Phonons are responsible for all thermal properties of a solid, such as its heat content and transport. The anharmonic part of lattice oscillations causes thermal expansion. All these are integral contributions of the phonon spectrum; only at low temperatures, where part of the spectrum can be frozenout, do they become partially spectrum selective. Thermal conductivity is determined by various mechanisms of phonon scattering.
KeywordsFrozen-out; heat content; phonon scattering; thermal conductivity; thermal expansion; transport
Heat CapacityThere are three macroscopic thermal properties originating from phonons:• Heat capacity • Thermal expansion • Thermal conductivity neglecting electronic contributions here. Since these are easily observable effects, they were studied early, and rather general, often semiempirical descriptions were given. These properties are of interest with regard to the interaction of electrons with phonons and the dissipation of energy, e.g., in optical excitation. The first set of processes is discussed in chapters "▶ Carrier Scattering at Low Fields" and "▶ Carrier Scattering at High Fields." The latter processes are kinetic in nature and are discussed in chapters "▶ Carrier Transit and Relaxation" and "▶ Phonon and Exciton Kinetics." In this chapter we will provide the essential basic information.Heat capacity is a measure of the content of thermal energy, which is stored in all active oscillations of atoms in the solid. The specific heat (or specific heat capacity) expresses the change *Email: pohl@physik.tu-berlin.de 1 Although the specific heat is measured more easily for constant pressure C P , the difference between C V and C P is very small for solids and is given bywhere a is the thermal expansion coefficient, k is the isothermal compressibility, and V m is the molar volume.Semiconductor Physics
