Progress in Research on Growth of Highly Crystalline Black Phosphorus Films on Silicon Substrates

Recently, Zhang Kai, a researcher at the Suzhou Institute of Nanotechnology and Nanobionics, Chinese Academy of Sciences, cooperated with Professor Pan Anlian of Hunan University and Professor Zhang Han of Shenzhen University to publish the title "Epitaxial nucleation and lateral growth of high-" in Nature Communications. A research paper on crystalline black phosphorus films on silicon reported a method for growing highly crystalline black phosphorus films on dielectric substrates such as silicon.

Black phosphorus is a two-dimensional layered semiconductor material with high carrier mobility, 0.3 ~ 1.5 eV adjustable band gap with thickness, anisotropy and other excellent properties. In the field of new electronic and optoelectronic devices, such as high Mobility field effect transistors, room temperature wide band infrared detectors, and multi-spectral high-resolution imaging have unique application advantages and have attracted widespread attention. However, the large-scale application development of black phosphorus has so far been limited to the preparation of large-area, high-quality thin films. Traditionally, black phosphorus can be prepared by methods such as high temperature and pressure, mercury catalysis, or recrystallization from bismuth solutions. The mineralizer-assisted gas phase transport (CVT) can further increase its yield and crystallinity. However, these methods can only obtain black phosphorus crystal bulk materials, and it is difficult to grow a black phosphorus thin film directly on the substrate. Recently, some researchers have attempted to directly grow black phosphorous films on dielectric substrates by pulsed laser deposition or drawing on high temperature and high pressure methods. However, most of the obtained films are amorphous, and the electrical properties such as small grain size and mobility are not ideal, which is far from the actual application requirements. Although many studies have made tremendous efforts, including Zhang Kai's team's previous continuous work attempts on black phosphorus growth, doping and compounding (Small 2016, 12, 5000; Adv. Funct. Mater. 27, 1702211, 2017; Nature Commun. 9, 4573, 2018), but how to achieve black phosphorus nucleation on the substrate and the controlled growth of highly crystalline films is still a challenge.

In this work, the author developed a new growth strategy, introducing the buffer layer Au3SnP7 as a nucleation point to induce the nucleation growth of black phosphorus on the dielectric substrate. In the CVT method reported in the past, Au3SnP7 is one of the important intermediate products when black phosphorus crystals are grown with Au or AuSn as a precursor. The author considered using Au3SnP7 to induce black phosphorus nucleation, mainly to note two points: one is that Au3SnP7 can exist very stably during the growth of black phosphorus; the second is the arrangement of phosphorus atoms on its (010) plane and black phosphorus (100 ) The face has a matching atomic structure. Based on this, the author controlled the nucleation and growth of black phosphorus by generating Au3SnP7 on the substrate. Among them, the formation of Au3SnP7 is obtained by heating the silicon substrate deposited with the Au thin film together with the red phosphorus, Sn, SnI4 precursors in a vacuum sealed tube, and its morphology is usually a regular-shaped crystal dispersed on the silicon substrate. One hundred nanometers. In the subsequent heat preservation process, the transition of P4 phase to black phosphorus phase occurred and epitaxial nucleation on the Au3SnP7 buffer layer. This assumption can be confirmed from the high-resolution cross-sectional TEM image, and it can be clearly seen that the orderly coexistence of black phosphorus and Au3SnP7 and the atomic smooth interface between them. Subsequently, during the continuous supply of phosphorus and cooling, the transitional black phosphorus nanosheet product and its growth and fusion on the silicon substrate will be observed, and a continuous black phosphorus film with a flat and clean surface will be finally obtained.

During the growth process, the excessive P4 vapor transport is not conducive to the control of the black phosphorus film morphology and thickness. In order to achieve controllable black phosphorus film growth, the authors designed several methods to reduce the P4 source involved in the phase transition transformation. First, the red phosphorus is placed on the low temperature side, and the growth of the black phosphorus film is placed on the high temperature side at the far end. As a result, the sublimated P4 molecules need to undergo thermodynamic transport against the temperature gradient to the end of the growing substrate, and the transport speed and the amount of participation in the reaction can be effectively controlled. In addition, a number of Au-coated silicon substrates are stacked, and the very narrow gap between the substrates is used to limit the amount of P4 molecules that diffuse into the substrate and actually participate in the growth on the Au3SnP7 buffer layer. Through these strategies, black phosphor films with adjustable thickness from a few nanometers to hundreds of nanometers can be grown on silicon substrates. As the thickness increases, the available film size increases accordingly. When the thickness is about 100 nm or more, it is easy to grow a black phosphorous film with a size of several hundred micrometers to sub-millimeters.

The grown black phosphorus film has good crystallinity and excellent electrical properties. The field-effect mobility and Hall mobility at room temperature exceed 1200 cm2V-1s-1 and 1400 cm2V-1s-1, respectively, and the switching ratio is as high as 106. It is equivalent to the nanosheets mechanically peeled from the black phosphorus crystals. In addition, what is more interesting is that the grown black phosphorus film also shows a unique layered microstructure, which is composed of a few nanometers thick (~ 5-10 nm) black phosphorus layer as an ordered stack of cells, and the cells are kept roughly equal. Measure nanometer-level tiny gaps. Such a specific microstructure makes the grown black phosphorous film also exhibit excellent optical properties compared to the conventional densely stacked black phosphorous film, and has enhanced infrared absorption and photoluminescence in the infrared band.

This work provides a new way for the controllable preparation of large-area, high-quality black phosphorus films, and further promotes the widespread application of black phosphorus in high-throughput device integration and the development of new optoelectronic devices. Related research results were published in the journal Nature-Communication (Nature Communications, DOI: 10.1038 / s41467-020-14902-z). This work was supported by the National Excellent Youth Science Foundation (61922082) and other funds, and the nano-vacuum interconnection experiment station (Nano-X) of Suzhou Institute of Nanotechnology has greatly assisted in characterization testing.

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