JP2001156395A - Surface emission semiconductor laser element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface emission semiconductor laser element in which oscillation transverse mode can be controlled. SOLUTION: A layer structure of a semiconductor material, where an emission layer 4 is sandwiched between upper and lower reflector layer structures 5, 2, is formed on a substrate 1. An opening located above the upper reflector layer structure 5 is coated with layers, e.g. dielectric films 8, 8A, transparent for the oscillation wavelength of laser light to form a laser light outgoing window 6A. Oscillation transverse mode of laser light can be controlled by varying the plan view of the outgoing window 6A.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface emitting semiconductor laser device, and more particularly, to a surface emitting semiconductor laser device capable of controlling a transverse mode of an oscillating laser beam.

[0002]

2. Description of the Related Art Recently, researches have been conducted to realize a large-capacity optical communication network or an optical data communication system such as optical interconnection and optical computing. Surface emitting semiconductor laser devices manufactured by using GaN have attracted attention.

FIG. 11 shows an example A of a basic layer structure of such a surface emitting laser device. In this element A, a lower reflector layer structure 2 is formed by alternately stacking thin layers of, for example, n-type AlGaAs having different compositions on a substrate 1 made of, for example, n-type GaAs. On the lower reflecting mirror layer structure 2, a lower cladding layer 3a made of, for example, i-type AlGaAs, a light emitting layer 4 made of a quantum well structure made of GaAs / AlGaAs, and an i-type AlGa
An upper cladding layer 3b made of As is sequentially laminated, and further, on this upper cladding layer 3b, for example, p
After the upper reflector layer structure 5 formed by alternately stacking thin layers of the AlGaAs type is formed, the upper reflector layer structure 5 is formed.
A p-type GaAs layer 6 is formed on the surface of the uppermost layer to form the entire layer structure. Then, at least a portion of the layer structure up to the upper surface of the lower reflecting mirror layer structure 2 is etched away, and a columnar layer structure is formed at the center.

Ga in a columnar layer structure located at the center
An annular upper electrode 7a made of, for example, AuZn is formed near the periphery of the upper surface of the As layer 6, and a lower electrode 7b made of, for example, AuGeNi / Au is formed on the back surface of the substrate 1.
Are formed. By covering the side surface of the columnar portion of the entire surface and the peripheral portion of the surface of the GaAs layer 6 located outside the upper electrode 7a with the dielectric film 8 made of, for example, SiNx, the GaAs is formed. A partial surface 6a of the layer 6, that is, a portion inside the upper electrode 7a is a circular opening 6C that functions as an emission window for laser light, and further covers the surfaces of the upper electrode 7a and the dielectric film 8. For example, an electrode lead pad 7c made of Ti / Pt / Au is formed.

In the laser element A, the lowermost layer of the upper reflector layer structure 5, that is, the layer 3c located closest to the light emitting layer 4 is formed of, for example, p-type AlAs. The outer peripheral portion of the AlAs layer 3c has an annular shape in plan view.
The insulating region 3d mainly composed of Al ₂ O ₃ formed by selectively oxidizing only the As layer forms a current confinement structure for the light emitting layer 4.

In this laser element A, the upper electrode 7
By operating the lower electrode 7a and the lower electrode 7b, laser oscillation occurs in the light emitting layer 4, and the laser light passes through the GaAs layer 6 and passes through the surface portion 6a (the emission window of the laser light) as shown by an arrow, 1 oscillates vertically upward.

[0007]

By the way, in order to incorporate a surface emitting semiconductor laser device as a light source in an optical transmission system, it is necessary to control an oscillation transverse mode of a laser beam oscillated from the laser device. For example, in the case of a data link using a multi-mode optical fiber, a laser element that stably oscillates in a higher-order transverse mode is required as a light source. In the case of a high-speed optical transmission system using a mode optical fiber, a laser device that oscillates in a fundamental transverse mode is required as a light source.

Conventionally, the oscillation lateral mode of the surface emitting semiconductor laser device having the above structure is controlled by the size of the current confinement structure shown in FIG. Specifically, by changing the width of the ring of the ring-shaped insulating region 3d, the current injection path 3e located at the center and having a circular shape in plan view is controlled by the magnitude of the current injection path 3e. For example, in the case of a laser device that oscillates in the fundamental transverse mode, the above-described current injection path 3e
Is required to have a diameter of about 5 μm or less.

However, if the diameter of the current injection path 3e is set to the above-mentioned small value, the resistance as a laser element eventually increases, which causes a problem that the operating voltage increases. In addition, the current injection path 3
Accurately controlling the diameter of e in the order of μm means that the width of the insulating region 3d, that is, the oxidation width of the AlAs layer 3c is accurately controlled. However, it is very difficult to control this oxidation width on the order of μm. As a result, the characteristics of the manufactured laser element vary, which causes a problem in reproducibility.

In the case of a laser device that oscillates in a multi-mode, if the diameter of the current injection path is increased, the oscillation transverse mode switches at the time of the current injection, so that noise is generated and the optical transmission characteristics deteriorate. The present invention includes a novel oscillation lateral mode control mechanism capable of solving the above-described problem in the conventional surface emitting semiconductor laser device in which the oscillation lateral mode is controlled by the size of the current injection path 3e. It is another object of the present invention to provide a surface emitting semiconductor laser device which is easier than that of the prior art.

[0011]

In order to achieve the above object, according to the present invention, a layer structure of a semiconductor material in which a light emitting layer is disposed between an upper reflecting mirror layer structure and a lower reflecting mirror layer structure is provided on a substrate. A surface emitting semiconductor laser device in which an upper electrode having a circular shape in plan view is formed above the upper reflecting mirror layer structure, and an inner side of the upper electrode is an opening. A surface emitting semiconductor laser device is provided, wherein a layer transparent to the oscillation laser light is formed by partially covering the surface of the opening.

Preferably, said transparent layer has at least one layer.
A current injection path is formed in the vicinity of the light emitting layer, and the position of the transparent layer formed in the opening is in the shape of the current injection path in plan view. The thickness of the transparent layer included in the surface emitting semiconductor laser element included in the laser light source is (2i + 1) of the oscillation wavelength of the laser light.
A surface emitting semiconductor laser device having a thickness corresponding to / 4n times (where n is the refractive index of the transparent layer and i represents an integer), or a thickness of 2i / 4n times the oscillation wavelength of the laser light ( Where n is the refractive index of the transparent layer and i is a natural number.
And a surface emitting semiconductor laser device in which a metal film is further formed on the transparent layer.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The surface emitting semiconductor laser device of the present invention reduces the reflectance of a portion where laser oscillation is not desired to occur in an upper reflector layer structure in which an opening is formed (ie, multiple reflection). To control the oscillation transverse mode, thereby forming a layer transparent to the oscillation wavelength of the oscillating laser light on a part of the surface of the opening functioning as an emission window for the laser light. (Hereinafter simply referred to as a transparent layer).

Here, the above-mentioned transparent layer is made of various materials in relation to the oscillation wavelength of the laser light, but is composed of SiNx, SiOx, AlOx, TiOx,
A preferable example is a dielectric film made of a dielectric material such as MgO or MgF. In addition, ITO (indium-tin oxide) can be used as a material for the transparent layer. When the transparent layer is formed of ITO, the transparent layer can function as an electrode. In the case of this dielectric film, it may be a single layer or a laminated structure of two or more layers.

Hereinafter, the surface emitting semiconductor laser device of the present invention will be described with reference to the drawings, taking as an example the case where the transparent layer is a dielectric film. FIG. 1 shows an example B of the laser device of the present invention.
1 and FIG. 2 shows another example B2. These elements B
In the case of 1 and B2, the layer structure is the same as that of the element A shown in FIG. However, since the dielectric film is also formed on the surface of the opening 6C functioning as the laser light exit window 6a described in the element A, the planar shape of the laser light exit window is different from that in FIG. ing.

The element B1 includes a dielectric film 8 having an annular shape in plan view at an opening located inside the upper electrode 7a.
By forming A, a new emission window 6A having a smaller diameter and a circular shape in plan view is formed. The element B2 has a circular shape in plan view because the dielectric film 8B having a circular shape in plan view is formed at the center of the GaAs layer 6 exposed from the opening 6C. This is a structure in which a new emission window 6B is formed.

Here, the thickness of the dielectric film 8A (8B) is λ × (2i + 1) / 4n times when the oscillation wavelength of the laser beam by the element B1 (B2) is λ. It is preferable that the thickness is approximately the same. In the above formula, n represents the refractive index of the dielectric constituting the dielectric film 8A (8B), and i represents an integer such as 0, 1, 2,.

If the dielectric film 8A (8B) having such a shape in a plan view and having such a thickness is formed on the uppermost surface, the upper portion located immediately below the dielectric film 8A (8B) is formed. The effective reflectance of the reflector layer structure decreases. As a result, laser oscillation occurs only immediately below the dielectric film 8A (8B), and the oscillation transverse mode is controlled.

The shape of the dielectric film in plan view is not limited to the type exemplified above, but may be an appropriate shape. When a metal film such as Au, Ti, or Cr is provided on the dielectric film having a thickness of λ × 2i / 4n, the above-described effect of lowering the reflectivity is improved and the oscillation transverse mode control of laser light is performed. It is preferable because it is more effectively exhibited.

As described above, since the dielectric film (transparent layer) formed in the opening 6C exhibits the above-described operation and effect, when the dielectric film (transparent layer) is projected downward, the position of the light emitting layer 4 It is preferable that the current injection path 3e that regulates laser oscillation is included in the plan view shape. For laser light oscillating via the current injection path 3e,
This is because the effective reflectivity of the upper reflecting mirror layer structure located immediately below the dielectric film is surely reduced, so that the reliability of controlling the oscillation transverse mode is improved.

[0021]

EXAMPLES (1) Structure of Laser Element The laser element shown in FIG. 1 was manufactured as follows. The oscillation wavelength of this laser device is designed to be 850 nm. On an n-type GaAs substrate 1, a 40 nm-thick n-type Al _0.2 Ga _0.8 As and a 50 nm-thick n are formed by MOCVD.
A thin layer of Al _0.9 Ga _0.1 As at the hetero interface with a thickness of 20
Alternately laminating with a composition gradient layer of 30 nm.
A lower reflector layer structure 2 composed of five pairs of multilayer films was formed. Next, a non-doped Al _0.3 Ga _0.7 As
Lower clad layer 3a (97 nm thick) composed of
aAs quantum well (thickness of each layer is 7 nm) and four layers of Al _0.2 G
A light emitting layer 4 having a quantum well structure composed of a _0.8 As barrier layers (thickness of each layer is 8 nm), and an upper cladding layer 3b (97 nm thick) made of non-doped Al _0.3 Ga _0.7 As are sequentially laminated. 40 nm thick p-type Al _0.2 Ga
Thin films of _0.8 As and 50 nm thick p-type Al _0.8 Ga _0.2 As are alternately laminated with a 20 nm thick composition gradient layer interposed at the heterointerface to form an upper reflector layer structure 5 composed of 25 pairs of multilayer films. did.

[0022] Then, p-type Ga on the p-type Al _0.2 Ga _0.8 As layer as the uppermost layer in the upper reflector layer structure 5
As layer 6 was laminated. The lowermost layer 3c of the above-mentioned upper reflector layer structure was made of p-type AlAs having a thickness of 50 nm. Next, after a SiNx film 8a is formed on the surface of the p-type GaAs layer 6 in these layer structures by a plasma CVD method, a circular resist mask 9 having a diameter of about 45 μm is formed thereon by photolithography using a normal photoresist. Formed (FIG. 3).

Next, after removing the SiNx film 8a other than the SiNx film immediately below the resist mask 9 by RIE using CF ₄ , the resist mask 9 is entirely removed, and the GaAs layer having a ring shape in plan view is formed. The surface of No. 6 was exposed. Then, using the SiNx film 8a as a mask, an etching treatment up to the lower reflector layer structure 2 was performed using an etchant composed of a mixture of phosphoric acid, hydrogen peroxide and water to form a columnar structure (FIG. 4). .

Then, this layer structure was heated in a steam atmosphere at a temperature of 400 ° C. for about 25 minutes. p-type AlAs
Only the outer peripheral portion of the layer 3c is selectively oxidized in an annular shape,
A current injection path 3e having a diameter of about 15 μm was formed at the center thereof (FIG. 5). Then, by RIE, SiNx
After the film 8a is completely removed, the entire surface is
Coating with a SiNx film 8 by the VD method,
SiNz formed on the upper surface of the 5 μm GaAs layer 6
The surface of the GaAs layer 6 is exposed by removing the film 8 into an annular shape having an outer diameter of 25 μm and an inner diameter of 15 μm, and AuZn is deposited thereon to form an annular upper electrode 7a. Is covered only with a SiNx film 8, and a pad 7c for extracting an electrode is further provided thereon with Ti / Pt /
It was formed of Au (FIG. 6).

The thickness of the SiNx film 8 at this time is
Since the refractive index of SiNx is 1.75 and the oscillation wavelength of the device is 850 nm, the above equation: (2i + 1) / 4n
Was set to the value when i = 0, that is, 121 nm. Then, S located inside the upper electrode 7a
By applying photolithography and RIE to the iNx film, a circular small hole having a diameter of 6 μm is formed in the center of the iNx film, and the surface of the GaAs layer 6 is exposed while leaving the other SiNx film 8A, and the emission window 6A is formed. To form the element B shown in FIG.
1 was produced. This is referred to as Example 1.

The exit window 6A was formed with a small hole diameter of 10 μm. This is Example 2. Further, the SiNx film located inside the upper electrode 7a was left in a circular shape having an outer diameter of 15 μm and an inner diameter of 6 μm by leaving a portion 8B having a diameter of 6 μm at the center and removing all other portions. The emission window 6B was formed, and the device B2 shown in FIG. 2 was manufactured. This is referred to as a third embodiment.

In any of the elements of the embodiment, after polishing the back surface of the substrate 1 to a total thickness of about 100 μm, AuGeNi / Au is vapor-deposited on the polished surface to form the lower electrode 7b.
Are formed. For comparison, the above-mentioned AlA
By setting the oxidation time of the s layer 3c to about 30 minutes,
The oxidation width of the lAs layer 3c is increased to make the diameter of the current injection path 3c about 5 μm, and the diameter of the current injection path 3c is about 1 μm on the GaAs layer 6.
An element A shown in FIG. 11 was manufactured by forming an opening 6C of 5 μm and using the opening 6C as an exit window 6a. This is referred to as Comparative Example 1. In the device of Example 1, a device A was manufactured in which the SiNx film located inside the upper electrode 7a was entirely removed to form an opening having a diameter of 15 μm, and this was used as the emission window 6a. This is referred to as Comparative Example 2. Table 1 collectively shows the dimensions of the current injection path and the exit window in the five types of elements.

[0028]

[Table 1]

(2) Evaluation of Characteristics of Laser Element 1) First, current-voltage characteristics and current-light output characteristics in Example 1 and Comparative Example 1 are shown in FIG. In the figure, the solid line is the current-
A voltage characteristic and a broken line represent current-light output characteristics, respectively. In each of Example 1 and Comparative Example 1, the oscillation mode was the basic transverse mode.

As is apparent from FIG. 7, the operating voltage of the first embodiment is lower than that of the first comparative example. For example, the operating voltage at 5 mA is about 1.8 V, which is smaller than the 2.4 V of the first comparative example. And quite low. This is because the diameter of the current injection path of the first embodiment is 15
This is because the resistance at that point is lower than that in Comparative Example 1 in which the diameter of the current injection path is 5 μm. Further, in the current-light output characteristics, although the threshold current is slightly higher in Example 1 than in Comparative Example 1,
The light output increases without saturation until the current reaches about 10 mA. This is considered to be an effect based on suppression of heat generation because the resistance near the current injection path is low.

2) Next, the current-voltage characteristics and the current-light output characteristics in Examples 2 and 3 and Comparative Example 2 are shown in FIG. 8, respectively. In the figure, the solid line represents current-voltage characteristics, and the broken line represents current-light output characteristics. As is clear from FIG.
There is no remarkable difference between the above-mentioned characteristics between Examples 2 and 3 and Comparative Example 2. Therefore, Examples 2, 3 and Comparative Example 2
For each of the devices, an emission near-field image was observed. FIG. 9 shows the result when the operating current is 7 mA, and FIG. 10 shows the result when the operating current is 15 mA.

9 and 10, the image a shows the case of Example 2, the image b shows the case of Example 3, and the image c shows the case of Comparative Example 2, respectively. As is clear from FIGS. 9 and 10, in the image c in the case of the comparative example 2, a large number of light emitting spots are recognized in the outer peripheral portion, and they are switched by current injection. In the case of the image a of the second embodiment, the number of light emission spots is small, and they are concentrated at the center. The switching behavior did not occur even if the current injection was increased.

In the case of the image b of the third embodiment, the number of light-emitting spots is larger than that of the second embodiment, and the light spots are diffused as a whole. However, also in the case of Example 3, no switching behavior occurred even if the current injection was increased.

[0034]

As is clear from the above description, the laser device of the present invention is a surface emitting semiconductor laser device capable of controlling the transverse oscillation mode, for example, a laser device which oscillates in the fundamental transverse mode at a low operating voltage. In an element that oscillates stably in a higher-order transverse mode, noise due to mode switching is also reduced. Therefore, this surface emitting semiconductor laser device can be used as a light source for parallel data transmission, thereby contributing to the construction of a high-speed optical data communication system.

The control of the oscillation transverse mode in the laser device of the present invention is not performed by controlling the size of the current confinement structure as in the prior art, but by controlling the dielectric film (transparent layer) formed on the entire upper surface. Since the control is performed in a plan view shape, the degree of design freedom is high, the manufacturing is easy, and the manufacturing cost is reduced.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a layer structure of an example B1 of a surface emitting semiconductor laser device of the present invention.

FIG. 2 shows another example B2 of the surface emitting semiconductor laser device of the present invention.
FIG. 3 is a cross-sectional view showing a layer structure of FIG.

FIG. 3 is a cross-sectional view showing a state in which a SiNx film and a resist mask are formed on a layer structure formed on a substrate.

FIG. 4 is a cross-sectional view showing a state where a columnar structure is formed on a substrate.

FIG. 5 is a cross-sectional view showing a state after an oxidation process is performed on the columnar structure of FIG. 4;

6 is a cross-sectional view showing a state in which an upper electrode and a dielectric film are formed on the structure of FIG.

FIG. 7 is a graph showing current-voltage characteristics and current-light output characteristics in Example 1 and Comparative Example 1.

FIG. 8 is a graph showing current-voltage characteristics and current-light output characteristics in Examples 2 and 3 and Comparative Example 2.

FIG. 9 is a luminescence near-field image of Examples 2 and 3 and Comparative Example 2 when the operating current is 7 mA.

FIG. 10 is a luminescence near-field image of Examples 2 and 3 and Comparative Example 2 when the operating current is 15 mA.

FIG. 11 is a sectional view showing an example A of a conventional surface emitting semiconductor laser device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Substrate 2 Lower reflecting mirror layer structure 3a Lower cladding layer 3b Upper cladding layer 3c AlAs layer 3d Insulating region 3e Current injection path 4 Light emitting layer 5 Lower reflecting mirror layer structure 6 GaAs layers 6a, 6A, 6B Laser light emission window 6C opening Part 7a Upper electrode 7b Lower electrode 7c Electrode leading pad 8, 8a, 8A, 8B Dielectric film (layer transparent to laser light oscillation wave) 9 Resist mask

Claims (7)

[Claims] 1. A layer structure of a semiconductor material having a light-emitting layer disposed between an upper mirror layer structure and a lower mirror layer structure is formed on a substrate, and a flat surface is formed above the upper mirror layer structure. In a surface emitting semiconductor laser device in which an upper electrode having a ring shape as viewed is formed and an inner side of the upper electrode is an opening, a part of a surface of the opening is covered to oscillate an oscillation laser beam. A surface emitting semiconductor laser device, wherein a layer transparent to a wavelength is formed. 2. The surface-emitting semiconductor according to claim 1, wherein by changing the shape of the transparent layer in a plan view, the shape of the emission window of the oscillation laser light in a plan view is changed to control the oscillation transverse mode of the laser light. Laser element. 3. A current injection path is formed in the vicinity of the light emitting layer, and a formation position of the transparent layer formed in the opening is included in a plan view shape of the current injection path. The surface emitting semiconductor laser device according to claim 1. 4. The surface emitting semiconductor laser device according to claim 1, wherein said transparent layer comprises at least one dielectric film. 5. The method according to claim 1, wherein the dielectric film is made of SiNx, SiOx,
5. The surface emitting semiconductor laser device according to claim 4, wherein the device is any one of AlOx, TiOx, MgO, and MgF. 6. The thickness of the transparent layer is a thickness corresponding to (2i + 1) / 4n times the oscillation wavelength of the oscillating laser light (where n is the refractive index of the transparent layer and i is an integer). The surface emitting semiconductor laser device according to claim 1, wherein: 7. The thickness of the transparent layer is equivalent to 2i / 4n times the oscillation wavelength of the oscillating laser light (where n is the refractive index of the transparent layer and i is a natural number). ,
6. The surface emitting semiconductor laser device according to claim 1, wherein a metal film is formed on the transparent layer.