What is Metal Organic Compound Chemical Vapor Deposition (MOCVD)?
MOCVD is a new vapor phase epitaxy growth technology developed on the basis of vapor phase epitaxy (VPE). It uses organic compounds of III and II elements and hydrides of V and VI elements as crystal growth source materials, and conducts vapor phase epitaxy on the substrate by thermal decomposition reaction to grow various III-V main groups, II-VI subgroup compound semiconductors and thin-layer single-crystal materials of their multicomponent solid solutions.
Abbreviation: Metal-organic Chemical Vapor Deposition （MOCVD）
Principle of MOCVD
MOCVD is a technology which use organic compounds of group III and group II elements and hydrides of group V and group VI elements as crystal growth source materials to grow various III-V, II-VI compound semiconductors and their thin-layer single-crystal materials with multiple solid solutions with vapor phase epitaxy(VPE) on substrate by thermal decomposition reaction.
Usually crystal growth in MOCVD systems is carried out in a cold-walled quartz (stainless steel) reaction chamber under normal pressure or low pressure (10-100 Torr) and passing through H2. The substrate temperature is 500-1200 °C, and after heating the graphite susceptor (the substrate substrate is above the graphite susceptor) with DC, H2 is bubbled through a temperature-controlled liquid source to carry the metal organics to the growth zone.
Because the source used for MOCVD growth is flammable, explosive, and highly toxic substances, and multi-component, large-area, thin-layer and ultra-thin-layer heterogeneous materials are to be grown. Therefore, in the design idea of MOCVD system, the tightness of the system should be considered, the flow and temperature control should be precise, the composition should be changed quickly, and the system should be compact. MOCVD equipment produced or assembled by different manufacturers and researchers is different. It is generally composed of source supply system, gas transportation and flow control system, reaction chamber and temperature control system, exhaust gas treatment and safety protection alarm system, automatic operation and electronic control system.
【MO source supply system】
Including the supply of group III metal organic compounds, group V hydrides and doping sources. The metal organic compounds are packed in a special stainless steel bubbler, and are carried to the reaction chamber by the incoming high-purity H2. In order to ensure that the metal organic compounds have a constant vapor pressure, the source bottle is placed in an electronic thermostat, and the temperature control accuracy can reach below 0.2 °C. The hydride is generally diluted with high-purity H2 to a concentration of 5% to 10%, and then loaded into a steel cylinder. When used, it is diluted with high-purity H2 to the desired concentration and transported to the reaction chamber.There are two types of doping sources, one is metal-organic compound and the other is hydride, and the transport methods are the same as those of metal-organic compound source and hydride source.
【Gas transportation system】
The gas transport tubes are all stainless steel pipes. To prevent storage effects, the inside of the tube is electropolished. The joints of the pipelines are connected by hydrogen arc welding or VCR and Swagelok, and positive pressure leak detection and Snoop liquid or He leak detection are performed to ensure that the reaction system has no leakage, which is one of the keys to MOCVD equipment assembly. Flow control is realized by mass flowmeters, solenoid valves, pneumatic valves, etc. with different ranges, fast response time and high precision. A filter is provided between the vacuum system and the reaction chamber to prevent oil or other particles from being sucked back into the reaction chamber. In order to rapidly change the reaction gas in the reaction chamber without causing a change in the pressure in the reaction chamber, set up "run" and "vent" pipes.
【Reaction chamber and heating system】
The reaction chamber is composed of a quartz tube and a graphite base. In order to generate compound semiconductor materials with uniform composition, ultrathin layers, and heterostructures, the researchers designed reaction chambers with different structures. The graphite base is made of high-purity graphite and wraps the SIC layer. Most of the heating uses high-frequency induction heating, and a few are radiant heating. The temperature is controlled by a thermocouple and a temperature controller, and the general temperature control accuracy can reach 0.2°C or lower.
【Exhaust gas treatment system】
Most of the reaction gas is thermally decomposed after passing through the reaction chamber, but some are not completely decomposed, so the tail gas cannot be directly discharged into the atmosphere and must be treated first. The treatment method is mainly to thermally decompose again with a high-temperature pyrolysis furnace, and then treat it with silicone oil or potassium permanganate solution; it is also possible to directly pass the exhaust gas into a suction filter bottle filled with H2SO4+H2O and a NaOH solution for treatment. In addition, passing the tail gas into a solid adsorbent for adsorption treatment, and rinsing the tail gas with water, etc., is also a method of tail gas treatment.
【Security protection and alarm system】
For safety, general MOCVD systems are also equipped with high-purity slave bypass systems, in case of failure of normal operation due to power failure or other reasons, pure N2 is introduced to protect the growing wafer or cleaning in the system. There is also a normally high-purity N2 protection system during growth arrest.
【Manual and automatic control system】
General MOCVD equipment has both manual and microcomputer automatic control operations. On the control system panel, there are valve switches, setting of gas flow in each pipeline, temperature and digital display. If there is any problem, it will automatically alarm, so that the operator can keep abreast of the operation of the equipment. In addition, MOCVD equipment is generally installed in a work room with strong exhaust air.
Advantages of MOCVD
1. Wide range of applications, almost all compounds and alloy semiconductors can be grown;
2. It is very suitable for growing various heterostructure materials;
3. Ultra-thin epitaxial layers can be grown, and a very steep interface transition can be obtained;
4. The growth is easy to control;
5. High purity material can be obtained by growth;
6. The large-area uniformity of the epitaxial layer is good;
7. Mass production is possible.
Development of MOCVD Systems
As the market for compound semiconductor devices such as GaAs MMICs, InP MMICs, and GaN blue LEDs continues to expand, the demand for MOCVD systems continues to grow. The world's leading MOCVD system manufacturers include: Germany's Aixtron, the US's Emcore (whose MOCVD has been merged by Veeco), and the UK's Thomass~ (merged by Aixtron in 1999). Since the most critical issue of MOCVD systems is to ensure the uniformity and repeatability of material growth, the main difference between MOCVD systems from different manufacturers is the structure of the reaction chamber. Aixtron uses Planetary Reactor, Emcore uses TurboDisc reaction chamber (the business has been sold to Veeco), Thomas Swan (the company was acquired by Aixtron in February 2003) uses Closed Coupled Showerhead (CCS) reaction chamber.
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