JP3869106B2 - Surface emitting laser device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a surface emitting laser device that is preferably used in communication, measurement, and the like, and particularly suitable for a multi-channel optical communication system.
[0002]
[Prior art]
As a technique for transmitting and processing a large amount of information such as image information at high speed, a parallel information processing system using a two-dimensional integrated optical device has been actively studied. In such a system, a surface emitting laser device capable of two-dimensional arrangement is particularly important.
[0003]
FIG. 2 is a block diagram showing an example of a conventional surface emitting laser device. This surface emitting laser device is described in a paper (IEEE PHOTONICS TECHNOLOGY LETTERS, vol.9 No.3, p280), and is sequentially formed on a substrate 5 made of GaAs with a mirror layer 4, an active region 3, and a mirror layer 2. Is formed using MOCVD (metal organic chemical vapor deposition) or the like, and the mirror layers 2 and 4 constitute an optical resonator having an optical axis along the layer thickness direction. Laser light is emitted in the normal direction.
[0004]
The lower mirror layer 4 is composed of 34.5 pairs of multilayer mirrors with an AlAs layer and an AlGaAs layer as a pair. The upper mirror layer 2 is composed of 20 pairs of multilayer mirrors, one pair of which is an AlAs layer and an AlGaAs layer. The active region 3 includes a quantum well layer made of InGaAs.
[0005]
The active region 3 and the mirror layer 2 are processed into a cylindrical shape by dry etching in order to limit the light emitting region and perform current confinement, and a current confinement layer 7 made of polyimide is filled around these regions. Yes. A ring-shaped electrode 1 is formed around the upper surface of the current confinement layer 7 and the upper surface of the mirror layer 2. An electrode 6 is also formed on the lower surface of the substrate 5.
[0006]
Next, the operation will be described. When a voltage is applied between the electrode 1 and the electrode 6, a current path of the mirror layer 2, the active region 3, the mirror layer 4 and the substrate 5 is formed, and electrons and holes are injected as carriers into the active region 3 to recombine carriers. Radiates light. Furthermore, when the amount of injected current is increased, stimulated emission starts, and laser oscillation starts between the mirror layers 2 and 4 constituting the optical resonator. The laser beam is output from the mirror layer 2 to the outside, and functions as a surface emitting laser device that can obtain an emission beam in the normal direction of the substrate 5 from the outside. With the configuration of FIG. 2, a laser beam with an output of several mW is obtained.
[0007]
FIG. 3 is a plan view showing an example of a surface emitting laser array. This laser array is a product already on the market (magazine "Optical Alliance" 1995.6 p43), and a plurality of surface emitting laser elements 9 are formed on a substrate 8 in a matrix arrangement of 8 rows by 8 columns, Individual electrodes 8a for individually supplying currents to the surface emitting laser elements 9 are drawn outward. A common electrode is formed on the back surface of the substrate 8. The inter-element pitch is 250 μm, an optical output of about 1 mW is obtained from one element, and the threshold current is 5 mA or less.
[0008]
[Problems to be solved by the invention]
In the surface emitting laser, the diameter of the resonator is generally set to several tens of μm or less in order to improve the degree of array integration and reduce the threshold current. However, if the diameter of the resonator is made too small, current and light losses increase, and the threshold current does not decrease as expected. This can be attributed to the fact that current is consumed by non-radiative recombination on the side surface of the mirror layer 2 processed into a cylindrical shape and that light scattering occurs due to processing errors. As such a countermeasure, a structure in which a current confinement layer is formed in a thin layer shape has been proposed.
[0009]
FIG. 4 is a block diagram showing another example of a conventional surface emitting laser device. In this surface emitting laser device, an active region composed of a mirror layer 15 and semiconductor layers 18 and 19 and a mirror layer 12 are sequentially formed on a substrate 16, and the mirror layers 12 and 15 formed of multilayer mirrors have a layer thickness. An optical resonator having an optical axis along the direction is configured. The active layer is formed between the semiconductor layers 18 and 19.
[0010]
An AlAs layer 13 is formed in the vicinity of the active region. Each layer is laminated, the active region and the mirror layer 12 are processed into a cylindrical shape by dry etching, and then annealed in water vapor so that the AlAs layer 13 is exposed from the outside. Oxidized to form an annular AlAs oxide layer. By controlling the degree of such oxidation, the inner diameter of the AlAs oxide layer can be controlled to a desired dimension. Since the central AlAs layer is conductive and the surrounding AlAs oxide layer is electrically insulating, the current passing through the AlAs layer 13 is concentrated in the central portion. Furthermore, since the refractive index of the AlAs oxide layer is lower than that of the AlAs layer, it performs the same function as the gradient index lens, and light traveling along the optical axis is condensed near the optical axis. Loss on the side surface of the mirror layer 12 is reduced. As a result, an optical output of 3 mW or more was obtained in the element of FIG. 4, and the threshold current was 1 mA or less.
[0011]
However, increasing the laser output is limited by thermal saturation. Thermal saturation is a phenomenon in which the element temperature rises due to Joule heat and other losses generated by laser oscillation, and thereby the gain of the active layer decreases, and oscillation cannot be maintained. The higher the electrical resistance and thermal resistance, the more prominent.
[0012]
In the active region of a surface emitting laser, a separate confinement heterostructure (SCH) is generally employed as in a normal edge emitting laser. In the SCH structure, a semiconductor layer having a sufficiently large forbidden band is formed in order to confine carriers in the vicinity of the active layer.
[0013]
When the surface emitting laser shown in FIG. 4 is made of, for example, an AlGaAs material, the forbidden band width increases as the Al composition increases in the same material, so that the semiconductor layers 18 and 19 have AlGaAs with an Al composition of 0.3 to 0.5. The forbidden band width on both sides of the active layer is increased. However, AlGaAs-based materials tend to increase electrical resistance and thermal resistance as the forbidden bandwidth increases. Therefore, an attempt to increase the carrier confinement effect in the active layer also increases electrical resistance and thermal resistance, resulting in generation of Joule heat and an increase in the temperature of the active layer, resulting in a decrease in laser gain and laser output due to thermal saturation. Will result.
[0014]
An object of the present invention is to provide a surface emitting laser device that can reduce the electrical resistance and thermal resistance of an optical waveguide layer and obtain high optical output.
[0015]
[Means for Solving the Problems]
The present invention comprises an active layer;
A pair of optical waveguide layers provided on both sides of the active layer and having a forbidden band width greater than or equal to the forbidden band width of the active layer;
Respectively provided between the active layer and both of the optical waveguide layer, a carrier blocking layer having a band gap greater than the band gap of the active layer and the optical waveguide layer,
A pair of mirror layers provided on both sides so as to sandwich the active layer, the optical waveguide layer and the carrier block layer, and for forming a resonator optical axis along the layer thickness direction;
A surface-emitting laser device comprising a current confinement layer provided between a carrier block layer and a mirror layer located on the opposite side of the active layer and for concentrating current near the resonator optical axis .
[0016]
According to the present invention, the carrier block layer positioned between the active layer and the optical waveguide layer performs the function of confining carriers in the active layer. Therefore, the composition and dimensions of the optical waveguide layer can be designed without much consideration of the carrier confinement function by the optical waveguide layer. Therefore, an optical waveguide layer with reduced electrical resistance and thermal resistance can be employed, and a surface-emitting laser with excellent thermal characteristics, high output and high reliability can be realized. For example, when a semiconductor laser is manufactured using an AlGaAs-based material, electrical resistance and thermal resistance can be reduced by forming the optical waveguide layer with a low Al composition.
[0017]
Furthermore, since the current confinement layer is provided between the carrier block layer and the mirror layer located on the opposite side of the active layer, it is possible to concentrate the current near the resonator optical axis, thereby improving the laser gain. The threshold current can be reduced. In addition, since the optical waveguide layer adjacent to the current confinement layer has a small current path area, the electrical resistivity of the optical waveguide layer is preferably as low as possible. In the present invention, by providing the carrier block layer, the optical waveguide layer in the vicinity of the current confinement layer can be formed of a semiconductor material having a low electrical resistivity.
[0018]
Further, since the heat resistance of the optical waveguide layer on the substrate side is reduced, the heat generated in the vicinity of the current confinement layer is smoothly transferred to the substrate side, so that the thermal saturation of the active layer can be suppressed.
[0019]
In the present invention, by providing each of the carrier blocking layer between the active layer and both of the optical waveguide layer, Ru can be more efficiently exhibited the carrier confinement function to the active layer.
[0026]
The present invention is also characterized in that the thickness of the carrier block layer is 50 nm or less.
[0027]
According to the present invention, the carrier block layer needs to have a certain thickness so that carriers existing in the active layer do not leak due to the tunnel effect. However, if the thickness is too large, the carrier injection efficiency and the light leaching efficiency are reduced. A layer thickness of 50 nm or less is preferred.
[0028]
According to the present invention, the thickness of the carrier block layer is 5 nm or more.
[0029]
According to the present invention, the carrier block layer needs to have a certain thickness so that carriers existing in the active layer do not leak due to the tunnel effect, but if the thickness is too thin, the band peak of the carrier block layer is relaxed. A carrier leakage due to a bending phenomenon or a tunnel effect occurs and the carrier blocking function is deteriorated, so that a layer thickness of 5 nm or more is preferable.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
FIG. 1A is a block diagram showing a first embodiment of the present invention, and FIG. 1B is a plan view. In the surface emitting laser device, a lower mirror layer 26, an active region, and an upper mirror layer 21 are sequentially formed on a substrate 27 made of GaAs using MOCVD (metal organic chemical vapor deposition) or the like. The layers 21 and 26 constitute an optical resonator having an optical axis along the layer thickness direction, and laser light is emitted from the mirror layer 21 in the normal direction of the substrate 27.
[0031]
Each mirror layer 21, 26 is formed of a multilayer mirror in which AlAs layers and AlGaAs layers are alternately stacked.
[0032]
The active regions are, in order from the bottom, a lower optical waveguide layer 25 made of Al _0.20 Ga _0.80 As, a carrier block layer 31 (thickness 15 nm) made of Al _0.50 Ga _0.50 As, a barrier layer of Al _0.20 Ga _0.80 As, and a GaAs layer. Active layer 30 composed of a quadruple well layer composed of a well layer, carrier block layer 29 (thickness 15 nm) composed of Al _0.50 Ga _0.50 As, upper first optical waveguide layer 24 composed of Al _0.20 Ga _0.80 As, AlAs A current confinement layer 23 (thickness 20 nm) including a layer and an AlAs oxide layer, and an upper second optical waveguide layer 22 made of Al _0.20 Ga _0.80 As.
[0033]
The thicknesses of the lower optical waveguide layer 25, the upper first optical waveguide layer 24, and the upper second optical waveguide layer 22 are appropriately set so that the thickness of the entire active region coincides with the oscillation wavelength λ. Satisfies the longitudinal mode condition.
[0034]
Regarding the conductivity type of each layer, if each layer from the active layer 30 to the lower optical waveguide layer 25 is n-type, each layer from the active layer 30 to the upper second optical waveguide layer 22 is p-type, and conversely the former is p-type Then, the latter is n-type.
[0035]
The active region and the mirror layer 21 are processed into a cylindrical shape by using chlorine reactive etching after the layers are stacked in order to limit the light emitting region and perform current confinement. The diameter of the active region (about 25 μm) is formed larger than the diameter of the mirror layer 21 (about 20 μm), and a ring-shaped electrode 28 is formed on the step surface. The other electrode 32 is formed on the substrate 27, and carriers are injected into the active region by supplying a current between the electrode 28 and the electrode 32.
[0036]
AlGaAs-based materials tend to increase the forbidden band width as the Al composition increases. In the present embodiment, the forbidden bandwidths of the optical waveguide layers 22, 24, and 25 are larger than the forbidden bandwidth of the active layer 30, and the forbidden bands of the carrier block layers 29 and 31 are larger than the optical waveguide layers 22, 24, and 25. The band width is larger.
[0037]
The current confinement layer 23 is formed of the AlAs layer 23a in the stacking process of each layer, and after the etching process of the active region and the mirror layer 21, the AlAs layer 23a is processed at a high temperature in a water vapor atmosphere at about 400 degrees. Oxidized from the outside, an annular AlAs oxide layer 23b is formed as shown in FIG. By controlling the degree of such oxidation, the inner diameter of the AlAs oxide layer 23b can be controlled to a desired dimension. Since the central AlAs layer 23a is conductive and the surrounding AlAs oxide layer 23b is electrically insulating, the current passing through the current confinement layer 23 is concentrated in the central AlAs layer 23a.
[0038]
Next, the operation will be described. When a voltage is applied between the electrode 28 and the electrode 32, the upper second optical waveguide layer 22, the current confinement layer 23, the upper first optical waveguide layer 24, the carrier block layer 29, the active layer 30, the carrier block layer 31, and the lower side A current path called the optical waveguide layer 25 is formed, electrons and holes are injected into the active layer 30 as carriers, and light is radiated by carrier recombination. Since the carriers in the active layer 30 are confined in the active layer 30 by the presence of the carrier block layers 29 and 31, the recombination efficiency is improved.
[0039]
Further, when the amount of injected current is increased, stimulated emission starts, and laser oscillation starts between the mirror layers 21 and 26 constituting the optical resonator. The laser light is output to the outside from the mirror layer 21 and functions as a surface emitting laser device that can obtain a light emission beam in the normal direction of the substrate 27 from the outside.
[0040]
In this embodiment, the provision of the carrier block layers 29 and 31 can significantly reduce the Al composition of each of the optical waveguide layers 22, 24 and 25 to 0.20, so that the overall electrical resistance and thermal resistance are remarkably increased. Get smaller.
[0041]
(Second Embodiment)
The structure of the second embodiment is the same as that of FIG. 1, but the layer material is different. Referring to FIG. 1, in a surface emitting laser device, a lower mirror layer 26, an active region, and an upper mirror layer 21 are sequentially formed on a substrate 27 made of GaAs by MOCVD (metal organic chemical vapor deposition). The mirror layers 21 and 26 constitute an optical resonator having an optical axis along the layer thickness direction, and laser light is emitted from the mirror layer 21 in the normal direction of the substrate 27.
[0042]
Each mirror layer 21, 26 is formed of a multilayer mirror in which AlAs layers and AlGaAs layers are alternately stacked.
[0043]
The active region is composed of a lower optical waveguide layer 25 made of GaAs, a carrier block layer 31 (thickness 25 nm) made of Al _0.30 Ga _0.70 As, a double layer double well made of a GaAs barrier layer and an InGaAs well layer in order from the bottom. Active layer 30 composed of layers, carrier block layer 29 (thickness 25 nm) made of Al _0.30 Ga _0.70 As, upper first optical waveguide layer 24 made of GaAs, current confinement layer 23 including AlAs layer and AlAs oxide layer ( The upper second optical waveguide layer 22 made of GaAs has a thickness of 20 nm.
[0044]
The thicknesses of the lower optical waveguide layer 25, the upper first optical waveguide layer 24, and the upper second optical waveguide layer 22 are appropriately set so that the thickness of the entire active region coincides with the oscillation wavelength λ. Satisfies the longitudinal mode condition.
[0045]
The current confinement layer 23 is formed of the AlAs layer 23a in the stacking process of each layer, and after the etching process of the active region and the mirror layer 21, the AlAs layer 23a is processed at a high temperature in a water vapor atmosphere at about 400 degrees. Oxidized from the outside, an annular AlAs oxide layer 23b is formed as shown in FIG.
[0046]
In the present embodiment, since the optical waveguide layers 22, 24, and 25 can be formed of GaAs having an Al composition of zero by providing the carrier block layers 29 and 31, the overall electric resistance and thermal resistance are further reduced. .
[0047]
In the above description, an example in which an AlGaAs-based semiconductor material is used has been described. However, a semiconductor material such as P (phosphorus) -based or GaN-based in which the relationship between the forbidden band width and the electrical resistance and thermal resistance is the same can be used. is there.
[0048]
The present invention is not limited to the quantum well layer made of GaAs or InGaAs, but can be applied to other materials such as a quantum well layer using InGaAsP or GaN, or a normal double heterostructure that is not a quantum well. is there.
[0049]
【The invention's effect】
As described above in detail, according to the present invention, since the carrier block layer located between the active layer and the optical waveguide layer performs the carrier confinement function in the active layer, the electrical resistance and the thermal resistance are reduced. An optical waveguide layer can be adopted, and a surface emitting laser with excellent thermal characteristics, high output and high reliability can be realized.
[Brief description of the drawings]
FIG. 1 (a) is a block diagram showing a first embodiment of the present invention, and FIG. 1 (b) is a plan view.
FIG. 2 is a block diagram showing an example of a conventional surface emitting laser device.
FIG. 3 is a plan view showing an example of a surface emitting laser array.
FIG. 4 is a block diagram showing another example of a conventional surface emitting laser device.
[Explanation of symbols]
21, 26 Mirror layer 22 Upper second optical waveguide layer 23 Current confinement layer 24 Upper first optical waveguide layer 25 Lower optical waveguide layer 27 Substrate 28, 32 Electrode 29, 31 Carrier block layer 30 Active layer

Claims (3)

An active layer,
A pair of optical waveguide layers provided on both sides of the active layer and having a forbidden band width equal to or greater than the forbidden band width of the active layer;
Respectively provided between the active layer and both of the optical waveguide layer, a carrier blocking layer having a band gap greater than the band gap of the active layer and the optical waveguide layer,
A pair of mirror layers provided on both sides so as to sandwich the active layer, the optical waveguide layer and the carrier block layer, and for forming a resonator optical axis along the layer thickness direction;
A surface emitting laser device comprising: a current confinement layer provided between a carrier block layer and a mirror layer located on the opposite side of the active layer, for concentrating current near the resonator optical axis. The surface emitting laser device according to claim 1, the layer thickness of the carrier blocking layer is characterized der Rukoto below 50nm. The surface emitting laser device according to claim 1 or 2 layer thickness of the carrier blocking layer is characterized der Rukoto than 5 nm.

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