An overview of photothermal deflection principles and applications is given. The modeling of temperature distribution and the calculation of deflection that is due to both the refractive-index gradient and the thermal deformation of the sample are presented. Three configurations usually employed are compared, and their respective advantages are discussed in relation to their application. The calibration for absolute measurement of absorption is detailed, showing that calibration limits the accuracy of measurement. Some examples of specific information obtained by photothermal mapping of absorption are given.

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Characterization of optical coatings by photothermal deflection.

The chapter demonstrates the fundamental differences between phonon heat transport in nanostructures and that in macrostructures. Interfaces and surfaces play a pivotal role for the thermal conductivity reduction. For heat flow parallel to interfaces, diffuse interface scattering is the key to reduce thermal conductivity along the film-wire plane. It discusses that for heat conduction perpendicular to interfaces, phonon reflection, particularly total internal reflection reduces the heat transfer and thermal conductivity. The chapter explores that defects, particularly dislocations, create a large thermal conductivity reduction. The mismatch in the lattice constants among different materials in nanocomposite structures creates high dislocation density that decreases the lattice thermal conductivity. Low-dimensionality offers the possibility to increase high figure-of-merit ZT through enhancing the power factor and reducing the lattice thermal conductivity. Opportunity windows may exist to simultaneously increase the power factor while decreasing the thermal conductivity.

Chapter 5 - Phonon Transport in Low-Dimensional Structures

Photothermal reflectance and photothermal displacement measurements of optical absorption and thermal conductivity are reported for electron-beam-(EB) deposited and ion-plated (IP) thin films of TiO(2), Ta(2)O(5), and ZrO(2). Of the particular set of samples investigated, the EB films have higher absorption than the IP films. The absorption of the EB samples decreases over a period of ~ 90 min on irradiations with an Ar-ion laser of 488-nm wavelength. By contrast, the absorption of the IP samples changes insignificantly or not at all. Photothermal displacement area scans of coating surfaces yield lower defect densities for the IP samples compared with the EB samples for all three oxide materials. The feasibility and limitations of photothermal measurements for thin-film optical and thermal characterizations are discussed.

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Absorption and thermal conductivity of oxide thin films measured by photothermal displacement and reflectance methods.

The normal component of thin-film thermal conductivity has been measured for the first time, to the best of our knowledge, for several advanced sputtered optical materials. Included are data for single layers of boron nitride, silicon aluminum nitride, silicon aluminum oxynitride, silicon carbide, and for dielectric-enhanced metal reflectors of the form Al(SiO2/Si3N4)n and Al(Al2O3/AlN)n. Sputtered films of more conventional materials such as SiO2, Al2O3, Ta2O5, Ti, and Si have also been measured. The data show that thin-film thermal conductivities are typically 10 to 100 times lower than conductivities for the same materials in bulk form. Structural disorder in the amorphous or fine-grained films appears to account for most of the conductivity difference. Conclusive evidence for a film–substrate interface contribution is presented.

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Thermal conductivities of thin, sputtered optical films

Due to continued miniaturization, the performance and reliability of electronic devices composed of multiple thin layers of material are highly dependent on effective thermal management. Since the thermal properties of thin films, such as SiO/sub 2/, can vary considerably from bulk values, the determination of those properties (as well as the interface resistance between SiO/sub 2/ and adjacent layers) is critical for the purposes of design. In this work, a transient thermo-reflectance system has been employed to measure the thermal characteristics of thin-film SiO/sub 2/ layers. Results show that for layers of SiO/sub 2/ in the range of 100-1000 /spl Aring/, the intrinsic thermal conductivity (TC) is independent of thickness and smaller than the traditionally reported value of bulk silicon dioxide (1.4 W/m-K). The intrinsic value was measured to be around 90% (1.27 W/m-k) and 75% (1.05 W/m-k) of the latter bulk value for thermally grown (TG) and ion beam sputtered (IBS) oxides, respectively. The thermal interface resistances of TG and IBS SiO/sub 2/ films were measured at 1.68 /spl times/ 10/sup -8/ m/sup 2/-K/W and 2.58 /spl times/ 10/sup -8/ m/sup 2/-K/W, respectively. If a chromium film of around 100 /spl Aring/ is deposited between the gold and SiO/sub 2/ layers, the interface thermal resistance improves to 0.78 /spl times/ 10/sup -8/ m/sup 2/-K/W for TG films and 1.15 /spl times/ 10/sup -8/ m/sup 2/-K/W for IBS films. Thus, the effective thermal resistance of SiO/sub 2/ thin-films (i.e., with interface effects) is up to one order of magnitude smaller than the values reported for bulk SiO/sub 2/.

Thermal transport properties of gold-covered thin-film silicon dioxide

A survey of experimental methods and data for thin-film thermal conductivity and heat
capacity is given. Simple microscopic theory is provided as a basis for understanding the
very small values of thin-film thermal conductivity observed. Interfacial resistance
effects, including some conjecture on origins, as well as applications of thermal property
data are discussed. The needs for future development of this important area of investigation
are summarized, and some suggestions for emphasis are provIded.

Donald L. Decker

Papers

Thermal properties of optical thin-film materials