This study analyzed the electronic properties of methyl-terminated germanane (GeCH3) flakes using surface photovoltage spectroscopy (SPS). Previous studies showed GeCH3 has a p-type response and band gap of 1.7 eV. The SPS data in this study showed GeCH3 surfaces can exhibit both p-type and n-type character, and humidity and oxidation can affect the results. The findings have implications for relating SPS features to sample structure to synthesize materials without defects. The study of defects in 2D GeCH3 is important because it is a direct bandgap semiconductor with applications in optoelectronics.

1. SPS data taken on Methyl-Germanane (GeCH3) 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 GeCH3 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 (GeCH3) electronic
band gaps and defects are important because GeCH3 is a 2D direct
bandgap semiconductor with applications in optoelectronics. GeCH3
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 GeCH3 Potential of GeCH3
MethodAbstract
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 GeCH3 Data
P-type GeCH3 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 GeCH3
• What are we studying?
o Methyl-Terminated Germanane
(GeCH3), 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 (CaGe2)
and de-intercalating with methyl
iodide (CH3I) 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