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A p-n junction is formed when p-type and n-type semiconductor materials are joined together. At the interface of the p-n junction, holes from the p-side and electrons from the n-side begin to diffuse into the opposite sides due to the concentration gradient. This diffusion of carriers leads to a region around the junction where there are no free charge carriers, known as the depletion region. The charge density within the depletion region for the n-side and p-side can be described by the...
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The vacuum level denotes the energy threshold required for an electron to escape from a material surface. It is usually positioned above the conduction band of a semiconductor and acts as a benchmark for comparing electron energies within various materials.
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Carrier generation is the process by which electron-hole pairs (EHPs) are created within the semiconductor. In direct-bandgap semiconductors, such as gallium arsenide (GaAs), this occurs efficiently when energy absorption prompts valence electrons to leap into the conduction band, leaving behind holes.
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The contact of metal and semiconductor can lead to the formation of a junction with either Schottky or Ohmic behavior.
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Types of Semiconductors

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Intrinsic semiconductors are highly pure materials with no impurities. At absolute zero, these semiconductors behave as perfect insulators because all the valence electrons are bound, and the conduction band is empty, disallowing electrical conduction. The Fermi level is a concept used to describe the probability of occupancy of energy levels by electrons at thermal equilibrium. In intrinsic semiconductors, the Fermi level is positioned at the midpoint of the energy gap at absolute zero. When...
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Quasiparticle Gap Renormalization Driven by Internal and External Screening in a WS_{2} Device.

Chakradhar Sahoo1, Yann In 't Veld2, Alfred J H Jones1

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Environmental screening and doping significantly alter two-dimensional semiconductor band gaps. In WS2 devices, stronger intrinsic doping in vacuum-exposed sections caused greater band gap renormalization than external screening in graphene-encapsulated sections.

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Area of Science:

  • Condensed Matter Physics
  • Materials Science
  • Nanoscience

Background:

  • The electronic band gap of 2D semiconductors is crucial for device performance.
  • Band gap tuning is influenced by dielectric environment and charge carrier doping.
  • Understanding these effects is key for designing advanced electronic devices.

Purpose of the Study:

  • To differentiate band gap renormalization effects from environmental screening and gate-induced doping in single-layer WS2.
  • To investigate the impact of encapsulation (graphene) versus vacuum exposure on WS2 electronic properties.
  • To correlate experimental observations with theoretical calculations.

Main Methods:

  • Microfocused angle-resolved photoemission spectroscopy (µARPES) for in situ analysis.
  • Fabrication of a WS2 device on hexagonal boron nitride with distinct exposed and encapsulated regions.
  • Gate-controlled doping to induce semiconductor-metal transitions.
  • GW calculations for theoretical validation.

Main Results:

  • Direct observation of doping-induced semiconductor-metal transition and band gap renormalization.
  • A larger band gap renormalization was observed in the vacuum-exposed WS2 section compared to the graphene-encapsulated section.
  • Experimental findings suggest intrinsic screening from doping can outweigh external environmental screening.

Conclusions:

  • The interplay between intrinsic doping and external dielectric screening dictates band gap renormalization in 2D semiconductors.
  • Graphene encapsulation does not necessarily lead to greater band gap renormalization compared to vacuum exposure.
  • This study provides critical insights for optimizing 2D semiconductor device design and performance.