Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/015—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on semiconductor elements having potential barriers, e.g. having a PN or PIN junction
  • G02F1/025—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on semiconductor elements having potential barriers, e.g. having a PN or PIN junction in an optical waveguide structure
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F2202/00—Materials and properties
  • G02F2202/06—Materials and properties dopant

Definitions

• the present invention relates to the fields of communication technologies and electronics.
• the present invention relates more particularly to a light modulator with optical telecommunications wavelengths (greater than 1.2 ⁇ m) made entirely of silicon.
• the modulator according to the present invention can be integrated into an optical waveguide, the section of which can be submicron, and can operate at high frequencies (of the order of several tens of GHz).
• the technology used is that of microphotonics on silicon, which is compatible with microelectronic technology.
• the modulator according to the invention consists of a silicon PIN diode, into which are introduced, at the heart of the intrinsic region (I), doping planes providing either electrons (N doping), or holes (P doping ), located unevenly in the structure.
• the assembly is placed in a submicron optical waveguide in silicon on insulator, the doping planes being centered on the mode of the guide.
• the free carriers are removed from the structure by polarization of the diode in reverse, which leads to a variation in the refractive index of the material, therefore to a phase variation of a light wave passing through it.
• This active area placed in an interferometric structure (like conventional devices of the Fabry-Perot cavity type or Mach-Zehnder interferometer) produced in the microguides of optical waves makes it possible to obtain a modulation of light intensity at frequencies several dozen Giga Hertz.
• the substrate is etched and the dopant is diffused therefrom from a side face of the etched region.
• the dopant profile is established in the horizontal direction, which makes it possible to control it. It is thus possible to obtain a uniform doping profile vertically ensuring a uniform current density in the vertical direction. If an anisotropic wet etching is carried out after the initial etching, a profile is obtained which concentrates the density of the current at a selected height of the substrate.
• an electro-optical device comprising a semiconductor layer in which a waveguide has been formed, a modulator formed through the guide waves comprising a p-doped region on one side and an n-doped region on the other side of the waveguide, at least one of the doped regions extending from the base of a recess formed in the semiconductor layer .
• the doped regions can extend deeper into the semiconductor layer and additionally prevent the charge carriers from leaking, without having to increase the diffusion distance of the dopant and subject the device to an additional thermal load.
• the doped region can extend to the insulating layer.
• the two p and n doped regions extend from the base of a recess, but this may prove unnecessary in some designs. Insulating layers can be used to ensure that the dopant extends only from the base of the recess, resulting in a more clearly defined doped region.
• the (or each) recess may have non-vertical sides, such as those formed by engravings of v-grooves, a combination of vertical side walls at the base of the recess and a non-vertical side wall at the level of the opening that can be used.
• phase modulator for waveguide in semiconductor.
• the invention which is the subject of this application relates to an optical phase modulator comprising an edge semiconductor waveguide (rib aveguide) having P and N doped zones forming a PN junction along the edge with electrodes serving to apply reverse bias to said junction in order to extend a desertion zone of carriers and, thus, to modify the refractive index.
• the PN junction is offset from the central axis of the edge, but once reverse polarization is applied, the carrier desertion zone extends over a central axis of the waveguide.
• the structure proposed in this document is therefore based on a variation in refractive index by depletion of carriers in an edge guide.
• the doped areas are narrow and easily centered on the guided optical mode.
• the solution according to the present invention makes it possible to control the number of carriers initially present and to evacuate them with applied voltages of only a few volts.
• Light modulators operating at frequencies of a few tens of GHz are currently made either of lithium niobate or with III-IV semiconductors on an InP substrate for the wavelengths of optical telecommunications (1.3 ⁇ m and around. 1.55 ⁇ m), therefore a priori not compatible with integration on a silicon substrate.
• the speed of silicon-based light modulators has been mostly limited by the physical process implemented at frequencies of a few hundred MHz. In fact, most of them work by injecting free carriers and not by desertion. The response time is then generally limited by the lifetime of the carriers and by the thermal effects linked to the passage of current.
• Recent scientific publications relating to all silicon modulators integrated in waveguides functioning by injection give response times which do not go below the nanosecond.
• the invention relates, in its most general sense, to an optoelectronic component for controlling an optical signal comprising an edge wave microguide in an SOI type substrate and an active area characterized in that said active area is formed of a plurality of very thin silicon layers either doped with N + type or doped with P + type ( ⁇ -doping), said zone being placed between an N + doped zone and a P + doped zone forming a PIN diode, connected with two electrodes arranged on either side of said active area making it possible to polarize the structure.
• said silicon layers are either all N + type doped, or all P + type doped.
• the doping planes are parallel to the substrate.
• the upper electrode is offset with respect to the edge of the waveguide which laterally delimits the active area.
• the electrical contact is ensured for example by the deposition of a layer of doped silicon, constituting the N + or P + zone of the diode, which is monocrystalline on the edge of the guide and polycrystalline on the insulator on both sides.
• the second electrode comes into contact with the lower doped silicon layer extending laterally beyond the active area.
• at least one of the two electrodes comes into contact with a layer of doped silicon (N + or P + ) of the PIN diode, via a layer of silicide.
• the doping planes are perpendicular to the substrate and the N + and P + zones of the diode are on each side of the guide, parallel to the edge.
• the invention also relates to the application of such an optoelectronic component for the production of an optoelectronic switch as well as for the production of an optoelectronic modulator
• the invention also relates to a method of manufacturing an optoelectronic component according to the invention characterized in that epitaxial growth is carried out on very thin layers of N + or P + doped silicon.
• doping assisted by laser is carried out to produce the N + or P + layers in the silicon of the active area.
• FIG. 1 illustrates an embodiment in vertical structure of the modulator according to the invention
• Figure 2 illustrates an embodiment in horizontal structure of the modulator according to the invention
• Figure 3 shows a use of the modulator according to the invention in an optical signal distribution in the context of a microelectronic integrated circuit.
• the transmission of information by optical means requires the coding of the light intensity, which can be done either by direct modulation of the source, or preferably for high frequencies using a light modulator.
• Optical telecommunications have used it for a long time, at frequencies of up to forty GHz, made either of lithium niobate, or based on InP and ternary or quaternary compounds (GalnAsP for example).
• SOI silicon on insulator
• the present invention relates to a new type of optoelectronic modulator entirely made of silicon and therefore integrable with CMOS integrated circuit technology and in SOI optical wave microguides.
• the modulators in the silicon sector are based on the variation in the number of free carriers (electrons or (and) holes) in the structure, which induces a variation in the refractive index of the material and thus leads to a phase variation of the light passing through it.
• This phase variation is then transformed into intensity variation by placing the active zone in an interferometric structure of the Fabry-Perot cavity or Mach-Zehnder interferometer type, the dimensions of which can be reduced by the use of photonic crystals.
• a accumulation layer leads to better performance but this layer is also very localized and therefore has a low overlap with the optical mode of a waveguide.
• the free carriers were holes, located near the center of the guide in SiGe / Si quantum wells, separated by silicon barriers comprising P-type doping planes providing these holes to balance.
• This structure is placed in a reverse polarized PIN diode to drive out the holes in the active area (operation by desertion).
• the initial trapping of the holes in the wells increases the response time which is of the order of 50 to 100 ps.
• the technology for components including quantum wells must not include steps at too high a temperature, which is a constraint for integration with microelectronics.
• the proposed modulator is entirely made of silicon and retains from the previous structure only the doping planes. This type of original structure also works by desertion, but either with holes or with electrons, depending on the type of doping used in the planes.
• the introduction of the doping planes is sufficient to locate the free carriers in their immediate vicinity by internal field effect, and the carriers no longer have a potential barrier to cross to be scanned from the active area. The response time is therefore much shorter, of the order of a few picoseconds.
• the expected refractive index variations are of the order of some 10 ⁇ 4 for applied voltages of a few volts, a little higher than with the SiGe / Si quantum wells. They are accompanied by a variation in absorption which can also be used for modulation. III-V semiconductor materials are fragile, expensive and the substrates are limited in size. These devices cannot be easily integrated monolithically on silicon substrates.
• a silicon modulator benefits from microelectronic technology, with which it can be integrated, with low cost and possible mass production on substrates from 200 to 300 mm in diameter.
• the modulators of the prior art in the silicon sector either any silicon operating by injection of carriers or by displacement of a plasma, or with SiGe / Si quantum wells in desertion, have operating frequencies too low for the intended applications .
• the present invention provides an all-silicon modulator, compatible with CMOS technology, which can be integrated into a silicon on insulator (SOI) wave microguide and with intrinsic response times of a few picoseconds.
• SOI silicon
• the substrate is of the SOI (Silicon On Insulator) type, the buried silica layer making it possible to confine the electromagnetic field in the surface silicon film which provides the guidance.
• SOI Silicon On Insulator
• the active structure is therefore inserted into a bowl etched in the silicon film of the SOI, thus leaving a minimum thickness of silicon for the resumption of epitaxy (typically ⁇ 30 nm).
• the entire structure is then produced by selective epitaxial growth.
• Different types of epitaxy can be used: it is thus possible to proceed by UHV-CVD (Ultra High Vacuum Chemical Vapor Deposition) or by RP-CVD (Reduced Pressure Chemical Vapor Deposition).
• the deposition is done only on silicon and not on silica, which may have served as a mask for etching the bowl. It is also possible to use chemical techniques leading to a non-selective deposit: the silicon is deposited everywhere and the structure is then leveled by chemical mechanical polishing (CMP) which leaves the epitaxial layers only in the bowl.
• CMP chemical mechanical polishing
• the P + layer is preferably deposited first, at the bottom of the bowl, but the structure can also be reversed with the N + layer at the bottom.
• Unintentionally doped silicon (crystalline by epitaxy) is then grown in which doping planes are inserted which are thin layers of silicon (typically 5 to 10 nm thick) doped either of type P or of type N , with a concentration of carriers of the order of some 10 18 cm -3 .
• the contact layers N + and P + must be thin enough and relatively little doped compared to the usual contact layers to limit the losses of the optical mode by absorption on the free carriers, as for the structure with the SiGe wells, as is mentioned in the publication Design of a SiGe-Si Quantum-Well Optical Modulator (Delphine Marris et al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 9, No. 2, May-June 2003).
• P doping is obtained with Boron in general, N doping with Arsenic or Phosphorus.
• the next step consists in etching a trench along the edge of the optical guide to make the electrical contact on the P + layer (N + for the inverted structure), to isolate the sides by a deposit silica. Windows are opened in the silica for the deposition of metal from the two electrodes.
• the electrical contact can advantageously be improved by the formation of a silicide, for example a nickel or platinum silicide.
• the metal is then deposited, then etched to eliminate it outside the zones constituting the electrodes.
• the doping planes can be produced by means of the laser doping technique.
• This technique makes it possible to incorporate the dopants over thicknesses of a few hundred nanometers and over a width which may be less than 100 nm.
• This technique is based on the localized fusion of Silicon in the presence of a gas such as di-borane (for example) for boron (P doping), arsine or phosphine for As or P (N doping).
• the contact zones P + and N + of the diode can be produced using the same technique.
• the following steps are to be implemented: • mask for P-type doping; • mask for N-type doping; • silica deposit; • openings for making contacts (formation of silicide and metallization).
• the structure could be produced by lateral epitaxial growth from a vertical face defined in the crystal direction (100)
• One of the advantages of the horizontal structure consists in the spacing of the contact zones relative to the optical mode , which causes a clear reduction in the optical propagation losses, the contacts being able to be more doped in order to decrease their resistance, which intervenes in the response time by time constants of RC type.
• the invention is described in the foregoing by way of example. It is understood that a person skilled in the art is able to produce different variants of the invention without going beyond the scope of the patent.

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• Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
• Optical Integrated Circuits (AREA)
• Semiconductor Lasers (AREA)

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• 2004
  • 2004-03-29 FR FR0450608A patent/FR2868171B1/fr not_active Expired - Fee Related
• 2005
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