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Complex Materials by Atomic Layer Deposition Adam M. Schwartzberg* and Deirdre Olynick are many excellent review articles coving the basics of the ALD process; therefore, only a brief overview will be given here.[2] The process, as shown in Figure 1, involves precursors that are highly reactive with surface species (such as hydroxyl groups), but inert to self-reactivity at the ALD temperature. In this way, as soon as the substrate has reacted completely with the ALD precursor and saturated any available chemistry, the reaction ceases, despite the presence of excess precursor, thus producing monolayer coverage and conformality. In the case of metal oxide ALD, the process begins with a hydroxyl-terminated surface and the introduction of a gasphase metal precursor (Figure 1a). When a precursor molecule encounters the surface, a rapid chemical reaction displaces one of the precursor ligands (nominally organics or halides) and a proton from the surface, resulting in a strong metal–oxygen bond. Over time, depending on the temperature, concentration, and reactivity, the surface will saturate and chemistry will cease. The deposition chamber is purged of reactive species, and the second reactive precursor is introduced (Figure 1b). This is commonly water, hydrogen peroxide, or ozone for the thermal ALD of metal oxides, but plasma-excited oxygen radicals are also used in plasma-assisted metal oxide ALD. Moreover, ammonia or hydrogen sulfide can typically be used in place of water to deposit the corresponding metal nitride or sulfide. The second precursor reacts with the surface-bound species, fully displacing existing ligands with the new chemistry to form the first layer of reacted material (Figure 1c). The chamber is purged again, and the second layer of deposition can begin (Figure 1a). The overall thickness is controlled by repeating deposition cycles until the desired thickness is achieved. The growth rate depends largely on the metal precursor, but is generally between 0.3 Å per cycle and 1.5 Å per cycle. It is particularly exciting that different materials can be used in each cycle, allowing one to systematically introduce new atoms at dopant, alloy, or layered-structure concentrations. ALD requires surface functional groups to initiate growth. Consequently, selective ALD can be accomplished by first patterning a substrate with non-reactive species.[3,4]
Complex materials are defined as nanostructured materials with combinations of structure and/or composition that lead to performance surpassing the sum of their individual components. There are many methods that can create complex materials; however, atomic layer deposition (ALD) is uniquely suited to control composition and structural parameters at the atomic level. The use of ALD for creating complex insulators, semiconductors, and conductors is discussed, along with its use in novel structural applications.
1. Introduction 1.1. Why is ALD Interesting? Atomic layer deposition (ALD), as the name implies, allows one to conformally deposit a broad library of materials one atomic layer at a time over tortuous surfaces. By controlling the deposition conditions, materials can be layered in a laminate fashion, or they can be mixed at the atomic level into heterogeneous solids that cannot be formed by conventional means. The ability to tune crystallinity, atomic composition, and thickness down to the single-atom level makes ALD one of the most flexible and powerful deposition methods currently available. Such control is critical to the advancement of new technologies and fundamental research. ALD stems from the more-mature field of vapor deposition, the prototypical example being chemical vapor deposition (CVD).[1] CVD is performed by combining inorganic or metal–organic precursors simultaneously in the gas phase in the presence of a substrate at temperatures high enough to crack the precursors and initiate material growth. The number of materials and varieties of structures that can be generated are truly staggering. Because the gas-phase precursors are introduced together, film growth rates are high and thickness can be roughly controlled by the deposition time. While these properties are advantageous, some applications demand higher precision in film thickness and/or near perfect conformality. This can be accomplished via ALD.
1.2. How Does ALD Work? To achieve precision film thickness and conformality, ALD splits the CVD process into self-limiting half-reactions. There
2. Complex Materials 2.1. Mixed Insulators
Dr. A. M. Schwartzberg, Dr. D. Olynick The Molecular Foundry Lawrence Berkeley National Laboratory Berkeley, CA 94720, USA E-mail: [email protected]
DOI: 10.1002/adma.201500699
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The most prevalent ALD processes are for single-component dielectric insulators and passivating layers, such aluminum oxide and hafnium oxide.[5–7] More recently, there has been interest in the development of novel insulators by ALD to improve both electronic and thermal properties of devices.
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Figure 1. ALD precursors are injected into vacuum chamber and react with hydroxyl terminated surface of substrate (a). The chamber is purged of precursor, and a secondary reactant is introduced that can react with the monolayer of deposited material (b). The chamber is purged again, leaving a clean, fully terminated monolayer of material (c). This process is repeated, each step leaving behind a known thickness of material.
A major challenge in micro- and nanoelectronics is to achieve simultaneously a high dielectric constant (high k) and a high resistance to electrical breakdown. In naturally occurring high-k materials, the formation of a conduction pathway is relatively unhindered, and the barrier to breakdown is low. Because ALD has the ability to create high-density, continuous, and conformal films, complex, layered materials can be created where each interface creates a barrier to direct conduction-pathway formation. Recently, Ding et al. found that a multilayer hafnia– alumina nanolaminate structure had vastly improved electronic properties than a single-species insulator (Figure 2a,b).[8] They compared a 5-layer nanolaminated 55 nm hafnia–alumina film (10nm/1 nm) multilayer laminate (shown schematically in Figure 2c)both to a single-element 55 nm hafnia film, a sandwich structure of 57 nm alumina–hafnia–alumina film (1 nm/55 nm/1 nm) (Figure 2d). The leakage current for the laminate was an order of magnitude smaller than the solid film (down to 1 × 10−9 A cm−2 at 3.3 V) with a greatly improved breakdown field of 3.3 MV cm−2. This improvement was attributed to the suppression of crystalline boundary defects. The leakage current and the conduction pathways that form at high fields are believed to originate at crystalline boundaries. The laminate structure suppressed crystallinity while the thin layers of alumina interrupted the current pathways through any remaining crystalline boundaries in the hafnia layer. Similar laminate insulators with enhanced properties are ideal for supercapacitors, where breakdown voltage and leakage current critically limit performance. ALD is also ideal for use in metal–insulator–metal (MIM) capacitors, needed for efficient storage of clean power. In order
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Figure 2. Examples of nanolaminate structures formed by ALD processes. a,b) Transmission electron microscopy images of alumina–hafnia nanolaminate insulator materials demonstrating the reduced crystallinity in ultra-thin layers of hafnia (a) versus thicker layers (b). c,d) The ultra-thin nanolaminate and sandwich structures shown in (a) and (b). e,f) Crosssectional images of metal–insulator–metal devices fabricated by ALD. a–d) Reproduced with permission.[5] Copyright 2004, IEEE. e,f) Reproduced with permission.[9] Copyright 2009, Macmillan Publishers Ltd.
to generate the high capacitance necessary for such devices, multiple capacitors with extremely thin films must be put together in close proximity. By coupling anodic aluminum oxide (AAO) nanoporous membranes with ALD-fabricated nanolaminates, Banerjee et al. recently demonstrated a capacitor device with power and energy densities matching those of electrostatic capacitors and electrochemical supercapacitors respectively.[9] The sandwich-type device layers are all produced by ALD. First, a conducting titanium nitride layer is deposited onto the AAO membrane. This is followed by an alumina insulating layer, and finally another titanium nitride layer. Figure 2e,f show the device; the conformal coating was successful in pores 50 nm in diameter and 10 µm in depth: an aspect ratio of 200:1. There are no other deposition methods available that can uniformly coat pores with such high aspect ratios. Incredible conformality and continuity is of the utmost importance in these devices, as any pinhole defects in the insulating film will result in shorting and poor or completely non-functioning capacitors. This type of device truly exemplifies the power of ALD techniques, especially when combined with the wide varieties of materials possible. A clear next step in MIM capacitors, is to replace the sandwich structure with nanolaminates to improve the breakdown properties toward a new breed of supercapacitors that can handle much higher fields. While these examples show the power of
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nanolaminate insulators for electrical-property enhancement, similar structures are also excellent thermal insulators. Reducing thermal conductivity is of the upmost importance for applications such as thermoelectric production and thermal-insulation in devices. It is well known that a high concentration of interfaces can reduce thermal conductivity by interrupting phonon transport[10] and many recent efforts in low-thermal-conductivity coatings have focused on colloidally produced films to increase the interfacial density. ALD-made materials offer a viable alternative for high-density interface structures, especially when conformal coatings of complex device structures are needed. Costescu et al. created such high-density interfaces using ALD to produce nanolaminates of tungsten and alumina. Their optimum interface density of 0.4 nm−1 yielded an extraordinarily low thermal conductivity of 0.6 W m−1 K−1,[11] a greater than threefold improvement over the already impressive thermal conductivity of amorphous alumina. Clearly ALD for thermal management is a field ripe for substantial growth.
2.2. Semiconductors 2.2.1. Quantum Wells Quantum-well structures are an excellent example of a complex material with properties greater than the sum of their parts. By stacking 2D quantum-confined thin films of semiconductors with thin barrier layers, it is possible to control the bandgap, carrier flow, and optical properties allowing for an impressive number of applications, including light-emitting diodes, quantum-cascade lasers, and sensitive detectors.[12,13] Conventionally, such devices have been fabricated by metal– organic CVD or molecular-beam epitaxy (MBE). In both cases, epitaxial growth is required, meaning high temperatures and growth from lattice-matched materials. Epitaxial growth has the advantage of high material quality and incredible control over composition and strain (bandgap control). However, it is highly constrained to the narrow band of material combinations that will allow for matched lattice growth. A new frontier for quantum-well structure development would be the ability to fabricate direct-bandgap semiconductors of high quality, with the required thickness control, but without the constraints of epitaxial growth. ALD has the potential to meet these requirements. However, as ALD growth is typically amorphous and/or polycrystalline, device quality may be degraded. As demonstrated in the previous section, ALD techniques allow for the deposition of an impressive array of metal oxides. The majority of metal oxides are indirect bandgap semiconductors or insulators; as such they have poor optoelectronic properties limiting their use in highly desired devices such as light-emitting diodes and photodetectors. ALD is not limited to oxides, the chemistry that controls the self-limiting reactivity is complex, and oxide chemistry is relatively simple and works with many pre-existing chemical vapor deposition precursors. Oxides have therefore dominated work on ALD during much of its early development. The use of ammonia- or nitrogencontaining plasmas allows metal nitride ALD, as mentioned
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briefly above with titanium nitride; however, nitrides are also dominantly indirect-bandgap semiconductors. In order to achieve direct-bandgap materials, it was recognized that metal chalcogenides, specifically sulfides, have the greatest potential and the most-achievable deposition chemistry. Chalcogenides (sulfides, selenides, and tellurides) have been of interest since the earliest work on ALD, and yet only 16 sulfides and a small handful of selenides and tellurides have been demonstrated, in contrast to the huge number of oxide materials known.[14,15] More recent work on metal sulfides has been demonstrated by the Prinz group, wherein the potential of ALD sulfide deposition is clear, as well as the complexity of working with these materials.[16,17] The sulfur precursor, hydrogen sulfide, is a highly toxic, flammable, corrosive gas that is not only dangerous to handle, but rapidly degrades the fast valves necessary for ALD. In order to reproducibly perform sulfide ALD, it was necessary to build a system specifically designed to handle and resist the precursors. In a great example of the power of producing directbandgap semiconductors with ALD, Dasgupta et al. demonstrated bandgap control in lead sulfide through quantum confinement by careful thickness tuning down to 2 nm.[16] The ultimate goal of this type of work is clearly the development of functional optoelectronic devices fabricated largely through ALD, thereby taking advantage of the low thermal requirements, conformal deposition, and high material control. To this end, Dasgupta et al. were able to produce lead sulfide/ zinc selenide quantum-well structures completely by ALD processing. This work points to the possibility of generating highly functional quantum-well device structures on arbitrary surfaces with arbitrary composition, completely opening up the possibility for bandgap tunability, carrier-flow control, and device customization.
2.2.2. Transition Metal Dichalcogenides One of the most exciting new directions in ALD semiconductors is in the formation of transition metal dichalcogenides (TMDCs), a unique class of semiconductors historically used for their bulk properties as lubricants. TMDCs are comprised of strongly bonded (ionic/covalent) chalcogen–metal–chalcogen layers which are held together by weak van der Waals forces and readily exfoliate into thin sheets. Recently, it has been discovered that, like graphene, TMDCs have extraordinary properties when isolated as single sheets. For instance, Mak, et al. reported that TMDCs convert from indirect- to direct-bandgap semiconductors when isolated as a single layer.[18] Their discrete bandgap, combined with high 2D quantum confinement of charge carriers, make them fascinating and potentially ideal for optoelectronic applications.[19] In addition, valley polarization has been identified in monolayer TMDCs which has sparked significant interest in multi-axis control of electronics and optoelectronics, allowing the reading and writing of both electronic and spin states of electrons.[20] Another promising use for single-layer TMDCs is in energy-harvesting applications, as they exhibit an optical bandgap ranging from the near infrared to the visible and strong light–matter interactions.[21] A bilayer heterojunction
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RESEARCH NEWS Figure 3. a) Synthesis procedure for the ALD-based WS2 nanosheets. b–d) Optical microscopy images of the transferred WS2 nanosheet on the SiO2 substrate for the mono-, bi-, and tetralayered thicknesses, respectively. e–g) AFM images and height profiles (inset) of the transferred WS2 nanosheet on the SiO2 substrate for the mono-, bi-, and tetralayered thicknesses, respectively. Reproduced with permission.[25] Copyright 2013, American Chemical Society.
device comprising single sheets of MoS2 and WSe2 has been shown to yield an unusually high external quantum efficiency of 1.5% considering the very low optical cross section of the approximately 2 nm device.[22] One of the major challenges in realizing practical devices has been the fabrication of highquality, single-layer materials over large areas. TMDC monolayers can be achieved through mechanical exfoliation of bulk materials, similarly to graphene from graphite, but have the same drawbacks, including size and scalability limitations. Because of this, much work has gone into vapor phase synthetic methods for depositing single-layer materials. These range from simply combing the raw component materials (e.g., sulfur and tungsten) in a tube furnace at high temperature, to induce CVD growth.[23] The drawback of these methods is the complexity of homogeneously forming single layers, often producing either discrete crystallites, or mixed-phase multilayer films. This becomes a major issue
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for large-scale (>10 µm) devices, as any component of the sample that contains more than one layer is effectively useless, switching the bandgap to indirect in these areas and interrupting in-plane electronic properties. More recently, a two-step process of sputtering metals, then annealing post-deposition in the presence of sulfur has been reported.[24] So far, this method has been constrained by the roughness of the metal film, which leads to mixed single and double layer films. Because uniformity and thickness control are of the utmost importance here, it is clear that ALD offers an ideal capability to create TMDC films either by post-deposition conversion, or direct synthesis of single-layer TMDCs. A recent demonstration of ALD-based TMDCs used 1 nm tungsten oxide films deposited by plasma-assisted ALD followed by exposure to hydrogen sulfide at 1000 °C, as shown in Figure 3a.[25] Because of the extremely high degree of thickness
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control afforded by the ALD process, it was possible to generate single-layer, 2-layer, and 4-layer WS2 over many square centimeters (Figure 3b–g). Despite being polycrystalline, the field-effect electron mobilities measured are quite high, on the order of 4 cm2 V−1 s−1, especially compared to mobilities obtained in large-scale CVD WS2 devices of 0.01 cm2 V−1 s−1.[26] This is in contrast to mobilities shown for micrometer-scale micromechanically cleaved single-crystal sheets of approximately 50 cm2 V−1 s−1, which is near the theoretical limit. There is clearly some reduced mobility due to the polycrystallinity of the ALD-synthesized materials; however, considering the large areas which can be coated conformally and the ease with which devices may be fabricated, this is an excellent first result for ALD-produced TMDCs. Ideally, one would want to produce TMDCs by ALD directly, without the need for an additional annealing/conversion step. Recently this has been demonstrated with MoS2 using molybdenum chloride and dimethyl disulfide (MoCl5 and H3CSSCH3).[27] The as-deposited films were annealed at high temperature to produce single-layer crystallites with high quality. This process was unable to demonstrate continuous films; however, it reveals a great potential for the synthesis of single-layer TMDCs directly by ALD, drastically reducing the complexity of forming large-area, high-efficiency devices of a wide variety of materials.
2.3.2. Plasmonics In the previous section we focused on transparent conductors; however, there is another class of conducting materials that are available by ALD, but not transparent in the visible. While not suited for transparent conductor applications, with their opacity comes plasmonic activity in or near the visible region. Plasmons are the collective oscillation of conduction-band electrons, which is generally considered a property of noble metal systems, but can also exist in highly doped semiconductor systems and some nitrides.[34,35] Plasmonic semiconductors accessible by ALD include AZO, which, when heavily doped, can possess a plasmon band in the near infrared, and titanium nitride, which has a plasmonic resonance in the visible region and has similar properties to gold.[30,36] Of particular interest is the fabrication of multilayer plasmonic metamaterials that can generate a negative index of refraction for applications such as perfect lensing and cloaking, and have been demonstrated with titanium nitride and AZO deposited by epitaxial growth methods.[37,38] These processes require lattice-matched substrates and dielectric spacing materials, which significantly hampers the modification and variety of structures possible. ALD has significant promise for creating new plasmonic metamaterials with significantly simplified processing requirements and the potential for creation of metamaterials on three-dimensionally nano- or micropatterned photonic-device structures.
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2.3.1. Transparent Conducting Oxides
2.4. Nanoscale Architecture
For modern optoelectronic devices, one of the most critical aspects to physically forming the device is the ability to create a transparent, conducting contact such that light can either exit (e.g., LEDs) or enter (e.g., photovoltaics) the device while allowing charges into or out of the system.[28] The standard transparent conducting materials have been tin-doped indium oxide (ITO) and fluorine-doped tin oxide (FTO), and are generally deposited by sputtering or physical vapor deposition. The drawbacks of these deposition methods include high surface roughness and poor conformality, which can hinder formation of a good contact between the conducting oxide and the active layer. This, of course, brings us back to ALD, which can create ultra-flat, conformal, tunable materials with a wide variety of processing conditions. Zinc oxide and aluminum-doped zinc oxide (AZO) were some of the first, high-performing transparent conducting oxide (TCO) materials deposited by ALD.[29–31] With resistivities as low as 1 µΩ cm, these materials are about 10× more resistive than conventionally prepared ITO or FTO at the same thickness; however, they have the advantage of avoiding indium, a relatively rare metal, and are deposited conformally and with very low roughness. More recently, TCOs without zinc have been developed. Zinc is a high-mobility metal, and one known to poison dielectric films. There have been several demonstrations of conducting tin oxide with and without doping.[32,33] Tin oxide films had better performance than AZO with resistances of 0.3 µΩ cm and avoided both zinc and rare materials.
There are new studies using ALD that extend beyond creating materials with specific optical, thermal, or electronic properties. This new area focuses on the modification and enhancement of mechanical properties. Similarly to how hollow tubes can be used to create high-strength-to-weight materials (versus a solid bar) at the meter scale, ALD can make nanoscale structures with similar advantages, but with the enhanced properties of low-dimensional materials. Meza et al. have demonstrated architectural lattice structures composed of hollow alumina tubes with wall thicknesses as low as 5 nm.[39] A schematic of these structures and their effective lattice structure can be seen in Figure 4a,b. These materials are fabricated through two-photon scanning photolithography, ALD, and oxygen plasma. The base structure is formed by scanning a tightly focused laser in a polymeric material, crosslinking, and solidifying the structure, effectively writing out the 3D structure one layer at a time. The resulting 3D pattern is then coated by a variety of methods by ALD, which is uniquely able to produce materials with conformality and excellent material properties. Once coated, the polymeric core material is removed by oxygen-plasma treatment, leaving behind hollow tubular structures, as seen in Figure 4c–e. Impressively, this new class of materials with densities as low as 6 kg m−3 have stiffnesses and strengths higher than any other material at similar densities (Figure 4g,h). Building upon this work, Zheng et al. recently demonstrated similarly impressive results, but over much larger areas by employing a modified 3D photolithographic method.[40] Unlike
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RESEARCH NEWS Figure 4. A) CAD image of the octet-truss design used in the study. The blue section represents a single unit cell. B) Cutaway of hollow octet-truss unit cell. C) Hollow elliptical cross section of a nanolattice tube. D) SEM image of alumina octet-truss nanolattice. E) Magnified section of the alumina octet-truss nanolattice. The inset shows an isolated hollow tube. F) Transmission electron microscopy (TEM) dark-field image with diffraction grating of the alumina nanolattice tube wall. G,H) Plots of the material properties : experimental stiffness and strength data against density for existing materials, showing that the materials created in this work reach a new niche in the high-strength and high-stiffness, lightweight material parameter space. Reproduced with permission.[39] Copyright 2014, AAAS.
the polymeric printing process, the ALD step, which actually provides the structural strength, was found to be completely scalable without any modification. New frontiers in this area may involve coating with ALD ceramic nanolaminates, similar to the insulator materials discussed above. Laminate structures are expected to have additional resistance to buckling and cracking because crack nucleation can be inhibited in such thin films.[41,42]
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3. Conclusion In this Research News article, we have discussed a wide range of complex material systems that are accessible by ALD, including insulators, semiconductors, conductors, and structural materials. Within these sections we have highlighted a few sub-topics of real novelty and described how ALD can produce complex material systems inaccessible by any other
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deposition method. In particular, the ability to control material composition, crystallinity, and thickness with atomic resolution, all while maintaining surface conformality opens up a new level of material complexity and applications that have yet to be imagined.
Acknowledgements The authors wish to acknowledge support by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02–05CH11231. Received: February 9, 2015 Revised: April 20, 2015 Published online: May 27, 2015
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Complex Materials by Atomic Layer Deposition Adam M. Schwartzberg* and Deirdre Olynick are many excellent review articles coving the basics of the ALD process; therefore, only a brief overview will be given here.[2] The process, as shown in Figure 1, involves precursors that are highly reactive with surface species (such as hydroxyl groups), but inert to self-reactivity at the ALD temperature. In this way, as soon as the substrate has reacted completely with the ALD precursor and saturated any available chemistry, the reaction ceases, despite the presence of excess precursor, thus producing monolayer coverage and conformality. In the case of metal oxide ALD, the process begins with a hydroxyl-terminated surface and the introduction of a gasphase metal precursor (Figure 1a). When a precursor molecule encounters the surface, a rapid chemical reaction displaces one of the precursor ligands (nominally organics or halides) and a proton from the surface, resulting in a strong metal–oxygen bond. Over time, depending on the temperature, concentration, and reactivity, the surface will saturate and chemistry will cease. The deposition chamber is purged of reactive species, and the second reactive precursor is introduced (Figure 1b). This is commonly water, hydrogen peroxide, or ozone for the thermal ALD of metal oxides, but plasma-excited oxygen radicals are also used in plasma-assisted metal oxide ALD. Moreover, ammonia or hydrogen sulfide can typically be used in place of water to deposit the corresponding metal nitride or sulfide. The second precursor reacts with the surface-bound species, fully displacing existing ligands with the new chemistry to form the first layer of reacted material (Figure 1c). The chamber is purged again, and the second layer of deposition can begin (Figure 1a). The overall thickness is controlled by repeating deposition cycles until the desired thickness is achieved. The growth rate depends largely on the metal precursor, but is generally between 0.3 Å per cycle and 1.5 Å per cycle. It is particularly exciting that different materials can be used in each cycle, allowing one to systematically introduce new atoms at dopant, alloy, or layered-structure concentrations. ALD requires surface functional groups to initiate growth. Consequently, selective ALD can be accomplished by first patterning a substrate with non-reactive species.[3,4]
Complex materials are defined as nanostructured materials with combinations of structure and/or composition that lead to performance surpassing the sum of their individual components. There are many methods that can create complex materials; however, atomic layer deposition (ALD) is uniquely suited to control composition and structural parameters at the atomic level. The use of ALD for creating complex insulators, semiconductors, and conductors is discussed, along with its use in novel structural applications.
1. Introduction 1.1. Why is ALD Interesting? Atomic layer deposition (ALD), as the name implies, allows one to conformally deposit a broad library of materials one atomic layer at a time over tortuous surfaces. By controlling the deposition conditions, materials can be layered in a laminate fashion, or they can be mixed at the atomic level into heterogeneous solids that cannot be formed by conventional means. The ability to tune crystallinity, atomic composition, and thickness down to the single-atom level makes ALD one of the most flexible and powerful deposition methods currently available. Such control is critical to the advancement of new technologies and fundamental research. ALD stems from the more-mature field of vapor deposition, the prototypical example being chemical vapor deposition (CVD).[1] CVD is performed by combining inorganic or metal–organic precursors simultaneously in the gas phase in the presence of a substrate at temperatures high enough to crack the precursors and initiate material growth. The number of materials and varieties of structures that can be generated are truly staggering. Because the gas-phase precursors are introduced together, film growth rates are high and thickness can be roughly controlled by the deposition time. While these properties are advantageous, some applications demand higher precision in film thickness and/or near perfect conformality. This can be accomplished via ALD.
1.2. How Does ALD Work? To achieve precision film thickness and conformality, ALD splits the CVD process into self-limiting half-reactions. There
2. Complex Materials 2.1. Mixed Insulators
Dr. A. M. Schwartzberg, Dr. D. Olynick The Molecular Foundry Lawrence Berkeley National Laboratory Berkeley, CA 94720, USA E-mail: [email protected]
DOI: 10.1002/adma.201500699
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The most prevalent ALD processes are for single-component dielectric insulators and passivating layers, such aluminum oxide and hafnium oxide.[5–7] More recently, there has been interest in the development of novel insulators by ALD to improve both electronic and thermal properties of devices.
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Figure 1. ALD precursors are injected into vacuum chamber and react with hydroxyl terminated surface of substrate (a). The chamber is purged of precursor, and a secondary reactant is introduced that can react with the monolayer of deposited material (b). The chamber is purged again, leaving a clean, fully terminated monolayer of material (c). This process is repeated, each step leaving behind a known thickness of material.
A major challenge in micro- and nanoelectronics is to achieve simultaneously a high dielectric constant (high k) and a high resistance to electrical breakdown. In naturally occurring high-k materials, the formation of a conduction pathway is relatively unhindered, and the barrier to breakdown is low. Because ALD has the ability to create high-density, continuous, and conformal films, complex, layered materials can be created where each interface creates a barrier to direct conduction-pathway formation. Recently, Ding et al. found that a multilayer hafnia– alumina nanolaminate structure had vastly improved electronic properties than a single-species insulator (Figure 2a,b).[8] They compared a 5-layer nanolaminated 55 nm hafnia–alumina film (10nm/1 nm) multilayer laminate (shown schematically in Figure 2c)both to a single-element 55 nm hafnia film, a sandwich structure of 57 nm alumina–hafnia–alumina film (1 nm/55 nm/1 nm) (Figure 2d). The leakage current for the laminate was an order of magnitude smaller than the solid film (down to 1 × 10−9 A cm−2 at 3.3 V) with a greatly improved breakdown field of 3.3 MV cm−2. This improvement was attributed to the suppression of crystalline boundary defects. The leakage current and the conduction pathways that form at high fields are believed to originate at crystalline boundaries. The laminate structure suppressed crystallinity while the thin layers of alumina interrupted the current pathways through any remaining crystalline boundaries in the hafnia layer. Similar laminate insulators with enhanced properties are ideal for supercapacitors, where breakdown voltage and leakage current critically limit performance. ALD is also ideal for use in metal–insulator–metal (MIM) capacitors, needed for efficient storage of clean power. In order
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Figure 2. Examples of nanolaminate structures formed by ALD processes. a,b) Transmission electron microscopy images of alumina–hafnia nanolaminate insulator materials demonstrating the reduced crystallinity in ultra-thin layers of hafnia (a) versus thicker layers (b). c,d) The ultra-thin nanolaminate and sandwich structures shown in (a) and (b). e,f) Crosssectional images of metal–insulator–metal devices fabricated by ALD. a–d) Reproduced with permission.[5] Copyright 2004, IEEE. e,f) Reproduced with permission.[9] Copyright 2009, Macmillan Publishers Ltd.
to generate the high capacitance necessary for such devices, multiple capacitors with extremely thin films must be put together in close proximity. By coupling anodic aluminum oxide (AAO) nanoporous membranes with ALD-fabricated nanolaminates, Banerjee et al. recently demonstrated a capacitor device with power and energy densities matching those of electrostatic capacitors and electrochemical supercapacitors respectively.[9] The sandwich-type device layers are all produced by ALD. First, a conducting titanium nitride layer is deposited onto the AAO membrane. This is followed by an alumina insulating layer, and finally another titanium nitride layer. Figure 2e,f show the device; the conformal coating was successful in pores 50 nm in diameter and 10 µm in depth: an aspect ratio of 200:1. There are no other deposition methods available that can uniformly coat pores with such high aspect ratios. Incredible conformality and continuity is of the utmost importance in these devices, as any pinhole defects in the insulating film will result in shorting and poor or completely non-functioning capacitors. This type of device truly exemplifies the power of ALD techniques, especially when combined with the wide varieties of materials possible. A clear next step in MIM capacitors, is to replace the sandwich structure with nanolaminates to improve the breakdown properties toward a new breed of supercapacitors that can handle much higher fields. While these examples show the power of
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nanolaminate insulators for electrical-property enhancement, similar structures are also excellent thermal insulators. Reducing thermal conductivity is of the upmost importance for applications such as thermoelectric production and thermal-insulation in devices. It is well known that a high concentration of interfaces can reduce thermal conductivity by interrupting phonon transport[10] and many recent efforts in low-thermal-conductivity coatings have focused on colloidally produced films to increase the interfacial density. ALD-made materials offer a viable alternative for high-density interface structures, especially when conformal coatings of complex device structures are needed. Costescu et al. created such high-density interfaces using ALD to produce nanolaminates of tungsten and alumina. Their optimum interface density of 0.4 nm−1 yielded an extraordinarily low thermal conductivity of 0.6 W m−1 K−1,[11] a greater than threefold improvement over the already impressive thermal conductivity of amorphous alumina. Clearly ALD for thermal management is a field ripe for substantial growth.
2.2. Semiconductors 2.2.1. Quantum Wells Quantum-well structures are an excellent example of a complex material with properties greater than the sum of their parts. By stacking 2D quantum-confined thin films of semiconductors with thin barrier layers, it is possible to control the bandgap, carrier flow, and optical properties allowing for an impressive number of applications, including light-emitting diodes, quantum-cascade lasers, and sensitive detectors.[12,13] Conventionally, such devices have been fabricated by metal– organic CVD or molecular-beam epitaxy (MBE). In both cases, epitaxial growth is required, meaning high temperatures and growth from lattice-matched materials. Epitaxial growth has the advantage of high material quality and incredible control over composition and strain (bandgap control). However, it is highly constrained to the narrow band of material combinations that will allow for matched lattice growth. A new frontier for quantum-well structure development would be the ability to fabricate direct-bandgap semiconductors of high quality, with the required thickness control, but without the constraints of epitaxial growth. ALD has the potential to meet these requirements. However, as ALD growth is typically amorphous and/or polycrystalline, device quality may be degraded. As demonstrated in the previous section, ALD techniques allow for the deposition of an impressive array of metal oxides. The majority of metal oxides are indirect bandgap semiconductors or insulators; as such they have poor optoelectronic properties limiting their use in highly desired devices such as light-emitting diodes and photodetectors. ALD is not limited to oxides, the chemistry that controls the self-limiting reactivity is complex, and oxide chemistry is relatively simple and works with many pre-existing chemical vapor deposition precursors. Oxides have therefore dominated work on ALD during much of its early development. The use of ammonia- or nitrogencontaining plasmas allows metal nitride ALD, as mentioned
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briefly above with titanium nitride; however, nitrides are also dominantly indirect-bandgap semiconductors. In order to achieve direct-bandgap materials, it was recognized that metal chalcogenides, specifically sulfides, have the greatest potential and the most-achievable deposition chemistry. Chalcogenides (sulfides, selenides, and tellurides) have been of interest since the earliest work on ALD, and yet only 16 sulfides and a small handful of selenides and tellurides have been demonstrated, in contrast to the huge number of oxide materials known.[14,15] More recent work on metal sulfides has been demonstrated by the Prinz group, wherein the potential of ALD sulfide deposition is clear, as well as the complexity of working with these materials.[16,17] The sulfur precursor, hydrogen sulfide, is a highly toxic, flammable, corrosive gas that is not only dangerous to handle, but rapidly degrades the fast valves necessary for ALD. In order to reproducibly perform sulfide ALD, it was necessary to build a system specifically designed to handle and resist the precursors. In a great example of the power of producing directbandgap semiconductors with ALD, Dasgupta et al. demonstrated bandgap control in lead sulfide through quantum confinement by careful thickness tuning down to 2 nm.[16] The ultimate goal of this type of work is clearly the development of functional optoelectronic devices fabricated largely through ALD, thereby taking advantage of the low thermal requirements, conformal deposition, and high material control. To this end, Dasgupta et al. were able to produce lead sulfide/ zinc selenide quantum-well structures completely by ALD processing. This work points to the possibility of generating highly functional quantum-well device structures on arbitrary surfaces with arbitrary composition, completely opening up the possibility for bandgap tunability, carrier-flow control, and device customization.
2.2.2. Transition Metal Dichalcogenides One of the most exciting new directions in ALD semiconductors is in the formation of transition metal dichalcogenides (TMDCs), a unique class of semiconductors historically used for their bulk properties as lubricants. TMDCs are comprised of strongly bonded (ionic/covalent) chalcogen–metal–chalcogen layers which are held together by weak van der Waals forces and readily exfoliate into thin sheets. Recently, it has been discovered that, like graphene, TMDCs have extraordinary properties when isolated as single sheets. For instance, Mak, et al. reported that TMDCs convert from indirect- to direct-bandgap semiconductors when isolated as a single layer.[18] Their discrete bandgap, combined with high 2D quantum confinement of charge carriers, make them fascinating and potentially ideal for optoelectronic applications.[19] In addition, valley polarization has been identified in monolayer TMDCs which has sparked significant interest in multi-axis control of electronics and optoelectronics, allowing the reading and writing of both electronic and spin states of electrons.[20] Another promising use for single-layer TMDCs is in energy-harvesting applications, as they exhibit an optical bandgap ranging from the near infrared to the visible and strong light–matter interactions.[21] A bilayer heterojunction
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RESEARCH NEWS Figure 3. a) Synthesis procedure for the ALD-based WS2 nanosheets. b–d) Optical microscopy images of the transferred WS2 nanosheet on the SiO2 substrate for the mono-, bi-, and tetralayered thicknesses, respectively. e–g) AFM images and height profiles (inset) of the transferred WS2 nanosheet on the SiO2 substrate for the mono-, bi-, and tetralayered thicknesses, respectively. Reproduced with permission.[25] Copyright 2013, American Chemical Society.
device comprising single sheets of MoS2 and WSe2 has been shown to yield an unusually high external quantum efficiency of 1.5% considering the very low optical cross section of the approximately 2 nm device.[22] One of the major challenges in realizing practical devices has been the fabrication of highquality, single-layer materials over large areas. TMDC monolayers can be achieved through mechanical exfoliation of bulk materials, similarly to graphene from graphite, but have the same drawbacks, including size and scalability limitations. Because of this, much work has gone into vapor phase synthetic methods for depositing single-layer materials. These range from simply combing the raw component materials (e.g., sulfur and tungsten) in a tube furnace at high temperature, to induce CVD growth.[23] The drawback of these methods is the complexity of homogeneously forming single layers, often producing either discrete crystallites, or mixed-phase multilayer films. This becomes a major issue
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for large-scale (>10 µm) devices, as any component of the sample that contains more than one layer is effectively useless, switching the bandgap to indirect in these areas and interrupting in-plane electronic properties. More recently, a two-step process of sputtering metals, then annealing post-deposition in the presence of sulfur has been reported.[24] So far, this method has been constrained by the roughness of the metal film, which leads to mixed single and double layer films. Because uniformity and thickness control are of the utmost importance here, it is clear that ALD offers an ideal capability to create TMDC films either by post-deposition conversion, or direct synthesis of single-layer TMDCs. A recent demonstration of ALD-based TMDCs used 1 nm tungsten oxide films deposited by plasma-assisted ALD followed by exposure to hydrogen sulfide at 1000 °C, as shown in Figure 3a.[25] Because of the extremely high degree of thickness
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control afforded by the ALD process, it was possible to generate single-layer, 2-layer, and 4-layer WS2 over many square centimeters (Figure 3b–g). Despite being polycrystalline, the field-effect electron mobilities measured are quite high, on the order of 4 cm2 V−1 s−1, especially compared to mobilities obtained in large-scale CVD WS2 devices of 0.01 cm2 V−1 s−1.[26] This is in contrast to mobilities shown for micrometer-scale micromechanically cleaved single-crystal sheets of approximately 50 cm2 V−1 s−1, which is near the theoretical limit. There is clearly some reduced mobility due to the polycrystallinity of the ALD-synthesized materials; however, considering the large areas which can be coated conformally and the ease with which devices may be fabricated, this is an excellent first result for ALD-produced TMDCs. Ideally, one would want to produce TMDCs by ALD directly, without the need for an additional annealing/conversion step. Recently this has been demonstrated with MoS2 using molybdenum chloride and dimethyl disulfide (MoCl5 and H3CSSCH3).[27] The as-deposited films were annealed at high temperature to produce single-layer crystallites with high quality. This process was unable to demonstrate continuous films; however, it reveals a great potential for the synthesis of single-layer TMDCs directly by ALD, drastically reducing the complexity of forming large-area, high-efficiency devices of a wide variety of materials.
2.3.2. Plasmonics In the previous section we focused on transparent conductors; however, there is another class of conducting materials that are available by ALD, but not transparent in the visible. While not suited for transparent conductor applications, with their opacity comes plasmonic activity in or near the visible region. Plasmons are the collective oscillation of conduction-band electrons, which is generally considered a property of noble metal systems, but can also exist in highly doped semiconductor systems and some nitrides.[34,35] Plasmonic semiconductors accessible by ALD include AZO, which, when heavily doped, can possess a plasmon band in the near infrared, and titanium nitride, which has a plasmonic resonance in the visible region and has similar properties to gold.[30,36] Of particular interest is the fabrication of multilayer plasmonic metamaterials that can generate a negative index of refraction for applications such as perfect lensing and cloaking, and have been demonstrated with titanium nitride and AZO deposited by epitaxial growth methods.[37,38] These processes require lattice-matched substrates and dielectric spacing materials, which significantly hampers the modification and variety of structures possible. ALD has significant promise for creating new plasmonic metamaterials with significantly simplified processing requirements and the potential for creation of metamaterials on three-dimensionally nano- or micropatterned photonic-device structures.
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2.3.1. Transparent Conducting Oxides
2.4. Nanoscale Architecture
For modern optoelectronic devices, one of the most critical aspects to physically forming the device is the ability to create a transparent, conducting contact such that light can either exit (e.g., LEDs) or enter (e.g., photovoltaics) the device while allowing charges into or out of the system.[28] The standard transparent conducting materials have been tin-doped indium oxide (ITO) and fluorine-doped tin oxide (FTO), and are generally deposited by sputtering or physical vapor deposition. The drawbacks of these deposition methods include high surface roughness and poor conformality, which can hinder formation of a good contact between the conducting oxide and the active layer. This, of course, brings us back to ALD, which can create ultra-flat, conformal, tunable materials with a wide variety of processing conditions. Zinc oxide and aluminum-doped zinc oxide (AZO) were some of the first, high-performing transparent conducting oxide (TCO) materials deposited by ALD.[29–31] With resistivities as low as 1 µΩ cm, these materials are about 10× more resistive than conventionally prepared ITO or FTO at the same thickness; however, they have the advantage of avoiding indium, a relatively rare metal, and are deposited conformally and with very low roughness. More recently, TCOs without zinc have been developed. Zinc is a high-mobility metal, and one known to poison dielectric films. There have been several demonstrations of conducting tin oxide with and without doping.[32,33] Tin oxide films had better performance than AZO with resistances of 0.3 µΩ cm and avoided both zinc and rare materials.
There are new studies using ALD that extend beyond creating materials with specific optical, thermal, or electronic properties. This new area focuses on the modification and enhancement of mechanical properties. Similarly to how hollow tubes can be used to create high-strength-to-weight materials (versus a solid bar) at the meter scale, ALD can make nanoscale structures with similar advantages, but with the enhanced properties of low-dimensional materials. Meza et al. have demonstrated architectural lattice structures composed of hollow alumina tubes with wall thicknesses as low as 5 nm.[39] A schematic of these structures and their effective lattice structure can be seen in Figure 4a,b. These materials are fabricated through two-photon scanning photolithography, ALD, and oxygen plasma. The base structure is formed by scanning a tightly focused laser in a polymeric material, crosslinking, and solidifying the structure, effectively writing out the 3D structure one layer at a time. The resulting 3D pattern is then coated by a variety of methods by ALD, which is uniquely able to produce materials with conformality and excellent material properties. Once coated, the polymeric core material is removed by oxygen-plasma treatment, leaving behind hollow tubular structures, as seen in Figure 4c–e. Impressively, this new class of materials with densities as low as 6 kg m−3 have stiffnesses and strengths higher than any other material at similar densities (Figure 4g,h). Building upon this work, Zheng et al. recently demonstrated similarly impressive results, but over much larger areas by employing a modified 3D photolithographic method.[40] Unlike
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RESEARCH NEWS Figure 4. A) CAD image of the octet-truss design used in the study. The blue section represents a single unit cell. B) Cutaway of hollow octet-truss unit cell. C) Hollow elliptical cross section of a nanolattice tube. D) SEM image of alumina octet-truss nanolattice. E) Magnified section of the alumina octet-truss nanolattice. The inset shows an isolated hollow tube. F) Transmission electron microscopy (TEM) dark-field image with diffraction grating of the alumina nanolattice tube wall. G,H) Plots of the material properties : experimental stiffness and strength data against density for existing materials, showing that the materials created in this work reach a new niche in the high-strength and high-stiffness, lightweight material parameter space. Reproduced with permission.[39] Copyright 2014, AAAS.
the polymeric printing process, the ALD step, which actually provides the structural strength, was found to be completely scalable without any modification. New frontiers in this area may involve coating with ALD ceramic nanolaminates, similar to the insulator materials discussed above. Laminate structures are expected to have additional resistance to buckling and cracking because crack nucleation can be inhibited in such thin films.[41,42]
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3. Conclusion In this Research News article, we have discussed a wide range of complex material systems that are accessible by ALD, including insulators, semiconductors, conductors, and structural materials. Within these sections we have highlighted a few sub-topics of real novelty and described how ALD can produce complex material systems inaccessible by any other
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deposition method. In particular, the ability to control material composition, crystallinity, and thickness with atomic resolution, all while maintaining surface conformality opens up a new level of material complexity and applications that have yet to be imagined.
Acknowledgements The authors wish to acknowledge support by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02–05CH11231. Received: February 9, 2015 Revised: April 20, 2015 Published online: May 27, 2015
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