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Chapter 4:
Thermal Conductivity and Thermal Distribution in Superlattice Structures
Optimization of high-power semiconductor light emitters requires knowledge of the thermal conductivity tensor of its various functional semiconductor layers from room temperature down to cryogenic temperatures. Optical devices such as long-wavelength (LWIR) and midwave infrared (MWIR) high-output light-emitting diodes (LEDs), quantum cascade lasers (QCLs), and interband cascade lasers (ICLs) perform best at low temperatures in continuous wave (CW) or are commonly operated not too much higher than room temperature in pulsed mode, and the lifetime of an optical device decreases exponentially with increasing temperature. Therefore, it is important to know the thermal conductivity of the active region and cladding layer in such structures to model the heat flow, and it is preferable to use a material with high thermal conductivity to deliver the heat out of the active region of the device and reduce the operating temperature. Furthermore, most of the optical devices nowadays are made of nanostructured materials, such as semiconductor superlattices, which can have anisotropic thermal conductivities, either by virtue of their anisotropic structure or their anisotropic shape with length scales below the phonon mean free path. The heat conduction model can be more accurate if both the in-plane and cross-plane thermal conductivity are calibrated.
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