three terminals optoelectronics devices integrated into a silicon on silicon waveguide

Optics and Lasers in Engineering 39 (2003) 317–332

Three terminals optoelectronics devicesintegrated into a silicon on silicon waveguide

Giuseppe Coppolaa,*, Andrea Iracea, Giovanni Breglioa,Mario Iodiceb, Luigi Zenic, Antonello Cutolod,

Pasqualina M. Sarroe

aDepartment of Electronic Enginnering and Telecommunications, Univerist "a di Napoli ‘‘Federico II’’,

Via Claudio 21, 80125 Napoli, Italyb Istituto di Ricerche Elettromagnetismo e Componenti Elettronici, Via Diocleziano 243, 80125 Napoli, Italy

cDepartment of Electronic Enginnering, Seconda Universit "a di Napoli, Aversa, ItalydUniversit "a del Sannio, Corso Garibaldi, 107 I- 82100 Benevento, Italy

eDIMES-Delft Institute of Microelectronics and Submicrontechnology, Electronic Components Technology

and Materials Group, Feldmannweg, 17, 2660 CD Delft, The Netherlands

Received 18 December 2000; received in revised form 10 May 2001; accepted 11 May 2001

Abstract

In this paper we describe two different kind of optoelectronic devices both based on a three

terminals active device and exploit the plasma dispersion effect to achieve the desired working.

The first device exploits this effect in order to obtain an optical modulation. The second device

is an optoelectronic router based on the mode-mixing principle together with the injection-

induced optical phase shift. Both devices are integrated into a Silicon on Silicon optical

channel waveguide which can be realized using a standard bipolar process. The possibility of

using standard, well-known technology presents several advantages with respect to III–V

Optoelectronics. The active three terminal device used is a Bipolar Mode Field Effect

Transistor (BMFET). Numerical simulation results are presented on both devices.

r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Optical modulator; Optoelectronic router; Integrated optoelectronic; Carrier injection

*Corresponding author. Tel.: +39-081-768-3128; fax:+39-081-593-4448.

E-mail address: [email protected] (G. Coppola).

0143-8166/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 1 0 7 - 5

1. Introduction

Unstrained pure crystalline silicon exhibits no linear electro–optic (Pockels) effect.Moreover, the Franz–Keldysh effect and the Kerr effect are very weak in silicon [1].For the Kerr effect, an applied field in the range 104–3� 105V/cm produces a changein refractive index of the order of 10�8–10�5 [1]. Therefore, high fields potentiallyclose to the breakdown of silicon are required to obtain a useful refractive indexchange. Most effective mechanisms to vary the refractive index/optical absorption oflight in pure silicon are the free carrier plasma dispersion effect and the thermo-opticeffect, which also have an added advantage of being polarization independent. Beingthe thermo-optic effect related to the thermal processes, it is rather slow and can beused for modulation at frequencies up to 1MHz in micrometric structures [2–6].Therefore, all the applications for higher modulation frequencies, up to fewhundreds of MHz, are based on the free carrier-dispersion effect. Moreover, thiseffect, that allows to change optical properties of Si electrically, and matured Sifabrication technology also make possible a monolithic integration of high-speed Sielectronic circuits for various applications [7].Accordingly, in this paper we review a previous numerical analysis both of an

optoelectronic modulator and of an optoelectronic router, based on the carrier-dispersion effect. Both the devices are integrated in an all-silicon channel-waveguidestructure, and have been designed to be made by means of a standard fullycompatible bipolar process. In order to achieve the desired change of the opticalproperties of both devices, an electron/hole plasma into the central waveguidingregion has to be produced. A three-terminal electrical structure carries out thisfunction. This kind of structure is preferred to a simple p2i2n diode. In fact, using astandard p2i2n diode the maximum switching frequency is limited by the injectionand depletion speed. In particular, an applied reverse bias allows to reduce thedepletion time constant, while the injection phase remains the slowest. To overcomethis limitation it is possible to use the third terminal to move the free carriers plasmainside the channel, without depletion but just re-arranging the electrons/holesdistribution applying a suitable electric field. This means that, for switchingpurposes, we exploit the free carriers drift, a faster phenomenon, instead of thediffusion.Finally, it is important to underline that an all-silicon waveguide is very cheap and

easy to make, when compared to the silicon-on-insulator structure where a p2i2ndiode can be easily realized. Moreover an all-silicon structure exhibits a very highthermal conductivity, this permits dissipation of the heat generated during theoperation, and reduces also undesired refractive index variation produced by themo-optic effect [8].

2. The plasma dispersion effect

Before entering into the details of the realization and the working principles of ourdevices, it is useful to recall some basics about the plasma-optic dispersion on which

G. Coppola et al. / Optics and Lasers in Engineering 39 (2003) 317–332318

the operation of both is based. According to the classical first-order Drude model,we can write:

Dn ¼e2l2

8p2c2e0nDNeme

þDNhmh

� �; ð1Þ

Dn ¼e2l2

8p2c2e0nDNem2eme

þDNhm2hmh

� �; ð2Þ

where Dn and Da are the real refractive index and the absorption coefficientvariations, respectively, e is the electron charge, e0 is the permittivity of free space, nis the refractive index of intrinsic Si, m is the effective mass, m is the free carriermobility, DN is the free carriers concentration variation. The subscript e and h referto electrons and holes contribution, respectively.Soref [3,4] experimentally investigated and found the refractive index and

absorption coefficient changes as a function of free electrons and holes concentra-tion, for both wavelength of interest in fiber optic systems, l ¼ 1:3 and l ¼ 1:55 mm,in a good agreement with the Drude model according to the relations

l ¼ 1:3 mm)Dn ¼ Dne þ Dnh ¼ 6:0�10�18 DNe þ 6:0�10�18ðDNhÞ

0:8�

Da ¼ Dae þ Dah ¼ 6:0�10�18 DNh þ 4:0�10�18 DNh;

(ð3Þ

l ¼ 1:55 mm)

Pn ¼ Dne þ Dnh ¼ 8:5�10�22 DNe þ 8:5�10�18ðDNhÞ

0:8�

Da ¼ Dae þ Dah ¼ 8:5�10�18 DNe þ 6:0�10�18 DNh:

(

ð4Þ

As final remark, it is important to spend some words about the thermo-opticeffect, which, for silicon at l ¼ 1:5 mm, is described by the relation qn=qT ¼þ1:86�10�4 K�1 [8]. In contrast to free carriers injection which reduces the refractiveindex, the thermo-optic effect increases the refractive index therefore it should betaken in the right account when designing a Silicon Optoelectronic Modulator.

3. The optoelectronic modulator

In this section we discuss the characteristics of an optical amplitude modulatorintegrated into an all-silicon channel waveguide and based on a three terminalelectronic device. The cross section of the structure is sketched in Fig. 1. Thisstructure has to be able, at the same time, to generate a complex refractive indexvariation, in order to modulate the infrared beam, and guarantee a goodconfinement of the optical radiation. This means that the modulator must be a 2-D optical channel waveguide too, although not optimized for this purpose. Themodulator is made up starting from a nþþ substrate (10mO cm); moreover anadditional implantation of arsenic is required in order to create a highly doped bufferlayer useful for light confinement. In fact, the higher doping difference between nþþ

buried layer and epilayer causes the refractive index difference enhancement, which

G. Coppola et al. / Optics and Lasers in Engineering 39 (2003) 317–332 319

ensures better vertical confinement for the infrared radiation propagating into theguiding film [9]. The two implanted p-type regions provide the lateral confinementand the upper shallow implanted n-type region acts as upper cladding. The devicehas been realized using the BIFET process DIMES-01 [10] of the Delft Institute ofMicroelectronics and Submicrontechnology (Delft, The Netherlands). In Fig. 2 aSEM picture of the device is reported.In order to achieve an optical modulation, an electron/hole plasma into the central

waveguiding region has to be injected. Both surface highly doped regions, togetherwith the nþþ buried layer, carry out this function. From the electrical point of view,the described structure is the elementary cell of a bipolar mode field effect transistor(BMFET) [11]. The two lateral p-type regions act as the gate, the central n-typeregion as the source, and the nþþ buried layer plays the role of the drain. Theforward biasing of the BMFET gate junctions causes the injection of minoritycarriers from the pþ regions to the channel, and gives rise to a conductivitymodulation of the epilayer with nEpbNEPI [12], similar to the high-injectionoperation of a power BJT. In this BMFET the variation in the charge density, andconsequently in the refractive index, can be very much larger than carrier densityresulting from injection of single polarity, like occurs in a standard FET.From the optical point of view, the modulator structure can be designed [11] to

show a single-mode operation, supporting only the first order modes for eachpolarization, i.e. the TE00 and TM00 modes. For l ¼ 1:55 mm, the propagationlosses of such a structure were evaluated using numerical techniques and measuredby cutback method; resulting attenuation is about 7 dB/cm [9]. Of course, this valueof propagating losses is referred to the off state that is when the optical channel isempty. In the active mode the injected free carriers fill completely the channel,increasing the propagating losses. For a pure absorption modulator, this is the effectwe exploit in order to obtain the modulation, so in this case it is not reasonable to

Fig. 1. Transverse cross section of the BMFET modulator.


speak in term of propagating losses. It is possible to minimize the undesired lossesincreasing using a longer modulator and/or a lower injection level.

3.1. Numerical simulation

The investigation of the electrical characteristics of the modulator, carried out bymeans of the two-dimensional semiconductor device simulation package MEDICI[13], shows a very good behavior in terms of uniformity of the injected plasma, whichfills the optical channel, as reported in Fig. 3. It is possible to find the bias conditionfor a useful injection level that produces a desired complex refractive index variation.The lateral section of the device consists of a pþ2n�2nþ structure between gate andsource transition and the epilayer. That structure acts as a p2i2n diode, where thegate is forward-biased with respect to the source and injects minority carriers (holes)into the lightly doped epilayer. Because of the small transverse dimensions of ourstructure, the minority carriers fill the whole low-doped n� region. Numericalelaboration of the data obtained in this way permits to calculate the correspondingspatial distribution of the refractive index and extinction coefficient into the channeland the overlap integral with the optical field. Afterwards, we get the inducedattenuation, related to the transverse cross section of the modulator by means of aFDM analysis.The second aspect of the discussion on the device operation is the analysis of the

switching characteristics and the advantages resulting from the presence of a thirdterminal. In particular it is convenient to apply a negative voltage to this terminal, in

Fig. 2. SEM photograph of a 500mm long modulator with bondpads.


order to attract the minority carriers (holes) to the epilayer-drain interface. Theconfinement of the free carriers in the lowest part of the optical channel is suitablebecause in this way the optical lateral confinement is guaranteed. On the contrary,pushing the holes away from the drain applying a positive bias, the free carriers fillthe region between the two gate diffusions, thus lowering the refractive index. Thistends to destroy the two-dimensional confinement property, turning the channelwaveguide into a slab waveguide.To sum up, numerical simulations of an idealized modulator indicate that the

proposed device shows an almost uniform refractive index variation over the regionin which the optical mode propagates, and that the device is highly efficient. Thepredicted performances are, for a 1000 mm long amplitude modulator, ME20%; fora switching power of 27.8mW, and a switching time of about 5.6 ns.

3.2. Experimental results

A large variety of BMFET modulators, different in length (500, 1000 and 2000 mm)and channel width (6 and 8 mm) have been realized. In this section we report somepreliminary experimental results obtained on these devices. In particular a 1000 mmlong modulator with an 8 mm wide optical channel has been tested. In following,main electrical and optical measurements are reported.The electrical characterization of the devices has been made in static conditions.

From an inspection of Fig. 4 we see that for different GATE currents we are able toinject free carriers into the SOURCE-DRAIN ohmic channel. This results in thesaturation of the DRAIN current and this tells us that the epilayer is completely

Fig. 3. Injection efficiency of the surface GATE-SOURCE diode.


filled with electron–hole pairs. In this case the electrical device is in the so called‘‘saturation’’ regime while the optical modulator is in the on state; its propagatinglosses have been increased and the light, eventually, has been switched off. Therefore,the modulator, in experimental set up for the measurement of optical modulationdepth, is electrically driven by means of a static biasing of the GATE-SOURCEdiode. For this kind of measurement, since we are going to explore the injectionefficiency of the surface p2i2n diode, only this configuration has to be exploited. InFig. 5, we report the modulation depth percentage versus the electrical power applied

Fig. 4. Electrical output characteristic of a 1000mm long BMFET modulator.

Fig. 5. Modulation depth versus Electrical power.


to the device in a quasi-static situation. It can be seen how with 0.5W of electricalpower it is possible to achieve 20% of modulation depth as correctly foreseen bynumerical simulations. Anyway, the slightly higher of electrical power needed forachieving this modulation depth can be due to the unavoidable presence of contactresistance in the real devices.Then the device has been tested in dynamic conditions. In Fig. 6 we see the GATE

current created by applying a 100 ns long voltage pulse to the GATE-SOURCEp2i2n diode. The switching behavior is typical of these kinds of devices even if itexhibits a very long storage time. This might be caused because the intrinsic epilayeris certainly characterized by a long value of the recombination lifetime. Anyway the20 ns measured value of the rise time is observed. Then the device has been switchedin a three-terminal way, that is the DRAIN-SOURCE channel has been biasedthrough a constant voltage source and a 100O resistor and a current pulse as beenapplied to the GATE. As it is possible to see in Fig. 6, while the GATE currentexhibits the same behavior as before, the DRAIN current mirrors the presence ofinjected free carriers into the channel. In this case a shorter fall-time is observed.

4. The optoelectronic router

In this section, we numerically investigate an optoelectronic Router [14–26] basedon the MMI principle [27] together with the injection-induced phase shift. Therouting switch is realized in an all-silicon rib waveguide, and a schematic view isreported in Fig. 7. As we can see it consists of three different stages: a single-modeinput optical waveguide, a bimodal active region and an output Y branch to separatethe two output channels. The input waveguide in asymmetrically coupled to the

Fig. 6. Dynamic behavior of the BMFET modulator when driven as a three terminal device.


active region. In this way, the fundamental mode coming from the input waveguidecan excite both the fundamental mode and the first higher order mode in activeregion, namely the quasi TE00 and TE01 modes, respectively (treatment would beanalogous for TM modes). These two modes spatially interfere when propagating inthe active region, leading to a localized output power maximum on one side or theother of the channel end-facet depending on their relative phase. Thus, the length ofthe active region is designed to ensure that in OFF state, the field pattern excited inthe active region can emerge on one of the two output sides. On the other hand, inON state, an additional p phase shift, between the two propagating modes in theactive region, is needed to steer the light on the other side of the output. The desiredphase shift is reached by means the change of the refractive index due to the freecarriers [2–6] which are injected and controlled by a BMFET [11]. By doing so, it ispossible to route the input power into one of the ports of the subsequent Y branch bysimply switching the BMFET between the OFF state and the ON state [14].The waveguides of the whole structure are rib waveguides, obtained from an 8 mm

thick epilayer, 1014 cm�3 n doped, grown on a heavily n doped 1018 cm�3 substrate.Before an epilayer growth, an additional implantation of arsenic is required in orderto create a highly doped buffer layer useful for light confinement [9]. In order toobtain the input waveguide an 8 mm wide and 2 mm etched rib is realized. The lateraletching of 2 mm has been chose in order to guarantee the single mode [28]. The lengthof the input waveguide (500 mm) is chosen in order to guarantee an optimumselection of the fundamental mode when excited by the output of an external opticalfiber. Numerical simulations have been carried out in order to evaluate insertionlosses, which result is around 0.3 dB.In the active region, a good confinement of both the fundamental mode (quasi

TE00) and the first higher mode (quasi TE01) has to be guaranteed. On this account,the width of the waveguide is doubled thus obtaining a 16� 8 mm2 rib waveguidewith a 2 mm etched rib. As we can see from Fig. 7, the input waveguide isasymmetrically coupled to the active region. In this way, the fundamental mode

Fig. 7. Schematic view of the optical router.


coming from the input waveguide can deliver enough power at both modes of theactive region.Finally, the output Y-branch is realized by two S-bend waveguides, which are able

to select one of the two output states of the active region. Both the waveguides of thebranch are designed to sustain only the fundamental mode as the input waveguide.Thus, these S-bend waveguides are 8� 8 mm2 rib waveguides with a lateral etching of2 mm, a radius of curvature of about 24 cm, da=0.41 and an offset of 13 mm (seeFig. 7).In order to determine the length of the active region, we have to ensure that in the

OFF state, the field pattern excited coming from input waveguide will emerge on oneof the two output sides. On the other hand, when the device is turned on, anadditional (shift is needed to steer the light on the other side of the output Y-branch.Thus, the following equations system has to be solved:

kDneffoffL ¼ mp;

kDneffOnL ¼ ðmþ 1Þp;ð5Þ

where DneffOFF and DneffON are the difference between effective indexes of the twopropagating modes in the OFF state and in the ON state, respectively. Furthermore,k is the vacuum propagation vector, L is the length of the active region and m is aninteger. The shortest value of L which solves system (5) is

L ¼1

k

pDneffON � DneffOFF

¼1

2

lDnON-OFF

; ð6Þ

having considered that k ¼ 2p=l; where l is the working wavelength of 1.55 mm andDnON-OFF ¼ neffON � neffOFF :As evident from Eq. (6), in order to obtain a shorter device, maximization of the

difference DnON-OFF has to be sought out. A first optimization of the structure can beobtained increasing the difference Dneff between the effective indexes of the twopropagating modes in the active region. In particular, a small tooth realized in thecenter of the rib, carries out this function. In fact, realizing a tooth 8 mm wide and2 mm thick, an increase by 20% of Dneff is obtained. In Fig. 8 the contour plot of thefundamental mode (solid line) and of the first higher mode (dashed line) of the crosssection are plotted. The optical losses for these two modes are estimated to be 2.0and 3.0 dB/cm, respectively.

4.1. Simulation results

To get a significant insight of the behavior of such a device, heavy interactionbetween optical and electrical simulation is required. The two-dimensionalsimulation package MEDICI has been used to predict the injected free carrierdensities in the active region of the device for different driving configuration. On theother hand, the three-dimensional simulation package SELENE-PROMETHEUS[29] has been employed to investigate both optical modes distribution in cross sectionof the waveguide and modes propagation along whole structure. In order tomaximize the overlap integral between electrical carriers an optical field, all the


optical analyses have been performed on the same mesh used by numerical devicesimulator MEDICI. This allows us to correctly take into account the non-uniformrefractive distribution due to doping profiles and injection.In our device an L ¼ 5000 mm and an applied voltage V=1.55V are considered.

Thus, when the gate is forward-biased with respect to the source, minority carriers(holes) have injected into lightly doped epilayer. This fact allows to obtain a uniformcarrier injection profile in the optical channel and consequently to achieve anuniform refractive index change across the waveguiding region both for thefundamental mode and for the first higher mode.Changes in the propagating characteristics due to the forward bias of the gate are

reported in Table 1. In Fig. 9, the spatially interference upon propagation in theactive region is shown in both OFF and ON states. As we can note that, in OFF statethe interference produces an image of the input field mirrored with respect to the

Fig. 8. Cross section of the active region and contour plot of the fundamental mode (solid line) and of the

first higher mode (dashed line).

Table 1

Propagation features of the fundamental mode and the first higher mode both in the OFF state and in ON

state

OFF state (VGS=0V) ON state (VGS=1.55V)

Fundamental mode neff00 ¼ nr � jnj 3.475157�j5.7� 10�6 3.47492�j1.3� 10�5

a ¼ ðnj 4pÞ=l �2.0 dB/cm �4.4 dB/cmFirst higher mode neff00 ¼ nr � jnj 3.474542�j8.5� 10�6 3.47394�j1.7� 10�5

a ¼ ðnj 4pÞ=l �3.0 dB/cm �5.9 dB/cmDneff ¼ neff00�neff01 6.1� 10�4 7.5� 10�4

DnON�OFF ¼ jDneffON ¼ DneffOFF j


Fig. 9. Beam propagation inside the active region in both (a) OFF state and (b) ON state.


plane y ¼ 0; whilst in ON state a direct replica of the input field is obtained. For theproposed configuration, the crosstalk, being the ratio of the desired to unwantedoutputs, can be calculated using the following expression:

crosstalkðdBÞ ¼ 10 log10ðPd=PuÞ;

where Pd and Pu are the power on the desired and on the unwanted output,respectively. So, in the OFF state, where Pd is the power on the left side of the outputY branch (see Fig. 9a), a crosstalk of �11.0 dB is obtained. On the other hand, inON state, where Pd is the power on the right side of the output Y branch (seeFig. 9b), a crosstalk of �9.7 dB is obtained.In Fig. 10 propagation losses along the structure are shown both for the OFF state

(solid line) and for the ON state (dashed line). We note that, the transition betweenthe OFF state and the ON state is characterized by propagation losses of 3 dB.In order to carry out the simulation of the dynamic operation of the proposed

three-terminal device a so-called plasma-drift configuration [30] has been used. Inthis configuration, the driving voltage is applied simultaneously to the gate and tothe drain terminal. By doing so, total charge present in the epilayer is fixed by thegate current, by means the series resistance R; to the amount required to have the pphase shift. On the other hand, the positive voltage pulse applied to the draindepletes the central region of the waveguide, as explained in the previous section forthe optical modulator. In this way, an increment of the phase shift between the twomodes is obtained. Thus, the positive voltage applied to the drain pushes minoritycarriers (holes) away from the epi-drain transition, giving rise to a drift region nearthe drain whose thickness depends upon the drain voltage. This results in a fasterswitching. The switching characteristics of the router have been evaluated using atransient solution. This involved initially applying a zero voltage to the device for a

Fig. 10. Propagation losses both in OFF state (solid line) and in ON state (dashed line).


time of 20 ns. A forward bias step of 5V, has been then applied for 50 ns, with a0.5 ns rise time and a 0.5 ns fall time. During the fall section of the step bias, thevoltage is stepped down to the initial voltage. The value of series resistance (1.67 kO)connected to the gate has been chosen in order to insure the required current densityof 2.1mA/mm. Thus, the steady state induce a p phase shift, between the twopropagating modes in the active region, in order to steer the light from one outputchannel to the other.In Fig. 11 we report the phase shift (solid line) and optical losses (dashed line)

induced by the injected plasma as a function of time when the device is driven by theabove-mentioned 50 ns–5V pulse on both gate and drain. For the proposed devicethe rise and fall times determined are tr ¼ 8:2 ns and tf ¼ 7:2 ns, respectively. Wherethe rise time tr (fall time tf ), is defined, as the time required for the induced phaseshift to change from 10% to 90% (from 90% to 10%) of the maximum value.Another interesting result can be drawn when we look at the optical losses induced

by the presence of the electron–hole plasma. It is clearly seen how the optical lossesincrease much slower when we drive the device in the plasma-drift configuration.This means that it is not necessary to wait the epilayer to be filled in order to get therequired phase shift between the two modes. Moreover, the optical losses can be keptlower by using the device at its speed limit.

5. Conclusions

Silicon-based Optoelectronics is definitely an advantage compared with III–Vtechnology because it can exploit standard technologies to realize both optical andelectrical operation on the same chip. In this paper we have shown how a standardbipolar process can be successfully used for the realization of three terminal devices:

Fig. 11. Phase shift switch in the plasma-drift operation mode (solid line) and optical losses in arbitrary

units (dashed line).


an optical modulator and an optical router. Both devices are embedded in Silicon onSilicon waveguides. The performances of these devices have been demonstrated to beattractive when the ‘‘need for speed’’ is not so urgent or when mass production isrequired.

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three terminals optoelectronics devices integrated into a silicon on silicon waveguide

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