a…main surface 14...first distributed Bragg reflector 14a…First lower distributed Bragg reflector 14b…Second lower distributed Bragg reflector 18...Spacer layer 20…Active layer 20a...Well layer 20b...Barrier layer 22...Spacer layer 24...Second distributed Bragg reflector 24a...Semiconductor layer 26…Current confinement layer 26a...Aperture section 26b...oxidized part 30…1st electrode 32...Wiring conductor 34...Pad electrode 40…Second electrode 42...Wiring conductor 44...Pad electrode 50...insulating layer 50a…Aperture 60...Bias power supply Ax1…1st axis Ax2…2nd axis L...laser light PS…Post R1...area R2…area TR...Trench

Claims

1. A vertical-cavity surface-emitting laser for emitting laser light having a wavelength of 830 nm or more and 910 nm or less, a substrate having a primary surface including GaAs; a first distributed Bragg reflector; an active layer; a second distributed Bragg reflector; Equipped with the substrate, the first distributed Bragg reflector, the active layer, and the second distributed Bragg reflector are arranged along a direction of a first axis intersecting the major surface; the main surface has an off angle of 6° or more with respect to the (100) plane, The active layer is In ｘ Al ｙ Ga １－ｘ－ｙ As (0<x<1, 0≦y<1), the active layer has strain; The absolute value of the distortion is 1.1% or more and 1.3% or less.

2. 2. The vertical-cavity surface-emitting laser according to claim 1, wherein the off-angle is 10 degrees or more.

3. a current confinement layer disposed between the active layer and the second distributed Bragg reflector; the current confinement layer includes an aperture portion and an oxidation portion surrounding the aperture portion, 3. The vertical-cavity surface-emitting laser according to claim 1, wherein the current confinement layer has a uniaxially symmetric or asymmetric shape in a cross section perpendicular to the direction of the first axis.

4. The active layer has a thickness of 3×10 １６ cm －３ 4. The vertical-cavity surface-emitting laser according to claim 1, having a carbon concentration of:

5. 5. The vertical-cavity surface-emitting laser according to claim 1, wherein the off-angle is 26 degrees or less.

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