JP3876918B2 - Surface emitting semiconductor laser device - Google Patents

Description

  The present invention relates to a surface-emitting type semiconductor laser device, and more particularly to a surface-emitting type semiconductor laser device that emits a stable laser beam in a higher-order mode.

A surface-emitting type semiconductor laser element is a semiconductor laser element that emits laser light in a direction orthogonal to a substrate surface, and is a semiconductor laser element that is attracting attention as a light source in various fields because of mounting advantages.
A surface-emitting type semiconductor laser device is provided between a pair of upper and lower reflecting mirrors (DBR: Diffractive Bragg Reflector) composed of a pair of compound semiconductor layers having different refractive indexes, and a pair of DBRs. And an active layer to be formed on the semiconductor substrate.
The surface emitting semiconductor laser device generally has a post-type mesa structure in which the upper DBR has a current confinement region, and a circular post-type mesa structure having a mesa diameter of about 30 μφ by etching the upper DBR by a method such as dry etching. In many cases, a current confinement structure by selective oxidation of an AlAs layer is provided in a circular post-type mesa structure for high-efficiency injection of current into the active layer (see, for example, Patent Document 1).

Here, the configuration of a conventional surface emitting semiconductor laser element having a post-type mesa structure will be described with reference to Patent Document 1 and FIG. FIG. 12 is a cross-sectional view showing the configuration of a conventional surface emitting semiconductor laser element disclosed in the above publication, that is, FIG.
According to the above-mentioned publication, the surface emitting semiconductor laser element 80 has a lower reflector layer structure (hereinafter referred to as an n-type semiconductor multilayer film) formed on an n-type GaAs substrate 82 in sequence, as shown in FIG. 84, a lower clad layer 86 made of non-doped AlGaAs, a light emitting layer (active layer) 88, an upper clad layer 90 made of non-doped AlGaAs, and an upper reflector layer structure made of a p-type semiconductor multilayer film (hereinafter referred to as upper part). DBR) 92, and a p-type GaAs cap layer 94.

The lower DBR 84 is a 30.5 pair semiconductor multilayer in which an n-type Al _0.2 Ga _0.8 As layer and an n-type Al _0.9 Ga _0.1 As layer are stacked with a composition gradient layer interposed at a hetero interface. The upper DBR 92 is composed of 25 pairs in which a p-type Al _0.2 Ga _0.8 As layer and a p-type Al _0.9 Ga _0.1 As layer are stacked with a composition gradient layer interposed at a hetero interface. It is comprised as a semiconductor multilayer film.

The cap layer 94, the upper DBR 92, the upper clad layer 90, the active layer 88, the lower clad layer 86, and the upper portion of the lower DBR 84 are etched and processed into a cylindrical columnar structure (mesa post) 96.
The compound semiconductor layer closest to the active layer 88 of the upper DBR 92 is formed by forming a p-type AlAs layer in place of the p-type Al _0.9 Ga _0.1 As layer, except for the central circular region. The AlAs layer is an oxide confinement type current confinement layer 98 in which Al is selectively oxidized and converted into an Al oxide layer.
The central circular region remains the original p-type AlAs layer and functions as a current injection path (current injection region) 98A, while the Al oxide layer is an oxide confinement type insulating region (current confinement region) having high electrical resistance. ) It functions as 98B.

A SiN _X film 100 is formed on the upper surface, the side surface, and the lower DBR 84 on both sides of the mesa post 96.
Further, the SiN _X film 100 on the upper surface of the mesa post 96 has an opening that is removed in a circular shape to expose the p-type GaAs cap layer 94, and an annular p-side electrode (upper electrode) 102 is formed along the periphery of the opening. Is formed. An n-side electrode (lower electrode) 104 is formed on the back surface of the n-type GaAs substrate 82. Reference numeral 106 denotes an extraction electrode of the p-side electrode 102.

A method for manufacturing the surface-emitting type semiconductor laser device 80 disclosed in the above publication will be described with reference to FIGS. FIGS. 13A and 13B are cross-sectional views showing respective manufacturing steps of the conventional surface emitting semiconductor laser element 80.
First, as shown in FIG. 13A, on an n-type substrate 82, a lower DBR 84, a lower cladding layer 86 made of non-doped AlGaAs, an active layer 88, an upper cladding layer 90 made of non-doped AlGaAs, an upper DBR 92, and A p-type GaAs cap layer 94 is laminated to form a laminated structure.

When the lower DBR 84 is stacked, 30.5 pairs of an n-type Al _0.2 Ga _0.8 As layer and an n-type Al _0.9 Ga _0.1 As layer with a composition gradient layer interposed at the heterointerface, When the upper DBR 92 is stacked, 25 pairs of p-type Al _0.2 Ga _0.8 As layers and p-type Al _0.9 Ga _0.1 As with a composition gradient layer interposed at the hetero interface are stacked. Laminate layers.
When the upper DBR 92 is stacked, the constituent layer closest to the active layer 88 or the constituent layer near the active layer 88 is replaced with a p-type Al _0.9 Ga _0.1 As layer. 108 are stacked.

Next, as shown in FIG. 13B, the p-type GaAs cap layer 94, the upper DBR 92, the AlAs layer 108, and the upper cladding layer are etched by etching using the SiN _X film mask 110 until the vicinity of the upper surface of the lower DBR 84 is reached. 90, the active layer 88, and a part of the lower cladding layer 86 are etched to form a mesa post 96.
Subsequently, the laminated structure having the mesa post 96 is heated in a water vapor atmosphere at a temperature of 400 ° C. for about 25 minutes, and only the p-type AlAs layer 108 is selectively oxidized from the side surface of the mesa post 96 toward the center.
Thereby, an annular current confinement region 98B made of an Al oxide layer surrounded by the side surface of the mesa post 96, and a circular current in which the p-type AlAs layer 108 surrounded by the current confinement region 98B remains without being oxidized. A current confinement layer 98 having an injection region 98A can be formed.

Subsequently, after forming a SiN _X film 100 on the entire surface, the SiN _X film 100 on the upper surface of the mesa post 96 to expose the p-type GaAs layer 94 is removed in a circular, a p-side electrode 102 there in an annular form To do. Further, the conventional surface emitting semiconductor laser element 80 can be completed by performing a process such as forming the n-side electrode 104 on the back surface of the n-type GaAs substrate 82.

  In the surface emitting semiconductor laser device having such a post-type mesa structure, the current confinement layer 98 can define the cross section of the current path injected into the active layer 88, so that the active layer 88 in the vicinity of the current injection region 98B Current is concentrated and injected into the region, so that efficient laser oscillation can be realized.

By the way, the conventional surface-emitting type semiconductor laser device generally oscillates in a multimode in which the transverse mode has a plurality of peaks when viewed in a far field pattern (FFP).
On the other hand, when a surface-emitting type semiconductor laser device is lens-coupled to an optical waveguide, for example, an optical fiber, in the field of communication or the like, the emitted laser light of the surface-emitting type semiconductor laser device is simply used to increase the optical coupling efficiency. A beam having a ridged Gaussian distribution shape, that is, a so-called single transverse mode laser beam is desirable.

In the current confinement structure of the oxide confinement type, the number of excited laser light modes is roughly proportional to the size of the current confinement diameter. Therefore, if the current injection region of the current confinement layer is narrowed to some extent, the active layer itself A single mode excited in a narrow area can be obtained as output light.
Therefore, in a conventional surface emitting semiconductor laser device having an oxide confinement type current confinement structure, the current confinement diameter (current injection region diameter) of the current confinement structure made of an Al oxide layer is reduced to emit light from the active layer. The area is reduced, and the single transverse single transverse mode is selectively oscillated.

Japanese Patent Laying-Open No. 2001-210908 (FIG. 1) JP 2002-359432 A JP 2001-24277 A JP-A-9-246660

  By the way, when trying to control the transverse mode to the single transverse mode by reducing the current confinement diameter of the current confinement structure, it seems to be reported to IEEE. Photon Tech. Lett. Vol.9 No.10 p1304 M. Grabherr et al. In addition, a very small constriction diameter of 4 μmφ or less is required as a current confinement diameter. However, when the current confinement diameter is 4 μmφ or less, the following problems occur.

First, even if an attempt is made to actually manufacture a surface emitting semiconductor laser device having such a small current confinement diameter, since the current confinement diameter is extremely small, the allowable range of manufacturing errors is small and the controllability is manufactured. This is a problem that it is difficult to perform, and the uniformity within the wafer surface is deteriorated, so that the yield is extremely deteriorated.
Secondly, since the current is squeezed and flowed through the current injection region (AlAs layer region) having an area one digit smaller than that of a normal device, the element resistance becomes high, for example, higher than 100Ω. As a result, the output, current / light output efficiency is lowered. In other words, since the single transverse mode property depends on the output, it is difficult to increase the output in the single transverse mode of the surface emitting semiconductor laser element.
Third, as a result of impedance mismatch due to resistance increase due to current confinement, even if an attempt is made to operate a surface-emitting type semiconductor laser device with high-frequency driving necessary for communication, there is a significant tendency to deteriorate high-frequency characteristics. It is difficult to apply to optical transmission of high frequency drive required in the field.

As a lateral mode control of the laser light of the surface emitting semiconductor laser element, for example, a method of stabilizing the lateral mode by processing the light emitting surface has been proposed. The purpose is different from that of the single mode of the transverse mode (see, for example, Patent Document 2).
In addition, it has been proposed to provide a reflectance distribution on the reflective surface opposite to the light emitting surface to stabilize the transverse mode. However, since it is a substrate emission type, it is difficult to apply to a surface emitting semiconductor laser device. Since it is premised on the proton injection type, it is difficult to apply to the oxidized constriction type (see, for example, Patent Document 3).
Furthermore, a method has been proposed in which a lens structure using a circular diffraction grating is provided inside the laser structure to stabilize the transverse mode, but the process is complicated and technical due to the necessity of regrowth of the compound semiconductor layer. And there is an economic problem (see, for example, Patent Document 4).

As described above, it is difficult to convert the laser light of the surface-emitting type semiconductor laser element into a single-peak transverse mode by the conventional technique, and it is difficult to emit a stable laser light in a higher-order mode.
Accordingly, an object of the present invention is to provide a surface emitting semiconductor laser element that emits a stable laser beam in a higher-order mode.

The present inventor repeated experiments in the research process, so that the oscillation lateral mode of the surface-emitting type semiconductor laser element is not only the current confinement diameter that has been attracting attention in the past, but also the optical configuration of the mesa upper surface that becomes the light emitting surface, For example, the present inventors have found that the structure of the mesa upper surface such as the refractive index distribution of the mesa upper surface and the electrode shape is greatly influenced.
Then, surface emitting semiconductor laser elements having different configurations and shapes on the mesa upper surface were manufactured as prototypes, and the relationship between the mesa upper surface configuration and the transverse mode was examined. It was found that not only the shape, refractive index, and film thickness, but also the structure of the electrode on the contact layer was greatly affected.

  As a result of further research, as shown in FIG. 14A, a contact layer extending on the upper DBR with a first opening exposing the upper DBR on the upper DBR, An electrode made of a metal film having a second opening that exposes the upper DBR inside the opening and extending annularly on the contact layer, and a third that exposes the contact layer outside the first opening A structure having an insulating film provided with an opening and interposed between the contact layer and the electrode, having a high impurity concentration in the current injection region, and improving the uniformity of the current injection density in that region is a simple structure. The present inventors have found that this contributes to the realization of a ridged transverse mode surface emitting semiconductor laser device.

With the configuration on the upper DBR as described above, the peripheral region of the second opening of the electrode, the peripheral region of the first opening of the contact layer, and the peripheral region of the third opening of the insulating film are the second region. A complex refractive index distribution structure is formed in which the complex refractive index isotropically changes from the center of the opening to the outside. The present inventors have found that the transverse mode can be controlled to be unimodal by this complex refractive index distribution structure.
It has also been found that the contact layer and the electrode constitute a complex refractive index distribution structure.
14A is a schematic cross-sectional view showing the configuration of the main part of the surface emitting semiconductor laser device according to the first invention, and FIG. 14B corresponds to FIG. 14A. It is a typical sectional view explaining the function.

  Based on the above knowledge, a surface emitting semiconductor laser element capable of controlling the transverse mode to be unimodal before the surface emitting semiconductor laser element according to the present invention will be described. In this example, a lower reflecting mirror made of a semiconductor multilayer film, an active layer, and an upper reflecting mirror made of a semiconductor multilayer film are sequentially provided on a substrate. In a surface-emitting type semiconductor laser device formed as a mesa post having a first semiconductor substrate, a compound semiconductor layer having a first opening exposing the upper reflecting mirror and extending on the upper reflecting mirror, and an inner side of the first opening And a metal film extending in a ring shape on the compound semiconductor layer, the metal film and the compound semiconductor layer extending outward from the center of the second opening. A complex refractive index distribution structure in which the complex refractive index changes is formed.

  In the surface emitting semiconductor laser element, preferably, the complex refractive index distribution structure is a structure in which the complex refractive index isotropically changes from the center of the second opening to the outside. This further facilitates the unimodality of the transverse mode.

As a surface emitting semiconductor laser element capable of controlling a higher-order mode, the surface emitting semiconductor laser element of the present invention is sequentially formed on a substrate from a lower reflector made of a semiconductor multilayer film, an active layer, and a semiconductor multilayer film. In a surface-emitting type semiconductor laser device formed as a mesa post having a current confinement layer in a region in the vicinity of the active layer of the upper or lower reflector, a first opening for exposing the upper reflector is provided. A compound semiconductor layer extending on the upper reflecting mirror, and a ring film extending in a ring shape on the compound semiconductor layer with a second opening exposing the upper reflecting mirror inside the first opening, It is characterized in that it comprises a metal film and a second upper reflector on the arranged isolated like islands islets shaped film in the opening.

  In a preferred embodiment of the present invention, the semiconductor device includes a third opening that exposes the compound semiconductor layer outside the first opening, and has an insulating film interposed between the compound semiconductor layer and the metal film, The metal film, the compound semiconductor layer, and the insulating film form a complex refractive index distribution structure in which the complex refractive index changes outward from the center of the second opening.

  In a specific embodiment of the present invention, the compound semiconductor layer constitutes a contact layer in ohmic contact with the metal film. In addition, a current injection region provided in the center of the current confinement layer is located below the first opening.

In the surface-emitting type semiconductor laser device capable of controlling the above-described transverse mode to be unimodal, as shown in FIG. 14A, a compound semiconductor layer such as a contact layer or an insulating layer is formed on the light emitting surface on the upper DBR. It consists of an electrical structure composed of three elements: a layer and an electrode. This electrical structure also has an optical function.
When the structure on the upper DBR is disassembled into optical elements, as shown in FIG. 14B, first, a contact layer having a first opening and extending in a ring shape, a third opening An insulating layer having a portion and extending in a ring shape on the contact layer is formed in a step shape. As a result, the complex refractive index increases outward from the center of the first opening, that is, the center of the light emitting surface, thereby forming a complex refractive index distribution structure that acts as a concave lens.
Second, an electrode made of a metal film formed on the light emitting surface and having a second opening smaller than the first opening has an aperture function with respect to light, and has a complex refractive index of the metal. Considering this, it is equivalent to a complex refractive index distribution structure in which a convex lens and an absorptive aperture are combined.

  That is, in this surface-emitting type semiconductor laser device, a composite optical system that combines an optical structure in which a convex lens and an absorptive aperture are combined and a concave lens is formed on the light emitting surface. In addition, the composite optical system functions as a part of the resonator structure because it is disposed in contact with the resonator structure of the surface emitting semiconductor laser element.

  In the surface-emitting type semiconductor laser device capable of controlling the transverse mode to be unimodal with the above configuration, as shown in FIG. 14B, out of the laser resonance modes selected to some extent by the current confinement layer, Higher-order modes with wide angles are scattered by the concave lens structure, absorbed by the absorptive aperture, and further converged by the convex lens, which is taken in as a resonance condition in the resonator, and the aperture effect of the current confinement layer It is thought that almost one mode is forcibly selected by the action and effect of the combination, resulting in unimodal oscillation.

This technical idea makes it possible to control more various transverse modes of the surface-emitting type semiconductor laser device, and to control higher-order modes.
That is, according to the present invention, an island-shaped metal film is provided in an annular metal film, and the island mode is based on the same technical idea as the surface-emitting type semiconductor laser device capable of controlling the above-mentioned transverse mode to be unimodal. By changing the shape of the film, the complex refractive index distribution can be adjusted, and the surface emitting semiconductor laser element can be oscillated in the desired transverse mode of the higher order mode.

  The compound semiconductor layer is composed of a plurality of compound semiconductor layers having different impurity concentrations, and the opening provided in each compound semiconductor layer is stepped from top to bottom of each layer. The compound refractive index distribution structure is formed in such a manner that the impurity concentration of each compound semiconductor layer decreases step by step from the upper layer to the lower layer in a stepwise manner. It may be formed.

In general, the compound semiconductor layer constitutes a contact layer in ohmic contact with the metal film.
Preferably, the non-oxidized current injection region provided in the central region of the current confinement layer is located below the first opening, and the impurity concentration of the current injection region is 5 × 10 ¹⁸ cm ⁻³ or more. Thus, the current injection density of the current injection region is uniform over the entire region. Thereby, the effect that the above-mentioned compound optical system functions as a part of the resonator structure can be further enhanced.

  The surface-emitting type semiconductor laser device capable of controlling the above-described transverse mode to be unimodal is a semiconductor multilayer film having a lower reflector made of a semiconductor multilayer film, an active layer, and a high Al content layer sequentially on a substrate. Forming a top reflecting mirror and a contact layer, etching a top reflecting mirror including a high Al content layer to form a mesa post, and steam oxidizing the high Al content layer of the mesa post to form a high Al Leaving a central region of the containing layer as a first current injection region made of a non-oxidized high Al-containing layer and forming a current confinement region made of an Al oxide layer surrounding the current injection region; and on the contact layer of the mesa post and Forming an insulating film on the side surface of the mesa post, providing an opening in the insulating film on the contact layer, exposing the contact layer, and opening an opening smaller than the insulating film in the contact layer A step of exposing the upper reflecting mirror, and a step of providing a metal film constituting the electrode on the upper reflecting mirror and the contact layer, and providing an opening smaller than the opening of the contact layer in the metal film to expose the upper reflecting mirror. And can be manufactured.

Further, in the step of forming a contact layer on the upper reflector, a plurality of contact layers whose impurity concentration is sequentially or continuously decreased stepwise from layer to layer from the upper layer to the lower layer are formed to insulate In the process of providing an opening smaller than the opening of the film in the contact layer and exposing the upper reflector, the impurity concentration of the contact layers of the plurality of layers decreases stepwise from layer to layer sequentially or continuously. Using the difference in the etching rate due to the above, openings are formed in each contact layer in such a manner that the opening diameter decreases stepwise sequentially or continuously from layer to layer from the upper layer to the lower layer.
A contact layer is formed so that the impurity concentration of each contact layer decreases stepwise from layer to layer in a stepwise manner from the upper layer to the lower layer, and the opening diameter of each layer from the upper layer to the lower layer By forming openings that gradually decrease in a stepwise manner or continuously decrease, a complex refractive index distribution structure can be easily formed.

Furthermore, in the step of forming a contact layer on the upper reflector, a plurality of contact layers are formed in which the Al composition decreases sequentially or continuously stepwise from layer to layer from the upper layer to the lower layer, In the process of providing an opening smaller than the opening of the insulating film in the contact layer and exposing the upper reflecting mirror, the Al composition of the multiple contact layers decreases stepwise from layer to layer sequentially from the upper layer to the lower layer By utilizing the difference in the etching rate due to this, openings are formed in each contact layer in such a manner that the opening diameter decreases stepwise sequentially or continuously from layer to layer from the upper layer to the lower layer.
A contact layer is formed so that the Al composition of each contact layer decreases stepwise from layer to layer in a stepwise manner from the upper layer to the lower layer, and the opening diameter of each layer from the upper layer to the lower layer By forming openings that gradually decrease in a stepwise manner or continuously decrease, a complex refractive index distribution structure can be easily formed.

  As described above, according to the present invention, the complex refractive index is changed outward from the center of the opening of the annular metal film by the annular metal film, the island-shaped metal film, and the annular compound semiconductor layer. By constructing the refractive index distribution structure on the upper reflecting mirror, it is possible to realize a surface emitting semiconductor laser element capable of controlling the transverse mode to a desired higher-order mode.

Hereinafter, the present invention will be described in more detail based on exemplary embodiments with reference to the accompanying drawings. It should be noted that the conductivity type, film type, film thickness, film forming method, and other dimensions shown in the following embodiments are examples for facilitating understanding of the present invention, and the present invention is limited to these examples. Is not to be done.
An example of a surface emitting semiconductor laser device capable of controlling the transverse mode to be unimodal First, prior to the description of the surface emitting semiconductor laser device according to the present invention, it is possible to control the above-described transverse mode to be unimodal. An example of a possible surface emitting semiconductor laser element will be described. FIG. 1 is a cross-sectional view showing the structure of the surface emitting semiconductor laser element, FIG. 2 is a top view of the surface emitting semiconductor laser element of FIG. 1, and FIG. FIG. 3B is a schematic cross-sectional view illustrating the function of the main part corresponding to FIG. 3A.
As shown in FIG. 1, the surface-emitting type semiconductor laser device 10 includes a lower reflecting mirror (hereinafter referred to as a lower DBR) 14 formed of an n-type semiconductor multilayer film sequentially formed on an n-type GaAs substrate 12. Al _0.3 Ga _0.7 As lower clad layer 16, GaAs light emitting layer (active layer) 18, Al _0.3 Ga _0.7 As upper clad layer 20, upper reflector layer made of p-type semiconductor multilayer film ( (Hereinafter referred to as upper DBR) 22 and a p-type GaAs contact layer 24 having a film thickness of 150 nm and an impurity concentration of 5 × 10 ¹⁸ cm ⁻³ .

The lower DBR 14 is configured as a semiconductor multilayer film having a total film thickness of about 4 μm in which 35 pairs of n-type AlAs layers and n-type GaAs layers are stacked, and the upper DBR 22 includes p-type Al _0.9 Ga _0.1 As layers and The p-type Al _0.1 Ga _0.9 As layer is configured as a semiconductor multilayer film of 25 pairs with a total film thickness of about 3 μm.
As shown in FIGS. 1 and 2, the contact layer 24, the upper DBR 22, the upper cladding layer 20, the active layer 18, the lower cladding layer 16, and the upper portion of the lower DBR 14 are cylindrically shaped mesa posts 26 having a mesa diameter of 40 μm by etching. It is formed as.

The layer closest to the active layer 18 of the upper DBR 22 is a circular AlAs layer region 26A having a diameter of 12 μm provided in the center, instead of the p-type Al _0.9 Ga _0.1 As layer, and a circular AlAs layer. This is an oxide confinement type current confinement layer 26 composed of an Al oxide layer region 26B provided annularly around the region, and the thickness of the AlAs layer region 26A is 30 nm.
The AlAs layer region 26A remains a p-type AlAs layer formed in place of the p-type Al _0.9 Ga _0.1 As layer, and the Al oxide layer region 26B is selectively made of Al in the p-type AlAs layer. It is oxidized to be converted into an Al oxide layer. The Al oxide layer region 26B is a region having a high electric resistance and functions as a current confinement region, while the central circular AlAs layer region 26A remains the original p-type AlAs layer, and the electric resistance is current confinement. It functions as a current injection region lower than the region 26B.

On the upper surface of the mesa post 26, the contact layer 24 is formed as an annular layer that opens a circular first opening 30 with an inner diameter of 20 μm in the central region and exposes the upper DBR 22 through the first opening 30. ing.
An insulating film, for example, a 300 nm-thickness SiO ₂ film 32 extends on the peripheral edge of the contact layer 24, the side surface of the mesa post 26, and the lower DBR 14 on both sides. The SiO ₂ film 32 on the contact layer 24 has a circular third opening 34 having an inner diameter of 35 μm larger than the first opening 30 to expose the contact layer 24 in an annular shape.

A p-side electrode 36 made of a Ti / Pt / Au metal laminated film having a thickness of 500 nm is formed on the contact layer 24 from the upper DBR 22 on the inner periphery of the first opening 30 of the contact layer 24 and then on the SiO 2 film 32. A circular second opening 38 having an inner diameter of 14 μm is formed on the upper DBR 22, and the upper DBR 22 is exposed from the second opening 38.
The diameter (12 μm) of the AlAs layer region (current injection region) 28A of the current confinement layer 28 is slightly smaller than the inner diameter (14 μm) of the third opening 38 of the p-side electrode 36, as shown in FIG.
An n-side electrode 40 made of AuGe / Ni / Au is formed on the back surface of the n-type GaAs substrate 12.

With reference to FIG. 3, the optical elements on the upper DBR 22 in this example will be schematically described. In the surface emitting semiconductor laser element 10, the contact layer 24, the SiO ₂ film 32, and the p-side electrode 36 on the upper DBR 22 have both an electrical function and an optical function.

When the structure described above is disassembled into optical elements, as shown in FIG. 3A, first, a contact layer 24 having a first opening 30 and extending in a ring shape, a third opening is provided. A SiO ₂ film 32 having a portion 34 and extending in a ring shape is formed on the contact layer 24 in a step shape. As a result, the complex refractive index isotropically increases outward from the center of the first opening 30, that is, the center of the light emitting surface, so that the complex lens that acts as a concave lens 42 is used as shown in FIG. A refractive index distribution structure is formed.
Second, the p-side electrode 36 having the second opening 38 has an aperture function with respect to light, and considering the complex refractive index of the metal of the p-side electrode 36, FIG. As shown, it has an optical function equivalent to a complex refractive index distribution structure in which an absorptive aperture 44 and a convex lens 46 are synthesized.
For example, the complex refractive index of gold (Au) is 0.2 for the real part of the refractive index and 5.6 for the imaginary part (absorption coefficient) for the laser light having a wavelength of 0.85 μm.

That is, in this surface emitting semiconductor laser element 10, the refractive index of the contact layer 24 having the first opening 30 is larger than the refractive index of the opening region. In addition, the absorption coefficient of the p-side electrode 36 having the second opening 38 is larger than the absorption coefficient of the opening region.
As a result, a composite optical system including the optical structure obtained by combining the convex lens 42 and the absorptive aperture 44 and the concave lens 46 is formed on the light emitting surface. In addition, since the composite optical system is disposed in contact with the resonator structure of the surface-emitting type semiconductor laser device 10, this structure functions as a part of the resonator structure.

With the above configuration, in the surface-emitting type semiconductor laser device 10, as shown in FIG. 3B, among the laser resonance modes selected to some extent by the current confinement action of the current confinement layer 28, a high-order output with a wide emission angle. The mode is scattered by the concave lens 42, absorbed by the absorptive aperture 44, and further converged by the convex lens 46.
In addition, these functions and effects are taken in as a resonance condition in the resonator, and in combination with the uniform light emission aperture effect of the current confinement layer 28, almost one transverse mode is forcibly selected and is unimodal. It oscillates.
Therefore, when the light output is increased, even if light oscillates in the multimode transverse mode, due to the action of the concave lens 42, the aperture 44 and the convex lens 46, almost one transverse mode is combined with the aperture effect of the current confinement layer 28. Since the mode is forcibly selected, the multimode transverse mode is unimodal and becomes a single transverse mode.

The surface-emitting type semiconductor laser device 10 having the above-described configuration was manufactured by the method described later, and the FFP full width at half maximum was measured to be 5.5 ° as shown in FIG. This is less than half the value of a conventional surface emitting semiconductor laser device having a constriction diameter of about 4 μm, and can be evaluated as a unimodal transverse mode. FIG. 4 is a graph showing an FFP (far field image) of the surface emitting semiconductor laser element 10. Here, the H waveform and the V waveform are intensity distributions of the outgoing beam in planes orthogonal to each other.
That is, in this example, the birefringence distribution that changes outward from the center of the second opening 38, that is, the center of the light emitting surface, by the stepped configuration of the contact layer 24, the SiO ₂ film 32, and the p-side electrode 40. And the transverse mode is changed to a single mode.

The surface-emitting type semiconductor laser device 10 having the above-described configuration can output the same optical output as the conventional multi-mode surface-emitting semiconductor laser device, and is the same as the conventional multi-mode surface-emitting semiconductor laser device. Since it is an electrical structure, resistance and impedance are equivalent.
Since the surface emitting semiconductor laser element 10 emits laser light in a single-peak transverse mode, the surface emitting semiconductor laser element 10 can be optically coupled to an actual optical fiber with high optical coupling efficiency.

Example of Manufacturing Method of Surface Emitting Semiconductor Laser Device This example is an example of a manufacturing method of a surface emitting semiconductor laser device capable of controlling the above-described transverse mode to be unimodal. FIGS. 5 (a) and 5 (b), FIGS. 6 (c) and (d), and FIGS. 7 (e) and 7 (f) show the surface emitting semiconductor laser device 10 manufactured according to the method of this example, respectively. It is sectional drawing explaining each process of these.
First, as shown in FIG. 5A, a lower DBR 14, a lower cladding layer 16, a light emitting layer (active layer) 18, an upper cladding layer 20, and an upper DBR 22 are sequentially formed on an n-type GaAs substrate 12 by MOCVD or the like. And a p-type GaAs contact layer 24 are formed.
When the upper DBR 22 is formed, an AlAs layer 26 having a thickness of 30 nm is formed as a layer closest to the active layer 18 of the upper DBR 22 in place of the p-type Al _0.9 Ga _0.1 As layer.

Next, as shown in FIG. 5B, the contact layer 24, the upper DBR 22, the upper cladding layer 20, the active layer 18, the lower cladding layer 16, and the upper portion of the lower DBR 14 are formed by a dry etching method using a chlorine-based gas. Etching is performed to form a cylindrical mesa post 26 having a mesa diameter of 40 μm.
Next, steam oxidation is performed at a temperature of 400 ° C. to selectively oxidize Al in the AlAs layer 26 from the outer periphery of the mesa post 26 to leave a circular AlAs layer region 26A having a diameter of 12 μm in the center and the AlAs layer region. An Al oxide layer region 26B is annularly formed around 26A to form an oxide confinement type current confinement layer.

Next, as shown in FIG. 6C, an SiO 2 film 32 is formed on the contact layer 24 of the mesa post 26, the side surface of the mesa post 26, and the lower DBR 14 beside the mesa post 26.
Next, as shown in FIG. 6D, the SiO 2 film 32 is etched to open an opening 34 having an inner diameter of 35 μm.

Subsequently, as shown in FIG. 7E, the contact layer 24 exposed from the opening 34 is etched to form an opening 36 having an inner diameter of 20 μm.
Next, as shown in FIG. 7F, a Ti / Pt / Au metal laminated film 39 is formed on the upper surface of the mesa post 26.
Further, the metal laminated film 39 is etched to provide an opening 38 to form the p-side electrode 34, the n-type GaAs substrate 12 is polished to a predetermined substrate thickness, and then the n-side electrode 38 is formed on the n-type GaAs substrate 12. The structure of the surface-emitting type semiconductor laser device 10 shown in FIG.

  As described above, the surface-emitting type semiconductor laser device 10 can be manufactured in the same manufacturing process as that of the conventional surface-emitting type semiconductor laser device only by changing the processing dimensions of the contact layer 24 and the p-side electrode 36.

FIG. 8 shows another example of a surface-emitting type semiconductor laser device capable of controlling the transverse mode to be unimodal. It is sectional drawing which shows this structure.
The main part 50 of the surface emitting semiconductor laser element is the surface emitting semiconductor laser element shown in FIG. 1 except that the configuration of the contact layer 52 is different and the configuration of the p-side electrode 54 is different. The same configuration as that of the example is provided.

As shown in FIG. 8, the contact layer 52 of this example is composed of an upper layer, an intermediate layer, and a lower layer of three contact layers 52A, 52B, and 52C in which the impurity concentration gradually decreases.
The lower contact layer 52C has, for example, an impurity concentration of 5 × 10 ¹⁸ cm ⁻³ , the lowest impurity concentration among the three layers, and the largest opening portion 56C among the three layers. . The intermediate contact layer 52B has, for example, an impurity concentration of 1 × 10 ¹⁹ cm ⁻³ , an impurity concentration higher than the lower layer, lower than the upper layer, and smaller than the lower opening 56C, and the upper opening 56A. A larger opening 56B is provided. The upper contact layer 52A has, for example, an impurity concentration of 3 × 10 ¹⁹ cm ⁻³ , the highest impurity concentration among the three layers, and the smallest opening portion 56A among the three layers. .
The p-side electrode 54 is formed in a step shape according to the shapes of the contact layers 52A, 52B, 52C and the respective openings 56A, 56B, 56C.

  With the configuration of the contact layer 52 and the p-side electrode 54 as described above, an effective complex refractive index distribution structure is configured, and the light focusing effect is enhanced, so that unimodality is further facilitated.

  In order to form the above-described three-stage opening of the contact layer 52, as shown in FIG. 9, an etching mask 58 is provided on the upper contact layer 52A in which the impurity concentration is lowered stepwise, and the etching rate is different due to the difference in impurity concentration. By utilizing the difference, the three contact layers 52A, 52B, and 52C are dry-etched under the same etching conditions, whereby the three-layer contact layers 52A, 52B, and 52C have openings 56A of a desired size, 56B and 56C can be opened.

  In this example, instead of the impurity concentration, a contact layer containing three layers of Al, an upper layer, an intermediate layer, and a lower layer, in which the Al composition of the contact layer decreases stepwise, is formed, and then etched due to the difference in Al composition By utilizing the difference in speed and dry etching the three contact layers under the same etching conditions, it is also possible to open openings in the three contact layers, the opening diameter of which gradually decreases from the upper layer to the lower layer.

Embodiment of Surface Emitting Semiconductor Laser Device According to the Present Invention Although the surface emitting semiconductor laser device capable of controlling the above-described transverse mode to be unimodal, the transverse mode is made to be a single mode. The technical idea of the invention enables more various transverse mode control of the surface emitting semiconductor laser element, and can also control higher-order modes.
The present embodiment is an example of an embodiment of a surface emitting semiconductor laser element according to the present invention. FIGS. 10A to 10C show the surface emitting semiconductor laser element of the present embodiment, respectively. It is sectional drawing which shows a structure, a top view, and the wave form diagram of transverse mode.
The surface-emitting type semiconductor laser device of this embodiment is a surface-emitting type semiconductor laser device that emits a TE ₀₁ mode (doughnut-like light emitting pattern), and its main part 60 is shown in FIGS. 10 (a) and 10 (b). As shown in FIG. 5, p is composed of an island-shaped film 64 made of a circular metal film and an annular film 68 made of an annular metal film provided in the same manner as in the first embodiment via an annular emission window 66. A side electrode 62 is provided.
The surface-emitting type semiconductor laser device according to this embodiment has the same configuration as that of the surface-emitting type semiconductor laser device 10 shown in FIG. 1 except that a circular island-shaped film 64 is provided.

The effect of the p-side electrode 62 composed of the contact layer 24 and the annular film 68 made of a metal film in the present embodiment is the same as the effect of the complex refractive index distribution structure described in the single mode surface emitting semiconductor laser device 10. Thus, the single fundamental mode lower than the desired higher order mode is suppressed, and at the same time, the higher order mode than the desired higher order mode is suppressed.
In this embodiment, the fundamental mode is absorbed and suppressed by the island-shaped film 64 made of a circular metal film (gold) provided in the central region of the light emitting surface, the aperture effect by the current confinement layer 28 (see FIG. 1), and the contact layer The TE ₀₁ mode is selectively emitted by scattering higher-order modes than the TE ₀₁ mode by the concave lens effect due to the complex refractive index distribution of 24.
In this example embodiment, if the oxide confinement diameter of the current confinement layer in TE ₀₁ mode transverse mode cutoff condition is satisfied except for, the higher the more TE ₀₁ mode selection effect.

In a conventional example related to higher-order mode control, for example, Japanese Patent Laid-Open No. 2002-35432 discloses a groove (or uneven shape) having a half-wavelength or a quarter-wave depth on the mesa surface, and an unnecessary excitation mode. It proposes to select a mode by canceling by interference or by enhancing a required mode.
However, this method can provide a function to the mesa in a post-processing process such as ion beam etching, but the workability is poor because each device is processed one by one, and even if etching is performed by patterning, it becomes an interference optical path difference. Since it is necessary to increase the required accuracy of the groove depth, it is considered difficult to apply to practical devices.

  On the other hand, in this embodiment, a mode contributing to laser resonance can be selected by providing a complex refractive index distribution structure at the top of the resonator structure. In addition, the complex refractive index distribution structure based on the shape and refractive index of the compound semiconductor layer, the insulating film, and the electrode on the top surface of the mesa can be processed without applying an additional process to the conventional manufacturing process. In addition, the fabrication accuracy of each part of the complex refractive index distribution structure is equivalent to the manufacturing accuracy of ordinary surface-emitting semiconductor laser elements, and high accuracy is not necessary, and the general-purpose process processing accuracy currently used is sufficient. Also, production reproducibility is good.

Comparative Example This comparative example is a comparative example of the embodiment of the surface emitting semiconductor laser device according to the present invention, and FIG. 11 is a cross-sectional view showing the configuration of the surface emitting semiconductor laser device of this comparative example.
As shown in FIG. 11, the main part 70 of the surface-emitting type semiconductor laser device of this comparative example includes a contact layer 72 having a scattering structure for scattering light randomly on the upper surface of the mesa and a fine uneven surface.
As a result, the oscillation mode is affected by scattering due to unevenness on the surface of the contact layer 72, and a large number of modes oscillate at random. This light emission includes a considerable number of modes, and a random pattern light emission can be obtained.

It is sectional drawing which shows the structure of an example of a surface emitting semiconductor laser element. FIG. 2 is a top view of the surface emitting semiconductor laser element of FIG. 1. FIG. 3A is a schematic cross-sectional view showing the configuration of the main part of an example of the surface emitting semiconductor laser element, and FIG. 3B is a schematic diagram illustrating the function of the main part corresponding to FIG. FIG. It is a graph which shows FFP (far-field image) of an example of a surface emitting semiconductor laser element. FIGS. 5A and 5B are cross-sectional views illustrating respective steps in manufacturing the surface emitting semiconductor laser element. FIGS. 6C and 6D are cross-sectional views for explaining each process in manufacturing the surface emitting semiconductor laser element, following FIG. 5B. FIGS. 7E and 7F are cross-sectional views illustrating respective steps in manufacturing the surface emitting semiconductor laser device, following FIG. 6D. It is sectional drawing which shows the structure of the other example of a surface emitting semiconductor laser element. It is sectional drawing explaining the process at the time of producing the other example of a surface emitting semiconductor laser element. 10A to 10C are a cross-sectional view, a plan view, and a transverse mode waveform diagram showing the configuration of the surface-emitting type semiconductor laser device according to the present invention, respectively. It is sectional drawing which shows the structure of the surface emitting semiconductor laser element of a comparative example. It is sectional drawing which shows the structure of the conventional surface emitting semiconductor laser element. 13A and 13B are cross-sectional views showing respective manufacturing steps of a conventional surface emitting semiconductor laser element. FIG. 14A is a schematic cross-sectional view showing the configuration of the main part of an example of the surface-emitting type semiconductor laser element, and FIG. 14B is a schematic cross-sectional view explaining the function of the main part.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Surface emitting semiconductor laser element, 12 ... n-type GaAs substrate, 14 ... lower DBR, 16 ... lower clad layer, 18 ... light emitting layer (active layer), 20 ... upper clad layer, 22 ... ... upper DBR, 24 ... p-type GaAs contact layer, 26 ... mesa post, 28 ... current confinement layer, 28A ... AlAs layer region, 28B ... Al oxide layer region, 30 ... opening, 32 ... SiO2 Membrane, 34... Opening, 36... P-side electrode, 38... Opening, 39... Metal laminated film, 40... N-side electrode, 42 ... concave lens, 44. 50... Main part of surface emitting semiconductor laser element, 52... Contact layer, 54... P-side electrode, 56. ... P-side electrode, 64 ... Insular film, 66 ...... Annular exit window, 68 ....... annular film, 70 .essential part of comparative surface emitting semiconductor laser element, 72... Contact layer, 80... Conventional surface emitting semiconductor laser element, 82. n-type GaAs substrate, 84 ... lower DBR, 86 ... lower clad layer, 88 ... light-emitting layer (active layer), 90 ... upper clad layer, 92 ... upper DBR, 94 ... p-type GaAs cap layer, 96 ... Mesa post, 98 ... Current confinement layer, 98A ... Current injection region, 98B ... Current confinement region, 100 ... SiNX film, 102 ... p-side electrode, 104 ... n-side electrode, 106 ... extraction electrode

Claims (8)

In a surface-emitting type semiconductor laser device comprising a lower reflecting mirror made of a semiconductor multilayer film, an active layer, and an upper reflecting mirror made of a semiconductor multilayer film on a substrate in order,
A compound semiconductor layer having a first opening exposing the upper reflector and extending on the upper reflector;
An annular film having a second opening that exposes the upper reflector inside the first opening and extending annularly on the compound semiconductor layer, and the upper reflector in the second opening the surface-emitting type semiconductor laser element characterized by comprising a metal film consisting of island-shaped arranged isolated islets like membrane on top.   2. The surface according to claim 1, wherein the metal film and the compound semiconductor layer form a complex refractive index distribution structure in which a complex refractive index changes outward from the center of the second opening. Light emitting semiconductor laser element. A third opening that exposes the compound semiconductor layer outside the first opening; and an insulating film interposed between the compound semiconductor layer and the metal film made of the annular film,
The metal film, the compound semiconductor layer, and the insulating film form a complex refractive index distribution structure in which a complex refractive index changes outward from the center of the second opening. 2. The surface emitting semiconductor laser device according to 1. The compound semiconductor layer is composed of a plurality of compound semiconductor layers having different impurity concentrations,
The opening provided in each compound semiconductor layer is formed as an opening in which the opening diameter is gradually reduced stepwise for each layer from the upper layer to the lower layer of the plurality of layers,
2. The surface emitting semiconductor laser device according to claim 1, wherein the impurity concentration of each of the compound semiconductor layers is sequentially decreased step by step from the upper layer to the lower layer of the plurality of layers.   2. The surface-emitting type semiconductor laser device according to claim 1, wherein the compound semiconductor layer constitutes a contact layer in ohmic contact with the metal film.   2. The surface emitting semiconductor laser device according to claim 1, further comprising a current confinement layer in a region near the active layer of the upper reflecting mirror or the lower reflecting mirror. Non-oxidized current injection region which is provided in a central region of the current confinement layer is positioned below the first opening, Ru der impurity concentration of 5 × 10 ¹⁸ cm ^-3 or more of said current injection region The surface-emitting type semiconductor laser device according to claim 6.   2. The surface emitting semiconductor laser device according to claim 1, wherein at least the upper reflecting mirror is etched to form a mesa post.

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