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Optical Characterization of Two Dimensional Semiconductors

  • Author(s): Odenthal, Patrick
  • Advisor(s): Kawakami, Roland
  • Tom, Harry
  • et al.
Abstract

The world is silicon based technology is about to come to a close as it rapidly approaches the quantum limit. New materials are required for further technological advances. The optical properties of two of these new materials, germanane (GeH) and molybdenum disulfide (MoS2) are studied in this thesis.

Germanane is synthesized using MBE co-deposition, resulting in atomic control of thickness with wafer scale size. Unfortunately, germanane is grown on germanium substrates which are conductive and opaque to light near the band edge. We have developed a large area transfer method using electrochemical delimitation (ie. Bubble transfer) to arbitrary substrates. Germanane films with thickness ranging from 1 nm to 600 nm and areas up to 1 cm2 have been reliably transferred. Characterization by photoluminescence, x-ray diffraction, and energy-dispersive X-ray pectroscopy invidicate that the films quality is preserved after transfer. The optical and electro-optical properties of germanane can now be measured. Temperature dependent photoluminescence excitation spectroscopy (PLE), absorption and photo conductivity show a band edge near 1.9 eV with a large Urbach tail. Photoluminescence (PL) measurements show a broad luminescence that is shifted by 700 meV from the absorption edge. We also observe an increase of the PL intensity of several orders of magnitude

from room temperature to 4 K. Time resolved photoluminescence (TRPL) reveal many time scales ranging from a few ns to a s.

Sputtering of MoS2 films of single-layer thickness by low-energy argon ions selectively reduces the sulfur content of the material without significant depletion of molybdenum. X-ray photoelectron spectroscopy shows little modification of the Mo 3d states during this process, suggesting the absence of significant reorganization or damage to the overall structure of the MoS2 film. Accompanying ab initio molecular dynamics simulations find clusters of sulfur vacancies in the top plane of single-layer MoS2 to be structurally stable. Measurements of the photoluminescence at temperatures

between 175 and 300 K show quenching of almost 80% for an 10% ecrease

in sulfur content.

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