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Berg, Morgann

Publications and source records attributed to Berg, Morgann.

At least 37 records · Page 2
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DOE OSTI2018

Local Electronic Structure Changes in Polycrystalline CdTe with CdCl 2 Treatment and Air Exposure

Postdeposition CdCl 2 treatment of polycrystalline CdTe is known to increase the photovoltaic device efficiency. However, the precise chemical, structural, and electronic changes that underpin this improvement are still debated. In this work, spectroscopic photoemission electron microscopy was used to spatially map the vacuum level and ionization energy of CdTe films, enabling the identification of electronic structure variations between grains and grain boundaries (GBs). In vacuo preparation and inert transfer of oxide-free CdTe surfaces isolated the separate effects of CdCl 2 treatment and ambient oxygen exposure. Qualitatively, grain boundaries displayed lower work function and downward band bending relative to grain interiors, but only after air exposure of CdCl 2 -treated CdTe. Analysis of numerous space charge regions at grain boundaries showed an average depletion width of 290 nm and an average band bending magnitude of 70 meV, corresponding to a GB trap density of 10 11 cm –2 and a net carrier density of 10 15 cm –3 . Finally, these results suggest that both CdCl 2 treatment and oxygen exposure may be independently tuned to enhance the CdTe photovoltaic performance by engineering the interface and bulk electronic structure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Work Function Variations in Twisted Graphene Layers

By combining optical imaging, Raman spectroscopy, kelvin probe force microscopy (KFPM), and photoemission electron microscopy (PEEM), we show that graphene’s layer orientation, as well as layer thickness, measurably changes the surface potential (Φ). Detailed mapping of variable-thickness, rotationally-faulted graphene films allows us to correlate Φ with specific morphological features. Using KPFM and PEEM we measure ΔΦ up to 39 mV for layers with different twist angles, while ΔΦ ranges from 36–129 mV for different layer thicknesses. The surface potential between different twist angles or layer thicknesses is measured at the KPFM instrument resolution of ≤ 200 nm. The PEEM measured work function of 4.4 eV for graphene is consistent with doping levels on the order of 10 12 cm -2 . Here, we find that Φ scales linearly with Raman G-peak wavenumber shift (slope = 22.2 mV/cm -1 ) for all layers and twist angles, which is consistent with doping-dependent changes to graphene’s Fermi energy in the ‘high’ doping limit. Our results here emphasize that layer orientation is equally important as layer thickness when designing multilayer two-dimensional systems where surface potential is considered.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Experimental Determination of the Ionization Energies of MoSe 2 , WS 2 , and MoS 2 on SiO 2 Using Photoemission Electron Microscopy

Here, the values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe 2 , WS 2 , and MoS 2 ) on SiO 2 using photoemission electron microscopy with deep ultraviolet illumination. The ionization energy displays a progressive decrease from MoS 2 , to WS 2 , to MoSe 2 , in agreement with predictions of density functional theory calculations. Combined with the measured energy positions of the valence band edge at the Brillouin zone center, we deduce that, in the absence of interlayer coupling, a vertical heterojunction comprising any of the three TMD monolayers would form a staggered (type-II) band alignment. This band alignment could give rise to long-lived interlayer excitons that are potentially useful for valleytronics or efficient electron–hole separation in photovoltaics.

36 MATERIALS SCIENCE↗