High-performance bilayer flexible resistive random access memory based on low-temperature thermal atomic layer deposition

We demonstrated a flexible resistive random access memory device through a low-temperature atomic layer deposition process. The device is composed of an HfO₂/Al₂O₃-based functional stack on an indium tin oxide-coated polyethylene terephthalate substrate. After the initial reset operation, the device exhibits a typical bipolar, reliable, and reproducible resistive switching behavior. After a 10⁴-s retention time, the memory window of the device is still in accordance with excellent thermal stability, and a 10-year usage is still possible with the resistance ratio larger than 10 at room temperature and at 85°C. In addition, the operation speed of the device was estimated to be 500 ns for the reset operation and 800 ns for the set operation, which is fast enough for the usage of the memories in flexible circuits. Considering the excellent performance of the device fabricated by low-temperature atomic layer deposition, the process may promote the potential applications of oxide-based resistive random access memory in flexible integrated circuits.     

Nonlinear growth of zinc tin oxide thin films prepared by atomic layer deposition

Zinc tin oxide (ZTO) thin films can be deposited by atomic layer deposition (ALD) with adjustable electrical, optical and structural properties. However, the ternary ALD processes usually suffer from low growth rate and difficulty in controlling film thickness and elemental composition, due to the interaction of ZnO and SnO₂ processes. In this work, ZTO thin films with different Sn levels are prepared by ALD super cycles using diethylzinc, tetrakis(dimethylamido)tin, and water. It is observed that both the film growth rate and atom composition show nonlinear variation versus [Sn]/([Sn]+[Zn]) cycle ratio. The experimental thickness measured by spectroscopic ellipsometry and X-ray reflectivity are much lower than the expected thickness linearly interpolated from pure ZnO and SnO_x films. The [Sn]/([Sn]+[Zn]) atom ratios estimated by X-ray photoelectron spectroscopy have higher values than that expected from the cycle ratios. Hence, to characterize the film growth behavior versus cycle ratio, a numerical method is proposed by simulating the effect of reduced density and reactivity of surface hydroxyls and surface etching reactions. The structure, electrical and optical properties of ZTO with different Sn levels are also examined by X-ray diffraction, atomic force microscope, Hall measurements and ultraviolet–visible–infrared transmittance spectroscopy. The ZTO turns out to be transparent nanocrystalline or amorphous films with smooth surface. With more Sn contents, the film resistivity gets higher (>1 Ω cm) and the optical bandgap rises from 3.47 to 3.83 eV.     

Improvement of carrier diffusion length in silicon nanowire arrays using atomic layer deposition

To achieve a high-efficiency silicon nanowire (SiNW) solar cell, surface passivation technique is very important because a SiNW array has a large surface area. We successfully prepared by atomic layer deposition (ALD) high-quality aluminum oxide (Al₂O₃) film for passivation on the whole surface of the SiNW arrays. The minority carrier lifetime of the Al₂O₃-depositedSiNW arrays with bulk silicon substrate was improved to 27 μs at the optimum annealing condition. To remove the effect of bulk silicon, the effective diffusion length of minority carriers in the SiNW array was estimated by simple equations and a device simulator. As a result, it was revealed that the effective diffusion length in the SiNW arrays increased from 3.25 to 13.5 μm by depositing Al₂O₃ and post-annealing at 400°C. This improvement of the diffusion length is very important for application to solar cells, and Al₂O₃ deposited by ALD is a promising passivation material for a structure with high aspect ratio such as SiNW arrays.     

Low thermal conductivity of atomic layer deposition yttria-stabilized zirconia (YSZ) thin films for thermal insulation applications

Thermal insulation applications have long required materials with low thermal conductivity, and one example is yttria (Y₂O₃)-stabilized zirconia (ZrO₂) (YSZ) as thermal barrier coatings used in gas turbine engines. Although porosity has been a route to the low thermal conductivity of YSZ coatings, nonporous and conformal coating of YSZ thin films with low thermal conductivity may find a great impact on various thermal insulation applications in nanostructured materials and nanoscale devices. Here, we report on measurements of the thermal conductivity of atomic layer deposition-grown, nonporous YSZ thin films of thickness down to 35 nm using time-domain thermoreflectance. We find that the measured thermal conductivities are 1.35–1.5 W m⁻¹ K⁻¹ and do not strongly vary with film thickness. Without any reduction in thermal conductivity associated with porosity, the conductivities we report approach the minimum, amorphous limit, 1.25 W m⁻¹ K⁻¹, predicted by the minimum thermal conductivity model.     

Effects of atomic layer deposition conditions on the formation of thin ZnO films and their photocatalytic characteristics

The use of photocatalysts in water treatment systems is regarded as an advanced technology. To ensure efficiency and stability, the optimization of photocatalyst immobilization is essential for application in water treatment processes. In this study, we investigated the effect of atomic layer deposition (ALD) conditions on the development of highly photocatalytically active thin ZnO films. Three different temperatures and three ALD cycles were employed to evaluate the photocatalytic activity of thin ZnO films (represented by the production rate of reactive oxygen species and the degradation rate of methylene blue). We found that the surface properties of the thin ZnO films, such as grain size and homogeneity, exerted a dominant influence on the photocatalytic activity. At a low temperature (50 °C), nanograins were not formed properly, while various nanograin shapes were obtained at a high temperature (250 °C). The optimized grain had a grain size of 20 nm and a (002)/(101) crystalline orientation ratio of 2.2. The UV light absorption increased in proportion to the film thickness, and a minimum film thickness (50 nm) was necessary to ensure high photocatalytic activity at the film surface. In addition, the increase in the photocatalytic activity was not significant as the thickness increased beyond the optimum thickness. These results will provide useful guidelines for the fabrication of thin ZnO films with excellent photocatalytic activity for water treatment.     

Al2O3/CrAlSiN multilayer coating deposited using hybrid magnetron sputtering and atomic layer deposition

In this study, Al₂O₃/CrAlSiN multilayer coatings with various periods were prepared using a hybrid process involving overlapping magnetron sputtering of CrAlSiN and atomic layer deposition (ALD) of Al₂O₃. The influence of the number of Al₂O₃ layers on the mechanical properties, corrosion behavior and oxidation characteristics of the coatings was studied using nano/micro indentation, electrochemical corrosion, and high temperature static oxidation tests. The results show that the multilayer structure can effectively prevent crack propagation during the coating and subsequently increase the coating toughness. A substantial improvement in the resistance to electrochemical and oxidation corrosion was observed in the Al₂O₃/CrAlSiN multilayer coatings and increasing the number of Al₂O₃ layers dramatically increases the corrosion durability. The Al₂O₃ ALD layers are expected to inhibit the diffusion of corrosive substances such as ions and oxygen and the increase of the Al₂O₃ layer number decreases the diffusion fluxes of the coating elements to the surface and limit the oxide growth, resulting in the evolution of the oxidation produces from irregular particles to nano-walls/fibers. It is supposed that the PVD/ALD hybrid process may open a new hard coating design concept by providing a superior toughness and corrosion/oxidation resistance.     

Nucleation and growth behavior of aluminum nitride film using thermal atomic layer deposition

Aluminum nitride (AlN) film possesses wide band gap energy (~6.2 eV) and a high dielectric constant (~9.2), and is resilient to thermal and chemical stimuli. It also exhibits several functionalities, such as piezoelectricity and pyroelectricity. Therefore, AlN film has been used for electronic and optoelectronic devices and micro-electromechanical systems (MEMSs). Among the various methods of AlN thin film growth, atomic layer deposition (ALD) can control film thickness at the nanoscale. Uniform and conformal film growth is possible at temperatures lower than that of chemical vapor deposition or molecular beam epitaxy. Because the ALD process relies on surface chemical reactions, it shows substrate dependency. To control film uniformity from the beginning, an understanding of nucleation and growth behavior on the substrate is necessary. Therefore, the nature of nucleation and growth behaviors on different substrates is investigated. In this study, AlN films are grown on bare Si and TiN substrates at 295–342 °C by thermal ALD using trimethyl aluminum (TMA) and ammonia. Facile nucleation and linear growth on the TiN substrate, and substrate-inhibited nucleation on the Si substrate, are observed. NH₃ pretreatment may enhance the growth rate at the nucleation stage. Therefore, the dissociation of NH₃ on the substrate is crucial to making uniform nuclei for the subsequent growth of AlN film.     

Atomic layer deposition and characterization of Bi1Se1 thin films

Van der Waals (vdWs) heterostructured materials have attracted considerable interest due to their intriguing physical properties. Here, we report on the deposition of BiSe by atomic layer deposition (ALD) using Bi(NMe₂)₃ and Se(SnMe₃)₂ as volatile and reactive Bi and Se precursors, respectively. The growth rate varies from 1.5 to 2.0 Å/cycle in the deposition temperature range of 90–120 °C. Higher deposition temperatures lead to increased grain sizes and enhanced crystallinity of resulting films. Further microstructure characterization reveals the formation of crystalline domains with varying orientations and nanotwinned boundaries. The presence of Bi-Bi zigzag bilayers and the formation of the BiSe phase were confirmed by the existence of the Bi-Bi binding energy peak in the XPS spectra and Raman spectra. Furthermore, the electrical conductivity of BiSe ranged from 1420 to 1520 S/cm due to the ultrahigh carrier concentration (2–3.5 × 10²¹ cm⁻³), which is the highest among undoped bismuth selenide-based materials.     

Atomic layer deposition of Al2O3 thin films on diamond

Atomic layer deposition (ALD) of aluminum oxide thin films on diamond was demonstrated for the first time, and the film properties as a gate insulator for diamond field effect transistor (FET) were examined. The interface between the aluminum oxide and the diamond was abrupt, and the ratio of aluminum to oxygen in the film was confirmed to be stoichiometric by Rutherford back scattering. Even a bumpy surface of polycrystalline diamond film was conformally covered by the Al₂O₃ films. To evaluate the feasibility of the film for FET gate insulator, the electrical characteristics of the Al₂O₃ films deposited by ALD on diamond were measured using metal–insulator–semiconductor structure. It was found that the Al₂O₃ films deposited by ALD were better than those deposited by conventional methods, which indicates that the ALD-Al₂O₃ films are feasible for gate insulators of diamond FETs.     

Modification of interparticle forces for nanoparticles using atomic layer deposition

Silica and titania nanoparticles were individually coated with ultrathin alumina films using atomic layer deposition (ALD) in a fluidized bed reactor. The effect of the coating on interparticle forces was studied. Coated particles showed increased interactions which impacted their flowability. This behavior was attributed to modifications of the Hamaker coefficient and the size of nanoparticles. Stronger interparticle forces translated into a larger mean aggregate size during fluidization, which increased the minimum fluidization velocity. A lower bed expansion was observed for coated particles due to enhanced interparticle forces that increased the cohesive strength of the bed. Increased cohesiveness of coated powders was also determined through angle of repose and Hausner index measurements. The dispersability of nanopowders was studied through sedimentation and z-potential analysis. The optimum dispersion conditions and isoelectric point of nanoparticle suspensions changed due to the surface modification. A novel atomic force microscope (AFM) technique was used to directly measure interactions between nanoparticles dispersed on a flat substrate and the tip of an AFM cantilever. Both Van der Waals and electrostatic interactions were detected during these measurements. Long and short range interactions were modified by the surface coating.     

Electrical and optical properties of Al-doped ZnO and ZnAl2O4 films prepared by atomic layer deposition

ZnO/Al₂O₃ multilayers were prepared by alternating atomic layer deposition (ALD) at 150°C using diethylzinc, trimethylaluminum, and water. The growth process, crystallinity, and electrical and optical properties of the multilayers were studied with a variety of the cycle ratios of ZnO and Al₂O₃ sublayers. Transparent conductive Al-doped ZnO films were prepared with the minimum resistivity of 2.4 × 10⁻³ Ω·cm at a low Al doping concentration of 2.26%. Photoluminescence spectroscopy in conjunction with X-ray diffraction analysis revealed that the thickness of ZnO sublayers plays an important role on the priority for selective crystallization of ZnAl₂O₄ and ZnO phases during high-temperature annealing ZnO/Al₂O₃ multilayers. It was found that pure ZnAl₂O₄ film was synthesized by annealing the specific composite film containing alternative monocycle of ZnO and Al₂O₃ sublayers, which could only be deposited precisely by utilizing ALD technology.     

Effect of atomic layer deposition temperature on the performance of top-down ZnO nanowire transistors

This paper studies the effect of atomic layer deposition (ALD) temperature on the performance of top-down ZnO nanowire transistors. Electrical characteristics are presented for 10-μm ZnO nanowire field-effect transistors (FETs) and for deposition temperatures in the range 120°C to 210°C. Well-behaved transistor output characteristics are obtained for all deposition temperatures. It is shown that the maximum field-effect mobility occurs for an ALD temperature of 190°C. This maximum field-effect mobility corresponds with a maximum Hall effect bulk mobility and with a ZnO film that is stoichiometric. The optimized transistors have a field-effect mobility of 10 cm²/V.s, which is approximately ten times higher than can typically be achieved in thin-film amorphous silicon transistors. Furthermore, simulations indicate that the drain current and field-effect mobility extraction are limited by the contact resistance. When the effects of contact resistance are de-embedded, a field-effect mobility of 129 cm²/V.s is obtained. This excellent result demonstrates the promise of top-down ZnO nanowire technology for a wide variety of applications such as high-performance thin-film electronics, flexible electronics, and biosensing.     

Chemistry of SiNx thin film deposited by plasma-enhanced atomic layer deposition using di-isopropylaminosilane (DIPAS) and N2 plasma

Silicon nitride (Si₃N₄) films have received great attention not only as dielectric materials for the gate dielectric of transistors and the insulator of capacitors, but also as a buffer layer and etch-stop layer for the semiconductor industry. As the applications of Si₃N₄ film increase, the necessity of investigating a novel deposition process applicable at low temperature has emerged. In this regard, the plasma-enhanced atomic layer deposition (PEALD) technique is attractive as a promising process; however, the Si₃N₄ film deposition process at growth temperatures less than 150?°C using PEALD has not been investigated. In this work, the growth behavior and chemistry of SiN_x (x?<?1.33) film deposited by the PEALD process at various growth temperatures were developed. Insufficient thermal energy from low growth temperature induces an unstable chemical state of deposited film due to the remaining unreacted ligand of adsorbed precursors. This state results in a further chemical reaction to SiO₂ formation by air exposure. Other chemical effects depending on chemical composition and electrical property were also examined in detail.     

Atomic layer deposition of TiO2 films on particles in a fluidized bed reactor

Atomic layer deposition (ALD) of controlled-thickness TiO₂ films was carried out on particle substrates in a fluidized bed reactor for the first time. Films were deposited on 550 nm SiO₂ spheres and 65 nm ZnO nanoparticles for enhanced optical properties. Nanoparticles were fluidized with the assistance of a magnetically-coupled stirring unit. The metalorganic precursor titanium tetraisopropoxide was used here followed by either H₂O or H₂O₂ to deposit TiO₂ at various substrate temperatures. Growth rates of 0.01 nm/cycle and 0.04 nm/cycle were achieved when using H₂O and H₂O₂ as the oxidizer, respectively. These conformal TiO₂ films were verified using HRTEM, ICP-AES, XPS and UV absorbance measurements. The specific surface area changed appropriately after the particle size increased by the deposition of films with a given density, which showed that primary particles were not agglomerated together due to the coating process. In situ mass spectrometry was used to monitor reaction progress throughout each ALD reaction cycle. Bulk quantities of powder were successfully functionalized by TiO₂ nanofilms without wasting excess precursor.     

Thermal conductivity of yttria-stabilized zirconia thin films grown by plasma-enhanced atomic layer deposition

Zirconia doped with yttrium, widely known as yttria-stabilized zirconia (YSZ), has found recent applications in advanced electronic and energy devices, particularly when deposited in thin film form by atomic layer deposition (ALD). Although ample studies reported the thermal conductivity of YSZ films and coatings, these data were typically limited to Y₂O₃ concentrations around 8 mol% and thicknesses greater than 1 μm, which were primarily targeted for thermal barrier coating applications. Here, we present the first experimental report of the thermal conductivity of YSZ thin films (∼50 nm), deposited by plasma-enhanced ALD (PEALD), with variable Y₂O₃ content (0–36.9 mol%). Time-domain thermoreflectance measures the effective thermal conductivity of the film and its interfaces, independently confirmed with frequency-domain thermoreflectance. The effective thermal conductivity decreases from 1.85 to 1.22 W m⁻¹ K⁻¹ with increasing Y₂O₃ doping concentration from 0 to 7.7 mol%, predominantly due to increased phonon scattering by oxygen vacancies, and exhibits relatively weak concentration dependence above 7.7 mol%. The effective thermal conductivities of our PEALD YSZ films are higher by ∼15%–128% than those reported previously for thermal ALD YSZ films with similar composition. We attribute this to the relatively larger grain sizes (∼23–27 nm) of our films.     

Thermal stability of MgO film on Si grown by atomic layer deposition using Mg(EtCp)2 and H2O

MgO films were deposited on Si via atomic layer deposition (ALD) using Mg(EtCp)₂ and H₂O precursors and their thermal stability was examined as a function of the post-deposition annealing (PDA) temperature. The characteristic self-limiting behavior of the ALD process was confirmed by changing several parameters, such as precursor pulsing times, deposition temperature, and number of cycles. The exceptional resulting step coverage was verified on a patterned wafer with a high aspect ratio. The band gap and dielectric constant of the as-deposited ALD-MgO film were extracted to be approximately 7.5 eV and 8.4, respectively, and were stable up to the PDA temperature of 700 °C. However, considerable outward diffusion of the underlying Si atoms toward MgO started to occur above 700 °C, and most of the MgO film was converted to an amorphous Mg-silicate phase at 900 °C with a thin layer of remaining MgO on top.     

Enhanced thermal stability of Bi2Te3-based alloys via interface engineering with atomic layer deposition

The ease of Te sublimation from Bi₂Te₃-based alloys significantly deteriorates thermoelectric and mechanical properties via the formation of voids. We propose a novel strategy based on atomic layer deposition (ALD) to improve the thermal stability of Bi₂Te₃-based alloys via the encapsulation of grains with a ZnO layer. Only a few cycles of ZnO ALD over the Bi₂Te_2.7Se_0.3 powders resulted in significant suppression of the generation of pores in Bi₂Te_2.7Se_0.3 extrudates and increased the density even after post-annealing at 500 °C. This is attributed to the suppression of Te sublimation from the extrudates. The ALD coating also enhanced grain refinement in Bi₂Te_2.7Se_0.3 extrudates. Consequently, their mechanical properties were significantly improved by the encapsulation approach. Furthermore, the ALD approach yields a substantial improvement in the figure-of-merit after the post-annealing. Therefore, we believe the proposed approach using ALD will be useful for enhancing the mechanical properties of Bi₂Te₃-based alloys without sacrificing thermoelectric performance.     

Effect of rapid thermal annealing on the properties of zinc tin oxide films prepared by plasma-enhanced atomic layer deposition

The effect of rapid thermal annealing treatments on the microstructure, surface morphology, and optical characteristics of zinc tin oxide (ZTO) films produced by plasma-enhanced atomic layer deposition was investigated. The ZTO films were annealed in oxygen atmosphere for 2 min at four selected temperatures from 500 to 800 °C. The X-ray diffraction showed that the annealing temperature has a great influence on the crystalline characteristics of ZTO films. The film shows complete amorphous structure for as-deposited ZTO film. Meanwhile, the spinel zinc stannate Zn₂SnO₄ was obtained for the samples annealed from 500 to 800 °C, which shows polycrystalline nature. The X-ray photoelectron spectroscopy proved that the annealing process in oxygen gas can effectively can reduce the oxygen vacancy defects in the films. In addition, the photoluminescence spectroscopy manifests an ultraviolet emission with a broad peak range from 345 to 385 nm. Moreover, the ultraviolet luminescence intensity increases continuously with the increase of annealing temperature. Spectroscopic ellipsometry analyses demonstrate that the refractive index of annealed films increases as the increase of annealing temperature, while the extinction coefficient decreases gradually with the increase of annealing temperature in the visible light range.     

Electrical characterization of ZnO-based thin film nanodiodes fabricated through atomic layer deposition: Effect of electron beam irradiation on interfacial property

ZnO thin films were deposited onto p-type (P-Si) Si wafers using atomic layer deposition. The rectifying performance of the deposited ZnO thin films was confirmed by current–voltage characteristics. P-Si/ZnO-based nanodiodes were subjected to electron irradiation. Depending on the irradiation conditions, the diode performance changed significantly. At 0.8 MeV, the diode was degraded in terms of both forward and reverse currents. At 2.5 MeV, the reverse current in the nanodiode decreased and the forward current increased, leading to significant enhancement in the current ratio. The electrical response was monitored using impedance spectroscopy. Impedance analysis indicated that depletion regions are significantly affected by electron irradiation.     

Femtosecond pulsed laser deposition of silicon thin films

Optimisation of femtosecond pulsed laser deposition parameters for the fabrication of silicon thin films is discussed. Substrate temperature, gas pressure and gas type are used to better understand the deposition process and optimise it for the fabrication of high-quality thin films designed for optical and optoelectronic applications.