Abstract:
The invention concerns a semiconductor opto-electronic component comprising at least two optically active structures ( 20, 30 ), at least one of which consists of a detector ( 30 ), characterized in that the detector or detectors ( 30 ) comprise a first active portion ( 33 ) able to detect a signal at a given wavelength and a second inactive portion ( 34 ) only slightly sensitive to the signal to be detected and exposed to the non-guided stray light conveyed in the component.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor opto-electronic component comprising at least two optically active sections each having a waveguide buried in a cladding layer. 
     For this type of multi-section opto-electronic component, it is important to have high electrical insulation between each section in order to prevent interactions between these during the functioning of the component. The invention relates more particularly to any opto-electronic component comprising at least one receiving element integrated with another element. 
     The objective of the present invention is to permit simultaneous functioning of the optically active structures without any interaction between the transmitter and receiver and/or between the different receivers detecting signals at different wavelengths. 
     FIG. 1 depicts a diagram in longitudinal section of a conventional in-line transceiver component, denoted 1D-TRD (“In-line Transmitter Receiver Device” in British and American literature), obtained by the monolithic integration of a laser  20  and detector  30  on the same substrate  10 . The laser  20  sends a signal towards an optical fibre, for example, whilst the detector  30  receives a signal coming from this same optical fibre. The emission wavelength of the laser  20  is less than the reception wavelength of the detector  30 . For example, the emission wavelength is equal to 1.3 μm whilst the reception wavelength is 1.55 μm. In this case, given that the emission wavelength is less than the reception wavelength, and that the laser  20  is situated close to the detector  30 , the laser can cause optical interference on the detector. This is because the laser also emits, in the direction of the detector, light at 1.3 μm which dazzles the latter. 
     To prevent this dazzling of the detector, the component has a third section, disposed between the laser  20  and detector  30 , forming an optical isolator  40 . This optical isolator absorbs the light emitted at 1.3 μm in the direction of the detector, so that the latter can detect the 1.55 μm optical signal coming from the optical fibre without being disturbed by the laser. 
     The substrate  10 , or bottom layer, can for example be n-doped InP. The waveguides, respectively  31  of the detector  30  and  21  of the laser  20  and of the optical isolator  40 , are etched in the form of ribbons and buried in a highly doped cladding layer  11 . The waveguides are said to be of the BRS (“Buried Ridge Structure” in British and American literature) type. The cladding material  11  is p+ doped when the substrate is n doped. Naturally, this type of ribbon is only one example. Other types of ribbon can be suitable. The n and p dopings of the different layers can also be reversed. 
     The composition and dimensions of the waveguides are of little importance. In the example in FIG. 1, the waveguide  31  of the detector  30  is for example produced from ternary material, whilst the waveguide  21  of the laser  20  and of the optical isolator  40  is produced in a structure with quantal wells. 
     In addition, metallic electrodes  22 ,  32 ,  42  and  14  are formed on the different sections and on the bottom of the component, so as to enable it to function. 
     An absorbent layer  12  doped with the same type of carrier as the cladding layer  11  is situated between the conductive layer  11  and the metallic electrodes  22 ,  32 ,  42  so as to afford good electrical contact and in order to collect the carriers which make it possible to detect the signal on the electrode  32  of the detector  30 . This absorbent layer  12  can consist of a ternary material, for example. 
     Because of the presence of conductive layers  11 , the component also has electrical isolation areas  50 , or resistivity areas, between the different sections  20 ,  30 ,  40  in order to prevent any electrical disturbance of one section vis-à-vis another during the functioning of the component. 
     This type of in-line transceiver, having a central part  40  for absorbing all the light flux sent at 1.3 μm to the detector, functions very well for all the light which is guided in the waveguide ribbons  21 . 
     However, not all the light emitted is entirely guided. This is because there exists also spontaneous light which is emitted throughout the volume of the component. In addition, some of the stimulated light can also be diffracted in the component because of the presence of defects in the waveguide  21 . 
     The curves in FIG. 2 show the penalties noted on the sensitivity of the detector, in dB, for different operating modes. Curve A represents a reception reference when the laser is off, curve B represents a reception reference when the laser is on continuously and curve C represents the simultaneous functioning of the laser and detector. A penalty of 4.5 dB is found between curve B and curve C, when the laser and detector are modulated simultaneously. This penalty is also increased by increasing the power of the laser. 
     This penalty is principally optical. It is caused by the non-guided light emitted at 1.3 μm, in all directions, which interferes with the detector at 1.55 μm. 
     This optical disturbance is depicted highly diagrammatically in FIG. 1 by the wave  60 . The metallic electrode  14 , disposed at the substrate/air interface, can play the role of an optical reflector in the substrate  10 . Some of the spontaneous light emitted in the volume of the component can therefore be reflected by the electrode  14  and be coupled with the waveguide  31  of the detector  30  from below. Likewise, some of the stray light  60  can also be reflected on the electrodes  42  and  32  since the absorbent layer  12  does not absorb all this stray light  60 . 
     Naturally, the disturbance of the detector  30  by the non-guided light  60  is in reality much more complex than a simple reflection. This is because some of the stray light can also undergo multiple reflections in the bottom layer  10  and top layer  11 . Another part of this stray light can also dazzle the detector in glancing incidence, for example. 
     Techniques have already been envisaged for combating the penalty of 4.5 dB found in the example given in FIG. 2, which occurs during the simultaneous functioning of the laser and detector. The techniques envisaged are essentially electronic techniques. 
     These techniques consist, for example, of taking part of the laser modulation signal, and then subtracting it in reception. The use of these electronic processing techniques has demonstrated a reduction of 2 dB in the penalty. However, they require the development, manufacture and adjustment of specific electronics for this type of particular transceiver component, so that they considerably increase the cost of this component. However, it is being sought to manufacture this type of component on a large scale and therefore to reduce its cost price to the maximum possible extent. Consequently these electronic processing techniques can not be used for the mass production of such a component. 
     In addition, an in-line transceiver is intended to be installed at subscribers and must be able to function between 0 and 70° C. without any temperature regulation. However, the reliability of these electronic techniques has not been demonstrated over this range of temperatures and it is not proved that they can automatically adjust themselves according to the temperature. 
     SUMMARY OF THE INVENTION 
     One aim of the present invention therefore consists of producing an inexpensive opto-electronic component including a detector and a parasitic element for this detector, such as a laser or any other element, the operating wavelength of the parasitic element being less than the reception wavelength of the detector, and in which the interference of 4.5 dB on the detector by the parasitic element (according to the example in FIG.  2 ), which occurs during their simultaneous operation, is considerably reduced. 
     To this end, the invention proposes to reduce the active proportion of the detector able to detect a signal, whilst the stray light illuminates the entire detector. 
     The invention concerns more particularly a method of manufacturing a semiconductor opto-electronic component comprising at least two optically active structures, at least one of which consists of a detector, characterised in that it comprises a step consisting of limiting the length of the active portion of the detector or detectors able to detect a signal at a given wavelength, the non-guided stray light conveyed in the component being distributed over the entire detector. 
     According to a first embodiment, the limitation of the active portion of the detector or detectors is achieved by implanting protons on the remaining portion of the detector. 
     According to a second embodiment, the limitation of the active portion of the detector or detectors is achieved by locating the detector contact on this portion. 
     According to a third embodiment, the limitation of the active portion of the detector or detectors is achieved by etching the remaining portion, and growing a passive layer on the latter. 
     According to a fourth embodiment, the detector is cleaved so as to limit its active portion. 
     The present invention also concerns the component obtained by such a method and more particularly a semiconductor opto-electronic component comprising at least two optically active structures, at least one of which consists of a detector, the said active structures being separated by an intermediate section, characterised in that the detector or detectors comprise a first active portion able to detect a signal at a given wavelength and a second inactive portion weakly sensitive to the signal to be detected and exposed to the non-guided stray light conveyed in the component. 
     According to a first variant, the second portion of the detector or detectors is implanted with protons. 
     According to a second variant, the detector contact is located on the first portion. 
     According to a third variant, each portion has different epitaxial layers, the active layer of the second portion having been removed by etching and replaced by a passive layer. 
     The invention also concerns a semiconductor opto-electronic component comprising at least two optically active structures, at least one of which consists of a detector, the said active structures being separated by an intermediate section, characterised in that the detector is cleaved so as to limit it to the length of the active portion able to detect a signal at a given wavelength. 
     According to one characteristic, the reception wavelength of the detector is greater than the operating wavelength or wavelengths of the other parasitic structure or structures. 
     According to one application, the component according to the invention constitutes an in-line transceiver. 
     According to another application, the component according to the invention constitutes an array of receivers. 
     The method according to the invention makes it possible to obtain a multi-section opto-electronic component in which the interference on the detector caused by the other elements is considerably reduced. 
     In addition, the embodiments described do not require the development of new manufacturing methods, and can consequently be implemented easily and rapidly. 
     In addition, the limitation of the active part of the detector to its front part limits the electrical field on the cleaved rear facet of the component, which has the direct consequence of limiting the risks of damage to the component due to defects on the facet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other particularities and advantages of the invention will emerge from a reading of the description given by way of illustrative example and made with reference to the accompanying figures, in which: 
     FIG. 1, already described, is a diagram of a conventional in-line transceiver, 
     FIG. 2, already described, illustrates curves for showing the operating penalties during a simultaneous modulation of the transmitter and receiver of the component of FIG. 1, 
     FIG. 3 is a diagram of an optical component according to a first embodiment of the invention, 
     FIG. 4 is a diagram of an optical component according to a second embodiment of the invention, 
     FIGS. 5 a  to  5   c  illustrate schematically the steps of producing an optical component according to a third embodiment of the invention, 
     FIG. 6 a  is a diagram of an optical component according to a fourth embodiment of the invention, 
     FIG. 6 b  depicts a plan view of FIG. 6 a.   
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 shows diagrammatically a first embodiment of a component according to the invention. It shows diagrammatically more particularly an in-line transceiver. However, the invention is not solely limited to in-line transceivers. It can apply to any integrated opto-electronic component with which cross-talk exists, that is to say to any component comprising a parasitic element and an element suitable for detecting for which the wavelengths emitted and received are compatible. 
     For example, in the case of an array of detectors, the stray light comes from the optical fibre and is conveyed throughout the component. 
     In the example illustrated, a transceiver is considered. The same references are used to designate the same elements as in the conventional transceiver shown diagrammatically in FIG.  1 . The laser  20  emits at a wavelength less than the reception wavelength of the detector  30 . The emission wavelength is for example 1.3 μm whilst the reception wavelength is 1.55 μm. 
     The spontaneous light emitted by the laser  20  and not guided by the waveguide  21  is emitted throughout the volume of the component. In addition, some of the stimulated light is diffracted in the component because of the presence of defects in the waveguide  21 . All these stray light waves emitted at 1.3 μm by the laser  20 , in all directions, disturb and dazzle the detector  30 , which can no longer correctly detect the wavelength at 1.55 μm. 
     These disturbances are shown diagrammatically simply by the wave  60  in FIG.  3 . They come from above and below the waveguides  21 ,  31  of the transceiver component. These disturbances give rise to a penalty of 4.5 dB (according to the example cited in FIG. 2) during simultaneous functioning of the laser  30  and detector  20 . 
     In order to minimise the impact of this non-guided stray light, the invention proposes to reduce the active portion  33  of the detector  30 . 
     This is because it has been found that only the first micrometers of the detector  30  absorb approximately 90% of the signal to be detected. Thus, for example, only 30 μm of a 80 μm long detector are enough for good detection of a signal. 
     However, the non-guided stray light is distributed over the entire waveguide  31  of the detector  30 . Thus the rear portion  34  of the detector  30  can collect more stray light  60  than signal. 
     This is why the active portion  33  able to detect a signal is reduced, the unguided stray light being distributed over the whole of the detector  30 . 
     Advantageously, the length of the active portion of the detector is fixed so that the said portion is able to detect, for example, approximately 90% of the signal. 
     The stray light coupled to the waveguide  31  in the active portion  33  is buried in the noise of the detector  30 , the signal being sufficiently strong to be correctly detected, whilst the stray light coupled to the waveguide  31  in the remaining portion  34  is more intense than the signal that has already almost completely been detected. Thus the remaining portion  34  of the detector  30  detects more stray light  60  than the signal and impairs the quality of the detector  30 . 
     Several means can be envisaged for limiting the active portion  33  of the detector  30 . 
     A first method is illustrated in FIG.  3  and consists of implanting protons in the rear portion  34  of the detector  30 . 
     This implantation of H +  protons, carried out in accordance with conventional techniques, has the effect of making the cladding layer  11  of the rear portion  34  insulating, that is to say incapable of collecting the carriers to the absorbent layer  12  in order to be gathered on the metallic electrode  32 . 
     The implantation of H +  protons can also make it possible to reduce the width of the active portion  33  for given applications. 
     A second method is illustrated in FIG.  4  and consists of locating the absorbent layer  12  and the metallic electrode  32  solely on the front active portion  33  of the detector  30 . 
     For this purpose, the absorbent layer  12  and part of the conductive layer  11  are etched on the rear portion  34  of the detector  30 . Thus only the front portion  33  is able to collect the carriers. The metallic electrode  32  will then be produced solely on the active portion  33  of the detector  30 . 
     A third method is illustrated in FIGS. 5 a  to  5   c  and consists of making an etching of the active layer  31  of the detector  30  so as to obtain an active waveguide  31  at the front  33  of the detector  30  and a passive area  35  at the rear  34  of the detector  30 . 
     By way of example, the diagrams in FIGS. 5 a  to  5   c  depict views in transverse section in the direction of the length of the optical component according to the invention during different steps of this etching method. 
     This method can consist, for example, firstly (FIG. 5 a ), of growing, on a substrate  10 , a first layer of quaternary material  31  able to fulfil the role of active waveguide of the detector  30 . 
     A local etching (FIG. 5 b ) of the waveguide  31  is then carried out, in accordance with a conventional etching method, in order to limit the active guide  31  to an active area  33 . Another epitaxy step then makes it possible to grow the waveguide on the other element  21 , as well as a passive area  35  on the rear  34  of the detector  30 . The structure of the passive area  35  is different from that of the active guide  31 . 
     For example, the passive area  35  can be a passive guide, or be of the same nature as the cladding layer  11 , or of any other composition. 
     These layers  21 ,  31  and  35  are then buried in a cladding layer made of InP  11 , which constitutes the previously described conductive layer, and an absorbent layer  12 . An electrode  32  is produced on the detector  30 , without necessarily being limited to the active portion  33  since no carrier will come from the rear portion  34  of the detector  30 . 
     A fourth method is illustrated in FIGS. 6 a  and  6   b  and consists of cleaving the detector  30  in order to limit it to the active portion  33 . 
     This FIG. 6 b  is a plan view of the optical component of FIG. 6 a.    
     The detector  30  being cleaved and reduced to the active portion  33 , the latter is too narrow to receive a metallic electrode  32 , whose dimensions are determined by the production of a hard-wired connection. The latter is generally achieved by thermocompression of a gold wire on the metallic electrode  32 , this welding being difficult to carry out on an electrode with a size of 30 μm. 
     It is consequently necessary to effect a transfer of metallisation for the metallic electrode  32 , that is to say to produce this electrode  32  on the central section  40  and to establish the contact with the active portion  33  of the detector  30  by means of a conductive bridge  36 .