Ultrathin Semiconductor Films Research Articles

Investigation of perfect narrow-band absorber in silicon nano hole array

In this paper, we proposed a triple layer structure consisting of the bottom silver layer, thin silicon oxide space layer, and ultrathin semiconductor silicon film with nano hole array achieving three absorption peaks with narrow band. The absorption spectrum can be easily controlled by adjusting the structural parameters including the radius and period of the nano hole array, and the maximal absorption can reach 99.0% and the narrowest full width of half maximum can reach about 6.5 nm in theory. We also clarified the physical mechanism of the proposed structure in details by finite-difference time-domain simulation, in which the three narrow band perfect adsorption peaks can be attributed to electric dipole resonance, magnetic dipole resonance and plasmonic resonance respectively. At the same time, we used a low-cost nanosphere lithography method to fabricate the proposed nano hole array in large area. In experiment, the absorption peak of the proposed triple layer structure can reach up to 98.3% and the narrowest full width of half maximum can reach up to about 10.1 nm. The highest quality factor Q can reach up to 98.4. This work can open a new avenue for high-quality factor narrow band perfect absorption using ultrathin semiconductor film and benefit for many fields such as infrared sensors, plasmonic filters, and hyperspectral imaging.

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.

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.

Microstructured Ultrathin Organic Semiconductor Film via Dip-Coating: Precise Assembly and Diverse Applications

ConspectusOrganic semiconductors (OSCs) have led to considerable progress in various fields owing to their intrinsic flexibility, adjustable chemical structures, multifunctionalities, and low processing cost. The optoelectronic properties of OSCs greatly depend on their aggregation state in molecular assemblies. To obtain ideal and reliable optoelectronic performances, a highly controlled assembly method for modulating OSC packing structures is indispensable. In particular, ultrathin OSC microstructures consisting of one to several molecular layers, known as microstructured ultrathin organic semiconductor films (MUOSFs), are of great importance for both fundamental research and applications of OSCs. However, most reported film thicknesses or molecular layer numbers of ultrathin OSC films/microstructures are “average values” rather than “real values” and are not molecularly precise. The lack of an effective and general assembly strategy seriously hinders the progress of MUOSFs. In this Account, we summarize our recent progress in exploring the assembly strategy, the underlying mechanism, and diverse applications of MUOSFs prepared via dip-coating under optimized conditions.Dip-coating, with an intrinsic three-phase contact line, has a great advantage to precisely assemble ultrathin OSC films/microstructures. As an evaporation-controlled method, the assembly processes used in dip-coating are susceptible to multiple factors, such as pulling speed, molecular structures, and solution properties. Under the guidance of our proposed “Balance Principle” for adjusting these factors, uniform and continuous MUOSFs can be obtained over a large area. Under optimized conditions, the number of molecular layers of MUOSFs can be customized with monolayer precision, which is significant for investigating the charge transport mechanism of OSCs. At the same time, the ultrathin and partial coverage characteristics of MUOSFs are beneficial to fabricate flexible and transparent devices for wearable electronics. Notably, the morphologies and coverage of ultrathin microstripes can be well tuned by the pulling speed, which is highly desired for high-performance bio/chemical sensors, because the large specific surface area provides abundant reactive sites.The feasibility and generality of our assembly method demonstrate that dip-coated MUOSFs have excellent advantages for fundamental research and applications of OSC materials, based on which many promising future works around MUOSFs can be extended to more related fields by engineering molecular orientations, packing structures, film morphologies, coverage, etc. The work on the assembly and applications of MUOSFs might offer the potential to promote the progress of organic electronics.

Cobweb-like, Ultrathin Porous Polymer Films for Ultrasensitive NO2 Detection.

Gas sensors based on polymer field-effect transistors (FETs) have drawn much attention owing to the inherent merits of specific selectivity, low cost, and room temperature operation. Ultrathin (<10 nm) and porous polymer semiconductor films offer a golden opportunity for achieving high-performance gas sensors. However, wafer-scale fabrication of such high-quality polymer films is of great challenge and has rarely been realized before. Herein, the first demonstration of 4 in. wafer-scale, cobweb-like, and ultrathin porous polymer films is reported via a one-step phase-inversion process. This approach is extremely simple and universal for constructing various ultrathin porous polymer semiconductor films. Thanks to the abundant pores, ultrathin size, and high charge-transfer efficiency of the prepared polymer films, our gas sensors exhibit many superior advantages, including ultrahigh response (2.46 × 106%), low limit of detection (LOD) (<1 ppm), and excellent selectivity. Thus, the proposed fabrication strategy is exceptionally promising for mass manufacturing of low-cost high-performance polymer FET-based gas sensors.

OFET chemical sensors: Chemical sensors based on ultrathin organic field‐effect transistors

AbstractIn the past decades, a variety of organic field‐effect transistor (OFET) based sensors have been reported, holding great promise in physical sensing, chemical sensing, biosensing, medical sensing and functionalized integrated sensor arrays, thanks to the superiorities of OFETs, including fabrication versatility and high sensitivity to external stimulus. However, the sensing performance still needs to be enhanced to meet practical application requirements, especially high sensitivity and short response time. Among all the effective strategies, ultrathin organic semiconductor (OSC) film devices show significantly promoted sensing performance, and the fabrication methods to obtain these ultrathin OSC films are of importance and diversity. Here, a brief review focused on ultrathin (including molecular monolayer) OSC film fabrication methods is presented, including high‐vacuum thermal evaporation, solution‐processable methods, and the approach of combining ultrathin films with heterojunctions. Additionally, applications of ultrathin OFET‐based chemical and biological sensors are summarized. Finally, the outlook and challenges ahead are discussed. We hope this review can offer guidance for the fabrication methods and potential applications of ultrathin OFET‐based chemical and biological sensors in the future. © 2020 Society of Industrial Chemistry

Elaboration and characterization of a 200 mm stretchable and flexible ultra-thin semi-conductor film

We describe a process for transferring a 200 nm thick, 200 mm wide monocrystalline silicon (mono c-Si) thin film from a silicon-on-insulator onto a flexible polymer substrate. The result is a stretchable and flexible ultra-thin semi-conductor film that can be subjected to tensile stress experiments. The process uses off-the-shelf 200 mm wafers and standard polymer temporary bonding techniques. The backside substrate and buried oxide are removed using grinding and wet etching processes. No cracks or wrinkles are observed on the film prior to the tensile stress experiments. The stretching of the flexible structure results in up to 1.5% uniaxial tensile elastic strain on the thin mono c-Si film.

Semiconductor-enhanced Raman scattering sensors via quasi-three-dimensional Au/Si/Au structures

Abstract We present a feasible way to strongly enhance Raman signals via introducing an ultra-thin dielectric film in the dual-layer plasmonic hotspots structure, which forms a quasi-three-dimensional structure. The Raman intensity was obtained with an enhancement factor of 735% for the dual-layer metal structure buffered with an ultra-thin silicon film. Moreover, the silicon layer based surface-enhanced Raman scattering (SERS) substrate provided a Raman signal two to five times larger than that of the silica buffered substrate. These distinct responses confirm that the ultra-thin high-index semiconductor film has the capability of additionally enhancing Raman scattering. Otherwise, the upper and lower metal clusters can support multiple kinds of plasmonic resonances, which produce a remarkable physical enhancement of the Raman signals. Besides these impressive optical properties, the substrates have prominent advantages on structural features, since the fabrication process can be fulfilled simply, suggesting a feasible way for a large-area and low-cost SERS platform. The findings may pave an avenue to achieve insights on the dielectric enhanced Raman scattering and hold potential applications in optoelectronics, such as environmental and health sensors.

Maximal Visible Light Energy Transfer to Ultrathin Semiconductor Films Enabled by Dispersion Control

AbstractIncreasing light absorption in an ultrathin semiconductor is critical for developing thin‐film photovoltaic devices. Here, it is shown that a maximal absorption of visible light is possible through controlling the dispersion of thin‐film materials. The ideal dispersion relation is determined for the permittivity of a thin film placed on a reflector with a dielectric spacer, and it is explained how the ideal dispersion relation can be realized for semiconductor materials possessing bandgaps. To experimentally verify dispersion control and maximal absorption, the permittivity of lead selenide (PbSe) thin film is tailored by controlling its polycrystallinity through the sputtering conditions. The measured reflectance of a dispersion‐controlled PbSe film (9 nm) deposited on an SiO2 (48 nm)/Al substrate shows a record level of absorbance for PbSe film of 88% taken as an average over the entire visible spectrum. This value is close to the theoretical maximum value of 95%. Overall, the dispersion control scheme offers promising avenues for thin‐film solar cell research.

Planar Metasurfaces Enable High-Efficiency Colored Perovskite Solar Cells.

The achievement of perfect light absorption in ultrathin semiconductor materials is not only a long‐standing goal, but also a critical challenge for solar energy applications, and thus requires a redesigned strategy. Here, a general strategy is demonstrated both theoretically and experimentally to create a planar metasurface absorber comprising a 1D ultrathin planar semiconductor film (replacing the 2D array of subwavelength elements in classical metasurfaces), a transparent spacer, and a metallic back reflector. Guided by derived formulisms, a new type of macroscopic planar metasurface absorber is experimentally demonstrated with light near‐perfectly and exclusively absorbed by the ultrathin semiconductor film. To demonstrate the power and simplicity of this strategy, a prototype of a planar metasurface solar cell is experimentally demonstrated. Furthermore, the device model predicts that a colored planar metasurface perovskite solar cell can maintain 75% of the efficiency of its black counterpart despite the use of a perovskite film that is one order of magnitude thinner. The displayed cell colors have high purities comparable to those of state‐of‐the‐art color filters, and are insensitive to viewing angles up to 60°. The general theoretical framework in conjunction with experimental demonstrations lays the foundation for designing miniaturized, planar, and multifunctional solar cells and optoelectronic devices.

Ultrathin semiconductor films for NH3 gas sensors prepared by vertical phase separation

Vertical phase separation of the polymer blendsis a facile method to prepare double-layered polymer films. In this study, ultrathin P3HT films prepared by the vertical phase separation of the P3HT and PMMA blends were reported. The film thickness of P3HT is simply controlled by varying the P3HT/PMMA blend ratio. The thickness of the as-formed ultrathin P3HT films can be as low as around 2.0 nm. The relationship between the P3HT film thickness and charge transport performance was studied. It was found that the mobility decreased slightly with the decrease of P3HT film thickness. The ultrathin P3HT films exhibit good organic field-effect transistor performance with a mobility of 2.3 × 10−2 cm2V-1s-1. The gas sensors based on the ultrathin P3HT films was also investigated. The performance of ammonia sensor improved rapidly as the film thickness decreased and the response of the P3HT ultrathin film with 2.0 nm thickness reaches 31.1%.

Solution Coating of Small Molecule/Polymer Blends Enabling Ultralow Voltage and High‐Mobility Organic Transistors

AbstractLow‐voltage organic field‐effect transistors (OFETs) are of great interest for organic electronics applications that require low power consumption such as wearable electronics, biomedical applications, or mobile electronics. In this work, an approach leading to transistors fabricated from solution with high charge carrier mobilities operating at voltages &lt; 1 V is presented. By blending the small‐molecule semiconductor 6,13‐bis(triisopropylsilyl‐ethynyl)pentacene (TIPS‐pentacene) with polystyrene it is possible to achieve good film coverage and uniformity as well as ultrathin semiconductor films. This reduction in thickness relative to neat films results in a high fraction of the high‐mobility polymorph of TIPS‐pentacene and excellent film morphologies with continuous highly crystalline domains. OFETs using SiO2 as the dielectric with average hole mobilities as high as 8.3 cm2 V−1 s−1 and maximum mobilities of up to 12.3 cm2 V−1 s−1 which favorably compares with the previous record for TIPS‐pentacene, especially when considering the simplicity of the approach, are demonstrated. By depositing the optimized semiconductor blends on solution‐based polymer dielectric layers of polyvinylphenol, cross‐linked with 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride, a record‐high mobility of 4.2 cm2 V−1 s−1 for solution‐processed, ultralow‐voltage OFET devices (operating at &lt;1 V) is obtained.

Ultra-thin MoO3 film goes wafer-scaled nano-architectonics by atomic layer deposition

From the technical and design points of view, it is quite difficult to maintain the integrity of nano-films during the deposition process to fabricate practical devices based on ultra-thin semiconductor films. Thus, defect-free wafer-scaled development of ultra-thin quasi two-dimensional (2D) oxide semiconductor films represents serious challenges. Plasma-enhanced atomic layer deposition (PE-ALD) made it possible to fabricate ultra-thin MoO3 nano-films (4.6nm) over the wafer-scaled granular Au electrode. The detailed ALD recipe for ultra-thin MoO3 film was established and verified. The C12H30N4Mo and O2 plasma were used as Mo precursor and oxygen source, respectively. The growth of crystalline phases was observed when the ALD temperature of 250°C was employed. Higher ALD temperature resulted in an increase of growth rate over Au substrate (1.21Ǻ/cycle). The precise recipe design enabled the scalable fabrication of environmental sensors based on ultra-thin MoO3 films with precise thickness controllability. Electrochemical sensors based on the fabricated MoO3 nanostructures demonstrated reliable performance to hydrazine (N2H4) detection.

III–V semiconductor resonators: A new strategy for broadband light perfect absorbers

Broadband light perfect absorbers (BPAs) are desirable for applications in numerous optoelectronics devices. In this work, a semiconductor-based broadband light perfect absorber (S-BPA) has been numerically demonstrated by utilizing plasmonlike resonances of high-index semiconductor resonators. A maximal absorption of 99.7% is observed in the near-infrared region. By taking the absorption above 80% into account, the spectral bandwidth reaches 340 nm. The absorption properties mainly originate from the optical cavity modes induced by the cylinder resonators and ultrathin semiconductor film. These optical properties and simple structural features can maintain the absorber platform with wide applications in semiconductor optoelectronics.

(Invited) Increasing the Bandwidth of Semiconductor Photodetectors through Hot Carrier Generation in Nanometer-Scale Optical Coatings

Nanometer scale optical coatings have recently been developed with tunable, high absorption through excitation of zeroth order Fabry-Perot (FP) modes. Such optical coatings have primarily increased above bandgap absorption in ultrathin semiconductor films with applications in thin film photodetection, photovoltaics and photocatalysis. We propose the use of zeroth order FP modes for expanding the bandwidth of semiconductor devices through sub-bandgap absorption in nanoscale metal contacts to semiconductors such as silicon. The absorption in the metal generates hot carriers that are injected into the semiconductor, enabling photocurrent generation from light with energy below the semiconductor bandgap energy. However, for efficient photocurrent injection, the metal must be highly absorptive at long wavelengths and ultra-thin in order for the hot carriers to reach the Schottky interface with enough energy to traverse the barrier. We show that by selecting materials with optical properties that satisfy a zeroth order FP mode (n ~ k for m = n + ik), it is possible to obtain ~80% absorption in materials as thin as 10-15 nm.

Plasmonics and Antenna Effect Mediated Super Absorption in Ultrathin Film via Core/Shell Design

The conventionally low-absorbing ultrathin film is recently shown to obtain a high absorption via a strong optical interference effect by coating the highly absorbing film onto a metal layer. In this letter, through finite-element simulation and modal analysis, we report that the optical performance of an ultrathin semiconductor film can be significantly improved by further nanostructuring the ultrathin semiconductor film. Through wrapping the film onto a metallic nanowire in either lying or vertical configuration, a high dielectric resonance antenna effect and a strong plasmonic resonance can be excited simultaneously, which enables an extremely high and broadband optical absorption. Especially, with introducing the vertical metal-core/semiconductor-shell design, the peak absorption can be up to >300% under an extremely high field-confinement effect, leading to an integrated photocurrent density, which can be 25 times higher than that of the planar system. The proposed scheme is useful for the realization of compact, low-cost, and highly absorbing systems for photon absorbing, solar-energy harvesting, and photodetection applications.

Carrier properties of B atomic-layer-doped Si films grown by ECR Ar plasma-enhanced CVD without substrate heating

The atomic-layer (AL) doping technique in epitaxy has attracted attention as a low-resistive ultrathin semiconductor film as well as a two-dimensional (2-D) carrier transport system. In this paper, we report carrier properties for B AL-doped Si films with suppressed thermal diffusion. B AL-doped Si films were formed on Si(100) by B AL formation followed by Si cap layer deposition in low-energy Ar plasma-enhanced chemical-vapor deposition without substrate heating. After fabrication of Hall-effect devices with the B AL-doped Si films on unstrained and 0.8%-tensile-strained Si(100)-on-insulator substrates (maximum process temperature 350°C), carrier properties were electrically measured at room temperature. Typically for the initial B amount of 2 × 1014 cm−2 and 7 × 1014 cm−2, B concentration depth profiles showed a clear decay slope as steep as 1.3 nm/decade. Dominant carrier was a hole and the maximum sheet carrier densities as high as 4 × 1013 cm−2 and 2 × 1013 cm−2 (electrical activity ratio of about 7% and 3.5%) were measured respectively for the unstrained and 0.8%-tensile-strained Si with Hall mobility around 10–13 cm2 V−1 s−1. Moreover, mobility degradation was not observed even when sheet carrier density was increased by heat treatment at 500–700 °C. There is a possibility that the local carrier (ionized B atom) concentration around the B AL in Si reaches around 1021 cm−3 and 2-D impurity-band formation with strong Coulomb interaction is expected. The behavior of carrier properties for heat treatment at 500–700 °C implies that thermal diffusion causes broadening of the B AL in Si and decrease of local B concentration.

New Insight into the Angle Insensitivity of Ultrathin Planar Optical Absorbers for Broadband Solar Energy Harvesting.

Two challenging problems still remain for optical absorbers consisting of an ultrathin planar semiconductor film on top of an opaque metallic substrate. One is the angle-insensitive mechanism and the other is the system design needed for broadband solar energy harvesting. Here, first we theoretically demonstrates that the high refractive index, instead of the ultrathin feature as reported in previous studies, is the physical origin of the angle insensitivity for ultrathin planar optical absorbers. They exhibit omnidirectional resonance for TE polarization due to the high complex refractive index difference between the semiconductor and the air, while for TM polarization the angle insensitivity persists up to an incident angle related to the semiconductor refractive index. These findings were validated by fabricating and characterizing an 18 nm Ge/Ag absorber sample (representative of small band gap semiconductors for photovoltaic applications) and a 22 nm hematite/Ag sample (representative of large band gap semiconductors for photoelectrochemical applications). Then, we took advantage of angle insensitivity and designed a spectrum splitting configuration for broadband solar energy harvesting. The cascaded solar cell and unassisted solar water splitting systems have photovoltaic and photoelectrochemical cells that are also spectrum splitters, so an external spectrum splitting element is not needed.

Applying low-energy ion implantation in the creation of nanocontacts on the surface of ultrathin semiconductor films

A new way for obtaining nanocontacts on the surface of semiconductors using Si (111) thin films as an example is proposed. The essence of the method lies in the fact that, first, cobalt-silicide nanofilms with a thickness of 45–50 A are formed by ion implantation in combination with annealing; then, atoms of the contact metal (Al) are sputtered. The specific resistance of the CoSi2 nanofilms is (2–3) × 10−5 Ω × cm.

Enhanced terahertz emission by coherent optical absorption in ultrathin semiconductor films on metals

We report on the surprisingly strong, broadband emission of coherent terahertz pulses from ultrathin layers of semiconductors such as amorphous silicon, germanium and polycrystalline cuprous oxide deposited on gold, upon illumination with femtosecond laser pulses. The strength of the emission is surprising because the materials are considered to be bad (amorphous silicon and polycrystalline cuprous oxide) or fair (amorphous germanium) terahertz emitters at best. We show that the strength of the emission is partly explained by cavity-enhanced optical absorption. This forces most of the light to be absorbed in the depletion region of the semiconductor/metal interface where terahertz generation occurs. For an excitation wavelength of 800 nm, the strongest terahertz emission is found for a 25 nm thick layer of amorphous germanium, a 40 nm thick layer of amorphous silicon and a 420 nm thick layer of cuprous oxide, all on gold. The emission from cuprous oxide is similar in strength to that obtained with optical rectification from a 300 μm thick gallium phosphide crystal. As an application of our findings we demonstrate how such thin films can be used to turn standard optical components, such as paraboloidal mirrors, into self-focusing terahertz emitters.