Semiconductor Films Research Articles

High-Performance Pure Aluminum Coatings on Stainless Steels by Cold Spray

Aluminum target material is an important target material and is widely used in preparations of semiconductor films, integrated circuits, display circuits, protective films, decorative films, etc. In this study, pure aluminum coatings were deposited on stainless steel substrates by cold-spray technology as part of an overall project to produce large-size pure aluminum sputtering target materials. The results show that pure aluminum coatings exhibit high adhesive strength (~98 MPa), high deposition efficiency (~95%), and low porosity (~0.3%) on stainless steel substrates. The bonding mechanisms of pure aluminum coatings on stainless steel are a combination of metallurgical and mechanical interlocking. The evolutions of microstructure and mechanical properties of pure aluminum coatings under different heat treatments were also studied. With the increase of heat treatment temperature, it is found that cold-sprayed aluminum coatings become more homogenous in microstructure, the microhardness is reduced, and the adhesive strength seems to be slightly reduced. Overall, this study demonstrates significant advantages of cold-spray technology in depositing high-performance pure aluminum coatings on stainless steels.

20.1: Invited Paper: Research on Oxide Thin Film Transistors for Wearable Sensors

With the rapid development of the Internet of Things, sensors as an important foundation in the era of smart interconnection, have a significant potential in the emerging fields of smart home, wearable devices, and smart mobile terminals. Flexible sensor has been attracting a great attention due to its portability, miniaturization and long endurance. The high consumption and signal crosstalk are urgent issues for the high‐integration sensing array. These issues could be effectively addressed by active‐matrix thin film transistors (TFT) backplane. In this work, the feasibility of oxide TFT for flexible sensor was discussed, in which high‐quality oxi de semiconductor films, oxide dielectric films, and printing Ag electrodes and wires were investigated, respectively. The device performance of flexible Si‐doped SnO2 TFT was optimized by modulating oxygen partial pressure, which could achieve a relative good electrical properties at room temperature. To implement low‐consumption application, high quality of high‐k ZrO2 dielectric film was investigated by solution‐processed method. The dielectric properties of ZrO2 film with the mircowave‐as sisted or deep ultraviolet irradiation‐assisted annealing process was similar to that of the thermal ann ealing process. The morphology and conductivity of Ag electrodes and wires printed by electrohydrodynamic (EHD) inkjet printing technology were studied. High conductivity of Ag electrodes and wires was achieved at a low curing temperature. These results imply that oxide TFT has a significant potential in the application of flexible sensor array.

A Methodology of Fabricating Novel Electrodes for Semiconductor Devices: Doping and Van der Waals Integrating Organic Semiconductor Films.

Electrodes are indispensable components in semiconductor devices, and now are mainly made from metals, which are convenient for use but not ideal for emerging technologies such as bioelectronics, flexible electronics, or transparent electronics. Here the methodology of fabricating novel electrodes for semiconductor devices using organic semiconductors (OSCs) is proposed and demonstrated. It is shown that polymer semiconductors can be heavily p- or n-doped to achieve sufficiently high conductivity for electrodes. In contrast with metals, the doped OSC films (DOSCFs) are solution-processable, mechanically flexible, and have interesting optoelectronic properties. By integrating the DOSCFs with semiconductors through van der Waals contacts different kinds of semiconductor devices can be constructed. Importantly, these devices exhibit higher performance than their counterparts with metal electrodes, and/or excellent mechanical or optical properties that are unavailable in metal-electrode devices, suggesting the superiority of DOSCF electrodes. Given the existing large amount of OSCs, the established methodology can provide abundant electrode choices to meet the demand of various emerging devices.

Liquid Ga–In–Sn Alloy Printing of GaInSnO Ultrathin Semiconductor Films and Controllable Performance Field Effect Transistors

Wide band gap semiconductor Ga2O3 is a high potential material for fabricating next-generation power electronics. However, the low conductivity and carrier mobility of Ga2O3 have been seen as large barriers for its practical application. For many years, the efficient and low cost doping process to enhance the conductivity of Ga2O3 has always been a technological challenge. Here, we report a one-step synthesis strategy to prepare Ga2O3 doped with In2O3 and SnO2 (GaInSnO) multilayers from the liquid Ga–In–Sn alloy surface. Large area, controllable thickness, and high conductivity GaInSnO multilayers can be facilely obtained by using van der Waals exfoliation at a low temperature of 200 °C. The printed GaInSnO multilayers are transparent and display band gaps above 4.5 eV. The field effect transistors (FETs) based on the printed GaInSnO multilayers show n-type switching with on/off ratio all exceeding 105, a maximum field-effect mobility (μeff) of 65.40 cm2 V–1 s–1, and a minimum subthreshold swing (SS) of 91.11 mV dec–1 at room temperature. With rising Ga concentration in GaInSnO multilayers, the μeff of a fabricated FET decreases, while the SS increases. The present method can be further extended to produce various doped Ga2O3 films and utilized to fabricate electronic and optoelectronic devices based on modified Ga2O3 in the coming time.

Optical characteristics of a monolayer of identical spherical particles in an absorbing host medium.

The problem of light interaction with a 2D ensemble of homogeneous spherical particles embedded into an unbounded homogeneous absorbing host medium is considered. Based on the statistical approach, the equations are derived to characterize optical response of such a system with taking into account multiple scattering of light. Numerical data are presented for the spectral behavior of coherent transmission and reflection, incoherent scattering, and absorption coefficients of thin dielectric, semiconductor, and metal films containing a monolayer of particles with various spatial organization. The results are compared with the characteristics of the inverse structure: particles consist of the host medium material and vice versa. Data for the redshift of the surface plasmon resonance of the monolayer of gold (Au) nanoparticles in the fullerene (C 60) matrix are presented as a function of the monolayer filling factor. They are in qualitative agreement with the known experimental results. The findings have potential applications in the development of new electro-optical and photonic devices.

Electrical conductivity and corrosion resistance of Mo/Ti/Mn-based composite conversion films on AZ91D magnesium alloy

In this reported study, conductive composite conversion films on an AZ91D magnesium alloy were prepared by adding Ga3+ to a deposition solution that contained molybdenum, titanium and manganese ions. The Ga3+ significantly improved the conductivity and corrosion resistance of the resulting films. The electrical contact resistance (ECR) of the Molybdenum/Titanium/Manganese (MTM) film decreased from 0.436 Ω/in2 to 0.106 Ω/in2 at 200 psi when Ga3+ was added to form the Molybdenum-Titanium-Manganese-Gallium (MTMG) film. In addition, the polarization resistance of the Ga3+ amended film increased from 633.1 Ω·cm2 to 1930.835 Ω·cm2, MTMG showed obvious passivation behavior. The Ga3+ doping appeared to increase the carrier concentration and narrow the band gap of the semiconductor film. The N-type semiconductor carrier concentration increased from 2.11 × 1019 cm−3 to 7.052 × 1019 cm−3 in the MTMG film vs the MTM film. The P-type semiconductor carrier concentration in the MTMG film increased from 2.727 × 1018 cm−3 to 3.480 × 1019 cm−3 compared to the MTM film. The MTM band gap decreased from 2.916 eV to 2.714 eV in the MTMG film, which was attributed to the cross-doping of Ga3+ with transition metals in the MTMG material. The MTMG film had a three-layer structure, which preferentially formed a thin film on the β-phase and subsequently grew layer by layer.

Field enhancement of epsilon-near-zero modes in realistic ultrathin absorbing films

Abstract Using electrodynamical description of the average power absorbed by a conducting film, we present an expression for the electric-field intensity enhancement (FIE) due to epsilon-near-zero (ENZ) polariton modes. We show that FIE reaches a limit in ultrathin ENZ films inverse of second power of ENZ losses. This is illustrated in an exemplary series of aluminum-doped zinc oxide nanolayers grown by atomic layer deposition. Only in a case of unrealistic lossless ENZ films, FIE follows the inverse second power of film thickness predicted by S. Campione, et al. [Phys. Rev. B, vol. 91, no. 12, art. 121408, 2015]. We also predict that FIE could reach values of 100,000 in ultrathin polar semiconductor films. This work is important for establishing the limits of plasmonic field enhancement and the development of near zero refractive index photonics, nonlinear optics, thermal, and quantum optics in the ENZ regime.

Complementary Inverter Based on n‐Type and p‐Type OFETs with the Same Ambipolar Organic Semiconductor and ITO S/D Electrodes

AbstractBottom‐gate and bottom‐contact n‐type and p‐type organic field‐effect transistors (OFETs) are simultaneously obtained by combining the ambipolar semiconductor film of diketopyrrolopyrrole‐based conjugated polymer (P4FTVT‐C32) with indium tin oxide (ITO) source/drain (S/D) electrodes. P4FTVT‐C32 thin film exhibits n‐type unipolar property with the low work functional (WF) ITO S/D electrodes modified by polyethylenimine ethoxylated (PEIE) and it exhibits p‐type unipolar property with the high WF ITO S/D electrodes modified by HCl:InCl3. Hence, complementary inverters with transition voltages near VDD/2 and the maximum gain of 138 converting “1” state input into “0” state output are achieved by two different modifications via screen printing on ITO electrodes and then, only one‐time bar coating of P4FTVT‐C32. To further improve the performance and the uniformity of the OFET devices, the modification of octadecyltrichlorosilane (OTS) is also introduced. This work provides an easy‐handling method for the fabrication of low‐cost, high performance organic electronic devices and integrated circuits.

An ab initio method on large sized molecular aggregate system: Predicting absorption spectra of crystalline organic semiconducting films.

Theoretical description of electronically excited states of molecular aggregates at an ab initio level is computationally demanding. To reduce the computational cost, we propose a model Hamiltonian approach that approximates the electronically excited state wavefunction of the molecular aggregate. We benchmark our approach on a thiophene hexamer, as well as calculate the absorption spectra of several crystalline non-fullerene acceptors, including Y6 and ITIC, which are known for their high power conversion efficiency in organic solar cells. The method qualitatively predicts the experimentally measured spectral shape, which can be further linked to the molecular arrangement in the unit cell.

Solid state dewetting of a metal –semiconductor bi-layers deposited onto c-Si substrate

A bi-layers stack consisting of a semiconductor thin film of a varied thickness and a very thin Pd layer (SiC/Pd/c-Si).was deposited onto c-Si by e-beam evaporation at room temperature. The multi-layers structure was subjected to a thermal annealing process at near eutectic temperature of the Si – Pd phase. It is noticed, through top view SEM and cross-section STEM analyses, that the sandwiched Pd metal layer dewets from the interface with the c-Si substrate in well dispersed nanoparticles and it diffuses inward onto the top few monolayers of the substrate; at times it permeates shallowly through the SiC semiconductor top layer. The size distribution of the nanoparticles was found to be closely linked to the thickness of the top semiconductor layer. On the other hand, the top SiC layer was found to form islands protruding above the surface, when the film was very thin. When thicker, the semiconductor SiC layer retained its integrity and remained unaffected. An optical model of the resulting metal-dielectric mixed layer is proposed.

The Modeling of Self-Consistent Electron–Deformation–Diffusion Effects in Thin Films with Lattice Parameter Mismatch

In our work, the model of self-consistent electron–deformation–diffusion effects in thin films grown on substrate with the mismatch of lattice parameters of the contacting materials is constructed. The proposed theory self-consistently takes into account the interaction of the elastic field (created by the mismatch of lattice parameters of the film and the substrate, and point defects) with the diffusion processes of point defects and the electron subsystem of semiconductor film. Within the framework of the developed model, the spatial distribution of deformation, concentration of defects, conduction electrons and electric field intensity is investigated, depending on the value of the mismatch, the type of defects, the average concentrations of point defects and conduction electrons. It is established that the coordinate dependence of deformation and the concentration profile of defects of the type of stretching (compression) centers, along the axis of growth of the strained film, have a non-monotonic character with minima (maxima), the positions of which are determined by the average concentration of point defects. It is shown that due to the electron–deformation interaction in film with a lattice parameter mismatch, the spatial redistribution of conduction electrons is observed and n-n+ transitions can occur. Information about the self-consistent spatial redistribution of point defects, electrons and deformation of the crystal lattice in semiconductor materials is necessary for understanding the problems of their stability and degradation of nano-optoelectronic devices operating under conditions of intense irradiation.

Effects of Post-UV/Ozone Treatment on Electrical Characteristics of Solution-Processed Copper Oxide Thin-Film Transistors.

To realize oxide semiconductor-based complementary circuits and better transparent display applications, the electrical properties of p-type oxide semiconductors and the performance improvement of p-type oxide thin-film transistors (TFTs) are required. In this study, we report the effects of post-UV/ozone (O3) treatment on the structural and electrical characteristics of copper oxide (CuO) semiconductor films and the TFT performance. The CuO semiconductor films were fabricated using copper (II) acetate hydrate as a precursor material to solution processing and the UV/O3 treatment was performed as a post-treatment after the CuO film was fabricated. During the post-UV/O3 treatment for up to 13 min, the solution-processed CuO films exhibited no meaningful change in the surface morphology. On the other hand, analysis of the Raman and X-ray photoemission spectra of solution-processed CuO films revealed that the post-UV/O3 treatment induced compressive stress in the film and increased the composition concentration of Cu-O lattice bonding. In the post-UV/O3-treated CuO semiconductor layer, the Hall mobility increased significantly to approximately 280 cm2 V-1 s-1, and the conductivity increased to approximately 4.57 × 10-2 Ω-1 cm-1. Post-UV/O3-treated CuO TFTs also showed improved electrical properties compared to those of untreated CuO TFTs. The field-effect mobility of the post-UV/O3-treated CuO TFT increased to approximately 6.61 × 10-3 cm-2 V-1 s-1, and the on-off current ratio increased to approximately 3.51 × 103. These improvements in the electrical characteristics of CuO films and CuO TFTs can be understood through the suppression of weak bonding and structural defects between Cu and O bonds after post-UV/O3 treatment. The result demonstrates that the post-UV/O3 treatment can be a viable method to improve the performance of p-type oxide TFTs.

Oxygen Vacancies Engineering in Thick Semiconductor Films via Deep Ultraviolet Photoactivation for Selective and Sensitive Gas Sensing

AbstractRoom‐temperature detection of volatile organic compounds in particle‐per‐billion concentrations is critical for the development of wearable and distributed sensor networks. However, sensitivity and selectivity are limited at low operating temperatures. Here, a strategy is proposed to substantially improve the performance of semiconductor sensors. Tunable oxygen vacancies in thick 3D networks of metal oxide nanoparticles are engineered using deep ultraviolet photoactivation. High selectivity and sensitivity are achieved by optimizing the electronic structure and surface activity while preserving the 3D morphology. Cross‐sectional depth analysis reveals oxygen vacancies present at various depths (≈24% at a depth of 1.13 µm), with a uniform distribution throughout the thick films. This results in ≈58% increase in the sensitivity of ZnO to 20‐ppb ethanol at room temperature while ≈51% and 64% decrease in the response and recovery times, respectively. At an operating temperature of 150 °C, oxygen‐vacant nanostructures achieve a lower limit of detection of 2 ppb. Density functional theory analysis shows that inducing oxygen vacancies reduces activation energy for ethanol adsorption and dissociation, leading to improved sensing performance. This scalable approach has the potential for designing low‐power wearable chemical and bio‐sensors and tuning the activity and band structure of porous, thick oxide films for multiple applications.

Ambient-aging process enables enhanced efficiency for wide-bandgap perovskite solar cells

Wide-bandgap perovskite is considered an ideal absorber for the top component cell of a tandem configuration, in addition to having proven advantageous for indoor light conversion and semitransparent solar cells. In this work, we demonstrate an ambient aging process that appears to further raise the performance of wide-bandgap perovskite solar cells. Systematic study shows that that secondary recrystallization arises during the aging process. The defect density is further attenuated by air passivation and the self-healing effect induced by halogen redistribution. As such, the nonradiative recombination in the perovskite films is suppressed, and the carrier lifetime is prolonged during the ambient aging. The champion wide-bandgap (Eg=1.74 eV) perovskite solar cell delivered a power-conversion efficiency as high as 20.12%, coupled with a high open-circuit voltage of 1.27 V, which is among the highest values reported for this type of wide-bandgap solar cells. This work reveals the impact of the air aging process on the properties of perovskite films from various aspects and demonstrates a scalable post-treatment strategy capable of producing state-of-the-art semiconducting films.

Resolving Complex Photoconductivity of Perovskite and Organic Semiconductor Films Using Phase-Sensitive Microwave Interferometry

Complex transient photoconductivity (Δσ) contains rich fingerprints of charge recombination dynamics in photoactive films. However, a direct measure of both real (Δσ′) and imaginary (Δσ″) components has proven difficult using conventional cavity-based time-resolved microwave conductivity approaches. Here, we present a novel approach to resolve Δσ′ and Δσ″ parts of Δσ by using a nonresonant coplanar transmission line and a microwave interferometric detection scheme. The use of a phase-sensitive microwave interferometer greatly increases the measurement sensitivity and eliminates the requirement of a resonant cavity. This broadband detection scheme allows for direct measurement of Δσ. The relationship between the experimental phase shift and Δσ′ and Δσ″ components is decoded through an in situ electron spin resonance (ESR) measurement. ESR line shape analysis is used to confirm the assignment of the transients to the Δσ′ and Δσ″ components. We demonstrate the utility of this technique on thin films of poly(3-hexylthiophene): [6,6]-phenylC61-butyric acid methyl ester (P3HT:PCBM) and perovskite MA0.85FA0.15PbI3 films on glass.

Tellurium Nanosphere/Hematite Hybrid Thin Films with Enhanced Light Absorption for Efficient Photocatalysis

As a semimetal, tellurium (Te) shows metallic and dielectric properties in the UV and visible ranges, respectively, and has potential as a broad-band light absorber. In this paper, plasmonic–semiconductor hybrid nanoscale films consisting of Fe2O3 thin films and Te nanospheres are designed and demonstrated. Te clusters are proved to have an enhancement of the electric near-field in a broad UV–vis–NIR spectrum, which facilitates light absorption and charge separation in photoelectrocatalysis. The thickness of Fe2O3 films is adjusted from 22 to 137 nm. The photocurrent density enhancement factor of Te-modified films is 13 times higher than that of 22 nm Fe2O3 films. Particularly, a Te-decorated Fe2O3 ultrathin film exhibits photoelectrochemical activity comparable to that of bulk Fe2O3, suggesting significant near-field enhancement from plasmon modes of the Te clusters. It is found that the Te cluster decoration led to a significant decrease of the charge transfer resistance, demonstrating that the Te clusters could boost the formation rate and suppress the recombination rate of electron–hole pairs. The hybridization of Te nanoparticles with nanoscale semiconductor films represents an efficient way to enhance the solar energy conversion efficiency.

Ultra-Violet-Assisted Scalable Method to Fabricate Oxygen-Vacancy-Rich Titanium-Dioxide Semiconductor Film for Water Decontamination under Natural Sunlight Irradiation

This work reports the fabrication of titanium dioxide (TiO2) nanoparticle (NPs) films using a scalable drop-casting method followed by ultra-violet (UV) irradiation for creating defective oxygen vacancies on the surface of a fabricated TiO2 semiconductor film using an UV lamp with a wavelength oof 255 nm for 3 h. The success of the use of the proposed scalable strategy to fabricate oxygen-vacancy-rich TiO2 films was assessed through UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The Ti 2p XPS spectra acquired from the UV-treated sample showed the presence of additional Ti3+ ions compared with the untreated sample, which contained only Ti4+ ions. The band gap of the untreated TiO2 film was reduced from 3.2 to 2.95 eV after UV exposure due to the created oxygen vacancies, as evident from the presence of Ti3+ ions. Radiation exposure has no significant influence on sample morphology and peak pattern, as revealed by the SEM and XRD analyses, respectively. Furthermore, the photocatalytic activity of the fabricated TiO2 films for methylene-blue-dye removal was found to be 99% for the UV-treated TiO2 films and compared with untreated TiO2 film, which demonstrated only 77% at the same operating conditions under natural-sunlight irradiation. The proposed UV-radiation method of oxygen vacancy has the potential to promote the wider application of photo-catalytic TiO2 semiconductor films under visible-light irradiation for solving many environmental and energy-crisis challenges for many industrial and technological applications.

Step‐Edge‐Induced Patterning and Orientation Control of Crystalline Organic Semiconductor Films

AbstractOrientation control over micropatterned crystalline organic semiconductor film is crucial to achieve high device performance with small variation in organic electronics. Here, using Au patterned SiO2 substrates, a method is reported to realize high‐resolution patterning and orientation control of crystalline organic thin films simultaneously. The vacuum deposited N,N‐dioctyl‐3,4,9,10‐perylene tetracarboxylic diimide (PTCDI‐C8) molecules can selectively diffuse to and nucleate on prepatterned Au stripes, resulting in patterned crystalline films with a preferential orientation distribution. Scanning tunneling microscope and high‐resolution atomic force microscope images reveal that the PTCDI‐C8 molecules first assemble on Au with perylene diimide cores in a laying down configuration, providing nucleation sites at the step edges for directional growth along the a‐axis of the molecule crystal. Subsequent deposition of molecules leads to orientated crystalline films growth due to the π packing of the perylene diimide cores, resulting in orientation control on micropatterned crystalline films. The finding provides a new strategy to grow oriented crystalline films for high device performance uniformity.

Conductivity maxima in electrolyte-gated transistors with molecular-doped semiconducting polymer films

Conductivity maxima in electrolyte-gated transistors with molecular-doped semiconducting polymer films

Hardware-Intrinsic Physical Unclonable Functions by Harnessing Nonlinear Conductance Variation in Oxide Semiconductor-Based Diode

With the advancement of the Internet of Things (IoT), numerous electronic devices are connected to each other and exchange a vast amount of data via the Internet. As the number of connected devices increases, security concerns have become more significant. As one of the potential solutions for security issues, hardware intrinsic physical unclonable functions (PUFs) are emerging semiconductor devices that exploit inherent randomness generated during the manufacturing process. The unclonable security key generated from PUF can address the inherent limitations of conventional electronic systems which depend solely on software. Although numerous PUFs based on the emerging memory devices requiring switching operations have been proposed, achieving hardware intrinsic PUF with low power consumption remains a key challenge. Here, we demonstrate that the process-induced nonlinear conductance variations of oxide semiconductor-based Schottky diodes provide a suitable source of entropy for the implementation of PUF without switching operation. Using a mild oxygen plasma treatment, the surface electron accumulation layer that forms naturally in oxide semiconductor film can be partially eliminated, resulting in a large variation of nonlinearity as an exotic entropy source. The mild plasma-treated Schottky diodes showed near ideal 50% average uniformity and uniqueness, as well as an ideal entropy value without the need for additional hardware area and power costs. These findings will pave the way for the development of hardware intrinsic PUFs to realize energy-efficient cryptographic hardware.