Microscale thermal phenomena in optical and optoelectronic thin film devices

This dissertation addresses the fundamental aspects of heat transfer in thin films and studies thermal phenomena in optical and optoelectronic thin-film devices.; Microscale heat transfer involves thermal energy transport processes in which heat carrier characteristic lengths (CLs) become comparable to each other or the device dimension. An order-of-magnitude analysis of these CLs reveals that two microscale heat conduction and three microscale thermal radiation regimes should be distinguished. Principles governing thermal energy transport in each regime are demonstrated.; An approach based on the coherence theory of light is introduced to describe the radiative properties of thin films. In many cases of practical interest, neither geometrical nor wave optics gives correct values for film reflectance and transmittance, because wave optics implies a coherent radiation field and geometrical optics an incoherent one, while real radiation fields are always partially coherent. The general expressions obtained from the new approach cover this intermediate range as well as the two extremes.; The temperature dependence of optical constants couples electromagnetic wave propagation and heat conduction during transient laser heating and leads to many interesting optical phenomena, such as optical bistability. For opaque thin films, the generated large temperature gradient stratifies the media optically such that the reflection from the surface depends on the temperature profile beneath it. Predictions of this internal reflection process agree well with available experimental data. Criteria are established to determine when internal reflection is important. For weakly absorbing films it is shown that the interference effects and the feedback from the temperature dependent absorption coefficient result in very non-uniform temperature fields inside the film. This non-uniformity yields the well-established Airy's formulae inapplicable for calculating the optical response of thin films.; Microscale heat conduction effects are investigated for quantum well (QW) structures. First, the equation of phonon radiative transport derived from the Boltzmann transport equation is used to predict the thermal conductivity (TC) of QW structures. The results show that the TC of QW structures can be an order of magnitude smaller than their corresponding bulk values because of phonon scattering at the QW interfaces. These results are then applied to the investigation of the temperature rise at the mirror of a QW laser. The study reveals that the thermal runaway in a QW laser is due to the absorption by the cladding media.; A new method is designed to measure thermophysical properties of thin films. After patterning a micro temperature sensor onto molecular-beam-epitaxy grown GaAs/AlGaAs surface-emitting-laser (SEL) structure of 7 {dollar}sim{dollar} 11 {dollar}mu{dollar}m thick, the substrate of the sample is removed by a selective etching technique. From the response of the temperature sensor, thermal diffusivity in both parallel and perpendicular directions is obtained. The results show an order of magnitude reduction of the thermal diffusivity of the SEL structure. (Abstract shortened by UMI.)...