Somayyeh Rahimi

Ultracompact and fabrication-tolerant integrated polarization splitter

Design and fabrication of a 2×2 two-mode interference (TMI) coupler based on-chip polarization splitter is presented. By changing the angle between the access waveguides, one can tune the effective TMI length for the mode with less optical confinement (transverse magnetic, TM) to coincide with the target TMI length for a desired transmission of the mode with higher optical confinement (transverse electric, TE). The fabricated 0.94 μm long 2×2 TMI splits the input power into TM (bar) and TE (cross) outputs with splitting ratio over 15 dB over 50 nm bandwidth. Fabrication tolerance analysis shows that the device is tolerant to fabrication errors as large as 60 nm.

Group-index independent coupling to band engineered SOI photonic crystal waveguide with large slow-down factor

Group-index independent coupling to a silicon-on-insulator (SOI) based band-engineered photonic crystal (PCW) waveguide is presented. A single hole size is used for designing both the PCW coupler and the band-engineered PCW to improve fabrication yield. Efficiency of several types of PCW couplers is numerically investigated. An on-chip integrated Fourier transform spectral interferometry device is used to experimentally determine the group-index while excluding the effect of the couplers. A low-loss, low-dispersion slow light transmission over 18 nm bandwidth under the silica light line with a group index of 26.5 is demonstrated, that corresponds to the largest slow-down factor of 0.31 ever demonstrated for a PCW with oxide bottom cladding.

Extremely High-Frequency Flexible Graphene Thin-Film Transistors

We have achieved 140-nm channel length graphene thin-film transistors (TFTs) on flexible glass with a 95-GHz intrinsic cutoff frequency and greater than 30-GHz intrinsic power frequency after standard de-embedding. The flexible glass substrate offers subnanometer surface smoothness as well as high thermal conductivity, 1 W/m · K, which can prevent thermomechanical failure, which is a limitation of plastic and rubber substrates. In addition, we developed a flexible 60-nm polyimide thin film as gate dielectric with low surface roughness less than 0.35 nm for optimal carrier transport and facilitate edge-injection contacts for low contact resistance. The maximum electron (hole) mobility is 4540 (1100) cm2/V · s, and the extracted contact resistance in the electron (hole) branch is 1140 (720) Ω · μm. The intrinsic cutoff frequency is 196% higher than our previous results on polymeric substrates. Importantly, the experimental saturation velocity of the graphene TFT is the highest for any flexible transistor on any material system reported so far.

Improvement of graphene field-effect transistors by hexamethyldisilazane surface treatment

We report the improvement of the electrical characteristics of graphene field-effect transistors (FETs) by hexamethyldisilazane (HMDS) treatment. Both electron and hole field-effect mobilities are increased by 1.5 × –2×, accompanied by effective residual carrier concentration reduction. Dirac point also moves closer to zero Volt. Time evolution of mobility data shows that mobility improvement saturates after a few hours of HMDS treatment. Temperature-dependent transport measurements show small mobility variation between 77 K and room temperature (295 K) before HMDS application. But mobility at 77 K is almost 2 times higher than mobility at 295 K after HMDS application, indicating reduced carrier scattering. Performance improvement is also observed for FETs made on hydrophobic substrate—an HMDS-graphene-HMDS sandwich structure. Raman spectroscopic analysis shows that G peak width is increased, G peak position is down shifted, and intensity ratio between 2D and G peaks is increased after HMDS application. We attribute the improvements in electronic transport mainly to enhanced screening and mitigation of adsorbed impurities from graphene surface upon HMDS treatment.

Growth and characterization of LuAs films and nanostructures

We report the growth and characterization of nearly lattice-matched LuAs/GaAs heterostructures. Electrical conductivity, optical transmission, and reflectivity measurements of epitaxial LuAs films indicate that LuAs is semimetallic, with a room-temperature resistivity of 90 μΩ cm. Cross-sectional transmission electron microscopy confirms that LuAs nucleates as self-assembled nanoparticles, which can be overgrown with high-quality GaAs. The growth and material properties are very similar to those of the more established ErAs/GaAs system; however, we observe important differences in the magnitude and wavelength of the peak optical transparency, making LuAs superior for certain device applications, particularly for thick epitaxially embedded Ohmic contacts that are transparent in the near-IR telecommunications window around 1.3 μm.

Growth and characterization of single crystal rocksalt LaAs using LuAs barrier layers

We demonstrate the growth of high‐quality, single crystal, rocksalt LaAs on III‐V substrates; employing thin well-behaved LuAs barriers layers at the III-V/LaAs interfaces to suppress nucleation of other LaAs phases, interfacial reactions between GaAs and LaAs, and polycrystalline LaAs growth. This method enables growth of single crystal epitaxial rocksalt LaAs with enhanced structural and electrical properties. Temperature-dependent resistivity and optical reflectivity measurements suggest that epitaxial LaAs is semimetallic, consistent with bandstructure calculations in literature. LaAs exhibits distinct electrical and optical properties, as compared with previously reported rare-earth arsenide materials, with a room-temperature resistivity of ∼459 μΩ-cm and an optical transmission window >50% between ∼3-5 μm.

Flexible 2D electronics using nanoscale transparent polyimide gate dielectric

We report NPI as a flexible dielectric for transistors based on 2D atomic sheets such as graphene and MoS2, which features high mechanical flexibility, stable electrical performances and low roughness. NPI offers the realistic prospects for highly flexible electronics beyond the typical 2% limitation of high-κ or ceramic gate dielectrics.

Temperature dependence of the electrical resistivity of LaxLu1-xAs

We investigate the temperature-dependent resistivity of single-crystalline films of LaxLu1-xAs over the 5–300 K range. The resistivity was separated into lattice, carrier and impurity scattering regions. The effect of impurity scattering is significant below 20 K, while carrier scattering dominates at 20–80 K and lattice scattering dominates above 80 K. All scattering regions show strong dependence on the La content of the films. While the resistivity of 600 nm LuAs films agree well with the reported bulk resistivity values, 3 nm films possessed significantly higher resistivity, suggesting that interfacial roughness significantly impacts the scattering of carriers at the nanoscale limit.

3D integrated monolayer graphene–Si CMOS RF gas sensor platform

Integration of a complementary metal-oxide semiconductor (CMOS) and monolayer graphene is a significant step toward realizing low-cost, low-power, heterogeneous nanoelectronic devices based on two-dimensional materials such as gas sensors capable of enabling future mobile sensor networks for the Internet of Things (IoT). But CMOS and post-CMOS process parameters such as temperature and material limits, and the low-power requirements of untethered sensors in general, pose considerable barriers to heterogeneous integration. We demonstrate the first monolithically integrated CMOS-monolayer graphene gas sensor, with a minimal number of post-CMOS processing steps, to realize a gas sensor platform that combines the superior gas sensitivity of monolayer graphene with the low power consumption and cost advantages of a silicon CMOS platform. Mature 0.18 µm CMOS technology provides the driving circuit for directly integrated graphene chemiresistive junctions in a radio frequency (RF) circuit platform. This work provides important advances in scalable and feasible RF gas sensors specifically, and toward monolithic heterogeneous graphene–CMOS integration generally.Chemical sensing: monolithic CMOS integration of chemi-resistive graphene junctionsThe combination of monolayer graphene with a silicon-based CMOS platform results in a monolithically integrated gas sensor. A team led by Deji Akinwande at the University of Texas at Austin developed a graphene-based gas sensing platform that leverages the remarkable gas sensitivity of graphene and the well-established advantages of silicon technology. Using a commercially available 0.18 μm CMOS radio-frequency (RF) circuit, the authors fabricated graphene chemi-resistive junctions atop the CMOS readout circuit, whereby the interaction between gas molecules and graphene can be read as an output frequency while minimizing the number of post-processing steps. The built-in RF functionalities enable a direct wireless connection with the transducer and offer reduced flicker noise, as opposed to direct-current sensors.

Towards wafer scale monolayer MoS 2 based flexible low-power RF electronics for IoT systems

There is a growing interest in the design of novel flexible electronics for future internet of things (IoT) systems [1]. IoT requires design of low power RF electronics operating at GHz frequency range. Molybdenum disulphide (MoS2) is the prototypical transitional metal dichalcogenide (TMD) affording a large semiconducting bandgap (1.8eV), high saturation velocity, good mechanical strength, high mobility (> 50cm2/Vs), high on/off ratio (> 106), good current saturation and GHz RF performance [2]. In this work, we demonstrate wafer scale monolayer MoS2 based flexible RF nanoelectronics that can be used for low power nanoelectronics and flexible IoT systems.