Plasma-assisted Atomic Layer Deposition Research Articles

Preparation of palladium-based catalyst by plasma-assisted atomic layer deposition and its applications in CO2 hydrogenation reduction

Supported Pd catalyst is an important noble metal material in recent years due to its high catalytic performance in CO2 hydrogenation. A fluidized-bed plasma assisted atomic layer deposition (FP-ALD) process is reported to fabricate Pd nanoparticle catalyst over γ-Al2O3 or Fe2O3/γ-Al2O3 support, using palladium hexafluoroacetylacetonate as the Pd precursor and H2 plasma as counter-reactant. Scanning transmission electron microscopy exhibits that high-density Pd nanoparticles are uniformly dispersed over Fe2O3/γ-Al2O3 support with an average diameter of 4.4 nm. The deposited Pd-Fe2O3/γ-Al2O3 shows excellent catalytic performance for CO2 hydrogenation in a dielectric barrier discharge reactor. Under a typical condition of H2 to CO2 ratio of 4 in the feed gas, the discharge power of 19.6 W, and gas hourly space velocity of 10000 h−1, the conversion of CO2 is as high as 16.3% with CH3OH and CH4 selectivities of 26.5% and 3.9%, respectively.

Annealing‐Free Ohmic Contacts to n‐Type GaN via Hydrogen Plasma‐Assisted Atomic Layer Deposition of Sub‐Nanometer AlOx

AbstractA plasma‐assisted atomic layer deposition (PE‐ALD) process is reported for creating ohmic contacts to n‐type GaN that combines native oxide reduction, near‐surface doping, and encapsulation of GaN in a single processing step, thereby eliminating the need for both wet chemical etching of the native oxide before metallization and thermal annealing after contact formation. Repeated ALD cycling of trimethyl aluminum (TMA) and high‐intensity hydrogen (H2) plasma results in the deposition of a sub‐nanometer‐thin (≈8 Å) AlOx layer via the partial transformation of the GaN surface oxide into AlOx. Hydrogen plasma‐induced nitrogen vacancies in the near‐surface region of GaN serve as shallow donors, promoting efficient out‐of‐plane electrical transport. Subsequent metallization with a Ti/Al/Ti/Au stack results in low contact resistance, ohmic behavior, and smooth morphology without requiring annealing. This electrical contracting approach thus meets the thermal budget requirements for Si‐based complementary metal–oxide–semiconductor structures and can facilitate the design and fabrication of advanced GaN‐on‐Si heterodevices.

Electrochemical Activation of Atomic-Layer-Deposited Nickel Oxide for Water Oxidation.

NiO-based electrocatalysts, known for their high activity, stability, and low cost in alkaline media, are recognized as promising candidates for the oxygen evolution reaction (OER). In parallel, atomic layer deposition (ALD) is actively researched for its ability to provide precise control over the synthesis of ultrathin electrocatalytic films, including film thickness, conformality, and chemical composition. This study examines how NiO bulk and surface properties affect the electrocatalytic performance for the OER while focusing on the prolonged electrochemical activation process. Two ALD methods, namely, plasma-assisted and thermal ALD, are employed as tools to deposit NiO films. Cyclic voltammetry analysis of ∼10 nm films in 1.0 M KOH solution reveals a multistep electrochemical activation process accompanied by phase transformation and delamination of activated nanostructures. The plasma-assisted ALD NiO film exhibits three times higher current density at 1.8 V vs RHE than its thermal ALD counterpart due to enhanced β-NiOOH formation during activation, thereby improving the OER activity. Additionally, the rougher surface formed during activation enhanced the overall catalytic activity of the films. The goal is to unravel the relationship between material properties and the performance of the resulting OER, specifically focusing on how the design of the material by ALD can lead to the enhancement of its electrocatalytic performance.

Optimization of photoelectric properties of transparent conductive B and Ga co-doped ZnO films for electrochromic applications

To improve the performance of electrochromic devices, it is urgently necessary to develop transparent indium-free conductive electrodes with high conductivity and transparency. In this study, thin films of Ga-doped ZnO (GZO) and B–Ga co-doped ZnO (BGZO) were prepared on glass substrates using the plasma-assisted atomic layer deposition (PEALD) technique, and their structural and optoelectronic properties were characterized. It is found that all thin films were consistent with hexagonal fibrous ZnO structure, and the surface morphology of the membrane was homogenous at nanoscale. UV spectroscopy analysis showed that the doping of B and Ga elements could increase the transparency of ZnO films. The electrical properties can be remarkably enhanced by doping with B and Ga elements. Further more, tungsten oxide (WO3) was deposited on the as-prepared BGZO films using magnetron sputtering method. We activated the sample by applying voltage and observed a color transition from transparent to dark blue. It has a modulation range of 65 % in the 633 nm spectrum and a coloring efficiency of 37.8 cm2C-1. This work opens up remarkable new opportunities for developing high-performance transparent conductive electrode in electrochromic devices.

De-Pinning Fermi Level and Accelerating Surface Kinetics with an ALD Finish Boost the Fill Factor of BiVO4 Photoanodes to 44.

With the rapid development of performance and long-term stability, bismuth vanadate (BiVO4 ) has emerged as the preferred photoanode in photoelectrochemical tandem devices. Although state-of-the-art BiVO4 photoanodes realize a saturated photocurrent density approaching the theoretical maximum, the fill factor (FF) is still inferior, pulling down the half-cell applied bias photon-to-current efficiency (HC-ABPE). Among the major fundamental limitations are the Fermi level pinning and sluggish surface kinetics at the low applied potentials. This work demonstrates that the plasma-assisted atomic layer deposition technique is capable of addressing these issues by seamlessly installing an angstrom-scale FeNi-layer between BiVO4 and electrolyte. Not only this ultrathin FeNi layer serves as an efficient OER cocatalyst, more importantly, it also effectively passivates the surface states of BiVO4 , de-pins the surface Fermi level, and enlarges the built-in voltage, allowing the photoanode to make optimal use of the photogenerated holes for achieving high FF up to 44% and HC-ABPE to 2.2%. This study offers a new approach for enhancing the FF of photoanodes and provides guidelines for designing efficient unassisted solar fuel devices.

Analysis and Classification of Charge Trapping States in β-Ga2O3 Mosfets Using I-V and C-V in Dark and Illumination Conditions

Beta-gallium oxide (β-Ga2O3) is a promising ultra-wide bandgap semiconductor for high power applications. Its breakdown electric field of 8 MV cm-1, a Baliga figure of merit 3-10 times larger than that of SiC and GaN, and the ability to be grown from the melt make it highly attractive for the semiconductor industry. Significant improvements have been made in β-Ga2O3 device performance for both high power and high frequency operation. Currently, aluminum oxide (Al2O3) is the preferred gate dielectric for β-Ga2O3 transistors due to its electron and hole barriers, and similar atomic structure. Process optimizations to improve the interface quality has been explored by chemical treatments1 to the β-Ga2O3 surface before dielectric deposition, during the deposition of Al2O3, and with post-deposition annealing. Therefore, there is a desire to understand and classify defects at and near the Al2O3/β-Ga2O3 interface. Here, we report on a study of the Al2O3/β-Ga2O3 interface and border traps in planar depletion-mode β-Ga2O3 MOSFETs using DC I-V and C-V measurements in dark and under illumination with wavelengths from 730 nm (1.7 eV) to 230 nm (5.4 eV). The MOSFETs are fabricated on a 50 nm Si-doped epitaxial layer with a target doping concentration of 2.4 x 1018 cm-3 grown on a (010) semi-insulating β-Ga2O3 substrate. A Ti/Al/Ni/Au metal stack is deposited and annealed to form the Ohmic contacts. Plasma-assisted atomic layer deposition (PE-ALD) is used to deposit the 20 nm Al2O3 gate dielectric. The fabricated transistor structures have a LG of 1.94 µm, LS of 0.5 µm, and LD varying from 0.5 µm, 5.5 µm, and 10.5 µm. Measurements were performed at room temperature. Most FETs exhibited a threshold voltage of approximately -4 V, high linearity, and ION/IOFF ratios between 107 – 109. Using the conductance method, we extract an interface trap state density, Dit, of 3 x 1011 eV-1 cm-2 up to 0.44 eV below the conduction band. Illumination is used to obtain deep trap concentration and behavior. We observe little to no change in IV and CV curves for wavelengths above 365 nm but see a significant voltage shift in the CV curves for illumination wavelengths between 365 nm (3.4 eV) to 230 nm (5.4 eV), indicating significant activated traps from 3.4 eV below the conduction band. We will present the analysis of photo-assisted CV and stress-CV curves at various wavelengths to obtain densities of fixed, fast, and slow trap states and discuss their impact on device performance.

Mechanisms of the Redistribution of Carbon Contamination in Films Formed by Atomic Layer Deposition

The depth distribution of carbon impurities in hafnium oxide films obtained by plasma-assisted atomic layer deposition is studied experimentally and theoretically. An analytical model is proposed that describes the dependence of the carbon impurity concentration profile in the film. The model takes into account the fact that the formation of a carbon impurity in the growing film may be caused by the incomplete oxidation of the organometallic precursor. The diffusion redistribution of impurities is determined by mech-anisms that take into account the presence of carbon in different kinds of states: an insoluble state (carbides, carbonates), a highly mobile state (CO, CO2), and an unstable state whose lifetime is longer than the film’s growth time. The possibility of controlling the carbon impurity both in deep and near-surface layers is shown. The predictions of the model are confirmed experimentally using the mass spectrometry of the secondary ions in the films obtained by atomic layer deposition. To test the theory, special structures of hafnium oxide are developed, consisting of several layers, in which the time of the plasma exposure to the sample is varied at the same dosage of the organometallic precursor. The layer with the shortest exposure time to the plasma is deposited on the substrate, then the same number of cycles on the next layer take place, provided that the exposure time is increased by a factor of n. This makes it possible to deepen the transition zones and thus pro-tect them from the effects of the atmosphere.

Nanostructures Stacked on Hafnium Oxide Films Interfacing Graphene and Silicon Oxide Layers as Resistive Switching Media.

SiO2 films were grown to thicknesses below 15 nm by ozone-assisted atomic layer deposition. The graphene was a chemical vapor deposited on copper foil and transferred wet-chemically to the SiO2 films. On the top of the graphene layer, either continuous HfO2 or SiO2 films were grown by plasma-assisted atomic layer deposition or by electron beam evaporation, respectively. Micro-Raman spectroscopy confirmed the integrity of the graphene after the deposition processes of both the HfO2 and SiO2. Stacked nanostructures with graphene layers intermediating the SiO2 and either the SiO2 or HfO2 insulator layers were devised as the resistive switching media between the top Ti and bottom TiN electrodes. The behavior of the devices was studied comparatively with and without graphene interlayers. The switching processes were attained in the devices supplied with graphene interlayers, whereas in the media consisting of the SiO2-HfO2 double layers only, the switching effect was not observed. In addition, the endurance characteristics were improved after the insertion of graphene between the wide band gap dielectric layers. Pre-annealing the Si/TiN/SiO2 substrates before transferring the graphene further improved the performance.

Synthesis of TiO2/Al2O3 Double-Layer Inverse Opal by Thermal and Plasma-Assisted Atomic Layer Deposition for Photocatalytic Applications.

In comparison to conventional nano-infiltration approaches, the atomic layer deposition (ALD) technology exhibits greater potential in the fabrication of inverse opals (IOs) for photocatalysts. In this study, TiO2 IO and ultra-thin films of Al2O3 on IO were successfully deposited using thermal or plasma-assisted ALD and vertical layer deposition from a polystyrene (PS) opal template. SEM/EDX, XRD, Raman, TG/DTG/DTA-MS, PL spectroscopy, and UV Vis spectroscopy were used for the characterization of the nanocomposites. The results showed that the highly ordered opal crystal microstructure had a face-centered cubic (FCC) orientation. The proposed annealing temperature efficiently removed the template, leaving the anatase phase IO, which provided a small contraction in the spheres. In comparison to TiO2/Al2O3 plasma ALD, TiO2/Al2O3 thermal ALD has a better interfacial charge interaction of photoexcited electron-hole pairs in the valence band hole to restrain recombination, resulting in a broad spectrum with a peak in the green region. This was demonstrated by PL. Strong absorption bands were also found in the UV regions, including increased absorption due to slow photons and a narrow optical band gap in the visible region. The results from the photocatalytic activity of the samples show decolorization rates of 35.4%, 24.7%, and 14.8%, for TiO2, TiO2/Al2O3 thermal, and TiO2/Al2O3 plasma IO ALD samples, respectively. Our results showed that ultra-thin amorphous ALD-grown Al2O3 layers have considerable photocatalytic activity. The Al2O3 thin film grown by thermal ALD has a more ordered structure compared to the one prepared by plasma ALD, which explains its higher photocatalytic activity. The declined photocatalytic activity of the combined layers was observed due to the reduced electron tunneling effect resulting from the thinness of Al2O3.

Plasma-assisted fluidized-bed atomic layer deposition of Pd-Cu nanoparticles on porous powder for CO2 hydrogenation

A novel plasma-assisted fluidized-bed atomic layer deposition process to synthesize Pd-Cu bimetallic nanoparticles is reported, using palladium hexafluoroacetylacetonate, copper(I)-N, N′-di-iso-propylacetamidinate and H2 plasma. The process allows us to uniformly deposit Pd-Cu nanoparticles in porous powder, which is a mixture of γ-Al2O3 (30 wt%), amorphous aluminum silicate (50 wt%) and molecular sieve (20 wt%) (ASM). With metal loadings of 13.1 and 2.5 mg g−1 for Pd and Cu, respectively, the afforded 13.1Pd-2.5Cu catalyst shows excellent catalytic performance for the hydrogenation of CO2 in a dielectric barrier discharge reactor with no intentional heating. Under the condition of discharge input power of 24.6 W, H2-to-CO2 ratio of 4 in feed gas, and gas hourly space velocity of 7595 h−1, the conversion of CO2 can reach as high as 38.0%, with the CH4 and CH3OH product selectivities of 6.7% and 12.8%, respectively. Density functional theory calculations are further employed to understand the associated CH3OH formation mechanism.

Plasma-Assisted Atomic Layer Deposition of IrO2 for Neuroelectronics

In vitro and in vivo stimulation and recording of neuron action potential is currently achieved with microelectrode arrays, either in planar or 3D geometries, adopting different materials and strategies. IrO2 is a conductive oxide known for its excellent biocompatibility, good adhesion on different substrates, and charge injection capabilities higher than noble metals. Atomic layer deposition (ALD) allows excellent conformal growth, which can be exploited on 3D nanoelectrode arrays. In this work, we disclose the growth of nanocrystalline rutile IrO2 at T = 150 °C adopting a new plasma-assisted ALD (PA-ALD) process. The morphological, structural, physical, chemical, and electrochemical properties of the IrO2 thin films are reported. To the best of our knowledge, the electrochemical characterization of the electrode/electrolyte interface in terms of charge injection capacity, charge storage capacity, and double-layer capacitance for IrO2 grown by PA-ALD was not reported yet. IrO2 grown on PtSi reveals a double-layer capacitance (Cdl) above 300 µF∙cm-2, and a charge injection capacity of 0.22 ± 0.01 mC∙cm-2 for an electrode of 1.0 cm2, confirming IrO2 grown by PA-ALD as an excellent material for neuroelectronic applications.

Atomic layer deposition of α-Al2O3 from trimethylaluminum and H2O: Effect of process parameters and plasma excitation on structure development

Aluminum oxide (Al2O3) thin films were deposited from trimethylaluminum (TMA) and H2O on Si substrates and α-Cr2O3 seed layers using thermal and plasma-assisted atomic layer deposition (ALD). All films deposited on Si were amorphous. On the α-Cr2O3 seed layers, α-Al2O3 was formed at temperatures ≥350 °C in the thermal ALD and at ≥325 °C in the plasma-assisted ALD processes. The Ar plasma used in the experiments most significantly contributed to crystallization when applied during the purge period following the H2O pulse or during the H2O pulse and the following purge. Application of plasma during the TMA pulse, during the purge following the TMA pulse, or only during the H2O pulse suppressed the crystal growth and could potentially be applied as a crystal growth inhibitor.

Plasma assisted atomic layer deposition NiO nanofilms for improved hybrid solid state electrochromic device

Alternate methods for deposition of electrochromic metal oxide thin films need to be explored to obtain an optimized one while designing solid state hybrid or all-inorganic electrochromic device. To continue this quest, electrochromic performance from a hybrid electrochromic device obtaining using plasma assisted atomic layer deposition (ALD) grown nickel oxide (NiO) thin film and viologen has been studied. A few tens of nanometers thick films of NiO were deposited first on silicon to optimize the deposition parameters prior to depositing on Fluorine-doped Tin Oxide (FTO)/glass substrates for fabricating the device. The structural and morphological properties of as-prepared films were analyzed by scanning electron spectroscopy (SEM), x-ray diffraction (XRD), Raman spectroscopy and x-ray photo electron spectroscopy (XPS) techniques. Thereafter, a solid state organic-inorganic hybrid electrochromic device was fabricated using Nickel oxide (NiO) and ethyl viologen (EV). The device changes its color between transparent and blue states under the potential window of ±1.7 V while taking less than 2s for toggling between the two-colored states. The device also exhibits an impressive electrochromic behavior with coloration efficiency of 118cm2/C, contrast ratio of 21% and a good cycle life/stability. The ALD grown films appeared to have advantage over other methods as it yields an atomically smooth and crack-less nano film so that proper power utilization take place for color switching. All these results open a new way to the fabrication of inorganic component to fabricate high-performance hybrid solid state electrochromic devices.

Plasma-Assisted Nanofabrication: The Potential and Challenges in Atomic Layer Deposition and Etching.

The growing need for increasingly miniaturized devices has placed high importance and demands on nanofabrication technologies with high-quality, low temperatures, and low-cost techniques. In the past few years, the development and recent advances in atomic layer deposition (ALD) processes boosted interest in their use in advanced electronic and nano/microelectromechanical systems (NEMS/MEMS) device manufacturing. In this context, non-thermal plasma (NTP) technology has been highlighted because it allowed the ALD technique to expand its process window and the fabrication of several nanomaterials at reduced temperatures, allowing thermosensitive substrates to be covered with good formability and uniformity. In this review article, we comprehensively describe how the NTP changed the ALD universe and expanded it in device fabrication for different applications. We also present an overview of the efforts and developed strategies to gather the NTP and ALD technologies with the consecutive formation of plasma-assisted ALD (PA-ALD) technique, which has been successfully applied in nanofabrication and surface modification. The advantages and limitations currently faced by this technique are presented and discussed. We conclude this review by showing the atomic layer etching (ALE) technique, another development of NTP and ALD junction that has gained more and more attention by allowing significant advancements in plasma-assisted nanofabrication.

Growth chemistry of cobalt nitride by plasma enhanced atomic layer deposition

State-of-the-art atomic layer deposition (ALD) and photoemission characterisation are applied to grow and characterise cobalt nitride, a material that has applications in renewable energy and semiconductor technologies. The growth process is characterised using an in situ cycle-by-cycle methodology to identify the main factors which underpin optimal material growth. The role of co-reactant dosing and substrate temperature is analysed in detail to demonstrate the impact these parameters have on the overall composition of the film. The in situ approach, combined with high-energy synchrotron-based photoemission studies of the resulting films, enables understanding of the bulk chemical properties without need for physical removal of material by sputtering. The results provide an insight into optimising plasma assisted ALD processes for deposition of cobalt nitride, and strategies for minimizing carbon incorporation into the film from the precursor ligands.

Atomic scale controlled tunnel oxide enabled by a novel industrial tube‐based PEALD technology with demonstrated commercial TOPCon cell efficiencies > 24%

AbstractIn this work, we report on a significant breakthrough in fabricating the critical tunnel oxide layer of tunnel oxide passivated contacts (TOPCon) high‐efficiency solar cells compatible with high‐volume manufacturing. We show that the tunnel oxide can be controlled at the atomic scale, enabled by an innovative tube‐type industrial plasma‐assisted atomic layer deposition (PEALD) method. In combination with an in situ doped poly‐Si (n+) layer grown by plasma‐enhanced chemical vapor deposition, a uniform, ultrathin ~1.3 nm SiOxlayer is obtained at thec‐Si/SiOx/poly‐Si (n+) interface. Extremely low recombination current densities down to 2.8 fA/cm2and an implied open‐circuit voltage (iVoc) as high as 759 mV are achieved, comparable to state‐of‐the‐art laboratory results. The developed tube‐type PEALD SiOxis applied to industrial TOPCon solar cells resulting in a solar cell efficiency and open‐circuit voltage of up to 24.2% and 710 mV, respectively. The tunnel oxide process window is about 2.4 Å, highlighting the importance of precisely controlling the tunnel oxide thickness at the atomic scale for TOPCon solar cells. The newly developed tube‐type industrial PEALD SiOxmethod opens up a promising new route toward mass production of high‐efficiency industrial TOPCon solar cells. Furthermore, the developed tube‐type PEALD method can easily be integrated with the industrial tube‐type plasma‐enhanced chemical vapor deposition (PECVD) method, thus enabling the deposition of all thin film layers in TOPCon solar cells in one integrated PEALD/PECVD system. This significantly simplifies manufacturing complexity and fosters the commercialization of next‐generation high‐efficiency industrial TOPCon solar cells.

Tube‐type plasma‐enhanced atomic layer deposition of aluminum oxide: Enabling record lab performance for the industry with demonstrated cell efficiencies >24%

AbstractIn this work, single‐side aluminum oxide (Al2O3) deposition enabled by a new tube‐type industrial plasma‐assisted atomic layer deposition (PEALD) technique is presented to meet the increasingly stringent requirements for high‐efficiency solar cell mass production. Extremely low emitter saturation current densities, J0e, down to 15 fA/cm2 are achieved on an industrial textured boron emitter with a sheet resistance of 104 Ω/sq, passivated by PEALD Al2O3/PECVD SiNx stack after firing. An implied open‐circuit voltage of up to 721 mV is obtained on symmetrical lifetime samples. The underlying passivation mechanisms of this new tube‐type PEALD Al2O3 are investigated by contactless corona‐voltage measurements. The results indicate that the superior passivation is mainly attributed to a low interface defect density down to 1.1 × 1011 cm−2 eV−1 and a high negative fixed charge density up to 4.5 × 1012 cm−2. Simulations show that the obtained J0e is close to its intrinsic limit. Lastly, the developed tube‐type PEALD Al2O3 is applied to industrial TOPCon solar cells achieving an average cell efficiency above 24% and a maximum Voc of 707 mV. This work shows that the record level of surface passivation available from lab‐scale PEALD reactors is now available in a flexible high‐throughput industrial PEALD platform, which opens a new route for mass production of high‐efficiency industrial TOPCon solar cells with a lean process at low costs.

Enhanced Surface Passivation by Atomic Layer-Deposited Al2O3 for Ultraviolet-Sensitive Silicon Photomultipliers

This article describes a superior passivation of p-type (p^&#x002B;) Si surface by an Al₂O₃ thin film that is synthesized by plasma-assisted atomic layer deposition (ALD) for ultraviolet-sensitive silicon photomultipliers (SiPMs), compared to conventional SiO₂ and SiN_<i>x</i> passivation schemes. The superiority of Al₂O₃ passivation is due not only to a sufficiently low interface defect density, but also to a high density of built-in negative charges. A 7-nm thin Al₂O₃ film can yield an emitter saturation current density of &#x007E;8 fA/cm² on a high sheet resistance p^&#x002B; layer, compared to &#x007E;60 and &#x007E;1480 fA/cm² for thermal SiO₂ and SiN_<i>x</i> passivation. This superior surface passivation allows the photon-generated carriers to have higher probabilities to reach the high-field region to trigger an avalanche event. In addition, the Al₂O₃ thin film provides very low values of effective surface recombination velocity on low resistivity n- and p-type Si surfaces, which can lead to well-passivated surface features on the guard ring and trench isolation regions of SiPM. These demonstrate the potential of the Al₂O₃ thin film passivation to improve the quantum efficiency and thus photo-detection efficiency (PDE) of ultraviolet-sensitive SiPM with p^&#x002B;/n^&#x2212;/n/n^&#x002B; structure.

Oxygen incorporation in AlN films grown by plasma-enhanced atomic layer deposition

Oxygen is often detected as an impurity in III-nitride films deposited by atomic layer deposition (ALD). The presence of oxygen has deep and unfavorable influences on the structural and optical properties of AlN. We have studied the oxygen incorporation in AlN films prepared by hollow cathode plasma-assisted ALD by alternating H2 plasma. We report a decrease in the O concentration in the film upon the addition of H2 plasma flow. However, increasing the H2 plasma flow does not further decrease the O incorporation. Film composition became almost constant at the surface and beneath the surface of AlN films deposited using N2/H2 plasma. Only samples grown with N2/H2 plasma showed decreases in oxygen concentration from the surface to the bulk film. Oxygen produces important modifications in the structural, chemical, and electrical properties. The Al 2p, N 1s, and O 1s high-resolution x-ray photoelectron spectra represent that AlN films are composed of Al–N, Al–O, and N–Al–O bonds. X-ray photoelectron spectroscopy data suggest that the composition of the AlN film changes to AlON + AlN when N2 plasma is used. The valence band maximum position of the AlN film is found to be at ∼1.7 eV for oxygen atomic concentration ∼20 at. %. The TO and LO phonon modes are present in AlN films with oxygen contamination ∼20 at. % and disappear at high oxygen contamination (∼55 at. %).

Enhanced Self-Assembled Monolayer Surface Coverage by ALD NiO in p-i-n Perovskite Solar Cells.

Metal halide perovskites have attracted tremendous attention due to their excellent electronic properties. Recent advancements in device performance and stability of perovskite solar cells (PSCs) have been achieved with the application of self-assembled monolayers (SAMs), serving as stand-alone hole transport layers in the p-i-n architecture. Specifically, phosphonic acid SAMs, directly functionalizing indium–tin oxide (ITO), are presently adopted for highly efficient devices. Despite their successes, so far, little is known about the surface coverage of SAMs on ITO used in PSCs application, which can affect the device performance, as non-covered areas can result in shunting or low open-circuit voltage. In this study, we investigate the surface coverage of SAMs on ITO and observe that the SAM of MeO-2PACz ([2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid) inhomogeneously covers the ITO substrate. Instead, when adopting an intermediate layer of NiO between ITO and the SAM, the homogeneity, and hence the surface coverage of the SAM, improve. In this work, NiO is processed by plasma-assisted atomic layer deposition (ALD) with Ni(MeCp)2 as the precursor and O2 plasma as the co-reactant. Specifically, the presence of ALD NiO leads to a homogeneous distribution of SAM molecules on the metal oxide area, accompanied by a high shunt resistance in the devices with respect to those with SAM directly processed on ITO. At the same time, the SAM is key to the improvement of the open-circuit voltage of NiO + MeO-2PACz devices compared to those with NiO alone. Thus, the combination of NiO and SAM results in a narrower distribution of device performance reaching a more than 20% efficient champion device. The enhancement of SAM coverage in the presence of NiO is corroborated by several characterization techniques including advanced imaging by transmission electron microscopy (TEM), elemental composition quantification by Rutherford backscattering spectrometry (RBS), and conductive atomic force microscopy (c-AFM) mapping. We believe this finding will further promote the usage of phosphonic acid based SAM molecules in perovskite PV.