EP0399033A1

EP0399033A1 - Method for producing semi-conductor lasers and lasers obtained by such method

Info

Publication number: EP0399033A1
Application number: EP90900897A
Authority: EP
Inventors: Claude Weisbuch; Baudouin De Cremoux; Jean-Paul Pocholle
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1988-12-09
Filing date: 1989-12-05
Publication date: 1990-11-28
Also published as: FR2640438B1; FR2640438A1; WO1990006608A1; US5055422A; JPH03502513A

Abstract

La présente invention concerne les procédés de réalisation de lasers semi-conducteurs. Le procédé selon l'invention se caractérise essentiellement par le fait qu'il consiste à réaliser une couche (1) d'un milieu actif semi-conducteur laser, à réaliser une cavité optique (2) associée à cette couche, à disposer, sur au moins une partie de la surface de la couche, des première (6) et deuxième (7) couches de matériaux d'impuretés de polarités opposées, à faire diffuser dans le milieu actif au moins une partie des deux matériaux d'impuretés pour réaliser dans la première couche un cylindre (8) d'axe sensiblement parallèle à l'axe de la cavité optique et formé de deux demi-coquilles (9, 10) semi-cylindriques d'impuretés de polarités opposées diffusées, et à relier deux conducteurs (12) de l'énergie électrique, respectivement aux deux demi-coquilles. Application à la réalisation d'une pluralité de diodes laser sur un même substrat support, pour donner naissance à un faisceau laser unique homogène et dense.The present invention relates to methods for producing semiconductor lasers. The method according to the invention is characterized essentially by the fact that it consists in producing a layer (1) of an active semiconductor laser medium, in producing an optical cavity (2) associated with this layer, in disposing, on at least part of the surface of the layer, of the first (6) and second (7) layers of impurity materials of opposite polarities, to cause at least a part of the two impurity materials to diffuse into the active medium to produce in the first layer a cylinder (8) with an axis substantially parallel to the axis of the optical cavity and formed of two semi-cylindrical half-shells (9, 10) of impurities of opposite polarities diffused, and to connect two conductors (12) electrical energy, respectively to the two half-shells. Application to the production of a plurality of laser diodes on the same support substrate, to give rise to a single homogeneous and dense laser beam.

Description

Method for producing semiconductor lasers and Lasers obtained by the method

The present invention relates to the methods of producing semiconductor lasers and the lasers thus produced.

A semiconductor laser is essentially constituted by an active medium of crystalline type based, for example, on Gallium Arsenide, on which an electrical energy called "pumping" is applied, by means of two electrodes respectively located on the side and on the other of the crystalline material.

Furthermore, to obtain stimulation of the energy of optical radiation, the active medium is placed, as for all lasers, in a resonant optical cavity formed by two reflecting mirrors, at least one of which is partially transparent to allow the light energy stimulated inside the optical cavity to emerge from this cavity.

In the field of semiconductor lasers, the mirrors of the optical cavity are formed, for example, of multi-dielectric layers placed directly on the crystalline active material. This method of producing the elements of a resonant optical cavity is moreover conventional and well known. However, there are others, for example the stack of semiconductor layers integrated into the structure of the crystal or of the substrate which supports the crystal.

Current techniques make it possible to respect all of these characteristics fairly well, but there are still a certain number of problems, in particular when placing the electrodes for supplying pumping electrical energy on the active medium. This problem arises more particularly in the case where a plurality of active media are implanted on a single base substrate, one next to the other essentially for the purpose of obtaining a plurality of light beams having substantially parallel emission directions. , especially since, to obtain a composite beam equivalent to a relatively dense and homogeneous single beam, it is: imperative that the active media are arranged one next to the other, the tightest possible and so that the axes of the resonant cavities are all oriented substantially perpendicular to the surface through which the beam is to be emitted. The problem posed is then the production of electrodes for supplying electrical energy, knowing that these electrodes can only be placed on the face on which at least one of the two mirrors of the optical cavity is exposed.

An embodiment of semiconductor lasers is already known in which the supply of electrical energy takes place substantially in the plane of one of the mirrors of the optical cavity of the laser. This realization is obtained in the following way. We begin by depositing, on a support substrate, a first set of layers constituting a first mirror of the optical cavity. On this first set of layers, a second set of crystal layers is deposited by epitaxy, for example Gallium Arsenide, over a thickness equal to that which is necessary to obtain the desired active laser medium, then a third set of layers , for example multi-dielectric, constituting the second mirror of the optical cavity, the lateral dimensions, length and width, of these three sets of layers being greater than those of the semiconductor laser as must be obtained in the end.

This multi-layer stacking being ^' finished, a circular groove known by men of 1' rt is dug under the term of "mesa", by removing part of the two or three sets of layers to make a sort of column.

In the groove, there are then, by any means, two different kinds of material, laterally surrounding the column and in contact with at least the second set of layers, which produces the two electrodes. These two materials are, for example, P-type and N-type semiconductor materials respectively, such as Gallium Arsenide and Aluminum doped with Zinc and Silicon, respectively. The two materials fill the groove by flush with the upper level of the third set of layers and, on their surface flush with this level, are respectively secured two conductors of electrical energy making an ohmic contact, for example based on Gold, Zinc, Nickel or Germanium.

This technique gives good results as regards the production of a laser element on a substrate. It nevertheless has certain drawbacks, essentially of a financial nature, due to the large number of operations it requires, and in terms of size, since it is difficult to produce grooves of very small dimensions which are suitable receiving the materials to constitute the electrodes for supplying electrical energy. It is therefore limited to being able to implant on a substrate of given dimensions only a relatively small number of laser elements, which is inconvenient for obtaining a wide laser beam, both dense and homogeneous.

The object of the present invention is to overcome the drawbacks mentioned above and to implement a process for producing semiconductor lasers which makes it possible to lower their cost price, and which allows integration, on a substrate of dimensions data, of a number of laser optical cavities in higher density than that allowed by the techniques known to date.

More specifically, the subject of the present invention is a method for producing semiconductor lasers, characterized in that it consists in: producing a layer of an active semiconductor laser medium having transverse dimensions greater than those of the medium active laser to obtain, to achieve a resonant optical cavity associated with said layer, said cavity being formed so that its optical axis is substantially perpendicular to the plane in which said transverse dimensions are defined, to be placed on at least part of the surface of said layer, first and second zones of impurity materials of opposite polarities, to diffuse in a part of said active medium at least a part of the two said impurity materials, to produce in said layer substantially a cylinder of axis substantially parallel to said axis of said optical cavity and formed of two substantially semi-cylindrical half-shells indric respectively of impurities diffused from opposite polarities, and to connect two conductors of electrical energy, respectively to the two half-shells.

The present invention also relates to a semiconductor laser obtained by the method according to the invention, characterized in that it comprises a resonant optical cavity, an active laser medium associated with said optical cavity, said active medium comprising two half -shells of impurities diffused with opposite polarities on, respectively, at least two portions of its lateral surface, and two conductors of electrical energy supply respectively connected to the two said half-shells.

Other characteristics and advantages of the present invention will appear during the following description given with reference to the drawings annexed by way of illustration, but in no way limiting, in which:

FIGS. 1A and 1B show, respectively in section and in perspective, a diagram of an embodiment of a semiconductor laser according to the invention, making it possible to explain a first embodiment of the method according to the invention, The FIG. 2 represents, seen in section and in perspective, a diagram of an embodiment of a semiconductor laser making it possible to explain a second embodiment of the method according to the invention, and

FIG. 3 represents, as an application, a set of semiconductor lasers integrated on the same support substrate making it possible to obtain a light beam of a large and relatively dense and homogeneous diameter.

The process for producing semiconductor lasers according to the invention consists, in a first step illustrated in FIG. 1A, of producing a layer 1 of an active semiconductor laser medium, advantageously on a support substrate 13, of transverse dimensions higher than those of the active laser medium when the laser is completed. By way of example, this active medium consists of a stack of thin layers of Gallium arsenide and Gallium arsenide and Aluminum.

To this active medium is associated a resonant optical cavity 2 having an optical axis direction 3 substantially perpendicular to the plane in which the transverse dimensions of the layer 1 are defined. This cavity is formed of two mirrors obtained, for example, by the deposition of a layer of dielectric material on each of the two opposite faces 4, 5 of., the layer of medium a _. ctif respectively contained in two planes parallel to that in which the transverse dimensions are defined.

In a possible implementation of the invention, the first step defined above can be carried out as follows: In a suitable reactor, sequential deposition of epitaxial layers is carried out, in the following order taking as reference the upper face of the base substrate 13 which serves as a support: production of a first so-called "lower" mirror, deposition of the active medium on this first mirror, and deposition of the second so-called "upper" mirror which, associated with the first, forms the cavity resonant optics. The active medium, like mirrors, can consist of a set of thin layers of semiconductors. The preferred materials are from the family of GaAIAs / GaAs or from the family of GalnAsP / InP. The mirrors are obtained, for example, by stacking semiconductor layers of alternating indices or of different indices.

In a second step of the method, for example by implantation or deposition, the first 6 and second directly on at least part of the surface of the layer 1 of active medium, or indirectly on the upper mirror of the optical cavity 2 7 zones of materials comprising impurities of opposite polarities, for example impurities of types P and N constituted by Zinc and Silicon. In the example illustrated, these two impurity zones 6, 7 are implanted indirectly on the face 5 perpendicular to the optical axis 3 opposite to the face 4 in contact with the support substrate 13, through the upper mirror of the optical cavity .

The next step in the process, in this case the third, consists in diffusing the materials of the two impurity zones 6, 7 into the active laser medium, in order to produce in the first layer 1, as illustrated in FIG. 1B, substantially a cylinder 8 with an axis parallel to the optical axis 3 of the resonant optical cavity 2. This cylinder 8 consists of two half-shells 9, 10 of diffused impurities of opposite polarities and delimiting a column 11 of the active semiconductor laser medium. The diffusion of the materials ^" of impurities in the layer 1 is carried out so that the height of the cylinder 8 is- substantially equal to the thickness of the layer of active medium taken between its two external faces 4, 5 perpendicular to the axis optical 3. By way of example, the diffusion is obtained by heat treatment, for example by subjecting the assembly to a temperature of the order of 450 degrees C. for approximately 30 minutes, using the example of implementation advantageous given above, the depth of the diffusion of the impurity atoms in the epitaxial structure is between one and ten microns.

When the diffusion in the layer 1 is finished, in a last step, two electric power supply conductors are respectively connected to the two half-shells 9, 10, only one, 12, having been shown in FIG. 1B.

In the example given above with reference to FIGS. 1A and 1B, the two zones of impurity materials of opposite polarities are arranged on one face of the layer of active medium perpendicular to the optical axis of the resonant cavity. However, for certain applications, in particular when the transverse dimensions of this layer are relatively large, it is possible, as shown in FIG. 2, to delimit, in the layer of active medium 20, a column 21 by digging a circular groove therein 22 The next step of the method then consists in placing on two portions of the cylindrical peripheral surface 23 of the column 21, respectively two impurity zones of opposite polarities 24, 25. This second embodiment of the method makes it possible to obtain higher resonant cavity heights than those obtained with the first mode described above. It is more advantageous to obtain semiconductor lasers having large transverse dimensions.

In fact, during the first mode of implementation of the method, the thickness of the half-shells widens away from the level of the first and second zones of impurities, which can lead to an increasing harmful lateral congestion. On the other hand, in the case of the production of semiconductor lasers having a large optical cavity, with the second embodiment of the method, the lateral widening of the half-shells described above is avoided, because it it is then possible to carry out a diffusion of the impurities in the active medium.

It is specified that the impurities are doping atoms of type "P" or "N", for example atoms of Zinc or of

-3 Silicon, and have concentrations always lower than 10, while the constituent elements of the semiconductors are Gallium (Ga), Aluminum (Al) and Arsenic (As) for the materials of the family GaAIAs / GaAs, and Gallium, Indium (In), Arsenic and Phosphorus (P) for those of the GaAsInP / InP family. The concentration of these constituent elements can take any value between 0 and 0.5, provided that there are as many atoms in column III of the classification of elements (Ga, Al, In) as there are in column V (As, P).

At the same time as the diffusion of impurities occurs, under the conditions mentioned below, the interdiffusion of the main constituent elements of the axial epi layers. More specifically, in this case, it is the process of interdiffusion assisted by the diffusion of impurities, during which the constituent elements of the layers are mixed when the impurities are diffused therein. If one starts from a regular stack of thin layers (^, 100 A) of different compositions (for example: GaAs / AlAs / GaAs / AlAs / ...), one can obtain, after diffusion, a mixture of formula average Ga _or Al ₀ As. Starting from an ordered material or super-alloy, we obtain a solid solution or disordered alloy.

The advantage of this process implemented for the production of the half-shells as shown in FIGS. 1B and 2 is that, as regards the lasers according to the invention, the disordered alloy has advantageous properties compared to those of the superalloy. Note however that, to obtain the interdiffusion effect whose scope is limited, it is necessary that the thickness of each of the thin layers which constitute the active region e is small, typically less than 100 A, while it this is not the case for the mirrors of the optical cavity. These diffusion and inter-diffusion are for example obtained by heating by means of electromagnetic radiation 26 like that given by a laser or the like, radiation of this type can easily plunge into the groove 22 in an oblique direction relative to the surface. lateral cylindrical 23.

Nevertheless, the widening of the thickness of the half-shells during the diffusion according to the first mode of implementation of the method can be limited by subjecting the layer of active laser medium to lateral stresses oriented towards the center of the active medium , that is to say substantially towards the optical axis of the resonant cavity and in such a way that the resultant of the pressure forces is substantially zero or parallel to this optical axis.

To avoid a significant widening of the thickness of the two half-shells, it may also be advantageous to produce the active laser medium in successive partial layers superposed. In this case, the implantation of the first and second zones of impurities is advantageously obtained by the successive localized implantation of impurities provided by beams of ions focused on each partial layer after it has been produced. FIG. 3 represents, as an advantageous application of the implementation of the method, a plurality of semiconductor lasers of the "laser diode" type 31, 32, 33 ... juxtaposed one beside the other in such a way that the mirrors of their resonant cavities are all substantially located in two same planes 34, 35 and that their optical axes 36, 37, 38, • .. are parallel. In this case, the active medium 40 is produced on a substrate 41 which serves as a support for all of the diodes. The laser diodes illustrated in FIG. 3 were produced according to the first embodiment of the method described above, that is to say from the implantation of two zones 41, 42 of impurities of polarities opposite on one of the two faces of the active medium perpendicular to all of the optical axes of the different cavities of the laser diodes, more particularly on the face opposite to that which is close to or in contact with the support substrate. Of course, such a set of laser diodes juxtaposed one beside the other could also be obtained by the second mode. implementing the method described with reference to FIG. 2.

It is apparent that the structure of a semiconductor laser obtained by the method according to the invention, of which two advantageous examples of implementation have been described above, comprises an active medium perfectly delimited laterally, which makes it possible to '' get a current

"With a relatively low threshold for triggering the laser effect, this delimitation being perfectly obtained by the spatial control of the diffusion and of the inter-diffusion, both in a vertical and transverse direction, that is to say say parallel or perpendicular to the optical axis of the cavity. This configuration also makes it possible to limit the transverse dimensions of each semiconductor laser, as developed above, and therefore to obtain a density of lasers on the same substrate. , for example of the "diode" type, greater than that which can be obtained with the methods according to the prior art.

Claims

DREAM ND ICATIONS

1. Method for producing semiconductor lasers, characterized in that it consists in: producing a layer of an active semiconductor laser medium (1,20,40) having transverse dimensions greater than those of the active medium of the laser to be obtained, to produce a resonant optical cavity (2) associated with said layer, said cavity being produced so that its optical axis (3) is substantially perpendicular to the plane in which said transverse dimensions are defined, to be placed, on at monk part (5,23) of the surface of said layer, the first (6,24,41) and second (7,25,42) zones of impurity materials of opposite polarities, to be diffused in a part of said active medium at least a portion of the two said impurity materials, in order to produce in said layer substantially a cylinder (8) with an axis substantially parallel to said axis of said optical cavity and formed of two half-shells (9,10-24, 25) substantially s semi-cylindrical respectively of impurities diffused with opposite polarities, and to connect two conductors (12) of electrical energy, respectively to the two half-shells.

2. Method according to claim 1, characterized in that said active medium layer (1,20,40) consists of a stack of thin layers chosen from the following two families: GaAs / GaAlAβ and GalnAsP / InP.

3. Method according to claim 2, characterized in that the thickness of said thin layers is at most equal to 100 A.

4. Method according to one of claims 1 to 3, characterized in that the first (6,24) and second (7,25) zones lesdiεes contain respectively P-type and N-type impurities.

5. Method according to one of claims 1 to 4, characterized in that said first and second zones respectively contain Zinc and Silicon.

6. Method according to one of claims 1 to 5, characterized in that the diffusion of impurities is obtained by heat treatment.

7. Method according to claim 6, characterized in that the heat treatment is obtained by application of a beam (26) of electromagnetic radiation.

8. Method according to one of claims 1 to 7, characterized in that said first and second zones (6,7) are arranged on one face (5) of said layer of active medium (1) perpendicular to said axis (3 ) of the resonant optical cavity (2).

9. Method according to one of claims 1 to 7, characterized in that said active medium is shaped substantially in the form of a column (21) delimited by a cylindrical lateral surface (23) and whose axis is substantially parallel to l axis of said resonant optical cavity, said first and second zones (24,25) being arranged on said cylindrical lateral surface (23).

10. Method according to one of claims 1 to 7, characterized in that, during the diffusion operation, said active medium is subjected to stresses substantially oriented towards the axis of said resonant optical cavity.

11. Method according to claim 1, characterized in that said active medium is produced by a stack of successive partial layers, said first and second zones being arranged by means of beams of impurity ions focused on each partial layer after 'it has been realized.

12. Method according to one of the preceding claims, characterized in that the operation consisting in placing, on at least part of the surface of said layer, first and second zones of impurity materials of opposite polarities, is chosen from the following two operations, deposit and implantation.

13. Semiconductor laser obtained by the method according to at least one of claims 1 to 12, characterized in that it comprises a resonant optical cavity (2), an active laser medium (1,20,40) associated with said optical cavity, said active medium comprising two half-shells (9,10) of impurities of opposite polarities diffused on, respectively, at least two portions of its lateral surface (5,23) , and two conductors (12) for supplying electrical energy respectively connected to the two said half-shells.