Halide method (Ga/AsCl3/H2 system)

Hydride method (Ga/HCl/AsH3/H2 system)

Metal organic epitaxy

Cooling method: 1) First contact the Ga cell with the GaAs solid source to make it saturated. 2) Place the Ga cell in contact with the substrate and cool down at a certain speed until the solution is supersaturated. 3) GaAs will precipitate on the substrate to reach the required thickness. 4) Separate the Ga pool from the substrate to stop growth. This method is suitable for growing thin single crystal layers.

Thermometry 1) The solution is in contact with the GaAs source chip above until equilibrium is reached. 2) Reduce the temperature of the lower part of the furnace and establish a certain temperature gradient (5~7℃/cm) in the Ga pool. 3) Push the boat to bring the Ga liquid into contact with the substrate below. 4) Since the upper and lower temperatures of the Ga liquid are different, they have different solubilities to GaAs. Therefore, the GaAs source chip dissolves at high temperatures, while GaAs will grow on the GaAs substrate at low temperatures. The epitaxial growth rate is determined by the temperature gradient in the Ga liquid. At the end of epitaxy, the Ga cell is pushed away from the substrate. This method can avoid the uneven distribution of impurities caused by cooling during the growth process, improve the longitudinal uniformity of impurities and crystal integrity, and the GaAs precipitation amount is not limited by the cooling range, making it suitable for growing thick epitaxial layers.

System self-doping: There are many factors that affect impurities in the epitaxial layer, such as the purity of the source and equipment, the sealing of the system, the contact temperature, the process conditions used, etc.

Measures to reduce self-doping: The equipment for liquid phase epitaxy must be made of high-purity graphite and treated with high vacuum and high temperature before epitaxy to remove Cr, Mn, Fe, Ni, Cu, zn, Se and other impurities. High-temperature baking is also helpful in removing oxygen that can create deep energy levels. In addition, the system must be strict, and the grinding joints of the epitaxial system should be protected with pure N2. These will help reduce oxygen contamination. Residual impurities also have an impact on the purity of the growth layer.

Dopants: n-type includes tellurium Te, tin Sn, and selenium Se; p-type uses Zn, Ge. n type: Sn has high solubility in GaAs, low vapor pressure, and small segregation coefficient (K≈10-4), and can grow a uniform n-type GaAs epitaxial layer with a wide doping concentration. The doping concentration can reach 8×1018/cm3 and is not affected by the growth temperature and substrate crystal orientation. It is the most commonly used N-type dopant; Te and Se have high vapor pressure and large segregation coefficient (K≈1 ,5), it is difficult to do uniform doping. Te and Se doping are generally not used. p type The vapor pressure, segregation coefficient and diffusion coefficient of Ge are lower than those of Zn, so it is the most commonly used dopant. Zn has a large solubility and segregation coefficient in GaAs. In order to obtain a steep impurity distribution and form a good electrode contact material , is also commonly used as a dopant. Semi-insulating GaAs can be obtained by doping Cr with a resistivity as high as 107Ω·cm.

It has been widely used to grow single crystal layers of III-V, II-VI, IV-VI and other compounds and their multicomponent compounds to produce various devices with complex structures and excellent performance.

Vacuum system

growth system

surveillance system

step:

Evaporation of source: The molecular beam used by MBE is obtained by installing a solid-state source in an emission furnace and evaporating it by heating. This is relatively simple for elements, but more complicated for compound semiconductors, such as a binary compound MX (M is a metal, X is a non-metal ), when the evaporation source is in thermal equilibrium, the beam current of volatile components is much larger than that of less volatile components, so it is more appropriate to use compounds as the source of volatile components. For example, using GaAs as the As source can provide a suitable Molecular beam, and Ga and doping elements generally use themselves as sources.

Adhesion coefficient: the ratio of the number of molecules grown on the substrate to the number of incident molecules Different types of molecules interact differently with the substrate surface. For example, Group III (Ga) atoms chemically adsorb with the GaAs substrate surface. Therefore, at a general growth temperature, their adhesion coefficient is 1. Group V (As) atoms (atoms) are first physically adsorbed, and after a series of physical and chemical processes, part of them are converted to chemical adsorption. Therefore, its adhesion coefficient is closely related to the molecular (atomic) state and temperature of the substrate surface. Related.

1. General growth process The polished GaAs substrate is loaded into the substrate pick-and-place chamber after routine cleaning. Vacuum is applied to prevent air from entering the growth chamber. The substrate pick-and-place chamber, storage and transfer chamber, and growth chamber are all under high vacuum conditions, and the substrate is sent into the growth chamber step by step. Heat exhaust treatment for all sources. After the vacuum reaches the required level, the substrate is processed. Because the substrate surface after routine cleaning was analyzed with an Auger spectrometer, oxygen and carbon contamination was found. Oxygen is easily removed by heating under high vacuum, but carbon removal is more difficult. Therefore, Ar sputtering treatment is used to remove carbon and other contamination before epitaxial growth. However, care should be taken to prevent new contamination caused by Ar sputtering, and heat treatment must be performed after sputtering to eliminate damage caused by sputtering.

If Ga and As are used as sources, GaAs is grown under the conditions of Ga:As beam current ratio of 1:10 and growth rate of 0.1~0.2nm/s, then the Ga furnace temperature is about 950°C, and the As furnace temperature is about 300°C, the Ga furnace temperature must be accurately controlled, and the substrate temperature is generally 500°C. Using Ga and As as sources, the beam current can be controlled independently, and the As source can be used for a long time. Although GaAs is used as the As2 source, although it is easier to control the beam than element As, its disadvantage is that As is quickly depleted. During the epitaxial growth process, the monitoring equipment in the system must work to obtain various information about the growth to ensure normal growth under microcomputer control and according to the program.

1. Self-doping grows an undoped GaAs epitaxial layer on a semi-insulating substrate. The background impurity concentration depends on the cleanliness of the epitaxial system, the residual impurities in the growth chamber and the purity of the source. The general impurity concentration is l×1015 /cm3, and is often high resistance.

2. Doping n-type dopants include Si, Ge, and Sn. Their adhesion coefficients are close to 1. Sn is often used, but the disadvantage is that there is a certain degree of segregation on the surface. However, Sn is easier to obtain a higher mobility than Ge, and Sn is also easier to process than Si. However, Si and Ge have strong amphoteric properties and can be both donors and acceptors. In addition to Sn, Si is also commonly used as a donor dopant.

The basic feature is to alternately supply two source gases so that the reactant forms a chemically adsorbed monolayer on the substrate surface, and then the other reactant source is also covered in a single layer through a chemical reaction. So alternately. When the surface coverage layer at each step is exactly one layer, the growth thickness is equal to the thickness of the single layer multiplied by the number of cycles. The growth thickness obtained by one cycle in the GaAs(100) direction is 0.283 nm. After years of research, ALE's experimental devices include horizontal and vertical devices, substrate rotation, air flow interruption, and some have illumination or laser induction devices.

advantage It can be known from the growth principle that the ALE method reacts through surface adsorption between the reactants and the substrate. When the reactant and the substrate react and adsorb for a sufficiently long time, no crystal growth will occur even if the reactant is supplied again. Therefore, the parameter that determines the growth thickness is the number of ALE cycles, so ALE is also called "digital epitaxy." Digital epitaxy has high repeatability, and the growth thickness can be accurately known from the number of cycles. For digital epitaxy, since it is the layer-by-layer growth of chemically adsorbed single-layer reactants, this mode has little to do with air flow distribution, temperature uniformity, etc., and does not require special attention to the boundary layer thickness, temperature distribution near the substrate, etc. parameter. As long as a layer of reactants is completely adsorbed on the substrate surface, a high degree of thickness uniformity will inevitably be achieved, and a high-quality mirror surface will be obtained, eliminating the inhomogeneity of the growth surface caused by different potential energies of the surface.

The main disadvantages are slow growth rate and long cycle time. Many people use many methods to reduce cycle times and increase growth speed, such as illumination, but compared with current common epitaxy methods, especially compared with MOCVD, the growth speed is very slow.

Basic principles and characteristics Electrochemical atomic layer epitaxy (ECALE) is a combination of electrochemical deposition and atomic layer epitaxy technology. Atomic layers of compound component elements are deposited cyclically and alternately under under-potential conditions, thereby directly generating compounds. This method was first proposed by Stickney, and its basic process is as follows:

The surface-limited reaction of ECALE is underpotential deposition (UPD). Underpotential deposition is the phenomenon whereby an element can be deposited on another substance at a potential that is more positive than its thermodynamically reversible potential. This is because the deposited atoms interact with the substrate in such a way that the deposition potential of the layer of atoms in direct contact with the substrate appears before the potential required for deposition on its own element. Theoretically, under UPD potential, a layer of ② atoms is deposited on the substrate on which a layer of ① atoms is deposited, as shown in Figure 2 (left).

Basic Principle The principle of atomic layer epitaxial growth is to use surface-limited reactions to form atomic layers of deposits one after another. The thin film formed by the alternate deposition of atomic layers one after another will become epitaxial two-dimensional growth. If the surface-confined reaction is extended to the deposition of single atomic layers of different elements in the compound, and the successive alternating deposition of single atomic layers of each element constitutes a cycle, then the result of each cycle is to generate a single layer of the compound, and the thickness of the deposited layer Determined by the number of cycles.

By combining underpotential deposition and atomic layer epitaxy technology, the advantages of both are combined, as follows: (1) Inorganic aqueous solutions are generally used to avoid carbon pollution; (2) The investment in process equipment is small, which reduces the cost of preparation technology; (3) Can be deposited on substrates with a set area or complex shape; (4) There is no need to use toxic gas sources and no pollution to the environment; (5) It grows layer by layer, avoiding the occurrence of three-dimensional deposition and achieving control at the atomic level. (6) It splits the deposition of compounds into consecutive steps of atomic deposition of each component element, and each step can be controlled independently, so the quality, repeatability, uniformity, thickness and stoichiometry of the film can be accurately controlled, while More mechanistic information can be obtained. (7) Room temperature deposition to minimize interdiffusion. At the same time, the internal stress caused by the difference in thermal expansion coefficient is avoided, ensuring the quality of the film. (8) The formula of each solution can be optimized separately, so that the selection of supporting electrolytes, pH values, additives or complexing agents can best meet the needs. (9) There is a wide range of reactants to choose from, and there are no special requirements for the reactants. As long as they are soluble substances containing the element, superlattices can generally be successfully prepared at lower concentrations.

Due to its unique advantages in the preparation of thin film materials, it has attracted the attention of many foreign material preparation experts. However, there is still very little research in this area in China. At present, there have been many reports on using the ECALE method to prepare semiconductor thin films and superlattices, mainly focusing on groups II-VI, III-V, and V-VI.

Most of them are modified from the source furnace of solid-state source MBE equipment. The control system of the MOVPE system is used in the pipeline where the gas source is input into the growth chamber. The specific structure varies depending on the researcher or manufacturer.

Although they are all collectively called CBE or gaseous source MBE, their growth mechanisms are different due to the combination of several sources (as listed in Table 7.4 of the textbook). Among them (1), solid metal is used as the group source, and group V hydride is thermally decomposed in advance into group V gaseous sources such as AS2 or P2, which are passed into the growth chamber and grown on the substrate. The growth mechanism of the solid-state source MBE is almost unknown What a difference. However, in the cases of (2), (3), and (4), it is different. Their Group III components are based on organic compounds of Group III elements. Before entering the growth chamber, the metal organic compounds are not heated. Instead, the molecular beam of the compound is directly shot onto the heated substrate surface for epitaxial growth.