https://research-information.bris.ac.uk/files/123523406/Full_text_PDF_accepted_author_manuscript_.pdf

Simultaneous determination of the lattice thermal conductivity and grain/grain thermal resistance in polycrystalline diamond

A novel transient thermal characterization technology is developed based on the principles of transient optical heating and Raman probing: time-domain differential Raman. It employs a square-wave modulated laser of varying duty cycle to realize controlled heating and transient thermal probing. Very well defined extension of the heating time in each measurement changes the temperature evolution profile and the probed temperature field at μs resolution. Using this new technique, the transient thermal response of a tipless Si cantilever is investigated along the length direction. A physical model is developed to reconstruct the Raman spectrum considering the temperature evolution, while taking into account the temperature dependence of the Raman emission. By fitting the variation of the normalized Raman peak intensity, wavenumber, and peak area against the heating time, the thermal diffusivity is determined as 9.17 × 10−5, 8.14 × 10−5, and 9.51 × 10−5 m2/s. These results agree well with the reference value of 8.66 × 10−5 m2/s considering the 10% fitting uncertainty. The time-domain differential Raman provides a novel way to introduce transient thermal excitation of materials, probe the thermal response, and measure the thermal diffusivity, all with high accuracy.

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Development of time-domain differential Raman for transient thermal probing of materials

In this paper, gate resistance thermometry (GRT) was used to determine the channel temperature of AlGaN/GaN high electron-mobility transistors. Raman thermometry has been used to verify GRT by comparing the channel temperatures measured by both techniques under various bias conditions. To further validate this technique, a thermal finite-element model has been developed to model the heat dissipation throughout the devices. Comparisons show that the GRT method averages the temperature over the gate width, yielding a slightly lower peak temperature than Raman thermography. Overall, this method provides a fast and simple technique to determine the average temperature under both steady-state and pulsed bias conditions.

Characterization of AlGaN/GaN HEMTs Using Gate Resistance Thermometry

The ability to fabricate AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrates has enabled the production of low cost high power electronics. To further enhance the performance of GaN electronics for high power conversion, the ability to maintain high off-state breakdown voltages with large electron densities is necessary. The use of Si substrates, however, limits the device's capabilities due to its weak electrical field strength. This limitation has been identified as the main cause for breakdown in HEMTs when the high electric field reaches the silicon substrate underneath the region between the gate and drain. To overcome this obstacle, removal of the Si substrate between the gate and drain region has shown to increase the device's breakdown voltage up to 3000 V. While removing the Si substrate extends the capabilities of GaN HEMTs for high voltage applications, the effects of the Si removal on the thermal performance during operation has not yet been investigated. Raman Thermometry, a well-developed technique, is used to compare the maximum temperature rise between a Local Substrate Removed (LSR) device and a non-LSR device. The application of nanoparticles (TiO2 and ZnO) for measuring surface temperatures via Raman spectroscopy is also investigated and applied to determine a more accurate temperature of the gate junction temperature. The LSR device was found to have a much higher thermal resistance than its non-LSR device counterpart limiting the maximum power dissipation the LSR device can achieve before severe degradation. Volumetric averaged residual stress mapping was also measured via Raman Spectroscopy and suggests the removal of the Si relaxes the stress in the GaN buffer layer and AlGaN barrier which can be exploited in designs to improve reliability. Methods to improve the thermal reliability of LSR devices are key to implementing such devices as future power switches.

The thermal effects of substrate removal on GaN HEMTs using Raman Thermometry

Environmental transmission electron microscopy was employed to probe transformations in the size, morphology, and composition of individual atmospheric particles as a function of temperature. Two d...

Heating-Induced Transformations of Atmospheric Particles: Environmental Transmission Electron Microscopy Study.

A new technique of high-resolution micro-Raman thermometry using anatase TiO2 microparticles (0.5–3 μm) is presented. These very high spatial resolution measurements (280 nm) reveal temperature gradients even within individual microparticles. Potential applications of this technique are demonstrated by probing the temperature distribution of a micro-fabricated heater consisting of a thin silicon nitride (Si-N) membrane with a gold coil on top of the membrane. Using TiO2 microparticle micro-Raman thermometry, the temperature from the outer edge of the coil to the inner portion was measured to increase by ∼40 °C. These high spatial resolution microscopic measurements were also used to measure the temperature gradient within the 20 μm wide Si-N between the gold heating coils. 2D numerical simulations of the micro heater temperature distribution are in excellent agreement with the experimental measurements of the temperatures. These measurements illustrate the potential to extend applications of micro-Raman thermometry to obtain temperature details on a sub-micrometer spatial resolution by employing microparticles.

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Peter A. F. Anastasi

Papers

High spatial resolution Raman thermometry analysis of TiO2 microparticles.