Temperature dependence of the growth of super-grain polycrystalline silicon by metal induced crystallization
Introduction
Polycrystalline silicon (poly-Si) on glass is a promising material for large-area electronics such as active-matrix liquid crystal displays, active-matrix organic light emitting diodes and solar cells. Common methods of crystallizing amorphous silicon (a-Si) are solid-phase crystallization (SPC), excimer laser annealing (ELA) and metal induced crystallization (MIC) [1]. The crystalline quality and crystallization temperature depend strongly on the crystallization method.
ELA of a-Si gives a good poly-Si on glass, where there is no ingrain defect, however, the grain size depends strongly on the laser intensity and its uniformity is still issue to be overcome. To have less energy sensitive and single-crystal like poly-Si on glass, silicon on glass, the sequential lateral solidification was proposed [2]. On the other hand, SPC poly-Si has ingrain defects and thus high temperature process is needed to reduce them [3]. To improve the material property of the SPC poly-Si, Si+ ion was implanted into the poly-Si to enlarge the grain size [4]. The in-grain defect and grain size were improved by the ELA of the SPC poly-Si, where large and disk-shaped grains of a preferable orientation of the {1 1 1} face or hexagonal grains have been demonstrated [5].
Recently, high-quality poly-Si on insulator by the metal induced lateral crystallization (MILC) has been demonstrated [6]. This needs a high temperature process to reduce the in-grain defects in the MILC poly-Si. The crystallization begins from one electrode and its speed is a few micrometers per hour at 500 °C.
To have large gains with uniform distribution and preferred orientation, a novel silicide mediated crystallization (SMC) poly-Si by rapid thermal annealing (RTA) of the a-Si has been proposed in this work. In the SMC, the a-Si is crystallized by the elapse of the NiSi2 precipitate that has a role of transforming a-Si to crystalline phase and this can be called the seed for crystallization or the nucleation site for crystallization. This means that the crystallization proceeds from the seed or nucleation site even though the role of the NiSi2 precipitates is different from the nucleation in the SPC of a-Si.
Section snippets
Experimental procedure
We deposited 50 nm thick a-Si:H film on glass by plasma enhanced chemical vapor deposition. Metal particles was scattered onto the a-Si:H by RF sputtering at a low power density. The samples were heated in a RTA system, where the rising, falling and keeping times were controlled. The first step is to heat the a-Si:H at 550 °C for 90 s for dehydrogenation. The hydrogen atoms in the a-Si:H are effused out and the film is changed to a hydrogen-free a-Si. The second step is heating the sample for
Results and discussion
Fig. 1 shows the optical microscopy images, showing the crystallization procedure. The Ni density of 4.6×1012 cm−2 on the a-Si was measured from SIMS depth profile. The a-Si was heated in a pulsed RTA at 700 °C for 10 s with various cycles of (a) 3; (b) 5; (c) 10; (d) 20. The growth of crystalline silicon can be seen from this figure. The Ni atoms on the a-Si aggregate together at first and then form NiSi2 precipitates [7], which can be the seeds for crystallization. From these nuclei, the
Conclusions
We have investigated the temperature dependence of the growth of super-grain poly-Si by MIC. From the XRD, TEM and optical microscopy, the recrystallization velocity of a-Si using Pulsed RTA is faster than that of RTA and FA. The number of NiSi2 precipitates required for inducing crystallization depends on the crystallization temperature. This result indicates that the grain size of the SMC poly-Si depends on the crystallization temperature. On the other hand, the XRD intensity decreases with
Acknowledgements
This work was supported by the National Research Lab Program of Korea.
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