JP4126878B2 - Gallium nitride compound semiconductor laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a structure of a ridge waveguide type semiconductor laser using a gallium nitride-based compound semiconductor (In _X Al _Y Ga _1-XY N, 0 ≦ X, 0 ≦ Y, X + Y ≦ 1). The present invention relates to an improvement of a waveguide structure for the purpose of improving the stability of a transverse mode in a ridge waveguide type semiconductor laser using a gallium compound semiconductor.
[0002]
[Prior art]
In semiconductor lasers using gallium nitride compound semiconductors (In _X Al _Y Ga _1-XY N, 0≤X, 0≤Y, X + Y≤1), a ridge waveguide structure is used to control the horizontal transverse mode. There are many cases to do. FIG. 4 is a perspective view showing an example of a general ridge waveguide structure. In FIG. 4, the substrate, the protective insulating film, etc. are omitted. Using a gallium nitride compound semiconductor, an n-type contact layer 2, an n-type cladding layer 4, an n-type light guide layer 6, an active layer 8, a p-type light guide layer 10, a p-type cladding layer 12, and a p-type contact layer 14 are sequentially laminated. The p-type contact layer 14 and the p-type clad layer 12 are removed by etching until the p-type clad layer 12 has a predetermined thickness, leaving a stripe-shaped region, and a stripe-shaped convex portion (= ridge portion) 20 is formed. Has been. A p-electrode 16 is formed on the p-type contact layer 14, and an n-electrode 18 is formed on the n-type contact layer 2 exposed by etching. Since the equivalent refractive index of the optical waveguide below the ridge portion 20 is higher than the equivalent refractive index of the regions on both sides thereof, light is confined in the lateral direction.
[0003]
[Problems to be solved by the invention]
However, a gallium nitride compound semiconductor laser using a general ridge waveguide structure has a problem that the kink phenomenon is remarkably generated because the stability of the transverse mode is not sufficient. An example of the kink phenomenon is shown in FIG. When the current value I of the laser is increased, the zero-order mode first oscillates in a current region exceeding the oscillation threshold value I _0. However, when the current value is further increased, the first-order mode oscillates at a certain current value I _1. As a result, the transverse mode is changed, and the current I-light output P characteristic is bent (= kink). When the kink phenomenon occurs, the linearity of the laser output characteristics is lost, and accurate control of the laser output becomes difficult.
[0004]
There is also a problem that pulsation is likely to occur as a result of the unstable lateral mode. In general, when a current is injected beyond the laser oscillation threshold, a decrease in carrier density due to laser oscillation and an increase in carrier density due to current injection are repeated, resulting in a “relaxation oscillation” in which the output of the laser light periodically increases and decreases. However, when the transverse mode is unstable, “pulsation” in which “relaxation oscillation” persists is likely to occur. When pulsation occurs, the output of the laser beam becomes unstable, and it becomes impossible to specify whether the laser beam is in an on state or an off state. When the current is injected, a change in the carrier density distribution occurs, which causes a change in the refractive index and changes in the light distribution, and a change in the transverse mode (higher order mode starts oscillation) further changes the carrier distribution density. It is thought to cause pulsation.
[0005]
Accordingly, an object of the present invention is to provide a new ridge waveguide structure capable of stabilizing a transverse mode in a gallium nitride compound semiconductor laser having a ridge waveguide structure.
[0006]
[Means for Solving the Problems]
In order to solve the above problems, a gallium nitride compound semiconductor laser according to the present invention includes an n-type gallium nitride compound semiconductor layer, an active layer made of a gallium nitride compound semiconductor layer, and a p-type gallium nitride compound semiconductor layer. In the gallium nitride compound semiconductor laser that performs light confinement below the ridge portion by sequentially stacking and partially increasing the thickness of the p-type gallium nitride compound semiconductor layer in a stripe shape to form a ridge portion The film thickness of the p-type gallium nitride compound semiconductor layer sandwiching the lower part of the ridge from both sides is periodically changed in the laser resonance direction.
[0007]
Thereby, the transverse mode in the ridge waveguide type gallium nitride compound semiconductor laser can be stabilized, and the kink phenomenon and pulsation can be suppressed. Although the mechanism by which the transverse mode is stabilized by periodically changing the film thickness of the p-type gallium nitride compound semiconductor layer beside the ridge is not clear, equivalent refraction in the peripheral region sandwiching the light confinement region below the ridge from both sides It is estimated that the rate periodically changes in the laser resonance direction.
[0008]
In order to periodically change the film thickness of the p-type gallium nitride compound semiconductor layer beside the ridge in the resonance direction of the laser, for example, the p-type gallium nitride compound semiconductor layer sandwiching the lower part of the ridge from both sides What is necessary is just to form a some recessed part for every fixed interval along the longitudinal direction of a part.
[0009]
In addition, when a p-type light guide layer and a p-type clad layer are sequentially stacked on the active layer as a p-type gallium nitride compound semiconductor layer, the recess is formed in a depth excluding a part of the p-type light guide layer. It is preferable to form it. Thereby, since the equivalent refractive index of the peripheral region can be changed at a position close to the active layer that is the center of laser emission, the transverse mode can be further stabilized.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
1 and 2 are a perspective view and a plan view showing an example of a ridge waveguide type gallium nitride compound semiconductor laser according to the present invention. In FIGS. 1 and 2, the substrate and the protective insulating film are omitted. On a heterogeneous substrate such as sapphire and SiC or a GaN substrate, an n-type contact layer 2 made of Al _x Ga _{1- x} N (0 ≦ x <1), Al _x Ga _{1- x} N (0 <x <1) n-type cladding layer ₄ made of nitride semiconductor layer _{comprising, In x Ga 1 - x n} (0 ≦ x <1) n -type optical guide layer made of _{6, In x Ga 1 - x} n (0 <x <1 ) active layer _{8 containing, In x Ga 1 - x N} (0 ≦ x <1 p -type optical guide layer 10 made _{of), Al x Ga 1- x N} (0 <x <1) the nitride semiconductor layer containing A p-type cladding layer 12 made of GaN and a p-type contact layer 14 made of GaN are sequentially stacked. A p-electrode 16 is formed on the p-type contact layer 14, an n-electrode 18 is formed on the n-type contact layer 2 exposed by etching, and almost the entire surface of the device excluding the p-electrode 16 and the n-electrode 18. In addition, a protective insulating film (not shown) such as ZrO ₂ or SiO ₂ is formed.
[0011]
The structure of the ridge waveguide in the present embodiment is as follows. First, a ridge portion 20 similar to the conventional one is formed in the p-type cladding layer 12. That is, the p-type clad layer 12 and the p-type contact layer 14 are removed by etching until the p-type clad layer 12 has a predetermined thickness, leaving a stripe-shaped region, and the ridge portion 20 that is a stripe-shaped convex portion is removed. Forming. A plurality of recesses 22 are formed at regular intervals along the ridge longitudinal direction on both sides of the ridge portion 20. The recess 22 is formed by etching from the upper surface of the p-type cladding layer 12 to a depth where the p-type light guide layer 10 remains by a predetermined thickness. The recess 22 is filled with a protective insulating film (not shown) such as ZrO ₂ or SiO ₂ .
[0012]
3A and 3B, the semiconductor laser element shown in FIGS. 1 and 2 has a position where the concave portion 22 is not formed (position along the line aa ′ in FIG. 2) and a concave portion 22 formed. It is a schematic diagram which shows the cross section of a position (position of the bb 'line | wire of FIG. 2). 3A and 3B, the protective film 19 is not omitted. In the region where the recess 22 is not formed, as shown in FIG. 3A, the p-type gallium nitride compound semiconductor layer is a p-type cladding layer in the peripheral region B sandwiching both sides of the region A where the optical waveguide is formed. Etching is performed so that a predetermined film thickness 12 remains, and a protective insulating film 19 is covered thereon. On the other hand, in the region where the recess 22 is formed, as shown in FIG. 3B, the p-type gallium nitride compound semiconductor layer is completely removed from the p-type cladding layer 12 in the peripheral region B, and the p-type light guide layer is formed. 10 is etched so as to remain a certain thickness (preferably about 400 to 1500 mm), and the formed recess 22 is filled with the protective insulating film 19. If the unevenness is relatively formed, the p-type light guide layer can be etched beyond the p-type cladding layer 12 in the region where the recess 22 is not formed (the region in FIG. 3A).
[0013]
According to the ridge waveguide structure of the present invention, the p-type nitride semiconductor layer (the p-type light guide layer 10 and the p-type clad layer 12) on the side of the ridge portion 20 is formed with irregularities, whereby a gallium nitride compound is formed. The transverse mode of the semiconductor laser can be stabilized and the occurrence of kink phenomenon and pulsation can be suppressed. Although the mechanism by which the transverse mode is stabilized by forming irregularities on the p-type gallium nitride compound semiconductor layer beside the ridge is not clear, as a result of the irregularities formation, the equivalent refractive index in the peripheral region B sandwiching the optical confinement region A from both sides is This is presumed to be due to periodic changes depending on the position of the laser resonance direction (here, the ridge longitudinal direction).
[0014]
That is, as shown in FIG. 3, the p-type nitride semiconductor in the peripheral region B is located at the position where the recess is not formed (FIG. 3A) and the position where the recess 22 is formed (FIG. 3B). Although the film thickness ratios of the layers (p-type light guide layer 10 and p-type cladding layer 12) and protective insulating film 22 are different, ZrO ₂ and SiO ₂ which are protective insulating films have a refractive index as compared with gallium nitride compound semiconductors. Since it is about 20-40% smaller, the equivalent refractive index can be periodically changed along the optical waveguide in the peripheral region B. For this reason, it is considered that a so-called distributed feedback is performed on the left and right sides of the active region, and the transverse mode of the laser is stabilized.
[0015]
The depth of the concave portion 22 is preferably deep as long as the active layer 8 is not exposed. This is because in order to stabilize the transverse mode of the laser, it is necessary to cause a periodic change in the equivalent refractive index in the peripheral region B at a position close to the active layer 8 that is the center of laser emission. In particular, when the p-type light guide layer 10 is provided between the active layer 8 and the p-type cladding layer 12 as in the present embodiment, the p-type light guide layer 10 remains in a range of 400 to 1500 mm. The depth is preferable.
[0016]
The width w _a of the concave portion 22 in the ridge orthogonal direction is formed to be larger than the range contributing to the light confinement in the lateral direction below the ridge portion 20. Further, the width w _b of the concave portion 22 in the ridge longitudinal direction and the interval w _c between the concave portions 22 are not particularly limited, but it is preferable that at least several cycles are repeated in the longitudinal direction of the ridge.
[0017]
The planar shape of the recess 22 is not particularly limited, and may be a circle, a rectangle, or a stripe shape orthogonal to the ridge. However, since the boundary portion where the recess 22 is in contact with the side surface of the ridge portion 20 has a particularly large step, if the outer periphery of the recess 22 is in contact with the side surface of the ridge portion 20 at an obtuse angle as shown in FIG. Is preferable.
[0018]
In the present embodiment, the film thickness is periodically changed by forming a plurality of recesses in the p-type gallium nitride compound semiconductor layer. However, the film thickness may be changed by another appropriate method. .
[0019]
【Example】
(Example 1)
A gallium nitride compound semiconductor laser having the structure shown in FIGS. 1 to 3 was fabricated by the following method.
On a sapphire substrate, a buffer layer (not shown) is grown to a thickness of about 200 mm from GaN using hydrogen as a carrier gas, ammonia and TMG (trimethyl gallium) as a source gas at a temperature of 510 ° C. Then, at 1050 ° C., TMA (trimethylaluminum), TMG, and ammonia gas are used as source gases, and undoped Al _0.05 Ga _0.95 N is grown to a thickness of 1 μm. Then, TMA, TMG, and ammonia gas are used, silane gas (SiH ₄ ) is used as impurity gas, and Al _0.05 Ga _0.95 N doped with Si 3 × 10 ¹⁸ / cm ³ is grown to a thickness of 3 μm. The n-type contact layer 2 was formed.
[0020]
Then the temperature to 800 ° C., TMG as the raw material gas, TMI using (trimethyl indium) and ammonia, using a silane gas impurity gas, _{an In} 0.08 _{Ga 0} was 5 × ¹⁰ 18 / cm ³ doped with _{Si. A} crack prevention layer (not shown) made of ₉₂ N was grown to a thickness of 1500 mm.
[0021]
Next, the temperature is set to 1050 ° C., TMA, TMG and ammonia are used as source gases, and an A layer made of undoped Al _0.14 Ga _0.86 N is grown to a thickness of 25 mm, and then TMA is grown. Then, a silane gas is used as an impurity gas, and a B layer made of GaN doped with Si at 5 × 10 ¹⁸ / cm ³ is grown to a thickness of 25 mm. Then, this operation is repeated 160 times to laminate the A layer and the B layer, and the n-type cladding layer 4 made of a multilayer film (superlattice structure) having a total film thickness of 8000 mm is grown. Then, at the same temperature, TMG and ammonia were used as source gases, and an n-type light guide layer 6 made of undoped GaN was grown to a thickness of 750 mm.
[0022]
Next, the temperature is set to 800 ° C., TMI, TMG, and ammonia are used as the source gas, silane gas is used as the impurity gas, and Si is doped with In _0.01 Ga _0.99 N doped with 5 × 10 ¹⁸ / cm ^3. A barrier layer is grown to a thickness of 100 mm. Subsequently, the silane gas is stopped and a well layer made of undoped In _0.11 Ga _0.89 N is grown to a thickness of 50 mm. This operation was repeated three times, and finally, an active layer 8 having a multi-quantum well structure (MQW) having a total film thickness of 550 mm and having a barrier layer laminated thereon was grown.
[0023]
Next, at the same temperature, TMA, TMG, and ammonia are used as source gases, Cp ₂ Mg (cyclopentadienylmagnesium) is used as an impurity gas, and Mg 1 × 10 ¹⁹ / cm ³ doped Al _0.4 A p-type electron confinement layer (not shown) made of Ga _0.6 N was grown to a thickness of 100 mm.
[0024]
Next, the temperature was set to 1050 ° C., TMG and ammonia were used as the source gas, and the p-type light guide layer 10 made of undoped GaN was grown to a thickness of 1500 mm. This p-type light guide layer 10 is grown as undoped, but due to the diffusion of Mg from the p-type electron confinement layer, the Mg concentration becomes 5 × 10 ¹⁶ / cm ³ and exhibits p-type.
[0025]
Next, at the same temperature, TMA, TMG and ammonia are used as source gases, an A layer made of undoped Al _0.1 Ga _0.9 N is grown to a thickness of 25 mm, and then TMA is stopped. Cp ₂ Mg is used as an impurity gas, and a B layer made of GaN doped with 5 × 10 ¹⁸ / cm ^{3 of} Mg is grown to a thickness of 25 mm. This operation was repeated 90 times, and the A layer and the B layer were laminated to grow the p-type cladding layer 12 made of a multilayer film (superlattice structure) having a total film thickness of 4500 mm. Then, at the same temperature, TMG and ammonia are used as the source gas, Cp ₂ Mg is used as the impurity gas, and the p-type contact layer 14 made of GaN doped with 1 × 10 ²⁰ / cm ^{3 of} Mg is formed with a thickness of 150 mm. Grown up.
[0026]
After the completion of the reaction, the wafer is annealed in a reaction vessel at 700 ° C. in a nitrogen atmosphere to further reduce the resistance of the p-type layer. After annealing, the wafer is taken out of the reaction vessel, a protective film made of SiO ₂ is formed on the surface of the uppermost p-side contact layer, etched with SiCl ₄ gas using RIE (reactive ion etching), and n-electrode The surface of the n-side contact layer 2 on which 18 is to be formed was exposed.
[0027]
Next, the ridge portion 20 was formed as follows. A first protective film made of Si oxide (mainly SiO ₂ ) is formed with a film thickness of 0.5 μm on almost the entire surface of the uppermost p-side contact layer 14 by a CVD apparatus to a stripe width of 1.8 μm. Pattern. Then, the p-side contact layer 14 and the p-type cladding layer 12 are etched by RIE using Cl ₂ and SiCl ₄ gas to form a ridge portion 20 having a stripe width of 1.8 μm. Etching was performed until the remaining thickness of the p-type guide layer 10 reached 1000 mm.
[0028]
Next, concave portions 22 on both sides of the ridge portion 20 were formed as follows. After the ridge portion 20 is formed, the concave portion pattern is patterned with a resist so that a plurality of concave portions are formed at regular intervals along the resonance direction in a portion sandwiching the lower side of the ridge portion 20 on both sides. Then, the recess 22 is formed by etching the semiconductor layer in the opening of the recess pattern using Cl ₂ and SiCl ₄ gas by RIE. Thereafter, the resist is removed. The depth of the recess 22 was 500 mm, and the size of the recess 22 was set such that the width wa in the ridge orthogonal direction was 4 μm, the width wb in the ridge longitudinal direction was 4 μm, and the interval wc was 16 μm.
[0029]
After forming the ridge stripe, the wafer is transferred to a PVD apparatus, the surface of the n-side contact layer 2 exposed by etching is covered with a resist, and a second protective film made of Zr oxide (mainly ZrO ₂ ) is applied to the first protective film. It was continuously formed with a film thickness of 0.2 μm on the protective film and the resist and on the p-side cladding layer 12 exposed by etching. It is preferable to form the Zr oxide in this way in order to stabilize the transverse mode. Then, the first protective film and the resist were removed by a lift-off method.
[0030]
Next, a p-electrode 16 made of Ni / Au is formed with a stripe width of 100 μm on the surface of the p-side contact layer 14 exposed by removing the first protective film, and Ti / Ni is formed on the surface of the n-side contact layer 2. An n electrode 18 made of Al was formed in a direction parallel to the stripe.
[0031]
Cleaved in a bar shape from the sapphire substrate side, a resonator was fabricated on the cleavage plane. A dielectric multilayer film made of SiO ₂ and TiO ₂ was formed on the cavity surface, and finally a bar was cut in a direction parallel to the p-electrode to obtain a laser element as shown in FIGS. The resonator length was 600 μm.
[0032]
(Example 2)
A semiconductor laser was fabricated in the same manner as in Example 1 except that the interval wc between the recesses was 10 μm.
(Example 3)
A semiconductor laser was fabricated in the same manner as in Example 1 except that the interval Wc between the recesses was set to 6 μm.
[0033]
Example 4
Etching to form the ridge portion 20 was performed so that 150 p of the p-type cladding layer 12 remained, and the depth of the concave portion 22 was changed to 1000 mm, and a semiconductor laser was fabricated in the same manner as in Example 1.
[0034]
(Example 5)
A semiconductor laser was fabricated in the same manner as in Example 4 except that the interval wc between the recesses was 10 μm.
(Example 6)
A semiconductor laser was fabricated in the same manner as in Example 4 except that the interval Wc between the recesses was set to 6 μm.
[0035]
(Comparative Example 1)
A semiconductor laser was fabricated in the same manner as in Example 1 except that the concave portions 22 were not formed on both sides of the ridge portion 20.
(Comparative Example 2)
A semiconductor laser was fabricated in the same manner as in Example 4 except that the concave portions 22 were not formed on both sides of the ridge portion 20.
[0036]
As for the semiconductor lasers of Examples 1 to 6 and Comparative Examples 1 and 2, the rising kinks and the lasing region kinks were measured. As for the semiconductor lasers of Examples 1 to 6, both the rising kinks and the lasing region kinks were comparative examples. It was suppressed more than the semiconductor lasers 1 and 2.
[0037]
【The invention's effect】
According to the present invention, it is possible to stabilize the transverse mode in the ridge waveguide type gallium nitride compound semiconductor laser device and suppress the occurrence of the kink phenomenon and pulsation.
[Brief description of the drawings]
FIG. 1 is a perspective view showing an example of a ridge waveguide type gallium nitride compound semiconductor laser according to the present invention.
FIG. 2 is a top view of the gallium nitride compound semiconductor laser shown in FIG.
FIGS. 3A and 3B are cross-sectional views showing cross sections taken along lines aa ′ and bb ′ of the gallium nitride compound semiconductor laser shown in FIG.
FIG. 4 is a perspective view showing an example of a conventional ridge waveguide type gallium nitride compound semiconductor laser.
FIG. 5 is a graph showing an example of current-light output characteristics of a gallium nitride-based compound semiconductor laser.
[Explanation of symbols]
2 n-type contact layer, 4 n-type cladding layer, 6 n-type light guide layer, 8 active layer, 10 p-type light guide layer, 12 p-type cladding layer, 14 p-type contact layer, 16 p-electrode, 18 n-electrode, 20 Ridge part, 22 concave part.

Claims (7)

An n-type gallium nitride-based compound semiconductor layer, an active layer composed of a gallium nitride-based compound semiconductor layer, and a p-type gallium nitride-based compound semiconductor layer are sequentially stacked, and the thickness of the p-type gallium nitride-based compound semiconductor layer is striped. In the gallium nitride compound semiconductor laser that performs light confinement below the ridge portion by partially thickening the ridge portion,
In the p-type gallium nitride compound semiconductor layer sandwiching the lower part of the ridge from both sides, recesses are periodically formed along the resonance direction of the laser,
A gallium nitride compound semiconductor laser, wherein a protective insulating film having a refractive index smaller than that of the gallium nitride compound semiconductor layer is formed on a surface of the p-type gallium nitride compound semiconductor layer including the inside of the recess. . _{2. The} gallium nitride compound semiconductor laser according to claim 1, wherein the protective insulating film is SiO ₂ or ZrO ₂ .   A p-type light guide layer and a p-type clad layer are sequentially stacked as the p-type gallium nitride compound semiconductor layer on the active layer, and the concave portion has a depth excluding a part of the p-type light guide layer. The gallium nitride compound semiconductor laser according to claim 2, wherein the gallium nitride compound semiconductor laser is formed.   4. The gallium nitride compound semiconductor laser according to claim 1, wherein the planar shape of the recess is a circle, a rectangle, or a stripe shape orthogonal to the ridge. 5.   5. The gallium nitride-based compound semiconductor laser according to claim 1, wherein an outer periphery of the concave portion is in contact with a side surface of the ridge portion at an obtuse angle. 6. The gallium nitride compound according to claim 1, wherein the p-type gallium nitride compound semiconductor layer includes a p-type contact layer, and a p-electrode is formed on the p-type contact layer. Semiconductor laser. 7. The gallium nitride-based material according to claim 1, wherein the protective insulating film is formed on a surface of the p-type gallium nitride-based compound semiconductor layer on a side surface of the ridge portion and on both sides of the ridge portion. Compound semiconductor laser.

Images