Synthesis of Doped, Ternary, and Quaternary Materials by Atomic

Subscriber access provided by University of Kansas Libraries

Review

Synthesis of Doped, Ternary, and Quaternary Materials by Atomic Layer Deposition: A Review Adriaan J. M. Mackus, Joel R. Schneider, Callisto MacIsaac, Jon G. Baker, and Stacey F. Bent Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02878 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on January 22, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Synthesis of Doped, Ternary, and Quaternary Materials by Atomic Layer Deposition: A Review Adriaan J. M. Mackus,1,2,# Joel R. Schneider,1,# Callisto MacIsaac,3 Jon G. Baker,1 Stacey F. Bent1,* 1Department 2Department

of Chemical Engineering, Stanford University, Stanford, California 94305, USA

of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, the Netherlands

3Department

of Chemistry, Stanford University, Stanford, California 94305, USA #co-first

*E-mail:

authors

[email protected]

Abstract In the past decade, atomic layer deposition (ALD) has become an important thin film deposition technique for applications in nanoelectronics, catalysis, and other areas due to its high conformality on 3-D nanostructured substrates and control of the film thickness at the atomic level. The current applications of ALD primarily involve binary metal oxides, but for new applications there is increasing interest in more complex materials such as doped, ternary, and quaternary materials. This article reviews how these multicomponent materials can be synthesized by ALD, gives an overview of the materials that have been reported in the literature to date, and discusses important challenges. The most commonly employed approach to synthesize these materials is to combine binary ALD cycles in a supercycle, which provides the ability to control the composition of the material by choosing the cycle ratio. Discussion will focus on four main topics: (i) the characteristics, benefits, and drawbacks of the approaches that currently exist for the synthesis of multicomponent materials, with special attention to the supercycle approach, (ii) the ACS Paragon Plus Environment

1

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 97

trends in precursor choice, process conditions, and characterization methods, as well as underlying motivations for these design decisions, (iii) the distribution of atoms in the deposited material and the formation of specific (crystalline) phases, which is shown to be dependent on the ALD cycle sequence, deposition temperature, and post-deposition anneal conditions, and (iv) the nucleation effects that occur when switching from one binary ALD process to another, with different explanations provided for why the growth characteristics often deviate from what is expected. This paper provides insight into how the deposition conditions (cycle sequence, temperature, etc.) affect the properties of the resultant thin films, which can serve as a guideline for designing new ALD processes. Furthermore, with an extensive discussion on the nucleation effects taking place during the growth of ternary materials, we hope to contribute to a better understanding of the underlying mechanisms of the ALD growth of multicomponent materials.

I.

Introduction

In the current era of nanotechnology, the thin film deposition technique of atomic layer deposition (ALD) has emerged as an essential element of the toolbox for processing materials at the atomic level. Although the technique was developed in the 1960s and 70s,1 it has only in the past decade acquired widespread attention, motivated first by its application for high-k gate dielectrics in state-of-the-art transistors, then later expanding to other applications in nanoelectronics.2 With critical dimensions of CMOS electronics currently approaching the sub-5 nm level, the unparalleled conformality and thickness control provided by ALD appear to be required not only for the gate dielectric, but also for the deposition of thin films at multiple locations in the CMOS device architecture. Furthermore, the required material properties for these films have become increasingly more demanding, as illustrated by the recent introduction of many new elements into semiconductor processing. The early industrial applications of ALD were almost all based on binary (i.e. two-element) metal oxide ALD processes.2 The next step will be the introduction of more complex materials including doped, ternary, and quaternary compounds. Moreover, the success of ALD as an essential technique for semiconductor manufacturing has recently triggered the exploration of ACS Paragon Plus Environment

2

Page 3 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


other applications in fields as diverse as catalysis3,4, energy storage,5,6 and photovoltaics.7–9 These new applications also often require complex materials with accurately tuned composition or specific doping profiles.10–12 Due to its atomic-level control of the growth, ALD allows for the creation of homogeneous multicomponent or multilayered thin films by depositing atomic layers of different materials in a specific sequence. ALD relies on the alternating exposure of a surface to a precursor and to a co-reactant, which react with the surface through self-limiting reactions.13 Because of the self-limiting nature of the surface reactions, the growth is not dependent on the precursor and co-reactant fluxes, resulting in deposition with high conformality even on nanostructured surfaces. Moreover, the cyclic pulsing of ALD provides subångström control of the film thickness. A binary ALD cycle consists of a precursor dose and a subsequent co-reactant dose, separated by purge steps. Precursors are typically metalorganic compounds containing the metal(s) or semimetal(s) of interest, such as trimethylaluminum, tetrakis(dimethylamino)tin, or tetraethyl orthosilicate. As co-reactants, H2O, O3, or O2 plasma are often employed for metal oxides such as Al2O3 and TiO2, NH3 or N2/H2 plasmas for metal nitrides such as TiN and SiNx, and H2S for metal sulfides such as ZnS.14 The most common approach for the synthesis of a doped or ternary material by ALD is to alternate between several cycles of two binary ALD processes in a so-called supercycle. By performing k cycles of binary process ACx and l cycles of binary process BCy, a ternary ABwCz film can be deposited, where C is a nonmetal atom (e.g. O, N, S). This approach allows for the tuning of the composition by choosing a certain cycle ratio 𝑘/(𝑘 + 𝑙). Doping can be achieved by performing supercycles with 𝑘 ≫ 𝑙, such that the ACx process deposits the host material and the BCy process the dopant. A quaternary or quinary material can be synthesized by combining three or more binary ALD processes into a supercycle. In the past decade, many processes for the deposition of doped, ternary, and quaternary materials have been reported. The literature on these materials clearly shows that the development of a ternary or doped process is more complex than just adding two binary processes together. Ideally, the growth rate and the composition should be a linear combination of the growth rate and composition of the employed ACS Paragon Plus Environment

3


Page 4 of 97

binary ALD processes, according to the rule of mixtures.15 However, the growth rate of a ternary process is often lower (and sometimes higher) than the growth rate expected from the binary growth rates and the composition typically deviates from the targeted properties, due to a variety of non-idealities including nucleation effects and precursor ligand interactions. Another important aspect to consider when depositing these materials is the spatial distribution of atoms in the deposited layer. For example, it is often desirable to synthesize a ternary material with a specific perovskite phase.16,17 At first sight, it appears best to switch as often as possible between the two binary processes, such that the elements are well-mixed; however, this also means that any of the nucleation effects that cause the non-idealities described above are amplified. Moreover, ALD at typical substrate temperatures of 100 – 400 ⁰C generally results in the deposition of amorphous materials,14 while many applications require crystalline phases. It is therefore often needed to perform an anneal after the deposition to facilitate the mixing of atoms or to crystallize the material. In this article, the deposition of doped, ternary, and quaternary materials by ALD is reviewed. There are many fine reviews of ALD in the literature;11–14,16–24 however, there is to our knowledge no comprehensive review with a focus on multicomponent (ternary and higher) materials. Given the growing importance of these materials, we have therefore written this review to compile the over 475 ALD processes for 207 ternary and 44 quaternary materials demonstrated to date and describe their common fabrication strategies; challenges regarding composition, phase, and nucleation; and future opportunities. The article will mainly focus on: (i) the characteristics, benefits and drawbacks of approaches currently available for the synthesis of multicomponent materials, (ii) the trends in precursor choice, process conditions, and characterization methods, (iii) intermixing of species and phase formation, as influenced by the ALD cycle sequence, deposition and anneal conditions, and (iv) the nucleation effects that occur when switching between binary ALD processes. These discussions will contribute to a better understanding of the mechanisms at work and to an enhanced ability to ACS Paragon Plus Environment

4

Page 5 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reproducibly deposit this class of materials, which can significantly broaden the application possibilities of ALD. This article is organized as follows: First, in Section II, general approaches for ALD of doped, ternary and quaternary materials are described. Second, in Section III, an overview of the ternary and quaternary materials synthesized by ALD and the processes used is presented. Section IV reviews the literature on intermixing and phase formation. In Section V, various sources of deviation in growth characteristics from those predicted by the rule of mixtures are reviewed, and the ZnAlxOy ALD process is presented as a case study to illustrate how several of these mechanisms can occur in concert.

II.

Approaches for deposition of doped, ternary, and quaternary materials

This section discusses various experimental strategies for producing multicomponent materials, together with their strengths and weaknesses. Relevant terminology is defined, and some literature studies are reviewed. A. The supercycle approach

The most common method for producing doped, ternary, and quaternary materials via ALD involves the combination of multiple binary ALD processes into supercycles. This approach takes the normal ALD cycles of sequential precursor and co-reactant pulses for each constituent process and combines them into a cycle of cycles, as illustrated in Figure 1. An ALD supercycle is defined as the minimum sequence of individual binary cycles that is repeated over the course of the ALD process. A simplified way of picturing this is that each cycle or block of cycles of a binary process results in the deposition of a sublayer of a specific material within the total film. As an example, ZnAlxOy is a ternary material formed by combining the ZnO and Al2O3 binary ALD processes in a supercycle. When the switching between Al2O3 and ZnO ALD occurs after only a small number of cycles as in Figure 1a, then a well-mixed ternary ZnAlxOy film may be formed. Since one ALD cycle leads to deposition of a sub-monolayer of material with a thickness of < 2 Å, regular switching assists the deposition of a compositionally uniform film. In cases where the ACS Paragon Plus Environment

5


Page 6 of 97

switching back and forth occurs less frequently, a multilayered film may be obtained with individual sublayers as in Figure 1b If one of the sublayers is much thinner than the other as in Figure 1c, then one binary process deposits a dopant and the other the host material, resulting in a doped films (e.g. Al-doped ZnO). The composition and properties of the film can be varied by changing two supercycle parameters: (i) the cycle ratio, and (ii) the bilayer period. The cycle ratio 𝐶𝑅 is defined as the number of cycles of one binary process in the supercycle divided by the total number of cycles in the supercycle. For instance, when a supercycle for ZnAlxOy ALD consists of 𝑘 Al2O3 ALD cycles and 𝑙 ZnO ALD cycles, the Al2O3 cycle ratio is 𝐶𝑅𝐴𝑙2𝑂3 = 𝑘/(𝑘 + 𝑙), and the ZnO cycle ratio is 𝐶𝑅𝑍𝑛𝑂 = 𝑙/(𝑘 + 𝑙) (such that 𝐶𝑅𝐴𝑙2𝑂3 +𝐶 𝑅𝑍𝑛𝑂 = 1). The cycle ratio can be varied by increasing the fraction of one binary ALD cycle relative to the other, allowing for tuning the composition of the ternary material in terms of the atomic percentage of the constituent metal atoms. A small cycle ratio (𝐶𝑅 ≪ 0.5) indicates a film that consists primarily of a single material with the other material acting as a dopant. The second parameter, the bilayer period 𝑃, is the total number of ALD cycles in a supercycle; in our ZnAlxOy example the bilayer period is equal to 𝑘 + 𝑙. Short bilayer periods are often required for forming a well-mixed film, as will be discussed in Section IV. A larger bilayer period can result in a laminar film with distinct ZnO and Al2O3 layers, which is better described as a multilayered film. The bilayer period also has a significant impact on the deposition of doped materials, with a small enough period being required to ensure optimal dopant distribution throughout the film. Figure 1 summarizes how a small bilayer period can yield a homogeneous film, a large bilayer period can result in a multilayered film, a small cycle ratio (𝐶𝑅 ≪ 0.5) with a small enough bilayer period can give a doped material. For a given cycle ratio, instead of adjusting the total number of binary cycles in each supercycle (the period), it is possible to modify the cycle sequence within the supercycle. For example, when considering a cycle ratio of 0.6, a cycle sequence of A, B, A, B, A could be employed as alternative to A, A, A, B, B. Adjusting

ACS Paragon Plus Environment

6

Page 7 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the frequency of switching between binary processes within a supercycle facilitates the mixing of the components. When depositing ternary materials by ALD, the growth rate and film composition should in theory follow the rule of mixtures,15 shown in Equation 1, 𝐺𝑟𝑜𝑤𝑡ℎ 𝑝𝑒𝑟 𝑠𝑢𝑝𝑒𝑟𝑐𝑦𝑐𝑙𝑒 = 𝐶𝑅𝐼𝑔𝐼 +𝐶𝑅𝐼𝐼𝑔𝐼𝐼 = 𝐶𝑅𝐼𝑔𝐼 + (1 ― 𝐶𝑅𝐼)𝑔𝐼𝐼

(1)

where 𝑔𝐼 and 𝑔𝐼𝐼 are the growth rates of the pure binary processes 𝐼 and 𝐼𝐼, respectively. The rule of mixtures predicts the growth per supercycle to be a linear combination of the two growth rates of the binary ALD processes, weighted with the cycle ratios. Following the rule of mixtures,15 the film composition is calculated as described in Equation 2, 𝜌𝐼 𝑔𝐼 𝐶𝑅𝐼

𝐴𝑡𝑜𝑚𝑖𝑐 𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛 (% 𝑜𝑓 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝐴) = 𝜌𝐼𝑔𝐼 𝐶𝑅𝐼

+ 𝜌𝐼𝐼 𝑔𝐼𝐼 𝐶𝑅𝐼𝐼

× 100%

(2)

where 𝜌𝐼, and 𝜌𝐼𝐼 are the atomic densities of the two binary materials. The resulting composition of the ternary material is predicted to be a mixture of the two pure materials, defined by the cycle ratios and scaled with the densities.25 Both Equations 1 and 2 assume that the growth rates of the individual binary ALD processes are constant throughout the deposition. Figure 2 shows how the cycle ratio and relative growth rates of the two binary ALD processes influence the composition according to the rule of mixtures. It can be seen that when the two binary processes have significantly different growth rates, there is a nonlinear relationship between film composition and cycle ratio. An alternative description of the relation between cycle ratios and composition was reported by Nilsen and coworkers in a series of articles.26–28 Their model expresses the growth rates of the binary components in a ternary ALD process in terms of area coverage of atoms deposited per cycle. This results in an expression for the composition of the deposited film as in Equation 3, 𝑈𝐼 𝐶𝑅𝐼

𝐷𝐼 = 𝑈𝐼 𝐶𝑅𝐼

+ 𝑈𝐼𝐼 𝐶𝑅𝐼𝐼

,

(3)

where 𝑈𝐼, and 𝑈𝐼𝐼 are described as “surface utilization coefficients” with units mol·mm-2·cycle-1 and 𝐷𝐼 is the fractional film composition of component 𝐼. The resulting equation has a similar form as Equation 2, with the main difference that the growth rate is expressed in atoms per area instead of in thickness, ACS Paragon Plus Environment

7


Page 8 of 97

which avoids having to make assumptions about the densities of the components. This facilitates making comparisons to X-ray fluorescence or Rutherford backscattering spectrometry (RBS) compositional analysis (typically measured in atoms per area) of deposited multicomponent ALD films. As in the rule of mixtures, Equation 3 can be used when designing a supercycle recipe to calculate which cycle ratios should be employed to achieve the desired composition. Similar expressions for the case of quaternary films can be found in Refs. 26,28. There are some challenges associated with the supercycle ALD approach. In most reactor designs, a successful multicomponent ALD process requires that the corresponding binary ALD processes must have overlapping temperature windows, otherwise decomposition or lack of reactivity would result in non-ideal ALD growth. Looking to the future, the development of reactor technologies capable of rapid thermal processing and temperature control or spatial separation of precursor/co-reactant dosing could allow for individual cycles to be performed at different temperatures. Importantly, depending on the employed cycle ratio, the mixing of the two ALD-grown materials into a homogeneous thin film is not guaranteed during deposition, a problem in many applications that will be addressed in Section IV and discussed in the specific case of doped materials in Section II.C. Moreover, non-ideal relationships in multicomponent ALD growth rates and composition have been reported for many materials, a topic which will be discussed in more detail in Section V. B. Alternative approaches to the synthesis of ternary films

A variety of alternative approaches to the supercycle method for ALD of ternary materials have been explored, as illustrated in Figure 3, and these are reviewed in this subsection. These methods solve some of the problems that exist within the supercycle paradigm; however, they possess their own disadvantages and chemical complexities. As previously discussed, if the two binary materials do not mix well, the supercycle strategy can result in a multilayered material. A potential solution to this problem is to co-inject the precursors by pulsing the two precursor vapor streams into the reaction chamber together (Figure 3b).29–32 Alternatively, ACS Paragon Plus Environment

8

Page 9 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the two precursors can be dissolved in a specific ratio in a solvent and then injected.33,34 Both of these codosing configurations result in an adsorbed monolayer containing two different precursor molecules, which then can be converted into a mixed multicomponent film by exposure to the co-reactant. The relative concentrations of the two metal atoms in the final film can, in this case, be controlled by varying the amounts of each precursor being dosed. This technique has been employed to synthesize the ternary material AlGaxNy30 and to produce ZnO with various dopants.29,31,32 Comparisons of Al-doped ZnO films deposited by the supercycle method to films prepared by co-dosing led Illiberi et al. to the conclusion that the films deposited using co-dosing have superior electrical and optical properties.31 However, care needs to be taken as the trimethylaluminum (TMA) precursor is approximately 25% more reactive to the surface sites than the diethylzinc (DEZ) precursor, which results in a nonlinear relationship between the film composition and the partial pressures of the precursors.29 The main drawback of the co-dosing approach is that the growth is dependent on competitive adsorption of the two precursors. Consequently, the ability to conformally coat a nanostructured surface with a compositionally uniform film, a hallmark of ALD, is potentially lost if the partial pressures of the precursor gasses are not perfectly homogeneous in complex architectures like trenches. Therefore, co-dosing has been employed mostly for spatial ALD on flat wafers.31,32,35–37 A variation on the co-dosing approach is to dose the two precursor molecules sequentially after each other without performing a co-reactant step in between, which results in an ABC-type ALD cycle. Instead of relying on competitive adsorption, how much of the second precursor adsorbs depends on the coverage of the first precursor and likely on the interaction between the two precursors. So far, this approach has only been investigated in a few studies38–43 as discussed in Section III. Another alternative approach for the ALD of multicomponent films is to use heteronuclear precursors (Figure 3c). Such precursors contain multiple metal centers—or a metal with a metalloid or nonmetal intended to be incorporated into the film (as in examples like WSixNy44 and SrBxOy45) —thereby guaranteeing the mixing of the atoms in the material and avoiding some of the problems inherent with the supercycle method. However, a major drawback of this method is that the composition of the deposited material is fixed by the structure of the precursor molecule. For example, ALD processes using either ACS Paragon Plus Environment

9


Page 10 of 97

Sr[Ta(OEt)5(OC2H4OCH3)]2 or SrTa2(OEt)10(dmae)2 precursors produced SrTaxOy films with little compositional variation,46,47 although the deposition temperature was found to have some impact on the composition of the film. In addition, BiSixOy from Bi(CH2SiMe3)3 and O3 were deposited with a tunable Si/Bi atomic ratio from ~1.5 - 5 by varying the temperature.48 Except for a few cases discussed further in Section III, 33,44–47,49–51 the use of heteronuclear precursors to deposit multicomponent materials by ALD is largely unexplored territory, but requires the development of new precursors. In practice, it will be difficult to develop heteronuclear precursors suitable for ALD, because these molecules also have to satisfy various other requirements to behave as a useful precursor, such as being sufficiently volatile and thermally stable while allowing for self-limiting adsorption. A third alternative approach to ternary ALD of metal oxides introduces the second metal atom as part of the co-reactant molecule. Namely, instead of using a conventional co-reactant such as H2O, a metal alkoxide can be used as the oxygen source (Figure 3d).52 The initial motivation for exploring such processes was to avoid the formation of an interfacial oxide with the substrate by eliminating traditional oxidants. However, since the metal alkoxide acts as both oxygen and metal source, it provides another approach for the synthesis of ternary films if the metal alkoxide co-reactant and the precursor contain different metal atoms. One possible process for this is the reaction between a metal alkoxide and metal halide to produce an oxide; a general scheme for this is shown in Equation 4, 𝑏𝑀(𝑂𝑅)𝑎 +𝑎𝑀′𝑋𝑏⇒𝑀𝑏𝑀′𝑎𝑂𝑎𝑏 +𝑎𝑏𝑅𝑋

(4)

where X represents a halide and 𝑎 and 𝑏 are the oxidation states of the two metal centers, M and M’, respectively. Several metal alkoxide processes were developed by Ritala et al. for ALD of ternary oxides with combinations of Al, Zr, Hf, Ti, and Si metal centers.52 In particular, the ternary materials HfAlxOy and HfZrxOy were observed to produce growth rates higher than traditional ALD, likely due to the incorporation of metal atoms during both half-cycles. Some small incorporation (>8%) of Cl in the films was observed, but the electrical characteristics were nevertheless deemed reasonable. A drawback of this ACS Paragon Plus Environment

10

Page 11 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


approach is that because the two precursors must be alternated in an one-to-one ratio, it provides little opportunity for compositional tuning. C. Approaches to the synthesis of doped films

The preparation of doped films by ALD has been explored for many material systems. Doped thin films are an increasingly important class of materials, as the addition of small amounts of dopants can dramatically affect the electrical or optical properties of a thin film. This makes doped materials invaluable in fields as varied as photovoltaics, optoelectronics, and catalysis.12,53 However the supercycle strategy of ALD can result in films that are not homogeneously doped, but contain concentration gradients.54 ALD-grown doped films are typically prepared by using supercycles with a small cycle ratio ( 𝐶𝑅 ≪ 0.5, see Figure 1c), such that the concentration of one of the components is low while the other functions as the host material. This means that the supercycle contains several binary ALD cycles of one process to deposit the host material and one binary ALD cycle of another process to deposit the dopant. There are several drawbacks to this approach. First of all, the dopant atoms may be spaced too far apart in the direction normal to the film surface. In addition, one cycle of the dopant ALD process may result in a too high density of dopant atoms in a given lateral plane. The effect of the distribution of the dopants on the material properties is captured by the term doping efficiency, which is defined as the number of active charge carriers per dopant atom.55 An increase in doping efficiency results in improved conductance, which can be accomplished either by decreasing defect sites that may trap, recombine, or scatter carriers, or by ensuring that the dopant molecules are within a certain critical distance of each other to promote carrier movement.54,56 Figure 4 illustrates how the distribution of dopant atoms influences the doping efficiency. In the case where the range of the effective field of the dopant atoms is smaller than the distance between two layers of dopant atoms as in Figure 4a, the carriers are trapped near the dopant atoms and cannot be transported across the layers, leading to a low doping efficiency. Conversely, having dopants too closely spaced (dopant clustering, as in Figure 4c) can result in lower doping efficiencies through overlapping effective fields or increased carrier scattering. In between these two cases, there is ACS Paragon Plus Environment

11


Page 12 of 97

an optimum as shown in Figure 4b, meaning that the distribution of the dopant atoms must be tuned accurately. While the heterogeneity of doped films prepared using the supercycle method has led to a variety of interesting architectures including surface-doped PbS-coated silica microwires57 and P-coated V2O5 oxidative catalysts,58 experimental effort has been focused on in situ and ex situ methodologies to produce more homogeneously-doped films. These methods can generally be grouped into two broad categories: (i) post-deposition annealing at temperatures high enough to result in dopant species migration,59–61 and (ii) decreasing the density of dopant atoms deposited per cycle. Annealing is an effective way to disperse dopant atoms within multicomponent thin films grown using ALD supercycles, the effects of which can be tuned by varying the bilayer thickness, the annealing temperature, and the annealing time.59–63 The influence of these parameters on the intermixing during annealing will be discussed in more detail in Section IV. In situ approaches to produce a homogeneously-doped material generally rely on reducing the amount of dopant atoms deposited during the dopant ALD cycle by exploiting the surface chemistry and ligand effects that dominate the in-plane growth of material. When the growth rate of the (one) dopant ALD cycle in the supercycle is reduced, the resulting lateral density of dopant atoms is lower. In addition, the supercycle can contain fewer binary cycles for depositing the host material for a certain volumedensity of dopants, such that the dopant atoms are spaced less far apart in the direction normal to the film surface. Taken together, a lower lateral density and a reduced distance between dopant layers gives a more uniform distribution of the dopant atoms (more closely representing Figure 4b). Some strategies to yield a less dense dopant layer have focused on utilizing sub-saturating dopant precursor exposures,56 but that approach sacrifices the conformality and layer-by-layer growth control of ALD, so we will not address it here. One chemical strategy to control the dispersion of dopant molecules across a surface involves tuning the steric bulkiness of the precursor ligands to achieve the desired density of adsorbed precursor molecules. This method was employed in the synthesis of Er-doped photoluminescent Y2O3, where the dopant distribution is especially important because Er-Er interactions can result in nonradiative decay.64 Tris(2,2,6,6-tetramethyl-3,5-heptanedionato)Er(III), a bulky βACS Paragon Plus Environment

12

Page 13 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


diketonate ligand, was used to spatially separate the Er atoms as they deposit on the surface, producing superior films (showing room-temperature photoluminescence). Similarly, the maximum doping efficiency of Al-doped ZnO was increased from approximately 10% to almost 60% by switching from the common ALD precursor TMA to the bulkier precursor dimethylaluminum isopropoxide (DMAI).65 As illustrated in Figure 5a, films deposited with the bulkier DMAI resulted in more spatially-dispersed Al atoms, and consequently substantially higher film conductivities.65 Although varying the ligands of the dopant precursor molecule can substantially change the amount of dopants deposited per ALD cycle, it is not a feasible option for all desired combinations of dopant metal atoms, dopant concentrations, and host materials. Yanguas-Gil et al. developed an alternative approach for producing homogeneous doped films that does not rely on altering precursor chemistry, but involves preconditioning the surface with so-called inhibitor molecules (e.g. ethanol, acetone) prior to the dopant ALD cycle.66–69 As illustrated in Figure 5b, these inhibitor molecules decrease the number of surface sites available for adsorption of the precursor, thereby influencing the lateral density of the deposited dopant atoms. The approach requires an inhibitor molecule that partly blocks the adsorption sites of the dopant precursor and is resistant to removal by the precursor pulse, while being completely detachable by the co-reactant. ABC-type cycles have been developed in which A is a pulse of the inhibitor molecule, B is a pulse of the dopant precursor, and C is a pulse of a co-reactant that removes both remaining precursor ligands and inhibitor molecules. Such ABCtype processes result in the deposition of a reduced amount of dopants atoms per dopant ALD cycle and, consequently, a higher doping efficiency. This method was successfully demonstrated for Al-doped ZnO using small alcohols like ethanol as saturating inhibitors prior to the TMA dopant pulses.69 The process resulted in a wide range of Al2O3 growth rates and led to Al-doped ZnO films with increased carrier densities. It is of note that the films grown were found to be free of additional impurities from the inhibitor molecules. Further research with a larger set of inhibitors including ketones, nitriles, and carbocyclic acids resulted in promising indications that the method could be applied to other metal oxide ALD processes like ZnO, TiO2, Er2O3, ZrO2, and MgO.68 While this approach offers a versatile and robust method to ACS Paragon Plus Environment

13


Page 14 of 97

increasing doping efficiency, it has the drawback of extending the ALD cycle by an extra step and potentially introducing contamination into the film. To summarize this section and illustrate the differences between ALD doping methodologies and other thin film depositional methods, we revisit the example process of ZnAlxOy. Literature on ZnAlxOy films grown using ALD supercycles have been plagued by low doping efficiencies (i.e. 10% or less).70 However, a variety of the ALD methods reviewed in this section have been employed to overcome this limitation: doping efficiencies can be increased to 60% by choosing larger precursors that optimize interdoping spacing65 and to 70% by co-dosing of precursors.31 For comparison, doping efficiencies of chemical vapor deposition (CVD) grown Al-doped ZnO are upwards of 86%, but it should be noted that the much higher substrate temperature of CVD facilitates a homogeneous profile of dopants in the final film. Few works outside of the field of ALD report performance in terms of doping efficiency, making it difficult to compare doping efficiencies of ALD-grown films to those of other synthesis methods. However, in addition to CVD, using methods like sol-gel or sputtering can result in highly mixed films so it could be predicted that these methods will have higher doping efficiencies. Further discussion of ALD mixing is presented in Section IV.

III.

Overview of ternary and quaternary ALD processes

There is a substantial body of work in the literature that examines a wide variety of ternary and quaternary materials synthesized by ALD. The span of this work is explored in this section, and common trends are discussed. Doped materials are not separately addressed but rather are included as ternary materials. A survey of ternary and quaternary materials synthesized by ALD is presented in Tables 1 to 5. Although many of the materials have been described in multiple reports, for brevity we have identified the reference representing what is to our knowledge the first paper to present the ALD synthesis of each material using a particular ALD process. Some constraints were imposed to limit the scope of the works included to those most relevant to this review. Molecular layer deposition (MLD)71–76 or hybrid ALDACS Paragon Plus Environment

14

Page 15 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MLD71,77–82 processes that use one precursor to deposit an entire molecule—usually an organic—are not covered here. As can be seen in Tables 4 and 5, ALD has been used to synthesize a number of different phosphates, carbonates, oxysulfides, carbonitrides, and other materials with multiple nonmetal elements. Nonmetal elements like carbon, nitrogen, and halogens are also common impurities found in ALD processes,83–91 so we have only included ALD processes here that were clearly designed such that these elements are intentionally incorporated as significant components of the films. Included in each process entry are the precursors and co-reactants, deposition temperature, postdeposition annealing conditions to yield crystallinity, and growth rates. The tables are organized alphabetically by metal atom contained in the film. Most ternary compounds synthesized by ALD have two metal atoms, A and B, and can be written as either ABxCy or BAzCy, so for brevity every material appears in the tables only once and is not duplicated in the sections corresponding to every constituent atom. Growth rates for supercycle processes can be reported in terms of growth per supercycle or growth per binary subcycle, and therefore for comparison all growth rates in Tables 1 to 5 are reported on a per binary subcycle basis. Many of the precursor ligands referenced are simple enough to include their entire chemical formula directly, but some ligands have been abbreviated. Me, Et, Pr, Bu, Ph, and Cp are used as abbreviations for methyl, ethyl, propyl, butyl, phenyl, and cyclopentadienyl groups, respectively, and all other abbreviations are given in Table 6. A. Categories of multicomponent processes Ternary metal oxides, consisting of two different metal atoms alloyed with oxygen, are by far the most prevalent materials in the class of ternary materials, and they are detailed in Table 1. The utility of ternary metal oxides in a variety of applications lends them well to multiple frontiers of research, and the number of studies exploring ternary metal oxides has grown dramatically in the last decade. Ternary metal nitrides, presented in Table 2, have been a much more recent development in the field of ALD. Thus far their applications have been primarily limited to the areas of optoelectronics92–95 and diffusion barriers,44,96–101 and the majority of the studies included in the tables have been published in the past five to ten years. ACS Paragon Plus Environment

15


Page 16 of 97

Even more recent has been the exploration of ternary metal sulfides by ALD, which are included in Table 3. Other ternary compounds such as Group III-V semiconductors, tellurides, selenides, phosphides, carbonates, and oxysulfides have been explored via ALD, but the body of work on these materials is much smaller, as can be seen in Table 4. Note that ternary oxides, nitrides, and sulfides containing metalloids (e.g. Si or Ge) or two nonmetals (e.g. carbonates or phosphates) instead of two different metal atoms are included in Table 4. Finally, ALD has also been used to synthesize quaternary materials, as listed in Table 5. These materials typically contain oxides, nitrides, or sulfides of three combined metal atoms, allowing for an additional dimension of compositional tuning to affect the properties of the resultant film. However, precise compositional control of these materials has proven challenging in many cases, particularly because the combination of three binary ALD processes provides many more opportunities for chemical phenomena to give rise to non-idealities in growth. We found only two examples of quinary (i.e. fivecomponent) ALD processes in the literature, one studying crystallization and diffusion in perovskites of KNawNbxTayOz, and the other investigating crystallization phenomena in solid electrolytes of LiLawZrxAlyOz.102,103 This sparse coverage could result from the fact that the process complexity and opportunity for non-ideal behavior observed in ternary and quaternary processes is present to an even greater extent in quinary processes. B. Applications The ternary and quaternary materials described in the tables cover a wide variety of applications, but several are of particular note. A large number of materials have been studied for use as high-k dielectrics,48,104–107 particularly Hf-,86,108–114 Ti-,115–122 and La104,123–127-containing compounds. Many of these studies have been dedicated to examining compositional effects of various metal atoms in ternary oxides in order to find materials with higher dielectric constants. Ultrahigh-k dielectric materials (e.g. SrTixOy) have also been studied extensively for memory applications.34,46,48,98,114,115,117,118,125,126,128–132 In addition, a number of materials are being explored for usage in buffer layers,90,100,128,130,133,134 and ACS Paragon Plus Environment

16

Page 17 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


diffusion barriers97–101,135–138 in electronic device manufacturing. Finally, of particular recent interest have been Li-containing compounds for application in Li-ion batteries as electrodes or solid electrolytes.39,89,139–145 This development is especially interesting given the increasing demand of Li-ion batteries and similar technologies, as well as the growing safety concerns of associated battery fires that could be addressed using solid electrolytes.89,135,139–141 Given the useful contributions the field of ALD has made in providing materials for these applications, together with the continued importance of effective energy storage and more compact processing devices,13,146,147 further development here is warranted to take advantage of these opportunities. C. Processes and precursors Several general observations can be made about the ALD strategies used to synthesize the ternary and quaternary films tabulated in Tables 1-5. Figure 6 illustrates a summary of all the ternary metal oxide processes included in Table 1. Across all of the metal atoms used, Al, Ti, Hf, and Zn are some of the metals employed to make the greatest number of different materials. This is perhaps due to the fact that these constituent binary processes are relatively popular, well-understood, and well-behaved. When processes are rendered more complex by combining the binary processes into ternary or quaternary processes, it is advantageous to have well-defined building blocks. Apart from these metal atoms, it can be seen in Figure 6 that La and Li are also used in a number of ternary and quaternary ALD studies. Neither process is as well-behaved, with Li2O and La2O3 both being hygroscopic126,148 and frequently requiring O3 to oxidize ligands in the ALD process. Even with O3 as the oxidizing agent, ternary oxides containing Li or La have been observed to contain carbon contamination via the formation of carbonates.88 La-based ALD is used nonetheless because of its promising applications in microelectronics and because the La atom size lends itself to forming perovskite crystal structures with other metal atoms.26,104,149 Figure 7 summarizes the ternary materials other than metal oxides synthesized thus far by ALD. Most of these are nitrides, sulfides, and phosphides, and Al and Zn are the most commonly used metals in them for similar reasons as stated above. ACS Paragon Plus Environment

17


Page 18 of 97

In the majority of cases found in Tables 1-4, the supercycle scheme is used to combine two separate binary ALD processes that share one element (e.g. O, N, S) into a single ternary process. Some studies implement deposition with heteronuclear precursors to introduce more than one atom into the film with a single precursor,33,40,44–46,48,50,51,150–168 and others eschew the supercycle pulsing strategy in other ways29,33,34,36–38,169–172 as discussed in Section II, but these are far less common amongst the works shown in the tables. Regarding the use of these alternative approaches, Si-containing compounds are of particular note as SiO2 itself is often difficult to grow on its own.34,40,44,48,106,173,174 A number of studies have successfully deposited Si-containing materials by ALD by using precursor strategies other than supercycles. These strategies include co-dosing multiple precursors simultaneously (Figure 3b),34 combining Si with another element in a heteronuclear precursor (Figure 3c),44,48,150,167,168 or using an alkoxide-containing precursor with no separate co-reactant (Figure 3d).40,52,175–185 These approaches have allowed for the synthesis of Si-containing compounds that might have otherwise been very challenging to grow via ALD, illustrating that these strategies are worth exploring for other interesting but difficult chemistries. Because of contamination concerns, however, the vast majority of the ternary and quaternary processes outlined above avoid halogenated precursors or precursors containing elements other than C, N, H, and O, which can limit the options for precursor chemistries in multicomponent processes. The choice of precursor in these multicomponent processes also depends on their relative reactivities. One can gain insight into the reactivities of many precursors by examining the ligands. In contrast to O3 and O2 plasma, H2O is not an oxidizing agent and is a source of protons, so ligand elimination can occur via a ligand exchange reaction, replacing the ligand with a hydroxyl group and releasing a protonated form of the ligand. In Tables 1-5, clear trends can be observed as to which precursors are sufficiently reactive to be used with H2O in a multicomponent ALD process. With few exceptions—like Mn(Cp)2186—most precursors with alkyl, phenyl, or amine ligands are used with H2O, whereas most precursors in the tables with cyclopentadienyl or diketonate ligands require O3 or O2 plasma to combust the ligands instead. While pKa is not a perfect descriptor of energetics in these ALD systems, alkyl chains, amines, and phenyl groups have significantly higher pKa values (≈35-50) than ACS Paragon Plus Environment

18

Page 19 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cyclopentadiene or beta-diketones (≈5-16).187,188 This in turn corresponds to a greater stability of the protonated eliminated alkyl, phenyl, and amine ligands and is reflected in the relative prevalence of H2O as the oxygen source with these precursors. An additional explanation for the difference in reactivity is that the negatively charged cyclopentadienyl and beta-diketonate ligands contain stabilizing aromaticity or resonance that must be broken in order for the ligand to be protonated and eliminated. As seen in Tables 1-5, however, halogenated and alkoxide precursors are almost always used with H2O despite the protonated ligands having a relatively low pKa (≈-8-17).187,188 By examining the processes listed in the tables where this is the case, it is apparent that these ligands are usually used with s-block (like LiOtBu or KOtBu), p-block (as in Si(OEt)4 or SnCl4), or early transition metal elements (such as TiCl4, VO(OiPr)3, or HfCl4) that tend to have a strong affinity for oxygen.189 This strong driving force for the formation of the oxide can be one of the reasons these classes of precursors nonetheless are used with H2O in multicomponent ALD processes. Furthermore, whereas HX (where X is a halogen) is a stable species that can be released upon reaction of a halogenated precursor with H2O, there is no analogous product of combustion of halogens by a co-reactant such as O3. While these examples come specifically from ALD literature, these principles for precursor reactivity and multicomponent processes can also be extended to a more general class of vapor phase processes. Given these trends in precursor reactivity, it can be seen in Tables 1-5 that when combining binary processes into a supercycle process, the precursors are frequently chosen such that the same co-reactant can be used in each binary process. This is particularly true for sulfides, where H2S is predominantly used. However, despite the flexibility in choice of oxygen source, processes are typically designed such that the two precursors have similar reactivities and thus can both be reacted with the same co-reactant, thereby simplifying the multicomponent process. This preference also arises because it is usually necessary for the temperature windows of the two constituent binary processes to be aligned. Two precursors of similar chemical structure have the potential to grow via comparable chemical reactions requiring similar conditions. Even in cases where the two binary precursors are quite different, some studies instead use O3 or O2 plasma for both processes if one of the constituent processes does not necessitate the use of a ACS Paragon Plus Environment

19


Page 20 of 97

stronger co-reactant. It has been shown that in some cases if a weak oxidizing agent is used, ligands may be left behind during ALD cycles, causing significant contamination in the resultant films. On the other hand, a stronger co-reactant can assist in fully removing ligands and activating the surface with reactive functional groups.87–89,190 However, this approach is not possible in some cases, e.g. with halogenated precursors, and materials like La2O3 or Li2O that prove incompatible with certain oxidizing agents as discussed above. These limitations are also discussed in more detail in Section V. Additionally, ternary and quaternary nitrides often use different plasmas containing NH3, N2, and/or H2 for different binary processes within the same ternary process.92,93,97,137 D. Process conditions and characterization

The breadth of ALD literature on multicomponent materials also covers a variety of process conditions and characterization strategies. In many of the works presented in Tables 1-5, the development of supercycle recipe typically begins with the characterization of the two binary ALD processes. In most studies presented here reporting supercycle ALD processes, it is assumed but not confirmed that the saturation conditions are the same as during the individual binary ALD processes. However, in a supercycle one must consider the additional factor of adsorption of precursor α (employed to deposit material A) on the material that is deposited with the other binary process, B, and vice versa. The surface of material B will be different from the surface of material A in active site density, roughness, or surface energy, and consequently the adsorption of a precursor may not reach saturation in the ternary process under the same conditions as in the binary process. Therefore, four additional saturation curves must be measured: precursor α on material B, precursor β on material A, and the same thing again for the coreactant half-reaction. In practice, however, it can be difficult to carry out such an extensive characterization of the ALD process (by measuring eight saturation curves). It has also been shown for several ALD processes that the reactor wall can influence the reactions occurring on the sample surface.191,192 While this can be avoided for binary processes by pre-coating the reactor wall, this is less effective for multicomponent supercycle ALD processes because the condition of the wall changes


20

Page 21 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


continuously during the deposition due to the switching between two binary ALD processes. In the absence of characterization to elucidate these effects, the ALD processes may be run under non-optimized conditions (e.g. sub-saturation) which can lead to deviation from the expected growth characteristics. Finally, the effect of temperature—which tends to be around 200-250°C—is rarely explored, and when it is, these more complex materials do not often show the presence of an ALD temperature window.38,87,88,169,193,194 These many potential sources of non-idealities and complex behavior makes studying multicomponent systems challenging. Finally, the characterization methods used to study the materials presented in Tables 1-5 tend to be ex situ techniques, with X-ray photoelectron spectroscopy (XPS), RBS, spectroscopic ellipsometry (SE), and X-ray reflectivity (XRR), being the most common. While these are often simpler to implement than many in situ techniques, they also can be limited in their efficacy for characterizing non-ideal process behaviors. In situ techniques such as XPS, SE, quartz crystal microbalance (QCM), Fourier transform infrared spectroscopy (FTIR), quadrupole mass spectrometry (QMS), and X-ray diffraction (XRD) can probe aspects of the ALD reactions themselves that are difficult or impossible to obtain via ex situ means. The rise in multicomponent ALD materials whose syntheses can often contain increasingly complex chemical interactions thus further stress the need of in situ analysis for efficient and complete understandings of the depositions of these materials.

IV.

Intermixing and phase formation

In this section, a deeper discussion of the distribution of atoms in the deposited material and the corresponding role of deposition temperature, bilayer period, and annealing conditions is presented. These topics provide insights into important characteristics of ALD-grown multicomponent materials. The formation of specific crystalline phases is also covered. A. Intermixing


21


Page 22 of 97

Most supercycle recipes result in homogeneously-mixed films, considering that only a sub-monolayer of material (< 2 Å in thickness) is deposited per binary ALD cycle. However, as compared to other techniques such as physical vapor deposition (PVD), the temperature employed during ALD is relatively low (100 – 400 °C), which limits the mobility of atoms after they are deposited on the surface. Therefore, in some cases multilayered structures can emerge instead of a homogeneously-mixed film. The scope of this discussion on intermixing is limited to ternary films prepared with the use of the supercycle strategy. Whether or not the two binary components of a ternary film mix depends strongly on the materials being deposited and on the deposition conditions (temperature, bilayer period, etc.). In general, intermixing can be achieved by using small bilayer periods, but a potential drawback of this approach is that a limited set of compositions can be achieved due to the number of subcycles being restricted to a small number per bilayer. Intermixing may become an issue in cases where utilizing small bilayer thicknesses is not possible or when energetic effects result in phase segregation.98,99,101,136 For the former case, intermixing may be achieved by diffusion. As discussed by Thimsen et al., the ratio of the diffusion length of the mobile species of interest into the adjacent sublayer and the thickness of the adjacent layer will provide an estimate of the degree of intermixing.195 This quantity is known as the mixing number and is defined in Equation 5195 𝐿𝑖

𝑀𝑗 = 𝑡𝑗 =

𝐷𝑖𝜏 𝑡𝑗

,

(5)

where 𝑀𝑗 is the mixing number in layer 𝑗, 𝐿𝑖 is the diffusion length of the mobile species 𝑖, 𝑡𝑗 is the thickness of the adjacent layer, 𝐷𝑖 is the diffusion coefficient of the mobile species in the adjacent layer, and 𝜏 is a characteristic time scale. Different cases representing the intermixing of a multilayered film deposited by ALD are illustrated in Figure 8. A general film is considered with sublayer thicknesses tA and tB, as prepared using the supercycle approach. The diffusion length LA describes the diffusion of metal atom A in BCy and LB describes the diffusion of metal atom B in ACx. Diffusion between the layers must occur in order to obtain ACS Paragon Plus Environment

22

Page 23 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a well-mixed film. Preferably, both metal atoms are mobile, meaning that two mixing numbers can be considered, MA and MB. The mixing numbers give an indication of how thin the sublayer thicknesses needs to be to ensure a well-mixed film. If 𝑀𝑗 ≫ 1 for a specific species, diffusion of that species into the adjacent sublayer readily occurs, which results in a well-mixed film.139,140,195,196 If 𝑀𝑗 ≈ 1, there is a gradient in concentration of the species at the interface, and if 𝑀𝑗 ≪ 1, the interface between sublayers is sharp and well-defined.197,198 These regimes are illustrated in Figure 8. Case 1 illustrates the regime in which both species have small diffusion lengths, resulting in a multilayered structure with sharp interfaces. Cases 2 and 3 illustrate the result that is obtained when only one of the two species has a small diffusion length, leading to regions of intermixed and of sub-stoichiometric material. Case 4 demonstrates the desired result of a well-mixed ternary material, which is obtained when the diffusion lengths of both mobile species are long compared to the sublayer thicknesses. In studies of HfAlxOy197 and TiAlxOy199 ALD systems, the effect of the bilayer period on intermixing was investigated. For HfAlxOy, the authors found that Hf and Al intermixing occurs on the scale of approximately 5 Å, which is roughly equal to the thickness of a bilayer prepared by 4 cycles of HfO2 and 4 cycles of Al2O3 ALD. When the bilayer thickness was increased, variation of composition with depth was detected, indicating a nanolaminate structure, similar to Case 1 in Figure 8. Similar results were obtained for multilayers of TiO2 and Al2O3.199 It was found that the bilayer thickness needs to be reduced below 8 Å to obtain a mixed TiAlxOy film. In cases where intermixing at the deposition temperature is not achieved, the primary method to increase intermixing is to increase the temperature of the deposition (if possible) or to perform a postdeposition anneal, as the diffusion coefficient is a strong function of temperature and in general can be modeled by the Arrhenius relationship shown in Equation 6, 𝐷 = 𝐷0𝑒

𝐸𝐴 𝐵𝑇

―𝑘

.

(6)

where 𝐷0 is the maximum diffusion coefficient, 𝐸𝐴 is the activation energy, 𝑘𝐵 is the Boltzmann constant, and 𝑇 is the temperature. ACS Paragon Plus Environment

23


Page 24 of 97

An example of how temperature can have a significant effect on the degree of mixing can be seen in study by Thimsen et al., where the degree of intermixing between a trilayer sulfide stack of ZnS, SnS2, and Cu2S deposited at 135 °C was investigated.195 The evolution of intermixing between the trilayer stack was measured using TOF-SIMS as a function of the annealing temperature. The depth profile of the asdeposited films indicated a diffuse interface between Cu2S and SnS2, whereas the interface between ZnS and SnS2 was much sharper. The diffuse interface arises because the diffusion length of Cu in SnS2 and Sn into Cu2S is on the same order of magnitude as the sublayer thicknesses. In the case of the sharp interface between ZnS and SnS2, the diffusion lengths of Zn in SnS2 and Sn in ZnS are an order of magnitude smaller than the sublayer thicknesses. Upon annealing the trilayer stack, substantial intermixing was observed. A temperature of 275 °C was required to obtain uniform Sn and Cu levels throughout the film, while 425 °C was needed to achieve a uniform compositional profile of Zn. While temperature is a major factor in determining diffusion behavior, atomic size and polarizability also play a role. For example, metal sulfide systems tend to have higher metal atom diffusion rates compared to metal oxide systems. 200,201 Sulfur is larger in size as compared to oxygen resulting in a larger lattice and higher polarizability. The larger lattice decreases repulsive overlap energy and the higher polarizability results in a decreased polarization energy as metal atoms move through the lattice.200,201 An example of the effect of the size and polarizability of cations on diffusion was studied for Fe, Co, and Ni metal atoms diffusing in a MgO lattice, and it was found that decreasing the ratio of the ionic radius to polarizability of the diffusing cations lead to faster diffusion.202 Because diffusion in metal oxides primarily occurs through defects or vacancies in the material, the overall diffusion rate of metal atoms in a material system can be influenced by crystal structure, composition, and defect concentration.203 B. Phase formation In ternary materials, multiple phases are often thermodynamically stable, but in many applications one specific crystalline phase is preferred to achieve the desired properties. Ternary or quaternary ACS Paragon Plus Environment

24

Page 25 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


materials grown by ALD are most commonly amorphous as-deposited, although this microstructure is less common for nitrides92–95,204 and for processes performed at higher temperatures.38,95,138,169,205–209 Consequently, most films must be annealed in order to induce crystallinity. Some general strategies to control the phase of multicomponent films deposited via ALD which will be discussed here include studying the bulk phase diagram of the material to appropriately tune composition and annealing conditions, tailoring atomic size and temperature to affect diffusion, and optimizing the supercycle recipe. Firstly, the bulk phase diagram can be used to ascertain which cycle ratio (to tune the composition) and annealing conditions (to tune mixing and phase) need to be employed. In most cases, ALD of a film with the correct stoichiometry does not directly result in a crystalline phase, and a post-deposition anneal is

required

(this

can

also

be

seen

in

Tables

1-5).26,33,44,46,86–88,90,96,104,107–114,117,119,121–

123,125,128,129,131,132,137,139,141,148,149,190,197,198,210–240 Typical annealing conditions are under nitrogen or oxygen

atmosphere or in air, and the temperature required to begin crystallization is generally between 600°C and 800°C, with the required temperature often varying depending on the composition of the film. However, while the phase diagram provides thermodynamic information on the bulk system, it has been shown that it often does not rigorously predict the formation of phases in ALD films.216,241 This phenomenon has been hypothesized to be a result of the layer-by-layer growth character of ALD, or of bulk material data not applying to the nanometer thickness scale of ALD films, but further investigation is required to unravel the effects. Discrepancies from the phase diagram are likely the result of strain and substrate effects in thin films, and these phenomena may also be present in other thin film, chemistry-based techniques. For ternary ALD, an example of the discrepancy between the thermodynamically-predicted and the experimentally-obtained phase was observed in ZnTixOy ALD in a study by Borgese et al.216 The films were deposited at a low temperature of 90 °C in an effort to deposit amorphous films, and then annealed at a mild temperature of 600 °C. Films with high Ti content behaved as predicted, forming TiO2 and ZnTiO3, but as the Zn content was increased only the inverse spinel Zn2TiO4 phase was observed. This observation is in contrast with the bulk phase diagram, which predicts ZnO or ZnTiO3 to coexist with the inverse spinel phase Zn2TiO4. Ageh et al. found that when the deposition temperature was increased to ACS Paragon Plus Environment

25


Page 26 of 97

200 °C, the ZnTiO3 phase was observed in as-deposited films with a high Zn content.217 Upon annealing, these films exhibited either only ZnTiO3 or a mixture of ZnTiO3 and the inverse spinel Zn2TiO4, which is in better agreement with the bulk phase diagram. Here, it is apparent that not only do films prepared by ALD supercycles and post-deposition annealing sometimes differ in phase from what the bulk phase diagram depicts, but also that the deposition conditions play a role. A second strategy for obtaining the desired crystalline phase of a multicomponent ALD film is to choose the deposition and annealing temperatures based on the atomic size, as predicted from fundamentals. Crystallization occurs through two dominant processes: nucleation of crystallites and growth of those crystallites. Based on classical nucleation theory,242–247 the driving force for nucleation is the added stability of the new crystalline phase, but there is an energy barrier to form stable nuclei (noted here as Δ𝐺 ∗ ) and a barrier for atoms to move through the solid and attach to the growing nucleus (noted here as Δ𝐺𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡). To provide a simplified view of the processes taking place, the rate of crystal growth can be considered with the Arrhenius relationship of Equation 7,

(

𝐶𝑟𝑦𝑠𝑡𝑎𝑙 𝐺𝑟𝑜𝑤𝑡ℎ 𝑅𝑎𝑡𝑒 ≈ 𝐴(𝑇)exp ―

𝛥𝐺 ∗ 𝑘𝑏𝑇

)exp ( ―

)

𝛥𝐺𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡 𝑘𝑏𝑇

(7)

where Δ𝐺 ∗ and Δ𝐺𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡 are both temperature-dependent.242–247 In as-deposited films, the growth rate of crystals is limited either by the barrier to form stable nuclei or by the transport barrier. The factor including Δ𝐺𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡 has a dependence on temperature similar to that of the diffusion coefficient (Equation 6), meaning that conditions favoring crystallization also lead to improved intermixing. Consequently, the formation of specific crystalline phases is often studied in the literature in order to investigate the intermixing of the material. The importance of Δ𝐺 ∗ and Δ𝐺𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡 in phase formation can be seen in a study by Myllymäki et al.,104 where ALD of a variety of rare earth scandate films was investigated. The study illustrated that with increasing metal atom size, the required temperature to form crystalline phases increased. For the smallest metal atom Lu, crystalline LuScxOy films with the cubic C-type phase were obtained as-deposited at a deposition temperature of 300 °C. The remaining films were amorphous as-deposited and required an ACS Paragon Plus Environment

26

Page 27 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


annealing step to achieve crystallinity. ErScxOy and YScxOy, containing the next larger metal atoms, required annealing to 600 °C and 1000 °C, respectively, while the rest of the rare earth scandate films needed to be annealed in the temperature range of 800-1000 °C. In the rare earth scandates, the C-type phase becomes more favorable with decreasing cation size, thus decreasing Δ𝐺 ∗ . In addition, decreasing ionic radius of the diffusing cation should result in decreased repulsive overlap energy, thus decreasing Δ 𝐺𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡. It has been reported for some material systems with fast diffusion kinetics that crystalline phases can form in the as-deposited films,38,139–141,196,219 but it is not guaranteed. For example, because of their small size, Li atoms typically have the property of long diffusion lengths in ternary materials as observed for LiCoxOy,139 LiAlxOy,196 LiTixOy,141 LiNbxOy,248 and LiMnxOy.140 However, whereas ALD of LiMnxOy140, LiVxOy140 and LiAlxOy196 at 225 °C have been reported to result in crystalline as-deposited films,196 ALD of LiCoxOy at 325 °C,139 and LiTixOy141 and LiNbxOy248 at 225 °C resulted in amorphous materials. This indicates that having high-mobility Li as one of the components of a ternary material may make the growth of crystalline phases more facile; however, barriers to formation of stable nuclei may still limit crystallization. In the case of hindered nuclei formation, a seed layer or favorable substrate may be used to initiate crystal growth.249,250 Finally, in ternary ALD, the construction of the ALD supercycle plays an important role in phase formation. Two approaches that have been used in constructing the supercycle are i) minimizing bilayer periods to obtain mixed as-deposited films (with bilayer thicknesses of approximately 2 nm) known as superlattices which are then annealed to enable intermixing and crystallization. 251–256

Both approaches are capable of producing high quality crystalline films. The first approach, which

is more commonly employed, may produce crystalline films without an annealing step, but in most cases is insufficient (see Table 1). The second approach addresses the two major drawbacks of the first approach: it enables fine tuning of composition, while minimizing nucleation effects (see Section V) due to the use of larger bilayer periods. BaTiO3,251 SrTiO3,252 and BiFeO3255 have been grown with the ACS Paragon Plus Environment

27


Page 28 of 97

superlattice approach resulting in single phase films. However, a potential drawback of superlattices is that it may result in incomplete mixing and undesired phases.253,254 An example of these unfavorable effects as a result of large bilayers can be seen in a study of ZnSnxOy (ZTO) ALD by Hägglund et al. where the effect of the bilayer period on the crystallographic phase after annealing at 800 °C was investigated.253 It was found that large bilayer periods lead to crystalline regions of ZnO and SnO2 but that films with shorter bilayer periods showed mostly ternary Zn2SnO4 diffraction patterns, the expected phase at this annealing temperature. The results highlight the importance of intermixing, by demonstrating that films with larger bilayer periods can crystallize into distinct phases (resembling Case 1 in Figure 8) rather than a single mixed phase. Systems in which sufficient diffusion is achieved at the crystallization temperature are better suited for the superlattice approach. Literature studies on this approach are quite limited however, so further research in this area should be pursued to develop ALD superlattice strategies.

V.

Nucleation effects during the growth of ternary materials

This section will investigate the various effects taking place during the deposition of ternary ALD films that can give rise to non-ideal growth behaviors. When two binary ALD processes I and II with growth rates 𝑔𝐼 and 𝑔𝐼𝐼 (Figure 9a) are brought together in a supercycle, the combined growth rate for the ternary ALD process is ideally expected to be the linear combination of the growth rates 𝑔𝐼 and 𝑔𝐼𝐼 (Figure 9b), as discussed in Section II. However, in practice, the overall growth rate of the ternary ALD process is often altered. This can be due to a nucleation delay when switching from one binary ALD process to the other (Figure 9c), or due to an increase in active site density on one material over the other (Figure 9d). Such effects would lead to films that contain less or more of one of the binary components than expected. In addition, both cation and anion exchange reactions have been observed during ALD, which can also lead to nucleation effects and deviating compositions. As mentioned in Section III, most of the ternary ALD studies reported in the literature that examine growth characteristics show growth that deviates from the rule of mixtures, and a growth curve similar to the schematic illustration of Figure 9b ACS Paragon Plus Environment

28

Page 29 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


appears to be an exception rather than the rule. Furthermore, as discussed in Section III, most studies do not characterize saturation in the multicomponent process, potentially introducing additional nonidealities from under-saturated conditions. In this section, the different explanations that have been put forth in the literature for these deviating trends will be discussed, grouped into three subsections: surface site density and reactivity, exchange reactions, and persisting species. The focus of this section is on ternary ALD materials (including doped materials), but similar effects also play a role during the growth of quaternary materials. In addition, a case study on Al-doped ZnO is presented, illustrating that several mechanisms causing deviating trends can occur at the same time. At the end of this section, we will also provide some general guidelines for how to avoid nucleation effects when designing a supercycle recipe. A. Surface site density and reactivity Differences in surface reaction site density between the two binary materials that form a ternary material have been pointed to as an explanation for the lower growth rate of ternary ALD processes. Tanskanen et al. performed density functional theory (DFT) calculations to investigate the modulation of reaction site density for the case of ZnSnxOy (ZTO) ALD from tetrakis(dimethylamido)tin (TDMASn), DEZ and H2O.257 For this ZnSnxOy ALD process, it was reported in an earlier study that there is a growth delay when ZnO is carried out after SnO2 ALD.258 This growth delay leads to a lower effective growth rate of the ternary ZnSnxOy process and films that are Sn-rich.258 For ZnO ALD, it has been established that the DEZ precursor adsorbs at a surface hydroxyl group by releasing one of its two ethyl ligands.259 Subsequently, one hydroxyl group is regenerated during the H2O pulse when the other ligand is eliminated. In contrast, with its four ligands, the TDMASn precursor might release one, two, or three ligands upon adsorption during the TDMASn pulse. The number of ligands eliminated during the precursor pulse has a large effect on the hydroxyl group density after the H2O pulse. When one ligand is eliminated, three hydroxyl groups are formed, which increases the number of reaction sites by two. Elimination of two ligands has no effect on the hydroxyl group density, while the elimination of three ligands reduces the number of hydroxyl sites by two. From the DFT calculations, it was concluded that ACS Paragon Plus Environment

29


Page 30 of 97

the number of eliminated ligands during the precursor step is close to (and not larger than) 3. Consequently, the surface reaction site density is reduced, which may contribute to the nucleation delay for ZnO ALD after SnO2 ALD. Elliott and Nilsen described a more general model for the growth of ternary materials,260 which can be considered as an extension of the model of Nilsen and coworkers described in Section II.149 Depending on the reactivity of the ligands, two different mechanisms were distinguished. When dealing with relatively inert ligands or when no hydroxyl groups are present on the growing surface, it can be assumed that no ligands are eliminated during the precursor pulses.260 Even for this simple case, there can be a nonlinear relationship between composition and the cycle ratio if there is a difference in metal cation charge in the two ALD precursors. For example, when homoleptic Mn(thd)3 and Ca(thd)2 precursors (charge on Mn is 3; Ca is 2) are used for CaMnxOy ALD,26 comparatively more Ca atoms are deposited per adsorbed thd ligand because of its lower charge, meaning that the deposited film should be Ca-rich as compared to a linear dependence on cycle ratio.260 The second mechanism also takes into account that for every ligand eliminated during the co-reactant pulse (e.g. by combustion), one hydroxyl group is formed, similar to the ZnSnxOy study described above. Unlike in the first mechanism where no ligands are eliminated during precursor pulses, these hydroxyl groups can subsequently serve as a source of protons for Brønsted elimination of ligands during the precursor half-reaction. The relative inertness of some ligands towards proton-assisted elimination as described in Section III was accounted for by defining a fraction of the hydroxyl groups that leads to Brønsted elimination as a fitting parameter. The model for the second mechanism was used to explain the relationship between composition and cycle ratio for previously reported data for LaFexOy and SrFexOy.149,260 Murray and Elliott proposed an alternative mechanism for the growth of ternary materials which also takes a ligand exchange process into account.261 The deposition of a ternary material involves two different metals, here referred to as M1 and M2. When metal oxide A is deposited on the surface of metal oxide B, it is possible that a ligand exchange reaction takes place from the absorbed M1-containing precursor to an M2 species at the surface. As a result, the ligand becomes bonded to an M2 surface atom. ACS Paragon Plus Environment

30

Page 31 of 97 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This ligand exchange reaction has an effect on the reactivity for the elimination of ligands by H2O because the elimination of ligands from the surface metal atom M2 has to be considered instead of from the metal center M1 of the incoming precursor. An interesting example highlighted by Murray and Elliott is the ALD of LaMnxOy, using La(thd)3 and Mn(thd)3 precursors and O3 as co-reactant. The binary ALD process for MnO2 was found to not be self-limiting; however, when combined with LaOx ALD, it functioned as a good ALD process for the deposition of ternary LaMnxOy.26 This is consistent with a proposed growth mechanism involving the transfer of thd ligands from Mn to La, which is supported by DFT calculations. Interestingly, this example illustrates that in some cases, it is not essential to use well-behaving binary processes as the starting point for the development of a ternary ALD process. In order to evaluate whether similar ligand exchange reactions play a role for other ternary ALD processes, additional DFT calculations should be carried out for a larger range of materials. B. Exchange reactions In several studies, exchange reactions that significantly influence the composition of the deposited film have been observed. Thimsen et al. reported on ion exchange during the growth of multilayers of Cu2S and ZnS.262 Unexpected mass changes were observed during the exposure of Cu2S to DEZ or during the exposure of ZnS to Cu2(dsbaa)2 precursor, and this was attributed to the following ion exchange reactions: 𝐶𝑢2𝑆(𝑠) +𝑍𝑛(𝐶2𝐻5)2(𝑔)→𝑍𝑛𝑆(𝑠) +2𝐶𝑢(𝑠) + 𝐶4𝐻10(𝑔)

(8)

𝑍𝑛𝑆(𝑠) + 𝐶𝑢2(𝑑𝑠𝑏𝑎𝑎)2(𝑔)→𝐶𝑢2𝑆(𝑠) +𝑍𝑛(𝑑𝑠𝑏𝑎𝑎)2(𝑔)

(9)

Equation 8 describes that when a sublayer of Cu2S is exposed to DEZ, two Cu+ cations in the layer exchange for Zn2+ from the incoming precursor resulting in ZnS formation. The Cu ions are then reduced to metallic Cu. The overall pathway of Equation 9 does not involve a redox reaction and instead shows how the Zn in the sulfide layer is replaced by Cu, while Zn leaves the surface as volatile Zn(dsbaa)2. Additional ellipsometry analysis suggested that a 54 nm thick ZnS film completely transforms into a 64 nm thick Cu2S film during exposure to a low partial pressure (> 2). (c) A doped material is obtained for small cycle ratios (CR > 2). (c) A doped material is obtained for small cycle ratios (CR

Synthesis of Doped, Ternary, and Quaternary Materials by Atomic

Recommend Documents