  We demonstrate a non-monotonic evolution of thermal conductivity of
large-area hexagonal boron nitride films with thickness. Wrinkles and
defects/impurities are present in these films. Raman spectroscopy, an
optothermal non-contact technique, is employed to probe the temperature and
laser power dependence property of the Raman active E2ghigh phonon mode, which
in turn is used to estimate the rise in the temperature of the films under
different laser powers. As the conventional Fourier law of heat diffusion
cannot be directly employed analytically to evaluate the thermal conductivity
of these films with defects and wrinkles, finite element modeling is used
instead. In the model, average heat resistance is used to incorporate an
overall defect structure, and Voronoi cells with contact resistance at the cell
boundaries are constructed to mimic the wrinkled domains. The effective thermal
conductivity is estimated to be 87, 55, and 117 W/m.K for the 2, 10, and 30
nm-thick films, respectively. We also present a quantitative estimation of the
thermal resistance by defects and wrinkles individually to the heat flow. Our
study reveals that the defects/impurities render a much higher resistance to
heat transfer in the films than wrinkles.
