Showing papers by "Samuel Graham published in 2014"

Highly Uniform Trilayer Molybdenum Disulfide for Wafer-Scale Device Fabrication

Abstract: Molybdenum disulfide (MoS2) is a layered semiconducting material with a tunable bandgap that is promising for the next generation nanoelectronics as a substitute for graphene or silicon. Despite recent progress, the synthesis of high-quality and highly uniform MoS2 on a large scale is still a challenge. In this work, a temperature-dependent synthesis study of large-area MoS2 by direct sulfurization of evaporated Mo thin films on SiO2 is presented. A variety of physical characterization techniques is employed to investigate the structural quality of the material. The film quality is shown to be similar to geological MoS2, if synthesized at sufficiently high temperatures (1050 °C). In addition, a highly uniform growth of trilayer MoS2 with an unprecedented uniformity of ±0.07 nm over a large area (> 10 cm2) is achieved. These films are used to fabricate field-effect transistors following a straightforward wafer-scale UV lithography process. The intrinsic field-effect mobility is estimated to be about cm2 V–1 s–1 and compared to previous studies. These results represent a significant step towards application of MoS2 in nanoelectronics and sensing.

Production of heavily n- and p-doped CVD graphene with solution-processed redox-active metal–organic species

Abstract: CVD graphene has been n- and p-doped using redox-active, solution-processed metal–organic complexes. Electrical measurements, photoemission spectroscopies, and Raman spectroscopy were used to characterise the doped films and give insights into the changes. The work function decreased by as much as 1.3 eV with the n-dopant, with contributions from electron transfer and surface dipole, and the conductivity significantly increased.

Reduced Graphene Oxide Thin Films as Ultrabarriers for Organic Electronics

Abstract: Encapsulation of electronic devices based on organic materials that are prone to degradation even under normal atmospheric conditions with hermetic barriers is crucial for increasing their lifetime. A challenge is to develop ultrabarriers that are impermeable, fl exible, and preferably transparent. Another important requirement is that they must be compatible with organic electronics fabrication schemes (i.e., must be solution processable, deposited at room temperature and be chemically inert). Here, a lifetime increase of 1300 h for poly(3-hexylthiophene) (P3HT) fi lms encapsulated by uniform and continuous thin ( ≈ 10 nm) fi lms of reduced graphene oxide (rGO) is reported. This level of protection against oxygen/water vapor diffusion is substantially better than conventional polymeric barriers such as Cytop, which degrades after only 350 h despite being 400 nm thick. Analysis using atomic force microscopy, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy suggest that the superior oxygen gas/moisture barrier property of rGO is due to the close interlayer distance packing and absence of pinholes within the impermeable sheets. These material properties can be correlated to the enhanced lag time of 500 h. The results provide new insight for the design of high-performance and solution-processable transparent ultrabarriers for a wide range of encapsulation applications.

Tailoring Electron-Transfer Barriers for Zinc Oxide/C60 Fullerene Interfaces

Abstract: The interfacial electronic structure between oxide thin films and organic semiconductors remains a key parameter for optimum functionality and performance of next-generation organic/hybrid electronics. By tailoring defect concentrations in transparent conductive ZnO films, we demonstrate the importance of controlling the electron transfer barrier at the interface with organic acceptor molecules such as C60. A combination of electron spectroscopy, density functional theory computations, and device characterization is used to determine band alignment and electron injection barriers. Extensive experimental and first principles calculations reveal the controllable formation of hybridized interface states and charge transfer between shallow donor defects in the oxide layer and the molecular adsorbate. Importantly, it is shown that removal of shallow donor intragap states causes a larger barrier for electron injection. Thus, hybrid interface states constitute an important gateway for nearly barrier-free charge carrier injection. These findings open new avenues to understand and tailor interfaces between organic semiconductors and transparent oxides, of critical importance for novel optoelectronic devices and applications in energy-conversion and sensor technologies.

A Numerical Study on Comparing the Active and Passive Cooling of AlGaN/GaN HEMTs

Abstract: In this paper, the power density capability of AlGaN/GaN high-electron mobility transistors (HEMTs) made on Si, SiC, and diamond substrates were compared with devices on Si and SiC with integrated microchannel cooling. A device temperature limit of 200 °C was used to define the power density. The numerical model accounts for heat transfer from channel of the AlGaN/GaN HEMTs to the heat sink, fluid flow rates, pressure drop, and pumping power required for liquid cooling. The diamond substrate was shown to be superior in reducing the junction temperatures in conventional passive cooling methods employing high thermal conductivity substrates. However, single-phase liquid cooling with microchannels integrated into a SiC substrate showed that it is possible to operate the devices at power densities higher than that on 200- $\mu $ m-thick diamond substrates, considering a maximum operational temperature of 200 °C. Microchannels integrated into the Si substrate also showed a slight increase in the power density compared with passively cooled devices on SiC. Overall, this methodology shows a promising alternative to expensive high thermal conductivity substrates for cooling AlGaN/GaN HEMTs.

Systematic reliability study of top-gate p- and n-channel organic field-effect transistors.

Abstract: We report on a systematic investigation on the performance and stability of p-channel and n-channel top-gate OFETs, with a CYTOP/Al2O3 bilayer gate dielectric, exposed to controlled dry oxygen and humid atmospheres. Despite the severe conditions of environmental exposure, p-channel and n-channel top-gate OFETs show only minor changes of their performance parameters without undergoing irreversible damage. When correlated with the conditions of environmental exposure, these changes provide new insight into the possible physical mechanisms in the presence of oxygen and water. Photoexcited charge collection spectroscopy experiments provided further evidence of oxygen and water effects on OFETs. Top-gate OFETs also display outstanding durability, even when exposed to oxygen plasma and subsequent immersion in water or operated under aqueous media. These remarkable properties arise as a consequence of the use of relatively air stable organic semiconductors and proper engineering of the OFET structure.

Analysis and characterization of thermal transport in GaN HEMTs on Diamond substrates

Abstract: The emergence of Gallium Nitride-based High Electron Mobility Transistor (HEMT) technology has proven to be a significant enabler of next generation RF systems. However, thermal considerations currently prevent exploitation of the full electromagnetic potential of GaN in most applications, limiting HEMT areal power density (W/mm 2 ) to a small fraction of electrically limited performance. GaN on Diamond technology has been developed to reduce near junction thermal resistance in GaN HEMTs. However, optimal implementation of GaN on Diamond requires thorough understanding of thermal transport in GaN, CVD diamond and interfacial layers in GaN on Diamond substrates, which has not been thoroughly previously addressed. To meet this need, our study pursued characterization of constituent thermal properties in GaN on Diamond substrates and temperature measurement of operational GaN on Diamond HEMTs, employing electro-thermal modeling of the HEMT devices to interpret and relate data. Strong agreement was obtained between simulations and HEMT operational temperature measurements made using two independent thermal metrology techniques, enabling confident assessment of peak junction temperature. The results support the potential of GaN on Diamond to enable a 3X increase in HEMT areal dissipation density without significantly increasing operational temperature. Such increases in HEMT power density will enable smaller, higher power density Monolithic Microwave Integrated Circuits (MMICs).

S2-T3: Next generation gallium nitride HEMTs enabled by diamond substrates

Abstract: This paper describes the thermal and electrical performance of GaN on Diamond devices, where the GaN on Diamond substrates are fabricated by taking epi from a host growth substrate and replacing it through direct growth of CVD diamond. We have found GaN on Diamond material improves thermal performance while maintaining electrical performance. This work demonstrates that GaN on Diamond technology can form the foundation of a next generation GaN device with 3X (or more) higher areal power density.

Photochemical Doping and Tuning of the Work Function and Dirac Point in Graphene Using Photoacid and Photobase Generators

Abstract: This work demonstrates that photochemical doping of CVD-grown graphene can be easily achieved using photoacid (PAG) and photobase (PBG) generators such as triphenylsulfonium perfluoro-1-butanesufonate (TPS-Nf) and 2-nitrobenzyl N-cyclohexylcarbamate (NBC). The TPS-Nf ionic onium salt photoacid generator does not noticeably dope or alter the electrical properties of graphene when coated onto the graphene surface, but is very effective at inducing p-doping of graphene upon exposure of the PAG-coated graphene sample. Likewise, the neutral NBC photobase generator does not significantly affect the electrical properties of graphene when coated, but upon exposure to ultraviolet light produces a free amine, which induces n-doping of the graphene. Electrical measurements show that the doping concentration can be modulated by controlling the deep ultraviolet (DUV) light exposure dose delivered to the sample. The interaction between both dopants and graphene is also investigated. The photochemical doping process is able to tune the work function of the single-layer graphene samples used in this work from 3.4 eV to 5.3 eV. Finally, a p–n junction is fabricated and analyzed, showing that it is possible to control the position of the two current minima (two Dirac points) in the ambipolar p–n junction.

The Impact of Noncontinuum Thermal Transport on the Temperature of AlGaN/GaN HFETs

Abstract: The effects of power density and heat generation zone size on the hotspot temperature of AlGaN/GaN HFET devices were predicted using an electrothermal modeling approach The thermal response was modeled using a multiscale model that accounted for ballistic-diffusive phonon transport effects in the heat generation zone near the gate and diffusive transport effects outside of this zone The Joule heating distribution was calculated using a hydrodynamic model in Sentaurus Device The hotspot temperatures at different biasing conditions were determined using the multiscale thermal model and compared with a fully diffusive transport model The results show that the hotspot temperature is higher when ballistic-diffusive transport effects are considered and this difference increases with increasing power density in the AlGaN/GaN HFETs

Electro-Thermo-Mechanical Transient Modeling of Stress Development in AlGaN/GaN High Electron Mobility Transistors (HEMTs) (Postprint)

Abstract: : In this paper, we present a coupled small-scale electrothermal model for characterizing AlGaN/GaN HEMTs under direct current (DC) and alternating current (AC) power conditions for various duty cycles. The calculated electrostatic potential and internal heat generation data are then used in a large-scale mechanics model to determine the development of stress due to the inverse piezoelectric and thermal expansion effects. The electrical characteristics of the modeled device were compared to experimental measurements for validation as well as existing simulation data from literature. The results show that the operating conditions (bias applied and AC duty cycle) strongly impact the temperature within the device and the stress fluctuations during cyclic pulsing conditions. The peak stress from the inverse piezoelectric effect develops rapidly with applied bias and slowly relaxes as the joule heating increases the device temperature during the on state of the pulse leading to cyclic stresses in operation of AlGaN/GaN HEMTs.

Formation of Air Stable Graphene p-n-p Junctions Using an Amine-Containing Polymer Coating

Abstract: An ultrathin layer of a polymer containing simple aliphatic amine groups, polyethylenimine ethoxylated (PEIE), is deposited on a back-gated field effect graphene device to form graphene p–n–p junctions. Characteristic I–V curves indicate the superposition of two separate Dirac points, which confirms an energy separation of neutrality points within the complementary regions. This is a simple approach for making graphene p–n–p junctions without a need for multiple lithography steps or electrostatic gates and, unlike, the destructive techniques such as substitutional doping or covalent functionalization, it induces a minor defect, if any, as there is no discernible D peak in the Raman spectra of the graphene films after creating junctions and degradation in the charge carrier mobilities of the graphene devices. This method can be easily processed from dilute solutions in environmentally-friendly solvents such as water or methoxyethanol and does not suffer any change upon exposure to air or heating at temperatures below 100 °C.

Evaporating device having porous media and method for manufacturing thereof

Abstract: Provided are an evaporating device including porous media and a method of manufacturing the evaporating device. The evaporating device includes a porous media and is configured to evaporate a working fluid, wherein the porous media is formed by sintering sinter particles and includes many pores, wherein a nanoparticle coating layer in which nanoparticles having a size of 10 nm to 30 nm are joined is formed on surfaces of the sinter particles. The method includes: sintering sinter particles to form porous media having many pores therein; and forming a nanoparticle coating layer on surfaces of the sinter particles forming the porous media by using nanoparticles having a size of 10 nm to 30 nm.

Dual-gated field-effect transistors made from wafer-scale synthetic few-layer molybdenum disulfide

Abstract: Molybdenum disulfide (MoS 2 ) has recently received significant attention because of its interesting thickness-dependent properties and its potential as a semiconducting substitute to graphene [1,2]. Most of the studies so far have focused on small ( 2 flakes [1-3]. For manufacturable electronics, it is essential to have large-area material that is compatible with standard fabrication processes for high yield and reproducibility. Though significant progress has been achieved using chemical vapor deposition (CVD) [4,5], the formation of high-quality wafer-scale MoS 2 of controlled thickness is still a challenge.