Understanding in-plane thermal transport in crystalline semiconducting thin films

Zhe Luo, Purdue University

Abstract

Crystalline thin films have found their applications in a broad range of industrial and research areas, such as integrated circuits, solar cells, optical coatings, thermal insulators, etc. In the past decade, the rapid emerging of novel materials such as two-dimensional crystals and topological insulators, which are typically in thin film forms, brought up even more intensive research interest. The in-plane thermal characterization of nanometer-thick thin films, which is a significant part of guiding the design and application of thin-film-based devices, remains challenging. The vulnerability of the nanoscale thin films, the minute amount and fast speed of energy transport, and the strong size effects, all require meticulous and accurate measurement platforms. In this dissertation, a non-contact thermal measurement system based-on Raman spectroscopy was developed and applied on various thin films, with some creative extensions or modifications of the thermometry system. Thermal conductivity of traditional semiconducting and amorphous thin films, novel two-dimensional materials, and topological insulator thin films were studied. Various mechanisms that determine the nanoscale heat transfer within these thin films were investigated, including phonon scattering, phonon dispersion, and bulk and surface state electron transport. The measurements on polycrystalline bismuth films reveal that the diffusive phonon scattering at the film surface greatly reduces the in-plane thermal conductivity, as well as grain boundary scattering of phonons. A Raman transducer method was developed to enable the micro-Raman thermometry to measure amorphous aluminum oxide films (and other thin films without Raman response), and the results match well with the predictions of the minimum thermal conductivity theories. A recently rediscovered two-dimensional material, black phosphorus, was prepared using polymer-based transfer method and measured with a stretched laser heat source which realized quasi-one-dimensional heat transfer. The measured in-plane thermal conductivity of few-layer black phosphorus films is highly anisotropic; with first-principles-based theoretical modeling, the anisotropy of thermal transport is attributed to the unique phonon dispersion of the material, which is anisotropic along the armchair and zigzag directions. The phonon scattering rate turned out to be almost isotropic. The Bi-Sb-Te-Se topological insulators were studied with a specific focus on electron contribution to the total in-plane thermal conductivity. The bulk electron thermal conductivity was studied on Bi0.1Sb1.9Te 3, and the Lorenz number was extracted. The surface state electron contribution to the in-plane thermal conductivity was investigated on ultra-thin Bi 2Te3 and Bi2Te2Se films. For Bi 2Te3 films, the surface state effect could be barely seen, while the results of Bi2Te2Se shows a thermal conductivity enhancement at small thicknesses, which is attributed to the topological surface states. In addition, light-polarization-dependent photocurrent measurements were conducted to study the spin-polarized surface electrons in BiSbTeSe 2.

Degree

Ph.D.

Advisors

Xu, Purdue University.

Subject Area

Mechanical engineering

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