Temperature-Dependent Thermal Conductivity and Heat Capacity of InGaAs and InAlAs Thin Films

While several works in the literature have experimentally examined the reduction in thermal conductivity (<i>k</i>) when two materials are alloyed together, there are comparatively fewer measurements related to the impact on the volumetric heat capacity (<i>C</i>_v) of such alloys systems. In most measurement scenarios utilizing periodic heating, <i>C</i>_v is an important input parameter and often estimated to be a compositional-average of the alloyed materials; sometimes referred to as a virtual-crystal approximation (VCA). New computational methods such as Green-Kubo modal analysis (GKMA) have been used to highlight the inability of the VCA to describe the temperature-dependent <i>k</i> of thin film InGaAs on the basis of the latter being unable to capture the true vibrational mode-character of the alloy. Given that the temperature-dependence of <i>C</i>_v is directly representative of the vibrational mode-character in a material without requiring knowledge and/or assumptions of phonon scattering rates, it is apparent that an ability to measure <i>C</i>_v(<i>T</i>) directly in thin film alloys would likely aid in answering several fundamental questions related to the mode-character in structurally-ordered, chemically-disordered alloy systems.<br/>This presentation chronicles our work measuring <i>k</i>(<i>T</i>) and <i>C</i>_v(<i>T</i>) of InGaAs and InAlAs thin films from 80 – 450 K. The sample set examined consists of alloy films deposited on InP substrates via metal-organic chemical vapor deposition (MOCVD) at In contents suitable for use as well and barrier layers in quantum cascade lasers (QCLs) and thicknesses ranging from ~100 – 500 nm. Several opto-thermal pump-probe measurement techniques including time-domain and steady-state thermoreflectance (TDTR and SSTR, respectively) were utilized to minimize the number and impact of various thermophysical parameters that could otherwise impact our ability to accurately determine <i>k</i>(<i>T</i>) and <i>C</i>_v(<i>T</i>). The resulting measurements of <i>C</i>_v(<i>T</i>) are compared to predictions from several flavors of the VCA employing both Debye-like and full phonon dispersions, highlighting the importance of these measurements for avoiding erroneous assumptions for <i>C</i>_v(<i>T</i>). This work provides important insight to aid in both the development and validation of future computational models focused on thermal transport processes in alloy material systems.