1.  Field of the Invention
The field of the invention is that of optical emission devices comprising an integrated component comprising at least one laser emission section and a section for modulating the optical power emitted by the laser.  These systems are also called ILMs, the acronym standing for Integrated Laser Modulator or EMLs, the acronym standing for Electro-absorption Modulated Laser.  The modulation section is generally also called an EAM, the acronym standing for Electro Absorption Modulator. 
These devices are used mainly in the field of high-throughput digital telecommunications.  The throughputs are typically from a few gigabits to several tens of gigabits per second. 
2.  Description of the Prior Art
It is possible to modulate a laser beam in three different ways.  A first method consists in directly modulating the source laser by controlling its feed current.  However, this technique does not make it possible to attain the performance necessary for high throughput.  It is also possible to use an external modulator, physically separate from the source laser and whose performance is not limited by that of the laser.  However, this method poses significant integration and positioning problems.  Finally, it is possible to produce an integrated component comprising, on the same substrate, a laser emission section and a section for modulating the optical power emitted by the laser.  The best compromise between the desired performance and the technological production and integration problems is thus obtained. 
The latter type of device, the ILM, is known and is described, for example, in French patent FR2 675 634.  By way of example, an ILM device is represented diagrammatically in FIG.  1.  It essentially comprises a laser section 1 and a modulation section 2.  As indicated in FIG.  1, the laser is driven by a current I and the modulation section 2 is an electro-absorbant section, on/off controlled by a voltage signal V. At the output of the ILM device, a high-frequency modulated optical signal is emitted (straight barred arrow of FIG.  1). 
Generally, the optical emission structure is a semi-conductor laser with a buried stripe also called a BRS structure, the acronym standing for Buried Ridge Stripe.  A diagram of such a structure is represented in FIG.  2.  It comprises essentially:                A first substrate 10 produced from n-doped semi-conductor material.  The first substrate is generally produced from Indium Phosphide (InP);        An active part 11 formed by a stripe of rectangular cross section, the lower face of this active part lying on the first substrate 10.  The active part has an optical index greater than that of the layers which surround it.  It is of small section, of the order of a micron or a few microns, and generally consists of a set of layers forming quantum wells and barriers.  The layers are conventionally produced from GaInAsP or from AlGaInAs;        A second substrate 12 produced from p-doped semi-conductor material.  This second substrate is also produced from InP and it completely covers the lateral faces and the upper face of the active part 10;        A lower electrode 13 disposed under the first substrate 10 and an upper electrode 14 disposed on the second substrate 12.  The electrodes convey the current necessary for the operation of the laser.         
This configuration makes it possible to ensure, at one and the same time:                Confinement of the carriers injected into the stripe if the difference in forbidden bandwidth between the material of the first substrate and that of the second substrate is sufficient;        Bidirectional guidance of light if the difference in optical index between the material of the first substrate and that of the second substrate is also sufficient.         
The benefit of this geometrical configuration is to obtain lasers with very low threshold current and with very high switching speed. 
Since the appearance of this type of structure in the 1980s, technological developments have made it possible to improve the performance of this type of laser.  Structures comprising semi-insulating layers are generally used.  Descriptions of this type of structure will be found in French patent FR 91 04636.  A diagram of such a structure is represented in FIG.  3.  It comprises essentially:                A first substrate 10 produced from n-doped semi-conductor material, generally produced from InP;        An active part 11 formed by a stripe of rectangular cross section, the lower face of this active part lying on the first substrate 10;        a lateral confinement layer 15 surrounding the lateral faces of the stripe.  This layer 15 is produced from semi-insulating semi-conductor material, generally produced from Fe-doped InP;        a vertical confinement layer 16 covering the upper face of the active part 10, the said layer 16 produced from p-doped semi-conductor material.  The doping is generally zinc.         A lower electrode 13 disposed under the first substrate and an upper electrode 14 disposed on the vertical confinement layer 16 and on the lateral confinement layer 15.  These electrodes convey the current necessary for the operation of the laser.         
This type of laser is also called SI BH, the acronym standing for Semi-Insulating Burried Heterostructure. 
Of course, in ILM devices, the laser section and the modulator section have the same buried structure.  Devices having very good performance in terms of thermal dissipation, optical losses, stability of the optical modes and reliability are thus obtained.  Moreover, the integration of the two emission and modulation functions in a single component makes it possible to substantially reduce costs. 
However, a laser of SI BH type exhibits significant lateral leakage current due to fast decay of the resistivity of the lateral confinement layer when a positive voltage is applied to the structure.  This effect is widely known and described.  It results from the interdiffusion of the p dopants of the vertical confinement layer and of the dopants of the semi-insulating lateral confinement layer during the step of producing the vertical confinement layer.  This leakage current appreciably degrades the performance of the laser. 
There exist various techniques for attempting to reduce this problem.  We shall cite:                The reducing of the surface area of the active structure during the step of lateral etching of the structure or the increasing of the thickness of the vertical confinement layer so as to retain a sufficiently resistive surface area.  These solutions do not, however, completely eliminate the leakage current;        The use of blocking layers, preventing the diffusion of the p dopants.  Unfortunately, in the case of an ILM device, this technology increases the stray capacitance of the modulation section which becomes excessive for use at high-throughput;        The use of other types of dopants such as Ruthenium which exhibit the drawbacks of being difficult to implement in epitaxial growth equipment and whose effectiveness has not been proved under laser conditions of use.         The use of other types of structures such as structures of PN—BH type, the acronym signifying P-type N-type Buried Heterostructure, comprising a truncated vertical confinement layer.  However, these structures which have good performance call upon a complex fabrication process comprising additional steps of epitaxial growth and of cropping of the vertical confinement layer, which are difficult to control fully.  This technology also leads to structures whose excessive stray capacitance does not permit high-throughput use. 