Showing papers by "Samuel Graham published in 2008"

Stability of Doped Transparent Carbon Nanotube Electrodes

Abstract: This paper evaluates the effectiveness of p-doping transparent single-walled carbon nanotube (SWNT) films via chemical treatment with HNO 3 and SOCl 2 . Stability of the improvement in electrical conductivity after doping is investigated for different doping treatments as a function of exposure time to air and as a function of temperature. Doped films were found to have a greater than twofold increase in conductivity with sheet resistance values as low as 105 Ω sq -1 with an optical transmittance of 80% at 550 nm. However, doping enhancements demonstrated limited stability in air and under thermal loading. The application of a thin capping layer of PEDOT/PSS is shown to stabilize the improvements in conductivity, evidenced by sustained lower sheet resistance in both air and under thermal loading.

Influence of Interfacial Mixing on Thermal Boundary Conductance Across a Chromium/Silicon Interface

Abstract: The thermal conductance at solid-solid interfaces is becoming increasingly important in thermal considerations dealing with devices on nanometer length scales. Specifically, interdiffusion or mixing around the interface, which is generally ignored, must be taken into account when the characteristic lengths of the devices are on the order of the thickness of this mixing region. To study the effect of this interfacial mixing on thermal conductance, a series of Cr films is grown on Si substrates subject to various deposition conditions to control the growth around the Cr/Si boundary. The Cr/Si interfaces are characterized with Auger electron spectroscopy. The thermal boundary conductance (h BD ) is measured with the transient thermoreflectance technique. Values of h BD are found to vary with both the thickness of the mixing region and the rate of compositional change in the mixing region. The effects of the varying mixing regions in each sample on h BD are discussed, and the results are compared to the diffuse mismatch model (DMM) and the virtual crystal DMM (VCDMM), which takes into account the effects of a two-phase region of finite thickness around the interface on h BD . An excellent agreement is shown between the measured h BD and that predicted by the VCDMM for a change in thickness of the two-phase region around the interface.

Micro-Raman thermometry in the presence of complex stresses in GaN devices

Abstract: 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 computat...

Temperature and doping dependence of phonon lifetimes and decay pathways in GaN

Abstract: The 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.

GaN HEMT thermal behavior and implications for reliability testing and analysis

Abstract: GaN HEMT reliability evaluation in a typical Arrhenius manner requires establishing peak junction temperature for a particular stress condition. Several new techniques have yielded promising results toward establishing peak temperature for these devices in combination with detailed physical modeling, particularly micro-Raman imaging. This paper compares results from finite element modeling to measurements by infrared imaging and micro-Raman imaging. The limitations of IR imaging were confirmed similar to earlier reports. Two techniques for establishing temperature from micro-Raman measurements were used to reveal excellent correlation to the model, and also provide insight into the relationship between temperature and structural change in the device. Temperature modeling data is reported for base plate temperature from 85°C to 250°C for practical GaN HEMT devices. Implications of the measurements for GaN HEMT reliability stress testing and analysis will be discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Polymer cell culture substrates with micropatterned carbon nanotubes.

Abstract: This article presents study of the interactions between cells and micropatterned carbon nanotubes on a polymer cell culture substrate. The polymer substrates with patterned carbon nanotubes were fabricated using an imprint process, whereby the nanotubes were pressed into a polymer layer at high temperature. The patterned substrates featured 28 different nanotube patterns of microscale lanes and circles, where the feature sizes ranged from 9 to 76 μm. Osteoblast-like cells were seeded on the substrates and cell alignment was quantified via fluorescent and electron microscopy. Many patterns were fabricated on each polymer substrate, allowing 28 different experiments on each cell culture substrate, which were tested over 10,000 cells. The cell response to the patterned nanotubes showed a maximum alignment to the microlane patterns of 55 ± 6% and no significant alignment to microcircle patterns. This work enables the study of cell response to a wider range of patterns featuring both the micro and nano length scales. © 2007 Wiley Periodicals, Inc. J Biomed Mater Res 2008

High-speed thermal analysis of high power diode arrays

Abstract: This paper explores high-speed image capture as a viable approach for non-invasive thermal analysis. The ability to monitor thermal transients and obtain accurate spatial information for miniature devices is important for many lighting applications. In this study, high power optoelectronic devices are stressed, and the impact of self-heating effects is examined. Results demonstrate that these effects lead to degraded device performance, reduced efficiency, and power loss.

Temperature Measurement of Microdevices using Thermoreflectance and Raman Thermometry

Abstract: Device temperature is often a primary factor in the proper operation, reliability, and lifetime of both MEMS and microelectronics. Thus, the measurement and verification of operational temperature is often an integral aspect the design and improvement of microdevices for commercial applications. Raman thermometry and thermoreflectance are two techniques commonly employed in the measurement of temperature at small length scales since they are noncontact in nature and their spatial and temporal resolution is on par with the needs of current device architectures. This work provides a summary in the physical basis, experimental methodology, and application of each of these techniques with respect to the analysis of microdevices.

The technical and broader societal impact of organic photovoltaics

Abstract: The need to develop alternative sources of energy has reached a critical level. Organic photovoltaics (OPVs) represent a next generation of solar cell technologies that will compete to be one of many viable solutions to this energy resource problem, with particular utility in low power consumption devices. To fully realize the technical and societal benefits of OPVs as an energy resource, research and development of the technical and performance status of OPVs is warranted along with a concomitant appreciation of the impact of policy and the environment.