Thermal analysis of GaN-based photonic membranes for optoelectronics

Semiconductor membranes are widely used in research fields that target medical, biological, environmental, and optical applications. Often such membranes derive their functionality from a nanopatterning, which challenges the determination of their optical, electronic, mechanical, and thermal properties. In this work, we demonstrate the noninvasive, all-optical thermal characterization of approximately-800-nm-thick and approximately-150-& micro;m-wide membranes that consist mainly of wurtzite GaN and a stack of In0.15Ga0.85N quantum wells as a built-in light source. Because of their application in photonics, e.g., for vertical-cavity surface-emitting lasers, such photonic membranes are bright light emitters, which challenges their thermal characterization by optical means. We combine top-view two-laser Raman thermometry (2LRT) with time-resolved photoluminescence spectroscopy to extract the in-plane thermal conductivity kappa in plane of these membranes, which represents a notable difference from previous studies on epitaxial GaN films. Thus, we can disentangle the entire laser-induced power balance. Thermal imaging by Raman spectroscopy yields kappa in plane = 165+16-14 W m-1 K-1 for the best membrane. This result compares well with kappa in plane = 177 W m-1 K-1 obtained by ab initio simulations based on a solution of the linearized phonon Boltzmann transport equation, including three-and four-phonon scattering, as well as phonon-isotope and phonon-boundary scattering. Furthermore, we study how kappa in plane is affected by a roughening of the membrane's back side and additional semiconductor layers. For the membrane with the roughest back side, we observe a reduction of kappa in plane by almost 40%, which is accompanied by an anisotropy of kappa in plane due to etch channel formation. Thanks to the 2LRT approach, such variations and anisotropies of kappa in plane become accessible to the experimentalist via highly spatially resolved temperature maps.