Polycrystalline silicon thin films on glass substrate

The fabrication of large-grain 1.25 μm thick polycrystalline silicon (poly-Si) films via two-stage aluminum-induced crystallization (AIC) for application in thin-film solar cells is reported. The induced 250 nm thick poly-Si film in the first stage is used as the seed layer for the crystallization of a 1 μm thick amorphous silicon (a-Si) film in the second stage. The annealing temperatures in the two stages are both 500°C. The effect of annealing time (15, 30, 60, and 120 minutes) in the second stage on the crystallization of a-Si film is investigated using X-ray diffraction (XRD), scanning electron microscopy, and Raman spectroscopy. XRD and Raman results confirm that the induced poly-Si films are induced by the proposed process.

A method for removing Si precipitations from the surface of AIC (aluminium-induced crystallisation) poly-Si films is introduced. The basic idea is to remove the aluminium oxide layer that is present between the poly-Si film and the Si precipitates and thus enable a lift-off process. Furthermore, a detailed structural investigation of the resulting smooth AIC poly-Si film prepared on SiN-coated planar glass is performed. Based on Raman, UV reflectance and plan-view TEM, the overall crystal quality is found to be excellent for a poly-Si film on glass. Especially the fact that 10-micron grains without dendritic growth pattern are obtained is encouraging. However, there is still room for improvement as both Raman and plan-view TEM reveal areas (/spl sim/10% of total surface area) of sub-micron grains and/or twinned grains. Work is in progress to understand the mechanism behind these results.

The formation of device-grade polycrystalline silicon (poly-Si) films on standard glass at low temperature (i.e., below the softening point of the glass) using simple and fast processes is one of the major challenges for low-cost thin-film solar cells. In this paper, we report what is believed to be the first successful realisation of thick (/spl sim/5 /spl mu/m), large-grained (/spl sim/5 /spl mu/m), continuous poly-Si films on glass at T<650/spl deg/C. This technological breakthrough has been achieved by using ion-assisted deposition at 630/spl deg/C for epitaxially thickening a thin (/spl sim/0.4 /spl mu/m) seeding layer made on glass by aluminium-induced crystallisation.

Polycrystalline silicon (pc-Si) thin films have been synthesized by aluminium induced crystallization (AIC) of amorphous silicon (a-Si) at low temperatures (≤500°C) on flexible metallic substrates for the first time. Different diffusion barrier layers were used to prepare stress free pc-Si films as well as to evaluate the effective barrier against substrate impurity diffusion. The layers of aluminum (Al) and then amorphous silicon with the thickness of 0.27 μm and 0.37 μm were deposited on barrier coated metal sheets by means of an electron beam evaporation and PECVD, respectively. The bi-layers were annealed in a tube furnace at different temperatures (400-500°C) under nitrogen flow for different time periods (1-10hours). The degree of crystallinity of the as-grown layers was monitored by micro-Raman and reflectance spectroscopies. Structure, surface morphology and impurity analysis were carried out by X-ray diffraction, scanning electron microscopy (SEM) and EDAX, respectively. The X-ray diffraction measurements were used to determine the orientation of grains. The results show that the AIC films on metal sheets are polycrystalline and the grains oriented in (100) direction preferentially. However, the properties of AIC films are highly sensitive to the surface roughness.

The effect of power density and thickness on aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) formed with plasma enhanced chemical vapor deposition (PECVD) was studied by using N2-protected conventional furnace reaction and optical microscopy. With the deposition power density ranging from 0.05 to 1.00 W/cm2 and the thickness from 500 to 5000Å, it was found that a low power density as well as a large a-Si thickness could result in a decrease of activation energy and therefore a significant reduction of the AIC reaction temperature. Scanning and transmission electron microscopy and X-ray diffraction were used to check the crystallinity and quality of the AIC thin films. High quality polysilicon thin films were achieved at an AIC reaction temperature as low as 120°C.

A polycrystalline silicon (pc-Si) thin film with large grains on a low-cost non-Si substrate is a promising material for thin-film solar cells. One possibility to grow such a pc-Si layer is by aluminum-induced crystallization (AIC) followed by epitaxial thickening. The best cell efficiency we have achieved so far with such an AIC approach is 8%. The main factor that limits the efficiency of our pc-Si solar cells at present is the presence of many intra-grain defects. These intra-grain defects originate within the AIC seed layer. The defect density of the layers can be determined by chemical defect etching. This technique is well suited for our epitaxial layers but relatively hard to execute directly on the seed layers. This paper presents a way to reveal the defects present in thin and highly-aluminum-doped AIC seed layers by using defect etching. We used diluted Schimmel and diluted Wright etching solutions. SEM pictures show the presence of intra-grain defects and grain boundaries in seed layers after defect etching, as verified by EBSD analyses. The SEM images of diluted Wright etched pc-Si seed layer shows that grain boundaries become much better visible than with diluted Schimmel etch.

Poly-crystalline silicon (poly-Si) films were fabricated by rapid thermal annealing (RTA) of amorphous silicon films which were deposited on quartz by hot wire chemical vapor deposition. An insertion of Cr layer can significantly suppress the peeling of Si films during the RTA process. The effect of RTA parameters on the structural properties of poly-Si films was investigated by Raman spectroscopy, X-ray diffraction and scanning electron microscopy. The results show that the crystallinity of the poly-Si films is increased with the increase of RTA temperature and duration. A sharp peak at about 520 cm−1 is observed in the Raman spectra of poly-Si films annealed at 900 and 1,100 °C for 15 s indicating the excellent crystallinity of the poly-Si films fabricated by RTA. Poly-Si films with high crystalline fraction of 97.3 % were obtained by RTA at 1,100 °C for 20 s.

Large-grained polycrystalline silicon on glass for thin-film solar cells

The aluminum-induced crystallization and layer exchange process shows great promise for converting a-Si into large-grained poly-Si for solar cell applications. To investigate the relationship between the grain size of Al and the final grain size of poly-Si, a series of samples were deposited by RF magnetron sputtering 165 nm of Al onto SiN/SiO 2 coated (100) silicon substrates. The Al grain size was varied by vacuum annealing prior to the deposition of 195 nm of a-Si. Completion of the layer exchange process resulted in poly-Si films which were then characterized with plan view TEM. The average Si grain size was found to increase as a function of increasing Al grain size, consistent with the grain-boundary nucleation model for this process. The largest average Si grain size of 4.9±1.92µm corresponded to the Al sample which was annealed for 24 hours at 550°C. The microstructure of the poly-Si film can therefore be manipulated by altering the properties of the as-deposited Al layer with an isothermal anneal.

Two-dimensional transition metal dichalcogenides (TMDCs) have the extensive application prospect in multifunctional electronics and photonics due to their unique electro-optical properties. In order to further expand their application scope in micro-nano optoelectronic devices and improve the performance of devices, the band-gap and defective engineering have been studied to tune the band-gap, morphology and structure of two-dimensional semiconductor materials. The tunning of the bandgap of MoS&lt;sub&gt;2(1-&lt;i&gt;x&lt;/i&gt;)&lt;/sub&gt; Se&lt;sub&gt;2&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; alloy has been typically achieved by controlling the Se concentration. Theoretical calculations revealed that layered stacked two-dimensional alloy materials with a larger aspect ratio, exposed edges and obvious edge dangling bonds show enhanced HER activity as compared with TMDCs. In this paper, the properties of stacked MoS&lt;sub&gt;2(1-&lt;i&gt;x&lt;/i&gt;)&lt;/sub&gt; Se&lt;sub&gt;2&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; alloy grown by the chemical vapor deposition method in a quartz tube furnace are investigated by using optical microscopy (OM), atomic force microscopy (AFM), scanning tunneling microscopy (SEM), Raman, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS). The OM and SEM images of the as-synthesized stacked MoS&lt;sub&gt;2(1-&lt;i&gt;x&lt;/i&gt;)&lt;/sub&gt;Se&lt;sub&gt;2&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; alloy show apparent interface between layers and their thickness is further acquired by AFM. Unlike most of single-layer or few-layer MoS&lt;sub&gt;2(1-&lt;i&gt;x&lt;/i&gt;)&lt;/sub&gt;Se&lt;sub&gt;2&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; alloys, stack-grown stepped MoS&lt;sub&gt;2(1-&lt;i&gt;x&lt;/i&gt;)&lt;/sub&gt; Se&lt;sub&gt;2&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; alloy materials all present the strong luminescence properties despite the thickness increasing from 2.2 nm (~3 layers) to 5.6 nm (~7 layers). And even till 100 nm, the emission spectrum with two luminescence peaks can still be observed. The two exciton luminescence peaks A and B are derived from the valence band splitting caused by the spin-orbit coupling, respectively. As the thickness increases, the two luminescence peaks are red-shifted and exhibit a band-bending effect that is only present when the alloy doping concentration is changed. As the sample thickness is 5.6 nm, a C-peak at 650 nm at the high energy end of the PL spectrum is observed, which may be attributed to the transition luminescence from the defect energy level introduced by Se (S) substitution, interstice or cluster. When the number of layers is small, the number of defects is small, so that the luminescence is not observed. As the number of layers increases, the defects increase to form a defect energy level. However, when the material thickness continuously increases until the bulk material is formed, the luminescence disappears in the PL spectrum because the band gap is reduced and the band gap is made smaller than the defect energy level. Raman spectroscopy gives two sets of vibration modes:like-MoS&lt;sub&gt;2&lt;/sub&gt; and like-MoSe&lt;sub&gt;2&lt;/sub&gt;. The Raman peak is almost unchanged as the thickness increases, but the two vibration modes E&lt;sub&gt;2g (Mo-Se)&lt;/sub&gt; and E&lt;sub&gt;2 g (Mo-S)&lt;/sub&gt; in the plane gradually appear and increase. At the same time, the intensity ratio and line width of Mo-Se related vibration mode E&lt;sub&gt;2g&lt;/sub&gt;/A&lt;sub&gt;1g&lt;/sub&gt; increase with thickness increasing, which indicates the enhancement of the Mo-Se in-plane vibration mode and the incorporation of randomness of Se into the lattice. Obviously, the defects and stress are the main factors affecting the electronic structure of stacked MoS&lt;sub&gt;2(1-&lt;i&gt;x&lt;/i&gt;)&lt;/sub&gt; Se&lt;sub&gt;2&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; alloy, which provides a meaningful reference for preparing the special functional devices and studying the controllable defect engineering.

2D β-Ga2O3 flakes on a continuous 2D graphene film were prepared by a one-step chemical vapor deposition on liquid gallium surface. The composite was characterized by optical microscopy, scanning electron microscopy, Raman spectroscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy (XPS). The experimental results indicate that Ga2O3 flakes grew on the surface of graphene film during the cooling process. In particular, tenfold enhancement of graphene Raman scattering signal was detected on Ga2O3 flakes, and XPS indicates the C-O bonding between graphene and Ga2O3. The mechanism of Raman enhancement was discussed. The 2D Ga2O3-2D graphene structure may possess potential applications.

Erbium oxide (Er2O3) coatings were deposited using filtered vacuum arc deposition (FVAD) and their structure and thermal stability were studied as a function of fabrication parameters. The coatings were deposited on silicon wafer and tantalum substrates with an arc current of 50 A and a deposition rate of 1.6 ± 0.4 nm/s. The arc was sustained on truncated cone Er cathodes. The influence of oxygen pressure (P= 0.40-0.93 Pa), bias voltage (Vb= -20, -40 or grounded) and substrate temperature (room temperature (RT) or 673K) on film properties was studied before and after post deposition annealing (1273K for 1 hour, at P~ 1.33 Pa). The coatings were characterized using X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Knoop Hardness. Optical microscope images indicated that the coatings had very low macroparticle concentration on their surface. The macroparticle diameters were less than 2.5 μm. The coatings were composed of only Er2O3 without any metallic phase under all deposition parameters tested. The coatings deposited on RT substrates were XRD amorphous and had a featureless cross-section microstructure. However, the coatings deposited on 673K heated substrates had a C-Er2O3 structure with (222) preferred orientation and weak columnar microstructure. The coating hardness varied with deposition pressure and substrate bias, and reached a maximum value of 10 GPa at P = 0.4 Pa and Vb = -40 V. The post-deposition annealing caused crystallization, and the coatings hardness dropped to 4 GPa with thermal treatment. However, after post-deposition annealing, no peeling or cracking appeared at the coating surface or the interface with the substrate.

To develop a multi-modal deep learning method for automatic classification of immune-mediated glomerular diseases based on images of optical microscopy (OM), immunofluorescence microscopy (IM), and transmission electron microscopy (TEM). We retrospectively collected the pathological images from 273 patients and constructed a multi-modal multi- instance model for classification of 3 immune-mediated glomerular diseases, namely immunoglobulin A nephropathy (IgAN), membranous nephropathy (MN), and lupus nephritis (LN). This model adopts an instance-level multi-instance learning (I-MIL) method to select the TEM images for multi-modal feature fusion with the OM images and IM images of the same patient. By comparing this model with unimodal and bimodal models, we explored different combinations of the 3 modalities and the optimal methods for modal feature fusion. The multi-modal multi-instance model combining OM, IM, and TEM images had a disease classification accuracy of (88.34±2.12)%, superior to that of the optimal unimodal model [(87.08±4.25)%] and that of the optimal bimodal model [(87.92±3.06)%]. This multi- modal multi- instance model based on OM, IM, and TEM images can achieve automatic classification of immune-mediated glomerular diseases with a good classification accuracy.

Snow is a divided medium that can adsorb atmospheric trace gases. Evaluating the impact of the snow cover on atmospheric chemistry therefore requires the knowledge of the specific surface area (SSA) of snow. This paper compares the results of three methods used to measure or estimate the SSA of four snow samples: CH4 adsorption at 77 K, optical microscopy (OM), and scanning electron microscopy (SEM, used only on two samples). Within error bars, CH4 adsorption and OM yield similar results on three of the four snow samples. Values for the 4th sample are within a factor of 2. For both samples where CH4 adsorption, OM, and SEM are used, all three methods yield similar results, but CH4 adsorption always has a better accuracy and a much better precision. Thus, despite its ease of use, estimates from OM images are often not accurate enough to monitor the evolution of snow SSA. The main sources of error in the OM method are the difficulty to determine snow crystal thicknesses and to take into account the topography of the snow crystal surface. The combination of CH4 adsorption and OM or SEM can provide useful information on the evolution of both the SSA and the shape of snow crystals. This will be useful to evaluate the respective contributions of adsorption/desorption and sublimation/condensation processes to the impact of the snow cover on atmospheric chemistry.

Polycrystalline silicon (poly-Si) films were prepared on glass substrates by the plasma- enhanced chemical vapor deposition method using SiH4/SiF4 mixtures as a function of deposition temperature, Td, from 150 to 400°C, and the structural properties were investigated. In addition, the effects of addition of H2 to the SiH4/SiF4 gas were also examined. The crystallinity and grain size of Si films with added were found to have maximum values at around Td = 250–300°C. However, poly-Si films without H2 addition contain numerous microvoids, and exhibit easy O contamination, and their crystallinity monotonically increased with Td. The change in the SiH and SiH2 bond density with H2 addition was consistently interpreted in terms of this model. As a result, H2 addition at low Td was suggested to suppress O contamination and improve the crystalline quality. By contrast, H2 addition at high Td is likely to deteriorate crystalline quality. The results were discussed in terms of fluorine and hydrogen chemistry.