Low-temperature Atomic Layer Deposition Research Articles

Nature Degradable, Flexible, and Transparent Conductive Substrates from Green and Earth-Abundant Materials

The rapid development of wearable and disposable electronic devices and the rising awareness of environmental sustainability impose growing new demands on the nature degradability of current electronic and energy systems. Here we report a new type of flexible transparent conductive paper completely made from green and earth abundant materials which are also fully degradable and recyclable. Aluminum-doped zinc oxide (AZO) was deposited by low-temperature atomic layer deposition (ALD) as the transparent conductive oxide (TCO) layer on transparent cellulose nanofibril (CNF) papers. The mesoporous structure of the CNF paper rendered strong adhesion of the AZO layer and exhibited excellent mechanical integrity and electrical conductivity within a wide range of tensile and compressive strains. The AZO-CNF paper could be completely dissolved in warm city water after one-hour stirring, demonstrating an excellent nature degradability. A flexible and transparent triboelectric nanogenerator (TENG) was further fabricated using such AZO-CNF papers with a performance that was comparable to other synthetic polymer-based systems. This work illustrated a new and promising strategy of utilizing 100% green and degradable materials in novel electronic and energy harvesting devices.

Fabrication and characterization of infrared-insulating cotton fabrics by ALD

To produce cotton fabrics with infrared-insulating properties, TiO2 nanoparticles are uniformly and firmly coated onto the fabric using low-temperature atomic layer deposition (ALD) technology at 90 °C. Cotton fabric samples were tested and the experimental data indicated that the fabrics treated by the ALD process had excellent infrared resistance and reflection properties, with the infrared and thermal insulation rates of 67.6 and 28.5% increasing to 86.7 and 63.9%, respectively, after 2000 ALD cycles. The thickness of the fabric increased with the number of ALD cycles, as determined using a fabric thickness gauge. The fabric surface morphology and fibre cross-sections were examined using a scanning electron microscope and transmission electron microscope, respectively. Energy-dispersive X-ray spectroscopy was used to confirm that TiO2 was the main component of the coating on the surface of cotton fabric. Beyond that, an infrared camera was used to detect the temperature of the upper and lower surfaces of the fabric and a thermal gravimetric analyser was used to show how the TiO2 particles were deposited on the cotton fabric.

Atomic Layer Deposition of Silica on Carbon Nanotubes

Low-temperature ozone-assisted atomic layer deposition (ALD) of SiO2 with four silane derivatives (3-aminopropyl)triethoxysilane (APTES), bis(diethylamino)silane (BDEAS), diphenylaminosilane (DPAS), and triethylsilane on carbon nanotubes (CNTs) leads to the one step formation of SiO2 nanotubes. In the process, CNTs act as templates and are removed during the ongoing deposition. From transmission electron microscopy images, the formation of a void between the CNTs surface and the SiO2 coating was observed, indicating an unexpected removal of carbon from the CNTs. This gap grows as the number of ALD cycles is increased, eventually leading to SiO2 nanotubes almost free of carbon. ATR-IR and EELS spectra proved the SiO2 formation. Depending on the CNTs templates used in this process, different morphologies of one-dimensional SiO2 nanostructures are obtained, including simple nanotubes, hollow wall nanotubes, tube-in-tube structures, and SiO2 nanowires. The application of this process on vertically aligned CNT...

Flexible thin film pH sensor based on low‐temperature atomic layer deposition

Flexible and transparent zinc oxide (ZnO) thin film field‐effect transistors (TF‐FET) for the use as small volume potentiometric pH sensors are developed. Low temperature atomic layer deposition (ALD) is used for the fabrication of the metal oxides ZnO and aluminum dioxide (Al2O3). Changing the deposition temperature of the ZnO from 150 to 100 °C allowed a significant increase in resistivity by four orders of magnitude. Hence, adjusting the controlled low carrier concentration for the field‐effect based sensor is demonstrated. ZnO TF‐FET pH sensors fabricated on silicon/silicon dioxide (Si/SiO2) substrates are compared with sensors based on flexible and transparent polyethylene naphthalate (PEN) foil substrates. Comparison of both types of pH sensors showed successful pH sensitivity for pH ranging from 5 to 10 in both cases.

Label-free SnO2 nanowire FET biosensor for protein detection

Novel tin oxide field-effect-transistors (SnO2 NW-FET) for pH and protein detection applicable in the healthcare sector are reported. With a SnO2 NW-FET the proof-of-concept of a bio-sensing device is demonstrated using the carrier transport control of the FET channel by a (bio-) liquid modulated gate. Ultra-thin Al2O3 fabricated by a low temperature atomic layer deposition (ALD) process represents a sensitive layer to H+ ions safeguarding the nanowire at the same time. Successful pH sensitivity is demonstrated for pH ranging from 3 to 10. For protein detection, the SnO2 NW-FET is functionalized with a receptor molecule which specifically interacts with the protein of interest to be detected. The feasibility of this approach is demonstrated via the detection of a biotinylated protein using a NW-FET functionalized with streptavidin. An immediate label-free electronic read-out of the signal is shown. The well-established Enzyme-Linked Immunosorbent Assay (ELISA) method is used to determine the optimal experimental procedure which would enable molecular binding events to occur while being compatible with a final label-free electronic read-out on a NW-FET. Integration of the bottom-up fabricated SnO2 NW-FET pH- and biosensor into a microfluidic system (lab-on-a-chip) allows the automated analysis of small volumes in the 400 μl range as would be desired in portable on-site point-of-care (POC) devices for medical diagnosis.

High‐Performance High‐Loading Lithium–Sulfur Batteries by Low Temperature Atomic Layer Deposition of Aluminum Oxide on Nanophase S Cathodes

This study examines the effects of nanophase S and surface coatings via atomic layer deposition (ALD) on high‐loading sulfur cathodes for developing high‐performance and high‐energy lithium–sulfur (Li–S) batteries. It is first verified that ball milling is an effective and facile route for nanoengineering microsized S powders and the resultant nanoscale S particles exhibit better performance. Using these ball milled nanoscale S cathodes, it is found that ALD Al2O3 performed at 50 °C yields deposits that evolve with ALD cycles from dispersed nanoparticles, to porous, connected films, and finally to dense and continuous films. Moreover, this low temperature ALD process suppresses S loss by sublimation. The ALD Al2O3 greatly improves sulfur cathode sustainable capacity and Coulombic efficiency. This study postulates two different mechanisms underlying the effects of ALD Al2O3 surface coatings depending on their morphology. ALD Al2O3 nanoparticles dispersed on the sulfur surface mainly function to adsorb polysulfides, thereby inhibiting S shuttling and improving sustainable capacity and Coulombic efficiency. By contrast, ALD Al2O3 films behave as a physical barrier to prevent polysulfides from contacting the liquid electrolyte and dissolving. The dispersed Al2O3 nanoparticles improve both sustainable capacity and Coulombic efficiency while the closed Al2O3 films improve Coulombic efficiency while decreasing the capacity.

Photocatalytic C60-amorphous TiO2 composites prepared by atomic layer deposition

Nanocomposites of TiO2 and single fullerene (C60) molecule are prepared by atomic layer deposition (ALD). To create nucleation sites for the ALD reaction, the bare fullerene is functionalized by H2SO4/HNO3 treatment, which results in C60-SO3H. After a NaOH washing step the intermediate hydrolyzes into C60OH. This process and the consecutive ALD growth of TiO2 are monitored with FTIR, TG/DTA-MS, EDX, Raman, FTIR, XRD, and TEM measurements. Although the TiO2 grown by ALD at 80 and 160°C onto fullerol is amorphous it enhances the decomposition of methyl orange under UV exposure. This study proves that amorphous TiO2 grown by low temperature ALD has photocatalytic activity, and it can be used e.g. as self-cleaning coatings also on heat sensitive substrates.

Abrasion resistance of ZnO and ZnO:Al films on glass substrates by atomic layer deposition

Scratches and constant abrasive wear can result in zinc oxide (ZnO) and aluminum doped zinc oxide (ZnO:Al) layers that render many devices impracticable to use. Currently, the ZnO and ZnO:Al layers are widely used as transparent conductivity oxide (TCO). The top and/or bottom electrodes based on ZnO and ZnO:Al were used in organic light-emitting diodes (OLED), organic and perovskite solar cells in order to replace expensive indium tin oxide (ITO). Consequently, knowledge of the frictional properties of the ZnO layer is a necessity for many scientists. In this study, the tribological properties of zinc oxide that was formed by the Low Temperature Atomic Layer Deposition (LT ALD) method were investigated. ALD-layers (ZnO and ZnO:Al) were deposited on a glass substrates in pre-selected conditions in order to achieve films appropriate for the application of photovoltaics. The results indicate a good adhesion of the films to the substrate. The layers were abraded during friction with Al2O3 ball and their wear intensity depend on hardness of the film/substrate system. The friction coefficient of ZnO film reached value 0.1 and was higher than friction coefficient of ZnO:Al film equal 0.08. The tribological properties of these films are compared with those reported for bulk ZnO and those films deposited by sputtering.

Low-Temperature Atomic Layer Deposition of MoS2 Films.

Wet chemical screening reveals the very high reactivity of Mo(NMe2 )4 with H2 S for the low-temperature synthesis of MoS2 . This observation motivated an investigation of Mo(NMe2 )4 as a volatile precursor for the atomic layer deposition (ALD) of MoS2 thin films. Herein we report that Mo(NMe2 )4 enables MoS2 film growth at record low temperatures-as low as 60 °C. The as-deposited films are amorphous but can be readily crystallized by annealing. Importantly, the low ALD growth temperature is compatible with photolithographic and lift-off patterning for the straightforward fabrication of diverse device structures.

Enhanced photoelectrochemical efficiency and stability using a conformal TiO2 film on a black silicon photoanode

Black silicon (b-Si) is a surface-nanostructured Si with extremely efficient light absorption capability and is therefore of interest for solar energy conversion. However, intense charge recombination and low electrochemical stability limit the use of b-Si in photoelectrochemical solar-fuel production. Here we report that a conformal, ultrathin, amorphous TiO2 film deposited by low-temperature atomic layer deposition (ALD) on top of b-Si can simultaneously address both of these issues. Combined with a Co(OH)2 thin film as the oxygen evolution catalyst, this b-Si/TiO2/Co(OH)2 heterostructured photoanode was able to produce a saturated photocurrent density of 32.3 mA cm−2 at an external potential of 1.48 V versus reversible reference electrode (RHE) in 1 M NaOH electrolyte. The enhanced photocurrent relative to planar Si and unprotected b-Si photoelectrodes was attributed to the enhanced charge separation efficiency as a result of the effective passivation of defective sites on the b-Si surface. The 8-nm ALD TiO2 layer extends the operational lifetime of b-Si from less than half an hour to four hours. Nanostructured black silicon can be used as a photoelectrode for solar-driven water splitting, but its high surface area can increase charge recombination and accelerate corrosion. Here the authors show that a thin, conformal film of TiO2 can increase both the photocurrent and the stability of black silicon.

Atomic Layer Deposition of Cubic and Orthorhombic Phase Tin Monosulfide

Tin monosulfide (SnS) is a promising light-absorbing material with weak environmental constraints for application in thin film solar cells. In this paper, we present low-temperature atomic layer deposition (ALD) of high-purity SnS of both cubic and orthorhombic phases. Using tin(II) 2,4-pentanedionate [Sn(acac)2] and hydrogen sulfide (H2S) as precursors, controlled growth of the two polymorphs is achieved. Quartz crystal microbalance measurements are used to establish saturated conditions and show that the SnS ALD is self-limiting over temperatures from at least 80 to 160 °C. In this temperature window, a stable mass gain of 19 ng cm–2 cycle–1 is observed. The SnS thin film crystal structure and morphology undergo significant changes depending on the conditions. High-resolution transmission electron microscopy and X-ray diffraction demonstrate that fully saturated growth requires a large H2S dose and results in the cubic phase. Smaller H2S doses and higher temperatures favor the orthorhombic phase. The op...

Dependence of electron mobility on gate voltage sweeping width and deposition temperature in MOSFETs with HfO2/Al2O3/InGaAs gate stacks

In this study, we fabricated MOSFETs with Al2O3/InGaAs or HfO2/Al2O3/InGaAs gate stacks. The surface was subjected to nitrogen plasma and trimethylaluminum cleaning prior to low-temperature atomic layer deposition. Electron mobility was extracted using the capacitance–gate voltage (C–VG) and drain current–gate voltage (ID–VG) characteristics. We determined that the mobility decreased when the gate voltage sweeping width increased during C–VG and ID–VG measurements. In addition, we determined that the lowering of the deposition temperature to 120 °C improved the mobility of MOSFETs with HfO2/Al2O3/InGaAs gate stacks as compared with that corresponding to deposition at 300 °C. Furthermore, HfO2/Al2O3/InGaAs gate stacks with various Al2O3 thicknesses were fabricated. When the number of Al2O3 deposition cycles was more than 4, the mobility of MOSFETs with HfO2/Al2O3/InGaAs gate stacks improved, reaching the value of the Al2O3/InGaAs gate stack.

Interface Engineering for Extremely Large Grains in Explosively Crystallized TiO2 Films Grown by Low-Temperature Atomic Layer Deposition

Functionality in thin films is closely linked with both the microstructure of the film as well as the innate nature of the thin film material. The engineering of microstructure, especially the grain size and its distribution in oxide thin films, is critical to designing functionalities targeting specific applications. However, the scope for manipulation of the microstructure of an oxide thin film is generally quite limited because of the lack of mobility of atoms at processing temperatures that are usually employed for the growth of these films. This work reports the observation of very large grains (∼100 times larger than the film thickness) in TiO2 films grown by atomic layer deposition at low temperature (<250 °C). Large grain sizes were observed in the as-grown films even in the absence of postdeposition annealing, wherein substrates with high surface energies were used. It is concluded that the surface energy engineering of substrate offers an efficient method to control the grain size of TiO2 films ...

Post-deposition-annealing effect on current conduction in Al2O3 films formed by atomic layer deposition with H2O oxidant

Atomic-layer-deposition (ALD) Al2O3 films are promising as gate insulators of non-Si semiconductor devices. Although they allow relatively small leakage currents just after deposition, ALD Al2O3 films formed at low temperatures are subject to high temperature during fabrication or operation of devices. Therefore, the effect of post-deposition annealing (PDA) on the properties of Al2O3 films is investigated in this study. ALD Al2O3 films formed using H2O oxidant at low temperatures are compacted by PDA, but their mass density and dielectric constant remain approximately unchanged or slightly decrease owing to the desorption of methyl groups contained in the films as impurities. In accordance with these results, the wet etching rate of Al2O3 films is not much reduced by PDA. The conduction current in ALD Al2O3 films formed on Si is reduced by PDA and becomes smaller than that in films formed at the same ALD temperatures as those of PDA. The conduction current for PDA temperatures above 250 °C, however, increases and, accordingly, spoils the merit of low-temperature ALD. Therefore, given that the dielectric constant of annealed films remains low, high-temperature ALD is practically more significant than applying PDA to low-temperature ALD Al2O3 films from the viewpoint of leakage current under the same thermal budget. Space-charge-controlled field emission analysis revealed that, at the aforementioned threshold temperature, PDA abruptly increases the Al2O3/SiO2 interfacial dipoles and simultaneously reduces the amount of the positive charge near the interface. The so-called negative-charge buildup by PDA might be caused by this decrease in the positive charge.

Chemical vapor deposition of monolayer MoS2 directly on ultrathin Al2O3 for low-power electronics

Monolayer MoS2 has recently been identified as a promising material for high-performance electronics. However, monolayer MoS2 must be integrated with ultrathin high-κ gate dielectrics in order to realize practical low-power devices. In this letter, we report the chemical vapor deposition (CVD) of monolayer MoS2 directly on 20 nm thick Al2O3 grown by atomic layer deposition (ALD). The quality of the resulting MoS2 is characterized by a comprehensive set of microscopic and spectroscopic techniques. Furthermore, a low-temperature (200 °C) Al2O3 ALD process is developed that maintains dielectric integrity following the high-temperature CVD of MoS2 (800 °C). Field-effect transistors (FETs) derived from these MoS2/Al2O3 stacks show minimal hysteresis with a sub-threshold swing as low as ∼220 mV/decade, threshold voltages of ∼2 V, and current ION/IOFF ratio as high as ∼104, where IOFF is defined as the current at zero gate voltage as is customary for determining power consumption in complementary logic circuits. The system presented here concurrently optimizes multiple low-power electronics figures of merit while providing a transfer-free method of integrating monolayer MoS2 with ultrathin high-κ dielectrics, thus enabling a scalable pathway for enhancement-mode FETs for low-power applications.

Highly Flexible and Transparent Ag Nanowire Electrode Encapsulated with Ultra-Thin Al2O3: Thermal, Ambient, and Mechanical Stabilities

There is an increasing demand in the flexible electronics industry for highly robust flexible/transparent conductors that can withstand high temperatures and corrosive environments. In this work, outstanding thermal and ambient stability is demonstrated for a highly transparent Ag nanowire electrode with a low electrical resistivity, by encapsulating it with an ultra-thin Al2O3 film (around 5.3 nm) via low-temperature (100 °C) atomic layer deposition. The Al2O3-encapsulated Ag nanowire (Al2O3/Ag) electrodes are stable even after annealing at 380 °C for 100 min and maintain their electrical and optical properties. The Al2O3 encapsulation layer also effectively blocks the permeation of H2O molecules and thereby enhances the ambient stability to greater than 1,080 h in an atmosphere with a relative humidity of 85% at 85 °C. Results from the cyclic bending test of up to 500,000 cycles (under an effective strain of 2.5%) confirm that the Al2O3/Ag nanowire electrode has a superior mechanical reliability to that of the conventional indium tin oxide film electrode. Moreover, the Al2O3 encapsulation significantly improves the mechanical durability of the Ag nanowire electrode, as confirmed by performing wiping tests using isopropyl alcohol.

Characterization of Al2O3 and ZnO multilayer thin films deposited by low temperature thermal atomic layer deposition on transparent polyimide

Recently, flexible and stretchable displays with organic light emitting diodes are one of hottest technical issues in electronic industries. Therefore, the application of polymer substrates is required because current glass-based materials cannot meet the required properties of flexible and stretchable displays. But there are some limitations for using polymers in display substrate such as inferior surface characteristics such as low scratch resistance, low hardness, and so on. Polymers have many additional technical issues like low stiffness, low heat resistance, low barrier properties, and high sensitivity to degradation compared with glass material. The low temperature atomic layer deposition (ALD) can be a good solution as a functional coating on the polymer substrate to resolve such surface characteristic issues. In this study, two oxide thin films, i.e., Al2O3 and ZnO, are deposited on a transparent polyimide (TPI) film substrate using a low temperature thermal ALD technique. To fabricate a reliable and high quality thin film, the pretreatment using O2 plasma was conducted on the surface of TPI. Two oxide thin films are deposited as multilayer thin films by layer-by-layer process to investigate the effect of multilayer deposition of oxide thin films on mechanical properties. Surface morphologies of the deposited thin films are observed using optical microscopy and scanning electron microscope. Chemical characteristics of the deposited thin film are analyzed using x-ray photoelectron spectroscopy. The physical and mechanical characteristics of the Al2O3 and ZnO multilayer thin film are investigated using nanoindentation and microtensile tests.

Distribution of oxygen functional groups of graphene oxide obtained from low-temperature atomic layer deposition of titanium oxide

The island-like distribution of the oxygen functional groups of graphene oxide was identified by deposition of TiO2 on the graphene oxide surface using low-temperature atomic layer deposition.

Low-temperature atomic layer deposition delivers more active and stable Pt-based catalysts.

We tailored the size distribution of Pt nanoparticles (NPs) on graphene nanoplatelets at a given metal loading by using low-temperature atomic layer deposition carried out in a fluidized bed reactor operated at atmospheric pressure. The Pt NPs deposited at low temperature (100 °C) after 10 cycles were more active and stable towards the propene oxidation reaction than their high-temperature counterparts. Crucially, the gap in the catalytic performance was retained even after prolonged periods of time (>24 hours) at reaction temperatures as high as 450 °C. After exposure to such harsh conditions the Pt NPs deposited at 100 °C still retained a size distribution that is narrower than the one of the as-synthesized NPs obtained at 250 °C. The difference in performance correlated with the difference in the number of facet sites as estimated after the catalytic test. Our approach provides not only a viable route for the scalable synthesis of stable supported Pt NPs with tailored size distributions but also a tool for studying the structure-function relationship.

Stabilizing organic photocathodes by low-temperature atomic layer deposition of TiO2

Low-temperature atomic layer deposition forms a compact TiO2 film atop a polymer light absorber for stable and efficient organic–inorganic photo-driven H2 evolution.