Gyungock Kim

Mechanism and observation of Mott transition in VO2-based two- and three-terminal devices

An abrupt Mott metal-insulator transition (MIT) rather than the continuous Hubbard MIT near a critical on-site Coulomb energy U/U_c=1 is observed for the first time in VO_2, a strongly correlated material, by inducing holes of about 0.018% into the conduction band. As a result, a discontinuous jump of the density of states on the Fermi surface is observed and inhomogeneity inevitably occurs. The gate effect in fabricated transistors is clear evidence that the abrupt MIT is induced by the excitation of holes.When holes of about 0.018% are induced into a conduction band (breakdown of critical on-site Coulomb energy), an abrupt first-order Mott metal–insulator transition (MIT) rather than a continuous Hubbard MIT near a critical on-site Coulomb energy U/Uc=1, where U is on-site Coulomb energy between electrons, is observed on an inhomogeneous VO2 film, a strongly correlated Mott insulator. As a result, discontinuous jumps of the density of states on the Fermi surface are observed and inhomogeneity inevitably occurs. The off-current and temperature dependences of the abrupt MIT in a two-terminal device and the gate effect in a three-terminal device are clear evidence that the abrupt Mott MIT was induced by the excitation of holes. Raman spectra measured by a micro-Raman system show an MIT without the structural phase transition. Moreover, the magnitude of the observed jumps ΔJobserved at the abrupt MIT is an average over an inhomogeneous measurement region of the maximum true jump, ΔJtrue, deduced from the Brinkman–Rice picture. A brief discussion of whether VO2 is a Mott insulator or a Peierls insulator is presented.

Raman study of electric-field-induced first-order metal-insulator transition in VO2-based devices

An abrupt first-order metal-insulator transition (MIT) without structural phase transition is first observed by current-voltage measurements and micro-Raman scattering experiments, when a DC electric field is applied to a Mott insulator VO_2 based two-terminal device. An abrupt current jump is measured at a critical electric field. The Raman-shift frequency and the bandwidth of the most predominant Raman-active A_g mode, excited by the electric field, do not change through the abrupt MIT, while, they, excited by temperature, pronouncedly soften and damp (structural MIT), respectively. This structural MIT is found to occur secondarily.An abrupt first-order metal-insulator transition (MIT) as a current jump has been observed by applying a dc electric field to Mott insulator VO2-based two-terminal devices. The size of the jumps was measured to be asymmetrical depending on the direction of the applied voltage due to heating effects. The structure of VO2 is investigated by micro-Raman scattering experiments. An analysis of the Raman-active Ag modes at 195 and 222cm−1, explained by pairing and tilting of V cations, and 622cm−1, shows that the modes below a low compliance (restricted) current do not change when the MIT occurs, whereas a structural phase transition above the low compliance current is found to occur secondarily, due to heating effects in the device induced by the MIT. The MIT has applications in the development of high-speed and high-gain switching devices.

Temperature Dependence of Silicon Nanophotonic Ring Resonator With a Polymeric Overlayer

In this paper, we investigate the temperature dependence of a silicon-on-insulator-based silicon nanophotonic ring resonator covered with a polymeric overlayer. Temperature-dependent wavelength shift is measured to be as low as 5 pm/degC for the TM mode in a silicon ring resonator composed of a 500 times 220 nm2 channel waveguide. We also show through simulations and experiments that the temperature dependence can be reduced for the TE mode or for both the TE and TM modes by adjusting the mode volume of a silicon nanophotonic waveguide.

Controlling temperature dependence of silicon waveguide using slot structure.

We show that the temperature dependence of a silicon waveguide can be controlled well by using a slot waveguide structure filled with a polymer material. Without a slot, the amount of temperature-dependent wavelength shift for TE mode of a silicon waveguide ring resonator is very slightly reduced from 77 pm/ degrees C to 66 pm/ degrees C by using a polymer (WIR30-490) upper cladding instead of air upper cladding. With a slot filled with the same polymer, however, the reduction of the temperature dependence is improved by a pronounced amount and can be controlled down to -2 pm/ degrees C by adjusting several variables of the slot structure, such as the width of the slot between the pair of silicon wires, the width of the silicon wire pair, and the height of the silicon slab in our experiment. This measurement proves that a reduction in temperature dependence can be improved about 8 times more by using the slot structure.

Low-voltage high-performance silicon photonic devices and photonic integrated circuits operating up to 30 Gb/s

We present high performance silicon photonic circuits (PICs) defined for off-chip or on-chip photonic interconnects, where PN depletion Mach-Zehnder modulators and evanescent-coupled waveguide Ge-on-Si photodetectors were monolithically integrated on an SOI wafer with CMOS-compatible process. The fabricated silicon PIC(off-chip) for off-chip optical interconnects showed operation up to 30 Gb/s. Under differential drive of low-voltage 1.2 V(pp), the integrated 1 mm-phase-shifter modulator in the PIC(off-chip) demonstrated an extinction ratio (ER) of 10.5dB for 12.5 Gb/s, an ER of 9.1dB for 20 Gb/s, and an ER of 7.2 dB for 30 Gb/s operation, without adoption of travelling-wave electrodes. The device showed the modulation efficiency of V(π)L(π) ~1.59 Vcm, and the phase-shifter loss of 3.2 dB/mm for maximum optical transmission. The Ge photodetector, which allows simpler integration process based on reduced pressure chemical vapor deposition exhibited operation over 30 Gb/s with a low dark current of 700 nA at -1V. The fabricated silicon PIC(intra-chip) for on-chip (intra-chip) photonic interconnects, where the monolithically integrated modulator and Ge photodetector were connected by a silicon waveguide on the same chip, showed on-chip data transmissions up to 20 Gb/s, indicating potential application in future silicon on-chip optical network. We also report the performance of the hybrid silicon electronic-photonic IC (EPIC), where a PIC(intra-chip) chip and 0.13μm CMOS interface IC chips were hybrid-integrated.

12.5 Gbps optical modulation of silicon racetrack resonator based on carrier-depletion in asymmetric p-n diode

We present a high speed optical modulation using carrier depletion effect in an asymmetric silicon p-n diode resonator. To optimize coupling efficiency and reduce bending loss, two-step-etched waveguide is used in the racetrack resonator with a directional coupler. The quality factor of the resonator with a circumference of 260 um is 9,482, and the DC on/off ratio is 8 dB at -12V. The device shows the 3dB bandwidth of approximately8 GHz and the data transmission up to 12.5Gbit/s.

Enhanced frequency response associated with negative photoconductance in an InGaAs/InAlAs avalanche photodetector

We report an InAlAs/InGaAs avalanche photodetector with the photocurrent–voltage characteristic exhibiting a negative conductance region. The frequency response of a device exhibits the internal rf-gain effect in the avalanche region, and the gain peak occurs at progressively higher frequencies as the applied voltage increases. The pulse response experiment in the time domain characterizes the formation process of the space-charge wave signal near the threshold voltage in the avalanche region. The experimental result, displaying a multivalued bandwidth curve with respect to the current-gain, shows that the avalanche process does not limit the speed of the avalanche photodetector, in contrast to the conventional one.

High-modulation efficiency silicon Mach-Zehnder optical modulator based on carrier depletion in a PN Diode

We present a high phase-shift efficiency Mach-Zehnder silicon optical modulator based on the carrier-depletion effect in a highly-doped PN diode with a small waveguide cross-sectional area. The fabricated modulator show a V(pi)L(pi) of 1.8 V x cm and phase shifter loss of 4.4 dB/mm. A device using a 750 microm-long phase-shifter exhibits an eye opening at 12.5 Gbps with an extinction ratio of 3 dB. Also, an extinction ratio of 7 dB is achieved at 4 Gbps for a device with a 2 mm-long phase shifter. Further enhancement of the extinction ratio at higher operating speed can be achieved using a travelling-wave electrode design and the optimal doping.

Si micro-ring MUX/DeMUX WDM filters

We demonstrate 3rd order micro-ring filters, 100 GHz-spaced 16 channels and 50 GHz-spaced 32 channels. Fabrication-induced resonant wavelength errors, σ = 0.237 nm, and temperature-dependent wavelength shift, 0.043 nm/°C tolerable to ΔT>10 °C, has been measured on filters based on the fundamental TM mode. The problem of CMOS-compatible photolithography is solved, while maintaining a small radius, R = 9 μm. As some dummy channels are arranged, it is shown that an on-chip optical network for many cores CPU can be constructed by 16 channel ring filters with the currently available technology.

Crosstalk Reduction in a Shallow-Etched Silicon Nanowire AWG

We introduced shallow-etched silicon nanowire into the arrayed waveguides to decrease the random phase error due to the core width fluctuation. A fairly improved crosstalk value of 18 dB was achieved in the fabricated arrayed-waveguide grating with the on-chip loss of 3 dB.