Epitaxial Silicon Thin Films by Low Temperature Aluminum Induced Crystallization of Amorphous Silicon (original) (raw)
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Understanding phenomena of thin silicon film crystallization on aluminium substrates
The realization of crystalline silicon thin films on foreign substrates is an attractive alternative to the ingot casting aiming at a reduction in total costs. The purpose of this work is to form polycrystalline silicon films using the crystallization of amorphous silicon deposited on aluminum (Al) substrates. The Al-substrate is used as a catalyzer for silicon crystallization but also as a conductive substrate and as a back reflector for the photovoltaic cell. The crystallization of 1-5 μm thick amorphous silicon films were carried out at a temperature of 550°C and for duration times from 10 to 80 min. The crystallized silicon films were then characterized by Raman spectroscopy, scanning electron microscopy and by electron backscatter diffraction. The analysis show that the annealed layer is composed of two distinct layers: a thin polysilicon film located just above the Al substrate and on the top a thicker layer made of a mix of silicon and aluminum. The thickness of the polysilicon film is found to increase with the annealing time. The crystallization of 5 μm thick amorphous silicon during 80 min resulted in a 1 μm thick polysilicon layer composed of grains of few micrometers in size. The mechanisms of accelerated crystallization are discussed. Such polysilicon films can be used as a seed layer for the growth of a thicker absorbing silicon film for photovoltaic applications.
2017
The effect of various annealing treatments on the structure properties of crystalline silicon (c-Si) produced by the inverted aluminium induced crystallization of amorphous silicon (a-Si) films was studied. The surface morphology and grain size of c-Si films were observed by optical microscope and scanning electron microscope. X-ray diffraction and Raman spectroscopy were used to study quantity of Si crystallization due to thermal annealing. Results showed that the c-Si with an average grain size of 54 nm in a (111) orientation was obtained by the thermal annealing at 350 o C for 1 h. Prolonged heat treatment improved Si crystallite quality and increased the average grain size.
2013
The crystallization of hydrogenated amorphous silicon layers (a-Si:H) [1,2] deposited by plasma enhanced chemical vapor deposition (PECVD) is of great interest. Generally, laser or metals are used to induce crystallization in aSi:H films. We have found that films deposited at high rf power (> 0.2 W/cm2) by PECVD technique shows some crystallites embedded in a-Si:H matrix and their after its vacuum thermal annealing at 250 and 300 C helps to further enhancement of crystallite size. These films were characterized using , UV-VIS spectrometry, Raman Spectra, of these films were measured as a function of temperature in the range of 300 C to 250 C. Keyword: Amorphous silicon, Thin Films, Growth PECVD.
EBSD analysis of polysilicon films formed by aluminium induced crystallization of amorphous silicon
Thin Solid Films, 2008
Among the methods for enlarging the grain size of polycrystalline silicon (poly-Si) thin films, aluminium induced crystallization (AIC) of amorphous silicon is considered to be a very promising approach. In the AIC process, a thin a-Si layer on top of an aluminium layer crystallizes at temperatures well below the eutectic temperature of the Al/Si system (T eu = 577°C). By means of electron backscattering diffraction (EBSD), we have mainly studied the effect of the aluminium layer quality varying the deposition system on the grain size, the defects and the preferential crystallographic orientation. We have found a strong correlation between the mean grain size and the size distribution with the Al deposition system and the surface quality. Furthermore, we show for the first time that more than 50% of the surface of the AIC films grown on alumina substrates are (103) preferentially oriented, instead of the commonly observed (100) preferential orientation. This may have important consequences for epitaxial thickening of the AIC layer into polysilicon absorber layers for solar cells.
Applied Surface Science, 2011
Amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) films were deposited by DC magnetron sputtering technique with argon and hydrogen plasma mixture on Al deposited by thermal evaporation on glass substrates. The a-Si/Al and a-Si:H/Al thin films were annealed at different temperatures ranging from 250 to 550 • C during 4 h in vacuum-sealed bulb. The effects of annealing temperature on optical, structural and morphological properties of as-grown as well as the vacuumannealed a-Si/Al and a-Si:H/Al thin films are presented in this contribution. The averaged transmittance of a-Si:H/Al film increases upon increasing the annealing temperature. XRD measurements clearly evidence that crystallization is initiated at 450 • C. The number and intensity of diffraction peaks appearing in the diffraction patterns are more important in a-Si:H/Al than that in a-Si/Al layers. Results show that a-Si:H films deposited on Al/glass crystallize above 450 • C and present better crystallization than the a-Si layers. The presence of hydrogen induces an improvement of structural properties of poly-Si prepared by aluminium-induced crystallization (AIC).
Low temperature growth of Amorphous Silicon thin film
2007
Western Cape The growth of amorphous hydrogenated silicon (a-Si:H) thin films deposited by hotwire chemical vapor deposition (HWCVD) has been studied. The films have been characterized for optical and structural properties by means of UV/VIS, FTIR, ERDA, XRD, XTEM and Raman spectroscopy. Low substrate heater temperatures in the range from 130 °C to 200 °C were used in this thesis because it is believed to allow for the deposition of device quality a-Si:H which can be used for electronic photovoltaic devices. Furthermore, low temperature allows the deposition of a-Si:H on any substrate and thus offers the possibility of making large area devices on flexible organic substrates. We showed that the optical and structural properties of grown a-Si:H films depended critically upon whether the films were produced with silane gas or silane diluted with hydrogen gas. Device quality a-Si:H can be deposited at low substrate heater temperature at optimized conditions. Finally, we also showed that it is possible to deposit crystalline materials at low temperature under high hydrogen dilution ratio of silane gas.
We have investigated epitaxial growth of poly crystalline intrinsic silicon film grown on glass and Si/SiO2 substrate using hot wire chemical vapour deposition technique. We have grown 20 nm nucleation layer at 400°C followed by an epitaxial growth of 200 nm thick at 600°C. Than 800 nm thick layer was grown on top using high hydrogen silane ratio of 15:5. Hydrogen soaking was performed for well passivated film. Evaluation of different layers was performed using cross sectional transmission electron microscopy. Poly crystalline as well as epitaxial and columnar growth regions were well observed.
Low Temperature Growth of Amorphous Silicon Thin Films
2007
Western Cape The growth of amorphous hydrogenated silicon (a-Si:H) thin films deposited by hotwire chemical vapor deposition (HWCVD) has been studied. The films have been characterized for optical and structural properties by means of UV/VIS, FTIR, ERDA, XRD, XTEM and Raman spectroscopy. Low substrate heater temperatures in the range from 130 °C to 200 °C were used in this thesis because it is believed to allow for the deposition of device quality a-Si:H which can be used for electronic photovoltaic devices. Furthermore, low temperature allows the deposition of a-Si:H on any substrate and thus offers the possibility of making large area devices on flexible organic substrates. We showed that the optical and structural properties of grown a-Si:H films depended critically upon whether the films were produced with silane gas or silane diluted with hydrogen gas. Device quality a-Si:H can be deposited at low substrate heater temperature at optimized conditions. Finally, we also showed that it is possible to deposit crystalline materials at low temperature under high hydrogen dilution ratio of silane gas.
Thin Solid Films, 2017
The fabrication of crystalline silicon thin films on foreign substrates is an attractive and alternative approach to the ingot casting aiming to the reduction of the total costs of photovoltaic cells and modules. The purpose of this work is to describe the CRYSTALSI process which aims at forming polycrystalline silicon films thanks to the thermal crystallization of amorphous silicon layer deposited on aluminium based substrates. The latest are used as a catalyzer for silicon crystallization but also as a back metal contact and reflector for photovoltaic solar cells. Two types of aluminium substrates were applied in these studies: a pure aluminium substrate (99.7% purity) and a silicon rich aluminium substrate containing about 12% of silicon. Silicon thicknesses between 1 and 10 μm were deposited and then annealed at temperatures of 490°C, 520°C and 550°C and for duration times from 5 min to 12 h. The crystallized silicon films were then characterized by Raman spectroscopy, by scanning electron microscopy and by electron backscatter diffraction. The analyses show that the resulting annealed film is composed of two distinct layers: a thin polycrystalline silicon film located just above the substrate and a thicker layer made of a mixture of silicon and aluminium. Contrary to the case of the pure aluminium substrate, the silicon rich aluminium substrate allow to obtain thick and continuous polycrystalline silicon layers due to a controlled diffusion of the silicon within the substrate. As a result, the crystallization at 550°C of 5 μm thick amorphous silicon on silicon rich aluminium substrate led to the formation of a thick polycrystalline silicon layer composed of grains of few micrometers in size. A low activation energy of about 2 eV is extracted suggesting that the silicon rich aluminium substrate is a catalyzer for the crystallization of amorphous silicon. As for the AIC process, it can be noticed that the limiting step of the CRYSTALSI process is the diffusion of the silicon in the aluminium. A chemical etching using a HNO 3 , HF, H 2 O (72.5 ml/1.5 ml/28 ml) solution is found to be appropriate to remove the residual top layer, in order to have access to the polycrystalline silicon layer. This work demonstrates that the CRYSTALSI process can lead to the formation of polysilicon films that can serve as a seed layer for the growth of a thicker absorbing silicon film for photovoltaic applications.
Poly-Silicon Thin Films Prepared by Low Temperature Aluminum-Induced Crystallization
Modern Physics Letters B, 2001
P-type poly-Si thin films prepared by low temperature Aluminum-induced crystallization and doping are reported. The starting material was boron-doped a-Si:H prepared by PECVD on glass substrates. Aluminum layers with different thicknessess were evaporated on a-Si:H surface and conventional thermal annealing was performed at temperatures ranging from 300 to 550°C. XRD, SIMS, TEM and Hall effect measurements were carried out to characterize the annealed films. Results show that a-Si:H contacted with adequate Al could be crystallized at temperature as low as 300°C after annealing for 60 minutes. This material has high carrier concentration as well as high Hall mobility can be used as a p-layer or seed layer for thin film poly-Si solar cells. The technique reported here is compatible with PECVD process.