JPH06112588A - Optical semiconductor element - Google Patents

Abstract

PURPOSE:To obtain an optical semiconductor element in which high output is realized while suppressing coupling loss and reflection. CONSTITUTION:In an optical semiconductor element wherein a mesa type second clad layer 35 is provided on a substrate 31 sequentially through a first clad layer 32, an optical waveguide layer 33, and a quantum well active layer 34 with the second clad layer 35 being embedded in a current block layer 40, the optical semiconductor element is sectioned into at least an active region and window regions on the opposite sides of the active region in the propagating direction of light. The active layer 34 is tapered on the opposite edges thereof at the border of the active region and window region and the tapered active layer 34 is coupled with a tapered semiconductor layer (output waveguide layer) 37 wider than the band gap of the active layer 34 where the semiconductor layer 37 is also tapered on the outside edge.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical semiconductor device,
In particular, it relates to a solid-state-laser-pumped high-power semiconductor laser and a high-power superluminescent diode for measurement / diagnosis.

[0002]

2. Description of the Related Art Conventionally, a super luminescent diode (hereinafter referred to as an SLD) has a conventional method of providing a low reflection coating on both end faces of a normal gain waveguide type semiconductor laser, or a stripe width with respect to a normal width of 5 μm. 30-40
There is a method of increasing the output by making the width as wide as .mu.m and inclining the angle of the stripe with respect to the resonator end surface by 5 degrees to make the end surface reflectance 10 ^-5 or less.

For example, FIG. 5 shows a schematic structure of a "high-power super luminescent diode" published in 1988 by Gerard et al., Quantum Electronics Division, Journal of the Institute of Electrical and Electronics Engineers, Volume 12, pages 2454-2457 (A. Gerard). , Et al: "High-P
ower Superluminescent Diodes "IEEE, J, Q
uantum. Electrics, vol24, No.12, pp2454-2457
(1988)).

Reference numeral 1 in the figure is an n-type GaAs substrate.
On this substrate 1, n-type Al _0.4 Ga _0.6 As clad layer 2, Al _0.06 Ga _0.94 As active layer 3, p-type Al
_{The 0.4} Ga _0.6 As clad layer 4, the n-type GaAs current blocking layer 5, and the insulating film 6 made of SiO ₂ are sequentially formed. The insulating film 6 is provided with a concave portion (stripe) 6a having an inclination angle (θ) with respect to the longitudinal direction of the substrate 1. A p-side electrode 7 connected to the current block layer 5 via the recess 6a is formed on the insulating film 6. An n-side electrode 8 is formed on the back surface of the substrate 1. In addition, 9 (hatched portion) in the drawing is Z formed in the current block layer 5 and the cladding layer 4 immediately below the stripe.
It is an n diffusion region.

In the diode having such a structure, n ₁
Where is the equivalent refractive index when the propagation constant of light around the active layer immediately below the stripe is converted into a refractive index, and n ₂ is the refractive index of the active layer portion other than the stripe, the tilted stripe 6a
The critical angle θ _c in which the Fabry-Perot mode exists is expressed by the following equation. sin θ _c = {1- (n ₂ / n ₁ ) ² } ^0.5

In the case of the prototype gain-guided laser of Gerrard et al., The refractive index difference is about 5 × 10 ⁻³ ,
θ _c becomes 3.13 degrees. By design, when both end faces are anti-reflection coated, the reflectance ρ (θ) in the stripe direction is 6 × 10 ⁻⁵ when the inclination angle θ of the stripe matches θ _c.
However, the Fabry-Perot mode cannot be completely suppressed. For angles larger than this critical angle θ _c , ρ
(Θ) becomes zero. In their prototype, θ = 5 degrees, resonator length = 400 μm, stripe width 5 μm (effective width is about 30 μm due to carrier diffusion), and maximum output is 2
Achieving 8mW.

On the other hand, in the high-power semiconductor laser, there is a method of preventing the absorption of the oscillated light at the end face by providing a window region transparent to the oscillation wavelength at the light emitting end face. Figure 6 shows 1982 Broubert et al. Applied Physics Letter Volume 40, 1029.
A cross-sectional view in the cavity direction of the long-cavity AlGaAs buried heterostructure window laser proposed in the page is shown (H. Bla.
uvelt, et al; "Large optical cavity AlGaA
s buried heterostructure window Lasers "Appl
． phys. Lett, vol40. p.1029).

Reference numeral 11 in the figure is an n-type GaAs substrate.
On this substrate 11, n-type Al _{x1 with the} vicinity of the end face removed
A Ga _1-x1 As clad layer 12 and an active layer 13 are formed. On the substrate 11 at both ends of both layers 12 and 13, p
-Type Al _z Ga _1-z As layer 14, n-type Al _z Ga _1-z A
The s layer 15 is formed so that the Al _z Ga _1-z As layer 15 is flush with the active layer 13. A p-type Al _y Ga _1-y is formed on the active layer 13 and the Al _z Ga _1-z As layer 15.
As (y <z) optical guide layer 16, p-type Al _x2 Ga _{1 -x2}
The As (x2> x1) clad layer 17 is sequentially formed. An insulating film 19 having an opening is formed on the clad layer 17. A p-side electrode 20 is formed in this opening. An n-side electrode 21 is formed on the back surface of the substrate 11. Reference numeral 22 in the figure denotes a Zn diffusion region formed in a part of the clad layer 17 and the light guide layer 16.

In the laser having such a structure, the active layer 13 has a smaller Al mixed crystal ratio than the cladding layer 12 and has a larger Al mixed crystal ratio than the optical guide layer 16 and has a high resistance p-type Al _{z in the} window region. It is embedded by a Ga _1-z As layer 14 and an n-type Al _z Ga _1-z As layer 15. Therefore, with respect to the maximum gain wavelength guided by the recombination of highly injected carriers in the central portion, there is no absorption in the vicinity of the end face of the resonator, and the optical guide layer 16 is continuous and is mode-converted. It is possible to increase the light output out of the window area without. However, in the laser having this configuration, the end face breakdown level is from several tens of mW to 150 mW.
Improved above.

[0010]

However, the conventional technique has the following problems.

The maximum output of the SLD when the non-reflective coating is provided on both end faces of the conventional semiconductor laser is a maximum of 5.
It is about mW. Further, the SLD proposed by Gerrard et al. In which the stripe direction is inclined 5 degrees with respect to the resonator direction has a high output of 28 mW at the maximum output, but since the stripe direction is inclined, the far-field pattern is From the center
It is 15 degrees apart from the horizontal and vertical. Further, in the case of tilting by 5 degrees, theoretically the Fabry-Perot mode (laser oscillation mode) is completely suppressed, but in reality, the diffusion effect of carriers in the lateral direction with respect to the junction surface, and Since the non-reflective coat has a significantly wider spectrum than LD,
It cannot cover all wavelengths, end face reflection occurs, laser mode exists, and FFP side mode exists. In the system using the reflected light, the returned light is mode-converted in the optical element, and the time phase information and the intensity information cannot be used.

In the window laser shown in FIG. 6, controllability of the layer thickness of the p-type Al _z Ga _1-z As layer 14 and the n-type Al _z Ga _1-z As layer 15 in the window region is required, and the window laser is activated. If it does not match the layer position, the coupling loss at the coupling portion increases, the threshold current increases, and it is difficult to obtain a large output. In particular,
The structure proposed by Broubert et al. Is unique to liquid phase growth technology that utilizes the property that re-growth on AlGaAs is extremely slow and only the window region where the GaAs substrate is exposed grows, making it difficult to control the layer thickness. It is difficult to produce with this growth technology.

In addition, when the window region is provided, a quantum well structure should be introduced into the active layer to remarkably increase the gain coefficient and achieve a low threshold and high efficiency operation.
However, this structure is more difficult than the layer thickness controllability, and is not suitable for the vapor phase growth technique in terms of mass productivity.

In the semiconductor laser, it is necessary to monitor the monitor output light with the light receiving element in the direction opposite to the emitting end face to stabilize the output. Also, two lasers are connected and one laser element is used as the monitor light receiving element. When used, a composite resonator is formed on both end faces to change the output of the laser element body, so that it becomes necessary to coat the monitor side with non-reflection.

The present invention has been made in consideration of such circumstances, and the coupling loss can be reduced by coupling the active region and the window region in a taper shape, and the reflection can be achieved by tapering the etching shape of the window region. It is an object of the present invention to provide an optical semiconductor device capable of suppressing the above-mentioned problem and increasing the output.

[0016]

SUMMARY OF THE INVENTION The present invention has the following features:
In an optical semiconductor device in which a mesa-shaped second cladding layer is provided via a first cladding layer, an optical waveguide layer, and a quantum well active layer in this order, and the second cladding layer is embedded with a current blocking layer, at least the active region and the light The active region is divided into window regions on both sides of the active region, and both end faces of the active layer are tapered at the boundary between the active region and the window region. Is an optical semiconductor device characterized in that a tapered semiconductor layer wider than the band gap of the active layer is coupled, and an end face of the semiconductor layer on the outer side is also tapered.

In the present invention, as means for forming the window region, an optical waveguide layer having a superlattice structure above and below the active layer, having a band gap wider than that of the active layer and narrower than that of the cladding layer, is provided. The window region is formed by disordering the quantum well structure by selectively injecting protons from the surface and injecting impurities.

In the present invention, as a means for a high power window type semiconductor laser, a method of cleaving one end face and performing high reflection coating, particularly when a GaAs substrate is used,
Under the active layer, the Al mixed crystal ratio is 0.3 or less, and the thickness is 0.2 μm.
The following optical waveguide layer may be introduced so that at least the optical waveguide layer remains in the first cladding layer (lower cladding layer) in the window region.

In the present invention, as an application of the window type laser, the active layer has a single quantum well structure and the stripe is divided into two taper portions on the emission side, and the active layer of one of the separated stripes is disordered light separation. It may be directly connected to the device.

[0020]

In the present invention, by tapering the connecting portion between the window region and the active region and connecting them,
It is possible to suppress mode conversion and reflection at the coupling portion. Further, when an optical waveguide is provided below the active layer and this layer remains in the window region, conversion of the filled distribution of light from the active region to the window region occurs smoothly. Furthermore,
In the GaAs system, the Al mixed crystal ratio of the optical waveguide is 0.3 or less,
When the thickness is 0.2 μm or less, it can play a role of suppressing the oxidation of the cladding layer due to the high Al mixed crystal ratio.

By the way, in the SLD, it is important to keep the reflectance of the end face of the semiconductor laser low. As a method thereof, a taper shape is used to suppress the reflectance. As another method, the Fabry-Perot mode can be suppressed by cleaving one end face of the window region and performing antireflection coating on the cleaved end face.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. Example 1 Reference is made to FIG.

Reference numeral 31 in the figure is an n-type GaAs substrate.
On this substrate 31, an n-type Al _0.45 Ga _0.55 As clad layer (first clad layer) 32 and an n-type Al _0.3 Ga _0.7 As
Optical waveguide layer 33, quantum well active layer 34, p-type Al _0.45 Ga
_0.55 As clad layer (second clad layer) 35, p-type Ga
The As cap layer 36 is sequentially formed. Here, the active layer 34 has a GaAs quantum well width of 10 nm and a quantum barrier A.
l _0.3 Ga _0.7 As formed of a double quantum well structure with a width of 5 nm. The spontaneous emission light generated by the recombination in the active layer 34 has a high efficiency due to the quantum well effect and its spectrum spread is narrower than that of the bulk active layer.

Both end faces of the active layer 34 along the light propagation direction (arrow X) are removed in a tapered shape. The active layer
On both ends of 34, 0.1 μm thick non-doped Al _0.4 G
The a _0.6 As output waveguide layer 37 is formed. An n-type Al _0.6 Ga _0.4 As current blocking layer 38 and an n-type GaAs layer 39 are formed on the output waveguide layer 37. The output waveguide layer 37 is tapered at a portion in contact with the active layer 34, and is also tapered at the other end side.
The second cladding layer 35 is filled with an n-type GaAs current blocking layer 40. This embedding forms an active region, and after removing both ends of the active layer 34 by etching, after re-growing the output waveguide layer 37, the current block layer 38 and the GaAs layer 39, the horizontal lateral light It is for suppressing the spread. In addition, in the process of removing the active layer and forming the window region, 10 nm is formed on the active layer.
Band cap is 0.5eV below the active layer with the following thickness
By providing a wide thin film as described above and continuing the etching step of leaving 0.2 to 0.3 μm on the active layer, the thin film on the active layer is used as a guide by a remarkably slow etching for removing the active layer. It is possible to determine whether or not to remove.

An n-type side electrode 41 is formed on the back surface of the substrate 31. A p-side electrode 42 is formed on the cap layer 36 and the current blocking layer 40. An insulating film 43 is formed on the current blocking layers 38, 40 and the GaAs layer 39. In order to increase the output, it is necessary to make the length of the active region longer than that of a semiconductor laser having a normal cavity length of 300 μm to increase the output and efficiently inject current into the active region. There is. Further, when a current is injected near the end face, there are dangling bonds, dislocations and vacancies that cause non-light emitting processes, the device temperature is raised, the threshold current is raised, and the optical output is lowered. Therefore, in order to suppress the temperature rise near the end face, the current non-injection region is set.

According to the first embodiment described above, both end faces of the active layer 34 and the output waveguide layer 37 are taper-bonded to each other at the boundary between the active region and the window region, and the optical waveguide layer 33 below the active layer 34.
Therefore, the filled distribution of light is mainly centered on the optical waveguide layer 33 by the output waveguide layer 37, the optical waveguide layer 33, and the current block layer 38. Further, the reflection can be suppressed only by making the etching shape of the window region tapered. (Example 2)

Referring to FIG. The window type semiconductor laser according to the second embodiment is characterized in that one end face of the window region is cleaved and a high reflection film 51 made of a dielectric multilayer film of SiO ₂ and amorphous Si is provided.

Thus, one end face has a tapered low reflection and the other end face has a high reflection (90% or more), and a high output laser can be used. Especially in high-power semiconductor lasers,
Since the energy distribution of light rises rapidly near the high-reflecting mirror, if there is a light absorption region in the light propagation direction, it is converted into heat and the band gap is narrowed as the temperature rises. As the optical absorption coefficient in the waveguide increases,
Since the gain coefficient is significantly reduced, the amount of injected carriers required for oscillation increases. In Example 2, the band gap with the Al _0.3 Ga _0.7 As layer directly coupled to the active layer in the window region was 300 meV or more, and the propagating light was not absorbed in the window region. (Example 3)

Referring to FIG. The semiconductor laser according to the third embodiment has a modulation-doped quantum well optical output waveguide layer 61 of 0.1 μm on the quantum well active layer 34 in which only the quantum barrier layer is p ^-type doped with an impurity concentration of 3 × 10 ¹⁷ cm ^−3. The window region has a structure in which at least the active layer 34 is disordered from above the insulating film 43 by proton injection. Here, as the configuration of the quantum well optical output waveguide layer 61,
Al _0.1 Ga _0.9 As quantum well width 10 nm, quantum barrier A
It is formed by a 5-period structure of l _0.5 Ga _{0.5 As} 10 nm. In the figure, 62 indicates a proton injection region.

In Example 3, by making the window region disordered, the effective composition of the active layer was Al.
Consistent with _0.1 Ga _0.9 As, quantum welled output waveguide layer 61
After the disordering of Al _0.3 Ga _0.7 As 0.1
Consistent with μm thick bulk. Therefore, the band gap difference between the window region and the active region is estimated to be 90 meV, and the window region becomes the non-absorption region with respect to the oscillation wavelength. Proton injection is performed from above the insulating film 43,
At least until the active layer is reached, annealing is performed after the implantation, so that disordering forms a taper-shaped widening of the band gap in the coupling portion due to thermal diffusion. (Example 4)

Referring to FIG. The semiconductor laser according to the fourth embodiment separates the TE wave and the TM wave of SLD or LD light. The quantum well active layer 34 is separated by a Y-shaped separation waveguide. With a mesa stripe width of 5 μm, a Y-shaped separation portion spread angle of 1 degree, and a quantum well width of 10 nm, one of the Y-shaped separation stripes is injected with protons to disorder the active layer until it reaches the GaAs substrate 31. . The end face is tapered to suppress end face reflection. In the fourth embodiment, the GaAs current blocking layer 40 fills the mesa stripe in the polarization separation region. However, this region can be left in a mesa stripe state by etching, and the same function can be performed in the ridge waveguide.

[0032]

As described above, according to the present invention, the coupling loss can be reduced by coupling the active region and the window region in a tapered shape, and the reflection can be reduced by tapering the etching shape of the window region. Can be suppressed,
By cleaving only one side to form a high reflection film, a high output laser can be realized, and an optical semiconductor device that can be expected to have a maximum output of 300 mW or more can be provided by horizontal horizontal mode control.

[Brief description of drawings]

FIG. 1 is a schematic perspective view of an SLD according to a first embodiment of the present invention.

FIG. 2 is a schematic perspective view of an LD according to a second embodiment of the present invention.

FIG. 3 is a schematic perspective view of an SLD according to a third embodiment of the present invention.

FIG. 4 is a schematic perspective view of an LD according to a fourth embodiment of the present invention.

FIG. 5 is a schematic perspective view of a conventional high output SLD.

FIG. 6 is a cross-sectional view of a conventional long cavity AlGaAs buried heterostructure window laser.

[Explanation of symbols]

31 ... GaAs substrate, 32, 35 ... Clad layer, 33 ...
Optical waveguide layer, 34 ... Quantum well active layer, 36 ... Cap layer, 37 ... Output waveguide layer, 38, 40 ... Current blocking layer, 39 ... GaAs layer, 41 ... N-side electrode, 42 ... P-side electrode, 43 ... Insulating film, 51 ... Highly reflective film,
61 ... Quantum well optical output waveguide layer, 62 ...
Proton injection area.

Claims (1)

[Claims] 1. An optical semiconductor in which a mesa-shaped second clad layer is provided on a substrate through a first clad layer, an optical waveguide layer and a quantum well active layer in this order, and the second clad layer is embedded with a current block layer. In the device, at least the active region and the window regions on both sides of the active region along the light propagation direction are sectioned, and both end surfaces of the active layer are tapered in a boundary between the active region and the window region. An optical semiconductor device characterized in that a tapered semiconductor layer wider than the band gap of the active layer is bonded to the removed tapered active layer, and an end face of the semiconductor layer on the outer side is also tapered. .