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Contact Engineering of Molybdenum Ditelluride Field Effect Transistors through Rapid Thermal Annealing Jiancui Chen, Zhihong Feng, Shuangqing Fan, Sigang Shi, Yuchen Yue, Wanfu Shen, Yuan Xie, Enxiu Wu, Chongling Sun, Jing Liu, Hao Zhang, Wei Pang, Dong Sun, Wei Feng, Yiyu Feng, Sen Wu, and Daihua Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06739 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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ACS Applied Materials & Interfaces

Contact Engineering of Molybdenum Ditelluride Field Effect Transistors through Rapid Thermal Annealing

Jiancui Chen1, 2, Zhihong Feng1, 2, Shuangqing Fan1, 2, Sigang Shi1, 2, Yuchen Yue3, Wanfu Shen2, Yuan Xie1, 2, Enxiu Wu1, 2, Chongling Sun1, Jing Liu*, 1, 2, Hao Zhang1, Wei Pang1, Dong Sun4, Wei Feng3,Yiyu Feng3, Sen Wu2, and Daihua Zhang*, 1, 2

1. State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Tianjin 300072, People’s Republic of China 2. College of Precision Instrument and Opto-electronics Engineering , Tianjin University, Tianjin 300072, People’s Republic of China 3. School of Materials Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China 4. International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, People’s Republic of China

1

ACS Paragon Plus Environment


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ABSTRACT Understanding and engineering the interface between metal and two-dimensional materials are of great importance to the research and development of Nano-electronics. In many cases the interface of metal and 2D materials can dominate the transport behavior of the devices. In this study, we focus on the metal contacts of MoTe2 (molybdenum ditelluride) FETs (field effect transistors) and demonstrate how to use post annealing treatment to modulate their transport behaviors in a controlled manner. We have also carried out low temperature and transmission electron microscopy studies to understand the mechanisms behind the prominent effect of the annealing process. Changes in transport properties are presumably due to anti-site defects formed at the metal-MoTe2 interface under elevated temperature. The study provides more insights into MoTe2 field effect devices and suggests guidelines for future optimizations.

KEYWORDS: MoTe2, Polarity control, Rapid Thermal Annealing, Antisite defects, Transport, TMDCs

INTRODUCTION The past decade has witnessed tremendous advancement in the field of two-dimensional (2D) materials. The trend has been driven by the great 2


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variety of layered materials and their interesting properties when reduced to few atomic layers. 1-3 Among them, transition metal dichalcogenides (TMDCs) are of particular interest as they exhibit very unique mechanical,4 electrical 5 and optical properties 6-8 and demonstrate great potential in numerous applications. 9, 10 The TMDC family has a rich set of materials. They can also be configured to different forms and phases to achieve versatile functions.

11, 12

Moreover, different materials can be

assembled together through van der Waals interaction 13, 14 or material engineering 15 to form inter- or intra- layer hetero-structures. This gives extra flexibility to band engineering both within and across the material interfaces. Because of the great versatility, field-effect devices based on TMDCs have been demonstrated in a wide variety of applications ranging from transistors, 16 diodes, 17, 18 to photo detectors and chemical sensors. 19

However, electronic devices made of TMDCs generally suffer consistency and repeatability issues due to various reasons. Subtle changes

during

material

synthesis,

device

fabrication,

and/or

post-treatment steps can result in very different outcomes, while most details have not been thoroughly understood up to this point. Taking MoTe2 FETs as an example, researchers from different groups have measured a wide range of carrier mobilities, on-off ratios, and even different types of carriers using similar devices and measurement settings. 3



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Some studies attributed the discrepancy to the metal electrodes and suggested their work functions could largely govern the transport behaviors. Concretely, large work functions would favor p-type transport and small work functions for n-type. However, the theory failed to explain the case that the same electrode material (e.g., Ti) could actually result in both p- and n-type transport according to different reports. 16, 20 A recent study by Yoo et al. added even more complexity to the discussion After experimenting with a wide work function range from 3.5 to 5.6 eV, they concluded that all metal electrodes should lead to p-type transport regardless of their work functions due to strong Fermi level pinning. 21

To better understand the contradictory observations and seek solutions for highly consistent, reproducible and stable MoTe2 FETs, we carried out studies with different metal electrodes and specifically focused on the Rapid Thermal Annealing (RTA) process. Our findings essentially agree with Yoo’s observation that all metals form p-type FETs, but we also found the conclusion held only for devices treated with RTA under sufficient temperatures. We have systematically experimented with different RTA conditions and observed a number of interesting phenomena, including large shift of charge neutrality point (CNP), sharp increase in carrier mobilities, and changes in Schottky barrier heights (SBHs) for both holes and electrons. The changes are presumably due to 4


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formation of Mo-Te anti-site defects near the metal - MoTe2 interface. The anti-site defects work as an acceptor-type dopant that pulls the Fermi level close to the valance band. Our work helps to clarify a number of important and controversial issues raised by previous studies, and more importantly, suggests guidelines for future device/process designs and optimizations.

RESULTS AND DISCUSSION Figure 1(a) shows the microscope image of a typical MoTe2 FET. The blue curve is an AFM height profile scan over the white dashed line on the image. Thickness of the MoTe2 film is about 4.8 nm. Among all devices used in the following studies, thicknesses of the MoTe2 films range from 4.2 to 7.8 nm. Before patterning the metal electrodes, we annealed the flake for 3 mins at 673 Κ in forming gas to reduce oxidization states on the surface and to remove air bubbles trapped under the film. The two XPS spectra in Fig.1 (b) indicate changes in chemical composition before (green curve) and after (red curve) annealing. The two peaks at 235.8 and 232.6 eV on the green curve are associated with oxidation states of Mo, 22, 23 and both peaks vanished after the treatment. Same happened to the two peaks of Te oxidation states at 587.6 and 576.9 eV.

22, 23

The absence of oxidation peaks in the red curve indicates

successful removal of the oxidation states from the MoTe2 film. 5



225

Intensity (a.u.)

Te d3/2

E1g

Mo d3/2

Ti/Au

4.8nm

before RTA after RTA

Mo d5/2

SiO2

(c) Te d5/2

(b) Normalized indensity (a.u.)

1 2 3 4 5 6 7 (a) 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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230

235

570

580

590

Eb (eV)

after RTA before RTA

A21g A1g

100

200

300

Raman Shift (cm-1)

Figure 1. (a) Optical microscope image of a typical MoTe2 FET with Ti/Au contacts; Scale bar is 5 µm. The blue curve is an AFM height profile taken along the white dashed line on the microscope image. (b) XPS spectra of α-MoTe2 films before (green) and after (pink) annealed at 673 Κ for 3 mins. (c) Raman spectra of the same MoTe2 flake before (purple) and after (orange) annealed at 673 Κ for 3 mins.

Figure 1(c) compares the Raman spectra before and after the annealing 1 treatment, Raman-active modes of A1g (170 cm-1), E2g (232 cm-1), and 1 B2g (287 cm-1) are originated from different phonon modes of the 2D

nanocrystal. 24 The Raman spectrum taken after annealing (red curve) perfectly overlaps with the original curve (blue curve), suggesting no structural changes are induced by the treatment. The XPS and Raman spectroscopy analysis proves that the annealing process is highly effective in oxidation removal and also sufficiently safe not to cause structural or major compositional changes to the nanocrystal. We note that this annealing process is a pre-treatment step, which should not be confused with the RTA process discussed in the following sections. In the context of this report, the RTA process specifically refers to the post-annealing step after metal electrodes are formed on the MoTe2 films. 6


400

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We also found that the pre-annealing step has negligible effect on the transport properties of the MoTe2 film. In S1, we observed very similar (b) n-type 10

-6

Pd Au Cr Ti Al SC

10-8 10-9 10-10 -80

-40

(c)

0 Vbg (V)

40

10

-8

10-10 10-12 -80

80

p-type

140 120 100 80 60

Sc

Ti

Cr Al Au contact metal

Pd

Pd Au Cr Ti Al SC -40

(d) n-type p-type

6

0 Vbg (V)

40

80

10-6 10-7

4

Ids (A)

mobility (cm2v-1s-1)

Ids (A)

Ids (A)

10-7

10

-6

shifts of CNP (V)

(a)

10-8 no RTA 373  473  573  673 

10-9

2

10-10 0

Sc

(e)

Ti

Cr Al Au contact metal

10-11 -80

Pd

0

10-8

-20

10-9

-40

0 Vbg (V)

10-10

40

3 min 6 min 9 min 12 min

10-12 300

400 500 600 RTA temperature ()

700

-80

80

RTA time:

10-11

-60 -80

-40

(f)

Ids (A)

CNP (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-40

0 Vbg (V)

40

80

Figure 2. Transfer characteristics of MoTe2 FETs with six different metal electrodes before (a) and after (b) RTA treatment. The inset of (b) summarizes the shifts of charge neutrality points in all six devices. Error bar represents the standard deviation of device signals. (c) Mobilities of majority carriers. The electron mobility of the n-type (before RTA) and hole mobility of the p-type device (after RTA) are represented by the blue and red bars, respectively. (d) Transfer characteristics of a Ti contacted MoTe2 FET after treated at different RTA temperatures. (e) The shift of CNP as a function of RTA temperature. (f) Transfer characteristics of the device after different durations (3, 6, 9, and 12 minutes) of RTA treatments. The RTA temperature was kept at 573 Κ. 7



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transfer characteristics from two devices with and without this treatment. This is in sharp contrast to the post-annealing (RTA) step that causes significant changes in transport behaviors. The following discussions will focus on the RTA effects and their explanations.

Fig. 2(a) summarizes the transfer characteristics of the MoTe2 FETs. Each measurement scans the gate voltage (applied to the n + doped silicon substrate) from -80 to 80 V under a drain-source bias of 𝑉𝑑𝑠 = 2 V. All six devices behave as n-type FETs regardless of their metal contacts. The threshold voltages range from - 50 to - 80 V. The n-type transport is very common in chalcogenides as chalcogen vacancies form donor sites which pin the Fermi level near the conduction band.25-27 The devices were then treated with RTA at 673 Κ for three minutes and reloaded in probe station for the same test. Fig. 2(b) plots the transfer characteristics after the treatment, showing drastic changes in all six devices. The charge neutrality points (CNPs) all shift to the right by a large amount, which convert the devices from n- to p-type. We plot the changes in CNPs for different metal electrodes in the inset of Fig 2(b). The metals are listed in order of their work functions from low to high. The shifts in CNP range from ~70 (Pd) to ~150V (Cr) depending on the metal contacts. The effect of RTA is qualitatively consistent with all electrodes used in the test. However, there appears no clear correlation between the CNPs and the 8


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metal work functions.

In Fig. 2(d) we single out the Ti electrode device and record its transfer curves after different RTA treatments. Each treatment lasts 3 minutes. The measurements give us a better idea how the transport behavior gradually transitions from n- to p- type as the annealing temperature increases from 373 to 673 Κ. In Fig. 2(e) we plot the CNPs as a function of the RTA temperature. The curve follows a monotonic trend and shows no sign of saturation up to 673 Κ. It is interesting to note that the RTA duration time makes negligible effect on the transport behavior. In a separate experiment, we kept the RTA temperature at 573 Κ and varied the annealing time from 3 to 12 minutes. The I𝑑𝑠 – Vg𝑠 curves taken after each treatment followed essentially the same pattern (Fig 2(f)). The observation suggests that each RTA temperature can lead the device to a separate metastable state. Extended heating time is unable to alter the equilibrium of a given state, and transitions between the states would always require the activation by higher thermal energies.

The following equation allows us to derive the field-effect mobilities from the measurements in Fig 2(a) and 2(b) 16: 𝜇 =

1

𝑑𝜎

Cg 𝑑Vbg

where 𝜇 is the field-effect mobility, Cg = ε0 εr /𝑑

(1) is the gate 9



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capacitance (ε0 is the permittivity in vacuum, εr is 3.9 for SiO2, and 𝑑 is the thickness of the dielectric layer (285 nm)), 𝜎 = (Ids /Vds )(L/w) is the two-terminal conductivity. We calculated the electron and hole mobilities before and after RTA using the data in Fig. 2(a) and 2(b), respectively. The average electron mobility dropped from 3.5 to 0.001 cm2V−1s−1 after annealing, while the average hole mobility increased from 0.001 to 6.8 cm2V−1s−1 by more than three orders of magnitude. The carrier mobilities vary among different electrode materials, but are not directly correlated with their work functions. We summarize the electron mobility of the n-type FETs and the hole mobility of the p-type’s in Fig. 2(c). The mobilities of minority carrier are not included in the figure as they are too small to plot in the same scale. We note that RTA treatments can rarely cause this level of mobilities change in other TMDCs FETs.28-30 The large effect on MoTe2 is likely related to its specific chemical structure, which we will discuss in details in a later section.

Fig. 3(a) and (b) plot the transfer curves of the Ti - MoTe2 device under different temperatures before and after the RTA treatment. The curves were taken at 290, 260, 230, 200, and 170 Κ, respectively. In both figures, positions of the CNPs keep essentially the same during the cooling process, while the current density drops by 1-2 orders of magnitude depending on the gate bias. The strong temperature dependence suggests 10


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(a)

(b)

10-6

10-6

10-8

10-8

T (K) 290 260 230 200 170

10-10 10-12 10-14 -60

-40

(c)

-20

0

Vbg (V)

20

40

Ids (A)

Ids (A)

that the carrier transport is primarily governed by thermionic emission,

10-10 10-12 10-14 -60

60

T (K) 290 260 230 200 170 -40

-20

(d)

0 20 Vbg (V)

0.25

-60 -40 -20 0 20 40 60

-20

-25

-30 3

4

5 6 1000/ (-1)

7

SB (ev)

Vbg (V) :

40

60

before after

0.30

-15

In (Ids)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.15 eV

0.20

0.10 eV

0.15

0.16 eV

0.10

0.13 eV

0.05 -60

-40

-20

0 Vbg

20

40

Figure 3. Transfer curves of the Ti - MoTe2 FET under different temperatures before (a) and after (b) RTA. (c) Arrhenius plots of the data in (b) and their linear fittings at six different gate biases from -60 to 60 V. (d) Effective SBH as a function of gate bias before (green curve) and after (red curve) RTA. The data were derived from linear fittings of Arrhenius plots at different gate biases with an error limit estimated to be 5%.

which relates the drain-source current (𝐼𝑑𝑠) to the effective Schottky barrier (ΦSB) through the following equation 5: 𝐼𝑑𝑠 = 𝐴𝐴∗ T 2 𝑒𝑥𝑝(𝑒ΦSB/𝑘𝐵 T)

(2)

where 𝐴 is the area of the Schottky junction, 𝐴∗ = 4𝜋𝑒𝑚∗ 𝑘𝐵2 ℎ−3 is the effective Richardson constant, 𝑒 is the elementary charge, 𝑘𝐵 is the 11


60


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Boltzmann constant, 𝑚∗ is the effective mass, and ℎ is the Planck constant. We can then use Arrhenius plot to derive the effective SBHs through linear fittings. Fig. 3(c) shows the plots from the post-RTA device at six different gate biases from -60 to 60 V. The fitting lines have different slopes therefore yielding different effective SBHs. We then apply the fitting process to all gate biases and to both the pre- and postRTA data. The estimated effective SBHs are plotted as a function of gate bias in Fig. 3(d). The green and red curves are from the pre- and postRTA data, respectively. Each curve consists of two branches corresponding to hole (left) and electron (right) conductions. On each branch, the segment near the CNP can be fitted with a straight line (as shown in the figure). These are the regimes in which thermionic emission dominates the carrier transport. Further away from the CNPs the curves start to deviate from linear fittings, marking the onset of electron/hole tunneling across the Schottky barriers. The SBH at the end point of the linear fitting characterizes the “True SBH”, which reflects the intrinsic energy barrier across the semiconductor-metal interface, defined as 21: Φ𝐵𝑛 = 𝑆(Φ𝑚 − Φ𝐶𝑁𝐿 ) + (Φ𝐶𝑁𝐿 − 𝜒)

(3)

Φ𝐵𝑝 = 𝑆(Φ𝑚 − Φ𝐶𝑁𝐿 ) + (𝜒 + 𝐸𝘨 Φ𝐶𝑁𝐿 )

(4)

where Φ𝐵𝑛 is n-type SBH, Φ𝐵𝑝 is p-type SBH, Φ𝑚 is metal work function , S is Fermi level pinning factor and Φ𝐶𝑁𝐿 is charge neutrality level. In our case, the True SBH of electrons increased from 0.10 to 0.16 12


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eV after RTA, whereas for holes the value dropped from 0.15 to 0.13 eV. The result explains the sharp increase in hole mobility and decrease in electron mobility as observed in Fig. 2. The data are consistent with the n- to p- transition triggered by RTA as well.

(b)

(a)

10-6 10-7

4 3 1

2

Ids (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10-8 10-9 10-10 10-11 -80

(c)

(d)

A pre-RTA B pre-RTA A post-RTA -40 0 Vbg (V)

40

80

Figure 4. (a) Optical image of the MoTe2 devices used in the control experiment. The scale bar is 5 µm. Electrode 1 and 2 were fabricated prior to RTA treatment; Electrode 3 and 4 were added after annealing. (b) Transfer characteristics of Device A (between Electrode 1 and 2) and Device B (between Electrode 3 and 4). (c) and (d) are HRTEM images of a MoTe2 film before and after RTA, the film has a discontinuous coating of Pd on the surface, equivalently 1 nm thick. The scale bar is 40 nm. Anti-site defects can be frequently found on post-annealing samples, but are absent on untreated MoTe2 films. Inset: The scale bar is 1 nm.

To confirm the changes were indeed originated from the MoTe 2 – metal 13



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contacts, we designed the experiment in Fig. 4(a). We first patterned two electrodes on a MoTe2 film (labeled as electrode 1 and 2) to form Device A. We then took its transfer curves before (green curve) and after (red curve) RTA (573 Κ) (Fig. 4(b)). The data are fully consistent with previous observations. The annealing resulted in a large shift of CNP accompanied by a clear n- to p- transition. In the next step, we went through the same fabrication process and added two more electrodes (labeled as 3 and 4 on Fig. 4(a)) to form Device B right next to the first device. Interestingly, even the MoTe2 film had already undergone the RTA treatment, Device B’s transfer characteristic (purple curve in Fig. 4(b)) remained the same as the pre-annealing state of Device A (green curve). The result proves that both the metal electrode and the MoTe2 film need to present for the RTA treatment to take effect. Changes in transport properties are merely related to the metal-MoTe2 contacts rather than modifications on the MoTe2 film itself.

The HRTEM images in Fig 4(c) and 4(d) allow us to take a close look at the metal-MoTe2 interface on the atomic level. The samples were prepared by using a liquid exfoliation method. 31 We first used a bath sonicator to shake small flakes of MoTe2 off from a bulk crystal into water solution. After centrifugation, the supernatant was carefully decanted onto TEM carbon grids for subsequent imaging. The images in 14


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Fig. 4(c) and (d) were taken on MoTe2 flakes with a discontinuous layer of Pd coating before and after RTA, respectively. The Pd layer is equivalently 10 Å thick and deposited through E-beam evaporation at a rate of 0.1 Å/s. Due to surface energy, the Pd atoms merge into clusters after landing on the MoTe2 surface. We chose heavy metals like Pd in this test as the material is relatively easy to image (we have also experimented with Au atoms with similar results shown in S2). Materials with small atomic numbers (e.g., Ti) make very poor image contrast and are hardly visible under HRTEM. The inset of Fig. 4(c) is a HRTEM image taken within an area not covered by Pd, as highlighted by the red box in the figure. The image shows a hexagonal array of alternating bright and dark spots with a lattice spacing of 3.8 Å, which is perfectly consistent with the lattice structure of 2H-MoTe2. According to the Z-contrast mechanism,32 atoms with larger atomic numbers appear brighter in a HRTEM image, therefore, the bright and dark spots correspond to the positions of the Te and Mo atoms, respectively. We inspected the MoTe2 surface with HRTEM at more than 30 different locations. The crystal lattices are clearly visible and free of defects in all images.

We can find two major differences by comparing the images before (Fig. 4(c)) and after (Fig. 4(d)) the RTA treatment. First, size of the Pd clusters grew due to further aggregation during the annealing process. They are 15



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averagely 3-10 times bigger after the treatment. Second, in most HRTEM images of the post-annealing sample, especially in areas close to the metal islands, there are a large number of structural defects such as the ones shown in Fig. 4(d) inset. The defects were presumably caused by the dissociation of Te atoms at elevated temperature which largely destructed the local crystal lattice. It is interesting to note that most defects appear in the vicinity of the metal islands. They are rarely detected in empty space far from the metal clusters, nor on bare MoTe2 films even after thermal treatments. The metal atoms appear to play an irreplaceable role in catalyzing the chemical reaction and formation of the structural defects. More specifically, it is likely the strong interaction between the Mo and metal’s d-orbitals that weakens the Mo-Te bonding and facilitates the dissociation of Te atoms.

33

The dissociated Te atoms are likely

evaporated according to the XPS spectra in S3. At the same time, the reaction requires sufficient thermal energy to activate the d-orbital interaction and to mobilize the Te and Mo atoms within the nanocrystal. This explains the absence of defects before the RTA treatment (Fig. 4(c)). We have also imaged MoS2 flakes with Au nanoparticles before and after the RTA treatment. Interestingly, no lattice defects were detected in the vicinity of the metal islands. We attribute the difference between MoS2 and MoTe2 to the small difference in electronegativity between the cations (Mo4+) and anions (Te2-) in MoTe2 34, 35, which results in very low 16


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energy barrier for the formation of antisite defects.

As suggested by previous studies,34-38 a large percentage of Te vacancies left by Te dissociation can be filled with neighboring Mo atoms and form anti-site defects (denoted as MoTe). This is presumably the root cause of the sharp change in transport behaviors.36 MoTe is an acceptor type defect and creates more holes in the nanocrystal. The reaction equation is as follows: •• ′′′′ • VTe + 2𝑒 ′ + MoMo → VMo + Mo′′′′ Te + 8ℎ

(5)

where VTe is the Te vacancy, MoMo is the host Mo atom, VMo is the Mo vacancy,

•

is a positive charge, ′ is a negative charge. As shown in

the equation, the Te vacancy results in 2 free electrons, and Mo vacancy results in 4 free holes according to Hund’s rule and other similar work. 34, 37, 38

On the other hand, the relocation of each Mo atom, from its original

position to the neighboring Te vacancy, contributes 4 holes. In sum, the reaction creates 6 holes per defect site due to the eventual electron– hole recombination. Hole doping at the metal-MoTe2 interface can effectively reduce the SBH for hole injection and lower the Fermi-level toward the valence band edge,

39-41

which accounts for the n- to p-

transition and large CNP shift as observed in Fig. 2. Our hypothesis borrows the idea from Kim’s work, 34, 38 where they use anti-site defects in Bi2Te3 nanocrystal to explain the changes in carrier concentrations. 17



(b)

(c)

10-7

10-7

0

Vbg (V)

40

0 min 10 min 20 min 30 min

10-11

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Figure 5. Transfer characteristics of an un-passivated MoTe2 FET before (a) and after (b) RTA treatment. Device stability degraded due to formation of anti-site defects and their interaction with air molecules. The experiment in (c) repeats the same measurements on a passivated MoTe2 FET. The passivation layer is 20 nm thick and made of Al2O3 through ALD.

As a side effect of anti-sites defects, the p-type devices became less stable compared to the n-type FETs prior to RTA. Fig. 5 (a) and 5 (b) compare the results of repetitive measurements on a MoTe2 FET before and after RTA. All measurements were done in air at room temperature. The n-type device in Fig. 5(a) showed very repeatable transport behavior with no detectable degradation within at least 7 days. However, the RTA-treated bipolar device kept shifting during a time span of merely 30 minutes, with its CNP gradually moved from -20 to 10 V. The continuous change in transfer characteristics is presumably due to adsorption of moisture and oxygen toward the anti-site defects near the metal-MoTe2 interface. The property is good for chemical sensing 42 but bad for electronic devices with strict stability requirements. To mitigate the problem, we coated a 20 nm layer of Al2O3 through atomic layer deposition (ALD) to seal the device, which effectively blocks its interaction with air molecules through the anti-site defects. Al2O3 itself is a chemically inert material that 18


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induces negligible change to the MoTe2 FETs. The passivated devices showed great improvement in stability (Fig. 5(c)). For this specific device in the figure, the passivation was actually done prior to the RTA treatment. This further proves that the CNP shift was primarily caused by antisites instead of oxidation (or moisture adsorption) at the defect sites. The RTA turns out to be a very versatile process step that can be applied either before or after the annealing treatment.

CONCLUSION In summary, our study focused on the RTA treatment and its effects on MoTe2 FETs. The treatment has a much more prominent effect on MoTe2 FETs compared to devices made of other 2D materials. It converts the MoTe2 transistors from n- to p-type with large shifts in CNPs. The changes in carrier mobilities are by 3-4 orders of magnitude and in opposite directions for electrons and holes. The observations were consistent among all six metal electrodes used in our study, ranging from very low (3.5 eV) to very high (5.8 eV) work functions. We have also carried out low temperature measurements and HRTEM studies to investigate the root cause of the RTA effects. It is suggested that the formation of MoTe anti-site defects at the metal-MoTe2 interface is responsible for the sharp changes in transport behaviors. The defects act as accepter type dopants and largely alter the Schottky barriers and carrier 19



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concentrations across the conduction channel. Our work provides a deep insight into the RTA effect and associates transport behaviors with microscopic

structural

and

compositional

changes

at

the

metal-semiconductor interface. We believe the data and discussions will be very valuable for future studies on TMDC-based electronic devices, especially those requiring contact engineering to achieve satisfactory functionality and performances.

EXPERIMENTAL SECTION Device Fabrication Flakes of 2H-MoTe2 were first exfoliated by scotch tape from a bulk crystal, then transferred to an n+ doped silicon wafer with 285 nm thick thermal oxide. To remove wrinkles and bubbles trapped under the flakes, we annealed the film in forming gas (98% N2 and 2% H2) at 573 Κ for 3 minutes using LABSYS’s RTA system RTP-1200. Electrodes were then patterned on the MoTe2 flake using E-beam lithography with a double-layer resist recipe.

43

Six electrode

materials with different work functions were used in the experiments, including Sc (3.5eV), Al (4.15 eV), Ti (4.3 eV), Cr (4.5 eV), Au (5.2 eV) and Pd (5.6 eV). 21, 33 The metal films were deposited on the substrate through E-beam evaporation, each with a thickness of approximately 5 nm. An Au film of 50 nm was then added on the top (without vacuum break) to passivate the metal surface. The deposition was followed by a 20


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standard lift-off process to complete the device fabrication.

Characterizations We used Atomic Force Microscopy (AFM) to characterize the thickness and morphology of the MoTe2 films. The AFM images were taken with a Bruker Dimension Icon. Their chemical composition was analyzed with X-ray photoelectron spectroscopy (XPS) (Thermo Scientific ESCALAB 250Xi) using 200 W Al radiation. The binding energies obtained in the spectral analysis were corrected for charging by referencing C1s to 284.6 eV. The 2H-phase structure was confirmed by Raman spectroscopy (Renishaw, Inc.) with a 532 nm excitation laser. The HRTEM images in Fig 4 were taken with JEOL JEM-2100F to compare the crystal structures before and after RTA treatments. The MoTe2 films had a 10 Å Pd coating on the surface, which was deposited through E-beam evaporation at a rate of 0.1 Å/s. The MoTe2 samples were obtained by liquid exfoliation method.31

The output and transfer characteristics of the MoTe2 FETs were inspected using semiconductor parameter analyzer Agilent B1500. The temperature dependent measurements in Fig. 3 used a low-temperature probe station (LINKPHYSICS CRX-4K) with liquid nitrogen cooling. In the demonstration of device passivation in Figure 5, we used Ultratech’s Atomic Layer Deposition (ALD) chamber to grow the Al2O3 layer. 21



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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. (J.L) *E-mail: [email protected]. (D.H)

ACKNOWLEDGEMENTS D. Zhang acknowledges support by the Tianjin Applied Basic Research and Advanced Technology (13JKYBJC37100). This work was financially supported the 111 Project (B07014).

ASSOCIATED CONTENT Supporting Information The control experiment data showing the effect of pre-annealing on the polarity behavior of the MoTe2 devices, on the chemical bond between the interface, and of the RTA on the MoS2 FETs.

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Contact Engineering of Molybdenum Ditelluride ... - ACS Publications

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