Temperature Atomic Layer Deposition Research Articles

The influence of tertiary butyl hydrazine as a co-reactant on the atomic layer deposition of silver

Ultra-thin conformal silver films are the focus of development for applications such as anti-microbial surfaces, optical components and electronic devices. In this study, metallic silver films have been deposited using direct liquid injection thermal atomic layer deposition (ALD) using (hfac)Ag(1,5-COD) ((hexafluoroacetylacetonato)silver(I)(1,5-cyclooctadiene)) as the metal source and tertiary butyl hydrazine (TBH) as a co-reactant. The process provides a 23°C wide ‘self-limiting’ ALD temperature window between 105 and 128°C, which is significantly wider than is achievable using alcohol as a co-reactant. A mass deposition rate of ∼20ng/cm2/cycle (∼0.18Å/cycle) is observed under self-limiting growth conditions. The resulting films are crystalline metallic silver with a near planar film-like morphology which are electrically conductive. By extending the temperature range of the ALD window by the use of TBH as a co-reactant, it is envisaged that the process will be exploitable in a range of new low temperature applications.

Aluminum oxide/titanium dioxide nanolaminates grown by atomic layer deposition: Growth and mechanical properties

Atomic layer deposition (ALD) is based on self-limiting surface reactions. This and cyclic process enable the growth of conformal thin films with precise thickness control and sharp interfaces. A multilayered thin film, which is nanolaminate, can be grown using ALD with tuneable electrical and optical properties to be exploited, for example, in the microelectromechanical systems. In this work, the tunability of the residual stress, adhesion, and mechanical properties of the ALD nanolaminates composed of aluminum oxide (Al2O3) and titanium dioxide (TiO2) films on silicon were explored as a function of growth temperature (110–300 °C), film thickness (20–300 nm), bilayer thickness (0.1–100 nm), and TiO2 content (0%–100%). Al2O3 was grown from Me3Al and H2O, and TiO2 from TiCl4 and H2O. According to wafer curvature measurements, Al2O3/TiO2 nanolaminates were under tensile stress; bilayer thickness and growth temperature were the major parameters affecting the stress; the residual stress decreased with increasing bilayer thickness and ALD temperature. Hardness increased with increasing ALD temperature and decreased with increasing TiO2 fraction. Contact modulus remained approximately stable. The adhesion of the nanolaminate film was good on silicon.

Influence of ALD process parameters on the physical and chemical properties of the surface of vascular stents

Studies of haemocompatibility of AISI 316LVM steel point to the need for nickel elimination from the surface and replacing it with other elements showing higher biotolerance. Such layers include titanium, carbon or silicon coatings. Therefore, the authors attempted to evaluate some selected physicochemical properties of TiO2 layers, grown by atomic layer deposition (ALD) method, on the surface of 316LVM steel at variable process temperature. ALD temperature has a major role in the final quality of the surface layer grown with the use of such method, regardless of the type of the base. It was observed that the growth of temperature had an adverse influence on corrosive resistance in the artificial plasma environment and contributed to formation of a double (porous) layer showing decreased tightness. Further on, assessment of the coating adhesion to the base showed that too low process temperature T=100°C had an adverse effect on mechanical properties, resulting in substantially reduced critical force. On the other hand, the performed surface wettability tests showed no influence of ALD temperature in the obtained angle values.

Formation of ZnO Nanoparticles by Atomic Layer Deposition for the Nonvolatile Memory Thin-Film Transistor Applications

Charge-trap-type memory device employing oxide-semiconductor charge-trap layers (CTL), such as ZnO and In-Ga-Zn-O (IGZO), have been intensively investigated for nonvolatile memory TFT applications [1]. Although encouraging device characteristics could be obtained for the proposed memory TFT, they can be expected to be far improved by using nano-particle (NP) mono-layers as alternative CTLs thanks to their isolated structures. So far, various methods such as spray pyrolysis and high-temperature-assisted self-assembly have been suggested to form discrete NP mono-layers [2]. However, low controllability in areal density of NPs and high process temperature limit device performance as well as process compatibility on flexible substrate.In this work, we propose a low-temperature atomic-layer-deposition (ALD) process for the formation of ZnO NPs with an island-growth mechanism and fabricated the memory TFTs using the ZnO-NP CTLs. We also investigate the effects of ALD conditions for the ZnO NPs on the device performance of the fabricated ZnO-NP memory TFTs (NP-MTFT). The gate-stack structure of top-gate NP-MTFTs were designed as In-Sn-O (ITO) gate electrode (150 nm)/Al2O3 blocking oxide (50 nm)/ZnO NP CTL/Al2O3 tunneling layer (10 nm)/IGZO active channel layer (20 nm)/ITO source-drain electrode (100 nm). ALD cycles for the ZnO NPs were varied to 5, 20, 30, and 40 and the deposition temperature was modulated from 100 to 190oC to investigate the effects of ALD conditions on the island formation of ZnO NPs. Diethylzinc (DEZn) and H2O was used as Zn and O precursors, respectively. The size and areal density of the ZnO NPs prepared at 160oC with 20 ALD cycles were estimated to be approximately 33 nm and 3×109 cm-2, respectively. It was interesting to note the changes in arithmetic average of surface roughness (Ra) at various ALD temperatures when the ALD cycles increased from 5 to 20 cycles. The variations in Ra were summarized to be from 0.24 to 0.28 nm at 100oC, from 0.26 to 0.40 nm at 130oC, from 0.98 to 2.13 nm at 160oC, and from 0.21 to 0.22 nm at 190oC. Consequently, it was found that the island growth of ZnO NPs could not be available at 100 and 190oC. On the other hand, at 160oC, the ZnO NPs could be successfully formed with appropriate deposition rate with increasing the ALD cycles.The NP-MTFTs employing these ZnO NP CTLs exhibited typical charge-trap nonvolatile memory device behaviors. The memory window (MW) width of the fabricated NP-MTFTs monotonously increased with increasing the ALD cycles from 0.6 to 14.6 V when VGS was swept in the forward and reverse directions in the range of ±20V at a drain bias (VDS) of 0.1 V. The MW (12.2 V) of the NP-MTFT drastically was increased for the 20-cycle device compared with that for the 5-cycle device. This result suggests that the areal density of ZnO NPs could be effectively modulated by controlling the ALD cycles of DEZ between 5 and 20 cycles. The on- and off-program characteristics of the NP-MTFTs were evaluated as a function of program voltage pulse width. The read-out operations were performed at VGS of -2 V and VDS of 0.1 V. The ratios of programmed IDS of on-states to that of off-states (Ion/Ioff) were obtained as more than 3×107 and did not experienced marked degradation when the program pulse width decreased from 1 s to 1 µs. These results suggest that the proposed NP-MTFT exhibited nonvolatile memory operations with wide memory window and high-speed program time. This is the first demonstration for the NP-MTFTs using the ALD-assisted ZnO-NP CTLs. Flexible memory device will be fabricated as future works and detailed device characteristics will be discussed at the presentation. [1] S. J. Kim, W. H. Lee, C. W. Byun, C. S. Hwang, and S. M. Yoon, IEEE Electron. Dev. Lett. 36 (11) 1153, 2015.[2] Z. Liu, C. H. Lee, V. Narayanan, G. Pei, and E. C. Kan, IEEE Trans. Electron. Dev. 49 (9), 1606, 2002.

Improvements in Sensing Responses to Ammonia Gas for the In-Ga-Zn-O Thin-Film Transistor Using Atomic-Layer-Deposited ZnO Nanoparticles as Gas Sensitizers

Introduction: Gas-sensing technologies have been actively investigated to monitor the quality of air in our daily life. Especially in the industrial environment, detection of toxic gas in the atmosphere is required to ensure the safety of the working place. Among several types of gas sensors categorized with various principles such as optical, electrical, magnetic operations, and so on, the electrical sensors with three-terminal transistor configurations are extensively studied owing to their technical advantages. So-called field-effect transistor (FET)-based sensors are promising candidates for low-cost and portable applications in various fields. Moreover, thanks to their device parameters, such as the field-effect mobility and threshold voltage (VTH), they could be used as sensor devices with many-fold parameters. In this work, a bottom-gate (BG) structured oxide semiconductor thin-film transistors (TFTs) were proposed as sensor TFTs, in which In-Ga-Zn-O (IGZO) thin films were used as channel layer as well as gas sensing material. In order to improve the device performances such as sensitivity, reliability, and reproducibility in sensing responses, the ZnO nanoparticles (NPs) were introduced on the top of the IGZO channel layer as sensitizers for the sensor TFTs. The ZnO has widely been used as a sensing agent by means of its drastic changes in electrical conductivity at the exposures to various gas molecules. Thus, the ZnO-NPs with island structure formed on IGZO channel can be expected to greatly enhance the sensing response of the IGZO sensing layer thanks to their higher reactivity and larger surface area. For the fabrication of sensor TFTs, the particle size and areal density the ZnO-NPs are optimized by process conditions of atomic-layer deposition (ALD) method. This approach can be very promising for realizing the high-performance gas sensors with three-terminal TFT configurations using the oxide semiconductors. Experiments: We fabricated sensor TFTs using IGZO channel and ZnO-NP sensitizer on the glass substrate. First, a 150-nm-thick In-Sn-O (ITO) gate electrodes were patterned by wet-etching process and the 100-nm-thick Al2O3 gate insulator films were deposited by ALD. The deposition and patterning of ITO S/D electrodes were followed by the formation of contact holes. Then, IGZO (20 nm) active layers were deposited by rf magnetron sputtering and were patterned without any protection layer, which is conventionally prepared during the IGZO patterning process for protecting the IGZO from the chemical damages. Finally, ZnO NPs with island structure were formed by controlling the ALD cycles and temperatures. The sensing responses of the fabricated sensor TFTs to NH3 were evaluated in vacuum chamber with following four sequential steps; (1) at air ambient to determine reference characteristics; (2) at 1% NH3/N2 gas mixture with various concentrations to obtain the sensing responses; (3) after 100-oC annealing for 1 h in air ambient to confirm the recovery characteristics; and (4) at the identical conditions to those in step (2) to confirm the reproducibility in sensing responses. All measurements were performed for both devices with and without ZnO-NP sensitizer. Results & Conclusions: First, the BG TFT characteristics were compared between before and after the formation of ZnO-NPs on the IGZO channel layers. The slight improvements in hysteric behaviors in IDS transfer characteristics could be confirmed for the devices using ZnO-NPs sensitizer. Most noteworthy change obtained after the introduction of ZnO-NPs was that both characteristics of the stability in TFT operations and the sensitivity to NH3 were drastically improved. To the contrary, without ZnO-NPs sensitizer, the sensor TFTs showed unstable transfer characteristics at repeatedly-performed measurements even in an air ambient. Eventually, with these unstable TFT operations, the reliable sensing response for the sensor TFTs cannot be guaranteed under the NH3 exposing conditions. The transition of VTH in a negative direction was clearly detected for the sensor TFTs employing the ZnO-NPs sensitizer and the VTH shift width was estimated to be as large as 3.3 V. Second, the recovery and reproducibility in the sensing responses to NH3 were systematically evaluated. After the 100-oC annealing process, the transfer curve and the VTH were almost recovered to initial values, during which the adsorbed NH3 gas molecules were desorbed from the ZnO-NPs sensitizer. Furthermore, it was also confirmed that the second exposure to NH3 after recovery caused almost the same amounts of VTH shift for the sensor TFT. In conclusion, the introduction of ZnO-NPs sensitizer was sufficiently effective for enhancing both stability and sensitivity of the proposed IGZO gas sensor TFTs. The fabricated devices showed the recovery and reproducibility in sensing responses to NH3.

(Invited) Mechanism of Low Temperature ALD of Al2O3 on Graphene Terraces

2-D materials are single or trilayer networks of atoms, such as carbon, hBN, and MoS2. 2D materials are attractive for future electronic devices because they provide ballistic transport of charge carriers as well as carrier confinement. These novel devices require ultrathin and defect-free dielectric layers as gate or channel insulators. However 2D semiconductors such as graphene, boron nitride, and metal dichalcogenides (MDCs) are un-reactive; thus, the dielectric layer selectively nucleates on defect sites or step edges. In the conventional atomic layer deposition (ALD) process on graphene or other 2D semiconductors, such non-uniformly grown oxides can create large leakage currents. In the present report, polar metal phthalocyanine molecules have been employed as an ALD template for high nucleation density deposition of dielectric layers. By combining in-situ scanning tunneling microscopy (STM) and fabrication graphene devices, the insulating property and growth mechanism of the dielectric layer have been elucidated. Metal Ptholocyanines have been used to deposit gate oxides on graphene, MoS2, and WSe2. To deposit an ultrathin dielectric onto WSe2, monolayer titanyl phthalocyanine (TiOPc) is deposited by molecular beam epitaxy (MBE) as a seed layer for atomic layer deposition (ALD) of Al2O3 on WSe2. TiOPc molecules are arranged in a flat monolayer with four-fold symmetry as measured by scanning tunneling microscopy (STM). ALD pulses of trimethyl aluminum and H2O nucleate on the TiOPc, resulting in a uniform deposition of Al2O3, as confirmed by atomic force microscopy and cross-sectional transmission electron microscopy. The field-effect transistor (FETs) formed using this process have a leakage current of 0.046 pA/µm2 at 1 V gate bias with 3.0 nm EOT, which is lower leakage current than prior reports. The n-branch of the FET yielded a subthreshold swing of 80 mV/decade.

The Influence of Hydrogen on Defects of In–Ga–Zn–O Semiconductor Thin-Film Transistors With Atomic-Layer Deposition of Al2O3

Hydrogen plays a crucial role in several oxide semiconductors, where the amount of hydrogen significantly influences the device performance. Thus, its manipulation in oxide semiconductors is important for device performance. In our investigation, we studied the effect of hydrogen on defects in In–Ga–Zn–O semiconductor thin-film transistors (TFTs), as it varies with Al2O3 atomic layer deposition temperature. We found that the total trap-density ( $N_{{\text {tot}}}$ ) extracted by the sub-threshold slope and the trap density ( $N_{t}$ ) measured by low-frequency noise (LFN) as well as the density-of-states analyzed by capacitance–voltage decreased with increasing amounts of hydrogen in the oxide semiconductor. Given that LFN data show that mobility fluctuation is the major origins of noise and the front channel of TFT is a major carrier transport region, our results indicate that hydrogen effectively passivates the defects in front channel of oxide semiconductor and contributes to achieving superior device performance.

Effect of atomic layer deposition temperature on current conduction in Al2O3 films formed using H2O oxidant

To develop high-performance, high-reliability gate insulation and surface passivation technologies for wide-bandgap semiconductor devices, the effect of atomic layer deposition (ALD) temperature on current conduction in Al2O3 films is investigated based on the recently proposed space-charge-controlled field emission model. Leakage current measurement shows that Al2O3 metal-insulator-semiconductor capacitors formed on the Si substrates underperform thermally grown SiO2 capacitors at the same average field. However, using equivalent oxide field as a more practical measure, the Al2O3 capacitors are found to outperform the SiO2 capacitors in the cases where the capacitors are negatively biased and the gate material is adequately selected to reduce virtual dipoles at the gate/Al2O3 interface. The Al2O3 electron affinity increases with the increasing ALD temperature, but the gate-side virtual dipoles are not affected. Therefore, the leakage current of negatively biased Al2O3 capacitors is approximately independent of the ALD temperature because of the compensation of the opposite effects of increased electron affinity and permittivity in Al2O3. By contrast, the substrate-side sheet of charge increases with increasing ALD temperature above 210 °C and hence enhances the current of positively biased Al2O3 capacitors more significantly at high temperatures. Additionally, an anomalous oscillatory shift of the current-voltage characteristics with ALD temperature was observed in positively biased capacitors formed by low-temperature (≤210 °C) ALD. This shift is caused by dipoles at the Al2O3/underlying SiO2 interface. Although they have a minimal positive-bias leakage current, the low-temperature-grown Al2O3 films cause the so-called blisters problem when heated above 400 °C. Therefore, because of the absence of blistering, a 450 °C ALD process is presently the most promising technology for growing high-reliability Al2O3 films.

(Invited) Mechanism of Low Temperature ALD of Al2O3 on Graphene Terraces

Uniform and defect-free Al2O3 films were grown on highly oriented pyrolytic graphite (HOPG) terraces by thermal atomic layer deposition (ALD) at low temperature (50 oC) without any functionalization of the surface. Controlling the pulse times of trimethylaluminum (TMA) and H2O while using short Ar purge times, 1-2 nanometer size Al2O3 particles were formed on the HOPG terraces. The particles provided a layer of nanoscale nucleation centers on the HOPG terraces. Capacitance-voltage measurements of Al2O3 films grown at 50 oC using 50 ALD cycles showed an areal capacitance 1.17 mF/cm2 with very small frequency dispersion, consistent with the absence of induction cycles and formation of a high quality interface. The leakage current of the Al2O3 was ~10-5 A/cm2 in large area devices (1900 mm2) which is comparable with results from devices prepared using an identical ALD process for Al2O3 on Si0.7Ge0.3(001) substrates, and is consistent with the absence of pinholes.

Formation of ZnO Nanoparticles by Atomic Layer Deposition for the Nonvolatile Memory Thin-Film Transistor Applications

A low-temperature atomic-layer-deposition (ALD) process for the formation of ZnO nanoparticles (NPs) was demonstrated, in which the ALD temperature and cycles for diethylzinc precursor were controlled to be 160 oC and 20 cycles. The size and areal density of ZnO NPs were confirmed to be approximately 33 nm and 3×109 cm-2, respectively. Nonvolatile memory thin-film transistors were fabricated by employing the ZnO NPs as charge-trap centers for memory operations. The memory window was found to increase from 0.6 to 17.8 V with increasing the ALD cycles from 5 to 20. On- and off-program operations were also confirmed even with voltage pulses as short as 1 µs.

Atomic layer deposited nanocrystalline tungsten carbides thin films as a metal gate and diffusion barrier for Cu metallization

Tungsten carbides (WCx) thin films were deposited on thermally grown SiO2 substrates by atomic layer deposition (ALD) using a fluorine- and nitrogen-free W metallorganic precursor, tungsten tris(3-hexyne) carbonyl [W(CO)(CH3CH2C ≡ CCH2CH3)3], and N2 + H2 plasma as the reactant at deposition temperatures between 150 and 350 °C. The present ALD-WCx system showed an ALD temperature window between 200 and 250 °C, where the growth rate was independent of the deposition temperature. Typical ALD characteristics, such as self-limited film growth and a linear dependency of the film grown on the number of ALD cycles, were observed, with a growth rate of 0.052 nm/cycle at a deposition temperature of 250 °C. The ALD-WCx films formed a nanocrystalline structure with grains, ∼2 nm in size, which consisted of hexagonal W2C, WC, and nonstoichiometric cubic β-WC1−x phase. Under typical deposition conditions at 250 °C, an ALD-WCx film with a resistivity of ∼510 μΩ cm was deposited and the resistivity of the ALD-WCx film could be reduced even further to ∼285 μΩ cm by further optimizing the reactant pulsing conditions, such as the plasma power. The step coverage of ALD-WCx film was ∼80% on very small sized and dual trenched structures (bottom width of 15 nm and aspect ratio of ∼6.3). From ultraviolet photoelectron spectroscopy, the work function of the ALD-WCx film was determined to be 4.63 eV. Finally, the ultrathin (∼5 nm) ALD-WCx film blocked the diffusion of Cu, even up to 600 °C, which makes it a promising a diffusion barrier material for Cu interconnects.

Non-Stoichiometric Titanium Dioxide Incorporation Onto Reduced Graphene Oxide By Atomic Layer Deposition for Oxygen Reduction Catalysis

Renewable energy devices such as fuel cells and metal air batteries harness their energy via lethargic oxygen reduction reactions (ORR) whose catalysis is largely relying on expensive and scarce noble metals. To address this issue, in particular, non-precious metal incorporated in carbon nanostructures have been widely explored. In this study, we focus on synthesizing inexpensive and efficient electrode material for ORR catalysis and energy storage using reduced graphene oxide (rGO) incorporated with non-stoichoimetric titanium dioxide by atomic layer deposition (ALD). Although stoichiometric TiO2 has been rarely considered as an effective ORR electrocatalyst, oxygen-deficient TiO2-x recently showed surprisingly competitive ORR activity and excellent durability[1]. The work was performed on a single crystalline TiO2. Motivated by the recent report and the advantages of TiO2 due to its abundance, safety and cost effectiveness, we probed the feasibility of applying this oxygen-deficient TiO2-x to the widely employed oxide nanoparticle-incorporated carbon nanostructures. This is to leverage extremely high surface area and excellent electronic conductivity of carbon-based 3D structures, which is highly advantageous for maximized catalytically active sites per mass. We used electrophoretically deposited rGO as the matrix structure. Graphene oxides prepared by the modified Hummer’s method are first deposited onto a stainless steel by electrophoresis. The resulting structure was deposited by highly dispersed nanoparticles or ultrathin films of TiO2-x via ALD. The degree of non-stoichiometry is controlled by either changes in ALD temperature or a separate post-thermal process. ALD is a newer variant of chemical vapor deposition in which materials are deposited on a surface by a multiple sequence of ‘self-limiting’ chemical reactions. This unique sequential process enables a highly uniform deposition of dots/films and accurate control over not only the size/thickness at the atomic scale but also chemical composition and location of deposition. In this presentation, we will present 1) the feasibility of using TiO2-x incorporated rGO as ORR electrode in terms of catalytic activity and cyclability and 2) the correlation among the ALD process parameters, resulting chemical composition/structure and ORR catalytic activity. This project is funded by the NASA MUREP Institutional Research Opportunity (MIRO) Program (Grant No. NNX15AQ01A). S.G. is supported by NASA Advanced STEM Training and Research (ASTAR) Fellowship. Figure 1

(Invited) Photoelectrochemical Investigation of Ta3N5 Films on TCO Substrates Synthesized Via Atomic Layer Deposition for Photocatalytic Water Splitting

Solar driven photoelectrochemical (PEC) water splitting is a promising method to store solar energy in chemical bonds. Tantalum nitride (Ta3N5) has recently emerged as a promising candidate for PEC water splitting. Most of the studies on Ta3N5 begin with a similar synthetic route, consisting of an initial oxidation of Ta(0) metal to Ta(IV), followed by ammonolysis at elevated temperatures ( > 850 °C). Despite the simplicity of this method, there are also several drawbacks. Importantly, the high temperature ammonolysis limits the utilization of transparent conductive oxide (TCO) substrates which are necessary for fundamental photoelectrochemical measurements. In addition, lack of viable TCOs thwart use of Ta3N5 as the top electrode in a tandem PEC cell. As a solution to the above problems, we will present the synthesis of Ta-doped TiO2 (TTO) as a TCO substrate via atomic layer deposition (ALD) which we found to be stable in reducing atmospheres. ALD was also used to directly deposit thin films of Ta3N5 on TTO. While initial attempts produced as-deposited films are primarily amorphous TaOxNy, these films can be nitridized to Ta3N5 at far more moderate conditions compared to all previous reports and ALD–deposited tantalum oxide analogue films. The photoelectrochemical properties of the TTO/Ta3N5 films were investigated and the PEC water oxidation performance was analyzed. The excellent material control reported here allowed for a detailed material structure – function relationship to be determined and a path to improved performance elucidated. Initial results of a modified procedure, including ALD precursor and temperature variations, which allowed the direct synthesis of crystalline Ta3N5 on TTO substrates, will also be presented.

Investigation on Alumina Passivation for Improved IGZO TFT Performance

A study on the influence of the back-channel interface on the operation of bottom-gate indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) is presented. TFTs without any passivation material deposited typically exhibit best-case initial results. However a passivation layer is required for device stability and process integration. The surface of IGZO is very sensitive to deposition conditions of passivation materials; in general additional oxygen vacancies are created which renders the IGZO conductive. Annealing in an oxidizing ambient may reverse this effect and result in devices which exhibit appropriate semiconducting properties. The impact of passivation using alumina deposited via electron beam evaporation and atomic layer deposition (ALD) has been investigated. A decrease in TFT channel mobility is attributed to the IGZO/alumina back-channel interface degradation associated with the alumina passivation process. Low temperature ALD (e.g. T ≤ 150°C) has been explored to minimize this degradation effect. Pre-passivation and post-passivation anneal treatments have also been explored to better understand the mechanisms involved. Improvements in the back-channel interface were shown to depend on the thickness and deposition method of the alumina passivation layer. Temperature-dependent measurements reveal differences in electrical characteristics including bias-stress stability. Modifications in the passivation and annealing procedures and process integration details have resulted in a marked improvement in the performance of passivated devices.

Systematic molecular engineering of Zn-ketoiminates for application as precursors in atomic layer depositions of zinc oxide.

Molecular engineering of seven closely related zinc ketoiminates, namely, [Zn(dapki)2], [Zn(daeki)2], [Zn(epki)2], [Zn(eeki)2], [Zn(mpki)2], [Zn(meki)2], and [Zn(npki)2], leads to the optimisation of precursor thermal properties in terms of volatilisation rate, onset of volatilisation, reactivity and thermal stability. The influence of functional groups at the imine side chain of the ligands on the precursor properties is studied with regard to their viability as precursors for atomic layer deposition (ALD) of ZnO. The synthesis of [Zn(eeki)2], [Zn(epki)2] and [Zn(dapki)2] and the crystal structures of [Zn(mpki)2], [Zn(eeki)2], [Zn(dapki)2] and [Zn(npki)2] are presented. From the investigation of the physico-chemical characteristics, it was inferred that all compounds are monomeric, volatile and exhibit high thermal stability, all of which make them promising ALD precursors. Compound [Zn(eeki)2] is in terms of thermal properties the most promising Zn-ketoiminate. It is reactive towards water, possesses a melting point of 39 °C, is stable up to 24 days at 220 °C and has an extended volatilisation rate compared to the literature known Zn-ketoiminates. It demonstrated self-saturated, water assisted growth of zinc oxide (ZnO) with growth rates in the order of 1.3 Å per cycle. Moreover, it displayed a broad temperature window from TDep = 175-300 °C and is the first report of a stable high temperature (>200 °C) ALD process for ZnO returning highly promising growth rates.

Self-limiting atomic layer deposition of conformal nanostructured silver films

The controlled deposition of ultra-thin conformal silver nanoparticle films is of interest for applications including anti-microbial surfaces, plasmonics, catalysts and sensors. While numerous techniques can produce silver nanoparticles, few are able to produce highly conformal coatings on high aspect ratio surfaces, together with sub-nanometre control and scalability. Here we develop a self-limiting atomic layer deposition (ALD) process for the deposition of conformal metallic silver nanoparticle films. The films have been deposited using direct liquid injection ALD with ((hexafluoroacetylacetonato)silver(I)(1,5-cyclooctadiene)) and propan-1-ol. An ALD temperature window between 123 and 128°C is identified and within this range self-limiting growth is confirmed with a mass deposition rate of ∼17.5ng/cm2/cycle. The effects of temperature, precursor dose, co-reactant dose and cycle number on the deposition rate and on the properties of the films have been systematically investigated. Under self-limiting conditions, films are metallic silver with a nano-textured surface topography and nanoparticle size is dependent on the number of ALD cycles. The ALD reaction mechanisms have been elucidated using in-situ quartz crystal microbalance (QCM) measurements, showing chemisorption of the silver precursor, followed by heterogeneous catalytic dehydrogenation of the alcohol to form metallic silver and an aldehyde.

Atomic layer deposition of NiS and its application as cathode material in dye sensitized solar cell

Nickel sulfide (NiS) is grown by atomic layer deposition (ALD) using sequential exposures of bis(2,2,6,6-tetramethylheptane-3,5-dionate)nickel(II) [Ni(thd)2] and hydrogen sulfide (H2S) at 175 °C. Complementary combinations of in situ and ex situ characterization techniques are used to understand the deposition chemistry and the nature of film growth. The saturated growth rate of ca. 0.21 Å per ALD cycle is obtained, which is constant within the ALD temperature window (175–250 °C). As deposited films on glass substrates are found polycrystalline without any preferred orientation. Electrical transport measurement reveals degenerative/semimetallic characteristics with a carrier concentration of ca. 9 × 1022 cm−3 at room temperature. The ALD grown NiS thin film demonstrates high catalytic activity for the reduction of I−/I3− electrolyte that opens its usage as cost-effective counter electrode in dye sensitized solar cells, replacing Pt.

Temperature dependent interfacial and electrical characteristics during atomic layer deposition and annealing of HfO2 films in p-GaAs metal–oxide–semiconductor capacitors**Project supported by the Advance Research Project of China (No. 5130803XXXX) and the National Natural Science Foundation of China (No. 61176070).

We have investigated the temperature dependent interfacial and electrical characteristics of p-GaAs metal–oxide–semiconductor capacitors during atomic layer deposition (ALD) and annealing of HfO2 using the tetrakis (ethylmethyl) amino hafnium precursor. The leakage current decreases with the increase of the ALD temperature and the lowest current is obtained at 300 °C as a result of the Frenkel-Poole conduction induced leakage current being greatly weakened by the reduction of interfacial oxides at the higher temperature. Post deposition annealing (PDA) at 500 °C after ALD at 300 °C leads to the lowest leakage current compared with other annealing temperatures. A pronounced reduction in As oxides during PDA at 500 °C has been observed using X-ray photoelectron spectroscopy at the interface resulting in a proportional increase in Ga2O3. The increment of Ga2O3 after PDA depends on the amount of residual As oxides after ALD. Thus, the ALD temperature plays an important role in determining the high-k/GaAs interface condition. Meanwhile, an optimum PDA temperature is essential for obtaining good dielectric properties.

Atomic layer deposited Co/γ-Al2O3 catalyst with enhanced cobalt dispersion and Fischer–Tropsch synthesis activity and selectivity

Co/γ-Al2O3 catalysts were prepared by atomic layer deposition (ALD) of Co(acac)2 and impregnation of Co(NO3)2, characterized by HRTEM, XRD, XPS, TPR, H2-TPD, O2 titration, and Raman spectroscopy methods, and used for Fischer–Tropsch synthesis (FTS). The ALD temperature window for self-limited chemisorption of Co(acac)2, rather than its condensation at lower or decomposition at higher temperatures, is 175–205°C. The period for Co(acac)2 saturated chemisorption and its complete reaction with ozone are also determined. Smaller cubic Co3O4 nanoparticles are formed in the ALD method, while spherical ones are developed in the impregnation method. The ALD catalysts have up to 2.3 times higher dispersions at the lower number of ALD cycles. However, more cobalt aluminates are formed on the ALD catalysts. In FTS, the catalyst with 6 ALD cycles shows 38% higher conversion, 30% higher C5+ selectivity, 28% less methane selectivity, and stabilizes at earlier time on stream, as compared to the impregnation catalyst with the same loading.

Low-temperature atomic layer deposition of copper(II) oxide thin films

Copper(II) oxide thin films were grown by atomic layer deposition (ALD) using bis-(dimethylamino-2-propoxide)copper [Cu(dmap)2] and ozone in a temperature window of 80–140 °C. A thorough characterization of the films was performed using x-ray diffraction, x-ray reflectivity, UV-Vis spectrophotometry, atomic force microscopy, field emission scanning electron microscopy, x-ray photoelectron spectroscopy, and time-of-flight elastic recoil detection analysis techniques. The process was found to produce polycrystalline copper(II) oxide films with a growth rate of 0.2–0.3 Å per cycle. Impurity content in the films was relatively small for a low temperature ALD process.