Amorphous Silicon Alloys Research Articles

Optoelectronic artificial synapse based on Si1-xSnx alloyed film

The emergence of big data has increased the requirements for computing and processing systems. Memristors–electrical components that simulate the synapses and neurons of the human brain—have been proposed to help meet these requirements. Optoelectronic synapses are excited by optical signals and have the advantages of low crosstalk, high bandwidth, and low power consumption. Herein, a MOS structure optoelectronic synapse device based on an amorphous silicon (a-Si1-xSnx; x = 0.0457) alloy thin film and an AlOy thin film was fabricated via oxygen vacancy energy band and heterostructure design. Significantly, the device exhibited ultralong decay time of 3576.07 s and a broadband response from the visible to the near-infrared-I region. A series of important synaptic functions, including excitatory postsynaptic current, paired-pulse facilitation, short-term memory to long-term memory transition, and learning experience behaviour, was successfully simulated on the device. We simulated a 3 × 3 optoelectronic synapse array, which can simulate the visual memory of images observed by the human eye. The effect of ‘pupil’ scaling on image memory was simulated by adjusting the read voltage. Based on these advantages, the optoelectronic synaptic device developed in this study has significant potential for application in the new generation of artificial synaptic systems.

Relationship between Mechanical and Electrochemical Property in Silicon Alloy Designed by Grain Size as Anode for Lithium-Ion Batteries

While silicon has a very high theoretical capacity but has relatively the stresses produced by volume changes during charge/discharge cycling lead to structural modifications (around 300%). To overcome this problem, many studies are being conducted to commercialize silicon. Herein, we produced amorphous silicon alloy using a melt-spinning method. Then, through annealing under various temperatures, we gradually recrystallized the silicon phase. Average silicon grain sizes were 70 and 130 nm for silicon alloys annealed at 800 and 873 K, respectively. The initial reversible capacities of silicon alloy-based anodes were 844.3 (800 K) and 865.1 mAh g−1 (873 K), and, after 100 cycles, capacity retention rates were found to be 68.5 (800 K) and 40.5% (873 K). At this stage, to elucidate the effect of grain sizes on cycle life retention rate, we determined mechanical hardness through nanoindentation. And, by measuring volume expansion values between cycles through in situ dilation, we could identify the relationship between electrochemical property and mechanical hardness of silicon alloy samples depending on recrystallized grain sizes. Thus, by analyzing the mechanical and electrochemical properties of silicon alloys depending on silicon grain sizes, we want to highlight the importance of controlling silicon grain size.

Mean Free Path of Photoelectronic Excitations in Hydrogenated Amorphous Silicon and Silicon Germanium Alloy Semiconductors

Hydrogenated amorphous silicon germanium alloy (a‐Si1−x Ge x :H) films are prepared by plasma enhanced chemical vapor deposition (PECVD) and characterized by in situ real time spectroscopic ellipsometry (RTSE). From complex dielectric function spectra extracted, the broadening width energy (Γ) of the primary absorption feature centered near 3.7 eV is quantified using the Cody–Lorentz oscillator model. Mean free path length of photoelectronic excitations is calculated from Γ based on reasonable estimates for speed of electron–hole photoexcitations. A model is applied with excited state lifetime assumed to be limited by scattering from network disorder and provides a relative measure of short‐range order. The simple model applied is an extension of that widely used to characterize broadening of optical transitions in polycrystalline semiconductors. Decreases in mean free path of up to ∼30% occur with germanium. Relative increases in mean free path with increased hydrogen during PECVD a‐Si:H, a‐Si0.73Ge0.27:H, and a‐Si0.60Ge0.40:H occur from ∼5% to ∼8% and with hydrogen plasma treatment of a‐Si:H by ∼6%. Mean free path changes are tracked with thickness to provide short range order evolution and the effect of the underlying material. Mean free path of photoexcitations in electronic quality a‐Si1−x Ge x :H is estimated at ∼3.5 Å, on the same order as interatomic spacing.

(Invited) Crystal Field Analysis of Eu3+ and Tb3+ Luminescence in Amorphous Silicon Alloys

Trivalent rare earth ions efficiently emit luminescence in amorphous silicon based alloy hosts. The luminescence details depend mostly on the immediate chemical neighborhood of the ions. In crystals the luminescence peaks at low temperatures can be adjusted by lorentzian lineshapes with energies determined by the symmetry of the well defined lattice sites. In disordered materials however not only the local symmetry can change but also the local stoichiometry is not well defined. As a result the luminescence peaks become gaussian in shape. Nevertheless, by carefully adjusting the luminescence spectra information can be obtained about the local properties. We studied the luminescence of Eu3+ and Tb3+ in amorphous silicon alloys at low temperatures. The results indicate that Eu3+ is mostly on distorted cubic symmetry sites, while for Tb3+ it is not possible to determine the symmetry reliably. In any case, the results are compatible with the rare earth ions coordinated to residual oxygen in the alloys.

Localization of vibrational modes leads to reduced thermal conductivity of amorphous heterostructures

We investigate the vibrational heat transfer mechanisms in amorphous Stillinger-Weber silicon and germanium-based alloys and heterostructures via equilibrium and nonequilibrium molecular dynamics simulations along with lattice dynamics calculations. We find that similar to crystalline alloys, amorphous alloys demonstrate large size effects in thermal conductivity, while layering the constituent materials into superlattice structures leads to length-independent thermal conductivities. The thermal conductivity of an amorphous ${\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ alloy reduces by as much as $\ensuremath{\sim}53%$ compared to the thermal conductivity of amorphous silicon; compared to the larger reduction in crystalline phases due to alloying, we show that compositional disorder rather than structural disorder has a larger impact on the thermal conductivity reduction. Our thermal conductivity predictions for a-Si/a-Ge superlattices suggest that the alloy limit in amorphous SiGe-based structures can be surpassed with interface densities above $\ensuremath{\sim}0.35\phantom{\rule{4pt}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}$. We attribute the larger reduction in thermal conductivity of layered Si/Ge heterostructures to greater localization of modes at and around the cutoff frequency of the softer layer as demonstrated via lattice dynamics calculations and diffusivities of individual eigenmodes calculated according to the Allen-Feldman theory [P. B. Allen and J. L. Feldman, Phys. Rev. B 48, 12581 (1993)] for our amorphous SiGe-based alloys and superlattice structures.

Development of Silicon and Carbon Based p-Type Amorphous Semiconductor Films with Optical Gap Variable for High-Efficiency Multi-Junction Solar Cells

P-type boron-doped amorphous silicon and carbon alloy (B-doped a-SixC1-x) thin films with wide optical gap selective from 1.80 to 2.50 eV were successfully deposited by radio frequency (r.f.) plasma-enhanced chemical vapor deposition (CVD) method using a mixed solution of tetramethylsilane (TMS) and trimethylborate (TMOB) as a liquid source. Optical gaps of the B-doped a-SixC1-x films could be controlled by changing Si/(C + Si) ratio of the film. From photo-electrochemical measurement under UV illumination, it was clarified that the B-doped a-SixC1-x film with an optical gap of 2.50 eV has the p-type semiconducting property and photoelectric conversion function with a quantum yield of 1.63 %. The rectifying action of a p-n heterojunction comprising p-type B-doped a-SixC1-x film and n-type Si (100) substrate was observed. The open circuit voltage (VOC) and short circuit current density (JSC) of the heterojunction were estimated to be 200 mV and 45 mA/cm2, respectively. The results indicate that p-type B-doped a-SixC1-x films with controllable optical gaps is a promising p-layer material for multi-junction solar cells.

(Invited) XAFS of Rare Earth Elements in Amorphous Silicon Alloys: What Do We Know about the Local Structure?

X-ray absorption spectroscopy (both XANES, X-ray Absorption Near-Edge Structure and EXAFS, Extended X-ray Absorption Fine Structure) is a powerful tool to determine the chemical environment of selected elements in solids. In this paper we present results obtained for the rare earth elements Eu and Tb in hydrogenated amorphous silicon alloys. Whenever possible, rare earth atoms coordinate to oxygen present in environments that depend on the host and can be very distorted. Annealing at high enough temperatures makes the coordination evolve towards oxide-like 6-fold environments. Europium is a special case. It may assume either 2+ or 3+ oxidation states. The X-ray absorption peaks can be used to determine the relative concentration of each. When the concentration is above 4 at% it is possible to completely oxidize the Eu2+ ions by annealing in an oxygen atmosphere. Efficient luminescence of the 3+ ions is associated with low coordinated oxygen environments. This environment does not change significantly at the relatively low annealing temperatures (up to 500°C) that maximize photoluminescence. The main effects of the annealing are not related to the rare earth chemical neighborhood.

(Invited) XAFS of Rare Earth Elements in Amorphous Silicon Alloys: What Do We Know about the Local Structure?

X-ray absorption spectroscopy (both XANES, X-ray Absorption Near-Edge Spectroscopy and EXAFS, Extended X-ray Absorption Fine Structure) is a powerful tool to determine the chemical environment of selected elements in solids. It is particularly well suited to study rare earth elements in amorphous silicon alloys. In this paper we review the technique and results obtained for different rare earth elements, especially Er, Eu and Tb. Whenever possible, the rare earth elements coordinate to any oxygen present in subcoordinated envitronments that vary with the host and can be very distorted. The details of the environment depend on the technique used to prepare the samples. Annealling above 400C makes the coordination evolve towards oxide-like 6-fold environments. Europium is a special case. It may assume either 2+ or 3+ oxidation states. The X-ray absorption peaks associated with the core electrons binding energy can be used to estimate the relative concentration of each. Annealling in oxidizing atmospheres can modify the Eu3+/Eu2+ ratio, affect the atomic surroundings and change the luminescence spectrum. In general, efficient luminescence of the 3+ ions is associated with low coordination oxygen environments. This environment does not change significantly at the relatively low annealling temperatures (up to 300C) that maximize the photoluminescence. The reason for the increase of the luminescence intensity with annealling not a modification of the local environment of the rare earth ions.

High damage tolerance of electrochemically lithiated silicon.

Mechanical degradation and resultant capacity fade in high-capacity electrode materials critically hinder their use in high-performance rechargeable batteries. Despite tremendous efforts devoted to the study of the electro–chemo–mechanical behaviours of high-capacity electrode materials, their fracture properties and mechanisms remain largely unknown. Here we report a nanomechanical study on the damage tolerance of electrochemically lithiated silicon. Our in situ transmission electron microscopy experiments reveal a striking contrast of brittle fracture in pristine silicon versus ductile tensile deformation in fully lithiated silicon. Quantitative fracture toughness measurements by nanoindentation show a rapid brittle-to-ductile transition of fracture as the lithium-to-silicon molar ratio is increased to above 1.5. Molecular dynamics simulations elucidate the mechanistic underpinnings of the brittle-to-ductile transition governed by atomic bonding and lithiation-induced toughening. Our results reveal the high damage tolerance in amorphous lithium-rich silicon alloys and have important implications for the development of durable rechargeable batteries.

Thin Film Solar Cell: Characteristics and Characterizations

In recent decades, due to some urgent and unavoidable issues, such as increasing energy demand, climate change, global warming, etc., the R&D of renewable energies have become inevitable to pave way the sustainable development of human society. In this regard, solar power is widely considered as the most appealing clean energy since there is no other one being as abundant as the sun. The amount of solar energy reaching our earth within one hour equals to the total annual energy need of all of humankind. Since the energy resources on Earth are being exhausted, solar energy have to serve as the main energy source in coming century and beyond. The photovoltaic solar cells developed so far have been based on silicon wafers, with this dominance likely to continue well into the future. The surge in manufacturing volume as well as emerging technologies over the last decade has resulted in greatly decreased costs. Therefore, several companies are now well below the USD 1 W−1 module manufacturing cost benchmark that was once regarded as the lowest possible with this technology. Thin-film silicon, such as hydrogenated amorphous silicon (a-Si), microcrystalline silicon (mc-Si) and related alloys, are promising materials for very low-cost solar cells. Here in this article, a brief description of thin film solar cell technologies followed by deferent state-of-art tools used for characterizing such solar cells are explored. Since characteristics of thin-film solar cells are the main ingredient in defining efficiency, the inherent properties are also mentioned alongside the characterizations.

Energy loss process analysis for radiation degradation and immediate recovery of amorphous silicon alloy solar cells

Performance degradation of a-Si/a-SiGe/a-SiGe triple-junction solar cells due to irradiation of silicon ions, electrons, and protons are investigated using an in-situ current–voltage measurement system. The performance recovery immediately after irradiation is also investigated. Significant recovery is always observed independent of radiation species and temperature. It is shown that the characteristic time, which is obtained by analyzing the short-circuit current annealing behavior, is an important parameter for practical applications in space. In addition, the radiation degradation mechanism is discussed by analyzing the energy loss process of incident particles (ionizing energy loss: IEL, and non-ionizing energy loss: NIEL) and their relative damage factors. It is determined that ionizing dose is the primarily parameter for electron degradation whereas displacement damage dose is the primarily parameter for proton degradation. This is because the ratio of NIEL to IEL in the case of electrons is small enough to be ignored the damage due to NIEL although the defect creation ratio of NIEL is much larger than that of IEL in the cases of both protons and electrons. The impact of “radiation quality effect” has to be considered to understand the degradation due to Si ion irradiation.

Comparison of silicon oxide and silicon carbide absorber materials in silicon thin-film solar cells

Since solar energy conversion by photovoltaics is most efficient for photon energies at the bandgap of the absorbing material the idea of combining absorber layers with different bandgaps in a multijunction cell has become popular. In silicon thin-film photovoltaics a multijunction stack with more than two subcells requires a high bandgap amorphous silicon alloy top cell absorber to achieve an optimal bandgap combination. We address the question whether amorphous silicon carbide (a-SiC:H) or amorphous silicon oxide (a-SiO:H) is more suited for this type of top cell absorber. Our single cell results show a better performance of amorphous silicon carbide with respect to fill factor and especially open circuit voltage at equivalent Tauc bandgaps. The microstructure factor of single layers indicates less void structure in amorphous silicon carbide than in amorphous silicon oxide. Yet photoconductivity of silicon oxide films seems to be higher which could be explained by the material being not truly intrinsic. On the other hand better cell performance of amorphous silicon carbide absorber layers might be connected to better hole transport in the cell.

Errata: Enhancement of light absorption efficiency of amorphous-silicon thin-film tandem solar cell due to multiple surface-plasmon-polariton waves in the near-infrared spectral regime

The reflectances of a thin-film solar cell were computed, using the rigorous coupled-wave approach, as functions of the angle of incidence and the free-space wavelength for illumination by linearly polarized plane waves. A tandem solar cell made of amorphous-silicon alloys was considered. The metallic back-reflector was taken to be periodically corrugated. Both the simple and the compound periodic corrugations of the metallic back-reflector (surface-relief gratings) were investigated. Low-reflectance bands in the reflectance spectrums were correlated with the solutions of the underlying canonical boundary-value problem to delineate the excitation of multiple surface-plasmon-polariton (SPP) waves. For the standard AM1.5 solar irradiance spectrum, we found that the light absorption efficiency in the near-infrared spectral regime can be increased up to 100% when multiple SPP waves of both linear polarization states are excited.

(Invited) Rare Earth Luminescence in Nanostructured Amorphous Silicon Alloys

Amorphous silicon and its alloys are widely used in semiconductor industry. Due to short range order, doping can be achieved by substituting phosphorus or boron for silicon. Moreover, the optical bandgap can be increased by alloying with C, N or O or decreased by alloying with Ge. Due to its amorphous nature the network can accommodate a variety of elements up to relatively high concentrations at the expense of an increased density of defects in the bandgap. Rare earth elements (RE) have been introduced in the form of trivalent ions in amorphous silicon or its wider bandgap alloys. They present characteristic intra-4f luminescence if their chemical neighborhood is non-centrosymetric and the electric dipole forbidden transitions become partially allowed. In the amorphous alloys excitation through the network can be efficiently achieved when a RE transition corresponds to roughly half the bandgap. The RE act as nucleation centers to induce formation of Si nanocrystals under high temperature annealing. Samples were prepared by reactive RF sputtering from a Si target partially covered with metallic RE platelets using appropriate reactive atmospheres. Annealing was used either to optimize the photoluminescence or to induce the formation of Si nanocrystals.We will present characterization by photoluminescence and EXAFS of Er, Nd, Eu and Tb in a-Si:H, a-SiOx:H and a-SiNx:H. The RE ions tend to occupy low coordination low symmetry sites in the network in the amorphous samples. This favors relatively strong wide photoluminescence lines. The RE present two luminescence lifetimes, the fast component determined by the host and the slow associated to the local symmetry of the RE ions.Models for the excitation of luminescence and recombination processes and their relation with the local nanostructure around the RE ions will be presented.

(Invited) Rare Earth Luminescence in Nanostructured Amorphous Silicon Alloys

Rare Earth (RE) doped amorphous silicon alloys can be prepared by reactive RF sputtering from a Si target partially covered with metallic or oxide RE platelets using appropriate reactive atmospheres. We have studied Er3+, Nd3+, Eu3+ and Tb3+ in a-Si:H, a-SiOx:H and a-SiNx:H. Annealing optimizes the photoluminescence and can induce the formation of Si nanocrystals. The RE act as nucleation centers. The RE ions present intense photoluminescence at room temperature. EXAFS measurements reveal a highly non-centrosymetric lattice site for Er in a-SiOx:H. This partially breaks the selection rule that forbids intra-4f transitions. The RE present two luminescence lifetimes, the fast component determined by the host and the slow component associated to the local symmetry ofthe ions. The amorphous and nanostructured hosts increase the cross section for RE excitation by a few orders of magnitude, making the materials practical for applications as phosphors and active amplifiers.

Impact of the hydrogen content on the photoluminescence efficiency of amorphous silicon alloys

This paper analyzes the impact of hydrogen on the photoluminescence (PL) efficiency of the three wide gap silicon alloys: silicon carbide (a-SiCx), silicon nitride (a-SiNx): silicon oxide (a-SiOx). All three materials behave similarly. The progression of the PL efficiency over the Si content splits into two regions. With decreasing Si content, the PL efficiency increases until a maximum is reached. With a further decrease of the Si content, the PL efficiency declines again. A comprehensive analysis of the sample structure reveals that the PL efficiency depends on the degree of passivation of Si and Y atoms (Y = C, N, O) with hydrogen. For samples with a high Si content, an effective passivation of incorporated Y atoms gives rise to an increasing PL efficiency. The PL efficiency of samples with a low Si content is limited due to a rising amount of unpassivated Si defect states. We find that a minimum amount of 0.2 H atoms per Si atom is required to maintain effective luminescence.

Enhancement of light absorption efficiency of amorphous-silicon thin-film tandem solar cell due to multiple surface-plasmon-polariton waves in the near-infrared spectral regime*

The reflectances of a thin-film solar cell were computed, using the rigorous coupled-wave approach, as functions of the angle of incidence and the free-space wavelength for illumination by linearly polarized plane waves. A tandem solar cell made of amorphous-silicon alloys was considered. The metallic back-reflector was taken to be periodically corrugated. Both the simple and the compound periodic corrugations of the metallic back-reflector (surface-relief gratings) were investigated. Low-reflectance bands in the reflectance spectrums were correlated with the solutions of the underlying canonical boundary-value problem to delineate the excitation of multiple surface-plasmon-polariton (SPP) waves. For the standard AM1.5 solar irradiance spectrum, we found that the light absorption efficiency in the near-infrared spectral regime can be increased up to 100% when multiple SPP waves of both linear polarization states are excited.

Optimization of an i-a-SiOx:H absorber layer for thin film silicon solar cell applications

Wide gap, high quality hydrogenated intrinsic amorphous silicon (i-a-Si:H) alloy films are essentially required for multi-junction solar cells fabrication to enhance the open-circuit voltage (Voc) of the top cell. In this paper, intrinsic hydrogenated amorphous silicon oxide films (i-a-SiOx:H) have been prepared using a very high frequency plasma-enhanced chemical vapor deposition technique for use as an absorber layer of the top cell for double-junction amorphous silicon thin film solar cells. We studied the effects of the carbon dioxide (CO2) to silane (SiH4) ratio on the properties of the i-a-SiOx:H films and also optimized these films for application in a-SiOx:H/a-Si:H double-junction solar cells. The Voc of the a-SiOx:H/a-Si:H double-junction solar cells was higher than that of conventional a-Si:H/a-Si:H double-junction solar cells. Using an optimized deposition condition, a-SiOx:H/a-Si:H double-junction solar cell with an initial cell efficiency (η) of 10.2% was produced.

Optimization of the absorption efficiency of an amorphous-silicon thin-film tandem solar cell backed by a metallic surface-relief grating

The rigorous coupled-wave approach was used to compute the plane-wave absorptance of a thin-film tandem solar cell with a metallic surface-relief grating as its back reflector. The absorptance is a function of the angle of incidence and the polarization state of incident light; the free-space wavelength; and the period, duty cycle, the corrugation height, and the shape of the unit cell of the surface-relief grating. The solar cell was assumed to be made of hydrogenated amorphous-silicon alloys and the back reflector of bulk aluminum. The incidence and the grating planes were taken to be identical. The AM1.5 solar irradiance spectrum was used for computations in the 400-1100 nm wavelength range. Inspection of parametric plots of the solar-spectrum-integrated (SSI) absorption efficiency and numerical optimization using the differential evolution algorithm were employed to determine the optimal surface-relief grating. For direct insolation, the SSI absorption efficiency is maximizable by appropriate choices of the period, the duty cycle, and the corrugation height, regardless of the shape of the corrugation in each unit cell of the grating. A similar conclusion also holds for diffuse insolation, but the maximum efficiency for diffuse insolation is about 20% smaller than for direct insolation. Although a tin-doped indium-oxide layer at the front and an aluminum-doped zinc-oxide layer between the semiconductor material and the backing metallic layer change the optimal depth of the periodic corrugations, the optimal period of the corrugations does not significantly change.

Near-Field Electrospray Microprinting of Polymer-Derived Ceramics

Ceramic microelectromechanical systems (MEMS) sensors are potentially game-changing devices in many applications in high-temperature and corrosive environments, where the use of conventional MEMS materials such as silicon is prohibited. However, the microfabrication of ceramic MEMS devices remains a major technical challenge. Here, we report a method to directly print micro ceramic patterns using near-field electrospray (ES) of polymer-derived ceramics (PDCs). We demonstrated that the viscous ceramic precursor liquids can be printed reliably without any clogging problem. The spray self-expansion due to Coulombic repulsion force among charged droplets can be suppressed by decreasing the droplet residence time in space. A spray expansion model is used to predict the line width, and the results are in decent agreement with the experiments. We demonstrated a 1-D printed polymer feature as narrow as 35 μm and a micro pentagram pattern. Moreover, after the pyrolysis of PDC at 1100 °C in nitrogen, amorphous alloys of silicon, carbon, and nitrogen (SiCN) are obtained. The samples show good integrity and adhesion to the substrate. The near-field ES PDC printing can become a useful addition to the toolbox of high-temperature MEMS.