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Wireless communications and optical fibre networks are rapidly expanding throughout the world, and are dependent on devices made of compound semiconductors grown by MBE.
Epitaxy is a crystal growth process in which thin layers
of compound semiconductor crystals are grown on the surface of a bulk crystal
material called the substrate. If the chemical composition of these surface
layers, also known as epitaxial layers, is similar enough to the crystal
structure of the substrate, the deposited layers of compound semiconductor
materials will form a single crystal structure replicating that of the substrate
material. Both the number and thickness of the epitaxial layers, as well
as their chemical composition and incorporated dopants, determine the performance
characteristics of the devices that are made from the resulting epitaxial
Molecular beam epitaxy (MBE) takes place in a reactor (also know as growth chamber) in which source materials are introduced in the form of molecular beams. Molecular beams are created by heating solid source materials, which are placed inside crucibles within containers known as effusion cells, until they vaporize. A gas source may be used instead of a solid source, in which case the source material is introduced into the reactor through a gas injector nozzle. Due to the UHV environment of the reactor, when the source materials escape from the crucibles their molecules form a series of directed beams that are able to travel without collision until they make impact with the wafer's surface. As the molecular beams collide with the surface of the wafer, their molecules decompose into the constituent atoms of the source materials. Because the wafer is heated during the process, there is sufficient kinetic energy for the atoms to rearrange themselves into a single crystal structure replicating the crystal structure of the underlying wafer (picture). The key advantages of MBE compared to other epi process technologies include:
MBE allows to grow epilayers with different chemical compositions to atomic layer accuracy (with the thickness of each surface layer being as thin as one or two atoms) and at the same time to ensure that uniformity across the wafer surface is maintained. The possibility of obtaining very high epilayer uniformities also allows to achieve yields as high as 95% from each epiwafer, which means that up to 95% of the epiwafer material can be processed to make compound semiconductor devices. The ability of MBE to produce abrupt transitions between layers of different semiconductor crystals also reduces electronic noise and distortion and increases power efficiency in devices.
Monitoring of epitaxial process
The UHV environment of the MBE growth chamber makes it possible to use electrons and light particles as probes to monitor with great precision the wafer's surface and epilayer quality during epitaxial growth. These monitoring processes facilitate the real-time control of the deposition and thereby provide a highly accurate quality control tool.
The UHV conditions within the MBE reactor allow for the rapid removal of unused source materials upon completion of a growing cycle, thereby decreasing the amount of time between growing cycles. This allows the MBE process to shift rapidly between wafer batches, which increases the potential for high throughput, a measurement of the number of epiwafers produced per unit of time.
Safety and ease of maintenance
The MBE process does not use the high volumes of toxic gases generally used in several competing epi processes (e.g. MOVPE), resulting in greater safety and ease of maintenance.
The compound semiconductor materials
Compound Semiconductors are materials used in a wide variety of electronic and optoelectronic (photonic) systems. Whereas silicon is the most popular element used today to make electronic devices, the compounds are considered a category of semiconductors that perform functions beyond the physical limits of the electronic properties of silicon. In addition to the pure element semiconductors, there are many different kinds and types of compound semiconductor materials. These may be formed from semiconducting or non-semiconducting elements. The particular advantage of compounds is that they provide the device engineer with a wide range of energy gaps and electron mobilities, so that materials are available with properties which exactly match specific requirements. One of the most important classes of compounds is the III-Vs, formed of elements from Groups IIIB and VB of the periodic table of elements. Typical examples of compound semiconductors are GaAs, InP, GaN, CdHgTe orSiGe. The first is one of the most versatile and useful semiconductors. Often, more than two elements are combined to make alloys such as GaAlAs (ternary) or GaInAsP (quaternary).
Gallium arsenide (GaAs) has superior electronic and optical properties, providing high electron mobility and low power consumption. GaAs components can operate at microwave frequencies whereas silicon devices cannot. In spite of its higher cost, relative to silicon, GaAs is the preferred material for monolithic microwave integrated circuits (MMICs). GaAs epiwafers are used in cellular phones, satellites, radar systems and various electronic devices. GaAs is also the material of choice for lasers and LEDs. The broad range of opto-electronic applications includes lasers for fiber optic communications, optical storage, solid state laser pumping, compact-disc players, and indicators in appliances, automobiles, traffic signals and commercial displays.
Indium phosphide (InP) is a compound material with properties which make it important for a number of opto-electronic and electronic applications. The band gap and lattice constant of InP allow the fabrication of semiconductor diode lasers whose wavelength is well matched to the optimum transmission in silica optical fibers. Due to the high mobility of carriers in InP, this material is also important for the manufacture of high speed and high frequency transistors, MMICs and other related products, such as single mode fiber-optic transmission systems.
Whether for digital players, laser printers, scanners, night-, general, and traffic lighting or even full-color TV displays, applying GaN lasers (picture) is now a realistic endeavor. Using violet and blue lasers in CDs and DVDs provides a significant technical advantage due to their typical 400-nm wavelength, which is significantly less than that of infrared-region lasers. Thus, violet and blue lasers can be focused to a smaller diameter to pick off information, enabling a fourfold increase in disk-storage capacity. Other applications arise from aesthetic and economic considerations: they are a cost-effective solution for automotive dashboards and for signboards. Another GaN market application that shows great promise is that of electronic device development.
Cadmium Mercury Telluride
Infrared vision systems (e.g., for vision at night or in space) are based on the deposition of narrow gap II-VI compounds enabling the fabrication of infra-red detectors. These devices operate upon different principles. They actually operate upon the same principle as cheap cadmium sulphide (CdS) photocells that are used in a range of household applications from camera-flash monitors, to street-lamps. The bandgap energy of CdS is suitable for the detection of visible light. Infra-red photons are of lower energy and so must be detected using a different semiconductor. The semiconductor mainly used for thermal imaging applications is cadmium mercury telluride (CdHgTe or CMT) although a range of other materials including indium antimonide (InSb) can also be used.
Silicon germanium (SiGe) epitaxy is a key material technology that allows bandgap engineering in the silicon process, and provides extremely interesting electronic (transport) and optical (luminescence) intrinsic device properties. Used to produce heterojunction bipolar transistors (HBTs), it can enable devices with higher operational frequencies, lower noise and lower power consumption. Development activities are underway to merge the advantages of CMOS technology in low-power, high-density digital signal processing with the high speed of the SiGe HBT into a BiCMOS technology (single chip), resulting in lower cost, lower power consumption and enhanced device performance.
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