Aluminum-induced Layer Exchange Research Articles

Fabrication of polycrystalline silicon thin films from a-SiOx via the inverted aluminum-induced layer exchange process

A novel fabrication method of polycrystalline silicon (poly-Si) films by aluminum-induced crystallization of amorphous silicon suboxide (a-SiO0.2) material via the inverted aluminum-induced layer exchange (ALILE) mechanism on glass substrates is presented. The presence of oxygen in the system prevented the formation of columnar crystalline Si precipitates (hillocks) in the bottom layer of the structure even at a process temperature of 550 °C. At the same time, this layer, consisting of Al, Si, and O atoms, contained nanocrystalline Si inclusions and a layer of initial a-SiOx with a thickness of about 60 nm, which was called the “a-SiOx residual layer”. The poly-Si formed had a preferred (1 1 1) crystal orientation and an average crystallite size of 4.5 µm.

Single grain growth of Si thin film on insulating substrate by limited region aluminum induced crystallization

Polycrystalline Si thin films on insulating substrates have been widely used to fabricate thin film transistors. However, since randomly induced grain boundaries in the polycrystalline thin films have been degrading transistor performance, application of thin film transistors has been limited to low speed processing units such as display drivers. Here, in this letter, we have tried to realize single grain growth of Si thin film at controlled position. Aluminum induced layer exchange crystallization of Si thin film was performed at limited region to suppress spontaneous nucleation during the crystal growth, and thus, single grain Si thin films have been successfully realized on quartz glass substrates. This technique will open up a way to realize high speed processing thin film transistors.

Nano-crystalline thin films fabricated by Si-Al co-sputtering and metal induced crystallization for photovoltaic applications

Fabrication of nano-crystalline thin film solar cell by co-sputtering of silicon and aluminum, and metal induced crystallization is reported in this paper. After a brief introduction on magnetron sputtering and aluminum-induced crystallization techniques, it discusses the drawbacks of “aluminum induced layer exchange” concept as the common method for fabricating poly-Si thin films; and proposes Si-Al co-sputtering as a simple alternative fabrication method. Then a three-steps process is introduced for the fabrication of “Al electrode/n-Si wafer/p-Si thin film/Al electrode” solar cell. Material and electrical characterizations of the fabricated p-Si thin films are done by Raman spectroscopy, X-ray diffraction, optical microscopy, scanning electron microscopy and Hall Effect measurement to study the crystallization ratio, grain size, morphology, carrier density and mobility of the prepared films, respectively. The best fabricated samples show a crystallization ratio of 88% and the largest grain size of 68 nm. The hole concentration in the films is in the order of 1018 to 1019 cm−3 with a maximum mobility value of 17 cm2/V s. Then, photovoltaic properties of the fabricated solar cells are determined by under dark and illumination J-V curves, and external quantum efficiency measurements. Dark J-V curve of the best sample represents an ideality factor of 1.39, rectification ratio of 1993 at ±1 V and reverse saturation current density of 62.1 nA/cm2 which confirms the formation of a good p-n junction. The illuminated J-V curve under AM1.5G conditions exhibits a photovoltaic conversion efficiency of 6.02% for the best sample, with a short circuit current density of 24.9 mA/cm2 and open circuit voltage of 456 mV. The external quantum efficiency of this sample reaches to more than 59% at the wavelength of 700 nm. Finally, a simple model is proposed to describe the formation of nano-crystalline Si thin film, deposited by Si-Al co-sputtering, during the thermal annealing.

Crystallization of silicon–germanium by aluminum-induced layer exchange

We have studied the crystallization of amorphous silicon–germanium (a-SiGe) by aluminum (Al)-induced layer exchange (ALILE) with a starting structure of glass/Al/Al oxide/a-SiGe. We examined ALILE at 450 °C, which is slightly higher than the eutectic temperature of Ge and Al, in order to shorten the ALILE time. We successfully produced c-SiGe films oriented in the (111) direction for 16 h without significant alloying. The thickness of Al layers should be 2800 Å or more to complete the ALILE for the a-SiGe layers of 2000–2800 Å thickness. When the Al layer is as thick as the a-SiGe layer, almost uniform c-SiGe is formed on the glass substrate. On the other hand, the islands of c-SiGe are formed on the glass substrate when the Al layer is thicker than the a-SiGe layer. The islands become smaller with thicker Al layers because more excess Al remains between the SiGe islands. The results indicate that the configuration of c-SiGe can be altered from a uniform structure to island structures of various sizes by changing the ratio of a-SiGe thickness to Al thickness.

In Situ Optical Monitoring of New Pathways in the Metal-Induced Crystallization of Amorphous Ge

We use high-resolution optical microscopy to characterize in situ the processes at play during the Al-induced crystallization of amorphous Ge. In addition to the well-established aluminum-induced layer exchange (ALILE) process, we demonstrate the existence of another crystallization mechanism with different kinetics and spatial extension using in situ monitoring. Further, ex situ characterizations show that both processes are active in our samples. The ALILE process is found to create a single Ge layer and 111-oriented crystallites in our growth conditions, while the other crystallization process yields a double Ge layer with mixed 111 and 100 orientations in the bottom layer, while the top layer remains amorphous. This work underlines the importance of in situ monitoring for the understanding and modeling of metal-induced crystallization.

Influence of surface roughness of ZnO layer on the growth of polycrystalline Si layer via aluminum-induced layer exchange process

Influence of surface roughness of ZnO layer on the growth of polycrystalline Si layer via aluminum-induced layer exchange process

Role of Al2O3 intermediate layer for improving the quality of polycrystalline-silicon film in inverted aluminum-induced layer exchange

A thin Al2O3 intermediate layer prepared by atomic layer deposition was introduced into inverted aluminum-induced layer exchange (inverted-ALILE) to form high-quality polycrystalline silicon (poly-Si) thin layer. It was demonstrated that the continuity and quality of poly-Si were obviously improved by the Al2O3 layer. The fraction of (100)-oriented crystals reached 93%, and the average grain size of 28μm with uniform surface morphology and low defect density were achieved at the optimal Al2O3 thickness of 4nm. It was also found that an a-AlOx layer always existed at the poly-Si/Al interface after inverted-ALILE process, which is independent on the original surface states. The results suggested that the thin poly-Si layer would be a promising epitaxial template for Si based thin film solar cells.

Impurity-driven multilayer formation in inverted aluminum-induced layer exchange of silicon

We report on the multilayer formation in inverted aluminum-induced layer exchange of silicon for samples with higher oxygen content in amorphous silicon. We find that the aluminum layer after the exchange contains silicon precipitates with a morphology that differed depending on oxygen contamination. The sample with the lower oxygen content shows columnar silicon precipitates, whereas the most contaminated sample shows a layered structure of Al/Si/Al. These results are interpreted in terms of aluminum and silicon diffusion influenced by the presence of oxygen, and a two-step layer exchange model is proposed for the formation of the layered structure.

Aluminum-induced crystallization for thin-film polycrystalline silicon solar cells: Achievements and perspective

Thin-film polycrystalline silicon (pc-Si) solar cell technology tries to combine the cost benefit of thin-film technology and the quality potential of crystalline Si technology. For this type of solar cells the challenge is to fabricate high quality coarse-grained (grain size of 1–100 µm) layers on non-silicon substrates in combination with superb light management. One possible material fabrication approach is metal induced crystallization (MIC). Many metals lower the crystallization temperature of a-Si but for photovoltaic (PV) solar cell applications aluminum seems to be the most interesting. Approximately a decade after the suggestion of Nast et al. to use aluminum induced layer exchange for thin-film solar cells, this paper will discuss the aluminum induced crystallization process itself as well as the implementation into workable solar cells. The record aluminum induced crystallization (AIC) solar cell today features an energy conversion efficiency of 8.5%, too low to be competitive with other PV technologies. The main problems and limitations as encountered today will be described. We believe that an energy conversion efficiency of 8.5% is not the limit of AIC solar cells. Different possible solutions to improve the AIC process as well as the resulting solar cells will be discussed together with our vision on the direction and future of AIC solar cell research.

Preparation of polycrystalline silicon thin film for solar cells on glass by aluminum-induced layer exchange

Polycrystalline silicon films were prepared by aluminum-induced layer exchange (ALILE). The films deposited by vacuum evaporation were initially amorphous and then transformed into polycrystalline state in the process of ALILE. The mechanism of ALILE was given briefly in the paper. The morphology of the samples was studied using AFM. It was found that substrate distances significantly affect the morphology of the samples and the optimum substrate distance was around 90mm. The crystalline quality of the films was examined using Raman spectroscopy, and it turned out that substrate temperature and annealing temperature were critical parameters for the crystalline volume fraction of the films. The crystalline volume fraction reached 89% at substrate temperature of 450°C and annealing temperature of 500°C.

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.

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.

Growth of polycrystalline silicon on glass for thin-film solar cells

Polycrystalline Si (poly-Si) thin-film solar cells on glass feature the potential to reach high single-junction efficiencies at low costs. However, the preparation is challenging because the process temperatures are limited by the glass to about 600 °C. There are several methods to prepare poly-Si films on glass. So far, the best poly-Si thin-film solar cells on glass have been prepared by solid phase crystallization (SPC) of amorphous Si (a-Si). In this paper, we give an overview on the formation of poly-Si films by both SPC and the aluminum-induced layer exchange (ALILE) process (which is based on aluminum-induced crystallization (AIC) of a-Si). Due to the fact that the utilization of ZnO:Al-coated glass is an attractive option for future poly-Si thin-film solar cells, we discuss the influence of an additional ZnO:Al layer on the formation of poly-Si films. Such an additional ZnO:Al layer leads to enhanced nucleation. Thus, the time necessary to form the poly-Si film (process time) and the resulting grain size of the poly-Si film are reduced.

Poly-Si films with low aluminum dopant containing by aluminum-induced crystallization

Typically, highly p-doped (2×1018 cm−3) poly-Si films fabricated by the aluminum induced layer exchange (ALILE) process are not suitable for solar cell absorber layers. In this paper, the fabrication of high-quality, continuous polycrystalline silicon (poly-Si) films with lower doping concentrations (2×1016 cm−3) using aluminum-induced crystallization (AIC) is reported. Secondary-ion-mass spectroscopy (SIMS) results showed that annealing at different temperature profiles leads to a variety of Al concentrations. Hall Effect measurements revealed that Al dopant concentration depends on the annealing temperature and temperature profile. Raman spectral analysis indicated that samples prepared via AIC contain some regions with small grains.

Silver-induced layer exchange for the low-temperature preparation of intrinsic polycrystalline silicon films

The preparation of large grained continuous polycrystalline silicon layers by metal-induced crystallization is reported. The macroscopic layer exchange of an amorphous silicon precursor layer in contact with a silver layer was observed for temperatures below the softening point of glass. This process is quite similar to the well-known aluminum-induced layer exchange. However, due to the use of silver as a catalyst, the recrystallized layers are electrically intrinsic rather than highly doped with Al acceptors. The resulting polycrystalline silicon layers show a good crystalline quality as deduced from Raman scattering, x-ray diffraction, and UV-reflectance measurements.

Hydrogen passivation of ultra-thin low-temperature polycrystalline silicon films for electronic applications

We have investigated the influence of hydrogen passivation on the electronic properties of ultra-thin polycrystalline silicon layers prepared by the aluminum-induced layer exchange process. Hall effect measurements reveal high hole carrier concentrations in the as-grown poly-Si layers up to several times 1019cm−3. We find a drastic increase of the resistivity after hydrogenation for very thin samples, which is attributed to a combination of two effects: (1) the reduction of free holes due to acceptor passivation and (2) compensation of free holes remaining after H-passivation by interface trap states. Temperature-dependent measurements show that the activation energy of the dark conductivity increases strongly after the hydrogenation process. The origin of the compensation was investigated by spin-dependent transport measurements. In addition, the potential of the passivated poly-Si layers for electronic applications was studied. We have demonstrated a normally ON back-gate depletion mode transistor with a two mask process, which exhibits a field-effect mobility of 21cm2/Vs for holes in the 20nm thin channel layer.

Ultra-thin polycrystalline Si layers on glass prepared by aluminum-induced layer exchange

In this work, we present studies of ultra-thin polycrystalline silicon layers (5–100nm) prepared by the aluminum-induced layer exchange process. Here, a substrate/Al/oxide/amorphous Si layer stack is annealed at temperatures below the eutectic temperature of the Al/Si system of 577°C, leading to a layer exchange and the crystallization of the amorphous Si. We have studied the process dynamics and grain growth, as well as structural properties of the obtained polycrystalline Si thin films. Furthermore, we derive a theoretical estimate of the grain density and examine characteristic thermal activation energies of the process. The structural properties have been investigated by Raman spectroscopy. A good crystalline quality down to a layer thickness of 10nm has been observed.

Mechanism of aluminum-induced layer exchange upon low-temperature annealing of amorphous Si/polycrystalline Al bilayers

The aluminum-induced layer exchange (ALILE) process occurring upon annealing amorphous Si/polycrystalline Al bilayers (a-Si/c-Al) has been observed at a temperature as low as 165 °C. The diffusion length of Si along Al grain boundaries is proposed as a tool for determining the annealing conditions, i.e., temperature and time, for the occurrence of the c-Al→c-Si layer exchange. Analysis of the local and global energy changes upon layer exchange reveals that a tiny driving force controls the kinetics of layer exchange and leads to a general interpretation of the mechanism of the ALILE process.