GB2318211A

GB2318211A - Optical waveguide

Info

Publication number: GB2318211A
Application number: GB9721489A
Authority: GB
Inventors: Andrew James Shields
Original assignee: Toshiba Cambridge Research Centre Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 1996-10-09
Filing date: 1997-10-09
Publication date: 1998-04-15
Anticipated expiration: 2017-10-09
Also published as: GB2318211B; GB9721489D0

Abstract

The waveguide comprises a semiconductor substrate (10), a guided region (23) for transmission of an optical beam therethrough and a confining semiconductor region (13), wherein said confining region (13) is interposed between the guided region (23) and the semiconductor substrate (10), the confining region (13) having a lateral variation in its refractive index such that the optical beam is confined to a predetermined section of the guided region (23). The waveguide may be used in an optical modulator or in a laser structure. The fabrication of a lateral pn junction laser using amphoteric doping of a layer grown on GaAs (100) and (311)A facets is also described (figure 3).

Description

Optical Waveguide Structure The present invention relates to an optical waveguide structure and devices incorporating such a structure. The present invention also relates to a method for fabricating such a structure.

Waveguides are used for confining electro-magnetic radiation in both passive structures, which simply channel the light, and in active devices, such as lasers, LEDs, modulators and detectors.

Waveguides work by confining a light beam within a layer of higher refractive index (n) than its surroundings, for example the core of an optical fibre. For semiconductors, the dielectric layers can be epitaxially grown from layers such as GaAs and AIxGal xAs (n decreases with Al content) or doped InP.

A slab waveguide is formed by sandwiching a layer of material with a refractive index of n1, between two cladding layers with a refractive index of n2, wherein n2 < nl. This can be regarded as the optical equivalent of the electronic quantum well, except that the optical confinement can be achieved by much thicker layers, of order of the light wavelength in the dielectric (-0.3 micron).

Several methods have been devised for producing optical confinement in a second direction. For example, the stripe-loaded waveguide where the upper cladding layer is etched so as to be thicker along a strip of thickness (0.5 - 5 microns). The etch is terminated in the cladding to avoid loss due to the roughness of the etched interface.

Light is confined under the stripe region, where the "barrier" to the light escaping is thickest.

The "effective index model" is one of the simplest and most powerful methods devised to solve the wave equation in such a structure. Here, one considers an array of vertical sections through the structure. For each section the l-d slab waveguide can be solved exactly to yield an effective refractive index. Sections taken through the stripe region have a thicker barrier layer and hence larger effective refractive index, than for those regions outside the stripe. One then regards this array of calculated effective indices, as forming a slab waveguide in the horizontal direction, in order to estimate the optical confinement in the horizontal direction.

Therefore, the previous methods of confining the optical mode involve surface processing of the waveguide structure. The present invention relates to devices wherein the optical mode is confined in the second direction due to characteristics of the dielectric layers embedded into the structure. The advantages of this are that there is no need to subject the whole device to etching after the layers have been formed. Another important advantage provided by the present invention is that the embedded layer can have another function. For example, the embedded layer can be a gate or an electrode for the injection of carriers. Other advantages relating to the fabrication of the structure will be discussed later in relation to the fabrication of the structure.

Therefore, in a first aspect, the present invention provides an optical device comprising a semiconductor substrate, a guided region for transmission of an optical beam therethrough and a confining region, wherein said confining region is interposed between the guided region and the semiconductor substrate, the confining region having a lateral variation in its refractive index such that the optical beam is confined to a predetermined region of the guided region, the term lateral defines a direction parallel to the plane of the guided region.

For the avoidance of doubt, the term layer is used to refer to a single layer. The term region is used to refer to both a single layers and a plurality of layers. The term section refers to a part of a layer or a group of layers. This term.is never used to describe a whole layer. The term 'overlying' means that a layer is formed on an opposing side of a another layer to the substrate. A layer which overlies another layer is not necessarily adjacent to that layer. The terms upper and lower are defined with reference to the substrate. The substrate being in the lower part of the device.

The ability to embed the layers which cause lateral confinement of the light beam opens up a wide range of devices which can be realised in accordance with the present invention. Essentially an inverted strip waveguide is formed. However, before specific devices are discussed, possible layer configurations of the structure will be discussed first. It is obvious to a man skilled in the art that these layer configurations can be applied to a wide range of devices.

A convenient way of producing a variation in the refractive index of the confining region can be provided by varying the thickness of the confining region, for example, by wet or dry etching. It is more preferable if the confining region comprises a patterned layer wherein the patterned layer has a variation is its thickness in a direction parallel to the upper surface of the confining region.

If the confining region has a layer with a higher refractive index than its surrounding layers, the optical mode will be confined to a section of the guided region which is closest to the thickest section of the patterned layer.

The term surrounding layers should be more clearly defined. It is used here to either refer to the substrate or to any other layer within the confining region. For example, if the substrate has a lower refractive index than the patterned layer, then the optical mode within the guided region should still be laterally confined. In many cases it is preferable if the confining region further comprises further layers formed on one or both sides of the patterned layer which have a lower refractive index than that of the patterned layer.

Similarly, the converse structure can also be fabricated, where the patterned layer has a lower refractive index than that of an adjacent surrounding layers and the optical mode is confined to a section of the guided region closest to that of the thinnest part of the patterned layer. It is preferable in this situation if a second surrounding layer is present.

This layer should have a refractive index equal to or lower than that of the first surround layer. Preferably, the second surround layer should be on an opposing side of first surround layer to the patterned layer.

In the above structures, it is important that the light beam confined in the guided region can 'see' the variation in the refractive index. Therefore, in the situation where there are layers formed between the active layer and the patterned layer, it is preferable if the total thickness of the intermediate layers is such that they can be penetrated by the optical mode.

For the avoidance of doubt, a layer with a thickness variation also includes layer where the thickness of the layer is zero in some parts. In many cases it is advantageous if the patterned layer is completely removed in some predetermined parts. This is because contact may be required to upper layers in the structure and these contacts may short to the patterned layer.

In the present invention, it is preferable if regions are formed over layers where there is a variation in thickness of the layer. It is important for subsequent layers formed to be continuous. Therefore, it is preferable if the layers with thickness variations comprise oblique side walls to aid smooth formation of subsequently grown layers The present invention also includes structures wherein the guided region is formed overlying the edges of a plurality of layers with differing refractive indices. A convenient way of achieving such confinement is to form a device with two layers of different refractive indices overlying one another. If the edges of the layers terminate in oblique facets then the guided region can be formed overlying the two layers. - The optical mode will be confined to a portion of the guided region which is closest to the layer with the largest refractive index.

Therefore, a second class of devices may be provided by the present invention wherein the confining region comprises an upper layer and a patterned layer, wherein the upper layer is provided overlying the patterned layer and the patterned layer has a higher refractive index than the upper layer, the confining region further comprising an oblique facet, the guided region being provided overlying the oblique facet such that the predetermined section of the guided region is located closest to the oblique facet edge of the patterned layer.

It is preferable for the substrate to have a refractive index lower than that of said patterned layer.

It is more preferable of the confining region further comprises a lower layer provided between said substrate and said patterned layer, and said lower layer has a refractive index lower than that of said patterned layer.

For fabrication of versatile devices, it is preferable if the confining region in any of the above structures performs another function. Therefore, it is preferable if the confining region comprises a first terminal. It is more preferable if the patterned layer is a terminal. For example, the patterned layer might function as a gate to modulate the carrier concentration of a remote layer or an electrode to modulate the electric field in a remote area. Or, it may function as an emitter or collector, wherein its purpose is to inject electrons or holes into the structure.

The confining region may in addition also comprise an ohmic contact to the guided region. This ohmic contact may be provided by the patterned layer.

To realise the any of the above, it is more preferable if the confining region comprises a doped semiconductor layer. In some cases, it may be more preferable if this doped semiconductor layer is a highly doped semiconductor layer.

The device, may also comprise a second terminal situated on the opposing side of the guided region to the substrate. This terminal may be in addition to or instead of the first terminal. For some devices, it may be preferable if the second terminal is a gate, or it might be an emitter, collector or it may be an ohmic contact.

It is preferable, if the guided region comprises a quantum well or a plurality of quantum well layers. It is more preferable, if the guided region if a upper cladding layer is provided overlying the guided region on an opposing side to the substrate. A lower cladding region on the same side of the substrate may also be provided. These cladding layers will have a lower refractive index than that of the guided region. In many situations, it may be preferable if the lower of these layers is also formed as a layer of the confining region.

The device may be provided with a doped layer which is provided on an opposing side of the guided region to the substrate. It is more preferable for making an isolated contact, if this layer comprises sections of opposing conductivity types. This can be achieved by using amphoteric dopants. The doped upper layer may conveniently have a refractive index lower than that of the guided region so as to serve as an upper cladding layer as well.

Another advantage is realised by forming the guided region over a layer with a thickness variation. As the change in the relief of the guided region formed overlying the lateral confinement region should result in further confinement of the optical mode to a predefined area of the guided region.

Furthermore, the growth rate of the layer which forms the guided region on the nonplanar region may be different to that on the planar regions. Therefore, the layers which form the guided region may experience a variation in thickness between the planar and non planar regions. This may enhance lateral optical confinement.

A planar region is defined as a layer or plurality of layers formed with the same plane as that of the substrate. Similarly, a non-planar region is defined as a layer or plurality of layers which are not parallel to the substrate plane.

The embedded waveguide structure can be used in a number of devices. A laser structure may be conveniently fabricated using either lateral or vertical carrier injection.

In the vertical injection structure, the device may further comprise a terminal located on an opposing side of the guided region to the substrate. The second terminal may be then conveniently be provided by the doped patterned layer. The first and second terminals can be easily fabricated so that they are of opposing conductivity types, thus forming a p-n junction across the guided region. A voltage may be applied two forward bias the junction and the device can exhibit LED or Laser action.

Alternatively, the first and second terminals can be of the same conductivity type, for formation of a unipolar interband light emitter (Laser or LED).

This structure would benefit from making an isolated contact to the top of the structure using a layer which comprises sections of opposing conductivity types as described above.

The above structure also lends itself to a lateral injection laser structure, this can be provided if the guided region which comprises section of opposing conductivity types.

The device may comprise ohmic contacts to the sections of opposing conductivity types. A laser action may be created if the p-n junction formed is driven into forward bias.

The present invention is also of great use in optical modulators, wherein considerable advantages are provided if the active region of the modulator can be controlled by two gates. In this situation the electric field across the guided region and the carrier concentration across the guided region can be independently changed. The provides advantages for optical modulators operating in accordance with the Moss Burstein effect. It will also provide advantages for optical modulators based on p-i-n structures where the modulators comprise a first and a second terminal where the first and second terminal comprises highly doped layers of opposing conductivity types. For instance, by reverse biasing the p-i-n structure an electric field can be developed across the guided region so as to change the intensity or phase of the optical mode.

In a second aspect, the present invention provides an optical modulator comprising a semiconductor substrate, a guided region for transmission of a beam therethrough and a lateral confinement region, the lateral confinement region is situated on the substrate side of the guided region, there is a variation in the thickness of said lateral confinement region so that an optical mode in said guided region is confined to a predetermined region of the guided region and the lateral confinement region has a different refractive index than that of a layer on the substrate side of the lateral confinement region.

For any of the above devices, the guided region may comprise a single- or multiquantum well for electrical confinement of the electrons and/or holes. The quantum wells could comprise Inzi.zAs layers sandwiched between GaAs or AIxGal xAs barriers, or GaAs quantum wells in AlyGal yAs barriers, for instance.

From a fabrication point of view, it is preferable if the device is formed by a regrowth technique. The regrowth technique of forming optical waveguides lends itself to producing optical circuits comprising a number of devices, for example, lasers, modulators, optical switches, polarisation elements, fibre couplers etc. On-chip integration is of particular advantage since assembly is often a major cost of producing such packages. Another possibility for this technology is the integration of optical logic elements in fast optical logic chips.

A structure in accordance with the present invention can be produced using multi-step epitaxial growth and standard processing techniques. A first series of dielectric layers are grown on a suitable substrate using an epitaxial technique such as MBE, MOCVD, MOVPE or LPE. A mask is then defined on the top surface of the structure using photo- or electron beam-lithography. Wet etching is used to expose certain facets outside the mask. The mask is then removed and a second series of dielectric layers are then "re-grown" to form a structure with a three dimensional variation in the refractive index.

The regrowth technique provides a novel method for producing waveguides and related optical devices, with advantages over the currently employed methods. It lends itself readily to on-chip integration of devices. Optical devices are ideally suited to photolithography, since they require typical length scales of order 0.1 - 10 microns.

Therefore, in a third aspect the present invention relates to a method of forming an optical device, the method comprising the steps of: forming a confining region overlying a semiconductor substrate; patterning said confining region, such that there is a variation in the refractive index of said confining region in a direction parallel to an upper surface of said confining region; forming a guided region overlying said confining region.

It is preferable if in the above method, the confining region comprises a patterned layer and the step of patterning said confining region comprises the step of etching said patterned layer to produce a variation in the thickness of said patterned layer in a direction normal to an upper surface of the confining region.

It is preferable if the semiconductor substrate and the patterned layer have different refractive indices.

It is also preferable if the method further comprises the step of forming a first surround layer before the step of forming the patterned layer on the semiconductor substrate.

For the structures where the optical mode is confined to region on the facet, it is preferable if the method further comprises the steps of: forming an upper surround layer overlying the patterned layer before the formation of the guided region, the upper surround layer having a lower refractive index than that of the patterned layer; etching through the pattered layer and the upper layer to expose an oblique facet at the sidewalls of the etch; the guided region being formed over the whole structure such that the optical mode is confined in the guided region formed on the oblique facet, closest to the patterned layer According to a third aspect of the present invention, the patterned layer is etched. Such an etch may be carried out by wet or dry etching means. It is preferable, if such an etch produces oblique facets at its sidewalls. To produce this, the patterned layer could be etched by a BHF/H202 based etch or a H3PO3 based etch.

It has been previously mentioned that there are advantages in forming isolated contacts if the device further comprises a doped layer which comprises section of opposing conductivity types. Such a layer may be easily formed if the patterned layer is etched to expose different crystallographic planes, as a dopant such as Si exhibits amphoteric behaviour in some crystal lattices and can dope n or p type on different crystal planes.

Therefore, it is preferable if the method of forming the device comprises the step of forming an amphoterically doped layer over the patterned layer.

The devices will be described with reference to the OaAs/AlxOaixAS material system.

However, many material systems could be used, for example: InGaAs/A1GaAs, InGaAs/InP, GaInP/(AlGa)InP, ZnCdSe/ZnSe, ZnSe/ZnMgSSe, ZnCdSe/ZnSSe, CdTe/CdZnTe, GaN/AIN, GaN/AIGaN, InGaN/GaN, InGaN/AIGaN and Si/SiGe.

When choosing a material system it is preferable if the material of the quantum well layer possess a direct band-gap.

The present invention will now be explained in more detail by reference of the following non-limiting preferred embodiments and with reference to the accompanying drawings, in which: Figure shows a conventional slab waveguide (Figure la) and a stripe loaded slab waveguide (Figure lib); Figure 2 shows a range of optical devices with a waveguide structure in accordance with the present invention; Figure 3 shows a lateral injection laser in accordance with the present invention; Figure 4 shows a vertical injection laser in accordance with the present invention; Figure 5 shows an on-facet (311 )A laser in accordance with the present invention; Figure 6 shows an on facet Laser structure similar to that of Figure 5 except that the laser structure is formed on the (100) facet; Figure 7 shows an optical modulator in accordance with the present invention; and Figure 8 shows a layer structure suitable for the device shown in Figure 7.

Figure la shows a convention, so called, Slab waveguide. Guided region 3, which is formed of a single layer 3 is formed adjacent and overlying surround layer 1. Layer 3 has a higher refractive index n1 than layer 1 n2. Surround layer 5 is formed adjacent and overlying layer 3 and has refractive index n2 which is again lower than nl. Light propagating along this structure is confined to propagate in layers 3 as this layer has a higher refractive index than surround layers 1 and 5.

However, in the structure as drawn, light is only confined in the z direction. For many device applications, it is necessary to confine the optical mode in both the z and x directions. Such confinement is achieved in the structure shown in Figure ib. Here, surround layer 5 is patterned to form a ridge 7. The formation of this ridge 7, serves to confine the optical mode 9 to the portion of guided region 3 closest to the ridge.

Basic device configurations with an embedded layer which causes confinement are shown in figures 2a to 2d. In these structures layers with four different refractive indices are used. Possible materials which could provide these characteristics are listed in examples 1 and 2.

Example 1 Example 2 n1 GaAs AlxGal.xAs n2 AlxGalxAs AlyGal.yAs (y > x) n3 GaAs or AlyGal yAs GaAs or AlwGa, wAs n4 AlzGai.zAS (z < y) AlzGal zAs (z < w) The layers shown in these examples are grown on a GaAs substrate and optionally GaAs or AlGaAs buffer layers.

In Figure 2a, patterned layer 13 is provided adjacent and overlying a semiconductor lower layer 11. The semiconductor lower layer may be the substrate or a layer formed overlying the substrate. Patterned layer 13 is patterned so that oblique facets 15 and 17, are formed at the sides of the patterned layer 13.

These facets are formed by etching a planar layer of the patterned layer 13. Here it can be seen that the etch has completely removed the patterned layer 13 in some parts e.g.

19 of the device. The etch has also proceeded into the lower layer 11, beyond the interface between the patterned layer 13 and the lower layer 11.

The lower layer 11 has a refractive index n4 and the patterned layer 13 has a refractive index of n3. The refractive index n3 is higher than n4.

A first cladding layer 21 is then formed adjacent and overlying the patterned layer 13 and the lower layer 11. A confinement region 23 is then formed overlying and adjacent the first cladding layer. A second cladding layer 25 is then formed overlying and adjacent the confinement region 23. The first and second cladding layers have a refractive index of n2. The confinement region has a refractive index of nl. Refractive index n1 is larger than refractive index n2. Such that the optical mode 27 is confined to the confinement region 25 between cladding layers 21 and 23.

However, the optical beam experiences a higher effective refractive index in the direction normal to the guided region when it is directly over patterned layer 13.

Therefore, the optical mode is confined to region 29 by the patterned layer 13.

Figure 2b shows a similar structure to that of 2a, except here, the patterned layer 13 is not completely etched away. The layer is just etched so that the central part of the layer (as seen of the Figure) is thicker. However, because the optical beam still experiences a variation in the effective refractive index within the confinement region 23, the optical mode 27 is still confined in the x direction.

Figure 2c basically shows another structure 2b. A semiconductor layer 31 is provided overlying and adjacent the lower layer 11. A patterned layer 33 is then provided overlying and adjacent the semiconductor layer 31. The patterned layers 33 is thinner in its centre (with reference to the figures). As with structures 2a and 2b the thinning of the patterned layer can be conveniently formed by etching the layer. As above, to ensure smooth overgrowth, the layer is etched so that it has sloping walls. The layer 31 has a higher refractive index n3 than the refractive index n4 of the lower layer 11 or the patterned layer 33.

Therefore, the optical mode is confined to the region 29 of the guided region 23 closest to the thinnest region of the patterned layer 33.

In the device shown in Figure 2d, the lateral confinement of the optical mode is achieved by forming the guided region over series of layers. A patterned layer 53 is formed adjacent and overlying a semiconductor lower surround layer 51. An upper surround layer 55 is then formed adjacent and overlying the patterned layer 53. The patterned layer 53 has a higher refractive index n3 than the refractive index n4 of the lower layer 51 and the upper surround layer 55.

The layers 51, 53 and 55 are then etched to expose an oblique facet. First cladding layer 21 followed by confinement region 23 and second cladding layer 25 are then formed overlying the etched structure. The optical mode is then further confined within the confinement region in the portion of the confinement region closest to the high refractive index patterned layer 53.

Figure 3 shows a LASER structure based on the layer configuration shown in Figure 2a.

The laser shown is a lateral carrier injection laser which employs the amphoteric nature of Si-doped MBE-grown AIxGal xAs.

Through control of the growth temperature and As4 pressure, Si dopants can incorporate as donors on the etched (100) facet and as acceptors on the (311)A and (111) facets.

Therefore, a lateral p-n junction can be realised by growing an Si doped layer over a structure which has been etched to expose the above facets.

In the device shown in Figure 3, an undoped AlyGai.yAs layer 63 is formed overlying an undoped GaAs substrate 61. An undoped AIwGal wAs patterned layer 65 is then formed overlying and adjacent layer 63. In the above, w > y. Therefore, the patterned layer 65 has a higher refractive index than that of layer 63. The layers are etched in the same manner as described in relation to Figure 2a. A suitable etch for such processing could be provided by H2O:H202:HF in the ratio of 60:x:6, wherein x is about 1 in order to etch a facet which approximately corresponds to the (100) plane is the etch is performed on a (311)A type substrate.

First and second cladding layers 67 and 71 and a confinement region are then formed overlying the etched structures. The cladding layers 67 and 71 are formed from undoped AlyGa1 yAs. The confinement region 69 has a Si doped or remotely doped quantum well so that the polarity of the carriers within the quantum well changes as the growth plane changes from the (111) plane (p-type) to the (311)A (p-type) to the (100) plane (n-type).

Ohmic contacts 73 and 75 are made to the n-type and p-type regions, using standard techniques. Forward biasing the p-n junction results in a current flow and light emission in the active area (79).

The optical mode (77) is confined to the active area because of the presence of the patterned layer directly underneath the active area (79).

Figures 4 to 6 are vertical carrier injection devices, i.e the electrons and holes are injected normal to the plane of the active layers. In these designs the patterned layer is additionally used as a bottom current-injection contact. The designs also employ the amphoteric nature of Si-doped MBE AlxGal xAs to define an isolated top contact.

Figure 4 shows a vertical carrier injection Laser structure. This structure is formed in a similar way to that shown in Figure 3. However, as the carriers are injected perpendicular into the quantum well, contacts are required above and below the quantum well. The top contact is provided on the surface, the lower contact also serves as the patterned layer 65.

This structure is formed on a (100) oriented substrate. Therefore, the device can be etched so that the facets are two (31 1)A which dope p-type and the top of the ridge 81 is n-type. This allows easy contact to be made to the top of the ridge e.g. by PdGe contacts.

Explicitly, the device is formed in an identical fashion to that described with reference to Figure 3. Except, here the patterned layer 65 is formed form either p-doped GaAs or p doped AIwGal wAs.

Also, the second cladding layer 71 is Si doped so that carriers can be injected from it into the quantum well.

Figure 5 also discloses a vertical injection Laser structure, except here, the confinement region is formed overlying a facet as in Figure 2d.

An undoped AlyGal yAs layer 103 is formed adjacent and overlying (100) orientation undoped GaAs substrate 101. N-doped patterned layer 105 is then formed adjacent and overlying said layer 103. In this example, the patterned layer may be formed from n GaAs or n doped AlwGai.wAs wherein w < y. Upper surround layer 107 is then formed overlying and adjacent patterned layer 105.

The plane of these layers is the (100) plane. These layers are then etched to expose the (311 )A plane which dopes p-type if overgrown under the correct conditions with an Si dopant. The etched structure is then overgrown with cladding layers 109 and 113 and guided region 111 which is sandwiched between the cladding layers 109 and 113.

The optical mode is confined to the section of the guided region closest to the patterned layer 105.

The patterned layer 105 provides the lower injection n-type contact and the top p-type contact is provided by the cladding layer 113. The cladding layer 113 is doped with Si so that is p-type on the facet and n-type elsewhere.

Figure 6 shows a very similar structure except here, the structure is formed on a (311)A type substrate and a (100) facet is exposed by the etch. Therefore, the top contact is ntype and a lower p-type contact is required. This is again provided by the patterned layer 105 which is now doped p-type.

The structure in figures 3 to 6 can also be employed as other electro-optic devices. For instance, by reverse biasing the p-i-n structure an electric field can be developed across the guided region so as to change the intensity or phase of the optical mode. If operated in this way, the devices can be used as optical modulators.

The device shown in Figure 7 has a patterned back-gate. An undoped buffer layer 152 is formed on the surface of the substrate 151, back gate layer 153 is then formed on an upper surface of the undoped buffer layer. The growth is then stopped and the back gate 153 is etched to produce the patterned back gate which with oblique facets 301.

The etched structure is then re-grown with a lower cladding layer 155, on the upper surface of the back gate 153 and the undoped buffer layer 152. The guided region 157 is then formed on an upper surface of the lower cladding layer 155. The upper cladding layer 159 is then formed on an upper surface of the guided region 157. The front gate 161, is then formed on an upper surface of the upper cladding layer 159. The two cladding layers have a lower refractive index than the guided region 157, so as to provide optical confinement. The back-gate layer 153, has a higher refractive index than buffer layer 152, so as to provide lateral optical confinement. Optionally, the upper cladding layer can be patterned to form a layer with a varying thickness to provide additional lateral optical confinement.

Figure 8 shows a possible layer structure for the device shown in Figure 7, fabricated using the GaAs/AlxGai.rAS material system. Here, the substrate is undoped GaAs 201.

Optionally GaAs buffer layers 203 may be formed on an upper surface of the substrate 201. Then an A1,GalAs cladding layer 205 is formed, the refractive index of this layer is lower than that of the back-gate 207 which is formed on an upper surface of this cladding layer 205. The back gate can be either formed from n type GaAs or AlzGai.zAs 207, where z < y. The growth is then stopped and the back-gate layer 207 is etched down to the substrate cladding layer 205 to form a patterned back-gate. To ensure the smooth growth of subsequently grown layers, the gate is etched to expose sloping sidewalls as shown in Figure 7. This can be conveniently achieved using a hydrogen peroxide/buffered hydrofluoric acid based etch.

The gate is patterned so that a portion of the gate extends away from the active region of the device so that ohmic contact can be made to the back-gate. The lower cladding layer may be fabricated from undoped AlyGa,."As 209. This is formed on an upper surface on the back-gate 207 and the substrate cladding layer 205, so that the back-gate 205 is surrounded by cladding layers.

The guided region may be formed from a plurality of layers. Starting with a lower barrier layer which is formed from AlxGa1xAS 211 (x < y) a quantum well layer 213 which is formed from undoped GaAs. An upper barrier layer which is formed from an undoped AlGaAs spacer layer 215 a doped barrier layer which is AlxGa1xAS 217 and an undoped AIxGa, xAs upper spacer layer 219. The upper cladding layer may be conveniently formed from AlyGajyAs 221 and the structure may be finished with a GaAs layer 223. It is on this layer that the front gate may be formed.

For the structure shown in Figure 7, there are some design considerations which must be taken into account if Si doping is used in the layers overlying the oblique facets. . It is important, that the doping of the layers formed overlying the oblique facets 301, is not too heavily compensated. Otherwise the situation might arise where the quantum well in the facet regions is intrinsic or the well might contain carriers of the opposite polarity to the quantum well formed on the planar regions. This can be avoided by careful selection of the growth parameters and/or the facet etch angle. Larger fact angles can be used if the facet corresponds to the (31 1)B plane, for which Si incorporates n-type.

In the light of this disclosure, modifications of the described embodiment, as well as other embodiments, all within the scope of the present invention as defined by the appended claims, will now become apparent to the person skilled in the art.

Claims

CLAIMS:

1. An optical device comprising a semiconductor substrate, a guided region for transmission of an optical beam therethrough and a confining semiconductor region, wherein said confining region is interposed between the guided region and the semiconductor substrate, the confining region having a lateral variation in its refractive index such that the optical beam is confined to a predetermined section of the guided region, the term lateral defines a direction parallel the plane of the guided region.

2. An optical device according to claim 1, wherein the confining region has a.

variation in its thickness in a direction normal to an upper surface of the confining region.

3. An optical device according to claim 2, wherein the confining region comprises a patterned layer, wherein the patterned layer has a variation is its thickness in a direction normal to an upper surface of the confining region.

4. An optical device according to claim 3, wherein the substrate has a different refractive index to that of the patterned layer.

5. An optical device according to either of claims 3 or 4, wherein the confining region further comprises a first surround layer adjacent the patterned layer, wherein the patterned layer and the first surround layer have different refractive indices.

6. An optical device according to any of claims 3 to 5, wherein a section of the patterned layer directly underneath the predetermined section of the guided region is thicker than the rest of the patterned layer and the patterned layer has a refractive index higher than that of the substrate.

7. An optical device according to any of claims 3 to 6, wherein a section of the patterned layer closest to the predetermined section of the guided region is thicker than the rest of the patterned layer and the patterned layer has a refractive index higher than that of the first surrounding layer.

8. An optical device according to any of claims 3 to 7, wherein a section of the patterned layer directly underneath the predetermined section of the guided region is thicker than the rest of the patterned layer and the patterned layer has a refractive index higher than that of the first and second surrounding layers.

9. An optical device according to any of claims 3 to 5, wherein a section of the patterned layer closest to the predetermined section of the guided region is thinner than the rest of the patterned layer and the patterned layer has a refractive index lower than that of the substrate.

10. An optical device according to any of claims 3 to 5 or 9, wherein a section of the patterned layer directly underneath the predetermined section of the guided region is thinner than the rest of the patterned layer and the patterned layer has a refractive index lower than that of the first surrounding layer.

11. An optical device according to claim 10, wherein the confining region further comprises a second surround layer, and the second surround layer having a refractive index equal to or lower than that of the first surround layer.

12. An optical device according to any of claims 3 to 11, wherein the patterned layer comprises oblique sidewalls.

13. An optical device according to claim 1, wherein the confining region comprises an upper layer and a patterned layer, wherein the upper layer is provided overlying the patterned layer and the patterned layer has a higher refractive index than the upper layer, the confining region further comprising an oblique facet, the guided region being provided overlying the oblique facet such that the predetermined section of the guided region is located closest to the oblique facet edge of the patterned layer.

14. An optical device according to claim 13, wherein the substrate has a refractive index lower than that of said patterned layer.

15. An optical device according to claim 13, wherein the confining region further comprises a lower layer provided between said substrate and said patterned layer, and said lower layer has a refractive index lower than that of said patterned layer.

16. An optical device according to any of claims 3 to 15, wherein any layers provided between the guided region and the patterned layer are sufficiently thin to allow penetration of the optical mode through them.

17. An optical device according to any of claims 3 to 16, wherein the patterned layer is a doped semiconductor layer

18. An optical device according to any of claims 3 to 17 wherein the patterned layer is a terminal.

19. An optical device according to any of claims 3 to 18, wherein the patterned layer is a gate.

20. An optical device according to any preceding claim wherein the confining region comprises a first terminal.

21. An optical modulator according to any preceding claim, wherein the confining region comprises a gate.

22 An optical device according to any preceding claim, wherein the device further comprises an upper terminal situated on an opposite side of the guided region to the substrate.

23. An optical device according to claim 22, wherein the upper terminal comprises a metal Schottky gate.

24. An optical device according to either of claims 22 or 23, wherein the upper terminal comprises a doped semiconductor layer.

25. An optical device according to any preceding claim, wherein an ohmic contact to the guided region is provided on an opposing side of the guided region to the substrate.

26. An optical device according to any preceding claim, wherein an ohmic contact to the guided region is provided within the confining region.

27. An optical device according to claim 26 when dependent on claim 3, wherein an ohmic contact to the guided region is provided by the patterned layer

28. An optical device according to any of claims 1 to 17, wherein the confining region comprises a first terminal and the device further comprises a second terminal, such that the first and second terminals are of opposing conductivity types.

29. An optical modulator according to any preceding claim, wherein the guided region comprises a quantum well.

30. An optical device according to any preceding claim, wherein the guided region further comprises two or more quantum wells.

31.. An optical device according to either of claims 29 or 30, wherein the quantum well layer possess a direct band-gap.

32. An optical device according to any preceding claim, wherein an upper cladding layer is provided overlying the said guided region, said upper cladding layer having a lower refractive index than that of said guided region.

33. An optical device according to claim 32, wherein the upper cladding layer is doped.

34 An optical device according to claim 33, wherein the upper cladding layer comprises section of opposing conductivity types.

35. An optical device according to any preceding claim, wherein the said guided region is provided overlying a lower cladding layer, said lower cladding layer having a lower refractive index than that of said active layer

36. An optical device according to any preceding claim, configured to operate as a laser.

37. An optical device according to any of claims 1 to 35, configured to work as an optical modulator.

38. An optical modulator comprising a semiconductor substrate, a guided region for transmission of a beam therethrough and a lateral confinement region, the lateral confinement region is situated on the substrate side of the guided region, there is a variation in the thickness of said lateral confinement region so that an optical mode in said guided region is confined to a predetermined region of the guided region and the lateral confinement region has a different refractive index than that of a layer on the substrate side of the lateral confinement region.

39. A method of forming an optical device, the method comprising the steps of: forming a confining region overlying a semiconductor substrate; patterning said confining region, such that there is a variation in the refractive index of said confining region in a direction parallel to an upper surface of the confining region; forming a guided region overlying said confining region.

40. A method of forming an optical device according to claim 39, wherein the confining region comprises a patterned layer and the step of patterning said confining region comprises the step of etching said patterned layer to produce a variation in the thickness of said patterned layer in a direction normal to the upper surface of said confining region.

41. A method of forming an optical device according to claim 40, wherein the method further comprises the step of forming a first surround layer before the step of forming the patterned layer overlying the semiconductor substrate.

42. A method of forming an optical device according to either of claims 40 to 41, wherein the step of etching the patterned layer produces oblique facets at the layer sidewalls.

43. A method of forming an optical device according to claim 40, wherein the method further comprises the steps of: of forming an upper surround layer overlying the patterned layer before the formation of the guided region, the upper surround layer having a lower refractive index than that of the patterned layer; and etching through the patterned layer and the upper layer to expose an oblique facet at the sidewalls of the etch, the guided region being formed over the whole structure such that the optical mode is confined in the guided region formed on the oblique facet, closest to the patterned layer

44. A method of forming an optical device according to any of claims 41 to 45, wherein an amphoterically doped layer is formed overlying the patterned layer such that sections of opposing conductivity types are formed within the doped layer.

45. An optical device substantially as hereinbefore described with reference to any of the accompanying figures.

46. An optical modulator substantially as hereinbefore described with reference to figures 7 and 8.

47. A method of forming an optical modulator, the method as substantially hereinbefore described with reference to any of the accompanying figures.