Device Fabrication Process Research Articles

Plasma-optimized contact for high-performance PdSe2 nanoflake-based field-effect transistors

Low-resistance contact has long been pursued in the two-dimensional (2D) electronic/optoelectronic device community. Still, an economy-efficient method highly compatible with the conventional 2D device fabrication process in laboratory remains to be explored. Herein, we report a plasma-optimized contact strategy for high-performance PdSe2 nanoflake-based field-effect transistors (FETs). Selenium vacancies created by air plasma can introduce p-type doping in the contact area, thus optimizing the device performance. The effect of plasma treatment on PdSe2 nanoflake is corroborated by high-resolution transmission electron microscopy, energy-dispersive x-ray spectroscopy spectrum, atomic force microscopy, and Kelvin probe force microscopy. The PdSe2 FET with plasma-optimized contact exhibits significantly improved field-effect carrier mobilities, current on/off ratios, and reduced contact resistance than that without plasma treatment fabricated from the same PdSe2 nanoflake. Moreover, this strategy has also been proven effective to prepare high-performance FETs based on 2D WSe2 and MoSe2 nanoflakes, further demonstrating its application prospect.

High Speed 1550 nm Indium Gallium Arsenide-Indium Phosphide Photodetector

This paper presents preliminary results of a high speed 1550 nm indium gallium arsenide (InGaAs)-based mesa-type modified uni-traveling carrier photodiode (M-UTC-PD) structure. Conventional UTC-PD refers to P-I-N type photodiodes which selectively use electrons as active carriers. Photons absorbed in the relatively thin P-type absorber create minority carriers which are field accelerated toward a depleted collector thereby establishing high velocity ballistic transport, making these structures applicable for high speed applications. The M-UTC-PD structure presented uses spatially tailored P-type absorber regions to limit minority carrier generation both in the lateral and axial dimensions. Utilizing an otherwise conventional UTC-PD epitaxial structure where the top P-type layers are undoped, the spatially tailored P-type regions are defined by closed ampoule Zinc diffusion techniques. The M-UTC-PD structure presented utilizes a series of nested p-doped rings within a mesa structure to limit dark current and reduce overall capacitance to improve high speed operation. Two photodiode structures will be investigated for this research project, a conventional UTC-PD structure and a modified structure, utilizing similar device designs, epitaxial designs and fabrication processes. The conventional structure will be utilized for fabrication process development, verification of epi quality and development of rapid prototyping approach toward chip-based testing and subsequent high speed RF testing procedures. Conventional UTC-PD device results will be used as a comparison to quantify the performance of the M-UTC-PD structure utilizing Zn-doped defined p-type absorber regions. Results are given for chip tests of UTC-PD chips verifying epitaxial quality and fabrication process, subsequent testing of packaged devices and RF analysis remains. Process development of the Zn-doped devices is underway, once completed, these devices will be compared to the base design to quantify performance enhancement associated with the modified design.

Terahertz Radiation from High Electron Mobility Avalanche Transit Time Sources Prospective for Biomedical Spectroscopy

A Schottky barrier high-electron-mobility avalanche transit time (HEM-ATT) structure is proposed for terahertz (THz) wave generation. The structure is laterally oriented and based on AlGaN/GaN two-dimensional electron gas (2-DEG). Trenches are introduced at different positions of the top AlGaN barrier layer for realizing different sheet carrier density profiles at the 2-DEG channel; the resulting devices are equivalent to high–low, low–high and low-high–low quasi-Read structures. The DC, large-signal and noise simulations of the HEM-ATTs were carried out using the Silvaco ATLAS platform, non-sinusoidal-voltage-excited large-signal and double-iterative field-maximum small-signal simulation models, respectively. The breakdown voltages of the devices estimated via simulation were validated by using experimental measurements; they were found to be around 17–18 V. Under large-signal conditions, the series resistance of the device is estimated to be around 20 Ω. The large-signal simulation shows that the HEM-ATT source is capable of delivering nearly 300 mW of continuous-wave peak power with 11% conversion efficiency at 1.0 THz, which is a significant improvement over the achievable THz power output and efficiency from the conventional vertical GaN double-drift region (DDR) IMPATT THz source. The noise performance of the THz source was found to be significantly improved by using the quasi-Read HEM-ATT structures compared to the conventional vertical Schottky barrier IMPATT structure. These devices are compatible with the state-of-the-art medium-scale semiconductor device fabrication processes, with scope for further miniaturization, and may have significant potential for application in compact biomedical spectroscopy systems as THz solid-state sources.

Influence of Different Sterilization Methods on the Surface Chemistry and Electrochemical Behavior of Biomedical Alloys.

Sterilization is a prerequisite for biomedical devices before contacting the human body. It guarantees the lack of infection by eliminating microorganisms (i.e., bacteria, spores and fungi). It constitutes the last fabrication process of a biomedical device. The aim of this paper is to understand the effect of different sterilization methods (ethanol-EtOH, autoclave-AC, autoclave + ultraviolet radiation-ACUV and gamma irradiation-G) on the surface chemistry and electrochemical reactivity (with special attention on the kinetics of the oxygen reduction reaction) of CoCrMo and titanium biomedical alloys used as prosthetic materials. To do that, electrochemical measurements (open circuit potential, polarization resistance, cathodic potentiodynamic polarization and electrochemical impedance spectroscopy) and surface analyses (Auger Electron Spectroscopy) of the sterilized surfaces were carried out. The obtained results show that the effect of sterilization on the corrosion behavior of biomedical alloys is material-dependent: for CoCrMo alloys, autoclave treatment increases the thickness and the chromium content of the passive film increasing its corrosion resistance compared to simple sterilization in EtOH, while in titanium and its alloys, autoclave and UV-light accelerates its corrosion rate by accelerating the kinetics of oxygen reduction.

Machine Learning Assisted Analysis, Prediction, and Fabrication of High‐Efficiency CZTSSe Thin Film Solar Cells

AbstractThe Earth‐abundant element‐based Cu2ZnSn(S,Se)4 (CZTSSe) absorber is considered as a promising material for thin‐film solar cells (TFSCs). The current record power conversion efficiency (PCE) of CZTSSe TFSCs is ≈13%, and it's still lower than CdTe and CIGS‐based TFSCs. A further breakthrough in its PCE mainly relies on deep insights into the various device fabrication conditions; accordingly, the experimental–oriented machine learning (ML) approach can be an effective way to discover key governing factors in improving PCE. The present work aims to identify the key governing factors throughout the device fabrication processes and apply them to break the saturated PCE for CZTSSe TFSCs. For realization, over 25,000 data points were broadly collected by fabricating more than 1300 CZTSSe TFSC devices and analyzed them using various ML techniques. Through extensive ML analysis, the i‐ZnO thickness is found to be the first, while Zn/Sn compositional ratio and sulfo‐selenization temperature are other key governing factors under thin or thick i‐ZnO thickness to achieve over 11% PCE. Based on these key governing factors, the applied random forest ML prediction model for PCE showed Adj. R2 = >0.96. Finally, the best‐predicted ML conditions considered for experimental validation showed well‐matched experimental outcomes with different ML models.

Sub-second synthesis of silver nanoparticles in 3D printed monolithic multilayered microfluidic chip: Enhanced chemiluminescence sensing predictions via machine learning algorithms

Futuristic microfluidics will require alternative ways to extend its potential in vast areas by integrating various facets such as automation of different subsystems, multiplexing, incorporation of cyber-physical capabilities, and rapid prototyping. On the rapid prototyping aspect, for the last decade, additive manufacturing (AM) or 3D printing (3DP) has advanced to become an alternative fabrication process for microfluidic devices, enabling industry-level abilities towards mass production. In this context, for the first time, this work demonstrates the fabrication of monolithic multilayer microfluidic devices (MMMD) from planar orientation (1 layer) to nonplanar (4 layers) monolithic microchannels. The developed MMM device was impeccable for synthesizing highly potentialized silver nanoparticles (AgNPs) in <1 s. Moreover, the transport of chemical species with laminar flow simulations was performed on the process along with the thorough characterizations of produced AgNPs, finding the mean AgNPs particle size of around 35 nm without any post-processing requirements. The well-known catalytic activity of AgNPs was leveraged to enhance weak chemiluminescence (CL) sensing signals by >1300 %, increasing CL sensitivity. Further, machine learning (ML) predictive models encouraged to obtain the experimental parameters without human intervention iterations for target-specific applications. The proposed methodology finds the potential to save resources, time, and enables automation with rapid prototyping, providing possibilities for mass fabrications.

Hybrid Device Fabrication Using Roll-to-Roll Printing for Personal Environmental Monitoring.

Roll-to-roll (R2R) printing methods are well known as additive, cost-effective, and ecologically friendly mass-production methods for processing functional materials and fabricating devices. However, implementing R2R printing to fabricate sophisticated devices is challenging because of the efficiency of material processing, the alignment, and the vulnerability of the polymeric substrate during printing. Therefore, this study proposes the fabrication process of a hybrid device to solve the problems. The device was created so that four layers, composed of polymer insulating layers and conductive circuit layers, are entirely screen-printed layer by layer onto a roll of polyethylene terephthalate (PET) film to produce the circuit. Registration control methods were presented to deal with the PET substrate during printing, and then solid-state components and sensors were assembled and soldered to the printed circuits of the completed devices. In this way, the quality of the devices could be ensured, and the devices could be massively used for specific purposes. Specifically, a hybrid device for personal environmental monitoring was fabricated in this study. The importance of environmental challenges to human welfare and sustainable development is growing. As a result, environmental monitoring is essential to protect public health and serve as a basis for policymaking. In addition to the fabrication of the monitoring devices, a whole monitoring system was also developed to collect and process the data. Here, the monitored data from the fabricated device were personally collected via a mobile phone and uploaded to a cloud server for additional processing. The information could then be utilized for local or global monitoring purposes, moving one step toward creating tools for big data analysis and forecasting. The successful deployment of this system could be a foundation for creating and developing systems for other prospective uses.

What MEMS Research and Development Can Learn from a Production Environment

The intricate interdependency of device design and fabrication process complicates the development of microelectromechanical systems (MEMS). Commercial pressure has motivated industry to implement various tools and methods to overcome challenges and facilitate volume production. By now, these are only hesitantly being picked up and implemented in academic research. In this perspective, the applicability of these methods to research-focused MEMS development is investigated. It is found that even in the dynamics of a research endeavor, it is beneficial to adapt and apply tools and methods deduced from volume production. The key step is to change the perspective from fabricating devices to developing, maintaining and advancing the fabrication process. Tools and methods are introduced and discussed, using the development of magnetoelectric MEMS sensors within a collaborative research project as an illustrative example. This perspective provides both guidance to newcomers as well as inspiration to the well-versed experts.

Self-powered solar-blind ultraviolet photodetectors with Ga2O3 nanowires as the interlayer

Solar-blind photodetectors are widely used in military, industrial and commercial fields. There is an urgent need to develop devices with high spectral selectivity, high performance and low energy consumption. Here, ultrafine β-Ga2O3 nanowires were synthesized on a n-Si substrate using chemical vapor deposition, and a self-powered solar-blind photodetector based on β-Ga2O3 nanowires cluster and n-Si were fabricated with PEDOT:PSS as a transparent conductive layer to spread the current. Additionally, PMMA was used as an insulating layer to prevent short circuiting of the device and the device fabrication process was described in detail. In the aforementioned PEDOT:PSS/Ga2O3NWs/Si double heterojunction device, Ga2O3 nanowires serve as vital spectral-selective materials for the solar-blind band. The significance of Ga2O3 nanowires as an interlayer within the device was explored by introducing nanowires with varying morphologies. The inclusion of Ga2O3 nanowires led to a substantial improvement in the device's responsivity within the solar-blind band, while the performance within the visible band remained relatively unchanged. Remarkably, at a wavelength of 250 nm, the device incorporating a cluster of Ga2O3 nanowires as the interlayer exhibited a responsivity of 26.8 mA/W, which is 44 times greater than that of the PEDOT:PSS/Si heterojunction device.

Transferred Polymer-Encapsulated Metal Electrodes for Electrical Transport Measurements on Ultrathin Air-Sensitive Crystals.

Owing to rapid property degradation after ambient exposure and incompatibility with conventional device fabrication process, electrical transport measurements on air-sensitive 2D materials have always been a big issue. Here, for the first time, a facile one-step polymer-encapsulated electrode transfer (PEET) method applicable for fragile 2D materials is developed, which showed great advantages of damage-free electrodes patterning and in situ polymer encapsulation preventing from H2 O/O2 exposure during the whole electrical measurements process. The ultrathin SmTe2 metals grown by chemical vapor deposition (CVD) are chosen as the prototypical air-sensitive 2D crystals for their poor air-stability, which will become highly insulating when fabricated by conventional lithographic techniques. Nevertheless, the intrinsic electrical properties of CVD-grown SmTe2 nanosheets can be readily investigated by the PEET method instead, showing ultralow contact resistance and high signal/noise ratio. The PEET method can be applicable to other fragile ultrathin magnetic materials, such as (Mn,Cr)Te, to investigate their intrinsic electrical/magnetic properties.

Low-temperature processed natural hematite as an electron extraction layer for efficient and stable perovskite solar cells

The mixed halide perovskite solar cells (PSC) have manifested as a contender to market-dominating silicon counterparts owing to their low-temperature solution processing and high efficiency. In PSCs, the electron extraction layer (EEL) plays a vital role as it controls charge transport/extraction from the perovskite absorber layer to the EEL and also determines interfacial recombination and charge accumulation at the EEL/perovskite interface. In this work, high-purity natural hematite (α-Fe2O3) is reported for the first time as the EEL in triple-cation perovskite (CsFAMA) solar cells. To show the cost-effectiveness of this novel EEL, the entire device fabrication process was carried out at a temperature below 150 °C. The optimal α-Fe2O3 EEL shows a mobility value of 9.5 × 10−4 cm−2 V−1 s−1 and trap densities of around 2.40 × 1016 cm−3; the latter is close to state-of-the-art SnO2 EEL (1.26 × 1016 cm−3). The PSCs employing optimal EEL concentration of 10 mg/mL demonstrated a power conversion efficiency (PCE) of 13.3 %, fill factor of 68 %, and VOC of 1.03 V. X-ray diffraction studies show a high crystallinity of natural hematite whereas the photoluminescence studies show a fast carrier extraction from the perovskite to the EEL. The natural α-Fe2O3-based PSCs exhibited superior shelf-life stability of over 30 days due to less charge recombination and smoother CsFAMA thin film deposited over α-Fe2O3 EEL. The low-temperature processed natural α-Fe2O3 EEL can therefore be a promising EEL material for low-cost efficient PSCs.

Improved characteristics of MoS2 transistors with selective doping using 1,2-dichloroethane

We demonstrate area-selective doping of MoS2 field-effect transistors using 1,2-dichloroethane (DCE) solution. In the device manufacturing process, area-selective chemical doping was used to implement contact engineering in the source/drain region. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy measurements were performed to confirm the blocked layer (BL) using a photoresist, which suppressed the doping effect of the DCE treatment. In the XPS results, the main core level of the MoS2 flake with BL did not shift, whereas that of the MoS2 flake without BL changed by approximately 0.24 eV. In the case of the MoS2 flakes with a BL, the vibrational modes of the Raman scattering did not shift. Conversely, the two Raman peaks of the MoS2 flake without BL red-shifted because of increasing electron–phonon scattering. The effect of area-selective doping was confirmed by electrical measurements. The field-effect mobility and the subthreshold swing were enhanced from 4.07 to 31.5 cm2 (V s)−1 and from 1.26 to 0.401 V/decade, respectively.

72‐2: Invited Paper: Color‐Conversion Liquid Crystal Display with In‐Cell Polarizer

An approach to developing a thin‐film polarizer with high polarization efficiency in liquid crystal cells is presented. The in‐cell polarizer facilitates the realization of novel color‐conversion liquid crystal displays, where advanced quantum color‐converters are used as photoluminescent color filter arrays. We experimentally demonstrated using azo‐dye‐based embedded polarizers for color‐conversion liquid crystal displays for the first time. We systematically designed the device structure and fabrication process. The display is color parallax free and can be driven at regular conditions with good contrast.

EIS capacitor sensor, with TiO2 dielectric, applied in the evaluation of phosphate in wastewater

This work presents the development of an Electrolyte-Insulator-Semiconductor (EIS) device with a built-in reference electrode to detect phosphate in wastewater. The idea of developing this device comes from the need to improve the use of water processed by the effluent treatment plant. After treatment process, a fraction of the water can be used as wastewater, this water cannot be consumed, however it can be used for other purposes including cleaning public spaces, such as streets and squares. In the fabrication process of the EIS device, titanium oxide (TiO2) deposited on a silicon substrate was used as a sensor element, and titanium nitride (TiN) was used for the reference electrode. Both materials were deposited by the reactive sputtering process. Aluminum (Al) was used for the electrode on the back of the slide deposited by evaporation. Preliminary results of structural and electrical characterizations indicate that the manufactured device has good sensitivity to phosphate ions (66 mV/ppm). This sensitivity is due to the good quality of the film, which is shown using Raman spectroscopy, atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques, in addition to electrical measurements.

Conceptualizing data-driven closed loop production systems for lean manufacturing of complex biomedical devices—a cyber-physical system approach

A model is presented for shifting the manual intensive manufacturing process of complex biomedical devices towards more lean and efficient production process via application of concepts of cyber physical systems in combination with big data and analytics in a closed loop manner. The concept model is capable of handling high product volumes and variety, has ability for self-adaptation and correction in various operating conditions, and offers real-time quality control. The approach acknowledges the challenge of these industries operating in a strict regulated environment and the higher standards of built-in quality required by developing a closed loop process, proposed to be built in accordance to the requirements of regulatory bodies and current Industry 4.0 practices. The proposed model illustrates that modern manufacturing methodologies and concepts can be integrated and adopted in such highly regulated manufacturing environments and that the model can be deployed to different production scenarios.

Unveiling the In Situ and Solvent Polymerization Engineering for Highly Efficient and Flexible Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes.

Thermally activated delayed fluorescence (TADF) polymer has great potential for the construction of flexible solution-processed organic light-emitting diodes (OLEDs). However, the relationship between polymerization engineering and device functions has rarely been reported. Here, two novel TADF polymers, P-Ph4CzCN and P-Ph5CzCN, with a small energy gap between the first excited singlet and triplet states (ΔEST; <0.16 eV) were newly developed by both solvent and in situ polymerization of a styrene component. Detailed device performance testing indicates that both polymerization strategies ensure that the TADF polymer achieves comparable high efficiencies in commonly rigid devices, and the maximum external quantum efficiencies (EQEmax) were 11.9%, 14.1%, and 16.2% for blue, green, and white OLEDs, respectively. Although in situ polymerization provides a simplified device fabrication process, which avoids the complicated synthesis and purification of the polymer, the inevitable high-temperature annealing makes it fail in a plastic substrate device. In contrast, P-Ph5CzCN achieved by solvent polymerization enables the successful fabrication of a flexible device on a poly(ethylene terephthalate) (PET) substrate, which was the first reported flexible OLED based on a TADF polymer. This work provides a strong guideline for the simple fabrication of TADF polymer devices and the application of TADF polymer materials in OLED flexible panels and flexible lighting.

A flexible self-powered UV–Vis photodetector based on Successive Ionic Layer Adsorption and Reaction (SILAR) deposited CdS on asymmetric contacts

Self-powered photodetectors exhibiting broadband spectral responses have drawn significant attention owing to their zero-power consumption, high sensitivity, and enhanced light-matter interaction. However, considering the existing complex fabrication processes for self-powered devices, facile fabrication schemes that can be performed at room temperature and are scalable are highly desirable. This work demonstrates a metal-semiconductor-metal (MSM) based flexible, self-powered photodetector employing Successive Ionic Layer Adsorption and Reaction (SILAR) deposition of CdS directly on a paper substrate with interdigitated Asymmetric contacts, namely of pencil-drawn graphite and silver paste. Upon light illumination, a built-in field is generated due to the asymmetry in the fermi levels of the metal contacts, supporting efficient carrier separation. The as-fabricated device extends its absorbance from Ultraviolet (UV) to Visible (Vis) spectra, with the highest peak absorption around 520 nm. Further, the device exhibits a responsivity of 6.56 mA/W and 1500 mA/W and detectivity of 2.3 × 108 Jones and 5.66 × 1010 Jones when illuminated with Visible (intensity-0.0125 mW/cm2) and UV (intensity-2.50 mW/cm2) lights respectively. Without any external power, the device showed response times of 0.11 s and 1.42 s when subjected to UV and Vis irradiations. This detector also exhibited superior resistance to bending stress and retained its performance even after multiple bending cycles (∼1000). Hence, the facile, low-cost, and room-temperature SILAR deposition technique-assisted photodetector fabrication paves the way for developing other high-performance and flexible photodetectors for various optoelectronic applications.

The fabrication of flexible and large‐size inverted metamorphic five‐junction solar cells

AbstractThe inverted metamorphic AlGaInP/AlGaAs/GaAs/InGaAs/InGaAs five‐junction solar cells with the bandgap energies of 2.0/1.67/1.42/1.18/0.83 eV were grown by metal–organic chemical vapor deposition on GaAs substrates. The mismatch stress between the two bottom (In0.17Ga0.83As and In0.46Ga0.54As) cells and GaAs substrate was nearly fully relaxed owing to the growth of AlInGaAs compositionally step‐graded buffers. By employing the low‐temperature epilayer transfer technology with only one‐time temporary bonding, the flexible five‐junction solar cells with the size of 12 cm2 were successfully fabricated. The conversion efficiency was 35.1% with the open‐circuit voltage of 4.73 V under AM1.5G spectrum. The excellent performance of the large‐size five‐junction solar cells is attributed to the low amount of mismatch dislocations in the material as well as the simple fabrication process of the flexible device.

Monolithic three‐dimensional integration of aligned carbon nanotube transistors for high‐performance integrated circuits

AbstractCarbon nanotube field‐effect transistors (CNT FETs) have been demonstrated to exhibit high performance only through low‐temperature fabrication process and require a low thermal budget to construct monolithic three‐dimensional (M3D) integrated circuits (ICs), which have been considered a promising technology to meet the demands of high‐bandwidth computing and fully functional integration. However, the lack of high‐quality CNT materials at the upper layer and a low‐parasitic interlayer dielectric (ILD) makes the reported M3D CNT FETs and ICs unable to provide the predicted high performance. In this work, we demonstrate a multilayer stackable process for M3D integration of high‐performance aligned carbon nanotube (A‐CNT) transistors and ICs. A low‐κ (~3) interlayer SiO2 layer is prepared from spin‐on‐glass (SOG) through processes with a highest temperature of 220°C, presenting low parasitic capacitance between two transistor layers and excellent planarization to offer an ideal surface for the A‐CNT and device fabrication process. A high‐quality A‐CNT film with a carrier mobility of 650 cm2 V–1 s–1 is prepared on the ILD layer through a clean transfer process, enabling the upper CNT FETs fabricated with a low‐temperature process to exhibit high on‐state current (1 mA μm–1) and peak transconductance (0.98 mS μm–1). The bottom A‐CNT FETs maintain pristine high performance after undergoing the ILD growth and upper FET fabrication. As a result, 5‐stage ring oscillators utilizing the M3D architecture show a gate propagation delay of 17 ps and an active region of approximately 100 μm2, representing the fastest and the most compact M3D ICs to date.image

ZnO photoconductive synaptic devices for neuromorphic computing

In the field of artificial intelligence, neuromorphic computing offers the allure of human brain simulation. The capacity of optoelectronic synapses to intelligently handle optoelectronic input signals makes them a key component of neuromorphic computing. In this work, we fabricated ZnO-based optoelectronic synapses and investigated their electronic and photonic synaptic plasticity. The device fabrication process flow is as follows: photolithography is performed on ITO glass, followed by hydrochloric acid etching and degumming to obtain interdigital electrodes on the glass substrate, and a ZnO film is grown on the interdigitated electrodes by magnetron sputtering. A good Ohmic contact is formed between the ZnO and ITO electrodes, and the device exhibits a linear relationship between current and voltage under dark field conditions, and exhibits excellent synaptic properties and sensitivity in UV switching response experiments. Additional processes were subsequently introduced through growing ZnO nanowires and annealing, and the synaptic properties of ZnO device were improved. The shift from short-term to long-term memory, short-term plasticity, long-term plasticity, paired-pulse facilitation, and learning to experience behavior are only a few of the synaptic activities of the nervous system that have been successfully described. The manufactured array device also includes a special memory recall feature that may retrieve previous optoelectronic data. This work will advance the field of artificial intelligence and stimulate additional study on optoelectronic synapses.