GB2436397A

GB2436397A - Monolithically integrated optoelectronic components

Info

Publication number: GB2436397A
Application number: GB0605751A
Authority: GB
Inventors: Kelvin Prosyk; Joan Haysom
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2006-03-23
Filing date: 2006-03-23
Publication date: 2007-09-26
Also published as: GB0605751D0; WO2007107782A1

Abstract

A monolithically integrated optoelectronic component comprises a substrate 48 and at least three optoelectronic sub-devices 41, 42, 43 integrated within a semiconductor layer on the substrate. Each of the sub-devices has an upper conductor 44 and a lower conductor 45, 46 separated by a common active region 49. Two sub-devices 41, 43 are electrically coupled to a conducting semiconductor sub-layer 47 within the semiconductor layer by their upper or lower conductors. Electrical isolation zones 54, 55, 57 are provided for electrically isolating a further one of the sub-devices 42 from the semiconductor sub-layer and the coupled conductors of the sub-devices, and for electrically isolating the other conductors of the sub-devices 41, 43 from one another. The combined use of such electrical isolation zones enables more complex electrical connection schemes to be used than has been possible with conventional integrated optoelectronic components. Such electrical connection schemes can be independent of the physical ordering of the sub-devices on the substrate which is of significance in terms of the optical path through the sub-devices.

Description

M&C Folio No P53814GB 1 Monolithically Integrated Optoelectronic

Components The present invention concerns monolithically integrated optoelectronic components, such as devices incorporating semiconductor laser diodes for example.

In some monolithically integrated optoelectronic devices some sub-sections may have separate electrical contacts and rely upon the physical separation of the contacts to reduce the electrical interconnection of the sub-sections, such an arrangement being described as completely unisolated. Such an arrangement is illustrated in the segmented electrodes of the tuneable laser disclosed in W003/012936, for example. However, such an arrangement may not be adequate for other devices, and it is known to use electrical isolation zones to achieve complete or partial electrical isolation of sub-sections of such devices.

Techniques for producing electrical isolation zones include dopant impurities to create regions of electrical isolation by means of melt growth, epitaxial growth (including selective etching or selective area growth), diffusion, implantation, and quantum well intermixing (QWI). By means of such techniques insulating layers or regions of layers that are parallel with the plane of the semiconductor wafer can be created. Also such techniques can be used to create barriers or partial barriers that are perpendicular to the plane of the wafer, and it is known to use layer and barrier features in combination.

In a simple combination of layer and barrier features deep implantation regions may be used in combination with a semi-insulating (SI) substrate to create complete electrical isolation between sub-devices, as in the arrangement shown Figure 1. In such an arrangement three sub-devices 1, 2 and 3 are formed within upper and lower cladding layers 4 and 5 applied to the substrate 6 with an active layer 7 therebetween. The three sub-devices 1, 2 and 3 have separate lower electrodes 8, 9 and 10 and upper electrodes 11, 12 and 13, and the three sub-devices 1, 2 and 3 are completely electrically isolated from one another by deep vertical isolation zones 14 and 15 formed by ion implantation and extending as far as the semi-insulating substrate 6.

M&C Folio No P53814GB 2 Being completely electrically isolated the sub-devices 1, 2 and 3 can be driven in the same or different bias directions with the same or different bias levels. However, in this arrangement, two contacts, which can be provided by wire bonds or electrical tracking on the semiconductor chip, are required for each sub-device. The contacts and associated wire bonds or electrical tracking increase manufacturing complexity and introduce unwanted electrical effects, such as resistance and capacitance, which will be subject to manufacturing variations resulting in performance variations in the device.

Furthermore, for an optoelectronic waveguide device, such as is shown Figure 2, in which the sub-devices 1, 2 and 3 are interconnected by conducting links 16 and 17, the isolation barriers create optical loss.

An example of a multi-section electro-optic device, in which each sub-device is completely electrically isolated, and in which the sub-devices are driven in series with the same bias direction, is described in the paper "Multistage Segmented 1.55um Lasers with Record Differential Efficiency, Low Thresholds, and 50 Ohm Matching", presented by J.T. Getty et al. of University of California, Santa Barbara (UCSB) at IEEE/LEOS Annual Meeting, paper number PDI.2, held Glasgow, Scotland, 10-14 November 2002.

An example of a device in which partial barriers are used to create partial electrical isolation between sub-devices is disclosed in WO2005/1 14307 and shown in Figure 3.

In such an arrangement three sub-devices 1', 2' and 3' are formed within upper and lower cladding layers 4' and 5' applied to a substrate 6' with an active layer 7' therebetween. The three sub-devices 1', 2' and 3' have a common lower electrode 8' electrically connected to the common lower cladding layer 5' of the three sub-devices, and three separate upper electrodes 9', 10' and 11' for the three sub-devices. The upper cladding layer 4' and the active layer 7' are divided into three regions by shallow vertical isolation zones 12' and 13' formed by ion implantation. It should be noted that, with such a device, it is possible to drive two or more of the sub-devices 1', 2' and 3' in parallel with identical biases, by connecting their electrodes together. The sub- devices can be driven in series, with equal and opposite biases, if the common lower electrode 8' is left free floating.

M&C Folio No P53814GB 3 It is also known to provide full or partial lateral electrical isolation of sub-devices by means of etched trenches. For example US2004/0042069 discloses such an arrangement using a semi-insulating substrate, and illustrates the manner in which the sub-devices can be electrically connected in series and in parallel. However, in such an etched arrangement, the optical mode that is common to both sub-devices passes out of the semiconductor material and then passes back into the semiconductor material as it moves between the sub-devices, causing degradation of the optical mode due to reflections andlor imperfections at the interfaces. US2004/0042069 also discloses an arrangement in which a laser and a SOA (semiconductor optical amplifier) are monolithically integrated and are connected in parallel by means of interconnected upper electrodes and a common lower electrode.

It is an object of the present invention to provide a monolithically integrated optoelectronic component.

According to the present invention there is provided a monolithically integrated optoelectronic component comprising a substrate, and at least three optoelectronic sub- devices integrated within a semiconductor layer on the substrate, each of the sub-devices having an upper conductor and a lower conductor separated by a common active region and two of the sub-devices being electrically coupled to a conducting semiconductor sub-layer within the semiconductor layer by their upper or lower conductors, wherein isolation zones are provided for electrically isolating a further one of the sub-devices from the semiconductor sub-layer and from said coupled conductors of said two sub-devices, and for electrically isolating the other conductors of said two sub-devices from one another.

The combined use of such zones of electrical isolation in a monolithically integrated optoelectronic component comprising at least three optoelectronic sub-devices integrated within a semiconductor layer on the substrate enables more complex electrical connection schemes to be used than has previously been possible with conventional integrated optoelectronic components of this type. Furthermore such electrical connection schemes can be independent of the physical ordering of the sub-devices on the substrate which is of course of significance in terms of the optical path M&C Folio No P53814GB 4 through the sub-devices. Such an arrangement also enables the component to have a reduced number of contacts as compared with a similar connection scheme implemented with an arrangement using complete isolation of the sub-devices. Such a connection scheme could not of course be implemented with an arrangement using only a single type of partial barrier isolation.

The arrangement of the present invention permits unconventional electrical connections to sub-devices on a common optical waveguide.

It will be appreciated that the common active layer may be a composite layer such as is provided, when monolithically integrating different devices, by selectively etching away an active area and regrowing that area with a different epitaxy that has different properties. With an etch and regrowth device the active cores may not align exactly or be of precisely the same thickness, provided that the transmitted optical modes in the two materials substantially optically align, at the interface, for reasons of coupling efficiency. The position of the optical mode is determined by the refractive index distribution of the different materials. However, typically the active cores do line up quite well.

Preferred embodiments of the invention use two different types of electrical isolation, that is (i) vertical isolation forming partial or full barriers extending transversely to the semiconductor layer on the substrate, and (ii) horizontal isolation forming partial isolation layers extending parallel to the semiconductor layer on the substrate. For example the isolation zones for electrically isolating the other conductors of the two sub-devices from one another and from the further sub-device may be formed by vertical barriers extending transversely to the semiconductor layer on the substrate.

The conducting semiconductor sub-layer may be part of a doped semiconductor wafer or may be epitaxially grown.

Preferably a conductive electrode is connected to one of the conductors of the further sub-device to allow an electrical connection to be made to the conductor of the further M&C Folio No P53814GB 5 sub-device separately from the two sub-devices. Furthermore a common conductive electrode is advantageously connected to the coupled conductors of the two sub-devices.

In one embodiment of the invention one of the other conductors of the two sub-devices is electrically coupled to one of the conductors of the further sub-device.

In order that the invention may be more fully understood, reference will now be made, by way of example, to the accompanying drawings, in which: Figures 1 to 3 are vertical sections through known integrated optoelectronic devices; Figure 4 is a vertical section through a first embodiment of integrated optoelectronic device in accordance with the invention; Figure 5 is a vertical section through a second embodiment of integrated optoelectronic device in accordance with the invention; Figure 6 is a vertical section through a third embodiment of integrated optoelectronic device in accordance with the invention; Figure 7 is a vertical section through a fourth embodiment of integrated optoelectronic device in accordance with the invention, Figure 8 being a diagrammatic perspective view of the embodiment; Figure 9 is a vertical section through a fifth embodiment of integrated optoelectronic device in accordance with the invention; Figure 10 is a vertical section through a sixth embodiment of integrated optoelectronic device in accordance with the invention; and Figure 11 is a vertical section through a seventh embodiment of integrated optoelectronic device in accordance with the invention.

M&C Folio No P53814GB 6 In the embodiment of Figure 4 four sub-devices 21, 22, 23 and 24 are formed within upper and first and second lower cladding layers 25, 26 and 27 applied on top of a lower conductive layer 28 on a semi-insulating substrate 29 with a common active layer 30 being provided between the upper cladding layer 25 and the second lower cladding layer 27. The first and third sub-devices 21 and 23 have a common lower electrode 31 electrically connected to a region of the lower cladding layer 28 common to the two sub-devices 21 and 23, and the third and fourth sub-devices 23 and 24 have a common upper electrode 32 electrically connected to a region of the upper cladding layer 28 common to the two sub-devices 23 and 24.

Furthermore the first two sub-devices 21 and 22 have separate upper electrodes 33 and 34, the upper cladding layer 25 being divided into three regions by vertical isolation zones 35 and 36 formed by ion implantation. In addition a horizontal barrier layer 37 (constituted by part of the first lower cladding layer 26 implanted with ions) is provided within the second sub-device 22 and serves to completely electrically isolate the sub-device 22 from the rest of the device. The second sub-device 22 is provided with a separate lower electrode 38 allowing an electrical connection to be made to the lower part of the sub-device 22 separately from the rest of the device. The fourth sub-device 24 is electrically isolated from the third sub-device 23, except in the region of the upper cladding layer 25, by a vertical isolation zone 40 formed by ion implantation, and is provided with a separate lower electrode 39.

In such an arrangement the sub-device 21 is only partially electrically isolated from the sub-device 23, with the lower cladding layer regions of the two sub-devices 21, 23 being electrically connected through the lower conductive layer 28, and the sub-device 24 is only partially electrically isolated from the sub-device 23, with the upper cladding layer regions of the two sub-devices 23, 24 being electrically connected through direct physical contact. It should be noted that, with such a device, it is possible to drive two or more of the sub-devices in parallel or in series. Either or both of the two long common electrodes 31 and 32 may be omitted.

M&C Folio No P53814GB 7 In a preferred method for fabricating such a device the lower conductive layer 28 and first lower cladding layer 26 are grown using low pressure metal-organic chemical vapour deposition (LP-MOCVD) to grow a Si-doped InP or InGaAsP layer (or combinations of layers of InP and InGaAsP). A patterned oxide mask with an opening in the area where the horizontal barrier layer 37 is to be formed is then used for ion implantation using an implantation species, energy and dose appropriate for the thickness of the first lower cladding layer 26. The oxide mask is of a thickness and composition suitable to shield the wafer from such implantation everywhere except where the semiconductor is to be exposed, and is removed after creation of the horizontal barrier layer 37 by such implantation. The second lower cladding layer 27 composed of Si-doped InGaAsP/InP and the common active layer 30 composed of undoped InGaAsP (or multiple layers) are provided with a patterned oxide mask with an opening in the area where the vertical isolation zone 40 is to be formed, and ion implantation is performed with an implantation species, energy and dose (or combination of multiple implantations) chosen to create an isolation region that extends from the active layer 30 through the second lower cladding layer 27, the first lower cladding layer 26 and the lower conductive layer 28, all the way to the semi-insulating substrate 29, after which the oxide mask is removed. The upper cladding layer 25 composed of Zn-doped InGaAsP/InP is then grown and provided with a patterned oxide mask with openings in the areas where the vertical isolation zones 35 and 36 are to be formed, and ion implantation is performed with an implantation species, energy and dose (or combination of multiple implantations) chosen to create an isolation region which extends from the surface of the wafer to the lower conductive layer 28, but not all the way to the semi-insulating substrate 29, after which the oxide mask is removed. The electrodes are then fabricated using standard deposition and photolithographic techniques.

Figure 5 shows a simpler embodiment of the invention in which three sub-devices 41, 42 and 43 are formed within upper and first and second lower cladding layers 44, 45 and 46 applied on top of a lower conductive layer 47 on a semi-insulating substrate 48 with an active layer 49 being provided between the upper cladding layer 44 and the second lower cladding layer 46. The sub-devices 41 and 43 have a common lower electrode 50 electrically connected to the lower conductive layer 47, and the three sub-M&C Folio No P53814GB 8 devices 41, 42 and 43 have separate upper electrodes 51, 52 and 53, the upper cladding layer 44, the active layer 49 and the first and second lower cladding layers 45 and 46 being divided into three regions by vertical isolation zones 54 and 55 formed by ion implantation. In addition a horizontal barrier layer 56 (constituted by part of the first lower cladding layer 45 implanted with ions) is provided within the second sub-device 42 and serves to completely electrically isolate the sub-device 42 from the rest of the chip. The second subdevice 42 is provided with a separate lower electrode 57. The electrode 50 may be omitted if required.

Figure 6 shows another simple embodiment of the invention (corresponding effectively to the previous embodiment but with the ordering of the layers reversed) in which three sub-devices 61, 62 and 63 are formed within an upper conductive layer 63A, first and second upper cladding layers 64 and 65, and a lower cladding layer 66 applied on top of on a semi-insulating substrate 67 with an active layer 68 being provided between the first upper cladding layer 64 and the lower cladding layer 66. The three sub-devices 61, 62 and 63 have a common upper electrode 69 electrically connected to the upper conductive layer 63A, and the three sub-devices 61, 62 and 63 have separate lower electrodes 70, 71 and 72, the lower cladding layer 66, the active layer 68 and the first and second upper cladding layers 64 and 65 being divided into three regions by vertical isolation zones 73 and 74 formed by ion implantation. In addition a horizontal barrier layer 75 (constituted by part of the second upper cladding layer 65 implanted with ions) is provided within the second sub-device 62 and serves to completely electrically isolate the subdevice 62 from the rest of the chip. The second sub-device 62 is provided with a separate lower electrode 76. The electrode 69 may be omitted if required.

It will be appreciated that, in each of the above described embodiments in accordance with the invention, by combining vertical and horizontal (planar) electrical isolation zones monolithic integration of the sub-devices is achieved in such a manner that at least one of the sub-devices is completely electrically isolated from the other sub-devices, and at least two other sub-devices are partially electrically isolated from one another.

M&C Folio No P53814GB 9 Our particular application of the invention is to enable unusual electrical interconnections in an optoelectronic device in which the sub-devices are connected on a common optical waveguide (or a circuit of optical waveguides). In a preferred embodiment of the invention shown in Figure 7 and based on the embodiment of Figure (like parts being designated by the same reference numerals in the two figures), the arrangement is applied to a monolithically integrated laser, semiconductor optical amplifier (SOA) and electro-absorption modulator (EA) in which the sub-devices 41, 42 and 43 are a laser, an SOA and an EA respectively. Without the complete isolation of either the laser or SOA, it would not be possible to drive them in series with both in forward bias. In this preferred embodiment, however, the upper electrode 51 of the laser and the lower electrode 57 of the SOA are interconnected by a conductive link 58 such that these two completely mutually electrically isolated sub-devices can be driven in series, both in forward bias, by a single DC drive signal. In addition the laser and the EA share a common lower electrode 50 which reduces the number of contacts required.

For optoelectronic chips and packages, the design limitations are sometimes the amount of current that can be provided by the signal driver and the number of wires available for supplying the drive signals to the package. Whilst a parallel configuration can reduce the number of wires to a chip, such a configuration further increases the current that needs to be supplied. By contrast a series configuration is sometimes preferred since it does not require the current to be increased, and the necessarily increased drive potential (to achieve the same power dissipation) is more readily achievable with common signal drivers. However, this requires one of the two sub-devices to be driven in series to be completely electrically isolated from the other.

Figure 8 is a diagrammatic perspective view of the embodiment of Figure 7 showing how the contacts to the different sub-devices might be made to the different electrodes of each sub-device. Although the vertical electrical isolation zones 54 and 55 are shown extending to the lateral edges of the chip, it will be appreciated that these isolation zones 54 and 55 could terminate before the lateral edges, and lateral implantation zones could be used to complete the isolation from the other sub-devices.

M&C Folio No P53814GB 10 Figure 9 illustrates a variant of the embodiment of Figure 7 (like parts being designated by the same reference numerals primed in Figure 9). In this case it is the first sub-device 41' constituting the laser (rather than the second sub-device 42' constituting the SOA) that is provided with a horizontal barrier layer 56' to completely electrically isolate the sub-device 41' from the rest of the chip, and that is provided with a separate lower electrode 57' linked to the upper electrode 52' of the second sub-device 42' by a conductive link 58'. In this embodiment a less deep electrical isolation zone 55' is required between the SOA and EA than in the embodiment of Figure 7.

Figure 10 shows a further embodiment of the invention in which three sub-devices 81, 82 and 83 are a laser, an EA and a PM (phase modulator) formed within upper and first and second lower cladding layers 84, 85 and 86 applied on top of a lower conductive layer 87 on a substrate 88 with an active layer 89 being provided between the upper cladding layer 84 and the second lower cladding layer 86. The EA and PM have a common lower electrode 90 electrically connected to the lower conductive layer 87, and the three sub-devices 81, 82 and 83 have separate upper electrodes 91, 92 and 93, vertical isolation zones 94 and 95 being formed between the sub-devices by ion implantation. In addition a horizontal barrier layer 96 is provided within the PM and serves to completely electrically isolate the PM from the rest of the chip, the PM is provided with a separate lower electrode 97, and the upper electrode 93 of the PM is linked to the upper electrode 92 of the EA by a conductive link 98. The separate upper electrodes 92, 93 of the EA and PM may alternatively be replaced by a single upper electrode that is common to both sub-devices. This invention is also applicable to a monolithically integrated device comprising a laser, Mach-Zehnder modulator and a phase modulator, and a similar arrangement to that described with reference to Figure can be used in such a device.

Figure 11 shows a variant of the embodiment of Figure 10 (like parts being designated by the same reference numerals primed in Figure 11) in which the upper electrodes 92' and 93' of the EA and the PM are omitted, and the electrical connection between the EA and the PM is achieved by ensuring that the vertical isolation zone 94' does not reach the top surface of the device.

M&C Folio No P53814GB 11 The ion implantation technique that is used for producing the vertical isolation zones in the above described embodiments of the invention typically involves implantation of a masked-off area of the substrate with ions at an appropriate stage in the normal fabrication process. The ions are typically H ions such as 3He, 4He or 0. The horizontal isolation zones can be produced by diffusion, by growth of an insulating layer that is patterned by etching, or by selective area growth (SAG).

In each of the described embodiments the first and second lower cladding layer may be portions of same layer. Also any of the layers may be composite layers. Other layers not mentioned above may also be included, such as grading layers that provide a non-abrupt transition between layers of different epitaxies.

In each of the described embodiments the isolation zones may comprise two or more different arrangements of isolating material, differing in thickness and/or position from the substrate, considered in a plane perpendicular to the substrate.

Claims

M&C Folio No P53814GB 12 CLAIMS: I. A monolithically integrated

optoelectronic component comprising a substrate, and at least three optoelectronic sub-devices integrated within a semiconductor layer on the substrate, each of the sub-devices having an upper conductor and a lower conductor separated by a common active region and two of the sub-devices being electrically coupled to a conducting semiconductor sub-layer within the semiconductor layer by their upper or lower conductors, wherein isolation zones are provided for electrically isolating a further one of the sub-devices from the semiconductor sub-layer and from said coupled conductors of said two sub-devices, and for electrically isolating the other conductors of said two sub-devices from one another.

2. The component of claim 1, wherein the isolation zone for electrically isolating the further sub-devices from the semiconductor sub-layer is formed by a horizontal barrier parallel to the semiconductor layer on the substrate.

3. The component of claim I or 2, wherein the isolation zones for electrically isolating the further sub-devices from said coupled conductors of said two devices arc formed by vertical barriers extending transversely to the semiconductor layer on the substrate.

4. The component of claim 1, 2 or 3, wherein the isolation zones for electrically isolating the other conductors of said two sub-devices from one another and from said further sub-device are formed by vertical barriers extending transversely to the semiconductor layer on the substrate.

5. The component of any preceding claim, wherein at least one of the isolation zones is produced by ion implantation.

6. The component of any preceding claim, wherein at least one of the isolation zones is formed from epitaxially grown semiconductor material doped so as to be substantially insulating.

M&C Folio No P53814GB 13 7. The component of any one of claims 1 to 6, wherein the conducting semiconductor sub-layer is part of a doped semiconductor wafer.

8. The component of any one of claims 1 to 6, wherein the conducting semiconductor sub-layer is epitaxially grown.

9. The component of any preceding claim, wherein a conductive electrode is connected to one of the conductors of said further sub- device to allow an electrical connection to be made to said conductor of said further sub-device separately from said two sub-devices.

10. The component of any preceding claim, wherein a common conductive electrode is connected to said coupled conductors of said two sub-devices.

11. The component of any preceding claim, wherein one of said other conductors of said two sub-devices is electrically coupled to one of the conductors of said further sub-device.

12. The component of any one of claims I to 11, wherein the sub-devices comprise a laser, a semiconductor optical amplifier (SOA) and an electro-absorption modulator (EA).

13. The component of any one of claims ito 11, wherein the sub-devices comprise a laser, an electro-absorption modulator (EA) and a phase modulator (PM).

14. A monolithically integrated optoelectronic component substantially as hereinbefore described with reference to Figures 4 to 11 of the accompanying drawings.