Влияние температуры отжига на кинетику процесса алюминий-индуцированной кристаллизации тонких пленок аморфного субоксида кремния

A polycrystalline silicon (pc-Si) film is produced by low-temperature LPCVD process on a poly silicon seed layer. The properties of such film are characterized and compared with the films on a glass substrate and a silicon wafer. The poly crystalline silicon seed layers are fabricated by aluminum-induced crystallization (AIC) process with a glass/Al/Al 2 O 3 /a-Si structure, in which the Al layer was deposited on the glass substrate by a DC magnetron sputter, and a-Si film by PECVD method, respectively. After the AIC process with 400°C for 150min, the poly silicon film has the average grain size of about 10μm. The poly silicon seed layer is thickened (i.e. the poly silicon film is deposited on the seed layer) by LPCVD process with 600°C for 60min. As results, it is observed that the quality of the poly silicon film on the seed layer is better than that on the glass and worse than that on the silicon wafer. But the deposited poly silicon absorber layer could not reflect the property of the poly silicon seed layer, due to the relatively low temperature process.

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.

Unique polycrystalline silicon (poly-Si) thin films, which were permeable to a concentrated hydrofluoric acid solution through their porous grain boundaries, were investigated to elucidate the formation mechanism of their microstructure. 0.1-μm-thick permeable poly-Si thin films were made through processes of amorphous silicon film formation by low pressure chemical vapor deposition, successive postannealing for crystallization, and excess phosphorus diffusion by a phosphorus oxichloride predeposition. At the grain boundaries, porous microstructures were formed after the films were cleaned in an SC1 solution (a 1:1:5 mixture of NH4OH:H2O2:H2O at 80 °C for 10 min), whereas segregated soluble precipitates observed by a field emission secondary electron microscope were present before the SC1 cleaning. Auger electron microscope revealed that the surface of the precipitates mainly consist of silicon (∼80 at. %) and oxygen (∼20 at. %). As a result of transmission electron microscope observation, it is concluded that enhancement of silicon atom mobility by the phosphorus doping process induced consequent segregation of the soluble precipitates at the grain boundaries.

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.

We correlated the off-state current in polycrystalline silicon (polysilicon) thin film transistor (TFT) with the peak electric field in the channel of the device. We investigated p-channel TFTs with different gate oxide thicknesses, at varying gate and drain biases. The electric fields in the channel of the devices under these experimental conditions were calculated using both process and device simulators, and the experimental off-state current was plotted against simulated fields. We found that in a wide range of electric fields the current exhibits a nearly exponential behavior. Different combinations of the drain and gate biases produce nearly the same off-state current, provided the calculated peak field is the same; moreover, curves corresponding to different gate oxide thicknesses overlay each other. Our results are compatible with the model of the off-state current as being due to the field emission at grain boundaries.

We investigated defect reduction in laser crystallized polycrystalline silicon (poly-Si) films by heat treatment with 1.3×106 Pa H2O vapor. The H2O vapor heat treatment at 260 °C for 6 h reduced the spin density in laser crystallized poly-Si films from 2.0×1018 (initial) to 6.5×1016 cm-3. The activation energy of the reaction for defect reduction was 0.26 eV. Photoconductivity under 532 nm light illumination at 100 mW/cm2 was increased from 2.7×10-6 (initial) to 3.3×10-5 S/cm by heat treatment for 1 h. The oxygen concentration in the silicon films was increased by 1.1×1019 cm-3 by heat treatment, although the hydrogen concentration was decreased by 1.4×1020 cm-3. This suggests that oxygen atoms have an important role in defect state reduction in polycrystalline silicon films.

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.

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.

Optical improved structure of polycrystalline silicon-based thin-film solar cell

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.

Conductivity of boron-implanted polycrystalline thin silicon films

The bending of silicon wafers by thin polycrystalline silicon film deposition and by film doping using boron diffusion

First AM conference joins MPIF's PM2014 and hardmetals events

A nanosecond time resolution pyrometer has been developed for measuring the transient temperature of thin polycrystalline silicon (p-Si) films irradiated by a pulsed excimer laser. The sample design structure and material optical properties allow direct measurement of the temperature at the solid–liquid phase change interface.

ABSTRACTFabrication of SiGe heterojunction bipolar transistors (HBTs) requires a low thermal budget to avoid relaxation of the strained SiGe base layer. Ion implantation is one of the most widely used techniques to achieve contacts. However, due to thermal budget constraints, low temperature rapid thermal annealing (RTA) cycles to activate these implants are insufficient to anneal out all of the implant damage. Polysilicon contacts provide an alternative to ion implantation, but are typically annealed at high temperatures (&gt;950°C) to achieve low sheet resistivity. In this study, amorphous silicon and polycrystalline silicon films were implanted with boron, arsenic, or phosphorus and RTA'd at temperatures from 800°C to 950°C and compared to single crystal silicon with identical implants and RTA cycles. The films were characterized using four-point probe, Hall measurements, TEM (transmission electron microscopy), and SIMS (secondary-ion mass-spectrometry). TEM analysis shows that the amorphous deposition produces larger grains upon RTA due to more rapid grain growth than the polycrystalline deposition. The sheet resistance for the amorphous deposited films is much lower than that of the polycrystalline deposition for all implant conditions. Activations of the implants indicate that the arsenic and phosphorus segregate to the grain boundaries, while the boron does not. The segregation is more significant for the polycrystalline films than for the amorphous films and can be explained by the grain boundary area. For contacts to the SiGe HBT, which requires a low thermal budget, an amorphous deposited silicon film is advantageous over a polycrystalline film at low annealing temperatures because it has lower sheet resistance, less segregation to the grain boundaries, and produces larger grains.