Si Thin Films Research Articles

Resonance colors in an amorphous-Si thin layer irradiated by an ultraviolet nanosecond laser

The optical resonances of nanoparticles have drawn considerable attention in academia and industry; however, the formation of uniform nanoparticles has been challenging, hindering their application. Herein, we propose that reflective-type color filters can be achieved using the optical resonance effect arising from Si nanoparticles created on an amorphous Si thin film via laser irradiation. Multiple equally spaced irradiations of an amorphous-Si surface by a Gaussian laser beam created Si nanoparticles with sizes of 100–200 nm at laser fluences in the range of 150–200 mJ/cm2. Furthermore, we demonstrated that the resonance colors could be tuned to red, green, and blue by adjusting the laser fluence and scan speed. The optical properties of the nanoparticles were analyzed using dark-field microscopy, scanning electron microscopy, and transmission electron microscopy. According to the measured nanoparticle geometries and spectral data, the Si nanoparticles are spheroidal shapes embedded in the Si layer, and simulations indicate that the resonance colors originate from electric and magnetic dipoles inside the Si nanoparticles. The results could inform the fabrication of color filters in reflective display device or sensor applications through a simple laser scanning process.

Effect of site disorder on the resonant microwave absorption in Co2Fe0.5Ti0.5Si Heusler alloy thin films

100 nm Co2Fe0.5Ti0.5Si (CFTS) thin films were grown on Si (100) substrate at different substrate temperatures TS to study the effect of site disorder on the magnetization relaxation processes and magnetic anisotropy in them. To this end, two different ferromagnetic resonance (FMR) techniques have been employed: broad-band (3 – 17 GHz) FMR at a fixed static magnetic field angle in the ‘in-plane’ (IP) sample geometry, and X-band (9.45 GHz) FMR at different polar field-angles, θH, in the ‘out-of-plane’ (OP) sample configuration. Increase in TS reduces the degree of disorder and improves crystalline order. Improvement in crystallinity, in turn, results in the decrease in the strength of magnetocrystalline anisotropy. Irrespective of the degree of disorder present, the OP anisotropy field, Hk⊥, is an order of magnitude higher than the IP counterpart, Hk‖. Out of the CFTS thin films studied, the film (TS500), deposited at TS = 500 °C, has the maximum L21 crystalline order, maximum saturation magnetization (MS = 779 G), and the least values for Hk‖ (5.3 Oe), Hk⊥ (17 Oe), Landé splitting factor, g = 2.02, and the Gilbert damping constant, α=5.5×10-3. These optimum values of MS, g and α, at room temperature, are conducive for spintronics applications. In the OP configuration, the FMR linewidth, ΔH⊥, mainly arises from the intrinsic Landau-Lifshitz-Gilbert damping and extrinsic angular spread of crystallite misorientation. However, the extrinsic two-magnon scattering contribution to ΔH⊥, is appreciable only in the angular range 70° ≤ θH ≤ 110° .

Compact Integration of Hydrogen–Resistant a–InGaZnO and Poly–Si Thin–Film Transistors

The low–temperature poly–Si oxide (LTPO) backplane is realized by monolithically integrating low–temperature poly–Si (LTPS) and amorphous oxide semiconductor (AOS) thin–film transistors (TFTs) in the same display backplane. The LTPO–enabled dynamic refreshing rate can significantly reduce the display’s power consumption. However, the essential hydrogenation of LTPS would seriously deteriorate AOS TFTs by increasing the population of channel defects and carriers. Hydrogen (H) diffusion barriers were comparatively investigated to reduce the H content in amorphous indium–gallium–zinc oxide (a–IGZO). Moreover, the intrinsic H–resistance of a–IGZO was impressively enhanced by plasma treatments, such as fluorine and nitrous oxide. Enabled by the suppressed H conflict, a novel AOS/LTPS integration structure was tested by directly stacking the H–resistant a–IGZO on poly–Si TFT, dubbed metal–oxide–on–Si (MOOS). The noticeably shrunken layout footprint could support much higher resolution and pixel density for next–generation displays, especially AR and VR displays. Compared to the conventional LTPO circuits, the more compact MOOS circuits exhibited similar characteristics.

Nanoscale laser flash measurements of diffuson transport in amorphous Ge and Si

The thermal properties of amorphous materials have attracted significant attention due to their technological importance in electronic devices. In addition, the disorder-induced breakdown of the phonon gas model makes vibrational transport in amorphous materials a topic of fundamental interest. In the past few decades, theoretical concepts, such as propagons, diffusons, and locons, have emerged to describe different types of vibrational modes in disordered solids. However, experiments can struggle to accurately determine which types of vibrational states carry the majority of the heat. In this study, we use nanoscale laser flash measurements (front/back time-domain thermoreflectance) to investigate thermal transport mechanisms in amorphous Ge and amorphous Si thin-films. We observe a nearly linear relationship between the amorphous film’s thermal resistance and the film’s thickness. The slope of the film’s thermal resistance vs thickness corresponds to a thickness-independent thermal conductivity of 0.4 and 0.6 W/(m K) for a-Ge and a-Si, respectively. This result reveals that the majority of heat currents in amorphous Si and Ge thin films prepared via RF sputtering at room temperature are carried by diffusons and/or propagons with mean free paths less than a few nanometers.

Experimental Activation Energy for Solid Phase Crystallization of Amorphous Silicon Thin Films at Elevated Temperatures Using Vertical-Cavity Surface-Emitting Laser-Based Infrared Heating

Vertical cavity surface-emitting lasers (VCSELs) were applied to the rapid heating of amorphous Si (a-Si) thin films using high-power infrared illumination at a wavelength of 980 nm, allowing for a high heating rate of up to 200 °C s−1, temperature control of less than 0.1 °C and temporal resolution of 0.1 s. The refined temperature control enabled us to accurately investigate the rapidly evolving period of crystallization in a-Si at a fast speed. The crystallinity and surface morphology were probed using Raman spectroscopy, UV–visible spectroscopy, and atomic force microscopy. The temperature-dependent crystallinity was quantified by fitting the annealing duration time with a sigmoidal curve. Based on the crystallization times in association with the Arrhenius relation, the activation energy for crystallization was calculated to be 2.6 eV, which is in excellent agreement with those obtained for low-temperature solid-phase crystallization.

Bonding performance in atomic diffusion bonding of wafers using amorphous Si thin films with smooth surface

Amorphous Si films with a low surface roughness of 0.13 nm were used to examine the bonding performance of atomic diffusion bonding of quartz glass wafers at room temperature in vacuum. The high bonding strength was achieved for films with thickness δ of 2–50 nm: the blade could not be inserted between the bonded wafers. Using a vacuum chamber with a base pressure of 1 × 10–6 Pa, the great bonding strength was maintained even with waiting time in vacuum of as long as 3.6 × 103 s from film deposition to bonding. The excellent bonding performance was almost equal to that achieved using Ti films.

Systematic investigations on doping dependent thermal transport properties of single crystal silicon by time-domain thermoreflectance measurements

Doped silicon (Si) is the key material in semiconductor industries. It is important to fully understand the doping effect on thermal transport properties of single-crystal Si, especially for the thermal management of Si-based electronic devices. In this work, we used a femtosecond-laser time-domain thermoreflectance method to characterize the thermal transport properties of Si with different doping concentrations (1013-1019 cm−3) and doping elements, boron (B) and phosphorus (P). Results show that the thermal conductivity of heavy doped Si (about 2 × 1019 cm−3) by both B and P decreases by about 22% compared with their pure counterparts. Theoretical calculations based on the Boltzmann transport equation were also carried out for comparison with measurement results, and different scattering terms were discussed to find out the main factors for suppressing the thermal transport in doped Si within different doping concentration ranges. Combining with a cryogenic system, the thermal conductivity of doped Si samples and interfacial thermal conductance between Si and aluminum thin films were also measured under low-temperature conditions (down to 160 K). Our systematic studies on the doping effect in the thermal transport of single-crystal semiconductors not only facilitate the understanding of the microscale thermal transport, but also provide references in industrial applications of doped semiconductors.

Enhanced Absorption in InP Nanodisk Arrays on Ultra-Thin-Film Silicon for Solar Cell Applications

The photovoltaic (PV) market today is dominated by silicon (Si)-based solar cells, which, however, can be improved in performance and cost by developing technologies that use less material. We propose an indium phosphide (InP) nanoresonator array on silicon ultra-thin film with a combined thickness of 0.5 μm to 2 μm as a solution to minimize cost and maximize power efficiency. This paper focuses on simultaneously achieving broadband antireflection and enhanced absorption in thin-film Si with integrated InP nanodisk arrays. Electromagnetic simulations are used to design and optimize the reflectance and absorption of the proposed design. By varying the height and radius of the InP nanodisks on the Si substrate, together with the array pitch, a weighted reflectance minimum, with respect to the AM1.5 solar spectrum, of 2.9% is obtained in the wavelength range of 400 nm to 1100 nm. The antireflective properties are found to be a combination of a Mie-resonance-induced strong forward-scattering into the structure and an effective index-matching to the Si substrate. In terms of absorption, even up to 2 μm from the Si surface the InP nanodisk/Si structure consistently shows superior performance compared to plain Si as well as a Si nanodisk/Si structure. At a depth of 500 nm from the surface of the substrate, the absorption values were found to be 47.5% for the InP nanodisk/Si structure compared to only 18.2% for a plain Si substrate. This shows that direct bandgap InP nanoresonator arrays on thin-film Si solar cells can be a novel design to enhance the absorption efficiency of the cell.

Realization of a cost-effective thermochromic solar absorber with a high emittance change based on VO2 and an infrared transparent intermediate layer

In this study, we present a thermochromic solar absorber coating that reaches a high thermal emittance change by using a thin, optically switching VOx film located on an infrared transparent interlayer (spacer) of Si or Ge with an optical thickness of λ/4 (for λ = 7 μm). Using this so-called “lambda/4-concept,” temperature-dependent reflection measurements in the spectral range between 2500 and 50 000 nm from an absorber with a 450 nm Si spacer and a VOx film oxidized from a 30-nm-thick V display an overall increase in emittance from ε(25 °C) = 12.2% to ε(150 °C) = 55.1%, resulting in a change of Δε = 42.9%. In addition, using an absorber with a 400 nm Ge spacer in combination with a VOx film oxidized from 17.5-nm-thick V, an increase in emittance from ε(25 °C) = 8.2% to ε(150 °C) = 49.2% with a change of Δε = 41.0% was achieved. In addition, the optical properties of Ge and Si thin films over a wide spectral range of 250–38 000 nm were determined using spectroscopic ellipsometry. Using this optical data and a simple optical model of the VOx film, reflectance simulations could be performed by using the ellipsometry analysis software WVASE©. It was shown by x-ray diffraction measurements that the optically switching VOx films oxidized from V in a belt furnace consist of the VO2 and the V2O5 phases. Using scanning electron microscopy images, the surface morphology of optically switching VOx films was compared with over-oxidized VOx films. A correlation between the surface morphology and the crystalline phases was revealed and applied to search for the optimal furnace parameters. This method could significantly reduce the time cost to achieve optically switching VOx coatings using an oxidation process.

Optical anisotropy of glancing angle deposited thin films on nano-patterned substrates

This study has demonstrated that 3D columnar micro-films/coatings can be deposited over pre-patterned surfaces with sub-micrometer periodic patterns. Four-angle polarisation analysis of thin (0.4 − 1~μm) Si and SiO2 films, evaporated via glancing angle deposition (GLAD) at 70° to the normal, was carried out in reflection mode using synchrotron infrared microspectroscopy at the Australian Synchrotron. The angular dependence of absorbance followed A(θ) ∝ cos 2θ, confirmed for Si substrates patterned by electron beam lithography and plasma etching, which were used to make checkerboard patterns of Λ = 0.4~μm period on Si. Retardance control by birefringence of a patterned SiO2 substrate coated by columnar SiO2 is promising for UV-visible applications due to the use of the same material to endow polarisation control.

Si epitaxy on Cl-Si(100)

Current atomically-precise fabrication methods in Si utilize a scanning tunneling microscope (STM) to pattern a monatomic resist adsorbed on Si(100). Recent interest in the use of Cl-based resists has necessitated further investigations on the impact of the tightly bound Cl to the overall device fabrication process. In particular, impacts to the crystallinity of the Si capping layer used to encapsulate the active device region can have dramatic effects on the functionality of atomic-scale quantum devices. Here, the epitaxial growth of Si films on Cl-Si(100) was studied to identify and characterize the Si thin film growth process. Adatoms diffusion on the surface was simulated based on the experimental results of the number density and distribution of Si adatom chains after <0.10 ML of Si deposition at temperatures ranging from 300–850 K. A diffusion barrier within 0.66–0.88 eV for Si adatoms on Cl-Si(100) was found. SIMS depth profiling of ∼25 nm thick Si films verified that Cl segregated towards the growth surface during Si deposition and was effectively removed below SIMS detection limits from both the buried interface and Si film at 850 K. TEM imaging confirmed good film crystallinity was achieved at temperatures as low as 600 K.

Evidence of sp2-like Hybridization of Silicon Valence Orbitals in Thin and Thick Si Grown on α-Phase Si(111)√3 × √3R30°-Bi.

One-monolayer (ML) (thin) and 5-ML (thick) Si films were grown on the α-phase Si(111)√3 × √3R30°-Bi at a low substrate temperature of 200 °C. Si films have been studied in situ by reflection electron energy loss spectroscopy (REELS) and Auger electron spectroscopy, as a function of the electron beam incidence angle α and low-energy electron diffraction (LEED), as well as ex situ by grazing incidence X-ray diffraction (GIXRD). Scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS) were also reported. The REELS spectra, taken at the Si K absorption edge (~1.840 KeV), reveal the presence of two distinct loss structures attributed to transitions 1s→π* and 1s→σ* according to their intensity dependence on α, attesting to the sp2-like hybridization of the silicon valence orbitals in both thin and thick Si films. The synthesis of a silicon allotrope on the α-phase of Si(111)√3 × √3R30°-Bi substrate was demonstrated by LEED patterns and GIXRD that discloses the presence of a Si stack of 3.099 (3) Å and a √3 × √3 unit cell of 6.474 Å, typically seen for multilayer silicene. STM and STS measurements corroborated the findings. These measurements provided a platform for the new √3 × √3R30° Si allotrope on a Si(111)√3 × √3 R30°-Bi template, paving the way for realizing topological insulator heterostructures from different two-dimensional materials, Bi and Si.

Thin films of the alpha-quartz Si_xGe_{1-x}O_2 solid solution

SiO_2 with the alpha-quartz structure is one of the most popular piezoelectrics. It is widely used in crystal oscillators, bulk acoustic wave (BAW) devices, surface acoustic wave (SAW) devices, and so on. GeO_2 can also be crystallized into the alpha-quartz structure and it has better piezoelectric properties, with higher piezoelectric coefficient and electromechanical coupling coefficients, than SiO_2. Experiments on bulk crystals and theoretical studies have shown that these properties can be tuned by varying the Si/Ge ratio in the Si_xGe_{1-x}O_2 solid solution. However, to the best of our knowledge, thin films of Si_xGe_{1-x}O_2 quartz have never been reported. Here we present the successful crystallization of Si_xGe_{1-x}O_2 thin films in the alpha-quartz phase on quartz substrates (SiO_2) with x up to 0.75. Generally, the films grow semi-epitaxially, with the same orientation as the substrates. Interestingly, the Si_{0.75}Ge_{0.25}O_2 composition grows fully strained by the quartz substrates and this leads to the formation of circular quartz domains with an ordered Dauphiné twin structure. These studies represent a first step towards the optimization of piezoelectric quartz thin films for high frequency (> 5 GHz) applications.

Crystallization of catalytic CVD hydrogenated n-a-Si films on textured glass substrates by flash lamp annealing

Flash lamp annealing (FLA) is a short-duration annealing technique that can crystallize amorphous silicon (a-Si) films for thin-film polycrystalline Si (poly-Si) solar cells. We investigated the crystallization of n-type hydrogenated a-Si (n-a-Si:H) films formed by catalytic chemical vapor deposition on Si nitride- (SiN x -) coated textured glass substrates. The n-a-Si:H films with a thickness of ∼2.7 μm were crystallized by FLA with no film peeling even without chromium adhesion layers. We also confirmed that the crystallization takes place through explosive crystallization (EC). The addition of phosphorous to the precursor a-Si:H slightly modifies the crystallization, resulting in different grain sizes and EC velocities compared to the case of EC of intrinsic a-Si:H.

Degradation Study of Thin-Film Silicon Structures in a Cell Culture Medium.

Thin-film silicon (Si)-based transient electronics represents an emerging technology that enables spontaneous dissolution, absorption and, finally, physical disappearance in a controlled manner under physiological conditions, and has attracted increasing attention in pertinent clinical applications such as biomedical implants for on-body sensing, disease diagnostics, and therapeutics. The degradation behavior of thin-film Si materials and devices is critically dependent on the device structure as well as the environment. In this work, we experimentally investigated the dissolution of planar Si thin films and micropatterned Si pillar arrays in a cell culture medium, and systematically analyzed the evolution of their topographical, physical, and chemical properties during the hydrolysis. We discovered that the cell culture medium significantly accelerates the degradation process, and Si pillar arrays present more prominent degradation effects by creating rougher surfaces, complicating surface states, and decreasing the electrochemical impedance. Additionally, the dissolution process leads to greatly reduced mechanical strength. Finally, in vitro cell culture studies demonstrate desirable biocompatibility of corroded Si pillars. The results provide a guideline for the use of thin-film Si materials and devices as transient implants in biomedicine.

Reactive force-field molecular dynamics simulation for the surface reaction of SiHx (x = 2–4) species on Si(1 0 0)-(2 × 1):H surfaces in chemical vapor deposition processes

Reactive force-field molecular dynamics simulation of Si thin film deposition in order to prove the availability of the simulation model including both chemical reactions and physical dynamics for a plasma-enhanced chemical vapor deposition process was performed. We simulated surface reactions of SiHx (x = 2–4) formed in the plasma on a Si(100)-(2 × 1) surface covered with H atoms and evaluated the surface reactions with different substrate temperature and H coverage. The existing potential parameter set was modified to fit the dissociation energies in the gaseous species. The gaseous species collided with the surface one by one, and then the surface reactions were classified into four events: reflection, desorption, chemisorption, and physisorption. Our results show that an increase in substrate temperature affects both the bond dissociation in the gaseous species and the desorption of gaseous species on the surface. The H atoms inhibit the chemisorption by terminating dangling bonds on the surface and promote reflection and desorption by thermal movement. This method should facilitate the simulation of much more complex systems such as SiC and SiGe.

Security labeling and optical information encryption enabled by laser-printed silicon Mie resonators.

Fighting against the falsification of valuable items remains a crucial social-threatening challenge stimulating a never-ending search for novel anti-counterfeiting strategies. The demanding security labels must simultaneously address multiple requirements (high density of the recorded information, high protection degree, etc.) and be realized via scalable and inexpensive technologies. Here, the direct reproducible femtosecond-laser patterning of thin glass-supported amorphous (α-)Si films is proposed for optical information encryption and the scalable and highly reproducible fabrication of security labels composed of Raman-active hemispherical Si nanoparticles (NPs). Laser printing conditions allow the precise control of the diameter of the formed NPs ensuring translation of their dipolar Mie resonance position within the entire visible spectral range. Two-temperature molecular dynamics simulations clarify the origin of α-Si NP formation by rupture of the molten Si layer driven by a negative GPa-range pressure near the liquid-solid interface. Arrangement of the laser-printed Mie-resonant NP allows the creation of hidden security labels offering several easy-to-realize information encryption strategies (for example, local laser-induced post-crystallization or mixing Mie-resonant and non-resonant NPs), additional protection modalities, facile Raman mapping readout and dense information recording (up to 60 000 dots per inch) close to the optical diffraction limit. The developed fabrication strategy is simple, inexpensive, and scalable and can be realized based on cheap Earth-abundant materials and commercially-available equipment justifying its practical applicability and attractiveness for anti-counterfeit and security applications.

Intrinsic, P-doped and B-doped nanocrystalline silicon thin films grown at room temperature on flexible substrates for photovoltaic applications

Nanocrystalline silicon (nc-Si) thin films in its intrinsic (i-nc-Si), B-doped (p-nc-Si) and P-doped (n-nc-Si) configurations were developed on the flexible substrates at ambient temperature (∼30 °C), using no additional H2 dilution in planar inductively coupled plasma chemical vapour deposition (ICP-CVD) system. A superior crystallinity (∼81%) with electrical conductivity (σD ∼10-7 S cm−1) was achieved for the intrinsic Si thin film by the virtue of the presence of atomic-H in high density within ICP-CVD, which initiates nucleation even at room temperature on the flexible substrates. Substitution of boron (B) dopants at the crystalline Si lattice maintains a high crystallinity (∼80%) and results significantly high electrical conductivity (6 x10-1 S cm−1) in the p-nc-Si thin film. On the contrary, during the growth of n-nc-Si films, a substantial amount of phosphorous (P) dopants, residing at the interstitial position, results in a moderate crystallinity (∼73%) and relatively lesser conductivity (3 x10-1 S cm−1) than the p-nc-Si films. Finally, in the present work i-nc-Si, p-nc-Si and n-nc-Si thin films all are successfully prepared on the flexible substrates at room temperature, with ample crystallinity, wide optical band gap (∼1.9–1.80 eV) and significant electrical conductivity, which appear suitable for the low-cost fabrication of flexible p-i-n solar cells.

Effect of vinylene carbonate electrolyte additive and battery cycling protocol on the electrochemical and cyclability performance of silicon thin-film anodes

The use of silicon as a replacement for graphite, the commonly utilised anode material, would help increase the energy density of lithium-ion batteries, as it has a significant specific capacity of 4200 mAh/g compared to only 372 mAh/g for graphite. However, the high electronic resistivity and low mechanical stability of silicon have hindered its commercial uptake. In this contribution, we have employed a multifaceted approach in order to enhance the capacity retention of pure silicon anodes. The effect of adding 5 vol. % vinylene carbonates to a standard electrolyte has been examined with respect to the performance of physical vapour deposited pure Si thin films. Changes in battery cycling parameters (i.e., lower and upper cut-off voltages and depth of discharge were examined using charge/discharge cycling, cyclic voltammetry, and electrochemical impedance spectroscopy, indicating that cycling stability and electrochemical performance of the anodes were heavily influenced by these changes. The effect of each procedure (i.e., electrolyte additive and battery cycling protocol) on the initial discharge capacity, initial coulombic efficiency, initial irreversible capacity, capacity retention, and the lithium-ion diffusion coefficient into the silicon anode was examined. The Si film with optimised: deposition conditions, electrolyte additives, and battery testing protocol had a discharge capacity of 1740 mAh/g and capacity retention of 92 % at a charge/discharge rate of C/2 after 1000 cycles.

Visualizing optically-induced strains by five-dimensional ultrafast electron microscopy

Ultrafast optical control of strain is crucial for the future development of nanometric acoustic devices. Although ultrafast electron microscopy has played an important role in the visualization of strain dynamics in the GHz frequency region, quantitative strain evaluation withnm × ps spatio-temporal resolution is still challenging. Five-dimensional scanning transmission electron microscopy (5D-STEM) is a powerful technique that measures time-dependent diffraction or deflection of the electron beam at the respective two-dimensional sample positions in real space. In this paper, we demonstrate that convergent beam electron diffraction (CBED) measurements using 5D-STEM are capable of quantitative time-dependent strain mapping in the nm × ps scale. We observe the generation and propagation of acoustic waves in a nanofabricated silicon thin plate of 100 nm thickness. The polarization and amplitude of the acoustic waves propagating in the silicon plate are quantitatively determined from the CBED analysis. Further Fourier-transformation analysis reveals the strain distribution in the momentum-frequency space, which gives the dispersion relation in arbitrary directions along the plate. Versatility of 5D-STEM-CBED analysis enables quantitative strain mapping even in complex nanofabricated samples, as demonstrated in this study.