Lisa Frammolino Final Defense

Abstract: 

The continued scaling of semiconductor devices necessitates novel material systems beyond traditional silicon. Two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising candidates due to their exceptional electronic, optical, and mechanical properties. Realizing their potential in nanoelectronics requires addressing two critical issues: (1) the impact of atomic point defects and their interactions, and (2) the Schottky or Ohmic nature of TMD–metal contacts.

The first part of this dissertation will report on the interaction of the two most common atomic point defects in tungsten disulfide (WS₂), namely the sulfur vacancies (V_S) and oxygen substituting sulfur vacancies(O_S). Utilizing scanning tunneling microscopy/spectroscopy (STM/S) technique, the appearance of a new occupied in-gap state (OIGS) at the V_S site resulting from its interaction with local O_S is revealed. The results from the STM measurements show that the energy location of the OIGS is a strong function of the local O_Sdensity and that the OIGS originates from the Γ-valley modification of the tungsten d- and sulfur p-orbital hybridization.

The next part investigates the nature of the contact between 2D semiconducting TMDs and semimetals.It has been predicted that by forming a van der Waals (vdW) junction with semimetal can greatly reduce Fermi level pinning effects and facilitate good Ohmic contact. Here, we fabricate atomically clean interfaces between monolayer TMDs (MoS₂, WSe₂, WS₂) and bulk semimetals (Bi and graphite) and directly probe the electronic structure across the vdW 2D-semiconductor/semimetal junction using a combination of STM/S and angle-resolved photoelectron spectroscopy techniques. We reveal n-type doping of TMDs when on Bi(111) with small-to-no Schottky barrier heights. Additionally, with field emission resonance STS measurements, work functions of both the semiconductor and semimetal for each heterostructure are determined individually, allowing for direct assessment of the validity of the Schottky-Mott rule (SMR). When TMD is on Bi(111), the SMR appears to hold, however, when on graphite the SMR breaks down completely.