Person:
Graham, Samuel

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Author: Christensen, Adam ×

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    Micro-Raman thermometry in the presence of complex stresses in GaN devices
    (Georgia Institute of Technology, 2008-06) Beechem, Thomas ; Christensen, Adam ; Graham, Samuel ; Green, D.
    Raman thermometry is often utilized to measure temperature in gallium nitride (GaN) electronics. However, the accuracy of the technique is subject to errors arising from stresses which develop during device operation as a result of both thermoelastic and inverse piezoelectric effects. To assess the implications of these stresses on Raman thermometry, we investigate the use of the Stokes peak position, linewidth, and Stokes to anti-Stokes intensity ratio to estimate the temperature of GaN devices during operation. Our results indicate that only temperature measurements obtained from the intensity ratio method are independent of these stresses. Measurements using the linewidth, meanwhile, were found to correspond well with those obtained from the intensity ratio through the use of a reference condition which accounted for the stress dependency of this spectral component. These results were then compared to a three dimensional finite element model which yielded a correlation to within 5% between the computational and experimental methods. The peak position method, in contrast, was found to underpredict temperature in all circumstances due to the stress distribution which is present during device operation.
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    Thermal transport properties of thin films of small molecule organic semiconductors
    (Georgia Institute of Technology, 2005-12) Kim, Namsu ; Domercq, Benoit ; Yoo, SeungHyup ; Christensen, Adam ; Kippelen, Bernard ; Graham, Samuel
    A series of harmonic Joule-heating experiments have been employed to determine the thermal conductivities of thin films of pentacene, N,N -diphenyl−N,N -di 3-methylphenyl − 1,1 -biphenyl -4,4 -diamine, and tris 8-hydroquinolinato aluminum, three widely used organic semiconductors. Room-temperature thermal conductivity values of 0.51, 0.24, and 0.48 W/mK were measured for films of these three compounds, respectively. These values are over two orders of magnitude lower than those of inorganic semiconductors. While amorphous films were found to display only small thermal conductivity changes over the temperature range of 228–350 K, pentacene exhibited stronger variations that are typical of phonon-phonon scattering observed in polycrystalline semiconductors.
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    Heat dissipation in high-power GaN electronics on thermally resistive substrates
    (Georgia Institute of Technology, 2005-08) Christensen, Adam ; Doolittle, William Alan ; Graham, Samuel
    The heat dissipation in GaN devices grown on low thermal conductivity lithium gallate (LGO) substrates was investigated. The thermal conductivity of single-crystal LGO was measured utilizing the 3ω technique for temperatures ranging from 100 K–500 K. For the GaN layer, the thermal conductivity was estimated using a phonon transport model which included dislocation density and temperature dependence. These data were then used in a finite element program to determine the thermal behavior of a heterojunction field-effect transistor. Based on a maximum junction temperature of 500 K, it was found that devices with a power dissipation of 1W/mm were possible if the primary heat dissipation path was through the low thermal conductivity substrate. However, in using a front side cooling scheme, results suggest that it may be possible to develop devices with power dissipation in the range of 10 W/mm.