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This process involves the deposition of atoms or molecules of the material layer by layer on the surface of the substrate to form a thin film with specific properties and structure, so its growth process directly affects the structure of the film as well as its final properties.

The epitaxial growth kinetics of thin films describes the evolution of various dynamic changes in the growth process of thin films, involving multiple key links such as surface diffusion, adsorption, desorption, and aggregation. The interaction between these links affects the structure, morphology, and properties of the film.

When atoms or molecules are shot at the substrate, they collide with the substrate surface, causing one part to be reflected and the other part to remain on the surface.

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Atoms and molecules that stay on the surface are affected by their own energy and the temperature of the substrate, and surface diffusion and migration occur. Some are detached from the surface, while others are partially adsorbed by the surface at high temperatures to form condensates. The entire condensation process includes steps such as nucleus formation, island formation, merging, and growth, culminating in the formation of a continuous thin film.

High-quality epitaxial films are the basis for making good devices, and in order to realize the fabrication of high-performance devices, it is necessary to comprehensively consider the properties of the materials, application requirements, growth conditions and other factors when selecting growth technologies to achieve precise control and high-quality growth of the films.

Here are a few common thin film epitaxy techniques:

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Magnetron sputtering technology

Magnetron sputtering is a physical deposition method. This type of equipment has a relatively simple structure, is easy to control the growth of thin films by adjusting parameters, and is suitable for the preparation of slightly larger film materials, and this technology is widely used in industry and laboratories.

The schematic diagram is shown below, mainly through the acceleration of electrons under the action of an electric field, hitting the Ar atom and ionizing the Ar atom into Ar+ and electrons.

When the high-speed argon ions hit the target, the target atoms gain enough momentum to break away from the target and fall on the substrate to form a dense film. Magnetron sputtering technology is divided into DC sputtering and radio frequency sputtering. Generally speaking, when the target is a material with poor conductivity, such as semiconductors and ceramics, the current source connected to the target is a radio frequency power supply; When the target is Au, Ti and other metal materials, the connected power supply is a DC source.

Chemical vapor deposition of organometallic compounds

MOCVD is a chemoepitaxial growth method. Since the 60s of the 20th century, this technology was proposed by Manasevit and others of Rockwell Company in the United States, and has now become the mainstream technology for mass preparation of semiconductor thin films. By transporting the reactants into the chamber through a carrier gas and undergoing a chemical reaction under suitable conditions, the preparation of Ga2O3 films is taken as an example:

The metal-organic source is triethylgallium (TEGa), oxygen is used as the reaction gas, and the inert gas argon is used as the carrier gas, and the metal-organic reaction source required for the experiment is transmitted to the reaction chamber in the form of gas through the carrier gas, and mixed with the oxygen in the reaction chamber, and finally the thermal decomposition reaction occurs on the high-temperature substrate to form a high-quality epitaxial film after precise control of the proportion of gas.

The reaction flow chart of MOCVD is as follows:

MOCVD technology has the following characteristics:

A wide variety of materials can be prepared: it can be used to prepare almost all compound semiconductor materials, such as silicides, nitrides, oxides, etc. Therefore, this technology has become a very important thin film preparation technology in the semiconductor industry.

2.The growth rate is continuously adjustable over a wide range, and it is suitable for the growth of ultra-thin layers of compound films. By adjusting and controlling the flow rate of the reactant gas stream, parameters such as the growth rate of the film and the doping concentration can be easily adjusted during the use of this technology. In addition, because the reaction gas in the reaction chamber can be switched at any time, this technology can make the material form an obvious interface during heteroepitaxial growth, which is conducive to the preparation of complex heterostructures.

3.The film prepared by it has good purity and uniformity, high repeatability, and a high degree of automation of the equipment, which makes it possible to mass produce a large area and is suitable for industrial production.

4.In-situ monitoring further ensures the quality and performance of the film during the growth process. With its unique advantages and characteristics, MOCVD technology occupies an important position in the field of semiconductor thin film preparation, and provides strong support for scientific research and industrial applications.

Laser molecular beam epitaxy system

Laser molecular beam epitaxy (LMBE) began to develop in the 90s of the last century, is a new high-precision film making technology, LMBE not only inherits the advantages of high efficiency, flexibility and suitable for a variety of materials in PLD preparation, but also realizes the precise regulation of the film growth process by introducing in-situ real-time monitoring technology in the growth process.

This real-time monitoring technology enables researchers to observe the growth status of the film in real time and adjust the growth parameters in time to ensure that the quality and performance of the film are at their best.

According to the characteristics of LMBE, this technology can be used to grow semiconductor superlattice materials, and is also suitable for the growth of multi-element, high-melting, and complex layered thin films, such as superconductors, optical crystals, ferroelectrics, piezoelectrics, ferromagnets, and organic polymers.

In addition, this method can also carry out basic research on the corresponding laser-matter interaction and the physics and chemistry of the film-forming process. The basic principle of LMBE is to use a high-energy laser to hit the target, so that the atoms on the target will fall off, reach the substrate, nucleate on the surface of the substrate and continue to aggregate, and gradually expand into a complete film.

The schematic diagram of the laser molecular beam epitaxy system is shown in the figure below.

This epitaxial method has the following characteristics：

1. High resolution of thin film structure: the growth rate is slow, generally about one atomic layer per second, so the film epitaxial by this growth method has uniform quality and excellent crystallinity, which is very suitable for the growth of superlattice and other thin films that need to be accurately controlled.

2. The growth process is carried out under ultra-high vacuum conditions, which can achieve high-purity epitaxial growth.

3. The growth process and growth rate can be strictly controlled, and can be monitored by RHEED, so real-time monitoring can be achieved to achieve accurate control of the film growth thickness.

4. Thin film characterization techniques usually use XRD, SEM, TEM, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible absorption spectroscopy to determine the crystal type, crystal quality, band gap, morphology characteristics, chemical composition and defects, as well as the formation and band structure of heterojunctions.

(1) X-ray diffractometer

XRD is a means to study the crystal structure and analyze the composition of materials. The main working principle is to use a beam of X-rays to irradiate the surface of the crystal structure to be measured, because the X-ray and the surface spacing in the crystal are similar, so the interference phenomenon will occur and produce strong diffraction fringes. The diffraction relation satisfies the Bragg diffraction formula:

This test method is widely used in condensed matter physics, materials science, mineralogy and other fields because it is convenient and fast and does not cause any damage to the material.

 

(2) Atomic force microscopy

AFM can analyze the structure and roughness of solid material surfaces. The working principle of AFM is mainly to apply the probe to fully contact the atoms on the surface of the sample to be measured, and to image the atomic force changes between the probe and the surface atoms by analyzing the nanometer resolution.

(3)Scanning electron microscopy

The application of SEM in semiconductors is mainly to observe the surface growth of samples, and cross-section SEM can observe the growth state and thickness analysis of multilayer samples. The basic principle is to use a beam of electrons to generate an enlarged image of the sample, scan the sample with a focused beam of electrons, and then probe the secondary electrons/backscattered electrons generated on the surface of the sample for imaging.

(4)Transmission electron microscopy

TEM is primarily used for high-magnification imaging of samples. The basic principle is that the electrons emitted by the electron gun are accelerated at high pressure, which is about 100-400 Kv, and then focused on the sample by a condenser lens. The sample must be thin enough for electrons to pass through. The transmitted electrons form a diffraction pattern in the back focal plane and a magnified microscope in the image plane.

With other lenses, microscopic images and diffraction patterns can be projected onto phosphor screens for observation or electrophotographic documentation. The diffraction pattern obtained by this method can give structural information about the sample. In a scanning transmission electron microscope (STEM), a beam with a diameter of about 0.1 nm is used to scan the test sample, and the objective lens detects the transported electrons at all points scanned by the beam and corresponds to a fixed area on the back focal plane.

Primary electrons in STEM also generate secondary electrons, backscattered electrons, X-rays, and light above the sample, just like in SEM. The inelastic scattering of electrons below the sample can be used to analyze the electron energy loss. This makes the device a true analytical electron microscope, and high-resolution TEM (HTEM) can give structural information of the order of atoms, also known as lattice imaging. This is an important means of interface analysis, especially in the development of semiconductor integrated circuits.

(5)X-ray photoelectron spectroscopy

XPS is a powerful surface analysis technique that can be used to study the surface chemistry of solid materials. When X-rays irradiate the surface of the material, the escaping photoelectrons are then captured by special detection equipment in the XPS system. By measuring the energy and quantity of these photoelectrons, a wealth of information can be obtained about the surface elements of the material. For example, different elements have different electron binding energies, so by analyzing the energy distribution of photoelectrons, it is possible to determine the type of element on the surface of the material. The obtained data results can be used as the abscissa with the electron binding energy as the abscissa and the relative intensity as the ordinate to plot the photoelectron spectrum of the material for the analysis of the sample element information.

(6)UV-Vis absorption spectroscopy

The molecule of a substance has the ability to absorb electromagnetic waves from the ultraviolet to the visible region (generally 190-800nm), resulting in the transition of its valence electrons from the ground state to the excited state, that is, the ultraviolet-visible absorption spectrum can be obtained. By analyzing the data from the UV-Vis spectrum, the main absorption bands of the material can be obtained. Combined with the Tauc formula, the band gap width of the material is inferred.