Control of polarity in multilayer MoTe₂ field-effect transistors by channel thickness

Abstract

In this study, electronic properties of field-effect transistors (FETs) fabricated from exfoliated MoTe₂ single crystals are investigated as a function of channel thickness. The conductivity type in FETs gradually changes from n-type for thick MoTe₂ layers (above ≈ 65 nm) to ambipolar behavior for intermediate MoTe₂ thickness (between ≈ 60 and 15 nm) to p- type for thin layers (below ≈ 10 nm). The n-type behavior in quasi-bulk MoTe₂ is attributed to doping with chlorine atoms from the TeCl₄ transport agent used for the chemical vapor transport (CVT) growth of MoTe₂. The change in polarity sign with decreasing channel thickness may be associated with increasing role of surface states in ultra-thin layers, which in turn influence carrier concentration and dynamics in the channel due to modulation of Schottky barrier height and band-bending at the metal/semiconductor interface.

INTRODUCTION

Recent advances and miniaturization in silicon based electronic devices are hindered by the limitation of short channel effects, high contact resistance and high leakage current. These limitations have motivated the researchers to search for alternate materials. Unique electronic and optoelectronics properties of graphene and beyond two-dimensional (2D) materials have shown the potential for next generation electronics.¹ Due to its gapless band structure, graphene-based FETs have a limited application for transistors or logic circuits for low-power applications. Advances and limitations in graphene research attracted researchers towards layered transition metal dichalcogenides (TMDCs). A broad range of bandgap values in TMDCs has paved the path to explore these materials for transistor applications. These layered materials have a common formula MX₂, where M is a transition metal from group IV-VII (Mo, W, Nb, etc.) and X is a chalcogen (S, Se or Te). One layer of MX₂ consists of a hexagonally packed sheet of M atoms that is sandwiched between two sheets of X atoms.² Weak van der Waals bonding between the layers enables thinning of TMDCs crystals down to a monolayer by mechanical or chemical exfoliation.

Among all TMDCs, MoTe₂ is the only material that can be grown in both semiconducting (2H) and semi-metallic (1T′) thermodynamically stable crystal structures and hence, it lends its potential usefulness for engineering of ohmic contacts and for resistance-switching applications. In the 2H semiconducting phase, band gap values for bulk and monolayer MoTe₂ are 0.81 eV (indirect) and 1.13 eV (direct), respectively,³ which makes it an attractive candidate for optoelectronic devices in the visible to near-infrared spectrum.^4–7 2H-MoTe2 is intrinsically p-doped but can also exhibit ambipolar behavior.⁸ Recently it was demonstrated that MoTe₂ FET polarity can be tuned by using dual top gates geometry or by selecting metals with appropriate work functions for drain and source contacts.^9–11

Here, we demonstrate control over n-, ambipolar and p-type conductivity in MoTe₂ back-gated FETs by reducing the channel thickness from thick (above ≈ 65 nm) to medium (between ≈ 60 nm and 15 nm) to thin (below ≈ 10 nm), respectively. The thickness modulated transport properties of MoTe₂ FETs open up possibilities for digital and analog circuits by providing a guidance for fabricating p-, ambipolar and n-type devices by tuning the channel thickness with no additional steps for extrinsic doping during material and device fabrication processes.

EXPERIMENTAL DETAILS

MoTe₂ flakes used in this work were mechanically exfoliated from bulk single crystals that were grown by chemical vapor transport (CVT) method using polycrystalline MoTe₂ powder and TeCl₄ (ca. 5 mg/cm³) transport agent sealed in evacuated quartz ampoules. Temperature in the hot and cold zones was 800 °C and 700 °C, respectively. Growth duration of MoTe₂ crystals was 140 h. This method produced pure 2H phase of MoTe₂ as confirmed by X-ray powder diffraction (XRD), Transmission Electron Microscopy (TEM) and Raman spectroscopy (see Fig. 1 for Raman and elsewhere¹² for XRD and TEM data).

Fig. 1.

Raman spectra of MoTe₂ flakes of different thickness exfoliated from bulk MoTe2 single crystals grown by CVT with TeCl₄ transport agent. Inset shows plan-view optical image of FET device with a 50 nm thick channel.

Flakes of different thickness, ranging from ≈ 5 nm to 78 nm, were transferred onto 300-nm-thick SiO₂/Si substrate, where silicon was used as a back-gate electrode and the oxide layer acted as a gate dielectric. Prior to exfoliation, the substrates were ultrasonically cleaned in acetone, 2-propanol and deionized water followed by oxygen plasma cleaning. The exfoliated flakes were located under an optical microscope and were mapped out to predefined alignment marks for source/drain contact fabrication. The source and drain contacts were patterned using conventional photolithography followed by e-beam deposition of Ti(40 nm)/Au(350 nm) bilayer and lift-off process. The devices were thermally annealed in vacuum at 350 °C for 5 min. AFM measurements were used to verify the thickness, smoothness and uniformity of MoTe₂ layers under investigation. All the fabricated devices were characterized by output and transfer characteristics using back-gated FET measurement at room temperature in ambient environment.

RESULTS AND DISCUSSION

The lattice vibrational modes of exfoliated 2H-MoTe₂ flakes were identified using Raman spectroscopy with a 532 nm laser source. The spectra for the 2H-MoTe2 (Fig. 1) exhibit characteristic A_1g at 170 cm⁻¹, E¹_2g at 235 cm⁻¹ and B¹_2g at ≈ 288 cm⁻¹ modes (the latter is active in thin layers only). Comparison of Raman spectra for varying thickness in Fig.1 is in agreement with Raman studies reported in the literature,⁴ which confirms the 2H phase and good crystalline quality of MoTe₂ layers used in FETs. Inset in Fig. 1 shows plan-view of a typical FET device.

To understand the effect of channel thickness on FET transport characteristics, we measured the output (drain current I_ds vs drain voltage V_ds) and transfer (I_ds vs back-gate voltage V_bg) characteristics. The results from output and transfer curve show that by adjusting the thickness of the MoTe₂ channel, it is possible to let holes or electrons dominate the channel conduction, resulting in p- or n- type FET behavior. The output curves at varying V_bg are shown in Fig. 2(a) for thin, 5.6 nm, and Fig. 2(b) for thick, 78 nm, channels. Clear p-type, i.e., decrease in I_ds with increasing V_bg, and n-type, increase in I_ds with decreasing V_bg, transport behavior is observed for thin vs. thick MoTe₂ FETs, respectively. Note that although the output characteristics appear to be nearly linear, a small Schottky barrier at the metal/semiconductor junction is expected due to difference between work function of Ti (4.33 eV) and electron affinity of MoTe₂ (4.3 ± 0.1 eV).^{10, 13}

Fig. 2.

Output characteristics of MoTe₂ FETs under different back-gate voltages, V_bg. FETs with channel thickness of (a) 5.6 nm and (b) 78 nm show p- and n- type conductivity, respectively.

We further analyzed the transfer behavior of the FETs by I_ds-V_bg curves at V_ds = 2 V, which showed n-, ambipolar and p- type behavior for thick, medium and thin channel, respectively. The gate leakage current I_gs in all the devices was negligible in the pA range. The maximum I_ds ON/OFF ratio was about 1×10³, which decreased with increasing channel thickness. The experimental results of transfer curves are shown in Fig. 3. Multiple devices were fabricated to reproduce the transfer characteristic as a function of MoTe₂ channel thickness. For channel thicker than ≈ 65 nm FETs showed consistent unipolar n-type behavior. For a medium channel thickness, from ≈ 60 nm to 15 nm, FET behavior was ambipolar, while FETs with very thin channel, from ≈ 10 nm down to 5 nm, were all p-type. Effect of channel thickness on various transport properties in FETs was previously described in black phosphorous, WSe₂, MoS₂, and MoTe₂ FETs.^14–18

Figure 3.

Transfer characteristics at V_ds = 2 V of (a) thin (p-type), (b) medium (ambipolar), and (c) thick (n-type) channel FETs. The drain current is normalized by the channel width.

In reference to n-type behavior, the n-doping in TMDCs has been observed when using TeCl₄ as the transport agent in the CVT growth¹⁹ and during post-growth chloride molecular doping of TMDC compounds.²⁰ In the present work, since SIMS measurements (not shown here) detected a presence of Cl impurity in the CVT grown MoTe₂ crystals, we assume that Cl doping is a dominant factor for n-type behavior in our thick channel devices. The switching of conductivity to p- type in thinner channel FETs suggests that the effectiveness of Cl doping diminishes upon MoTe₂ layer thinning. This can be associated with increasing role of surface defects and adsorbates in ultra-thin layers, a phenomenon which is exemplified in²¹ for atomically thin MoS₂ FETs. In addition to possible detrimental effect of reduced MoTe₂ channel thickness on the n-type doping efficiency, we also speculate that the n- to p- polarity switching in thinner layers may be caused by the modulation of Schottky barrier height and corresponding band alignment and band-bending at the metal/MoTe₂ interface.

In this study, we also investigated the thickness-dependent field-effect carrier mobility (μ_FE) of the fabricated devices, which is extracted from the transfer characteristics shown in Fig. 3. Following equation defines the μ_FE of the MoTe₂ FET:

$${\mu }_{FE}=\frac{d{I}_{DS}}{d{V}_{BG}}\left(\frac{L}{W{C}_{OX}{V}_{DS}}\right),$$ (1)

where C_ox is a silicon oxide capacitance per unit area, L is the channel length, and W is the channel width. Fig.4 shows the overall trend of mobility increasing with increasing channel thickness. The influence of thickness in MoTe₂ devices can be related to Coulomb scattering and quantum confinement.^22,23 The scattering of the carriers is weakened by the Coulomb interaction resulting in higher mobility in thicker vs. thinner channel as it was demonstrated for both MoTe₂¹⁴ and MoS₂²¹ FETs.

Fig. 4.

Field effect mobility of electrons and holes vs channel thickness for MoTe₂ FETs. Encircled pairs of data points correspond to the ambipolar devices that exhibit ambipolar conductivity.

CONCLUSION

In this study, we have demonstrated a simple, yet effective way to control the conductivity type in MoTe₂ FETs by tuning the channel thickness. FETs were fabricated from the CVT-grown and mechanically exfoliated MoTe₂ single crystals. The transport properties were examined for varying channel thickness using the output and transfer characteristics. FETs showed a change in polarity from n-type through ambipolar to p-type with reducing the channel thickness from 78 nm to 5 nm. The n-type conductivity in thick MoTe₂ layers is attributed to chlorine doping from TeCl₄ transport agent used in the CVT growth. The switch of polarity by thinning the FET channel may be associated with increasing role of surface states in ultra-thin layers, which can influence charge carrier concentration and dynamics in the channel by modulating Schottky barrier height and associated band alignment and band-bending at the metal/semiconductor interface.