IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 4, NO. 6, NOVEMBER/DECEMBER 1998 997

Amorphous Silicon-Based Guided-WavePassive and Active Devices for Silicon

Integrated OptoelectronicsGiuseppe Cocorullo,Member, IEEE, Francesco G. Della Corte,Member, IEEE,

R. De Rosa, Ivo Rendina, A. Rubino, and E. Terzini

Abstract— Waveguides and interferometric light amplitudemodulators for application at the 1.3- and 1.55-�m fibercommunication wavelengths have been fabricated with thin-filmhydrogenated amorphous silicon and its related alloys. Thetechnique adopted for the thin-film growth is the plasma-enhanced chemical vapor deposition, which has been shown togive the lowest defect concentration in the film. Consequentlythe proposed waveguiding structures take advantage of the lowoptical absorption shown by a-Si:H at photon energies belowthe energy gap. In addition a good radiation confinement can beobtained thanks to the bandgap tailoring opportunity offered bythis simple and inexpensive technology.

In particular rib waveguides, based on a a-SiC:H/a-Si:H stack,have been realized on crystal silicon, showing propagation lossesas low as 0.7 dB/cm. The same structure has been utilized forthe fabrication of thermooptic Fabry-Perot modulators withswitching times of 10�s. Modulators based on the alternativewaveguiding configuration ZnO/a-Si:H, giving comparableresults, are also presented.

Index Terms—Amorphous silicon, integrated optics, modula-tors, silicon optoelectronics, thermooptic effect, waveguides.

I. INTRODUCTION

CRYSTALLINE SILICON (c-Si) promises to acquire animportant role in future low-cost optoelectronic technol-

ogy [1]. The potential advantages offered by this material arewell known. Besides the aspect of the continuous technologicaldevelopments carried by VLSI manufacturers, some interestingoptical characteristics of Si are in fact clearly outstanding.In particular the low absorption at photon energies belowbandgap, the plasma dispersion effect, the thermooptic effect,and the capability to electroluminesce when hosting Erions,have all been exploited to fabricate passive (waveguides) andactive (photodetectors, modulators and LED’s) optoelectronicdevices for IR fiber-optic communication purposes. In contrast,

Manuscript received May 5, 1998; revised August 8, 1998. This work wassupported by Regione Campania, Assessorato alla Ricerca Scientifica underGrant L.R. 41/94 and by the C.N.R. under Grant P.F. MADESS II (Sensorsand Microsystems Sub-Project).

G. Cocorullo is with the Istituto di Ricerca per l’Elettromagnetismo ed iComponenti Elettronici—CNR, I-80124 Naples, Italy. He is also with DEIS,Universita della Calabria, I-87036 Rende, Cosenza, Italy.

F. G. Dalla Corte and I. Rendina are with the Istituto di Ricerca perl’Elettromagnetismo ed i Componenti Elettronici—CNR, I-80124 Naples,Italy.

R. De Rosa, A. Rubino, and E. Terzini are with ENEA—Centro Ricerche,I-80055 Portici, Naples, Italy.

Publisher Item Identifier S 1077-260X(98)08615-8.

very little has been done to assess the potentiality of hydro-genated amorphous silicon (a-Si:H) in this field. Since thediscovery of the possibility of inducing the modification of theFermi level by doping [2], this semiconductor has been con-sidered almost exclusively for low-cost wide-area photovoltaicapplications. This has induced a huge number of scientiststo concentrate merely on the study of the optoelectronicproperties of a-Si in the visible portion of the spectrum, whereit exhibits a good quantum efficiency. Comparatively very littlehas been done to assess its behavior outside this range, andin particular at the IR wavelengths of interest in the fiber-optic communication area. Recently, however, a new interestfor a-Si as an optoelectronic material has to be recognized.For instance, light emitting diodes [3], [4], photodetectors [5],[6], and optocouplers [7] have been fabricated making useof technologies compatible with the standard microelectronicprocesses, thus offering new opportunities for the integrationof optoelectronic tasks on a silicon chip.

In this paper, we present recent results concerning therealization and characterization of some a-Si:H based, guidedwave, optoelectronic devices designed for use at the fiber opticcommunication wavelengths of 1.3 and 1.55m.

II. A MORPHOUSSILICON FOR OPTOELECTRONICS:TECHNOLOGY AND CHARACTERISTICS

Thin films of a-Si can be deposited either by physicaldeposition techniques, like sputtering or evaporation, or bychemical vapor deposition (CVD) techniques. In both casesthe process takes place at low temperatures, usually below300 C, and therefore layers of this material can be virtuallydeposited on any substrate. However, the electronic and opticalproperties of the deposited films are deeply influenced by thekind of technology adopted, and on the particular processparameters. Sputtered a-Si, for instance, is characterized by ahigh density of states in the forbidden band, and therefore theintroduction of doping atoms is usually ineffective in movingthe Fermi level toward the valence or conduction band. Itsphoton absorption tail extends well beyond the energy gap, sothat optical absorption coefficients in excess of 1000 cmaretypical at 1.3 eV [8]. As a consequence this semiconductor hasbeen never considered for waveguiding purposes.

Remarkably, lower absorption coefficients, especially at theinfrared wavelengths in the range of 1.3 and 1.55m, are

1077–260X/98$10.00 1998 IEEE

998 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 4, NO. 6, NOVEMBER/DECEMBER 1998

Fig. 1. Modification of the optical energy gap of the a-SiC:H alloy as afunction of the CH4 flowed into the reactor during the plasma deposition.Eg04 is defined as the energy corresponding to an optical absorption of 104

cm�1.

generally shown by silicon thin films deposited by plasma-enhanced CVD (PECVD). In this case, the chemical reactionthat forms the silicon layer is sustained by a plasma dischargeassisted decomposition of the main process gas molecules,usually SiH . This technique allows the incorporation ofhydrogen atoms in the film, which saturate the silicon danglingbonds and clean the semiconductor forbidden band from aconspicuous amount of undesired states. The resulting a-Si:Halloy can easily show energy gaps spanning from 1.6 to 1.9 eV,depending on the H atom concentration, and therefore on thedegree of saturation of the silicon bonds [9]. The effectivenessof the hydrogen atoms in saturating the dangling bonds in a-Si:H depends on the deposition temperature, as we showed in[10].

Another interesting opportunity offered by PECVD is thatby mixing the main process gas, i.e., SiH, with other com-ponents like CH, CO or NO during the plasma reaction,it is possible to obtain semiconductor alloys with widerenergy gaps, while the opposite change is achieved by addingGeH . This makes bandgap tailoring at hand with this simpletechnology. In Fig. 1 the value of the energy gap , definedas the photon energy corresponding to an absorption coefficient

of 10 cm , has been measured for a-SiC:H as a functionof the CH gas percentage in the CH-SiH deposition gasmixture. The other deposition process parameters are the samewe reported in [10]. Variation of from 1.87 eV forpure a-Si:H to about 2.3 eV for 80% of CHare obtained,corresponding to refractive index values spanning from about3.4 to 2.9.

The absorption spectra of pure a-Si:H and of a-SiC:H,deposited in our PECVD system, are plotted in Fig. 2. Thelatter film has been obtained for 70% of CHin the CH -SiH deposition gas. The measurements have been carriedout by optical transmittance-reflectance experiments at higherenergies and by photothermal deflection spectroscopy [11] inthe lower energy range. At the photon energy of 0.95 eV,corresponding to the wavelength of 1.3m, the absorptioncoefficient of a-Si:H is about 0.1 cm, a value for which asemiconductor can be considered transparent and useful forintegrated optics applications [1]. This sets a lower limit ofapproximately 0.4 dB/cm at 1.3m for a waveguide realizedwith this material, if no other loss mechanisms are acting. The

Fig. 2. Absorption spectra of the undoped a-Si:H and a-SiC:H films de-posited in our plasma reactor. Undoped crystal silicon absorption spectrum isshown for comparison.

Fig. 3. Schematic of the fabricated a-Si:H/a-SiC:H rib waveguide.

absorption becomes even lower at higher wavelengths (i.e.,m). At these wavelengths a-SiC:H is characterized

by higher values of , but never exceeding 1 cm. In thesame figure the absorption spectrum of intrinsic crystallinesilicon (c-Si) is reported for comparison.

III. W AVEGUIDES AT 1.3 AND 1.55 m

The previously discussed characteristics of PECVD hydro-genated amorphous silicon-based alloys have been exploitedfor the realization of low-loss channel waveguides at

and m. The waveguides consist of an a-Si:H/a-SiC:H heterostructure deposited on a c-Si wafer. The lowdeposition temperature of the amorphous films ensures the fullcompatibility with the standard microelectronic processes.

A schematic of the realized rib waveguides is shown inFig. 3. The a-SiC:H undercladding layer has a thickness of 0.4

m, while the a-Si:H core is 3m thick. The overcladding isair. The fabrication involved the following steps. First, thenative oxide layer was removed from a heavily Sbdoped silicon wafer by a short dip in oxide-etch solution.The wafer was then loaded into the deposition system, wherea final vacuum of 10 torr was reached before the plasmaprocess could start. The a-SiC:H undercladding was formed bythe RF assisted decomposition of SiHand CH , which wereintroduced into the chamber at flow rates of 20 sccm (standardcubic centimeters per minute) and 47 sccm respectively. Dur-ing this process the substrate temperature was held at 180C.To form the a-Si:H core layer, only 42 sccm of SiHwereflowed, while the substrate temperature was risen to 220C.The process pressure was 700 mtorr in both cases, whilethe 13.56-MHz RF power was 17 mW/cmand 23 mW/cmrespectively. More details about the deposition technique canbe found in [10].

After photolithographic patterning of the planar amorphousstack, the rib waveguides were defined by plasma etching,

COCORULLO et al.: AMORPHOUS SILICON-BASED GUIDED-WAVE PASSIVE AND ACTIVE DEVICES 999

TABLE IMEASURED PROPAGATION LOSSES INa-Si:H/a-SiC:H RIB WAVEGUIDES

using photoresist as an etch mask. The process gas mixturewas 8% O in CF , flowed at 30 sccm at a pressure of 0.3mtorr, and the RF power density was 98 W/cm. During theetch, the substrate was held at room temperature. The final ribheight was 1.2 m.

A set of waveguides with widths of 15, 12, 10, 8, and 6mhave been examined to evaluate their respective propagationlosses. Due to the comparable etching rates of a-Si:H andphotoresist, and to some kind of under-etching, which ispresently under study, the rib definition process revealed un-reliable, and structures narrower than 6m showed, therefore,frequent interruptions. For this reason, they have not beentaken into account. As a consequence, all the waveguidesunder test are multimode. However, in accordance with theconsideration reported in [12], numerical simulations show thatthe fabrication of single-mode waveguides with our presenttechnology is possible, but requires in general smaller ribheights, to be calculated separately for each waveguide width.

Waveguides of various lengths were obtained by cleavageof the crystalline substrate. The radiation of a 1-mW 1.3-mlaser diode, pigtailed to a 5-m-core monomode fiber, was buttcoupled to each waveguide for testing. The transmitted lightwas detected at the output by means of an InGaAs photodiode.The same measurements were also carried out at a wavelengthof 1.55 m.

The propagation losses have been estimated with the cut-back technique, from a set of four points at least for each singlestructure. The measured losses at m are reported inTable I. The technique is affected by errors coming from thefluctuation of the output signal, which in turn depends on thedegree of success in the fiber-to-waveguide coupling procedureat each point. This fluctuation was limited to less then 20%in all of our measurements. This value was used to estimatethe errors reported in the same table. The largest waveguides(15 m) show the best performance (0.7 dB/cm). Lossesincrease rapidly for narrower ribs, reaching an average of 10dB/cm for the 6- m-wide waveguides. Measurements made at

m gave the same results within the experimentalerror. For all of our geometries, numerical simulations basedon the effective index method predict losses between 0.4 and1 dB/cm for the first guided modes. These values, which are inagreement with the theoretical attenuation due to the intrinsicabsorption of the core material, well fit the experimentalvalues measured in 15-m-wide waveguides. The divergencebetween experimental and theoretical data for narrower ribsfinds explanation mainly in the unevenness of the waveguides,which clearly affects more the narrower devices.

Another loss mechanism concerns the coupling of a radi-ation fraction into the heavily doped substrate. This tail in-creases for smaller cross-section area waveguides, and, there-fore, for narrower ribs [13] where it determines higher losses.

Although we believe that this effect has only a minor influ-ence on our structures, it could be circumvented by slightlyincreasing the thickness of the low refractive index a-SiC:Hundercladding.

IV. a-Si:H-BASED MODULATOR STRUCTURES

AND FABRICATION

Light switching systems are required for the construction ofoptical communication links in local area networks (LAN’s)and also in photonic intermodule connections. In those ap-plications where high bit rates are not required, such as infiber-to-the-home networks and automotive products, the useof robust and low-cost optical components, compatible withthe present microelectronic technology, is greatly preferred tothat of the high-performance high-cost III–V optoelectronicdevices. For this reason, in the last few years, an increasinginterest has been devoted to the fabrication of all-silicon lightswitches or modulators.

Among the various techniques explored to realize activedevices, those based on interference principles have beenshown to be more effective [14]. In particular, the thermoopticeffect (TOE) has been exploited to fabricate c-Si based lightmodulators. The first prototype was developed by Treyz in1991 [15]. The device, a Mach–Zehnder guided-wave modula-tor realized in silicon-on-insulator (SOI) technology, exhibiteda bandwidth of a few tens of kilohertz when heated bymeans of an electrical power dissipating in a resistive layercovering one of the two arms of the interferometric structure.An analogous modulator, but exploiting a guided-wave filmstructure in GeSi on Si, was proposed in 1992 and showedbandwidths up to about 90 kHz [16]. An optimization, mainlyof the waveguiding characteristics, of the Treyz’s device wasthen proposed by Fisheret al. [17]. They in particular reportedswitching times of about 5 s in a large cross-section SOI ribguided-wave single-mode structure.

Recently, we have reported the encouraging results ofa micromachined all-crystal-silicon Fabry–Perot thermoopticmodulators that extend the capability of thermally controlledswitches at bandwidths beyond 1 MHz [18]. Unfortunatelythese devices, based on a silicon-on-silicon waveguiding struc-ture, showed high insertion losses, due to the poor confinementof the radiation obtained by doping the cladding layers.

In order to overcome this problem, the superior opticalcharacteristics of the bandgap engineered a-Si:H based waveg-uiding structures have been exploited for the fabrication oftwo optical modulators. The device operation is still basedon the TOE. Our hydrogenated amorphous silicon has beenpreviously characterized for this effect, and a thermoopticcoefficient K at m hasbeen measured [10]. This value is comparable to that reportedfor c-Si, and is much higher than for other thermoopticmaterials, like LiNbO (5.3 10 K ) or soda-lime glasses(1 1.5 10 K ).

The modulators consist again of Fabry–Perot interferom-eters. They are based on the rib-like structures sketched inFig. 4(a) and (b). Modulation is achieved by a phase shift inthe cavity induced by TOE. The structure heating is obtained

1000 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 4, NO. 6, NOVEMBER/DECEMBER 1998

(a)

(b)

Fig. 4. Schematic cross section of the two realized a-Si:H-based guided waveinterferometric Fabry–Perot modulators.

by applying a current pulse to the resistive tungsten film layingon top of the rib. The light is guided through the 3-m-thick a-Si:H layer. In the modulator type A, a 400-nm-thicklower refractive index a-SiC:H buffer layer effectively screensthe radiation from the highly light absorbing, n-doped, c-Sisubstrate. The deposition of the a-SiC:H cladding and thea-Si:H follows the process reported in [10]. For the devicecharacterization we refer to [19].

Instead of a-SiC:H, the modulator type B uses a ZnO film,deposited by sputtering, as undercladding. Details about thistechnology can be found in [20]. This semiconductor oxideshows a refractive index close to 2.0, which has allowed toreduce the thickness of the undercladding down to 100 nmwithout significantly affecting the waveguiding characteristicsof the modulator. This structure also presents a 100-nm-thickZnO cladding film deposited on top of the guiding a-Si:H layer.

A 100-nm-thick tungsten layer was then deposited by-beam evaporation over both the stacked structures. This

film was patterned by photolithography to define theresistive heater. In particular, a selective etch based onKH PO /KOH/K Fe(CN) (0.25/0.24/0.1 M) was used fortungsten.

The rib of device A was defined by a partial etch of thea-Si:H layer. This was accomplished in a plasma etchingreactor with the same technique described for waveguides inthe previous section. To obtain the rib of structure B, the etchof the ZnO top layer was performed in water diluted HCl. Thelast step was the evaporation and definition of the 1-m-thickAl bond pads. The SEM picture of a 30- and a 40-m-widemodulators of the type B is shown in Fig. 5. In this case thetungsten film between the pads has a resistance of600 .

The Fabry–Perot interferometric modulators were obtainedby substrate cleaving. In order to perform the modulation

Fig. 5. SEM micrograph of a portion of a chip containing an array ofmodulators. The square regions are the aluminum bond pads necessary tocontact the heating tungsten film on top of the waveguide.

Fig. 6. Modification of the modulation depth in 30-�m-wide 2-mm-longmodulators of type A (a-Si:H/a-SiC:H) and B (ZnO/a-Si:H) as a functionof the energy delivered by the current pulses applied to the resistive tungstenfilm.

tests, the dies were bonded onto TO39 metal cases, andthe integrated devices were used as reflected light intensitymodulators.

The m radiation of a DFB fiber pigtailed laserdiode was butt-coupled to the first input arm of a3 dB Ybranched, single-mode fiber coupler. The single output of thefiber coupler was applied to the modulator, while the secondinput arm was used to monitor the intensity of the reflectedradiation. The DFB laser module included an optical isolatorto avoid stray coupling with the external Fabry-Perot cavity.

The optical modulation depth (1) versus the drivingelectrical pulse energy is reported in Fig. 6 for two 30-

m-wide 2-mm-long devices of type A and B. This plotdemonstrates that device type B in general requires a higherenergy to show the same modulation depth of device A.In fact, due to the 1.2-m-thick rib, the active volume ofdevice A is smaller, and therefore requires less heat to reach agiven temperature. The maximum modulation depthof about 60%, corresponding to a cavity phase shift, hasbeen observed for gate energies of 500 nJ. The idealpredicted by theory for these structures, by assuming a 0.3reflectivity at the etalon mirrors (viz., that one of an ideal Si-airinterface), is about 70%. The agreement between experimental

1The modulation depthM is defined as(Imax�Imin)=Imax, whereImax

andImin are the maximum respectively the minimum reflected light intensity.

COCORULLO et al.: AMORPHOUS SILICON-BASED GUIDED-WAVE PASSIVE AND ACTIVE DEVICES 1001

Fig. 7. ZnO/a-Si:H device response to a 500-ns-long 300-nJ-energy currentpulse. The switchoff time is about 10�s.

and theoretical values indicates the good optical quality ofthe Fabry–Perot end-facets obtained by substrate cleaving.It should be pointed out that the modulation depth may bedegraded in a multimode cavity by the superposition of asmany Airy’s functions as the number of the propagating modes[21]. All of these modes will be characterized by slightlydifferent effective refractive indexes, and will therefore resultout of phase each other. Although this effect could even leadto a severe distortion of the modulation pattern, with reductionof the modulation depth, there was no dramatic evidence of itduring the measurements. This problem, however, can be fullyovercome by the adoption of single-mode structures.

The device response to a 500-ns-long 300-nJ-energy elec-trical driving pulse is reported in Fig. 7. A switchoff timeof about 10 s is measured. This value is two order ofmagnitude shorter than that obtainable in silica thermallycontrolled switches, and comparable to that reported in muchmore expensive epitaxial SOI and SiGe–Si based structures.Moreover, it is worthwhile pointing out that the device speedcan be easily improved by reducing its transverse dimensions.In fact, the large cross section area of the waveguide (303

m ) is still far from the cut-off limit.

V. CONCLUSION

The simple and inexpensive technology of amorphous sil-icon and related alloys has been utilized to fabricate guidedwave passive and active devices for the IR communicationwavelengths of 1.3 and 1.55m. In particular, we presenteda set of waveguides showing propagation losses as low as0.7 dB/cm, and two interferometric thermooptical modulatorsrealized with similar technologies. Thanks to their smallvolume, the modulators have shown fast thermal transients,and maximum switching times of the order of 10s havebeen measured.

The low temperature (200 C) required for the depositionof the amorphous films makes this technology suitable for theintegration of optoelectronic functions on VLSI chips realizedwith standard microelectronic techniques.

ACKNOWLEDGMENT

The authors wish to thank Dr. A. Antonaia for the ZnOfilm deposition.

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[5] J. Ho, Y. K. Fang, K. Wu, and C. S. Tsai, “High-gainp-i-n infraredphotosensors with Bragg reflectors on amorphous silicon-germaniumalloy,” Appl. Phys. Lett., vol. 70, no. 7, pp. 826–828, 1997.

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[15] G. V. Treyz, “Silicon Mach-Zehender waveguide interferometers oper-ating at 1.3�m,” Electron. Lett., vol. 27, no. 2, pp. 118–120, 1991.

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[18] G. Cocorullo, M. Iodice, I. Rendina, and P. M. Sarro, “All-siliconthermo-optic micro-modulator,” inProc. 25th Eur. Solid State DeviceRes. Conf., H. C. de Graaff and H. van Kranenburg, Eds., The Hague,The Netherlands, 1995, pp. 651–654.

[19] G. Cocorullo, F. G. Della Corte, R. De. Rosa, I. Rendina, A. Rubino, andE. Terzini, “Amorphous silicon based waveguides and light modulatorsfor silicon low-cost photonic integrated circuits,” inProc. MRS FallMeet., Boston, MA, 1997, vol. 486, pp. 113–117, paper no. H 10.3.

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1002 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 4, NO. 6, NOVEMBER/DECEMBER 1998

Giuseppe Cocorullo(M’93), for a biography, see this issue, p. 988.

Francesco G. Della Corte(M’98), for a biography, see this issue, p. 988.

R. De Rosais a Physicist at working at ENEA as a Researcher since 1987. Heis currently the Head of the Amorhous Silicon Section at the ENEA CentroRecerche Portici. His research emphasis in amorphous silicon (a-Si) thin filmranges from the material deposition and study to the device applications. Inthe area of photovoltaic (PV) devices he achieved, in 1988, the first Italian p-i-n junction having an efficiency conversion of 10%. In 1995, he managed theactivity of device scale-up to large area that led to the European efficiencyrecord of 9.1% for a-Si tandem junction on 900 cm2 area. He is activelyinvestigating the extensive exploitation of a-Si material for optoelectronicdevices realization.

Ivo Rendina, for a biography, see this issue, p. 989.

A. Rubino received the Doctor degree in physics from the University“Federico II,” Naples, Italy.

In 1988, he won a fellowship of Ansaldo S.p.A. and was involved in thestudy of diffusion of gallium and aluminum in silicon. From 1990, he gained atwo-year fellowship at ENEA Centro Recerche Portici on electrical and opticalcharacterization of a-Si:H alloys. Since 1992, he has had a permanent positionin the same Institute. Since 1994, he has been in charge of the R&D of small-area a-Si:H photovoltaic devices. In 1988, he became a Project Manager ofa-Si-based optoelectronic devices at ENEA-CR Portici.

E. Terzini received the Doctor degree in physics and the Ph.D. degree insolid state physics from the University “Federico II,” Naples, Italy, in 1992and 1996, respectively.

Since 1994, he has had a permanent position as a Researcher at ENEACentro Recerche Portici. He was involved in research and development ofamorphous silicon-based (a-Si) optoelectronic devices. Since 1996, his mainresearch interest has been in the field of a-Si/c-Si heterojunctions for large-areaphotovoltaic applications.