Structural Analyses of Phase Stability in Amorphous and Partially Crystallized Ge-Rich GeTe Films Prepared by Atomic Layer Deposition.

The method of atomic layer deposition (ALD) is considered one of the primary candidates for the uniform and conformal deposition of ultrathin films vital for the continuous miniaturization in the semiconductor industry and related high-technology markets. By the virtue of two selflimiting surface reactions, the ALD technique yields an ultimate control of film growth in the sense that a submonolayer of material is deposited per so-called ALD cycle. With established materials being at the verge of industrial implementation, efforts are continuously undertaken to optimize and develop new ALD configurations and processes. So far, the main emphasis within the field of ALD has been on the materials characterization of the films by means of ex situ analysis. The research described in this thesis aims at the development of the relatively new configuration of plasma-assisted ALD and at in situ diagnostics studies of the (plasmaassisted) ALD processes. In plasma-assisted ALD, a plasma is used to activate the reactants in the gas phase in order to supply additional reactivity to the ALD chemistry. Plasma-assisted ALD is researched to provide benefits in the development of new ALD processes and materials. In particular, the opportunities to improve and tune the film properties, and to deposit films at reduced substrate temperatures have been addressed in this thesis. This work has been accompanied by studies using various in situ diagnostics, from which fundamental insight into the reaction mechanisms governing the ALD processes can be obtained. Moreover, in situ techniques provide the opportunity to monitor, optimize, and control the ALD process. In this work the use of in situ spectroscopic ellipsometry, transmission infrared spectroscopy, mass spectrometry, and optical emission spectroscopy has been demonstrated in studies of the plasma-assisted ALD processes of metal nitrides and metal oxides. The results of the film characterization obtained by these techniques have been corroborated and complemented by extensive ex situ analysis. In particular, the combination of in situ spectroscopic ellipsometry and the layer-by-layer ALD growth has been explored comprehensively. The merits of this in situ technique during ALD have been demonstrated by addressing various aspects relevant to ALD processes and materials. A large part of the work has concentrated on the plasma-assisted ALD process of the metal nitrides TiN and TaN. The merits of plasma-assisted ALD were observed in the deposition TiN films with excellent conductivity and low impurity content, even at low deposition temperatures. Furthermore, it was shown that by variation of the plasma condition in the ALD process of TaN, the film properties could be tailored from conductive, cubic TaNx;x??1 to semiconductive, amorphous Ta3N5. These aspects were clearly demonstrated by in situ spectroscopic ellipsometry, where the transition in TaNx phase could be distinguished by monitoring the energy dispersion in the optical constants. For the conductive films, the light absorption by free conduction electrons could be probed and that enabled extraction of the electrical film properties from the ellipsometry data. The latter was valuable to demonstrate electron-impurity scattering and finite size effects in TiN films. Furthermore, fundamental insight into the reaction mechanisms of plasma-assisted ALD process of TaN was obtained by detection of the volatile reaction by-products by mass spectrometry and optical emission spectroscopy. The possibilities for plasma-assisted ALD to improve the material properties and to deposit at reduced temperatures have been demonstrated for the process of Al2O3. The Al2O3 films were deposited at substrate temperatures down to room temperature and these films yielded good moisture permeation barrier properties as relevant for encapsulation purposes. The fundamental reaction mechanisms of this plasma-assisted ALD process were elucidated by transmission infrared spectroscopy in order to understand and further improve the film properties obtained at these reduced deposition temperatures. It was established that the surface chemistry is ruled by –CH3 and –OH surface groups created by the Al(CH3)3 precursor adsorption and the combustionlike reactions during the O2 plasma step, respectively. Moreover, infrared spectroscopy provided insight into the influence of deposition temperature on the material properties. It was shown that by prolonging the plasma exposure, i.e., by supplying more plasma reactivity to the ALD process, the surface chemistry at low temperatures was enhanced and the impurity content in the Al2O3 was reduced. In conclusion, the knowledge gained through the in situ diagnostic studies in this work is relevant to further develop the ALD technique. The insight obtained into the reaction mechanisms and the material properties of the ALD films in this work are particularly useful to further exploit the possibilities and opportunities of the plasma-assisted ALD technique in the synthesis of novel (complex) materials.

Atomic layer deposition : from reaction mechanisms to 3D-integrated micro-batteries

Atomic layer deposition (ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions, reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

The analysis of the local structure of covalent glass is one of the major challenges for analyzing the amorphous structure. Usually, people use a cutoff distance to determine the coordinated atoms and relevant structural information, such as coordination number and bond angles. Recently, the electron localization function (ELF) has been used to analyze the local structure of amorphous Ge2Sb2Te5. But how to determine the EFL threshold and cutoff distance has not been reported. Here, according to the ab-initio calculations, we systematically investigate the relationship between the bond number and the ELF threshold, and also the cutoff distance in amorphous GeTe. The reasonable value of the ELF threshold and the cutoff distance are determined according to the inflection point and slope change of the bond number with ELF value respectively. Furthermore, the minimal ELF value distributions of Ge-Ge, Ge-Te and Te-Te bonds are presented. The comparison shows that the majority of removed bonds in structural analysis are weak Ge-Te bonds due to the low localization degree of electron. In contrast, the stronger Ge-Ge bonds are almost unchanged when changing the ELF threshold value from 0.58 to 0.63 because of the high localization degree of electron. The average minimal ELF value of Ge-Te bonds in crystalline GeTe is calculated, and it is close to the ELF threshold that is determined by the inflection point. t is easy to find that the Ge-Te bonds which are removed by increasing the ELF threshold are relatively weak. Therefore, these weaker bonds should be removed in structure analysis, which also means that the ELF threshold determined by the inflection point are reasonable value. Finally, based on the EFL threshold value, the coordination number and bond angle distribution of Ge in amorphous GeTe are obtained. The analysis of the coordination number of the Ge atoms shows that as the ELF threshold increases from 0.58 to 0.63, the 5- fold Ge atoms almost disappear because they are against the (8-N) rule. Furthermore, when the ELF threshold value is 0.58, the bond angle distribution analysis of Ge atoms shows that the local structure is a configuration that is mainly defectively octahedral (3-fold Ge) and distorted tetrahedral (4-fold Ge), but it remains unchanged when the threshold value increases to 0.63. It further demonstrates that all the removed chemical bonds are weaker ones as the ELF threshold increases. This approach is useful to improve the accuracy of amorphous structure analysis by obtaining the more reasonable inter-atomic bonding information. And it should be applied to the structural analyses of other systems generally.

The study aims to expand the application of the proportion integral derivative (PID) algorithm and improve the practical application of the PID algorithm to the atomic layer deposition (ALD) process. First, the ALD process is analyzed, and the application method of the PID algorithm is determined. Second, the research conditions of the PID algorithm based on the ALD process are designed. Finally, the temperature control operation of the PID algorithm in the ALD reaction chamber is modeled and experimentally studied under different research conditions. The results show that temperature significantly impacts the reaction chambers of stainless steel and aluminum. When the heating temperature increases, the temperature of the stainless steel chamber will also change, and the maximum difference between the chamber and the heating temperature is about 33°C. In contrast, the temperature of the aluminum chamber varies little with the heating temperature. The maximum difference between the chamber temperature and heating temperature is about 350°C, which shows that the temperature of the stainless steel chamber is better controlled and is more practical under the same temperature conditions. The pressure change has little effect on the temperature change of the reaction chamber of the two materials. The temperature curves of the two chambers show that the PID temperature control system can be used normally and has strong practicability. The study provides technical support for improving the PID temperature control system and the rational use of the PID temperature control algorithm in the ALD process.

With ever-growing industry demand for more uniform and conductive metal or metal oxide thin film coatings, an improvement to the techniques that produce these materials must be done in parallel. Two techniques that have shown great utility in this regard are referred to as chemical vapour deposition (CVD) and atomic layer deposition (ALD). While both techniques offer highly modular reactor designs and robust precursor-substrate chemistries (e.g., the benchmark Al2O3 process), many processes result in non-uniformity issues, ultimately leading to poor device performance. One such challenging process has been the fabrication of high-purity and uniform noble metal thin films such as gold. Equally as challenging is the ability to characterize metal CVD and ALD processes when the metallic film is just beginning to nucleate. This “nucleation delay”, or induction process,is especially common in metal ALD processes and techniques such as ellipsometry lack sensitivity in the low-cycle regime of ALD (or pulsed CVD) processes. The present work addressed in this thesis introduces, for the first time, the use of tilted fiber Bragg grating (TFBG) sensors for accurate, real-time, and in-situ characterization of CVD and ALD processes for noble metals, but with a particular focus on gold due to its desirable optical and plasmonic properties. Through the use of orthogonally-polarized transverse electric (TE) and transverse magnetic (TM) resonance modes imposed by a boundary condition at the cladding-metal interface of the optical fiber, polarization-dependent resonances excited by the TFBG are easily decoupled. It was found that for ultrathin thicknesses of gold films from CVD (~6-65 nm), the anisotropic property of these films made it non-trivial to characterize their effective optical properties such as the real component of the permittivity. Nevertheless, the TFBG introduces a new sensing platform to the ALD and CVD community for extremely sensitive in-situ process monitoring. We later also demonstrate thin film growth at low (<10 cycle) numbers for the well-known Al2O3 thermal ALD process, as well as the plasma-enhanced gold ALD process. Finally, the use of ALD-grown gold coatings has been employed for the development of a plasmonic TFBG sensor with ultimate refractometric sensitivity (~550 nm/RIU).

Since the first report of ferroelectricity in HfxZr1−xO2 (HZO) in 2011 [1], a lot of attention has been devoted for future non-volatile memory device applications due to its scalability (< 10 nm), low thermal budget (< 400°C) and matured atomic layer deposition (ALD) technique [2]. The thermal ALD process using H2O or O3 is generally employed for the fabrication of ferroelectric HZO thin films. On the other hand, the effect of the plasma-enhanced ALD process using O2 plasma gas on the crystallinity and ferroelectricity has not been systematically clarified. In this paper, we report the importance of ALD oxidant gases for HZO films using H2O or O2 plasma on the ferroelectricity of metal–ferroelectric–metal (MFM) capacitors.MFM capacitors with the H2O- and O2 plasma-based HZO films were fabricated as follows. A 10-nm-thick HZO films were deposited on a TiN bottom-electrode (BE-TiN) by ALD at 300°C using H2O or O2 plasma as oxidant gases and (Hf/Zr)[N(C2H5)CH3]4 (Hf:Zr = 1:1) cocktail precursor. The Hf/Zr ratios in both types of HZO films were estimated to be 0.4/0.6 using X-ray photoelectron spectroscopy (XPS). Next, a post-deposition annealing (PDA) process was carried out at 400 °C for 1 min under a N2 atmosphere. Finally, a TiN top-electrode was deposited by DC sputtering.First, the crystallinity and remanent polarization (2P r = P r + − P r −) of the H2O- and O2 plasma-based HZO films were investigated. Nanocrystals with ferroelectric orthorhombic (O), tetragonal (T), cubic (C) phases partially existed in the O2 plasma-based film after the ALD process, while an amorphous structure dominantly existed in the H2O-based film, analyzed by the grazing-incidence X-ray diffraction analysis and cross-sectional transmission electron microscopy images [3]. This could be because of a strong oxidation power and high energy ion/electron bombardment of O2 plasma. After the PDA process, the O phase was formed more in the O2 plasma-based HZO film than the H2O-based film. Therefore, the nanocrystal grains in the as-grown O2 plasma-based HZO film could play an important role as nuclei for the crystallization and formation of O phase [3,4]. From the polarization–electric field hysteresis curves, the PDA-treated O2 plasma-based capacitor exhibited 2P r of 20 µC/cm2, which was 1.5 times higher than that (13 µC/cm2) of the H2O-based capacitor. Thus, these results indicate that the crystallization and O phase formation of HZO films were promoted by using O2 plasma gas as an ALD oxidant, resulted in a higher 2P r.Next, endurance properties of these MFM capacitors were evaluated. In the wake-up state, both capacitors showed almost the same increase rate (~8%) of switching polarization (P sw). In the fatigued state, on the other hand, the O2 plasma-based capacitor exhibited less P sw degradation (~33%) after 106 cycles, whereas the P sw of H2O-based capacitor decreased by ~47%. To clarify these different fatigue properties, the TiOxNy interface reaction layer (IRL) formation at the HZO/BE-TiN interface was analyzed using synchrotron hard X-ray photoelectron spectroscopy (HAXPES). For the HAXPES Ti 2p spectra, the Ti-O peak intensity for the O2 plasma-based capacitor was larger than that of the H2O-based capacitor [5]. Therefore, an oxygen-rich TiON-IRL could be formed at the HZO/BE-TiN interface during ALD process due to the stronger oxidizing power of O2 plasma. Additional oxygen vacancies (VO) were reported to formed in HfO2-based films during field cycling due to the interface reaction between the HfO2-based film and TiN electrode. Moreover, one of the origins of the P sw degradation was thought to be domain pinning caused by VO [6]. Therefore, an oxygen-rich TiON-IRL for the MFM capacitor with the O2 plasma-based HZO film could prevent the interface reaction and formation of additional VO in HZO films, led to superior fatigue properties.In summary, the MFM capacitor with the O2 plasma-based HZO film showed superior 2Pr and fatigue resistance compared to those of the H2O-based one. Based on these experimental results, it is important to consider the selection of an ALD oxidant for fabrication of HZO-based MFM capacitors.This work was partly supported by JSPS KAKENHI (Nos. JP21J01667, JP20H02189, and JP18J22998) and MEXT Leading Initiative for Excellent Young Researchers (No. JPMXS0320220213). The HAXPES measurements were performed under approval of the NIMS Synchrotron X-ray Station (2020A4602 and 2020A4651).[1] J. Müller et al., Appl. Phys. Lett. 99, 112901 (2011).[2] J.-H. Kim et al., ACS Appl. Electron. Mater. 5, 4726 (2023).[3] T. Onaya et al., APL Mater. 10, 051110 (2022).[4] T. Onaya et al., Microelectron. Eng. 215, 111013 (2019).[5] T. Onaya et al., Solid State Electron. 210, 108801 (2023).[6] M. Pešić et al., Adv. Funct. Mater. 26, 4601 (2016).

Recently, Prussian blue analogues (PBAs) have been reported to exhibit a low voltage charge/discharge behavior with high capacity (300–545 mAh/g) in lithium-ion secondary batteries (LIBs) [...]

This paper investigates the purge flow rate in a reactor scale simulation of an Atomic Layer Deposition (ALD) process. A three-dimensional numerical analysis approach was implemented in the ALD process to fabricate thin films of aluminium oxide (Al2O3). Despite the abundance of literature on the specific use of, and increase in deposited material through the process of ALD, limited studies exist on the physical and chemical processes that occur during the growth of ALD. Previous literature has indicated that purging has presented a major challenge in the effective deposition rate of the ALD process. The precise purge flow rate has also been greatly contended. The importance of the purge sequence within the ALD process cannot be overemphasized. The term purge sequence refers to the essential property that defines the ALD advanced nano-fabrication technique in producing ultra-thin film. Therefore, this study focused on the purge flow rate effects of the ALD process. The reactants employed in the simulation process were trimethyl-aluminium (TMA) and ozone (O3) as the metal and oxidant precursors, respectively, and inert argon as the purge gas. Numerical simulations were carried out at a stable operating pressure of 1 torr, with a substrate temperature of 200°C, and three purge flow rates of 20, 10 and 5 sccm, respectively. An extended ozone exposure is crucial to in providing an adequately oxidized substrate. It is discovered that the 5 sccm flow rate shows, superior mass fractions, unity surface coverage and a time extensive surface deposition rate. The 20 sccm, 10 sccm and 5 sccm purge flow rate growth obtained a 0.58, 0.92, and 1.6 Å/cycle, respectively. The findings revealed close similarities to experimental behaviours and recorded growth.

Colloidal quantum dots (QDs) are a promising material for optoelectronic applications. Typically, device integration requires QDs to be embedded in a host material. Atomic layer deposition (ALD) is often considered as a deposition technique for such purposes. However, it is known that ALD and vacuum processes often influence the optical properties of QDs in a negative way. Here, we describe an in situ photoluminescence (PL) measurement setup and use it to monitor the PL of QDs under vacuum and during ALD. For CdSe-based core/shell QDs, a reduction in the QD PL was observed upon exposure to vacuum. Water was identified as crucial for maintaining a high PL as evidenced by re-exposure to different gases. Furthermore, we addressed the influence of vacuum, different plasmas (O2, H2O, H2, H2S/Ar, and Ar), precursors (trimethylaluminum, diethylzinc, tetrakis(dimethylamido)titanium, and tetrakis(ethylmethylamido)hafnium), reactants (H2O, H2S, and O3), and ALD processes (Al2O3, TiO2, HfO2, and ZnS) on QDs. We observed a PL reduction by up to 90% upon plasma treatments. Furthermore, we found that trimethylaluminum and diethylzinc reduced the PL efficiency by more than 70% while exposure to tetrakis(dimethylamido)titanium and tetrakis(ethylmethylamido)hafnium lowered the PL by only 10-20%. Surprisingly, tetrakis(dimethylamido)titanium and H2O, which by themselves had only a minor influence on the QD PL, still caused an 80% drop of the PL efficiency when combined as an ALD process. On the other hand, ALD growth of HfO2 by combining tetrakis(ethylmethylamido)hafnium and O3 preserved 80% of the initial PL quantum yield, making it a promising process for QD embedding. These results put forward in situ PL measurements as a versatile technique to identify suitable precursors, reactants and ALD processes for QD embedding and investigate the interaction between QDs and reactive gaseous species in general.

In this study, we investigate the atomic layer deposition (ALD) process on all-inorganic CsPbBr3 perovskite nanocrystals (PNCs) to introduce an inorganic electron transport layer (ETL) in light-emitting diode (LED) devices. Two types of CsPbBr3 PNCs were synthesized with oleate (OA) and oleylammonium (OLA) ligands on the surface. We found that CsPbBr3 PNCs with Cs oleate surfaces experienced severe photoluminescence (PL) quenching after the ALD process, while those with oleylammonium bromide surfaces did not show any significant PL drop. Transmission electron microscopy and X-ray photoelectron spectroscopy revealed that significant Pb metal formation and Ruddlesden-Popper planar faults, linked to uncoordinated Pb2+ ion defects, were generated in CsPbBr3 PNCs terminated with Cs oleate after ALD ZnO. Finally, we fabricated LEDs using PNCs with an ALD ZnO process to introduce inorganic ZnMgO nanoparticles as the ETL. The devices processed with ALD exhibited superior luminance and external quantum efficiency compared to those without the ALD process. This research provides crucial insights into the surface-dependent chemistry of PNCs and the surface-dependent performance of perovskite-based optoelectronic devices.

This paper studies the adverse environmental impacts of atomic layer deposition (ALD) nanotechnology on manufacturing of Al2O3 nanoscale thin films. Numerical simulations with detailed ALD surface reaction mechanism developed based on density functional theory (DFT) and atomic-level calculations are performed to investigate the effects of four process parameters including process temperature, pulse time, purge time, and carrier gas flow rate on ALD film deposition rate, process emissions, and wastes. Full-cycle ALD simulations reveal that the depositions of nano thin films in ALD are in essence the chemisorption of the gaseous species and the conversion of surface species. Methane emissions are positively proportional to the film deposition process. The studies show that process temperature fundamentally affects the ALD chemical process by changing the energy states of the surface species. Pulse time is directly related to the precursor dosage. Purge time influences the ALD process by changing the gas–surface interaction time, and a higher carrier gas flow rate can alter the ALD flow field by accelerating the convective heat and mass transfer in ALD process.

Atomic layer deposition (ALD) is a vapor phase thin film deposition technique based on self-limited surface reaction. ALD processes consist of two (or more than) half-reactions. The first half-reaction is the self-limiting adsorption of precursor molecules which contain core metal atoms; the second half reaction is the self-limiting reaction between surface adsorbed precursor molecules and reactants. Since ALD could deposit thin films with high quality, good uniformity, high conformality, and sub-nanometer thickness controllability, [1-3] ALD has been regarded as one of the most suitable deposition technologies for semiconductor device fabrication. Since thin films of alumina (Al2O3) have wide range of applications such as high-k dielectric material for electronic devices, mechanical and chemical protective coatings, diffusion barriers, and optical coatings, [4-7] ALD of Al2O3 process is the mostly and thoroughly studied. For the deposition of Al2O3, trimethylaluminum (TMA) have been the most widely used for Al precursor due to its high vapor pressure and reactivity. And water (H2O) is widely utilized as the oxygen source in ALD Al2O3 processes, because since H2O often shows facile ligand exchange reaction during ALD. However, ALD Al2O3 processes with H2O reactant showed undesirable substrate oxidation issue. For example, there was an unwanted interface oxide between ALD deposited Al2O3 film and Si substrate. The interface oxide could reduce the dielectric constant of the deposited thin films and increase leakage current density. [8-9] Especially, the oxidation of substrate is critical issue for 2 dimensional (2D) transition-metal dichalcogenides (TMDCs) based field effect transistors (FETs). From our previous results, the oxidation of MoS2 by H2O considerably degraded device performance. [10-11] To avoid the oxidation of substrates and improve device performance, it is necessary to develop a new ALD process by using oxidants with lower oxidation potential than that of H2O. Despite its technical importance, ALD Al2O3 processes with weaker oxidants such as alcohols have rarely been investigated. [8] For this reason, the chemical reaction mechanism between surface adsorbed precursor and reactant has not been clearly identified. In this work, we fundamentally investigated ALD process Al2O3 on Si substrate, using TMA and various alcohol oxidants (methanol (MeOH), ethanol (EtOH), and n-propanol (n-PrOH)). Furthermore, we investigate the reaction mechanism of various alcohol oxidants during ALD of Al2O3 with TMA. Density functional theory (DFT) calculations at B97D3 level of theory were performed using Gaussian 09 suite of programs. Our developed ALD processes showed typical ALD growth characteristics. The saturated growth rates with MeOH, EtOH and n-PrOH were 0.10, 0.96, and 0.74 Å/cycle, respectively. From the calculation results, we revealed that the beta-hydrogen transfer reaction of EtOH and n-PrOH could easily oxidize surface methyl group into surface hydroxyl. The results could be applicable to highly integrated semiconductor devices fabrication and 2D TMDC based FET fabrication processes.

Atomic layer deposition (ALD) is a thin film deposition technique, in which substrate surface is exposed to reactants in cyclic manner separated by discrete purging steps. Thus one atomic layer of material is deposited per ALD cycle. This cycle is repeated until desired thickness is achieved. The bottom up nature of this deposition process facilitates excellent conformality, uniformity and thickness and composition tunability.Patterning of ALD films for various device applications is commonly performed by removing the specific part of the film or by selectively depositing films on specific areas of the surface. In the former method, patterning is done with the use of extra processing steps such as etching, lithography etc. The latter method involves the use of masking layers to prevent ALD on specific areas.1, 2 Maskless ALD, i.e., surface selective atomic layer deposition of thin films has recently garnered considerable interest due to its simplicity. This method of patterning ALD films is unique in that no surface mask is required. Materials nucleate on different surfaces to different extents. This difference in nucleation time is used to achieve selective atomic layer deposition (SALD). In our earlier studies on SALD of HfO2 and TiO2, we used water as the oxygen source.3, 4 In this study, ALD of HfO2 was carried out using tetrakis(diethylamino)hafnium and a primary alcohol (ethanol) in a custom built ALD reactor (Fig. 1). Unlike common ALD processes, here we used ethanol as the oxygen source. Ethanol reacts with the hydroxyl (-OH) groups on silicon surface and forms alkoxides that serve as a starting surface for the ALD film growth. Thus the film growth is obtained on the Si surface. Figure 2 shows the thickness tunability of the HfO2 ALD process on Si surface using ethanol as the oxygen source. HfO2 was found to grow at a rate of 0.05 nm/cycle, without any growth inhibition on the silicon surface. Linear dependence indicates self-limited growth of HfO2 thin film, typical to the ALD process. On the other hand, significant growth delay was observed on copper surface. The use of ethanol reduces surface copper oxide into metallic copper and reduces the number of potential ALD nucleation sites. Thus selective deposition of HfO2 was achieved on silicon surface over copper surface. Such selectivity depends on many ALD processing parameters and substrate pretreatment conditions. In this report, the effect of these parameters on the extent of selectivity will be discussed in detail along with copper diffusion barrier characteristics of such selectively deposited ultrathin (1-2 nm-thick) HfO2 layers. The growth rate on silicon side was measured using spectral ellipsometry. X-ray photoelectron spectroscopy was used to probe the presence of HfO2 on copper surface. Results of this study can be used to selectively deposit ultra-thin layers of HfO2 on silicon surface over copper surface for copper diffusion barrier applications. This work was supported in part by the National Science Foundation (CBET 1067424 and EEC 1062943). We also thank Dr. Gregory Jursich for many helpful discussions.

Transition metal sulfides have recently aroused great attention for a variety of applications in energy conversion and storage devices. Many transition metal sulfides, such as cobalt, nickel, and iron sulfides, have shown superb electrochemical properties, so they are highly promising as candidate active materials for supercapacitors, batteries, and water-splitting electrocatalysis. Atomic layer deposition (ALD) is a well-known nanotechnology for preparing uniform, conformal coating films on complex 3D structures. Recently, ALD has gained particular attention in energy conversion and storage applications, as ALD has been demonstrated as an effective approach to uniformly and conformally load active materials onto complex 3D nanostructured electrodes. For most electrodes, their electrochemical properties are largely determined by the surface properties, and therefore conformally coating the surface by ALD is of particular importance for the device performance, as the optimization of the nanostructured geometry of the electrodes can be decoupled from the modification of the surface properties. On the other hand, however, ALD of metal sulfides is much less explored, as compared to oxides. The ALD processes for many of the important sulfides, such as CoSx, NiSx, and FeSx, are still not well established, which therefore could seriously hinder their applications in energy devices. Herein, we will present our recent progress on the ALD of CoSx, NiSx, and FeSx. The metal sulfides were all deposited using metal amidinates as the metal precursors and H2S as the sulfur source. The saturation behaviors of these ALD processes were carefully studied. Typical film properties, such as microstructure, purity, and morphology, were evaluated by various techniques including TEM, AFM, SEM, RBS, and XPS. With optimal deposition conditions, our ALD processes were able to produce high-quality, pure, smooth, and well-crystallized films of CoSx, NiSx, and FeSx. In addition, all these sulfide films were able to uniformly and conformally cover deep narrow trenches with high aspect ratio of 10:1, which demonstrated the excellent conformality of our ALD processes. We will also present a few examples to demonstrate the promising applications of these ALD sulfides in energy conversion and storage devices. ALD CoSx showed excellent electrochemical redox kinetics in alkaline aqueous solution, and therefore it was a suitable candidate active material for supercapacitors. By conformally depositing CoSx thin film on porous nickel foam electrodes, the synthesized ALD-coated electrodes showed remarkable supercapacitor performance with high specific capacitance, good rate performance, and good cycling stability. Meanwhile, ALD NiSx was found to show excellent electrocatalytic performance toward OER, and therefore it was promising for electrocatalysis and metal-air batteries. Further studies showed that the ALD NiSx converted to porous nickel oxyhydrate during electrochemical aging, and the aged product exhibited superior OER performance with good stability.