Control Of Film Composition Research Articles

Recent Developments in Atomic Layer Deposition of Functional Overlayers in Perovskite Solar Cells.

Over the last decade, research in organic-inorganic lead halide perovskite solar cells (PSCs) has gathered unprecedented momentum, putting the technology on the brink of full-scale commercialization. A wide range of strategies have been implemented for enhancing the power conversion efficiency of devices and modules, as well as improving stability toward high levels of irradiation, temperature, and humidity. Another key element in the path to commercialization is the scalability of device manufacturing, which requires large-scale deposition of conformal layers without compromising the delicate structure of the perovskite film. In this context, atomic layer deposition (ALD) tools excel in depositing high-quality conformal films with precise control of film composition and thickness over large areas at relatively low processing temperatures. In this commentary, we will briefly outline recent progress in PSC technology enabled by ALD tools, focusing on layers deposited above the absorber layer. These interlayers include charge transport layers, passivation layers, buffer layers, and encapsulation techniques. Additionally, we will discuss some of the challenges and potential avenues for research in PSC technology underpinned by ALD tools.

Physical vapor deposition of Yb-doped Cs2AgSbBr6 films

Ytterbium-doped halide double perovskites of the form Cs2AgBX6 (B = Bi, Sb, In and X = Cl, Br) are being explored as potential lead-free UV/blue to near-infrared (NIR) downconversion materials. Of the various B and X combinations, Cs2AgSbBr6 has only recently been synthesized and never yet deposited as a film using vapor deposition. Here, we report the deposition of undoped and Yb-doped Cs2AgSbBr6 thin films via thermal evaporation from CsBr, AgBr, SbBr3, and YbBr3. Film composition control is difficult due to the high volatility of SbBr3. We explored various strategies, including co-evaporation and sequential deposition of layers, followed by postdeposition annealing. The formation of Cs2AgSbBr6 was confirmed with x-ray diffraction and optical absorption, although impurity phases such as Cs2AgBr3 were often present because the films easily became Sb-deficient due to volatile SbBr3 leaving the film. NIR photoluminescence quantum yields of up to 12% were achieved with this material for the first time. The optical properties and reported bandgap transitions are critically reviewed and assessed in light of new optical absorption data from thin films. The Cs2AgSbBr6 film has an indirect bandgap at 1.95 ± 0.05 eV followed by a direct transition at 2.5 ± 0.05 eV.

Atomic Layer Deposition for Memory Applications

The semiconductor industry has continuously adopted more complex architectures and 3D geometries, while simultaneously down scaling the critical dimensions of the devices. This presentation will briefly review the evolution of the memory device architectures, their impact on material/processing requirements, and outline how atomic scale processing has helped to overcome integration challenges. We will first present some key atomic scale processes which have enabled the fabrication of 3D memory devices, with particular focus on V-NAND and DRAM. Next, we will introduce new atomic scale processes, their implementation in emerging memory concepts and the related challenges on the path to productization. We will present the recent advances in ALD of ferroelectric materials, showing how a precise control of film composition, dopant incorporation and morphology are key to control ferroelectricity in doped HfZrO.

Rational Compositional Control of Electrodeposited Ag-Fe films.

Some alloys are very difficult to electrodeposit, due to problems such as the large difference in the equilibrium potentials and/or deposition kinetics of the alloy components, as well as the bath instability due to the spontaneous reactions in the bulk electrolyte. The Ag-Fe system is one of those. In this work, a novel alkaline citrate-dimethylhydantoin (DMH) complex has been used to synthesize thermodynamically immiscible Ag-Fe alloy films. The large difference in standard potentials and deposition kinetics of Ag and Fe is partially resolved by complexing Ag(I) with DMH, while the instability caused from spontaneous reduction of Ag(I) by Fe(II) was partially resolved by adding 1% Fe3+, based on the philosophy of the mixed potential theory. Furthermore, a paradigm for control of film composition is developed, based on the mass-transfer coefficient ratio, tested with various bath constitutions. Uniform and high-quality films are obtained, with a composition error of <4 at. % comparing with the predicted values. Electrochemical reactions involved in the deposition baths were systematically investigated using cyclic voltammetry, showing satisfactory agreement with the predicted deposition potentials calculated by equilibrium thermodynamics. The films deposited at high overpotential are proven to be biphasic, containing Ag-fcc and Fe-bcc phases, with a trend of decreasing crystallinity at increasing overpotentials when deposited at constant deposition time. A narrow transition potential range (<0.05 V) from the onset of Fe deposition to its limiting current condition was observed.

Additive Manufacturing of Mechanically Isotropic Thin Films and Membranes via Microextrusion 3D Printing of Polymer Solutions

Polymer extrusion additive manufacturing processes, such as fused filament fabrication (FFF), are now being used to explore the fabrication of thin films and membranes. However, the physics of molten polymer extrusion constrains achievable thin film properties (e.g., mechanical isotropy), material selection, and spatial control of film composition. Herein, we present an approach for fabrication of functional polymer thin films and membranes based on the microextrusion printing of polymer solutions, which we refer to as "solvent-cast printing" (SCP). Constructs fabricated via SCP exhibited a 43% reduction in anisotropy of tensile strength relative to those fabricated using FFF. The constructs fabricated via SCP exhibited a lesser extent of visible layering defects relative to those fabricated by FFF. Further, the swelling dynamics of the films varied depending on the membrane fabrication technique (i.e., SCP vs manual drop casting). The opportunity for expanding material selection relative to FFF processes was demonstrated by printing poly(benzimidazole), a high-performance thermoplastic with high glass-transition temperatures ( Tg ∼ 400 °C). Results from this work indicate that our new approach could facilitate the manufacture of mechanically isotropic thin films and membranes using currently unprintable high-performance thermoplastics.

Metal Fluorides as Lithium-Ion Battery Materials: An Atomic Layer Deposition Perspective

Lithium-ion batteries are the enabling technology for a variety of modern day devices, including cell phones, laptops and electric vehicles. To answer the energy and voltage demands of future applications, further materials engineering of the battery components is necessary. To that end, metal fluorides could provide interesting new conversion cathode and solid electrolyte materials for future batteries. To be applicable in thin film batteries, metal fluorides should be deposited with a method providing a high level of control over uniformity and conformality on various substrate materials and geometries. Atomic layer deposition (ALD), a method widely used in microelectronics, offers unrivalled film uniformity and conformality, in conjunction with strict control of film composition. In this review, the basics of lithium-ion batteries are shortly introduced, followed by a discussion of metal fluorides as potential lithium-ion battery materials. The basics of ALD are then covered, followed by a review of some conventional lithium-ion battery materials that have been deposited by ALD. Finally, metal fluoride ALD processes reported in the literature are comprehensively reviewed. It is clear that more research on the ALD of fluorides is needed, especially transition metal fluorides, to expand the number of potential battery materials available.

Nonuniform Composition Profiles in Amorphous Multimetal Oxide Thin Films Deposited from Aqueous Solution.

Metal oxide thin films are ubiquitous in technological applications. Often, multiple metal components are used to achieve desired film properties for specific functions. Solution deposition offers an attractive route for producing these multimetal oxides because it allows for careful control of film composition through the manipulation of precursor stoichiometry. Although it has been generally assumed that homogeneous precursor solutions yield homogeneous thin films, we recently reported evidence of nonuniform electron density profiles in aqueous-deposited films. Herein, we show that nonuniform electron densities in lanthanum zirconium oxide (LZO) thin films are the result of inhomogeneous distributions of metal components. Specifically, La aggregates at the film surface, whereas Zr is relatively evenly distributed throughout single-layer films. This inhomogeneous metal distribution persists in stacked multilayer films, resulting in La-rich interfaces between the sequentially deposited layers. Testing of metal-insulator-semiconductor devices fabricated from single and multilayer LZO films shows that multilayer films have higher dielectric constants, indicating that La-rich interfaces in multilayer films do not detrimentally impact film properties. We attribute the enhanced dielectric properties of multilayer films to greater condensation and densification relative to single-layer films, and these results suggest that multilayer films may be preferred for device applications despite the presence of layering artifacts.

Superconducting niobium titanium nitride thin films deposited by plasma-enhanced atomic layer deposition

NbTiN has a variety of superconducting applications, ranging from RF cavities to single-photon detectors. Here, we systematically investigated the plasma-enhanced atomic layer deposition (PEALD) of NbxTi with the organometallic precursors (t-butylimido) tris(diethyamido) niobium(V) and tetrakis (dimethylamido) titanium in conjunction with a remote H2/N2 plasma. Deposited film properties have been studied as a function of the ratio of Nb to Ti precursor pulses within each ALD supercycle. PEALD NbTiN films were characterized with spectroscopic ellipsometry (thickness, optical properties), four point probe (resistivity), x-ray photoelectron spectroscopy (composition), x-ray reflectivity (density and thickness), x-ray diffraction (crystallinity), and superconductivity measurements. The PEALD process has shown distinct advantages over deposition of superconducting films via thermal ALD or sputtering, for example a lower processing temperature and more efficient control of film composition. This control of film composition enabled the tuning of electrical and superconducting properties, such as varying the superconducting critical temperature TC between 6.9 and 13.2 K.

Atmospheric spatial atomic layer deposition of Zn(O,S) buffer layer for Cu(In,Ga)Se2 solar cells

Zinc oxysulfide has been grown by spatial atomic layer deposition (S-ALD) and successfully applied as buffer layer in Cu(In, Ga)Se2 (CIGS) solar cells. S-ALD combines high deposition rates (up to nm/s) with the advantages of conventional ALD, i.e. excellent control of film composition and superior uniformity over large-area and even non-flat substrates. Diethylzinc, water and hydrogen sulfite (H2S) have been used as zinc, oxygen and sulfur precursor, respectively. The S/(S+O) ratio in the film is accurately controlled by exposing the substrate simultaneously to both H2O and H2S precursors, which are pre-mixed and co-injected in the same deposition zone. The optoelectronic and morphological properties of Zn(O,S) are characterized as a function of the S/(S+O) ratio. Zn(O,S) buffer layers with different values of S/(S+O) ratio are applied in CIGS solar cells. An optimum value of S/(S+O) ratio of about 0.4 is found for which both the short circuit current density (Jsc=34.2mA/cm2) and cell efficiency (η=15.9%) increase, as compared to reference cells with CdS buffer layer (Jsc=32.1mA/cm2, η=15.5%).

Spatial Atmospheric ALD of Functional Layers for CIGS Solar Cells

Spatial Atmospheric Atomic Layer Deposition combines the advantages of temporal ALD, i.e. excellent control of film composition and uniformity over large area substrates, with high growth rates (up to nm/s). In this paper we present a short overview of our research activity carried out on S-ALD of functional thin films for the front window of copper indium gallium di-selenide (CIGS) solar cells. Zn(O,S) and ZnO:Al are grown by co-injecting the vaporized precursors of the cationic (O, S) and anionic (Zn, Al) elements, respectively. An insight on the growth characteristics and properties of Zn(O,S) and ZnO:Al is presented.

Spatial Atmospheric ALD of Functional Layers for CIGS Solar Cells

Copper Indium Gallium di-Selenide (CIGS) solar cells are a promising approach in photovoltaic technology, having low production costs, high conversion efficiencies (> 20 %), as well as the possibility to manufacture them on flexible substrates. State-of-the-art in CIGS solar cells manufacturing is to use a stack of CdS, intrinsic ZnO (i-ZnO) and an Al-doped ZnO TCO on top of the CIGS film. Replacement of CdS by a non-toxic Cd-free layer with wider band gap (> 2.4 eV) would a) decrease the production cost by avoiding the expensive treatment of toxic wastes and b) increase the overall cell efficiency by enhancing the quantum efficiency in the blue range. Moreover, the use of a “soft” and highly conformal deposition technique is preferred to improve the electrical properties of the buffer layer/CIGS interface. In this paper we present spatial atmospheric atomic layer deposition of a Zn(O,S) buffer layer as CdS replacement for CIGS solar cells. Spatial ALD is emerging as an industrially scalable deposition technique at atmospheric pressure which combines the advantages of temporal ALD, i.e. excellent control of film composition and uniformity on large area substrates, with high growth rates (up to nm/s). Films are grown by sequentially exposing the substrate to oxygen and sulfur precursors (H2O, H2S) and the zinc metal precursor (i.e., DEZn). By controlling the kinetics of surface reactions between evaporated precursors and reactive sites at the film surface, the composition of Zn(O,S) can be precisely tuned. The incorporation of S into ZnO results in a bowing of the band gap in the range from 3.3 eV (ZnO) to 2.7 (S/O+S ~ 0.5) and 3.4 eV (ZnS), as measured by spectrophotometry. The morphology of the Zn(Ox-1,Sx) films varies from polycrystalline (for 0<x<30 and 70<x<100) to amorphous (30<x<70), as measured by X-ray diffraction. CIGS solar cells with a Zn(O,S) buffer layer show an increased spectral response around 400 nm compared to solar cells with a CdS buffer layer. The solar cells with the Zn(O,S) buffer layer had an efficiency of 15.9 %, compared to 15.5 % for the reference solar cells with a CdS buffer layer.

Graphene as an efficient interfacial layer for electrochromic devices.

This study presents an interfacial modification strategy to improve the performance of electrochromic films that were fabricated by a magnetron sputtering technique. High-quality graphene sheets, synthesized by chemical vapor deposition, were used to modify fluorine-doped tin oxide substrates, followed by the deposition of high-performance nanocomposite nickel oxide electrochromic films. Electrochromic cycling results revealed that a near-complete monolayer graphene interfacial layer improves the electrochromic performance in terms of switching kinetics, activation period, coloration efficiency, and bleached-state transparency, while maintaining ∼100% charge reversibility. The present study offers an alternative route for improving the interfacial properties between electrochromic and transparent conducting oxide films without relying on conventional methods such as nanostructuring or thin film composition control.

LAYER-BY-LAYER ASSEMBLY OF POLYMERIC COMPLEXES

The layer-by-layer(LbL) assembly has been demonstrated to be a convenient and versatile method to fabricate composite film materials with precise control of film composition and structure.Polymeric complexes represent a large variety of supramolecular assemblies hold together by multiple weak interactions,which include polyelectrolyte-polyelectrolyte complexes,polyelectrolyte-surfactant complexes,non-charged polymer-polymer complexes,polymeric-inorganic hybrid complexes and so forth.We demonstrate herein that the LbL assembly is unique to fabricate functional films by using polymeric complexes as building blocks:(1) It enables rapid fabrication of micrometer-thick films because of the large dimension of the polymeric complexes.(2) It facilitates the flexible control of film structures because the structure of polymeric complexes can be easily tailored in solution or in LbL assembled films.(3) The LbL assembly of polymeric complexes can build up novel functional films which are difficult or impossible to fabricate by LbL assembly of uncomplexed polymers.

Electrodeposition of PVA-protected PtCo electrocatalysts for the oxygen reduction reaction in H2SO4

In this paper we report the electrosynthesis of PVA-protected PtCo films (PVA=poly(vinylalcohol)) and their activities towards the oxygen reduction reaction (ORR). PtCo electrodeposits were potentiostatically obtained in the presence and absence of PVA at distinct potentials. The film morphology and composition were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), which revealed that the use of PVA in the electrodeposition of PtCo films was decisive to achieve better film composition control. Cyclic voltammetry for PVA-protected PtCo films showed that the electrochemical surface area is dependent on the electrodeposition potentials and suggested different adsorption strengths of oxygen-containing species. Films produced in the presence of PVA presented the following activity order towards ORR as a function of the electrodeposition potential (vs. Ag/AgCl): −0.9V>−0.8V>−1.0V>−0.7V. In contrast, PtCo films electrodeposited in the absence of PVA displayed very similar activities regardless of the electrodeposition potential. The simplicity of the electrodeposition method combined with its effectiveness enabled the production of “model electrodes” for investigating the fundamental aspects of the reactions taking place in the fuel cell cathodes.

The Consequences of Kesterite Equilibria for Efficient Solar Cells

Copper-zinc-tin-chalcogenide kesterites, Cu(2)ZnSnS(4) and Cu(2)ZnSnSe(4) (CZTS(e)) are ideal candidates for the production of thin film solar cells on large scales due to the high natural abundance of all constituents, a tunable direct band gap ranging from 1.0 to 1.5 eV, a large absorption coefficient, and demonstrated power conversion efficiencies close to 10%. However, Sn losses through desorption of SnS(e) from CZTS(e) at elevated temperatures (above 400 °C) impede the thorough control of film composition and film homogeneity. No robust and feasible fabrication process is currently available. Here we show that understanding the formation reaction of the kesterite absorber is the key to control the growth process and to drastically improve the solar cell efficiency. Furthermore, we demonstrate that this knowledge can be used to simplify the four-dimensional parameter space (spanned by the four different elements) to an easy and robust two-dimensional process. Sufficiently high partial pressures of SnS(e) and S(e) (a) prevent the decomposition reaction of the CZTS(e) at elevated temperatures and (b) introduce any missing Sn into a Sn-deficient film. This finding enables us to simplify the precursor to a film containing only Cu and Zn, whereas Sn and S(e) are introduced from the gas phase by a self-regulating process.

An electron impact emission spectroscopy flux sensor for monitoring deposition rate at high background gas pressure with improved accuracy

Electron impact emission spectroscopy (EIES) has been proven to be a critical tool for film composition control during codeposition processes for the fabrication of multicomponent thin film materials including the high-efficiency copper-indium-gallium-diselenide photovoltaic cells. This technique is highly specific to atomic species because the emission spectrum of each element is unique, and the typical width of atomic emission lines is very narrow. Noninterfering emission lines can generally be allocated to different atomic species. However, the electron impact emission spectra of many molecular species are often broadband in nature. When the optical emission from an EIES sensor is measured by using a wavelength selection device with a modest resolution, such as an optical filter or monochromator, the emissions from common residual gases may interfere with that from the vapor flux and cause erroneous flux measurement. The interference is most pronounced when measuring low flux density with the presence of gases such as in reactive deposition processes. This problem is solved by using a novel EIES sensor that has two electron impact excitation sources in separate compartments but with one common port for optical output. The vapor flux is allowed to pass through one compartment only. Using a tristate excitation scheme and appropriate signal processing technique, the interfering signals from residual gases can be completely eliminated from the output signal of the EIES monitor for process control. Data obtained from Cu and Ga evaporations with the presence of common residual gases such as CO2 and H2O are shown to demonstrate the improvement in sensor performance. The new EIES sensor is capable of eliminating the effect of interfering residual gases with pressure as high as in the upper 10−5Torr range.

Epitaxial growth of the metastable phase ytterbium monoxide on gallium nitride surfaces

Molecular beam deposition systems allow for unparalleled control of film composition and structure. This article addresses the capacity for controlling metal and oxidant fluxes in the Yb/O 2 system to access the metastable phase ytterbium monoxide (YbO). Experiments exploring the growth of polycrystalline YbO x films by molecular beam deposition demonstrate that a 2:1 molar ratio of Yb:O 2 fluxes is necessary to achieve preferential growth of the divalent oxide. Applying similar deposition conditions to a (0 0 1) GaN surface leads to the growth of epitaxial (1 1 1) YbO films. Similar to other rocksalt oxides grown on GaN surfaces, YbO films display a 3D growth mechanism that leads to a grainy morphology with crystallites of 50 nm lateral dimensions. Rocking curves in ω and φ have full-width half-maximum values of 1.77° and 4.1°, respectively; further improvements in crystal quality appear to be limited by the thermal stability of the YbO phase.

Modelling and Optimization for Deposition of SiOxNyFilms by Radio-Frequency Reactive Sputtering

SiOxNy films are deposited by reactive sputtering from aSi target in Ar/O2/N2 atmospheres. In order to achievethe control of film composition and to keep a high deposition rate atthe same time, a new sputtering model based on Berg's work is providedfor the condition of double reactive gases. Analysis based on thismodel shows that the deposition process can easily enter thetarget-poisoning mode when the preset gas flow (N2 in this work)is too high, and the film composition will change from nitrogen-rich toSiO2-like with the increase of oxygen supply while keeping theN2 supply constant. The modelling results are confirmed in thedeposition process of SiOxNy. Target self-bias voltagesduring sputtering are measured to characterize the different sputteringmodes. FTIR-spectra and dielectric measurements are used to testify themodel prediction of composition. Finally, an optimized sputteringcondition is selected with the O2/N2 flow ratio varyingfrom 0 to 1 and N2 supply fixed at 1 sccm. Average depositionrate of 17 nm/min is obtained under this selected condition, which hassuggested the model validity and potential for industry applications.

Property control of expanding thermal plasma deposited textured zinc oxide with focus on thin film solar cell applications

Property control of expanding thermal plasma deposited textured zinc oxide is demonstrated considering intrinsic, i.e. bulk, and extrinsic transparent conducting oxide quality relevant for application in thin film amorphous silicon pin solar cells. Particularly the interdependence of electrical conductivity, film composition and film morphology, i.e. structure, feature shape and roughness of the surface, is addressed. Control of film composition is mainly governed by plasma production and gas phase chemistry inherently inducing a significant contribution to film morphology, whereas control of film morphology solely is governed by near-substrate conditions. Especially the ratio of zinc to oxygen and the reactor chamber pressure appear to be determinative in obtaining zinc oxide exhibiting the appropriate intrinsic and extrinsic quality, i.e. a high electrical conductivity, a high transmittance, a textured rough surface morphology and a strong hydrogen plasma resistance. The solar cell performance of appropriate undoped and aluminium doped textured zinc oxide inherently obtained during deposition is comparable with respect to Asahi U-type fluorine-doped tin oxide.

Binary and ternary NiTi-based shape memory films deposited by simultaneous sputter deposition from elemental targets

The most challenging requirement for depositing NiTi-based shape memory thin films is the control of film composition because a small deviation can strongly shift the transformation temperatures. This article presents a technique to control film composition via adjustment of the power supplied to the targets during simultaneous sputter deposition from separate Ni, Ti, and X (e.g., Hf) targets. After optimization of sputter parameters such as working gas pressure, target-substrate distance, and target power ratio, binary Ni100−xTix thin films were fabricated and characterized by energy dispersive x-ray spectroscopy in a scanning electron microscope (to measure the film composition and uniformity), in situ x-ray diffraction (to identify the phase structures), and differential scanning calorimetry (to indicate the transformation and crystallization temperatures). To explore the possibility of depositing ternary shape memory NiTi-based thin films with a high temperature transformation &amp;gt;100°C, a Hf target was added to the NiTi deposition system. Annealing was carried out in a high vacuum furnace slightly above the films’ crystallization temperatures (500 and 550 °C for NiTi and NiTiHf films, respectively). Differential scanning calorimetry (DSC) results of free-standing films illustrated the dependence of transformation temperatures on film composition: Ap and Mp (referring to the austenitic and martensitic peaks in the DSC curve) were above room temperature in near equiatomic NiTi and Ti-rich films, but below it in Ni-rich films. In NiTiHf films, the transformation temperatures were a function of Hf content, reaching as high as 414 °C (Ap) at a Hf content of 24.4 at. %. Atomic force microscopy revealed nanostructure surface morphology of both NiTi and NiTiHf films. Detailed characterization showed that the film properties were comparable with those of NiTi and NiTiHf bulk alloys.