JPH0582758B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a surface-emitting semiconductor laser, and more particularly to a surface-emitting semiconductor laser in which the reflective surfaces of an active layer and a resonator are improved. [Technical background of the invention and its problems] Semiconductor lasers have features common to semiconductor light emitting devices such as small size, high efficiency, light weight, and resistance to mechanical vibrations, as well as the ability to perform high-speed direct modulation and the ability to connect with optical fibers. Due to its features such as high efficiency coupling, it has been put into practical use as a light source for optoelectronics in recent years, but in order to further expand its field of use, it is necessary to improve the light output and significantly reduce the cost. is necessary. By the way, as a conventional semiconductor laser, -
It is known that a double heterojunction structure is formed using a crystal of a group compound semiconductor, and the cleaved plane perpendicular to the junction plane obtained by cleaving the crystal is used as a reflecting plane. As an example, the diffused stripe type laser shown in FIG. 4 is shown. 1 in the figure is an n-type semiconductor substrate made of - group compound semiconductor, 2, 3, 4
are an n-type cladding layer, an active layer, and a p-type cladding layer for forming a double heterojunction, which are successively laminated on the substrate 1. 5 is a Zn diffusion stripe provided on the p-type cladding layer 4 to limit the current; 6 is a positive electrode layer provided on the p-type cladding layer 4; 7 is a Zn diffusion strip on the back surface of the substrate 1. A negative electrode layer is provided. Further, 8a and 8b in the figure are cleavage planes formed by cleaving the - group compound semiconductor crystals 1 to 4 along the crystal planes, respectively, and these cleavage planes 8a and 8b are double heterojunction planes. Because it is vertical and extremely flat,
These serve as reflective surfaces and constitute a Fabry-Perot resonator. In the semiconductor laser having such a structure, when a forward bias DC current is applied between the electrodes 6 and 7, carriers are injected into the active layer 3 and light is emitted. At this time, since the active layer 3 has a higher refractive index than the cladding layers 2 and 4, the light is confined in the active layer 3 and is repeatedly reflected at the cleavage planes (reflecting surfaces) 8a and 8b, causing laser oscillation. . A portion of the generated laser light passes through the cleavage plane 8a or 8b and is emitted to the outside. However, in the above-mentioned semiconductor laser, the reflective surface is formed by cleaving a semiconductor crystal, a process in which it is extremely difficult to control its performance, that is, with low reproducibility, so the rate of obtaining a reflective surface with good performance is low. , there was a problem of poor manufacturing yield. In addition, the reflectance of the cleavage plane is low at about 30%.
Considering that the output of a semiconductor laser depends on the reflectance of the reflective surface, there was also a problem in improving the output. On the other hand, recently some research has been carried out on surface-emitting semiconductor lasers that emit laser light in a direction perpendicular to the double heterojunction surface, but in semiconductor lasers with such a structure, the reflective surface is formed by cleavage. It is impossible to do so. For this reason, surface-emitting semiconductor lasers are manufactured using epitaxial growth technology. However, this method has the problem of not being able to achieve sufficient flatness of the reflecting surface, and of being able to control the resonator length to only about 0.1 μm at most. In surface-emitting semiconductor lasers, the spacing between the reflective surfaces is narrow, for example, about 10 μm, so mass production is impossible unless the position of the reflective surfaces can be controlled to the order of 10 Å. Conventional technology has not been able to produce reflective surfaces that satisfy the above order. No means of forming this have been developed. Also,
Conventional surface-emitting semiconductor lasers have extremely short resonators, so they have problems such as higher threshold current density than normal semiconductor lasers, making it difficult to operate at high output, or having low reliability during output. . [Object of the Invention] The present invention aims to provide a surface-emitting semiconductor laser that enables highly reliable output operation and highly efficient and stable oscillation. According to the invention, -
In a surface-emitting type semiconductor laser having an active layer surrounded by a cladding layer having a composition with a relatively large bandgap on a group compound semiconductor substrate, a plurality of the active layers exist in a multilayer structure, and an active layer is provided. It is possible to obtain a surface-emitting semiconductor laser in which the layers are optically coupled and in which a reflective surface with excellent flatness and high reflectance is formed with good controllability. [Summary of the Invention] The present invention provides a surface-emitting semiconductor laser having an active layer surrounded by a cladding layer having a composition with a relatively large bandgap on a - group compound semiconductor substrate, in which the active layer has a plurality of layers, Due to the solid phase reaction product with the metal that exists in a multilayer structure and the reflective surface of the resonator reacts with the group or group element of the - group compound semiconductor or both to produce a compound having a stoichiometric composition. It is characterized by the fact that it is formed. Examples of metals that react with the above groups or elements of the groups or both to produce compounds having a chemical equivalent composition include Ti, Fe, Co, Ni, Rh, Pd, W,
Examples include Os, Ir, Pt, and lanthanum-based rare earth elements, all of which are transition metals. especially,
Ni, Pd, and Pt are preferred. When one or more of these metals is attached to the surface of a - group compound semiconductor and a solid phase reaction is caused, a solid phase reaction product is obtained. The surface of the - group compound semiconductor of this solid phase reaction product becomes a surface with extremely excellent flatness close to a mirror surface. This is because the solid phase reaction mentioned above is -
This is because the interface between the compound semiconductor and the solid phase reaction product becomes flat on the order of the lattice constant. This situation will be explained in detail with reference to FIGS. 1a to 1c. First, as shown in FIG. 1a, - group semiconductor 1
A metal layer 12 of Ni, Pd, Pt, etc. is deposited on top of the metal layer 12 by vacuum evaporation, sputtering, or the like. In addition,
Reference numeral 13 denotes a natural oxide and mechanically damaged layer that is normally formed on the surface of the - group compound semiconductor 11. Next, heat treatment is performed to form the compound semiconductor 1.
1 and the metal layer 12 to cause a solid phase reaction. This reaction proceeds through intermediate steps as shown in FIG. 1b. That is, between the two, a compound 14 of the metal layer 12 and the group element of the compound semiconductor is formed on the side of the compound semiconductor 11, and a compound 15 of the metal and the group element of the compound semiconductor is formed on the side of the metal layer 12.
As shown in FIG. 1B, this reaction proceeds while eroding the native oxide and the mechanically damaged layer 13, forming a flat interface. The reaction proceeds until the deposited metal layer 12 disappears, as shown in FIG. 1c. Therefore, a compound 14 of the group element of the semiconductor and the metal is finally formed in contact with the - group compound semiconductor 11,
A compound 15 of a semiconductor group element and the metal is formed on the outside thereof. Since the solid phase reaction described above proceeds in such a way that the low-index plane of the - group compound semiconductor appears, the interface of the compound (reaction product) 14 in contact with the semiconductor 11 becomes flat on the order of the lattice constant. The above-mentioned solid phase reaction involves the deposition of metal and -
Although the reaction rate varies somewhat depending on the type of compound semiconductor, the reaction can generally be carried out at a temperature that does not cause thermal denaturation of the compound semiconductor, for example, 250 to 450°C. The above is the reason why an extremely flat surface can be obtained by the solid phase reaction between a - group compound semiconductor and a metal. The solid phase reaction product thus obtained is
Optically, it has properties similar to metals, so it is difficult to use conventional cleaved surfaces (reflectance of about 30%) or chemically etched surfaces (reflectance of about 30%).
(15%), the reflectance is significantly improved (at least 60% or more). This is extremely effective for lowering the threshold current of a surface-emitting semiconductor laser. Furthermore, the position of the interface between the solid phase reaction product and the compound semiconductor (plane A in FIGS. 1b and 1c) can be controlled with extremely high precision. Figure 2 shows
This figure shows the relationship between the heat treatment time and heat treatment temperature required to form an interface at a constant depth from the surface of GaAs by solid phase reaction after depositing a certain amount of Pt on GaAs. As is clear from this characteristic diagram, the surface position of the solid phase reaction product formed inside the compound semiconductor can be controlled by several tens of Å by controlling the thickness of the deposited metal film, heat treatment time, and heat treatment temperature. It can be seen that extremely accurate control can be achieved on the order of . In this way, the solid-state reaction product of a - group compound semiconductor and a metal that reacts with the group or group elements or both to form a compound having a stoichiometric composition is extremely flat at the interface with the - group compound semiconductor. Furthermore, the surface has a high reflectance and the surface position can be easily controlled, making it extremely suitable as a reflective surface for a surface-emitting semiconductor laser. [Embodiments of the Invention] Hereinafter, an example in which the present invention is applied to a GaAs-based surface-emitting semiconductor laser will be described in detail, together with the manufacturing method shown in FIGS. 3a to 3. First, as shown in Figure 3a, semi-insulating GaAs
For example, an intermediate cladding layer 22 made of n-type GaAlAs with a thickness of 2 μm, a GaAs active layer 23 with a thickness of 1 μm, and a GaAs active layer 23 with a thickness of 2 μm are formed on a substrate 21 by epitaxial growth.
p-type GaAlAs layer 24, 1 μm thick GaAs active layer 25, 2 μm thick n-type GaAlAs layer 26, 1 μm thick.
GaAs active layer 27 and p-type GaAlAs with a thickness of 2 μm
A multilayer structure 29 was formed by sequentially depositing surface cladding layers 28 consisting of: Next, as shown in Figure b, photo-etching was performed.
After forming a resist pattern 30 in which the expected n ⁺ type region is opened, a portion extending from the surface cladding layer 28 to the surface of the intermediate cladding layer 22 is selectively etched using the resist pattern 30 as a mask to form an etched portion 31. did. Subsequently, after peeling off the resist pattern 30, GaAlAs doped with tin (Sn) is selectively epitaxially grown on the etched portion 31 to form the n ⁺ type region 32.
was formed (as shown in figure c). Next, as shown in Figure d, the CVD method is used to
After depositing a SiO ₂ film 33 and selectively etching the SiO ₂ film 33 using a resist pattern (not shown) as a mask to open a diffusion window 34, Zn is thermally diffused using the SiO ₂ film 33 as a mask. A p ⁺ type region 35 was selectively formed in a region extending from the surface cladding layer 28 to the surface of the intermediate cladding layer 22. Subsequently, the n ⁺ type region 32 and the p ^{+ type} region 32 are formed by photolithography.
A resist pattern 36 is formed in which the region between the mold regions 35 and a portion where the positive electrode is to be formed is opened, and the SiO ₂ film 33 is exposed using the resist pattern 36 as a mask.
is selectively etched to form a reflective surface forming window 37.
After opening, Pt films 38 ₁ and 38 ₂ were deposited on the entire surface including the resist pattern 36 by vacuum evaporation or sputtering. At this time, as shown in figure e, the exposed part of the resist pattern 36 is
Pt film 38 ₁ and Pt film 38 ₂ on resist pattern 36
and are separated by the step of the resist pattern 36. Subsequently, the resist pattern 36 was removed and the Pt film ₃₈₂ thereon was lifted off. As a result, the _Pt film pattern 39 forms p ⁺
A positive electrode 40 made of Pt is placed on a part of the mold region 35.
were formed respectively. Next, a resist pattern 41 in which a negative electrode planned portion is opened is formed by photolithography, and the SiO ₂ film 33 is selectively etched using the resist pattern 41 as a mask to open an electrode extraction window 42, and then the resist pattern 41 is opened. Au-Ge-Ni films 43, 43 ₁ ,
43 ₂ was deposited. _At this time _, as shown in FIG. They are separated by 41 steps. Subsequently, the resist pattern 41 was removed, and the Au-Ge-Ni film ₄₃₂ thereon was lifted off. As a result, a negative electrode 44 made of Au--Ge--Ni was formed on a portion of the n ⁺ -type region 32 exposed through the electrode extraction window 42 of the SiO ₂ film 33, as shown in FIG. Next, as shown in FIG. 3I, the back surface (lower surface) of the substrate 21 was polished to a desired thickness, and then a resist pattern 45 having openings corresponding to the windows 37 was formed by photolithography. Next, the same figure j
As shown in FIG. 3, an opening 46 was formed by selectively etching the semi-insulating GaAs substrate 21 using the resist pattern 45 as a mask until the intermediate cladding layer 22 on the upper surface thereof was exposed. At this time, the center line of the opening 46 is the reflection surface forming window 3 of the SiO ₂ film 33.
It is desirable to match the center line of No. 7. Next, after peeling off the resist pattern 45,
A Pt film 47 was deposited on the back surface of the substrate 21 including the opening 46 by a vacuum deposition method or a sputtering method, and was further heat-treated at 400° C. for 10 minutes. At this time, as shown in FIG.
Solid phase reaction products 48a and 48b were formed at the interface between the Pt film 47 and the intermediate cladding layer 22, respectively. Solid phase reaction products 48a, 48b thus formed
is the reflective surface of the Hubble-Perot resonator, but since the laser light will be extracted in the direction perpendicular to it through one solid phase reaction product 48b, the Pt film 47 for forming the solid phase reaction product 48b is ,
After the vapor deposition thickness is made sufficiently thin or a solid phase reaction product is formed, a part of it may be removed by etching. By the heat treatment described above, the positive electrode 40 made of Pt, the p ⁺ type region 35, and
Negative electrode 44 and n ⁺ type region 32 made of Au-Ge-Ni
and were connected to each other. Thereafter, cutting was performed as shown in FIG. 1 to manufacture a surface-emitting semiconductor laser 49 . The surface-emitting semiconductor laser 49 of this embodiment has an active layer 2 on a semi-insulating substrate 21, as shown in FIG.
It has a multilayer structure 29 in which three layers of 3, 25, and 27 are laminated, and an n ⁺ type region 32 and a p ⁺ type region are electrically isolated from each other in a region extending from the surface cladding layer 28 to the surface of the intermediate cladding layer 22. 35 and further
At the interface between the surface cladding layer 28 exposed through the window 37 of the SiO ₂ film 33 and the Pt film pattern 39, Ga,
Solid phase reaction product with As or GaAs 48a, 4
8b, and the p ⁺ type region 35 and n ⁺
The structure includes a positive electrode 40 of Pt and a negative electrode 44 of Au--Ge--Ni which are ohmicly connected to the mold region 32, respectively. Controlling the oscillation wavelength of the semiconductor laser 49 having such a structure requires not only processing time but also
This is possible depending on the thickness of the Pt film to be deposited and the processing temperature. However, it is extremely difficult to achieve this with a conventional surface-emitting semiconductor laser, that is, one whose reflecting surface is an epitaxially grown layer or a metal thin film simply deposited on the substrate surface, or a multilayer film mirror such as GaAs/GaAlAs. Furthermore, since the surface emitting semiconductor laser 49 of this embodiment has three active layers, the oscillation output can be significantly improved compared to a conventional surface emitting semiconductor laser having a single active layer. Further, the p ⁺ type region 35 and the n ⁺ type region 32 are connected through the positive electrode 40 and the negative electrode 44.
is larger than the forbidden band width of GaAs, and GaAlAs
When a voltage smaller than the forbidden band width is applied, the current is p-type GaAlAs layer 24 → GaAs active layer 23 → n
Intermediate cladding layer 22 of type GaAlAs, p-type
GaAlAs layer 24 → GaAs active layer 25 → n-type
GaAlAs layer 26, p-type GaAlAs surface cladding layer 28 → GaAs active layer 27 → n-type GaAlAs layer 26
flows through three routes. Thus the current is 3
During the process of flowing through two paths, holes and electrons recombine and generate light, which can significantly reduce the threshold current. In addition, in the above embodiment, the use of a semi-insulating substrate,
With the structure described in detail, the electrode for current injection is provided only on one surface of the semiconductor crystal, which is convenient for complex integration of devices or device assembly. However, the present invention is not necessarily limited to this. For example, a conductive substrate such as Si-doped n-GaAs is used as the substrate, and a p-(norp)-n layer centered on the active layer is formed. active layer,
Stacking with p + p−(norp)− as the basic unit, or p ⁺ p−(norp)−
A laminated structure having nn ⁺ as a basic unit may be provided, and a structure may be adopted in which a current flows in the thickness direction of the semiconductor crystal. In the above example, the mirror surface of the solid phase reaction product is formed on the GaAlAs cladding layers above and below the active layer, but when adding a GaAs buffer layer, cap layer, etc., the solid phase reaction product is formed on the solid phase reaction product. Of course, a product may also be formed. Further, although both of the reflective surfaces of the laser are made of solid-phase reaction products, one of the reflective surfaces may be made of a metal such as Au or the like deposited by vapor deposition. In the above example, epitaxial growth technology and impurity diffusion technology are used together to form the n ⁺ type region and the p ⁺ type region, but both regions can also be formed using only one of the techniques. Of course it is possible. In the above embodiment, the composition is GaAlAs-based and the oscillation wavelength is 0.7 to 0.9 μm, but the present invention is not necessarily limited to this. For example,
A semiconductor laser having an InGaAsP composition and an oscillation wavelength in the 1.1 to 1.7 μm band can also be used. The number of active layers is not limited to three, but may be two or more. [Effects of the Invention] As detailed above, according to the present invention, a reflective surface having a multilayered active layer, excellent flatness, and high reflectance can be formed with good controllability, and mass production is possible. A surface-emitting semiconductor laser can be provided.

[Brief explanation of the drawing]

Figures 1 a to c are cross-sectional views showing the production process of solid phase reaction products, Figure 2 is the treatment temperature in the solid phase reaction,
A diagram showing the relationship between processing time and interface penetration depth. FIGS. 3a to 3 are cross-sectional views showing the manufacturing process for obtaining a surface-emitting semiconductor laser using a GaAs-based compound semiconductor in an embodiment of the present invention. , FIG. 4 is a perspective view showing a conventional semiconductor laser. 21...Substrate, 22...Intermediate cladding layer, 2
3, 25, 27...GaAs active layer, 24...p type
GaAlAs layer, 26... n-type GaAlAs layer, 28...
Surface cladding layer, 29...Multilayer structure, 33...
SiO ₂ film, 37...window for forming reflective surface, 39...Pt
Membrane pattern, 40... Positive electrode, 44... Negative electrode,
46...Opening, 47...Pt film, 48a, 48
b...Solid phase reaction product, 49 ...Surface-emitting semiconductor laser.

Claims (1)

[Claims] A surface-emitting semiconductor laser having an active layer surrounded by a cladding layer having a composition with a relatively large bandgap on a 1-group compound semiconductor substrate, wherein the active layer has a plurality of layers and a multilayer structure. and the reflective surface of the resonator is formed by a solid-phase reaction product with a metal that reacts with a group or a group element of the group compound semiconductor, or both, to produce a compound having a stoichiometric composition. A surface-emitting semiconductor laser characterized by: 2 A plurality of active layers are sandwiched between two buried cladding layers which form part of the cladding layer and have opposite conductivity types, and are connected to the plurality of active layers through the two buried cladding layers. 2. A surface-emitting semiconductor laser according to claim 1, wherein current injection is performed. 3. The surface-emitting semiconductor laser according to claim 2, wherein both positive and negative electrodes for current injection are provided on one surface of the semiconductor crystal. 4. The surface-emitting semiconductor laser according to claim 1, wherein the metal is a transition metal. 5. The surface-emitting semiconductor laser according to claim 1, wherein the metal is one of Ni, Pd, and Pt.