Thin film growth atomic layer deposition

Sammelselg, A., Rosental, A., Tarre, L., Niinisto, K., Heiskanen, K., Dmonen, L.-S., Johansson, T., Uustare, V., 1998. TiO thin films by atomic layer deposition a case of uneven growth at low temperature. Appl. Surf. Sci. 134, 78—86. 

The fifth contribution by M. Putkonen and L. NiinistO presents an overview of Organometallic Precursors for Atomic Layer Deposition (ALD). The key principle of ALD in contrast to CVD is the exclusion of any gas-phase prereaction allowing the thin film growth to be fully controlled by surface reactions and adsorption/desorption kinetics. ALD is thus ideally suited for the growth of ultra-thin layers and atomically abrupt interfaces needed in future nanoelectronic devices. While CVD and ALD have many aspects in common, precursors suitable for ALD generally need to be much more reactive than those used for CVD. Another challenge is to combine low steric demand with very high selectivity of the surface reactions. 

This chapter is intended to cover major aspects of the deposition of metals and metal oxides and the growth of nanosized materials from metal enolate precursors. Included are most types of materials which have been deposited by gas-phase processes, such as chemical vapor deposition (CVD) and atomic layer deposition(ALD), or liquid-phase processes, such as spin-coating, electrochemical deposition and sol-gel techniques. Mononuclear main group, transition metal and rare earth metal complexes with diverse /3-diketonate or /3-ketoiminate ligands were used mainly as metal enolate precursors. The controlled decomposition of these compounds lead to a high variety of metal and metal oxide materials such as dense or porous thin films and nanoparticles. Based on special properties (reactivity, transparency, conductivity, magnetism etc.) a large number of applications are mentioned and discussed. Where appropriate, similarities and difference in file decomposition mechanism that are common for certain precursors will be pointed out. 

J. Aarik, A. Aidla, T. Uustare, V. Sammelselg, Morphology and structure of Ti02 thin films grown hy atomic layer deposition . Journal of Crystal Growth, 148, 268-275, (1995). 

Here, m andp are the oxidation states of UPD metal M and the more noble metal P. The factors b, 9, and q are introduced to accurately express the amount of deposited metal P in ML units with respect to atomic areal density of the substrate S h, k, /). They represent, respectively, the number of full UPD MLs, the UPD ML coverage, and the packing density of M atoms in complete UPD ML with respect to the substrate S h, k, /). The subscripts s and solv indicate the physical state of the metal (solv = solution phase and s = deposited). If sequence A-E (Fig. 7) is repeated an arbitrary number of times, a multilayer homo- or heteroepitaxial films can be obtained. The thin film growth using this method can be completely automated with experimental apparatus for Electrochemical Atomic Layer Epitaxy developed by Stickney et al. ... 

Torndahl, T. Ottosson, M. Carlsson, J.-O., Growth of copper metal by atomic layer deposition using copper(l) chloride, water and hydrogen as precursors. Thin Solid Films, 2004 458 129-136. 

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). 

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. 

Also, thin films of semiconducting compounds were formed on a gold electrode. To obtain them, a methodology called electrochemical ALE (electrochemical atomic layer epitaxy) has been developed. This procedure is based on the formation of individual atomic layers of particular elements, which may further form a compound. Accordingly, in each cycle, a controlled formation of a monolayer of the particular compound occurs. The advantage of this methodology is that three-dimensional growth of one-elemental deposit is inhibited. 

Atomic layer epitaxy (electrochemical) — Electrochemical atomic layer epitaxy (ECALE) is a self-limiting process for the formation of structurally well-ordered thin film materials. It was introduced by Stickney and coworkers [i] for the layer by layer growth of compound semiconductors (CdTe, etc.). Thin layers of compound semiconductors can be formed by alternating - underpotential deposition steps of the individual elements. The total number of steps determines the final thickness of the layer. Compared to flux-limited techniques... 

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