JP5553035B2 - Gallium nitride semiconductor laser device - Google Patents

Description

  The present invention relates to a gallium nitride based semiconductor laser device.

  Conventionally, there has been known a semiconductor laser device in which an n-type cladding layer, an n-type light guide layer, an active layer, an electron block layer, a p-type light guide layer, and a p-type cladding layer are laminated in this order on a semiconductor substrate ( For example, see Non-Patent Document 1 below). The active layer of the semiconductor laser device described in Non-Patent Document 1 has a multiple quantum well structure (MQW structure) in which well layers and barrier layers are alternately stacked.

Journal of Applied physics, 107, 023101 (2010)

  By the way, the emission characteristics of a gallium nitride based semiconductor laser device having an active layer in which a plurality of well layers are stacked via a barrier layer are different from those of a gallium nitride based semiconductor laser device having an active layer consisting of a single well layer. Tend to be better. However, in a gallium nitride based semiconductor laser device having an active layer in which a plurality of well layers are stacked via a barrier layer, the well is compared with a gallium nitride based semiconductor laser device having an active layer composed of a single well layer. Since the band offset between the layer and the barrier layer is large, a large voltage drop (for example, a voltage drop of 3 V or more) occurs, and the operating voltage for obtaining a predetermined amount of current tends to increase. Therefore, it is required to reduce the operating voltage for a gallium nitride based semiconductor laser device having such an active layer.

  The present invention is intended to solve the above-described problem, and a gallium nitride based semiconductor laser device capable of reducing an operating voltage while having an active layer in which a plurality of well layers are stacked via a barrier layer. The purpose is to provide.

  As a result of intensive studies to solve the above problems, the present inventor has formed a quantum well structure of a well layer / barrier layer / well layer capable of generating light of 510 to 550 nm on a semipolar plane of a substrate. In the element having the quantum well structure, it is easy to form a band structure having excellent carrier mobility as compared with the element having the quantum well structure on the polar surface of the substrate via the light guide layer. We found that it was easy to reduce.

  Furthermore, the present inventor adjusted the band gap of the barrier layer to be smaller than the band gap of the light guide layer in the quantum well structure disposed on the semipolar plane of the substrate via the light guide layer. The present inventors have found that a band structure further excellent in carrier mobility can be formed and the operating voltage can be reduced.

  That is, a gallium nitride based semiconductor laser device according to the present invention has a substrate having a semipolar principal surface, a light guide layer disposed on the semipolar principal surface, and a quantum well structure capable of generating light of 510 to 550 nm. And a quantum well structure having a first well layer made of InGaN, and a barrier layer made of a gallium nitride based semiconductor and disposed on the first well layer. And a second well layer made of InGaN disposed on the barrier layer, and the band gap of the barrier layer is smaller than the band gap of the light guide layer.

  In the gallium nitride based semiconductor laser device according to the present invention, the well layer / barrier layer / well layer quantum well structure disposed on the semipolar main surface of the substrate via the light guide layer can generate light having a wavelength of 510 to 550 nm. The band gap of the barrier layer is smaller than the band gap of the light guide layer. As a result, a band structure having excellent carrier mobility is formed, and carrier movement is promoted. Therefore, in the gallium nitride based semiconductor laser device according to the present invention, an operating voltage for obtaining a predetermined amount (for example, 200 mA) of current is obtained while having an active layer in which a plurality of well layers are stacked via a barrier layer. Can be reduced.

  The semipolar main surface is preferably inclined in the m-axis direction of the gallium nitride semiconductor from the plane orthogonal to the c-axis of the gallium nitride semiconductor. In this case, the operating voltage can be further reduced.

  The inclination angle of the semipolar main surface in the m-axis direction is preferably 63 ° or more and less than 80 °. In this case, the operating voltage can be further reduced.

  The film thickness of at least one of the first well layer or the second well layer is preferably 1.0 to 4.0 nm. In this case, the operating voltage can be further reduced.

  In the present invention, at least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 1.12 eV and not more than 1.14 eV, and the barrier layer The film thickness is preferably 1.0 to 2.2 nm. In this case, a semiconductor laser element that can further reduce the operating voltage can be realized.

  In the present invention, at least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 1.07 eV and not more than 1.12 eV, and the barrier layer The film thickness is preferably 1.0 to 2.4 nm. In this case, a semiconductor laser element that can further reduce the operating voltage can be realized.

  In the present invention, at least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 1.02 eV and 1.07 eV or less, and the barrier layer The film thickness is preferably 1.0 to 2.9 nm. In this case, a semiconductor laser element that can further reduce the operating voltage can be realized.

  In the present invention, at least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 0.97 eV and not more than 1.02 eV, and the barrier layer The film thickness is preferably 1.0 to 3.3 nm. In this case, a semiconductor laser element that can further reduce the operating voltage can be realized.

  In the present invention, at least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 0.92 eV and not more than 0.97 eV, and the barrier layer The film thickness is preferably 1.0 to 4.1 nm. In this case, a semiconductor laser element that can further reduce the operating voltage can be realized.

  In the present invention, at least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 0.87 eV and not more than 0.92 eV, and the barrier layer The film thickness is preferably 1.0 to 4.8 nm. In this case, a semiconductor laser element that can further reduce the operating voltage can be realized.

  In the present invention, at least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 0.82 eV and not more than 0.87 eV, and the barrier layer The film thickness is preferably 1.0 to 6.5 nm. In this case, a semiconductor laser element that can further reduce the operating voltage can be realized.

  In the present invention, at least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is 0.77 eV or more and 0.82 eV or less, and the barrier layer The film thickness is preferably 1.0 to 8.3 nm. In this case, a semiconductor laser element that can further reduce the operating voltage can be realized.

  According to the present invention, a gallium nitride based semiconductor laser device capable of reducing an operating voltage for obtaining a predetermined amount of current while having an active layer in which a plurality of well layers are stacked via a barrier layer. Can be provided. According to the present invention, a gallium nitride capable of achieving an operating voltage equivalent to that of an element having an active layer composed of a single well layer while having an active layer in which a plurality of well layers are stacked via a barrier layer. A semiconductor laser device can be provided.

1 is a schematic cross-sectional view showing a gallium nitride based semiconductor laser device according to an embodiment of the present invention. FIG. 2 is a schematic diagram of an energy band in a part of the gallium nitride based semiconductor laser element shown in FIG. 1. It is drawing which shows the simulation result of an energy band. It is drawing which shows an electric current-voltage characteristic. It is drawing which shows the relationship between an operating voltage and In composition. It is drawing which shows the relationship between the allowable film thickness of a barrier layer, and In composition. It is drawing which shows the relationship between the band gap difference of a well layer and a barrier layer, and In composition of a barrier layer.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of a gallium nitride based semiconductor laser device according to the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a schematic cross-sectional view showing a gallium nitride based semiconductor laser device according to this embodiment. A semiconductor laser (gallium nitride based semiconductor laser element) 1 according to this embodiment includes a semiconductor substrate 10, a semiconductor region 20, an active layer 30, and a semiconductor region 40.

  The semiconductor substrate 10 is a gallium nitride based semiconductor substrate such as a GaN substrate, for example. The semiconductor substrate 10 has a front surface 10a and a back surface 10b facing each other, and the front surface 10a of the semiconductor substrate 10 is, for example, from the plane perpendicular to the c-axis of the gallium nitride semiconductor to the m-axis direction of the gallium nitride semiconductor. It is a semipolar main surface inclined in the direction. The inclination angle of the surface 10a in the m-axis direction is preferably from 63 ° to less than 80 °, more preferably from 70 ° to less than 80 °, and still more preferably from 71 ° to 79 ° from the viewpoint of further reducing the operating voltage.

  The semiconductor laser 1 is formed by epitaxially growing the semiconductor region 20, the active layer 30, and the semiconductor region 40 on the surface 10a of the semiconductor substrate 10 in the normal direction of the surface 10a. The main surface of each constituent layer of the semiconductor region (semiconductor region 20, active layer 30, semiconductor region 40) epitaxially grown on the surface 10a of the semiconductor substrate 10 tends to inherit the crystal orientation of the surface 10a.

  The semiconductor region 20 is composed of one or a plurality of gallium nitride based semiconductor layers, and is composed of a gallium nitride based semiconductor layer made of GaN, AlGaN, InGaN, InAlGaN or the like. The semiconductor region 20 includes at least one light guide layer, and includes, for example, a buffer layer 20a, a lower cladding layer 20b, a lower light guide layer 20c, and a lower light guide layer 20d.

The buffer layer 20a is disposed on the surface 10a of the semiconductor substrate 10 and joined to the surface 10a. The buffer layer 20a is, for example, an n-type gallium nitride semiconductor layer (GaN layer or the like) containing Si or the like as an n-type dopant. The n-type dopant concentration of the buffer layer 20a is, for example, 3 × 10 ¹⁸ / cm ³ . The film thickness of the buffer layer 20a is, for example, 1.1 μm.

The lower cladding layer 20b is disposed on the buffer layer 20a. For example, an n-type gallium nitride based semiconductor layer (In _0.03 Al _0.14 Ga _0.83 N layer or the like) containing Si or the like as an n-type dopant is used. ). The n-type dopant concentration of the lower cladding layer 20b is, for example, 3 × 10 ¹⁸ / cm ³ . The film thickness of the lower cladding layer 20b is, for example, 1.2 μm.

The lower light guide layer 20c is disposed on the lower cladding layer 20b and is, for example, an n-type gallium nitride based semiconductor layer (GaN layer or the like) containing Si or the like as an n-type dopant. The n-type dopant concentration of the lower light guide layer 20c is, for example, 2 × 10 ¹⁸ / cm ³ . The film thickness of the lower light guide layer 20c is, for example, 0.250 μm.

The lower light guide layer 20d is disposed on the semiconductor substrate 10 via the buffer layer 20a, the lower cladding layer 20b, and the lower light guide layer 20c. The lower light guide layer 20d is, for example, an n-type In _x Ga _1-x N semiconductor layer containing Si or the like as an n-type dopant. The In composition x is preferably 0.025 ≦ x ≦ 0.06. The n-type dopant concentration of the lower light guide layer 20d is, for example, 5 × 10 ¹⁷ to 2 × 10 ¹⁸ / cm ³ . The film thickness of the lower light guide layer 20d is, for example, 0.115 μm.

  The active layer 30 is disposed on the lower light guide layer 20d and joined to the main surface 20s of the lower light guide layer 20d. The active layer 30 has a multiple quantum well structure (MQW structure) capable of generating light of 510 to 550 nm. The active layer 30 includes a well layer (first well layer) 30a, a barrier layer 30b disposed on the well layer 30a, and a well layer (second well layer) 30c disposed on the barrier layer 30b. MQW structure.

The well layers 30a and 30c are, for example, non-doped In _y Ga _1-y N semiconductor layers. The In composition y is preferably 0.25 ≦ y ≦ 0.30. The In composition y of the well layer 30a and the In composition y of the well layer 30c may be the same as or different from each other.

  The film thickness of at least one of the well layer 30a or the well layer 30c is preferably 1.0 to 4.0 nm, and more preferably 2.0 to 3.0 nm. Moreover, it is preferable that both of the film thicknesses of the well layers 30a and 30c are 1.0 to 4.0 nm. If both the thicknesses of the well layers 30a and 30c are less than 1.0 nm, the light emission efficiency tends to decrease. If both the thicknesses of the well layers 30a and 30c exceed 4.0 nm, the strain tends to increase and the crystal tends to break easily.

The barrier layer 30b is made of a gallium nitride semiconductor having a smaller band gap than the lower light guide layer 20d, and is, for example, a non-doped In _z Ga _1-z N semiconductor layer. The In composition z is preferably 0.025 ≦ z ≦ 0.10, more preferably 0.025 <z ≦ 0.10, and still more preferably 0.03 ≦ z ≦ 0.06. If the In composition z is less than 0.025, the voltage drop in the barrier layer 30b tends to increase. If the In composition z exceeds 0.10, the barrier layer 30b tends to be difficult to grow. The In composition z is preferably greater than or equal to the In composition x of the lower light guide layer 20d, and more preferably greater than the In composition x of the lower light guide layer 20d. The In composition z is preferably smaller than the In composition y of the well layers 30a and 30c.

  The film thickness of the barrier layer 30b is preferably 1.0 nm or more. If the thickness of the barrier layer 30b is less than 1.0 nm, crystal growth tends to be difficult.

  The semiconductor region 40 is composed of one or a plurality of gallium nitride based semiconductor layers, and is composed of a gallium nitride based semiconductor layer made of GaN, AlGaN, InGaN, InAlGaN or the like. The semiconductor region 40 includes, for example, an upper light guide layer 40a, an electron block layer 40b, an upper light guide layer 40c, an upper light guide layer 40d, an upper cladding layer 40e, and a contact layer 40f.

The upper light guide layer 40 a is disposed on the active layer 30 and joined to the main surface 30 s of the active layer 30. The upper light guide layer 40a is, for example, a non-doped gallium nitride based semiconductor layer (such as an In _0.025 Ga _0.975 N layer). The film thickness of the upper light guide layer 40a is, for example, 0.075 μm.

The electron block layer 40b is disposed on the upper light guide layer 40a and is, for example, a p-type gallium nitride based semiconductor layer (GaN layer or the like) containing Mg or the like as a p-type dopant. The electron block layer 40b has a p-type dopant concentration of, for example, 1 × 10 ¹⁹ / cm ³ . The film thickness of the electron block layer 40b is, for example, 20 nm.

The upper light guide layer 40c is disposed on the electron block layer 40b. The upper light guide layer 40c is, for example, a p-type gallium nitride semiconductor layer (such as an In _0.025 Ga _0.975 N layer) containing Mg or the like as a p-type dopant. The p-type dopant concentration of the upper light guide layer 40c is, for example, 5 × 10 ¹⁸ / cm ³ . The film thickness of the upper light guide layer 40c is, for example, 0.050 μm.

The upper light guide layer 40d is disposed on the upper light guide layer 40c and is, for example, a p-type gallium nitride based semiconductor layer (GaN layer or the like) containing Mg or the like as a p-type dopant. The p-type dopant concentration of the upper light guide layer 40d is, for example, 5 × 10 ¹⁸ / cm ³ . The film thickness of the upper light guide layer 40d is, for example, 0.250 μm.

The upper clad layer 40e is disposed on the upper light guide layer 40d, and for example, a p-type gallium nitride based semiconductor layer (In _0.03 Al _0.14 Ga _0.83 N containing Mg or the like as a p-type dopant). Layer). The p-type dopant concentration of the upper cladding layer 40e is, for example, 1 × 10 ¹⁹ / cm ³ . The film thickness of the upper cladding layer 40e is, for example, 0.40 μm.

The contact layer 40f is disposed on the upper cladding layer 40e, and is, for example, a p ⁺ -type gallium nitride based semiconductor layer (GaN layer or the like) containing Mg or the like as a p-type dopant. The contact layer 40f has a p-type dopant concentration of 3 × 10 ¹⁹ / cm ³ , for example. The film thickness of the contact layer 40f is, for example, 0.050 μm.

FIG. 2 is a schematic diagram of an energy band in a part of the semiconductor laser 1, and is a band diagram from the lower light guide layer 20d to the upper light guide layer 40a. The vertical axis represents band energy, and the horizontal axis represents coordinates in the stacking direction. A solid line E _c indicates the lower end of the conduction band, and a solid line E _v indicates the upper end of the valence band.

  According to the knowledge of the present inventor, the operating voltage can be reduced by adjusting the band gap BG1 of the barrier layer 30b to be smaller than the band gap BG2 of the lower light guide layer 20d. Further, the operating voltage can be further reduced by adjusting the band gap BG1 of the barrier layer 30b to be smaller than the band gap BG3 of the upper light guide layer 40a. The size of the band gap of each semiconductor layer can be adjusted by, for example, the In composition of each semiconductor layer, and the band gap tends to decrease as the In composition increases.

  Further, according to the knowledge of the present inventor, after adjusting the band gap BG1 to be smaller than the band gap BG2, (i) reducing the band offsets BO1 to BO4 between the well layers 30a and 30c and the barrier layer 30b. (Ii) The operating voltage can be further reduced by reducing the film thickness T of the barrier layer 30b. The band offsets BO1 to BO4 can be adjusted by, for example, the In composition of the well layers 30a and 30c and the barrier layer 30b.

  In the present embodiment, from the viewpoint of realizing a semiconductor laser device capable of further reducing the operating voltage, the band gap difference between the well layers 30a and 30c and the barrier layer 30b (that is, between the well layers 30a and 30c and the barrier layer 30b). It is preferable to adjust the film thickness of the barrier layer 30b according to the band offset. For example, when the band gap difference between the well layers 30a and 30c and the barrier layer 30b is large, it is preferable to use the barrier layer 30b having a small thickness from the viewpoint of improving the tunneling action of carriers in the barrier layer 30b.

  When at least one of the band gap difference between the well layer 30a and the barrier layer 30b or the band gap difference between the well layer 30c and the barrier layer 30b is greater than 1.12 eV and less than or equal to 1.14 eV, the thickness of the barrier layer 30b is 1 It is preferable that it is 0.0-2.2 nm.

  When at least one of the band gap difference between the well layer 30a and the barrier layer 30b or the band gap difference between the well layer 30c and the barrier layer 30b is more than 1.07 eV and not more than 1.12 eV, the thickness of the barrier layer 30b is 1 It is preferable that it is 0.0-2.4 nm.

  When at least one of the band gap difference between the well layer 30a and the barrier layer 30b or the band gap difference between the well layer 30c and the barrier layer 30b is more than 1.02 eV and 1.07 eV or less, the film thickness of the barrier layer 30b is 1 It is preferable that it is 0.0-2.9 nm.

  When at least one of the band gap difference between the well layer 30a and the barrier layer 30b or the band gap difference between the well layer 30c and the barrier layer 30b is more than 0.97 eV and 1.02 eV or less, the film thickness of the barrier layer 30b is 1 It is preferable that it is 0.0-3.3 nm.

  When at least one of the band gap difference between the well layer 30a and the barrier layer 30b or the band gap difference between the well layer 30c and the barrier layer 30b is more than 0.92 eV and not more than 0.97 eV, the thickness of the barrier layer 30b is 1 It is preferable that it is 0.0-4.1 nm.

  When at least one of the band gap difference between the well layer 30a and the barrier layer 30b or the band gap difference between the well layer 30c and the barrier layer 30b is more than 0.87 eV and not more than 0.92 eV, the thickness of the barrier layer 30b is 1 It is preferable that it is 0.0-4.8 nm.

  When at least one of the band gap difference between the well layer 30a and the barrier layer 30b or the band gap difference between the well layer 30c and the barrier layer 30b is more than 0.82 eV and less than 0.87 eV, the thickness of the barrier layer 30b is 1 It is preferable that it is 0.0-6.5 nm.

  When at least one of the band gap difference between the well layer 30a and the barrier layer 30b or the band gap difference between the well layer 30c and the barrier layer 30b is 0.77 eV or more and 0.82 eV or less, the thickness of the barrier layer 30b is 1. It is preferable that it is 0-8.3 nm.

  In the semiconductor laser 1, the quantum well structure of the well layer 30a / barrier layer 30b / well layer 30c arranged on the surface 10a which is a semipolar main surface via the lower light guide layer 20d can generate light having a wavelength of 510 to 550 nm. The band gap of the barrier layer 30b is smaller than the band gap of the lower light guide layer 20d. In this case, as will be described later with reference to FIG. 3, a band structure having excellent carrier mobility is formed, so that carrier movement is promoted. Therefore, although the semiconductor laser 1 has an active layer in which a plurality of well layers are stacked via a barrier layer, the operating voltage for obtaining a predetermined amount of current can be reduced. In such a semiconductor laser 1, light emission characteristics superior to that of the element can be obtained while achieving an operating voltage equivalent to that of an element having an active layer composed of a single well layer.

  The present invention is not limited to the above-described embodiment, and various modifications can be made. For example, in the above-described embodiment, the surface 10a of the semiconductor substrate 10 is inclined in the direction of the m-axis of the gallium nitride semiconductor from the plane orthogonal to the c-axis of the gallium nitride semiconductor. It may be inclined in the direction of the a-axis of the gallium nitride based semiconductor from the plane orthogonal to. A ridge portion extending in the optical waveguide direction of the semiconductor laser element may be formed on the surface side of the semiconductor region 40.

  In the semiconductor laser 1, the upper light guide layer 40a is non-doped, but may be a p-type gallium nitride based semiconductor layer containing Mg or the like as a p-type dopant. In the semiconductor laser 1, the electron block layer 40b is disposed between the upper light guide layer 40a and the upper light guide layer 40c. However, the single upper light guide layer is not disposed without the electron block layer 40b. May be formed.

  Next, the results of device simulation using the following semiconductor laser element models A to C will be described. The semiconductor laser element model A has the same configuration as that of the semiconductor laser 1. Details of the configurations of the semiconductor laser element models A to C are as follows.

(Semiconductor laser element model A)
Semiconductor substrate 10 (non-doped GaN substrate, main surface: {20-21} plane (inclination angle from c-plane to m-axis direction: 75 °)
Buffer layer 20a (n-type GaN layer, dopant: Si, dopant concentration: 3 × 10 ¹⁸ / cm ³ , film thickness: 1.1 μm)
Lower cladding layer 20b (n-type In _0.03 Al _0.14 Ga _0.83 N layer, dopant: Si, dopant concentration: 3 × 10 ¹⁸ / cm ³ , film thickness: 1.2 μm)
Lower light guide layer 20c (n-type GaN layer, dopant: Si, dopant concentration: 2 × 10 ¹⁸ / cm ³ , film thickness: 0.250 μm)
Lower light guide layer 20d (n-type In _0.025 Ga _0.975 N semiconductor layer, dopant: Si, dopant concentration: 5 × 10 ¹⁷ / cm ³ , film thickness: 0.115 μm)
Active layer 30 (multiple quantum well structure: well layer 30a / barrier layer 30b / well layer 30c, composition of well layers 30a, 30c: In _0.30 Ga _0.70 N, thickness of well layers 30a, 30c: 3 nm, the composition of the barrier layer _{30b: in z Ga 1-z} N semiconductor layers (z: 0.025,0.050,0.10,0.15), a barrier layer 30b having a thickness of: 1nm, 2nm, 5nm, 10nm )
Upper light guide layer 40a (non-doped In _0.025 Ga _0.975 N semiconductor layer, film thickness: 0.075 μm)
Electron blocking layer 40b (p-type GaN layer, dopant: Mg, dopant concentration: 1 × 10 ¹⁹ / cm ³ , film thickness: 20 nm)
Upper light guide layer 40c (p-type In _0.025 Ga _0.975 N layer, dopant: Mg, dopant concentration: 5 × 10 ¹⁸ / cm ³ , film thickness: 0.050 μm)
Upper light guide layer 40d (p-type GaN layer, dopant: Mg, dopant concentration: 5 × 10 ¹⁸ / cm ³ , film thickness: 0.250 μm)
Upper clad layer 40e (p-type In _0.03 Al _0.14 Ga _0.83 N layer, dopant: Mg, dopant concentration: 1 × 10 ¹⁹ / cm ³ , film thickness: 0.40 μm)
Contact layer 40f (p ⁺ type GaN layer, dopant: Mg, dopant concentration: 3 × 10 ¹⁹ / cm ³ , film thickness: 0.050 μm)

(Semiconductor laser element model B)
The semiconductor laser element model B has the same configuration as the semiconductor laser element model A except that the main surface of the semiconductor substrate 10 is a {0001} plane (c-plane).

(Semiconductor laser element model C)
The semiconductor laser element model C has the same configuration as the semiconductor laser element model A except that the active layer 30 is a single layer of the well layer 30a.

FIG. 3 is an energy band diagram provided by simulation using the semiconductor laser element models A and B, and is an energy band diagram from the lower light guide layer 20d to the barrier layer 30b. 3A shows a simulation result using the semiconductor laser element model A, and FIG. 3B shows a simulation result using the semiconductor laser element model B. The vertical axis represents band energy (eV), and the horizontal axis represents coordinates in the stacking direction. Solid lines E _c1 and E _c2 indicate the lower end of the conduction band, and solid _lines E _v1 and E _v2 indicate the upper end of the valence band. Dashed lines F _c1 and F _c2 indicate electron Fermi levels, and broken _lines F _v1 and F _v2 indicate hole Fermi levels.

Referring to FIG. 3A, in the element having the quantum well structure of the lower light guide layer 20d / well layer 30a / barrier layer 30b on the semipolar plane of the semiconductor substrate, the well layer 30a at the lower end E _c1 of the conduction band. The energy level EL _{c11 at} the / barrier layer 30b interface has a higher band energy than the energy level EL _c12 at the well layer 30a / lower light guide layer 20d interface, and the well layer 30a at the upper end E _v1 of the valence band The energy level EL _{v11 at} the / barrier layer 30b interface has higher band energy than the energy level EL _v12 at the well layer 30a / lower light guide layer 20d interface.

On the other hand, referring to FIG. 3B, in the element having the quantum well structure of the lower light guide layer 20d / well layer 30a / barrier layer 30b on the polar surface of the semiconductor substrate, the well layer is formed at the lower end E _c2 of the conduction band. The energy level EL _{c21 at} the 30a / barrier layer 30b interface has a lower band energy than the energy level EL _c22 at the well layer 30a / lower light guide layer 20d interface, and the well layer at the lower end E _v2 of the valence band. 30a / barrier layer 30b is band energy lower than the energy level _{EL v22} in the well layer 30a / the lower optical guide layer 20d interfaces towards the energy level _{EL v21} at the interface.

  For this reason, the height of the band barrier that must be overcome when electrons are injected into the well layer 30a is relatively low in the case of semipolarity, so that an element having a quantum well structure on the semipolar plane is polar. Since it has a band structure excellent in carrier mobility compared to an element having a quantum well structure on the surface, the operating voltage for obtaining a predetermined amount of current in the semiconductor laser element can be lowered.

  Using the semiconductor laser element model A, device simulation of current-voltage characteristics was performed by changing the film thickness and In composition of the barrier layer 30b. FIG. 4 is a diagram illustrating current-voltage characteristics provided by simulation, and is a diagram illustrating current-voltage characteristics when the thickness of the barrier layer 30b is 10 nm as an example of a simulation result. The vertical axis represents current (mA), and the horizontal axis represents voltage (V).

  FIG. 5 is a diagram showing results obtained by deriving an operating voltage for obtaining a current of 200 mA at each film thickness and each In composition of the barrier layer 30b based on the simulation result of the current-voltage characteristics of FIG. It is drawing which shows the relationship with In composition. The vertical axis represents the operating voltage Vf (V) for obtaining a current of 200 mA, and the horizontal axis represents the In composition (%) of the barrier layer 30b.

  Referring to FIG. 5, it can be confirmed that the operating voltage for obtaining a current of 200 mA decreases as the In composition increases, regardless of the thickness of the barrier layer 30b. As the In composition of the barrier layer 30b increases, the band gap of the barrier layer 30b becomes smaller than the band gap of the lower light guide layer 20d, and the band offset between the barrier layer 30b and the well layers 30a and 30c decreases. As a result, it is presumed that a band structure more excellent in carrier movement is formed and the operating voltage is reduced.

  Referring to FIG. 5, it is confirmed that the operating voltage with the same In composition decreases as the thickness of the barrier layer 30b decreases. As the film thickness of the barrier layer 30b becomes smaller, it is presumed that the operating voltage is reduced by improving the tunneling action of carriers in the barrier layer 30b.

  Further, when a device simulation of current-voltage characteristics was performed using the semiconductor laser element model C having an active layer composed of a single well layer, the operating voltage Vf (V) for obtaining a current of 200 mA was 6. It was confirmed to be 3V. In a semiconductor laser device, the operating voltage tends to increase as the number of well layers increases, and the amount of increase in operating voltage for obtaining a current of 200 mA is an element having an active layer composed of a single well layer. The operating voltage Vf (V) for obtaining a current of 200 mA is preferably 7.3 V or less.

  FIG. 6 shows the result of calculating the film thickness at which the operating voltage becomes 7.3 V (hereinafter referred to as “allowable film thickness”) within the range of 2.5% to 10% of the In composition based on the result of FIG. It is drawing and is a drawing which shows the relationship between the allowable film thickness of the barrier layer 30b, and In composition. The vertical axis represents the allowable film thickness (nm) of the barrier layer 30b, and the horizontal axis represents the In composition (%) of the barrier layer 30b.

  In addition, using the semiconductor laser element model A, a device simulation was performed on the band gap difference between the well layer 30a and the barrier layer 30b. FIG. 7 is a drawing showing the relationship between the band gap difference between the well layer 30a and the barrier layer 30b and the In composition of the barrier layer 30b. The vertical axis represents the band gap difference (eV) between the well layer 30a and the barrier layer 30b, and the horizontal axis represents the In composition (%) of the barrier layer 30b. The band gap of the well layer 30a was 2.2 eV. The band gap difference monotonously decreased as the In composition increased, and an approximate curve “y = −0.0497x + 1.2675 (x: In composition, y: band gap difference)” was obtained.

  Next, based on the results of FIGS. 6 and 7, the relationship between the allowable film thickness of the barrier layer 30b and the band gap difference between the well layer 30a and the barrier layer 30b was derived. For example, in FIG. 7, in the range where the band gap difference is greater than 1.12 eV and less than or equal to 1.14 eV, when the barrier layer 30b has an allowable film thickness at 1.14 eV, the operating voltage is in any of the ranges. Can be 7.3 V or less. In FIG. 7, when the band gap difference is 1.14 eV, the In composition is 2.5 (%). When the In composition is 2.5 (%), the allowable film thickness in FIG. 6 is 2.2 nm. Therefore, when the band gap difference is greater than 1.12 eV and less than or equal to 1.14 eV, the operating voltage for obtaining a current of 200 mA is reduced to 7.3 V or less by reducing the film thickness of the barrier layer 30 b to 2.2 nm or less. can do.

  By the same procedure, the allowable film thickness is calculated to be 2.4 nm or less in FIG. 6 when the band gap difference exceeds 1.07 eV and is 1.12 eV or less in FIG. In the range where the band gap difference exceeds 1.02 eV and 1.07 eV or less, the allowable film thickness is calculated to be 2.9 nm or less. When the band gap difference is more than 0.97 eV and 1.02 eV or less, the allowable film thickness is calculated to be 3.3 nm or less. In the range where the band gap difference exceeds 0.92 eV and is 0.97 eV or less, the allowable film thickness is calculated to be 4.1 nm or less. In the range where the band gap difference exceeds 0.87 eV and is 0.92 eV or less, the allowable film thickness is calculated to be 4.8 nm or less. In the range where the band gap difference exceeds 0.82 eV and 0.87 eV or less, the allowable film thickness is calculated to be 6.5 nm or less. In the range where the band gap difference is 0.77 eV or more and 0.82 eV or less, the allowable film thickness is calculated to be 8.3 nm or less.

  DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser (gallium nitride semiconductor laser element), 10 ... Semiconductor substrate, 10a ... Surface (semipolar main surface), 20d ... Lower light guide layer, 30 ... Active layer, 30a ... Well layer (first well layer) 30b ... barrier layer, 30c ... well layer (second well layer).

Claims (13)

A substrate having a semipolar main surface;
A light guide layer disposed on the semipolar main surface;
An active layer having a quantum well structure capable of generating light of 510 to 550 nm and disposed on the light guide layer,
The semipolar principal surface is inclined in the m-axis direction of the gallium nitride semiconductor from a plane orthogonal to the c-axis of the gallium nitride semiconductor;
An inclination angle of the semipolar main surface in the m-axis direction is 63 ° or more and less than 80 °,
The quantum well structure includes a first well layer made of InGaN, a barrier layer made of a gallium nitride semiconductor and disposed on the first well layer, and a second well made of InGaN and placed on the barrier layer. And a well layer,
The band gap of the barrier layer is rather smaller than the band gap of the light guide layer,
At least one of the first well layer or the second well layer is made of In _y Ga _1-y N (0.25 ≦ y ≦ 0.30),
The gallium nitride based semiconductor laser device , wherein the gallium nitride based semiconductor of the barrier layer is an In _z Ga _1-z N semiconductor (0.025 ≦ z ≦ 0.10) .   The gallium nitride based semiconductor laser device according to claim 1, wherein the semipolar principal surface is a {20-21} plane. Wherein the light guide layer is made of _{In x Ga 1-x N (} 0.025 ≦ x ≦ 0.06), gallium nitride semiconductor laser device according to claim 1 or 2. The light guide layer contains a n-type dopant, a gallium nitride-based semiconductor laser device according to any one of claims 1-3. The gallium nitride based semiconductor laser device according to any one of claims 1 to 4 , wherein a film thickness of at least one of the first well layer or the second well layer is 1.0 to 4.0 nm. At least one of the band gap difference between the first well layer and the barrier layer, or the band gap difference between the second well layer and the barrier layer is more than 1.12 eV and not more than 1.14 eV,
The thickness of the barrier layer is 1.0～2.2Nm, gallium nitride semiconductor laser device according to any one of claims 1-5. At least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 1.07 eV and not more than 1.12 eV,
The thickness of the barrier layer is 1.0～2.4Nm, gallium nitride semiconductor laser device according to any one of claims 1-5. At least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 1.02 eV and not more than 1.07 eV,
The thickness of the barrier layer is 1.0～2.9Nm, gallium nitride semiconductor laser device according to any one of claims 1-5. At least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 0.97 eV and not more than 1.02 eV,
The thickness of the barrier layer is 1.0～3.3Nm, gallium nitride semiconductor laser device according to any one of claims 1-5. At least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 0.92 eV and not more than 0.97 eV,
The thickness of the barrier layer is 1.0～4.1Nm, gallium nitride semiconductor laser device according to any one of claims 1-5. At least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 0.87 eV and not more than 0.92 eV,
The thickness of the barrier layer is 1.0～4.8Nm, gallium nitride semiconductor laser device according to any one of claims 1-5. At least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is more than 0.82 eV and not more than 0.87 eV,
The thickness of the barrier layer is 1.0～6.5Nm, gallium nitride semiconductor laser device according to any one of claims 1-5. At least one of the band gap difference between the first well layer and the barrier layer or the band gap difference between the second well layer and the barrier layer is 0.77 eV or more and 0.82 eV or less,
The thickness of the barrier layer is 1.0～8.3Nm, gallium nitride semiconductor laser device according to any one of claims 1-5.

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