GB2040552A

GB2040552A - Semiconductor Laser

Info

Publication number: GB2040552A
Application number: GB8004283A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1979-02-13
Filing date: 1980-02-08
Publication date: 1980-08-28
Also published as: FR2449348A1; IT8019822A0; DE3003667A1; JPS55111192A; AU5537180A; IT1140545B; SE8001054L; NL7901122A

Abstract

In a semiconductor laser having a heterojunction layer structure and a p-n junction (10), the substrate (2) comprises a cavity (6) from which laser radiation (7) emanates normal to the surface. The active volume (4A) of the laser is determined by the p-n junction, and the bottom of the cavity is formed by an optically flat interface (12) between the substrate (2) and a passive layer (3). The interface (12) is exposed by etching. The reflectors of the resonator are formed by the interface (12) and the interface between the layer (5) and the layer (13). The laser can be readily coupled to an optical fibre. <IMAGE>

Description

SPECIFICATION Semiconductor Lasers and their Manufacture The invention relates to semiconductor lasers and to methods of manufacturing such lasers.

A semiconductor laser is known from IEEE transactions on Electron Devices, Vol. ED 24, No.

7, July 1977, pages 995 to 1000, in particular Figure 7. This is a double heterojunction laser of the conventional type having on a substrate an active layer which is situated between two passive layers. The active layer has a smaller forbidden bandwidth and a composition differing from that of the passive layers. The reflective side facets of the semiconductor crystal constitute the resonator and current injection takes place from a strip-shaped electrode on the second passive layer. For measuring purposes, a cavity is provided in the substrate so that radiation which is normal to the plane of the active layer can be observed. The p-n junction across which charge carriers are injected into the active layer is formed between the active layer and one of the passive layers and extends entirely parallel to the direction of the layer.

In conventional double heterojunction lasers which, with the exception of the cavity in the substrate, usually correspond to the abovedescribed laser serving for measuring purposes only, the laser radiation emanates parallel to the active surface and the resonator is formed by reflective parallel cleavage surfaces forming the side facets of the semiconductor crystal. These lasers have an active volume which in one direction has a large length, in the order of a few hundred microns, and hence gives rise to longitudinal Fabry-Pérot modes of oscillation with comparatively small mutual wavelength differences so that operation in only one longitudinal mode is not possible. Moreover, the provision of cleavage surfaces is a delicate operation which requires accurate crystal orientation.

Furthermore, in these known lasers the possibility that crystal defects occur in the long strip-shaped active region is fairly large, while the radiation beam emanating parallel to the active layer shows a rather large divergence due to the small active layer thickness. Furthermore, the beam often is fairly strongly astigmatic.

According to the invention a semiconductor laser having a semiconductor body comprising a substrate of a first conductivity type, a first passive layer of the first conductivity type situated on the substrate, an active layer which is present on the first passive layer, and a second passive layer situated on the active layer, which active layer has a smaller forbidden bandwidth than the passive layers and is situated at least partly within a resonator formed by two reflective surfaces of the body, in which laser the substrate comprises a cavity through which radiation emanates, said cavity extending through the whole thickness of the substrate to the first passive layer, the substrate and the second passive layer are each connected electrically to a connection conductor, and a p-n junction is present between said connection conductors for injecting charge carriers into the active layer, is characterized in that a region of the second conductivity type extends from a surface of the semiconductor body through at least the whole thickness of the second passive layer and forms with the adjoining part of the semiconductor body of the first conductivity type a p-n junction which, viewed from said surface, at least laterally entirely surrounds a semiconductor region situated above the cavity and determines an active volume of the active layer, in that on said surface an electrode layer forming one of said connection conductors is provided which electrode layer adjoins the region of the second conductivity type, and in that one of the reflective surfaces of the resonator is formed by the optically flat interface between the substrate and the first passive layer which interface is exposed by and forms the bottom of the cavity.

Thus, at least one of the reflective surfaces of the rsonator in a semiconductor laser in accordance with the invention is formed by the original crystallographic interface between two adjacent semiconductor layers of the device instead of by a cleavage surface. Because of this the lasers which are manufactured simultaneously on one semiconductor wafer can simply be severed for example, by sawing or cracking. Due to the small active volume, the possibility of crystal defects in this volume is small. Furthermore, a glass fibre light conductor can be provided simply in the cavity while only little divergence occurs as a result of the emanating area which is comparatively large with respect to the active volume.Since the dimensions of the active volume can be chosen to be small, operation in only one mode of radiation is possible; this applies in particular to the longitudinal mode since the emanating beam is normal to the active layer. Furthermore the threshold current can be kept comparatively low and the lasers can be tested during manufacture while they are still interconnected via the semiconductor wafer.

In one form of a semiconductor laser in accordance with the first aspect of the invention the active layer and the second passive layer are both formed from layers of the first conductivity type and that the region of the second conductivity type extends through the active layer and, with the active layer, forms a p-n junction which extends transversely through said layer, said junction surrounding a region of the first conductivity type comprising a part of the active layer and a part of the second passive layer situated above the cavity.

In another form of a semiconductor laser in accordance with the invention the p-n junction entirely surrounds the region of the second conductivity type within the semiconductor body and for its major part extends substantially parallel to the active layer.

In accordance with a further aspect of the invention there is provided a method of manufacturing a semiconductor laser in accordance with the first aspect of the invention, which method includes the steps of growing successively on a semiconductor substrate of a first conductivity type at least a first passive semiconductor layer, an active semiconductor layer and a second passive semiconductor layer, all of the first conductivity type, the material of the passive layers having a larger forbidden bandwidth than that of the active layer, forming a semiconductor region of the second conductivity type which extends from the surface of the body situated opposite the substrate at least to the active layer so as to form with the adjoining region of the first conductivity type a p-n junction which at least laterally surrounds a surfaceadjoining region, forming a cavity in the substrate by means of an etching process in which the material of the first passive layer is not attacked, continuing said etching process until a part of the interface between the substrate and the first passive layer has been exposed to form the bottom of the cavity, which cavity is formed opposite said region surrounded by the p-n junction, and providing the substrate and the second region with electrode layers.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a diagrammatic cross-sectional view of a semiconductor laser in accordance with the invention; Figures 2 to 5 are diagrammatic crosssectional views of successive stages in the manufacture of the semiconductor laser shown in Figure 1, and Figure 6 is a diagrammatic cross-sectional view of another semiconductor laser in accordance with the invention.

The Figures are diagrmmatic and not drawn to scale, in particular the dimensions in the thickness direction are exaggerated for clarity.

Corresponding parts are generally referred to by the same reference numerals; semiconductor regions of the same conductivity type are shaded in the same direction.

Figure 1 is a diagrammatic cross-sectional view of a semiconductor laser in accordance with the invention. The laser has a semiconductor body 1 comprising a substrate 2 of a first conductivity type, in this example an n-type substrate of gallium arsenide (GaAs). On said substrate 2 there is provided a first passive layer 3, also of the nconductivity type, in this example a 10 micron thick layer of n-type gallium aluminium arsenide in which 30 atoms% of the Group Ill element is aluminium, so with a composition GaO 7Aio 3As and with a doping concentration of sox 1017 tin atoms per cm3. An active layer 4, in this example of gallium arsenide, having a thickness of 0.5 micron is present on the first passive layer 3, which active layer is present partly within a resonator to be described hereinafter.

A second passive layer 5 having a composition GaO 7Alo 3As and a thickness of 2 microns is present on the active layer 4. The two passive layers 3 and 5 have a larger forbidden bandwidth than the active layer 4 so that the injected charge carriers within the semiconductor body are restricted substantially to the active layer.

The substrate 2 has a cavity 6 through which the radiation emanates in the direction of the arrow 7. The cavity 6 in this example is rotationally symmetrical about the line MM' so that the cross-section of the cavity parallel to the direction of the layer is circular. The cavity 6 extends through the whole thickness of the substrate up to the first passive layer 3. The substrate 2 and the second passive layer 5 are each connected electrically to a connection conductor, namely the substrate 2 is connected to an electrode layer 8 and the second passive layer 5 is connected to an electrode layer 9. A p-n junction 10 for injecting charge carriers, in this case holes, into the active layer 4 is present between the connection conductors 8 and 9.

In accordance with the invention, a region of the second conductivity type, in this example a ptype region 11, extends from the surface of the semiconductor body through at least the whole thickness of the second passive layer 5. The region 11 forms a p-n junction 10 with the adjoining part of the semiconductor body of the first, n-conductivity type. The junction 10, viewed from the surface, laterally entirely surrounds a semiconductor region which is situated above the cavity 6 and in this example is also situated rotationally symmetrically about the line MM' and is formed by the active layer 4 and the second passive layer 5. The junction 11 determines an active volume 4A of the active layer 4.

Furthermore, in accordance with the invention, an electrode layer 9 is provided on the surface and adjoins the region 1 The resonator is formed by the surface of the second passive layer 5 and by the reflective optically flat interface 12 between the substrate 2 and the first passive layer 3. The interface 12 is exposed by and forms the bottom of the cavity 6. In the semiconductor laser described the active layer 4 and the second passive layer 5 are both formed from layers of the first conductivity type, so in this example of the nconductivity type. The layer 4 in this example has a doping concentration of sox 1017 tin atoms per cm3. The layer 5 also has a doping concentration of 5x 1017 tin atoms per cm3. The region 11 extends through the active layer 4 and with the layer 4 forms a p-n junction 10 extending transversely through the layer.

In this embodi:ment, the semiconductor body also comprises a semiconductor contact layer 13 of p-type gallium arsenide having a thickness of approximately 1 micron. The contact layer 1 3 is present on the second passive layer 5 and comprises an aperture which is situated above the cavity 6 and which extends down to the second passive layer 5. The region 11 extends through the whole contact layer 1 3.

In this embodiment the substrate 2 is in the form of an 80 microns thick n-type supporting body 14 having a doping concentration of 1018 silicon atoms per cm3 on which a 5 microns thick epitaxial layer 1 5 of the same material and the same conductivity type, in this example having a doping concentration of 5 x 1017 tin atoms per cm3, is provided. The interface 12 which is used as a reflector is an interface between two epitaxial layers and hence has less defects than the interface between a supporting body and the first epitaxial layer grown thereon. However, it is possible instead to use a substrate 2 in the form of a monocrystalline semiconductor wafer without an epitaxial layer.

The surface of the part 5A of the layer 5 in this example is covered with a dielectric layer 1 6 and an electrode layer 9 is provided on the contact layer 13 and the insulating layer 1 6. As a resulting of the presence of the layer 1 6 the surface of the region 5A cannot be damaged by alloying effects during provision of the electrode layer 9 and the associated heating so that an optimally flat surface is maintained. The layer 1 6 preferably has a thickness which is substantially equal to half a wavelength of the laser radiation in the material of the layer 16. The reflection at the electrode layer 9 is then substantially equal to that which would be obtained if the layer 9 was provided directly on the semiconductor surface.

The layer 16 may consist, for example, of silicon oxide or silicon nitride or of a succession of sublayers having different compositions.

In the Figure 1 embodiment, however, it is not necessary for the electrode layer 9 to be provided above the region 5A since the current supply takes place via the region 11. The surface of the region 5A may be entirely exposed, if desired, or be covered only with a reflection-increasing layer, while finally in the Figure 1 embodiment the layer 1 6 may be omitted entirely.

The bottom 12 of the cavity 6 is provided with a reflection-increasing layer 1 7. This may be a dielectric layer formed from one or a plurality of successive layers, or be a very thin metal layer, for example a gold layer, having a thickness of, for example, 10 nm, which is sufficiently permeable to the emanating radiation.

The largest dimension of the active volume 4A of the active layer 4 in any direction parallel to the direction of the layer, so in this example the diameter of the disc-shaped part 4A, is in this case 10 microns and generally is preferably at most 100 microns. With such a small active volume the possibility of crystal defects occurring therein is substantially negligible.

When applying a voltage in the forward direction between the electrode layers 8 and 9 above a given threshold voltage the semiconductor laser described provides a beam 7 of radiation having very little divergence and also little astigmatism due to the fact that the path of the radiation through the active layer is very short, thus giving rise to only negligible phase differences between different parts of the beam cross-section. Coupling to a light conductor, for example, a glass fibre light conductor, can be achieved in a simple manner by mounting it with one end in the cavity 6.

Since the distance between the reflective surfaces 12 and 18 is only 13 microns, only one longitudinal Fabry-P6rot mode of radiation occurs in the semiconductor laser described in contrast with most known lasers in which the distance between the reflective surfaces generally is a few hundred microns.

A further advantage of a semiconductor laser in accordance with the invention is that the thickness of the active and passive layers plays a less important role than in known semiconductor lasers. As regards the active layer in fact a certain compensation occurs in that, with the same current through the laser, with a thinner active layer the concentration of charge carriers in the active volume, hence also the amplification, is larger but this amplification occurs over a shorter range (the thickness of the active layer), and conversely. Although these oppositely acting effects do not depend to the same extent on the thickness of the active layer, the tolerance for the layer thickness nevertheless is larger due to this compensation than in known lasers radiating in the direction of the layer.As regards the thickness of the passive layers: this determines, together with the thickness of the active layer, the wavelength distance of adjacent longitudinal modes and has a rather wide tolerance.

The threshold current density of a laser in accordance with the invention is comparatively high. This is caused inter alia by the fact that the region over which amplification occurs, that is to say the layer thickness, is only very small and the concentration of charge carriers in this active region must hence be very large.

Since the dimensions of the active volume are small, however, the threshold current will nevertheless be low; with a contact diameter of 5 microns this will be in the order of 40 mA in the absence of the layers 1 6 and 1 7. Due to the action of said reflection-increasing layers the threshold current can still be reduced considerably.

As already said above, the thickness of the active and passive layers is comparatively less important for the operation of a semiconductor laser in accordance with the invention. The overall thickness of the active and passive layers 3, 4 and 5, however, which constitutes the length of the laser between the reflective surfaces 12 and 18, is preferably chosen to be at least equal to 5 microns and at most equal to 20 microns. When the overall thickness is larger, the wavelengths of the longitudinal Fabry-Pérot modes are so close together that the possibility arises of the occurrence of more than one longitudinal radiation mode.When the overall thickness is smaller than approximately 5 microns, the wavelengths of adjacent modes become separated to such an extent that a single mode may not fall within the amplification profile (which has a "width" in the order of 20 nm). The mode distance AA for an overall layer thickness of 1 3 microns in the embodiment described is approximately 7 nm; the wavelength of the emitted radiation is approximately 900 nm. In accordance with the invention, the semiconductor laser shown in Figure 1 may be manufactured as follows (see Figures 2 to 5). The starting material, for example, is a semiconductor wafer 14 of gallium arsenide of a first conductivity type, in this example the n-conductivity type, having a doping concentration of 10'8 silicon atoms per cm3 and a thickness of approximately 300 microns.On this wafer is provided, for example by growing from the liquid phase, a 5 micron thick layer 1 5 of ntype GaAs with a doping concentration of sox 1017 tin atoms per cm3. The successive growth of epitaxial layers from the liquid phase is a generally known technique; for this purpose reference may be made, for example, to the book by D. Elwell and J. J. Scheel, Crystal Growth from High Temperature Solutions, Academic Press 1975, pages 433 to 467.

On the n-type gallium arsenide substrate 2 thus obtained and consisting of the layers 14 and 15 are provided successively and without removing the wafer from the growth apparatus a layer 3 of n-type GaO 7Alo 3As to a thickness of 10 microns and a doping concentration of 5x1017 tin atoms per cm3, a layer 4 of n-type GaAs to a thickness of 0.5 micron and a doping concentration of sox 1017 tin atoms per cm3, a layer 5 of n-type GaO ,Alo 3As to a thickness of 2 microns and a doping concentration of sox 1017 tin atoms per cm3, and a layer 13 of n-type GaAs to a thickness of 1 micron and a doping concentration of sox 1017 tin atoms per cm3.

A semiconductor region 11 of the second (p-) conductivity type is then formed, see Figure 2.

This may be done in various manners. In this example, an island 20 of silicon nitride is provided using known deposition and photolithographic etching methods. Zinc is then indiffused to form the p-type region 1 the silicon nitride 20 serving as a mask against the diffusion. This diffusion may be carried out, for example, in an evacuated capsule with Zn As2 as a source at approximately 8000C. Alternatively, however, it is possible to provide the region 11 in a different manner, for example, at the area where the region 11 is to be formed the material of the layers 3, 4, 5 and 13 may be removed and be replaced by p-type semiconductor material, for example p-type gallium aluminium arsenide, obtained by epitaxial growth.

In this example an aperture is then provided in the layer 13, although this is not strictly necessary. For this purpose the nitride 20 is removed by means of hot phosphoric acid after which a mask 21 of silicon oxide is provided by known deposition and photolithographic etching methods (see Figure 3). By etching, for example, with a solution consisting of 25 cm3 of hydrogen peroxide of 30 vol.%, 25 cm3 of water and 0.5 cm3 of NH40H of 30 vol.% which attacks gallium arsenide but does not attack gallium aluminium arsenide, an aperture is etched in the layer 1 3 the bottom of which coincides with the interface between the layers 5 and 13 and is optically flat.

A silicon nitride layer 1 6 is then provided over the surface, see Figure 3. This layer 1 6 has a thickness of approximately 0.23 micron, which corresponds to approximately half a wavelength of the laser radiation within the nitride. The oxide layer 21 and the parts of the layer 1 6 present thereon are then removed by etching after which a metal 9 is deposited (see Figure 4) which in this example is composed of a 50 nm thick chromium layer, a 100 nm thick platinum layer present thereon and a 50 nm thick gold layer on the platinum layer. However, the layer 9 may consist of other metals.

As stated above, the layer 1 6 may be omitted, if desired, but this is at the expense of a surface which reflects less well.

The metal layer 9 is preferably baked at 3500C for a few minutes. On the substrate side the thickness of the semiconductor wafer is reduced to an overall thickness of approximately 100 microns by lapping, polishing and etching. An electrode layer 8 (see Figure 5) consisting, for example, of a gold-germanium-nickel alloy is then provided on the surface on the substrate side and is baked at 4250C for a few minutes. A cavity 6 is formed in the substrate. For this purpose, after masking the electrode layer 8 with a mask of photolacquer the layer 8 is removed to form an aperture having a diameter of approximately 1 50 microns at the area of the cavity to be provided. A fresh photolacquer mask having a smaller aperture is then provided and the GaAs substrate 2 is removed by etching with the same etchant as was used for etching the layer 13.Since the etchant does not attack gallium aluminium arsenide a part of the interface 12 between the substrate 2 and the layer 3 at the bottom of the cavity is exposed. The cavity 6 is provided opposite the aperture etched in the layer 13, and so it is opposite the region of the layers 4 and 5 surrounded by the p-n junction 10.

After provided a reflection-increasing layer 1 7 as stated above within the aperture 6 on the surface 12, if this should be desired, the semiconductor laser shown in Figure 1 is obtained.

It will be obvious that many lasers can be provided simultaneously on one and the same semiconductor wafer. The lasers are then separated from each other, for example, by sawing, cracking or etching. A particular advantage of the invention is that the lasers can be tested during the manufacture while still connected together.

Instead of the structure described so far, a semiconductor laser in accordance with the invention may have a different structure. Another embodiment is shown in the cross-sectional view of Figure 6. In this embodiment the region 11 of the second (p-) conductivity type is provided so that the p-n junction 10 entirely surrounds the region 11 within the semiconductor body. The p-n junction 10, for its major part, extends substantially parallel to the active layer 4. The region 11 extends at least down to the active layer and, as shown in Figure 6, may extend into the active layer, or even through the active layer into the layer 3.In this embodiment the electrode layer 9 outside the region 11 is separated from the semiconductor surface by an insulating layer 19, for example a silicon oxide layer, while in this case the active layer has a higher doping concentration than that described with reference to Figure 1 since the injection of electrons from the layer 4 into the region 11 determines the laser action, in particular when the region 11 extends throughout the thickness of the layer 4, in which case the active volume is entirely p-type conductive. In this case the doping concentration of the layer 4 therefore is, for example, 5 x 1018 tellurium atoms per cm3. The dimensions, layer thicknesses and remaining doping concentrations may the same, if desired, as described with reference to Figure 1.

In the present embodiment in which the region 11 does not extend over the whole thickness of the layer 4, the active volume consists of a p-type part in which electrons are injected from the layer 4 and therebelow an n-type part in which holes are injected from the region 11.

In Figure 6 the device is also assumed to be rotationally symmetrical about the line NN'.

Corresponding parts are referred to by the same reference numerals as in Figure 1. In this case also the p-n junction, in particular the part thereof extending parallel to the direction of the layer, determines the active volume of the laser.

It will be obvious that the invention is not restricted to the embodiments described but that many variations are possible to those skilled in the art without departing from the scope of this invention. For example, it is not necessary for the cavity 6 and the aperture in the contact layer 1 3 to be rotationally symmetrical, although this is to be preferred if a symmetrical beam of radiation is desired. The layer thicknesses and the doping concentrations, as well as the materials of which the various layers consist, may also be varied at will by those skilled in the art. Quite a lot of semiconductor materials suitable for laser manufacture are known from the technical literature from which those skilled in the art can make a selection. Furthermore, of course, all conductivity types may (simultaneously) be replaced by their opposite types. The electrode layers 8 and 9 may also be replaced by other ohmic contact metallizations which may consist of several metal layers provided one on top of the other.

Claims

1. A semiconductor laser having a semiconductor body comprising a substrate of a first conductivity type, a first passive layer of the first conductivity type situated on the substrate, an active layer which is present on the first passive layer, and a second passive layer situated on the active layer, which active layer has a smaller forbidden bandwidth than the passive layers and is situated at least partly within a resonator formed by two reflective surfaces of the body, in which laser the substrate comprises a cavity through which radiation emanates, said cavity extending through the whole thickness of the substrate to the first passive layer, the substrate and the second passive layer are each connected electrically to a connection conductor, and a p-n junction is present between said connection conductors for injecting charge carriers into the active layer, characterized in that a region of the second conductivity type extends from a surface of the semiconductor body through at least the whole thickness of the second passive layer and forms with the adjoining part of the semiconductor body of the first conductivity type a p-n junction which, viewed from said surface, at least laterally entirely surrounds a semiconductor region situated above the cavity and determines an active volume of the active layer, in that on said surface an electrode layer forming one of said connection conductors is provided which electrode layer adjoins the region of the second conductivity type, and in that one of the reflective surfaces of the resonator is formed by the optically flat interface between the substrate and the first passive layer which interface is exposed by and forms the bottom of the cavity.

2. A semiconductor laser as claimed in Claim 1, characterized in that the largest dimension of the active volume of the active layer in a direction parallel to the layer is at most 100 microns.

3. A semiconductor laser as claimed in Claim 1 or Claim 2, characterized in that the overall thickness of the semiconductor layers between the bottom of the cavity and the oppositely located surface of the body is at least 5 microns and at most 20 microns.

4. A semiconductor laser as claimed in any of the preceding Claims, characterised in that the cavity has a substantially circular cross-section.

5. A semiconductor laser as claimed in any of the preceding Claims, characterized in that at least the bottom of the cavity is covered with a reflection-increasing layer.

6. A semiconductor laser as claimed in any of the preceding Claims, characterized in that the substrate consists of a supporting body and an epitaxial layer of the same material and the same conductivity type grown thereon, which epitaxial layer forms the interface with the first passive layer.

7. A semiconductor laser as claimed in any of the preceding Claims, characterized in that the active layer and the second passive layer are both formed from layers of the first conductivity type and that the region of the second conductivity type extends through the active layer and, with the active layer, forms a p-n junction extending transversely through said layer, said junction surrounding a region of the first conductivity type comprising a part of the active layer and a part of the second passive layer situated above the cavity.

8. A semiconductor laser as claimed in Claim 7, characterized in that an insulating layer is provided on the surface of the regions of the first conductivity type surrounded by the p-n junction.

9. A semiconductor laser as claimed in Claim 8, characterized in that the thickness of the insulating layer is substantially half a wavelength of the emitted radiation in the insulating layer.

10. A semiconductor laser as claimed in Claim 8 or Claim 9, characterized in that the electrode layer adjoining the region of the second conductivity type also extends on the insulating layer.

11. A semiconductor laser as claimed in any of Claims 7 to 10, characterized in that the semiconductor body comprises a semiconductor contact layer provided on the second passive layer, said contact layer comprising an aperture which is situated above the cavity and which extends down to the second passive layer, said region of the second conductivity type extending through the whole thickness of the contact layer.

12. A semiconductor device as claimed in any of Claims 1 to 6, characterized in that the p-n junction entirely surrounds the region of the second conductivity type within the semiconductor and for its major part extends substantially parallel to the active layer.

13. A method of manufacturing a semiconductor laser as claimed in any of the preceding Claims, including the steps of growing successively on a semiconductor substrate of a first conductivity type at least a first passive semiconductor layer, an active semiconductor layer and a second passive semiconductor layer, all of the first conductivity type, the material of the passive layers having a larger forbidden bandwidth than that of the active layer, forming a semiconductor region of the second conductivity type which extends from the surface of the body situated opposite the substrate at least to the active layer so as to form a p-n junction with the adjoining region of the first conductivity type, which junction at least laterally surrounds a surface-adjoining region, forming a cavity in the substrate by means of an etching process in which the material of the first passive layer is not attacked, continuing said etching process untii a part of the interface between the substrate and the first passive layer has been exposed to form the bottom of the cavity, which cavity is formed opposite said region surrounded by the p-n junction, and providing the substrate and the second region with electrode layers.

14. A method as claimed in Claim 1 which further includes the steps of growing a semiconductor contact layer on the second passive layer, and forming an aperture in said contact layer above the cavity by a selective etching process which does not attack the material of the second passive layer so that within the aperture an optically flat part of the interface between the second passive layer and the contact layer is exposed.

1 5. A semiconductor laser substantially as herein described with reference to Figure 1 or Figure 6 of the accompanying drawings.

16. A method of manufacturing a semiconductor laser substantially as herein described with reference to Figures 1 to 5 of the accompanying drawings.