EBSD analysis of polysilicon films formed by aluminium induced crystallization of amorphous silicon (original) (raw)
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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.
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.
Journal of Crystal Growth, 2007
Aluminium-induced crystallization of amorphous silicon (a-Si) allows the formation of polysilicon layers at low temperature (o577 1C) on non-Si substrates such as glass or ceramics. The aim of this work is two-folds: (i) to study the growth kinetics of the polycrystalline Si (poly-Si) film, (ii) to identify defects present inside the isolated grains grown by the electron backscattering diffraction (EBSD) analysis technique and to discuss the effect of the annealing temperature on the crystalline orientation and defect density. The analysis shows that major defects present in such polysilicon are twins and low-angle boundaries. The comparison of optical microscopy and EBSD maps shows that the increased grain size observed with diminishing the annealing temperature is compensated by an increase in the twin density. A link is found between (1 0 0)-oriented grains and ''absence'' of twins.
2010
Large-grained, n + n-type polycrystalline silicon (poly-Si) films were obtained on alumina substrates by combining the aluminium induced crystallization (AIC) process of amorphous silicon and chemical vapour deposition (LPCVD) at high temperature (1000 1C) for the epitaxial thickening. The n + seed layer was obtained by phosphorus doping of the AIC layer. The electron backscattering diffraction (EBSD) technique was used for the crystallographic analysis of the poly-Si thin films. Seed layers with an average grain size of 7.6 mm were obtained on alumina substrates by exchange annealing at 475 1C for 6 h. Heterojunction emitter (HJE) solar cells were fabricated on such layers and their characteristics were monitored. IQE measurements show that n-type material based solar cells led to a much higher current collection over a large part of the spectrum compared to p-type cells. Accordingly a high effective diffusion length of about 2 mm for n-type heterojunction solar cells was obtained while it is about 0.9 mm for the p-type cell. As a result, the first n-type solar cells showed efficiencies above 5%, which is a very promising result considering that no optimization nor texturing have been applied so far.
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.
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.
MRS Proceedings, 2006
Aluminum-induced crystallization of hydrogenated amorphous silicon was used to fabricate epitaxial silicon films through solid phase epitaxy. Silicon wafers of (100) orientation were used as the starting crystalline structure for the epitaxial thin film growth. A configuration of c-Si/Al/a-Si:H was used to produce these films through the phenomenon of layer inversion. A thin layer of aluminum (300 nm) was deposited on a silicon wafer by sputtering. On top of this layer, a 300 nm amorphous silicon film was deposited using plasma-enhanced chemical vapor deposition. After annealing the samples at 475°C for 40 minutes, a continuous film of crystalline silicon was formed on the silicon substrate. X-ray diffraction, scanning electron microscopy, and cross-sectional transmission electron microscopy were used to characterize the films. Auger depth profiling indicated the formation of a Si/Al mixed phase within the first few minutes of annealing. A proposed model of the growth mechanism is presented.
IEE Proceedings - Circuits, Devices and Systems, 2003
Thin film polycrystalline silicon solar cells on foreign substrates are viewed as one of the most promising approaches to cost reduction in photovoltaics. To enhance the quality of the film, the use of 'seeding layers' prior to deposition of active material is being investigated. It has been shown that a phenomenon suitable to create such a seeding layer is the aluminium-induced crystallisation of amorphous silicon. Previous work mainly considered glass as the substrate of choice, thereby introducing limitations on the deposition temperature. Results concerning the application of such a technique to ceramic substrates (allowing the use of high-temperature CVD) are described. Also, the first reported results of a solar cell made in silicon deposited on these seeding layers are presented.
Journal of Applied Physics, 2009
Polycrystalline silicon ͑pc-Si͒ thin-films with a grain size in the range of 0.1-100 m grown on top of inexpensive substrates are economical materials for semiconductor devices such as transistors and solar cells and attract much attention nowadays. For pc-Si, grain size enlargement is thought to be an important parameter to improve material quality and therefore device performance. Aluminum-induced crystallization ͑AIC͒ of amorphous Si in combination with epitaxial growth allows achieving large-grained pc-Si layers on nonsilicon substrates. In this work, we made pc-Si layers with variable grain sizes by changing the crystallization temperature of the AIC process in order to see if larger grains indeed result in better solar cells. Solar cells based on these layers show a performance independent of the grain size. Defect etching and electron beam induced current ͑EBIC͒ measurements showed the presence of a high density of electrically active intragrain defects. We therefore consider them as the reason for the grain size independent device performance. Besides dislocations and stacking faults, also ⌺3 boundaries were electrically active as shown by combining electron backscattered diffraction with EBIC measurements. The electrical activity of the defects is probably triggered by impurity decoration. Plasma hydrogenation changed the electrical behavior of the defects, as seen by photoluminescence, but the defects were not completely passivated as shown by EBIC measurements. In order to reveal the origin of the defects, cross section transmission electron microscopy measurements were done showing that the intragrain defects are already present in the AIC seed layer and get copied into the epitaxial layer during epitaxial growth. The same types of intragrain defects were found in layers made on different substrates ͑alumina ceramic, glass ceramic, and oxidized silicon wafer͒ from which we conclude that intragrain defects are not related to the relatively rough alumina ceramic substrates often used in combination with high temperature epitaxy. Further improvement of the material quality, and hence device performance, is therefore not simply achieved by increasing the grain size, but the intragrain quality of the material also needs to be taken into account. For pc-Si layers based on AIC and epitaxial growth, the seed layer has a crucial impact on the intragrain defect formation.