JP4134366B2 - Surface emitting laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device, and a surface emitting semiconductor laser (hereinafter referred to as a surface emitting laser) having a resonator that oscillates laser light in a direction substantially perpendicular to a semiconductor layer constituting the semiconductor laser device, that is, a so-called vertical resonator. About.
[0002]
[Prior art]
A surface emitting laser emits laser light in the vertical direction from the surface of the substrate crystal, enabling two-dimensional parallel integration, and can be applied to systems such as optical interconnection and parallel optical information processing, or to optical fiber communications. Expected. Furthermore, since the inspection of the element and the coupling with the optical fiber are easy, the system can be provided at a low cost. Further, by providing a current confinement layer, current can be locally injected from a portion called an aperture, and laser oscillation can be generated in a small volume active region. Therefore, it is suitable for low threshold current, that is, low power consumption.
[0003]
As the current confinement layer, the use of an AlOx layer in which AlAs is selectively oxidized has recently been proposed, and a significant reduction in threshold current has been achieved. The details are described in, for example, IEEE J. Selected Topics in Quantum Electronics, Vol. 3, pages 893-904, 1997.
[0004]
[Problems to be solved by the invention]
The AlOx current confinement layer is produced by etching an AlAs layer produced by a crystal growth method into a mesa shape, and selectively oxidizing the AlAs layer from the periphery of the mesa into an AlOx layer. The central portion of the mesa remaining unoxidized in the AlAs layer becomes an aperture layer into which current is injected. In order to mass-produce a surface emitting laser with a low threshold current, the size of the aperture layer is set to only a few μm, and the size needs to be accurately controlled. However, the rate of oxidation of the AlAs layer is highly scattered, and it is difficult to accurately control the size of the aperture layer, and low yield is a problem in the practical application of a surface emitting laser having an AlOx current confinement layer. ing. Normally, a GaAs layer is disposed above and below the AlAs aperture layer (and the AlOx current confinement layer), but a hetero barrier occurs at the interface between the AlAs aperture layer and the GaAs layer, resulting in series resistance and deterioration of electrical characteristics. Also, since the thermal expansion coefficient of AlOx insulator and semiconductor GaAs differ greatly, physical delamination may occur at the interface between the AlOx current confinement layer and the GaAs layer, reducing device yield and device life. There are problems such as shortening.
[0005]
An object of the present invention is to provide an inexpensive application system such as a laser light transmission module and an optical interconnection, or an optical fiber communication using the surface emitting laser with excellent characteristics as a light source by producing a surface emitting laser with excellent characteristics. is there. Another object is to provide a surface emitting laser with low series resistance. A further object is to provide a surface emitting laser having a long wavelength band (wavelength: 1.2 to 1.6 μm) having excellent characteristics.
[0006]
[Means for Solving the Problems]
In order to obtain a laser beam from the active layer that generates light on the substrate crystal and the light generated from the active layer, it has a resonator structure in which the upper and lower sides of the active layer are sandwiched by reflecting mirrors, and the light is perpendicular to the substrate crystal. In a surface emitting laser that emits light, a semiconductor having a band gap of 2 eV or more such as AlInP, AlGaInP, or AlAs, that is, a so-called wide gap semiconductor is used as a current confinement layer, and a photolithography process (opening in the current confinement layer) and a regrowth process (opening) The yield can be greatly improved by forming the aperture layer in the aperture (opening) provided in the current confinement layer by the semiconductor crystal growth). The wide gap semiconductor discussed below in this specification is a semiconductor material having a band gap (forbidden band width) of 2 eV or more, regardless of the type, and the above-described AlInP and AlGaInP satisfy this band gap value. It is required to have such a composition.
[0007]
On the other hand, the material of the aperture layer formed so as to fill the opening formed in the current confinement layer is preferably GaAs or AlGaAs having a band gap smaller than that of the current confinement layer. Further, the series resistance can be reduced by forming a current introduction layer doped at a high concentration on the current confinement layer. Furthermore, by using GaInNAs in the active layer, it can be applied to the 1.3 μm band or 1.55 μm band used in optical fiber communications (especially when a semiconductor layer other than the active layer is formed of GaAs, AlGaAs, AlAs, AlInP) The active layer composition is desirable from the viewpoint).
[0008]
With this configuration, current can be injected around the portion of the active layer facing the aperture layer, and the light generated thereby is a resonance composed of a reflecting mirror facing the aperture layer. The stimulated emission light is generated by reciprocating the vessel. Here, the reflecting mirror refers to a film or a laminated structure of a plurality of films made of a substance having a higher reflectivity than the layer (semiconductor layer) bonded to the active layer side, and the material is a semiconductor or a dielectric, etc. Chosen from. The reflectivities of the reflecting mirror formed on the substrate side and the reflecting mirror formed on the opposite side of the substrate are set according to the specifications of the resonator.
[0009]
The surface emitting laser of the present invention can be used in systems such as optical interconnection and optical fiber communication. In that case, it is preferable to use the surface emitting laser as a laser light transmission module packaged together with an IC or an optical fiber component for driving the surface emitting laser.
[0010]
The structure, manufacturing method, and operation of the surface emitting laser of the present invention will be described with reference to FIG. In the figure, 1 is an n-type GaAs substrate, 2 is an n-type semiconductor multilayer mirror, 3 is a first GaAs spacer layer, 4 is an active layer, 5 is a second GaAs spacer layer, 6 is an AlInP current confinement layer, and 7 is p. GaAs current introduction layer, 8 is a regrowth interface, 9 is a third GaAs spacer layer, 10 is a multilayer reflector, 11 is a p-side electrode, and 12 is an n-side electrode. First, layers 2 to 7 are produced by a crystal growth method. Next, in order to form the aperture layer, the p-type GaAs current introduction layer 7, the AlInP current confinement layer 6, and a part of the second GaAs spacer layer 5 (a region including a portion provided for laser oscillation) are shown in FIG. As shown, it is removed by etching (this removes the current confinement layer from the optical axis for oscillating the laser beam). Thereafter, the p-type GaAs is regrown by a crystal growth method to produce a third GaAs spacer layer 9 (in other words, the opening is filled with GaAs). A portion of the third GaAs spacer layer 9 sandwiched between the current confinement layers 6 serves as an aperture layer. The size of the aperture layer can be accurately controlled because it is produced by a photolithography process. By setting the doping concentration of the aperture layer, that is, the third GaAs spacer layer 9 to about p = 1 × 10 ¹⁸ cm ⁻³ , sufficient conductivity can be obtained, and the optical loss to the laser beam is sufficient. Can be reduced. Finally, the multilayer reflector 10, the p-side electrode 11, and the n-side electrode 12 are formed to complete the structure.
[0011]
The current is injected from the p-side electrode 11 and guided to the aperture layer through the p-type third GaAs spacer layer 9 and the p-type GaAs current introduction layer 7. When the current flows parallel to the current confinement layer 6, the p-type GaAs current introduction layer 7 is doped at an extremely high concentration of p = 1 × 10 ²⁰ cm ⁻³ , so that the resistance is very low and most of the current is this layer. Flowing. Further, since the third GaAs spacer layer 9, the p-type GaAs current introduction layer 7, the aperture layer, and the second GaAs spacer layer 5 are all GaAs, series resistance due to the hetero barrier is not generated. Therefore, a voltage drop hardly occurs between the p-side electrode 11 and the active region immediately below the aperture. In addition, since the series resistance of the n-type semiconductor multilayer film reflecting mirror 2 is small, a surface emitting laser having a very low element resistance can be manufactured.
[0012]
Here, the action of current confinement by AlInP will be described with reference to FIG. AlInP can be lattice-matched with a GaAs substrate, and a good quality crystal can be obtained, so that optical degradation such as laser light scattering is not caused. FIG. 3A shows a band lineup of n-type AlInP and p-type GaAs, and FIG. 3B shows a band line-up of p-type AlInP and p-type GaAs. Even if a hole indicated by + in the figure tries to move from GaAs to AlInP, there is a heterobarrier in the valence band (a line drawn downward in the figure) and it cannot move. That is, no current flows. The size of the heterobarrier is about 2 eV when AlInP is n-type, and about 0.4 eV or more even when AlInP is p-type. In the structure of the present invention, since almost no voltage drop occurs between the p-side electrode 11 and the active region immediately below the aperture as described above, the AlInP layer 6 has a sufficiently large hetero barrier even in the case of the p-type although the n-type is preferable. And acts as a current confinement layer for the p-type GaAs current introduction layer 7. The material for the current confinement layer may be AlGaInP or AlAs, for example, as long as the band gap that can be formed on the GaAs substrate is a wide gap semiconductor of 2 eV or more because the same effect as the present invention can be obtained. The second GaAs spacer layer 5 is preferably a p-type or non-doped layer in order to prevent electrons from flowing into the AlInP current confinement layer 6. In the present invention, since the current is confined by the hetero barrier of the current introduction layer and the wide gap semiconductor, it is more suitable that the surface emitting laser has a low operating voltage. That is, it is better that the active layer 4 has a smaller band gap and an oscillation wavelength longer than 0.85 μm. Specifically, GaInAs and GaInNAs are suitable as the material for the active layer. When the active layer is GaInNAs, a long-wavelength surface emitting laser with excellent characteristics can be manufactured. When the oscillation wavelength is shorter than 0.85 μm, the same effect can be obtained by replacing the GaAs layer with an AlGaAs layer in the above description.
[0013]
Further, since the current confinement layer 6 is a semiconductor, it has substantially the same thermal expansion coefficient as that of the upper and lower GaAs layers, and has a feature that the device life is long without peeling at the heterointerface.
[0014]
Therefore, the surface emitting laser of the present invention has excellent characteristics and a long lifetime, and also has a high yield.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to Examples 1 and 2 and FIGS.
[0016]
<Example 1>
In this example, a surface emitting laser having an emission wavelength of 0.98 μm was manufactured. FIG. 1 shows a sectional view of the structure. 1 is an n-type GaAs substrate (n = 1 × 10 ¹⁸ cm ⁻³ , thickness: d = 100 μm), 2 is an n-type GaAs / AlAs semiconductor multilayer mirror (n = 1 × 10 ¹⁸ cm ⁻³ ), 3 is n-type first GaAs spacer layer (n = 1 × 10 ¹⁶ cm ⁻³ , d = ¼ wavelength thickness in semiconductor), 4 is a non-doped GaInAs / GaAs strained quantum well active layer, 5 is a p-type second GaAs spacer layer (p = 1 × 10 ¹⁶ cm ⁻³ , d = 1/8 wavelength thickness in semiconductor), 6 is an n-type Al _0.5 In _0.5 P current confinement layer (n = 1 × 10 ¹⁷ cm) lattice-matched to the GaAs substrate ^-3 , d = one wavelength thickness in semiconductor), 7 is p-type GaAs current introduction layer (p = 1 × 10 ²⁰ cm ^-3 , d = 1/2 wavelength thickness in semiconductor), 8 is regrowth The interface, 9 is a p-type third GaAs spacer layer (p = 1 × 10 ¹⁸ cm ⁻³ ), 10 is a dielectric multilayer reflector, 11 is a p-side electrode, and 12 is an n-side electrode. As the active layer 4, a strained quantum well layer having a band gap of 1.27 eV (wavelength: 0.98 μm) is used by separating three 7 nm-thick GaInAs well layers by a 10 nm-thick GaAs barrier layer. In the semiconductor multilayer mirror 2, the high refractive index GaAs layer of 1/4 wavelength thickness in the semiconductor and the low refractive index AlAs layer of 1/4 wavelength thickness in the semiconductor are alternately laminated. In order to achieve a reflectance of 99.5% or more, the number of mirror layers laminated was 20 pairs.
[0017]
The semiconductor layer 2-7 was crystal-grown continuously in a high vacuum of 1 × 10 ⁻⁵ Torr using an actinic radiation epitaxy apparatus. Metal group aluminum, gallium and indium were used for Group III materials, and phosphine and arsine were used for Group V materials. Si, Be, and CBr ₄ were used as dopant materials. The wafer is taken out, and the p-type GaAs current introduction layer 7 and the n-type AlInP current confinement layer 6 are selectively etched sequentially with a sulfuric acid-based etching solution and a hydrochloric acid-based etching solution as shown in FIG. An aperture was formed. The wafer is returned to the actinic radiation epitaxy apparatus, and a portion of the p-type second GaAs spacer layer 5 is etched by 10 nm as shown in FIG. 1 using trisdimethylaminoarsine to form a regrowth interface 8 having good crystallinity. did. Thereafter, the third GaAs spacer layer 9 was regrown. A portion of the third GaAs spacer layer 9 sandwiched between the current confinement layers 6 serves as an aperture layer. The size of the aperture layer can be accurately controlled because it is produced by a photolithography process. The thickness of the third GaAs spacer layer 9 is set to 3 + 1/4 wavelength in the semiconductor together with the thickness of the second GaAs spacer layer 5, and finally, a 3.5 wavelength resonator is formed together with the first GaAs spacer layer 3 (first The growth rate of the 3GaAs spacer layer 9 is the same on the regrowth interface 8 and on the p-type GaAs current introduction layer 7). Since the outer portion of the aperture layer deviates from the resonance condition, laser oscillation does not occur and a single transverse mode oscillation is obtained. After forming the ring-shaped p-side electrode 11 having an inner diameter of 10 μm and an outer diameter of 15 μm by the lift-off method, the dielectric multilayer film reflecting mirror 10 was formed by the sputtering method. The dielectric multilayer reflector 10 is formed by alternately laminating a quarter-wave thickness high refractive index amorphous Si layer in a dielectric and a quarter wavelength thickness low refractive index SiO ₂ layer in a dielectric. Produced. In order to achieve a reflectance of 99% or more, the number of layers was set to 5 pairs. In this example, the material system of the amorphous Si layer and the SiO ₂ layer was used for the dielectric multilayer reflector of the surface emitting laser, but the dielectric multilayer reflector is alternately laminated with a high refractive index layer and a low refractive index layer. Therefore, other material systems such as SiN and SiO ₂ , amorphous Si and SiN, or TiO ₂ and SiO ₂ may be used. Thereafter, as shown in FIG. 1, the outside of the dielectric multilayer film reflecting mirror 10 was etched by Cl-based reactive ion beam etching to expose the p-side electrode 11. Finally, the n-side electrode 12 was formed.
[0018]
When current was injected into the surface emitting laser, laser oscillation occurred at a threshold current of 10 μA. The laser light was emitted from the dielectric multilayer film reflecting mirror side, and the oscillation wavelength was 0.98 μm at room temperature. This surface emitting laser had a long element lifetime of 100,000 hours or more. Also, the yield with a 3-inch wafer was as high as 80% or more.
[0019]
<Example 2>
In this example, a surface emitting laser having an emission wavelength of 1.3 μm was manufactured. FIG. 2 shows a sectional view of the structure. 21 is an n-type GaAs substrate (n = 1 × 10 ¹⁸ cm ⁻³ , d = 300 μm), 22 is an n-type GaAs / AlInP semiconductor multilayer mirror (n = 1 × 10 ¹⁸ cm ⁻³ ), 23 is a non-doped first 1 GaAs spacer layer (d = 1/2 wavelength thickness in semiconductor), 24 is undoped GaInNAs / GaAs strained quantum well active layer, 25 is undoped second GaAs spacer layer (d = 2 wavelength thickness in semiconductor), 26 is P-type Al _0.3 Ga _0.2 In _0.5 P current confinement layer lattice matched to GaAs substrate (p = 1 × 10 ¹⁶ cm ^-3 , d = 3/8 wavelength thickness in semiconductor), 27 is p-type GaAs current introduction layer (P = 1 × 10 ¹⁹ cm ⁻³ , d = one wavelength thickness in the semiconductor), 28 is the regrown interface, 29 is the p-type third GaAs spacer layer (p = 1 × 10 ¹⁸ cm ⁻³ ), 30 is A non-doped GaAs / AlInP semiconductor multilayer mirror, 31 is a p-side electrode, and 32 is an n-side electrode. As the active layer 24, a strained quantum well layer having an effective band gap of 0.95 eV (wavelength: 1.3 μm) in which one 7 nm thick GaInNAs well layer is sandwiched between 10 nm thick GaAs barrier layers was used. The semiconductor multilayer mirror 22 includes an Al _0.5 In _0.5 P layer lattice-matched with a high refractive index GaAs layer of 1/4 wavelength thickness in a semiconductor and a low refractive index GaAs substrate of 1/4 wavelength thickness in a semiconductor. Alternatingly stacked. In order to achieve a reflectance of 99.5% or more, the number of mirror layers laminated was 20 pairs.
[0020]
The semiconductor layers 22-27 were continuously grown in a vacuum of 50 Torr using a metal organic vapor phase epitaxy apparatus. Metals trimethylaluminum, trimethylgallium and trimethylindium were used as Group III materials, and dimethylhydrazine, phosphine and arsine were used as Group V materials. Disilane and dimethylzinc were used as dopant materials. As shown in FIG. 2, the wafer is taken out and a part of the p-type GaAs current introduction layer 27, the p-type AlInP current confinement layer 26, and the second GaAs spacer layer 25 is etched with a bromine-based etchant as shown in FIG. A 5 μm aperture was formed. The wafer was returned to the metalorganic vapor phase epitaxy apparatus, and a part of the second GaAs spacer layer 25 was further etched by 10 nm using HCl to form a regrowth interface 28 having good crystallinity. Thereafter, the third GaAs spacer layer 29 was regrown. A portion of the third GaAs spacer layer 29 sandwiched between the current confinement layers 26 becomes an aperture layer. The size of the aperture layer can be accurately controlled because it is produced by a photolithography process. The thickness of the third GaAs spacer layer 29 was set to 3.5 wavelengths in the semiconductor together with the thickness of the first GaAs spacer layer 25, and finally the four-wavelength resonator was formed together with the GaAs spacer layer 23. Since the outer portion of the aperture layer deviates from the resonance condition, laser oscillation does not occur and a single transverse mode oscillation is obtained. Subsequently, a non-doped GaAs / AlInP semiconductor multilayer mirror 30 was grown. The semiconductor multilayer mirror 30 is formed by alternately laminating a high refractive index GaAs layer having a quarter wavelength thickness in a semiconductor and an AlInP layer lattice-matching with a low refractive index GaAs substrate having a quarter wavelength thickness in the semiconductor. . In order to make the reflectivity 99% or more, the number of mirror layers was 15 pairs. Thereafter, as shown in FIG. 2, the outside of the semiconductor multilayer reflector 10 was etched to the second GaAs spacer layer 29 with a bromine-based etchant. Finally, a ring-shaped p-side electrode 31 and an n-side electrode 32 having an inner diameter of 7 μm and an outer diameter of 15 μm were formed.
[0021]
When current was injected into the surface emitting laser, laser oscillation occurred at a threshold current of 100 μA. The laser light was emitted from the dielectric multilayer film reflecting mirror side, and the oscillation wavelength was 1.3 μm at room temperature. This surface emitting laser had a long element lifetime of 100,000 hours or more. Also, the yield with a 3-inch wafer was as high as 70% or more. The above performance is very excellent as a long wavelength surface emitting laser.
[0022]
【The invention's effect】
According to the present invention, since a wide gap semiconductor having a band gap of 2 eV or more such as AlInP, AlGaInP, or AlAs is used for the current confinement layer, and the aperture layer is formed by the photolithography process and the regrowth process, the yield can be greatly improved. Therefore, a large amount of surface emitting lasers having excellent characteristics and long life can be provided at low cost. Therefore, the surface emitting laser can be used in laser light transmission modules and application systems such as optical interconnection or optical fiber communication.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a surface emitting laser according to Embodiment 1 of the present invention.
FIG. 2 is a sectional view of a surface emitting laser according to Example 2 of the present invention.
FIG. 3 is a diagram showing a band lineup (a) of n-type AlInP and p-type GaAs and a band lineup (b) of p-type AlInP and p-type GaAs.
[Explanation of symbols]
1 ... n-type GaAs substrate, 2 ... n-type semiconductor multilayer mirror, 3 ... n-type first GaAs spacer layer, 4 ... active layer, 5 ... p-type second GaAs spacer layer, 6 ... AlInP current confinement layer, 7 ... p Type GaAs current introduction layer, 8 ... regrowth interface, 9 ... p-type third GaAs spacer layer, 10 ... multilayer reflector, 11 ... p-side electrode, 12 ... n-side electrode.

Claims (5)

Reflective mirrors (2, 10) above and below the active layer (4) in order to obtain laser light from the active layer (4) that generates light on the GaAs substrate (1) crystal and the light generated from the active layer (4) In a surface emitting laser that has a resonator structure sandwiched between and emits light perpendicular to the substrate (1) crystal,
A current confinement layer made of a wide gap semiconductor having a band gap of 2 eV or more, and an aperture layer made of GaAs or AlGaAs provided on the inner side in a two-dimensional section from the current confinement layer;
AlInP current introduction layer doped with impurities at a high concentration is provided only on the current confinement layer,
One of the reflecting mirrors is a dielectric multilayer film reflecting mirror (10) provided on the aperture layer and straddling the current introduction layer,
The current introduction layer and the aperture layer are of the same conductivity type,
A surface-emitting laser, wherein a P-type electrode (11) is provided outside the dielectric multilayer mirror (10) as viewed in the two-dimensional section.   2. The surface emitting laser according to claim 1, wherein the current confinement layer is one selected from the group consisting of AlInP, AlGaInP, and AlAs.   3. The surface emitting laser according to claim 1, wherein GaInAs or GaInNAs is used for the active layer.   4. The surface emitting laser according to claim 1, wherein the wavelength of the laser beam is longer than 0.85 [mu] m.   4. The surface emitting laser according to claim 1, wherein the wavelength of the laser light is in a 1.3 [mu] m band or 1.55 [mu] m band used in optical fiber communication.

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