Engineering Defect States and Charge Puddles in Topological Insulator Thin Films for Enhanced Spintronic Functionality

Sharma, Kanav (2026) Engineering Defect States and Charge Puddles in Topological Insulator Thin Films for Enhanced Spintronic Functionality. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Kanav Sharma (20RS108)) 20RS108.pdf - Submitted Version Restricted to Repository staff only Download (67MB)

Abstract

Topological insulators (TIs) have emerged as a transformative class of quantum materials which are characterized by an insulating bulk and metallic surface states. The spin-momentum locking is the key feature of these surface states. This means an electron’s spin orientation is strictly dictated by its propagation direction. This unique chiral property suppresses 180◦ backscattering from non-magnetic impurities. This property of TIs helps to produce highly efficient, ultra low power spintronic devices such as spinorbit torque memories, pure spin current generators, and spin to charge converters. Also when proximity coupled to a conventional s-wave superconductor, TI surface states are predicted to host Majorana zero modes. These modes are non-Abelian quasi-particles that hold immense promise as the foundational building blocks for fault tolerant topological quantum computation. Despite this great potential of TIs, intrinsic defects and residual bulk conduction frequently obscure their surface transport properties. This thesis presents a comprehensive investigation into the growth, defect-state engineering, and quantum transport phenomena of ternary (Bi,Sb)₂Te₃ (BST) thin films and their Indiumdoped counterparts (IBST). In particular this thesis will tell what types of intrinsic and extrinsic defects create the major problems to our TIs. Intrinsic defects, such as vacancies and anti-site defects, simply shifts the chemical potential of TIs into the bulk bands. This in turn increases the bulk conduction and obscures the topological surface states. To isolate the surface contributions one need to make the bulk highly insulating. In this work, we achieve this through Indium doping and investigate its impact on the material’s impurity band states. Using low-frequency noise spectroscopy, we reveal the generation-recombination noise that arises from these dominant impurity bands. Indium doping successfully enhances bulk insulation through compensation. But it also introduces a high density of charged impurities. These impurities cause the band structure to meander, leading to the formation of nanoscale charge puddles. Consequently, we examine the direct interplay between these charge puddles and carrier mobility. Finally, we report the observation of resistance switching in spin pumping experiments using a bilayer NiFe/BST, a phenomenon distinctively isolated at low drive currents. Ultimately, this research quantitatively establishes the limits of topological protection against impurity-driven scattering. This outlines a practical approach for utilizing surface states in electrically tunable spintronic devices.

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