Abstract
1. Introduction
Beginning with the successful exfoliation of monolayer graphene, two-dimensional (2D) materials, including h-BN, transition metal dichalcogenides (TMDs), Xenes and MXenes, have attracted significant interest in the past dozen years due to their unique physical and chemical properties. As a specific Xene branch, 2D atomic sheets of group VA elements possess tunable electrical, thermal, optical properties compared to their bulk state, thus having a broad prospect in the field of CMOS applications, such as field-effect transistors (FET), sensors, optoelectronics, thermoelectrics, topological insulators and so on. Group VA consists of nitrogen, phosphorus, arsenic, antimony and bismuth, all of which have a puckered or buckled layered structure except for nitrogen. Black phosphorus (BP) is the most thermodynamically stable allotrope of phosphorus under standard conditions[
Figure 1.(Color online) (a) Atomic structure of α phase P (left) and
As the most intensively studied 2D Xene material, BP is known for its tunable direct band gap (0.3 eV for bulk and ~2 eV for monolayer), anisotropic transport properties, thickness-dependent anisotropic optical response[
2D monolayer arsenene possesses carrier mobility as high as 635 cm2V–1s–1 for electrons and 1700 cm2V–1 s–1for holes as calculated[
As the last element of the VA group, β-bismuth possesses the same rhombohedral structure as arsenic and antimony[
With the rapid increase in 2D materials research, many emerging 2D materials such as graphene, BP and TMDs, have been demonstrated to be applicable to CMOS devices, including transistors, memories and inverters, benefiting from their high carrier mobility or tunable bandgap[
Starting from the structure and properties of bismuth, this review summarizes various physical vapor deposition of 2D bismuth, followed by its existing or potential CMOS applications. Besides, bismuth derivatives are mentioned as a strategy to enhance the performance of 2D bismuth. Finally, a brief summary on the current challenges and future prospects are discussed. Besides, it should be noted that in this review, “bismuthene” refers to bismuth from monolayer to 4 nm, “2D bismuth” indicates 4–30 nm (critical thickness for observing quantum confinement effect[
2. Structure and properties of bismuthene
As a cousin of phosphorene, the unique structure of bismuthene gives rise to extraordinary electronic properties. To utilize the properties in potential applications, it is necessary to understand the relationship between the atomic, band structure and electronic properties of 2D bismuth.
2.1. Atomic structure
Bismuth possesses a rhombohedral A7-type structure, namely β phase as shown in Fig. 1(a), space group R
2.2. Band structure and electronic properties
Bismuth has an electron configuration of 6s2 6p3[
In general, monolayer bismuthene possesses a moderate direct bandgap ranging from 0.43 to 0.99 eV depending on different computation methods[
Figure 2.(Color online) (a) Band structure of bismuthene is calculated without (up) and with (down) the SOC. Reproduced with permission from Ref. [
Strong SOC effect leads to a splitting of the states in heavy Bi atoms, and the large energy difference between the 6p1/2 and 6p3/2 is 1.5 eV after splitting[
3. Physical vapor deposition of 2D bismuth
Material synthesis or growth is the first step to perform experimental study on the aforementioned exotic properties of bismuthene. To date, a number of methods have been developed to fabricate 2D bismuth, including physical vapor deposition, liquid exfoliation[
3.1. Molecular beam epitaxy (MBE)
Under ultra-high vacuum (~10–8 Pa) and small mismatch, atoms arrange and form high-quality single crystal films with controllable thickness, the surface being very smooth with roughness lower than 1 nm, even picometer level[
The interaction between the substrate and 2D bismuth has a crucial effect on film growth, and substrate-dependent electronic properties and orientations on various substrates are extensively studied, e.g. SiC[
Figure 3.(Color online) (a) Sketch and (b) STM image of flat honeycomb bismuthene epitaxy on SiC (0001). Reproduced with permission from Ref. [
Furthermore, Meyer et al. reported a sudden and massive generation of misfit dislocations at a critical thickness of 4 nm in Bi (111)/Si (001) for strain relaxation in Fig. 3(g)[
Nevertheless, the MBE method has a high requirement on facility with high cost and a long experimentation period, preventing it from large-scale 2D bismuth preparation.
3.2. Pulsed laser deposition (PLD)
The PLD method uses a high energy laser to vaporize and dissociate the material under 10–5–10–6 Pa. High ion energy promotes the adatom mobility[
An unusual epitaxial growth occuring on Bi/Si (100) at the beginning of the PLD process was first reported in 1999[
Figure 4.(Color online) Morphology and X-ray-diffraction patterns (XRD) patterns of 2D bismuth deposited by PLD. (a) SEM micrographs of Bi films deposited by PLD at 185 °C on glass (left) and Si (100) (right). (b) Thickness-dependent XRD patterns of Bi films deposited at 20 °C on Si (100). Reproduced with permission from Ref. [
Substrate temperature and laser energy seriously affect the roughness, grain size and orientation of PLD 2D bismuth[
Moreover, the low ionic energy (~110 eV) deposited samples have a pure (111) orientation in Fig. 4(c), while the high ionic energy (~270 eV) presented a (110) preferential orientation without substrate heating as Rodil reported[
Compared with MBE, PLD has advantages of high deposition rates, low temperature, low cost and unlimited targets, whereas it has not demonstrated few-layer or even bilayer bismuthene yet.
3.3. Electron-beam (e-beam) evaporation
In addition to a high energy pulse laser, the electron beam can also be focused onto the surface of source material in an e-beam evaporation method. Only a small part of the source material is heated by the precisely positioned electron beam, which can minimize the evaporation of crucible materials or other possible contaminations. Precisely controlled temperature and rate allow a convenient control on thickness and properties of grown 2D bismuth.
Jankowski et al. reported the controllable growth of 4–20 nm 2D bismuth films with a deposition rate of ~0.02 Å/s on α-Al2O3(0001) insulating substrate by electron beam evaporation[
Figure 5.(Color online) (a) X-ray reflectivity (XRR) curves for a 14 nm Bi (110) film grown at 40 K, measured at 300, 400 and 450 K. At 400 K, the onset of orientation transition towards a Bi (111) film is seen where at 450 K the entire film is transformed. (b) XRR curves for increasing Bi film thickness, grown at RT. Reproduced with permission from Ref. [
For room temperature-deposited bismuth, Bi (110) domains are grown within the first 4 nm, followed by Bi (111) domains starting around 6 nm in Fig. 5(b)[
In contrast, Rodil et al. deposited ~180 nm Bi (111) thin films on not deliberately heated glass substrates at a high rate of ~18 Å/s by e-beam evaporation, different from low rate growth of Jankowski, and (110) orientation shows up as the thickness increases as illustrated in Fig. 5(c)[
Generally, e-beam evaporation provides a cost-effective scalable method to prepare 2D bismuth for device applications. It needs further improvements on surface roughness and precise control for few-layer bismuthene.
3.4. Thermal evaporation
In view of the low melting point of bismuth (271 °C), thermal evaporation is suitable for deposition of 2D bismuth[
Figure 6.(Color online) (a, b) Schematic diagram of the tube employed for the synthesis and atomic force microscopy image of 2D bismuth. Reproduced with permission from Ref. [
3.5. Magnetron sputtering
The magnetron sputtering method can achieve a high deposition rate under base pressure of 10–4–10–5 Pa, which is suitable for depositing bismuth thin films with thickness of hundreds to thousands of nanometers[
3.6. Flash vaporization
Flash vaporization sends fine powders bit by bit to the high temperature evaporation source where powders can realize complete evaporation in a short time. Therefore, it is easy to obtain films with the same composition as the source material.
Polycrystalline bismuth thin films with thickness of 40–160 nm was prepared via flash evaporation on preheated (453 K) glass substrates under a vacuum of 2 × 10−6 Torr for the first time[
Regarding the orientation of physical vapor deposited films, it can be summarized as follows: 1) for epitaxial 2D bismuth, the puckered-layered (110) phase is favored due to the higher cohesive energy for small thickness films. As thickness increasing, the situation is reversed, resulting in (111) dominated (Fig. 7). The critical thickness is 4–6 layers according to Nagao et al.[
Figure 7.(Color online) Schematic of 2D bismuth preferred orientation corresponding to different physical vapor deposition.
Above-mentioned physical vapor deposition methods can realize large area continuous 2D bismuth (methods and corresponding parameters are summarized in Table 1.), which are also easy to control the size and thickness, thus, providing platforms to obtain 2D bismuth compatible with CMOS technology. Still, more attention needs to be focused on growth mechanisms since there is still a challenge to balance between low cost and high quality. In short, the MBE method has the greatest advantages in quality and controllability, while the high cost limits the possibility of large-scale preparation for CMOS applications at present. Magnetron sputtering and thermal evaporation are common methods with low cost systems, but for the deposition of ultra-thin 2D bismuth for nanoelectronic devices, the shortcomings in crystallinity and roughness are obvious. In comparison, e-beam evaporation and PLD are not as expensive as MBE, and the products are relatively smooth and high-quality films with controllable thickness, thus holding great potential for the realizing of large area 2D bismuth for device integration in the future.
4. 2D bismuth devices and applications
2D bismuth has been explored in a variety of CMOS relevant applications, due to good air stability and outstanding electronic properties. Below is a survey on theoretical and experimental implementations about 2D bismuth devices including field-effect transistors, sensors, photodetectors, optical devices, thermoelectric devices, topological and spintronic applications, magnetic devices and memory devices.
4.1. Field-effect transistors
A field-effect transistor is an essential building block nowadays of CMOS devices. Modern electronic chips contain billions of FETs per square millimeter[
Figure 8.(Color online) Transport properties characterization of PLD grown Bi (111) films. Reproduced with permission from Ref. [
4.2. Chemical sensors
Because of the large specific surface area and its conductance changing with the extent of surface adsorption, 2D materials are widely utilized in sensors. Several calculation works have demonstrated that 2D bismuth is a promising candidate to detect some molecules. Bhuvaneswari et al. reported that a bismuthene nanosheet (BiNS) was calculated to show a response to the G series which is a kind of nerve agents, revealing the application in biosensors[
4.3. Photodetectors and optical devices
Bi is favored for ultra-broadband and high-responsive photodetectors due to its metallic surface state and small bulk gap. Yao et al. used PLD-grown Bi thin film to prepare a photodetector shown in Fig. 9(a)[
Figure 9.(Color online) (a) Three-dimensional schematic view of the Bi photodetector. (b) Time-dependent switching behavior of the photocurrent. Device area: 0.32 × 0.32 mm2. Power density: 300 mW/cm2. (c) Normalized responsivity as a function of illumination wavelength. Device size: 2 × 1.2 mm2. (d) Operating mechanism of the Bi photodetector. Reproduced with permission from Ref. [
Bismuthene was proved to be applied in nonlinear optical applications. Lu et al. characterized its nonlinear optical response at the visible band by Z-scan and cross-phase modulation (XPM) methods[
Figure 10.(Color online) (a−c) Formation process of all-optical switching based on XPM using 532 nm laser with intensity at 5.48 W/cm2. (d) Schematic experimental setup of XPM. Reproduced with permission from Ref. [
4.4. Thermoelectric devices
As a post-transition metal with a carrier mobility of up to ~ 20 000 cm2V–1s–1[
Fortunately, according to the quantum confinement effect of Hicks & Dresselhaus [
Recently, Yang et al.[
There are still great challenges and prospects for further research on the thermoelectric properties of 2D bismuth in the future, such as avoiding the oxidation during growth, controlling the surface roughness of thin films and unifying the instrument standard for the thermoelectric measurement of 2D materials.
4.5. Topological and spintronic device
Koroteev et al. found strong spin-orbit splitting on Bi surfaces[
Figure 11.(Color online) (a) The second derivative of the Landau-level (SDLL) pattern of the 4.0-nm-thick film as a function of B. Acquisition conditions: –150 mV and 4 nA, modulation of 1.0 mV by root mean square. Reproduced with permission from Ref. [
Quantum spin Hall (QSH) material has edge conductance channels, which can prevent some types of scattering, and is a hope for a revolutionary device without loss of spin current[
It is inspiring to apply 2D bismuth to spintronic and topological devices, which can reduce power consumption and increase the running speed of CMOS.
4.6. Magnetoresistance and memory applications
High-performance magnetic and current sensors are widely used in integrated circuits and magnetoresistance (MR) materials are promising in this field. The giant magnetoresistance (GMR) head is more sensitive than the MR head. In other words, the same magnetic field change can cause a greater resistance value change for GMR head. It can achieve a higher storage density, holding a great prospect in CMOS. The following work reveals the giant magnetoresistance property of 2D bismuth and its potential applications in resistive memories.
Yang et al. fabricated 1–20 μm thick single crystal bismuth thin film by electrodeposition and suitable annealing and observed the MR up to 250% at 300 K and 380 000% at 5 K[
Figure 12.(Color online) (a, b) Calculated temperature dependence of the magnetoresistance ratio for 29 nm and 193 nm bismuth thin films. Reproduced with permission from Ref. [
Xu et al. reported the reversible and nonvolatile tuning of electronic transport properties of the Bi-based heterostructures[
To sum up, 2D bismuth has been studied in many aspects of the CMOS field including high mobility FET, ultra-broadband and high-responsive photodetectors, optical, thermoelectric and topological devices. But there is a lack of experimental study on sensors and spintronic devices, urgently needing further research. Besides, the giant magnetoresistance of bismuth will become a fascinating application in the near future. It can be optimistically predicted that 2D bismuth will hold a bright prospect in CMOS.
5. 2D bismuth derivatives (Bi–X)
Even though 2D bismuth has many peculiar properties, there will still be limitations for an elemental 2D material to realize various applications. Many strategies can be performed to enrich the properties and applications of 2D bismuth including doping, defect, strain, alloying and so on. The alloying method can be viewed as doping new atoms into Bi crystal[
Bi and Sb share the same rhombohedral structure, so they could form a continuous solid solution, namely Bi1–xSbx. Bulk Bi1–xSbx is a small gap topological insulator (bandgap < 20 meV) when 0.07 < x< 0.22 whereas a semimetal whenx< 0.07 orx> 0.22[
Bismuth telluride (Bi2Te3) or bismuth selenide (Bi2Se3) have attracted immense interest due to their exceptional thermoelectric and optoelectronic properties. Bi2Te3 and Bi2Se3 also possess the rhombohedral crystal structure with the space group
In bulk form, Bi2Te3 and Bi2Se3 single crystal have more excellent thermoelectric performance compared with Bi. Here, we mainly focus on the figure of merit zT, which is a critical parameter to characterize the performance of thermoelectric materials. Researchers have founded that the zT values of bulk Bi2Se3and Bi2Te3 are 0.11[
Furthermore, Bi2Te3 and Bi2Se3 are expected to be important photoelectric materials for high-performance terahertz to infrared applications, due to the small bandgap, thickness and size-dependent light absorption, and tunable surface bandgap characteristics[
Bismuth derivatives indicate that composite element is an effective method to tune the properties for more versatile device performance.
6. Summary and perspectives
In this review, CMOS technology compatible physical vapor deposition methods of 2D bismuth are summarized especially focusing on epitaxy, evaporation and sputtering as well as preferred orientation of vapor deposited films. This sort of method provides a platform for synthesizing 2D bismuth with more flexible and controllable capability. Benefiting from fascinating electrical and thermal properties, 2D bismuth has a great potential in next-generation nanodevices according to experimental and theoretical investigations, which can be applied to CMOS such as transistors, sensors, optical modulators, spintronic and memory devices. In addition, an alloying strategy (Bi–X, X = Sb, Te, Se) is mentioned for further enhanced performance. Emerging 2D Xene, like bismuthene in this review, has drawn tremendous attention in CMOS related nanodevices. With atomic precision in layer control and scalability, physical vapor deposition techniques have advantages in realization of large-area, uniform and high-quality 2D bismuth for future CMOS compatible technology.
Despite the rapid development and promising achievements of 2D bismuth in recent years, there are remaining challenges for experimental study and practical applications. First, a low-cost and high-quality deposition method should be testified for industrial production of wafer-scale 2D bismuth. The recognized high-quality molecular beam epitaxy method, is costly and time consuming, making it limited to laboratory research. Traditional evaporation or sputtering methods need to be improved to yield smoother films with better quality for CMOS devices. Many potential applications are still in the stage of theoretical simulations. In-depth experimental research on material properties and device integration are necessary to narrow the gap between theoretical and experimental research on 2D bismuth. Moreover, advanced characterization techniques for 2D materials should be developed to obtain more precise results about the growth mechanism, properties and device performance. Nano scale device integration techniques also need improvement to accommodate the smaller size and higher integration density in the CMOS technique. Rational design and modification approaches such as alloying, doping, external magnetic field or strain, play an important role in determining electronic, chemical and physical properties of 2D bismuth, which can further enhance the performance and give birth to novel device applications.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51602051), Jiangsu Province Innovation Talent Program, Jiangsu Province Six-Category Talent Program (No. DZXX-011).
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