OPTOELECTRONIC AND TRANSPORT PROPERTIES OF LOW-DIMENSIONAL LAYERED MATERIALS FOR PHOTONIC AND SPINTRONIC DEVICE APPLICATIONS

Abstract

Layered materials are an inimitable category of materials, which are known to have strong covalent bonding between atoms in the two-dimensional (2D) plane. The each single layer in these materials is weakly bonded to the neighboring layers through a much weaker van der Waals interactions, perpendicular to the 2D plane. This unique bonding environment for the atoms and layers of materials unfurls remarkable opto-electronic and transport properties. These materials can be transformed into single atomic layers, two-dimensional materials, or few-layer stacks simply by exfoliation techniques due to weak interlayer interaction. Their opto-electronic and transport properties change as the number of layer in the stacks changes. Also, a change in bonding environment due to change in relative position of one or more layers has substantial impacts on the properties. Such fascinating features invite immense research activities on the study of opto-electronic and transport properties of layered materials and their heterostructure. Several layered materials have been recognized as 2D materials such as black phosphorus (bP), graphene, and transition metal di-chalcogenides (TMDs) such as MoS2, WS2, VS2, etc. have been found to exhibit substantial change in their electronic band structure and consequently a change also in their opto-electronic and transport properties. Most of the studied materials have either wide or zero electronic band gap and hence are active only under the incidence of short wavelength light. Since the current research trend is gradually migrating from visible to mid infrared (MIR), the band gap in the MIR regime is highly desirable in the realm of photonic devices. However, the study of materials having inherent band-gap in MIR wavelength is still demanding. On the other hand, synthesis of materials at large scale having high electrical conductivity, required for high charge-spin conversion efficiency, and mobility are not widely explored. Recently, black arsenic (bAs) has been resurfaced as a 2D layered material having electronic band gap in MIR region and exotic opto-electronic and transport properties, similar to bP. On the other hand, platinum di-chalcogenides, an emerging class of TMD materials, have been found to exhibit ultra-high conductivity and mobility. In view of the above, this thesis work is based on the opto-electronic and transport properties of layered materials for photonic and spintronic device applications. Among several layered materials, black arsenic and platinum di-chalcogenides have drawn significant attention of the scientific community owing to their tunable band gap from visible to mid-infrared (MIR) regimes, intrinsic anisotropic crystal structure, etc. Experimental results on the opto-electronic and transport properties of layered materials for photonic and spintronic device applications have been presented in this dissertation. The following six chapters comprise the thesis. In Chapter 1, brief introduction of layered materials, in particular black arsenic, hexagonal boron nitride, and platinum di-chalcogenides, is presented including their energy band gap, crystal structure, significant highlights of the discovery and history followed by the potential and challenges. Along with this, different types of deposition and growth techniques for layered materials are also discussed. In Chapter 2, the results on the thickness dependent opto-electronic properties of black arsenic are presented. Black arsenic flakes of different thicknesses were transferred on the hBN/SiO2/Si substrate. The devices were fabricated using laser lithography in same process as in chapter 2. The thickness of flakes were measured using AFM. It is realized from the optoelectronic characterization of devices that the devices of different channel thickness are showing different photoresponse. A 60 nm-thick black arsenic channel device is found to show a peak photoresponsivity of 244 A/W at 3.00 μm. However, a lower thickness (~ 25 nm) was found to exhibit maximum detectivity of 6.14 × 109 Jones, along with an excellent (i.e., the least) noise equivalent power of ∼89 fW/Hz1/2. In Chapter 3, we have fabricated bAs devices and studied their optoelectronic properties. For the device fabrication, the bulk layers of black arsenic from its crystal were transferred on the silicon-on-insulator (SOI) substrate. A heterostructure of black arsenic and Si was pattern using MicroTech 405 laser writer. The metal electrodes Au/Cr (100/5 nm) were deposited using RF-sputtering under high vacuum. The fabricated devices were characterized for opto-electronic properties of black arsenic on Si. Under the atmospheric conditions, visible to mid infrared light was found to be detected through fabricated black arsenic – Si heterostructure. In Chapter 4, platinum di-telluride is synthesized using CVD technique. Different substrates and thicknesses of platinum di-telluride were used to check the effect of substrate and channel thickness on the transport properties of platinum di-telluride. Negative magnetoresistance was also observed in some of the devices. The magnetoconductance data has been fitted to HLN equation to verify the chiral anomaly in the devices. Chapter 5 presents the work on large-scale synthesis of nickel sulfide using CVD method. Synthesized nickel sulfide was characterized using different techniques such as Raman spectroscopy, XRD, SEM, and XPS. The obtained nickel sulfide was utilized to fabricate interdigitated electrodes based devices to check its electrical properties. Source drain current is observed of the order of a few μA, which is comparable to the other transition metal di-chalcogenide multifunctional materials. Chapter 6 summarizes all the findings and presents future scope.