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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.


(a) Current Ids in a logarithmic scale as extracted from the same WSe2 FET in Fig. 2 at T = 105 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 (magenta), 68 (dark yellow), and 90 mV (brown). Notice that the ON/OFF ratio still approaches 106. (b) Conductivity σ as a function of Vbg for several values of Vds. Notice that even at lower Ts all the curves collapse on a single curve. Notice how the threshold gate voltage Vtbg for conduction increases from ~0 V at 300 K to ~15 V at 105 K. Below, we argue that the observation of, and the increase of Vtbg as T is lowered, corresponds to evidence for charge localization within the channel. (c) Field effect mobility μFE = (1/cg) dσ/dVbg as a function of Vbg. (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 ≈ 665 cm2/Vs.
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f3: (a) Current Ids in a logarithmic scale as extracted from the same WSe2 FET in Fig. 2 at T = 105 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 (magenta), 68 (dark yellow), and 90 mV (brown). Notice that the ON/OFF ratio still approaches 106. (b) Conductivity σ as a function of Vbg for several values of Vds. Notice that even at lower Ts all the curves collapse on a single curve. Notice how the threshold gate voltage Vtbg for conduction increases from ~0 V at 300 K to ~15 V at 105 K. Below, we argue that the observation of, and the increase of Vtbg as T is lowered, corresponds to evidence for charge localization within the channel. (c) Field effect mobility μFE = (1/cg) dσ/dVbg as a function of Vbg. (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 ≈ 665 cm2/Vs.

Mentions: Figures 3a, b, c, and d show respectively, Ids as a function of Vbg for several values of Vds, the corresponding conductivities σ as a function of Vbg, and the resulting field-effect mobility as previously extracted through Figs. 2c and d. All curves were acquired at T = 105 K. As seen, at lower temperatures σ(T, Vbg) still shows a linear dependence on Vds although lower Ts should be less favorable for thermally activated transport across Schottky barriers. In fact, we collected similarly linear data sets at T < 105 K. At T = 105 K, μFE displays considerably higher values, i.e. it surpasses 650 cm2/Vs (accompanied by a reduction in the SS down to ~140 mV per decade). However, as seen in Fig. 3a, lower temperatures increase the threshold gate voltage Vtbg required for carrier conduction. Below we argue that this is the result of a prominent role played by disorder and/or charge traps at the interface between WSe2 and SiO2 instead of just an effect associated with the Schottky barriers. Large Schottky barriers are expected to lead to non-linear current Ids as a function of the excitation voltage Vds characteristics, with a sizeable Ids emerging only when Vds surpasses a threshold value determined by the characteristic Schottky energy barrier ϕ, as seen for instance in Ref. 28. But according to Figs. 2b and 3b, σ is basically independent on Vds above a threshold gate voltage, even at lower temperatures.


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 the same WSe2 FET in Fig. 2 at T = 105 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 (magenta), 68 (dark yellow), and 90 mV (brown). Notice that the ON/OFF ratio still approaches 106. (b) Conductivity σ as a function of Vbg for several values of Vds. Notice that even at lower Ts all the curves collapse on a single curve. Notice how the threshold gate voltage Vtbg for conduction increases from ~0 V at 300 K to ~15 V at 105 K. Below, we argue that the observation of, and the increase of Vtbg as T is lowered, corresponds to evidence for charge localization within the channel. (c) Field effect mobility μFE = (1/cg) dσ/dVbg as a function of Vbg. (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 ≈ 665 cm2/Vs.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: (a) Current Ids in a logarithmic scale as extracted from the same WSe2 FET in Fig. 2 at T = 105 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 (magenta), 68 (dark yellow), and 90 mV (brown). Notice that the ON/OFF ratio still approaches 106. (b) Conductivity σ as a function of Vbg for several values of Vds. Notice that even at lower Ts all the curves collapse on a single curve. Notice how the threshold gate voltage Vtbg for conduction increases from ~0 V at 300 K to ~15 V at 105 K. Below, we argue that the observation of, and the increase of Vtbg as T is lowered, corresponds to evidence for charge localization within the channel. (c) Field effect mobility μFE = (1/cg) dσ/dVbg as a function of Vbg. (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 ≈ 665 cm2/Vs.
Mentions: Figures 3a, b, c, and d show respectively, Ids as a function of Vbg for several values of Vds, the corresponding conductivities σ as a function of Vbg, and the resulting field-effect mobility as previously extracted through Figs. 2c and d. All curves were acquired at T = 105 K. As seen, at lower temperatures σ(T, Vbg) still shows a linear dependence on Vds although lower Ts should be less favorable for thermally activated transport across Schottky barriers. In fact, we collected similarly linear data sets at T < 105 K. At T = 105 K, μFE displays considerably higher values, i.e. it surpasses 650 cm2/Vs (accompanied by a reduction in the SS down to ~140 mV per decade). However, as seen in Fig. 3a, lower temperatures increase the threshold gate voltage Vtbg required for carrier conduction. Below we argue that this is the result of a prominent role played by disorder and/or charge traps at the interface between WSe2 and SiO2 instead of just an effect associated with the Schottky barriers. Large Schottky barriers are expected to lead to non-linear current Ids as a function of the excitation voltage Vds characteristics, with a sizeable Ids emerging only when Vds surpasses a threshold value determined by the characteristic Schottky energy barrier ϕ, as seen for instance in Ref. 28. But according to Figs. 2b and 3b, σ is basically independent on Vds above a threshold gate voltage, even at lower temperatures.

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.