Mobility Extraction in 2D Transition Metal Dichalcogenide Devices—Avoiding Contact Resistance Implicated Overestimation

Abstract

Schottky barrier (SB) transistors operate distinctly different from conventional metal‐oxide semiconductor field‐effect transistors, in a unique way that the gate impacts the carrier injection from the metal source/drain contacts into the channel region. While it has been long recognized that this can have severe implications for device characteristics in the subthreshold region, impacts of contact gating of SB in the on‐state of the devices, which affects evaluation of intrinsic channel properties, have been yet comprehensively studied. Due to the fact that contact resistance (RC) is always gate‐dependent in a typical back‐gated device structure, the traditional approach of deriving field‐effect mobility from the maximum transconductance (gm) is in principle not correct and can even overestimate the mobility. In addition, an exhibition of two different threshold voltages for the channel and the contact region leads to another layer of complexity in determining the true carrier concentration calculated from Q = COX * (VG–VTH). Through a detailed experimental analysis, the effect of different effective oxide thicknesses, distinct SB heights, and doping‐induced reductions in the SB width are carefully evaluated to gain a better understanding of their impact on important device metrics.

Toward Low-Temperature Solid-Source Synthesis of Monolayer MoS2

“…The field-effect mobility values reported from MOCVD growths (at or below 500°C for 8 to 30 hours) are marked in blue. [36][37][38][39][40][41] For this simple comparison we benchmark two-probe mobility measurements, although more complex four-probe measurements with a threshold voltage correction can yield more accurate mobility values, 68 if current shunt paths through the (invasive) inner voltage probes are avoided. 7 In general, we have found no correlation between carrier mobility and growth temperature, only between growth temperature and MoS2 crystallite size or substrate adhesion.…”

“…The CVD films in this work were grown at 560 °C for 50 min, while MOCVD films were grown at or below 500 °C for 8 to 30 h, − as labeled. The μ eff from sheet resistance (TLM) measurements tend to be more reliable, while μ FE could be under- or overestimated depending on the V GS -dependence of gated contacts. ,, All mobility data are reported at room temperature. During review, we became aware of recent MOCVD films grown at 320 °C for over 14 h; , a summary of μ FE data for these films (lighter blue triangles) is shown as the box-and-whisker plot at 320 °C (average μ FE of 20.4 cm 2 V –1 s –1 in the blue box).…”

“…It is also possible that μ FE is overestimated in some studies depending on the gate-voltage-dependence of the contacts. In other words, a sharp turn-on of back-gated contacts can lead to an overestimated peak transconductance g m and μ FE . ,, Due to these effects of gated contacts, effective mobility (μ eff ) from sheet resistance (TLM) measurements (particularly when reported as a function of the carrier density n ) tend to be more reliable and are preferable, rather than μ FE , as the main figure of merit for MoS 2 .…”

“…During review, we became aware of recent MOCVD films grown at 320 °C for over 14 h; 37,38 a summary of μ FE data for these films (lighter blue triangles) is shown as the box-and-whisker plot at 320 °C (average μ FE of 20.4 cm 2 V −1 s −1 in the blue box). benchmark two-probe mobility measurements, although more complex four-probe measurements with a threshold voltage correction can yield more accurate mobility values 68 if current shunt paths through the (invasive) inner voltage probes are avoided. 7 In general, we have found no correlation between carrier mobility and growth temperature, only between growth temperature and MoS 2 crystallite size or substrate adhesion.…”

“…The CVD films in this work were grown at 560 °C for 50 min, while MOCVD films were grown at or below 500 °C for 8 to 30 h, 36−41 as labeled. The μ eff from sheet resistance (TLM) measurements tend to be more reliable, while μ FE could be under-or overestimated depending on the V GS -dependence of gated contacts 68,73,74. All mobility data are reported at room temperature.…”