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We demonstrate a non-monotonic evolution of thermal conductivity oflarge-area hexagonal boron nitride films with thickness. Wrinkles anddefects/impurities are present in these films. Raman spectroscopy, anoptothermal non-contact technique, is employed to probe the temperature andlaser power dependence property of the Raman active E2ghigh phonon mode, whichin turn is used to estimate the rise in the temperature of the films underdifferent laser powers. As the conventional Fourier law of heat diffusioncannot be directly employed analytically to evaluate the thermal conductivityof these films with defects and wrinkles, finite element modeling is usedinstead. In the model, average heat resistance is used to incorporate anoverall defect structure, and Voronoi cells with contact resistance at the cellboundaries are constructed to mimic the wrinkled domains. The effective thermalconductivity is estimated to be 87, 55, and 117 W/m.K for the 2, 10, and 30nm-thick films, respectively. We also present a quantitative estimation of thethermal resistance by defects and wrinkles individually to the heat flow. Ourstudy reveals that the defects/impurities render a much higher resistance toheat transfer in the films than wrinkles.

Decoupling the Roles of Defects/Impurities and Wrinkles in Thermal Conductivity of Wafer-scale hBN Films