Layer Exchange Process Research Articles

Al-induced crystallization of amorphous [formula omitted] (0 [formula omitted] x [formula omitted] 1): Diffusion, phase development and layer exchange

Al-induced crystallization (AIC) of amorphous SixGe1−x (a-SixGe1−x) solid solutions, with compositions over the entire range of the isomorphous Si–Ge system, was investigated. The crystallization progress was monitored in detail, at low temperatures from 150 to 400°C, by in situ X-ray diffraction while gradually increasing the annealing temperature until crystallization was completed, allowing for an evaluation of the crystallization kinetics. The formation of single-phase crystalline SixGe1−x solid solutions upon AIC of a-SixGe1−x (0⩽x⩽1) was observed at all annealing temperatures. Concentration–depth profiling by Auger electron spectroscopy provided a detailed insight into the diffusion processes occurring in the early stage of the low-temperature AIC process and the intriguing phenomenon of layer exchange of the Al and a-SixGe1−x sublayers. On the basis of evaluation of the Si and Ge diffusion coefficients, the results suggest that, upon AIC, both Si and Ge atoms diffuse at comparable rates into the Al layer in the low temperature range investigated. The degree of completeness of the layer-exchange process after full crystallization is a function of the a-SixGe1−x composition; complete layer exchange occurs for all Al/a-SixGe1−x specimens containing more than 27at.% Si.

Coordination Reactions and Layer Exchange Processes at a Buried Metal–Organic Interface

The reactive interface between phthalocyanine (2HPc) and a Cu(111) surface was investigated with X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption/reaction (TPD/TPR). 2HPc reacts with Cu(111) to form copper(II) phthalocyanine (CuPc). For 2HPc submonolayers, the reaction starts already below 240 K. Thin 2HPc multilayers (4 monolayers) are completely converted into CuPc at elevated temperatures (500 K). To understand how molecules can react even though they are initially not in contact with the Cu surface, TPD/TPR studies with a NiPc interlayer between Cu and 2HPc were performed. These studies reveal the operation of an exchange mechanism, by which CuPc or NiPc molecules from the first layer are replaced by 2HPc molecules from higher layers. Once a 2HPc molecule comes into contact with the Cu surface, the molecule can react with the surface, forming CuPc, which can then be replaced by another unreacted 2HPc molecule. These findings have important implications for metal–organic int...

Influence of the Surface Roughness of a ZnO Layer on the Growth of a Polycrystalline Si-layer by Using an Aluminum-induced Layer-exchange Process

Influence of the Surface Roughness of a ZnO Layer on the Growth of a Polycrystalline Si-layer by Using an Aluminum-induced Layer-exchange Process

Effect of Ge/Al thickness on Al‐induced crystallization of amorphous Ge layers on glass substrates

AbstractLow‐temperature (350 °C) crystallization of amorphous Ge films on SiO2 was investigated using Al‐induced layer exchange (ALILE) process. Thicknesses of Ge and catalytic Al layers were varied in the range of 30‐300 nm, which strongly influenced the ALILE growth morphology. Based on the study, the Ge thickness was adjusted to 40 nm while the Al thickness was adjusted 50 nm. This sample satisfied both of the surface coverage of polycrystalline‐Ge and the annihilation of randomly oriented Ge regions. Moreover, the enhancement of the heterogeneous Ge nucleation improved the (111) orientation and the grain size. As a result, the area fraction of the (111)‐orientation reached as high as 97% and the average grain size as large as 70‐μm diameters. This (111)‐oriented Ge layer with large‐grains promises to be the high‐quality epitaxial template for various functional materials to achieve next‐generation devices. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Double-Layered Ge Thin Films on Insulators Formed by an Al-Induced Layer-Exchange Process

A large-grained (111)-oriented Ge thin film is achieved on a SiO2 glass substrate using an Al-induced crystallization (AIC) technique. Low-temperature (350 °C) AIC of an amorphous Ge thin film (50 nm thickness) resulted in a self-organized double-layered structure of polycrystalline Ge layers. The top Ge layer consisted of randomly oriented small grains (1 μm diameter) with a high defect density; in contrast, the bottom Ge layer was of high quality. Based on the growth model, we annihilated the top Ge layer using preferential etching. The bared bottom Ge layer provided a (111)-oriented area fraction of 99% and an average grain size of 90 μm. This Ge layer appears promising for use in Ge-based thin-film devices, as well as a heteroepitaxial seed layer for group III–V compound semiconductors, aligned nanowires, and other advanced materials.

Aluminum-Induced Crystallization of p<sup>+</sup> Silicon Pinholes for the Formation of Rear Passivation Contact in Solar Cell

Formation of nano-crystalline p+ silicon (Si) in pinholes through a silicon dioxide layer was achieved by pinning of aluminum through the thin silicon dioxide (SiO2) layer. In addition to opening holes of SiO2 layer by aluminum (Al) pining, amorphous silicon (a-Si) was subsequent deposited on the Al layer and another heated at low temperature (500°C) to allow solid- phase epitaxial growth of p+ Si in the pinholes due to the Al induced layer exchange process. The poly-crystalline p+ Si obtains lower effective surface recombination than the Al back surface field (BSF). The technique demonstrated to result in ohmic contacts with low contact resistance. The evaluation of Al-induced crystallization of a-Si in a-Si/Al bilayer was studied by X-ray diffraction. In this paper, the influence of a-Si/Al thickness ratio on the specific conductivity value and crystalline grain size of the p+ Si thin film is discussed. The obtained results are helpful for a further design of the rear passivation contact in solar cell.

Large-Grained Polycrystalline (111) Ge Films on Insulators by Thickness-Controlled Al-Induced Crystallization

Low-temperature (350°C) crystallization of amorphous Ge films on SiO2 was investigated using Al-induced layer exchange (ALILE) process. Thicknesses of Ge and catalytic Al layers were varied in the range of 30–300 nm, which strongly influenced the ALILE growth morphology. Based on the study, the Ge thickness was adjusted to 40 nm while the Al thickness was adjusted 50 nm. This sample satisfied both of the surface coverage of polycrystalline-Ge and the annihilation of randomly oriented Ge regions. Moreover, the enhancement of the heterogeneous Ge nucleation improved the (111) orientation and the grain size. As a result, the area fraction of the (111)-orientation reached as high as 97% and the average grain size as large as 70-μm diameters. This (111)-oriented Ge layer with large-grains promises to be the high-quality epitaxial template for various functional materials to achieve next-generation devices.

Fabrication of large-grained thin polycrystalline silicon films on foreign substrates by titanium-assisted metal-induced layer exchange

Metal-induced layer exchange (MILE) is a well-known method to grow large-grained high quality polycrystalline silicon on foreign substrates. We have modified the commonly used layer stack by an additional titanium interfacial layer (substrate/metal/titanium/oxide/amorphous silicon). The resulting layer exchange process is called titanium-assisted metal-induced layer exchange (Ti.MILE). For the investigated metals, Al (Ti.ALILE) and Ag (Ti.AgILE), the additional Ti layer does not affect the overall layer exchange process but results in a strong enlargement of the grains in the resulting polycrystalline silicon layer up to 250 μm. We have investigated the influence of the titanium interfacial layer on the process dynamics and grain growth. Furthermore, the structural and optical properties of the resulting polycrystalline silicon layer are investigated by means of different analysis methods.

Polycrystalline silicon films prepared by metal-induced crystallisation using pre- and post-deposited aluminium on amorphous silicon

We report on the successful fabrication of polycrystalline silicon films by aluminium-induced crystallisation (AIC) of Radio frequency (rf) plasma-enhanced chemical vapour deposited (PECVD) a-Si films. The effects of annealing at different temperatures (300 and 400°C), below the eutectic temperature of the Si–Al binary system, on the crystallisation process have been studied. This work emphasises the important role of the position of the Al layer with respect to the Si layer on the crystallisation process. The properties of the crystallised films were characterised using X-ray diffraction, Raman spectroscopy, ellipsometry, field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). With an increase in the annealing temperature, it was found that the degree of crystallisation of annealed a-Si/Al and Al/a-Si films increased. The results showed that the arrangement where the Al was on top of the a-Si had a more prominent effect on crystallisation enhancement than when Al was below the a-Si. The interfacial layer between the Al and a-Si film is crucial because it influences the layer-exchange process during annealing. The oxide layer formed between the Al and the a-Si layers greatly retards the crystallisation process in the case of the Al/Si arrangement. Our investigations suggest that polycrystalline Si films formed by AIC can be used as a seed layer in solar cell fabrication.

On the growth mechanism of polycrystalline silicon thin film by Al-induced layer exchange process

We attempted to clarify controlling mechanisms of Al-induced layer exchange process of Al and amorphous silicon (a-Si) and microstructures in resultant polycrystalline silicon (poly-Si) thin film by utilizing in situ observation of the growth process. Introduction of a few nm-thick germanium adlayer remarkably reduces crystallization time and affects the grain size of poly-Si films. This is likely to be accompanied by the growth mode transition as suggested by change of Avrami constant. Control of the Al/a-Si interface is of crucial importance to control the growth process.

Back Cover: Epitaxial upward transport of Al at the beginning of the Al-induced layer exchange process (Phys. Status Solidi RRL 5-6/2011)

The Rapid Research Letter by Birajdar et al. on pp. 172–174 reports on the Al transport during the early stage of Al-induced layer exchange (ALILE) and crystallization reaction, a promising process for the fabrication of polycrystalline Si films to be used in thin film transistors and solar cells. The authors report that not only vertical but also lateral redistribution of Al occurs during the ALILE and crystallization reaction. This yields Al deficient dendritic cell centers surrounded by an about 10 μm wide Al excess zone containing epitaxial islands of “pushed-up” Al whose number density and size decrease with increasing distance from the reaction front as illustrated in the Al elemental map acquired in the transmission electron microscope. The corresponding sketches in the cross-section geometry are shown on the right.

Epitaxial upward transport of Al at the beginning of the Al‐induced layer exchange process

AbstractAl‐induced layer exchange (ALILE) and crystallization in a stack of amorphous Si (a‐Si, 100 nm)/Al (50 nm)/quartz substrate, annealed at 450 °C, have been investigated at different length scales by optical microscopy as well as analytical scanning and transmission electron microscopy. Significant lateral and vertical redistribution of Al was observed, yielding Al deficient dendritic cell centers surrounded by an about 10 µm wide Al excess zone containing epitaxial islands of “pushed‐up” Al whose number density and size decreases with increasing distance from the cell boundary. This new finding is likely to be a key step forward in the understanding of the complex redistribution of Al and Si during the ALILE process. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Metal induced crystallization of amorphous silicon for photovoltaic solar cells

A silicon thin-film technology could lead to less expensive modules by the use of less silicon material and by the implementation of monolithic module processes. A technology based on polycrystalline-silicon thin-films with a grain size between 1 μm and 1 mm (pc-Si), seems particularly promising since it combines the low-cost potential of a thin-film technology with the high efficiency potential of crystalline silicon. One of the possible approaches to fabricate pc-Si absorber layers is metal induced crystallization (MIC). For solar cell applications mainly aluminium is investigated as metal because 1) it forms a eutectic system with silicon instead of a silicide-metal system like e.g. Ni 2) only shallow level defects are formed in the forbidden bandgap of silicon and 3) a layer exchange process can be obtained in combination with a-Si. Aluminum induced crystallization (AIC) of a-Si on non-silicon substrates can results in grains with a preferential (100) orientation and a maximum grain sizes above 50 micrometer. These layers can act as seed layers for further epitaxial growth. Based on this two-step approach (AIC + epitaxial growth) we made solar cells with an energy conversion efficiency of 8%. Based on TEM, EBIC, SEM, defect etch and EBSD measurements we showed that the efficiency is nowadays mainly limited by the presence of electrical intragrain defects.

Au-Induced Low-Temperature (∼250°C) Crystallization of Si on Insulator Through Layer-Exchange Process

Au-Induced Low-Temperature (∼250°C) Crystallization of Si on Insulator Through Layer-Exchange Process

Study on amorphous silicon thin film by aluminum-induced crystallization

With the rapid development photovoltaic industry, crystallization techniques play an important role in silicon thin film solar cells. In this paper, Amorphous silicon thin film and aluminum films were prepared by electron beam evaporation technique. The films after different annealing crystallization processes were characterized by means of X-ray diffraction, Raman spectra and scanning electron microscopy. We have the emphasis on discussing the influence parameters for the amorphous silicon crystallization quality and growth mechanism. In addition, Al/Si interface was also studied. A layer exchange process occurs between Al layer and Si layer.

다결정 실리콘 태양전지 제조를 위한 비정절 실리콘의 알루미늄 유도 결정화 공정 및 결정특성 연구

Polycrystalline silicon (pc-Si) films are fabricated and characterized for application to pc-Si thin film solar cells as a seed layer. The amorphous silicon films are crystallized by the aluminum-induced layer exchange (ALILE) process with a structure of glass/Al//a-Si using various thicknesses of layers. In order to investigate the effects of the oxide layer on the crystallization of the amorphous silicon films, such as the crystalline film detects and the crystal grain size, the layer thickness arc varied from native oxide to 50 nm. As the results, the defects of the poly crystalline films are increased with the increase of layer thickness, whereas the grain size and crystallinity are decreased. In this experiments, obtained the average pc-Si sub-grain size was about at relatively thin layer thickness ( 16 nm). The preferential orientation of pc-Si sub-grain was .

Two-step crystallization during the reverse aluminum-induced layer exchange process

In this work, the reverse aluminum-induced layer exchange (R-ALILE) process with an initial layer stack of substrate/amorphous Si/Si-oxide/Al was studied in detail. The influence of the annealing temperature on the sample properties was investigated by optical reflection/transmission measurements and Raman spectroscopy. In addition, focused ion beam measurements were conducted to elucidate the inner structure of the layers. Two steps during crystallization were observed: at first a substrate/Al–Si composite/closed poly-Si layer structure is formed with an activation energy EApoly-Si=1.1 eV, which can be transferred to the stable configuration of substrate/Al+Si-islands (hillocks)/poly-Si by extended annealing or a high temperature step (EAhillocks=2.4 eV). Both processes are basically independent at low annealing temperatures due to the large difference in activation energy. The transformation of the Al–Si composite to the Al/Si-hillock structure involves the crystallization of a-Si regions and their subsequent coalescence, different to the feedback mechanism suggested for the normal ALILE process, where hillock and closed poly-Si growth are believed to influence each other. This insight into the process leads to the possibility to prepare poly-Si layers on pure Al back contacts by R-ALILE, possibly improving efficiencies of solar cells prepared by epitaxial overgrowth of poly-Si seed layers.

Thin Film Solar Cells Prepared on Low Thermal Budget Polycrystalline Silicon Seed Layers

In this work, we present data from solar cells with Si grown by plasma-enhanced chemical vapour deposition as the absorber material prepared on polycrystalline silicon seed layers. For the seed layer preparation, the reverse aluminum-induced layer exchange (R-ALILE) process is used. In contrast to the conventional ALILE process, the R-ALILE results in a smooth top surface of the polycrystalline silicon and to the automatic formation of an Al-back contact, which both are beneficial for solar cell preparation. We found that the proper treatment of the seed layers prior to the absorber layer deposition is crucial for a good solar cell performance. Here, we investigated different wet chemical methods (HF-solution, Al-etch) and the influence of an H2-plasma treatment. Furthermore, we studied the influence of an additional Ag/indium tin oxide (ITO)-back contact on the solar cell performance. We find that solar cell efficiencies over 5% can be obtained using the presented seed layer concept. The results obtained in this work can help to improve the epitaxial overgrowth of seed layers.

Mechanism of Aluminum Induced Lateral Crystallization of Amorphous Silicon

The aluminium-induced lateral crystallization (AILC) of amorphous silicon (a-Si) on a glass substrate has been investigated. By means of a photoresist-based process, Al islands (100 nm) were thermally evaporated using 15 V, 3.5 A, and 25 V, 5.6 A on the a-Si layer (100 nm), which was deposited on a glass substrate. SEM examinations indicated that the Al islands exhibited smooth and crystalline-grain morphology. Annealing processes were carried out at 748 and 823 K for various times. After annealing, AILC could be clearly observed in the crystalline-grain sample, but not in the smooth sample. TEM analyses showed that the mechanism of AILC resulted from two layer exchange processes. First, the Al islands exchanged with the underlying a-Si layer vertically during AIC, and then the generation of Al particles accompanying AIC caused a lateral layer exchange with the remaining a-Si layer with further annealing.

A kinetic simulation study of the mechanisms of aluminum induced layer exchange process

The aluminum induced layer exchange (ALILE) process allows the formation of thin polycrystalline Si (poly-Si) layers of large grain size on foreign substrates such as glass at low process temperatures. This paper is devoted to a computer simulation study of the kinetics of the ALILE process taking into account the mechanisms of its separate stages: Si diffusion in the AlOx membrane, nucleation and growth of grains, and the formation of preferential (100) orientation. The characteristics of the ALILE process are explained based on the evolution of the Si concentration within the Al layer. In particular it is demonstrated that the characteristic suppression of nucleation after short annealing times results from a decrease in the Si concentration in the Al layer due to the growth of existing grains. A number of important parameters of ALILE process are estimated comparing the results of simulation to the experimental data.