JP2005243918A - Surface light emitting semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface light emitting semiconductor laser which can project laser beam of a further narrow radiation angle. <P>SOLUTION: The surface light emitting semiconductor laser 100 comprises a substrate 101, a first mirror 102 provided above the substrate, an active layer 103 provided above the first mirror 102, a second mirror 104 provided above the active layer 103, and a refractive index matching layer 120 provided above the second mirror 104. The second mirror 104 consists of a semiconductor multilayer film, and the refractive index of the refractive index matching layer 120 is lower than at least the refractive index of the uppermost layer of the second mirror 104. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a surface emitting semiconductor laser.

  A surface emitting semiconductor laser is a semiconductor laser that emits laser light perpendicular to a semiconductor substrate, and is easier to handle than an end surface semiconductor laser and has a circular radiation pattern. Expected to be a light source. When a surface emitting semiconductor laser is used for various sensors or optical communication, it may be desirable to obtain a laser beam having a radiation pattern with a narrow radiation angle.

  An object of the present invention is to provide a surface emitting semiconductor laser capable of emitting laser light having a narrower emission angle.

The surface emitting semiconductor laser according to the present invention is:
A substrate,
A first mirror provided above the substrate;
An active layer provided above the first mirror;
A second mirror provided above the active layer;
A refractive index matching layer provided above the second mirror,
The second mirror is made of a semiconductor multilayer film,
The refractive index matching layer has a refractive index lower than at least the refractive index of the uppermost layer of the second mirror.

  According to the surface emitting semiconductor laser of the present invention, the radiation angle of the laser beam can be narrowed. That is, by providing a refractive index matching layer above the second mirror, laser light is emitted from the refractive index matching layer into, for example, air. Accordingly, since the discontinuity of the refractive index can be reduced as compared with the case where laser light is emitted from the second mirror into the air, the radiation angle of the laser light can be narrowed.

  A typical configuration of the surface emitting semiconductor laser according to the present invention is as follows.

(1) In the surface emitting semiconductor laser according to the present invention,
The refractive index matching layer has a plurality of layers,
The refractive indexes of the plurality of layers can be lowered upward from the second mirror side.

(2) In the surface emitting semiconductor laser according to the present invention,
The refractive index matching layer is
A first refractive index matching layer provided above the second mirror;
A second refractive index matching layer provided above the first refractive index matching layer,
The first refractive index matching layer is made of titanium dioxide,
The second refractive index matching layer may be made of silicon dioxide.

(3) In the surface emitting semiconductor laser according to the present invention,
The second mirror has an AlGaAs layer;
The refractive index matching layer is a compound semiconductor containing at least Al, and the composition ratio of Al can be increased upward from the second mirror side.

(4) In the surface emitting semiconductor laser according to the present invention,
The second mirror has two types of AlGaAs layers having different Al composition ratios,
The uppermost layer of the second mirror is provided with an AlGaAs layer having a high Al composition ratio among the two types of AlGaAs layers,
The refractive index of the refractive index matching layer may be at least higher than the Al composition ratio of the uppermost layer of the second mirror.

(5) In the surface emitting semiconductor laser according to the present invention,
When the wavelength of the laser beam in vacuum is λ, and the refractive index of the refractive index matching layer is n,
The film thickness of the refractive index matching layer may be mλ / 4n (m is an odd number).

  According to the surface emitting semiconductor laser according to the present invention, the reflectance of the laser light of the refractive index matching layer can be increased by defining the film thickness as compared with the case of other film thicknesses.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

1. Device Structure FIG. 1 is a plan view schematically showing a surface-emitting type semiconductor laser 100 according to a first embodiment to which the present invention is applied, and FIG. 2 is a cross section taken along the line AA in FIG. FIG.

  As shown in FIGS. 1 and 2, the surface emitting semiconductor laser 100 according to the present embodiment includes a semiconductor substrate (GaAs substrate in the present embodiment) 101 and a vertical resonator (hereinafter referred to as a “resonator”) formed on the semiconductor substrate 101. 140, a first electrode 107, a second electrode 109, and a refractive index matching layer 120. The resonator 140 includes the first mirror 102, the active layer 103, and the second mirror 104.

  Next, each component of the surface emitting semiconductor laser 100 will be described.

The resonator 140 includes, for example, a first mirror 102 that is a 40-pair distributed Bragg reflection mirror (DBR) in which n-type Al _0.9 Ga _0.1 As layers and n-type Al _0.15 Ga _0.85 As layers are alternately stacked, GaAs An active layer 103 including a well layer and an Al _0.3 Ga _0.7 As barrier layer and including a quantum well structure including three well layers, a p-type Al _0.9 Ga _0.1 As layer, a p-type Al _0.15 Ga _0.85 As layer, And a second mirror 104 which is a distributed Bragg reflection type mirror (DBR) composed of 25 pairs of semiconductor multilayer films alternately stacked. Note that the composition and the number of layers constituting each of the first mirror 102, the active layer 103, and the second mirror 104 are not limited thereto.

  The second mirror 104 is made p-type by doping with C, Zn, Mg, or the like, for example, and the first mirror 102 is made n-type by doping with Si, Se, or the like, for example. Yes. Therefore, a pin diode is formed by the second mirror 104, the active layer 103 not doped with impurities, and the first mirror 102.

  The second mirror 104, the active layer 103, and a part of the first mirror 102 constitute a columnar semiconductor deposited body (hereinafter referred to as “columnar portion”) 130. The side surface of the columnar portion 130 is covered with a buried insulating layer 106.

  An insulating layer 105 that functions as a current confinement layer may be formed in a region close to the active layer 103 among the layers constituting the columnar section 130. The insulating layer 105 can have a ring shape along the periphery of the columnar portion 130. The current confinement insulating layer 105 is made of, for example, aluminum oxide.

  In the surface emitting semiconductor laser 100 according to the present embodiment, the buried insulating layer 106 is formed so as to cover the side surface of the columnar portion 130. As the resin constituting the embedded insulating layer 106, for example, a polyimide resin, a fluorine resin, an acrylic resin, an epoxy resin, or the like can be used. In particular, from the viewpoint of ease of processing and insulation, a polyimide resin or a fluorine resin can be used. A resin is desirable.

  A first electrode 107 is formed on the columnar part 130 and the buried insulating layer 106. The opening on the columnar portion 130 in the first electrode 107 serves as a laser beam emission surface 108. The first electrode 107 is made of, for example, a laminated film of an alloy of Au and Zn and Au. Further, a second electrode 109 is formed on the back surface of the semiconductor substrate 101. The second electrode 109 is made of, for example, a laminated film of an alloy of Au and Ge and Au. That is, in the surface emitting semiconductor laser 100 shown in FIGS. 1 and 2, the first electrode 107 is joined to the second mirror 104 on the columnar portion 130, and the second electrode 109 is joined to the semiconductor substrate 101. . Current is injected into the active layer 103 by the first electrode 107 and the second electrode 109.

  The materials for forming the first and second electrodes 107 and 109 are not limited to those described above. For example, Cr, Metals such as Ti, Ni, Au, or Pt, and alloys thereof can be used.

  The refractive index matching layer 120 includes a first refractive index matching layer 122 and a second refractive index matching layer 124. The first refractive index matching layer 122 is formed on the second mirror 104. The second refractive index matching layer 124 is provided on the first refractive index matching layer 122. The refractive index of the first refractive index matching layer 122 is lower than the refractive index of at least the uppermost layer of the second mirror 104. The refractive index of the second refractive index matching layer 124 is lower than the refractive index of the first refractive index matching layer 122.

FIG. 3 is an enlarged view of a portion indicated by a symbol B in FIG. The second mirror 104 has the p-type Al _0.9 Ga _0.1 As layer 112 and the p-type Al _0.15 Ga _0.85 As layer 114 as described above, and the p-type Al _0.9 Ga _0.1 As layer 112 and the p-type Al _0.15 Ga _0.85. 25 pairs of distributed Bragg reflection type mirrors in which As layers 114 are alternately stacked. In the present embodiment, a p-type Al _0.9 Ga _0.1 As layer 112 is formed on the uppermost layer of the second mirror 104. The p-type Al _0.9 Ga _0.1 As layer 112 has a higher Al composition ratio than the p-type Al _0.15 Ga _0.85 As layer 114. The higher the Al composition ratio, the lower the refractive index of the AlGaAs layer.

Further, a first refractive index matching layer 122 is provided above the p-type Al _0.9 Ga _0.1 As layer 112. The refractive index of the first refractive index matching layer 122 is preferably lower than the refractive index of the p-type Al _0.9 Ga _0.1 As layer 112. In the surface-emitting type semiconductor laser 100 according to the present embodiment, the first refractive index matching layer 122 is made of TiO ₂ having a lower refractive index than the p-type Al _0.9 Ga _0.1 As layer 112, and the second refractive index matching. layer 124 is composed of SiO _2.

When the refractive index in vacuum is λ and the refractive index of the first refractive index matching layer 122 is n ₁ , the thickness of the first refractive index matching layer 122 is mλ / 4n ₁ (m is an odd number). It is desirable that Similarly, when the refractive index of the second refractive index matching layer 124 is n ₂ , the thickness of the second refractive index matching layer 124 is desirably mλ / 4n ₂ (m is an odd number).

  By defining the film thickness in this way, the reflectance of the laser light of the refractive index matching layer 120 can be increased and the light output can be prevented from being lowered as compared with other film thicknesses.

  The refractive index matching layer 120 of the surface emitting semiconductor laser 100 according to the present embodiment is composed of two layers of a first refractive index matching layer 122 and a second refractive index matching layer 124. One layer may be sufficient and it may consist of three or more layers.

The refractive index matching layer 120 is made of TiO ₂ (n = 2.2) and SiO ₂ (n = 1.46), Ta ₂ O ₅ (n = 2.1), SiN (n = 2.0). ) Or ZnO (n = 1.9) and AlGaN (n = 2.5) InGaN (n = 2.5). Further, it may be made of AlGaAs or AlAs having a high Al composition.

  In the case where there are a plurality of refractive index matching layers 120, it is desirable that the refractive index matching layers 120 be formed so that the refractive index of each layer decreases stepwise from the second mirror 104 side upward.

  By forming the refractive index of each layer to be stepwise lower, when the laser light is emitted into the air, the discontinuity between the refractive index of the second mirror 104 and the refractive index of the air is made smaller. can do.

  When the refractive index matching layer 120 is made of an AlGaAs layer, the Al composition ratio in the semiconductor is preferably increased upward from the second mirror 104 side. In this case, the Al composition ratio may increase stepwise from the second mirror 104 side upward, or may increase continuously. As the Al composition ratio of the semiconductor increases, the refractive index of the AlGaAs layer decreases.

1-2. Device Operation A general operation of the surface emitting semiconductor laser 100 according to the present embodiment will be described below. The following driving method of the surface emitting semiconductor laser 100 is an example, and various modifications can be made without departing from the gist of the present invention.

  First, when a forward voltage is applied to the pin diode between the first electrode 107 and the second electrode 109, recombination of electrons and holes occurs in the active layer 103, and light emission due to such recombination occurs. Stimulated emission occurs when the generated light reciprocates between the second mirror 104 and the first mirror 102, and the light intensity is amplified. When the optical gain exceeds the optical loss, laser oscillation occurs, and the semiconductor substrate 101 passes through the first refractive index matching layer 122 and the second refractive index matching layer 124 from the emission surface 108 on the upper surface of the columnar portion 130. A laser beam is emitted in a direction perpendicular to the direction.

  The surface-emitting type semiconductor laser 100 according to the present embodiment includes the first refractive index matching layer 122 and the second refractive index matching layer 124, thereby providing the first refractive index matching layer 122 and the second refractive index. Compared with the case where laser light is emitted from the second mirror 104 into the air without passing through the rate matching layer 124, the radiation angle of the emitted laser light can be narrowed. That is, the first refractive index matching layer 122 and the second refractive index matching layer 124 having a refractive index between the refractive index (n = 3.5) of the second mirror 104 and the refractive index of air (n = 1). , The discontinuity of the refractive index in the laser beam emission direction can be reduced. Thus, by reducing the discontinuity of the refractive index, the radiation angle of the laser beam can be narrowed.

1-3. Device Manufacturing Method Next, an example of a method of manufacturing the surface-emitting type semiconductor laser 100 according to the first embodiment to which the present invention is applied will be described with reference to FIGS. 4 to 7 are cross-sectional views schematically showing manufacturing steps of the surface-emitting type semiconductor laser 100 of the present embodiment shown in FIGS. 1 to 3, and each correspond to the cross section shown in FIG.

(1) First, the semiconductor multilayer film 150 is formed on the surface of the semiconductor substrate 101 made of n-type GaAs by epitaxial growth while modulating the composition, as shown in FIG. Here, the semiconductor multilayer film 150 includes, for example, 40 pairs of first mirrors 102 in which n-type Al _0.9 Ga _0.1 As layers and n-type Al _0.15 Ga _0.85 As layers are alternately stacked, a GaAs well layer, and Al _0.3 Ga _0.7. 25 pairs in which an active layer 103 including a quantum well structure composed of an As barrier layer and including three well layers, and p-type Al _0.9 Ga _0.1 As layers and p-type Al _0.15 Ga _0.85 As layers alternately stacked The second mirror 104. By laminating these layers on the semiconductor substrate 101 in order, the semiconductor multilayer film 150 is formed.

Here, the uppermost layer of the second mirror 104 is preferably composed of a p-type Al _0.9 Ga _0.1 As layer having a lower refractive index. In the second mirror 104, an AlAs layer may be used instead of the p-type Al _0.9 Ga _0.1 As layer.

  When the second mirror 104 is grown, at least one layer in the vicinity of the active layer 103 can be formed on an AlAs layer or an AlGaAs layer that is oxidized later to become the insulating layer 105 for electrode confinement. The AlGaAs layer serving as the insulating layer 105 has an Al composition of 0.95 or more. Further, it is desirable that the outermost layer of the second mirror 104 has a high carrier density and facilitates ohmic contact with the electrode (first electrode 107).

  The temperature at which the epitaxial growth is performed is appropriately determined depending on the growth method and raw material, the type of the semiconductor substrate 101, or the type, thickness, and carrier density of the semiconductor multilayer film 150 to be formed. Preferably there is. Further, the time required for performing the epitaxial growth is also appropriately determined in the same manner as the temperature. In addition, as a method for epitaxial growth, a metal-organic vapor phase epitaxy (MOVPE) method, an MBE (Molecular Beam Epitaxy) method, an LPE (Liquid Phase Epitaxy) method, or the like can be used.

  Subsequently, after applying a resist on the semiconductor multilayer film 150, the resist is patterned by a lithography method to form a resist layer R100 having a predetermined pattern as shown in FIG. The resist layer R100 is formed above the region where the columnar portion 130 (see FIGS. 1 and 2) is to be formed. Next, using this resist layer R100 as a mask, the second mirror 104, the active layer 103, and a part of the first mirror 102 are etched by, for example, a dry etching method, and as shown in FIG. (Columnar part) 130 is formed. Thereafter, the resist layer R100 is removed.

  Subsequently, as shown in FIG. 6, the Al composition in the second mirror 104 is changed by introducing the semiconductor substrate 101 on which the columnar portion 130 is formed by the above process into a water vapor atmosphere of about 400 ° C., for example. A high layer (a layer having an Al composition of 0.95 or more) is oxidized from the side surface to form an insulating layer 105 for current confinement. The oxidation rate depends on the furnace temperature, the amount of steam supplied, the Al composition of the layer to be oxidized and the film thickness.

  (2) Next, as shown in FIG. 7, the buried insulating layer 106 surrounding the columnar portion 130, that is, a part of the first mirror 102, the active layer 103, and the second mirror 104 is formed.

  Here, a case where polyimide resin is used as a material for forming the buried insulating layer 106 is described. First, a precursor (polyimide precursor) is applied onto the semiconductor substrate 101 having the columnar portion 130 using, for example, a spin coating method to form a precursor layer. At this time, the precursor layer is formed to have a thickness greater than the height of the columnar section 130. As a method for forming the precursor layer, known techniques such as a dipping method, a spray coating method, and a droplet discharge method can be used in addition to the spin coating method described above.

  Next, the semiconductor substrate 101 is heated using, for example, a hot plate to remove the solvent, and then placed in a furnace at, for example, about 350 ° C. to imidize the precursor layer, thereby almost completely curing polyimide. A resin layer is formed. Subsequently, as shown in FIG. 7, the upper surface 130 a of the columnar part 130 is exposed, and the buried insulating layer 106 is formed. As a method for exposing the upper surface 130a of the columnar section 130, a CMP method, a dry etching method, a wet etching method, or the like can be used. Alternatively, the buried insulating layer 106 can be formed using a photosensitive resin. The buried insulating layer 106 can be patterned by lithography or the like as necessary.

  (3) Next, a process of forming the first electrode 107, the second electrode 109, and the laser beam emission surface 108 (see FIG. 2) for injecting current into the active layer 103 will be described.

  First, before forming the first electrode 107 and the second electrode 109, the exposed upper surfaces of the columnar portion 130 and the semiconductor substrate 101 are cleaned using a plasma treatment method or the like as necessary. Thereby, an element having more stable characteristics can be formed. Next, as shown in FIG. 2, for example, a laminated film of, for example, an alloy of Au and Zn and Au is formed on the upper surface of the buried insulating layer 106 and the columnar portion 130 by, for example, vacuum deposition, and then the columnar portion is formed by lift-off method. A portion where the laminated film is not formed is formed on the upper surface of 130. This portion becomes the emission surface 108. In the above process, a dry etching method or a wet etching method can be used instead of the lift-off method.

  Further, a laminated film of, for example, an alloy of Au and Ge and Au is formed on the exposed surface of the semiconductor substrate 101 by, for example, a vacuum deposition method. Next, annealing is performed. The annealing temperature depends on the electrode material. In the case of the electrode material used in the present embodiment, it is usually performed at around 400 ° C. Through the above steps, the first electrode 107 and the second electrode 109 are formed.

  (4) Next, a process of forming the first refractive index matching layer 122 and the second refractive index matching layer 124 (see FIGS. 2 and 3) will be described.

A TiO ₂ film is laminated on the emission surface 108 portion on the second mirror 104 by sputtering. Further, a SiO ₂ film is laminated on the TiO ₂ film by sputtering. After laminating the TiO ₂ film and the SiO ₂ film, a resist is applied and patterned so that the resist remains only on the emission surface 108. Subsequently, the TiO ₂ film and the SiO ₂ film stacked on the first electrode 107 are removed by wet etching using hydrofluoric acid. In this way, the first refractive index matching layer 122 and the second refractive index matching layer 124 are formed.

As a method for removing the TiO ₂ film and the SiO ₂ film stacked on the first electrode 107, dry etching can be used in addition to the above-described wet etching method using the resist as a mask. Further, instead of etching, the first refractive index matching layer 122 and the second refractive index matching layer 124 may be formed by using a lift-off method.

  Through the above process, the surface emitting semiconductor laser 100 shown in FIGS. 1 and 2 is obtained.

  When the refractive index matching layer 120 is made of a semiconductor, the refractive index matching layer 120 is formed by growing the second mirror 104 epitaxially and further growing a semiconductor layer thereabove.

2. Modified Example Hereinafter, a surface emitting semiconductor laser 200 according to a modified example will be described with reference to FIG. Since the basic structure is the same as that of the surface-emitting type semiconductor laser 100 according to this embodiment described above, only different points will be described.

2-1. Device Structure FIG. 8 is an enlarged view of a portion corresponding to the portion B of FIG. 2 according to a modification. The surface emitting semiconductor laser 200 according to the modification is different from the surface emitting semiconductor laser 100 described above in that the contact layer 124 is formed on the uppermost layer of the second mirror 104. The contact layer 124 is made of, for example, a p-type GaAs layer. The contact layer 124 on the emission surface 108 is removed, and the p-type Al _0.9 Ga _0.1 As layer 112 is exposed. A first refractive index matching layer 122 and a second refractive index matching layer 124 are stacked on the p-type Al _0.9 Ga _0.1 As layer 112.

2-2. Device Manufacturing Method Among the manufacturing methods according to the modification, 1-3. Of the manufacturing method according to the present embodiment. Since (1) to (3) are the same, description thereof is omitted.

A contact layer 126 is laminated on the uppermost layer of the second mirror 104. Subsequently, the contact layer 126 on the emission surface 108 is removed by wet etching the contact layer 126. Here, it is preferable to use a mixed solution of NH ₃ , H ₂ O ₂ , and H ₂ O as an etching solution. Furthermore, it is preferable to use a mixed solution of NH ₃ : H ₂ O ₂ : H ₂ O = 1: 10: 100.

  The contact layer 126 other than on the emission surface 108 is not removed by etching because the first electrode 107 is formed on the contact layer 126.

  After the contact layer 126 is etched, the first refractive index matching layer 122 and the second refractive index matching layer 124 are stacked by the same process (1-3. (4)) as the process according to the present embodiment. .

  When the contact layer 126 is made of a p-type GaAs layer, the refractive index of the uppermost layer of the second mirror is high. Therefore, according to the surface emitting semiconductor laser 200 according to the modification, the contact layer 126 on the emission surface 108 is removed and the first refractive index matching layer 122 and the second refractive index matching layer 124 are stacked. The discontinuity of the refractive index can be reduced, and the radiation angle of the laser beam can be reduced.

1 is a plan view of a surface emitting semiconductor laser according to an embodiment. Sectional drawing of the surface emitting semiconductor laser shown in FIG. FIG. 3 is an enlarged view of a portion B of the surface emitting semiconductor laser shown in FIG. 2. Sectional drawing which shows the manufacturing method of the surface emitting semiconductor laser which concerns on this Embodiment. Sectional drawing which shows the manufacturing method of the surface emitting semiconductor laser which concerns on this Embodiment. Sectional drawing which shows the manufacturing method of the surface emitting semiconductor laser which concerns on this Embodiment. Sectional drawing which shows the manufacturing method of the surface emitting semiconductor laser which concerns on this Embodiment. The enlarged view of the part corresponded to B part of the surface emitting semiconductor laser which concerns on a modification.

Explanation of symbols

100 surface emitting semiconductor laser, 101 semiconductor substrate, 102 first mirror, 103 active layer, 104 second mirror, 105 insulating layer, 106 buried insulating layer, 107 first electrode, 108 exit surface, 109 second electrode, 120 refraction Index matching layer, 122 First index matching layer, 124 Second index matching layer

Claims (6)

A substrate,
A first mirror provided above the substrate;
An active layer provided above the first mirror;
A second mirror provided above the active layer;
A refractive index matching layer provided above the second mirror,
The second mirror is made of a semiconductor multilayer film,
The surface emitting semiconductor laser, wherein a refractive index of the refractive index matching layer is lower than at least a refractive index of an uppermost layer of the second mirror. In claim 1,
The refractive index matching layer has a plurality of layers,
The surface emitting semiconductor laser, wherein the plurality of layers have a refractive index that decreases upward from the second mirror side. In claim 1 or 2,
The refractive index matching layer is
A first refractive index matching layer provided above the second mirror;
A second refractive index matching layer provided above the first refractive index matching layer,
The first refractive index matching layer is made of titanium dioxide,
The surface-emitting type semiconductor laser, wherein the second refractive index matching layer is made of silicon dioxide. In claim 1 or 2,
The second mirror has an AlGaAs layer;
The surface-emitting type semiconductor laser, wherein the refractive index matching layer is a compound semiconductor containing at least Al, and the Al composition ratio increases upward from the second mirror side. In claim 4,
The second mirror has two types of AlGaAs layers having different Al composition ratios,
The uppermost layer of the second mirror is provided with an AlGaAs layer having a high Al composition ratio among the two types of AlGaAs layers,
The surface emitting semiconductor laser, wherein a refractive index of the refractive index matching layer is higher than at least an Al composition ratio of an uppermost layer of the second mirror. In any of claims 1 to 5,
When the wavelength of the laser beam in vacuum is λ, and the refractive index of the refractive index matching layer is n,
The surface-emitting type semiconductor laser, wherein the film thickness of the refractive index matching layer is mλ / 4n (m is an odd number).

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