Atomic Layer Deposition Precursors Research Articles

(Invited) Seebeck Coefficient Measurement of Pbsete / PbTe Nanolaminate Structures

Thermoelectric (TE) materials can be applied for solid state power generation or heating/cooling systems that convert heat into electricity and vice-versa. The green renewable technology related to thermoelectrics could play an important role to decrease the energy consumption from fossil fuels and decrease greenhouse gas emission. The main challenge for thermoelectric devices produced currently is their low efficiency. Therefore, it is essential to develop high efficiency TE materials and devices with superior performance. The efficiency of TE materials is expressed by the dimensionless thermoelectric figure of merit, ZT = S2σT/ k, where S is the Seebeck coefficient, σ is the electrical conductivity, and kis the thermal conductivity [1]. Previous work has demonstrated two main TE materials have higher ZT values. The first TE material with a complex crystal structure called a phonon glass electron crystal in which the voids, defects, and rattlers could scatter phonons to decrease the lattice thermal conductivity, and thus to increase ZT. Another one is called nanomaterials: including quantum wells, quantum dots, superlattice, quantum wires and nanolaminates, which introduce a large density of interfaces to enhance phonon scattering. The thermal conductivity of these nanomaterials was effectively reduced, and the ZT of TE materials was improved significantly. The IV-VI semiconducting lead chalcogenides, such as PbTe and PbSe, have been investigated due to their high figure of merit, good chemical stability, low vapor pressure and high melting point. Therefore, they are promising thermoelectric materials for intermediate temperature applications ranging from 600 to 800 K. There are different techniques that can be used to synthesis PbTe and PbSe, including pulsed laser deposition (PLD), metal-organic chemical vapor deposition (MO-CVD), magnetron sputtering deposition, molecular beam epitaxy (MBE) and atomic layer deposition (ALD). The ALD is a self-limiting technique that allows atomic layer growth each time. ALD can precisely control the film layer thickness, stoichiometry, composition, uniformity, and sharp interface. ALD also shows perfect conformal coverage when it is deposited on complex surface structures. Therefore, ALD is considered as a novel and competitive method to deposit PbTe/PbSe nanolaminate structures. It is possible to generate the reproducible and well-defined nanolaminate structures. In addition, the ALD deposition can be operated at rather low temperature compared to other techniques. In this work we report on the Seebeck coefficient measurements in PbSeTe / PbSe nanolaminate structures. The multiple PbTe/PbSeTe nanolaminates were deposited on silicon substrates by a thermal ALD system. This new nanolaminate structure was achieved by first depositing a very thin layer of the PbSeTe ternary compound with binary PbTe layers. The PbSeTe ternary compound is formed by nucleation of PbSe quantum dots between binary PbTe layers, and followed by an annealing process step. It is noted that the formation of the PbSe quantum dots is facilitated by the prevailing Volmer-Weber island growth mode. Lead bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (Pb(C11H19O2)2), (trimethylsilyl) telluride ((Me3Si)2Te) and (trimethylsilyl) selenide ((Me3Si)2Se) were employed as the chemical ALD precursors for lead, telluride and selenide, respectively. 20 sccm N2 was used as a carrier gas to transport the chemical precursors into the ALD reaction chamber. The ALD growth temperature was 170 oC. The solid lead precursor was volatilized at a temperature of 140 oC, the liquid Te precursor required heating to 40 oC, and the liquid Se precursor was kept at room temperature. The chamber base pressure was kept at 30 mTorr. Several physical characterization techniques have been employed to determine the ALD nanolaminate formation. The crystal structure was analyzed by X-ray diffraction (XRD). The film morphology and structure of the products were determined by field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM).The surface roughness was analyzed by atomic force microscopy (AFM). The analysis of the composition and stoichiometry of the ternary and binary layers were carried out by X-ray photoelectron spectroscopy (XPS) and Energy dispersive X-ray spectroscopy (EDS). The Seebeck coefficient of the sample was also measured in order to calculate ZT value using Seebeck coefficient measurement equipment of MMR SB100. Figure 1 shows an FE-SEM image of surface morphplogy of PbTe/PbSe nanolaminate films on silicon substrate.

Targeted Single-Site MOF Node Modification: Trivalent Metal Loading via Atomic Layer Deposition

Postsynthetic functionalization of metal organic frameworks (MOFs) enables the controlled, high-density incorporation of new atoms on a crystallographically precise framework. Leveraging the broad palette of known atomic layer deposition (ALD) chemistries, ALD in MOFs (AIM) is one such targeted approach to construct diverse, highly functional, few-atom clusters. We here demonstrate the saturating reaction of trimethylindium (InMe3) with the node hydroxyls and ligated water of NU-1000, which takes place without significant loss of MOF crystallinity or internal surface area. We computationally identify the elementary steps by which trimethylated trivalent metal compounds (ALD precursors) react with this Zr-based MOF node to generate a uniform and well characterized new surface layer on the node itself, and we predict a final structure that is fully consistent with experimental X-ray pair distribution function (PDF) analysis. We further demonstrate tunable metal loading through controlled number density of t...

Al2O3 on Black Phosphorus by Atomic Layer Deposition: An in Situ Interface Study.

In situ "half cycle" atomic layer deposition (ALD) of Al2O3 was carried out on black phosphorus ("black-P") surfaces with modified phosphorus oxide concentrations. X-ray photoelectron spectroscopy is employed to investigate the interfacial chemistry and the nucleation of the Al2O3 on black-P surfaces. This work suggests that exposing a sample that is initially free of phosphorus oxide to the ALD precursors does not result in detectable oxidation. However, when the phosphorus oxide is formed on the surface prior to deposition, the black-P can react with both the surface adventitious oxygen contamination and the H2O precursor at a deposition temperature of 200 °C. As a result, the concentration of the phosphorus oxide increases after both annealing and the atomic layer deposition process. The nucleation rate of Al2O3 on black-P is correlated with the amount of oxygen on samples prior to the deposition. The growth of Al2O3 follows a "substrate inhibited growth" behavior where an incubation period is required. Ex situ atomic force microscopy is also used to investigate the deposited Al2O3 morphologies on black-P where the Al2O3 tends to form islands on the exfoliated black-P samples. Therefore, surface functionalization may be needed to get a conformal coverage of Al2O3 on the phosphorus oxide free samples.

Growth of TixAl1 − xOy films by atomic layer deposition using successive supply of metal precursors

Atomic layer deposition (ALD) of thin ternary films from TiCl4, trimethylaluminum (TMA), AlCl3 and H2O was investigated at different successions of precursor pulses with the aim to explore passivation effect of TiCl4 to the chemisorption of TMA and AlCl3. Concentrations of Ti and Al in the films grown on silicon substrates at 100–500°C in different ALD precursor sequences were studied. In order to obtain additional information about the deposition mechanisms, the behavior of the quartz crystal microbalance (QCM) signal during ALD cycles was characterized and analyzed at a deposition temperature of 200°C. Post-deposition analysis revealed up to 2 times lower Al2O3 deposition rate in the TiCl4-TMA-H2O and TiCl4-AlCl3-H2O processes than that in the TiCl4-H2O-TMA-H2O and TiCl4-H2O-AlCl3-H2O processes, respectively. Despite of the markedly reduced growth rate of Al2O3 in the TiCl4-TMA-H2O and TiCl4-AlCl3-H2O processes, films with Al/(Al+Ti) ratios exceeding 0.5 were obtained. Consequently, the surface intermediate species, which stopped further adsorption of TiCl4, limited the adsorption of AlCl3 and TMA only partially. Enhanced growth rate of TiO2 was observed in films grown in the TiCl4-TMA-H2O and TiCl4-H2O-TMA-H2O processes. QCM revealed more efficient chlorine release and thereby reduced effect of steric hindrance in those processes. In addition, marked increase of Al2O3 deposition rate was observed in the TiCl4-H2O-TMA-H2O and TiCl4-TMA-H2O processes with the increase of deposition temperature from 300°C to higher temperatures. As no increase of this kind was recorded in the TMA-H2O process, the effect was related to the influence of Ti-containing surface layer on the adsorption of TMA.

Impact of the atomic layer deposition precursors diffusion on solid-state carbon nanotube based supercapacitors performances

A study on the impact of atomic layer deposition (ALD) precursors diffusion on the performance of solid-state miniaturized nanostructure capacitor array is presented. Three-dimensional nanostructured capacitor array based on double conformal coating of multiwalled carbon nanotubes (MWCNTs) bundles is realized using ALD to deposit Al2O3 as dielectric layer and TiN as high aspect-ratio conformal counter-electrode on 2 μm long MWCNT bundles. The devices have a small footprint (from 100 μm2 to 2500 μm2) and are realized using an IC wafer-scale manufacturing process with high reproducibility (≤0.3E-12F deviation). To evaluate the enhancement of the electrode surface, the measured capacitance values are compared to a lumped circuital model. The observed discrepancies are explained with a partial coating of the CNT, that determine a limited use of the available electrode surface area. To analyze the CNT coating effectiveness, the ALD precursors diffusions inside the CNT bundle is studied using a Knudsen diffusion mechanism.

Improved high temperature integration of Al2O3 on MoS2 by using a metal oxide buffer layer

We deposited a metal oxide buffer layer before atomic layer deposition (ALD) of Al2O3 onto exfoliated molybdenum disulfide (MoS2) in order to accomplish enhanced integration. We demonstrate that even at a high temperature, functionalization of MoS2 by means of a metal oxide buffer layer can effectively provide nucleation sites for ALD precursors, enabling much better surface coverage of Al2O3. It is shown that using a metal oxide buffer layer not only allows high temperature ALD process, resulting in highly improved quality of Al2O3/MoS2 interface, but also leaves MoS2 intact.

The Mechanical Behavior of ALD‐Polymer Hybrid Films Under Tensile Strain

Atomic layer deposition (ALD) is increasingly becoming commonplace to deposit high density, conformal films, as novel deposition techniques allow improved deposition rates on various substrates. Diffusion and miscibility of ALD precursors with polymers are known to affect film nucleation. To assess the influence of polymeric underlayer on the growth and mechanical behavior of ALD films, we deposited aluminum oxide (Al2O3) and titanium dioxide (TiO2) films on three different polymers with known reactivity to the ALD precursors and compared them to films on polyethylene terephthalate (PET). We present the trends in nucleation and penetration of the ALD films on the various polymers as well as the in situ mechanical behavior under tensile strain. X‐ray photoelectron spectroscopy (XPS) revealed that Cytop, a fluorinated polymer and polyethyleneimine ethoxylated (PEIE) delay the nucleation of Al2O3 and TiO2 films. Time‐of‐flight mass spectrometry (ToFSIMS) shows the extent of diffusion of the ALD precursors into the polymer underlayer, delaying nucleation. Tensile tests show that while the crack onset strain remained unchanged despite the various polymer underlayers, the crack saturation density decreased for ALD films on PEIE and polyhydroxystyrene (PHOST). Films on Cytop showed no crack saturation limit until the nucleation delay was overcome indicating that judicious choice of the ALD film and polymer underlayer will allow tailoring of the length of the diffusion zone and crack saturation, but not the crack onset strain, which will be addressed in future studies.

Characterization of Ru thin films from a novel CVD/atomic layer deposition precursor “Rudense” for capping layer of Cu interconnects

The authors have succeeded in development of a novel Ru precursor, Ru(EtCp)(η5-CH2C(Me)CHC(Me)O) [Rudense], for CVD and atomic layer deposition (ALD) processes under nonoxidative condition. Rudense has sufficient vapor pressure and good thermal stability (decomposition temperature = ca. 230 °C). Ru thin films were grown on Pt, Ru, Si, and SiO2 substrates using Rudense and NH3 as Ru precursor and reactant, respectively. Rudense gave the conformal, low-impurity (<1021 atoms/cc), and low-resistivity (16 μΩ cm) Ru thin films. Moreover, Rudense showed substrate selectivity; therefore, Rudense will be a candidate for area-selective CVD and ALD precursor for Ru capping layers of Cu interconnects.

Hydrophobicity of Rare Earth Oxides Grown by Atomic Layer Deposition

Rare earth oxide (REO) atomic layer deposition (ALD) processes are investigated for hydrophobic coatings. Thermal and plasma-enhanced ALD (PE-ALD) Er2O3 and Dy2O3 are developed using the newly synthesized Er and Dy precursors bis-methylcyclopentadienyl-diisopropyl-acetamidinate-erbium and bis-isopropylcyclopentadienyl-diisopropyl-acetamidinate-dysprosium, with H2O and O2 plasma counter oxidants. The Er and Dy precursors show typical ALD growth characteristics with no nucleation incubation, indicating that they are suitable ALD precursors. The hydrophobicities of ALD-grown Er2O3 and Dy2O3 are investigated, together with those of ALD-grown Y2O3, La2O3, and CeO2 that were previously developed for high-k applications. All the ALD-grown REOs show high hydrophobicity, with water contact angles as high as 90°. After annealing at 500 °C in air for 2 h, hydrophobicity is degraded depending on the kind of material; this degradation is related to the hygroscopy of REOs. In addition, we demonstrate the fabrication of a superhydrophobic surface by depositing highly conformal ALD REO films on 3D Si nanowire nanostructures. The Si NWs are conformally coated with ALD Y2O3, yielding a surface with a water contact angle of about 158°. The ALD REOs reported herein should find widespread applicability in the fabrication of robust hydrophobic coatings.

Size-controlled platinum nanoparticles prepared by modified-version atomic layer deposition for ethanol oxidation

This study adopts a modified atomic layer deposition (ALD) process to prepare size-controlled Pt nanoparticles on the surface of carbon black, showing superior catalytic activity toward ethanol oxidation. Two types of ALD precursors, (methylcyclopentadienyl) trimethyl platinum (MeCpPtMe3) and oxygen (O2), were used to grow Pt deposits at 250 °C. For 30 ALD cycles, the pulse period of MeCpPtMe3 serves as a key factor in controlling the particle size and the weight loading of Pt deposits. The Pt growth rates over the carbon support can be attributed to the surface coverage of Pt–O* sites, diffusion rate of MeCpPtMe3, and lateral interaction between each active site. Since the MeCpPtMe3 dose strongly affects the Pt particle size and the deposit density, the growth of ALD–Pt can be assumed as being diffusion-limited. Because of the surface-catalyzed reaction steps, the small-sized ALD-Pt catalysts offer better catalytic activity, CO tolerance, and long-term stability as compared with the large-sized ones. On the basis of the results, the modified ALD technique exhibits a great potential for tuning the Pt particle size and weight loading onto the carbon support for fuel cell application.

Conformal atomic layer deposition of alumina on millimeter tall, vertically-aligned carbon nanotube arrays.

Atomic layer deposition (ALD) can be used to coat high aspect ratio and high surface area substrates with conformal and precisely controlled thin films. Vertically aligned arrays of multiwalled carbon nanotubes (MWCNTs) with lengths up to 1.5 mm were conformally coated with alumina from base to tip. The nucleation and growth behaviors of Al2O3 ALD precursors on the MWCNTs were studied as a function of CNT surface chemistry. CNT surfaces were modified through a series of post-treatments including pyrolytic carbon deposition, high temperature thermal annealing, and oxygen plasma functionalization. Conformal coatings were achieved where post-treatments resulted in increased defect density as well as the extent of functionalization, as characterized by X-ray photoelectron spectroscopy and Raman spectroscopy. Using thermogravimetric analysis, it was determined that MWCNTs treated with pyrolytic carbon and plasma functionalization prior to ALD coating were more stable to thermal oxidation than pristine ALD coated samples. Functionalized and ALD coated arrays had a compressive modulus more than two times higher than a pristine array coated for the same number of cycles. Cross-sectional energy dispersive X-ray spectroscopy confirmed that Al2O3 could be uniformly deposited through the entire thickness of the vertically aligned MWCNT array by manipulating sample orientation and mounting techniques. Following the ALD coating, the MWCNT arrays demonstrated hydrophilic wetting behavior and also exhibited foam-like recovery following compressive strain.

Heteroleptic Precursors for Atomic Layer Deposition

As the atomic layer deposition (ALD) method is based on sequential, self-limiting surface reactions the precursor chemistry is the key to a successful processing of conformal high quality thin films. ALD precursor chemistry has traditionally been based on homoleptic compounds such as, but not limited to, metal halides, alkylamides and alkoxides. However, these precursors sometimes have drawbacks such as possible halide contamination and low thermal stabilities. Consequently, heteroleptic precursors have been investigated as alternatives to the existing homoleptic counterparts, leading to the development of several advantageous processes. Here, examples of heteroleptic precursors for ALD processes of transition metals and their oxides are given. Special focus is given to oxides of the rare earths and groups 4 and 5. Trends in the properties of heteroleptic precursors are discussed. Several examples of our recent results are shown, including introduction of novel processes based on amidinate-cyclopentadienyl complexes for ALD of rare earth oxides.

Fluorinated graphene and hexagonal boron nitride as ALD seed layers for graphene-based van der Waals heterostructures

Ultrathin dielectric materials prepared by atomic-layer-deposition (ALD) technology are commonly used in graphene electronics. Using the first-principles density functional theory calculations with van der Waals (vdW) interactions included, we demonstrate that single-side fluorinated graphene (SFG) and hexagonal boron nitride (h-BN) exhibit large physical adsorption energy and strong electrostatic interactions with H2O-based ALD precursors, indicating their potential as the ALD seed layer for dielectric growth on graphene. In graphene-SFG vdW heterostructures, graphene is n-doped after ALD precursor adsorption on the SFG surface caused by vertical intrinsic polarization of SFG. However, graphene-h-BN vdW heterostructures help preserving the intrinsic characteristics of the underlying graphene due to in-plane intrinsic polarization of h-BN. By choosing SFG or BN as the ALD seed layer on the basis of actual device design needs, the graphene vdW heterostructures may find applications in low-dimensional electronics.

Modeling precursor diffusion and reaction of atomic layer deposition in porous structures

Atomic layer deposition (ALD) is a technique for depositing thin films of materials with a precise thickness control and uniformity using the self-limitation of the underlying reactions. Usually, it is difficult to predict the result of the ALD process for given external parameters, e.g., the precursor exposure time or the size of the precursor molecules. Therefore, a deeper insight into ALD by modeling the process is needed to improve process control and to achieve more economical coatings. In this paper, a detailed, microscopic approach based on the model developed by Yanguas-Gil and Elam is presented and additionally compared with the experiment. Precursor diffusion and second-order reaction kinetics are combined to identify the influence of the porous substrate's microstructural parameters and the influence of precursor properties on the coating. The thickness of the deposited film is calculated for different depths inside the porous structure in relation to the precursor exposure time, the precursor vapor pressure, and other parameters. Good agreement with experimental results was obtained for ALD zirconiumdioxide (ZrO2) films using the precursors tetrakis(ethylmethylamido)zirconium and O2. The derivation can be adjusted to describe other features of ALD processes, e.g., precursor and reactive site losses, different growth modes, pore size reduction, and surface diffusion.

Manganese Precursor Selection and the Thermal Atomic Layer Deposition of Copper/Manganese Alloy Films

Our laboratory seeks to develop the growth of metallic first row transition metal thin films using atomic layer deposition (ALD). The microelectronics industry is calling for the growth of metallic first row transition metal films by the ALD method for a variety of applications, including copper metallization, seed layers for copper metallization, copper/manganese alloys for self-forming copper diffusion barriers, and magnetic alloys. The ALD growth of noble metal thin films has been explored extensively in the past ten years, due to the positive electrochemical potentials of these metal ions and relative ease of reduction to the metallic state. The low temperature ALD of high purity, low resistivity Cu films has been described, but ALD routes to the other metallic first row transition metal films remain poorly developed, largely because of the negative electrochemical potentials of most of the ions and a corresponding lack of powerful reducing co-reagents that can convert precursors in positive oxidation states to the metals. We will describe the synthesis, structure, and properties of a large series of new first row transition metal ALD precursors that combine high volatilities, high thermal stabilities, and high reactivities toward reducing agents. We will also report borane and other reducing agents that can react with the metal precursors to afford metallic films. Additionally, we will overview the thermal growth of metallic copper, nickel, cobalt, iron, manganese, and chromium thin films from these new precursors. Key advances include development of optimized ALD precursors with very similar ligands and chemistry, and identification of new reducing co-reagents that can rapidly reduce the positive oxidation state precursors to the metals.

ALD Synthesis of Ternary PbSeTe Layers Alternating with the Binary PbTe in Nanolaminate Structures for High Zt Thermoelectric Materials

Thermoelectric materials have attracted much attention for their potential in conversion between heat and electricity and consequently could be used for heating and cooling systems. The efficiency of a thermoelectric material is expressed by the dimensionless thermoelectric figure of merit, ZT = S2σT/ k, where S is the Seebeck coefficient, σ is the electrical conductivity, and kis the thermal conductivity [1]. Previous work demonstrated alloying was used to decrease the lattice thermal conductivity and thus to increase ZT owning to mass difference enhanced phonon scattering. In order to further progress in the development and commercialization of the next generation of thermoelectric devices it is necessary to enhance the ZT value. Most recently research focused on low dimensional nanostructures, including quantum wells, quantum dots, superlattice, quantum wires and nanolaminates to improve ZT. In superlattice structures, reduced lattice thermal conductivity is mainly due to the phonon scattering at multiple interface boundaries. Significant efforts have been made to investigate the IV-VI semiconducting lead chalcogenides, such as PbTe and PbSe due to their high figure of merit, good chemical stability, low vapor pressure and high melting point. Therefore they are promising thermoelectric materials for intermediate temperature applications ranging from 600 to 800 K.PbTe and PbSe films have been deposited by different techniques, such as pulsed laser deposition (PLD), metal-organic chemical vapor deposition (MO-CVD), magnetron sputtering, molecular beam epitaxy (MBE) and atomic layer deposition (ALD). Among these, ALD is considered as a novel and competitive method to deposit PbTe/PbSe nanolaminate structures. The ALD is a self-limiting atomic layer reaction in each ALD cycle introduce. It can precisely control the film layer thickness, stoichiometry, composition, uniformity, and sharp interface. ALD also can be used to deposit conformal films onto very complex structures. It is possible to generate the reproducible and well-defined nanolaminate structures. Furthermore, the deposition temperature of ALD is rather low in comparison with other growth techniques.In this work we report on the successful synthesis of multiple PbTe/PbSeTe nanolaminates on silicon substrates by a thermal ALD system. In this new nanolaminate structure a very thin layer of the PbSeTe ternary compound is formed by nucleation of PbSe quantum dots between binary PbTe layers, which is followed by an annealing process step. It is noted that the formation of the PbSe quantum dots is facilitated the prevailing Volmer-Weber island growth mode. Lead bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (Pb(C11H19O2)2), (trimethylsilyl) telluride ((Me3Si)2Te) and (trimethylsilyl) selenide ((Me3Si)2Se) were employed as the chemical ALD precursors for lead, telluride and selenide, respectively. 20 sccm N2 was used as a carrier gas to transport the chemical precursors into the ALD reaction chamber. The ALD growth temperature was 170 oC. The solid lead precursor was volatilized at a temperature of 140 oC, the liquid Te precursor required heating to 40 oC, and the liquid Se precursor was kept at room temperature. The chamber base pressure was kept at 30 mTorr.Several physical characterization techniques have been employed to determine the ALD nanolaminate formation. The crystal structure was analyzed by X-ray diffraction (XRD). The film morphology and structure of the products were determined by field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM).The surface roughness was analyzed by atomic force microscopy (AFM). The analysis of the composition and stoichiometry of the ternary and binary layers were carried out by X-ray photoelectron spectroscopy (XPS) and Energy dispersive X-ray spectroscopy (EDS).

Flash-Lamp-Enhanced Atomic Layer Deposition of Thin Films

Atomic Layer Deposition (ALD) is a particular thin film deposition technology which is based on alternating saturated surface reactions. As a result, the film growth proceeds in a self-limiting manner enabling the deposition of thin films with excellent thickness control, uniformity, and conformity. Although a large number of materials have been deposited by ALD so far for various applications, there are still some challenges in ALD.The deposition temperatures in ALD are typically lower compared to CVD due to the limited thermal stability of ALD precursors. As a consequence of the lower energy available for film formation the films may not meet the properties needed for application. In these cases a post deposition annealing is required to improve the film properties, e.g. to obtain the desired film structure, density, or purity. However, this high temperature processing is often impracticable due to a restricted thermal budget of the substrate, in particular when coating temperature sensitive substrates. Secondly, the reactants of an ALD process, e.g. oxygen, may react with the substrate itself leading to the formation of a parasitic interfacial layer. In order to avoid this issue, the proper choice of reactants or the use of an alternative deposition technique is essential. Furthermore, many ALD processes suffer from substrate inhibited film growth accompanied by inefficient precursor consumption and the formation of films with unfavorable properties. Finally, there are materials of interest, e.g. titanium, which so far can not be deposited by thermal ALD at all. These limitations may be overcome by the application of flash lamp annealing (FLA) in ALD.In FLA the substrate is exposed to a light flash with durations typically in the millisecond range. The light energy is absorbed within the top layers of the sample causing a rapid heating of the surface near region. On the contrary, the bulk material experiences no or only moderate heating. Consequently, FLA is a suitable technology to power high temperature processes even on temperature sensitive substrates.The film growth in flash lamp enhanced ALD is induced by this effect. Thereby, each process cycle consist of both a precursor pulse and the irradiation of the substrate with a light flash. During each single flash the surface temperature exceeds the threshold temperature which is required to achieve the thermal decomposition of adsorbed precursor molecules or to activate chemical reactions between the adsorbed precursor molecules and a second reactant. The film growth proceeds step-by-step and thus the film thickness can be controlled by varying the number of cycles. In addition, FLA in each cycle results in the periodical annealing of the already grown film and hence may lead to an improved film quality. Consequently, flash lamp enhanced ALD has a high potential for the realization of single-source processes, for the reduction of growth delay in the initial phase of film growth, for the deposition of high purity thin films, and for the deposition of new materials.In this work the principle of flash lamp enhanced ALD will be presented in detail, the technology will be reviewed and classified. Thereafter, we will give an overview about our studies on the flash enhanced ALD of aluminum-, ruthenium-, and tantalum-based thin films. These depositions were realized by flashing periodically on a substrate during the precursor pulses. We will show that the film growth is induced by the flash heating and the processes exhibits typical ALD characteristics. The obtained relations between flash parameters and film growth parameters will be discussed with the use of simulation results illustrating the temperature profile during the FLA treatment. Moreover, this work addresses the potentials of this technology as well as the technical challenges.

ALD Synthesis of PbSe Thin Films inside Porous Si Templates for Innovative Thermoelectric Applications

The IV-VI lead chalcogenides are considered as prospective thermoelectric materials which have their potential applications in green renewable energy by waste heat recovery and infrared devices. Among these materials, PbSe demonstrates a remarkable thermoelectric performance, The thermoelectric figure of merit ZT exceeds 1 for both n-type and p-type PbSe at high temperature. ZT is defined as ZT=σS2T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity, respectively. In addition, Se element is more abandant and less costly than Te which is a scarce element [1].PbSe thin films on Si substrates have been synthesized by various methods. However, a careful literature search reveals, there are few if any reports on the synthesis of PbSe thin films by atomic layer deposition (ALD). The ALD technique exhibits self-limiting surface reactions in each ALD cycle which consequently results in precise layer thickness control, stoichiometry, composition, uniformity of large area on the substrate. ALD also can be used to deposit conformal film onto very complex substrates or nano-porous template surfaces. This is the one advantageous property of ALD which renders it more competitive and promising in porous template nano-structures. In contrast, magnetron sputtering and PLD cannot be used to coat complex 3D structures with PbSe. Furthermore, the growth temperature of ALD is rather low in comparison with other fabrication processes.In this work we report on the successful synthesis of multiple PbSe thin films on silicon substrates and inside microporous Si templates by thermal ALD, using lead bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (Pb(C11H19O2)2) and (trimethylsilyl) selenide ((Me3Si)2Se) as the chemical ALD precursors for lead and selenide, respectively. The repetitive structure of nano-porous Si templates offers an additional degree of periodicity in the quest for high ZT thermoelectric films.The XRD and FE-SEM results reveal that the Lead selenide thin films are polycrystalline at deposition temperatures of 170 °C based on Volmer Weber Island growth mode and have characteristic FCC rock-salt crystal structure as shown in Figure (1), (2) and (3).

Heteroleptic Precursors for Atomic Layer Deposition

As the atomic layer deposition (ALD) method is based on sequential, self-limiting surface reactions the precursor chemistry is the key to a successful processing of conformal high quality thin films. ALD precursor chemistry has traditionally been based on homoleptic compounds such as, but not limited to, metal halides, alkylamides and alkoxides. However, these precursors may have drawbacks such as possible halide contamination and low thermal stabilities in the case of the alkylamides and alkoxides. Consequently, heteroleptic precursors have been investigated as alternatives to the existing homoleptic counterparts, leading to the development of several advantageous processes.In the recent years, increasing research effort has been dedicated in development of novel ALD processes focusing especially on transition metals and their oxides using heteroleptic precursors. Interestingly, heteroleptic compounds often show different properties than their homoleptic counterparts. One type of ligand can provide reactivity and another stability, thus widening the process window with comparable film growth rate.In this presentation heteroleptic precursors for ALD processes of transition metals and their oxides will be reviewed. Special focus is given to oxides of the rare earths and groups 4 and 5. Trends in the properties of heteroleptic precursors are discussed. Several examples of our recent results are shown, including introduction of novel processes based on amidinate-cyclopentadienyl complexes for ALD of rare earth oxides.

Atomic Layer Deposition of Groups 4 and 5 Transition Metal Oxide Thin Films: Focus on Heteroleptic Precursors

The atomic layer deposition (ALD) process, an alternative to CVD, is universally appreciated for its unique advantages such as excellent repeatability, conformity, and thickness control at the atomic level. ALD precursor chemistry has mainly been based on homoleptic compounds such as, but not limited to, metal halides, alkylamides, and alkoxides, however these precursors have drawbacks such as possible halide contamination and low thermal stabilities in the case of the alkylamides and alkoxides. Consequently, heteroleptic precursors have been investigated as alternatives to the existing homoleptic counterparts, leading to the development of several advantageous processes. Nevertheless, there is no thematic review dedicated to the heteroleptic precursors and their properties, and it seems that no coherent strategy has been adopted for the development of heteroleptic precursors. This review gives a brief description of ALD and presents studies on the deposition of thin films of groups 4 and 5 metal oxides using ALD. A description of the general ALD properties of homoleptic precursors, in addition to a review on the thermal ALD of groups 4 and 5 metal oxides from heteroleptic precursors, is provided. Trends in the properties of heteroleptic ALD precursors, based on the literature review and recent experimental data, are discussed.