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Uniform Atomic Layer Deposition of Al 2 O 3 on Graphene by Reversible Hydrogen Plasma Functionalization

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ABSTRACT

A novelmethod to form ultrathin, uniform Al2O3 layerson graphene using reversible hydrogen plasma functionalizationfollowed by atomic layer deposition (ALD) is presented. ALD on pristinegraphene is known to be a challenge due to the absence of danglingbonds, leading to nonuniform film coverage. We show that hydrogenplasma functionalization of graphene leads to uniform ALD of closedAl2O3 films down to 8 nm in thickness. Hallmeasurements and Raman spectroscopy reveal that the hydrogen plasmafunctionalization is reversible upon Al2O3 ALDand subsequent annealing at 400 °C and in this way does not deterioratethe graphene’s charge carrier mobility. This is in contrastwith oxygen plasma functionalization, which can lead to a uniform5 nm thick closed film, but which is not reversible and leads to areduction of the charge carrier mobility. Density functional theory(DFT) calculations attribute the uniform growth on both H2 and O2 plasma functionalized graphene to the enhancedadsorption of trimethylaluminum (TMA) on these surfaces. A DFT analysisof the possible reaction pathways for TMA precursor adsorption onhydrogenated graphene predicts a binding mechanism that cleans offthe hydrogen functionalities from the surface, which explains theobserved reversibility of the hydrogen plasma functionalization uponAl2O3 ALD.

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Mobility of graphene determined by Hallmeasurements, after transfer,after plasma treatment, after 100 cycles Al2O3 ALD, and after 400 °C anneal for pristine, O2, andH2 plasma treated graphene.
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fig5: Mobility of graphene determined by Hallmeasurements, after transfer,after plasma treatment, after 100 cycles Al2O3 ALD, and after 400 °C anneal for pristine, O2, andH2 plasma treated graphene.

Mentions: Hall mobilitymeasurements were performed to investigate the effectof the O2 and H2 plasma treatments on the electricalproperties of graphene (Figure 5). The mobility values of the pristine graphene samples usedin this study range between 1300 and 1800 cm2/(V s) (indicatedby the black bars in Figure 5) which is typical for large area (1 × 1 cm2) CVD graphene.3,45,46 The deposition of Al2O3 on pristine grapheneresults in a mobility increase to 117% of its initial value (1520cm2/(V s)). This increase could be caused by several effects:(1) Al2O3 can passivate defects present in thegraphene;47 (2) Al2O3 can act as a barrier preventing H2O and O2 reaching the graphene surface which would otherwise degrade thecarrier mobility of graphene;48 (3) TheAl2O3 layer can also help to screen chargedimpurities, present in the SiO2 substrate, which wouldnormally act as scattering centers for the electrons and holes inthe graphene.49 Charge screening couldalso explain why the mobility is further increased to 140% of itsinitial value (1860 cm2/(V s)) after the sample is annealedat 400 °C. This is because annealing Al2O3 at 400 °C generally gives the highest Al2O3 built-in charge,50 resulting in maximumpassivation and an increased mobility of the graphene after annealing.It should be noted though that the Al2O3 layeron pristine graphene is not closed and therefore not suited for applications,for example as a gate dielectric.


Uniform Atomic Layer Deposition of Al 2 O 3 on Graphene by Reversible Hydrogen Plasma Functionalization
Mobility of graphene determined by Hallmeasurements, after transfer,after plasma treatment, after 100 cycles Al2O3 ALD, and after 400 °C anneal for pristine, O2, andH2 plasma treated graphene.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5384478&req=5

fig5: Mobility of graphene determined by Hallmeasurements, after transfer,after plasma treatment, after 100 cycles Al2O3 ALD, and after 400 °C anneal for pristine, O2, andH2 plasma treated graphene.
Mentions: Hall mobilitymeasurements were performed to investigate the effectof the O2 and H2 plasma treatments on the electricalproperties of graphene (Figure 5). The mobility values of the pristine graphene samples usedin this study range between 1300 and 1800 cm2/(V s) (indicatedby the black bars in Figure 5) which is typical for large area (1 × 1 cm2) CVD graphene.3,45,46 The deposition of Al2O3 on pristine grapheneresults in a mobility increase to 117% of its initial value (1520cm2/(V s)). This increase could be caused by several effects:(1) Al2O3 can passivate defects present in thegraphene;47 (2) Al2O3 can act as a barrier preventing H2O and O2 reaching the graphene surface which would otherwise degrade thecarrier mobility of graphene;48 (3) TheAl2O3 layer can also help to screen chargedimpurities, present in the SiO2 substrate, which wouldnormally act as scattering centers for the electrons and holes inthe graphene.49 Charge screening couldalso explain why the mobility is further increased to 140% of itsinitial value (1860 cm2/(V s)) after the sample is annealedat 400 °C. This is because annealing Al2O3 at 400 °C generally gives the highest Al2O3 built-in charge,50 resulting in maximumpassivation and an increased mobility of the graphene after annealing.It should be noted though that the Al2O3 layeron pristine graphene is not closed and therefore not suited for applications,for example as a gate dielectric.

View Article: PubMed Central - PubMed

ABSTRACT

A novelmethod to form ultrathin, uniform Al2O3 layerson graphene using reversible hydrogen plasma functionalizationfollowed by atomic layer deposition (ALD) is presented. ALD on pristinegraphene is known to be a challenge due to the absence of danglingbonds, leading to nonuniform film coverage. We show that hydrogenplasma functionalization of graphene leads to uniform ALD of closedAl2O3 films down to 8 nm in thickness. Hallmeasurements and Raman spectroscopy reveal that the hydrogen plasmafunctionalization is reversible upon Al2O3 ALDand subsequent annealing at 400 °C and in this way does not deterioratethe graphene’s charge carrier mobility. This is in contrastwith oxygen plasma functionalization, which can lead to a uniform5 nm thick closed film, but which is not reversible and leads to areduction of the charge carrier mobility. Density functional theory(DFT) calculations attribute the uniform growth on both H2 and O2 plasma functionalized graphene to the enhancedadsorption of trimethylaluminum (TMA) on these surfaces. A DFT analysisof the possible reaction pathways for TMA precursor adsorption onhydrogenated graphene predicts a binding mechanism that cleans offthe hydrogen functionalities from the surface, which explains theobserved reversibility of the hydrogen plasma functionalization uponAl2O3 ALD.

No MeSH data available.