Anomalous Increase of Grain Size in Ni-Mediated Crystallization of Amorphous Silicon

Abstract

We studied the grain growth in Ni-mediated crystallization of amorphous silicon ͑a-Si͒ films with various thicknesses. The Ni particles with density of 3.39 ϫ 10 13 cm Ϫ2 was deposited on a-Si and this was annealed at 580°C for 15 min. The grain size of the crystallized poly-Si increases from 25 to 104 m as the thickness of a-Si increases from 20 to 300 nm. The grain size has a linear relationship with the a-Si thickness. This can be understood on the basis of the cylindrical seed formation and subsequent cylindrical, lateral grain growth. A cylindrical grain was found in atomic force microscope image of a partially crystallized a-Si after the Secco etch of surrounding a-Si phase.Polycrystalline silicon thin-film transistors ͑poly-Si TFTs͒ fabricated on glass are of increasing interest for use in active-matrix liquid crystal display ͑AMLCD͒ and active-matrix organic light emitting diode ͑AMOLED͒. Recently, reducing the process time and lowering the process temperature for the poly-Si TFTs have become the main issues to increasing the throughput of TFT array production and also for use of glass substrate. Among many crystallization methods exist for low-temperature poly-Si, Ni silicide-mediated crystallization ͑SMC͒ has become of increasing interest because it does not use the laser and the TFT performance with the SMC poly-Si is almost the same as that of a good poly-Si by excimer laser annealing. 1 Therefore, low-cost production of low-temperature poly-Si TFT is possible using SMC.When an ultrathin metal is deposited onto amorphous silicon ͑a-Si͒, the a-Si crystallizes at a lower temperature than its solid phase crystallization ͑SPC͒ temperature. Among many metals used for crystallization, Ni is used mainly because the resulting poly-Si has a good material property. 2-4 The precipitates of Ni disilicide, formed at Ͻ400°C, act as seeds for crystallization, which then leads to lowering crystallization temperature. 5 Hayzelden et al. demonstrated the grain growth from the isolated nickel disilicide precipitates in Ni-implanted a-Si. 6 The performance of a TFT can be improved by reducing the a-Si thickness in metal induced lateral crystallization ͑MILC͒. 7,8 However, the grain size is independent of the film thickness for the MILC poly-Si. 9 Note that the poly-Si thickness for TFTs is ϳ50 nm, which is widely used also in excimer laser crystallization of a-Si. There is no literature indicating the relationship between the a-Si thickness and grain size for SMC and MILC, but the performance of TFT made by MILC poly-Si depends on a-Si thickness. 7,8,10

Effect of Ni Thickness on Off-State Currents of Poly-Si TFTs Using Ni-Induced Lateral Crystallization of Amorphous Silicon

“…The disk-shaped grains meet with neighboring grains that were independently grown from the different nucleation sites, resulting in grain boundaries. 14 Figure 3 shows the transfer characteristics of the MILC poly-Si TFTs using various Ni area densities of 3.4 ϫ 10 13 cm −2 ͑a͒, 1.4 ϫ 10 14 cm −2 ͑b͒, 7.3 ϫ 10 14 cm −2 ͑c͒, and 9.2 ϫ 10 15 cm −2 ͑d͒ on a-Si. The channel width ͑W͒ and the length ͑L͒ of the TFTs were 8 and 8 m, respectively.…”

Formation of (001)-textured grain in (111) polycrystalline silicon film

“…Then, the (001) grains grow from the seeds and nucleation of (111) textured nuclei appears as can be seen in (b). The Ni diffuses from the center of grain to its grain boundary with increasing crystallization time because the growth model of Nimediated crystallization is related with the trace of NiSi 2 crystallites [20]. The grains of (111) and (001) orientations grow, but the speed of (111) grains is higher.…”

Growth of (100)-Oriented Polycrystalline Si Film by Ni-Mediated Crystallization of Thin Amorphous Silicon

“…It was found that the crystallization behavior was the same when we changed the bias polarity. 9,12 The structural properties of the poly-Si films were examined through scanning electron microscopy ͑SEM͒ and transmission electron microscopy ͑TEM͒ measurements. The crystallographic properties of the poly-Si films were examined by electron-beam backscattered diffraction ͑EBSD͒.…”