Ultrathin Amorphous Si Research Articles

Transparent photovoltaic-based photocathodes for see-through energy systems

Solar energy is an abundant clean energy source, which may support current and future energy demands as well as sustainable growth. The increasing energy demand in modern society promotes the commercialization of solar cells with high power conversion efficiency and low costs. However, the intermittent nature of solar power limits its practical uses, especially in solar cells and in the corresponding hydrogen production systems. Solar-powered devices and their water-splitting associated hydrogen generation offer clean and continuous energy. We propose transparent photovoltaics (TPVs) combined with the transparent photoelectrochemical cells (TPECs) uniting electric power and hydrogen generation in the domestic framework. A transparent photocathode based on an ultrathin amorphous Si (a-Si) solar absorber is applied in a TPV–TPEC energy system with good transparency, generating a high open-circuit voltage (0.852 V), and short-circuit current density (3.744 mA/cm2), noticeably device efficiency (1.66%). The transparent photocathode shows a maximum photocurrent density of 4 mA/cm2 at −0.5 V vs. RHE, driving stable solar hydrogen production. We demonstrate the promising potential of the TPV–TPEC integrated system for the unstoppable generation, supply, and alteration of energy in see-through platforms. Hereupon, the integration of this system in building windows will enable the continuous production of green energy without the loss of external vision.

Argon Ion Beam Current Dependence of Si-Si Surface Activated Bonding.

In order to optimize the process parameters of Si-Si wafer direct bonding at room temperature, Si-Si surface activated bonding (SAB) was performed, and the effect of the argon ion beam current for surface activation treatment on the Si-Si bonding quality was investigated. For the surface activation under the argon ion beam irradiation for 300 s, a smaller ion beam current (10~30 mA) helped to realize a lower percentage of area covered by voids and higher bonding strength. Especially with the surface activation under 30 mA, the bonded Si-Si specimen obtained the highest bonding quality, and its percentage of area covered by voids and bonding strength reached <0.2% and >7.62 MPa, respectively. The transmission electron microscopy analyses indicate that there exists an ultrathin amorphous Si interlayer at the Si-Si bonding interface induced by argon ion beam irradiation to Si wafer surfaces, and its thickness increases as the argon ion beam current rises. The investigation results can be used to optimize the SAB process and promote the applications of SAB in the field of semiconductor devices.

All Electrochemical Process for Synthesis of Si Coating on TiO2 Nanotubes As Durable Negative Electrode Material for Li-Ion Batteries

Although silicon is a promising negative electrode material for Li-ion batteries, its practical application has been hindered by the huge volume expansion resulting poor cyclic stability. It has been demonstrated that ultra-thin (< 50 nm) amorphous Si shown an excellent cyclic stability. However, the energy density delivered from the ultra-thin film is not sufficient for high energy application and increasing the thickness leads to the fracture of the film. Therefore, it is a key to design a conductive substrate with high surface area to enhance adhesion and Si active mass before reaching the fracture thickness. In this work, amorphous silicon (a-Si) film electrodeposited on self-organized TiO2 nanotubes is investigated as durable negative electrode material for Li-ion batteries. The nanostructured composite electrodes were fabricated by a two-step cost effective electrochemical process. Firstly, TiO2 nanotube arrays were synthesized by anodizing of Ti foil. Subsequently, thanks to room temperature ionic liquid, conformal Si film was electrochemically coated on the TiO2 nanotubes for the fabrication of nanostructured a-Si/TiO2 nanotube composite negative electrodes. Compared to other strategies where Si is used as a form of thin films, the TiO2 nanotube arrays provide several advantages such as a strong mechanical support to buffer Si volume expansion, a high surface area to increase Si active mass, and the direct contact of the nanotubes with the Ti current collector facilitates 1D electron transport and avoids the need of adding inactive binders or conductive additives. The influence of the Si loading as well as the crystallinity of the TiO2 nanotubes have been studied in terms of capacity and cyclic stability of the composite electrode. For an optimized Si loading, it is shown that the amorphous state for the TiO2 nanotubes enables to get stable lithiation and delithiation with a total areal charge capacity of about 0.32 mA h cm-2 with improved capacity retention of about 84% after 50 cycles. The specific capacity contribution of Si in the composite is estimated to be about 900 mAh/g after 50th cycle. However, the Si deposited on crystalline TiO2 nanotubes showed poor cyclic stability independently from the Si loading. Fig. 1 SEM images of TiO2 nanotubes formed by anodizing Ti foil (left) and after Si coating of the nanotubes via electrodeposition from room temperature ionic liquids (right). Figure 1

Influences of ultrathin amorphous buffer layers on GaAs/Si grown by metal–organic chemical vapor deposition

In this work, a technique for the growth of GaAs epilayers on Si, combining an ultrathin amorphous Si buffer layer and a three-step growth method, has been developed to achieve high crystalline quality for monolithic integration. The influences of the combined technique for the crystalline quality of GaAs on Si are researched in this article. The crystalline quality of GaAs epilayer on Si with the combined technique is investigated by scanning electron microscopy, double crystal X-ray diffraction (DCXRD), photoluminescence, and transmission electron microscopy measurements. By means of this technique, a 1.8-µm-thick high-quality GaAs/Si epilayer was grown by metal–organic chemical vapor deposition. The full-width at half-maximum of the DCXRD rocking curve in the (400) reflection obtained from the GaAs/Si epilayers is about 163 arcsec. Compared with only using three-step growth method, the current technique reduces etch pit density from 3 × 106 cm−2 to 1.5 × 105 cm−2. The results demonstrate that the combined technique is an effective approach for reducing dislocation density in GaAs epilayers on Si.

Red-luminescence band: A tool for the quality assessment of germanium and silicon nanocrystals

We present the photoluminescence (PL) emission of Silicon and Germanium nanocrystals (NCs) of different sizes embedded in two different matrices. Formation of the NCs is achieved via solid-state dewetting during annealing in a molecular beam epitaxy ultra-high vacuum system of ultrathin amorphous Si and Ge layers deposited at room temperature on SiO2. During the dewetting process, the bi-dimensional amorphous layers transform into small pseudo-spherical islands whose mean size can be tuned directly with the deposited thickness. The nanocrystals are capped either ex situ by silicon dioxide or in situ by amorphous Silicon. The surface-state dependent emission (typically in the range 1.74eV–1.79eV) exhibited higher relative PL quantum yields compared to the emission originating from the band gap transition. This red-PL emission comes from the radiative transitions between a Si band and an interface level. It is mainly ascribed to the NCs and environment features deduced from morphological and structural analyses. Power dependent analysis of the photoluminescence intensity under continuous excitation reveals a conventional power law with an exponent close to 1, in agreement with the type II nature of the emission. We show that Ge-NCs exhibit much lower quantum efficiency than Si-NCs due to non-radiative interface states. Low quantum efficiency is also obtained when NCs have been exposed to air before capping, even if the exposure time is very short. Our results indicate that a reduction of the non-radiative surface states is a key strategy step in producing small NCs with increased PL emission for a variety of applications. The red-PL band is then an effective tool for the quality assessment of NCs based structures.

High Photoelectrochemical Oxygen and Hydrogen Evolution Performance by Using a-Si/c-Si Heterojunction Solar Cells as Photoelectrodes

Si heterojunction (SHJ) solar cells, consisting of ultrathin amorphous Si (a-Si) layers on crystalline Si (c-Si), are designed as a photoanode or a photocathode by simply switching the polarity. Critical to enhance the performance of photoelectrodes is good surface passivation. Good surface passivation reduces photocarriers recombination, directly improving the open-circuit voltage (V OC). Moreover, the optical losses by catalysts can be solved by separating the PV from the water redox reaction. The low recombination of photoelectrodes is critical to successfully achieve this design to avoid the photogenerated carrier recombination loss during the period of transport through the whole devices to participate in water redox reaction. The thin a-Si:H layers of SHJ solar cells served as not only the power generating layers but also good passivation layers, including chemical passivation and field-effect passivation. In this feature, we demonstrated the operation of an efficient and stable SHJ photoanode and photocathode. The SHJ photocathode displays excellent hydrogen evolution performance with a of 0.640 mV, of 33.49 mA/cm2, where is the onset potential measured at water reduction current density of 1 mA/cm−2, is the equilibrium water reduction potential in 1 M H2SO4 solution, and is the current density at , and a solar to hydrogen conversion efficiency (SHCE) of 13.26%, which is the highest ever reported for Si-based photocathode.

Highly Porous Silicon Membranes Fabricated from Silicon Nitride/Silicon Stacks

Nanopore formation in silicon films has previously been demonstrated using rapid thermal crystallization of ultrathin (15 nm) amorphous Si films sandwiched between nm-thick SiO2 layers. In this work, the silicon dioxide barrier layers are replaced with silicon nitride, resulting in nanoporous silicon films with unprecedented pore density and novel morphology. Four different thin film stack systems including silicon nitride/silicon/silicon nitride (NSN), silicon dioxide/silicon/silicon nitride (OSN), silicon nitride/silicon/silicon dioxide (NSO), and silicon dioxide/silicon/silicon dioxide (OSO) are tested under different annealing temperatures. Generally the pore size, pore density, and porosity positively correlate with the annealing temperature for all four systems. The NSN system yields substantially higher porosity and pore density than the OSO system, with the OSN and NSO stack characteristics fallings between these extremes. The higher porosity of the Si membrane in the NSN stack is primarily due to the pore formation enhancement in the Si film. It is hypothesized that this could result from the interfacial energy difference between the silicon/silicon nitride and silicon/silicon dioxide, which influences the Si crystallization process.

Broadband antireflection and absorption enhancement by forming nano-patterned Si structures for solar cells

In this letter, we report the antireflection and light absorption enhancement by forming sub-wavelength nano-patterned Si structures via nano-sphere lithography technique. It is found that the surface reflection can be significantly suppressed in a wide spectral range (400-1000 nm) and the weighted mean reflection is less than 5%. Meanwhile, the broad band optical absorption enhancement is achieved consequently. Heterojunction solar cells are prepared by depositing ultrathin amorphous Si film on the nano-patterned Si structures, the short circuit current density increases to 37.2 mA/cm(2)and the power conversion efficiency is obviously improved compared to the reference cell on flat Si substrate.

Ultra-Thin Silicon Carbon Nitride Film: a Promising Protective Coating for Read/Write Heads in Magnetic Storage Devices

Ultra-thin amorphous Si–C–N films, down to 2nm, have been synthesized by MW-ECR, plasma enhanced unbalanced magnetron sputtering. The friction coefficient of the film is only 0.11, determined in dry friction tests against the GCr15 ball at a load of 400 mN for 20 min. The films exhibit good protection against corrosion when they are immersed in a more severe corrosion environment of 0.1 mol/L oxalic acid for 12 h compared to the usual conditions (0.05 mol/L, 4 min) used in current computer industries. These good properties can be attributed to the smooth, dense and pore free structure of the film. These indicate that the Si–C–N film synthesized by the present technique may be a promising protective coating for read/write heads and other magnetic storage devices.

Electroluminescent devices based on amorphous SiN/Si quantum dots/amorphous SiN sandwiched structures

A single layer of dense Si quantum dots with average size of 4 nm sandwiched in amorphous SiN layers was prepared by laser crystallization of ultrathin amorphous Si film followed by subsequently thermal annealing. The electroluminescent diodes were fabricated by evaporating Al electrodes on back sides of p-Si substrates and the top surface of samples. Room temperature electroluminescence can be detected with applying the negative voltage around 10V on the top gate electrode and the luminescent intensity is increased with increasing the applied voltage. It was found that the integrated luminescent intensity is linearly proportional to the injection current which suggested the intensity depends on the concentrations of injected carriers after Fowler-Nordheim tunneling through amorphous SiN barriers. The influence of the amorphous SiN with different band gap on the device performance was also discussed briefly.

Formation of high-density Si nanodots by agglomeration of ultra-thin amorphous Si films

High-density and similarly-sized Si nanodots were formed by annealing ultra-thin amorphous Si (a-Si) films deposited on SiO 2/Si substrates in vacuum. Dependences of density and diameter of the Si nanodots on the a-Si film thickness and, annealing temperature and time were investigated by scanning electron microscopy. It is found that drastic increase (decrease) in the density (diameter) occurred at an a-Si thickness of 1 nm. By agglomeration of sub-nanometer thick a-Si films, a density larger than 10 12 cm − 2 , an average diameter smaller than 5 nm, and a dispersion of diameter less than 15% were achieved.

Electrically driven luminescence of nanocrystalline Si/SiO 2 multilayers on various substrates

Amorphous-Si:H/SiO 2 multilayers were fabricated on various substrates by alternatively changing the ultra-thin amorphous Si films deposition and in situ plasma oxidation processes. Uniform silicon nanodots with controllable size were formed by combining the rapid thermal annealing and furnace annealing techniques as revealed by cross-sectional transmission electron microscopy. Light-emitting diodes based on Al/(nc-Si/SiO 2) multilayers/Si substrate/Al structures were fabricated and electrically driven luminescence was demonstrated at room temperature. It was found that the substrate had a significant impact on the carrier injection efficiency and the luminescence intensity. The turn-on voltages of the device on p +-Si and p-Si substrate were 2.2 and 8 V, respectively. Meanwhile, electroluminescence intensity of the device on p +-Si substrate was five times higher than that on p-Si substrate under the same power supply, which means the heavily doped substrate is helpful for enhancing the carrier injection efficiency.

Electron field emission of nanocrystalline Si prepared by laser crystallization

The electron field emission characteristics of nanocrystalline Si thin films prepared by KrF excimer laser crystallization of ultrathin amorphous Si films and subsequent thermal annealing is reported. Stable and reproducible field emissionbehavior can be observed for the crystallized Si films. The turn-on electric field is reduced from 17V/μm for the as-deposited sample to 85V/μm for the crystallized one, and the emission current density can reach as high as 01mA/cm2 The improvement in field emission characteristics is attributed to both the change of film surface and the formation of high-density nanocrystalline Si, which induces the enhancement of internal local electric field.

Enhanced electron field emission from dense Si nano-dots prepared by laser crystallization of ultrathin amorphous Si films

Dense Si nano-dots with a surface area density of >10 10 cm −2 were fabricated by excimer laser induced crystallization of 15 nm-thick amorphous Si thin films. The enhanced electron field emission characteristics were found from laser irradiated samples. The threshold electric field is as low as 9.8V/μm and the field enhancement factor can reach as large as 719, which is compatible with the other good cold cathode materials. The improvements in field emission behavior can be associated with the change in the surface morphology after laser irradiation as well as the enhanced internal electric field due to the formation of Si nano-dots within the films.

Enhancement of electroluminescence in p–i–n structures with nano-crystalline Si/SiO2 multilayers

Nano-crystalline Si/SiO2 multilayers were prepared by alternately changing the ultra-thin amorphous Si film deposition and the in situ plasma oxidation process followed by the post-annealing treatments. Well-defined periodic structures can be achieved with 2.5 nm thick SiO2 sublayers. It is shown that the size of formed nano-crystalline Si is about 3 nm. Room temperature electroluminescence can be observed and the spectrum contains two luminescence bands located at 650 nm and 520 nm. In order to improve the hole injection probability, p–i–n structures containing a nanocrystalline Si/SiO2 luminescent layer were designed and fabricated on different p-type substrates. It is found that the turn-on voltage of p–i–n structures is obviously reduced and the luminescence intensity increases by 50 times. It is demonstrated that the use of a heavy-doped p-type substrate can increase the luminescence intensity more efficiently compared with the light-doped p-type substrate due to the enhanced hole injection.

Laser Induced Surface Nanostructuring on Ultra-Thin Amorphous Si Films

A new approach to obtain Si nanostructures on insulating layer is proposed by laser irradiation on ultra-thin hydrogenated amorphous silicon (a-Si:H) films with subsequently thermal annealing. It was found that the surface nanostructuring was occurred when the laser fluence exceeded the threshold value as revealed by AFM images. The size and area density of formed Si nanostructures were depended on the laser fluence and film thickness while thermal annealing played an important role in the size and its distribution. The results showed that a high density (&gt;1011cm-2) Si nanostructures with average lateral size of 10-20nm can be achieved by the present technology.

Crystallization kinetics of ultrathin amorphous Si film induced by Al metal layer under thermal annealing and pulsed laser irradiation

The crystallization kinetics of ultrathin a-Si induced by Al under thermal annealing and pulsed laser irradiation has been studied. Under thermal annealing, the crystallization temperature and activation energy for crystallization of a-Si with a thin Al metal layer was reduced to around 340°C and 3.3eV, respectively. The reaction exponent was determined to vary from 1.5 to 1.8, corresponding to a crystallization process in which grain growth occurs with nucleation, and the nucleation rate decreases with the progress of grain growth. Under high power pulsed laser irradiation, the crystallization and reamorphization of a-Si were found to take place sequentially in a-Si∕Al. The reamorphization of a-Si in contact with a thin Al metal layer can be attributed to the melting of a-Si∕Al initiated at the interface, due to the low melting temperature of Si–Al alloy and the rapid solidification that followed. Considering only the crystallization process, the activation energy for crystallization of a-Si induced by Al, estimated to be about 0.22eV, was nearly an order of magnitude lower than that under thermal annealing. This may be explained by the explosive crystallization of a-Si by mechanical impact with a high power pulsed laser. In the meantime, the reaction exponent, determined to range from 1.9 to 2.2, was slightly higher than that under thermal annealing, indicating that the decrease of nucleation rate with the progress of grain growth during crystallization was slower, and the crystallization process became more nucleation dominant.

Si Nanocrystal Memory Devices Self-Assembled by In Situ Rapid Thermal Annealing of Ultrathin a-Si on SiO[sub 2

Si-nanocrystal memories have been fabricated using the thermal agglomeration of an ultrathin (0.9-3.5 nm) amorphous Si (a-Si) film. The a-Si was deposited by electron beam evaporation followed by in situ annealing at 850°C for 5 min. Hemispherical nanocrystals were obtained with a dot density up to 3.9 X 10 11 cm -2 , and a stored charge density of 4.1 × 10 12 cm -2 (electron + hole) was achieved. The data retention characteristics of nanocrystal-embedded devices have shown a 1 V memory window after 100 h. Well-separated Si nanocrystals with a 23-32% surface-coverage ratio have been successfully demonstrated. The process compatibility of the proposed technique was also discussed.

Visible light emission from single layer Si nanodots fabricated by laser irradiation method

A single layer of dense (&amp;gt;1011cm−2) Si nanodots on an insulating a-SiN layer was fabricated by the method combining the laser irradiation on the ultrathin amorphous Si films and subsequent thermal annealing. Raman scattering spectroscopy, planar and cross-section transmission electron microscopy were employed to characterize the formation of Si nanodots. It was found that the size of formed Si nanodots is strongly influenced by the initial amorphous Si film thickness. Visible light emission was observed from the obtained Si nanodots at room temperature and the luminescence peak is varied from 660to725nm with increasing the amorphous Si film thickness. The variable luminescence can be attributed to the interface state assisted radiative recombination rather than the quantum size effect.

Ultrathin amorphous Si layer for the growth of strain relaxed Si0.75Ge0.25 alloy layer

We propose a method for the growth of strain relaxed and smooth Si0.75Ge0.25 alloy layers on a Si(001) substrate. In this method, we have used an ultrathin amorphous Si (UTA-Si) layer as a buffer layer and implemented a two-step process to grow the top alloy layer. High-resolution x-ray diffraction studies show that the alloy layers are highly relaxed. Topographic studies by contact mode atomic force microscopy show that the surfaces are very smooth. UTA-Si works as a strain adjuster and helps to reduce residual strain introducing dislocation in the buffer and substrate regions. However, it was observed that the residual strain and the surface morphology depend on the thickness of the UTA-Si buffer layers and also on the growth mode of the alloy layer.