Person:
Graham,
Samuel
Graham,
Samuel
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ItemMicro-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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ItemTemperature and doping dependence of phonon lifetimes and decay pathways in GaN(Georgia Institute of Technology, 2008-05) Beechem, Thomas ; Graham, SamuelThe lifetimes of polar optical phonons are known to affect both the electrical and thermal performances of gallium nitride (GaN) based devices. Hence, understanding the dynamical behavior of these phonons in GaN is integral to the elucidation of carrier drift velocities, hot phonon effects, and temperature localization in these nitride semiconductors. To investigate this dynamic behavior, temperature dependent phonon lifetimes were acquired through utilization of the linewidth of the Raman response for GaN samples having various doping types and concentrations. The temperature dependent lifetimes of the four examined phonon modes were then correlated with the Klemens decay model modified to account for four-phonon processes to deduce the decomposition of the zone center phonons. A graphical method that maps this decomposition in the high symmetry directions of the Brillouin zone is also presented. From the variation in lifetime with free carrier concentration, dominant scattering mechanisms are subsequently found for each of four different phonon modes. It is observed that the phonon-carrier interaction directly determines the lifetimes of the polar optical A1 and E1(LO) modes, while the transverse modes into which these longitudinal phonons decay are independent of this interplay. These results indicate that temperature localization likely arises due to the continual emission and reabsorption between the LO phonon modes and the free carriers rather than the persistence of lattice/carrier interaction throughout the entirety of the energy cascade.
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ItemStress relaxation in GaN by transfer bonding on Si substrates(Georgia Institute of Technology, 2007-12-17) Hsu, S. C. ; Pong, B. J. ; Li, W. H. ; Beechem, Thomas ; Graham, Samuel ; Liu, C. Y.The stress state of GaN epilayers transferred onto Si substrates through a Au–Si bonding process was studied by micro-Raman scattering and photoluminescence techniques. By increasing the Au bonding thickness from 1 to 40 µm, the high compressive stress state in GaN layer was relieved. A 10 µm Au bonding layer thickness is shown to possess the maximum compressive stress relief and also the deformation potential of the quantum well was found to be ∼ 85 meV. A nonlinear parabolic relation between luminescent bandgap and the biaxial stress of the transferred GaN epilayer in the compressive region was observed.
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ItemInvited Article: Simultaneous mapping of temperature and stress in microdevices using micro-Raman spectroscopy(Georgia Institute of Technology, 2007-06) Beechem, Thomas ; Graham, Samuel ; Kearney, Sean P. ; Phinney, Leslie M. ; Serrano, Justin R.Analysis of the Raman Stokes peak position and its shift has been frequently used to estimate either temperature or stress in microelectronics and microelectromechanical system devices. However, if both fields are evolving simultaneously, the Stokes shift represents a convolution of these effects, making it difficult to measure either quantity accurately. By using the relative independence of the Stokes linewidth to applied stress, it is possible to deconvolve the signal into an estimation of both temperature and stress. Using this property, a method is presented whereby the temperature and stress were simultaneously measured in doped polysilicon microheaters. A data collection and analysis method was developed to reduce the uncertainty in the measured stresses resulting in an accuracy of ±40 MPa for an average applied stress of −325 MPa and temperature of 520 °C. Measurement results were compared to three-dimensional finite-element analysis of the microheaters and were shown to be in excellent agreement. This analysis shows that Raman spectroscopy has the potential to measure both evolving temperature and stress fields in devices using a single optical measurement.
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ItemRole of interface disorder on thermal boundary conductance using a virtual crystal approach(Georgia Institute of Technology, 2007-01-05) Beechem, Thomas ; Graham, Samuel ; Hopkins, Patrick ; Norris, PamelaAn analytical method is presented to estimate the effects of structural disorder on the thermal boundary conductance (TBC) between two materials. The current method is an extension of the diffuse mismatch model (DMM) where the interface is modeled as a virtual crystal of finite thickness with properties derived from those of the constituent materials. Using this approximation, the TBC for a series of chromium/silicon interfaces is modeled and shown to be within 18% of experimentally obtained values. The methodology improves upon the predictive capabilities of the DMM and allows for quick estimation of the impact of interface mixing on TBC.