DE102004007251A1 - Electro-optical modulator for use in waveguide applications has a waveguide that forms an electrical capacitor with two oppositely doped semiconductor areas separated by a non-conducting capacitor insulation layer - Google Patents

Abstract

Electro-optical modulator has at least a waveguide (300) that forms an electrical capacitor and has at least two wave-guiding semiconductor areas (310, 320) that are separated by a non-conducting capacitor insulation layer (340).

Description

The The invention is based on an electro-optical modulator.

One Previously known electro-optical modulator is for example in the Reference "Low Power Consumption Short-Length and High-Modulation-Depth Silicon Electrooptic Modulator "(Carlos Angulo Barrios, Vilson Rosa de Almeida, Michal Lipson; Journal of Lightwave Technology, Vol 21, no. 4, April 2003). Of the Previously known modulator has a waveguide based on Silicon On Insulator (SOI) material on. The waveguide forms a rib structure. Adjacent to the rib structure of the waveguide are arranged a p-doped and an n-doped zone. By Applying a "forward voltage" will be with the two doped zones charge carriers injected into the waveguide region or into the rib structure, whereby the refractive index is lowered in the waveguide. In this Way can be change the optical behavior of the waveguide and that in the waveguide modulate propagating light. The achievable modulation frequency of Previously known modulator is injected through the life of the Minority carrier limited.

Of the Invention is based on the object, an electro-optical modulator be specified, with the reach particularly high cut-off frequencies to let.

These The object is achieved by a Electrooptical modulator with the characterizing features of Patent claim 1 solved. advantageous Embodiments of the electro-optical modulator according to the invention are in dependent claims specified.

After that is inventively provided that at least one waveguide of the electro-optical modulator forms electrical capacitor. For this purpose, the waveguide at least two wave leading Semiconductor areas formed by a non-conductive layer from each other are electrically isolated. The non-conductive layer forms a Capacitor insulation layer of the electric capacitor.

One significant advantage of the electro-optical modulator according to the invention can be seen in the fact that with this very high cutoff frequencies in the modulation of the propagating light in the waveguide achieve. This is specifically due to the existing in the waveguide achieved electrical capacitor structure, with which electrical charges build up in the waveguide very quickly and degrade. Because the Refractive index and the absorption coefficient in semiconductor materials due to the plasma effect of the respective charge carrier density depend, leads a change the carrier density to a modulation of the light in the waveguide; Here is the modulation the phase is usually dominant. The cutoff frequency of the modulator according to the invention is essentially by the RC element formed by the capacitor limited. The RC element results from the ohmic resistance the two semiconductor regions of the waveguide and by the Capacitance value, which is determined by the capacitor insulation layer. A 3dB cutoff frequency over 30 gigahertz let yourself with the modulator according to the invention easily reach.

in the Difference to the aforementioned, previously known Modulator, where the modulation frequency through the lifetime limited the injected into the pin diode structure charge carriers is is in the inventive modulator the cutoff frequency of the lifetime of the charge carriers in the independent of both semiconductor regions.

When Semiconductor material for the wave-leading Semiconductor regions are, for example, silicon, SiGe alloys, III-V semiconductor material or II-VI semiconductor material suitable.

Prefers the two semiconductor regions of the waveguide lie on one common carrier layer on. The carrier layer may for example be electrically insulating and / or waveguide (z. B. vertical waveguide) serve. The carrier layer may be formed for example by a substrate.

The backing and the capacitor insulation layer are preferably one predetermined angle to each other. For example, the capacitor insulation layer perpendicular to the carrier layer be arranged. Alternatively, the capacitor insulation layer also parallel to the carrier layer.

The The capacitor insulation layer preferably has a "meandering" structure in cross section, and a "meandering" structure increases the efficiency of the modulator considerably, because the overlap of the optical field of light in a waveguide with charges in forming Charge layers of the capacitor is significantly increased.

Especially economical is silicon semiconductor material, so it is considered advantageous when the two semiconductor regions of the waveguide made of silicon consist. In the manufacture of the electro-optical modulator can then resorted to the well-known silicon semiconductor technology be, so that extremely cost-effective modulators can be produced.

Around to achieve that the capacitor charges to the capacitor insulation layer fed quickly (or removed from this) can be the two semiconductor regions are doped differently. This means that one of the two semiconductor regions p-doped and the other of the two semiconductor regions is n-doped. The driving of the capacitor is then such that to the p-doped semiconductor region a greater voltage potential as applied to the n-doped semiconductor region. For example the n-doped semiconductor region is grounded, and the p-doped semiconductor region is controlled by applying a positive voltage. This type of activation of the capacitor is achieved that the capacitor charges on both sides of the capacitor insulation layer by majority carriers of the respective Semiconductor region are formed.

Preferably it is the waveguide of the electro-optical modulator to a single-mode waveguide to an optimal "modulation depth" when modulating to reach the light propagating in the waveguide.

Preferably the capacitor insulation layer is arranged in the waveguide in this way, that the electrical capacitor charges that in the capacitor insulation layer adjacent semiconductor zones or semiconductor layers of the two Semiconductor areas are "accumulated" in the area maximum optical field intensity of the optical waveguide. To optimize is therefore the degree of overlap between the carriers of the capacitor and the optical field in the waveguide, with it optimum modulation efficiency is achieved.

Especially easy and inexpensive For example, rib waveguides can be produced so that it is considered advantageous if the waveguide is a ridge waveguide is. For vertical wave guidance For example, a layer with a lower refractive index than the refractive index used in the rib waveguide. The layer for the vertical wave guide can by the way also have a dual function and the above already mentioned electrically insulating carrier layer form.

If it is the wave leading Material of the rib waveguide is silicon, so can the Layer for vertical wave guidance for example, porous Consist of silicon. Alternatively, the layer may be covered by an oxide layer (eg Buried Oxide: buried oxide layer) may be formed; in this Trap is in the rib waveguide to a so-called SOI (Silicon On Insulator) rib waveguide.

In order to obtain an amplitude modulation from a phase modulation of the light guided in the waveguide, the usual optical arrangements are available:
For example, the electro-optic modulator may have an MZI (MZI: Mach-Zehnder interferometer) waveguide structure if amplitude modulation is to be generated from a phase modulation.

alternative or additionally For example, the modulator can be based on resonant structures that are mirrored on both sides or partially mirrored, and / or in the longitudinal direction of the waveguide a lattice structure, in particular a Bragg lattice structure. This will both increases the effective optical interaction length without sacrificing the electrical capacity to increase, and the device becomes wavelength selective.

to explanation of the invention show:

1 a first embodiment of an electro-optical modulator according to the invention,

2 the waveguide structure of the electro-optical modulator in cross-section in detail,

3 the refractive index distribution in the waveguide according to 2 .

4 the lateral intensity distribution of the light in the waveguide cross-section according to 2 .

5 A second embodiment of an electrooptical modulator according to the invention with a Bragg grating structure,

6 the optical behavior of the modulator according to 5 depending on the wavelength of the light propagating through the modulator and

7 to 14 Further embodiments of an electro-optical modulator according to the invention.

In the 1 is an MZI modulator 10 shown. The MZI modulator 10 has an input waveguide 20 , is irradiated into the optical wave with the light intensity P _a on. To the input waveguide 20 closes a first Y branch 30 on, on the output side, two waveguides 40 and 50 - hereinafter called waveguide arms - are connected.

The two waveguide arms 40 and 50 terminate in a second Y branch 60 , where an Aungangswellenleiter 70 followed. At the end of the output waveguide 70 passes through the MZI modulator 10 modulated light wave with the intensity P _off out.

The two waveguide arms 40 and 50 in each case two pairs of electrodes are assigned. Concretely lying next to the waveguide arm 40 the two electrodes 100 and 110 , Adjacent to the waveguide arm 50 are the electrodes 120 and 130 ,

By applying a voltage to the electrodes 100 and 110 respectively. 120 and 130 can the refractive index in the two waveguide arms 40 and 50 modulate, so that inevitably the light signal in the output waveguide 70 is modulated.

The waveguide structure of the two waveguide arms 40 and 50 is in the 2 by way of example in the 1 right waveguide arm 50 shown. One recognizes in the 2 the two electrodes 120 and 130 parallel to the waveguide arm 50 are arranged. The waveguide arm 50 is through an SOI rib waveguide 300 educated.

The rib waveguide 300 includes two semiconductor regions 310 and 320 , In the embodiment according to the 2 is the semiconductor area 310 n-doped and the semiconductor region 320 p-doped.

The two semiconductor areas 310 and 320 lie on a common electrically insulating carrier layer 330 on, the z. B. by a buried SiO ₂ layer of the SOI rib waveguide 300 is formed. The carrier layer 330 serves in the embodiment according to the 2 both for the electrical isolation of the two semiconductor regions 310 and 320 as well as the optical waveguide in the vertical direction.

For lateral waveguide has the SOI rib waveguide 300 in the horizontal direction a rib, which is made for example by etching.

In the 2 you can see that the two semiconductor areas 310 and 320 through a non-conductive layer 340 are separated, which forms a capacitor insulating layer. The capacitor insulation layer 340 separates the two semiconductor regions 310 and 320 electrically complete.

The thickness of the capacitor insulation layer 340 is in the 2 denoted by the reference d. The thickness d is, for example, d = 10 nm.

The thickness of the capacitor insulation layer 340 is preferably chosen so small that between the two semiconductor regions 310 and 320 an optical coupling is maintained. Namely, the thickness of the capacitor insulation layer 340 chosen too thick, the two semiconductor areas 310 and 320 form separate optical waveguides; only with a sufficiently thin capacitor insulation layer in the nanometer range (<500 nm, preferably <50 nm) it is ensured that the two semiconductor regions 310 and 320 cause only an electrical, but no optical separation.

The length of the SOI rib waveguide 300 For example, L = 750 microns. The rib width of the SOI rib waveguide 300 is in the 2 denoted by the reference w. The width is for example w = 200nm. The overall height H of the SOI rib waveguide 300 is for example H = 200nm.

In the 2 are also two layers 400 and 410 characterized; at the shift 400 it is a negative, that is formed by electrons charge layer when applying a positive voltage U in the n-doped semiconductor region 310 at the interface to the capacitor isolation layer 340 is trained.

The electrically negative charge layer 400 opposite is an electrically positively charged charge layer 410 formed by "holes." The two charge layers 400 and 410 So are by majority carrier of the two semiconductor regions 310 and 320 educated. The thickness of the charge layers 400 and 410 is in the 2 denoted by the reference signs d _P and d _n .

By switching on and off the at the two electrodes 120 and 130 applied positive voltage U can thus be the charge layers 400 and 410 build up or dismantle.

As can be seen, for example, in the publication "Electrooptical Effects in Silicon" (Richard A. Soref, Brian R. Bennett, IEEE Journal of Quantum Electronics, Volume QE-23, No. 1, January 1987), the refractive index in silicon is added Presence of additional charge carriers lowered 2 drawn positive voltage U is thus reduced due to the induced capacitor charges, the refractive index, thereby causing a phase modulation of the light in both the charge layer 400 as well as in the charge layer 410 comes.

In summary, therefore, the voltage U causes the carrier density to be in the charge layers 400 and 410 and hence the effective refractive index in the SOI waveguide 300 modulate.

The capacity C of the two half conductor areas 310 and 320 and the capacitor insulation layer 340 formed capacitor is calculated as follows: C = ε 0 · ε SiO2 · H · L / d = 5.1810 -13 F

Here, ε ₀ denotes the dielectric constant in vacuum ε and _SiO2, the relative dielectric constant of SiO ₂ (ε _SiO2 3.9). Alternatively, for the capacitor insulation layer 340 instead of SiO ₂ , another dielectric may also be used, preferably one with a high dielectric constant ("high-k-dielectric").

The electrical "lead resistance" R to that through the two semiconductor regions 310 and 320 and the capacitor insulation layer 340 formed capacitor is calculated as follows: R = 2 × ρ × a / (L × h) = 3.44 Ω << 10 Ω, where ρ _Si denotes the electrical conductivity of silicon. The electrical conductivity of silicon is at a doping of 1 · 10 ¹⁸ cm ^-3 in the case of a p-doping psi = 0.013 Ωcm. As web height h of the rib waveguide, h = H / 2 was used. For the resistance R is substantially the distance a between the electrodes 120 respectively. 130 and the capacitor insulation layer 340 relevant; this distance a is, for example, a = 1 μm.

As The last-mentioned formula is the electrical resistance R of the RC element formed by the capacitor significantly smaller as 10 Ω.

The result is thus a 3dB cutoff frequency of about: f 3dB = 1 / (2π * R * C) = 31 GHz, for R = 10 Ω

The in the two charge layers 400 and 410 Area-based carrier density N _2D can be calculated as follows: N 2D = ρ F / e = Q / (e * A) = C * U / (e * H * L) = 4.31 * 10 12 1 cm 2 where e is the elementary charge and ρ _{F is} the area-related charge in the charge layers 400 and 410 designated. It is based on a voltage U = 2 volts.

The volume-related carrier density N _3D in the two charge layers 400 and 410 then calculated according to: N 3D = N 2D / d P = 4.31 x 10 18 1 cm 3 , where d _n and d _{P were} each assumed to be about 10 nm.

In the 3 is the horizontal refractive index distribution in the SOI rib waveguide 300 according to 2 shown. The refractive index profile can be seen in a section parallel to the carrier layer 330 ; the cut runs in the upper rib area of the rib waveguide 300 ,

It can be seen that the two semiconductor regions 310 and 320 have a refractive index of about 3.5, whereas the capacitor insulation layer 340 has a refractive index of about 1.45. The refractive index of the capacitor insulation layer 340 thus substantially corresponds to the refractive index of the insulating material, which is the rib area of the SOI rib waveguide 300 surrounds. This insulation material is in the interests of clarity 1 and 2 not shown. The insulating material may for example consist of silicon dioxide, plastic or other dielectrics.

In the 4 is the intensity distribution of the in the SOI ridge waveguide 300 propagating light wave shown in section. The section according to the 3 corresponds to the section according to the 2 ,

One recognizes that in the border areas 1 and 7 virtually no light is led. The main parts of the light become in the semiconductor areas 310 and 320 led the areas 2 . 3 . 5 and 6 correspond.

Despite the low refractive index in the capacitor insulation layer 340 a significant proportion of light in this layer is performed. The light fraction Γ ₄ is about 4.9%. The intensity components Γ ₁ to Γ _{7 of} the individual areas according to 3 have the following numerical values
Γ ₁ = 8.1% (outer dielectric)
= ₂ = 34.4% (semiconductor area 310 )
Γ ₃ = 5.0% (charge layer 400 )
= ₄ = 4.9% (capacitor insulation layer 340 )
Γ ₅ = 5.0% (charge layer 410 )
Γ ₆ = 34.4% (semiconductor area 310 )
Γ ₇ = 8.1% (outer dielectric).

The proportion of light in the two charge layers 400 and 410 is led - these are in the 4 the areas 3 and 5 -, is thus: Γ = (Γ 3 + Γ 5 ) = 10%.

At a voltage U = 2 V, there is thus a change Δn in the effective refractive index in the SOI rib waveguide 300 according to: Δn = ΓΔn Mat Δn _Mat denotes the change in the refractive index due to the charge carriers in the two charge layers 400 and 410 , Δn _Mat calculates according to: .DELTA.n Mat = δn / δN · N 3D = 10 -21 cm -3 · 4.31 · 10 18 cm -3 = 4.31 x 10 -3 where δn / δN indicates the refractive index change in silicon due to the charge carrier density change (cf the already mentioned document "Electrooptical Effects in Silicon").

For Δn we get: Δn = ΓΔn Mat = 10% x 4.31 x 10 -3 = 4.31 x 10 -4

With this refractive index change (eg with differential control with ± 2 V and a DC bias voltage or offset voltage of +2 V), a phase shift ΔΦ results between the two waveguide arms 40 and 50 from ΔΦ = 2 · Δn · (2 · π · L / λ) = 3, 12 ≈ π, what at the output of the modulator 10 leads to destructive interference (extinction).

Reachable is a 3dB cutoff frequency of 31 GHz, without it being a "traveling Wave Concepts "needs.

In the 5 is shown a further embodiment of an electro-optical modulator. This modulator carries the reference numeral 600 and has a resonant structure.

As reflected in the 5 reveals a waveguide points 610 of the modulator 600 in its longitudinal direction a lattice structure 615 on. This grid structure 615 is formed by the rib height of the SOI rib waveguide 610 is varied stepwise in the longitudinal direction. Otherwise, the waveguide corresponds 610 from its construction in connection with the 2 explained SOI rib waveguide 300 ; This also means that the waveguide 610 comprises an integrated capacitor structure with a capacitor insulation layer.

Due to the lattice structure 615 indicates the modulator 600 a wavelength dependent behavior on: Will now be next to the rib waveguide 610 located electrodes 620 and 630 applied a voltage, so can the reflection behavior and thus the transmission behavior of the electro-optical modulator 600 vary. This shows the 6 :
In the 6 one recognizes the reflection behavior R of the electro-optical modulator 600 depending on the optical wavelength λ of the SOI ridge waveguide 610 passing light. By applying a voltage U to the two electrodes 620 and 630 can the reflection behavior or the characteristic curve be shifted.

How is the 6 can be increased at a wavelength of about 1301.75 nm by applying a voltage of 2 V, the reflection value of R = 0 to about R = 0.9 can be increased. The modulator 600 can thus be used as an on-off switch.

Due to the resonated design of the modulator 600 If the interaction length is the same, the geometric length can be reduced to 300 μm (see Modulator length of the modulator 10 according to the 1 to 4 : L = 750 μm). This increases the 3 dB cutoff frequency to 78 Ghz.

In the 7 to 13 further embodiments of a modulator are shown. In the 7 to 13 the same reference numerals are used for identical or comparable elements.

The 7 shows a modulator 700 that's like the modulator 10 on a SOI ribbed fiber 705 based. Unlike the modulator 10 the capacitor insulation layer runs 710 however, parallel to the buried oxide layer 715 of the SOI material, so that an upper waveguiding semiconductor region 720 and a lower waveguiding semiconductor region 725 be formed.

electrodes 730 and 735 for contacting are accordingly on the rib 740 or in the bridge area 745 of the waveguide 705 ,

The upper semiconductor region 720 is formed by poly-silicon, for example, on the capacitor insulation layer 710 (eg, oxide layer) is deposited. The lower semiconductor region 725 is formed for example by the Si cover layer of the SOI material.

The electrode 730 may result in undesirably high attenuation in the waveguide 705 guided light waves, for example, when a metal electrode is used. In order to reduce the attenuation, may be between the upper semiconductor region 720 (eg poly-silicon) and the electrode 730 an intermediate layer 750 (For example, made of porous silicon) are arranged (see. 8th ), which has a lower refractive index than the upper semiconductor region 720 having. The intermediate layer 750 causes the optical light wave from the electrode 730 is disconnected, so that the attenuation through the electrode 730 is reduced.

The optical attenuation due to the electrode 730 can also be reduced in other ways: for example, the geometry of the upper semiconductor region 720 be configured such that in the waveguide 705 guided wave the electrode 730 hardly "sees." An example of such Geometry shows the 9 , In this example, the upper semiconductor region 720 a groove-like structure on, and the electrode 730 is in two parts.

Alternatively, the upper semiconductor region 720 also a variety of parallel semiconductor lands 760 have, by dividing webs 765 are separated. The dividers 765 have a smaller refractive index than the refractive index of the upper semiconductor region 720 on. The dividers 765 For example, they may be made of plastic (eg, BCB). A corresponding structure shows the 10 , In the embodiment according to 10 are the electrodes 730 separated from each other; Alternatively, the electrodes 730 also form a single overall electrode that extends over the entire top of the rib (see 10A ).

The capacitor insulation layer 710 may also have a "meandering" structure in cross-section A "meandering" structure significantly increases the efficiency of the modulator since the overlap of the optical field of the light in the waveguide with the charges in the charge layers 770 and 780 is significantly increased (see. 11 ).

The 12 shows an embodiment with a meandering cross-section capacitor insulation layer 710 and an upper semiconductor region 720 with a large number of parallel semiconductor bridges 760 and dividers 765 , Also in this embodiment, the above applies to the 10 Executed accordingly: this means that the electrodes 730 can also be connected to each other and over the dividers 765 can extend. This shows the 12A ,

In the 13 an embodiment for a modulator is shown in which the capacitor insulation layer 710 obliquely so that, accordingly, the charge layers 770 and 780 run obliquely through the waveguide. The upper semiconductor region 720 For example, as already mentioned above, it consists of poly-silicon, which is on the capacitor insulation layer 710 is deposited.

In the 14 an embodiment for a modulator is shown in which the waveguide 705 has a quasi diamond-shaped cross-section. The illustrated structure can be made by etching a silicon wafer using conventional etching techniques. The in the 14 shown buried oxide layer 715 is not necessary for waveguiding and could also be omitted.

To the mechanical stability of the waveguide 705 can increase, a top coat 800 be applied; this is in the 14 by a dashed line 805 indicated. The cover layer 800 must have a smaller refractive index than the upper semiconductor region 720 exhibit.

10

MZI modulator

20

Input waveguide

30

First Y-branch

40

waveguide arm

50

waveguide arm

60

Second Y-branch

70

Output waveguides

100,110,

120.130

electrodes

300

SOI ridge waveguide

310

N-doped Semiconductor region

320

P-doped Semiconductor region

330

SiO ₂ carrier layer

340

Capacitor insulating layer

400

stratified

410

stratified

600

modulator

610

waveguides

615

lattice structure

620.630

electrodes

700

modulator

705

waveguides

710

Capacitor insulating layer

715

buried Oxide layer of the SOI material

720

upper wave leading Semiconductor region

725

lower wave leading Semiconductor region

730

electrode

735

electrode

740

rib

745

web region

750

interlayer

760

Semiconductor webs

765

dividers

770

stratified

780

stratified

800

topcoat

805

line

Claims (15)

Electro-optical modulator ( 10 . 600 . 700 ), characterized in that at least one waveguide ( 40 . 50 . 300 . 610 ) of the modulator forms an electrical capacitor and for this purpose at least two wave-guiding semiconductor regions ( 310 . 320 ), which is protected by a non-conductive, a capacitor insulation layer ( 340 ) forming layer are electrically isolated from each other. Electro-optical modulator according to claim 1, characterized in that the two semiconductors areas ( 310 . 320 ) on a common, in particular electrically insulating carrier layer ( 330 ) rest. Electro-optical modulator according to claim 2, characterized in that the carrier layer ( 330 ) as well as the capacitor insulation layer ( 340 ) are arranged at a predetermined angle to each other. Electro-optical modulator according to claim 3, characterized in that the capacitor insulation layer ( 340 ) perpendicular to the carrier layer ( 330 ) is arranged. Electro-optical modulator according to claim 3, characterized characterized in that the capacitor insulation layer is parallel to backing is arranged. Electro-optical modulator according to one of the preceding claims, characterized in that the capacitor insulation layer ( 710 ) has a meandering structure in cross-section. Electro-optical modulator according to one of the preceding claims, characterized in that one of the two semiconductor regions ( 320 ) p-doped and the other of the two semiconductor regions ( 310 ) is n-doped. Electro-optical modulator according to one of the preceding claims, characterized in that the two semiconductor regions ( 310 ) and ( 320 ) consist of silicon. Electro-optical modulator according to one of the preceding claims, characterized in that the waveguide ( 50 ) is single-mode. Electro-optical modulator according to one of the preceding claims, characterized in that the capacitor insulation layer ( 340 ) is arranged such that in the adjacent semiconductor regions ( 310 . 320 ) caused electrical charges ( 400 . 410 ) in the region of maximum optical field intensity of the optical waveguide ( 300 ) lie. Electo-optical modulator according to one of the preceding claims, characterized in that the waveguide ( 300 ) is a rib waveguide. Electo-optical modulator according to one of the preceding claims 2 to 11, characterized in that the carrier layer ( 330 ) a lower refractive index than the two waveguiding semiconductor regions ( 310 . 320 ) having. Electro-optical modulator according to claim 12, characterized characterized in that the rib waveguide consists of SOI material. Electro-optical modulator according to one of the preceding Claims, characterized in that the modulator comprises an MZI waveguide structure. Electro-optical modulator according to one of the preceding claims, characterized in that the modulator ( 600 . 700 ) in the longitudinal direction of the waveguide ( 610 ) a lattice structure ( 615 . 710 ), in particular a Bragg grating structure.