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Having mastered the technology of epitaxial deposition of crystalline thin films (i.e. homo and heteroepitaxy) on crystalline substrates has already been found providing better device designs with numerous advantages in the development of microelectronics devices and circuits. Consequently, mass-scale production of epitaxial thin films could successfully be developed and used in fabricating discrete devices and integrated circuits (ICs) using silicon/compound semiconductors commercially. Especially, realizing the hetero-epitaxial interfaces possessing two-dimensional electron/hole gas (2DEG/2DHG) sheets could offer very-high electron/hole mobilities for producing high-electronmobility transistors (HEMTs) and amplifiers for microwave/millimeter-wave communication systems. However, the major limitation of this technology was its requirement of extremely high cost infrastructures. Subsequently, the rising demands of the technologies to produce large-size displays/electronics systems, and large-numbers of sensors/ actuators in Internet of Thing (IoT) made it imminent for the researchers to explore replacing the existing cost intensive technologies by more affordable ones. In such an endeavor, developing a simpler and alternate epitaxial technology became imminent to look for. Incidentally, electrodeposition based epitaxy attracted the attention of the researchers by employing potentiostatic set-up for understanding the growth kinetics of the ionic species involved. While going through these studies, starting with the deposition of metallic/semiconducting thin films, atomic-layer epitaxial depositions could be successfully made and named as electrochemical atomic layer deposition (EC-ALD). Despite numerous attempts made for almost two decades in this fascinating field the related technology is not yet ready for its commercial exploitations. Some of the salient features of this process (i.e. commonly known as EC-ALD or EC-ALE) are examined here with recent results along with future prospects. Indexing Terms: Vapor Phase Epitaxy (VPE), Liquid Phase Epitaxy (LPE), Atomic Layer Deposition, Atomic Layer Epitaxy (ALE), and Molecular Beam Epitaxy (MBE); Electrochemical Atomic Layer Deposition (EC-ALD) Introduction A variety of thin over-layers have been produced commercially using chemical vapor depositions (CVDs) in the later half of the 20th Century for their extensive applications in microelectronics devices and integrated circuits of different complexities [1]. Out of numerous types of such thin films, those of monocrystalline semiconducting ones deposited after extensive refinement of the general CVD-processes were noted to play a significant role in extending the utility of crystalline substrates in realizing optimal functionalities from the fabricated devices and circuits [2]. Epitaxial growth in combination with oxide masking and diffusion has given the device designer extremely flexible tools for making an almost unlimited structural options. After 1964 epitaxial growth remained an important technique in semiconductor device fabrication and the demand for improved device yield per wafer, still higher device operating frequencies, and more sophisticated device structures did need continuing innovation and development [3-5]. While exploring better CVD processes, yet another improved version of atomic-layer deposition (ALD) was discovered to produce atomically thin layers that were put to use in producing discrete devices, integrated circuits (ICs), and photovoltaic solar cells with improved performances [6]. Further, the applications of various kinds of CVD thin films included many other forms of polycrystalline Si in photovoltaic devices, wear-resistant coatings of carbides/nitrides, in addition to incorporating hydrophobic, and weather-resistant features. Recently developed crystalline nanoporous materials using metal-organic frameworks (MOF) are expected to produce good gas sensors, and low-k dielectrics [7]. CVD-over-layers of monocrystalline semiconductors are deposited on to a substrate of the same or the other materials with almost matching lattice constant within few-percent come under the category of homo or hetero epitaxial thin films. Such semiconducting thin films in combination with those of the dielectric and metal thin films have been put to maximum use in realizing the required Volume 2 | Issue 1 | 1 of 20 Journal of Applied Material Science & Engineering Research doping profiles of a multi-layered device structures to sustain the corresponding electric field profile capable of offering the desired charge carrier transport across such structures [8]. In a typical thin film CVD, one or more gaseous precursors are mixed with a nonreactive carrier gas. Such compositions are allowed to pass over a heated substrate placed appropriately inside a reactor. These precursors react and/or decompose on the substrate surface resulting in deposition of the corresponding thin films. Volatile reaction byproducts are subsequently removed from the reactor with the help of carrier gas used. The CVD over-layers so deposited included monocrystalline, polycrystalline, amorphous, as well as epitaxial thin films. These also included a big list of materials like various combinations of silicon (e.g. oxide, carbide, nitride/oxynitride), germanium, carbon as single/multi-walled nanotubes, graphene, and diamond; fluorocarbons; W, Ti, TiN, and various high-k dielectric materials, and many more [2,8]. In order to meet the structural specifications for producing the active device interfaces having targeted I-V characteristics, Si CVD was refined to the extent of depositing monocrystalline thin films in commercial high throughput reactors by taking right combinations of temperature, pressure, and appropriate silicon bearing precursors. This kind of vapor phase epitaxy (VPE) could be used in realizing specific combinations of the substrate along with a combination of over-layers using special deposition conditions including a typical design of reaction chamber, substrate holder, and the gas manifold to introduce different precursor species in gas phase. These reactant precursors diffuse from the flowing carrier gas stream and reach the heated substrate. Subsequently, they participate in chemical reactions producing atomic and molecular species that help in growing 2-D epitaxial thin film on the entire surface. A number of Si bearing chemical precursors that are often used in silicon epitaxy includes silicon tetrachloride (SiCl4), di-chlorosilane (SiH2Cl2), trichlorosilane (SiHCl3) and silane (SiH4) [2,8]. Another variant of CVD epitaxy called molecular beam epitaxy (MBE) was subsequently added to the existing list of thin film depositions by considerably modifying the surface reaction conditions. For instance, thermal evaporations of the precursors were used to grow epitaxial layers instead of deposition through dissociation and chemical reactions employed in the conventional CVD process. Although, the method has been known since the early 1960s, it has only recently been possible to develop the production worthy processes meeting the requirements of silicon device fabrications in the manufacturing environment. In MBE process, the Si along with dopants are thermally evaporated from effusion cells before transporting them at a relatively higher velocity in an ultra high vacuum (UHV) system to strike against a heated and rotating substrate resulting in condensation of the species on a relatively lower-temperature substrate without any contamination. Usually, Si MBE is performed under UHV conditions for eliminating the unwanted contaminant species as far as it is possible. Because of several stringent conditions to be met, it took quite some time to develop commercial MBE reactors particularly suited to the mass scale production of epitaxial wafers. Although, the process has comparatively lower throughput and is cost intensive due to essential requirement of UHV environment inside the deposition chamber. The MBE process, however, does have a number of inherent advantages over conventional CVD techniques in meeting the precise structures for fabricating over layers for advanced semiconductor devices [2,8]. In yet another variant called liquid phase epitaxial (LPE) growth of semiconductor thin films, the substrate is brought into contact with a saturated solution of the semiconducting material to be deposited at an appropriate temperature. Withdrawal of the dipped substrate resulting in substrate cooling at a suitable rate could allow growing monocrystalline films as observed in case of compounds and III-V alloy semiconductors. Though, LPE could cost much less in producing layers with low defect density and very precise control of stoichiometry, the process suffered from being applicable only in case of a restrictive list of materials besides being difficult to control the morphology with poor surface quality [2,8]. Atomic layer epitaxy (ALE), also called atomic layer deposition (ALD), is another recently developed novel epitaxial deposition process in which the alternate monolayers of two/more elements are deposited onto an underlying substrate in vacuum in a selflimiting mode. The crystal lattice structure achieved in this process is uniform, and aligned with the lattice structure of the underlying substrate. The reactants are brought to the substrate in the form of alternate pulses with a suitable time-gap in between. In ALD, the incoming material species bond strongly until all the sites available for chemisorption are occupied. The time-gap between the two subsequent steps is used to flush out the unreacted materials present there. It is mostly used in semiconductor fabrication to grow thin films of thickness of the atomic order [2,3,8]. After going through these developments that took place in the area of epitaxial growth of semiconductor thin films for device applications, one common point that is noted is the presence of heated

Electrochemical Atomic Layer Deposition (EC-ALD) Low Cost Synthesis of Engineered Materials a Short Review