Showing papers by "Samuel Graham published in 2009"

A hybrid encapsulation method for organic electronics

Abstract: We report a thin-film encapsulation method for organic electronics that combines the deposition of a layer of SiOx or SiNx (100 nm) by plasma enhanced chemical vapor deposition followed by a layer of Al2O3 (10–50 nm) by atomic layer deposition and a 1-μm-thick layer of parylene by chemical vapor deposition. The effective water vapor transmission rates of the encapsulation was (2±1)×10−5 g/m2 day at 20 °C and 50% relative humidity (RH). The encapsulation was integrated with pentacene/C60 solar cells, which showed no decrease in conversion efficiency after 5800 h of exposure to air demonstrating the effectiveness of the encapsulation methodology.

Specific contact resistance at metal/carbon nanotube interfaces

Abstract: In this report, the specific contact resistance between a thin film single wall carbon nanotube electrode and a deposited silver contact was measured. The specific contact resistance was found to be 20 mΩ cm2, which is an order of magnitude higher than typically observed in standard Si photovoltaic technology. We demonstrate that when utilized as the transparent anode in organic photovoltaics, the specific contact resistance has the potential to induce non-negligible resistive power losses. Thus, specific contact resistance will adversely affect the performance of these systems and should therefore be addressed.

Assessment of stress contributions in GaN high electron mobility transistors of differing substrates using Raman spectroscopy

Abstract: The capability of gallium nitride (GaN) high power transistors arises, in large part, due to piezoelectric polarizations that induce the formation of a carrier rich two-dimensional electron gas. These polarizations, in turn, are directly related to the strain and hence stress that is present within the transistor. As a consequence, the stress load, as well as its measurement, is extremely important to the optimization of this device class. In response, this study demonstrates a technique to quantify the magnitude of operational thermoelastic stress that evolves in a GaN transistor through simultaneous use of the Raman signal’s Stokes peak position and linewidth. After verifying the technique through comparison with a finite element model, the method is then utilized in the analysis of high electron mobility transistors grown on silicon (Si) and silicon carbide (SiC) substrates. For each series of device, the major stress contributors—thermoelastic, converse piezoelectric, and residual—are acquired and com...

The fabrication of high aspect ratio carbon nanotube arrays by direct laser interference patterning

Abstract: High aspect ratio periodic carbon nanotube arrays were fabricated using direct laser interference patterning with a frequency tripled Nd:YAG (YAG: yttrium aluminium garnet) laser (λ = 355 nm) emitting 10 ns laser pulses. The depth of the fabricated arrays could be adjusted by controlling the number of laser pulses. Application of successive laser pulses (10–20) induced pattern distortion, resulting in the formation of conical organized arrays. The number of laser pulses necessary to produce this distortion is proportional to the spatial period of the interference pattern. Raman spectroscopy analyses confirmed that the chemical form of the carbon nanotubes was preserved after patterning.

Multiscale Modeling of Hot Spots in GaN High Electron Mobility Transistors

Abstract: A coupled Lattice Boltzmann (LB)-Finite Difference (FD) method is used to solve for the heat transport in a 6 finger GaN high electron mobility transistor. The LB method is used to capture relevant phonon physics near a microscopic heat generation region by solving the Boltzmann Transport Equation, while an FD model is used to capture the thermal transport at the macroscopic level. The coupling region between the LB and FD domains, which enables multiscale modeling, is discussed. The results of the multiscale models were compared to results generated from other numerical methods. An increasing departure from diffusion theory is observed with increasing dissipated power under the gray phonon model. This difference is attributed to a combination of boundary scattering effects as well as phonon confinement within the small dimensions of the hot spot.© 2009 ASME