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SPS data taken on Methyl-Germanane (GeCH 3 ) flakes. Pervious study of this material show a p-type response, multiple defect states and a band gap energy of 1.7 eV. Center of Emergent Materials, The Ohio State University Our SPS data shows that GeCH 3 surfaces can have both p-type and n- type character. Both humidity and oxidation can affect these results so that careful testing is required. This has significant future implications for relating SPS electronic features to sample structure in order to synthesize materials without defects The study of 2D Methyl-Terminated Germanane (GeCH 3 ) electronic band gaps and defects are important because GeCH 3 is a 2D direct bandgap semiconductor with applications in optoelectronics. GeCH 3 shows a duality in electronic response using surface photovoltage spectroscopy. These findings have implications that can impact material synthesis Conclusions The REU program is part of an NSF Materials Research Science and Engineering Center (MRSEC) supported under NSF Award Number DMR-1420451 Topography of GeCH 3 Potential of GeCH 3 Method Abstract o Defects present in semiconductors occur when the crystal lattice has been disrupted. o Surface localized states are created by a small electric field created to achieve thermal equilibrium between the bulk and the surface. These create defect states within the bandgap. • Why are we doing studying defects? o Defect states can present a number of problems semiconductor application like transistors, integrated circuits, microchips, and optoelectronics. o Defects decrease mobility of electrons, effect efficiency of solar cells for example Results Data on the same flake sample has also shown n-type behavior. At hn = 0.8 eV, decrease surface electrons, band bending, and work function. Bandgap at 1.6eV N-type GeCH 3 Data P-type GeCH 3 Data Background and Introduction • Surface Photovoltage Spectroscopy (SPS) o SPS tracks changes in material’s Fermi level, corresponding to changes in work function due to band bending measured as contact potential difference (DCPD). Bands can have downward bending (n-type) with an excess of negative charge on the surface or upward bending (p-type) with depletion of negative charge at surface. At bandgap energy, all charge removed and bands are flat. o Surface electrons that absorb photons at energies below bandgap energy can get excited to: At hn = 0.8eV, increase in surface state and band bending, decrease in work function. At hn = 1.1eV decrease in surface state and band bending, increase in work function. Bandgap at 1.8eV. 0.8 eV 1.1 eV • Atomic Force Microscopy (AFM) and Kelvin Probe Force Mircoscopy (KPFM) o AFM scan maps out surface morphology and the KPFM scan tracks surface potential.These two scans are taken simultanously using a circuit involving a Kelvin probe that vibrates at two set frequencies. The experimental process for collecting data included: Error Signal of GeCH 3 • What are we studying? o Methyl-Terminated Germanane (GeCH 3 ), one of few 2D materials to have a direct band gap o Ball and stick diagram shows Ge (blue) with Carbon (black) and Hydrogen (grey) groups o These material is synthesized by taking calcium germanide (CaGe 2 ) and de-intercalating with methyl iodide (CH 3 I) and washed in HCl to creating single layer crystal structures Depopulating surface state— excitation of electrons from the surface state into the conduction band Results—Flattening of band bending because of less repulsion experienced by the conduction and valence band Population surface state— excitation of electrons from the valence band into the surface state Results— Increased band bending due to the increase in negative charge at the surface
