JPS625354B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser having a structure effective for controlling oscillation modes.

To use a semiconductor laser as a light source for optical communications and optical information processing, it is necessary to oscillate in a stable fundamental transverse mode and single-axis mode when driven with pulses or direct current, regardless of the magnitude of the current. required. Stabilization of the fundamental transverse mode is achieved by providing a built-in refractive index difference in the direction parallel to the active layer, ie in the transverse direction. The most basic type of semiconductor laser is a buried hetero laser in which the active region is surrounded by a semiconductor having a lower refractive index from all directions other than the laser emitting end face. While this structure can reliably stabilize the transverse mode, its manufacturing method is complicated, as it involves suspending crystal growth, forming an active region by etching, and then burying this active region by growing the crystal again. In particular, since crystal growth is required twice, crystal defects are introduced due to thermal deterioration of the substrate, and these become non-radiative recombination centers for carriers, increasing the oscillation value threshold current and shortening the operating life. There is. Furthermore, in a buried hetero laser, in order to obtain fundamental transverse mode oscillation, the width of the active layer must be controlled to about 1 μm, and the spot size of the light must be small, which reduces the optical energy density at the light output end face. It becomes large and optical damage on the end face is significant, making high output operation difficult. Furthermore, since the spot size is small, the laser radiation angle becomes large, resulting in a problem that the coupling efficiency with, for example, an optical fiber becomes low. On the other hand, a semiconductor laser has been proposed that solves the above problems and is formed by one-time crystal growth, and also has a structure in which a difference in refractive index is created in the lateral direction. This type of semiconductor laser performs stable fundamental transverse mode oscillation similar to that of a buried hetero laser without reducing the differential quantum efficiency of laser oscillation. In a double heterojunction semiconductor laser with an electrode stripe structure, there is an active layer with a uniform layer thickness as a region for guiding light, and a layer adjacent to this active layer that is thicker at the center of the groove width than at the edge of the groove. A plano-convex waveguide has a structure in which an optical guide layer whose thickness changes in the direction parallel to the active layer, that is, in the lateral direction, whose refractive index is lower than that of the active layer, and whose forbidden band width is large.
Waveguide (PCW) laser (hereinafter referred to as PCW laser) has been proposed (26th Applied Physics Association Conference, Lecture Proceedings 1979 ((1979)) Spring Page 170, Lecture number 27a- W-6, patent application 54-
(See 4961 “Semiconductor Laser”). With PCW laser,
Since the cross section of the light guide layer is plano-convex, a gentle effective refractive index difference Δn is created in the lateral direction, and the light guide layer oscillates in a stable fundamental transverse mode with a spot size of 2 to 3 μm in width. Therefore, it can operate up to a higher output than a buried hetero laser and at a smaller radiation angle. However, this PCW laser
It has the following drawbacks. That is, during direct current operation, the oscillation occurs in a substantially single axial mode, but during pulse operation, the oscillation increases to at least two to three axial modes, and the intensity of each of these modes is unstable. Such axial mode instability is a common problem in semiconductor lasers unless some kind of improvement is made. Hitherto, a buried heterostructure semiconductor laser has been proposed in which the width of the active layer is varied along the optical waveguide direction, that is, along the resonator length direction, in order to stabilize the axial mode. (Refer to Japanese Patent Application Laid-Open No. 52-152182). Figure 1 shows a perspective view of a semiconductor laser designed to stabilize this axial mode, and Figures 2a and b show two lasers with different widths of the active layer.
FIG. 3 is a cross-sectional view of two parts perpendicular to the resonator length direction.
The GaAs active layer 3 has a wide part and a small part, and is an n-type with a lower refractive index than the GaAs active layer 3.
The _surfaces other than the resonator end surfaces are surrounded by the Al _{0.3 Ga 0.7} As layer ₂ and _the p-type Al _0.3 Ga _0.7 As layer 4. 1 is an n-type GaAs substrate, 5 is an n-type GaAs layer, and a Zn planar diffusion part 6 for current confinement is provided directly above the GaAs active layer 3.
is provided. In this conventional semiconductor laser, light is confined in the active layer 3 and guided, so that it oscillates in the fundamental transverse mode. On the other hand, the effective refractive index difference Δn around the periphery for the guided light is small in the large width portion of the active layer, and on the contrary, large in the small width portion. Therefore, this structure can be regarded as a combination of optical waveguides guiding different modes in series, or equivalently, a laser resonator combined with another laser resonator. Therefore, a certain single axial mode of the light is selected and oscillated. However, this conventional buried hetero laser with axial mode selectivity has the following drawbacks. That is, the axial mode selectivity can be enhanced by, and only by, increasing the variation in the effective refractive index in the lateral direction and thus in the width of the GaAs active layer 3. However, if the width is significantly larger than the mode size of the guided light, the injected current will be wasted, resulting in lower luminous efficiency and higher oscillation threshold current.

An object of the present invention is to provide a semiconductor laser that avoids two crystal growths by epitaxially growing on a substrate with grooves of changed shape, performs fundamental transverse mode oscillation, and has axial mode selectivity. It is to be.

The main structure of the semiconductor laser of the present invention is as follows. In other words, it is a semiconductor laser with a double heterojunction structure formed on a semiconductor substrate having grooves with varying widths, and has a single active layer forming an active region, or an active layer with a refractive index lower than that of the active layer. It has a layered structure in which the semiconductor layer forming the active region is sandwiched between two optical confinement layers having a lower refractive index than either of the semiconductor layers. At least one of the semiconductor layers forming the active region in the striped region directly above the groove is convexly bulged toward the semiconductor substrate, and the width of this convex portion is The thickness is in the direction in which the groove extends,
This is a semiconductor laser in which light is guided in the direction in which the groove extends.

According to the present invention, the shape of the active region, that is, the thickness and width are both changed in the length direction of the resonator by forming the output end face that becomes the Fabry-Perot resonator perpendicular to the direction in which the groove extends. A semiconductor laser is obtained.

Next, embodiments of the present invention will be described with reference to the drawings. First, FIG. 3 shows a semiconductor substrate used in an embodiment of the present invention in which axial mode selectivity is imparted to the aforementioned PCW laser, and FIG. 4 shows a perspective view of a semiconductor laser formed on the semiconductor substrate. Fifth
The figure shows a cross-sectional view parallel to the length direction of the laser resonator. Figures 6 1 to 5 are cross-sectional views perpendicular to the laser cavity length direction at a portion where the groove width is large;
10 to 10 are diagrams showing the main manufacturing steps of the semiconductor laser of the present invention, also shown by cross-sectional views of portions with small groove widths. This structure is similar to the conventional
This corresponds to a PCW laser in which the shape of the optical guide layer, that is, the layer thickness and the width of the convex portion, change in the cavity length direction. The structure of the semiconductor laser of the present invention will be explained below with reference to its manufacturing process. First, a photoresist film having striped windows with varying widths was formed on the {100} plane of the n-type GaAs layer 7 shown in FIGS. 1 and 6, and using this as a mask, the width was changed by chemical etching. Form a groove. The remaining photoresist film is removed and thoroughly cleaned to obtain a semiconductor substrate with grooves as shown in FIG.
Figures 2, 7). Next, by liquid phase epitaxial growth, n-type for light and carrier confinement is sequentially grown.
Al _{0.3 Ga 0.7} As optical confinement layer 8C Fig. 6 3 _, 8),
n-type for light guide and carrier confinement
Al ₀ . ₁ Ga ₀ . ₉ As optical guide layers 9, 9' (Fig. 6 4,
9) Form a p-type GaAs active layer 10, a p-type Al _{0.3 Ga 0.7} As optical confinement layer ₁₁ , and a p-type GaAs contact facilitation layer 12 (FIGS. 6, 5 and 10). At this time, since the growth rate is high in areas where the groove is narrow, the cooling rate and growth time are adjusted so that the growth ends with the central part of the optical confinement layer 2 concave, as shown in FIG. 6 and 8, even in such areas. Control. Next, SiO ₂ with a band-shaped window is formed on the surface of the p-type GaAs contact facilitating layer 12.
An insulating film 13 is formed so that the window is located directly above the groove of the n-type GaAs substrate 7, and then a p-type electrode 14 is deposited to provide a striped electrode along the groove. Further, an n-type electrode 15 is attached to the back surface of the n-type GaAs substrate 7, and a resonator reflecting surface is formed perpendicular to the stripe direction of the striped p-type electrode 14, thereby obtaining the semiconductor laser shown in FIG. According to this embodiment,
As shown in FIGS. 5 and 6, an active layer 10 forming an active region and light guide layers 9, 9'
The shape of the optical guide layers 9 and 9' changes in the laser resonator length direction, and therefore the optical waveguide whose effective refractive index changes in the light waveguide direction becomes the active region of the semiconductor laser. There is. In the semiconductor laser of the present invention manufactured in this way, the width and thickness of the active region change in the light waveguide direction. The thickness of the active region is
Since it is usually small, about a fraction of the width of the active region, the regulation of the optical waveguide mode is extremely strong in the thickness direction.
That is, in the present invention, by changing the thickness and utilizing this stronger waveguide mechanism, it is possible to easily obtain an effective refractive index change for the axial mode. This is also extremely advantageous when considering scattering loss when light is guided. That is, in the conventional buried hetero laser with axial mode selectivity described above, scattering loss at the side surface of the GaAs active layer 3 whose width changes cannot be ignored, but in the case of the present invention, such scattering loss can be avoided due to the strong light waveguide. becomes less. On the other hand, in the present invention, the effective refractive index difference Δn is provided in the direction parallel to the active layer, similar to the above-mentioned PCW laser, so that the transverse mode is also stable in the fundamental mode up to high output. Furthermore, in the present invention, since the effective refractive index difference Δn in the lateral direction is small in the portions of the active region where the width of the convex portion is wide, the portions where the width of the convex portions is large, that is, the portions where the groove width of the semiconductor substrate is wide, Similarly, by adjusting the combination with the small portion, Δn in the lateral direction can be adjusted, which has the advantage of increasing the degree of freedom in element design that takes into account not only the stability of the axial mode but also the fundamental transverse mode.
Furthermore, in this embodiment, if the resonator end face is formed so that the groove width is large near the laser emitting end face, light will seep out from the active layer into the optical guide layer in this part, spread in the thickness direction, and be guided. Therefore, a semiconductor laser with a small laser radiation angle in the direction perpendicular to the rectifying junction and excellent coupling efficiency with an optical fiber is obtained.

As described above, the present invention provides a semiconductor laser that oscillates in a stable fundamental transverse mode, has axial mode selectivity, and can be easily manufactured in a single crystal growth process without thermal deterioration of the substrate. can get.

The embodiments described above use the active layer 10 as the active region.
This is a so-called PCW laser with a different groove width, which has optical guide layers 9 and 9', but the same effect and function can be achieved even if the active region has only an active layer and no optical guide layer. has. In this case, the GaAs active layer has a shape similar to that of the optical guide layers 9 and 9' in Figure 4, so the lateral refractive index difference is larger than that of the PCW laser described above, resulting in a smaller spot size and a stronger laser beam. Obtain axis mode selectivity. Also, in these two examples,
Although the width was large in the thick part of the active region, the effect and function of the present invention can be achieved even if the convex part is thick in the small width part.

In the above embodiment, the case was described where it was formed on a GaAs substrate, but the optical guide layer and the active layer were formed using I _o P as the substrate, and a quaternary crystal such as I _ox Ga _1-x As _y P _1-y was used. Needless to say, it's good to have it. In this case, since the I _o P substrate has a smaller refractive index and a larger forbidden band width than the I _o GaAsP that forms the active layer or optical guide layer, the I _o P substrate with the groove formed
The substrate itself becomes an optical confinement layer. Therefore I _o
The functions and effects of the present invention can be obtained by forming grooves with varying widths and depths on a P substrate and using InGaAsP as a semiconductor layer for forming an active region. However, even if an InP substrate with grooves of varying width is used, the functions and effects of the present invention can still be achieved by forming an InP or InGaAsP optical confinement layer and then forming an InGaAsP layer as a light guide layer. Needless to say.

In the embodiment, straight grooves are used, but it goes without saying that curved grooves may be used as long as the width of the grooves is varied.

[Brief explanation of the drawing]

Figure 1 is a perspective view of a conventional semiconductor laser; Figure 2 is a perspective view of a conventional semiconductor laser;
Figures a and b are cross-sectional views of a conventional semiconductor laser;
The figure is a perspective view of a semiconductor substrate of a semiconductor laser of the present invention, FIG. 4 is a perspective view of a semiconductor laser of the present invention,
FIG. 5 is a main cross-sectional view in a direction parallel to the laser resonator length direction of the semiconductor laser of the present invention, and FIG. 6 is a diagram showing the main manufacturing steps of the semiconductor laser of the present invention. Each code indicates 1... n-type GaAs substrate, 2
...n-type _Al0.3Ga0.7As layer, 3... _{GaAs active layer} ,
4...p _- type _Al0.3Ga0.7As layer, 5... _n - _type GaAs
Layer, 6... Zn planar diffusion part, 7... Semiconductor substrate, 8, 11... Light confinement layer, 9, 9'... Light guide layer, 10... Active layer, 12... Contact facilitation layer, 13 ...SiO ₂ insulating film, 14...p-type electrode,
15...corresponds to an n-type electrode.

Claims (1)

[Claims] 1. An active region consisting of a single active layer or an active layer and a light beam having a lower refractive index and a larger forbidden band width than the active layer are formed on a semiconductor substrate in which a groove whose width changes along the light waveguide direction is provided. A layer structure is formed in which an active region consisting of a guide layer is sandwiched between two optical confinement layers having a lower refractive index than any of the semiconductor layers constituting the active region, and At least one of the layers has a shape that is convex toward the semiconductor substrate side in a striped region directly above the groove, and the convex portion is thickest in the central portion of the groove, and is thickest near the edge of the groove. A semiconductor laser characterized by being thin and having a width and thickness that change in the waveguide direction.