Organic electro-optic modulator using transparent conducting oxides as electrodes

A novel organic electro-optic (EO) modulator using transparent conducting oxides (ZnO and In2O3) as electrodes is demonstrated for the first time. The modulator employs the poled guest-host chromophore/polymer material AJL8/APC having r33=35pm/V and is able to achieve a low Vπ=2.8 V switching voltage for an 8mm-long device at a wavelength of 1.31μm. This corresponds to a Vπ=1.1 V switching voltage for a 1cm-long device in a push-pull configuration. The push-pull VπL figure of merit normalized for the EO coefficient is thus 1.1V-cm/(35 pm/V), which is 3-4x lower than that can be achieved with a convetional modulator structure. The bottom electrode is ZnO grown by Metal-Organic Vapor Deposition (MOCVD) and top electrode In2O3 deposited using ionbeam assisted deposition. The top electrode is directly deposited on the guest-host organic material. The design and fabrication considerations for the modulator are also discussed. ©2005 Optical Society of America OCIS codes: (190.4710) Optical nonlinearities in organic materials; (250.2080) Electro-optic polymers; (250.7360) Waveguide modulators. References and links 1. N. Dagli, “Wide-Bandwidth Lasers and Modulators for RF Photonics,” IEEE Trans. Microwave Theory Tech. 47, 1151-1171 (1999). 2. L. R. Dalton, W. H. Steier, B. H. Robinson, C. Zhang, A. Ren, S. Garner, A. Chen, T. Londergan, L. Irwin, B. Carlson, L. Fifield, G. Phelan, C. Kincaid, J. Amend and A. Jen, “From molecules to opto-chips: organic electro-optic materials,” J. Mater. Chem. 9, 1905-1920 (1999). 3. Y. Shi, C. Zhang, H. Zhang, J. H. Bechtel, L.R. Dalton, B. Robinson, W. H. Steier, “Low (sub-1-Volt) half voltage polymeric electro-optic modulators achieved y controlling Chromophore shape,” Science, 288, 119122 (2000). 4. P. Zhu, H. Kang, M. E. van der Boom, Z. Liu, G. Xu, J. Ma, D. Zhou, S. Ho, and T. J. 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Introduction
Recently, there has been great interest in reducing the switching voltage of electro-optic (EO) modulators to <1V for RF photonics applications [1,2].Low switching voltages improve link gain of RF signals.Many effort has been put into reducing EO modulator switching voltage by synthesizing higher EO coefficient materials and by implementing improved device structures [2][3][4][5][6][7][8][9][10][11].New organic [4,5,8] and inorganic materials [10] with high EO coefficients have been developed as have organics with long-term orientational stability [5].In regarding to device design, the push-pull Mach-Zehnder configuration has been introduced to reduce the modulator switching voltage while RF transmission lines have been designed to optimize the RF and optical lightwave velocity match at high speeds.Although conducting polymers represent one means to reduce the voltage drop in the cladding layers [11,12].The effectiveness of this approach is limited by polymers conductivity having the requisite optical properties (~ 10 -5 S/cm [11]), which is too low to benefit high frequency operation.
In this letter, we demonstrate for the first time a novel low-voltage EO modulator structure using transparent conducting oxide (TCO) materials [13,14] as electrodes.Compared with conventional metallic electrodes, TCO electrodes have the advantages of low optical loss and adjustable conductivity.In this new structure, the thin film nonlinear EO material is sandwiched directly between conductive TCO electrodes and there is no additional voltage drop from the top and bottom cladding, thus significantly increasing the electric modulation field applied on the nonlinear EO material.As a result, the modulator switching voltage can be reduced significantly.Below, we refer to this type of modulator as a TCO modulator.
The application of TCOs as electrodes in waveguide devices requires that the optical loss of the TCO-based structure to be low while maintain useful conductivity for practical device fabrication.We have simulated that high-speed (Gbit/s) operation of TCO modulator will also be possible by engineering appropriately TCO's conductivity σ versus optical loss α (σ/α) ratio [15].In this paper, we will describe the benefits of using TCO electrodes in an EO modulator and a side conduction structure to avoid TCO index mismatch and high optical loss followed by the fabrication and characterization of the TCO modulator.Conventional modulators utilize metals as electrodes.Typical structural parameters are summarized in Table 1.To avoid the metal-induced optical loss in the waveguide, the thickness of the top and bottom cladding layers must be sufficiently thick that the optical field has nearly completely decayed before touching the metal electrodes.For example, for the EO modulator structure shown in Table 1, the typical thicknesses of the top and bottom claddings must be greater than 2μm and 2.5 μm, respectively, to reduce the metal loss and substrate coupling loss to less than 0.1dB/cm.The required thick cladding layers substantially increase the modulator switching voltage.The voltage drop across the cladding layer can be eliminated by placing TCO electrodes directly against the EO material.The device layer characteristics are summarized in Table 2.

Benefit of TCO electrode in EO modulator
For the TCO modulator, the distance between the electrodes is now given by the EO layer.Importantly, for the same applied voltage, the TCO modulator gives a 3-4 times greater electric field across the EO layer, resulting in 3-4 times lower switching voltage compared to the conventional modulator structure.The exact magnitude of the field enhancement for the TCO modulator vs. the conventional modulator will depend on the structure of the nature of the TCO materials comprising the top and bottom electrodes and the EO modulator structure.

Side conduction structure
Currently, TCO materials are widely used in display applications.The optical loss of the TCO layer is not of great concern due to its small thickness used in display applications.However, in TCO modulator applications , the optical loss of the TCO layer is one of the most important factors since the modulator length will be in centimeters level typically.Another potential limitation of TCO materials is its refractive index mismatch with the organic waveguide.TCO materials usually have higher refractive indices (~ 2.0) compared to most organic materials (~ 1.5) in the wavelength 1.3-1.5 μm range.To avoid the large optical loss and the refractive index mismatch introduced by the TCO layer, a side conduction modulator structure is suggested shown in cross-section in Fig. 1.The TCO layer thickness must be sufficiently thin so that the refractive index mismatch can be avoided and optical loss in the TCO layers made small.In present TCO modulator, the thickness of the bottom TCO electrode is 50nm ZnO and top TCO electrode is 20nm In 2 O 3 .The optical confinement for the top and bottom TCO electrode is 0.7% and 0.2% respectively.The bulk optical loss coefficients of the ZnO and In 2 O 3 TCO electrodes used in the fabrication are about 20/cm and 1000/cm, respectively, measured using the usual waveguide cutback method and a Y-splitter dual-waveguide-arm technique with TCO film as waveguide cladding only along one of the arms to obtain the differential absorption loss due to the TCO material.The growth method of the TCO electrodes will be discussed in section of fabrication.So the total optical loss of TCO electrodes is about 2/cm.With development of low refractive index and less optical loss TCO materials, the TCO modulator structure may eventually use TCO as the entire cladding layer.
In that case a top-down conduction structure would further reduce switching voltage.

High-speed performance simulation of the TCO modulator
The modulation bandwidth with different TCO conductivity for the structure in Fig. 1 was estimated using the traveling-wave model used in Ref. [16].The TCO layers were modeled as a series resistance R s to the modulator.The modulator is model as a capacitor with capacitance of C m .The series resistance per unit length of the TCO layer can be expressed as: R s = L TCO /(σ TCO t TCO ), where L TCO is the distance between the waveguide and the metal transmission line, σ TCO is the TCO material conductivity and t TCO is the thickness of the TCO layer, as shown in Fig. 1.In the simulation, the thickness of the TCO layers is assumed to be 50nm and L TCO =5 μm.The device length is assumed to be 8mm.Typical values of the metal transmission line conduction resistance R con =3.5 Ωmm -1 GHz -1/2 and the metal inductance L m =0.1-1nH/mm are used in the simulation [16].Fig. 2 shows the dependence of the simulated modulation bandwidth versus the conductivity of the TCO electrode.It can be seen that with TCO conductivity σ>50 S/cm, the TCO modulator can operate above 20GHz.This magnitude of conductivity can be achieved using the TCO materials we developed as shown in section 3.

TCO modulator fabrication
Details of the TCO modulator fabrication are described in this section.A 2.8μm thick SiO 2 layer forming the bottom cladding was first deposited on a GaAs substrate wafer by PECVD.The refractive index of the SiO 2 layer was measured to be n=1.45 at the wavelength of 1.3 μm.The required SiO 2 thickness was determined to keep the substrate coupling loss to less than 0.1dB/cm.Followed by the bottom TCO electrode: 50nm ZnO grown by low-pressure metal-organic chemical vapor deposition (MOCVD) at 450 o C. The precursor for Zn was Zn(hfa) 2 (N,N'-DEEDA) which was purified by multiple vacuum-sublimation [13].The conductivity, mobility, and carrier concentration of these ZnO films are measured to be 54.1 S/cm, 11.7 cm 2 /Vs, 2.87x10 19 /cm 3 , respectively, by four-probe Hall effect measurements.The optical loss of these ZnO films are measured to be ~ 20/cm.The bottom ZnO electrode was patterned using standard photolithography and wet etching techniques.Nonlinear organic EO material AJL8/APC (25wt% AJL8 in amorphous poly(carbonate) (APC) host [5]) is then directly spin-coated on bottom ZnO electrode.The thickness of the EO layer is ~1.5 μm.A poling protective layer of 1.5μm poly-(4-vinylphenol) (PVP) is next spin-coated on before depositing the gold electrode for electrical field poling process.The film is then poled at 140 °C with a poling voltage of 300 V for 5min, then cooled down to room temperature while maintaining the electrical field applied to the sample.After the poling process, the top metal electrode is etched away using conventional wet etching.The poling protective layer (the PVP layer) is then washed away with methanol.Subsequently, a 20nm In 2 O 3 top electrode is deposited using an Ion-Beam Assisted Deposition (IAD) system.The top TCO electrode pattern is defined by a shadow mask.The In 2 O 3 layer is deposited at room temperature at a pressure of 4mT with a deposition rate of 2.4nm/min.The carrier concentration is controlled via the O 2 partial pressure during deposition.The conductivity, mobility, and carrier concentration of the In 2 O 3 film deposited are measured to be 70 S/cm, 30 cm 2 /Vs, and 1.5x10 19 /cm 3 respectively.The bulk optical loss of the IAD-deposited In 2 O 3 is somewhat greater than that of the MOVCD deposited ZnO, ~ 1000/cm.Above the top electrode, a rib of spin-coated UV-cured NOA74 with thickness of 0.8 μm and width of 3μm is formed by O 2 RIE plasma etching.Photoresist is used as an etching mask.

Results and discussion
The switching voltage of the TCO modulator fabricated herein was measured using a single straight waveguide configuration.Light with λ=1.31μm emitted from a DBF laser diode was coupled into the modulator waveguide using an optical lens.Two polarizers with polarization perpendicular to each other were used to analyze the phase change of the TE/TM mode after modulator.Details of the measurement setup can be found in Ref. [4].The optical insertion loss of the 8mm-long modulator is 15dB (~9dB due to un-optimized coupling).Fig. 3 shows the typical EO response of the TCO modulator at a wavelength of 1.31μm.Traces 1 and 2 represent the applied voltage signal and response signal of the modulator, respectively.The half-wave voltage V π is measured to be 2.8V for a 8mm long waveguide.For electro-optical modulator, the relationship of switching voltage V π and EO coefficient r is given by Eq. ( 1): where λ is the wavelength of the input light, d is the distance between the lower and the upper electrodes, n is the effective optical refractive index, L is the length of the electro-optic interaction region, and Γ is the optical mode overlap of the nonlinear material in the waveguide.The EO coefficient of the poled polymer is estimated to be ~35 pm/V using the above device parameters.Using the same poling conditions, the EO coefficient was measured to be 68 pm/V immediately after poling for controlled sample.The reduction in EO coefficient of the modulator sample is a result of the thermal exposure in post-poling process, e.g.metal deposition and photolithography.By optimization of the processing or using a higher T g host material, we believe that this thermal relaxation can be reduced.For the conventional structure as shown in Table 1, using a polymer with the same EO coefficient, the switching voltage will be 3-4 times higher.The next step for TCO modulator development will be the demonstration of high-speed performance and characterizing the dependence of the modulation performance on the properties of the TCO materials.

Conclusions
We have realized a low-voltage organic electro-optical modulator for the first time using transparent conducting oxide (TCO) as electrodes.With this new structure, the switching voltage can be reduced by 3-4 times compared to a conventional modulator structure with the same EO-active material.Our initial modeling indicates that an optimized TCO modulator could potentially achieve a modulation frequency of 10-20GHz.The TCO modulator fabricated has a switching voltage of 2.8V (linear geometry) for an organic EO material with EO coefficient r = 35pm/V and a device electrode length of only 8mm.These results show that a modulator with switching voltage of <0.5V should be achievable in a push-pull Mach-Zehnder configuration with a typical 2cm long device using TCO electrodes and optimized poling conditions.

Fig. 1 .
Fig. 1.Structure of a TCO modulator using poled AJL8/APC as the EO layer and ZnO and In 2 O 3 as the electrodes.

Fig. 2 .
Fig. 2. Dependence modulation bandwidth on the conductivity of the TCO electrode.

Fig. 3 .
Fig. 3. Modulation response of the TCO modulator.Trace 1 is the applied voltage and trace 2 the optical intensity at the output side of the modulator.

Table 1 .
Layer structure of a conventional EO modulator a .

Table 2 .
Layer structure of a TCO EO modulator.