Poly-Silicon Thin Films Prepared by Low Temperature Aluminum-Induced Crystallization (original) (raw)
Related papers
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.
Materials
In this paper, the analysis, synthesis and characterization of thin films of a-Si:H deposited by PECVD were carried out. Three types of films were deposited: In the first series (00 process), an intrinsic a-Si:H film was doped. In the second series (A1–A5 process), n-type samples were doped, and to carry this out, a gas mixture of silane (SiH4), dihydrogen (H2) and phosphine (PH3) was used. In the third series (B1–B5 process), p-type samples were doped using a mixture of silane (SiH4), dihydrogen (H2) and diborane (B2H6). The films’ surface morphology was characterized by atomic force microscopy (AFM), while the analysis of the films was performed by scanning electron microscopy (SEM), and UV–visible ellipsometry was used to obtain the optical band gap and film thickness. According to the results of the present study, it can be concluded that the best conditions can be obtained when the flow of dopant gases (phosphine or diborane) increases, as seen in the conductivity graphs, where...
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.
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.
Large grain poly-Si thin films by metal induced crystallization of a-Si:H
Conference Record of the Thirty First Ieee Photovoltaic Specialists Conference 2005, 2005
Large grain poly-Si thin fiIms on glass substrates have been successfully fabricated. The film thickness was 400 nm and the grains sizes were in excess of several microns. The films were fabricated using aluminum-induced crystallization of a-Si:H in the presence of a silicon oxide layer at the Alla-Si:H interface. The aSi:H film was deposited on glass substrates and the annealing temperatures and annealing times were kept below 450°C and 30 minutes, respectively. The resulting poly-Si was heavily Al-doped.
Thin Solid Films, 2019
Aluminium induced crystallization (AIC) technique can be used to form the high-quality and large-grained polycrystalline silicon (poly-Si) thin films, which are with the thickness of ~200nm and used as a seed layer, on silicon nitride coated glass substrate. Thanks to aluminium metal in AIC process, the natural doping of AIC thin films is p + type (~2×10 18 cm-3). On the other hand, recombination of carriers can be controlled by partial doping through the defects that may have advantages to improve the thin film quality by the overdoping induced passivation. In this study, boron (B) and phosphorus (P) doped AIC seed layers were thicken to ~2μm by solid phase epitaxy (SPE) technique at 800°C for 3 hours under nitrogen flow in a tube furnace. During the crystallization annealing, exodiffusion of dopants was formed through the SPE film from the AIC seed layer. Optical microscope and electron back scattering diffraction technique (EBSD) were used to analyse the structural quality of the Si films. The poly-Si layer with an average grain size value of ~32µm was formed by AIC+SPE technique for P doped samples while EBSD analysis gave no results for B doped samples due to the quite deterioration on the surface of the films. AIC+SPE films were analysed in terms of structural properties by using micro-Raman Spectroscopy and X-ray diffraction systems. The results showed that the crystallinity of compressive stress formed AIC+SPE films reached up to 98.55%. Additionally, the Raman analysis pointed out that no temperature-induced stress were generated in the AIC+SPE films while compressive stress was induced by increasing the annealing duration for doped AIC film. For all samples, the preferred orientation was <100>, and the crystallite size up to 44.4nm was formed by phosphorus doping of AIC films. The doping efficiency was determined by time-of-flight secondary ion mass spectroscopy for doped samples. A graded n + n doping profile was obtained by exo-diffusion of phosphorus from the overdoped seed layer during the epitaxial thickening while boron doping of SPE film has failed with exo-diffusion of boron from AIC seed layer into SPE film. Finally, high-quality n + n type poly-Si films were fabricated on glass substrate by using AIC+SPE technique.
Thin Solid Films, 2006
Thin films of poly-Si on low cost substrates such as glass are attractive for thin film solar cells and other large-area electronic devices. The controlled and reproducible preparation of polycrystalline silicon thin films by aluminium-induced crystallisation (AIC) needs optimization of both the process of crystallisation and the deposition conditions of the precursor silicon and aluminium layers. In this work, the influence of the hydrogen concentration in the precursor amorphous silicon layer and the deposition temperature of the aluminium layer on the structural properties of poly-Si thin films obtained by AIC of un-hydrogenated (a-Si) and hydrogenated amorphous silicon films (a-Si:H) are studied. Stacks of glass/a-Si:H/ Al were prepared for performing the AIC process. The aluminium and amorphous silicon films are deposited by rf magnetron sputtering. The hydrogen concentration in the a-Si:H films is varied from 0 to 18 at.%. The substrate temperature during a-Si (a-Si:H) deposition is kept constant at 250-C. The Al film is deposited on top of the a-Si (a-Si:H). The deposition temperature of the aluminium films (T Al S) is varied from room temperature (RT) to 500-C. The samples are isothermally annealed in air at T an = 530-C for 7 h. The structural properties of the poly-Si films are studied by Raman spectroscopy and optical microscopy. The results indicate that the hydrogen concentration in the precursor a-Si:H is a very important parameter. It is also observed that T Al S influences the structural properties of the poly-Si films. It is found that films with better crystalline structure are obtained when the a-Si:H precursor layers contain 9 at.% hydrogen and the aluminium deposition temperature is about 350-C.
Polycrystalline silicon on glass thin-film solar cell research at UNSW the seed layer concept
Clinical Nutrition Supplements, 2003
A novel seed layer-based poly-Si solar cell concept on glass-ALICIA (aluminium-induced crystallisation, ion-assisted deposition)-is presently being developed at the University of New South Wales (UNSW). The first key feature of ALICIA solar cells is a large-grained p/sup +/-doped poly-Si seed layer made on the glass by means of aluminium-induced crystallisation (AIC) of amorphous silicon. The other key feature is the
Polycrystalline silicon thin film solar cells prepared by PECVD-SPC
International Journal of Hydrogen Energy, 2008
Among the most promising technological alternatives for the development of photovoltaic modules and cells of a low cost, good energetic conversion and feasibility for mass production, polycrystalline silicon thin film solar cells deposited directly on a transparent substrate are currently being considered the best. We have developed in our laboratory a PECVD reactor capable of producing the deposition of amorphous hydrogenated silicon at rates of above 2 nm/seg, allowing a significant production per line on the plant. Discharge gas is silane, to which diborane or phosphine is added so as to form the cell. Basically, work is done on a structure of cell type TCO/n+/pÀ/p+/M, which has 2 mm of total thickness. Schott AF-37 glass is used as a substrate, for their ability to withstand temperatures of up to 800 1C. The amorphous cell is subsequently annealed at gradual temperatures of 100 1C to achieve dehydrogenation up to 650-700 1C for 12 h until their complete crystallization is achieved. Our results show a complete crystallization of silicon with a grain size of less than a micron, with a dehydrogenation process at 500 1C, leaving a remainder of less than 1% in hydrogen as monohydrate. The parameters of the cell estimated from the IV curve yield low values, FFo0.55, Icc o200 mA and Voco420 mV. The high series resistance is due to the grain size and defect density, which will be attempted to be improved by post-hydrogenation and rapid thermal annealing (RTA) methods at high temperatures.
Crystalline silicon thin-film solar cells on foreign substrates: The European project METEOR
The European project METEOR aims at the development of large-grained poly-Si thin-film solar cells on foreign substrates. A two step process has been used to form the poly-Si films: (1) a thin large-grained poly-Si film (seed layer) is prepared by the aluminium-induced layer exchange (ALILE) process and (2) this seed layer is subsequently used as a template for an epitaxial thickening process. Two different concepts have been investigated: (i) a low-temperature approach using glass substrates (T < 600°C) and (ii) a high-temperature approach using ceramic substrates (T > 1000°C). The surface roughness of the ceramic substrates has a negative impact on the ALILE process. The surface roughness can be reduced by the deposition of an additional oxide layer. At high temperatures thermal CVD has successfully been used for epitaxy. At low temperatures about 73% of the area under investigation have been epitaxially thickened by a 400nm ECRCVD grown film because of the preferential (100) orientation of the seed layer. First poly-Si thin-film solar cells have been prepared at low and high temperatures. The best solar cell so far has reached an open circuit voltage of 428mV and an efficiency of 4.2%.