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Ladungstransport durch Graphenschichten und GaAs-Nanodrähte untersucht mit einem Multispitzen-Rastertunnelmikroskop
von Stefan KorteThis work describes the use of the combination of a scanning electron microscope
(SEM) and a multitip scanning tunneling microscope (STM) with four tips as a nanoprober.
Electrical measurements on graphene layers and freestanding gallium arsenide
(GaAs) nanowires were conducted. Four-probe-measurements are necessary to measure
the resisitvity of such one- and two-dimensional conductors. Due to unknown
voltage drops at contacts that carry currents, additional contacts have to be employed
for current-free potential measurements. Therefore, the multitip scanning tunneling
microscope with its four individually controllable tips has been upgraded with extended
electronics, enabling us to use it as a flexible nanoprober.
Graphene layers on insulating SiO2 and hexagonal boron nitride (h-BN), prepared by
mechanical exfoliation, were contacted with the multitip STM. Tunneling current could
not be used as feedback when approaching the first tip. Therefore, a contrast change in
the SEM image upon contacting a graphene flake with a tip was used. Once contacted,
flakes were scanned by RTM and electrical measurements were conducted. Graphene
transferred to h-BN showed bubbles, wrinkles and contaminations. Still, STM images
of clean areas revealed a moiré pattern, proving that the atomically thin graphene lay flat
on the atomically flat h-BN surface. Four point measurements of these samples showed
a poor conductivity of 1=s = 16kW= and a low field effect mobility of m = 300cm2=Vs.
The reason for this might be the contaminations from the transfer process, as well as
effects from prolonged irradiation with electrons from the SEM.
Freestanding p-doped GaAs nanowires, grown by metal-organic vapor-phase-epitaxy
in the vapor-liquid-solid-growth mode, in a process with two temperature steps, were
contacted with the multitip STM. Using three tips as well as the substrate as contacts,
four point measurements were performed. It showed that elastic deformation of these
flexible nanowires has no significant influence on their conductivity. The high spatial
resolution of the combination of a SEM with a multitip STM made it possible to record
resistance profiles of freestanding nanowires by performing four point measurements
along a nanowire. The main segment of the nanowires, grown at 400C for better crystal
quality exhibits a resisitivity of a few kW=m, in agreement with literature values.
The nanowire base, grown at 450C to facilitate better nucleation, shows an increased
resisitvity of several MW=m. The resistance of the nanowire base is relevant especially
for future opto-electronical components based on freestanding nanowires and thus has
to be understood. Comparing profiles of nanowires grown by an identical process on different
substrates showed that the substrate is not the cause of the increased resistance.
From the measured resistivities the dopant concentrations, as well as the thickness of
the space charge layer at the surface of the GaAs nanowires were calculated. The nanowire
segments grown at 400C have a dopant concentration of roughly 1019cm??3, those
grown at 450C about 21017 cm??3. In the base the space charge layer poses a considerable
constriction to the conduction. A qualitative explanation for the temperature
dependence of the dopant concentration is given.
(SEM) and a multitip scanning tunneling microscope (STM) with four tips as a nanoprober.
Electrical measurements on graphene layers and freestanding gallium arsenide
(GaAs) nanowires were conducted. Four-probe-measurements are necessary to measure
the resisitvity of such one- and two-dimensional conductors. Due to unknown
voltage drops at contacts that carry currents, additional contacts have to be employed
for current-free potential measurements. Therefore, the multitip scanning tunneling
microscope with its four individually controllable tips has been upgraded with extended
electronics, enabling us to use it as a flexible nanoprober.
Graphene layers on insulating SiO2 and hexagonal boron nitride (h-BN), prepared by
mechanical exfoliation, were contacted with the multitip STM. Tunneling current could
not be used as feedback when approaching the first tip. Therefore, a contrast change in
the SEM image upon contacting a graphene flake with a tip was used. Once contacted,
flakes were scanned by RTM and electrical measurements were conducted. Graphene
transferred to h-BN showed bubbles, wrinkles and contaminations. Still, STM images
of clean areas revealed a moiré pattern, proving that the atomically thin graphene lay flat
on the atomically flat h-BN surface. Four point measurements of these samples showed
a poor conductivity of 1=s = 16kW= and a low field effect mobility of m = 300cm2=Vs.
The reason for this might be the contaminations from the transfer process, as well as
effects from prolonged irradiation with electrons from the SEM.
Freestanding p-doped GaAs nanowires, grown by metal-organic vapor-phase-epitaxy
in the vapor-liquid-solid-growth mode, in a process with two temperature steps, were
contacted with the multitip STM. Using three tips as well as the substrate as contacts,
four point measurements were performed. It showed that elastic deformation of these
flexible nanowires has no significant influence on their conductivity. The high spatial
resolution of the combination of a SEM with a multitip STM made it possible to record
resistance profiles of freestanding nanowires by performing four point measurements
along a nanowire. The main segment of the nanowires, grown at 400C for better crystal
quality exhibits a resisitivity of a few kW=m, in agreement with literature values.
The nanowire base, grown at 450C to facilitate better nucleation, shows an increased
resisitvity of several MW=m. The resistance of the nanowire base is relevant especially
for future opto-electronical components based on freestanding nanowires and thus has
to be understood. Comparing profiles of nanowires grown by an identical process on different
substrates showed that the substrate is not the cause of the increased resistance.
From the measured resistivities the dopant concentrations, as well as the thickness of
the space charge layer at the surface of the GaAs nanowires were calculated. The nanowire
segments grown at 400C have a dopant concentration of roughly 1019cm??3, those
grown at 450C about 21017 cm??3. In the base the space charge layer poses a considerable
constriction to the conduction. A qualitative explanation for the temperature
dependence of the dopant concentration is given.