Evaluation of the effective work-function of monolayer graphene on silicon dioxide by internal photoemission spectroscopy

Thin Solid Films 674 (2019) 39–43

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Evaluation of the effective work-function of monolayer graphene on silicon dioxide by internal photoemission spectroscopy

T

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Vadim Trepalina, , Inge Asselberghsb, Steven Bremsb, Cedric Huyghebaertb, Iuliana Radub, Valeri Afanas'eva, Michel Houssaa, Andre Stesmansa a b

Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, Leuven B-3001, Belgium IMEC, Kapeldreef 75, Leuven B-3001, Belgium

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphene Internal photoemission Interface barrier Effective work function Uncapped graphene

Internal photoemission of electrons from uncapped monolayer graphene to insulating SiO2 has been observed in samples prepared by water-intercalation based graphene transfer. The barrier height between the graphene Fermi level and the oxide conduction band bottom was reproducibly found to be 4.1–4.2 eV. Moreover, this value was weakly sensitive to the contacting metal work function (Al, Cu, Au). This barrier height corresponds to an effective work function of graphene close to 5.0 eV, which is nearly 0.5 eV higher than the usually reported vacuum value.

1. Introduction The interest in the family of two-dimensional (2D) materials is growing due to their unique physical properties, in particular, electronic characteristics and intrinsic thickness of one or more atomic layers [1]. The potential to stack different materials via van der Waals bonding permits attractive combined properties of individual compounds. The resulting heterostructures are intended for application in nanosized devices [1,2]. Accordingly, 2D materials will probably be applied in next generation electronics. For example, transistors made of 2D materials are promising substitutes for Si-based devices, which will avoid shortchannel effects hampering their downscaling. In principle, a 2D-based transistor will comprise transition metal dichalcogenides as the channel material, hexagonal boron nitride as gate or tunnel insulator and graphene as the optimal stable contact material [1–5]. Combining these materials in a heterostructure improves electrostatic channel control. In addition, changing the layer number to tune band diagram can finely adjust built-in voltages of devices for low-voltage operation [6]. However, designing such a device requires precise knowledge about the relative positions of the Fermi level and energy bands of channel and gate electrode layers combined in a particular heterostructure. As graphene is a perfect candidate material for application as a contact or gate electrode in 2D transistors and interconnects [3], knowing its effective work function (EWF) is crucial, as it determines the contact potential difference with other materials.

⁎

From theoretical calculations it was found that the vacuum work function (WF) of graphene is around 4.5 eV, i.e., close to the graphite WF [7,8]. However, numerous experimental studies on actual samples, where graphene is exposed to air or brought into contact with a metal or insulator, consistently reveal the EWF to be 0.3-0.7 eV higher than the vacuum value [9–12]. One of the most extensively used methods to determine the graphene EWF is capacitance-voltage (CV) measurements on metal/oxide/ silicon (MOS) capacitors foreseen with a graphene electrode. The EWF values obtained using this technique range from 4.7 eV to 5.2 eV [10–12]. Since graphene is usually transferred on top of the oxide film, a dipole layer between the graphene layer and oxide stemming from terminating OH-groups may affect the EWF of graphene making it to differ from the vacuum value [13]. Internal photoemission (IPE) of electrons represents the most straightforward method to measure the EWF of conducting materials at the interfaces with insulators and semiconductors [14]. Several attempts have already been made to analyze graphene using IPE measurements [15–17]. In early papers the IPE signal from graphene could not be detected [18] and the film was suggested to be used only as a transparent electrode due to its low optical absorption [19]. Later, the EWF of the transferred graphene was reported to be close to the vacuum value, i.e., around 4.5 eV [17], which is obviously inconsistent with the numerous electrical CV results mentioned above. The reason for such ambiguity may be related to the complicated sample structure in which graphene was additionally capped with a high-k dielectric and a

Corresponding author. E-mail address: [email protected] (V. Trepalin).

https://doi.org/10.1016/j.tsf.2019.01.036 Received 15 August 2018; Received in revised form 16 January 2019; Accepted 17 January 2019 Available online 23 January 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.

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Evac

semitransparent metal (Pt) top electrode [17]. This additional interface may provide a contribution to the detected IPE current as suggested by exactly the same electron IPE threshold energy values found at Pt/ Al2O3 or Pt/HfO2 interfaces [20]. Therefore, in order to clarify the EWF issue and determine it in a reliable way, one needs to detect IPE from the uncapped graphene monolayer.

χ(SiO2)

Φe

2. Experimental 2.1. Sample fabrication

EFermi

Monolayer graphene was synthesized by chemical vapor deposition on Pt foil and then transferred onto a target substrate — thermal SiO2 (50 nm)/p-type Si (100). The initial Pt template was grown on a c-plane of a sapphire crystal to achieve high quality Pt foil with low density of grain boundaries. Then a graphene monolayer was synthesized on the Pt template at 1070 °C using a gas mixture of CH4/H2 = 8/850 for 30 min. In order to transfer graphene from the Pt growth template to a target substrate, water intercalation between graphene and Pt layers was used. This was followed by the standard procedure of layer transfer involving 4 consequent steps: polymethyl metacrilate (PMMA) spin coating, graphene-on-PMMA peeling off from the template, transfer onto a target Si/SiO2 substrate followed by PMMA removal. Excess water was eliminated by annealing at 400 °C for 4 h in forming gas (N2 + 10% H2). The resulting samples have area of about 4 cm2 fully covered with monolayer graphene of high crystallinity. Finally, to enable electrical measurements, optically nontransparent contact metal pads were thermoresistively evaporated through a shadow mask on top of the graphene layer. These pads measured 100 nm thick with an area of 0.01 mm2. As indicated in Table 1, metals with different WF values were used to avoid ambiguity and ensure that the measured IPE current originates from optical excitation in graphene. All the metals used are considered chemically inert with respect to the graphene layer, no significant change of its band structure is expected [21]. An aluminum blanket contact layer was evaporated on the backside of the p-type Si substrate. Thus, the analyzed graphene monolayer remains intact after being transferred on top of SiO2 film.

Graphene

Photocurrent

hν eΔVG

Graphene

Y (hν ) =

Y (hν )~(hν − Φe ) p

WF, eV

5.1

4.5

4.1

(2)

(3)

Therefore, the IPE spectral threshold energy can be found by linear extrapolation of the Y1/3 vs. hν plot to zero yield value. In addition, it should be taken into account that the image force interaction between the electron entering the oxide and the polarized photoemitter leads to the field-dependent barrier lowering (the Schottky effect) as schematically shown in Fig. 3.

Table 1 Metals used as top contacts and corresponding values of vacuum work function. Al

IPC (hν ) e ·nph (hν )

In case of IPE from semiconductors, the spectral dependence of the quantum yield (Y) in the near-threshold spectral range can be approximated by a power function with exponent factor p=3 [23]:

A corresponding band diagram is schematically shown in Fig. 1. To measure the IPE current the graphene/SiO2/Si sample was connected to a Keithley 617 electrometer in a circuit enabling to apply a

Cu

p-Si

bias voltage of required polarity. The photoexcited electrons with sufficiently high energy can escape from the emitter and be swept by the applied field towards the opposite Si electrode. A schematic representation of this process is shown in Fig. 2. The measured photocurrent is converted to quantum yield, Y, in order to account for the spectral dependence of the incident photon flux (nph):

(1)

Au

SiO2

Fig. 2. Schematic representation of the IPE process under negative applied gate coltage ΔVG.

IPE represents the phenomenon of electron photoemission from the occupied states in an emitter material (in our case graphene) into the unoccupied states in the conduction band (CB) of the collector material (insulating oxide, SiO2). Once the energy of photons absorbed by electrons in the emitter becomes sufficient to overcome the potential barrier (hν > Φe) at the interface, charge injection in the oxide occurs which is detected by photocurrent measurements. In case of the graphene/SiO2 interface this IPE barrier is equal to the energy difference between the graphene Fermi level and the oxide CB bottom edge. Then, the graphene EWF can be found by adding the well-known value of oxide's electron affinity (χ(SiO2) = 0.9 eV [22]) to the inferred Φe barrier:

Metal

p-Si

Fig. 1. Band diagram of the samples studied, showing the Fermi level of graphene (EFermi), barrier energy (Φe), vacuum level (Evac), and oxide electron affinity (χ).

2.2. Measurement technique

EWF = Φe + χ (SiO2)

SiO2

40

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p-Si

SiO2

4

2

ΔΦ(F) Φ0

(IPE Yield)1/3 (a.u.)

Energy (eV)

3

Φ(F)

1 0 -1 -2 0

1

2

8x10-3

-7 V -3

4x10

-0.1 V

3

Distance (nm)

0

Fig. 3. Image force interface barrier illustrating the Schottky effect in the case of the p-type Si/SiO2 interface. Φ0 energy barrier for electrons in absence of external electric field, Φ(F) corresponds to the spectral threshold of IPE from the silicon valence band in the case of non-zero field F, ΔΦ = Φ0 − Φ(F), represents the image-force barrier lowering.

4.5

5.0

Fig. 5. IPE spectra in Y1/3 vs. hν coordinates for a gold contact pad sample. Data are recorded at different bias voltages applied to the contact pad on top of graphene monolayer starting from −0.1 V (blue points) to −7 V (yellow points). The straight lines represent linear fits to the spectral data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4.3

e  4.2 eV

IPE threshold (eV)

(4)

IPE current measurements with sensitivity down to the fA range were performed in the dc current mode using a Keithley 617 electrometer and extensive signal averaging (more than 100 points at each photon energy). The IPE spectra were taken in the photon energy range 2 < hν < 5.5 eV using monochromatized light from a 150 W Xe arc lamp with constant spectral resolution of 2 nm. The measurement circuit is schematically shown in Fig. 4. 3. Results and discussion 3.1. Gold contact pad

4.2

4.1

4.0

3.9 0.0

The IPE spectra of a graphene MOS capacitor with gold contact pad were recorded under application of different top gate voltages ranging from −0.1 V to −7 V. The corresponding spectra in Y1/3 vs. hν coordinates are presented in Fig. 5. The observed almost perfect linear increase of the yield in Y1/3 vs. hν coordinates is consistent with the photocurrent being originating from the linearly increasing density of states in graphene. In case of photoemission from metallic gold, the quantum yield would follow the Fowler function Y(hν) ∼ (hν − Φe)2 [23]. There is also a well-pronounced red shift of the barrier energy with increasing applied voltage,

0.2

0.4

0.6

0.8

1.0

1.2

(Electric Field)1/2 (MV/cm)1/2 Fig. 6. Field dependence (Schottky plot) of extracted IPE threshold values Φ(F) for the sample with gold contact pads. The red straight line represents a linear fit to the data used to determine Φe by extrapolation to zero field. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

indicative of a significant Schottky effect. The IPE spectral threshold values inferred as the onset of IPE yield are plotted as a function of the square root of electric field strength (F) in the oxide layer as shown in Fig. 6. The obtained IPE threshold field dependence closely follows the Schottky law. Extrapolating this dependence to a 0 MV/cm electric field value results in Φe ≈ 4.20 ± 0.05 eV. Then, a value of 5.1 eV is obtained for graphene EWF, by adding the electron affinity of silicon dioxide χ(SiO2), equal to 0.9 eV. As Table 1 shows, this value cannot be immediately accepted, since the gold WF has the same value. To ensure the photocurrent has no component related to the photoemission of electrons at the gold contact pad edges, experiments with a low WF metals are conducted.

Metal

Thick pad

ΔVG

4.0

Photon energy (eV)

In the ideal case, the value of the barrier energy lowering, ΔΦ, is proportional to the square root of the electric field in the oxide. Thus, to find the barrier in absence of the external field, Φ0, one needs to measure the IPE spectral threshold at different applied voltages and extrapolate this field dependence to zero electric field in the oxide.

ΔΦ = Φ0 − Φ(F )~ F

3.5

Graphene SiO p-Si

Fig. 4. The schematics of the IPE measurement circuit and the incident light beam (yellow), demonstrating the geometry of the experiment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 41

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2x10-2

(IPE Yield)1/3 (a.u.)

(IPE Yield)1/3 (a.u.)

2x10-

-7 V

10-2

-7 V

10-2

5x10-3

-0.5 V

-0.2 V 0

0

3.5

4.0

4.5

5.0

3.5

4.0

4.5

5.0

Photon energy (eV)

Photon energy (eV)

Fig. 9. IPE spectra in Y1/3 vs. hν coordinates for a copper contact pad sample as measured under different voltages applied to the contact pad on top of the graphene monolayer, ranging from −0.5 V to −7 V. The straight lines represent linear fit to the spectral data.

Fig. 7. IPE spectra in Y1/3 vs. hν coordinates for an aluminum contact pad sample as measured under different voltages applied to the contact pad on top of graphene monolayer starting from −0.2 V to −7 V. The straight lines represent linear fits to the spectral curves.

3.3. Copper contact pad 3.2. Aluminum contact pad The third metal to be used as a contact pad material was copper. Its WF has a value of around 4.5 eV — different from that of both gold and aluminum. The IPE yield spectra of this sample were recorded using the top gate voltages ranging from −0.5 V to −7 V. The results are illustrated in Fig. 9. Again, the spectral plots are very similar to those obtained on the samples with Au and Al electrodes, assuring that electron states of the metal contact pads give no contribution to the measured IPE currents. Determination of the interface barrier height using linear extrapolation of the Schottky plot is illustrated in Fig. 10. The graphene/SiO2 electron barrier height in the sample with the copper contact pad appears to be the same as in the sample with the Al contact, i.e., Φe ≈ 4.10 ± 0.05 eV, which corresponds to the graphene EWF of 5.0 eV. Again, this value is significantly higher than the corresponding WF of copper, further affirming that the origin of the photocurrent is the graphene layer. The analysis of IPE spectra obtained from three different samples with gold, aluminum and copper contact pads, allows us to conclude that IPE of electrons from the occupied electron states below the Fermi

4.1

e  4.1 eV

e  4.1 eV

IPE threshold (eV)

IPE threshold (eV)

The second metal used as contact pad was aluminum with, as known, a WF ≈ 4.1 eV. IPE spectra obtained from this sample were recorded using top gate voltages ranging from −0.2 V to −7 V and are presented in Fig. 7. The similar linear increase of the quantum yield in Y1/3 vs. hν coordinates and comparable yield magnitude as observed in the sample with the Au contact provide significant evidence for the same origin of the photocurrent, i.e., electron IPE from graphene. The corresponding field dependence of the IPE spectral threshold is shown in Fig. 8. Extrapolation of the Schottky plot for the sample with the aluminum pad to zero field yields Φe ≈ 4.10 ± 0.05 eV and a graphene EWF ≈ 5.0 eV. This value is considerably higher than the aluminum WF which excludes the latter metal as potential source of photoelectrons. On the other hand, the IPE thresholds and the obtained graphene EWF are close to the values obtained on the sample with the gold pad supporting the earlier assignments.

4.0

3.9

3.8 0.0

4.1

4.0

3.9

3.8

0.2

0.4

0.6

(Electric Field)

1/2

0.8

(MV/cm)

1.0

0.0

1/2

0.2

0.4

0.6

0.8 1/2

(Electric Field)

Fig. 8. Field dependence (Schottky plot) of extracted IPE thresholds for the sample with aluminum contact pad. The red straight line represents a leastsquare linear fit of the data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

1.0

1.2

1/2

(MV/cm)

Fig. 10. Schottky plot of the extracted IPE threshold values for the sample with copper contact pad. Thered straight line represents a linear fit of the data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 42

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level of graphene constitute the dominant source of the photocurrent. The extracted graphene EWF is found to be approximately 5.0 eV and marginally sensitive to the contact metal WF. These IPE results appear to be within the range of EWFs obtained by other the experimental methods such as CV and Kelvin Probe Force Microscopy. Therefore, we may conclude that the EWF of monolayer graphene transferred on SiO2 is close to 5.0 eV, which is 0.5 eV higher than the vacuum WF of graphene [9–12,24]. We hypothesize that this difference is related to the hydroxyl groups present at the SiO2 surface [25], which provide a dipole-like contribution to the interface barrier due to electron transfer along the polar O-H bond [26]. This may become a significant factor of interface barrier instability because the areal density of silanol (Si-OH) groups on the silica surfaces is known to be strongly temperature dependent [27]. In turn, this EWF value will directly be reflected in the threshold voltage of the transistors if graphene is used as a gate electrode.

[6]

[7]

[8]

[9]

[10]

[11] [12]

4. Conclusions [13]

The IPE experiments to determine the graphene EWF have been carried out on samples with uncapped monolayer graphene electrodes transferred on top of thermally grown SiO2, using three different contact metals (Au, Al, Cu). An energy barrier between graphene Fermi level and oxide conduction band bottom was consistently found in the 4.1-4.2 eV range. This interface barrier value corresponding to the EWF of graphene transferred on top of SiO2, is measured at approximately 5.0 eV, independent of the contact metal, and is 0.5 eV higher than the corresponding WF obtained in vacuum. The difference is tentatively ascribed to the interface dipole formation due to silanol (SiOH) groups forming on the oxide surface.

[14]

[15]

[16]

[17]

Acknowledgments [18]

This work had received partial support from Flanders Innovation & Entrepreneurship [2Dfun (2D functional MX2/graphene hetero-structures), an ERA-NET project in the framework of the EU Graphene Flagship] and from KU Leuven Internal Funding (project C14/16/061).

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