Atomic Layer Epitaxy ALE

In molecular beam epitaxy (MBE), the constituent elements of the desired film in the form of molecular beams are deposited epitaxially onto a heated crystalline substrate. These molecular beams are typically from thermally evaporated elemental sources (e.g., evaporation of elemental As produces molecules of As2, As3, and As4). A refinement of this is atomic layer epitaxy (ALE) (also known as atomic layer deposition, ALD) in which the substrate is exposed alternately to two... 

The deposition of a wide range of materials using beams of elemental sources in high-vacuum apparatus (10-4—10-8 torr), essentially by physical methods, is known as molecular beam epitaxy (MBE)8 12 and atomic layer epitaxy (ALE). These methods will be mentioned where there is an overlap with CVD techniques, but will not be fully reviewed. (They are mentioned also in Chapter 9.15). 

Historically, EC-ALE has been developed by analogy with atomic layer epitaxy (ALE) [76-82], ALE is a methodology used initially to improve epitaxy in the growth of thin-films by MBE and VPE. The principle of ALE is to use surface limited reactions to form each atomic layer of a deposit. If no more than an atomic layer is ever deposited, the growth will be 2-D, layer by layer, epitaxial. Surface limited reactions are developed for the deposition of each component element, and a cycle is formed with them. With each cycle, a compound monolayer is formed, and the deposit thickness is controlled by the number of cycles. 

Oznuluer and Demir [88] have employed electrochemical atomic layer epitaxy (ALE) to the investigations of kinetics of structural changes occurring within initial monolayers of thin Bi2S3 films on Au(l 11). 

There are numerous materials, both metallic and ceramic, that are produced via CVD processes, including some exciting new applications such as CVD diamond, but they all involve deposition on some substrate, making them fundamentally composite materials. There are equally numerous modifications to the basic CVD processes, leading to such exotic-sounding processes as vapor-phase epitaxy (VPE), atomic-layer epitaxy (ALE), chemical-beam epitaxy (CBE), plasma-enhanced CVD (PECVD), laser-assisted CVD (LACVD), and metal-organic compound CVD (MOCVD). We will discuss the specifics of CVD processing equipment and more CVD materials in Chapter 7. 

This was recknognized by Suntola11 in the late 1970 s, when he developed a new coating technique Atomic Layer Epitaxy (ALE). 

It is possible by the ML method to carry out chemical assembly step by step (monolayer by monolayer), by repeated surface reactions. The first nanolayers by the ML method were synthesized about 40 years ago in Saint-Petersburg Technological Institute at the Department of Chemistry of solids. There have been many publications in western countries detailing the analogs of the ML method, namely atomic layer epitaxy (ALE), and atomic layer deposition (ALD), since this techniques inception by Valentine Aleskovski and members of his science school. 

An understanding of gas-phase and surface chemistry is particularly important to the next generation of MOVPE processes involving selective epitaxy [18] and atomic layer epitaxy (ALE) [19]. In the first process, the compound semiconductor is deposited selectively on substrate areas opened in a suitable masking material (e.g., SiOz). This is achieved by operating under conditions where nucleation occurs only on the substrates. Slight variations in processing environment and the presence of impurities can cause nucleation on the mask and result in loss of selectivity. 

The potential benefits of CVD over other film deposition techniques are that CVD-derived films can be deposited under conditions that give conformal coverage, they can be deposited at low temperatures, there can be a high level of compositional control, thin layers can be deposited, the technique can be scaled to coat large areas uniformly, and there is also the possibility for area-selective deposition13 as a result of the chemical nature of this process. The details of CVD and related chemical deposition processes such as atomic layer epitaxy (ALE), organometallic vapor-phase epitaxy (OMVPE), and others have been described elsewhere.6... 

Blackman, et al used the gas-phase technique, also known as atomic layer epitaxy (ALE), to develop controlled loadings of Co on silica using a Co(acac)3 precursor (Fig. 18). By this technique they obtained loadings of Co from 5.7 to... 

In the period mentioned above the most known world centers of the studies of chemical reactions on silica and alumina surfaces were USSR, Germany, Bulgaria. But since 1977 after the patent [26] had been published and later in the eightieth as well as at present the number of publications in this field has increased in various countries [27-39]. It should be noted, that some authors use another names for the synthesis like the ML method for example, atomic layer epitaxy (ALE) [30], chemical surface coating [31], grafting [32],elementary surface reactions of CVD method [33]. 

This paper describes ongoing studies of the electrodeposition thin films of the compound semiconductors CdTe and InAs, using the method of electrochemical atomic layer epitaxy (ALE). Surface limited electrochemical reactions are used to form the individual atomic layers of the component elements. An automated electrochemical flow deposition system is used to form the atomic layers in a cycle. Studies of the conditions needed to optimize the deposition processes are underway. The deposits were characterized using X-ray diffraction, scanning probe microscopy, electron probe microanalysis and optical/infrared absorption spectroscopy. 

In certain instances, reaction rates substantially decrease with increasing temperature. A classic example is atomic layer epitaxy (ALE). This is a result of adversely modifying the rate of desorption of the reactants to become greater than the concomitant increase in the surface reaction rate accompanying the increase in substrate temperature. One potential approach to address this specific issue is to move to alternate precursors (see Sect. 1.8). 

Very recently, Viirola and Niinisto have applied atomic layer epitaxy (ALE) to tin oxide films using SnCL [165]. The thickness increased linearly with the number of cycles yielding typical values of 0.35 A per cycle. This is a rather low value, which can be explained by a fractional monolayer growth mode instead of layer-by-layer 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 preparation method of the thin layers and the electroluminescent materials available are reviewed in Ref. [28J. Among the teehniques used are sputtering, vacuum evaporation, metal-organic chemical-vapor deposition (MOCVD) and atomic layer epitaxy (ALE). 

This chapter concerns the state of development of electrochemical atomic layer epitaxy (EC-ALE), the electrochemical analog of atomic layer epitaxy (ALE). EC-ALE is being developed as a methodology for the electrodeposition of compound semiconductors with nanoscale control. ALE is based on the formation of compounds, one monolayer (ML) at a time, using surface-limited reactions. An atomic layer of one element can be electrodeposited at a potential under that needed to deposit the element on itself, and this process is referred to as underpotential deposition (UPD). EC-ALE is the use of UPD for the surface-limited reactions in an ALE cycle. 

One way to prepare precise surface structures on catalyst supports is to utilize adsorption controlled methods. Adsorption covers a variety of interactions from physisorption (weak interaction) to chemisorption (strong interaction). The term control, however, restricts the adsorption to strong interactions. The requirements for adsorption controlled catalyst processing have been shown to be met in both liquid and gas phase. In this paper we focus on the use of a gas phase method, called Atomic Layer Epitaxy (ALE), in the preparation of heterogeneous... 

Figure 1. The Atomic Layer Epitaxy (ALE) method in relation to other methods of processing catalysts. The other methods are classified aceording to the International Union of Pure and Applied Chemistry (lUPAC) [10].

Atomic layer epitaxy (ALE), molecular precursors - chemisorFrtion from gas phase... 

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