Understanding the impact of deviations from perfect periodicity on the motion of electrons in a periodic crystal, in other words, understanding the effects of crystalline disorder, has been a central problem in studying condensed matter bulk materials. The extensive study of disorder in atomically thin two-dimensional (2D) van der Waals (vdW) materials, such as graphene and transition metal dichalcogenides, has unveiled crucial insights into how defects, impurities, and structural imperfections influence their electronic and optical properties. However, there is a notable gap in the research concerning the impact of disorder in thin films of three-dimensional (3D) vdW crystals, specifically those with thicknesses on the order of a few hundred nm to a few microns. This intermediate scale, neither truly 2D nor bulk 3D, presents unique challenges and opportunities for understanding the disorder because crystalline disorder can become relevant. This thesis aims to bridge this gap by investigating the effects of disorder in MoTe2, a 3D Weyl semimetal, and HfTe5 and ZrTe5, both (weakly-gapped) 3D Dirac semimetals, within this thickness range. By employing advanced characterization techniques, such as high-resolution scanning photocurrent microscopy and Raman spectroscopy, the goal is to elucidate how different mesoscopic defects, such as layer stacking disorder and local strain inhomogeneity, alter the properties of these materials. The gained insight may facilitate novel applications in electronics and optoelectronics, where control over disorder at this scale could be advantageous. MoTe2 can be realized in three different crystal phases: 2H (semiconducting), 1T' (metallic), and Td (topological Weyl semimetal). Consequently, MoTe2 has garnered significant research attention, partly focused on elucidating the topological phase transition from the metallic 1T' phase to the topological Td phase. Using photocurrent microscopy, we spatially probed the temperature-dependent phase transition from 1T' to symmetry-broken Td-MoTe2. We found a highly disordered photocurrent response in the intermediate temperature regime when the films are neither in 1T' phase nor Td phase, with domain sizes of a few microns. This highly disordered intermediate phase also resulted in ultrafast helicity-dependent photocurrents, which are otherwise symmetry-forbidden. In recent years, HfTe5 and ZrTe5 have emerged as another highly versatile platform for delving into unconventional quantum and topological transport phenomena of low density 3D semimetals. Their impressive carrier mobilities and remarkably low carrier densities have facilitated the observation of 3D quasi-quantum and fractional Hall effects in transport under exceptionally weak magnetic fields. Moreover, positioned at the brink between trivial and topological phases, these materials exhibit an electronic structure highly responsive to external stimuli such as magnetic fields, layer thickness, or strain. Motivated by the ongoing debate on the origin of the quasi-quantized 3D Hall effect, we set out to elucidate the role of possible edge transport in exfoliated HfTe5 films. Unexpectedly, we found that the photocurrent response of the HfTe5 is governed by a hitherto hidden, anomalous photo-Nernst effect of massive Dirac fermions. In addition, we investigated the impact of contact fabrication on the local electronic properties of HfTe5. This study is motivated by the seemingly inconsistent reports on the electronic properties of HfTe5 and ZrTe5 in the recent literature, where nominally identical samples can show vastly different transport signatures. We identified signatures of local, strain-induced inhomogeneities in these highly sensitive films using spatially resolved and strain-dependent Raman spectroscopy. These local strain fields could be traced back to the contact fabrication process. Such microscale strain can locally drive HfTe5 and ZrTe5 film from trivial into topological phase (and vice versa), potentially leading to a mesoscopically disordered potential landscape. We hypothesize that the latter is the origin of the often inconsistent transport results in the literature.
