Film Stack Research Articles

Correlated Disorder Substrate‐Integrated Nanodisk Scatterers for Light Extraction in Organic Light Emitting Diodes

AbstractA major loss mechanism in organic light emitting diodes (OLEDs) is the coupling of the emitter molecule light field to waveguide modes in the OLED thin film stack. In this work, a disordered 2D array of TiO2 nanodisk scatterers is integrated into the OLED substrate to enable efficient light extraction from these waveguide modes. Fabrication of the nanodisks is based on a bottom‐up, colloidal lithography technique and subsequent pattern transfer into high refractive index TiO2 via reactive ion etching. The substrates are completed by spin‐coating a polymer planarization layer before applying the OLED thin film stack. This ensures reproducible optoelectronic properties of the OLED through leaving the electrically active layers planar. Simultaneously, the nanodisks in close vicinity to the thin film stack ensure efficient out‐of‐plane scattering of waveguide modes. In a monochromatic OLED (center wavelength λ0 = 520 nm), a 44.2%rel increase in external quantum efficiency is achieved in comparison to a device without scattering structure. An in‐depth numerical analysis reveals that this significant enhancement is only partly due to the out‐coupling of waveguide modes. Additional enhancement is suspected to result from out‐coupling of substrate modes through scattering by the nanodisks. Further improvements to the scattering structure are numerically evaluated.

Polythiophenes with alkylthiophene side chains for efficient polymer solar cells

We synthesized two polythiophene derivatives with alkylthiophene side chains, named PT4T-0F and PT4T-2F. The introduction of alkylthiophene side chains endowed PT4T-0F with more ordered edge-on stacking in film, resulting in higher hole mobility of 0.45 cm2V−1s−1 in organic field-effect transistors in comparison to P3HT. Further introducing fluorine atoms, a mainly face-on orientation was achieved for PT4T-2F neat film, which was facilitated to charge transport and collection in polymer solar cells (PSCs). Using F8IC as acceptor, the PSC devices based on PT4T-2F achieved the champion efficiency of 10.48%, owing to the appropriate film morphology with moderate phase separation and favorable face-on molecular orientation. Such efficiency greatly outperformed those of PT4T-0F (2.85%) and P3HT (0.35%) based devices.

Inline metrology of high aspect ratio hole tilt and center line shift using small-angle x-ray scattering

High aspect ratio (HAR) structures found in three-dimensional nand memory structures have unique process control challenges. The etch used to fabricate channel holes several microns deep with aspect ratios beyond 50:1 is a particularly challenging process that requires exquisitely accurate and precise control. It is critical to carefully analyze multiple aspects of the etch process, such as hole profile, tilt, uniformity, and quality during development and production. X-ray critical dimension (XCD) metrology, which is also known as critical dimension small-angle x-ray scattering, is a powerful technique that can provide valuable insights on the arrangement, shape, and size of periodic arrays of HAR features. XCD is capable of fast, non-destructive measurements in the cell-area of production wafers, making XCD ideal for in-line metrology. Through several case studies, we will show that XCD can be used to accurately and precisely determine key properties of holes etched into hard mask, multilayer oxide/nitride film stacks and slit trenches. We show that the measurement of hole and slit tilt can be achieved without the aid of a structural model using a Fast Tilt methodology that provides sub-nanometer precision. Measurements were performed across several production wafers to determine the etch uniformity and quality. Particular attention was given at the edge of the wafers to account for large variations observed. In addition, we used a detailed physical model to characterize the HAR structures beyond linear tilt. This approach provides a more complete picture of the etch quality.

Green Reflector with Predicted Chromatic Coordinates

The color reflector with multiple-layer thin film scheme has attracted much attention because of the potential for massive production by wafer-scale deposition and the possibility to integrate with photonics (semiconductor) devices. Here, an angle-insensitive green reflector with a simple multilayer dielectric thin film structure was reported, with predicted chromatic coordinates based on CIE 1931 standard. The SiN/SiO2 multilayer thin film stack, including a special silicon-rich nitride material with ultrahigh refractive index, was grown alternatively by an inductively coupled plasma chemical vapor deposition (ICPCVD) system at a low stage temperature of 80 °C. The green reflector showed a maximum reflectivity of 73% around 561 nm with a full width at half maximum (FWHM) of 87 nm in the visible wavelength range, which contributed significantly to its color appearance. The measurement by an angle-resolved spectrometer under the illumination of p/s-polarized light wave with a variable angle of incidence indicated that the reflectance spectrum blue-shifted slightly with the increasing of incident angle such that the green color could be kept.

A critical overview of thin films coating technologies for energy applications

We report on several state of the art thin films coating technologies including physical vapor deposition (PVD) and solution process deposition techniques. Such techniques have their own significance to develop the energy efficiency devices. Choosing the right technique has become very critical in recent days as the scaling varies significantly with the technology. Thus, it becomes obvious to pick-up the right deposition procedure according to the needs where substrate size, film thickness, and required surface roughness are critically involved. Among all techniques, PVD offers various advantages, which include full control over growth, pristine film quality, large-scale fabrication, stacking of thin films, co-deposition flexibility, and controlling of the deposition temperature. In addition, such techniques can be adapted in small-scale research based labs with capping in terms of budget. Such vapor deposition processes also result in pin-hole free, homogeneous and continuous thin films which are highly desirable parameters for many energy related applications. Solution process techniques such as dip, spin, blade, and roll coating techniques are also discussed to provide an overall view. This report aims to give in-depth cross sectional review of all deposition techniques considering both positive and negative sides. PVD growth processes are very efficient where pattering of films are very significantly required. Hence, in this work, we have captured many deposition techniques where the properties of the films are very crucial to end up with different morphologies. The goal is to compare various techniques as a key parameter to control the fine formation of thin films. We are convinced that our report will show substantial importance to a wider group of materials scientists, chemists, physicists, and to the wider photovoltaic community to pick the best-suited vehicle for thin film growth purposes.

High-Performance Shortwave Infrared Detector Based on Multilayer Carbon Nanotube Films.

Carbon nanotube (CNT) is an ideal candidate material for shortwave infrared (SWIR) detectors due to its large band gap tunability, strong infrared light absorption, and high mobility. Furthermore, the photodetectors based on CNT can be prepared on any substrate using a low-temperature process, which is conducive to three-dimensional (3D) integration. However, owing to the absorption limitation (<2%) of a single-layer network CNT film with low density, the photodetectors of CNT film show low photocurrent responsivity and detectivity. In this paper, we optimize the thickness of the high-purity semiconducting network CNT films to increase the photocurrent responsivity of the photodetectors. When the thickness of network CNT film is about 5 nm, the responsivity of the zero-bias voltage can reach 32 mA/W at 1800 nm wavelength. Then, using stacked CNT films and contact electrode design, the photodetectors exhibit a maximum responsivity of 120 mA/W at 1800 nm wavelength. The photodetectors with stacked CNT films and local n-type channel doping demonstrated a wide response spectral range of 1200-2100 nm, a peak detectivity of 3.94 × 109 Jones at room temperature, and a linear dynamic range over 118 dB. Moreover, the peak detectivity is over 2.27 × 1011 Jones when the temperature is 180 K. Our work demonstrates the potential of the CNT film for future SWIR imaging at a low cost.

Development of high uniformity Al1-xScxN piezoelectric film stack dry etching process on 8-inch silicon wafers

This paper have investigated the basic properties of inductively coupled plasma reactive ion etching (ICP-RIE) of scandium doped aluminum nitride (Al1-xScxN) thin films on 8-inch silicon wafers, and presented a pretreatment scheme which can effectively improve the etching uniformity and wafer yield in the mass fabrication of Al1-xScxN-MEMS (micro-electromechanical systems) devices. Based on BCl3/Ar pretreatment and Cl2/Ar main etching scheme, an etching rate of 128 nm/min is obtained, with the low nonuniformity, and selectivity of 1.8 with respect to the loss of molybdenum (Mo) electrode is achieved, respectively. The relationship between etching parameters and etching uniformity is revealed in detail by the 5-step Al1-xScxN etching model. Meanwhile, to optimize the etching compatibility in the fabrication of Al1-xScxN-piezoelectric MEMS devices, a variety of etching masks (e.g. photoresist and silicon dioxide) are respectively used to obtain the consistent sidewall profile at different locations, with photoresist of 24° and silicon dioxide of 57°.

Importance of the Buffer Layer Properties for the Performance of Perovskite/Silicon Tandem Solar Cells

The power conversion efficiency of two-terminal perovskite/silicon tandem solar cells recently surpassed 30%, demonstrating the great potential of this technology. The most common electron-selective contact is a stack of a buckminsterfullerene film, a tin oxide buffer layer, and a transparent conductive oxide. While research is conducted on improving the fullerene/perovskite interface and on removing the buffer layer, less focus is put on tuning the buffer layer properties. In this work, the interplay of the doping densities in the fullerene film and the tin oxide layer as well as the impact of the tin oxide electron affinity on the device performance is investigated using electrical numerical simulations (technology computer-aided design). The simulated power conversion efficiency increases with higher doping densities in the fullerene film and the tin oxide layer, while an increasing conduction band offset between the two materials leads to a decreasing device performance. The results of this work imply that the properties of the buffer layer directly influence the performance of the tandem solar cell. In addition, the importance of measuring various material and interface characteristics in order to derive an improved device understanding is stressed.

2D Ti3C2Tx MXene-derived self-assembled 3D TiO2nanoflowers for nonvolatile memory and synaptic learning applications

Two-dimensional (2D) semiconducting materials and transition-metal oxides are promising materials for nonvolatile memory and brain-inspired neuromorphic computing applications. However, it remains challenging to obtain high-quality stacked 2D films with low energy consumptions (or drive currents) because of their high interfacial resistance. In this study, we synthesized 2D Ti3C2Tx MXene-derived three-dimensional (3D) TiO2 nanoflowers (NFs) as a feasible resistive switching (RS) material with outstanding electronic properties and synaptic learning capabilities. The electrical and optical characteristics of the synthesized material were determined through density functional theory calculations. Electrical measurements of the Al/Ti3C2Tx-TiO2 NF/Pt memory device indicated the occurrence of forming-free switching phenomena with extremely low switching voltages (0.68–0.53 V), stable ON/OFF ratio (2.3 × 103), and retention greater than 105 s. The Holt–Winters exponential smoothing technique was used for modeling and predicting the switching voltages of the RS device. The mechanism underlying the reliable RS was confirmed by observing the dense conductive filaments through conductive atomic force microscopy. Interestingly, the 2D Ti3C2Tx MXene-derived 3D TiO2 NF-based RS device mimicked the potentiation/depression and spike-time-dependent plasticity of a biological synapse. Finally, a convolutional neural network was implemented based on the observed synaptic weights of Al/Ti3C2Tx-TiO2 NF/Pt for image-edge detection.

A flexible proton beam imaging energy spectrometer (PROBIES) for high repetition rate or single-shot high energy density (HED) experiments (invited).

The PROBIES diagnostic is a new, highly flexible, imaging and energy spectrometer designed for laser-accelerated protons. The diagnostic can detect low-mode spatial variations in the proton beam profile while resolving multiple energies on a single detector or more. When a radiochromic film stack is employed for "single-shot mode," the energy resolution of the stack can be greatly increased while reducing the need for large numbers of films; for example, a recently deployed version allowed for 180 unique energy measurements spanning ∼3 to 75MeV with <0.4MeV resolution using just 20 films vs 180 for a comparable traditional film and filter stack. When utilized with a scintillator, the diagnostic can be run in high-rep-rate (>Hz rate) mode to recover nine proton energy bins. We also demonstrate a deep learning-based method to analyze data from synthetic PROBIES images with greater than 95% accuracy on sub-millisecond timescales and retrained with experimental data to analyze real-world images on sub-millisecond time-scales with comparable accuracy.

Large inverse spin Hall effect in BiSb topological insulator for 4 Tb/in2 magnetic recording technology

It is technically challenging to shrink the size of a tunneling magnetoresistance reader to below 20 nm for magnetic recording technology beyond 4 Tb/in2 due to its complex film stack. Recently, we proposed a reader architecture based on the inverse spin Hall effect to resolve those challenges, referred below as spin–orbit torque (SOT) reader, whose structure consists of a SOT layer and a ferromagnetic layer. However, the heavy metal-based SOT reader has small output voltage and low signal-to-noise ratio (SNR) due to the limited spin Hall angle θSH (&amp;lt; 1) of heavy metals. In this Letter, we demonstrate the integration of BiSb topological insulator with strong inverse spin Hall effect into the SOT reader that can significantly improve the output voltage and SNR. First, we theoretically calculate the noises in a 20 × 20 nm2 BiSb-based SOT reader to establish the relationships between SNR and θSH at various bias currents. We then demonstrate proof-of-concept BiSb-based SOT readers using CoFe/MgO/BiSb stack, which show large output voltages up to 15 mV at an input current of 9.4 kA/cm2 at room temperature. We project a giant θSH = 61 for BiSb. Our work demonstrates the potential of BiSb for SOT reader beyond 4 Tb/in2 magnetic recording technology.

Fabrication-conscious neural network based inverse design of single-material variable-index multilayer films.

Multilayer films with continuously varying indices for each layer have attracted great deal of attention due to their superior optical, mechanical, and thermal properties. However, difficulties in fabrication have limited their application and study in scientific literature compared to multilayer films with fixed index layers. In this work we propose a neural network based inverse design technique enabled by a differentiable analytical solver for realistic design and fabrication of single material variable-index multilayer films. This approach generates multilayer films with excellent performance under ideal conditions. We furthermore address the issue of how to translate these ideal designs into practical useful devices which will naturally suffer from growth imperfections. By integrating simulated systematic and random errors just as a deposition tool would into the optimization process, we demonstrated that the same neural network that produced the ideal device can be retrained to produce designs compensating for systematic deposition errors. Furthermore, the proposed approach corrects for systematic errors even in the presence of random fabrication imperfections. The results outlined in this paper provide a practical and experimentally viable approach for the design of single material multilayer film stacks for an extremely wide variety of practical applications with high performance.

Hot-filament CVD diamond coatings for optical applications

The demand for highly scratch resistant transparent coatings is motivated by the industrial sector of consumer goods for example for the protection of displays, touch-screens, watches, and all kinds of optical instruments. In a wide variety of applications, scratch-free and highly wear-resistant surfaces are the enabling factor for improvement of lifetime and thus sustainability of the products. Diamond with its unique properties such as the highest known hardness, low thermal expansion coefficient and chemical resistance (to acids and bases) [1,2] has outstanding potential for optical applications [3]. The use of thin ultrahard diamond layers with adapted optical properties is expected to enhance the resistance against abrasive wear to a new level. At Fraunhofer IST large-area Hot-Filament Chemical Vapor Deposition (HFCVD) processes have been developed [4,5] and now utilized for depositing highly uniform and transparent diamond coatings on optical components with sizes of up to 200 mm × 200 mm. The use of single-layer diamond coatings is investigated as well as the integration of diamond films into multilayer film stacks combining anti-reflective properties with mechanical stability (strength). Four-layer anti-reflective coatings (Si3N4/SiO2/Diamond/SiO2) entirely deposited with hot-filament/hot-wire processes and their optical and mechanical performance will be presented.

High-performance thermoplastic polyaryletherketone/carbon fiber composites: Comparison of plasma, carbon nanotubes/graphene nano-anchoring, surface oxidation techniques for enhanced interface adhesion and properties

This investigation highlights the development and performance comparison of carbon fiber (CF)-polyaryletherketone (PAEK) thermoplastic laminates prepared using CF, surface-modified with plasma, multi-walled carbon nanotubes (MWCNT), graphene, acid, and rare earth elements. Analysis of surface-treated CF by Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) revealed development of functional groups and changes in elemental composition on the CF surface. Significant lowering of contact angle and improvement in wettability was achieved by plasma and MWCNT/graphene treatment on CF. Field emission scanning electron microscopy (FESEM) analysis gave insight into the surface irregularities caused by various surface treatment techniques. The CF-PAEK composites were prepared by film stacking and compression moulding. The plasma-treated PAEK-CF composites exhibited the highest tensile properties, flexural strength, fracture toughness, and interlaminar shear strength. The superior properties are credited to better interfacial adhesion between CF and PAEK arising from mechanical interlocking over the roughened surface and covalent bonds by functional groups grafted on CF surface by plasma. MWCNT and graphene-anchored CF-PAEK composites also showed significant improvement in mechanical properties attributed to the bridging effect and interfacial interaction arising from increased surface area and functional groups. The surface-treated CF-PAEK composites exhibited a positive shift in the glass transition temperature, which is evidence of better adhesion. In dynamic loading conditions, MWCNT anchored CF-PAEK composite presented the highest storage modulus and energy absorption characteristics. Examination of the tensile fracture surface and fiber pull-out gave insight into the failure pattern and interfacial adhesion mechanism for the individual surface modification techniques.

Magnetic properties of (Ni0.81Fe0.19/SiO2)n multilayers for high frequency on-wafer inductor applications

We investigated the magnetic properties of multilayer stacks of Ni81Fe19 thin films separated with SiO2 intended for use in inductors operating at sub-GHz frequencies. We measured saturation magnetization, anisotropy field, coercivity, and complex permeability of sputtered multilayer structures consisting of thin film stacks of 5, 10, 15, 20, and 25 layers, ranging from 160 to 1455 nm in total magnetic material thickness. Our analysis indicates that multilayer stacks composed of repeated layers thinner than 115 nm generally maintained the desired magnetic properties found in individual thin films, such as high permeability, low anisotropy, and low coercive field. Moreover, the multilayer stacks did not exhibit unwanted characteristics such as stripe domain formation and decreased value of real permeability, which commonly occurred in single thicker films. These properties make them suitable for on-wafer inductor applications.

Different Production Processes for Thermoplastic Composite Materials: Sustainability versus Mechanical Properties and Processes Parameter

Up to now, fiber-reinforced composites with thermoplastic matrix have seen limited fields of use in the structural scope due to their high viscosity in the molten state, which results in poor impregnability of the reinforcement, leading to mechanical properties of the finished product that are not comparable to those of thermosets. Although the latter still dominate the various sectors of automotive, aerospace, transportation and construction, new applications involving the production of thermoplastic composites are growing rapidly, offering new approaches to the solution of this problem. The aim of this work is to study and evaluate the state of the art on the manufacturing processes of thermoplastic matrix composite, analyzing the parameters that come into play and that most influence the process and material performance. The advantages of film stacking and powder impregnation techniques are contrasted by the versatility of hybrid fabrics and, at the same time, parameters such as pressure and temperature must be carefully considered. A description of different thermoplastic composite processes such as powder impregnation, film stacking molding, hybrid woven fabric, hybrid yarn and products follows, which represent the current possibilities to move from a thermosetting matrix composite to a thermoplastic one, upon which the concept of sustainability is based. This article wants to present an overview of research that has been done in manufacturing thermoplastic reinforced composites and will serve as a baseline and aid for further research and development efforts.

Fabrication error analysis of nonperiodic-multilayer-dielectric gratings

We analyzed fabrication errors of nonperiodic-multilayer-dielectric gratings that were designed to have a high reflective diffraction efficiency for one wavelength and a high transmittance for another wavelength at the same time. Significant deviations were found between the measured and calculated efficiency values although the groove profile parameters were very closed to the design values. The source of error was attributed to coating errors of the film stack. To explain the deviation well, we estimated the parameters of actual coating stack by reverse calculations and using scanning electron microscope. We recalculated the diffraction efficiencies and the results showed that the actual film stack parameters were inverted well. This provided a foundation for us to reoptimize grating groove parameters on the basis of actual coating stack.

Measurement method for laser-accelerated multi-hundred-MeV protons utilizing multiple Coulomb scattering in an emulsion cloud chamber

We have developed a measurement method for laser-accelerated multi-hundred-MeV-class protons using an emulsion cloud chamber (ECC), which consists of a stack of nuclear emulsion films and scatterer plates. Based on multiple Coulomb scattering (MCS) theory depending on the incident energy of protons, we have constructed a regression model to inversely estimate the incident energy by means of support vector regression (SVR) with a radial basis function. The prediction accuracy was approximately 10% (standard deviation) for protons with energies up to 0.3 GeV in a 25 layer ECC with 0.5 mm thick tungsten plates, corresponding to a 2 cm thick compact detector.

Implementation of high-performance and high-yield nanoscale hafnium zirconium oxide based ferroelectric tunnel junction devices on 300 mm wafer platform

In this work, hafnium zirconium oxide (HZO)-based 100 × 100 nm2 ferroelectric tunnel junction (FTJ) devices were implemented on a 300 mm wafer platform, using a baseline 65 nm CMOS process technology. FTJs consisting of TiN/HZO/TiN were integrated in between metal 1 (M1) and via 1 (V1) layers. Cross-sectional transmission electron microscopy and energy dispersive x-ray spectroscopy analysis confirmed the targeted thickness and composition of the FTJ film stack, while grazing incidence, in-plane x-ray diffraction analysis demonstrated the presence of orthorhombic phase Pca21 responsible for ferroelectric polarization observed in HZO films. Current measurement, as a function of voltage for both up- and down-polarization states, yielded a tunneling electroresistance (TER) ratio of 2.28. The device TER ratio and endurance behavior were further optimized by insertion of thin Al2O3 tunnel barrier layer between the bottom electrode (TiN) and ferroelectric switching layer (HZO) by tuning the band offset between HZO and TiN, facilitating on-state tunneling conduction and creating an additional barrier layer in off-state current conduction path. Investigation of current transport mechanism showed that the current in these FTJ devices is dominated by direct tunneling at low electric field (E &amp;lt; 0.4 MV/cm) and by Fowler–Nordheim (F–N) tunneling at high electric field (E &amp;gt; 0.4 MV/cm). The modified FTJ device stack (TiN/Al2O3/HZO/TiN) demonstrated an enhanced TER ratio of ∼5 (2.2× improvement) and endurance up to 106 switching cycles. Write voltage and pulse width dependent trade-off characteristics between TER ratio and maximum endurance cycles (Nc) were established that enabled optimal balance of FTJ switching metrics. The FTJ memory cells also showed multi-level-cell characteristics, i.e., 2 bits/cell storage capability. Based on full 300 mm wafer statistics, a switching yield of &amp;gt;80% was achieved for fabricated FTJ devices demonstrating robustness of fabrication and programming approach used for FTJ performance optimization. The realization of CMOS-compatible nanoscale FTJ devices on 300 mm wafer platform demonstrates the promising potential of high-volume large-scale industrial implementation of FTJ devices for various nonvolatile memory applications.

Isofilm thickness control of hydrostatic bearings under piezoelectric stacked film restrictors

Isofilm thickness control of hydrostatic bearings under piezoelectric stacked film restrictors