Electrical Characteristics of Multilayer MoS2 FET's with MoS2

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Electrical Characteristics of Multilayer MoS FET’s with MoS/Graphene Hetero-Junction Contacts 2

Joon Young Kwak, Jeonghyun Hwang, Brian Calderon, Hussain Alsalman, Nini Munoz, Brian Schutter, and Michael G Spencer Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl5015316 • Publication Date (Web): 30 Jun 2014 Downloaded from http://pubs.acs.org on July 1, 2014

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Electrical Characteristics of Multilayer MoS2 FET’s with MoS2/Graphene Hetero-Junction Contacts Joon Young Kwak,* Jeonghyun Hwang, Brian Calderon, Hussain Alsalman, Nini Munoz, Brian Schutter, and Michael G. Spencer School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853, United States

ABSTRACT: The electrical properties of multilayer MoS2/graphene hetero-junction transistors are investigated. Temperature dependent I−V measurements indicate the concentration of unintentional donors in exfoliated MoS2 to be 3.57 × 1011 cm-2 while the ionized donor concentration is determined as 3.61 × 1010 cm-2. The temperature dependent measurements also reveal two dominant donor levels; one at 0.27 eV below the conduction band and another located at 0.05 eV below the conduction band. The I−V characteristics are asymmetric with drain bias voltage and dependent on the junction used for the source or drain contact. I−V characteristics of the device are consistent with a long channel one-dimensional FET model with Schottky contact. Utilizing devices, which have both graphene/MoS2 and Ti/MoS2 contacts, the Schottky barrier heights of both interfaces are measured. The charge transport mechanism in both junctions was determined to be either thermionic-field emission or field emission depending on bias voltage and temperature. Based on a thermionic field emission model, the barrier height at

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the graphene/MoS2 interface was determined to be 0.23 eV, while the barrier height at the Ti/MoS2 interface was 0.40 eV. The value of Ti/MoS2 barrier is higher than previously reported values, which did not include the effects of thermionic field emission.

KEYWORDS:

Hetero-junction, graphene, MoS2, donor level, ionized impurity, barrier height

TEXT: Because of the unique electrical, mechanical and optical properties of graphene, interest has grown exponentially since the material was successfully exfoliated onto SiO2 in 2004.1-6 Other two dimensional (2D) crystal materials like Molybdenum Disulfide (MoS2) and hexagonal Boron Nitride (hBN) singularly or in combination with graphene also have intriguing properties. For example, it has been reported that when graphene is transferred onto hBN (a 2D insulating material) mobility is improved to 200,000 cm2/V·s due to the atomic flatness, low interface charge density and small lattice mismatch (with graphene).7 Single-layer and multilayer MoS2 transistors show high current on-off ratios of 1×108 and ultralow standby power dissipation.8 2D hetero-junction structures of graphene in combination with other 2D materials are driving new types of applications. Initial studies are reported on vertical hetero-junctions of hBN-graphene,911

transition metal dichalcogenides (TMDC)-graphene,12,13 TMDC-TMDC,14 and graphene-

silicon.15-17 Applications of 2D hetero structures for memory (using MoS2-graphene junctions1820

) and as photodetectors (utilizing MoS2-graphene21,22 or graphene-silicon23) have been

proposed. In this work, we study the electrical characteristics of multilayer MoS2 FETs with


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MoS2/graphene hetero-junction contacts. By studying multilayer material it is possible to determine the electrical properties, the role of contacts and background doping of the material without additional complications due to backside dielectric junction. The principal results of this study are 1) The I−V characteristics of multilayer MoS2/graphene hetero-junction FET’s can be modeled with a one-dimensional FET model (similar to E. Dubois et. al.24), 2) Due to charge transfer from MoS2, graphene makes a low barrier height contact to MoS2, and 3) The electrostatics of the transistor can be self-consistently explained by two background donor levels present in the exfoliated MoS2 where the total donor concentration is found to be ~ 3.57 × 1011 cm-2. A free electron concentration of ~ 3.61 × 1010 cm-2 in the material is due to partial ionization of these donors. It is believed that the donors in MoS2 transfer charge to the graphene in agreement with Sachs, et al..25 Hetero-junction devices were fabricated on insulator/p+-Si substrate; back gate biases were applied to Si (Figure 1a, b). Drain and source biases were applied to the device through two contacts; either the graphene or Ti/Au, which serve as either drain or source depending on bias conditions. A graphene FET was formed between contacts #1 and #2, and a MoS2 FET was formed between contacts #3 and #4 for additional characterization. Channel lengths of the devices are 10-20 µm implying that the FET’s are operating in the long channel regime. The MoS2/graphene hetero-junction devices were fabricated with exfoliated MoS2 and transferred graphene. The fabrication starts either with 100 nm thermally grown SiO2 or deposition of 50 nm thickness of Al2O3 using plasma Atomic Layer Deposition (ALD) at 200 ºC. Using the “scotch-tape” technique, MoS2 flakes were exfoliated onto the insulator/p+-Si substrates producing an active channel. The extraneous MoS2 flakes were removed by CF4 plasma. Metal contact pads consisting of Ti/Au (15 nm/150 nm) were deposited on the MoS2 flakes using e-


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beam evaporation. Titanium was used as an adhesion layer for gold contact metal on the flakes. Atomic Force Microscope measurements (AFM) were made to determine the thickness of the MoS2 flakes. After MoS2 characterization, CVD grown graphene on copper foil was transferred onto the surface using a wet transfer method.26,27 Metal contact pads of Ti/Au (15 nm/150 nm) were fabricated on the transferred graphene side by evaporation. To form a hetero-junction channel graphene was patterned with oxygen plasma etching at 150 ºC, while the MoS2 flake was not etched at this condition. Finally, the device was annealed at 300 ºC in 10% forming gas of Ar/H2 for 2 hours to remove photoresist residue on the surface. To confirm the presence of graphene and MoS2, Raman spectra measurements were performed on the hetero-junction using a confocal Raman microscope. The Raman signature of MoS2 (E12g peak at 383 cm-1 and A1g peak at 408 cm-1) 28,29 and graphene (D peak at 1351 cm-1, G peak at 1580 cm-1, and 2D peak at 2700 cm-1) 30-32 are clearly present (Figure 1c). All the Raman spectra indicate that the MoS2 peaks are stronger than graphene peaks and high-resolution scans were done to confirm the presence of graphene. Direct wafer probing was used for electrical characterizations of the devices. Measured electrical properties are shown in Figure 2. Room temperature current-voltage curves are measured on 80 nm thick MoS2 transferred to 100 nm thick SiO2 on p+-Si. The transferred graphene is monolayer thick as verified by Raman spectroscopy. The fabricated device has a MoS2 channel length of 20 µm and the channel width of 50 µm. The IDS−VDS curves show different electrical behavior depending on which contact (graphene or Ti/Au) is grounded (Figure 2a, b). Mobility of MoS2 determined from Equation (1) is found to be 11 cm2/V·s,

=

⋯ (1)


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where gm = dIDS/dVGS is the transconductance, L and W are the channel length and the channel width, respectively, Cox is the capacitance of the gate dielectric and VDS is the drain to source bias voltage. The IDS−VDS curves (Figure 2a) show FET like behavior with gate bias dependency. For small drain bias (inset of Figure 2a), the curves look diode like. When the MoS2 side is grounded (Figure 2b) the IDS−VDS curves show FET like behavior but the contacts are clearly affecting the curves (inset of Figure 2b). The magnitude of the drain current is reduced and there is a “knee” in the curve. The IDS−VGS curves with graphene source contacts and MoS2 source contacts are shown in Figure 2c and Figure 2d, respectively. At lower gate biases the curves show a quadratic dependence, while at higher gate biases (greater than VGS = 1 V) the current increment slows down. Band diagram of the MoS2/dielectric interface is shown in Figure 3a. Electron affinity and the energy bandgap of multi-layer MoS2 are 4.1 eV and 1.3 eV.33 The work function of monolayer graphene at the Dirac point is 4.5 eV34-36 and dielectrics, silicon dioxide (SiO2) and aluminum oxide (Al2O3), have bandgaps of 8.5 eV and 8.8 eV with electron affinities of 0.95 eV and 1.25 eV.37 Electron affinity and bandgap of silicon are 4.05 eV and 1.12 eV.38 Measured devices are normally ON and can be biased into accumulation or depletion, indicating the presence of background charge, which prevents unintentional carrier depletion of unbiased devices. The background donor charge density is determined to be ~ 3.57 × 1011 cm-2 (Supporting Information S1). The energy band diagram models and a simplified one-dimensional FET model with Schottky contacts are used to explain the back gate modulation of a hetero-junction device (Figure 3a, b). Changing the back gate voltage modulates the thickness of the conducting layers. As the back gate voltage further increases, electrons are accumulated near the MoS2/dielectric interface,


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which will screen the back gate electric field (VGS > VFB case in Figure 3a). The total ionized mobile charge density at VDS = 3 V and VGS = 0 V is calculated for the device of Figure 2 using Equation (2), = ()( − ) " ⋯ (2) ! where ID is the drain current, µ is the mobility calculated using Equation (1), tMoS2 (80 nm) is the thickness of the MoS2 (i.e. maximum channel depth), XD is the depleted channel depth at a gate bias of zero volt and W/L (2.5) is the width to length ratio of the device. The XD for the device was calculated as 6 nm using Equation (S1) in the Supporting Information. The calculated ionized mobile charge density, n, is ~ 3.61 × 1010 cm-2 at room temperature, which means about 10% of the total donor density, ND ~ 3.57 × 1011 cm-2, is ionized and contributes to the current. Table S1 shows the variation in ionized charge as a function of temperature. Figure 4 shows a plot of log [IDS] vs. 1/kT from the data of Table S1. Two different slopes can be fitted implying two donor levels are present. Previous studies have discussed the nature of impurities in MoS2. The possible role of Rhenium impurities and Sulfur vacancies has been investigated.25,39 We speculate that the measured donor levels are from one or two (intrinsic or extrinsic) donor sources. The two different donor levels and concentrations are calculated from Fermi-Dirac (F-D) statistics using the following equations = $1 − &('( , ' )*+( + $1 − &(' , ' )*+ ⋯ (3) +( + + = + ⋯ (4)


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where f(En,EF) is the F-D function for a donor level of energy En, n is the total ionized mobile charge density from Table S1, ND1 and ND2 are donor concentrations for the two donor levels and ND is the total donor density. The Fermi level in Equation (3) is determined from the measured values of n using an effective density of states for MoS2 of 1.2 × 1013 cm-2.40 Using an iterative process we found optimum values of ND1, ND2, E1 and E2, which fit the data. The optimum values of deep donor density (ND1) and shallow donor density (ND2) are found to be 3.2 × 1011 cm-2 and 3.3 × 1010 cm-2, respectively, the deep donor level and the shallow donor level are found to be 0.27 eV and 0.05 eV measured from the conduction band edge. The high donor concentrations in MoS2 can explain the doping of graphene by MoS2.25 The graphene Fermi level shift from the Dirac point is determined to be 0.17 eV in thermal equilibrium, and this value is obtained from the graphene/MoS2 junction barrier height (discussed below) and the bulk Fermi level location in MoS2. The Fermi level shift in graphene is due to charge transfer from MoS2. We calculate a graphene doping concentration of 2 × 1012 cm-2 based on the graphene Fermi level position.41 The high concentration of deep donors effectively “pins” the interface Fermi level. Figure 4b summarizes our findings. Figure 2a shows the output characteristics obtained when the graphene terminal is grounded. For positive VDS the electrons see the graphene/MoS2 interface as the barrier to injection at the source region but no barrier to collection at the drain region. On the other hand, for negative VDS the electrons see the Ti/MoS2 interface as the barrier to injection at the drain region, but no barrier to collection at the source region (Figure 5a). The asymmetry of the current behavior in low bias region is due to the barrier height difference between the graphene/MoS2 heterojunction and the Ti/MoS2 junction. In order to extract the Schottky barrier heights, temperature dependent IDS−VDS (VGS = 0 V) measurements were made (Figure 5b,c). At low drain bias


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voltages the current is strongly temperature dependent, while at high voltages IDS is temperature independent. We conclude that at low VDS the resistance of the device is dominated by thermionic or thermionic-field emission of the interface barrier (strongly temperature dependent), while at high VDS the barrier transport is by field emission (temperature independent). In the field emission region, the series resistance of the channel dominates. The data of Figure 5b,c is replotted to show the total resistance vs. VDS with dominating resistance regions labeled (Figure S3). In the previous discussion on donor levels, the high bias region (VDS = 3 V) was used where the field emission (or channel resistance) dominates, while a low bias (VDS = −0.1 V) is used to estimate the Schottky barrier heights. Two different techniques are used to analyze the temperature dependent IDS (Figure S4, S5); 1) Thermionic emission model42 utilizing the slope of log [|IS|/T2] vs. 1/kT yields a value of 0.08 eV for the graphene/MoS2 interface and 0.14 eV for the Ti/MoS2 barrier and 2) Thermionic field emission model43 utilizing the slope of log [|IS|cosh(E00/kT)/T] vs. 1/E0 where E00 = qh/4π(ND/m*ε)1/2 and E0 = E00coth(E00/kT) yields a barrier height of 0.23 eV for the graphene/MoS2 interface and 0.40 eV for the Ti/MoS2 interface. Due to the large concentration of deep donors and the resulting high electric field at the interface, we believe that the thermionic field emission model represents the Schottky barrier heights more accurately. As a result, the 0.40 eV barrier height predicted for the Ti/MoS2 interface is higher than that predicted by Das, et al..44 When the ground contacts are switched, the current characteristics of the device are dramatically changed. For positive VDS the electrons will now see the Ti/MoS2 Schottky barrier to injection at the source region but no barrier to collection at the drain region. On the other hand, for negative VDS the electrons will see the graphene/MoS2 Schottky barrier to injection at the drain region but no barrier to collection at the source region. As in the case of the grounded graphene contact


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above, the asymmetry of the current behavior is due to the barrier height difference between the graphene/MoS2 hetero-junction and the Ti/MoS2 junction. Hetero-junction transistors with multilayer MoS2/graphene were designed and fabricated to study the behavior of electrical transport properties and the operation mechanism of thick heterojunction devices. The concentration of unintentional donors in exfoliated MoS2 is determined to be ~ 3.57 × 1011 cm-2 with two dominant donor levels one at 0.27 eV and the other at 0.05 eV below the conduction band. The high concentration of deep donors effectively “pins” the interface Fermi level. The electronic behavior of the device is strongly influenced by the contacts. The charge transport mechanism in both junctions was determined to be either thermionic-field emission or field emission depending on bias voltage and temperature. Based on a thermionic field emission model, the barrier height at the graphene/MoS2 interface was determined to be 0.23 eV, while the barrier height at the Ti/MoS2 interface was 0.40 eV. The value of the Ti/MoS2 barrier is higher than previously reported values, which did not include the effects of thermionic field emission. This study provides some understanding of basic physics of electrical characteristics of hetero-junctions that is essential to properly design practical heterojunction devices for various applications.

ASSOCIATED CONTENT Supporting Information Detailed descriptions of Donor density calculation (S1), Table of total ionized mobile charge density (S2), Table of the ionized mobile charge density from each donor level (S3), Total


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resistance graph of a device (S4) and Schottky barrier height extraction methods (S5). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Cornell Center for Materials Research with funding from the NSF MRSEC program (DMR-1120296). This work was also performed in part at the Cornell NanoScale Facility, a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation (Grant ECCS-0335765).

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FIGURE CAPTIONS: Figure 1. Device structure and characterization of a MoS2/graphene hetero-junction device. (a) Schematic illustration of a MoS2/graphene hetero-junction device.

(b) Optical image of a

graphene/MoS2 hetero-junction device. The labeled numbers indicate as follows: 1) graphene with the red dotted line, 2) hetero-junction, 3) MoS2, 4) SiO2 and 5) SiO2 with CF4 plasma etched. A slight color difference between 4) and 5) is due to a slight different thickness of SiO2, which was etched by CF4 plasma. (c) Raman spectrum of the hetero-junction area. Both graphene and MoS2 Raman signatures are observed. Because the graphene Raman peaks are much weaker than MoS2 Raman peaks, it is magnified in y-axis. The inset Raman spectrum shows the original magnitude of MoS2 Raman peaks at less than 500 cm-1. Figure 2. Current-Voltage curves (IDS−VDS and IDS−VGS) of a back gated hetero-junction device with 80 nm thick MoS2 on 100 nm thick SiO2 at room temperature. (a) IDS−VDS of the device when graphene side is grounded, i.e. source on graphene side. The inset shows the currentvoltage curves in a small bias region. (b) IDS−VDS of the device when MoS2 side is grounded, i.e. source on MoS2 side. The inset shows the current-voltage curves in a small bias region. (c) IDS−VGS of the device when graphene side is grounded. (d) IDS−VGS of the device when MoS2 side is grounded. Figure 3. Energy band diagrams with different back gate bias conditions. (a) Energy band diagrams of the MoS2 channel region not covered by graphene (from left to right: MoS2Dielectric-Si) under various back gate voltages. A back gate bias (VGS) changes the size of the flat region in MoS2 (i.e. the conducting channel depth). The red dotted curve near the interface of the MoS2/dielectric describes the accumulations of carriers. (b) One-dimensional FET model


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to describe the change of channel depth by a back gate bias with a VDS applied. The arrow indicates the movement of channel depth by the applied back gate bias. The length change of the flat region in MoS2 in the energy band diagram of (a) corresponds to the depth change of the channel. Figure 4. Two donor levels measurement and energy band diagram model. (a) The log [IDS] vs. 1/kT plot of the measured current and the best fit calculated current with different temperatures (120K-400K) at a bias condition, VDS = 3 V and VGS = 0 V. The blue dotted guide lines indicate two different slopes (i.e. two different donor levels) are present; one is a deep donor level and the other is a shallow donor level. The calculated data show the best fit to the measured data. The optimum values of the deep donor density (ND1) and the shallow donor density (ND2) are 3.2 × 1011 cm-2 and 3.3 × 1010 cm-2, respectively, and the deep donor level and the shallow donor level from the conduction band are 0.27 eV and 0.05 eV, respectively. (b) Energy band diagram of the MoS2 covered by graphene (from left to right: graphene-MoS2). The small circles in MoS2 indicate two dominant donor levels in MoS2. The deep donor level (blue circle) is located at 0.27 eV from the conduction band and the shallow donor level (red circle) is located at 0.05 eV from the conduction band. Figure 5. Energy band diagrams with different drain bias conditions and log-log scale plots of IDS−VDS in a reverse bias and a forward bias (VGS = 10 V) condition. (a) Energy band diagrams (from left to right: graphene-MoS2-Ti/Au) in a reverse drain bias and in a forward drain bias condition, respectively, when the graphene side is grounded.

(b) IDS−VDS curves with

temperature dependent measurement when graphene is grounded. The energy band diagram (inset) shows the Ti/MoS2 Schottky barrier in a reverse bias region. (c) IDS−VDS curves with


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temperature dependent measurement when MoS2 is grounded. The energy band diagram (inset) shows the graphene/MoS2 Schottky barrier in a reverse bias region.


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For Table of Contents Only:


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Figure 2 146x144mm (300 x 300 DPI)


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Figure 4 80x129mm (300 x 300 DPI)


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Electrical Characteristics of Multilayer MoS2 FET's with MoS2

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