Heat-Transport Mechanisms in Superlattices

Superlattices are important structures for thermoelectric applications because of their potential for achieving high efficiency for thermoelectric energy conversion. Despite numerous theoretical and experimental studies, basic understanding of the thermal conductivity L of superlattices is incomplete. In semiconductors, heat is carried by wave-like lattice vibrations, i.e., phonons, with a broad distribution of mean-free-paths ‘. For long-period semiconductor superlattices (i.e., ‘ < h, where h is the thickness of the individual layers in the superlattice), L is reduced by the finite transmission coefficient of phonons across interfaces; this fundamental transport property of the interface is typically referred to as the thermal conductance of the interface. This simple picture cannot, however, explain heat transport in shortperiod superlattices (i.e., ‘ > h) because the thermal conductivities of short-period superlattices do not decrease linearly with superlattice period. Even generic understanding of which lattice modes dominate heat transport is lacking. For example, optical phonons which do not contribute significantly to the heat conduction in bulk crystals may be the dominant heat carriers in short-period superlattices through tunneling and mode conversions. To complicate matters further, coherent reflections of long-wavelength acoustic phonons from multiple interfaces may result in the formation of phonon minibands, in which phonons transmit across interfaces with high probability but with reduced group velocity. As a result, theory predicts that L of superlattices with atomically smooth and abrupt interfaces should increase with decreasing period for superlattices with short periods. Indeed, some researchers reported an increase of L in short-period superlattices, but other researchers working on similar materials reported a decrease ofL. This inconsistency suggests that, in addition to temperature and period, other parameters, such as interface roughness and lattice mismatch, might be important. In this paper, we identify the heat transport mechanisms in superlattices from thermal conductivity L measurements of (AlN)4 nm–(GaN)y superlattices over awide range ofGaN thickness, 2 nm< y< 1000nm, and temperature, 90<T< 600K. We affirm thatheat conduction in long-periodsuperlattices is controlledby the thermal conductance of interfaces. To elucidate the dominant heat carriers in short-period superlattices, we created point defects in an (AlN)4.1 nm–(GaN)4.9nmsuperlatticebybombardmentwith2.3MeV Arþ ions. As high-frequency acoustic and optical phonons are strongly scattered by point defects, the small reduction inL that we observe in these ion-bombarded superlattices implies that heat is transported predominantly by long-wavelength acoustic phonons; these long-wavelength phonons are relatively weakly scattered by the superlattice interfaces. Our study of the thermal transport physics of superlattices is facilitated by our capability for growing GaN-based superlattices with high structural quality. AlN/GaN interfaces grown by molecular beam epitaxy (MBE) are chemically abrupt at monolayer scale with interface roughness of a fewmonolayers. Our results are also technologically important. GaN-based heterojunctions and superlattices are emerging material F U LL P A P E R www.afm-journal.de