JPH1027941A - Normal radiation semiconductor laser device and method of fabricating the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a normal radiation semiconductor laser device, which comprises an electrode having a low ohmic contact resistance and in which an oscillation threshold current is small and the transverse mode is stabilized. SOLUTION: This semiconductor laser has an embedding structure of a double hetero connection of a first cladding layer 23 made of n-InP formed on an n-InP substrate 21, an active layer 24 made of GaInAsP, and a second cladding layer 27 of p-InP. In an electrode area, a cap layer has a two-layer structure of a first cap layer 11 made of p-GaInAsP and a second cap layer 12 which is made of p-GaInAsP formed on the first cap layer and has a band gap narrower than that of the first cap layer and high impurity concentration. Further, a metal electrode 13 is formed on the second cap layer. In a resonator area, a cap layer comprises only the first cap layer. On the first cap layer, a transparent electrode 14 and a dielectric multilayer reflection mirror 15 are overlapped in a part of a metal electrode 13 and are formed so as to be electrically connected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface emitting semiconductor laser device, and more particularly, to a surface emitting semiconductor device having an electrode having a small ohmic contact resistance, a small oscillation threshold current, and a stable transverse mode. Type semiconductor laser device, in particular 1.
The present invention relates to a surface-emitting type semiconductor laser device which is optimal as a laser device having an oscillation wavelength in the range of 2 to 1.6 μm.

[0002]

2. Description of the Related Art Semiconductor laser devices are light-emitting devices having a resonator for confining light therein, and are classified into surface-emitting type and other types of semiconductor laser devices according to the structure of the resonator. The surface-emitting type semiconductor laser device has a resonator structure having a pair of reflecting mirrors sandwiching an active layer, and emits laser light perpendicular to the substrate surface. FIG. 2 is a sectional view showing an example of the configuration of a conventional surface-emitting type semiconductor laser device. The conventional surface-emitting type semiconductor laser device 20 shown in FIG.
N- layers sequentially formed on a substrate 21 made of InP.
Etching stop layer 22 made of InGaAs, n-In
A double heterojunction having a laminated structure composed of a first cladding layer 23 made of P, an active layer 24 made of i-GaInAsP, and a second cladding layer 27 made of p-InP is provided.

A cap layer 28 of p-InGaAs having a high impurity concentration is formed in an electrode region on the active layer 24 adjacent to the resonator region. Since the cap layer 28 made of p-InGaAs has a higher impurity concentration and a smaller band gap energy than the active layer 24 made of i-GaInAsP, the light absorption loss is large. Therefore, in order to reduce light absorption loss in the resonator, the cap layer 28 is removed in the resonator region.

A p-side electrode 30 is formed on the cap layer 28. Further, an n-side electrode 29 is formed on the lower surface of the InP substrate 21 corresponding to the p-side electrode 30. In order to form a pn reverse bias layer with a compound semiconductor having a wider band gap than the active layer 24 and perform current confinement, p-In
A first current confinement layer 25 made of P and a second current confinement layer 26 made of n-InP are formed on both sides of the active layer 24. The reflecting mirrors 31 and 32 constituting the resonator are:
It is formed on the upper cladding layer 27 in the resonator region and under the lower cladding layer 23 in a through hole 33 provided in the substrate 21 correspondingly. Reflecting mirrors 31, 32
Has a structure in which dielectrics having different refractive indexes, for example, a Si layer and an SiO ₂ layer are alternately stacked at a thickness of λ (oscillation wavelength) / 4 optical length. Therefore, no current flows through the reflecting mirror.

When a forward bias is applied between the electrodes 29 and 30 and the current value exceeds a threshold value, an active region is formed in the active layer 24, a laser beam is generated, and a reflection mirror 31 is formed. Laser light oscillated by a Fabry-Perot resonator formed between the laser beam and the reflecting mirror 32 is emitted through the through-hole 33. The surface emitting semiconductor laser device shown in FIG. 2 operates continuously at a low threshold current of 2.4 mA and at room temperature.

[0006]

In the above-described conventional surface-emitting type semiconductor laser device, the mobility of carriers in the upper cladding layer 27 made of p-InP is small. The current path does not spread in the lateral direction, the current amount concentrates near the edge of the resonator region (indicated by X in FIG. 2), and the phenomenon that the current amount is small at the center portion of the resonator region is likely to occur. For this reason, there has been a problem that the multi-mode of the transverse mode occurs. Japanese Patent Application Laid-Open No. 1-282882 discloses that a cap layer is formed on a resonator region, and a transparent electrode and a metal electrode (Au- Zn
-Au) are sequentially formed, and a surface-emitting type semiconductor laser device in which they function as an electrode and a mirror is proposed.

However, this method has the following problems. First, since the cap layer is formed in the resonator region, there is a problem in that absorption loss of laser light is large. The second problem is that the ohmic contact resistance of the cap layer is large. To reduce the ohmic contact resistance, it is necessary to increase the impurity concentration. However, in order to suppress laser light absorption loss and facilitate laser oscillation, the impurity concentration is suppressed and the band gap wavelength of the cap layer is reduced. Must be larger than the band gap wavelength of the active layer. Therefore, it is difficult to satisfy both at the same time. Third, there is a problem that the reflectance of the reflecting mirror is low. Although it is not easy to obtain a reflectance of 99% or more even with the Au reflection film alone, the reflectance is further reduced because Zn is added to the p-side metal electrode to improve ohmic contact. In particular, in a surface-emitting type semiconductor laser device having an oscillation wavelength of 1.3 to 1.6 μm using InP as a substrate, it is extremely difficult to achieve a reflectance of 99% or more required for continuous oscillation at room temperature. . Fourth, this method has a problem that it is difficult to form a dielectric multilayer mirror capable of obtaining a high reflectance because the ohmic contact resistance is reduced by a laminated structure of a transparent electrode and a metal electrode. There is also.

In view of the above problems, an object of the present invention is to provide a surface emitting semiconductor laser device having an electrode having a low ohmic contact resistance, a small oscillation threshold current, and a stable transverse mode. It is to be.

[0009]

Means for Solving the Problems The present inventor has devised the electrode structure so that the current can be uniformly injected into the active region, thereby reducing the threshold current and unifying the transverse mode. The present invention has been completed by focusing on the fact that ohmic contact resistance can be reduced. In order to achieve the above object, based on the above findings, a surface-emitting type semiconductor laser device according to the present invention has a reflection mirror region, a reflection mirror region adjacent to a reflection mirror region on a laser light oscillation structure formed on a substrate. A surface-emitting type semiconductor laser device comprising: a first cap layer made of a compound semiconductor layer having a conductivity type different from the conductivity type of the substrate; and an electrode region provided on the first cap layer. A cap layer and a metal electrode having a smaller band gap than the first cap layer, a higher impurity concentration, and a two-layer structure with a second cap layer made of a compound semiconductor layer of the same conductivity type are sequentially laminated. In the reflector region, a first cap layer, a transparent electrode, and a dielectric multilayer reflector are sequentially laminated and formed, and the transparent electrode partially overlaps the metal electrode in the electrode region. Connected electrically It is characterized in Rukoto.

In order to apply the present invention to the configuration of a surface-emitting type semiconductor laser device having an oscillation wavelength in the 1.3 to 1.6 μm band, the surface-emitting type semiconductor laser device must be formed on an n-InP substrate. -InP first cladding layer, GaInAs
An active layer made of P and a second clad layer made of p-InP. The first cap layer formed on the second clad layer has a band gap wavelength λ _g of λ _g <1.2 μm and an impurity concentration of approximately The second cap layer is made of 1 × 10 ¹⁸ cm ⁻³ p-GaInAsP and has an impurity concentration of about 1 × 10 ¹⁹ cm 3.
^-3 of p-InGaAs or band gap wavelength λ _g is λ
_g > 1.4 μm and an impurity concentration of about 1 × 10 ¹⁹ cm ⁻³
-GaInAsP, and the transparent electrode layer is made of ITO (Indi
um Tin Oxide) or a metal obtained by adding at least one of Al, In and Si to ZnO. As a result, the present invention specifies an optimum configuration as a surface-emitting type semiconductor laser device having an oscillation wavelength of 1.2 to 1.6 μm.

In the present invention, the cap layer in the electrode region of the surface-emitting type semiconductor laser device has a relatively large band gap and a low impurity concentration, and a first cap layer provided on the first cap layer. It has a two-layer structure with a smaller band gap, a higher impurity concentration, and a second cap layer of the same conductivity type, and the cap layer in the reflector region is only the first cap layer. The impurity concentration of the first cap layer is about 1 × 10 ¹⁸ cm ⁻³ to reduce light absorption loss, while the impurity concentration of the second cap layer is 1 × 10 ¹⁸ cm ⁻³ to reduce ohmic contact resistance. It is about ¹⁹ cm ^-3 . Accordingly, in the electrode region, the ohmic contact resistance can be reduced by the laminated structure of the second cap layer having a high impurity concentration and a high mobility and the metal electrode having a good thermal conductivity. On the other hand, in the reflector region, a metal electrode is electrically connected to the metal electrode and has a laminated structure of a transparent electrode having almost no light absorption loss and a first cap layer having a small light absorption loss. Current can be injected from the metal electrode through the transparent electrode under the voltage applied to the substrate.

Therefore, in the surface emitting semiconductor laser device according to the present invention, in addition to the direct injection of the current from the metal electrode in the electrode region, the current is also injected through the transparent electrode in the reflector region. Current can be injected into the active region at a uniform density. As a result, the transverse mode is unified, the heat conduction is good, the ohmic contact resistance is low, and the threshold current applied between the p-side and n-side electrodes is also reduced.

The method for fabricating a surface emitting semiconductor laser device having an oscillation wavelength of 1.2 to 1.6 μm according to the present invention comprises the steps of:
A first cladding layer made of n-InP, an active layer made of GaInAsP, and a p-InP
Forming a double-heterojunction buried structure including a second cladding layer made of a first cap layer and a second cap layer on the second cladding layer, and forming a second cap layer on the second cladding layer. A mask pattern for partitioning the reflecting mirror region is formed on the cap layer by photolithography technology, a metal electrode film is deposited on the entire surface of the second cap layer while maintaining the mask pattern, and then the metal on the reflecting mirror region is formed by lift-off technology. Removing the electrode film to form a metal electrode in the electrode region, and using a selective etching solution to etch away the second cap layer in the reflector region while leaving the first cap layer in the region Process, a transparent electrode layer and a dielectric multilayer mirror are sequentially formed, and the transparent electrode layer and the dielectric multilayer are further overlapped so that the transparent electrode partially overlaps the metal electrode and is electrically connected. It is characterized by comprising a step of patterning the reflector.

[0014]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. This embodiment example, only one example of the surface-emitting type semiconductor laser device according to the present invention, the oscillation wavelength is 1.2~1.6μm
This is an example in which the present invention is applied to a surface-emitting type semiconductor laser device in the range described above. FIG. 1 is a schematic sectional view showing the layer structure of the surface emitting semiconductor laser device of the present embodiment. The surface-emitting type semiconductor laser device 10 of this embodiment (hereinafter simply referred to as the semiconductor laser 10) has a p-side reflecting mirror region, that is, a p-side resonator region and a p-side reflecting mirror region adjacent to the p-side reflecting mirror region. Except for the structure of the side electrode region, it has the same configuration as the conventional semiconductor laser 20 shown in FIG.

In the electrode region, the cap layer includes a first cap layer 11 made of GaInAsP having a band gap wavelength λ _g of λ _g <1.2 μm, and a first band gap wavelength of λ _g <1.2 μm.
InG smaller than the band gap wavelength of the cap layer 11
It has a two-layer structure with the cap layer 12 made of aAs. For the second cap layer 12, instead of InGaAs,
GaI of the band gap wavelength λ _g is λ _g> 1.40μm
nAsP can also be used. Second cap layer 12
Is increased to about 1 × 10 ¹⁹ cm ^{−3 in} order to greatly reduce the ohmic contact resistance. On the other hand, the first cap layer 11 has an impurity concentration of about 1 × 10 ¹⁸ cm ^{−3 in} order to reduce light absorption loss. On the second cap layer 12, a laminated metal electrode 13 made of a metal having a low ohmic contact resistance, for example, Au / Zn or Ti / Pt / Au is formed.

In the resonator region, the cap layer includes only the first cap layer 11 of the same layer as the first cap layer 11 formed in the electrode region. Further, on the first cap layer 11, a transparent electrode 14 having a very small light absorption loss is provided.
Are formed so as to partially overlap with the metal electrode 13 to be electrically connected. The transparent electrode 14 is transparent and has a very small light absorption loss, for example, ITO (Indium Tin Oxide).
Or an electrode formed of a metal obtained by adding at least one of Al, In, and Si to ZnO to have a thickness of λ / 4 or an odd-number multiple of the optical length. On the transparent electrode 14, a reflecting mirror 15 in which a pair of dielectric high reflectivity films having different thicknesses of λ / 4 optical length, for example, a pair of a Si film and a SiO ₂ film or a pair of a Si film and an Al ₂ O ₃ film are stacked. Are formed.

In this embodiment, the oscillation wavelength λ is 1.25 μm
As described above, light absorption loss in the first cap layer 11 having a band gap wavelength smaller than the oscillation wavelength hardly occurs. Further, since the electrode 14 is formed of a transparent electrode, there is almost no light absorption loss, and the electrode 14 operates as a part of a mirror. In the semiconductor laser 10 of the present embodiment, the current is supplied to the metal electrode 13, the second cap layer 12, and the first cap layer 1.
1 is injected into the active region through the transparent electrode 14 and the first cap layer 11 in addition to being injected into the active region through 1, so that the density of the injected current is uniform in the plane of the active region. Due to the above effects, in this embodiment, the threshold current was reduced to 1 mA, and the single transverse mode was achieved.

In the following, p of the semiconductor laser 10 of the present embodiment will be described.
A method for manufacturing the side electrode unit will be described. First, as in the conventional case, a first n-InP substrate is formed on an n-InP substrate 21.
A cladding layer 23, an active layer 24 made of GaInAsP,
Then, a double hetero junction buried structure including the second cladding layer 27 made of p-InP is formed. Next, by an epitaxial crystal growth method such as a liquid phase epitaxial growth method, a metal organic chemical vapor deposition method, and a molecular beam epitaxial growth method,
The first cap layer 11 and the second cap layer 12 are formed on the second clad layer 27. Subsequently, the second cap layer 12
A resist mask pattern for defining a reflecting mirror region is formed thereon by photolithography, and a metal electrode film is deposited on the second cap layer 12 while holding the mask pattern. Next, the metal electrode film on the reflector region is removed by a lift-off technique, and the metal electrode 14 is formed in the electrode region.

Next, using a selective etching solution such as a mixed solution of tartaric acid and hydrogen peroxide, the second cap layer 12 in the region of the reflector is removed by etching while leaving the first cap layer 11 in the mirror region. . Next, the transparent electrode layer 14 and the dielectric multilayer mirror 15 having an optical thickness of λ / 4 or an odd multiple thereof
Are sequentially formed, and the transparent electrode layer 1 is further overlapped with the metal electrode 13 so as to be electrically connected.
4 and the dielectric multilayer mirror 15 are patterned to obtain the structure shown in FIG.

[0020]

According to the structure of the present invention, the cap layer in the electrode region of the surface-emitting type semiconductor laser device has a relatively large band gap and a low impurity concentration, a first cap layer and a band gap smaller than the first cap layer. , A two-layer structure with a second cap layer of the same conductivity type having a high impurity concentration, and the cap layer in the reflector region is only the first cap layer. Thus, in the electrode region, the ohmic contact resistance can be reduced by the laminated structure of the second cap layer having a high impurity concentration and a high mobility and the metal electrode having a good thermal conductivity. On the other hand, in the reflective mirror region, a laminated structure of a transparent electrode which is electrically connected to the metal electrode and has little light absorption loss and a first cap layer having a small light absorption loss,
A current can be injected from the metal electrode through the transparent electrode under a voltage applied to the metal electrode while suppressing light absorption loss. Therefore, in the surface emitting semiconductor laser device according to the present invention, in addition to the direct injection of the current from the metal electrode in the electrode region, the current is also injected through the transparent electrode in the reflecting mirror region. Current can be injected into the active region at a density. As a result, the transverse mode is unified, the heat conduction is good, and the ohmic contact resistance is low, so that the threshold current is lower than in the conventional case. The present invention is particularly suitable as a configuration of a surface emitting semiconductor laser device having an oscillation wavelength of 1.2 to 1.6 μm.

[Brief description of the drawings]

FIG. 1 is a configuration of a surface emitting laser according to the present invention.

FIG. 2 is a configuration of a conventional surface emitting laser.

[Explanation of symbols]

 Reference Signs List 10 Example of surface emitting semiconductor laser device according to present invention 11 First cap layer made of p-GaInAsP 12 Second cap layer made of p-InGaAs 13 p-side metal electrode 14 p-side transparent electrode 15 mirror 20 conventional 21 Surface-emitting type semiconductor laser device 21 Substrate made of n-InP 22 Etching stop layer made of n-InGaAs 23 First clad layer made of n-InP 24 Active layer made of i-GaInAsP 25 First current made of p-InP Narrowing layer 26 Second current narrowing layer made of n-InP 27 Second cladding layer made of p-InP 28 Cap layer made of p-InGaAs 29 n-side electrode 30 p-side metal electrode 31, 32 mirror

Claims (3)

[Claims] 1. A surface-emitting type semiconductor laser device comprising a reflector region and an electrode region adjacent to the reflector region on a laser light oscillation structure formed on a substrate, wherein the electrode region has A first cap layer made of a compound semiconductor layer having a conductivity type different from the conductivity type of the substrate, and a band gap smaller than the first cap layer, a higher impurity concentration, and the same conductivity type provided on the first cap layer. A cap layer and a metal electrode having a two-layer structure with a second cap layer made of a compound semiconductor layer are sequentially laminated and formed, and a first cap layer, a transparent electrode and a dielectric A surface-emitting type semiconductor laser device characterized in that a body multilayer reflector is sequentially laminated and formed, and the transparent electrode partially overlaps and is electrically connected to the metal electrode in the electrode region. 2. A surface emitting semiconductor laser device comprising: n-In
A first clad layer made of n-InP formed on a P substrate, an active layer made of GaInAsP, and a second clad layer made of p-InP; the first cap layer formed on the second clad layer is band gap wavelength λ _g is λ _g <
1.2 μm p-G with an impurity concentration of approximately 1 × 10 ¹⁸ cm ^-3
consists aInAsP, p-InGaAs or bandgap wavelength lambda _g impurity concentration at lambda _g> 1.4 [mu] m of the second cap layer is an impurity concentration approximately 1 × 10 ¹⁹ cm ^-3 is approximately 1
× 10 ¹⁹ cm ^-3 of p-GaInAsP, and the transparent electrode layer is made of ITO (Indium Tin Oxide) or ZnO with Al, I
2. The surface-emitting type semiconductor laser device according to claim 1, comprising a metal to which at least one of n and Si is added. 3. The method for manufacturing a surface emitting semiconductor laser device according to claim 2, wherein a first cladding layer made of n-InP, an active layer made of GaInAsP, and a p-type semiconductor layer are formed on an n-InP substrate. -InP
Forming a double-heterojunction buried structure including a second cladding layer made of a first cap layer and a second cap layer on the second cladding layer, and forming a second cap layer on the second cladding layer. A mask pattern for partitioning the reflecting mirror region is formed on the cap layer by photolithography technology, a metal electrode film is deposited on the entire surface of the second cap layer while maintaining the mask pattern, and then the metal on the reflecting mirror region is formed by lift-off technology. Removing the electrode film to form a metal electrode in the electrode region; and using a selective etchant to etch away the second cap layer in the reflector region while leaving the first cap layer in the region. Forming a transparent electrode layer and a dielectric multilayer mirror in order, and furthermore, the transparent electrode layer and the dielectric so that the transparent electrode partially overlaps the metal electrode and is electrically connected. Manufacturing method of the surface emitting semiconductor laser device, characterized in that it comprises a step of patterning the layer film reflecting mirror.