M. Muske

Aluminum-induced crystallization of amorphous silicon

We investigated the aluminum-induced crystallization of amorphous silicon (a-Si) during the aluminum-induced layer exchange (ALILE) process, in which a stack of glass/Al/a-Si is transformed into a glass/polycrystalline silicon (poly-Si)/Al(Si) structure by an annealing step well below the eutectic temperature of the Al/Si system. Our experiments resulted in continuous large-grained poly-Si films on glass substrates. The nucleation and the growth of the crystalline phase during the ALILE process was observed using an optical microscope. We found an activation energy of 1.8 eV for the nucleation process and we related this energy to a large barrier at the a-Si/Al interface.

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.

Temperature stability of ZnO:Al film properties for poly-Si thin-film devices

The crystallization of thin silicon films at temperatures between 425 and 600°C was investigated on glass substrates coated with Al-doped zinc oxide (ZnO:Al). Bare ZnO:Al layers degrade at the crystallization temperatures used. A silicon layer on top, however, efficiently prevents deterioration. The resistivity was even found to drop from 4.3×10−4Ωcm for the as deposited ZnO:Al to 2.2×10−4Ωcm in the case of aluminium induced crystallization and to 3.4×10−4Ωcm for solid phase crystallization. The temperature-stable conductivity of ZnO:Al films coated with Si opens up appealing options for the production of polycrystalline silicon thin-film solar cells with transparent front contacts.

Depletion regions in the aluminum-induced layer exchange process crystallizing amorphous Si

Annealing of aluminum/amorphous silicon bilayers below the eutectic temperature of the aluminum/silicon system leads to an exchange of the layer positions and a concurrent crystallization of silicon (aluminum-induced layer exchange). This letter discusses a model for the self-limited suppression of nucleation during the process. This characteristic feature is the reason why large grain sizes can be obtained. In our experiments, we combine nucleation caused by supersaturation and undercooling. Si depletion regions around existing grains are made visible. These experiments give direct proof of the idea that the suppression of nucleation occurs by Si depletion in the aluminum-induced layer exchange process.

Polycrystalline silicon on glass by aluminum-induced crystallization

We prepared thin polycrystalline silicon (poly-Si) films on glass by an aluminum-induced layer exchange (ALILE) process which is based on the aluminum-induced crystallization (AlC) of amorphous silicon (a-Si). During the ALILE process a glass/Al/a-Si stack is transformed into a glass/poly-Si/Al+Si structure. We investigated both the growth of the poly-Si layer and the final Al+Si layer on top of the poly-Si layer. Furthermore, we carried out the ALILE process on large glass substrates and on metal-coated glass substrates.

Large-grained polycrystalline silicon thin-film solar cells using AIC seed layers

Large-grained polycrystalline silicon (poly-Si) films were prepared on foreign substrates by epitaxial thickening of seed layers. The seed layers were formed by the aluminum-induced layer exchange (ALILE) process which is based on aluminum-induced crystallization (AIC) of amorphous silicon (a-Si). The epitaxial thickening was carried out at two different temperature regimes (low- and high-temperature approach). Using these large-grained poly-Si films first thin-film solar cells have been prepared. The best poly-Si thin-film solar cell obtained so far has reached an efficiency of 4.5% (high-temperature approach).

Suppression of nucleation during the aluminum-induced layer exchange process

ABSTRACT Formation of polycrystalline silicon (poly-Si) thin films on inexpensive glass substrates is of great interest for large area electronic devices. Large grain sizes are desirable to reduce grain boundary effects. In the aluminum-induced layer exchange process Al/a-Si bi-layers exchange their positions with a concurrent crystallization of the amorphous Si (a-Si) in a simple annealing step. The process is characterized by the self regulated suppression of nucleation by existing grains resulting in large grain sizes above 10 µm. This paper deals with the mechanism responsible for this nucleation suppression. INTRODUCTION Polycrystalline silicon (poly-Si) thin films on inexpensive foreign substrates like glass are of great interest for large area electronic devices such as displays, sensors and solar cells. The use of glass substrates limits process temperatures to below 600°C. Fabrication of large grained silicon films at these low temperatures remains a big challenge. Solid phase crystallization (SPC) of amorphous silicon (a-Si) layers is one of the options for obtaining poly-Si films at low temperatures, but it suffers from two mayor problems. Crystallization takes a long time and the grain sizes obtained are small (<1 µm). Metal-induced crystallization (MIC) of a-Si is known to reduce the crystallization time and lower the crystallization temperature [1]. The aluminum-induced layer exchange (ALILE) process has been demonstrated to lead to very large grain sizes [2]. The ALILE process was first discovered by Nast et al. [3]. In this process a glass/Al/a-Si layer stack is annealed at temperatures below the eutectic temperature of the Al/Si system (577°C). During annealing the Al and Si layer exchange their position with a concurrent crystallization of the silicon. It was found that a thin Al oxide layer is needed on top of the Al layer separating both layers throughout the process [4]. The basic model for the ALILE process derived by O. Nast is shown in Fig. 1. (1) Upon heating silicon diffuses from the a-Si into the Al layer across the Al oxide interface. (2) Silicon diffuses within the Al layer. (3) Silicon nuclei are formed within the Al layer and grow in all directions until confined within the Al layer between glass substrate and Al oxide layer. (4) After confinement the silicon grains grow laterally until neighboring grains form a continuous poly-Si film on the glass substrate. (5) Al is displaced and diffuses across the Al oxide interface into the initial a-Si layer. A characteristic feature of the ALILE process is the suppression of nucleation leading to grain diameters of well above 10 µm [2]. The purpose of this paper is to explain the origin of the nucleation suppression.

Aluminium-induced crystallisation of amorphous silicon: parameter variation for optimisation of the process

Thin large-grained polycrystalline silicon (poly-Si) films can be formed on glass by aluminium-induced crystallisation (AIC) of amorphous silicon (a-Si) in an annealing process below the eutectic temperature of the Al/Si system. The influence of increasing the temperature during the anneal as well as the modification of the initial aluminium layer was investigated. In both cases similar or even larger silicon grains were achieved while reducing the process time.