Waveguides and modulators in 3C-SiC Adrian Kewella, Adrian Vonsovici", Graham T. Reeda Alan G.R. EvanSC, aUniversity of Surrey, School ofElectronics, Information Technology and Mathematics, Guildford GU2 5XH, UK 'Bookham Technology Plc, 90 Milton Park, Abingdon, OXON 0X14 4RY, UK,
'tJniversity of Southampton, Department ofElectronics and Computer Science, Highfield, Southampton 5017 1BJ, UK ABSTRACT We have designed and fabricated waveguide optical modulators using cubic silicon carbide-(3C-SiC)-on-insulator rib waveguides. A refractive index change is induced in the rib via the plasma dispersion effect. These types ofdevices have potential for relatively high-speed silicon-based photonics compatible with silicon processing technology, as compared to pure silicon. Furthermore, the wide bandgap (2.2 eV) of 3C-SiC makes the devices suitable for use over the visible and near infrared spectrum range as well as the longer communication wavelengths. We have demonstrated waveguiding in 3C-SiC, fabricating the waveguides by ion implantation ofoxygen into a silicon carbide layer. We have also established a processing recipe for the SiC wafers which enables fabrication of3-dimensional devices. The work reported here describes the fabrication ofthe devices and presents preliminary experimental results for the waveguide losses and the modulation of the refractive index as a function of applied current. An efficient waveguide modulator for a single polarisation is reported.
Keywords: Si-based optoelectronics, silicon on insulator, SiC planar waveguides, SiC rib waveguides,
1. INFRODUCTION In recent years Si-based optoelectronics has received significant attention due to potential high performance at moderate cost, notably for applications such as fiber-to-the-home (FTFH) and Dense Wavelength Division Multiplexing (DWDM). Since the linear electro-optic effect (Pockels effect) is not present in crystalline silicon due to the centrosymmetric crystal structure, active devices such as modulators and switches must be designed using the free carrier dispersion effect [1]. Furthermore, the transparent wavelength range of Si is limited to the region of above 1.2 .tm, and therefore applications in the visible wavelength range are excluded. There is a great demand for photonic integrated devices compatible with silicon technology and the use of silicon based alloys (SiGe, SiGeC, SiC) offer a solution for some applications. SiC has been considered for high-power and high-temperature devices because of their attractive electronic properties. The cubic polytype (3C or ) of the silicon carbide (SiC) has a wide bandgap (2.2eV), transparent in the range O.54-2im and therefore suitable for waveguiding over the visible and near-infrared wavelength range as well as the longer communication wavelengths. SiC is a crystal with a zincblende structure and possesses a 43m point-group lattice structure which is noncentrosymmetric. Hence the electro-optic effect is observable, and the electro-optic coefficient, is more than 70% higher than that of GaAs [2]. Optical devices fabricated from n-SiC can be operated in the visible range, offering much greater scope for a variety of applications including shorter wavelength interferometers with better resolution, visible signal routing for optical data storage, and high temperature sensing. Other important characteristics including high mechanical strength, chemical inertness and excellent thermal conductivity make. However, in this work we have considered the relatively simple modulation technique of carrier injection, similar to that utilised in pure silicon. 3-SiC waveguides, formed by attaching a SiC film to a sapphire substrate, were experimentally studied by Tang et a!. [3]. Prucnal and Liu [4] proposed planar SiC waveguides on Si02, in structures similar to silicon-on-insulator ones. Recently, waveguiding in SiC-on insulator (SICOI) and SiC-on 501 (SICSOI) was thoroughly investigated in planar waveguides [5]. The SiCOI waveguides were made using a process similar to the SIMOX fabrication method using a one step high-energy implantation of oxygen. The losses were reasonably low for both SICOI and SICSOI materials (8.8-9dB/cm).
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[email protected] -Fax 44-(O)1483-534 139. Silicon-based and Hybrid Optoelectronics Ill, David J. Robbins, John A. Trezza, Ghassan E. Jabbour, 54
Editors, Proceedings of SPIE Vol. 4293 (2001) 2001 SPIE . 0277-786X101/$1 5.00
In this paper we investigate both planar and rib-waveguides made using SiCOI, as these have the best potential for optical modulation. Measurements have been made at wavelengths of I .3 and I .55pm. Following on work of reference [5], we have improved the fabrication of the SICOI material by using a two-step high-energy implantation of oxygen. A thicker buried layer was obtained reducing the loss by leakage toward the substrate for the TE polarization where the previous sIcol material had losses of approximately 26dB/cm at I .55gm.
Reactive Ion Etching method (RIE) was used for the fabrication of rib waveguides which is fully compatible with silicon processing technology, offering the opportunity to develop high-speed, low cost photonic circuits. Carrier injection into a SiC rib waveguide was achieved by making ohmic contacts either side ofthe rib structure.
2. FABRICATION OF PLANAR WAVEGUIDES A n-SiC epilayer (-2m thick), grown using CVD by Cree Research Inc. (USA) on a Si substrate, has been used as a substrate for fabricating the SICOI waveguides. The SiC layer has a high-resistivity (3O}cm) and therefore little freecarrier absorption is expected from it. The buried oxide layer was produced by a two-step high-energy ion implantation of oxygen. High-energy (1 .6 and 2MeV) oxygen ions were implanted to a dose of 1O'8cnf2. This gives a predicted range of approximately I . 14 and 1.3prn respectively and an Rstraggje of about 85 and 95nm respectively (using the ion implantation simulation software TRIM [6]). The superposition of the two implantation profiles gives a range of about 1 .23im and a Rstraggie of about 175nm. Therefore we expect a guiding SiC layer of approximately I .O5im and a buried layer of approximately O.35im. During the implantation the sample was maintained at 6OO°C by using a heated sample stage to minimize the implantation damage [7]. A full-analysis of the buried layer formed in a similar process but using lower implantation energy (200keV), was made by XTEM and RBS. While the Si concentration at the 0-rich layer is reduced leading to a silicon dioxide layer, the C is practically all ejected from this layer. The RBS results indicate a predominantly Siox (x2.04) layer with a thickness equal to approximately 2Rsragge. The Si02/bulk-SiC interface is sharper than the surface-SiC/Si02 one. Furthermore, channeling results show that crystalline quality of the SiC surface layer is good with minimum yield Xn141 The dual implant process has been used to reduce leakage loss from the waveguides reported in our previous work [5]. y
1Y air
L_.____.1
n-SiC t=1 O5m n=2.57
Si02 t0.35Rm n145 a-SiC t=O.6im n=2.57
Si n3.505
_______ _____ n
Figure 1 . The schematic cross-sectional view ofthe SiCO! waveguide formed by ion implantation. The associated refractive
index profile is also shown.
3. WAVEGUIDE LOSS MEASUREMENTS Waveguide insertion loss measurements for planar and rib waveguides were carried out at 1 .3 and l.55tm for both the TE and TM polarizations. For these experiments, the SiC waveguides were cleaved and then polished at the ends to form the waveguide facets. Insertion loss results from coupling loss due to the mismatch between the field profile of the input beam and guided mode, propagation loss of the guided mode and Fresnel reflection loss at both endfaces. Assuming a normally incident beam the total Fresnel reflection loss was estimated to be 1.9dB. The experimental setup used for insertion loss measurements is shown in figure 2. The first light source is a pigtailed DFB laser diode emitting at 1.3 tm with an output power of 2mW, and the second is a pigtailed FP laser diode operating at 1 .S5pni with a nominal power of 1.5mW at the output of the fibre. An in-line fibre polarization controller is used for adjusting for both TE and TM guided mode excitation. The fibre output is collimated and focused onto the waveguide input using two objective lenses. The advantage of this setup is the ability to change the polarization without changing the Proc. SPIE Vol. 4293
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mechanical positioning at the input ofthe waveguide. The output of the waveguide was imaged using an objective lens onto an infrared Ge detector. Using a beamsplitter and an infrared video camera positioned at 9Ø0to the optical axis the image of the waveguide could be observed to optimise waveguide coupling. Both the input and the output objective lenses were mounted on precision positioning stages that allow precise control of the alignment with the waveguide input-output. The waveguide mount is fixed on top of a precision 4-axis waveguide manipulator that allows supplementary flexure rotations for optimum tilting and alignment ofthe waveguide with the exciting laser beam. Three different lengths ofwaveguides were used for the loss measurement. This is known generically as the cutback method
[8]. As the net propagation loss varies exponentially with the length of the waveguide, using at least three different waveguide lengths we could eliminate coupling losses and Fresnel losses (assumed to be the same for all waveguides) from the total insertion loss measured. The averaged error was estimated to be Since there may be the possibility of inconsistent coupling to multimode waveguides, the experiments were repeated several times for each set of waveguides, and the mean of the results taken, and the spread giving an indication of the uncertainty associated with the measurements. It should be noted that due to the leaky nature ofthe SICOI planar waveguides, modes of higher order than the fundamental will have leakage loss greater than 50dB/cm (e.g TE 1 order mode 60dB/cm at 1.3im). This is a very different situation from that of mode excitation in truly multimode waveguides with no leakage. The higher order leaky-modes are "stripped" from the waveguide after a short distance ('-2-3mm) so that the measured value is related to the fundamental mode ofthe waveguides, although any light initially coupled into the higher order modes will contribute to the overall loss.
Ge
Polarisation controller
1.3tm and 1.55im pigtailed laser diodes
BS
63X objective lens
WG
63Xobjective lens
I
detector
\
Figure 2. Schematic ofthe experimental setup.
4. RESULTS AND DISCUSSIONS 4.1 Loss mechanisms in SiCOI planar waveguides. Losses of SICOI planar waveguides can originate from three main causes. - Leakage into the substrate for waveguides fabricated on a high-refractive index substrate (leaky-waveguides).
-Interface scattering losses resulting from imperfections of interfaces in the waveguide geometry after the fabrication process.
-Bulk material losses, due to imperfections in the refractive index of the guiding layer material and to absorption of freecarriers. The residual absorption of the SiC has been neglected, as it has been shown that a-SiC is highly transparent in the 0.54-2 un range [9], [10]. These loss mechanisms have been reviewed previously for SiC waveguides in [5].
The results obtained for SICOI waveguide propagation loss (ae,,) for TE and TM polarizations at 1.3 and 1.55pm are summarized in Table I, together with predicted leakage loss (a1J, the latter being discussed further below. Whilst these losses are not low when compared to silica waveguides ('-0.0 1dB/cm) or silicon waveguides('-0.5dB/cm), they are the lowest losses yet reported for silicon carbide thin film waveguides, and the lowest of these quoted losses (6.9dB/cm at l.3tm TM polarization) is tolerable for active devices 2-3mm long. 56
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The degree of leakage loss is determined by the buried oxide thickness as the light propagating in such a waveguide could escape into the substrate by an effect analogous to the tunnel effect in quantum mechanics. For the SICOI planar waveguide, fabricated in a 2pm SiC layer on a silicon substrate, using a two-step implant, we expect a waveguide with about 1.O5pm of SiC, separated by 0.35 pm of buried oxide from a remaining 0.6 pm SiC layer and the underlying substrate (see figure 1). In figure 3 we have shown the predicted losses by leakage into the substrate as a function of the buried oxide thickness, for TE0 and TM0 modes, at 1.3pm and 1.55pm respectively [11]. A 0.35pm buried oxide layer is sufficiently thick in order to ensure less than 0.7dB/cm leakage loss at l.3pm for a 1.05pm SiC layer. A loss of approximately 4dB/cm is predicted for TE polarisation and 2dB/cm for TM polarisation at 1.55pm. —TE 1.3 TMI.3 —A—TE 1.55 -TM 1.55 15
... 5
0.25
0275
0.3
0.325
0.375
0.35
0.4
Figure 3. Predicted leakage into the substrate loss at 1.3 and 1.55pm for TE,TM polarisations.
The remaining loss can be attributed to bulk and interface scattering [5]. However, the important conclusion is that by using additional ion implantation the buried oxide layer can be thickened, thus reducing the loss due to leakage. A significant reduction of the leakage loss is observed compared to the one-step implanted SICOI waveguides reported previously [5], which had leakage losses greater than 5dB/cm for a TE polarization and about 2dB/cm for a TM polarization at 1.3 p.m. Total waveguide losses of less than 10dB/cm were measured at 1.55pm, a significant improvement compared to the onestep implanted SICOI waveguides that were very lossy (15-27dB/cm) due to the leakage. The losses due to propagation (leakage) have been reduced to less than 4dB/cm at 1 .55p.m before any additional scattering loss is considered. Furthermore, it is reasonable to expect that losses of the order of 6dB/cm can be obtained at 1.3 and 1.55 p.m by increasing the thickness of the buried oxide layer. Table I. Experimental total loss for SICOI waveguides and the estimated loss by leakage toward the substrate.
a0(dB/cm)
a1(dB/cm)
TE
7.4
0.7
TM
6.9
0.3
TE
9.8
4.0
TM
7.5
2.0
Wavelength(p.m)/Polarization 1.3
1.55
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4.2 Rib waveguides and optical modulators. Rib waveguides were fabricated based on the SiCOI planar waveguides described above. Standard silicon processing technology was used to fabricate the rib waveguides. Firstly a thick PECVD oxide layer (-4pin) was deposited on top of the substrates, and standard lithography was used to pattern parallel straight rib waveguides. The patterns were etched in the oxide using a CHF3+(2%)Ar RIE. The oxide mask was needed as a supplementary mask for the next step of dryetching of the SiC using a mixture of CHF3+33%02 [12]. The etching of the SiC has a very slow rate (152Onm/min) but excellent anisotropy. Therefore, smooth and almost vertical (
