In order to break through the bottleneck of Moore's Law, researchers introduce two-dimensional materials as channel layers into field effect transistors. Two-dimensional semiconductor materials such as molybdenum sulfide have the characteristics of atomic thickness, inert surface without dangling bonds, high carrier concentration and mobility. Theoretically, this material can reduce the size of the field effect transistor without reducing the performance of the device. However, it is a challenging task to introduce two-dimensional materials into micro-nano electronic devices, because it is necessary not only to prove that the devices after introducing two-dimensional materials show higher performance advantages, but also to overcome the challenges of preparing two-dimensional materials and the negative effects of integrating two-dimensional materials into silicon-based devices, such as flicker noise, hysteresis, long-term drift caused by bias-temperature instability, low mobility and sub-threshold fluctuation.
Recently, Professor Mario Lanza's team published a summary entitled "Calcium fluoride as a high-k electronic product of 2D electronics" in the Review of Applied Physics, which was selected as an article recommended by the editor. Wen Chao, a master student at Soochow University, is the first author of the paper, and Mario Lanza, a professor at King Abdullah University of Science and Technology, is the correspondent. In this paper, the feasibility of calcium fluoride thin films as dielectric materials for two-dimensional micro-nano electronic devices is systematically studied from three aspects: material synthesis method, electrical properties and current application, and possible solutions to the challenges faced by calcium fluoride thin films in the future research are put forward.
In this paper, the synthesis status of calcium fluoride dielectric thin films is reviewed, and the quality of calcium fluoride thin films synthesized by various methods (atomic layer deposition, chemical vapor deposition, thermal evaporation and molecular beam epitaxy) is compared. It is considered that molecular beam epitaxy is the best method to grow calcium fluoride thin films at this stage. The calcium fluoride film grown on silicon (11) substrate by this method has high crystallinity, no defect clusters, smooth surface and low defect density with the substrate. At the same time, the crystal plane (1 1 1) of the calcium fluoride thin film has no dangling bonds and is chemically inert. The good interface between the surface of calcium fluoride (11) grown by molecular beam epitaxy and two-dimensional materials can be measured in situ by reflected high energy electron diffraction. However, the molecular beam epitaxy system is expensive and the operation is relatively complicated, and the growth rate of calcium fluoride thin films is low (1.3 nm/min). The quality of calcium fluoride thin films grown by common industrial synthesis methods (chemical vapor deposition, atomic layer deposition and thermal evaporation) is not as good as that of molecular beam epitaxy in crystallinity and hanging bond density, but the lower cost of these methods may further synthesize and optimize calcium fluoride dielectric thin films in industry. In addition, different electronic devices need different defect concentrations, and the calcium fluoride thin films synthesized by these methods can be used for other types of micro-nano electronic devices.
Figure 1. (a) Morphology of calcium fluoride thin films grown by molecular beam epitaxy at high temperature; (b) Morphology of calcium fluoride films grown by molecular beam epitaxy at low temperature (250℃); (c) growing a calcium fluoride thin film on a copper substrate by molecular beam epitaxy at 300K; (d) a calcium fluoride thin film grown by chemical vapor deposition; (e) growing a calcium fluoride film by thermal evaporation at high temperature.
Secondly, the electrical properties of calcium fluoride thin films are discussed from theoretical calculation and experimental measurement, including band gap, dielectric constant and dielectric strength. Firstly, the author summarizes the numerical values of various parameters in the literature, discusses the reliability of the measurement methods for obtaining these parameters, and compares the electrical parameters of calcium fluoride with those of other dielectrics used as micro-nano electronic devices. By comparison, it is concluded that calcium fluoride is an ideal dielectric material with high band gap, high dielectric constant and high dielectric strength, which can effectively prevent leakage current.
Figure 2. (a) A schematic diagram of measuring the nano-scale electrical properties of calcium fluoride thin films by using a conductive atomic force microscope. (b-d) Voltammetric characteristic curves of four dielectric films measured at different positions.
Then, the author discusses the application of calcium fluoride thin films in micro-nano electronic devices such as crystal field effect transistors, organic thin film transistors, optical sensors and diodes. Because of its excellent electrical properties and expansibility of synthesis methods, calcium fluoride thin films show excellent integration potential in different solid-state electronic devices. At the same time, the high-quality van der Waals-like interface formed between the crystal faces of calcium fluoride films (1 1) grown on silicon (1) substrates by molecular beam epitaxy can reduce electron scattering, overcome negative effects and slow down the degradation process of devices, thus improving the performance of two-dimensional micro-nano electronic devices.
Figure 3. (a) schematic diagram of mos 2/caf 2 (11)/n type silicon field effect transistor. Characteristic curve of field effect transistor. Aluminum/calcium fluoride/diamond field effect tube. Voltammetric characteristic curve of (e-f)d pattern field effect transistor.
Finally, the author summarizes some challenges in the integration of two-dimensional micro-nano electronic devices and calcium fluoride thin films, and puts forward potential solutions:
Paper link:
https://aip.scitation.org/doi/ 10. 1063/5.0036987