Xiang
Tan‡
ab,
Shu
Wang‡
a,
Qiaoxuan
Zhang‡
c,
Juxing
He
a,
Shengyao
Chen
ae,
Yusong
Qu
a,
Zhenzhou
Liu
af,
Yong
Tang
c,
Xintong
Liu
c,
Cong
Wang
*d,
Quan
Wang
*b and
Qian
Liu
*ae
aCAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology & University of Chinese Academy of Sciences, Beijing 100190, China. E-mail: liuq@nanoctr.cn; wangcongphysics@mail.buct.edu.cn; wangq@ujs.edu.cn
bZhenjiang key laboratory of advanced sensing materials and devices, Jiangsu University, Zhenjiang 212013, PR China
cHebei University of Water Resources and Electric Engineering Electrical Automation Department, 061001, Cangzhou, Hebei, China
dCollege of Mathematics and Physics, Beijing University of Chemical Technology, Beijing, 100029, China
eMOE Key Laboratory of Weak-Light Nonlinear Photonics, TEDA Applied Physics Institute, School of Physics, Nankai University, Tianjin 300457, China
fSchool of Physical Science and Technology, Inner Mongolia University, Inner Mongolia 010000, China
First published on 3rd May 2023
The number of excellent 2D materials is finite for nano optoelectric devices including transistors, diodes, sensors, and so forth, thus the modulation of 2D materials is important to improve the performance of the current eligible 2D materials, and even to transform unqualified 2D materials into eligible 2D materials. Here we develop a fine laser doping strategy based on highly controllable laser direct writing, and investigate its effectivity and practicability by doping multilayer molybdenum ditelluride (MoTe2). Power-gradient laser doping and patterned laser doping, for the first time, are presented for designable and fine doping of 2D materials. The laser-induced polar transition of MoTe2 indicates good controllability of the method for the carrier concentration distribution in MoTe2. Multiple devices with finely tuned energy band structures are demonstrated by means of power-gradient laser doping and patterned laser doping, further illustrating the design capability of a precise energy band in 2D materials.
Doping, as an important means of modulating semiconductor materials and devices, is a critical issue in almost all semiconductor fabrication. At present, thermal diffusion doping and ion implantation doping are widely used in industries to modulate the properties of traditional semiconductor devices.5 However, for 2D materials, these mature doping strategies will lead to lattice distortion and unfavorable defects,6 which might not be fully compatible with 2D materials. In recent years, studies have shown that plasma, chemical reaction, and heating can also induce doping in 2D materials,7 but these doping methods realize designable and local modulation in 2D materials with difficulty, and some might even induce annoying contamination. Photothermal doping, as a newly developed method, is a pollution-free method and can complete doping in the desired area. For 2D materials, the photothermal effect will merge the O2 and H2O in the atmosphere into the material,8 which is quite similar to heating in the atmosphere.9 Based on this route, it is possible to control the hole carrier concentration and even the polarity shift in 2D materials to develop high-performance junctions, memory, low-contact barriers, and logic devices.10
The laser direct writing (LDW) technique has gathered plenty of attention and made great progress because of its flexibility and compatibility in nano–micro fabrication.11 LDW not only ensures high precision, but also has the advantages of low cost and low practical operation difficulty with electron beam doping. More importantly, LDW can be photothermally doped according to the designed pattern, and can remove the pollutants attached to the surface. Therefore, LDW provides a feasible tool to perform ultrafine photothermal doping on 2D materials.12 At the same time, it is critical for 2D-material-based electronics13 to explore the LDW doping of 2D materials, including the doping features and mechanism, and detailed composition variation in ultrafine doping.
In this work, a modified LDW system with super-resolution fabrication ability is adopted to perform ultrafine photothermal doping. Here, taking MoTe2 as an example, ultrafine laser doping has been deeply investigated with regard to modulation controllability, doping features, and mechanism. The results show that the local p-doping level in MoTe2 can be well controlled by modulating laser parameters such as laser power, irradiation time, etc. Moreover, for the first time, we present power gradient laser doping and patterned laser doping of 2D materials. The corresponding ultrafine tuning of the carrier distribution and energy band structure is also clarified through simulation and calculation. Owing to the simplicity and flexibility of our LDW doping method, it will provide a powerful means for ultrafine photothermal doping, and will open many new application opportunities for 2D-material-based nanoelectronics.
The Au (25 nm)–Cr (5 nm) bottom electrode was prepared on SiO2 (300 nm)/Si substrate by magnetron sputtering (Kurt J. Lesker PVD 75) for the 2D-material devices. The multilayer MoTe2 was mechanically transferred onto the electrodes and then photothermally doped using LDW. Heat transfer simulation of laser treatment was performed using “COMSOL Multiphysics Software”, and modelling and parameters were referenced from experiments. The electrical measurements of the devices were performed using a four-probe electrical test system with a three-channel sourcemeter (Keithley 2635 and 2602). Density functional theory (DFT) was performed using the Vienna ab initio Simulation Package (VASP).
To understand composition variations of MoTe2 after laser action at different powers, EDS analysis along the horizontal yellow line in Fig. 1a was performed, as shown in Fig. 1b. The Te elemental mass (blue curve) gradually decreases with increasing laser power and tends toward a stable value, while the Mo elemental mass (orange curve) is almost constant. The decrease in Te atoms should be attributed to the adsorption of O2 and H2O due to an increase in thermal motion of the Te atoms under laser action, and eventually leads to Te vacancy defects. The four typical morphologies of the 2H-MoTe2 (I, 0 mW; II, 4.4 mW; III, 8.3 mW; IV, 25.8 mW) are shown in Fig. 1c. Moreover, the Raman spectra (Fig. 1d and Fig. S4a†) show 1T′-MoTe2, 2H-MoTe2 and Te for different powers. When the laser power gradually increases from I to II, the intensities of two characteristic peaks of 2H-MoTe2 at 171 cm−1 and 232 cm−1 gradually weaken, indicating the entrance of foreign molecules. Te peaks also begin to appear at this stage at 90 cm−1, 123 cm−1, and 141 cm−1, corresponding to the E1, A1, and E2 modes.15 Due to the high atomic number and large electron polarization rate of Te, the peaks of Te are more prominent, leading to the signal of the A1g peak of 2H-MoTe2 disappearing. From II to III, holes (Fig. 1cIII) started to be generated on the surface of the MoTe2, the E2g peak of 2H-MoTe2 disappeared, and the Ag peak of 1T′-MoTe2 appeared at 248 cm−1, indicating that a phase transition occurred in the MoTe2 and the intensity of the Te peak increased. As the laser power further increased (from III to IV), the MoTe2 surface was damaged severely (Fig. 1cIV), the Ag peak of 1T′-MoTe2 gradually weakened and eventually disappeared, and only the Te peak remained. The peak intensity variation (Fig. S4b†) was calculated through the Raman results, which indicated that the 2H-MoTe2 was transformed into 1T′-MoTe2 under a low-power laser, and the 1T′ structure was destroyed when the power was over 8.3 mW, while Te gradually precipitated and eventually formed a Te cluster on the MoTe2 surface.
These results present changes in morphology and elements of the MoTe2 under laser irradiation. The destruction of the material is unfavorable in doping, thus the doping power used for laser doping is set beneath the threshold of the phase change (4.4 mW). The surface potential of the MoTe2 after laser irradiation from 0 to 6.6 mW was measured by KPFM, which can reflect the variation of the Fermi level as shown in Fig. 1e and f. The surface potential at the irradiated region firstly remains steady and then increases with increasing laser power. When the laser power is between 2.8 mW and 4.4 mW, the surface potential increases (the Fermi surface moves downward) linearly with the laser power.10 The result proves the ability of laser doping to finely control the Fermi level and the doping level. Based on this effect, planar MoTe2 devices with various types of ultrafine doping modes were designed and tested to verify their applicability in complex electronic circuits.
To reveal the mechanism of the polarity shift of the MoTe2 induced by laser doping, energy band calculations of different doping situations were conducted, as shown in Fig. 2d–f. According to previous works, laser irradiation on MoTe2 mainly involves two types of doping, vacancy doping and O2 cluster doping.10 Raman measurement further proves that the oxygen exists as clusters instead of MoO3 or other compounds such as MoO2 (Fig. S7†), indicating that O2 was adsorbed into MoTe2 and that no substitution doping of Te atoms occurred. Thus, we followed the characterization results and built the device model. To study the effect of the defect states, we took a 10 nm-length channel FET (with three layers of MoTe2 and six layers of Au electrodes) as an example, and considered three cases (Fig. S8†): (i) no vacancies in the channel region; (ii) Te vacancies in all the three layers in the channel region; and (iii) Te vacancies and O2 cluster absorption in the channel region. The total local density of states (LDOS) in each case is illustrated in Fig. 2d–f. Metal hybridization in the band gap of the pure Te defected case (ii) is obviously stronger in the channel region compared with the no defect case (i) and obviously lower than the O2 cluster adsorption case (iii). Moreover, the position of the Fermi level relative to the valence band maximum in the channel is obviously lower in cases (ii) and (iii) compared with the original case (i). This is due to the Te vacancies and exposure to air of the MoTe2 layers inducing hole (p-type) doping. According to the LDOS of the channel region from three layers of MoTe2 FETs with Au electrodes, the effective SBH (0.24 eV) can be extracted (Fig. S9†). The simulation results are well consistent with the measurements and clearly demonstrate the mechanism of the doping effect, which enables us to perform more complicated and delicate tuning of MoTe2.
In the first case where the laser irradiates the whole channel to produce doping (Fig. 3a), the energy band shifts upward, and the device conductivity is slightly enhanced due to the increased number of hole carriers, but no rectification is observed (Fig. 3b). However, when the laser irradiates the half channel rather than whole channel to produce doping (Fig. S10†), the I–V curves of the device show that the rectification ratio is sensitive to laser doping, and such strong rectification is a direct demonstration of the laser-induced pn junction.10 In order to confirm the applicability of the laser-functionalized p-doping, the excellent electrical features in devices with integrated n–n, n–p, p–p, and n–p–n junctions (Fig. S11†) were further demonstrated. The rectification ratio of the MoTe2 in-plane p–n junction prepared by laser doping is 102, which is much smaller than that of the traditional silicon-based p–n junction. In addition, laser doping in semiconductor materials can be designed in pattern because the LDW can controllably dope according to designed pattern. Laser doping of two-dimensional materials to prepare p–n junctions has the obvious advantages of no pollution, low cost, strong designability and universality.
For the second situation (Fig. 3c), the laser still irradiates the whole channel by adopting a gradient laser power to dope. This will result in the hole concentration distribution of the 2D MoTe2 tending to be uphill and the energy band being warped, so that a unidirectional barrier is introduced. The device modulated using the method mentioned above exhibits a rectification effect and the switching ratio reaches 102 (Fig. 3d).
In the third and fourth situations, shape-dependent local laser dopings (rectangular and triangular) are carried out at the center of the device channel (Fig. 3e and g). Rectangular pattern doping forms a homogeneous junction with symmetric energy band bending of potentials across the channel, introducing a transverse potential barrier difference between the electrodes and thus lowering the conductivity, as shown in Fig. 3f. Triangular pattern doping forms a unidirectional potential barrier, leading the device to have a strong rectification effect with a switching ratio of 102 (Fig. 3h).
These experiments show that band structure in 2D material devices can be modulated flexibly by different laser doping modes. And some new features we obtained in the devices indicate that local and patterned laser doping as well as gradient-power laser doping are feasible and have great potential in the energy band modification of 2D materials.
To further verify the capacity of patterned laser doping and power-gradient laser doping, a triangular laser-doped junction was adopted for further experiments. As shown in Fig. 4a, the rectification ratio of the device can be regulated by the gate voltage, and the corresponding diode model is also constructed in a simulation program with integrated circuit emphasis (SPICE) based on the characteristic parameters of the device at Vgs = 0, with an average relative error MRE of less than 9.3%. Finally, the basic logic gates “OR” and “AND” (Fig. 4c) are built and verified using this model. The input and output of the logic gates show a steady performance (Fig. 4d), which further proves the applicability of the laser doping modulation. It should be stressed that there is often a large voltage drop when two-dimensional materials are applied to logic gate circuits. This means the performance of two-dimensional materials for electronic devices needs to be improved with regard to crystal quality and contact between electrodes and 2D materials, and so on.
Compared with other 2D material doping techniques, the designability of laser doping is irreplaceable. Diverse and flexible laser-doping modes can be realized by modulating the laser power and laser irradiation time, and by gradient-power doping and shape-dependent doping. In principle, the laser doing method is suitable for any 2D materials, but the doping effect may be different for the same laser parameters and the same number of layers due to different crystal structures. It is believed that the laser fine-tuning method of 2D materials provides not only new ideas for the development of doping strategies but also many new application opportunities for 2D-material-based devices including resistors, memristors and diodes. In the future, capacitors and inductors will be realized by changing the target materials, laser parameters and laser direct writing methods, which is of significance for the development of future electronics.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nr00808h |
‡ These authors contributed equality to this work |
This journal is © The Royal Society of Chemistry 2023 |