Composition dependence of phonon scattering rates in InAs/AlSb superlattices: application toward quantum well hot carrier solar cells

In photovoltaic devices, hot electrons generated by the absorption of solar energy typically thermalize by dissipating energy to phonons. The presence of hot optical phonons can inhibit the thermalization of electrons, thus facilitating a stable hot electron population. The significantly larger phonon band gap between optical and acoustic phonons in AlSb (relative to InAs) suppresses the scattering of optical phonons by lower frequency phonons, thus diminishing energy loss from higher energy phonons, creating the potential of hot phonons. By modifying the InAs/AlSb superlattice composition to be AlSb dominant, similar scattering behavior in the superlattice can be achieved, through a larger effective “phonon frequency gap” in AlSb-rich superlattice. In this work, we study phonon scattering rates in InAs/AlSb superlattices as a function of superlattice composition using first-principles methods based on deriving the harmonic and anharmonic interatomic force constants from density-functional theory and using them in a solution of the phonon Boltzmann transport equation. Computations are performed for four superlattices with compositions ranging from AlSb-rich to InAs-rich (93.75% AlSb, 87.5% AlSb, 12.5% AlSb, and 6.25% AlSb). Average phonon scattering rates of high-energy phonons through decay into lower energy phonons are found to be lower in the AlSb-rich superlattices. These lower scattering rates in the AlSb-rich superlattices can have important implications for the design of high-efficiency hot carrier solar cells.