Temperature Atomic Layer Deposition Research Articles

Atomic layer deposition and application of group III nitrides semiconductor and their alloys

Group III nitride semiconductors, such as GaN, AlN, and InN, are an important class of compound semiconductor material, and have attracted much attention, because of their unique physicochemical properties. These semiconductors possess excellent characteristics, such as wide direct bandgap, high breakdown field strength, high electron mobility, and good stability, and thus are called third-generation semiconductors. Their alloy materials can adjust their bandgaps by changing the type or proportion of group III elements, covering a wide wavelength range from near-ultraviolet to infrared, thereby achieving wavelength selectivity in optoelectronic devices. Atomic layer deposition (ALD) is a unique technique that produces high-quality group III nitride films at low temperatures. The ALD has become an important method of preparing group III nitrides and their alloys. The alloy composition can be easily controlled by adjusting the ALD cycle ratio. This review highlights recent work on the growth and application of group III nitride semiconductors and their alloys by using ALD. The work is summarized according to similarities so as to make it easier to understand the progress and focus of related research. Firstly, this review summarizes binary nitrides with a focus on their mechanism and application. In the section on mechanism investigation, the review categorizes and summarizes the effects of ALD precursor material, substrate, temperature, ALD type, and other conditions on film quality. This demonstrates the effects of different conditions on film growth behavior and quality. The section on application exploration primarily introduces the use of group III nitride films in various devices through ALD, analyzes the enhancing effects of group III nitrides on these devices, and explores the underlying mechanisms. Additionally, this section discusses the growth of group III nitride alloys through ALD, summarizing different deposition methods and conditions. Regarding the ALD growth of group III nitride semiconductors, there is more research on the ALD growth of AlN and GaN, and less research on InN and its alloys. Additionally, there is less research on the ALD growth of GaN for applications, as it is still in the exploratory stage, while there is more research on the ALD growth of AlN for applications. Finally, this review points out the prospects and challenges of ALD in preparation of group III nitride semiconductors and their alloys.

Influence of Increasing Deposition Temperature during Atomic Layer Deposition on Electrical Properties and Reliability in Al2O3 - Doped ZrO2 Based Two-Dimensional and Three-Dimensional Metal-Insulator-Metal Decoupling Capacitors

The influence of deposition temperature (TD) on thin film layers of high-k dielectrics used as insulators in embedded Metal-Insulator-Metal (MIM) decoupling capacitors is critical to their future behavior in terms of electrical properties and reliability. These layers are deposited via atomic layer deposition (ALD), which is known for its high uniformity in any type of structural topology. High-k dielectrics, such as Zr, inside high aspect ratio three-dimensional (3-D) structures are a challenging task even in the case of ALD processing. Al2O3 doping is often used within the high-k layer stack during ALD to optimize electrical performance of MIM structures with respect to leakage currents and reliability. In this context, appropriate atomic layer deposition temperature (TD) of the Al2O3-doped ZrO2 high-dielectric stack must be established to enable optimum electrical properties. Electrical data of decoupling capacitors must fulfill the industrial requirements, which demand that, for example, the leakage current density (J) must not exceed a certain level (<1µA/cm²) and that long-term lifetimes under different operating temperatures of a minimum of 10 years at accumulated fail rate of 10 ppm must be achieved.In this work, we report on a study of the influence of ALD TD on Al2O3-doped ZrO2 thin films fabricated on 300mm wafers. We evaluate the impact of TD on high aspect ratio substrates based on electrical properties and reliability parameters. To achieve this, we use 3 different deposition temperatures (293°C, 313°C, and 333°C) and two physical topologies (planar and 3-D capacitors with cavities-trenches) without compromising the chemical stability of the ALD precursors. For metal precursors, TEMAZ (Tetrakis-ethylmethylamino-zirconium) and TMA (Tri-methyl-aluminum) are used for Zr and Al, respectively. Ozone is used as reactant agent and argon as carrier/purge gas. The ZrO2 cycle consists of pulsing with TEMAZ, purging with Ar gas, oxidizing with O3, and purging with Ar gas. In a similar way, the dopant (TMA) is added, followed by purging steps with Ar, O3 and Ar again. After a few ZrO2 cycles, an Al2O3 cycle is followed, so that nano-laminate films of Al2O3 are formed on top of ZrO2 films. By repeating the ZrO2 and the Al2O3 cycles several times, the desired film thickness is achieved. For the MIM stacks, TiN is used as an electrode material on both sides. There was no post deposition anneal to maintain the amorphous state of the insulator with low defect density and high structural uniformity.The planar capacitors show minimal changes in terms of capacitance density with increasing TD. On the other hand, the 3-D samples show a 17% increase in capacitance density with a TD increase of 20°C from 293°C to 313°C. This increase becomes much smaller (2.3%) for a further TD increase of 20°C at 333°C. However, with increasing TD, the E-field linearity of 3-D samples, expressed by the quadratic E-field capacitor coefficient α, increases by 2% and 17% for TD=313°C and TD=333°C, respectively. The significant improvement in field linearity indicates the highest TD provides the best and most stable capacitance behavior. The J(E)-curves of planar and 3-D devices exhibit different behavior with respect to current density and dielectric breakdown. Specifically, planar devices show greater variation in J(E)-curves over temperature, with decreasing breakdown field (EBD) as TD increases. On the other hand, 3-D samples show little variation in EBD with temperature. Furthermore, planar devices show lower symmetry in their J(E)-curves over the field polarity compared to 3-D capacitors, which could be attributed to differences in conduction mechanisms. Despite having a lower EBD, planar devices reveal improved lifetime and reliability with increasing TD, possibly due to reduced carbon impurities during the insulator deposition. For instance, increasing TD from 293°C to 313°C leads to a 47% improvement in extrapolated lifetime, while a further increase in TD, improves lifetime by an additional 14%. A 10-year lifetime goal can only be achieved at 1MV/cm with TD=333°C and for samples exposed to any temperature conditions (25-150°C). In contrast, 3-D capacitors exhibit a better degradation slope at lower deposition temperatures but require the highest TD to achieve an 18% improvement in lifetime. However, due to their lower EBD, they cannot reach the 10-year lifetime goal at 1MV/cm, except at low applied temperatures (25°C-50°C).In conclusion, the highest TD of 333°C without any chemical decomposition of the metal precursor (TEMAZ) is the optimal deposition temperature for both planar and 3-D Al2O3-doped ZrO2 based MIM decoupling capacitors, providing auspicious results in terms of all relevant parameters. Figure 1

Study of Nb2O5 High-k Dielectric Material Deposited By Atomic Layer Deposition for Metal-Insulator-Metal Capacitor

Metal-Insulator-Metal (MIM) capacitors are important components in many electronic devices, including memory devices, filters, and oscillators. The performance of these devices depends strongly on the electrical properties of the dielectric layer. The application of new high-k materials can improve the properties of modern capacitors such as leakage current and capacitance density. However, it is necessary to understand the deposition conditions of both electrode and dielectric materials in order to master the dielectric properties in future MIM capacitors.One potentially interesting high-k material is niobium pentoxide (Nb2O5) with a wide band gap (3.6 eV) and high refractive index (n=2.3 @633 nm) deposited by Atomic Layer Deposition (ALD) using niobium (V) ethoxide as niobium precursor at 180°C and H2O as reaction agent and oxygen source. Nb2O5 films were deposited directly on a silicon (Si 100) substrate or on a 10nm of titanium nitride (deposited by ALD on Si substrate, n=1.4@633 nm). The Nb2O5 films were deposited with a good film thickness uniformity on the 6-inches of the wafers with a thickness ranging from 20 to 60 nm on each sample. The deposition was done under a suitable ALD temperature of 200-300 °C. In order to investigate the film growth, structure, morphology and optical properties, the samples were characterized by spectroscopic ellipsometry (SE), scanning electron microscopy (SEM), X-Ray diffraction (XRD) and atomic force microscopy (AFM). The as-deposited films were then annealed in a furnace at temperature ranging from 500 to 650°C under N2 atmosphere.Subsequently, a MIM structure (Al/Nb2O5/TiN) was produced by adding a second aluminum layer deposited at 250°C on a Nb2O5/TiN/Si stack by the sputtering method. The surface of the second electrodes was aimed to have 0.1-1.0 mm2 to confirm the surface dependency of the capacitance value. The electrical properties of Nb2O5 dielectric layer such as dielectric constant, breakdown voltage and leakage current at possible user voltage were measured on this last structure. The low leakage current density is desirable for MIM capacitors as it helps to minimize energy loss and increase the energy storage capabilities of the capacitors. The obtained J-E curve was compared to some electrical conduction mechanisms to identify its electrical conduction behaviors. The barrier height of the F-N tunneling mechanism was determined by fitting the experimental data to the F-N equation. These measurements indicate that the electrical properties of the Nb2O5 material produced by ALD with specific settings are promising for the application in capacitors. These results provide new insight into our understanding of the application of Nb2O5 realized by ALD for the MIM capacitor.

(Invited) Atomic Layer Processing of MoS2

Molybdenum disulfide (MoS2) is one of several transition metal dichalcogenides consisting of a layer of transition atoms sandwiched between layers of chalcogens. Interest in MoS2 has been driven by its 1.8 eV direct electronic band gap in monolayer form and its moderate electron mobility (10-100 cm2/Vs) even when only three atoms (~6.5 Å) thick. These properties offer potential for retaining or improving speed and efficiency in scaled electronic devices.Advancing MoS2 and other 2D materials into high volume manufacturing of semiconductor devices requires scalable deposition and etching processes. The process of atomic layer deposition (ALD) has been used to deposit 2D materials and can deposit films at lower temperatures for back-end-of-line (BEOL) compatibility. However, ALD relies on covalent surface reactions. When these reactions occur at high density during film nucleation, the resulting film is typically amorphous or nanocrystalline and requires thermal annealing to form a more well-ordered layered crystal structure. A key challenge is to establish a low temperature ALD process that achieves a layered crystal structure with low defectivity. In terms of material removal, MoS2 films have been etched by plasma-based atomic layer etching (ALE). ALE is analogous to ALD except that cyclic surface reactions promote volatility and lead to film removal rather than deposition. Together, atomic layer deposition and atomic layer etching constitute complementary facets of atomic layer processing and enable new pathways for nanomanufacturing.Here, we describe our recent progress in thermal ALD and thermal ALE of MoS2 films. For ALD of MoS2, we use cycles of MoF6 and H2S between 150-350 °C. Nucleation on metal oxide surfaces shows a strong temperature dependence, which is attributed in part due to the temperature dependence of surface hydroxyl groups. Density functional theory (DFT) calculations support the role of hydroxyls in promoting nucleation and reveal that MoF6 dissociates on the surface rather than participating in a ligand exchange reaction. The dissociation reaction leads to the formation of a metal fluoride interface between the oxide surface and the MoS2 film. Surface species following MoF6 include MoOF4 and MoO3, both of which convert to MoS2 during the H2S pulse, which releases H2O and HF byproducts. Films deposited at 200 °C and below are amorphous but convert to a layered structure upon annealing in H2S. Deposition at 250 °C yields films with a layered crystal structure without post-deposition annealing.Thermal ALE of MoS2 films deposited by ALD can be achieved using alternating exposures of MoF6 and H2O. It was found that MoS2 films are fluorinated by the MoF6 and then oxygenated by H2O. Volatile byproducts are MoF2O2, H2S, and HF, which are primarily released during the H2O half-cycle. Thermal ALE of MoS2 is temperature dependent and follows Arrhenius behavior. At 200 °C, the mass loss per cycle is 6 ng/cm2 (~0.5 Å/cycle) and reaches 271 ng/cm2 at 300 °C. Etch stop behavior was observed once the etched film approached the metal fluoride interface of the growth surface. Layered MoS2 deposited by ALD with post-deposition annealing exhibited layer-by-layer etching at 0.2 Å/cycle at 250 °C.The ALD results provide key insights into the nucleation and growth of MoS2 films using low temperature ALD. Further work is required to control basal plane orientation but achieving crystalline films well within BEOL thermal budgets is promising. The results from the ALE study provide insights into the etching reactions for MoS2. Combining the two processes offers greater control over MoS2 films. Using ALD followed by ALE and post-deposition annealing, we achieved few-layer MoS2 films. These combined thermal processes represent a pathway for integration of MoS2 films into device manufacturing.

Tailoring crystallisation of anatase TiO2 ultra-thin films grown by atomic layer deposition using 2D oxides as growth template

TiO2 ultrathin films are required in many material research areas. The anatase phase was found to be more stable in case of atomic layer deposition growth, nevertheless a critical thickness of around 10 nm appears necessary to obtain crystallization on native silicon oxide on Si (001). This work focuses on direct liquid injection Atomic Layer Deposition (ALD) of TiO2 films with thicknesses about some nanometers, using titanium tetra-isopropoxide as precursor and H2O as oxidant. A particular care to the treatment of ellipsometric measurements is employed. Below the threshold value, films remain amorphous unless the growth starts onto a crystalized surface, a thick TiO2 layer in our case. Here we propose and show the efficiency of 2-dimensional (2D) [Ca2Nb3O10]− and [Ti0.865O2]0.54− nanosheets as seeds at the surface for the crystallization and epitaxy of ultra-thin films of TiO2 below the critical thickness. Furthermore, we show that the structure of the 2D nanosheets determines the growth orientation of the epitaxial anatase, namely (010) for [Ti0.865O2]0.54−, and (100) for [Ca2Nb3O10]−. These results, based on the tailoring of crystal anisotropy via adapted 2D seed nanosheets and low temperature ALD, opens the way for opto-electronics applications of titanium oxide layers of a few unit cells.

Plasma-enhanced atomic layer deposition of molybdenum carbide and carbonitride films using bis(isopropylcyclopentadienyl)molybdenum(IV) dihydride and an H2/N2/Ar plasma

Molybdenum carbide (MoC) and molybdenum carbonitride (MoCN) films were successfully deposited by plasma-enhanced atomic layer deposition (PEALD) using bis(isopropylcyclopentadienyl)molybdenum(IV) dihydride [(iPrCp)2MoH2] as the Mo precursor at temperatures of 200−400 °C. To obtain the MoC and MoCN films, 4%H2/96%Ar (H2/Ar) and 4%H2/96%N2 (H2/N2) plasmas were selectively used as co-reactants, respectively. PEALD MoC and MoCN exhibited atomic layer deposition temperature windows of 200−400 and 250−300 °C with growth per cycle of 0.012 and 0.047 nm/cycle, respectively. X-ray photoelectron spectroscopy revealed that the 300 °C-grown MoC film prepared using an H2/Ar plasma contained Mo–C bonds and an atomic composition of MoC0.77. In contrast, the 300 °C-grown MoCN film prepared using an H2/N2 plasma exhibited Mo–C and Mo–N bonds, with an atomic composition of MoC0.31N0.23. The atomic composition of the PEALD MoCN films varied depending on the deposition temperature; at 200 °C, the carbon-rich MoC0.52N0.16 film was obtained, whereas the MoC0.23N0.23 film with a carbon-to-nitrogen ratio of 1 was grown at a higher temperature of 400 °C. The 300 °C-grown MoC film was crystallized into a cubic δ-MoC phase, whereas the PEALD MoCN film showed diffraction peaks corresponding to the hexagonal MoC and molybdenum nitride (MoN) structures. The as-deposited PEALD MoC and MoCN films at 300 °C exhibited resistivities of 600 and 3038 μΩ cm, respectively, and post-deposition annealing at 700−800 °C resulted in significantly low resistivities of 37−203 μΩ cm due to the formation of metallic Mo films.

Interplay of Precursor and Plasma for The Deposition of HfO2 via PEALD: Film Growth and Dielectric Properties

AbstractHfO2 thin films are appealing for microelectronic applications such as high‐κ dielectric layers, memristors, and ferroelectric memory devices. To fulfill the different requirements of each application, the properties of the deposited material need to be tuned accordingly. In this context, plasma‐enhanced atomic layer deposition (PEALD) is a powerful processing route to tailor the properties of HfO2 thin films, especially at low temperatures. Herein, a comprehensive bottom‐up approach is presented, ranging from the synthesis of molecularly engineered Hf precursors to the development of a HfO2 PEALD process and a detailed evaluation where plasma can be exploited to tune the dielectric properties. With the example of the newly synthesized bis‐(dialkylamido)‐bis‐(formamidinato) Hf(IV) precursor, [Hf{η2‐(iPrN)2CH}2(NMe2)2] which is reactive, thermally robust and volatile, successful implementation in a PEALD process for HfO2 at low temperatures is demonstrated. The typical atomic layer deposition (ALD) characteristics of precursor saturation, linearity, and ALD temperature window are demonstrated with constant growth of 0.7 Å per cycle from 125 to 200 °C, yielding high‐purity layers. The effect of plasma pulse duration on the chemical composition alongside structural, topographical, as well as dielectric properties of the films is investigated. For the latter, the films are incorporated in metal‐insulator semiconductor (MIS) structures.

Atomic Layer Deposition of Ultra-Thin Crystalline Electron Channels for Heterointerface Polarization at Two-Dimensional Metal-Semiconductor Heterojunctions

Atomic layer deposition (ALD) has emerged as a promising technology for the development of the next generation of low-power semiconductor electronics. The wafer-scaled growth of two-dimensional (2D) crystalline nanostructures is a fundamental step toward the development of advanced nanofabrication technologies. Ga2O3 is an ultra-wide bandgap metal oxide semiconductor for application in electronic devices. The polymorphous Ga2O3 with its unique electronic characteristics and doping capabilities is a functional option for heterointerface engineering at metal-semiconductor 2D heterojunctions for application in nanofabrication technology. Plasma-enhanced atomic layer deposition (PE-ALD) enabled the deposition of ultra-thin nanostructures at low-growth temperatures. The present study used the PE-ALD process for the deposition of atomically thin crystalline ß-Ga2O3 films for heterointerface engineering at 2D metal-semiconductor heterojunctions. Via the control of plasma gas composition and ALD temperature, the wafer-scaled deposition of ~5.0 nm thick crystalline ß-Ga2O3 at Au/Ga2O3-TiO2 heterointerfaces was achieved. Material characterization techniques showed the effects of plasma composition and ALD temperature on the properties and structure of Ga2O3 films. The following study on the electronic characteristics of Au/Ga2O3-TiO2 2D heterojunctions confirmed the tunability of this metal/semiconductor polarized junction, which works as functional electron channel layer developed based on tunable p-n junctions at 2D metal/semiconductor interfaces.

Fabrication of Bilayer Stacked Antiferroelectric/ Ferroelectric HfxZr1-xO2 FeRAM and FeFET With Improved Leakage Current and Robust Reliability by Modifying Atomic Layer Deposition Temperatures

We fabricated FeRAM and FeFET with a bilayer Hf <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (HZO), which comprises 5-nm-thick antiferroelectric HZO and 5-nm-thick ferroelectric HZO. Higher orthorhombic phase and large grain size were shown in grazing-incidence x-ray diffraction and TEM results, respectively. By using low ALD temperature, the leakage current (< 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> under ±2V), gate control ability, the memory window (> 1.5V) and the endurance (> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cycles) have been improved. These results suggest that a low atomic layer deposition temperature is a promising process for use in non-volatile memory devices.

Optimum temperature of atomic layer deposition of alumina on CsPbBr3 quantum-dot for optical performance and environmental stability

Atomic layer deposition (ALD) plays a pivotal role in raising the long-term stability of perovskite quantum dot (PQD). The deposition temperature of excellent alumina film prepared by ALD is generally higher than 200 °C. In such a temperature environment, the lattice morphology of PQD is seriously damaged. In this work, to effectively protect PQD, the optimum temperature for depositing alumina on the surface of CsPbBr3 PQD is investigated. The CsPbBr3 PQD is deposited on a transparent glass substrate. Subsequently, an alumina layer is deposited on the surface of PQD via ALD at a deposition temperature of 40–120 °C. In our experiment, the significant degradation of PQD is observed when the deposition temperature of ALD is set to 80 °C and above. Lowering the deposition temperature can effectively improve the optical performance of PQD. The samples with alumina deposited at 75 °C show excellent stability after undergoing aging tests, and therefore 75 °C is considered the best ALD temperature. Besides, white light-emitting diode (W-LED) is produced by a method that a stack structure consisting of CsPbBr3 coated via ALD at 75 °C and CdSe QD is excited using blue Micro-LED, and the color gamut of W-LED accounts for the wide area of 94.00% NTSC (1953) and 86.20% Rec.2020 displayed wide color gamut features. The experiment provides practical guidance for selecting the optimal deposition temperature for ALD-packaged PQD-based devices.

Direct growth and interface reactions of ferroelectric Hf0.5Zr0.5O2 films on MoS2

The integration of ferroelectric HfO2 films into two-dimensional layered-material-based devices is expected to provide significant functionality for future electronics. In this study, Hf0.5Zr0.5O2 (HZO) films are directly grown on single-crystalline MoS2 flakes by atomic layer deposition (ALD) with varying deposition temperatures using H2O or O3 oxidants. According to density functional theory calculations and Raman measurements, O3-based ALD oxidizes the MoS2 surface at the atomic layer level, in contrast to the H2O-based ALD process, thus facilitating the conformal deposition of an HZO film (10 nm) without any surface treatment of MoS2 at an elevated ALD temperature of 260 °C. Annealing the O3-based HZO film with a Mo capping layer at 600 °C significantly improves the ferroelectric properties with a symmetrical hysteresis loop on MoS2. However, distinct S out-diffusion accompanied by Hf, Zr, and O diffusion toward the Mo capping layer occurs because of the thermal dissociation of MoS2 at an additional atomic layer level, thereby leading to the subsequent reduction of weak HZO bonds by the released S atoms along the grain boundaries.

Enhancing microbiostatic properties of silicone catheters with Al2O3 coatings deposited by atomic layer deposition

Catheter-associated infections are a concern in healthcare and the leading cause of hospital-acquired infections, which can be caused by a variety of microorganisms, including bacteria and fungi. To prevent severe complications, developing materials with antimicrobial and/or microbiostatic properties is critical. Atomic layer deposition (ALD) is a thin-film deposition technique that precisely controls the thickness and composition of deposited metallic oxide films. ALD is promising for creating biomedical devices, drug delivery systems, and tissue engineering scaffolds with complex geometries. Given the microbiostatic properties of some metallic oxides-coatings, which act hampering the adherence of microorganisms, this work investigated the ALD of Al2O3 thin films on the surface and lumen of silicone catheters. Herein, we showed that Al2O3-coated silicone catheters with a thickness of 100 nm inhibited the formation of biofilm towards Staphylococcus aureus, Escherichia coli and Candida albicans, while reducing the contamination by 1 Log. The low temperature ALD approach presented in this investigation paves the way for further studies on active coatings aiming at biomedical devices with improved microbiostatic properties towards different microorganisms.

Combined effects of the deposition temperature and metal electrodes on ferroelectric properties of atomic-layer-deposited Hf0.5Zr0.5O2 films

The combined effects of the atomic-layer-deposition (ALD) temperature (220 °C–280 °C) and metal electrodes (TiN and Mo) on the ferroelectric properties of Hf0.5Zr0.5O2 films were studied. Regardless of the metal electrode, a tetragonal–orthorhombic–monoclinic phase evolution sequence was observed with increasing ALD temperature after post-metallization annealing. However, the phase transition temperature slightly changed depending on the metal electrode, which was predetermined based on the as-deposited states. Additionally, the out-of-plane orientation of the final orthorhombic grains was highly dependent on the crystallographic alignment of the metal electrode grains, resulting in different values of the maximum remanent polarization. In terms of long-term reliability, the fatigue characteristics were highly dependent on the electrode characteristics, and the wake-up characteristics were mainly affected by the initial phase distribution determined by both the ALD temperature and metal electrode.

Tuning the charge carrier polarity of roll-to-roll gravure printed carbon nanotube-based thin film transistors by an atomic layer deposited alumina nanolayer.

Charge carrier polarity tuning in printed thin film transistors (TFTs) is a crucial step in order to obtain complementary printed devices. In this work, we studied the effect of an Al2O3 passivation layer on printed single-walled carbon nanotube (SWCNT) based TFTs to tune the charge carrier polarity. By varying the atomic layer deposition (ALD) temperature and Al2O3 layer thickness, we can tune the doping degree of Al2O3 to tailor the polarity of printed SWCNT-based TFTs (SWCNT-TFTs). The precise control of threshold voltage (Vth) and polarity from p-type to well-balanced ambipolar, and n-type SWCNT-TFTs is successfully demonstrated with high repeatability by optimizing the ALD temperature and Al2O3 layer thickness based on 20 printed samples per test. As a proof-of-concept, inverter logic circuits using the SWCNT-TFT with different polarity types are demonstrated. The ambipolar device-based inverter exhibits a voltage gain of 3.9 and the CMOS-based inverter exhibits a gain of approximately 4.3, which is comparable to the current roll-to-roll (R2R) printed inverter circuits. Different thicknesses of Al2O3 layer, coated by the ALD at different temperatures and thicknesses, provide a deep understanding of the device fabrication and control process to implement the tailored doping method to efficiently realize R2R printed SWCNT-TFT-based complementary electronic devices.

Analysis on Contact Resistance and Effective Channel Length of Thin Film Transistors Using Composition-Modified In–Ga–Zn-O Active Channels Prepared with Atomic Layer Deposition and Various Electrode Materials

To verify the effects of process conditions for the TFTs employing In–Ga–Zn-O (IGZO) channels prepared by atomic layer deposition (ALD), such as cationic composition of channel and/or source/drain (S/D) electrodes, the device characteristics including the field-effect mobility (μFE), the contact resistance (RC), and the channel length deviation (ΔL) were systematically investigated for the ALD IGZO thin film transistors (TFTs) fabricated with controlling the ALD subcyclic ratios and with variations in S/D electrodes of indium–tin-oxide (ITO) and molybdenum (Mo). The ALD temperature was fixed at 200 °C. When the subcyclic ratios of precursors (triethyl indium: In–Ga bimetallic: diethyl zinc) were modulated to 0:2:2, 1:2:2, and 2:2:2, the cationic compositions (In:Ga:Zn) were determined to be 1.0:2.8:5.4, 1.0:1.1:2.3, and 1.0:0.7:1.5, respectively. As the the indium (In) contents within the IGZO channel increased from 5% to 14%, the μFE’s obtained from the TFTs using ITO and Mo S/D showed dramatic improvement from 16.1 to 36.9 cm2/(V s) and 2.8 to 20.1 cm2/(V s), respectively. For both sets of devices, the ΔL’s were estimated to be sensitively varied with channel compositions, and the RC markedly decreased owing to the reduction of barrier height formed at electrode/channel interfaces with increasing the In contents. The types of S/D electrodes also have a critical effect on the device performance. The devices using Mo S/D exhibited marked degradations in device characteristics rather than the devices using ITO S/D, which was suggested to originate from the additional formation of MoO3 layer at Mo/IGZO interfaces during the ALD process for the preparation of IGZO channel layers. Based on these findings, the intrinsic values of μFE for the ALD IGZO TFTs could be determined with considerations of RC and ΔL, especially for the devices with shorter channel lengths.

Growth mechanism study of boron nitride atomic layer deposition by experiment and density functional theory

We investigated the fundamental mechanisms determining the limit of the atomic layer deposition (ALD) temperature window by density functional theory (DFT) simulations. Boron nitride (BN) ALD experiments were performed using BCl3 or tris-dimethylaminoborane (TDMAB) and NH3 on Si(100) substrates to check whether the ALD temperature window exists or not. With increasing the growth temperature, an increase in growth per cycle (GPC) between 700 and 900 °C was relatively small in the BCl3 and NH3 system, while the GPC exponentially increased in the TDMAB and NH3 system. Based on the experimental results, we performed DFT simulations to investigate the reaction mechanisms determining the lower ALD temperature window in the BCl3 and NH3 systems. The two initial reactions and two cyclic reactions were numerically analyzed: (a) BCl3 reactions on OH-terminated Si(100) surface, (b) NH3 reactions on Cl-terminated Si(100) surface, (c) BCl3 reactions on H-terminated BN surface, and (d) NH3 reactions on Cl-terminated BN surface. We found activation energies for reaction (c) is 30.0 kcal/mol, determining the lower limit of the ALD temperature window. The activation temperature of reaction (c) is obtained 789 °C which is just in the ALD temperature window by our experiments.

Nanocrystallite Seeding of Metastable Ferroelectric Phase Formation in Atomic Layer-Deposited Hafnia-Zirconia Alloys.

Hafnia-based ferroelectric thin films are promising for semiconductor memory and neuromorphic computing applications. Amorphous, as-deposited, thin-film binary alloys of HfO2 and ZrO2 transform to the metastable, orthorhombic ferroelectric phase during post-deposition annealing and cooling. This transformation is generally thought to involve formation of a tetragonal precursor phase that distorts into the orthorhombic phase during cooling. In this work, we systematically study the effects of atomic layer deposition (ALD) temperature on the ferroelectricity of post-deposition-annealed Hf0.5Zr0.5O2 (HZO) thin films. Seed crystallites having interplanar spacings consistent with the polar orthorhombic phase are observed by a plan-view transmission electron microscope in HZO thin films deposited at an elevated ALD temperature. After ALD under conditions that promote formation of these nanocrystallites, high-polarization (Pr > 18 μC/cm2) ferroelectric switching is observed after rapid thermal annealing (RTA) at low temperature (350 °C). These results indicate the presence of minimal non-ferroelectric phases retained in the films after RTA when the ALD process forms nanocrystalline particles that seed subsequent formation of the polar orthorhombic phase.

Fabrication of PtIrPd Noble Metal Medium Entropy Alloy Thin Film by Atomic Layer Deposition

Noble metal medium entropy alloy (MEA) thin films have recently attracted enormous research interests recently because of their great potential in the catalytic application. However, conventional bottom‐up fabrication methods for MEA thin film such as magnetron sputtering face enormous challenges including precise control over film thickness and consumption of costly high‐purity noble metal targets. Herein, a facile and tunable approach is developed coupling sequential atomic layer deposition (ALD) of noble metal layers with electric Joule heating (EJH) alloying process to grow platinum‐iridium‐palladium (PtIrPd) MEA thin films. The PtIrPd MEA thin film with a thickness of ≈20 nm and an atomic ratio of 35:35:30 is successfully fabricated. The effect of EJH processing temperature on the film morphology and structure evolution is investigated. ALD provides flexibility in the metal deposition sequence with meticulous thickness control and atomic composition precision, while the ultrafast EJH ramping/cooling rates promote homogeneous alloying of MEA combinations inhibiting phase separation. Furthermore, the integrated method circumvents a major obstacle of finding a common ALD temperature window for different noble metal precursors as well as opens new pathways for the controllable synthesis of multicomponent metal alloy thin films with potential catalytic, magnetic, and optical properties.

Surface Acidity-Induced Inherently Selective Atomic Layer Deposition of Tantalum Oxide on Dielectrics

Selective deposition shows a great perspective for the downscaling of nanoelectronics. In this work, inherently selective atomic layer deposition (ALD) of tantalum oxide was studied on a series of oxide substrates. The Ta2O5 films linearly grow on acidic oxides of MnO2, SiO2, and Ta2O5, while there are long nucleation delays on basic oxides such as Al2O3 and HfO2. The inherent selectivity is induced through acidity differences which influence the reaction path, and the H-transfer reaction is a key factor. The Ta(OEt)5 precursor with alkaline ligands tends to chemically adsorb on acidic SiO2 surfaces through H-transfer reaction between surface hydroxyls and precursor molecules. Through H-transfer reaction, the ligands of the Ta(OEt)5 precursor are dissociated into ethanol molecules and the remaining intermediates are strongly chemisorbed on the surface, initiating the following nucleation. However, it is hard to nucleate on basic Al2O3 and HfO2 substrates because the H-transfer reaction is blocked. Moreover, robust selectivity only exists within the ALD temperature window, which highlights that tuning deposition temperature is also an effective way to amplify selectivity. The obstruction of H-transfer reaction on basic oxides provides a new strategy for inherently selective ALD, which will expand the selective toolbox of nanofabrication for next-generation nanoelectronic applications.

Atomic Layer Deposition of MoS2 Decorated TiO2 Nanotubes for Photoelectrochemical Water Splitting

AbstractA thermal atomic layer deposition (ALD) process to fabricate MoS2 thin films is successfully demonstrated by using cycloheptatriene molybdenum tricarbonyl (C7H8Mo(CO)3) and H2S as precursors at an ALD temperature below 300 °C. The process is systematically investigated, showing a typical self‐limiting characteristic within an ALD temperature window of 225–285 °C and a high growth‐per‐cycle of 0.11 nm. The as‐deposited films are amorphous while they can be crystallized in situ by sulfurization with H2S at a low temperature of 300 °C. A prototypical application of the developed ALD process is demonstrated by constructing a MoS2/TiO2 heterostructure through depositing MoS2 onto anodized TiO2 nanotubes for photoelectrochemical water splitting. The MoS2/TiO2 heterostructures exhibit approximately three times superior photoelectrochemical performance than the pristine TiO2 nanotubes. This is attributed to an enhanced visible light‐harvesting ability of MoS2 and an improved separation of the photo‐generated charge carriers at the heterostructure interface, which is affirmed by a staggering gap (type II) between MoS2 and TiO2 as probed by ultraviolet photoelectron spectroscopy.