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- μ m -wide membranes that consist mainly of wurtzite GaN and a stack of In 0.15 Ga 0.85 N 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 κ 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 κ in plane = 165 − 14 + 16 W m − 1 K − 1 for the best membrane. This result compares well with κ in plane = 177 W m − 1 K − 1 obtained by 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 κ 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 κ in plane by almost 40%, which is accompanied by an anisotropy of κ in plane due to etch channel formation. Thanks to the 2LRT approach, such variations and anisotropies of κ in plane become accessible to the experimentalist via highly spatially resolved temperature maps.