Showing papers by "Samuel Graham published in 2011"

A Review of Carbon Nanotube Ensembles as Flexible Electronics and Advanced Packaging Materials

Abstract: The exceptional electronic, thermal, mechanical, and optical characteristics of carbon nanotubes offer significant improvement in diverse applications such as flexible electronics, energy conversion, and thermal management. We present an overview of recent research on the fabrication, characterization and modeling of carbon nanotube (CNT) networks or ensembles for three emerging applications: thin-film transistors for flexible electronics, interface materials for thermal management and transparent electrodes for organic photovoltaics or light emitting diodes. Results from experimental measurements and numerical simulations to determine the electrical and thermal transport properties and characteristics of carbon nanotube networks and arrays used in the above applications are presented. The roles heterogeneous networks of semiconducting and metallic CNTs play in defining electrical, thermal, and optical characteristics of CNT ensembles are presented. We conclude with discussions on future research directions for electronics and packaging materials based on CNT ensembles.

Electro-thermo-mechanical modeling of GaN-based HFETs and MOSHFETs

Abstract: A multiphysics model was developed to describe the evolution of the electrical and thermal behavior along with the mechanical stress fields in AlGaN/GAN heterostructure field-effect transistors and metal-oxide semiconductor heterostructure field-effect transistors under operational conditions. The electric field and power dissipation were obtained from Maxwell's equations. A one-way coupling procedure between the electrical and thermo-mechanical model was utilized to estimate the peak temperature and thermal stresses in these devices. By coupling the solutions, the direct impact of the electrical performance on the thermal stresses in these devices was analyzed.

Change in electronic states in the accumulation layer at interfaces in a poly(3-hexylthiophene) field-effect transistor and the impact of encapsulation.

Abstract: The electrical properties of organic field-effect transistors (OFETs) are largely determined by the accumulation layer that extends only a few molecular layers away from the gate dielectric/organic semiconductor interface. To understand degradation processes that occur within the device structure under ambient conditions, it is thus essential to probe the interface using an architecture that minimizes the effects of bulk transport of contaminating species through upper layers of material in a thick film device. Using FETs designed with multiple voltage probes along the conducting channel and an ultrathin film of the active material, we found that the charge carrier density and the FET mobility decrease, and further, the contact and channel properties are strongly correlated. FET devices prepared with an ultrathin film of P3HT become significantly contact limited in air due to a hole diffusion barrier near the drain electrode. Encapsulation of the device with a layered organic/inorganic barrier material co...

Lattice boltzmann and discrete ordinates methods for phonon transport modeling: A comparative study

Abstract: The description of heat transport at small length scales is very important in understanding a wide range of micro and nanoscale systems. In systems where coherent phonon transport effects are negligible, the Boltzmann transport equation (BTE) is often employed to describe the distribution and propagation of thermal energy in the lattice. The phonon distribution function depends not only on the temporal and spatial coordinates, but also on polarization and wave vector, making fully-resolved simulations very expensive. Therefore, there is a need to develop accurate and efficient numerical techniques for the solution of the BTE. The discrete ordinates method (DOM) and more recently, the lattice Boltzmann method (LBM) have been used for this purpose. In this work, a comparison between the numerical solution of the phonon BTE by the LBM and DOM is made in order to delineate the strengths and weaknesses of these approaches. Test cases are chosen with Knudsen (Kn) numbers varying between 0.01–100 to cover the full range of diffusive to ballistic phonon transport. The results show that solutions obtained from both methods converge to analytical results for the 1 dimensional phonon transport in a slab. Solutions obtained by two methods converge to analytical solutions of 2 dimensional problems at low Kn. However, solution accuracy is strongly determined by angular resolution for moderate to high Kn. Since the number of propagation directions in LBM are limited, significant errors are engendered in multi-dimensional acoustically-thin problems. DOM also suffers errors at low angular resolutions for high Kn, but yields accurate solutions when sufficient angular resolution is employed.Copyright © 2011 by ASME

The development of thin film barriers for encapsulating organic electronics

Abstract: In this work, we investigate several approaches to the development of encapsulation of organic electronics. A combination of plasma enhanced chemical vapor deposition, atomic layer deposition, and physical vapor deposition are used to make single layer and multilayer thin films and study the impact of structure on effective water vapor transmission rates. It was found that multilayer thin films consisting of organic and inorganic layers as well as inorganic nanolaminates provided the highest performance barrier films with effective water vapor transmission rates less than 5 × 10−5 g/m2/day. Materials such as atomic layer deposition deposited Al 2 O 3 also showed excellent initial performance, but were found to be susceptible to corrosion from water. Combining alumina with other materials was found to improve the long-term performance of the alumina films. Integration of these films into organic solar cell platforms was shown to effectively maintain shelf lifetime performance for more than 7000 h.

Temperature- and Doping-Dependent Anisotropic Stationary Electron Velocity in Wurtzite GaN

Abstract: The influences of temperature, dopant density, free-carrier density, and field orientation on the electron drift mobility and velocity-field relationship in wurtzite c -axis GaN are quantified by means of theoretical investigation. Electron velocity perpendicular to the growth plane is uniformly lower than that parallel to the growth plane for field strengths below 500 kV/cm, although anisotropy within the basal plane itself is found to be insignificant. The calculated low-field electron mobility is demonstrated to be consistent with recent Hall measurements over a range of dopant densities. Low-field mobility is enhanced under the influence of free-carrier densities above the background doping due to both increased screening of ionized impurities and a reduction in A1-LO phonon lifetime through the plasmon-phonon interaction.

Traveling dipole domains in AlGaN/GaN heterostructures and the direct generation of millimeter‐wave oscillations

Abstract: A new paradigm is presented for the direct generation of self-sustaining millimeter-wave oscillations, based on the piezo- and ferroelectric properties of wurtzite III-nitride materials. In contrast to Gunn diodes which exploit a bulk-like active region, periodic oscillation is achieved in the proposed structures through the creation, propagation and collection of traveling dipole domains supported by fixed polarization charge and the associated two-dimensional electron gas along the plane of a polar heterojunction. State-of-the-art numerical simulation based on the synchronous full-band ensemble Monte Carlo method is applied to study induced oscillations in a simple triode structure commonly used for AlGaN/GaN high electron mobility transistors. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Development of reliable mW level powers in pseudomorphic ultraviolet light emitting diodes on bulk aluminum nitride substrates

Abstract: The use of low defect density bulk AlN substrates has led to reliable pseudomorphic ultraviolet light emitting diodes capable of mW level power outputs from packaged devices emitting from 250–275 nm.

Characterization of Metallically Bonded Carbon Nanotube-Based Thermal Interface Materials Using a High Accuracy 1D Steady-State Technique

Abstract: The next generation of Thermal Interface Materials (TIMs) are currently being developed to meet the increasing demands of high-powered semiconductor devices. In particular, a variety of nanostructured materials, such as carbon nanotubes (CNTs), are interesting due to their ability to provide low resistance heat transport from device to spreader and compliance between materials with dissimilar coefficients of thermal expansion (CTEs). As a result, nano-Thermal Interface Materials (nTIMs) have been conceived and studied in recent years, but few application-ready configurations have been produced and tested. Over the past year, we have undertaken major efforts to develop functional nTIMs based on short, vertically-aligned CNTs grown on both sides of a thin interposer foil and interfaced with substrate materials via metallic bonding. A high-precision 1-D steady-state test facility has been utilized to measure the performance of nTIM samples, and more importantly, to correlate performance to the controllable parameters. Nearly 200 samples have been tested utilizing myriad permutations of such parameters, contributing to a deeper understanding and optimization of CNT growth characteristics and application processing conditions. In addition, we have catalogued thermal resistance results from a variety of commercially-available, high-performance thermal pads and greases. In this paper, we describe our material structures and the parameters that have been investigated in their design. We report these nTIM thermal performance results, which include a best to-date thermal interface resistance measurement of 3.5 mm2 -K/W, independent of applied pressure. This value is significantly better than all commercial materials we tested and compares favorably with the best results reported for CNT-based nTIMs in an application-representative setting.Copyright © 2011 by ASME

A correlation study between the total permeated water vapor and lifetime of an encapsulated OPV

Abstract: Barrier films play a critical role in the packaging and protection of solar cells, especially organic photovoltaics. While high performance barrier films are needed to limit the ingress of water vapor and oxygen into the cells, little has been done to correlate the performance of such cells with their barrier films. In this work, a study is presented with correlates the shelf lifetime of an encapsulated organic solar cells with barrier films having differing water vapor transmission rates. The study shows that overall barrier performance of multilayer thin films and the extended shelf lifetime of encapsulated organic solar cells can be related through the calculation of the total permeated water vapor through the barrier films. The total permeated water vapor was calculated separately in the transient and steady-state regions considering the lag time effect of the multilayer barriers. The efficiency of pentacene/C 60 -based solar cells encapsulated with one or two pairs of SiN X /parylene dropped to 50% after permeation of about 1.63 g/m2 of water vapor regardless of transmission rate. From these calculations, the lifetime of encapsulated devices with three pairs are predicted.