Effects of the thickness and laser irradiation on the electrical properties of e-beam evaporated 2D bismuth

Xinghao Sun a, Hanliu Zhao ab, Jiayi Chen a, Wen Zhong a, Beibei Zhu *a and Li Tao *ab
aSchool of Materials Science and Engineering, Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing, 211189, China. E-mail: tao@seu.edu.cn; 101012333@seu.edu.cn
bCenter for 2D Materials, Southeast University, Nanjing, 211189, China

Received 20th August 2020 , Accepted 31st December 2020

First published on 2nd January 2021


Abstract

Two-dimensional (2D) bismuth is expected to yield exotic electrical properties for various nanoelectronics, despite the difficulty in large-area preparation and property tuning directly on a device substrate. This work reports electron beam (e-beam) evaporation of large-area 2D bismuth directly on SiO2/Si with an electrical conductivity of ∼105 S m−1 and a field effect carrier mobility of ∼235 cm2 V−1 s−1 at room temperature, comparable to those of the molecular beam epitaxy (MBE) counterparts with a similar thickness. Interestingly, the electrical conductivity of 2D bismuth changes when exposed to laser irradiation that possibly induced an increase of the defect concentration, indicating a potential photo-sensor application. The electrical response of 2D bismuth can be modified either by laser irradiation or by varying the layer thickness. Due to the dimension and surface state effects in 2D bismuth, the layer thickness has a strong influence on the carrier concentration and mobility. Inspiringly, a simultaneous increase of the electrical conductivity and the Seebeck coefficient was achieved in 2D bismuth, which is preferred for thermoelectric performance but rarely reported. Our results provided a more accessible platform than MBE to produce decent quality 2D bismuth and similar Xenes with tunable electrical properties for various nanoelectronics.


1. Introduction

Xenes, buckled or puckered elemental two-dimensional (2D) materials mostly from IVA and VA, have attracted increasing research attention due to the tunable band gap between graphene and transition metal dichalcogenides (TMDs). Emerging representatives of Xenes include phosphorene and silicene. Phosphorene, often referred to as black phosphorus (BP), is a unique anisotropic 2D material for optoelectronics and electronics,1 owing to a high on-current with decent field-effect mobility and on/off ratio.2 Similarly, silicene3 presents some excellent properties which resemble those of graphene,4 with a measured room-temperature mobility of ∼100 cm2 V−1 s−1 (ref. 5) and a tunable band gap predicted by density functional theory calculations.6 However, such Xenes face the serious challenges of environmental stability7,8 and a small band gap which limit their application in electronic devices. In contrast, bismuthene, a 2D atomic sheet of bismuth (Bi), has excellent air stability and a tunable band gap9,10 at room temperature (RT), holding prospects in nanoelectronics, optoelectronics, spintronics and other devices.

Bi forms a three-dimensional (3D) hexagonal crystal that exhibits quite a lot of eye-catching characteristics.10 Bi crystallizes with a rhombohedral layered structure, called β-phase, which is the most and only stable allotropic form under atmospheric pressure,11 as shown in Fig. 1a. In addition, Fig. 1b displays the top view of hexagonal β-phase Bi, exhibiting a honeycomb structure. The side views along different directions shown in Fig. 1c and d reveal a buckled bilayer structure. Recently, Bi droplets prepared from a Bi thin film by post-annealing showed a phase transition induced by laser irradiation, demonstrating the potential for controlling the optical functionalities of materials on a nano/micro-scale.12 However, such a form of Bi is hard to synthesize and research on the electrical properties of solid-state 2D bismuth after laser irradiation has been lacking. Besides laser irradiation, phase transition and the corresponding properties are also thickness-dependent. For example, when Bi undergoes transition from 3D to a single layer, the 2D hexagonal lattice is compressed and the semimetal–semiconductor (SMSC) transition happens. The critical thickness dz for this transition is predicted by most calculations to fall somewhere in the range of 23–32 nm.13 In addition to SMSC transition, a pseudo-cubic (012) allotrope phase to (001) phase transition above a critical thickness of 2–3 nm was also reported by using a scanning tunneling microscope and first-principles calculations.14


image file: d0nr06062c-f1.tif
Fig. 1 Crystal structure of Bi. (a) Bulk Bi with a rhombohedral structure. (b) Top view and (c and d) side views of the atomic structure of layered Bi.

Bi possesses several unusual physical properties. For instance, Bi has a strong spin–orbit coupling effect15,16 due to the heavy atomic mass.17 Besides, a large gap of ∼0.8 eV was experimentally detected in monolayer bismuthene on SiC (0001) at RT, making it a candidate for a high-temperature quantum spin Hall material.18,19 In addition, owing to the small electron effective mass (<0.03m0)20 and extremely long mean free path,21 Bi displays a large magnetoresistance effect. Meanwhile, single-crystal bulk bismuth has a high mobility (∼106 cm2 V−1 s−1),22 suggesting its potential application in field effect transistors, but a low carrier density (∼1017 cm−3)23 due to a small band overlap that can be tuned by the thickness. Such a combination of high mobility24,25 and low carrier density allows 2D bismuth to decouple the transport coefficients,26–28e.g. the Seebeck coefficient and the electrical conductivity that have opposite charge carrier concentration dependence. Hence, it is possible to realize a simultaneous improvement in the electrical conductivity and the Seebeck coefficient, which is highly preferred in thermoelectric effects.

Currently, 2D bismuth can be grown via two approaches: chemical and physical methods. For instance, chemical reaction29 can provide ultrathin Bi nanosheets. Sonochemical exfoliation supplies few-layer bismuthene for ultrafast photonics.30 Besides, ice-bath sonication and liquid phase exfoliation31 can prepare few-layer and monolayer Bi nanosheets. Besides chemical solution methods, physical vapor deposition approaches, such as vapor phase deposition,32 thermal evaporation,33 pulsed laser deposition (PLD),9 molecular beam epitaxy (MBE)34 and so on, have the advantage to produce a large-area continuous Bi thin film on device substrates. MBE can provide high-quality single-crystal bismuthene due to commensurable growth with the least lattice defects at high facility and time costs. Vapor phase deposition and thermal evaporation methods are more accessible but suffer from low quality of grown samples. Therefore, there is a high demand to employ a cost-effective and convenient method to produce high-quality 2D bismuth with device compatibility. Here, we report our exploration on 2D bismuth via electron beam (e-beam) evaporation with emphasis on the electrical performance under the dimension effect and laser irradiation, holding great potential for innovative nanoelectronics.

2. Experimental methods

2.1 E-beam evaporation of 2D bismuth

2D bismuth was grown on SiO2/Si or Si (111) substrates in an electron-beam evaporator (VZS 600 Pro) with an in situ thickness meter. Before the deposition, Si (111) and SiO2/Si substrates were cleaned by ultrasonication in acetone for 10 min and rinsed with isopropyl alcohol and deionized water. The deposition of Bi was initiated from a 99.999% pure Bi source in a boron nitride crucible at a rate of 0.01 Å s−1 under 4.0 × 10−4 Pa pressure. Annealing was performed in a tube furnace at 423–523 K under a 100 sccm Ar atmosphere at ∼100 Pa pressure for 1–1.5 h.

2.2 Phase and structural characterization of 2D bismuth

The surface morphology of 2D bismuth was measured using a Dimension ICON atomic force microscope (AFM) and an FEI Sirion scanning electron microscope (SEM). The crystal structure and crystallinity of the samples with different thicknesses and diverse annealing parameters were determined using an Ultima IV X-ray diffraction (XRD) system. A LabRAM HR UV-Visible Raman spectrometer equipped with a 532 nm laser was employed for phase characterization of 2D bismuth and measuring the thickness. High-resolution imaging and selected area electron diffraction (SAED) were performed using an FEI G2 20 transmission electron microscope (TEM). A typical PMMA assisted transfer process was employed to prepare the 2D bismuth sample on a copper mesh for TEM.

2.3 Electrical characterization of the 2D bismuth FET

The evaporated 2D bismuth, as the channel material, on SiO2/Si or Si (111) substrates, was then subjected to a field effect transistor (FET) fabrication process via lithography and Ti/Au electrode evaporation. The thickness of the Ti layer and the Au layer was 5 nm and 45 nm, respectively. The length and width of the FET channel were 100 μm and 150 μm, respectively. Electrical characterization, such as output characteristics and the transfer curve of the FET based on 2D bismuth with different thicknesses, was performed on a Cascade® EPS150 probe station with a Keysight® 2902 analyzer in a dark environment at RT. The Hall effect was measured using the HET-RT system. The Seebeck coefficient was measured using a portable Joule Yacht PTM-3 apparatus at RT.

3. Results and discussion

A typical Raman spectrum of our 2D bismuth on SiO2/Si, as shown in Fig. 2a, has two first-order Raman bands at ∼66.2 cm−1 and ∼93.4 cm−1 associated with two characteristic optical phonon modes Eg (in-plane) and A1g (out-of-plane) of the rhombohedral lattice, respectively. This suggests pristine 2D bismuth, which is also available on the bare Si substrate (Fig. S1), without detectable oxidation peaks (115–130 cm−1, 280–350 cm−1 and 445–485 cm−1).35 Raman mapping for the intensity ratio of Eg/A1g over a 25 × 25 μm2 area (Fig. 2b) indicates a highly continuous and uniform surface of 2D bismuth.
image file: d0nr06062c-f2.tif
Fig. 2 Phase characterization of e-beam evaporated 2D bismuth. (a) Raman spectrum with Eg and A1g characteristics. (b) Raman intensity mapping image of Eg/A1g over a 25 × 25 μm2 area as shown in the optical image as an inset. (c) XRD and (d) SAED patterns of 20 nm 2D bismuth.

As shown in Fig. 2c (or Fig. S2), 2D bismuth on SiO2/Si (or bare Si) presents strong X-ray diffraction peaks (003) and (006), which indicates the preferred orientation along the (001) family of planes in a hexagonal structure. Considering the rhombohedral crystal structure of 2D bismuth, it can be inferred that Bi grows along the (111) direction. This is consistent with the SAED pattern (Fig. 2d), which has a representative 6-fold symmetry, suggesting an obvious hexagonal structure of evaporated 2D bismuth. The calculated inter-planar spacing of 0.44 nm is larger than the standard value of 0.395 nm for the (003) plane spacing in PDF#44-1246, indicating a lattice expansion. Importantly, there is no detectable peak of bismuth oxide in the XRD pattern, suggesting elemental 2D bismuth has been obtained, which is consistent with Raman characterization result.

2D bismuth in this work shows good coverage and uniformity, evidenced by SEM, energy dispersive spectroscopy (EDS) and AFM characterization. A mostly continuous surface with few tiny pinholes is observed on 5 nm 2D bismuth (Fig. 3a) and a pin-hole free uniform surface is guaranteed for a thickness of 10 nm (Fig. 3b) and above. This is distinguished from the discontinuous near-percolation structure in ultrathin bismuth films prepared by other methods.36 In addition, the EDS mapping of 20 nm 2D bismuth (Fig. S3) verified a uniform distribution of the Bi element. Moreover, AFM images and profiles (Fig. 3d–f) indicate that the average crystallite size becomes larger as the thickness increases, also seen in PLD Bi films.37 It is worth mentioning that annealing was conducted to increase the crystallite size of 2D bismuth (Fig. S4) and thus improve the electrical conductivity (Table S1). 2D bismuth shows an average root-mean-square (RMS) surface roughness of approximately 1 nm over a 600 × 600 nm2 area, presenting a highly flat surface as shown in the optical photograph (Fig. S5). E-beam evaporation with annealing resulted in smooth and homogeneous crystalline 2D bismuth.


image file: d0nr06062c-f3.tif
Fig. 3 Surface morphology of e-beam evaporated 2D bismuth. SEM images of (a) 5 nm, (b) 10 nm and (c) 20 nm 2D bismuth. AFM images and profiles of typical (d) 5 nm, (e) 10 nm and (f) 20 nm 2D bismuth.

2D bismuth FETs on SiO2/Si were fabricated (Fig. 4a with details in the Experimental methods section) to characterize the electrical properties of our e-beam evaporated 2D bismuth. The electrical conductivity was measured by performing the measurement of output characteristics when the gate voltage (Vg) is 0 in a three-terminal configuration with a typical channel length of 100 μm and a width of 150 μm. Fig. 4b reveals the linear relationship between drain–source voltage (Vds) and drain–source current (Ids), suggesting a good ohmic contact between the Ti/Au electrode and 2D bismuth. To probe the charge transport property, IdsVg measurement was conducted at fixed Vd. The field-effect mobility (μFE) was obtained by using the following formula in the linear region:

 
image file: d0nr06062c-t1.tif(1)
where L and W are the channel length and width, respectively, and Ci refers to the capacitance per unit area of SiO2. With an applied Vds value of 0.1 V and ∂Ids/∂Vg (slope of the linear fit curve in Fig. 4c) of −1.22 × 10−6, a typical μFE value is around 235 cm2 V−1 s−1 for 2D bismuth.


image file: d0nr06062c-f4.tif
Fig. 4 Electrical characterization of e-beam evaporated 2D bismuth. (a) The schematic diagram of the 2D bismuth FET. (b) IdsVds and (c) IdsVg charge transport curves of the same 20 nm 2D bismuth FET. (d) Comparison of the carrier mobility of 2D bismuth with those of Xenes5,38 and the Bi film prepared by other methods.9,39,40

Nonetheless, the on/off ratio was approximately 2.5 which is quite small compared with graphene and black phosphorus.41 As seen in the IdsVg curve, our e-beam evaporated 2D bismuth is P-type, which is consistent with the literature reports.9,42Fig. 4d surveys the mobility of Xenes, including BP,38 silicene5 and 2D bismuth prepared by different approaches, such as radio frequency magnetron sputtering,39 PLD9 and MBE.40 The electrical property of e-beam evaporated 2D bismuth is comparable to those of the MBE counterparts with a similar thickness at RT.

2D bismuth in this work displays photo-responsive properties. Multiple consecutive Raman spectra measurements were conducted and the results of three 15 nm 2D bismuth samples (I–III) are presented in Fig. 5a and b. It should be highlighted that the intensity of A1g increases after exposing to a laser beam for 10 seconds, resulting in the decrease of the intensity ratio of Eg/A1g from 1.25 to 0.72 and 1.16 to 0.88 for samples I and II, respectively, and 1.23 to 0.98 after a longer laser irradiation time in sample III. Considering that Eg and A1g represent the transverse and longitudinal modes, respectively, a stronger A1g peak indicates that the out-of-plane (⊥ (001)) phonons took the initiative after laser flashing. This is due to the formation of defects, e.g. the long-range disorder in the crystal lattice introduced during laser exposure.12


image file: d0nr06062c-f5.tif
Fig. 5 Laser irradiation-induced changes of the Raman spectra and electrical conductivity. (a) Multiple consecutive Raman spectra measurements of three 2D bismuth samples. (b) Raman intensity ratio and FWHM of the Eg and A1g modes of three 2D bismuth samples under 0.5 mW laser irradiation for different times. (c) The schematic diagram of the laser irradiation on the 2D bismuth channel of the FET. (d) Laser-induced change of the IdsVds curve.

Besides the intensity ratio and peak position, the full width at half maximum (FWHM) of Eg and A1g shows a remarkable change after laser irradiation. According to Fig. 5b, the FWHM of Eg became wider after 10s laser irradiation, whereas that of A1g became narrower. The wider the FWHM values of the peak, the higher the percentage of actual vibration that deviates from the theoretical value, resulting in possible instability. For this reason, Raman spectroscopy is sensitive to probe the disorder in the Bi crystal lattice induced by the laser. These changes of Eg and A1g after laser flashing suggest the decrease of crystallinity and will influence the electrical property of 2D bismuth. Thus, laser irradiation was conducted at the channel of the FET as shown in Fig. 5c. As shown in the IdsVds curve in Fig. 5d, an obvious decline of the slope reveals a higher resistance after laser irradiation of the 2D bismuth channel. The conductivity significantly dropped from 1.28 × 105 S m−1 to 2.3 × 103 S m−1 because of laser-induced defects and disorder, which suggests a potential photo-sensing application.

The above blue shift of Raman peak positions and the increase of Eg/A1g also happened when the thickness of 2D bismuth decreased from 30 nm to 5 nm (Fig. 6a and b). The thickness of the samples in this article has been cross-checked by AFM as shown in Fig. S7 and Table S2. For instance, the Eg and A1g mode bands shift around 11 cm−1 and 6.4 cm−1, respectively; the intensity ratio of Eg/A1g for both 5 nm and 8 nm 2D bismuth is less than 1, but greater than 1 for a thickness above ∼10 nm. It may be due to the low crystallinity (partially amorphous) which can be inferred from the TEM images shown in Fig. S8. The XRD patterns of 2D bismuth with different thicknesses are shown in Fig. 6c. It is found that the peaks are broadened when the thickness is reduced, indicating smaller crystallites in thinner films with consistent observation in AFM images (Fig. 3). Moreover, strain can be another factor to induce a blue shift. The Bi atoms collectively oscillate within the monolayer due to covalent bonding. When more layers of Bi are bonded by van der Waals forces, the oscillation of Bi is inhibited.43 Therefore, the Raman energy of thinner films is smaller, leading to the blue shift. The result confirms the lattice expansion compared with bulk Bi which is relevant to the SAED pattern shown in Fig. 2d. The blue shift with a decreased thickness in our 2D bismuth was absent for Raman characteristic peaks in PLD Bi films.9 Besides the blue shift, strain can also make the phonon energy for the in-plane mode higher than that for the out-of-plane mode, producing an intensity ratio of Eg/A1g > 1. Thus, Raman spectroscopy provides an effective method to roughly measure the thickness of Bi. For instance, as shown in Fig. 6b, a ratio smaller (larger) than 1.0 reveals a Bi film below (above) 10 nm.


image file: d0nr06062c-f6.tif
Fig. 6 Thickness-dependent physical properties of e-beam evaporated 2D bismuth. (a) Raman spectra of 2D bismuth with various thicknesses. The trends of the peak shift are indicated by dashed lines for eye-guide. (b) Thickness-dependent Raman intensity ratio of Eg/A1g in e-beam evaporated 2D bismuth. (c) XRD patterns of 2D bismuth with various thicknesses. (d) Conductivity of the FET based on 2D bismuth with different thicknesses.

It is also noticed that the (003)/(012) intensity ratio in XRD patterns (Fig. 6c) increases sharply as the thickness increases, which is totally opposite to the work reported by Rodil et al.44 due to our lower evaporation rate and lower thickness.45 The increase of the intensity ratio can be explained by a change in the crystal orientation. When the thickness is less than a critical value, the structure is pseudo-cubic with the (012) preference,14 and 2D bismuth gradually grows toward the Bi (001) preference46 with increasing thickness.

Both laser irradiation and thickness change can induce Raman spectra changes. Since laser flashing has led to the decrease of the conductivity, the dependence of the electrical properties on the thickness was also examined as shown in Fig. 6d. The electrical conductivity is proportional to the thickness, which agrees with the fact that bulk Bi is a semimetal. Besides, as the thickness increases, less boundary scattering from a larger crystallite size leads to the increase of the conductivity as shown in Table 1. From the maximum value of the results, the conductivity for 30 nm 2D bismuth can reach 2.3 × 105 S m−1, which is in the same order of magnitude as that of the Bi film prepared by MBE (2.2 × 105 S m−1 for the 7 nm sample).47

Table 1 Average size and conductivity of 2D bismuth crystallites with different thicknesses
Film thickness (nm) 8 10 20 30
Crystallite size (nm) 6.38 8.10 16.58 28.88
Conductivity (S m−1) 1.27 × 104 2.05 × 104 8.20 × 104 1.86 × 105


To find out the reasons behind thickness-dependent electrical conductivity, Hall effect measurements with various thicknesses were carried out. The electrical conductivity with different thicknesses measured by the Hall effect method shows a similar pattern to that of the field-effect method as shown in Fig. 7a. According to Fig. S9, the Hall coefficients (RH) of all the samples are positive, indicating a clear P-type feature. The ratio of RH to the thickness measured is quite impressive, revealing a high sensitivity of the Hall effect device prepared with evaporated 2D bismuth. The carrier concentration (n) and Hall mobility (μH) can be calculated by the following equations:

 
image file: d0nr06062c-t2.tif(2)
 
image file: d0nr06062c-t3.tif(3)
where e represents the electronic charge and ρ is the resistivity. Hence, μH and n of the samples of different thicknesses are obtained as shown in Fig. 7b. They change in the opposite way and the Hall mobility dominates the electrical conductivity. The carrier concentration is on the order of 1020 cm−3 which is higher than that of bulk Bi (∼1017–1018), owing to the apparent surface contributions in 2D bismuth. A high surface metallic state of 2D bismuth results in a high carrier concentration as the thickness range we studied is <30 nm.13,48 When the thickness decreases, quantum confinement may push the surface state band above the Fermi level, resulting in an increased carrier concentration49 (Fig. 7b). The smaller mobilities in thinner 2D bismuth can be explained by the large carrier effective mass in the surface state which has been experimentally proved in low-dimensional bismuth.50 With the increase of the thickness, the influence of surface states decreases and the carrier concentration drops subsequently. Nevertheless, the conductivity becomes larger due to the increase of the carrier mobility, which can be attributed to the decreased electron scattering from the reduced charge carrier concentration, and less grain boundary scattering from the increased grain size as shown in Fig. 3 and Table 1.


image file: d0nr06062c-f7.tif
Fig. 7 Thickness-dependent electrical and thermoelectric properties: (a) conductivity, (b) Hall mobility and carrier concentration, (c) Seebeck coefficient value and (d) power factor of 2D bismuth on the SiO2/Si substrate.

The reduction of n results in the increased absolute value of the Seebeck coefficient when the thickness increases at RT as illustrated in Fig. 7c. Furthermore, the Seebeck coefficient and the electrical conductivity exhibit simultaneous improvement when the thickness increases as the unique surface states induce high mobility and low carrier concentration. The resultant power factor (PF) was obtained as shown in Fig. 7d, showing two orders of magnitude enhancement with the increase of the thickness. Since the thermal conductivity will decrease in a lower thickness Bi film due to the enhanced phonon scattering from grain boundaries,51,52 an optimal thermoelectric figure of merit might be obtained by optimizing the thickness of the bismuth thin film, demonstrating its potential in thermoelectric applications.

4. Conclusions

This work investigated large-area e-beam evaporated 2D bismuth with decent and tunable electrical properties. Our 2D bismuth on SiO2/Si exhibited a field-effect mobility of ∼235 cm2 V−1 s−1 comparable to those of the MBE counterparts with a similar thickness at RT. There is a modification in the electrical response of 2D bismuth induced by laser irradiation that seems to create defects, enabling potential photo-sensor applications. Besides, for 2D bismuth <30 nm, a transition from pseudo-cubic (012) to (001) is preferred and an increased crystallite size was observed with increasing thickness. These could boost the conductivity from ∼104 to ∼105 S m−1. Unlike many other 2D materials, the electrical conductivity and the Seebeck coefficient increase simultaneously with the thickness due to the decreasing carrier concentration, which is favorable for thermoelectric applications. Furthermore, the Eg and A1g modes show a blue shift and the intensity ratio of Eg/A1g changes when the thickness decreases owing to the change of the crystal orientation and tensile stress supported by SAED, providing an effective method to estimate the thickness of 2D bismuth using Raman spectroscopy. This study paves the way to produce large-area and high-quality 2D bismuth with tunable properties for innovative nanoelectronics, such as photo-sensors and thermoelectric devices.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51602051), the Jiangsu Province Innovation Talent Program, the Jiangsu Province Six-Category Talent Program (DZXX-011) and the Fundamental Research Funds for the Central Universities (2242020K40008).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nr06062c
Equal contribution to this work.

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