Michael E. Ramón

CMOS-compatible synthesis of large-area, high-mobility graphene by chemical vapor deposition of acetylene on cobalt thin films.

We demonstrate the synthesis of large-area graphene on Co, a complementary metal-oxide-semiconductor (CMOS)-compatible metal, using acetylene (C(2)H(2)) as a precursor in a chemical vapor deposition (CVD)-based method. Cobalt films were deposited on SiO(2)/Si, and the influence of Co film thickness on monolayer graphene growth was studied, based on the solubility of C in Co. The surface area coverage of monolayer graphene was observed to increase with decreasing Co film thickness. A thorough Raman spectroscopic analysis reveals that graphene films, grown on an optimized Co film thickness, are principally composed of monolayer graphene. Transport properties of monolayer graphene films were investigated by fabrication of back-gated graphene field-effect transistors (GFETs), which exhibited high hole and electron mobility of ∼1600 cm(2)/V s and ∼1000 cm(2)/V s, respectively, and a low trap density of ∼1.2 × 10(11) cm(-2).

Three-Gigahertz Graphene Frequency Doubler on Quartz Operating Beyond the Transit Frequency

We demonstrate a 500-nm graphene frequency doubler with a record 3-GHz bandwidth, exceeding the device transit frequency by 50%, a previously unobserved result in graphene, indicating that graphene multiplier devices might be useful beyond their transit frequency. The maximum conversion gain of graphene ambipolar frequency doublers is determined to approach a near lossless value in the quantum capacitance limit. In addition, the experimental performance of graphene transistor frequency detectors is demonstrated, showing responsivity of 25.2 μA/μW. The high-frequency performance of these gigahertz devices is enabled by top-gate device fabrication using synthesized graphene transferred onto low capacitance, atomically smooth quartz substrates, affording carrier mobilities as high as 5000 cm2/V ·s.

Self-aligned inversion-type enhancement-mode GaAs metal-oxide-semiconductor field-effect transistor with Al2O3 gate dielectric

In this letter, we report fabrication of self-aligned inversion-type enhancement-mode GaAs metal-oxide-semiconductor (MOS) field-effect transistors with atomic layer deposition of Al2O3 gate dielectric directly on GaAs substrates using a simple ex situ wet clean of GaAs. Thermal stability of the gate stack was examined by monitoring the frequency dispersion behavior of GaAs MOS capacitors under different annealing conditions. A maximum drive current of ∼4.5μA∕μm was obtained for a gate length of 20μm at a gate overdrive of 2.5V. The threshold voltage and subthreshold slope were determined to be ∼0.4V and ∼145mV∕dec from the corresponding Id-Vg characteristics.

Self-aligned graphene field-effect transistors with polyethyleneimine doped source/drain access regions

We report a method of fabricating self-aligned, top-gated graphene field-effect transistors (GFETs) employing polyethyleneimine spin-on-doped source/drain access regions, resulting in a 2X reduction of access resistance and a 2.5X improvement in device electrical characteristics, over undoped devices. The GFETs on Si/SiO2 substrates have high carrier mobilities of up to 6300 cm2/Vs. Self-aligned spin-on-doping is applicable to GFETs on arbitrary substrates, as demonstrated by a 3X enhancement in performance for GFETs on insulating quartz substrates, which are better suited for radio frequency applications.

Hall mobility measurements in enhancement-mode GaAs field-effect transistors with Al2O3 gate dielectric

We report the direct measurement of the inversion charge density and electron mobility in enhancement-mode n-channel GaAs transistors using gated Hall bars. The Hall data reveal the existence of a reduced mobile charge density in the channel due to significant charge trapping. The peak electron mobility was found to be relatively high (∼2140 cm2/V s), in agreement with inherent high carrier mobility of electrons in III-V materials.

Origin of shape anisotropy effects in solution-phase synthesized FePt nanomagnets

Controlling the morphology of inorganic nanocrystals is important because many of their electronic attributes are highly sensitive to shape and aspect ratio. FePt nanocrystals have potential as advanced magnetic materials for ultrahigh-density memory. This is due to their high shape and/or magnetocrystalline anisotropy, which allows bits as small as 3 nm to be thermally stable over typical data storage periods of 10 years. Herein, nanocrystals were simply fabricated by simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl in properly chosen conditions of solvent/surfactant proportions and temperature for rational design of their shape and magnetic properties. This work has combined magnetometry measurements and micromagnetic simulations to illustrate the role of the external shape on the rotation of the magnetization vector for colloidal assemblies.

Graphene frequency doubler with record 3GHz bandwidth and the maximum conversion gain prospects

We report a 500nm graphene field-effect transistor operating at the Dirac point for frequency doubling with maximum output power of −23dBm and a record bandwidth of 3GHz, 2× higher than the state-of-the-art. The experimental device exceeds its ft and fmax by about 50%. Contact resistance degrades the performance of the experimental GFET. In the limit of negligible non-idealities and maximum gate capacitance, the conversion gain approaches lossless frequency doubling. The record performance of the graphene doubler is enabled by the growth of high-quality graphene affording carrier mobilities as high as 5000cm2/V-s and 2200cm2/V-s on smooth quartz and flexible substrates respectively.

Fast and slow transient charging in various III-V field-effect transistors with atomic-layer-deposited-Al2O3 gate dielectric

We report measurement of fast transient charging effects (FTCE) in enhancement-mode n-channel GaAs, InP, and In0.53Ga0.47As field-effect transistors (FETs) using Al2O3 as the gate dielectric. The FTCE data reveal superior drive current and enhanced threshold voltage stability for In0.53Ga0.47As FETs. We further report charge pumping measurements for In0.53Ga0.47As transistors, revealing that the majority of interface traps are donor traps, as well as an increased trap density within the Al2O3 bulk. Such data, together with FTCE data, reveal that drain current degradation observed during pulsed I-V measurements is predominantly due to slow oxide traps, underscoring their significance within III-V/high-κ metal-oxide-semiconductor FETs.

Accurate inversion charge and mobility measurements in enhancement-mode GaAs field-effect transistors with high-k gate dielectrics

Recently, extensive studies have been conducted [1–5] in order to realize enhancement-mode III–V MOSFETs by improving the interface between gate oxide and III–V channel. The carrier mobility in advanced substrates is generally regarded as a figure of merit in benchmarking high-mobility channel materials against bulk Si substrates. However, charge trapping can be a source of error in mobility calculation using split C-V method for Si MOSFETs with high-k dielectrics [6]. On the other hand, magnetotransport measurements are suitable for direct measurement of inversion charge density (Ninv) and mobility in MOS devices with high-k gate dielectrics, where significant charge trapping makes evaluation of inversion charge density using split C-V method inaccurate. In this work, we employ gated-Hall-bar (GHB) structures to directly measure the inversion charge and mobility in a GaAs MOSFET.

Impact of contact and access resistances in graphene field-effect transistors on quartz substrates for radio frequency applications

High-frequency performance of graphene field-effect transistors (GFETs) has been limited largely by parasitic resistances, including contact resistance (RC) and access resistance (RA). Measurement of short-channel (500 nm) GFETs with short (200 nm) spin-on-doped source/drain access regions reveals negligible change in transit frequency (fT) after doping, as compared to ∼23% fT improvement for similarly sized undoped GFETs measured at low temperature, underscoring the impact of RC on high-frequency performance. DC measurements of undoped/doped short and long-channel GFETs highlight the increasing impact of RA for larger GFETs. Additionally, parasitic capacitances were minimized by device fabrication using graphene transferred onto low-capacitance quartz substrates.