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Hall and field-effect mobilities in few layered p-WSe₂ field-effect transistors.

Pradhan NR, Rhodes D, Memaran S, Poumirol JM, Smirnov D, Talapatra S, Feng S, Perea-Lopez N, Elias AL, Terrones M, Ajayan PM, Balicas L - Sci Rep (2015)

Bottom Line: Here, we present a temperature (T) dependent comparison between field-effect and Hall mobilities in field-effect transistors based on few-layered WSe2 exfoliated onto SiO2.The hole Hall mobility reaches a maximum value of 650 cm(2)/Vs as T is lowered below ~150 K, indicating that insofar WSe2-based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides.The gate capacitance, as extracted from the Hall-effect, reveals the presence of spurious charges in the channel, while the two-terminal sheet resistivity displays two-dimensional variable-range hopping behavior, indicating carrier localization induced by disorder at the interface between WSe2 and SiO2.

View Article: PubMed Central - PubMed

Affiliation: National High Magnetic Field Laboratory, Florida State University, Tallahassee-FL 32310, USA.

ABSTRACT
Here, we present a temperature (T) dependent comparison between field-effect and Hall mobilities in field-effect transistors based on few-layered WSe2 exfoliated onto SiO2. Without dielectric engineering and beyond a T-dependent threshold gate-voltage, we observe maximum hole mobilities approaching 350 cm(2)/Vs at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm(2)/Vs as T is lowered below ~150 K, indicating that insofar WSe2-based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides. The gate capacitance, as extracted from the Hall-effect, reveals the presence of spurious charges in the channel, while the two-terminal sheet resistivity displays two-dimensional variable-range hopping behavior, indicating carrier localization induced by disorder at the interface between WSe2 and SiO2. We argue that improvements in the fabrication protocols as, for example, the use of a substrate free of dangling bonds are likely to produce WSe2-based FETs displaying higher room temperature mobilities, i.e. approaching those of p-doped Si, which would make it a suitable candidate for high performance opto-electronics.

No MeSH data available.


Related in: MedlinePlus

(a) Current Ids in a logarithmic scale as extracted from a WSe2 FET at T = 300 K and as a function of the gate voltage Vbg for several values of the voltage Vds, i.e. respectively 5 (dark blue line), 26 (red), 47 (blue), 68 (magenta), and 90 mV (brown), between drain and source contacts. Notice that the ON/OFF ratio approaches 106 and subthreshold swing SS ~250 mV per decade. We evaluated the resistance Rc of the contacts by performing also 4 terminal measurements (see Fig. 7 a below) through Rc = Vds/Ids – ρxx l/w, where ρxx is the sheet resistivity of the channel measured in a four-terminal configuration. We found the ratio Rc/ρxx ≈ 20 to remain nearly constant as a function of Vbg. (b) Conductivity σ = S l/w, where the conductance S = Ids/Vds (from (a)), as a function of Vbg and for several values of Vds. Notice, how all the curves collapse on a single curve, indicating linear dependence on Vds. As argued below, this linear dependence most likely results from thermionic emission across the Schottky-barrier at the level of the contacts. (c) Field effect mobility μFE = (1/cgdσ/dVbg as a function of Vbg, where cg = εrε0/d = 12.789 × 10−9 F/cm2 (for a d = 270 nm thick SiO2 layer). (d) Ids as a function of Vbg, when using an excitation voltage Vds = 5 mV. Red line is a linear fit whose slope yields a field-effect mobility μFE ≈ 300 cm2/Vs.
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f2: (a) Current Ids in a logarithmic scale as extracted from a WSe2 FET at T = 300 K and as a function of the gate voltage Vbg for several values of the voltage Vds, i.e. respectively 5 (dark blue line), 26 (red), 47 (blue), 68 (magenta), and 90 mV (brown), between drain and source contacts. Notice that the ON/OFF ratio approaches 106 and subthreshold swing SS ~250 mV per decade. We evaluated the resistance Rc of the contacts by performing also 4 terminal measurements (see Fig. 7 a below) through Rc = Vds/Ids – ρxx l/w, where ρxx is the sheet resistivity of the channel measured in a four-terminal configuration. We found the ratio Rc/ρxx ≈ 20 to remain nearly constant as a function of Vbg. (b) Conductivity σ = S l/w, where the conductance S = Ids/Vds (from (a)), as a function of Vbg and for several values of Vds. Notice, how all the curves collapse on a single curve, indicating linear dependence on Vds. As argued below, this linear dependence most likely results from thermionic emission across the Schottky-barrier at the level of the contacts. (c) Field effect mobility μFE = (1/cgdσ/dVbg as a function of Vbg, where cg = εrε0/d = 12.789 × 10−9 F/cm2 (for a d = 270 nm thick SiO2 layer). (d) Ids as a function of Vbg, when using an excitation voltage Vds = 5 mV. Red line is a linear fit whose slope yields a field-effect mobility μFE ≈ 300 cm2/Vs.

Mentions: Figure 2a shows the extracted field-effect current Ids as a function of the back gate voltage Vbg for several fixed values of the voltage Vds across the current contacts, i.e. when using a 2-terminal configuration. From initial studies7, but in contrast with Refs. 25,26, WSe2 is expected to show ambipolar behavior, i.e. a sizable current resulting from the accumulation of either electrons or holes at the WSe2/SiO2 interface due to the electric field-effect. Although we have previously observed such a behavior, all FETs studied here show a rather modest electron current (i.e. saturating at ~10−8 A) at positive Vbg values in contrast also with samples covered with Al2O3, see Ref. 26. Therefore our samples behave as if hole-doped (i.e. sizeable currents only for negative gate voltages). At room temperature the minimum current is observed around Vbg ≈ 0 V while the difference in current between the transistor in its “ON”-state with respect to the OFF- one (on/off ratio) is >106. For all measurements, the maximum channel current was limited in order to prevent damaging our FETs. The subthreshold swing SS is found to be ~250 mV per decade, or ~3.5 times larger than the smallest values extracted from Si MOSFETs at room temperature. Figure 2b shows the conductivity σ = Ids l/Vdsw (from a), as a function of Vbg for several values of Vds. As indicated in the caption of Fig. 1 the separation between the current contacts, is l = 15.8 μm while the width of the channel is w = 7.7 μm. As seen, all curves collapse on a single curve indicating linear behavior, despite the claimed role for Schottky barriers at the level of contacts19. See also the Supplemental Information section for linear current-voltage characteristics for the range of excitation voltages used. Figure 2c: the field-effect mobility μFE can be evaluated in the standard way by normalizing by the value of the gate capacitance (cg = 12.789 × 10−9 F/cm2) the derivative of the conductivity with respect to Vbg. As seen, μFE increases sharply above Vbg ≈ 2 V reaching a maximum of ~305 cm2/Vs at Vbg ~−20 V, decreasing again beyond this value. Alternatively, the mobility can be directly evaluated through the slope of Ids as a function of Vbg in its linear regime, and by normalizing it by the sample geometrical factors, the excitation voltage Vbg and the gate capacitance cg, yielding a peak value μFE ≈ 302 cm2/Vs. We have observed μFE values as high as 350 cm2/Vs (see results for sample 2 below). These values, resulting from two-terminal measurements, are comparable to those previously reported by us for multi-layered MoS2, where we used a four-terminal configuration to eliminate the detrimental role played by the less than ideal contacts27.


Hall and field-effect mobilities in few layered p-WSe₂ field-effect transistors.

Pradhan NR, Rhodes D, Memaran S, Poumirol JM, Smirnov D, Talapatra S, Feng S, Perea-Lopez N, Elias AL, Terrones M, Ajayan PM, Balicas L - Sci Rep (2015)

(a) Current Ids in a logarithmic scale as extracted from a WSe2 FET at T = 300 K and as a function of the gate voltage Vbg for several values of the voltage Vds, i.e. respectively 5 (dark blue line), 26 (red), 47 (blue), 68 (magenta), and 90 mV (brown), between drain and source contacts. Notice that the ON/OFF ratio approaches 106 and subthreshold swing SS ~250 mV per decade. We evaluated the resistance Rc of the contacts by performing also 4 terminal measurements (see Fig. 7 a below) through Rc = Vds/Ids – ρxx l/w, where ρxx is the sheet resistivity of the channel measured in a four-terminal configuration. We found the ratio Rc/ρxx ≈ 20 to remain nearly constant as a function of Vbg. (b) Conductivity σ = S l/w, where the conductance S = Ids/Vds (from (a)), as a function of Vbg and for several values of Vds. Notice, how all the curves collapse on a single curve, indicating linear dependence on Vds. As argued below, this linear dependence most likely results from thermionic emission across the Schottky-barrier at the level of the contacts. (c) Field effect mobility μFE = (1/cgdσ/dVbg as a function of Vbg, where cg = εrε0/d = 12.789 × 10−9 F/cm2 (for a d = 270 nm thick SiO2 layer). (d) Ids as a function of Vbg, when using an excitation voltage Vds = 5 mV. Red line is a linear fit whose slope yields a field-effect mobility μFE ≈ 300 cm2/Vs.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4355631&req=5

f2: (a) Current Ids in a logarithmic scale as extracted from a WSe2 FET at T = 300 K and as a function of the gate voltage Vbg for several values of the voltage Vds, i.e. respectively 5 (dark blue line), 26 (red), 47 (blue), 68 (magenta), and 90 mV (brown), between drain and source contacts. Notice that the ON/OFF ratio approaches 106 and subthreshold swing SS ~250 mV per decade. We evaluated the resistance Rc of the contacts by performing also 4 terminal measurements (see Fig. 7 a below) through Rc = Vds/Ids – ρxx l/w, where ρxx is the sheet resistivity of the channel measured in a four-terminal configuration. We found the ratio Rc/ρxx ≈ 20 to remain nearly constant as a function of Vbg. (b) Conductivity σ = S l/w, where the conductance S = Ids/Vds (from (a)), as a function of Vbg and for several values of Vds. Notice, how all the curves collapse on a single curve, indicating linear dependence on Vds. As argued below, this linear dependence most likely results from thermionic emission across the Schottky-barrier at the level of the contacts. (c) Field effect mobility μFE = (1/cgdσ/dVbg as a function of Vbg, where cg = εrε0/d = 12.789 × 10−9 F/cm2 (for a d = 270 nm thick SiO2 layer). (d) Ids as a function of Vbg, when using an excitation voltage Vds = 5 mV. Red line is a linear fit whose slope yields a field-effect mobility μFE ≈ 300 cm2/Vs.
Mentions: Figure 2a shows the extracted field-effect current Ids as a function of the back gate voltage Vbg for several fixed values of the voltage Vds across the current contacts, i.e. when using a 2-terminal configuration. From initial studies7, but in contrast with Refs. 25,26, WSe2 is expected to show ambipolar behavior, i.e. a sizable current resulting from the accumulation of either electrons or holes at the WSe2/SiO2 interface due to the electric field-effect. Although we have previously observed such a behavior, all FETs studied here show a rather modest electron current (i.e. saturating at ~10−8 A) at positive Vbg values in contrast also with samples covered with Al2O3, see Ref. 26. Therefore our samples behave as if hole-doped (i.e. sizeable currents only for negative gate voltages). At room temperature the minimum current is observed around Vbg ≈ 0 V while the difference in current between the transistor in its “ON”-state with respect to the OFF- one (on/off ratio) is >106. For all measurements, the maximum channel current was limited in order to prevent damaging our FETs. The subthreshold swing SS is found to be ~250 mV per decade, or ~3.5 times larger than the smallest values extracted from Si MOSFETs at room temperature. Figure 2b shows the conductivity σ = Ids l/Vdsw (from a), as a function of Vbg for several values of Vds. As indicated in the caption of Fig. 1 the separation between the current contacts, is l = 15.8 μm while the width of the channel is w = 7.7 μm. As seen, all curves collapse on a single curve indicating linear behavior, despite the claimed role for Schottky barriers at the level of contacts19. See also the Supplemental Information section for linear current-voltage characteristics for the range of excitation voltages used. Figure 2c: the field-effect mobility μFE can be evaluated in the standard way by normalizing by the value of the gate capacitance (cg = 12.789 × 10−9 F/cm2) the derivative of the conductivity with respect to Vbg. As seen, μFE increases sharply above Vbg ≈ 2 V reaching a maximum of ~305 cm2/Vs at Vbg ~−20 V, decreasing again beyond this value. Alternatively, the mobility can be directly evaluated through the slope of Ids as a function of Vbg in its linear regime, and by normalizing it by the sample geometrical factors, the excitation voltage Vbg and the gate capacitance cg, yielding a peak value μFE ≈ 302 cm2/Vs. We have observed μFE values as high as 350 cm2/Vs (see results for sample 2 below). These values, resulting from two-terminal measurements, are comparable to those previously reported by us for multi-layered MoS2, where we used a four-terminal configuration to eliminate the detrimental role played by the less than ideal contacts27.

Bottom Line: Here, we present a temperature (T) dependent comparison between field-effect and Hall mobilities in field-effect transistors based on few-layered WSe2 exfoliated onto SiO2.The hole Hall mobility reaches a maximum value of 650 cm(2)/Vs as T is lowered below ~150 K, indicating that insofar WSe2-based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides.The gate capacitance, as extracted from the Hall-effect, reveals the presence of spurious charges in the channel, while the two-terminal sheet resistivity displays two-dimensional variable-range hopping behavior, indicating carrier localization induced by disorder at the interface between WSe2 and SiO2.

View Article: PubMed Central - PubMed

Affiliation: National High Magnetic Field Laboratory, Florida State University, Tallahassee-FL 32310, USA.

ABSTRACT
Here, we present a temperature (T) dependent comparison between field-effect and Hall mobilities in field-effect transistors based on few-layered WSe2 exfoliated onto SiO2. Without dielectric engineering and beyond a T-dependent threshold gate-voltage, we observe maximum hole mobilities approaching 350 cm(2)/Vs at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm(2)/Vs as T is lowered below ~150 K, indicating that insofar WSe2-based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides. The gate capacitance, as extracted from the Hall-effect, reveals the presence of spurious charges in the channel, while the two-terminal sheet resistivity displays two-dimensional variable-range hopping behavior, indicating carrier localization induced by disorder at the interface between WSe2 and SiO2. We argue that improvements in the fabrication protocols as, for example, the use of a substrate free of dangling bonds are likely to produce WSe2-based FETs displaying higher room temperature mobilities, i.e. approaching those of p-doped Si, which would make it a suitable candidate for high performance opto-electronics.

No MeSH data available.


Related in: MedlinePlus