Epitaxial atomic processes

Pyrophoric A compound that reacts with air in a way that results in spontaneous ignition. m-V compound semiconductor A semiconductor that in pure form is composed of a mixture of atoms of one or more elements from Column III and an equal number of atoms of one or more elements from Column V of the Periodic Table. The Column III atoms are arranged on one sublattice and the Column V atoms are on another. Vapor-phase epitaxy (VPE) An epitaxial growth process that only uses chemical precursors that are delivered to the growing surface in the vapor-phase. 

Atomic layer deposition (also termed atomic layer epitaxy) a process in which alternate pulses of two volatile precursors are passed over the substrate to promote layer-by-layer film growth... 

Figure 4-41. Representation of the atomic layer epitaxial (ALE) process, (i) TMGa exposure, leaving a Ga layer (ii), (iii) arsine exposure, leaving an As layer (iv), (v) the cycle begins to repeat.

The mobility or diffusion of die atoms over the surface of die substrate, and over the film during its formation, will occur more rapidly as the temperature increases since epitaxy can be achieved, under condition of ctystallographic similarity between die film and the subsuate, when the substrate temperamre is increased. It was found experimentally that surface diffusion has a closer relationship to an activation-dependent process than to the movement of atoms in gases, and the temperamre dependence of the diffusion of gases. For surface diffusion the variation of the diffusion coefficient widr temperature is expressed by the Anhenius equation... 

We have so far assumed that the atoms deposited from the vapor phase or from dilute solution strike randomly and balHstically on the crystal surface. However, the material to be crystallized would normally be transported through another medium. Even if this is achieved by hydrodynamic convection, it must nevertheless overcome the last displacement for incorporation by a random diffusion process. Therefore, diffusion of material (as well as of heat) is the most important transport mechanism during crystal growth. An exception, to some extent, is molecular beam epitaxy (MBE) (see [3,12-14] and [15-19]) where the atoms may arrive non-thermalized at supersonic speeds on the crystal surface. But again, after their deposition, surface diffusion then comes into play. 

Chemical vapor deposition may be defined as the deposition of a solid on a heated surface from a chemical reaction in the vapor phase. It belongs to the class of vapor-transfer processes which is atomistic in nature, that is the deposition species are atoms or molecules or a combination ofthese. Beside CVD, they include various physical-vapor-deposition processes (PVD) such as evaporation, sputtering, molecular-beam epitaxy, and ion plating. 

MBE growth of very thin layer of boron and silicon. The problems associated with boron implant and laser anneal can be overcome by growing a very thin (5 nm) layer of silicon with boron atoms on the backside of the thinned CCD (1% boron, 99% silicon). The growth is applied by molecular beam epitaxy (MBE) machines. This process was developed by JPL and MIT/LL. 

The focus of the work described here is on understanding the mechanisms of compound electrodeposition and how to control structure, morphology and composition. The primary tool for understanding compound electrodeposition and for improving control over the process has been the methodology of electrochemical atomic layer epitaxy (EC-ALE) [29, 73-75],... 

The use of UPD layers can in principle generate deposits with composition modulated on the atomic scale, and Pauling et al. have produced what they call hetero-structured ultra-thin films containing Ag, Pd and T1 by this method [158], Stickney and coworkers have assembled multilayered deposits of CdTe and GaAs by addition of one atomic layer of the individual components at a time, a process they call electrochemical atomic-layer epitaxy [159 162], The essential controlling feature in the UPD mechanism is that the deposited layers are allowed to reach equilibrium. Hence, the process represents an extreme of local, reversible control. 

The electrochemical atomic layer epitaxy (ECALE) technique, also known as electrochemical atomic layer deposition (EC-ALD), is based on layer-by-layer electrodeposition. Each constituent of the thin him are deposited separately using underpotential deposition (UPD) of that element. UPD is a process wherein an atomic layer of one element is deposited on the surface of a different element at a potential under that needed to deposit the element on itself. ECALE has been used to grow mainly II-VI and III-V compounds. A thorough review of ECALE research has been published by Stickney.144 A summary of the materials deposited using ECALE are given in Table 8.4, with a more detailed discussion for a few select examples given below. 

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