• Photonics Research
  • Vol. 9, Issue 4, 615 (2021)
Wei-Che Hsu, Erwen Li, Bokun Zhou, and Alan X. Wang*
Author Affiliations
  • School of Electrical Engineering and Computer Science, Oregon State University, Corvallis, Oregon 97331, USA
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    DOI: 10.1364/PRJ.416656 Cite this Article Set citation alerts
    Wei-Che Hsu, Erwen Li, Bokun Zhou, Alan X. Wang. Characterization of field-effect mobility at optical frequency by microring resonators[J]. Photonics Research, 2021, 9(4): 615 Copy Citation Text show less

    Abstract

    A novel characterization method is proposed to extract the optical frequency field-effect mobility (μop,FE) of transparent conductive oxide (TCO) materials by a tunable silicon microring resonator with a heterogeneously integrated titanium-doped indium oxide (ITiO)/SiO2/silicon metal–oxide–semiconductor (MOS) capacitor. By operating the microring in the accumulation mode, the quality factor and resonance wavelength shift are measured and subsequently used to derive the μop,FE in the ultra-thin accumulation layer. Experimental results demonstrate that the μop,FE of ITiO increases from 25.3 to 38.4 cm2?V-1?s-1 with increasing gate voltages, which shows a similar trend as that at the electric frequency.

    1. INTRODUCTION

    Metal–oxide–semiconductor (MOS) capacitors are one of the most prevailing electronic device structures, and have laid the foundation of modern transistors that have transformed the entire industry of microelectronics [1]. In recent years, MOS devices have also gained increasing utility in photonic applications, which could pave the way for a new generation of hybrid electronic–photonic systems [2,3]. MOS-driven silicon photonic devices in particular have rapidly become one of the most promising building blocks for future optical interconnect systems due to their enhanced performance in electro-optic (E-O) modulation and scalability of fabrication [46]. Photonic devices based on the MOS structure usually operate in the accumulation mode. When a negative bias voltage (Vg) is applied, it induces the field effect and modifies the refractive indices of the semiconductor materials through the plasma dispersion effect so that an optical phase shift is induced to the guided light. In addition to their intrinsic advantages, MOS structures provide feasibility of heterogeneous integration with other materials such as graphene, III-V, and transparent conductive oxides (TCOs) on silicon photonics [79]. Of these heterogeneously integrated photonic devices, an MOS device with a TCO gate can achieve unity-order refractive index changes in the accumulation layer [10]. Several ultra-efficient Si-TCO photonic devices have been reported using a Mach–Zehnder interferometer, an electro-absorption modulator, a photonic crystal nanocavity, and a microring resonator (MRR) [9,1113].

    Carrier mobility is one of the most pivotal properties of semiconductors, as it can determine the performance of solid-state devices. Carrier mobility represents the velocity of electrons or holes under certain electric fields, and therefore it determines the conductivity and frequency response of electronic devices such as transistors. Thus, the high mobility of semiconductors is critical to achieving high bandwidth and low power dissipation [14]. For photonic devices, the impact of carrier mobility reaches even further. As described by the Drude model, the collision frequency [Eq. (1c)], which is the collision process between free carriers and ionized impurities in TCOs, is inversely proportional to the carrier mobility at the optical frequency [15,16]. Furthermore, the optical loss due to free carrier absorption is determined by the imaginary part of the complex permittivity [Eq. (1a)], which is influenced by the collision frequency as well. Hence, high-mobility semiconductors are critical to low optical loss waveguides. For instance, previous research has shown that high-mobility TCOs can significantly enhance the performance of photonic modulators by increasing the extinction ratio, improving the energy efficiency and quality factor (Q factor) [13,17,18].

    The carrier mobility of semiconductors at electrical frequency (DC or RF) is usually measured by the Hall effect. It actually measures the bulk mobility (μbulk), which is the average mobility of the entire thin film layer [19,20]. For many electronic devices, field-effect mobility (μFE) is even more critical to determining the device performance. When a bias Vg is applied to the gate, the field effect induces accumulation or inversion layer at the surface of the semiconductor with the insulator, forming a channel of free charges that are drastically different than those in the bulk materials [10,21]. The carrier mobility in the accumulation or inversion layer, which is also called the field-effect mobility μFE, is generally higher than μbulk because the high concentration of free carriers in the channel layer brings an electrostatic screening effect that reduces impurity coulomb scattering [22]. This phenomenon has been verified by thin-film transistors (TFTs) [23], and TFTs have been used to measure the electric frequency μFE. The measurements of the gate voltage, drain voltage, and drain current are used to extract the electric frequency μFE. For example, experimental results show that the electric frequency μFE of TCOs increases as the Vg increases [2428].

    In contrast to electric frequency mobility, which is limited by ionized-impurity scattering and grain-boundary scattering, the optical frequency mobility (μop) is insensitive to grain-boundary scattering. It is only determined by ionized-impurity scattering because the average electron path length, which is in the range of a few nanometers and under the application of a rapidly oscillating electric field, is much smaller than the grain size [29]. By comparing the difference between electrical and optical frequency carrier mobility, we can observe the contribution from the grain-boundary scattering and ionized-impurity scattering separately [30]. The optical frequency bulk mobility (μop,bulk) of a semiconductor film on a thick substrate is usually characterized by a spectroscopic ellipsometry [31]. However, ellipsometry cannot effectively measure the optical frequency field-effect mobility (μop,FE) due to the ultrathin accumulation layer (1  nm). The accumulation layer is only around 0.1% of the probing wavelength used in the ellipsometry, which cannot induce meaningful light–matter interaction to calculate the film’s refractive index and thickness. Therefore, a fundamentally different method is needed for the measurement of μop,FE in the ultra-thin accumulation layer.

    In this paper, we propose a novel characterization method to extract the μop,FE of TCO materials using an MRR on a silicon-on-insulator (SOI) wafer. This method works for all TCOs and can even be applied to other types of semiconductor materials. In this paper, titanium-doped indium oxide (ITiO) is used in the experiment for μop,FE characterization due to its potential for high mobility. An ITiO-SiO2-Si MOS-driven MRR is fabricated through heterogeneous integration, which can provide orders of magnitude stronger light–matter interaction compared with ellipsometry measurement. By operating the MRR in the accumulation mode with negative Vg, the Q factors and resonance wavelength shift (Δλ) values are measured and subsequently used to derive the μop,FE in the ultra-thin accumulation channel. Experimental results in this work demonstrate that the μop,FE of ITiO increases from 25.3 to 38.4  cm2V1s1 with increasing negative Vg. This proposed μop,FE measurement technique will provide an effective characterization method for field-effect electro-optic devices, especially for heterogeneously integrated silicon photonic devices.

    2. DESIGN AND PRINCIPLE

    A. Design of ITiO-gated MOS MRR

    (a) 3D schematic of ITiO-Si-SiO2 MOS-driven MRR. (b) Cross-sectional schematic in the active region. With the negative Vg, it induces the carrier accumulation and refractive index modulation in the ITiO and Si layers. (c) Simulated results with different models: quantum-moment model plots in dashed lines and uniform model plots in solid lines. Q factor (blue line, left y axis) and resonance wavelength shift Δλ (red line, right y axis) as a function of Vg.

    Figure 1.(a) 3D schematic of ITiO-Si-SiO2 MOS-driven MRR. (b) Cross-sectional schematic in the active region. With the negative Vg, it induces the carrier accumulation and refractive index modulation in the ITiO and Si layers. (c) Simulated results with different models: quantum-moment model plots in dashed lines and uniform model plots in solid lines. Q factor (blue line, left y axis) and resonance wavelength shift Δλ (red line, right y axis) as a function of Vg.

    The plasma frequency (ωp) is related to the carrier concentration (Nc) by ωp=Nce2ε0m*,where e is the electron charge, ε0 is the vacuum permittivity, and m* is the effective mass of charge carriers.

    The plasma collision frequency (γ) is related to the μop by γ=em*μop.The cross-sectional schematic in the active region of the device is shown in Fig. 1(b). Applying a negative Vg on the ITiO gate induces electron accumulation at the ITiO/SiO2 interface and hole accumulation at the p-Si/SiO2 interface. This field effect changes the optical permittivities of ITiO and Si, which influences the resonance wavelengths (λres) and Q factors of the silicon MRR. The Q factor can be written as [33] Q=πngLraλres(1ra),where r is the self-coupling coefficient, a is the single-pass amplitude transmission, L is the circumference of a ring, and ng is the group index of the ring waveguide.

    The value of a is related to the loss α by a2=eαL.The values of r and a are crucial to the Q factor. r is determined by the coupling between the bus waveguide and microring and can be adjusted by changing the waveguide gap or coupling length [34,35]. a is affected by the loss from the accumulation layers of ITiO and p-Si when the negative Vg is applied. Hence, applying a moderate Vg changes a while not affecting r. At the critical coupling (r=a) condition, the transmission at λres decreases to zero [33,36]. When the loss of the MRR is fixed, the Q factor can be improved by working at the critical coupling condition [37].

    The Δλ can be calculated by the change of the effective index (neff): Δλ=Δneffneffλres.

    As shown in Fig. 1(a), the ITiO does not cover the whole ring. Therefore, the neff depends on the length of the microring covered by the ITiO electrode, which can be written as neff=P×neff,active+(1P)×neff,coupling,where neff,active is the effective index in the active region covered by ITiO and the neff,coupling is the effective index in the coupling region without the coverage of ITiO. P is the ITiO coverage percentage on the MRR.

    To understand how the Q factor and Δλ are affected by Vg, we simulated an ITiO-gated MOS MRR with a radius of 6 μm by the finite-difference-eigenmode (FDE) solver in Lumerical MODE software. The carrier concentration distribution is simulated by Silvaco and imported into Lumerical MODE. The simulation results are plotted with dashed lines in Fig. 1(c). When a negative Vg is applied, it increases the Nc, and changes the relative permittivity [Eqs. (1a) and (1b)] of ITiO, which will further modulate the effective index neff of the guided mode in the microring waveguide calculated by Lumerical. The reduction of the real part of neff blueshifts the resonance wavelength as given in Eq. (3a), while the increase of the imaginary part of the neff increases the optical loss and reduces the Q factor as explained in Eqs. (2a) and (2b). Figure 1(c) shows the downward trend of the Q factor and blueshift of Δλ by applying the Vg.

    B. Model Setup

    (a) Simulation model includes the p-Si layer, SiO2 layer, and the ITiO, consisting of the bulk material and 1 nm accumulation channel. (b) Simulated cross-sectional electric field intensity (|E|2) distribution of the ITiO-gated MOS bending waveguide with a 17 nm SiO2 layer and a 17 nm ITiO layer. (c) Q factor maps, with respect to μop,FE and Δλ, in different bulk conditions.

    Figure 2.(a) Simulation model includes the p-Si layer, SiO2 layer, and the ITiO, consisting of the bulk material and 1 nm accumulation channel. (b) Simulated cross-sectional electric field intensity (|E|2) distribution of the ITiO-gated MOS bending waveguide with a 17 nm SiO2 layer and a 17 nm ITiO layer. (c) Q factor maps, with respect to μop,FE and Δλ, in different bulk conditions.

    When a negative Vg is applied, it induces the field effect and changes carrier concentration in the accumulation layer. We can sweep different ΔN to simulate different external Vg. We have already known that the electric frequency μFE increases under the field effect because an electrostatic screening effect reduces the ionized-impurities scattering when the concentration of accumulated free carriers increases [22]. As the μop,FE is also affected by ionized-impurities scattering, we expect that the μop,FE also changes under the field effect. Hence, we can sweep ΔN and μop,FE in the simulation, which will induce different α, ng, and neff while running the FDE solver. α and ng are used to calculate the Q factor with Eqs. (2a) and (2b), and the Δλ can be obtained from Eq. (3). After Q factors and Δλ are obtained from the simulation, we can plot the Q factor map with respect to μop,FE and Δλ, as shown in Fig. 2(c). However, we can see that the Q factor maps are influenced by the initial conditions, i.e., Nc and μbulk. Therefore, the final Q factor map will be known when the initial condition is measured from the experiment. Finally, we can measure the experimental Q factor and Δλ from the tunable MRR with negative Vg to derive the μop,FE by mapping the Q factor with the simulation results. Also, we can observe how the field effect changes the μop,FE.

    3. FABRICATION AND CHARACTERIZATION

    A. Fabrication Processes and Testing

    (a) Scanning electron microscope (SEM) image of the fabricated passive Si-MRR with false colors. The microring has a radius of 6 μm. (b) Zoom-in SEM image of microring to show the side-wall roughness. (c) The experimental transmission spectrum of the passive MRR, which is fitted by the Lorentzian function, has a high Q factor of ∼13,000. (d) Optical image of the fabricated ITiO-gated MOS MRR. The ITiO gate, which is highlighted by the white dashed line, covers the active region of the microring except the coupling region to the bus waveguide. The active region covers ∼83% of the MRR. The gate electrode lies on ITiO, and the ground electrodes are connected to the p-Si microring through a partially etched Si slab.

    Figure 3.(a) Scanning electron microscope (SEM) image of the fabricated passive Si-MRR with false colors. The microring has a radius of 6 μm. (b) Zoom-in SEM image of microring to show the side-wall roughness. (c) The experimental transmission spectrum of the passive MRR, which is fitted by the Lorentzian function, has a high Q factor of 13,000. (d) Optical image of the fabricated ITiO-gated MOS MRR. The ITiO gate, which is highlighted by the white dashed line, covers the active region of the microring except the coupling region to the bus waveguide. The active region covers 83% of the MRR. The gate electrode lies on ITiO, and the ground electrodes are connected to the p-Si microring through a partially etched Si slab.

    Next, a 17 nm thick SiO2 layer is formed by dry oxidation at 1000°C, and a 17 nm ITiO gate is deposited by radio frequency (RF) sputtering at room temperature, followed by a lift-off photolithography process. The ITiO is characterized by Hall effect measurement, which has the Nc of 2.63×1019  cm3 and μbulk of 26.5  cm2V1s1. The SiO2 layer on the Si contact region is etched by hydrofluoric (HF) acid. Finally, the Ni/Au electrodes are thermally evaporated and patterned by regular photolithography. For characterization of the ITiO-gated MOS MRRs, the input and output fibers have a tilt angle of 8°, and the polarization controller is used to make the input light in the TE mode. The light is coupled into and out from the silicon bus waveguide through the waveguide grating couplers. The gate voltage is applied through the GSG electrodes from the GSG probe. Finally, the transmission spectra with different Vg are detected by an optical spectrum analyzer.

    B. Experimental Results

    In this work, the initial condition of ITiO is measured, which has the Nc of (2.624±0.014)×1019  cm3 and μbulk of 26.5±0.15  cm2V1s1. Hence, we can build the experimental Q factor map with these parameters (Nc and μbulk), and this Q factor map can be used to derive the μop,FE with the experimental results.

    (a) Lorentzian fitted experimental transmission spectra of ITiO-gated MOS MRR with different Vg. (b) Experimental Q factor (blue line, left y axis) and Δλ (red line, right y axis). (c) μop,FE extraction from experimental Q factor and Δλ with errors. (d) Capacitance as a function of Vg for the ITiO-gated MOS MRR.

    Figure 4.(a) Lorentzian fitted experimental transmission spectra of ITiO-gated MOS MRR with different Vg. (b) Experimental Q factor (blue line, left y axis) and Δλ (red line, right y axis). (c) μop,FE extraction from experimental Q factor and Δλ with errors. (d) Capacitance as a function of Vg for the ITiO-gated MOS MRR.

    Since this method is an indirect method to estimate the μop,FE, we need to discuss its accuracy. The major error sources come from the experimental results in Fig. 4(b) with the simulation in Fig. 1(c). For the wavelength tunability, the experiment (48.5 pm/V) matches the simulation (51.9 pm/V) with a standard deviation of 7%. For the Q factor, we can first compare it at the initial condition (Vg=0  V) because it does not have any change of ΔN and μop,FE in the accumulation layer. Therefore, we can directly see the difference between experiment and simulation when we use the same parameters. The experiment matches very well with the simulation at Vg=0  V, which only has an error of <1%. Even though the Q factor error increases when a larger gate bias is applied, it is still less than 5%. The error from the mismatch causes the error of μop,FE (Δμop,FE), which is 2.5  cm2V1s1. The other source of errors comes from the experiment measurement. The experimentally measured Q factors are Q±50. This standard deviation can cause a Δμop,FE of 3  cm2V1s1. However, in the small Vg region, it can even be as large as Δμop,FE of 5-10  cm2V1s1 due to the small relative change. In addition, the λres may have ±2  pm difference during the measurement, which induces ΔμFE of 1  cm2V1s1. The measurement errors from Nc and μbulk are minor and cause Δμop,FE of 0.7 and 1  cm2V1s1, respectively. The overall error of Δμop,FE is combined with Δμop,FE(all)=Δμop,FE(mismatch)2+Δμop,FE(Q)2+Δμop,FE(Δλ)2+Δμop,FE(Nc)2+Δμop,FE(μbulk)2.Finally, the error bars are plotted in Fig. 4(c) together with the mobility results. Figure 4(c) results show that the μop,FE has a large fluctuation in the small Vg region (0 to 2  V). We can determine the accumulation mode region (Vg<2  V) from Fig. 4(b) since the Δλ is linearly proportional to Vg in the accumulation mode [39]. The flat band voltage (VFB) is also found to be around 2  V from the capacitance–voltage curve of the device, as shown in Fig. 4(d). This method can only achieve meaningful results when the field-effect is obvious. When the Vg is small (0 to 2  V), the change of the carrier concentration is relatively minor compared to the bulk concentration. Therefore, the change of the Q factor and Δλ are difficult to measure accurately, which induces a large fluctuation when Vg is low. In the obvious accumulation mode region (Vg<2  V), when the ΔN becomes larger, the measurement error does not have a significant influence. Hence, we can see that the μop,FE increases steadily in the moderate to strong accumulation mode. It shows a trend of increasing μop,FE from 25.3 to 38.4  cm2V1s1 as the negative Vg increases. Interestingly, a similar phenomenon is also mentioned in the TFT measurement when measuring the electric frequency μFE, and it shows a stable growth of electric frequency μFE in accumulation mode but not in the depletion mode [40,41]. When the larger negative Vg is applied, it has a higher μop,FE in the accumulation layer, reducing the optical absorption loss. Therefore, it can help the ITiO-gated MOS MRR maintain a good Q factor even though a larger negative Vg is applied.

    4. CONCLUSION

    In conclusion, we invented a new characterization method for quantifying the μop,FE in the accumulation channel by a tunable ITiO-SiO2-Si MOS-driven MRR. The proposed integrated photonic platform provides dramatically stronger light–matter interaction compared with the traditional ellipsometry measurement. By constructing a comprehensive numerical model, we generated the contour map of the Q factor of the MRR with respect to μop,FE and Δλ by sweeping ΔN and μop,FE in the simulation. Experimental results of the Q factor and Δλ were measured under the negative Vg and subsequently used to derive the μop,FE by mapping the data into the simulation results. Our experimental results demonstrated that the μop,FE of ITiO increases from 25.3 to 38.4  cm2V1s1 with increasing Vg, which shows a similar trend in the electric frequency μFE. This method provides a novel pathway to precisely obtain the in-device μop,FE from an integrated photonics platform that has never been explored. Our approach fills the gap of existing carrier mobility characterization methods for field-effect electro-optic devices, especially for heterogeneously integrated silicon photonic devices.

    Acknowledgment

    Acknowledgment. The authors would like to acknowledge the Oregon State University Materials Synthesis and Characterization Facility (MASC) and Electronic Microscopy Facility for their support in device fabrication, and Prof. Janet Tate at the Department of Physics for the Hall measurement.

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    Wei-Che Hsu, Erwen Li, Bokun Zhou, Alan X. Wang. Characterization of field-effect mobility at optical frequency by microring resonators[J]. Photonics Research, 2021, 9(4): 615
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