JP2009032716A - Surface emitting laser, and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface emitting laser which has an enlarged waveguide diameter, even with the diameter, a fundamental transverse mode light propagation can be obtained by making small a refractive index difference between an oxidation region and a non-oxidation region constituting a current constriction structure, and thereby light output of the surface emitting laser is increased, and to provide a manufacturing method thereof. <P>SOLUTION: The surface light emitting laser is equipped with semiconductor multilayer film reflecting mirrors and has the current constriction structure having the oxidation region and the non-oxidation region formed by selectively oxidizing an Al<SB>x</SB>Ga<SB>1-x</SB>As layer being a semiconductor layer constituting one semiconductor multilayer film reflecting mirror, wherein the oxidation region has a structure in which the refractive index difference from the non-oxidation region is made small by making the refractive index of the oxidation region large by increasing the Ga density through a heat treatment. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a surface emitting laser having a selective oxidation structure.

Surface emitting lasers have many advantages such as low threshold and low power consumption, circular spot shape, easy coupling with optical elements, and arraying. Applications are expected as a light source for optical recording and electrophotography.
Among such surface-emitting lasers, a selective oxidation surface-emitting laser having a current confinement structure manufactured by selectively oxidizing a part of the layers constituting the multilayer mirror is particularly low in threshold current. Known to achieve high power conversion efficiency.

As one of such selective oxidation type surface emitting lasers, Patent Document 1 discloses one using an Al _x Ga _1-x As oxide layer for current confinement and lateral mode control.
Specifically, this has a structure as shown in FIG.
In FIG. 5, 5110 is a resonator, 5112 is an active layer, 5114 is an upper spacer layer, 5116 is a lower spacer layer, 5120 is a p-type semiconductor multilayer reflecting mirror, and 5122 is a non-oxidized layer.
Reference numeral 5160 denotes an n-type semiconductor multilayer reflection mirror, 5162 denotes a high refractive index, and 5164 denotes a low refractive index layer.
Here, an oxide layer 5250 is formed in the layer structure of the p-semiconductor multilayer reflective mirror 5120 constituting the surface emitting laser, and the current flowing through the active layer 5112 is controlled by utilizing the insulating property of the oxide layer. It is taken.
US Pat. No. 5,903,589

By the way, when the Al _x Ga _1-x As oxide layer is used for current confinement as in the selective oxidation surface emitting laser of Patent Document 1 as the conventional example, from the viewpoint of the manufacturing process, the Al composition ratio X is: A thickness of about 0.98 and a thickness of about 20 nm to 30 nm are suitable.
On the other hand, when this oxide layer is regarded as a transverse mode control layer, it can be seen that the refractive index difference between the oxide layer and the surrounding laser material is very large.
In an optical waveguide such as an optical fiber, it is known that when the relative refractive index difference between the core and the clad is large, the core diameter at which the fundamental transverse mode light propagation is obtained becomes small by increasing the total reflection angle.
Similarly, also in the selective oxidation surface emitting laser, when the relative refractive index difference between the oxide layer and the surrounding material increases, the core diameter, that is, the laser waveguide diameter decreases.
Therefore, the subject that the optical output of a laser is restrict | limited arises.

In view of the above problems, the present invention can increase the waveguide diameter at which the fundamental transverse mode light propagation can be obtained by reducing the difference in refractive index between the oxidized region and the non-oxidized region constituting the current confinement structure,
An object of the present invention is to provide a surface emitting laser capable of increasing the light output and a manufacturing method thereof.

The present invention provides a surface emitting laser configured as follows and a method for manufacturing the same.
The surface emitting laser according to the present invention includes an oxide region formed by selectively oxidizing an Al _x Ga _1-x As layer, which is a semiconductor layer constituting one of the semiconductor multilayer reflection mirrors, including a semiconductor multilayer reflection mirror And a surface emitting laser having a current confinement structure with a non-oxidized region,
The oxidized region has a structure in which the Ga concentration is increased by heat treatment to increase the refractive index, and the difference in refractive index from the non-oxidized region is reduced.
Further, in the surface emitting laser of the present invention, the increase in the refractive index by the heat treatment of the oxidized region is selectively performed on a region adjacent to the non-oxidized region constituting a partial region of the oxidized region, Only the adjacent region has a high refractive index.
The surface emitting laser of the present invention is characterized in that the Ga concentration in the region by the heat treatment has a Ga concentration of 5 to 25 times that of the region by the non-heat treatment.
Further, the surface emitting laser of the present invention, the interface between the Al _x Ga _1-x As layer and the other semiconductor layer in the current confinement structure, constituting the semiconductor multilayer reflection mirror,
Zn is added in an amount of 5 × 10 ¹⁸ cm ⁻³ to 5 × 10 ¹⁹ cm ⁻³ .
The method for manufacturing a surface-emitting laser according to the present invention is a method for manufacturing a surface-emitting laser in which a current confinement structure is formed in a semiconductor layer constituting a semiconductor multilayer reflective mirror,
In order to form the current confinement structure, an Al _x Ga _1-x As layer, which is a semiconductor layer constituting one of the semiconductor multilayer film reflection mirrors, is selectively oxidized to form a current confinement structure by an oxidized region and a non-oxidized region. Forming, and
Performing a heat treatment on the oxidized region, increasing a Ga concentration by interdiffusion with a peripheral semiconductor layer, and forming a structure in which a difference in refractive index from the non-oxidized region is reduced by increasing the refractive index; ,
It is characterized by having.
In the surface emitting laser manufacturing method of the present invention, the heat treatment may be a selective heat treatment using a lamp, and the heat treatment using the lamp may be performed with the non-oxidized region constituting a partial region of the oxidized region. It is characterized in that it is selectively performed on adjacent areas. In the method of manufacturing a surface emitting laser according to the present invention, the heat treatment is a selective heat treatment using a pulse laser, and the heat treatment by the pulse laser forms the partial region of the oxidized region. It is characterized in that it is selectively performed on the adjacent region.
Further, in the method of manufacturing the surface emitting laser according to the present invention, the interface between the Al _x Ga _1-x As layer and another semiconductor layer in the current confinement structure, which constitutes the semiconductor multilayer reflection mirror,
The method further includes a step of adding 5 × 10 ¹⁸ cm ⁻³ to 5 × 10 ¹⁹ cm ^{−3 of} Zn to accelerate the mutual diffusion.

  According to the present invention, by reducing the difference in refractive index between the oxidized region and the non-oxidized region constituting the current confinement structure, the waveguide diameter capable of obtaining fundamental transverse mode light propagation can be increased, and the light output can be increased. It can be increased.

Next, an embodiment of the present invention will be described.
In FIG. 1, as an example of the selective oxidation surface emitting laser of this embodiment, the entire oxidized region constituting the current confinement structure is increased in refractive index by increasing the Ga concentration by heat treatment, and the refractive index with respect to the non-oxidized region. The schematic diagram explaining the structural example which has the structure which made the difference small is shown.

In FIG. 1, 110 is a resonator, 112 is an active layer, 114 is an upper spacer layer, 116 is a lower spacer layer, 120 is a p-type semiconductor multilayer reflective mirror, 122 is a non-oxidized layer, and 124 is a heat-treated oxide layer. .
Reference numeral 160 denotes an n-type semiconductor multilayer film reflecting mirror, 162 denotes a high refractive index layer, and 164 denotes a low refractive index layer.

The selective oxidation surface emitting laser according to the present embodiment is basically a structure in which two high reflection mirrors, a p-type semiconductor multilayer reflection mirror 120 and an n-type semiconductor multilayer reflection mirror 160, are sandwiched between laser resonators 110 including an active layer 112. It has a structure.
Furthermore, a current confinement structure for performing laser oscillation with a low threshold current has an oxide region formed by selectively oxidizing an Al _x Ga _1-x As layer, which is a semiconductor layer constituting one of the semiconductor multilayer reflection mirrors, and And a non-oxidized region.
Specifically, this current confinement structure was obtained by further heat-treating an oxide layer formed by steam-oxidizing the non-oxidized layer 122 composed of Al _0.98 Ga _0.02 As in the lateral direction from the periphery of the mesa. The heat treatment oxide layer 124 is provided.
The heat-treated oxide layer 124 is obtained by interdiffusion with surrounding materials as will be described in detail later.
The oscillation wavelength is controlled by spacer layers 114 and 116 provided above and below the resonator 110.
Furthermore, the reflectivity of the p-type semiconductor multilayer film reflection mirror 120 and the n-type semiconductor multilayer film reflection mirror 160 is controlled by a high-refractive-index ¼ wavelength film (high refractive index layer) 162 containing a large amount of aluminum.
Further, it is controlled by the refractive index and the number of quarter wavelength films (low refractive index layers) 164 on the low refractive index side with less aluminum.

An important parameter for simultaneously performing current confinement and transverse mode control in this heat-treated oxide layer 124 is:
The refractive index of the heat-treated oxide layer 124, the thickness of the heat-treated oxide layer 124, the position of the heat-treated oxide layer, the resistivity of the heat-treated oxide layer, the aperture diameter of the non-oxidized layer, the refractive index of the non-oxidized layer, and the like.
From the viewpoint of current confinement, the position of the heat-treated oxide layer is preferably closer to the active layer, but from the viewpoint of transverse mode control, it is necessary to emphasize equivalent refraction.
That is, the VCSEL-specific equivalent refractive index considering the refractive index of the heat-treated oxide layer and the refractive index of the non-oxidized layer, the respective thicknesses, as well as the intensity distribution in the oscillation axis direction determined by the resonator length, is obtained.
By setting the maximum aperture diameter that can be maintained in the basic mode, it is possible to realize high output of the VCSEL.

In the current confinement in the conventional selective oxidation surface emitting laser, the refractive index of the non-oxidized layer is 3.3 and the refractive index of the oxidized layer is about 1.6. As a result, the relative refractive index difference is about 48%. The maximum aperture diameter calculated from this is about 7 μmφ.
On the other hand, in the selective oxidation type surface emitting laser of this embodiment, as described in the next example, for example, the refractive index of the heat-treated oxide layer is different from that of the non-oxidized layer. Can be reduced to a maximum of about 2.5 to reduce the refractive index difference.
As a result, the diameter of the waveguide from which the fundamental transverse mode light propagation can be obtained can be increased, and the light output of the laser can be increased.
In the above description, a configuration example in which the entire oxidized region constituting the current confinement structure is increased in refractive index has been described, but the present invention is not limited to such a configuration example.
For example, a high refractive index by heat treatment of the oxidized region is selectively performed on a region adjacent to a non-oxidized region that constitutes a partial region of the oxidized region, and only the adjacent region is increased in refractive index to increase the refractive index. You may comprise so that the refractive index difference with an oxidation area | region may be made small.

Examples of the present invention will be described below.
[Example 1]
In Example 1, a selective oxidation type surface emitting laser in which the entire oxidized region constituting the current confinement structure is increased in refractive index and the difference in refractive index from the non-oxidized region is reduced will be described.
Since the basic structure of the present example is not fundamentally different from the selective oxidation surface emitting laser of the present embodiment described above, it will be described with reference to FIG.

In the surface emitting laser of this embodiment, the active layer 112 is constituted by a quantum well active layer, spacer layers 114 and 116 above and below it, and p-type semiconductor multilayer reflection mirror 120 and n-type semiconductor multilayer reflection above and below them. The mirror 160 is formed by stacking.
In addition, AlInGaP is used as the spacer layers 114 and 116, and the thickness is adjusted so that a resonator having an optical thickness of one wavelength is obtained.
Note that the emission wavelength, the number of wells, or the spacer layer can be adjusted as necessary.
The quantum well active layer 112 used here includes, for example, three InGaP well layers. In this case, in the example, the emission center wavelength was set to 660 nm, and the resonance wavelength of the resonator 110 was set to 670 nm.

In the multilayer film reflection mirror, low refractive index layers 164 and high refractive index layers 162 are alternately stacked.
The low refractive index layer 164 is made of Al _0.9 Ga _0.1 As, and the high refractive index layer 162 is made of Al _0.5 Ga _0.5 As.
The optical thicknesses of the low refractive index layer and the high refractive index layer are both 1/4 wavelength optical thicknesses.
The number of pairs was set so that the reflectance at the oscillation wavelength was 99.98% and 99.6% on the substrate side and the emission side. Specifically, 60 pairs and 36 pairs are set.

The current confinement is composed of an Al _0.98 Ga _0.02 As oxide (Al ₂ O ₃ ) region indicated by 124 and a non-oxidized region of Al _0.98 Ga _0.02 As indicated by 122.
The thickness of Al _0.98 Ga _0.02 As is 30 nm, is configured as a part of the multilayer reflective mirror, and is combined with the low refractive index layer 164 and the high refractive index layer 162 so as to satisfy the above-mentioned quarter wavelength condition. .

In addition, the position of the heat-treated oxide layer 124 that performs current confinement is located in the first layer from the boundary of the upper semiconductor multilayer film reflecting mirror 120 from the resonator 110 toward the upper semiconductor multilayer film reflecting mirror.
Since the main component of the current confinement layer after oxidation is Al _x O _y , the refractive index is greatly reduced from about 3.3 to about 1.6 in the Al _0.98 Ga _0.02 As layer 122.
Therefore, the current confinement layer has a great influence not only on the current confinement effect but also on the optical waveguide characteristics due to an effective refractive index change.

Conventional oxide VCSELs are difficult to propagate in a single transverse mode (0th order) at a large aperture due to such a large refractive index difference, and the light output is also typically less than 1 mW in red. .
In the present embodiment, the tip portion which is a part of the oxidized current confinement layer, that is, only the region where the light distribution becomes large, is within the Al _x O _y layer from the surrounding Al _x Ga _1-x As layers 200, 220, 230. The refractive index is increased by diffusion of Ga and As.
Thereby, the difference in refractive index from the current injection layer 200 is reduced, and the above-described problem is improved.

In this example, the current confinement layer was designed to have a distance of about 110 nm from the active layer, an opening size of about 4 to 10 μmφ, a diffusion region length of 2 to 5 μm, and a thickness of 30 nm to 100 nm.
In the present embodiment, as a method of increasing the refractive index of the entire oxide layer, it is possible to increase the speed of mutual diffusion between the oxide layer using Zn diffusion and its peripheral portion.
The refractive index of the oxide layer is increased by causing mutual diffusion between the oxide layer and the peripheral Al _x Ga _1-x As layer by heat treatment.
For example, when heat treatment is performed at 650 ° C. to 900 ° C. for about 1 to 60 minutes, the As and Ga concentrations in the region by heat treatment are about 5 times or more and 25 times or less compared to the region by non-heat treatment. It can be expected to have a concentration.

Furthermore, Zn may be used as a method for increasing the mutual diffusion.
In the present embodiment, for example, a region having a width of 10 nm centering on the interface between the heating oxidation region 200 and the Al _x Ga _1-x As layers 270 and 230 immediately above the region is 5 × 10 ¹⁸ cm ⁻³ to 5 × 10 ¹⁹ cm ^{−. The} above results can be expected by selectively adding ³ Zn.
In the case of the above heating conditions, the refractive index is such that the mutual diffusion proceeds to a state where there is no difference in concentration between the heat-treated oxide layer 124 and the non-oxide layer 122.
If the refractive index of the heat-treated oxide layer 124 is 1.6 and the refractive index of the non-oxidized layer 122 is 3.3, and a linear approximation can be expected, the refractive index of the heating region is 2.5 at maximum.
As a result, the maximum diameter of single transverse mode propagation can be increased by 40% compared to the conventional case, and the area can be increased by 196%, and assuming that the optical output is proportional to the area, the single transverse mode light output is 196% compared with the conventional case.

Next, a specific manufacturing method of the selective oxidation surface emitting laser shown in FIG. 1 will be described.
In this embodiment, an organic metal compound vapor phase growth method (MOCVD method) is used here as a crystal growth method.
If there is a method capable of producing a high-quality semiconductor crystal with high accuracy, the method is not limited to the MOCVD method, and a molecular beam epitaxy method or the like can be used.
As the formation of the current confinement structure, a selective steam oxidation method of Al _x Ga _1-x As is used.
After the crystal growth, the mesa shown in FIG. 1 is formed by dry etching.

Thereafter, the Al _0.98 Ga _0.02 As layer is oxidized from the side surface of the mesa by heating in water vapor.
This makes it possible to obtain a configuration capable of concentrating the current injected into the surface emitting laser in the central portion of the mesa by utilizing the fact that the oxidized region (Al _x O _y ) is insulative.
In addition, since the oxidation rate is proportional to the amount of aluminum contained in Al _x Ga _1-x As, it is selected by creating a region where the Al concentration is higher than the surrounding area in a specific layer of the upper semiconductor multilayer reflective mirror 120. Thus, an oxidized region can be formed.
After performing the above selective oxidation, a protective film for preventing As loss is formed on the crystal surface, and a heat treatment is performed in a nitrogen atmosphere at 650 ° C. to 900 ° C. for 1 minute to 60 minutes.
As a result, a structure in which the Ga concentration is increased to increase the refractive index and the difference in refractive index from the non-oxidized region is reduced is formed.

[Example 2]
In the second embodiment, a configuration example of a selective oxidation type surface emitting laser in which only a portion of the oxidized region constituting the current confinement structure is increased in refractive index to reduce the difference in refractive index from the non-oxidized region will be described. .
FIG. 2 is a schematic diagram illustrating a configuration example of the selective oxidation surface emitting laser according to the present embodiment.
In FIG. 2, reference numeral 200 denotes an Al _x Ga _1-x As layer, 230 denotes an Al _x Ga _1-x As layer, 250 denotes an oxide layer, and 270 denotes an Al _x Ga _1-x As layer.

In the selective oxidation surface emitting laser of the present embodiment, the heated oxidation region 200 has Ga and As atoms increased and oxygen atoms decreased compared to the non-oxidized layer 122.
This is what Al _x Ga _1-x As layer of the heating oxidized region 200 and the peripheral portion 122,270,230 has occurred by causing mutual diffusion by heating.
For example, when a heat treatment is performed at 650 ° C. to 900 ° C. for about 1 to 60 minutes, As and Ga can be expected to be about 5 to 25 times as a concentration difference from the oxide layer 250 in the heated oxidation region 200.
Furthermore, Zn can also be used as a method for increasing interdiffusion.

Next, a method for forming the selective heating region of this embodiment will be described.
In FIG. 3, the selective heating process using the lamp heating by the reflective film mask is performed selectively on the adjacent area to the non-oxidized area constituting the partial area of the oxidized area, thereby forming the selectively heated area. The schematic diagram explaining the method is shown.
FIG. 4 illustrates a method of forming a selective heating region by performing selective heating processing using a pulse laser selectively on a region adjacent to a non-oxidized region constituting a partial region of the oxidized region. A schematic diagram is shown.

In this embodiment, lamp heating and pulsed laser heating using the reflective film mask 310 can be used as a method for forming the selective heating region.
In the case of the selective heat treatment using lamp heating, as shown in FIG. 3, the heat ray from the lamp 300 is reflected by the reflective film mask formation region, and the region 320 without the reflective film mask in which the protective film 330 is formed. Only selectively heat.
As a heating condition, heating is performed at 400 to 900 ° C. for about 1 to 60 minutes in an atmosphere of an inert gas such as nitrogen or argon.
In the case of heat treatment using a pulse laser, as shown in FIG. 4, first, alignment is performed by an XYZ stage 400 in order to control the position and depth of the heating region.
When a GaAs substrate is used, the light source 410 having a wavelength of 860 nm or longer that is not absorbed by the crystal layer is condensed by the optical system 420 and heated at 430.
For example, it is considered that the effect of interdiffusion can be expected with irradiation of 1 to 10 times at a peak power density of (0.1 to 1 GW / cm ² ) with 100 femtosecond pulses.
In the case of a circular irradiation pattern, about 15 spots are formed with a 10 μm oxidation aperture.

Example of selective oxidation type surface emitting laser in which the entire oxidized region constituting the current confinement structure is made high in refractive index and the difference in refractive index from the non-oxidized region is reduced in the embodiment of the present invention and in Example 1. FIG. Example of the selective oxidation surface emitting laser according to the second embodiment of the present invention, in which only a portion of the oxidized region constituting the current confinement structure is increased in refractive index to reduce the difference in refractive index from the non-oxidized region. FIG. The schematic diagram explaining the formation method of the selective heating area | region using the lamp heating by the reflecting film mask in Example 2 of this invention. The schematic diagram explaining the formation method of the selective heating area | region using the pulse laser in Example 2 of this invention. The schematic diagram explaining the surface emitting laser which has the selective oxidation structure in a prior art example.

Explanation of symbols

110: Resonator 112: Active layer 114: Upper spacer layer 116: Lower spacer layer 120: p-type semiconductor multilayer reflection mirror 122: Non-oxidized layer 124: Heat-treated oxide layer 160: n-type semiconductor multilayer reflection mirror 162: High refractive index layer 164: low refractive index layer _{200: Al x Ga 1-x} As layer (thermal oxidation region)
230: Al _x Ga _1-x As layer 250: Oxide layer 270: Al _x Ga _1-x As layer 300: Heat source lamp 310: Reflective film mask 320: Area without reflective film 330: Protective film 400: X—Y— Z stage 410: pulsed laser light source 420: optical system 430: heating region

Claims (8)

Current narrowing by an oxidized region and a non-oxidized region formed by selectively oxidizing an Al _x Ga _1-x As layer, which is a semiconductor layer constituting one of the semiconductor multilayered film reflecting mirrors, including a semiconductor multilayered film reflecting mirror A surface emitting laser having a structure,
The surface-emitting laser characterized in that the oxidized region has a structure in which the Ga concentration is increased by heat treatment to increase the refractive index, and the difference in refractive index from the non-oxidized region is reduced.   The increase in the refractive index by heat treatment of the oxidized region is selectively performed on a region adjacent to the non-oxidized region that constitutes a partial region of the oxidized region, and only the adjacent region is increased in refractive index. The surface emitting laser according to claim 1, wherein   3. The surface emitting laser according to claim 1, wherein the Ga concentration in the region by the heat treatment has a Ga concentration of 5 to 25 times that of the region by the non-heat treatment. At the interface between the Al _x Ga _1-x As layer and the other semiconductor layer in the current confinement structure that constitutes the semiconductor multilayer film reflection mirror,
4. The surface emitting laser according to claim 1, wherein Zn is added in an amount of 5 × 10 ¹⁸ cm ⁻³ to 5 × 10 ¹⁹ cm ⁻³ . A method of manufacturing a surface emitting laser in which a current confinement structure is formed in a semiconductor layer constituting a semiconductor multilayer reflection mirror,
In order to form the current confinement structure, an Al _x Ga _1-x As layer, which is a semiconductor layer constituting one of the semiconductor multilayer film reflection mirrors, is selectively oxidized to form a current confinement structure by an oxidized region and a non-oxidized region. Forming, and
Performing a heat treatment on the oxidized region, increasing a Ga concentration by interdiffusion with a peripheral semiconductor layer, and forming a structure in which a difference in refractive index from the non-oxidized region is reduced by increasing the refractive index; ,
A method for manufacturing a surface emitting laser, comprising:   The heat treatment is a selective heat treatment using a lamp, and the heat treatment by the lamp is selectively performed on a region adjacent to the non-oxidized region constituting a partial region of the oxidized region. A method for manufacturing a surface emitting laser according to claim 5.   The heat treatment is a selective heat treatment using a pulse laser, and the heat treatment by the pulse laser is selectively performed on a region adjacent to the non-oxidized region constituting a partial region of the oxidized region. A method of manufacturing a surface emitting laser according to claim 5. At the interface between the Al _x Ga _1-x As layer and the other semiconductor layer in the current confinement structure that constitutes the semiconductor multilayer film reflection mirror,
8. The method according to claim 5, further comprising the step of adding 5 × 10 ¹⁸ cm ⁻³ to 5 × 10 ¹⁹ cm ^{−3 of} Zn to accelerate the interdiffusion. The manufacturing method of the surface emitting laser of description.

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