arXiv subjects

Kousik Bera

Publications and source records attributed to Kousik Bera.

2 recordsLinked to original sources

arXiv2023

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

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.

cond-mat.mtrl-sci↗

arXiv2019

Strain distribution and thermal strain relaxation in MOVPE grown hBN films on sapphire substrates

Recently, hexagonal boron nitride (hBN) layers have generated a lot of interest as ideal substrates for 2D stacked devices. Sapphire-supported thin hBN films of different thicknesses are grown using metalorganic vapour phase epitaxy technique by following a flow modulation scheme. Though these films of relatively large size are potential candidates to be employed in designing real devices, they exhibit wrinkling. The formation of wrinkles is a key signature of strain distribution in a film. Raman imaging has been utilized to study the residual strain distribution in these wrinkled hBN films. An increase in the overall compressive strain in the films with an increase in the layer thickness has been observed. To find whether the residual lattice strain in the films can be removed by a thermal treatment, temperature dependent Raman measurements of these films are carried out. The study demonstrates that the thermal rate of strain evolution is higher in the films of lower thickness than in the thicker films. This observation further provides a possible explanation for the variation of strain in the as-grown films. An empirical relation has been proposed for estimating the residual strain from the morphology of the films. We have also shown that the residual strain can be partially released by the delamination of the films.

cond-mat.mtrl-sci↗