Epitaxy molecular beam

Molecular-beam epitaxy is a technique that has mainly been used by the semiconductor industry for producing thin films of compound semiconductors (e.g., GaAs and InP) used in the fabrication of LEDs, laser diodes, etc. Because these inorganic semiconductors are ceramics it should not be surprising that the technique can also be used to grow other ceramic thin films, such as the high-temperature superconductor YBaiCusO . In fact, MBE is ideal for ceramics that have layered structures because it allows precise sequential deposition of single monolayers. 

A diagram of an MBE system is shown in Eigure 28.5. The materials to be deposited are usually evaporated from 

Molecular-beam epitaxy of semiconductors requires the use of an ultrahigh vacuum (UHV) chamber (background pressure 10 -10 ° torr). For oxide ceramics background pressures of 10 torr are more common. The high vacuum requirement of MBE presents a problem for the growth of 

In addition to the requirement of high vacuum or UHV environments, other features of MBE limit its use  

The equipment is expensive ( 1 million) so the value added must be high. 

An atomic or molecular beam of Ga, As, Al, and/or other elements is created thermally in a ultrahigh-vacuum chamber and directed at a specific, well-defined, atomically flat facet of a single-crystal substrate. A very low deposition rate, associated with a precisely controlled substrate temperature and various real-time techniques for monitoring the state of the surface, permit one to find growth conditions that preserve a coherent crystalline order or minimize the density of structural 

Quantum wires are less commonly encountered, since their fabrication procedures are even more complicated one method exploits the step-flow growth of vicinal surfaces [3.129]. 

Self-assembly in specific MBE-type situations leads to the formation of wire structures. Thus, straight atomic lines of Si have been grown on a /S-SiC(lOO) surface [3.130]. 

Clusters of various metals can be formed by use of a dewetting (Vollmer-Weber) type of epitaxial growth this has been done mainly on highly oriented pyrolytic graphite (HOPG). Eormation of 3-D islands has been reported for Cu, Ag, Au [3.134], Pt [3.135], Co [3.136], Mo, [3.137], and Pd [3.138]. 

In some specific situations, a 3-D stacking of quantum dot layers has been achieved [3.139]. 

An additional advantage of molecular beam epitaxy over chemical vapor deposition is that lower substrate temperatures are used in molecular beam epitaxy. The high temperatures required to effect a chemical reaction in chemical vapor deposition are not needed in molecular beam epitaxy. Given the extremely thin nature of the films, atomic diffusion is kept to small distances, and hence the small diffusion coefficients do not seriously retard the overall reaction rate. The difficulty presented by small diffusion coefficients with respect to chemical reactions between bulk solids is discussed in Chapter 5. 

Physical Chemistry for the Chemical and Biological Sciences, University Science Books, Sausalito, CA, 2000, pp. 64-65. 

Eerden, M. Tietema, R. Krug, T. Hovsepian, P. E. In Proc. 48th Annual Technical Conference of the Society of Vacuum Coaters, 2005, 575-579. 

Ghorayeb, A. M. Coleman, C. C. Yoffe, A. D. J. Phys. C Solid State Phys. 1984, 17(21), L715-L719. 

IUPAC Committee on Colloid and Surface Chemistry. Pure Appl. Chem. 1972, 51(4), 577-638. 

Another evaporation technique is molecular beam epitaxy (MBE). MBE produces extremely pure and very thin films with abrupt composition changes.l l Deposition rate however is very slow and the process is still considered experimental. It has been used for the deposition of AIN and SiC films.(75][76] 

An MBE deposition system for ZnO growth consists essentially of a conventional MBE chamber, but with added equipment, such as a compact RF source or H2O2 or O3 for oxygen source. Because of O environment, the heating elements, particularly that for the substrate, are constructed out of Pt. Other measures must also be taken to 

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A. Schmidt, L. K. Chau, A. Back, and N. R. Armstrong, Epitaxial Phthalocyanine Ultrathin Films Grown by Organic Molecular Beam Epitaxy (OMBE), in Phthalo-cyanines, Vol. 4, C. Leznof and A. P. B. Lever, eds., VCH Publications, 1996. 

Panish M B and Temkin H 1993 Gas Source Molecular Beam Epitaxy (New York Springer)... 

The growth of a well ordered fullerene monolayer, by means of molecular beam epitaxy, has been used for the controlled nucleation of single crystalline thin films. The quality and stability of molecular thin films has been shown... 

In practical applications, gas-surface etching reactions are carried out in plasma reactors over the approximate pressure range 10 -1 Torr, and deposition reactions are carried out by molecular beam epitaxy (MBE) in ultrahigh vacuum (UHV below 10 Torr) or by chemical vapour deposition (CVD) in the approximate range 10 -10 Torr. These applied processes can be quite complex, and key individual reaction rate constants are needed as input for modelling and simulation studies—and ultimately for optimization—of the overall processes. 

GaAs, GaAlAs, and GaP based laser diodes are manufactured using the LPE, MOCVD, and molecular beam epitaxy (MBE) technologies (51). The short wavelength devices are used for compact disc (CD) players, whereas the long wavelength devices, mostly processed by MBE, are used in the communication field and in quantum well stmctures. 

Silicon Epitaxy. A critical step ia IC fabricatioa is the epitaxial depositioa of sdicoa oa an iategrated circuit. Epitaxy is defined as a process whereby a thin crystalline film is grown on a crystalline substrate. Silicon epitaxy is used ia bipolar ICs to create a high resistivity layer oa a low resistivity substrate. Most epitaxial depositioas are doae either by chemical vapor depositioa (CVD) or by molecular beam epitaxy (MBE) (see Thin films). CVD is the mainstream process. 

Molecular beam epitaxy is a non-CVD epitaxial process that deposits silicon through evaporation. MBE is becoming more common as commercial equipment becomes available. In essence, silicon is heated to moderate temperature by an electron beam in a high vacuum... 

Molecular beam epitaxy (MBE) is a radically different growth process which utilizes a very high vacuum growth chamber and sources which are evaporated from controlled ovens (15,16). This technique is well suited to growing thin multilayer stmctures as a result of very low growth rates and the abihty to abmpdy switch source materials in the reactor chamber. The former has impeded the use of MBE for the growth of high volume LEDs. 

Epitaxial crystal growth methods such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have advanced to the point that active regions of essentially arbitrary thicknesses can be prepared (see Thin films, film deposition techniques). Most semiconductors used for lasers are cubic crystals where the lattice constant, the dimension of the cube, is equal to two atomic plane distances. When the thickness of this layer is reduced to dimensions on the order of 0.01 )J.m, between 20 and 30 atomic plane distances, quantum mechanics is needed for an accurate description of the confined carrier energies (11). Such layers are called quantum wells and the lasers containing such layers in their active regions are known as quantum well lasers (12). 

Fig. 4. Schematic of a high vacuum molecular beam epitaxy (MBE) chamber containing four effusion (Knudsen) cells. Also shown is a high energy electron...

Electrical Properties. Generally, deposited thin films have an electrical resistivity that is higher than that of the bulk material. This is often the result of the lower density and high surface-to-volume ratio in the film. In semiconductor films, the electron mobiHty and lifetime can be affected by the point defect concentration, which also affects electromigration. These effects are eliminated by depositing the film at low rates, high temperatures, and under very controUed conditions, such as are found in molecular beam epitaxy and vapor-phase epitaxy. 

Physics and chemistry researchers approach III—V synthesis and epitaxial growth, ie, growth in perfect registry with the atoms of an underlying crystal, differently. The physics approach, known as molecular beam epitaxy (MBE), is essentially the evaporation (14—16) of the elements, as illustrated in Figure 4. The chemistry approach, organometaUic chemical vapor deposition (OMCVD) (17) is exemplified by the typical chemical reaction ... 

Fig. 4. Schematic of an ultrahigh vacuum molecular beam epitaxy (MBE) growth chamber, showing the source ovens from which the Group 111—V elements are evaporated the shutters corresponding to the required elements, such as that ia front of Source 1, which control the composition of the grown layer an electron gun which produces a beam for reflection high energy electron diffraction (rheed) and monitors the crystal stmcture of the growing layer and the substrate holder which rotates to provide more uniformity ia the deposited film. After Ref. 14, see text.

For construction of suitable samples molecular beam epitaxy was selected, the method of choice for the production of complicated epitaxial layer systems with different materials. As substrates Si wafer material (about 20x20 mm-, thickness 1 mm) and SiO, discs (diameter 30 mm, thickness 3 mm) were used. Eight layered structures (one, two and three layers) were built up with Al, Co, and Ni, with an indicated thickness of 70 nm, each. 

Recent applications of e-beam and HF-plasma SNMS have been published in the following areas aerosol particles [3.77], X-ray mirrors [3.78, 3.79], ceramics and hard coatings [3.80-3.84], glasses [3.85], interface reactions [3.86], ion implantations [3.87], molecular beam epitaxy (MBE) layers [3.88], multilayer systems [3.89], ohmic contacts [3.90], organic additives [3.91], perovskite-type and superconducting layers [3.92], steel [3.93, 3.94], surface deposition [3.95], sub-surface diffusion [3.96], sensors [3.97-3.99], soil [3.100], and thermal barrier coatings [3.101]. 

The composition must be controlled to give the required emission wavelength. Techniques utilized include molecular-beam epitaxy (MBE) and liquid-phase epitaxy (LPE). 

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