Frequency-Dependent Thermal Conductivity in Time Domain Thermoreflectance Analysis of Thin Films
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Frequency-Dependent Thermal Conductivity in Time Domain Thermoreflectance Analysis of Thin Films Gilles Pernot1*, Hélène Michel1, Bjorn Vermeersch1, Peter Burke2, H. Lu2, Jean-Michel Rampnoux3, Stefan Dilhaire3, Younès Ezzahri4, Arthur Gossard2 and Ali Shakouri1 *
[email protected]
1
Department of Electrical Engineering, University of California Santa Cruz, Santa Cruz, CA 95064, U.S.A. 2 Materials Department, University of California Santa Barbara, Santa Barbara, CA 93106, U.S.A. 3 LOMA, Université de Bordeaux 1, 33405 Talence, France. 4 Institut Pprime, CNRS-Université de Poitiers-ENSMA, 86022 Poitiers, France. ABSTRACT Over the past three decades, ultrashort laser pulses have been demonstrated to be a very powerful tool to investigate materials properties at the nanoscale. A key driving force is the hightime resolution required to study heat transfer across interfaces and in thin films. The TimeDomain Thermoreflectance (TDTR) is now widely used. This optical technique offers an interesting alternative to electrical approaches such as the 3ω method. We present a complete study of the TDTR signals. We investigate the influence of the modulation frequency on the measured signals and we show how this experimental parameter could be set to enhance or reduce the sensitivity to a specific thermal parameter. The dependence of the measured “apparent” thermal conductivity of the thin film as a function of the modulation frequency is discussed. Results are applied to investigate thermal properties of a series of InGaAs samples with embedded ErAs nanoparticles. INTRODUCTION Pump-Probe Time Domain Thermoreflectance (TDTR) has been demonstrated as a powerful technique to investigate acoustic and thermal properties at very short time and length scales. Nowadays, TDTR has proven its ability and is now fully accepted as a reference technique to measure the thermal properties1,2, in the same way as the well-known 3ω method. In 2007, Koh and Cahill observed a significant reduction of the thermal conductivity in alloy materials by varying the frequency of the heat source at the MHz scale3. Such a reduction was theoretically predicted by Volz five orders of magnitude higher4. The mechanics of this reduction, the competition between ballistic and diffusive effects and their impact on TDTR measurements of the thermal conductivity are still not fully understood. In this work, we first present an analysis of the TDTR signal, we show the influence of the frequency and the waveform of the modulated heat source. Their impact on the TDTR signal and the sensitivity of certain thermal parameters are discussed. We present the experimental results on different semiconductor bulk materials and nanocomposite thin layers of InGaAs doped with ErAs nanoparticles. The frequency dependence of the thermal conductivity will be discussed.
TIME DOMAIN THERMOREFLECTANCE PRINCIPLE Time Domain Thermoreflectance technique is based on a femtosecond pumpprobe set-up where an ultrashort laser "pump" pulse generates a transient thermal response, and a weake
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