Cross-Plane Thermal Conductivity in III-V Epitaxial Superlattices: The Role of Composition

Epitaxial superlattices (SL) offer an interesting model-system to study the dependence of thermal transport on the characteristics of interfaces, also thanks to the great advancement in characterization techniques that now allow the study of these mechanisms at the sub-µm scale.[1] A deeper comprehension of phonon transport, together with the development of high ZT materials, is highly desirable to allow the scalability of devices into local thermoelectric (TE) coolers integrated into optoelectronic devices.[2] III-V compounds are appealing from both an applicative point of view, for thermal management of III-V optoelectronic devices, and a fundamental one, since they offer the possibility to study several parameters (interface roughness, atomic interdiffusion, strain) by changing the SL composition and growth conditions. We present the study of InAs/GaAs and AlAs/GaAs SLs grown on (100) GaAs substrates by Molecular Beam Epitaxy with identical SL period; the results are compared to those obtained on InxGa1-xAs and AlxGa1-xAs alloys with composition equivalent to that of the SLs (x = 0.058). The structures' properties are studied by low-temperature photoluminescence (PL) and high-resolution X-ray diffraction (HRXRD) to evaluate their strain state, the atomic interdiffusion (typical of InAs/GaAs SLs due to In segregation [3]) and the interface abruptness (typical of AlAs/GaAs SLs). The cross-plane thermal conductivity (?) of the structures is measured with the 3? technique in the 200 K <= T <= 380 K range. Both types of SLs show a reduction of ? compared to the corresponding bulk binaries and, in both cases, ? is not sensibly dependent on the number of periods composing the SLs, suggesting that the phonon transport is incoherent.[1] When comparing the SLs to the corresponding alloys, we observe that only the InAs/GaAs system allows a reduction of ? with respect to the InGaAs alloy, achieving a value of 8 W/(m*K) with only 5.8% of In in the structure. Similarly to what happens in the SiGe system,[1] this behavior can be attributed to the effect of both the SL interfaces and the atomic interdiffusion due to the large segregation of In in GaAs. The AlAs/GaAs system, not characterized by the segregation phenomenon, does not show a distinct reduction of ? (15-20 W/(m*K)) with respect to the alloy. As a result of our work, we suggest that III-V SLs with reduced ? can be effectively prepared through suitable designs that aim at enhancing the atomic interdiffusion in the structures. [1] P. Chen et al. Phys Rev Lett 111, 115901 (2013) [2] I. Chowdhury et al. Nature Nanotech 4, 235 (2009) [3] L. Seravalli et al. J Phys D-Appl Phys 46, 315101 (2013)