GB2444279A

GB2444279A - Optoelectronic device

Info

Publication number: GB2444279A
Application number: GB0623943A
Authority: GB
Inventors: Kelvin Prosyk; Joan Haysom; Ronald Moore; Michael Scheer
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2006-11-30
Filing date: 2006-11-30
Publication date: 2008-06-04
Also published as: GB0623943D0

Abstract

An optoelectronic device comprises an active layer (203), a p-doped cladding (214) formed above the active layer (203), an n-doped cladding (202) formed below the active layer (203) and an n-doped delta layer (205) formed between the active layer (203) and the p-doped cladding (214). The p-doped cladding (214) and the delta layer (205) comprise a III-V semiconductor material, the p-doped cladding (214) comprises a p-type dopant comprising zinc and the delta layer (205) comprises an n-type dopant comprising silicon. A dose of silicon in the delta layer (205) and a concentration of zinc in at least a portion of the p-doped cladding (214) fulfill specific relations. The dose of silicon is defined as an amount of silicon in the delta layer per area. The delta layer acts as a diffusion barrier for the p-type dopant.

Description

OPTOELECTRONIC DEVICE

The present invention generally relates to optoelectronic devices, more particularly to optoelectronic devices having an active layer comprising at least one optic property

which may be influenced by an electric field.

In modern telecommunication networks, signals may be transmitted by means of light pulses guided by optical fibers. To this end, light is created by a light source such as a laser. In particular, semiconductor lasers adapted to create light in the near infrared range have gained popularity for use in telecommunication networks. The light created by the light source is modulated to vary an intensity of the light in accordance with the signal to be transmitted. The modulated light is then coupled into an optical fiber connecting the transmitter and the receiver. At the receiver location, the transmitted signal is determined by measuring variations of the intensity of light obtained via the optical fiber.

In order to modulate the light, optoelectronic devices such as electro-absorption modulators or variable optical attenuators may be used.

In the following, an electro-absorption modulator according to the state of the art will be described with reference to Figs. 1 a to 1 c.

Fig. 1 a shows a schematic cross-sectional view of an electro-absorption modulator 100 according to the state of the art.

The electro-absorption modulator 100 comprises a substrate 101. On the substrate 101, an n-doped cladding 102 is formed. Over the n-doped cladding 102, an active layer 103 and an intermediate layer 104 are formed. The active layer 103 and the intermediate layer 104 may be substantially intrinsic (i.e. not intentionally doped). A p-doped cladding is provided over the active layer 103 and the intermediate layer 104. A first electrode 106 and a line 109 electrically connect the substrate 101 and the n-doped cladding 102 formed thereon with a driver circuit 111. Similarly, electrical connection between the driver circuit 111 and the p-doped cladding 105 is provided by a second electrode 107 and a line 108.

The active layer 103 comprises a material having a transmission for light in the near infrared region which may be influenced by the presence of an electric field.

In some examples of prior art electro-absorption modulators, the active layer 103 comprises a bulk Ill-V semiconductor material, for example In0 93Ga037As0 80P020 known to persons skilled in the art as "01.47". In the absence of an electric field, 01.47 produces photoluminescence at a wavelength of 1.47 pm. Therefore, in the absence of an electric field, there is an absorption edge at a wavelength of about 1.47 pm. While light having a wavelength shorter than the wavelength of the absorption edge is absorbed, light having a wavelength longer than the wavelength of the absorption edge is transmitted to a large extent by the active layer 103. Under the influence of an electric field, the band structure of Q 1.47 is altered which entails a narrower bandgap and a shift of the absorption edge to greater wavelengths. Therefore, under the influence of an electric field, light having a wavelength greater than about 1.47 pm, which is transmitted in the absence of the electric field, may be absorbed by the active layer 103. This is known as the "Franz-Keldysh effect". In particular, light having a wavelength of about 1.55 pm, which is frequently used in telecommunication networks, may be absorbed in the presence of moderately high electric fields. Semiconductor materials other than Qi.47 may be used in order to obtain absorption edges at different wavelengths.

As an alternative to bulk semiconductor materials, the active layer 103 may comprise a multi-quantum well material. As persons skilled in the art know, such materials may comprise an absorption edge which is shifted towards longer wavelengths in the presence of an electric field due to the quantum-confined Stark effect.

The substrate 101, the n-doped cladding 102 and the p-doped cladding 105 may comprise a semiconductor material other than the material of the active layer 103 (i.e. forming a heterostructure). In examples of prior art electro-absorption modulators wherein the active layer 103 comprises 01.47, the substrate 101 and/or the n-doped cladding 102 and the p-doped cladding 105 may comprise indium phosphide (lnP). Due to the different materials in the active layer 103, the n-doped cladding 102 and the p-doped cladding 105, the active layer 103 may have a higher index of refraction and a narrower bandgap than the n-doped cladding 102 and the p-doped cladding 105 such that the active layer 103 acts as a guide for light 110 impinging on the electro-absorption modulator 100.

The intermediate layer 104 may be provided between the active layer 103 and the p-doped cladding 105 in order to reduce an exit barrier for charge carriers such as electrons entering the p-doped cladding 105 from a single large step in the band edge to a series of smaller steps. To this end, the intermediate layer may comprise a plurality of sub-layers comprising both the material of the active layer 103 and the material of the p-doped cladding 105. Sub-layers in the vicinity of the active layer 103 comprise a greater concentration of the material of the active layer 103 than sub-layers in the vicinity of the p-doped cladding 105 (i.e. the sub-layer may have a graduated composition).

Alternatively, a continuous transition between the material composition of the active layer 103 and the material composition of the p-doped cladding 105 can be provided.

Due to the different doping of the n-doped cladding 102 and the p-doped cladding 105, and the substantially intrinsic conductivity of the active layer 103 and the intermediate layer 104, the electro-absorption modulator 100 comprises a p-i-n diode structure. In the vicinity of the intrinsic region provided by the active layer 103 and the intermediate layer 104, electrons from the n-doped cladding 102 and holes from the p-doped cladding 105 recombine with each other. Thus, in the vicinity of the active layer 103 and the intermediate layer 104, depletion regions 102' and 105' form in the n-doped cladding 102 and the p-doped cladding 105, respectively In the depletion region 102', there is a lower density of electrons than in the rest of the n-doped cladding 102. Similarly, in the depletion region 105', there is a lower density of holes than in the rest of the p-doped cladding 105. Therefore, a space charge is present in the depletion regions, which leads to the formation of an electrical field in the active layer 103 and the intermediate layer 104.

Fig. lb shows schematic diagrams 120, 121 illustrating the space charge and the electric field in the electro-absorption modulator 100 along a line z in the absence of an electric voltage applied between the electrodes 106, 107. In the diagram 120, a vertical axis 122 denotes values of the space charge. In the diagram 121, a vertical axis 123 denotes absolute values of the electric field. The space charge along the line z is represented by a first curve 124 and the absolute value of the electric field along the line z is represented by a second curve 125. A dashed line 126 indicates a boundary between the depletion region 102' and the rest of the n-doped cladding 102. Similarly, the boundary between the depletion region 105' and the rest of the p-doped cladding 105 is represented by a dashed line 129. Further dashed lines 127, 128 represent interfaces between the n-doped cladding 102 and the active layer 103 and an interface between the intermediate layer 104 and the p-doped cladding 105, respectively.

In the depletion region 102' of the n-doped cladding 102, negatively charged electrons which have recombined with holes from the p-doped cladding 105 are missing.

Therefore, a positive space charge builds up in this region. Similarly, in the depletion region 105' of the p-doped cladding 105, a negative space charge is created by the absence of holes which have recombined with electrons from the n-doped cladding 102.

The absolute value of the electric field increases in the depletion region 102', assumes a substantially constant value in the intrinsic active layer 103 and the intermediate layer 104 and decreases to zero in the depletion region 105'. Therefore, the active layer 103 is exposed to an electrical field even in the absence of a voltage applied between the electrodes 106, 107.

If a voltage is applied between the electrodes 106, 107, the thickness of the depletion regions 102', 105' and the electrical field in the active layer 103 are altered. In the following, a voltage between the electrodes 106, 107 will be denoted as "positive voltage", if the positive pole of a power source is connected with the electrode 107 on the p-doped cladding 105 and the negative pole of the power source is connected to the electrode 106 connected to the n-doped cladding 102 via the substrate 101. Conversely, a voltage will be denoted as "negative voltage" if the negative pole of the power source is connected to the electrode 107 and the positive pole is connected to the electrode 106.

If a positive voltage is applied to the electro-absorption modulator 100, electrons are injected into the depletion region 102' and holes are injected into the depletion region 105'. Therefore, the depletion regions 102', 105' shrink, which entails a reduction of the electric field in the active layer 103. Conversely, in case a negative voltage is applied, electrons are removed from the depletion region 102' and holes are removed from the depletion region 105', which entails an increase of the width of the depletion regions 102', 105' and an increase of the electric field strength in the active layer 103.

Thus, the electric field in the active layer 103 which, as detailed above, may have an influence on the absorption of light in the active layer 103, can be varied by controlling the voltage applied between the electrodes 106, 107.

Fig. ic shows a schematic drawing 140 illustrating the dependence of the transmittance of the active layer 103 for light having a wavelength of interest, for example, light having a wavelength of about 1.55 rim, as a function of the voltage applied between the electrodes 106, 107. A first coordinate axis 141 represents a negative voltage applied between the electrodes 106, 107. Hence, positive voltages are represented by the left side of the first coordinate axis 141 and negative voltages are represented by the right side of the first coordinate axis 141. A second coordinate axis 142 represents the transmittance of the active layer 103. Conveniently, the second coordinate axis 142 has a logarithmic scale, representing the transmittance in the unit "decibel" well known to persons skilled in the art.

The transmittance is represented by a curve 143. At relatively large positive voltages, a high transmittance of the active layer 103 is obtained, corresponding to the transparency of the material of the active layer 103 at low electric field strength. As the voltage applied between the electrodes 106, 107 is decreased, the transmittance of the active layer 103 becomes smaller for voltages smaller than a corner voltage V. Since, as detailed above, the active layer 103 is exposed to an electric field created by the p-i-n transition in the electro-absorption modulator 100 even in the absence of an external voltage, the corner voltage Vc may be a positive voltage.

For positive voltages smaller than the corner voltage V, and for negative voltages, the transmittance of the active layer 103 decreases. Thus, a low transmittance is obtained for moderately large negative voltages.

Prior art driver circuits 111 are typically adapted to provide an "on"-voltage V0, and an "oW-voltage VOft which are applied in order to create high and low intensity light pulses to be transmitted via an optical fiber connected to the output of the electro-absorption modulator 100. Typically, the "on"-voltage has a value in a range from about 0 to -0.7 V and the "ofr-voltage has a value in a range from about -2.3 to about -3.OV. A customer drive specification of the driver circuit 111 may typically be about -1.5 V -1.0 V. As shown in Fig. 1 c, at the "on"-voltage V0, a first transmittance T0 of the active layer 103 is obtained, while a second transmittance T0 is obtained at the "oW-voltage VOff. The second transmittance T0 obtained at the uoffflvoltage V0 is lower than the first transmittance T0. Thus, an intensity of light transmitted by the electro-absorption modulator 100 may be modulated in accordance with a sequence of voltages V0 and V0 applied between the electrodes 106, 107. Since the "on"-voltage V0 is smaller than the corner voltage V, the transmittance of the active layer 103 in the "on"-state of the electro-absorption modulator is lower than the maximum transmittance which may be obtained at moderately large positive bias. This entails an undesirable loss of intensity of light pulses.

Although, in principle, it might be possible to design a driver circuit adapted to apply "on" and "oW-voltages having values other than those described above, this may lead to undesirably high costs, since common driver circuits 111 adapted to operate in the voltage range specified above and having a good frequency response are available at -6--low costs. Moreover, such common driver circuits may have a relatively compact configuration.

It is, therefore, a problem of electro-absorption modulators according to the state of the art that a transmittance of the active layer which can be obtained at "on"-voltages provided by common driver circuits may be considerably lower than a maximum transmittance of the active layer.

A further problem of the prior art electro-absorption modulator 100 s that zinc, which is commonly used as p-type dopant for the p-doped cladding 105, may diffuse into the intermediate layer 104 or even the active layer 103. In particular, such diffusion may occur in processing steps performed in the manufacturing of the electro-absorption modulator wherein the electro-absorption modulator 100 is exposed to temperatures exceeding a value of about 400 C. Such high temperatures may, in particular, occur during epitaxial deposition processes or high temperature annealing steps. The presence of zinc in the active layer 103 and/or the intermediate layer 104, however, may adversely affect the performance of the electro-absorption modulator 100 and may also lead to a reduction of the yield of the manufacturing process.

In the prior art is has been suggested to provide an n-doped delta layer between the active layer 103 and the p-doped cladding 105. The delta layer may comprise the same material as the p-doped cladding, for example indium phosphide and may be doped with silicon which acts as an n-type dopant in Ill-V semiconductor materials. It has been found that the silicon prevents the diffusion of zinc through the delta layer into the active region 103, thus acting as a diffusion barrier. The mechanism by which the silicon prevents unwanted zinc diffusion is not fully understood yet (cf. C. Blaauw et. at., J. AppI. Phys. 66 (2), July 15, 1989). However, it is considered that there is a correlation between the zinc concentration and the silicon dose (being defined as the amount of silicon present in the delta layer per unit area) that is required in order to adequately prevent diffusion of zinc out of the p-doped cladding 105.

The presence of the n-doped delta layer within the p-i-n diode structure of the electro-absorption modulator 100, however, can alter the electric field in the vicinity of the active layer 103, which, in turn, may also affect the transmission of the active layer 103. In particular, the total thickness W0 (Fig. 1 b) of the depletion region, between the boundary of the depletion region 102' and the rest of the n-doped cladding 102 and the boundary of the depletion region 105' and the rest of the p-doped cladding 105, may be increased.

This may lead to a reduction of the slope of the curve 143 indicating the dependence of the transmission of the active layer 103 on the applied voltage at voltages greater than the corner voltage Vc.

Moreover, the presence of the n-doped delta layer may lead to a redistribution of the space charge in the electro-absorption modulator 100. This may shift the corner voltage V to more negative voltage values. Although this may lead to a greater transmission of the active layer 103 at the "on"-voltage Von, in combination with the reduction of the slope of the curve 143, this may lead to an undesirable increase of the transmission T0 at the "oW-voltage V0.

Therefore, in the prior art, it has been recommended to apply relatively low doses of silicon (about 1.2. 1012 cm2, corresponding to a 30 nm thick layer comprising a silicon concentration of about 4 1 O'7 cm3) in the delta layer, whereas higher silicon doses of about 2.6 1012 cm2 have been disclosed as detrimental for the functionality of the electro-absorption modulator. Hence, a problem of the electro-absorption modulator 100 according to the state of the art is that a reduction of the diffusion of zinc into the active layer 103 may be obtained only at the cost of a decrease of the performance of the electro-absorption modulator 100.

It is an object of the present invention to solve some or all of the problems detailed above.

According to an aspect of the present invention, an optoelectronic device comprises an active layer, a p-doped cladding formed above the active layer, an n-doped cladding formed below the active layer, and an n-doped delta layer formed between the active layer and the p- doped cladding. The p-doped cladding and the delta layer comprise a Ill-V semiconductor material. The p-doped cladding comprises a dopant comprising zinc and the delta layer comprises a dopant comprising silicon. A dose of silicon in the delta layer and a concentration of zinc in at least a portion of the p-doped cladding fulfill the following relations: n /1012 cm2 = 0.25 on / 1012 cm2 = -5.0 p / i' cm3 + 3.75 Sn / 1012 cm2 = 5.0 p11018 cm3 + 0.25 wherein n denotes the dose of silicon and p denotes the concentration of zinc, the dose of silicon being defined as an amount of silicon in the delta layer per area.

The relatively high dose of silicon in the delta layer provides a corner voltage of the optoelectronic device at negative voltages applied between the n-doped cladding and the p-doped cladding which may be about equal to or slightly greater than the typical "on"-voltages provided by common driver circuits. Thus, a relatively high transmission of the active layer at "on"-voltages provided by a common driver circuit and a substantially flat transmission function at the "on"-voltage can be obtained. Moreover, the concentration of zinc in the p-doped cladding or a portion thereof, respectively, is adapted to the silicon dose such that its adverse effect upon the device performance may be avoided or at least reduced.

In some embodiments of the present invention, the p-doped cladding may comprise a first portion and a second portion. A distance between the first portion and the active layer is smaller than a distance between the second portion and the active layer. A concentration of zinc in the first portion is greater than a concentration of zinc in the second portion. The relations between the silicon dose in the delta layer and the concentration of zinc are fulfilled for the concentration of zinc in the first portion of the p-doped cladding.

A zinc concentration in the first portion fulfilling the above relations provides a narrow depletion region, which forms in the p-i-n diode structure provided by the n-doped cladding, the active layer and the p-doped cladding. Thus, a rapid increase in the absorption of light in the active layer can be obtained, as the negative voltage applied between the n-doped cladding and the p-doped cladding is increased. The lower dopant concentration in the second sub-layer of the p-doped cladding provides a reduced absorption of light by the p-type dopants, thus reducing the optical loss of the device.

Thus, the present invention allows a relatively high transmission of light in the on-state and a relatively low transmission of light in the off-state at "on"-voltages and "oW-voltages, which may be provided by common driver circuits in an optoelectronic device comprising a delta layer.

According to another aspect of the present invention, an optoelectronic device comprises an active layer, a p-doped cladding formed above the active layer, an n-doped cladding formed below the active layer and an n-doped delta layer formed between the active layer and the p- doped cladding. The p-doped cladding comprises a first portion and a second portion. A distance between the first portion and the active layer is smaller than a distance between the second portion and the active layer. A dopant concentration in the first portion is greater than a dopant concentration in the second portion.

The presence of the n-doped delta layer provides an optoelectronic device having a corner voltage in the vicinity of the "on"-voltage of conventional driver circuits. Thus, advantageously, a high transmission of the optoelectronic device in the "on"-state may be obtained. Additionally, the delta layer may provide a diffusion barrier for p-type dopants in the p-doped cladding. A relatively large dopant concentration in the first portion provides a narrow depletion region, which forms in the p-i-n diode structure provided by the n-doped cladding, the active layer and the p-doped cladding. Thus, a rapid increase in the absorption of light in the active layer can be obtained, as the negative voltage applied between the n-doped cladding and the p-doped cladding is increased. The lower dopant concentration in the second sub-layer of the p-doped cladding provides a reduced absorption of light by the p-type dopants, thus reducing the optical loss of the device. Thus, the present invention allows a relatively high transmission of light in the on-state and a relatively low transmission of light in the off-state at "on"-voltages and uoffvoltages, which may be provided by common driver circuits in an optoelectronic device comprising a delta layer.

In some embodiments, the p-doped cladding and the delta layer comprise a Ill-V semiconductor material. The p-doped cladding comprises a dopant comprising zinc and the delta layer comprises a dopant comprising silicon. In case the p-doped cladding is doped with zinc, a delta layer comprising silicon may be particularly efficient in preventing a diffusion of zinc from the p-doped cladding into the active layer.

In some embodiments of the present invention, a dose of silicon in the delta layer and a concentration of zinc in the first portion of the p-doped cladding fulfill the following relations: n/10'2cm2 =0.25 on /1012 cm2 = -5.0 p / 1018 cm3 + 3.75 On / 1012 cm2 = 5.0 p / 1018 cm3 + 0.25 wherein On denotes the dose of silicon and p denotes the concentration of zinc, the dose of silicon being defined as an amount of silicon in the delta layer per area.

The dose of silicon in the delta layer and the concentration of zinc in a portion of the p-doped cladding, for example, the first portion thereof, may further fulfill at least one of the following relations: on / 1012 cm2 = 0.75 On /1012 cm2 = 4.0 OnflO'2cm2 =5.O p/1018cm3-3.75 p/ 10 cm3 0.425 p/ 1018cm3 = 1.0.

Values of the silicon dose and the zinc concentration fulfilling one or more of these relations allow further optimization of the performance of the optoelectronic device.

A thickness of the first portion of the p-doped cladding may be less than 40% greater than a thickness of a depletion region forming in the first portion of the p-doped cladding when an "off"-voltage is applied between the p-doped cladding and the n-doped cladding, the "ofr-voltage having a value in a range from about -2.3 V to about -3 V. Thus, the depletion region forms only in the first portion of the p-doped cladding having a higher dopant concentration than the second portion, while undesirable absorption of light caused by high concentrations of p-type dopant may be reduced.

In some embodiments of the present invention, the thickness of the first portion of the p-doped cladding may be less than 10% greater than the thickness of the depletion region with the "off"-voltage applied. Thus, a more exact adaptation of the thickness of the first portion of the p-doped cladding to the thickness of the depletion region can be obtained.

The active layer can be an intrinsic active layer.

In some embodiments, the optoelectronic device may further comprise an intermediate layer between the p-doped cladding and the active layer. Thus, an exit barrier for charge carriers entering the p-doped cladding from the active layer may be reduced.

The intermediate layer can be an intrinsic intermediate layer. -11 -

The intermediate layer can be located between the delta layer and the active layer. Thus, the dopants in the p-doped cladding can be maintained at a distance to the active layer.

This reduces a risk of a diffusion of the dopants into the active layer.

Alternatively, the intermediate layer may be provided between the delta layer and the p-doped cladding. Thus, a distance between depletion zones in the p-doped cladding and/or the delta layer and the active layer may be reduced. This may help to further improve the performance of the optoelectronic device.

Moreover, the intermediate layer may comprise a first portion and a second portion, the delta layer being located between the first and the second portion of the intermediate layer. Hence, a distance between depletion region in the p-doped cladding and the delta layer and the active layer may be reduced, while the portion of the intermediate layer between the delta layer and the active layer may act as a further diffusion barrier preventing dopants from the p-doped cladding which are not captured by the delta layer from entering the active layer.

In embodiments of the present invention, the p-doped cladding and the n-doped cladding comprise a first semiconductor material and the active layer comprises a second semiconductor material. The intermediate layer comprises both the first semiconductor material and the second semiconductor material. Thus, an abrupt alteration of the band structure at the interface between the first and the second semiconductor material may be avoided.

The p-doped cladding and the n-doped cladding may comprise indium phosphide.

The delta layer may have a thickness in a range from O.Snm to lOOnm, and preferably is between lOnm and 5Onm. A thinner delta layer may help to provide a lower separation between the dopants in the p-doped cladding and the active region, which may help improve the performance of the optoelectronic device.

The optoelectronic device may comprise at least one of an electro-absorption modulator and a variable optical attenuator.

Embodiments of the present invention will now be described with reference to the accompanying drawings, wherein: Fig. 1 a shows a schematic cross-sectional view of an electro-absorption modulator according to the state of the art; -12-Figs. lb and ic show schematic diagrams illustrating properties of the electro-absorption modulator according to the state of the art, shown in Fig. la.

Fig. 2a shows a schematic cross-sectional view of an electro-absorption modulator according to an embodiment of the present invention; Figs. 2b and 2c show schematic diagrams illustrating the operation of the electro-absorption modulator shown in Fig. 2a; Fig. 3 shows a diagram illustrating a concentration of a p-type dopant in a p-doped cladding and a dose of an n-type dopant in a delta layer in an electro-absorption modulator according to the present invention; Fig. 4 shows a schematic cross-sectional view of an electro-absorption modulator according to another embodiment of the present invention; Fig. 5 shows a schematic cross-sectional view of an electro-absorption modulator according to yet another embodiment of the present invention; and Fig. 6 shows a schematiccross-sectional view of an electro-absorption modulator according to a still further embodiment of the present invention.

Fig. 2a shows a schematic cross-sectional view of an electro-absorption modulator 200 according to an embodiment of the present invention. The electro-absorption modulator comprises a substrate 201. On the substrate 201, an n-doped cladding 202, an active layer 203 and an intermediate layer 204 are formed. On the intermediate layer 204, an n-doped delta layer 205 and a p-doped cladding 214 are provided. Electrodes 208, 209 are formed under the substrate 201 and over the p-doped cladding 214, respectively. The electrodes 208, 209 provide an electrical contact between the electro-absorption modulator 200 and a driver circuit 212 via wires 210, 211.

The electro-absorption modulator 200 comprises a substrate 201, which may be a semiconductor substrate. In some embodiments of the present invention, the substrate 201 may comprise a Ill-V semiconductor material, for example indium phosphide (lnP).

On the substrate 201, the n-doped cladding 202 is formed. The n-doped cladding 202 may comprise a semiconductor material, for example a Ill-V semiconductor material such as lnP. The n-doped cladding 202 can be formed by means of deposition techniques well known to persons skilled in the art such as metal-organic chemical vapor deposition. -13-

In metal-organic chemical vapor deposition, the substrate 202 is maintained at an elevated temperature of about 400 C or more within a reactor vessel. Gaseous reactants comprising chemical elements present in the n-doped cladding 202, for example reactants comprising indium and/or phosphor are supplied to the reactor vessel.

In addition to the reactants, one or more carrier gases such as hydrogen and/or nitrogen can be supplied to the reactor vessel. On the surface of the substrate 201, or in the vicinity thereof, a chemical reaction occurs between the reactants. In the chemical reaction, the material to be deposited is created and forms a layer on the surface of the substrate 201.

The n-doped cladding 202 comprises an n-type dopant. In the case that the n-type cladding 202 comprises a Ill-V semiconductor material, the dopant may be silicon. When incorporated into the crystal lattice of a Ill-V semiconductor material, silicon acts as an electron donator, thus being an n-type dopant. In some embodiments of the present invention, the dopant may be introduced into the n-type cladding 202 while the n-type cladding is formed. To this end, in metal-organic chemical vapor deposition, a reactant comprising the dopant may be supplied to the reactor vessel. In other embodiments, the dopant may be introduced after the deposition of a substantially undoped material layer comprising the material of the n-doped cladding 202, for example by means of ion implantation.

On the n-doped cladding 202, the active layer 203 is formed. Similar to the formation of the n-doped cladding 202, this may be done by means of known methods such as metal organic chemical vapor deposition.

The active layer 203 may comprise a Ill-V semiconductor material other than the material of the n-doped cladding 202. In embodiments of the present invention wherein the n-doped cladding 202 comprises indium phosphide, the active layer 203 may comprise a quaternary indium phosphide based alloy such as lnGaAsP, for example 1n0.63Ga037As080P020, which is also known to persons skilled in the art as Qi.47. Other materials may also be employed.

The active layer 203 need not be a bulk material layer comprising a substantially homogeneous semiconductor material. In other embodiments of the present invention, the active layer 203 may have a substructure. For example, the active layer 203 may comprise a multi-quantum well material of a type known to persons skilled in the art.

-14 -The active layer 203 may be an intrinsic semiconductor (i.e. not intentionally doped). To this end, the active layer 203 may be substantially undoped. As persons skilled in the art know, semiconductor layers formed by known methods may comprise small amounts of impurities which are incorporated inadvertently during the formation of the semiconductor layer. Such impurities may stem from contaminants present in a reactor vessel wherein the semiconductor layer is formed, or from contaminants present in reactants supplied in the formation of the semiconductor layer. In accordance with the common language used by persons skilled in the art, in the following, a semiconductor layer shall be denoted as "substantially undoped" if no measures have been performed in order to deliberately introduce dopant materials such that the concentration of any dopant present in the layer is smaller than dopant concentrations in other, deliberately doped layers.

Over the active layer 203, an intermediate layer 204 is formed. The intermediate layer 204 may comprise both the material of the active layer 203 and a material of the p-doped cladding 214. In the intermediate layer 204, a concentration of the material of the active layer 203 may be greater in portions of the intermediate layer 204 in the vicinity of the active layer 203 than in portions of the intermediate layer 204 being provided at a greater distance to the active layer 203. Conversely, a concentration of the material of the p-doped cladding may be greater in portions of the intermediate layer 204 in the vicinity of the p-doped cladding. While in some embodiments of the present invention, the composition of the intermediate layer 204 may vary in a step-wise manner such that the intermediate layer 204 comprises a plurality of sub-layers having a different composition, in other embodiments, the composition of the intermediate layer 204 may vary in a continuous manner in the thickness direction of the intermediate layer 204.

The intermediate layer 204 may be formed by means of metal organic chemical vapor deposition. A variation of the composition of the intermediate layer 204 can be obtained by varying the composition of a reactant gas supplied in the metal organic chemical vapor deposition in a step-wise or continuous manner.

An n-doped delta layer 205 can be formed over the intermediate layer 204. The delta layer 205 may comprise the same material as the p-doped cladding 214. In other embodiments, a different material may be used. For example, the delta layer 205 may comprise a Ill-V semiconductor material such as indium phosphide. The delta layer 205 may comprise an n-type dopant. In embodiments wherein the delta layer 205 comprises a Ill-V semiconductor material, the n-type dopant may be provided in form of silicon. -15-

Similar to other material layers in the electro-absorption modulator 200, the delta layer 205 can be formed by means of metal organic chemical vapor deposition.

The present invention, is not restricted to embodiments comprising an intermediate layer 204. In other embodiments, the intermediate layer 204 may be omitted and the delta layer 205 may be formed on the active layer 203.

Over the delta layer 205, the p-doped cladding 214 is formed. The p-doped cladding 214 can be formed by means of methods known to persons skilled in the art such as metal-organic chemical vapor deposition. The p-doped cladding 214 comprises a p-type dopant. In embodiments of the present invention wherein the p-doped cladding 214 comprises a Ill-V semiconductor material such as indium phosphide, the p-type dopant may be zinc. Other dopants may also be used.

In the operation of the electro-absorption modulator 200, light impinging on the edge of the electro-absorption modulator 200 as indicated by arrow 213 in Fig. 2a is guided by the active layer 203 which may have a greater index of refraction than the n-doped cladding 202 and the p-doped cladding 214. However, the electromagnetic field of the light in the active layer 203 may also extend at least partially into the n-doped cladding 202 and the p-doped cladding 214. The dopant concentration in the p-doped cladding 214 may be relatively high, as will be explained in more detail below.

Finally, the electrodes 208, 209 are formed below the bottom surface of the substrate 201 and over the second sub-layer 207 of the p-doped cladding 214 and the electrodes 208, 209 can be connected to a driver circuit 212 by means of wires 210, 211. In the formation of the electrodes 208, 209, deposition techniques known to persons skilled in the art may be employed. The electrodes 208, 209 need not be formed directly over the substrate 201 and the p-doped cladding 214. Alternatively, intermediate material layers which may, for example, comprise doped semiconductor materials, may be formed between the electrodes 208, 209 and other portions of the electro-absorption modulator 200.

Figure 2b shows a schematic diagram 220 illustrating the space charge in the electro-absorption modulator 200 along a line z extending through the electro-absorption modulator 200 in a vertical direction as shown in Fig. 2a, and a schematic diagram 221 illustrating the absolute value of the electrical field in the electro-absorption modulator along the line z. Both diagrams 220, 221 show the configuration of the electro-absorption modulator 200 in the absence of an external voltage applied to the electrodes -16- 208, 209. A dashed line 227 indicates an interface between the n-doped cladding 202 and the active layer 203. Dashed lines 228, 229 indicate an interface between the intermediate layer 204 and the delta layer 205 and an interface between the delta layer 205 and the p-doped cladding 214, respectively.

Since the delta layer 205 is n-doped and the p-doped cladding 214 is p-doped, there is a p-n-transition between the delta layer 205 and the p-doped cladding 214. At the interface between the p-doped cladding 214 and the delta layer 205, electrons from the delta layer 205 and holes from the p-doped cladding 214 recombine with each other. The recombination of electrons and holes creates a depletion region having a positive space charge in the delta layer 205 and a depletion region having a negative space charge in the p-doped cladding 214. In Fig. 2a, the depletion region in the p-doped cladding 214 is indicated by reference numeral 214'. In Fig. 2b, dashed line 230 indicates the location of an interface between the depletion region 214" and the rest of the p-doped cladding 214.

The space charge creates an electrical field in the delta layer 205 and the p-doped cladding 214, as shown by curve 225 in drawing 221, wherein a vertical coordinate axis 123 denotes absolute values of the electric field in the electra-absorption modulator 200.

A thickness 1 of the depletion region 214' in the p-doped cladding 214 depends on the number of holes which recombine with electrons from the delta layer 205. The number of recombining holes, in turn, depends on the amount of n-type dopant in the delta layer 205.

A quantitative measure for the amount of n-type dopant present in the delta layer 205 is provided by the dose on of n-type dopant which is defined as the number of dopant atoms per unit area of the delta layer 205 with respect to a plane perpendicular to the z-direction (e.g. the interface between the delta layer 205 and the p-doped cladding 214).

In embodiments of the present invention wherein there is a substantially equal concentration of dopant atoms throughout the delta layer 205, the dopant dose is equal to a product On = & n, wherein tz denotes the thickness of the delta layer 205 and n denotes the concentration of n-type dopant in the delta layer 205.

In other embodiments, the concentration of dopants in the delta layer 205 may vary along the direction z. In such embodiments, the dopant dose On equals an integral of the dopant concentration over the extension of the delta layer 205 in the z-direction, = wherein n(z) denotes a concentration of the n-type dopant along the line z and zi, z2 indicate the location of the interface between the intermediate layer 204 and the delta layer 205 and the location of the interface between the delta layer 205 and the p-doped cladding 214, respectively.

The thickness I of the depletion region 214' further depends on the concentration of p-type dopant in the p-doped cladding 214. A greater concentration of p-type dopant in the p-doped cladding 214 entails a lower thickness T of the depletion region 214'.

Conversely, a lower concentration of p-type dopant entails a greater thickness T of the depletion region 214'.

At moderately high concentrations of p-type dopant in the p-doped cladding 214, and moderate or high doses of the n-type dopant in the delta layer 205, the holes provided by the p-type dopant recombine with electrons provided by the n-doped delta layer 205.

Therefore, in the absence of an external electric field, formation of a depletion region in the n-doped cladding 202 need not occur. At very high concentrations of the p-type dopant in the p-doped cladding 214 and/or a low concentration of n-type dopant in the delta layer 205, however, a depletion region may also form in the n-doped cladding 202.

In some embodiments of the present invention, the amount of n-type dopant in the delta layer 205 and the concentration of the p-type dopant in the p-doped cladding 214 are adapted such that substantially no depletion region is formed in the n-doped cladding 202 in the absence of an external electric field. Advantageously, in such embodiments, the active layer 203 is substantially not exposed to an electric field if no electric voltage is applied between the electrodes 208, 209. In other embodiments of the present invention, however, a strength of the electric field in the active layer 203 in the absence of an electric voltage applied between the electrodes 208, 209 may be controlled by adapting the concentration of p-type dopant in the p-doped cladding 214 and the dose of n-type dopant in the delta layer 205 such that a depletion region is formed in the n-doped cladding 202.

If an electric voltage is applied between the electrodes 208, 209, the thickness of the depletion regions in the delta layer 205 and the p-doped cladding 214 is altered. In the following, a voltage between the electrodes will be denoted as "positive voltage", if the positive pole of a power source is connected with the electrode 209 adjacent the p-doped cladding 214 and the negative pole of the power source is connected with the electrode 208 adjacent the substrate 201. A voltage will be denoted as "negative voltage" if the negative pole of the power source is connected to the electrode 209 and the positive pole is connected to the electrode 208.

If a positive voltage is applied, the depletion regions in the delta layer 205 and the p-doped cladding 214 shrink. In particular, the thickness T of the depletion region 214' is reduced. Conversely, if a negative voltage is applied, the depletion regions become thicker. In particular, the thickness T of the depletion region 214' increases. Moreover, even in embodiments of the present invention wherein there is no depletion region in the n-doped cladding 202 in the absence of an external field, a depletion region may form in the n-doped cladding 202 if a negative voltage is applied between the electrodes 208, 209.

In the operation of the electro-absorption modulator 200, light 213 is directed to the active layer 203, as indicated by arrow 213 in Fig. 2a. Then, transmission of the light 213 through the active layer 203 is varied by applying a voltage between the electrodes 208, 209.

Fig. 2c shows a diagram 240 illustrating the dependence of the transmission of the active layer 203 on the voltage applied between the electrodes 208, 209. A first coordinate axis 241 denotes negative voltage. A second coordinate axis 242 denotes transmission of light 213 through the active layer 203. Note, that the second coordinate axis 242 has a logarithmic sca'e, wherein values of the transmission are expressed in the unit decibel well known to persons skilled in the art. A curve 243 represents the dependence of the transmission of the active layer 203 on the applied voltage. It should be appreciated that the curve 243 is schematic. In some embodiments of the present invention, a graph illustrating the dependence of the transmission of the active layer 203 on the applied external voltage may comprise non-linearities not shown in Fig. 2c, depending on the active layer 203 design.

Similar to the prior art electro-absorption modulator 100 described above with reference to Figs. 1 a to 1 c, at positive voltages (represented by the left side of the coordinate axis 241), high values of the transmission of the active layer 203 are obtained. Since, as detailed above, due to the presence of the delta layer 205, the presence of an electric field in the active layer 203 in the absence of an external voltage may be avoided or an electric field strength in the active layer 203 may be reduced compared to the prior art -19-electro-absorption modulator 100, the transmission of the active layer 203 may remain high at zero voltage or even at relatively small negative voltages below a corner voltage V. For negative voltages having a greater absolute value than the corner voltage V, the transmission of the active layer 203 decreases. For negative voltages having an absolute value greater than that of V, the more negative the applied voltage, the smaller is the transmission of the active layer 203.

The corner voltage V may be controlled by varying the dose of the n-type dopant in the delta layer 205. The greater the dose of the n-type dopant, the greater is the negative voltage which must be applied in order to create a depletion region in the n-doped cladding 202 and a moderately strong electric field in the active layer 203. Hence, a greater dose of the n-type dopant may lead to a greater corner voltage V. The decrease of the transmission of the active layer 203 at moderately large negative voltages may be controlled by varying the concentration of the p-type dopant in the p-doped cladding 214. A greater concentration of the p-type dopant may lead to a smaller thickness of the depletion region 206' and, thus, to a greater electric field strength in the active layer 203. Thus, the transmission of the active layer 203 may decrease more quickly at moderate negative voltages if a greater concentration of the p-type dopant is provided in the p-doped cladding 214.

The driver circuit 212 which may be connected to the electro-absorption modulator 200 can be adapted to provide an uonvoltage V0, in order to provide a high transmission of the active layer 203 and an uofrvoltage V0 in order to provide a low transmission of the active layer 203. Similarly to the control circuit 111 in the electro-absorption modulator according to the state of the art described above with reference to Figs. 1 a to 1 C, the uon.voltage V0 can be in a range from about 0 to -O.7V and the "oW-voltage V0 can be in a range from about -2.3 to about -3.0 V. A key indicator for the quality of the modulation provided by the electro-absorption modulator 200 is the so-called extinction ratio ER, which is the ratio between the transmission of the active layer 203 in the "on"-state with V0 applied, and the transmission T0 of the active layer 203 in the "oW-state with V0 applied. When T0 and T0fi are expressed in the logarithmic unit decibel, ER is given by ER =T-T0ff -20 -A further measure for the performance of the electro-absorption modulator 200 is the voltage efficiency VE which, when and T0ff are expressed in decibel, reads VE= Ton-Toff Von -Voff Large values of the extinction ratio ER and the voltage efficiency VE may be obtained if the transmission of the electro-absorption modulator 200 in the uonnstate is high and the transmission in the off-state is low. A high transmission in the on-state may be advantageous in long-range data transmission wherein a high intensity of the light injected into the optical fiber is desirable. The extinction ratio ER and the voltage efficiency VE are also influenced by the average slope of the curve 243 in the voltage range from V0 to V0, wherein a greater slope corresponds to a greater extinction ratio ER and a greater voltage efficiency V0. Moreover, it may be desirable to have a corner voltage V that is less than the on-voltage V0. Besides helping to provide a high transmission of the active layer 203 with the on-voltage V0 applied, a flat transmission function at V0 may help to reduce a sensitivity of the electro-absorption modulator 200 with respect to minor manufacturing variations.

Fig. 3 shows a diagram 300 illustrating values of the dose of the n-type doparit in the delta layer 205 and the concentration of the p-type dopant in the p-doped cladding 214 which may be useful in obtaining desirable values of the extinction ratio ER and the voltage efficiency VE for V0, and V0ff having values in the ranges disclosed above.

A first coordinate axis 301 denotes values of the concentration p of the p-type dopant in the p-doped cladding 214 and a second coordinate axis 302 denotes values of the dose of the n-type dopant in the delta layer 205. In embodiments of the present invention, the concentration p of the p-type dopant and the dose on of the n-type dopant may fulfill the following relations: On /1012 cm2 = 0.25 On / 1012 cm2 = -5.0 p/10'8 cm3 + 3.75 On / 1012 cm2 = 5.0 p / 1018 cm3 + 0.25 In the diagram shown in Fig. 3, values of the concentration p and the dose On fulfilling these relations lie in a region 303. While such p-type dopant concentrations and n-type dopant doses may be particularly advantageous in embodiments of the present invention -21 -wherein the p-doped cladding 214 and/or the delta layer 205 comprise indium phosphide, the p-type dopant in the p-doped cladding 214 comprises zinc and the n-type dopant in the delta layer 205 comprises silicon, such values for the p-type dopant concentration p and the n-type dopant dose 6n may also be employed in other embodiments wherein different materials are used.

Particularly advantageous properties of the electro-absorption modulator 200 according to the present invention may be obtained if the concentration p of the p-type dopant in the p-doped cladding 214 and the dose on of the n-type dopant in the delta layer 205 additionally fulfill one or more of the following relations: On/1012cm2 =0.75 On /1012 cm2 = 4.0 On/1O'2cm2 =5.O pIlO'8cm3-3.75 p1 1018 cm3 0.425 p/1018cm3 = 1 In Fig. 3, values of the concentration p and the dose On fulfilling each of these relations lie in a region indicated by reference numeral 304.

The delta layer 205 may have a thickness in a range from 0.5nm to lOOnm, and is preferably between lOnm and 5Onm.

Fig. 4 shows a schematic cross-sectional view of an electro-absorption modulator 400 according to another embodiment of the present invention. The electro-absorption modulator 400 comprises a substrate 201, an n-doped cladding 202, an active layer 203, an intermediate layer 204, a delta layer 205, a p-doped cladding 414, and electrodes 208, 209. The electro-absorption modulator 400 may be connected to a control circuit 212 by lines 210, 211.

The p-doped cladding 414 comprises a first portion 206 and a second portion 207. While the first portion 206 is formed on the delta layer 205, the second portion 207 is formed on the first portion 206. Thus, a distance between the first portion 206 and the active layer 203, measured in a vertical direction perpendicular to an interface between the active -22 -layer 203 and the n-doped cladding 202, is greater than a distance between the second portion 207 and the active layer 203.

The concentration of p-type dopant in the first portion 206 of the p-doped cladding 414 can be greater than the concentration of p-type doparit in the second portion 207. p-type dopants such as zinc may absorb light in the near infrared range having wavelengths which are commonly used in optical telecommunication networks. In order to reduce an undesirable attenuation of light caused by absorption of light by p-type dopants, a relatively low concentration of the p-type dopant may be provided in the second portion 207 of the p-doped cladding 414.

The dopant concentration in the first portion 216 of the p-doped cladding 414 may be relatively high in order to provide a narrow depletion zone, as explained above for the embodiments described with reference to Figs. 2a to 2c and 3. In some embodiments of the present invention, the dopant concentration in the first portion 216 of the p-doped cladding 414 may have properties corresponding to those of the p-doped cladding 214 in the embodiments described above with reference to Figs. 2a to 2c and 3, while a lower concentration of p-type dopant may be provided in the second portion 217.

Due to the lower dopant concentration in the second portion 207 of the p-doped cladding 414, a relatively small amount of light is absorbed in the second portion 207. Thus, an absorption of light may advantageously be reduced compared to embodiments of the present invention wherein a high dopant concentration is provided in the whole p-doped cladding. Dopants in the first portion 206 of the p-doped cladding 414 may also absorb light. In the first portion 206 of the p-doped cladding 414, however, a relatively high concentration of the p-type dopant may help to obtain a large increase of the absorption of light in the active layer 203 if a negative voltage is applied to the electro-absorption modulator 200.

In order to reduce an absorption of light by p-type dopants in the first portion 206 of the p-doped cladding 414, a thickness of the first portion 206 may be adapted to the maximum thickness of the depletion region 214' which is obtained in the operation of the electro-absorption modulator 400. As detailed above, the thickness T of the depletion region 214' increases if a negative voltage is applied between the electrodes 208, 209.

The greater the absolute value of the negative voltage, the greater the thickness of the depletion region 214'. Hence, in the operation of the electro-absorption modulator 400, the greatest thickness of the depletion region 214' is obtained when the Moff"-voltage V0 -23 - is applied, which in some embodiments of the present invention may have a value in a range from about -2.3 V to about -3 V. The thickness of the first portion 206 of the p-doped cladding 414 may be less than 40% greater than a thickness of the depletion region 214' that is obtained when the "off"-voltage V0 is applied between the electrodes 208 and 209. In some embodiments, the thickness of the first portion 206 of the p-doped cladding 414 may be less than 10% greater than the thickness of the depletion region 214' which is obtained when the "off'-voltage V0 is applied between the n-doped cladding 202 and the p-doped cladding 414.

The thickness of the depletion region 214' in the presence of the "off-voltage V0n may be measured by means of methods known to persons skilled in the art such as a CV measurement, wherein the capacitance of the electro-absorption modulator is measured as a function of the applied voltage. Alternatively, electron beam induced current (EBIC) techniques or scanning probe microscopy techniques such as scanning voltage microscopy (SVM) or scanning spreading resistance microscopy (SSRM) using test structures comprising a sequence of layers similar to that in the electro-absorption modulator 400 may be employed. Alternatively, the width of the depletion region 214' may be calculated theoretically using methods known to persons skilled in the art. As persons skilled in the art know, a quantitative calculation of the thickness of the depletion region 214' as a function of the applied voltage may comprise numerically soMng the spatially-dependent Fermi-Dirac integral and Poisson equations.

In some embodiments of the present invention, the first portion 206 of the p-doped cladding 414 may have a thickness in a range from 50 nm to 70 nm, or may have a thickness of about 60 nm.

Similar to the embodiments described above with reference to Figs. 2a -2c and 3, in some embodiments of the present invention, the electro-absorption modulator 400 may comprise a substantially constantdopant concentration throughout the p-doped cladding 414.

Fig. 5 shows a schematic cross-sectional view of a semiconductor structure 500 according to another embodiment of the present invention.

The electro-absorption modulator comprises a substrate 201, an n-doped cladding 202, an active layer 203, a p-doped cladding 414 having a first portion 206 and a second portion 207 and electrodes 208, 209. The electro-absorption modulator 200 may be -24 -connected to a control circuit 212 by lines 210, 211. For convenience, in Fig. 2a, Fig. 4 and Fig. 5, like reference numerals have been used to denote like components.

On the active layer 203, a delta layer 501 is formed. On the delta layer 501, an intermediate layer 502 is formed. Similar to the intermediate layer 204 in the embodiments of the present invention described above with reference to Fig. 2a, the intermediate layer 502 may comprise both a material of the active layer 203 and a material of the p-doped cladding 414, wherein a concentration of the material of the active layer 203 decreases with increasing distance from the active layer 203 and the concentration of the material of the p-doped cladding increases with increasing distance from the active layer 203.

The delta layer 501 may comprise substantially the same material as the active layer 203, or may comprise both the material of the active layer 203 and the material of the p-doped cladding 414. The delta layer 501 comprises an n-type dopant. The atomic species of the dopant and the dose thereof in the delta layer 501 may correspond to those in the delta layer 205 described above with reference to Figs. 2a to 2c.

Advantageously, providing the delta layer 501 on the active layer 203 may enable the reduction of the separation between the distal boundaries of the depletion region in the p-doped cladding 414 and the more distant of the depletion regions in the delta layer 501 and/or the n-doped cladding 202, which may be advantageous for device performance.

Thus, a more rapid decrease of the transmission of the active layer 203 with a negative voltage applied between the electrodes 208, 209 may be obtained.

Similar to the embodiments described above with reference to Figs. 2a-2c and 3, in some embodiments of the present invention, the electro-absorption modulator 500 may comprise a substantially constant dopant concentration throughout the p-doped cladding 414.

Fig. 6 shows a schematic cross-sectional view of an electro-absorption modulator 600 according to another embodiment of the present invention. Similar to the embodiments of the present invention described above with reference to Figs. 2a to 2c, 3, 4 and 5, the electro-absorption modulator 600 comprises a substrate 201, an n-doped cladding 202, an active layer 203, a p-doped cladding 414 having a first portion 206 and a second portion 207 and electrodes 208, 209. The electro-absorption modulator 600 may be connected to a control circuit 212 by lines 210, 211. For convenience, in Figs. 2a, 4, 5 and 6, like components are denoted by like reference numerals.

-25 -An intermediate layer 604 is formed between the active layer 203 and the p-doped cladding 414. The intermediate layer 604 comprises a first portion 601 and a second portion 603. Between the first portion 601 and the second portion 603, an n-doped delta layer 602 is formed. Similar to the embodiments of the present invention described above with reference to Figs. 2a to 2c, the intermediate layer 604 may comprise both a material of the active layer 203 and a material of the p-doped cladding 414. Portions of the intermediate layer 604 in the vicinity of the active layer 203 may comprise a greater concentration of the material of the active layer 203 than portions at a distance to the active layer 203, while portions of the intermediate layer 604 in the vicinity of the p-doped cladding 414 comprise a greater concentration of the material of the p-doped cladding 414. In particular, a concentration of the material of the active layer 203 may be greater in the first portion 601 of the intermediate layer 604 than in the second portion 603 of the intermediate layer 604.

The delta layer 602 may comprise both the material of the active layer 203 and the material of the p-doped cladding 414. An atomic species of the n-type dopant present in the delta layer 602 and the dose thereof may correspond to those in the delta layer 205 described above with reference to Figs. 2a-2c and 3.

Providing the delta layer 602 between the first portion 601 and the second portion 603 of the intermediate layer 604 allows a reduction in the separation between the depletion regions forming in the p-doped cladding 414 and the delta layer 602 and the active layer 203, while the first portion 601 of the intermediate layer 604 may act as a further diffusion barrier preventing dopants from the p-doped cladding 414 which are not captured by the delta layer 602 from entering the active layer 203.

Similar to the embodiments described above with reference to Figs. 2a -2c and 3, in some embodiments of the present invention, the electro-absorption modulator 600 may comprise a substantially constant dopant concentration throughout the p-doped cladding 414.

Furthermore, the present invention is not restricted to electro-absorption modulators. In other embodiments, an optoelectronic device according to the present invention may comprise a variable optical attenuator.

Claims

-26 - CLAIMS

1. Optoelectronic device comprising: an active layer; a p-doped cladding formed above said active layer; an n-doped cladding formed below said active layer; and an n-doped delta layer formed between said active layer and said p-doped cladding; wherein said p-doped cladding and said delta layer comprise a Ill-V semiconductor material, said p-doped cladding comprises a dopant comprising zinc and said delta layer comprises a dopant comprising silicon; characterized in that a dose of silicon in said delta layer and a concentration of zinc in at least a portion of said p-doped cladding fulfill the following relations: 6n / 1012 cm2 = 0.25 on / 1012 cm2 = -5.0 p / 1018 cm3 + 3.75 On /1012 cm2 = 5.0 p/1018 cm3 + 0.25 wherein On denotes said dose of silicon and p denotes said concentration of zinc, said dose of silicon being defined as an amount of silicon in said delta layer per area.

2. Optoelectronic device according to claim 1, wherein said p-doped cladding comprises a first portion and a second portion, a distance between said first portion and said active layer being smaller than a distance between said second portion and said active layer, wherein a concentration of zinc in said first portion is greater than a concentration of zinc in said second portion, and wherein said relations are fulfilled for said concentration of zinc in said first portion of said p-doped cladding.

3. Optoelectronic device comprising: an active layer; a p-doped cladding formed above said active layer; -27 -an n-doped cladding formed below said active layer; and an n-doped delta layer formed between said active layer and said p-doped cladding; characterized in that said pdoped cladding comprises a first portion and a second portion, a distance between said first portion and said active layer being smaller than a distance between said second portion and said active layer, wherein a dopant concentration in said first portion is greater than a dopant concentration in said second portion.

4. Optoelectronic device according to claim 3, wherein said p-doped cladding and said delta layer comprise a Ill-V semiconductor material, said p-doped cladding comprises a dopant comprising zinc and said delta layer comprises a dopant comprising silicon.

5. Optoelectronic device according to claim 4, wherein a dose of silicon in said delta layer and a concentration of zinc in in said first portion of said p-doped cladding fulfill the following relations: n / 10 cm2 = 0.25 Sn /1012 cm2 = -5.0 p/1018 cm3 + 3.75 Sn! 1012 cm2 = 5.0 p / 1018 cm3 + 0.25 wherein Sn denotes said dose of silicon and p denotes said concentration of zinc, said dose of silicon being defined as an amount of silicon in said delta layer per area.

6. Optoelectronic device according to claim 1, wherein said dose of silicon in said delta layer and said concentration of zinc in said at least one portion of said p-doped cladding further fulfill at least one of the following relations: Sn /1012 cm2 = 0.75 Sn / 1012 cm2 = 4.0 Sn / 1012 cm2 = 5.0 p / 1018 cm3 -3.75 p/1018cm3 =0.425 p/1018cm3 = 1.

-28 -

7. Optoelectronic device according to claim 5, wherein said dose of silicon in said delta layer and said concentration of zinc in said first portion of said p-doped cladding further fulfill at least one of the following relations: On /1012 cm2 0.75 on / 1012 cm2 = 4.0 On / 1012 cm2 = 5.0 p /1018 cm3 -3.75 p / 1018 cm3 = 0.425 p/10'8cm3 = 1.

8. Optoelectronic device according to any of claims 2 and 3, wherein a thickness of said first portion of said p-doped cladding is less than 40% greater than a thickness of a depletion region forming in said first portions of said p-doped cladding if an off-voltage is applied between said p-doped cladding and said n-doped cladding, said off-voltage having a value in a range from -2.3 V to -3 V.

9. Optoelectronic device according to claim 8, wherein said thickness of said first portion of said p-doped cladding is less than 10% greater than said thickness of said depletion region.

10. Optoelectronic device according to any of claims 1 to 9, wherein said active layer is an intrinsic active layer.

11. Optoelectronic device according to any of claims 1 to 10, further comprising an intermediate layer between said p-doped cladding and said active layer.

12. Optoelectronic device according to claim 10, wherein said intermediate layer is an intrinsic intermediate layer.

13. Optoelectronic device according to any of claims Ii and 12, wherein said intermediate layer is located between said delta layer and said active layer.

14. Optoelectronic device according to any of claims 11 and 12, wherein said intermediate layer is located between said delta layer and said p-doped cladding.

-29 -

15. Optoelectronic device according to any of clainis 11 and 12, wherein said intermediate layer comprises a first portion and a second portion, said delta layer being located between the first and the second portion of said intermediate layer.

16. Optoelectronic device according to any of claims 11 to 15, wherein said p-doped cladding and said n-doped cladding comprise a first semiconductor material and said active layer comprises a second semiconductor material, and wherein said intermediate layer comprises both said first semiconductor material and said second semiconductor material.

17. Optoelectronic device according to any of claims 1 to 15, wherein said p-doped cladding and said n-doped cladding comprise indium phosphide.

18. Optoelectronic device according to any of claims 1 to 17, wherein said delta layer has a thickness in a range from O.5nm to lOOnm.

19. Optoelectronic device according to claim 18, wherein said delta layer has a thickness in a range from lOnm to 5Onm.

20. Optoelectronic device according to any of claims 1 to 19, wherein said optoelectronic device comprises at least one of an electro-absorption modulator and a variable optical attenuator.