Optimisation of low-dimensional materials for thermal management applications and novel passivation layers in devices

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Low-dimensional materials (LDMs) have been identified as candidates for next-generation electronics, with the potential to extend device scaling limits and enable effective heat spreading due to promising thermal properties. However, difficulties producing high-quality, stable materials for devices currently limit applications. Strategies to optimise the thermal properties and stability of LDMs compatible with their ultra-thin nature and sensitivity to external perturbation by residues, substrates or impurities are required.

Thermal conductivity of single-layer graphene and 11-layer h-BN were studied using an opto-thermal Raman method. The effect of polymer residues on graphene thermal conductivity was quantified and values as high as those reported for clean graphene were recovered (3100 W/mK) by maximising the separation between residue clusters. The effect of isotopic disorder on h-BN was examined by comparing the thermal conductivity of the natural (20% 10B, 80% 11B) and monoisotopic (>99% 10B) material. An enhancement factor of 56% was found and the measured thermal conductivity for monoisotopic h-BN was almost as high (630 W/mK) as that previously reported for 1-3 layer natural samples. Recommendations for improving the accuracy of LDM opto-thermal Raman measurements are put forward based on thorough analysis of measurement uncertainty.


A heterostructure approach to passivation of air-unstable layered material GaTe was investigated using graphene and h-BN as top or bottom encapsulation layers. Sandwiching GaTe between two graphene layers slowed the degradation of GaTe significantly, resulting in only a 17.6% reduction in photoluminescence response of the material after two weeks, compared with 100% for unencapsulated flakes. Raman monitoring of degradation was employed to reveal substrate wettability as a key factor in controlling GaTe stability. Hybrid GaTe/graphene photodetector devices on SiO2/Si or h-BN were fabricated, with the latter exhibiting improved stability under bias. Responsivity and external quantum efficiency of 4×106A/W and 9×108% were measured, exceeding all previously reported values for GaTe-based photodetectors.
Date of Award28 Sep 2021
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorJames W Pomeroy (Supervisor) & Martin H H Kuball (Supervisor)

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