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Ultra-Short Pulsed Laser Annealing Effects on MoS_{2} Transistors with Asymmetric and Symmetric Contacts

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## Abstract

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_{2}thin film transistors (TFTs) without thermal damage on plastic substrates. However, there has been insufficient investigation into how much improvement can be brought about by the laser process. In this paper, we observed how the parameters of TFTs, i.e., mobility, subthreshold swing, I

_{on}/I

_{off}ratio, and V

_{th}, changed as the TFTs’ contacts were (1) not annealed, (2) annealed on one side, or (3) annealed on both sides. The results showed that the linear effective mobility (μ

_{eff_lin}) increased from 13.14 [cm

^{2}/Vs] (not annealed) to 18.84 (one side annealed) to 24.91 (both sides annealed). Also, I

_{on}/I

_{off}ratio increased from $2.27\times {10}^{5}$ (not annealed) to $3.14\times {10}^{5}$ (one side annealed) to $4.81\times {10}^{5}$(both sides annealed), with V

_{th}shifting to negative direction. Analyzing the main reason for the improvement through the Y function method (YFM), we found that both the contact resistance (R

_{c}) and the channel interface resistance (R

_{ch}) improves after the pulsed laser annealings under different conditions. Moreover, the R

_{c}enhances more dramatically than the R

_{ch}does. In conclusion, our picosecond laser annealing improves the performance of TFTs (especially, the R

_{c}) in direct proportion to the number of annealings applied. The results will contribute to the investigation about correlations between the laser annealing process and the performance of devices.

## 1. Introduction

_{2}(transition metal dichalcogenides (TMDs)) by weak van der Waals force) can be a good candidate for solving these problems due to their properties. For example, single/multilayer molybdenum disulfide (MoS

_{2}) has the interesting features: high surface to volume ratio, relatively large bandgap (1.2–1.9 eV), high mobility at room temperature (~100 cm

^{2}/Vs), low subthreshold swing (SS, ~70 mV/decade), an absence of dangling bond, and high mechanical strength (a higher Young’s modulus than that of steel) [4,5,6,7,8]. Furthermore, it can be deformed by up to 11% without fracture [9,10] and can be bent to a bending radius of 0.75 nm without losing its electrical properties [11,12]. However, when metal‒MoS

_{2}junctions form, electrical contacts can disrupt the seamless flow of charge carriers to obtain the expected maximum performance [13]; to achieve high performance of electronic devices with MoS

_{2}, the contact resistance (R

_{c}) should be considered. Some methods have been suggested to reduce the R

_{c}: a highly-doped interface [14], scandium electrode [15], and thermal annealing [7]. However, there are some obstacles to using them. Highly-doped interfaces have trouble maintaining a constant high doping rate and doping effect. Scandium electrode is inappropriate to use commercially because of its scarcity. Conventional thermal annealing to heat the entire wafer at a high temperature can damage flexible substrates with a low thermal budget (< 200 °C).

_{2}junction [17]. In these previous experiments, we annealed both source- and drain-channel contact regions. However, neither of these annealings can provide detailed information about the degree and the trend of the effect made by each, and both, at the source and drain contact regions. Therefore, in this paper, we examine the degree of improvement in one-side annealing where only one of the source or the drain is annealed and the contacts thereby becoming asymmetric (“asymmetric contacts”) and in both-side annealing where both of the source and the drain are annealed and hence contacts becoming symmetric (“symmetric contacts”). Then, we probe into the primary reason for the differences in the degree of the improvement under asymmetric- and symmetric-contact conditions.

## 2. Experiments and Setup

_{2}TFT. The devices were fabricated by the following process: (1) a 250-nm-thick SiO

_{2}dielectric layer was deposited by sputtering on the heavily doped Si substrate, (2) a multilayer MoS

_{2}flake (~50 nm) was transferred from bulk MoS

_{2}crystal (SPI Supplies, West Chester, PA, USA), and (3) Ti of 10 nm and Au of 300 nm as source and drain electrodes were formed by conventional photolithography, lift-off techniques, and e-beam evaporation. Here, we note that the lowest layer would be a flexible substrate, but this laser annealing process did not affect the substrate in our previous results [17,18] or the convenience of fabrication, so we used the rigid Si substrate instead of flexible films in the experiment. Also, the MoS

_{2}flakes were obtained from mechanical exfoliation because they had a larger electrical mobility value than MoS

_{2}grown by chemical vapor deposition (CVD). Note that the studies on the reason why artificially obtained MoS

_{2}has low mobility are still ongoing; here are some possible reasons: (1) the twisted stacked structure causing a change in the bandgap by interlayer electronic coupling, (2) a change in the unit cell structure of MoS

_{2}from trigonal prismatic (2H) to octahedral (1T), and (3) a lot of fabrication defects [19,20,21]. Moreover, the Ti/Au electrodes we use can make up for the drawbacks of Au without a Ti layer, which has a relatively high work function and bad adhesion to the substrate [6]. Figure 1b shows the representative top-view image (filmed with an optical microscope) of the fabricated MoS

_{2}transistor of 11.2 μm (channel width) by 7 μm (channel length). For an accurate comparison of the improvement between asymmetric and symmetric contacts, we tried to select and use MoS

_{2}transistors having almost the same width on both sides of the electrodes.

_{2}contacts because a large part of the electron is injected through them [22]. Also, the selected wavelength (355 nm) matched well the absorption coefficient of Au‒MoS

_{2}; the absorption spectrum of MoS

_{2}‒Au has a peak near 355 nm [23]. A beam of light with Gaussian profile from ps laser went to the target point of a sample on a high-resolution X-Y linear stage (Aerotech, Pittsburgh, PA, USA) after passing the dichroic mirror and the objective lens (39x) that modulated the beam spot size as 1.5 μm diameter at a 1/e

^{2}peak irradiance (Figure 2b). A small amount of the light beam toward the CCD camera allowed us to adjust the focus of the laser at the accurate target point by confirming the position of the spot.

## 3. Results and Discussion

#### 3.1. Thermal Analysis

^{−10}~10

^{−12}s). Then, the generated heat moves to the direction of the higher thermal conductivity. Unfortunately, there is as of yet no way of directly measuring the temperature produced by the ultra-short pulsed laser. However, we can predict the transient temperature distribution at a specific position where a ps pulsed laser is irradiated through a three-dimensional (3D) Finite-Element Method (FEM) using COMSOL Multiphysics

^{®}. In more detail, we can estimate the generated heat profiles and flows when the pulsed laser beam irradiates the top of the Au electrode. The thermal analysis generally has many variables and it is very difficult to enter information to accurately mirror the real-world environment. To resolve these difficulties, we made a rough estimate of the results through the equilibrium temperature approximation. The estimation applies to the classical Fourier conduction model:

_{p}is the specific heat, k is the thermal conductivity, and Q is the Gaussian heat source term, expressed as

_{0}, R, A

_{c}, and ${\mathsf{\sigma}}_{\mathrm{x},\mathrm{y}}$ means the total input power, the reflectivity, the absorption coefficient, and the x, y widths of the spatial irradiation distribution, respectively. Also, for the accuracy of the analysis, we performed an actual experiment for finding the laser-induced damage threshold and extracted the parameters from the results of the real test. Figure 3a shows the computed temperature distribution while the eight laser pulses (the selected laser power of 20 mW, see Appendix A for the results of the parametric study in detail) irradiate at the surface of the Au electrode. The first pulse was absorbed to the top of the surface and a sharp heat spike occurred. Due to the high repetition rate (80 MHz), the generated spike heat was not able to cool down to the initial temperature before the next pulse (i.e., the heat accumulates) [24]. Therefore, we can observe two parts in Figure 3a: a pulsed laser-affected region (red) and a heat-accumulated region (blue). Moreover, the heat was confined to the shallow surface due to the ultra-short pulse duration (12 ps), as shown in Figure 3b. The aforementioned heat accumulation effect lasted up to 0.2 s until the moving laser beam (with a spot size of 1.5 μm diameter and a scan speed of 10 μm/s) was completely out of a specific point. As more and more pulses were absorbed, the bottom temperature of the electrode became the same as the surface temperature (Figure 3b). After the effect of the spike peaks disappeared, the heat dissipated entirely at 2.5 s. In a continuous time, we can restructure the graph for the overall accumulated temperature profile of point 1 (the top of the Au electrode) and point 2 (the bottom of the Au electrode) as shown in Figure 3c. From the simulated results, we were able to have enough heat (around 520 K) for the generally performed annealing process through the ultra-short pulsed laser at a power of 20 mW. Furthermore, this heat conducted to the bottom electrode (point 2) with minor energy loss due to the relatively high thermal conductivity of the electrode; this conducted heat mainly created thermal annealing consequences at the contact interface (between MoS

_{2}and electrode). Changes in electrical characteristics suggest that the effect of temperature contributes to performance improvement.

#### 3.2. Electrical Characteristics

_{2}‒metal junction at thermal equilibrium and the equivalent circuit of them. At the MoS

_{2}and metal junction, a Schottky barrier can exist because of the difference of the metal‒vacuum work function and the semiconductor‒vacuum electron affinity and can make the R

_{c}at the metal‒MoS

_{2}junction, which is expressed as R

_{d}and R

_{s}[25]. Then, the total resistance of the equivalent circuit will be R

_{s}+ R

_{d}+ 1/g

_{m}: The sum of R

_{c}and channel interface resistance (R

_{ch}). To investigate the difference in the degree of the improvement under asymmetric and symmetric contacts, we firstly measured the transfer curve (the source-drain current (I

_{ds}) and the gate-source voltage (V

_{gs})) and the output curve (I

_{ds}-V

_{ds}) of not annealed (Before, reference), one side annealed (Asymmetric), and both sides annealed (Symmetric) devices on the log (left) and linear-scale (right) in Figure 4b,c. The I

_{ds}-V

_{gs}in Figure 4b was measured at low ${\mathrm{V}}_{\mathrm{ds}}$ (0.4 V) to guarantee that the shape of the conductive channel can be uniformly formed (linear regime) and for applying the Y function method (YFM) with the assumption that it be used for extracting the contact parameters later [26,27,28]. Also, we used the devices with a relatively large contact barrier as a sample for observing considerable changes; the I

_{ds}-V

_{ds}showed a non-linear output curve in a low V

_{ds}regime (Figure 4c). From the curves, the field-effective mobilities in linear regime (μ

_{eff_lin}) were extracted: 13.17 cm

^{2}/Vs (Before), 18.84 cm

^{2}/Vs (Asymmetric contact), and 24.91 cm

^{2}/Vs (Symmetric contacts). In a qualitative sense, an increase in μ

_{eff_lin}($=\frac{{\mathsf{\mu}}_{0}}{1+\mathsf{\theta}({\mathrm{V}}_{\mathrm{g}}-{\mathrm{V}}_{\mathrm{th}})}$, where θ is the mobility attenuation factor caused by the sum of the scattering factor, θ

_{0}and the contact resistance factor, θ* ($=\frac{\mathrm{W}}{\mathrm{L}}{\mathrm{C}}_{\mathrm{ox}}{\mathsf{\mu}}_{0}{\mathrm{R}}_{\mathrm{c}}$) and μ

_{0}is the intrinsic mobility) can be based on the decrease of θ value and/or on the decrease of V

_{th}value. In a quantitative sense, the linear increment of the μ

_{eff_lin}could mean that a dominant factor of the variation could be also changed linearly. Due to the enhancement of the μ

_{eff_lin}, the ON current level appears to have a similar tendency. Therefore, I

_{on}/I

_{off}ratio has increased: $2.27\times {10}^{5}$ (Before), $3.14\times {10}^{5}$ (Asymmetric contact), and $4.81\times {10}^{5}$ (Symmetric contact). As we expected, we observed a negative shift of V

_{th}(${\mathsf{\varphi}}_{\mathrm{MS}}-\frac{{\mathrm{Q}}_{\mathrm{f}}}{{\mathrm{C}}_{\mathrm{ox}}}+2{\mathsf{\phi}}_{\mathrm{B}}+\frac{\sqrt{{\mathrm{qN}}_{\mathrm{a}}2{\u03f5}_{\mathrm{s}}2{\mathsf{\phi}}_{\mathrm{B}}}}{{\mathrm{C}}_{\mathrm{ox}}}$, where ${\mathsf{\varphi}}_{\mathrm{MS}}$ is the work function difference between the gate and the channel material and ${\mathsf{\phi}}_{\mathrm{B}}$ is the Fermi level offset. This indicates that the surface and interface charges (Q

_{f}) could be decreased by the decline of the coulomb scattering through the lower interface traps. This fact also agreed with the changes of the subthreshold swings (SS): 3.80 V/decade (Before), 3.53 V/decade (Asymmetric contact), and 3.21 V/decade (Symmetric contact). We know that this result comes from the diminishing of the C

_{it}from the relational formula of SS, $(\mathrm{ln}10)(\frac{\mathrm{kT}}{\mathrm{q}})(\frac{{\mathrm{C}}_{\mathrm{ox}}+{\mathrm{C}}_{\mathrm{p}}+{\mathrm{C}}_{\mathrm{it}}}{{\mathrm{C}}_{\mathrm{ox}}})$ [V/decade] (C

_{p}: Depletion layer capacitance, C

_{it}: Channel interface–trap capacitance). The degree of the decline of SS was proportionate to the number of annealings; the laser process could have a positive effect on the reduction of traps of the channel interface. By all accounts, the laser process can improve the performance of the MoS

_{2}TFTs by reducing the R

_{c}as well as making a better channel interface. However, it is still hard to decide what accounts for the changes; we just know that improving contact and interface quality can contribute to enhanced electrical characteristics after the laser annealing.

_{c}and MoS

_{2}channel interface, we compared the μ

_{eff_lin}and the μ

_{0}through the Y function method (YFM). Generally, a transfer-line method (TLM) with various channel lengths and uniform contact on the same channel body is engaged to extract the contact parameters. However, TLM is difficult to apply in our case, in which the devices are arbitrarily obtained and have an irregular shape and small size. On the other hand, YFM does not need any additional geometry except the low V

_{ds}condition (linear regime). In a linear regime, I

_{ds}is defined by, ${\mathrm{I}}_{\mathrm{ds}}={\mathrm{g}}_{\mathrm{ds}}\times {\mathrm{V}}_{\mathrm{ds}}=\frac{\mathrm{W}}{\mathrm{L}}{\mathrm{Q}}_{\mathrm{ch}}{\mathsf{\mu}}_{\mathrm{eff}\_\mathrm{lin}}\times {\mathrm{V}}_{\mathrm{ds}}$ (g

_{ds}: the channel conductance, Q

_{ch}: the channel charge per unit area). I

_{ds}can be rewritten as

_{0}because the other parameters are known. As shown in Figure 5a, the slopes ($4.28\times {10}^{4}$ (Before), $4.57\times {10}^{4}$ (Asymmetric contact), and $4.75\times {10}^{4}$ (Symmetric contact)) were obtained and the values of extracted μ

_{0}are shown in Figure 5b (black filled rectangular points). Furthermore, we can calculate the μ

_{eff_lin}from the square-law model (see black unfilled rectangular points in Figure 5b). Here, we note that the μ

_{eff_lin}includes the contact and interface effects, but μ

_{0}contains only the interface factor. Therefore, the root cause of the discrepancy between μ

_{0}and μ

_{eff_lin}can be the R

_{c}, and the difference decreased through the consecutive laser annealing process from 47.04% to 33.55% to 18.67% (Figure 5b). Also, the μ

_{0}has risen; the value of μ

_{0}in the case of both annealed contacts went up 18.8% from the initial case (no annealing) and 7.44% from the case of one side annealed contact. Since μ

_{0}only exhibits the effect of R

_{ch}, this can come down to the improvement of the interface quality after the laser treatment. The degree of the enhancement for the R

_{c}and R

_{ch}was linearly varied along the process, and the parameters made an impact on the linear advancement of the electrical performance. However, the improvement of the R

_{c}is more dominant than that of the R

_{ch}; the difference in proportions (the size of the area of each dominated region) and in the increase rates between μ

_{0}and μ

_{eff_lin}can be seen in Figure 5b.

## 4. Summary and Outlook

_{2}TFTs and the simulated temperature distribution showed that the process was compatible with a fabrication for flexible/wearable devices with a low thermal budget. The μ

_{eff_lin}, μ

_{0}, I

_{on}/I

_{off}ratio, SS, and V

_{th}in the direction of high performance changed whenever the number of laser processes increased. (μ

_{eff_lin}: 13.14 [cm

^{2}/Vs] (Before), 18.84 (asymmetric contact), and 24.91 (symmetric contact), μ

_{0}: 24.87 [cm

^{2}/Vs] (Before), 28.35 (asymmetric contact), and 30.63 (symmetric contact), I

_{on}/I

_{off}ratio: $2.27\times {10}^{5}$ (Before), $3.14\times {10}^{5}$ (asymmetric contact), and $4.81\times {10}^{5}$(symmetric contact), SS: 3.80 [V/decade] (Before), 3.53 (asymmetric contact), and 3.21 (symmetric contact), and V

_{th}: shifted to negative direction.) Furthermore, we traced the main reason for the improvement through the YFM. The laser annealing process can improve the R

_{c}as well as the R

_{ch}, and the two factors were linearly proportional to the contact conditions, such as the number of laser processes. Also, reducing the R

_{c}had a more major effect than promoting the channel interface. These results can be expected to help us develop an in-depth understanding of the interaction between the laser irradiation and materials. Furthermore, we expect that the irradiation of a pulsed laser with high energy density and short wavelength onto metal as an electrode can lead to a thermal annealing effect at a locally confined small area that needs high temperature without severe thermal damage to the entire plastic substrate; this process can be compatible with the soft materials often used in applications for flexible/wearable electronics.

## Author Contributions

## Funding

## Conflicts of Interest

## Appendix A

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**Figure 1.**Bottom-gated multilayer MoS

_{2}thin-film transistors (TFTs): (

**a**) conceptual schematic of the structure of MoS

_{2}TFTs; (

**b**) top-view image of the optical microscope image for the finished MoS

_{2}TFTs fabricated by a mechanical exfoliation.

**Figure 2.**The ultra-short pulsed laser annealing system: (

**a**) the visualized diagram of the laser annealing process; (

**b**) configuration of optical setup for the picosecond laser system.

**Figure 3.**Thermal analysis of transistors during picosecond laser annealing process: (

**a**) temperature distribution by laser annealed heat at sample; (

**b**) the timeline view of all laser annealing process; (

**c**) the overall temperature profile of point 1,2 (point 1 is the top of Au electrode and point 2 is the bottom of Au electrode) as a function of time.

**Figure 4.**Electrical properties of MoS

_{2}TFTs by laser annealing process: (

**a**) the energy band diagram of the metal-MoS

_{2}junction at thermal equilibrium condition and the equivalent circuit of them; (

**b**) I

_{ds}-V

_{gs}characteristics for a MoS

_{2}transistors on low V

_{ds}( = 0.4 V) in linear scale (solid) and log scale (dot); (

**c**) I

_{ds}-V

_{ds}characteristics of both sides annealed contacts (solid), one side annealed contact (dash), and before annealing (dot) for MoS

_{2}transistors in linear scale.

**Figure 5.**Extracted mobility values from Y function method: (

**a**) Y function values of both sides annealed contacts (red), one side annealed contact (blue), and before annealing (black) with respect to V

_{gs}; (

**b**) comparison of the intrinsic mobility (μ

_{0}) and the linear effect mobility (μ

_{eff}) during each laser process.

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## Share and Cite

**MDPI and ACS Style**

Kwon, H.; Baik, S.; Jang, J.E.; Jang, J.; Kim, S.; Grigoropoulos, C.P.; Kwon, H.-J.
Ultra-Short Pulsed Laser Annealing Effects on MoS_{2} Transistors with Asymmetric and Symmetric Contacts. *Electronics* **2019**, *8*, 222.
https://doi.org/10.3390/electronics8020222

**AMA Style**

Kwon H, Baik S, Jang JE, Jang J, Kim S, Grigoropoulos CP, Kwon H-J.
Ultra-Short Pulsed Laser Annealing Effects on MoS_{2} Transistors with Asymmetric and Symmetric Contacts. *Electronics*. 2019; 8(2):222.
https://doi.org/10.3390/electronics8020222

**Chicago/Turabian Style**

Kwon, Hyeokjin, Seunghun Baik, Jae Eun Jang, Jaewon Jang, Sunkook Kim, Costas P. Grigoropoulos, and Hyuk-Jun Kwon.
2019. "Ultra-Short Pulsed Laser Annealing Effects on MoS_{2} Transistors with Asymmetric and Symmetric Contacts" *Electronics* 8, no. 2: 222.
https://doi.org/10.3390/electronics8020222